Synthesis and Characterization of New Zintl Phases for Thermoelectrics

第一部分： 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现有能源的能量转换，并产生大量的废热。 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特性解耦，这是一种称为Zintl相的化合物非常适合的任务。 这些化合物的独特和复杂的结构提供了热导率低的材料，因此可以专注于调整电传输性能。该项目由材料研究部的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Zintl相化合物。 该项目确定了新的和现有的具有低热导率的晶体结构，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力的利用。下一代科学家接受材料合成和结构-性能相关性发展的基础培训。本科生和研究生提供实践培训，他们的科学和沟通技能是通过研讨会和个性化的指导发展。 本科实验室和课堂课程的开发也被纳入该项目。 第二部分： 该提案集中于被描述为Zintl相的新型热电材料的合成和朝向高效能量转换技术的结构-性质相关性的开发。热电性基于三个重要的输运参数：塞贝克系数、电阻率和热导率。热电材料的效率由无单位的品质因数zT描述，其取决于所有这三个相互依赖的性质。Zintl相被认为是金属间化合物的一个亚组，其中金属阳离子和阴离子或聚阴离子形成复合结构并且是半导体。Zintl采用电子计数来理解复杂固态结构的基本思想提供了对键合的洞察，并允许系统地优化性能。因此，Zintl相是热电应用的主要候选者，因为它们是具有大的可调谐性的半导体，因为它们含有离子键和共价键。等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节性质。在该项目中，由材料研究部门的固态和材料化学计划支持，针对预期具有低热导率的特定结构类型，并测试Zintl计数规则的应用，以调整电子特性。目标包括进一步优化Zintl结构和合成新化合物，这些化合物的结构预计会产生低热导率和有效的热电能量转换。该项目的重点是尚未研究的混合阳离子和含轻元素的相。新的化合物通过熔剂和冶金途径合成为单晶和高纯度粉末，并通过放电等离子烧结制备完全致密的球团。学生和下一代科学家提供动手培训，他们的科学和沟通技能是通过讲习班和个性化的指导发展。 该研究将在国家和国际会议上发表，研究结果将发表在同行评审期刊上。该奖项反映了NSF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.