Alumina belongs to the rhombohedral system with a melting point of 2054 ℃ and a boiling point of 2980 ℃. Due to the difference of steam partial pressure, temperature and pressure, there are many kinds of Al2O3 isomorphs before Al (OH) 3 is completely dehydrated to α-Al2O3 at high temperature. It is reported that there are 12 kinds of crystal structures, such as α, β, γ, η, δ, κ, θ, and ρ, the most common of which are α, β and γ. Their density, specific surface area, porosity and acidity vary due to different dehydration conditions and initial aluminum hydroxide.
α-Al2O3 is a rhombohedral system, also known as corundum. The O _ 2-in its Al2O3 crystal is packed closely in hexagonal, while the smaller particle spacing makes the crystal firm and stable. Among all the alumina, α-Al2O3 has the highest density and the most stable crystal phase. Its density is related to impurities and remains about 3.99g/cm3. α-Al2O3 is mainly found in rubies, sapphires and natural corundum. Although the activity of α-Al2O3 is low, it has excellent properties such as high hardness, high strength, wear resistance, chemical corrosion resistance and good dielectric properties, so it is widely used in refractories and ceramics.
As a kind of alkaline aluminate, the density of β-Al2O3 is about 3.30g/cm3. The stability of β-Al2O3 is poor, and α-Al2O3 and Na2O will be decomposed from it after heating. The interior of Na- β-Al2O3 crystal has a layered structure. Na+ can move and ion exchange freely between layers, so that it has relaxation polarization and ionic conductivity, while Na+ can not be diffused parallel to the C-axis, so that it has no conductivity in the parallel C-axis direction. Using this property, β-Al2O3 can be used as a diaphragm material in sodium-sulfur battery, and widely used in electronic, medical, mechanical and other fields.
As a transition phase Al2O3, the density of γ-Al2O3 is about 3.42g/cm3. The O2-in γ-Al2O3 crystal structure is distributed at the vertex of cubic lattice, which is closely packed as cubic, and Al3+ is distributed in the packing gap of O2 -. γ-Al2O3 has strong loose adsorption, easy to absorb water and can be dissolved by acid and alkali. Its large specific surface area makes it have high surface energy and high activity. At the same time, γ-Al2O3 will be transformed into stable α-Al2O3 with the increase of temperature, and its volume density increases and volume shrinks at the same time, about 14.3%. The conversion rate and degree of conversion depend on the purity of alumina, preparation conditions and particle size.
Aluminum oxide properties
Alumina has extremely high temperature stability and can withstand atmospheric temperatures of 2900 degrees Fahrenheit or 1650 degrees Celsius and vacuum temperatures of up to 3000 degrees Fahrenheit or 2000 degrees Celsius. Parts made of alumina maintain 50% of their room temperature tensile strength at 1000 ℃.
Wear is the behavior of friction and wear material, changing its structure and reducing its applicability under the required conditions. Alumina has a very high hardness, making it very resistant to wear and other types of mechanical wear. This means that parts and components made of alumina will maintain their desired performance and shape for longer.
Alumina is chemically inert and unreactive, which means that it is highly tolerant to different types of chemicals, including bases and acids. Alumina does not cause surface and structural damage due to exposure to salt and solvent solutions at high temperatures, which further prolongs the durability and life of different parts.
Alumina is composed of fine particles with no voids in the material. the less the voids, the higher the density and volume. The density of alumina is different at different temperatures, and the standard density is 3.965g/cm3 at 25 ℃.
The mechanical properties of materials are related to their strength, which refers to the strain and stress that the material can withstand before fracture. Alumina has a very high strength, only a higher purity can be further improved.
Dielectric and thermal properties.
Alumina is very suitable for insulation because it prevents current from passing through it. In addition, it is very resistant to thermal shock, which is also one of the characteristics of this technical ceramic, which is further improved with the improvement of purity.
Hardness refers to the ability of a material to withstand mechanical wear and wear. In this respect, alumina is harder than tungsten carbide and steel tools. It has the hardness of HRA80-90, which only diamonds can surpass.
Aluminum oxide Uses
Alumina has excellent physical and chemical properties, so it is widely used in ceramics, abrasives, refractories, petrochemical and many other fields. In order to meet the needs of different industries, alumina with different particle size, specific surface area and shape can be obtained by special treatment of Al (OH) 3 or γ-Al2O3, such as roasting, grinding, hydraulic separation and so on. For ultra-fine active alumina (γ-Al2O3 or η-Al2O3), it is generally used as adsorbent or catalyst carrier; columnar or spherical alumina is usually used as carrier or ceramic membrane filter; high temperature series alumina is mainly prepared by calcination of industrial alumina with co-solvent and is generally used in ceramics and refractories; polished alumina is mainly used in glass, metal and non-oxide ceramics firing aids.
Aluminum oxide Price
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