Research Progress of Hyperstoichiometric UO2 Crystals

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.