Method and device for detecting particulate matter contained in a gas to be measured

A particulate matter detection element includes a capacitance component disposed in parallel with a detected resistance RSEN. A direct current-power source that supplies a direct current (IDC) for particulate matter detection, and an alternating-current power source that supplies an alternating current (IAC) for disconnection detection are provided.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2010-242162, filed Oct. 28, 2010, the entire contents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and device for detecting an amount of particulate matter contained in a gas to be measured, such as an exhaust gas from vehicles, and in particular, the method and apparatus detect an amount of the particulate matter based on electrical resistance caused by the particulate matter accumulating between electrodes. The present invention also relates to a method of manufacturing a particulate matter detection element used in the particulate matter detecting device.

2. Description of the Related Art

The exhaust gas of a diesel engine of an automobile and the like may include environmental pollutants, particularly particulate matter (hereinafter referred to accordingly as “PM”) mainly composed of soot particles and soluble organic fractions (SOF). A diesel particulate filter (hereinafter referred to accordingly as “DPF”) is provided on an exhaust gas path to collect the PM. The DPF is made of a porous ceramic having excellent heat resistance. The DPF captures the PM as a result of the exhaust gas passing through a partition wall having numerous fine pores.

When the amount of collected PM exceeds an allowable amount, the DPF becomes clogged. Pressure loss may increase. Alternatively, the amount of PM escaping through the DPF may increase. Therefore, collection capability is recovered by a regeneration process being periodically performed.

In general, increase in the differential pressure across the DPF caused by increase in the amount of collected PM is used for determining the regeneration timing. Therefore, a differential pressure sensor is provided that detects the difference in pressure upstream and downstream from the DPF.

The regeneration process is performed by high-temperature exhaust gas being introduced into the DPF through heating using a heater, by post-injection, or the like, and the PM being removed by burning.

On the other hand, a sensor capable of directly detecting the PM in the exhaust gas has been proposed. The PM sensor is, for example, provided downstream from the DPF and measures the amount of PM escaping through the DPF. The PM sensor can be used in an on-board diagnosis (OBD) device to monitor an operating state of the DPF or to detect abnormalities such as cracks and damage.

Use of the PM sensor in place of the differential pressure sensor to determine the regeneration timing of the DPF is also being discussed. In this instance, the PM sensor is provided upstream from the DPF and measures the amount of PM entering the DPF.

As a basic configuration of a PM sensor such as that described above, JP-A-S59-197847 discloses an electrical-resistance-type smoke sensor. The smoke sensor is configured such that a pair of conductive electrodes are formed on a front surface of a substrate having insulating properties, and a heating element is formed on a back surface of or within the substrate. The smoke sensor takes advantage of smoke (particulate carbon) having conductivity, and detects changes in electrical resistance value occurring as a result of smoke accumulating between the electrodes that serve as a detection section.

In a particulate matter detecting device such as that described above, when a certain amount of particulate matter or more is accumulated between detection electrodes, the detected resistance no longer changes. The amount of particulate matter within gas to be measured can no longer be detected.

Therefore, the heating element that generates heat as a result of being energized is provided. The detection section is heated by being directly heated by a heater. Alternatively, the detection section is heated by post-injection or the like, by exhaust gas, serving as the gas to be measured, being heated to a high temperature. As a result, the particulate matter accumulated between the detection electrodes is removed by burning. Detection capability is thereby recovered.

In addition, WO 2008/031654 discloses an example of a particulate matter detection element, such as that described above, and a control method. The particulate matter detection element in WO 2008/031654 is configured such that a resistance layer is connected in parallel to electrical resistance formed by particulate matter accumulated between detection electrodes. The resistance layer is provided between a substrate and a pair of detection electrodes. The resistance layer is formed by a conductive layer containing zirconia or the like. As a result of the resistance layer being formed, damage and deterioration of the electrodes can be detected.

However, in a conventional electrical-resistance-type particulate matter detecting device, such as that described in JP-A-S59-197847, when the particulate matter accumulated between detection electrodes is heated and removed, the resistance between the detection electrodes becomes extremely high, causing an almost insulated state.

Therefore, it may be difficult to use the value of the detected resistance to differentiate between a state in which particulate matter is not accumulated between the detection electrodes, and a state in which a disconnection abnormality has occurred in an electrical wire of a signal line connecting a detection element and a detection circuit or the like.

As described in WO 2008/031654, energization is performed between the detection electrodes by the conductive layer. Therefore, output is detected even in a state in which particulate matter is not accumulated, if the resistance value of the conductive layer is too low. As a result, malfunction may occur. Furthermore, the resistance value of the conductive layer is required to be adjusted with high accuracy, leading to increase in manufacturing cost.

Moreover, the metal configuring the detection electrodes inevitably becomes dispersed in the conductive layer as a result of extended use. The resistance value of the conductive layer changes, thereby causing instable output.

SUMMARY

The present invention has been achieved in light of the above-described issues. An object of the present invention is to provide an electrical-resistance-type particulate matter detecting device used to detect particulate matter within exhaust gas of an internal combustion engine. The particulate matter detecting device is highly reliable, having high detection accuracy and being capable of detecting disconnection abnormality. The present invention also provides a method of manufacturing a particulate matter detection element used in the particulate matter detecting device. The present invention also provides a method of detecting disconnection in a particulate matter detecting device.

According to a first aspect, A detecting device is provided that detects an amount of particulate matter included in a gas to be measured, the detecting device having:

a detection element having a detection section that has electrical resistance in relation to the particulate matter against a direct current and a capacitance component;

a detection circuit having an alternating-current power source that supplies an alternating current to the detection element and an alternating-current detector that detects the alternating current flowing through the detection element.

According to the first aspect, the capacitance component becomes a predetermined value proportional to a relative permittivity of the dielectric layer and the area of parallel plate conductors and inversely proportional to the film thickness of the dielectric layer. The capacitance component indicates impedance of a certain range in relation to the alternating current supplied from the alternating current power source.

Even in a state in which the particulate matter is not accumulated between the detection electrodes, the direct current applied by the direct current power source is not flowing through the particulate matter detection element, and output from a direct-current detector cannot be detected, the alternating-current detector can detect the alternating current transmitted via the capacitance component.

Therefore, when disconnection abnormality occurs between the detection circuit and the particulate matter detection element connected via a signal line, the alternating current detected by the alternating-current detector changes. The disconnection abnormality can be promptly detected.

As a result of the direct-current detector detecting the direct current flowing to the particulate matter detection element, detection resistance formed by the particulate matter accumulated between the detection electrodes can be accurately measured.

At this time, as a result of an insulating ceramic being used as the dielectric layer, the direct current resistance between the parallel plate conductors can be increased and insulation properties can be ensured in the capacitance component. Therefore, the amount of particulate matter accumulated in the detection section can be stably detected without the direct current flowing to the capacitance component and without the output of the detected resistance detected by the direct-current detector being affected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A particulate matter detecting device according to various preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.

First Embodiment

A first embodiment will be described with reference toFIG. 1toFIGS. 9A and 9B, andFIG. 13.

A particulate matter detecting device100according to the first embodiment of the present embodiment is provided on an exhaust gas flow path of an internal combustion engine. The exhaust gas serves as the gas to be measured. The particulate matter detecting device100detects the amount of particulate matter within the gas to be measured. Combustion control of the internal combustion engine, regeneration of an exhaust emission control device, abnormality diagnosis, and the like are performed using the detection result.

The particulate matter detecting device100includes at least a particulate matter detection element10and a detection circuit20. The particulate matter detection element10is provided with a pair of detection electrodes110and120serving as a detection section11. The pair of detection electrodes110and120are provided such as to oppose each other with a predetermined amount of space therebetween on a front surface of an insulating substrate101. The detection circuit20is connected to the particulate matter detection element10by a pair of signal lines116and126(seeFIG. 4A). The detection circuit20detects electrical resistance formed by particulate matter PM accumulated in the detection section11as detected resistance RSEN. In the particulate matter detecting device100that detects the amount of particulate matter PM included within the gas to be measured, the particulate matter detection element10includes a capacitance component13disposed in parallel with the detected resistance RSEN. Furthermore, the detection circuit20includes a direct-current power source21, an alternating-current power source22, a direct-current detector23, and an alternating-current detector24. The direct-current power source21supplies a direct current IDCto the particulate matter detection element10. The alternating-current power source22supplies an alternating current IAChaving a predetermined frequency f and a predetermined amplitude. The direct-current detector23detects the direct current IDCflowing to the particulate matter detection element10. The alternating-current detector24detects the alternating current IAC. The capacitance component13is formed by an insulating ceramic serving as a dielectric layer150and a pair of parallel plate conductors130and140. The dielectric layer150has a predetermined film thickness d and a predetermined relative permittivity ∈r. The pair of parallel plate conductors130and140have a predetermined area S and are disposed such as to oppose each other with the dielectric layer150therebetween.

In a state in which the particulate matter is not accumulated between the detection electrodes110and120, the capacitance component13is connected in series between the detection electrodes110and120. As a result, disconnection D1in the detection section11can be detected in addition to disconnection D2between the detection section11and the detection circuit20.

An overview of the particulate matter detection element10will be described with reference toFIG. 1.

The particulate matter detection element10has the insulating substrate101and the detection section11. The particulate matter detection element10has the pair of detection electrodes110and120, a pair of detection lead sections111and121, and detection terminal sections112and122. The pair of detection electrodes110and120are formed on the front surface of the insulating substrate101. The pair of detection lead sections111and121are formed connected to the detection electrodes110and120. The detection terminal sections112and122are formed respectively connected to the detection lead sections111and121.

The insulating substrate101is formed into a rough plate shape by a known method, such as a doctor blade method, using an insulating heat-resistant material such as alumina.

The detection electrodes110and120, the detection lead sections111and121, and the detection terminal sections112and122are formed by a known method, such as thick film printing, using a conductive material such as platinum.

Furthermore, the capacitance component13that is a main section of the present embodiment is formed such as to be layered on the rear-surface side of the insulating substrate101.

The capacitance component13has the dielectric layer150and the pair of plate conductors130and140. The dielectric layer150is formed in a rough plate shape, having a predetermined film thickness d. The dielectric layer150is formed using an insulating ceramic having a predetermined relative permittivity ∈r, such as alumina. The pair of plate conductors130and140are formed such as to oppose each other and have a predetermined area S. The pair of plate conductors130and140are formed on either side of the dielectric layer150such as to sandwich the dielectric layer150. The plate conductors130and140are respectively connected to conductor lead sections131and141. The plate conductors130and140are also connected to the detection lead sections111and121via the conductor lead sections131and141, through-hole electrodes132,142, and143that pass through the dielectric layer150and the insulating substrate101. As a result, the capacitance component13is parallel with the detected resistance RSEN.

The plate conductors130and140configure parallel plate conductors that oppose each other with the dielectric layer150having a thickness d therebetween, thereby forming the capacitance component13.

Capacitance C13of the capacitance component13is C13=∈r∈0S/d, when relative permittivity is ∈r, vacuum permittivity is ∈0, the area of the plate conductors130and140is S, and the thickness of the dielectric layer150is d.

An alternating current impedance Z of the capacitance component13is expressed by Z=1/jωC13(j being an imaginary unit). An absolute value |Z| of the alternating current impedance Z is expressed by 1/ωC13=1/(2π·f·C13) (f being a frequency of the alternating current that is applied).

According to the first embodiment, the area S of the plate conductors130and140and the film thickness d of the dielectric layer150are formed such that the absolute value of the alternating current impedance Z is 200 kΩ or less.

In other words, when the area S of the plate conductors130and140is 10 (mm2), the relative permittivity ∈rof alumina configuring the dielectric layer150is 11.2, and the frequency f of the alternating current applied by the alternating-current power supply22, described hereafter, is 20 kHz, C13is 1000/8/π=39.8 pF or more. In other words, the film thickness d is formed to be 11.2×8.854×10/39.8=24.9 μm or less.

In addition, according to the first embodiment, the dielectric layer150is formed having a film thickness d such that direct current resistance R150of the dielectric layer150is 1 MΩ or more, using an insulating ceramic having a volume resistivity p that is 1.4×1011(Ωm) or more at a temperature of 600° C.

In other words, according to the first embodiment, a relationship is established in which d≧7 (μm).

Here, a procedure for forming the capacitance component13that is a main section of the method of manufacturing the particulate matter detection element10of the present embodiment will be described.

The dielectric layer150is formed by an insulating ceramic mixing and dispersing procedure and a dielectric layer forming procedure being performed. In the insulating ceramic mixing and dispersing procedure, insulating ceramic powder having a predetermined relative permittivity ∈rand a predetermined volume resistivity p, a predetermined dispersion medium, a predetermined binder, and a predetermined plasticizer are mixed and dispersed into a slurry state or a paste state. In the dielectric layer forming procedure, the ceramic slurry or the ceramic paste obtained by the insulating ceramic mixing and dispersing procedure is used to form the dielectric layer150by coating or printing.

According to the first embodiment, the roughly plate-shaped dielectric layer150having a predetermined film thickness d is formed by the doctor blade method, using a slurry formed by the insulating ceramic being dispersed in a predetermined dispersing medium or the like.

In a first plate conductor forming procedure, on one surface of the dielectric layer150formed in a rough plate shape, the first plate conductor130having a predetermined area S and the first conductor lead section131are formed by printing, using a metal paste composed of Pt or the like. Furthermore, in a second plate conductor forming procedure, on the other surface of the dielectric layer150, the second plate conductor140having a predetermined area S and the second conductor lead section141are formed by printing, using a metal paste composed of Pt or the like.

The through-hole electrode132is formed by a through-hole being filled with an electrode paste composed of Pt or the like. The through-hole is provided such as to pass through the rear-surface side and the front-surface side of the insulating substrate101to connect the first conductor lead section131and the detection lead section111.

In addition, the through-hole electrode143is formed by a through-hole being filled with the electrode paste. The through-hole is provided such as to pass through the rear-surface side and the front-surface side of the insulating substrate101to connect the second conductor lead section141and the detection lead section121. Furthermore, the through-hole electrode142is formed by a through-hole being filled with an electrode paste composed of Pt or the like. The through-hole is provided such as to pass through the rear-surface side and the front-surface side of the dielectric layer150.

As a result of the foregoing components being integrally layered, the pair of parallel plate conductors are formed such that the plate conductor130and the plate conductor140oppose each other with the dielectric layer150therebetween. The plate conductors130and140are respectively connected to the detection lead sections111and121. The capacitance component13is connected in parallel to the detected resistance RSEN.

Furthermore, an insulating substrate102is disposed such as to be layered on the rear-surface side of the dielectric layer150. A heating element160and a pair of heating element lead sections161aand161bconnected to the heating element160are formed on the insulating substrate102. An insulating substrate to ensure insulation between the second plate conductor140and the heating element lead sections161aand161bis formed in a rough plate shape in a manner similar to the insulating substrate101on the insulating substrate101, for example by printing alumina paste, so that the heating element160and the heating element lead sections161aand161bare covered. Furthermore, a pair of through-hole electrodes162aand162bare formed such as to pass through the insulating substrate102. The pair of through-hole electrodes162aand162bare connected to the heating element lead sections161aand161b. A pair of heating element terminal sections163aand163bare formed on the rear-surface side of the insulating substrate102such as to be connected to the through-hole electrodes162aand162b.

Furthermore, a protective layer103is formed such as to be layered on the detection electrodes110and120and to cover the detection lead sections111and121. The protective layer103is formed using heat-resistant glass and insulating ceramic. An opening section104from which the detection section11is exposed is provided in the protective layer103.

The protective layer103protects the detection lead sections111and121, and prevents malfunction caused by accumulation of the particulate matter PM in areas other than the detection section11.

The integrated particulate matter detection element10is completed by the compact having a laminated structure obtained by the above-described manufacturing procedures being fired. When manufacturing is performed as described above, the capacitance component13can be integrally formed with significant ease when forming the particulate matter detection element10. For example, unlike in the above-described manufacturing method, the capacitance component may be mounted once the particulate material detection element is formed. However, when the above-described manufacturing method is used, the number of manufacturing procedures and manufacturing cost can be reduced.

In addition, the relative permittivity of the insulating ceramic to be used, the film thickness of the dielectric layer150to be formed, and the area of the parallel plate conductors130and140can be arbitrarily set. Therefore, the alternating current impedance of the particulate matter detection element10can be easily controlled to a desired value.

A particulate matter detection sensor1provided in the particulate matter detection element10of the present embodiment will be described with reference toFIG. 2.

The particulate matter detection sensor1includes a roughly cylindrical insulator185and a housing17. The particulate matter detection element10is inserted and held within the insulator185. The housing17is fixed to a flow path wall50of the flow path through which the gas to be measured flows. The housing17holds the insulator185and holds the detection section11of the particulate matter detection element10in a predetermined position within a measuring flow path500. Furthermore, the particulate matter detection sensor1includes a cover body190and a roughly cylindrical casing19. The cover body190is provided on the tip-end side of the housing17and protects the detection section11of the particulate matter detection element10. The casing18is provided on the base-end side of the housing17. A pair of signal lines115and125are inserted into the casing18via a sealing member182. The signal lines115and125are connected to the detection terminal sections112and122of the particulate matter detection element10by connection fittings113,114,123, and124. The signal lines115and125transmit the detected electrical resistance RSENbetween the detection electrodes110and120to the external detection circuit20. The detected electrical resistance RSENchanges depending on the amount of PM collected and accumulated in the detection section11. In addition, a pair of conduction lines166aand166bare inserted into the casing18. The conduction lines166aand166bare connected at one end to the heating element160within the particulate matter detection element10, via the heating element terminal sections163aand163band connection fittings164a,164b,165a, and165b. The conduction lines166aand166bare connected at the other end to a heating element control device30.

Measured gas inlet/outlet holes192and193are formed accordingly in the cover body190. The gas to be measured that includes the PM is introduced into the detection section11through the measured gas inlet/outlet holes192and193. A flange section191provided on a base-end side of the cover body190is clumped and fixed by a clumping section174provided at the tip-end side of the housing17.

According to the first embodiment, the capacitance component13is included in the particulate matter detection element10and disposed in a position within the insulator185where the temperature is stable at 500° C. or below. As a result of the capacitance component13being disposed in the position where the temperature is 500° C. or below, insulation resistance of the insulating ceramic configuring the dielectric layer150does not decrease to 1 MΩ or less by receiving heat from ambient temperature. The configuration is therefore preferable.

The gas to be measured that includes the particulate matter PM flows through the measured gas flow path500, and is introduced from the measured gas inlet/outlet holes192provided in the cover body190. The gas to be measured comes into contact with the front surface of the detection section11of the particulate matter detection element10that exposes the detection section11to the gas to be measured. As a result, the PM accumulates between the detection electrodes110and120.

An overview of the overall particulate matter detecting device100using the particulate matter detection element10of the present embodiment will be described with reference toFIG. 3.

With the electrical resistance formed by the particulate matter accumulated between the detection electrodes120and130configuring the detection section11of the particulate matter detection element10serving as the detected resistance RSEN, the capacitance component13is connected in parallel to the detected resistance RSEN.

The detection circuit20is provided with the direct-current power source21, the alternating-current power source22, the direct-current detector23, and the alternating-current detector24. The direct-current power source21applies the direct current IDCto the particulate matter detection element10. The alternating-current power source22applies the alternating current IAC. The direct-current detector23detects the direct current IDCflowing to the detected resistance RSEN. The alternating-current detector24detects the alternating current IACflowing via the capacitance component13.

When the particulate matter PM accumulates between the detection electrodes110and120, and the detected resistance RSENis formed, the direct current IDCbased on the detected resistance RSENflows in relation to a direct current voltage VDCapplied by the direct-current power source21. The direct-current detector23detects the direct current IDC. The amount of particulate matter PM accumulated in the detection section11can be calculated from the change in direct current IDC.

Furthermore, the alternating current IACflows via the capacitance component13in relation to an alternating current voltage VACapplied by the alternating-current power source22. The alternating-current detector24detects the alternating current IAC. As a result, a disconnection abnormality can be detected that occurs between the particulate matter detection element10and the detection circuit20connected by the detection lead sections111and121, the detection terminal sections112and122, the connection fittings113,114,123, and124, and the signal lines115and125.

The heating element160is connected at one end to a drive power source31from the heating element lead section161avia the conduction line166a. The heating element160is grounded at the other end via the heating element lead section161b, the conduction line166b, an open/close element320, and a current detecting means330. The open/close element320is provided in the heating element control device30and is controlled such as to open and close by a driving section32. The current detecting means330detects the current flowing to the heating element160. A temperature detection section33detects the temperature of the heating element160based on resistance of the heating element160detected by the current detecting means330. Temperature control of the heating element160is performed using the detected temperature.

A semiconductor, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), is used as the open/close element320. The open/close element320is opened and closed in adherence to drive signals sent from the driving section32. The open/close element320supplies a pulsed current to the heating element160, and adjusts the amount of generated heat by controlling the duty ratio of energization pulses. The amount of generated heat to be adjusted is set based on the temperature of the heating element160detected by the temperature detection section33and the like.

A first test conducted to confirm the effects of the present embodiment will be described with reference toFIG. 4AtoFIG. 4D.

As shown inFIG. 4A, an open/close switch SW is provided between the signal lines116and126connecting the particulate matter detection sensor1including the particulate matter detection element10of the present embodiment and the detection circuit20. The open/close switch SW simulates a disconnection. The changes in the direct current IDCand the alternating current IACdetected when the switch SW is opened and closed were examined. The direct current IDCand the alternating current IACflowing through the particulate matter detection element10are changed into voltage by the direct-current detector23and the alternating-current detector24, and the direct-current detector23and output voltage from the alternating-current detector24are obtained as result of the first test.

As shown inFIG. 4B, in a normal state, complex impedance of the detected resistance RSENand the capacitance C13of the capacitance component13changes based on the changes in the amount of PM accumulated in the detection section11of the particulate matter detection element10and the temperature of the particulate matter detection element10. The output voltage VACdetected by the alternating-current detector24changes between about 1.4V to 5V. On the other hand, during disconnection, the output voltage VACis about 0.8V to 1.0V.

Then, the heating element160was energized. In a state in which the PM is not accumulated in the detection section11, the open/close switch SW simulating disconnection was changed from a closed state to an open state. As a result, as shown inFIG. 4C, the output from the alternating-current detector24indicates 1.55V in the state in which the switch SW is closed and a normal connection is simulated. In a state in which the switch is opened and disconnection is simulated, the output changes to 0.9V. Normal connection and disconnection are clearly differentiated. On the other hand, as shown inFIG. 4D, the output from the direct-current detector23in a state in which the PM is not accumulated in the detection section11was not detected, regardless of the opening and closing of the switch SW, because conduction does not occur between the detection electrodes110and120.

Confirmation has been made that, when the particulate matter detection element10of the present embodiment is used in this way, disconnection abnormality occurring in the signal lines116and126can be clearly detected by detection of the alternating current IAC, regardless of the accumulation state, including the state in which the PM is not accumulated in the detection section11.

FIG. 13is an example of an operational flow for disconnection detection and particulate matter detection using the particulate matter detection element10. First, an alternating current is applied to the particulate matte detection element10by the alternating-current power source22(Step S101). The alternating-current detector24detects the alternating current (such as the alternating current voltage value) (Step S102). Then, based on the detected value, a calculating section6performs comparison with a threshold value for judging whether disconnection has occurred that has been determined in advance, and judgment regarding whether disconnection has occurred is made (Step S103). For example, disconnection is judged to have occurred when an effective value of the voltage is lower than the predetermined threshold value.

Next, a direct current is applied to the particulate matter detection element10by the direct-current power source at a predetermined timing (such as when judged that disconnection has not occurred at Step S103) (Step S104). The direct-current detector23detects the direct current (Step S105). Based on the detected direct current, the calculating section26calculates the amount of particulate matter (Step S106).

The application of the present embodiment is not limited to the above-described operational flow. For example, the direct current and the alternating current may be superimposed. Judgment regarding disconnection and particulate matter detection may be simultaneously performed.

A control section27inFIG. 3outputs operation commands at predetermined timings to the direct-current power source21, the alternating-current power source22, the direct-current detector23, and the alternating-current detector24. The control section27is configured by an oscillator or the like. An analog-to-digital (AD) converting section25converts the detected values from the direct-current detector23and the alternating-current detector24to digital signals, and outputs the digital signals to the calculating section26.

A test conducted on the insulating ceramic used in the dielectric layer150of the particulate matter detection element10according to the first embodiment will be described with reference toFIG. 5AtoFIG. 9B.

FIG. 5Ashows the results of a preliminary test conducted in an instance in which the dielectric layer150is composed of alumina. In the preliminary test, a plate conductor of which the area S is 1.1 cm2was formed on both surfaces of an alumina substrate of which the relative permittivity ∈ris 11.2 and the thickness d is 0.5 mm. Sweeping was performed using alternating currents of 10 kHz and 20 kHz. The changes in alternating current impedance |Z| when measurement temperature is changed from room temperature to 700° C. were examined using an impedance analyzer.FIG. 5Bshows the results of a preliminary test conducted in an instance in which the dielectric layer150is composed of zirconia. In the preliminary test, a plate conductor of which the area S is 1.1 cm2was formed on both surfaces of an zirconia substrate of which the relative permittivity ∈ris 12.5 and the thickness d is 0.5 mm. Sweeping was performed using alternating currents of 10 kHz and 20 kHz. The changes in alternating current impedance |Z| when measurement temperature is changed were examined.

As shown inFIG. 5AandFIG. 5B, in both instances, a decrease in alternating current impedance was observed in accompaniment with temperature increase.

On the other hand, when zirconia is used, if the alternating current used for sweeping is 20 kHz, the alternating current impedance is 200 kΩ or less in all temperature ranges even when the thickness d is 0.5 mm. However, when the temperature becomes higher than 300° C. at which the particulate matter detection element10is used, the alternating current impedance may become too low and detection of the alternating current or voltage may no longer be possible, regardless of frequency.

FIG. 6AandFIG. 6Bshow temperature changes in alternating current impedance in an instance in which the dielectric layer150is composed of alumina, and the thickness d of the dielectric layer150is 25 μm or 12.5 μm.

As shown inFIG. 6A, when the thickness d is set to 25 μm and the frequency used for sweeping is 20 kHz, the alternating current impedance becomes 200 kΩ or less in the temperature range of room temperature to 700° C. Detection is expected to be easily performed.

As shown inFIG. 6B, when the thickness d is set to 12.5 μm, regardless of the frequency used for sweeping, the alternating current impedance becomes 200 kΩ or less in the temperature range of room temperature to 700° C. Detection is expected to be easily performed.

On the other hand, in an instance in which zirconia is used, when the thickness d of the dielectric layer150is 25 μm or 12.5 μm, as shown inFIG. 7AandFIG. 7B, detection is possible from room temperature to 300° C. in both instances. However, when the temperature becomes higher than 300° C. at which the particulate matter detection element10is used, the impedance becomes too low and detection of the alternating current or voltage is no longer possible.

Results of further examination of the effects in an instance in which alumina is used in the dielectric layer150will be described with reference toFIG. 8AandFIG. 8B.

FIG. 8Ais a characteristics chart showing temperature characteristics of volume resistivity of a representative alumina.FIG. 8Bis a characteristics diagram of changes in direct current resistance RALbetween the first plate conductor130and the second plate conductor140when the thickness d of the dielectric layer150is changed.

As shown inFIG. 8A, the volume resistivity p of alumina at 600° C. has a difference of about 4×1010Ωm to 1.4×1011Ωm depending on alumina content.

As shown inFIG. 8B, for example, in an instance in which 96 mass percent alumina is used, when the thickness d of the dielectric layer150is 7 μm or more, the direct current resistance RALof 1 MΩ or more is ensured even at 600° C.

Therefore, for example, in an instance in which 96 mass percent alumina is used, when the dielectric layer150is set to 7 μm or more and 25 μm or less, the alternating current impedance becomes 200 kΩ or less. A direct current resistance of 1 MΩ or more at 600° C. can be ensured. As a result of the direct current resistance being 1 MΩ or more, insulating properties of the capacitance component13can be ensured. Direct current does not flow from the direct-current power source21to the capacitance component13. Direct current detection accuracy of the direct-current detector23is improved. Therefore, disconnection detection is expected to be facilitated without PM detection being affected.

The results of a test conducted to confirm the effects regarding detection of disconnection abnormality occurring within the particulate matter detection element10of the present embodiment will be described with reference toFIG. 9AandFIG. 9B.

As shown inFIG. 9A, as dielectric layers150(A)and150(Z)respectively configuring capacitance components13(A)and13(Z), particulate matter detection elements10(A)and10(Z) respectively using alumina and zirconia were made. Furthermore, a sample intentionally simulating disconnection within the particulate matter detection element was made by the detection lead sections111and121being disconnected. The changes in output detected by the alternating-current detector24when measurement temperature TEXis heated from room temperature to 400° C. at which the particulate matter detection element is ordinarily used were examined. The results of the examination are shown inFIG. 9B.

As indicated by150(A)inFIG. 9B, when alumina is used, no alternating current is detected even when the measurement temperature rises to 400° C. The disconnected state can be detected.

On the other hand, as indicated by 150(Z)inFIG. 9B, when zirconia is used, direct current resistance R(Z)between the plate conductors130and140gradually decreases with the increase in measurement temperature. The output voltage gradually increases when 500 seconds have elapsed from the start of measurement.

This is assumed have occurred because conductivity is generated in accompaniment with temperature increase as a result of the semiconductor properties of zirconia.

Therefore, at temperatures near 400° C. that is the usage environment, zirconia, in which the alternating current impedance is low, detection is difficult, and sufficient insulation cannot be ensured, and therefore it has been found to be unsuitable for use in the dielectric layer150of the particulate matter detection element10of the present embodiment.

On the other hand, alumina actualizes sufficient insulation and alternating current impedance facilitating detection when the thickness d of the dielectric layer150is formed to be 7 μm or more and 25 μm or less. Therefore, alumina has been found to be effective as the dielectric layer150of the particulate matter detection element10of the present embodiment.

As insulating ceramic materials other than alumina as the material for the dielectric layer150, any material selected from beryllia, calcia, magnesia, thoria, and spinel, or a composite ceramic composed of these materials are expected to be favorably used.

A material preferably meets the following conditions when used as the material for the dielectric layer150of the particulate matter detection element10of the present embodiment.

In other words, the material has a predetermined volume resistivity by which the direct current resistance of the dielectric layer150becomes 1 MΩ or more at 600° C. The area S of the plate conductors130and140and the thickness d of the dielectric layer150are set such that the alternating current impedance of the dielectric layer150becomes 200 kΩ or less, and the direct current resistance of the dielectric layer150at 600° C. becomes 1 MΩ or more.

The relationship of C13=∈r·∈0·S/d is established among the relative permittivity ∈rof the insulating ceramic configuring the dielectric layer150, the vacuum permittivity ∈0, the thickness d of the dielectric layer150, the area S of the plate conductors130and140, and the capacitance C13of the capacitance component13. The relationship Z=1/(j·ω·C13)=1/(j·2π·f·C13) (j being an imaginary unit), and therefore |Z|=1/(2π·f·C13) are established among the alternating current impedance Z, the capacitance C13, and the sweeping frequency f.

In addition, the direct current resistance R13of the capacitance component13is calculated from the volume resistivity ρ (Ωm) of the insulating ceramic material at 600° C. and the thickness d of the dielectric layer150. The direct current resistance R13is preferably 1 MΩ or more.

In other words, the volume resistivity p or the thickness d of the dielectric layer150is set such that ρ·d≧1 (MΩ)

Second Embodiment

A particulate matter detection element10aaccording to a second embodiment of the present embodiment will be described with reference toFIG. 10. In the second embodiment and subsequent embodiments, configurations similar to those according to the first embodiment are given the same reference numbers. Explanations thereof are omitted.

According to the first embodiment, an example is described in which the dielectric layer150is formed in a rough plate shape by the doctor blade method or the like. The dielectric layer150is layered on the rear-surface side of the insulating substrate101configuring the detection section11. As an alternative, as described according to the second embodiment, a capacitance element may be formed on the front-surface side of the insulating substrate101on which the detection electrodes110and120are provided. Specifically, a first plate conductor130aand a first conductor lead section121amay be formed on the front surface of the insulating substrate101on which the detection electrodes110and120are provided. A printing paste using insulating ceramic is made. A dielectric layer105ais formed by printing such as to cover the first plate conductor130a. Furthermore, a second plate conductor140aand a second conductor lead section141ais formed such as to be layered on the dielectric layer150a.

As a result of a configuration such as that described above, the first conductor lead section131aand the detection lead section11can be connected, and the second conductor lead section141aand the detection lead section121can be connected directly without the through-hole electrodes132,142, and143therebetween. Therefore, manufacturing of the particulate matter detection element10aof the present embodiment is facilitated.

The thickness d of the dielectric layer150athat can be formed by thick film printing is about several μm to 20 μm. Therefore, a suitable thickness d can be easily formed through adjustment of printing pressure and printing frequency, such that a capacitance component13ahas desired alternating current impedance Z13and direct current resistance R13.

Third Embodiment

A particulate matter detection element10baccording to a third embodiment of the present embodiment will be described with reference toFIG. 11andFIG. 12.

According to the first and second embodiments, a configuration is described in which the detection section11is a plurality of detection electrodes110and120opposing each other such as to be arrayed in a comb-shape. As an alternative, as described according to the third embodiment, detection electrodes110band120bthat extend linearly may be disposed opposing each other as opposing electrodes. The detection electrodes110band120bmay be covered by an insulating protective layer103bprovided with an opening section104such that the detection section11bis exposed.

According to the third embodiment as well, in a state in which the particulate matter is accumulated between the detection electrodes110band120b, a capacitance component13bis connected in parallel to the detected resistance RSEN. In a state in which the particulate matter is not accumulated between the detection electrodes110band120b, the capacitance component13bis connected in series between the detection electrodes110and120.

The results of a third test conducted to confirm the effects of the particulate matter detection element10baccording to the third embodiment of the present embodiment will be described with reference toFIG. 12AandFIG. 12B. The method of testing is similar to the above-described method of the second test. A particulate matter detection element according to the third embodiment is used.

As shown inFIG. 12A, according to the third embodiment of the present embodiment as well, the test was conducted with an intentional disconnection formed within the particulate matter detection element10b. As a result, in a manner similar to that according to the first embodiment, as indicated by150b(A)inFIG. 12B, in an instance in which alumina is used as a dielectric layer150b(A)configuring a capacitance component13b(A), disconnection within the detection section11bcan be detected with certainty. On the other hand, as indicated by150B(Z)inFIG. 12B, in an instance in which zirconia is used as a dielectric layer150b(Z)configuring a capacitance component13b(A), insulation resistance RZof the dielectric layer150b(Z)decreases in accompaniment with the increase in temperature TEX. Disconnection can no longer be detected.

Therefore, in the particulate matter detection element10baccording to the third embodiment as well, through use of alumina or the like having a predetermined volume resistivity by which the direct current resistance of the dielectric layer150b(A)at 600° C. becomes 1 MΩ or more, the area S of the parallel plate conductors130and140, and the thickness d of the dielectric layer150b(A)are set such that the alternating current impedance of the dielectric layer150b(A)becomes 200 kΩ or less, and the direct current resistance of the dielectric layer150b(A)at 600° C. becomes 1 MΩ or more. As a result, disconnection abnormality is confirmed to be detected with certainty.