Multiscale evaluation of microstructural worthiness based on the physical–analytical matching (PAM) approach

By selecting the appropriate spatial and temporal variables, physical abnormalities at the microscale can be limited to the essentials such that effective analytical solutions can be made available and identified with high resolution TEM and SEM micrographs. A catalogue of physical–analytical pairs can be stored electronically for the evolution of material damage from micro (or nano) up to macroscopic failure. High speed digital processing can organize such information and make forecast on potential failure in the cyberspace. The micro/macro line cracking model is used to illustrate the use of the PAM (physical–analytical matching) technique although it is still at the early stage of development. Three essential parameters describing the inhomogeneity of the material serve as the basis of the approach while specific microstructural details can be accounted for by using the incidental variables as programmed by PAM in the future. Demonstration is made by using the dual scale micro/macro line crack model where closed form asymptotic solution can be obtained from singularity representation. No generality is lost from the line crack configuration. This is analogous to taking the shape of an atom to be spherical since the exact shape is not relevant. The essential quantities are the energy density and characteristic length associated with the equivalent crack length defined with reference to the spatial and temporal variables under consideration.For a macroscopic tensile specimen containing a micro/macrocrack, multiple microcracking patterns are generated. A priori assumptions related to the grain geometries and/or cohesive force laws to create branching will not be made. Instead, values of the essential parameters are selected to obtain multiple minima of the volume energy density functions. These minima are very closely spaced. This implies that the initiation of multiple microcracking is probable even though dynamic effects are not present. This is in contrast to macrocrack branching where the crack velocity had to approach that of the Rayleigh wave speed. The formulation also shows that empirical approach blind folds the details of microscopic and scaling effects. The double singularity line crack model is used to illustrate that different multiple microcracking patterns can be predicted from the volume energy density fracture criterion that has been used extensively for examining the initiation of macrocracking. The criterion relies on identifying the locations of the stationary values of the energy density function with the potential threshold sites dominated by dilatation or distortion without assuming that the two energy density components are the linear sum, a condition invoked in linear elasticity. To reiterate, three essential parameters are defined to describe the non-homogeneous behavior of the material while two incidental variables are used for the double singularity line crack to account for specific microstructural effects. It is the ease with which asymptotic closed form solution can be obtained and identified with observed damage patterns that suggests the possibility to develop the PAM technique in the cyberspace.     

Correlation of optical and microstructural properties of Gd2O3 thin films through phase-modulated ellipsometry and multi-mode atomic force microscopy

Gadolinium oxide thin films have been prepared by electron beam evaporation with different reactive oxygen pressures at low ambient substrate temperature. These thin films have been analyzed with phase-modulated spectroscopic ellipsometry and multi-mode atomic force microscopy (AFM). The distinct influences of oxygen pressure on surface topographies, microstructures and refractive indices of the thin films have been observed from the results of these advanced measurements. Both these techniques have displayed very strong co-relationships in the characterisation results acquired through respective modes of measurements and data analysis. Unequivocally, these two techniques indicated an optimum value of the oxygen pressure leading to best optical and structural properties of such a novel optical coating material.     

新疆喀什河吉林台水电站地区稳定性研究

新疆喀什河吉林台水电站地区是地壳现代构造运动仍十分活跃的地区，工程的安全有可能会受到断层新活动的威胁，必须提供有关区域稳定性资料作为建设的依据，本文应用显微构造分析，同位素法测年，石英形貌法研究综合确定可能威胁工程的断层最后一次较强烈活动的时间为中更新世晚期，其活动的下限时间是13-20万年，上限时间为9-15万年。结合模拟实验的结果指出，在继续遭受到区域构造应力的作用下，工程区会作整体的抬升，不会产生新的断裂系统和应力集中区，因此选取的工程场址是优良的。     

Microstructural modeling of asphalt concrete using a coupled moisture–mechanical constitutive relationship

The coupled effect of moisture diffusion and mechanical loading on the microstructure of asphalt concrete is studied. The traditional Continuum Damage Mechanics (CDM) framework is modified to model detrimental effects of moisture and mechanical loading. Adhesive/cohesive moisture-induced damage constitutive relationships are proposed to describe the time-dependent degradation of material properties due to moisture. X-ray two-dimensional (2D) computed tomography-imaging technique is used to construct finite element (FE) microstructural representation of a typical dense-graded asphalt concrete. After being calibrated against pull-off experiments, the proposed moisture-induced damage constitutive relationship, which is coupled to thermo-viscoelastic–viscoplastic–viscodamage mechanisms, is used to simulate the microstructure of asphalt concrete. Simulation results demonstrate that the generated 2D FE microstructural representation along with the coupled moisture–mechanical constitutive relationship can be effectively used to model the overall thermo-hygro-mechanical response of asphalt concrete.     

Effect of Thermal Cycles on the Thermal Expansion Behavior of T700 Carbon Fiber Bundles

The relationships between the coefficient of thermal expansion(CTE) of T700 carbon fiber bundles(CFBs) and the thermal cycles were investigated. The microstructure of T700 CFBs was analyzed with Raman spectra and XRD before and after the thermomechanical test. The results indicated that the T700 CFBs exhibited negative expansion in the direction of parallel fibers in the temperature range of -150-150℃. The thermal strain that occurred during the heating and the cooling thermal cycle had an unclosed curve that served as the loop. When the experimental load was the same, the position of strain loop tended to move upward, and the length of the specimen increased continuously with the thermal cycles increasing. The microstructural analysis suggested that the degree of structural order and the degree of orientation along the fiber axis were improved with the increase of thermal cycles. The change of microstructure parameters could be the primary cause of the negative CTE's variation within the T700 CFBs.     

Loss mechanism and lifetime estimation of Ce–W cathode in high temperature thermionic emission electrostatic precipitation

Rare earth tungsten, 2.2% cerium–tungsten (Ce–W), was used as emission cathode material in thermionic emission electrostatic precipitation. The microstructures and emission properties were investigated. A model was established to estimate cathode lifetime in high temperature and oxygen-bearing atmosphere. Results show that there were not great changes in the inner microstructure of emission materials. But on the surface, tungsten was oxidized to tungsten trioxide and then tungsten trioxide and rare earth oxides evaporated and lost. Decrease of oxygen concentration was effective to prolong operation time of cathode. The increase of temperature reduced cathode lifetime remarkably.     

Microstructure characterization and cation distribution of nanocrystalline cobalt ferrite

Nanocrystalline cobalt ferrite has been synthesized using two different methods: ceramic and co-precipitation techniques. The nanocrystalline ferrite phase has been formed after 3 h of sintering at 1000 °C. The structural and microstructural evolutions of the nanophase have been studied using X-ray powder diffraction and the Rietveld method. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is partially an inverse spinel. The transmission electronic microscope analysis confirmed the X-ray results. The magnetic properties of the samples were characterized using a vibrating sample magnetometer.     

Influence of slight microstructural gradients on the surface properties of Ti6Al4V irradiated by UV

Ti6Al4V alloy is one of the most widely used materials for biomedical implants. Among its properties, it is remarkable the photoactivity displayed by its passive layer, which is mainly composed by titanium dioxide. However, variations in the processing conditions may yield to differences in the microstructure which can be reflected on the surface properties of the machined product. From contact angle measurements taken on different zones of samples removed from a commercial bar of Ti6Al4V, it has been shown that the modifications of the surface Gibbs energy suffered by the alloy under UV irradiation have a radial dependence. This behaviour is related to slight microstructural changes of the alloy, particularly with an increase in the volume fraction of the β-phase when moving to the interior of the sample, which alters the composition and/or microstructure of the passive layer along its radius. This study shows that gradients in the microstructure and physical properties are sample size dependent and are likely related to thermal gradients during processing.     

Heat-induced changes in microstructure and magnetic properties of Co0.75Ni0.75[Fe(CN)6] · XH2O

Polycrystalline Co_0.75Ni_0.75[Fe(CN)₆]?·?XH₂O was prepared by coprecipitation. The coprecipitated powder was annealed in vacuum at 80°C, 100°C, and 130°C. Variation of microstructural and magnetic properties with different annealed temperatures was studied by Fourier-transform infrared, X-ray diffraction, and magnetization measurements. The differences in magnetic phase transition temperature, coercivity, remanence, and effective magnetization were studied in detail. The magnetic contribution mainly results from Fe^III–CN–Co^II/Ni^II and Fe^III–NC–Co^II/Ni^II because Fe^II–CN–Co^III/Ni^II carries no net spin. After annealing at 130°C, the microstructures Fe^III–CN–Co^II/Ni^II and Fe^III–NC–Co^II/Ni^II convert to Fe^II–CN–Co^III/Ni^II. Differences in magnetic properties may be attributed to heat-induced microstructural changes.     

Highly processable ester-functionalized polythiophenes as valuable multifunctional and post-functionalizable conjugated polymers

A series of newly ester-substituted polyalkylthiophenes (PATEs) was prepared. All polymers were fully soluble in common organic solvents, which allowed for a wide range of physico-chemical measurements, including solvato- and thermo-chromic, thermal, microstructural and electrical (in neutral and doped state) analyses. Marked differences were observed in the material characteristics of these polyalkylthiophene derivatives. It was thus possible to identify and describe the different properties ascribable not only to the different esteric groups in side chain but also to the different average degrees of functionalization of the thiophenic repeating units. In neutral state the PATEs of this study proved to be thermostable and soluble, while at the same time exhibiting high ordered molecular assemblies leading to high average conjugation lengths, electrical conductivities and stabilities. These polymers can also be easily and quantitatively transformed in other PAT derivatives using simple nucleophilic reactions either in solution or in solid (film) state. In view of the above and given that these polymers can be profitably and inexpensively synthesized, their future use for innovative applications in the field of synthetic metals appears feasible as from now.