Formation of sodium aluminates at high temperatures under the influence of Mg and Ca

Abstract

The formation of sodium aluminates at high temperatures under the influence of Mg and Ca was studied to obtain insight into lifetime processes that occur in high pressure sodium (HPS) discharge lamps with translucent alumina envelopes. Powders with molar composition Na 2CO 3-5Al 2O 3, Na 2CO 3-9Al 2O 3, Na 2CO 3-MgO-5Al 2O 3 and 2Na 2CO 3-CaCO 3-5Al 2O 3 were thoroughly mixed and fired at temperatures up to 1600°C for top-level times up to 100 hrs. Quantitative X-ray diffraction was used to study the composition of the quenched mixtures. In the systems Na 2CO 3-5Al 2O 3 and Na 2CO 3-9Al 2O 3 the compound Na 2O.Al 2O 3 is formed first at low temperatures, coincident with the decomposition of carbonate. This compound subsequently reacts with Al 2O 3 to form a mixture of β- and β′−Al 2O 3 in a weight ratio of 1:6. At temperatures above 1400°C β′−Al 2O 3 transforms to β-Al 2O 3. Remarkably this transformation is most complete in the Na 2CO 3-5Al 2O 3 mixture. In the Na 2CO 3-MgO-5Al 2O 3 mixture the compound Na 2O.Al 2O 3 is also formed first, directly followed by MgO.Al 2O 3. β- and β′−Al 2O 3 are formed subsequently in a weight ratio of about 1:1.6. At temperatures above 1400°C stabilized β′−Al 2O 3 is then formed almost completely. The reactions in the system 2Na 2CO 3-CaCO 3-5Al 2O 3 are very complex. The compound Na 2O.Al 2O 3 is formed first, directly followed by the ternary compound Na 2O.CaO.2Al 2O 3 which forms solid solutions with Na 2O.Al 2O 3, starting at 1150°C. At about 1000°C the compound Na 2O.3CaO.8Al 2O 3 starts to form with a maximum around 1200°C. At higher temperatures it is consumed again, resulting in a complete disappearance at 1600°C. β/β′−Al 2O 3 also starts to form at 1000°C and is still present at 1600°C, together with a solid solution of Na 2O.CaO.2Al 2O 3 and Na 2O-Al 2O 3. Conclusions can be drawn from the present results that are relevant for an understanding of lifetime processes in HPS lamps.