Gel technology in ceramics, glass-ceramics and ceramic-ceramic composites

Abstract

For many years, ceramists have been interested in sintering at low temperatures to manufacture devices that incorporate different materials sintering together, to deposit films on a substrate, to make fibers or ceramic-ceramic composites and to achieve fine-grained microstructures. Today submicronic microstructure control is needed for many applications and the use of sol-gel routes opens up new possibilities. The purpose of this overview is to present the various trends in gel technology. Sol-gel routes have been widely used for many centuries in earthenware and china production and history points out that progress is due to the ‘wedding’ of (inorganic and organic) chemistry with ceramics. In this paper the different sol-gel methods (‘physical’ or ‘chemical’ gels, homo- or heterometallic alkoxide hydrolysis-polycondensation, homogeneous or heterogeneous ways, monolithic or powder routes, etc.) are discussed. The present knowledge of gel structures, sol/gel-, gel/glass- and glass/ceramic transitions is reviewed. Special emphasis is given to the discussion of advantages (heterogeneity control, microstructure design, displacement of the sintering temperature, porosity and crystallinity control, etc.) and drawbacks (need of sol-gel formers, cost, toxicity, solvent recycling, carbon traces, abnormal grain growth, etc.).