Research progress of thermally conductive organosilicon materials

Research progress of thermally conductive silicone materials
Abstract: Although silicone materials have excellent high and low temperature resistance, electrical insulation, weatherability, water repellency, corrosion resistance, etc., but its thermal conductivity is poor, it is difficult to meet the aerospace, electrical and electronic, high-frequency communications and other fields of high-performance equipment and miniaturization needs. In recent years, the use of a variety of thermally conductive fillers to modify silicone materials to give their thermal conductivity has become one of the hot spots of research. This paper introduces the mechanism of thermal conductivity of silicone materials, focuses on the review of recent years, thermal conductive silicone grease and thermal conductive silicone rubber research progress, and looks forward to the direction of its development.
Keywords: thermal conductivity mechanism, silicone grease, silicone rubber, graphene, carbon nanotube
00 Introduction
Silicone materials are polymer materials with silicone-oxygen bonding as the main chain, which have excellent high and low temperature resistance, electrical insulation, weatherability, water repellency and chemical resistance, as well as excellent processing performance and easy molding, and have become a class of important polymer materials for a wide range of applications, and have played a very important role in promoting the rapid development of China’s aviation and aerospace, electrical and electronic electronics, and high-frequency communications and other key fields. However, the key electronic equipment in these fields tends to be high performance and miniaturization, resulting in a large amount of heat generated and rapidly accumulated during operation. Studies have shown that, for electronic devices, the temperature rises 2 ℃, its reliability decreased by 10%; and transformer winding temperature rises 6 ℃, its life expectancy will be shortened by half. Although the comprehensive use of silicone materials with excellent performance, but its thermal conductivity is only 0.2W / (mk), heat transfer efficiency is very low. Therefore, it is necessary to modify silicone materials to make them have good thermal conductivity in order to meet the development needs of high performance and miniaturization of equipment. This paper introduces the mechanism of thermal conductivity of silicone materials, focusing on a review of recent years, thermal conductivity of silicone materials, especially thermal conductive silicone grease and thermal conductive silicone rubber research progress, in order to engage in the development of thermally conductive silicone materials for researchers to provide reference.
01 Organo-silicone material thermal conductivity mechanism
Like most polymer materials, silicone materials have no free electrons and phonons, and their heat conduction is mainly realized through the vibration of molecular chains, which is a poor conductor of heat. Therefore, the need to add thermally conductive fillers to give silicone materials good thermal conductivity. Commonly used thermal conductivity filler can be divided into three categories: one is a metal material, such as silver, copper, aluminum, etc.; the second is a carbon material, such as graphite, diamond, carbon nanotubes, carbon fibers, graphene, etc.; the third is a ceramic material, such as alumina, boron nitride, aluminum nitride, silicon carbide, zinc oxide, etc.. Common thermal conductivity filler thermal conductivity is shown in Table 1.
Table 1. Thermal conductivity of common thermally conductive fillers

For the same kind of thermally conductive filler, the amount, shape and dispersion is the main factor affecting the thermal conductivity of silicone materials. When the amount of filler is low, the filler particles in the silicone matrix contact less, difficult to form a thermally conductive pathway, the thermal conductivity of the silicone material is basically no improvement. When the amount of thermally conductive filler increased to a certain value, the particles contact each other more, forming a thermally conductive pathway, the silicone material from a poor conductor of heat into a good conductor of heat, this transformation is called “overdiffusion”. With the increase in the amount of thermally conductive filler, the system will form more thermally conductive pathways, the thermal conductivity of silicone materials to further improve. When the thermal pathway and the direction of heat flow parallel to the thermal conductivity will be significantly improved. When the amount of thermally conductive filler continues to increase, the thermal conductivity network in the system will gradually reach saturation, the filler will increase the mutual accumulation of phonon scattering, thus generating thermal resistance, so that the increase in thermal conductivity of the material slows down, such as
Influence of filler amount on the thermal conductivity of polymers
Compared with zero-dimensional thermally conductive fillers, the same amount of carbon nanotubes, carbon fibers and other one-dimensional materials and graphene, hexagonal boron nitride, flake alumina and other two-dimensional materials in the silicone matrix to form a larger contact area, which is conducive to the construction of thermal conductivity network. Compared with the small particle size thermally conductive filler, large particle size filler in the silicone matrix interfacial contact less, lower interfacial thermal resistance, better thermal conductivity. However, it is difficult to form a close buildup between large-size thermally conductive fillers, which is not conducive to the formation of thermally conductive pathways. In general, the use of different particle size of the thermal conductivity of the filler can be used together to obtain good thermal conductivity. Most of the thermal conductive fillers are polar, while the silicone material is non-polar, so the thermal conductive fillers tend to accumulate in the silicone material, and it is difficult to form an effective thermal conductive pathway. The use of silane coupling agent surface modification of thermally conductive filler, can realize its uniform dispersion in the silicone material, while reducing the filler and the interface between the silicone matrix thermal resistance.
02 Thermally conductive silicone materials research progress
At present, thermally conductive silicone materials are mainly thermally conductive silicone grease and thermally conductive silicone rubber. Among them, thermally conductive silicone grease is a paste containing silicone oil grease, can be filled into the small gaps in the electronic devices to form a good contact, thereby reducing the contact thermal resistance, improve the heat dissipation effect; thermally conductive silicone rubber in the form of crosslinked solid form exists, divided into room temperature vulcanization and hot vulcanization type, the product has a thermal pads, heat sinks, potting adhesive and so on, is mainly used in the electronic and electrical industries, such as heat dissipation, insulation, sealing, and so on.
2.1 Thermally Conductive Silicone Grease

Thermally conductive silicone grease, also known as thermal grease, thermal grease, etc., is a class of methyl silicone oil, methyl phenyl silicone oil and other silicone oil matrix in the addition of thermally conductive fillers and thickening agents, lubricants and other additives, and through the mixing of thermally conductive silicone products. Thermally conductive silicone grease has the appearance of a paste-like viscous liquid, can be filled with a variety of gaps, mainly used in high-power heating components and heat sinks, heat sinks, heat sinks and other heat dissipation facilities between the contact surfaces, play a role in heat transfer, moisture, dust, corrosion, vibration and so on.
The thermal conductive silicone grease with a thermal conductivity of 5.7 W/(mk) was prepared by combining γ⁃methacryloyloxypropyltrimethoxysilane (KH570) modified spherical Ag powder, flaky Ag powder, and spherical aluminum powder with poly(methylphenylsiloxane) and hydroxyl-capped poly(dimethylsiloxane) as the bases to construct a thermal conductivity pathway (Fig. 2). In addition, due to the hydrogen bonding in the hydroxyl silicone oil and the interfacial effect of the thermal conductive nano-fillers to hinder the movement of the silicone chain segments, the silicone grease also has a good resistance to oil seepage. Under the simulated spacecraft environment, the grease shows excellent heat dissipation, and the temperature of the heating element is reduced from 39.4 ℃ to 37.1 ℃ without cracking or oil seepage under high and low temperature thermal cycling conditions. Although the thermally conductive silicone grease has a high thermal conductivity, but because of the use of silver powder as a thermally conductive filler, the cost is high.
Thermal conductivity pathway modeling with different shapes of thermal conductive fillers
Comparatively speaking, the thermal conductivity of metal oxides, nitrides and carbides is not as good as that of metallic silver, but they are inexpensive and thus favored by researchers. Lei Shucao et al. prepared a series of thermally conductive silicone grease by adding Al2O3, 1 μm ZnO and 2 μm aluminum powder with different particle sizes and dosages, using dimethyl silicone oil as the base material. When the mass ratio of dimethyl silicone oil, 30 μm Al2O3, 10 μm Al2O3, 2 μm Al2O3, ZnO and Al is 1∶4.5∶2.25∶0.75∶2.5∶0.31, the resulting silicone grease has both fluidity, high thermal conductivity and good insulating properties, with a viscosity of 57000 mPa-s and a thermal conductivity of 3.12 W/(mk). Its viscosity is 57000mPa-s, thermal conductivity is 3.12 W/(mk), and volume resistivity is 1.35×1014Ω-m at 500V. Xixiang chose dimethyl silicone oil as the base material, adding 80% of AlN, to produce thermal conductivity of 1.218W/(mk) of thermal conductivity of silicone grease. On this basis, the thermal conductivity of the silicone grease reaches 1.623W/(mk) with the introduction of the thermal conductive fillers of flake graphite and micron silver rods, and the thermal conductivity of the silicone grease reaches 1.623W/(mk) when the mass ratio of AlN, graphite and silver rods is 10∶6∶1. Kang et al. used zinc stearate to surface treat different thermally conductive fillers (including Al2O3, AlN, SiC and graphite) and mixed with dimethyl silicone oil to prepare thermally conductive silicone grease. It was found that after the modification of AlN by zinc stearate, the dispersion and compatibility of AlN in silicone oil were improved, and the thermal conductivity of silicone grease was increased. Compared with the pre-modification, the modified SiC and graphite decrease the thermal conductivity of the silicone grease, which is mainly due to the fact that the zinc stearate changes the filler surface from lipophilic to oleophobic, which makes it difficult for the thermally conductive filler to combine with the silicone grease, and the thermal resistance increases. In addition, the highest thermal conductivity of 0.95 W/(mk) was obtained when the mass ratio of spherical Al2O3 to dimethicone was 7:1 and the particle size was 40 μm. Ge et al. first modified spherical BN particles in ethanol by using MQ silicone resin to produce MQ silicone resin-coated BN particles, and then added it to polydimethylsiloxane to prepare a thermally conductive silicone resin with thermal conductivity of 1.22 W/((mk)) and thermal resistance of 1.22 W/((mk)), and the thermal resistance of 1.22 W/((mk)). Then it was added into polydimethylsiloxane to produce a thermal conductivity of 1.22 W/(mk) and thermal resistance of 0.49 ℃/W, which is mainly used in the field of battery thermal management.

Among all the thermally conductive fillers, graphene and carbon nanotubes as the representative of nanocarbon materials have the best thermal conductivity, and can give good thermal conductivity to the silicone grease with less dosage.Yu et al. synthesized graphene nanoparticles (GNP) and reduced graphene oxide (RGO) chemically from natural graphite flakes (NFG), and added them into silicone grease by mechanical colloidal milling method to produce two kinds of thermally conductive silicone greases. Two kinds of thermally conductive silicone greases, GNP⁃SO and RGO⁃SO, were prepared, and compared with the silicone grease with NFG as the thermal conductive filler (NFG⁃SO). It is found that when the volume fraction of the thermally conductive filler is 1%, the thermal conductivity of the silicone grease is most obviously improved by RGO, and the thermal conductivity of RGO ⁃SO reaches 0.31 W/(mk), while the thermal conductivity of GNP ⁃SO and NFG ⁃SO is 0.26 W/(mk) and 0.17 W/(mk), respectively. However, when the volume fraction of the thermally conductive filler exceeds 1.25%, the viscosity of RGO⁃SO increases sharply and loses its fluidity, while the viscosity of GNP⁃SO does not increase significantly, and its thermal conductivity reaches 1.03 W/(mk) at 4.25% of the amount of GNP. Guo et al. prepared silicone greases with multi-walled carbon nanotubes (MWCNTs) as the thermally conductive filler by the hot-pressing method, and found that the thermal conductivity of silicone greases decreases with the shortening of the length of MWCNTs, and the thermal conductivity of the greases increases with the shortening of the length of the MWCNTs. It was found that the thermal conductivity of the silicone grease increased with the shortening of MWCNT length. This is mainly due to the fact that when MWCNT is long (50-60 μm), it is easy to be entangled with each other to form clusters and aggregates, and the distribution is not directional (see Fig. 3), and the thermal conductivity of the silicone grease produced is only 0.57 W/(mk). When the length of MWCNT is shortened to 2-3 μm, the above mentioned tangles are significantly reduced, and the thermal conductivity of silicone grease reaches 2.112 W/(mk) with the enhancement of the directional distribution of thermally conductive fillers. The thermal conductivity of the silicone grease reaches 2.112 W/(mk), and the dispersion of the fillers in the silicone grease becomes more homogeneous after the surface treatment of MWCNTs with strong acids and bases, which leads to the construction of more thermal conduction paths, and the thermal conductivity of the silicone grease is further increased to 4.267 W/(mk).
Thermal conductivity modeling of silicone grease with MWCNTs addition
Thermally conductive silicone grease was prepared by dispersing micron-sized copper metal particles and KH570-modified commercially available graphene into dimethicone oil using a ball mill. It was found that the thermal conductivity of the silicone grease was 0.96 W/(mk) when the small-size copper powder accounted for 20% of the total copper powder volume. Compared with unmodified graphene, the dispersion of modified graphene in dimethyl silicone oil was improved. On the basis of copper powder as a thermally conductive filler, continue to add 2% modified graphene mass fraction, the thermal conductivity of the silicone grease increased to 2.3 W / (mk).
2.2 Thermally conductive silicone rubber
Unlike paste-like thermally conductive silicone grease, thermally conductive silicone rubber is solid and highly elastic after vulcanization, and is widely used in the aerospace, electrical and electronic, instrumentation and other industries for elastic bonding, heat dissipation, insulation, sealing and vibration damping, and is mainly used in the form of heatsinks, pads, and sealant or potting adhesive products. Thermally conductive silicone rubber is mainly composed of silicone rubber matrix, thermally conductive filler, reinforcing agent, vulcanizing agent, etc., in the process will also be used in diluent, vulcanization accelerator, structure control agents, plasticizers, crosslinking agents and other additives. Silicone rubber matrix is mainly dimethyl silicone rubber, methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber and so on. Similar to the thermal conductivity of silicone grease, used to improve the thermal conductivity of silicone rubber filler including metals, metal oxides, nitrides, carbides, carbons and their compounds.
Methyl vinyl silicone rubber as a substrate, respectively, aluminum powder, nickel powder, silver-plated copper powder and other metal powders as thermally conductive fillers, the preparation of different thermally conductive silicone rubber. It was found that the thermal conductivity of the silicone rubber could only be increased to 0.4 W/(mk) by increasing the amount of metal powder to more than 300 parts, so the thermal conductivity of the silicone rubber prepared with metal powder as the thermally conductive filler was not satisfactory. Liu et al. used stearic acid, KH570 and vinyl tris (2⁃methoxyethoxy) silane (A⁃172) modified ZnO nanometer as thermal conductive filler, and dispersed them uniformly in the addition of room temperature vulcanization silicone rubber and vulcanization molding to produce thermally conductive silicone rubber. At the dosage of 30 parts of thermally conductive filler, the silicone rubber prepared by A⁃172 modified ZnO showed the best thermal conductivity of 0.47 W/(mk), which was higher than that of the silicone rubber made from stearic acid and KH570 modified ZnO system [0.31 W/(mk) and 0.43 W/(mk), respectively]. Liao et al. prepared thermally conductive and insulating silicone rubber with methyl vinyl silicone rubber as the matrix and γ⁃aminopropyltriethoxysilane modified hexagonal flake BN as the thermally conductive filler by the refining-opening-molding process. It was found that the thermal conductivity and dielectric properties of the silicone rubber increased gradually with the increase of the amount of modified BN. When the amount of BN was 50%, the thermal conductivity and relative dielectric constant of the silicone rubber were 1.13 W/(mk) and 3.87, respectively.Wu et al. prepared polypyrrole-coated alumina as a thermally conductive filler and dispersed it into methyl vinyl rubber by using the near-micellar polymerization reaction.
The polypyrrole-coated alumina was prepared by Wu et al. using a near micellar polymerization reaction, and dispersed as a thermally conductive filler into a mixture of methyl vinyl silicone rubber, silica, and hydroxyl silicone oil. It was found that polypyrrole was beneficial to improve the interfacial compatibility between the thermally conductive filler and the silicone rubber. When the mass fraction of polypyrrole-coated alumina was 83%, the thermal conductivity of the silicone rubber was 1.98 W/(mk), and the tensile strength was 2.9 MPa. Yang et al. synthesized a strawberry shaped alumina ⁃ poly(catechol ⁃ polyamine) ⁃ silver hybrid thermally conductive filler by surface modification of alumina particles with poly(catechol ⁃ polyamine) (PCPA) and generating silver nano-particles on the PCPA by electroplating method. A strawberry-shaped alumina ⁃ poly(catechol ⁃ polyamine ⁃ silver) hybrid thermally conductive filler was synthesized and added into silicone rubber to prepare thermally conductive silicone rubber. PCPA can effectively reduce the thermal resistance, and the thermal conductivity of thermally conductive silicone rubber is 0.4367 W/(mk) when the volume fraction of hybridized thermally conductive filler is 30%. As can be seen from the above examples, the use of metal powders, metal oxides, nitrides and their modifiers to prepare thermally conductive silicone rubber