Abstract: Three-dimensional finite element (FE) methods are used to predict the Young’s modulus
of two types of 2D woven carbon/carbon composites. Tensile tests are performed to validate the
predictions. The results indicate that a novel image-based route in generating FE meshes gave
strong agreement with experimental data, while a comparative unit cell FE model of the structure
was found to be poorer. The differences between the image-based and unit cell methodologies were
the consideration of the finer architectures of the composites and their porosity. The image-based
approach highlighted true porosity in the structure due to meshes forming directly from X-ray
tomographic data. However, the finer fibre architectures of the composites were compromised
because of limitations in the pixel resolutions employed during the initial scanning process. In
comparison, the unit cell models were based solely on idealisations of the composite microstructure,
in which porosity was neglected.
Abstract: A Pulse Plasma Sintering (PPS) process was employed to manufacture Cu-diamond composites with a 50% volume fraction of each constituent. Pure and Cr (0.8wt.%) alloyed copper matrices were used and commercial diamond powders. The composites were sintered at temperature of 900°C for 20 min and under pressure of 60 MPa. In these sintering conditions diamond becomes thermodynamically unstable. Cu0.8Cr-diamond and Cu-diamond composites with relative densities of 99,7% and 96% respectively were obtained. The thermal conductivity of Cu0.8Cr-diamond composite is equal to 640 W(mK)-1 whereas that of Cu-diamond is 200 W(mK)-1. The high thermal conductivity and relative density of Cu0.8Cr-diamond composite is due to the formation of a thin chromium carbide layer at the Cu-diamond interface.
Abstract: Fe- or Mn-powders were mixed with the diamonds and the mixtures were heat treated under different gas atmospheres like hydrogen or argon gas and at varying temperatures in order to roughen the surface of the diamond particles. Subsequently the reaction layers are removed by the addition of aqueous solutions of HCl. The modified surface structure after the etching process is investigated by electron microscopy showing increased roughening of the formerly flat faces of the diamond particles with increasing heat treatment temperature. After drying the diamonds, composites were prepared by gas pressure infiltration with pure Al. For treatments in the temperature range from 750 to 850°C the thermal conductivity can be improved by up to 20 percent compared to composites based on un-treated powders.
Abstract: . A baseline electroless deposition processes for Cu on CNTs has been developed. This process results in the formation of copper particles of few tens of nanometres. Using this process in a CNT loaded solution it is possible to obtain a homogeneous distribution of CNTs and Cu, even for volume fraction of CNTs as high as 17 v%. By the application of wet chemical processing it is possible to penetrate the natural felt-like structure of the CNTs and to fill the gaps with copper particles. Variations of the baseline deposition process have been established, allowing adding small amount of nickel on the CNT prior to the copper deposition to strengthen the interfacial bonding between matrix and CNTs. Hot pressing of the highly CNT loaded metal matrix composites has been developed; it allows producing bulk material that can be handled. The microstructure of these materials has been investigated and samples have been machined for further testing, i.e. mechanical characterization and thermo-physical properties.
Abstract: The mechanical properties of a SiC-fiber/copper matrix composite, reinforced with SCS-0 SiC-fibers ( 140µm, Specialty Materials), can significantly be increased by applying a Ti-Ta-C multilayer between fiber and matrix. This interlayer is deposited with a magnetron sputter device directly on the single fibers. By changing the deposition parameters of this sputter process the Ti-Ta-C interlayer can be optimized regarding fiber strength and fiber/matrix adhesion. Experiments with different deposition pressures, bias voltages and layer thickness’ were performed to increase the bond strength and the ultimate tensile strength when compared to the Ti-Ta-C reference sample.
Abstract: In view of power electronics applications, baseplates made from metal diamond composites have been manufactured and characterised. The surface contours of the baseplates were measured during thermal loads up to 180°C starting at room temperature with help of the TherMoiré technique. X-ray analysis investigation was performed to detect porosity and local inhomogeneities of the baseplates. Al- and Cu-based diamond composite baseplates were Ni-plated and used for manufacturing of 3.3 kV IGBT modules. The solder layer between AlN AMB (active metal brazing) substrates and baseplates was investigated by ultrasonic and X-Ray analyses. Thermal resistance of the manufactured IGBT modules was characterised and compared to that of IGBT modules with AlSiC or Cu baseplates. The influence of thermal cycling on the solder layer and thermal resistance of the manufactured module was investigated.
Abstract: This paper examines the effect of anisotropy on the effective composite thermal conductivity, Kc, of metal matrix composites containing carbon-based inclusions. Added effects of thermal interface conductance, hbd, and size have also been considered. It has been found that at high hbd values, the effective thermal conductivity of the composite is limited by inclusion anisotropy. At lower hbd values and smaller inclusion sizes, this effect is greatly diminished due to the more dominant effect of limited heat flow across the interface.
Abstract: Copper matrix composites reinforced with silicon carbide fibres (SiCf/Cu) are considered as heat sink materials for the divertor of DEMO as they combine high thermal conductivity and good mechanical strength at high temperature. A new method was developed to synthesise a metal matrix composite (MMC) consisting of about 3-6 unidirectional reinforced layers (UD-layers). The UD-layers were prepared by two subsequent electroplating processes which allow to adjust various fibre volume fractions. These single UD layers were stacked with different relative fibre orientations (0°/0° and 0°/90°) and consolidated by vacuum hot pressing to form the MMC specimen. The thermal conductivity perpendicular to fibre direction was obtained by laser flash apparatus (LFA) measurements. It is about 310 Wm-1K-1 for electroplated copper (Cu) and above 200 Wm-1K-1 for MMC specimens with a fibre volume fraction of 8-13%. Due to the manufacturing process, boundaries within the matrix were found resulting in a reduction of the values. In addition, DSC (differential scanning calorimetry) measurements were performed which gave similar results.
Abstract: A micromechanical model is employed to investigate the influence of the interface between the fibre and the matrix of a metal matrix composite with long fibre, which is elaborated through finite element method. Also, transverse properties of composite are studied in the present work. The interface, between the fibre and the matrix, is studied employing cohesive elements. These elements employ a cohesive zone model, which follows a bilinear law.
Abstract: The purpose of this work is to investigate and understand the thermomechanical behaviour of copper matrix composites with long fibres. The effects of the volume fraction of fibres are considered in this analysis. A micromechanical finite element model has been used to study the behaviour of the copper matrix composites reinforced with tungsten fibres, which undergo thermal cycling conditions. Furthermore, regular and random arrangements of fibres are considered