Transparent ceramics for lighting

Abstract

The development of ceramic arc lamps for optical applications requires consideration of materials other than sintered polycrystalline alumina (PCA). Regular PCA is translucent, not transparent. Except small-grained, regular PCA cannot be used for high luminance applications such as required by projection systems. Silica lamps are currently operating close to their limit in highly loaded discharge lamps. These may be replaced with ceramic lamps so the Hg pressure may be elevated and/or higher powers achieved. Cubic materials can be polished to transparency for use as optical sources of short arcs. The current paper surveys the composition, structure, and properties of transparent ceramic lamp tube materials including small-grained PCA, sapphire, aluminum oxynitride, yttrium aluminate garnet, and dysprosium oxide. The challenges beyond the optical transparency are to achieve (1) strong bonding between the transparent ceramic and electrode system to complete the discharge enclosure, (2) satisfactory characteristics including thermo-mechanical properties in order to withstand the rapid heating and cooling cycles encountered in both the discharge tube and seal, (3) durability to resist the attack from lamp chemicals at high temperatures, and (4) stability to maintain the optical quality throughout the life. Performance, energy efficiency, environmental sustainability, and economics are driving the development of ceramic envelopes in lighting products. Transparent ceramics offer opportunities to push the limits of envelope materials for improved lamps. The paradigm used during the course of transparent ceramics research exemplifies advancement of new and improved materials.