Electrospinning a Thermoelectric Polymer

ENMA490 Spring 2012 By: Natan Aronhime Jason Thomen Sepi Parvinian Chris Wolfram Eric Epstein Kevin Mecadon Komal Syed Matt Widstrom

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Motivation � People using too

much energy

� Need renewable sources

� A tremendous amount of energy is wasted as heat

http://www.heatingoil.com/category/blog/opec-blog/

http://www.savewaveenergy.com/store/alternative_energy

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Thermoelectrics

� Bismuth Telluride most common thermoelectric �  Expensive

� Polymers �  Good electrical conductivity �  Naturally poor thermal conductivity

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Design Goals � Design a thermoelectric device using electrospun

conductive polymer fibers � Predict thermoelectric properties of our device

�  Target: Power factor on order of PEDOT:PSS thin film �  Power Factor = 4.8 x 10-6 W/mK2 �  Efficiency = .006%

� Predict size of electrospun fibers based on easily measurable processing parameters to within 9% error

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Intellectual Merit � Extend dimensional analysis (Helgeson et.

al. 2008) to electrospinning with carrier polymer and asses accuracy/applicability

� Compare conductivity and thermoelectric parameters of fibers to bulk

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Broader Impacts and Ethics �  Wasted heat is almost everywhere

�  Efficient conversion of thermal gradient to electricity ΔTà ΔV

�  Optimization of Thermoelectric properties/minimizing cost

�  Increase in fiber alignment by stretching during electrospinning

�  Many thermoelectric devices are either not cost effective or use complicated processing techniques

�  Some of our solvents can be harmful if released in large quantities

�  Electrospinning uses very high voltages

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Basic Design

High V Power Source

Syringe with electrospinning solution

Glass slide with two grounded Cu electrodes

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PEDOT:PSS

� General Properties: �  Conductive polymer �  Soluble in water

� Thermoelectric Properties: �  Electrical Conductivity (thin film) = 850 S/cm

� Bi2Te3 =1100 S/cm (M. Takelishi et al., Japan Symposium on Thermophysical properties, 2006.)

�  Seebeck coefficient (thin film) = ~10 μV/K � Bi2Te3 =287 μV/K (Proceedings of SPIE. 5836, 711)

http://clevios.com/media/webmedia_local/media/datenblaetter/81076210_Clevios_PH_1000_20101222.pdf

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Material Selection �  Electrospin PEDOT:PSS

�  Good electrical properties

�  Research reveals pure PEDOT:PSS very difficult to electrospin

�  Carrier polymer required for electrospinning: Polyacrylonitrile (PAN)

�  Dopant: Sorbitol

�  Copper electrodes �  Lower workfunction than PEDOT-ohmic contact

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Materials Science Aspects

Material Properties

Processing

Materials Chemistry

Solid State Physics

Electrospinning voltage Electrospinning flow rate Shear thinning

Electrical conductivity Thermal conductivity Seebeck coefficient Viscosity Surface tension Dielectric constant Polymer entanglement

Density of states Fermi level shift

Weight percent calculations Solution miscibility Solubility Carrier polymers

Characterization

SEM Tensiometer Viscometer LCR meter Nanovoltmeter

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Technical Approach: VRH � We will test Variable Range Hopping � Electrical conductivity:

�  𝜎= 𝜎↓0 exp[−( 𝑇↓0 /𝑇 )↑𝛾 ] � PEDOT:PSS and PAN can be arranged

either in parallel or in series within the fiber �  𝜎↓𝑃𝑎𝑟𝑎𝑙𝑙𝑒𝑙 = 𝜎↓𝑃𝐴𝑁 𝜑↓𝑃𝐴𝑁 + 𝜎↓𝑃𝐸𝐷𝑂𝑇 𝜑↓𝑃𝐸𝐷𝑂𝑇   

�  𝜎↓𝑆𝑒𝑟𝑖𝑒𝑠 = 1/𝜑↓𝑃𝐴𝑁 /𝜎↓𝑃𝐴𝑁  + 𝜑↓𝑃𝐸𝐷𝑂𝑇 /𝜎↓𝑃𝐸𝐷𝑂𝑇   

�  Lichtenecker equation is an in-between case

�  log(𝜎↓𝑇 )= 𝜑↓𝑃𝐸𝐷𝑂𝑇 log(𝜎↓𝑃𝐸𝐷𝑂𝑇 )+ 𝜑↓𝑃𝐴𝑁 log(𝜎↓𝑃𝐴𝑁 )

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Technical Approach: VRH � Seebeck coefficient

�  𝑆= 𝑘↓𝐵↑2 /2𝑒 (𝑇↓0 𝑇)↑1/2  ( 𝑑𝑙𝑛𝑁(𝐸)/𝑑𝐸 )↓𝐸=𝐸𝑓 

ST = Sa * σa + Sb * σb

σa + σb

σa>>σb

ST = Sa

Annika Lenz, Hans Kariis, Anna Pohl, Petter Persson, Lars Ojamae. (2011). The Electronic Structure and Reflectivity of PEDOT:PSS from Density Functional Theory. Chemical Physics, 384, 1-3

Park, Sungeun, Sung Ju Tark, and Donghwan Kim. "Effect of Sorbitol Doping in PEDOT:PSS on the Electrical Performance of Organic Photovoltaic Devices." Current Applied Physics 11.6 (2011): 1299-301

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Technical Approach: VRH

Using image J DOS information was extracted around the Fermi energy. Optimal Seebeck Coefficient: 6.20E-04 V/K corresponds to a Fermi energy shift of 0.075 eV and 1.8% sorbitol.

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Electrospinning: Solutions Prepared

� Design (Ideal) Solution

-PEDOT:PSS and sorbitol in EG:NMP -PAN in DMF -Individual solutions mixed

� Clevios -  Successfully

electrospun -  7.1 wt% Clevios

(water + PEDOT) -  0.08% wt%

PEDOT -  8 wt% PAN -  DMF solvent

� Diluted Solution

- Ratio PEDOT:PSS to PAN reduced from 5% to 3.34% - Attempted to electrospin: Failed

Background: Electrospinning

� Underlying theory based on “electrohydrodynamics” (Taylor & Melcher, 1966)

�  In 2001, Hohman et al. predict 3 modes that apply to electrospinning/electrospraying: �  Rayleigh mode

� Dominated by surface tension effects � Applies when low/no electric field

�  Axisymmetrical conducting mode �  Whipping conducting mode

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Dominate due to surface charge effects at high field strengths

Whipping vs. Axisymmetrical Modes

�  Take-home message: Balance of electromechanical stresses with intrinsic stresses within fluid (viscosity/capillary stresses) lead to formation of stable jet

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Decreasing Applied Electric Field POLYMER ENGINEERING AND SCIENCE, 2005, 705

Electrospin Modeling: Dimensional Analysis (Helgeson et. al. 2008)

�  captures the jet dynamics during the streching regime

𝛱=   (𝜀− 𝜀↓0 ) 𝐸↑2 𝑅↓𝑗↑3 /𝜋𝜂↓𝑒 𝑄 

𝑂ℎ= 𝜂↓0 /(𝜌𝛾𝑅↓𝑗 )↑0.5  

�  captures free resistance to jet breaking up into droplets and surface flow disturbances which lead to capillary breakup

Simplified expression (Helgeson et. al. 2007)

Electrospin Modeling: Experimental Observations

𝑂ℎ=𝐻𝛱1↑−𝑚  Rj  ∝  solution  parameters

(Figure from Helgeson et. al. 2007)

PEO-water PEO-water-ethanol poly(ethylene terephthalate- co-ethylene isophthalate)-chloroform- Dimethylformamide poly(methyl methacrylate)-dimethylformamide

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Electrospin Modeling: Experimental Observations

𝑂ℎ=𝐻𝛱↑−𝑚  Rj  ∝  solution  parameters

(Figure from Helgeson et. al. 2008)

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Electrospin Modeling: Determination of Rj and Rf relation

�  Usually, this approximation made:

�  Derivation from volume conservation:

�  Wang et. al. 2007 determined experimentally:

𝑅↓𝑓 = 𝑅↓𝑗 𝑤↓𝑝↑0.5 

𝑤↓𝑝 = 𝜋𝑅↓𝑓↑2 𝑙↓𝑓 /𝜋𝑅↓𝑗↑2 𝑙↓𝑗  →𝑤↓𝑝 = 𝑅↓𝑓↑2 𝑙↓𝑓 /𝑅↓𝑗↑2 𝑙↓𝑗  

𝑅↓𝑓 = 𝑅↓𝑗 𝑤↓𝑝↑0.9653 = 𝑅↓𝑗 𝑤↓𝑝↑0.5 𝑤↓𝑝↑0.4653   ;  √𝑙↓𝑗 /𝑙↓𝑓    = 𝑤↓𝑝↑0.4653 

Extra term inverse extension ratio, dominated by solvent evaporation

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Electrospin Modeling: Determination of Rj and Rf relation

(Data from El-Aufy 2004) Limited data, differing solvent evaporation ratesà No smooth fit possible

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Electrospin Modeling: Determination of Rj and Rf relation

Estimate solvent evaporationà qualitatively correct fit at low wp (region of interest for design)à use fit in calculationsà assume (Wang et. al. 2007) b/c PAN in DMF is dominating the fit curve

𝑅↓𝑓 = 𝑅↓𝑗 𝑤↓𝑝↑0.9653 

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Electrospin Modeling: Dimensional Analysis Helgeson 2007 Plot

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Electrospin Modeling: Dimensional Analysis Helgeson 2008 Plot

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Effect of viscosity:

�  Too low viscosity à jet breaks up to polymer droplets à electrospraying

�  Low viscosity causes formation of beaded fiber �  Very high viscosity à unable to pump the solution �  There is a viscosity range which electrospining is

effective �  As viscosity increases so does the average fiber

diameter

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Viscosity of Diluted Solution:

� Viscosity was measured by cone and plate viscometer

� Data was inaccurate due to excessive noise

� Qualitative analysis: Electrospinning failed due to very low viscosity �  Diluted “solution” turned out to

be suspension of polymer, not solution

�  Fluid broke up into droplets rather than forming stable Taylor cone

Viscosity Tests on Clevios Solution: �  Viscosity was measured by cone and plane viscometer �  Shear-Thinning Fluid : �  Still within the window of electrospinning

σ= kɵᵒn n=0.43 (power law index)

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Electrospinning Result: Fibers from Clevios Solution

Fiber diameter range: 100-250nm

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TE Modeling Results: σll (S/cm)

σLi (S/cm)

PFll (W/mK2)

PFLi (W/mK2)

ηll (%) ηLi (%)

Ideal Solution

.125 1.48 E-11 4.79 E-6 5.67 E-16 .006 7.33 E-13

Diluted Solution

.103 1.21 E-11 3.98 E-6 4.67 E-16 .005 6.03 E-13

Clevios .024 5.81 E-12 9.21 E -7 2.23 E-16 .001 2.88 E-13

�  Goal: �  PF = 4.8 x 10-6 W/mK2 � η = .006 %

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TE Measurements on Prototype � Seebeck Coefficient: 𝑆=− ∆𝑉/∆𝑇  � Measure using voltmeter and thermocouple

while heating � Measured a value of 6.36 𝝁𝐕/𝐊 � Attempted to measure electrical conductivity

using four point probe: 𝑅=   𝑙/𝜎𝐴  � Unable to obtain measurements using 4 pt.

probe

Conclusions: Thermoelectric Goals and Insight from Results � Goal: to fabricate nanofiber TE device

that rivaled that of PEDOT:PSS thin films in terms of Power Factor and efficiency

�  Insight from Results: If percolation is reached, can obtain similar efficiency to a doped PEDOT:PSS thin film �  (Less conductive polymer required)

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Conclusion: Electrospinning Goals and Results

�  Goal: Predict diameter of electrospun fibers based on easily measurable processing parameters

�  Results: Helgeson model does not fully agree to experimental observations of complex systems

�  Insight from Results: Dimensional analysis still useful as engineering design tool

�  Future work: Design electrospinning solution using PEO as carrier polymer �  Cleaner environmental impact: use water as solvent �  Obtain accurate trends of dielectric constants, viscosities,

and surface tension in polymer solutions to make more robust model

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Facilities �  Dr. Hu – electrospinning device & SEM

�  Dr. Rabin – electrical conductivity and Seebeck coefficient

�  Physics Machine Shop (Setup prepared) and LCR

meter in Undergraduate Teaching Lab(KIM)- Dielectric Constant Measurements

�  Dr. Calabrese and Dr. Raghvan for rheology measurements

�  Dr. Phaneuf’s lab for solution preparation

Acknowledgements �  First and foremost, we would like to thank Dr.

Phaneuf for helping and guiding us throughout the semester with this project. In addition, we would like to thank all the faculty members who have met with us, helped us, and let us borrow their labs and grad students. Without these resources, we would not have been able to accomplish what we did. Lastly, we would like to extend our sincerest appreciation to the entire faculty of the Materials Science department. It is the education and guidance that you provided over the years that enabled us to come this far.

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