JPS63295958A - Measurement of gas density using solid electrolyte - Google Patents

Abstract

PURPOSE:To measure the density of a gas accurately, by measuring a threshold current in an atmosphere of a low pressure with a threshold current type gas sensor so that a gas to a sensor element is diffused in a Knudsen's manner. CONSTITUTION:A solid electrolyte 1 of stabilized zirconia is provided with an anode 2 and a cathode 3 on both sides and is heated with a heater 6 up to several 100 deg.C. When a voltage is applied to both electrodes 2 and 3, oxygen in an atmosphere is turned with the cathode 3 to oxygen ions and transferred to the anode 2 through the solid electrolyte 1, causing a current to flow. The diffusion of oxygen flowing form a gas passage hole 5 of a gas diffusion control body 4 is controlled to be flat at an area where the applied voltage exists, developing a threshold current. here, the atmosphere gas is brought down to a low pressure to cause a Knudsen's diffusion. In the atmosphere of Knudsen's diffusion, the threshold current is linearly proportional to the concentration of oxygen. Thus, the knudsen's diffusion is caused under the atmosphere of a low pressure, thereby enabling highly accurate measurement of the density of a gas.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring the concentration of a specific gas contained in a gas.

(Prior art) There are various methods in practice for measuring gas concentration, especially oxygen concentration, in gas, and one known method is to use a limiting current type oxygen sensor using a solid electrolyte. ,
Examples of prior art by the inventors of the present application (Japanese Patent Application No. 61-5050
No. 61-237985, etc.). As shown in FIG. 1, the oxygen sensor element of this oxygen sensor has an anode 2 and a cathode 3 on both sides of a solid electrolyte 10 made of, for example, stabilized zirconia, and is heated to a high temperature of several hundred degrees Celsius by a heater 6. When a voltage is applied between both electrodes of the solid electrolyte 1, the oxygen contained in the gas is transferred to the cathode 3.
The oxygen ions are reduced to oxygen ions, and the oxygen ions are transferred to the anode 2 via the oxygen ion vacancies in the solid electrolyte, so that a current using the oxygen ions as carriers flows. This current generates a flat portion in a region where the applied voltage is present because oxygen diffusion is controlled by minute gas flow holes 5 provided in a cap-shaped gas diffusion control body 4 that is placed on the cathode side. . This voltage-current characteristic is shown in Figure 2, and the current value at the flat part is called the so-called limiting current, but as seen in the figure, it varies depending on the oxygen concentration, and the relationship between the limiting current value and the oxygen concentration is shown in Figure 3. The limiting current value depends on the oxygen concentration in the gas. For this reason, a method has been adopted in which the oxygen concentration is measured from the limiting current value using the oxygen sensor described above.

The oxygen concentration measurement method using a limiting current type oxygen sensor using a solid electrolyte as described above assumes measurement in an environment where the pressure of the atmospheric gas is normal pressure, that is, around 1 atm. The characteristics shown in FIG. 3 are actually measured values under 1 atmosphere, but the voltage-current characteristics in this case are based on so-called normal diffusion of oxygen gas from the gas flow holes 5. On the other hand, when the pressure of the atmospheric gas is low, the so-called Knudsen expansion 11k becomes dominant.
Whether the gas diffusion is dominated by normal diffusion or Knudsen diffusion depends on the so-called mean free path (m
(ean free path) λ and the diameter d of the gas flow hole 5 of the sensor. That is, d) Normal diffusion when λ (1) Knudsen diffusion when d≦λ (2) As is well known, λ is (
3) Given by Eq.

3.06X10 bow T λ=, t p Cp) (3) Here, σ is the gas impingement diameter (in), and T is the sensor's absolute temperature (
K), P is the total pressure of the atmospheric gas (atm). σ
is 3.47 people for pure oxygen, 3.80 people for nitrogen, and 3.71 people for air. For example, sensor temperature T-4
At 50°C (723K) and P=1 atm, λ is 0.18a for oxygen, 0.15u for nitrogen, and 0.16.1511 for air. Since d of a normal sensor is 30 tm, equation (1) is satisfied. That is, normal diffusion becomes dominant under 1 atm. As is well known, the limiting current value ILIO at this time is expressed by the following equation (4).

IL(OX) = ””' b (I
t) ) (4) Tl where, F: Faraday constant, D: oxygen gas diffusion coefficient in normal diffusion, R: gas constant, T: absolute temperature of sensor, S: gas flow hole area, 1: gas flow hole length, P
: total pressure of the atmospheric gas; Here, the gas diffusion coefficient of oxygen is generally expressed by the following equation (5).

o = o, <”=>”・”-(5')73P Here, Ds is 273K, gas diffusion coefficient under 1 atmosphere, α is a constant (usually 1.5 to 2 and varies depending on the type of gas) It is. Therefore, from equations (4) and (5), in the normal diffusion region, the limiting current value IL(Of) does not depend on the total pressure P of the atmospheric gas and is independent of the oxygen mole fraction, that is, the oxygen concentration. It becomes a straight line and draws a curve as shown in Figure 3 (.

(Problem to be solved by the invention) When normal diffusion is dominant as described above, from equation (4), when pure oxygen, that is, oxygen is 100 The limiting current value IL (021) cannot be measured.As shown in the actual measured value in Figure 2, when the oxygen concentration is high, a high voltage is required for the current value to reach the limiting current value.However, if the voltage is too high If the voltage is multiplied by
According to the formula, the limiting current region is not observed at all (or, even if it is observed, it is in a narrow voltage width region.Also, the applied voltage to obtain the limiting current value cannot be set constant). When the oxygen concentration is high, it is necessary to increase the applied voltage as described above, and there is a difficulty in determining the limiting current value because the flat part of the current is narrow. The relationship between oxygen concentration and oxygen concentration is non-linear, and in practice it is suitable when the oxygen concentration is approximately 30% or less, where the limiting current value and oxygen concentration have a nearly proportional relationship. % or more, the accuracy is poor and if the concentration approaches 100%, it cannot be measured, which is not very preferable.If the atmospheric gas is a gas other than oxygen, the same as above is also applied.

(Means for Solving the Problems) The present invention has been made in order to solve the above problems. As mentioned above, the normal pressure of 1 atm (760 mm1
In the case of an atmospheric gas of the order of g), oxygen diffusion is by normal diffusion, but if the pressure of the atmospheric gas is
It is known that at moderate pressures, oxygen diffusion occurs through so-called Knudsen diffusion. Therefore, the present invention provides a method for measuring the concentration of a specific gas species in the atmosphere by measuring the limiting current value of the atmospheric gas to be measured at a pressure that causes Knudsen diffusion. Since Knudsen diffusion usually occurs at a lower pressure than normal diffusion, Knudsen diffusion cannot be achieved in the measurement gas at normal pressure under the same pressure conditions. However, in this case, Knudsen diffusion can be easily achieved by sucking the atmospheric gas to be measured using a pump or the like and further adjusting the suction amount to a low pressure using a needle valve or the like.

(Function) In the above formula (3), P = 1 mmHg = 1/7'60a
When tn+, the mean free path λ of gas is 14 when oxygen
0I11n, 117jm1 for nitrogen, 122 for air
It becomes p. Therefore, the diameter d of the gas flow hole of a normal sensor
is 30u, but if d is 100p or less, the above (2)
Since the equation is satisfied, Knudsen diffusion becomes dominant. In this case, the limiting current value IL (02) kn is expressed by the following equation (7).

IL Tori k+s=Sejun 辷-0X(oz)
(7) Tl Here, DkR is the gas diffusion coefficient of oxygen in Knudsen diffusion, and the other symbols are the same as above. This gas diffusion coefficient, unlike the equation (5) for normal diffusion, does not depend on the total pressure P of the atmospheric gas but depends on the temperature, and is expressed by the following equation (8).

D m, 1 = 0.486d 117r
(8”) Here, d is the diameter of the gas flow hole of the sensor, and 8 represents the average molecular weight of the gas. Therefore, equation (7) and (
From equation 8), if the structure of the sensor and the composition of the atmospheric gas are constant, the limiting current value IL (Of□7 is the oxygen molar fraction
(ox), and can be measured even in the case of 100% oxygen.

(Example) A limiting current type oxygen sensor as shown in FIG.
s + mHg, and the sensor is heated to 4 with heater 6.
The voltage-current characteristics were measured when heated to 50° C. and the oxygen concentration of the atmosphere gas was varied, and the results shown in FIG. 4 were obtained. In this case, of course, as mentioned above, Knudsen diffusion is dominant, and when the relationship between the limiting current value and the oxygen concentration is determined from FIG. 4, it is linearly proportional as in the above theory. From this, it can be seen that measurement is possible even when the oxygen concentration is 100%, and as can be seen from FIG. 4, it is possible to measure any oxygen concentration by setting the applied voltage to a certain constant value.

In this case, as can be seen from equation (7) above, the limiting current in the Knudsen diffusion region depends on the pressure of the atmospheric gas, so this pressure needs to be known.

The above shows that the pressure of the atmospheric gas is l where Knudsen diffusion occurs.
The measurement results are for mm11g, but if the pressure of the atmospheric gas is high and normal diffusion becomes dominant, the atmospheric gas is sucked in with a pump, etc., and the suction amount is adjusted to a low pressure with a needle pulp, etc. to prevent Knudsen diffusion. If this is allowed to occur, the oxygen concentration can be measured even if the pressure of the atmospheric gas is high.

In addition, if the operating temperature of the oxygen sensor as mentioned above is T, it is expressed as follows, where R is the gas constant and Q is the oxygen ion (0"
−”) in the solid electrolyte, μ is the electrochemical potential between the electrodes, X is the thickness of the solid electrolyte, and ■ is the limiting current value.

As can be seen from equation (9), if μ is constant (determined by the applied voltage), the smaller 1 is, the lower the sensor operating temperature T will be. The gas diffusion coefficient Dkn during Knudsen diffusion is 1 to 2 orders of magnitude smaller than the gas diffusion coefficient during normal diffusion, which is clear by comparing equations (4) and (7) and Figures 2 and 4. As shown above, the limiting current value IL (Oゎ□) during Knudsen diffusion is one to two orders of magnitude smaller than the limiting current value IL (oゎ) during normal diffusion, so in the Knudsen diffusion region, the sensor operates from (9) 2 It can be seen that the temperature T becomes small, that is, the oxygen sensor can operate at low temperatures according to the measuring method of the present invention.

By the way, in actual measurements, it is often difficult to achieve complete Nudsen diffusion due to various conditions, but even if Knudsen diffusion and normal diffusion are mixed, Knudsen diffusion is dominant. Therefore, if Knudsen diffusion can be substantially realized, high gas concentration measurement is possible.

Here, the limiting current type oxygen sensor is a general term for sensors that are provided with a means to limit the supply of oxygen molecules (ions) (or to limit the rate of diffusion) to the solid electrolyte electrode surface having oxygen ion conductivity. A solid electrolyte with electrodes formed on both sides is covered with a hollow capsule in which minute gas flow holes are opened between the outside air, and the limiting current characteristics caused by the gas diffusion resistance of the gas flow holes are In addition to the above-mentioned shape to be used, a part of the capsule is provided with a porous material (a porous material having a large number of fine through-holes, for example, a porous ceramic) instead of a gas flow hole that causes diffusion resistance;
A porous substance is formed to surround one or both sides of a solid electrolyte, or the entire solid electrolyte, a porous diffusion control body is provided on the electrode of the solid electrolyte, and a dense material is further placed on the solid electrolyte to prevent diffusion. A solid electrolyte plate is formed on a part or the entire surface, or a dense layer is formed directly on the electrode, or a solid electrolyte plate with electrodes formed on at least one side on both sides with a slight gap is arranged. Types that utilize the gas diffusion resistance effect due to gaps, types that have electrodes on the inner and outer surfaces of a cylindrical solid electrolyte with one end closed, and a type that has a diffusion control body as described above on one of the electrodes, etc. It is possible to use a system in which the concentration is measured directly by voltage-current characteristics or indirectly by other measurement methods by limiting (rate-limiting) the oxygen ion transport phenomenon in the solid electrolyte. However, when a porous material is used as the diffusion control body, the gas flow pore diameter alone cannot be considered as a factor in the occurrence of Knudsen diffusion, as shown in the above example.
It is determined by a combination of various parameters such as porosity, pore shape, and thickness.

Furthermore, if a gas ion conductor other than the oxygen ion conductor is used, the concentration of other gas species can be measured. For example, when attempting to measure hydrogen gas concentration, an H+ ion conductor is used. Since H1 ions are positive ions, the direction of voltage application and the direction of ion transport are the same, so the diffusion control body is provided on the anode side, contrary to the case of oxygen concentration measurement.

(Effects of the Invention) According to the method for measuring gas concentration according to the present invention, even if the pressure of the atmospheric gas is high as described above, the gas concentration can be zero if it is known.
It is possible to measure the concentration of a specific gas species in the atmosphere in the range of ~100%, and since there is an almost linear proportional relationship between the limiting current value and the gas concentration in this entire range, it can be measured with high accuracy. . Furthermore, the applied voltage can be set to a lower constant voltage, and the heating temperature of the sensor can also be lowered.

[Brief explanation of the drawing]

Fig. 1 (a) is a cross-sectional view of the limiting current type oxygen sensor used in the present invention, Fig. 2 is a graph showing the voltage-current characteristics of the conventional oxygen sensor under normal atmospheric pressure (latm), and Fig. 3 is a graph showing the voltage-current characteristics of the conventional oxygen sensor under normal atmospheric pressure (latm). Figure 4 is a graph showing the relationship between the limiting current value and oxygen concentration obtained from the figure. Figure 4 is a graph showing the voltage-current characteristics of the oxygen sensor under low pressure (1 mmHg) according to the present invention.
The figure is a graph showing the relationship between the limiting current value obtained from FIG. 4 and the oxygen concentration. 1: solid electrolyte, 2 near node, 3: cathode', 4:
Gas diffusion control body, 5: gas flow hole. Agent Patent Attorney Takeuchi Mori Proceedings (formula) July 1, 1988 Director General of the Patent Office Yoshi 1) Takeshi Moon 1, Indication of the case Patent Application No. 032455 of 1988 2, Title of the invention Gas concentration measurement method using solid electrolyte 3, relationship with the case of the person making the amendment Patent applicant address: 5-1 Kiba-chome, Koto-ku, Tokyo Name:
(51B) Fujikura Electric Wire Co., Ltd. Representative Makoto Kagaya - 4, Agent 101 Address 2-15-13-6 Uchikanda, Chiyoda-ku, Tokyo
Column 7 of “Brief explanation of drawings” of specification S subject to amendment, content of amendment

Claims (1)

[Claims] Porous electrodes are provided on both sides of a solid electrolyte with gas ion conductivity, one of which is used as an anode and the other as a cathode.
Using a limiting current type gas sensor equipped with a gas diffusion controller that covers one of the electrodes, in a low-pressure atmosphere where the diffusion of a specific gas species in the atmospheric gas to the sensor element is essentially Knudsen diffusion. A gas concentration measuring method using a solid electrolyte, characterized in that the concentration of a specific gas species in an atmospheric gas is measured by measuring a limiting current.