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Large diameter crystals grown by a variety of techniques have been manufactured for many years. The perameters for optimising growth conditions are well established. This paper will attempt to identify these conditions and illustrate the problems likely to be encountered. Having selected a method of growth suitable for generating crystals for optical applications, furnace design should be considered. The size and type of the optical component will influence furnace design in several areas, e.g. heating, control and atmosphere. The areas in need of careful control will be discussed, namely power input, temperature control, cooling water, pressure. Crucible material is an important feature, influencing the purity of the melt. With many materials to select from the choice is often limited by the application the crystal is put to, the compatibility of raw material and crucible material and, finally, cost. The purity of raw material needs to be considered carefully, identifying and limiting those impurities likely to have an adverse effect on the finished component. The temperature gradient in the furnace and crucible are critical to the formation of crystals that exhibit good lattice structure, establishing the usefulness of the end product. Internal stresses set up in the growing crystal must be kept to a minimum. This requires a knowledge of heat flow patterns inside the furnace and crucible. Equally important in producing stress free uniform structured crystals are the growth rates used, care must be taken in cooling the grown crystal down to ambient temperature and annealing and cooling cycles need to be produced for each type of material grown. Processing of crystaline materials pose their own problems. In general most are both mechanically and thermally weak, requiring specially developed techniques for cutting and shaping. Grinding and polishing crystaline materials follows generally recognised optical practice but special attention must be given to overcome the frailty and delequesence of some crystals.
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