(Ce0.9Nd0.1)1-xMoxO2-δ(0.00≤x≤0.10)的合成、表征与电性能

采用溶胶-凝胶法合成(Ce0.9Nd0.1)1-xMoxO2-δ(x=0.00、0.02、0.05、0.10)氧化物,通过X射线衍射(XRD)、场发射扫描电镜(FESEM)等手段对氧化物进行结构表征,交流阻抗谱测试电性能.结果表明:所有样品均为单一萤石立方结构;少量MoO3的加入提高了材料的致密性,降低了材料的总电阻、晶界电阻和晶界电阻在总电阻中所占比例,提高了材料的电导率.1200 ℃烧结样品24 h,测试温度700℃时,(Ce0.9Nd0.1)1-xMoxO2-δ(x=O.00)总电导率和晶界电导率分别为0.05和O.19 S·m-1,掺Mo材料(Ce0.9Nd0.1)1-xMoxO2(x=0.02)的总电导率和晶界电导率分别为2.42和3.96 S·m-1.     

Influence of sintering atmosphere and thermobaric treatment (TBT) on dielectric behaviors of CaCu3Ti4O12 ceramics

Copper titanate (CaCu₃Ti₄O₁₂, or CCTO) ceramics sintered in oxygen and vacuum at 1100°C for 12 h have been treated by thermobaric treatment (TBT) at 9 GPa and 1000°C for 10 min and then quenched in liquid nitrogen (LN₂). Pure cubic body-centered perovskite-related structure was confirmed from XRD results. Besides, after TBT and quenching, additional minor peaks of TiO₂ and Cu were indexed, as well as a little cell volume expansion. A little reduction of the grain size as well as fuzzy grain boundaries can be observed in FE-SEM after TBT. The dielectric constant ε' of CCTO ceramics sintered in oxygen (～4600 @ 100 Hz) were a half of those treated by TBT and quenching (～9600 @ 100 Hz), whereas the value decreased almost by an order of magnitude (from 10⁵ to 10⁴) after TBT and quenching for the samples sintered in vacuum. The complex impedance spectra at high temperature showed three semicircles so a three RC model was used to explain the different relaxation regions consisting of grain, grain boundary and domain boundary. Besides, two relaxation peaks appeared in the frequency dependence of the imaginary part of the electric modulus formalism $M^{?}$ and the complex impedance $Z^{?}$. Moreover, the contributions of grain boundaries can be figured out by the relaxation activation energy $E_{re}$ (338–629 meV) and the conduction one $E_{dc}$ (396–823 meV) fitted by $M^{?}$ and $Z^{?}$ plots separately. As a result, the disorder and heterogeneity typical of grain boundaries should be responsible for the giant dielectric relaxation characteristics of CCTO ceramics, which can be affected by the sintering atmosphere as well as the extreme treating conditions.     

Incipient plasticity and voids nucleation of nanocrystalline gold nanofilms using molecular dynamics simulation

In this study, the incipient plasticity and voids nucleation of nanocrystalline gold were investigated using a molecular dynamics simulation. The effects of mean grain size and temperature were evaluated in terms of the material's stress-strain diagram, Young's modulus, yield strength, common-neighbor analysis, slip vectors, and deformation behaviors. From the stress-strain diagram, at 300?K, the maximum stress value corresponding to a grain size of 3.2?nm was much lower and the stress curve was clearly different from those corresponding to other grain sizes. Young's modulus increased with increasing mean grain size. The inverse Hall–Petch relation was observed. The slip was the main deformation behavior at a mean grain size of 3.2?nm. Moreover, the internal stress was more pronounced with increasing temperature. At 700?K, the main deformation area range was concentrated in the lattice at the middle of the samples, resulting in an almost force–induced structural transformation phenomenon in the middle. Void damage occurred at the junction of three–grain boundaries during the tensile process. With decreasing mean grain size, the less internal differential slip was generated under the same temperature and strain conditions.     

Microstructure damage at ensemble of points for grain composites

Fracture concentration zones are considered in microstructure elements of grain composites. Mathematical model of micro-heterogeneous medium with random properties of elements is used for calculations. The distribution laws for the modules of elasticity and ultimate strengths in the elements as well as the tensor of macroscopic deformations for the composite serve as the initial data. Different types of stresses are evaluated. Correlation functions for micro stresses are obtained by the Green’s tensor method.Random microstructure strength condition is a difference between the stress and the ultimate strength at any point of an ensemble with a particular configuration. The probability of simultaneously exceeding the ultimate strength in this set of elements determines the likelihood of failure of this ensemble of points and the relative damage at the micro level.The damage is calculated using multivariate normal distribution. Structure of correlation matrix of distribution depends on the type of fracture concentration zones. Correlation functions of microstructure strength condition depend on the distance between the points of the ensemble. Calculations of multipoint damage are provided for several configurations of points, in particular, for the three points on a straight line segment, and for the five points in the vertices and the center of a tetrahedron. For two-dimensional distribution density, the smoothing surface formulas are derived, taking into account the moments of stresses up to and including the fourth order.The influence of microstructure properties and the type of ensemble of points on composite damage is demonstrated. Study of microstructure damage enables the prediction of early stages of construction material failure.     

Plasticity of metal wires in torsion: Molecular dynamics and dislocation dynamics simulations

The orientation dependent plasticity in metal nanowires is investigated using molecular dynamics and dislocation dynamics simulations. Molecular dynamics simulations show that the orientation of single crystal metal wires controls the mechanisms of plastic deformation. For wires oriented along , dislocations nucleate along the axis of the wire, making the deformation homogeneous. These wires also maintain most of their strength after yield. In contrast, wires oriented along and directions deform through the formation of twist boundaries and tend not to recover when high angle twist boundaries are formed. The stability of the dislocation structures observed in molecular dynamics simulations are investigated using analytical and dislocation dynamics models.     

Temperature-dependent texture, stress and resistivity in melt spun Cu0.95Co0.05 ribbon

Ribbon samples of Cu_0.95Co_0.05 were prepared by melt spinning method to perform systematic investigations on structure and transport properties as a function of annealing temperature. X-ray diffraction study shows that the ribbon is polycrystalline with a strong 2 0 0 texture along the surface normal of the as-quenched Cu_0.95Co_0.05 ribbon and the degree of texture is enhanced upon annealing. The compressive stress, which relaxes upon annealing, is observed in as-quenched ribbon. The resistivity, which is higher in as-quenched ribbon, decreases toward the bulk value of Cu upon annealing. The compressive stress and higher resistivity in as-quenched ribbon are attributed to the incorporation of Co atoms/particles in Cu matrix. The decrement of the stress and resistivity upon annealing is due to the precipitation of Co atoms from the Cu matrix, segregating as Co or Co-rich Cu grains as observed from the transmission electron microscopy measurements.     

Simulation of dc magnetic effects due to geometrically defined grain boundaries in type-II superconductors

A Bean model-based program (“Trazacorrientes”^®) has been used to simulate the current distribution in the saturated remanent state of type-II superconducting bicrystal-like squared samples. The grain boundary was modeled by a set of periodically spaced holes geometrically defining the current transparency. Current simulations performed as a function of the boundary transparency, width and geometry are analyzed. Current distributions agree qualitatively with previously reported imaging measurements, while quantitative results can be obtained with an accuracy of 5% due to present computing resolution limits. Thanks to “Trazacorrientes”^® easy way of implementing irregular defects, meandering grain boundaries formed by straight facets of different local transparency could be simulated. The advantages and disadvantages of the program for the simulation of type-II superconductors with defects, among which GB’s, are discussed.     

Effects of sulfur content and slab reheating temperature on the magnetic properties of fully processed nonoriented electrical steels

The effects of sulfur content and slab reheating temperature on the magnetic properties of four fully processed nonoriented electrical steels have been investigated. Four slabs of nonoriented electrical steels with sulfur content in the range of 0.0006–0.0126 wt% were reheated to 1100, 1200, and 1300 °C, respectively. Then, they were hot rolled and annealed at 700 °C, cold rolled at the same condition and annealed at 820 °C in the salt bath furnace for 1 min to simulate continuous annealing. The ac core loss, dc hysteresis loss, and ac and dc permeability were measured at 15 kG inductions. It was found that the amount of inclusions in the hot-rolled bands increased with increasing slab reheating temperature and increasing sulfur content in steels. After final annealing, grain sizes of cold-rolled steel sheets decreased with increasing sulfur content and increasing slab reheating temperature. The main preferred orientations in the final annealed steel sheets were (0 1 1) 〈1 0 0〉 and (1 1 1) 〈u v w〉 γ fiber texture. Steel sheets containing 0.0032 and 0.0060 wt% sulfur developed a more stronger (0 1 1)〈1 0 0〉 texture than other steel sheets. However, steel sheets containing 0.0126 wt% sulfur had the weakest (1 1 1)〈u v w〉 texture during slab reheating at temperatures higher than 1200 °C. Both ac core loss and dc hysteresis loss increased with increasing slab reheating temperature and increasing sulfur content in steel sheets. Both ac and dc permeability decreased with increasing slab reheating temperature and increasing sulfur content in steel sheets. If sulfur content decreased from 0.0060 to 0.0032 wt%, there were great improvements in ac core loss, dc hysteresis loss, and ac and dc permeability. However, eddy current loss was almost independent of the sulfur content and slab reheating temperature.     

Investigation of three-dimensional aspects of grain-scale plastic surface deformation of an aluminum oligocrystal

This is a study of plastic strain localization, surface roughening and of the origin of these phenomena in polycrystals. An oligocrystal aluminum sample with a single quasi-2D layer of coarse grains is plastically deformed under uniaxial tensile loading. During deformation, the history of strain localization, surface roughening, microstructure and in-grain fragmentation is carefully recorded. Using a crystal plasticity finite element model, corresponding high-resolution simulations are conducted. A series of comparisons identifying aspects of good and of less good match between model predictions and experiments is presented. The study suggests that the grain topology and microtexture have a significant influence on the origin of strain heterogeneity. Moreover, it suggests that the final surface roughening profiles are related both to the macro strain localization and to the intra-grain interaction. Finally slip lines observed on the surface of the samples are used to probe the activation of slip systems in detail. The study concludes with an assessment of the limitations of the crystal plasticity model.     

Size effects in indentation response of thin films at the nanoscale: A molecular dynamics study

The indentation response of Ni thin films of thicknesses in the nanoscale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. A series of simulations were performed in films in the [1 1 1] orientation with thicknesses varying from 4 to 12.8 nm. The study included both single crystal films and films containing low angle grain boundaries perpendicular to the film surface. The simulation results for single crystal films show that as film thickness decreases larger forces are required for similar indentation depths but the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The low angle grain boundaries can act as dislocation sources under indentation. The mechanism of preferred dislocation emission from these boundaries operates at stresses that are lower as the film thickness increases and is not active for the thinnest films tested. These results are interpreted in terms of a simple model.