Adhesive bonding and miscellaneous chemical manufacture – Delaminating processes adapted for specified product – Delaminating in preparation for post processing recycling step
Patent
1988-07-26
1990-04-10
Doll, John
Adhesive bonding and miscellaneous chemical manufacture
Delaminating processes adapted for specified product
Delaminating in preparation for post processing recycling step
156601, 156608, 1566201, 1566205, 156DIG61, 156DIG89, C30B 1514
Patent
active
049157736
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE ART
The present invention relates to growing crystals from melts and, more specifically, to a process for growing single crystals of refractory optically transparent metal compounds, more particularly, to a process for growing shaped single crystals.
PRIOR ART
The quality of shaped single crystals, namely the accuracy of dimensions, electric-breakdown resistance, value of integral light transmission, crystallographic disorientation of single-crystal blocks and mechanical strength, depends on the temperature conditions in the zone of crystallization and pulling of the single crystal, as well as on the conditions of cooling of the grown single crystal. Therefore, the main factor defining the quality of crystals is the character of the temperature field in the "melt-single crystal" system. Melting of the starting materials should be effected under a minimum possible overheating, causing no spontaneous crystallization of the melt. This diminishes its dissociation and interaction with the material of the crucible, which result in an increase of foreign inclusions and microblisters in the single crystal, that impair the light transmission and electric breakdown resistance of single crystals.
The temperature field in the zone of crystallization of the single crystal should be of such a character that would ensure the obtaining of a shaped single crystal with the predetermined accuracy of geometrical dimensions; for example, it should be uniform over the single crystal cross-section.
The conditions of cooling of the single crystal should provide minimum temperature stresses therein and preclude the formation of cracks.
Growing of shaped single crystals of refractory optically transparent metal compounds is a complicated multi-staged process. The manner of carrying out each step of this process considerably influences the yield of suitable crystals and service life of the process equipment, which are among the main factors defining their production costs.
The most important criterion of the process for growing single crystals is the yield of the final product, which is inferred from the compliance of the criteria of the crystal quality with the required values. For example, for shaped single crystals of sapphire in the form of tubes employed in high-pressure sodium lamps, the yield of acceptable single crystals is determined by the geometry of tubes (deviation of the inside diameter of a tube is .+-.0.2 mm), electric breakdown resistance (not less than 50 kV/mm), value of integral light transmission (at least 92%), crystallographic disorientation of blocks in a tube (mechanical strength and service life of tubes become sharply reduced at a disorientation of more than 20.degree.). In some cases the number of blocks in a tube is also rated.
Known in the art is a process for growing shaped single crystals of refractory optically transparent metal compounds (DE, B, 2325104) comprising melting, in an inert gas atmosphere, a starting material of a respective refractory metal compound by heat evolved by a heater, fusing of a seed in the crystallization zone, building-up of a single crystal and pulling thereof from the crystallization zone to the required length under a continuous supply of the melt to the crystallization zone through a capillary zone of a shaping unit and variation of the heater power, whereafter the single crystal is cut off from the melt and cooled.
To maintain the required accuracy of the geometrical dimensions of a shaped single crystal, in pulling use is made of an optical control of the height of the melt column between the end face of the shaping unit and the crystallization front, the constant height of the column being adjusted by varying the heating unit power. In so doing, the melt overheating in the crucible may occur, which impairs the physical properties of the single crystal.
It should also be noted that after cutting-off the single crystal from the melt the heater is immediately switched off and the crystal cooling proceeds as a natural process, thus resulting in a thermal
REFERENCES:
patent: 3715194 (1973-02-01), Plooster
patent: 3853489 (1974-12-01), Bailey
patent: 3870477 (1975-03-01), Labelle
patent: 3915656 (1975-10-01), Mlavsky et al.
patent: 3953174 (1976-04-01), Labelle
patent: 3961905 (1976-06-01), Rice
Alishoev Alexandr L.
Averyanov Viktor V.
Egorov Leonid P.
Freiman Efim A.
Kravetsky Dmitry Y.
Doll John
Kunemund Robert M.
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