thermal decomposition

Many ceramic powders are not made by combining things but by breaking one thing apart with heat. Thermal decomposition is a reaction in which a single solid, when heated, splits into a new solid plus a gas that escapes. Bake limestone and it turns to lime, breathing out carbon dioxide; heat aluminium hydroxide and it becomes alumina, driving off water vapour. The gas leaves, the solid transforms, and you are left with the oxide you wanted.

The textbook case is the calcination of a carbonate: CaCO3 heated gives CaO plus CO2 gas. A hydroxide behaves the same way — Al(OH)3 or boehmite decomposing to Al2O3 plus H2O. Two things make these reactions distinctive. First, they are driven by the escape of the gas: removing the gas product (or lowering its partial pressure) pulls the reaction forward, and each has a temperature above which the gas pressure it wants to produce exceeds the surroundings, so decomposition runs. Second, they leave a fingerprint on the solid: as the gas rushes out of what was a dense grain, it tears open a highly porous, high-surface-area product — a reactive powder ideal for the next sintering step. The mass loss is often large; calcining CaCO3 loses about 44 percent of its weight as CO2.

Thermal decomposition (calcination) is a standard early step in ceramic processing: it converts a cheap, easily purified precursor — a carbonate, hydroxide, nitrate, or oxalate — into the oxide, burns off volatiles, and sets the phase and surface area of the powder before forming and firing. The honest cautions are practical. The reaction is often controlled by how fast the gas can escape through the product layer (a diffusion-and-transport limitation), so heating too fast can trap gas and bloat or crack a body; and decomposition temperatures quoted in tables assume the gas can freely leave, which is not always true deep inside a packed powder bed.