JP3078485B2 - Contact combustion type gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalytic combustion type gas sensor.

[0002]

2. Description of the Related Art Conventionally, a contact combustion type gas sensor has been known as a means for detecting the presence of a combustible gas in order to detect incomplete combustion or misfire of a gas appliance. Compared with a semiconductor gas sensor that detects a gas by a change in resistance of a semiconductor due to gas desorption on a semiconductor surface, the contact combustion gas sensor has a stable gas detection characteristic over a long period of time. Widely used for detecting carbon monoxide.

[0003] The conventional contact combustion type gas sensor uses an electric resistor made of a platinum wire whose surface is coated with platinum or palladium as a gas detecting element. When such a gas detecting element is brought into contact with a flammable gas while being maintained at a predetermined temperature, the flammable gas burns using the platinum or palladium as a catalyst, and the heat of combustion heats the electric resistor, and The resistance changes. Therefore, the electric resistor is energized, and the concentration of the combustible gas is detected by detecting the amount of change in the electric resistance of the electric resistor when the electric resistor is heated.

The contact combustion type gas sensor is designed to maintain the temperature of the gas detecting element at a temperature of 300 ° C. or higher in order to increase the catalytic activity when the combustible gas comes into contact with the gas detecting element to facilitate combustion. It has become. However, the contact combustion type gas sensor detects a gas when a hair spray containing silicone oil is used in a room where a gas appliance is installed, or when a silicone resin is used as a sealant for a gas pipe of the gas appliance. There is a problem that the sensitivity is significantly reduced.

[0005] The problem is that a low polymerization degree silicone resin such as silicone oil is brought into contact with the gas detecting element maintained at a temperature of 300 ° C. or more to cause a polymerization reaction, thereby producing a high polymerization degree silicon resin. Alternatively, it is considered that the surface of the catalyst is covered with vitreous silicon dioxide generated by further oxidizing the silicon resin at a high temperature, and the contact between the catalyst and the combustible gas is interrupted. Then, in order to solve the above-mentioned problem, it is conceivable to use the contact combustion type gas sensor so that the temperature of the gas detecting element does not become 300 ° C. or more.

However, in a conventional contact combustion type gas sensor using a gas detector as an electric resistor made of a platinum wire whose surface is coated with platinum or palladium, when the temperature of the gas detector is lower than 300.degree. There is an inconvenience that a sufficient amount of change in electrical resistance cannot be obtained to detect the presence of the electric current.

[0007]

SUMMARY OF THE INVENTION The present invention solves such a disadvantage, and has a sufficient electric resistance to detect the presence of flammable gas even when the temperature of the gas detecting element is low, that is, lower than 300 ° C. An object of the present invention is to provide a contact combustion type gas sensor capable of obtaining a change amount.

[0008]

To achieve SUMMARY OF THE INVENTION The above objects, a catalytic combustion type gas sensor of the present invention, electrostatic consisting of iron
The surface of the resistance element is coated with nickel, and the nickel coating is applied.
Gas detection element coated with platinum or palladium
And combusts the combustible gas contacted with the gas detection element maintained at a predetermined temperature using the platinum or palladium as a catalyst, and detects a change in the electric resistance of the electric resistor due to the heat of combustion of the combustible gas. and detecting the concentration of combustible gas by.

According to the present invention , since iron having a higher rate of change in electrical resistivity with temperature than platinum is used as the electrical resistor, even if the temperature of the gas detecting element is set to a low temperature of less than 300 ° C., it becomes flammable. The change in electrical resistance sufficient to detect the presence of a reactive gas can be obtained.

The electric resistor is formed by forming a thin wire into a coil shape.
When the electric resistor is iron, the surface is coated with nickel, and the nickel coating is further coated with platinum or palladium. Can be prevented from being broken due to heat generated during energization.

Further, the contact combustion type gas sensor according to the present invention comprises:
Since it is a sensor for detecting carbon monoxide, hydrogen and flammable gas, it is suitably used for detecting incomplete combustion or misfiring of gas appliances using city gas or liquefied petroleum gas (LPG) as fuel.

[0012]

Next, an embodiment of the present invention will be described in more detail with reference to the accompanying drawings.
FIG. 1A is a circuit diagram showing a circuit configuration of the catalytic combustion type gas sensor of the present invention, FIG. 1B is a perspective view showing a partial cross section of the gas detection element shown in FIG. 1A, and FIG. 4 is a graph showing the relationship between the electrical resistivity of a metal serving as a resistor and temperature.

As shown in FIG. 1 (a), a catalytic combustion type gas sensor 1 of the present invention is used for city gas or liquefied petroleum gas (LP).
G) is used to detect incomplete combustion or misfire of a gas appliance using G) as a fuel. The circuit configuration is such that the gas detecting element 2 and the temperature compensating element 3
a, 4b, and a power supply 6 for supplying electric power to the bridge 5, a gas concentration indicator 7 for indicating a gas concentration detected by the gas detecting element 2, and a gas concentration indicator 7 for detecting a predetermined gas concentration. Alarm means 8 for issuing an alarm when the standard is exceeded. The gas concentration indicator 7 is connected to the fixed contact A and the movable contact B of the bridge 5, and the alarm means 8 is connected in parallel with the gas concentration indicator 7.

As shown in FIG. 1 (b), the gas detecting element 2 comprises a platinum or palladium coating layer formed by electroplating on the surface of an electric resistor 9 formed by forming a thin metal wire into a coil shape. 10 is provided. In the catalytic combustion type gas sensor 1 of the present invention, iron (F) is used as the electric resistor 9.
e) or nickel (Ni) is used.

[0015] As shown in FIG.
At a temperature of 0 ° C. or higher, the electrical resistivity changes more greatly with temperature than that of platinum (Pt), so that it is suitable for the electric resistor 9 of the gas detection element used at a temperature of less than 300 ° C. Examples of iron used for the electric resistor 9 include those having a purity of 99.5%. Also,
The nickel used for the electric resistor 9 may be, for example, one having a purity of 99.7%.

The copper (Cu) shown in FIG. 2 is shown for reference, is a good conductor, and has a smaller change in electric resistivity with temperature than platinum. Not preferred.

When the electric resistor 9 is made of iron, it tends to be oxidized by repeated use to increase the resistance. Therefore, in order to prevent disconnection due to heat generation during energization, its surface is coated with nickel. The platinum or palladium is coated on the nickel coating. The nickel coating is stable and prevents surface oxidation, so the electrical resistance of iron does not change with repeated use. The nickel coating on the iron of the electric resistor 9 is performed by, for example, electroless plating.

In order to prevent the breaking of the iron,
As with the nickel coating, a coating capable of preventing oxidation may be formed on the iron surface, and the platinum or palladium coating serving as a catalyst may be formed, for example, by forming a thin film of palladium or platinum on the surface, and further forming a surface thereof. The catalyst may be formed by adhering platinum black or palladium black as a catalyst.

Next, the operation of the contact combustion type gas sensor 1 will be described. First, the contact combustion type gas sensor 1
While the gas detection element 2 is kept at a predetermined temperature by an external heater or the like (not shown), power is supplied from the power supply 6 to the bridge 5 to energize the electric resistor 9 made of the thin metal wire of the gas detection element 2. I do. Since the gas detecting element 2 is composed of the electric resistor 9 having the coating layer 10 of platinum or palladium on its surface as described above, the gas detecting element 2 maintained at a predetermined temperature as described above When the combustible gas comes into contact, the combustible gas burns using the platinum or palladium of the coating layer 10 as a catalyst, and the resistance of the electric resistor 9 changes due to the heat of combustion of the combustible gas.

Therefore, the contact combustion type gas sensor 1 moves the movable contact B of the bridge 5 and detects the amount of change in the electric resistance of the electric resistor 9 as a voltage difference between the contacts A and B of the bridge 5. Has become. Since the amount of change in electric resistance of the electric resistor 9 detected as described above is proportional to the gas concentration, the gas concentration converted from the amount of change in electric resistance is directly indicated on the gas concentration indicator 7 and the alarm means is provided. 8 is output. The alarm means 8 includes control means (not shown). When the detected gas concentration exceeds a predetermined reference, the control means determines that incomplete combustion or misfire has occurred in the gas appliance. The warning is issued through an auditory means such as an electronic sound or a visual means such as a photoelectric display.

Next, examples and comparative examples of the present invention will be described.

[0022]

Embodiment 1 FIG. 3 is an explanatory sectional view showing a configuration of an experimental apparatus for measuring gas detection performance of a gas detection element used in a contact combustion type gas sensor, and FIGS. 4 to 6 show embodiments of the present invention. And is a graph showing the relationship between the carbon monoxide concentration and the amount of change in the electrical resistance value in the gas detection element of the comparative example,
FIG. 4 shows the case of the gas detection element of the first embodiment, and FIG.
FIG. 6 shows the case of the gas detecting element of the comparative example. FIGS. 7 to 10 are graphs showing the relationship between the concentration of carbon monoxide and the amount of change in the electrical resistance value of the catalytic combustion type gas sensors of each of Examples and Comparative Examples at a predetermined temperature, and FIG. FIG. 8 shows 300 ° C.
9 shows the case of 350 ° C., and FIG. 10 shows the case of 400 ° C.

In this embodiment, the diameter is 0.1 mm and the length is 10 mm.
A 0 mm coiled nickel wire (purity 99.7%) was prepared, and its surface was coated with platinum black.

The coating with platinum black was carried out by electroplating as follows. First, the nickel wire was immersed in acetone, ultrasonically cleaned for 1 minute, degreased, and then immersed in room temperature water for 1 minute to wash with water. Next, the nickel wire was immersed in chromium-mixed acid at room temperature for 1 minute and pickled, and then immersed in room temperature water for 1 minute and washed with water.

Next, an aqueous solution of hexachloroplatinic acid at room temperature is supplied as a plating solution to an electroplating bath provided with electrodes, and the nickel wire washed as described above is immersed in the plating solution to form a nickel plating solution. An operation of applying a voltage of 5 V between the electrodes and applying a current of 0.4 to 15.5 mA for 3 minutes was performed twice, thereby covering the surface of the nickel wire with platinum black. Next, the nickel wire coated with platinum black was immersed in water at room temperature for 1 minute or more, and washed with water.

Next, 1N sulfuric acid is supplied as an electrolytic solution to an electrolytic cell provided with an electrode, and the nickel wire coated with platinum black is immersed in the electrolytic solution, so that the nickel wire and the electrode are interposed between the nickel wire and the electrode. The nickel wire was electrolytically cleaned by applying a voltage of 5 V and applying a current of 0.5 to 15 mA for 1 minute twice.

Next, the nickel wire coated with platinum black was immersed in water at room temperature for 1 minute or more, washed with water, the water adhering to the nickel wire was replaced with acetone, and dried with hot air. . 0.9-1.1 mg of platinum black is adhered to the surface of the nickel wire by the electroplating.

Next, the nickel wire coated with the platinum black was subjected to a heat treatment at 400 ° C. for 1 hour under a reduced pressure of 10 ⁻³ Torr to produce the gas detecting element 2 shown in FIG. 1B.

Next, the gas detection performance of the gas detection element 2 was evaluated using the apparatus shown in FIG. The apparatus shown in FIG. 3 assumes that the gas detection element 2 is used for the contact combustion type gas sensor 1 shown in FIGS. 1A and 1B, and the gas detection element 2 is maintained at a predetermined temperature. It measures carbon monoxide detection performance, and comprises a sealed mullite tube 11 holding the gas detection element 2 and an electric furnace 12 accommodating the mullite tube 11.

The mullite tube 11 has a carbon monoxide introducing means 13 and an air introducing means 14 as a combustible gas at one port 11a, and a carbon monoxide analyzer 15 at the other port 11b.
Is connected. Further, a thermometer 16 for measuring the temperature of the gas detection element 2 is connected to one port 11a.

The electric furnace 12 includes a PID temperature controller 1
7 so that the temperature inside the furnace can be adjusted.
Further, the gas detecting element 2 is connected to the port 11 of the mullite tube 11.
A constant current source 18 is connected via a and 11b, and an ammeter 19 is connected in series with the gas detection element 2, and a voltmeter 20 is connected in parallel.

The gas detecting performance of the gas detecting element 2 is as follows.
First, the voltage at both ends of the gas detection element 2 when a constant current of 10 mA flows from the constant current source 18 to the gas detection element 2 in the air while maintaining the temperature at 300 ° C., 350 ° C., and 400 ° C. The electrical resistance (R ₁ ) of the electrical resistor 9 at each of the above temperatures was determined by measuring with the total 20.

Next, at each of the above temperatures, carbon monoxide was introduced by the carbon monoxide introducing means 13 and the air introducing means 14 at 500 ppm, 1000 ppm and 1500 pp, respectively.
m, 2,000 ppm, 2500 ppm mixed air is introduced into the mullite tube 11, the atmosphere in the mullite tube 11 is sufficiently replaced with air of each concentration of carbon monoxide, and then the inflowing gas is shut off. The electric resistance (R ₂ ) of the electric resistor 9 corresponding to each carbon monoxide concentration was determined. Then, for each of the temperatures, the difference between the electric resistance (R ₂ ) of the electric resistor 9 in the air containing carbon monoxide and the electric resistance (R ₁ ) in the air containing no carbon monoxide. , And the amount of change in electric resistance of the electric resistor 9 corresponding to the concentration of carbon monoxide (ΔR = R ₂ −R ₁ , unit: mΩ) was obtained. The results are shown in Table 1 below and FIG.

[0034]

[Table 1]

[0035]

Embodiment 2 In this embodiment, the diameter is 0.1 mm and the length is 10 mm.
A 0 mm coiled iron wire (99.7% purity) was prepared, the surface of which was first coated with nickel, and then a platinum black coating was further applied on the nickel coating.

The coating with nickel was performed by the electroless plating method as follows. First, the iron wire was immersed in acetone, ultrasonically cleaned for 1 minute, degreased, and then immersed in room temperature water for 1 minute to wash with water. Next, the iron wire was immersed in 20% hydrochloric acid at room temperature for 1 minute and pickled, then immersed in water at room temperature and washed within 1 minute.

Next, the surface of the iron wire was coated with nickel by immersing the iron wire in an electroless nickel plating solution at 80 ° C. for 1 hour and performing electroless plating. Next, the iron wire coated with nickel is immersed in water at room temperature for 5 minutes or more and washed with water, and the water adhering to the iron wire is replaced with acetone for 2 minutes, followed by hot air drying, An iron wire coated with nickel was obtained. By the electroless plating,
On the surface of the iron wire, 0.4 mg of nickel is 1.2 μm.
It has a thickness of m and adheres smoothly.

Next, the surface of the nickel-coated iron wire was coated with platinum black by the same electroplating method as the nickel wire of Example 1. 0.9 to 1.1 mg of platinum black is adhered on the nickel coating by the electroplating.

Next, an iron wire further coated with platinum black on the nickel coating was placed under reduced pressure of 10 ^-3 torr under 400
After performing a heat treatment at 1 ° C. for 1 hour, annealing was performed at 400 ° C. to produce the gas detection element 2 shown in FIG. 1A.

Next, the gas detection performance of the gas detection element 2 was evaluated in exactly the same manner as in Example 1 using the apparatus shown in FIG. The results are shown in Table 2 below and FIG.

[0041]

[Table 2]

[0042]

Comparative Example In this example, the diameter was 0.1 mm and the length was 100 mm.
mm coiled platinum wire was prepared and its surface was
Was coated with platinum black by the same electroplating method as the nickel wire. 0.9 to 1.1 mg of platinum black is adhered to the surface of the platinum wire by the electroplating.

Next, the platinum wire coated with the platinum black was
Heat treatment was performed at 400 ° C. for 1 hour under a reduced pressure of 0 ⁻³ Torr to produce the gas detection element 2 shown in FIG.

Next, the gas detection performance of the gas detection element 2 was evaluated in the same manner as in Example 1 using the apparatus shown in FIG. The results are shown in Table 3 below and FIG.

[0045]

[Table 3]

From the data in Tables 1 to 3 and FIGS.
According to the gas detection element 2 of each of the above-described Examples and Comparative Examples, the amount of change in the electric resistance of the electric resistor 9 substantially corresponds to the concentration of carbon monoxide. It is clear that when the concentration is small and the concentration of carbon monoxide is high, the amount of change in electric resistance is large, and it can be clearly used for a catalytic combustion type gas sensor.

The data of Tables 1 to 3 and FIGS. 4 to 6 are summarized in FIGS. From FIG. 7, according to the gas detection elements 2 of the first and second embodiments, 2
Even when maintained at a temperature of less than 300 ° C. of 30 ° C., the amount of change in the electrical resistance of the electrical resistor 9 is larger than that of the gas detection element 2 (comparative example) in which the electrical resistance 9 is made of platinum for 2500 ppm or less of carbon monoxide. It is clear that it can be obtained. Therefore, according to the gas detection elements 2 of the first and second embodiments, a combustible gas such as carbon monoxide can be detected in a temperature range in which the polymerization reaction of the silicon resin is difficult to proceed.

Further, from FIGS. 8 to 10, it can be seen from the first embodiment.
And according to the gas detection element 2 of Example 2, except for the case of carbon monoxide of 1000 ppm or less when maintained at 300 ° C., the change amount of the electric resistance larger than that of the gas detection element 2 of Comparative Example is larger. It is clear that it can be obtained.

Although the electric resistor 9 is covered with platinum (platinum black) in each of the above embodiments, it may be covered with palladium.

[Brief description of the drawings]

FIG. 1A is a circuit diagram showing a circuit configuration of a catalytic combustion type gas sensor of the present invention, and FIG. 1B is a perspective view showing a partial cross section of the gas detection element shown in FIG. 1A.

FIG. 2 is a graph showing the relationship between the electrical resistivity of a metal serving as an electrical resistor and temperature.

FIG. 3 is an explanatory sectional view showing a configuration of an apparatus for measuring gas detection performance of a gas detection element.

FIG. 4 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in the electric resistance value in the gas detection element of Example 1 of the present invention.

FIG. 5 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in the electric resistance value in the gas detection element according to the second embodiment of the present invention.

FIG. 6 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in electric resistance in a conventional gas detection element 2.

FIG. 7 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in electric resistance when the gas detection element is maintained at 230 ° C.

FIG. 8 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in electric resistance when the gas detection element is maintained at 300 ° C.

FIG. 9 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in electric resistance when the gas detection element is maintained at 350 ° C.

FIG. 10 is a graph showing the relationship between the concentration of carbon monoxide and the amount of change in electric resistance when the gas detection element is maintained at 400 ° C.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Contact combustion type gas sensor 2 ... Gas detection element 9
... electric resistor, 10 ... platinum or palladium.

Claims (2)

(57) [Claims] 1. The surface of an electric resistor made of iron is made of nickel.
Coating, platinum or palladium on the nickel coating
A gas detection element having a coating is provided, and the combustible gas in contact with the gas detection element held at a predetermined temperature is burned using the platinum or palladium as a catalyst, and the electric resistor is heated by combustion heat of the combustible gas. A contact combustion type gas sensor that detects the concentration of combustible gas by detecting a change in electrical resistance. 2. The catalytic combustion type gas sensor according to claim 1 , wherein said gas sensor detects incomplete combustion or misfire of a gas appliance.