It is well known that alumina is a major player among thermally conductive fillers. α-Al₂O₃ has a thermal conductivity of approximately 33–36 W/(m·K) and a volume resistivity of about 10¹⁰ Ω·cm, making it an excellent thermally conductive and electrically insulating filler. The particle morphologies of α-Al₂O₃ include spherical, flake, irregular multi-angular, worm-like, ellipsoidal, and other forms; its particle size ranges from nanoscale, submicron, to micron scale; and its particle structure can be polycrystalline or single-crystalline. These different microstructures have a significant impact on the performance of thermally conductive materials.
Ordinary alumina, due to its surface containing a large number of hydroxyl groups and being highly polar, can only be filled in silicone oils or epoxy resins at low loadings, making it difficult to meet industrial demands. In comparison, spherical alumina allows for a considerably higher filling fraction. Its high sphericity and low specific surface area enable high-density packing, yielding mixtures with low viscosity and good fluidity that are easy to disperse uniformly. Moreover, it possesses excellent electrical insulation, high thermal stability, low thermal expansion coefficient, and good mechanical properties, making it suitable for high-performance applications such as electronic packaging.
Of course, if thermal conductivity alone is considered, flake alumina is the optimal choice. Flake particles readily contact each other to form "face-to-face" thermal conduction pathways with low resistance and high thermal transfer efficiency. At the same time, their smooth surfaces easily bond with polymer macromolecular chains to form cross-linked structures that facilitate heat transfer. Such flake alumina also has an adjustable thermal expansion coefficient, which helps improve the dimensional stability of thermal interface materials.
However, in actual production, flake alumina tends to increase the viscosity of the polymer system, affecting processing performance, and is prone to sedimentation, leading to material stratification. Therefore, it demands higher forming process requirements, which is the main reason for its relatively limited application in thermally conductive silicone pads.
Thus, based on the requirements for stability, high performance, and low cost in thermally conductive alumina systems with different morphologies, composite thermally conductive alumina filler materials-achieved by densely packing spherical, quasi-spherical, and angular alumina fillers to construct a thermal conduction network and thereby enhance the thermal conductivity of thermal interface materials-are expected to gain widespread application, with growing market demand.
In terms of purity, standard alumina typically has a purity of about 99.8%. However, when refined to 99.99% high-purity alumina, its thermal conductivity further improves. This is because impurities and defects in alumina interfere with the smooth flow of phonons in lattice vibrations. A common impurity that significantly affects thermal conductivity is sodium; as an alkali metal, sodium causes distortion of the crystal structure, thereby hindering heat flow. Conventional alumina may have a sodium content exceeding 100 ppm, whereas high-purity alumina produced via efficient and sustainable processes can keep sodium content below 10 ppm. Owing to its low impurity content and uniform structure, high-purity alumina enables better phonon transport and delivers superior thermal performance.
But when the structure of alumina changes to a porous architecture, it exhibits excellent thermal insulation and refractory properties. Why is this? The main reasons are as follows:
On one hand, the thermal conductivity of the same material varies with temperature. As the temperature in industrial kilns rises, the thermal conductivity of alumina continuously decreases; at 1200°C, its thermal conductivity is roughly half of that at 400°C. Nevertheless, alumina's thermal conductivity at room temperature is 20–30 W/(m·K), and even at 1200°C in a high-temperature kiln, it can reach nearly 10 W/(m·K)-still higher than many other materials. Therefore, the insulating behavior of alumina is fundamentally due to changes in its structure.
When alumina is used as a thermally conductive material, it generally requires high density and high purity to minimize the glassy phase components arising from disordered atomic or ionic arrangements. However, the most distinctive feature of alumina as a thermal insulator is its porous, low-density structure. Its low thermal conductivity, high mechanical strength, and high service temperature make it an ideal choice for refractory linings in industrial kilns.
Thermal insulation materials effectively suppress the pathways of heat conduction, convection, and radiation through their unique compositional structure, thereby achieving efficient insulation. Heat transfer in insulating materials occurs mainly through three routes: solid-phase thermal conduction, gas-phase thermal conduction, and radiative heat transfer. Among these, solid-phase conduction is the most important; even at relatively high temperatures, heat transferred through the solid phase still accounts for about 70%.
In such porous structures, solid-phase heat conduction proceeds mainly via two paths: one is heat transfer through the gas within pores, which includes air convection, radiative transfer, and heat transfer due to gas molecular motion; the other is still solid-phase transfer, but the direction of heat flow is altered, greatly extending the overall heat transfer path. Therefore, porous alumina ceramics can, to a certain extent, impede both thermal conduction and thermal radiation, effectively providing thermal insulation.
Classification of nanoporous alumina thermal insulation materials
Nanoporous alumina thermal insulation materials mainly include alumina aerogel insulation materials and nano-alumina pressed insulation materials, among others. They feature low density, low thermal conductivity, and high-temperature resistance, and have broad application prospects in industries and aerospace.
Alumina aerogel insulation materials
The preparation process of alumina aerogels mainly includes sol formation, gelation, aging, and drying steps.
In the preparation of alumina aerogels, aluminum salts are typically first converted into a solution or sol, which then undergoes chemical reactions to form a gel, and finally the gel is dried to obtain the alumina aerogel. This process is centered on the sol-gel method, combined with different drying techniques to preserve the nanoporous network structure. Oxide aerogels prepared via the sol-gel method offer strong process flexibility, superior sample properties, and more mature process development, making this the primary direction for alumina aerogel preparation.
Nano-alumina pressed insulation materials
Nano-alumina pressed insulation materials consist of a porous structure formed by the packing of nano-alumina particles. The abundant pores effectively block gas molecular heat conduction; the solid skeleton has low thermal conductivity; and the nanoscale particle size significantly increases heat scattering, reducing thermal conductivity.
The preparation method for nano-alumina pressed insulation materials mainly involves combining nano-alumina powder particles as the matrix with reinforcing materials to develop high-performance insulation materials. The preparation techniques can be broadly divided into dry forming and wet forming.
Dry forming: Dry forming is carried out without liquids or with only a small amount of liquid, where the nano-matrix and reinforcing materials are mixed and then dry-pressed. This method is mild, simple to operate, and environmentally friendly. Specifically, nano-powders, reinforcing materials, binders, and other solid raw materials are uniformly mixed by mechanical means, then the mixture is loaded into a mold and pressed under specific conditions using a press to obtain the nano-composite material.
Wet forming: Wet forming uses nano-oxide powders as raw materials. During the mixing process, a liquid such as deionized water is added as a solvent. Through mechanical stirring, the raw materials are mixed into a slurry with a certain fluidity. The slurry is then poured into a mold, and the solvent is removed by filtration, drying, heat treatment, or other methods to obtain the nano-oxide insulation material.
Nano-alumina pressed insulation materials produced by wet forming have room-temperature thermal conductivity similar to that of dry-formed materials. Moreover, the presence of solvent allows for more uniform mixing of raw materials, providing better dispersibility; it is easier to control the slurry formulation and precisely adjust parameters such as concentration.