Breakthroughs in spherical alumina preparation by researchers at home and abroad |
At home and abroad, the production of alumina with good particle size has become a hot issue. Different synthetic methods were used to prepare morphological aluminas powders. These included fibrous, sand and plate, rods, and porous alumina membranes. In recent years, due to the rapid globalization of the industry, spherical aluminum powder has undergone extensive research. Synthetic spherical Alumina is mainly used as abrasives, ceramic powders, catalysts and carriers and in chemical mechanical polishing.
Different crystal forms, unique physical and chemical characteristics of alumina, and its hydrates, have led to their wide use in a variety of fields, including petrochemicals, electronic refractory ceramics, aerospace, pharmaceuticals, and abrasives. The morphology and particle size of raw materials have a direct relationship to application performance. Powder particles with different shapes have a spherical shape, which has a regular morphology. It also has a low specific surface, high bulk density, and good bulk density. The flow characteristics can significantly improve the performance of the material.
Presently, the following methods are reported as being used to prepare ultrafine spherical Alumina: ball-milling, homogeneous precipitation, sol-emulsion gel method, dropping ball, template, aerosol decomposition and spraying. The particle sizes of the spherical aluminum produced by these methods range from nanometers to microns.
Most commonly, ultrafine alumina is prepared by ball milling. Ball mills are used to grind and stir the material, which is impacted and re-milled by the abrasive. Lu Baiping has studied the factors that affect the ultra-fine alumina particle size. The ball milling process is prolonged and the speed of the ball milling increased to reduce the particle sizes. Grinding aid can be added during the ball milling process to improve the uniformity. It is possible to prepare ultrafine powder by ball milling. This method is low-cost, easy to use, and produces high yields. However, there are some limitations. These include an uneven distribution of particle size, a mechanically restricted minimum particle size, and the difficulty in obtaining spherical alumina particles.
In a homogeneous mixture, the precipitation process is nucleation followed by aggregation, growth and then precipitation. This process is usually non-equilibrium. However, if the precipitant concentration in the homogeneous mixture is reduced or even slowed down, it will result in a uniform formation of many tiny crystal nuclei. The fine precipitated particles that are formed will also be evenly dispersed in the solution and maintain equilibrium for a long time. It is homogeneous.
This method was developed based on the sol-gel technique. Initially, the sol gel method was mainly used to make alumina sol and to study the structure. Gradually this method developed into a superfine dust. For spherical particle powders, it is necessary to create a small droplet using the interfacial friction between the oil and water phases. Lastly, spherical precipitated particles are obtained. Takashi Ogihara et al. Sol-gel was used to prepare spherical aluminum powder using the aluminum alkoxide. The entire hydrolysis process is complex, with acetonitrile as the solvent, octanol in aluminum aluminum dispersed, and butyl and octanol water being dispersed. The alcohols accounted for 9% and 1%, respectively, and hydroxypropylcellulose was used as a dispersing agent to obtain spherical γ-alumina powder having a very good sphericity.
Drip Ball Method
Dropping the alumina into the oil layer is usually done using mineral oil or paraffin. Surface tension will form spherical sol particle by the action. Then, the sol particles will be gelled in an aqueous ammonia solutions, and finally, the gel particles obtained. This is a method for drying and calcining the alumina to produce spherical particles. This method improves the Sol-Emulsion-Gel method by applying emulsion to the aging of the sol. The oil phase is kept stationary and this eliminates the separation of powder from oily reagent. This method is usually used to prepare large spherical particles of alumina, which are then applied to adsorbents or catalyst carriers.
The templating technique uses a material that is spherical to control the morphology of the process. Usually, the resultant product has a hollow core or shell structure. Jin Lu used a microsphere of carbonaceous material enriched with carboxylates to prepare hollow spherical Alumina.
Aerosol decomposition is usually done using aluminum alkoxide. It uses the properties of high temperature pyrolysis and easy hydrolysis to vaporize aluminum alkoxide. Contact with water vapor then hydrolyzes and atomizes. After high temperature drying, or direct high temperatures pyrolysis the gas-liquid phase transformation or gas solid phase is achieved. The key to this method is a complex experiment consisting of both an atomized part and a reacting portion.
The spraying method of preparation of spherical Alumina is to achieve phase transformation within a short period of time. Surface tension is then used to spheroidize the product. The spraying method can be divided according to the phase transformation. It includes spray pyrolysis and drying, as well as spray melting. M. Vallet-Regi et al. AIC1, Al(SO), and AI(NO3)3 were atomized to form a spherical solution. A spherical dust was then formed after high-temperature combustion. This process is energy-intensive and requires a decomposition temperature at 900 °C.
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