Phase equilibria in the Nb-Si-Ge phase diagram

Niobium silicide-based in-situ composites have the potential to supersede nickel-based superalloys due to their excellent high temperature mechanical properties and low density. A thermodynamic database is being developed using the CALPHAD method to aid in alloy development. The addition of small amounts of germanium into these systems is of particular interest as it can significantly improve oxidation resistance. For example, germanium is reported to benefit high temperature oxidation resistance of coatings used on refractory silicide alloys by the formation of a glassy GeO2.SiO2 phase which fills cracks and is impermeable to further oxygen penetration. The effect of germanium on the phases formed in bulk niobium silicide-based in-situ composites is not particularly well understood, and limited data exists in the literature. To understand the effect of germanium on alloys, a thermodynamic description of the ternary Nb-Si-Ge phase diagram has been developed using the Calphad method. To support thermodynamic modelling samples were produced along the Nb5Ge3-Nb5Si3 pseudo binary and assessed using XRD. Experimental results show that germanium stabilises the high temperature Nb5Si3 (W5Si3 prototype) to low temperatures. The thermodynamic assessment will be presented and compared to experimental data from the current work and the literature

Characterisation and disposability assessment of multi-waste stream in-container vitrified products for higher activity radioactive waste

Materials from GeoMelt® In-Container Vitrification (ICV)™ of simulant UK nuclear wastes were characterised to understand the partitioning of elements, including inactive surrogates for radionuclide species of interest, within the heterogeneous products. Aqueous durability analysis was performed to assess the potential disposability of the resulting wasteforms. The vitrification trial aimed to immobilise a variety of simulant legacy waste streams representative of decommissioning operations in the UK, including plutonium contaminated material, Magnox sludges and ion-exchange materials, which were vitrified upon the addition of glass forming additives. Two trials with different wastes were characterised, with the resultant vitreous wasteforms comprising olivine and pyroxene crystalline minerals within glassy matrices. Plutonium surrogate elements were immobilised within the glassy fraction rather than partitioning into crystalline phases. All vitrified products exhibited comparable or improved durability to existing UK high level waste vitrified nuclear wasteforms over a 28 day period

Sustainable iron-rich cements:raw material sources and binder types

Abstract The bulk of the cement industry’s environmental burden is from the calcareous source. Calcium is mostly available naturally as limestone (CaCO₃), where almost half of the mass is eventually released as CO₂ during clinker manufacture. Iron (Fe) is the fourth most common element in the Earth’s crust surpassed only by oxygen, silicon, and aluminium; therefore, potential raw materials for alternative cements can contain significant amounts of iron. This review paper discusses in detail the most abundantly available Fe-rich natural resources and industrial by-products and residues, establishing symbiotic supply chains from various sectors. The discussion then focusses on the impact of high iron content in clinker and on ferrite (thermo)chemistry, as well as the importance of iron speciation on its involvement in the reactions as supplementary cementitious material or alkali-activated materials, and the technical quality that can be achieved from sustainable Fe-rich cements