열전 응용을 위한 도핑된 티탄산 스트론튬과 비스무스 텔루라이드의 제조

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2017. 2. 현택환.Enhancement of thermoelectric performance is of key importance in the practical application of thermoelectric materials. Recently, the use of nanostructured bulk materials has been proposed to exploit the reduction in the thermal conductivity and consequently increase in the dimensionless figure-of-merit ZT. In this dissertation, I will discuss synthesis of nanocrystals, their compaction to nanostructured bulk materials, and the thermoelectric property characterization. Amongst many thermoelectric candidate materials, I will focus on nanostructured bulk materials of La-doped SrTiO3 and K-doped Bi2Te3. First, I will report on the synthesis of La-doped SrTiO3 nanoparticles with controlled La doping levels and their enhanced thermoelectric properties. The nanoparticles were sintered using a spark plasma sintering process to fabricate nanostructured bulk materials. The ZT value reaches a maximum of ~0.37 at 973 K, which is ~25% higher than that of the single-crystal bulk material. The increased ZT is due to the reduction of the thermal conductivity while maintaining the electrical conductivity and Seebeck coefficient. In addition, the nanostructured bulk La-doped SrTiO3 exhibits thermal stability even after heat treatment at 973 K, demonstrating their suitability for high-temperature applications. Second, I will report on the preparation of new nanostructured bulk materials of bismuth telluride having non-equilibrium phase compositions. Furthermore, potassium cations, which act as unconventional electron donors, were successfully doped into the bulk bismuth telluride. According to the results of scanning transmission electron microscope with electron energy loss spectroscopy studies and density functional theory calculations, K cations occupy the interlayer and interstitial sites. The resulting compounds with high tellurium and potassium concentrations show increased electrical conductivity and the Seebeck coefficient. Consequently, a high power factor of ~43 μW cm–1 K–2 and the ZT >1.1 at 350 K, which are among the highest values observed for n-type bismuth telluride materials, are achieved in the thermoelectric measurements.Chapter 1. Introduction: Bulk Nanostructured Materials for Enhanced Thermoelectric Efficiency and Dissertation Overview 1 1.1 Introduction to Thermoelectrics 1 1.2 Improvement in Thermoelectric Materials 10 1.3 Nanostructured Thermoelectric Materials with Low Lattice Thermal Conductivities 15 1.3.1 Nanostructuring with Different Phases 20 1.3.2 Nanostructured Bulk from Bottom-Up Process 26 1.3.3 Chemically Synthesized Colloidal Nanocrystals for Nanostructured Bulk Assembly 30 1.3.4 Doping the Nanostructured Bulk using Chemically Synthesized Nanocrystals 34 1.4 Dissertation Overview 36 1.5 References 37 Chapter 2. Enhancement of Thermoelectric Efficiency in Nanostructured Bulk of La-doped SrTiO3 44 2.1 Introduction 44 2.2 Experimental Section 47 2.3 Results and Discussion 50 2.4 Conclusion 81 2.5 References 82 Chapter 3. Extraordinary Off-Stoichiometric Bismuth Telluride for Enhanced n-Type Thermoelectric Performance 88 3.1 Introduction 88 3.2 Experimental Section 92 3.3 Results and Discussion 102 3.4 Conclusion 158 3.5 References 159 Biblography 165 국문 초록 (Abstract in Korean) 171Docto

인터넷 쇼핑몰에서 성공한 제품의 공통요인 연구

학위논문(석사)--서울대학교 대학원 :산업공학과,2000.Maste