Alumina ceramics usually refer to ceramic materials with an α-Al₂O₃ content of 70% or more. Alumina ceramics with high α-Al₂O₃ content are sometimes called corundum after the mineral name of its main crystalline phase.

Compared with ordinary ceramics, alumina ceramics in the composition and manufacturing process is very different. Ordinary ceramics are mostly raw materials preparation, billet forming and kiln firing three processes to complete, while most of the alumina ceramics are manufactured using powder sintering technology method.

Alumina ceramics are structurally stable and are the most widely used and produced ceramic material among oxide ceramics. Generally speaking, the higher the alumina content in alumina ceramics, the better its performance.

Alumina is called a ceramic mainly because it has the basic properties and structural characteristics of a ceramic material. Although alumina is a specific compound (Al₂O₃), when it is subjected to a ceramic preparation process (e.g., molding, sintering), it forms a material with ceramic properties.

Alumina ceramics have the basic properties of traditional ceramics such as high hardness, high wear resistance, high temperature resistance, corrosion resistance, and also excellent electrical insulation properties. These properties make alumina materials meet the broad definition of ceramics.

Like modern high-tech ceramics, alumina ceramics are made from high-precision raw materials, which is why they are also known as precision ceramics, specialty ceramics or high-tech ceramics. Its ability to precisely control composition and have anisotropic properties distinguishes it from conventional ceramics.

Alumina ceramics and glass with inorganic non-metallic materials, but the two have significant differences: ceramics belong to polycrystalline materials, including crystalline phase and a small amount of glass phase, while ordinary glass is a typical amorphous material; ceramics are more anisotropic, excellent mechanical properties (high hardness, wear resistance, high flexural strength), while the glass isotropic, tensile and folding performance is weak, prone to brittle fracture; in the chemical stability of alumina ceramics Chemical stability, alumina ceramics are more resistant to acid and alkali corrosion than general glass; transparency, general ceramics are opaque, only high-purity dense alumina can be made semi-transparent, while the glass is naturally transparent but poor mechanical properties.

Aluminum oxide and zirconium oxide (ZrO₂) are two common engineering ceramics that each have their own characteristics in terms of composition, performance and cost:

• Composition and Appearance: Alumina is mainly composed of Al₂O₃, white or yellowish in pure form; Zirconia is ZrO₂, usually milky white, close to white in high purity, both of which can be added with coloring agent (e.g., chromium) to make colored ceramics.
• Density: Zirconia is much denser than aluminum oxide (about 6.0 g/cm³ vs. 3.9 g/cm³), about 1.3-1.5 times denser than the latter, so the same volume of zirconia is heavier.
• Mechanical properties: alumina is harder (Mohs hardness ≈ 9) and wear-resistant, but the toughness is general; zirconia has excellent toughness and can be toughened by phase change to make the material obtain high fracture toughness. Usually zirconia compressive strength and flexural strength is higher than alumina, suitable for occasions subject to impact; while alumina is slightly inferior to zirconia in terms of flexural strength. Industry comparisons suggest that “alumina typically has higher hardness and wear resistance, while zirconia has higher flexural strength and toughness.”
• High temperature resistance: Zirconium oxide has a melting point of about 2715°C, which is significantly higher than aluminum oxide (2050°C). This makes zirconia more stable in very high temperature environments, but in practice alumina is sufficient to cover most high temperature needs.
• Thermal and electrical insulation: Aluminum oxide has a higher thermal conductivity than zirconium oxide and is an excellent electrical insulator; zirconium oxide has a lower thermal conductivity but excellent thermal shock and chemical stability.
• Cost: Alumina is relatively inexpensive due to its abundant source of raw materials and mature preparation process; zirconia is more expensive due to its high preparation cost (stabilization, e.g., rare-earth doping). Overall, both are good at what they do, and the choice needs to be weighed against material performance and cost.

The machining of alumina ceramics is a precision technology. Due to the high hardness of the ceramic material, it is difficult to cut and process asymmetric products with complex shapes.

Raw material preparation: alumina powder purity, particle size and distribution and activity are important factors affecting the alumina ceramic process and product performance. Commonly used alumina powder can be divided into ordinary alumina powder for industrial use and high purity, ultrafine active powder.

Sintering process: pure alumina powder in the sintering process, accompanied by the growth of grains, and some grains grow abnormally (called the second recrystallization), which will cause product cracking or pores are encapsulated into the grain to form defects and can not reach the full densification.

The addition of additives such as MgO inhibits secondary recrystallization, bringing the alumina ceramics close to the theoretical density and making them translucent.

Atmosphere and pressure control: Even Al₂O₃ ceramics with added MgO and rare-earth oxides can only approach theoretical densities when sintered in air; only sintering in a vacuum or hydrogen provides better light transmission.

99 alumina ceramics and 95 alumina ceramics are two common high-purity alumina ceramic materials that differ in properties and applications.

• 99 Alumina Ceramics: refers to alumina ceramics with Al₂O₃ content around 99%. This material is used to make high-temperature crucibles, refractory furnace tubes and special wear-resistant materials, such as ceramic bearings, ceramic seals and water valve pieces.
• 95 alumina ceramics: refers to the Al₂O₃ content of 95% or so alumina ceramics. Mainly used as corrosion-resistant, wear-resistant parts. 85 porcelain in the part of the talc is often doped to improve the electrical properties and mechanical strength, can be sealed with molybdenum, niobium, tantalum and other metals, some used as an electric vacuum device devices.

High-purity alumina ceramics refers to ceramic materials with Al₂O₃ content of 99.9% or more, which are generally made into fused glass to replace platinum crucibles due to their sintering temperature of up to 1,700°C and transmission wavelengths of 1 to 6 μm.

With technological advances, new electronic ceramic materials are constantly being developed. Alumina ceramics, as one of the most widely used electronic ceramics, are also being improved and innovated.

Alumina ceramics are not only traditional refractory materials, engineering ceramics, but also the most widely used electronic ceramics and a bioceramic. Its intrinsic brittleness is being improved by compounding with ZrO₂, CeO₂, TiC, SiC, etc., and a kind of complex-phase ceramics with good toughness and resistance to mechanical impacts, but good high-temperature strength is emerging.

In recent years, with the growing demand for transparent alumina ceramics and sapphire crystals, the preparation technology of high-purity ultrafine and even nano alumina powders has been rapidly developed.

Transparent alumina ceramics are prepared using high-purity ultrafine powders with a purity of 4N (99.99%) and a particle size of less than 100 nm, while sapphire crystal growth requires powders with a purity of 5N (99.999%).

Selection of alumina ceramics should examine the manufacturer's production process capabilities, raw material purity control, quality management level and its experience in specific applications. High-performance alumina ceramics require fine powder formulation and high-temperature sintering process, the manufacturer's preparation equipment, technical level and R & D innovation directly affect product performance.

At ZhiHao Ceramics, we specialize in the development and production of high-performance alumina ceramic wafers, and are able to provide customers with a full range of product solutions from 95 alumina ceramics to 99.8 alumina ceramics.