Grain growth similarity

On elevation of the temperature unstable allotropes become converted more rapidly than at low temperatures into the stable modification and the increased mobility of atoms at high temperatures produces a similar modification in the rapidly cooled conglomerates or amorphorus substances this conversion into the crystalline accompanied by a grain growth or a conversion of the micro-crystalline into the macrocrystalline state is termed sintering the preliminary stage in annealing. 

The factors that determine crystal size, a topic of particular relevance to this chapter, have been discussed to some extent in Section 3.4. There are two main factors that generally affect crystal size for any particular material the deposition mechanism and the deposition temperature. The hydroxide cluster mechanism is expected to give a crystal size similar to that of the original hydroxide cluster (with some growth possible as deposition proceeds). That size depends mainly on temperature, both because higher temperatures allow more grain growth and, possibly more important, lower temperatures kinetically stabilize very small nuclei in solution that are thermodynamically unstable. For example, in the hydroxide cluster mechanism, where crystal size is believed to be controlled mainly by the size of the Cd(OH)2 colloids, the relevant equilibria are... 

Hot-pressed material has a grain size of about 2 /mr which can be increased by annealing at 1200 °C in a PbO atmosphere provided by burying the specimen in powder of similar composition. Grain growth follows the relation... 

---