1. Brief introduction of flake alumina powder
Flaky alumina powder is a functional fine powder material with excellent performance, which belongs to α-Al2O3. It has obvious scaly structure characteristics and a large diameter-thickness ratio. At present, the radial size of flaky alumina grains is generally 5-50 μm, and the thickness is generally between 100-500 nm. The finely-developed particles also exhibit regular hexagonal morphology.
The diameter-to-thickness ratio of the flake alumina fine powder synthesized by the artificial method is adjustable, and has the advantages of high purity, smooth and smooth surface, and good dispersibility in water. The particle size of industrial grade flaky alumina powder can reach micron size, and the thickness reaches nanometer or near nanometer scale. This not only makes it have significant light reflection ability and shielding effect, but also makes it reflect good surface activity and superior adhesion. It has been widely used in coatings, cosmetics, polishing powders, automotive finishes, toughened ceramics, etc. field. At the same time, because its price is dozens of times that of metallurgical grade alumina, the flake alumina preparation process has great research significance.
2. Preparation method of flake alumina powder
(1) Hydrothermal (alcoholic) method
Hydrothermal crystallization is a common method for preparing flake aluminum oxide in the early days. Yasuo et al. Ground alumina with a ball mill into submicron particles (particle size ≤ 1 μm), and hydrothermally crystallized it in an alkaline solution to prepare alumina platelets with a thickness of less than 0.1 μm.
Although the flaky alumina powder prepared by the hydrothermal method has high purity, good dispersibility, and has a thin flat plate structure and uniform and excellent crystal morphology, hydrothermal synthesis not only has a long cycle, but also requires high temperature and pressure during the reaction The reactor has a strong dependence on the equipment, and the phase transition temperature of the crystal is relatively high (> 400 ° C).
(2) molten salt method
The basic principle of the molten salt method is to dissolve the reactant in one or more low-melting salts at a certain temperature and synthesize the target product at a lower temperature. Preparation of powders by the molten salt method is mainly divided into two processes: the process of grain formation and grain growth.
Nitaa et al. First used the molten salt method for the preparation of flaky alumina. They used sulfate as the molten salt and a small amount of titanium dioxide and phosphate as additives to induce the crystallization of the soluble aluminum salt. The colloid produced by the reaction was 1200 ° C. It was calcined for 5 hours to obtain a flake-shaped hexagonal powder having a particle size of 3 to 22 μm and a thickness of 0.2 to 0.5 μm.
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 powder, good crystal morphology, and high phase purity. However, in the preparation process of the molten salt method, The decomposition of toxic and harmful gases will cause great damage to the calcining equipment. After calcination, the molten salt, powder and crucible form a complex solid mixture, and subsequent processing is more difficult.
(3) Sol-gel method
The sol-gel method uses a soluble salt as the aluminum source (aluminum sulfate or aluminum hydroxide), hydrolyzes and condenses in a liquid phase system, and crystallizes. After drying, the precursor γ-Al2O3 can be prepared and then sintered in a high temperature environment. The solidification is completed from the γ phase to the α phase. This method can overcome the shortcomings of the reaction kettle that requires high temperature and high pressure in the hydrothermal process, so the requirements on the equipment are relatively low, and it is widely used. The disadvantage is 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 flaky alumina by the sol-coating method is similar to the preparation of high-purity and ultra-fine alumina by the sol-gel method. This method first prepares an oxide precursor into a sol, and then coats the sol on a smooth substrate. After the film is placed in a dryer, the oxide or hydroxide flakes can be obtained. After further calcination, the desired flake alumina powder can be obtained.
The products prepared by this method are easy to control various process parameters, such as particle size, thickness, chemical composition, etc. In addition, the required equipment is relatively simple and easy to operate, but the mechanical strength of the obtained product is not high and the particle size distribution is uneven. Products that require a range of particle sizes require further screening.
(5) Mechanical law
The mechanical method is to mix a certain proportion of powder under the action of mechanical force. Generally, ball milling, stirring mill, colloid mill, and vibration mill are used. During long-term operation, the powder is repeatedly impacted by the grinding medium. After repeated extrusion, cold welding and pulverization, it becomes a method of dispersing superfine ions.
This method is easy to operate and low in cost, but the powder obtained is low in purity. In addition, the mechanical force is complicated and changeable, which makes the powder structure difficult to control, and it is easy to produce a considerable number of broken platelets. Polishing may cause nicks and surfaces. Under the damage.
Hibbert et al. Used a wet method to grind the raw materials by mechanical methods to prepare flaky alumina powders with a crystal particle size of 3-20 μm. The powder prepared by this method has good dispersibility, but there are still many Fine fragment crystals. Wet ground powder can be used to remove fine particles by sedimentation classification, and the obtained powder can be used as a polishing powder for microelectronic substrates.
(6) Carbon fraction method
The carbon fractionation method belongs to the hydrolysis method of sodium aluminate. Generally, carbon dioxide is used to decompose the sodium aluminate solution, and then an appropriate crystal form control agent is added.
(7) High temperature sintering method
Kebbede et al. Studied the microstructure of alumina prepared by adding titanium dioxide, composite doped titanium dioxide, and silica. The single-doped titanium dioxide sample sintered at 1450oC for 2h was equiaxed, while the double-doped sample obtained platelets, but the crystal size distribution was uneven and the diameter-to-thickness ratio was only 3.4.
3. Application of flake alumina powder
(1) Abrasive polishing liquid
Compared with the conventional nano-alumina, the flat and smooth flake-shaped surface is not easy to be scratched by the object being polished (such as semiconductor silicon wafers, smartphone casings, etc.), and the qualified rate of the product can be increased by 10% to 15%. . Therefore, flake alumina has become the new favorite of high-precision microelectronics industry, gem processing industry and cermet industry.
(2) Pearlescent pigment
Flaky aluminum oxide has stable chemical properties, wide coverage area, high refractive index, and its unique optical properties enable pigments to have a strong pearlescent effect. Therefore, flaky aluminum oxide has been used as a substrate material for pearlescent pigments, which has become a superior development property. New hotspots of pearlescent pigments and industry economic growth points.
(3) Inorganic filler
Flaky alumina is an indispensable filler in industrial production. It is used in functional ceramics, plastics and rubber products to increase hardness and stiffness, and to adjust shrinkage and thermal expansion coefficient.
(4) Cosmetics
Flake aluminum oxide has a narrow particle size distribution, a large diameter-thickness ratio, stable chemical properties, a smooth and flat surface, and can be well dispersed in water. Most importantly, this powder is non-toxic and tasteless, so it is also widely used in cosmetics. field. 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 when attached to the skin surface. It has excellent spreadability and adhesion properties, which can effectively prevent powder makeup from falling off due to oil secretion or skin sweating.
(5) Functional coating
The flaky alumina with excellent performance has no agglomeration phenomenon, and has good adhesion, and is easy to be combined with other functional fine powders to prepare a variety of attractive new functional coating films. The aluminum oxide coating used for the capillary can significantly improve its selectivity and stability for reversing electroosmotic performance and target analysis; the functional coating film for stealth aircraft can absorb radio waves to prevent it from being searched by radar; in addition, the aluminum oxide coating It also blocks ultraviolet rays and performs photocatalytic functions, so it also has a good application prospect on solar panels.