JP2014209128A - Gas sensor and manufacturing method for sensor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor that realizes high-accuracy measurement by having high responsiveness and also having strength enough to prevent a sensor element from being broken due to stress generated when assembled and used.SOLUTION: A gas sensor includes a sensor element 101 that is constituted using an oxygen-ion-conducting solid electrolyte as a main component, and detects a prescribed gas component included in measured gas. The sensor element 101 comprises: an internal gap 20 that introduces the measured gas from an external space; and a pump cell that includes both a first electrode 22 formed on a surface of the internal gap and a second electrode 23 formed in a space different from the internal gap and is provided in a manner capable of pumping out oxygen in the internal gap by applying prescribed voltage between the first electrode 22 and the second electrode 23. The thickness of the internal gap is 50 μm or more but 180 μm or less.

Description

  The present invention relates to a gas sensor for measuring a predetermined gas component in a gas component to be measured and a method for manufacturing the sensor element.

Conventionally, various measuring devices have been used to know the concentration of a desired gas component in a gas to be measured. For example, as an apparatus for measuring the NOx concentration in a gas to be measured such as combustion gas, an electric device configured by forming a Pt electrode and an Rh electrode on a solid electrolyte layer having oxygen ion conductivity such as zirconia (ZrO ₂ ) Gas sensors having chemical pump cells are known.

  Such a gas sensor is manufactured, for example, by performing predetermined processing and printing of a circuit pattern on ceramic green sheets corresponding to each layer, then laminating them, and firing and integrating them. For example, a gas sensor in which six green sheets are stacked and fired and integrated is known (see, for example, Patent Document 1).

Japanese Patent No. 3272215

  In order to enable measurement with high accuracy in the gas sensor as described above, it is possible to increase the responsiveness, that is, to perform concentration measurement that quickly follows the change in concentration of a predetermined gas component in the gas to be measured. It is necessary that the sensor element has a strength that does not break due to various stresses generated during assembly and use.

  For this purpose, a vacant space is formed so that the pumping capacity of the pump cell is maximized, or it is caused by, for example, partial cooling due to heating or disturbance (water adhesion) due to high-temperature gas to be measured. It is required to prevent cracks caused by thermal stress caused by a steep temperature gradient from occurring in the sensor element.

  However, in the case of a conventional gas sensor, it has been developed on the premise that it is manufactured by laminating a plurality of green sheets having the same thickness as disclosed in Patent Document 1 in order to reduce costs and simplify manufacturing. Therefore, there is a limit to the improvement in response characteristics and the increase in strength as described above.

  Accordingly, the present invention has been made to solve such a problem, and an object thereof is to provide a gas sensor and a method for manufacturing the sensor element, which can measure with high accuracy by having high response and high strength. And

  The invention of claim 1 is a gas sensor having a sensor element composed mainly of a solid electrolyte having oxygen ion conductivity, and detecting a predetermined gas component in a gas to be measured, wherein the sensor element is an external space. A first internal space for introducing a gas to be measured from a second internal space communicating with the first internal space under a predetermined diffusion resistance, and a first internal space An inner pump electrode formed on the surface and an outer pump electrode formed in a space different from the first inner space, and a predetermined voltage is applied between the inner pump electrode and the outer pump electrode A main pump cell provided so as to be able to pump out oxygen in the first internal space, an auxiliary pump electrode formed on the surface of the second internal space, and the outer pump electrode, Between the auxiliary pump electrode and the outer pump electrode. And an auxiliary pump cell provided so that oxygen in the second internal space can be pumped out by applying a predetermined voltage to the first internal space, and the first and second internal spaces are in the longitudinal direction of the sensor element. And the thickness of the first and second internal spaces is 50 μm or more and 180 μm or less, and the thickness of the solid electrolyte layer between the inner pump electrode and the outer pump electrode is 220 μm or more and 600 μm. It is as follows.

  According to a second aspect of the present invention, in the gas sensor according to the first aspect, a third electrode that detects a voltage value corresponding to the amount of oxygen present in the second internal space in the second internal space. And determining the concentration of the predetermined gas component according to the voltage value.

  According to a third aspect of the present invention, there is provided a gas sensor having a sensor element composed mainly of an oxygen ion conductive solid electrolyte and detecting a predetermined gas component in a gas to be measured, wherein the sensor element is an external space. A first internal space that introduces a gas to be measured from a second internal space that communicates with the first internal space under a predetermined diffusion resistance, and a second internal space A third internal space that communicates under a predetermined diffusion resistance, an inner pump electrode formed on the surface of the first internal space, and a space different from the first internal space A main pump cell provided so as to be able to pump out oxygen in the first internal space by applying a predetermined voltage between the inner pump electrode and the outer pump electrode, An auxiliary pump electrode formed on the surface of the second internal cavity; An external pump electrode, and an auxiliary pump cell provided so as to pump out oxygen in the second internal space by applying a predetermined voltage between the auxiliary pump electrode and the outer pump electrode. The first to third internal cavities communicate with each other along the longitudinal direction of the sensor element, and the thickness of the first to third internal cavities is not less than 50 μm and not more than 180 μm, The thickness of the solid electrolyte layer between the inner pump electrode and the outer pump electrode is 220 μm or more and 600 μm or less.

  According to a fourth aspect of the present invention, in the gas sensor according to the third aspect, the third electrode that detects a voltage value corresponding to the amount of oxygen present in the third internal space in the third internal space. And determining the concentration of the predetermined gas component according to the voltage value.

  According to a fifth aspect of the present invention, in the gas sensor according to the fourth aspect, the third electrode is exposed in the third internal space.

  A sixth aspect of the present invention is a method of manufacturing a sensor element provided in the gas sensor according to any one of the first to fifth aspects, wherein a predetermined processing is performed and a predetermined circuit pattern is formed, and the thicknesses are different. A preparation step of preparing a plurality of ceramic green sheets, a lamination step of laminating the plurality of ceramic green sheets to form a laminate, a cutout step of cutting out the laminate, and a laminate cut out in the cutout step A firing step for firing the body.

  According to the first to sixth aspects of the invention, it is possible to realize a gas sensor having high responsiveness and capable of measuring with high accuracy.

  In addition, according to the first to sixth aspects of the present invention, it is possible to realize a gas sensor capable of measuring with high accuracy by having a strength that does not damage the sensor element due to stress generated during assembly and use.

It is a fragmentary sectional view showing roughly an example of composition of gas sensor 100 concerning an embodiment of the invention. It is the figure which showed roughly the AA 'cross section of the sensor element 101 shown in FIG. 1 seen from the reference | standard gas introduction space side. It is the figure which showed the relationship between the thickness of an internal space, response time, and diffusion resistance. It is the figure which showed the relationship between the thickness of a pump layer, the destruction water droplet amount ratio, and the relationship with an impedance. It is the cross-sectional schematic diagram which showed schematically the structure of the sensor element 201 which has three internal spaces. It is the cross-sectional schematic diagram which showed schematically the structure of the sensor element 301 which has three internal spaces. It is the cross-sectional schematic diagram which showed schematically the structure of the sensor element 401 which has three internal spaces. It is a figure which shows the relationship between the thickness x1 of the internal space about the sensor element 201, and response time. It is a figure which shows the relationship between the thickness x1 of the internal space about sensor element 301, and response time. It is a figure which shows the relationship between the thickness x1 of the internal space about sensor element 401, and response time.

<Embodiment>
<Schematic configuration of gas sensor>
First, a schematic configuration of the gas sensor 100 will be described.

FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100. The sensor element 101 includes a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, and a first solid electrolyte layer 4 each made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO ₂ ). A long and slender plate having a structure in which six layers of a spacer layer (cavity layer) 5 and a second solid electrolyte layer (pump layer) 6 are laminated in this order from the bottom in the drawing. It is a body-shaped element. The solid electrolyte forming these six layers is dense and airtight. The sensor element 101 is manufactured, for example, by performing predetermined processing and circuit pattern printing on a ceramic green sheet corresponding to each layer, stacking them, and firing and integrating them.

  One end of the sensor element 101, and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, is a gas inlet 10, a first diffusion rate limiting unit 11, and a buffer space. 12, the second diffusion rate limiting part 13, the first internal space 20, the third diffusion rate limiting part 30, and the second internal space 40 are adjacently formed in such a manner that they communicate in this order.

  The gas introduction port 10, the buffer space 12, the first internal space 20, and the second internal space 40 are provided on the lower surface of the second solid electrolyte layer 6 with the upper portion provided in a state in which the spacer layer 5 is cut out. The space inside the sensor element 101 is defined by the lower part being the upper surface of the first solid electrolyte layer 4 and the side parts being the side surfaces of the spacer layer 5.

  Each of the first diffusion rate controlling unit 11, the second diffusion rate controlling unit 13, and the third diffusion rate controlling unit 30 is provided as two horizontally long slits (the opening has a longitudinal direction in a direction perpendicular to the drawing). . In addition, the site | part from the gas inlet 10 to the 2nd internal space 40 is also called a gas distribution part.

  Further, at a position farther from the front end side than the gas circulation part, the side part is partitioned by the side surface of the first solid electrolyte layer 4 between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5. The reference gas introduction space 43 is provided at the position. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

  The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is formed so as to cover the reference electrode 42.

  The reference electrode 42 is an electrode formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, the reference electrode 42 leads to the reference gas introduction space 43. An air introduction layer 48 is provided. Further, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 using the reference electrode 42.

  In the gas circulation part, the gas inlet 10 is a part opened to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas inlet 10.

  The first diffusion control unit 11 is a part that provides a predetermined diffusion resistance to the gas to be measured taken from the gas inlet 10.

  The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate controlling unit 11 to the second diffusion rate controlling unit 13.

  The second diffusion rate limiting unit 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20.

  When the gas to be measured is introduced from the outside of the sensor element 101 to the inside of the first internal space 20, the pressure fluctuation of the gas to be measured in the external space (exhaust pressure pulsation if the gas to be measured is an automobile exhaust gas) ), The gas to be measured that is suddenly taken into the sensor element 101 from the gas inlet 10 is not directly introduced into the first internal space 20, but the first diffusion control unit 11, the buffer space 12, the second After the concentration variation of the gas to be measured is canceled through the diffusion control unit 13, the gas is introduced into the first internal space 20. As a result, the concentration fluctuation of the gas to be measured introduced into the first internal space 20 becomes almost negligible.

  The first internal space 20 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured introduced through the second diffusion rate limiting unit 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

  The main pump cell 21 includes an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, and an upper surface of the second solid electrolyte layer 6. An electrochemical pump cell comprising an outer pump electrode 23 provided in a manner exposed to the external space in a region corresponding to the ceiling electrode portion 22a, and a second solid electrolyte layer 6 sandwiched between these electrodes. is there.

  The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first inner space 20, and the spacer layer 5 that provides side walls. Yes. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom portion is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Spacer layers in which the electrode portions 22b are formed and the side electrode portions (not shown) constitute both side walls of the first internal space 20 so as to connect the ceiling electrode portions 22a and the bottom electrode portions 22b. 5 is formed on the side wall surface (inner surface), and is disposed in a tunnel-shaped structure at the portion where the side electrode portion is disposed.

  The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, cermet electrodes of Pt and zirconia containing 1% of Au). The inner pump electrode 22 in contact with the gas to be measured is formed using a material that has a reduced or no reducing ability for the NOx component in the measured gas.

  In the main pump cell 21, a desired pump voltage Vp 0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and the pump current is positive or negative between the inner pump electrode 22 and the outer pump electrode 23. By flowing Ip0, oxygen in the first internal space 20 can be pumped into the external space, or oxygen in the external space can be pumped into the first internal space 20.

  In order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte 6, the spacer layer 5, the first solid electrolyte 4, The third substrate layer 3 and the reference electrode 42 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for main pump control.

  By measuring the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known. Further, the pump current Ip0 is controlled by feedback control of Vp0 so that the electromotive force V0 is constant. Thereby, the oxygen concentration in the first internal space 20 can be kept at a predetermined constant value.

  The third diffusion control unit 30 provides a predetermined diffusion resistance to the gas under measurement whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20, and the gas under measurement is supplied to the gas under measurement. This is the part that leads to the second internal space 40.

  The second internal space 40 is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured introduced through the third diffusion control unit 30. The NOx concentration is measured mainly in the second internal space 40 in which the oxygen concentration is adjusted by the auxiliary pump cell 50, and further by measuring the pump cell 41 for measurement.

  In the second internal space 40, after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal space 20 in advance, the auxiliary pump cell 50 further supplies the gas to be measured introduced through the third diffusion control unit. The oxygen partial pressure is adjusted. Thereby, since the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, the gas sensor 100 can measure the NOx concentration with high accuracy.

  The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, and an outer pump electrode 23 (outer pump electrode 23). The auxiliary electrochemical pump cell is configured by the second solid electrolyte layer 6 and the sensor element 101 and an appropriate electrode on the outside.

  The auxiliary pump electrode 51 is disposed in the second internal space 40 in the same tunnel configuration as the inner pump electrode 22 provided in the first internal space 20. That is, the ceiling electrode portion 51 a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40 is formed on the first solid electrolyte layer 4. The bottom electrode part 51b is formed, and the side electrode part (not shown) connecting the ceiling electrode part 51a and the bottom electrode part 51b is provided on the spacer layer 5 that provides the side wall of the second internal space 40. It has a tunnel-type structure formed on both wall surfaces.

  Note that the auxiliary pump electrode 51 is also formed using a material having a reduced reduction ability or no reduction ability with respect to the NOx component in the gas to be measured, like the inner pump electrode 22.

  In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal space 40 is pumped to the external space, or It is possible to pump into the second internal space 40 from the space.

  Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte. The layer 4 and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an auxiliary pump control oxygen partial pressure detection sensor cell 81.

  The auxiliary pump cell 50 performs pumping by the variable power source 52 that is voltage-controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. Thereby, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

  At the same time, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and the electromotive force V0 is controlled, so that the third diffusion rate limiting unit 30 controls the second internal space. The gradient of the oxygen partial pressure in the gas to be measured introduced into the gas 40 is controlled so as to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

  The measurement pump cell 41 measures the NOx concentration in the gas to be measured in the second internal space 40. The measurement pump cell 41 includes a measurement electrode 44 provided on a top surface of the first solid electrolyte layer 4 facing the second internal space 40 and spaced from the third diffusion rate-determining portion 30, an outer pump electrode 23, The electrochemical pump cell is constituted by the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

  The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the second internal space 40. Further, the measurement electrode 44 is covered with a fourth diffusion rate controlling part 45.

The fourth diffusion rate-determining part 45 is a film composed of a porous body mainly composed of alumina (Al ₂ O ₃ ). The fourth diffusion control unit 45 plays a role of limiting the amount of NOx flowing into the measurement electrode 44 and also functions as a protective film for the measurement electrode 44.

  In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the generated amount can be detected as the pump current Ip2.

  In order to detect the partial pressure of oxygen around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, The reference electrode 42 constitutes an electrochemical sensor cell, that is, a measurement pump control oxygen partial pressure detection sensor cell 82. The variable power supply 46 is controlled on the basis of the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82.

The gas to be measured introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion rate-determining unit 45 under the condition where the oxygen partial pressure is controlled. Nitrogen oxide in the gas to be measured around the measurement electrode 44 is reduced (2NO → N ₂ + O ₂ ) to generate oxygen. The generated oxygen is pumped by the measurement pump cell 41. At this time, the variable power source is set so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes constant. The voltage Vp2 is controlled. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxide in the gas to be measured, the nitrogen oxide in the gas to be measured using the pump current Ip2 in the measurement pump cell 41. The concentration will be calculated.

  If the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detecting means as an electrochemical sensor cell, the measurement electrode The electromotive force according to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around 44 and the amount of oxygen contained in the reference atmosphere can be detected, whereby the NOx component in the gas to be measured It is also possible to determine the concentration of.

  The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83. The oxygen partial pressure in the gas to be measured outside the sensor can be detected by the electromotive force Vref obtained by the sensor cell 83.

  In the gas sensor 100 having such a configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx). A gas to be measured is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas is determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41 in proportion to the NOx concentration in the measurement gas. You can know.

  Furthermore, the sensor element 101 includes a heater unit 70 that plays a role of temperature adjustment for heating and maintaining the sensor element 101 in order to increase the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

  The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to an external power source, power can be supplied to the heater unit 70 from the outside.

  The heater 72 is an electric resistor formed in a form sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater electrode 71 through the through-hole 73, generates heat when power is supplied from outside through the heater electrode 71, and heats and keeps the solid electrolyte forming the sensor element 101.

  The heater 72 is embedded over the entire area from the first internal space 20 to the second internal space 40, and the entire sensor element 101 can be adjusted to a temperature at which the solid electrolyte is activated. ing.

  The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 by an insulator such as alumina. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72.

  The pressure dissipating hole 75 is a portion that is provided so as to penetrate the third substrate layer 3 and communicate with the reference gas introduction space 43, and is for the purpose of alleviating the increase in internal pressure accompanying the temperature increase in the heater insulating layer 74. Formed.

<Internal void size and location>
Next, the size and formation position of the first internal space 20 and the second internal space 40 in the sensor element 101 will be described.

  2 shows an AA ′ cross section (cross section perpendicular to the longitudinal direction of the sensor element 101 passing through the first internal space 20) of the sensor element 101 shown in FIG. It is the figure shown schematically.

  In FIG. 2, the length of the first internal space 20 (the thickness of the first internal space 20) in the short direction (the thickness direction of the sensor element 101) of the cross section AA ′ of the sensor element 101 (FIG. 1). Is represented as x1. The thickness direction of the sensor element 101 is also the short direction in the AA ′ cross section of the first internal space 20. However, when the sensor element 101 has a plurality of internal spaces provided with pump cells, the thickness (the length in the thickness direction of the sensor element 101) of any of the internal spaces is x1, and hence the following description. In x, x1 is simply referred to as the thickness of the internal space. In FIG. 2, the distance from the upper surface of the sensor element 101 to the first internal space 20 in the thickness direction of the sensor element 101 is represented by x2, but the distance x2 corresponds to the thickness of the pump layer 6. Henceforth, x2 is also called the thickness of a pump layer. Below, the range of x1, x2 which prescribes | regulates the suitable magnitude | size and position of an internal space is demonstrated.

(About the thickness of the internal space in the sensor element)
First, a preferable range of the thickness of the internal space, which is a value that defines the size of the internal space in the sensor element 101, will be described.

  FIG. 3 is a diagram showing the relationship between the thickness x1 of the internal space and the response time and the relationship with the diffusion resistance. Specifically, FIG. 3 shows the results of measuring 10 response sensors and diffusion resistances for each of a plurality of types of gas sensors 100 with different internal void thicknesses. The circles and triangles in FIG. 3 indicate the average values of the response time and diffused resistance measured under the same conditions. The lines above and below the circles and triangles indicate the maximum values under each measurement condition. And shows the minimum value. Strictly speaking, the response time and the diffusion resistance vary depending on the size of the region other than the pump electrode (inner pump electrode 22 in the first internal space 20) provided in each internal space. However, for example, the thickness of the inner pump electrode 22 and the auxiliary pump electrode 51 (the length in the thickness direction of the sensor element 101) is about 10 μm to 15 μm, which is smaller than the thickness x1 of the internal space. For simplicity, the thickness of the pump electrode provided in the internal space is negligible except in some exceptional cases.

  As for the response time, when the NOx concentration in the gas to be measured was changed instantaneously, the sensor output (Ip2) corresponding to 10% of the sensor output (Ip2) corresponding to the changed NOx concentration was detected. It was determined by measuring the time from when the sensor output (Ip2) corresponding to 90% was detected.

  On the other hand, the diffusion resistance was obtained by calculating from the Nernst equation using the limiting current value obtained from the current-voltage curve of each electrode.

  As shown in FIG. 3, when the thickness x1 of the internal space was less than 50 μm, the diffusion resistance (average value) and the variation in diffusion resistance increased rapidly. The diffusion resistance increases in this manner because the upper electrode (ceiling electrode portion 22a in the first inner space 20) and the lower electrode (bottom electrode portion 22b in the first inner space 20) are reduced when the inner space is made thinner. This is considered to be due to the fact that the distance between the upper electrode and the lower electrode becomes shorter and the diffusion resistance imparted by the region between the upper electrode and the lower electrode increases. Further, the diffusion resistance variation is increased because the influence of the pump electrode provided in the internal space cannot be ignored due to the thin internal space, and the variation due to the pump electrode is superposed. Specifically, because the cross-sectional shapes of the inner pump electrode 22 and the auxiliary pump electrode 51 that are porous cermet electrodes are not accurate rectangles, the cross-sectional area of the space between the electrodes and the diffusion path length vary. it is conceivable that.

  In this way, when the diffusion resistance received by the gas under measurement in the internal space varies, the total diffusion resistance applied from when the gas under measurement is taken in from the gas inlet 10 until it reaches the measurement electrode 44 is reduced. Variations occur, which makes high-precision measurement difficult. Therefore, the thickness x1 of the internal space is preferably 50 μm or more.

  On the other hand, as shown in FIG. 3, the response time increases as the thickness x1 of the internal space decreases. This is because the thickness x1 of the internal space is reduced and the volume of the internal space is reduced, so that the volume of the space controlled by the main pump cell 21 and the auxiliary pump cell 50 (pumped volume) is reduced. This is probably because the pumping capacity (oxygen concentration controllability) of the pump cell was improved. In general, there is no practical problem if the response time is 1500 ms or less when the response time is obtained under the same conditions, but it is preferably 1200 ms for more precise engine control. In FIG. 3, the response time is 1200 ms or less when the value of x1 is 180 μm or less. Therefore, it can be said that the thickness x1 of the internal space is preferably 180 μm or less.

  In view of the above, in the gas sensor 100 according to the present embodiment, it is preferable that the thickness of the internal space of the sensor element 101 is 50 μm to 180 μm. By satisfying such requirements, the gas sensor 100 according to the present embodiment has high responsiveness and can perform measurement with high accuracy.

(Internal void location)
Next, a preferable range of the distance x2 from the upper surface of the sensor element 101 to the internal space, which is a value defining the position of the internal space, will be described.

  FIG. 4 is a diagram showing the relationship between the thickness x2 of the pump layer 6 and the amount of broken water droplets that serves as an index of the breaking strength of the sensor element 101, and the impedance. Specifically, FIG. 4 shows the results of preparing 10 types of gas sensors 100 with different thicknesses of the pump layer 6 and measuring the amount of broken water droplets and impedance. However, the amount of broken water droplets is shown as a ratio to the value when the thickness of the pump layer 6 is 200 μm. The circles and triangles in FIG. 4 indicate the average values of the measured values under the same conditions for the destruction water droplet amount ratio and impedance, and the lines above and below the circles and triangles indicate the maximum values for each measurement condition. Values and minimum values are shown.

  The amount of breaking water droplets was determined by performing a water droplet dropping test. Specifically, the sensor element 101 is driven while being heated at a predetermined temperature, and water droplets are dropped on a predetermined location of the sensor element 101 (for example, on the tip of the element, the side surface of the element, or the outer electrode). It was confirmed whether cracks occurred. Then, the same test is performed by increasing the amount of water droplets to be dropped until a crack occurs in the sensor element 101, and when the crack occurs in the sensor element 101, the driving of the sensor element 101 is stopped. The amount of water droplets was taken as the amount of water droplets destroyed.

  On the other hand, the impedance was obtained by measuring a current-voltage curve between the inner pump electrode 22 and the outer pump electrode 23 and calculating the slope of the straight line portion.

  As shown in FIG. 4, when the thickness x2 of the pump layer 6 was larger than 400 μm, the impedance value increased rapidly. This is considered to be because the impedance of the pump layer 6 existing between the inner pump electrode 22 and the outer pump electrode 23 increases as the thickness of the pump layer 6 increases.

  Thus, when the impedance in the pump cell becomes large, it becomes difficult to control the pumping of oxygen by the pump cell with high accuracy, which makes it difficult to perform high-precision measurement, which is not preferable. In general, when the impedance is 90Ω or less when the impedance is obtained under the same conditions, the measurement accuracy is not affected. In FIG. 4, the impedance is 90Ω or less when the value of x2 is 600 μm or less. Therefore, the thickness x2 of the pump layer is preferably 600 μm or less. More preferably, it is 400 μm or less.

  On the other hand, as shown in FIG. 4, the larger the thickness x2 of the pump layer 6, the greater the amount of water droplets to break. That is, the thicker the pump layer 6, the greater the breaking strength of the sensor element 101. In general, there is no practical problem with the breaking strength as long as the amount of breaking water droplets when the water droplet dropping test is performed under the same conditions is equivalent to the breaking water droplet amount of the current sensor. However, in order to ensure reliability in a higher temperature environment or a temperature change environment and to realize a highly accurate sensor, the amount of water droplets is 1.3 times or more that of the current sensor. It is preferable to enlarge it. Therefore, it is preferable that the thickness x2 of the pump layer is 220 μm or more at which the ratio of broken water droplets is 1.3 or more in FIG.

  In view of the above, in the gas sensor 100 according to the present embodiment, it is preferable that the thickness of the pump layer 6 of the sensor element 101 is 220 μm to 600 μm. By satisfying such requirements, the gas sensor 100 according to the present embodiment includes the high-strength sensor element 101, and as a result, measurement with high accuracy is possible.

<Manufacturing process of sensor element>
Next, a process for manufacturing the sensor element 101 having the above-described shape will be described. In the present embodiment, a sensor element 101 is formed by forming a laminate composed of a green sheet containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component, and cutting and firing the laminate.

  In the case of producing the sensor element 101 composed of the six layers shown in FIG. 1, the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer Six green sheets corresponding to the layer 5 and the second solid electrolyte layer 6 are prepared.

  First, a blank sheet corresponding to each layer is prepared. Here, in the completed sensor element 101, a blank sheet having a thickness of 55 μm to 200 μm is used for the spacer layer 5 in order to satisfy the requirements of the thickness x1 of the internal space and the thickness x2 of the pump layer 6 described above. For the 2-solid electrolyte layer 6, a blank sheet having a thickness of 240 μm to 720 μm is used.

  Next, processing, pattern printing, and drying are performed on each blank sheet. A known screen printing technique can be used for printing a pattern or an adhesive. Also, a known drying means can be used for the drying process after printing. When the pattern printing is finished, printing and drying processing of an adhesive paste for laminating and bonding the green sheets corresponding to the respective layers are performed.

  Subsequently, the green sheets to which the adhesive has been applied are stacked in a predetermined order, and subjected to pressure bonding by applying predetermined temperature and pressure conditions, thereby performing a pressure bonding process to form a single laminate. When the laminated body is obtained, a plurality of portions of the laminated body are cut and cut into individual units (referred to as element bodies) of the sensor element 101. And the sensor element 101 which satisfy | fills the range of the thickness x1 of the above-mentioned 1st internal space 20 and the thickness x2 of the pump layer 6 is produced | generated by baking the cut-out element body on predetermined conditions.

  As described above, according to the present embodiment, it is possible to obtain a gas sensor capable of measuring with high accuracy by realizing high responsiveness and high strength of the sensor element.

<Modification>
In the above-described embodiment, for a gas sensor in which the sensor element has two internal spaces, a preferred range of the thickness x1 of the internal space in the sensor element and the distance x2 from the upper surface of the sensor element 101 to the internal space. However, the configuration of the sensor element to which the preferable range of x1 and x2 is applied is not limited to this.

  FIG. 5, FIG. 6, and FIG. 7 are schematic cross-sectional views schematically showing configuration examples of the sensor elements 201, 301, and 401 having three internal spaces, respectively. Among the constituent elements included in the sensor elements 201, 301, and 401, the same constituent elements as those included in the sensor element 101 according to the above-described embodiment are denoted by the same reference numerals as those of the sensor element 101 and description thereof is provided. Is omitted. In addition, the sensor elements 201, 301, and 401 have the same heater unit 70 as the sensor element 101, but the heater unit 70 is simplified in FIGS.

  In the sensor element 201 shown in FIG. 5, the gas inlet 10 to the second internal space 40 are provided in the same manner as the sensor element 101, and further, the fourth diffusion rate limiting unit 60 is provided in the second internal space 40. And the third internal space 61 communicate in this order. The fourth diffusion rate controlling unit 60 has two horizontally long (opening in the direction perpendicular to the drawing in the longitudinal direction), like the first diffusion rate controlling unit 11, the second diffusion rate controlling unit 13, and the third diffusion rate controlling unit 30. It is provided as a slit.

  However, only the auxiliary pump electrode 51 is provided in the second internal space 40, and in the sensor element 101, the measurement electrode 44 and the fourth diffusion rate controlling unit 45 provided in the second internal space 40 are the sensors. The element 201 is not provided in the second internal space 40. In the sensor element 201, the measurement electrode 44 is provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61 so as to be exposed to the third internal space 61. That is, the sensor element 201 employs a configuration in which a slit-like fourth diffusion rate-limiting unit 60 is provided instead of the configuration in which the measurement electrode 44 is covered with the fourth diffusion rate-limiting unit 45 that is used in the sensor element 101. It can be said that.

  In addition, in the sensor element 201, a protective layer 90 made of a porous body is provided on the second solid electrolyte layer (pump layer) 6. Note that the protective layer 90 may also be provided in the sensor element 101 according to the above-described embodiment.

  In addition, the sensor element 301 shown in FIG. 6 is the same as the sensor element 201 except that the buffer space 12 is omitted and the first diffusion rate limiting unit 11 and the second diffusion rate limiting unit 13 are a single diffusion rate limiting unit 14. It has the same configuration.

  In the sensor element 401 shown in FIG. 7, the introduction part 10 is omitted, the first diffusion rate-determining part 11 is directly an opening to the external space, and the second internal space in the longitudinal direction of the sensor element 401 is provided. The sensor element 201 has the same configuration except that the size of the point 40 is large.

  Similarly to the sensor element 101, the sensor elements 201, 301, and 401 having the above-described configuration can be obtained by setting the value of x1 to 50 μm to 180 μm and the value of x2 to 220 μm to 600 μm. High strength of the elements and high responsiveness in the gas sensor provided with each element are realized, and consequently, high accuracy of measurement of the gas sensor is realized. For example, the relationship between the internal space thickness x1 and the response time for the sensor elements 201, 301, and 401 shown in FIGS. 8, 9, and 10 is the relationship for the sensor element 101 shown in FIG. The response time is 1200 ms or less when x1 is 150 μm or less.

DESCRIPTION OF SYMBOLS 20 1st internal space 21 Main pump cell 22 Inner pump electrode 22a Ceiling electrode part 22b Bottom electrode part 23 Outer pump electrode 40 2nd internal space 41 Measurement pump cell 44 Measurement electrode 50 Auxiliary pump cell 51 Auxiliary pump electrode 100 Gas sensor 101,201 , 301, 401 Sensor element

Claims (6)

A gas sensor having a sensor element composed mainly of an oxygen ion conductive solid electrolyte and detecting a predetermined gas component in a gas to be measured,
The sensor element is
A first internal space for introducing the gas to be measured from the external space;
A second internal space communicating with the first internal space under a predetermined diffusion resistance;
An inner pump electrode formed on a surface of the first inner space; and an outer pump electrode formed in a space different from the first inner space; the inner pump electrode and the outer pump electrode A main pump cell provided so as to be able to pump out oxygen in the first internal space by applying a predetermined voltage between
The auxiliary pump electrode formed on the surface of the second inner space and the outer pump electrode, and the second pump electrode by applying a predetermined voltage between the auxiliary pump electrode and the outer pump electrode. An auxiliary pump cell provided so as to be able to pump oxygen in the internal space of
With
The first and second internal spaces communicate with each other along the longitudinal direction of the sensor element;
The thickness of the first and second internal spaces is 50 μm or more and 180 μm or less,
The thickness of the solid electrolyte layer between the inner pump electrode and the outer pump electrode is 220 μm or more and 600 μm or less,
A gas sensor characterized by that. The second internal space further comprises a third electrode that detects a voltage value corresponding to the amount of oxygen present in the second internal space;
The gas sensor according to claim 1, wherein the concentration of the predetermined gas component is obtained according to the voltage value. A gas sensor having a sensor element composed mainly of an oxygen ion conductive solid electrolyte and detecting a predetermined gas component in a gas to be measured,
The sensor element is
A first internal space for introducing the gas to be measured from the external space;
A second internal space communicating with the first internal space under a predetermined diffusion resistance;
A third internal space communicating with the second internal space under a predetermined diffusion resistance;
An inner pump electrode formed on a surface of the first inner space; and an outer pump electrode formed in a space different from the first inner space; the inner pump electrode and the outer pump electrode A main pump cell provided so as to be able to pump out oxygen in the first internal space by applying a predetermined voltage between
The auxiliary pump electrode formed on the surface of the second inner space and the outer pump electrode, and the second pump electrode by applying a predetermined voltage between the auxiliary pump electrode and the outer pump electrode. An auxiliary pump cell provided so as to be able to pump oxygen in the internal space of
With
The first to third internal spaces communicate with each other along the longitudinal direction of the sensor element;
A thickness of the first to third internal spaces is not less than 50 μm and not more than 180 μm;
The thickness of the solid electrolyte layer between the inner pump electrode and the outer pump electrode is 220 μm or more and 600 μm or less,
A gas sensor characterized by that. A third electrode for detecting a voltage value corresponding to the amount of oxygen present in the third internal space in the third internal space;
The gas sensor according to claim 3, wherein the concentration of the predetermined gas component is obtained according to the voltage value.   The gas sensor according to claim 4, wherein the third electrode is exposed in the third internal space. A method for manufacturing a sensor element provided in the gas sensor according to any one of claims 1 to 5,
A preparatory step of preparing a plurality of ceramic green sheets having different thicknesses, wherein predetermined processing is performed and a predetermined circuit pattern is formed;
A laminating step of laminating the plurality of ceramic green sheets to form a laminate;
A cutting step of cutting out the laminate;
And a firing step of firing the laminate cut out in the cut-out step.

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