Materials Science Forum Vols. 654-656

Abstract: First-principles density functional calculations were performed to investigate mechanical properties of ZnO nanowires and the size effects. Structural optimizations were performed first, and a series of strains were applied to the nanowires in the axial direction. The ground state energies were calculated and the elastic moduli of ZnO nanowires were obtained from the energy versus strain curves. It is found that the elastic moduli of the ZnO nanowires with three different diameters (1.2, 1.5 and 1.8nm) are 136.3, 138.7 and 138.0 GPa, respectively, and that of bulk ZnO along [0001] direction is 140.1 GPa. The elastic modulus of ZnO nanowire is slightly lower than that of the bulk and it decreases as the diameter decreases. Comparisons to experimental results and theoretical predications are made.

Abstract: The NdOx phase formed at the Nd/Nd-Fe-B interface in Nd-sputtered Nd-Fe-B sintered magnets is paid rather attention recently due to its important role in coercivity generation of surface Nd-Fe-B grains. Its crystal structures have been reported to vary with the change of the oxygen concentration, and the disorder fcc phase derived from Nd2O3-C-type structure to be the main form of existence. To understand the formation mechanism of this fcc-NdOx interfacial phase, the stability of all oxygen concentration range of Nd-O system has been investigated from the first principles. Based on LSDA+U calculations for selected ordered phases at various oxygen concentration in Nd-O, the Cluster Expansion Method (CEM) is applied to evaluate the formation energy, density of states and other properties of disorder phase.

Abstract: The electronic structure and magnetic properties of the (Co1-xFex)Tet(CoxFe2-x)OctO4 spinels (x is defined as the degree of inversion) scenario are investigated theoretically from first-principles, using generalized gradient approximation (GGA) method for the systems with strong coulomb correlations, which gives a correct description of the electronic structure. The GGA+U method gives an improved qualitative result compared with the GGA not only for the excited-state properties such as energy gaps but also for the ground-state properties such as magnetic moments and crystal parameters. The nominal valence of the transition metal elements and the ground state structure have been established based on the study of variation of the cation distribution (x=0.0, 0.25, 0.5, 0.75 and 1.0) over the tetrahedral and octahedral sites. The site-preference calculation on bulk systems indicates that Co2+ ions strongly prefer the octahedral B sites, and the electronic structure and magnetic properties of cobalt ferrites highly depend on the cation distributions even though the chemical composition of the compound does not change. The results are in good agreement with the available experimental data and most of the other theoretical results.

Abstract: We numerically evaluate the accumulation of dislocations in periodic structure of the shallow trench isolation (STI) type ULSI cells which has generally been adopted as the latest semiconductor device structure. STI type ULSI cells with gate length less than 62 nm and various trench depths are employed and subjected to a temperature drop from the initial value of 1000 °C. Dislocation accumulation is evaluated by a technique of crystal plasticity analysis. Relations between the geometry of the STI type ULSI cells and dislocation accumulation are discussed.

Abstract: Metallic aerospace components are commonly painted with a primer to improve their corrosion resistance. The primer contains a polymer matrix with embedded corrosion inhibitor and filler particles. Its performance is determined by the microscopic distributions of the particles. Various techniques have been used to quantify such distributions, including X-ray micro-computed tomography (CT). However, its success is sometimes limited by factors such as different particles having similar X-ray CT absorption properties and their size being smaller than the resolution of micro-CT. In this paper, we have performed two X-ray CT measurements on a paint primer sample consisting of SrCrO4 corrosion inhibitor particles and UV-absorbing TiO2 filler particles. Fe and Ti targets were used as X-ray sources with different spectral distributions. The measured CT data sets were used as constraints for a data-constrained microstructure modeling (DCM) prediction of the sample’s microscopic structures. DCM model predictions were compared with experimental elemental surface maps and showed reasonable degree of agreement, suggesting X-ray micro-CT combined with DCM modeling would be a powerful technique for detailing the dynamics of chromate-inhibited primers and other multiphase systems where the components are sensitive to incident X-ray energy.

Abstract: The electronic properties of the solar cells were greatly influenced by the aluminum atomic concentration in Al-BSF region under that the Al-BSF is doped heavily. The effects of the dopincg profile in heavily-doped Al-BSF on electronic properties of n+pp+ monocrystalline solar cells were investigated by PC1D. The results show that the electronic properties of solar cells are almost independent of the doping profile of the Al-BSF, but are more or less affected by the BSF profile if the solar cell back surface is passivated well with the BSRV less than ~105cm/s. When the sheet resistance is about between 5 and 30Ω/□, the conversion efficiency can reach the maximum value. And the optimum thickness of Al-BSF is about between 10~15μm.

Abstract: As quasi-thermodynamic equilibrium plasma, DC Arc Plasma has the advantage of very high gas temperature and thus the very high degree of activation of the precursors for diamond film deposition. The present paper reviews the progresses in the R&D of the novel high power dc arc plasma jet CVD system with rotating arc and operated at gas recycling mode for large area high quality diamond film deposition, developed at the University of Science and Technology Beijing (USTB) in the mid 1990s of the 20th century. Thanks to the continuous efforts made in the technological improvement in the past 15 years, considerable progresses have been achieved in the commercialization of this high power dc arcjet CVD system, which is now capable of mass production of large area high quality freestanding diamond films for optical, thermal, and mechanical (tool) applications. The present status in the commercialization and the property level of the resultant diamond films in optical, thermal, mechanical, dielectric, oxidation resistance, sand erosion resistance, and laser damage threshold etc. are presented. Based on the same high power dc arcjet technology, a novel high current extended dc arc plasma (HCEDCA) CVD system has been developed which successfully changed the diamond film deposition mode from 2D planar deposition in to 3D deposition (as confined by two hollow (virtue) columns). It is demonstrated to be advantageous for mass production of diamond thin film coated WC-Co cutting tools. Recent results in the R&D of thin diamond film coated WC-Co drills and end mills, and the results in field tests are discussed.

Abstract: High current pulsed electron beam (HCPEB) is a fairly new technique for improving surface properties such as corrosion and wear resistances. One of the negative effects induced by HCPEB is the potential formation of craters on the surface of the HCPEB treated materials. These changes can impair the corrosion-resistance by promoting pitting. The mechanisms of nucleation and growth are detailed and the effect of the number of pulses on crater formation is discussed.

Abstract: High temperature superconducting YBa2Cu3O7 films have been prepared by pulsed laser deposition on top of single crystal substrates and metallic templates. Films have been investigated in order to determine the influence of microstructural peculiarities on the critical current density. It has been found that there is an inherent mechanism limiting the superconductive current flow in YBa2Cu3O7 film deposited on metallic template. This mechanism is associated with YBa2Cu3O7 film architecture, fabrication procedure and following low temperature usage.

Abstract: Aluminum nitride (AlN) thin films were deposited on (100) oriented silicon wafers substrates by Hollow Cathode Electron Beam Vapor Deposition system (HCEBVD) under various Ar/N2 mass flow ratio. The films were characterized by Atomic Force Microscopy (AFM), Glancing Incident X-ray Diffraction (GIXRD) techniques and Ultraviolet/Visible Spectrophotometer (UV/VIS). It was found that the thin films are polycrystalline and have a hexagonal wurtzite structure with (002) preferred orientation, as revealed by GIXRD. AFM analysis indicates that the surface of the thin films is smooth, with average RMS (Root Mean Square) roughness Ra of 0.7nm, which is suitable for application in surface acoustic wave devices. The film thickness and optical refractory properties of the AlN thin films were investigated.