Influence of the initial crystallinity on the high pressure–high temperature phase transition in boron nitride

Effect of the BN content on the thermal shock resistance and properties of BN/SiO2 composites fabricated from mechanically alloyed SiBON powders

“…Hence, improvements in the sinterability and mechanical properties of hexagonal boron nitride have been the main issues in the past decades. [9][10][11] Adding compounds such as SiO 2 , Al 2 O 3 , MAS (magnesium aluminum silicate) and YAG (yttrium aluminum garnet) is an effective way to improve the sinterability of BN ceramics. [12][13][14] Among these, fused silica is the excellent one, notably improving the sintering characteristics, with rather low and stable dielectric constant (3 < 3.5), and very low loss tangent, low coefficient of thermal expansion, and high melting point (1700 C).…”

Observations of nanocrystalline cubic boron nitride formed with plasma spraying

“…In-situ high pressure/high temperature XRD observations by Lorenz et al demonstrated that having an amorphous structure reduces the temperature and pressure (1200 C and 7 GPa) needed to transform a-BN to c-BN [38]. By comparison, the ordered t-BN to c-BN formation proceeds through an intermediate h-BN step at relatively higher temperature and pressure (1450 C and 8 GPa) and is accompanied by a lower conversion rate from h-BN to c-BN [19,38]. These references coupled with our observations suggest that using metastable precursors with weak bonding and a high number of structural imperfections facilitates the phase transition to c-BN thus reducing the required processing pressures and temperatures to those accessible by plasma spray processing.…”

Thermodynamic Considerations on Solid Structural Transition Temperatures of Nanocrystals

“…Formation of nanocrystals is also a potential tool for the preparation of metastable phases of bulk, which otherwise would not be attainable. These phenomena have been found in CdSe, [31][32][33] ZnO, 34 35 ZrO 2 , 36 ZnS, 37 Al 2 O 3 38 TiO 2 , 39 Fe 2 O 3 , 40 H 2 O, 41 BN, 42 Au, 43 44 Ag, 45 46 and C. 47 The reduced size of nanocrystals gives also a useful handle for dealing with the kinetics since some experimental phase transitions are also dictated by kinetic effects, such as the processes of nucleation and growth. Due to the limitation of space however, this review neglects them and only concentrates on thermodynamic equilibrium conditions.…”