This study concerns new Si 3 N 4 -graphene composites manufactured using the hot-pressing method. Because of future applications of silicon nitride for cutting tools or specific parts of various devices having contact with high temperatures there is a need to find a ceramic composite material with good mechanical and especially thermal properties. Excellent thermal properties in the major directions are characteristic of graphene. In this study, the graphene phase is added to the silicon nitride phase in a quantity of up to 10 mass%, and the materials are sintered under uniaxial pressure. The mixture of AlN and Y 2 O 3 is added as sintering activator to the composite matrix. The studies focus on thermal stability of produced composites in argon and air conditions up to the temperature of 1,000°C. The research also concerns the influence of applied uniaxial pressure during the sintering process on the orientation of graphene nanoparticles in the Si 3 N 4 matrix. The study also presents research on anisotropy of thermal diffusivity and following thermal conductivity of ceramic matrix composites versus the increasing graphene quantity. Most of the presented results have not been published in the literature yet.

Nanorods of the wide-bandgap semiconductor silicon carbide belong to a promising group of onedimensional materials with potential applications extending from reinforcement of composites to applications as building blocks that can be logically assembled into appropriate two-(and three-) dimensional architectures, permitting researchers to exploit their unusual electronic, optical, and other properties. Specific to the most common silicon carbide polytypes are a low intrinsic carrier concentration, an exceptionally high breakdown electric field, high thermal conductivity, high-temperature stability, and resistance to an aggressive environment. This should permit one to develop even submicron-level SiC-based devices operating under high-temperature, high-power, and/or high-radiation conditions, under which conventional semiconductors cannot function. Detailed control of the conditions favorable for the nucleation and growth processes of nanorods of a given SiC polytype is necessary because the electrical and optical properties of each SiC polytype are very different. Therefore, a systematic investigation of factors that primarily influence the morphology and polytype of a vapor-phase-grown SiC has been made in the present work. These factors were the temperature, the flow rates of the gaseous precursors, and the Si/C molar ratio in the gas phase. In order to investigate the role of these factors, the "cold gas-hot substrate" chemical vapor deposition (CVD) method has been applied, because it permits them to be closely controlled in a wide range. While in the overwhelming majority of previous investigations nanorods of the 3C SiC polytype have been grown, the present work delineates conditions that are favorable for the growth of single-phase 2H, 3C, 15R, and 6H SiC nanorods, respectively.

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The presence of h-BN phase in the composites, in which it is fulfilling a role of a solid lubricant, causes the appearance of the anisotropy of the elastic properties and thermal properties. In order to assess the extent of the phenomenon, velocity changes of ultrasonic wave propagation were measured and thermal conductivity changes were researched. Measurements were carried out on composites of alumina matrix, which had h-BN introduced in an amount of 0-30 vol.%. The study included measurements of the speed of propagation of ultrasound waves and thermal conductivity, which were made in two directions, namely, perpendicular and parallel directions to the axis of compression. Strong dependence of the thermal conductivity anisotropy of the phase composition of materials was revealed.