An investigation of the fabrication and characterisation of nanocomposite thermal interface materials

Contemporary electronic devices such as microprocessors feature high component-level heat-fluxes and tight temperature constraints. Thermal interface materials (TIMs) are a critical element in the path for heat transfer from devices to ambient. The objectives of this thesis are: to design, build and commission a thermal test facility capable of measuring the thermal properties of TIM pads with an acceptable level of uncertainty; and to manufacture and characterise nanomaterial-polymer composites and novel nanos- tructures as viable TIM pads for electronic cooling applications. A bench top test facility was designed and commissioned according to ASTM D 5470 Standard Test Method in order to measure the thermal impedance and e ective thermal conductivity of both commercially-available and newly developed TIM pads under a wide range of clamping pressures and input powers. The calibration process for this facility was carried out using stainless steel disks as benchmarks with a thermal conductivity of 14.85 Wm 1K1. The discrepancy was found to be within 1.08%, which confirmed the successful design and implementation of the experimental setup. Dry-joint thermal impedance tests were also conducted under a wide range of pressures for averaged input powers based on the energy balance between the two meter bars: the heat losses were in all the cases within 4%. At 2.5 MPa, a thermal impedance value of 0.525 Ccm2W-1 was measured with a calculated uncertainty of approximately 4.18%. Repeatability tests showed variations within 1.26% and reproducibility tests were shown to be in good agreement with previous studies. Two principal types of nanocomposites were manufactured and characterised: Batches of carbon nanotubes (CNTs), Ag nanowires (AgNWs) and Ag akes were randomly dispersed in a silicone elastomer matrix by both mechanical and solution mixing at di erent weight percentages of the llers. Thermal impedances between 12{38 Ccm2W-1 resulted in one order of magnitude above the state-of-the-art of thermal pads. It is believed that an aligned orientation of high-aspect-ratio fillers, coupled with a reduced bond line thickness (BLT) of the pads, could lead to an improved performance in terms of thermal impedance. AgNW arrays embedded inside polycarbonate (PC) templates were fabricated by electrodeposition. The vertically-aligned nanowires' protrusion from the mem- brane surface appeared to enable them to conform to the surface roughness of the contacting surfaces, resulting in better thermal contact. By depositing a 30 nm Au layer on top of the as-received composite, a thermal impedance value of 0.93 Ccm2 W-1 was measured with a calculated uncertainty of 3.4% at a contact pressure of 1 MPa. In order to reduce further the thermal impedance of the samples, double-sided, ultra-long, high-density AgNW arrays were fabri- cated using a nanoporous anodised aluminium oxide (AAO) template and their feasibility as TIM pads was assessed. The new composite showed a reduction of thermal impedance of about 37% at 0.95 MPa when compared to that of AgNW- PC. By wetting the double-sided AgNW array with mineral oil, a value of thermal impedance as low as 0.245 Ccm2W-1 was measured at 1 MPa with an uncertainty of 8.47%. The double-sided AgNW array matched well with the state-of-the-art of commercially-available thermal pads. The ndings of the thesis are of practical relevance to the development of AgNW arrays as a viable solution for component-level thermal management applications, where thermal interface impedance is a critical factor.