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A fiber-optic evanescent-wave hydrogen gas sensor using palladium-supported tungsten oxide
Sensors and Actuators B 66 Ž2000. 142–145 www.elsevier.nlrlocatersensorb
A fiber-optic evanescent-wave hydrogen gas sensor using palladium-supported tungsten oxide S. Sekimoto a,) , H. Nakagawa a , S. Okazaki a , K. Fukuda a , S. Asakura a , T Shigemori b, S. Takahashi b a
Faculty of Engineering, Yokohama National UniÕersity, Yokohama 240-8501, Japan b New Cosmos Electric Co. Ltd., Osaka 532-0036, Japan
Received 30 July 1998; received in revised form 22 February 1999; accepted 14 January 2000
Abstract A new optical-fiber hydrogen sensor has been developed. The sensor utilizes the absorption change of the evanescent field in the clad region. The platinum- or palladium-supported tungsten oxide was used as sensing media. Two different approaches were adopted for the fiber fabrication. One used PdrWO 3 containing silicone resin as the clad. The other utilized the sol–gel process to form a thin PdrWO 3 clad. In the presence of hydrogen, strong evanescent-wave absorption was observed as a result of the formation of tungsten bronze. The sensor sensitively and immediately responded to hydrogen. It was found that the characteristics of the sensor were easily controlled by the amount of catalysts. The sensor developed in this study has potential to measure the spatial distribution along the fiber line, unlike the traditional hydrogen sensors that measure the concentration of a certain spatial point. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Hydrogen sensor; Fiber optics; Tungsten oxide; Evanescent wave
1. Introduction Fossil fuels such as the petroleum or the natural gas, which we have used as the main energy source, are finite. Atmospheric pollutants, like CO 2 , which is one of the major sources for the global warming, are generated when they were burnt. In recent years, much attention has been paid for the utilization of hydrogen as a clean energy source. Hydrogen turns into water upon burning and the raw material may be produced from water. On the other hand, hydrogen has the characteristics of colorlessness, odorlessness, inflammableness and the large flame propagation velocity. The range of combustion for hydrogen is 4.0–74.2%, much wider than that for methane of 5.0– 15.0%. It is also easy to leak out because of the smallest molecule. The technological development for the safe and economical transportation and storage to handle the vast amount of hydrogen is required before hydrogen becomes one of the major alternative energy source. A hydrogen leakage sensor that monitors rather a wide area sensitively )
Corresponding author. Fax: q81-45-339-3981. E-mail address: [email protected] ŽS. Sekimoto.
would be needed. A three-dimensional measurement is most desirable to detect a leakage of hydrogen quickly and certainly. However, it is difficult to realize such a measurement using current semiconductor gas sensors or the other spot type sensors. An optical-fiber method utilizing the evanescent-wave absorption has potential to monitor a leakage over a wide area. For instance, the sensor of this type can be used by winding it around a huge storage tank or a pipeline. The sensor utilizes the absorption change of the evanescent field in the clad region near the surface of the core. An optical fiber is used so that the location of the leakage point can be measured by Optical Time Domain Reflectometry ŽOTDR. w1x, and the sensor of this type is quite immune to an electromagnetic noise. In this study, a fiber-optic hydrogen gas sensor, which has the platinum- or palladium-supported tungsten oxide thin film has been developed, and the characteristics are discussed.
2. Experimental A 200 Žcore.r230 Žclad. mm quarts-corerplastic-cladding fiber was used. The sensing portion of the plastic
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 3 3 0 - 0
S. Sekimoto et al.r Sensors and Actuators B 66 (2000) 142–145
143
cladding was removed by mechanical rubbing with the help of a solvent Žethanolamine.. Tungsten oxides supporting palladium or platinum were used as hydrogen-sensitive media. They are greenish yellow substances, but their colors changed to blue when hydrogen reduces them to tungsten bronze. Nishizawa et al. w2x developed an optical waveguide hydrogen sensor with this material and evanescent field. But their sensor is of spot-type due to unavailability of long waveguide. The reaction may be described as follow:
m H WO
WO 3 q x Hqq xey
x
3
Ž 0 - x - 1. .
Ž 1.
Pd and Pt are catalysts for dissociation of hydrogen. The above reaction would proceed even at room temperature with the coexistence of Pd or Pt. Two different coatings of the WO 3 had been developed. The first one was that the transparent silicone resin in which the palladium-supported tungsten oxide powder Žthe grain diameter is about 3 mm. was dispersed. The palladium-supported tungsten oxide was prepared as follows. Five grams of the tungsten oxide was dissolved into 50 ml of 2.25 = 10y4 M PdCl 2 and impregnated for 3 h, and then heated at 3008C for 3 h in the air. The PdrWO 3 dispersed silicone resin was applied to the core to form the clad. The cross-section of the sensor device is shown in Fig. 1a. The second one utilizes the sol–gel process. Tungsten oxide sols are formed from the sodium tungstate aqueous solutions of various concentrations containing hydrogen tetrachloropalladate ŽII. acid Žor chloroplatinic acid. w3x. The solutions could be acidified when it passed through a proton exchange resin. The solution obtained after the protonation was yellowish transparent moiety. The dipcoating method was used to obtain the films. The fiber whose clad had been removed was dipped in the sol and pulled at almost constant rate to achieve a uniform film thickness. Then the fiber was placed in a furnace and calcined at 5008C for 3 h in atmosphere. The WO 3 films
Fig. 1. Schematic diagram of the sensor devices, utilizing the silicone resin Ža. and prepared PtrWO 3 by the sol–gel process Žb.. The thickness of the films are exaggerated, not proportional to the actual dimensions.
Fig. 2. Schematic diagram of the experiments using fiber-optic transmission probe.
were cubic or monoclinic crystals when calcined at 5008C w4x. The schematic diagram of the cross-section is shown in Fig. 1b. A spectrophotometer ŽCary 5E., with the optional fiber-optic probe adopters, which can extract and re-introduce the monochromatic light beam from the instrument, was used for the spectroscopy measurement. The configuration for the transmittance measurement is shown schematically in Fig. 2. The fiber-optic hydrogen sensor was set between the probes, and absorption spectra were measured. The reflectance measurement was also performed with the utilization of the different fiber-optic probe adopters.
3. Results and discussion 3.1. Properties of the hydrogen-sensitiÕe films Reflectance spectra of the PdrWO 3 dispersed in the silicone resin are shown in Fig. 3. The ordinate, which is expressed in the unit of absorbance against the reference ray set in the spectrophotometer, indicates intensity of the reflected light by the thin film. Although the spectra were
Fig. 3. Reflectance spectra of PdrWO 3 in the silicone resin. The reflectance is converted into the absorbance to compare with other figures.
144
S. Sekimoto et al.r Sensors and Actuators B 66 (2000) 142–145
somewhat noisy because of the poor optical coupling between fibers and the instrument, the absorption in the longer wavelength region was increased when the sample was exposed to hydrogen. The experimental results agreed with the visual observation that the color of the hydrogensensitive material changes to blue in the presence of hydrogen gas. The small peak around 730 nm is due to the absorption of the material of fiber-optic probes or the apparatus inside of the spectrometer, and is not characteristics of hydrogen sensor. The PtrWO 3 thin film prepared by the sol–gel process shows absorption spectra similar to those of the PdrWO 3 in the silicone resin. It was revealed that the catalyst-supported tungsten trioxide retained the characteristics to be reduced as in Eq. Ž1. even in the silicone resin or in the thin film prepared by the sol–gel process. 3.2. The sensor characteristics The transmission spectra of the sensor whose core was coated with the PdrWO 3-dispersed silicone resin is shown in Fig. 4. The decrease in the absorbance was observed in the wavelength range above 500 nm, upon the fiber exposure to hydrogen. However, the decrease in the absorbance is in contradiction with the experimental result shown in Fig. 3, where the color of the hydrogen-sensitive material changes to blue on visual observation. The phenomena was tentatively attributed to the change of dielectric constant in WO 3 and H xWO 3 , since the certain structures of crystalline WO 3 have been reported to have very high dielectric constants w5x. When hydrogen reduced the crystalline WO 3 to tungsten bronze, the dielectric constant became small. The reflective indices of the core and the clad greatly influence the transmission characteristics of a fiber w6x. In the above experimental result, the change of the refractive index might seriously affect the transmission light in fiber optics. The response time of the sensor is slow, and about 1 h was required for the 90% absorbance change upon hydro-
Fig. 4. The transmission spectra of the sensor using PdrWO 3 dispersed in silicone resin.
Fig. 5. Typical response of the sensor at H 2 concentration 100%.
gen exposure. When the sensor was exposed to air again, more than 10 h were required to return to the absorbance of the initial value completely. The sensor detects the variation of the evanescent field at the boundary of the core and the clad. The diffusion speed of hydrogen within silicone-resin film to the evanescent field could be the reason for the slow response. The response of the sensor that the tungsten oxide film was prepared by the sol–gel process is shown in Fig. 5. The absorbance increased immediately after the exposure of hydrogen gas and reached the steady state. It took about 7 min for the 90% absorbance change. The response time was greatly improved by preparing the thin hydrogen-sensitive film with the sol–gel process. The transmission spectra of the sensor, as shown in Fig. 6, was different from the one with the silicone resin-dispersed PdrWO 3 ŽFig. 4.. The increase in the absorbance was observed in the wavelength range above 530 nm, and the decrease was observed below that wavelength. The observation is in accordance with the fact that the film changed to blue as a result of the formation of the tungsten bronze. The film deposited by one dip-coating is approximately 1 mm. It is assumed that the tungsten oxide film in this order cannot form the high dielectric domain to exhibit
Fig. 6. The transmission spectra of the sensor prepared by the sol–gel process.
S. Sekimoto et al.r Sensors and Actuators B 66 (2000) 142–145
145
very slow response. The response time was significantly improved by the sol–gel process. The sensor has potential to measure the distribution along the fiber line, unlike the traditional hydrogen sensors that measure at a certain spatial point. The fiber sensor of the present research formed by the sol–gel process is very promising for wide area monitoring.
References
Fig. 7. Calibration curve for the fiber-optic hydrogen sensor.
ferroelectricity, and the characteristic of the color change appeared strongly in the transmission spectra without interfering effect of dielectric constant change. To solidify the postulate, the thicker films were deposited by the sol–gel process using thick solution of 1.0 M Na 2WO4 . The transmission spectra became similar to Fig. 4, i.e. decrease in absorbance in the long wavelength range upon exposure to hydrogen. It may be considered that the tungsten oxide film has the enough thickness to form the high dielectric film, and the influence of refractive index strongly affected the transmission spectra. Concentration dependence of the sensor is shown in Fig. 7. The sensor showed high sensitivity in the low concentration range, but the response tends to saturate above 40%. In these experiments, the length of the sensing part was 7.5 cm. It is theoretically expected to increase the sensitivity by making it longer. The effect of the amount of the catalysts on the response to hydrogen gas on 10- and 20-cm sensing length fibers was examined. It was clear that the increase in the Pt content makes not only the sensitivity high, but also the response time fast. Also, the increase in sensitivity with the increase in sensing length was apparent. When the sensing length varied from 10 to 20 cm at the same amount of catalysts, the change in absorbance was approximately doubled. Sensitivity in the low concentration range and the response time depend on the thickness and quality of the hydrogen-sensitive film.
4. Conclusions The sensors prepared by using the silicone resin and the sol–gel process had been developed. The platinum- or palladium-supported tungsten oxides were used as the hydrogen sensitive material. The catalysts-doped tungsten oxides were reduced to blue tungsten bronze upon exposure to hydrogen even at room temperature. However, the sensor using the silicone resin-dispersed PdrWO 3 had
w1x Y. Suematsu, K. Iga, Hikarifaiba Tuushin Nyuumon, Ohmsha, pp. 227, 228, in Japanese. w2x K. Nishizawa, E. Sudo, M. Maeda, T. Yamasaki, Optical waveguide type hydrogen sensor using the change of absorbance of Pd–WO 3 layers, OQE85-63, in Japanese. w3x P. Judeinstein, J. Livage, Electrochromic properties of sol–gel derived WO 3 coatings, SPIE 1328 Ž1990. Sol–Gel Optics. w4x T. Nishide, F. Mizukami, Control of refractive index of sol–gel tungsten oxide films, J. Sol–Gel Sci. Technol. 6 Ž1996. 263–267. w5x S. Tanisaki, J. Phys. Soc. Jpn. 15 Ž1960. 573. w6x P.H. Paul, G. Kycakoff, Appl. Phys. Lett. 51 Ž1987. 12.
Biographies Shinjiro Sekimoto received his B. Eng. degree from Yokohama National University in 1998. He joined the master course of Yokohama National University in 1998. Hidemoto Nakagawa obtained his M. Appl. Sci. and PhD degrees from the University of Toronto in 1974 and 1978, respectively. His research centers on various sensors and their applications since he joined Yokohama National University in 1993. He is also interested in electrochemistry and micromachining. Shinji Okazaki received his B. Eng and M. Eng. degrees from Yokohama National University in 1991 and in 1993. He joined Yokohama National University as a research associate in 1997. His major fields are electrochemistry and sensor engineering. Kenzo Fukuda received his MS and PhD degrees from the University of Tokyo in 1967 and 1972, respectively. He is a project manager of the WE-NET Center. His fields of interest are energy technologies, sensors and catalyses. Shukuji Asakura received his M. Eng. and PhD degrees from the University of Tokyo in 1965 and 1968, respectively. In 1972, he joined Yokohama National University, and became a professor in 1988. His fields of interest are safety engineering, corrosion science and chemical sensors. Tesshi Shigemori received his BS and MS degrees from Hokkaido University in 1966 and 1968, respectively. Then he joined New Cosmos Electric and was promoted to a senior director. His fields of interest are gas sensors and their applications. Sachio Takahashi graduated from Osaka Prefecture University in 1959. He joined the Osaka National Research Institute in the field of battery engineering and obtained PhD In 1996, he joined New Cosmos Electric. He is supervising the research division of the company as a managing director.
A fiber-optic evanescent-wave hydrogen gas sensor using palladium-supported tungsten oxide S. Sekimoto a,) , H. Nakagawa a , S. Okazaki a , K. Fukuda a , S. Asakura a , T Shigemori b, S. Takahashi b a
Faculty of Engineering, Yokohama National UniÕersity, Yokohama 240-8501, Japan b New Cosmos Electric Co. Ltd., Osaka 532-0036, Japan
Received 30 July 1998; received in revised form 22 February 1999; accepted 14 January 2000
Abstract A new optical-fiber hydrogen sensor has been developed. The sensor utilizes the absorption change of the evanescent field in the clad region. The platinum- or palladium-supported tungsten oxide was used as sensing media. Two different approaches were adopted for the fiber fabrication. One used PdrWO 3 containing silicone resin as the clad. The other utilized the sol–gel process to form a thin PdrWO 3 clad. In the presence of hydrogen, strong evanescent-wave absorption was observed as a result of the formation of tungsten bronze. The sensor sensitively and immediately responded to hydrogen. It was found that the characteristics of the sensor were easily controlled by the amount of catalysts. The sensor developed in this study has potential to measure the spatial distribution along the fiber line, unlike the traditional hydrogen sensors that measure the concentration of a certain spatial point. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Hydrogen sensor; Fiber optics; Tungsten oxide; Evanescent wave
1. Introduction Fossil fuels such as the petroleum or the natural gas, which we have used as the main energy source, are finite. Atmospheric pollutants, like CO 2 , which is one of the major sources for the global warming, are generated when they were burnt. In recent years, much attention has been paid for the utilization of hydrogen as a clean energy source. Hydrogen turns into water upon burning and the raw material may be produced from water. On the other hand, hydrogen has the characteristics of colorlessness, odorlessness, inflammableness and the large flame propagation velocity. The range of combustion for hydrogen is 4.0–74.2%, much wider than that for methane of 5.0– 15.0%. It is also easy to leak out because of the smallest molecule. The technological development for the safe and economical transportation and storage to handle the vast amount of hydrogen is required before hydrogen becomes one of the major alternative energy source. A hydrogen leakage sensor that monitors rather a wide area sensitively )
Corresponding author. Fax: q81-45-339-3981. E-mail address: [email protected] ŽS. Sekimoto.
would be needed. A three-dimensional measurement is most desirable to detect a leakage of hydrogen quickly and certainly. However, it is difficult to realize such a measurement using current semiconductor gas sensors or the other spot type sensors. An optical-fiber method utilizing the evanescent-wave absorption has potential to monitor a leakage over a wide area. For instance, the sensor of this type can be used by winding it around a huge storage tank or a pipeline. The sensor utilizes the absorption change of the evanescent field in the clad region near the surface of the core. An optical fiber is used so that the location of the leakage point can be measured by Optical Time Domain Reflectometry ŽOTDR. w1x, and the sensor of this type is quite immune to an electromagnetic noise. In this study, a fiber-optic hydrogen gas sensor, which has the platinum- or palladium-supported tungsten oxide thin film has been developed, and the characteristics are discussed.
2. Experimental A 200 Žcore.r230 Žclad. mm quarts-corerplastic-cladding fiber was used. The sensing portion of the plastic
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 3 3 0 - 0
S. Sekimoto et al.r Sensors and Actuators B 66 (2000) 142–145
143
cladding was removed by mechanical rubbing with the help of a solvent Žethanolamine.. Tungsten oxides supporting palladium or platinum were used as hydrogen-sensitive media. They are greenish yellow substances, but their colors changed to blue when hydrogen reduces them to tungsten bronze. Nishizawa et al. w2x developed an optical waveguide hydrogen sensor with this material and evanescent field. But their sensor is of spot-type due to unavailability of long waveguide. The reaction may be described as follow:
m H WO
WO 3 q x Hqq xey
x
3
Ž 0 - x - 1. .
Ž 1.
Pd and Pt are catalysts for dissociation of hydrogen. The above reaction would proceed even at room temperature with the coexistence of Pd or Pt. Two different coatings of the WO 3 had been developed. The first one was that the transparent silicone resin in which the palladium-supported tungsten oxide powder Žthe grain diameter is about 3 mm. was dispersed. The palladium-supported tungsten oxide was prepared as follows. Five grams of the tungsten oxide was dissolved into 50 ml of 2.25 = 10y4 M PdCl 2 and impregnated for 3 h, and then heated at 3008C for 3 h in the air. The PdrWO 3 dispersed silicone resin was applied to the core to form the clad. The cross-section of the sensor device is shown in Fig. 1a. The second one utilizes the sol–gel process. Tungsten oxide sols are formed from the sodium tungstate aqueous solutions of various concentrations containing hydrogen tetrachloropalladate ŽII. acid Žor chloroplatinic acid. w3x. The solutions could be acidified when it passed through a proton exchange resin. The solution obtained after the protonation was yellowish transparent moiety. The dipcoating method was used to obtain the films. The fiber whose clad had been removed was dipped in the sol and pulled at almost constant rate to achieve a uniform film thickness. Then the fiber was placed in a furnace and calcined at 5008C for 3 h in atmosphere. The WO 3 films
Fig. 1. Schematic diagram of the sensor devices, utilizing the silicone resin Ža. and prepared PtrWO 3 by the sol–gel process Žb.. The thickness of the films are exaggerated, not proportional to the actual dimensions.
Fig. 2. Schematic diagram of the experiments using fiber-optic transmission probe.
were cubic or monoclinic crystals when calcined at 5008C w4x. The schematic diagram of the cross-section is shown in Fig. 1b. A spectrophotometer ŽCary 5E., with the optional fiber-optic probe adopters, which can extract and re-introduce the monochromatic light beam from the instrument, was used for the spectroscopy measurement. The configuration for the transmittance measurement is shown schematically in Fig. 2. The fiber-optic hydrogen sensor was set between the probes, and absorption spectra were measured. The reflectance measurement was also performed with the utilization of the different fiber-optic probe adopters.
3. Results and discussion 3.1. Properties of the hydrogen-sensitiÕe films Reflectance spectra of the PdrWO 3 dispersed in the silicone resin are shown in Fig. 3. The ordinate, which is expressed in the unit of absorbance against the reference ray set in the spectrophotometer, indicates intensity of the reflected light by the thin film. Although the spectra were
Fig. 3. Reflectance spectra of PdrWO 3 in the silicone resin. The reflectance is converted into the absorbance to compare with other figures.
144
S. Sekimoto et al.r Sensors and Actuators B 66 (2000) 142–145
somewhat noisy because of the poor optical coupling between fibers and the instrument, the absorption in the longer wavelength region was increased when the sample was exposed to hydrogen. The experimental results agreed with the visual observation that the color of the hydrogensensitive material changes to blue in the presence of hydrogen gas. The small peak around 730 nm is due to the absorption of the material of fiber-optic probes or the apparatus inside of the spectrometer, and is not characteristics of hydrogen sensor. The PtrWO 3 thin film prepared by the sol–gel process shows absorption spectra similar to those of the PdrWO 3 in the silicone resin. It was revealed that the catalyst-supported tungsten trioxide retained the characteristics to be reduced as in Eq. Ž1. even in the silicone resin or in the thin film prepared by the sol–gel process. 3.2. The sensor characteristics The transmission spectra of the sensor whose core was coated with the PdrWO 3-dispersed silicone resin is shown in Fig. 4. The decrease in the absorbance was observed in the wavelength range above 500 nm, upon the fiber exposure to hydrogen. However, the decrease in the absorbance is in contradiction with the experimental result shown in Fig. 3, where the color of the hydrogen-sensitive material changes to blue on visual observation. The phenomena was tentatively attributed to the change of dielectric constant in WO 3 and H xWO 3 , since the certain structures of crystalline WO 3 have been reported to have very high dielectric constants w5x. When hydrogen reduced the crystalline WO 3 to tungsten bronze, the dielectric constant became small. The reflective indices of the core and the clad greatly influence the transmission characteristics of a fiber w6x. In the above experimental result, the change of the refractive index might seriously affect the transmission light in fiber optics. The response time of the sensor is slow, and about 1 h was required for the 90% absorbance change upon hydro-
Fig. 4. The transmission spectra of the sensor using PdrWO 3 dispersed in silicone resin.
Fig. 5. Typical response of the sensor at H 2 concentration 100%.
gen exposure. When the sensor was exposed to air again, more than 10 h were required to return to the absorbance of the initial value completely. The sensor detects the variation of the evanescent field at the boundary of the core and the clad. The diffusion speed of hydrogen within silicone-resin film to the evanescent field could be the reason for the slow response. The response of the sensor that the tungsten oxide film was prepared by the sol–gel process is shown in Fig. 5. The absorbance increased immediately after the exposure of hydrogen gas and reached the steady state. It took about 7 min for the 90% absorbance change. The response time was greatly improved by preparing the thin hydrogen-sensitive film with the sol–gel process. The transmission spectra of the sensor, as shown in Fig. 6, was different from the one with the silicone resin-dispersed PdrWO 3 ŽFig. 4.. The increase in the absorbance was observed in the wavelength range above 530 nm, and the decrease was observed below that wavelength. The observation is in accordance with the fact that the film changed to blue as a result of the formation of the tungsten bronze. The film deposited by one dip-coating is approximately 1 mm. It is assumed that the tungsten oxide film in this order cannot form the high dielectric domain to exhibit
Fig. 6. The transmission spectra of the sensor prepared by the sol–gel process.
S. Sekimoto et al.r Sensors and Actuators B 66 (2000) 142–145
145
very slow response. The response time was significantly improved by the sol–gel process. The sensor has potential to measure the distribution along the fiber line, unlike the traditional hydrogen sensors that measure at a certain spatial point. The fiber sensor of the present research formed by the sol–gel process is very promising for wide area monitoring.
References
Fig. 7. Calibration curve for the fiber-optic hydrogen sensor.
ferroelectricity, and the characteristic of the color change appeared strongly in the transmission spectra without interfering effect of dielectric constant change. To solidify the postulate, the thicker films were deposited by the sol–gel process using thick solution of 1.0 M Na 2WO4 . The transmission spectra became similar to Fig. 4, i.e. decrease in absorbance in the long wavelength range upon exposure to hydrogen. It may be considered that the tungsten oxide film has the enough thickness to form the high dielectric film, and the influence of refractive index strongly affected the transmission spectra. Concentration dependence of the sensor is shown in Fig. 7. The sensor showed high sensitivity in the low concentration range, but the response tends to saturate above 40%. In these experiments, the length of the sensing part was 7.5 cm. It is theoretically expected to increase the sensitivity by making it longer. The effect of the amount of the catalysts on the response to hydrogen gas on 10- and 20-cm sensing length fibers was examined. It was clear that the increase in the Pt content makes not only the sensitivity high, but also the response time fast. Also, the increase in sensitivity with the increase in sensing length was apparent. When the sensing length varied from 10 to 20 cm at the same amount of catalysts, the change in absorbance was approximately doubled. Sensitivity in the low concentration range and the response time depend on the thickness and quality of the hydrogen-sensitive film.
4. Conclusions The sensors prepared by using the silicone resin and the sol–gel process had been developed. The platinum- or palladium-supported tungsten oxides were used as the hydrogen sensitive material. The catalysts-doped tungsten oxides were reduced to blue tungsten bronze upon exposure to hydrogen even at room temperature. However, the sensor using the silicone resin-dispersed PdrWO 3 had
w1x Y. Suematsu, K. Iga, Hikarifaiba Tuushin Nyuumon, Ohmsha, pp. 227, 228, in Japanese. w2x K. Nishizawa, E. Sudo, M. Maeda, T. Yamasaki, Optical waveguide type hydrogen sensor using the change of absorbance of Pd–WO 3 layers, OQE85-63, in Japanese. w3x P. Judeinstein, J. Livage, Electrochromic properties of sol–gel derived WO 3 coatings, SPIE 1328 Ž1990. Sol–Gel Optics. w4x T. Nishide, F. Mizukami, Control of refractive index of sol–gel tungsten oxide films, J. Sol–Gel Sci. Technol. 6 Ž1996. 263–267. w5x S. Tanisaki, J. Phys. Soc. Jpn. 15 Ž1960. 573. w6x P.H. Paul, G. Kycakoff, Appl. Phys. Lett. 51 Ž1987. 12.
Biographies Shinjiro Sekimoto received his B. Eng. degree from Yokohama National University in 1998. He joined the master course of Yokohama National University in 1998. Hidemoto Nakagawa obtained his M. Appl. Sci. and PhD degrees from the University of Toronto in 1974 and 1978, respectively. His research centers on various sensors and their applications since he joined Yokohama National University in 1993. He is also interested in electrochemistry and micromachining. Shinji Okazaki received his B. Eng and M. Eng. degrees from Yokohama National University in 1991 and in 1993. He joined Yokohama National University as a research associate in 1997. His major fields are electrochemistry and sensor engineering. Kenzo Fukuda received his MS and PhD degrees from the University of Tokyo in 1967 and 1972, respectively. He is a project manager of the WE-NET Center. His fields of interest are energy technologies, sensors and catalyses. Shukuji Asakura received his M. Eng. and PhD degrees from the University of Tokyo in 1965 and 1968, respectively. In 1972, he joined Yokohama National University, and became a professor in 1988. His fields of interest are safety engineering, corrosion science and chemical sensors. Tesshi Shigemori received his BS and MS degrees from Hokkaido University in 1966 and 1968, respectively. Then he joined New Cosmos Electric and was promoted to a senior director. His fields of interest are gas sensors and their applications. Sachio Takahashi graduated from Osaka Prefecture University in 1959. He joined the Osaka National Research Institute in the field of battery engineering and obtained PhD In 1996, he joined New Cosmos Electric. He is supervising the research division of the company as a managing director.