JP2013140175A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor that can suppress a change in sensor output associated with a change in temperature of a sensor element or a gas to be measured, and can further suppress a reduction in measurement accuracy due to the change in sensor output.SOLUTION: A sensor element comprises: a reference electrode 42 that is disposed at a first principal surface side of a predetermined solid electrolyte layer and is brought into contact with a reference gas serving as a reference of oxygen concentration when a predetermined gas component is measured; and a measuring electrode 44 that is disposed at a second principal surface side of the solid electrolyte layer and can reduce an oxide gas component in the predetermined gas. As viewed from a thickness direction of the solid electrolyte layer, centers of gravity of the measuring electrode 44 and the reference electrode 42 are separated from each other in a width direction while a sensor output that sets a potential difference between the reference electrode 42 and the measuring electrode 44 to a control index is being obtained. When a former area is represented by S1 and an area of an overlapping region is represented by S2, an overlapping ratio R=(S2/S1)×100 is set to be 10 or greater and 77 or less.

Description

  The present invention relates to a gas sensor or an oxygen sensor that measures the concentration of oxide gas in a gas to be measured, such as a nitrogen oxide (NOx) sensor, and a gas sensor having a sensor element that detects a predetermined gas component in the gas to be measured. .

Conventionally, various measuring devices are used to know the concentration of a desired gas component in the gas to be measured. For example, a sensor element (hereinafter referred to as a Pt electrode and a Rh electrode) formed on a solid electrolyte layer having oxygen ion conductivity such as zirconia (ZrO ₂ ) as an apparatus for measuring the NOx concentration in a measurement gas such as combustion gas. A gas sensor having a simple structure is also known (see, for example, Patent Document 1 and Patent Document 2).

JP-A-8-271476 JP 2004-37473 A

  In measuring the concentration of the NOx component in the gas to be measured using the gas sensor as described in Patent Document 1 and Patent Document 2, the sensor element activates the oxygen ion conductivity of the solid electrolyte constituting the sensor element. It will be heated to temperature. In the sensor element heated to a predetermined temperature, NOx is reduced by the measurement electrode formed on the solid electrolyte layer, and a current proportional to the amount of oxygen generated thereby is detected as the sensor output. Then, the concentration of the NOx component in the gas to be measured is obtained based on the detected sensor output.

  On the other hand, in the measurement of the concentration of a predetermined gas component performed using such a gas sensor, the sensor output is conventionally increased by heating the sensor element, that is, by increasing the temperature of the sensor element itself or the temperature of the gas to be measured inside the element. There is a problem of lowering. And the fall of a sensor output will lead to the fall of the measurement accuracy of a gas sensor.

  The present invention has been made in view of the above problems, and can suppress a change in sensor output accompanying a temperature change of a sensor element or a gas to be measured, and can suppress a decrease in measurement accuracy due to the change in the sensor output. An object is to provide a gas sensor.

In order to solve the above problems, the invention of claim 1 has a sensor element composed mainly of a solid electrolyte having oxygen ion conductivity, and the sensor element is heated to a temperature at which the solid electrolyte is activated. A gas sensor that detects the current flowing through the solid electrolyte according to the concentration of a predetermined gas component in the gas to be measured using the oxygen ion conductivity of the solid electrolyte after being kept warm, and the sensor element includes: A reference electrode provided on the first main surface side of the predetermined solid electrolyte layer and in contact with a reference gas used as a reference for oxygen concentration when measuring the concentration of the predetermined gas component; and on the second main surface side of the solid electrolyte layer Provided with a measurement electrode capable of reducing an oxide gas component in the predetermined gas, a sensor output using a potential difference between the reference electrode and the measurement electrode as a control index is obtained, and the solid When viewed from the thickness direction of the electrolyte layer, the measurement electrode and the reference electrode have their centroids shifted in the width direction, and the area of the measurement electrode viewed from the thickness direction of the solid electrolyte layer Is defined as S1, and the area of the region where the reference electrode overlaps the measurement electrode when viewed from the thickness direction of the solid electrolyte layer is defined as S2, and the overlapping rate R, which is a parameter representing the degree of overlapping,
R = (S2 / S1) × 100
The overlap rate R is 10 or more and 77 or less.

  A second aspect of the present invention is the gas sensor according to the first aspect, wherein the overlap rate R is 50 or more and 77 or less.

  According to a third aspect of the present invention, in the gas sensor according to the first or second aspect, the predetermined gas component is nitrogen oxide, and the sensor element is mainly composed of zirconia.

  According to the first to third aspects of the invention, since the positions of the measurement electrode and the reference electrode are overlapped, the temperature difference between the measurement electrode and the reference electrode can be reduced. Thereby, since the thermoelectromotive force generated due to the temperature difference between the electrodes can be reduced, the influence of the thermoelectromotive force on the sensor output can be reduced. That is, a decrease in measurement accuracy due to thermoelectromotive force can be suppressed.

2 is a schematic cross-sectional view schematically showing the configuration of the gas sensor 100. FIG. It is a partial explanatory view showing typically the form of the I-II section in Drawing 1. It is a figure which shows the other example of overlap with the measurement electrode 44 and the reference electrode. It is a figure which shows the relationship between the overlapping rate R and thermoelectromotive force which were obtained in the Example.

<Configuration of gas sensor>
FIG. 1 is a schematic cross-sectional view schematically showing a configuration of a gas sensor 100 which is an example of a gas sensor according to the present invention. The gas sensor 100 detects a predetermined gas component in the gas to be measured (measurement gas) and further measures the concentration thereof. In the present embodiment, the case where the gas sensor 100 is a NOx sensor using nitrogen oxide (NOx) as a detection target component will be described as an example. The gas sensor 100 includes a sensor element 101 made of a solid electrolyte having oxygen ion conductivity, such as zirconia (ZrO ₂ ), used for detection of a predetermined gas component in the gas to be measured.

  A sensor element 101 illustrated in FIG. 1 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 a solid electrolyte having oxygen ion conductivity. The six layers of the spacer layer 5 and the second solid electrolyte layer 6 are laminated in this order from the lower side in the drawing (the vertical direction in the drawing, that is, the direction perpendicular to each solid electrolyte layer is also referred to as the lamination direction). This is a long plate-like element having the above-described structure. These six layers are densely formed by a solid electrolyte. The sensor element 101 is manufactured, for example, by performing predetermined processing or circuit pattern printing on a ceramic green sheet corresponding to each layer, stacking and integrating them, and further cutting and firing the element unit. Is done.

  A gas inlet 10 and a first diffusion rate-determining unit 11 are provided at one end (element tip) of the sensor element 101 between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The 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 formed adjacent to each other in this order. It becomes. 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 lower part is an internal space partitioned by the upper surface of the first solid electrolyte layer 4 and the side part by the side surface 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, the other end of the sensor element 101 (the other end of the element) is located farther from the tip of the element than the gas flow part, and is between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5. In addition, a reference gas introduction space 43 is provided at a position where the side portion is partitioned by the side surface of the first solid electrolyte layer 4. 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 reference electrode 42 is an electrode formed in such a manner that it is sandwiched between the lower surface (first main surface) of the first solid electrolyte layer 4 and the upper surface of the third substrate layer 3. An air introduction layer 48 connected to the reference gas introduction space 43 is provided around the reference electrode 42, and thereby, the reference gas is introduced into the reference electrode 42. Further, as will be described later, the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 can be measured 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 rate-determining unit 11 is a part that imparts a predetermined diffusion resistance to the measurement gas taken in 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 provides 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 pressure fluctuation 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 pressure fluctuation of the gas to be measured introduced into the first internal space 20, that is, the concentration fluctuation becomes almost negligible.

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

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

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

The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (cermet electrodes of Pt and ZrO ₂ containing 1% of Au). The inner pump electrode 22 in contact with the gas to be measured is formed using a material that has weakened or has no reducing ability with respect to the NO 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 by a variable power supply 24 provided outside the sensor element 101, so that the inner pump electrode 22 and the outer pump electrode 23. It is possible to pump oxygen in the first internal space 20 into the external space or pump oxygen in the external space into the first internal space 20 by flowing the pump current Ip0 in the positive or negative direction between It has become.

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

  In the main pump control oxygen partial pressure detection sensor cell 80, the oxygen concentration in the first internal space 20 can be determined by measuring the electromotive force V0 generated in the sensor cell. Further, the pump current Ip0 can be controlled by feedback controlling the pump voltage Vp0 so that the measured value of the electromotive force V0 is constant. As a result, the oxygen concentration in the first internal space 20 is maintained at a predetermined constant value.

  The third diffusion control unit 30 gives 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 measurement gas is supplied to 2 A part that leads to the 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 measurement of the NOx concentration is performed by the operation of the measurement pump cell 41 in the second internal space 40 in which the oxygen concentration is adjusted by the auxiliary pump cell 50.

  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 constituted by the second solid electrolyte layer 6 and a suitable electrode outside the sensor element 101 is sufficient.

  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 51c (not shown) connecting the ceiling electrode part 51a and the bottom electrode part 51b provides a side wall of the second internal space 40. It has a tunnel-type structure formed on both sides of the.

  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, a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 through the variable power supply 46 provided outside the sensor element 101, thereby allowing the atmosphere in the second internal space 40 to be maintained. The oxygen can be pumped into the external space, or can be pumped into the second internal space 40 from the external 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 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 an upper surface (second main surface) of the first solid electrolyte layer 4 facing the second internal space 40 and a measurement electrode 44 provided at a position separated from the third diffusion rate limiting unit 30. This is an electrochemical pump cell constituted by the outer pump electrode 23, 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 having a substantially rectangular shape in plan view. 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 layer 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 is pumped out, and the generated amount is pump current Ip 2 (the pump current Ip 2 is an output of the sensor obtained in the gas sensor 100. Hereinafter, in the measurement pump cell 41, the current Ip2 generated by pumping out oxygen around the measurement electrode 44 can also be detected as a sensor output Ip2.

  Further, in order to detect the oxygen partial pressure 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, An electrochemical sensor cell, that is, an oxygen partial pressure detecting sensor cell 82 for controlling a measurement pump is constituted by the reference electrode 42. The variable power source 46 is controlled based on an electromotive force V2 (hereinafter also referred to as a control voltage 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 46 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.

  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. ing. The oxygen concentration in the measurement gas outside the sensor element 101 can be detected by 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 electrical resistor that is formed in such a manner that it is 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 pressure rise accompanying the temperature rise in the heater insulating layer 74. Is formed.

<Positional relationship between measurement electrode and reference electrode>
Next, the positional relationship between the measurement electrode 44 and the reference electrode 42 will be described. In the sensor element 101, as described above, the heater unit 70 for heating and keeping the sensor element 101 for the purpose of activating the solid electrolyte is formed. Further, a gas introduction port 10 for taking in the gas to be measured into the sensor element 101 is provided at one end of the sensor element 101, and a reference gas introduction space 43 for taking in a reference gas such as the atmosphere is provided at the other end. ing. As described above, the sensor element 101 is heated to a predetermined temperature by the operation of the heater unit 70, while it is connected to a gas to be measured outside the element that is normally lower than the temperature of the heater 72 and a reference gas such as the atmosphere. There is contact. In the sensor element 101, the temperature distribution is biased (temperature gradient) due to the temperature difference of the heater 72 and the gas to be measured inside the element.

  Further, the sensor element 101 includes a solid electrolyte layer (first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, spacer layer 5, 2 solid electrolyte layer 6), each electrode such as measurement electrode 44 and reference electrode 42 containing components such as noble metal, heater insulating layer 74 made of an insulator such as alumina, and further formed inside sensor element 101. This is constituted by a space (the first internal space 20, the second internal space 40, the reference gas introduction space 43), etc., and the difference in these thermal conductivities causes a bias in the temperature distribution inside the sensor element 101. It has become.

  A temperature difference is generated between the electrodes (for example, between the measurement electrode 44 and the reference electrode 42) due to the deviation of the temperature distribution inside the element. Furthermore, a thermoelectromotive force is generated between the electrodes due to a temperature difference generated between the electrodes.

  The control voltage V2 is an electromotive force generated by the difference between the oxygen concentration in the vicinity of the measurement electrode 44 and the oxygen concentration taken from the reference gas introduction space 43 and reaching the reference electrode 42 through the atmosphere introduction layer 48 (reference gas). If the difference in oxygen concentration between the vicinity of the measurement electrode 44 and the periphery of the reference electrode 42 is large, the control voltage V2 becomes a large value, and if the difference in oxygen concentration is small, the control voltage V2 becomes a small value. Then, the voltage Vp2 of the variable power supply 46 is controlled so that the control voltage V2 becomes constant, and pumping by the measurement pump cell 41 is performed.

  When a thermoelectromotive force is generated between the measurement electrode 44 and the reference electrode 42, the voltage obtained by adding the thermoelectromotive force to the electromotive force due to the oxygen concentration difference between the measurement electrode 44 and the reference electrode 42 is the control voltage V2. Will be detected. Since the detected control voltage V2 includes an electromotive force generated by heat, when the voltage is made constant by the variable power source 46, the electromotive force generated by the oxygen concentration difference becomes lower than the electromotive force that is originally controlled. That is, the oxygen concentration difference between the measurement electrode 44 and the reference electrode 42 is reduced. Therefore, the oxygen concentration in the second internal space 40 is higher than the originally controlled oxygen concentration. When the oxygen concentration in the second internal space 40 is high, the NOx reduction reaction is suppressed. In particular, since the oxygen concentration is high in the vicinity of the measurement electrode 44, the reduction reaction of NOx is suppressed. Further, as the ratio of the thermoelectromotive force in the control voltage V2 increases, the degree to which the NOx reduction reaction at the measurement electrode 44 is suppressed increases.

  As described above, since the reduction reaction of NOx is suppressed by the thermoelectromotive force generated between the measurement electrode 44 and the reference electrode 42, the sensor output (pump current Ip2) is reduced, so that the measurement accuracy of the sensor is improved. It will fall.

  In order to suppress such a decrease in sensor output and a decrease in measurement accuracy due to a decrease in sensor output, the relative position between the measurement electrode 44 and the reference electrode 42 is suitable in the present invention. That is, when the sensor element 101 is viewed from the stacking direction of the solid electrolyte layer (when the sensor element 101 is viewed from the first substrate layer 1 side or the second solid electrolyte layer 6 side), the measurement electrode 44 and the reference electrode 42 are These electrodes are arranged so as to have a predetermined degree of overlap (hereinafter also simply referred to as overlap).

  FIG. 2 is a cross-sectional explanatory diagram of the gas sensor 100 taken along the line I-II in FIG. 1 to illustrate the state of overlap between the measurement electrode 44 and the reference electrode 42. FIG. 2 schematically shows the second internal space 40 formed in the spacer layer 5 as viewed from the direction perpendicular to the upper surface of the second solid electrolyte layer 6. In FIG. 2, the reference electrode 42 formed at a position sandwiched between the third substrate layer 3 and the first solid electrolyte layer 4 is indicated by a dotted line. Note that the illustration of the fourth diffusion rate controlling part 45 and the auxiliary pump electrode 51 in the second internal space 40 is omitted. As shown in FIG. 2, in this embodiment, a case where the area of the reference electrode 42 is larger than the area of the measurement electrode 44 will be described as an example.

  As shown in FIG. 2, in the sensor element 101, the reference electrode 42 and the measurement electrode 44 are arranged so as to overlap each other. A region D1 in the figure indicates a region (overlapping region) where the measurement electrode 44 and the reference electrode 42 overlap when viewed from the stacking direction of the elements. The overlapping region D1 indicates the overlapping of the measurement electrode 44 with respect to the entire surface.

  Regarding the width direction of the sensor element 101, if the reference electrode 42 and the measurement electrode 44 have an overlap, the change in the thermoelectromotive force value between the electrodes is small even if the reference electrode 42 and the measurement electrode 44 are shifted in the width direction. Has been confirmed by.

  As shown in FIG. 2, in the configuration in which the measurement electrode 44 and the reference electrode 42 are overlapped, that is, in the configuration in which the area of the overlap region D1 is not 0, the temperature difference between the measurement electrode 44 and the reference electrode 42 However, the thermoelectromotive force is suppressed because it is smaller than the case where the electrodes do not overlap. That is, a decrease in sensor accuracy due to the thermoelectromotive force can be suppressed.

  Thus, in the present invention, the thermoelectromotive force is reduced by reducing the temperature difference between the electrodes by disposing the measurement electrode 44 and the reference electrode 42 at a position where they overlap each other. Sensor output change is suppressed.

  Here, R is defined as a parameter amount indicating the degree of overlap. The overlap rate R is a value defined by the following (formula 1), where the area of the measurement electrode 44 is the area S1, and the area of the reference electrode 42 that overlaps the measurement electrode 44 (overlap area) is the area S2. is there.

R = (S2 / S1) × 100 (Formula 1)
That is, it can be said that the overlapping rate R is an amount representing the ratio of the area of the area overlapping the reference electrode 42 in the area S1 of the measuring electrode 44. For example, in the sensor element 101, the overlapping rate R is 100% in the positional relationship between the measurement electrode 44 and the reference electrode 42 shown in FIG.

  In such a gas sensor 100, in order to measure the concentration of a predetermined gas component with high accuracy, the thermoelectromotive force is preferably about 10 mV or less. And in order that thermoelectromotive force is 10 mV or less, it is suitable that an overlapping rate is about 10% or more. More preferably, the overlapping rate is 50% or more, and in this case, the thermoelectromotive force is about 2 mV. More preferably, the overlap rate is 100% (corresponding to the positional relationship as shown in FIG. 2).

  In addition, regarding the thickness direction of the sensor element 101, it has been confirmed that the temperature difference becomes smaller as the measurement electrode 44 and the reference electrode 42 are closer to each other. However, the insulating layer (atmosphere introduction layer 48) covering the reference electrode 42, the thickness of the reference electrode 42 itself, and a solid between the measurement electrode 44 and the reference electrode 42 to form the measurement pump cell 41 are solid. Since there are restrictions such as the need to form an electrolyte layer, it is necessary to determine the positional relationship in the thickness direction in consideration of the relationship between these and the temperature distribution.

  In addition, when the reference electrode 42 is moved to the reference gas introduction space 43 side or when the reference electrode 42 is enlarged, the inventors have confirmed that the measurement accuracy of the sensor is lowered.

  In the gas sensor 100 having the above-described configuration, the thermoelectromotive force between the electrodes caused by the uneven temperature distribution inside the sensor element 101 is suppressed by making the position of the electrodes suitable, and is caused by the temperature rise of the element. The influence of the sensor output due to the temperature distribution inside the element can be reduced. As a result, the gas sensor 100 can measure the concentration of the predetermined gas with high accuracy.

<Another example of the positional relationship between the measurement electrode and the reference electrode>
Next, another aspect of the positional relationship between the measurement electrode 44 and the reference electrode 42 will be described. FIG. 3 is a diagram illustrating the overlapping state of the measurement electrode 44 and the reference electrode 42 as an example. FIG. 3 is a cross-sectional view taken along line I-II in the gas sensor 100 shown in FIG. 1 in which the positions of the measurement electrode 44 and the reference electrode 42 are different from those shown in FIGS. 1 and 2. 3 is the same as that shown in FIGS. 1 and 2 except that the position of the reference electrode 42 is different.

  Also in FIG. 3, as shown in FIG. 2, the reference electrode 42 and the measurement electrode 44 are arranged so as to overlap each other. A region D2 in the figure indicates a region where the measurement electrode 44 and the reference electrode 42 overlap when viewed from the stacking direction of the elements. Also in FIG. 3, the area of the reference electrode 42 is larger than the area of the measurement electrode 44, and the overlapping region D <b> 2 is a partial region of the surface of the measurement electrode 44 on the second solid electrolyte layer 6 side.

  Even in the overlap mode as shown in FIG. 3, the measurement electrode 44 and the reference electrode 42 have an overlap, so that the temperature difference generated between these electrodes due to the bias of the temperature distribution inside the element is reduced. be able to.

<Modification>
In this embodiment, the sensor element 101 in which the heater 72 (and the heater unit 70 including the heater 72) is integrally formed has been described. However, the application of the present invention is not limited to this. The present invention can also be applied to a sensor element in which a heater is formed separately. Even in this case, the temperature difference between the measurement electrode and the reference electrode due to heating and heat retention of the heater, and the temperature difference. Thus, the influence of the thermoelectromotive force on the sensor output can be suppressed.

  Moreover, although the case where the measurement electrode 44 and the reference electrode 42 are planar rectangular cermet electrodes has been described, the shape of these electrodes is not limited to a planar rectangular shape.

  Further, the gas sensor 100 (and sensor element 101) for measuring the NOx concentration has been described. However, the application of the present invention is not limited to the case where NOx is a target gas component for concentration measurement, and oxygen and oxide gases are concentrated. The present invention can also be applied to the case where the measurement target gas component is used. Also in this case, similarly to the embodiment described above, it is possible to suppress a decrease in measurement accuracy due to the thermoelectromotive force.

  Further, although the case where the area of the measurement electrode 44 is smaller than the area of the reference electrode 42 has been described, the size relationship between the measurement electrode 44 and the reference electrode 42 is not limited thereto.

(Example)
As an example, the gas sensor 100 is disposed in the space, the sensor element 101 is heated to about 800 ° C. by the heater unit 70, and the value of the thermoelectromotive force generated between the measurement electrode 44 and the reference electrode 42 is measured. did. The thermoelectromotive force was measured for a plurality of gas sensors 100 (test sensors 1 to 8). All test sensors were tested under dry air conditions of 8 L / min.

For the measurement of the thermoelectromotive force, the positional relationship between the measurement electrode 44 and the reference electrode 42 shown below was used. The center of gravity of the reference electrode 42 is at a position of 8.0 mm in the longitudinal direction from the tip of the element on the gas inlet 10 side to the other end of the element on the reference gas introduction space 43 side. The area of the reference electrode 42 viewed from the stacking direction is 6.0 × 10 ⁻³ cm ² . The center of gravity of the measurement electrode 44 is located at a position of 8.05 mm in the longitudinal direction from the tip of the element on the gas inlet 10 side to the other end of the element on the reference gas introduction space 43 side. That is, the center of gravity of the reference electrode 42 is located at a position of 0.05 mm from the center of gravity of the measurement electrode 44 in the longitudinal direction on the element tip side on the gas inlet 10 side (hereinafter referred to as the center of gravity position C). The area of the measurement electrode 44 viewed from the stacking direction is 4.0 × 10 ⁻³ cm ² . The area of the overlapping region between the reference electrode 42 and the measuring electrode 44 was 3.1 × 10 ⁻³ cm ^2, that is, the test was performed using a sensor element having an overlapping rate R of 77%. The distance in the width direction between the measurement electrode 44 and the center of gravity of the reference electrode 42 was within a range of 1.3 mm.

(Comparative example)
On the other hand, as a comparative example, similarly to the embodiment, the gas sensor 100 is arranged in the space, the sensor element 101 is heated to about 800 ° C. by the heater unit 70, and the center of gravity of the reference electrode 42 is measured. Between the reference electrode 42 and the measurement electrode 44 at each position when moved 1.2 mm and 4.0 mm in the longitudinal direction from the center of gravity of the element to the gas inlet 10 side of the element (hereinafter referred to as the center of gravity positions B and C). The value of thermoelectromotive force was measured. The thermoelectromotive force was measured using the same test sensors 1 to 8 according to the examples. Moreover, the test sensor was tested on the conditions of dry air 8L / min similarly to the Example.

(Comparison of Example and Comparative Example)
Table 1 shows the results of the thermoelectromotive force generated between the measurement electrodes 44 and the center of gravity positions A, B, and C of the measurement electrodes 44 and the reference electrodes 42 measured by the test sensors 1 to 8 as described above.

  Table 1 shows, for the test sensors 1 to 8, the thermoelectromotive force between the center positions A, B, and C of the measurement electrode 44 and the reference electrode 42 and the center positions A, B, It is a table | surface which shows the relationship with the distance of C. Since the measurement electrode 44 overlaps with the reference electrode 42, the overlap rate R is also shown, and the overlap rate R is 77.0%.

  As shown in Table 1, it was confirmed that the value of the thermoelectromotive force generated between these electrodes increases as the center of gravity of the measurement electrode 44 is moved in the longitudinal direction on the gas inlet 10 side. That is, it was confirmed that in the region from the first internal space 20 to the second internal space 40, the value of the thermoelectromotive force gradually increases as the position from the measurement electrode 44 increases.

  The magnitude of the thermoelectromotive force generated between the measurement electrode 44 and the center of gravity position A of the reference electrode 42 (overlap ratio 77.0%) is the thermoelectromotive force generated between the center of gravity positions B and C of the measurement electrode 44. It is very small compared to the size of. In particular, the thermoelectromotive force generated between the measurement electrode 44 and the reference electrode 42 having an overlap was about 10 mV or less. In order to obtain high measurement accuracy in the gas sensor, it is desirable that the magnitude of the thermoelectromotive force generated between the measurement electrode 44 and the reference electrode 42 is about 10 mV or less.

  Table 2 extracts the result of the thermoelectromotive force generated between the gravity center positions A, B, and C of the measurement electrode 44 and the reference electrode 42 from the results shown in Table 1 above, and the heat of the test sensors 1 to 8 is extracted. The value which calculated the average value of the electromotive force is shown.

  As shown in Table 2, the average value of the thermoelectromotive force of the test sensors 1 to 8 is 12 mV at the centroid position B of the reference electrode 42 and 1.5 mV at the centroid position A of the reference electrode 42. On the other hand, as described above, the measurement electrode 44 and the reference electrode 42 have an overlap rate R of 77.0%.

  FIG. 4 is a diagram showing the relationship between the overlap ratio R and the average value of the thermoelectromotive force. Compared with the average value of the thermoelectromotive force between the center of gravity position B of the reference electrode 42 having an overlap rate R of 0% and the measurement electrode 44, the position of the center of gravity A of the reference electrode 42 having an overlap rate R of 77.0% is measured. It was confirmed that the average value of the thermoelectromotive force between the electrodes 44 was smaller. Further, the inventors have confirmed that the value of the thermoelectromotive force gradually decreases as the overlap ratio increases.

DESCRIPTION OF SYMBOLS 1 1st board | substrate layer 2 2nd board | substrate layer 3 3rd board | substrate layer 4 1st solid electrolyte layer 5 Spacer layer 6 2nd solid electrolyte layer 41 Pump cell for measurement 42 Reference electrode 44 Measuring electrode 70 Heater part 100 Gas sensor 101 Sensor element Ip2 pump Current

Claims (3)

It has a sensor element composed mainly of a solid electrolyte having oxygen ion conductivity, and the sensor element is heated to a temperature at which the solid electrolyte is activated and kept warm, and then the oxygen ion conductivity of the solid electrolyte is increased. A gas sensor that detects a current flowing through the solid electrolyte according to a concentration of a predetermined gas component in a gas to be measured,
The sensor element is
A reference electrode provided on a first main surface side of a predetermined solid electrolyte layer, and in contact with a reference gas serving as a reference for oxygen concentration when measuring the concentration of the predetermined gas component;
A measurement electrode provided on the second main surface side of the solid electrolyte layer and capable of reducing an oxide gas component in the predetermined gas;
With
While obtaining a sensor output using the potential difference between the reference electrode and the measurement electrode as a control index,
When viewed from the thickness direction of the solid electrolyte layer, the measurement electrode and the reference electrode, the center of gravity of each other is shifted in the width direction,
The area of the measurement electrode viewed from the thickness direction of the solid electrolyte layer is S1, and the area of the region where the reference electrode overlaps the measurement electrode when viewed from the thickness direction of the solid electrolyte layer is S2. The overlapping rate R, which is a parameter representing the degree of overlapping,
R = (S2 / S1) × 100
When defined by the above, the overlapping rate R is 10 or more and 77 or less,
A gas sensor characterized by that. The gas sensor according to claim 1,
The overlap ratio R is 50 or more and 77 or less,
A gas sensor characterized by that. The gas sensor according to claim 1 or 2,
The gas sensor, wherein the predetermined gas component is nitrogen oxide, and the sensor element is mainly composed of zirconia.

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