JPH01131034A - Solid electrolyte for sensor - Google Patents

Abstract

PURPOSE:To obtain the title solid electrolyte having a high glass transition temperature and ionic conductivity, by vitrifying AgI, Ag2O and WO3 components of a specific composition. CONSTITUTION:An aqueous solution of AgNO3 is mixed with aqueous solutions of K2WO4 and KI to provide precipitates, which are then washed, separated by filtration and dried to afford a mixture of AgI with Ag2WO4. WO3 is subsequently added and blended with the above-mentioned mixture and the obtained blend is pulverized, melted by heating and vitrified to provide a solid electrolyte, consisting of 35-58mol% AgI, 19-30mol% Ag2O and 23-35mol% WO3, having 130-190 deg.C glass transition temperature and 10<-2>-10<-3>Scm electric conductivity (ambient temperature) and useful as sensors. The resultant electrolyte is cut into the form of a thin sheet to provide a substrate 1. The second electrode 6 consisting of an LaF3 layer 2 and air-permeable platinum layer is then formed on the specular surface side of the substrate 1 and the first electrode 3 having a lead wire 4 secured to the other surface is simultaneously formed. Other surfaces are molded with a resin 5, leaving the LaF3 layer 2. A copper wire 7 is bonded onto the resin 5 to constitute a sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a solid electrolyte with high ionic conductivity suitable for constructing a solid-state chemical sensor.

(Prior art) A chemical sensor using a solid electrolyte basically has a reference electrode on one surface of an electrolyte substrate, and a sensitive material layer and a detection electrode on the other surface. This solid electrolyte electrochemically couples the oxidation or reduction reactions at the two poles by ionic conduction, and also has physical controls to prevent the active substances at the poles from mixing with each other. It also functions as a bulkhead.

For example, in a zirconia oxygen sensor, a detection electrode is provided on one surface of a substrate made of a stabilized zirconia sintered body, and an air reference electrode is provided on the other surface, and the electromotive force based on the oxygen partial pressure at the detection electrode is detected. It is composed of However, in order for the stabilized zirconia sintered body, which is a solid electrolyte, to exhibit ionic conduction, it requires a temperature of 600°C or higher, which is not only troublesome to handle, but also requires a heating mechanism, resulting in structural problems. The problem is that it complicates things.

In order to solve these problems, we are using polycrystalline solid electrolytes such as A9I and CaF2, which exhibit high ionic conductivity at relatively room temperature (near temperature), to lower operating temperatures, downsize, and improve response speed. Chemical sensors have also been proposed, but their ionic conductivity is still low and has not yet reached the practical level.

In order to meet these demands, recently A9I-A92
MOO4, A9I A92 Cr207, A9I-A
Highly ionic conductive glasses containing A9 It as a main component, such as 92 As04, have been developed and are excellent in exhibiting high ionic conductivity.

However, in order to miniaturize and improve the functionality of sensors, the formation of sensitive material layers and electrode layers relies on vapor deposition and ion blating, and the smoothness and thermal stability of the solid electrolyte substrate and the thin film The processability is an important factor,
The temperature at which glass becomes liquid, the so-called glass transition point, is still low, making it difficult to maintain a sufficient substrate temperature during vapor deposition or ion blating processes, and processing is difficult.

(Purpose) The present invention was made in view of these problems, and its purpose is to provide a solid electrolyte that has both a high glass transition temperature and high ionic conductivity, and which can be processed by grinding etc. Our goal is to provide the following.

(Summary of the invention) That is, the present invention is characterized in that silver iodide 35
The point is that components having a composition of 58 mol % to 58 mol %, silver oxide 19 to 30 mol %, and tungsten oxide 23 to 35 mol % are vitrified.

(Example) Therefore, the details of the present invention will be explained below based on an example.

Prepare 0.5 mol/A aqueous solutions of A 9 N O3, 2W O−+ and KI each, and add 2W○ to KI and 2W○.
A 9 N O 3 aqueous solution was slowly added to the aqueous solution of 4 to form a precipitate, and the precipitated 89 and A92WO,I were washed with distilled water 10 to 15 times, then with methanol, and filtered through a glass filter. After drying the filtered material for several days under reduced pressure with an aspirator, it was dried for several days under reduced pressure.
It was dried at ℃ for about 6 hours.

The △9 process synthesized by the above process, A92W0.1 (or commercially available A920), and commercially available WO3a were accurately weighed to the desired molar ratio, and then ground and mixed in an agate mortar (A92WO4 has a molar ratio of Te A 920: WO3
= 1: 1).

This mixed powder was placed in a heat-resistant glass tube, preheated at a temperature of 400°C for 1 hour in a nitrogen atmosphere, and then heated at 600°C for 5 hours to melt it. A vitreous body was obtained.

Next, samples were prepared by the method described above while changing the mixing ratio of the three components of A9I-A920 WO2, and the presence or absence of vitrification was investigated, and the results shown in FIG. 1 were obtained.

That is, Ac+1 content 35-58m01%, A920
Content 19-19-3O%, 2S-35mol containing WO3 fluorescein
It was found that materials in the range along the composition line of %W O3 / A 920 = 1 were vitrified by natural air cooling (in the figure, ○ indicates those that were vitrified, and C indicates those that were only partially vitrified. , . represents completely crystalline material, and .mu. represents silver precipitated material).

Table 1 shows the electrical conductivity and glass transition temperature of the vitrified sample, and the electrical conductivity is 1 at room temperature.
There was no big difference, showing high values of 0-2 to 10-10-3S+, but 7A9I-6A920-7W03 (3
5-30-35 mo1%) showed the highest glass transition temperature (T9) of 190° C. and was found to be most suitable as a substrate for vapor deposition.

Next, a method for constructing a sensor using the solid electrolyte thus obtained will be described.

Cut the bulk glass body mentioned above into a piece with a length and width of approximately 5 mm using a cutting tool.
It was cut into a thin plate measuring 5 mm x 2 mm, and one side was wrapped to give it a mirror surface.

The substrate 1 thus formed was placed in a Herjar with the mirror side facing downward, and a heater was placed on the other side to heat the substrate to 125°C, which is a temperature suitable for vapor deposition, and powdered fluorine was applied to the substrate. LaFsi is housed in a molybdenum port, the inside of the Pelger is evacuated to the order of 1O-5 Torr, and powdered fluoride LaF3 is evaporated to form crystalline fluoride with a thickness of 1L1m only on the mirror side surface of the substrate 1. A lanthanum layer 2 was formed.

During this vapor deposition process, the substrate 1 was heated to a temperature of 125° C., but it still maintained its glassy state without inducing crystallization.

Applying silver dotite to the surface opposite to the surface on which the lanthanum fluoride layer 2 is formed to form the first electrode 3,
A lead wire 4 was fixed to this electrode 3. Leaving only the surface of the fluorinated porcelain layer 2 intact, the other surfaces were molded with epoxy resin 5. At the same time, a copper wire 7 for maintaining electrical contact with the second electrode 6 was bonded onto the epoxy resin 5 near the fluorinated titanium layer.

Using platinum as a target material in an argon gas atmosphere, a voltage of 2.4 KV is applied to the surface of the fluorinated titanium layer 2 and the Riet wire.
6 intermittently for 30 seconds at an ionization current of 5 mA by applying
The second electrode 6 was formed by repeating sputtering to form a platinum layer having air permeability.

In this example, while maintaining the substrate at a temperature of 25°C, an atmosphere consisting of a mixture of 02 gas and N2 gas was injected, and the electromotive force of the sensor relative to the oxygen concentration was measured, as shown in Figure 3. (where, El is a constant, n is oxygen gas 1 at the sensing electrode
Each represents the number of electrons (mol number) involved in electrode response including mol. ) It showed a response with good linearity expressed by the Nernst equation.

Next, we investigated the change in the electromotive force of the sensor by increasing the oxygen partial pressure in stages, and found that the time required for a 90% response was approximately 9 minutes, regardless of the oxygen partial pressure, as shown in Figure 4. Ta.

In the sensor configured in this way, at the interface between the metal silver constituting the first electrode 3 and the substrate 1, A9=Aq''+e
-2 Furthermore, at the interface between the substrate 1 and the lanthanum fluoride layer 2, A9'' +F-=A9F...3 reaction occurs.

However, LaF3 crystal has a very large number of 5-chottky defects at room temperature because the activation energy for 5-chottky defect generation is as low as 0.069 ev.

In addition, there are three sites in F in LaF3 that are not equivalent in energy, and 152 of them are dominated by covalent bonds, while the other 3rd site forms a layer and has a 60%
This is due to ionic bonds of 40% and ■ bonds of 40%, and the other 2
The binding is weaker than that of two sites. For this reason, fluorine ion holes are likely to be generated at the third site, and it is thought that F- ions rapidly move using these holes as carriers.

On the other hand, oxygen 02 passes through the electrode 6 and at the interface between the electrode 6 and the lanthanum fluoride layer 2, Q2+2Fp +2e=2
0ax+2F----4 (0γ is the ○- ion that has entered the F-site of LaF3, Fi represents the fluoride ion F-7a at the regular lattice site in lanthanum fluoride) Through the electrode reaction, lanthanum fluoride and react.

The generation of oxide ion ○- from oxygen atom ○ requires 8.8 eV less energy than the generation of oxide ion ○-2, and the radius of nodide ion F- is 1.33
Since the radius of the oxide ion 〇- is relatively small (1.76 people), it is thought that the substitution between the fluoride ion and the oxide ion 〇- takes place without significantly distorting the lattice of LaF3. .

Due to this substitution, the produced fluoride ions move through the vacancies in the lanthanum fluoride to the glass-lanthanum fluoride interface, and according to equation 3, AqF! arise.

Therefore, between the first electrode 3 and the second electrode 4, the total reaction in the sensor is 2A9+02 +2FW = 20ax+2AqF -5
Since the electromotive force reaction based on the two-electron reaction occurs, the electromotive force of the sensor is 6 (where E2 is a constant and a represents each activity).

By the way, although some oxygen ions are already irreversibly dissolved in the LaF3 lattice near the interface between the second electrode 6 and the lanthanum fluoride layer 2, the electrode reaction as shown in No. 28 Then, since its composition does not substantially change, ao
F and aFg are constant regardless of oxygen partial pressure (aAq, aA
Since 9F is constant since 9F and A9F are solid phases whose compositions do not change, the electromotive force is as follows, which is expressed by Equation 1 and the experimental characteristics can be explained.

In this way, the substrate 1 is 10-2 to 10-2 at room temperature.
It is made of silver ion conductive glass having an extremely high ion conductivity of 33 cm-', and metal silver is disposed on one side of the substrate, and the interface between the two is expressed by the formula 2 above. Due to the synergistic effect of the reversible electrode reaction (which occurs rapidly), the interface forms a stable solid reference electrode that has low interfacial resistance and quickly returns to an equilibrium state even in the face of external disturbances. This results in an extremely high response speed.

] 1 In other words, - In boat fishing, the total resistance of an element using a solid electrolyte is often controlled by the interfacial resistance, but since the solid electrolyte of the present invention can reduce the interfacial resistance, the internal resistance of the sensor The resistance (approximately 10'Ω) is the same as the conventional sensor (approximately 10
This results in a reduction to 1/1000 compared to Ω).

Further, using the above substrate, a first electrode is formed on one air-permeable surface of the substrate, a second electrode is formed on the other surface, and molded with only the first electrode exposed. When we constructed a sensor and measured the electromotive force while changing the concentration of chloride ions by immersing the sensor in water, we were able to obtain an electromotive force proportional to the concentration of chloride ions.

Furthermore, when we measured halogen gases such as iodine and bromine using the same sensor, we were able to obtain an electromotive force proportional to the partial pressure of the halogen gas.

From this, it was found that the solid electrolyte is suitable for measuring the partial pressure of halogen compound ions.

Furthermore, a layer of sulfide such as silver sulfide is interposed between the substrate and the first electrode, so that the first electrode has a thickness of 1F! :N was used to construct a sensor, and when this sensor was exposed to hydrogen sulfide gas, an electromotive force proportional to the concentration of hydrogen sulfide gas was generated.

Similarly, silver carbonate, silver sulfide,
By providing a layer of silver cyanide or the like, it was possible to obtain an electromotive force proportional to the partial pressure of carbon dioxide gas, sulfur dioxide gas, and hydrogen cyanide gas, respectively.

(Effects) As explained above, according to the present invention, the composition has a composition of 35 to 58 mol% silver iodide, 9 to 30 mol% silver oxide, and 23 to 35 mol% tungsten oxide, and therefore has high ionic conductivity. It is possible to obtain a solid electrolyte that exhibits a high glass transition temperature, which improves the response speed of sensors, and simplifies the deposition of sensitive materials and electrode metals on the substrate. This makes it possible to simplify processing into thin substrates.

In addition, the substrate formed with this material can be heated to 1
It has an extremely high ionic conductivity of 0-2 to 10-38 cm-', and when metal silver is placed on one side, the electrode reaction occurs quickly and reversibly at the interface, which is synergistic. Due to the action, the internal resistance at the interface is low,
In addition, since it quickly returns to an equilibrium state even in response to disturbances, it is possible to realize a sensor with excellent response at room temperature.

[Brief explanation of the drawing]

Figure 1 shows the composition of the solid electrolyte used in the present invention, Figure 2 is a sectional view showing an example of an oxygen sensor constructed using the solid electrolyte described above, and Figure 3 shows the response of the above device. It is a diagram showing the characteristics. ] ... Substrate 2 ... Fluoride ion conductor 3 ... First electrode 4 ... Second electrode 7 ... Molded

Claims (1)

[Claims] A solid electrolyte for a sensor made by vitrifying components having a composition of 35 to 58 mol% silver iodide, 19 to 30 mol% silver oxide, and 23 to 35 mol% tungsten oxide.