Refractory tube production - for high pressure sodium halide lamp envelopes modified zone melting of pre formed sintered tube

Abstract

Abstract In the mfr. of a refractory tube starting from a preformed tube of refractory material round which a concentrated heating zone is arranged by means of radiant energy of uniform intensity in such a way that a molten ring of uniform height is formed in, and is displaced along, the tube, the height being such that the surface tension forces and gravitational force hold the molten ring connected to the solid parts of the tube on each side thereof while the relative translatory motion between the refractory tube and the heating zone is regulated so as to displace the molten ring along the tube, (a) the radiant energy is formed by laser energy and the heating zone is formed by a series of laser beams of semicircular form uniformly distributed round the tube and each beam is focussed on the refractory tube to form the molten ring therein, and (b) the focussing is carried out in such a way that the energy density distribution of each beam is adjusted to give equal absorption of the laser energy round the whole surface of the tube within the molten ring. Enables the virtually full theoretical density to be achieved by eliminating any porosity introduced by e.g. impurities. Also permits, when appropriate, the production of a tube as a single crystal, so that limitations arising from the presence of grain boundaries, e.g. chain of atmosphere during treatment, are no longer of importance. The technique is particularly suitable for the production of Al2O3 envelopes for sodium halide-especially NaI- lamps, which operate at high pressure and temperature.

Inventors

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