JPS6359461B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oxygen concentration detector that can accurately and quickly detect the oxygen concentration in gas at a relatively low temperature with a fast response speed and low impedance output.

2. Description of the Related Art Conventionally, oxygen concentration detectors have been known that use an oxygen ion conductive solid electrolyte and measure the oxygen concentration in exhaust gas discharged from an internal combustion engine or the like based on the principle of an oxygen concentration battery. And this oxygen concentration detector is
For example, it is common to use itria-doped zirconia porcelain as a solid electrolyte and platinum as an electrode. Therefore, there are problems such as the impedance as an oxygen concentration detector becomes high, the signal becomes unstable due to the influence of noise, and the response speed tends to be slow. The lower limit was about 350°C. However, the exhaust gas temperature of internal combustion engines, etc. may be around 200℃ to 300℃ below this temperature when starting or idling, and therefore conventional oxygen concentration detectors do not work well for low-temperature gas. There were flaws. In addition, in order to accurately measure the oxygen concentration in a gas to be measured based on the Nernst equation using the principle of an oxygen concentration battery using an oxygen ion conductive solid electrolyte, it is necessary to heat the solid electrolyte to a temperature of approximately 600°C or higher. Yes, electric furnace for heating,
This method requires a temperature controller, etc., and has the drawback that the measuring device is large and complicated.

The present invention has been made to solve these conventional drawbacks, and provides an oxygen concentration detector that can accurately and quickly measure oxygen concentration with good responsiveness and low impedance even in low-temperature atmospheric gases. In an oxygen concentration detector that detects the partial pressure of oxygen in a gas by attaching an electrode to an oxygen ion conductive solid electrolyte and forming an oxygen concentration battery, A first heating means that generates self-heating of the solid electrolyte by applying an alternating voltage with a frequency whose polarization is mainly that of the solid electrolyte, and a second heating means disposed around or in the solid electrolyte. It is a concentration detector.

To explain the configuration of the present invention in more detail, the oxygen concentration in exhaust gas from an internal combustion engine or the like is measured using an oxygen ion conductive solid electrolyte based on the principle of an oxygen concentration battery. Oxygen concentration detectors are required to withstand high temperatures and exhibit fast response speeds even at low temperatures.
For the electrode on the side of the gas to be measured, a metal having a large catalytic ability and a high melting point, such as platinum metal, is used.

The frequency characteristic of the impedance (Z) due to the polarization of alternating current in this oxygen concentration battery is a complex impedance representation of Z-Z'-jZ'', as shown in Figure 1, in the form of two series of circular arcs, and its impedance characteristic is approximately expressed as the equivalent circuit shown in FIG.

In Figure 2, R ₁ is the polarization resistance at the interface between the electrode and the solid electrolyte, R ₂ is the resistance at the grain boundaries of the solid electrolyte, R ₃ is the resistance within the crystal grains of the solid electrolyte, and C ₁ is the resistance between the electrode and the solid electrolyte. The capacitance due to the polarization of the interface, _C2 is the capacitance of the grain boundaries of the solid electrolyte. In FIG. 1, the value at point A corresponds to R ₁ +R ₂ +R ₃ in FIG. 2, the value at point B corresponds to R ₂ +R ₃ , and the value at point C corresponds to R ₃ . In addition, the polarization of the oxygen concentration battery from point A to point B is mainly based on R ₁ and C ₁ , and from point B to C
Up to this point, it is mainly based on R ₂ , R ₃ , and C ₂ .
The relationship between each point and the frequency is that at point A, the frequency is direct current, and the frequency increases as you move along the arc toward point B, and the frequency further increases as you move along the next arc toward point C.

The arc from point A to point B changes greatly depending on the electrode attachment condition, long-term use, etc., so when applying an AC voltage with a frequency in this range, it is not possible to stably apply the power necessary for heating. This is difficult, and the large polarization voltage at the interface between the electrode and the solid electrolyte causes peeling of the electrode, deterioration of the solid electrolyte, etc., and a slight difference in the polarization characteristics of the two electrodes causes bias in the DC component.

The first heating means of the present invention is one in which an alternating current with a frequency on an arc from point B to point C is passed through the solid electrolyte, in other words, an alternating current with a frequency where the polarization of the alternating current component is mainly the polarization of the solid electrolyte. However, when these currents are passed, even if the current is large enough to heat the solid electrolyte, the electrodes will not peel off, the solid electrolyte will not change in quality, and the DC component will not be biased. There is no such thing. That is B
When an AC voltage with a higher frequency than the point is applied, most of the polarization is applied inside the solid electrolyte corresponding to R ₂ , C ₂ , and R ₃ , but inside the solid electrolyte, the polarization is uniform in the thickness direction of the solid electrolyte. Because it is dispersed, deterioration due to electricity is unlikely to occur, whereas deterioration usually occurs.
This is thought to be because almost no polarization occurs at the interface between the electrode and the solid electrolyte corresponding to R ₁ and C ₁ and there is no effect on the interface.

Furthermore, the frequency on the arc from point B to point C is determined by the characteristics of the solid electrolyte itself, so it is less affected by the adhesion state of the electrodes, changes due to durability, etc. When applying an AC voltage with a frequency where the polarization of the solid electrolyte is mainly due to the components of the solid electrolyte, the impedance becomes low and stable, ranging from a few times to several tenths of that of the DC resistance. can be heated stably.

In other words, the value of R ₁ usually decreases as the temperature decreases.
R ₂ and R ₃ suddenly become higher than that, and this limits the lower limit of the operating temperature of the oxygen concentration detector.
In the present invention, as a means to reduce the value of R ₁ , an AC voltage is applied at a frequency at which the polarization of the AC component of the oxygen concentration battery is mainly the polarization of the solid electrolyte, that is, a frequency on a circular arc from point B to point C. Therefore, regardless of the value of R ₁ , a current is passed through R ₂ +R ₃ or R ₃ caused by the polarization of the solid electrolyte to heat it.

Note that even if the polarization of the AC component is within the frequency range mainly due to the polarization of the solid electrolyte, it is preferable to use C ₂ in FIG. 2 to prevent local heating.
It is best to heat at a frequency where the impedance of R2 is smaller than _R2 . Furthermore, in the present invention, R ₂ , C ₂ , and R ₃ inside the solid electrolyte are not single resistors or capacitors, but have negative temperature coefficients of resistance, as shown schematically in an enlarged view of one specific example in Figure 3. For example, the temperature of one of these particles R _' Even if for some reason increases, the resistance decreases and current flows easily, that current 1' will connect to that specific R' ₃ C' ₂
The value determined by the applied voltage v′ and frequency
i′=2πC′ ₂ v′ or more does not flow, and 1
Since the voltage v' applied to a point and the local C' ₂ are extremely small, local concentration of current is eventually prevented.For example, when direct current is simply passed through a conventional solid electrolyte, or from point A to point B in Figure 1. There is no localized heating that occurs when low-frequency alternating current corresponding to the range is applied, and even when electrodes are provided on both sides of a flat plate, the entire plate can be heated to a uniform temperature.

Note that the frequency between _point B and point C varies depending _on the composition, temperature, shape of the _solid electrolyte, shape of the electrode, etc., and is not constant. On the other hand, in an oxygen concentration battery with platinum electrodes attached to the inner and outer surfaces of porcelain with an outer diameter of 3.5 mm at the tip, an effective length of 10 mm, and a thickness of 0.75 mm, with 3 parts of clay added, the B point is 10 Hz at 350°C. , point C is approximately 50KHz or higher.

In addition, solid electrolytes such as zirconia porcelain have negative resistance-temperature characteristics, and at room temperature, the resistance R ₂ of the crystal grain boundaries of the solid electrolyte and the resistance R ₃ within the crystal grains of the solid electrolyte shown in Figure 2 are significantly large. In order to generate self-heating by applying AC voltage, the temperature of the solid electrolyte must be set to the temperature T _S shown in Figure 4.
(About 2000℃ for zirconia solid electrolyte)
It is necessary to preheat the solid electrolyte by means of a second heating means, which is an auxiliary heating source provided around or in the solid electrolyte.

Once the solid electrolyte self-heats due to the application of AC voltage and its resistance decreases, heating by the second heating means is either stopped or Even if it is weakened, it can continue heating even at temperatures lower than T _S due to self-heating caused by the supply of alternating current, which is the first heating means, and is a low-voltage power source that requires extremely little power and has little high-frequency induction interference to the surrounding area. can be used to operate the oxygen concentration detector.

Further, a specific example of the present invention will be explained based on FIG. 5. An internal electrode 4 made of platinum or the like is provided on the inner surface of a bottomed cylindrical solid electrolyte 3 made of zirconia porcelain doped with ittria, etc. An external electrode 5 is provided on the surface to form an oxygen concentration cell in pair with the internal electrode 4, and the portion in contact with the exhaust gas is preferably covered with a porous protective layer (not shown), similar to known oxygen concentration detectors. ) is covered. The internal resistor 4 and the external resistor 5 are connected via an AC power supply 7 that applies an AC voltage with a frequency where the polarization of the AC component is mainly due to the polarization of the solid electrolyte, and a DC component blocking capacitor 8. A DC voltage detector 9 is connected between the electrode 4 and the external electrode 5. Further, a heating wire 6 serving as a second heating means is inserted inside the internal electrode 4 inside the bottomed cylindrical solid electrolyte tube, and is connected via a DC power source 12 and a switch 13 to form an oxygen concentration detector.

Note that the electrode for applying an AC voltage provided on the solid electrolyte 3 may serve both as the internal electrode 4 and the external electrode 5 of the oxygen concentration battery, as shown in FIG. Alternatively, either one of the electrodes may be used also.

In addition to the example shown in FIG. 5, as the second heating means, a heating element 10 such as a nichrome wire or silicon carbide may be arranged around the outer periphery of the solid electrolyte 3 as shown in FIGS. 6 to 8. The metallized layer 11 and the like may be placed on the surface of the solid electrolyte 3 or buried in the solid electrolyte 3. Further, the second heating means is not limited to a heating element using an electric current, and may be heating using, for example, infrared rays, flame, electromagnetic induction, or the like.

Next, examples of the present invention will be described.

Example 1 A mixture consisting of 95 mol% ZrO ₂ and 5 mol% Y ₂ O ₃
The outer diameter of the tip of the element is 1.5 mm, the inner diameter is 1.5 mm, and the tip is made of zirconia porcelain made by adding 3 parts by weight of clay to 100 parts by weight.
As shown in FIG. 5, an internal electrode 4 and an external electrode 5 are provided on the inner and outer surfaces of a bottomed cylindrical solid electrolyte of mm in size, and a porous layer of spinel is attached to the outer surface, and the inner electrode 4 is A heating wire 6 made of a nichrome wire is inserted into the electrode, and an AC power source 7, a DC component blocking capacitor 8, and a DC voltage detector 9 are connected between the inner and outer electrodes 4 and 5, respectively, as shown in FIG. A detector was prepared. Then, an oxygen concentration detector is installed in the exhaust gas at 20°C, and the heating wire 6, which is the second heating means, is energized, and the solid electrolyte is heated to approx.
Preheated to 350°C. Thereafter, a solid current of 100 KHz 0.5 A at a frequency in which the polarization of the alternating current component, which is the first heating means, mainly corresponds to the polarization of the solid electrolyte, was then applied to the solid electrolyte, thereby causing the solid electrolyte to self-heat.
Thereafter, heating by the heating wire 6, which is the second heating means, was stopped. As a result, solid electrolytes consume less power
It continued to self-heat at 5W and stabilized at 700℃, and with the solid-state concentration battery's DC resistance of 30Ω, it was possible to stably and accurately measure the oxygen concentration in exhaust gas for a long time.

As described above, the present invention provides a first method for self-heating a solid electrolyte by applying an AC voltage at a frequency such that the polarization of the AC component is mainly the polarization of the solid electrolyte to the electrodes provided on the oxygen ion conductive solid electrolyte. This oxygen concentration detector has a heating means and a second heating means placed around or in the solid electrolyte, so it can stably operate the oxygen concentration battery even when in contact with low temperature gas. This is an oxygen concentration detector that can accurately and quickly detect oxygen concentration with good responsiveness and low impedance based on the principle of It is a useful oxygen concentration detector.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the complex impedance characteristics of the oxygen concentration battery, Fig. 2 is an explanatory diagram showing the equivalent circuit of the oxygen concentration battery, and Fig. 3 is an explanatory diagram showing the relationship between the microstructure of the solid electrolyte and the equivalent circuit. FIG. 4 is an explanatory diagram showing the relationship between the ambient temperature of exhaust gas etc. and the temperature of the solid electrolyte, FIG. 5 is an explanatory diagram showing a specific example of the oxygen concentration detector of the present invention, and FIGS. FIG. 8 is an explanatory diagram showing a different specific example of the oxygen concentration detector of the present invention. 1... Fine particles having a negative temperature coefficient of resistance, 2...
...High resistance region layer, 3... Solid electrolyte, 4... Internal electrode, 5... External electrode, 6, 10, 11... Heating element, 7... AC power supply, 8... Capacitor, 9...
...DC voltage detector, 12...DC power supply, 13...
Switch.

Claims (1)

[Claims] 1. In an oxygen concentration detector that detects the partial pressure of oxygen in a gas by attaching an electrode to an oxygen ion conductive solid electrolyte to configure an oxygen concentration battery, the polarization of the alternating current component is mainly caused by the polarization of the alternating current component at the electrode provided on the solid electrolyte. a first heating means that applies an AC voltage having a frequency corresponding to the polarization of the solid electrolyte to self-heat the solid electrolyte;
An oxygen concentration detector having a second heating means disposed around or in the solid electrolyte.