JPH05107214A - Gas detecting element - Google Patents

Abstract

PURPOSE:To strengthen adhesion between a gas detecting element and a substrate without deteriorating a detection characteristic, by a method wherein an adherent layer of which the main material is iron oxide is interlaid between the substrate and a gas-sensitive layer being the gas detecting element. CONSTITUTION:An adherent layer 4 of which the main material is iron oxide is provided between a gas-sensitive layer 4 and an alumina substrate 1. This adherent layer 4 is formed on the substrate 1 beforehand by a thick-film printing method before the gas-sensitive layer 2 is laminated, and it is prepared by baking at a temperature of 1250 deg.C to 1550 deg.C, preferably 1300 deg.C to 1450 deg.C. By laminating the gas-sensitive layer 2 thereon and baking it subsequently, a gas sensor wherein the gas-sensitive layer 2 and the substrate 1 are made to adhere strongly to each other and which has an excellent sensitivity characteristic is obtained. By interlaying an adherent layer prepared by adding one kind or more of platinum or palladium to the iron oxide between the substrate 1 and a heater, moreover, the adhesion of the heater is improved and also deterioration of the characteristic due to a heat cycle load for a long period is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thick film type gas detecting element which can be easily miniaturized and has high productivity. More specifically, the present invention relates to a gas detecting element in which a substrate and a sensitive part and a substrate and a heater are firmly adhered to each other. The present invention relates to an element and a manufacturing method thereof.

[0002]

2. Description of the Related Art It is known that a gas sensor made of iron oxide or a humidity sensor may be used as a means for detecting gas concentration and humidity in the atmosphere. Since humidity is one of the gas components in the following description, the humidity sensor is included in the meaning of the gas detecting element. As such a sensor, there is known a sensor in which an electrode is applied to bulk iron oxide and heated by a heater coil from the outside. However, from the viewpoint of productivity and cost, a thick film type gas sensor has been studied in recent years. For example, as shown in FIG. 4, an electrode 3 is provided on the upper surface of the substrate 1, and a gas sensitive portion made of iron oxide is provided on the electrode 3. This element is not only excellent in cost and productivity, but also has excellent characteristics such as fast response. However, on the other hand, there is a drawback that the substrate and the element are easily separated. Moreover, the adhesion of the thick film after printing and firing is not only weak. Generally, when a sensing element using iron oxide is used for a long period of time in normal air, it is heated and cleaned to remove the moisture and miscellaneous gas adsorbed on the element surface. ~ 500 ℃
Continuous heating is carried out at the optimum temperature in the range.

For the above-mentioned reasons, the gas sensing element is often exposed to a high temperature for a long period of time or is subjected to a sudden thermal cycle load. Under such a harsh condition, a crack or the like occurs between the substrate and the sensitive section due to the difference in thermal expansion coefficient between the two, and this causes a problem that the sensitive section is easily separated from the substrate. If the adhesive strength between the sensitive part and the substrate is weak, it may easily peel off due to mechanical vibration or shock.
The reliability problem due to the adhesion between the sensitive part and the substrate has been one of the most important problems in putting the thick film sensor into practical use.

Similar problems have also been pointed out regarding the adhesion between the thick film heater and the substrate, and studies have been conducted to improve these problems. For example, in the manufacturing process of the sensing element, it is conceivable that the sensitive portion is baked at a high temperature to generate a reaction layer at the interface between the substrate and the sensitive layer, thereby strengthening the adhesion to the substrate. However, in this method, a fairly high temperature is required to firmly adhere the sensitive part and the substrate, so that due to the progress of sintering of the sensitive structure or the diffusion of impurities from the substrate,
There is a problem that the detection characteristics deteriorate. Also,
In Japanese Patent Application No. 61-93944, it has been considered to improve the adhesion between the substrate and the detection part by providing artificial irregularities on the substrate surface. For example, a thick film electrode or When a heater is printed to form an element, unevenness on the surface of the substrate causes disconnection and defective conduction, which is a great practical difficulty.

[0005] In addition, in Japanese Patent Application No. Sho 57-50648 Patent and Japanese Patent Application Sho 60-142240, for example, the adhesion layer comprising a glass component such as _{S i O 2, B 2 O} 3 between the substrate and the sensitive part A method has been proposed to improve the adhesion between the substrate and the sensitive part by interposing a glass. However, in the case of an adhesive layer containing such a glass component, the adhesion strength between the substrate and the sensitive part is On the other hand, there is a problem that the sensitivity is deteriorated due to the reaction between the glass component and the sensitive portion during firing, and the sensitivity is also lowered due to the diffusion of the glass component in long-term reliability. For these reasons, it is difficult for the conventional thick film type sensing element to simultaneously solve practically sufficient sensitivity characteristics, long-term sensitivity maintenance, and mechanical strength and reliability against thermal shock such as thermal cycling. Met.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object thereof is the adhesion between a sensitive portion containing iron oxide as a main material and a substrate without deteriorating detection characteristics. It is to provide a gas detection element with a solid structure, and another purpose is to improve the adhesion between the thick film heater for heating and the substrate, thereby dramatically improving the life of the heater. The object is to provide a gas detection element that operates stably over a long period of time.

[0007]

In order to achieve the above object, the inventors of the present invention have diligently studied to improve the adhesion between the sensitive portion and the substrate in the gas detection element made of iron oxide. As a result, good adhesion and practically high enough sensitivity were confirmed by interposing an adhesion layer made of iron oxide, which is the main material of the sensitive area, between the substrate and the sensitive area. The adhesion layer made of iron oxide is formed on the substrate in advance by a thick film printing method before laminating the sensitive part, and is 1250 ° C to 1550 ° C, preferably 1300 ° C.
Prepared by baking at ℃ ~ 1450 ℃. Then, by laminating and firing the sensitive portion thereon, it was possible to obtain a gas sensor having the sensitive portion firmly adhered to the substrate and having good sensitivity characteristics.

Here, the baking temperature of the adhesion layer is 12
If the temperature is below 50 ° C, sufficient adhesion strength cannot be obtained for the adhesion layer.
On the other hand, if the temperature exceeds ℃, the reactivity with the substrate progresses on the contrary and the substrate is apt to warp. Further, if the sinterability of the iron oxide forming the adhesion layer is too advanced, the reactivity between the reaction part and the adhesion layer will be extremely small when the reaction part is laminated and fired after this, and sufficient adhesion will be obtained. I can't get sex. As described above, the feature of the present invention resides in that iron oxide, which is the main material of the sensitive portion, is provided as an adhesion layer between the sensitive portion and the substrate in order to bring the sensitive portion and the substrate into close contact with each other. For this reason, good adhesion was achieved, and no diffusion of components degrading the characteristics to the sensitive part or formation of reaction products was observed, resulting in obtaining sufficiently high sensitivity characteristics in practical use.

Further, the present inventors have made efforts to improve the reliability of the heating heater provided in the gas detection element. As a result, by interposing an adhesion layer in which one or more of platinum or palladium is added to iron oxide between the substrate and the heater, the adhesion of the heater is remarkably improved and the characteristics are deteriorated even by a long-term thermal cycle load. We have completed a heater for heating a gas sensor that does not do so. Here, platinum or palladium is used because it has a high melting point, does not melt or remarkably sinter at the baking temperature of the adhesive layer, and can maintain a uniform and fine particle state dispersion. For example, when gold having a melting point of 1063 ° C. is used, remarkable grain growth of gold occurs, and improvement in adhesion cannot be obtained.

The added metal is chemically stable, and its oxidation resistance is important especially because it is fired in the air. It is known that platinum hardly changes even when heated in air, and palladium is also stable within the range of the main firing temperature. However, for example, even if the same precious metal material is used, it is known that when rhodium is heated to 1100 ° C. or higher, weight loss occurs due to formation of volatile oxide R _h O ₂ , and iridium and ruthenium are also volatile at high temperatures. It produces oxides. Therefore, when rhodium or the like is used,
Although firing must be performed in a non-oxidizing atmosphere, under such conditions, the iron oxide forming the adhesion layer is reduced and becomes electrically conductive. Therefore, it cannot be used as the adhesion layer of the heater part. Thus, in the present invention, it is important to add and disperse platinum or palladium to iron oxide.

Here, if the amount of platinum or palladium added is less than 2 Vol% with respect to the iron oxide forming the adhesion layer, long-term reliability under a thermal cycle load cannot be obtained, and 40 Vol% is required. If it exceeds, conduction will occur in the adhesion layer and a short circuit of the heater will occur. For this reason,
The range of addition amount is 2 to 40 Vol%, preferably 5 to 30 Vol%
Is. Next, if the firing temperature of this adhesion layer is lower than 1250 ° C, the reactivity between the main material iron oxide and the substrate is not sufficient,
The strength of the adhesive layer cannot be obtained. If the firing temperature exceeds 1600 ° C., on the contrary, the reactivity between the iron oxide and the substrate will be excessively advanced, and the substrate will be warped, which is not practical. Therefore, the firing temperature range is 1250 ° C to 1550 ° C, preferably 1300 ° C to 1450 ° C. The composition of the present invention also includes a composition obtained by adding a sintering aid or the like to iron oxide within a range that does not impair the sensitivity characteristics and the characteristics of the heater. Further, in the adhesion layer to which platinum or palladium is added, it is also included that a part of the adhesion layer is added with iridium, ruthenium, rhodium, tungsten or the like to be alloyed and used.

[0012]

Embodiments will be described below with reference to the drawings. Figure 1
FIG. 1 is a configuration diagram of an embodiment of a gas detection element according to the present invention. In FIGS. 2, 3, and 4, the same parts as those in FIG. 1 are designated by the same reference numerals. Reference numeral 4 denotes an adhesion layer containing iron oxide as a main material, which is provided between the gas sensitive layer 2 and the alumina substrate 1. On the other hand, FIG. 2 and FIG. 3 are configuration diagrams of another embodiment, and the adhesion layer 4-1 is made by adding 2 to 40 vol% of platinum or palladium to iron oxide. Here, 5 is a heater containing platinum. Examples will be described below.

Example 1 In this example, a paste prepared by mixing a vehicle with iron oxide added with 0 or 20 vol% of platinum or palladium was printed as an adhesion layer on an alumina substrate having a thickness of 0.35 mm and dried at 1200 ° C. Baking for 1 h at ~ 1600 ° C. The iron oxide used here is a powder produced by neutralizing and precipitating an aqueous solution of iron nitrate with sodium carbonate by a coprecipitation method, washing it thoroughly with water, freeze-drying it, and calcining it at 500 ° C. for 1 hour in the atmosphere. The iron oxide powder thus produced has an extremely small average particle diameter of 200 Å to 1000 Å, and thus has excellent sinterability. The average particle size of platinum and palladium is 0.5 μm. Subsequently, a gas-sensitive layer containing iron oxide as a main component or a heater layer containing 50% by volume of platinum was printed on the substrate provided with the adhesion layer, dried, and then baked at 650 ° C. for 1 hour. When the adhesion between the substrate and the gas-sensitive layer and the heater layer was examined by a tape peeling test, the results shown in FIG. 5 were obtained. As shown in FIG. 5, it can be seen that the adhesion between the substrate and the gas-sensitive layer is improved by providing the adhesion layer. In addition, in the figure, the mark ◯ indicates that there is no peeling, and the mark x indicates that there is peeling. It was also confirmed that the adhesion between the substrate and the heater layer was further increased by adding platinum or palladium to the adhesion layer. The firing temperature of the adhesion layer is 1200 ℃
Peeled off due to insufficient strength of the adhesion layer, and warped on the substrate at 1600 ° C. Therefore, the firing temperature of the adhesion layer is preferably in the range of 1250 ° C to 1550 ° C.

Example 2 In this example, an iron oxide paste containing 0 to 100 Vol% platinum or palladium was printed as an adhesion layer on an alumina substrate, dried, and then baked at 1400 ° C. for 1 hour. Then, the electrode for measuring the electrical conductivity of the adhesion layer was baked at 800 ° C. for 1 hour. FIG. 6 shows the measurement results of the resistance value of the adhesive layer. As shown in FIG. 6, the resistance value of the adhesion layer tended to decrease as the platinum or palladium content increased. Thus, the content of platinum or palladium in the adhesion layer is 40%.
If it exceeds Vol%, the adhesion layer becomes conductive, so
The content of platinum or palladium in the adhesion layer is 40
Vol% or less is desirable.

Example 3 An iron oxide paste containing 0 or 20 Vol% platinum or palladium was printed as an adhesion layer on an alumina substrate, dried, and then baked at 1300 ° C. for 1 hour. Subsequently, the heater layer was printed, dried, and then baked at 650 ° C. for 1 hour to form the heater element shown in FIG. A heater voltage was cycle-applied to this heater, and a thermal cycle test was conducted in which the sensing element was heated to 350 ° C and 50 ° C. The relationship between the room temperature resistance of the heater layer and the number of thermal cycles is shown in FIGS. 7 and 8. In FIGS. 7 and 8, circles indicate an adhesion layer containing 20 Vol% of platinum or palladium, squares indicate an underlayer containing no platinum or palladium, and triangles indicate a comparison example without an adhesion layer. This is the case. As shown in the figure, the room temperature resistance of the heater layer without the adhesion layer increases with the number of thermal cycles,
At the end, I broke the wire. However, the heater formed on the substrate provided with the adhesion layer containing no platinum showed a resistance change but no disconnection. Furthermore,
The resistance change of the heater layer was hardly seen in the one in which platinum or palladium was added to the adhesion layer. According to the above example, the reliability of the heater layer could be remarkably improved by adding platinum or palladium to the adhesion layer.

Example 4 In this example, a thermal cycle test was conducted on an element in which a heater and a gas sensitive layer were laminated on an alumina substrate provided with an adhesion layer of iron oxide containing 0 to 40 Vol% platinum or palladium. The procedure was as follows. First, an iron oxide paste containing 0 to 40 Vol% platinum or palladium is printed as an adhesion layer on an alumina substrate, dried, and then dried at 1300 ° C for 1 hour.
Baked. Subsequently, a heater layer, an insulating layer for insulating the heater layer, an electrode layer for extracting an electric signal of the gas sensitive layer, and a gas sensitive layer for detecting gas are laminated and printed, and after drying,
Simultaneous firing was performed at 700 ° C. for 1 hour to form the sensing element shown in FIG. A heater voltage is cycled to the heater of this sensing element, and the sensing part is cycle heated to 350 ° C and 50 ° C,
A heat cycle test was performed. The relationship between the room temperature resistance of the heater layer and the number of thermal cycles is shown in FIGS. 9 and 10. Note that FIG.
Shows the case of an adhesion layer containing platinum, and FIG. 10 shows the case of an adhesion layer containing palladium. From the results shown in the figure, in the case where the heater and the gas-sensitive layer have a laminated structure, it can be seen that the platinum or palladium content in the adhesion layer is 1 Vol% or less, the wire breaks even after 10,000 thermal cycles, Different results were obtained than when the heater alone was operated. This is when the heater has a laminated structure and when it is operated independently.
This is probably because the stress applied by thermal shock is different.
However, if platinum or palladium is used in the adhesion layer,
With 40 Vol% added, no wire breakage was observed even after tens of thousands of thermal cycles. The rate of change in the heater resistance tended to decrease as the amount of platinum or palladium added to the adhesion layer increased. According to the above embodiment, when the heater has a laminated structure, the heater life is dramatically increased by adding 2 to 40 Vol% of platinum or palladium to the adhesion layer interposed between the substrate and the thick film heater. It is possible to provide a gas detection element that can be improved and that operates stably over a long period of time.

[0017]

As described above, according to the present invention, an adhesive layer containing iron oxide, which is a gas-sensitive element component, as a main material is interposed between the substrate and the gas-sensitive element. It is possible to provide a gas detection element having sufficient adhesion without damaging the gas detection element. Further, by interposing the adhesion layer containing platinum or palladium between the heater for heating and the substrate, it is possible to dramatically improve the life of the heater and provide a gas detection element that operates stably over a long period of time.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a gas detection element according to the present invention.

FIG. 2 is a configuration diagram of another embodiment.

FIG. 3 is a configuration diagram of still another embodiment.

FIG. 4 is a diagram illustrating a conventional example.

FIG. 5 is a diagram showing the results of a tape peeling test on a substrate and a gas-sensitive layer and a heater layer.

FIG. 6 is a diagram showing the electric conductivity of an adhesion layer.

FIG. 7 is a diagram showing a thermal cycle test result of an adhesion layer containing platinum.

FIG. 8 is a diagram showing a thermal cycle test result of an adhesion layer containing palladium.

FIG. 9 is a diagram showing the results of a heat cycle test in the case of containing platinum.

FIG. 10 is a view showing a thermal cycle test result when palladium is contained.

[Explanation of reference numerals] 1 substrate 2 gas sensitive layer 3 electrode 4 adhesion layer 5 heater 6 insulating layer

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[Procedure amendment]

[Submission date] February 26, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

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[Correction method] Change

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Example 3 An iron oxide paste containing 0 or 20 Vol% platinum or palladium was printed as an adhesion layer on an alumina substrate, dried, and then baked at 1300 ° C. for 1 hour. Then, by printing a heater layer, dried, and it constitutes a heater element of Figure 2 with 1h calcined at 650 ° C.. A heater voltage was cycle-applied to this heater, and a thermal cycle test was conducted in which the sensing element was heated to 350 ° C and 50 ° C. The relationship between the room temperature resistance of the heater layer and the number of thermal cycles is shown in FIGS. 7 and 8. In FIGS. 7 and 8, circles indicate an adhesion layer containing 20 Vol% of platinum or palladium, squares indicate an underlayer containing no platinum or palladium, and triangles indicate a comparison example without an adhesion layer. This is the case. As shown in the figure, the room temperature resistance of the heater layer without the adhesion layer increases with the number of thermal cycles,
At the end, I broke the wire. However, the heater formed on the substrate provided with the adhesion layer containing no platinum showed a resistance change but no disconnection. Furthermore,
The resistance change of the heater layer was hardly seen in the one in which platinum or palladium was added to the adhesion layer. According to the above example, the reliability of the heater layer could be remarkably improved by adding platinum or palladium to the adhesion layer.

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 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroki Okumura 5310 Mikajiri, Kumagaya, Saitama Prefecture Chichibu Cement Co., Ltd. Huain Ceramics Division

Claims (7)

[Claims] 1. A gas detecting element made of iron oxide provided on a substrate, wherein an adhesion layer containing iron oxide as a main material is interposed between the substrate and the gas detecting element. .. 2. The firing temperature of the adhesion layer mainly composed of iron oxide is
The method for producing a gas detection element according to claim 1, wherein the temperature is 1250 ° C to 1550 ° C. 3. A heating heater, wherein an adhesion layer containing iron oxide as a main material is interposed between the substrate and the heater. 4. The gas detecting element according to claim 1, wherein an adhesion layer containing iron oxide as a main material is interposed between the substrate and the heater. 5. The adhesion layer according to claim 3, wherein at least one of platinum and palladium is added to iron oxide. 6. The composition according to claim 5, wherein the amount of platinum or palladium added to the adhesion layer is 2 to 40 Vol%. 7. The method for producing an adhesion layer according to claim 5, wherein the firing temperature is 1250 ° C. to 1550 ° C.