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High temperature reactions between molybdenum and metal halides
Applied Surface Science 252 (2006) 8309–8313 www.elsevier.com/locate/apsusc
High temperature reactions between molybdenum and metal halides ´ . Bo¨ro¨czki a, G. Dobos b,*, V.K. Josepovits b, Gy. Ha´rs b A a b
General Electric Consumer & Industrial, Va´ci Street 77, Budapest H-1340, Hungary Budapest University of Technology and Economics, Department of Atomic Physics, Surface Physics Laboratory, Budafoki Street 8, Budapest H-1111, Hungary
Received 30 August 2005; received in revised form 3 November 2005; accepted 7 November 2005 Available online 7 December 2005
Abstract Good colour rendering properties, high intensity and efficacy are of vital importance for high-end lighting applications. These requirements can be achieved by high intensity discharge lamps doped with different metal halide additives (metal halide lamps). To improve their reliability, it is very important to understand the different failure processes of the lamps. In this paper, the corrosion reactions between different metal halides and the molybdenum electrical feed-through electrode are discussed. The reactions were studied in the feed-through of real lamps and on model samples too. X-ray photoelectron spectroscopy (XPS) was used to establish the chemical states. In case of the model samples we have also used atomic absorption spectroscopy (AAS) to measure the reaction product amounts. Based on the measurement results we were able to determine the most corrosive metal halide components and to understand the mechanism of the reactions. # 2005 Elsevier B.V. All rights reserved. Keywords: Lamp; Corrosion; Metal halide; Molybdenum; XPS; AAS
1. Introduction Metal halide lamps represent one of the basic types of general-purpose high intensity discharge (HID) lamps. In these lamps the light is produced by a high pressure gas discharge, generated between two electrodes of refractory metal material. In contrast to the mercury vapour discharge lamps that contain noble gas and mercury, metal halide lamps also contain different kinds of metal halide salt additives. By means of these metal halide constituents, the light emitting properties of the discharge plasma are considerably improved [1,2]. Metal halide salts are highly reactive materials, especially at the extremely high operating temperatures. Because of this, harmful chemical corrosion can take place between them and the structural materials of the lamps. These chemical reactions cause the gradual degradation of the arc tube components, which finally leads to lamp failure. The corrosion of the wall of the discharge tube [3] and the corrosion of tungsten cathodes [4] are widely discussed in the literature. In case of ceramic arc tubes the corrosion of the electrical feed-through is also thoroughly studied: new halide
* Corresponding author. Tel.: +36 30 471 4483; fax: +36 1 463 4357. E-mail address: [email protected] (G. Dobos). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.035
resistant sealing frits and novel feed-through constructions have been developed, to prevent corrosion of the niobium feedthrough wire and the sealing frit [5,6]. However, the reactions in the electrical feed-through structure of metal halide lamps with quartz discharge tube are not discussed. In these lamps molybdenum foils serve as feed-through electrodes. The wall of the arc tube is pinched to the foils producing vacuum-tight bonds. Practical experience shows, that the metal halide additives can diffuse into the electrical feed-through structure, and corrode the molybdenum foil. The accumulation of reaction products on the molybdenum–quartz interface weakens the bond between them, which can lead to leakage and lamp failure. In our study, we have investigated this phenomenon both on real lamps and on model samples.
2. Materials and methods In metal halide lamps different salt mixtures are used to produce different colour temperature light sources. In our study we have compared the effects of three different salt mixtures. The first mixture (A) consisted of sodium and scandium iodide. The second (B) was a mixture of sodium, thallium and indium iodide, also containing traces of scandium and neodymium iodide. The last one (C) was pure indium iodide.
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In the course of experiments with real lamps we have analysed two lamps from each type: one, which was operated only for a couple of hours (15 h in case of the A mixture and 2 h in case of B and C mixtures), and one, which was operated for a much longer time (approximately 1000 h). The arc tubes were removed from the molybdenum foils by selective etching in HF, which can dissolve the quartz material of the arc tube without corroding the molybdenum. After the etching process the surface of the foils has been analysed with XPS. Surface atoms were excited by Mg Ka X-ray irradiation from a VG Microtech XR3E2 X-ray source. A VG Microtech CLAM2 truncated hemispherical analyzer, having an energy resolution of 0.3 eV, was used for electron energy analysis. The background pressure in the analytical chamber was held below 3 10 7 Pa during the experiments. However, the HF did not deteriorate molybdenum it removed not only the quartz, but also the reaction products from its surface. Therefore, the changes in the bonding state of molybdenum could only be detected. The amount of reaction products could not be measured. Because of this, further experiments on model samples were carried out. Our model samples were small capsules made of fuse silica (quartz) tubes. Molybdenum foils and metal halide salt mixtures were added into the capsules, and they were sealed at both ends under argon atmosphere. The salt mixtures and the molybdenum foils (PLANSEE MY-ESS) were identical to the ones used in real lamps. The samples were heat treated at 1000 8C for 15, 300 and 1300 h. At this temperature, the salt additives vaporised, and condensed to the surface of the molybdenum foils when cooled down, producing an overlayer, which hindered surface analytical measurements on the molybdenum surface. This overlayer was so thick, that it could not be removed by in situ ion beam sputtering. Because of this we used ultrasonic cleaning in water before the measurements. This cleaning process effectively removed the loosely bound macroscopic contamination, and made possible to carry out the XPS measurements. During the heat treatment the metal halides react with the molybdenum, producing some kind of molybdenum compound. Besides this, some metal halides (or the components of the metal halides) can diffuse to the surface of the molybdenum, producing a solid state solution. Our early experiments have shown that the cleaning process, which has been used to remove the metal halide overlayer, was effective to remove the reaction products too. Because of this we have measured the molybdenum content of the cleaning water by atomic absorption spectroscopy to establish the quantity of the reaction products. In addition, depth profiles were taken from the molybdenum surface by XPS to study the diffusion of the additives in the molybdenum. (XPS measurements on model samples were carried out in the same equipment and with the same parameters as the analysis of the real lamp feed-throughs.) After taking surface spectra, 3 keV Ar+ ion beam sputtering was applied (sputtering speed on SiO2 30 nm/h), and further spectra were taken. The process was repeated until the components of the metal halide salts have been removed.
3. Results and discussion 3.1. XPS measurements on the electrical feed-throughs of real lamps After the etching process in HF we did not reveal any components of the metal halide additives on the surface of the feed-through – foils. However, we could detect changes in the bonding state of the molybdenum. In case of the lamps with A and B mixture another doublet appeared 3.9–4.1 eV away from the metallic peaks (Fig. 1). These peaks could not be associated to stochiometric molybdenum-oxides. (The energy shift for MoO2 is approximately 1.6–1.7 eV while it is around 4.7– 4.9 eV for MoO3 [7,8].) Because of this, it is a substantiated assumption, that this bonding state is related to some kind of reaction product. This reaction product could not be a iodine, thallium, indium or silicon compound, because we did not find any traces of these elements on the surface. However, the overlapping of the Na 1s XPS peak with the Mo KLL Auger peak and the Sc XPS peaks with the Mo XPS peaks makes it hard to identify a low amount of these elements besides molybdenum. It is well possible, that one (or both) of them was presented on the surface in a very low concentration, which could not be detected. According to the literature data [7,8], the energy shift for Na2MoO4 (4.1–4.3 eV) is approximately equal with the shift, we found. In addition, practical experiences show that corrosion reactions are most intense in case of the sodiumcontaining salt mixtures. This strongly suggests that the reaction product, found on the surface were sodium-molybdate. 3.2. AAS measurements on model samples Table 1 shows the molybdenum contents of cleaning water samples. Obviously in a surface reaction the amount of reaction products is proportional to the surface of the sample. Therefore,
Fig. 1. Mo 3d spectrum of the electric feed-through foil of a lamp with the ‘A’ mixture.
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Table 1 Molybdenum amounts in the washing water Code of salt mixture A 300 h Molybdenum concentration in the washing water (mg/l) Mass of the molybdenum foil (g) Normalised amount of molybdenum (mg/g) a
B a
0.155 0.0321 48.29
1300 h
a
0.075 0.0242 30.99
C
300 h
1300 h
300 h
1300 h
0.019 0.0289 6.57
0.050 0.0289 17.30
0.008 0.0294 2.72
0.008 0.0287 2.79
Duration of heat treatment.
it is necessary to normalise the measured amounts with the surface areas. Since the thicknesses of molybdenum foils were equal, their mass is proportional to their surfaces, which make it possible to carry out the normalisation by means of their masses, which can be measured for more accurately. For the 15 h samples the amount of molybdenum was under the detection limit. In case of the other samples the normalised amount of reaction products was the highest for the ‘A’ mixture, while it is around the detection limit for the ‘C’. Because the ‘C’ additive did not cause any corrosion, difference could not be found with the duration of the heat treatment on these samples. In the case of the ‘B’ mixture the amount or reaction products significantly increased with the duration, as expected. In the case of the ‘A’ mixture the amount of reaction products is about one and a half times higher on the sample which was heat treated for 300 h, relative to the other. Presumably in this case the reaction was so fast, that it has saturated in less then 300 h.
In contrast to the real lamp experiments, on the surface of the model samples we have also found some sodium, but no scandium. This confirms that the 3.9 eV energy shift of the Mo 3d peak is related to Na2MoO4. According to the literature data [8,9] the higher energy shift (4.6 eV) might be related to
3.3. XPS measurements on model samples XPS measurements also show signs of reaction between molybdenum and metal halides. On the Mo 3d spectrum taken from the surface of the sample which was heated with the ‘A’ mixture for 1300 h the molybdenum appeared in different chemical states. Besides the elementary, metallic Mo 3d peak (Mo 3d 5/2 at 227.4 eV), the previously mentioned bonding state has appeared at 3.9 eV higher binding energy (Fig. 2). On the spectrum, taken after 5 min of ion beam sputtering, the peak shifted to 230.6 eV, which is 3.2 eV away from the elementary peak, and its intensity have significantly decreased. Besides this, another doublet appeared approximately 1 eV away from the elementary peak, which might be related to MoO2 (Fig. 3). On the third spectrum taken after repeated 15 min long ion beam sputtering only the elementary molybdenum peak appeared. On the surface of the sample which were heated for 300 h with the ‘A’ mixture the elementary Mo 3d 5/2 peak was found at 227.4 eV and another doublet at 4.6 eV higher bonding energy. After 5 min of ion beam sputtering only the elementary peak remained. Similar results were obtained from the samples, which were heated for 300 and 1300 h with the ‘B’ mixture. On the surface of the samples heated with the ‘C’ mixture only the elementary doublet appeared.
Fig. 2. Mo 3d spectrum from the surface of the molybdenum foil, heated for 1300 h with the ‘A’ mixture.
Fig. 3. Mo 3d spectrum taken after 5 min of ion beam sputtering from the molybdenum foil, heated for 1300 h with the ‘A’ mixture.
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Table 2 Sodium concentrations on the surface of the molybdenum foils
Table 4 Iodine concentrations on the surface of the molybdenum foils
Sputtering time (min)
Sputtering A B C time (min) 15 h 300 h 1300 h 15 h 300 h 1300 h 15 h 300 h 1300 h
0 5 20 45
A
B
15 h
300 h
1300 h
15 h
300 h
1300 h
1.1 1.1 0.9 0
5.3 0.5 0 0
0.9 1.3 0 0
3.8 3.9 2.2
3.4 0.9 0 0
0.3 0 0 0
Na2MoO42H2O, which is well possible, since the cleaning process took place in water. The lower energy shift (3.2 eV) represents lower oxidation state of the molybdenum, presumably another sodium molybdate with higher sodium, or lower oxygen content. Taking into consideration, that on the spectrum, where this bonding state appeared we have also detected MoO2, it is well possible that the ion beam sputtering has shattered the sodium molybdate to molybdenum-oxide and another sodium molybdate with lower oxygen content. On the samples, which were heated for a long time with the sodium containing salt mixtures (A and B), the amount of sodium were smaller, than on those which where heated only for 15 h (Table 2). It is also a significant difference, that the sodium could be removed from these samples by 5–10 min long ion beam sputtering, while in the case of 15 h samples a much longer time was necessary for this. It is very interesting that the amount of reaction products on the surface of the molybdenum compared to the amounts in the washing water show an inversing tendency with the duration of the heat treatment. It well may be, that during a short heat treatment there were no significant reactions between the molybdenum and the sodium iodide. Sodium iodide only adsorbed on the molybdenum and dissolved in the surface region so the ultrasonic cleaning could not remove it. However, during a longer heat treatment, the corrosion reactions have produced sodium molybdates, which were loosely bound to the surface, so the cleaning process could remove them, and only traces remained which could be removed by a short ion beam sputtering. In case of the samples which were heated with the ‘C’ mixture (pure InI) the amount of indium was significantly smaller on the sample which was heated only for 15 h, than on the others (Table 3). Taking into consideration, that the atomic absorption spectroscopy measurements did not show virtually any reaction products in the washing water, we might conclude, that InI did not corrode the molybdenum, only dissolves in it, Table 3 Indium concentrations on the surface of the molybdenum foils Sputtering time (min) 0 5 20 45 115 235
B
C
15 h
300 h
1300 h
15 h
300 h
1300 h
11.9 7.9
7.3 4.3 0.9 0
9.5 4.3 0.6 0
20.3 14.6 12.8 10.5 8.4
25.3 36.1 36.5 36.5 34.6 23.6
17.9 31.3 30.6 25.6 24.7 10.6
1.7
0 5 20 45 115 235
9.3 2.3 1.1 0.7
2.8 0.9 1.1 0.6
1.6 1.2 0.5 0
0.8 0.7 0.9
3.4 0.5 0.1 0
1.9 0.3 0 0
5.8 3.6 3.7 3.4 3.2
19.7 18.8 16.9 14.7 13.3 7.6
15.2 17.2 12.7 8.9 9.6 3.8
and the amount of indium dissolved obviously increases with duration of the heat treatment. On the other hand in the case of the samples heated with the ‘B’ mixture (which contained both NaI and InI) the amount of indium was the highest on the sample which was heated for 15 h only. Presumably during the longer heat treatments the NaI component corroded the molybdenum, transforming the surface region to a loosely bound layer, which cold be washed down easily together with its indium content. The amount of iodine also confirms this theory (Table 4). On the samples heated with the ‘C’ mixture (pure InI) the amount of iodine increases parallel with the duration of the heat treatment (similarly to the amount of indium). However, in the case of the samples heated with the ‘A’ mixture opposite tendency shows up. In case of the samples heated with the ‘B’ mixture (which contained NaI and InI too) on the surface there is an inverse relation relative to the deeper layers in terms of iodine concentration as function of heat treatment duration. The amount of iodine on the surface is higher after the long heat treatments; as opposed to the deeper layers where the highest iodine concentration was found on the 15 h long treated sample. After a short heat treatment some indium iodide diffused to the molybdenum, causing low iodide content on the surface and also in the deeper layers. During a longer heat treatment presumably more indium iodide diffused to the molybdenum, but the surface was also corroded by the sodium iodide, so the effected layer was washed down during the cleaning process. Despite of this, the traces of this layer which remained on the surface still contained a relatively high amount of iodine, but these traces could easily be removed by a short ion beam sputtering. However, ‘A’ mixture contains a significant amount of scandium, XPS measurements did not show any sign of it on the surface of the samples. The overlapping of their peaks always makes it difficult to study scandium besides molybdenum, but even though we are able to at least detect it, if it were be presented. (The scandium 2p peaks lie between the molybdenum 3d 5/2 and 3/2 peaks, so they used to be clearly visible, though the peak fitting might be difficult.) We have also failed to detect thallium on the surfaces of the samples which were heated with the ‘B’ mixture. Based of this, we can conclude, that these components does not cause any reactions with the molybdenum.
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4. Conclusion
References
Based on the XPS and atomic absorption spectroscopy results we can conclude that the harmful corrosion reactions are caused by the sodium iodide content of the salt mixtures. This component can react with the molybdenum, producing some kind of sodium molybdates (Na2MoO4, Na3MoO4 or Na4MoO4), which are loosely bound to the molybdenum surface. This may cause the separation of the molybdenum feed-through foil from the arc tube, which finally leads to leakage and lamp failure. By model experiments it has been also shown that the indium iodide component can dissolve in the molybdenum, but without harmful corrosion reactions. In the case of other metal halides (scandium and thallium iodide) we did not find any traces of such reactions.
[1] J.R. Coaton, A.M. Marsden, Lamps and Lighting, fourth ed., Edward Arnold, 1997. [2] J.F. Waymouth, Electric Discharge Lamps, The M. I. T. Press, 1971. [3] T. Markus, U. Niemann, K. Hilpert, J. Phys. Chem. Sol. 66 (2005) 372–375. [4] L. Cifuentes, G.M. Forsdyke, N.W. O’Brien, Corr. Sci. 33 (1992) 1581– 1592. [5] A.S.G. Geven, M.L.P. Renardus, P.A. Seinen, J.A.J. Stoffels, C. Wiejenberg, H.R. Dielis, Philips Electronics N.V., European Patent 0528428 (1993). [6] J. Fitzgerald, C.A.J. Jacobs, M.H.A. van de Weijer, Philips Corp., US Patent 4475061 (1984). [7] D. Briggs, M.P. Seah, Practical Surface Analysis, Wiley, 1990. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, 1979. [9] S.O. Grim, L.J. Matienzo, Inorg. Chem. 14 (1975) 1014–1018.
High temperature reactions between molybdenum and metal halides ´ . Bo¨ro¨czki a, G. Dobos b,*, V.K. Josepovits b, Gy. Ha´rs b A a b
General Electric Consumer & Industrial, Va´ci Street 77, Budapest H-1340, Hungary Budapest University of Technology and Economics, Department of Atomic Physics, Surface Physics Laboratory, Budafoki Street 8, Budapest H-1111, Hungary
Received 30 August 2005; received in revised form 3 November 2005; accepted 7 November 2005 Available online 7 December 2005
Abstract Good colour rendering properties, high intensity and efficacy are of vital importance for high-end lighting applications. These requirements can be achieved by high intensity discharge lamps doped with different metal halide additives (metal halide lamps). To improve their reliability, it is very important to understand the different failure processes of the lamps. In this paper, the corrosion reactions between different metal halides and the molybdenum electrical feed-through electrode are discussed. The reactions were studied in the feed-through of real lamps and on model samples too. X-ray photoelectron spectroscopy (XPS) was used to establish the chemical states. In case of the model samples we have also used atomic absorption spectroscopy (AAS) to measure the reaction product amounts. Based on the measurement results we were able to determine the most corrosive metal halide components and to understand the mechanism of the reactions. # 2005 Elsevier B.V. All rights reserved. Keywords: Lamp; Corrosion; Metal halide; Molybdenum; XPS; AAS
1. Introduction Metal halide lamps represent one of the basic types of general-purpose high intensity discharge (HID) lamps. In these lamps the light is produced by a high pressure gas discharge, generated between two electrodes of refractory metal material. In contrast to the mercury vapour discharge lamps that contain noble gas and mercury, metal halide lamps also contain different kinds of metal halide salt additives. By means of these metal halide constituents, the light emitting properties of the discharge plasma are considerably improved [1,2]. Metal halide salts are highly reactive materials, especially at the extremely high operating temperatures. Because of this, harmful chemical corrosion can take place between them and the structural materials of the lamps. These chemical reactions cause the gradual degradation of the arc tube components, which finally leads to lamp failure. The corrosion of the wall of the discharge tube [3] and the corrosion of tungsten cathodes [4] are widely discussed in the literature. In case of ceramic arc tubes the corrosion of the electrical feed-through is also thoroughly studied: new halide
* Corresponding author. Tel.: +36 30 471 4483; fax: +36 1 463 4357. E-mail address: [email protected] (G. Dobos). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.035
resistant sealing frits and novel feed-through constructions have been developed, to prevent corrosion of the niobium feedthrough wire and the sealing frit [5,6]. However, the reactions in the electrical feed-through structure of metal halide lamps with quartz discharge tube are not discussed. In these lamps molybdenum foils serve as feed-through electrodes. The wall of the arc tube is pinched to the foils producing vacuum-tight bonds. Practical experience shows, that the metal halide additives can diffuse into the electrical feed-through structure, and corrode the molybdenum foil. The accumulation of reaction products on the molybdenum–quartz interface weakens the bond between them, which can lead to leakage and lamp failure. In our study, we have investigated this phenomenon both on real lamps and on model samples.
2. Materials and methods In metal halide lamps different salt mixtures are used to produce different colour temperature light sources. In our study we have compared the effects of three different salt mixtures. The first mixture (A) consisted of sodium and scandium iodide. The second (B) was a mixture of sodium, thallium and indium iodide, also containing traces of scandium and neodymium iodide. The last one (C) was pure indium iodide.
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In the course of experiments with real lamps we have analysed two lamps from each type: one, which was operated only for a couple of hours (15 h in case of the A mixture and 2 h in case of B and C mixtures), and one, which was operated for a much longer time (approximately 1000 h). The arc tubes were removed from the molybdenum foils by selective etching in HF, which can dissolve the quartz material of the arc tube without corroding the molybdenum. After the etching process the surface of the foils has been analysed with XPS. Surface atoms were excited by Mg Ka X-ray irradiation from a VG Microtech XR3E2 X-ray source. A VG Microtech CLAM2 truncated hemispherical analyzer, having an energy resolution of 0.3 eV, was used for electron energy analysis. The background pressure in the analytical chamber was held below 3 10 7 Pa during the experiments. However, the HF did not deteriorate molybdenum it removed not only the quartz, but also the reaction products from its surface. Therefore, the changes in the bonding state of molybdenum could only be detected. The amount of reaction products could not be measured. Because of this, further experiments on model samples were carried out. Our model samples were small capsules made of fuse silica (quartz) tubes. Molybdenum foils and metal halide salt mixtures were added into the capsules, and they were sealed at both ends under argon atmosphere. The salt mixtures and the molybdenum foils (PLANSEE MY-ESS) were identical to the ones used in real lamps. The samples were heat treated at 1000 8C for 15, 300 and 1300 h. At this temperature, the salt additives vaporised, and condensed to the surface of the molybdenum foils when cooled down, producing an overlayer, which hindered surface analytical measurements on the molybdenum surface. This overlayer was so thick, that it could not be removed by in situ ion beam sputtering. Because of this we used ultrasonic cleaning in water before the measurements. This cleaning process effectively removed the loosely bound macroscopic contamination, and made possible to carry out the XPS measurements. During the heat treatment the metal halides react with the molybdenum, producing some kind of molybdenum compound. Besides this, some metal halides (or the components of the metal halides) can diffuse to the surface of the molybdenum, producing a solid state solution. Our early experiments have shown that the cleaning process, which has been used to remove the metal halide overlayer, was effective to remove the reaction products too. Because of this we have measured the molybdenum content of the cleaning water by atomic absorption spectroscopy to establish the quantity of the reaction products. In addition, depth profiles were taken from the molybdenum surface by XPS to study the diffusion of the additives in the molybdenum. (XPS measurements on model samples were carried out in the same equipment and with the same parameters as the analysis of the real lamp feed-throughs.) After taking surface spectra, 3 keV Ar+ ion beam sputtering was applied (sputtering speed on SiO2 30 nm/h), and further spectra were taken. The process was repeated until the components of the metal halide salts have been removed.
3. Results and discussion 3.1. XPS measurements on the electrical feed-throughs of real lamps After the etching process in HF we did not reveal any components of the metal halide additives on the surface of the feed-through – foils. However, we could detect changes in the bonding state of the molybdenum. In case of the lamps with A and B mixture another doublet appeared 3.9–4.1 eV away from the metallic peaks (Fig. 1). These peaks could not be associated to stochiometric molybdenum-oxides. (The energy shift for MoO2 is approximately 1.6–1.7 eV while it is around 4.7– 4.9 eV for MoO3 [7,8].) Because of this, it is a substantiated assumption, that this bonding state is related to some kind of reaction product. This reaction product could not be a iodine, thallium, indium or silicon compound, because we did not find any traces of these elements on the surface. However, the overlapping of the Na 1s XPS peak with the Mo KLL Auger peak and the Sc XPS peaks with the Mo XPS peaks makes it hard to identify a low amount of these elements besides molybdenum. It is well possible, that one (or both) of them was presented on the surface in a very low concentration, which could not be detected. According to the literature data [7,8], the energy shift for Na2MoO4 (4.1–4.3 eV) is approximately equal with the shift, we found. In addition, practical experiences show that corrosion reactions are most intense in case of the sodiumcontaining salt mixtures. This strongly suggests that the reaction product, found on the surface were sodium-molybdate. 3.2. AAS measurements on model samples Table 1 shows the molybdenum contents of cleaning water samples. Obviously in a surface reaction the amount of reaction products is proportional to the surface of the sample. Therefore,
Fig. 1. Mo 3d spectrum of the electric feed-through foil of a lamp with the ‘A’ mixture.
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Table 1 Molybdenum amounts in the washing water Code of salt mixture A 300 h Molybdenum concentration in the washing water (mg/l) Mass of the molybdenum foil (g) Normalised amount of molybdenum (mg/g) a
B a
0.155 0.0321 48.29
1300 h
a
0.075 0.0242 30.99
C
300 h
1300 h
300 h
1300 h
0.019 0.0289 6.57
0.050 0.0289 17.30
0.008 0.0294 2.72
0.008 0.0287 2.79
Duration of heat treatment.
it is necessary to normalise the measured amounts with the surface areas. Since the thicknesses of molybdenum foils were equal, their mass is proportional to their surfaces, which make it possible to carry out the normalisation by means of their masses, which can be measured for more accurately. For the 15 h samples the amount of molybdenum was under the detection limit. In case of the other samples the normalised amount of reaction products was the highest for the ‘A’ mixture, while it is around the detection limit for the ‘C’. Because the ‘C’ additive did not cause any corrosion, difference could not be found with the duration of the heat treatment on these samples. In the case of the ‘B’ mixture the amount or reaction products significantly increased with the duration, as expected. In the case of the ‘A’ mixture the amount of reaction products is about one and a half times higher on the sample which was heat treated for 300 h, relative to the other. Presumably in this case the reaction was so fast, that it has saturated in less then 300 h.
In contrast to the real lamp experiments, on the surface of the model samples we have also found some sodium, but no scandium. This confirms that the 3.9 eV energy shift of the Mo 3d peak is related to Na2MoO4. According to the literature data [8,9] the higher energy shift (4.6 eV) might be related to
3.3. XPS measurements on model samples XPS measurements also show signs of reaction between molybdenum and metal halides. On the Mo 3d spectrum taken from the surface of the sample which was heated with the ‘A’ mixture for 1300 h the molybdenum appeared in different chemical states. Besides the elementary, metallic Mo 3d peak (Mo 3d 5/2 at 227.4 eV), the previously mentioned bonding state has appeared at 3.9 eV higher binding energy (Fig. 2). On the spectrum, taken after 5 min of ion beam sputtering, the peak shifted to 230.6 eV, which is 3.2 eV away from the elementary peak, and its intensity have significantly decreased. Besides this, another doublet appeared approximately 1 eV away from the elementary peak, which might be related to MoO2 (Fig. 3). On the third spectrum taken after repeated 15 min long ion beam sputtering only the elementary molybdenum peak appeared. On the surface of the sample which were heated for 300 h with the ‘A’ mixture the elementary Mo 3d 5/2 peak was found at 227.4 eV and another doublet at 4.6 eV higher bonding energy. After 5 min of ion beam sputtering only the elementary peak remained. Similar results were obtained from the samples, which were heated for 300 and 1300 h with the ‘B’ mixture. On the surface of the samples heated with the ‘C’ mixture only the elementary doublet appeared.
Fig. 2. Mo 3d spectrum from the surface of the molybdenum foil, heated for 1300 h with the ‘A’ mixture.
Fig. 3. Mo 3d spectrum taken after 5 min of ion beam sputtering from the molybdenum foil, heated for 1300 h with the ‘A’ mixture.
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Table 2 Sodium concentrations on the surface of the molybdenum foils
Table 4 Iodine concentrations on the surface of the molybdenum foils
Sputtering time (min)
Sputtering A B C time (min) 15 h 300 h 1300 h 15 h 300 h 1300 h 15 h 300 h 1300 h
0 5 20 45
A
B
15 h
300 h
1300 h
15 h
300 h
1300 h
1.1 1.1 0.9 0
5.3 0.5 0 0
0.9 1.3 0 0
3.8 3.9 2.2
3.4 0.9 0 0
0.3 0 0 0
Na2MoO42H2O, which is well possible, since the cleaning process took place in water. The lower energy shift (3.2 eV) represents lower oxidation state of the molybdenum, presumably another sodium molybdate with higher sodium, or lower oxygen content. Taking into consideration, that on the spectrum, where this bonding state appeared we have also detected MoO2, it is well possible that the ion beam sputtering has shattered the sodium molybdate to molybdenum-oxide and another sodium molybdate with lower oxygen content. On the samples, which were heated for a long time with the sodium containing salt mixtures (A and B), the amount of sodium were smaller, than on those which where heated only for 15 h (Table 2). It is also a significant difference, that the sodium could be removed from these samples by 5–10 min long ion beam sputtering, while in the case of 15 h samples a much longer time was necessary for this. It is very interesting that the amount of reaction products on the surface of the molybdenum compared to the amounts in the washing water show an inversing tendency with the duration of the heat treatment. It well may be, that during a short heat treatment there were no significant reactions between the molybdenum and the sodium iodide. Sodium iodide only adsorbed on the molybdenum and dissolved in the surface region so the ultrasonic cleaning could not remove it. However, during a longer heat treatment, the corrosion reactions have produced sodium molybdates, which were loosely bound to the surface, so the cleaning process could remove them, and only traces remained which could be removed by a short ion beam sputtering. In case of the samples which were heated with the ‘C’ mixture (pure InI) the amount of indium was significantly smaller on the sample which was heated only for 15 h, than on the others (Table 3). Taking into consideration, that the atomic absorption spectroscopy measurements did not show virtually any reaction products in the washing water, we might conclude, that InI did not corrode the molybdenum, only dissolves in it, Table 3 Indium concentrations on the surface of the molybdenum foils Sputtering time (min) 0 5 20 45 115 235
B
C
15 h
300 h
1300 h
15 h
300 h
1300 h
11.9 7.9
7.3 4.3 0.9 0
9.5 4.3 0.6 0
20.3 14.6 12.8 10.5 8.4
25.3 36.1 36.5 36.5 34.6 23.6
17.9 31.3 30.6 25.6 24.7 10.6
1.7
0 5 20 45 115 235
9.3 2.3 1.1 0.7
2.8 0.9 1.1 0.6
1.6 1.2 0.5 0
0.8 0.7 0.9
3.4 0.5 0.1 0
1.9 0.3 0 0
5.8 3.6 3.7 3.4 3.2
19.7 18.8 16.9 14.7 13.3 7.6
15.2 17.2 12.7 8.9 9.6 3.8
and the amount of indium dissolved obviously increases with duration of the heat treatment. On the other hand in the case of the samples heated with the ‘B’ mixture (which contained both NaI and InI) the amount of indium was the highest on the sample which was heated for 15 h only. Presumably during the longer heat treatments the NaI component corroded the molybdenum, transforming the surface region to a loosely bound layer, which cold be washed down easily together with its indium content. The amount of iodine also confirms this theory (Table 4). On the samples heated with the ‘C’ mixture (pure InI) the amount of iodine increases parallel with the duration of the heat treatment (similarly to the amount of indium). However, in the case of the samples heated with the ‘A’ mixture opposite tendency shows up. In case of the samples heated with the ‘B’ mixture (which contained NaI and InI too) on the surface there is an inverse relation relative to the deeper layers in terms of iodine concentration as function of heat treatment duration. The amount of iodine on the surface is higher after the long heat treatments; as opposed to the deeper layers where the highest iodine concentration was found on the 15 h long treated sample. After a short heat treatment some indium iodide diffused to the molybdenum, causing low iodide content on the surface and also in the deeper layers. During a longer heat treatment presumably more indium iodide diffused to the molybdenum, but the surface was also corroded by the sodium iodide, so the effected layer was washed down during the cleaning process. Despite of this, the traces of this layer which remained on the surface still contained a relatively high amount of iodine, but these traces could easily be removed by a short ion beam sputtering. However, ‘A’ mixture contains a significant amount of scandium, XPS measurements did not show any sign of it on the surface of the samples. The overlapping of their peaks always makes it difficult to study scandium besides molybdenum, but even though we are able to at least detect it, if it were be presented. (The scandium 2p peaks lie between the molybdenum 3d 5/2 and 3/2 peaks, so they used to be clearly visible, though the peak fitting might be difficult.) We have also failed to detect thallium on the surfaces of the samples which were heated with the ‘B’ mixture. Based of this, we can conclude, that these components does not cause any reactions with the molybdenum.
A´. Bo¨ro¨czki et al. / Applied Surface Science 252 (2006) 8309–8313
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4. Conclusion
References
Based on the XPS and atomic absorption spectroscopy results we can conclude that the harmful corrosion reactions are caused by the sodium iodide content of the salt mixtures. This component can react with the molybdenum, producing some kind of sodium molybdates (Na2MoO4, Na3MoO4 or Na4MoO4), which are loosely bound to the molybdenum surface. This may cause the separation of the molybdenum feed-through foil from the arc tube, which finally leads to leakage and lamp failure. By model experiments it has been also shown that the indium iodide component can dissolve in the molybdenum, but without harmful corrosion reactions. In the case of other metal halides (scandium and thallium iodide) we did not find any traces of such reactions.
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