Hydrothermal synthesis of quartz

Abstract

Large single salt crystals can be grown at atmospheric pressure from suitable seed crystals by careful control of the supersaturation of an aqueous solution. Quartz is not sufficiently soluble in aqueous solution at atmospheric pressure for this type of synthesis to be successful. The method of quartz synthesis to be described in this thesis, permits variation of the pressure at which the crystal is grown, and thus the temperature range for growth may be increased. The resulting changes in solubility allow the growth of large single crystals which would be extremely difficult to grow in any other way. This technique, called the hydrothermal method of crystal growth relies on the use of an aqueous solution at high temperatures and pressures to dissolve a material called the nutrient, in one part of the system, transport it to another part of the system and deposit it on the seed. This process is usually carried out in a sealed vertical pressure vessel, with a temperature difference (ΔT) established between the top and bottom of a vessel. Usually the nutrient is placed in the lower, hotter part of the cylindrical pressure vessel, and the seeds are mounted in the cooler, upper part on a suitable silver framework, separated from the nutrient by a perforated silver disc, called a baffle. The baffle is useful in localizing the temperature difference at the baffle region, producing isothermal dissolving and growth zones necessary for uniform growth on all seeds.