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Transparent ceramic lamp envelope

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)

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Please note that terms and conditions apply. INSTITUTE OF 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 of similar compositions have been developed in recent years, as a result of the improved understanding of powder-processing-fabrication- -property inter-relationships. These high- materials with a range of thermal and mechanical properties are candidate envelopes for focused-beam, short-arc lamps containing various fills operating at higher than quartz. This paper reviews the composition, structure and properties of transparent ceramic lamp envelope materials including , small-grained polycrystalline alumina, oxynitride, aluminate , aluminate 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 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- (HPS) [2] and -halide size: converting PCA to sapphire through abnormal grain lamps [3]. PCA combines the attributes of resistance to fill growth, a -state conversion (SSCC) process [5] attack, optical transmission, mechanical properties and the and (2) very small : achieving transparent submicron- ability of being fabricable into thin-wall structures, making grained structure through sinter-HIP () it suitable to serve as housing for high-temperature corrosive of compacts of nearly nano-sized alumina powder [6]. 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 at translucency (not transparency) of regular PCA. If the arc grain boundaries. The 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 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 (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 and 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 [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, and translucency followed by out-diffusion of the MgO 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 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 . 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 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].

100 C, wt ° 10

1

1 MgO solid solubility at 1250

0.0 0.0 1 1 ∞ Average grain size (micron)

Figure 4. MgO solid solubility versus grain size in MgO-doped alumina [22]. function of the grain size [22] (figure 6). Thus, during the initial a background of boundary motion, precipitation of spinel stage of densification of compacts of MgO-doped, submicron second phase and diffusional flux of the dopant. As grains Al2O3 powders, the MgO dopant appears to be all dissolved, grow approaching 1.2 µm, the enrichment factor and boundary mostly in the grain-boundary region. During grain growth width increase. At >0.7 µm grain size, even with an increased and sintering of MgO-doped alumina, the enriched MgO width and saturation limit at the boundaries, the boundary level at grain boundaries, boundary width and equilibrium interface region could no longer accommodate all the MgO MgO content in the lattice undergoes dynamic evolution in dopant. The reduced grain-boundary area (associated with the

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Temperature (C)

1900 1800 1700 1500 1200 1000

Grain size = 2µ m Ref. 27

Grain size = 6µ m Grain size = 0.5µ m Ref. 25

Grain size = 0.7-1.2µ m

100 Grain size = 0.5-1.2µ m Ref. 22

Single crystal Ref. 26 MgO Solid Solubility in alumina (ppm)

10 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 1/Temperature (K-1)

Figure 5. MgO solid solubility in alumina versus temperature [22, 25–27].

10000

1000 = 2nm) δ (

100 MgO enrichment factor at boundaries 024681012 Grain size, micron

Figure 6. MgO enrichment factor at grain boundaries in alumina versus grain size [22].

(a) (b)

pore

Spinel

0.24 µm 0.25 µm

Figure 7. Transmission electron micrographs of 220 ppm MgO-doped, submicron-grained alumina before (a) and after anneal at 1150˚C for 2000 h. Pores in (a) and spinel in (b) are indicated by arrows. larger grains) available for the MgO dopant partition makes fracture strength of submicron alumina was, reportedly, nearly the excess MgO dopant appear as MgAl2O4 spinel second- doubled by using MgO and Yb2O3 co-dopants [29]. phase precipitates (figure 7), with an overall reduced solid solubility. Various strategies [28], including adjustment of the 2.3. YAG (yttrium aluminate garnet) microchemistry of grain boundaries via co-doping of MgO, Er2O3,Y2O3 and La2O3, were proposed in order to enhance Transparent YAG ceramic is one of the most remarkable the stability of submicron alumina, making it suitable for long- developments in recent years [30–32]. YAG ceramics is term use in high-temperature burners. The high-temperature of interest from the standpoints of optical and mechanical

3060 Transparent ceramic lamp envelopes properties. It has a cubic symmetry with an isotropic (a) thermal expansion and no birefringence effect at the grain boundaries. YAG ceramic, therefore, can have much higher in-line transmittance [31] than translucent PCA, which consists of bi-refringent alumina grains of hexagonal symmetry. The isotropic thermal expansion means no residual stresses in the YAG body when compared with the expansion-anisotropy 0.5mm induced residual stresses at grain boundaries of PCA. The absence of residual stresses in YAG ceramic means a higher mechanical strength (at a given flaw size) than that of PCA [31]. Early studies of translucent, sintered YAG ceramic involved (b) MgO [10]orSiO2 sintering aids [30], while recent studies reported transparent YAG ceramics without dopants [31]. The pure, transparent YAG bodies have been used as envelopes in lamps in addition to their use as host rods in Nd–YAG [31, 32]. However, resistance of YAG to rare-earth halides was pointed out as a potential concern [33, 34]. Thus, one challenge is to take full advantage of the excellent optical properties while minimizing corrosion risks by developing new fills. A lifetime of 10 000 h was reported for Hg-Dy-Tl-Ba-Br-I fills in a YAG tube of which the inside surface was coated Figure 8. (a) Optical micrograph of etched, polished section of with Tm (or Yb or Lu )Al O [35]. YAG tubes of 25 W transparent Y2O3–La2O3.(b) Photograph of (from left to right) 3 3 3 5 12 polished disc (∼25 mm diameter), as-sintered disk and as-sintered size (0.5 mm wall thickness) containing Hg-Na-Tl-In-I fills had tubes of Y2O3–La2O3. lifetimes of 2000 h [36]. High pressure (>6 MPa) Hg lamps using 150 W YAG tubes, were also reported [37]. Other rare- earth aluminate garnet ceramics were also developed [30–32]. an etched, polished section. A similar transparent ceramic based on the Y2O3 system, such as ThO2-doped Y2O3,was Polycrystalline Er3Al5O12 tubes with Hg-Dy-Tl-Na-I fills had ∼90% maintenance at 5000 h [38]. also reported [7]. The drawback of La2O3-doped Y2O3 is its low thermal conductivity and mechanical strength and high solubility in rare-earth molten salts [34], limiting lamp design 2.4. AlON (aluminum oxynitride) flexibility. AlON ceramics with cubic symmetry have been fabricated into transparency for applications as transparent armour 2.7. Properties and (IR) [11, 38]. The mechanical strength and thermal expansion properties of AlON [39] are close Figure 9 shows the room-temperature, in-line transmittance to those of PCA [2], so that AlON should be able to windows that have been achieved in several transparent survive the stresses in ceramic metal-halide lamps. In- ceramic lamp envelope materials relative to that of quartz. line transmittance values as good as that of sapphire have The in-line transmittance refers to spectrophotometer specular been achieved in polished AlON ceramics [39]. However, transmittance, which is the transmitted light of a normal thermodynamic calculations suggested that AlON was stable incident beam collected in a 2˚ cone of scattering angle. The in a small range of partial pressure at temperatures data were taken from the [12, 18, 31, 32]. The UV cutoff is below 1640˚C [40]. determined by electronic transition and band gap, while the IR absorption is controlled by multi-phonon edge. As temperature 2.5. MgAl2O4 (magnesium aluminate) increases, both edges move in and the transmission narrows. The in-line transmittance values of transparent A transparent, cubic MgAl O spinel ceramic with excellent 2 4 ceramics are generally higher than regular, translucent PCA, in-line transmittance for application in IR windows [9]was and some are close to that of quartz. Most transparent ceramics developed. This material, however, reacted preferentially with exhibit sharp edges in the UV and IR. Residual pores, if Na vapour in HPS lamps [41]. Its compatibility with rare-earth present in significant amount, could tilt the UV edge and halides was of concern [42]. decrease the in-line transmittance in the visible range. The intrinsic peaks in the IR edge of YAG are due to multi-phonon 2.6. Y2O3 or Y2O3–La2O3 (yttria or yttria–lanthana) interactions. The in-line transmittance of submicron alumina

La2O3-doped Y2O3 was developed using a transient, gradually decreases from a nearly theoretical value in the IR solid, second-phase sintering mechanism for applications in to a relatively low value in the UV, owing to scattering of grain materials such as lamp envelopes and IR domes or windows, boundaries and residual pores. The transmittance windows of by taking advantage of its long IR cutoff and low emissivity [8]. Y2O3–La2O3 and Y2O3 are nearly identical; the long IR cutoff The structure is cubic and excellent in-line transmittance gives a relatively low emissivity and high efficacy in lamps [8]. was achieved in ∼100 µm-grained La2O3-doped Y2O3 [8]. The rapid heating and cooling cycles encountered by the Figure 8 shows an image of discs and arc tubes consisting envelopes in lamps must be considered; a satisfactory thermal of 100 µm-grained La2O3-doped Y2O3 and a micrograph of shock resistance is required. The resistance of ceramics to

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Figure 9. Room-temperature spectrophotometer in-line transmittance windows of polished discs (0.8 mm thick) of transparent ceramic lamp envelope materials versus quartz. Data calculated and compiled from [2,6,12,17,18,31,32].

Table 1. Properties and calculated R2 of various transparent ceramic lamp envelope materials. Property values from [2, 6, 12, 17, 18,31,32,46,51,52]. Coefficient Fracture −1 Property at Fracture Elastic of thermal Thermal R2(W m ) −1/2 500˚C/ strength constant expansion conductivity R2 = (1 − ν) (MPa m ) material (MPa) (GPa) (10−6/˚C) (W m−1 K−1) ×λσ/Eα at 23˚C Sapphire 745.0 420.0 7.30 12.50 1820 2.4 ( a-axis) Sapphire 351.7 369.3 8.30 13.40 1018 1.2 ( c-axis) Spinel 117.0 320.6 7.90 5.80 185 1.9 Y2O3–La2O3 95.3 186.2 7.40 4.00 191 0.7 Quartz 103.0 77.0 0.90 2.10 2528 0.8 AlON 319.0 317.0 7.00 7.00 755 1.4 YAG 500.0 260.0 7.70 5.00 874 1.2 Submicron PCA 620.0 420.0 7.80 12.00 1612 2.6 PCA 240.0 420.0 7.80 12.00 624 1.8 thermal shock cracking is represented by a parameter called possibly important as well, owing to the presence of the R2 [43], where electrical field and temperature gradient in the discharge λσ lamp. Chemical reactions with the rare-earth halide fills R2 = (1 − ν) , (1) Eα (Dy-Tm-Tl-Ho-Ca-Na-I) similar to equation (2) can also occur where ν is Poisson’s ratio, λ thermal conductivity, σ fracture in ceramic tube materials as an alternative to regular PCA. strength, E elastic constant and α thermal expansion The corrosive reactions over a long time might result in a coefficient. Most of the properties exist in the literature. Many leaky spot in the wall or consume high levels of rare-earth of the properties are also temperature dependent [12]. For species in the fills, such that the lamp could not be ignited. example, the room-temperature thermal conductivity values of Such reactions would dictate the life of the ceramic lamp, two materials could be quite different but the high-temperature assuming corrosion in the frit seals located at the cold end of the values could be close to each other. Table 1 lists the calculated vessel is relatively insignificant. The above consideration leads R2 (at 500˚C) values of various transparent ceramics. Owing to a parameter: upper limit temperature for a given ceramic −1 to anisotropy, the R2 value of sapphire (1820 W m ) along tube material. The upper limit temperature is defined as the −1 the a-axis is significantly higher than that (1018 W m ) along average wall temperature at which the lamps can maintain a −1 the c-axis. The latter is actually lower than that (1612 W m ) life of 10 000 h. For example, regular PCA tubes containing of submicron alumina (table 1). rare-earth halide fills typically have a life of 10000 h when Chemical compatibility between the arc tube and fill is operated at a maximum wall temperature of 1200˚C [4]. Thus, important of the long lives expected lamps. As the kinetics of the upper limit temperature for regular alumina is taken as the corroding reactions [44,45] such as 1200˚C. Sapphire is thought to be able to operate at a slightly higher temperature (1250˚C) than regular PCA owing to the 3DyI (l) + 4Al O ↔ Dy Al O (s) + 3AlI (g), (2) 3 2 3 3 5 12 3 absence of grain boundaries. Quartz devitrifies at 900˚C, are sufficiently slow, the PCA metal-halide lamps are practical. which is its upper limit temperature in lamps. For transparent Molten salts of rare-earth halides can dissolve PCA, which ceramic materials that have not been tested for sufficient could then deposit or precipitate at cooler places inside the time statistically, estimation has to be made based on the burner where the solubility is lower. It is speculated that consideration of short-time data or thermodynamics of the the flux-line and upward drilling corrosion process commonly stability of the ceramics and their reactions with the rare-earth occurring in the lining of melting fills. For example, La2O3-doped Y2O3 is limited to a wall is also operative here. Electrochemical effects are temperature of <1100˚C because of its high solubility in

3062 Transparent ceramic lamp envelopes

1400 C ° 1300 Sapphire (//a-axis) Sapphire (//c-axis) 1200 PCA

La2O3-doped Y2O3 1100 Submicron-grained PCA AlON Spinel YAG 1000

900 Quartz Wall temperature limit for rare earth fills,

800 100 1000 10000 ° R2 (resistance to thermal shock cracking) at 500 C, W/m

Figure 10. Upper limit temperature for rare-earth fills versus calculated thermal shock resistance parameter, R2 [23], for various transparent ceramic lamp envelope materials. R2 is (1 − ν)λσ/Eα where ν is Poisson’s ratio, λ thermal conductivity, σ fracture strength, E elastic constant and α thermal expansion coefficient. Material properties values used in R2 calculation were taken from [2, 6, 12, 17, 18,31,32,46,51,52]. molten rare-earth salts [34]. The upper limit temperature stresses of 21 MPa and 10 MPa, respectively. Such creep for submicron alumina doped with MgO is estimated to rates suggest a significant amount of and creep be 1100˚C owing to the grain growth and microstructural cavitation (formation of micro-cavities at grain boundaries), instability discussed in section 2.2. could possibly occur under the stresses at that temperature, Figure 10 shows a plot of the experimental or estimated which in turn would result in an opaque body and lower lamp 7 upper limit temperature versus R2 (calculated from 500˚C output, over the 3000 h (1 × 10 s) life. In addition, high- properties) for various materials. In practice, quartz has by temperature (1200˚C) fracture strength of sapphire and regular far the best thermal shock resistance owing to its low thermal PCA is ∼30% lower than that at room-temperature [12,49]. expansion and elastic constant. The calculated R2 for quartz The fracture strength of submicron alumina doped with is indeed quite high. Submicron-grained Al2O3 and sapphire MgO is expected to have similar temperature dependence, (parallel to the a-axis) have high values of R2 because of their further limiting the design of the lamp. High-temperature high mechanical strength. The R2 value of sapphire depends creep behaviour of other transparent ceramics was less on the direction of the stresses; sapphire is more resistant to extensively studied when compared with PCA. The creep thermal shock fracture in the direction parallel to the a-axis rate of polycrystalline 2 µm-grained YAG was reportedly [50] than in the direction of the c-axis. The R2 values of AlON and lower than that of PCA [47]. YAG are slightly higher than those of PCA. MgAl2O4 spinel Other properties such as (KIC) where and Y2O3–La2O3 have relatively low R2 values owing to low 1/2 thermal conductivity and strength. Physical properties such KIC ∼ (E · γ) , (4) as mechanical strength, elastic constant, thermal expansion, thermal conductivity and emissivity, along with the geometry, where γ is the fracture energy and E the , which size and thickness of transparent ceramics, are important is related to fracture strength (σ ), and critical flaw size (c), parameters for assessing their resistance to thermal shock and where K stresses in lamps. σ ∼ IC , (5) 1/2 In addition to thermal shock resistance, creep rate (c) (∂ε/∂t) is, should also be considered. The values of room-temperature ∂ε ∼ D · (grain size)−N, (3) fracture toughness were included in table 1. The fracture ∂t toughness data of sapphire (parallel to the a-axis and where D is diffusivity, N = 2 for lattice-diffusion controlled the c-axis), YAG, submicron alumina and other materials deformation, N = 3 for boundary-diffusion controlled were from [12, 17, 51–53]. Fracture toughness is a measure deformation [43] and creep rupture strength at high of resistance of crack propagation; a higher value of KIC temperatures under high are important material gives a longer time to failure [54]. One challenge in the parameters. Creep deformation and rupture in submicron development of transparent ceramics has been to toughen Al2O3 doped with MgO are of concern owing to the high the material without sacrificing optical transmission [12]. strain rate brought about by the small grain size, equation (3). Fracture toughness of transparent ceramics decreases with High-temperature creep data reported for ∼0.5 µm-grained increasing temperature [12,52]. Here again, wall temperature alumina doped with MgO [47, 48] gave an estimated creep is a significant parameter for the lamp’s life, from the viewpoint rate of 2 × 10−7s−1 and 8 × 10−8 s−1 at 1200˚C under tensile of crack propagation. For example, at the same

3063 GCWei intensity and critical flaw size, the velocity of crack growth References in regular PCA increases by four orders of magnitude (104) when the temperature increases from 1200˚C to 1400˚C [55]. [1] Coble R L 1961 J. Appl. Phys. 32 793 [2] de Groot J J and van VlietJAJM1986 The High-Pressure Therefore, because both the corrosion of ceramic tubes by Sodium Lamp (London: MacMillan) fills and the survivability (crack growth) of ceramic envelopes [3] Seinen P A 1995 Proc. 7th Int. Symp. on Science and under thermal stresses, are functions of temperature, the of Light Sources (Kyoto: Illuminating design and control of the wall temperature in the lamps Society of Japan) p 101 [4] Juengst S, Lang D and Galvez M 2004 Light Sources 2004 are important. Because of the brittle nature of ceramics, (Proc. 10th Int. Symp. on Science and Technology of Light statistically meaningful lamp experiments on mechanical Sources, Toulouse, 18–22 July 2004 ) Institute of Physics reliability, typically, requires that 30 samples [43]be Conf. Ser. vol 182, ed G Zissis (Bristol: Institute of Physics tested. Publishing) p 1014 [5] Scott C et al 2002 J. Am. Cer. Soc. 85 1275 [6] Hayashi K et al 1991 Mater. Trans. JIM 32 1024 [7] Greskovich C and Chernoch J P 1973 J. Appl. Phys. 44 4599 3. Conclusions and summary [8] Rhodes W 1981 J. Am. Cer. Soc. 64 13 [9] Roy D 1981 Proc. SPIE 29713 PCA envelopes have been vital for the widespread construction [10] de With G and van Dijk H 1984 Mater. Res. Bull. 19 1669 of contemporary HPS and ceramic metal-halide lamps in [11] McCauley J M and Corbin N D 1979 J. Am. Cer. Soc. 62 476 [12] Harris D 1999 Materials for Infrared Windows and Domes place of quartz. Lamp scientists have constantly been (Bellingham, WA: SPIE) searching for new focused-beam light-sources, using improved [13] LaBelle H E 1980 J. Cryst. Growth 50 9 transparent ceramic envelopes for better energy efficiency, [14] RigdenSAR1965 G.E.C. J. 32 37 colour properties, life, cost and environmental renewability. [15] Wei G, Kramer J, Walsh J and Lapatovich W 1997 US Patent No 5621295 Many studies in the literature aimed at producing improved [16] Morinaga K, Torikai T, Nakagawa K and Fujino S 2000 Acta lamp envelopes, involved sintering of ceramic materials to Mater. 48 4735 transparency for tube, window, IR dome and applications. [17] Krell A, Blank P, Ma H, Hutzler T, van BruggenMPBand While melt-grown single crystals such as sapphire are Apetz R 2003 J. Am. Cer. Soc. 86 12 restricted to straight tubes, technological advancements in [18] Apetz R and van BruggenMPB2003 J. Am. Cer. Soc. 86 480 [19] Nagayama H 1995 European Patent No EP650184 ceramic powder synthesis, forming, sintering and HIP have [20] Miyanaga S, Ikeuchi M, Mori K and Tagawa Y 2000 European made possible a wide range of compositions (e.g. Y2O3,Y2O3– Patent No EP0987736 La2O3, MgAl2O4, YAG, small-grained Al2O3 and AlON, and [21] Yagi H, Yamazaki H and Kubo H 2001 Japan Patent shapes (cylindrical, bulgy, spherical and elliptical), presenting No JP2001-199761 [22] Wei G C 2004 J. Cer. Soc. Japan. 112 S1 flexibility in the designing of lamps and fill chemistry. [23] Yeh T and Sacks M 1988 J. Am. Cer. Soc. 71 841 Significant advances in developing the optical properties have [24] Xue L and Chen I 1990 J. Am. Cer. Soc. 73 3518 resulted from an improved understanding of the powder- [25] Greskovich C and Brewer J A 2001 J. Am. Cer. Soc. 84 420 processing-microstructure-property inter-relationships over [26] Ando K and Momoda M 1987 Yogyo-Kyokaishi 95 381 [27] Roy S and Coble R L 1968 J. Am. Cer. Soc. 51 1 the years. However, the rapid heating and cooling cycles [28] van Bruggen M, Kop T and Keursten T 2004 International encountered by the envelopes in lamps must be considered; Patent No WO2004/007397 a satisfactory thermal shock resistance is required. Quartz has [29] Mitsuoka T, Yamamoto H and Iio S 2003 Key Eng. Mater. 247 excellent thermal shock resistance owing to its low thermal 349 [30] Ikesue A and Kamata K 1996 J. Am. Cer. Soc. 79 1927 expansion and elastic constant. Physical properties such [31] Yanagitani T, Imagawa S, Yagi H and Kubo H 1998 European as mechanical strength, elastic constant, thermal expansion, Patent No EP0926106 thermal conductivity and emissivity, along with the geometry, [32] Lu J et al 2000 App. Phys. Lett. 77 3707 size and thickness of transparent ceramics are important [33] Maekawa K 1995 Proc. 7th Int. Symp. on Science and parameters for assessing their resistance to withstand thermal Technology of Light Sources (Kyoto: Illuminating Engineering Society of Japan) p 293 shock and stresses in lamps. The corrosive nature of [34] Rhodes W 1998 Proc. 8th Int. Symp. on Science and lamp-fill liquid and vapour at high temperatures requires Technology of Light Sources (Greifswald: Kiebu-Druck) that all lamp components be carefully chosen to meet the p 109 target life. The economics could be hard to justify a new [35] Yanagitani T, Imagawa S, Yagi H and Kubo H 1999 European Patent No EP0926106 transparent ceramic for improved performance (efficacy) in [36] Honda H et al 1999 European Patent No EP0935278 well-developed applications such as HPS lamp envelopes. The [37] Mitsuhashi K and Mori K 1998 Japan Patent No JP10-188893 wide range of new transparent ceramic envelope materials [38] Yanagitani T et al Japan Patent No JP11-147757 represents opportunities for advanced light-sources such as [39] Hartnett T M and Gentilman R L 1984 SPIE 505 15 [40] Willems H X, Hendrix M, Metselarr R and de Wit D 1992 beamers and automotive headlights of new designs and fill J. Eur. Cer. Soc. 10 327 chemistries. [41] Hing P 1976 J. Mater. Sci. 11 1919 [42] Caruso N 1995 International Patent No WO 95/28733 [43] Kingery W D, Bowen H K and Uhlmann D R 1976 Acknowledgments Introduction to Ceramics (New York: Wiley) [44] van Erk W 2000 Proc. HTMC X Conf. 267 (Julich:¨ Forchungszentrum Julich)¨ The author acknowledges the support from his colleagues at [45] Markus T, Niemann U and Hilpert K 2003 Proc. HTMC XI OSRAM SYLVANIA, USA and OSRAM, Germany. Conf. (Tokyo: University of Tokyo) p 28

3064 Transparent ceramic lamp envelopes [46] Wei G 1988 SPIE Proc. 929 50 [52] Newcomb S and Tressler R 1994 J. Am. Cer. Soc. [47] Chen I and Xue L 1990 J. Am. Cer. Soc. 73 2585 77 3030 [48] Gruffel et al 1990 Proc. Riso Symp. [53] Rice R 2000 Mechanical Properties of Ceramics and (Denmark: Riso) p 305 Composites (New York: Dekker) p 76 [49] Charles R and Shaw R 1962 GE Report 62-RL-3081M [54] Evans A 1973 Ceramics for High-Performance Applications [50] French J et al 1994 J. Am. Cer. Soc. 77 2857 (Ohio: and Ceramic Information Center.) [51] Iwasa M and Bradt R 1984 Adv. Ceram. 10 767 [55] Evans A 1976 Prog. Mater. Sci. 303 (Oxford: Pergamon)

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