Catalytic combustion type gas sensor, sensing element and compensating element for the same

A sensing element for catalytic combustion type gas sensor includes: a heat conducting layer (12); a heater coil (13) buried in the heat conducting layer (12); and a combustion catalyst layer (11) coating or carried by a surface of the heat conducting layer (12) and causing combustion of a detection target gas brought into contact therewith, wherein the combustion catalyst layer (11) and the heat conducting layer (12) are both made of a firing material whose main component is tin oxide. Preferably, the heat conducting layer (12) is made of fine powder of the firing material, the firing material being produced by firing at a higher temperature than a temperature for producing the firing material of the combustion catalyst layer (11). Accordingly, durability and waterproof performance of the gas sensor itself can be enhanced, and without the need to take measures such as special environment setting, mounting thereof on a FCV can be realized.

TECHNICAL FIELD

The present invention relates to a sensing element and a compensating element for a catalytic combustion type gas sensor detecting the leakage of various kinds of gases, and a catalytic combustion type gas sensor including these elements.

BACKGROUND TECHNOLOGY

As a sensor detecting flammable gas such as hydrogen gas and methane gas, a catalytic combustion type gas sensor has conventionally been known. As shown inFIG. 13, for instance, the catalytic combustion type gas sensor uses a sensing element20in which a heater coil23is buried in a heat conducting layer (catalyst carrier)22whose surface is coated with or carries a combustion catalyst layer21causing combustion of a detection target gas brought into contact therewith, and the heater coil23of the sensing element20is heated to a predetermined temperature in advance by supplying electricity therethrough, and when flammable gas comes into contact with the combustion catalyst layer21to burn, a change in resistance value of the heater coil23occurs due to a temperature increase caused by the combustion and is detected as voltage, whereby the presence of the flammable gas is detected (for example, see a patent document 1).

Further, the same patent document 1 describes a gas sensor as shown inFIG. 14in whose sensor main body25, a compensating element26(resistance value RC) is serially connected to the sensing element20(resistance value RD) in order to cancel the influence by a change in ambient temperature, and this series circuit is connected in parallel to a series circuit composed of two serial-connected fixed resistance elements27(resistance value R1) and28(resistance value R2), thereby forming a Wheatstone bridge circuit. Direct-current voltage is applied by a power source29between both ends of the parallel circuit, and output voltage Vout between a connection point a of the sensing element20and the compensating element26and a connection point b of the two resistance elements27,28is detected. As the compensating element26in this case, used is a element in which a heater coil having the same electric characteristic as that of the sensing element20is buried in a heat conducting layer coated with a compensating material layer instead of the combustion catalyst layer.

The aforesaid output voltage Vout of the Wheatstone bridge circuit is dependent on a balance of the electrify resistances (RDand RC) of the sensing element20and the compensating element26. In a clean atmosphere, the resistance value RDand the resistance value RCare determined in a balanced state between heat generation amounts of the heater coils in the sensing element20and the compensating element26and amounts of heat dissipated into the atmosphere, and the output voltage Vout shows a zero-point value. When the detection target gas comes into contact with the sensing element20, the temperature of the sensing element20increases due to catalytic combustion, and thus only the resistance value RDincreases, resulting in an increase in the output voltage Vout, and the target gas is detected based on an amount of this increase.

By the way, in recent years, the practical application of a fuel cell vehicle (hereinafter, abbreviated as “FCV”) which does not use fossil fuel such as gasoline and thus does not involve a possibility of causing air pollution due to exhaust gas has been started, and it is stipulated as compulsory that the FCV is equipped with a gas sensor for high-sensitivity detection of hydrogen leakage.

However, automobile components go through severe tests, for example, dew condensation is made to occur in the components and dew is frozen, or the components are repeatedly ON-OFF operated while exposed to 90° vapor. In all of these tests, moisture is given to the automobile components to check their durability, and components mounted on the FCV are also required to have the same durability.

The catalytic combustion type gas sensor is considered as one of sensors having an operation principle with high total adaptability as a hydrogen sensor for FCV, but cannot be said to have high waterproof performance because of its structure.

The reason for this will be briefly described. A sensing element and a compensating element in a conventional catalytic combustion type gas sensor has a cross-sectional structure as shown inFIG. 13, and the sensing element20has a sponge-like cross-sectional structure allowing the permeation of hydrogen so that hydrogen combustion activity can be obtained also in a thickness direction of the combustion catalyst layer21, and thus steam, minute water droplets, and the like can enter the combustion catalyst layer21.

Further, materials forming the combustion catalyst layer21are “tin oxide+iron oxide+platinum fine powder+palladium fine powder+others”, and the combustion catalyst layer21exhibits a high characteristic with respect to a retaining capability of water droplets and the like owing to a hydrophilic property of these materials.

The compensating material layer of the compensating element does not have a sponge-like cross sectional structure, but it has a hydrophilic property as in the sensing element because its constituent materials are “tin oxide+copper oxide+others”.

On the other hand, the heat conducting layers positioned on inner sides of the combustion catalyst layer and the compensating material layer of the sensing element and the compensating element have a dense structure and their constituent materials are “alumina+titania (titanium oxide)+boron nitride+bismuth oxide glass+others” and so on, and the heat conducting layers exhibit a hydrophobic property owing to these factors.

Therefore, these constituent layers peel off each other in a high-humidity environment due to a difference in hydrous property therebetween. In particular, when dew condensation occurs in the sensing element and the compensating element and the dew is frozen, the combustion catalyst layer and the compensating material layer drop from surfaces of the heat conducting layers. Consequently, the electrify resistance values of the sensing element and the compensating element change from initial values and accordingly, a zero point output value of the output voltage of the Wheatstone bridge circuit greatly fluctuates, resulting in error detection.

Therefore, some FCV makers take measures which involve high cost factors, such as setting a special environment for protecting a hydrogen sensor from water, and improvement in waterproof performance of the hydrogen sensor itself is required in order to promote wider use of the FCV.

As a technique for enhancing durability of such a conventional gas sensor, it has been proposed in, for example, a patent document 2, that a heater heating a detection target gas is provided adjacent to and on an upstream side of a gas sensor installation position in a channel through which the detection target gas flows, thereby preventing dew condensation in the gas sensor.

Patent document 1: JP H 3-162658A

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, preventing the occurrence of the dew condensation in the gas sensor by the heater heating the detection target gas is only a symptomatic treatment means even if this measure is adopted in the catalytic combustion type gas sensor and cannot enhance durability and waterproof performance of the catalytic combustion type gas sensor itself.

The invention was made in view of such a background, and its object is to enhance durability and waterproof performance of a catalytic combustion type gas sensor itself and to thereby make it possible to mount the catalytic combustion type gas sensor on a FCV without taking measures such as special environment setting. To attain the object, there is provided a sensing element and a compensating element for catalytic combustion type gas sensor which have high durability and waterproof performance, and there is further provided a catalytic combustion type gas sensor including the sensing element and the compensating element.

Means for Solving the Problems

A sensing element for catalytic combustion type gas sensor according to the invention is a sensing element which includes a heat conducting layer, a heater coil buried in the heat conducting layer, and a combustion catalyst layer coating or carried by a surface of the heat conducting layer and causing combustion of a detection target gas brought into contact therewith, and in order to attain the above objects, the combustion catalyst layer and the heat conducting layer are both made of a firing material whose main component is tin oxide.

Preferably, the heat conducting layer is made of fine powder of the firing material whose main component is tin oxide, the firing material being produced by the firing at a higher temperature than a temperature for producing the firing material of the combustion catalyst layer.

More preferably, at least one of platinum and palladium is added to the firing material of the heat conducting layer.

Desirably, the fine powder of the firing material forming the heat conducting layer is bound by bismuth oxide-based low-melting point glass.

A compensating element for catalytic combustion type gas sensor according to the invention is a compensating element which includes a heat conducting layer, a heater coil buried in the heat conducting layer, and a compensating material layer coating or carried by a surface of the heat conducting layer, and in order to attain the above objects, the compensating material layer and the heat conducting layer are both made of a firing material whose main component is tin oxide.

Preferably, the heat conducting layer is made of fine powder of the firing material whose main component is tin oxide, the firing material being produced by the firing at a higher temperature than a temperature for producing the firing material of the compensating material layer.

Desirably, the fine powder of the firing material forming the heat conducting layer is bound by bismuth oxide-based low-melting point glass.

To attain the above objects, a catalytic combustion type gas sensor according to the invention includes the above-described sensing element and compensating element for catalytic combustion type gas sensor according to the invention, and a first series circuit in which the heater coil of the sensing element and the heater coil of the compensating element are serially connected and a second series circuit in which a first resistance element and a second resistance element are serially connected are connected in parallel to form a Wheatstone bridge circuit.

Further, a direct-current voltage is applied between connection points of the first series circuit and the second series circuit, and a voltage between a connection point of the sensing element and the compensating element and a connection point of the first resistance element and the second resistance element is output as a detection signal of a detection target gas.

The detection target gas can be any of various flammable gases, but the invention is especially effective for detecting hydrogen gas.

Effect of the Invention

According to the catalytic combustion type gas sensor of the invention, it is possible to enhance durability and waterproof performance of the sensing element and the compensating element, and accordingly durability and waterproof performance of the sensor itself can be enhanced, which makes it possible to mount the catalytic combustion type gas sensor on a FCV without taking measures such as special environment setting.

REFERENCE OF NUMERALS

1: sensor main body of catalytic combustion type gas sensor

3: mount base

7: first resistance element

8: second resistance element

9: power source

11: combustion catalyst layer

12,12′: heat conducting layer of sensing element

13: heater coil of sensing element

31: compensating material layer

32: heat conducting layer of compensating element

33: heater coil of compensating element

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out the invention will be concretely described based on the drawings.

First, an embodiment of a sensing element and a compensating element for catalytic combustion type gas sensor according to the invention will be described by usingFIG. 1toFIG. 6.

FIG. 1is a cross-sectional view of the sensing element taken along its longitudinal direction andFIG. 3is a cross-sectional view taken along the A-A line inFIG. 1. In the sensing element10, a bead section of a heater coil13is buried in a heat conducting layer12, and a surface of the heat conducting layer12is coated with a combustion catalyst layer (also called an oxidation catalyst layer)11which is a catalyst layer causing oxidation combustion of a detection target gas brought into contact therewith.

The heater coil13is made of a platinum wire or a wire of a platinum-based alloy such as a platinum-rhodium alloy. Preferably, a portion to be the bead section is formed as a double-wound coil in such a manner that, for example, an original wire with a wire diameter of about 10 μm to about 50 μm, more preferably, about 20 μm to about 30 μm is wound around a core bar to form a single-wound coil, and this coil is wound around a core bar again. This structure increases a contact area between the heater coil13and the heat conducting layer12and further increases a resistance value, realizing high gas sensitivity. Further, since lead sections13a,13bare also single-wound coils and thus can absorb external impact, realizing a highly impact-resistant sensor. However, this structure is not restrictive, and a generally used conventional heater coil, that is, a heater coil whose bead section is a single-wound coil and whose lead sections are linear may be used.

The heat conducting layer12and the combustion catalyst layer11can have an improved waterproof property in a high-humidity environment when they have a small difference in hydrous property and have physical affinity. However, in realizing this, no change is made in the combustion catalyst layer11in view of a gas sensitivity property, and a material of the heat conducting layer12is changed to the material of the combustion catalyst layer11.

Therefore, the heat conducting layer12and the combustion catalyst layer11of the sensing element10are both made of a firing material whose main component is tin oxide.

More preferably, the combustion catalyst layer11, similarly to that of the conventional sensing element, is made of a firing material which contains as its main component tin oxide (SnO2) and additionally contains iron oxide when necessary, with platinum (Pt) and palladium (Pd) in fine powder form dispersed as a catalyst and which is produced by firing a mixture of these components in the atmosphere at a temperature of about 600° C., and this combustion catalyst layer11coats the surface of the heat conducting layer12.

As the heat conducting layer12, tin oxide is used instead of conventionally used alumina and titania. However, preferably, tin oxide for the heat conducting layer is firing in the atmosphere at a high temperature of 1100° C. to 1200° C. unlike tin oxide for the combustion catalyst layer, and has a lower content of water of crystallization than tin oxide for the combustion catalyst layer which is firing at 600° C., and fine powder of the firing material is used to form the heat conducting layer12.

Further, preferably, at least one of platinum and palladium, preferably both are added to tin oxide for the heat conducting layer. This eliminates a need for an intermediate layer of platinum-palladium which is set on an interface between the heat conducting layer and the combustion catalyst layer of the conventional sensing element and moreover, can improve gas detection sensitivity. Conventionally, in order to set the intermediate layer, in a manufacturing process of the sensing element, a platinum-palladium solution is applied on a surface of the alumina-titania-based heat conducting layer and this solution is heat-treated, whereby the intermediate layer is formed on the surface of the heat conducting layer. According to this embodiment, a process of forming the intermediate layer can be eliminated.

Further, in this embodiment, bismuth oxide-based low-melting point glass is mixed in the fine powder of the aforesaid tin oxide-based firing material for the heat conducting layer to bind the fine powder. Consequently, minute pores are distributed inside, which allows the detection target gas to enter the heat conducting layer12.

Processes for manufacturing such a preferable material for the heat conducting layer used in the sensing element of the invention will be described with reference to the flowchart shown inFIG. 6.

First, tin salt and iron salt are dissolved in water, and the resultant water solution is dried and burnt. Platinum salt and palladium salt are added to a thus produced firing material made of tin oxide and iron oxide, and the resultant is firing again, and its fine powder and bismuth oxide-based low-melting point glass are mixed, thereby producing the material for the heat conducting layer.

The reason why the fine powder is used as the material of the heat conducting layer is to avoid the formation of a void layer in the heat conducting layer, thereby preventing deterioration of the heat conducting performance.

As a result of changing the heat conducting layer in the above-described manner, the conventionally occurring drop of the combustion catalyst layer from the surface of the heat conducting layer is eliminated even when dew condensation occurs in the sensing element10and dew is frozen, and as a result, durability and waterproof performance of the sensing element for catalytic combustion type gas sensor are greatly enhanced.

In a case where neither platinum nor palladium is added to the material of the heat conducting layer12, combustion activity, though high in the combustion catalyst layer11, is low in the heat conducting layer12as shown inFIG. 4, so that catalytic combustion of flammable gas does not occur in the heat conducting layer12. On the other hand, adding at least one of platinum and palladium to the material of the heat conducting layer12results in higher combustion activity also in the heat conducting layer12as shown inFIG. 5. Consequently, the catalytic combustion of flammable gas occurs in the whole area of the combustion catalyst layer11up to the inside of the heat conducting layer12, resulting in improved sensitivity for detecting flammable gas.

FIG. 2is a cross-sectional view showing an embodiment of the compensating element for catalytic combustion type gas sensor according to the invention, taken along the longitudinal direction.

In the compensating element30, a bead section of a heater coil33is buried in a heat conducting layer32and a compensating material layer31coats a surface of the heat conducting layer32.

The heater coil33and the heat conducting layer32of the compensating element30are made of the same materials as those of the heater coil13and the heat conducting layer12of the sensing element10and are equal in heat capacity to the heater coil13and the heat conducting layer12. Therefore, the heater coil33is made of a platinum wire or a wire of a platinum-based alloy such as a platinum-rhodium alloy.

The heat conducting layer32is made of a firing material whose main component is tin oxide instead of conventionally used alumina and titania.

Similarly to the combustion catalyst layer11of the sensing element10, the compensating material layer31is made of a firing material whose main component is tin oxide, with copper oxide or the like being added thereto, and is formed to be equal in heat capacity to the combustion catalyst layer11. However, neither platinum nor palladium as an oxidation catalyst is added.

Preferably, as the firing material of the heat conducting layer32, whose main component is tin oxide, used is fine powder of a firing material whose main component is tin oxide and which is produced by firing at a higher temperature than a temperature for producing the firing material of the compensating material layer31. Further, preferably, the fine powder of the firing material forming the heat conducting layer is bound by bismuth oxide-based low-melting point glass. These points are the same as in the aforesaid heat conducting layer12of the sensing element10, and therefore detailed description thereof will be omitted.

In this compensating element30, it is also possible to realize physical affinity of the heat conducting layer and the compensating material layer by changing the material of the heat conducting layer, and therefore, even when dew condensation occurs and dew is frozen in the compensating element30, the conventionally occurring drop of the compensating material layer from the surface of the heat conducting layer does not occur, resulting in great enhancement of durability and water proof performance of the compensating element for catalytic combustion type gas sensor.

Next, an embodiment of the catalytic combustion type gas sensor according to the invention will be described with reference toFIG. 7andFIG. 8.FIG. 7is a front view of a sensor main body.

The sensor main body1shown inFIG. 7has a plate-shaped mount base3made of ceramic or resin, and the above-described sensing element10for catalytic combustion type gas sensor according to the invention is fixed in such a manner that lead sections13a,13b, which project from both ends, of its heater coil13are attached to electrode pins4,5for external connection penetrating through the mount base3.

Further, the above-described compensating element30for catalytic combustion type gas sensor according to the invention, though not shown, is fixedly set in line with the sensing element10, similarly by having both lead sections of the heater coil33attached to another pair of electrode pins.

The sensing element10and the compensating element30are surrounded by the mount base3and a not-shown gas permeable explosion-proof structure made of a wire gauze or a sintered body of metal powder or ceramic powder.

FIG. 8shows a basic circuit configuration of the catalytic combustion type gas sensor according to the invention, and is a diagram of a circuit including a Wheatstone bridge circuit and a power source applying a direct-current voltage to the Wheatstone bridge circuit.

A first serial circuit in which the above-described sensing element (D element)10and compensating element (C element)30of the sensor main body1are serially connected and a second series circuit in which a first resistance element7and a second resistance element8are serially connected are connected in parallel to form the Wheatstone bridge circuit. A power source9applies the direct-current voltage between connection points of the first series circuit and the second series circuit, and a voltage Vout between a connection point a of the sensing element10and the compensating element30and a connection point b of the first resistance element7and the second resistance element8is output as a detection signal of the detection target gas.

Here, the output voltage Vout of the Wheatstone bridge circuit is zero volt (0 mV) when R1×R2=RD×RC, where RD, RC, R1, and R2are electrify resistance values of the sensing element10, the compensating element30, the first resistance element7, and the second resistance element8respectively. Desirably, as the first resistance element7and the second resistance element8, those having the same resistance value and the same temperature characteristic are used.

When the power source9which can vary output voltage applies a rated voltage to the series circuit composed of the heater coil of the sensing element10and the heater coil of the compensating element30, an operating temperature is generated in the sensing element10and the compensating element30, so that the output voltage Vout is obtained, the output voltage Vout being dependent on the electrify resistance values RD, RCin a balanced state between heat supply components from the heater coils in the respective elements and a heat release component dissipated to the atmosphere in a clean atmosphere. The output voltage at this time is also called “a zero point value”. If the detection target gas is present and it comes into contact with the combustion catalyst layer11of the sensing element10shown inFIG. 1to burn, the temperature of the heater coil13increases and only its energizing resistance value RDincreases, and therefore, an increase in the output voltage Vout to a plus (+) side by an amount corresponding to gas sensitivity occurs and can be detected.

Examples of the detection target gas are methane gas, hydrogen gas, LP gas (liquefied petroleum gas), propane gas, butane gas, ethylene gas, and carbon monoxide gas, or organic component gas such as ethanol and acetone.

The catalytic combustion type gas sensor, when mounted on a fuel cell vehicle (FCV), can detect leakage of hydrogen gas with high sensitivity.

The catalytic combustion type gas sensor has remarkably improved durability and waterproof performance owing to the use of the sensing element10and the compensating element30according to the invention, and thus can pass a severe test for automobile components.

Table 1, Table 2,FIG. 9, andFIG. 10show the results of freezing tests, and Table 3, Table 4,FIG. 11, andFIG. 12show the results of power ON-OFF tests in the presence of dew condensation. As the detection target gas, hydrogen (H2) gas is used and a unit of sensor output variation is [H2/ppm].

Table 1 andFIG. 9are data showing the result of the freezing test conducted on catalytic combustion type gas sensors each using a sensing element and a compensating element whose heat conducting layers have the specifications according to the invention, and even when the number of freezing times reaches 600 times, the sensor output variation is within ±500 [H2/ppm] even for a sample with the maximum sensor output variation.

In contrast, Table 2 andFIG. 10are data showing the result of the freezing test conducted on catalytic combustion type gas sensors each using a sensing element and a compensating element whose heat conducting layers have the conventional specifications, and when the number of freezing times exceeds 400 times, it is seen that the sensor output variation rapidly increases to exceed ±500 [H2/ppm] and the breakage of one of or both of the sensing element and the compensating element starts to occur.

Table 3 andFIG. 11show data of the power ON-OFF test in the presence of dew condensation, conducted on catalytic combustion type gas sensors each using a sensing element and a compensating element whose heat conducting layers have the specifications according to the invention, and it is seen that, when the number of power ON times in the presence of dew condensation reaches 6000 times, the sensor output variation is within ±500 [H2/ppm] even for a sample with the maximum sensor output variation.

Table 4 andFIG. 12show data of the power ON-OFF test in the presence of dew condensation, conducted on catalytic combustion type gas sensors each using a sensing element and a compensating element whose heat conducting layers have the conventional specifications, and it is seen that, when the number of power ON times in the presence of dew condensation exceeds 4000 times, the sensor output variation rapidly increases to exceed ±500 [H2/ppm], and the breakage of one of or both of the sensing element and the compensating element starts to occur.

It should be noted that the structures, shapes, materials, manufacturing methods, and so on of the catalytic combustion type gas sensor according to the invention and its sensing element and compensating element are not limited to those in the above-described embodiment, and it goes without saying that appropriate changes can be made in matters except the matters described in the claims.

TABLE 1number ofheat conducting layer with the specifications according tofreezingthe inventiontimesabcde0000001000−37−12920083−3210−3113300172−7842−8840400249−11391−10585500312−198109−134121600367−220153−181186

TABLE 2(unit: H2/ppm)number offreezingheat conducting layer with the conventional specificationstimesfghij000000100−32−62−12−8251200−292−126−281−6193300−203−85−386−94328400−408−145−426−117434500−528−916−1052−2821144600−600−1126−1545−11921810

TABLE 3heat conducting layer with the specifications according tonumber ofthe inventionON timesklmno00000010001623−35−220009141−1817−17300018969−3222−21400022398−4938−345000289130−6644−496000307164−8767−62

TABLE 4(unit: H2/ppm)number ofheat conducting layer with the conventional specificationsON timespqrst0000001000−8430870−58−1452000−343171283−21923000−481154413−79554000−5081534671764065000−70011211097−11210106000−76914201548−9821534

INDUSTRIAL APPLICABILITY

The sensing element, the compensating element, and the catalytic combustion type gas sensor according to the invention are widely applicable as a gas leakage detector in apparatuses and systems using various kinds of flammable gases or in rooms where they are installed, and so on. In particular, a fuel cell whose rapid practical application in future is desired uses flammable hydrogen gas as its fuel, and thus needs be equipped with a sensor detecting hydrogen leakage, and it is compulsory to install a hydrogen gas sensor in each section inside a fuel cell vehicle. Further, it is indispensable to provide a hydrogen gas sensor also in a fuel cell system and the like used as an auxiliary power source for industrial use or for home use, and the application of the invention is extremely effective in these hydrogen gas sensors.