Vacancy-driven stress relaxation in layers

The role of vacancies together with their generally non-ideal sources and sinks has been investigated previously in the context of multi-component diffusion. This concept is applied to a thin layer on a substrate subjected to an initial eigenstress state. The surface of the layer is assumed to act as an ideal source and sink for vacancies. The interface may act either as an ideal or as no source and sink for vacancies. Non-ideal sources and sinks are supposed in the bulk. The formation of a vacancy site fraction profile across the layer due to active sources and sinks and due to diffusion is studied, provoking a relaxation of the residual eigenstress state. A set of two nonlinear partial differential equations for both the vacancy site fraction and the eigenstress distributions is developed. The problem is reformulated in dimension-free quantities. The relation between the diffusivity and the activity of sources and sinks for vacancies in the bulk is characterized by a vacancy intensity parameter k. Numerical solutions are presented, and the influence of nonlinear terms in the evolution equations is evaluated. The simulations provide the evolution of the vacancy site fraction and of the hydrostatic stress profiles in the layer. The development of a film of newly deposited or removed atoms at the layer surface and of the total thickness of the layer is also calculated. The results of simulations are discussed.     

Photoconductors based on ZnxCd1−xS and CdSe1−ySy thin films, fabricated with multilayer structure

Photoconductors based on evaporated Zn_xCd_1−xS and CdSe_1−yS_y thin films, with a novel multilayer structure, were fabricated and characterized through spectral response measurements. The device structure is constituted by several layers of different energy gaps, sequentially deposited side by side and interconnected in series or in parallel. The influence of the preparation conditions, number of layers and electrode configuration, on the performance of the photoconductors was determined. Photoresistors with a detection range between 400 and 750 nm were developed, using four layers with energy gaps varying between 1.8 eV and 2.8 eV and interconnected in parallel.     

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。     

Semi-empirical atomistic study of point defect properties in BCC transition metals

We have constructed a set of embedded atom method (EAM) potentials for Fe, Ta, W and V and used them in order to study point defect properties. The parametrizations of the potentials ensure that the third order elastic constants are continuous and they have been fitted to the cohesive energies, the lattice constants, the unrelaxed vacancy formation energies and the second order elastic constants. Formation energies for different self-interstitials reveal that the split dumbbell is the most stable configuration for Fe while for Ta, W and V we find that the split dumbbell is preferred. Self-interstitial migration energies are simulated using the nudged elastic band method and for Fe and W the migration energies are found to be in good agreement with experimental and ab initio data. Migration energies for Ta and V self-interstitials are found to be quite low. The calculated formation, activation and migration energies for monovacancies are in good agreement with experimental data. Formation energies for divacancies reveal that the second nearest neighbor divacancy is more energetically favorable than nearest neighbor divacancies and the migration energies indicate that nearest neighbor migration paths are more likely to occur than second nearest neighbor migration jumps. For Fe, we have also studied the influence of the pair potential behavior between the second and third nearest neighbor on the stability of the split dumbbell, which revealed that the higher the energy level of the pair potential is in that region, the more stable the split dumbbell becomes.     

Raman spectra of TiN/AlN superlattices

TiN (4.5 nm)/AlN (3, 6, 22 nm) superlattices deposited by DC magnetron sputtering on MgO(001) at a temperature of 850°C exhibit Raman signals. They indicate N and Ti vacancies (as in thick TiN) in TiN_1−x layers (x=3±2%). x is higher for the sample with 3-nm thick AlN layers, which is ascribed to N diffusion from AlN (standing close to the TiN interfaces) to TiN. In comparison to Raman peaks of thick AlN, there are split signals of wurzite AlN phase, and a signal from another phase, which might be defective rocksalt AlN standing close to the TiN interfaces. The Raman signals clearly show interactions between AlN and TiN layers.     

Hydrogen adsorption around lithium atoms anchored on graphene vacancies

Density functional theory and molecular dynamics were used to study the interaction of a lithium atom with a vacancy inside a graphene layer. It was found that the lithium atom is adsorbed on this vacancy, with a binding energy much larger than the lithium cohesive energy. Then, the adsorption of hydrogen molecules around lithium atoms was studied. We found that at 300 K and atmospheric pressure, this system could store up to 6.2 wt.% hydrogen, with average adsorption energy of 0.19 eV per molecule. Thus, this material satisfies the gravimetric capacity requirements for technological applications. A complete desorption of hydrogen occurs at 750 K. However, a multilayer of this system would be required for practical reasons. Under atmospheric pressure and at 300 K, we found that a system made of multiple layers of this material is stable. The storage capacity remained at 6.2 wt.%, but all adsorbed molecules were dissociated. The average adsorption energy becomes 0.875 eV/H.     

Promoting the reduction reactivity of magnetite by introducing trace-K-ions in hydrogen direct reduction

Hydrogen metallurgy exhibits considerable potential for application in the steel industry. However, it is restricted by the low utilization efficiency of hydrogen owing to the poor kinetic conditions and low reduction efficiency, which limit the development of ironmaking via hydrogen direct reduction. Therefore, improving the reduction efficiency of hydrogen direct reduction is critical. In this study, the reactivity of magnetite was considered. The impregnation of trace K⁺ (0.8 wt%) was found to significantly promote the reduction efficiency, and a possible mechanism was proposed. The increase in the metallization ratio from 88.51% to 99.43% indicated that the 0.8 wt% K⁺-promoted magnetite exhibited an improved reaction efficiency due to its higher reactivity. Furthermore, modified magnetite reacted under a broader range of gas conditions, e.g., the partial pressure of hydrogen could be reduced to 57%. According to the refined X-ray diffraction patterns, the promotion of the reduction reactivity could mainly be attributed to the increased Fe–O bond polarities, in addition to the solid-state diffusion of O²⁻, due to the formation of more oxygen vacancies. An enhanced reduction efficiency during hydrogen direct reduction was realized, which could provide promising guidance for the green, sustainable development of hydrogen direct reduction.