Critical Assessment 5: Thermodynamic data for vacancies

Abstract

The thermodynamic properties of crystalline phases are generally represented well in terms of the compound energy formalism and the model has been implemented in all the major software packages used to calculated phase equilibria. The formalism is particularly successful in modelling defects in crystals, both the formation of holes or vacancies on lattice sites, and the dissolution of atoms on interstitial sublattices. In the former case the formalism may require the definition of data for pure vacancy itself. In this assessment we investigate the possible values for the Gibbs energy of vacancy to be used in the compound energy formalism and show that a value of zero or negative will always lead to unwanted catastrophic stability of the phase.     

Use of fractals and kinetic equations to model thermally induced hillock formation and growth in thin metal films

We investigated the applicability of a model based on fractals and the Smoluchowski kinetic equations to describe hillock formation in thin metal films. We have previously used this model to analyze cluster and ultrafine particle production. We show how to extract two parameters from measured hillock size distributions which may reveal the scaling of the mobility of clusters and vacancies in films with varying hillock size. On the basis of our application of this model to certain data taken from the literature, the model shows considerable potential for being able to provide an internally consistent quantitative basis for monitoring thermally driven mass redistribution processes in metal films.     

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.     

Characterization of lattice defects in metallic materials by positron annihilation spectroscopy: A review

Positron is an excellent probe of lattice defects in solids. A thermallized positron delocalized in lattice can be trapped at open volume defects, e.g. vacancies, dislocations, grain boundaries etc. Positron annihilation spectroscopy is a non-destructive technique which enables characterization of open volume lattice defects in solids on the atomic scale. Positron lifetime and Doppler broadening of annihilation photo-peak are the most common observables related to positron annihilation process. Positron lifetime spectroscopy enables to identify defects in solids and to determine their concentrations while coincidence measurement of Doppler broadening provides information about local chemical environment of defects. This article provides a review of the state-of-art of defect characterization in bulk metallic materials by positron annihilation spectroscopy. Advanced analysis of positron annihilation data and recent developments of positron annihilation methodology are described and discussed on examples of defect studies of metallic materials. Future development in the field is proposed as well.     

Investigations of the divalent metal ions Zn2+, Co2+, Ni2+ doped ultra-wide temperature stable BBNN system

Multi-component BaTiO₃–Bi_0.5Na_0.5TiO₃–Nb₂O₅ (BBNN) system doped with divalent metal ions (Zn²⁺, Co²⁺, Ni²⁺) was prepared by the conventional solid–state method.The X-ray diffraction patterns revealed all samples exhibited perovskite (P4mm) single phase. The dielectric properties and micro-mechanisms were discussed and validated. Novel theories, based on the characteristics of the different kinds of dielectric polarization, are proposed to explain the dielectric anomalies in the dielectric system. The relationships between microstructure and the dielectric properties were investigated systematically for the first time. The samples doped with 15 mol% Zn²⁺/Co²⁺presented good dielectric properties of an ultra-broad temperature stable range (from −50 and >300), high dielectric constant (ε~1925 for Zn²⁺/ ε~1341 for Co²⁺) and low dielectric loss (tan δ<0.02) were obtained. These features made the ceramics system have high practical values for miniaturization and harsh environments applications.     

Chemical composition of Na0.5Bi0.5TiO3 solid solutions with Sr0.7Bi0.2TiO3 on a local level

Oxygen vacancies in Na_0.5Bi_0.5TiO₃ are of high demand as they stimulate the ionic conductivity essential to solid fuel cell applications. However, their available concentration, which is directly related to introduced A-site vacancies, is very limited. The objective of the present research is to study how the concentration of A-site vacancies may be increased by forming solid solutions with Sr_0.7Bi_0.2TiO₃. A thorough study of heterogeneity and chemical composition of (1-y)Na_0.5Bi_0.5TiO₃-ySr_0.7Bi_0.2TiO₃ ceramics on a local level is conducted and accompanied by impedance measurements. It is shown that the chemical composition of the matrix grains is stable and follows the designed formula, allowing formation of A-site vacancies in a wide concentration range and, in limited amount, also oxygen vacancies. Inclusions, the content and character of which depend on Sr_0.7Bi_0.2TiO₃ concentration, are detected and discussed. Complex impedance studies suggest that the compositions with y < 0.5 preserve low ionic conductivity up to 600–700 °C, but it significantly increases at higher y.     

A DFT study of dopant (Zr,Nb) and vacancies on the dehydrogenation on MgH2 (001) surface

Ab initio calculations were used to study dehydrogenation energy in a set of systems, pure, Nb- or Zr-doped, free of defects and containing vacancy-like defects MgH₂ (001) surface. The preferential location site for dopants was determined by means of occupation energy analysis. Zr atom prefers substitutional and interstitial sites while Nb locates on interstitial site close to the surface. The effect of vacancies in the geometric structure and in dehydrogenation energy was considered. It was found that MgH₂ containing a Mg vacancy doped with Zr, in an interstitial site and the same system containing Mg–H complex vacancy, modify the surface geometry and weakens the Mg–H bonds making easier the dehydrogenation process. When the surface is doped with Nb despite any vacancy generation, the dehydrogenation is less probable energetically in comparison with Zr-doped.     

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.