JPH04268448A - Gas sensor - Google Patents

Abstract

PURPOSE:To obtain a gas sensor by using a joint material which enables a stem and a cap to be joined at a low cost as well as in the atmospheric air. CONSTITUTION:A gas sensor has a sensor element 4 which is sealed up into a sealing member made up of a stem 2 comprising ceramics and a cap 6 comprising a porous ceramics. The stem 2 and the cap 6 are joined by a joint material with an expansion coefficent thereof approximate thereto. The joint material is an inorganic adhesion 7 which contains at least an inorganic oxide and presents acidity or alkalescence. The gas sensor 1 thus obtained allows the joining of the stem 2 and the cap 6 firmly in the atmospheric air and under a temperature of room temperature to about 150 deg.C without affecting a lead 5 and the sensor element 4 adversely thereby achieving a reduction in cost with higher workability.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor, and more particularly to a gas sensor in which a stem and a cap, which are enclosing members of the gas sensor, are bonded together with an inorganic adhesive.

0002

2. Description of the Related Art In recent years, various gas sensors have been proposed and put into practical use. Among them, a limiting current type oxygen sensor using a solid electrolyte is made of pure zirconia (ZrO2).
A divalent or trivalent metal oxide such as yttria (Y2O3), magnesia (MgO), or calcia (CaO) is dissolved in a solid solution of several mol% to maintain a fluorite-shaped cubic crystal structure even at low temperatures. It has various features such as being able to detect a wide range of oxygen partial pressures, fast response speed, stable electromotive force, and can be used in high-temperature atmospheres. This sensor is suitable for a variety of purposes, including concentration measurement, combustion management for engines and boilers, and pollution measurement.

[0003] In general, in this oxygen sensor, a sensor element for detecting oxygen concentration is sealed inside an enclosure member, and the enclosure member is composed of a stem made of ceramic and a cap made of porous ceramic. is bonded with amorphous glass (bonding material) whose expansion coefficient is similar to that of these.

[0004] The material of the stem is alumina (A
Ceramics such as 12O3) are preferably used. In addition, the cap is mainly made of porous ceramics that have air permeability, and the material is Al2O3, which is extremely easy to handle in mass production and has excellent heat resistance.
or steatite ((Mg3Si4O10(OH)2), etc. are preferably used. Note that if the structure of the oxygen sensor does not need to be explosion-proof, a stainless steel mesh cover may be used instead of the cap. be.

[0005]

[Problems to be Solved by the Invention] Incidentally, in the above-mentioned oxygen sensor, it is common to use amorphous glass, which has a coefficient of expansion similar to that of the stem and cap, to join the stem and cap. When joining using glass, it is necessary to heat the amorphous glass to at least about 400° C. to melt the amorphous glass. In this case, since the stem pin holding the sensor element will be oxidized if heated in the atmosphere, it is common to heat it in a neutral atmosphere using an inert gas such as nitrogen (N2).

[0006] However, when heating in a neutral atmosphere, there is a drawback that running costs are high because it is necessary to keep N2 gas flowing at all times. Further, in this case, it is necessary to control the oxygen (O2) concentration in the N2 gas on the order of ppm, and there is also the problem that controlling the O2 concentration is extremely difficult. Therefore, it is extremely important for oxygen sensor manufacturers to search for bonding materials that do not require O2 concentration control, can be bonded firmly by heating in the atmosphere, and are low-cost. It became a development theme.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a gas sensor using a bonding material that allows a stem and a cap to be bonded in the atmosphere at low cost.

[0008]

[Means for Solving the Problems] In order to solve the above problems, the present invention employs the following gas sensor. That is, in a gas sensor, a sensor element is enclosed within an enclosure member constituted by a stem made of ceramics and a cap made of porous ceramics, and the stem and cap are joined by a joining material whose expansion coefficient is similar to that of the stem and cap. The bonding material is characterized in that it is an inorganic adhesive that contains at least an inorganic oxide and exhibits acidity or weak alkalinity.

[0009]

[Operation] In the gas sensor of the present invention, the step of joining the stem and the cap is by using an inorganic adhesive containing at least an inorganic oxide and exhibiting acidity or weak alkalinity as a joining material for joining the stem and the cap. Heating and bonding in the atmosphere becomes possible, and there is no need to use N2 gas or control O2 concentration as in the past.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a front sectional view showing an oxygen sensor 1 which is an embodiment of the gas sensor of the present invention. In this oxygen sensor 1, the lead wires 5 of the sensor element 4 are connected to the tips 3a of four stem pins 3, etc. provided perpendicularly to the stem 2, and the sensor element 4 has air permeability. It is covered with a cap 6 made of porous ceramic. The cap 6 is bonded to the stem 2 with an inorganic adhesive 7.

[0011] The above-mentioned inorganic adhesive 7 is, for example, Al2O
3. Contains inorganic oxides such as SiO2 and exhibits acidity or weak alkalinity.

Next, the stem 2 is attached using various inorganic adhesives.
The results of an experiment in which the cap 6 and the cap 6 were joined will be explained. Table 1 shows the composition ratio and properties of the inorganic adhesive used in the experiment, and Table 2 shows the experimental results.

[Table 1]

[Table 2] Here, the stem 2 was made of materials such as Al2O3 and steatite ((Mg3Si4O10(OH)2), and the cap 6 was made of porous Al2O3 with air permeability. The conditions were 110°C for 30 minutes.

As is clear from Table 2, No. A (using a weakly alkaline inorganic adhesive) and No. In case B (using an acidic inorganic adhesive), the bond between the stem 2 and the cap 6 was extremely strong. Furthermore, no embrittlement was observed in the lead wire 5, and no abnormality was observed in the characteristics of the sensor. However, No. C~No. For F (using strong alkaline inorganic adhesive), stem 2
Although the bond between the lead wire 5 and the cap 6 was strong, the surface of the lead wire 5 had lost its luster and was severely brittle. in this case,
Problems also arose regarding sensor characteristics.

[0014] From the above results, the acidic or weakly alkaline inorganic adhesive of the present invention has a
It was found that the stem 2 and the cap 6 were firmly joined under the low-temperature heating conditions of 30 minutes at °C, there was no embrittlement of the lead wire 5, and no abnormalities were observed in the characteristics of the sensor.

As explained above, according to the above embodiment, the stem 2 and the cap 6 are bonded using an inorganic adhesive that contains an inorganic oxide such as Al2O3 or SiO2 and is acidic or weakly alkaline. Therefore, the stem and the cap can be firmly joined in the atmosphere at a temperature of about room temperature to 150° C. without adversely affecting the lead wire 5 and the sensor element 4. Therefore, it can be easily carried out using a general-purpose dryer or the like, improving workability and reducing costs. Further, there is no need for a conventional neutral atmosphere furnace, and there is no need to supply N2 gas or control O2 concentration. The above makes it possible to provide a gas sensor that can improve workability and reduce costs.

[0016]

As described in detail above, according to the gas sensor of the present invention, the sensor element is enclosed within an enclosure member composed of a stem made of ceramics and a cap made of porous ceramics, and In a gas sensor in which the cap and cap are bonded using a bonding material with a similar expansion coefficient to these, the bonding material is an inorganic adhesive that contains at least an inorganic oxide and exhibits acidity or weak alkalinity, so that Moreover, the stem and the cap can be firmly joined at temperatures ranging from room temperature to about 150° C. without adversely affecting the lead wires and the sensor element. Therefore, it can be easily carried out using a general-purpose dryer or the like, improving workability and reducing costs. Further, there is no need for a conventional neutral atmosphere furnace, and there is no need to supply N2 gas or control O2 concentration. The above makes it possible to provide a gas sensor that can improve workability and reduce costs.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of an oxygen sensor that is an embodiment of the present invention.

[Explanation of symbols]

1 Oxygen sensor 2 Stem 3 Stem pin 4 Sensor element 5 Lead wire 6 Cap 7 Inorganic adhesive

Claims (1)

[Claims] 1. A sensor element is enclosed within an enclosure member constituted by a stem made of ceramics and a cap made of porous ceramics, and the stem and the cap are joined by a joining material having a coefficient of expansion similar to that of the stem and the cap. A gas sensor characterized in that the bonding material is an inorganic adhesive containing at least an inorganic oxide and exhibiting acidity or weak alkalinity.