PECULARITIES OF THE STRUCTURE PARTICLES OF BORON NITRIDE OBTAINED BY CARBOTHERMAL SYNTHESIS

V.I.Lyaschenko,
   
E.Prilutskiy,
 
I.Uvarova,
 
V.Vereschaka
 

I. M. Frantsevich Institute for Problems of Materials Science of the NAS of Ukraine, Omeliana Pritsaka str.,3, Kyiv, 03142, Ukraine
Electron Microscopy and Strength of Materials - Kiev: Frantsevich Institute for Problems of Materials Science NASU, 2008, #15
http://www.materials.kiev.ua/article/327

Abstract

Transmission electron microscopy was used to investigate boron nitride produced by carbothermal synthesis (using sucrose as a carbon-containing component) in the temperature range 1000-1450 0C. The structure types of formed BN have been classified according to 1) crystal-morphology features; 2) mechanisms of formation and 3) synthesis temperatures. In the first classification approach, three basic structure types of particles are distinguished: onion-like particles, cylindrical and faceted tubes, and plates. Each of these structures, in turn, is characterized by a number of morphological peculiarities and is presented in the synthesis product with elements of different scale, from several units to several hundreds of nm. In the classification according to the second feature, the structure types of particles are divided into i) structures of growth, that is, those that are formed through nucleation and ii) structures of breakdown that are formed as a result of structure transitions. Onion-like and faceted particles and tubes are formed at the initial stages in the condensed synthesis product, and at T≥1100-1200оC their further growth from a gaseous phase takes place. The data obtained make it possible to draw a conclusion that the formation of onion-like particles in a condensed phase may be regarded as a mechanism of turbostratic BN ordering. Faceted tubes have been established to be composed of the rhombohedral BN modification. At T≥1350оC, the rhombohedral-to-hexagonal phase transition occurs in them with the participation of packing defects and formation of multilayered prototypes. The conclusion has been made that destruction of tubes of both modifications at T≥1350оC is predominantly caused by significant anisotropy of the coefficient of thermal expansion of graphite-like modifications of boron nitride.


BORON NITRIDE, GRAPHITE-LIKE, NANOTUBES, ONION, TURBOSTRATIC