Ivb Transition Metal Nitrides First Principles Study

The IVB transition metal nitrides, namely, TiN, ZrN, and HfN, are a group of ceramic materials with special properties. These compounds not only possess excellent electric and thermal conductivity but also share the properties of covalent compounds, such as high melting point, high hardness, and high corrosion resistance. And the optical properties also indicate an unusual combination of metallic and dielectric properties. These compounds have attracted considerable attention among researchers. In the field of industrial application, IVB transition metal nitrides mostly serve as cutting materials, ultra-wear-resistant materials, solar absorbing selectively absorbing layer and thermal barrier coating. Thus, these compounds have a significant function in basic research and technical research. The researching of mechanical, optical and thermodynamic properties has very important significance for the application development of these materials.Using the first-principle method, this research studies the structure parameter and elastic constants of TiN, ZrN, and HfN. A detailed investigation on the mechanical, optical and thermodynamic properties of these materials as well as anisotropic conditions and solar-optical selectivity is also performed.The research has found that the electronic structure of IVB transition metal nitrides shows covalence, ionicity and metallicity, respectively, and it is mainly metallicity at ground state. The values of elastic modulus and hardness of the TiN, ZrN, and HfN are calculated and analyzed in detail. The obvious numerical differences of elastic modulus along different crystal orientation suggests typical elastic anisotropy, which increases in the order of TiN�'ZrN�'HfN. This phenomenon results in inevitable lattice distortion and microcracks. The tensile stress strain curves along [100]、[110]、[111] orientation have confirmed the anisotropy of mechanical properties:the materials have favorable toughness along [110], while show typical brittleness along both [100] and [111]. The highest idea tensile strength are appeared in the [111] orientation.In optical properties research, the computed results of complex refractive index, conductivities, absorptions, reflectivities, transmissivities and loss functions of the three materials are analyzed in terms of band structures, which are performed based on complex dielectric functions. Analysis results show that the optical properties of these materials in low-energy regions are metallic because of the free electrons intraband-transition, and the transit to semiconducting properties in high-energy area is caused by valence electrons interband-transition. It is worth noting that the sharp peaks of the reflectivity and transmissivity spectra indicate excellent optical selectivity in the visible light area(1.64eV<E<3.19eV).The thermodynamic properties of TiN, ZrN, and HfN present the structural and mechanical stability in the conditions of high temperature and high pressure. The minimum value of thermal conductivity of the polycrystalline system decreases in the order of TiN�'ZrN�'HfN. The thermal conductivity of single crystals in each crystal orientation presents a nearly anisotropic thermodynamic property. Considering all the thermal properties, HfN is the ideal thermal barrier materials due to its low thermal conductivity and excellent structural and mechanical stability in the limiting conditions.Finally, with red shift of pseudogap, the metallic properties of TiN, ZrN, and HfN improve in turn. And the elastic and optical properties change along with metallic properties improving:brittleness reduces, the anisotropy degree of mechanical and thermal properties increases; the critical energy between intraband and interband transitions increases; solar-optical selectivity decreases. So doping with metal atoms(Ti, Zr, Hf) or C, N atoms to change the portion of conduction electrons is an effective method to control and improve relevant properties of these materials.