Dilatometry Characterization of CeO2 Ceramic

Discs as a Function of Temperature and Atmosphere D.P. Kramer, S.M. Goodrich, C.D. Barklay,

and C.E. Whiting

University of Dayton Research Institute

Nuclear and Emerging Technologies for Space - NETS

February 2015

University of Dayton Research Institute Shaping the technology of tomorrow

A chemical characteristic of PuO2 and CeO2 is that they readily release oxygen atoms under reducing conditions

Tend to release oxygen atoms under reducing conditions forming sub-oxides CeO2-x or PuO2-x

CeO2 exhibits a cubic crystal structure which is maintained as it is reduced down to Ce4O7 CeO2 can be reduced even further to Ce2O3 which is hexagonal The reduction of CeO2 to Ce2O3 with hydrogen has been shown to consist of a number of intermediates which exhibit a range of colors: - Over thirty intermediate compositions between CeO2 and Ce2O3 whose

color palette ranges from pale yellow, grey-blue, dark blue, blue-black, black, to olive green Bevan, D.J.M., “Ordered Intermediate Phases in the System CeO2-Ce2O3,” J.

Inorg. and Nuc. Chem., v. 1, Pergamon Press Ltd., London, 1955

Since PuO2/CeO2 reduction is complex is there a technique which can be employed to help our “ceramics” understanding?

Thermal expansion is an important characteristic of a ceramic which can provide information on many properties of a material including: - Thermal shock resistance - Crystallinity/amorphous - Crystal changes as a function of

temperature/time/atmosphere

Dilatometry is a relatively quick and inexpensive technique for measuring expansion of a sample as a function of processing parameters

Changes in PuO2-x stoichiometry can impact ceramic related physical properties Comparison of percent linear thermal expansion as a function

of temperature and stoichiometry

Touloukian, Y.S., Thermophysical Properties of Matter, v.13, IFI/Plenum, NY, 1977

Advantages of a non-radioactive surrogate such as CeO2 for performing various support endeavors

Reduced personnel exposures Significant reduction in cost

CeO2 is one candidate surrogate material for 238PuO2

Similar chemical properties i.e. Oxygen reduction

Similar crystalline characteristics i.e. Face-Centered Cubic

Ceramic properties? i.e. Microstructure/Processing

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Dilatometer experiments were performed as a function of time, temperature, and atmosphere on CeO2 sintered discs

Since PuO2 and CeO2 have many similar properties a set of thermal expansion experiments were performed employing CeO2 as a surrogate for PuO2 CeO2 (99.9% trace metals basis) powder was obtained from Sigma-Aldrich with a reported particle size of <5µm In fabricating test specimens the CeO2 powder was first loaded into a ~2.54 cm (~1”) diameter steel die and it was cold pressed using a hydraulic press After the green disc was removed from the steel die it was then sintered in air typically ~1300 - 1600oC for up to several hours Theoretical densities of the sintered discs were in the 70% to 90+% range which was easily controlled by the careful selection of the processing parameters (i.e. green density, sintering temp., and soak time)

Example of a cold pressed + air sintered CeO2 disc and a typical sintered microstructure

Fine grain “uniform” CeO2 phase with inter-granular and some

intra-granular porosity regions ~2.2cm diameter cold pressed + air sintered CeO2 disc

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Dilatometer specimen preparation ensured that test specimens had very similar densities/thermal history

Dilatometer specimens <6 mm (0.24”) in diameter to fit unit Diamond core drill used to “cut-out” smaller diameter specimens

from a single large diameter cold press + air sintered disc Obtained 4-5 small CeO2 dilatometer specimens per large disc

Expansion experiments were performed in a Linseis 75 Duel Push Rod Vertical Research Dilatometer System

High temperature furnace - Up to 1500oC

Expansion measuring module - <<1µ resolution

Gas management manifold - Constant gas flow - Can automatically switch

gasses during a run Computer acquisition

system

Dual push rod system allows either 1 sample + 1 standard or 2 samples to be run concurrently during the same experiment

High Temp

Furnace

Dual push rods

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General dilatometer experimental parameters

Specimens - Cold Press + Air sintered (1400oC) discs - Core drilled to dilatometer dimensions

Dilatometer temperatures - 1000oC - 1400oC

Gases - Air - Argon/5ppm Oxygen - 95% Argon/5% Hydrogen (95/5) - Single Gas or Multi-gas experiments

Time for the reduction to occur at the two temperatures can be used to provide information on the reaction kinetics

• 1000oC experiment • Reduction not completed after ~2 hours • Total expansion ~45µ • ~Δ120%

• 1400oC experiment • Reduction completed

after only ~30 minutes • Total expansion ~48µ • ~Δ160%

Temperature

Expansion

Air 95% Ar/5% H2

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A set of three additional dilatometer experiments were run with and without the presence of a graphite ring

1) Vacuum 2) Argon/5ppm O2

3) 95%Ar/5%H2

Same t-T cycle - 1400oC - 4 hrs soak

CeO2 core drilled discs from one large dia. disc

Without graphite ring With graphite ring

Example of a machined duel diameter graphite ring and

a core drilled CeO2 specimen

CeO2 disc specimen before and after a dilatometer experiment using a 95%Ar/5%H2 cover gas

CeO2 disc specimen inside the graphite ring prior to a dilatometer run

Core drilled disc specimen after a dilatometer run exhibited a change in

color in comparison to its “parent” air sintered CeO2 disc

OUO – ITAR

238PuO2 Fuel/Pellet Related Studies – Expansion Studies Related to Pellet Stability/Cracking

Dilatometer results on CeO2 discs heated with/without a graphite ring Vacuum Argon/5ppmO2 95%Ar/5%H2

+Graphite

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+Graphite

+Graphite

+Graphite

Dilatometer results on CeO2 discs heated under 95%Ar/5%H2 with and without a graphite ring

+ Graphite

+ Graphite

Several of the dilatometer specimens were mounted and polished for microstructural examination Microstructural examination is an important tool for

understanding how changes in processing parameters impact a specimen’s ceramic microstructure

Sectioning of the pellets was performed using a slow speed diamond saw - The pellets were cut dry with no application of any cutting

fluids which could tend to infiltrate into pores or any cracks in the structure of the pellet making further examination more difficult

After sectioning, a pellet was vacuum impregnated cold mounted using a two part epoxy

Standard grinding and polishing techniques were employed to obtain polished mounted specimens for examination

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Specimen microstructure characteristics as a function of dilatometer cover gas with a graphite ring (500x)

Argon/5ppm O2 Vacuum 95%Ar/5%H2

Two Phase

Regions

Microstructures obtained on CeO2 dilatometer specimen heated to 1400oC/4hrs/95%Ar/5%H2 with a graphite ring

Area 1 Area 2

Area 3 Area 4 Area 5

1

2

3 4

5

Microscopy confirms the presence of two distinct phases via polished surface height variations (Hardness)

Area 5

Height profile scan - from left to right

From left to right (µ)

Cra

ck “Softer” lighter

color phase “Harder” darker

color phase

Previous CeO2 dilatometer specimen heated to 1400oC/4hrs/95%Ar/5%H2 a with graphite ring

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Lessons Learned/Summary

Dilatometer experiments were performed on ceramic test specimens produced from commercially available CeO2 powder as a surrogate for PuO2

“Identical” dilatometer test specimens were core drilled from a single larger diameter ceramic disc which were first green pressed + air sintered using standard ceramic processing techniques

Thermal expansion vs. temperature graphs can be employed to provide information on reduction reaction kinetics

The expansion characteristics of CeO2 is a function of a number of factors including; Temperature/Cover gas/Crystalline phases

Dilatometry is a relatively quick and inexpensive technique for measuring the expansion of a sample as a function of possible ceramic processing parameters

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Acknowledgements

This research was conducted under U.S. Department of Energy contract DE-NE0000422

The technical support of Mr. Dirk Cairns-Gallimore (U.S. DOE Office of Space & Defense Power Systems) is greatly appreciated