JP5070627B2 - Gas sensor - Google Patents

Description

  The present invention relates to a gas sensor, and more particularly to a gas sensor that prevents abnormal operation due to miscellaneous gas or water.

A conventional gas sensor has a structure (see Patent Document 1) that includes a moisture absorbing member that has an action of absorbing and releasing water vapor that has entered the hydrogen detection unit, and water vapor in a space that houses the gas detection unit. Some have an adsorbent and have a structure (see Patent Document 2) having an action of evaporating adsorbed water vapor by heating with a heater.
JP 2007-040757 A JP 2006-284498 A

  However, the above gas sensor has problems such as the influence of output fluctuations due to miscellaneous gases other than hydrogen, the fact that the sensor tends to be large with many components, and the power consumption of the heating mechanism also increases at the same time. is there.

  Therefore, in view of the above-described problems, the present invention provides a gas sensor that can eliminate the influence of miscellaneous gas with a simpler structure than conventional ones, and prevent moisture from entering the gas detection unit to prevent abnormal operation. It is aimed.

  The gas sensor according to claim 1, which has been made to solve the above problems, includes housing members 11, 20 constituting a gas detection chamber 11 a, a gas detection unit 14 disposed in the gas detection chamber 11 a, The gas detection unit 14 is provided so as to cover the gas detection part 14 via the gap 22, and is filled in the gas detection / permeation membrane 16 for introducing only the detected gas in the outside air into the gas detection chamber, the gas detection chamber 11a, A support 17 having an inert gas 21 sealed by a gas separation permeable membrane 16, a cavity portion 17a covering the gas separation permeable membrane 16, and a plurality of vent holes 18 ventilating the cavity portion 17a; And a moisture removal layer 19 formed in the plurality of air holes 18.

  The invention described in claim 2 made to solve the above-mentioned problems is characterized in that, in the gas sensor described in claim 1, the moisture removing layer 19 is formed of a hydrophobic or hydrophilic material.

  The invention according to claim 3 made to solve the above-mentioned problems is characterized in that in the gas sensor according to claim 2, the hydrophobic or hydrophilic material is zeolite.

  According to a fourth aspect of the present invention, there is provided the gas sensor according to any one of the first to third aspects, wherein the housing member has an insulating film 12 formed thereon. A substrate 11 having a cavity portion 11a formed so that a part of the insulating film 12 becomes a diaphragm 12a at the center thereof, the gas detection portion 14 being formed on the diaphragm 12a, and the cavity portion 11a being the gas It is a detection chamber.

  According to a fifth aspect of the present invention, there is provided the gas sensor according to the fourth aspect, wherein the substrate 11 is a silicon substrate, and the gas detection unit 14 diffuses impurities into the silicon substrate 11. A resistance temperature detector comprising an impurity diffusion layer formed on the silicon substrate 11 or a metal thin film formed on the silicon substrate 11, wherein gas detection and heating of the gas separation permeable film 16 are performed simultaneously. To do.

  Note that the reference numerals in the description of the means for solving the above-described problems correspond to the reference numerals of the constituent elements in the following description of the embodiments of the invention, but these limit the interpretation of the claims. Not what you want.

  According to the present invention, a gas separation / permeation membrane for introducing only a sensed gas in the outside air into the gas detection section in the gas detection chamber is provided, and a gas to be sensed is provided in the gas separation / permeation membrane via the moisture removal layer. Since the outside air containing air is ventilated, it is possible to eliminate the influence of miscellaneous gas with a simpler structure than before, prevent water from entering the gas detection unit, and prevent abnormal operation due to water. Further, it can be used in an environment of −30 ° C. to + 120 ° C. In addition, a gas sensor can be obtained in which malfunction of sensor output due to moisture hardly occurs even when continuous in a humid atmosphere. Further, since the path from the gas flow path to the gas detection unit is short and has a simple structure, it is small and can respond quickly.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, a case where the gas sensor of the present invention is applied to a hydrogen sensor for a vehicle equipped with an FC (fuel cell) will be described.

  FIG. 1 is a schematic cross-sectional view showing a configuration of a gas sensor according to an embodiment of the present invention, FIG. 2 is a partial plan view, and FIG. 3 is a perspective view.

In the gas sensor 1, an insulating film 12 is formed on a substrate 11, and a heater 14 is formed as a gas detection unit on a diaphragm 12a located in a cavity 11a formed by anisotropic etching in the center of the substrate 11. Has been. A plurality of openings (holes) 12b are formed in the diaphragm 12a. The substrate 11 is, for example, silicon (Si). The insulating film 12 is a thin film made of, for example, silicon dioxide (SiO ₂ ) or silicon nitride (Si ₃ N ₄ ).

  The heater 14 is a resistance temperature detector composed of an impurity diffusion layer in which impurities such as boron are diffused, and functions as a gas detection unit. A technique using a resistance temperature detector as a gas detection unit is known, and is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-101156.

  A hydrogen permeable gas separation permeable membrane 16 is formed on the heater 14 so as to cover the heater 14 through the gap 22. The gas separation and permeable membrane 16 is made of a palladium alloy membrane or the like that can transmit only hydrogen. As a film material that can transmit only hydrogen, an alloy of palladium (Pd) and silver (Ag), silicon carbide (SiC), or the like is used in addition to a palladium (Pd) alloy. This gas separation permeable membrane 16 separates only hydrogen from the mixed gas in the atmosphere in which the gas sensor 1 is installed and guides it to the heater 14 which is a gas detection unit, eliminates the influence of miscellaneous gas, and improves the sensitivity and durability of the sensor. There is an effect to raise.

  A support body 17 is disposed on the insulating film 12. The support 17 is, for example, a glass structure having a hollow portion 17a in the center and formed with a plurality of fine ventilation holes 18, and is manufactured collectively from a glass wafer by an imprint technique or a fine cutting technique, It arrange | positions so that the heater 14 which is a gas detection part, and the gas separation permeable film 16 may be covered with the cavity part 17a. The dimension of the vent hole 18 is set to an order of a sufficient 1 millimeter, for example.

  A moisture removal layer 19 is formed in the vent hole 18. The moisture removal layer 19 has a role of preventing water droplets flowing through the pipe where the sensor is installed or generated by condensation from entering the heater 14 which is a gas detection unit. The moisture removal layer 19 is formed of zeolite or the like. As the zeolite, a hydrophilic material such as LTA (Linde Type A) zeolite, silicalite which is a hydrophobic zeolite, or the like can be used.

  The gas separation permeable membrane 16 is opposed to the heater 14 which is a gas detection part through a slight gap 22. The gas separation permeable membrane 16 is heated by the heater 14 and exhibits a hydrogen separation function. Due to this effect, the gas that permeates the gas separation permeable membrane 16 is almost completely only hydrogen gas. The hydrogen partial pressure on the cavity 17a side and the hydrogen partial pressure on the cavity 22 side are the same. The concentration of the permeated hydrogen gas is measured by a change in heat conduction using the heater 14 disposed on the perforated diaphragm 12a. The heater 14 also serves as a heater as a gas detection unit that detects the hydrogen concentration and a heater that heats the gas separation permeable membrane 16.

  The cavity 11a of the substrate 11 is fully filled with an inert gas (nitrogen, xenon, radon, etc.) whose thermal conductivity is considerably smaller than that of hydrogen gas in advance, and the gas separation / permeable membrane 16 and the substrate 11 (of the housing member) Sealed with a sealing body 20 (corresponding to a part of the housing member) joined to the part), whereby the cavity 11a serves as a gas detection chamber, and hydrogen that has permeated the inert gas. The concentration of hydrogen as the measurement target gas is detected from the change in thermal conductivity with the gas.

  Next, a method for manufacturing the gas sensor of FIG. 1 will be described with reference to FIGS. Each material is collectively manufactured in a wafer state by a MEMS process, an imprint process, or the like. The zeolite used as the moisture removal layer 19 is filled in the vent hole 18 by a method such as hydrothermal synthesis.

First, the silicon processing process shown in FIG. 4 will be described.
Step (A) The Si wafer (substrate 11) is cleaned.
Step (B) The surface is oxidized to form an SiO ₂ layer as an insulating film, and patterning is performed (11b, 11c).
Step (C) Impurities such as boron are diffused to form a heater pattern that becomes the heater 14 of the resistance temperature detector composed of the impurity diffusion layer.
Step (D) The SiO ₂ layer (11b) created in step (B) is removed.
Step (E) An insulating film 12 such as Si ₃ N _{4 which} is a material of the diaphragm is formed and patterned by a method such as CVD.
Step (F) A sacrificial layer 30 such as SiO _{2 serving as} a void of a gas separation / permeation membrane such as a palladium (Pd) alloy is formed and patterned.
Step (G) A gas separation / permeable membrane 16 such as a palladium (Pd) alloy is formed and patterned.
Step (H) A pad (electrode 15), a metal pattern (not shown) to be a temperature sensor, and the like are formed on the surface. The back surface is etched to form the cavity 11a and the diaphragm 12a.
Step (I) The sacrificial layer 30 is removed from the back surface, and a gap 22 is formed between the gas separation permeable membrane 16 and the gas separation permeable membrane 16.

Next, the glass processing / joining process shown in FIG. 5 will be described.
Step (A) The glass wafer (support 17) is cleaned.
Step (B) The cavity 17a is formed by a thermal imprint process.
Step (C) The air holes 18 are formed by a thermal imprint process or machining.
Step (D) Zeolite (moisture removal layer 19) is formed in the vent hole 18 to close the hole.
Step (E) In an inert gas atmosphere, glass (support 17) -silicon (substrate 11) -glass (sealing body 20) is bonded by an anodic bonding apparatus or the like. An anodic bonding device adheres glass and silicon in an arbitrary gas atmosphere such as air, nitrogen, and vacuum, applies temperature and voltage, diffuses cations in the glass, and generates electrostatic attraction. In addition to encouraging the chemical reaction, it is strongly bonded by chemical reaction. Therefore, when glass with a cavity and flat silicon are overlapped and joined in an inert gas atmosphere, the inert gas is contained in the glass cavity while being sealed.

  In the gas sensor manufactured in this way, the gas sensor 1 in a state where the heater 14 is energized so as to have a heating temperature (for example, 200 degrees) at which the constituent members are not damaged by heat is discharged from the fuel cell, for example. When placed in an atmosphere containing hydrogen, such as off-gas, a change in the thermal conductivity of hydrogen gas when contacting the heater 14 through the moisture removal layer 19 and the gas separation permeable membrane 16 formed in the vent hole 18. As a result, the resistance value of the heater 14 changes according to the hydrogen concentration.

  Here, the principle of detection of the hydrogen concentration will be described below. The heater 14 on the thin film is deprived of heat by the surrounding gas. As a result, the temperature (resistance value) of the heater 14 decreases. When hydrogen having high thermal conductivity is introduced into the cavity 11a as the gas detection chamber through the gas separation permeable membrane 16, more heat is generated than when the surrounding gas is only an inert gas (nitrogen). Is deprived of hydrogen gas. Thereby, the temperature of the heater 14 further decreases. Since the thermal conductivity of hydrogen is much higher than that of nitrogen or the like, the hydrogen concentration can be found by calculating the amount of heat taken from the heater. That is, (energy given to the heater 14) − (current heater 14 energy (corresponding to the temperature of the heater 14) = (stolen energy), and this stolen energy is energy transmitted to hydrogen. This is because it depends on the hydrogen concentration.

  There are various gases other than hydrogen which is a gas to be detected. However, since the sensor wants to measure only the concentration of a specific gas (hydrogen in this embodiment), in the present invention, a special metal filter (hydrogen separation / permeation membrane) that allows only hydrogen gas to pass through is used as the gas separation / permeation membrane 16. Use as In this embodiment, a palladium alloy is used.

  However, if only hydrogen is permeated, if the inert gas 21 is not sealed in the cavity 11a, the hydrogen concentration before permeation is always 100% regardless of whether the hydrogen concentration before permeation is 10% or 50%. As a result, the hydrogen concentration before permeation cannot be measured, and it does not become a densitometer.

  Therefore, an inert gas (nitrogen, xenon, radon, or the like) is previously filled in the hollow portion 11a serving as a gas detection chamber. Since this inert gas has the gas separation / permeation membrane (hydrogen separation / permeation membrane) 16, it can never go out of the cavity 11 a. The hydrogen that has permeated through the gas separation / permeation membrane (hydrogen separation / permeation membrane) 16 reaches equilibrium when the hydrogen partial pressures before and after permeation (hydrogen concentration in all gases) become equal, and the concentration does not change beyond that.

  Therefore, the amount of hydrogen gas mixed in the inert gas can be regarded as being equal to the hydrogen partial pressure of the pre-permeation gas. Of course, since the amount of the inert gas sealed in the cavity 11a is known, the amount of hydrogen gas can be found by subtracting the amount of the inert gas from the thermal conductivity of all the gases. Thereby, the density | concentration of only hydrogen gas can be measured, without receiving to the influence of other surrounding gas (miscellaneous gas).

  Based on the principle described above, the temperature (resistance value) of the heater 14 changes according to the hydrogen concentration. Therefore, for example, the hydrogen concentration can be detected by detecting the resistance value as a current change amount or a voltage change amount with a detection circuit (not shown) configured by a Wheatstone bridge including the resistance of the heater 14.

  Water vapor and condensed water adhering to the gas sensor 1 are removed by the moisture removal layer 19 not only when the heater 14 is energized but also when it is not energized, preventing water from entering and contacting the heater 14 serving as a gas detection unit. The

  Further, in synchronism with the energization of the heater 14 (or at an appropriate time), the substrate 11 is heated by a separately provided heater, whereby the entire gas sensor 1 is heated and absorbed and stored in the moisture removal layer. Water can be removed by evaporation.

  As described above, according to the present invention, the gas separation permeable membrane for introducing only the gas to be detected in the outside air into the gas detection portion in the gas detection chamber is provided, and the moisture separation layer is interposed in the gas separation permeable membrane. Since the outside air containing the gas to be detected is vented, the structure is simpler than before, eliminating the effects of miscellaneous gases, preventing moisture from entering the gas detector, and preventing abnormal operation due to water. it can. Further, it can be used in an environment of −30 ° C. to + 120 ° C. In addition, even when continuously used in a humid atmosphere, it is possible to obtain a gas sensor in which malfunction of sensor output due to moisture hardly occurs. Since batch production is possible by the MEMS process, it is inexpensive. Further, since the path from the gas flow path to the gas detection unit is short and has a simple structure, it is small and can respond quickly.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this, A various deformation | transformation and application are possible.

For example, in the embodiment described above, the insulating film 12, although more and then the silicon nitride (Si ₃ N _4), two-layer laminated silicon dioxide (SiO ₂₎ and silicon nitride (Si ₃ N ₄₎ is from bottom to top A structure may be employed, or a three-layer structure in which silicon dioxide (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), and hafnium oxide (HfO ₂ ) are sequentially laminated from the bottom may be employed.

  The heater 14 may be formed by forming a metal thin film such as platinum (Pt) on the substrate 11. In addition, when the heater 14 is formed of an impurity diffusion layer in which impurities such as boron are diffused into silicon, it is more advantageous in terms of heat resistance than a heater made of only a metal material such as platinum (Pt).

  Moreover, although the case where it applied to the hydrogen sensor was demonstrated in the above-mentioned embodiment, this invention is not limited to this, VOC sensor, CO sensor, etc. in which moisture permeation and contact with a gas detection part are a problem It can be applied to other gas sensors. The present invention is applicable not only to hydrogen sensors that measure the hydrogen concentration in piping of FC (fuel cell) -equipped vehicles, but also to other fuel cell systems including a stationary type that generates a large amount of water vapor.

It is a schematic sectional drawing which shows the structure of the gas sensor which concerns on embodiment of this invention. It is a fragmentary top view which shows the structure of the gas sensor which concerns on embodiment of this invention. It is a perspective view which shows the structure of the gas sensor which concerns on embodiment of this invention. (A)-(I) is a figure which shows the process of the manufacturing method of the gas sensor of FIG. (A)-(E) are figures which show the process of the manufacturing method of the gas sensor of FIG.

Explanation of symbols

1 Gas sensor 11 Substrate (part of housing member)
11a Cavity (gas detection chamber)
12 Insulating film 12a Diaphragm 14 Heater (gas detector)
16 Gas separation permeable membrane 17 Support 17a Cavity 18 Vent 19 Moisture removal layer 20 Sealed body (part of housing)
21 Inert gas 22 Void

Claims (5)

A housing member constituting a gas detection chamber;
A gas detector disposed in the gas detection chamber;
A gas separation and permeable membrane that is provided so as to cover the gas detection part via a gap, and introduces only the gas to be detected in the outside air into the gas detection chamber;
An inert gas filled in the gas detection chamber and sealed with the gas separation permeable membrane;
A support having a cavity covering the gas separation permeable membrane, and a plurality of vent holes ventilating the cavity;
A moisture removal layer formed in the plurality of ventilation holes;
A gas sensor comprising: The gas sensor according to claim 1, wherein
The gas sensor, wherein the moisture removing layer is formed of a hydrophobic or hydrophilic material. The gas sensor according to claim 2, wherein
The gas sensor according to claim 1, wherein the hydrophobic or hydrophilic material is zeolite. The gas sensor according to any one of claims 1 to 3,
The housing member includes a substrate on which an insulating film is formed, and a substrate in which a hollow portion is formed so that a part of the insulating film becomes a diaphragm at the center, and the gas detection unit is disposed on the diaphragm. A gas sensor formed, wherein the cavity is the gas detection chamber. The gas sensor according to claim 4, wherein
The substrate is a silicon substrate, and the gas detection unit is a resistance temperature detector heater formed of an impurity diffusion layer formed by impurity diffusion in the silicon substrate or a metal thin film formed on the silicon substrate. A gas sensor, wherein gas detection and heating of the gas separation permeable membrane are performed simultaneously.

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