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The effect of alumina filler for thermal silica gel is poor?

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With the development of the electronics industry, electronic components and integrated circuits tend to be dense and miniaturized, and "heat" has become the prime enemy of the operation of electrical appliances. In order to avoid electrical failures caused by poor heat dissipation to the greatest extent, thermally conductive silica gel is generally applied to the contact surface between the heating element of the electronic product and the heat dissipation facility.

Thermally conductive silica gel is a product that has a certain degree of flexibility, excellent insulation, compressibility, and natural viscosity on the surface. It is specially produced for the design of using the gap to transfer heat. In addition to filling the gap and completing the heat transfer between the heating part and the heat dissipation part, it also plays the role of insulation and shock absorption, meeting the design requirements of equipment miniaturization and ultra-thinness, and the thickness is applicable to a wide range, so it is a kind of extreme Good thermal conductivity filler materials are widely used in electronic and electrical products.

However, ordinary silica gel is a poor conductor of heat, so it is necessary to add suitable thermally conductive fillers to improve its thermal conductivity. Among the inorganic non-metallic thermally conductive insulating powder fillers, alumina not only has good insulation properties, but also its thermal conductivity is not low. (The thermal conductivity at room temperature is 30W/m·K). With various advantages, it has become one of the most commonly used thermal conductive fillers.

However, it is said that the thermal conductivity is good. In fact, it is still a long way from "good" (for example, the thermal conductivity of aluminum nitride is 150W/m·K, which is at least five times that). The best place for alumina should be It's cost-effective. In order not to lose this advantage, and at the same time to improve the application advantages of alumina, it is necessary to find ways to improve the thermal conductivity of silica gel under the premise of the same raw material. There are two main options, one is that the filler can form a heat-conducting chain or a heat-conducting network in the matrix; the other is to improve the thermal conductivity of the alumina filler itself.

1. The formation of thermal network

According to thermodynamics, heat conduction is the transfer of thermal energy in the form of vibrational energy, that is, the collision and transfer of microscopic particles within the substance. Since silica gel is a polymer material composed of asymmetric polar chain links, the entire molecular chain cannot move completely freely, and only the vibration of atoms, groups or chain links can occur, so the thermal conductivity is very low, and it is a poor conductor of heat. , Only by filling high thermal conductivity fillers to increase the thermal conductivity of the material.

When the amount of filler added is small, the distribution of the filler in the silica gel matrix is approximately in the form of islands, and the thermal conductivity is not greatly improved at this time. In order to improve the thermal conductivity of the thermally conductive insulating silica gel, the amount of alumina filled in the silica gel must be increased, so that the alumina particles form a thermal conduction channel inside the material. However, blindly increasing the filling amount of alumina will affect the process performance of the silica gel system and the performance of the product. Generally speaking, when alumina is filled into the thermally conductive material, the tensile strength of the thermally conductive material will increase with the increase of the filling amount. And the hardness gradually increases, while the flexibility of the material gradually deteriorates, and its elongation at break continues to decrease. This is because the alumina is filled into the polymer composite, and the alumina powder strengthens the matrix.

Therefore, when preparing high thermal conductivity insulating silica gel materials, one cannot simply rely on increasing the filling amount to increase thermal conductivity, because thermally conductive insulating silica gel not only requires the thermal conductivity of the material, but also has requirements for viscosity, compressibility, and flexibility. If you want to further improve the thermal conductivity of the thermally conductive silica gel material, you have to improve the performance of the alumina filler itself. In addition, the use of thermally conductive fillers of different particle sizes and shapes and different types of thermally conductive fillers can also take advantage of the characteristics of various fillers, improve the thermal conductivity of the material, and reduce costs.

For thermal silica, viscosity, compressibility, and flexibility are also important

2. Improve the thermal conductivity of alumina itself

To increase the thermal conductivity of alumina itself, the crystallinity and density of the crystals must be improved. Therefore, the alumina filler must have a high α-phase content. This is because α-phase alumina has a hexagonal structure, which is the most important of the alumina variants. Dense structure. In addition, α-phase nano-alumina also has the characteristics of high hardness, high strength, heat resistance, corrosion resistance, etc., and its preparation has a variety of process routes, mainly using the following:

1. Chemical pyrolysis

Chemical pyrolysis methods include ammonium alum pyrolysis method, ammonium aluminum carbonate pyrolysis method and spray pyrolysis method.

① The ammonium alum pyrolysis method uses ammonium aluminum sulfate solution to react with ammonium sulfate to prepare ammonium alum, which is then heated to decompose into nano-alumina. The process is simple, but the production cycle is long and it is difficult to achieve large-scale;

②The ammonium alum pyrolysis method is improved to form the ammonium aluminum carbonate pyrolysis method. At present, it has been reported that the ammonium alum and ammonium bicarbonate are reacted to prepare the ammonium dawsonite precursor precipitation, and then burned at 1200°C to produce Obtained alumina powder with a particle size of 15nm;

③The spray pyrolysis method is to spray the metal salt solution into a high temperature atmosphere in the form of a mist, so that the water in it evaporates, the metal salt is thermally decomposed, the solid phase is precipitated, and the nano alumina ceramic powder is directly prepared.

2. Amorphous crystallization method

This method is mainly to crystallize the amorphous compound after annealing. This method can produce nano-materials with accurate composition, and does not need to undergo a molding process, and nano-alumina can be directly prepared from an amorphous state. The plasticity of the nanostructured materials produced by this method is very sensitive to the size of the crystal grains. Only when the particle size is small, the plasticity is better, otherwise the material becomes very brittle. This method has simple equipment and technology, high yield, low cost, and small environmental pollution, but the product has uneven particle size distribution and is easy to agglomerate.

3. Sol-gel method

This method is widely used in the preparation of oxide nanopowders. The chemical process is to hydrolyze the raw materials to generate active monomers, then polymerize them into sols, and then generate gels with a certain structure, and finally dry and heat treatment to obtain nanoparticles. The whole reaction is: molecular state-polymer-sol-gel-crystalline (or amorphous) process.

The sol-gel method has low reaction temperature, controllable product crystal shape and particle size, high particle uniformity, high purity, easy control of the reaction process, few side reactions, but the problem of product agglomeration is significant, and when organic materials are used as raw materials, the toxicity is high and the price is high. high.

4. Liquid phase precipitation method

The liquid-phase precipitation method is to precipitate the active ingredients in the raw materials through a chemical reaction in the state of a solution, and then filter, wash, dry, and thermally decompose to prepare nanoparticles. It includes direct precipitation method, co-precipitation method and homogeneous precipitation method.

① Direct precipitation method is to prepare nanoparticles from solution through precipitation reaction;

②The co-precipitation method is to add the precipitant to the mixed metal salt solution to make the components mixed and precipitate, and then decompose by heating to obtain ultrafine particles;

③The uniform precipitation method is a method in which the material that is easily hydrolyzed slowly is used as the precipitation agent, and the growth rate of the particles is controlled by the hydrolysis rate to obtain nanoparticles. It can reduce agglomeration, the product size is uniform, the particle size distribution is narrow, and the purity is high.

The precipitation method has simple operation, short process flow and low cost, but the reaction is easily controlled by solution components, concentration, temperature, time, etc., and it is not easy to form dispersed particles. However, in recent years, through the introduction of freeze-drying, azeotropic drying, supercritical drying and other processes, the problem of hard agglomeration has been effectively solved, and high-quality nanoparticles can be produced.

5. Reverse micelle microemulsion method

In this method, one of the two immiscible solutions is dispersed in the other phase (oil phase) in the form of tiny droplets (water phase) to form a microemulsion (w/o type), and the water phase is used as the oxidation In the micro-reactor generated by the compound or hydroxide, the precipitation reaction occurs, and then the nano-alumina powder is obtained by washing, drying, and calcination.

The reverse micelle microemulsion method is simple to operate, and the particle size can be controlled by changing the raw material components, and the particle size distribution is narrow. The uniform multiphase inorganic compound powder produced is of great significance to the production of functional ceramic materials. However, the product particles are too fine, which increases the difficulty of the subsequent separation process.

6. Solvent evaporation method

In this method, the metal salt solution is made into tiny droplets, the solvent is quickly evaporated, and the solute is precipitated into nano particles. Solvent evaporation methods include direct drying, spray drying, freeze drying, and supercritical drying.

①The drying method has low acid efficiency and poor quality, and its application is limited;

②The spray drying method uses aluminum nitrate and ammonium aluminum carbonate as raw materials, and the operation is simple, but the aluminum nitrate releases nitrogen oxides, which may pollute the environment;

③The freeze-drying method has good product uniformity, but the cost is high;

④Supercritical fluid drying technology uses aluminum nitrate as raw material, and the alumina produced in the inorganic salt-organic solvent system has small particle size, large pore size, low density, high surface energy, and great product application potential.

In addition, the morphology of alumina also has varying degrees of influence on the thermal conductivity. According to different firing control, the crystal morphology of alumina can show worm-like, flake-like, spherical (quasi-spherical) morphology, etc. At present, the morphology of alumina filled in high thermal conductivity insulating materials is mainly spherical (quasi-spherical). ) Mainly. There are also some research institutions that use flake alumina to make high thermal conductivity and insulating silicone rubber composite materials.

Spherical alumina and flake alumina

In short, as the filler alumina for thermally conductive insulating silica gel, its crystal morphology and particle size distribution will have a great impact on the thermal conductivity of the thermally conductive insulating silica gel material and product performance. Therefore, in order to improve the thermal conductivity of composite materials, controlling the performance indicators of alumina is a very critical task.

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