JP6347976B2 - Hydrogen gas sensor and hydrogen gas detection method - Google Patents

Description

  The present invention relates to a gas detection element comprising a gas sensitive part, a detection electrode connected to the gas sensitive part, and hydrogen gas based on a change in resistance value of the gas sensitive part detected by the detection electrode. The present invention relates to a hydrogen gas sensor provided with a detection circuit for detection and a hydrogen gas detection method.

Conventionally, hydrogen is colorless, tasteless and odorless at room temperature, and is the lightest gas among gases, and has excellent properties such as diffusibility and reducibility, so it can be used in fields such as the semiconductor industry, electronics industry, and chemical industry. It is widely used in the field of space engineering such as for space rocket fuel.
In such a field, hydrogen is transported and stored in a liquefied state from the viewpoint of efficiency. However, since the boiling point of liquid hydrogen is as low as −250 ° C. or less, it is easily vaporized by the heat of the surrounding environment. In order to prevent this, the liquid hydrogen tank has a structure including a vacuum heat insulating layer around the liquid hydrogen container. There is also a triple-shell tank in which the outer shell of the vacuum heat insulating layer is filled with nitrogen or liquid nitrogen.

Hydrogen has a thermal conductivity about 7 times higher than that of air and is easy to transfer heat. Therefore, if even a small amount of liquid hydrogen leaks into the vacuum heat insulation layer, the heat insulation performance of the vacuum heat insulation layer is reduced by the vaporized hydrogen. There is a risk that external heat is conducted to the container and liquid hydrogen in the container is vaporized (boiled off).
In order to avoid such a risk, the leakage of hydrogen to the vacuum insulation layer is detected at an early stage, and if there is a leakage of hydrogen, measures such as exhausting the vacuum insulation layer immediately are taken. There is a need.

  On the other hand, the countermeasure when air outside the tank flows into the vacuum heat insulation layer is not so urgent. This is because the outer wall of the container is at a very low temperature, and even if a small amount of air flows in from the outside, it is liquefied by the outer wall, so that the heat insulating performance of the vacuum heat insulating layer does not rapidly decrease. .

From the viewpoint of avoiding the danger of boil-off of liquid hydrogen at an early stage, a technique for selectively detecting hydrogen gas in the vacuum heat insulating layer, that is, in a vacuum, is required.
Further, in the space transportation field and the manned space flight field, a technique for selectively detecting hydrogen gas in outer space, that is, in vacuum, is required in order to enhance safety.
In these applications, it is required to selectively detect hydrogen gas in a pressure range from atmospheric pressure to 1 × 10 ⁻³ Torr.

Common gas sensors include a catalytic combustion type gas sensor, a semiconductor type gas sensor, a MOSFET type gas sensor, and the like. However, these gas sensors are based on the principle of utilizing an oxidation reaction of a reducing gas such as hydrogen gas on the surface of the element, and therefore do not function in a vacuum, that is, in a low oxygen partial pressure environment.
As other gas sensors, there are an ultrasonic gas sensor, a gas heat conduction gas sensor, and a gas sensor using Raman scattering using an ultraviolet laser as a light source. However, the ultrasonic gas sensor does not propagate ultrasonic waves in a vacuum. In the gas heat conduction type gas sensor, in the pressure range of 0.1 Torr or less, the mean free path of gas molecules becomes larger than the element size, and the heat transfer efficiency of the elements due to the gas molecules decreases. In a system using an ultraviolet laser as a light source and utilizing Raman scattering, the molecular density is low in vacuum and the scattering intensity is reduced. That is, none is suitable for use in a vacuum.

On the other hand, there is a vacuum gauge for detecting gas molecules (gas) in vacuum.
Vacuum gauges are based on gas ionization phenomena, such as diaphragm vacuum gauges that measure pressure based on mechanical phenomena, Pirani vacuum gauges that measure pressure based on gas transport phenomena, thermocouple vacuum gauges, etc. It is classified according to the principle of measuring pressure, such as Penning vacuum gauge, ionization vacuum gauge, mass spectrometer, etc. that measure pressure, each vacuum gauge is limited in the pressure range that can be measured based on the respective principle,・ Used properly according to purpose. Since the principle and characteristics of each vacuum gauge are generally well-known techniques, prior art documents are not described.

However, these vacuum gauges capture the density of gas molecules in the gas phase, but they do not detect the type of gas separately except for a mass spectrometer. Further, the usable pressure range of the mass spectrometer is limited to a high vacuum pressure range of 1 × 10 ⁻⁵ Torr or less.

  As described above, there is no sensor that can selectively detect hydrogen gas in various pressure ranges with a single unit.

  The present invention has been made in view of the above problems, and has a simple structure and a hydrogen gas sensor and hydrogen that can selectively detect hydrogen gas with high sensitivity in various pressure ranges without depending on pressure. An object is to provide a gas detection method.

  In order to solve the above problems, a first characteristic configuration of a hydrogen gas sensor according to the present invention is a gas sensing element including a gas sensitive part and a sensing electrode connected to the gas sensitive part, and the sensing electrode senses A hydrogen gas sensor comprising a detection circuit for detecting hydrogen gas in a detection space in which an oxygen partial pressure is constant based on a change in the resistance value of the gas sensitive portion, wherein the gas sensitive portion has oxide ion conductivity. The detection circuit is based on a change in the resistance value of the gas sensitive part that changes in accordance with a change in the partial pressure ratio between the oxygen partial pressure and the hydrogen partial pressure. And a detector for detecting the hydrogen gas.

  Through inventor's diligent research, mixed conductive metal oxides such as cerium oxide and yttria-stabilized zirconia, which have both oxide ion conductivity and electron conductivity properties, and hydrogen gas as the gas to be detected In the meantime, it was found that a bulk control type reaction in which lattice oxygen in the bulk of the metal oxide is supplied to hydrogen gas proceeds, and the resistance value of the metal oxide is in a low resistance state.

  Cerium oxide, an example of a mixed-conducting metal oxide that has both oxide ion conductivity and electron conductivity, is a non-stoichiometric compound, and it is in the crystal lattice depending on the surrounding oxygen concentration (partial pressure). The oxygen vacancies generated by releasing oxygen into the detection space promote the movement of oxide ions and change the electron conductivity.

  At this time, if a highly reducible substance such as hydrogen gas exists in the detection space, the charge transfer of cerium oxide is facilitated by the adsorption of the hydrogen gas, and the electron conductivity changes drastically. This is because when hydrogen gas is adsorbed on the surface of cerium oxide, oxygen vacancies in the crystal lattice of hydrogen gas and cerium oxide attract oxygen atoms, which apparently increases the oxygen vacancies and facilitates the movement of electrons. .

  In this way, mixed oxide metal oxides that have both oxide ion conductivity and electron conductivity properties, such as cerium oxide, have electronic conductivity due to the movement of oxide ions caused by contact with hydrogen gas. Therefore, it is convenient as a gas sensitive part of the gas detection element.

That is, a detection circuit is connected to a gas detection element that constitutes a gas sensitive part using only the metal oxide as a component, and the detection part provided in the detection circuit responds to a change in the partial pressure ratio of oxygen partial pressure and hydrogen partial pressure. By monitoring the change in the resistance value of the gas sensitive part that changes in this manner, the hydrogen gas in the detection space can be detected.
As described above, a hydrogen gas sensor capable of selectively detecting hydrogen gas with high sensitivity can be realized with a simple structure.

  The second characteristic configuration is that the detection circuit calculates a concentration of hydrogen gas detected based on the concentration storage unit in which a hydrogen gas concentration table corresponding to the partial pressure ratio is stored in advance, and the partial pressure ratio. The point is that it has a part.

  The slope of the resistance value relative to the hydrogen partial pressure ratio in the detection space where oxygen is present at a constant partial pressure is affected by the oxygen partial pressure ratio. That is, there is a proportional relationship between the resistance value and the hydrogen partial pressure / oxygen partial pressure. Accordingly, the hydrogen gas concentration can be calculated from the change in the resistance value of the gas sensitive unit by previously having a hydrogen gas concentration table corresponding to the partial pressure ratio in the concentration storage unit.

  A characteristic configuration of the hydrogen gas detection method according to the present invention is a hydrogen gas detection method for detecting hydrogen gas in a detection space where the oxygen partial pressure is constant, and both the oxide ion conductivity and the electron conductivity are detected in the detection space. Detecting a resistance value of the gas sensitive part detected by the detection electrode of a gas detection element comprising a gas sensitive part comprising only a metal oxide and a detection electrode connected to the gas sensitive part And a step of detecting hydrogen gas based on the change in the resistance value. According to the gas detection method, hydrogen gas is selectively selected in various pressure ranges without depending on pressure. It became possible to detect.

Illustration of gas detection element Illustration of hydrogen gas sensor Explanatory diagram of heater control circuit Graph showing the relationship between sensor temperature and sensor resistance in vacuum Graph showing the relationship between air pressure, heater current value and heater temperature Explanatory drawing of experimental equipment Graph showing the relationship between atmospheric pressure and sensor resistance of each gas Graph showing the relationship between hydrogen partial pressure and sensor resistance at each atmospheric pressure Graph showing the relationship between hydrogen partial pressure and sensor resistance at each oxygen partial pressure The graph which shows the relationship between the partial pressure ratio of hydrogen partial pressure and oxygen partial pressure, and sensor resistance value in each oxygen partial pressure Graph showing the relationship between hydrogen concentration and sensor resistance when oxygen concentration is different under atmospheric pressure

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view of a gas detection element 1 provided in a hydrogen gas sensor 100 according to the present invention.
The gas detection element 1 includes a pair of detection electrodes 4a and 4b formed on the upper surface of the insulating substrate 3, and both oxide ion conductivity and electron conductivity provided so as to cover the detection electrodes 4a and 4b. A gas sensitive layer 2 composed only of cerium oxide, which is an example of a metal oxide, and a thin film heater 5 on the lower surface of the insulating substrate 3, and a change in the resistance value of the gas sensitive layer 2 detected by the sensing electrode 4 Based on the above, hydrogen gas as a gas to be detected is detected.

  The insulating substrate 3 can be preferably applied to those used in conventional substrate type gas detection elements, and the size, shape, etc. are not particularly limited. The material of the insulating substrate 3 may be an insulating material, and for example, alumina, silica, glass, etc. can be applied. Among these, the use of alumina as the insulating substrate 3 is because the surface is not completely smooth and has nano-order irregularities, and therefore, the bonding with the detection electrodes 4a and 4b and the thin film heater 5 can be strengthened by the anchor effect. ,preferable.

  As the detection electrodes 4a and 4b, those used in conventional gas detection elements can be preferably applied. The shape of the detection electrodes 4a and 4b is not particularly limited. Although FIG. 1 shows an example in which a pair of comb-shaped detection electrodes 4a and 4b is provided, other shapes such as a parallel plate type and a spiral type can be adopted. The material of the detection electrodes 4a and 4b is not particularly limited, and for example, a noble metal such as platinum or gold, a platinum palladium alloy, or the like can be provided by vapor deposition or the like. In particular, platinum is an extremely durable material and can be preferably applied to the detection electrodes 4a and 4b.

  In the present embodiment, the inter-electrode distance between the pair of detection electrodes 4a and 4b is 5 to 100 μm. Since cerium oxide constituting the gas sensitive layer 2 is a high resistance material, the distance between the electrodes is set shorter than that of the conventional gas sensing element 1.

  The gas sensitive layer 2 is composed of only cerium oxide as a component, and is provided so as to cover the pair of detection electrodes 4a and 4b. The gas sensitive layer 2 may be a mixed conductive metal oxide having both the properties of oxide ion conductivity and electron conductivity. For example, only yttria-stabilized zirconia may be used as a component.

  The thickness of the gas sensitive layer 2 is set in the range of 10 to 100 μm. When the thickness of the gas sensitive layer 2 is smaller, a change in the resistance value of the gas sensitive layer 2 due to the reaction between hydrogen gas and the metal oxide constituting the gas sensitive layer 2 occurs at a position closer to the detection electrode. For this reason, the sensitivity to hydrogen gas can be increased, but if the thickness of the gas sensitive layer 2 is made thinner than 10 μm, uniform film formation becomes difficult. On the other hand, if the thickness of the gas sensitive layer 2 is greater than 100 μm, cracks and the like are likely to occur due to uneven distribution of internal stress in the film and heating stress, and the conductivity in the gas sensitive layer 2 may be reduced. There is a fear. Therefore, by setting the thickness of the gas sensitive layer to 10 to 100 μm, both sensitivity and durability can be improved.

In the present embodiment, the gas detection element 1 is formed by forming a platinum thin film comb-shaped detection electrodes 4a and 4b on the upper surface of an alumina ceramic insulating substrate 3 and forming a thin film heater 5 on the lower surface by sputtering. 4b is integrally manufactured by applying and sintering cerium oxide on 4b.
The gas detection element 1 manufactured in this way is small and has low power consumption, and a redundant design including a plurality of gas detection elements 1 in the same space is possible. For example, one of the plurality of gas detection elements 1 can be used as the main gas detection element 1 and the other can be used to evaluate the soundness of the main gas detection element 1. In addition, when installing such a gas detection element 1 in a vacuum such as a vacuum heat insulating layer, it is difficult to replace the gas detection element 1 when a failure occurs. Therefore, a plurality of gas detection elements 1 are installed in advance as spares. You can also

Cerium oxide, a metal oxide with both oxide ion conductivity and electron conductivity, is a non-stoichiometric compound, and has the characteristic of occluding and releasing oxygen in the crystal lattice according to the surrounding oxygen concentration (partial pressure). Oxygen vacancies generated by releasing oxygen into the detection space promote the movement of oxide ions, changing the electron conductivity.
At this time, if a highly reducible substance such as hydrogen gas exists in the detection space, the charge transfer of cerium oxide is facilitated when the hydrogen gas reaches the gas sensitive layer 2, and the electron conductivity changes drastically. .

  The reaction between cerium oxide and hydrogen gas proceeds by a so-called bulk control type reaction mechanism. According to this bulk control type reaction mechanism, the reaction with the hydrogen gas extends not only to the surface of the gas sensitive layer 2 but also to the bulk. That is, the hydrogen gas does not react with the adsorbed oxygen existing on the surface of the gas sensitive layer 2 as in a semiconductor gas sensor well known as a conventional gas sensor, but the hydrogen gas is a gas sensitive component containing only cerium oxide. Reacts with lattice oxygen present in the bulk of layer 2. Electrons flow in the gas sensitive layer 2 by diffusing oxygen defects generated in the bulk by this reaction.

  When hydrogen gas is adsorbed on the surface of cerium oxide, oxygen vacancies in the crystal lattice of hydrogen gas and cerium oxide attract oxygen atoms, and oxygen vacancies appear to increase, and electrons move in the gas sensitive layer 2. It becomes easy, that is, the resistance value of the gas sensitive layer 2 is in a low resistance state.

  Based on the above characteristics, the oxide ion conductivity and the electron conductivity of cerium oxide reflect the movement of oxide ions generated by contact with hydrogen gas in the electron conductivity. It is convenient as a material. As described above, the gas detection element 1 that can selectively detect the hydrogen gas with high sensitivity in various pressure ranges without depending on the pressure can be realized with a simple structure.

FIG. 2 shows a hydrogen gas sensor 100 including the gas detection element 1, the detection circuit 10, and the heater control circuit 20.
The detection circuit 10 is connected in parallel to the load resistance R0 connected in series with the gas detection element 1, the power supply unit E that applies a predetermined operating voltage to the gas detection element 1 and the load resistance R0, and the load resistance R0. And a voltmeter V for detecting an operating voltage applied to the load resistor R0.
As described above, the gas detection element 1 has a low resistance value when hydrogen gas is present in the detection space. Therefore, by detecting the operating voltage applied to the load resistor R0 with the voltmeter V, the resistance value of the gas detection element 1 or the change in the output can be indirectly calculated.

The thin film heater 5 is provided to maintain the operating temperature of the gas detection element 1.
The material of the thin film heater 5 is not particularly limited, and for example, a noble metal such as platinum or gold, a platinum palladium alloy, or the like can be provided by vapor deposition or the like. In particular, platinum is an extremely durable material and can be preferably applied to the thin film heater 5.

  FIG. 3 shows the heater control circuit 20 of the thin film heater 5. The heater control circuit 20 controls the thin film heater 5 so that the temperature of the gas detection element 1 becomes a predetermined temperature in the range of 200 ° C. to 1000 ° C., preferably in the range of 400 ° C. to 650 ° C.

The electron mobility of the cerium oxide constituting the gas sensitive layer 2 depends on the operating temperature of the gas sensing element 1, and the operating resistance decreases and the electron conductivity increases as the gas sensing element 1 is operated at a higher temperature. However, when the operating temperature of the gas detection element 1 is higher than 1000 ° C., it is preferable because the mobility of oxide ions is excessively increased, the electron conductivity is excessively improved, and the change width when hydrogen is detected is not sufficient. Absent.
On the other hand, considering the heat resistance of cerium oxide constituting the gas sensitive layer 2, the operating resistance value increases and the electron conductivity decreases as the gas detecting element 1 is operated at a lower temperature, but the lifetime is extended. However, if the operating temperature of the gas detection element 1 is lower than 200 ° C., the mobility of oxide ions is limited, and the electron conductivity is excessively lowered, which is not preferable.

FIG. 4 shows the relationship between the operating temperature and the operating resistance value of the gas sensing element 1 when hydrogen gas is present and when it is not present in a vacuum of 1.2 × 10 ⁻⁴ Torr.
As can be seen from FIG. 4, when hydrogen gas is present, the change ratio of the operating resistance value tends to increase as the operating temperature decreases. From the viewpoint of life, a temperature range of 400 ° C. to 650 ° C. is preferably employed as described above.

By the way, when the atmospheric pressure in the detection space changes, the heat conduction effect of the gas changes, so the temperature of the gas detection element 1 changes.
FIG. 5 shows the relationship between the atmospheric pressure, the heater current, and the heater temperature.
As shown in FIG. 5, when the atmospheric pressure in the detection space decreases, the operating temperature of the thin film heater 5 increases. This is because the lower the pressure, the fewer gas molecules that serve as a heat transfer medium. Conversely, when the atmospheric pressure in the detection space increases, the operating temperature of the thin film heater 5 decreases. This is because the higher the pressure, the more gas molecules that become the heat transfer medium. Therefore, the atmospheric pressure change in the detection space can be detected by monitoring the operating temperature of the thin film heater 5.

In other words, if the operation resistance value of the thin film heater 5 is made constant so that the operating temperature of the thin film heater 5 does not change according to the change in atmospheric pressure, the thin film heater 5 is reduced when the atmospheric pressure in the detection space decreases. The operating current may be small. Conversely, when the atmospheric pressure in the detection space increases, the operating current of the thin film heater 5 increases. That is, by monitoring the change in the operating current value of the thin film heater 5 and correcting the environmental temperature in combination with a separately provided temperature sensor for measuring the temperature of the detection space, a change in the atmospheric pressure in the detection space can be detected. become. The operating voltage value applied to the thin film heater 5 may be monitored.
In addition to the thermistor and resistance temperature detector, the temperature sensor to be used in combination also includes the insulating substrate 3 of the gas detection element 1 and the same insulating substrate 3 sealed with air or inert gas in a sealed container. can do. In this case, since the temperature characteristics of the gas detection element 1 and the temperature sensor are close, correction is easy.

Returning to FIG. 3, a feedback circuit having a Wheatstone bridge is used for the heater control circuit 20 in order to keep the operating resistance value of the thin film heater 5 constant.
The resistors R1 and R3 constituting the Wheatstone bridge have the same resistance value, and the resistors R2 and the thin film heater 5 have the same resistance value.
The unbalanced voltage across these resistors is differentially input to the operational amplifier A1, the output is received by the emitter follower of the power transistor TR1, and applied to the Wheatstone bridge so that the resistance value of the thin film heater 5 is always the same value as the resistor R2. Adjust the operating voltage automatically.
In addition, as a monitor part which monitors the operating current or operating voltage value applied to the thin film heater 5, a voltmeter connected in parallel to the thin film heater 5 or an ammeter connected in series to the thin film heater 5 is used. .

Using the hydrogen gas sensor 100 including the gas detection element 1, the detection circuit 10, and the heater control circuit 20 configured as described above, various experiments relating to detection of hydrogen gas in a vacuum were performed.
In the experiment, a vacuum chamber 30 as shown in FIG. 6 was prepared.
The vacuum chamber 30 has a capacity of about 30 L, and leaks 31 and 32 for supplying hydrogen, oxygen, and nitrogen into the vacuum chamber 30, a vent valve 33, and a vacuum in the vacuum chamber 30. The vacuum pump 34 is provided, and the gas detection element 1 is installed inside.

  First, an experiment was conducted to confirm the state of the resistance value of the gas detection element 1 when hydrogen gas is present in vacuum and when hydrogen gas is not present.

In the experiment, after supplying air into the vacuum chamber 30, the resistance value _RA of the gas detection element 1 when the vacuum pump 34 is started and the pressure is reduced from about 1 × 10 ² Torr to 1 × 10 ⁻⁵ Torr, After supplying oxygen into the vacuum chamber 30, the vacuum pump 34 is started and the resistance value R _O of the gas detection element 1 when the pressure is reduced from about 1 Torr to 1 × 10 ⁻⁵ Torr and nitrogen is supplied into the vacuum chamber 30. after, the resistance value R _N of the gas sensing element 1 when the vacuum from about 1Torr start the vacuum pump 34 to 1 × 10 ^-5 Torr, after supplying the hydrogen gas into the vacuum chamber 30, a vacuum pump 34 The resistance value _RH of the gas detection element 1 was obtained when the operation was started and the pressure was reduced from about 1 Torr to 1 × 10 ⁻⁵ Torr. The result is shown in FIG.

As can be seen from FIG. 7, the resistance value _RA gradually decreases in a vacuum. This is because oxygen in the crystal lattice of cerium oxide constituting the gas sensitive layer 2 is released according to the surrounding oxygen concentration (partial pressure), and the resulting oxygen vacancies promote the movement of oxide ions. It is thought that.
Similar trends were confirmed even resistance R _O, and the resistance value R _N when the supply nitrogen when oxygen was supplied to the vacuum chamber 30. The resistance value R _A, the resistance value R _O, from a comparison of the resistance value R _N, the gas-sensitive layer 2 it can be seen that also in response to the oxygen.
On the other hand, the resistance value R _H when supplying hydrogen gas into the vacuum chamber 30, the resistance R _A, the resistance value R _O, as compared with the resistance value R _N, and greatly reduced to about 1/100, the tendency It was confirmed that the inside of the vacuum chamber 30 was maintained until the pressure was reduced to 1 × 10 ⁻⁵ Torr.
This is because the hydrogen gas is not oxidized on the surface of the cerium oxide constituting the gas sensitive layer 2, but oxygen in the lattice is attracted by the adsorption of the hydrogen gas to the cerium oxide, and the movement of oxygen vacancies is easy. This is considered to be the result.

  As described above, the electronic conductivity of cerium oxide constituting the gas sensitive layer 2 is influenced by oxygen in the environment. Therefore, for the purpose of evaluating the influence of oxygen on the resistance value characteristic with respect to the hydrogen partial pressure, an experiment for confirming the tendency of the electric resistance value when nitrogen is supplied into the vacuum chamber 30 and the hydrogen partial pressure ratio is changed under no oxygen. Went.

In this experiment, nitrogen was supplied from the leak valve 31 into the vacuum chamber 30 while starting the vacuum pump 34, and the pressure was adjusted to a predetermined equilibrium pressure (1 × 10 ⁻² Torr).
Next, hydrogen gas is supplied from the leak valve 32 step by step so that the predetermined conditions (hydrogen partial pressure ratios of about 1%, about 4%, 51%, 83%) are obtained, and the resistance value when the pressure is stabilized is I got it.
The same experiment was performed under different pressure conditions (1 × 10 ⁻³ Torr, 1 × 10 ⁻⁴ Torr). When the equilibrium pressure is 1 × 10 ⁻³ Torr, hydrogen gas is supplied from the leak valve 32 in a stepwise manner (hydrogen partial pressure ratios of 3.8%, 35%, 66%, 83%). The resistance value was obtained when the pressure was stabilized. When the equilibrium pressure is 1 × 10 ⁴ Torr, hydrogen gas is supplied from the leak valve 32 in a stepwise manner (hydrogen partial pressure ratio 9%, 33%, 53%, 68%, 80%). The resistance value was obtained when the pressure was stabilized. The result is shown in FIG.

As can be seen from FIG. 8, in a nitrogen balance environment that is in an oxygen-free state, even if the water level partial pressure ratio is low, a large change in resistance value is confirmed as compared with the case where hydrogen gas is not present. The resistance values with respect to the hydrogen partial pressure ratio are substantially the same regardless of the pressure conditions of 1 × 10 ⁻² Torr, 1 × 10 ⁻³ Torr, and 1 × 10 ⁻⁴ Torr. It can be seen that it reacts with hydrogen gas.
Note that the detection limit concentration of hydrogen gas obtained by extrapolation with an approximate expression based on the pressure condition data of 1 × 10 ⁻³ Torr is estimated to be 1000 ppm or less.
In the gas detection element 1 including the gas sensitive layer 2 composed of only cerium oxide as a component, the change in electron conductivity accompanying the adsorption of hydrogen gas is used in a vacuum, so that the hydrogen gas can be detected with high sensitivity. it can.

Next, an experiment was conducted to confirm the influence of the difference in oxygen partial pressure ratio on the detection of hydrogen gas under a certain vacuum environment.
For the purpose of evaluating the influence of the oxygen partial pressure ratio on the resistance value, oxygen is supplied into the vacuum chamber 30 so as to have a partial pressure ratio different from that of nitrogen, and the electric resistance value when the hydrogen partial pressure ratio is changed at each oxygen partial pressure ratio. An experiment was conducted to confirm the tendency.

In this experiment, nitrogen of each oxygen concentration (oxygen partial pressure ratio 1%, 5%, 10%, 50%) is supplied from the leak valve 31 into the vacuum chamber 30 while the vacuum pump 34 is started, and a predetermined equilibrium pressure is obtained. It adjusted to (1 * 10 < ^-3 > Torr).
Next, hydrogen gas was supplied from the leak valve 32 step by step so as to satisfy a predetermined condition, and a resistance value when the pressure was stabilized was obtained. The result is shown in FIG.

  As can be seen from FIG. 9, as a result of comparing the resistance value against hydrogen gas under the environment of each oxygen partial pressure ratio, it was confirmed that the slope of the resistance value relative to the hydrogen partial pressure ratio is affected by the oxygen partial pressure ratio.

  Therefore, as shown in FIG. 10, when the correlation between the resistance value and the hydrogen partial pressure / oxygen partial pressure was confirmed, it was confirmed that they were in a proportional relationship. That is, the resistance value depends on the partial pressure ratio between the hydrogen partial pressure and the oxygen partial pressure.

  From the results shown in FIGS. 9 and 10, it can be seen that the resistance value of the gas sensitive layer 2 maintained at a constant temperature is determined by the oxygen partial pressure and the hydrogen partial pressure without depending on the total pressure. If it does not depend on the total pressure, the same resistance change can be obtained even under atmospheric pressure. Therefore, an experiment was conducted to confirm the relationship between the resistance value at atmospheric pressure and the hydrogen concentration. The experimental method is that when the oxygen concentration (partial pressure) is adjusted to 0.1% or 1.0% in the nitrogen balance, and hydrogen is injected there to change the hydrogen concentration (partial pressure). The resistance value was obtained. The result is shown in FIG.

As can be seen from FIG. 11, the relationship between the hydrogen concentration and the resistance value has a characteristic that the hydrogen concentration changes abruptly with a certain threshold as a boundary, and the hydrogen concentration at which the resistance value rapidly decreases as the oxygen concentration increases. It can be seen that it becomes higher. The characteristics of the region where the hydrogen concentration is high after the resistance value suddenly decreases are consistent with the tendency confirmed in a vacuum of 1 × 10 ⁻² Torr, and support that the same characteristics are exhibited even at atmospheric pressure. It becomes. From this result, it is understood that the resistance value is determined by the oxygen partial pressure (concentration) and the hydrogen partial pressure (concentration) in the environment and does not depend on the total pressure.

  By detecting the voltmeter V provided in the detection circuit 10, hydrogen gas is detected based on the change in the resistance value of the gas sensitive layer 2 that changes according to the change in the partial pressure ratio between the oxygen partial pressure and the hydrogen partial pressure. Can do. That is, the voltmeter V constitutes a detection unit.

  Further, if the detection circuit 10 includes a concentration storage unit that stores a hydrogen gas concentration table corresponding to the partial pressure ratio in advance and a concentration calculation unit that calculates a hydrogen gas concentration based on the partial pressure ratio, The hydrogen gas concentration can be calculated based on the pressure ratio.

Incidentally, the resistance value of the gas sensitive layer 2 changes according to the atmospheric pressure of the detection space. For example, the resistance value of the gas sensitive layer 2 is higher in the atmosphere than in a vacuum. Therefore, if the load resistances arranged in series with the sensor have the same resistance value, the change width of the voltage monitored by the voltmeter V is different, and it becomes difficult to detect hydrogen gas.
Therefore, a variable type is adopted as the load resistance R0 connected in series with the gas detection element 1, and the voltmeter is switched by switching the resistance value of the load resistance R0 of the detection circuit 10 between the vacuum and the atmosphere. The change width of the voltage detected by V can be changed.
For example, by changing to 1 kΩ when in a vacuum, and changing to 500 kΩ when in the air, hydrogen can be detected accurately in the range of 0 to 4% vol%.

The change in atmospheric pressure that triggers switching of the resistance value of the load resistor R0 can be linked to the heater control circuit 20 of the thin film heater 5.
That is, by monitoring a voltmeter connected in parallel to the thin film heater 5 or an ammeter connected in series to the thin film heater 5, a change in atmospheric pressure can be detected.

  A resistance switching unit that switches the resistance value of the load resistor R0 based on the change in the atmospheric pressure detected in this way, and a predetermined atmospheric pressure threshold value that triggers switching of the resistance value of the load resistor R0 by the resistance switching unit, For example, a current value or voltage value corresponding to 1 Torr is set in advance, and when the current value or voltage value indicates that the atmospheric pressure is less than the predetermined atmospheric pressure threshold, the resistance value is decreased, If it shows that there exists, it should just control to raise resistance value. Note that the predetermined atmospheric pressure threshold value may be one or plural. By calculating a preferable resistance value in accordance with the atmospheric pressure and changing the resistance value in accordance with the atmospheric pressure of the detection space, the detection sensitivity of the gas to be detected can always be adjusted to an optimum value.

  As described above, it is possible to realize a hydrogen gas sensor and a hydrogen gas detection method that can selectively detect hydrogen gas with high sensitivity in various pressure ranges without depending on pressure, although the structure is simple.

  Each of the above-described embodiments is an example of the present invention, and the present invention is not limited by the description. The specific configuration of each part can be appropriately changed and designed within the range where the effects of the present invention are exhibited. Needless to say.

1 Gas detection element 2 Gas sensitive layer (gas sensitive part)
3 Insulating substrate 4 Detection electrode 10 Detection circuit 20 Heater control circuit 100 Hydrogen gas sensor

Claims (3)

A gas sensing element comprising a gas sensitive part and a sensing electrode connected to the gas sensitive part;
A hydrogen gas sensor comprising a detection circuit that detects hydrogen gas in a detection space that is vacuum and has a constant oxygen partial pressure based on a change in the resistance value of the gas sensitive part detected by the detection electrode,
The gas sensitive part is composed only of a metal oxide having both oxide ion conductivity and electronic conductivity,
The detection circuit includes a detection unit that detects the hydrogen gas based on a change in a resistance value of the gas sensing unit that changes in accordance with a change in a partial pressure ratio between the oxygen partial pressure and the hydrogen partial pressure. A hydrogen gas sensor. The detection circuit includes:
A concentration storage unit in which a hydrogen gas concentration table corresponding to the partial pressure ratio is stored in advance;
The hydrogen gas sensor according to claim 1, further comprising a concentration calculation unit that calculates a concentration of hydrogen gas detected based on the partial pressure ratio. A hydrogen gas detection method for detecting hydrogen gas in a detection space that is vacuum and has a constant oxygen partial pressure,
The detection of a gas detection element comprising a gas sensitive part composed only of a metal oxide having both oxide ion conductivity and electronic conductivity in the detection space and a detection electrode connected to the gas sensitive part Detecting a resistance value of the gas sensitive part detected by the electrode;
A hydrogen gas detection method comprising a step of detecting hydrogen gas based on a change in the resistance value.

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