UNIVERSITY OF CALIFONIA SANTA CRUZ HIGH TEMPERATURE EXPERIMENTAL CHARACTERIZATION OF MICROSCALE THERMOELECTRIC EFFECTS A dissertation submitted in partial satisfaction of the requirements for the degree of DOCTOR OF PHILOSOPHY in ELECTRICAL ENGINEERING by Tela Favaloro September 2014 The Dissertation of Tela Favaloro is approved: Professor Ali Shakouri, Chair Professor Joel Kubby Professor Nobuhiko Kobayashi Tyrus Miller Vice Provost and Dean of Graduate Studies Copyright © by Tela Favaloro 2014 This work is licensed under a Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 International License Table of Contents List of Figures ............................................................................................................................ vi List of Tables ........................................................................................................................... xiv Nomenclature .......................................................................................................................... xv Abstract ................................................................................................................................. xviii Acknowledgements and Collaborations ................................................................................. xxi Chapter 1 Introduction .......................................................................................................... 1 1.1 Applications of thermoelectric devices for energy conversion ................................... 1 1.2 Physical Effects ............................................................................................................ 6 Seebeck Coefficient ........................................................................................................... 6 Peltier Effect ...................................................................................................................... 8 Joule heating ..................................................................................................................... 9 1.3 Thermoelectric material properties governing efficiency ......................................... 10 Thermoelectric module design ....................................................................................... 10 Material Figure of Merit.................................................................................................. 12 1.4 Advances in material synthesis ................................................................................. 14 Interdependence of thermoelectric material properties ............................................... 14 Material nanostructuring theory .................................................................................... 16 1.5 Advanced materials in this work ............................................................................... 18 III-V films with embedded nanostructures ..................................................................... 18 Nitride superlattices ........................................................................................................ 20 Nanostructured bulk Mg2Si ............................................................................................. 21 Vanadium dioxide nanobeams ....................................................................................... 22 1.6 High temperature material characterization methodologies and scope of work ..... 24 In-plane and cross-plane measurements........................................................................ 24 Thermoreflectance imaging microscopy ........................................................................ 25 Transient Harman technique .......................................................................................... 27 High temperature considerations ................................................................................... 28 Chapter 2 Characterization techniques for thermoelectric material and device properties 29 2.1 In-plane measurements .......................................................................................... 30 Van der Pauw method for electrical conductivity .......................................................... 30 Differential Seebeck coefficient measurement .............................................................. 33 iii Consolidation of in-plane measurements ....................................................................... 39 2.2 Cross-plane measurements .................................................................................... 41 3ω method for measurement of thermal conductivity .................................................. 41 Extension to measurement of the cross-plane Seebeck coefficient .............................. 48 Chapter 3 Thermoreflectance Thermal Imaging.................................................................. 52 3.1 General Principle ..................................................................................................... 53 4-bucket approach to thermal imaging: homodyne method ......................................... 55 8-bucket technique: simultaneous imaging of Peltier and Joule .................................... 59 Transient thermal imaging: pulsed boxcar averaging ..................................................... 61 3.2 Applications of thermal mapping: Peltier and Joule surface thermal profiles for high temperature imaging .................................................................................................. 63 3.3 High temperature experimental calibration method ............................................. 69 Room temperature calibration measurements .............................................................. 69 High temperature calibration methodology ................................................................... 72 Gold ................................................................................................................................. 79 Platinum .......................................................................................................................... 84 Aluminum ........................................................................................................................ 86 Equations governing temperature dependence and the effect of passivation .............. 88 Chapter 4 Transient Harman technique for direct measurement of the cross-plane figure of merit of thin films at high temperatures ............................................................................ 95 4.1 Theory and measurement requirements .................................................................. 96 4.2 Optimization of device design ................................................................................... 98 4.3 Measurement Details and Results .......................................................................... 103 Application of the transient Harman technique ........................................................... 103 Thermoreflectance imaging .......................................................................................... 107 High temperature experimental results and cross-plane Seebeck ............................... 109 4.4 Finite Element Modelling ........................................................................................ 114 Chapter 5 High magnification imaging of vanadium dioxide nanobeams .......................... 121 5.1 VO2 material background information ...................................................................... 121 5.2 Measurement Specifics ............................................................................................. 123 5.3 Measurement results: periodic domain organization .............................................. 125 5.4 Measurement results: irregular domain configuration ............................................ 129 5.5 Discussion .................................................................................................................. 133 Chapter 6 Design and fabrication of high temperature vacuum thermostat ................... 139 iv 6.1 General Considerations ......................................................................................... 139 6.2 Bulk measurement stage ...................................................................................... 141 6.3 Chamber for optical measurement ....................................................................... 144 Chapter 7 Conclusions ......................................................................................................... 147 High temperature thermoreflectance imaging and transient Harman characterization of thermoelectric energy conversion devices ....................................................................... 150 Characterization of the temperature dependence of the thermoreflectance coefficient for conductive thin films ......................................................................................................... 151 Direct observation of nanoscale Peltier and Joule effects at metal-insulator domain walls in vanadium dioxide nanobeams ...................................................................................... 152 References ............................................................................................................................ 155 v List of Figures Figure 1.1: Radioisotope thermoelectric generator used by NASA to power the Curiosity Mars rover. Right hand side is a picture taken by the rover