Qun Hui

First-principles study on the stability and electronic structure of Mg/ZrB2 interfaces

The geometric optimizations, values of the ideal work of adhesion, interface energies and electronic structures of Mg(001)/ZrB2(001) interfaces with different stacking sequences (top, center and bridge) were studied by the plane wave pseudopotential method based on the first-principles density functional theory (DFT). The results show that the B-terminated top-site (top1 and top2) interfaces have little change and the B-terminated bridge-site interface transforms into a new B-terminated center-site interface, and both the Zr-terminated top- and bridge-site interfaces transform into new Zr-terminated center-site interfaces after geometry optimizations. The bond lengths of Mg-B, interfacial distances and values of the ideal work of adhesion of the newly formed center-site interfaces and the optimized original center-site interfaces are close to each other. The B-terminated center-site interface is the most stable as it has the largest value of the ideal work of adhesion and the smallest interfacial distance. The values of the ideal work of adhesion of the sub-interface regions indicate that the interfaces can improve the bond strengths of the sub-interfaces in Mg side while weaken those in ZrB2 side. The B-terminated (Zr-terminated) center-site interface has negative interface energy and can be formed spontaneously in B-rich (poor) environment. The B-terminated center- and topsite interfaces have both ionic bonds and covalent bonds which exhibit strong directionality in the B-terminated center-site interface. ZrB2 particles are suitable to be used as effective nucleants to refine the grain size of Mg alloy or as reinforcements to prepare Mg matrix composites due to the strongly bonded Mg/ZrB2 interfaces.摘要本文采用平面波密度泛函理论研究了Mg(001)/ZrB2(001)界面的分离功、界面能和电子结构. 首先通过界面结构优化可知, B终端的顶位界 面结构几乎不发生改变; 而B终端的桥位界面结构转变为B终端的中心位界面结构. 另外, Zr终端的顶位界面结构和Zr终端的桥位结构都转变为 Zr终端的中心位界面结构, 新形成的B终端中心界面结构中的Mg–B键的长度、界面距离、分离功与初始的B终端中心界面结构几乎相同. B终端 中心位界面结构有最大的分离功, 同时有最小的界面距离, 因此B终端中心位界面结构是最稳定的界面结构. 次界面位置的分离功表明Mg(001)/ ZrB2(001)界面增强了Mg端的结合强度, 削弱了ZrB2端的结合强度. B终端中心位的界面结构和Zr终端中心界面可以自发形成是由于它们在富B 和在贫B环境中分别有负的界面能. 在B终端中心位和顶位界面结构中, 界面处有共价键和离子键形成, 而且B终端中心位界面结构的共价键方 向性更强. 结果表明, ZrB2颗粒可以用来增强Mg合金.

First-principles study on the stability and electronic structure of Mg/ZrB2 interfaces@@@Mg/ZrB2界面稳定性和电子结构第一性原理研究

The geometric optimizations, values of the ideal work of adhesion, interface energies and electronic structures of Mg(001)/ZrB2(001) interfaces with different stacking sequences (top, center and bridge) were studied by the plane wave pseudopotential method based on the first-principles density functional theory (DFT). The results show that the B-terminated top-site (top1 and top2) interfaces have little change and the B-terminated bridge-site interface transforms into a new B-terminated center-site interface, and both the Zr-terminated top- and bridge-site interfaces transform into new Zr-terminated center-site interfaces after geometry optimizations. The bond lengths of Mg-B, interfacial distances and values of the ideal work of adhesion of the newly formed center-site interfaces and the optimized original center-site interfaces are close to each other. The B-terminated center-site interface is the most stable as it has the largest value of the ideal work of adhesion and the smallest interfacial distance. The values of the ideal work of adhesion of the sub-interface regions indicate that the interfaces can improve the bond strengths of the sub-interfaces in Mg side while weaken those in ZrB2 side. The B-terminated (Zr-terminated) center-site interface has negative interface energy and can be formed spontaneously in B-rich (poor) environment. The B-terminated center- and topsite interfaces have both ionic bonds and covalent bonds which exhibit strong directionality in the B-terminated center-site interface. ZrB2 particles are suitable to be used as effective nucleants to refine the grain size of Mg alloy or as reinforcements to prepare Mg matrix composites due to the strongly bonded Mg/ZrB2 interfaces.摘要本文采用平面波密度泛函理论研究了Mg(001)/ZrB2(001)界面的分离功、界面能和电子结构. 首先通过界面结构优化可知, B终端的顶位界 面结构几乎不发生改变; 而B终端的桥位界面结构转变为B终端的中心位界面结构. 另外, Zr终端的顶位界面结构和Zr终端的桥位结构都转变为 Zr终端的中心位界面结构, 新形成的B终端中心界面结构中的Mg–B键的长度、界面距离、分离功与初始的B终端中心界面结构几乎相同. B终端 中心位界面结构有最大的分离功, 同时有最小的界面距离, 因此B终端中心位界面结构是最稳定的界面结构. 次界面位置的分离功表明Mg(001)/ ZrB2(001)界面增强了Mg端的结合强度, 削弱了ZrB2端的结合强度. B终端中心位的界面结构和Zr终端中心界面可以自发形成是由于它们在富B 和在贫B环境中分别有负的界面能. 在B终端中心位和顶位界面结构中, 界面处有共价键和离子键形成, 而且B终端中心位界面结构的共价键方 向性更强. 结果表明, ZrB2颗粒可以用来增强Mg合金.

Hydrogen Storage Mechanism of Fe3O4: A First-Principles Study

The adsorption and dissociative adsorption of a hydrogen molecule on Fetet1- terminated Fe3O4 (111) surfaces are investigated within the framework of density function theory. The surface Fetet1 atom site with the parallel mode of hydrogen molecule is found to be the most stable equilibrium adsorption state. The most possible dissociative adsorption pathway of a hydrogen molecule needs an energy barrier of 2.85 or 2.87 eV at the surface Oa or Oc atom site, and generates a water molecule, remarkably in agreement with Otsuka.K’s conclusion on the reaction condition and hydrogen storage mechanism of Fe3O4 from microscopic point of view. The calculation of the electronic properties of the termination states with the adsorption and dissociative adsorption of a hydrogen molecule shows strong interactions between atomic hydrogen and Fe3O4 (111) surface.

First-Principle Calculations of the Electronic Structure and Elastic Constants of Arsenic Doped β-SiC

The electronic structure and elastic constants of arsenic doped β-SiC have been studied by first principles density functional theory (DFT) calculations. The band structure, bulk modulus, and density of states were calculated. We have demonstrated that both the top of the valence and the bottom of the conduction band shift to lower energy levels. However, arsenic doped β-SiC exhibits a non-monotonic variation of the band gap and bulk modulus with the concentration of As. Keywords: arsenic doped β-SiC, electronic structure, elastic constant, first principles