Diffusion bonding of silicon nitride to

In this article, the results are presented of an investigation dealing with diffusion bonding of hot-pressed silicon nitride to austenitic stainless steel AISI 316 without the use of a metallic interlayer. Experiments were carried out in vacuum, and it was found that under specific con- ditions, permanent joints can be obtained. Optical microscopy and electron probe microanalysis (EPMA) reveal that the reaction layer formed during the diffusion bonding process consists of a porous zone extending into the ceramic and a diffusion zone extending into the steel. Both zones are similar in chemical composition and contain mainly Fe, Ni, Cr, and Si, whereas the pores appear to contain molecular nitrogen. The formation of the reaction layer can be explained in terms of the decomposition of the silicon nitride, which is promoted under diffusion bonding conditions in vacuum. The free silicon generated by the decomposition reaction diffuses into the steel, whereas the nitrogen is trapped in pores. It appears that the strength of the joint is determined by the residual stresses which develop as a result of the thermal mismatch between the ceramic and the steel. The porous zone has a beneficial influence on the strength, because it partly accommodates the residual stresses.     

Preparation of pure silicon by electrowinning in a bytownite-cryolite melt

Pure silicon was prepared by electrowinning in a bytownite-cryolite molten mixture at about 970 °C. In the electrolysis cell, the bottom of a graphite crucible was used as the anode. The cathode was also made of graphite. The reaction was carried out at the cell voltage of about 2.7 V, and the cathode current density was either 0.027 or 0.053 A/cm² for 10, 50, and 100 pct of the time for all Si deposited. The obtained silicon, which was analyzed by electron microprobe analysis (EMPA), contained 99.79 to 99.98 pct Si, which is a specially pure quality. CO₂ gas formed at the anode flushed silicon deposited at the cathode and purified it by oxidizing the impurities that ordinarily would be deposited at the cathode.     

Solid-state diffusion bonding of silicon nitride using titanium foils

This article presents an effective way to control the interfacial reaction during solid-state diffusion bonding of silicon nitride (Si₃N₄) using titanium foils. The interfacial structure and its growth kinetics were analyzed in detail with scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). The actual phase sequence of the joint interfaces bonded at temperatures between 1473 and 1673 K is concluded to be Si₃N₄/Ti₅Si₃(N)/α-Ti(N)+Ti₅Si₃(N), which is different from the phase sequence observed at room temperature after bonding. The joints are very weak due to the formation of a brittle Ti₅Si₃(N) layer at the interface. To suppress the growth of the Ti₅Si₃ layer, a nitrogen-solution treatment of titanium foils prior to each bonding experiment is implemented. Although a perfect prevention of the Ti₅Si₃(N) layer formation is not achieved with this treatment, it is shown that the growth of the layer is effectively suppressed enough to improve the joint strength to a level 3 times higher than the case in which pure titanium is employed.