Getting the right thermal paste for your application is a crucial part of any electronics design. But, many engineers are often unclear about the most effective way to get the thermal conductivity they need, and this article discusses the key steps to creating a spherical alumina thermal paste that will do the job.
Abstract
Various synthetic parameters affecting the morphology of the alumina particles and the rate of nitridation of the AlN particles were investigated. It was found that the rate of nitridation increased with the temperature. The formation rate of liquid Ca-aluminates was higher than the nitridation rate. In addition, the alumina particles produced were spherical. This facilitated the material transport through the liquid phase.
It was found that the thermal conductivity of the roundish alumina particles produced according to the present invention was significantly improved. This could be due to the fact that the particles assume the shape of coarse corundum particles, which exhibit favorable flow characteristics. Moreover, they can be incorporated into high-thermal-conductivity rubber or plastic.
In addition, the presence of the roundness enhancer in the coarse alumina particles promotes the roundness of the particles. This roundness enhancer acts synergistically with other agents to enhance the flow characteristics of the coarse alumina particles. This enhancer promotes the growth of AlN particles via the dissolution-precipitation mechanism. The small AlN particles promote the growth of the larger AlN particles via the same mechanism.
In addition, the presence of the two-dimensional graphene sheets can increase the thermal conductivity of the alumina particles. This two-dimensional graphene can provide faster pathways for phonon transport. It can also decrease the thermal boundary resistance of the alumina particles.
The amount of agents to be added in the production process varies with the particle size of the alumina employed. It is preferably between 3 and 20 mass %. Various synthetic parameters, such as the type of heating furnace and the residence time of the material, have a major effect on the particle size.
The amount of aluminum hydroxide that is added to the alumina particles preferably falls within the range of 5 to 300 mass %. It can be combined with the alumina particles in the rubber/plastic composition to enhance thermal conductivity.
Methods
Various resins, such as polyolefin, phenol and silicone resins, can be made with high thermal conductivity by using spherical alumina powder of the present invention. This powder is suitable for use as a resin filler and has good insulating property. Moreover, it has low alpha dose and uranium content. These properties can prevent deterioration of the resin's mechanical properties. Therefore, spherical alumina powder is suitable for use as a cooling member in electronic parts and as a filler in resin.
The present invention describes a method for producing spherical alumina powder by feeding an aluminum hydroxide powder slurry into a flame. The powder is fed through a raw material feed pipe. The flame is composed of combustible gas and combustion supporting gas. During feeding, thermal decomposition of surface treating agent causes an inorganic oxide layer to form on the surface of the powder. The powder is then collected and dried.
The method of the present invention enables the production of high-quality spherical alumina powder with excellent productivity and high collection efficiency. The specific surface area of the powder is also improved. The specific surface area of the resulting powder is approximately 0.6 m2/g. The spherical alumina powder has an average particle diameter of D50 of about 2.8 mm.
The particle diameter distribution of the powder is very sharp. The average particle diameter D50 can be as high as 70 mm. Generally, the spherical alumina powder in the present invention has a ratio of D50 to Dbet of 2.7 to 10. The sphericity of the powder is preferably greater than 0.90.
The maximum thermal conductivity of the resulting powder is 7 +- 0.3 W/m*K. However, the thermal conductivity increases less when the particle size of the powder is reduced. Hence, the sphericity of the powder should be 0.90 or greater for particle diameter range from 3 mm to 20 mm.
The spherical alumina particle of the present invention has a low uranium content. The content of uranium is about 10 ppb or less. It is preferably used for encapsulation materials of semiconductors. The uranium content can be quantified by glow-discharge mass spectrometry.
Results
Various processes for producing alumina particles have been developed and employed in various fields. In some fields, alumina particles are used as fillers, sealing materials for electronic parts, finish lapping material and aggregates in refractory materials. In other fields, alumina particles are used as an additive for composites, especially composites used for sealing. Alumina has excellent electrical conductivity and thermal conductivity. Various types of alumina particles are used in the fields of glass ceramics, seals, sealing materials and high thermal conductive heat sinks.
In order to produce spherical alumina particles, various techniques have been developed. The alumina particles are derived from the chemical synthesis of AlN powders. The powders were synthesized at 1800degC and under different N 2 pressures. Afterwards, the particles were pulverized. The pulverized particles have a mean particle size of less than 120 mm. In addition, they have excellent flow characteristics.
In order to promote the growth of AlN particles, the powders were subjected to the dissolution-precipitation mechanism. Small AlN particles reprecipitated on the surface of the larger particles. Hence, the morphology of the AlN particles changed at 1800degC. The morphology of the AlN particles was spherical under N 2 pressure of 1 Mpa. However, the AlN particles were not smooth. This resulted in a considerable wear on the kneader.
The particles are then subjected to a high temperature for a brief period. The products are then crushed with a known pulverization technique. Generally, the thermal conductivity of the particles increases with the volume percentage. At 15%, the thermal conductivity reaches 6.5 +- 0.03 W/m*k. The particles are spherical with the lowest surface free energy.
The thermal conductivity of the particles increases with the concentration of added agents. However, the amount of agents to be added varies depending on the type of heating furnace and the residence time in the furnace. Generally, the effective concentration of the agents is 3-5 mass %. Besides, the amount of agents to be added mainly depends on the particle size of the employed sintered alumina.
Besides, the alumina particles produced by the present invention preferably are incorporated into rubber or plastic. The use of the particles produces a high-thermal-conductivity rubber or plastic composition.
Discussion
Using alumina as filler additives and two-dimensional graphene, thermal conductivity of thermal grease was improved. In addition to improving thermal conductivity, the combination of alumina and graphene can improve phonon transport and thermal boundary resistance. The two-dimensional structure is compact and provides additional pathways for heat flow.
The thermal conductivity of the thermal grease increased as the concentration of the solid phase increased. The addition of 5 vol% of copper powder improved thermal conductivity by 20 %. The maximum thermal conductivity of the thermal grease reached 3.45 W/m*K when the addition of graphene was only 1 wt%.
A commercially available thermal grease was prepared by mixing alumina and copper powder. The thermal conductivity of alumina with copper powder was higher than alumina without copper powder. The addition of graphene and copper powder increased thermal conductivity by 18 to nearly 106 %. Moreover, thermal conductivity was improved by mixing copper nano powders with silicon oil.
Thermal conductivity of alumina and graphene enhanced by the addition of copper powder increased by 4.5 W/m*K over the silicon base. In addition, the thermal conductivity of alumina and graphene containing alumina increased by 3.2 W/m*K.
The nLM-THEMs prepared from aluminum plate showed Ga and In. They were stable at 60 degC and had a high thermal diffusivity. They also displayed good electrical insulation properties. Moreover, they were stable in humid conditions. They also demonstrated stable anti-corrosion effect. They also showed no corrosion response to aluminum, glass and plastic.
The nLM-THEMs exhibit stable electrical insulating properties and passive heat exchange through rapid heat dissipation. They also demonstrate stable thermal conductivity with humidity. However, a high amount of AlN will lead to a higher viscosity of the composite. The addition of over 80 wt% Al 2 O 3 will deteriorate the mechanical properties of the composite.
Furthermore, the combination of two-dimensional graphene and alumina can form a compact thermal network structure that provides additional pathways for heat flow. The addition of two-dimensional graphene and boron nitride can improve thermal conductivity. Moreover, the alumina filler particles can hinder the aggregation of graphene. This is one reason why the thermal grease has low fluidity.
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