High Total Transmittance Alumina Discharge Vessels Having Submicron Grain Size

Abstract

The present invention uses a post-sinter-HIP anneal to increase the total transmittance of ceramic discharge vessels comprised of a submicron-grained alumina doped with MgO. After the anneal, the submicron-grained alumina discharge vessels have high values of both total and in-line transmittance, and are thus suitable for use in focused-beam, short-arc lamps. In particular, the total transmittance of the discharge vessel is increased to greater than 92% in the wavelength range from about 400 nm to about 700 nm.

Description

BACKGROUND OF THE INVENTION [0001] Translucent polycrystalline alumina (PCA) ceramic has made possible present-day high-pressure sodium (HPS) and ceramic metal halide lamps. The arc discharge vessels in these applications must be capable of withstanding the high temperatures and pressures generated in an operating lamp as well be resistant to chemical attack by the fill materials. In addition, the discharge vessels are typically required to have >92% total transmittance in the visible wavelength region from about 400 nm to about 700 nm in order to be useable in commercial lighting applications. [0002] In HPS lamps, the discharge vessels are tubular, whereas for ceramic metal-halide lamps discharge vessels can range from a cylindrical shape to an approximately spherical shape (bulgy). Examples of these types of arc discharge vessels are given in European Patent Application No. 0 587 238 A1 and U.S. Pat. No. 5,936,351, respectively. The bulgy shape with its hemispherical ends yield...

AI Technical Summary

Benefits of technology

[0005] The present invention overcomes the problem of low total transmittance in MgO-doped, submicron-grained alumina discharge vessels by a post-sinter-HIP anneal. The anneal is believed to alter the ionization and association states of the color centers, but not significantly change the total population of oxygen vacancies. The temperature, time, and partial pressure of oxygen of the annealing atmosphere are controlled so that significant portions of light-absorbing oxygen vacancies are converted to non-light-absorbing oxygen vacancies while at the same time a stable microstructure is maintained (no significant grain growth or precipitation of magnesium aluminate spinel second phase particles). As a result, the post-sinter HIP anneal effectively brings the total transmittance up to >92% in the range from about 400 nm to about 700 nm, a level suitable for commercial lamp applications.

[0006] In accordance with an aspect of the invention, there is provided a method of making a ceramic discharge vessel comprising: (a) forming the discharge vessel with a submicron-grained alumina powder doped with MgO; (b) sinter-HIPing the discharge vessel; and (c) annealing the discharge vessel to increase the total transmittance of the discharge vessel to >92% in a wavelength range from about 400 nm to about 700 nm.

[0008] It is expected that additional co-doping of the PCA with ZrO2, Y2O3, Er2O3, Yb2O3, Sc2O3, etc., may alter the relative population of the point defects, but the problem of low total transmittance is expected to still exist because of the presence of MgO. Thus, the method of this invention should be useful to increase the total transmittance of such co-doped alumina ceramics.

Problems solved by technology

It is expected that additional co-doping of the PCA with ZrO2, Y2O3, Er2O3, Yb2O3, Sc2O3, etc., may alter the relative population of the point defects, but the problem of low total transmittance is expected to still exist because of the presence of MgO.

Images