Sintered corundum can be also produced from fused corundum, which is crushed and sintered with auxiliary agents such as glass phase agents [KLOC05a, p. 25].
2.1.3 Manufacture of Corundum by Sol-Gel Process
Sol-Gel corundum is produced by the sol-gel procedure and a subsequent pressure-less sintering procedure (Fig. 2.8) [KLOC05a, p. 25]. The crystal sizes are below 500 nm [ENGE02, p. 7]. Growth inhibitors or nucleating agents themselves can still be present in the sol-gel corundum grits.
Bauxite is purified by the Bayer process and then transformed through the Ziegler process into boehmite, which is у-aluminum oxide hydroxide (y-AlOOH) [JACK11, p. 29]. Powdered boehmite (y-AlOOH) is transformed into a clear sol by mixing with water. Modifying agents are added, such as grain growth inhibitors (MgO, ZrO2, TiO2, lanthanum and other noble earths) or nucleating agents (Al2O3 or Fe2O3) [KLOC03]. Adding the nucleating agents is called seeding. Sol-gel corundum can either be produced with or without seeding [KOMA07].
Then a peptisator, being nitric acid (HNO3) in most cases, is added. Agglomerates are dissolved and the sol is stabilized. The boehmite is now finely dispersed and homogeneously distributed. After adding of a diluted acid or nitrate solution, the sol reacts to a gel including dehydration and polymerisation. The gel is rolled into thin layers or lines and dryed at about 80-100 °C. The resulting solid and brittle body is milled and sieved if necessary. Heating at about 450-550 °C for
|
|
about 16 h transforms the boehmite grits into y-Al2O3. During this so called calcination, nitrogen (NOx) and water are set free. Afterwards, the y-Al2O3 is sintered pressure-less for about 30 min. Generally, seeded sol-gel material is sintered at 1300-1400 °C, nonseeded sol-gel at temperatures that are about 100 °C higher [ENGE02, p. 11, KLOC05a, p. 26, KOMA07]. With additional extrusion processes, needle-shaped, extruded sol-gel corundum is manufactured with aspect ratios of 8:1 [MARI07, p. 113].