HYDROLYSIS AND THERMOBARIC BEHAVIOR OF AMORPHOUS BN: STRUCTURAL CHANGES AND CUBIC BN FORMATION

Abstract

The aim of this work was to study the thermobaric behavior of the original X-ray amorphous a-BN in the state of supply with elucidation of pressure-temperature conditions for the cubic BN formation without traditional solvent catalysts. An a-BN was obtained using a modified urea process in combination with thermomechanical treatment of the charge. The technology allows reducing the synthesis temperature almost 2 times, while significantly increasing the yield of the BN product (> 96%). Extremely small size of the structural elements of the cluster structure of the final product (<2 nm) cause the exceptional activity of this precursor. It was found that a-BN is significantly reactive with humidity in the air resulting in a partial hydrolysis of the compound with increasing of the temperature. The fusible reaction products, including orthoboric acid H₃BO₃, ammonium pentaborate NH₄B₅O₈×4H₂O and nitrates NH₄NO₃, played a key role in the processes of structural transformation and recrystallization of a-BN into cBN. HP-HT experiments were performed using toroidal type high-pressure apparatus at pressure of 6.5 and 8 GPa with accuracy of  0.3 GPa. The temperatures range of p,T-treatment was 1350–2200 °C (± 50°C) and duration did not exceed of 60 s. The XRD method using an Ultima IV (Rigaku, Japan) diffractometer and SEM studies (FEI Verios 400L XHR SEM, USA) were applied to investigate the structural features of the samples. It was found that at pressure of 6.5 GPa full conversion of a-BN with the formation of spacious segregation of cBN nanoparticles (crystals) occurs even at 1400 °C for 45 s of p,T-action. This behaviour was associated with the catalytic effect of fluid phases, which probably appear from the above-mentioned products of hydrolysis. No high density cBN polycrystalline structures with the perfect intergranular boundaries of a recrystallization origin were obtained at any parameters of HP-HT experiments. The obvious reason for this behaviour consists in a high content of impurity phases in the material. From the point of view of scientific and applied potential of the obtained results, they can be considered as a certain basis for synthesis development of the nanodispersed cBN powders.