JPS63180847A - Pressure measurement using solid electrolyte - Google Patents

Abstract

PURPOSE:To enable measurement of pressure of an atmospheric gas due to current flowing with gas ions as the carrier, by a method wherein porous electrodes are provided on both sides of a solid electrolyte with one thereof as anode and the other as cathode and a gas diffusion control body is provided covering the cathode to apply a voltage between both the electrodes under a low pressure. CONSTITUTION:An anode 12 and a cathode 13 are provided on both surfaces of a solid electrolyte 11; when a voltage is applied between both the electrodes of the solid electrolyte 11 heated with a heater 16, oxygen contained in a gas is reduced with the cathode 13 to become oxygen ions, which are transferred to the anode 12 through an oxygen ion vacancy in the solid electrolyte 11 to make current flow with the oxygen ions as the carrier. A plateau is generated at an area where the voltage is applied with the control of the diffusion of oxygen through a gas passage hole 15 provided on a gas diffusion control body 14. The so-called threshold current value IL at the plateau is proportional to the concentration of oxygen in a gas, thereby enabling an oxygen sensor to detect the concentration of oxygen from the current value IL.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method of measuring the pressure of a low-pressure atmospheric gas using a pressure sensor, and in particular to a solid electrolyte that can measure pressure at high temperatures. The present invention relates to a method of measuring pressure using a pressure sensor.

(Prior art) There are various methods for detecting pressure, but in recent years semiconductor pressure sensors using semiconductor elements have been widely used in various fields because they have the advantages of small size, high sensitivity, and high accuracy. has been done. Moreover, many pressure detection structures use the displacement of a diaphragm as a medium. An example thereof is an absolute pressure type semiconductor pressure sensor as shown in FIG. In the same figure, 1 is a stem, 2 is a glass pedestal,
3 is a silicon single crystal plate, and this silicon single crystal plate 3
A portion of the lower surface of the diaphragm 4 is etched away to form a circular diaphragm 4. A vacuum or constant pressure is maintained between the diaphragm 4 and the pedestal 2, and four P-type resistors are formed at the periphery of the upper surface of the diaphragm 4 by diffusion or ion implantation techniques. 5
is the gold wire connected to the above resistor, 6 is the lead pin,
7 is a case. When the diaphragm 4 curves downward due to the pressure Pa of the medium to be measured, strain occurs in the resistor formed in the diaphragm 4 accordingly. Then, the pressure Pa of the medium to be measured is detected from the change in the resistance value of the resistor based on this strain. In addition, there are differential pressure type semiconductor pressure sensors, and improved versions of these sensors.

(Problems to be Solved by the Invention) The usable temperature range of a semiconductor pressure sensor mainly composed of a semiconductor element as described above is -10°C.
It is very narrow at about ~50°C, and cannot withstand practical use at temperatures higher than this, and the low-pressure diaphragm has the drawback of being mechanically very fragile.

(Means for solving the problem) Conventionally, an oxygen sensor mainly composed of a solid electrolyte as shown in FIG. 1(a) is known. and cathode 13
is provided, and when a voltage is applied between the two electrodes of the solid electrolyte 11 heated to a high temperature of several hundred degrees Celsius by the heater 16, the oxygen contained in the gas is reduced to oxygen ions at the cathode 13, and these oxygen ions Oxygen ions are transferred to the anode 12 via the vacancies in the solid electrolyte, so that a current using the oxygen ions as carriers flows. This current produces a flat portion in a region where the applied voltage is present, as the diffusion of oxygen is controlled by minute gas flow holes 15 provided in the cap 14 placed on the cathode side. This voltage-current characteristic is shown in Figure 1 (b), and the current value I at the flat part is what is called the limiting current, but since this (1) is proportional to the oxygen concentration in the gas, the above The oxygen sensor has a limiting current value■
1 to detect the oxygen concentration. It has been found that this limiting current value changes depending on the pressure under low pressure of about 100 mmHg or less.

Therefore, the present invention provides a method of measuring pressure in an environment where the oxygen concentration is known by measuring the limiting current value using a pressure sensor having the same structure as a so-called limiting current type oxygen sensor using a solid electrolyte. It is.

(Function) In the voltage-current characteristics shown in FIG. 1 (B) of the limiting current type pressure sensor shown in FIG. Flow hole 1
This is because gas diffusion is rate-limited by 5. The voltage-current characteristics of such a normal limiting current type sensor are based on so-called normal diffusion of oxygen gas from the gas flow hole 15. The limiting current value IL at this time is
As is well known, is expressed by the following equation (1).

I L (n) = ”'Kizuna・6 (1=u%) (1)
RTA P Here, F: Faraday constant, D gas diffusion coefficient of oxygen in binormal diffusion, R: gas constant, T: absolute temperature of sensor, S: gas flow hole area, β: gas flow hole length, P
: total pressure of atmospheric gas, PO2: partial pressure of oxygen in atmospheric gas (here, PO2/P means oxygen concentration in atmospheric gas). However, in the case of normal diffusion, the gas diffusion coefficient of oxygen is generally expressed by the following equation (2).

o = os (-i-"j'・"-(2')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) ). Therefore, from equations (1) and (2), in the normal diffusion region, if the oxygen concentration PO2/P is constant, the limiting current value IL (I+1 does not depend on the total pressure P of the atmospheric gas. I understand.

This normal diffusion occurs when the total pressure P of the atmospheric gas, that is, the pressure is high, but conversely, when the pressure is low, the sensor output becomes dominated by so-called Knudsen diffusion. In this case, the limiting current value ILnr is expressed by the following equation (3).

4FD・・SPo・ (3) T L (k
l ”” RT , 2 where Dkn is the gas diffusion coefficient of oxygen in Knudsen diffusion, and the other symbols are the same as above. Unlike the equation (2) for normal diffusion, this gas diffusion coefficient Ri does not depend on the total pressure P of the atmospheric gas, but depends on the temperature, and is expressed by the following equation (4).

Dk, l = 0.486d, / T/IJ
(4) Here, d is the diameter of the gas flow hole of the sensor, and Q is the average molecular weight of the gas. Therefore, from equations (3) and (4), the sensor has a gas diffusion hole diameter d at a constant temperature.
It can be seen that if the structure of the sensor except for KI-d-PO2 (Kl: constant) (5
) Furthermore, if the gas diffusion hole diameter d is constant, ILfA>=2.Po2 (K2: constant) (6). Therefore, when formula (6) is modified by setting the total pressure of the atmospheric gas as P, it becomes h<*r=Kz·(Po□/P)·P(7), where Po2/P is the oxygen concentration. Therefore, under a constant oxygen concentration, the limiting current value IL(k) is proportional to the total pressure of the atmospheric gas, so this limiting current value ■Lck,
By knowing, the pressure P can be determined.

However, whether normal diffusion or Knudsen diffusion is dominant depends on the so-called mean free path (mea) of the gas.
n free path) λ and the sensor gas flow hole diameter d. That is, d) Normal diffusion when λ (8) Knudsen diffusion when d≦λ (9) Here, λ is <
It is given by equation (10).

λ=jμ　−(-) (10) σ2P where σ is the gas impingement diameter (person), T is the absolute temperature of the sensor (K), and 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, when the sensor temperature is 450°C (723K) and the total pressure of the atmospheric gas is 1 mm).
In the case of Ig (P = 1/760 atm), λ is 140 for pure oxygen, 117P for nitrogen, and 122J for air.
rIn, and the sensor's normal gas flow hole diameter d=30
-, it satisfies equation (9) and becomes a region of Knudsen diffusion. However, when the total pressure of the atmospheric gas becomes very high, Equation (8) is satisfied, and as mentioned above (normal diffusion becomes dominant, but when the total pressure of the atmospheric gas is intermediate in magnitude, d and λ The relationship with is intermediate between equations (8) and (9), and since normal diffusion and Knudsen diffusion coexist, the limiting current (11LIt is an intermediate value between IL(7, and IL~). The relationship with the total pressure of the atmospheric gas, that is, the pressure, will draw a saturation curve.Therefore, if the relationship between the pressure of the atmospheric gas and the limiting current is determined in advance at various oxygen concentrations, the limit will be determined within the range where this relationship is not saturated. Current value I
By measuring L, the pressure of the atmospheric gas can be determined.

(Example) Using a limiting current type pressure sensor as shown in FIG. When heated to 450℃, the voltage-current characteristics were determined for the pressure of various atmospheric gases.
The 0□-N2 system and the 40% 0□-N2 system are shown in Figures 2 (a) and (b), respectively.

The relationship between the current in the flat part of the characteristic curve in the same figure, that is, the limit current value (2), and the pressure of the atmospheric gas is as shown in FIG. 2 (c). Therefore, if the relationship between the limiting current value and the pressure of the atmospheric gas at various oxygen concentrations of the atmospheric gas is determined, the pressure of the atmospheric gas can be determined by measuring the limiting current value at a certain oxygen concentration.

In addition, the voltage-current characteristics (actual measurements at 25°C and 50% R1) when the air contains moisture as an atmospheric gas have two flat parts as shown in Figure 3 (a). Regarding the current characteristics, this is due to the current applied due to the decomposition of water in the flat area of the second stage, and the current due to the ionization of oxygen in the flat area of the first stage. Using the current value as the limiting current value, we were able to obtain the relationship with the pressure of the atmospheric gas in Figure 3 (b). Therefore, if moisture is included, the applied voltage should be selected appropriately (in the case of Figure 3, it is approximately 1 volt), the pressure of the atmospheric gas can be determined by measuring the limiting current value of the flat portion of the first stage.

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 shapes to be used, capsules in which a porous material (a porous material with many fine through-holes, e.g., porous ceramic) is provided in a part of the capsule in place of gas flow holes that cause diffusion resistance, and solid electrolytes. A porous substance is formed to surround one or both sides of the solid electrolyte, or the solid electrolyte has a porous diffusion control body on the electrode, and a dense layer that prevents diffusion is further provided on the solid electrolyte electrode. A solid electrolyte plate formed on a part or the entire surface, or a dense layer formed directly on the electrode, or a solid electrolyte plate with electrodes formed on both sides with a slight gap between them. Some types of solid electrolytes include those that utilize the diffusion resistance effect of solid electrolytes, and types that have electrodes on the inner and outer surfaces of a cylindrical solid electrolyte with one end closed, and a diffusion control body as described above on one of the electrodes. 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. However, when a porous material is used as a diffusion control material, the gas flow pore size alone cannot be considered as a factor for Knudsen diffusion as shown in the above example, and various factors such as porosity, pore shape, thickness, etc. It is determined by a combination of parameters.

(Effects of the Invention) As described above, by measuring the limiting current value in an atmospheric gas of known gas concentration using a pressure sensor having the same structure as a gas sensor based on the present invention, the pressure of the atmospheric gas can be easily determined. . Moreover, the sensor used can withstand high temperatures, so it is possible to perform measurements at high temperatures, and it is durable as it does not have fragile parts such as a diaphragm. Note that this is effective when this pressure is approximately 100 mmHg or less.

It goes without saying that if a hydrogen ion conductive solid electrolyte is used, the method can also be applied to pressure measurement in a hydrogen atmosphere.

[Brief explanation of the drawing]

Figure 1 (a) is a cross-sectional view of the pressure sensor used in the present invention, Figure 1 (b) is a graph showing the tendency of the voltage-current characteristics of the sensor, Figures 2 (a), (b) and 3 Figure (a) is a graph showing the actual measured values of the voltage-current characteristics of the pressure sensor, and Figures 2 (c) and 3 (b) are graphs showing the relationship between the limit current value of the pressure sensor and the pressure of the atmospheric gas. , FIG. 4 is a sectional view of a conventional semiconductor pressure sensor. 11: solid electrolyte, 12 near node, 13: cathode,
14: Microscopic gas flow hole, 15: Gas diffusion control body. Agent Patent Attorney Takeuchi 9 Figure 1 (A) (B) Voltage Figure 2 (B) Figure 2 (C) Power (mmHg) Figure 3 (A) Electric power (V) Special opening aG3-180847 (6) Northern (mmHg)

Claims (1)

[Claims] Porous electrodes are provided on both sides of a solid electrolyte with ion conductivity, one of which is an anode and the other is a cathode, a gas diffusion control body is provided that covers the cathode, and a voltage is applied between both electrodes under low pressure to generate gas ions. A pressure measurement method using a solid electrolyte, characterized in that the pressure of an atmospheric gas is measured by a current flowing as a carrier.