Vacancy effects on structural and electronic properties of 4d transition-metal carbides

We present a study of the effect of the vacancies on the structural and electronic properties in substoichiometric NbC_x and MoC_x in the NaCl type structure using ab-initio full-potential linear augmented plane wave method (FP-LAPW). A model structure of 8 and 16 atom supercell with ordered vacancies within the carbon sublattices is used. We find that the lattice parameters of the studied stoichiometries in both MoC_x and NbC_x are smaller than that of ideal stoichiometric MoC and NbC. Our results are found to be in good agreement with experiment and other theoretical ab-initio calculations.     

Effect of DC electric field on the tensile deformation of ultrafine-grained 3Y-TZP at 1450–1600 °C

Two test types were employed to determine the effect of DC electric field E = 5–8.3 V cm⁻¹ applied in the loading direction on the tensile flow stress of 3Y-TZP (d_o = 0.44 μm) at 1450–1600 °C. In type a, the field was applied continuously throughout the tensile test; in type b, the field was alternately applied and removed during the course of the tensile test. The field reduced the flow stress in both test types, but with an appreciably larger effect in the type a tests. The field, however, had no discernable effect on the stress exponent $n=\mathrm{\partial }\log \dot{\epsilon }/\mathrm{\partial }\log \sigma$ determined by strain rate change tests. The reduction in flow stress in the type b tests reversed upon removal of the field. It was interpreted to result from both Joule heating and a decrease in the energy to form the rate-controlling Zr⁴⁺ vacancies in the space charge zone adjacent to the grain boundaries. The calculated width of the space charge zone λ (=1.5–7.8 nm) and the ratio of the applied electric potential to the reduction in stress Δϕ_a/Δσ (=18–19 μV/MPa) were in accord with predictions. The larger reduction in the flow stress in the type a tests is attributed to a greater retardation of grain growth by the field.     

Structural,electrical and electrochemical characterizations of SrNb0.1Co0.9O3−δ as a cathode of solid oxide fuel cells operating below 600 °C

SrNb_0.1Co_0.9O_3−δ (SNC) perovskite oxide has been prepared by high-energy ball milling followed by calcination at 1100 °C. According to oxygen temperature-programmed desorption and thermogravimetry analysis results, highly charged Nb⁵⁺ successfully stabilizes the perovskite structure to avoid order-disorder phase transition. The electrical conductivity reaches 550 S cm⁻¹ at 300 °C in air and as high as 106 S cm⁻¹ under P(O₂) = 1 × 10⁻⁵ atm at 900 °C. The high electrical conductivity is beneficial in improving the charge-transfer process for the oxygen reduction reaction on the cathode. Based on the defect chemical analysis, the Nb-doping in SrCoO_3−δ perovskite facilitates the formation of Co²⁺, which increases oxygen nonstoichiometry and, subsequently, the mixed valence of [Co²⁺]/[Co³⁺] under lower oxygen partial pressure. A relatively low thermal expansion coefficient of 19.1 × 10⁻⁶ K⁻¹ in air was achieved. All above properties show SNC to be a promising cathode material in the practical application of low-temperature solid oxide fuel cells.     

第一性原理研究PDP放电单元MgO保护层各种空缺对二次电子发射系数的影响

基于密度泛函理论的第一原理赝势法,研究了PDP放电单元中MgO保护层在形成氧空缺后的电子结构的变化.通过对能带结构和态密度分布的计算,可以看到MgO形成氧空缺后在禁带中引入了能级.本文计算了完整MgO以及含F、F~+、F~(2+)空缺的MgO晶体,得到不同能带结构和态密度分布,同时计算了相应的二次电子发射系数.结果表明空缺的形成,可有效提高二次电子发射系数,其中形成F空缺的MgO晶体的二次电子发射系数最大.     

Monte Carlo simulation on the cation diffusion via vacancies in simple spinels

The correlation factors of cations and vacancies for diffusion in simple spinels were calculated by using the Monte Carlo simulation technique. Since the predominant point defects are cationic vacancies at high temperature and especially at high oxygen partial pressures in many spinels, the cations are considered to move primarily via cationic vacancies in this study. Several input parameters including the lattice size, the number of jumps per cation, etc., were optimized to calculate the correlation factors. Then the correlation factors of cations moving via vacancies in simple spinels were calculated in different types of exchange jumps, i.e., diffusion that occurs on tetrahedral or octahedral or both cationic sites. Results were obtained with satisfactory accuracy. The influence of the vacancy concentration on the correlation factors was also analyzed.     

Gold/hydroxyapatite catalysts: Synthesis, characterization and catalytic activity to CO oxidation

This work reports the synthesis, characterization and catalytic activity for CO oxidation of gold catalysts supported on calcium hydroxyapatite. On both, the hydroxyapatite support and the gold-supported hydroxyapatite catalyst, the CO conversion shows a peak near 100% of conversion at room temperature. The generation of structural vacancies by interaction of CO with the solid provokes the formation of peroxide species in the presence of gaseous oxygen, which seems to be responsible of this high conversion of CO at room temperature. Moreover, the influence of the pre-treatment temperature on the activity has been observed and related with the elimination of carbonate species and the generation of structural defects in the apatite structure, which are able to modify the gold oxidation state.     

Defect structure transformation during substitution in quadruple perovskite CaCu3-xTi4-xFe2xO12 studied by positron annihilation spectroscopy

Positron annihilation spectroscopic studies have been performed to investigate the defect structural evolution in polycrystalline CaCu_3-xTi_4-xFe_2xO₁₂ (x = 0.0, 0.1, 0.3, 0.5 and 0.7) cubic perovskite samples. The positron lifetimes, relative intensities and Doppler-broadened annihilation lineshape parameters all indicate about the annulment of crystalline vacancies from the structure and attending porous ceramic structure at higher concentration of substitution. The compositional dependence of the intensity of the defect specific positron lifetime component clearly suggests that, in the initial stages of substitution, positron annihilates in vacancy-type defects. But, for higher concentrations of Fe³⁺, the materials are devoid of such defects and positron trapping takes place within the pores. The results of coincidence Doppler broadening spectroscopy measurements support such transformation in which the positron trapping sites are relocated to the ceramic pores leading to the modification of the material properties.     

Muon kinetics in heat treated Al (–Mg)(–Si) alloys

Al–Mg–Si alloys are heat-treatable and rely on precipitation hardening for their mechanical strength. We have employed the technique of muon spin relaxation to further our understanding of the complex precipitation sequence in this system. The muon trapping kinetics in a material reveals a presence of atom-sized defects, such as solute atoms (Mg and Si) and vacancies. By comparing the muon kinetics in pure Al, Al–Mg, Al–Si and Al–Mg–Si when held at different temperatures, we establish an interpretation of muon trapping peaks based on different types of defects. Al–Mg–Si samples have a unique muon trapping peak at temperatures around 200 K. This peak is highest for samples that have been annealed at 70–150 °C, which have microstructures dominated by a high density of clusters/Guinier–Preston zones. The muon trapping is explained by the presence in vacancies inside these structures. The vacancies disappear from the material when the clusters transform into more developed precipitates during aging.     

Nanostructured CuO: Facile synthesis,optical absorption and defect dependent electrical conductivity

Nanostructured CuO with different average crystallite sizes in the range ~11–48 nm is synthesized by thermolysis of carbonate precursor at different temperatures. Structural characterization of the samples is done using X-ray powder Diffraction and Transmission Electron Microscopy techniques. Analysis of UV–Visible absorption spectra of the samples reveal direct band gaps in the range 1.49–1.79 eV which is blue shifted in comparison with that corresponding to single crystalline CuO. DC electrical conductivity of the samples is found to increase with increase in crystallite size/decomposition temperature. The conduction mechanism is found to be defect dependent with holes associated with uncompensated Cu²⁺ vacancies being the charge carriers. A comparison of the DC electrical conductivities in vacuum and air ambience reveal that there is a decrease in the concentration of O^2- vacancies with increase in the decomposition temperature. X-ray photoelectron spectral studies confirm an increase in the concentration of Cu³⁺ ions in samples with higher electrical conductivity. Analysis of Raman spectra indicates a decrease in the concentration of O^2- vacancies with increase in the decomposition temperature confirming the proposed role of Cu²⁺ and O^2- vacancies in determining the electrical conductivity.     

钨中氘热脱附特性的定量研究

金属钨(W)是未来聚变堆反应环境中面向等离子体的主要候选材料之一,开展W中氢同位素输运和滞留行为的定量研究,对评估钨的服役性能及聚变堆燃料的物料平衡至关重要。本工作采用热脱附谱方法定量研究了多晶W经能量为100 eV/D、注量为3.8×10²⁴ D/m²的D⁺辐照作用后,在不同升温速率下氘的热脱附特性。研究发现,氘的热脱附量在不同升温速率下均为10²² D₂/m²量级,随着升温速率的增大,氘的热脱附峰峰位向高温方向移动。多晶W中氘的热脱附行为符合一级反应特征,钨中空位是氘在钨中的主要俘获态,D原子的热脱附能为1.04 eV。