Papers by Keyword: Phase Development

Abstract: Due to material and structural issues, fire-assaying crucibles used for analyzing precious metals in ores have encountered challenges related to poor thermal cycling in Ghana’s sub-Saharan region. This study aimed to enhance the crucibles by analyzing aluminosilicate minerals' multiphase development using X-ray diffraction and understanding the effects of composition determined by X-ray fluorescence on thermal behavior and water absorption observed through optical microscopy. The improved crucible design exhibited enhanced thermal cycling stability and lower permeability to the assay charge. Analysis showed that Fosu Clay (FC) demonstrated promise with a favorable Al₂O₃:SiO₂ ratio and low impurities; mullite was identified as the primary phase formed at high temperatures, with quartz and cristobalite also present. Introducing 6% CSM dopant to FC increased the mullite content while supporting the transformation from quartz to cristobalite. The optimal crucible sample included coarse and fine-doped grog with an FC-clay binder, demonstrating excellent thermal stability, adequate porosity, and water absorption. Adjusting the percentage of doped grog further increased mullite content while reducing silica content; this suggests that locally produced improved crucibles are feasible through sintering commercial clay with mullite doping and precise composition adjustments.

Abstract: Functional, high performance mortars such as Self-Leveling Underlayments (SLU), Tile Adhesives (TA) or Grouting Mortars (GM) are composed, besides complex binders, of mineral and organic fillers, and most importantly admixtures. Amongst the organic fillers, re-dispersible powders produced by spray drying from emulsions, are added to improve flexibility, adhesion and water resistance. These powders, however, also influence fresh properties such as workability and plastic shrinkage or sedimentation. Additionally they are prone to interfere with the hydration kinetics thus playing a role in the very early structuring process of a cement-based mortar.In this study, the influence of re-dispersible powders on sedimentation, plastic as well as chemical shrinkage in ternary binder composites has been investigated for the first 24 hours. The structuring process has been additionally characterized by the hydrate phase development. Ternary binders are defined in this study as compositions of Calcium Aluminate Cement (CAC), Ordinary Portland Cement (OPC) and Calcium Sulfate (CS̅).The addition of re-dispersible powders showed not only the well-known retardation in hydration kinetics, longer setting times and increased gel times but also changed dimensional stability. Dimensional changes occurring during the first 24 hours were monitored by different test devices in order to separate different shrinkage/expansion mechanisms. The measurements are compared to the in-situ measurement of phase development by XRD.

Abstract: In order to study the effect of coexistence of Nd2O3 and Sm2O3 on phase development in Ba4 (Sm0.7Nd0.3)9.33Ti18O54 ceramics, the calcination process was applied in one-step and two-step way. From the XRD patterns of one-step calcinated powders, the solid-state reactions were deduced. And the results indicated the coexistence of Nd2O3 and Sm2O3 would be helpful to form secondary phase Nd2Ti2O7, which would worsen the properties. The final tested results of sintered samples confirmed the advantages of two-step calcination. The εr was improved from 72.1 to 82.4 and Q.f was improved from 7200 GHz to 8300 GHz, while τf was still near zero.

Abstract: The solidification behaviour of pure Alloy 625, and Alloy 625 enriched respectively in iron and carbon, was investigated in situ by hot-stage light optical microscopy. Using this technique planar front solidification for distances of several hundred microns was facilitated. After solidification, the material that experienced planar front solidification corresponded accurately to that of dendrites tens of microns in width adopting an equivalent dendrite arm width approach. Eventually, the planar solidification front broke down, where after the residual liquid solidified eutectic-like. This material contained γ-phase, Laves phase and, if carbon was dissolved in the liquid, niobium rich carbides formed. Molybdenum and niobium showed strong tendencies to segregate. Their segregation was balanced by inverse segregation of nickel and iron. The chromium concentration remained almost constant in γ in the entire matrix material. Addition of carbon did not cause detectable alterations of the material that experienced planar front solidification. However, it promoted the formation of niobium rich carbides in the material that solidified eutectic-like. Thus, this material differed from that of the pure sample in constitution, and consequently in γ-phase composition. Niobium rich carbides formed prior to Laves phase; in carbon rich volumes only the carbides form. As compared to the pure sample, the sample enriched in iron had decreased global minimal solute concentration in the material that experienced planar front solidification. However, once the concentrations were corrected with respect to the dilution simply caused by the presence of iron, the solidification behaviour in this material was identical to that of the pure sample. The constitution in the material that solidified eutectic-like was γ and Laves phase. As compared to the pure sample, the Laves phase was enriched in iron.