Development of smart heat sink composites using hybrid fillers
Heat dissipation efficiency is a critical parameter in the design of electrical circuits. If the amount of heat generated in electrical devices is allowed to accumulate, it is significant enough to cause irreversible damage. Even if circuits are below temperatures that cause damage, thermal cycling may cause fatigue and reduce the overall device lifespan, thereby underscoring the importance of efficient heat transfer in electronic materials.
A popular method of ensuring efficient heat dissipation is the incorporation of fillers with high thermal conductivity, such as aluminum nitride (AlN) and boron nitride (BN) in polymer matrices.1 However, since thermal conductivity of the composites decreases with increasing temperature, the heat generated is often accumulated, and causes a rapid temperature rise in the device during operation.2 In order to delay the temperature increase, heat-sink systems have been adopted using phase transition materials like paraffin wax, which absorb the generated energy through its latent heat of melting.3 However, these phase change materials are not thermally stable and can decompose under repeated thermal cycling.
Here, we use a tin-indium (Sn/In) alloy as a phase change material for developing a smart thermal-barrier composite system.4 The Sn/In alloy is an ideal heat-sink filler because it absorbs energy through its latent heat of melting at 125°C—a critical temperature for the safety of most electronic devices.
The composite system that we have developed demonstrates heat sink capability and high thermal conductivity throughout repeated thermal cycling experiments. As shown in Figure 1(a), the high-density metal particles were successfully dispersed into a polymer matrix without sinking by incorporating inorganic aluminum nitride and boron nitride particles. The samples were also heated to and held at 140°C for twenty minutes; after three such heating cycles, the samples were imaged, and Figure 1(b) shows that the Sn/In particle retains its circular shape and remains encapsulated in the epoxy matrix. This demonstrates the reversibility of the thermal-barrier capability in the composite system. The differential scanning calorimetry (DSC) thermogram in Figure 1(c) demonstrates that the latent heat of melting does not change over 10 cycles of heating and cooling, indicating that the Sn/In particles are well encapsulated in the composite cavity.
When the composite is heated, the temperature increase in the system is also delayed by the endothermic melting of Sn/In particles. In Figure 2(a), the range of the delayed temperature difference (ΔTDTD) becomes broader with increased heating rates, and the maximum ΔTDTD may depend on the incorporated Sn/In content in composites. The latent heat of melting from the fusible metal particles appears to suppress the temperature changes. As shown in Figure 2(b), the actual composite temperature, TD, is lower than that programmed in the melting region due to the absorption of external heat from the melting of the Sn/In particles. This demonstrates that the developed composite could be used as a smart heat-sink system that reversibly delays the temperature increase in the system and could prevent instantaneous overheating of a device caused by an external thermal shock.
The thermal barrier capability of a composite material comprised of fusible metal particles and inorganic fillers was investigated experimentally. Results indicate that the composite is robust over repeated thermal cycling and delays temperature increases when subjected to heating, demonstrating the potential of this material in the development of smart heat dissipation systems. Future work will involve the mechanical characterization of these composite materials.
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- W. J. Yao and N. Wang, Latent heats of fusion and crystallization behaviors of Co–Si
binary alloys, J Alloy. Compd. 487, pp. 354-357, 2009.
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