Key Engineering Materials Vols. 609-610

Abstract: We reported here a convective assembly process for the formation of large-area self-assembled monolayers of silica microspheres on silicon and glass substrates. Uniformly coated monolayers of silica spheres were achieved on silicon wafers with and without coated SiN₂ of 3 inch of diameter and large glass substrate of 6 × 6 in² in size. The coating of large-area uniform monolayers of silica microspheres was characterized with scanning electron microscopy and optical microscopy. The mechanism of the convective assembly has been explained by the convective flux that is generated by capillary immersion force caused from the solution evaporation and hydrodynamic drag force. The patterns of silica microspheres were transferred to the silicon substrates using a deep reactive ion etching technique. It is found that textured silicon reduced the reflectance of silicon substrate from 52.2% to 33.2% around 400 nm and from 33.9% to 19.5% around 1,100 nm. The rapid self-assembled monolayer with silica microspheres provided a glimpse at the wide range of coating and photonic device applications where convective assembly can be used.

Abstract: The collection efficiency (CE) of an aerosol sampler is usually assessed dependently by using a sampler with higher CE and higher sampling accuracy or comparing the grain size distribution, concentration and/or other characteristics of the collected dust to that of the original dust, instead of the sampler itself. To establish a simple method for the assessment on the collection efficiency (CE) of an aerosol sampler, a self-dependent method was derived to calculate the CE of an aerosol sampler, which was patented with the number of ZL200910233001.X by the State Intellectual Property Office of China. According to the patent method, two or more uniform aerosol samplers of the same model were connected in series the inlet of a sampler was connected directly with the outlet of another sampler. The CE (η) of the aerosol sampler can be calculated by a simple equation as: η=1-m₂/m₁, in which m₁ and m₂ is the weight of the aerosol particles collected by sampler 1# and sampler 2# in the connection sequence, respectively. A cascade impactor sampler was used to sample in a micrometer particle (d₅₀=2.5 μm) aerosol environment and a nanoparticle (d₅₀=42 nm) aerosol environment which were formed artificially in a glove box, as well as a workplace environment which manufactured nanometer powders. The sampling test results indicated that the cascade impactor sampler showed relative high CE (99.51%) for micrometer aerosol but a little bit low CE (95.2%) for nanoparticle aerosol. However, a low CE (93.93%) was calculated out by the method because of low concentration aerosol nanoparticles in the workplace environment, which result to big testing errors. It was found that the assessment result on collection efficiency of a sampler is highly affected by the subsequent analytical methods and detection accuracies after the sampling process. If the precision of the electronic balance was improved to a reasonable higher order of magnitude, the cascade impactor sampler can hopefully show much higher collection efficiency on nanoparticle aerosols.

Abstract: This paper reports a sealing process for reliable hermetic wafer-scale packaging among three wafers forming sandwich structure by glass frit. The process could be applied to Si, quartz crystal, SiC wafers and so on. The device wafer is bonded to the capping wafer in the first step and to another capping wafer forming vacuum-cavities in the second step. Experimental results indicate that the average bonding strength of packaging structure reached as high as 0.85kg/mm2 fulfilling MIL-STD-833E standard. The average leakage rate fits the standard of GJB-548A.This paper also realizes glass frit hermetic bonding in the case of Au/Cr and Pt/Ti film feed-through deposited on the device wafer.Experimental results indicate that Pt/Ti film feed-through hold up to bonding temperature cycling and avoid dissolution and adherence reduction compared with Au/Cr film feed-through

Abstract: The microstructure and material properties of AZ31 magnesium alloy are very sensitive to process parameters, which directly determine the service properties. To explore and understand the deformation behavior and the optimization of the deformation process, the microstructure evolution during equal channel angular pressing was predicted by using the DEFORM-3D software package at different temperature. To verify the finite element simulation results, the microstructure across the transverse direction of the billet was measured. The results show that the effects strain and deformation temperatures on the microstructure evolution of AZ31 magnesium during ECAP process are significant, and a good agreement between the predicted and experimental results was obtained, which confirmed that the derived dynamic recrystallization mathematical models can be successfully incorporated into the finite element model to predict the microstructure evolution of ECAP process for AZ31 magnesium.

Abstract: In order to understand the fracture mechanisms of Cu/CuWC_P layered composites. An in-situ experimental study was carried out to investigate the behavior of the composites under uniaxial tensile loading. The specimens were manufactured by vacuum hot-pressed sintering technique, microscopic observations displayed that the microstructure of Cu/CuWC_P layered composites distribute uniformly, and have no visible defect at interface. In situ tensile tests were performed in a scanning electron microscope (SEM) and the tensile strengths, failure modes of composites were measured. From the in situ experiments, the stages of nucleation, growth and coalescence of cracks in the vicinity of particles are well observed and understood. The results indicated that microcrack initiation happens at particle agglomeration and the matrix-particle interface because bond strength is weak,. With the density of microcracks increaseing, macrocrack formed, and finally cause failure of CuWC_P layer, however, the Cu layer is not fracture during the whole testing.

Abstract: Wrinkle evolution under temperature and compression was experimentally explored in a metal film deposited on a compliant polymer. After wrinkle formation, the samples undergo thermal annealing in different temperature circumstances. In the range of 75-150°C, the wavelength and amplitude of wrinkles became both smaller as the temperature increased, indicating that annealing effectively releases residual stress and promote material modification. In contrast, compression only changed wrinkle rearrangement, and did not alter the wavelength and amplitude. These results suggest that external stimulation can transform wrinkled surfaces. This work provides the guidance for service conditions of surface wrinkling.

Abstract: The secondary phases such as carbonitrides in the microalloyed steel play very important roles in retarding the austenitic grains growth during reheating. The dissolving process of carbonitrides containing Nb, Ti, Mo will affect the austenitic grain sizes directly. In the present work, the dissolving behaviors of secondary phases in low carbon microalloyed steel during isothermal holding at different temperatures were investigated by electrolytic experiment, carbon extraction replicas, TEM and EDX analysis. Meanwhile, the austenitic grain sizes were measured corresponding to the temperatures. The experimental results indicate that there are two types of carbonitrides in as-forged steel. One is the coarsened Ti-rich precipitates originated from solidification, and the other is the finer Nb-rich particles attributed to strain-induced process. The strain-induced precipitates disappear after being held for 1 h at 1000°C. At 1000~1220 °C, the austenitic grains grow obviously due to rapid dissolving of the carbonitrides containing Nb and Mo. However, some undissolved Nb, Ti carbidenitrides still hinder the grain boundary migration. When the reheating temperature rises to 1270°C, the grain size grows abnormally after being held for 2h. At the holding temperature, few Nb-bearing TiN precipitates can be stable while the pinning effect weakens markedly.

Abstract: The thermo-simulation test and transmission electron microscopy (TEM) were applied to investigate the evolution of dislocation configuration and strain induced precipitation behavior during relaxation at 850°C in a deformed Fe-40Ni-Ti alloy. The stress relaxation curve can be divided into three stages, namely, the process of incubation, nucleation and growth, and the coarsening of strain-induced precipitates. The highly dense and twisted dislocations formed during the deformation develop into dislocation cells and finally, the sub-grains can be observed when relaxing to 1000s. The strain induced precipitates occur both onto the dispersed dislocations and dislocation cells. The precipitates pin the dislocations which results in retarding the progress of dislocation configuration evolution. As precipitates start to coarsen, the pinning effect weakens and the dislocations get rid of the pinning though bypassing mechanism. Adopting the same simulation test to bainitic steel, the optimum refinement could be obtained at 60-200s during relaxation processing, corresponding to the perfect dislocation cells formation of Fe-40Ni-Ti alloy.

Abstract: Viscous dissipation is the key factor impacting flowing characteristics of polymer melt. In order to study the difference between micro scale and macro scale, experimental studies of viscous dissipation at various shear rate were investigated with several polymers, including PMMA and HDPE, at different temperature when melts flow through 1000μm,500μm,350μm diameter channels of identical aspects ratio in the paper. The results indicate that the temperature rises caused by viscous dissipation increase with increasing shear rate and the temperature rise for some shear rate decreases with increasing melts temperature. The temperature rises decrease significantly with the reduction of the characteristic size of micro channel at the same shear rate. However, the average temperature rises per unit length increase when the character size of channel decreases. This indicates the shear friction gradually increases with the decrease of channel characteristic size. Therefore polymer melt viscous dissipation effects of micro scale dimensions are different from that of macro-scale dimensions.

Abstract: The fast-growing Eucalyptus (E. Europhylla) was used as the raw materials to prepare for micro/nanocellulose fibrils. The morphology changes of cellulose by sodium hydroxide linkage ultrasonic energy treatment was discussed. The properties of treated cellulose was evaluated by X-ray , scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy. By the degree of crystallinity of the experimental test results showed that: a high concentration (17.5%, mass fraction) NaOH solution swelling with ultrosonication chemical pretreatment of cellulose prepared micro/nanofibrils change in crystal form, that is transformed cellulose I into cellulose II. However, the cellulose micro/nanofibrils remained crystalline cellulose I type after treated by a low concentration (2%, mass fraction) NaOH solution swelling with ultrosonicaion chemical pretreatment. High alkali activation sound chemical pretreatment increased the crystallinity of obtained micro/nanofibrils, the corresponding values were 89.2% and 86.3%. Observed by the scanning electron microscope that: a low concentration alkaline with ultrosonication chemical pretreatment increased the degree of sub-wire broom, the fiber surface area increased accordingly, and the fiber is more "open", so that the reaction activity of the cellulose fibers improved. The infrared spectrum showed that: the chemical changes between cellulose micro/nanofibrils and NaOH occurred after mercerization.