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usheenthermal
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As the power density of electronic devices continues to escalate, heat dissipation issues are becoming increasingly critical. Thermal Interface Materials (TIMs), acting as a bridge connecting heat sources and heat sinks, directly influence the overall heat dissipation efficiency. However, achieving perfect contact at the thermal interface is often challenging in practice, and the interfacial thermal resistance has become a key bottleneck limiting heat dissipation performance. Ideally, heat should transfer unimpeded from the heat source to the heat sink. Yet, real-world interfaces are not atomically smooth; microscopic roughness and waviness exist, preventing perfectly tight contact and leading to the formation of microscopic voids. These voids are typically filled with air, which has a very low thermal conductivity (approximately 0.026 W/m·K), significantly lower than most solid materials. Consequently, when heat passes through the interface, it must circumvent these high thermal resistance air gaps, causing heat flux lines to constrict and bend, resulting in additional thermal resistance known as interfacial thermal resistance. The magnitude of interfacial thermal resistance is complexly influenced by various factors, primarily including material surface roughness, contact pressure, material mechanical properties, TIM thermal conductivity, TIM thickness, and the operating environment. Therefore, to effectively reduce interfacial thermal resistance and enhance heat dissipation performance, several key strategies can be employed.
Firstly, surface treatment and precision machining are crucial optimization approaches. Improving the surface finish of contact surfaces, such as by grinding and polishing the contact surfaces of heat sources and heat sinks, can significantly reduce surface roughness, minimize microscopic voids, and increase the actual contact area. Nanoscale polishing techniques can even achieve surfaces approaching atomic smoothness, maximizing the reduction of contact thermal resistance. Furthermore, surface coating modification is also an effective method. Depositing a thin film coating on the contact surface, such as self-assembled monolayers (SAMs) with low surface energy or nano-coatings, can improve interface wettability, promoting better spreading of TIMs and filling of microscopic voids. Some specialized coatings can even reduce interfacial phonon scattering, further enhancing heat transfer efficiency. Another cutting-edge strategy is to construct micro-nanostructured surfaces on the contact areas, such as micro-pillar arrays, nanowire arrays, and bio-inspired structures. These structures can increase the actual contact area and utilize elastic deformation capabilities to better adapt to interface unevenness, reducing contact thermal resistance. For instance, micro-pillar array structures can undergo elastic deformation under pressure, increasing the contact area, while their internal voids can serve as heat transfer channels, further lowering thermal resistance.
Secondly, optimizing the intrinsic properties of the thermal interface material itself is paramount. Selecting high thermal conductivity fillers is key to enhancing the intrinsic thermal conductivity of TIMs. Materials like boron nitride (BN), aluminum oxide, carbon nanotubes (CNTs), graphene, and metal powders (such as copper and silver powder) are commonly used high thermal conductivity fillers. The choice of filler requires a comprehensive consideration of its thermal performance, dispersibility, cost, and compatibility with the base material. Going a step further, optimizing the arrangement and orientation of fillers can create more efficient heat conduction pathways. Through specialized preparation processes, such as magnetic field alignment and shear force field assistance, oriented alignment of fillers within the TIM matrix can be achieved, significantly improving thermal performance. For example, directional alignment of CNTs or graphene can markedly increase their thermal conductivity in specific directions. Moreover, the development of adaptive interface materials is a significant direction for advancement. Developing TIMs with good deformability and adaptability, such as thermal gels, liquid metals, and phase change materials (PCMs), enables them to fully fill microscopic voids at the interface even under low pressure, achieving a higher actual contact area and effectively reducing contact thermal resistance. For example, thermal gels exhibit excellent flexibility and deformability, allowing them to conform well to irregular surfaces. Liquid metals, on the other hand, possess extremely high thermal conductivity and very low contact thermal resistance, but their application is limited by factors such as cost and reliability.
Finally, system-level optimization strategies should not be overlooked. Within the system design’s allowable limits, appropriately increasing the contact pressure between the heat source and heat sink can effectively reduce interfacial gaps and lower contact thermal resistance. However, care must be taken to avoid excessive pressure that may damage devices or cause TIM squeeze-out. Optimizing interface design is also crucial; structurally, efforts should be made to ensure the flatness and matching of contact interfaces. For instance, employing high-precision machining processes to manufacture the contact surfaces of heat sinks and heat sources can reduce surface unevenness. In specific applications, such as high-vacuum electronic devices or devices sensitive to oxidation, operating in a vacuum or inert gas environment can be considered to eliminate the influence of air gaps, significantly reducing contact thermal resistance. To accurately evaluate interfacial thermal resistance, common testing methods include the steady-state thermal resistance test method, transient thermal resistance test method, and infrared thermography. The steady-state thermal resistance test method calculates thermal resistance by measuring temperature differences and heat flux density under steady-state conditions, the transient thermal resistance test method analyzes temperature responses during transient heating or cooling, and infrared thermography visually observes interface temperature distribution.
In summary, interfacial thermal resistance is a key factor limiting the heat dissipation performance of thermal interface materials. By implementing surface treatment and precision machining, optimizing the intrinsic properties of TIMs, and adopting system-level optimization strategies, interfacial thermal resistance can be effectively reduced, and heat dissipation efficiency improved. In the future, as the power density of electronic devices continues to increase, the performance requirements for thermal interface materials will become even more demanding. Research directions will focus on developing new TIMs with higher thermal conductivity, lower contact thermal resistance, and greater reliability and stability, combined with advanced micro-nanofabrication techniques and system-level optimization design, to collectively break through the heat dissipation bottleneck and meet the cooling needs of future electronic devices.
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