高氧空位氧化锌晶体的制备及光催化性能

利用气相沉积法,在低氧气氛下制备高缺陷的ZnO晶体。分别将样品在800℃、900℃、1000℃下通氧退火1h,对ZnO晶体做表面修饰。PL光谱实验和光催化降解亚甲基蓝实验表明ZnO晶体的氧空位、表面态和光催化活性间存在内在联系。1000℃下退火的样品表面缺陷程度多于800℃和900℃下退火样品,光催化活性也优于后两者。     

Investigation of ordering and disordering in B-doped Ni76Al24 by residual resistometry

Homogeneous long-range ordering (LRO) kinetics in Ni₇₆Al₂₄ + 0.19 at.% B was studied by residual resistometry. The LRO relaxations were composed of two simultaneous processes showing dramatically different rates strongly dependent on temperature. The resulting very high activation energy of the processes was in agreement with previous results indicating its correlation with the value of the “order-disorder” transition temperature. The results were compared with the literature data and discussed in terms of the microscopic image of vacancy migration in the L1₂ superstructure.     

Charge trapping during constant current stress in Hf-doped Ta2O5 films sputtered on nitrided Si

Metal-insulator-silicon structures containing Hf-doped Ta₂O₅ dielectric films sputtered on rapid thermally nitrided Si are shown to have very good reliability properties. Stress-induced leakage currents are low, both at low and at high-fields. It is found that charge trapping during the stress is the dominant wear-out mode for very long stress times of 500 s even for injected current densities J_s as high as 100 mA/cm². Stress curves approach saturation at long stress time, indicating that the trap generation rate is very low, even compared to the observed reduced trapping at pre-existing traps.Applying a trapping kinetics model, two trapping sites with characteristic trapping times τ₁ = 3.2 s and τ₂ = 49 s were determined and attributed to pre-existing defects in the bulk Hf:Ta₂O₅ layer and not in the interfacial SiO_xN_y layer. It was found that both τ₁ and τ₂ do not depend on J_s, which may be explained by the presence of a mechanism of charging the active sites through field activated emission of charge from them.     

SearchFill: A stochastic optimization code for detecting atomic vacancies in crystalline and non-crystalline systems

We present an implementation of a stochastic optimization algorithm applied to location of atomic vacancies. Our method labels an empty point in space as a vacancy site, if the total spatial overlap of a “virtual sphere”, centered around the point, with the surrounding atoms (and other vacancies) falls below a tolerance parameter. A Metropolis-like algorithm displaces the vacancies randomly, using an “overlap temperature” parameter to allow for acceptance of moves into regions with higher overlap, thus avoiding local minima. Once the algorithm has targeted a point with low overlap, the overlap temperature is decreased, and the method works as a steepest descent optimization.Our method, with only two free parameters, is able to detect the correct number and coordinates of vacancies in a wide spectrum of condensed-matter systems, from crystals to amorphous solids, in fact in any given set of atomic coordinates, without any need of comparison with a reference initial structure.     

The influence of carbon vacancies on the stacking fault energy of TiC

The influence of carbon vacancies on the stacking fault energy (SFE) of TiC has been studied. It is found that SFE of TiC is depending on positions and concentration of the carbon vacancies. The closer carbon vacancies to the stacking faults (SFs) and the higher the concentration are, the smaller SFE will be. In other words, carbon vacancies in TiC with SFs prefer to segregate around SFs. When TiC with a few carbon vacancies, carbon vacancies tend to be uniformly distributed around SFs, but if a lot of carbon vacancies exist, they will be long-range ordered around SFs. The study of the diffusion of carbon vacancies demonstrates that carbon vacancies can be more easily diffused in TiC with SFs, especially in the SF layers, which further confirms the above results.     

Pt diffusion in B2-type ordered NiAl intermetallic compound and its diffusion mechanisms

Diffusion of platinum element which increase protectivity of NiAl layer for hot corrosion and oxidation of Ni-base superalloys was investigated in NiAl compound with compositions of 43.7–51.9 at% Al at 1373–1673 K by the diffusion couple method. The diffusion coefficients, D_Pt, exhibit a large concentration dependence of valley type-shape; the D_Pt has the minimum value quite near the stoichiometric composition and it increases with deviation from the stoichiometry. On the other hand, the activation energy, Q, shows the maximum value near the stoichiometry. Q slightly decreases with Ni composition in Ni-rich side, while it decrease dramatically with Al composition in Al-rich side. Diffusion of platinum element in NiAl occurs mainly by the Triple defect mechanism at stoichiometry, by the anti-site atom bridge mechanism in considerably off-stoichiometry in Ni-rich side and by the next nearest neighbor-jump mechanism with constitutional Ni vacancies in quite off-stoichiometry in Al-rich side.     

Cation vacancies in perovskites doped with La and Gd

Abstract

Whereas doping CaTiO₃ or SrTiO₃ with ～0·05 formula units (f.u.) of trivalent rare earth ions substituted for Ca can yield majority positron annihilation lifetime τ₁ increases of up to 30% due to the presence of cation vacancies, similar doping of BaTiO₃ produces increases of only ～10%. The difference is attributed to hole trapping around cation vacancies in BaTiO₃. The absence of significant changes to τ₁ for La doped CaTiO₃ when oxidised samples are reduced by sintering in a hydrogenous atmosphere is attributed to the formation of oxygen vacancies that co-exist with the original cation vacancies. Solid state nuclear magnetic resonance showed that a significant fraction of the La ions in BaTiO₃ containing 0·04 f.u. of La were in cubic coordination, irrespective of the charge compensation scheme employed. Doping of sintered SrTiO₃ with La to produce up to 0·2 f.u. of Sr cation vacancies had no significant effect on Sr leaching in deionised water at 90°C.     

Diffusion in mercury cadmium telluride—an update

The various diffusion coefficients (self-diffusivity, chemical self-diffusivity compositional diffusivity, impurity diffusivity) are defined. The conditions required for the observation of a Kirkendall effect are described. There is good agreement in the results for Hg and for Cd self-diffusion. The evidence suggests that the self-diffusivity is largely independent of P_Hg above ~300°C but shows an increase as Hg saturation is approached (for both Hg and Cd). Good agreement is also found in the Arrhenius parameters describing the movement of the p to n conversion boundary. Modeling of this diffusion boundary does, however, raise problems which are discussed. The situation in compositional interdiffusion is more complex: above ~400°C, reasonable agreement exists between various workers for large x_Hg but not for small values; below 400°C, substantial disagreement is evident. In impurity diffusion exhibits both erfc and non-erfc profiles for reasons which are unclear. Good agreement is found between In diffusivity measurements at high In concentrations: at low concentrations, the diffusivity decreases dramatically. As diffusion yields erfc profiles with good agreement again being found: a notable feature is the P_Hg dependence of the As diffusivity. Where appropriate, diffusion models are discussed.     

Some effects of an electric field on the plastic deformation of metals and ceramics

 The external parameters generally considered in the plastic deformation of metals and ceramics are the temperature, pressure or stress and time. Usually neglected are the effects of electric and magnetic fields. However, such fields can often have a significant influence, especially when applied concurrently with the more common parameters. Some examples of the effects of an electric field on the plastic deformation of metals and ceramics found by the author and his coworkers are presented. Included are the following: (a) the influence of electropulsing on the flow stress of metals at 78–300 K, (b) the effect of an external electric field (surface charge) on the superplastic deformation of the 74754 Al alloy, (c) the influence of an electric field on the flow stress and ductility of polycrystalline NaCl at 0.28–0.75 T_M and (d) the effect of an electric field on the superplastic deformation of 3Y-TZP. Mechanisms responsible for the observed effects are considered. Received: 1 January 1998 / Accepted: 1 March 1998     

Majority and minority intrinsic defects in lithium and sodium halides

We present new findings on the enthalpies of formation of vacancies and interstitial ion defects on either sublattice in the alkali halide crystals MX (X=F, Cl, Br and M=Li, Na) using the MPPI model. The model uses a minimum fit to crystal properties and is fully consistent with the dielectric response. The role of the non-Coulombic short range forcesvis-a-vis polarization fields in the stabilization of the defect formation process is examined and discussed in detail. It is shown that it is important to treat the electrostatic polarization caused by the point defect carefully and variation in potential modelling has a comparatively minor effect on the calculations of enthalpies of formation.