Yttrium aluminium oxides

Some oxide-type minerals have been found to luminesce when irradiated. A simple example is ruby (aluminium oxide with chromium activator), which emits bright-red light. The phosphors are incorporated into colour television screens to emit the colours blue (silver-activated zinc sulphide), green (manganese-activated zinc orthosilicate), and red (europium-activated yttrium vanadate). 

The preparation of yttrium aluminium garnet (Y3AI5O12, YAG) illustrates a variation of the hydrothermal method used if the starting materials have very different solubilities from each other. In this case, yttrium oxide (Y2O3) was placed in a cooler section of the autoclave and aluminium oxide as sapphire in a hotter section to increase its solubility (Figure 3.8). YAG forms where the two zones meet. 

Single oxide ceramics, e.g. aluminium oxide (AI2O3, alumina) and zirconium dioxide (Zr02, zirconia), are bioceramics of an inert nature. An inert ceramic does not form a bonding to bone similar to those bioceramics of bioactive nature. Alumina bioceramics are in the pure aluminium oxide form, whereas zirconia bioceramics are partially stabilized by additional oxides, e.g. yttrium oxide, calcium oxide or magnesium oxide. 

Yttria. yttrium oxide Y2O3 (q.v.). Yttrium Aluminium Garnet. Y3AI5O12 YAG occurs as a grain-boundary phase... 

M. Wilson and P. F. McMillan. Interpretation of X-ray and neutron diffraction patterns for amorphous yttrium and lanthanum aluminium oxides from computer simulation, Phjs. Rev. B 69, 54206-54217 (2004). 

G. N. Greaves, M. C. Wilding, L. Heimet, W. Bras, O. MaJ s, S. Fean, and F. Kagl, Liquid-liquid transitions, crystallisation and long range fluctuations in supacooled yttrium oxide-aluminium oxide, J. Non-Cryst Solids ZSS, 715-721 (2009). 

G. N. Greaves, M. C. Wilding, D. Fearn, D. Langstaff, F. Kargl, S. Cox, Q. Vu Van, 0. Majems, C. J. Benmore, J. K. R. Weber, C. M. Martin, and L. Hennet, Detection of first order liquid-liquid phase transitions in yttrium oxide-aluminium oxide melts. Science 322, 566-570 (2008). 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

The exceptional properties of the alloy are due in no small way to the yttrium component which together with the aluminium forms a stable and firmly bound oxide layer that exhibits excellent resistance to exhaust gas emissions at high temperatures over prolonged periods.( ) At the same time, it provides an ideal surface to receive another coating of metal or metal oxide which, in the context of catalyst applications, is most essential. At the present time most catalytic convertors utilise ceramic substrates which are prone to damage by both mechanical and thermal shock. 

Scandium (Sc, at. mass 44.96) occurs in its compounds exclusively in the III oxidation state. Some of its chemical properties resemble those of the lanthanides and yttrium. Scandium hydroxide Sc(OH>3 precipitates at a pH as low as 4.8 and dissolves in alkaline medium in this respect scandium resembles aluminium. 

Oxides of the type RjO arc formed by all rare earth elements, by the ignition of the hydroxide, nitrate, sulfate, carbonate, or oxalate. In general the basicity decreases with increase of atomic weight, though yttrium and scandium are exceptions to the rule. The basicity of lanthanum approaches that of the alkaline earths, while scandium is about as basic as aluminium. 

The electronic configuration of scandium can be expressed as [Ar] 3d 4s leading to oxidation states of 3+ (most common), 2+, and 1+. Its oxide is therefore given as SC2O3. Scandium is considered a rare earth element, but this is firstly because it was discovered together with several other lanthanides. Secondly, scandium resembles yttrium and the rare earth elements more than it resembles aluminium or titanium (Hammond 2015). On the other hand, it also much resembles the ferromagnesian transition elements, although in aqueous systems it behaves more like the REE (see Chap. 1, and McLennan 2012). 

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