JP2583209B2 - Solid electrolyte type oxygen sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oxygen sensor using a solid electrolyte.

(Prior art) An oxygen sensor using a solid electrolyte has a detection electrode on one side of a substrate made of stabilized zirconia and a reference electrode on the other side, and detects an electromotive force based on oxygen partial pressure on both sides of the substrate. However, it is necessary to heat the solid electrolyte to a high temperature of 600 ° C, where the solid electrolyte exhibits ionic conduction, which is not only troublesome to handle, but also complicates the heating mechanism and increases the size of the device. There's a problem.

In order to solve such a problem, a solid-state oxygen sensor using a fluoride ion conductor, for example, a single crystal of lanthanum fluoride as a detection substance has been proposed. According to this, although operation at room temperature is possible, there is a disadvantage that it takes about 90 minutes to reach 90% response and the response speed is extremely low.

(Object) The present invention has been made in view of such a problem, and an object of the present invention is to provide a solid electrolyte type oxygen sensor which has a quick response without requiring a special heating mechanism. is there.

(Summary of the Invention) That is, the feature of the present invention is that silver iodide 35
Fluoride ion conductor layer is provided on one surface of a substrate having a composition of about 30 to about 58 mol%, silver oxide of about 19 to 30 mol%, and tungsten oxide of about 23 to 35 mol%, and air permeability is provided on the surface of the ion conductor layer. And the second electrode is formed by providing a metal silver layer on the other surface of the substrate.

(Embodiment) Therefore, the details of the present invention will be described below based on an illustrated embodiment.

FIG. 1 shows an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes 35 to 58 mol% of silver iodide, and 19 to 30 of silver oxide.
This is a glass substrate having a composition of so-called high silver ion conductivity, having a composition of 23 mol% and 23 to 35 mol% of tungsten oxide. A lanthanum fluoride (LaF ₃ ) layer 2 is provided on one surface of the glass substrate by vapor deposition or the like. A first electrode 3 having air permeability is provided on the surface thereof, and a metal silver layer is provided on the other surface of the substrate 1 to form a second electrode 4, and only the first electrode 3 is communicated with the outside air. A seal 5 made of a polymer material or the like is provided on the periphery so as to perform the operation. Reference numerals 6 and 7 in the figure denote lead wires connected to the first and second electrodes 3 and 4, respectively.

In this embodiment, while maintaining the temperature of the substrate 1 at 25 ° C., an atmosphere composed of a mixture of O ₂ gas and N ₂ gas is injected,
When the electromotive force of the sensor with respect to the oxygen concentration was measured,
As shown in FIG. (Where E ₁ is a constant, n is 1 mol of oxygen gas at the sensing electrode)
Represents the number of electrons (mol number) involved in the electrode response. ) A good linearity response represented by the following Nernst equation of about n = 2 was obtained.

Next, when the oxygen partial pressure was increased stepwise and the change in the electromotive force of the sensor was investigated, the time required for 90% response was about 9 minutes regardless of the oxygen partial pressure, as shown in FIG. Was.

In the sensor configured as described above, at the interface between the metallic silver forming the second electrode 4 and the substrate 1, Ag = Ag ⁺ + e 2. At the interface between the substrate 1 and the lanthanum fluoride layer 2, The reaction Ag ⁺ + F ⁻ = AgF (3) occurs between the silver ion Ag ⁺ generated in the process (Equation 2) and the fluoride ion F ^{− in} LaF ₃ that has migrated to the interface.

By the way, LaF ₃ crystal has a low activation energy of Schottky defect generation of 0.069 eV.
Has a y defect.

In addition, F in LaF ₃ has three sites that are not equivalent in energy, two of which are dominated by covalent bonds, while the other third site forms a layer and forms a 60% ion Due to the bond and 40% of the π bond, the bond is weaker than the other two sites. For this reason, fluoride ion vacancies are easily generated at the third site, and it is considered that F ⁻ ions move quickly using the vacancies as carriers.

On the other hand, at the interface between the oxygen O ₂ passes through the electrode 3 electrode 3 and the lanthanum fluoride layer ^{_{2, O 2 + 2F F X}} + 2e = 2O F X + 2F - the ... 4 (O _F ^X is LaF ₃ F ^- Site O entered the ^- ion, F _F ^X is fluoride ions F regular lattice sites in the lanthanum fluoride ^- reacts with lanthanum fluoride by representing the) consisting electrode reaction. Oxide ions O from oxygen atom O ^- generation of, O ^-2 corner less energy is only 5.8eV than when generating the ions, also fluoride ions F ^- radius is 1.33A, and oxide ions
Since the radius of O ⁻ is relatively small at 1.76 °, it is considered that the replacement of the fluoride ion and the oxide ion O ⁻ is performed without significantly distorting the lattice of LaF ₃ .

By this substitution, the generated fluoride ions move through the vacancies in the lanthanum fluoride to the glass-lanthanum fluoride interface, and AgF is generated according to Equation 3.

Thus, between the first electrode 3 and the second electrode 4, as the total reaction in the sensor, since the electromotive force based reactions ^{_{2Ag + O 2 + 2F F X}} = 2O F X + 2AgF ... 5 that the two-electron reaction occurs, The electromotive force of the sensor is (However, E ₂ represents a constant, and a represents each activity.)

By the way, the LaF ₃ lattice near the interface between the first electrode 3 and the lanthanum fluoride layer 2 already contains a certain amount of oxide ions.
Although O ^- is irreversibly dissolved in the solid solution, its composition does not substantially change in the electrode reaction as shown in Formula 8, Is constant irrespective of the oxygen partial pressure (a _Ag and a _AgF are constant because Ag and AgF are solid phases in which the composition does not change). Becomes

Thus, when the substrate 1 is at room temperature, 10 ^{−2 to} 10 ⁻³ Scm
^-1 and a silver ion conductive glass having an extremely high ion conductivity, and metallic silver is disposed on one surface of the substrate.
The interface forms a solid reference electrode with a low interface resistance and a stability that returns to an equilibrium state quickly even with disturbance due to the synergistic effect with the extremely rapid reversible occurrence of the electrode reaction shown in Therefore, the response speed is extremely high as compared with the case where only LaF ₃ is used.

That is, in general, the total resistance of the device using the solid electrolyte is often governed by the interface resistance. However, according to the above-described embodiment, the interface resistance can be reduced. 10 ⁺⁴ Ω) is reduced to one thousandth of that of a conventional sensor using only lanthanum fluoride single crystal (about 10 ⁺⁷ Ω).

In this embodiment, the surfaces other than the detection electrodes are sealed with a mold. However, it is apparent that the same effect can be obtained by sealing with a case having one end opened.

Example A 0.5 mol / water solution of each of AgNO ₃ , K ₂ WO ₄ and KI was prepared, and an aqueous solution of AgNO ₃ was slowly added to the aqueous solution of KI and K ₂ WO ₄ to form a precipitate. The precipitated AgI and Ag ₂ WO ₄ was washed with distilled water 10 to 15 times, then with methanol and filtered through a glass filter. The filtrate was dried for several days while reducing the pressure with an aspirator, and then dried at 120 ° C. for about 6 hours under reduced pressure.

AgI, Ag ₂ WO ₄ (or commercially available A
g ₂ O) and commercially available WO ₃ were precisely weighed to a desired molar ratio, and then pulverized and mixed with an agate milk stick (Ag ₂ WO ₄ was Ag ₂ O: WO ₃ = 1: 1 in molar ratio). Can be converted as). This mixed powder is placed in a heat-resistant glass tube, preheated in a nitrogen atmosphere at a temperature of 400 ° C. for 1 hour, then heated and melted at 600 ° C. for about 5 hours, and the melt is allowed to cool in the air to form a bulk glass body. I got

This bulk glass body was cut into a thin plate of about 5 mm × 5 mm × 2 mm in width and width, and one surface was wrapped to obtain a mirror surface.

The substrate thus formed is placed in a bell jar with the mirror surface facing downward, and a heater is arranged on the other surface to maintain the substrate temperature at 125 ° C., and powdered lanthanum fluoride LaF _{3 is used} to make a molybdenum boat. And inside the bell jar 10 ^-5
Evacuation was performed to the order of Torr, and powdery lanthanum fluoride LaF ₃ was evaporated to form a crystalline lanthanum fluoride layer having a thickness of 1 μm only on the mirror surface of the substrate.

A second electrode was formed by applying silver dootite on the surface opposite to the surface on which the lanthanum fluoride layer was formed, and a lead wire was fixed to this electrode. The other surface was molded with epoxy resin, leaving only the lanthanum fluoride layer surface.
At the same time, a copper wire for maintaining electrical contact with the first electrode was bonded onto the epoxy resin near the lanthanum fluoride layer.

Applying a voltage of 2.4 KV to the surface of the lanthanum fluoride layer and the lead wire portion in an argon gas atmosphere with platinum as a target material, and sputtered intermittently 6 times at an ionization current of 5 mA for 30 seconds each to form a breathable platinum layer. Thus, a first electrode was formed.

When the above-described standard gas was measured while changing the temperature of the sensor thus configured in the range of 5 to 80 ° C., the electromotive force was expressed by the equation 1 with respect to the oxygen concentration, as shown in FIG. Based on this, the measurement results changed. From this,
In practice, it was found that operation was possible by keeping the solid electrolyte at 5 ° C. or higher.

Further, the thickness of the lanthanum fluoride LaF ₃ vapor deposition film formed on the substrate was changed from 0.05 μm to 100 μm to form a sensor, and the standard gas was measured at a substrate temperature of 5 to 80 ° C. As shown in FIG. Almost the same measurement results were obtained. From this, if the thickness of the lanthanum fluoride layer is 0.05 to 100 μm, the temperature is 5 to 8 μm.
It has been found that it can operate within the range of 0 ° C.

Next, by changing the mixing ratio of the three components of _{AgI-Ag 2 O-WO 3} , to create a sample by the method described above, was investigated whether the glass ratio, results as shown in FIG. 4 I got

That, AgI content 35~58mol%, Ag ₂ O content 19~30mol
%, A range along the series of WO ₃ / Ag ₂ O = 1 having a WO ₃ content of 23 to 35 mol% was found to be vitrified by natural air cooling. Represents a partially vitrified material, ● represents a completely crystalline material, and ▲ represents a silver precipitate.

Table 1 shows the electrical conductivity and the glass transition temperature of the vitrified sample. In terms of the electrical conductivity, the values are as high as 10 ^-2 to 10 ^-3 Scm ^-1 at room temperature, and there is no significant difference. However, the composition of 7AgI-6Ag ₂ O-7WO ₃ (35-30-35 mol%) showed the highest value of glass transition temperature (Tg) of 190 ° C. and was found to be the most suitable as a deposition substrate. did.

Furthermore, instead of lanthanum fluoride, lead fluoride (PbF ₂ )
When a sensor was manufactured by a similar process using a typical fluoride ion conductor such as lead tin fluoride (PbSnF ₄ ) and calcium fluoride (CaF ₂ ), the same sensor as that manufactured by lanthanum fluoride was used. Performance was confirmed.

(Effects) As described above, according to the present invention, one surface of a substrate having a composition of silver iodide 35 to 58 mol%, silver oxide 19 to 30 mol%, and tungsten oxide 23 to 35 mol% Since the fluoride ion conductor layer was provided, the first electrode having air permeability on the surface of the conductor layer, and the metal silver layer was provided on the other surface of the substrate to form the second electrode, The substrate has an extremely high ionic conductivity of 10 ^{-2 to} 10 ^-3 Scm ^-1 at room temperature while utilizing the oxygen detection characteristics at room temperature of lanthanum fluoride, and metallic silver is disposed on the second electrode side Since the internal resistance is low at the interface and the equilibrium state is quickly returned to disturbance due to the synergistic effect with the rapid and reversible occurrence of the electrode reaction at the interface, the response characteristics at room temperature can be improved. Can be planned.

[Brief description of the drawings]

FIG. 1 is a sectional view of an apparatus showing one embodiment of the present invention,
FIG. 3 is a diagram showing the response characteristics of the above device, and FIG. 4 is a diagram showing the composition of the solid electrolyte used in the present invention. DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Fluoride ion conductor 3 ... 1st electrode 4 ... 2nd electrode 7 ... Mold

Claims (1)

(57) [Claims] (1) 35 to 58 mol% of silver iodide, 19 to 30 of silver oxide
A fluoride ion conductor layer is provided on one surface of a substrate having a composition of 23 mol% and 23 to 35 mol% of tungsten oxide, and a first air-permeable layer is formed on the surface of the conductor layer.
A solid electrolyte type oxygen sensor, wherein a second electrode is formed by providing the electrode described above and a metal silver layer on the other surface of the substrate, and only the first electrode is communicated with the outside air.