Surface Modification of Aluminum Nitride (AlN) Powders
Aluminum nitride (AlN), with the chemical formula AlN, is a covalent bonded nitride consisting of [AlN₄] tetrahedra as structural units. It belongs to the hexagonal crystal system and has characteristics such as low molecular weight, strong atomic bonding, simple crystal structure, and high lattice vibration coordination.
Due to the unique crystal parameters, AlN exhibits excellent properties such as high thermal conductivity, high strength, high volume resistivity, high insulation withstand voltage, low dielectric loss, and a thermal expansion coefficient that matches silicon. These properties make AlN an ideal material for high thermal conductivity ceramic electronic substrates and packaging materials. It is often considered the most ideal electronic substrate material.
Tip 1: Thermal Conductivity and Other Applications of AlN
The theoretical thermal conductivity of AlN single crystals is 320 W/(m·K), while the thermal conductivity of polycrystalline AlN ceramics can range from 140 to 200 W/(m·K), which is approximately 10 times that of conventional resin substrates and alumina ceramics. In addition, AlN has a direct bandgap structure and can theoretically emit light across a wide range of wavelengths, from deep ultraviolet to deep infrared. It is an indispensable material in GaN-based light-emitting diodes, field-effect transistors, and other devices.
1. Why Modify the Surface of Aluminum Nitride Powders?
The surface of AlN powders is highly reactive and easily reacts with moisture in the air. AlN initially transforms into an amorphous AlOOH phase, which may further convert into Al(OH)₃ under certain temperature, pH, and ion activity conditions. This leads to the formation of Al(OH)₃ or AlOOH (aluminum hydroxide) films on the surface of the powders. The increase in oxygen content significantly reduces the thermal conductivity of AlN ceramics. Due to this characteristic, handling, storage, transportation, and post-processing of AlN powders can be problematic.
Tip 2: Methods for producing Aluminum Nitride Powders
The primary methods for producing high-purity, fine-grained, and narrowly distributed AlN powders include the direct nitridation of aluminum powder, the carbothermic reduction of aluminum oxide, self-propagating high-temperature synthesis (SHS), chemical vapor deposition (CVD), and high-energy ball milling. Industrial production mainly uses the first two methods, while the others are primarily employed in experimental research.
Tip 3: Methods for Determining the Hydrolysis Degree of AlN Powders
The degree of hydrolysis of AlN powders can be determined through various techniques, including X-ray diffraction (XRD) analysis to examine phase changes, scanning electron microscopy (SEM) to observe changes in the shape of AlN powder particles before and after hydrolysis, and transmission electron microscopy (TEM) to analyze the crystal structure of the products. Additionally, the hydrolysis degree can be determined by monitoring the ammonia gas produced during hydrolysis, which results in the formation of NH₄⁺ and OH⁻ ions, leading to changes in the pH of the solution.
Moreover, the primary forming method for AlN-based ceramic substrates, often referred to as the "king of ceramic substrates," is the slip casting method. This method is efficient and cost-effective. However, the hydrolysis of AlN powders severely hinders the development of water-based slip casting processes for AlN ceramics. Non-water-based slip casting methods, while viable, are expensive, lead to poor uniformity of samples, and generate organic vapors that cause environmental pollution.
In addition, new applications of AlN, such as in thermally conductive plastics and thermally conductive adhesives, require AlN powders with improved hydrolysis resistance, compatibility with organic materials, and low interface thermal resistance.
Thus, improving the hydrolysis resistance and stability of AlN powders has become a hot topic in research on surface modification methods for AlN powders. The following will briefly explore the methods of surface modification for AlN.
2. Surface Modification Methods of Aluminum Nitride (AlN)
There are various surface modification techniques for AlN powder, with the basic principle being to perform physical adsorption or chemical treatment on the powder's surface. This results in the coating of AlN particles or the formation of a thin reactive layer, preventing AlN powder from hydrolyzing when it interacts with water. The main methods include coating modification, surface chemical modification, and heat treatment, among others.
1. Coating Modification Method of AlN
Coating modification is a traditional method that has been applied for a long time. It involves coating AlN powder with inorganic or organic compounds to reduce or shield particle agglomeration. Furthermore, the coating generates steric hindrance, making it difficult for the particles to re-agglomerate, thus achieving surface modification. The modifiers used for coating include surfactants, inorganic materials, dispersants, etc.
a. Surfactant Method
Surfactant modification relies on the surface charge characteristics of AlN particles. By adding cationic or anionic surfactants, the interfacial tension between the powder dispersion system (gas-liquid, solid-liquid) is altered, and a coating layer is formed on the powder surface with carbon-oxygen chains extending outward. The surfactant's hydrophilic group can adsorb onto the solid surface, alter chemical reactivity, and lower surface tension, which can control the hydrophilicity, lipophilicity, and surface activity of the nanomaterial. This process modifies the surface properties of the powder or imparts new characteristics to the powder.
This is reflected in three aspects:
The hydrophilic group reacts with the surface group to form a new structure, which provides new activity to the powder surface.
The reduction of the surface energy stabilizes the powder.
The hydrophobic groups of the surfactant form steric hindrance on the surface, preventing powder re-agglomeration, thereby improving the dispersion of the nanomaterial in different media.
Example: Research by Guo Xingzhong et al. found that AlN powder modified with organic carboxylic acids and polyethylene glycol showed no significant Al(OH)3 phase after soaking in water for 48 hours, indicating that the organic carboxylic acids coated the surface of the AlN powder, thereby preventing water molecules from eroding the AlN powder surface.
b. Inorganic Coating Modification
Inorganic surface modification of AlN powder involves depositing inorganic compounds or metals on the surface through specific methods to form a coating film or a core-shell composite particle, improving surface properties. This process utilizes physical or chemical adsorption principles to ensure that the coating material is uniformly attached to the coated object, forming a continuous and complete coating layer. The modified powder's surface then exhibits the properties of the coating material.
c. Dispersant Method
Dispersants, which have amphiphilic structures similar to traditional surfactants, use anchoring groups and solvated chains to replace the hydrophilic and hydrophobic groups of surfactants. The anchoring groups can strongly adsorb onto the particle surface through ionic bonds, covalent bonds, hydrogen bonds, or van der Waals forces, either at single or multiple points. The solvated chains are selected by varying the polymer monomer or adjusting the copolymer composition to regulate compatibility with the dispersion medium. Additionally, increasing the molecular weight of the solvated chain ensures the formation of a sufficiently thick space layer on the particle surface.
When selecting dispersants, two main factors are considered:
The polarity of the dispersion medium and its solubility for the solvated chains of the dispersant. Typically, a medium with a high ability to dissolve the solvated chain and a low ability to dissolve the anchoring group is preferred.
The surface polarity, surface functional groups, and acid-base properties of the particles to be dispersed. Low-polarity particles require dispersants with multiple anchoring groups. Different functional groups have varying reactivity and interaction methods, and the absorption sites on the particle surface can select anchoring groups based on their acid-base properties.
Surface Chemical Modification
Surface chemical modification is accomplished through chemical reactions or adsorption between surface modifiers and the particle surface. Polymer long chains are grafted onto the surface of the powder, and the long chains containing hydrophilic groups in the polymer extend and form a steric barrier in aqueous media. This helps to disperse and stabilize the AlN powder in the medium, relying not only on electrostatic repulsion but also on steric hindrance, which is highly effective.
The selection of surface modifiers must aim to lower the surface energy of the particles, eliminate surface charges, and reduce surface attraction. To achieve good surface modification, organic substances used for modification should also provide the maximum degree of wetting with the particles, forming a uniform and dense coating. This is primarily dependent on the physical and chemical adsorption of the organic modifier on the particle surface. Physical adsorption occurs through van der Waals forces, electrostatic attraction, and other physical interactions between the modifier and the particles. Chemical adsorption relies on the reaction between functional groups on the particle surface and the modifier to achieve the surface coating of the particles by the surfactant.
Thermal Spray Method for Surface Coating of AlN Powder
The surface properties of AlN particles differ significantly from those of organic matrices, and common surface chemical modifiers (such as organosilicon, titanates, and aluminates) are used to modify the surface of AlN to increase its compatibility with the matrix. After treatment, the powder exhibits strong non-wettability to water. These small particles, with their non-wetting nature, float in water like an oil film without sinking. Based on this phenomenon, the activation index is used to characterize the degree of hydrophobicity. A higher activation index indicates better hydrophobicity, while a lower index indicates poorer hydrophobicity. When the unmodified nano-AlN powder is strongly hydrophilic, it sinks entirely when in contact with water, resulting in an activation index of R = 0. The activation index R is defined as the mass of the floating portion of the sample divided by the total mass of the sample.
Coupling Agent Modification of AlN
A coupling agent is a compound that contains both a polar group that can react with the surface of inorganic particles and an organic functional group that can react with or be compatible with organic materials. The role of the coupling agent is that one end can bind to the surface of the powder, while the other end can strongly interact with the dispersion medium. This improves the affinity of AlN powder with polymer materials, facilitating the dispersion of the powder in the polymer matrix.
Common coupling agents include the following types:
a) Silane Coupling Agents: Organic silane coupling agents are the most commonly used and widely applied coupling agents. Their general structural formula is Y-(CH2CH2-Si)-X3, where n is typically 2-3. In this structure, Y is the organic functional group, such as vinyl, methacryloxy, epoxy, amino, or hydrophobic groups, and X is the functional group bound to the silicon atom. Silane coupling agents are often classified based on the X group, which includes types like hydrolyzed silane, peroxysilane, and polysilane.
b) Titanate Ester Coupling Agents: These coupling agents are a new type developed by Kenrich Petroleum Company in the mid-1970s. They exhibit good modification effects for many inorganic particles.
c) Aluminate Ester Coupling Agents: Aluminate ester coupling agents are a new class of coupling agents.
b) Hydrophobization Treatment: Hydrophobization treatment involves selecting organic substances with hydrophobic groups (such as long-chain alkyl, aliphatic hydrocarbon, or cycloalkyl groups) to surround the surface of AlN powder. These hydrophobic groups firmly bond to the powder surface, resulting in a strong hydrophobicity.
c) Surface Grafting Modification: Surface grafting polymerization is a chemical method that links high-molecular polymers to the surface of AlN powder, significantly improving the dispersion of particles in organic solvents or polymer matrices.
d) Inorganic Acid Modification: Inorganic acids, such as phosphoric acid or mono-dihydrogen phosphate, are used to treat the surface of AlN powder. This treatment not only enhances the hydrolysis resistance of AlN but also improves the dispersion of the powder. The relationship between the suspension stability of AlN and time, as well as the stability of AlN in water, depends on the specific inorganic acid used.
Heat Treatment Method
The heat treatment method involves heating the powder to induce oxidation on its surface, forming a dense alumina protective film, thereby enhancing its hydrolysis resistance. Li Yawei and other researchers studied the effect of heat treatment on the hydrolysis resistance of aluminum nitride (AlN) in the temperature range of 700-1050°C in air. They found that AlN started to oxidize at 700°C in the air, and as the temperature increased, further oxidation occurred. At 1050°C, the AlN was completely oxidized. After heat treatment, the hydrolysis resistance of the AlN powder was found to be temperature-dependent, with the resistance decreasing as the water temperature increased.
Other Modification Methods
There are several other modification methods, such as high-energy treatments, ultrasound, and encapsulation, which can also be used to modify the surface of AlN powders. Typically, combining these methods with others yields better surface modification results.
Factors Affecting the Surface Modification of AlN Powder
Several factors influence the surface modification of AlN powder, such as temperature, time, and the amount of modifier used. The modifier works by interacting with the surface groups of the nano powder to achieve modification. However, the chemical structure of the modifier and the length of the molecular chains can impact the dispersion of the nano powder in the polymer matrix. The molecular weight of the modifier has a significant effect on surface modification. If the molecular weight is too low, the coating layer will be thin and unable to provide sufficient steric hindrance, leading to poor dispersion of the modified powder. On the other hand, a higher molecular weight modifier will form a thicker coating on the surface, which can better interact with the organic matrix, as shown in Figure 4.
When using silane coupling agents to modify the surface of AlN powder, a small amount of anhydrous ethanol or other solvents is often added to accelerate the reaction between the coupling agent and the powder. However, the presence or absence of the solvent can affect the interaction between the AlN powder and the coupling agent.
Reference: Baidu Wenku