JPS6024442A - CO sensor - Google Patents

Abstract

PURPOSE:To obtain a sensor, which can detect the CO concentration in combustible gas highly sensitively with stable aging property, by utilizing a sintered body or sintered film, in which a specified rate of Au is included in CdO, as a sensitive body. CONSTITUTION:CdO is added in an aqueous solution of HAuCl4.4H2O, and powder, in which 0.1-10wt% of Au is included with respect to CdO, is obtained. Two Pt wires are embedded in the powder. After pressurization and molding, the powder is burned in air, and a porous sintered body 1 is formed. Meanwhile, an NaOH solution is dropped in a CdSO4 solution, and the yielded precipitate at pH9 is filtered and washed. Then the powder is dried. An HAuCl4 solution is mixed to the dry powder by a dry method, and triethanolamine is added so as to obtain a paste state. A pair of Au electrodes 2 in a comb shape is formed on a substrate 1 comprising Al2O3 or the like and coated by above described material. The material is burned and a sensitive film 5 is formed. By using the sintered body 1 or the film 5, e.g., a heater 2 is attached to the sintered body 1. Said device is fixed to a header 13. A wire gauze 14 is attached to the header and a CO sensor is formed. Thus the sensor with good durability is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a Co sensor using a composite metal oxide semiconductor used for detecting combustible gases.

Conventional Structures and Their Problems In recent years, various research and developments have become active regarding materials for sensing elements for flammable gases. This is strongly attributable to the fact that explosions and poisoning accidents caused by flammable gases occur frequently, mainly in households and in various factories, and have become a major social problem. Among these, in particular, those that detect propane gas or city gas have been developed and put into practical use with considerably high levels of sensitivity and reliability. These films are widely applied, for example, to various gas leak alarms.

On the other hand, as another social need for gas disaster prevention, CO
Detection has become a hot topic. This is largely due to the spread of various gas appliances and the airtightness of housing structures.

That is, there is a problem of incomplete combustion of gas appliances or poisoning due to CO generated from new building materials in the early stages of a fire.

In particular, the latter is an extremely important social problem because it accounts for the majority of deaths caused by fire. However, at present, there is no inexpensive and simple gas sensor that can accurately detect CO, and the above-mentioned social needs are not fully met. The reason is,
In the case of a sensor for general combustible gases, the concentration of combustible gas to be detected is a fraction of the lower explosive limit, but in the case of a sensor for CO This is because extremely small amounts of CO must be detected.

That is, in the case of sensors for other combustible gases, the purpose is to prevent gas explosions, whereas in the case of CO sensors [1d, the main purpose is to prevent Go poisoning.

This is because the amount must be detected at an extremely small amount compared to the lower explosive limit.

In combustible gas sensors that are inexpensive and reliable, oxide semiconductors that are kept at high temperatures are often used to detect changes in their resistance. Several types of oxide semiconductors have been found that are highly sensitive or selectively sensitive to CO, but unfortunately, sensors with sufficient reliability have not yet been obtained. be.

The purpose of the invention was made in view of this situation, and C
A highly sensitive and highly reliable CO sensor is realized.

Structure of the Invention The present invention was discovered while studying the effects of additives on a gas sensing element using cadmium oxide (CaO) as a gas sensitive material.

That is, the CO sensor of the present invention uses CdO to which 0.1 to 10% by weight of Au is added as a gas sensitive material.This improves the gas sensitive characteristics and its reliability. This was achieved based on the discovery that the sensitivity was dramatically improved and that it was possible to realize a sufficiently high sensitivity for practical use even to the minute amount of CO mentioned above.

Description of examples The present invention will be explained in detail below.

In Example 1, commercially available CdO was used.

The effect of the amount of Au added on this will be described.

[Example 1] Commercially available cadmium oxide (CaO) was mixed with commercially available chloroauric acid (
HAuCl4・4H20) is dissolved in water and the concentration is 1.
00-4t solutions were added as shown in Table 1, respectively. The respective powders were then dry mixed for 3 hours using a mixer. Two platinum wires were embedded in this powder, which was press-molded into a cylindrical shape with two diameters and three heights, and was fired in air at 75Q°C for 1 hour. The obtained porous sintered body was attached to a sensing element header, and a coil-shaped heater was placed in place of one of the sintered bodies.
CO sensor t [′f
? -O Figure 1 shows the structure of the QcO sensor.

In the figure, 1 is a sintered body in which electrodes 3 and 4 made of two platinum wires are embedded. Reference numeral 2 denotes a heater for heating the sintered body 1, and power is supplied to the heater from heater pins 11 and 12 through heater frames 7 and 8. The resistance of the sintered body 1 is configured to be measured between the pins 9 and 1o from the electrode 3.4 through the frame 6.6t. Heater pins 11 and '12 and pins 9 and 1o are header 1
3, and a stainless steel wire mesh 14 is attached to the heggoni.

Regarding the GO sensor obtained as described above, the gas sensitivity characteristics and the energized life at normal usage temperature (4so° C.) were investigated.

The method for measuring gas sensitivity characteristics is to apply a current to the heater part of the CO sensor in advance, adjust the temperature of the sensitive body to 360°C, and then insert it into a measurement box with a known volume. , test gas (Co gas) with a syringe
A mixed gas of 5.0% and N2 (96.0%) and N2 gas (99% or more)) are injected into the measurement box, and when the concentration of CO or N2 reaches 0.01% by volume (100F)K. The resistance value of the sintered sensitive body was measured. The gas concentration to be measured was chosen to be 1100 ppm because the permissible concentration of CO for industrial hygiene is 100 ppm, so it was necessary to be sensitive to at least this concentration or lower.

The gas sensitivity characteristics are as follows: (1) Gas sensitivity e (resistance value in air (R
a)/resistance value in gas (Rg)), and (11) rate of change in resistance over time ΔR (rate of change in resistance value relative to the initial value when the sensitive body is held at a temperature of 450° C. for 2000 hours). Table 1 and Figure 3 show the gas sensitivity (Ra/Rg) when an additive (MU) is added and the rate of change in resistance over time (ΔR).
shows.

Table 1 Comparative Example Table 1 and Figure 3 show that adding 1.0 to 10% by weight of MuU to CdO shows extremely high activity towards CO, and is stable over time. As a result, it can be seen that extremely high gas sensitivity and reliability can be achieved.

In Example 1, the sensitive body is a sintered body, and the additive amount of Au is described when a commercially available reagent is used as the base material CdO. In Example 2, which will be described below, we will discuss the effect of adding Mu when the sensitive body is a sintered film and the base material CdO is a powder obtained by a precipitation method.

[Example 2] The starting material was commercially available cadmium sulfate (CdS04, rH
20) 100g of the solution was dissolved in water from step 11 and stirred while maintaining the temperature at 60°C. While keeping the temperature of this solution at 50°C, add 2N sodium hydroxide (NaOH) solution to this solution.
The solution was added dropwise at a rate of 1 minute until the hydrogen ion concentration of the solution reached 9. Immediately after the dropwise addition was completed, the precipitate was suction-filtered. Furthermore, water was repeatedly poured over this precipitate,
Washed thoroughly.

The powder thus obtained was heat treated in air at 110° C. for 1 hour. to this heat-treated powder.

A solution prepared by dissolving commercially available chloroauric acid (HAuCl4.4H20) in water to a concentration of 1001 q/ml was added as shown in Table 2. Then, the respective powders were dry mixed for 3 hours using a sieve machine.

This powder was sized to a size of 60 to 100 microns and presented to Trieno. In the figure, the surface of an alumina substrate 1 with a thickness of 5 mm and a length of 5 mm each as a substrate for a CO sensor is Q and 5 mm.
A pair of comb-shaped gold electrodes 2 were formed between the dragons. A gold electrode 3 for the resistor was also formed on the back side at the same time, and during this time a glaze resistor 4 was printed and baked to serve as a heater.Next, the above paste was printed on the surface of the reed board to a thickness of about 7071mm. After air drying at room temperature, it was gradually heated in a trench to a temperature of 750° C. and maintained at this temperature for 1 hour. At this stage, the paste evaporated and became a sintered film 6. The thickness of this gas sensitive body was approximately 65μ. In this way C
Obtained an O sensor.

Table 2 - Comparative Example The gas sensitivity characteristics of each CO sensor were measured in the same manner as in Example 1. %CdO in Table 2 and Figure 4
The gas sensitivity (Ra71g) and resistance change rate (ΔR) are shown when MuU is added to the sample.

As is clear from Table 2 and FIG. 4, even if the sensitive body is a sintered film or a powder prepared by the CdO precipitation method, the results obtained in Example 1 are The same characteristics are obtained. Further, the rate of change in resistance value over time is also very small, as in Example 1.

Moreover, from Table 2 and FIG. 4, if the amount of Au added is less than 0.1% by weight, there is no effect, and the effect of the present invention cannot be expected. On the other hand, if the amount added exceeds 10.0% by weight, it becomes impractical in terms of reduced gas sensitivity or stability of characteristics. The amount of Au contained in the gas sensing element of the present invention is
The reason why the amount added to CdO is limited to 0.1 to 10% by weight is based on the above-mentioned reason.

By the way, in Example 1, a commercially available oxide reagent was used as cadmium oxide, and in Example 2, cadmium sulfate was used as the starting material for cadmium. Regarding Au Vc, we have described the one using commercially available chloroauric acid, but in the present invention, the final composition of the sensitizer may be within the above-mentioned range.
This does not limit the starting materials or manufacturing process in any way.

As described in detail, the CO sensor of the present invention uses a sintered body or a sintered film in which Au is added to C(10) as a sensitive body. It is possible to realize high sensitivity to Co gas.This is due to the fact that there are many accidents caused by CO poisoning due to incomplete combustion of gas appliances or early stages of fires. It accurately responds to the needs of society as society's demands become more modest, and its effects are extremely significant.One advantage of the present invention is the lifespan characteristics, especially the change in resistance value over time due to energization. This is a significant reduction in variation.In other words, this makes an extremely significant contribution to improving the reliability of the device, which is the most important element of any sensing device.

Figures 3 and 4, which show the structure of the sensor, show the amount of additives in the CO sensor and the amount of CO and H2 in an embodiment of the present invention.
FIG. 2 is a characteristic diagram showing the relationship between sensitivity (Ha/Rg) and resistance change rate over time (ΔR).

1... Sintered body.

Name of agent: Patent attorney Toshio Nakao (1 person)
Low to W& pulse old masu palm dam

Claims (2)

[Claims] (1) Cadmium oxide (CdO) VC gold (MU) is 0.
A Co sensor characterized in that a Co sensor containing 1 to 10% by weight is used as a gas sensitive material. (2) Claim (1) characterized in that the gas sensitive body is a sintered body obtained by pressure molding and firing, or a sintered film obtained by printing and firing a paste. Co sensor as described.