Hydrogen sensor, production method thereof, measuring device, and method for measuring hydrogen concentration

Abstract

The invention relates to a hydrogen sensor (8) and a method for producing the same, a measuring device (2), and a method for measuring the hydrogen concentration. The hydrogen sensor (8) for measuring the hydrogen concentration in an environment (4) comprises a substrate (10) on which a hydrogen-absorbing sensor medium (14) in the form of a thin film is applied in a sensor area (12) in communication with the environment, wherein the sensor medium (14) changes its volume depending on the hydrogen concentration in the sensor medium (14), and the change in the volume causes a change in the mechanical strain introduced by the sensor medium (14) in the substrate (10). A further development of the hydrogen sensor (8) consists in that at least the sensor area (12) of the substrate (10) is a piezoresistive semiconductor.

Description

Technical Field

[0001] The present invention relates to a hydrogen sensor for measuring the hydrogen concentration in an environment, comprising a substrate on which a hydrogen-absorbing sensor medium in the form of a thin film is applied in a sensor region communicating with the environment, wherein the sensor medium changes its volume depending on the hydrogen concentration in the sensor medium, and the change in the volume causes a change in the mechanical strain introduced into the substrate by the sensor medium. Furthermore, the present invention relates to a measuring device for measuring the hydrogen concentration in an environment, comprising a measuring unit and such a hydrogen sensor. Furthermore, the present invention relates to a method for producing such a hydrogen sensor. Finally, the present invention relates to a method for measuring the hydrogen concentration in an environment. Background Art

[0002] Hydrogen sensors for measuring the hydrogen concentration in gases such as air, for example, are known in different variants. A large number of commercially available hydrogen sensors are based on few measurable physical effects. One of these physical effects is the ability of different materials to absorb hydrogen. Depending on the amount of hydrogen dissolved in the material used as the sensor, one or more measurable physical properties change.

[0003] From DE 10 2006 018 767A1, a hydrogen sensor using a change in the optical properties of a sensor material is known. In the case of the sensor known from this document, the sensor medium is irradiated with electromagnetic radiation, and the transmission coefficient of the sensor medium is measured by means of a detector. Thus, a material is used as the sensor medium, the transmission coefficient of which changes depending on the hydrogen concentration in the measurement environment of the sensor medium. The measured transmission coefficient is used as a measure of the hydrogen concentration in the environment.

[0004] In US 7,791,150B1, a hydrogen sensor is disclosed in which a structured electrode is applied on a silicon wafer having a thermally generated oxide layer on its upper side. A hydrogen-sensitive semiconductor oxide film (In2O3 doped with SnO2) is applied thereon. The hydrogen-sensitive film is applied, for example, by a sol-gel method. In addition, a hydrogen-selective film is provided on the upper side of the film. The change in the electrical characteristics of the semiconductor oxide film is used as a quantitative measure of the hydrogen concentration.

[0005] Another hydrogen sensor is known from KR 20180074100. A film system is provided, which consists of a polymer film serving as a substrate, an adhesion-promoting film or layer, and a palladium film applied thereon. If the palladium film is exposed to a hydrogen-containing environment, the palladium film thereby changes its volume. This change in volume causes a mechanical strain, which gradually increases in the above-mentioned film system. The film system is designed as a bending bar. Furthermore, the mechanical strain causes the bending bar to bend more or less strongly depending on the hydrogen concentration.

[0006] Another hydrogen sensor is known from CN 108872314, in which a film made of palladium is applied to zinc oxide (ZnO) as a substrate. By this sensor, the above effect that palladium undergoes a volume change when absorbing hydrogen is utilized to apply a mechanical force to zinc oxide. Since zinc oxide exhibits a piezoelectric effect, a passive sensor can be provided. The piezoelectric strain on zinc oxide is a direct measure of the hydrogen concentration to which the palladium film is exposed.

[0007] The switchable physical properties of palladium are caused by the interaction between metal atoms and hydrogen, and hydrogen dissolves in the metal at the atomic level in the interstitial sites. Depending on the hydrogen concentration, i.e., the amount of hydrogen dissolved in the metal lattice, the lattice constant of palladium changes, which is related to the volume expansion of the material. This effect is reversible. Summary of the Invention

[0008] The object of the present invention is to provide an enhanced hydrogen sensor, an enhanced measuring device, and an enhanced method for measuring hydrogen concentration, and an enhanced method for producing such a hydrogen sensor.

[0009] This object is solved by a hydrogen sensor for measuring the hydrogen concentration in an environment, the hydrogen sensor comprising a substrate, on which a hydrogen-absorbing sensor medium is applied as a thin film in a sensor area communicating with the environment, wherein the sensor medium changes its volume depending on the hydrogen concentration in the sensor medium, and the change in the volume causes a change in the mechanical strain introduced into the substrate by the sensor medium, wherein the development of the hydrogen sensor lies in that at least the sensor area of the substrate is a piezoresistive semiconductor.

[0010] Using the hydrogen sensor according to aspects of the present invention, the hydrogen concentration in a large measurement range can be measured. In addition, the sensor does not have any cross-sensitivity to other gases and exhibits a fast response time. The sensor utilizes the volume change of the sensor medium caused by hydrogen absorption. The sensor medium introduces a mechanically variable strain (meaning different levels or intensities of strain) into the substrate depending on the hydrogen concentration. Since the piezoresistive properties of the semiconductor are also used, the mechanical strain can be easily and very precisely measured, thereby measuring the hydrogen concentration. Advantageously, the mechanical strain in the sensor area of the piezoresistive semiconductor can be measured electrically, for example, by providing a resistive structure on the semiconductor in the form of a Wheatstone bridge.

[0011] According to another embodiment, it is provided that the sensor medium is a metal or a metal alloy.

[0012] According to another embodiment, it is provided that the piezoresistive semiconductor is silicon. In other words, at least the sensor area of the substrate of the hydrogen sensor is composed of silicon.

[0013] Furthermore, it is specifically provided that the sensor medium includes palladium, yttrium, scandium, lanthanides, actinides, tungsten oxide, and / or vanadium oxide.

[0014] According to another embodiment, it is provided that the sensor medium is a palladium alloy, or a mixture, alloy, or compound containing one or more of the following materials: palladium, yttrium, scandium, lanthanides, actinides, tungsten oxide, and vanadium oxide.

[0015] According to another advantageous embodiment, it is provided that the sensor medium is an alloy composed of palladium and gold (Pd x Au y alloy), or an alloy composed of palladium and nickel (Pd x Ni y alloy), wherein, in particular, the portion of gold or nickel is between 0.5 at% and 50 at% (Pd 0.5 Au 99.5 alloy to Pd 50 Au 50 alloy, or Pd 0.5 Ni 99.5 alloy to Pd 50 Ni 50 alloy).

[0016] The sensor medium absorbs hydrogen, where hydrogen atoms are introduced into the atomic lattice of the sensor medium at interstitial sites. This applies in particular to the use of palladium or palladium alloys as the sensor medium and to all other materials mentioned.

[0017] During the introduction of hydrogen atoms into the atomic lattice, various physical effects that are known per se occur. However, one of these effects that is not specifically used in the hydrogen sensor according to the present invention is the α-phase to β-phase transition. This is also the case when using palladium or palladium alloys as the sensor medium. The α-phase is opaque on the metal, while the β-phase of palladium is transparent, or at least more transparent than the α-phase. This phase transition can be used to optically measure the hydrogen concentration in palladium used as the sensor medium. However, this has the technical disadvantage that the α-phase to β-phase transition must occur before the measurement signal can be obtained. The same applies to measurements based on the physical effect that the α-phase is conductive on the metal and the β-phase is semiconductive. The phase transition must also occur in sensors using this effect. For example, the α-phase in palladium exists up to a hydrogen concentration of 1.68 at%. This means that in the case of such sensors using the above phase transition, low hydrogen concentrations cannot be measured, or are difficult to measure.

[0018] The above-mentioned technical disadvantages are advantageously overcome by using a sensor according to aspects of the present invention. For example, when palladium is in the alpha phase, i.e., at low hydrogen concentrations, a volume change of the sensor medium occurs due to the hydrogen atoms introduced at the gaps. This change in volume associated with a change in the lattice constant leads to a mechanical strain on the exposed substrate of the sensor membrane. This strain can be measured using the piezoresistive effect in the substrate. Therefore, using a sensor according to aspects of the present invention, low hydrogen concentrations can be detected with high accuracy.

[0019] The above physical and technical effects are explained using palladium only as an example. They also occur to a greater or lesser extent in the other materials mentioned.

[0020] A further development of the hydrogen sensor is advantageous in that the sensor medium, which is designed as a thin film, has a film thickness of less than 500 nm, and in particular a film thickness of between 5 nm and 100 nm, and more particularly a film thickness of between 5 nm and 20 nm.

[0021] The above-mentioned film thicknesses have proven to be advantageous in practice. On the one hand, with sensor films having a film thickness in the indicated range, sufficiently large mechanical strains can be generated so that measurable effects occur in the sensor region of the substrate. On the other hand, the effort required to produce the film is manageable. Delamination of the applied sensor film, which is usually observed with large film thicknesses, does not occur.

[0022] Furthermore, the hydrogen sensor according to another embodiment is further developed in that the sensor medium is a thin film produced by means of sputtering deposition.

[0023] As an alternative to sputter deposition, which is preferably produced by means of DC magnetron sputter deposition, other thin film techniques can be used, such as physical vapor deposition, laser deposition, etc.

[0024] According to another embodiment, the hydrogen sensor is further developed in that the covering film is present on the upper part of the sensor medium. The covering film is arranged on the surface of the sensor medium which, in the absence of the covering film, is in contact with the environment. The covering film is permeable to hydrogen. For example, the covering film can be a SiO2 film produced by CVD (chemical vapor deposition). The covering film can be used as a protective film against aggressive components in the environment. It can also be used to coordinate the elastic mechanical properties of the substrate and the hydrogen absorbing film consisting of the sensor medium.

[0025] Furthermore, the object is solved by a measuring device for measuring the hydrogen concentration in an environment, the measuring device comprising a measuring cell, wherein the measuring device is developed as a hydrogen sensor according to one or more of the aforementioned embodiments, the sensor medium of the hydrogen sensor being connected to the environment, wherein the measuring cell is configured to measure the ohmic resistance of the substrate, in particular the ohmic resistance in a sensor region of the substrate, and to determine the hydrogen concentration in the environment from the value of the measured ohmic resistance.

[0026] In fact, in such a measuring device, the ohmic resistance or the difference thereof of a resistance formed or present in the sensor region of the substrate is used, for example, a Wheatstone bridge circuit is used, to infer the mechanical strain introduced into the substrate by the sensor film. The volume expansion / change of the sensor film that causes the mechanical strain can be inferred from the value of the mechanical strain. The change in volume is in turn directly related to the amount of hydrogen absorbed by the sensor medium. Under the valid assumption that the hydrogen concentration in the sensor medium is in equilibrium with the environment in which the hydrogen concentration is measured, the hydrogen concentration present in this environment can be directly inferred using the measured ohmic resistance.

[0027] In addition, the same or similar advantages as those already mentioned above with respect to the hydrogen sensor apply to the measuring device.

[0028] In addition, this object is solved by a method for producing a hydrogen sensor according to one or more of the foregoing embodiments, wherein the method is developed in that a sensor medium is deposited as a thin film on a substrate by means of sputter deposition, in particular by means of magnetron sputter deposition or by physical vapor deposition.

[0029] It has been proven that producing the sensor layer by means of sputter deposition is an efficient method.

[0030] This object is also solved by a method for measuring the hydrogen concentration in an environment, and the method is developed by the following steps:

[0031] - Exposing a hydrogen-absorbing sensor medium to the environment, wherein the sensor medium is applied as a thin film in the sensor region of the substrate, and at least the sensor region of the substrate is a piezoresistive semiconductor,

[0032] - Measuring the ohmic resistance of the substrate, in particular the ohmic resistance within the sensor region of the substrate,

[0033] - Determining the hydrogen concentration in the environment from the value of the measured ohmic resistance.

[0034] The same or similar advantages as those already mentioned with respect to the hydrogen sensor also apply to the method for measuring the hydrogen concentration.

[0035] According to other embodiments, the following materials are used as piezoresistive semiconductors, and the k value of the materials is ≥2, particularly ≥5, more particularly ≥10, and even more particularly ≥25. In the context of this specification, it should be understood that the k value is the proportionality factor between the length change related to expansion and the initial length, and the resistance change related to expansion and the initial resistance, on the resistance measurement path of the semiconductor. This relationship is also described by the following equation:

[0036] ΔR / R = k * ΔL / L

[0037] In the above equation, R is the ohmic resistance of the piezoresistive semiconductor in the sensor area measured along the length L. ΔR is the change in the ohmic resistance caused by the change in length ΔL. The change in length ΔL is the result of the deformation of the substrate in the sensor area, which is the result of the mechanical strain introduced into this area by the sensor medium.

[0038] Other features of the present invention will become apparent from the description of the embodiments according to the present invention, as well as from the claims and the drawings. Embodiments according to the present invention can implement a single feature or a combination of multiple features.

[0039] Within the scope of the present invention, features designated as "special" or "preferred" are understood as optional features. Description of the Drawings

[0040] Without limiting the general concept of the present invention, the present invention will be described below based on exemplary embodiments with reference to the drawings. Therefore, for all details according to the present invention that are not described in more detail in the context, we explicitly refer to the drawings. In the figures:

[0041] Figure 1 A measuring device with a hydrogen sensor is shown in a schematically simplified cross-sectional view, where the sensor medium is not exposed to the hydrogen concentration.

[0042] Figure 2 The measuring device is also shown in a schematically simplified cross-sectional view, where the sensor medium is exposed to the hydrogen concentration.

[0043] Figure 3 A schematically simplified plan view of the hydrogen sensor is shown.

[0044] Figure 4 A schematically simplified perspective view of another hydrogen sensor is shown.

[0045] Figure 5 A simplified circuit diagram of the circuit measuring the resistance within the sensor area of the piezoresistive semiconductor connected in the form of a Wheatstone bridge circuit is shown, and

[0046] Figures 6-8 An example of the measurement of different hydrogen concentrations in the environment depending on time using a hydrogen sensor according to an aspect of the present invention is shown. Detailed Description of the Invention

[0047] In the drawings, the same or similar elements and / or components are provided with the same reference numerals in each case; therefore, repeated descriptions will always be omitted.

[0048] In a schematically simplified cross-sectional view, Figure 1A measuring device 2 for measuring the hydrogen concentration in an environment 4 is shown. The environment 4 is in contact with another environment or atmosphere in which the hydrogen concentration is to be measured via a channel, connection, etc. For this purpose, appropriate measures are taken in the measuring device 2 to enclose the environment 4 and couple it to other measuring environments via a connection. The measuring device 2 further includes a measuring unit 6, such as a computer, a microcontroller or suitable software means implemented in another unit. The measuring unit is configured to measure the ohmic resistance and in this regard particularly includes, for example, a voltage source and a voltage measuring device. The measuring device 2 further includes a hydrogen sensor 8, and the hydrogen sensor 8 is configured to measure the hydrogen concentration in the environment 4. The hydrogen sensor 8 includes a substrate 10, and on the substrate 10, a hydrogen-absorbing sensor medium 14 in the form of a thin film is applied in a sensor area 12 that communicates with the environment 4.

[0049] By way of example only, the environment 4 is arranged on the lower side of the sensor area 12. It is also provided that the sensor medium 14 is applied as a thin film on the upper side of the sensor area 12, and the environment 4 in which the hydrogen concentration is measured is accordingly located on the upper side. However, it is necessary to stipulate that the sensor film 14 is only located on one side of the sensor area 12, for example configured as a silicon film, and this side, i.e., the sensor film communicates with the hydrogen-containing environment 4.

[0050] The sensor medium 14 is, for example, a metal or a metal alloy. The sensor medium 14 is, for example, a thin film including palladium, yttrium, scandium, lanthanides, actinides, tungsten oxide or vanadium oxide, and alloys and mixtures of these materials are also provided. In particular, it is stipulated that the sensor medium 14 is an alloy (PdAu) composed of palladium and gold or an alloy (PdNi) composed of palladium and nickel produced by co-sputtering deposition. For this purpose, the substrate 10 is introduced into the receiver of the sputtering system, and the PdAu or PdNi film is directly applied onto the substrate 10.

[0051] The film thickness d of the sensor medium 14 applied as a thin film onto the substrate 10 is, for example, less than 500 nm; further, for example, between 5 nm and 100 nm, and still further, for example, between 5 nm and 20 nm.

[0052] The sensor medium 14 is a material that changes its volume depending on the hydrogen concentration present in the sensor medium 14. This change in volume changes the mechanical strain introduced by the sensor medium 14 into the substrate 10 in the sensor area 12. Figure 1 and 2 The comparison between shows this effect.

[0053] Figure 2The measuring device 2 is also shown in a schematically simplified sectional view, in which the sensor medium 14 is exposed to the concentration of hydrogen present in the environment 4. This means that the sensor medium 14 expands, and this expansion causes a schematic bending in the sensor region 12 of the substrate 10, which is shown enlarged for clarity. If the sensor medium 14 were located on the opposite upper side of the substrate 10, it would bend in the opposite direction. At least the sensor region 12 of the substrate 10 is a piezoresistive semiconductor. The mechanical strain generated in the sensor medium 14 exposes at least the sensor region 12 of the substrate 10 to mechanical strain. These mechanical strains can be detected by the measuring unit 6 by using the piezoresistive effect. For this purpose, the measuring unit 6 contacts the substrate 10 via an electrical connection not shown in the figure.

[0054] The measuring unit 6 measures the ohmic resistance of the substrate 10, or respectively the change in the ohmic resistance, in at least the sensor region 12. The hydrogen concentration in the environment 4 can be inferred from the value of the measured ohmic resistance.

[0055] Figure 3 A schematically simplified plan view of the hydrogen sensor 8 is shown. As an example, Figure 3 A plan view of one side of the substrate 10 of the hydrogen sensor 8 with the sensor medium 14 applied in the sensor region 12 is shown. The substrate 10 includes contact pads 16, and the hydrogen sensor 8 contacts the measuring unit 6 via the contact pads 16. By means of the contact pads 16, the changes in the resistances R1, R2 connected in the form of a Wheatstone bridge circuit can be determined.

[0056] Figure 4 A schematically simplified perspective view of another hydrogen sensor 8 is shown. Inside the sensor region 12, this hydrogen sensor includes the sensor medium 14 applied from the lower side, as indicated by the arrow, and the sensor medium 14 is exposed to the hydrogen-containing environment 4. The mechanical strain caused by the sensor medium 14 is detected by means of resistance measurement, where there is a tapping at the contact pads 16, from which the ohmic resistance of the substrate 10 can be measured in the sensor region 12. Furthermore, the circuit can be, for example, in the form of a Wheatstone bridge circuit. In this way, the resistance between the contact pads 16a is measured as the resistance R2, and the resistance between the contact pads 16b is measured as the resistance R1.

[0057] Figure 5 A schematically simplified circuit diagram of the circuit of the ohmic resistances R1 and R2 connected in the form of a Wheatstone bridge circuit and measured, for example, inside the sensor region 12 of a piezoresistive semiconductor is shown. A voltage is applied to two terminals 18, and the diagonal voltage or bridge voltage Ua of the resistances R1, R2 connected as a voltage divider is measured. The change in the bridge voltage Ua is a measure of the change in the resistances R1, R2 and thus a direct measure of the mechanical strain introduced into the sensor region 12 of the substrate 10 by the sensor medium 14. Therefore, the hydrogen concentration in the environment 4 can be directly read from the bridge voltage Ua.

[0058] Figures 6 to 8 Shows an example of the measurement of different hydrogen concentrations in the environment 4 depending on the time t. The vertical axis represents the voltage in volts, while the horizontal axis represents the time t in seconds. The voltage shown on the vertical axis is a direct measure of the dominant mechanical strain in the sensor area 12 and thus also a direct measure of the hydrogen concentration in the environment 4. It can be clearly seen that the hydrogen sensor 8 provides very constant measurement results over a large measurement range. In Figure 6 , the measurement starts with a hydrogen concentration of 1 vol% in air and increases to 10 vol%. In Figure 7 the measurements shown, the hydrogen concentrations of 1 vol% and 10 vol% in air were also measured. Figure 6 and 7 the measurements in are each carried out for a maximum duration of 4400 seconds, corresponding to 73 minutes. Figure 8 Shows another measurement in which the hydrogen concentration is measured between 1 vol% and 10 vol%. The hydrogen sensor 8 exhibits high sensitivity, fast response behavior ( Figure 6 ), stable measurement values over time ( Figure 6 ) and highly reproducible measurement results ( Figure 7 and 8 ).

[0059] All the features mentioned, including the features obtained from the drawings themselves and the individual features disclosed in combination with other features, are considered essential to the present invention, either individually or in combination. Embodiments according to the present invention can be achieved by a single feature or a combination of multiple features.

[0060] Reference Signs

[0061] 2 Measuring device

[0062] 4 Environment

[0063] 6 Measuring unit

[0064] 8 Hydrogen sensor

[0065] 10 Substrate

[0066] 12 Sensor area

[0067] 14 Sensor medium

[0068] 16, 16a, 16b Contact pads

[0069] 18 Terminals

[0070] d Film thickness

[0071] R1, R2 Resistance

[0072] Ua Bridge Voltage

Claims

1. A measuring device (2) for measuring the hydrogen concentration in an environment (4), characterized in that, It includes a measuring unit (6) and a hydrogen sensor (8) for measuring the hydrogen concentration in the environment (4). The hydrogen sensor (8) includes a substrate (10), on which a hydrogen-absorbing sensor medium (14) in the form of a thin film is applied in a sensor area (12) in communication with the environment. The sensor medium (14) changes its volume depending on the hydrogen concentration in the sensor medium (14), and the change in volume causes a change in the mechanical strain introduced by the sensor medium (14) in the substrate (10). At least the sensor area (12) of the substrate (10) is a piezoresistive semiconductor. The sensor medium (14) is in communication with the environment (4). The measuring unit (6) is configured to measure the ohmic resistance within the sensor area (12) of the substrate (10), and the hydrogen concentration in the environment (4) is determined from the value of the measured ohmic resistance.

2. The measuring device (2) according to claim 1, characterized in that, The sensor medium (14) is a metal or a metal alloy.

3. The measuring device (2) according to claim 1 or 2, characterized in that, The sensor medium (14) includes palladium, yttrium, scandium, lanthanides, actinides, tungsten oxide, and / or vanadium oxide.

4. The measuring device (2) according to claim 1 or 2, characterized in that, The sensor medium (14) is a palladium alloy or a mixture, alloy, or compound containing one or more of the following materials: palladium, yttrium, scandium, lanthanides, actinides, tungsten oxide, and vanadium oxide.

5. The measuring device (2) according to claim 1 or 2, characterized in that, The sensor medium (14) is an alloy composed of palladium and gold, i.e., Pd x Au y alloy, or an alloy composed of palladium and nickel, i.e., Pd x Ni y alloy.

6. The measuring device (2) according to claim 5, characterized in that, The portion of gold or nickel is between 0.5 at% and 50 at%, namely Pd 0.5 Au 99.5 alloy to Pd 50 Au 50 alloy, or Pd 0.5 Ni 99.5 alloy to Pd 50 Ni 50 alloy.

7. The measuring device (2) according to claim 1, characterized in that, The sensor medium (14) is a thin film with a film thickness (d) of less than 500 nm.

8. The measuring device (2) according to claim 7, characterized in that, The film thickness (d) is between 5 nm and 100 nm.

9. The measuring device (2) according to claim 8, characterized in that, The film thickness (d) is between 5 nm and 20 nm.

10. The measuring device (2) according to claim 1, characterized in that, The sensor medium (14) is a thin film made by sputter deposition.

11. A method for measuring the hydrogen concentration in an environment (4), characterized by the following steps: - Expose the hydrogen-absorbing sensor medium (14) to the environment (4), where the sensor medium (14) in the form of a thin film is applied in the sensor area (12) of the substrate (10), and at least the sensor area (12) of the substrate (10) is a piezoresistive semiconductor. - Measure the ohmic resistance within the sensor area (12) of the substrate (10). - Determine the hydrogen concentration in the environment (4) from the value of the measured ohmic resistance.

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