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The wettability of sapphire, polycrystalline alumina, and quartz by molten metal halide salts
Journal of Physics and Chemistry of Solids 66 (2005) 484–487 www.elsevier.com/locate/jpcs
The wettability of sapphire, polycrystalline alumina, and quartz by molten metal halide salts Lori R. Brock* Osram Sylvania Research and Development, 71 Cherry Hill Drive, Beverly, MA, USA Accepted 15 June 2004
Abstract The sessile drop technique has been used to measure the wetting angle of molten metal halide salts on polycrystalline alumina, sapphire (crystalline alumina), and quartz substrates. The metal halide salts studied are pure NaI, and a typical metal halide lamp chemistry containing NaI, TlI, DyI3, HoI3, and TmI3. Measurements were made in argon filled quartz ampoules at temperatures varying from the melting point of the salts to a maximum of 1050 8C. In all the cases, the measured wetting angles were less than 908 indicating strong wettability of the substrates by the molten salts. q 2004 Elsevier Ltd. All rights reserved. Keywords: Wetting angle; A. Inorganic compounds; A. Surfaces; A. Ceramics
1. Introduction The most important components of a metal halide lamp are the arc tube and the metal halides. The arc tube is the envelope in which the metal halides are held, and from which the light is generated. Arc tubes within metal halide lamps were traditionally fabricated from quartz (silica), which can be operated at temperatures up to approximately 900 8C. More recently, arc tubes made of new materials, such as polycrystalline alumina (PCA) and sapphire have seen commercial use as metal halide lamps. One advantage of PCA and sapphire arc tubes is that they can be operated at higher temperatures, up to approximately 1200 8C [1]. Typical chemical fills within these arc tubes contain combinations of metal iodides that may include alkali and alkaline earth iodides (such as NaI, CsI, CaI2), rare-earth iodides (such as DyI3, HoI3, TmI3), and a variety of other iodides (such as ScI3, ZnI2) [2]. The chemical composition for a particular metal halide lamp is chosen to yield desired photometric properties, such as a specific color temperature, lumen output, and long lamp life. * Fax: C1 978 750 1799. E-mail address: [email protected]. 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.086
The metal halide additives must have a high enough vapor pressure at the temperature of the arc tube wall for visible radiation to be generated by the high pressure discharge within the lamp. Higher vapor pressures are achieved with higher arc tube wall temperatures. At temperatures above 600 8C, the additives are liquid, and a pool of molten metal iodides is formed within the arc tube. Interactions between the molten metal iodides and the arc tube wall material at high temperature are critical for lamp performance. For example, the contact between the molten salt and the arc tube wall often leads to corrosion of the vessel. For this reason, the wettability of the different arc tube materials by the molten metal halide fills have been investigated. We have measured the wetting angles of NaI and a typical metal halide lamp fill containing NaI, TlI, DyI3, HoI3, and TmI3, on PCA, sapphire and quartz substrates, as a function of temperature, using the sessile drop method [3,4]. This method involves placing a solid pellet on a flat substrate and raising the temperature gradually until a molten drop forms. A photograph is recorded from the side, and the wetting angle is measured from the profile of the drop. The wetting angle (also known as contact angle), q, of a liquid drop on a solid substrate is the included angle
L.R. Brock / Journal of Physics and Chemistry of Solids 66 (2005) 484–487
between the substrate surface and the tangent plane to the surface of a drop of liquid at liquid-substrate point of contact [3,4]. The wettability of a surface by a liquid is described by the contact angle. When q!908, the liquid is said to wet the solid, and when qO908, the liquid is considered nonwetting. The limit of qZ08 is full wetting, and of qZ1808 is complete nonwetting.
2. Experimental The cubic ampoules used in these experiments were fabricated from square quartz tubing (25!25 mm) with optical quality quartz windows fused onto each end. A plate of PCA, quartz, or sapphire was sealed in each ampoule. The 20!20 mm plates that served as the PCA substrates were made in-house. The PCA was sintered to translucency with the addition of 200 ppm MgO as a grain-growth inhibiting agent [5]. The mean grain diameter was approximately 40 mm. The quartz plates were purchased from Quartz Plus (Brookline, NH), and the non-oriented polished sapphire (crystalline Al2O3) plates were purchased from Guild Optical Associates (Amherst, NH). Within a glove box, one hemispherical salt pellet was sealed in each ampoule with an atmosphere of 50 Torr of dry argon. The ampoules were sealed using the same process as a metal halide lamp arc tube. Each pellet weighed approximately 200 mg. Table 1 has a description of the salt compositions. A Lindberg/Blue M single zone clamshell style furnace was used to melt the metal halide pellets and maintain the drop formed at the desired temperature G2 8C. The temperature in the furnace was increased by increments of 50 8C to a maximum of 1050 8C. Photographs were taken every 50 8C after the furnace temperature had stabilized for 15 min. No changes of wetting angle with elapsed time were observed after 15 min. A Canon EOS D60 digital single-lens reflex camera with 6.3 million effective pixels and a microscopic zoom lens system was utilized to take pictures of the sessile drops. The recorded image size was 3072 by 2048 pixels. Interfacing the camera to a computer allowed the images to be recorded directly on the hard drive as JPG files. After the completion of each experiment, the drop images were loaded into the ImageJ analysis software [6]. The profile of the drop was converted to xy coordinates, and the coordinates were fit to calculate the wetting angle.
485
3. Results and discussion We have measured the wetting angles of pure NaI and a typical lamp chemistry, on planar PCA, sapphire, and quartz substrates, as a function of temperature. Two typical sessile drop images, labeled as A and B, are shown in Fig. 1. The substrate is quartz and the drop is of typical lamp chemistry, as described in Table 1. Analysis of image A, taken at 415 8C, resulted in a wetting angle of 868, and of B, at 700 8C, resulted in a wetting angle of 388. An increase in condensation on the ampoule window is observed as the temperature is increased. This behavior is expected because the metal halide lamp additives must have high vapor pressures at the temperature of the arc tube wall in order to contribute to the visible radiation emitted from the lamp. As vaporization and condensation increase within the ampoule, it becomes more difficult to photograph the sessile drop. This results in increased error bars at higher temperatures, especially for the ampoules containing pure NaI. In all cases the measured wetting angle, q, was less than 908 indicating the wettability of PCA, quartz, and sapphire by the molten salts is strong. The wetting angles were generally highest at the lower temperatures, and decreased as the temperature was raised. A plot of wetting angle vs. temperature for a typical metal halide lamp chemistry on quartz, sapphire and PCA substrates is shown in Fig. 2. The largest differences between substrates are apparent at the lowest temperatures measured. For example, at 415 8C, qZ868 on quartz, and qZ348 on sapphire. At the lowest temperature measured (500 8C) on PCA, qZ438, only half the value measured on quartz.
Table 1 The compositions of the metal iodide salts shown in weight percent Salt composition Typical lamp chemistry Pure NaI
NaI 32.5 100.0
TlI
DyI3
HoI3
TmI3
9.0
19.5
19.5
19.5
–
–
–
–
Fig. 1. Two sessile drop images, A and B. The substrate is quartz, and the salt is typical lamp chemistry (see Table 1). Analysis of image A, taken at 415 8C, resulted in a wetting angle of 868, and of B, at 700 8C, resulted in a wetting angle of 388. The base of drop A has a radius of 2.85 mm, and B has a radius of 4.05 mm. An increase in condensation on the ampoule window is observed in B.
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L.R. Brock / Journal of Physics and Chemistry of Solids 66 (2005) 484–487
Fig. 2. A plot of wetting angle vs. temperature for a typical metal halide lamp chemistry on quartz, sapphire and PCA substrates.
The value of the wetting angle on sapphire varies only a few degrees (between 31 and 348) from near the melting point at 415 8C to a temperature of 750 8C. The variation of q with temperature on PCA is somewhat stronger, decreasing from 428 at 500 8C to 298 at 750 8C. On quartz, however, q rapidly decreases from 86 to 348 over the same temperature range. At 750 8C, wetting angle values of 29–348 are observed for quartz, PCA and sapphire. For all three substrates, q approaches 08 at 900 8C. Metal halide lamps are typically operated with arc tube walls in the 700–900 8C temperature range. Therefore, the lamp chemistry should equivalently
wet arc tubes made with these three materials. Similar wetting behavior is, in fact, observed in metal halide lamps with both quartz and PCA arc tubes filled with this chemistry. The major component of the typical lamp chemistry is NaI (32.5 wt %). For this reason, the wetting of PCA, sapphire and quartz by pure NaI was studied. A plot of wetting angle vs. temperature for NaI on quartz, sapphire and PCA substrates is shown in Fig. 3. Condensation on the ampoule window created some difficulty in photographing the drop, and therefore error bars around G58 are present.
Fig. 3. A plot of wetting angle vs. temperature for NaI on quartz, sapphire and PCA substrates. Condensation on the ampoule window created difficulty in photographing the drop, and therefore error bars around G58.
L.R. Brock / Journal of Physics and Chemistry of Solids 66 (2005) 484–487
The wetting of sapphire and PCA by NaI is similar to what was observed for the typical lamp chemistry, as shown in Fig. 2. The wetting of quartz, however, is much different. As solid NaI melts on quartz, it instantly and completely wets the surface, yielding qZ08 at only 650 8C. This behavior suggests the occurrence of an interfacial reaction, although the interface remained macroscopically planar. No evidence of a reaction was observed upon examination of the substrate. Interestingly, the wetting angle on quartz for the typical lamp chemistry drops sharply from 66 to 428 over the same temperature region (600–650 8C). The chemistry of sapphire (crystalline alumina) and polycrystalline alumina are similar, with the exception of the MgO dopant (and therefore the grains) in the polycrystalline material. The comparable wettability of sapphire and PCA by NaI and the typical lamp chemistry is, therefore, not surprising. Although the grain boundaries of some polycrystalline solids have been shown to enhance wetting [4], in this case, the grain structure does not greatly influence the wettability. The chemical composition of quartz (silica) is very different from alumina. Therefore, the observed differences between alumina and silica can be accepted.
487
wetting angles at 650 8C and higher, with similarly decreasing wetting angles toward 08 at 900 8C. Since metal halide lamps are typically operated at wall temperatures above 650 8C, the wetting of quartz, PCA or sapphire arc tubes by this melt is expected to be comparable. Similar wetting behavior is observed in metal halide lamps with both quartz and PCA arc tubes filled with this chemistry.
Acknowledgements The technical expertise of Joanne Browne, Arlene Hecker, Joe Lima, Dr Warren Moskowitz, and David Wentzel are gratefully acknowledged. Helpful discussions with Dr Dieter Lang and Dr Stefan Ju¨ngst are also appreciated. Some associated wetting experiments were performed in the laboratory of Professors William Wilcox and Liya Regel, with guidance from Arun Kota, of Clarkson University (Potsdam, New York). I thank them for their instruction and hospitality during my two visits.
References 4. Conclusions The wettability of sapphire (crystalline alumina) and polycrystalline alumina by NaI and a typical metal halide lamp chemistry (NaI, TlI, DyI3, HoI3, and TmI3) are comparable over the temperature range of 500–900 8C. The PCA grain structure does not greatly influence the wettability. Furthermore, the wetting of sapphire, PCA and quartz by typical lamp chemistry all have comparable
[1] K.E. Parker, D.T. Evans, in: J.R. Coaton, A.M. Marsden (Eds.), Lamps and Lighting, Arnold, London, 1997, p. 119. [2] K. Hilpert, U. Niemann, High temperature chemistry in metal halide lamps, Thermochimica Acta 299 (1997) 49–57. [3] Ju. V. Naidlich, in: D.A. Cadenhead, J.F. Danielli (Eds.), Progress in Surface and Membrane Science, Academic, New York, 1981, p. 353. [4] N. Eustathopoulous, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Pergamon, Amsterdam, 1999. [5] R.L. Coble, US Patent Number 3026210, Transparent Alumina and Method of Preparation, 1962. [6] W. Rasband, ImageJ, version 1.28u, http://rsb.info.nih.gov/ij/
The wettability of sapphire, polycrystalline alumina, and quartz by molten metal halide salts Lori R. Brock* Osram Sylvania Research and Development, 71 Cherry Hill Drive, Beverly, MA, USA Accepted 15 June 2004
Abstract The sessile drop technique has been used to measure the wetting angle of molten metal halide salts on polycrystalline alumina, sapphire (crystalline alumina), and quartz substrates. The metal halide salts studied are pure NaI, and a typical metal halide lamp chemistry containing NaI, TlI, DyI3, HoI3, and TmI3. Measurements were made in argon filled quartz ampoules at temperatures varying from the melting point of the salts to a maximum of 1050 8C. In all the cases, the measured wetting angles were less than 908 indicating strong wettability of the substrates by the molten salts. q 2004 Elsevier Ltd. All rights reserved. Keywords: Wetting angle; A. Inorganic compounds; A. Surfaces; A. Ceramics
1. Introduction The most important components of a metal halide lamp are the arc tube and the metal halides. The arc tube is the envelope in which the metal halides are held, and from which the light is generated. Arc tubes within metal halide lamps were traditionally fabricated from quartz (silica), which can be operated at temperatures up to approximately 900 8C. More recently, arc tubes made of new materials, such as polycrystalline alumina (PCA) and sapphire have seen commercial use as metal halide lamps. One advantage of PCA and sapphire arc tubes is that they can be operated at higher temperatures, up to approximately 1200 8C [1]. Typical chemical fills within these arc tubes contain combinations of metal iodides that may include alkali and alkaline earth iodides (such as NaI, CsI, CaI2), rare-earth iodides (such as DyI3, HoI3, TmI3), and a variety of other iodides (such as ScI3, ZnI2) [2]. The chemical composition for a particular metal halide lamp is chosen to yield desired photometric properties, such as a specific color temperature, lumen output, and long lamp life. * Fax: C1 978 750 1799. E-mail address: [email protected]. 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.086
The metal halide additives must have a high enough vapor pressure at the temperature of the arc tube wall for visible radiation to be generated by the high pressure discharge within the lamp. Higher vapor pressures are achieved with higher arc tube wall temperatures. At temperatures above 600 8C, the additives are liquid, and a pool of molten metal iodides is formed within the arc tube. Interactions between the molten metal iodides and the arc tube wall material at high temperature are critical for lamp performance. For example, the contact between the molten salt and the arc tube wall often leads to corrosion of the vessel. For this reason, the wettability of the different arc tube materials by the molten metal halide fills have been investigated. We have measured the wetting angles of NaI and a typical metal halide lamp fill containing NaI, TlI, DyI3, HoI3, and TmI3, on PCA, sapphire and quartz substrates, as a function of temperature, using the sessile drop method [3,4]. This method involves placing a solid pellet on a flat substrate and raising the temperature gradually until a molten drop forms. A photograph is recorded from the side, and the wetting angle is measured from the profile of the drop. The wetting angle (also known as contact angle), q, of a liquid drop on a solid substrate is the included angle
L.R. Brock / Journal of Physics and Chemistry of Solids 66 (2005) 484–487
between the substrate surface and the tangent plane to the surface of a drop of liquid at liquid-substrate point of contact [3,4]. The wettability of a surface by a liquid is described by the contact angle. When q!908, the liquid is said to wet the solid, and when qO908, the liquid is considered nonwetting. The limit of qZ08 is full wetting, and of qZ1808 is complete nonwetting.
2. Experimental The cubic ampoules used in these experiments were fabricated from square quartz tubing (25!25 mm) with optical quality quartz windows fused onto each end. A plate of PCA, quartz, or sapphire was sealed in each ampoule. The 20!20 mm plates that served as the PCA substrates were made in-house. The PCA was sintered to translucency with the addition of 200 ppm MgO as a grain-growth inhibiting agent [5]. The mean grain diameter was approximately 40 mm. The quartz plates were purchased from Quartz Plus (Brookline, NH), and the non-oriented polished sapphire (crystalline Al2O3) plates were purchased from Guild Optical Associates (Amherst, NH). Within a glove box, one hemispherical salt pellet was sealed in each ampoule with an atmosphere of 50 Torr of dry argon. The ampoules were sealed using the same process as a metal halide lamp arc tube. Each pellet weighed approximately 200 mg. Table 1 has a description of the salt compositions. A Lindberg/Blue M single zone clamshell style furnace was used to melt the metal halide pellets and maintain the drop formed at the desired temperature G2 8C. The temperature in the furnace was increased by increments of 50 8C to a maximum of 1050 8C. Photographs were taken every 50 8C after the furnace temperature had stabilized for 15 min. No changes of wetting angle with elapsed time were observed after 15 min. A Canon EOS D60 digital single-lens reflex camera with 6.3 million effective pixels and a microscopic zoom lens system was utilized to take pictures of the sessile drops. The recorded image size was 3072 by 2048 pixels. Interfacing the camera to a computer allowed the images to be recorded directly on the hard drive as JPG files. After the completion of each experiment, the drop images were loaded into the ImageJ analysis software [6]. The profile of the drop was converted to xy coordinates, and the coordinates were fit to calculate the wetting angle.
485
3. Results and discussion We have measured the wetting angles of pure NaI and a typical lamp chemistry, on planar PCA, sapphire, and quartz substrates, as a function of temperature. Two typical sessile drop images, labeled as A and B, are shown in Fig. 1. The substrate is quartz and the drop is of typical lamp chemistry, as described in Table 1. Analysis of image A, taken at 415 8C, resulted in a wetting angle of 868, and of B, at 700 8C, resulted in a wetting angle of 388. An increase in condensation on the ampoule window is observed as the temperature is increased. This behavior is expected because the metal halide lamp additives must have high vapor pressures at the temperature of the arc tube wall in order to contribute to the visible radiation emitted from the lamp. As vaporization and condensation increase within the ampoule, it becomes more difficult to photograph the sessile drop. This results in increased error bars at higher temperatures, especially for the ampoules containing pure NaI. In all cases the measured wetting angle, q, was less than 908 indicating the wettability of PCA, quartz, and sapphire by the molten salts is strong. The wetting angles were generally highest at the lower temperatures, and decreased as the temperature was raised. A plot of wetting angle vs. temperature for a typical metal halide lamp chemistry on quartz, sapphire and PCA substrates is shown in Fig. 2. The largest differences between substrates are apparent at the lowest temperatures measured. For example, at 415 8C, qZ868 on quartz, and qZ348 on sapphire. At the lowest temperature measured (500 8C) on PCA, qZ438, only half the value measured on quartz.
Table 1 The compositions of the metal iodide salts shown in weight percent Salt composition Typical lamp chemistry Pure NaI
NaI 32.5 100.0
TlI
DyI3
HoI3
TmI3
9.0
19.5
19.5
19.5
–
–
–
–
Fig. 1. Two sessile drop images, A and B. The substrate is quartz, and the salt is typical lamp chemistry (see Table 1). Analysis of image A, taken at 415 8C, resulted in a wetting angle of 868, and of B, at 700 8C, resulted in a wetting angle of 388. The base of drop A has a radius of 2.85 mm, and B has a radius of 4.05 mm. An increase in condensation on the ampoule window is observed in B.
486
L.R. Brock / Journal of Physics and Chemistry of Solids 66 (2005) 484–487
Fig. 2. A plot of wetting angle vs. temperature for a typical metal halide lamp chemistry on quartz, sapphire and PCA substrates.
The value of the wetting angle on sapphire varies only a few degrees (between 31 and 348) from near the melting point at 415 8C to a temperature of 750 8C. The variation of q with temperature on PCA is somewhat stronger, decreasing from 428 at 500 8C to 298 at 750 8C. On quartz, however, q rapidly decreases from 86 to 348 over the same temperature range. At 750 8C, wetting angle values of 29–348 are observed for quartz, PCA and sapphire. For all three substrates, q approaches 08 at 900 8C. Metal halide lamps are typically operated with arc tube walls in the 700–900 8C temperature range. Therefore, the lamp chemistry should equivalently
wet arc tubes made with these three materials. Similar wetting behavior is, in fact, observed in metal halide lamps with both quartz and PCA arc tubes filled with this chemistry. The major component of the typical lamp chemistry is NaI (32.5 wt %). For this reason, the wetting of PCA, sapphire and quartz by pure NaI was studied. A plot of wetting angle vs. temperature for NaI on quartz, sapphire and PCA substrates is shown in Fig. 3. Condensation on the ampoule window created some difficulty in photographing the drop, and therefore error bars around G58 are present.
Fig. 3. A plot of wetting angle vs. temperature for NaI on quartz, sapphire and PCA substrates. Condensation on the ampoule window created difficulty in photographing the drop, and therefore error bars around G58.
L.R. Brock / Journal of Physics and Chemistry of Solids 66 (2005) 484–487
The wetting of sapphire and PCA by NaI is similar to what was observed for the typical lamp chemistry, as shown in Fig. 2. The wetting of quartz, however, is much different. As solid NaI melts on quartz, it instantly and completely wets the surface, yielding qZ08 at only 650 8C. This behavior suggests the occurrence of an interfacial reaction, although the interface remained macroscopically planar. No evidence of a reaction was observed upon examination of the substrate. Interestingly, the wetting angle on quartz for the typical lamp chemistry drops sharply from 66 to 428 over the same temperature region (600–650 8C). The chemistry of sapphire (crystalline alumina) and polycrystalline alumina are similar, with the exception of the MgO dopant (and therefore the grains) in the polycrystalline material. The comparable wettability of sapphire and PCA by NaI and the typical lamp chemistry is, therefore, not surprising. Although the grain boundaries of some polycrystalline solids have been shown to enhance wetting [4], in this case, the grain structure does not greatly influence the wettability. The chemical composition of quartz (silica) is very different from alumina. Therefore, the observed differences between alumina and silica can be accepted.
487
wetting angles at 650 8C and higher, with similarly decreasing wetting angles toward 08 at 900 8C. Since metal halide lamps are typically operated at wall temperatures above 650 8C, the wetting of quartz, PCA or sapphire arc tubes by this melt is expected to be comparable. Similar wetting behavior is observed in metal halide lamps with both quartz and PCA arc tubes filled with this chemistry.
Acknowledgements The technical expertise of Joanne Browne, Arlene Hecker, Joe Lima, Dr Warren Moskowitz, and David Wentzel are gratefully acknowledged. Helpful discussions with Dr Dieter Lang and Dr Stefan Ju¨ngst are also appreciated. Some associated wetting experiments were performed in the laboratory of Professors William Wilcox and Liya Regel, with guidance from Arun Kota, of Clarkson University (Potsdam, New York). I thank them for their instruction and hospitality during my two visits.
References 4. Conclusions The wettability of sapphire (crystalline alumina) and polycrystalline alumina by NaI and a typical metal halide lamp chemistry (NaI, TlI, DyI3, HoI3, and TmI3) are comparable over the temperature range of 500–900 8C. The PCA grain structure does not greatly influence the wettability. Furthermore, the wetting of sapphire, PCA and quartz by typical lamp chemistry all have comparable
[1] K.E. Parker, D.T. Evans, in: J.R. Coaton, A.M. Marsden (Eds.), Lamps and Lighting, Arnold, London, 1997, p. 119. [2] K. Hilpert, U. Niemann, High temperature chemistry in metal halide lamps, Thermochimica Acta 299 (1997) 49–57. [3] Ju. V. Naidlich, in: D.A. Cadenhead, J.F. Danielli (Eds.), Progress in Surface and Membrane Science, Academic, New York, 1981, p. 353. [4] N. Eustathopoulous, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Pergamon, Amsterdam, 1999. [5] R.L. Coble, US Patent Number 3026210, Transparent Alumina and Method of Preparation, 1962. [6] W. Rasband, ImageJ, version 1.28u, http://rsb.info.nih.gov/ij/