Advances in the synthesis and characterization of boron nitride

We review the progress in the experiments and understanding of high pressure and high temperature (HPHT) induced phase transformation in boron nitride. The HPHT induced phase transformation is significantly enhanced by refining the microstructure of the starting material, e.g. by ball-milling hexagonal boron nitride (h-BN) to a defective, nanocrystalline or even amorphous state. For example, cubic boron nitride (c-BN) forms from nanocrystalline or amorphous BN (a-BN) matrix at 900 degreesC and complete a-BN to c-BW phase transformation occurs at 1350 degreesC under 7.7 GPa. These temperatures and pressures are significantly lower than required to transform coarse-grained crystalline h-BN to c-BN. High resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) revealed that the c-BN phase nucleates directly from the sp(3) hybridized amorphous matrix, which is originally induced by ball milling and is therefore responsible for the lower HPHT requirements. This c-BN nucleation mechanism is completely different from the so-called diffusionless "puckering" mechanism that operates in the nucleation of c-BN from coarse-grained h-BN in HPHT experiments, but very similar to one of the proposed mechanisms involved in the chemical vapor deposition (CVD) of diamond and c-BN. HRTEM also shed new light on the phase transformation of slightly deformed coarse-grained h-BN under HPHT conditions. The interface structures among h-, w- and c-BN reveal that the phase transformation can proceed by different routes including h -->w -->. h -->g -->c, h -->w --> 6H'-->c, h -->w --> 2H --> 2H'-->c, h --> am -->c and h -->c, where h, w, c, g, 6H'. 2H' and am represent h-BN, w-BN, c-BN, g-BN, 6H'-BN, 2H'-BN and a-BN, respectively. irrespective of these different transformation routes, all the phase transformations follow the same orientation relationships, i.e. [11(2) over bar 0](h)//[11(2) over bar 0](w)//[1(1) over bar 0](c) and (1(1) over bar 00)(h)//(1(1) over bar 00)(w)//(1 1 1)(c). The phase transformations also follow a general rule: at low temperatures, the formation of c-BN is always preceded by the formation of intermediate phases such as w-BN, 2H' and 6H', and the transformation is martensitic in nature: at higher temperatures. there is a direct h-BN to c-BN transformation, and the transformation is diffusional dominated. The transformation from w-BN to c-BN is achieved by introducing periodic stacking-faults (SFs) in w-BN. Based on the HRTEM images. three new; BN poivh pe phases i.e., g-BN 2H'-BN and 6H'-BN, were proposed for the first time. HRTEM also revealed that hall-milling introduces significant defects such as SFs, twins, Frank dislocations, delamination, rotating and shearing of the sp(2) layers, and disordering in the h-BN lattice. These defects are found to promote the subsequent hexagonal to cubic transformation.