Patent Application
20220057372
The present invention relates to a hydrogen sensor using a Membrane-type Surface stress Sensor (hereinafter referred to as MSS), and particularly to a hydrogen sensor using an amorphous palladium-copper-silicon alloy (hereinafter referred to as PdCuSi) thin film as a sensitive film thereof. The present invention also relates to a method for detecting hydrogen using such a hydrogen sensor.
In order to reduce environmental load, for example, research and development for utilization of hydrogen as energy sources has been advanced, and practical utilization has been achieved in some cases. One problem in the wide utilization of hydrogen is a possibility of explosion due to the leakage of hydrogen from a container, piping, or the like. Since hydrogen is light and diffusive, in an open space, hydrogen is less prone to accumulate and it can be said that the risk of explosion is not so high. On the other hand, when the leakage of hydrogen occurs in a nearly closed compartment, the retention of hydrogen may occur. When hydrogen is retained and the hydrogen concentration in the air enters a wide region (explosion range) of 4 to 75 volume % (hereinafter, hydrogen concentration is expressed by a volume ratio unless otherwise specified), ignition causes explosion. Therefore, it is very important to discover the leakage or accumulation of hydrogen at an early stage while the hydrogen concentration is sufficiently lower than 4% in view of ensuring the safety of hydrogen-related equipment or facilities.
Hydrogen sensors based on various operation principles proposed so far include those in which a thin film of a hydrogen storage material is formed on one side of a micro cantilever and the hydrogen concentration in a sample gas is measured from a change in the amount of deflection of the cantilever (displacement of the cantilever end) caused by the expansion and contraction of the thin film due to the storage and release of hydrogen by the thin film (for example, Non Patent Literature 1 and Non Patent Literature 2). This type of sensor (cantilever-type sensor) can selectively detect specific substances in a liquid or gas sample by appropriately selecting a film (referred to as a sensitive film, receptor layer, or the like) of a substance adhered to the surface of the cantilever, and thus various applications thereof have been proposed for detecting trace components.
Since hydrogen has a risk of explosion even at a low concentration of 4% as described above, it is required to increase the sensitivity of the hydrogen sensor as possible in order to detect the accumulation of hydrogen due to leakage or the like sufficiently prior to its concentration reaches the explosion range. Particularly, in a closed region having a complex internal structure or shape, the distribution of hydrogen concentration in the region may largely change due to a position where the leakage of hydrogen occurs, and thus the sensitivity of the hydrogen sensor is required to have a further allowance in order to deal with such a case. However, since about 0.5 ppm of hydrogen is typically contained in the air (Non Patent Literature 3), such a high sensitivity that can detect a value close to 0.5 ppm is not necessary in use for the purpose of detecting a small amount of hydrogen leakage.
The inventors of the present application have found that the above-mentioned objective is achieved by an MSS using a thin film of a hydrogen storage material such as a hydrogen storage metal or alloy such as palladium having a thickness or 30 nm or less as a sensitive film, and a patent application has been filed as Japanese Patent Application No. 2017-155808 (JP 2019-35613 A).
While the hydrogen sensor for which the patent application has already been filed has a feature of being capable of detecting hydrogen at a sufficient high sensitivity and capable of hydrogen detection in the absence of oxygen, it is desirable to improve the S/N ratio of measurement data and the stability of measurement results particularly in a low concentration range by further improving the sensitivity. In addition, with typical hydrogen storage materials, the detection speed is low due to slow storage and release of hydrogen, and it is rather difficult to handle response signals due to hysteresis characteristics of storage and release, for which improvements are desired.
According to an aspect of the present invention, a hydrogen sensor having a sensitive film on a surface for receiving surface stress of a membrane-type surface stress sensor is provided, wherein the sensitive film is an amorphous palladium-copper-silicon alloy thin film.
Here, an atomic ratio between palladium, copper, and silicon contained in the amorphous palladium-copper-silicon alloy may be 65<x<90, 3<y<20, and 3<z<20 wherein the alloy is represented as PrxCuySiz, with x, y, and z being percentage values.
The sensitive film may have a thickness of greater than 0 nm and less than 100 nm.
The sensitive film may have a thickness of 50 nm or less.
The sensitive film may have a thickness of 15 nm or more.
The sensitive film may have a thickness of 5 nm or more.
According to another aspect of the present invention, a method for detecting hydrogen is provided, comprising alternately supplying a target gas containing hydrogen and a purge gas to any of the hydrogen sensor above and measuring a hydrogen concentration in the target gas based on an output signal from the hydrogen sensor.
Here, an arithmetic treatment may be performed on the output signal.
The arithmetic treatment may be time differentiation.
In addition, a process of calculating the hydrogen concentration may be performed based on a peak value of the output signal on which the time differentiation has been performed.
Alternatively, a process of obtaining the hydrogen concentration may be performed based on a signal waveform following a peak value of the time differentiated output signal.
The process of calculating the hydrogen concentration based on the signal waveform following the peak value of the time differentiated output signal may be selectively performed based on the peak value of the time differentiated output signal.
The present invention provides a hydrogen sensor having a high sensitivity and low hysteresis characteristics while having a simple configuration and low cost, and a method for detecting hydrogen using the hydrogen sensor.
As a result of intensive studies, the inventors of the present application have found that an MSS using an amorphous PdCuSi thin film as a sensitive film achieves the above-mentioned objective, and brought the present invention into completion.
Note that it is conventionally known that an amorphous Pd-based alloy exhibits a hydrogen storage property, and hydrogen sensors using this property have been proposed. For example, Patent Literatures 4 to 6 and Non Patent Literature 4 disclose hydrogen sensors that use change in electrical resistance of amorphous PdCuSi. In addition, Patent Literature 7 and Non Patent Literature 5 disclose hydrogen sensors that use change in capacitance caused by the displacement of a PdCuSi film due to the film bending when storing hydrogen. However, since such hydrogen sensors using change in electrical resistance involve passing currents in PdCuSi, there is a problem of high power consumption during operation of the sensor. In addition, hydrogen sensors using change in capacitance as described above require a stacking structure of PdCuSi film in a direction perpendicular to the film surface, which leads to complexity in structure and difficulty in manufacture. In addition, it is difficult to reduce the thickness of a PdCuSi film in terms of its structure (according to the left column of page 316 in Non Patent Literature 5, the thickness of the PdCuSi film is 500 nm), and thus the amount of use of Pd, which is an expensive noble metal, is increased.
The present invention provides a hydrogen sensor that exhibits a high sensitivity and relatively good response characteristics even with an amorphous PdCuSi thin film having a very small thickness, has a low hysteresis and a simple structure, and can reduce power consumption by using an MSS as a surface stress sensor.
First, the MSS will now be described. As seen from the schematic diagram shown in
In the MSS, in detecting surface stress in the surface of the thin plate-shaped member (sensor body surface), two orthogonal components of surface stress generated on the sensor body surface by the sensitive film (mentioned as “hydrogen sensing film” in
Types of cantilever-type sensors currently used include light reading cantilevers and piezo-resistive cantilevers, and in comparison for sensitivity, MSS≥light reading cantilevers>>piezo-resistive cantilevers. The reason for which an MSS even using a piezo-resistive element as a stress detecting member can have a much higher sensitivity than piezo-resistive cantilevers is that it is possible to utilize the fact that, in the case of using a (001) face of a p-type silicon monocrystal for the stress detecting members (and the thin plate-shaped member formed integrally with them) of the MSS, piezo-resistivities when current flows on this surface in the [110] direction have opposite signs in the [110] and [1/10] directions (where “/1” indicates a symbol of 1 with an overbar). That is, in this case, assuming a coordinate system where the above-mentioned two directions are set as x and y axes, an infinitesimal change dR in a piezo-resistance value R is proportional to the difference, σx−σy, between an x-direction stress and a y-direction stress. Thus, by configuring the four stress detecting members such that currents flow in the same direction ([110] direction) in the piezo-resistive elements of these stress detecting members (note that the black bold lines indicated at the four stress detecting parts in the MSS structure shown in
The MSS is well known to those skilled in the fields of surface stress sensors and their applications, and will not be described in more detail. If more detailed information is required, see Patent Literatures 1 to 3 and Non Patent Literature 6, which describe the MSS, for example.
In the hydrogen sensor of the present invention, a thin film of amorphous PdCuSi, which is a kind of hydrogen storage material, is used as the sensitive film of the MSS. As described in the following example, examining the relationship between a gas (mixture gas of hydrogen and nitrogen) with a very wide range of hydrogen concentration from 4% on the high-concentration side to 0.25 ppm on the low-concentration side, which is even ½ of the concentration of hydrogen typically contained in the air, about 0.5 ppm, as described above, and the detection output of the MSS, an effect beyond expectation is obtained showing that, even in the case of using an amorphous PdCuSi thin film with a very small thickness of less than 100 nm, specifically 30 nm, which is in a range for which inspection has not been made in Non Patent Literatures 4 and 5 or the like, it is possible to detect a very low concentration of hydrogen at the lower limit of this range with a sufficiently high S/N ratio. Although the example also involves an experiment for the case of using an amorphous PdCuSi thin film having a thickness of 50 nm as the sensitive film, as can be understood theoretically and from the experiment results of the example, the intensity of an output signal is smaller as the thickness is smaller in a thickness range of this level, and therefore the experiment of the example is performed by mainly using the amorphous PdCuSi thin film having the smaller 30 nm thickness, which imposes stricter conditions. Although an experiment for the case where the amorphous PdCuSi thin film is thinner than 30 nm is not performed, the sensitivity does not largely change due to the thickness in a range of thickness of a few tens of nm, as discussed in Non Patent Literature 9, and therefore the sensitivity does not become ten times or 1/10 even when the thickness becomes twice or half, for example. Therefore, considering the fact that hydrogen at a concentration of 0.25 ppm can be detected with a sufficiently high S/N ratio when the thickness is 30 nm, it is considered that hydrogen to a concentration of about 0.25 ppm can be sufficiently detected even when the thickness is decreased to about 20 nm, and if the detection of a concentration of about 0.5 ppm is required, it is possible to further reduce the thickness to about 15 nm.
If a further lower detection sensitivity is allowed, it is possible to further reduce the thickness. However, depending on the deposition method and deposition conditions of the PdCuSi thin film, if the average thickness is reduced to about 5 nm, there is a tendency that the actually formed film would be in a state that is difficult describe as a film in a usual meaning such as minute particles scattered on the base or a large number of minute island-like regions discontinuous with each other formed on the base at spaces. Since in such a state, surface stress generated on the detector body surface due to storage and release of hydrogen by the thin film is largely decreased as compared to the case of considering a model in which a continuous and uniform film is formed on the base, it is considered that a continuous and uniform film results in a higher sensitivity of hydrogen detection. In this context, it can be said that the lower limit of the thickness of the sensitive film is about 5 nm unless a special film formation technique or the like is used to form a highly continuous and very thin film. On the other hand, as can be understood from Non Patent Literature 8, it is considered that a significant reduction in sensitivity of the surface stress sensor does not occur simply because of discontinuity of the sensitive film. Thus, not only a completely continuous film but also a discontinuous film with breaks or the like is allowed as the sensitive film, and in this context, it should be understood that a thickness mentioned herein refers to an average thickness. According to the same literature, reduction in sensitivity becomes significant when a state where small regions such as island-like or particulate are distributed at spaces from each other in a discontinuous minute structure of the sensitive film, that is, a ratio of coverage of the sensitive film on the sensor body surface is substantially lower than 1.
In addition, as described in the following example, although the storage of hydrogen is a phenomenon that generally requires relatively long time despite the fact that the storage of hydrogen by PdCuSi is more rapid than other hydrogen storage metals, if the sensitive film of hydrogen storage material is formed to be very thin, the amount of storage of hydrogen in the sensitive film approaches a state of equilibrium in a short time when the supply of hydrogen-containing gas (sample gas) to be measured to the MSS is started. Thus, it is possible to detect the hydrogen concentration in the sample gas with a high accuracy by using output signals from the MSS only at the beginning of the supply of the sample gas and in its vicinity. Although no limitation is intended of course, it is possible to easily extract hydrogen concentration information included in such signals near the beginning of the supply by performing operations such as time differentiation on the output signal from the MSS. More specifically, it is possible to determine the hydrogen concentration with a high accuracy only by observing peak values of the time derivative of the output signals from the MSS. In addition, there is a tendency that the storage of hydrogen slowly reaches equilibrium when the hydrogen concentration is particularly low, and in that case, it is difficult to determine the hydrogen concentration with a high accuracy only from the peak values. Even in such a case, differences in hydrogen concentration are reflected in waveforms and values (hereinafter referred to as “waveforms and the like”) of output signals while some length of time elapses from the beginning of the supply of the target gas or of those after operations such as time differentiation. Therefore, it is possible to determine the hydrogen concentration with a high accuracy even in a low concentration range by using information of such waveforms and the like. In addition, it is also possible to improve the detection accuracy in a low hydrogen concentration range by performing division into cases according to hydrogen concentration (specifically, peak values obtained, for example) and, when the detected hydrogen concentration is low, determining the hydrogen concentration by additionally considering information of waveforms and the like as described above, and to determine the hydrogen concentration in a short time by using only the peak values in a range where the hydrogen concentration can be obtained with a high accuracy without such information.
In the example, an example of amorphous PdCuSi having a composition where the atomic ratios of Pd:Cu:Si are 75%:10%:15% is used; however, the present invention is not limited thereto. Based on the triangular phase diagram in Non Patent Literature 4 and Non Patent Literature 5, the range of atomic ratios in which PdCuSi can take an amorphous structure is represented by PrxCuySiz, where x, y, and z are expressed by percentage as 65<x<90, 3<y<20, and 3<z<20.
In the following, a hydrogen sensor in which a thin film (with thicknesses of 50 nm and 30 nm as shown in
On the MSS, Pd75Cu10Si15 was deposited onto the flat member for receiving surface stress by ternary simultaneous sputtering. As described above, two thicknesses of 30 nm and 50 nm are illustrated in
Here, an experiment was conducted for specifically comparing the sensor operations of the hydrogen sensor composed of the MSS using the amorphous Pd75Cu10Si15 thin film (30 nm thick) sensitive film in this example and the hydrogen sensor composed of the MSS using the Pd thin film (20 nm thick) in Japanese Patent Application No. 2017-155808 described above. Although the Pd thin film has a smaller thickness, the Pd thin film contains a larger amount of Pd of these two films, and it is therefore considered that the comparison is not particularly against the hydrogen sensor using the Pd thin film.
In the comparative experiment, a nitrogen gas in which 4% of hydrogen is mixed and a pure nitrogen gas were repeatedly introduced into the two hydrogen sensors mentioned above to measure sensor outputs. Data of the hydrogen sensor of the example of the present invention and the hydrogen sensor using the Pd thin film are shown in the graphs in
As a result of the above comparison, the hydrogen sensor of the example of the present invention has a larger response speed, better reproducibility (lower hysteresis), and higher signal intensities, and this result is associated with the fact that Pd in PdCuSi is in the form of minute particles and in the state of alloy and thus exhibits good adhesion to the MSS substrate.
As described above, the present invention is expected to have high industrial applicability such as availability for ensuring high safety of a fuel cell, hydrogen-related facilities or the like by using a hydrogen sensor with a simple configuration.
Patent Literature 1: JP 2015-45657 A
Patent Literature 2: JP WO2013/157581 A1
Patent Literature 3: JP WO2011/148774 A1
Patent Literature 4: JP 2008-8869 A
Patent Literature 5: JP 2009-139106 A
Patent Literature 6: JP 2010-181282 A
Patent Literature 7: JP 2017-215170 A
Non Patent Literature 1: S. Okuyama et al., Jpn. J. Appl. Phys., 39(2000)3584.
Non Patent Literature 2: Yen-I Chou et al., Tamkang J. Sci. Eng., 10(2007)159.
Non Patent Literature 3: JIS W 0201:1990 Standard Atmosphere
Non Patent Literature 4: Susumu Kajita, Panasonic Technical J. Vol. 61(2015)61.
Non Patent Literature 5: Hiroaki Yamazaki et al., IEEJ Trans. Sens. Micro. 138(2018)312.
Non Patent Literature 6: G. Yoshikawa et al., Nano Lett., 11(2011)1044.
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Non Patent Literature 8: G. Imamura et al., Anal. Sci., 32(2016)1189.
Non Patent Literature 9: G. Yoshikawa, Applied Physics Letters 98, 173502 (2011).
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-040136 | Mar 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/005694 | 2/14/2020 | WO | 00 |
