MNiSn基half-Heusler合金微波固相合成机理及其热电性能研究

The development of basic research of thermoelectric material synthesis holds greater realistic significance for us to respond to energy crisis and environmental challenges. Aiming to solve the problems of e.g. complicated and costly preparation route, difficulties in optimization of the thermal and electrical transport performances of the promising intermediate temperate thermoelectric material of MNiSn(M=Ti, Zr, Hf) based half-Heusler (HH) alloys, this project tries to establish microwave-hot pressing new route with Ti, Zr, Hf, Ni, Sn, etc. metal powders as raw materials and microwave heating as synthesis approach. Based on the investigations on the interaction of metal powders and microwave, the comparison of microwave synthesis and arc smelting methods were investigated. The effects of influencing factors such as stoichiometric ratio, microwave power and irradiation time on microstructure characteristics such as phase compositions, element distributions, lattice parameters, and grain size will be investigated. Then the microwave solid-phase synthesis mechanisms i.e. promotion of ions diffusion, homogenization of phase, and in-situ precipitation of nano crystalline structure will be revealed. The thermal and electrical transport properties such as seeback coefficient, electric conductivity, thermal conductivity will be charactirized. Furthermore, the relatioship between microstructure and thermal and electrical transport properties will be defined. On the basis of above studies, the optimization of thermoelectric performances were studied to establish the regulation mechanism of thermoelectric figure of merit of HH alloys via microwave-hot pressing. The successful realization of this project will enrich scientific understanding of microwave chemistry, and bring important academic value and scientific significance to promote the technical progress in synthesis of thermoelectric materials through microwave heating.

开展热电材料基础研究对我国应对能源和环境危机具有极其重要的现实意义。针对中温热电材料MNiSn(M=Ti,Zr,Hf)基HH合金制备工艺复杂、成本昂贵、热电性能优化困难等问题，本项目拟以Ti、Zr、Hf、Ni、Sn等金属粉为原料，建立微波固相合成-热压烧结制备MNiSn基HH热电合金新方法。在阐述金属与微波作用规律基础上，采用微波合成与电弧熔炼对比，探索原料配比、微波功率、辐射时间等影响因素对产物相组成、杂质分布、晶格参数、晶粒大小等微观结构特征的影响规律,揭示微波强化离子扩散、促进相组成均匀化、促使原位析出纳米晶体结构的合成机理，阐明产物塞贝克系数、电导率、热导率等热、电传输性能与微观结构特征之间的内在关系, 进行热电性能调控研究，建立微波固相合成-热压烧结制备高热电优值HH合金的调控机制。项目的开展对丰富微波化学科学内涵，推动微波在热电材料领域的技术发展具有重要的学术价值和科学意义。

由于日益严重的化石能源枯竭危机和环境污染恶化，对废热进行回收正变得越来越引人瞩目。热电转化技术通过Seebeck和Peltier效应，可将废热直接转化成可利用的电能，为应对现代能源危机提供可持续发展途径。半赫斯勒合金是具有MgAgAs型晶体结构（SG 216 F-43 m）和xyz化学计量组成的18电子金属间化合物，这里x和y属于过渡金属如Ti、Zr、Hf、Ni、Co等，z属于金属或类金属元素如Sn、Sb等。它是一种极具潜力的中高温热电材料，具有较好的导电性、较高的塞贝克系数。半赫斯勒电材料的制备通常采用电弧熔炼法、悬浮熔炼法、固相合成法和机械合金化等制备出前驱体，并采用72 h~2 weeks退火来细化晶粒并使组织均一化，最后采用放电等离子体烧结或热压烧结来获得致密化块体。存在工艺复杂、生产周期长、成本昂贵等突出问题。在项目资助下，我们提出采用微波合成和微波烧结来制备TiNiSn基热电合金块体的技术思路。首次采用粉末冶金与微波合成、微波烧结的全微波制备流程，在4~5 min内即获得了单一相纯度很高的ZrNiSn、TiNiSn合金，并采用微波无压烧结20~30min制备半赫斯勒热电块体，相比传统制备路线，微波加热极大程度缩短制备时间，无需退火。在高效制备的前提下，利用微波场中极大的加热速率，获得了基体晶粒小于等于10μm，原位析出烧结孔和少量热分解第二相尺寸约100~500nm，这些纳米结构强化了量子限域效应，增强了声子界面散射机制，使制备的非掺杂ZrNiSn、TiNiSn热电块体在热性能方面显著优于文献报道，其热导率分别为2.86~3.96 W•m-1•K-1和1.96~2.45 Wm-1K-1。针对电阻率较大的问题，在z位掺杂Sb元素，在保持晶格热导率较低（1.76~3.77 Wm-1K-1）的条件下，使电阻率降低一个数量级，获得的TiNiSb0.05Sn0.95最高功率因子为4042μWm-1K-2（473K）；针对热电优值提高不大的问题，在x位掺杂较重元素来优化成分和微结构，通过调整能带结构来提高综合电性能，获得的Zr0.3Ti0.7NiSn最高无量纲热电优值为0.60（673K）。项目的开展对丰富微波化学科学内涵，推动微波在热电材料领域的技术发展具有重要的学术价值和科学意义。