Question

When snytheis the YBa2Cu3O7 in the tube furnace, why the rates of heating and cooling can significantly influence the purity and crystallinity of products of solid-state reactions?

Answer 1

Solution: During the heating and/or cooling of a crystalline compound like YBa₂Cu₃O₇ at a specific temperature, phase transformations occur, which are accompanied by the absorption and release of heat. If the compound is cooled/heated uniformly and rapidly to the desired crystallization temperature, all of the mass will crystallize at the same condition at the same time. This would lead to the maximum production of the nuclei and the narrow distribution of the crystallite size. On the other hand if the cooling/heating rate is slow or non-uniform, crystallization will also be non-uniform. The regions that cool/heat more rapidly, nucleation will take place first and these crystals will grow as the rest of the mass crystallizes. The nuclei that formed first become larger than the other crystals and this may result in a coarse texture. Moreover, as the compound is cooled/heated uniformly to the desired crystallization temperature, there would be maximum production of the nuclei and the narrow distribution of the crystallite size or uniform crystal size which will lead to the maximum purity of the crystals.