JP2971167B2 - Ceramic heater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic heater mounted on a sensor or the like, and more particularly, to a ceramic heater for heating an oxygen sensor for detecting, for example, an oxygen concentration in exhaust gas of an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, for example, an oxygen sensor is mounted on an exhaust pipe of an internal combustion engine and used to detect the oxygen concentration in exhaust gas. A ceramic heater for heating the element portion is used.

As the ceramic heater, for example, a plate-like or cylindrical heater is used in accordance with the shape of the sensor. Among them, the cylindrical ceramic heater is
For example, a green sheet on which a heat generation pattern is formed is laminated on a surface of a cylindrical ceramic base made of alumina (Al ₂ O ₃ ), and is formed by integrally firing.

[0004]

However, when the above-described ceramic heater is used for heating an oxygen sensor which is exposed to a high temperature for a long time, the following problems may occur.

That is, if the oxygen sensor is exposed to a high temperature for a long period of time, the heat generation pattern may deteriorate during use and its resistance may increase, thereby breaking the heat generation pattern or protecting the heat generation pattern. Cracks sometimes occurred in the layer. Further, in the worst case, the protective layer may collapse, thereby shortening the heater life. In this case, the vicinity of the heat generation pattern, which is close to the cathode in appearance, is darkened, causing a so-called blackening phenomenon.

For this reason, it has been practiced to maintain the durable life of the heater by detecting the use condition of the heater and energizing only when necessary. In this case, the detecting means and the energizing control means are used. There is a problem that the device is separately required and the device becomes complicated, and a failure of the detection means may cause a new cause of shortening the service life, so that it cannot be a fundamental solution. there were.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has as its object to provide a ceramic heater having a long service life even in a high-temperature environment without requiring any additional means such as a detecting means. .

[0008]

In order to achieve this object, the present invention provides a first ceramic layer, a second ceramic layer laminated on the surface of the first ceramic layer, and the first and second ceramic layers. in the ceramic heater having a heating pattern consisting of heating unit arranged on the boundary surface of the layer and the conductive portion, a heating portion of the heating pattern is a line
A high heat generating portion having a high resistance value and a high temperature;
A low heat generating portion having a lower wire resistance value and a lower temperature than the high heat generating portion;
And the high heat generating portion is provided with the ceramic heater.
Is disposed in a portion where the temperature is high, and the low heat generation portion is the high temperature portion.
The gist of the present invention is a ceramic heater characterized in that the ceramic heater is disposed in a portion where the temperature is lower than a portion where the temperature becomes lower .

Incidentally, the ceramic heater of the present invention may have, for example, a sensor element, an insulating layer, or the like as a configuration other than the above. As the first and the material for forming the second ceramic layer, for although Al ₂ O ₃ is preferred, for high-temperature strength material particularly excellent in thermal conductivity, Al ₂
O ₃ preferably has an average crystal grain size of 10 μm or less and a relative theoretical density of 94% or more.

[0010] The first ceramics layer may include:
Ceramic substrate formed in various shapes such as cylindrical, rod, plate, etc., depending on the shape of the sensor to be heated, or formed by firing a green sheet so as to cover the surface of the ceramic substrate Can be adopted. Also, the second
As the ceramic layer, a layer formed by firing a green sheet can be employed.

The ceramic substrate may be a high-temperature, high-strength ceramic such as alumina-like ceramic such as mullite or spinel, in addition to Al ₂ O ₃ . Also, the ceramic layer formed from the green sheet is
The heating pattern is protected in a high-temperature environment, and the bonding property between the ceramic substrate and the heating pattern is improved. The heating pattern is preferably positioned so as to include at least the heating pattern.

As a material of the heat generation pattern, tungsten (W) or molybdenum (Mo) is mainly used, and further, a high melting point metal component such as platinum (Pt) or rhodium (Rh) is mixed with these components. Good. Further, Pt or Rh may be used alone to improve resistance characteristics. In addition, as long as it does not adversely affect, an oxide or the like made of the same material as the ceramic layer may be slightly present.

The heat generation pattern generates heat when energized.
It is composed of a heating section and a conducting section including a lead section and a terminal section for supplying electricity to the heating section. In particular, the heat generating portion includes a high heat generating portion having high resistance, and a low heat generating portion having a line resistance value lower than the line resistance value of the high heat generating portion by, for example, about 1/5. For example, at 20 ° C., the wire resistance value of the high heat generating part is 0.2.
In the case of 1 to 0.2 Ω / mm, the wire resistance of the low heat generating portion is 0.2.
It is set to 02 to 0.04 Ω / mm. In order to reduce the line resistance value of the low heat generation part, for example, the width of the low heat generation part is about five times wider than that of the high heat generation part.

The outline of the method for manufacturing the ceramic heater is as follows. As a raw material, for example, the main component Al ₂
A powder obtained by wet mixing powder of O ₃ is prepared.
In order to obtain a dense, high-temperature, high-strength material, a high-purity powder having a purity of 90% or more is used as the raw material powder, and the particle size is preferably 2 μm or less.

However, the firing promoting components SiO ₂ and Mg
O, CaO, and B ₂ O ₃ are oxides in the sintering process, and thus those having a predetermined network structure, for example, hydroxides,
You may mix | blend as a salt (for example, carbonate etc.).

The molding of the compounded powder can be performed by various methods such as pressure molding (for example, hydrostatic molding or doctor blade molding) or extrusion molding. In this molding, of course, a predetermined solvent, a binder and the like are appropriately blended.

For the formation of the heat generation pattern, various means such as plating and vapor deposition (for example, sputtering and vapor deposition) can be adopted. In particular, when a heat generation pattern is formed by a metal paste, a predetermined pattern is formed on the formed green sheet by, for example, screen printing, the pattern printing surface side is covered with a green sheet, and a predetermined pattern is formed thereon. Thereafter, it is preferable to further cover the sheet with a green sheet and to provide the sheet with a ceramic substrate. this is,
This is because if the metal pattern is directly joined to the base material, mutual adhesion becomes insufficient, and there is a possibility that a cause of oxidation (a cause of disconnection) of the heat generation pattern component due to pore generation may occur.

In the firing, it is preferable to simultaneously fire the ceramic substrate and each ceramic layer in order to enhance the mutual adhesion. As a firing method, a mold pressing (HP, HI
P) Various types such as sintering, atmospheric pressure sintering, and reaction sintering can be adopted, and the sintering temperature is preferably selected from the range of 1450 to 1600 ° C. The atmosphere is an inert gas (for example, A
r, N ₂ ), an oxidizing atmosphere (for example, in the air), or a reducing atmosphere (for example, H ₂ gas).

The ceramic heater thus obtained is
The terminals of the heat generation pattern are metallized, and leads from the power supply are connected by brazing. The ceramic heater according to the present invention is particularly suitable as a heater for heating an oxygen sensor for controlling the air-fuel ratio of an internal combustion engine that is used for a long time at a high temperature. In this case, the ceramic heater
It may be inserted into the test tube type solid electrolyte oxygen sensor element or may be attached to the oxygen sensor element.

[0020]

The present inventors have analyzed the causes of sensor deterioration when used at high temperatures and completed the present invention based on the analysis. Hereinafter, the analysis and consideration will be described in detail.

The mechanism of the disconnection phenomenon of the heater is as follows, as already disclosed in Japanese Patent Application No. 63-48721. FIG. 5 schematically shows the result of EPMA (elemental analysis) on the appearance after disconnection of a conventional heater.
FIG. 5A is a cross-sectional view of FIG.
The results revealed the following facts (a) and (b). (A) Heat generation part P2 on the cathode side in heat generation pattern P1
Is locally changed from white (the normal color of Al ₂ O ₃ ) to black. (B) Of the heat generation pattern P1, the heat generation part P2 on the anode side
The area around has cracks locally.

The heater was placed in an air atmosphere at 1000 ° C., a current was applied by continuously applying a DC voltage of 17 V, and the change in the resistance value of the heat generating portion was examined.
There was found. (C) As shown in FIG. 6, the resistance of the first pattern portion P3 (FIG. 5A), which is the heat generating portion P2 closest to the anode side,
Significant increase compared to other pattern parts. FIG. 6 shows the total resistance of the heat generation pattern P1 and the first resistance.
The resistance of the pattern portion P3 and the resistance of the other pattern portions are changed over time.

The theoretical considerations made to elucidate the above facts are as follows. (A) Consideration Since the alumina base material constituting the alumina heater contains various metal oxides as a sintering promoting component together with Al ₂ O ₃ as a main component and is sintered, the sintered body is made of Al.
These metal oxides exist as a glass phase at the ₂ O ₃ grain boundary.

When a direct current is applied to such an alumina heater at a high temperature, magnesium (M
g) and calcium (Ca) atoms become cations and move to the cathode side. On the other hand, oxygen (O) atoms existing in the vicinity of the component become oxygen ions and move to the anode side in order to maintain electrical neutrality. For this reason, the Mg and Ca components are deposited near the cathode side terminal as a simple substance or an oxide, and the portion is blackened. That is, the application of the direct current causes the flux components in the glass phase at the Al ₂ O ₃ grain boundary to undergo electrolysis.

Consideration (b) Further, the material of the heat generating pattern P1, for example, tungsten (W) is oxidized by the oxygen ions moved to the anode side,
Increase the resistance value at that location.

Consideration of (c) Due to the oxidation reaction of (b), the heat generation pattern P1 expands in volume, causing a break in the heat generation pattern P1 and a stress being applied to the protective layer P4 to cause cracks.
Note that a part of the oxidized heat generating pattern P1 material moves to the protective layer P4 and further to the outside by diffusion, and in this sense also increases the resistance value.

Therefore, when such an alumina heater is continuously exposed to a high temperature, the material of the heat generating pattern P1 is explosively oxidized by external air oxygen that has entered from cracks in the protective layer P4, causing further volume expansion, and the protective layer P4 Peeling off
It is considered to lead to collapse.

That is, the root causes of the disconnection mechanism of the heat generation pattern P1 are considered to be the movement (migration) of Mg ²⁺ and Ca ^{2+ to} the lower potential side and the movement of O ^{2− to} the higher potential side.

In order to prevent disconnection of the heater due to the mechanism described above, the present inventors have formed a low heat generating part which is lower in temperature than the high heat generating part in a part of the heat generating part of the heat generating pattern. As a result, accumulation of Mg ²⁺ and Ca ^{2+ in} the cathode portion of the heat generating portion due to the movement of Mg ²⁺ and Ca ^{2+ to} the lower potential side can be prevented, and the movement of O ^{2− to} the higher potential side can be prevented. Heating part by
The oxidation of the heat generation pattern material (W) at the anode side portion was reduced.

That is, since the temperature of the low heat generating portion of the present invention is lower than that of the high heat generating portion, the movement of Mg ²⁺ and Ca ²⁺ becomes difficult, and as a result, the anode moves from the anode side to the cathode side along the heat generation pattern. It is considered that it works as a barrier for moving Mg ²⁺ and Ca ²⁺ . In other words, the low heat generating portion provided in a part of the heat generating portion has an effect of dividing the movement of ions to both sides thereof, and disperses the deterioration which has conventionally been concentrated in the heat generating pattern. The disconnection of the heater will be greatly reduced.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the ceramic heater and the method of manufacturing the same according to the present invention will be described below.

As shown in FIG. 1, a ceramic heater 1 according to the present embodiment has a
Ceramic layers 3 and 4 are formed, and a heat generating pattern 7 is formed on a boundary surface 5 between the first ceramic layer 3 and the second ceramic layer 4.

The heating pattern 7 is shown in an exploded perspective view of FIG.
As shown in the figure, a heating portion 6 meandering many times on the tip side of the ceramic heater 1 and a narrow high-frequency portion of the heating portion 6 are generated.
A heating portion 8; a low heating portion (deterioration prevention pattern) 10 formed in a U-shape at the center of the heating portion 6 ; and an anode-side terminal disposed at the rear end of the ceramic heater 1 and connected to a power source 9a and a cathode side terminal 9b (collectively referred to as a terminal portion 9), and lead portions 11a and 11b connecting the heating portion 8 and the terminal portion 9 to each other.
It is composed of

The low heat generating portion 10 is formed to be about 5 times wider than the high heat generating portion 8 and has a line resistance of about 1/1 / the width of the heat generating portion.
It is set to 5 times.

Next, a W paste prepared in advance is applied to the surface of the second green sheet 4a by a thick film printing method.
Screen printing is performed to a size of 0 to 30 μm to form a print pattern 7 a to be the heat generation pattern 7.

Further, a first green sheet 3a having a thickness of 0.05 to 0.10 mm to be the first ceramic layer 3 and formed by the same method as that for the second green sheet 4a is pressed onto the printed surface, Form a laminated sheet. (D) Integration of base material, first and second green sheets and heat generation pattern To the blended powder produced in (a) above, 25% of polyvinyl butyral, 8% of DBP, and 30% of butyl carbidol were added to form a paste. Product, and paste this material into
It is applied to the surface of the first green sheet 3a of the laminated sheet obtained in the above (c).

Next, the laminated sheet is wound around the base material 2 so as to be used for bonding the coated surface to the base material 2 and pressed and adhered. Next, after the resin is removed at 250 ° C., it is baked at 1500 to 1600 ° C. in a hydrogen furnace atmosphere to form the integrated ceramic heater 1. Thereafter, the ends of the terminal portions 10 and 11 of the ceramic heater 1 are plated with Ni, and are joined to a lead wire lead-out terminal (not shown) using a brazing material.

Next, an experimental example performed to confirm the effect of the ceramic heater 1 manufactured as described above will be described. (Experimental Example 1) A high-temperature durability test was performed using the ceramic heater 1 of this embodiment.

The experiment was performed under a heating atmosphere of 1000 ° C. for 3 hours.
A current of 17 V DC was applied to the heat generating pattern 7 having a resistance value of Ω, and a change with time of the resistance value was measured. As a comparative example, a high-temperature durability test was similarly performed on a conventional heater (FIG. 4B) having a heat generation pattern without a low heat generation portion. The results are shown in FIG. 3, where the vertical axis represents the rate of resistance change (%) and the horizontal axis represents the endurance time (Hr). Incidentally, in the experiment, as the present example and the comparative example, 5
Experiments were performed on the individual samples.

As is apparent from FIG. 3, the ceramic heater 1 of this embodiment has excellent high-temperature durability with little change in resistance over time as compared with that of the comparative example. On the other hand, the wire of the comparative example is disconnected in less than 100 hours, is inferior in high-temperature durability, and is unsuitable.

(Experimental Example 2) Next, the state of the ceramic sensor 1 of this embodiment (FIG. 4A) and the ceramic sensor of the comparative example (FIG. 4B) after 200 hours of durability were examined. That is, the accumulation state of Mg and Ca in the heat generating portion of the sensor, the oxidation state of W, and the like were examined. The other conditions were the same as those in Experimental Example 1.

As a result, as shown in FIG. 4, in the ceramic sensor 1 according to the present embodiment, the slight heat generation portion 8x on the left side of the drawing and the low heat generation portion 10 side of the high heat generation portion 8y on the right side are slightly affected. Although only accumulation of Mg and Ca was observed, in the ceramic sensor of the comparative example, a large amount of M
g and Ca were accumulated, and W was oxidized on the anode side of the heat generating portion, and a collapse phenomenon was observed around it.

As described above, the ceramic heater 1 of the present embodiment has a low heat generating portion 1 having a small wire resistance at the center of the heat generating portion 6.
0, the low heat generating portion 10 allows Mg
The movement of ²⁺ and Ca ^{2+ to} the lower potential side and the movement of O ^{2− to} the higher potential side are prevented. As a result, an increase in the resistance value due to the deterioration of the heating pattern 7 can be prevented, so that the life of the ceramic heater 1 can be improved.

Therefore, when the ceramic heater 1 is applied to, for example, an oxygen sensor for controlling the air-fuel ratio which is exposed to a high temperature for a long period of time, the heat generation pattern 7 is hardly deteriorated and the resistance thereof can be prevented from increasing. , Heat generation pattern 7
Can be prevented from being broken or cracks can be generated in the protective layer. As a result, even when the oxygen sensor is used at a high temperature, there is a remarkable effect that the ceramic heater 1 can be suitably used for a long period.

[0045]

As described above, in the ceramic heater according to the present invention, since a low heat generating portion is provided in a part of the heat generating portion, M
This prevents movement of g ²⁺ , Ca ²⁺ , and O ²⁻ , thereby preventing deterioration of the heat generation pattern. In particular, the composition of the present invention has a small change in resistance value even under conditions exposed to high temperatures, can maintain stable heating characteristics for a long time, and is excellent in durability. Moreover, the structure is simple without the need for detecting means and the like, and the manufacture is easy. Therefore, it can be suitably applied as a heater for various sensors, and is extremely useful.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a ceramic heater according to an embodiment of the present invention, partially cut away.

FIG. 2 is an exploded perspective view showing the ceramic heater of the embodiment.

FIG. 3 is a graph showing the results of a durability test of the ceramic heaters of the example and the comparative example.

FIG. 4 is an explanatory view showing a state after endurance of the ceramic heaters of an example and a comparative example.

FIG. 5 is an explanatory view showing a problem of a conventional ceramic heater.

FIG. 6 is a graph showing a change in resistance value of a conventional ceramic heater.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ceramic heater 2 ... Ceramic base material 3, 4 ... Ceramic layer 5 ... Boundary surface 7 ... Heat generation pattern 6 ... Heat generation part 8 ... High heat generation part 10 ... Low heat generation part (deterioration prevention pattern)

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H05B 3/10-3/18

Claims (1)

(57) [Claims] A first ceramic layer, a second ceramic layer laminated on a surface of the first ceramic layer, and a heat generating portion and a conductive portion disposed at an interface between the first and second ceramic layers. And a heating portion of the heating pattern, wherein a heating portion of the heating pattern has a high wire resistance and a high temperature.
A high heat generating portion, and a wire resistance lower than the high heat generating portion.
A low-heat-generating portion that is lower in temperature than the hot portion, and the high-heat-generating portion is a high-temperature portion of the ceramic heater.
And the low heat generating portion is higher than the high temperature portion.
A ceramic heater, wherein the ceramic heater is arranged in a portion where the temperature becomes low .