Advanced materials science for metal heat sinks: from electronic heat conduction mechanism to multiphase microstructure regulation
Introduction: Multi-scale physical images of metal heat conduction
The essential function of metal heat sinks is to achieve efficient heat transport. But the answer to "why copper conducts heat two orders of magnitude faster than stainless steel" is rooted in the transport mechanism of heat-carrying particles in condensed matter physics. In metallic crystals, heat conduction is carried out by free electron gas and lattice vibrating phonons; the contribution of free electrons dominates (the Wiedemann-Franz law verifies the proportional relationship between electron thermal conductivity and electrical conductivity). This means that any microscopic defects that affect electron mobility - point defects, dislocations, grain boundaries, second-phase particles - scatter electrons and phonons simultaneously, reducing thermal conductivity.
The design of high-performance heat sinks is essentially to minimize the scattering cross-section of the microstructure on heat conduction carriers under the premise of meeting the engineering requirements of strength, machinability, and corrosion resistance. This requires material scientists to go deep into the atomic scale to design alloy compositions and heat treatment regimes.
Limits and contradictions of pure metal systems
The thermal conductivity of industrial pure copper (Cu≥99.9%) is about 398 W/(m · K) at room temperature, and pure aluminum (Al≥99.5%) is about 237 W/(m · K). However, the mechanical properties of pure metals are extremely poor: the yield strength of pure copper is only about 70 MPa, and that of pure aluminum is less than 50 MPa. In heat sinks that need to withstand mechanical assembly stress, vibration shock or thread connection, pure metals can easily deform and slip. Therefore, practical heat sinks use alloying solutions without exception.
The cost of alloying is the introduction of solid-solution atoms. When 0.5% tin is dissolved in copper (to form bronze), the thermal conductivity plummets to about 150 W/(m · K); when 5% silicon is dissolved in aluminum (cast aluminum alloy), the thermal conductivity drops to about 150-180 W/(m · K). This attenuation is due to the local lattice distortion caused by the size mismatch between the solute atoms and the matrix atoms, which produces strong scattering of the propagating electron waves. Quantitatively, according to the Mattison rule, the total resistivity of the alloy can be decomposed into the sum of the matrix resistivity and the residual resistivity caused by the scattering of impurities, and the thermal conductivity decreases approximately linearly with the increase of impurity concentration.
III. Microstructural engineering of aluminum alloy grades
6063 aluminum alloy is currently the absolute main force of extrusion heat sinks. Its composition design revolves around the formation of Mg and Si to strengthen the phase of Mg -2 Si. After rapid quenching after solid solution heat treatment (520 ° C insulation), Mg and Si atoms are "frozen" to form a supersaturated solid solution in the aluminum lattice. At this time, the alloy has moderate strength but the lowest thermal conductivity (about 180 W/(m · K)). The subsequent artificial time effect (175 ° C insulation for 8 hours) prompts Mg -2 Si to disperse and precipitate in the form of nanoscale precipitates. On the one hand, the solute atoms in the lattice are consumed during the precipitation process (partial restoration of electron transport), and on the other hand, the precipitated phase itself becomes an obstacle to the movement of dislocations (increasing intensity). On the aging curve, there is a peak aging point (the highest intensity) and an overaging point. Heat sink designers often choose the over-aging state: although the strength is slightly reduced, the purity of the matrix is improved after more solute atoms are precipitated, the thermal conductivity can be increased from 180 to 210 to 230 W/(m · K), and the stress corrosion sensitivity is also reduced.
Similarly, 6061 aluminum alloy (containing Cu, Mn, etc.) is stronger, but the thermal conductivity is only about 167 W/(m · K), which is suitable for structural parts with extremely high mechanical requirements and secondary heat dissipation requirements. 1070 pure aluminum (thermal conductivity about 230 W/(m · K)) has little strengthening ability and is only used for pure aluminum layers in heat-conducting gaskets or composite heat sinks.
IV. Engineering trade-offs for copper alloys
High thermal conductivity copper alloys are mainly divided into two categories: C11000 pure copper (highest thermal conductivity) and C18200 chromium-zirconium copper. While retaining more than 80% pure copper thermal conductivity, chromium-zirconium copper increases the tensile strength to more than 350 MPa by precipitating intermetallic compounds of Cr and Zr, and the softening temperature is as high as 500 ° C (much higher than the 250 ° C of pure copper). This property makes it the first choice for heat dissipation substrates that need to withstand high temperature soldering or reflow processes, such as the copper layer on the bottom layer of DBC (direct copper cladding) ceramic substrates in power modules.
V. Permeation design of multiphase composites
In order to solve the contradiction between "high thermal conductivity" and "low density/low price", academia and industry have explored metal matrix composites. For example, the introduction of diamond particles in the aluminum matrix (natural thermal conductivity can reach 2000 W/(m · K)), Al-diamond composites formed by powder metallurgy or squeeze casting can exceed 550 W/(m · K), and the thermal expansion coefficient can be adjusted to match the chip (Si or SiC), greatly reducing thermal stress. However, the interface thermal resistance between diamond particles and aluminum is a bottleneck - carbide-forming elements such as Ti and Cr need to be coated on the surface to improve phonon matching.
Graphene/aluminum composites are even more advanced. Although the in-plane thermal conductivity of single-layer graphene is extremely high, the in-plane thermal conductivity of graphene in the composite is distributed in a disordered orientation, and the in-plane thermal conductivity advantage is difficult to exert. The thermal conductivity of the composite jumps significantly only when the graphene content exceeds the percolation threshold (about 2-5 vol%) and forms a connected network. After adding 5% reduced graphene oxide to the aluminum matrix at the highest level in the current laboratory, the thermal conductivity reaches 380 W/(m · K). However, this is still a triple challenge of dispersion uniformity, interfacial bonding, and cost.
Intrinsic thermal resistance and optimization of thermal interface materials
The heat sink must be in contact with the chip through the TIM. Even the best TIM (sintered silver, liquid metal) cannot completely eliminate the contact thermal resistance. Among them, the thermal conductivity of liquid metals (such as Ga-In alloy) can reach 30~ 40 W/(m · K), but the corrosion and surface tension problems are serious; although the filling coefficient of thermal conductive silicone grease is high, the silicone oil evaporates to form dry cracks after long-term aging, and the thermal resistance soars several times. The industry trend is to use phase change TIM: solid state at room temperature, the chip is melted into a liquid state after heating up to 45~ 50 ° C, filled with microscopic bumps, and solidified again after cooling. It has both easy installation and low thermal resistance (
VII. Conclusion
From pure aluminium to graphene/aluminium composites, the development of heat sink materials has always revolved around one core: minimising the scattering of heat-carrying particles while maintaining engineering suitability. The next generation of breakthroughs is likely to come from the structural design of phonon transport "metamaterials" - rather than relying solely on composition adjustment. This requires a deep intersection of heat transfer, solid state physics and powder metallurgy.
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