Experiments on Synthetic Mica Crystal Growing. Crystal Growing with Carbon Granule Resistance Furnace

Abstract

Abstract Synthetic mica crystal growing experiments were carried out with a graphite crucible and a well-sintered high-aluminous clay crucible in a carbon granule resistance furnace of 30 kg. melting capacity, specially designed for this purpose. In experiments with a graphite crucible, the melt often became supercooled and crystallized rapidly, to result in small crystals once crystallization started at the supercooled temperature. With a clay crucible, several experiments failed because of cracks in the crucible or because of corrosion of the crucible by the melt. Only with a well-sintered high-aluminous clay crucible and under the condition of small loss of fluorine and potassium in the melt were perfect large single crystals measuring about 5 cm. by 5 cm. formed even at a mean cooling rate of 2∼5°C/hr. in the range of 1360°C to 1300°C. The yield of large single crystals was very small, doubtless because only the few crystals which had formed earlier had enough time and space to be in contact with the melt and thus to grow to large size. If the growing crystals come into contact or collide with other growing crystals, the crystals are either prevented from growing further or become strained or bent. Consequently, for crystals to grow to a large size, they must be in contact with the melt for a long time without being disturbed by other crystals. This can be done when crystals grow in the same direction. It was found that the temperature gradient of 4∼5°C/cm. was necessary for crystals to grow in one direction. Among many factors affecting the growth of crystals, the rate of cooling should be controlled to secure the rate of crystal growth at less than 0.06 mm./min. in order to grow perfect and flawless single crystals of mica. Needless to say, a sufficient volume of melt is necessary to grow large crystals, i. e. a large quantity of melt gives more opportunity for large crystals to grow. Crystals measuring 5 cm. by 5 cm. were never obtained before the 30 kg. melting experiments were carried out.