JP2992748B2 - Contact combustion type gas sensor and manufacturing method thereof - 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 capable of detecting a combustible gas and a method of manufacturing the same.
More specifically, a new element that can increase the sensitivity to flammable gas to detect low-concentration gas, reduce the size of the element to reduce power consumption, improve the response speed, and resist vibration and shock. The present invention relates to a contact combustion type gas sensor having a structure and a manufacturing method thereof.

[0002]

2. Description of the Related Art A catalytic combustion type gas sensor is a sensor which converts reaction heat generated when a combustible gas reacts (combustes) with oxygen into an electric signal and senses it. As shown in FIG. 1, the conventional contact combustion type gas sensor has a combustion section 1 in which combustible gas burns, a heating section that heats the combustion section at an appropriate temperature, and a resistance that changes due to the gas combustion heat of the combustion section 1. And a heater section 2.

[0003] The combustion section 1 is made of Pd or Pd on an alumina insulator.
A noble metal catalyst such as t is included to promote the combustion of combustible gas. The heater unit 2 mainly uses a platinum wire having a high temperature resistance coefficient. Fig. 3
Is connected to a part of the Wheatstone bridge to form a measurement circuit, and detects a flammable gas by measuring a change in resistance of a platinum wire due to heat of combustion of the gas.

That is, in the measurement circuit, a contact combustion type sensor is connected to one side of a bridge circuit, and the temperature and humidity characteristics are almost the same as those of the contact combustion type sensor. Compensating element 3 that does not react to gas
Is connected to one side adjacent to one side of the bridge circuit to which the combustion type sensor is connected, fixed resistors 4 and 5 are connected to the other two sides of the bridge circuit, and a bridge balance adjustment variable resistor 6 is connected to these. It is configured to be connected in parallel. In the figure, VR is a variable resistor, and a switch (S
It is configured to regulate the voltage (E) coupled via W). The measurement circuit described above adjusts the variable resistor 6 so that the bridge portion maintains an equilibrium state when the sensor element is heated at 300 to 600 ° C. Thereafter, when the combustible gas touches the sensor element, the gas burns, and the heat of combustion increases the resistance value of the platinum coil of the sensor element. On the other hand, since the compensating element 3 does not burn with the combustible gas, it does not generate combustion heat, and the resistance of the platinum coil does not change due to the combustion heat. Therefore, a difference occurs between the resistance of the sensor element and the resistance of the compensating element 3, and a bridge voltage is output. This bridge voltage is output in proportion to the concentration of the combustible gas, and FIG. 4 shows an example of the output characteristic with respect to the methane gas.

On the other hand, in the above-described conventional method for manufacturing a contact combustion type sensor, as shown in FIG. 2, a platinum wire is formed in a coil shape to manufacture a heater portion of the sensor, and at the same time, a ceramic containing alumina as a main component is used. After the carrier is made into a sol or paste form, it is manufactured in a bead shape of a certain size, and attached to a heater portion of a platinum coil to form a combustion portion. Next, the formed beads are placed in a firing furnace and subjected to a heat treatment, and then the beads are added to platinum (Pt) or palladium (Pd).
And the like. Then, heat treatment in a firing furnace,
After forming a sensing element in which a catalyst is contained in an alumina carrier in a bead form, it is sealed by bonding to a terminal pin of a package.

However, the above-mentioned conventional catalytic combustion type gas sensor and its manufacturing method have the following problems.
That is, in general, assuming that the concentration of the combustible gas is c and the temperature rise due to the heat of combustion of the gas is ΔT, the resistance increase ΔR of the sensor is as shown in Equation 1. ΔR = a · ΔT = a · b · c · Q / h where a is the temperature coefficient of resistance of the heater coil, Q is the heat of combustion of the flammable gas, h is the heat capacity of the element, and b is the catalyst added to the element. Is the factor attributable to this. As is apparent from the above equation, in order to obtain a sensor with high sensitivity, the resistance temperature coefficient of the heater must be large and the heat capacity of the element must be small. In addition, the amount of combustion heat lost to the outside is largely lost by conduction along the platinum wire and loss due to radiation into the air. There must be. This has a close relationship with heater power consumption besides sensor sensing. Therefore, in order to reduce the loss of combustion heat, the diameter of the platinum wire must be reduced, and the volume of the combustion portion must be reduced. The conductivity must be high and the distance of the heat transfer must be as short as possible. However, the conventional sensor has a bead-shaped (spherical) combustion part, and the spherical bead has a structure covering the platinum coil. Significantly affects sensitivity. Therefore, there is a limit to reducing the size of the bead, and a limit to reducing the diameter of the platinum coil to support a bead of a fixed size and weight.

Further, the conventional sensor in which the size of the bead and the thickness of the platinum coil are selected so as to obtain sufficient sensor sensitivity is relatively susceptible to shock and vibration, and the heater for heating the bead at a high temperature is used. Power consumption is also high. As well,
The contact combustion type gas sensor has a structure in which a combustible gas is burned by a bead, the heat of combustion is transmitted to a platinum coil, and the resistance of the coil changes.The sensor sensitivity and response speed are determined based on the transmission effect and distance. Therefore, there is a problem that the structure of the conventional gas sensor has a limit in improving the sensitivity and the gas response speed.

Further, in the conventional method for manufacturing a sensor, an alumina sol or paste manufactured for forming a bead is attached to a coil and dried and fired. However, the forming method is performed by a manual operation and a similar method. It is difficult to finely control the bead size of each element. In addition, mass production is low because the processes such as baking process after bead formation, catalyst impregnation process, and wire bonding are performed on a device basis, and the compensating device is manufactured in a separate process from the sensing device to form a package. There is a problem that the manufacturing cost is almost doubled from the manufacturing process because of the mounting.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has been made to reduce the driving power consumption of the sensor element by reducing the size thereof, is resistant to vibration and shock, and has a high sensitivity. It is an object of the present invention to provide a contact combustion type gas sensor having a high response speed and a high response speed. Another object of the present invention is to provide a method for manufacturing a contact combustion type gas sensor which has reproducibility and mass productivity and can be manufactured at low cost.

[0010]

According to a first aspect of the present invention, there is provided a contact combustion type gas sensor having a coil-shaped heat generating portion and a film-shaped combustion portion in which a catalyst is dispersed on the surface of the coil. The sensing element, the coil-shaped heat generating portion, and the compensation element formed in a film shape without containing a catalyst on the surface of the coil are simultaneously packaged.

The method for manufacturing a contact combustion type gas sensor according to the present invention includes the steps of bonding two platinum or an alloy thereof wound in a coil shape to a package electrode, respectively.
A step of forming a metal film by electroplating on platinum or an alloy thereof, a step of thermally oxidizing the metal film to form a porous metal oxide film, and a step of forming a noble metal on the porous metal oxide film used as a sensing element. And a step of drying and calcining after impregnation of the catalyst.

[0012]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 5A is a diagram schematically illustrating the structure of the catalytic combustion type gas sensor of the present embodiment, FIG. 5B is a diagram schematically illustrating the cross section of the sensing element and the compensating element, and FIGS. It is a figure which shows typically the cross section and plane at the time of packaging the gas sensor of this embodiment. As shown in FIG. 5A and FIG. 5B, the sensing element 10 includes a heater section 11 wound in a coil shape of platinum or an alloy thereof, and a porous oxide semiconductor having pores on the surface of the heater section 11. A film is formed, and the film is formed by the combustion unit 12 in which a catalyst such as platinum or palladium is dispersed on the surface. On the other hand, the compensating element 13
In the same manner as in the sensing element 10, platinum or an alloy thereof is wound in a coil shape to form a heater portion 14, and a porous oxide semiconductor film 15 is formed on the surface of the heater portion 14. It is configured not to include a catalyst such as palladium. Sensing element 1 thus formed
The coil 0 and the compensating element 13 are respectively coupled to two electrode terminal pins 17 of the package 16, and the coil-shaped sensing element 10 and the compensating element 13 are arranged side by side in one package, and are electrically connected to the electrode terminal pins 17. And are covered by a cover 18 (see FIGS. 6c and 6d).

A voltage of 2 V is applied to the gas sensor of the present embodiment configured as described above, and the temperature of the element is set to 500 to 60.
Heating was performed at 0 ° C., and the fixed resistors 4 and 5 of the bridge circuit in FIG. 3 were set to 100 ohms, respectively, and the sensor output for C ₄ H ₁₀ and CH ₄ gas was measured. As shown in FIG. It is possible to detect highly flammable gas of about 2000 ppm with high sensitivity, the sensor response speed is much faster than several seconds, and the power consumption can be reduced to several tens mW or less depending on the size of the sensor.

Such a measurement result is obtained for the following reasons. The combustion part is made of an oxide semiconductor having a thermal conductivity better than that of alumina, and the combustion part is arranged on the surface of the coil, so that heat of combustion by gas is directly transmitted to almost the entire coil. . On the other hand, in a conventional bead-type sensing element in which a catalyst is contained in alumina, the combustible gas is once burned in the bead, and the combustion heat is conducted to the coil through the alumina. Therefore,
In the case of the present embodiment, the heat transfer efficiency is significantly higher than that of the conventional one. Moreover, in the present embodiment, the combustion portion containing the catalyst is formed of an n-type oxide semiconductor such as tin oxide, which has a high electrical conductivity and a high thermal conductivity when exposed to the gas. When touching the surface, it is burned to generate heat of combustion, and the thermal conductivity of the oxide semiconductor increases to transfer the generated heat of combustion to the coil, so the heat transfer efficiency is extremely high, Sensor sensitivity is high. Furthermore, a combustion part is formed in the shape of a film on the surface of the coil,
Since the distance that the combustion heat is transmitted to the coil is short, the resistance change speed of the coil due to the combustion heat, that is, the sensor response speed is fast, and the diameter of the coil can be reduced, so that heat loss due to conduction is small and power consumption is reduced. Can be.
Further, in the present embodiment, since the burning portion 12 of the sensing element 10 is formed of the porous oxide semiconductor film coated on the coil surface, the weight and bulk are small, and the device is resistant to vibration and impact.

Next, a method of manufacturing the contact combustion type gas sensor according to this embodiment will be described with reference to FIG. First, using a winding machine, a platinum wire having a diameter of 0.005 to 0.06 mm,
The coil is wound into a coil having an inner diameter of 0.1 to 1 mm and an overall size of 0.3 to 1.2 mm as 5 to 20 turns (S
101). The two winding coils wound as above are
The two package terminal pins used as the respective electrode pins are attached by a resistance welding method or a high frequency fusion method, and are arranged side by side in the package (S102). The two coils mounted on the package are immersed in a tin plating solution, a cathodic voltage is applied to the terminal pins to which the coils are adhered, and an anodic voltage is applied to tin (Sn) in the solution, while simultaneously applying an applied electrode The plating time is appropriately adjusted, and the surface of the platinum coil is plated with tin to form a film having a predetermined thickness (S103). At this time, the thickness of the plating layer varies depending on the size of the coil and the diameter of the platinum wire, but is preferably 0.5 to 5 times the diameter of the platinum wire.

After washing and drying after the plating step, a voltage is applied to both ends of the terminal pins to maintain the tin-plated coil heated at several hundred degrees centigrade to oxidize the plated tin. The tin oxide (S
nO ₂ ). Of the two thin-film coils formed with tin oxide, a compound containing a noble metal, such as chloroplatinic acid or palladium chloride, is added to a suitable solution at a constant concentration in the porous oxide film of a platinum coil used as a sensing element. And a fixed amount is discharged by a fixed amount discharger (dispensor) (S105). After the solution is dried, when a certain voltage is applied to the coil and heated as in the oxidation step, the compound is decomposed, and only a certain amount of noble metal remains in the porous tin oxide.
The noble metal impregnation amount is adjusted by the concentration of the solution and the discharge amount. The heating temperature at this time is 800 ° C. or higher (S106).
After completion of the predetermined process, the sensor is manufactured by assembling the cover on the package.

According to the manufacturing method as described above, unlike the conventional method in which each sensor is placed in a firing furnace for each element and heat-treated, the heat treatment is performed by applying a voltage in a package state without a firing furnace. In addition, the heat treatment method can be easily performed at low cost. In general, tin oxide itself is difficult to be formed in a film shape on a platinum coil because the sintering temperature of the oxide itself is high. On the other hand, in the present embodiment, tin (Sn) is oxidized after plating with tin (Sn). Since the method of forming tin is used, tin oxide can be easily obtained. Further, unlike another vapor deposition method for forming a film, a plating method is used, so that mass productivity is good, economical, and a thick film can be easily obtained. Then, the formed tin oxide increases its bulk when phase-changed, so that it is thicker than the thickness of the tin plating layer and is porous.
In addition, it exhibits the characteristics of an n-type oxide semiconductor, and has a property of increasing thermal conductivity when exposed to a combustible gas. in this way,
In the method of the present embodiment, since the sensing element and the compensating element are processed through an electrical method in one package, the thickness and the size are constant, and the reproducibility, mass productivity, and economy are excellent. The heat treatment is simple and the heat treatment temperature can be finely adjusted.

[0018]

As described above, in the contact combustion type gas sensor of the present invention, since the combustion portion is formed in a film shape on the surface of the coil, the entire film comes into contact with the gas, burns, and directly contacts the coil. Since the heat is transferred, the sensitivity as a gas sensor is very high, the response speed of the sensor is fast, and the power consumption is low.
Not only that, the combustion part is small in thickness and bulk, so it is suitable for miniaturization and is very resistant to vibration and impact. When the membrane is a porous membrane, the membrane can be made lighter, more suitable for miniaturization, and more resistant to vibration and impact. According to the manufacturing method of the present invention for manufacturing such a sensor, the sensing element and the compensating element can be obtained by an oxide semiconductor film obtained by metal plating, and the steps can be performed in one package via an electrical method. Therefore, the thickness and size are constant, the reproducibility, mass productivity, and economic efficiency are excellent. Also, the heat treatment method can be easily performed at low cost, and the heat treatment temperature can be finely adjusted. There is.

[Brief description of the drawings]

FIG. 1 is a diagram showing the structure of a conventional catalytic combustion type gas sensor.

FIG. 2 is a flowchart showing a manufacturing process of a conventional catalytic combustion type gas sensor;

FIG. 3 is a circuit diagram for measuring a combustible gas using a contact combustion type gas sensor,

FIG. 4 is a graph showing sensitivity characteristics when measuring methane gas using the circuit of FIG. 3;

5A is a diagram schematically showing the structure of the gas sensor according to the embodiment of the present invention, FIG. 5B is a diagram schematically showing a cross section of the sensing element and the compensating element,

FIG. 6 is a diagram schematically showing a cross section and a plane when the gas sensor according to the embodiment of the present invention is packaged;

FIG. 7 is a manufacturing process diagram of the gas sensor of the present invention,

FIG. 8 is a graph showing a sensor output for the gas sensor of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Sensing element 11, 14 Heater part 12 Burning part 13 Compensation element 15 Porous oxide semiconductor film 16 Package 17 Terminal pin

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Gyu Jen Lee South Korea, Seoul, Seochoek, Bampo 4-Don (No Address) Mid Apartment 309-601 (72) Inventor Hyun Gi Hong South Korea, Gyeonggi-do Gacheon-Shi-Brim-Dong-41-Sugong-Apartment 903-105 (56) References JP-A-61-246660 (JP, A) JP-A-7-190980 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/16

Claims (6)

(57) [Claims] 1. A sensing element in which a combustion section made of a porous oxide semiconductor film in which a catalyst is dispersed is formed on the surface of a coil-shaped heating section, and the surface of the coil-shaped heating section is a film containing no catalyst. A catalytic combustion type gas sensor in which a compensating element having a covered shape is packaged simultaneously. 2. The catalytic combustion type gas sensor according to claim 1, wherein the coil is made of platinum or an alloy thereof and has a diameter of 0.005 to 0.06 mm. 3. The combustion section made of the porous oxide semiconductor film has a thickness of 0.01 to 0.2 mm thermally oxidized from a metal.
The contact combustion type gas sensor according to claim 1, wherein the gas sensor is formed of a film . 4. The catalytic combustion type gas sensor according to claim 3, wherein the porous oxide semiconductor film is made of an n-type oxide such as tin oxide or zinc oxide whose thermal conductivity is increased by a gas or a mixture thereof. . 5. A step of bonding two platinum or an alloy thereof wound in a coil shape to a package electrode, a step of forming a metal film on the platinum or alloy alloy coil by electroplating, and A step of forming a porous metal oxide film by thermal oxidation, a step of impregnating the porous metal oxide film of platinum or its alloy used as a sensing element with a noble metal, and a step of drying and firing after the impregnation of the noble metal catalyst A method for manufacturing a contact combustion type gas sensor comprising: 6. The noble metal catalyst impregnating step comprises dissolving a compound containing a noble metal such as chloroplatinic acid or palladium chloride in a suitable solution at a predetermined concentration in a porous metal oxide film used as the sensing element. 6. The method for manufacturing a contact combustion type gas sensor according to claim 5, wherein a fixed amount is discharged by a fixed amount discharger.