Electric Field-Assisted Additive

Manufacturing Polyaniline Based

Composites for Thermoelectric Energy

Conversion

Bruce Y. DeckerDepartment of Mechanical Engineering,

College of Engineering,

California State Polytechnic University,

Pomona, 3801 West Temple Avenue,

Pomona, CA 91768

e-mail: bydecker@cpp.edu

Yong X. Gan1

Mem. ASME

Department of Mechanical Engineering,

College of Engineering,

California State Polytechnic University, Pomona,

3801 West Temple Avenue,

Pomona, CA 91768

e-mail: yxgan@cpp.edu

Polyaniline (PANi) based composites were made by electric forceassisted nanocasting. The PANi matrix was mixed with thermo-electric Bi–Te alloy nanoparticles. The uniform dispersion of thenanoparticles in the polymer was achieved via electric fieldassisted casting. The nanoparticles can enhance the thermoelec-tricity, specifically increase the Seebeck coefficient. Structureanalysis and Seebeck effect experiments were performed. Themicrostructure of the composite materials was studied by the useof electron microscopy. The preliminary results show that thenanocomposites are n-type with an average Seebeck value of30 lV/K. The electrical resistance of the composites is about35 MX. [DOI: 10.1115/1.4029398]

Introduction

Semiconducting polymers have been extensively studied due totheir unique properties and advantages over other ceramic semicon-ducting materials such as light weight and easy to processing [1,2].PANi is a typical semiconducting polymer with a conjugated struc-ture. It has excellent stability in air. PANi is stable at elevated tem-peratures. It has been used for sensors, batteries, super capacitors,etc. It is also considered for thermoelectric energy conversion [3]and explosive decomposition [4]. There is an increasing interest inresearching on PANi because its performances could be improvedsignificantly by incorporating additives [5].

Nanocasting has the capability of generating well-ordered, veryfine entities such as nanofibers or nanorods, coaxial nanotubes,and porous architectures. Traditional casting refers to the solidifi-cation process, whereby a molten metal is transferred into the cav-ity of a mold [6]. Casting is considered as a cost-effectivetechnique for manufacturing various parts [7]. Casting can becombined with rolling, which is the semisolid casting. The moltenmetal is cast into a large ingot. The ingot is subjected to a hot-rolling process. In some cases, the molten metal is cast into thespace of water cooled dies so that the metal is in a mixed liquidand solid state. The semisolid slurry is under the subsequent

extrusion or rolling; thus, segregation is inhibited. Chemical com-position and mechanical properties are more uniform [8]. Inspiredby the casting technique, researchers have considered makingnanoscale components or features using the similar process. Thisis the origin of nanocasting technology. Nanocasting deals mainlywith nonmetallic material processing. For example, oligomericand block copolymers were used to cast nanoporous materials[9–13]. Porous silica materials were obtained via nanocasting on apseudopolyrotaxanes template synthesized from a-cyclodextrinand polyamines [14]. Nanocasting two-dimensionally (2D) or-dered porous arrays using monolayer colloidal crystal templateshave the advantage of generating hierarchical structures at microand nanometer length scales. Polystyrene (PS) beads with the sizeof 1 lm were used to form 2D ordered arrays [15]. The arrayswere served as the casting molds to make Co3O4 hierarchicalstructures.

The external force-assisted nanocasting or spinning concept hasbeen proposed for years [16–19]. This technique has been studiedfor making polymer fibers [16,18,20]. The principles have alsobeen explored for manufacturing ceramic fibers [16,19]. Byextending the external force-assisted nanocasting process conceptto various material systems, it is possible to synthesize fibers assuitable organic–inorganic composites. Up to now, there is verylimited research work done on electric force assisted centrifugalnanocasting thermoelectric composite materials. The challengesof this new technology include: the use of a single step process tomanufacture large scale fiber reinforced materials; increasing theproduction rate of the nanostructures while keeping the cost asso-ciated with the manufacturing process reasonable; and mostimportantly, to manufacture heterogeneous material-supportedcomposites with controlled architectures for thermoelectric energyconversion. It is important to understand the fundamentals of elec-tric force assisted nanocasting manufacturing process and to usethis new approach to make organic–inorganic composites.

Scalable and low cost manufacturing process for making highperformance nanomaterials has caught big attention. A fast, scal-able, and low cost manufacturing process for making high per-formance thermoelectric energy conversion composite materialsemerges as one of the challenge problems to be tackled. Specifi-cally, there is a need to develop a manufacturing process in whichcoupled electromagnetic and mechanical forces are applied. Theapplication of electromagnetic forces in metal forming and weld-ing was reported and quality improvement on material processingwas discussed [21]. In this technical note, it is shown that centrifu-gal casting with a newly designed rotary machine can generate themechanical forces needed. The electric force is added by using ahigh voltage direct current (DC) power source. Magnetic force isimposed using permanent magnets. Thermoelectric nanoparticle-containing semiconducting polymers are successfully cast intomultilayer composite materials. The thermoelectric property ofthe composite materials is characterized in view of the Seebeckcoefficient.

Materials and Methods

All the materials used in this work were purchased from AlfaAesar. Figure 1(a) shows the schematic of the electric forceassisted nanocasting experimental setup. A precision auto lapping/polishing machine was used as the main part. This machine con-tains a rotating platform, whose speed can be well controlled. AShimpo tachometer was used to measure its rotating speed. TheTiO2 nanotube specimen was put at the two ends of a plastic pipebeing fixed on the rotating platform. High voltage of 15 kV wasapplied across the aniline solution. Aniline was electrochemicallypolymerized in the TiO2 nanotubes because the titanium plates, asanodes, were connected by two carbon fiber brushes to the outerring which serves as the positive electrode of the DC powersource. Under the combined electric and mechanical forces, thepolymerized PANi was cast into nanofibers within the TiO2 nano-tubes. The centrifugal casting principle is shown in Fig. 1(b).

1Corresponding author.Contributed by the Manufacturing Engineering Division of ASME for publication

in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript receivedMarch 28, 2014; final manuscript received December 10, 2014; published onlineJanuary 20, 2015. Assoc. Editor: Joseph Beaman.

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The Seebeck coefficient of the nanofiber composite was meas-ured using a self-build measurement system containing a Talboysheat platform with temperature control and a mode 410 ExtechMultimeter. The nanofiber composite was clamped onto two stripsof aluminum tape for voltage measurement. The temperaturedifference was imposed at the two ends of the specimen. Theabsolute values of the Seebeck coefficients obtained at differentmeasuring temperature ranges were obtained and plotted. Theresistance of the composite material was also measured by thetwo point method using a CHI 600E electrochemical workstationrunning under the linear scanning mode.

Results and Discussion

Without combined electric and centrifugal forces, the electro-chemically polymerized PANi started growing just at the surfaceof the titanium dioxide nanotubes as revealed by both the scan-ning electron microscopic (SEM) image of Fig. 2(a) and the trans-mission electron microscopic (TEM) image of Fig. 2(b). Becausethe aniline solution is hydrophobic, it tends to stay on the top sur-face of the titania nanotubes which are hydrophilic. Therefore,electrocentrifugal nanocasting has to be used to draw the PANiinto the nanotubes under coupled electric and mechanical forces.Basically, the electric action is the driving force for polymeriza-tion. The aniline monomer is polymerized by the supplied elec-trons at the cathode because the electrochemical polymerizationof aniline is an oxidation process. The mechanical force due tothe centrifugal effect generated the instability of the PANi suspen-sion, which facilitates the PANi materials move toward thetitanium dioxide nanotube electrode.

The thermoelectric nanoparticles were made by the Galvanicdisplacement method similar to that as reported in Refs. [22–24].In the previous work [22,23], Ni–Fe alloy was used as the raw

material. But in this work, Ni and Co nanoparticles were used asthe precursors to form core shell particles due to the better con-trollability. The surface layer was BiTe alloy and the core was Nior Co. BiTe alloy is used because it is considered as one of thebest materials with high value of Seebeck coefficient in a widetemperature range. Since Ni or Co tends to be attracted by themagnetic force, the BiTe/Ni shell-core nanoparticle clusters (asshown in Fig. 3(a)) in the aniline solution held by the plastic con-tainer (as shown in Fig. 3(b)) move outwardly toward the TiO2

nanotubes. The size of the nanoparticles, 4 nm, as seen fromFig. 3(a), is much smaller than the inner diameter of the nano-tubes, which is about 120 nm as can be seen from Fig. 3(c). Obvi-ously, the external field helped the thermoelectric nanoparticlesgo into the nanotubes.

It is found that the stable absolute value of Seebeck coefficientfor the PANi nanofiber is about 30 lV/K as shown in Fig. 4. Thematerial shows n-type behavior. As compared with the inorganicsemiconducting material, bulky silicon crystalline material, thenanofiber has lower value of Seebeck coefficient. The measuredSeebeck value for silicon bulk material is as high as 40 lV/Kunder the same measurement condition. Therefore, furtherimprovement on the thermoelectric property of the polymericnanomaterial is needed. The electrical resistance of the nanocastPANi nanofiber was measured at the room temperature of 25 �C.The material shows the resistance of 35 MX, which is in the rangeof those typical semiconducting materials.

The Seebeck coefficient is the time dependent at the initialstage. This is because with the transient heat conduction at themoment of contact, the effective temperature difference fluctuatesat the beginning. The heat induced electron ejection behavior sta-bilizes after several minutes. Therefore, the measured Seebeckcoefficient changes with time. After 3 min, the absolute values ofthe Seebeck coefficient are stable as shown in Fig. 4.

Fig. 1 Electric force assisted nanocasting experimental setup and the working principle.(a) Nanocasting unit on a rotating platform and (b) nanocasting under external forces.

Fig. 2 Images of PANi nanofibers on titanium dioxide nanotubes. (a) SEM and (b) TEM.

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Conclusions

Polymer based composite nanomaterial can be processed underthe assistance of high electric field. The nanocomposite showsn-type property with an average absolute value of Seebeck coeffi-cient of 30 lV/K. The electrical resistance of the composite isabout 35 MX. The preliminary results from this work show thepromising of the electric force assisted nanocasting. Future workwill be on developing a commercially viable, scalable manufac-turing process ensuring high process yield, process and productrepeatability and reproducibility, along with optimized qualitycontrol. The scalable manufacturing process should be furthertested for producing functional nanocomposites for high effi-ciency thermoelectric energy conversion.

Acknowledgment

This work was supported by the U.S. National Science Founda-tion (NSF) under Grant No. CMMI-1333044 (Program Manager:Dr. Z. J. Pei) and the Kellogg Legacy Project No. 060650. We alsoacknowledge the support from the Chancellor’s Office, CaliforniaState University through the Campus-as-a Living-Lab Program.

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Fig. 3 (a) TEM image of a Bi–Te/Ni shell-core nanoparticle cluster generated by Galvanic displacement, (b) Bi–Te/Ni in aniline solution, and (c) SEM image of titanium dioxide nanotubes

Fig. 4 The absolute value of the Seebeck coefficient for thePANi composite material

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