JP4533159B2 - Ceramic heater and manufacturing method thereof - Google Patents

Description

  The present invention relates to a ceramic heater and a manufacturing method thereof, and more particularly to a ceramic heater suitable for a plate-like oxygen sensor element used for detecting an oxygen concentration in automobile exhaust gas and the like and a manufacturing method thereof.

In recent years, environmental issues have been highlighted, and efforts are being made to put the global environment as a top priority in each industry. In particular, in the automobile industry, as represented by the exhaust gas regulations of California in the United States, reducing the amount of CO ₂ , CO, HC and NOx in exhaust gas year by year has become a trend of the world. . Among them, in order to further reduce the gas in the exhaust gas, it is important how to burn the fuel efficiently. For that purpose, the residual oxygen amount in the exhaust gas is instantaneously measured and the information is obtained. There is a growing demand for oxygen sensors that can provide fast feedback to the combustion system.

  Up to now, oxygen sensors have used the heat of exhaust gas to raise the temperature of a cup-shaped sensor and develop the sensor function. However, until the sensor function appeared, the exhaust gas was flowing down, and it has become impossible to meet the recent strict exhaust gas regulations. Therefore, an oxygen sensor that actively heats the cup-shaped sensor with a heater was developed. By using such an oxygen sensor, the sensor function can be expressed quickly, and information can be fed back with better response.

  However, in the cup-shaped sensor as described above, the size is inevitably increased, and the distance between the heater and the sensor is increased, so that the speed of the sensor function is limited. Therefore, recently, a plate-shaped oxygen sensor element that has a sensor part that is made into a plate shape, and that is further molded by integrating the sensor part and the heater to increase the rate of temperature rise so that the sensor function can be expressed more quickly. Equipped oxygen sensors (plate-like oxygen sensors) are being developed.

  A heater that is integrally formed with the sensor is often used in a severe temperature cycle in which the temperature is raised and lowered repeatedly. For this reason, the heater is required to have durability against the temperature cycle.

  Patent Document 1 discloses a ceramic heater that simultaneously fires a heater substrate made of ceramics and a heater electrode made of a conductive material and a ceramic additive, wherein the ceramic additive of the heater electrode is an alkali metal and / or an alkaline earth metal. A ceramic heater using an oxide whose content is less than or equal to a predetermined amount, and a ceramic heater using a material having a higher sintering activity than that of the ceramic of the heater substrate is described as a ceramic additive for the heater electrode. .

  According to this heater, when a ceramic heater is used at a high temperature and under a high DC voltage, the phenomenon of migration of ions such as alkali metals contained in alumina as impurities, so-called migration, can be prevented. It is described that can be prevented.

  However, although it is possible to prevent disconnection of the heater electrode due to the occurrence of migration, it cannot be said that the durability is necessarily sufficient in severe durability conditions in which the temperature rise and temperature fall are repeated. There is a possibility that the resistance of the heater electrode increases and the heater electrode is disconnected.

JP-A-5-51275

  An object of the present invention is to repeatedly raise and lower the temperature of a ceramic heater composed of a heater base made of ceramics and a heater electrode made of a conductive material and a ceramic co-material and provided on or inside the heater base. It is an object of the present invention to provide a ceramic heater and a method for manufacturing the same that can prevent an increase in resistance of a heater electrode even under severe durability conditions.

  As a result of intensive studies to solve the above problems, the inventors of the present invention have a lower sintering activity of the ceramic co-material constituting the heater electrode than the sintering activity of the ceramic constituting the heater base in the ceramic heater. In this case, since the resistance of the heater electrode can be prevented from increasing, the new knowledge that the heater electrode can be prevented from breaking even under severe endurance conditions in which the temperature is raised and lowered repeatedly. The headline and the present invention were completed.

That is, the ceramic heater and the manufacturing method thereof according to the present invention have the following configurations.
(1) A ceramic heater comprising a heater base made of ceramics and a heater electrode made of a conductive material and a ceramic co-material and provided on the surface of or inside the heater base, the ceramics constituting the heater electrode A ceramic heater characterized in that the sintering activity of the common material is lower than the sintering activity of the ceramic constituting the heater base.
(2) The ceramic heater according to (1), wherein the ceramic co-material constituting the heater electrode and the ceramic constituting the heater base are the same kind of material.
(3) The ceramic heater according to (1) or (2), wherein the specific surface area of the ceramic co-material constituting the heater electrode is smaller than the specific surface area of the ceramic constituting the heater base.
(4) The specific surface area of the ceramic co-material constituting the heater electrode is 10 to 80% with respect to the specific surface area of the ceramic constituting the heater base, according to any one of (1) to (3) Ceramic heater.
(5) The ceramic heater according to any one of (1) to (4), which is for a gas sensor.
(6) At least a step of mixing a conductive material and a ceramic co-material to produce a print paste for a heater electrode, a step of printing this print paste on a green sheet made of ceramic and forming a heater electrode, A step of closely laminating another green sheet made of ceramics on the surface of the green sheet on which the heater electrode is formed, and a step of simultaneously firing the green sheet laminated in close contact, and as a ceramic co-material in the printing paste, A method for producing a ceramic heater, wherein a sintering activity is lower than the sintering activity of the ceramic in the green sheet .

  According to the ceramic heater and the manufacturing method thereof of the present invention, since the sintering activity of the ceramic co-material of the heater electrode is made lower than the sintering activity of the ceramic of the heater base, it is a severe durability condition in which the temperature rise and fall are repeated. However, it is possible to prevent the resistance of the heater electrode from increasing and to suppress the disconnection of the heater electrode. In addition, since a plurality of green sheets and heater electrodes are fired at the same time, the number of times of firing is only one, and the cost can be reduced.

Hereinafter, a ceramic heater of the present invention and a manufacturing method thereof will be described in detail with reference to the drawings.
<Ceramic heater>
FIG. 1 is a schematic sectional view showing an example of the ceramic heater of the present invention. As shown in FIG. 1, the ceramic heater 1 includes a heater electrode 2 and a heater base 3 formed so as to embed the heater electrode 2 therein.

  The heater electrode 2 is made of a conductive material and a ceramic co-material. As the conductive material, various known conductive metal materials can be used. For example, platinum, tungsten, or an alloy of platinum and one kind selected from the group consisting of rhodium, palladium, lutetium, and gold is used. Is possible. In the present invention, platinum, tungsten, or the like can be preferably used because it can be sintered simultaneously with the ceramic material such as the heater base 3. Examples of the ceramic co-material include alumina, forsterite, zirconia, mullite, titania, silicon nitride, silicon carbide and the like. The ceramic co-material is preferably contained in the heater electrode 2 in a ratio of 20 to 60% by volume with respect to the total amount of the conductive material and the ceramic co-material.

  The ceramic of the heater base 3 is not particularly limited as long as it is an insulating ceramic. For example, insulating alumina, forsterite, zirconia, mullite, titania, silicon nitride, silicon carbide, or the like is used. be able to. In the present invention, it is particularly preferable that the ceramic co-material constituting the heater electrode 2 and the ceramic constituting the heater base 3 are the same type of material. Thereby, the difference in thermal expansion between the ceramic co-material constituting the heater electrode 2 and the ceramic constituting the heater base 3 can be reduced.

  The conductive material preferably has an average particle size of 0.1 to 2.0 μm, and the ceramic co-material preferably has an average particle size of 0.1 to 2.0 μm. Moreover, it is preferable that the average particle diameter of the ceramic material of the heater base | substrate 3 is 0.1-2.0 micrometers.

  In the ceramic heater 1 according to the present invention, the sintering activity of the ceramic co-material constituting the heater electrode 2 is lower than the sintering activity of the ceramic constituting the heater base 3. Thereby, it is possible to prevent the resistance of the heater electrode 2 from increasing even under severe durability conditions in which the temperature rise and the temperature fall are repeated. This is because the use of the ceramic co-material having the above-described relationship with respect to the sintering activity reduces the Young's modulus of the heater electrode 2 and generates stress even when distortion caused by the thermal expansion difference due to temperature rise or fall occurs. It is presumed that this acts to prevent the resistance of the heater electrode 2 from increasing. On the other hand, if the sintering activity is not in the above relationship, the durability against the temperature cycle decreases. In the present invention, “sintering activity” means sinterability at the time of firing, “high sintering activity” means that it is easy to sinter, and “low sintering activity” means “sintering activity”. It means that it is hard to tie.

  In order to make the sintering activity in the above relationship, it is preferable that the specific surface area of the ceramic co-material constituting the heater electrode 2 is smaller than the specific surface area of the ceramic constituting the heater base 3. Specifically, the specific surface area of the ceramic co-material constituting the heater electrode 2 is preferably 10 to 80% with respect to the specific surface area of the ceramic constituting the heater base 3. On the other hand, if the specific surface area of the ceramic co-material constituting the heater electrode 2 is less than 10% with respect to the specific surface area of the ceramic constituting the heater base 3, the particle size becomes coarse, and the heater electrode 2 is homogeneous. However, if it exceeds 80%, the durability of the heater electrode 2 with respect to the temperature cycle decreases, which is not preferable.

The specific surface area is generally called a BET specific surface area by nitrogen adsorption, the BET specific surface area of the ceramic co-material constituting the heater electrode 2 is 5 to 30 m ² / g, and the ceramic BET constituting the heater base 3 The specific surface area is preferably ⁵ to 30 m ² / g, and the specific surface area may be in the above relationship within these ranges.

  In the ceramic heater of the present invention, the heater electrode may be provided on the surface of the heater base. Even with this configuration, an increase in the resistance of the heater electrode can be prevented.

<Manufacturing method of ceramic heater>
Next, an example of the method for manufacturing the ceramic heater of the present invention will be described based on the exploded view of FIG. In FIG. 2, through holes for electrode connection and the like are omitted. First, a ceramic material constituting the heater base 3 and an organic binder such as a butyral resin, an organic solvent, a medium, and the like are mixed to prepare a slurry, and this slurry is formed by, for example, tape molding using a doctor blade method or the like. The green sheets 11 and 12 are obtained by molding and drying by a known molding method. The thickness of the green sheets 11 and 12 is preferably 50 to 500 μm.

  Next, the conductive material constituting the heater electrode and the ceramic co-material are mixed to produce a print paste for the heater electrode. When preparing this paste, it is preferable to use an organic binder such as butyral resin and an organic solvent such as terpineol. Further, the mixing method is not particularly limited, and for example, roll mixing such as three rolls, mill mixing using a mill, or the like can be used.

  The heater electrode printing paste prepared above is printed on one side of the green sheet 11 by a method such as screen printing to form the heater electrode pattern 13. Then, an electrode pad 14 for the extraction electrode is formed on the other surface of the green sheet 11 through a through hole by a known method. Next, the green sheet 12 is adhered and laminated on the surface of the green sheet 11 on which the electrode pattern 13 is formed to obtain a green body (green sheet laminate). In addition, it is preferable to carry out the pressure bonding at about 30 to 100 MPa, and the adhesion lamination may be heated while being pressure bonded. Thereby, generation | occurrence | production of the joining defect of ceramics can be suppressed. In order to adjust the thickness of the ceramic heater, a green sheet on which the electrode pattern 13 is not printed may be further laminated on the green body.

  After the green body is cut into a predetermined size with a hot knife or the like, the green body is simultaneously fired to obtain a ceramic heater. Thereby, a baking process can be carried out once and cost reduction can be aimed at. The firing temperature can be appropriately selected depending on the relationship with the ceramic material, the electrode material, and the like.

  The ceramic heater in the present invention is a gas sensor (ceramic heater structure) integrated with other members in order to heat various elements such as an oxygen sensor, NOx sensor, and CO sensor in addition to heating means in various structural parts. Body). When the ceramic heater is applied to a gas sensor, for example, a detection electrode, a reference electrode, and an electrode pad are formed on a solid electrolyte green sheet in the same manner as the heater electrode pattern 13 is formed. A gas sensor can be formed by adhering and laminating together with the green sheet 11 formed with 13 and firing in the same manner as described above.

<Oxygen sensor>
The case where the ceramic heater of the present invention is used for an oxygen sensor will be described in detail with reference to the drawings. FIG. 3 is a schematic cross-sectional view showing an example when the ceramic heater of the present invention is used for a plate-like oxygen sensor.

  As shown in FIG. 3, the plate-like oxygen sensor 4 includes a heater electrode 2, a ceramic base 5 formed so as to embed the heater electrode 2 therein, a solid electrolyte layer 6 forming a sensor portion, and a detection electrode. 7 and a reference electrode 8, a hollow portion 9 that is hollow so as to be in contact with the reference atmosphere, and a protective layer 10 for protecting the detection electrode 7. And this plate-like oxygen sensor 4 is integrally formed by baking.

  For the solid electrolyte layer 6, a general solid electrolyte material can be used. For example, zirconia, titania-based ceramics and the like can be suitably used, and it is particularly preferable to contain a stabilizing material such as yttria. Examples of the protective layer 10 include a ceramic porous layer made of zirconia, alumina, magnesia, spinel, or the like.

  In the plate-like oxygen sensor 4, the heater electrode 2 is energized and the solid electrolyte layer 6 is heated to about 400 to 1000 ° C., the reference atmosphere (oxygen) is introduced into the cavity 9, and the detection electrode 7 is made of exhaust gas or the like Used in a measurement atmosphere. As a measurement method, for example, a concentration cell type in which an electromotive force generated between the detection electrode 7 and the reference electrode 8 is measured to measure the oxygen concentration in the exhaust gas, a current is detected by applying a constant voltage. For example, a limiting current type for measuring the oxygen concentration may be used.

  Hereinafter, although the ceramic heater of the present invention and the manufacturing method thereof will be described in more detail with reference to Examples and Comparative Examples, the present invention is not limited to the following Examples.

<Production of ceramic heater>
(Green sheet)
Alumina powder (average particle size 0.4 μm, BET specific surface area value 12 m ² / g), butyral binder, solvent and media were mixed and stirred for 48 hours to obtain a slurry. The slurry obtained above was molded by doctor blade molding and dried to produce a green sheet having a thickness of 200 μm.

(Heater electrode)
As a ceramic co-material, 40% by volume of alumina powder having an average particle diameter of 0.7 μm having a BET specific surface area value shown in Table 1 and 60% by volume of platinum powder having an average particle diameter of 1 μm are mixed, and then a butyral system. A binder and terpineol were mixed, and the mixture was mixed by passing 10 times with three rolls, and then diluted with terpineol to obtain a heater paste having a viscosity adjusted. This heater paste was formed by screen printing on the green sheet obtained above and dried to obtain a green sheet on which a heater electrode was formed.

(Ceramic heater)
Various green sheets having a total number of laminated layers of 6 including the green sheet on which the heater electrode was formed were positioned and heated and pressed by thermocompression to obtain a heater green body (laminated body). Then, it cut with the hot knife, the said heater green body was baked at 1450 degreeC for 2 hours, and the ceramic heater was produced.

  The ceramic heater obtained above was evaluated for the average resistance change rate and paste plate slippage. Each evaluation method is shown below, and the results are also shown in Table 1. The “BET value” in Table 1 represents the BET specific surface area value of the alumina powder used as the ceramic co-material.

<Average resistance change rate>
An accelerated intermittent durability test was conducted at 1150 ° C. for 1 minute and 1 minute, and the resistance after 15000 cycles was measured to determine the resistance change rate. The rate of change in resistance was calculated by dividing the resistance after 15000 cycles by the resistance before the accelerated intermittent durability test and dividing it by 100 minutes. The evaluation was performed with 10 ceramic heaters, and the average value was defined as the average resistance change rate.
<Paste release of paste>
Based on the following evaluation criteria, the printing ability of the paste was evaluated by visually observing the printed surface with a binocular microscope.
○: Good △: Slightly good (practically no problem level)

[Examples 2 to 5 and Comparative Example 1]
A ceramic heater was produced in the same manner as in Example 1 except that alumina powder having a BET specific surface area value shown in Table 1 was used as the ceramic co-material. Subsequently, the average resistance change rate and the paste detachment property were evaluated for the obtained ceramic heater in the same manner as in Example 1. The results are also shown in Table 1.

  From Table 1, it can be seen that Examples 1 to 5 have a small average resistance change rate. From this, it can be seen that the prevention of the increase in heater electrode resistance is achieved even under severe endurance conditions in which the temperature rise and fall are repeated. On the other hand, Comparative Example 1 showed a result with a large average resistance change rate.

It is a schematic sectional drawing which shows the ceramic heater in this invention. It is a lamination | stacking exploded view which shows the manufacturing method of the ceramic heater in this invention. It is a schematic sectional drawing which shows an example at the time of using the ceramic heater of this invention for a plate-shaped oxygen sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ceramic heater 2 Heater electrode 3 Heater base 4 Plate-shaped oxygen sensor 5 Ceramic base 6 Solid electrolyte layer 7 Detection electrode 8 Reference electrode 9 Cavity part 10 Protective layer 11,12 Green sheet 13 Heater electrode pattern 14 Electrode pad

Claims (6)

A ceramic heater composed of a heater base made of ceramics and a heater electrode made of a conductive material and a ceramic co-material and provided on the surface of or inside the heater base,
A ceramic heater characterized in that the sintering activity of the ceramic co-material constituting the heater electrode is lower than the sintering activity of the ceramic constituting the heater base.   The ceramic heater according to claim 1, wherein the ceramic co-material constituting the heater electrode and the ceramic constituting the heater base are the same material.   The ceramic heater according to claim 1 or 2, wherein a specific surface area of the ceramic co-material constituting the heater electrode is smaller than a specific surface area of the ceramic constituting the heater base.   The ceramic heater according to any one of claims 1 to 3, wherein a specific surface area of the ceramic co-material constituting the heater electrode is 10 to 80% with respect to a specific surface area of the ceramic constituting the heater base.   It is for gas sensors, The ceramic heater in any one of Claims 1-4. At least a step of mixing a conductive material and a ceramic co-material to produce a printing paste for a heater electrode, a step of printing this printing paste on a green sheet made of ceramics to form a heater electrode, the steps of close contact laminate the other green sheet made of ceramic on the surface forming the heater electrode and a step of co-firing a green sheet the adhesive laminate, as the ceramic common material in the printing paste, sintering activity A method for producing a ceramic heater, wherein the green sheet is lower than the sintering activity of the ceramics .

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