Thermal and Thermoelectric Transport Studies in Nanomaterials Using Microfabricated Testbench

Open Access
Author:
Kim, Duksoo
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 04, 2014
Committee Members:
  • Srinivas A Tadigadapa, Dissertation Advisor
  • Jerzy Ruzyllo, Committee Member
  • Suman Datta, Committee Member
  • Gerald Dennis Mahan, Committee Member
Keywords:
  • Thermal
  • Thermoelectric
  • Nanomaterials
  • Testbench
Abstract:
This dissertation presents development of a microfabricated device (testbench) for measuring thermal conductivity and Seebeck coefficient of low-dimensional materials in addition to electrical conductivity and discusses the thermal and thermoelectric transport in nanoscale systems based on measurements performed using testbench. Despite the potential and obvious advantages of thermoelectric transduction, widespread use of such systems for energy harvesting and refrigeration remains limited to niche and specialized applications due to low efficiency. The thermoelectric efficiency is represented by a dimensionless figure of merit ZT = σS2T/κ, where σ is electrical conductivity, S is Seebeck coefficient, κ is thermal conductivity, and T is absolute temperature. Low-dimensional structures are expected to have improved power factor (σS2) and suppressed lattice thermal conductivity, and can therefore exhibit higher ZT than their bulk counterparts. Thus, there is a critical need for understanding thermal and thermoelectric transport in low-dimensional materials. However, in the case of low-dimensional materials, the accurate measurement of the thermal and thermoelectric properties is extremely challenging because of their small size. In order to address this need, we have developed a microfabricated testbench that can be used to measure two-probe electrical conductivity, thermal conductivity, and Seebeck coefficient of a variety of low-dimensional materials. Using the testbench, the thermoelectric efficiency of bismuth telluride nanotubes has been investigated. The bismuth telluride is one of the most efficient thermoelectric materials. Although the efficiency of bismuth telluride nanowires and nanoplates was already reported by other groups, no enhancement in the ZT in these materials was observed. This has been primarily attributed to the unexpectedly small Seebeck coefficient caused by unintentional doping or surface band bending. Owing to small wall thickness (< 15nm) and nanocrystalline nature, bismuth telluride nanotubes synthesized by solution-phase method showed ZT of 0.75 at T = 300 K which is 88 % larger than bulk bismuth telluride. As a further application of the microfabricated testbench, studies on the effects of magnetic field on the thermoelectric transport in GaAs/MnAs core/shell nanowires and Co nanowires have been carried out in this work. The thermoelectric transport in magnetic nanoscale systems has been the subject of intense investigation since the recent observation of spin-Seebeck effect in ferromagnetic thin films. By measuring Seebeck coefficient and the electrical conductivity of ferromagnetic nanowires under the influence of magnetic field, it has been shown that the absolute value of Seebeck coefficient is increased by electron-magnon scattering whereas it is decreased by s-d scattering (scattering of spin-up 4s electrons into spin-down 3d states)