Synthesis and Characterization of New Zintl Phases for Thermoelectrics

第1部分：非技术概述热电材料将温度梯度转化为电能，并可能影响来自太阳能或具有大量废热的现有能源的能量转换。一种材料的转换效率要求它具有像金属一样的良好导电性和像绝缘体一样的低导热性。主要的挑战是将这些相互关联的性质解耦，这是一种被称为Zintl相的化合物非常适合的任务。这些化合物独特而复杂的结构提供了导热系数低的材料，因此可以专注于调整电传输特性。该项目由材料研究部固态和材料化学项目支持，重点是合成含有过渡金属和稀土元素的新型Zintl相化合物。该项目确定了新的和现有的具有低导热性的晶体结构，并为提高有效能量转换的电子特性提供了路线图。这项研究影响了我们未来能源需求的技术，特别是废热转化为电能的利用。下一代科学家在材料合成和结构-性能相关性的发展方面接受基础训练。为本科生和研究生提供实践培训，并通过研讨会和个性化指导培养他们的科学和沟通技能。本科实验室和课堂课程的发展也纳入该项目。本提案的重点是合成新的热电材料，描述为Zintl相和结构-性能相关的高效能量转换技术的发展。热电是基于三个重要的传输参数：塞贝克系数，电阻率和导热系数。热电材料的效率由无单位的优点zT来描述，它依赖于所有这三个相互依赖的特性。锌相被认为是金属间化合物的一个亚群，其中金属阳离子和阴离子或多阴离子形成复杂的结构，是半导体。Zintl的基本思想是利用电子计数来理解复杂的固态结构，为键合提供了洞察力，并允许系统地优化性质。因此，Zintl相是热电应用的主要候选者，因为它们是具有较大可调性的半导体，因为它们含有离子和共价键。同价和异价元素的固溶体提供了一种改变载流子浓度的方法，从而改变电子结构以调整性质。在该项目中，由材料研究部的固态和材料化学项目支持，针对预期具有低导热性的特定结构类型，并测试Zintl计数规则的应用，以调整电子性能。目标包括进一步优化Zintl结构和合成具有低导热率和高效热电能量转换结构的新化合物。该项目的重点是混合阳离子和含轻元素的相，这些尚未被研究过。新化合物通过熔剂和冶金路线合成为单晶和高纯度粉末，并通过火花等离子烧结制备全致密球团。为学生和下一代科学家提供实践培训，并通过研讨会和个性化指导培养他们的科学和沟通技能。这项研究将在国内和国际会议上发表，研究结果将发表在同行评议期刊上。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARY Thermoelectric materials convert a temperature gradient into electricity and could impact energy conversion from solar or current energy sources with significant waste heat available. The conversion efficiency of a material requires that it have good electrical conductivity like a metal and low thermal conductivity like an insulator. The primary challenge is decoupling these interrelated properties, a task that a type of compound known as Zintl phases are well suited for. These compounds' unique and complex structures provide materials where thermal conductivity is low and therefore it is possible to focus on tuning the electrical transport properties. This project, supported by the Solid State and Materials Chemistry program within the Division of Materials Research, is focused on the synthesis of new Zintl phase compounds containing transition metals and rare earth elements. This project identifies new and existing crystal structures with proposed low thermal conductivity and provides a road map for enhancing the electronic properties for efficient energy conversion. This research impacts technology for our future energy needs and specifically the utilization of waste heat conversion into electricity. Next generation scientists receive fundamental training in materials synthesis and development of structure-property correlations. Undergraduate and graduate students are provided hands-on training and their scientific and communication skills are developed through workshops and individualized mentoring. Undergraduate laboratory and classroom curriculum development is also incorporated in the project. PART 2: TECHNICAL SUMMARY This proposal is focused on synthesis of new thermoelectric materials described as Zintl phases and development of structure-property correlations toward efficient energy conversion technologies. Thermoelectricity is based on three important transport parameters: the Seebeck coefficient, electrical resistivity, and thermal conductivity. The efficiency of thermoelectric materials is described by the unitless figure of merit zT, which is dependent upon all three of these interdependent properties. Zintl phases are considered a subgroup of intermetallics where metal cations and anions or polyanions form complex structures and are semiconductors. The foundational idea of Zintl to employ electron counting to understand complex solid state structure provides insight into bonding and allows for systematic optimization of properties. Therefore, Zintl phases are prime candidates for thermoelectric applications as they are semiconductors with large tunability because they contain both ionic and covalent bonding. Solid solutions of iso- and alio-valent elements provide a means to change carrier concentration and therefore the electronic structure to tune properties. Within this project, supported by the Solid State and Materials Chemistry program within the Division of Materials Research, specific structure types that are expected to have low thermal conductivity are targeted and the application of Zintl counting rules is tested in order to tune the electronic properties. Goals include both further optimization of Zintl structures and the synthesis of new compounds with structures that are predicted to give rise to low thermal conductivity and efficient thermoelectric energy conversion. The project focuses on mixed cations and light element containing phases that have not yet been investigated. New compounds are synthesized as single crystals and high purity powders via flux and metallurgical routes and fully dense pellets will be prepared via spark plasma sintering. Students and next generation scientists are provided hands-on training and their scientific and communication skills are developed through workshops and individualized mentoring. The research will be presented at national and international meetings and the findings published in peer-review journals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.