Crucibles, electric mixtures

In 1808 Sir Humphry Davy tried in vain to decompose zirconia with the electric current, but Berzelius (36) finally obtained the metal in 1824 by heating a dry mixture of potassium and potassium zirconium fluoride in a very small closed iron tube placed inside a platinum crucible. After the quiet reaction had taken place, he cooled the tube and placed it in distilled water, whereupon, to use his own words, There fell from the tube a black powder as fast as the salt dissolved, and at the same time there was evolved a small quantity of hydrogen.. . . The zirconium obtained in this manner is easily deposited. It can be washed with water without oxidizing. Washed and dried, it forms a black powder resembling charcoal, which cannot be compressed nor polished like a metal (15). 

Tungsten Subphosphide, W4P3.—By strongly heating a mixture of phosphoric acid and tungstic anhydride in a carbon crucible, lustrous steel-coloured crystals of this phosphide are obtained. The crystals are si.x-sided prisms of density 5-21. It conducts electricity well, is very" stable in air even at higher temperatures, and is very resistant to acids. When heated in a current of oxygen it bums brilliantly. It dissolves in a fused mixture of alkali carbonate and nitrate. 

In 4.2.6. (see above), we illustrate the t5T)ical setup for the crucible and melt in the Czochralski apparatus. We place the Ir crucible containing the mixture of oxides on a circular Zr02 platform. This acts as a thermal barrier for the bottom of the apparatus. We may then place a larger Zr02 cylinder around the crucible for further insulation (Note that the choice of a thermal insulator is not critical except that it must be able to withstand the anticipated temperatures to be used). An R.F. coil is placed around the outside of the insulation in a position where it can electrically couple with the metal crucible. Finally, an outside wall of insulation is put into place and atop cover plate is put into position. At 2000 °C., the outer wall thickness of Zr02 needs to be at least 2.5-3.0 cm. The whole is then... 

An intimate mixture of 14.4 g. of LigCOa, 6.44 g. of Be (OH) 3 and 50 g. of H3BO3 (all finely powdered) is fused in a Pt crucible until no further COs is evolved andthe melt has become transparent. The melt is poured into a graphite crucible preheated to 600°C the crucible is placed in an electric muffle furnace preheated to 600°C and the furnace is allowed to cool to room temperature overnight. 

IZnO-.lFesOa, taking into account the water content. The mixture is then screened and weighed out. The powder is mixed for four hours in a Pyrex bottle on a mechanical shaker. Following this, 4-g. portions of the mixture are placed in an open platinum crucible, which is then set in an electric furnace. The reaction may be carried out either at 800°C in a stream of dry air or at 1000°C in the absence of such an air flow. In either case, one hour is required for the reaction. 

A tantalum crucible (height 20 cm.,diameter 2.5cm.) containing a mixture of 20 g. of Sm303 and 20 g. of freshly prepared La turnings is heated for 30 minutes at 1450°C in an electric furnace at a pressure of 0.001 mm. The upper part of the crucible projects... 

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