international workshop on mechanics of energy materials...

Program and Book of Abstracts

International Workshop on

Mechanics of Energy Materials

(IWMEM 2017)

Edited by

Dr. Yixiang Gan (The University of Sydney, Australia)

Professor Changqing Chen (Tsinghua University, China)

Professor Weibang Lu (SINANO CAS, Suzhou, China)

Professor Kejie Zhao (Purdue University, USA)

8-11 November 2017,

The University of Sydney Centre in China, Suzhou

Industrial Park, Jiangsu, PR China.

International Workshop on Mechanics of Energy Materials (IWMEM 2017) 8-11 Nov 2017, The University of Sydney Centre in China, Suzhou, China.

_______________________________________________________________________________________________________________

2

Venue and Map Workshop Venue: The University of Sydney Centre in China (CiC USYD) Address Room 1202, Wisdom Centre Tower A, 10 Yueliangwan (Moon Bay) Road, Suzhou Industrial Park, Jiangsu, China. Phone: +86 512 6298 1900 中国江苏省苏州工业园区月亮湾路 10号慧湖大厦 A幢 1202室

Hotel: Suzhou Four Points by Sheraton

苏州福朋喜来登酒店，中国江苏省苏州工业园区月亮湾路 8号


_______________________________________________________________________________________________________________

3

Program (Day 1, Wednesday, 08 November 2017)

Registration and Reception (CiC USYD) 15:00 – 18:00

Dinner and Social Events (CiC USYD) 18:00 – 21:30

Program (Day 2, Thursday, 09 November 2017) Registration 08:30 – 09:00

Opening and Welcome 09:00 – 09:20

Keynote 1 (Chair: Professor Marc Kamlah) 09:20 – 09:50 (T01) Mohamed A. Abdou (UCLA) Environmentally Responsible Energy Security Future Requires Diversified Portfolio Approach 09:20 – 09:50

Session 1 Battery Materials I (Chair: Professor Marc Kamlah) 09:50 – 10:40 (T02) Bai-Xiang Xu (TU Darmstadt) Phase-field Modeling of Electro-chemo-mechanical Behavior of Lithium-ion Batteries 09:50 – 10:15

(T03) Narasimhan Swaminathan (IIT Madras) Molecular dynamics studies of primary damage in β-Li2TiO3 10:15 – 10:40

Morning Tea 10:40 – 11:00

Session 2 Battery Materials II (Chair: Professor Bai-Xiang Xu) 11:00 – 12:40 (T04) Oleg Birkholz (KIT) Modeling the effective electrical conductivity of electrode structures in lithium-ion batteries using DEM 11:00 – 11:25

(T05) Chris D Ling (USYD) Quasielastic Neutron Scattering Reveals Diffusion Pathways in Solid-state Oxide-ionic Conductors 11:25 – 11:50

(T06) Kejie Zhao (Purdue University) Mechanical and structural degradation of LiNixMnyCozO2 cathode in Li-ion batteries 11:50 – 12:15

(T07) Tao Zhang (KIT) A Nonlocal Species Concentration Theory for Diffusion and Phase Changes 12:15 – 12:40

Lunch 12:40 – 13:45 Keynote 2 (Chair: Professor Changqing Chen) 13:45 – 14:15 (T08) Huiling Duan (Peking University) Constitutive Equations Of Irradiated Metallic Materials 13:45 – 14:15

Session 3 Nuclear Materials I (Chair: Professor Changqing Chen) 14:15 – 15:30 (T09) Shurong Ding (Fudan University) The effects of internal pressure on the hydrogen-pickup-induced multi-field coupling behavior in the zircaloy cladding

14:15 – 14:40

(T10) Ratna Kumar Annabattula (IIT Madras) Modelling hydrogen embrittlement of steels using stress-diffusion coupling 14:40 – 15:05

(T11) Dalin Zhang (Xi’an Jiaotong) Thermal and mechanical analyses of the CLAM steel in supercritical-water cooled ceramic breeder blanket for CFETR

15:05 – 15:30

Afternoon Tea 15:30 – 16:00

Lab Tours (SINANO) 16:00 – 18:00

Dinner (Four Points) 19:00 – 22:00


_______________________________________________________________________________________________________________

4

Program (Day 3, Friday, 10 November 2017)

Keynote 3 (Chair: Professor Kejie Zhao) 09:00 – 09:30 (T12) Sulin Zhang (PSU, USA) Mechanics in Electrochemistry: From tiny to huge batteries; from energy storage to harvesting 09:00 – 09:30

Session 4 Energy Resources (Chair: Professor Kejie Zhao) 09:30 – 10:45 (T13) Jie Wang (ZJU, China) Strain-induced improper ferroelectricity in Ruddlesden–Popper perovskite halides for photovoltaic applications. 09:30 – 09:55

(T14) Qun Li (Xi’an Jiaotong) Microscopic study on the mechanism of damage and micro-crack propagation in the polysilicon solar cell 09:55 – 10:20

(T15) Abraham C.F. Chiu (Hohai University) Mechanical behavior of methane hydrate bearing coarse-grained sediment 10:20 – 10:45

Morning Tea 10:45 – 11:00

Session 5 Materials and Modelling I (Chair: Mingchao Liu) 11:00 – 12:40 (T16) Changqing Chen (Tsinghua University) Patterning Curved Three-dimensional Structures with Programmable Kirigami Designs 11:00 – 11:25

(T17) Kejie Zhao (Purdue University) Operando nanoindentation: A perfect platform to measure the mechanical properties of electrodes 11:25 – 11:50

(T18) Dorian Hanaor (TU Berlin, Germany) Harnessing polymorphic pyroxenes as geomimetic phase change materials 11:50 – 12:15

(T19) Dayu Zhou (DUT) Electrocaloric effects of silicon-doped hafnium oxide ferroelectric and anti-ferroeletcric thin films. 12:15 – 12:40

Lunch 12:40 – 13:45 Keynote 4 (Chair: Professor Huiling Duan) 13:45 – 14:15 (T20) Marc Kamlah (KIT) Discrete-Element-Modeling of the Thermomechanics of Ceramic Pebble Breeder Beds 13:45 – 14:15

Session 6 Nuclear Materials II (Chair: Professor Huiling Duan) 14:15 – 15:30 (T21) Baoping Gong (Southwestern Institute of Physics) Numerical investigation of the pebble bed packing structures for HCCB TBM 14:15 – 14:40

(T22) Songlin Liu (IPP, CAS) Numerical and Experimental Study of pebble bed flow characteristics for Ceramic Breeder blanket of CFETR 14:40 – 15:05

(T23) Cong Wang (IPP, CAS) Numerical and Experimental Study on Effective Thermal Conductivity of Binary Pebble Bed 15:05 – 15:30

Afternoon Tea 15:30 – 15:50

Session 7 Materials and Modelling II (Chair: Dr Dorian Hanaor) 15:50 – 17:30 (T24) Weijing Dai (USYD) Topological evolution of granular hard sphere beds packed in confined cylindrical containers under vibration 15:50 – 16:10

(T25) Mingchao Liu (Tsinghua University) Multiscale modeling of effective elastic properties of fluid-filled porous materials 16:10 – 16:30

(T26) Pengyu Huang (USYD) Modelling Multiphase Interactions Using Coarse-Grained Molecular Dynamics 16:30 – 16:50

(T27) Yanyao Bao (USYD) A smoothed particle hydrodynamics formulation for simulating dynamic behaviours of water within SOFCs 16:50 – 17:10

(T28) Chongpu Zhai (UNSW) The electrical contact at rough-to-rough spherical interface. 17:10 – 17:30

Closing and Socialisation (CiC USYD) 17:30 – 18:30


_______________________________________________________________________________________________________________

5

Program (Day 4, Saturday, 11 November 2017)

Gathering and Social Events (Four Points) 08:30 – 12:00

Lunch and Culture Activities (Pingjiang Road) 12:00 – 15:00


_______________________________________________________________________________________________________________

6

Sponsors

Suzhou BSE Air Conditioning Co., Ltd.

Global Mainstream Dynamic Energy Technology Ltd.

The University of Sydney China Study Centre and Centre in China


_______________________________________________________________________________________________________________

7

Environmentally Responsible Energy Security Future Requires

Diversified Portfolio Approach

Mohamed A. Abdou1, 2, *

1. Distinguished Professor and Director of the Center for Energy Science and Technology, UCLA

2. Founding President, Council of Energy Research and Education Leaders (CEREL), USA * Corresponding author. E-mail: [email protected]

The increase in world population, the rapid expansion of technological products, and the increase in per capita income have led to three important challenges facing humanity in the 21st century and beyond: water, energy, and the environment. These three challenges are strongly interrelated. The water problem can be solved if we have enough energy for desalination. The quality of the environment can be vastly improved if we develop sources to produce energy in an environmentally–responsible manner. We must determine how to satisfy the global population’s insatiable appetite for energy while reducing greenhouse-gas emissions and other debilitating environmental concerns. Reducing our dependence on fossil fuel from the current 80% of world energy production is an imperative that cannot be met by renewables alone. Solving the energy problem requires a diversified portfolio and pursuing several approaches:

§ Improve energy efficiency § Expand use of existing “clean” energy sources (e.g. nuclear and renewable

sources – solar, wind, etc.) § Develop technologies to reduce impact of fossil fuels use (e.g. carbon capture

and sequestration) § Develop major new (clean) energy sources (e.g. fusion)

§ Develop alternate (synthetic) fuels and electrical energy storage for transportation

This lecture will summarize the energy situation then address the options for solving the energy problem while favorably resolving the climate change issue. Current worldwide efforts on energy efficiency, renewable energy, nuclear technologies, and future fusion power will be summarized.


_______________________________________________________________________________________________________________

8

Phase-field Modeling of Electro-chemo-mechanical Behavior of

Lithium-ion Batteries

Bai-Xiang Xu1, *, Ying Zhao1, and Peter Stein1

1. Division Mechanics of Functional Materials, Department of Materials Science, Technical University of Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany

* Corresponding author. E-mail: [email protected]

Li-ion batteries are among the most important energy storage devices in our daily life and are the most attractive solution for the instability of renewable solar and wind energy. In pursuit of larger capacity, potential battery systems with new electrode materials are introduced. However, many of these electrodes experience irreversible mechanical degradation already after few charge/discharge cycles due to high stresses. These stresses arise from changes in lattice dimensions and crystal structures, which are associated with overall volume changes and phase separation. Phase separation often occurs during (de-)lithiation in the electrode particles made of materials such as crystalline silicon (Si), tin (Sn), antimony (Sb) and their oxides for anodes, as well as cathode material, i.e. LixFePO4 and LixMn2O4. In return, the chemically induced mechanical situation, such as elevated stress and fracture, can have an impact on the battery behavior on different levels, including active particle behavior, particle-particle interaction, particle-composite interaction, and eventually the cell performance. To understand electro-chemo-mechanical behavior of phase-separating materials, phase field simulations which couples Cahn–Hilliard-type diffusion model and large deformation mechanics are carried out for the active particles and electrode composites. Thereby the electrochemical reaction is modeled through a mechanically informed Butler–Volmer equation to account for the influence of the phase change and mechanical stresses on the reaction fronts. Moreover, phase-field fracture model is also employed to account for the crack prorogation during de-lithiation and thermal treatment of electrode materials. Furthermore, on the basis of the particle model, a modified single-particle battery cell model is employed to describe the cell performance under different discharging C-rates. For the numerical simulation, novel finite element methods— Isogeometric Analysis and the Finite Cell method—are employed for a direct treatment of the fourth-order Cahn–Hilliard equation and a flexible representation of the particle geometry. Based on the outlined models, finite element simulations were carried out, and a serials of results were obtained on electrode particles [3, 5, 1], interconnected particle networks [4], composite electrodes [2] and battery cells. Figure 1 shows illustrative simulation examples. Results reveal the electrochemical reactions on particle surfaces, phase interfaces and crack surfaces, as shown in Fig. 1a. It demonstrates that during delithiation, initial crack notches propagates due to core-shell phase separation and crack branches when phase interface overtakes the crack tip. Reaction rate is increased since newly cracked surfaces are exposed to electrolyte. In composite electrodes, concentration distribution and deformation inside electrode particles and matrix have strong interactions with each other during charge and discharge processes


_______________________________________________________________________________________________________________

9

(Fig. 1b). Particle geometry also plays a crucial role in electrochemical behavior of electrodes. In phase-separating materials, interconnectivity across particles can largely influence concentration distribution across particle networks, resulting in current “hot spots” (Fig. 1c) as shown by X-ray microscopy (STXM) images on V2O5 nanowires. Representative examples based on STXM images are simulated to discuss the factors that will influence phase separation during non-equilibrium lithiation and delithiation, as well as relaxation towards equilibrium. The simulation reveals that particles with a slight advance during (de-)lithiation at the beginning will strengthen their advance at the expense of neighboring particles, in a fashion of winner-takes-all. Simulation results show very good agreement with experimental observations of STXM.

Figure 1: Simulation results of (a) crack propagation in an electrode particle during delithiation, (b) lithiation in composite electrodes and (c) lithiation across interconnected particle network.

Acknowledgements

The author Xu would particularly like to thank the Adolf Messer Foundation for awarding her the Adolf Messer Price and for the financial support.

References

1. Bai-Xiang Xu, Ying Zhao, and Peter Stein. Phase field modeling of electrochemically induced fracture in Li-ion battery with large deformation and phase segregation. GAMM- Mitteilungen, 39(1):92–109, 2016.

2. Ying Zhao, Dominik Schillinger, and Bai-Xiang Xu. Variational boundary conditions based on the Nitsche method for fitted and unfitted isogeometric discretizations of the mechanically coupled CahnHilliard equation. Journal of Computational Physics, 340:177–199, July 2017.

3. Ying Zhao, Peter Stein, and Bai-Xiang Xu. Isogeometric analysis of mechanically coupled Cahn–Hilliard phase segregation in hyperelastic electrodes of Li-ion batteries. Computer Methods in Applied Mechanics and Engineering, 297:325–347, 2015.

4. Ying Zhao, Bai-Xiang Xu, Peter Stein, and Dietmar Gross. Modeling of phase separation across interconnected electrode particles in lithium-ion batteries. RSC Advances. Accepted.

5. Ying Zhao, Bai-Xiang Xu, Peter Stein, and Dietmar Gross. Phase-field study of electrochemical reactions at exterior and interior interfaces in Li-ion battery electrode particles. Computer Methods in Applied Mechanics and Engineering, 312:428–446, December 2016.


_______________________________________________________________________________________________________________

10

Molecular Dynamics Studies of Primary Damage in β-Li2TiO3

Mohammed Suhaila1, *, Paritosh Chaudhuri2, Ratnakumar Annabattula1, and

Narasimhan Swaminathan1

1. Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, 600036, India

2. Institute for Plasma Research, Bhat, Gandhinagar – 382428, India * Corresponding author. E-mail: [email protected] Monoclinic lithium meta titanate (Li2TiO3) is one of the materials used to breed tritium in fusion reactors because of its high thermal conductivity, high tritium release and low chemical reactivity. Experimental studies have confirmed that the release of tritium is significantly affected due to the presence of radiation defects. However, the kind of defects generated, their concentration in material, and how they might affect diffusion processes have not yet been ascertained. Therefore, for the first time, classical molecular dynamics simulations have been performed on Li2TiO3 to study point defects, point defect clusters and the diffusion of Li, Ti and O in the presence of point defects. Collision cascades were initiated with either Li, Ti or O as the primary knock on atom (PKA) with an energy of 2 keV. Furthermore, cascades were conducted in three crystallographic directions [100], [010] and [001]. The irradiated simulations cells were checked for point defects and point defect clusters using computational techniques. As expected, Ti PKA produced maximum damage (largest number of defects) due to its higher mass. Most of the point defects showed a strong dependence on the direction of the PKA with [100] having the highest damage. However, Li and Ti antisites numbers seemed to be independent of PKA direction. Amongst all the atoms, Li defects were highest followed by O and Ti. It was also seen that no defect clusters were produced in the material at the PKA energies studied in this work. Furthermore, it was also found that the diffusion of Li was enhanced in the presence of radiation defects, while Ti hardly diffused. Keywords: Irradiation, Collision, Cascade, Defects, Cluster


_______________________________________________________________________________________________________________

11

Modeling the Effective Electrical Conductivity of Electrode

Structures in lithium-ion Batteries Using Discrete-Element-Methods

Oleg Birkholz1, *, Yixiang Gan1, and Marc Kamlah1

1. Karlsruhe Institute of Technology, Institute of Applied Materials 76131 Karlsruhe, Germany * Corresponding author. E-mail: [email protected] The performance of lithium-ion batteries (LIB) is strongly influenced by the composition and fabrication of the electrode structures. On the one hand, the used material and its microstructure plays an important role. On the other hand, the mechanical densification processes, such as calendering or sintering, impact on the quality of the battery. Due to the low electrical conductivity of the active material (AM) inside a LIB, carbon black (CB) powder is being added to overcome this drawback. A variety of tools have been proposed in the past to model electrode structures, see [4]. First, an initial structure can be generated according a given distribution of AM and CB using the Random-Close- Packing algorithm (RCP), where a randomly distributed, densely packed and overlap-free assembly of spheres can be created. Secondly, the given assembly can be densified by either simulating compression using the Discrete-Element-Method (DEM) or mimicking sintering processes with a Numerical-Sintering algorithm (NS). Lastly, percolated clusters of the conducting phase can be identified and translated into an equivalent electrical circuit, where the Resistor-Network method (RN) can be used to calculate an effective conductivity - or performance for that matter - of the given electrode structure. The crucial part inside the RN concerns the calculation of the electrical resistance between each individual particle. Therefore, the method was extended according to [3] to account for particles with varying radius- and conductivity-ratios.

Figure 1: Radius size distribution of active material and carbon black of a LFP-electrode

taken from [2] (without pore size distribution)

An application for the methodology described above is given here: In the first step, the distribution of a real electrode structure with LiFePO4 `(LFP) was taken provided


_______________________________________________________________________________________________________________

12

· ·

by [2] using FIB-SEM tomography. The AM and CB phases were reconstructed with RCP where the volume fraction of the latter ranged within ΦCB = 0.00...0.15. In the second step, the assemblies had been densified a little using NS to establish contacts between the particles, such that RN could be used to calculate effective conductivities κeff,AM and κeff,AM+CB of the AM or the CB phase, respectively.

Figure 2: Influence of Carbon Black (CB) on the effective conductivity

The remarkable results are presented in figure 2, where one can see that the mere presence of CB - though never percolating - with a very large bulk conductivity κbulk,CB = 3.0·104S/m [1] compared to the one of AM κbulk,CB = 1.0·10−7S/m [5] increases the effective conductivity up to a factor of almost 3, compared to the values where no CB is available.

References

1. A. Awarke, S. Lauer, S. Pischinger, and M. Wittler. Percolation-tunneling modeling for the study of the electric conductivity in LiFePO4 based li-ion battery cathodes. 196(1):405–411.

2. M. Biton, V. Yufit, F. Tariq, M. Kishimoto, and N. Brandon. Enhanced imaging of lithium ion battery electrode materials. 164(1): A6032–A6038.

3. J. Gan, Z. Zhou, and A. Yu. Effect of particle shape and size on effective thermal conductivity of packed beds. 311:157–166.

4. J. K. Ott. Modeling the microstructural and micromechanical influence on effective properties of granular electrode structures: with regard to solid oxide fuel cells and lithium ion batteries / von.

5. G. Qiu, A. S. Joshi, C. R. Dennison, K. W. Knehr, E. C. Kumbur, and Y. Sun. 3-d pore-scale resolved model for coupled species/charge/fluid transport in a vanadium redox flow battery. 64:46–64.


_______________________________________________________________________________________________________________

13

Quasielastic Neutron Scattering Reveals Diffusion Pathways in Solid-

state Oxide-ionic Conductors

Chris D Ling1, *, Julia Wind1, and Richard Mole2

1. The University of Sydney, School of Chemistry, Sydney 2006, Australia. 2. ANSTO, Australian Centre for Neutron Scattering, Menai 2234, Australia. * Corresponding author. E-mail: [email protected] The search for new and improved solid-state ionic conductors (SSICs) is one of the most active and important fields in materials chemistry, driven by the key role they play in solid-state battery and fuel cell technologies. The paradoxical chemical and structural requirements of SSICs are challenging: long-range order to provide a mechanically stable framework, together with short-range disorder so that selected atoms can migrate through it. Rational design and optimisation of SSICs depends on a detailed understanding of the atomic-scale mechanisms by which this paradox is resolved. Inelastic neutron scattering is the only experimental technique that simultaneously probes ionic diffusion (as quasielastic neutron scattering, QENS) and lattice dynamics (as a generalised density of states, GDOS) in SSICs.1 When the diffusing species has a predominantly incoherent neutron scattering cross section, key parameters describing diffusion can be extracted directly by modelling the form of the QENS. In SSICs where the diffusing atoms have predominantly incoherent neutron scattering cross sections, key parameters describing diffusion can be extracted directly by modelling the form of the QENS. The archetypal case is hydrogen, an almost purely incoherent scatterer. QENS analysis has been successfully applied to hydrogen dynamics in solid-oxide fuel cell materials2, organic–inorganic perovskite solar cells3, porous organic cages4 and even biological systems5,6. QENS analysis is far more complex (and unresolved) problem when the diffusing atoms have significant coherent neutron scattering cross-sections. Oxygen and lithium (and, in fact, most isotopes of most elements) fall into this category. Consequently, while INS studies on oxygen (an almost purely coherent scatterer) SSICs have noted the appearance of QENS in the ionic conduction temperature regime, none have been able to model it. Instead, they used the experimental GDOS to validate computer simulations7-9 – i.e., the key insights into diffusion come indirectly from simulations rather than directly by fitting data. Similarly, INS studies of lithium SSICs have modelled the incoherent contribution to the QENS signal, but not the equally significant coherent contribution, resulting in imperfect fits and incomplete results10-12. A quantitative empirical model for coherent QENS in solids would be a powerful tool to study solid-oxide fuel cell and lithium-ion battery materials, but is yet to be developed. We have now developed and tested such a model by starting with the ideal case: an SSIC where the diffusing atom is an almost purely coherent scatterer (oxygen), the structure is simple, and the QENS signal is strong. This is the high-temperature cubic form of bismuth oxide, δ-Bi2O3, which has the highest known solid-state oxide ionic conductivity (1–2 S cm-1)13,14 over its narrow stability range 729–817°C. We fit the oscillating form of the QENS broadening using a modified


_______________________________________________________________________________________________________________

14

version of a jump-diffusion model previously reserved for liquid ionic conductors. Fit parameters include a quantitative jump distance and a semi-quantitative diffusion coefficient. The results show that diffusion is isotropic (liquid-like) even though some directions present shorter oxygen-vacancy distances, an insight corroborated by computational dynamics simulations. More broadly, the results show for the first time that QENS can be directly analysed to yield insights into the atomic-scale diffusion mechanisms of SSICs, even when the diffusing species is a coherent neutron scatterer such as oxygen. This makes it a powerful tool for studying energy materials, notably for solid-oxide fuel cells and, ultimately, lithium-ion batteries.

References 1 Hempelmann, R. Quasielastic neutron scattering and solid state diffusion. (Clarendon

Press, 2000). 2 Karlsson, M. Proton dynamics in oxides: insight into the mechanics of proton conduction

from quasielastic neutron scattering. Phys Chem Chem Phys 17, 26-38, doi:10.1039/c4cp04112g (2015).

3 Leguy, A. M. et al. The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells. Nat Commun 6, 7124, doi:10.1038/ncomms8124 (2015).

4 Liu, M. et al. Three-dimensional protonic conductivity in porous organic cage solids. Nat Commun 7, 12750, doi:10.1038/ncomms12750 (2016).

5 Schiro, G. et al. Translational diffusion of hydration water correlates with functional motions in folded and intrinsically disordered proteins. Nat Commun 6, 6490, doi:10.1038/ncomms7490 (2015).

6 Gabel, F. et al. Protein dynamics studied by neutron scattering. Quarterly Reviews of Biophysics 35, 327-367, doi:10.1017/s0033583502003840 (2002).

7 Auckett, J. E. et al. Combined Experimental and Computational Study of Oxide Ion Conduction Dynamics in Sr2Fe2O5 Brownmillerite. Chemistry of Materials 25, 3080-3087, doi:10.1021/cm401278m (2013).

8 Ling, C. D. et al. Local Structure, Dynamics, and the Mechanisms of Oxide Ionic Conduction in Bi26Mo10O69. Chemistry of Materials 24, 4607-4614, doi:10.1021/cm303202r (2012).

9 Mashkina, E., Magerl, A., Ollivier, J., Göbbels, M. & Seifert, F. Oxygen mobility in the perovskite SrTi1−xFexO3−δ (x=0.8). Physical Review B 74, doi:10.1103/PhysRevB.74.214106 (2006).

10 Nozaki, H. et al. Li diffusive behavior of garnet-type oxides studied by muon-spin relaxation and QENS. Solid State Ionics 262, 585-588, doi:10.1016/j.ssi.2013.10.014 (2014).

11 Kartini, E. et al. Structure and dynamics of solid electrolyte (LiI)0.3(LiPO3)0.7. Solid State Ionics 262, 833-836, doi:10.1016/j.ssi.2013.12.041 (2014).

12 Myrdal, J. S. G., Blanchard, D., Sveinbjörnsson, D. & Vegge, T. Li-ion Conduction in the LiBH4:LiI System from Density Functional Theory Calculations and Quasi-Elastic Neutron Scattering. The Journal of Physical Chemistry C 117, 9084-9091, doi:10.1021/jp311980h (2013).

13 Takahashi, T. & Iwahara, H. Oxide ion conductors based on bismuthsesquioxide. Materials Research Bulletin 13, 1447-1453, doi:10.1016/0025-5408(78)90138-1 (1978).

14 Harwig, H. A. & Gerards, A. G. Electrical properties of the α, β, γ, and δ phases of bismuth sesquioxide. Journal of Solid State Chemistry 26, 265-274, doi:10.1016/0022-4596(78)90161-5 (1978).


_______________________________________________________________________________________________________________

15

Mechanical and Structural Degradation of LiNixMnyCozO2

Cathode in Li-ion Batteries

Kejie Zhao1, * 1. School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United

States * Corresponding author. E-mail: [email protected] LiNixMnyCozO2 (NMC) is the current choice of cathode for Li-ion batteries. The structural and mechanical stability of NMC plays a vital role in determining the electrochemical performance of batteries. However, the dynamic mechanical properties of NMC during Li reactions are widely unknown because of the microscopic heterogeneity of composite electrodes as well as the challenge of mechanical measurement for air-sensitive battery materials. We employ instrumented nanoindentation in an inert environment to measure the elastic modulus, hardness, and interfacial fracture strength of NMC of a hierarchical meatball structure as a function of the state of charge and cycle number. The mechanical properties significantly depend on the lithiation state and degrade as the electrochemical cycles proceed. The results are further compared with the properties of bulk NMC pellets. We perform first-principles theoretical modeling to understand the evolution of the elastic property of NMC on the basis of the electronic structure. This work presents the first time systematic mechanical measurement of NMC electrodes which characterizes damage accumulation in battery materials over cycles.


_______________________________________________________________________________________________________________

16

A Nonlocal Species Concentration Theory for Diffusion and Phase

Changes: Application to phase-separating Lithium ion Battery

Electrode Particles Tao Zhang1, * and Marc Kamlah1

1. Institute for Applied Materials, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany

∗ Corresponding author. E-mail: [email protected] A nonlocal species concentration theory is introduced from a nonlocal free energy density to describe diffusion and phase changes in a material showing phase segregation, and it can be interpreted as a generalization of the Cahn-Hilliard theory. In principle, nonlocal effects beyond an infinitesimal neighborhood are taken into account. This theory incorporates two second-order partial differential equations involving second-order spatial derivatives of species concentration and an additional variable called nonlocal species concentration, and is computationally less demanding than the fourth-order Cahn-Hilliard equation. In this theory, the nonlocal free energy density is split into the penalty energy density and the variance energy density. The thickness of the interface between two phases in phase segregated states of a material is controlled by a normalized penalty energy coefficient and a characteristic interface length scale. We implemented the theory in COMSOL Multiphysics® for a spherically symmetric boundary value problem of lithium insertion into a LixMn2O4 cathode material particle of a lithium ion battery. The two above mentioned material parameters controlling the interface are determined for LixMn2O4 and the evolution of the interface between segregated phases of this material is studied. Comparison to the Cahn-Hilliard theory shows that nonlocal species concentration theory is more accurate when simulating problems where the dimensions of the microstructure such as phase boundaries are of the same order of magnitude as the problem size. This is typically the case in nanosized particles of phase separating electrode materials in lithium ion batteries. For example, the nonlocality of nonlocal species concentration theory turns out to make the interface of the local concentration field thinner than in Cahn-Hilliard theory. Keywords: Nonlocality; Phase field approach; Phase segregation; Interface; Lithium ion batteries


_______________________________________________________________________________________________________________

17

Constitutive Equations of Irradiated Metallic Materials

Xiazi Xiao1, 2, Long Yu1, Lirong Chen1, 2, and Huiling Duan1, 2, *

1. State Key Laboratory for Turbulence and Complex System, Department of Mechanics and

Engineering Science, College of Engineering, Peking University, Beijing 100871, P.R. China 2. CAPT, HEDPS and IFSA Collaborative Innovation Center of MoE, Peking University,

Beijing 100871, P.R. China ∗ Corresponding author. E-mail: [email protected] Abstract A multi-scale crystal plasticity model is proposed to model the mechanical behaviors of irradiated face-centered cubic (FCC) polycrystalline material with interface effect. At the grain level, both the effects of irradiation-induced defects and interface effect (grain boundaries (GBs) and twin boundaries (TBs)) are considered for the plastic behaviors of individual grains. At the polycrystal level, the elastic-viscoplastic self-consistent (EVPSC) method is adopted to bridge the property of microscopic individual grains to macroscopic behaviors of polycrystals. The efficiency and feasibility of the model have been validated by comparing the numerical results with experimental data. It is believed that the proposed theory can offer a valid way to study the irradiation effect on polycrystals, nanocrystals and nanotwins. Introduction Designing materials that can withstand severe irradiation environments is a great challenge for next generation nuclear reactors. High-energy particle impact can remove atoms from their original lattice sites during the collision cascade and result in the formation of irradiation-induced defects, which include interstitials, vacancies, defect loops and stacking fault tetrahedrons (SFTs), etc. These immovable defects can lead to irradiation hardening and embrittlement, which are the main reasons of material failure. It has been noticed recently that some special microstructures (like GBs and TBs) can act as significant sinks for irradiation-induced defects [1,2]. Therefore, it is believed that the study of irradiation effects on mechanical properties of materials with microstructures would be critical for the design of next-generation irradiation-resistant materials. However, to the author’s knowledge, the corresponding theoretical model to analyze the mechanical behaviors of irradiated metals with microstructures is scarce. In this work, a single crystal model together with the self-consistent method will be applied to study both the effect of irradiation and microstructures on the macroscopic behaviors of polycrystals. Constitutive equations According to the standard power law, the plastic shearing rate γ!is expressed as:

γ! = γ!!!

!!"##!

!!sign(τ!),

where τ! and γ! denote the resolved shear stress (RSS) and a reference shearing rate and m is the strain rate sensitivity. The critical resolved shear stress (CRSS) τ!"##! is related with microstructure of grain, which consists of the contributions of the


_______________________________________________________________________________________________________________

18

irradiation-induced defect [3], grain boundary [4], twin boundary [5], network dislocation, lattice friction. In the macro-scale, a multiphase elastic-viscoplastic self-consistent [Xiao et al., 2010] (EVPSC) framework is developed to analyze the elastic-viscoplastic behavior of irradiated polycrystalline materials. And the local mechanical behavior of different inclusions is related to the macroscopic behavior through the strain rate concentration relation, which can be derived from the secant self-consistent scheme:

ε! = A!!!: E− E!" + A!!

!: E!" + S!: S!: (C!: ε!!! − C!:A!!

!:E!")

σ! = C!:A!!!: [S!: Σ+ S! − I! : ε!

!" − A!!!:E!" ]

where SE is the well-known Eshelby tensor. Results and conclusions

Figure 1. Macroscopic stress–strain relations of NC copper under uniaxial tension regarding

different grain sizes.

Figure 2. Stress-strain relationship for irradiated nt ultrafine polycrystalline copper. The defect

density is 𝑁!"# = 1.3×10!"m!!.

In this work, a multi-scale crystal plasticity model is proposed to model the mechanical behaviors of irradiated face-centered cubic (FCC) polycrystalline material with interface effect. At the grain level, the microstructure of is considered to study the plastic deformation of grain according to the crystal plasticity theory. At the polycrystal level, the elastic-viscoplastic self-consistent (EVPSC) method is adopted to bridge the property of microscopic individual grains to macroscopic behaviors of polycrystalline materials. References 1. Bai, X. M., Voter, A. F., Hoagland, R. G., Nastasi, M. and Uberuaga, B. P., 2010.

“Efficient annealing of radiation damage near grain boundaries via interstitial emission”, Science 327, 1631–1634.

2. Yu, K. Y., Bufford, D., Sun, C., Liu, Y., Wang, H., Kirk, M. A., Li, M. and Zhang, X., 2013. “Removal of stacking-fault tetrahedra by twin boundaries in nanotwinned metals”, Nature Comm. 4, 1377-1377.


_______________________________________________________________________________________________________________

19

3. Xiao, X. Z., Song, D. K., Xue, J. M., Chu, H. J. and Duan, H. L., 2015. “A self-consistent plasticity theory for modeling the thermo-mechanical properties of irradiated FCC metallic polycrystals”, J. Mech. Phys. Solids 78, 1-16.

4. Xiao, X. Z., Song, D. K., Chu, H. J., Xue, J. M. and Duan, H. L., 2015. “Mechanical properties for irradiated face-centred cubic nanocrystalline metals”, Proc. R. Soc. A 471, 20140832.

5. Xiao, X. Z., Song, D. K., Chu, H. J., Xue, J. M. and Duan, H. L., 2015. “Mechanical behaviors of irradiated FCC polycrystals with nanotwins”, Int. J. Plasticity. 74, 110-126.


_______________________________________________________________________________________________________________

20

The Effects of Internal Pressure on the Hydrogen-pickup-induced

Multi-field Coupling Behavior in the Zircaloy Cladding

Bingzhong Wang1, Jingyu Zhang1, and Shurong Ding1, *

1. Institute of Mechanics and Computational Engineering, Department of Aeronautics and Astronautics, Fudan University, Shanghai 200433, China

∗ Corresponding author. E-mail: [email protected], [email protected] Zircaloy is widely used as nuclear cladding materials in the pressurized water reactors (PWR) because of its high strength, high corrosion resistance and low neutron absorption cross section. The cladding tube in a fuel rod is the first barrier to prevent nuclear leakage. It is very important to keep the structural integrity of zircaloy cladding tubes during the in-pile operation stage and also the long-term storage period of spent fuels. In the high-temperature and high-pressure PWR service environment, zircaloy cladding tubes will suffer some irradiation damage effects under the attack of fast neutrons, presented as irradiation creep, irradiation hardening and irradiation growth in the macro scale. It should be noted that zirconium is easy to react with water to produce hydrogen atoms. A fraction of them can be absorbed by cladding tubes as solid solutes, and they will diffuse within the tubes under the gradients of temperature, hydrostatic stress and hydrogen concentration. When the hydrogen concentration in some areas reaches the terminal solid solution (TSS), the subsequent arrivals will be precipitated as hydrides, leading to heat release and transformation strains. This is a multi-field coupling process. Above all, hydrides will cause hydride-induced hardening and embrittlement to severely degrade the fracture toughness of zirconium alloys, or even initiate delayed hydride cracking (DHC). First of all, the coupling behavior of hydrogen diffusion, hydride precipitation, non-mechanical energy flow and mechanical deformation should be simulated. In this study, the computational flow chart for the coupled fields is designed, and the finite element procedures and the related iteration schemes for each field are written. With the obtained and verified programs on Finite Element Program Generator (FEPG) platform, the multi-field coupling behavior is simulated for a cladding tube with cracks and different internal pressures. The effects of internal pressure are analyzed. The research results indicate that (1) the stress concentration occurs in the crack tip; the temperature variation induced by precipitation of hydrides can be ignored; (2) the hydrostatic stress gradient toward the crack tip is enhanced with increasing the internal pressure; and the hydrostatic stress and its gradient are positive under high internal pressure, which will drive the surrounding hydrogen atoms diffuse towards the crack tip, the hydride volume fraction increases with time in the crack tip and the hydride behind it dissolves; (3) higher positive hydrostatic stresses results in lower TSS, which explains the fact that low concentration of hydrogen atoms in solid solution appears in the crack tip for the case with a high internal pressure. This study lays a foundation for the future research on DHC. Acknowledgements


_______________________________________________________________________________________________________________

21

The authors thank for the supports of National Natural Science Foundation of China (No. 11772095, 11572091) and the support of the National Key Research and Development Program of China (2016YFB0700100).


_______________________________________________________________________________________________________________

22

Modelling Hydrogen Embrittlement of Steels Using Stress-diffusion

Coupling

Aditya Venkatraman1, Uday Girajavagol1, and Ratna Kumar Annabattula1, *

1. Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai - 600036, INDIA

∗ Corresponding author. E-mail: [email protected] Hydrogen embrittlement is an environmentally assisted failure phenomenon. The growth and profitability of the hydrogen economy necessitate reliable design of systems for the storage and transport of hydrogen. Hence, there is great impetus for the study of the failure of vessels that store and pipelines that transport hydrogen. Several theories have been proposed as to how diffusion of hydrogen in metals causes their failure. Hydrogen Enhanced Localized Plasticity (HELP) is one such mechanism. In group V-B metals especially (and many F.C.C metals), under conditions that don't favour hydride formation [1], HELP has been proposed as a viable mechanism. Delafosse and Magnin. et. al [2] were able to gather experimental evidence of strain localization in cathodic alloy specimens when placed under a hydrogen environment. The conventional explanation that hydrogen decreases cohesive energy between slip surfaces, causing fracture is insufficient. Therefore, a more ‘enhanced’ plasticity is proposed as being necessary. Hydrogen gas, when present inside metals is either stored in NILS (Normal interstitial Lattice Sites) or traps like dislocations, grain boundaries, etc… Ability of trap sites to contain hydrogen depends upon hydrogen binding energy and activation energy for hydrogen release. Depending upon binding energy, trapping sites are divided into two categories, reversible and irreversible traps. Grain boundaries, carbides are irreversible whereas dislocations are reversible. Experimental observations yield relationships between crack-tip plasticity and hydrogen concentration (both at NILS and at trap sites), but these relations are empirical and vary from system to system. So, a physics-based model is necessary to incorporate all the processes that the metal undergoes when being strained in a hydrogen environment. With that in mind, this paper aims to develop a physics-based model to study the coupling between diffusion and mechanical properties of martensitic steels. The works of Sofronis and McMeeking et. al. [3], Krom et. Al [4], and Sofronis, Y.Liang and Aravas et. al. [5] serves as a basis for the FE modeling of diffusion which is done using Fick’s law as the base constitutive equation. Heat transfer analogy is used to determine a modified flux and internal energy term for calculating the pressure effect on diffusion. The effect of plastic strain on number of trap sites and the concentration of hydrogen at the trap sites is taken into account as well. These constitutive relations are incorporated using the Abaqus subroutine UMATHT, which requires user definitions for potential energy, flux and their variation with temperature and temperature gradient. This computational study focuses on the interaction of mechanics and diffusion in certain steel specimens. Therefore, the effect on hydrogen diffusion on mechanics of steel is incorporated. Hydrogen diffusion in steel causes two kinds of change. One is the straining of the lattice, which is purely dilatational in nature and the other is a marked reduction in


_______________________________________________________________________________________________________________

23

yield strength, which is the central aspect of HELP. This is due to hydrogen decreasing barriers to dislocation motion [6-7]. These effects are studied using a UMAT subroutine in Abaqus, making use of a modified flow rule that incorporates the elasto-plastic hardening as well as the dilatational straining and softening effects. Using this, the effect of hydrogen diffusion on stress around crack-like defects in pre-cracked specimens as well as deep-notched specimens is studied. Comparison between cases with and without Hydrogen diffusion is done. The growth of Hydrogen concentration around these defects have also been illustrated. The study has been conducted for both monotonic and cyclic loading. These results are compared with analytical and experimental data for validation. A seam crack has been modeled to study the effect of Hydrogen diffusion on energy release rates. A python code is written to calculate the J-integral by extrapolating the stress and strain values and the results are compared. The effect of texture on hydrogen diffusion and shear localization is investigated using anisotropic material behaviour. The anisotropy is modelled in Abaqus, by re-orienting the stiffness tensors appropriately depending on the grain orientation w.r.t loading axis. The study is done for various angles of the grain boundary. The clustering of grains where hydrogen concentration is above nominal is investigated in order to discern some information on influence of hydrogen on the above process. The influence of hydrogen-plasticity interactions on clustering are reported. References 1. Birnbaum HK, Sofronis P. Hydrogen enhanced localized plasticity a mechanism

for hydrogen-related fracture, Material Science and Engineering A 1994;176:191202.

2. Delafosse D, Magnin T. Hydrogen induced plasticity in stress corrosion cracking of engineering systems, Engineering Fracture Mechanics 2001;68:693729.

3. P. Sofronis, R.M. McMeeking, Numerical analysis of hydrogen transport near a blunting Krom AHM, Koers RWJ, Bakker A., Hydrogen transport near a blunting crack tip, Journal of the Mechanics and Physics of Solids, 1999; 47:971-992.

4. P. Sofronis, Y. Liang, N.Aravas, Hydrogen induced shear localization of the plastic flow in metals and alloys, European Journal of Mechanics-A/Solids, 2001; 20: 857-872.

5. Rozenak P, Robertson IM, Birnbaum HK. HVEM studies of the effects of hydrogen on the deformation and fracture of AISI type 316 austenitic stainless steel, Acta Metall. Mater., 1990;38:2031-40

6. Robertson IM, Birnbaum HK. An HVEM study of hydrogen effects on the deformation and fracture of nickel, Acta Metall. Mater., 1986;34:353-66.


_______________________________________________________________________________________________________________

24

Thermal and Mechanical Analyses of the CLAM Steel in

Supercritical-water Cooled Ceramic Breeder Blanket for CFETR

Dalin Zhang1, *, Shijie Cui1, Jie Cheng1, 2, Wenxi Tian1, and G.H. Su1

1. State Key Laboratory of Multiphase Flow in Power Engineering, Shaanxi Key Lab. of Advanced Nuclear Energy and Technology, School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China

2. Department of Mechanical Engineering, University of Nevada, Las Vegas, Las Vegas NV 89154, USA

∗ Corresponding author. E-mail: [email protected] The Chinese Fusion Engineering Test Reactor (CFETR) is a new test Tokamak device which is now being designed to make the transition from the International Thermonuclear Experimental Reactor (ITER) to the future fusion power plant (FPP) and realize the fusion engineering application in China. As one of the core components of CFETR, breeding blanket is of vital importance for the function of tritium self-sufficiency and fusion energy collection. Recently, a conceptual design scheme of supercritical-water cooled ceramic breeder (SCCB) blanket has been proposed as the potential blanket candidate for CFETR. China Low Activation Martensitic (CLAM) steel, which is one of the reduced activation ferritic/martensitic steels (RAFMs), has been selected as the only structure material for this blanket concept. The structure material mainly accounts for accommodating the internal flowing coolant, separating different kinds of pebble beds and keeping the structure integrity at the same time. In this study, both the thermal-hydraulic and thermal-mechanical performances of the SCCB blanket concept are analyzed in detail. The calculation results indicate that the maximum temperatures of all blanket components can be effectively cooled below the corresponding material temperature limits, and the maximum stress of all the blanket structure material is also within the corresponding stress limit of CLAM steel even with additional thermal stress applied to the blanket. This work preliminarily verifies the reasonability of this conceptual design scheme and can provide a valuable reference for the further analysis and optimization design of the CFETR SCCB blanket.


_______________________________________________________________________________________________________________

25

Mechanics in Electrochemistry: From tiny to huge batteries; from

energy storage to harvesting

Sulin Zhang1, *

1. Department of Engineering Science and Mechanics Department of Biomedical Engineering, The Pennsylvania State University, University Park, PA 16802, United States

∗ Corresponding author. E-mail: [email protected] Rechargeable batteries function by reversible ion shuttling between the electrodes through the electrolyte. To enable high-energy and high-power batteries, as many ions as possible and as fast as possible the ions can be inserted and extracted from the electrodes. However, large amount, high rate ion insertion induces large deformation in electrode materials, leading to material failure, and consequently irreversible capacity decay and poor cyclability. How do mechanics and electrochemistry reciprocally influence one another in battery charge-discharge cycling? How might the mechanics-electrochemistry reciprocity be harnessed and regulated for energy storage and energy harvesting, and how might it be unharnessed and dysregulated during battery degradation? These questions are stimulating new understandings and developments at the interface between mechanics and electrochemistry. This talk will highlight a set of exciting electro-chemo-mechanical phenomena of rechargeable batteries, enabled by advanced in-situ transmission electron microscopy and corroborated by multiscale, multiphysical modeling. Emphasis will be placed on the fundamental principles of mechanics and electrochemistry that underlie materials, designs, and devices.


_______________________________________________________________________________________________________________

26

Strain-induced Improper Ferroelectricity in Ruddlesden–Popper

Perovskite Halides for Photovoltaic Applications

Yajun Zhang1, 2 and Jie Wang1, 2, *

1. Department of Engineering Mechanics, School of Aeronautics and Astronautics, Zhejiang University, Zheda Road 38, Hangzhou 310027, China

2. Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Zheda Road 38, Hangzhou 310027, China

∗ Corresponding author. E-mail: [email protected] Activating multiple symmetry modes and promoting a strong coupling between different modes by strain are indispensable to stabilize a polar ferroelectric phase from a non-polar perovskite. The prototypical strategy to engineering ferroelectric phase in perovskite oxide thin films by epitaxial strain is either to induce an out-of-plane polarization by a compressive strain or to generate an in-plane polarization through a tensile strain. Herein, we demonstrate from first-principles calculations that an experimentally accessible compressive strain can induce an in-plane polarization in perovskite halide thin films with a Ruddlesden-Popper structure, resulting in an unusual paraelectric to ferroelectric phase transition. The detailed analysis on structure and energy shows that the unusual ferroelectric phase transition in the perovskite halides stems from the strong coupling between strain and antiferrodistortive mode. Moreover, it is found that the strained ferroelectric thin film possesses a suitable band gap of 1.6 eV for photovoltaic application. These findings not only unfold a novel nonpolar-to-polar transition in halides, but also open a new avenue to design optimal ferroelectric semiconductors for solar cell applications.


_______________________________________________________________________________________________________________

27

Microscopic Study on the Mechanism of Damage and Micro-crack

Propagation in the Polysilicon Solar Cell

Yangbo Zhang1, Qun Li1, *, and Hong Zuo1

1. State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace, Xi’an Jiao-tong University, Xianning, West Road 28#, P.R. China

∗ Corresponding author. E-mail: [email protected] Abstract Damage and micro-crack in the photovoltaic panel will lead to a deeply loss in the electric generation and lifetime reduction of the photovoltaic module, the main part of solar cell. Thus, the mechanisms for the initiation and propagation of these kinds of damage and micro-crack is investigated experimentally and numerically. In detail, a se-ries of nano-indentation tests are designed and carried out. The damage characteristic of polysilicon solar cell is investigated by means of the scanning electron microscope (SEM). After that, the relevant numerical simulations are implemented. According to the experiment, the material parameters, such as Young`s modulus and micro-hardness are obtained directly. In the end, the fracture toughness obtained from the test is also used to discuss the mechanism of the micro-crack propagation in the numerical simulation. Introduction The damage and micro-crack in the solar cell, the key part in the solar power system, have a strong influence on the lifetime of photovoltaic module and efficiency of electric generation of the system. Inhibiting the formation of damage and propagation of micro-crack is a one of the most effective method to improve the efficiency and lifetime of photovoltaic module. Aimed at polysilicon, the main material of solar cell, we have completed the experimental research and numerical simulation in the microscopic scales to explore the mechanism of damage and micro-crack propagation. Results Nano indentation is used as the loading mode to explore the rule of micro-crack initiation and propagation micro-crack. The material properties of the polysilicon are directly obtained through the Nano indentation at first. Sample of the polysilicon solar cell sent by the CSI is prepared by amounted and polished method because of its tough surface. With the result of load-displacement curve (Figure 1) of the Nano indentation test, the elastic modulus and hardness measured are about 151.75GPa and 11.97GPa, which can be used as the model parameter for the FEA. And also the microscopic characteristic of polysilicon solar cell (Figure.2) can be studied through the observation of indentation under scanning electron microscope (SEM). Micro-crack occurred and propagated in the corner of the indentation when the load of Berkovich tip is large enough. With the length of the micro-crack and the maximum load of the indentation, the critical stress intensity factor Kc can be calculated as 0.280 MPa⋅m1⁄2. According to the parameters of the polysilicon solar cell obtained before, the numerical simulation of the indentation process have been done. Stress distribution of


_______________________________________________________________________________________________________________

28

the polysilicon solar cell have been analyzed. Obviously the position of stress concentration is at the edge of the Berkovich tip. Considering the stress state in the location of indentation, the numer-ical simulation with cohesive element have been done to explore the mechanism of damage and micro-crack propagation.

Figure 1. The load-displacement curve of Nano indentation

Figure 2. Microscopic characteristic of Nano indentation under scanning electron

microscope(SEM)

Conclusions For the fragility of the polysilicon solar cell, which is difficulty to determine the material properties, Nano indentation have been used to get the material parameters. The elastic modulus and hardness measured are about 151.75GPa and 11.97GPa, which can be used as the model parameter for the FEA. And also with the length of the Micro-crack and the maximum load of the indentation, the critical stress intensity factor Kc can be calculated as 0.280 MPa⋅m1⁄2. Considering the stress state in the location of indentation, the numerical simulation has been done to explore the mechanism of damage and micro-crack propagation. Acknowledgements The authors would like to acknowledge the financial and technical support of the EDF and CSI. References 1. Paggi M, Corrado M, Rodriguez MA. A multi-physics and multi-scale numerical

approach to microcracking and power-loss in photovoltaic modules. Compos Struct 2013;95:630–8

2. Al Ahmar J, Wiese S. Analysis and simulation of cracks and micro cracks in PV cells[C]// In-ternational Conference on Thermal, Mechanical and Multi-Physics Simulation and Experi-ments in Microelectronics and Microsystems. 2013:1 - 4.


_______________________________________________________________________________________________________________

29

3. Kulshreshtha P K, Youssef K M, Rozgonyi G. Nano-indentation: A tool to investigate crack propagation related phase transitions in PV silicon[J]. Solar Energy Materials & Solar Cells, 2012, 96(96):166-172.

4. Y.S. Jenq, S.P. Shah. Two parameter fracture model for concrete, J. Eng. Mech. 1985;111:1227-1241.


_______________________________________________________________________________________________________________

30

Mechanical Behavior of Methane Hydrate Bearing Coarse-grained

Sediment in the Framework of Equivalent Granular Void Ratio

Abraham C.F. Chiu1, * and J. Shen1

1. Key Laboratory of Geomechanics and Embankment Engineering of the Ministry of Engineering, Hohai University, Nanjing, China

∗ Corresponding author. E-mail: [email protected] Methane hydrate, a crystalline material made up of a water cage surrounding a methane molecule, forms naturally in regions of permafrost and deep marine sediment where there are appropriate pressure and temperature conditions and sufficient methane gas (Kvenvolden 1999, Collett 2002). The occurrences of natural methane hydrate have been verified through core drilling at locations all over the world, such as in Mallik-Mackenzie Delta, Gulf of Mexico, Nankai Trough, Northern South China Sea and Ulleung Basin. The large amount of reserves of methane hydrate has prompted it to be considered as a potential fuel for the future as it offers a substantial source for the ever-increasing demand for energy (Holder and Kamath 1984, Grauls 2001, Dallimore 2005, Ruppel 2007). Considering the economic costs and technical feasibility, the ideal methane hydrate-bearing sediments (MHBS) should be coarse-grained for successful recovery of methane gas from the deep subsurface (Boswell 2009). During the production of methane from hydrates, hydrate dissociation may induce a variety of geological disasters, such as subsea landslides, casing deformation, and production platform collapse (Mienert et al. 2005, Nixon and Grozic 2007). Thus, it is important to study the mechanical properties of MHBS for safe extraction of methane from hydrate reservoirs. The present paper focuses on modeling the constitutive behavior of MHBS. The proposed model consists of two parts: (i) a generalized plasticity model for the uncemented granular skeleton, and (ii) an elastic damage model for the hydrate bonds. In the generalized plasticity model, the state-dependent dilatancy is adopted to take into account the effects of density and stress on the volumetric behavior of MHBS. Furthermore, the model is formulated using the framework of equivalent granular void ratio to describe uniquely the effects of pore-filling and load-bearing accumulation habits of hydrate on the mechanical behavior of MHBS. In the elastic damage model, the degradation of the hydrate bonds with the development of deformation is considered by proposing a nonlinear damage function for the stiffness of MHBS. Compared to the basic generalized plasticity model for the host sediment, only 3 additional parameters are required to model the mechanical behavior of MHBS. The proposed model is applied to simulate the laboratory shear test results of MHBS synthesized from a wide range of host sediments. It is demonstrated that the proposed model can satisfactorily capture the stress–strain and volume change behavior of MHBS under different hydrate saturations, confining pressures and void ratios.


_______________________________________________________________________________________________________________

31

Patterning Curved Three-dimensional Structures with

Programmable Kirigami Designs

Fei Wang1, Xiaogang Guo1, Jingxian Xu1, Yihui Zhang1, and C.Q. Chen1, * 1. Department of Engineering Mechanics, CNMM & AML, Tsinghua University, Beijing

100084, China * Corresponding author. E-mail: [email protected] (C.Q. Chen). Originated from the art of paper cutting and folding, kirigami and origami have shown promising applications in a broad range of scientific and engineering fields. Developments of kirigami-inspired inverse design methods that map target three-dimensional (3D) geometries into two dimensional (2D) patterns of cuts and creases are desired to serve as guidelines for practical applications. In this paper, using programmed kirigami tessellations, we propose two design methods to approximate the geometries of developable surfaces and non-zero Gauss curvature surfaces with rotational symmetry. In the first method, a periodic array of kirigami pattern with spatially varying geometric parameters is obtained, allowing formation of developable surfaces of desired curvature distribution and thickness, through controlled shrinkage and bending deformations. In the second method, another type of kirigami tessellations, in combination with Miura origami, is proposed to approximate non-developable surfaces with rotational symmetry. Both methods are validated by experiments of folding patterned thin copper films into desired 3D structures. The mechanical behaviors of the kirigami designs are investigated using analytical modeling and finite element simulations. The proposed methods extend the design space of mechanical metamaterials, and are expected to be useful for kirigami-inspired applications.


_______________________________________________________________________________________________________________

32

Operando Nanoindentation: A Perfect Platform to Measure the

Mechanical Properties of Electrodes During Electrochemical

Reactions

Kejie Zhao1, *

1. School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States

* Corresponding author. E-mail: [email protected] Abstract: We present an experimental platform of operando nanoindentation that probes the dynamic mechanical behaviors of electrodes during real-time electrochemical reactions. The setup consists of a nanoindenter, an electrochemical station, and a custom fluid cell integrated in an inert environment. We evaluate the influence of the argon atmosphere, electrolyte solution, structural degradation and volumetric change of electrodes upon Li reactions, as well as the surface layer and substrate effects by control experiments. Results inform on the system limitations and capabilities, and provide guidelines on the best experimental practices. Furthermore, we present a thorough investigation of the elastic-viscoplastic properties of amorphous Si electrodes, during cell operation at different C-rates and at open circuit. Pure Li metal is characterized separately. We measure the continuous evolution of the elastic modulus, hardness, and creep stress exponent of lithiated Si and compare the results with prior reports. Operando indentation will provide a perfect platform to understand the fundamental coupling between mechanics and electrochemistry in energy materials.


_______________________________________________________________________________________________________________

33

Harnessing Polymorphic Pyroxenes as Geomimetic Phase Change

Materials

Dorian A. H. Hanaor1, *

1. Fachgebiet Keramische Werkstoffe, Technische Universität Berlin, Germany * Corresponding author. E-mail: [email protected] Phase change materials (PCMs) are solid compounds that facilitate system functionality through changes in atomic arrangement. Owing to the latent heat and volumetric changes associated with such transformations, PCMs are of significant value in applications of thermal storage and mechanical toughening. To overcome the stagnation in the field of ceramic PCMs we turn to the field of mineralogy for inspiration. For applications in impact dissipation and energy conversion, functionally polymorphic ceramics inspired by diffusionless phase changes in pyroxene silicates that exhibit displacive transformations induced by pressure and temperature are targeted. Building on previous reports, pyroxenes are further attractive towards applications as bioactive materials with high strength. This is particularly relevant as current transformation-toughening bio-ceramics (partially stabilised ZrO2) do not exhibit bioactivity, which is in contrast achievable in certain pyroxenes. Polymer precursor synthesis can be used to fabricate granular and bulk metastable pyroxenes with controlled crystallography, composition, and morphology that enhance their tendency to undergo phase transformations under localised conditions of mechanical and thermal loading. In particular, the effects of dopant elements on phase stability, solubility, and transformation kinetics will be investigated by combined experimental and theoretical studies. Towards the development of applied materials, computational methods can be used to determine how composition, dopants, growth habit and granular morphology influence thermo-mechanically induced phase transformations. Such polymorphism can enhance the mechanical strength of granular systems under conditions of shock loading and will represent a new approach to transformation toughening in ceramics. By bringing together disparate concepts of mineralogy, ceramic materials, granular matter, and interface science, this cross-disciplinary project brings new multi-scale and multi-disciplinary perspectives towards applied phase change materials.


_______________________________________________________________________________________________________________

34

Electrocaloric Effects of Silicon-doped Hafnium Oxide Ferroelectric

and Anti-ferroeletcric Thin Films Dayu Zhou1, *, Xiaohua Liu1, Ali Faizan1, Jin Xu2, Johannes Müller3, Tony Schenk4,

Uwe Schroeder4, Fei Cao5, and Xianlin Dong5

1. Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China

2. Department of Electronic Engineering, Dalian Neusoft University of Information, Dalian 116023, China

3. Fraunhofer IPMS-CNT, Koenigsbruecker Strasse 178, 01109 Dresden, Germany 4. NaMLab gGmbH/TU Dresden, Noethnitzer Strasse 64, 01187 Dresden, Germany 5. Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of

Ceramics, Chinese Academy of Sciences, Shanghai 200050, China * Corresponding author. E-mail: [email protected] Electrocaloric effect (ECE) is the converse of the pyroelectric effect, and is defined as the isothermal entropy or adiabatic temperature change of a dielectric material when an electric field is applied or removed. The ECE may provide an efficient means to realize solid-state cooling devices for a broad range of applications such as on-chip cooling and temperature regulation for sensors and electronic devices. The application potentials have led to a great deal of research efforts being expended in investigation of the ECE in various ferroelectric (FE), relaxor ferroelectric, and anti-ferroelectric (AFE) materials, in which the electric field induced polarization change is associated with a large entropy change. In this work, we report the ECE of silicon-doped hafnium oxide (Si:HfO2) ferroelectric (∼5 mol.% Si) and anti-ferroelectric (∼6 mol.% Si) thin films prepared by atomic layer deposition. The polarization hysteresis behavior of TiN /Si:HfO2/TiN planar capacitors were measured over a wide range of applied fields from 0.5 to 4.0 MV/cm and in a wide temperature range from 100 K to 400 K. Based on the measurement data, reversible adiabatic changes in temperature (ΔT) and entropy (ΔS) were calculated using Maxwell equations. The maximum adiabatic temperature changes (∆T) of -15.9 K and 14.9 K were obtained for the ferroelectric and anti-ferroelectric samples, respectively. The underlying mechanisms for positive and negative ECE were discussed.

Figure 1. Temperature dependent polarization hysteresis loops of 10 nm thick Si:HfO2 ferroelectric and anti-ferroelectric thin films doped with 5 mol.% (a) and 6 mol.%. (b) Si, respectively. The arrow in the figure indicates the evolution of the P-E curve with increase of temperature from 100 K to 400 K.


_______________________________________________________________________________________________________________

35

Figure 2. Adiabatic temperature change (∆T) and entropy change (∆S) as a function of measuring temperature for 10 nm thick Si:HfO2 ferroelectric thin films doped with 5 mol.% Si (a) and anti-ferroelectric thin films doped with 6 mol.% Si (b). The change of electric field magnitude (∆E) is 2 MV/cm.


_______________________________________________________________________________________________________________

36

Discrete-Element-Modeling of the Thermomechanics of Ceramic

Pebble Breeder Beds

Marigrazia Moscardini1, Simone Pupeschi1, Yixiang Gan1, Ratna Kumar Annabattula1, and Marc Kamlah1, *

1. Karlsruhe Institute of Technology, Institute for Applied Materials, Germany * Corresponding author. E-mail: [email protected] The first part of this talk deals with the Discrete-Element-Simulation of pebbles beds consisting of ellipsoidal particles [1]. The fabrication process of tritium breeding ceramics pebbles has an influence on the sphericity of the pebbles. Sometimes it is quite high (i.e. needle type particles) but sometimes the shape is more close to an oblate spheroid. A Discrete Element Method (DEM) code previously developed in the Institute of Applied Materials (IAM) for perfectly spherical particles was extended in order to simulate the mechanical behavior of ellipsoidal particles. The method to represent non-spherical particles was the multi-sphere (MS) approach. It is based on the union of several spheres to obtain the required shape. The MS method leaves the possibility to continue to use the same algorithms developed for spherical particles. In agreement with previous studies carried on assemblies of spherical pebbles, the initial packing factor is found to noticeably affect the mechanical response of the investigated assemblies. Moreover, a remarkable influence of the shape on the mechanical behavior of the simulated assemblies is observed. Therefore it is concluded that for production techniques that result in poor sphericity, DEM simulations with non-spherical particles are necessary to reproduce realistic stress-strain behavior of pebble beds.

In the second part of this talk, the effect of cyclic loading on the mechanical behaviour of pebble bed assemblies is investigated by DEM [2]. The numerical simulations are compared to experimental outcomes. The numerical simulations show that the pebble size distribution affects noticeably the stress-strain behaviour of the assemblies. A good qualitative agreement between experimental and simulation results was found in terms of difference between residual strains of consecutive cycles. An increase of the oedometric modulus with the compressive load was observed for all investigated compositions in both experimental and DEM simulations. The numerical results show an increase of the oedometric modulus (E) with progressive compaction of the assemblies due to the cyclic loading, while no significant influence of the pebbles size distribution was observed.


_______________________________________________________________________________________________________________

37

In the breeder blanket both tritium and heat are generated. Therefore, the effective thermal properties of packed beds are of major importance. In KIT an in-house thermal-DEM code was developed to evaluate the effective thermal conductivity of packed granular assemblies [3]. A 3D random network model was implemented to determine the heat exchange in packed systems under an imposed thermal gradient. In the network, particles are interconnected by thermal resistors defined by the two types of thermal contact, namely contact area and separation gap. In the third part of this talk, we present parametric studies to investigate the influence of temperature, compressive stress, type and pressure of the interstitial gas and the grain materials on the effective thermal conductivity of the assembly. The Smoluchowski effect has been implemented in the thermal-DEM code allowing simulation of the effect of the gas pressure on the effective thermal conductivity in the Knudsen domain. Furthermore, to study the influence of the compressive stress on the effective thermal conductivity the thermal code was coupled with the mechanical DEM code of KIT. The stresses generated in the pebble bed due to the mechanical constraint were simulated by means of uniaxial compressions and the variation of the effective thermal conductivity was evaluated. Numerical results are compared with existing experimental data in literature and recent experiments carried out in KIT [4]. Reference 1. Moscardini, M.; Gan, Y.; Annabattula, R. K.; Kamlah, M.: A Discrete Element

Method to simulate the mechanical behavior of ellipsoidal particles for a fusion breeding blanket. Fusion Engineering and Design 121(2017), 22 - 31.

2. Pupeschi, S.; Knitter, R.; Kamlah, M.; Gan, Y.: Numerical and experimental characterization of ceramic pebble beds under cycling mechanical loading. Fusion Engineering and Design 112(2016), 162 - 168.

3. Moscardini, M.; Gan, Y.; Pupeschi, S.; Kamlah, M.: Discrete element method for effective thermal conductivity of packed pebbles accounting for the Smoluchowski effect. Submitted, Fusion Engineering and Design, 2017.

4. Pupeschi, S.; Knitter, R.; Kamlah, M.: Effective thermal conductivity of advanced ceramic breeder pebble beds. Fusion Engineering and Design 116(2017), 73 - 80.


_______________________________________________________________________________________________________________

38

Numerical Investigation of the Pebble Bed Packing Structures for

HCCB TBM Baoping Gong1, *, Yongjin Feng1, Hongbin Liao1, Xiaoyu Wang1, and Kaiming Feng1

1. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, China *Corresponding author. E-mail: [email protected] In future fusion reactor, the tritium breeder blankets play a crucial role on the function of tritium self-sufficiency. The tritium breeder (Li4SiO4, Li2TiO3, and Li2O, etc, Lithium based ceramic) and neutron multiplier (Be, BeO, and Be12Ti ) in solid tritium breeding blanket are used in form of pebbles [1]. For the China helium-cooled ceramic breeder test blanket module (HCCB TBM), the Li4SiO4 ceramic pebbles with the diameter of ~1mm and Beryllium pebbles with the diameter of ~0.2 and 2mm are selected as the tritium breeder and the neutron multiplier respectively [2]. These individual pebbles are packed in the tritium breeder region and the neutron multiplication region to form the tritium breeders pebble bed and the neutron multiplier pebble bed. Due to the discrete characterization of the pebbles, the pebble beds can be treated as granular materials. In solid tritium breeding blanket, the packing structures of pebble beds are important to estimate the thermal properties of pebble bed [3-4], the thermal mechanical of pebble bed [5-6], the flow characteristics of purge gas [7] and the tritium breeding ratio [1]. The packing structure of pebble bed depends on lots of factors, such as pebble size, pebble shape, container dimension, pebble bed height, packing history and so on. Different packing structure can be obtained under different conditions. However, little works on U-shaped pebble bed is available. Thus their packing behaviors and packing structures of U-shaped pebble bed for HCCB TBM still needs to be fully investigated before they can be applied. So as to obtain a reasonable and feasible pebble bed for HCCB TBM. In this work, the discrete element method (DEM) was applied to simulate the pebble packing structures of mono-sized and binary-sized pebble bed, respectively, in a cubic and a U-shaped container. The current results obtained in this study obviously show that with the increase of aspect ratios the average packing factors can be significantly increased both in cubic pebble bed and cylindrical pebble bed. The results in this work were obviously comparable to the experiment results, as shown in Fig.1. The pebble size distributions have great influence on the packing structures of pebble bed, as shown in Fig.2. By optimizing the pebble size component, a higher packing factor can be obtained in binary-size pebble bed with a higher size ratio of larger pebbles diameter to smaller pebbles. And the peak values appear at the volume fraction 70% of larger pebbles, as shown in Fig.3. Furthermore, the pebble packing structures in U-shaped pebble beds were simulated and analyzed, as shown in Fig.4 and Fig.5. The local packing factor distribution shows an oscillating and damping characteristic. In the near wall region the local packing factor changed obviously. In the inner bulk region, the local packing factor gradually approach to average packing factor. The oscillation of local packing factor is limited within about 5d close to wall. And decreasing the pebble size can reduce the wall effect region of the U-shaped pebble bed and a more uniform pebble bed can be obtained.


_______________________________________________________________________________________________________________

39

0 5 10 15 20 25 30 350.50

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66(a)

Ave

rage

Pac

king

Fac

tor

Aspect ratio ( αcylinder, D/d )

experiment, A. Khalil [9] experiment, D. Nemeca [21] experiment, G.E. Mueller [48] experiment, P. Langston [49] simulation, P. Langston [49] simulation, In this work

0 5 10 15 20 25 30 35 40 45 50 550.50

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66(b)

Aver

age

Pack

ing

Fact

or

Aspect ratio (αcubic, a/d) Fig.1. Effects of the aspect ratios on average packing factor: a, cylinder pebble bed; b, cubic

packed bed.

Fig.2. Effect of pebble size distribution on

average packing factor Fig.3. Packing factor versus volume fraction of

large pebbles with different size ratio dL/dS

0 4 8 12 16 200.0

0.2

0.4

0.6

0.8

1.0

Loca

l pac

king

fact

or

Radial distance to inside wall (mm)

Case 1 Case 2 Case 3

(a)

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

loca

l pac

king

fact

or

Axial distance to bottom wall (mm)

Case 1 Case 2 Case 3

(b)

0 2 4 6 8 10 12 14 160.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

loca

l pac

king

fact

or


Fig.4. local packing factor distribution in these three U-shaped pebble bed

0 2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

Loca

l pac

king

fact

or

Radial distance to inside wall (mm)

Packed with 1mm pebbles Packed with 2mm pebbles

(a)

0 10 20 30 40 50 60 70 80 90 1000.0

0.2

0.4

0.6

0.8

1.0

Loca

l pac

king

fact

or


Packed with 1mm pebbles Packed with 2mm pebbles

(b)

0 2 4 6 8 10 12 14 160.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Loca

l pac

king

fact

or


Fig.5. Local packing factor distribution in U-shaped Li4SiO4 pebble bed with 1mm and 2mm pebbles


_______________________________________________________________________________________________________________

40

Keywords: Packing factor; Pebble bed; Test blanket module; Discrete element method References 1. K. Feng, X. Wang, Y. Feng, et al., Current progress of Chinese HCCB TBM

program, Fusion Eng. Des. 109-111 (2016) 729-735. 2. Z. Zhao, B. Zhou, Q. Wang, et al, Preliminary verification of structure design for

CN HCCB TBM with 1×4 configuration, Fusion Eng. Des. 103 (2016) 152-159. 3. D. Mandal, D. Sathiyamoorthy, M. Vinjamur, Void fraction and effective thermal

conductivity of binary particulate bed, Fusion Eng. Des. 88 (2013) 216-225. 4. Z. Zhao, K. Feng, Y. Feng, Theoretical calculation and analysis modeling for the

effective thermal conductivity of Li4SiO4 pebble bed, Fusion Eng. Des. 85 (2010) 1975-1980.

5. Y. Gan, M. Kamlah, Thermo-mechanical modelling of pebble bed-wall interfaces, Fusion Eng. Des. 85(2010) 24-32

6. A. Ying, J. Reimann, L. Boccaccini et al, Status of ceramic breeder pebble bed Thermo-mechanics R&D and impact on breeder material mechanical strength, Fusion Eng. Des. 87 (2012) 1130-1137

7. A. Abou-Sena, F. Arbeiter, L. V. Boccaccini et al, Experimental study and analysis of the purge gas pressure drop across the pebble beds for the fusion HCPB blanket, Fusion Eng. Des. 88 (2013) 243-247


_______________________________________________________________________________________________________________

41

Numerical and Expermental Study of Pebble Bed Flow Characterics

for Water Cooled Ceramic Breeder Blanket of CFETR

Songlin Liu1, Youhua Chen2, Lei Chen1, and Cong Wang1

1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, 230031, China. 2. University of Science and Technology of China, Hefei, Anhui, 230026, China Corresponding author. E-mail: [email protected] (Songlin Liu) The beryllium pebble bed and Li2TiO3/Be12Ti mixed pebble bed are selected to realize neutron multiplication and breeding in the Water-cooled ceramic breeder blanket (WCCB) of China Fusion Engineering Test Reactor (CFETR). Helium (mixed with 0.1% content of H2) is used as the purge gas to realize tritium extraction by flowing through the pebble beds. In order to evaluate and improve the performance of WCCB, studies of the purge gas flow characteristics of the concerned pebble beds are necessary.

In the current research, a numerical model will be developed by using distinct element method (DEM) and computational fluid dynamics (CFD) method to analyze flow characteristics (including porosity distribution, velocity distribution and pressure drop) of prototypical blanket pebble beds (see Fig. 1). Meanwhile, an experimental equipment is constructed to measure the pressure drop of the concerned pebble beds (see Fig. 2). The suitability and validity of the current numerical model will be evaluated by comparing with previous empirical formula and the current experimental data. According to the current results, both the velocity profile in both mono-sized and multi-sized pebble beds shows fluctation trend in the near-wall region. And the pressure drop of binary pebble bed increases greatly with the increase of the packing factor and it was much larger than mono-sized pebble bed.

Fig. 1. Simulation of purge gas flow characteristics in unitary pebble bed (UPB) and binary

pebble bed (BPB)


_______________________________________________________________________________________________________________

42

(a) unitary pebble bed (dia.=2.00mm, Pinlet=0.2MPa) (b) binary pebble bed (dia.=5.00/3.00mm, Pinlet=0.2MPa)

Fig. 2. Measurement of purge gas pressure drop in pebble beds


_______________________________________________________________________________________________________________

43

Numerical and Experimental Study on Effective Thermal

Conductivity of Binary Pebble Bed

Cong Wang1, Lei Chen2, and Songlin Liu1, *

1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, 230031, China. 2. University of Science and Technology of China, Hefei, Anhui, 230026, China Corresponding Author. E-mail: [email protected] (Songlin Liu) Water-cooled ceramic breeder blanket (WCCB) is one of candidate blankets for China Fusion Engineering Test Reactor (CFETR). The Li2TiO3/Be12Ti mixed pebble bed is assembled in WCCB to improve tritium breeding ratio. In order to obtain the effective thermal conductivity (ETC) of Li2TiO3/Be12Ti mixed pebble bed and thus evaluate the thermal performance of WCCB, both numerical study and experimental measurement of ETC for binary pebble beds are firstly performed in the current study. In this work, an experimental platform (Fig. 1) for studying thermo-mechanics and purge gas flow characteristics of the concerned pebble beds will be introduced. First, the suitability and validity of the experimental results on ETC will be evaluated by comparing with the previous experimental data. Second, different ways of mixing the binary pebble bed will be compared by using X-ray tomography. Besides, numerical modeling of binary pebble beds is developed by using DEM-CFD coupling method. Finally, the comparison of binary pebble bed between the experimental results and calculation results by DEM-CFD model will be discussed. According to the discussion, the DEM-CFD model will be updated to estimate the ETC of Li2TiO3/Be12Ti mixed pebble bed.

Fig. 1: Experimental platform. The platform includes pebble bed test section, thermal power

control system, purge gas flow system, measuring system and other auxiliary systems.


_______________________________________________________________________________________________________________

44

Topological Evolution of Granular Hard Sphere Beds Packed in

Confined Cylindrical Containers Under Vibration

Weijing Dai1 and Yixiang Gan1, *

1. The School of Civil Engineering, The University of Sydney, Sydney, Australia Corresponding Author. E-mail: [email protected] Hard sphere systems are the simplest but most versatile representation of granular media, both theoretically and experimentally. Because of the concision of identical hard sphere systems, models based on these systems establish foundation of granular theories. Additionally, quantitative characterization on these systems in experiments draws a framework to explain various granular phenomena and paves the way to verify granular theories. One of the distinct behaviour of granular media is the phase transition along with the varied properties caused by external athermal perturbation like shearing, tapping and vibrating. Such phase transition is attributed to the topological evolution of granular media subjected to external athermal perturbation. In this study, a hard sphere system confined in a cylindrical container is simulated by the discrete element method to investigate the dynamic of the system under vibration. For a hard sphere system perturbed by relative weak vibration, the increase of packing factor, i.e. compaction is the macroscopic process, a slow relaxation. In this process, spheres move to adjacent vacancies or compress neighbour voids to rearranging their positions. Nevertheless, the origin of the rearrangement has been fully understood. The free volume theory has been adopted to roughly explain this process but the topological evolution has not been clearly demonstrated. In order to reveal the detail of the topological evolution of a hard sphere system under continuous vibration, bond orientation order parameters are used to quantitatively characterise the occurrence and development of the symmetry structure. Due to the confinement of the cylindrical wall, the evolution of the topology presented in the boundary region is different from the core region in the container. This influence of boundaries can be used to elucidate the inconsistency among different experiments. This statistical analysis of the bond orientation order parameters of this system will provide insight into the in situ mechanism of compaction process from a topology perspective. It will be a good counterpart alongside with the mechanical aspect of granular media.


_______________________________________________________________________________________________________________

45

Multiscale Modeling of Effective Elastic Properties of Fluid-filled

Porous Materials

Mingchao Liu1, 2, Jian Wu1, Yixiang Gan2, *, and C.Q. Chen1, *

1. Department of Engineering Mechanics, CNMM & AML, Tsinghua University, Beijing, China 2. School of Civil Engineering, The University of Sydney, Sydney, NSW, Australia Corresponding authors. E-mail: [email protected]; [email protected] Graphic abstract

Abstract Fluid-filled porous materials are widely available in nature and modern industry. A mutual understanding of their effective elastic properties is of fundamental importance in fulfilling their applications. In this work, a two-dimensional micromechanical model with double-porosity is employed to investigate the overall elastic response of fluid-filled porous materials. Based on the one-scale-porosity model, by adopting a geometrical factor, the effective elastic properties of porous materials with ordered pressurized pores are obtained theoretically. The predictions are compared with finite element simulations. Good agreement is obtained for a wide range of porosities. Furthermore, according to the double-porosity model, the evolution of effective properties induced by the diffusion of fluid from large pores to small pores in matrix is investigated. For the cases of constant and varying fluid pressure, the time-dependent elastic properties are derived. It is found that the presence and the diffusion of the pressurized pore fluid can significantly affect the elastic response of porous materials. Therefore, they must be taken into account when modeling this kind of materials. The proposed theoretical model is expected to shed light on the understanding of the elastic response of fluid-filled porous materials and facilitate the applications in carbon capture and storage and hydraulic fracture problems in which porous media are of fundamental importance. Keywords: Porous material; elastic properties; micromechanical model; double-porosity; fluid diffusion


_______________________________________________________________________________________________________________

46

Modelling Multiphase Interactions Using Coarse-Grained

Molecular Dynamics

Pengyu Huang1, Luming Shen1, *, and Yixiang Gan1

1. School of Civil Engineering, The University of Sydney, NSW 2006, Australia Corresponding author. Email: [email protected] Many energy materials and reservoirs consist of two or more phases (fluids and solid phases). For example, in energy harvesting, the reverse electrowetting-on-dielectric (REWOD) method transfers the mechanical energy into electrical energy, by changing the interfacial contacting area between conductive liquid bridge and the dielectric-coated conductive substrates [1]. For solid oxide fuel cell, the numerical modelling of multiphase interactions at the microscale is used to study the effect of heterogeneity of electrode [2]. Further, the hydraulic fracturing method can be used to enhance the permeability of the geothermal and hydrocarbons reservoirs [3]. These examples demonstrate the needs of multiphase interactions studies at the microscale in energy engineering. A few numerical methods have been proposed to study the multiphase interactions system: smoothed particle hydrodynamics (SPH) [4], Lattice Boltzmann method (LBM) [5], dissipative particle dynamics (DPD) [6] and the volume-of-fluid (VOF) method [7], and some of them can couple with discrete element method (DEM) to simulate the solid particles in flows, while the deformation or fracture of the solid particles is not allowed [8, 9]. In this work, a framework based on coarse-grained molecular dynamics (CGMD) method is developed for modelling multiphase interactions numerically at the microscale. The CGMD method is to use one particle to represent a group of molecules or atoms, in order to achieve a greater length and time scale than in MD simulations. All three phases are modelled with coarse-grained particles. Their motions and interactions obey the classical mechanics. The conservative part of many-body dissipative particle dynamics (MDPD) is used to model the liquid-liquid/liquid-vapour inter-particle interaction and a Morse potential function is used to model the solid-solid inter-particle interaction. The solid-liquid inter-particle interaction is modelled using Lennard-Jones potential function. To demonstrate the capacity of the method, the thermodynamic and mechanical properties of water and fused silica are matched. For water, the density and surface tension at room temperature are matched with experimental values, and for fused silica, the density, tensile strength and Young’s modulus are matched. In addition, the trend of surface tension against temperature over a wide range of temperature (around 293 K to 600 K) is recovered, by choosing a proper set of MDPD parameters. The scaling methods for liquid and solid models, which calculate the parameters of inter-particle potential functions from the particle size at one length scale to the particle size at another length scale, are validated separately. The liquid-vapour-solid system is modelled by placing a microscale water droplet on a solid substrate. The resulting contact angle values depend on the inter-particle interaction energy between liquid and solid particles. This framework can be applied to study the wettability properties of multiphase systems, such as the contact angle hysteresis, and the behaviour of partially saturated porous media under various environmental conditions.


_______________________________________________________________________________________________________________

47

References 1. Krupenkin, T. and Taylor, J.A., Reverse electrowetting as a new approach to high-

power energy harvesting. Nature communications, 2011. 2: p. 448. 2. Xu, H. and Dang, Z., Numerical investigation of coupled mass transport and

electrochemical reactions in porous SOFC anode microstructure. International Journal of Heat and Mass Transfer, 2017. 109: p. 1252-1260.

3. McCartney, J.S., Sánchez, M., and Tomac, I., Energy geotechnics: Advances in subsurface energy recovery, storage, exchange, and waste management. Computers and Geotechnics, 2016. 75: p. 244-256.

4. Tartakovsky, A., Trask, N., Pan, K., Jones, B., Pan, W., and Williams, J., Smoothed particle hydrodynamics and its applications for multiphase flow and reactive transport in porous media. Computational Geosciences, 2015: p. 1-28.

5. Chen, S. and Doolen, G.D., Lattice Boltzmann method for fluid flows. Annual review of fluid mechanics, 1998. 30(1): p. 329-364.

6. Groot, R.D. and Warren, P.B., Dissipative particle dynamics: Bridging the gap between atomistic and mesoscopic simulation. Journal of Chemical Physics, 1997. 107(11): p. 4423.

7. Huang, H., Meakin, P., and Liu, M., Computer simulation of two‐phase immiscible fluid motion in unsaturated complex fractures using a volume of fluid method. Water resources research, 2005. 41(12).

8. Cook, B.K., Noble, D.R., and Williams, J.R., A direct simulation method for particle-fluid systems. Engineering Computations, 2004. 21(2/3/4): p. 151-168.

9. Jing, L., Kwok, C., Leung, Y., and Sobral, Y., Extended CFD–DEM for free‐surface flow with multi‐size granules. International Journal for Numerical and Analytical Methods in Geomechanics, 2015. 40(1): p. 62-79.


_______________________________________________________________________________________________________________

48

A Modified Smoothed Particle Hydrodynamics Formulation for

Simulating Dynamic Behaviours of Water within SOFCs

Yanyao Bao1, Ling Li1, Luming Shen1, and Yixiang Gan1, *

1. School of Civil Engineering, The University of Sydney, NSW 2006, Australia Corresponding author. E-mail: [email protected] The solid oxide fuel cell (SOFC) and electrolyzer demonstrate great potential for a clean energy solution. Although scientific advances have been achieved over the last few decades, one of the major obstacles for commercialization of SOFC relates to the problem of excessive water on the cathode side, which can have deleterious effects on cell performance. In this work, a smoothed particle hydrodynamics (SPH) model with pairwise force is adopted for modelling dynamic phenomena of water within SOFCs. In this model, an inter-particle force was implemented to reproduce the relevant interfacial properties, including the surface tension and wettability between different phases, i.e., wetting, non-wetting and solid phases. For frictional condition at solid-liquid interface, the conventional treatment includes arbitrary tangential force or dragging force, which may not contain direct links to physical quantities involved during complex flow conditions. In this study, an interfacial viscous force is introduced to reflect the real dynamic behaviour of fluid interacting with solid. This formulation is established based on a micromechanical model considering the shear deformation of viscous fluid contained within a narrow near-wall region, i.e., the gap between SPH particles in wetting fluid and solid phases. To validate this newly added formulation of interfacial forces, we conducted two series of dynamic simulations with and without viscous force, for the following scenario: (a) gravity-driven flow in a tube, and (b) channel flow at various flow rates. The results demonstrate the effectiveness of this added interfacial force, and the necessity to include this formation to produce realistic velocity profile across the channel. Next, we studied the dynamic contact angle resulting from different contact line moving speed, i.e., at various capillary number. Utilising this modified model, we have developed an empirical equation to describe and predict the relationship between the dynamic contact angle and capillary number. The results demonstrate that our model is able to successfully reproduce the rate-dependent behaviours of the moving contact line, and can be thus employed to tackle the complex multiphase interaction problems within SOFCs.


_______________________________________________________________________________________________________________

49

The Electrical Contact at Rough-to-rough Spherical Interface

Chongpu Zhai1, Dorian Hanaor2, and Yixiang Gan1, *

1. School of Civil Engineering, The University of Sydney, Sydney, Australia. 2. Institute for Material Science and Technologies, Technical University of Berlin, Berlin,

Germany Corresponding author. E-mail: [email protected]

In this paper, we study the electrical contact of a rough interface between two spheres under compression. In the proposed numerical model for the rough-to-rough contact, the rough interface is described as the coating layer with depth-gradient elastic module governed by the truncation area. The interacting curved rough surfaces are generated with prescribed respective power spectra of surface structures. Fast-Fourier-Transform-based boundary method was employed to obtain the dependence of the gradient elastic module on contact area. The macroscopic electrical contact resistance of the rough interface under increasing normal load is determined by the total overlapping area at varying surface interferences. Shoulder-to-shoulder asperity contacts are incorporated in calculating the total contact area by considering the height-width ratio of contacting zones between interacting asperities. The dependence of electrical contact resistance obtained from the proposed numerical approach show good agreement with experimental observations for four types of rough spheres with different surface treatments. The electrical contact resistance shows a unified power-law scaling with respect to the applied normal load for both real and simulated interfaces with distinct sets of roughness descriptors. The exponent of the power-law relationship is found to correlate positively to the fractal dimension while its amplitude is inversely correlated to the surface roughness amplitude. This study provides an easily implemented and computationally efficient method to connect electro-mechanical behaviour with multi-scale surface structures, which can be utilized in design and optimization of engineering applications involving curved rough-to-rough contacts. Keywords: electrical contact resistance; functional gradient materials; rough surfaces; fractal dimension, nanoindentation.

international workshop on mechanics of energy materials...

Documents