The classification of purity of alumina ceramics and their applications after processing

The purity of alumina ceramics determines their performance: The 99.9% high-purity type can withstand temperatures up to 1650°C and has excellent light transmittance, making it suitable for semiconductor substrates and laser devices; the 95% ceramic has excellent wear resistance and can be used in industrial valves with a lifespan of up to 100,000 cycles; the 85% ceramic can be metal-sealed and is suitable for medical tools. In the future, it will make breakthroughs in nanocomposite and green manufacturing.

The performance and application of alumina ceramics (Al₂O₃) are closely related to their purity. The higher the purity, the better the properties such as hardness, high-temperature resistance, and electrical insulation. Depending on the purity level, alumina ceramics are mainly classified into high-purity type, ordinary type, and various other purity grades. After different processing techniques, they are widely applied in multiple fields.

High-purity alumina ceramics (Al₂O₃ ≥ 99.9%) have extremely high purity (99.9%-99.99%), with sintering temperatures ranging from 1650 to 1990℃, excellent light transmittance (transmitting wavelengths of 1-6μm), and a thermal conductivity of up to 32 W/(m·K). These high-purity ceramics perform exceptionally well in high-temperature device applications, such as serving as an alternative platinum crucible for molten glass and sodium lamp tubes. They are favored due to their resistance to alkali metal corrosion.

In the semiconductor and electronics fields, high-purity alumina ceramics are used in integrated circuit substrates, high-frequency insulating materials, and the lining of semiconductor equipment chambers. These applications require low particle contamination and excellent vacuum sealing. Additionally, its applications in the optical field are also very extensive, such as laser reflectors and sapphire crystal substrates. These applications fully utilize the combined advantages of its light transmission and high hardness.

The common types of alumina ceramics include 99 porcelain (Al₂O₃ ≈ 99%), 95 porcelain (Al₂O₃ ≈ 95%), and 85 porcelain (Al₂O₃ ≈ 85%). The flexural strength of 99 porcelain is 310 MPa, its thermal conductivity is 29 W/(m·K), and it has a yellowish color. It is mainly used in high-temperature equipment (such as high-temperature crucibles and refractory furnace tubes) and wear-resistant components (such as ceramic bearings, mechanical seals, and water valve plates). The flexural strength of 95 porcelain is 280 MPa, its density is 3.7-3.8 g/cm³, it is milky white, and is commonly used in industrial wear-resistant parts (such as ball valves, grinding media, and chemical pump accessories) as well as electronic packaging (such as circuit boards and corrosion-resistant insulating components).

The 85 porcelain type enhances mechanical strength by adding talcum powder and can be sealed with metals (such as molybdenum and niobium). It is widely used in electrical vacuum devices (such as vacuum switch tubes and electronic tube components) and medical tools (such as corrosion-resistant surgical instrument parts). Additionally, there is 96%-97% porcelain, which is mainly used in the medical field (such as artificial joints and blood valves) as well as precision nozzles; 92% porcelain is used for industrial lining bricks and grinding media; 75% porcelain is used as a low-cost refractory material, but it is gradually being phased out.

The processing technology has a significant impact on the application scenarios of alumina ceramics. Dry pressing is suitable for simple geometric parts (such as ceramic bearings), with an efficiency of up to 50 pieces per minute; injection molding is suitable for complex thin-walled parts (such as ceramic membranes and artificial bones); additive manufacturing (3D printing) can be used to manufacture porous structures or customized irregular parts (such as bone scaffolds). In terms of sintering technology, atmospheric pressure sintering is suitable for cost-effective mass production (such as 95 porcelain substrates), while hot isostatic pressing (HIP) is used for high densification (such as 99.5% porcelain semiconductor substrates with a density of ≥ 99.5%). Post-processing techniques include precision machining (such as diamond grinding, with a roundness error of ceramic bearings ≤ 0.1 μm) and surface metallization (such as magnetron sputtering gold/silver plating, used for circuit wiring on electronic packaging substrates).

Alumina ceramics perform exceptionally well in several typical application fields. In the mechanical industry, the cutting speed of its cutting tools is 50% higher than that of hard alloys, and the lifespan of its wear-resistant ball valves can reach 100,000 times. In the electronic and power sector, the dielectric loss of its high-frequency insulation substrates is ≤ 0.0002, and the light transmittance of transparent ceramics for LED packaging is ≥ 85%. In the medical field, the wear rate of artificial joints is only 1/10 that of metals, and dental restoration materials are also widely used due to their excellent performance. In the chemical and energy sectors, its corrosion-resistant pumps and valves and lithium battery separator coatings (with nano-alumina enhancing safety) perform outstandingly. In the aerospace field, its transparent covers and high-temperature sensor components require heat shock resistance, and alumina ceramics can also meet these requirements.

In the future, the development trends of alumina ceramics will include further improvement in purity (such as the expansion of high-purity ceramics with a purity of over 99.9% in the semiconductor and nuclear industries), composite toughening (through ZrO₂ phase transformation or SiC whisker reinforcement, with the target fracture toughness reaching 8 MPa·m¹/²), and green manufacturing (such as a circular economy model with a waste recycling rate of ≥ 90%).

In conclusion, the diverse purity classifications of alumina ceramics (ranging from 75% to 99.99%) enable them to exhibit high adaptability in fields such as mechanics, electronics, and medicine. With the breakthroughs in technologies like nano sintering and intelligent hot pressing, their application boundaries will further expand to cutting-edge areas such as new energy and quantum devices.