Alumina / aluminum oxides

Alumina is the oxygen compound of the chemical element aluminum. In the technical field, alumina is referred to as electrocorundum (ELK).

Alumina

Extraction and presentation

From bauxite aluminum hydroxide is obtained by digestion in sodium hydroxide solution (Bayer process). By removing the water, for example by firing, sintering or calcination, alumina is obtained.

The preparation of alumina can also be done by careful dehydration of gibbsite (hydrargillite) or boehmite.

Aluminum oxide is also formed during the combustion of aluminum powder with ammonium perchlorate in solid rockets.

The pure metal aluminum has a thin spontaneous aluminum oxide layer (self-passivation) after storage in air, which protects it from corrosion. By means of an electrolytically applied aluminum oxide layer, the aluminum surfaces of everyday objects are provided with an extremely hard (such as Mohs hardness 9) and corrosion-resistant protective layer by anodizing. For the production of electrolytic capacitors to use these anodized aluminum grades.

The global production of alumina increased from 108 million tonnes in 2014 to an estimated 118 million tonnes in 2015. The three largest producers of alumina were 2014 China (47,8 million tonnes), Australia (20,5 million tonnes) and Brazil (10,6 million tonnes). The USGS quoted average US import prices for aluminum oxide at 410 USD / tonne in 2015, while import prices for bauxite in the same year were at 28 USD / tonne.

As a by-product of the production of alumina, red mud, containing corrosive caustic soda and toxic heavy metals, is stored worldwide in open landfills or discharged into rivers.

Features
modifications

The most important modifications of the alumina are:

the cubic γ-Al2O3 (clay, starting material for ceramic and aluminum production)
the rhombohedral (trigonal) α-Al2O3 (known as mineral corundum, sapphire or - for chromium doping - ruby, as an abrasive and alumina ceramic)
Likewise, the β-alumina (β-Al2O3) is known, this is a historical error. This is the combination of Na2O and Al2O3 to Na2Al22O34 (Na2O · 11Al2O3), also known under the mineral name Diaoyudaoit

Electrical Properties

Al2O3 is a very good insulator and has a very high dielectric strength of 35 kV / mm. The specific resistance at 20 ° C is 1012 Ω m, at 1000 ° C it drops to 107 Ω · m. The relative permittivity is 9-10 at 100 MHz, the loss factor is about 10-4.

Thermal properties

Under standard conditions, the thermal conductivity due to the phonon resonance is at a relatively high value of 35,6-39 for ceramic materials W · m-1 · K-1 (single crystal corundum: 40 W · m-1 · K-1, dense ceramic with 96% Al2O3 25 W · m-1 · K-1), which rises sharply with decreasing temperature and decreases with increasing temperature at 1000 ° C to about 5 W · m-1 · K-1.

The coefficient of expansion is in the range 6,5-8,9 · 10-6 K-1.

The melting temperature is 2072 ° C, therefore the application temperature of high purity alumina ceramics should be below 1900 ° C.

Chemical properties

Al2O3 is an amphoteric salt, that is, it can react as an acid (in combination with a base) or as a base (in combination with an acid).

The γ-Al2O3 is a hygroscopic, white, loose powder that is not soluble in water, but in strong acids and bases. Already as of 800 ° C, the γ-Al2O3 changes into the α-Al2O3 which is generally insoluble in acids such as bases.

γ-Al2O3 is a porous material whose surface structure can be strongly influenced by the manufacturing process or its temperature. In chromatography, it is used as a stationary phase.

Aluminum oxide forms aluminates with various metal oxides.

Mechanical properties

The mechanical properties of the alumina ceramics depend on the purity and microstructure of the ceramic produced. The purer the variety is, the better the properties are usually achieved, but the more complex the entire production process. In addition to the properties listed in the table below, alumina ceramics are also characterized by very good tribological properties and very good friction and wear behavior:

Usage

About 70% of world 120 annual production million tonnes of 2016 aluminum oxide were used in the production of metallic aluminum (Hall-Héroult process).

Hybrid circuit on an alumina ceramic substrate

High pressure sodium discharge tubes with discharge tubes (these are the dull bars inside) made of transparent alumina ceramics

The α-Al2O3 has a Mohs hardness of 9 to 9,5 and is processed among other things to bearing stones of measuring instruments and watches, as well as to abrasives. The basis for this is often the by-product of aluminothermy accumulating Alundum.

Calcined aluminas are used in ceramics (eg in sinks, hotel ware, bulletproof clothing) or in the broadest sense as polishing agents (eg in glass ceramic cleaners, car care products, brake pads, toothpastes). Furthermore, sintered α-Al2O3 (sintered corundum) serves as refractory material in furnace linings or laboratory equipment.

Contaminated by small amounts of Cr2O3 or TiO2, the corundum forms the gems ruby ​​(watch stones, drawing stones, ruby ​​laser) and sapphire.

Ti2O3-doped Al2O3 single crystals form the heart of the Titan: sapphire laser.

γ-Al2O3 serves as an adsorbent and as a catalyst support, as well as a catalyst itself.

In electrical engineering alumina ceramic is used as a dielectric because of its low dielectric loss factor. The main field of application is the realization of strip lines and capacitors in high-frequency technology. Aluminum oxide ceramic plates also serve as a substrate for thick film technology, thin film technology and platinum temperature sensing resistors (see PT100). The good metallizability of this ceramic also allows the direct soldering of electronic components such as resistors or LEDs. The ceramic also acts as a heat sink at the same time. These ceramic electronic systems are just as effective as systems containing metallic heat sinks. Aluminum oxide is also used for the production of securing bodies.

The high dielectric strength and maximum operating temperature of up to 1900 ° C make aluminum oxide the ideal insulator for spark plugs.

Aluminum oxide ceramics are used in plant and machine construction, in particular for wear and corrosion protection. For example, transport channels and chutes, drum mills and mixers are lined with high-performance ceramic tiles to increase the service life of the systems. The corrosion resistance of glass surfaces can be significantly increased by a coating of aluminum oxide. Also in plasma welding, nozzles made of alumina have proven. Due to the good tribological properties in particular components such as sealing and regulating wheels, bearing bushes and shafts, thread guides in the textile industry as well as hip joint balls and pans in endoprosthetics have proven themselves. Also innovative is the use of ceramic studs in the inrun track of ski jumps.

High-purity, large-crystalline and therefore transparent alumina ceramic is used to produce burner tubes of high-pressure gas discharge lamps (sodium vapor lamps, metal halide lamps). It used to be used as an ultraviolet transparent window material for EPROMs.

Latest sintering processes make it possible to use alumina for the production of extremely strong nanoscale glass-ceramics, for. B. Wrist Watch Glasses.

Al2O3 ceramics have also recently been used in armor of vehicles. The ceramic tiles are glued to an aramid or Dyneema fabric. This type of armor achieves twice the protective effect of armor steel with the same basis weight. The ceramic fragments the projectile, and the aramid fibers then catch the fragments.

Al2O3 is marketed as corundum, semi-precious corundum and normal corundum known as corundum (ELK). In an electric oven, it is made at about 2.000 ° C. The resulting melt cake is smashed and sieved according to DIN specified grain sizes. Noble corundum is used in the art as an abrasive in making abrasive wheels. It is also used as a blasting agent and as a polishing powder.

Other aluminas

In addition to the trivalent aluminum oxide, two other aluminum oxides in lower oxidation states, aluminum (I) oxide and aluminum (II) oxide are known. However, these are stable only at high temperatures in the gas phase

Prices of aluminum oxides

Price for aluminum oxides -> prices for special metals