JP2000193637A - Hydrocarbon sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrocarbon sensor having a satisfactory yield and high detection accuracy. SOLUTION: This sensor is equipped with a solid electrolyte layer 1 comprising barium/cerium-based oxides, a cathode 2 and an anode 3, a pair of electrodes installed on the solid electrolyte layer 1, a ceramic substrate 4 having a thermal expansion coefficient roughly equal to that of the solid electrolyte layer 1, and a heater 7 installed on the ceramic substrate 4. In this case, the solid electrolyte layer 1 and the ceramic substrate 4 are jointed together.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrocarbon sensor.

[0002]

2. Description of the Related Art Conventionally, a hydrocarbon sensor using a barium-cerium-based oxide as a solid electrolyte has been proposed (Japanese Patent Laid-Open No. 9-127055).

FIG. 6 shows a schematic structure of this hydrocarbon sensor. Reference numeral 51 denotes a solid electrolyte layer made of a barium-cerium-based oxide.
Are formed by a thick film printing process. The solid electrolyte layer 51 includes a ceramic substrate 5 made of forsterite.
Reference numeral 4 denotes an adhesive made of an inorganic material (hereinafter abbreviated as an inorganic adhesive) 58, which is bonded and fixed. On the surface of the ceramic substrate 54, a heater 59 is formed by a thick film printing process. A diffusion-controlling hole 61 (portion indicated by a dotted line in FIG. 6) into which a hydrocarbon gas (hereinafter, referred to as HC) is introduced is provided in a part of the adhesive layer joined by the inorganic adhesive 58. I have.

Next, the operation of the hydrocarbon sensor will be described. As shown in FIG. 6, a constant voltage is applied to the cathode 52 and the anode 53. Also, an ammeter for output detection is provided in the circuit. The heater 59 is used to activate the solid electrolyte layer 51. When HC reaches the anode 53 through the diffusion control holes 61 in this state, the HC is decomposed, and protons are conducted in the solid electrolyte layer 51. As a result, a current I flows through the circuit. The magnitude of this current I depends on the amount of protons, that is, HC
Increases in proportion to the concentration of Therefore, the concentration of HC can be detected from the output of the ammeter.

[0005] The hydrocarbon sensor having such a structure not only provides a linear output with respect to the HC concentration, but is not affected by the coexistence of oxygen when the HC concentration is low. That is, a hydrocarbon sensor having good gas selectivity can be obtained.

[0006]

However, when such a hydrocarbon sensor is applied to the detection of HC concentration in automobile exhaust gas, the solid electrolyte layer 51 must be activated. For this reason, it has been found that when a current is passed through the heater 59, the ceramic substrate 54 provided with the heater 59 receives a large stress, and the yield decreases. This can be for the following reasons.

1) The ceramic substrate 54 (forsterite) provided with the heater 59 has a coefficient of thermal expansion of 1
1 to 11.5 × 10 ⁻⁶ / ° C., but the solid electrolyte layer 51
Has a thermal expansion coefficient of about 10 × 10 ⁻⁶ / ° C., which is considered to be due to the difference between the two values by 1 × 10 ⁻⁶ / ° C. or more.

2) HC in the exhaust gas is a reducing atmosphere, and when the ceramic substrate 54 is exposed to the reducing atmosphere, part of the composition of the ceramic substrate 54 (MgO in forsterite) is reduced and the strength is reduced. it is conceivable that.

3) It is considered that since the thick platinum paste for forming the heater 59 has a porous and uneven structure, current flows intensively in these portions, and the temperature is locally increased. .

4) The heater 59 is a ceramic substrate 54
Is formed on only one side, and the formed surface has an exposed structure. In this state, when an electric current is applied to the heater 59, the surface between the heater formed and the back surface (adhesion surface with the solid electrolyte layer) is formed. It is considered that the temperature difference suddenly occurs and a large stress is generated.

Another problem with the conventional hydrocarbon sensor is that when a hydrocarbon sensor is installed in the exhaust gas of an automobile, the temperature of the exhaust gas changes depending on the operating condition of the engine.
The temperature of the hydrocarbon sensor will be 620 ± 30 ° C. That is, the temperature fluctuation range was as high as 60 ° C. Since the output current of the hydrocarbon sensor is affected not only by the HC concentration but also by the temperature, this temperature fluctuation contributes to a decrease in the accuracy of the hydrocarbon sensor. This is because the temperature adjustment accuracy of the heater 5 was insufficient.

The present invention solves this problem,
It is an object of the present invention to provide a hydrocarbon sensor having good yield and high detection accuracy.

[0013]

In order to solve this problem, a hydrocarbon sensor according to the present invention comprises: a solid electrolyte layer made of a barium-cerium-based oxide; a pair of electrodes provided on the solid electrolyte layer; A ceramic substrate having substantially the same thermal expansion coefficient as the solid electrolyte layer, and a heater provided on the ceramic substrate are provided, and the solid electrolyte layer and the ceramic substrate are joined.

Further, an auxiliary substrate having a thermal expansion coefficient substantially equal to that of the ceramic substrate is provided on the heater forming surface side of the ceramic substrate.

Further, the heater has a control means for controlling the heater to be turned on and off, and a control means for comparing a resistance value of the heater with a target resistance value of the heater defined in advance corresponding to the temperature. A comparison means is connected to a judgment means for suppressing an output from the control means in accordance with a signal from the comparison means.

With these configurations, a hydrocarbon sensor having good yield and high detection accuracy can be obtained.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention comprises a solid electrolyte layer made of a barium-cerium-based oxide, a pair of electrodes provided on the solid electrolyte layer, A ceramic substrate having the same thermal expansion coefficient and a heater provided on the ceramic substrate are provided, and the solid electrolyte layer and the ceramic substrate are joined. This has the effect of reducing the stress on the heater due to the difference in expansion coefficient and improving the yield as a hydrocarbon sensor.

According to a second aspect of the present invention, in the first aspect of the invention, since the ceramic substrate is made of partially stabilized zirconia, the strength of the substrate is not reduced even when used in exhaust gas in a reducing atmosphere. It has the effect of reducing the damage to the heater.

According to a third aspect of the present invention, in the first aspect of the present invention, since the heater is densely formed by a thick film forming method using a platinum paste, the heater portion is not only inexpensive but also locally formed. This has the effect of eliminating unevenness in the resistance value and suppressing the deterioration of the heater.

According to a fourth aspect of the present invention, in the first aspect of the present invention, an auxiliary substrate having a thermal expansion coefficient substantially equal to that of the ceramic substrate is provided on the heater forming surface side of the ceramic substrate. Therefore, when the heater is energized, heat is also diffused to the auxiliary substrate, so that a rapid temperature difference between the front and back of the ceramic substrate can be prevented, and the effect of reducing damage to the heater can be obtained.

According to a fifth aspect of the present invention, in the fourth aspect of the invention, since the ceramic substrate and the auxiliary substrate are joined by an adhesive made of an inorganic material, the temperature of the heater becomes high. Also has the effect that both substrates can be securely joined.

According to a sixth aspect of the present invention, in the first or the fourth aspect of the present invention, the heater has a driving means, a control means for controlling on / off of the heater, and a resistance value of the heater. A comparing unit for comparing a target resistance value of the heater predetermined in accordance with a temperature, and a judging unit for suppressing an output from the control unit according to a signal from the comparing unit. Is connected, so that the heater can be highly controlled to a target temperature, and furthermore, the temperature of the solid electrolyte layer for detecting the HC concentration can be controlled with high accuracy, and the structure can be relatively simplified. Has an action.

The invention described in claim 7 is the invention according to claims 4 and 5.
In the invention described in the above, the detection element portion comprising the solid electrolyte layer, the ceramic substrate, and the auxiliary substrate is fixed to a flat portion provided at one end of the ceramic column with an adhesive made of an inorganic material, and housed in a metal case. With such a configuration, it has an effect that it is resistant to high-temperature and oxygen-reducing atmosphere and high reliability can be secured.

According to an eighth aspect of the present invention, in the invention according to the seventh aspect, since the thermal expansion coefficient of the ceramic cylinder is substantially equal to the thermal expansion coefficient of the ceramic substrate or the auxiliary substrate, This has the effect of preventing the substrate or the auxiliary substrate from receiving a large stress from the ceramic cylinder and reducing the damage to the heater.

According to a ninth aspect of the present invention, there is provided the heater according to the seventh aspect of the invention, which is configured so that the thermal expansion coefficient of the adhesive made of an inorganic material and the thermal expansion coefficient of the ceramic substrate are substantially equal to each other. Even when the temperature rises, not only can the auxiliary substrate forming the detecting element portion and the flat portion provided at one end of the ceramic cylinder be securely bonded, but also the effect of reducing the damage to the heater can be obtained.

According to a tenth aspect of the present invention, in the invention of the seventh aspect, since the metal case is made of heat-resistant stainless steel, it is not only easy to process than ceramics, but also has a high temperature. However, it has the effect that the ceramic cylinder can be securely fixed.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG. (Embodiment 1) FIG. 1 is a schematic sectional view of a detection element portion of an embodiment of a hydrocarbon sensor according to the present invention. FIG. 2 is a pattern diagram of the heater. FIG. 3 is a schematic perspective view mainly illustrating a flat portion provided on a ceramic cylinder holding a detection element portion. FIG. 4 is an exploded perspective view of a metal case holding a ceramic column. FIG. 5 is a block diagram for explaining a heater driving circuit.

Reference numeral 1 denotes a 10 mm square solid electrolyte layer made of barium, cerium and gadolinium oxide having a thickness of 0.5 mm, and a pair of electrodes consisting of a cathode 2 and an anode 3 having a thickness in contact with the surface thereof. After membrane printing
It is formed by firing. An aluminum paste was used for forming the cathode 2 and a platinum paste was used for forming the anode 3.

A partially stabilized zirconia ceramic substrate 4 having substantially the same dimensions as the solid electrolyte layer 1 is bonded to the anode 3 side of the solid electrolyte layer 1 with an inorganic adhesive 5. Here, a glass paste was used as the inorganic adhesive 5, and both the solid electrolyte layer 1 and the ceramic substrate 4 were printed and fired, and then they were overlapped and fired again. The coefficient of thermal expansion of the partially stabilized zirconia ceramic substrate 4 was 10 × 10 ⁻⁶ / ° C., which was substantially equal to the coefficient of thermal expansion of the solid electrolyte layer 1 (about 10 × 10 ⁻⁶ / ° C.).

As shown in FIG. 1, a diffusion controlling hole 6 is formed by opening a part of the inorganic adhesive 5.

On the surface of the ceramic substrate 4 opposite to the surface joined to the solid electrolyte layer 1, a platinum paste (for example,
Printed the product number TR709) made by Tanaka Kikinzoku, 1100 ° C
By baking, the heater 7 is formed thickly and densely. The pattern of the heater 7 was a serpentine type shown in FIG. 2A or a concentric type shown in FIG. 2B.

The ceramic substrate 4 on which the heater 7 is formed
An auxiliary substrate 8 made of partially stabilized zirconia having substantially the same dimensions as the ceramic substrate 4 is bonded thereon with an inorganic adhesive 5. Therefore, the thermal expansion coefficients of the ceramic substrate 4 and the auxiliary substrate 8 can be matched. As the inorganic adhesive 5, a silicon-based adhesive (for example, Ceramabond (trade name of Alemco), part number 685) is used, and its thermal expansion coefficient is 10.8 × 10 ⁻⁶ / ° C.
Therefore, even when compared with the thermal expansion coefficient of the partially stabilized zirconia, the thermal expansion coefficient difference is 0.8 × 10 ⁻⁶ / ° C.,
They are almost equivalent.

The bonding is performed under the condition of heating at 371 ° C. for 2 hours for curing. Detection element section 9 completed as described above
Is fixed to the flat part 11 provided at one end of the ceramic column 10 by the inorganic adhesive 5 as shown in FIG. 3 (a) or 3 (b). The ceramic cylinder 10 must be shaped as shown in FIGS. 3 (a) and 3 (b). Further, since it is necessary to provide a through hole for passing a lead wire (described later) inside the ceramic cylinder 10, a material having a free-cutting property, for example, a silica-based vitreous ceramic (a product of Macor (a product of Corning Incorporated)) Here, the thermal expansion coefficient of macol is 9.
4 × 10 ⁻⁶ / ° C., and the coefficient of thermal expansion of the partially stabilized zirconia (10 × 10 ⁻⁶ /
° C). As the inorganic adhesive 5, the same ceramer bond as described above was used.

Inside the ceramic cylinder 10, lead wires 11 for sensor output and heater are passed and electrically connected to each other.

The ceramic cylinder 10 is housed in a metal case 13 and fixed by a metal lid 14. As shown in FIG. 4, a vent hole 15 is formed in the metal case 13 at a position corresponding to the detection element section 9 for passing exhaust gas. Also, the metal case 13
The metallic cover 14 is made of heat-resistant stainless steel in consideration of use in exhaust gas from automobiles. In this embodiment, JIS SUS430 is used as the heat-resistant stainless steel.
Many heat-resistant stainless steels other than the standard SUS310 can be used.

Since the principle of detection of the hydrocarbon sensor in this embodiment is basically the same as that of the conventional example, the configuration and method for driving and controlling the heater 7 will be described in detail below.

In FIG. 5, 16 is a driving means for driving the heater 7, 17 is a control means for on / off control, 18 is a reference resistance for defining a target resistance value,
19 is a comparing means, 20 is a judging means, 21, 22, and 23 are resistors.

Since the heater 7 uses a platinum resistor, the resistance temperature coefficient is constant, and a proportional relationship is established between the temperature and the resistance value. Therefore, the temperature (for example, 700
C), it is possible to select a reference resistor 18 corresponding to a target resistance value of the heater 7 specified in advance.

For example, when 12 V is supplied from a power supply to the driving means 16 for driving the heater 7, a repetition pulse of ON / OFF (pulse frequency is about 4 Hz) is supplied from the control means 17 until the target temperature is reached. Judgment means 20
And is continuously supplied to the heater 7 via the AND circuit.

However, when the temperature of the heater 7 reaches the target temperature, the resistance value of the heater 7 and the resistance value of the reference resistor 18 match, so that the output from the comparing means 19 becomes a low level. Next, when this output is inputted to the judging means 20, the judging means 20 no longer provides the driving means 16 with a repetition pulse of ON / OFF.
Is stopped.

The hydrocarbon sensor having the above configuration was actually tested and examined in automobile exhaust gas. As a result, the yield after energization of the heater 7 achieved 100% (the number of test pieces 1).
0).

This is because the heater 7 has the same thermal expansion coefficient as that of the solid electrolyte layer 1 and the ceramic substrate 4, and further has an auxiliary substrate 8.
It is considered that the stress received by the subject was remarkably reduced.

In order to evaluate and evaluate the temperature control function of the heater 7 in detail, the following experiment was conducted. A sample in which a thermocouple as a temperature sensor was adhered on the solid electrolyte layer 1 with an inorganic adhesive to monitor the temperature of the detection element section 9 was prepared, and the above-described sample was arranged in exhaust gas of an automobile. Such temperature control was implemented. As a result, when the operating condition of the engine is changed, the conventional temperature fluctuation range is 60
However, by performing the temperature control as described in the present embodiment, the temperature can be suppressed to only 8 ° C., and it has been demonstrated that the temperature can be controlled with high accuracy.

In order to control the temperature of the heater 7, a temperature sensor is provided in a part of the detecting element section 9 so that the detecting element section 9 is always controlled.
It is expected that the temperature can be controlled with high accuracy by monitoring the temperature of the heater 7 and controlling the current of the heater 7. But,
As a result of an actual study, no particular temperature control characteristic was obtained compared to a configuration in which the temperature was obtained from the resistance value of the heater 7 and controlled. Therefore, the temperature control method described in the present embodiment utilizing the characteristics of the resistance value of the heater 7 itself does not require a separate temperature sensor, does not increase the number of lead wires, and has a simple structure. There are many benefits.

In the temperature control using the characteristic of the resistance value of the heater 7 itself described in the present embodiment, other configurations (for example, feedback control) for performing various controls can be considered.

Also, the two types of heater 7 patterns as shown in FIGS. 2A and 2B have been described, but from the viewpoint of temperature control, the same high accuracy can be realized in either case. . The difference between the two is that there is only a difference in the resistance value of the heater 7 when the pattern has the same width, so that the optimum resistance value is obtained in consideration of the required temperature and power consumption. What is necessary is just to select a shape. Further, the width and interval of the pattern of the heater 7, the number of twists, the number of concentric circles, and the like may be arbitrarily selected in consideration of the required temperature, power consumption, and the like, so that the optimum resistance value can be obtained. 2A and 2B are not limited at all.

Further, the shape of the ceramic cylinder is shown in FIG.
(A) and FIG. 3 (b) show two types, but either may be used in forming a hydrocarbon sensor. However, FIG.
The flat portion 11 shown in FIG. 1 has an advantage that the heat capacity is small because it is thin and the heating time when the heater 7 is heated is short. On the other hand, there is a disadvantage that it is difficult to form the flat portion 11 because it is thin. Further, since the flat portion 11 shown in FIG. 3B is formed by cutting only a part of a cylinder, it has an advantage that it is easy to process, but has a large heat capacity of the flat portion 11 and a slightly longer temperature rise time. There are disadvantages. Therefore, both FIG. 3A and FIG. 3B have contradictory advantages and disadvantages. Therefore, the optimum shape and dimensions may be determined in consideration of the required heating time, processing cost, and the like.

Further, in the present embodiment, the description has been made mainly on the configuration in which the auxiliary substrate 8 is provided. However, this is not necessarily essential when the required temperature, power consumption and detection accuracy are comprehensively considered. However, even in this configuration,
The technical idea of the present invention has been fully implemented.

It should be noted that the specific material names, trademarks, and product numbers described in the present embodiment are all examples of constituting a hydrocarbon sensor, and the present invention is not limited to these materials.

As described above, a hydrocarbon sensor having good yield and high detection accuracy can be realized.

[0051]

As described above, the present invention provides a solid electrolyte layer made of a barium-cerium-based oxide, a pair of electrodes provided on the solid electrolyte layer, and a thermal expansion coefficient substantially equal to that of the solid electrolyte layer. Since the ceramic substrate has a ceramic substrate and a heater provided on the ceramic substrate, and the solid electrolyte layer and the ceramic substrate are joined, a hydrocarbon sensor having a good yield can be obtained.

The heater has a control means for controlling the heater to be turned on and off, and a control means for comparing a resistance value of the heater with a target resistance value of the heater defined in advance corresponding to the temperature. Since the comparison unit and the determination unit for suppressing the output from the control unit in accordance with the signal from the comparison unit are connected, a hydrocarbon sensor with high detection accuracy can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a detection element section of an embodiment of a hydrocarbon sensor of the present invention.

FIG. 2A is a pattern diagram of a heater according to the present embodiment; FIG.

FIG. 3A is a schematic perspective view mainly illustrating a flat portion provided on a ceramic column in the present embodiment; FIG.

FIG. 4 is an exploded perspective view of a metal case in the present embodiment.

FIG. 5 is a block diagram for describing a heater driving circuit according to the present embodiment.

FIG. 6 is a schematic sectional view of a detection element section of a conventional hydrocarbon sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solid electrolyte layer 2 Cathode 3 Anode 4 Ceramic substrate 5 Inorganic adhesive 6 Diffusion controlling hole 7 Heater 8 Auxiliary substrate 9 Detecting element part 10 Ceramic column 11 Flat part 12 Lead wire 13 Metal case 14 Metal lid 15 Vent hole 16 Driving means 17 Control means 18 Reference resistance 19 Comparison means 20 Judging means 21, 22, 23 Resistance

Claims (10)

[Claims] 1. A solid electrolyte layer made of a barium-cerium-based oxide, a pair of electrodes provided on the solid electrolyte layer, a ceramic substrate having a thermal expansion coefficient substantially equal to that of the solid electrolyte layer, and the ceramic substrate Wherein the solid electrolyte layer and the ceramic substrate are joined. 2. The hydrocarbon sensor according to claim 1, wherein the ceramic substrate is made of partially stabilized zirconia. 3. The hydrocarbon sensor according to claim 1, wherein the heater is densely formed by a thick film forming method using a platinum paste. 4. The hydrocarbon sensor according to claim 1, wherein an auxiliary substrate having a thermal expansion coefficient substantially equal to that of the ceramic substrate is provided on the side of the ceramic substrate on which the heater is formed. 5. The hydrocarbon sensor according to claim 4, wherein the ceramic substrate and the auxiliary substrate are joined with an adhesive made of an inorganic material. 6. The heater includes a driving unit, a control unit for controlling on / off of the heater, and a comparison between a resistance value of the heater and a target resistance value defined in advance corresponding to the temperature. 5. The hydrocarbon sensor according to claim 1, further comprising: a comparison unit configured to perform the operation, and a determination unit configured to suppress an output from the control unit according to a signal from the comparison unit. 6. 7. A detecting element portion comprising a solid electrolyte layer, a ceramic substrate and an auxiliary substrate is fixed to a flat portion provided at one end of a ceramic column with an adhesive made of an inorganic material, and housed in a metal case. The hydrocarbon sensor according to claim 4. 8. The hydrocarbon sensor according to claim 7, wherein a coefficient of thermal expansion of the ceramic cylinder is substantially equal to a coefficient of thermal expansion of the ceramic substrate or the auxiliary substrate. 9. The hydrocarbon sensor according to claim 7, wherein the thermal expansion coefficient of the adhesive made of an inorganic material is substantially equal to the thermal expansion coefficient of the ceramic substrate. 10. The hydrocarbon sensor according to claim 7, wherein the metal case is made of heat-resistant stainless steel.