JPH0711499B2 - Ozone sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ozone sensor, and more particularly to an ozone sensor used for ozone concentration control or ozone detection in an ozone generator or equipment utilizing ozone.

[0002]

2. Description of the Related Art Ozone has a strong oxidizing action and is used in many fields such as water and sewage treatment, medical treatment, and food industry for the purpose of deodorization and sterilization. However, on the other hand, although it has such a useful utility value, even if a very small amount of ozone has a harmful effect on the human body, it is necessary to reliably control the amount of ozone generated and detect leaked ozone.

Under such circumstances, the redox titration method, the absorptiometry method, the ultraviolet absorption spectrum method and the like have been used more and more for the purpose of detecting ozone and measuring the ozone concentration. On the other hand, recently, a simpler ozone concentration measuring method has been desired, and a small resistance change type ozone sensor using a metal oxide such as In ₂ O ₃ has been proposed as one method.

[0004]

However, the methods conventionally used generally have a drawback that they require a large-scale device, complicated operations, and are expensive and therefore cannot be easily used.

On the other hand, in the case of an ozone sensor using a metal oxide, which has been proposed as a simple ozone concentration measuring method, a reducing gas such as CO or alcohol or N in the measurement atmosphere is used.
O _X, the oxidizing gas such as SO _X is present, the electrical resistance of the sensor changes according to the amount of the gas. Therefore, the sensor output changes, which often causes malfunction of the ozone detection system or the control system attached thereto. There is also a slight problem with thermal stability. The present invention solves such a problem, and provides an ozone sensor that reduces the influence of a reducing interfering gas or an oxidizing interfering gas to a negligible level and also has thermal stability. To aim.

[0006]

The ozone sensor of the present invention solves the above-mentioned problems by providing a pair of electrodes formed on a substrate and mainly a metal oxide formed between the pair of electrodes. And a MnO ₂ , Fe ₂ O ₃ , Co ₃ O ₄ , and NiO so as to cover the gas sensitive body composed of
The coating layer is made of a material mainly containing at least one oxide selected from the above and MgO.

[0007]

With this configuration, when the interfering gas species is a reducing gas, the ozone sensor of the present invention is oxidized and inactivated by the oxidation catalytic action of oxides such as MnO ₂ , so that it has an effect on the sensitizer. Will be almost negligible. On the other hand, if the interfering gas species is an oxidizing gas such as NO _x ,
It reacts with MgO contained in the coating layer to produce nitrate or basic nitrate. As a result, NO _X for the sensor
The effect of is greatly reduced, and the characteristic change of the sensor hardly occurs. The nitrated MgO is easily thermally decomposed to become MgO again and the function is regenerated. Further, since MgO, which is a constituent material of the coating layer, has an effect of preventing the coating layer itself and the sensitive body from being sintered, the thermal stability of the sensor is improved. Since the coating layer is effective even if it is thin to some extent, the reduction in size and weight of the sensor element is not impaired.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An ozone sensor according to an embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 1 is a substrate made of alumina, and 2 is an In ₂ O ₃ (95 wt) film formed by screen printing using an ink prepared mainly from an organometallic compound of In and Sn and baked at 500 ° C.
%) And SnO ₂ (5 wt%) gas sensitive body (thickness about 1000 Å), 3 is Co ₃ O ₄ about 40 wt%, M
Coating layers (thickness: about 10 μm) 4 composed of gO about 40 wt% and Al ₂ O ₃ about 20 wt% are platinum electrodes formed on the substrate 1 in advance. As a comparative example, an ozone sensor (comparative example 1) having a sensitizer having the same composition as that of the example and having a coating layer containing only Co ₃ O ₄ formed thereon, Mg
An ozone sensor having a coating layer containing only O (Comparative Example 2), an ozone sensor having a coating layer containing neither Co ₃ O ₄ nor MgO (Comparative Example 3), and an ozone sensor having no coating layer (Comparative Example) 4) was prepared respectively. The substrate and other shape and heat treatment conditions were all the same.

Using these ozone sensors, the effects of CO and NO ₂ on the ozone response characteristics were evaluated by the following method. First, the ozone sensor was fixed to the heater for heating the sensor element, set in the measurement box, and the heater was energized to heat and maintain the sensor element temperature at 350 ° C. Then 1p
Pm ozone, 2 ppm CO, and 1 ppm NO ₂ were individually or simultaneously injected into the measurement box and brought into contact with the ozone sensor, and the electrical resistance of the ozone sensor in each case was measured. The results are shown in FIGS. 2 (a), (b), (c),
Each is shown in (d). The electric resistance is shown on an arbitrary scale common to each drawing. FIG. 2A shows the case of ozone single injection. The examples and comparative examples (1 to 4) show almost the same electric resistance change. 2 (b) is C
The case where only O is injected is shown. The electric resistances of the sensors of Examples and Comparative Example 1 were close to the reference level (air level), but the electric resistances of the sensors of Comparative Examples 2, 3, and 4 were much lower than the reference level. It is shown that the sensors of Comparative Examples 2, 3, and 4 are sensitive to CO. In addition, FIG.
The case where only O _{2 is} injected is shown. Example and Comparative Example 2
The electric resistance change of the sensor of No. ₂ was small and almost equal to the reference level, but the sensors of Comparative Examples 1, 3 and 4 showed a large electric resistance change in response to NO ₂ . Further, as shown in FIG. 2 (d), when ozone and a mixed gas composed of CO and NO ₂ were injected, the sensor of the example showed a change in electrical resistance almost equal to the ozone level. The electric resistances of the sensors of Comparative Examples (1 to 4) each showed a level change as shown in the figure.

Generally, semiconductor gas sensors have a drawback that they have poor gas selectivity. In the case of the sensor of the example, since the sensitizer is an n-type semiconductor, when the reducing gas comes into contact with it, the electrical resistance decreases contrary to the case of ozone contact. Therefore, when ozone and reducing gas coexist,
The change in electrical resistance of the sensor is small. On the other hand, when the oxidizing gas comes into contact, the electric resistance of the sensor increases as in the case of ozone. Therefore, when ozone and oxidizing gas coexist, the electric resistance change becomes larger. When the sensitive body is a p-type semiconductor, the direction of change in the electrical resistance of the sensor depending on the gas species is opposite, but the above discussion holds in a similar manner. As a result of such an operation, an output signal may be obtained even when ozone to be originally detected does not exist, or a different concentration may be erroneously determined in the case of concentration detection. In order to avoid such a situation, it is necessary to suppress the influence of the gas serving as an interfering signal source on the sensor output signal of the gas to be originally detected as much as possible. For this reason, forming a coating layer for blocking an interfering gas on the sensor sensitive body can be a very effective means. In the case of the sensor of this example, it is effective to provide a coating layer containing Co ₃ O ₄ and MgO, and it is clear that the influence of CO and NO ₂ can be suppressed to an extremely low level. did.

Next, the thermal stability of this sensor was confirmed. The ozone sensor of the above-mentioned example and the comparative example (1-
The ozone sensor of 4) was continuously left in an air atmosphere of 450 ° C., taken out every 200 hours, set in the measurement box in the same manner as above, and the electric resistance of the ozone sensor when ozone having a concentration of 1 ppm was injected was measured. Then, the sensor sensitivity was obtained. The sensitivity of the sensor is the ratio (R) of the electrical resistance (R _G ) of the sensor during ozone injection and the electrical resistance (R _A ) in the air.
_G / _RA ) was used. The result is shown in FIG. The sensitivity of the sensor of the example and the sensor of the comparative example 2 (containing MgO in the coating layer) hardly changed with time, but the respective sensors of the comparative examples 1, 3 and 4 showed a tendency of lowering the sensitivity. Since this sensor is mainly composed of metal oxides, thermal stability can be basically secured to some extent, but the effect of MgO used as the constituent material of the coating layer is particularly large, and the sensor and coating It contributes to prevention of sintering of the layer itself. Therefore, it is considered that the gas diffusion rate and the reaction rate hardly change, and the temporal stability of the sensor sensitivity is obtained.

As described above, the ozone sensor of this embodiment is extremely stable and has excellent characteristics. In the examples, the case where Co ₃ O ₄ and MgO are contained in the coating layer has been described, but even if MnO ₂ or Fe ₂ O ₃ or NiO is used instead of Co ₃ O ₄ , almost the same effect can be obtained. You may mix them in multiple numbers. The coating layer is Co ₃ O ₄ and Mg
Although a single layer of a mixture of O and a layer containing Co ₃ O ₄ and Mg
A two-layer structure with a layer containing O may be used. The same applies when other oxides are used. As a method of forming the coating layer, various methods such as coating, printing, vapor deposition, and thermal spraying can be used depending on the shape of the sensor element. As the interfering gas, CO and NO ₂ have been explained.
The same effect is exhibited in the case of O, SO _X , hydrocarbons, etc. In the example, In ₂ O ₃ (95 wt.
%) + SnO ₂ (5 wt%) has been described, but the same can be applied to other composition ratios or other materials. Further, although only the case of the thin film type sensor has been described as the form of the sensor, it can be similarly applied to the thick film type sensor and the sintered type sensor, and the effect can be obtained in the same manner. The structure and shape of each part of the sensor element, the substrate material, and the electrode material can be freely designed or used without departing from the spirit of the invention.

[0013]

As is apparent from the above description of the embodiments, the ozone sensor of the present invention has excellent gas detection characteristics, thermal stability, small size, light weight, and low cost. It is also suitable for use in ozone concentration control or ozone detection in equipment using ozone.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic configuration of an ozone sensor according to an embodiment of the present invention.

FIG. 2A is a graph showing a change in electric resistance of the ozone sensor when injected with ozone alone; FIG. 2B is a graph showing a change of electric resistance when injected with CO alone of the ozone sensor; A graph showing a change in electric resistance when the sensor is injected with NO ₂ alone (d) is a graph showing a change in electric resistance when the mixed gas of ozone, CO, and NO ₂ of the ozone sensor is injected

FIG. 3 is a graph showing the thermal stability of the ozone sensor.

[Explanation of symbols]

 1 substrate 2 gas sensor 3 coating layer 4 electrode

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Kunio Kimura Inventor Kunio City, Osaka Prefecture 1006 Kadoma, Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-63-298149 (JP, A) JP-A-4-152258 (JP, A) JP-A-4-152259 (JP, A) JP-A-4-1563 (JP, A)

Claims (1)

[Claims] 1. A pair of electrodes formed on a substrate, a gas sensitive body mainly composed of a metal oxide formed between the pair of electrodes, and formed so as to cover the gas sensitive body. An ozone sensor having at least one oxide selected from MnO ₂ , Fe ₂ O ₃ , Co ₃ O ₄ , and NiO, and a coating layer made of a material mainly containing MgO.