As electronic devices develop toward high power, miniaturization, and integration, thermal management has become a core factor determining product performance, reliability, and lifespan. Aluminum nitride (AlN) possesses properties such as high thermal conductivity, high electrical resistance, low dielectric loss, low expansion, excellent thermal shock resistance, and good mechanical properties, making it one of the key materials for high-end heat dissipation solutions.
Aluminum nitride (AlN) is a strongly covalent-bonded compound with a hexagonal wurtzite structure. Its crystal consists of an Al-N covalent bond network: Al atoms occupy octahedral interstitial sites, and N atoms form a tetrahedral coordination structure, in which Al atoms are located in the octahedral voids, while N atoms occupy the positions of the tetrahedral voids to form a tetrahedral structure. Its crystal structure is similar to materials such as BeO and SiC and features highly directional covalent bonding, so AlN exhibits excellent physical and chemical properties such as high thermal conductivity.
In aluminum nitride ceramics, heat is mainly transferred through lattice waves in the form of lattice vibrations. According to quantum theory, the energy of lattice vibrations is quantized, and its basic energy unit is called a phonon. The propagation and scattering processes of phonons in the crystal determine the material's heat conduction capability. For ideal single-crystal aluminum nitride, its thermal conductivity is mainly limited by the vibration characteristics of the lattice itself and can approach the material's intrinsic thermal conductivity. However, in practical polycrystalline aluminum nitride ceramics, phonons are continuously affected by various scattering effects during propagation, thereby shortening their propagation distance and making the actual thermal conductivity significantly lower than the single-crystal level. Therefore, the key to improving the thermal conductivity of aluminum nitride ceramics lies in reducing scattering sources and improving the continuity of the crystal structure so that phonons can transfer heat more effectively.
In current practical applications, the application of aluminum nitride in the high thermal conductivity field is mainly concentrated in two aspects: packaging substrates and thermally conductive fillers.
01 Aluminum nitride substrates:
High-end heat dissipation foundation in the computing era
Power electronic devices are continuously developing toward high power density and high integration. The heat flux density of current chips has exceeded 200 W/cm2, placing severe requirements on heat dissipation substrate materials. Aluminum nitride ceramic substrates, due to their excellent thermal conductivity, low dielectric constant, and coefficient of thermal expansion that is highly matched with silicon wafers, can effectively solve problems such as heat dissipation of high-power devices and thermal stress mismatch, and have become a key foundational material for high-performance thermal management systems.
Aluminum nitride ceramic substrate, image source: Zhongci Electronics
High-quality aluminum nitride powder is a prerequisite for obtaining high-thermal-conductivity aluminum nitride ceramic substrates. The preparation technology for high-end AlN powder is basically monopolized by developed countries such as Japan, the United States, and Germany. In recent years, domestic enterprises have achieved mass production of AlN powder, but in the short term, the situation in which domestic aluminum nitride powder for high-thermal-conductivity aluminum nitride ceramic substrates relies on imports will be difficult to change.
The main preparation methods for aluminum nitride powder include the carbothermal reduction nitridation method, direct nitridation method, self-propagating high-temperature synthesis method, chemical vapor deposition method, and plasma vapor phase preparation method. At present, industrial production mainly adopts the carbothermal reduction nitridation method and the direct nitridation method. Among them, the carbothermal reduction nitridation method occupies most of the market share and is currently a commonly used method for preparing aluminum nitride in industry at home and abroad.
The carbothermal reduction nitridation method uses Al2O3 powder and high-purity carbon black (C) as precursors, fully mixes them, and then places them in a flowing N2 atmosphere for reaction at a high temperature of 1400°C-1800°C. Al2O3 is first reduced to elemental metallic aluminum by high-purity carbon black, and then Al undergoes a secondary nitridation reaction with N2 to form AlN. AlN powder synthesized by this method has advantages such as uniform particle size distribution, low degree of agglomeration, and relatively high purity.
The direct nitridation method usually promotes the reaction of Al with the atmosphere under a continuously flowing nitrogen (N2) and ammonia (NH3) atmosphere at a temperature above 900°C, thereby obtaining AlN powder. This method has advantages such as low cost, short cycle, large output, and suitability for large-scale production.
Sintering technology determines the microstructure evolution path and upper limit of heat transport performance of high-thermal-conductivity aluminum nitride ceramics. Common sintering processes mainly include hot pressing sintering, microwave sintering, and pressureless sintering. During the sintering process, in addition to the process and atmosphere affecting product performance, the selection of sintering aids is also particularly important. A balance must be sought between promoting densification and controlling grain growth, while also considering the impact of the aids on the material's thermal conductivity.
The surface quality and processing precision of aluminum nitride ceramics have an important impact on device performance and service life. Fully sintered aluminum nitride ceramics have high hardness (Mohs hardness 7-8) and high brittleness and are typical hard and brittle materials. At present, single precision processing technologies still have some problems. To improve the processed surface quality and processing efficiency of aluminum nitride ceramic substrates, adopting multiple processing methods, such as ultrasonic vibration-assisted grinding and ultrasonic-assisted fixed abrasive chemical mechanical polishing for composite polishing technology, may be a feasible approach.
The explosion of AI computing power has spurred optical module upgrades and server heat dissipation demand, bringing development opportunities for aluminum nitride ceramic substrates. In recent years, China's aluminum nitride ceramic substrate industry has continued to develop, but there are still relatively few enterprises with full-industry-chain production capacity. Domestic enterprises are continuing to tackle key problems and gradually fill shortcomings in links such as powder, substrates, and deep processing.
02 Aluminum nitride fillers
Unlocking the ultimate heat dissipation potential of TIM
Thermal interface materials (TIM) are an important component of thermal management systems. They fill the space between heat-generating devices and heat-dissipating devices, can significantly reduce contact thermal resistance, and improve heat transfer efficiency. TIM mainly includes thermal conductive silicone grease, thermal conductive gel, thermal conductive phase change materials, and thermal conductive pads. Their basic composition is a polymer matrix and thermally conductive fillers, and thermally conductive fillers are the core component of TIM.
Image source: Zhenjing New Materials
The theoretical thermal conductivity of AlN is as high as 320 W/(m·K), and in practical applications as a filler it still performs far better than conventional aluminum oxide. At the same time, because it has excellent electrical insulation and its coefficient of thermal expansion is highly matched with silicon, it has also become a top-level upgrade solution that combines thermal conductivity and insulation. In addition, the advantages of AlN are also reflected in:
Excellent dielectric properties: Aluminum nitride has a low dielectric constant and dielectric loss, which means that in high-frequency and high-speed circuits (such as 5G base stations and RF devices) it can not only dissipate heat quickly but also effectively reduce signal delay and electromagnetic loss.
High phonon transport efficiency: Unlike metals that conduct heat through electrons, the heat conduction mechanism of aluminum nitride is phonon conduction. If lattice defects are few and purity is high, the heat conduction path at the microscopic level will be extremely unobstructed.
Lightweight and structural adaptability: Compared with some metal fillers, aluminum nitride has a moderate density and, under the premise of achieving the same heat dissipation effect, helps reduce the overall weight of the equipment.
However, aluminum nitride faces some challenges in practical applications.
AlN powder easily undergoes hydrolysis reaction with water in humid environments, generating aluminum hydroxide (Al(OH)3) or aluminum oxyhydroxide (AlOOH), resulting in loss of nitrogen content and increase in oxygen content, which reduces the thermal conductivity of ceramics and thermal interface materials prepared from it and hinders its industrial application. The core idea of anti-hydrolysis modification is to construct a "physical barrier" or "chemical passivation layer" on the AlN surface to block contact between water molecules and AlN and proton transfer.
Chemical modification involves chemical reactions between AlN surface groups (such as Al-OH) and modifiers to form a covalently bonded protective layer, which has the advantages of strong bonding force and high stability and is currently the most widely studied technical route. Common modifiers include silane coupling agents, inorganic or organic acids, polymers, and small molecules. Physical modification refers to forming a physical barrier on the AlN surface through physical means such as mechanical mixing, coating, or heat treatment. It has the advantages of simple process and no chemical pollution and is suitable for scenarios sensitive to chemical modification. It mainly includes liquid-phase coating, physical vapor deposition, heat treatment modification, and liquid metal coating.
Conclusion
As a new-generation electronic packaging and thermal management material, aluminum nitride's excellent thermal properties and compatibility with semiconductor materials give it an irreplaceable position in the field of high-power devices. As the industrialization of aluminum nitride continues to mature and domestic manufacturers continue to make breakthroughs in supply-side bottlenecks, aluminum nitride, with its core advantages such as high thermal conductivity, is expected to become the first choice for pursuing ultimate heat dissipation performance in the future.