Papers by Keyword: Thermal Conductivity (TC)

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 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: 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: Diamond-based metal matrix composites have been made based on pure Al and eutectic Ag-3Si alloy by gas pressure infiltration into diamond powder beds with the aim to maximize thermal conductivity and to explore the range of coefficient of thermal expansion (CTE) that can be covered. The resulting composites covered roughly the range between 60 and 75 vol-% of diamond content. For the Al-based composites a maximum thermal conductivity at room temperature of 7.6 W/cmK is found while for the Ag-3Si based composites an unprecedented value of 9.7 W/cmK was achieved. The CTE at room temperature varied as a function of the diamond volume fraction between 3.3 and 7.0 ppm/K and 3.1 and 5.7 ppm/K for the Al-based and the Ag-3Si-based composites, respectively. The CTE was further found to vary quite significantly with temperature for the Al-based composites while the variation with temperature was less pronounced for the Ag-3Si-based composites. The results are compared with prediction by analytical modeling using the differential effective medium scheme for thermal conductivity and the Schapery bounds for the CTE. For the thermal conductivity good agreement is found while for the CTE a transition of the experimental data from Schapery’s upper to Schapery’s lower bound is observed as volume fraction increases. While the thermophysical properties are quite satisfactory, there is a trade-off to be made in these materials between high thermal conductivity and low CTE on the one side and surface quality and machinability on the other.

Abstract: Tungsten is the main candidate material for the armor of plasma facing components for ITER and future fusion devices [1]. Plasma spraying is an alternative method for manufacturing tungsten-based coatings, including composites and graded layers, having a number of advantageous features [2]. On the other hand, the main limitation to application of these coatings on high heat flux components, is their low thermal conductivity, originating in the layered structure [3]. This paper is focused on four methods of improving the coatings’ thermal conductivity. First method consists in modification of the basic spraying parameters, which have a direct influence on the coating structure and therefore properties. The other three methods involve post-processing of the coatings: molten copper infiltration, laser remelting and densification by HIPping. The latter encompasses also tungsten-copper composites of various compositions. Experimental results, including structural and thermal characterization, are presented for each method. Finally, the applicability of these methods, from the point of view of manufacturing the plasma facing components, is discussed.

Abstract: Thermal properties of SiC at the micrometer-scale were measured quantitatively with a thermal microscope using thermo-reflectance and periodic heating techniques. In this study, SiC single crystal and polycrystal were investigated. The small values of standard deviation suggest that the SiC single crystal had constant thermal conductivities. For the single crystal, the average value of the thermal conductivity at the micrometer-scale was in good agreement with the macro-scale thermal conductivity value obtained by the laser flash technique. On the other hand, thermal conductivity of the polycrystal was heterogeneous at the micrometer-scale. An average thermal conductivity value of 257 Wm-1K-1 was obtained within an area of 50 m ×100 µm. The highest and lowest values of the thermal conductivity from the polycrystal were 300 and 220 Wm-1K-1, respectively.

Abstract: –sialons (Si6-zAlzOzN8-z, z=3) synthesized by mechanically activated combustion synthesis (MA-CS) at a low N2 pressure of 1 MPa, were sintered by Spark Plasma Sintering (SPS) and thermal conductivity was measured at room temperatures. Specimens were fully densified at 1600oC for 10 mins. and showed only –sialon phases confirmed by x-ray diffraction patterns though un-reacted Si was present as impurities after MA-CS. Thermal conductivities increased with sintering temperature and had a maximum value 5.49 W m-1 K-1 for specimens sintered at 1700oC.

Abstract: The thermal conductivity at the micrometer-scale of AlN ceramics with eliminated grain boundary phase was measured by the thermoreflectance technique with periodic heating. Thermal conductivities were ranged from 160 to 260 W/m•K and an average value of 201 W/m•K was obtained from a 22 m2 area. The variation in the thermal conductivities was related to the individual AlN grains and grain boundary characteristics.

Abstract: The high thermal conductivity of Aluminum nitride, coupled with its high electrical resistivity and nontoxic nature, makes it a very promising material for electronic substrate. In this study, microstructural characterization on the thermal conductivity of AlN ceramics was investigated. An AlN ceramic was prepared with a dopant Y2O3 under a reducing nitrogen atmosphere with carbon. In order to obtain high thermal conductivity, cooling rate control and after-heat treatment was carried out. Morphology of the second phase was characterized using scanning electron microscopy (SEM). SEM studies showed that the microstructural change caused by after-heat treatment have a major influence on the thermal conductivity.

Abstract: Metal impurities are known to degrade dramatically the performances of silicon-based devices, even at concentrations as low as 1012 cm-3. A specific process, named proximity gettering, has been optimised by some authors in order to reduce the influence of these impurities [1]. This process consists in the building of a favourable impurity trapping zone in a non-active area of the device, by introducing implantation defects. This paper reports on the application of introducing such gettering sites as an approach to control phonon properties in 4H-SiC epilayer, and increase the thermal conductivity.