Synthesis and Thermoelectric Properties of Bismuth Telluride-based Materials

Abstract

본 연구의 제 1장은 Cu_(0.07)Bi_(2-x)Sb_(x)Te_(x), Cu_(0.07)Bi₂Te_(3-x)Se_(x)과 Cu_(x)Bi₂Te₃ 합성을 통한 Bi₂Te₃ 기반 물질에 도핑된 구리와 열전 특성 변화에 관한 연구이다. 단결정의 Cu_(0.07)Bi₂Te₃가 300 K에서 ZT ~1.15에 도달한 기존 결과를 따라 p-type Cu_(0.07)Bi₂Sb_(2-x)Te₃와 n-type Cu_(0.07)Bi₂Te_(3-x)Se_(x)합성부터 시작하여 열전 효율 향상을 위해 구리 함량을 변화시키며 n-type Cu_(x)Bi₂Te_(2.5)Se_(0.5)를 합성하였다. 합성된 Bi₂Te₃ 기반 물질의 구조와 조성은 PXRD를 통해 분석하였고 온도에 따른 전기전도도, 제벡 상수와 열전도도가 측정되었다. 측정 결과 실온에서 p-type Cu_(0.07)Bi₁Sb₁Te₃의 power factor가 24 μW/cmK², n-type Cu_(0.07)Bi₂Te_(2.5)Se_(0.5)의 power factor가 13 μW/cmK²에 도달했다. 단결정의 Cu_(x)Bi₂Te_(2.5)Se_(0.5)의 경우 425 K에서 최대 ZT 0.83으로 열전 특성이 향상되었다. 제 2장은 Cr이 첨가된 Cr_(x)Bi₂Te₃와 Cr이 치환된 Cr_(x)Bi_(2-x)Te₃의 열전특성에 관한 것이다. 합성된 물질은 PXRD를 통해 구조와 조성을 분석 했고 온도에 따른 전기전도도, 제벡 상수, 열전도도가 측정되었다. 첨가 및 치환된 Cr은 전기전도도를 크게 향상시켰고 제벡 상수는 유지되었으며, 열전도도는 감소시켰다. 결과적으로 실온에서 1% Cr이 첨가된 Bi₂Te₃는 최고 ZT 0.8에, 1% Cr이 치환된 Bi₂Te₃는 최고 ZT 0.65로 순수한 Bi₂Te₃보다 향상되었다.;In chapter 1, in order to study the effect of Cu on the thermoelectric properties of Bi₂Te₃-based materials, polycrystalline Cu-intercalated Bi_(2-x)Sb_(x)Te₃and Bi₂Te_(3-x)Se_(x) were synthesized. Single crystalline Cu_(x)Bi₂Te_(2.5)Se_(0.5) was also prepared by using Bridgman method to investigate the anisotropic effect of these materials. In previous research, an impressive high ZT of ~1.15 at 300 K was reported in the single crystalline bulk Cu_(0.07)Bi₂Te₃system. Such improvement was achieved by reducing the thermal conductivity by intercalated nanostructured Cu precipitates in the bulk system. In order to tailor these materials for high ZT, we have synthesized and characterized p-type Cu_(0.07)Bi_(2-x)Sb_(x)Te₃ and n-type Cu_(0.07)Bi₂Te_(3-x)Se_(x)solid solution by varying the Bi/Sb and Te/Se ratio, respectively. Furthermore, Cu_(x)Bi₂Te_(2.5)Se_(0.5) solid solution was also synthesized to optimize the thermoelectric properties. The crystal structure and composition of the samples are characterized by using PXRD. The temperature dependence of electrical conductivity and Seebeck coefficient of p-type Cu_(0.07)Bi_(2-x)Sb_(x)Te₃ and n-type Cu_(x)Bi₂Te_(2.5)Se_(0.5) samples are measured. The power factor of p-type Cu_(0.07)Bi₁Sb₁Te₃shows the highest value of 24 μW/cmK² at 300K, and n-type Cu_(x)Bi₂Te_(2.5)Se_(0.5) reaches to 13 μW/cmK² at 300 K. To investigate the anisotropic physical properties of single crystalline Cu_(x)Bi₂Te_(2.5)Se_(0.5) were also measured. Cu_(0.01)Bi₂Te_(2.5)Se_(0.5) sample obtained enhancement of thermoelectric property. The dimensionless of figure of merit (ZT) of Cu_(0.01)Bi₂Te_(2.5)Se_(0.5) reaches to 0.83 at 423 K. In chapter 2, Cr-intercalated Cr_(x)Bi₂Te₃ and Cr-substituted Cr_(x)Bi_(2-x)Te₃were prepared using Bridgman method. The phase composition and structure of the samples are characterized by using powder XRD. The temperature dependence of thermal conductivity, electrical conductivity, and Seebeck coefficient are measured. Effect of intercalation and substitution with Cr improved the electrical conductivity and reduced the thermal conductivity, but there is no effective enhancement of Seebeck coefficient. As a result of an improvement, the peak ZT value of 0.8 and 0.65 near room temperature were achieved for 1% Cr-intercalated and 1% Cr-substituted Bi₂Te₃, respectively.