超化学计量比氧化铀晶体的研究进展

Research Progress of Hyperstoichiometric UO2 Crystals

氧化铀不仅是重要的核材料,也是潜在的多功能材料。UO2晶体具有优异的半导体性能和抗辐照能力,其禁带宽度(1.3 eV)与Si(1.1 eV)相近,塞贝克系数是常用热电材料BiTe的4倍,对太阳光的全吸收使其成为高效的太阳能电池材料,在半导体、太阳能和热电等领域具有巨大的应用潜力。但是UO2随着环境变化会出现从缺氧到过氧的价态变化(UO2±x,x=–0.5~1),即超化学计量比特性,给材料制备和性能控制等方面带来很多问题。本文从相图出发,总结了各种铀氧化物的结构及其稳定性,重点聚焦UO2晶体的研究进展。理想化学计量比UO2被认为是最好的Mott绝缘体,其电导率是相对稳定的;超化学计量比氧化铀则具有半导体特性,其电导率、热导率、扩散系数以及光学性能都与x密切相关。目前,UO2晶体生长主要采用化学气相输运法(CVT)、冷坩埚法、水热法、升华法、助熔剂法等,晶体尺寸和质量还不理想,冷坩埚法和水热法被认为是最有潜力的生长技术。氧化铀单晶生长研究不仅有助于深入了解UO2材料特性,也为其在太阳能电池、热电器件以及未来电子学领域的应用提供可能性。

Uranium dioxide is a potential multi-functional material as well as nuclear rod.It exhibits excellent semiconductor performance and anti-irradiation ability.It has the similar band gap(1.3 eV)of silicon crystal(1.1 eV),its Seebeck coefficient is 4 times of the commercial thermoelectric material BiTe,and it shows higher conversion efficiency of solar cells due to its nearly full absorption.These properties make it great potential applications in the fields of semiconductor,solar energy and thermoelectricity.However,the U atoms in uranium dioxide(UO2±x)can vary from–0.5 to 1,which is called hyperstoichiometric characteristics,resulting in some problems in crystal growth and property homogeneity.In this paper,we analyzed the structure and chemical stability of uranium oxides according to U-O phase diagrams,summarized recent research progress on crystal growth and physical properties of UO2 crystals.UO2 is an ideal Mott insulator with a stable electric conductivity,while the hyperstoichiometric UO2±x crystals are semiconductors,and their physical properties,including electric conductivity,thermal conductivity and diffusion coefficient,and optical properties,are closely related to x.So far,UO2 crystals have grown via several methods,such as chemical vapor transport(CVT),sublimation,skull melting,hydrothermal and flux.The skull melting and hydrothermal techniques are expected to improve crystal dimensions and quality in future.The growth of UO2 crystals is expected to enhance the understanding of the material and provide the possibility of great potential applications in solar cells,thermoelectric devices and future electronics.