In the fields of precision machinery and optics, product surface quality is often critical, affecting not only aesthetics but also production efficiency and ease of use. To meet this demand, physical and chemical methods are widely used to reduce surface roughness, a process known as "polishing." Polished products often undergo a dramatic change in surface appearance. Take liquid crystals as an example; unpolished LCD products exhibited diffuse reflection, resulting in dull colors. Today, large-screen LCD TVs, high-end monitors, and phone screens all utilize polishing technology, significantly improving contrast and making colors far more vibrant than unpolished screens.

In LCD polishing, the key is using high-speed rotating fine abrasives, known as "polishing powder," to abrade the surface, making it smooth. These powders typically consist of alumina, silica, zirconia, ceria, etc., each with different hardness and chemical properties in water, leading to varied applications.

For example, SiO2 polishing powder offers good light transmission but relatively slow cutting speed; diamond or SiC powders have high hardness and fast cutting but may leave scratches. Alumina polishing powder, with its moderate hardness, excellent light transmission, fast cutting speed, simple production process, and low cost, is widely used in machinery manufacturing, optical processing, jewelry processing, and automotive repair and maintenance, bringing significant economic and social benefits to China.
Composition and Preparation of Alumina Polishing Powder

The main component of alumina polishing powder is α-Al2O3. During synthesis, various auxiliary components are added, such as dispersants, suspending agents, lubricants, and emulsifiers, to further enhance polishing performance.
The structure and morphology of alumina polishing powder vary depending on the application. In chemical mechanical polishing, spherical and flake-like morphologies are commonly used. This choice is based on their properties, effectively meeting different polishing needs to ensure optimal results.
① In chemical mechanical polishing, using spherical alumina (especially ultrafine powder) significantly improves removal rate and polishing speed, while preventing micro-scratches and achieving high gloss. This is ideal for fine processing of high-end products such as semiconductor single crystals, quartz crystals, and precision optical components.

② When using flake-like alumina, its excellent dispersion stability and ordered parallel arrangement allow its upper and lower surfaces to remain largely parallel to the polished material's surface. This minimizes subsurface damage, ensuring a mirror-smooth finish, particularly suitable for ceramic polishing and fine processing of glass surfaces such as optical microscopes.
Controllable Synthesis of Alumina Powder

Given the significant impact of microstructure on alumina's properties, researchers have focused on morphology control techniques. By adjusting raw materials, additives, and process parameters like temperature, the microstructure and crystal structure can be finely tuned to obtain ideal products for specific needs.

Preparation Methods for Spherical Alumina Powder

① Precipitation Method: Raw materials are dissolved, and a precipitant is added to induce aluminum salt precipitation. After dehydration, drying, and heat treatment, ultrafine spherical alumina powder is obtained.

② Sol-Emulsion-Gel Method: This method cleverly utilizes interfacial tension between oil and water phases to form tiny spherical droplets. Inside the droplets, sol particle formation and gelation are strictly controlled, yielding spherical alumina precipitate particles.
 

2. Preparation Methods for Flake-like Alumina Powder

① Mechanical Method: Ball milling, stir milling, ultrafine airflow pulverization, and vibration milling are used to mix and pulverize raw materials into ultrafine particles. Prolonged grinding subjects particles to impact, friction, and shear forces, leading to refinement.

② Coating Method: A precursor is prepared into a sol and uniformly coated onto a smooth substrate. After drying, the film is peeled off to obtain flake-like alumina powder. Heat treatment can further adjust the film's properties.

③ Hydrothermal Method: Raw materials are placed in a sealed container (e.g., autoclave) at high temperature and pressure, with water as the reaction medium. After dissolving in the molten salt, alumina grains form and grow into a flake-like structure.

④ Molten Salt Method: Since the mass transfer rate in molten salt is much higher than in solid-state reactions, this method significantly reduces reaction temperature and time. It also effectively controls grain size and shape, synthesizing powders with specific morphologies.

The molten salt method effectively controls the grain size and shape of flake-like alumina powder, making it advantageous for preparing powders with specific morphologies. It also lowers reaction temperature and time, improving efficiency.
With rapid advancements in high technology, the demand for polishing powders is increasing. Predictions show that the global market size will surge from $140 million in 2018 to $250 million by 2025 in most countries, presenting vast opportunities. Alumina polishing powder preparation technology will likely evolve towards lower cost and higher performance, posing significant challenges. Therefore, we must prioritize research and development in this area.