Synthesis and characterization of lithium silicate ceramic for the test blanket module (TBM) in fusion reactors

Abstract

Lithium-based ceramic, Li4SiO4, is being considered as promising solid breeder materials in the tritium breeding blanket of thermonuclear fusion reactors, because of its high Li- density, high thermal conductivity and prominent tritium release rate at low temperatures between 300 and 500oC, its low activation characteristics, low thermal expansion coefficient and high thermal conductivity. For tritium recovery purpose samples having 85-90% of true density with open porosity (around 5%) is required. Uniform small grain size distribution (having diameter between 2-4μm) is preferable as activation energy for tritium diffusion through grain is higher compared to grain boundary. Solid state method requires higher calcination temperature, producing coarser particle and impurity, which adversely affects in achieving high sintered density (above 85% of theoretical density) and forms large grain size with entrapped closed pore inside the grain.
In the current work Li4SiO4 powder was synthesized by solution combustion technique using cheaper precursor of silica i.e. from rice husk ash (an agricultural waste). We found that by controlling the metal to citrate ratio of the solution and calcination temperature of the as-burnt powder phase purity can be achieved. The particle size of Li4SiO4 powder (prepared at M/C=1.4) was found to be 100-200 nm with low surface area (2m2/gm). It was found that Li4SiO4 powder can be sintered at a temperature as low as 900oC with a density ~ 85% of the theoretical density. Phase stability in the sintered sample was studied. Attempts were made to minimize the lithium loss from the sintered specimens. Moisture reactivity studies were conducted together with FTIR spectroscopy on calcined powder to find out the moisture affinity of the samples. Kinetics of the initial stage of sintering from the shrinkage data has been analyzed to find out the sintering mechanism. Thermal diffusivity measured by laser flash method. Thermal conductivity value depends on the density of the sample. AC impedance method has been used to characterize electrical property of the sintered sample as tritium diffusion is related to the Li+ ion conductivity in Li4SiO4.