JPH0220121B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high temperature thermistor that is not easily affected by gas. Recently, high-temperature thermistors have been used for overheating prevention devices for catalysts in internal combustion engine exhaust gas purification devices and temperature compensation for exhaust gas oxygen sensors, and stabilized ZrO ₂ is the main component of high-temperature thermistors used at temperatures between 300 and 1000℃. However, there was a problem in that it required high-temperature firing at 1,650°C or higher, and the resistance value easily changed depending on the oxygen partial pressure in the atmosphere. Also, M(Al _x , _Cry ,
The spinel system, which has the general formula of Fe _z ) ₂ O _4, also requires high-temperature firing along with ZrO ₂ , and the platinum wire of the embedded electrode becomes brittle due to high-temperature firing, so a thick electrode wire with a diameter of 0.5 mm or more is required. There was a problem in making the elements expensive. Improve these shortcomings,
In order to obtain a thermistor that can be fired at temperatures below 1600°C, has minimal weakening of the electrode wire, allows the use of thin electrode wires of 0.4 mmφ or less, and has good durability and a stable temperature coefficient, the applicant first developed Cr ₂ O ₃ An application was filed for a high-temperature thermistor composition consisting of the three components -V ₂ O ₅ -TiO ₂ (Japanese Patent Application No. 110932/1982, Japanese Patent Application No. 37269/1983). However, when the thermistor using this composition adheres to the surface of the element, which is generated when the air-fuel ratio of the fuel mixture used for combustion is in a rich state, The carbon film also acted as an electrode, and had the problem that the thermistor resistance decreased as the apparent electrode area increased. High-temperature thermistors containing titania as a main component are also known. This titania had the disadvantage that when it reached a high temperature of around 600°C, the titania itself became active against oxygen, causing a change in resistance. Therefore, JP-A No. 56-84541, 55-
Nos. 158548 and 55-162046 propose providing a glass layer on the titania surface. This glass layer on the titania surface blocks exhaust gas and prevents resistance changes due to titania's gas sensitivity. As a result, titania can be used as a high-temperature thermistor. However, due to the mismatch in thermal expansion coefficients between glass and titania, cracks are likely to occur in the glass due to thermal shock. Another problem is that the glass flows at high temperatures, which increases the resistance change of the thermistor. The inventors of the present invention have conducted intensive studies on high-temperature thermistors that do not have the above-mentioned problems, and have solved the above-mentioned conventional problems at once by applying an insulating coating with sufficiently high specific resistance to the surface of the thermistor element as the outermost layer. However, they have discovered that temperature can be measured accurately and quickly, and have completed the present invention. In other words, the gist of the present invention is that a porous coating layer having a resistivity 100 times higher than the material constituting the thermistor element is provided as the outermost layer on the surface of a thermistor element in which a pair of electrode wires are embedded. This is a high temperature thermistor that is characterized by the following. Next, the present invention will be explained in detail with reference to the drawings. Fig. 1 shows one embodiment of the thermistor element used in the high temperature thermistor of the present invention. Zirconium oxide (ZrO ₂ ), spinel having the general formula of M(Al _x , _Cry , Fe _z ) ₂ O ₄ etc. can also be used, but the present applicant disclosed it in JP-A-56-37269. Preferably, it consists of the following composition: That is, the composition _is (X) _Cr2O3- (Y)
In the ternary composition diagram in Figure 2, which is represented by V ₂ O ₅ -(Z)TiO ₂ , X=99.5 Y=0.5 Point A at Z=0 X=70 Y=30 Point B at Z=0 X= Quadrilaterals A, B, formed by line segments connecting point C at 0.5 Y=30 Z=69.5 and point D at X=0.5 Y=0.5 Z=99.0.
A composition within the area of C, D or 100 parts by weight of the composition,
This is a high temperature thermistor composition comprising 20 parts by weight or less of one or more selected from Al ₂ O ₃ , SiO ₂ , CaO, BaO, MgO and MnO ₂ . Reference numeral 2 denotes an electrode wire whose tip is embedded in the thermistor element 1, and on the surface of the thermistor element 1, a porous coating layer 3 whose specific resistance is 100 times or more higher than the material constituting the thermistor element 1 is formed. It is provided as the outermost layer. Since the porous film layer 3 has such a high specific resistance, it has the effect of blocking surface leakage of current due to carbon attached to the surface. In order to obtain this effect more effectively, the specific resistance of the porous coating layer 3 is preferably 1000 times or more the specific resistance of the material constituting the thermistor element 1. In addition, since the thermistor shown in the figure only has a porous film layer 3 on the outside of the thermistor element 1, the thermistor element 1 is protected from the object to be measured such as gas.
heat is transferred almost unimpeded. Therefore, this thermistor responds sharply to changes in the temperature of the object to be measured. Furthermore, the porous coating layer 3 is less likely to crack due to its porous nature. That is, even if a thermal gradient occurs and, as a result, one part expands more than another, much of the expansion in that part occurs in a way that reduces the volume of the pores, and is therefore due to distortion due to the thermal gradient. Cracks are less likely to occur. As a specific material for the porous coating layer 3, ceramics such as alumina, silica, forsterite, and spinel are preferable. Among these, alumina has high specific resistance and heat resistance, and is a material that constitutes many thermistor elements 1. Thermal expansion characteristics are similar to that of the same material, making it an excellent coating material. If the thickness is 5μ or more, the influence of carbon on the surface can be excluded. Therefore, there is no need to make it very thick, and a thickness of 500 μm or less is sufficient. The thickness and porosity of the porous film layer 3 affect its ability to prevent surface leakage of current and its own resistance to cracking, but when both are taken into consideration comprehensively, , for alumina, a porous membrane of 10-400μ and a porosity of 10-40% gives good results. It is considered that the same range is preferable for materials other than alumina as long as the film eliminates the surface leakage. For example, the porous coating layer 3 can be provided on the thermistor element 1 in the following manner. First, powder of the material constituting the thermistor element 1 is placed in a mold, the electrode wires 2 are embedded in the powder, and both are pressed into a predetermined shape. Next, the coating material is sprinkled in water to form a slurry, and the obtained molded article is dipped in the slurry to coat the surface of the molded article with the coating material. In this case, the coating thickness can be adjusted by the number of dips. The molded body coated with such a coating material will be fired in a later step, but before firing, the coating material is not as porous as it is after firing. By firing, the particles that make up the coating material grow and bond with each other, forming appropriate pores and becoming porous. However, by adjusting the concentration of the slurry that is the raw material for the coating material, the size of the particles in the slurry, etc., the filling rate of particles per unit volume of the coating material can be adjusted appropriately, and the desired porosity can be achieved after firing. The porous coating layer 3 is made to be as follows. In this way, various methods such as spraying can be used to form the coating material, as long as the filling rate of particles per unit volume of the coating material can be adjusted appropriately.
Next, by sintering at 1350 to 1450° C. for 1 to 2 hours using an electric furnace or the like, the thermistor element 1 and coating layer 3 can be fired at the same time. The thermistor element 1 provided with the coating layer 3 in this manner is bonded to an insulator tube 5 made of alumina or the like using an adhesive 4 such as heat-resistant cement, for example, as shown in FIG. The electrode wire is led out from the opposite end of the insulator tube 5 to form a high temperature thermistor. As mentioned above, the high temperature thermistor of the present invention has the following characteristics:
A porous film layer with high specific resistance is provided on the surface of the element. Therefore, even if carbon, which is generated when the air-fuel ratio is in an excessively rich state, adheres, it blocks surface leakage, so it does not act as an electrode and does not increase the apparent electrode area. In addition, the thermistor of the present invention only has a porous coating layer on the outside of the thermistor element, so
Heat is transmitted from the object to be measured, such as gas, to the thermistor element almost unhindered. Therefore, the thermistor of the present invention responds sharply to changes in the temperature of the object to be measured, and is capable of accurate and rapid temperature measurement. It is also possible to measure small and rapid changes in the temperature of the object to be measured. Furthermore, since the protective coating layer is porous, a considerable amount of expansion due to heat contributes to a reduction in the volume of the pores, so that cracks due to thermal strain are less likely to occur. In order to manufacture the high-temperature thermistor of the present invention, the method described above involves simultaneously sintering the element and the porous coating layer. Therefore, after firing the thermistor, there is no need for a complicated process such as recoating the film layer, and since the film layer and thermistor base material are sintered simultaneously, they are firmly bonded to each other, and the film layer does not change during use. Will not peel off. The present invention will be explained in more detail below using experimental examples, but the present invention is not limited by the following experimental examples unless it exceeds the gist thereof. Experimental example Cr ₂ O ₃ 80% (weight%, same below), TiO ₂ 15%,
A thermistor composition consisting of 5% V ₂ O ₅ was placed in a mold and powder pressed together with a 0.3φ PR wire into the shape shown in FIG. The surface of the press molded body is immersed in a slurry in which 92% Al ₂ O ₃ , 8% borosilicate glass, and 3% polyvinyl alcohol are dispersed in water to provide an alumina coating layer on the surface. The film thickness was controlled by the number of dips. The thermistor element and the alumina coating layer were fired in an electric furnace at 1400°C for 1 hour. For comparison, one part of the thermistor element was coated with another insulating layer on its surface after firing. The thermistor element is fixed to the insulator pipe as shown in Figure 3, and the air-fuel ratio A/F is adjusted in the exhaust pipe of the 2000 c.c. engine.
Carbon deposits were deposited on the device surface at 11,400°C for 10 hours. The resistance value of the thermistor element of each sample was measured at 230℃±
Table 1 shows the results measured at 2°C. Note that the specific resistance of the thermistor element without the coating layer was approximately 1×10 Ω·cm.

[Table] From the results in Table 1, the resistance value of Sample 1, which does not have a coating layer, decreases significantly when carbon deposits are deposited, and when the coating layer is a glass layer like Sample 8, the coating layer It was found that cracks were likely to occur in the layer. It has also been found that if a coating material is applied to the thermistor element after it has been fired, the coating layer tends to peel off.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an example of a thermistor element used in the high-temperature thermistor of the present invention, and FIG. 2 shows an example of a thermistor composition that can be used in the high-temperature thermistor of the present invention. composition diagram,
FIG. 3 is a partially sectional plan view showing an embodiment of the high temperature thermistor of the present invention. 1... Thermistor element, 2... Electrode wire, 3...
...Coating layer.

Claims (1)

[Claims] 1. A porous film layer having a resistivity 100 times or more higher than the material constituting the thermistor element is provided as the outermost layer on the surface of a thermistor element in which a pair of electrode wires are embedded. A high temperature thermistor characterized by: 2 The material constituting the thermistor element is Cr ₂ O ₃
The high-temperature thermistor according to claim 1 _, which contains _-TiO2 - _V2O5 as a main component. 3. The high temperature thermistor according to claim 1, wherein the porous coating layer contains porous alumina as a main component.