Nanpu Cheng

Electrochemical hydrogen property improved in nano-structured perovskite oxide LaFeO3 for Ni/MH battery

Perovskite oxide LaFeO3, as a novel candidate for the electrode of Ni/MH battery, holds high specific capacity and good cyclical durability at elevated temperatures. However, the poor electrochemical kinetics is a bottleneck for the application of this type of material. By use of nano-structured materials, there are greatly enhanced values of exchange current density I0 and hydrogen diffusion coefficient D, which resulted in an improvement of electrochemical kinetics, a much higher specific capacity and excellent stability during cycling for nano-structured LaFeO3. In theory, there is a significant possibility of further advancing the hydrogen reaction kinetics of perovskite type oxides for Ni/MH battery.

Tunable Bandgap and Optical Properties of Black Phosphorene Nanotubes

Black phosphorus (BP), a new two-dimensional material, has been the focus of scientists’ attention. BP nanotubes have potential in the field of optoelectronics due to their low-dimensional effects. In this work, the bending strain energy, electronic structure, and optical properties of BP nanotubes were investigated by using the first-principles method based on density functional theory. The results show that these properties are closely related to the rolling direction and radius of the BP nanotube. All the calculated BP nanotube properties show direct bandgaps, and the BP nanotubes with the same rolling direction express a monotone increasing trend in the value of bandgap with a decrease in radius, which is a stacking effect of the compression strain on the inner atoms and the tension strain on the outer atoms. The bending strain energy of the zigzag phosphorene nanotubes (zPNTs) is higher than that of armchair phosphorene nanotubes (aPNT) with the same radius of curvature due to the anisotropy of the BP’s structure. The imaginary part of the dielectric function, the absorption range, reflectivity, and the imaginary part of the refractive index of aPNTs have a wider range than those of zPNTs, with higher values overall. As a result, tunable BP nanotubes are suitable for optoelectronic devices, such as lasers and diodes, which function in the infrared and ultra-violet regions, and for solar cells and photocatalysis.

Electronic, Optical, and Lattice Dynamical Properties of Tetracalcium Trialuminate (Ca4Al6O13)

The electronic, optical, and lattice dynamical properties of tetracalcium trialuminate (Ca4Al6O13) with a special sodalite cage structure were calculated based on the density functional theory. Theoretical results show that Ca4Al6O13 is ductile and weakly anisotropic. The calculated Young’s modulus and Poisson ratio are 34.18 GPa and 0.32, respectively. Ca4Al6O13 is an indirect-gap semiconductor with a band gap of 5.41 eV. The top of the valence band derives from O 2p states, and the bottom of conduction band consists of Ca 3d states. Transitions from O 2p, 2s states to empty Ca 4s, 3d and Al 3s, 3p states constitute the major peaks of the imaginary part of the dielectric function. Ca4Al6O13 is a good UV absorber for photoelectric devices due to the high absorption coefficient and low reflectivity. The lattice vibration analysis reveals that O atoms contribute to the high-frequency portions of the phonon spectra, while Ca and Al atoms make important contributions to the middle- and low-frequency portions. At the center of the first Brillouin zone, lattice vibrations include the Raman active modes (E, A1), infrared active mode (T2), and silentmodes (T1, A2). Typical atomic displacement patterns were also investigated to understand the vibration modes more intuitively.

Intermetallic Growth and Interfacial Properties of the Grain Refiners in Al Alloys

Al3TM(TM = Ti, Zr, Hf, Sc) particles acting as effective grain refiners for Al alloys have been receiving extensive attention these days. In order to judge their nucleation behaviors, first-principles calculations are used to investigate their intermetallic and interfacial properties. Based on energy analysis, Al3Zr and Al3Sc are more suitable for use as grain refiners than the other two intermetallic compounds. Interfacial properties show that Al/Al3TM(TM = Ti, Zr, Hf, Sc) interfaces in I-ter interfacial mode exhibit better interface wetting effects due to larger Griffith rupture work and a smaller interface energy. Among these, Al/Al3Sc achieves the lowest interfacial energy, which shows that Sc atoms should get priority for occupying interfacial sites. Additionally, Sc-doped Al/Al3(Zr, Sc) interfacial properties show that Sc can effectively improve the Al/Al3(Zr, Sc) binding strength with the Al matrix. By combining the characteristics of interfaces with the properties of intermetallics, the core-shell structure with Al3Zr-core or Al3Zr(Sc1-1)-core encircled with an Sc-rich shell forms.

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.