Platinum Metals Rev., 1977, 21, (1), 17
Novel Electronic Materials
Their Dependence On The Platinum Group Metals
Many of the electronic devices which have emerged during the last one and a half decades rely for their operation on the use of single crystal oxides, which possess high structural and chemical perfection. The melt growth temperatures of these oxides range from 738°C for lead germanate to 2105°C for magnesium aluminate (spinel). The high melting points of both the single metal-oxide compounds and mixed systems, coupled with their high reactivity and their growth dependence on an oxygen or neutral atmosphere, severely limit the choice of materials available for containing their melts.
The chemical stability of platinum and iridium at elevated temperatures, 1500 and 2200°C respectively, and the chemical compatibility of these two platinum group metals with most oxide systems make them an obvious choice as container materials for the growth of oxides.
At the “Chemistry in Industry—the Way Ahead” Conference held at Wembley in November 1976, P.M. Welch of Johnson Matthey Chemicals Limited reviewed the use of platinum group metals in crystal growing in a paper entitled “High Purity Chemicals for Electronic Applications”.
One development in platinum metal metallurgy, highlighted in the paper, which could be of paramount importance to crystal growers—in particular for laser and electrooptic materials with melting points below 1500°C—is the introduction of a zirconia grainstabilised grade of platinum. This material has increased strength at elevated temperature and better contamination resistance compared with pure platinum.
One category of magnetic materials that promises to be a major outlet for single crystals in the next decade is bubble domain devices used as a data storage and manipulation medium. The devices comprise a single crystal insulating substrate of Gd3Ga5O12 (GGG), on to which is deposited an epitaxial layer, 3 to 6 μm thick, of a ferromagnetic garnet of the type RxY3−xFe5−yGayO12, where R is one or more rare earth ions.
The GGG has a melting point of 1725°C, and is grown by the Czochralski technique at a typical pulling rate of 5 to 8 mm/h from an iridium crucible under a slightly oxidising gaseous environment. GGG single crystal boules are now being produced commercially up to 7.5 cm in diameter, and weighing about 4 kg.
The magnetic thin films are grown at about 1000°C in platinum from a supersaturated solution of the garnet in a PbO/B2O3 flux.