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The Introduction and Summary with Respect to Advantages and Disadvantages on Different Insulative Fillers with Thermal Conductivity

    Thermal conductive filler definition is added for increasing material coefficient of thermal conductivity of filler in a matrix material, which commonly uses thermal conductive filler with alumina, magnesium oxide, zinc oxide, aluminum nitride, boron nitride, silicon carbide and others. Especially micron sized alumina, silicon powder as the main body, nano aluminum oxide nitride powder in the field of high thermal conductivity fillers, and zinc oxide mostly for thermal paste (thermal grease filler). 
     All of these are used in aviation, aerospace, LEDs, precision electronic instruments, such as special areas of high thermal conductive fillers fibrous high thermal conductivity toner, scaly toner high thermal conductivity, and high thermal conductivity cloth. The Comparison of thermal conductivity of different fillers are showed in the following.

Note: The above data is from the following three papers:
1. Aluminum oxide in the thermal conductivity of the application of polymer composite materials, Li Bing, plastic additives, in 2008 third, pages 14~16
2. Metal substrate with high thermal conductive adhesive film research, fan Wang Kong, Guangdong Shengyi science and technology, the 11th CCL market technology seminar paper set, pages 101~106
3. Composite insulation thermal conductive adhesive research, Zhou Wenying and other Chinese adhesives in November 2006 fifteenth, Volume 11, pages 22~25

The following sections are viewpoints from journal papers, and its content is mostly from product engineers.
Analysis of advantages and disadvantages:
1. Aluminum Nitride-AlN.  Its advantage is very high thermal conductivity. The disadvantages are expensive, usually RMB 1000 or more per kilogram, and Aluminum Nitride moisture absorption will react with water AlN+3H2O=Al(OH)3+NH3, products produced by hydrolysis of Al(OH)3, thus allowing conduction path interruption by affecting phonon transmission, reducing thermal conductivity. Also, there is no guarantee that 100% filler surface is covered by silane coupling agent. Although Aluminum Nitride has a high thermal conductivity, the system viscosity is very high, and this severely limits product applications.
2. Boron Nitride-BN. Its advantages are thermal conductivity coefficient is very high and its stability. The disadvantages are its high price, ranging from several hundred to thousands of US dollars (according to different quality levels), and although Boron Nitride achieves higher thermal conductivity, which is similar to Aluminum Nitride, its viscosity rises very severely when filling a large quantity of systems, therefore limiting its product applications. There are foreign manufacturers producing spherical BN having large size, smaller surface area, and a high filling rate. However, it is not easy to increase viscosity, and its very high price.
3. Silicon carbide-SiC. Its advantage is high thermal conductivity. The disadvantage is carbon and graphite are difficult to remove in the synthesis process, which results in lower product purity and high electrical conductivity. Thus, it is not suitable for plastics. However, it is suitable in epoxy adhesives due to its high density and easy precipitation in organic silicone rubber layers.
4. Magnesium Oxide-MgO. Its advantage is low price. The disadvantages are ease to absorb moisture in air, high viscosity, and its filling limitations. In general, MgO is easily corroded by acid, which limits its application in acidic environments.
5. Alpha - alumina (needle). Its advantage is low price. The disadvantage is its added nominal amount. For example, ordinary aluminum oxide needle maximum amount is generally about 300-copies in liquid silica gel, which restricts product thermal conductivity.
6. Alpha alumina (spherical). Its advantage is a large amount of filling in liquid silica gel, spherical alumina maximum can be added to 600~800-copies, which results in high product thermal conductivity. The disadvantage is its expensive price, but it's still less than the Boron Nitride and Aluminum Nitride.
7. Zinc Oxide-ZnO. Its advantages are good uniformity particle size, and ZnO is suitable for thermal grease production. The disadvantages are its unstable thermal conductivity, which makes it inappropriate for product assembly due to lacking consistency, and its high viscosity. Hence, it's not suitable for potting.
8. Quartz powder (crystallization). Its advantages are high density, suitable for sealing; low price, and suitable for a wide filling application range, thus reducing cost. The disadvantages are its low thermal conductivity, which means it's not suitable for high thermal conductivity products, and its high density, which may produce stratification.
9. High thermal conductivity fibrous toner. Its advantages are its very high heat transfer coefficient, thermal conductivity fiber direction is 2-3 times of copper, reaching a maximum of 700 W/m*K, it has good mechanical properties, such as excellent conduction, heat radiation capacity, and its thermal design orientation. The disadvantages are its high price and it's not easily mixed with plastics.
10. High thermal conductivity flake toner. Its advantages are high heat conductivity coefficient, nano scale particle size, and its high filling rate capacity. The disadvantages are its bulk density and high price, but less than fibrous high thermal conductivity carbon.
       To sum up, different fillers have specific characteristics. The filler choice should make full use of its intrinsic advantages. Several fillers have mixed usages, which creates a synergistic effect, thereby achieving high thermal conductivity and effectively reducing cost while ensuring filler miscibility and silicone matrix.

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