Influence of porosity on the transport properties of Bi 2 Te 3 -based alloys by field-assisted sintering
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ter A. Sharma Materials Physics, Sandia National Laboratories, Livermore, California 94551-0969
Nancy Yang Energy Nanomaterials Sciences, Sandia National Laboratories, Livermore, California 94551-0969
Enrique J. Lavernia Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California 95616 (Received 19 November 2012; accepted 1 April 2013)
Retention of a nanostructure in thermoelectric materials through rapid sintering (e.g., field-assisted sintering) is generally associated with leaving certain amounts of porosity due to short sintering times. In this study, the influence of porosity on the thermoelectric transport properties in Bi2Te3-based alloys was studied by changing the sintering pressure during spark plasma sintering. N-type Bi2Te3 and p-type (Bi0.2Sb0.8)2Te3 were sintered at 673 K using pressures from 50 to 300 MPa to obtain different levels of porosity. Electrical resistivity, thermal conductivity, Seebeck coefficient, carrier concentrations, and Hall mobility were measured and characterized. The results show that increasing sintering pressure is effective in reducing porosity, which lowers electrical resistivity and increases the carrier concentrations. The transport properties were fitted to general effective medium equations and demonstrate that in p-type (Bi0.2Sb0.8)2Te3 sintered at high pressures, decreases in electrical resistivity and lattice thermal conductivity exceeded the Seebeck coefficient reduction, improving the thermoelectric figure of merit. I. INTRODUCTION
Direct conversion of thermal and electric energies through thermoelectric effect has attracted extensive interest in areas of waste heat recovery and environmentally friendly energy harvesting. The efficiency of a thermoelectric material is given by the dimensionless figure of merit ZT, which is defined as ZT 5 S2T/(qj), where S is the Seebeck coefficient, q is the electrical resistivity, and j is the thermal conductivity. These transport parameters are interdependent and related to the microstructure, acoustic phonon scattering at the grain boundary, atomic defects, as well as the mobility and concentration of carriers. For these reasons, maximization of ZT requires an optimization of these interdependent parameters of S, q, and j.1,2 Nanostructured Bi2Te3-based materials have shown improvements to the figure of merit ZT and this was attributed to the thermal conductivity reduction by scattering of phonons at increased amounts of interfaces between nanoscale grains.3–7 In our previous study on nanostructured pure Bi2Te3 prepared via ball milling and the field-assisted sintering technique (FAST), the sintered samples, however, contained a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.99 J. Mater. Res., 2013
nanoscale porosity and exhibited high electrical resistivity and low carrier concentration that offset the reduction in thermal conductivity and increase in the Seebeck coefficient and hence compromised ZT.6 These observations raise the question on how
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