势垒过滤优化典型Half Heusler材料热电性能的研究

In this project, based on the stratagem of low energy carriers filtering, the potential barriers rising from grain boundaries and interfaces between bulk and nano composites would be designed and tuned to improve the thermoelectric properties of typical Half Heusler (HH) materials and refine the related transportation model via theoretical and experimental research. The band structure, density of states, doping behavior and transportation mechanism of typical HH and composite materials would be explored by the first principles calculation theoretically and the four coefficient methods experimentally. By matching the band structure and density of states between the bulk HH and nano composites, barrier with optimized height even in high temperature range can be designed. Various methods of the barrier tuning, such as controlling of the Fermi level of the bulk HH and nano composites through doping, modification of the grain boundaries and changing of the grain size, will be explored for the thermoelectric properties improving, and an full tuning technics of the potential barriers in HH nano composites. Combining the low energy carriers filtering effects on transportation and thermoelectric properties through the potential barriers originated from grain boundaries and nano composites, a refined model considering barriers distribution will be provided in this project.

本项目拟结合理论与实验手段，应用势垒过滤策略，设计、调控晶界和纳米复合颗粒在典型Half Heusler (HH)材料中所形成的势垒，优化热电性能，完善理论模型。采用第一原理方法，计算典型HH和复合相的能带结构、态密度、掺杂行为；用四参数法研究"HH+纳米复合颗粒"体系的能带结构和输运特性。匹配基体与复合相的能带结构、态密度，设计具有最优势垒高度且能保持到高温的HH热电材料体系；制备典型HH材料体系的单晶和多晶纳米复合材料，控制基体与复合相费米能级、改变晶界成分和晶粒尺寸以调控势垒高度，探索多晶HH材料中势垒调控方法与手段，优化HH材料热电性能；了解势垒分布特征对输运过程、热电性能的影响,综合考虑晶界势垒和纳米复合颗粒势垒的载流子过滤效果，完善已有势垒过滤模型。

本项目研究了制备势垒结构，过滤低能载流子，提高赛贝克同时保持适当电导率来提高功率因子，即势垒过滤实现热电性能的优化。理论方面，在单抛物线能带和单非抛物线能带基础上，项目组建立了包含简单晶界过滤，考虑势垒隧穿，考虑能量量子化导致共振隧穿的载流子输运模型。实验方面，针对Half- Heusler材料，单掺杂和双掺杂虽未能有效引入势垒结构，但源于掺杂导致的散射作用，依然对体系热电优值有所提高。Sb位Sn掺杂ZrCoSb，ZrCoSb0.7Sn0.3在973 K时最大ZT值为0.52；Sb位Sn掺杂NbCoSb，NbCoSb0.8Sn0.2在973 K时最大ZT值为0.56，比未掺杂NbCoSb高~40%；Nb位Ta，V双掺杂NbCoSb，Nb0.44V0.44Ta 0.12CoSb在973 K时最大值为0.5，比未掺杂NbCoSb高~25%。针对构造核壳结构引入势垒提高热电优值方面，我们以n型Bi2Te3基材料和p型Mg2X材料为对象，通过阴离子置换原理成功构筑微纳核壳结构基元，并作为添加物添加到基体材料中以形成复合热电块体材料。结果表明，研磨热压法制得的Mg2Si@Mg1.92Li0.08Ge0.4Sn0.6微纳核壳结构可使最高ZT值达到0.75，相比p型Mg1.92Li0.08Ge0.4Sn0.6提升50%；溶剂法和研磨热压法制得的Bi2S3@Bi2(Te0.93Se0.07)3(TeI4)0.005微纳核壳结构可使最高ZT值分别达到0.78和0.99，相比于纯n型Bi2(Te0.93Se0.07)3(TeI4)0.005（ZT=0.65）分别提升了20%和52%；核壳结构提高热电优值的原因可能来自于下面两个方面： 其一，微纳核壳结构增强了声子散射，降低晶格热导率；其二，微纳核壳结构引起低能载流子过滤，达到提高功率因子、降低电子热导率的作用。因此，核壳结构引入的势垒过滤效果是一种广泛适用的提高热电性能的策略。