JP2941395B2 - Composite gas sensing element - Google Patents

Description

[Detailed Description of the Invention] (Object of the invention) (Industrial application field) The present invention relates to a composite gas detection element.

(Prior art) Conventionally, as a gas detection element for detecting a reducing gas in the atmosphere, a gas detection element using a sintered body of a metal oxide semiconductor such as SnO ₂ , ZnO or Fe ₂ O ₃ exhibiting n-type semiconductor characteristics has been known. Are known.
These gas sensing elements utilize a phenomenon that when a metal oxide semiconductor comes into contact with a reducing gas, the electrical conductivity thereof increases. In general, the reaction between a metal oxide semiconductor and a reducing gas is understood as an oxidation reaction caused by atmospheric oxygen adsorbed (negative ion adsorbed) on the surface of the metal oxide semiconductor. Since this reaction is a reversible reaction, when the concentration of the reducing gas in the atmosphere decreases, the element resistance returns to the value in the clean air. Therefore, the sensor can maintain its characteristics as long as physical properties such as the surface or crystal structure of the metal oxide semiconductor do not change.

With a gas detection element using SnO ₂ and ZnO, it is difficult to obtain practical detection sensitivity only with a semiconductor. For this reason, noble metal catalysts such as platinum (Pt) and palladium (Pd) or oxide catalysts are usually used as sensitizers. These catalysts may be added to the sintered body of the semiconductor, or may be formed as a catalyst layer on the surface of the semiconductor while being supported on a carrier such as alumina. The catalyst not only functions as a sensitizer, but also has an effect of improving selectivity for a gas to be detected. Factors that improve gas selectivity include the operating temperature in addition to the catalyst material.

The reason why the catalyst material acts as a sensitizer and exhibits an effect of improving gas selectivity is as follows. First, the catalyst promotes adsorption of atmospheric oxygen to the semiconductor for negative ions. As a result, the resistance value of the semiconductor increases, and the sensitivity (rate of change in resistance) to a certain concentration of reducing gas increases due to an increase in O 2 ^- ions. In addition, the catalyst is
It also promotes the reaction with O ^- ions. However, depending on the type of catalyst and the reaction temperature (element operating temperature), the oxidation reaction of the reducing gas may not cause a change in the resistance of the semiconductor. For this reason, gas selectivity occurs depending on the catalyst type and the element operating temperature.

As for the element structure of the gas detection element, at present, the mainstream is one in which a heating heater coil is inserted inside a cylindrical sintered body element. However, in recent years, research on a thin-film miniaturized sensor has been promoted in response to a demand for application of a gas detection element to a system and multifunctionality having a plurality of detection functions.

The thin-film gas sensing element has a metal oxide semiconductor film formed on the surface of the substrate, a resistor such as Pt is printed on the back of the substrate as a heater, or a metal oxide semiconductor is formed on the surface using an insulating substrate with a heater inside. After the product semiconductor film is formed, it is mounted using a technique such as wire bonding. The thin film type sensor is a so-called flat type element, unlike the conventional sintered body type, and the element can be integrated.

(Problems to be Solved by the Invention) In a semiconductor-type gas detection element, it is impossible to identify a plurality of gas types with a single element. It is necessary to integrate a plurality of types of sensitive devices. However, at present, multifunctionalization has not been achieved.

SUMMARY OF THE INVENTION An object of the present invention is to provide a composite gas sensing element that can identify a plurality of gas types with high reliability.

[Composition of the Invention] (Means and Actions for Solving the Problems) The composite gas detection element of the present invention comprises CH ₄ , H ₂ , C ₃ H ₈ , CO 2
And a C ₂ H ₅ OH gas detecting element for detecting the presence of CH ₄ and H ₂ formed on a ceramic substrate having a built-in heater. Semiconductor gas detection element, a second oxide semiconductor gas detection element for detecting the presence of C ₃ H ₈ and C ₂ H ₅ OH, H ₂ , C ₃ H ₈ , CO and
C ₂ H ₅ the third oxide semiconductor gas detecting element for detecting the presence of OH, and C ₂ H ₅ OH fourth oxide four oxide semiconductor gas detection comprising a semiconductor gas sensing element for detecting the presence of And a means for judging the outputs of the four types of gas detection elements.

In the present invention, four types of oxide semiconductor gas sensing elements having different gas selectivities can be formed by using, for example, four different types of catalysts. As means for determining the outputs of the four types of gas detection elements, means for detecting the presence of a specific gas by inputting ON / OFF of the four types of gas detection elements to the CPU can be cited.

In the present invention, the heater is localized on a part of the substrate,
In addition, it is preferable that a notch is provided in the substrate to divide the surface of the substrate so that a temperature difference is generated between the gas detection elements formed in the divided region. With such a configuration, the gas selectivity of each gas detection element can be improved.

In the present invention, it is preferable to provide an oxygen sensor in addition to the four types of oxide semiconductor gas detection elements. In general, a sensor using stabilized zirconia is known as an oxygen sensor using a solid electrolyte. In this method, a detection electrode is provided on one surface of a zirconia substrate and a reference electrode is provided on the other surface, and an electromotive force due to a difference in oxygen partial pressure on both surfaces is detected. However, since zirconia exhibits ionic conduction at a high temperature of about 600 ° C. or higher, the power consumption of the heating heater is high. On the other hand, an oxygen sensor using a fluoride ion conductor is preferable because the operating temperature is low. By providing the oxygen sensor in this manner, the oxygen concentration in the atmosphere can be monitored, and the state of oxygen deficiency can be detected.

In the present invention, it is preferable to provide a resistor for temperature compensation in addition to the four types of oxide semiconductor gas sensing elements. By providing such a resistor, it is possible to correct a change in characteristics of each gas detection element due to a change in ambient temperature.

Embodiment An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a plan view of a composite gas detection element in which four types of gas detection elements having different gas selectivities, an oxygen sensor, and a platinum resistor are integrated on a heater substrate. FIG. 2 is a side view showing the composite gas detecting element with a part cut away. 3 and 4 are plan views showing the composite gas sensing element in the order of the manufacturing process.

The rectangular heater substrate 1 has a built-in internal heater 2 localized at one corner. In the heater substrate 1, four notches 3 are provided along two short sides from two corresponding portions of two long sides to near the center, and the surface of the heater substrate 1 is partitioned into six regions. These notches 3
This has the function of preventing the heat generated from the internal heater 2 from being transmitted to other areas, and causing a temperature difference between the gas detection elements formed in the divided areas. The peripheral portion of the heater board 1, the bonding pads ₄₁ to ₁₁ are provided.
Bonding pads 4 _6, 4 ₇ is connected to the internal heater 2. Bonding pad ₄₁ to ₅ is connected to the counter electrode 5 ₁ to 5 ₅ of each gas sensing element will be described later. Among the counter electrode 5 ₅ has a common electrode of the four gas sensing element.
Counter electrode 5 ₁ is formed inside the heater 2 above, 5 on the ₅ first gas sensing element (for city gas detection) 6 is formed. First into compartment 3 adjacent to the compartments gas sensing element 6 is formed, respectively counter electrode 5 _2, 5 second gas sensing element on a ₅ (propane alcohol detection) 7, the counter electrode 5 ₃ , on 5 ₅ third gas sensing element 8 (for miscellaneous gas detection),
Counter electrode 5 _4, 5 on the ₅ fourth gas sensing element (alcohol detection) 9 is formed. An oxygen sensor 10 is formed in a section adjacent to the section in which the third gas detection element 8 is formed. The oxygen sensor 10 has a structure in which an Ag electrode, a LaF ₃ layer, and a Pt electrode are sequentially formed on an alumina substrate, as described later. Ag electrode and Pt electrodes are connected to the bonding pad 4 _8, 4 ₉ respectively by gold wires 11, 11. In a section adjacent to the section in which the fourth gas detection element 9 is formed, platinum resistors 12 are provided with bonding pads 4 ₁₀ and 4 _11.
Is formed.

Thus each bonding pad ₄₁ to ₁₁ of the heater board 1, such a configuration, the lead 13, respectively, ... are connected. Each of the leads 13,... Is connected to each of the lead pins 15,. Stem 14
On the top, a net-made protective cap 16 is formed.

A method for manufacturing the composite gas sensing element will be described with reference to FIGS. 3, 4, and 1. FIG.

The heater substrate 1 was manufactured as follows. A small amount of SiO ₂ , MgO, CaO, etc., a solvent, a plasticizer, and a resin were added to Al ₂ O ₃ as a main raw material, and mixed with a ball mill to prepare a slurry. From this slurry, a green sheet having a thickness of about 0.4 mm was produced by a doctor blade method. The green sheet has an area where a plurality of heater substrates can be formed. An internal heater 2 was formed by printing a thick-film resistor (Pt-W) for a heater on a part of the surface of the first green sheet by a screen printing method. On the surface of this green sheet, a second layer green sheet having a via hole connected to the internal heater 2 was laminated by thermocompression bonding. 2
The conductive paste was filled into the via holes of the green sheet of the layer by screen printing. At this stage, the notch 3 was provided by punching out the green sheet using a jig. Further, the platinum resistor 12 was formed by a screen printing method. Approximately 1550 ° C under a reducing atmosphere
Was fired. After firing, the surface of the substrate, and printed pattern of the counter electrode 5 ₁ to 5 ₅ and the bonding pads ₄₁ to ₁₁ (Au) by a screen printing method. One sheet was divided into a plurality of pieces to produce the heater substrate 1. The dimensions of the manufactured heater substrate 1 are 14 mm × 10.4 mm × 0.4 mm.

As a semiconductor material constituting each of the gas detection elements 6 to 9, a SnO ₂ -based semiconductor thin film was used. First, Nb resin salt was added to Sn metal soap (for example, tin 2-ethylhexanoate), and about 1% of Nb was added as an impurity to Sn at an atomic ratio. These raw material mixtures were dissolved in n-butanol, and Sn
A solution having a concentration of about 20% by weight of metallic soap was prepared. In order to apply this solution to a predetermined location (about 2 mm square) on the surface of the heater substrate 1, the periphery of the predetermined location is hardened with epoxy resin,
A predetermined amount of the solution was dropped using a dispenser. After leaving it in the air for about 1 hour, baking it at about 600 ° C for 30 minutes, about 500 hours
A 0 ° thin film was formed. As a raw material of Sn, in addition to metal soap, a resin salt, an alkoxide, or an organic compound containing Sn may be used. As impurities, Sb in addition to Nb
May be used. The amount of impurities added is the atomic ratio to Sn
0.5-5% is preferred. The firing temperature is preferably in the range of 400 to 700 ° C.

A catalyst layer was formed on these SnO ₂ -based semiconductor thin films as follows. As a catalyst layer, Pt (1.0 wt%)-Al ₂ O ₃ is used for the first gas detecting element (for detecting city gas) 6, and Rh is used for the second gas detecting element (for detecting propane / alcohol) 7.
(1.0 wt%)-Al ₂ O ₃ , Cu (1.0 wt%)-W (5.0 wt%)-Al ₂ O ₃ , fourth gas detecting element (for miscellaneous gas detection) 8
Cu (1.0wt) for the gas detection element (for alcohol detection) 9
%) - were used Al ₂ O _3.

H ₂ PtCl is used as a raw material for Pt among the components constituting the catalyst layer.
RhCl ₃ .H ₂ O was used as a raw material for ₆ · H ₂ O and Rh, copper sulfate was used as a raw material for Cu, and ammonium tungstate was used as a raw material for W. First, a Pt catalyst powder, a Rh catalyst powder, and a Cu catalyst powder were prepared as follows. An aqueous solution of each raw material was prepared, and the aqueous solution was sampled and mixed with alumina so that a predetermined amount of the catalyst was supported on the alumina carrier. 1 to
After drying under reduced pressure for 2 hours and further drying and solidifying at 100 ° C., the mixture was sufficiently ground in a mortar. This powder was calcined at about 500 ° C. for 1 hour to prepare each catalyst powder. Further, the prepared Cu catalyst powder and a predetermined amount of an aqueous solution of ammonium tungstate were mixed, and a Cu-W catalyst powder was prepared in the same manner as described above. In addition, as a raw material of Pt and Rh, an ammonium salt such as (NH ₄ ) ₂ PtCl ₅ and (NH ₄ ) ₃ RhCl ₆ may be used in addition to the above-mentioned chloride. The calcination temperature of the Pt catalyst powder and the Rh catalyst powder is preferably 400 to 800 ° C. The firing temperature of the Cu catalyst powder and the Cu-W catalyst powder is preferably from 400 to 600 ° C.

Each prepared catalyst powder was slurried using an alcohol-based organic binder or an Al resin salt. A predetermined amount of each of these slurries was dropped on the SnO ₂ -based semiconductor thin film using a dispenser. After drying at about 100 ° C. for 30 minutes, it was baked at about 500 ° C. for 5 minutes and baked to form each catalyst layer.

The oxygen sensor 10 was manufactured as follows. LaF ₃ was used as a low-temperature operation type fluoride ion conductor constituting the oxygen sensor 10. Ag was vapor-deposited on a 0.2 mm-thick alumina substrate to a thickness of 2000 mm so as to have a 1.5 mm square shape as a reference electrode. LaF ₃ powder was slurried using an organic binder such as PVA. The slurry was applied on an Ag electrode by a spin coating method and baked at about 300 ° C. for 1 hour to form a LaF ₃ layer. It was sputtered to a thickness of about 1000Å Pt as sensing electrode LaF ₃ layer on. The size of the element
The element was scribed out of the substrate so as to be 2 mm square, and this element was adhered and fixed on the heater substrate 1 with an inorganic adhesive. Approximately 30μm gold wire 11 was used as the lead wire for both electrodes, and both were bonded with conductive paste.
4 _8, 4 is connected to the _9.

Using pure nickel lead frame, by a parallel gap welder, the bonding pads ₄₁ to ₁₁ and the leads 13, after connecting the ... and lead from the lead frame 1
3, ... was disconnected. Next, the respective leads 13,... And the respective lead pins 15, embedded in the stem 14 were connected by a parallel gap welder. Further, the protective cap 16 made of net was attached to the stem 14 by caulking.

The operation of the composite gas detection element of this embodiment will be described. In this composite gas detection element, the internal heater 2 is energized,
When the surface temperature of the first gas detecting element 6 becomes about 450 ° C., the surface temperatures of the other gas detecting elements become about 400 ° C. for the second gas detecting element 7 and about 400 ° C. for the third gas detecting element 8. 350 ℃, 4th
Is about 300 ° C. Regarding the oxygen sensor 10, the thickness of the inorganic adhesive is adjusted so that the surface temperature of the oxygen sensor 10 becomes approximately 150 ° C. when the surface temperature of the first gas detection element 6 is approximately 450 ° C. This oxygen sensor 10
Indicates an output value of about 100 mV when O _{2 is} 21%. Also, the change in O ₂ concentration indicates an output change according to the Nernst equation, and the change rate is about 20 mV / decade. Platinum resistor 1
Regarding 2, the surface temperature of the first gas detection element 6 is about 450
In the case of ° C., the surface temperature is about 200 ° C., and the resistance at this time is 140 to 150 Ω. The rate of change of the resistance with respect to the temperature change is about 5Ω / 10 ° C.

In general, the gas detection sensitivity of a gas detection element is usually represented as a ratio (Rair / Rgas) between the resistance value (Rair) in the atmosphere and the resistance value (Rgas) in the atmosphere containing a predetermined concentration of gas. Table 1 shows the sensitivity of each gas detection element of this embodiment. As is clear from Table 1, each device shows high sensitivity to the following gases. 6: CH ₄ and H ₂ , 7: C ₃ H ₈ and C ₂ H
_{5 OH, 8: H 2,} C 3 H 8, CO and _{C 2 H 5 OH, 9:} C 2 H 5 OH.

Here, the gas detection level of each element is set as follows.

6: 1000 ppm of H ₂ , 7: 500 ppm of C ₂ H ₅ OH, 8: 300 ppm of CO, 9: 500 ppm of C ₂ H ₅ OH.

Thus in the case of setting the gas detection level, gas which can be detected by the gas detection element, as shown in Table 2, 6: CH ₄ and _{H 2, 7: C 3 H} 8 and C ₂ H _{5 OH, 8: H 2, C} 3 H 8, C
O and C ₂ H ₅ OH, 9: C ₂ H ₅ OH.

Therefore, from the output of each gas detection element, for example, the fifth
According to the flowchart shown in the figure, CH ₄ , H ₂ , C ₃ H ₈ , CO
And C ₂ H ₅ OH can be detected. These detection results are displayed by, for example, turning on an LED.

In addition, the oxygen concentration in the atmosphere can be monitored by the oxygen sensor 10, and an oxygen deficiency state can be detected. Further, the platinum resistor 12 can correct the characteristic change of each of the gas detection elements 6 to 9 due to the change of the ambient temperature.

[Effects of the Invention] As described above in detail, the composite gas detection element of the present invention has CH ₄ , H ₄ by integrating four types of gas detection elements.
The five gases of ₂ , C ₃ H ₈ , CO and C ₂ H ₅ OH can be distinguished with high reliability, and their industrial value is extremely large.

[Brief description of the drawings]

FIG. 1 is a plan view of a composite gas detecting element according to an embodiment of the present invention, FIG. 2 is a side view showing the composite gas detecting element with a part cut away, and FIG. 3 and FIG. FIG. 5 is a plan view showing a method of manufacturing the gas detection element in the order of steps, and FIG. 5 is a flowchart showing a method of detecting five types of gases by the composite gas detection element. 1 ... heater substrate, 2 ... internal heater, 3 ... notch,
₄₁ to ₁₁ ...... bonding pads, 5 ₁ to 5 ₅ ...... counter electrode, 6 ...... first gas sensing element (for city gas detection), 7
... Second gas detecting element (for detecting propane / alcohol), 8... Third gas detecting element 8 (for detecting miscellaneous gas),
9 ... 4th gas detection element (for alcohol detection), 10 ...
... Oxygen sensor, 11 ... Gold wire, 12 ... Platinum resistor, 13 ...
Lead, 14 ... Stem, 15 ... Lead pin, 16 ... Protective cap.

Claims (1)

(57) [Claims] 1. A composite gas detecting element for detecting five kinds of gases of CH ₄ , H ₂ , C ₃ H ₈ , CO and C ₂ H ₅ OH, each of which is formed on a ceramic substrate having a built-in heater. A first oxide semiconductor gas detection element for detecting the presence of CH ₄ and H _2, a _second oxide semiconductor gas detection element for detecting the presence of C ₃ H ₈ and C ₂ H ₅ OH, H ₂ , A _third oxide semiconductor gas sensing element for detecting the presence of C ₃ H ₈ , CO and C ₂ H ₅ OH, and C ₂ H ₅ OH
Characterized by comprising: four kinds of oxide semiconductor gas detecting elements each comprising a fourth oxide semiconductor gas detecting element for detecting the presence of a gas; and means for judging outputs of the four kinds of gas detecting elements. Type gas detection element.