KR20220109988A - Limiting current type oxygen sensor - Google Patents

Abstract

The present invention provides an oxygen sensor of a limiting current type in which an influence by moisture is suppressed. According to the present invention, the oxygen sensor of a limiting current type comprises: a solid electrolyte capable of pumping oxygen ions; an anode electrode and a cathode electrode formed on an upper surface and a lower surface of the solid electrolyte; a diffusion barrier provided on the cathode electrode side, and including a gas diffusion hole forming a diffusion space on a lower portion of the cathode electrode and introducing gas of which the oxygen concentration is to be measured to the diffusion space side; and a control unit controlling a voltage (hereafter, pumping voltage) applied between the cathode electrode and the anode electrode. The control unit allows a pumping voltage of a level for suppressing steam decomposition in the gas at the cathode electrode to be applied.

Description

Limiting current type oxygen sensor

The present invention relates to a limiting current type oxygen sensor used for measuring oxygen concentration.

Oxygen sensors are widely used for the purpose of measuring air pollution, reducing fuel and improving thermal efficiency in combustion devices such as automobiles and boilers, reducing harmful components of exhaust gas, and controlling oxygen concentration in manufacturing facilities that require oxygen.

The oxygen sensor has been developed into a limiting current type, an oxide semiconductor type, a liquid electrolyte type, a voltage measuring type, and the like. Among them, the limiting current type oxygen sensor can measure oxygen concentration in a wider range compared to the oxide semiconductor type that observes resistance change due to oxygen adsorption, and does not cause electrolyte loss due to evaporation, a problem of liquid electrolyte type oxygen sensor. There are advantages. In addition, compared to the voltage measurement type oxygen sensor, a reference electrode is unnecessary and it has the advantage that it can be used at a relatively low temperature. In addition, it is widely used among various types of oxygen sensors because of its simple structure and small device-type sensor fabrication, as well as low price and excellent reproducibility in mass production.

Conventionally, a limiting current type oxygen sensor using a solid electrolyte made of Yttria Stabilized Zirconia (YSZ) with yttria (Y ₂ O ₃ ) as an additive has been known. 1 is a schematic diagram showing the structure of a limiting current type oxygen sensor widely used in the prior art.

As shown in FIG. 1 , the conventional limiting current type oxygen sensor 10 has a structure in which electrodes 13 and 15 are formed at both ends of a solid electrolyte 11 and a voltage (hereinafter, a pumping voltage) is applied.

The solid electrolyte 11 has conductivity with respect to oxygen ions (O ² -). When a voltage V is applied to both electrodes 13 and 15 , oxygen around the cathode electrode 15 acquires electrons from the cathode electrode 15 and is reduced to oxygen ions. In addition, oxygen ions move toward the anode electrode 13 through the solid electrolyte 11 , give electrons to the anode electrode 13 , and are oxidized into oxygen molecules. The above process is called electrochemical ion pumping, and the movement of oxygen ions through ion pumping causes a current to flow in the circuit connected between the cathode electrode 15 and the anode electrode 13 .

In order to measure the limiting current in the limiting current type oxygen sensor 10, a diffusion barrier is formed on the cathode electrode 15 side to control the diffusion amount of oxygen supplied to the cathode electrode 15 to show dependence on the oxygen concentration. do. The limiting current type oxygen sensor 10 illustrated in FIG. 1 includes a cap structure 17 on the cathode electrode 15 side as an example of a diffusion barrier. The cap structure 17 forms a predetermined diffusion space 19 under the cathode electrode 15 and includes a gas diffusion hole 21 for introducing a gas for which oxygen concentration is to be measured into the diffusion space 19 side. include

Ion pumping through the solid electrolyte 11 reaches an equilibrium state within a short time, and the magnitude of the current thereafter is limited to a constant magnitude even if the magnitude of the pumping voltage is changed. The magnitude of this limiting current varies depending on the diffusion amount of oxygen supplied to the cathode electrode 15 , that is, the partial pressure of oxygen and the temperature of the solid electrolyte 11 . Therefore, when the magnitude of the limit current is measured under the condition that the temperature of the solid electrolyte 11 is fixed, the oxygen concentration of the diffusion space 19 to which the cathode electrode 15 is exposed can be measured.

In the case of diffusion through the gas diffusion hole 21, if the size of the gas diffusion hole 21 is much smaller than the mean free path of the gas, the effect between the gas and the outer wall of the gas diffusion hole 21 affects the diffusion rather than the effect between the gases. This time is called Nussen diffusion.

When a constant voltage is applied to an oxygen sensor by Nussen diffusion, the limit current is directly proportional to the oxygen concentration (oxygen partial pressure). Accordingly, in the limiting current type oxygen sensor 10 , the partial pressure of oxygen shows a pattern of linearly increasing according to the magnitude of the limiting current. However, when moisture is introduced, there is a problem in that the oxygen concentration is measured excessively due to the decomposition reaction of moisture.

The present invention has been devised in recognition of the above-described problems of the prior art, and an object of the present invention is to provide a limiting current type oxygen sensor in which the effect of moisture is suppressed.

A limiting current type oxygen sensor according to the present invention for achieving the above technical problem, a solid electrolyte capable of pumping oxygen ions; an anode electrode and a cathode electrode respectively formed on the upper surface and the lower surface of the solid electrolyte; a diffusion barrier provided on the cathode electrode side, the diffusion barrier forming a diffusion space under the cathode electrode and including a gas diffusion hole for introducing a gas to be measured oxygen concentration into the diffusion space side; and a control unit for controlling a voltage (hereinafter referred to as a pumping voltage) applied between the cathode and anode electrodes, wherein the control unit applies a pumping voltage having a magnitude to suppress decomposition of water vapor in the gas at the cathode electrode. do it with

The controller may allow a pumping voltage equal to or less than a maximum applied voltage calculated in consideration of an allowable value of an oxygen concentration error generated by the inflow of water vapor to be applied.

In this case, the allowable value of the oxygen concentration error may be 10 ppm.

The control unit obtains an amount of oxygen concentration overestimation from the difference between the formula for the limit current expressed including the concentration of oxygen gas generated in consideration of the steam decomposition reaction and the formula for the oxygen concentration and the limit current in the gas, and performs the steam decomposition reaction By combining the formula for the limiting current expressed including the concentration of oxygen gas generated in consideration and the thermodynamic equilibrium reaction equation for water vapor decomposition, a function between the pumping voltage and the water vapor decomposition amount is obtained, and the oxygen concentration overestimated amount and the oxygen The relation between the concentration error and the pumping voltage may be obtained, and the maximum applied voltage may be calculated from the relation.

)와 측정가스 중 수증기 농도(C_H2O) 조건하에서 허용하는 절대 오차(

)의 함수로서, 누센 확산일 때 다음 수식The maximum applied voltage (V _max ) is the oxygen concentration to be measured (

) and the _absolute error (

) as a function of Nussen's diffusion,

을 따를 수 있다.

can follow

The resolution of the limiting current type oxygen sensor may be 1 to 2 ppm.

In the present invention, further comprising a sensor substrate attached face-to-face to the side of the cathode electrode, at an interface between the solid electrolyte and the sensor substrate, a flat void region exposing at least a portion of the cathode electrode and the void region A line-shaped gas diffusion passage opened through the sidewall of which the interface is exposed is formed to communicate with, so that the flat void area becomes a diffusion space of the diffusion barrier and the line-type gas diffusion passage becomes a gas diffusion hole of the diffusion barrier it could be this

Here, the line-shaped gas diffusion passage and the flat void region may be left as traces as the combustible screen print pattern is burned.

According to one aspect of the present invention, the influence of moisture can be suppressed by controlling the level of the pumping voltage applied to the limiting current type oxygen sensor. Accordingly, accurate oxygen detection is possible and precise measurement is possible by applying high detection limit standards. Since a separate structure for error correction is not required, there is an effect that the sensor can be made light, thin and compact.

According to another aspect of the present invention, a diffusion barrier structure of a limiting current type oxygen sensor having a structure including a line-type gas diffusion passage communicating with each other and a flat pore region can be proposed and formed by a simple process. Since the space occupied by the diffusion barrier structure is minimized, the oxygen sensor can be lightweight and compact. In addition, the manufacturing cost of the oxygen sensor can be reduced by simplifying the process.

The following drawings attached to this specification illustrate preferred embodiments of the present invention, and together with the detailed description of the present invention serve to further understand the technical spirit of the present invention, the present invention is should not be construed as being limited to In the drawings, like reference numerals indicate like members.
1 is a schematic diagram showing the structure of a limiting current type oxygen sensor widely used in the prior art.
2 is a cross-sectional view illustrating a structure of a limiting current type oxygen sensor according to an embodiment of the present invention.
3 is a cross-sectional view illustrating a structure of a limiting current type oxygen sensor according to another embodiment of the present invention.
4 is a graph showing the decomposition amount of water vapor and overestimated measurement error according to the applied voltage under the condition of 630° C. and 10 ppm oxygen, assuming Nussen diffusion.
5 is a graph showing the measurement relative error according to the applied voltage under the condition of 630° C. and 3.7% water vapor.
6 is a graph of measured oxygen concentration according to applied voltage at 630° C., oxygen concentration of 25 ppm and water vapor concentration of 4.5, 3.1, 1.5, and 0% at 14.8 ppm.
7 shows an overestimated error of the measured oxygen concentration when the applied voltage is changed under the condition of 4.5% water vapor at 630° C. and an oxygen concentration of 25ppm, and an overestimated error calculated under the Nussen diffusion condition.
8 shows an overestimated error of the measured oxygen concentration when the applied voltage is changed under a condition of 3.1% water vapor at an oxygen concentration of 25ppm at 630°C, and an overestimated error calculated under the Nussen diffusion condition.
9 shows an overestimated error in oxygen concentration measured when an applied voltage is changed under a condition of 1.5% water vapor at an oxygen concentration of 25ppm at 630°C, and an overestimation error calculated under Nussen diffusion conditions.

Hereinafter, a limiting current type oxygen sensor according to the present invention will be described in detail with reference to the accompanying drawings. The drawings introduced below are provided as examples in order to sufficiently convey the spirit of the present invention. Accordingly, the present invention is not limited to the drawings presented below and may be embodied in other forms. Also, like reference numerals refer to like elements throughout.

Unless otherwise defined in technical and scientific terms used in this specification, it is apparent that those of ordinary skill in the art to which this invention pertains have a meaning commonly understood. In addition, in the following description and accompanying drawings, descriptions of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

2 is a cross-sectional view illustrating a structure of a limiting current type oxygen sensor according to an embodiment of the present invention. 3 is a cross-sectional view illustrating a structure of a limiting current type oxygen sensor according to another embodiment of the present invention.

First, referring to FIG. 2, the limiting current oxygen sensor 100 according to an embodiment of the present invention includes a solid electrolyte 121 and an anode electrode 123 made of a porous material for applying a pumping voltage to both surfaces thereof and A cathode electrode 125 is included. The solid electrolyte 121 is a material capable of pumping oxygen ions. The anode electrode 123 is formed on the upper surface of the solid electrolyte 121 and the cathode electrode 125 is formed on the lower surface of the solid electrolyte 121 . An anode lead 123a and a cathode lead 125a may be connected to each of the electrodes 123 and 125 .

A diffusion barrier DB is provided on the cathode electrode 125 side. The diffusion barrier DB forms a diffusion space A ₁ under the cathode electrode 125 and includes a gas diffusion hole A ₂ for introducing a gas to be measured oxygen concentration into the diffusion space A ₁ side. do.

The diffusion barrier DB has a cap structure shape. This is the sidewall portion DB_a extending downward from a point spaced a predetermined distance from the edge of the cathode electrode 125 and the lower end of the sidewall portion DB_a is vertically bent toward the center of the limiting current type oxygen sensor 100. and a bottom portion DB_b extending to the gas diffusion hole A ₂ . Here, the gas diffusion hole A ₂ is drilled in a direction perpendicular to the cathode electrode 125 .

In order to form the diffusion space (A ₁ ) and the gas diffusion hole (A ₂ ) using the cap structure shape inside the limiting current type oxygen sensor 100 , a groove is dug in the sheet substrate constituting the diffusion barrier DB to form a sidewall After the part DB_a and the bottom part DB_b are formed, a process of drilling the gas diffusion hole A ₂ in the central portion of the bottom part DB_b may be performed. Instead, the bottom portion DB_b in which the gas diffusion hole A ₂ is formed in the central portion and the sidewall portion DB_a in the form of a spacer to be attached near the cathode electrode 125 are independently manufactured, and then the cathode electrode 125 side It is also possible to perform a process of vertically stacking on the.

On the other hand, the solid electrolyte 121 of the limiting current type oxygen sensor 100 needs to be heated to a temperature that can cause ion pumping (hereinafter referred to as a reaction temperature), for example, a temperature of several hundred degrees. To this end, the thin film heater 131 is provided under the bottom portion DB_b of the diffusion barrier DB. The thin film heater 131 receives power from a separate DC power supply DC1 through the heater lead wire 133 to generate resistance heat up to a temperature of several hundred degrees. Then, heat is conducted toward the solid electrolyte 121 so that the temperature of the solid electrolyte 121 rises to a temperature that can cause ion pumping.

In the present invention, the solid electrolyte 121 is not particularly limited as long as it is a material capable of pumping oxygen ions. Preferably, the solid electrolyte 121 may be made of YSZ material, which is conventionally widely used as a solid electrolyte of an oxygen sensor. As another example, it may be made of YSZ/alumina, GDC (Ceria (CeO ₂ ) doped with Gadolium (Gd)), or LSGM ((La,Sr) (Ga,Mg)O ₃ ) materials.

The thickness of the solid electrolyte 121 is determined by the dimension of the sensor, and may be appropriately selected in the range of 50-300um, for example.

The anode electrode 123 and the cathode electrode 125 are not particularly limited as long as they are porous conductive materials. Preferably, they may be formed of a porous thin film made of a noble metal such as platinum (Pt) or gold (Au). Alternatively, the anode electrode 123 and the cathode electrode 125 may be formed of a composite material in which at least one of glass particles and YSZ particles is mixed with a noble metal as a matrix. For various examples, the anode electrode 123 and the cathode electrode 125 may be LSCF, LSCF/GDC, LSM, or LSM/YSZ.

The thickness of the anode electrode 123 and the cathode electrode 125 is determined by the dimensions of the sensor, and may be appropriately selected in the range of 5-40 μm, for example.

The anode lead 123 and the cathode lead 125a are not particularly limited as long as they are made of a metal material having low resistance and electrical conductivity. Preferably, they may be made of a noble metal wire such as platinum (Pt) or gold (Au).

The thin film heater 131 is connected to a DC power source DC1 through a heater lead wire 133 to generate resistance heat, and the generated heat is transferred to the solid electrolyte 121 side. Then, the temperature of the solid electrolyte 121 rises to a temperature at which oxygen ions can be pumped (reaction temperature). Here, the reaction temperature varies depending on the type of the solid electrolyte, and is, for example, a temperature of about 400-700 degrees.

Preferably, the specifications of the DC power supply DC1 and the material, thickness, length, etc. of the thin film heater 131 may be appropriately selected so that the solid electrolyte 121 can be heated to a temperature of 400-700 degrees.

As an example, the DC power supply DC1 may have an operating voltage of about 3-24V. In addition, the thin film heater 131 may be made of a platinum (Pt) material.

Next, referring to FIG. 3 , the limiting current oxygen sensor 100 ′ according to another embodiment of the present invention includes a solid electrolyte 121 capable of transporting oxygen ions through ion pumping, and the solid electrolyte 121 . An anode electrode 123 and a cathode electrode 125 in the form of a porous thin film formed respectively on the upper and lower surfaces of the The sensor substrate 127 attached face-to-face to the electrode 125 side, and a flat void area (B) of a substantially rectangular shape exposing a partial surface of the cathode electrode 125 and the surface of the sensor substrate 127 opposite thereto ₁ ), and a line-type gas diffusion passage (B ₂ ), one side communicating with the flat void region (B ₁ ) and the other side opening to the outside air through the sidewall of the limiting current type oxygen sensor ( 100 ′).

Meanwhile, although not essential, a portion 125 ′ of the cathode electrode 125 is exposed to the outside through a cutout formed on one side of the sensor substrate 127 , and a cathode lead wire 125a is formed on the exposed portion 125 ′. This can be connected In addition, the surface of the exposed portion 125 ′ may be completely covered with an insulating material, for example, glass.

In one embodiment, the void region B ₁ and the line-type gas diffusion passage B ₂ are formed between the solid electrolyte 121 and the sensor substrate 124 during the manufacturing process of the limiting current type oxygen sensor 100 ′. The combustible sheet fragments sandwiched at the interface and the combustible wire may be traces from burning during the co-firing process. In this case, the void region B ₁ has a spatial structure of a shape corresponding to the combustible sheet fragment, and the line-type gas diffusion passage B ₂ may also be called a line-type pinhole, and corresponds to the shape of the combustible wire. It has a cylindrical inner wall structure.

In addition, the boundary between the solid electrolyte 121 and the ceramic crystal grains constituting the sensor substrate 124 may be exposed on the inner wall of the pore region B ₁ and the line-type gas diffusion passage B ₂ .

In addition, a very small amount of carbon component that is not visually confirmed may be present on the inner wall of the void region B ₁ and the line-type gas diffusion passage B ₂ . The trace amount of carbon component may be derived from combustion of the combustible sheet fragment and the combustible wire.

Preferably, the combustible sheet fragment is made of a thin film having a shape such as a square plate or a circular plate. In addition, the material of the combustible sheet fragment is not particularly limited as long as it is a material that can be burned in the simultaneous firing process. As an example, the combustible sheet fragment may be made of a paper material, a synthetic resin such as polyethylene (PE) or polypropylene (PP), or a carbon material.

Preferably, the combustible wire has a wire shape extending in a straight line and having a circular cross section. The material constituting the combustible wire is not particularly limited as long as it is a material that can be combusted in the simultaneous firing process. For example, paper processed in the form of a synthetic resin yarn or wire such as polyethylene (PE) or polypropylene (PP) yarn or animal fiber yarn, or carbon fiber.

In another example, the void region B ₁ and the line-type gas diffusion passage B ₂ are the interface between the solid electrolyte 121 and the sensor substrate 127 during the manufacturing process of the limiting current type oxygen sensor 100 ′. The combustible screen print pattern sandwiched between the two may be traces caused by combustion in the co-firing process. The combustible print pattern includes a pattern for forming a void region and a pattern for forming a gas diffusion passage. In this case, the void region (B ₁ ) is formed to have a spatial structure of a shape corresponding to the pattern for forming the void region, and the line-type gas diffusion passage (B ₂ ) corresponds to the shape of the pattern for forming the gas diffusion passage It is formed to have an inner wall structure, such as an inner wall structure of a rectangular or square cross section.

In the inner wall of the void region B ₁ and the line-type gas diffusion passage B ₂ , a very small amount of carbon component that is not visually confirmed may be present. The trace amount of carbon may be derived from combustion of the screen print pattern.

Preferably, the pattern for forming the void region may be formed in a pattern having a shape such as a square plate or a circular plate, and the pattern for forming the gas diffusion passage may be formed in a band-shaped pattern. The pattern for forming the gas diffusion passage may have a band shape extending in a straight line and having a rectangular or square cross-section. The pattern for forming the void region and the pattern for forming the gas diffusion passage may be simultaneously formed in the same process through a screen printing method. That is, it is not necessary to separately prepare a member for forming the void region B ₁ and a member for forming the line-shaped gas diffusion passage B ₂ . There is also no need to align each member or manage to have the appropriate combustion temperature for each other.

In addition, the material of the screen print pattern including the pattern for forming the void region and the pattern for forming the gas diffusion passage is not particularly limited as long as it is a material that can be burned in the simultaneous firing process. As an example, the screen print pattern may be formed of a paste including graphite or carbon black. The paste may further include a solvent, a binder, a plasticizer, a dispersant, and the like to have properties such as viscosity, coatability, and density suitable for forming by screen printing. For example, a preferred example of the paste may include starch, graphite, carbon black, and PMMA as a solid component, and further include a solvent, a binder, a plasticizer and a dispersant. In this case, the solvent may be, for example, any one of ethanol, methanol, acetone, isopropyl alcohol, toluene, and alpha terpineol. The binder may be any one of methyl cellulose, polyvinyl alcohol, ethylene glycol, and ethyl cellulose. The plasticizer may be any one of dibutyl phthalate and polyethylene glycol. The dispersant may be any one of fish-oil, kd-1, and kd-6.

In either case, the line-shaped gas diffusion passage B ₂ is formed in the cathode electrode 125 in a horizontal direction. And since the line-type gas diffusion passage (B ₂ ) communicates with the void region (B ₁ ), an external gas for which an oxygen concentration is to be measured passes through the line-type gas diffusion passage (B ₂ ) through the void region (B) ₁ ) can spread.

In the limiting current type oxygen sensor 100 ′, a diffusion barrier is provided toward the cathode electrode 125 between the solid electrolyte 121 and the sensor substrate 127 . In the limiting current oxygen sensor 100 ′, the flat void region B ₁ corresponds to the diffusion space A ₁ of the limiting current oxygen sensor 100 shown in FIG. 2 , and the limiting current oxygen sensor 100 '), the line-type gas diffusion passage (B ₂ ) corresponds to the gas diffusion hole (A ₂ ) of the limiting current type oxygen sensor 100 . Here, the line-type gas diffusion passage (B ₂ ) is a point in the horizontal direction to the cathode electrode 125 , the gas diffusion hole (A ₂ ) in the limiting current type oxygen sensor 100 is a point in the vertical direction to the cathode electrode 125 . is different from

The limiting current type oxygen sensor 100 ′ may further include a heater substrate 129 attached to the lower surface of the sensor substrate 127 , and a thin film heater 131 attached to the lower surface of the heater substrate 129 . have. Although the thin film heater 131 is illustrated as being formed on the lower surface of the heater substrate 129 , it may be formed on the interface between the heater substrate 129 and the sensor substrate 127 .

The sensor substrate 127 is not particularly limited as long as it has insulating properties and has the same or similar coefficient of thermal expansion to the solid electrolyte 121 . Preferably, the sensor substrate 127 may be made of the same material as the solid electrolyte 121 , for example, YSZ material. Of course, it may be made of YSZ/alumina, GDC, LSGM material.

Since the sensor substrate 127 serves to mechanically support the upper structure, it is preferable that the sensor substrate 127 be thicker than the solid electrolyte 121 . As an example, the thickness of the sensor substrate 127 may be appropriately selected in the range of 50-800um.

In the limiting current type oxygen sensor 100 ′ disclosed in FIG. 3 , a diffusion barrier is formed by a line-type gas diffusion passage B ₂ , and a height diagram of the void region B ₁ provides a space for the reduction reaction of oxygen to occur. Since it is substantially the same as the height of the type gas diffusion passage (B ₂ ), the thickness is considerably thinner than that of the conventional oxygen sensor. Therefore, it is possible to reduce the weight and size of the limit current type oxygen sensor. In addition, the void region (B ₁ ) and the line-shaped gas diffusion passage (B ₂ ) are formed by burning a combustible sheet fragment and a combustible wire or screen print pattern sandwiched between the solid electrolyte 121 and the sensor substrate 124 . Therefore, the manufacturing process is very simple. Therefore, the structure of the limiting current type oxygen sensor is very advantageous in reducing manufacturing cost.

In addition, the limiting current type oxygen sensor according to the present invention may have various structures. The schematic structure of the limiting current type oxygen sensors 100 and 100' according to the embodiments of the present invention has been described above. The oxygen sensing of the limiting current type oxygen sensors 100 and 100' shown in FIGS. 2 and 3 will now be described in more detail.

DC power supply that applies a pumping voltage of several volts to the anode lead 123a and the cathode lead 125a in the limiting current type oxygen sensors 100 and 100' so that oxygen ions are pumped through the solid electrolyte 121 ( DC2) is connected. Here, the pumping voltage varies depending on the type of the solid electrolyte 121 and the reaction temperature at which the ion pumping occurs. The controller 140 controls this pumping voltage.

When the solid electrolyte 121 is heated to a reaction temperature and a pumping voltage of an appropriate level is applied to the anode electrode 123 and the cathode electrode 125, an electrochemical reaction condition in which ion pumping occurs is established. In this case, oxygen diffused into the diffusion space A ₁ or the void region B ₁ through the gas diffusion hole A ₂ or the line-shaped gas diffusion passage B ₂ is converted into the diffusion space A ₁ or the void region B 2 . B ₁ ) is reduced on the surface of the cathode electrode 125 exposed to, converted into oxygen ions, and then pumped to the anode electrode 123 side through the solid electrolyte 121 . In addition, oxygen ions reaching the anode electrode 123 are oxidized while losing electrons by an oxidation reaction, are converted back into oxygen gas, and are discharged to the outside air.

When the oxygen ions are pumped, a current flows in the closed loop circuit where the anode lead 123a and the cathode lead 125a are connected, and when the ion pumping reaches a parallel state within a short time, the magnitude of the current is also constantly limited. At this time, if the limit current is measured through the shunt resistor (R), the oxygen concentration in the outside air can be measured.

Here, the controller 140 controls the voltage applied between the cathode electrode 125 and the anode electrode 123 from the DC power source DC2, that is, the pumping voltage. In particular, in the present invention, the controller 140 applies a voltage having a magnitude to suppress the decomposition of water vapor in the cathode electrode 125 . Hereinafter, the voltage tuning principle of the control unit 140 will be described in detail based on the embodiment.

Theoretical background

When the electrodes 123 and 125 are formed at both ends of the solid electrolyte 121 such as yttria-stabilized zirconia (YSZ), which is an oxygen ion conductor, and a voltage is applied, the oxygen concentration (or partial pressure) at the cathode electrode 125 (anode). , activity) is expressed by the following Nernst equation.

[Formula 1]

,

, V, F, R, T는 각각 캐소드 전극(125)에서의 산소 농도, 측정 가스 중의 산소 농도, 인가 전압, 페러데이 상수, 가스상수, 절대온도를 의미한다. 이 때, 캐소드 전극(125)의 산소 농도는 인가 전압에 의해 측정 가스 중 산소 농도보다 매우 작은 값 (

≫

)로 일정하게 유지된다. 이 때문에 측정가스 중 산소는 Fick's 1^st law에 따라 외부에서 가스 확산 구멍(A₂) 또는 라인형 가스 확산 통로(B₂)를 통해 확산 공간(A₁) 또는 공극 영역(B₁)에 노출된 캐소드 전극(125) 방향으로 유입된다. 누센 확산 조건하에서 한계 전류와 산소 농도와의 관계는 다음의 수식으로 표현될 수 있다. here,

,

, V, F, R, and T mean an oxygen concentration in the cathode electrode 125, an oxygen concentration in the measurement gas, an applied voltage, a Faraday constant, a gas constant, and an absolute temperature, respectively. At this time, the oxygen concentration of the cathode electrode 125 is a very small value (

≫

) is kept constant. For this reason, oxygen in the measurement gas is exposed to the diffusion space (A ₁ ) or the void region (B ₁ ) through the gas diffusion hole (A ₂ ) or the line-type gas diffusion passage (B ₂ ) from the outside according to ^Fick 's 1st law. It flows in the direction of the cathode electrode 125 . The relationship between the limiting current and the oxygen concentration under the Nussen diffusion condition can be expressed by the following equation.

[Equation 2]

가 성립하므로 수식 2는 하기와 같은 수식 3으로 표현 가능하다.Here, I _LK and k _LK mean the limiting current and the proportional constant, respectively. In addition,

Since is established, Equation 2 can be expressed as Equation 3 as follows.

[Equation 3]

That is, it can be said that the relationship between the limiting current and the oxygen concentration is in direct proportion as shown in Equation 3.

On the other hand, when moisture exists in the diffusion space (A ₁ ) or the void region (B ₁ ) inside the cathode electrode 125 , excess oxygen gas is generated by the following steam decomposition reaction.

[Chemical Reaction Formula 1]

Since the oxygen concentration in the cathode electrode 125 is fixed constant by Equation 1, oxygen gas generated from water vapor decomposition is pumped out from the cathode electrode 125 to the anode electrode 123 (cathode) and exits. The generated hydrogen escapes from the inside of the diffusion space A ₁ or the void region B ₁ to the outside through the gas diffusion hole A ₂ or the line-shaped gas diffusion passage B ₂ . After all, the limit current expressed including the concentration of oxygen gas generated in consideration of the steam decomposition reaction is expressed by the following equation.

[Equation 4]

는 각각 비례 상수와 수증기 분해 반응에 의해 생성된 산소 가스의 양을 의미한다. 또한, 한 개의 수증기 분자는 화학반응식 1을 통해 한 개의 수소분자와 0.5개의 산소분자를 생성하므로 수식 4를 다음과 같이 수증기 분해 반응에서 분해된 수증기의 농도를 이용해 표현한 한계 전류로 나타낼 수도 있다.where k _humidity ,

denotes the proportional constant and the amount of oxygen gas produced by the steam decomposition reaction, respectively. In addition, since one water vapor molecule generates one hydrogen molecule and 0.5 oxygen molecules through Chemical Reaction Equation 1, Equation 4 can also be expressed as a limiting current expressed using the concentration of water vapor decomposed in the steam decomposition reaction as follows.

[Equation 5]

는 수증기 분해 반응에서 분해된 수증기의 농도이며

의 2배와 같다. here,

is the concentration of water vapor decomposed in the steam decomposition reaction

equal to twice the

On the other hand, oxygen and water vapor flowing in from the outside of the diffusion space (A ₁ ) or the pore region (B ₁ ) move through the same gas diffusion hole (A ₂ ) or the line-type gas diffusion passage (B ₂ ) and move under an isothermal condition Since the Nussen diffusion coefficient is inversely proportional to the square root of the molecular weight under

[Equation 6]

Here, M _X means the molecular weight of the gas species X.

Therefore, by combining Equations 4 to 6, the limiting current can be expressed with the oxygen concentration and the water vapor concentration consumed by the water vapor decomposition reaction as shown in Equation 7 below.

[Equation 7]

를 구할 수 있고, 이는 다음과 같이 표현된다.Comparing Equation 7 for the limiting current considering the steam cracking reaction with Equation 3 for the limiting current not considering the steam cracking reaction, it can be seen that the steam cracking reaction increases the magnitude of the limiting current, leading to an overestimation of the oxygen concentration. can Therefore, the oxygen concentration overestimate from the difference between Equation 7 and Equation 3

can be obtained, which is expressed as

[Equation 8]

As a result, suppressing the decomposition of water vapor in the cathode electrode 125 is a method of increasing the measurement accuracy by reducing the error of the limiting current type oxygen sensors 100 and 100 ′.

In the present invention, it is proposed to control the level of the pumping voltage through the control unit 140 in order to suppress the decomposition of water vapor in the cathode electrode 125 .

Determination of applied voltage to suppress water vapor decomposition

When Chemical Reaction 1 for water vapor decomposition is expressed as a thermodynamic equilibrium reaction equation, it is expressed as Equation 9 below.

[Equation 9]

는 일정하게 유지되고 수소 생성량과 수증기 분해량은 동일하므로, 수식 9와 결합해, 수식 5를 수증기 분해량의 함수와 인가 전압의 함수로 나타내면 다음 수식 10이 된다.Here, ΔG _f,H2O means the formation free energy of water vapor, and C _X means the concentration of gas species X when the total gas pressure is 1 atmosphere. Oxygen concentration in the cathode electrode 125 by Equation 1

is kept constant and the hydrogen production amount and water vapor decomposition amount are the same, so when combined with Equation 9 and expressing Equation 5 as a function of the water vapor decomposition amount and the applied voltage, the following Equation 10 is obtained.

[Equation 10]

The error corresponding to the oxygen concentration overestimation due to the inflow of water vapor in the limiting current sensor due to Nussen diffusion can be expressed as the following inequality using Equations 8 and 10.

[Equation 11]

[Equation 12]

Equation 11 expresses the absolute error because it is the overestimated oxygen concentration itself. Equation 12 expresses the relative error because it is an overestimated oxygen concentration compared to oxygen in the measured gas.

)와 측정가스 중 수증기 농도(C_H2O) 조건하에서 허용하는 절대 오차(

)의 함수로서, 누센 확산 조건하에서 다음과 같이 표현된다.Therefore, if the allowable error level is determined, it is possible to determine the maximum applied voltage V _max , which is the voltage V that allows the equal sign to be established in Equations 11 and 12 . The maximum applied voltage (V _max ) is the oxygen concentration (

) and the _absolute error (

), under Nussen diffusion conditions, it is expressed as

[Equation 13]

)와 측정가스 중 수증기 농도(C_H2O) 조건하에서 허용하는 상대오차(

)의 함수로 나타내면 다음과 같다. In addition, the oxygen concentration to measure the maximum applied voltage (V _max ) (

) and the _allowable relative error (

) as a function of

[Equation 14]

Therefore, in the limiting current sensor (100, 100') of the present invention, the controller 140 sets an allowable error in consideration of the case where Nussen diffusion occurs, and accordingly, the maximum applied voltage determined by Equation 13 or Equation 14 or less. to make it accredited.

Experimental example

, 단위: ppm), 우측 세로축은 수증기 분해량(

, 단위: ppm)이다. 4 is a graph showing the hydrogen production amount and overestimated measurement error according to the applied voltage under the condition of 630° C. and 10 ppm oxygen, assuming Nussen diffusion, and is a graph of the calculation result according to Equation 11. FIG. In FIG. 4 , the horizontal axis represents the applied voltage (unit: mV), and the left vertical axis represents the overestimated oxygen concentration (absolute error).

, unit: ppm), the vertical axis on the right is the amount of water vapor decomposition (

, unit: ppm).

When 20 ppm of water vapor is decomposed, 10 ppm of oxygen produced according to Chemical Reaction Equation 1 is produced. This appears as an overestimated oxygen concentration (green dashed line in FIG. 4). And, the error increases as the concentration of water vapor increases (increasing in the order of 1%, 3.7%, and 50%) and as the applied voltage increases (the value of the horizontal axis in the graph increases).

If the magnitude of the voltage (intersection with the green dotted line) when the error is 10ppm on a straight line having a water vapor concentration of 50%, 3.7%, and 1% in FIG. 4 is read, it is 390, 493, and 543 mV, respectively. In other words, 390, 493, and 543 mV are calculated values by substituting 10 ppm of tolerance in Equation 12.

As such, if the pumping voltage applied to the sensor is 493 mV when the water vapor concentration is 3.7%, an error of about 10 ppm occurs. If the pumping voltage applied to the sensor is determined to be 390 mV or less, the error due to the inflow of water vapor can be reduced to 10 ppm or less even when the water vapor concentration is 50%. Therefore, if the allowable error size is 10 ppm, the controller 140 controls the DC power supply DC2 so that the applied voltage is 493 mV or less when the water vapor concentration is 3.7%. Similarly, if the allowable error level is 10 ppm, the controller 140 controls the DC power supply DC2 so that the applied voltage becomes 390 mV or less when the water vapor concentration is 50%.

, 단위: %)이다. 도 5에서 실선은 산소 10ppm, 점선은 산소 1000ppm일 때를 가리킨다. 5 is a graph showing a measurement relative error according to an applied voltage under a condition of 630° C. and 3.7% water vapor, and is a graph of a calculation result according to Equation 13. FIG. In FIG. 5 , the horizontal axis represents the applied voltage (unit: mV), and the vertical axis represents an overestimated oxygen concentration (relative error).

, unit: %). In FIG. 5 , a solid line indicates 10 ppm of oxygen, and a dotted line indicates a case of 1000 ppm of oxygen.

Referring to FIG. 5 , a relative error according to an applied voltage is confirmed when measuring 10 ppm and 1000 ppm oxygen concentrations under the conditions of 630° C. and 3.7% water vapor. It can be seen that even when measuring 1000 ppm oxygen, an error of 20.1% or less occurs when 700 mV is applied under the condition of 3.7% water vapor.

Measurement example

In the experimental example, a limiting current type oxygen sensor having the structure as shown in FIG. 2 was constructed, and the measured oxygen concentration was observed by changing the water vapor concentration and applied voltage at an oxygen concentration of 25 ppm, and dry conditions without water vapor at an oxygen concentration of 14.8 ppm. The measured oxygen concentration was observed by changing the applied voltage under

6 is a graph of measured oxygen concentration according to each applied voltage when the water vapor concentration is 4.5, 3.1, 1.5, and 0% at 630° C., oxygen concentrations of 25 ppm (black, red, blue solid lines) and 14.8 ppm (green solid lines).

As shown in FIG. 6 , in the case of the black, red, and blue solid lines, the actual oxygen concentration is the same as 25 ppm, but the higher the water vapor concentration and the higher the applied voltage, the overestimated and measured. In addition, when measuring 14.8 ppm of oxygen without water vapor, it was found that the measured value appeared constant regardless of the applied voltage.

7 to 9 are compared with the numerical values calculated using Equation 11.

7 to 9 show the overestimated error (black dots) and Nussen diffusion (black dots) of the measured oxygen concentration when the applied voltage is changed under the conditions of 4.5%, 3.1%, and 1.5% water vapor concentration at 630° C. and 25 ppm oxygen concentration, respectively. (solid line) shows the overestimation error calculated under the condition.

7 to 9, the error calculated by Equation 11 fits well with the calculated value and the measured value at several ppm or more. The resolution of the sensor used for the measurement was about 1 to 2 ppm.

In the above, the present invention has been illustrated and described with reference to specific preferred embodiments, but the present invention is not limited to the above-described embodiments, and those of ordinary skill in the art to which the invention pertains within the scope not departing from the spirit of the present invention Various changes and modifications will be possible by the person.

100, 100': limiting current type oxygen sensor 121: solid electrolyte
123: anode electrode 125: cathode electrode
127: sensor board 129: heater board
131: thin film heater 133: heater lead wire
123a: anode lead wire 125a: cathode lead wire
A ₁ : diffusion space A ₂ : gas diffusion hole
B ₁ : Flat void area B ₂ : Line-shaped gas diffusion passage
140: control unit

Claims (8)

a solid electrolyte capable of pumping oxygen ions;
an anode electrode and a cathode electrode respectively formed on the upper surface and the lower surface of the solid electrolyte;
a diffusion barrier provided on the cathode electrode side, the diffusion barrier forming a diffusion space under the cathode electrode and including a gas diffusion hole for introducing a gas to be measured oxygen concentration into the diffusion space side;
A control unit for controlling a voltage (hereinafter referred to as a pumping voltage) applied between the cathode electrode and the anode electrode,
The limiting current type oxygen sensor, characterized in that the control unit applies a pumping voltage of a magnitude for suppressing decomposition of water vapor in the gas to the cathode electrode. The limiting current type oxygen sensor according to claim 1, wherein the control unit applies a pumping voltage equal to or less than a maximum applied voltage calculated in consideration of an allowable value of an oxygen concentration error generated by the inflow of water vapor. The limiting current type oxygen sensor according to claim 2, wherein the allowable value of the oxygen concentration error is 10 ppm. The method of claim 2, wherein the control unit obtains the oxygen concentration overestimation amount from the difference between the formula for the limit current expressed including the concentration of oxygen gas generated in consideration of the steam decomposition reaction and the formula for the oxygen concentration and the limit current in the gas, ,
The function between the pumping voltage and the amount of hydrogen produced is obtained by combining the formula for the limiting current expressed including the concentration of oxygen gas generated in consideration of the steam cracking reaction and the thermodynamic equilibrium reaction formula for steam cracking,
Obtaining the relation between the oxygen concentration error and the pumping voltage using the oxygen concentration overestimate and the function,
A limiting current type oxygen sensor, characterized in that the maximum applied voltage is calculated from the relational expression. According to claim 2, wherein the maximum applied voltage (V _max ) is the oxygen concentration to be measured (

) and the _absolute error (

) as a function of Nussen's diffusion,

A limiting current type oxygen sensor, characterized in that it follows. The limiting current oxygen sensor according to claim 5, wherein the resolution of the limiting current oxygen sensor is 1 to 2 ppm. According to claim 1, further comprising a sensor substrate attached to the side of the cathode electrode, at an interface between the solid electrolyte and the sensor substrate, a flat void region exposing at least a portion of the cathode electrode and the void An open line-type gas diffusion path is formed through the sidewall with the interface exposed to communicate with the region,
The limiting current type oxygen sensor according to claim 1, wherein the flat void region becomes a diffusion space of the diffusion barrier and the line-shaped gas diffusion passage becomes a gas diffusion hole of the diffusion barrier. 8. The limiting current type oxygen sensor according to claim 7, wherein the line-shaped gas diffusion passage and the flat void area are left as traces of the combustible screen print pattern being burned.

Images