Aluminium oxides corundum

The ruby laser, invented in 1960, was the first device to put the ideas just described into practice. Rubies are crystals of alumina (aluminium oxide, corundum, AI2O3), containing about 0.5 % chromium ions, Cr +, in place of aluminium ions, Al " ". Ruby is a dilute solid solution, and the Cr " " ions form substitutional defects. The laser action involves only the Cr " " ions and is due to the transition of electrons from the ground state to higher energy levels among the 3d orbitals. 

Transitions between orbitals localised on atoms e.g. d-d transitions of transition metal salts, f—f transitions of lanthanide ions. Such metal-centred (MC) transitions are ubiquitous in transition metal and lanthanide complexes. They are relatively weak because they are symmetry (Laporte) forbidden. Although they may not be the important transitions for any particular application of transition metal photochemistry, they will almost always be present. These are the transitions that give many transition metal salts their characteristic colour and are foimd in some gemstones and minerals. For example, the red colour in ruby is due to the d-d transitions in chromium (III) present at certain sites in an aluminium oxide (corundum) crystal. 

Aluminium group Aluminium hydroxide, bayerite type Aluminium hydroxide, nordstrandite type Aluminium oxide, amorphous type Aluminium oxide, corundum type Bayerite Boehmite Calcium aluminium oxide Chromium aluminium cobalt oxide Cobalt aluminium oxide Corundum Diaspore Gibbsite Hercynite Lead aluminium oxide Nordstrandite Ochre Cobalt blue Emery Satin white Spinel pigments Turkish green... 

Aluminium oxide occurs naturally as emery (an impure form) and as corundum. Corundum is a crystalline form which may be coloured by traces of impurity, for example as ruby (red) and sapphire (blue). Small synthetic rubies and sapphires have been made by heating alumina with the colouring oxide in an oxy-hydrogen flame. 

Aluminium oxide is the oldest ceramic material used in medicine. Bauxite and corundum are the main natural sources of aluminium oxide. Bauxite is a mixture of diaspore, gibbsite, iron hydroxides, clay minerals and quartz. It is formed by the tropical weathering of silicate rocks during which quartz and the elements sodium, calcium, magnesium and potassium are largely washed away. This is the reason why the remaining material becomes richer in the resistant elements titanium, iron and aluminium. The latter is extracted from this mixture in the form of aluminium hydroxide. In its turn this compound is converted into aluminium oxide by heating the mixture to 1200-1300 °C, this is called calcination. The hydroxide is thus made anhydrous. 

Corundum (a-aluminium oxide), specified to contain > 99% AI2O3, purchased from Union Carbide (USA). The sample was washed six times in deionized water in order to eliminate the traces of some interfacial impurities. It was subsequently dried at 433 K in a vacuum dryer for 30 hrs [54]. 

The structural relations between the many crystalline forms of aluminium oxide and hydroxide are exceedingly complex but they are of exceptional scientific interest and immense technological importance. The principal structural types are listed in Table 7.8 and many intermediate and related structures are also known. AI2O3 occurs as the mineral corundum (ot-Al203, cJ 4.0gcm ) and as emery, a granular form of corundum contaminated with iron oxide and silica. Because of its great hardness (Mohs... 

AgF SILVER FLUORIDE 8 AI203 ALUMINIUM OXIDE (ALPHA,CORUNDUM) 48... 

XRD-results have shown that aluminium oxide was formed at 600-700 °C. It consists of nano-dimensional grains and have the y-structure providing the high reactivity of fibrous material. When fibres of aluminium oxide were heated to 900 °C, its crystal structure varied from y- to 0-phase, and above 1200 °C it changed to a-corundum. It is illustrated by the patterns in Fig. 1. 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid state structure of 0t-Al2O3 consists of an hep array of ions with cations occupying two-thirds of the octahedral interstitial sites. ot-Alumina is extremely hard and is relatively unreactive (e.g. it is resistant to attack by acids). Its density (4.0gcm ) exceeds that of Y-AI2O3 (3.5gcm ) which has a defect spinel structure (see Box 13.6 and Section 21.10). The a-form is made by dehydrating... 

Surface hardened steels show a different reaction to the standard treatment of mild steel. Normal grit blasting with chilled iron will only polish the surface and not remove the scale adequately. Aluminium oxide, or corundum, is the best choice for cleaning these materials. 

Associated minerals These mineral constituents do not impart plasticity to clay. Examples include silicate minerals like micas, quartz, feldspars etc. iron oxides and hydroxides like magnetite, hematite, maghemite, goethite, lepidoerocite etc. and aluminium oxides and hydroxides like corundum, gibbside, boehmite, diaspore etc. 

The same considerations apply to the surfaces of the many crystalline modifications of aluminium oxide (Boehm 1966). Besides a-A Os (corundum) there are at least eight oxides mostly classed under the term of Y-AI2O3 (cf. Ginsberg et al. 1957). Some of these forms may be non-stoechiometric oxide-hydroxides. 

Important aluminium compounds include aluminium oxide AI2O3, which is commonly called alumina (in powder form) or corundum (in coarse crystalline structure), and aluminium hydroxide Al(OH)3 ( hydrated alumina, usually extracted from bauxite in the Bayer process). 

The widespread applications of Al are summarized in Fig. 13.5a. Its strength can be increased by alloying with Cu or Mg. Aluminium oxide (see Section 13.7) has many important uses. Corundum (a-alumina) and emery (corundum mixed with the iron oxides magnetite and haematite) are extremely hard and are used as abrasives. Diamond is... 

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