Pavol Štefánik

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.

Effect of thermal cycling on the microstructure of continuous carbon fibre reinforced copper matrix composites

Abstract Copper reinforced by continuous carbon fibres is a candidate material for heat sinks of electronic modules. The thermal expansion can be matched to the adjacent ceramics and the thermal conductivity is higher than that of alternative materials. Because the material is expected to work in cyclic thermal conditions during application its structure must withstand such a load without any severe damage. The structure of unidirectionally and cross-ply fibre reinforced samples was observed before and after thermal cycling by optical microscopy and Scanning Electron Microscopy (SEM). SEM was used for examining the fibre-matrix interface before and after thermal cycling. Optical microscopy showed the arrangement of individual monolayers. The influence of adding copper foils between individual monolayers on the structure of the material is reflected in a reduction in the number of cracks in monolayers.

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.

Thermal expansion of copper matrix composite with spiral arrangement of carbon fibres

Abstracts are not published in this journal

Galvanic deposition of Co-Mo layers and their influence on tensile strength of carbon fibers

Aluminum and copper are of interest for application in the electronic industry due to high thermal and electric conductivity, but their coefficient of thermal expansion is too high for some applications. The use of carbon fibers with these metals as a composite material can decrease the thermal expansion and the density of the material [1–7]. The problem with using copper–carbon fibers (Cu/CF) composite is the low adhesion between copper and carbon fibers. Due to poor wettability and low adhesion between the composite components the transfer of load from matrix to fibers is poor and a very low value of shear strength in unidirectional composite is obtained. To increase the adhesion between the composite components, either carbide forming elements (for example Cr, W, Mo, Ti, etc.) are added to copper matrix, or carbon fibers are coated with layers containing these elements [4–6, 8]. The large interface layer formed by these reactions reduces carbon fibers properties. Higher content of these elements in copper considerably decreases some properties of the composite, mainly the thermal conductivity. The carbon fibers are also used as reinforcement in aluminum matrix composites [1–3]. To prevent the formation of aluminum carbide the carbon fibers are usually coated by a thin carbide or oxide protective layer. Galvanic metal coating is a widely used method in which different thicknesses can be obtained, from about several tenths of micrometer up to micrometers, which should be sufficient for the matrix to produce satisfaction composites. In our experiments we choose molybdenum as the carbide forming element. Unfortunately, it is not possible to deposit this metal alone either from aqueous or organic solutions. Therefore, in this letter we present a method of deposition of Co-Mo layers onto carbon fiber tow from an alkaline Co-Mo plating bath in which the metals were presented as a complex carbonate [9]. High strength carbon fibers (Torayca T300) with 3000 monofilaments in tow (diameter of a filament is about 7 μm) were used for the present study. All galvanic layers were deposited on carbon fibers in the continuous coating apparatus schematically described in Fig. 1. The sizing from the original C-fibers was removed in the furnace (2) at a temperature about 550 ◦C. To obtain active surface nuclei the commercial Sn (3) and Pd (4) baths were used (the same as at the chemical copper plating [10]). The individual fibers (original and heat treated at 700 ◦C for 30 s in argon atmosphere) were glued onto a paper frame with gauge length 20 mm and inserted into a tensile testing machine with a load cell of maximum force 0.5N. To explain some facts we used the Weibull statistical method [11]. The fibers were galvanically coated in aqueous electrolyte (5) with Na2MoO4 and CoCl2 at 90 ◦C. The deposition conditions which assured the forming of a continuous layer around each monofilament with the highest amount of Mo were optimized. The thickness of coating layer on carbon fibers was from 0.1 to 0.2 μm. On some carbon fibers tows with CoMo layer, in an other galvanic bath (6), copper was deposited with thickness approximately five times higher than the first layer. Because it is difficult to determine Mo content in such a thin layer directly onto the fiber, we deposited (at similar conditions as the fiber coating) the Co-Mo layer on a carbon plate with thickness of several micrometers. Energy dispersive X-rays analysis measurements showed that the alloy consisted of ∼40 wt.% of molybdenum and ∼60 wt.% of cobalt. From scanning electron microscopy observation one can see that all fibers are uniformly coated. The structure of Co-Mo layer on carbon fibers is fine-grained (Fig. 2), whereas the surface of the Cu layer (Fig. 3) is coarse. The tensile strength of the coated carbon fibers is practically the same as that of the original T300 fibers— neither galvanic deposition conditions nor thin deposited layers influence the surface of the fibers. Heat treatment of all types of coated fibers at 700 ◦C has different effect on their strength (Fig. 4). Whereas the strength of the original fibers and that, of the one with

Al-Limestone and Al-Granite Composites Prepared by Cold Spraying

Two types of aluminium metal matrix composites (Al-MMCs) were prepared by cold spray process. The first Al-MMC was reinforced with granite rock and the second was reinforced with limestone rock particles. Al powder and rock powders (granite or limestone) were mixed to homogeneous mixtures and sprayed onto the Al substrate. The microstructure of as-sprayed composites was compared with microstructure of MMCs reinforced with commercially available Al2O3. The microstructures of Al-MMCs reinforced with granite and limestone were affected by hardness of rocks, particle sizes and compositions of the mixture. The coating is formed through high velocity impact of solid powders. The diameter of Al powder was about 30 μm and diameters of rock powders were from 5 to100 μm. Rock particles are distributed uniformly through the coating, maintaining their irregular morphologies. However, some large sized particles cracked and fragmented. The powder porosity was approximately 2.3 % for both types of Al-MMCs. Porous microstructure leads to lower critical velocity. The results indicate that, introducing irregular morphologies and/or pores into the feedstock. High quality metallic coatings can be more easily deposited by cold spray.

Ni-NiO Porous Preform with Controlled Porosity Using Al 2 O 3 Powder

The Ni-NiO skeleton seems to be a good candidate for various applications in industry such as corrosion-proof filters or components in refrigerating systems and as preforms for reactive infiltration with molten metals.The present work was focused on preparation of Ni-NiO composite with higher, controlled porosity. Sintering of pure Ni powder always leads to a substantial closed porosity in almost whole sample volume [1,2]. To eliminate this, we added Al2O3 particles with diameter of-32 +20 μm into the Ni powder (-75 +45 μm diameters) and sintered this loose powder mixture (Ni + 25 vol. % Al2O3) in air by progressive heating up to 800 °C followed by 2 hours isothermal exposure. As a control, pure Ni powder was sintered under the same conditions. Thermal oxidation of loose powder samples performed in alumina crucible indicates that the strongest oxidation occurred in the top part of sample, while the bottom part was the least oxidized. Therefore, it was necessary to run the thermal oxidation once more, but out of the crucible, to ensure the sufficient diffusion of oxygen to the whole volume of sample.

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.

Thermal Cycling of Copper Based Composite Reinforced with High Modulus Carbon Fibres

The thermal expansion behaviour of Cu-1Cr/C composite subjected to 5 thermal cycles in the temperature range 30 - 1000 °C was investigated. The coefficients of thermal expansions (CTEs) as low as 0.7 x 10-6 K-1 in longitudinal and as large as 24.0 x 10-6 K-1 in transversal direction were obtained. Electron microscopy observations confirmed the high structural stability of the thermally cycled composite as no signs of disintegration were observed within the applied thermal cycling conditions.