Polycrystalline bodies and means for producing them

A method for producing a polycrystalline material of high optical quality having a grain growth size in the range of about 0.10 to about 10.0 millimeters from a batch of crystalizable metal halide material which comprises

• heating said batch sufficiently to convert said batch to a homogeneous noncrystallized molten mass
• thereafter cooling the mass to a temperature which is within the polynucleation temperature range of the mass, the polynucleation temperature range of a substance being defined as the temperature range below the melting point of the substance at which a rapid uniform removal of heat results in the formation of numerous nucleation or independent crystallization centers which form without substantial growth at the expense of other crystals, by removing heat in predetermined directions of outflow therefrom, at a removal rate which is substantially equal in each direction of outflow
• thereby to
• initiate freezing of said mass as a plurality of individual crystals each of which cohere with others of said crystals in random crystal orientation, and induce substantially uniform contraction of the frozen material in directions in correspondence to the directions of heat outflow,
• the heat removal being effected
• at a controlled rate to maintain sufficient of said batch of material molten to flow into any voids produced in the frozen portion by contraction of the same, and
• through said frozen portion at a rate which is at least substantially equal to the rate of liberation of latent heat of fusion and specific heat from the remaining molten material as the latter freezes, and
• continuing heat removal at said rate until said mass is completely frozen into a solid polycrystalline material.