Manufacturing and characterization of nanomaterials for low-temperature sintering and electronics thermal management applications

Licentiate thesis, 2020

Nanotechnology is expected to have a significant impact on the long-term development within and across many disciplines. While offering potential improvements in energy efficiency and reduced energy and materials consumption, nanomaterials are at the centre of the cutting-edge technologies and sustainable manufacturing processes era with near half of the products in the next period of 10 years expected to embed nanoscale solutions. The condensed substance of nanosized dimension has shown excellent properties that offer new possibilities due to their surface area to volume ratio. The non-negligible surface energy was proven to induce melting temperature depression, low sintering activation energy and high electronic density that contributes to thermal transport. In this work, we investigate the possibility to take advantage of the advanced nanoscale properties as a sintering aid for low-temperature manufacturing and as thermal dissipation materials for electronics cooling. By combining the high surface energy of the nanomaterial with the tailoring of the local chemical composition, an extra degree of freedom is expected to enable further tuning of their physical and chemical properties. Furthermore, due to the chemical stability of the carbon-based materials and their outstanding physical properties, graphene is explored as a nanoscale coating for nanoparticles and applied herein as a potential nanofiller for nanofluids cooling approach. An additional effort is made to use the graphene/metal composite at a microscale level as high porosity material for heat dissipation.

In this thesis, a novel approach for nanoscale materials production was exploited to manufacture multi-elements alloyed iron nanoparticle. Using spark erosion, low carbon steel nanopowder was produced in order to tune the chemical composition of the nanoparticles in order to combine size effect with composition effect and tailor their performances. A melting depression recorder, while the sintering behaviour of the powder indicated an early activation of the diffusion at temperatures higher than 150°C.  The results allow such materials to be used as a sintering aid and lower the sintering temperature of iron powders.

Secondly, graphene-coated copper nanoparticles were developed as additives for nanofluids. The nanocomposite fillers of the copper core with multilayers graphene shell were added to water as the base-fluid. The presence of the graphene coating acted as oxidation protection for the metallic particles. Besides, it was found that the presence of the graphene as a local coating on the spherical metallic nanoparticles resulted in a proportional increase in the thermal conductivity of the fluid as the temperature and the concentration of the nanoparticles increased. Such an approach was found promising in the use of graphene-coated nanoparticles as fillers for nanofluids with good heat dissipation.

Finally, graphene has been used as a three-dimensional (3D) foam structure with sintered silver nanoparticles. The sintering of the metallic particles allowed a pressure-free attachment of the high porosity and lightweight material on the back of a chip as a heat sink. The thermal properties of the graphene foam were investigated and found to reach a thermal conductivity of 319 W/mK. The addition of a layer of coating of silver on the 3D graphene foam material improved further its thermal properties with a 54% enhancement in its effective thermal conductivity. The high porosity fraction was later gradually filled with paraffin as a phase change material. As a result, the maximum temperature of the chip was proportionally lowered and delayed. Most importantly, a CFD model was developed to study the contribution of the secondary microchannels in the heat dissipation process and revealed a positive and non-negligible effect of the additional microporosity present in the case of the graphene foam structure.