Phase Equilibria, Thermodynamic and Transport Properties of Thermoelectric Alloys

技术概述：热电材料和器件可以将热能转化为电能，反之亦然，是解决某些能源需求的可行方案。与传统发电系统相比，其主要缺点是效率较低。热电器件的低效率主要是由于热电合金的低品质系数（ZT）。对于发电应用，高温下的高ZT是必不可少的。该项目的目标是开发在高温下具有高优值的新型热电（TE）合金。性能系数是材料实现高热电转换效率的关键性能。为了提高性能值，需要在保持高热电功率的同时降低晶格导热系数和电阻率。二元热电合金的合金化导致无序的单元胞结构，从而降低了热导率的晶格贡献，从而导致更高的ZT。晶体结构中的晶格无序性可以通过TE合金的熵变增加来评价。由于较新的热电合金通常是多组分合金，因此将对此类系统进行热力学研究，以更好地了解TE合金的性能。该研究项目旨在开发ABX和ABX2型高温热电合金的基本数据（热力学性质，相平衡和输运性质）：其中A和B是过渡金属（Ti, Nb, Co, Mn）， X是硼或硅。利用固态原电池法研究热电合金的热力学性质。确定了化合物的吉布斯生成能和过量的吉布斯混合能。测定合金体系在不同温度下的相稳定性。采用TEM、SEM和XRD等技术对合金相进行表征。热导率、电阻率和塞贝克系数等各种输运特性将通过现有的实验装置来确定。本研究中产生的相稳定性、过量吉布斯混合能、化合物的吉布斯形成能、化合物的可用晶体结构数据将用于新热电合金的相平衡建模。这项工作有望产生新的热力学性质数据，可用于开发新的热电合金。非技术总结：由于能源需求的增加、可燃化石燃料的高成本和耗尽以及当前技术对环境的影响，对替代、可持续能源转换和发电的需求迫在眉睫。热电材料和设备可以将热能转化为电能，反之亦然，是满足能源需求的可行解决方案。热电器件具有静音、可靠、可扩展和无运动部件等优点。然而，与传统发电系统相比，其主要缺点是效率较低。热电器件的低效率主要是由于热电合金的低品质系数（ZT）。对于发电应用，高温下的高ZT是必不可少的。该项目的目标是开发在高温下具有高优值的新型热电（TE）合金。这项研究的结果将提高我们对热电合金的基本认识。生成的热力学性质和相平衡数据将为开发新的热电合金奠定基础。该计划将提供广泛的专业知识（教学和研究），丰富科学知识，并向学生灌输新的创造性和批判性思维。通过拟议的研究项目，该大学的研究和教育将得到加强，特别是少数民族学生。通过hbcu和高中的合作，拟议的研究项目将促进K-12学生和高中教师的经验。将强调基础科学和用于有效利用各种高温过程余热的新型TE合金。研究结果将通过在有关期刊上发表报告和出版物的方式传播给广泛的科学人员网络。

TECHNiCAL SUMMARY:Thermoelectric materials and devices, that can convert heat energy to electrical energy and vice versa, are viable solutions to some energy needs. Compared to conventional power generation systems, their main disadvantage is lower efficiency. The lower efficiency of thermoelectric devices is primarily attributed to the low figure of merit (ZT) of thermoelectric alloys. For power generation applications, high ZT at high temperature is essential. The goal of this project is to develop newer thermoelectric (TE) alloys that have high figure of merit at high temperature. The figure of merit is the key material property that can achieve high heat-to-electricity conversion efficiency. In order to increase the figure of merit, the lattice thermal conductivity and electrical resistivity needs to be lowered while maintaining high thermoelectric power. Alloying the binary thermoelectric alloys leads to disordered unit cell structure thereby reducing the lattice contribution of the thermal conductivity which results in higher ZT. The lattice disorder in the crystal structure can be assessed in terms of increase in entropy change of the TE alloys. Since the newer thermoelectric alloys are typically multi-component alloys, a thermodynamic study of such systems will be investigated to better understand the properties of TE alloys. This research project aims to develop fundamental data (thermodynamic properties, phase equilibria and transport properties) for newer high ZT at high temperature thermoelectric alloys of type ABX and ABX2: where A and B are transition metals (Ti, Nb, Co, Mn) and X is Boron or Silicon. Thermodynamic properties of thermoelectric alloys using solid-state galvanic cell method will be investigated. Gibbs energy of formation of compounds and excess Gibbs energy of mixing will be determined. Phase stability in the alloy systems at different temperatures will be determined. The alloy phases will be characterized using several techniques such as TEM, SEM and XRD. Various transport properties such as thermal conductivity, electrical resistivity and Seebeck coefficient will be determined with already existing experimental setup. Experimental data generated in this study for phase stability, excess Gibbs energy of mixing, Gibbs energy of formation of the compounds, available crystal structural data of the compounds will be used in modeling the phase equilibria of the newer thermoelectric alloys. This work is expected to generate new thermodynamic properties data that can be used in development of newer thermoelectric alloys. NON-TECHNICAL SUMMARY:The demand for alternative, sustainable energy conversion and power generation is imminent due to the increasing demand for energy, high cost and exhaustion of combustible fossil fuels and the environmental impact of current technology. Thermoelectric materials and devices, that can convert heat energy to electrical energy and vice versa, is a viable solution to the energy needs. Thermoelectric devices offer the advantages of being silent, reliable, and scalable, and having no moving parts. However, compared to conventional power generation systems, their main disadvantage is lower efficiency. The lower efficiency of thermoelectric devices is primarily attributed to the low figure of merit (ZT) of thermoelectric alloys. For power generation applications, high ZT at high temperature is essential. The goal of this project is to develop newer thermoelectric (TE) alloys that have high figure of merit at high temperature. The outcome of this research would improve our fundamental understanding of thermoelectric alloys. Thermodynamic properties and phase equilibria data generated will form a basis for the development of newer thermoelectric alloys. The program would provide a wide variety of expertise (teaching and research), enrichment of scientific knowledge and to instill new creative and critical thinking among students. Integration of research and education at the university will be enhanced, particularly minority students through the proposed research project. With the collaboration of the HBCUs and high schools, the proposed research program will facilitate experience for K-12 students and high school teachers. Fundamental science and newer TE alloys used in efficiently harnessing the waste heat from various high temperature processes will be emphasized. The research results will be disseminated to a wide network of scientific personnel via presentations and publications in relevant journals.