JP2008240631A - Honeycomb structure and particulate sticking quantity measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a honeycomb structure usable for an onboard diagnostic sensor for particulate. <P>SOLUTION: This honeycomb structure has a large number of flow holes penetrating in the axial direction by being partitioned by a partition wall. The honeycomb structure has an electrode core of forming an electrode by sticking a conductive substance to one end surface vertically formed to the axial direction, and an electrode parallel to the electrode core on the other end surface or inside the honeycomb structure. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a honeycomb structure and a method for measuring the amount of fine particles attached to the honeycomb structure, and more particularly, measurement of particulate matter (PM) emission in an internal combustion engine exhaust system and on-board diagnosis (OBD) for particulate (PM) regulations. The present invention relates to a honeycomb structure that can be used for the present invention and a method for measuring the amount of adhered fine particles.

  Emission regulations for harmful substances contained in automobile exhaust gas are becoming stricter, and it is said that in-vehicle diagnostic equipment for emissions will be essential in the United States in 2010.

  There is also a report that even if it is a gasoline vehicle, if the fuel is directly injected into the cylinder, more particulates (PM) are discharged than a diesel vehicle equipped with a particulate filter (DPF). .

  Patent Document 1 pays attention to the fact that the particulates in the exhaust gas are composed of conductive particles, and utilizes the fact that the electrical resistance of the electrical insulating member adhering or adsorbing the conductive particulates decreases. A particulate detection element and a particulate detection filter are disclosed.

  Further, in Patent Document 2, light emitted from a light source is transmitted through an exhaust gas flow and received by a light receiving unit, and light of an exhaust gas having a certain correlation function with a particulate concentration is not transmitted. A technique is disclosed in which the degree is detected and converted to a particulate concentration by a control means.

JP 59-060018 A Japanese Patent Laid-Open No. 04-203413

  In the type that detects a change in resistance due to fine particles adhering to a small sensor such as the invention disclosed in Patent Document 1, since the fine particles move along the gas flow, the fine particles do not easily adhere due to the wraparound action of the gas. There is a problem that the detection accuracy is low because the amount of particles is small in the downstream of the particulate collection filter.

  Further, in the method of transmitting light through the exhaust gas pipe and measuring the imperviousness as in the invention disclosed in Patent Document 2, the entire exhaust gas flow can be measured, but the window of the light emitting / receiving section is dirty. There is a problem that accuracy deteriorates gradually.

  For these techniques, an electrode is disposed on a honeycomb structure that is not substantially plugged, the change in the electrical characteristics of the honeycomb structure is measured, and the amount of change in the measured electrical characteristics is used for the honeycomb structure. A method for calculating the amount of fine particles adhering to the structure is useful. By adopting a honeycomb structure having substantially no plugging, it is possible to cover the entire surface of the exhaust gas pipe perpendicular to the gas flow direction. Further, in this case, since there is substantially no plugging, the pressure loss is small, and it can be manufactured more easily and cheaply than the particulate collection filter.

  FIG. 8 is a schematic perspective view showing a honeycomb structure with an electrode according to the related art. The honeycomb structure 1 is composed of a plurality of flow holes, and has two end faces 2 in the axial direction of the flow holes and side surfaces 3 formed by partition walls of the outermost flow holes. Electrodes 5 are formed on the side surface 3 so as to face each other.

  FIG. 9 is a schematic perspective view showing a usage state of the honeycomb structure 1. The honeycomb structure 1 with an electrode is stored in a can body 9 together with a particulate collection filter (DPF) 8. If this can 9 is disposed in the gas flow path so that the DPF 8 is upstream of the flow path, and the measurement of impedance is continued by passing an alternating current between the electrodes while passing the exhaust gas, it is included in the gas flow 10 Conductive fine particles adhere to the honeycomb structure 1, and the impedance between the electrodes disposed on the side surface of the honeycomb structure changes. Since there is a certain correlation between the change in impedance and the amount of attached fine particles, the amount of attached fine particles can be determined from the change in impedance.

  FIG. 10 is a graph showing the relationship between the absolute value | Z | of the impedance measured as described above and the elapsed time. As shown in FIG. 10, there is a problem that the amount of change in the absolute value | Z | of the impedance is small (a portion surrounded by a dotted line in FIG. 10) and the detection sensitivity of the fine particles is low for a predetermined time after the start of measurement. It was.

  The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide a highly sensitive fine particle quantification means.

  In the method for measuring the amount of adhering fine particles of the honeycomb structure shown in FIG. 9, it is presumed that the impedance changes between the electrodes installed on the side surfaces of the honeycomb structure due to adhesion of the conductive fine particles to the honeycomb structure. . However, the impedance change at this time is a change in the resistance component due to the adhesion of the conductive particles, and since the electrodes are installed on the side surfaces, the distance between the electrodes is long, and the impedance changes if a large amount of fine particles do not adhere to the honeycomb. There was a fault that could not be detected.

  The inventors have found that the fine particles hardly adhere to the inside of the honeycomb structure and concentrate and adhere to the upstream end face of the gas flow path in the honeycomb structure. Therefore, in view of the fact that the fine particles in the exhaust gas are conductive and the impedance is inversely proportional to the electrode area, by using the fine particles attached to the end face of the honeycomb structure as an electrode, not only the resistance component changes, Change capacitance (Z = 1 / C = d / (2πfεS); where Z is impedance, C is capacitance, d is distance between electrodes, f is frequency, ε is dielectric constant, and S is electrode area) I found out that I can make it. Then, by measuring the impedance due to this resistance component and capacitance change, it was found that the amount of attached fine particles can be determined with good sensitivity, and the present invention has been completed. Change in the area of the conductive part on the end face of the honeycomb structure due to adhesion of conductive fine particles to the end face of the honeycomb structure, rather than the change in impedance between the electrodes on the side face of the honeycomb structure due to the attachment of conductive fine particles to the honeycomb structure The capacitance change due to is much larger, and the impedance change can be increased with respect to the amount of attached fine particles.

  That is, according to the present invention, the following honeycomb structure is provided.

[1] A honeycomb structure having a large number of flow holes penetrating in the axial direction partitioned by partition walls and having two end faces in the axial direction, and a conductive substance adheres to one end face A honeycomb structure comprising an electrode nucleus forming an electrode and an electrode parallel to the electrode nucleus on the other end face or inside the honeycomb structure.

[2] The honeycomb structure according to the above [1], wherein the electrode nucleus has an area of 50% or less of the area of the electrode parallel to the electrode nucleus.

[3] The honeycomb structure according to [1] or [2], wherein the electrode core is electrically isolated at least partially to form an isolated electrode core.

[4] In a honeycomb structure having a number of flow holes penetrating in the axial direction partitioned by partition walls and having two end faces in the axial direction, an electrode is formed by attaching a conductive substance to one end face. An electrode core to be formed and an electrode parallel to the electrode core are formed on the other end face or inside the honeycomb structure, and the end face on which the electrode core is formed is upstream of the gas flow path. The gas flow path is arranged so that a gas containing conductive fine particles is passed through the gas flow path, an alternating current is passed between the electrode core and the electrode, the impedance is measured, and the measured impedance A method for measuring the amount of attached fine particles of a honeycomb structure, wherein the amount of conductive fine particles attached to the honeycomb structure is determined.

  The honeycomb structure of the present invention can measure the amount of attached fine particles with higher sensitivity. In particular, the sensitivity at the initial measurement stage can be dramatically improved.

  BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described below, but the present invention is not limited to the following embodiment, and is based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention. It should be understood that modifications and improvements as appropriate to the following embodiments also fall within the scope of the present invention.

  FIG. 1 is a schematic perspective view of a honeycomb structure showing an embodiment of the present invention. The honeycomb structure 1 has two end faces 2 in the axial direction of flow holes serving as fluid flow paths and side faces 3 formed by partition walls of the outermost flow holes. On one end face of the end faces 2, an electrode core 4 is formed in a star-shaped planar shape in which two crosses are shifted and overlapped as shown in FIG. An electrode 5 is formed on the entire other end face. Since the end face of the honeycomb structure has a shape in which a lattice is fitted inside the circle, the electrode 5 also has a shape in which a lattice is fitted inside the circle (not shown). The gas flow 10 flows from the end surface on the side where the electrode core 4 is formed to the end surface on the side where the electrode 5 is formed. At this time, the conductive fine particles contained in the gas flow adhere to the end face on the side where the electrode core 4 is formed and are electrically connected to the electrode core 4 to work as an electrode.

  2 is a schematic side view of the honeycomb structure shown in FIG. 1, and FIG. 3 is a schematic plan view of the honeycomb structure shown in FIG. As shown in FIGS. 2 and 3, it is preferable that L> d, where L is the maximum distance between the adjacent electrode nuclei 4 and d is the distance between the electrode nuclei 4 and the electrodes 5.

  FIG. 4 is a schematic plan view of a honeycomb structure showing another embodiment of the present invention. The electrode core 4 may have a pattern shown in FIG. In this case, the maximum distance L between the adjacent electrode nuclei 4 is as shown in FIG.

  FIG. 5 is a schematic plan view of a honeycomb structure showing still another embodiment of the present invention. In this embodiment, the electrode core 4 comprises an isolated electrode core 6 that is electrically isolated. When conductive fine particles adhere between the electrode core 4 and the isolation electrode core 6, the electrode core 4 and the electrode core 6 become conductive. At this time, the area of the electrode formed by the electrode core 4 and the conductive fine particles spreads abruptly when the electrode core 6 conducts, and the impedance changes abruptly, which is useful for improving the detection sensitivity. .

[Honeycomb structure]
In the present invention, the honeycomb structure means a structure having a large number of flow holes (cells) penetrating in the axial direction partitioned by partition walls.

  In the present invention, the particulate collection filter (DPF) is a staggered one end of each cell on the end face of the honeycomb structure in which a plurality of cells serving as gas flow paths are defined by porous partition walls. It is what was alternately plugged so as to be in the shape.

  In the present invention, it is preferable to use a honeycomb structure that is not substantially plugged. Here, the fact that the plugging is not substantially performed is not limited to the case where the plugging is not performed at all, but includes that which is plugged about 10%. The particulate collection efficiency at this time is about 50% or less, and is difficult to use as a filter. This is different from DPF in that the gas does not collect fine particles when passing through the pores in the partition walls.

  In the present invention, the material of the substrate of the honeycomb structure (excluding the electrode) is not particularly limited, but silicon carbide, cordierite, alumina titanate, sialon, mullite, silicon nitride, zirconium phosphate, zirconia, titania, alumina or A material composed of a ceramic or a combination thereof, or a material mainly composed of a sintered metal is suitable.

  In the present invention, the honeycomb structure may scrape the center part of one or both of the end faces to form a countersink. With this configuration, the gas flow can be concentrated at the central part of the honeycomb structure, and control can be performed so that the fine particles adhere to the electrode cores, thereby improving the detection sensitivity of the fine particles. Can do.

The honeycomb structure used in the present invention may or may not have a catalyst supported on a substrate. In the present invention, the catalyst may be a three-way catalyst, an oxidation catalyst, a NO _X selective reduction catalyst (SCR), a NO _X storage reduction catalyst (LNT), or the like.

[Electrode core]
The electrode core in the present invention refers to a member that functions as an electrode by adhering a conductive substance. Ideally, if the electrode core itself does not have an area, and the electrode core alone does not function as an electrode, it is possible to detect adhesion of a very small amount of conductive fine particles to the honeycomb structure. It is preferable. However, in reality, the electrode core may be a member having a conductive area. The electrode nucleus is preferably 0% to 50% of the area of the electrode parallel to the electrode nucleus.

  In order to efficiently detect the conductive material adhering to the entire end face of the honeycomb structure, it is preferable that the electrode core is stretched over the entire end face of the honeycomb structure. However, as described above, the electrode nucleus actually has an area, and the electrode nucleus alone functions as an electrode. Also, when an alternating current is applied between the electrodes, the electric field has a spatial spread, so adjacent electrodes If the nuclei are too close to each other, it apparently works as an integrated electrode without a gap, and the adhesion of the conductive material to the end face of the honeycomb structure cannot be detected effectively. Therefore, it is preferable to set the maximum electrode nucleus distance L at the end face of the honeycomb structure so that a relationship of L> d is established with respect to the electrode nucleus-electrode distance d.

  In the present invention, the material of the electrode core is not particularly limited, but is preferably composed of any one of a sintered body of conductive paste, conductive ceramics, or metal.

In the present invention, the method for forming the electrode core is not particularly limited. However, when a method of applying a conductive paste such as a silver paste to the end face of the honeycomb structure and baking it by heating is used, the formation of the electrode core is easy. This is preferable because it is firmly bonded to the structure. In addition, it is preferable to select each material so that the difference in thermal expansion coefficient between the honeycomb structure and the electrode core may be 20 × 10 ⁻⁶ / ° C. or less. For example, when the honeycomb structure of the present invention is used directly under an engine, the honeycomb structure is exposed to a high temperature environment during use, so if the difference in thermal expansion coefficient between the honeycomb structure and the electrode core is too large, The honeycomb structure may be damaged or the electrode core may be peeled off due to the difference in thermal expansion. However, if the difference in thermal expansion coefficient between the two is 20 × 10 ⁻⁶ / ° C. or less, such a problem may occur. Low.

[electrode]
In the present invention, the electrode is preferably formed on the end face of the honeycomb structure where the electrode nucleus is not formed or on the inside of the honeycomb structure.

  In the present invention, the material of the electrode is not particularly limited, but is preferably composed of any one of a sintered body of conductive paste, conductive ceramics, or metal.

  In the present invention, the method for forming the electrode is not particularly limited. However, when a method of applying a conductive paste such as silver paste to the end face of the honeycomb structure and heating and baking it is easy to form, the electrode has a honeycomb structure. It is preferable because it is firmly bonded to the body.

In addition, it is preferable to select each material so that the difference in thermal expansion coefficient between the honeycomb structure and the electrode may be 20 × 10 ⁻⁶ / ° C. or less. For example, when the honeycomb structure of the present invention is used directly under an engine, it is exposed to a high temperature environment during use. Therefore, if the difference in the thermal expansion coefficient between the honeycomb structure and the electrode is too large, There is a risk that the honeycomb structure may be damaged or the electrode may be peeled off due to the difference in expansion. However, if the difference in thermal expansion coefficient between the two is 20 × 10 ⁻⁶ / ° C. or less, such a problem may occur. Lower.

[Fine particle measurement]
In the present invention, the amount of collected fine particles is determined by utilizing the electrode core and the electrode of the honeycomb structure. Specifically, since the fine particles in the exhaust gas are conductive and the impedance is inversely proportional to the electrode area, the fine particles attached to the end face of the honeycomb structure are used as electrodes to measure the impedance. Determine the amount. That is, in this honeycomb structure, the area of the conductive part on the end face of the honeycomb structure due to the adhesion of the fine particles to the honeycomb structure by measuring the AC impedance between the electrode core and the electrode provided in the honeycomb structure. Can be measured. The area of the conductive part in the honeycomb structure changes according to the absolute amount of the conductive fine particles attached to the honeycomb structure. Therefore, the amount of fine particles attached to the honeycomb structure is uniquely determined from the measurement data of electrical characteristics such as AC impedance. Can be determined. Specifically, the electrical characteristics such as alternating current impedance are measured by graphing the relationship between the mass of the attached fine particles and the electrical characteristics such as alternating current impedance in advance based on the actual measurement values. Determine the amount of fine particles attached at the time of measurement.

  In the present invention, it is possible to determine the adhesion amount of fine particles from the measured value of AC impedance in this way, but in order to determine the adhesion amount of fine particles with higher accuracy, a coil (inductance) is provided in the impedance measurement circuit. Are preferably connected. By connecting the coils in this way, the AC impedance of the circuit including the coil and the filter having the electrostatic capacitance is the resonance condition Lω = 1 / Cω (L: inductance, C: electrostatic capacitance, ω: 2πf (f: Since the frequency a)) suddenly approaches 0, the change in AC impedance with respect to the change in the amount of attached fine particles becomes sharp, and the amount of attached fine particles can be determined with higher accuracy.

  The inductance value of the coil to be connected may be set so as to satisfy the resonance condition in the target fine particle adhesion amount. That is, by using a variable inductance, the value of the inductance is controlled in advance so that when the amount of adhered fine particles reaches a predetermined value, the AC impedance suddenly approaches zero. In the present invention, it is possible to adjust resonance conditions by connecting capacitance, DC resistance, etc. in series or in parallel in the impedance measurement circuit in addition to such inductance.

  In the honeycomb structure of the present invention, the frequency of the alternating current when measuring the alternating current impedance is preferably 100 Hz to 10 MHz. If the frequency is less than 100 Hz, the impedance value of the honeycomb structure becomes extremely high, and thus it is necessary to handle a very small current in impedance measurement. Therefore, it becomes easy to be affected by noise, and the measurement accuracy of fine particles is lowered. On the other hand, when the frequency exceeds 10 MHz, the influence of the inductance included in the entire measurement system including the signal extraction line from the honeycomb structure cannot be ignored, and the resonance with the capacity regulated by the honeycomb structure and wiring is induced. Detection becomes difficult and measurement accuracy decreases. Moreover, since the sensitivity to a minute amount is better at the low frequency, the frequency of the alternating current when measuring the alternating current impedance is preferably 1 kHz to 100 kHz.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

(Example)
Talc, kaolin, calcined kaolin, alumina, aluminum hydroxide, the respective powders of silica, SiO ₂ is 42 to 56 wt%, _Al 2 _{O 3} is 0 to 45 wt%, MgO is the chemical composition of 12 to 16 wt% To the cordierite forming raw material prepared at a predetermined ratio so as to fall within the range of 15 to 25% by mass of graphite as a pore-forming agent, 5 to 15% by mass in total of synthetic resins such as PET, PMA, and phenol resin, Further, methylcelluloses and a surfactant were added in predetermined amounts, and water was added thereto and kneaded to obtain a clay. Next, after vacuum deaeration of this clay, it was extruded into a honeycomb structure, dried by microwave drying and hot air drying, and then fired at a maximum temperature of 1400 to 1435 ° C., whereby a cell wall thickness of 0.05 mm, A honeycomb structure made of porous ceramics (cordierite) having a cell density of 900 cpsi, a diameter of 106 mm, and a length of 50 mm was manufactured. A center portion of one end face of the honeycomb structure is scraped to form a countersink portion, and the shape shown in FIG. 6 is obtained. On the one end face of this honeycomb structure, silver paste is applied in the pattern shown in FIG. 3, while on the other end face, silver paste is applied to the entire surface of the countersink portion as shown in FIG. 6 and fired. Electrode nuclei and electrodes were formed. Here, electrode core area / electrode area <0.3, and L (maximum electrode core distance) = 30 (mm) × 2 × sin 22.5 ° ≈21.6 mm> d = 15 (mm). Next, an impedance measurement circuit for measuring the AC impedance between the electrodes was connected. This electrode-attached honeycomb structure was housed in a can together with a particulate collection filter.

Flowing diesel engine exhaust gas containing fine particles (soot) into this can body and measuring the AC impedance between the electrode core and electrode while adhering the fine particles to the honeycomb structure with electrodes, the elapsed time and the measured alternating current The relationship with the rate of change in impedance is as shown in FIG. The engine conditions were a diesel engine with a displacement of 2 liters, a rotational speed and torque of 1650 rpm / 55 Nm, and an EGR valve opening of 29%. The exhaust gas conditions at this time were 216 ° C., a flow rate of 1.3 Nm ³ / min, and a fine particle concentration of 3.8 mg / m ³ . FIG. 7 shows that there is a correlation between the amount of fine particles attached to the honeycomb structure and the change in impedance.

(Comparative Example 1)
Instead of forming electrodes and electrode nuclei on the end face of the honeycomb structure produced in the above embodiment, the electrodes facing each other as shown in FIG. 8 on the side face of the honeycomb structure are extended. Paired. Using this honeycomb structure, the impedance between the electrodes was measured while attaching fine particles under the same conditions as described above. The results are shown as (b) in FIG.

(Comparative Example 2)
A honeycomb structure with an electrode was produced in the same manner as in Comparative Example 1 except that no countersink was formed, and the impedance between the electrodes was measured using this honeycomb structure while adhering fine particles. . The results are shown as (c) in FIG.

  As is clear from FIG. 7, the honeycomb structure (Example) of the present invention of (a) has a large impedance change under the same conditions as compared with Comparative Example 1 of (b) and Comparative Example 2 of (c). The detection sensitivity of the attached conductive fine particles is high.

  The present invention can be used for particulate collection filters such as DPF and on-vehicle diagnostic sensors for particulates.

1 is a schematic perspective view of a honeycomb structure according to an embodiment of the present invention. Fig. 2 is a schematic side view of the honeycomb structure of Fig. 1. Fig. 2 is a schematic plan view of the honeycomb structure of Fig. 1. FIG. 3 is a schematic plan view of a honeycomb structure according to another embodiment of the present invention. FIG. 6 is a schematic plan view of a honeycomb structure according to still another embodiment of the present invention. FIG. 3 is a schematic perspective view of a honeycomb structure according to an example. 4 is a graph showing a relationship between impedance change and time of honeycomb structures of Examples, Comparative Examples 1 and 2; FIG. 3 is a schematic perspective view of a honeycomb structure according to related technology. It is a typical perspective view which shows the use condition of the honeycomb structure which concerns on related technology. It is a graph which shows the relationship between the impedance change of a honeycomb structure which concerns on related technology, and time.

Explanation of symbols

1: honeycomb structure, 2: end face, 3: side face, 4: electrode core, 5: electrode, 6: isolation electrode core, 7: counterbore, 8: DPF, 9: can body, 10: gas flow

Claims (4)

A honeycomb structure having a large number of flow holes penetrating in the axial direction partitioned by partition walls, and having two end faces in the axial direction,
An electrode core that forms an electrode by attaching a conductive substance to one end face;
A honeycomb structure comprising an electrode parallel to the electrode core on the other end face or inside the honeycomb structure.   The honeycomb structure according to claim 1, wherein the electrode core has an area of 50% or less of the area of the electrode parallel to the electrode core.   The honeycomb structure according to claim 1 or 2, wherein the electrode core is electrically isolated at least partially to form an isolated electrode core. In a honeycomb structure having a large number of flow holes penetrating in the axial direction partitioned by partition walls, and having two end faces in the axial direction,
An electrode core that forms an electrode by attaching a conductive substance on one end face, and an electrode parallel to the electrode core on the other end face or inside the honeycomb structure,
The honeycomb structure is disposed in the gas flow path so that the end surface on which the electrode core is formed is on the upstream side of the gas flow path,
Let a gas containing conductive fine particles pass through the gas flow path,
An alternating current is passed between the electrode core and the electrode to measure impedance,
A method for measuring the amount of attached fine particles of a honeycomb structure, wherein the amount of conductive fine particles attached to the honeycomb structure is determined from the measured impedance.

Images