CdGeAs2晶体的结构、弹性和电子性能的模拟计算和分析

利用基于密度泛函理论的平面波赝势法对CdGeAs2晶体的结构,弹性和电子性能进行了研究.CdCeAs2晶体具有6个独立的弹性常数,利用计算的弹性性能可以判断晶体具有机械稳定性、延性和弹性各向异性等力学特点.通过总的和部分态密度分析了不同能带的贡献成分.计算的结构参数及弹性常数与实验值基本吻合.CdGeAs2晶体具有直接...     

硫靛染料发色体结构与性质的泛函理论研究

在密度泛函理论(DFT)的B3LYP/6-31 G*水平上对硫靛染料发色体的几何结构进行优化计算,在获得基态稳定结构的基础上,应用含时密度泛函理论(TD—DFT)方法在相同水平计算电子吸收光谱。探讨了给电子基团对电子吸收光谱的影响。结果表明:给电子基团给电子能力的增强使光谱产生一定的红移。通过对前线轨道组成进行自然布居分析,揭示了硫靛染料的发光均源自分子中HOMO-LUMO(π→π*)电子跃迁。     

Elastic,mechanical, electronic,and defective properties of Zr–Al–C nanolaminates from first principles

By means of first principles calculations, Zr–Al–C nanolaminates have been studied in the aspects of chemical bonding, elastic properties, mechanical properties, electronic structures, and vacancy stabilities. Although the investigated Zr–Al–C nanolaminates show crystallographic similarities, their predicated properties are very different. For (ZrC)_nAl₃C₂ (n = 2, 3, 4), the Zr–C bond adjacent to the Al–C slab with the C atom intercalated in the Zr layers is the strongest, but the one with the C atom intercalated between the Zr layer and Al layer is the weakest. In contrast, the situation for (ZrC)_nAl₄C₃ (n = 2, 3) is just the opposite. For Zr–Al–C nanolaminates, the calculated bulk, shear and Young's modulus increase in the sequence of Zr₂AlC < Zr₃AlC₂ < Zr₂Al₄C₅ < Zr₃Al₄C₆ < Zr₂Al₃C₄ < Zr₃Al₃C₅ < Zr₄Al₃C₆. The (ZrC)_nAl₃C₂ (n = 2, 3, 4) series exhibit the most outstanding elastic properties. In the presence of the external pressure, the bulk and shear moduli exhibit a linear response to the pressure, except for Zr₂AlC and Zr₃AlC₂, both of which belong to the so‐called MAX phases. The two materials also exhibit very distinct properties in the strain‐stress relationship, electronic structures and vacancy stabilities. As the intercalated Al layers increase, the formation energy of V_Zr and V_Al increases, while the formation energy of V_C decreases.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

新型抗肿瘤药物Epothilones的电子结构研究

采用量子化学和分子力学方法寻找一类新型促微管蛋白聚合的大环内酯类抗肿瘤药(Epothilones)的优势构象并进行定量构效关系的研究。通过相关性分析得知,分子的偶极矩和核-核排斥能对分子的活性差异性有显著的影响,可以通过改变C12和C15侧链来改变分子的偶极矩从而提高Epothilones的生物活性。此外,C7上的O原子和C15侧链噻唑基团上的N原子的负电荷增加,对Epothilones的生物活性的增大有利。     

非晶态合金Co80P20体系局域结构及稳定性的DFT研究

利用含相关校正能的DFT方法在B3LYP/Lan12dz水平下,对原子簇模型Co<,4>P<,30>余种可能存在构型在二、四、六重态下进行全参数优化和频率计算,确定了8种稳定构型.从其能量、结合能、几何参数、过渡态及成键等方面对其稳定构型分析,结果显示,8种稳定构型主要是三角双锥和类螺旋桨两种结构,在二重态和四重态中最...     

Electronic Properties and Molecular Simulations of Polyoxometalates

Taking classical polyoxometalates such as Lindqvist, Keggin, Dawson, and Preyssler anions as the starting point, a walk is taken through the electronic and structural properties of polyoxoanions in solution. We will discuss nucleation mechanisms, redox properties, and isomerism of polyoxometalates. The effects of confinement on water molecules and cation distributions inside nanocapsules are other topics discussed in the present review.     

Preparation and Characterization of UltraHigh‐Temperature Ternary Ceramics Ta4HfC5

Tantalum hafnium carbide (Ta₄HfC₅) powders were synthesized by solvothermal treatment and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride, hafnium chloride, and phenolic resin were used as the sources of tantalum, hafnium, and carbon, respectively. Pyrolysis of the complexes at 1000°C/1 h initiated the carbothermal reduction to result in multiplex phases including tantalum carbide and hafnium oxide which after heat treatment at 1400°C–1600°C transformed to single‐phase solid solution Ta₄HfC₅ by solid solution reaction. The mean crystallite size of Ta₄HfC₅ particles was less than 80 nm, and the composition of Ta, Hf, and C elements was near stoichiometric and homogeneously distributed in the powder samples. XRD pattern for Ta₄HfC₅ powders was analyzed.     

Properties and structures of nonstoichiometric (K,Na)NbO3‐based lead‐free ceramics

The 0.968[(K_0.48Na_0.52)]Nb_0.95+xSb_0.05O₃–0.032(Bi_0.5Na_0.5)ZrO₃ [KNN_xS–BNZ] lead‐free ceramics with nonstoichiometric niobium ion were fabricated via conventional solid‐state sintering technique and their piezoelectric, dielectric and ferroelectric properties were investigated. When x = 0.010, enhanced piezoelectric properties (d₃₃ ≈ 421 pC/N and k_p ≈ 0.47) were obtained due to the construction of rhombohendral—tetragonal phase boundary near room temperature. The KNN_xS–BNZ ceramics possesses enhanced Curie temperature (T_c) with improved piezoelectric constant. A large d₃₃ of ~421 pC/N and a high T_c ~256°C can be simultaneously induced in the ceramics with x = 0.010. Especially, good thermal stability was observed in a broad temperature range. The results indicated that our work could benefit development of KNN‐based ceramics and widen their application range.     

Combined experimental and DFT study on high-temperature thermochromism of Co-doped LaCrO3 coating for thermal indicator

Noncontact temperature measurement using thermochromic materials is vital in the field of temperature indication, but the related mechanisms behind thermochromic behavior are diverse and high complexity, and the color-temperature correspondence is poorly explored. This paper systematically studied the thermochromic mechanisms and color-temperature correspondence in the cobalt-doped LaCrO₃ coatings via experiment and density functional theory study. The coatings appear reversible thermochromism from green to black with temperature and composition, functions from room temperature to 700°C. This thermochromism is attributed to the lattice expansion and bandgap reduction as the temperature increase, and a model of temperature effect bandgap was proposed. Meanwhile, the coating temperature can be inferred from the color change because the CIE chromaticity coordinate of the coating varies linearly with temperature, and the mean relative error of thermochromic measurements is 8.28%. Furthermore, the cobalt doped introduces impurity energy levels and enhances the interaction between photons and carriers, which reduce the bandgap and increase the absorption in the visible spectrum resulting in darker colors. This work provides a stable and efficient high-temperature thermochromic coating that has a wide thermochromic temperature range and clear color-temperature correspondence, which shows broad application prospects in the field of thermal indication at high temperatures.     

Remarkable piezoelectricity and stable high‐temperature dielectric properties of quenched BiFeO3–BaTiO3 ceramics

Effects of quenching process on dielectric, ferroelectric, and piezoelectric properties of 0.71BiFeO₃?0.29BaTiO₃ ceramics with Mn modification (BF–BT?xmol%Mn) were investigated. The dielectric, ferroelectric, and piezoelectric properties of BF–BT?xmol%Mn were improved by quenching, especially to the BF–BT?0.3 mol%Mn ceramics. The dielectric loss tanδ of quenched BF–BT?0.3 mol%Mn ceramics was only 0.28 at 500°C, which was half of the slow cooling one. Meanwhile, the remnant polarization P_r of quenched BF–BT?0.3 mol%Mn ceramics increased to 21 μC/cm². It was notable that the piezoelectric constant d₃₃ of quenched BF–BT?0.3 mol%Mn ceramics reached up to 191 pC/N, while the T_C was 530°C, showing excellent compatible properties. The BF–BT?xmol%Mn system ceramics showed to obey the Rayleigh law within suitable field regions. The Rayleigh law results indicated that the extrinsic contributions to the dielectric and piezoelectric responses of quenched BF–BT?xmol%Mn ceramics were larger than the unquenched ceramics. These results presented that the quenched BF–BT?xmol%Mn ceramics were promising candidates for high‐temperature piezoelectric devices.     

Ablation properties of C/C-UHTCs and their preparation by reactive infiltration of K2MeF6 (Me = Zr,Ti) molten salt

Ultra-high temperature ceramic-modified C/C composites (C/C-UHTCs) were prepared by the reactive infiltration of K₂MeF₆ (Me = Zr, Ti) mixed with Si and Zr-Si powders. Molten salt infiltration can be divided into two stages: salt ion melt and Me-Si alloy melt. In the temperature range below 1400 °C, Zr and Si dissolve in the molten salt, are carried by the ion melt, and precipitate at the PyC interface to form carbides. Above 1400 °C, a large amount of molten salt volatilises and thermally decomposes. The Me-Si alloy forms a melt and infiltrates the C/C matrix, and finally forms C/C-ZrC-SiC, C/C-Ti₃SiC₂-SiC, and C/C-ZrC-TiC-SiC composites. The C/C-ZrC-SiC composite with the highest ZrC content exhibited the lowest mass rate (2.6 ± 0.02 mg/s) and linear ablation rate (0.82 ± 0.04 μm/s), which were reduced by 43.5 and 50.8 %, respectively, compared to the unmodified C/C-ZrC-SiC composite.     

Synthesis,densification, and microstructure of TaC‐TaB2‐SiC ceramics

The usual way to prepare TaC‐TaB₂ ceramics by adding B₄C to TaC leads to formation of residual C, which degrades samples’ mechanical properties. To eliminate the residual C, we suggest incorporating Si together with B₄C into TaC ceramics, resulting in new ultrahigh‐temperature ceramics (TaC‐TaB₂‐SiC). Dense ceramics (>99%) with SiC volume fraction ranging from 15.86% to 41.04% were fabricated by reactive spark plasma sintering at 1900°C for 5 minutes. The formation of SiO₂‐based transient liquid phase was evidenced by the “film” in intermediate products, which can promote densification. The fine‐grained microstructure in final products was found to be associated with the in situ formed SiC, which impeded TaC and TaB₂ grains from coarsening by the pinning effect. Besides, ultrafine TaB₂ grains (~100 nm) produced during the reaction and then rearranged in liquid also contributed to grain refinement. Compared to TaC‐TaB₂(‐C) ceramics prepared from TaC and B₄C, the acquired composites exhibit better mechanical properties, due to their fine‐grained microstructures and the elimination of residual C.     

Catalytic Sulfation of Betulin with Sulfamic Acid: Experiment and DFT Calculation

Betulin is an important triterpenoid substance isolated from birch bark, which, together with its sulfates, exhibits important bioactive properties. We report on a newly developed method of betulin sulfation with sulfamic acid in pyridine in the presence of an Amberlyst^®15 solid acid catalyst. It has been shown that this catalyst remains stable when being repeatedly (up to four cycles) used and ensures obtaining of sulfated betulin with a sulfur content of ~10%. The introduction of the sulfate group into the betulin molecule has been proven by Fourier-transform infrared, ultraviolet-visible, and nuclear magnetic resonance spectroscopy. The Fourier-transform infrared (FTIR) spectra contain absorption bands at 1249 and 835–841 cm⁻¹; in the UV spectra, the peak intensity decreases; and, in the nuclear magnetic resonance (NMR) spectra, of betulin disulfate, carbons С3 and С28 are completely shifted to the weak-field region (to 88.21 and 67.32 ppm, respectively) with respect to betulin. Using the potentiometric titration method, the product of acidity constants K₁ and K₂ of a solution of the betulin disulfate H⁺ form has been found to be 3.86 × 10^–6 ± 0.004. It has been demonstrated by the thermal analysis that betulin and the betulin disulfate sodium salt are stable at temperatures of up to 240 and 220 °C, respectively. The density functional theory method has been used to obtain data on the most stable conformations, molecular electrostatic potential, frontier molecular orbitals, and mulliken atomic charges of betulin and betulin disulfate and to calculate the spectral characteristics of initial and sulfated betulin, which agree well with the experimental data.     

靛族染料化合物结构与性能的密度泛函理论研究

在密度泛函理论(DFT)的B3LYP／6-31G^*水平上对一系列靛族染料化合物的几何构型进行优化计算：在获得基态稳定结构的基础上，应用含时密度泛函理论(TD-DFT)在相同水平下计算其电子吸收光谱，探讨了不同给电子基团和发色体系的延伸对电子吸收光谱的影响，得到了与实验基本一致的变化规律。结果表明，给电子能力的增强和发色体系的纵向延伸均使光谱产生一定红移，通过对前线轨道组成进行自然布居分析，揭示了靛族染料的发光均源自分子中HOMO-LUMO(П→П^*)电子跃迁。     

氮杂双环硝胺类含能材料的分子结构与性能的关系

使用Gaussian03软件,采用密度泛函理论(DFT)在B3LPY/6-31++G(d,p)计算精度下,计算并研究了9个具有不同官能团以及母环结构的氮杂双环硝胺类含能化合物的分子结构,分析了其结构与性能的关系,优化了几何构型,通过计算得到了分子体积、能量等;以此为基础计算了该系列材料的密度、爆轰性能以及感度等数据。通过比较分子结构,从密度、生成热、爆轰性能以及撞击感度等方面,研究了不同官能团以及环系结构对该类含能材料性能的影响;通过对比得到了对双环硝胺化合物有利的官能团及结构。结果表明,双环硝胺结构有利于稳定结构以及形成紧凑的空间排布,并提高化合物的撞击感度及密度;同时,羰基的引入以及接近零氧平衡是提高该类含能材料爆炸性能的技术途径。     

Influence of the Electronic Structure of the Metal at the Polymer/Metal Interface: A Tentative Interpretation on the Basis of Lewis Acid-Base Reactions

In the present paper, we examine the link between theoretical qualitative predictions made on the grounds of the Lewis acid-base concept and the actual interface built between polyamide-6,6 (PA-6,6) and copper, and PA-6,6 and platinum. By using a combination of very-surface-sensitive photoemission spectroscopies and model polymers, it becomes possible to obtain important information about the chemical nature of the polymer/metal interface. Our experiments show a complete dissociative chemisorption of the polymer on Pt, while PA-6,6 retains its chemical integrity on copper.     

含能材料分子N8H8结构与性质的理论研究

采用密度泛函理论的b3lyp方法在6-311＋＋G＊＊基组上对30种N8H8链状异构体进行了理论计算,并应用分子中的原子理论（Atoms In Molecules,AIM）和自然键轨道理论（Nature Bond Orbital,NBO）分析了这些化合物相对稳定性和成键特征,G3MP2方法计算了各异构体的能量及生成热。结果表明：N8H8链状异构体中,含有N=N双键特征的异构体稳定性较好,影响氮氮键键长变化的主要因素是N原子孤对电子到相邻的氮氮键的超共轭作用;所有异构体中α3的生成热最大,β15的密度最大。     

High-temperature oxidation behavior of monolithic Zr3[Al(Si)]4C6 and its composite incorporating 30vol% ZrB2-SiC: Providing a basis for broadening the service temperature

To provide a basis for the high-temperature oxidation of ultra-high temperature ceramics (UHTCs), the oxidation behavior of Zr₃[Al(Si)]₄C₆ and a novel Zr₃[Al(Si)]₄C₆-ZrB₂-SiC composite at 1500 °C were investigated for the first time. From the calculation results, the oxidation kinetics of the two specimens follow the oxidation dynamic parabolic law. Zr₃[Al(Si)]₄C₆ exhibited a thinner oxide scale and lower oxidation rate than those of the composite under the same conditions. The oxide scale of Zr₃[Al(Si)]₄C₆ exhibited a two-layer structure, while that of the composite exhibited a three-layer structure. Owing to the volatilization of B₂O₃ and the active oxidation of SiC, a porous oxide layer formed in the oxide scale of the composite, resulting in the degradation of its oxidation performance. Furthermore, the cracks and defects in the oxide scale of the composite indicate that the reliability of the oxide scale was poor. The results support the service temperature of the obtained ceramics.     

Organometallic molecular modelling – the computational chemistry of metallocenes: a review

The application of ab initio, density functional theory and molecular mechanical methods to the modelling of metallocenes, particularly ferrocene, is reviewed. Approaches to the representation of the metal‐ring π‐bonding in molecular mechanics and the quality of computed results from ab initio and density functional theory serve to illustrate the utility of modelling studies in organometallic chemistry. © 1999 Society of Chemical Industry