JP5058224B2 - NOx sensor - Google Patents

Description

  The present invention relates to a NOx sensor that detects a nitrogen oxide (NOx) concentration and an ammonia concentration in a gas to be measured.

In recent years, a so-called urea SCR (Selective Catalytic Reduction) system has been developed as a system for purifying NOx contained in vehicle exhaust gas.
The urea SCR system adds urea into the exhaust system upstream of a selective reduction type NOx purification catalyst (SCR catalyst) provided in the exhaust system. Thereby, ammonia (NH ₃ ) generated by the decomposition of the added urea selectively reduces NOx in the exhaust gas in the NOx purification catalyst and purifies the exhaust gas.

In such a system, if the amount of urea added is insufficient, NOx will not be sufficiently purified. Conversely, if the amount of urea added is excessive, ammonia will be discharged together with the exhaust gas. Therefore, it is necessary to control the addition amount of urea to an appropriate amount, and for this purpose, it is necessary to accurately measure the ammonia concentration in addition to the NOx concentration in the exhaust system.
Therefore, the exhaust system is provided with a NOx sensor for detecting the nitrogen oxide (NOx) concentration and the ammonia concentration in the exhaust gas.

  As an apparatus for detecting NOx concentration and ammonia concentration in exhaust gas, there is one disclosed in Patent Document 1. In this apparatus, ammonia in the gas to be measured introduced into the internal space of the sensor element is oxidized to generate NOx, and this NOx concentration is measured by the sensor element.

JP-A-9-33512

  However, in the above apparatus, ammonia may not be sufficiently oxidized, and as a result of reaching the sensor cell, it may be difficult to accurately measure the ammonia concentration. That is, there is a problem that sufficient sensitivity to ammonia cannot be obtained and the measurement accuracy of ammonia concentration is low.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a NOx sensor excellent in measurement accuracy of ammonia concentration in a gas to be measured.

The first invention of the present application is a gas chamber to be measured into which a gas to be measured is introduced,
A diffusion resistance portion provided in the introduction portion of the measurement gas in the measurement gas chamber;
An oxygen ion conductive solid electrolyte body for a sensor, and a pair of sensor electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for the sensor, A sensor cell for detecting the NOx concentration in the gas chamber;
An oxygen ion conductive solid electrolyte body for a pump, and a pair of pump electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for pump, A pump cell for adjusting the oxygen concentration in the gas chamber;
The temperature of the diffusion resistance portions have a temperature control means for controlling the above 700 ° C. during use,
The temperature control means includes a heater that generates heat when energized, and a temperature detection means that directly or indirectly detects the temperature of the diffusion resistance portion.
The temperature detecting means measures an impedance between a pair of measurement electrodes provided on the sensor solid electrolyte body or the pump solid electrolyte body, and detects the temperature of the diffusion resistance section based on the measured value. The NOx sensor is characterized in that at least one of the pair of measurement electrodes is in contact with the diffusion resistance portion .

The NOx sensor has a temperature control means for controlling the temperature of the diffusion resistance portion during use to 700 ° C. or higher. Therefore, when the gas to be measured containing ammonia passes through the diffusion resistance portion, the reaction with oxygen contained in the gas to be measured is promoted to sufficiently oxidize the ammonia, so that NOx derived from ammonia is reduced. Fully generated.
As a result, this NOx is detected in the sensor cell, and a sensor output corresponding to the NOx concentration, that is, a sensor output corresponding to the ammonia concentration in the gas to be measured can be obtained accurately.

In addition, since ammonia is oxidized in the diffusion resistance portion that is the introduction portion of the gas to be measured into the gas chamber to be measured, the oxygen concentration in the gas chamber to be measured can be kept low. In other words, ammonia can be oxidized if oxygen is present in the diffusion resistance portion that is the introduction portion without increasing the oxygen concentration in the measurement gas chamber. Therefore, it becomes easy to suppress the offset error of the sensor output due to the oxygen concentration.
Further, in the present invention, the temperature control means includes a heater that generates heat when energized, and a temperature detection means that directly or indirectly detects the temperature of the diffusion resistance portion.
Therefore, the temperature control in the diffusion resistance portion can be easily and accurately performed.
The temperature detection means measures the impedance between a pair of measurement electrodes provided on the sensor solid electrolyte body or the pump solid electrolyte body, and detects the temperature of the diffusion resistance unit based on the measured value. And at least one of the pair of measurement electrodes is in contact with the diffusion resistance portion.
Therefore, it is possible to measure the impedance at the portion of the solid electrolyte body for sensor or the solid electrolyte body for pump that is in contact with the diffusion resistance portion, and the temperature of the diffusion resistance portion can be measured with higher accuracy.
As described above, according to the first aspect of the present invention, it is possible to provide a NOx sensor that is excellent in measurement accuracy of the ammonia concentration in the gas to be measured.

  Next, the second invention of the present application is a gas chamber to be measured into which a gas to be measured is introduced,
  A diffusion resistance portion provided in the introduction portion of the measurement gas in the measurement gas chamber;
  An oxygen ion conductive solid electrolyte body for a sensor, and a pair of sensor electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for the sensor, A sensor cell for detecting the NOx concentration in the gas chamber;
  An oxygen ion conductive solid electrolyte body for a pump, and a pair of pump electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for pump, A pump cell for adjusting the oxygen concentration in the gas chamber;
  Temperature control means for controlling the temperature of the diffused resistor portion in use to 700 ° C. or higher,
  The temperature control means includes a heater that generates heat when energized, and a temperature detection means that directly or indirectly detects the temperature of the diffusion resistance portion.
  The temperature detecting means is a NOx sensor comprising a thermocouple arranged in contact with the diffusion resistance portion (claim 2).
  The NOx sensor has a temperature control means for controlling the temperature of the diffusion resistance portion during use to 700 ° C. or higher. Therefore, when the gas to be measured containing ammonia passes through the diffusion resistance portion, the reaction with oxygen contained in the gas to be measured is promoted to sufficiently oxidize the ammonia, so that NOx derived from ammonia is reduced. Fully generated.
  As a result, this NOx is detected in the sensor cell, and a sensor output corresponding to the NOx concentration, that is, a sensor output corresponding to the ammonia concentration in the gas to be measured can be obtained accurately.
  In addition, since ammonia is oxidized in the diffusion resistance portion that is the introduction portion of the gas to be measured into the gas chamber to be measured, the oxygen concentration in the gas chamber to be measured can be kept low. In other words, ammonia can be oxidized if oxygen is present in the diffusion resistance portion that is the introduction portion without increasing the oxygen concentration in the measurement gas chamber. Therefore, it becomes easy to suppress the offset error of the sensor output due to the oxygen concentration.
  The temperature detecting means is composed of a thermocouple disposed in contact with the diffusion resistance portion.
  Therefore, it is possible to directly measure the temperature in the diffused resistor portion, and it is possible to measure the temperature with high accuracy.
  As described above, according to the second aspect of the invention, it is possible to provide a NOx sensor that is excellent in measurement accuracy of the ammonia concentration in the gas to be measured.

The longitudinal cross-sectional view of the NOx sensor in the reference example 1 . The explanatory view of the urea SCR system in reference example 1 . In Reference Example 1, and the NOx concentration and NH ₃ concentration in the exhaust gas downstream of the SCR catalyst, diagram showing the relationship between the urea addition amount. In experimental example graph showing the relationship of NH ₃ / NO sensitivity ratio and the temperature of the diffusion resistance portion. 2 is a longitudinal sectional view of a NOx sensor in Embodiment 1. FIG. FIG. 6 is a longitudinal sectional view of a NOx sensor in the second embodiment. FIG. 6 is a longitudinal sectional view of a NOx sensor in Embodiment 3 . FIG. 6 is a longitudinal sectional view of a NOx sensor in Example 4 . The longitudinal cross-sectional view of the NOx sensor in the reference example 2 .

  In the present invention, examples of the gas to be measured include an exhaust gas of an internal combustion engine such as an automobile engine.

Further, the temperature control means preferably controls the temperature of the diffused resistor portion above 800 ° C. during use (claim 3).
In this case, ammonia in the gas to be measured is more easily oxidized, and the ammonia sensitivity of the NOx sensor can be further improved.

Further, the temperature control means, that Yusuke a heater for generating heat by energization, and a temperature detection means for detecting directly or indirectly the temperature of the diffusion resistance portion.
Therefore , the temperature control in the diffusion resistance portion can be easily and accurately performed.

In addition , the said temperature detection means can also measure the impedance in the said sensor cell or the said pump cell, and can also detect the temperature of the said diffused resistance part based on the measured value.
In this case, by measuring the impedance of the sensor cell or the pump cell that changes depending on the temperature, the temperature of the diffusion resistance portion that directly or indirectly contacts these can be measured indirectly. In this case, since the sensor electrode or the pump electrode can be used as a part of the temperature detecting means, the NOx sensor can be easily simplified, reduced in size, and reduced in cost. be able to.

Further, the temperature detecting means measures the impedance in the pumping cell, it is also possible to detect the temperature of the diffusion resistance section based on the measured value.
In this case, the temperature of the diffused resistor portion can be measured more accurately. In other words, the pump cell is preferably arranged at a position closer to the measurement gas introduction portion, that is, the diffusion resistance portion than the sensor cell, in order to adjust the oxygen concentration in the measurement gas chamber. From this point of view, the temperature of the diffused resistor portion can be measured with higher accuracy by measuring the impedance in the pump cell that is more easily disposed near the diffused resistor portion.

In the first invention, the temperature detecting means measures an impedance between a pair of measurement electrodes provided in the sensor solid electrolyte body or the pump solid electrolyte body, and based on the measured value, the diffusion resistance section configured to detect a temperature, at least one of the pair of measurement electrodes that are in contact with the diffusion resistance portion.
Therefore, Ki out to measure the impedance at the portion in contact with the diffusion resistance portion in the solid electrolyte body or for the pump solid electrolyte body for the sensor, it is possible to more precise temperature measurement diffusion resistance portion.

Then, in the second invention, the temperature detecting means, ing from the thermocouple in contact disposed on the diffusion resistance portion.
In this case, since the temperature in the diffused resistor portion can be directly measured, highly accurate temperature measurement is possible.

Incidentally, the temperature detecting means measures the resistance of the heater, may be based on the measurement value, to detect the temperature of the diffusion resistance portion.

( Reference Example 1 )
Per according NOx sensor to a reference example of the present invention will be described with reference to FIGS.
As shown in FIG. 1, the NOx sensor 1 of this example includes a measurement gas chamber 2 into which a measurement gas is introduced, a diffusion resistance portion 3 provided in a measurement gas introduction portion in the measurement gas chamber 2, and A sensor cell 4 for detecting the NOx concentration in the gas chamber 2 to be measured, a pump cell 5 for adjusting the oxygen concentration in the gas chamber 2 to be measured, and a temperature control for controlling the temperature of the diffusion resistance unit 3 during use to 700 ° C. or higher. Means 6.

The sensor cell 4 includes a pair of sensor electrodes 421 disposed on the surface opposite to the surface facing the gas chamber 2 to be measured in the sensor solid electrolyte body 41 having oxygen ion conductivity. 422.
The pump cell 5 includes a pair of pump electrodes 521 disposed on an oxygen ion conductive solid electrolyte body 51 for pumping and a surface opposite to the surface facing the gas chamber 2 to be measured in the solid electrolyte body 51 for pumping. 522.

  The sensor solid electrolyte body 41 and the pump solid electrolyte body 51 are made of, for example, yttria-stabilized zirconia. The sensor electrodes 421 and 422 and the pump electrodes 521 and 522 are made of, for example, a cermet material containing a zirconia component in a platinum alloy. In particular, the sensor electrode 421 facing the measured gas chamber 2 is made of a NOx active electrode material made of a Pt—Rh (platinum-rhodium) alloy or the like, and the pump electrode 521 facing the measured gas chamber 2. Is made of a NOx inactive electrode material made of a Pt—Au (platinum-gold) alloy or the like.

The gas chamber 2 to be measured is formed between the sensor solid electrolyte body 41 and the pump solid electrolyte body 51. That is, a spacer layer 121 having an opening is interposed between the sensor solid electrolyte body 41 and the pump solid electrolyte body 51, and the sensor solid electrolyte body 41 is provided in the opening of the spacer layer 121. The gas chamber 2 to be measured is formed between the pump and the solid electrolyte body 51 for a pump.
Further, the diffusion resistance portion 3 is disposed on a part of the spacer layer 121 on the distal end side (left side in FIG. 1) of the gas chamber 2 to be measured. The diffusion resistance unit 3 is made of porous alumina ceramics, and communicates the measured gas chamber 2 and the outside of the NOx sensor 1 at the tip of the NOx sensor 1.

  A first atmosphere chamber 111 for introducing the atmosphere is formed on the surface of the sensor solid electrolyte body 41 opposite to the gas chamber 2 to be measured. One sensor electrode 422 of the sensor cell 4 faces the first atmospheric chamber 111. The first atmospheric chamber 111 is formed between the sensor solid electrolyte body 41 and the spacer layer 122 and the cover layer 123 laminated thereon.

Further, a second atmosphere chamber 112 for introducing the atmosphere is formed on the surface of the pump solid electrolyte body 51 opposite to the gas chamber 2 to be measured. One pump electrode 522 of the pump cell 5 faces the second atmosphere chamber 112.
The ceramic heater 13 is laminated on the pump solid electrolyte body 51 via a spacer layer 124. The ceramic heater 13 is configured by providing, between a pair of heater substrates 131 made of alumina, a heat generating portion 132 that generates heat by energization and a lead portion 133 for supplying current to the heat generating portion 132.
The heat generating part 132 is made of, for example, platinum or a platinum alloy.
The spacer layers 121, 122, and 124 and the cover layer 123 are made of alumina.

  The sensor solid electrolyte body 41 may be provided with a monitor cell for monitoring the oxygen concentration in the measured gas chamber 2. In this case, for example, a monitoring electrode made of a Pt—Au (platinum-gold) alloy inert to NOx is provided on the sensor solid electrolyte body 41 so as to face the gas chamber 2 to be measured (not shown).

The temperature control unit 6 includes the ceramic heater 13 and a temperature detection unit 61 that indirectly detects the temperature of the diffusion resistance unit 3.
The temperature detection means 61 is configured to measure the impedance in the pump cell 5 and detect the temperature of the diffusion resistance unit 3 based on the measured value. That is, the temperature measuring unit 61 measures the impedance between the pair of pump electrodes 521 and 522 in the pump cell 5. The impedance of the pump cell 5 increases as the temperature of the pump cell 5 increases. The pump cell 5 and the diffusion resistance unit 3 are close to each other, and the solid electrolyte body for pump 51 and the diffusion resistance unit 3 constituting the pump cell 5 are in contact with each other, so there is a slight temperature difference. Also, the temperature of the pump cell 5 and the temperature of the diffused resistor 3 are approximated.

That is, since there is a predetermined relationship between the impedance and temperature of the pump cell 5 and between the temperature of the pump cell 5 and the temperature of the diffusion resistance unit 3, the impedance of the pump cell 5 and the temperature of the diffusion resistance unit 3 are There is also a predetermined relationship between the two. By grasping this relationship in advance, the temperature of the diffused resistor portion 3 can be detected based on the measured value of the impedance of the pump cell 5. The relationship between the impedance of the pump cell 5 and the temperature of the diffusion resistance unit 3 can be measured in advance using another NOx sensor of the same specification in which a thermocouple is arranged in the diffusion resistance unit 3.
Here, as described above, since the pump cell 5 is close to the diffusion resistance unit 3, the temperatures of both are approximated. As a result, the temperature of the diffusion resistance unit 3 is accurately estimated by using the impedance in the pump cell 5. be able to.

  The temperature control means 6 controls the temperature of the diffused resistor section 3 detected in this way so as to maintain a predetermined temperature of 700 ° C. or higher (for example, 750 ° C. ± 20 ° C.). That is, the temperature control unit 6 includes the temperature detection unit 61 and the ceramic heater 13, and controls energization and non-energization of the ceramic heater 13 (heating element 132) based on the measurement result of the temperature detection unit 61. A control signal is sent to the heater power supply 134.

As described above, the concentration of NOx and ammonia in the gas to be measured is measured by the sensor cell 4 while maintaining the temperature of the diffusion resistance unit 3 at a predetermined temperature of 700 ° C. or higher.
That is, the gas to be measured is introduced into the gas chamber 2 to be measured through the diffusion resistance unit 3 maintained at a high temperature of 700 ° C. or higher. At this time, ammonia contained in the gas to be measured is oxidized in the diffusion resistance unit 3 to generate NOx. Therefore, in the measurement gas introduced into the measurement gas chamber 2, NOx originally contained in the measurement gas and NOx derived from ammonia may exist.

The gas to be measured may contain oxygen, but if this oxygen concentration changes, the output in the sensor cell 4 will fluctuate, so that the oxygen concentration in the gas chamber 2 to be measured is kept constant by the pump cell 5. Oxygen pumping is performed between the measured gas chamber 2 and the second atmospheric chamber 112. Here, the oxygen concentration is preferably kept at 0%.
The pump cell 5 is formed at a position closer to the diffusion resistance portion 3, that is, the introduction portion of the measured gas into the measured gas chamber 2 than the sensor cell 4. Therefore, the gas to be measured in a state where the oxygen concentration is adjusted to a predetermined value by the pump cell 5 is guided to the sensor cell 4.

  Then, in the sensor electrode 421 of the sensor cell 4, NOx in the measurement gas is decomposed to generate oxygen ions. A predetermined voltage is applied between the pair of sensor electrodes 421 and 422, and oxygen ions generated in the sensor electrode 421 are conducted through the sensor solid electrolyte body 41 and move to the other sensor electrode 422. By doing so, a sensor output based on the NOx concentration is obtained.

As shown in FIG. 2, the NOx sensor 1 is used in a urea SCR (Selective Catalytic Reduction) system 7 for purifying NOx contained in vehicle exhaust gas.
In the urea SCR system 7, a selective reduction type NOx purification catalyst (SCR catalyst 72) is provided downstream of the particulate filter 71 provided in the exhaust system for removing PM (particulate matter) in the exhaust gas. ), Urea is added into the exhaust system. As a result, ammonia generated by the decomposition of the added urea selectively reduces NOx in the exhaust gas in the SCR catalyst 72 to purify the exhaust gas. In terms of apparatus, the urea water stored in the urea tank 74 is sent to the injector 76 by the pump 75, and the urea water is injected into the exhaust gas from the injector 76 on the upstream side of the SCR catalyst 72.

In such a system, if the amount of urea added is insufficient, NOx will not be sufficiently purified, and if the amount of urea added is excessive, ammonia will be discharged together with the exhaust gas. An oxidation catalyst 73 for oxidizing and detoxifying ammonia is disposed on the downstream side of the SCR catalyst 72. However, since the amount of ammonia that can be rendered harmless by the oxidation catalyst 73 is limited, it is necessary to prevent excess ammonia from being discharged from the SCR catalyst 72.
Therefore, in order to control the addition amount of urea to an appropriate amount, the NOx sensor 1 of this example is disposed on the downstream side of the SCR catalyst 72, and the ammonia in the exhaust gas after the NOx purification is performed with ammonia. Concentration is measured in addition to NOx concentration.

That is, the exhaust gas (gas to be measured) after purification of NOx in the urea SCR system 7 is introduced into the gas chamber 2 to be measured of the NOx sensor 1 through the diffusion resistance unit 3 as described above.
Here, if the amount of urea added is appropriate, ammonia will purify NOx without any excess, so both the ammonia concentration and the NOx concentration become substantially zero.
However, when the amount of urea added is too small, NOx cannot be completely purified, and NOx remains in the exhaust gas (measured gas). This NOx concentration is measured in the sensor cell 4 of the NOx sensor 1. In this case, feedback is made so as to increase the amount of urea added.

On the other hand, when the amount of urea added is too large, substantially the entire amount of NOx in the exhaust gas (measured gas) can be purified, but excess ammonia that could not react with NOx remains. When this ammonia passes through the diffusion resistance part 3 maintained at a high temperature of 700 ° C. or higher in the NOx sensor 1, it is oxidized to generate NOx. The measurement gas containing NOx is measured in the sensor cell 4. That is, it can be said that most of the NOx concentration measured as the sensor output at this time is the concentration of ammonia-derived NOx.
Then, feedback is performed so as to reduce the amount of urea added based on the ammonia concentration.

Note that the NOx concentration measured in the NOx sensor 1 can be said to be the total amount of NOx (nitrogen oxide) and NH ₃ (ammonia) in the gas to be measured before being introduced into the NOx sensor 1. Then, it may be fed back to the adjustment of the urea addition amount so that the total amount is minimized.

That is, the NOx concentration and the ammonia (NH ₃ ) concentration change as shown in FIG. 3 depending on the amount of urea added. That is, when the urea addition amount is small, the NOx concentration is high, and when the urea addition amount is large, the ammonia concentration is high. When the urea addition amount is an appropriate amount x, the total concentration of NOx and ammonia is minimized. What is necessary is just to feed back adjusting the urea addition amount aiming at this appropriate amount x.
Therefore, it is not always necessary to distinguish between the NOx concentration and the ammonia concentration in the NOx sensor 1.

  However, whether it is the concentration of NOx in the gas to be measured before being introduced into the NOx sensor 1 or the concentration of NOx derived from ammonia is identified by the increase or decrease of the sensor output when the urea addition amount is increased. Is possible. That is, when the sensor output increases when the urea addition amount is increased, the measured NOx concentration is derived from ammonia, and conversely, when the sensor output decreases when the urea addition amount is increased, the measured NOx concentration. Is derived from NOx in the gas to be measured before being introduced into the NOx sensor 1.

Next, the effect of the reference example 1 will be described.
The NOx sensor 1 has temperature control means 6 for controlling the temperature of the diffusion resistance unit 3 during use to 700 ° C. or higher. Therefore, when the gas to be measured containing ammonia passes through the diffusion resistance portion 3, the reaction with oxygen contained in the gas to be measured is promoted to sufficiently oxidize the ammonia, so that NOx derived from ammonia is reduced. Fully generated.
As a result, this NOx is detected in the sensor cell 4, and a sensor output corresponding to the NOx concentration, that is, a sensor output corresponding to the ammonia concentration in the gas to be measured can be obtained accurately.

  In addition, since ammonia is oxidized in the diffusion resistance portion 3 that is the introduction portion of the gas to be measured into the gas chamber 2 to be measured, the oxygen concentration in the gas chamber 2 to be measured can be kept low. . In other words, even if the oxygen concentration in the gas chamber 2 to be measured is not increased, ammonia can be oxidized if oxygen is present in the diffusion resistance portion 3 that is the introduction portion. Therefore, it becomes easy to suppress the offset error of the sensor output due to the oxygen concentration.

Moreover, since the temperature control means 6 has the ceramic heater 13 and the temperature detection means 61, the temperature control in the diffusion resistance part 3 can be performed easily and accurately.
Moreover, the temperature detection means 61 measures the impedance in the pump cell 5, and detects the temperature of the diffusion resistance part 3 based on the measured value. Therefore, by measuring the impedance of the pump cell 5 that varies depending on the temperature, it is possible to indirectly measure the temperature of the diffusion resistance unit 3 that directly or indirectly contacts these. In this case, since the pump electrodes 521 and 522 can be used as a part of the temperature detection means 61, the NOx sensor 1 can be easily simplified, downsized, and reduced in cost. it can.

  In particular, since the temperature detecting means 61 measures the impedance in the pump cell 5 instead of the sensor cell 4, the temperature of the diffusion resistance unit 3 can be measured more accurately. That is, the pump cell 5 is arranged closer to the measurement gas introduction part, that is, the diffusion resistance part 3 than the sensor cell 4 in order to adjust the oxygen concentration in the measurement gas chamber 2. Therefore, the temperature of the diffusion resistance unit 3 can be measured with higher accuracy.

As described above, according to the present reference example 1 , it is possible to provide a NOx sensor having excellent measurement accuracy of the ammonia concentration in the gas to be measured.

( Experimental example )
In this example, as shown in FIG. 4, the relationship between the temperature of the diffusion resistance unit 3 and the sensitivity of the NOx sensor 1 to ammonia is examined.
The sensitivity of the NOx sensor 1 to ammonia was evaluated by the following index “NH ₃ / NO sensitivity ratio”. The “NH ₃ / NO sensitivity ratio” is a ratio of sensor outputs obtained when two types of gas to be measured containing NO and ammonia (NH ₃ ) of the same concentration are supplied to the NOx sensor 1. . This ratio has the same value as the ratio of ammonia oxidized in the diffusion resistance portion 3. That is, for example, when all ammonia is oxidized in the diffusion resistance unit 3 to generate NO, the “NH ₃ / NO sensitivity ratio” is 100%, and when half is oxidized to generate NO, “NH ₃ / NO The “sensitivity ratio” is 50%, and 0% when NO is not generated without oxidation.

It was examined how this “NH ₃ / NO sensitivity ratio” changes depending on the temperature of the diffusion resistance section 3.
Specifically, a temperature detection thermocouple is embedded in the diffusion resistance portion 3 of the NOx sensor 1 of Reference Example 1 , and the temperature of the diffusion resistance portion 3 is changed by changing the input power to the ceramic heater 13 of the NOx sensor 1. The sensor output (current flowing through the sensor cell 4) for NO gas and NH ₃ gas with respect to the temperature of the diffusion resistance unit 3 was measured.
First, a mixed gas of NO gas concentration 100 ppm, O ₂ concentration 5% and balance gas N ₂ was prepared, and the sensor output was measured using this mixed gas (NO mixed gas) as a measurement gas.
Similarly, a mixed gas of NH ₃ gas concentration of 100 ppm, O ₂ concentration of 5% and balance gas N ₂ was prepared, and the sensor output was measured using this mixed gas (NH ₃ mixed gas) as a measurement gas.

These measurements were performed by changing the temperature of the diffused resistor portion 3 in the range of 500 to 900 ° C. NH ₃ is the ratio of the NH ₃ sensitivity to the NO sensitivity at an O ₂ concentration of 5% based on the sensor output for the NO mixed gas and the NH ₃ mixed gas at each temperature of the diffusion resistance unit 3 measured in this way. The relationship between the ₃ / NO sensitivity ratio (%) and the temperature of the diffusion resistance portion 3 was determined. The result is shown in FIG. FIG. 4 shows data when the O ₂ concentration is 5%. However, even when the O ₂ concentration in the gas to be measured is changed in the range of 1 to 20%, the change has almost no effect. It was.

In FIG. 4, the actual measurement values are plotted with “□”, and an approximate curve L connecting them is drawn. As can be seen from the figure, the higher the temperature of the diffused resistor portion 3, the higher the NH ₃ / NO sensitivity ratio. When the temperature of the diffused resistor portion 3 is 700 ° C. or higher, the NH ₃ / NO sensitivity ratio exceeds 50% and rapidly increases. Furthermore, when the temperature of the diffused resistor portion 3 was 800 ° C. or higher, the NH ₃ / NO sensitivity ratio showed a high value of 85% or higher and was almost saturated.

From this result, by controlling the temperature of the diffusion resistance unit 3 to 700 ° C. or higher, it is possible to oxidize more than half of ammonia in the diffusion resistance unit 3 to make NOx, so that the NOx sensor 1 has sufficient sensitivity to ammonia. It can be seen that can be obtained.
Furthermore, by controlling the temperature of the diffusion resistance unit 3 to 800 ° C. or more, most of the ammonia can be oxidized to NOx in the diffusion resistance unit 3 and the sensitivity of the NOx sensor to ammonia can be further increased. I understand. Therefore, it can be said that it is more desirable from the viewpoint of ammonia detection accuracy to control the temperature of the diffusion resistance unit 3 during use to a predetermined temperature of 800 ° C. or higher (for example, 850 ° C. ± 20 ° C.).

(Example 1 )
In this example, as shown in FIG. 5, the configuration of the temperature detecting means 61 is changed from that of the first embodiment. That is, the temperature detection means 61 is configured to measure the impedance between the pair of measurement electrodes 611 and 612 provided on the solid electrolyte body 51 for the pump, and detect the temperature of the diffusion resistance unit 3 based on the measured value. One measurement electrode 611 is in contact with the diffused resistor portion 3.
The other measurement electrode 612 is an electrode common to the pump electrode 521 facing the gas chamber 2 to be measured.
Others are the same as in Reference Example 1 .

In the case of this example, since the impedance at the portion in contact with the diffusion resistance portion 3 in the solid electrolyte body 51 for pump can be measured, the temperature of the diffusion resistance portion 3 can be measured with higher accuracy.
In addition, the same effects as those of Reference Example 1 are obtained.

(Example 2 )
In this example, as shown in FIG. 6, the temperature detection means 61 is configured by a temperature detection cell 62 including a pair of measurement electrodes 621 and 622 and a pump solid electrolyte body 51. One measurement electrode 621 is formed in contact with the diffused resistor portion 3, and the other measurement electrode 622 is formed on the opposite side of the one measurement electrode 621 with the pump solid electrolyte body 51 interposed therebetween.
In such a configuration, the temperature of the diffusion resistance unit 3 is detected by measuring the impedance of the temperature detection cell 62.

Also in this example, the relationship between the impedance of the temperature detection cell 62 and the temperature of the diffusion resistance unit 3 is measured in advance using another element of the same specification in which a thermocouple for temperature detection is arranged in the diffusion resistance unit 3. deep. Then, the temperature of the diffusion resistance unit 3 can be estimated from the impedance of the temperature detection cell 4, and the ceramic heater 13 can be controlled so that the diffusion resistance unit 3 has a desired temperature.
Others are the same as in Reference Example 1 .

In the case of this example, since the temperature detection cell 4 is formed in contact with the diffusion resistance unit 3, the temperature of the diffusion resistance unit 3 can be controlled more accurately.
In addition, the same effects as those of Reference Example 1 are obtained.

(Example 3 )
In this example, as shown in FIG. 7, the temperature detecting means 61 is configured by a temperature detecting cell 63 including a pair of measuring electrodes 631 and 632 and a sensor solid electrolyte body 41. One measurement electrode 631 is formed in the vicinity of the diffused resistor portion 3, and the other measurement electrode 632 is formed on the opposite side of the one measurement electrode 631 across the sensor solid electrolyte body 41.

Further, in the NOx sensor 1 of the present example, oxygen composed of the sensor electrode 422 exposed to the atmosphere as the reference oxygen concentration gas, the measurement electrode 632 exposed to the gas to be measured, and the sensor solid electrolyte body 41. A sensor cell 14 is configured. An electromotive force generated by the oxygen concentration cell is generated between the sensor electrode 422 and the measurement electrode 632 based on the Nernst equation, whereby the oxygen concentration in the gas to be measured can be detected.
Others have the same configuration as that of the second embodiment, and have the same effects.

(Example 4 )
In this example, as shown in FIG. 8, the temperature detecting means 61 is an example configured by a thermocouple 64 arranged in contact with the diffused resistor portion 3.
That is, the thermocouple 64 is arranged in the diffusion resistance unit 3, thereby directly measuring the temperature of the diffusion resistance unit 3 and controlling the ceramic heater 13 so that the diffusion resistance unit 3 has a desired temperature. For the thermocouple 64, for example, a combination of Pt and Pt-Rh can be used.
Others are the same as in Reference Example 1 .

In the case of this example, since the temperature in the diffusion resistance unit 3 can be directly measured by the thermocouple 64, it is possible to measure the temperature with high accuracy and to control the temperature of the diffusion resistance unit 3 more accurately.
In addition, the same effects as those of Reference Example 1 are obtained.

( Reference Example 2 )
In this example, as shown in FIG. 9, the temperature detection unit 61 is configured by a heater resistance detection unit that detects the temperature of the diffusion resistance unit 3 by measuring the heater resistance of the ceramic heater 13.
Here, the heater resistance is a resistance value of the heating element 132 in the ceramic heater 13.
The temperature relationship between the heater resistance and the diffusion resistance unit 3 is measured in advance using another element of the same specification in which a thermocouple for temperature detection is arranged in the diffusion resistance unit 3. Then, by measuring the heater resistance in each NOx sensor 1, the temperature of the diffusion resistance unit 3 can be estimated from the heater resistance, and the temperature of the diffusion resistance unit 3 can be grasped. Then, the ceramic heater 13 can be controlled so that the temperature of the diffusion resistance unit 3 becomes a desired temperature.
Others are the same as in Reference Example 1 .

Moreover , it has the same effect as Reference Example 1 .

DESCRIPTION OF SYMBOLS 1 NOx sensor 2 Gas chamber to be measured 3 Diffusion resistance part 4 Sensor cell 41 Sensor solid electrolyte body 421, 422 Sensor electrode 5 Pump cell 51 Pump solid electrolyte body 521, 522 Pump electrode 6 Temperature control means

Claims (3)

A gas chamber to be measured into which the gas to be measured is introduced;
A diffusion resistance portion provided in the introduction portion of the measurement gas in the measurement gas chamber;
An oxygen ion conductive solid electrolyte body for a sensor, and a pair of sensor electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for the sensor, A sensor cell for detecting the NOx concentration in the gas chamber;
An oxygen ion conductive solid electrolyte body for a pump, and a pair of pump electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for pump, A pump cell for adjusting the oxygen concentration in the gas chamber;
The temperature of the diffusion resistance portions have a temperature control means for controlling the above 700 ° C. during use,
The temperature control means includes a heater that generates heat when energized, and a temperature detection means that directly or indirectly detects the temperature of the diffusion resistance portion.
The temperature detecting means measures an impedance between a pair of measurement electrodes provided on the sensor solid electrolyte body or the pump solid electrolyte body, and detects the temperature of the diffusion resistance section based on the measured value. A NOx sensor comprising: a pair of measurement electrodes, at least one of which is in contact with the diffusion resistance portion .   A gas chamber to be measured into which the gas to be measured is introduced;
  A diffusion resistance portion provided in the introduction portion of the measurement gas in the measurement gas chamber;
  An oxygen ion conductive solid electrolyte body for a sensor, and a pair of sensor electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for the sensor, A sensor cell for detecting the NOx concentration in the gas chamber;
  An oxygen ion conductive solid electrolyte body for a pump, and a pair of pump electrodes disposed on a surface opposite to the surface facing the gas chamber to be measured in the solid electrolyte body for pump, A pump cell for adjusting the oxygen concentration in the gas chamber;
  Temperature control means for controlling the temperature of the diffused resistor portion in use to 700 ° C. or higher,
  The temperature control means includes a heater that generates heat when energized, and a temperature detection means that directly or indirectly detects the temperature of the diffusion resistance portion.
  The NOx sensor, wherein the temperature detecting means comprises a thermocouple disposed in contact with the diffusion resistance portion. 3. The NOx sensor according to claim 1 or 2, wherein the temperature control means controls the temperature of the diffused resistor portion during use to 800 ° C. or higher.

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