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What are the factors that affect the performance of thermal paste?
The thermal conductivity of the filled adhesive mainly depends on the resin matrix, the thermal conductive filler and the interface formed by the two. The type, amount, particle size, geometry, hybrid filling and surface modification of the thermal conductive filler will conduct heat to the adhesive. Performance has an impact.
1) Type and amount of thermal adhesive filler
The type and amount of filler will affect the thermal conductivity of the adhesive. When the filler is small, the filler is completely encapsulated by the matrix resin, and most of the filler particles are not directly contacted; at this time, the adhesive matrix becomes a heat flow barrier between the filler particles, and the transfer of the filler phonon is suppressed, so no matter what is added What kind of filler does not significantly improve the thermal conductivity of the adhesive. As the amount of filler increases, the filler gradually forms a stable thermal conduction network in the matrix. At this time, the thermal conductivity increases rapidly, and filling the high thermal conductivity filler is more beneficial to improve the thermal conductivity of the adhesive. However, too much thermal conductivity of the filler is not conducive to the improvement of the thermal conductivity of the system. Studies have shown that when the ratio of the thermal conductivity of the filler to the matrix resin exceeds 100, the increase in thermal conductivity of the composite is not significant.
The data shown in the previous study example is used to illustrate the relationship between the amount of filler and heat transfer performance. After the addition of a highly thermally conductive filler to the adhesive, the thermal conductivity of the composite increases significantly as the amount of filler increases. Studies have shown that when w (synthetic diamond SD) = 20% (relative to the epoxy EP quality), the thermal conductivity is 0.335 W (/ m · K); when w (SD) = 50%, the heat The conductivity is 1.07 W(/m·K), which is 3.5 times higher than that of pure resin. When w(SD)<20%, the thermal conductivity of the system increases slowly; when w(SD)>20%, the system The thermal conductivity rises rapidly. This is because when w(SD)>20%, the particles begin to contact each other and gradually form a heat-conducting chain; when w(SD)=50%, a large amount of contact between the particles forms a heat conduction network, so the thermal conductivity is remarkable. improve
2) Particle size and geometry of thermal paste
When the amount of the thermal conductive filler is the same, the nanoparticles are more favorable than the microparticles to improve the thermal conductivity of the adhesive. The quantum effect of the nanoparticles increases the number of grain boundaries, so that the specific heat capacity increases and the covalent bond becomes a metal bond. The heat conduction changes from molecular (or lattice) vibration to free electron heat transfer, so the thermal conductivity of the nanoparticles is relatively more. At the same time, the particle size of the nanoparticles is small and the number is large, so that the specific surface area is large, and an effective heat conduction network is easily formed in the matrix, so that the thermal conductivity of the adhesive is improved.
For micro-particles, when the amount of filler is the same, the large-diameter heat-conductive filler has a small specific surface area and is not easily wrapped by the adhesive, so the probability of connecting to each other is large (it is easier to form an effective heat-conducting path), which is beneficial to the improvement of the thermal conductivity of the adhesive. .
In a specific case, the study shows that the thermal conductivity of the Al2O3 system with 30 nm is the highest when the amount of filler is the same, the thermal conductivity of the Al2O3 system with 20 μm is second, and the thermal conductivity of the Al2O3 system with 2 μm is relative. lowest. This is because when the amount of filler is the same, the specific surface area of the nanoparticles is larger than that of the microparticles, and the large specific surface area makes the formation of the heat conduction network higher than that of the microparticles; for the 20, 2 μm Al2O3 filled system, the smaller particle size It has a larger specific surface area and more phase interface with the substrate, so it is more easily wrapped by the substrate and cannot form an effective heat conduction network. Therefore, the thermal conductivity of the 2 μm Al2O3 filling system is relatively low.
When the amount of filler is the same, the heat transfer network has different probabilities in the same kind of fillers of different geometries. The heat transfer filler with larger aspect ratio is more likely to form a heat conduction network, which is more beneficial to improve the thermal conductivity of the matrix. On the number, studies have shown that when φ (nanoscale silver wire) = 26% (relative to the epoxy resin EP adhesive volume) reaches the percolation threshold, the thermal conductivity increases from 5.66 W (/ m·K) to 10.76 W (/ m·K); the percolation threshold is reached when φ (nano-scale silver rod) = 28%, φ (nano-scale silver block) = 38%; the larger the aspect ratio, the smaller the percolation threshold. Compared with silver rods and silver blocks, the silver wire with a large aspect ratio increases the probability of forming a heat-conductive mesh chain in the resin system due to its orientation, and a higher thermal conductivity can be achieved when the filler is small.
3) Mixed filling of thermal adhesive filler
Compared with the single-particle filler filling system, the hybrid filling of different particle sizes and the same kind of filler is more beneficial to improve the thermal conductivity of the adhesive. Hybrid filling of different forms of the same kind of filler is easier to obtain a high thermal conductivity adhesive than filling with a single spherical filler. Hybrid filling is also superior to a single type of packing when the different types of fillers are properly proportioned. This is attributed to the fact that the above-mentioned hybrid filling is relatively easy to form a close-packed structure, and the high aspect ratio particles in the hybrid filling tend to bridge between the spherical particles, thereby reducing the contact thermal resistance, thereby making the system relatively higher. Thermal conductivity. Studies have shown that when w (AlN) = 80% (relative to the quality of silicone rubber), the particle size is 15, 5 μm, respectively, the thermal conductivity of the system is 1.83, 1.54 W (/ m · K); When the total amount of AlN is constant and the mass ratio of the two particle sizes is 1:1, the thermal conductivity of the system is 1.85 W(/m·K). The size and particle size doping has higher thermal conductivity than the single particle size. This is because when the size particle size is doped, the small particle size particles are more easily filled into the voids of the large particle size (increasing density), so that the particles are The contact between the two is more tight, and the packing density of the filler inside the substrate is increased (the contact thermal resistance is reduced), thereby increasing the thermal conductivity of the system.
4) Surface modification of thermal adhesive filler
There is a difference in polarity between the inorganic particles and the resin matrix interface, resulting in poor compatibility between the two, so that the filler is easily aggregated in the resin matrix (not easily dispersed). In addition, the large surface tension of the inorganic particles makes the surface difficult to be wetted by the resin matrix, and there are voids and defects between the phase interfaces, thereby increasing the interface thermal resistance. Therefore, the modification of the surface of the inorganic filler particles can improve the dispersibility, reduce the interface defects, enhance the interfacial adhesion strength, suppress the scattering of phonons at the interface, and increase the propagation free path of the phonons, thereby contributing to the improvement of the heat of the system. Conductivity.