Štefan Kavecký

Thermal conductivity of unidirectional copper matrix carbon fibre composites

Abstract The thermal conductivity of the unidirectional copper matrix–carbon fibre composite is characterised and analysed in directions parallel and transverse to the carbon fibre orientation. Unidirectional samples with different fibre content were produced by diffusion bonding of continuous copper-coated carbon fibre tows. The thermal diffusivity was measured in two main orientations to the fibre direction by the laser-flash technique. The longitudinal and transverse thermal conductivity was then calculated and results were compared with simple analytical models. Measurements revealed decreasing thermal conductivity as the fibre volume content in the composite increased and the transverse thermal conductivity of the unidirectional samples presented much lower values in comparison to the longitudinal one.

Silicon carbide powder synthesis by chemical vapour deposition from silane/acetylene reaction system

Abstract Amorphous fine silicon carbide powders have been prepared via the chemical vapour deposition from reaction mixture SiH 4 –C 2 H 2 in a vertical tubular flow reactor in the temperature range 900–1250°C. Powder particles prepared at temperature 1100°C and C 2 H 2 /SiH 4 mol ratio 1.2 are equiaxial, quasispherical and agglomerated. The mean particle size of the powder is approx. 0.1–0.2 μm. The maximum agglomerate size is about 0.3 μm.

Thermal expansion of cross-ply and woven carbon fibre–copper matrix composites

Abstract This paper presents thermal expansion data for cross-ply and woven copper matrix–carbon fibre composites (Cu–C f MMCs) that were prepared by diffusion bonding. Thermal expansion was measured in two perpendicular in-plane directions of plate samples. For cross-ply samples (57 vol.%fibres) the mean coefficient of thermal expansion (CTE) between −20 and 300°C changed from approximately 6.5×10 −6 /°C to 3.5×10 −6 /°C during heating/cooling. The in-plane CTE increases with decreasing fibre content. Composites with woven arrangement of carbon fibres show a slightly higher CTE at elevated temperature.

Preparation of fine amorphous silicon nitride powder in the system SiH4-Ar-NH3

Abstract Amorphous silicon nitride powders have been prepared in a tube reactor in the system silane-argon-ammonia in the temperature range 500–1100 °C. The powders prepared at 500 °C show the presence of elementary silicon. The powders prepared above 650 °C are near stoichiometric in composition, isometric in morphology and the particle size distribution is in the range of 0.05–0.2 μm. The powders prepared at 1100 °C are nearly monodispersive, having the particle size 0.1 μm. The influence of molar ratio of ammonia to silane shows that silicon nitride powder of near stoichiometric composition can be prepared above 11.62 and below this ratio the powders contain elementary silicon. The change in total gas flow rate shows that below 300 cm 3 min −1 the powders again contain elementary silicon and with further increase in flow rate the powders are near stoichiometric at 400 cm −3 min −1 and little hyperstoichiometric in nitrogen content at and above the flow rate of 500 cm −3 min −1 . Surface characterization by X-ray photo-electron spectroscopy indicates that most of the oxygen is confined on the surface and is present in the form of SiO 2 .

Microstructure and Erosion Resistance of Zirconium Diboride Ceramics Infiltrated by Pure Copper

Cu/ZrB2 composite was prepared by gas pressure infiltration of molten metal into ceramic preform. Microstructure and erosion resistance of composite was investigated. The microstructure was analysed by light microscopy and scanning electron microscopy. The chemical compositions were analysed using energy dispersive X-ray spectroscopy. Good penetration of copper along the grain boundaries of the 60% porosity sintered ceramics was analysed in the whole volume of composite. The interfacial morphology shows the regular interfaces without any macroscopic reactions [1]. Cu/ZrB2 composite was subjected to 60 spark discharges to investigate the ablation resistance. Linear dependence of the amount of loss material on the number of electrical discharge analytical cycles for Cu/ZrB2 composite was determined.

Microstructure and Thermal Expansion of Hybrid - Copper Alloy Composites Reinforced with both Tungsten and Carbon Fibres

Tungsten as refractory material and high thermal conductive carbon fibres are promising candidates for production of copper matrix composites for high temperature applications. Three types of rod-like samples were prepared by gas pressure infiltration of different carbon/tungsten fibre preforms with copper and/or copper alloy (Cu-0.5Cr) respectively. The fibres are aligned parallel to rod axis and were combined with the tungsten wire cloth. The microstructure of prepared hybrid composites was examined. The samples were thermally cycled 3 times up to 550 °C at a relatively high heating/cooling rate (10 K/min) to touch real condition in applications where high heat is formed during short time. The thermal expansion behaviour in radial direction was also analysed. Results show that a combination of both types of reinforcements in rod-shapes samples insures good protection against composite disintegration during high temperature thermal loading.

CARBON NANOTUBES AND SHORT HIGH MODULUS CARBON FIBRES COMBINATION TO COMPOSITE PREPARATION

The improvement of the thermal properties of high conductive metal matrix composites is expected with suitable coefficient of thermal expansion CTE by using short carbon fibres Granoc XN-100 having axial thermal conductivity of 9 00 W/m.K upon its combination with carbon nanotubes (CNTs). From this reason a parametric study of the synthesis of multi wall carbon nanotubes (MWCNTs) by catalytic decomposition of acetylene over catalyst precursor based on nanometer sized hematite particles was performed. The nanoparticles were applied on the surface of commercial short high modulus (HM) carbon Granoc XN-100 fibres and MWCNTs were grown by catalytic chemical vapour deposition (CCVD) method. SEM and TEM images showed that multi wall CNTs was prepared but coaxial cylindrical graphene sheets and bamboo like microstructure was observed. The variation the CNTs synthesis parameters e.g. reaction time and surface covering density of hematite catalyst on HM carbon fibres resulted in production of desired quantity and quality of CNTs.

Thermal Expansion of Advanced Materials for High Temperature Fusion Application

In this paper, two different technologies for preparation of the composite are presented: vacuum diffusion bonding and gas pressure infiltration. Samples contained different volume fractions of tungsten fibres (10%, 50% and 78%) that were arranged in unidirectional, cross-ply and circular architecture. The coefficient of thermal expansion (CTE) of Cu/W MMC samples was measured in the longitudinal and transverse direction to the fibre orientation and it strongly depended on arrangement of reinforcing phase and direction of measurement. For the unidirectional material the CTE was 4 - 6 ppm.K-1 and 11-12 ppm.K-1 in directions parallel and transverse to the fibre orientation, respectively. Samples in a shape of a ring contained reinforcement that was wounded around the sample centre (circumferential winding) or was cut-out from the Cu/W MMC plate. In the second case, the reinforced fibres had the woven arrangement. The CTE of ring measured in radial direction showed values from 5 ppm.K-1 to 20 ppm.K-1 for composites containing 72 and 10 vol. % of W-fibres, respectively.

Carbon Nanotubes Prepared by CVD

Carbon nanotubes have been prepared by CVD from acetylene-argon gaseous system in tubular flow reactor. Acetylene gas was used as the carbon source needed for carbon nanotube growth. Several types of high porosity foam-like Ni substrate were used as catalyst material at pyrolysis temperatures from 673 to 973 K. Influence of the surface morphology of the foam-like Ni substrates on the carbon nanotubes morphology and growth have been studied.