Decreasing Powder Size Research Articles

Effect of α‐Si3N4Particle Size on the Microstructural Evolution of Si3N4Ceramics

The effects of initial particle size on the microstructure of silicon nitride ceramics produced by pressureless sintering have been investigated. The microstructures of the silicon nitride ceramics varied considerably with the size of the initial powder. With decreasing powder size, abnormal grain growth was enhanced, which resulted in significant bimodal distribution of grain size. The observed results are discussed in relation to the two‐dimensional nucleation and growth theory for faceted crystals.

Solidification behaviour of Pd–Rh droplets during spray atomization

Solidification microstructure in spray-atomized Pd–10 wt% Rh powders using high-pressure gas atomization was studied. The solidification cooling rate and the solidification front velocity were investigated using a transient heat-transfer finite element method. Two different atomization gases, nitrogen and helium, were considered in the modelling studies. On the basis of the results obtained, it was found that gas atomization using helium gas led to solidification cooling rates and solidification front velocities which were two times higher than those obtained using nitrogen gas. Moreover, the cooling rate and the solidification front velocity increased with decreasing powder size for both types of atomization gas. The numerically estimated solidification front velocity using finite element analysis for nitrogen gas atomization was found to be smaller than the analytically determined absolute stability velocity that is required to promote a segregation-free microstructure. This was noted to be consistent with the segregated microstructure that was experimentally observed in nitrogen gas atomized powders. In the case of helium gas atomization, however, the increased cooling rate and solidification front velocity are anticipated to promote the formation of a segregation-free microstructure in the gas-atomized powders.

Characterization of centrifugally atomized Al-12Si-5Fe-3Cu-1Mg(wt%) alloy powder and extruded rod

The Al-12Si-5Fe-3Cu-1Mg(wt%) alloy was rapidly solidified by centrifugal atomization. The microstructural characteristics of rapidly solidified powder and the microstructure changes with heat treatment were investigated in terms and related to powder size. The microstructures of the powder consisted of dendritic α-Al, eutectic phase, Cu-rich phase, and needle-like intermetallic compounds. These phases were much finer than that of ingot cast structure and the size decreased with increasing cooling rate. The X-ray diffraction of the atomized powders revealed the presence of non-equilibrium 3-(AlFeSi) intermetallic phase. This phase appeared to transform to an equilibrium β-(AlFeSi) phase by heating at temperatures above 470°C. The extruded rod which was hot extruded at 360°C with an extrusion ratio of 40:1 also revealed the presence of the β-(AIFeSi) intermetallic phase. Using DSC, the exothermic peak due to precipitation from the supersaturated α-Al matrix was observed in the range of 200–250°C during continuous heating of atomized powder, and the size of the peaks increased with decreasing powder size.

Voiding in BGAs

Voiding in BGA assembly using SN63 solder bumps is primarily introduced at board‐level assembly stage. On the pretinned PCBs, voiding of BGA joints increases with increasing solvent volatility, increasing metal content and increasing reflow temperature, and with decreasing powder size. This can be explained by a viscosity dictated flux‐exclusion‐rate model. In this model, a higher viscosity in the fluxing medium at reflow temperature could hinder the exclusion of flux from the interior of the molten solder, hence increase the chance of outgassing due to the increasing amount of entrapped flux, and consequently result in higher voiding in BGA assembly. Flux activity and reflow atmosphere appear to have a negligible effect on voiding when the solderability of the immobile metallisation is not a concern. An increase in void content is accompanied by an increase in the fraction of large voids. This suggests that, similar to voiding phenomena in the SMT process, factors causing voiding in BGA will have an even greater impact on joint reliability than shown by the total‐void‐volume analysis results.

ＹＢａ２Ｃｕ３Ｏｘ磁気特性の粒径依存性

Magnetic properties of Y1Ba2Cu3Ox. powders pulverized into different particle size were measured by using Vibrating Sample Magnetometer(VSM). The lower critical magnetic field H cl increased with decreasing powder size corresponding to view of thermodynamics. The external field and particle size dependence was obtained from the time decay rate of magnetization and zero-field cooling (Shielding Effect) and field cooling (Meissner Effect) measurement. It showed presence of low field region where the gradient of magnetic field density in grain is rather clear.

ＳｉＣウィスカ強化アルミニウム複合押出し成形品の強度に及ぼす粉末粒度および熱処理の影響

For each matrix powder size, the tensile strength of composite increased proportionally with increasing volume fraction of the whisker, Vf, up to a critical value, (Vf)c, beyond which the strength was holded. (Vf)c increased with decreasing powder size. Thus, the composite produced with finer matrix powder was effectively strengthened by the whisker up to higher Vf. The amount of whisker per unit surface area of matrix powder calculated for each size of powder at (Vf)c coincided approximately with each other. The specific amount of whisker was considered to be a limitation to effectively reinforce the matrix. As for the effect of heat treatment on the strength of composite, it was 20% increased by heating at 913 K for about 30ks. On the other hand, the prolonged heat treatment reduced the strength by causing remarkable reaction between the whisker and the matrix.

Electroless Nickel Plating of Iron Powders

This work illustrates the technical feasibility of the electroless nickel plating of iron powders. The coated powders exhibit a high level of homogeneity and the sintered compacts have good mechanical properties. Decreasing powder size increases the nickel concentration and improves diffusion by particle-to-particle contact. Alloying is promoted by reducing diffusion distances.

光音響分光法によるスラグの状態分析に関する基礎的研究

Application of the photoacoustic spectroscopy is considered to be one of the most usefull methods to characterize the chemical structures of slags, since visible spectra due to transition metals and the optical basicity in slags can be easily measured with this technique.The influences of measuring conditions on the photoacoustic spectra have been investigated by using Cr6+ absorption spectrum observed in CaO-Al2O3-SiO2 glasses doped with 0.2 mol%Cr2O3. It was found that the phase difference of lock-in amplifiers (\varphi) was the most important parameter in a double beam photoacoustic spectrometer used in this study. When \varphi was π/2 rad, the photo-acoustic spectra showed good agreement with the spectra obtained by the conventional transmittance method, and its intensities became maximum. The modulation frequency (ω), another parameter, had strong influence on the intensity of spectrum, which was propotional to ω−3⁄2 in transition spectra of glasses. On the other hand, the intensity of spectrum increased with increasing sample mass and with decreasing powder size of the sample. Especially when the mean diameter of sample was less than 2.0×10−3 m, the intensity of spectrum became steeply large.The optical basicities of alkaline silicate glasses were measured by using Pb2+ as the indicator ion. These results qualitatively agreed with the theoretical optical basicities proposed by Duffy and Ingram. Na2CO3 melted at 1473 K in Al2O3 or MgO crucible showed two optical basicities, 0.68 and 1.11. It suggests that Na2CO3 partly decomposes to Na2O and CO2 at 1473 K, thus 0.68 being the optical basicity of Na2CO3 and 1.11 being that of Na2O.