Abstract: The ideal thermal management material working as heat sink and heat spreader should have a high thermal conductivity combined with a reduced and tailorable thermal expansion. To meet these market demands copper composites reinforced with diamond particles were fabricated by a powder metallurgical method (powder mixing with subsequent pressure assisted consolidation).
In order to design the interfacial behaviour between copper and the reinforcement different alloying elements, chromium or boron, were added to the copper matrix. The produced composites exhibit a thermal conductivity up to 700 W/mK combined with a coefficient of thermal expansion (CTE) of 7-8 x 10-6/K. The copper composites with good interfacial bonding show only small decrease in thermal conductivity and a relatively stable CTE after the thermal cycling test.
Abstract: Thermal management materials frequently have multi-phase composite character with complex architecture of the constituents. As a result, design rules are needed which can be used in selection of the phases and optimizing their volume fractions. The study shows that such are provided by finite element modeling of these composites. This is demonstrated for a diamond-SiC-Si-(Al) composites, which have been optimized in terms of the volume fraction of SiC, contact area between the components and presence of open porosity.
Abstract: Particle reinforced metal matrix composites are developed for heat sink applications. For power electronic devices like IGBT modules (Insulated Gate Bipolar Transistor) a baseplate material with high thermal conductivity combined with a low coefficient of thermal expansion is needed. Commonly AlSiC MMC are used with a high volume content of SiC particles (~ 70 vol.%). To improve the performance of these electronic modules particle reinforced materials with a higher thermal conductivity are developed for an advanced thermal management. For this purpose highly conducting diamond particles (TC ~ 1000 W/mK) are embedded in an Al matrix. These new diamond reinforced MMC were investigated concerning their thermal fatigue mechanisms compared to the common AlSiC MMC. Differences in reinforcement architecture and their effects on thermal fatigue damage were studied by in situ synchrotron tomography during thermal cycling.
Abstract: The designed plasma facing materials for the divertor-components in ITER are up to now carbon (graphite or CFC) and tungsten. The heat flux loading can result in temperatures of up to 550°C at the interface between the plasma facing material and the CuCrZr heat sink, an operation temperature which is too high for CuCrZr. Additionally the temperature gradient and the mismatch in thermal expansion (CTE) of both parts of the divertor result in high stresses at the interface. A tungsten-copper composite material with a gradation from pure copper on the one side to pure tungsten on the opposite side is supposed to support the stress reduction. The tungsten contributes to the strength of the composite, whereas the copper provides the required thermal conductivity (TC) of at least 200 W/mK. The graded W/Cu layers were produced by the “chemical mixing” method and subsequent liquid phase sintering. Here the use of sub-µm W-particle sizes offers an opportunity to gain well dispersed W-particles with high Cu-containing composites resulting in lower CTE values and higher thermal conductivities compared to coarser W-powders. Brazing of the functionally graded W/Cu interlayers to CuCrZr and CFC materials resulted in adjoined mock-ups.
Abstract: In Plasma Facing Components (PFCs) for nuclear fusion reactors, the protective material, carbon based or tungsten, has to be joined to the copper alloy heat sink for optimum heat transfer. High temperature vacuum brazing is a possible joining process as long as a proper interlayer is introduced to mitigate the residual stresses due to the mismatch of thermal expansion coefficient (CTE). Pure copper can act as plastic compliant layer, however for carbon based materials a proper structuring of the joining surface is necessary to meet the thermal fatigue lifetime requirements. In this work pure molybdenum and tungsten/copper Metal Matrix Composites (W-wires in Cu-matrix) interlayers have been studied as alternative to pure copper for carbon based protective materials in flat tile configuration. Finite element simulations of the brazing process have been performed to evaluate the expected residual stress reduction near the metal-carbon interface. In fact it has been demonstrated that stiff low CTE interlayers can shift the peak stresses from the weak carbon-metal interface to the strongest metal-metal one. Relevant samples have been manufactured and subjected to preliminary metallographic and thermal shock tests. Results obtained so far are encouraging and active cooled mock-ups are being prepared for high heat flux testing. Research work is in progress as regards monoblock configuration with both Wf/Cu MMC and graded Cu/W plasma sprayed and HIPped layers.
Abstract: Thin graded W/Cu coatings plus subsequent heat treatment at 800°C are used to improve the interfacial adhesion between W and Cu. Specimen with ~500 nm thick graded W/Cu coatings were characterized and analyzed after thermal treatment at 550°C, 650°C and 800°C. At 800°C a significant change in the nanostructure is observed. The better adhesion is caused by W/W grain boundary diffusion processes and Oswald ripening of the nanometre-sized grains leading to their interconnection between each other and the W substrate. The phase and texture analysis of graded W/Cu indicates grain growth and diffusion processes of pure W and Cu. The stress analysis shows that the changes in the nanostructure of the W/Cu coatings correlates to the stress relieve of W at temperature starting from 650°C. After cooling of the coating to RT the residue intrinsic tensile stress is caused by thermal mismatches of the substrate and the 100% W layer of the coating.
Abstract: The effect of the shape, size and kind of distribution of the silicon inclusions in Al-Si alloys of hypoeutectic, eutectic, and hypereutectic compositions (6–22 % Si) on the quality of oxide layer formed by micro-arc oxidation (MAO) is studied. The hitherto unknown phenomenon of oxide layer growth inhibition by the silicon particles under MAO is discovered and researched. The filling of near-surface pores by oxidation products in MAO process is observed also. The discovered ability to increase the properties of oxide layers obtained by the MAO due to prior directed modifica¬tion of the initial structure provides the good quality of oxide layers for aluminium alloys with high level of silicon content up to hypereutectic compositions of Al-Si alloys. The physical opportunity to get the high quality oxidized layers by MAO on structurally modified Al-Si alloys is confirmed for large-sized Al-Si alloys castings (including cylinder block for engine).
Abstract: A graphite sheet has been successfully brazed to a Nimonic 105 superalloy using a commercial TiCuSil paste. A chromium layer was deposited on the graphite surface by sputtering and controlled heat treatments were employed in order to develop a suitable microstructure. Scanning electron microscopy measurements showed rough, crack-free interfaces between the filler metal and both the graphite and nimonic parts. From metallographic examination a well defined layered structure of the metallic elements close to the filler/graphite interface has been found. The metallic elements transport from the interface to the carbon bulk where they fill all the graphite pores up to a depth up of 50 μm and form a layered structure within the pores.
Abstract: One of the key problems in copper-diamond composites is the interface between the metal matrix and the diamond reinforcement. In order to take advantage of the high thermal conductive diamond filler in a composite the design of the interface is crucial. One approach to minimize the thermal contact resistance between metal and diamond reinforcement is to coat the diamonds with functional layers, e.g. Mo or W. For coating of diamonds PVD and CVD have been used followed by characterization of coating thickness by different methods. The coated diamonds were used for composite manufacturing and the thermal diffusivity of the compacted materials was measured.
Abstract: Copper matrix nanocomposites (materials for coatings) and reinforcements from
nanodiamonds, nanosilica or diatomite were fabricated by mechanical alloying, and coatings were
produced by the friction cladding method. Investigations included the study of material at all steps
of the technological scheme: formation of granules during mechanical alloying; distribution of
nanoparticles in the copper matrix after consolidation of granules; structure of coatings and
distribution of reinforcements in the coatings. As the result of these investigations, optimal
technological conditions of treatment were developed.