Interface Reactions Responsible for Run-Out in Active Brazing: Part 3

This study examined the interface reaction between sessile drops of the Ag-xAl filler metals having x = 0.2, 0.5, and 1.0 wt-% and KovarTM base material as an avenue to understand the run-out phenomenon observed in active filler metal braze joints. The brazing conditions were combinations of 965°C (1769°F) and 995°C (1823°F) temperatures and brazing times of 5 and 20 min. All brazing was performed in a vacuum of 10–7 Torr. Microanalysis confirmed that a reaction layer developed ahead of the filler metal to support spontaneous wetting and spreading activity. However, run-out was not observed with the sessile drops because the additional surface energy created by the sessile drop free surface constrained wetting and spreading. The value of z in the reaction layer composition, (Fe, Ni, Co)yAlz, increased with x of the Ag-xAl sessile drops for both brazing conditions. Generally, the values of z were lower for the more severe brazing conditions. Also, the reaction layer thickness increased with the Al concentration in the filler metal but did not increase with the severity of brazing conditions. These behaviors indicate that the interface reaction was controlled by the chemical potential rather than the rate kinetics of a thermally activated process. The determining metrics were filler metal composition (Ag-xAl) and brazing temperature. The findings of the present study provided several insights toward developing potential mitigation strategies to prevent run-out.

Competitive Ligand Binding and Photosensitivity Studies of Iron(III)-thiocyanate in presence of Glutamate ion

Addition of a chelating ligand (glutamate ion) to [Fe(SCN)]2+ solution leads to change in the colour. On increasing the glutamate ion concentration in iron thiocyanate complex solution, the colour of [Fe(SCN)]2+ disappears with the emergence of a new peak at lower wavelength due to the formation of [Fe(Glu)]2+complex. The conductance of [Fe(SCN)]2+ complex ion in solution is high while on addition of different concentrations of glutamate ion to iron thiocyanate complex, their conductance value decreases due to formation of [Fe(Glu)]2+. Photosensitivity studies of a series of solutions prepared by the addition of glutamate ion of varying concentrations to ferric chloride-ammonium thiocyanate/potassium thiocyanate solution in the short UV region demonstrate the better stability of [Fe(Glu)]2+compared to [Fe(SCN)]2+ and the rate kinetics of decomposition has been reported.

Geopolymer leaching in water and acetic acid

Elemental leaching of metakaolin based geopolymers was investigated by immersing hardened paste specimens in a solution. For this, pure water and 0.1 molar acetic acid solutions were replenished ten times distributed over 56 days in total. Dissolution and diffusion of the elements through and from the geopolymer paste into the surrounding solutions was investigated on cross-sections of specimens by SEM-EDS microscopy, indentation, X-ray powder diffraction analysis and measuring the eluted elements by ICP-MS when replenishing the solution over time. The presented new methodological approach thus combines the dissolution rate kinetics obtained via wet chemistry (ICP) with the complementary solid state characterisation methods to gain new insights into the complex geopolymer dissolution mechanisms. Results indicated a relatively small leachability of geopolymers, limited only to the surface layer which is directly exposed to the aggressive solution, while the more inner parts of the geopolymer framework remain intact. Elemental maps revealed dissolution of aluminates that occurred across the outermost surface parts of the sections, while potassium leached out gradually but reached deeper inner parts. However, there was still a high portion of potassium being left bonded inside the geopolymer, even for the harsh acidic conditions, limited by the diffusion-reaction mechanism which took place within the geopolymer. The obtained experimental results represent a first approach towards feeding reactive transport numerical modeling approaches still to be developed for simulating leaching and degradation of geopolymer materials when exposed to water or acidic solutions.