Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.Comment: 29 pages, 5 figure

Cubic and hexagonal boron nitride phases and phase boundaries

Phase stability of boron nitride (BN) polymorphs at elevated temperature is perplexing due to their complex nucleation and growth kinetics, nevertheless, holds great significance in fundamental science and technology. Therefore, the phase-transformation of three-dimensional cubic BN (3D c-BN) to a two-dimensional hexagonal BN (2D h-BN) or vice versa, remains an exciting domain to explore. Here, we used temperature-dependent spark plasma sintering on 3D c-BN, enabling phase transformations to a mixed phase of 3D/2D c-BN/h-BN material and ultimately to 2D h-BN. The phase transformed 2D h-BN ceramic features an extremely high density reaching ∼90% of the theoretical limit, and exhibits excellent room temperature thermal conductivity and mechanical properties. Our findings provide valuable fundamental insights into the complex phase diagram, the relative stability regimes and boundaries of 3D c-BN and 2D h-BN phase, with the exhibition of functional properties, pivotal for extreme environments sustainable material-based technology.</p

Cubic and hexagonal boron nitride phases and phase boundaries

Phase stability of boron nitride (BN) polymorphs at elevated temperature is perplexing due to their complex nucleation and growth kinetics, nevertheless, holds great significance in fundamental science and technology. Therefore, the phase-transformation of three-dimensional cubic BN (3D c-BN) to a two-dimensional hexagonal BN (2D h-BN) or vice versa, remains an exciting domain to explore. Here, we used temperature-dependent spark plasma sintering on 3D c-BN, enabling phase transformations to a mixed phase of 3D/2D c-BN/h-BN material and ultimately to 2D h-BN. The phase transformed 2D h-BN ceramic features an extremely high density reaching ∼90% of the theoretical limit, and exhibits excellent room temperature thermal conductivity and mechanical properties. Our findings provide valuable fundamental insights into the complex phase diagram, the relative stability regimes and boundaries of 3D c-BN and 2D h-BN phase, with the exhibition of functional properties, pivotal for extreme environments sustainable material-based technology.</p