Transparent Ceramic Lamp Envelope Materials
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Home Search Collections Journals About Contact us My IOPscience Transparent ceramic lamp envelope materials This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2005 J. Phys. D: Appl. Phys. 38 3057 (http://iopscience.iop.org/0022-3727/38/17/S07) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 128.157.160.13 The article was downloaded on 14/06/2010 at 20:16 Please note that terms and conditions apply. INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 38 (2005) 3057–3065 doi:10.1088/0022-3727/38/17/S07 Transparent ceramic lamp envelope materials GCWei OSRAM SYLVANIA, 71 Cherry Hill Drive, Beverly, MA 01915, USA E-mail: [email protected] Received 16 February 2005, in final form 5 April 2005 Published 19 August 2005 Online at stacks.iop.org/JPhysD/38/3057 Abstract Transparent ceramic materials with optical qualities comparable to single crystals of similar compositions have been developed in recent years, as a result of the improved understanding of powder-processing-fabrication- sintering-property inter-relationships. These high-temperature materials with a range of thermal and mechanical properties are candidate envelopes for focused-beam, short-arc lamps containing various fills operating at temperatures higher than quartz. This paper reviews the composition, structure and properties of transparent ceramic lamp envelope materials including sapphire, small-grained polycrystalline alumina, aluminium oxynitride, yttrium aluminate garnet, magnesium aluminate spinel and yttria–lanthana. A satisfactory thermal shock resistance is required for the ceramic tube to withstand the rapid heating and cooling cycles encountered in lamps. Thermophysical properties, along with the geometry, size and thickness of a transparent ceramic tube, are important parameters in the assessment of its resistance to fracture arising from thermal stresses in lamps during service. The corrosive nature of lamp-fill liquid and vapour at high temperatures requires that all lamp components be carefully chosen to meet the target life. The wide range of new transparent ceramics represents flexibility in pushing the limit of envelope materials for improved beamer lamps. (Some figures in this article are in colour only in the electronic version) 1. Introduction This paper reviews the progress made in transparent ceramics in recent years. In order to improve transparency Translucent polycrystalline alumina (PCA), discovered in the or in-line transmittance, recent developments in PCA have early 1960s [1], has evolved and improved so as to be useful pushed in two opposite directions: (1) extremely large grain for both high-pressure sodium (HPS) [2] and metal-halide size: converting PCA to sapphire through abnormal grain lamps [3]. PCA combines the attributes of resistance to fill growth, a solid-state crystal conversion (SSCC) process [5] attack, optical transmission, mechanical properties and the and (2) very small grain size: achieving transparent submicron- ability of being fabricable into thin-wall structures, making grained structure through sinter-HIP (hot isostatic pressing) it suitable to serve as housing for high-temperature corrosive of compacts of nearly nano-sized alumina powder [6]. plasma and chemicals at a maximum wall temperature of Ceramics of cubic symmetry are of interest owing to their ∼1250˚C in HPS and ∼1100˚C in cylindrical [3] or round [4] transparency in the polished state. These are Y2O3 [7], ceramic metal-halide lamps. These ceramic lamps are Y2O3–La2O3 [8], MgAl2O4 [9], Y3Al5O12 (yttrium aluminate typically used for either wide-area and outdoor, or interior garnet (YAG)) [10] and Al23O27N5 (AlON) [11], in which and spot-illumination applications, owing to the nature of the cubic symmetry limits birefringent light scattering at translucency (not transparency) of regular PCA. If the arc grain boundaries. The thermal expansion values [12]of tubes are transparent (beyond translucent), the lamps could Y2O3,Y2O3–La2O3, MgAl2O4, YAG and AlON, are all allow focused-beam applications such as projection lamps and very close to that of PCA. Therefore, the feedthrough automotive headlights. scheme [3] developed for PCA metal-halide lamps is readily 0022-3727/05/173057+09$30.00 © 2005 IOP Publishing Ltd Printed in the UK 3057 GCWei (a) (b) Figure 1. (a) Optical micrograph of etched, polished cross-section of a PCA tube wall (0.8 mm thick) partially converted to sapphire at edge/corner. (b) Scanning electron micrograph (SEM) of fractured surface of a PCA tube converted to sapphire in interior of the wall. applicable to arc tubes consisting of these transparent strength are of interest for focused-beam lamps. The highest ceramics. in-line transmittance reportedly achieved in this material (0.8 mm thick) was about 70% of sapphire, with a narrow 2. Transparent ceramic materials angular dependence of the scattered signals (>99% of the transmitted light is within a cone of 5˚ scattering angle) [18]. 2.1. Sapphire It was reportedly used in Hg-Tl-Na-In-I lamps with graded cermet and frit seals operating to 8000 h [19]. Others Straight sapphire tubes are typically grown by the edge-defined used 10–75 W, 0.6 mm-wall, small-grained alumina tubes film-feed growth (EFG) method [13]. It involves a floating containing Tl-Nd-Dy-I [20]. Fills based on Dy-Tl-Na-Br-I orifice through which the sapphire tube is grown, plus capillary were used in submicron alumina lamps showing 78% action to bring the molten liquid to the flat edge of the die, maintenance at 1500 h [21]. The wall temperatures were not which defines the outer diameter of the tube. Sapphire tubes reported. Microstructural and transmittance stability of this often have a fine layer of Mo precipitates (from dissolution of type of materials at elevated temperatures was a concern [17]. the Mo die in the molten alumina) along with pores caused In submicron-grained alumina doped with MgO sintering by solidification shrinkage in the near-surface region. The aid, growth of grains and pores occur at temperatures as EFG method is limited to straight tubes. Such sapphire tubes low as 1150˚C [22]. For example, an average grain size of were used in early HPS lamps [14] before the discovery of the 0.47 µm grew to about 0.71 µm (figures 2(a) and (b)), and cost-effective translucent PCA. Some recent designs of burners the spectrophotometer in-line transmittance value at 600 nm involved straight sapphire tubes directly bonded to shrinking dropped about 25%, after 2000 h, at 1150˚C. Figure 3 shows PCA hats during sintering without frits [15]. grain growth versus time at 1150–1250˚C in submicron- In the SSCC process [5], the PCA doped with MgO was grained alumina, along with values reported in [23, 24]. heat treated to achieve a state of equiaxed grain structure of The difference is ascribed to different powders, dopants and translucency followed by out-diffusion of the MgO dopant residual pores. This grain-growth behaviour is distinctly to <60 ppm so as to bring about a high rate of intrinsic different from that of the regular, 10–30 µm-grained PCA, grain growth resulting in transparent sapphire shapes. The which do not show any grain growth during 20 000 h of service, conversion typically takes place in the near-surface region at 1250˚C, in HPS lamps. The observed grain growth at 1150˚C where out-diffusion of MgO sintering aid readily occurs, is related to the extremely small size of the grains, since the figure 1(a). A gradient in MgO dopant distribution produced a microstructure consisting of a large, converted single crystal grain growth rate is inversely proportional to the grain size and in the inside bounded with small-grained, unconverted near- proportional to the product of boundary mobility and boundary surface region, figure 1(b). One needs to combine these energy. two cases. The conversion method could potentially produce Such microstructural instability of the submicron-grained curved complex shapes of sapphire. The issues are the ability alumina is tied to the fact that the solid solubility of MgO in to consistently control where nucleation starts and the degree alumina is a function of grain size in the range of submicron ∼ and speed of the transformation. to 2 µm[22] (figure 4). The solubility data in figure 4 were calculated from bulk chemical analysis of the MgO level and electron microprobe mapping of the number, density and 2.2. Small-grained alumina size of MgAl2O4 spinel particles of polished cross-sections of PCA of submicron-grain size and high in-line transmittance samples annealed at various temperatures [22]. Figure 5 shows (very low residual porosity and small grains) was reportedly the solid solubility of MgO in ∼10–30 µm-grained alumina accomplished using sinter-HIP of compacts consisting of is ∼95–100 ppm at 1800˚C [25–27], which extrapolated to nearly nano-sized alumina particles [6]. The mechanical ∼1 ppm at 1250˚C is, much lower than the >220 ppm solid strength was as high as sapphire, owing to the small solubility of MgO in submicron-grained alumina [22]. The grain size [16, 17]. Such high-in-line transmittance and grain-boundary enrichment factor and width also appear to be a 3058 Transparent ceramic lamp envelopes (a) (b) Figure 2. SEM micrographs of submicron alumina before (a) and after (b) anneal at 1150˚C for 2000 h. Average grain size grew from 0.47 to 0.71 µm during the anneal. 10 Ref. 22 1250°C 1150°C Ref. 24 1 Average grain size (micron) Average Ref. 23 0.1 0.1 1 10 100 1000 10000 Time (h) Figure 3. Average grain size versus time for submicron-grained alumina [22–24].