Particulate matter detection sensor

A sensor element which has a pair of positive and negative detection electrodes disposed on a surface of an insulation body as a detecting portion and a cover body configured to cover an opening of a cylindrical housing. The cover body is provided with gas inlet and outlet holes via which the measuring gas is introduced and discharged. The pair of detection electrodes have a plurality of wire electrodes. The wire electrodes electrically connected to the positive electrode and the wire electrodes electrically connected to the negative electrode are alternately arranged in parallel. Any one of a first insulation layer which is a narrow electrode interval Dn and a second insulation layer which is a wide electrode interval Dw, arranged between adjacent wire electrodes, and the first insulation layer arranged in a center part of the detecting portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International Application No. PCT/JP2016/077034 filed on Sep. 14, 2016 which designated the U.S. and claims the benefit of priority of earlier Japanese Patent Application No. 2015-182218 filed on Sep. 15, 2015, and Japanese Patent Application No. 2016-142329 filed on Jul. 20, 2016, the entire contents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a particulate matter detection sensor which detects particulate matter contained in a measuring gas, and more particularly relates to a sensor which detects particulate matter in an exhaust gas emitted from an internal combustion engine.

RELATED ART

Conventionally, an electrical resistance type particulate matter detection sensor is used as a particulate matter detection sensor, in order to detect an amount of particulate matter (specifically PM) in an exhaust gas emitted from an internal combustion engine.

For example, a particulate matter detection sensor disclosed in JP literature 1 is provided with a laminate structured insulating body, at least one part of the insulating body having a detection electrode embedded in the insulating body, and a sensor element which has a surface in which detection electrodes are exposed thereon as a detecting portion.

The sensor element is maintained inside a cover body which is provided with exhaust gas an inlet-holes. Detection electrodes of different polarities are alternately disposed with an insulation layer intervened therebetween on the detecting portion of the sensor element, into which the exhaust gas flows. Once an electrostatic field is formed by application of a voltage, charged particulate matter is attracted and this particulate matter accumulates between electrodes. An amount of particulate matter contained in the exhaust gas may be thus detected from a change in a resistance value between the electrodes. A comb-shaped electrode formed by printing on a surface of the insulating body may also be used as the detecting portion.

The particulate matter detection sensor described above is mounted on an exhaust pipe of a diesel engine, for example, and is used for malfunction diagnosis of an exhaust gas purifying apparatus equipped with a diesel particulate matter filter (referred to as DPF hereon).

CITATION LIST

Patent Literature

A particulate matter detection sensor has an insensitive period which exists at a start-up point of a sensor until the sensor output has reached a predetermined value by accumulation of particulate matter between electrodes of a detecting portion. As a result, the shorter the start-up time is the earlier the detection of particulate matter may be performed. That is, intervals between the detection electrodes may be narrow in order to increase a sensitivity of the sensor, for the particulate matter detection sensors used for malfunction diagnosis. However, when only electrode intervals are configured to be narrow without changing the size or the number of the detection electrodes, an essential detection area becomes smaller, and if a position in which exhaust gas flows is misaligned when the exhaust gas is introduced inside the cover body, the sensitivity of the sensor will in contrast decrease. In this regard, effects of dimensional precision of the cover body and assembly precision of the sensor are increased, thus, a difference in the sensitivity occurring between sensors also increases.

In order to avoid such issues mentioned above, if a number of detection electrodes provided is increased and a detection area is configured to be larger, a production cost will also increase due to a higher number of laminate layers and an increased usage of electrode materials.

On the other hand, particulate matter adhered to a wall inside an exhaust pipe may detach therefrom, for example, and coarse particles having a larger particle diameter than the usual size may be formed and emitted. In this case, if the electrode interval between detection electrodes is narrow, an acute increase of the sensor output will occur frequently. As a consequence, a precision of diagnosing malfunctions of a DPF (Diesel particulate filter) decreases, and there is a concern of DPF malfunction and erroneous diagnosis occurring.

In view of the above issues, the present disclosure aims to provide a particulate matter detection sensor which has a good sensor sensitivity, a difference in sensitivity between sensors is small, and a probability of the occurrence of a sensor output changing due to adherence of particulate matter is low, with superior productivity and reliability.

SUMMARY

Solution to Problem

A mode of the present disclosure is a particulate matter detection sensor provided with a sensor element for detecting particulate matter contained a measuring gas. The sensor element is provided with one pair of detection electrodes which consist of a positive electrode and negative electrode, the detection electrodes being disposed on a surface of an insulating body the insulation body being a detecting portion, and a cover body configured to cover an opening of a cylindrical housing which accommodates the sensor element. The cover body is provided with a gas inlet/outlet holes. The measuring gas being introduced and discharged through the gas inlet/outlet holes.

Each of the detection electrodes composing the one pair of the detection electrodes is provided with a plurality of wire electrodes exposed on a front surface of the detecting portion. The wire electrode being electrically connected to the positive electrode and the wire electrode being electrically connected to the negative electrode are alternately disposed in parallel to each other. Either one of a first insulation layer and a second insulation layer is disposed between two mutually adjacent electrodes, among the wire electrodes, the first insulation layer configuring an electrode interval Dn as an interval between two mutually adjacent electrodes of the detection electrodes, and the second insulation layer configuring an electrode Dw. The electrode interval Dw is a wider interval than the electrode interval Dn which is a narrow interval. The second insulation layer provided with a plurality of insulation layers having different layer thicknesses. The first insulation layer is provided in a center part of the detecting portion11.

It is to be understood that symbols in the summary and claims are used to show a corresponding relation between specific means as a mode described in preferred embodiments described herein after and do not limit a technical scope of the disclosure.

The particulate matter detection sensor is provided with the pair of the electrodes of the detecting portion of the sensor element into which the measuring gas is introduced, and any one of the first insulation layer and the second insulation layer intervened between a plurality of the wire electrodes which are mutually adjacent to each other. The interval between the two wire electrodes adjacent to each other is either one of the electrode interval Dn being the narrow interval and the electrode interval Dw which is wider electrode interval than the narrow electrode interval Dn.

Since the center part of the detecting portion has the narrow electrode interval Dn, once the measuring gas which flows from the gas inlet/outlet holes of the cover body is introduced thereto, particulate matter is instantly detected. Additionally, in providing the wide electrode interval Dw section, a detection area is enlarged even when a position in which the measuring gas is introduced (specifically, a gas flow position) is misaligned, and a decrease in the sensitivity of the sensor is thus suppressed. Also, if the sensor is configured with only the narrow intervals Dn, electricity is easily conducted between the pair of electrodes and an acute increase of the sensor output also occurs easily when coarse particles flow. However, since the electrode interval Dw which is wider than the electrode interval Dn is provided, the acute increase of the sensor output is suppressed.

According to the mode, a desirable sensor sensitivity is maintained, and while suppressing a difference in the sensitivity between sensors, an output variation due to coarse particles is also decreased, and a detection precision may be enhanced. Furthermore, a probability of the sensor output changing due to adhesion of the coarse particles is decreased without an increase of man-hours and an amount of materials used for electrodes, thus a particulate matter detection sensor having superior productivity and reliability may be actualized.

EMBODIMENTS

First Embodiment

Next, an embodiment of a particulate matter detection sensor will be described with reference to the figures. InFIGS. 1 and 2, a basic configuration of the particulate matter detection sensor S according to a first embodiment is provided with a laminate-type sensor element1which has a detecting portion11configured on a front end thereof. The sensor1detects particulate matter contained in a measuring gas. The measuring gas is combustion exhaust gas emitted from an internal combustion engine, for example, a diesel engine, which contains minute particulate matter (referred to as PM hereon), for example, soot which has conductivity. The particulate matter detection sensor S is mounted on a wall W of an exhaust pipe of the internal combustion engine, and configures a malfunction diagnosis system of an exhaust gas purifying apparatus equipped with a DPF, for example.

As shown inFIG. 1, the sensor element1has a rectangular shaped insulating body2and a pair of detection electrodes3and4which consists of a positive electrode and a negative electrode as the detecting portion11disposed on the front end of the sensor element1. For example, the detection electrode3is the positive electrode and the detection electrode4is the negative electrode. Each of the detection electrodes3and4are provided with a plurality of wire electrodes3aand4awhich are exposed on a front surface of the detecting portion11.

It is noted a length wise direction of the insulating body2is an element length direction X, a line length direction of the wire electrodes3aand4aof the detecting portion11is an element width direction Y, and a lamination direction which is orthogonal to the Y direction is an element thickness direction Z.

The plurality of wire electrodes3aand4aare arranged so that the wire electrode3aelectrically connected to the detection electrode3which is the positive electrode and the wire electrode4aelectrically connected to the detection electrode4which is the negative electrode are alternately arranged in parallel. The pair of wire electrodes3aand4awhich are adjacent to each other are formed in plurality. Either one of a first insulation layer21or a second insulation layer22is disposed between the two adjacent wire electrodes3aand4a. A layer thickness of the first insulation layer21is formed to be thinner than a layer thickness of the second insulation layer22. As a result, the two adjacent wire electrodes3aand4awith the first insulation layer21intervened therebetween have an electrode interval Dn. Additionally, the two adjacent wire electrodes3aand4awith the second insulation layer22intervened therebetween have an electrode interval Dw. The electrode interval Dw is a wider interval than the electrode interval Dn which is a narrow interval. It is noted that, the electrode Dn which is a narrow interval among the electrode intervals Dn and Dw may also be referred to as a first interval, and the interval Dw which is wider than the interval Dn may also be referred to as a second electrode interval. A third insulation layer23is arranged on an outer-side of the pair of electrodes3and4of the detecting portion11, in the element thickness direction Z. A configuration of the detecting portion11is described in detail hereafter.

The particulate matter detection sensor S coaxially accommodates the sensor element1inside a cylindrical housing H, and the detecting portion11arranged inside a front opening H1of the cylindrical housing H is protected by a cover5body which is mounted so that the front opening H1of the cylindrical housing H is covered, as shown inFIG. 2. The particulate matter detection sensor S is fixed to a thread hole W1provided on the exhaust pipe wall W of the internal combustion engine, for example, by a thread member H2which is provided on an outer circumference of the cylindrical housing H.

The cover body5is a double container shape consisting of co-axially disposed outer cover51and inner cover52. A plurality of gas inlet and outlet holes5aand5bare provided to surround an axis with an equal distance therebetween on a bottom portion and a side surface of each of the covers51and52. The gas inlet and outlet holes5aare provided on the outer cover51in a plurality of positions on a lower side surface and a bottom section of an outer circumference thereof. The gas inlet and outlet holes5bare provided on the inner cover52in a plurality of positions on an upper side surface and in 1 position in a center part of a bottom section thereof.

A flow direction g in which the combustion exhaust gas flows into the exhaust pipe (a left to right direction in the figure) is a direction which is orthogonal to the element length direction X (a vertical direction in the figure). The length direction X is an axial direction of the particulate matter detection sensor S. The combustion exhaust gas flows from the gas inlet/outlet holes5aof the lower side surface of the outer cover51to an inside of the cover body5. Thereafter, the gas flows via a route formed between the outer cover51and the inner cover52, and passes through the gas inlet/outlet hole5bon the upper side surface of the inner cover52to be guided into the detecting portion11from a tip end entrance of the cylindrical housing H which opposes the detecting portion11.

The particulate matter detection sensor S is mounted downstream of the DPF mounted in the exhaust pipe, for example, not shown in figures, and detects particulate matter which slips through the DPF. The particulate matter detection sensor S may configure a part of the DPF malfunction diagnosis system.

A shape of the cover body5and disposed position of the gas inlet/outlet holes5aand5b, shown inFIG. 2, are one example and may be suitably changed. The cover body5may be configured so that combustion exhaust gas flowing inside from gas inlet/outlet holes5bof the inner cover52is guided towards a center part of the detecting portion11. In general, the sensor1is preferably configured so that the gas inlet/outlet hole5bof the inner cover52is positioned near to a lower section of the detecting portion11and the combustion exhaust gas flowing from the gas inlet/outlet hole5bpasses through the lower section of the center part of the detecting portion11, as shown. Additionally, the respective gas inlet/outlet holes5aand5bare configured with a distance from each other in an axial direction or radial direction, to avoid gas directly flowing from the gas inlet/outlet hole5aof the outer cover51to the gas inlet/outlet-hole5bof the inner cover52.

The sensor element1is configured of the insulating body2of laminated ceramic green sheets2ato2cwhich have electric insulating properties and the wire electrodes3aand4aalternately disposed between the green sheets2ato2cas shown inFIG. 3. The wire electrodes3aand4aare the detection electrodes3and4. The green sheet2awhich is the first insulation layer21or the ceramic green sheet2bwhich is the second insulation layer is arranged between the adjacent wire electrodes3aand4a. At this point, the ceramic green ceramic sheet2aand the ceramic green sheet2b, for example, are disposed to be alternately laminated in the element thickness direction Z, in order to provide a configuration shown inFIG. 1. The ceramic green sheet2acorresponds to the narrow electrode interval Dn. A sheet thickness of the ceramic green sheet2ais formed thinner than a sheet thickness of ceramic green sheet2bwhich corresponds to the wide electrode interval Dw.

The plurality of ceramic green sheets2cwhich form the third insulation layer23are disposed on a respective top layer and bottom layer of the laminate body. A heater electrode6aand leading electrode6bare arranged between the plurality of green sheets2con a lower-layer side to configure a heater6. Terminal electrodes31and41are formed on a top surface of the top layer ceramic green sheet2cat an end portion which opposes a side in which the wire electrodes3aand4aare disposed. Terminal electrodes61and62used for the heater6are formed on a lower surface of the bottom ceramic green sheet2c. The heater electrode6ais provided to correspond with the wire electrodes3aand4a, and configured to heat an entire detecting portion11. The particulate matter detection sensor S supplies power to the heater6when the sensor element1is operating, eliminates water and particulate matter on the surface of the detecting portion11and prevents erroneous detection.

The wire electrodes3aand4aare formed on top of the ceramic green sheets2ato2cby screen printing and extended to another end side by leading electrodes3band4b. The leading electrodes3band4bare formed along a side end section of the ceramic green sheets2aand2b. It is noted that the wire electrodes3aand4aare preferably configured so that a portion which is exposed to a surface of the insulating body2is formed as a linear shaped electrode. For example, one side of the wire electrodes3aand4amay be configured as a rectangle or trapezoid shaped electrode film which is embedded between the ceramic green sheets2ato2c. The leading electrodes3band4bare provided on different surfaces of side surface edge sections of the ceramic green sheets2ato2cand connected to the top terminal electrodes31and41through a conductor portion which is not shown in the figures. The conductor portion is formed on another end side thereof in the element thickness direction Z. A position in which the leading electrodes3band4bare connected is shown with a broken line in the figures.

Insulation materials, for example, alumina, magnesia, titania and mullite, or known ceramic materials, for example, dielectric body materials for example, titanic acid or valium which have a high permittivity, mixed with alumina or zirconia for example, can be used as materials used to form the ceramic green sheets2ato2c. Metal materials, for example, aluminum, gold, platinum and tungsten, or metal oxide materials, for example, ruthenium oxide, or known conductive material, for example, perovskite structured conductive oxide materials are used for the wire electrodes3aand4a.

The ceramic green sheets2aand2bare a same rectangular shape, which may be formed by changing the sheet thickness of the same material. As shown inFIG. 4, the first insulation layer21and the second insulation layer22may be configured using one type of ceramic green sheet2a. In this case, the second insulation layer22corresponding to the wide electrode interval Dw is formed by combining a plurality of thin ceramic green sheets2a(for example 3) which have the same sheet thickness as ceramic green sheet2b, shown inFIG. 3. Other configurations are as shown inFIG. 3, details of which are omitted here. The narrow electrode interval Dn is formed from ceramic green sheets which may be formed using screen printing, for example.

The sensor element1is configured of the wire electrodes3aand4aand the leading electrodes3band4b, the terminal electrodes31and41, the heater electrode6a, the leading electrode6b, and the terminal electrode61and62, for example, on the ceramic green sheets2ato2c. The above-mentioned elements may also be laminated as shown in eitherFIG. 3orFIG. 4, and sintered for unification thereof. More specifically, the insulating body2consists of an insulation layer which includes the first insulation layer21to the third insulation layer23. The third insulation layer23is arranged to cover the outer-side of the pair of detection electrodes3and4in the element thickness direction Z of the detecting portion11. The first to third insulation layer21to23have a constant width in the element width direction Y of the detecting portion11, which is sufficiently larger than a wire length of the wire electrodes3aand4a. The first to third insulation layers21to23are crimped to each other on both sides of the pair of wire electrodes3aand4a, without going through the wire electrodes3aand4a. In this way, the insulation layers (specifically, the first to third insulation layers21to23) incorporate the outer-side of the pair of detection electrodes3and4which are exposed on the surface at the front end surface of the insulating body2which forms the detecting portion11. As a result, detachment of the detection electrodes3and4is prevented.

A layer thickness of the third insulation layer23is usually formed to be thicker than the thickness of the first insulation layer21, for example, and the insulation layer23is formed to have a layer thickness which is the same as or greater than the second insulation layer22. The third insulation layer23is preferably a thick layer, as a prevention measure against detachment of the detection electrodes3and4, to secure insulation properties of the insulation layers, and maintain constant electrode intervals. However, in this regard, since a region in which the detection electrodes3and4may be formed becomes narrow, the layer thickness of the third insulation layer23is preferably in a range of three times greater or less than the thickness of the second insulation layer22.

When the third insulation layer23on the top layer is too thin, a stiffness thereof is reduced, and detachment of the third layer23may occur, therefore, the third insulation layer23is formed to be thicker than the first insulation layer21. However, if the third insulation layer23is formed to have a thickness which is excessively thick, a size of the insulating body and cost thereof increases, therefore, the thickness is desirably three times greater or less than the thickness of the second insulation layer22.

As a result, the detecting portion11is configured of the pair of wire electrodes3aand4aopposed to each other with the first insulation layer21intervened therebetween, and the pair of wire electrodes3aand4aopposed to each other with the second insulation layer intervened therebetween, each of the above mention pair of wire electrodes alternately arranged in the element thickness direction Z, at the front end surface of the insulating body2, as shown in the reference example inFIG. 5. At this point, the first insulation layer21is positioned in a gas flow position G1in the center part of the detecting portion11, and the first insulation layer21and the second insulation layer22are preferably arranged symmetrically, on both sides thereof (specifically, on a respective upper-side and lower-side thereof). The disposed position of the first insulation layer21and the second insulation layer22is exemplified inFIG. 5and also exemplified inFIG. 6toFIG. 8, which will be described hereinafter.

The detecting portion11is provided with the first insulation layer21in the center part. The first insulation layer21in the center part is intervened between the second insulation layer22and the first insulation layer21alternately disposed in this order on both sides thereof, as shown inFIG. 5. For example, four second insulation layers22are disposed between five first insulation layers21, and the first insulation layer21disposed on the most outer-side thereof is positioned near to an edge section of the detecting portion11in the Z direction. In this way, by arranging the pair of wire electrodes3aand4ahaving the narrow electrode interval Dn in the center of the detecting portion11which corresponds to the gas area position G1of combustion exhaust gas, minute particulate matter which flows via the gas inlet/outlet holes5aand5bto the detection electrodes is promptly detected, thus the sensitivity of the sensor is enhanced. Additionally, by providing the pair electrode wires3aand4awhich have the wide electrode interval Dw arranged in plurality and increasing a detection area, for example, particulate matter may be detected, even if the combustion gas flow is misaligned with the center of the detecting portion11and is guided to flow to the gas area G2position. As a result, particulate matter is further captured between the wire electrodes3aand4adisposed on the outer-side thereof, even in a case of the cover body5having variable dimensions or an assembly of the cover body2on sensor element1varies and the position of the gas flow area is misaligned from the center of the detecting portion11. As a further result, a decrease of the sensitivity of the sensor is avoided. A difference in sensitivity between sensors is also prevented, thus a detection precision is enhanced.

Additionally, since the pair of wire electrodes3aand3bof the wide electrode interval Dw and the pair of wire electrodes3aand3bof the narrow electrode interval are alternately disposed, a wide area of the electrode interval is increased. For example, even if particulate matter accumulated in the exhaust pipe detaches therefrom and flows as coarse PM particles (that is, coarse PM shown in the figures), a probability of conduction between the wire electrodes3aand4ais decreased, since a wide section between the electrodes is formed. As a result, an acute increase of the sensor output due to coarse PM particles is suppressed, and precision of detection is enhanced.

In contrast, a conventional sensor element shown inFIG. 9is provided with electrode intervals between the detection electrodes3and4which have a constant electrode intervals, and the wire electrodes3aand4aarranged in parallel between a plurality of first insulation layers21in a center part of a detecting portion11. In this way, by providing only narrow electrode intervals Dn, the sensitivity of a sensor is increased, however, a detection area of the detecting portion11is decreased and a surrounding area without the detection electrodes3and4disposed thereon is also increased. As a result, if the gas flow position G1is misaligned with the center part thereof, a large part of the peripheral gas flow area G2will then be out of range with the detection electrodes3and4, thus the difference in sensitivity between sensors will increase and the detection precision also decrease. When a coarse PM particle which is larger than the electrode interval Dn (specifically, the coarse PM particle shown in the figure) adheres to the detection electrodes3and4, the output of the sensor will sharply increase. Therefore, when the detection sensor is used as a DPF malfunction diagnosis apparatus, there is a concern that a normal DPF may be determined to be abnormal.

The narrow electrode interval Dn, specifically the layer thickness of the first insulation layer of the detecting portion11is usually set in a range of 1 μm to 60 μm, and preferably between 5 μm to 60 μm. It has been confirmed that particulate matter size is generally distributed, for example, in a range of 10 nm to 100 nm, with a center particle diameter of approximately 40 nm. In order to promptly detect the above mentioned particulate matter, the narrower the electrode interval Dn is configured to be the better. However, if the layer thickness of the first insulation layer21is thin, time and labor needed to manufacture the detecting portion11with a desired dimensional precision, and there is a concern of the frequency of an acutely increased output due to adherence of the coarse PM particles also increasing.

In contrast, the wide electrode interval Dw, specifically the layer thickness of the second insulation layer22is usually set in a range of 20 μm to 300 μm, and more preferably in a range of 20 μm to 100 μm. It is estimated that a size of the coarse PM particles is usually in a range of several μm to 100 μm, provided with a center particle diameter of approximately 20 μm. Therefore, by providing the electrode interval Dw at 20 μm or more, a preventive effect of the acute increased output due to the adherence of the coarse PM particles is enhanced. As described above, the layer thickness of the third insulation layer23is greater than a layer thickness of the first insulation layer21and three times or less than the thickness of the second layer22, (specifically a thickness of more than 1 μm and 900 μm or less) which is preferably set in a range of 100 μm to 400 μm.

As the sensor11is provided with the detection electrodes3and4arranged in a larger region, the sensitivity of the sensor is increased and a response thereof may also be enhanced. Specifically, when the element width of the sensor element1(that is, a length of the element width direction Y of the front end surface being the detecting portion11) is in a range of 3 mm to 5 mm, for example, a width of the detection electrodes3and4(that is, a linear length of the wire electrodes3aand4aof the element width direction Y) is set in a range of 2 mm to 4 mm, for example.

At this point, if a length of the first to third insulation layers21to23positioned on the outer-side of the wire electrodes3aand4ais in a range of 0.4 mm to 1 mm in total for both ends, in the element width direction Y (specifically, between 0.2 mm to 0.5 mm for one end) a crimping performance between the insulation layers is secured, and detachment, for example, thereof may also be prevented. The element thickness (that is, a length of the element thickness in the Z direction of the front end surface which is the detecting portion11) is in a range of 1 mm to 3 mm, for example, and a desired number of the pairs of the detection electrode3and4may be disposed with a desired interval therebetween, in the element thickness direction Z, with a position and layer thickness of the first insulation layer21or the second insulation22which is preferably set in the range described hereinabove.

The detecting portion11may be configured with the second insulation layer22which has of a plurality of insulation layers221and222combined, which have different layer thicknesses. The second insulation22forms the electrode interval Dw which is wider than the first insulation layer21, as shown inFIG. 6andFIG. 7. In providing the plurality of insulation layers221and222, electrode intervals Dw1and Dw2formed between the wire electrodes3aand4aare both configured wider than the electrode interval Dn, and may be appropriately set within the same range as the electrode interval Dw of the second insulation layer22. At this point, for example, the electrode interval Dw2of the insulation layer222is formed wider than the electrode interval Dw1of the insulation layer221. The electrode interval Dw1of the insulation layer221is formed narrower than the electrode interval Dn, thus the detection area is configured the same as the area of the detecting portion11shown inFIG. 5(Specifically, Dn<Dw1<Dw<Dw2).

InFIG. 6, the insulation layer222providing the electrode interval Dw2which is wider than the electrode interval Dw is arranged on both sides of the first insulation layer21of the center part, between the wire electrodes3aand4a. The insulation layer221is arranged on an outer-side of the insulation layer222via the first insulation layer21. In this way, as the insulation layer222of the wider electrode interval Dw2is arranged near to the center first insulation layer21, the increased output due to the adhesion of the coarse PM particles may be suppressed, for example, when coarse PM particles easily flow to the detecting portion11. If the PM detection sensor is adapted as a malfunction detection system, an effect of preventing erroneous detection is enhanced.

As shown inFIG. 7, the insulation layer221which has a smaller thickness may be arranged on both sides with the first insulation layer21provided in the center, intervened therebetween, on the detecting portion11. The insulation layer222which has a greater thickness is disposed on an outer-side of the insulation layer221via the first insulation layer21. For example, when a configuration is such that the gas flow position G1is easily misaligned from the center part due to an effect of the varying dimension of the cover body5, for example, the insulation layer221which has a comparatively narrow electrode interval Dw1is positioned near to the center first insulation layer21, and the pairs of wire electrodes3aand4agathered so that the sensitivity of the sensor may be enhanced.

Additionally, as shown inFIG. 8, in providing the plurality of first insulation layers21disposed in both the center part and peripheral area of the sensor11, the number of the wire electrodes3aand4amounted may be increased. The second insulation layer22is disposed in the center part and the peripheral area thereof. The wire electrodes3aand4aare alternately arranged with the first insulation layer21intervened therebetween at the center part and the peripheral areas thereof, and each the second insulation layers22is disposed in between the three layers of the first insulation layers21. Since the pair of wire electrodes3aand4aof the electrode interval Dn are formed in pluralities at the center part and the peripheral area thereof, a difference in sensitivity levels between sensors may be prevented, even when the gas area position is easily misaligned with the center part thereof. Additionally, since the pair of wire electrodes3aand4aof the electrode interval Dw are also formed therebetween, the decrease in the sensitivity of the sensor may be suppressed, even when coarse PM particles flow into the detecting portion11.

The sensor element1shown inFIG. 5toFIG. 9were manufactured as follows. The sensor element1was evaluated using a diesel engine bench test machine, and a relation of a disposed position of the electrodes of the sensor11and the sensor output was investigated. The sensor element1had an element width of 4 mm, an element thickness of 1.6 mm and an electrode width of 3.2 mm. Alumina green sheets were produced with an adjusted thickness and used as the ceramic green sheets2ato2c. Alumina green sheets were obtained by adding a solvent, for example, ethanol, and a binder solvent to alumina powder to form a slurry. The slurry was then formed into a sheet shape using a known Doctor Blade method and dried thereafter. The obtained alumina green sheets were then cut into a predetermined size, and the wire electrodes3aand4awhich are the detection electrodes3and4, and the leading electrodes3band4brespectively formed at predetermined positions using screen printing method, for example. In the same manner, the heater electrodes6aand leading electrode6bwas also formed at respective predetermined positions on the alumina green sheets which is the heater6, using screen printing, for example. Additionally, the terminal electrodes31,41,61and62were formed on the alumina green sheet disposed on either one of the top layer or the bottom layer of the sensor element.

The alumina green sheets were laminated in a predetermined order, crimped by pressing using uniaxial pressing or cold isostatic pressing method, for example, after which they were subjected to delipidation and then sintered (for example, at 1450° C. for 2 hours). Thereafter, the detection electrodes3and4of the detecting portion11were exposed by sanding the surface of the insulating body2. Additionally, the leading electrodes3band4bexposed to a side surface of the insulating body2were connected to respective terminal electrodes31and41via a conductive section using conductive paste, for example. In the same manner, the leading electrode6bof the heater6was connected to the terminal electrodes61and62and the sensor element1was obtained.

At this point, the thickness of the alumina green sheets forming the respective first insulation layer21and the second insulation layer22was adjusted, and the sensor element1provided with the detecting portion11shown inFIG. 5toFIG. 8, was produced by changing a disposed position of the wire electrodes3aand4aand a laminated order of the insulation layers. Each of sensor elements1was referred to as a respective sensor elements S1to S4. A sensor element1provided with a conventional detecting portion11shown inFIG. 9referred to as sample 0, was also produced for comparison.

The sensor element1obtained was accommodated inside the cylindrical housing H, the cover body5was assembled thereto and the result configuration given as the particulate matter detection sensor S. A DPF which was opened so that the particulate matter in combustion gas slipped through the DPF was mounted on an exhaust pipe of a diesel engine, and the particulate matter detection sensor S assembled to a wall of the exhaust pipe in a position which was 1000 mm downstream of the DPF. The particulate matter detection sensor S was mounted in order to be exposed to the combustion exhaust gas which was the measuring gas. The front end-side of the sensor element protected by the cover body5was inserted and positioned inside the exhaust pipe. A radius of the exhaust pipe was ϕ 55 mm and the combustion gas was introduced into the exhaust pipe at a flow rate 40 m/s, a PM concentration 5 mg/m3and a temperature of 200° C. A predetermined capturing voltage was applied between the detection electrodes3and4of the sensor element1and the particulate matter that passed down stream of the DPF filter was detected. A probability of increased output which occurred as a result of the coarse PM particles and a start-up time of the sensor output was measured.

Results are shown inFIG. 10andFIG. 11. The probability of the increased output occurrence due to the coarse PM particles was calculated from an occurrence rate of a number of times in which the output of the sensor was acutely increased due to the adherence of coarse particulate matter, using the particulate matter detection sensor S, when the sensor output start-up time was measured a predetermined number of times (for example 30 times) described later in detail. Additionally, the start-up time of the sensor output is a time from when the capturing voltage is applied until a predetermined sensor output (for example, 15 μA) is reached, in which an average value and a difference thereof is calculated (for example, when performed 30 times). The start-up time is specifically after the electricity is supplied to the heater6, to regenerate the sensor1performed by removing particulate matter adhered to the sensor element.

As shown inFIG. 10andFIG. 11, the probability of the increased output occurring due to the coarse PM particles exceeded 25% and the start-up time of the sensor output had a high variance for the sample 0 which was configured with constant electrode intervals Dn between the detection electrodes3and4of the detecting portion11. In contrast, the probability of the increased output was lower than 15% and the start-up time had a low variance for any one of the samples S1to S4. In particular, the probability of the increased output was largely decreased to 10% or less for the samples S1to S3configured with the wide electrode intervals Dw, Dw2and Dw1, on both sides of the electrode interval Dn of the wire electrodes3aand4aprovided in the center part of the detecting portion11. It was found that the wider the electrode intervals Dw, Dw1and Dw2were configured (specifically, Dw1<Dw<Dw2) the lower the effect of the coarse particulate matter was. In contrast, the narrower the electrodes intervals Dw, Dw1and Dw2were configured, the lower the variance was of the sensor output time, as a result, a difference in the sensitivity between sensors was also small. The sample 4 which is provided with the plurality of wire electrode pairs3aand4aof the electrode interval Dn in the center of the detecting portion11had the shortest sensor startup time and a high sensitivity.

As a result, the disposed position of the detection electrodes3and4of the detecting portion11may be suitably adjusted according to a sensor needs and usage. For example, in an environment where the effect of coarse PM particles is comparatively large, for example, electrodes which have a disposed arrangement of sample 1 or sample 2 is selected. Specifically, the disposed arrangement of the electrodes which suppresses a changing output and has a small variance in sensitivity of the sensor as shown inFIG. 5(specifically, sample 1) or which elicits a high suppression effect for an output variation, as shown inFIG. 6(for example, sample S2) is selected. In an environment where the effect of coarse PM particles is comparatively small, electrodes which have a disposed arrangement as in sample S3and sample S4are selected. Specifically, the disposed arrangement of the electrodes (specifically sample S3) shown inFIG. 7which can both decrease varying sensitivity and suppress the changing output of the sensor or the disposed arrangement of the electrodes (specifically sample 4) which further enhances the sensitivity of the sensor as shown inFIG. 8may be selected.

Second Embodiment

In the first embodiment, the front-end surface of the laminate-type sensor1is the detecting portion11configured with the plurality of the wire electrodes3aand4aembedded in the insulating body2. The pair of detection electrodes3and4may also be formed on a surface other than the front-end surface of the insulating body2, as shown inFIG. 12andFIG. 13for a second embodiment. A basic configuration of the particulate matter detection sensor S according to the second embodiment is the same as the first embodiment, therefore the difference between the embodiments is mainly described herein below. The sensor element1is provided with a flat shape insulating body2, and the pair of detection electrodes3and4disposed as the detecting portion11on a front-end surface of a plate surface. The detection electrodes3and4are connected to a terminal electrode, which is not shown in the figures, by the leading electrodes3band4b, formed in the element length direction X on the surface of the insulating body2. Each of the plurality of wire electrodes3aand4aare a comb shaped electrode connected to one end-side thereof. Each of the wire electrodes3aand3bof the detection electrodes3and4, and the leading electrodes3band4bare formed by a known screen printing, for example.

InFIG. 12, the wire electrode3aelectrically connected to the detection electrode3which is the positive electrode, and the wire electrode4aelectrically connected to the detection electrode4which is the negative electrode are alternately disposed in parallel in the element width direction Y, and the pair of the wire electrodes3aand4awhich are adjacent to each other are formed in pluralities. The intervals of the adjacent wire electrodes3aand4aare provided in an order of the pair of wire electrodes3aand4aof the narrow electrode intervals Dn pluralities (for example, three pairs) in a center part of the element width direction Y, and-the pair of wire electrodes3aand4aof the wide electrode interval Dw, and the pair of wire electrodes3aand4aof the narrow electrode interval Dn disposed on both sides of the pluralities of the wire electrodes3aand4a. The first insulation layer21is formed between the wire electrodes3aand4aof the narrow electrode interval Dn, and the second insulation layer22is formed between the wire electrodes3aand4aof the wide electrode interval Dw. An electrode length and an electrode width of the wire electrodes3aand4amay be appropriately set. It is noted that, in the present embodiment, the element width direction Y is a width direction of the plate surface formed by the detecting portion11of the insulating body2.

In this way, by providing the detection electrodes3and4as print-formed electrodes, adjustment of the electrode interval Dn and the electrode interval Dw is easily performed. As shown inFIG. 13, a configuration in which the wire electrode3aelectrically connected to the detection electrode3which is the positive electrode, and the wire electrode4aelectrically connected to the detection electrode4which is the negative electrode are alternately disposed in parallel in the element length direction X on the detecting portion11, may also be provided. In this configuration, the pair of wire electrodes3aand4aof the narrow electrode interval Dn are disposed in pluralities (for example, 3 pairs) in the center part of the element length direction X. Additionally, the pair of wire electrodes3aand4aof the wide electrode interval Dw and the pair of wire electrodes3aand4aof the narrow electrode interval Dn are disposed in this order on both sides of the center part thereof. The heater6is disposed on a side which opposes the detecting portion11of the insulating body2at the front-end surface thereof. Specifically, the heater6, in which the heater electrode6ais disposed, is configured to cover a region formed by the wire electrodes3aand4aof the detection electrodes3and4. The heater6and the leading electrode6bare formed using a known screen printing method, for example. The heater6may also be combined with the arranged position of the electrodes shown inFIG. 12. In this case, the heater6is preferably configured to cover an entire region in which the wire electrodes3aand4aare formed.

As a result, the detecting portion11shown inFIG. 13has a larger detection area which corresponds to the wire electrodes3aand4a, compared to a conventional configuration of the detecting portion11which is provided with the detection electrodes3and4formed with constant electrode intervals, as shown inFIG. 14. Furthermore, the sensitivity of the sensor may be enhanced without increasing the number of electrodes. If the wire electrodes3aand4aare disposed to extend in the length direction X of the element, the detection area is easily enlarged without changing the electrode intervals Dn and Dw or the number of electrodes provided, as shown in the detecting portion11inFIG. 12.

It is noted that, unless specifically shown, the same symbols for configuring elements described in the first and second embodiments are used hereinafter.

Third Embodiment

In the first and the second embodiments, the wire electrodes3aand4awhich form the detection electrodes3and4of the sensor1are configured so that each pair of electrodes are provided with constant wide electrode intervals Dw, Dw2and Dw1or narrow electrode intervals Dn, however, the disposed position of the electrodes (specifically, the length direction of the wire electrodes3aand4a) may also be changed to the element width direction Y. The laminate formed sensor element1shown inFIG. 15is an example of configuration which may be adapted for a third embodiment. A basic structure of the detecting portion11is the same as the first embodiment shown inFIG. 5. A difference (between the two configurations) will mainly be described, hereafter.

As shownFIG. 15, the detecting portion11provided on the front end surface of the sensor1has the pair of wire electrodes3aand4aof the wide electrode interval Dw and the pair of wire electrodes3aand4aof the narrow electrode interval Dn alternately positioned. The pair of wire electrodes3aand4aof the narrow electrode interval Dn are disposed in the center part of the detecting portion11, and the pair of electrodes of the wide electrode interval Dw in addition to the pair of wire electrodes3aand4aof the narrow electrode interval are symmetrically disposed in this order on both sides thereof, in the element thickness direction Z. Additionally, the pair of wire electrodes3aand4aof the narrow electrode interval Dn are provided with a small width section12on both ends, in the element width direction Y. The small width section12has an electrode interval Dn1which is narrower than other parts thereof.

Specifically, a main part of each pair of narrow electrode intervals Dn is arranged to have a constant narrow electrode interval Dn, from the center part of the element width direction Y to both end ends thereof, among the wire electrodes3aand4aarranged on the detecting portion11. Both tip ends which continue from the main part are provided to face both respective ends, so that the interval of opposing wire electrodes3aand4agradually becomes narrow to form a taper shaped electrode arrangement, and the small width section12is formed as a narrowest interval part at both tip ends. InFIG. 15, one side of the tip end is shown enlarged. Additionally, when a virtual line (specifically, broken line inFIG. 16) which is extended from opposed ends of the wire electrode3aand4ais set, edges which oppose the both tip ends which form the small interval section12are each positioned on the inner side thereof, pass the virtual line.

Among the wire electrodes3aand4aof the detecting portion11, a large width section13is formed on both ends of each pair of the wide electrode intervals Dw which are adjacent to the narrow interval Dn. The large width section13of the wide electrode interval Dw3which is wider than other sections thereof is thus formed. That is, each pair of the wide electrode intervals Dw has a major part from the center part of the element width direction Y to both ends thereof formed as a constant wide electrode interval Dw. Both tip ends provided to continue from the main part thereof are configured so that the interval between the opposed wire electrodes3aand4agradually widens to form a taper shaped electrode disposed arrangement, and the large width section12formed on the both ends thereof.

The sensor element1described above has the pair of wire electrodes3aand4aof the narrow electrode interval Dn which is configured with the small width section12being further narrowed on the both ends thereof. As a result, the small width sections12can detect smaller particles. Additionally, since the sensor element1is also provided with the wide width section13adjacent to the small width section12, an effect of suppression of a sudden increase of the sensor output due to the coarse particles is enhanced.

As shown inFIG. 17, the particulate matter sensor S provided with the sensor element1described above has the cylindrical housing H which is mounted to the wall W of the exhaust pipe, and the double container shaped cover5surrounding an outer circumference of the sensor element1, configured in the same way as the particulate matter detection sensor S according to the first embodiment, shown inFIG. 2. The cover body5is configured so that the combustion gas is introduced to the detecting portion11of the sensor element1, via the plurality of gas inlet/outlet holes5bof the inner cover52from the plurality of inlet/outlet-holes5aof the outer cover51. The gas inlet/outlet-holes5bof the inner cover52are positioned slightly lower than the front end of the sensor element1, for example, 8 gas inlet/outlet holes5bare equally disposed to surround the sensor11, as shown inFIG. 18.

At this point, according to the flow of combustion gas introduced from the outer cover51, the combustion gas is introduced from more than one of any of the 8 gas inlet/outlet holes, to an inside of the inner cover52, and flows along the surface of the detecting portion11towards the gas inlet/outlet hole5bon an opposed side thereof. By positioning of the sensor element1so that a direction of the gas flow and the element width direction Y of the detecting portion11(specifically, the length direction of the wire electrodes3aand4a) are the same, the combustion gas reliably passes through the surface which includes an end of the detecting portion11during which time the particulate matter is captured.

A mechanism of the above described is shown inFIG. 19. As shown with the broken lines in the enlarged diagram, an interval of equipotential lines is blocked and an electric field occurs at small interval sections12provided on both ends of the wire electrodes3aand4aof the narrow electrode interval Dn. As a result, a Coulomb's force works and particulate matter is easily gathered at the small interval section12. As a result, smaller particulate matter can be rapidly captured at side ends of the detecting portion11. A decrease of the sensitivity due to the wide electrode intervals Dw is thus suppressed, and the detection sensitivity of an entire sensor is enhanced.

When this type of sensor element1is manufactured, as was described in the first embodiment, the wire electrodes3aand4amay be alternately disposed between the green ceramic sheets2ato2cto form the detection electrodes3and4, shown inFIGS. 20 and 21. Specifically, electrodes films which are the respective wire electrodes3aand4aare disposed on both a top and bottom surface of the ceramic green sheet2a, so that the ceramic green sheet2awhich corresponds to the narrow electrode interval Dn is intervened therebetween. The ceramic green sheet2bwhich corresponds to the wide electrode interval Dw is further disposed on a top and bottom of the wire electrodes3aand4awhich are disposed as described above. At this point, on a side of the electrode films which are the wire electrode3aand4a(specifically, on an outer-side of the element width direction Y) a gap having a same film thickness as the electrode film is formed between the respective ceramic green sheets2aand2b.

Thereafter, as shown with an arrow A inFIG. 20, the entire formed structure is pressed in the element thickness direction Z, so that the green ceramic sheets2ato2care pressed with the electrode films which are the wire electrodes3aand4aintervened therebetween, to be unified into one. During this process, a main part of the electrode films which are the wire electrodes3aand4aare absorbed into adjacent ceramic green sheets2aand2b. In contrast, since the side of the wire electrodes3aand4aare not absorbed between the ceramic green sheets2aand2b, a density of the sides of the wire electrodes3aand4aremains low, compared to other parts. As are result, if further pressing is applied thereto, the ceramic green sheets2aand2bbreak more easily, and the opposed ends deform to become closer to each other, forming the small width section12on the both ends of the wire electrodes3aand4a. The large width section13is also formed on an outside of the small width section12, which is omitted from the figure.

Fourth Embodiment

As a fourth embodiment, when a configuration is such that the small width section12is not formed, a ceramic green sheet2dwhich has the same film thickness may be formed on a side of the electrode films which are the wire electrodes3aand3a, by the manufacturing method of the third embodiment as shown inFIG. 22. If pressure is applied to the state shown in theFIG. 22, the small width section12and the large width section13are not formed since the gap is not formed between the ceramic green sheets2aand2b, as exemplified in the third embodiment. Specifically, in this case, a configuration in which constant narrow electrode interval Dn is provided between the wire electrodes3aand4a, as shown in the first embodiment, can be provided.

According to the manufacturing method described, since the green ceramic green sheet2dis disposed between the ceramic green sheets2aand2b, pressing is desirably applied without forming a level difference. As a result, insulation layers which surround the detection electrodes3and4of the detecting portion11are formed, and adherence is enhanced. The ceramic green sheets2a,2band2dare adhered together and ripping between the detection electrodes3and4is prevented. As a result, insulation properties and durability are enhanced.

Also, when the ceramic green sheet2acorresponding to the narrow electrode interval Dn has a comparatively large thickness, an effect of the film thickness of the wire electrodes3aand4ais decreased. Specifically, when the ceramic green sheets2aand2bare laminated, the level difference due to the film thickness of sufficiently thin electrode wires3aand4ais almost not formed. In this case, the configuration of the first embodiment may be achieved without providing the ceramic green sheet2d.

The start-up time of the sensor output, described in the first experiment 1, was investigated in the same way for the configurations of the third and fourth embodiment. For example, the start-up time of the sensor output of 180 seconds when the small width section12is not configured was shortened to 160 seconds when the small width section12is configured, as comparatively shown inFIG. 23. In this way, it was confirmed that by providing the detecting portion11with the small width section12, a sensitivity of the entire sensor element1may be enhanced.

In the third embodiment, the laminate-type sensor element1is adapted, however, a configuration in which the plurality of wire detection electrodes3aand4aprovided with small width sections12and large width sections13for the printed sensor element1of the second embodiment may also be adapted. Additionally, a configuration in which only a small width section12is provided on one end, without a large width section13, may also be provided. As shown in the first embodiment, when either one or both of the electrode intervals Dw1and Dw2are configured in addition to the wide electrode interval Dw or in substitute of the wide electrode Dw, the large width section13may also be provided on each of the ends or on one of the ends thereof. In this case, the same electrode intervals or different electrode intervals may be configured on each of the large width sections13.

As described above, the particulate matter detection sensor S is configured with the detecting portion11of the sensor element S. The detecting portion11is provided with the pair of the detection electrodes3and4which have the plurality of the electrode intervals Dn and Dw. As a result, the sensitivity of the sensor is maintained and variation in sensitivity may be decreased, without largely changing a structure of the detecting portion and manufacturing method. The frequency of an acute output due to the coarse particulate matter particles is decreased thus a detection precision may be enhanced.

As described in the fourth embodiment, the particulate matter detection sensor S is described as sensor which detects the particulate matter contained in the combustion gas of the internal combustion engine. However, the sensor S may be adapted for other engines, as long as the measuring gas contains particulate matter. The particulate matter detection sensor S is not presupposed to be used for DPF malfunction diagnosis. That is, the particulate matter detection sensor may be adapted for various usages. An internal combustion engine is not limited to a diesel engine and may also be used in gasoline engine for example.

The particulate matter detection sensor S is not limited to the above described embodiments, and may be modified without departing from the scope of the disclosure. For example, the cover body5which protects the sensor element1is preferably configured so that the measuring gas is introduced into the detecting portion11of the sensor element1. The shape of the outer cover51and the inner cover52, a size and number of the gas inlet/outlet holes, and an arranged position, for example, may be appropriately set. Additionally, the sensor element1is configured so that the detecting portion11has the detection electrodes3and4arranged on the surface of the insulating body2, and a shape and material, for example of the insulating body2may be appropriately modified.

SYMBOLS