Conductive adhesive (as a substitute for Pb/Sn solder) has great potential for installation in micro-components, bonding of complex lines, etc. [3]; however, electronic components are moving toward miniaturization and lightness. It will accumulate a large amount of heat during operation (if the heat cannot be dissipated in time, it is easy to form local overheating), which will reduce the reliability and working life of the system [4-6]. Therefore, the development of high thermal conductivity, comprehensive performance of thermal insulation (or thermal conductive) adhesive is the key to achieving heat dissipation of electronic components.


With the advancement and development of science and technology, the contemporary electronic information industry has developed rapidly, and people's requirements for the power of electronic devices are constantly improving. Fast and efficient heat dissipation and upgrade of electronic and electrical cooling systems have become the key to modern miniaturized electronic products. For every 2 °C increase in temperature, the stability drops by 1/10, and the temperature rises at 50 °C, which is 1/6 when the temperature rises by 25 °C [1]. The luminous efficiency of LEDs is higher than that of ordinary light sources, but the energy utilization rate is still less than 20%, which means that more than 80% of the energy is not converted into the type of energy that humans need, and it is lost in the form of unusable heat. The heat-conducting medium has metal, metal oxide and some non-metal materials, and has good thermal conductivity, but has a large specific gravity, is difficult to process, and is not resistant to corrosion. The thermal conductivity of common fillers is shown in Table 1.






The thermal conductive adhesive is mainly composed of a resin matrix [EP (epoxy resin), silicone and PU (polyurethane), etc.) and a thermally conductive filler. This study reviews the effects of the type, amount, geometry, particle size, hybrid filling and modification of the thermally conductive filler on the thermal conductivity of the thermal conductive adhesive, and aims to provide a reference for future research.


1 Thermal conduction principle of thermal adhesive


The solid internal heat conducting carrier is mainly electrons and phonons. There are a large number of free electrons inside the metal, which can transfer heat through the collision between the electrons; the inorganic non-metal crystals conduct heat through the thermal vibration of the aligned crystal grains, which is usually described by the concept of phonons [7]; Forming very fine crystals, so amorphous heat conduction can also be analyzed by the concept of phonons, but its thermal conductivity is much lower than that of crystals [8]; most polymers are saturated systems, no free electrons exist, so their heat conduction Mainly through lattice vibration. The thermal conductivity of a polymer depends primarily on the crystallinity and orientation direction (ie, the degree of scattering of the phonons). The random entanglement of polymer molecular chains and the large M (r relative molecular mass) lead to low crystallinity, while the molecular size is not equal and the polydispersity of Mr makes the polymer unable to form intact crystals [9]; Molecular chain vibration, resin interface and defects can cause phonon scattering, so the thermal conductivity of the polymer is poor.


The addition of a highly thermally conductive filler to the adhesive is the primary means of improving its thermal conductivity. The heat conductive filler is dispersed in the resin matrix and contacts each other to form a heat conduction network, so that heat can be rapidly transferred along the "heat conduction network", thereby achieving the purpose of improving the thermal conductivity of the adhesive.


2 Factors affecting the thermal conductivity of the adhesive


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.


2.1 Types and dosages of thermal conductive fillers


The type and amount of filler will affect the thermal conductivity of the adhesive. When the filler is small, the filler is completely wrapped 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 [10]. However, too much thermal conductivity of the filler is not conducive to the improvement of the thermal conductivity of the system. Zhou Wenying et al. [9] showed that when the ratio of the thermal conductivity of the filler to the matrix resin exceeds 100, the thermal conductivity of the composite is not significantly improved. Therefore, the rational use of thermally conductive fillers is essential to improve the thermal conductivity of the adhesive. He Bingbing et al [11] doped the same amount of alumina (Al2O3) and boron nitride (BN) into the epoxy resin (EP) potting compound. Studies have shown that when φ (filler) <15% (relative to the total volume of potting compound), the thermal conductivity of BN/EP and Al2O3/EP is not much different; when φ (filler) is >15%, BN The thermal conductivity of /EP is much larger than the thermal conductivity of Al2O3/EP, and the difference between the thermal conductivity of the two increases with the amount of filler; when φ(BN)=35%, the thermal conductivity of BN/EP It is 2.12 W/(m·K). The thermal conductivity of synthetic diamond (SD), BN, and silica (SiO2) are 2 000, 250, and 1.5 W/(m·K), respectively, while Firdaus et al. [12] fills EP with nanoscale SD, BN, and SiO2. . Studies have shown that SD/EP has the highest thermal conductivity, flexural strength and modulus when the amount of filler is the same.


The thermal conductivity of different kinds of fillers varies greatly. 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. Lin Xuechun [13] et al. added SD (particle size 10 μm) to EP. Studies have shown that when w(SD) = 20% (relative to EP mass), the thermal conductivity is 0.335 W / (m · K); when w (SD) = 50%, the thermal conductivity is 1.07 W / (m·K), which is 3.5 times higher than pure resin; when w(SD)<20%, the thermal conductivity of the system increases slowly; when w(SD)>20%, the thermal conductivity of the system 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. Park et al [14] prepared xGIC (expanded graphite intercalation compound) / UV (ultraviolet light) cross-linked acrylate resin PSA (pressure sensitive adhesive). Studies have shown that when w(xGIC) = 20% (relative to the quality of PSA), the thermal conductivity of PSA is 0.46 W / (m·K), which is 2.89 times higher than that of pure acrylate resin. This is because xGIC has a high thermal conductivity and a large aspect ratio, which is conducive to the formation of a heat conduction path.


2.2 Particle size and geometry of thermally conductive filler


When the amount of 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. High [15]; 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. . Yan Changlong et al. [16] added Al2O3 with a particle size of 0.030, 20, and 2 μm to the silicone resin. The results show that the thermal conductivity of Al2O3 system with 30 nm is the highest when the amount of filler is the same, the thermal conductivity of Al2O3 system with 20 μm is second, and the thermal conductivity of Al2O3 with 2 μm is the 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 lowest.


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. Xia Yanping [17] added nanoscale silver wires, silver bars and silver blocks with length to diameter ratios of 33, 15, and 1, respectively, to the EP adhesive. Studies have shown that the percolation threshold is reached when φ (nanoscale silver wire) = 26% (relative to the EP adhesive volume), and the thermal conductivity increases from 5.66 W/(m·K) to 10.76 W/(m·K). When the φ (nano-scale silver rod) = 28%, φ (nano-scale silver block) = 38%, the percolation threshold is reached; 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. Fu et al [5] added graphene, graphite nanosheets and graphite powder to EP to prepare thermal paste. Studies have shown that when w (graphene) = 10.10% (relative to EP mass), the thermal conductivity of graphene / EP is as high as 4.01 W / (m · K), 22 times that of pure EP, which is 16.81%. Graphite nanosheets/EP are 2.2 times, which is 2.4 times that of 44.3% graphite powder/EP. This is because graphene has high thermal conductivity and ultra-thin layered structure, and the ultra-thin layered structure of graphene helps to form a three-dimensional heat conduction network in the resin matrix, thereby obtaining high thermal conductivity; thickness of graphite nanosheets. It is 10 times that of graphene, so the heat conduction path formed by graphite nanosheets is less than graphene, and graphite powder is difficult to form a three-dimensional heat conduction network due to the thick layered structure, so only some heat conduction chains can be formed.


2.3 Hybrid filling of thermally conductive 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.


Zhu Ningning [18] added AlN (aluminum nitride) to silicone rubber. Studies have shown that when w(AlN)=80% (relative to the quality of silicone rubber) and the particle size is 15 and 5 μm, respectively, the thermal conductivity of the system is 1.83, 1.54 W/(m·K), respectively. 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. Chiang et al. [19] used alicyclic epoxy (UVR-6110), hexahydrophthalic anhydride as curing agent, 1-cyanoethyl-2-ethyl-4-methylimidazole as accelerator, and different shapes of micron silver. A filler is used to prepare an electrically conductive thermally conductive silver paste for bonding LED components. Studies have shown that when w(EP) = 100%, w (anhydride) = 85%, w (accelerator) = 1%, w (spherical silver with a particle size of 1.25 μm) = 40% and w (particle size 15 μm) When the flake silver) = 40% (both relative to the quality of the silver paste), the performance of the hybrid filled silver paste is relatively optimal [thermal conductivity up to 3.2 W / (m·K), resistance is 1.11 × 10 - 4 Ω ·cm]. The small-diameter spherical silver is filled in the pores of the large-sized flaky silver to form a dense stacked structure, which increases the contact area between the silver particles, thereby forming an effective heat conduction network, so the thermal conductivity of the adhesive is high.


Draman et al. [20] prepared a p-toluenesulfonic acid modified polypyrrole/epoxy fiber-based thermal conductive adhesive. Studies have shown that when w(SiC) = 5% (relative to the quality of the adhesive), m (SiC): m (Si3N4) = 80: 20 or 20: 80, the thermal conductivity is 0.528, 0.636, 0.572 W, respectively. / (m·K). The synergistic effect of the hybrid fillers promotes the interaction between the fillers to form a structural network, which is helpful for heat conduction. The hybrid fillers facilitate the network-type dispersion of the fillers in the matrix, so the thermal conductivity of the hybrid filling system is better than that of the single filling system. In addition, the m(SiC):m(Si3N4)=80:20 system is superior to the m(SiC):m(Si3N4)=20:80 system due to the smaller particle size, larger specific surface area and absorption at the interface. Can be higher.


Yuan et al [21] filled the EP with AlN/graphene oxide (GO) composite filler. Studies have shown that when w(AlN) = 50%, w(GO) = 6% (relative to EP mass), the thermal conductivity of the system [up to 2.77 W / (m·K)] is much higher than w ( AlN) = 70% of the thermal conductivity of a single filler filled with EP. This is because when mixed and filled, the sheet-like GO having a large aspect ratio acts as a bridge between the AlN particles to form an effective heat conduction path, and the AlN suppresses the agglomeration of the GO, thereby increasing the interface area, so the interface The thermal resistance is reduced; in addition, the functional group of GO reacts with EP to form a covalent bond, which increases the interface strength, so the interface thermal resistance decreases.


2.4 Surface modification of thermally conductive 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.


Li et al. [22] prepared a three-layer structure for LED heat dissipation composed of two substrates and an adhesive, and a BN-filled EP adhesive modified with a hydroxylated-silane coupling agent (KH-550). Studies have shown that when w(BN) = 75% (relative to EP adhesives)
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