Effect of thermal insulation on microstructural homogeneity and onset temperature of flash sintered materials

The present work aimed to reduce the microstructure heterogeneity inherent to flash sintering by using alumina blankets as a thermal insulator around ZnO cylindrical samples during the sintering process, under different electric field conditions. Thermal insulation significantly reduced the flash onset temperature and the grain size heterogeneity. For higher electric fields, a temperature reduction as high as 480 °C was observed, which also led to lower densification. These findings were discussed in terms of changes in the heat loss dynamics coupled with the adsorbed water retention, both promoted by the applied thermal insulation. A model to estimate the temperature at stage III of flash sintering was proposed. The final temperature reached with thermal insulation did not differ significantly from the ones without it. Thus, thermal insulation could represent an alternative route to flash sinter materials with lower furnace temperatures with energy savings up to 78 % and a more homogeneous microstructure.     

Image-based extended finite element modeling of thermal barrier coatings

Further advances in Thermal Barrier Coating (TBC) design are linked with the evolution of numerical models for TBCs. The present paper, therefore, enhances the idea of a currently available FEM package (OOF) that has been designed for microstructural level simulations. The approach of Extended FEM (XFEM) is incorporated in an in-house developed program to account for the existence of cracks in TBCs; both for stress-strain analysis and for heat transfer analysis. The new XFEM program is then employed to carry out the analyses of a YSZ deposit and a multilayered TBC to predict the effective Young's moduli, the overall thermal conductivities, and to assess the fracture behavior of the coating.     

Thermophysical properties of a Ti–44%Al–8%Nb–1%B alloy in the solid and molten states

The families of titanium aluminide intermetallic alloys have attractive high temperature mechanical properties which make them potential candidate materials for a wide range of applications, particularly in the aeronautic and automobile sectors. The development of appropriate manufacturing techniques is an essential stage in the engineering exploitation of these materials, e.g., Induction Skull Melting is one of the techniques which needs to be optimised for the casting of titanium aluminides. Research is underway to develop a computer model of this process but data are required for the key thermophysical properties. Pulse-heating techniques have been used to measure properties for the Ti–44Al–8Nb–1B system. Rectangular samples have been prepared and are resistively heated as part of a fast capacitor discharge circuit. Time-resolved measurements with sub-μs resolution of currents through the specimen were made with a Pearson probe current monitor using the induction principle. Voltages across the specimen were determined with knife-edge contacts and voltage dividers, and radiance temperatures of the sample were measured with a pyrometer. These measurements allow the calculation of specific heat and dependencies between enthalpy, electrical resistivity and temperature of the alloy up into the liquid phase. Data for thermal diffusivity have been obtained by using the Wiedeman–Franz relation. The results are compared with those obtained using DSC and the four-probe method to measure the temperature dependence of the resistivity.     

Thermal error measurement and modelling in machine tools.: Part I. Influence of varying operating conditions

Thermal error in machine tools has been observed to be closely linked to the temperature of critical elements of the machine. It was found, in the experiments conducted herein, that there was a significant increase in the axis positioning error on account of an increase in the temperature of the machine elements due to continuous operation. However, it was also observed that the specific operating parameters of the test cycles carried out also significantly affected the positioning error. Different sets of operating parameters generated significantly different error values even though the temperature of the machine elements generated by those operating conditions was similar. As such, this observation forms a very important consideration in the development of a generic thermal error compensation system. It appears to indicate that such a system needs to be capable of evaluating the compensation depending upon the temperature values of the machine elements as well as account for the effect that different operating parameters induce upon the positioning error under the same thermal condition of the machine. This paper attempts to analyse the thermal behaviour of a three-axis vertical machining centre under the influence of various operating parameters and, through the experimental results obtained, point out and explain the effect of these parameters on the axis positioning error. This behaviour forms the basis of an improved modelling methodology that is presented separately in Part II of this paper.     

Effects of laser beam energy and incident angle on the pulse laser welding of stainless steel thin sheet

This paper reports an experimental study of laser spot welding on stainless steel sheets. A pulsed Nd:YAG laser was used to weld the stainless steel specimen in the range of laser energy 0.6–1.2 J and incident angle 30–75° (the angle of the laser beam incident direction to the sheet surface). Metallography was applied to measure the cross-sectional size and shape of the welded spot. From the experimental results, it is found that as the laser energy increases, the penetration depth, bead length, and bead width of the welded spot increase. As the laser incident angle increases, the penetration depth and the bead width increase while the bead length decreases. The results illustrate that the shape and size of the welded spot depend not only on the laser energy, but also on the incident angle of laser beam.     

Atomic force microscopy study of biaxially oriented polypropylene films

Atomic force microscopy (AFM) uses a very sharp pointed mechanical probe to collect real-space morphological information of solid surfaces. AFM was used in this study to image the surface morphology of a biaxially oriented polypropylene film. The polymer film is characterized by a nanometer-scale, fiberlike network structure, which reflects the drawing process used during the fabrication of the film. AFM was used to study polymer-surface treatment to improve wettability by exposing the polymer to ozone with or without ultraviolet (UV) irradiation. Surface-morphology changes observed by AFM are the result of the surface oxidation induced by the treatment. Due to the topographic features of the polymer film, the fiberlike structure has been used to check the performance of the AFM tip. An AFM image is a mixture of the surface morphology and the shape of the AFM tip. Therefore, it is important to check the performance of a tip to ensure that the AFM image collected reflects the true surface features of the sample, rather than contamination on the AFM tip.     

国外武器装备电磁环境适应性试验与评价技术及能力发展综述

武器装备的电磁环境适应性已成为制约武器装备能打仗、打胜仗的一个重大问题;西方军事强国非常重视面向复杂电磁环境的武器装备试验与评价技术研究和能力建设,通过相关投资计划的长期持续投入,支持突破其中的关键技术、方法,以及构建相应的试验与评价设施能力;文章从标准规范和研制程序、技术方法、试验设施、科研和能力建设投资计划等几方面综述了国外武器装备复杂电磁环境适应性试验与评价技术和能力发展情况,并在此基础上,提出了对我国武器装备电磁环境适应性试验与评价能力的发展建议.     

Epsilon-near-zero or mu-near-zero materials composed of dielectric photonic crystals

We theoretically demonstrate that at certain frequencies two-dimensional dielectric photonic crystals （PCs） may be regarded as either epsilon-near-zero or mu-near-zero materials. We show that the transmission through a slab of such materials upon normal incidence is normally non-unity and decays with slab thickness. However, when the incident angle increases slightly, the transmittance experiences a dramatic increase due to the Brewster effect. The combination of the tunneling and resonance effects makes such materials good candidates for almost perfect bending waveguides and cloaking in waveguides. The zero index also enables applications of focusing and directive emission. At last, the distinction between the single-zero and double-zero media is discussed. In all of the above results, the numerical simulations perfectly match with theoretical predictions from the effective medium analysis.     

宽带钢厚度模型的研究与优化

介绍莱钢宽带钢厚度模型的优化,以改善板形,提高控制精度.     

一种消除阶梯效应的图像分解新模型

针对TV-L1分解模型在进行图像分解时所得到的结构部分有阶梯效应的问题,本文提出一种改进的图像分解模型即GJTV-L1模型。该模型首先得到结构部分的切向量场,由切向量场可得到结构部分的法向量场,然后拟合法向量场得到重构图像,即结构部分。通过仿真实验,验证了GJTV-L1模型和算法的合理性及有效性。