Introduction to oxide ceramics in advanced structural ceramics

Oxide ceramics mainly include alumina ceramics, zirconia ceramics and Beryllium oxide ceramics. Here's a detailed introduction!

① Alumina ceramics

Alumina ceramic is a ceramic material with the main crystal phase of mouth A1203. The content of A12 03 is generally between 75% and 99% Between 9%. Usually classified based on the Al2 03 content in the ingredients. The content is about 75% for "75" porcelain, 85% for "85" porcelain, 95% for "95" porcelain, and 99% for "99" porcelain. Alumina mainly has three crystal forms of d, B and y. B-Al203 is a Aluminate containing alkaline earth metal or alkaline metal, hexagonal lattice (a=0 56nm, c=2.25nm), density 3.3-3.63g/cm: 31400-l500 ℃ begins to decompose, and transforms into d-A12 03 at 1600 ℃,; R-Al2O3 belongs to the spinel type (cubic) structure, with oxygen atoms densely packed in cubes and aluminum atoms filled in gaps. Its density is relatively small, ranging from 3.42 to 3.47g/cm. It is unstable at high temperatures and has poor mechanical and electrical properties. It is rarely used as a separate material and can be converted into a-Al2 () 3 at 1500 ℃; Among several crystal forms, d. Al2() 3 is the most stable and high-temperature form. Its stable temperature is as high as the melting point, and its density is 3 96 ～ 4. Olg/cms, Hexagonal crystal family, corundum structure, "-0.476nm, c=1.299nm. In nature, it exists in minerals such as natural Gangwang, Ruby, Sapphire, etc. The a-A1203 has the most compact structure, low activity, and high temperature stability. It has the best electrical properties, excellent electromechanical properties, and a Mohs scale hardness of 9. Alumina porcelain has a series of characteristics such as high mechanical strength, high volume resistivity, good electrical insulation, high strength, wear resistance, and oxidation resistance, and is widely used as structural components and work Capable of installing ceramic components, such as wear-resistant and corrosion-resistant components used in the fields of machinery and chemical technology; Refractory materials used in crucibles, protective tubes, and metallurgical industries; Ceramic components for electronic industries such as substrates, insulators, radar radomes, and microwave materials. Alumina ceramics are one of the advanced ceramics that have been studied earlier, widely used, and matured.

② Zirconia ceramic

Zirconia ceramic is a newly developed and important structural ceramic, second only to alumina ceramic. There are three types of zirconia. At room temperature, it is monoclinic with a density of 5 65g/cm3, heated to around 1170 ℃ and transformed into a tetragonal crystal with a density of 6.lg/cm3, heated to 2370. C transforms into a cubic crystal form with a density of 6 27g/cm3, melted to around 2700 ℃. The above changes are reversible. The transformation between monoclinic and tetragonal crystal forms is accompanied by a volume change of about 7%. When heated, the southern monoclinic Zr() p transforms into tetragonal ZrO2, resulting in volume contraction. When cooled, it transforms from tetragonal ZrOz to monoclinic Zr() 2, resulting in volume expansion. However, this shrinkage and expansion do not occur at the same temperature, with the former at around 1200 ℃ and the latter at around 1000 ℃, accompanied by crystal transformation and thermal effects. Zirconia has the characteristics of high melting point, hardness, strength and toughness, low Specific heat capacity and thermal conductivity, and can form oxygen vacancy defect Solid solution. It is widely used as structural ceramics and functional ceramics, such as tools, mechanical parts, advanced refractories, high-temperature anion conductors, oxygen sensors, etc. However, a major drawback of zirconia ceramics is the severe attenuation of their strength and toughness at high temperatures, which limits their application under high temperature conditions.

③ Beryllium oxide ceramics

Beryllium oxide belongs to the Hexagonal crystal family, which has the same type of crystal structure as Wurtzite. Its structure is stable and amorphous. Very dense. The melting point of BeO is as high as (2570 ± 30) ℃, with a density of 3.028g/cm3 and a Mohs degree of 9. Be0 ceramics have a thermal conductivity similar to that of metals, which is A12 (). 15-20 times. Therefore, it can be used as a heat dissipation device; Be0 ceramics have good high-temperature insulation performance, high dielectric constant, and increase with increasing temperature. The dielectric loss is small, and also slightly increases with increasing temperature. Therefore, it can be used to manufacture insulation materials with high temperature to volume resistance. Be0 ceramics can resist the erosion of alkaline substances (except for caustic soda) and can be used as crucibles for melting rare metals and high-purity gold filings such as beryllium, platinum, and vanadium. Be () ceramics have good nuclear properties, strong neutron deceleration ability, and high penetrating power to a-rays, and can be used as Neutron moderator and radiation shielding materials for atomic reactors. In addition, the coefficient of thermal expansion of Be0 is not high, and the mechanical strength is not high, about 1/4 of the mouth AI203, but it does not decrease significantly at high temperatures. Be0 is highly toxic, which is caused by dust and steam. Precautions must be taken during operation. However, sintered Bc0 ceramics are non-toxic and safety precautions should be taken during production.

④ Mullite ceramics

Mullite is the only stable compound in the Al 203-Si02 binary Meta-system. Its melting point is 1800 ℃, and its composition is uncertain, generally between 2Al 203. Si02 and 3Al 203. Si02. It is generally believed that the stoichiometric formula of mullite is 3Al 203 · 2Si02. Mullite has advantages such as good high-temperature mechanical properties, low thermal conductivity, coefficient of thermal expansion, density, and good creep resistance. However, its disadvantages are poor mechanical properties at room temperature and difficulty in sintering. Mullite ceramics are widely used in the fields of high-temperature structural ceramics and refractory materials and have shown good potential. Mullite ceramics can be used to manufacture thermocouple protection tubes, electrical insulation tubes, high-temperature furnace linings, as well as to manufacture polycrystalline mullite fibers and components of high-frequency device ceramics, such as high-frequency and high-voltage insulators, coil frames, capacitor shells, high-voltage switches, bushings, and other large device components. In addition, due to its surface microstructure, it can also be used as a substrate for carbon film resistors. Composite with other ceramics is one of the main ways to improve its room temperature mechanical properties and expand its application range.

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