Aluminum nitride has high strength, high volume resistivity, high insulation withstand voltage, and a thermal expansion coefficient that matches silicon. It can be applied to structural ceramics as a reinforcing phase or sintering aid, particularly in the production of ceramic packaging materials and electronic substrates.
Aluminum nitride has far superior and more comprehensive performance than aluminum oxide. However, AlN powder readily forms aluminum hydroxide with hydroxyl groups in water in a humid environment, as well as an aluminum oxide layer on its surface. AlN's thermal conductivity is decreased as a result of the significant amount of oxygen dissolved in the alumina lattice, which also alters the material's physical and chemical properties and makes AlN powder application challenging.
Hydrolysis Mechanism of Aluminum Nitride Ceramic Powder
AlN+3H2O→Al(OH)3+NH3
AlN+2H2O→AlOOHamorph+NH3
NH3+H2O→NH4++OH-
The main way to prevent the hydrolysis of AlN powder is to coat a substance with chemical bonds or physical adsorption on the surface of the AlN particles, isolating it from water and preventing the hydrolysis reaction. Methods for inhibiting aluminum nitride hydrolysis are described below:
Surface Chemical Modification to Inhibit Aluminum Nitride Ceramic Powder Hydrolysis
Surface chemical modification is the chemical reaction of AlN particles with a surface modifier to form a protective layer and passivate the surface in order to improve the surface properties of AlN. Surface chemical modification methods for AlN powder primarily include coupling agent modification, coupling graft copolymerization modification, surface oxidation modification, and surfactant modification.
(1) Coupling Agent Modification
Coupling agent modification is the result of a chemical coupling reaction between the particle surface and the coupling agent. In addition to the interaction of the van der Waals force, hydrogen bond, or coordination bond between the two components, the combination of ionic or covalent bonds exist as well. The coupling agent molecule needs to have two groups—one for chemical reaction with inorganic particle surface or the precursor for making nanoparticles, and the other should be reactive or compatible with organic matrix.
One of the most popular coupling agents is silane, which has the general formula RSiX3. R is an organic group and X is a group that is simple to hydrolyze. The hydroxyl groups on the surface of AlN particles can react with the X group to form an Al-Si covalent bond between the silane and the AlN matrix, effectively improving the AlN powder's hydrolysis resistance.
(2) Coupling Graft Copolymerization Modification
Coupling graft copolymerization involves first treating the surface of AlN with a coupling agent, followed by graft copolymerization between polymers. Graft copolymerization is the reaction that occurs when a specific monomer is polymerized in the presence of a polymer component, and a branch polymer component is chemically bonded to another branch on the backbone polymer. Graft copolymers are typically formed through the polymerization of monomers via free radical, ion addition, or ring-opening in the presence of reactive macromolecules.
(3) Surface Oxidation Modification
An Al2O3 protective film is created by oxidizing the surface of AlN particles through heat treatment and grinding, which can increase the AlN powder's resistance to hydrolysis.
(4) Surfactant Modification
Typically, surfactants consist of hydrophilic group and hydrophobic group. The hydrophilic group is adsorbed on the surface of AlN particles, and the hydrophobic group extends into the solvent to produce a steric hindrance effect, preventing the AlN particles from contacting water and thus improving the AlN particles' hydrolysis resistance.
Surface Physical Coating Modification to Inhibit Aluminum Nitride Ceramic Powder Hydrolysis
(1) Liquid phase coating modification
A coating layer is formed on the surface of the AlN powder by adding a modifier and mechanically stirring the suspension. The AlN particle's surface and the coating do not interact chemically; instead, they are joined by adsorption or the van der Waals force.
(2) Vapor deposition modification
To increase the hydrolysis resistance of AlN powder, a specific substance that readily sublimates when heated is used; this substance is then allowed to condense and deposit on the surface of the AlN particles. For instance, the AlN powder can be modified by sublimating SiO solid powder into a gaseous state. Next, the AlN powder, SiO, carbon felt, and graphite plate are loaded into an Al2O3 crucible. The temperature is then raised to cause the SiO to sublimate, resulting in the formation of a protective layer on the surface of the AlN particles.
Additionally, another method for modifying AlN powder involves using a strong acid modifier and combining it with AlN powder via mechanical ball milling. This method does not require high temperatures and also has good repeatability. It can significantly improve the hydrolysis resistance of AlN powder, and can also make AlN powder have better dispersibility and stability in water, which is advantageous for obtaining AlN ceramic slurry with high solid content.