Doping with silver bromide has been found to be an effective technique for enhancing the optical homogeneity and other qualities of single crystals of lead bromide. These crystals are grown from melts by directional solidification. The doping suppresses cleavage that would otherwise occur during cooling after directional solidification.
Like other lead halides, silver bromide is potentially useful in acousto-optical and photoluminescent devices; the combination of optical and acoustical properties of the lead halides may be particularly suitable for extending the performances of optical signal-processing systems. Lead halides exhibit very high acousto-optical figures of merit; for example, the acousto-optical figure of merit of lead bromide is about 550 times that of quartz. Lead halides also exhibit a transparency range wider than that of most other commercially available materials that could be used for the same purposes.
To realize the full potential of lead halides, it is necessary to grow high-quality crystals without cracks. In the case of highly pure lead bromide, a second-order solid-state phase transformation occurs near the melting temperature during cooling after growth of a crystal from a melt. This phase transition changes the symmetry of the crystal, evolves a large amount of energy, and is associated with a disordering reaction. The resulting stress is large enough to cleave the crystal.
Doping with silver bromide reduces the energy of the phase transformation and thereby reduces the stress, making it possible to grow a crystal of better optical quality. Experiments have shown that doping to a level between 300 and 1,500 parts per million (ppm) is sufficient to prevent both microcracking and cleavage. Further experiments have shown that doping to a level as high as 3,500 ppm does not change acoustical or optical properties.
The figure illustrates a quartz ampoule and cartridge designed to ensure proper seeding and vacuum conditions during growth of a crystal of doped lead bromide by the Bridgman directional-solidification technique. The top of the ampoule is sealed into the cartridge with cap 2. The bottom of the ampoule tapers down to a seed tube. Cap 1 seals the vacuum in the ampoule. To keep the melt in contact with the seed in the microgravity environment or in a tilted orientation in normal gravity, a spring pushes down on a quartz piston. The top and bottom ends of the spring are attached to cap 1 and the piston, respectively, via quartz beads. During the crystal growth, as the material melts and the volume changes, the spring force on the piston keeps the melt in contact with the seed.
The directional-solidification process can be implemented by translation of either the ampoule or the furnace used to melt the doped lead bromide contents along part of the length of the ampoule. The cartridge is suitable for either implementation. The dimensions shown in the figure are typical and can be changed to grow wider or narrower crystals. The lengths of the ampoule and cartridge can similarly be chosen to fit the length of the crystal to be grown.
This work was done by N. B. Singh, T. Henningsen, R. Ma-zelsky, M. Gottlieb, J. J. Conroy, R. H. Hopkins, Walter M. B. Duval, G. Santoro, Thomas E. Haley, and Ronald D. Hamacher of Westinghouse Electric Corp. for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category. Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Lewis Research Center, Commercial Technology Office, Attn:
Tech Brief Patent Status, Mail Stop 7-3, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-15539.