Impact of Brownian motion on the particle settling in molten metals

Understanding the settling behavior of nanoparticles in molten metals/alloys is important as it will aid in achieving uniform dispersions of reinforcement particles in metal matrix nanocomposites. Uniform dispersions are necessary to activate the Orowan strengthening mechanism, which can increase yield strength without significant diminishment of ductility. In this work, an analytical model of particle size effects on settling is described that takes into account both deterministic Stokes’ law and stochastic Brownian motion. The model shows a clear transitional behavior where settling velocity follows Stokes’ law for large particles and then drops to zero for small particles implying that Brownian motion predominates. It indicated that, in the Brownian motion regime, where the discrete nature of the liquid must be considered, the random motion imparted by unbalanced collisions can overwhelm the motions normally imposed by forces such as gravity, viscous drag, and thermal/concentration gradients.     

高合金化Al-Zn-Mg-Cu系7049A合金的连续冷却析出图

研究了所有有技术价值的冷却速率范围内7049A铝合金的析出行为。冷却速率从接近平衡冷却时的慢速条件变化到形成完全超饱和固溶体的高速率,跨越了7个数量级（0．0005到5000K／s）。7049A铝合金连续冷却析出行为采用差热分析（DSC）、扫描电镜（SEM）和维氏硬度测量相结合的方法记录。冷却速率在0．0005到4K／s时,对高合金化、高强度和高淬火敏感性的变形铝合金7049A从固溶温度下的淬火析出行为采用传统的DSC方法研究。在此冷却速率范围内,至少观察到了两个放热反应：一个是在很窄的温度区间430～450℃内的高温反应;另外一个是最低到200℃且范围很宽的低温反应。这两个反应的强度随着冷却速率的增高而降低。采用快速差热分析（DFSC）和差分再加热方法（DRM）对合金淬火冷却速率从慢速到数千K／s时的析出行为进行了研究。该合金不析出沉淀相时的临界淬火速率为100～300K／s。     

铸态Al-7Si-0．3Mg合金的连续冷却析出图

采用冷却速率为0．01～3K／s的差示扫描量热法（DSC）和冷却速率更高的淬火膨胀法研究铸态Al-7Si-0．3Mg合金淬火的析出行为。在合金冷却的过程中发生了两种析出反应,高温反应开始于淬火起始温度540℃,低温反应始于400℃左右。3K／s的淬火冷却速率已经显著抑制淬火过程中相的析出。合金T6态的硬度随着淬火速率的增快而增加,这是由合金过饱和固溶度增加而导致的。通过膨胀实验和硬度实验的结果可以估计临界冷却速率大约为60K／s。通过光学显微镜观察淬火态的铸态Al-7Si-0．3Mg合金的显微组织。结果表明：根据淬火冷却速率的不同合金的显微组织由铝一硅共晶组织、铝固溶体枝晶及枝晶间的析出相组成。     

Performance evaluation of three optical particle counters with an efficient “multimodal” calibration method

The sizing accuracies of two widely used yet hitherto unevaluated optical particle counters (OPCs—Grimm Model 1.109 and Palas Model WELAS 2100) as well as one high-resolution, non-commercial OPC were evaluated. The measured data were compared to scattering intensity calculations based on Mie theory. Additionally, the counting efficiency for all three counters was measured, as was the influence of coincidence effects for the OPC with the lowest (manufacturer specified) upper concentration limit.Beside the traditional polystyrene latex calibration, a little-known, very fast and precise “multimodal” calibration method was used, which is based on the simultaneous generation of up to eight sharp multiple-charge modes from polydisperse di-ethyl-hexyl-sebacate (DEHS) particles by electrical mobility classification.     

Varistor Behavior at Twin Boundaries in ZnO

The electrical behavior of commercial ZnO varistor devices has been examined with voltage contrast microscopy and point contact dc electrical measurements. Nonlinear voltage-dependent behavior has been observed across both of the major crystalline boundary types present in the system: Bi₂O₃ layer containing ZnO grain boundaries (or grain boundaries) and antimony spinel layer internal ZnO inversion twin boundaries (or twin boundaries). Twin boundaries, which bisect practically every grain in a typical commercial device, possess potential barriers with higher average breakdown voltages than do grain boundaries. Certain zinc antimonate spinel (Zn₇Sb₂O₁₂) grains are electrically isolated from the matrix, whereas others are conductive within the matrix.     

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m^?1 K^?1 for EB-PVD YSZ coatings to about 0.7 W m^?1 K^?1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni₃Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.     

Analysis and specification of the crash behaviour of plastics/metal-hybrid composites by experimental and numerical methods

Plastics materials are nowadays used in many structural applications for the substitution of metals with respect to weight reduction. In order to utilize the high freedom of design and the light-weight potential of plastics materials in crash-relevant structural parts, so-called hybrid composites which combine the high rigidity and strength of steel with the advantages of plastics materials are investigated in the outlined research. Thereby, the joining of both materials as well as the design by means of numerical methods such as the finite element analysis (FEA) are challenges which have to be met. A new approach in joining is based on the modified arc welding process where metal pin structures are formed in one working step and subsequently welded onto the surface. The pins are formed with ball-shaped, cylindrical or spiky ends and produced directly from the welding wire without requiring additional pre-fabricated components such as studs or similar. This allows the small-scale surface structuring of metal components that can be adapted optimally for a form fit on the respective plastics structure. Subsequently, injection molding is used for the application of the plastics material onto the pin-structured metal part in order to generate a positive fit between metal and plastics in an intrinsic joining process. An additional joining process, which is carried out after injection molding, is not required. Within the framework of the research presented, comprehensive mechanical tests are presented to illustrate the suitability of pin-structured metal-hybrid composites in crash applications. In comparison to structures which are in particular exposed to static loads and therefore designed to exhibit maximum component strength, crash applications are designed to fail in a continuous process to achieve maximum energy consumption. The outlined research illustrates the enhanced failure behavior of pin-structured plastics/metal-hybrid composites and the increased energy consumption under impact loading. Moreover, a comparison between pin structuring and laser structuring with regard to the obtainable mechanical properties under impact loading is given. Concluding, the current potential and weak points in the simulation of plastics/metal-hybrid structures using FEA is presented and discussed.     

Shape forming alumina slurries via Colloidal IsoPressing

The relative density, deformation behavior, and dynamic rheology of bodies consolidated from aqueous alumina slurries, formulated at pH 4 and containing NH₄Cl, were studied as a function of the salt concentration (0.1–0.75 M). The smallest salt concentration produced the highest relative density (0.596), which was independent of consolidation pressure. Nearly identical relative densities were obtained for the other salt concentrations, ranging from 0.57 to 0.59 for consolidation pressures of 5–100 MPa, respectively. The consolidated bodies exhibited either a plastic or an elastic behavior when tested in compression. It was observed that the transition from a plastic to an elastic behavior occurred at a critical consolidation pressure. This critical consolidation pressure was directly dependent on the salt concentration. The storage (elastic) modulus for the fluidized, plastic bodies increased with salt concentration. The storage modulus was also observed to be dependent on the time the bodies were allowed to rest after they were subjected to a high shear rate. A pre-consolidated and fluidized slurry formulated to contain 0.5 NH₄Cl was extruded into a rubber mold cavity and further consolidated by isopressing at 200 MPa for 2 min. The body not only increased its relative density (0.58–0.62), but it also became elastic to enable its removal from the rubber cavity without shape distortion. No linear shrinkage was detected after the isopressed bodies were dried. Because of the lack of shrinkage, rapid drying was possible. For example, bar specimens (6.4 mm × 12.7 mm × 60 mm) would survive a 34 °C/min heating rate to 300 °C; the specimen was completely dried within 20 min. Because shrinkage does not occur during drying, bodies could be placed directly into a furnace and heated to 1500 °C for 0.5 h to achieve a relative density of >0.99 and an average grain size of 1.6 μm.     

Fabrication of Photonic/Microfluidic Integrated Devices Using an Epoxy Photoresist

Using a single layer of SU‐8 photoresist to fabricate optical waveguide cores and microfluidic channels on Pyrex glass is an ideal way to achieve photonic/microfluidic integration on a single chip. To address the problem of poor bonding, a thin nanoscale intermediate polymer layer was applied to reduce the stress generated from the material processing while maintaining strong adhesion between the patterning polymer layer and Pyrex. It was found that a 186–600 nm thick intermediate layer of a specialty epoxy photoresist effectively served the purpose without deteriorating the optical performance of the involved waveguides. Quality photonic/microfluidic integrated devices with satisfied optical performance were fabricated.

     

Vaporization of Protective Oxide Films into Different Gas Atmospheres

Oxidation of Metals - Vaporization often accompanies high-temperature oxidation and corrosion. In this review, vaporization under a vacuum as well as static gas and flowing gas conditions is...