Surface treatment of aluminum nitride(aln) powder

    Aluminum nitride powder has high thermal conductivity and insulating property, adding it into the thermal conductive interface material can significantly improve the thermal conductivity of the material.Although AlN is good, but it has a disadvantage - it is a material that is extremely easy to absorb moisture and oxygen, once it comes into contact with moisture and oxygen, it will be hydrolyzed and oxidized, and it will lose its thermal conductivity and dissipation of heat performance characteristics. And AlN powder without surface treatment and modification, it is difficult to mix evenly with polymer materials, so that it is difficult to form a good thermal conductivity channel, interpenetrating network.

Hydrolysis process of aluminum nitride: AlN+3H2O=Al(OH)3↓+NH3↑

    Obviously, in order to expand the application of AlN in thermal conductive materials, the first thing to be solved is the problem of its hydrolysis, oxidation, difficult to disperse. The current mainstream method is to carry out the corresponding physical adsorption or chemical treatment on the surface of the powder, coating or forming a thin reaction layer on the surface of AlN powder to prevent the hydrolysis reaction between AlN powder and water.

    The main methods include coating modification method, surface chemical modification method, heat treatment method and so on.

1. Coating modification method

    Coating modification is a traditional modification method that has been applied for a long time, which is to use inorganic compounds or organic compounds to coat the surface of AlN powder, and play a role in weakening or shielding the agglomeration of particles. The modifiers used for coating modification include surfactants, inorganic compounds and hyperdispersants.

    ① Surfactant method: according to the nature of the surface charge of AlN particles, the addition of cationic or anionic surfactants, change the gas-liquid, solid-liquid interfacial tension in the powder dispersion system, the formation of carbon and oxygen chains on the surface of the powder outwardly stretching with a certain thickness of the coating layer.

    ② inorganic coating modification: AlN powder inorganic surface modification is the inorganic compounds or metals through certain means of deposition on the surface to form a coating film, or the formation of a core-shell composite particles, so that the modified surface of the powder presents the nature of the coating material.

    ③ Hyperdispersants: Hyperdispersants are similar to traditional surfactants in terms of amphiphilic structure, but the hydrophilic and lipophilic groups of surfactants are replaced with anchoring groups and solventized chains. Anchoring groups can be firmly adsorbed on the particle surface through ionic bonding, covalent bonding, hydrogen bonding and van der Waals forces and other interactions in the form of single-point or multi-point anchoring, and its solubilized chain can be adjusted to its compatibility with the dispersing medium through the selection of different polymerization monomers or changing the ratio of copolymerization monomers.

2. Surface chemical modification

    Surface chemical modification is accomplished by chemical reaction or chemical adsorption between the surface modifier and the particle surface. The polymer long links are branched on the surface of the powder. The polymer containing hydrophilic groups in the long chain through the hydration stretch in the water medium to play a three-dimensional barrier, so that the dispersion of AlN powder in the medium in addition to relying on electrostatic repulsion and rely on the space resistance. The effect is very obvious.

    ① coupling agent modification: coupling agent is a kind of compound that has both polar groups that can react with the surface of inorganic particles and organic functional groups that are reactive or compatible with organic matter. Its role is that one end can be combined with the surface of the powder and the other end can have a strong interaction with the dispersing medium, so it can improve the affinity of AlN powder and polymer materials, to achieve the dispersion of the powder in the polymer material.

    ② Hydrophobic treatment: hydrophobic treatment is to choose organic materials with hydrophobic groups (such as long-chain alkyl, chain hydrocarbon group and cycloalkyl, etc.) around the surface of the AlN powder, so that the alkyl and other solidly bonded to the surface of the powder, presenting a strong hydrophobicity.

    ③ Surface grafting modification method Surface graft polymerization is to graft polymer links to the surface of AlN powders by chemical reaction, which can significantly improve the dispersion of particles in organic solvents or polymers.

    ④ Inorganic acid modification using phosphoric acid, dihydrogen phosphate, etc. on the surface of AlN powder, it was found that not only can make AlN hydrolysis-resistant and can also enhance the dispersion of the powder. the relationship between the value of AlN suspension with time and the stability of AlN in water depends on the inorganic acid reagents used.

3. Heat treatment and other methods

    The heat treatment method is to produce hydrolysis resistance by heat treatment of the powder so that oxidation occurs on the surface to produce a dense protective film of aluminum oxide.

    Other modification methods, such as high-energy treatment, ultrasonic, encapsulation modification can also be surface modification of AlN powder. Usually, these methods are combined with other methods to achieve a better effect on the surface modification of the powder.