Interaction of Alkali Metal and Halide Vapors with Ceramic Materials

Abstract

The effect of minor constituents in alumina arc tubes on lamp performance has been investigated. It is found that arc tubes containing calcia are less resistant to sodium vapor and accelerate early failures. Microstructural studies reveal extensive formation of ß-alumina phases and sodium penetration across the wall of the reacted alumina. A number of other interactions, including calcia-doped MgAI 2 0 4 spinel have been studied. Mechanisms of sodium migration in terms of structural defects and chemical constitution of the grain boundary are presented and discussed.

Reactions causing loss of sodium vapor in HPS lamps

“…The X-ray diffraction measurements indicated the following phases: a-Al 2 O 3 (PDF 00-10-173) and NaAlO 2 (PDF 00-33-1200) in the sample sintered at 1100 8C for 8 h. The formation of the NaAlO 2 is confirmed by different literature data [3,9]. The literature does not mention the actual crystal system of the evolved NaAlO 2 during the operation of the lamp.…”

“…2, the comparison of two experimental series produced from Reynolds-alumina can be seen. Beside the NaAlO 2 an other Na-O-Al compound such as NaAl 11 O 17 (00-45-1451) [9,10] with hexagonal structure was identified in the sintered powder mixture of sample 3 heated at 800 8C for 4 h then at 1100 8C for 24 h. The comparison of the two experimental series suggest that NaAlO 2 with orthorhombic structure formed can be the stable product of this solid state reactions. Only after longer sintering time (24 h) at 1100 8C another Na-O-Al species, NaAl 11 O 17 appears in smaller amount.…”

“…The formation of NaAl 23 O 35 with hexagonal structure is not mentioned by other authors [3,6,7,9], who dealt with the sodium loss reaction in HPS lamps. This NaAl 23 O 35 phase (PDF 01-73-1717) was the first suggestion on the composition and structure of a Na-b-alumina (NaAl 11 O 17 ).…”

Translucent Polycrystalline Alumina with Improved Resistance to Sodium Attack

“…YAG scavenges calcium and silicon impurities, 13 which are known to be detrimental to sodium resistance. 8,10,19 Additionally, yttrium was found to segregate to grain boundaries by TEM/EDXA, 20 with an enrichment ratio of 7.1%-9% (cation sites) quantified via SIMS imaging. 21 This was reported to decrease the grain-boundary transport rates in Al 2 O 3 , as exhibited by a lower creep rate, 22,23 and a lower densification rate.…”

“…This could be attributed to (i) the segregation of yttrium at grain boundaries to slow down the fast-path transport and (ii) the presence of YAG phase that could absorb calcium and silicon impurities 13 known to reduce sodium resistance. 19 Segregation of ions such as Zr 4ϩ and Tb 3ϩ are also believed to retard the grain-boundary transport. The sodium resistance of the sintered AMZY, AMYZ, and AMT could in part be attributed to such segregation at boundaries.…”