Water and Ammonium Compounds

When water is added to the h-BN starting material, c-BN can be produced at 5 GPa and 1770 K or at 6 GPa and 870-970 K [171, 172], These are rather low temperature and low pressure conditions compared to other syntheses. The c-BN obtained is rather fine-grained and suitable for sintering. NH4B508 was identified as the flux in this process [173], Essential for the conversion from h-BN to c-BN is the ratio NH 7B203. 

Unusual flux precursors like urea, ammonium nitrate, boric acid, or ammonium boron-hydride have also been studied [173], In all these cases derivatives of amino borate form the liquid phase. 

The limits for the synthesis parameters are lower temperature limit 1070-1270 K lower pressure limit 4.8 GPa for H20/4.3 GPa for urea/4.6 GPa for boric acid. 

A rather interesting discovery was made by Kobayashi et al. [174]. At 1470-1870 K and 4.2 GPa, c-BN was transformed into h-BN. According to the earlier BN phase diagrams this should not have happened, because these conditions are located in the region where c-BN should be stable. 

Synthesis in the B-B203-hBN system at temperatures 2300 K and in the pressure range of 4-6.5 GPa resulted in w-BN and c-BN phases [175]. 

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