Preparation And Application Of Flaky Alumina Powder

1. Introduction to flaky alumina powder
Flaky alumina powder is a kind of functional micro-powder material with excellent performance, which belongs to α- Al2O3 has obvious scale structure characteristics and large diameter to thickness ratio. At present, the radial size of flaky alumina grain is generally 5~50 μ m. The thickness is generally between 100 and 500 nm, and the particles with well-developed crystal form also show regular hexagonal morphology.
The diameter and thickness ratio of flake alumina powder synthesized by artificial method can be adjusted, and it has the advantages of high purity, smooth surface and good dispersion in water. The particle size of industrial flake alumina powder can reach micron size, and the thickness can reach nanometer or close to nanometer scale. This not only enables it to have significant light reflection and shielding effects, but also reflects good surface activity and excellent adhesion. It has been widely used in coatings, cosmetics, polishing powder, automotive topcoats, toughened ceramics, and other fields. At the same time, because its price is tens of times that of metallurgical grade alumina, the preparation process of flake alumina has great research significance.
2. Preparation method of flaky alumina powder
(1) Hydrothermal method
Hydrothermal crystallization is a common method for the early preparation of flaky alumina. Yasuo et al. grind alumina into submicron particles (particle size ≤ 1 μ m) And hydrothermal crystallization in alkaline solution to prepare a thickness less than 0.1 μ M of alumina flake.
Although the flaky alumina powder prepared by hydrothermal method has high purity, good dispersion, thin plate-like structure and uniform and excellent crystal morphology, hydrothermal synthesis not only has a long cycle, but also requires a high-temperature and high-pressure reactor in the reaction process, which has a strong dependence on the equipment, and the crystal phase transition temperature is high (>400 ℃).
(2) Molten salt method
The basic principle of molten salt method is to dissolve the reactant in one or more low melting point salts at a certain temperature, and synthesize the target product at a lower temperature. The powder preparation by molten salt method is mainly divided into two processes: grain formation and grain growth.
Nitaa and others first used the molten salt method for the preparation of flaky aluminum oxide. They used sulfate as molten salt, and a small amount of titanium dioxide and phosphate as additives to induce the crystallization of soluble aluminum salt. The colloid prepared by the reaction was calcined at 1200 ℃ for 5 hours to obtain a particle size of 3~22 μ m. Flaky hexagonal powder with thickness between 0.2 and 0.5 microns.
Compared with the conventional solid phase method, this method has the advantages of simple process, low synthesis temperature, short holding time, uniform chemical composition of the synthesized powder, good crystal morphology and high phase purity. However, during the preparation process of molten salt method, toxic and harmful gases are often decomposed, which has great damage to the calcination equipment. After calcination, molten salt, powder and crucible form a complex solid mixture, which is difficult for subsequent treatment.
(3) Sol gel method
The sol gel method uses soluble salts as the aluminum source (aluminum sulfate or aluminum hydroxide), hydrolyzes and condenses in the liquid phase system, and crystallizes. After drying, the precursor can be prepared γ- Al2O3 is sintered and solidified in high temperature environment γ Opposite α The process of phase transformation. This method can overcome the shortcomings of the reactor requiring high temperature and high pressure in the hydrothermal process, so it has low requirements for equipment and is widely used. Its disadvantages are that the powder particle size distribution is large, the raw materials prepared are expensive, and some are harmful to the environment.
(4) Sol-coating method
The preparation of flake alumina by sol-gel method is similar to the preparation of high-purity and ultra-fine alumina by sol-gel method. This method first prepares the precursor of oxide into sol, then coating the sol on a smooth substrate, drying the coating in a dryer to obtain thin sheets of oxide or hydroxide, and further calcining to obtain the required flake alumina powder.
The product prepared by this method is easy to control various process parameters, such as particle size, thickness, chemical composition, etc. In addition, the required equipment is relatively simple and the operation is relatively easy, but the mechanical strength of the product is not high and the particle size distribution is not uniform, and further screening is needed to obtain the product with the required particle size range.
(5) Mechanical method
The mechanical method is to mix a certain proportion of powder under the action of mechanical force. Generally, the methods of ball milling, stirring milling, colloid milling and vibration milling are used. During the long time operation, the powder is repeatedly squeezed, cold-welded and crushed under the repeated impact of the grinding medium to become the dispersed and distributed ultra-fine ions.
This method is easy to operate and low in cost, but the purity of the powder is low. In addition, the mechanical force is complex and changeable, resulting in the powder structure is not easy to control, which is easy to produce a considerable number of broken flakes, which may cause nicks and subsurface damage during polishing.
Hilbert et al. grinded raw materials by mechanical method and wet grinding, and prepared crystals with a particle size of 3~20 μ M of flaky alumina powder, the powder prepared by this method has good dispersion, but there are still many fine flakes. The fine particles can be removed from the wet-grinded powder by sedimentation classification, and the obtained powder can be used as polishing powder for microelectronic substrate.
(6) Carbon method
The carbon separation method belongs to the hydrolysis method of sodium aluminate. Usually, carbon dioxide is used to decompose the sodium aluminate solution, and then appropriate crystal form control agent is added to calcine the aluminum hydroxide to decompose and produce crystal form transformation, and finally obtain flakes ɑ- Al2O3。
(7) High temperature sintering method
Kebbede et al. studied the microstructure of alumina prepared by adding titanium dioxide and co-doping titanium dioxide and silicon dioxide. The mono-doped titanium dioxide sample sintered at 1450 ℃ for 2 hours is equiaxed crystal, while the double-doped sample is lamellar, but the crystal size distribution is uneven, and the diameter-thickness ratio is only 3.4.
3. Application of flaky alumina powder
(1) Abrasive polishing fluid
Compared with conventional nano-alumina, the flat and smooth chip surface is not easy to scratch the ground objects (such as semiconductor silicon chips, smart phone shells, etc.), and the qualified product rate can be increased by 10% to 15%. Therefore, flaky alumina has become a new favorite in high-precision microelectronics industry, gem processing industry and cermet industry.
(2) Pearlescent pigment
The flake alumina has stable chemical properties, wide coverage area, high refractive index, and its unique optical properties enable the pigment to produce a strong pearlescent effect. Therefore, taking flake alumina as the substrate material of pearlescent pigment has become a new hot spot and industry economic growth point for developing superior pearlescent pigment.
Image source: Internet
(3) Inorganic filler
Flaky alumina is an indispensable filler in industrial production. It can increase hardness and rigidity, and can adjust shrinkage and thermal expansion coefficient when applied in functional ceramics, plastics and rubber products.
(4) Cosmetics
Flaky alumina has narrow particle size distribution, large diameter to thickness ratio, stable chemical properties, smooth and smooth surface, and can be well dispersed in water. Most importantly, this powder is non-toxic and odorless, so it is also widely used in cosmetics. As an additive that can improve the thermal conductivity of cosmetics, flaky aluminum oxide can optimize the gloss and color of cosmetics, and because of its good adhesion, it has a good comfort on the skin surface. It has excellent spreading and adhesion properties, and can effectively prevent the powder from falling off caused by oil secretion or skin sweating.
(5) Functional coating
The flaky alumina with excellent performance has no agglomeration phenomenon, and has good adhesion. It is easy to combine with other functional micro-powders to prepare a variety of promising new functional coatings. Alumina coating used for capillary can significantly improve its reverse electroosmosis performance and selectivity and stability of target analysis; The functional coating used for stealth aircraft can absorb radio waves to prevent being detected by radar; In addition, the aluminum oxide coating film also has the functions of blocking ultraviolet rays and carrying out photocatalysis, so it also has a good application prospect in solar panels.