JP2010122187A - Nitrogen oxide concentration detector and method of manufacturing gas sensor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce detection error (correction error) of NOx concentration caused by temperature dependency of the current Ip1 in a gas concentration detector 1 which corrects the current Ip2 having oxygen concentration dependency while being detected according to the amount of decomposition of NOx with the current Ip1 according to the oxygen concentration in exhaust gas. <P>SOLUTION: When the diffusion resistance of a first porous part 151 is defined as D1 and the diffusion resistance of a first measuring room 150 where exhaust gas is induced through the first porous part 151 is defined as D2, these are constituted so as to satisfy 0.5<D1/(D1+D2)≤0.9. The first porous part 151 is constituted so that the diameters of pores are involved in the range of 10-4,000 nm, Thereby, the variation of flow-in amount of the introduced exhaust gas with variation of temperature is mostly prevented in the first porous part 151. With this, the flow-in amount of the exhaust gas is stabilized (temperature dependency is reduced) so that the correction error of the current Ip2 with the current Ip1 is reduced even when the temperature of the first porous part 151 varies. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a nitrogen oxide concentration detection device that detects a nitrogen oxide concentration in exhaust gas components of a combustor or an internal combustion engine, and a method for manufacturing a gas sensor element in the nitrogen oxide concentration detection device.

  Conventionally, nitrogen oxide (NOx) concentration in exhaust gas from an engine or the like is measured to control the engine or the catalyst. In this field, the NOx concentration is configured such that the oxygen concentration contained in the exhaust gas is reduced in advance, and then the NOx in the gas with the reduced oxygen concentration is decomposed, and the NOx concentration is detected from the current flowing according to the decomposition amount. A concentration detection device is known (see, for example, Patent Documents 1 to 3).

  More specifically, in this type of NOx concentration detector, oxygen is pumped (pumped or pumped) into the first measurement chamber into which the exhaust gas is introduced through the first porous portion and the exhaust gas in the first measurement chamber. A first pump cell for performing oxygen, a second measurement chamber into which oxygen pumped or pumped gas is introduced through the second porous portion, and NOx in the second measurement chamber is decomposed. The NOx concentration is detected using a NOx sensor having a second pump cell through which a current (according to the amount of movement of oxygen ions) flows according to the amount of oxygen ions after decomposition. That is, the NOx concentration detection device detects the NOx concentration from the value of the current flowing through the second pump cell (hereinafter also referred to as the second pump cell current).

  By the way, in the case of such a NOx concentration detection device, there is a problem that even if the NOx concentration is constant, if the oxygen concentration in the exhaust gas varies, the second pump cell current varies accordingly.

  For example, when the oxygen concentration in the exhaust gas is 0 (zero), there is no oxygen pumped out by the first pump cell, so the gas concentration ratio (ratio of each component contained in the exhaust gas) flowing into the second measurement chamber does not change. . On the other hand, as the oxygen concentration in the exhaust gas increases, the amount of oxygen pumped out by the first pump cell increases, and as a result, the gas concentration ratio flowing into the second measurement chamber increases as the oxygen concentration decreases. To do. That is, as the oxygen concentration in the exhaust gas is higher, the concentration of nitrogen oxides in the second measurement chamber increases, and the amount of current in the second pump cell that is proportional to the amount of oxygen generated by the decomposition of nitrogen oxides increases. The sensitivity of the sensor to nitrogen oxide concentration increases. Therefore, in order to calculate a more accurate nitrogen oxide concentration, such an increase in sensitivity of the sensor cannot be ignored and must be corrected.

Thus, for example, in Patent Documents 2 and 3, the NOx concentration obtained based on the second pump cell current is corrected in accordance with the amount of oxygen pumped by the first pump cell (the amount of current flowing through the first pump cell). .
JP-A-8-271476 JP 2002-116180 A Japanese Patent Laid-Open No. 11-23528

  However, the amount of current in the first pump cell varies depending on the temperature change from the entrance of the first porous portion to the entrance of the second porous portion. That is, with the temperature change, the flow rate of the exhaust gas flowing from the entrance of the first porous portion to the entrance of the second porous portion varies. The pore diameter of the first porous part is the average free path of each gas diffusion species (each of the various particles contained in the exhaust gas) (the average value of the distance that can travel without being disturbed by scattering sources such as molecules and electrons). Gas diffusion is dominated by molecular diffusion and is approximately proportional to temperature only. Further, since the first measurement chamber is a space, it can be considered substantially the same as the first porous portion. Therefore, in the NOx concentration detection device, the temperature from the entrance of the first porous portion to the entrance of the second porous portion also fluctuates under temperature fluctuations, so that the gas diffusion amount changes. For this reason, a large error may occur between the inflow amount of the exhaust gas estimated based on the current amount of the first pump cell and the actual inflow amount of the exhaust gas depending on temperature fluctuations. Note that the temperature from the entrance of the first porous portion to the entrance of the second porous portion is the temperature distribution of the heater attached to the NOx concentration detection device in order to activate the NOx concentration detection device, or the temperature of the NOx concentration detection device. It varies depending on the configuration, the temperature of the attachment jig of the NOx concentration detection device, the exhaust gas temperature, the gas flow rate, and the like.

  And, for example, in the NOx concentration detecting device that corrects the NOx concentration according to the amount of oxygen pumped out by the first pump cell as in Patent Documents 2 and 3, the above-mentioned NOx concentration is constant although the actual NOx concentration is constant. There is a case where an error occurs in the correction due to the influence of the temperature dependency as described above and an error occurs in the NOx concentration output.

  The present invention has been made in view of these problems, and an object of the present invention is to reduce a detection error of a nitrogen oxide concentration accompanying a temperature change in a nitrogen oxide concentration detection device that detects a nitrogen oxide concentration of a measurement target gas. To do.

  The invention according to claim 1, which has been made to achieve the above object, includes a first measurement chamber into which a measurement target gas is introduced through a first porous portion, a first solid electrolyte body, and the first solid electrolyte. A pair of first electrodes formed on the body, wherein one of the pair of first electrodes is disposed in the first measurement chamber, and oxygen with respect to the measurement target gas introduced into the first measurement chamber A first pump cell that pumps out or pumps in, a second measurement chamber into which the measurement target gas into which oxygen has been pumped out or pumped in the first measurement chamber is introduced via a second porous portion, A second solid electrolyte body and a pair of second electrodes formed on the second solid electrolyte body, wherein one of the pair of second electrodes is disposed in the second measurement chamber, 2 Current flows according to the nitrogen oxide concentration in the measurement chamber The second pump cell and a pair of third electrodes, wherein one electrode of the pair of third electrodes is in the first measurement chamber and / or the second measurement chamber, and the flow path in which the measurement target gas flows An oxygen concentration detection cell disposed between one electrode and the second electrode and generating a voltage in the third electrode in accordance with an oxygen concentration in the first measurement chamber and / or the second measurement chamber; In the nitrogen oxide concentration detecting device, comprising: a correcting means for correcting the nitrogen oxide concentration obtained based on the current flowing through the second pump cell according to the oxygen concentration obtained based on the pump cell. When the diffusion resistance of the part is D1 and the diffusion resistance in the first measurement chamber is D2, it is configured to satisfy 0.5 <D1 / (D1 + D2) ≦ 0.9, and the first porous part The range of the pore diameter is 10 to 4000 nm It is configured to include a nitrogen oxide concentration detection apparatus according to claim.

  In this nitrogen oxide concentration detector, oxygen is pumped or pumped into the measurement target gas (exhaust gas) in the first measurement chamber by the first pump cell, and the oxygen concentration of the exhaust gas is set to the target concentration. Specifically, the oxygen in the first measurement chamber is such that the voltage generated in the oxygen concentration detection cell approaches a predetermined target value (predetermined target value so that the oxygen concentration in the first measurement chamber becomes constant). Adjust the density.

  The exhaust gas whose oxygen concentration is set to the target concentration is introduced into the second measurement chamber through the second porous portion. Nitrogen oxide is decomposed in the second measurement chamber, and the second pump cell has a current (hereinafter, second pump cell) corresponding to the amount of oxygen ions generated by the decomposition of nitrogen oxide (that is, according to the concentration of nitrogen oxide). Current).

  Incidentally, the second measurement chamber contains oxygen ions generated by the decomposition of nitrogen oxides and oxygen ions generated by the decomposition of oxygen remaining from the beginning. For this reason, the second pump cell current corresponding to the sum of the two flows through the second pump cell. That is, the second pump cell current varies depending on the oxygen concentration (residual oxygen).

  Further, as the oxygen concentration of the exhaust gas increases, the concentration of nitrogen oxides occupying the second measurement chamber increases, and the second pump cell current proportional to the amount of oxygen generated by the decomposition of nitrogen oxides increases.

  The correction means corresponds to the residual oxygen and the oxygen concentration obtained based on the current of the first pump cell (hereinafter also referred to as the first pump cell current) (in other words, the amount of discharged oxygen obtained based on the first pump cell current). And means for correcting the nitrogen oxide concentration obtained based on the second pump cell current.

  Furthermore, in the nitrogen oxide concentration detection device according to the first aspect of the present invention, it is possible to reduce the detection error of the nitrogen oxide concentration caused by temperature dependency. This will be described below.

  First, the diffusion resistance represents the difficulty (or the ease of flow) of fluid flow. The diffusion resistance in the first measurement chamber is the diffusion resistance in the exhaust gas path from the inner wall of the first porous portion to the inner wall of the second porous portion on the first measurement chamber side, for example. The diffusion resistance affects the flow rate per unit time of the exhaust gas flowing from the first porous portion into the first measurement chamber. For example, it is considered that the flow rate per unit time of the exhaust gas flowing from the first porous portion into the first measurement chamber decreases as the diffusion resistance increases (that is, the fluid does not flow easily). It is considered that the flow rate per unit time of the exhaust gas flowing into the first measurement chamber from the first porous portion increases as the fluid flows more easily (that is, the fluid flows more easily).

  And it is thought that the site | part with a large diffusion resistance becomes large with respect to the stability with respect to the stability of gas inflow. Therefore, it becomes a problem when the temperature dependency (the amount of fluctuation of the gas inflow amount due to temperature) of the portion where the diffusion resistance is large is large.

  In this respect, in the nitrogen oxide concentration detection apparatus according to the first aspect of the present invention, when the diffusion resistance of the first porous portion is D1 and the diffusion resistance in the first measurement chamber is D2, 0.5 <D1 / (D1 + D2) By configuring so as to satisfy ≦ 0.9 (hereinafter referred to as diffusion resistance condition), the diffusion resistance of the first porous portion is increased. It is possible to sufficiently pump out oxygen in the first measurement chamber by introducing gas into the first measurement chamber in a state where the gas diffusion amount is restricted by the first porous portion.

  Moreover, the problem of temperature dependence is reduced by configuring the first porous portion so that the pore diameter is included in the range of 10 to 4000 nm (so that the pore diameter is smaller than the conventional one). In this regard, first, when the pore diameter is larger than 4000 nm, the temperature dependency becomes so large that it cannot be ignored. One reason for this is that, since the pore diameter is sufficiently larger than the mean free path of the gas diffusion species, the molecular diffusion is dominant in the gas diffusion and the temperature dependency is increased. According to the first aspect of the present invention, the pore diameter is set to 4000 nm or less, and such a temperature dependency problem can be reduced.

  If the pore diameter is smaller than 10 nm, the inflow of exhaust gas becomes extremely slow and the responsiveness decreases. According to the first aspect of the present invention, the pore diameter is set to 10 nm or more to avoid the problem of responsiveness degradation.

  In such a nitrogen oxide concentration detection apparatus according to claim 1, since the temperature dependence of the flow rate of the exhaust gas in the first porous portion is reduced, it is estimated according to the oxygen concentration output (first pump cell current). The error between the new inflow amount of exhaust gas and the actual inflow amount can be reduced. Therefore, the correction error of the nitrogen oxide concentration output (second pump cell current) due to the oxygen concentration output (first pump cell current) can be reduced. That is, it is possible to reduce the detection error of the nitrogen oxide concentration that accompanies the temperature change (according to the temperature change of the first porous portion).

  Meanwhile, another parameter that affects the temperature dependency is the porosity of the first porous portion.

  Therefore, as in claim 2, the first porous portion is preferably configured so that the porosity is included in the range of 5 to 45 vol%.

  For example, if the porosity is higher than 45%, the pores are connected to each other, which causes a substantial increase in the pore diameter, which may increase the temperature dependency. If the porosity is less than 5%, inflow of exhaust gas from the first porous portion to the first measurement chamber is extremely limited, and the responsiveness is lowered.

  In this regard, according to the configuration of claim 2, such a problem can be avoided.

  Next, the nitrogen oxide concentration detection apparatus according to claims 1 and 2 is preferably configured as in claim 3.

  That is, the first porous portion has a dimension (hereinafter referred to as a depth dimension) from the surface on the measurement target gas introduction side to the first measurement chamber side surface in the first porous portion of 0.1 to 0.1. It is good to comprise so that it may be contained in the range of 5.0 mm.

  If the depth dimension is to be less than 0.1 mm, a slight error in the dimension cannot be ignored. For example, even if the dimensions differ slightly from product to product, the performance variation from product to product increases. Further, if the depth dimension is to be made larger than 5.0 mm, the overall size of the nitrogen oxide concentration detecting device becomes large. In this regard, according to the configuration of claim 3, such a problem can be avoided.

  Next, the invention of claim 4 is directed to a first measurement chamber into which a measurement target gas is introduced through the first porous portion, a first solid electrolyte body, and a pair of the first solid electrolyte body formed on the first solid electrolyte body. A first electrode is provided, and one of the pair of first electrodes is disposed in the first measurement chamber, and pumps out or pumps oxygen into the measurement target gas introduced into the first measurement chamber. A pump cell; a second measurement chamber into which the measurement target gas into which oxygen has been pumped or pumped in the first measurement chamber is introduced via a second porous portion; a second solid electrolyte body; 2 having a pair of second electrodes formed on the solid electrolyte body, one of the pair of second electrodes being disposed in the second measurement chamber, and the concentration of nitrogen oxides in the second measurement chamber A second pump cell in which a current corresponding to the current flows and a pair of third electrodes And one electrode of the pair of third electrodes is in the first measurement chamber and / or the second measurement chamber, and between the first electrode and the second electrode in the flow path through which the measurement target gas flows. An oxygen concentration detection cell that generates a voltage in the third electrode in accordance with the oxygen concentration in the first measurement chamber and / or the second measurement chamber, the method comprising: Paste material was prepared by adding 25 to 45 vol% of carbon powder having a particle size of 0.05 to 4.0 μm to alumina powder having a diameter of 0.1 μm to 0.7 μm, and the prepared paste material was laminated and laminated. A method for manufacturing a gas sensor element, comprising: baking a paste material to form the first porous portion.

  By baking the paste material as described above, the carbon powder is burned out, and for example, a first porous portion having a pore diameter in the range of 10 to 4000 nm can be obtained. Moreover, the porosity of a 1st porous part can be included in the range of 5-45 vol%. Moreover, a gas sensor element can be comprised so that the diffusion resistance conditions mentioned above may be satisfy | filled by the structure of such a 1st porous part. If such a gas sensor element is used, the nitrogen oxide concentration detection apparatus of claim 1 can be configured. Therefore, the same effect as described in claim 1 can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a block diagram showing an outline of a gas concentration detection apparatus 1 of the present embodiment.

  The gas concentration detection device 1 includes a sensor element 10, a sensor control circuit 200, and a correction circuit 300.

  The sensor element 10 includes a first pump cell 110 for detecting the oxygen concentration and a second pump cell 130 for detecting the NOx concentration as main components.

  The sensor control circuit 200 includes an oxygen concentration detection unit 210 and a NOx concentration detection unit 220.

  The oxygen concentration detection means 210 is connected to a first pump cell electrode (not shown) of the sensor element 10 and has a function of applying a voltage to the first pump cell 110 and detecting a first pump cell current corresponding to the oxygen concentration. The oxygen concentration detection means 210 outputs the detected oxygen concentration as a signal S1 to the correction circuit 300 and an external device (not shown) (for example, engine control ECU).

  The NOx concentration detection means 220 is connected to a second pump cell electrode (not shown) of the sensor element 10 and has a function of applying a voltage to the second pump cell 130 and detecting a second pump cell current corresponding to the NOx concentration. The NOx concentration detection means 220 outputs the detected NOx concentration to the correction circuit 300 as a signal S2.

  The correction circuit 300 receives the signal S1 from the oxygen concentration detection unit 210 and the signal S2 from the NOx concentration detection unit 220, and reflects the NOx current error due to the oxygen concentration as a reflection of the fact that the NOx concentration output has oxygen concentration dependency. to correct. Then, the correction circuit 300 outputs the corrected NOx concentration output as a signal S3 to an external device (for example, an engine control ECU).

  Next, the gas concentration detection apparatus 1 will be described more specifically with reference to FIGS. 3 is a cross-sectional view taken along the line AA in FIG. Hereinafter, description will be made mainly with reference to FIG.

  The gas concentration detection device 1 has a configuration in which a sensor element 10 in the form of an elongated and long plate-like body is held in a housing (not shown) for mounting on an engine exhaust pipe (not shown). A signal line for extracting a signal output from the sensor element 10 is drawn out from the sensor element 10, and is electrically connected to a sensor control device 5 attached at a position away from the sensor element 10.

  First, the sensor element 10 will be described. The sensor element 10 has a structure in which three plate-like solid electrolyte bodies 111, 121, 131 are formed in layers with sandwiching insulators 140, 145 made of alumina or the like. Further, on the outer side (lower side in FIG. 2) on the solid electrolyte body 131 side, sheet-like insulating layers 162 and 163 mainly composed of alumina are laminated, and a heater pattern 164 mainly composed of Pt is embedded therebetween. An element 161 is provided.

  The solid electrolyte bodies 111, 121, and 131 are made of zirconia, which is a solid electrolyte, and have oxygen ion conductivity. Porous electrodes 112 and 113 are provided on both surfaces of the solid electrolyte body 111 in the stacking direction of the sensor element 10 so as to sandwich the solid electrolyte body 111, respectively.

  The electrodes 112 and 113 are made of Pt, a Pt alloy, or cermet containing Pt and ceramics. In addition, a porous protective layer 114 made of ceramic is provided on the surfaces of the electrodes 112 and 113 to protect the electrodes 112 and 113 from being exposed to a toxic gas (reducing atmosphere) contained in the exhaust gas. Yes. Thereby, deterioration of an electrode is prevented.

  In the solid electrolyte body 111, an electric current is passed between the electrodes 112 and 113, whereby the atmosphere in contact with the electrode 112 (atmosphere outside the sensor element 10) and the atmosphere in contact with the electrode 113 (atmosphere in the first measurement chamber 150 described later). ) Can be pumped out and pumped in (so-called oxygen pumping). The solid electrolyte body 111 and the electrodes 112 and 113 correspond to the first pump cell 110 of FIG. Hereinafter, the first pump cell 110 is referred to as an Ip1 cell 110.

  Next, the solid electrolyte body 121 is disposed so as to face the solid electrolyte body 111 with the insulator 140 interposed therebetween. Porous electrodes 122 and 123 are provided on both surfaces of the solid electrolyte body 121 in the stacking direction of the sensor element 10 so as to sandwich the solid electrolyte body 121. The electrodes 122 and 123 are made of Pt, a Pt alloy, or cermet containing Pt and ceramics.

  Further, a first measurement chamber 150 as a small space is formed between the solid electrolyte body 111 and the solid electrolyte body 121, and the electrode 113 on the solid electrolyte body 111 side and the electrode 122 on the solid electrolyte body 121 side. Are arranged in the first measurement chamber 150. The first measurement chamber 150 is a small space in which the exhaust gas flowing through the exhaust passage is first introduced into the sensor element 10, and the first measurement chamber 150 has a partition inside and outside the first measurement chamber 150, A porous first porous portion 151 that limits the flow rate of exhaust gas per unit time into the first measurement chamber 150 is provided (see also FIG. 3).

  Next, as a partition between the first measurement chamber 150 and a second measurement chamber, which will be described later, on the rear end side (right side of the drawing) of the sensor element 10 of the first measurement chamber 150, the flow rate of exhaust gas per unit time is limited. A second porous portion 152 is provided.

  The solid electrolyte body 121 and the electrodes 122 and 123 are mainly composed of an atmosphere separated by the solid electrolyte body 121 (the atmosphere in the first measurement chamber 150 in contact with the electrode 122 and the reference oxygen chamber 170 to be described later in contact with the electrode 123). Electromotive force can be generated according to the oxygen partial pressure difference between the (atmosphere). Hereinafter, the solid electrolyte body 121 and the electrodes 122 and 123 will be referred to as a Vs cell 120.

  Next, the solid electrolyte body 131 is disposed so as to face the solid electrolyte body 121 with the insulator 145 interposed therebetween. On the surface of the solid electrolyte body 131 on the solid electrolyte body 121 side, porous electrodes 132 and 133 made of cermet containing Pt, Pt alloy, or Pt ceramic are provided.

  A reference oxygen chamber 170 as an isolated small space is formed at a position where the electrode 132 is formed. In the reference oxygen chamber 170, the electrode 123 of the Vs cell 120 is disposed. The reference oxygen chamber 170 is filled with a ceramic porous body. In addition, a second measurement chamber 160 as an independent small space is formed between the reference electrode chamber 170 and the reference oxygen chamber 170 at a position where the electrode 133 is formed. The solid electrolyte member 121 and the insulator 140 are provided with openings 125 and 141 so as to communicate with the second measurement chamber 160. As described above, the first measurement chamber 150 and the opening 141 are provided. Are connected with the second porous portion 152 interposed therebetween.

  The solid electrolyte 131 and the electrodes 132 and 133 are separated by an insulator 145 (atmosphere in the reference oxygen chamber 170 in contact with the electrode 132 and atmosphere in the second measurement chamber 160 in contact with the electrode 133). It can pump out oxygen. The solid electrolyte body 131 and the electrodes 132 and 133 correspond to the second pump cell 130 of FIG. Hereinafter, the second pump cell 130 will be referred to as an Ip2 cell 130.

  Next, the configuration of the sensor control device 5 electrically connected to the sensor element 10 will be described.

  The sensor control device 5 includes a microcomputer 60, an electric circuit unit 58, and the like. The microcomputer 60 and the electric circuit unit 58 correspond to the sensor control circuit 200 and the correction circuit 300 in FIG.

  The microcomputer 60 includes a CPU 61 that executes various calculations, a RAM 62 that stores calculation results and the like, and a ROM 63 that stores programs executed by the CPU 61. In addition, the A / D converter 65 is connected to the electric circuit unit 58 via the A / D converter 65, and the signal input / output unit 64 communicates with an electronic control unit (hereinafter referred to as ECU) 90. And a timer clock (not shown).

  The electric circuit section 58 includes a reference voltage comparison circuit 51, an Ip1 drive circuit 52, a Vs detection circuit 53, an Icp supply circuit 54, an Ip2 detection circuit 55, a Vp2 application circuit 56, and a heater drive circuit 57, and is controlled by the microcomputer 60. In response, the sensor element 10 is used to detect the NOx concentration in the exhaust gas.

  The electrode 113 on the first measurement chamber 150 side of the Ip1 cell 110, the electrode 122 on the first measurement chamber 150 side of the Vs cell 120, and the electrode 133 on the second measurement chamber 160 side of the Ip2 cell 130 are connected to the reference potential. ing. One electrode of the heater element 161 is grounded.

  The Icp supply circuit 54 supplies current Icp between the electrodes 122 and 123 of the Vs cell 120 to pump out oxygen from the first measurement chamber 150 into the reference oxygen chamber 170.

  The Vs detection circuit 53 is a circuit for detecting the voltage Vs between the electrodes 122 and 123, and outputs the detection result to the reference voltage comparison circuit 51.

  The reference voltage comparison circuit 51 is a circuit for comparing the voltage Vs between the electrodes 122 and 123 of the Vs cell 120 detected by the Vs detection circuit 53 with a reference voltage (for example, 425 mV) as a reference. Is output to the Ip1 drive circuit 52.

  The Ip1 drive circuit 52 is a circuit for supplying a current Ip1 between the electrodes 112 and 113 of the Ip1 cell 110. The magnitude and direction of the current Ip1 is based on a reference voltage in which the voltage between the electrodes 122 and 123 of the Vs cell 120 is set in advance based on the comparison result of the voltages between the electrodes 122 and 123 of the Vs cell 120 by the reference voltage comparison circuit 51. It has been adjusted to approximately match. As a result, oxygen is pumped from the first measurement chamber 150 to the outside of the sensor element 10 or oxygen is pumped from the sensor element 10 to the first measurement chamber 150 by the Ip1 cell 110. In other words, the Ip1 cell 110 adjusts the oxygen concentration in the first measurement chamber 150 so that the voltage between the electrodes 122 and 123 of the Vs cell 120 is maintained at a constant value (reference voltage value). .

  Further, the Vp2 application circuit 56 is a circuit for applying a voltage Vp2 (for example, 450 mV) between the electrodes 132 and 133 of the Ip2 cell 130, whereby oxygen from the second measurement chamber 160 to the reference oxygen chamber 170 is supplied. Is pumped out.

  The Ip2 detection circuit 55 is a circuit that detects the value of the current Ip2 flowing from the electrode 133 of the Ip2 cell 130 to the electrode 132.

  The heater drive circuit 57 is controlled by the CPU 61 and applies a voltage to the heater pattern 164 of the heater element 161 to heat the solid electrolyte bodies 111, 121, 131 (in other words, the Ip1 cell 110, the Vs cell 120, and the Ip2 cell 130). And a circuit for keeping the temperature of the solid electrolyte bodies 111, 121, 131 at a predetermined temperature.

  The heater pattern 164 is a single electrode pattern connected within the heater element 161 and is connected to the heater drive circuit 57. The heater drive circuit 57 performs PWM energization control of the heater pattern 164 so that the solid electrolyte bodies 111, 121, 131 (specifically, the solid electrolyte body 121 in the present embodiment) have a target temperature, and controls the heater. The pattern 164 is configured to be controlled to apply a voltage.

  The sensor control device 5 having such a configuration detects the NOx concentration in the exhaust gas using the sensor element 10. The operation when detecting the NOx concentration will be described.

  The solid electrolyte bodies 111, 121, and 131 constituting the sensor element 10 shown in FIG. 2 are heated and activated as the heater pattern 164 to which a drive current is supplied from the heater drive circuit 57 is heated. As a result, the Ip1 cell 110, the Vs cell 120, and the Ip2 cell 130 are operated.

  The exhaust gas flowing in the exhaust passage (not shown) is introduced into the first measurement chamber 150 while being limited in the flow rate by the first porous portion 151. Here, a weak current Icp flows from the electrode 123 side to the electrode 122 side in the Vs cell 120 by the Icp supply circuit 54. For this reason, oxygen in the exhaust gas can receive electrons from the electrode 122 in the first measurement chamber 150 on the negative electrode side, flows as oxygen ions in the solid electrolyte body 121, and moves into the reference oxygen chamber 170. To do. That is, oxygen in the first measurement chamber 150 is sent into the reference oxygen chamber 170 by causing the current Icp to flow between the electrodes 122 and 123.

In the Vs detection circuit 53, the voltage between the electrodes 122 and 123 is detected, compared with the reference voltage (425 mV) by the reference voltage comparison circuit 51, and the comparison result is output to the Ip1 drive circuit 52. Here, if the oxygen concentration in the first measurement chamber 150 is adjusted so that the potential difference between the electrodes 122 and 123 is constant around 425 mV, the oxygen concentration in the exhaust gas in the first measurement chamber 150 becomes a predetermined value ( For example, 10 ⁻⁸ to 10 ⁻⁹ atm).

  Therefore, in the Ip1 drive circuit 52, when the oxygen concentration of the exhaust gas introduced into the first measurement chamber 150 is lower than a predetermined value, the current Ip1 is passed through the Ip1 cell 110 so that the electrode 112 side becomes a negative electrode, To pump oxygen into the first measurement chamber 150. On the other hand, when the oxygen concentration of the exhaust gas introduced into the first measurement chamber is higher than a predetermined value, the Ip1 drive circuit 52 supplies the current Ip1 to the Ip1 cell 110 so that the electrode 113 side becomes a negative electrode, The oxygen is pumped from the sensor element 10 to the outside.

Thus, the exhaust gas whose oxygen concentration is adjusted in the first measurement chamber 150 is introduced into the second measurement chamber 160 through the second porous portion 152. NOx in the exhaust gas in contact with the electrode 133 in the second measurement chamber 160 is decomposed into N ₂ and O ₂ on the electrode 133 by applying a voltage Vp2 between the electrodes 132 and 133 by the Vp2 application circuit 56. The (reduced) and decomposed oxygen flows into the solid electrolyte body 131 as oxygen ions and moves into the reference oxygen chamber 170. At this time, the residual oxygen pumped out in the first measurement chamber 150 similarly becomes a current that moves into the reference oxygen chamber 170 by the Ip2 cell 130. For this reason, the current flowing through the Ip2 cell 130 becomes a current derived from NOx and a current derived from residual oxygen. Here, since the concentration of residual oxygen remaining in the first measurement chamber 150 is adjusted to a predetermined value as described above, the current derived from the residual oxygen can be regarded as substantially constant, and the current derived from NOx. Therefore, the current flowing through the Ip2 cell 130 is proportional to the NOx concentration. In the sensor control device 5, the current Ip2 flowing through the Ip2 cell 130 is detected by the Ip2 detection circuit 55, and a correction calculation process of a known offset current derived from residual oxygen is performed based on the current value to detect the NOx concentration in the exhaust gas. Do.

  The following correction is also performed. That is, as the oxygen concentration of the exhaust gas increases, the amount of oxygen pumped out by the Ip1 cell 110 increases, and as a result, the gas concentration ratio flowing into the second measurement chamber 160 increases by the amount of decrease in oxygen concentration. That is, as the oxygen concentration of the exhaust gas is higher, the nitrogen oxide concentration in the second measurement chamber 160 is increased, and the current amount of the Ip2 cell proportional to the amount of oxygen generated by the decomposition of the nitrogen oxide is increased. Therefore, the current Ip2 (NOx concentration output) is corrected based on the current Ip1 (oxygen concentration output).

  Next, a manufacturing example of the sensor element 10 will be described.

  FIG. 4 is a diagram for explaining a manufacturing example and a layout of the sensor element 10. In FIG. 4, the main part is mainly shown.

In FIG. 4, a ZrO ₂ green sheet and various pastes are laminated in the order of upper left → lower left → upper right → lower right in the figure to produce the sensor element 10. Various pastes are laminated by screen printing on a predetermined ZrO ₂ green sheet.
[ZrO ₂ green sheet]
The solid electrolyte bodies 111, 121, and 131 are configured.

The ZrO ₂ powder is calcined in an atmospheric furnace, for example, at 600 ° C. for 2 hours. The calcined ZrO ₂ powder is put in a trommel together with a dispersant, an organic solvent and a spherulite, and mixed for about 50 hours to make it uniform. To the mixture, a solution in which an organic binder is dissolved in an organic solvent is added, and further mixed for 20 hours to prepare a slurry having a viscosity of about 10 Pa · s. A ZrO ₂ green sheet (denoted as ZrO ₂ -GS in FIG. 4) having a thickness of about 0.4 mm is prepared from this slurry by a doctor blade method and dried in an atmosphere of 100 ° C. for 1 hour, for example.
[Print paste]
(1) For electrodes 112, 123, 132, 133:
Platinum powder, ZrO ₂ powder, and an appropriate amount of organic solvent are put into a raking machine (or pot mill) and mixed for 4 hours to make it uniform. To this mixture, a solution obtained by dissolving an organic binder in an organic solvent is added, and a viscosity modifier is also added, and further mixed for 4 hours to prepare a paste having a viscosity of about 150 Pa · s.

(2) For electrodes 113 and 122:
Platinum powder, ZrO ₂ powder, gold powder, and an appropriate amount of organic solvent are placed in a raking machine (or pot mill) and mixed for 4 hours to make uniform. To this mixture, a solution obtained by dissolving an organic binder in an organic solvent is added, and a viscosity modifier is also added, and further mixed for 4 hours to prepare a paste having a viscosity of about 150 Pa · s.

(3) For insulators 140 and 145, insulating layers 162 and 163, and protective layer 114:
Alumina powder and an appropriate amount of organic solvent are put into a raking machine (or pot mill), mixed for 12 hours and dissolved, and then 20 g of viscosity modifier is added. Thereafter, the paste is further mixed for 3 hours to prepare a paste having a viscosity of about 150 Pa · s.

(4) For the first porous portion 151:
Alumina powder, carbon powder, organic binder, and organic solvent are put in a raking machine (or pot mill) and mixed to make uniform. A viscosity modifier is added to the mixture. Thereafter, the paste is further mixed for 4 hours to prepare a paste having a viscosity of about 400 Pa · s.

Specifically, an alumina powder (α-Al ₂ O ₃₎ having a particle size of 0.3 [mu] m, and each respectively the same amount of alumina powder (α-Al ₂ O ₃₎ having a particle size of 0.5 [mu] m, the particle diameter 0.1μm The carbon powder was mixed so that the mixing ratio of the alumina powder and the carbon powder was 75 vol% alumina powder and 25 vol% carbon powder. This composition is an example.

(5) For carbon coating:
Carbon powder, organic binder, and organic solvent are put into a raking machine (or pot mill) and mixed uniformly. A viscosity modifier is added to the mixture and further mixed for 4 hours to produce a paste. Incidentally, by forming the carbon coat by printing, for example, electrical contact between the electrodes can be prevented. The carbon coat is used to form the first measurement chamber 150 (and the second measurement chamber 160). Since carbon burns away during firing, the carbon coat layer does not exist in the fired body.

(6) For the second porous portion 152 and the reference oxygen chamber 170:
Alumina powder with an average particle size of about 2μm, organic binder, and organic solvent are placed in a roughing machine (or pot mill), mixed for 1 hour, granulated, and pressed at about 2t / cm ² with a die press. A molded body (green state) is produced. The green state press-molded body is disposed at a predetermined position of the laminated structure and then fired to form the second porous portion 152 and the reference oxygen chamber 170 in the sensor element 10.
[ZrO ₂ lamination method]
One to three layers of ZrO ₂ green sheets are pressure-bonded at a pressure of 5 kg / cm ² and a pressing time of 1 minute.
[Binder removal and firing]
The pressure-bonded molded body is debindered for 2 hours in an atmosphere of 400 ° C., for example, and fired for 1 hour in an atmosphere of 1500 ° C., for example.

  According to such a manufacturing example, the sensor element 10 is created.

  By the way, in the present embodiment, as described above, correction calculation processing of a known residual oxygen-derived offset current is performed, and the oxygen concentration dependency is reflected according to the current Ip1 (oxygen concentration output). The current Ip2 (NOx concentration output) is corrected.

  When the former is correctly corrected by the correction calculation process, the amount of residual oxygen needs to be stable, but in order to stabilize the amount of residual oxygen, the electrodes 112 and 113 have a larger area (for example, longer length). Longer). This is to reliably perform oxygen pumping. Therefore, also in this embodiment, as shown in FIG. 1, the lengths of the electrodes 112 and 113 are made longer. As a result, the first measurement chamber 150 also has a shape that is long horizontally in accordance with the length of the electrodes 112 and 113.

  On the other hand, since the first measurement chamber 150 is a space, molecular diffusion is dominant and the temperature dependence of gas diffusion is large. Furthermore, when the length of the first measurement chamber 150 is large, the path length through which the exhaust gas flows becomes large, and thus the diffusion resistance increases. That is, since the proportion of the diffusion resistance of the first measurement chamber 150 occupied by the sensor element 10 also increases, the temperature dependence of the gas diffusion amount increases.

  Further, in the latter case where the current Ip2 (NOx concentration output) is corrected according to the current Ip1 (oxygen concentration output), in the case of correct correction, according to the current Ip1 (oxygen concentration output / oxygen discharge amount). Although the amount of exhaust gas that newly flows in needs to be stable, the amount of exhaust gas that flows in has a temperature dependency that varies depending on the temperature of the first porous portion 151.

  From such a point, by optimizing the diffusion resistance of the first porous portion 151 and the diffusion resistance of the first measurement chamber 150, the influence of temperature dependency can be reduced, and the first porous portion 151 can be reduced. To reduce the temperature dependence of the.

  For this reason, the present applicant has solved the above problem by devising the composition of the first porous portion 151. Specifically, the first porous portion 151 was formed by the manufacturing example as described above to solve the above problem. Hereinafter, this point will be described together with the contents and results of experiments conducted by the applicant of the present application.

  FIG. 6 shows the relationship between the composition of the first porous portion 151 and the temperature dependence.

  FIG. 5 is a flowchart showing a control process for evaluating temperature dependency.

  First, FIG. 5 will be described. As shown in FIG. 5, the temperature dependency is determined by controlling the sensor element 10 at a high temperature (temperature of the first porous portion 151: 850 ° C.) from normal control (temperature of the first porous portion 151: 750 ° C.). Evaluation was performed based on the amount of fluctuation of the current Ip1 when the control was changed to the above. For example, it can be seen that the greater the fluctuation amount of the current Ip1, the higher the temperature dependency (the inflow amount of the exhaust gas is likely to fluctuate due to the temperature fluctuation).

  In the control process of FIG. 5, first, energization of the heater pattern 164 is started in S110.

  Next, the process proceeds to S120, and it is determined whether or not Rpvs as the resistance value of the sensor element 10 (resistance value of the Vs cell 120) is less than 300Ω. As the temperature of the sensor element 10 increases, Rpvs as the resistance value of the sensor element 10 (resistance value of the Vs cell 120) decreases.

  If it is determined in S120 that it is not less than 300Ω (S120: NO), the process of S120 is repeated again.

  On the other hand, if it is determined in S120 that Rpvs is less than 300Ω, the sensor element 10 is assumed to be activated, and the process proceeds to S130.

  In S130, the current Ip1 when Rpvs becomes the target value of 250Ω is acquired. At this time, the temperature of the first porous portion 151 is about 750 ° C., and the temperature of the Vs cell 120 is about 680 ° C.

  Next, in S140, the current Ip1 when Rpvs becomes the next target value of 150Ω is acquired. At this time, the temperature of the first porous portion 151 is about 850 ° C., and the temperature of the Vs cell 120 is about 780 ° C.

  In the present embodiment, the temperature dependence is evaluated by comparing the current Ip1 obtained when the Rpvs is 250Ω and the current Ip1 obtained when the Rpvs is 150Ω, and by examining the amount of variation between them. To do.

  Next, FIG. 6 will be described. As shown in FIG. 6, the alumina composition, the carbon composition, and the mixing ratio of alumina and carbon were changed, and the temperature dependency (variation amount of current Ip1) at that time was evaluated.

  Also, here, it is configured to satisfy the following conditions. Specifically, when the diffusion resistance of the first porous portion is D1 and the diffusion resistance in the first measurement chamber is D2, 0.5 <D1 / (D1 + D2) ≦ 0.9 is satisfied.

  The value D1 / (D1 + D2) was calculated as follows.

  Specifically, the value Ip1D2 of the current Ip1 detected when the first porous portion 151 is not provided and the value Ip1 (D1 + D2) of the current Ip1 detected when the first porous portion 151 is provided. Define.

Next, since the current value Ip1D2 and the current value Ip1 (D1 + D2) are proportional to the inverse of D2 and (D1 + D2),
Ip1D2∝1 / D2, Ip1 (D1 + D2) ∝1 / (D1 + D2).

From this, (D1 + D2) ∝1 / Ip1 (D1 + D2), D1∝1 / Ip1 (D1 + D2) −1 / Ip1D2,
The value of D1 / (D1 + D2) is
D1 / (D1 + D2) = 1-Ip1 (D1 + D2) / Ip1D2
It is calculated by.

  Comparison (A): First, carbon having a particle diameter of 5 μm was mixed with alumina having a particle diameter of 2 μm (100%). The mixing ratio was 35 vol% alumina and 65 vol% carbon. In this case, the temperature dependency was 15%.

  Comparison (B): Next, carbon having a particle size of 5 μm was mixed with alumina having a particle size of 0.3 μm (50%) and a particle size of 0.5 μm (50%). The mixing ratio was 65 vol% alumina and 35 vol% carbon. In this case, the temperature dependency was 12%.

  Under the conditions of the comparisons (A) and (B), the temperature dependency was a high value exceeding 10%.

  Countermeasure (a): Next, carbon having a particle diameter of 4 μm was mixed with alumina having a particle diameter of 0.3 μm (50%) and a particle diameter of 0.5 μm (50%). The mixing ratio was 55 vol% alumina and 45 vol% carbon. In this case, the temperature dependency was 9%.

  Countermeasure (b): Next, carbon having a particle diameter of 0.1 μm was mixed with alumina having a particle diameter of 0.3 μm (50%) and a particle diameter of 0.5 μm (50%). The mixing ratio was 55 vol% alumina and 45 vol% carbon. In this case, the temperature dependency was 8%.

  Countermeasure (c): Next, carbon having a particle diameter of 4 μm was mixed with alumina having a particle diameter of 0.3 μm (50%) and a particle diameter of 0.5 μm (50%). The mixing ratio was 65 vol% alumina and 35 vol% carbon. In this case, the temperature dependency was 4%.

  Countermeasure (d): Next, carbon having a particle diameter of 0.1 μm was mixed with alumina having a particle diameter of 0.3 μm (50%) and a particle diameter of 0.5 μm (50%). The mixing ratio was 65 vol% alumina and 35 vol% carbon. In this case, the temperature dependency was 2%.

  Countermeasure (e): Next, carbon having a particle diameter of 0.05 μm was mixed with alumina having a particle diameter of 0.3 μm (50%) and a particle diameter of 0.5 μm (50%). The mixing ratio was 65 vol% alumina and 35 vol% carbon. In this case, the temperature dependency was 1%.

  Countermeasure (f): Next, carbon having a particle diameter of 0.1 μm was mixed with alumina having a particle diameter of 0.3 μm (50%) and a particle diameter of 0.5 μm (50%). The mixing ratio was 75 vol% alumina and 25 vol% carbon. In this case, the temperature dependency was 1%.

  Thus, it can be seen that the temperature dependency is improved under the conditions of measures (a), (b), (c), (d), (e), and (f). And in the 1st porous part 151 formed on the conditions of this countermeasure (a), (b), (c), (d), (e), (f), a pore diameter is 10-4000 nm. It has become. In particular, it can be seen that the temperature dependence is greatly improved under the conditions of measures (d), (e), and (f). And in the 1st porous part 151 formed on the conditions of this countermeasure (d), (e), (f), the pore diameter is the range of 10-200 nm. The porosity is in the range of 5 to 45 vol%.

  The smaller the temperature dependency, the smaller the fluctuation amount of the inflow amount of the exhaust gas accompanying the temperature change in the first porous portion 151. Therefore, the error between the oxygen concentration of the exhaust gas estimated from the first pump cell current and the actual oxygen concentration is small even under temperature fluctuations. That is, the temperature dependence of the NOx concentration output can be reduced, and the NOx concentration can be detected with higher accuracy.

  In the present embodiment, the first pump cell (Ip1 cell) 110 corresponds to the first pump cell in the claims, the electrodes 112 and 113 correspond to the pair of first electrodes, and the solid electrolyte body 111 corresponds to the first solid. The second pump cell (Ip2 cell) 130 corresponds to the second pump cell in the claims, the electrodes 132 and 133 correspond to a pair of second electrodes, and the solid electrolyte body 131 corresponds to the second solid electrolyte. The Vs cell 120 corresponds to an oxygen concentration detection cell, the electrodes 122 and 123 correspond to a pair of third electrodes, the correction circuit 300 corresponds to correction means, and the current Ip1 is a current flowing through the first pump cell. The current Ip2 corresponds to the current flowing through the second pump cell.

  As mentioned above, although one Embodiment of this invention was described, this invention is not limited to the said embodiment, A various form can be taken within the technical scope of this invention.

  In the above embodiment, the depth dimension of the first porous portion 151 (the dimension from the outer surface to the surface on the first measurement chamber 150 side: see FIG. 3) is in the range of 0.1 to 5.0 mm. You may take any value.

1 is a schematic configuration diagram of a gas concentration detection device 1. FIG. 1 is a detailed configuration diagram of a gas concentration detection device 1. FIG. 1 is a cross-sectional view of a sensor element 10. FIG. 2 is a diagram illustrating a manufacturing example of the sensor element 10. It is a flowchart showing the control processing for evaluation of temperature dependence. 4 is a diagram illustrating a composition example of a first porous portion 151.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Gas concentration detection apparatus, 5 ... Sensor control apparatus, 10 ... Sensor element, 51 ... Reference voltage comparison circuit, 52 ... Ip1 drive circuit, 53 ... Vs detection circuit, 54 ... Icp supply circuit, 55 ... Ip2 detection circuit, 56 DESCRIPTION OF SYMBOLS ... Vp2 application circuit, 57 ... Heater drive circuit, 58 ... Electric circuit part, 60 ... Microcomputer, 64 ... Signal input / output part, 65 ... A / D converter, 90 ... ECU, 110 ... 1st pump cell (Ip1 cell), 111, 121, 131 ... solid electrolyte body, 112, 113, 122, 123, 132, 133 ... electrode, 114 ... protective layer, 120 ... Vs cell, 125 ... opening, 130 ... (second pump cell) Ip2 cell, 140 ... Insulator 141 ... Opening part 145 ... Insulator 150 ... First measurement chamber 151 ... First porous part 152 ... Second porous part 160 ... Second measurement Chamber, 161 ... heater element, 162 ... insulating layer, 164 ... heater pattern, 170 ... reference oxygen chamber, 200 ... sensor control circuit, 210 ... oxygen concentration-detecting means, 220 ... NOx concentration detection means, 300 ... correction circuit.

Claims (4)

A first measurement chamber into which a measurement target gas is introduced via the first porous portion;
A first solid electrolyte body and a pair of first electrodes formed on the first solid electrolyte body, one electrode of the pair of first electrodes being disposed in the first measurement chamber, the first measurement A first pump cell for pumping or pumping oxygen to the measurement target gas introduced into the chamber;
A second measurement chamber into which the measurement target gas into which oxygen has been pumped or pumped in the first measurement chamber is introduced via a second porous portion;
A second solid electrolyte body and a pair of second electrodes formed on the second solid electrolyte body, and one electrode of the pair of second electrodes is disposed in the second measurement chamber; A second pump cell through which a current corresponding to the nitrogen oxide concentration in the two measurement chambers flows;
A pair of third electrodes, and one electrode of the pair of third electrodes is in the first measurement chamber and / or the second measurement chamber, and the first electrode and the second electrode in the flow path through which the measurement target gas flows An oxygen concentration detection cell disposed between two electrodes and generating a voltage in the third electrode in accordance with the oxygen concentration in the first measurement chamber and / or the second measurement chamber;
Correction means for correcting the nitrogen oxide concentration obtained based on the current flowing through the second pump cell according to the oxygen concentration obtained based on the first pump cell;
In a nitrogen oxide concentration detection apparatus comprising:
When the diffusion resistance of the first porous portion is D1, and the diffusion resistance in the first measurement chamber is D2, the structure is configured to satisfy 0.5 <D1 / (D1 + D2) ≦ 0.9,
And the said 1st porous part is comprised so that a pore diameter may be contained in the range of 10-4000 nm. The nitrogen oxide concentration detection apparatus characterized by the above-mentioned.   The said 1st porous part is comprised so that a porosity may be contained in the range of 5-45 vol%, The nitrogen oxide concentration detection apparatus of Claim 1 characterized by the above-mentioned.   The first porous portion includes a dimension from the measurement target gas introduction side surface to the first measurement chamber side surface in the first porous portion in a range of 0.1 to 5.0 mm. The nitrogen oxide concentration detection device according to claim 1 or 2, wherein the nitrogen oxide concentration detection device is configured as described above. A first measurement chamber into which a measurement target gas is introduced via the first porous portion;
A first solid electrolyte body and a pair of first electrodes formed on the first solid electrolyte body, one electrode of the pair of first electrodes being disposed in the first measurement chamber, the first measurement A first pump cell for pumping or pumping oxygen to the measurement target gas introduced into the chamber;
A second measurement chamber into which the measurement target gas into which oxygen has been pumped or pumped in the first measurement chamber is introduced via a second porous portion;
A second solid electrolyte body and a pair of second electrodes formed on the second solid electrolyte body, wherein one of the pair of second electrodes is disposed in the second measurement chamber, A second pump cell through which a current corresponding to the nitrogen oxide concentration in the two measurement chambers flows;
A pair of third electrodes, wherein one electrode of the pair of third electrodes is in the first measurement chamber and / or the second measurement chamber, and the first electrode and the second electrode in the flow path through which the measurement target gas flows An oxygen concentration detection cell disposed between two electrodes and generating a voltage in the third electrode in accordance with the oxygen concentration in the first measurement chamber and / or the second measurement chamber;
A gas sensor element manufacturing method comprising:
A paste material is prepared by adding 25 to 45 vol% of carbon powder having a particle size of 0.05 to 4.0 μm to alumina powder having an average particle size of 0.1 to 0.7 μm, and the prepared paste material is laminated, A method for producing a gas sensor element, comprising: firing a laminated paste material to form the first porous portion.

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