Microstructure evolution and mechanical properties of a submerged friction-stir-processed AZ91 magnesium alloy

Journal of Materials Science - AZ91 casting alloy is subjected to friction stir processing (FSP) in air (NFSP) and under water (SFSP). The thermal histories of the two FSP procedures are measured,...     

Seismic performance of raised floor system by shake‐table excitations

Damage of expensive equipment installed in the raised floor system of high‐tech fabs was often observed during past earthquakes in Taiwan. This resulted in a huge loss of manufacturing functions and properties for the high‐tech industry. Therefore, there is an urgent need to understand the dynamic characteristics of the raised floor system for future seismic protection. This paper explores the seismic performance of a raised floor system under shake‐table excitations. The raised floor used was a pedestal–stringer frame structure supporting the simulated equipment. This raised floor system was the typical system frequently used in semi‐conductor fabs of Taiwan. The input motions for the shake‐table tests were the waffle‐slab floor accelerations of a typical semi‐conductor fab to simulated ground motions. The ground motions were simulated according to the phase spectrum and the maximum potential earthquake of the site located at Hsin‐Chu Science Park, Taiwan. The dynamic characteristics of the raised floor system were studied and discussed. This study also employed the finite element package to carry out numerical simulation on seismic responses of raised floor systems. Comparison with the experimental data showed that the proposed simulation model achieved excellent performance. Finally, the effectiveness of base isolation for reducing the acceleration of the system was also studied. Copyright © 2011 John Wiley & Sons, Ltd.     

Ultimate Bearing Capacity Analysis and Sizing Optimization of a Cracked Reactor Pressure Vessel in the Coupled Thermo-Mechanical Field

Pressurized thermal shock (PTS) can subject a crack surface to a very high tensile stress. Also the material toughness is obviously decreased in the cooling process, so it is necessary to study the influence of PTS on the ultimate bearing capacity of a reactor pressure vessel with defects. A 3-D finite element model is established for the beltline region around an inner crack. The FEM is used to reveal the transient temperature field and stress field, and the XFEM is adopted to simulate the ductile crack propagation. To ensure that the strength requirement is satisfied, the ultimate internal pressures of vessels with different crack sizes and different wall thicknesses are obtained. The result shows that the ultimate bearing capacity of the base wall with shallow surface cracks at high temperature is mainly controlled by tensile strength, while it is also affected by the fracture toughness of the material under the severe PTS. The stress in the early stage of the PTS is mainly the thermal stress, and later is the thermo-mechanical coupling stress. The impact of the crack depth on the bearing capacity of the structure is much greater than that of the crack length.     

Microstructure evolution of nanoprecipitates in half-Heusler TiNiSn alloys

The microstructures of thermoelectric TiNiSn half-Heusler alloys have been studied in detail. For concentration ratios that are slightly rich in Ni, a high density of Heusler-phase nanosized precipitates tended to precipitate within a half-Heusler matrix. The morphology and average size of the Heusler nanoprecipitates were very sensitive to the Ni concentration ratio in the half-Heusler matrix of the alloys. Smaller Heusler nanoprecipitates with coherent ellipsoidal (<5 nm) and disc (<10 nm wide) morphologies tended to precipitate within the matrices of alloys with slightly elevated Ni concentration ratios (Ti:Ni:Sn = 1.0:1.1:1.0). However, much larger coherent discs (<45 nm wide and <5 nm thick), semicoherent platelets (up to 1 μm long and <30 nm thick) and spheres (up to 1 μm wide) were observed in the matrices of the alloys with larger Ni concentration ratios (Ti:Ni:Sn = 1.0:1.2:1.0). Tetragonal structures were observed in the coherent Heusler nanoprecipitates. The formation of such structures was closely associated with the size, morphology and interface coherency of the nanoprecipitates. Moreover, most of the coherent Heusler nanoprecipitates were preferentially oriented parallel to the cubic {0 0 1}_HH orientations. Interfacial defects between the Heusler and half-Heusler phases, as well as lattice point defects, Ni antisites and vacancies, were found to be closely related to the formation of the Heusler nanoprecipitates. A mechanism has been proposed in this study to describe the coarsening of the Heusler nanoprecipitates via the formation of lattice point defects and interfacial defects.     

Dependence of electrical and optical properties of ZnO films on substrate temperature

ZnO films were prepared by filtered cathodic vacuum arc technique with Zn target at different substrate temperatures. The crystallinity is enhanced with increasing substrate temperature and preferably oriented at (1 0 3) direction when the substrate temperature is higher than 230°C. The PL emission corresponding to the exciton transition at 3.37 eV can be observed at room temperature, which indicates that high-quality films have been obtained by this technique. The Hall mobility, which increases with substrate temperature, is dominated by grain boundary scattering.     

The compatible and mechanical properties of biodegradable poly(Lactic Acid)/ethylene glycidyl methacrylate copolymer blends

The Fourier transform infrared results suggest that the carboxylic acid groups of poly(lactic acid) (PLA) molecules react with the epoxy groups of molecules of Ethylene Glycidyl Methacrylate Copolymer (EGMC) during the reactive extrusion processes of PLA_xEGMC_y specimens. The tensile and tear strength values of PLA_xEGMC_y blown-film specimens in machine and transverse directions improve significantly, and reach their maximal values as their EGMC contents approach an optimum value of 6 wt.%. The melt shear viscosity values of PLA_xEGMC_y resins, measured at varying shear rates, are significantly higher than those of the PLA resin, and increase consistently with their EGMC contents. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) of PLA and PLA_xEGMC_y specimens reveal that the percentage crystallinity, peak melting temperature, and onset re-crystallization temperature values of PLA_xEGMC_y specimens reduce gradually as their EGMC contents increase. In contrast, the glass transition temperatures of PLA_xEGMC_y specimens increase gradually in conjunction with their EGMC contents. Demarcated porous morphology with several connected fungi-decomposed cavities was found on the surfaces of the PLA_xEGMC_y specimens after being buried for specific amounts of time, in which the sizes of the fungi-decomposed cavities found on the surfaces of buried PLA_xEGMC_y specimens reduce significantly as their EGMC contents increase. Further DMA and morphological analysis of PLA_xEGMC_y specimens reveal that the EGMC molecules are compatible with PLA molecules at EGMC contents equal to or less than 2 wt.% because no phase-separated EGMC droplets and tan δ transitions were found on fracture surfaces and tan δ curves of PLA_xEGMC_y specimens, respectively. The possible reasons for these remarkable properties of the PLA/EGMC specimens are proposed in this study.     

Filler effect and pozzolanic reaction of ground palm oil fuel ash

This research examines the compressive strength of mortar and how the filler effect and pozzolanic reaction of ground palm oil fuel ash (POFA) contribute to this strength. POFA and river sand were ground to three different particle sizes and used to replace Type I Portland cement at 10–40% by weight of binder to cast the mortar. The compressive strengths of ground POFA and ground river sand mortars were determined at various ages between 7 and 90 days. The results showed that the compressive strength of mortar due to the filler effect of ground river sand was nearly constant during the 7–90 day period for a specified replacement rate of cement. However, the compressive strength of mortar due to the filler effect tended to increase slightly with increased cement replacement. The pozzolanic reaction of ground POFA increased with increasing particle fineness of ground POFA, replacement rate of cement, and age of the mortar. The compressive strength contribution from the pozzolanic reaction of ground POFA was much more pronounced than the contribution from the filler effect when the smallest sizes of both materials were considered.     

Crosslinking kinetics of polycarbosilane precursor in ozone atmosphere and the formation mechanism of continuous hollow SiC fiber

Crosslinking is favorable for increasing the ceramic yield of polycarbosilane precursor (PCS), so PCS fiber with crosslinked skin-uncrosslinked core structure may contribute to formation of hollow SiC fiber. In this study, diffusion-controlled ozone oxidation crosslinking method was adopted. EDS verified the oxygen element contributing to crosslinking mainly distributed in the outer layer of ozone-crosslinked PCS fiber (O ? PCS_f). After pyrolysis, SEM confirmed the formation of continuous hollow fiber and XRD indicated the presence of β-SiC structure. N₂ adsorption-desorption measurement demonstrated that a lot of micropores and mesopores, which were the main escape paths for degradation product of uncrosslinked core part of O ? PCS_f and contributed to the formation of continuous hollow SiC fiber, formed along fiber during pyrolysis and disappeared after the pyrolysis temperature was increased. In addition, by adjusting ozone oxidation time and applying vacuum pyrolysis method, the morphology of resulting hollow SiC fiber could be further modified.     

学风问题的消费主义根源与对策分析

在当今消费主义思潮的冲击下,高等教育消费主义已经渗透到高等学校的各个方面。作为高校校风核心和归宿的学风也不可避免地受到消费主义的影响,越来越具有功利化的倾向。本文对此进行了分析,并提出了建议。     

活性污泥胞外聚合物EPS的影响因素研究

胞外聚合物EPS对活性污泥的表面特性、生物絮凝、沉降及脱水性能等具有重要的影响,主要对污泥负荷Ns,DO,温度,Ca2+,pH及废水水质等因素对EPS的影响进行了研究。试验结果表明:Ns,pH及废水水质对EPS的含量、成分有显著的影响;DO,温度对EPS的影响较小;同时随着进水Ca2+浓度的增加,污泥的EPS,脱氢酶及OUR增加。