The present invention relates to a hydrogen sensor for detecting hydrogen gas.
From a viewpoint of preventing carbon dioxide emissions into the atmosphere, hydrogen has been attracting attention as an energy source. There is, however, a risk of explosion if hydrogen gas leaks into an atmosphere. Thus, the development of a hydrogen sensor capable of quickly detecting leaked hydrogen gas has been being advanced. As such hydrogen sensor, a semiconductor sensor using tin oxide has been developed. The operating temperature of this semiconductor sensor is, however, as high as about 400° C. Thus, in using this semiconductor sensor, it is necessary to take a preventive measure against explosion. Consequently, a hydrogen gas leak detector using this semiconductor sensor has a drawback that it is complicated in structure.
In this situation, there have been developed hydrogen sensors in which a thin film layer of a magnesium-nickel alloy or the like formed on the surface of a substrate of glass or the like is quickly hydrogenated in the presence of hydrogen gas under the action of a catalyst layer of palladium or the like, and thus undergoes changes in material properties. A hydrogen sensor of this type is disclosed in Unexamined Japanese Patent Publication No. 2005-83832 (hereinafter referred to as Patent Document 1), for example. The hydrogen sensor disclosed in Patent Document 1 can detect hydrogen gas that has leaked into an atmosphere by detecting a change in optical reflectance (hereinafter, sometimes referred to simply as “reflectance”) of the thin film layer caused by hydrogenation. The hydrogen sensor disclosed in Patent Document 1, in which the thin film layer is reversibly hydrogenated at normal temperatures, has also an advantage that it can detect leaked hydrogen gas safely and quickly.
In the hydrogen sensor disclosed in Patent Document 1, however, the catalyst layer, which is directly exposed to the atmosphere, is prone to oxidation, because magnesium, which is a constituent of the thin film layer, diffuses, deposits or the like (hereinafter, sometimes the word “diffuse” is used to cover this meaning) in the catalyst layer, as the hydrogenation and dehydrogenation are repeated (hereinafter, wording “repetition of hydrogenation and dehydrogenation” is sometimes simplified into wording “repetition of hydrogenation”). Thus, there is a risk that the oxidation of the catalyst layer entails oxidation of the thin film layer, resulting in a smaller change in reflectance, and therefore, a decrease in leaked hydrogen gas detection sensitivity of the thin film layer (which means deterioration of the hydrogen sensor).
Thus, from the result of measurement shown in
In order to solve the above-mentioned problem, the present invention provides a hydrogen sensor comprising a substrate, a thin film layer formed over the substrate, a buffer layer formed over the thin film layer, and a catalyst layer formed over the buffer layer, which, by being contacted by hydrogen gas in an atmosphere, hydrogenates the thin film layer, thereby changing optical reflectance of the thin film layer, wherein the buffer layer contains a constituent that combines with a constituent of the thin film layer which diffuses from the thin film layer into the catalyst layer, thereby restraining oxidation of the catalyst layer.
In other words, the characteristic feature of this hydrogen sensor is that between the catalyst layer and the thin film layer, there is formed a buffer layer containing a constituent that combines with a constituent of the thin film layer which diffuses from the thin film layer into the catalyst layer. Thus, in this hydrogen sensor, the constituent of the thin film layer diffuses into the buffer layer and further diffuses into the catalyst layer, together with the constituent of the buffer layer, and both constituents combine with each other by inter-atomic bonding or the like, for example, within the catalyst layer, at the surface of the catalyst layer and elsewhere. The elements combined in this manner are less prone to oxidation than uncombined elements. Thus, the oxidation of the catalyst layer after repetition of hydrogenation is restrained, so that the oxidation of the thin film layer is also restrained. Here, the buffer layer may be formed of either a single constituent (one metal element or the like, for example) or a plurality of constituents (an alloy, for example).
Specifically, the thin film layer may be formed of a magnesium alloy or magnesium, for example, while the catalyst layer may be formed to contain palladium or platinum, for example. The thin film layer formed of such substance can undergo a change in optical reflectance by reversible hydrogenation. The catalyst layer may be formed of any of palladium, platinum, a palladium alloy and a platinum alloy.
More specifically, the thin film layer may be formed of a magnesium-nickel alloy, a magnesium-titanium alloy, a magnesium-niobium alloy, a magnesium-cobalt alloy or a magnesium-manganese alloy, for example. The thin film layer formed of such substance can more quickly undergo reversible hydrogenation.
When the thin film layer is formed of a magnesium alloy or magnesium, the buffer layer is formed to contain a constituent that combines with magnesium that diffuses from the thin film layer into the catalyst layer, thereby restraining the oxidation of the catalyst layer attributed to the magnesium.
In this case, the buffer layer may, specifically, be formed to contain nickel, titanium, niobium or vanadium, for example.
For an improvement in hydrogen gas detection sensitivity, it is desirable that a constituent of the catalyst layer be diffused in the thin film layer. There is however a possibility that, while preventing the oxidation of the catalyst layer as mentioned above, the buffer layer prevents the constituent of the catalyst layer from diffusing into the thin film layer, resulting in the thin film layer difficult to hydrogenate.
However, if, in the above hydrogen sensor, the thickness of the buffer layer is in a preferable range of 1 to 5 nm, such phenomenon hardly occurs while the oxidation of the catalyst layer is restrained by the buffer layer.
Preferably, in the above hydrogen sensor, as another preferable measure against the above phenomenon, a thin film activation layer may be interposed between the substrate and the thin film layer and/or between the buffer layer and the thin film layer. The thin film activation layer contains a constituent which, by being contacted by hydrogen, hydrogenates the thin film layer, thereby changing the optical reflectance of the thin film layer.
In the hydrogen sensor structured as described above, even if the buffer layer prevents the constituent of the catalyst layer from diffusing in the thin film layer, the above constituent of the thin film activation layer diffuses in the thin film layer and promotes the hydrogenation and dehydrogenation of the thin film layer, resulting in a further improved hydrogen detection sensitivity of the hydrogen sensor. The thin film activation layer may be formed of either a single constituent (one metal element or the like, for example) or a plurality of constituents (an alloy, for example), as long as it contains a constituent which, by being contacted by hydrogen, hydrogenates the thin film layer, thereby changing the optical reflectance of the thin film layer.
Specifically, in the above hydrogen sensor, the thin film activation layer may be formed to contain the same constituent as the catalyst layer contains. In this case, the constituent having the same catalytic function as the catalyst layer diffuses from the thin film activation layer into the thin film layer, and promotes the hydrogenation and dehydrogenation of the thin film layer. Consequently, desirable hydrogen gas detection sensitivity is maintained, while the oxidation of the catalyst layer is restrained by the buffer layer.
More specifically, in the above hydrogen sensor, the thin film activation layer may be formed to contain palladium or platinum. In this case, palladium or platinum having a catalytic function diffuses from the thin film activation layer into the thin film layer. Consequently, desirable hydrogen gas detection sensitivity is maintained, while the oxidation of the catalyst layer is restrained by the buffer layer.
With reference to the drawings, embodiments of the present invention will be described below.
First, referring to
The hydrogen sensor 10a shown in
The buffer layer 13 less than 1 nm in thickness results in a reduction in the amount of titanium (Ti) or the like diffusing from the buffer layer 13 into the catalyst layer 14, and therefore difficulty in preventing the oxidation of the catalyst layer 14. The buffer layer 13 more than 5 nm in thickness, on the other hand, makes it difficult for a constituent of the catalyst layer 14, such as palladium (Pd), to diffuse into the thin film layer 12, and therefore makes hydrogenation of the thin film layer 12 difficult, which possibly results in a decrease in hydrogen gas detection sensitivity. In the hydrogen sensor 10a, the thickness of the buffer layer 13 is therefore determined by taking account of the balance between the beneficial and adverse effects of the buffer layer 13, i.e., prevention of oxidation of the catalyst layer 14 and decrease in hydrogen gas detection sensitivity.
The buffer layer 13 may be formed of a substance capable of combining with magnesium (Mg) that diffuses from the thin film layer 12 into the catalyst layer 14 to thereby restrain the oxidation of the catalyst layer 14 attributed to the magnesium (Mg). For example, the buffer layer 13 may be formed of nickel, niobium, vanadium or the like. Preferably, the thickness of the catalyst layer 14 may be within a range of 1 nm to 100 nm.
The thin film layer 12, the buffer layer 13 and the catalyst layer 14 can each be formed by sputtering, vacuum evaporation, electron beam evaporation, plating or the like. The substrate 11 may be an acrylic resin sheet, a polyethylene sheet (polyethylene film) or the like.
When exposed to an atmosphere with a hydrogen concentration of about 100 ppm to 1% or more, the hydrogen sensor 10a structured as described above exhibits a visible (visualizable) change in optical reflectance of the thin film layer 12, quickly, namely in several to ten sec or so.
Thus, the diagram shows that in spite of 60 times of hydrogenation, magnesium (Mg) constituting the thin film layer 12 hardly diffuses to the catalyst layer 14, so that very little magnesium (Mg) is present in the region of etching times from 0 to about 20 sec. This means that the oxidation of the catalyst layer 14 can be prevented. Both before and after the repetition of hydrogenation, the maximum atomic percentage of titanium (Ti) is present in the region of etching times from 25 to about 50 sec, which means that the buffer layer 13 is stable. Further, both before and after the repetition of hydrogenation, magnesium (Mg) increases from the etching time about 20 sec, which means that the thin film layer 13 is rather stable. Palladium (Pd) in the catalyst layer 14 does not exhibit a remarkable change, which means the catalyst layer 14 is also stable.
Thus, it is recognized that, in the hydrogen sensor 10a, in spite of repetition of hydrogenation, the buffer layer 13 prevents the oxidation of the catalyst layer 14, and therefore, prevents a decrease in hydrogen detection sensitivity caused by the oxidation of the catalyst layer 14. Incidentally, Si in
The hydrogen sensor with the thin film layer not hydrogenated (dehydrogenated) exhibits low transmittance (thus high reflectance), and the transmittance increases (reflectance decreases) with hydrogenation of the thin film layer. Since the width of variation of transmittance with variation of hydrogen gas concentration (hereinafter, sometimes referred to simply as “variation width of transmittance”) determines the hydrogen detection sensitivity of the hydrogen sensor, it is desired that the variation width of transmittance stay constant. Further, for stable detection of leaked hydrogen gas present at low concentrations, it is desired that the transmittance with the thin film layer not hydrogenated stay constant. Thus, in the hydrogen sensor, it is desired that the variation width of transmittance as well as the transmittance with the thin film layer not hydrogenated stay constant.
As
In contrast, as
As seen in
In contrast, as seen in
Next, referring to
In the hydrogen sensor 10b shown in
The substrate 11, the thin film layer 12, the buffer layer 13 and the catalyst layer 14 of the hydrogen sensor 10b are similar in elemental composition as well as formation to those of the hydrogen sensor 10a according to the first embodiment. The first thin film activation layer 15p contains palladium (Pd) that can act as a catalyst in the catalyst layer 14. Like the hydrogen sensor 10a, the hydrogen sensor 10b includes the buffer layer 13, which can prevent the oxidation of the catalyst layer 14 and the thin film layer 12, thereby preventing a decrease in hydrogen detection sensitivity. In the hydrogen sensor 10b, further, palladium (Pd) diffuses from the first thin film activation layer 15p interposed between the substrate 11 and the thin film layer 12 into the thin film layer 12 and promotes hydrogenation/dehydrogenation of the thin film layer 12. This leads to an improvement in hydrogen detection sensitivity of the hydrogen sensor 10b. Thus, in the hydrogen sensor 10b, while the buffer layer 13 prevents the oxidation of the catalyst layer 14 and the thin film layer 12, the first thin film activation layer 15p compensates for the decrease in hydrogen detection sensitivity attributed to the presence of the buffer layer 13.
In the present embodiment, the thickness of the first thin film activation layer 15p is 2 nm. The first thin film activation layer 15p is however provided to make palladium (Pd) diffuse into the thin film layer 12, thereby promoting the hydrogenation of the thin film layer 12. Thus, as long as this object can be achieved, the thickness of the first thin film activation layer 15p is not restricted to that in the present embodiment. Further, since the first thin film activation layer 15p is provided to make a metal acting as a catalyst diffuse into the thin film layer 12, the constituent of the first thin film activation layer 15p is not restricted to palladium (Pd).
Next, referring to
The hydrogen sensor 10c shown in
The hydrogen sensor 10d shown in
It is without saying that the hydrogen sensor according to the present invention is not restricted to the above-described embodiments, but can be modified appropriately without departing from the spirit and scope of the present invention.
Number | Date | Country | Kind |
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2007-148001 | Jun 2007 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2008/059833 | 5/28/2008 | WO | 00 | 12/3/2009 |