JPH0332740B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for manufacturing a sensor that indicates the equivalent composition of combustion gas, and is capable of reducing oxidizing gases such as NO _x , SO _x , O ₂ and hydrocarbons, CO, alcohol, H ₂ and the like. It has a catalytic effect that allows a chemical equilibrium reaction (combustion reaction) to proceed rapidly in an atmospheric gas mixed with a reactive gas (fuel), and has high resistance in a region where reducing gas is excessive at the equivalent gas composition of combustion. It is an object of the present invention to easily obtain a responsive, long-life sensor that exhibits low metal conductivity in a region where oxidizing gas is excessive.

There is a need for inexpensive and highly reliable sensors for controlling the air-fuel ratio of automobile engines, controlling the air-fuel ratio of burner combustion, and operating safety devices for preventing incomplete combustion in gas and petroleum equipment. For these,
Conventionally, there has been an oxygen concentration detection sensor in which Pt electrodes 2 are arranged on both sides of a stabilized zirconia solid electrolyte 1 as shown in Fig. 1, and the electromotive force generated between the two electrodes 2 changes as the gas composition changes. It is commercially available.
In FIG. 1, 3 is a Pt film lead, and 4 is a porous Al ₂ O ₃ protective film. When this sensor is exposed to a gas such as oxygen-deficient combustion exhaust gas containing a mixture of oxidizing and reducing gases, the Pt of the electrode
The chemical equilibrium reaction proceeds rapidly, and the partial pressure of oxygen in the atmospheric gas changes greatly after reaching the equivalence point of combustion of both gases, resulting in a large change in the electromotive force. However, it had the disadvantage of being expensive due to the use of a large amount of Pt in the electrodes and the complicated shape.

On the other hand, an oxygen concentration detection sensor has been proposed that utilizes resistance change and has a sensor base made of SnO ₂ , TiO ₂ or MgCoO ₄ . This sensor uses SnO ₂ , TiO
Since MgCoO ₄ itself has almost no action as a combustion catalyst, it has the disadvantage of not being able to obtain a sharp change in resistance across the equivalence point of combustion unless an expensive precious metal catalyst is added separately. It has the disadvantage that when exposed to gas, the components change, resulting in poor reproducibility.

The present invention allows the sensor base to be constructed from inexpensive materials,
It exhibits the action of a combustion catalyst, exhibiting extremely low resistance in oxidizing gases, but high resistance in reducing gas atmospheres, eliminating the drawbacks of conventional devices and improving low-output devices such as thermoelectromotive elements. It also makes it possible to operate a safety valve by inserting and connecting it in series to a power source.

In addition, a sensor using a perovskite compound material with a composition similar to that of the present invention is used to detect particles contained in the air.
Methods that measure the concentration of gases such as CO, hydrocarbons, alcohol, and propane gas have been proposed, but none utilize changes in the oxygen partial pressure range, which is much larger than the gas composition targeted by the present invention. The operating mechanism is no different from a gas sensor using a normal oxide semiconductor, and it takes advantage of the fact that the relationship between resistance R and gas concentration C changes continuously as logR∝±logC. Among these, those having a composition similar to that of the present invention, for example those having Co at the B site, have a catalytic effect for chemical equilibrium reactions, and thus operate in the same manner as the present invention. However, if these materials are exposed to an atmosphere with an excess of reducing gases, they may no longer show the original low resistance value even when returned to an atmosphere with an excess of oxidizing gases, or structural damage may occur due to long-term exposure. I was feeling tired.

The present invention uses LnCoO ₃ (Ln:
We discovered that the resistance of compounds consisting of the first rare earth elements (from La to Nd) changes markedly at the combustion equivalent gas composition, and that the resistance of Co at the B site of LnCoO ₃
A part of the A site is at least one element selected from the group consisting of Fe, Mn, V, and Ti, and Ln at the A site is at least one element selected from the group consisting of Ca, Sr, and Ba. It was discovered that the compounds obtained by substituting each element with the above elements can overcome these drawbacks that occur when there is an excess of reducing gases, and that the resistance can be made extremely small in the region where there is an excess of oxidizing gases. Based on.

First, a sensor manufactured by the manufacturing method of the present invention will be described below with reference to the drawings. Figure 2 a and b are
1 shows a pellet structure sensor manufactured in an embodiment of the present invention. In the figure, 11
is the sensor base. As a raw material, oxide powder of each metal element of the compound for the sensor may be used as a starting material, but if the reaction is carried out at as low a temperature as possible, a sensor with a large specific surface area can be obtained.
In order to improve responsiveness, a predetermined amount of water-soluble salts such as acetates, oxalates, and nitrates are dissolved in water, and the powder is stirred and evaporated in a rotary evaporator for approximately 450 min in air. After heating to ℃ and pyrolysis oxidation, pressure molding and molding in air at 850 to 950℃ for 5 to 10 hours.
A powder that has been pre-calcined and reacted is used. Note that for V and Ti among the metal elements in the compound for the reactor, oxide powders are used from the beginning. If a mixed oxide is used as a starting material, a single product cannot be obtained and the raw material powder will be coarse unless calcined at a high temperature of about 300°C. Add an aqueous solution containing 20 to 40% methylcellulose to the powder thus obtained, mix well, and heat at 50°C to remove water so that the water content is 45% by volume or less. The powder was molded and fired at 1000-1200°C for 2 hours. It should be noted that if the water content exceeds 45% by volume, coarse particles may be produced during firing, which is not preferable. Reference numeral 12 denotes a sensor lead made of an alloy of Pd group or Pt group and Ag or Cu, which is embedded in the sensor base 11 during press molding and serves as both an electrode and a lead. In order to prevent it from easily coming off from the sensor base 11, a flattened tip is used. Reference numeral 13 denotes a lead made of a heat-resistant metal and electrically connected to the sensor lead 12. 14 is a ceramic tube through which the lead 13 is passed. 1
5 is a ceramic cement for fixing the lead 13 to the ceramic tube.

Figures 3a and 3b show a sensor manufactured in another embodiment of the invention. The difference from the structure shown in FIG. 2 is that the direction of embedding the alloy wire 12, which also serves as an electrode and a lead, is different. Figure a is a view of the sensor from above, and figure b is a side view. Other symbols are the same as those explained in FIG.

Next, characteristics will be explained. Here, the second
Since there is no major difference in characteristics between the structures shown in FIG. 3 and FIG. 3, the structure shown in FIG. 3 will be used as a representative for explanation. In this case, the sensor diameter is 4mmφ and the thickness
2.5mm, Pd wire outer diameter 0.3mmφ, distance between wire centers 1.0mm
It is.

The sensor was placed in a tube furnace maintained at 850℃, and _N2 containing 100PPm of oxygen was first introduced, and then
The cycle of switching to _N2 containing 100 ppm CO was repeated. The ventilation time for each gas is 1 hr.

An increase in resistance due to exposure to an atmosphere containing an excess of reducing gas will not occur instantaneously unless all of the current paths between the electrodes are exposed. For this purpose, the passageway made of the sintered body of the sensor must be narrow.
Alternatively, it seems necessary that a thin portion exists. Using La _0.4 Sr _0.6 Co _0.8 Fe _0.2 O ₃ as an example, differences in responsiveness due to differences in manufacturing methods will be explained.

Figure 4 shows the case where all oxide powders are used as raw materials (line 47) and the case where aqueous solution mixing is performed (line 48).
This figure shows the relationship between reaction temperature and response time. Methyl cellulose was added as a 30% by weight solution in water, and the firing temperature was kept constant at 1100°C. For all samples, there is a reaction temperature at which the response time is the fastest, below which a single pervskite structure cannot be obtained, and it was observed that as the reaction temperature increases, the particles become larger. It is thought that the effect of aqueous solution mixing is that since the component elements are mixed uniformly, a pervskite single phase is easily formed even at a low reaction temperature, and coarsening of the particles can be prevented.

FIG. 5 shows a comparison between 40% by weight of methyl cellulose mixed as a powder without dissolving it in water (line 49) and adding it dissolved in water (line 53). The same figure shows the effect of the amount of methylcellulose added when it is dissolved in water (lines 50 to 54). The raw material powders were all mixed with an aqueous solution and reacted at 900°C. The responsiveness is better when methylcellulose is dissolved in water, but we think this is because methylcellulose is more uniform around the particles and uniform sintering can be achieved when it is dissolved in water. The optimum amount of methylcellulose added is between 20 and 40% by weight (lines 51-53).
Below that level (addition amount: 10% by weight, line 50), the problem worsens because the porosity of the base material decreases, inhibiting the diffusion of gas from the outside; The reason why the line 54) deteriorates is considered to be because the porosity becomes too large and sintering is inhibited.

As described above, since the present invention includes a step of mixing each aqueous solution of a metal salt containing a specific metal element, the component elements used as starting materials are uniformly mixed,
Even if the reaction temperature is low, a perovskite single phase is easily formed, particles can be prevented from becoming coarse, and the specific surface area of the sensor can be increased, so the responsiveness of the sensor can be improved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an oxygen concentration detection sensor, which is one of the conventional sensors, FIGS. Figures a and b are a top view and a side view of a sensor manufactured in another example of the present invention, and Figure 4 is a comparative example using oxide powder and an aqueous solution mixture as an example of the present invention. Figure 5 shows the relationship between reaction temperature and response time for each case in which methyl cellulose is dissolved in water and mixed with raw material powder in an example of the present invention. FIG. 2 is a diagram showing a comparison with that obtained by mixing without dissolving in water. 11...Sensor base, 12...Sensor lead.

Claims (1)

[Claims] 1 Part of Co at the B site of LnCoO ₃ is selected from the group consisting of at least one element of Fe or Mn, and a part of Ln at the A site is selected from the group consisting of Ca, Sr and Ba. A method for producing a sensor that has a compound substituted with at least one element, respectively, as a sensitive body, and that indicates the equivalent composition of a combustion gas, such that the electrical resistance of the sensitive body changes greatly with the equivalent composition of the combustion gas as a boundary. a step of mixing each aqueous solution of a metal salt containing a metal element contained in the compound with each other and thermally decomposing the mixture, and then performing a heating reaction in air to obtain a raw material powder for the sensitive body; The equivalent composition of the combustion gas is specified by adding melted methyl cellulose, mixing, drying, molding, and burning in air at a temperature of 1000 to 1200°C to obtain the reactor. How to manufacture the sensor. 2. A method for manufacturing a sensor for indicating the equivalent composition of combustion gas according to claim 1, characterized in that a heating reaction is carried out in air at a temperature of 850 to 950°C. 3. The method for manufacturing a sensor for indicating the equivalent composition of combustion gas according to claim 1, wherein the metal salt is any one of acetate, oxalate, and nitrate. 4 20-40% by weight of methylcellulose in raw material powder
2. A method for manufacturing a sensor for indicating the equivalent composition of combustion gas according to claim 1, characterized in that: 5. The method for manufacturing a sensor for indicating the equivalent composition of combustion gas according to claim 1, characterized in that the methyl cellulose is mixed and then dried so that the water content is 45% by volume or less.