Synthesis and Processing of Boron Nitride

Hexagonal Boron Nitride (h-BN) The industrial-scale synthesis of boron nitride is accomplished by reacting B(OH)3, either chemicaUy synthesized or as the mineral sassolite, with gaseous ammonia at elevated temperatures. The two-stage process proceeds according to  

Boron trioxide can also be reacted at T 1000 °C with other nitrogen carriers such as urea, guanidine, dicyandiamide or melamine, in a nitrogen atmosphere, to yield  

A third synthesis route which is used today begins from calcium hexaboride, according to  

The resultant h-BN is well-crystallized, and shows platelets with a dimension of 1-10 gm and a thickness of 0.1-0.5 gm. Purification may be achieved by reheating the product to temperatures in excess of 1500 °C, so as to evaporate off any remaining boric oxide. 

Starting from as-synthesized powder, dense sintered h-BN can be obtained by axial hot pressing in graphite molds at a temperature of about 2200 °C, to yield monolithic ceramic bodies of 88-97% theoretical density. Any residual oxygen in the BN at the grain boundaries will form B2O3 that, subsequently, will act as an efficient sintering aid. The typical anisotropic properties of uniaxially hot pressed 

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