JP3152699B2 - Limit current type gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiting current type gas sensor for measuring a gas concentration depending on a magnitude of a limiting current.

[0002]

2. Description of the Related Art When a gas flow is applied to a limiting current type sensor body including a stabilized zirconia plate, a platinum negative electrode and a positive electrode, and a ceramic heater, the temperature of the stabilized zirconia plate decreases, and the temperature of the gas to be measured decreases. Even if there is no change in the oxygen concentration, the oxygen ion conductivity decreases, the current value decreases, and accurate gas concentration cannot be measured. For this reason, conventionally, a limiting current type gas sensor in which the sensor main body is surrounded by a tubular (closed tip) metal mesh filter and the influence of gas flow is reduced without significantly reducing responsiveness has been proposed.

[0003]

However, this limiting current gas sensor has the following disadvantages. (A) The effect of preventing the influence of the gas flow is inferior, and as shown in FIG. 9, even if the oxygen concentration in the gas to be measured does not change, the current value increases when the gas flow velocity becomes 0.8 m / S or more. Decrease
Accurate gas concentration cannot be measured. (I) When the metal mesh filter is doubled, the effect of preventing the influence of the gas flow is increased (up to about 2 m / S), but the size of the limiting current type gas sensor is increased and the material cost is greatly increased. An object of the present invention is to provide a limiting current type gas sensor which can prevent the influence of gas flow at low cost.

[0004]

In order to solve the above-mentioned problems,
The present invention relates to a rod-shaped probe, a solid electrolyte plate having good conductivity of oxygen ions, and a voltage applied to the solid electrolyte plate, which is disposed in close contact with the solid electrolyte plate and provides a limit current having a size corresponding to the oxygen concentration in the gas to be measured. A porous negative electrode and a positive electrode to be provided, an electric heater for heating the solid electrolyte plate located at a detection portion of these electrodes, and gas supply restricting means for restricting diffusion of the gas to be measured with respect to the negative electrode. A sensor main body protruding from the distal end surface of the probe, a sensor body surrounding the sensor main body, an open end fixed to the distal end of the probe, and a small vent for ventilation formed at a position away from the detection section; And a porous filter having an open end fixed to the tip of the probe and surrounding the heat-resistant protector.

[0005]

[Function] A limiting current type gas sensor is arranged in a gas to be measured,
When the electric heater is energized, the oxygen ion conductivity of the solid electrolyte plate located at the detection part of the cathode and the anode increases. The porous filter makes the flow rate of the gas to be measured entering the limiting current type gas sensor substantially zero, captures fine dust in the gas to be measured, and prevents the intrusion. The gas to be measured that has entered the porous filter enters the inside of the protector through the small hole for ventilation, and reaches the sensor body. Here, since the small holes for ventilation of the heat-resistant protector are formed at positions away from the detection part of the negative electrode and the positive electrode, when the gas to be measured enters the inside of the protector, the temperature drop near the detection part almost occurs. Do not let. Oxygen in the gas to be measured diffuses into the detection section of the negative electrode, where it is ionized into oxygen ions,
Pumped to the positive electrode according to the applied voltage. When the applied voltage is set to a certain value, the amount of oxygen diffusion into the detection unit of the negative electrode reaches a plateau at a value determined according to the oxygen concentration in the gas to be measured, due to the gas supply restricting means. The current value does not increase (limit current). The oxygen concentration in the gas to be measured is determined based on the magnitude of the limiting current.

[0006]

According to the present invention, even if the limiting current type gas sensor is exposed to the gas flow of the gas to be measured, the temperature in the vicinity of the detecting portion hardly drops. For this reason, when the oxygen concentration in the gas to be measured does not change, the oxygen ion conductivity of the solid electrolyte plate located at the detection part of the cathode and the anode also keeps a substantially constant value, and the limiting current value and the gas flow are zero. Is almost the same as in the case of Therefore, the limiting current gas sensor can accurately detect the oxygen concentration in the measured gas regardless of the flow rate of the measured gas. Since only a small hole for ventilation is formed in the cylindrical heat-resistant protector with a closed end, the cost for materials and processing is reduced, and the probe can be easily fixed. For this reason, the manufacturing cost of the limiting current gas sensor is not significantly increased.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIGS. As shown in FIGS. 1 and 2, the limiting current type gas sensor A includes a round bar-shaped probe 1 and a tip surface 11 of the probe 1.
Sensor main body 2 inserted in a protruding state to sensor body 2 and sensor main body 2
And a sintered metal filter 4 surrounding the protector 3.

The probe 1 is formed of a hollow metal, and has disk-shaped fitting portions 12 and 13 for fitting the protector 3 and the sintered metal filter 4 at the tip.

The sensor body 2 is, as shown in FIGS.
It has a stabilized zirconia plate 21, a porous negative electrode 22 and a positive electrode 23, a ceramic heater 24 for heating the stabilized zirconia plate 21 located at the detecting portions 221 and 231 of these electrodes, and a gas supply restricting means 25.

The stabilized zirconia plate 21 shown in FIG.
A solid electrolyte having good conductivity of oxygen ions obtained by adding yttrium oxide as a stabilizer to zirconium oxide and forming a solid solution. In this embodiment, the length is 5 mm, the width is 7 mm, and the thickness is 0.3 m.
The size of m is used.

As shown in FIGS. 3 and 4, the negative electrode 22 and the positive electrode 23 are porous platinum electrodes and are juxtaposed on the upper surface of the stabilized zirconia plate 21 in close contact with the drawing. In the present embodiment, the thickness of the negative electrode 22 and the positive electrode 23 is 20 μm, and the size of the detection units 221 and 231 is 2.
5 mm, and the size of the connection portions 222 and 232 is 1 width.
mm and a length of 2 mm.

The ceramic heater 24 shown in FIG. 3 has a structure in which tungsten is buried in a ceramic mainly composed of alumina.
Locally heat to 0 ° C to 700 ° C. Incidentally, reference numeral 241 denotes an exposed portion for energization, and 242 denotes a vent for improving the heating efficiency of the stabilized zirconia plate 21.

The gas supply restricting means 25 includes an alumina porous layer 251 covering a part of the connecting portion 222 of the cathode 22 and the detecting portion 221, and an alumina porous layer 251 so that the gas to be measured does not directly enter the detecting portion 221. Is formed based on the glaze layer 252 covering the periphery of the detection unit 221 including the

Here, a method for manufacturing the sensor main body 2 will be described. On the green sheet that becomes the stabilized zirconia plate 21 after firing, a platinum paste that becomes the negative electrode 22 and the positive electrode 23 after firing is printed and integrally fired at 1500 ° C. On a stabilized zirconia plate 21 on which the negative electrode 22 and the positive electrode 23 are formed, a paste obtained by mixing glass with alumina powder (to become an alumina porous layer 251 after firing) is partly connected to the connecting portion 222 of the negative electrode 22. Then, a coating is applied so as to cover the detection unit 221, and further, glass powder (which becomes the glaze layer 252 after firing) is scattered thereon and baked at 850 ° C. to 900 ° C. to manufacture a sensor body chip. Vent 24 after firing
A heater pattern 245 is printed with a tungsten paste on the upper surface of a 96% by weight alumina green sheet 244 having a window 243 formed therein. A similar green sheet 249 on which a paste made of ruthenium oxide serving as the electrodes 247 and 248 is printed is covered, and this is fired and integrated to manufacture the ceramic heater 24 (see FIG. 5). Sensor chip and ceramic heater 24
And about 800 ° C. using a sealing glass (about 800 ° C.).
° C). The sensor electrodes 247 and 248 and the connection section 22
2, 232 and brazing.

The protector 3 is located at a side position spaced a predetermined distance (in the present embodiment, 2 mm from the periphery of the hole) rearward from a rear end (a line PP 'in the drawing) of a portion requiring heating at a constant temperature. A stainless-steel bottomed cylindrical body having four small holes for ventilation with a diameter of 1 mm at intervals of 90 °;
The open end 31 is fixed to the fitting portion 12 of the probe 1 with an inorganic adhesive.

Metal sintered filter 4 (pore diameter 70 μm)
Has a bottomed cylindrical shape surrounding the protector 3, and has an open end 41 inserted into the fitting portion 13 of the probe 1 and fixed.

Next, the operation of the limiting current type gas sensor A will be described. The limiting current type gas sensor A is arranged in the gas to be measured, and a current is supplied to the ceramic heater 24 to apply a voltage between the positive electrode 23 and the negative electrode 22. Detection unit 221 of cathode 22
The oxygen inside is ionized into oxygen ions, and the oxygen in the gas to be measured is pumped from the negative electrode 22 to the positive electrode 23 according to the applied voltage V. At this time, the detecting unit 221
Only the stabilized zirconia plate 21 is locally heated, and the connection 2
Since the stabilized zirconia plate 22 of FIG. 22 is not heated sufficiently to exhibit oxygen ion conductivity, oxygen is exposed from the exposed portion of the connection portion 222 which is not covered by the glaze layer 252.
And diffuses into the detection unit 221 covered with the light. Here, the output current I flowing between the positive electrode 23 and the negative electrode 22 changes as shown in FIG. When the applied voltage V is between the voltage values V1 and V2, the amount of oxygen diffusion into the detection unit 221 is controlled by the gas supply restricting means 25 of the negative electrode 22, and is limited according to the oxygen concentration in the gas to be measured. The amount of diffusion is limited, and the current value is also limited accordingly, resulting in a diffusion-limited current value _IL1 .
Becomes the flat portion F1. The applied voltage V is the diffusion limiting current value I _L1
Is higher than the obtained voltage value V2, the water vapor (moisture) in the gas to be measured is decomposed, and oxygen ions generated by the decomposition are pumped to the positive electrode 23, so that the water vapor is also covered by the glaze layer 252. The non-exposed portion of the connection portion 222 diffuses into the detection portion 221 covered with the glaze layer 252, and the current value increases according to the diffusion amount. When the applied voltage V is further increased to a voltage value V3 to V4, the output current value I
Is further increased according to the water vapor concentration, but the amount of diffusion of water vapor is restricted by the gas supply restricting means 25 of the negative electrode 22, and the current value is also restricted accordingly, resulting in a diffusion limited current value _IL2 corresponding to the water vapor concentration, The second flat portion F2 is shown. Here, if a voltage of V1 to V2 is applied between the positive electrode 23 and the negative electrode 22, and the diffusion limiting current value _IL1 is measured, the oxygen concentration can be detected from the magnitude of the diffusion limiting current value _IL1 . Also, V3 ~
By measuring the diffusion limiting current value I _L2 by applying the voltage V4, the humidity can be detected from the magnitude of the diffusion limiting current value I _L2 .

Next, the relationship between the vent hole 32 and the limit current and response will be described. The inventor manufactured the protector 3 in which the drilling position, the number of holes n, and the hole diameter d of the small holes for ventilation 32 were variously changed, and the small holes for ventilation 32 shown below were manufactured.
The optimal range was found. (1) The rear end of the portion requiring heating at a constant temperature (P-
When the distance from the P ′ line) to the periphery of the hole is less than 2.0 mm, even when the number of holes n of the small holes for ventilation 32 is small and the hole diameter d is small, when the gas to be measured enters through the small holes for ventilation 32,
The temperature of the stabilized zirconia plate 21 at the positions of the detection units 221 and 231 tends to decrease. (2) Total opening area of ventilation small holes 32 ｛n × π × (d /
2) From the area (L × πD) of the outer surface occupied by the effective length L of the protector 3, ² ｝ is calculated as the rear end (PP ′ line in FIG.
From the Q 'line), the length m, which is 2 mm each before and after in the axial direction
(10 mm in this embodiment) occupies the area of the outer surface (mx
πD) Area of outer surface of protector 3 minus π × D
× (L−m)｝, where R is 4.5 ×
If it is 10 ^-2 , the diffusion limiting current value is hardly reduced for gas flows up to a flow velocity of 3 m / S (see FIG. 7). Note that R
= 4.5 × 10 ⁻² is the hole diameter d = 3 m in this embodiment.
This is equivalent to three small holes 32 for ventilation. (3) If R ≧ 4 × 10 ⁻⁴ , as shown in FIG. Note that R = 4 × 10
^In the present embodiment, ^-4 corresponds to a case where one small ventilation hole 32 having a hole diameter d = 0.5 mm is formed.

The operation and effect of the limiting current type gas sensor A will be described below. (A) If the small holes for ventilation 32 satisfy all of the above conditions (1) to (3), responsiveness that cannot be practically used can be obtained, and diffusion is limited for gas flows up to a flow velocity of 3 m / S. It has been found that an effect of not lowering the current is obtained. (A) Since it is not necessary to reduce the pore diameter of the metal sintered filter 4, clogging of the metal sintered filter 4 hardly occurs. (C) Since the protector 3 is simply a bottomed cylindrical body, there is no need to increase the diameter or length of the metal sintered filter 4 to be loosely fitted with the protector 3, and the physical size of the limiting current type gas sensor A is eliminated. Does not lead to an increase in In addition, since the protector 3 made of stainless steel is merely added, the cost is slightly increased.

The present invention includes the following embodiments in addition to the above embodiment. a. In the above embodiment, the small holes 32 for ventilation are connected to the protector 3.
However, it may be formed on the front side of the protector 3. b. In the above embodiment, the metal sintered filter 4 is used as the porous filter, but a polymer filter such as glass or Teflon, a metal mesh filter, or the like may be used. c. The limiting current type gas sensor A may measure the CO ₂ and NO ₂ concentrations in addition to the oxygen concentration and the water vapor amount. d. The negative electrode and the positive electrode may be embedded in a solid electrolyte plate. e. As shown in FIG. 10, the sensor body 2 of the limiting current type gas sensor has a minute hole 261 for limiting the diffusion of the gas to be measured.
(Gas supply restricting means) may be provided on the negative electrode 22 side, and may include a housing 260 having a cavity 262.

[Brief description of the drawings]

FIG. 1 is an assembly diagram of a limiting current type gas sensor according to an embodiment of the present invention.

FIG. 2 is a structural explanatory view of the limiting current type gas sensor.

FIG. 3 is a structural explanatory view of a sensor main body of the limiting current type gas sensor.

FIG. 4 is a sectional view of the sensor main body.

FIG. 5 is an explanatory diagram for explaining a method of manufacturing the ceramic heater of the limiting current type gas sensor.

FIG. 6 is a graph illustrating the operation of the limiting current type gas sensor.

FIG. 7 is a graph showing the effect of the limiting current gas sensor to prevent the influence of gas flow.

FIG. 8 is a graph showing the response of the limiting current type gas sensor.

FIG. 9 is a graph showing the effect of a conventional limiting current gas sensor to prevent the influence of gas flow.

FIG. 10 is a structural explanatory view of another embodiment of the sensor main body of the limiting current type gas sensor according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Probe 2 Sensor main body 3 Protector (heat-resistant protector) 4 Sintered metal filter (porous filter) 11 Tip surface 21 Stabilized zirconia plate (solid electrolyte plate) 22 Negative electrode 23 Positive electrode 24 Ceramic heater (electrothermal heater) 25 Gas supply limiting means 31 and 41 open end eyelet for 32 vent 221, 231 detecting section A limiting current type gas sensor I _L1, I _L2 diffusion limiting current value (limit current)

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-57-72163 (JP, A) JP-A-61-25051 (JP, A) JP-A-1-305350 (JP, A) JP-A-3-305 138559 (JP, A) Fully open sho 59-194059 (JP, U) Fully open sho 60-107757 (JP, U) Really open sho 62-91237 (JP, U) (58) Fields investigated (Int. ⁷ , DB name) G01N 27/41

Claims (1)

(57) [Claims] 1. A rod-shaped probe, a solid electrolyte plate having good oxygen ion conductivity, and a voltage which is disposed in close contact with the solid electrolyte plate and which can obtain a limit current having a magnitude corresponding to the oxygen concentration in the gas to be measured. A porous negative electrode and a positive electrode to be applied, an electric heater for heating the solid electrolyte plate located at a detection section of these electrodes, and gas supply restricting means for restricting diffusion of a gas to be measured with respect to the negative electrode. A sensor main body protruding from the distal end surface of the probe, and surrounding the sensor main body, an open end is fixed to the distal end of the probe, and a small vent for ventilation is formed at a position distant from the detection unit. A limiting current gas sensor comprising: a tubular heat-resistant protector having a closed end; and a porous filter having an open end fixed to the tip of the probe and surrounding the heat-resistant protector.