2 - Conventional routes to ceramics

Summary

Introduction

Precipitation from solution, powder mixing and fusion are all conventional techniques for synthesis of traditional and advanced ceramics on both the laboratory and industrial scale. These methods, with selected examples, are described in this chapter together with limitations of their use for advanced ceramic materials. There is increasing demand for alternative routes to ceramic materials that impart superior properties compared with those attainable from conventional syntheses and the reasons for this continuing search for other synthetic pathways are also described here.

Precipitation from solution

Alumina occurs as the mineral bauxite and is refined in the Bayer process whereby ore is initially dissolved under pressure in sodium hydroxide so that solid impurities (SiO₂, TiO₂, Fe₂O₃) separate from sodium aluminate solution (Evans & Brown, 1981). This solution is either seeded with gibbsite crystals (α-Al₂O₃.3H₂O) or undergoes auto- precipitation to bayerite (β-Al₂O₃.3H₂O) after its neutralisation with CO₂ gas. Temperature, alumina supersaturation and amount of seed affect particle size during crystallisation but, as for other precipitation reactions, the product is agglomerated (section 9.3). Alumina production from the Bayer process was 31.9 × 10⁶ Mg in 1985; 90 weight % of this was reduced to metal and only 5 weight % found ceramic and refractory applications (MacZura, Carbone & Hart, 1987).