Boron Nitride and Boron Carbon Nitride Nanosheets Synthesis Using Inductively Coupled Thermal Plasma

Book excerpt: "This thesis presents a study on the use of a radio frequency inductively coupled thermal plasma (RF-ICP) system coupled to a conical reactor in order to generate boron nitride nanosheets (BNNS) and boron carbon nitride nanosheets (BCNNS) and provide extended materials characterization. BNNS are analogous to graphene in their two-dimensional (2D) structures consisting of boron and nitrogen atoms replacing the carbon atoms in the stacked hexagonal sheet-like structures. Contrary to graphene, which is electrically conductive, BNNS is an insulator with a wide energy band gap. Two routes for generating the BNNS powder structures are described in the thesis, one based on heterogeneous nucleation and growth, and the other through homogeneous nucleation. In the first scenario of BNNS generation, the 2D BNNS structures are synthesized from relatively large boron particles and nitrogen gas. The synthesis is carried out in argon plasma conditions. With these precursors, BNNS structures nucleate and grow heterogeneously on the boron particles following a particle spheroidization process. The growth, which follows a base-growth mechanism, starts as BN nanowalls and grow further upon collisions of active nitrogen species on the boron liquid layer of the particles. The 'height' of these BN nanowalls from the boron particle can be controlled to some extent by varying the operating pressure of the system. The nanowall growth itself is made at the expense of the boron particle which feeds the BNNS nanowall with boron. At the optimum pressure, the formation of the BN phase is maximized, while lower or higher pressures are minimizing the formation of the BN phase. In general, the overall yield is low but improves upon the increase of the nitrogen gas loading in the plasma system at the optimum pressure.The second BNNS synthesis route follows homogeneous nucleation and growth of the material involving solid ammonia borane and nitrogen gas as the precursors. Ammonia borane melts, vaporizes and dissociates rapidly in the plasma zone. Then, clusters of BxNyHz nucleate to form particles of critical sizes that extend laterally while releasing H2. A very good control over the dimensions of the sheets as well as the overall purity of the product is achieved through varying the operating pressure. At the optimum pressure, the product exhibits a very high purity. Below the optimum pressure, the sheets are smaller in size but with practically the same thickness. At these pressures, the purity of product is compromised by the formation of BNH-based polymers. Operating above the optimum pressure results in increasing the sheet size and thickness. However, this comes at the cost of forming large boron particles as contaminants.Thermodynamic equilibrium calculations for the heterogeneous process (B particles and N2 gas) predict that the temperature of BN nucleation and growth in all pressure cases are very close. However, the formation of a liquid boron phase on the boron particles is necessary for BNNS growth. It is proposed in this case that B(liq), B, N, N2, and BN are the major precursors for the BNNS growth. The homogeneous formation of BNNS takes place within very similar temperature ranges. However, liquid boron is not a prerequisite. Major precursors in this process are BN, N, NH, BH, and BxNyHz. Computational fluid dynamic simulations are used to estimate the axial cooling rates and residence times in all the pressure scenarios. It has been speculated that the deviation from certain values of these parameters leads to the controllability over the sheets dimensions and the overall purity of the products"--