Zhiqian Chen

Study of Electronic Structure, Thermal Conductivity, Elastic and Optical Properties of α, β, γ-Graphyne

In recent years, graphyne was found to be the only 2D carbon material that has both sp and sp2 hybridization. It has received significant attention because of its great potential in the field of optoelectronics, which arises due to its small band gap. In this study, the structural stability, electronic structure, elasticity, thermal conductivity and optical properties of α, β, γ-graphynes were investigated using density functional theory (DFT) systematically. γ-graphyne has the largest negative cohesive energy and thus the most stable structure, while the β-graphyne comes 2nd. Both β and γ-graphynes have sp-sp, sp-sp2 and sp2-sp2 hybridization bonds, of which γ-graphyne has shorter bond lengths and thus larger Young’s modulus. Due to the difference in acetylenic bond in the structure cell, the effect of strain on the electronic structure varies between graphynes: α-graphyne has no band gap and is insensitive to strain; β-graphyne’s band gap has a sharp up-turn at 10% strain, while γ-graphyne’s band gap goes up linearly with the strain. All the three graphynes exhibit large free carrier concentration and these free carriers have small effective mass, and both free carrier absorption and intrinsic absorption are found in the light absorption. Based on the effect of strain, optical properties of three structures are also analyzed. It is found that the strain has significant impacts on their optical properties. In summary, band gap, thermal conductivity, elasticity and optical properties of graphyne could all be tailored with adjustment on the amount of acetylenic bonds in the structure cell.

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.

Lead-free K0.5Na0.5NbO3–Bi0.5Li0.5ZrO3–BiAlO3 ternary ceramics: Structure and piezoelectric properties

Lead-free perovskite (0.995–x)(K0.5Na0.5)NbO3–x(Bi0.5Li0.5)ZrO3–0.005BiAlO3 ternary piezoelectric ceramics were projected and prepared by a conventional solid-state method. A research was conducted on the effects of (Bi0.5Li0.5)ZrO3 content on the structure and piezoelectric properties of the ceramics. By combining the X-ray diffraction patterns with the temperature dependence of dielectric properties, a rhombohedral–orthorhombic–tetragonal phase coexistence was identified for the ceramics with 0.02xa0≤xa0xxa0≤xa00.025, and a rhombohedral–tetragonal phase boundary was determined in the composition xxa0=xa00.03. Upon further increasing the (Bi0.5Li0.5)ZrO3 content, the rhombohedral–tetragonal phase boundary transformed to a single rhombohedral structure with xxa0≥xa00.035. An obviously improved piezoelectric activity was obtained for the ceramics with compositions in and around the rhombohedral–tetragonal phase boundary, among which the composition xxa0=xa00.025 exhibited the maximum values of piezoelectric constant d33, and planar and thickness electromechanical coupling coefficients (kp and kt), of 252 pC/N, 0.366, and 0.466, respectively. In addition, the ceramic with xxa0=xa00.025 was found to possess a relatively high Curie temperature of 368xa0°C, suggesting it may have a prospect for applications at elevated ambient temperatures.

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.

Deformation Modes and Anisotropy of Anti-Perovskite Ti3AN (A = Al, In and Tl) from First-Principle Calculations

Deformation modes were studied for Ti3AN (A = Al, In and Tl) by applying strain to the materials using first-principle calculations. The states of the bonds changed during the deformation process, and the Ti-N bonds remained structurally stable under deformation. The elastic anisotropy, electronic structures, hardness, and minimum thermal conductivity of anti-perovskite Ti3AN were investigated using the pseudo potential plane-wave method based on density functional theory. We found that the anisotropy of Ti3InN was significantly larger than that of Ti3AlN and Ti3TlN. All three compounds were mechanically stable. The band structures of the three compounds revealed that they were conductors. The minimum thermal conductivities at high temperature in the propagation directions of [100], [110], and [111] were calculated by the acoustic wave velocity, which indicated that the thermal conductivity was also anisotropic. It is indicated that Ti3InN is a good thermal barrier material.

Elastic properties, hardness, and anisotropy in baddeleyite IVTMO2 (M=Ti, Zr, Hf)

In this article, we used plane-wave density functional theory to investigate the elasticity, anisotropy, and minimum thermal conductivities of baddeleyite type the IVTMO2 (m-TiO2, m-ZrO2, and m-HfO2). The elastic constants and modulus, Poisson’s ratio, hardness, sound speed, Debye temperature, and minimum thermal conductivities at high temperature were calculated. These calculations show that m-MO2 is not superhard, with a hardness range of about 8–13 GPa. Among these materials, m-TiO2 is the hardest, while m-HfO2 is the least hard. Their elastic and plastic anisotropy are given in detail. Moreover, the m-HfO2 thin film is the most likely to develop microcracks during preparation because it has the highest elastic anisotropy. Among the three dioxides, m-HfO2 is the best thermal barrier because it has the lowest thermal conductivity.中文摘要本文用平面波密度泛函理论研究了斜锆石型IVTMO2 (m-TiO2, m-ZrO2和m-HfO2)的弹性, 各向异性以及最小热导率. 通过 计算给出了弹性常数及其模量、泊松比、硬度、声速及德拜温度高温下的最小热导率. 结果表明, m-MO2不是超硬材料, 其硬度范围 为8−13 GPa. m-TiO2是其中最硬的, 而m-HfO2的硬度最小. 同时还对弹性及塑性的各向异性进行了详细的分析, 表明由于m-HfO2具有 最强的各向异性, 在制作薄膜时最容易产生微裂纹. 值得指出的是, 在三种氧化物中, m-HfO2由于具有最小的高温热导率而最有可能作 为热障材料应用.

Composition dependence of phase structure and piezoelectric properties in (0.98 − x )(K 0.4 Na 0.6 )NbO 3 –0.02CaZrO 3 – x Bi 0.5 Na 0.5 HfO 3 ternary ceramics

Lead-free perovskite (0.98u2009−u2009x)(K0.4Na0.6)NbO3–0.02CaZrO3–xBi0.5Na0.5HfO3 (KNN–CZ–xBNH) ceramics were prepared by a conventional solid-state reaction method, and the connections between their composition, phase structure, and electrical properties were investigated. The X-ray diffraction results, together with the temperature dependence of dielectric properties, showed that a rhombohedral–tetragonal (R–T) phase boundary was successfully constructed in the composition range of 0.04u2009≤u2009xu2009≤u20090.045. An enhancing effect of R–T phase boundary on the piezoelectric activity was observed, and the ceramics with the composition xu2009=u20090.045 exhibited the highest piezoelectric properties, possessing a piezoelectric constant d33 of 320xa0pC/N and a planar electromechanical coupling coefficient of 0.40. In addition, it was found that while adding a small amount of Bi0.5Na0.5HfO3 could result in very large grain sizes in the (K,Na)NbO3-based ceramics, adding too much of it would lead to extremely fine grain sizes. In general, our experimental results indicate that the KNN–CZ–xBNH ceramics are promising candidate materials for lead-free piezoelectric applications.

Elasticity, slowness, thermal conductivity and the anisotropies in the Mn3Cu1−xGexN compounds

We perform the first-principles to systematically investigate the elastic properties, minimum thermal conductivity and anisotropy of the negative thermal expansion compounds Mn3Cu1−xGexN. The elastic constant, bulk modulus, shear modulus, Young’s modulus and Poisson ratio are calculated for all the compounds. The results of the elastic constant indicate that all the compounds are mechanically stable and the doped Ge can adjust the ductile character of the compounds. According to the values of the percent ratio of the elastic anisotropy AB, AE and AG, shear anisotropic factors A1, A2 and A3, all the Mn3Cu1−xGexN compounds are elastic anisotropy. The three-dimensional diagrams of elastic moduli in space also show that all the compounds are elastic anisotropy. In addition, the acoustic wave speed, slowness, minimum thermal conductivity and Debye temperature are also calculated. When the ratio of content for Cu and Ge arrived to 1:1, the compound has the lowest thermal conductivity and the highest Debye tempe...

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.