JPH0529024A - Hydrogen ion detecting element and lead acid battery provided with the same - Google Patents

Abstract

PURPOSE:To display hydrogen ion concentration speedily in the form of voltage by forming a niobium oxide thin film on a metallic surface or on a gate insulating film of an electric field effect type transistor FET, and combining this film and a suitable reference electrode with each other. CONSTITUTION:FET has a source l, a drain 2 and a gate 3, and a niobium oxide Nb2O5 thin film is formed on insulation of the gate 3 by means of a plasma CVD method, and a temperature compensating transistor 4 is arranged on the other edge. This FET having ion selectivity and a double junction type silver/silver chloride reference electrode are combined with each other, and it is soaked into various kinds of acid solutions, and a relationship between voltage and hydrogen ion concentration in aqueous solution is found. In this case, voltage between the drain and the source is set to be 2.5V, and electric current of the drain is set to be 100muA. The relationship between the voltage and logarithm of the hydrogen ion concentration shows a linear relationship in a wide range of concentration regardless of kinds of acid. A time until electric potential of the FET is stabilized is considerably short such as within about 5 seconds, so that response speed becomes fast.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element for measuring hydrogen ion concentration in an aqueous solution and a lead storage battery equipped with this element.

[0002]

2. Description of the Related Art As a means for measuring the hydrogen ion concentration in an aqueous solution, there is a method using a hydrogen ion electrode, and a pH meter using a glass electrode is widely known. Also,
Molybdophosphoric acid << Old Carpenter, Murata, Ikeda, Proceedings of 34th Annual Meeting of Japan Society for Analytical Chemistry, 2D05, P489 (1985) >> and dodecylamine << D.Ammann et.al., Anal.Chem., 53,2267 (1981) ) >> and other ion-sensitive substances, for example, so-called polymer-immobilized so-called solid membrane hydrogen ion electrodes have also been proposed. Furthermore, as a hydrogen ion sensitive substance, SiO ₂ , Al ₂ O ₃ ,
Ta ₂ O ₅ oxide and Si ₃ N _4, such as "Matsuo, Esashi, electrochemical, 50, 64 (1982)", or titanium nitride (TiN) "Wakita, Makabe, Mochizuki, Yamane, East, the Chemical Society of Japan An ion selective field effect transistor using the 58th Spring Annual Meeting Proceedings 1,3IG17, P360 (1989) >> has been proposed.

On the other hand, in a lead storage battery, the charge / discharge reaction of the battery follows the following equation.

[Chemical 1]

That is, as the battery discharges, sulfuric acid (H ₂ SO ₄ ) in the electrolytic solution is consumed and its concentration decreases, and conversely, as the charge progresses, sulfuric acid is generated and its concentration increases. . In other words, the consumption amount or production amount of these sulfuric acids is proportional to the discharge electricity amount or the charge electricity amount, respectively, so that the charge / discharge state or the remaining capacity of the lead storage battery can be known by measuring the sulfuric acid concentration. .

On the basis of such a principle, a float type hydrometer and a photorefractive hydrometer have been put to practical use in order to know the charge / discharge state of a lead storage battery. However, these pycnometers are generally large in size and are not used by being integrally attached to a lead acid battery, but rather a portion of the electrolyte solution is once pumped out of the lead acid battery and then its specific gravity is measured. The method has been adopted. Therefore, such a hydrometer cannot be used especially when the lead acid battery is a sealed type.

[0006] Even in a sealed lead-acid battery, as a method for measuring the concentration of the electrolyte, for example, West German Patent 2,254,2
As shown in No. 07 (1973), focusing on the fact that the water vapor pressure in the space above the sulfuric acid solution in vapor-liquid equilibrium with the sulfuric acid solution depends on the concentration of the sulfuric acid solution, the humidity sensor The method of measuring the concentration of sulfuric acid aqueous solution by measuring, and further developing this principle, the humidity sensor measures the partial pressure of water vapor diffused through the pores of the porous polypropylene membrane. Method 《JLWe
ininger et.al., J. Electrochem. Soc. , 129 , 2409 (1982)
], Or an electrode-type hydrometer (for example, JP-A-60-62066) based on the principle that the potential difference between a lead dioxide electrode and a lead electrode depends on the concentration of sulfuric acid.

[0007]

Hydrogen ion electrodes such as pH meters have hitherto been generally called so-called acid error or alkali error in a region beyond the so-called pH measurement region (2 to 12). Since it deviates significantly from the Nernst type straight line, it cannot be used in a strong acid or strong alkaline aqueous solution. The concentration of sulfuric acid aqueous solution used in lead acid batteries is usually in the range of specific gravity of about 1.28 to 1.05 (concentration of about 5.
Since there is 2 mol / l to 0.8 mol / l), there has been no proposal to use a hydrogen ion electrode as a sulfuric acid concentration sensor for a lead storage battery.

As the only exception, in the case of the solid film type hydrogen ion sensor in which molybdophosphoric acid is bound with a binder as described above, the Nernst equation must be obeyed in the concentration range of the sulfuric acid aqueous solution used in this lead acid battery. However, this method is currently under development and is inferior in reliability.

In addition, tantalum oxide (Ta ₂ O ₅ ) exhibits the most excellent characteristics among the methods for measuring the hydrogen ion concentration using a field effect transistor having an oxide or nitride film attached. Reported 《Matsuo, Esashi Applied Physics 49 596 (1980)》. However, this system has the drawback that the potential becomes unstable within a few months if it is immersed in sulfuric acid at the concentration used for the electrolyte of lead acid batteries. In addition, in the case of the method of measuring the sulfuric acid concentration using the conventional humidity sensor, the response is generally slow, and the vapor of sulfuric acid corrodes the humidity sensor. In the case of the above-mentioned electrode type specific gravity sensor, since self-discharge of the electrodes occurs, it is necessary to flow an electric current from the outside to perform electrolytic oxidation-reduction, so that there is a drawback that it is extremely troublesome in operation.

Moreover, in the case of a sealed lead-acid battery, a very small amount of sulfuric acid aqueous solution is generally held in a glass mat which is a separator and an electrolyte holding agent at the same time. Since it does not exist, it is necessary to miniaturize the hydrogen ion detection element so that the sulfuric acid concentration in the glass mat portion can be measured.

[0011]

The present invention relates to niobium oxide (Nb ₂ O ₂₎ as an ion-sensitive substance of a hydrogen ion concentration detecting element.
_The above problem is solved by combining the hydrogen ion electrode having the gate insulating film surface or the metal surface of the field-effect transistor covered with niobium oxide with the reference electrode using ₅ ).

Further, by mounting this hydrogen ion detecting element on a lead storage battery, the concentration of sulfuric acid as an electrolytic solution can be detected, and the charging / discharging state of the lead storage battery can be always known through this. is there.

[0013]

The present inventors have found that a hydrogen ion detection element in which niobium oxide (Nb ₂ O ₅ ) is used as a hydrogen ion sensitive material and the gate insulating film surface or metal surface of a field effect transistor is covered with niobium oxide is Ion concentration is 10 ^-6 mol
In the range of / l to 2 mol / l, a linear relationship was shown between the potential and the logarithm of the hydrogen ion concentration, and it was discovered that its sensitivity (potential change when the concentration was changed 10 times) was about 60 mV.

Although the details of the potential response mechanism of niobium oxide and hydrogen ions are not clear, the ion-selective field-effect transistor or the hydrogen ion electrode using niobium oxide as the hydrogen ion sensitive substance was measured with hydrogen ions. When placed in a solution, the potential E and the hydrogen ion activity [a _H +
] The following Nernst equation holds between mol / l. E = 2.303 x (RT / F) x log [a _H +]

Therefore, the hydrogen ion activity in the solution to be measured can be determined by measuring the voltage between the electrode and a suitable reference electrode. On the other hand, since the relationship between the hydrogen ion activity and the concentration in various acid solutions is known, the concentration can be known from the voltage. Moreover,
This ion selective field effect transistor or hydrogen ion electrode has a hydrogen ion concentration of 10 ^-6 mol / l to 2 mol / l.
Not only the usual thin concentration range, but also the concentration range of sulfuric acid, which is the electrolyte of lead-acid batteries, of 5.2 mol / l to 0.8 mol /
It was discovered that the above Nernst equation holds even in a wide range such as l, and that the potential characteristics do not change even when immersed in sulfuric acid of such a concentration for a long period of more than half a year, leading to the present invention. It was As described above, considering that it is common knowledge that the hydrogen ion electrode generally deviates from the Nernst equation at such a high acid concentration,
First of all, it should be understood that this finding is of great significance.

Niobium oxide is not only highly sensitive to hydrogen ions, but also is not affected by high-concentration sulfuric acid.
This hydrogen ion electrode is one of the most important factors that can be used in lead acid batteries.

As the reference electrode, a conventionally known silver-silver chloride electrode may be used, or it may be incorporated in a field effect transistor together with a hydrogen ion electrode which is a measuring electrode.

In particular, when the above field effect transistor system is used for the hydrogen ion electrode, its cross section is 0.23 × 0.
Since it can be made as thin as about 5 mm, it is easy to install in lead acid batteries, especially sealed lead acid batteries containing a very small amount of electrolyte. It should be understood that this can be understood by considering that the diameter of the cross section of the solid film type hydrogen ion electrode in which molybdophosphoric acid is fixed with polyvinyl chloride can hardly be made to be 10 mm or less.

A specific mounting location is suitable between the electrodes of the lead storage battery and the separator, but is not limited to this. Further, as a lead storage battery, the effect of the present invention is more remarkable in the case of the sealed type, but of course, the effect can be exhibited even in the non-sealed type.

[0020]

EXAMPLES The present invention will be described below with reference to preferred examples. [Example 1] Field effect transistor (FET, size: 0.5 x
A niobium oxide (Nb ₂ O ₅ ) thin film was formed on the gate insulating film of 6.5 × 0.23 mm) by the plasma CVD method. Niobium pentachloride (NbCl ₅ ) and carbon dioxide (CO ₂ ) are used as source gases, and hydrogen (H ₂ ) is used as carrier gas.
It was used. The substrate temperature was set to 800 ° C., RF of 13.56 MH _Z
Plasma was used. Niobium oxide is obtained by the following reaction formula. 2NbCl ₅ + 5CO ₂ + 5H ₂ → Nb ₂ O ₅ + 5CO + 10HCl The thickness of the obtained niobium oxide thin film was about 2000 Å. FIG. 1 shows the structure of the produced ion selective field effect transistor. In the figure, 1 is a source, 2 is a drain, 3 is a gate, 4 is a temperature compensation transistor, and 5 is on the gate. It is a niobium oxide (Nb ₂ O ₅ ) thin film formed on.

Next, this ion-selective field-effect transistor was combined with a double-junction type silver-silver chloride reference electrode and immersed in various acid solutions to determine the relationship between the hydrogen ion concentration in the aqueous solution and the voltage. . As an acid solution,
Each concentration was changed using each aqueous solution of sulfuric acid, hydrochloric acid, and nitric acid. At this time, the measurement condition of the ion selective field effect transistor is drain-source voltage (V _DS ) = 2.50
V and drain current ( _ID ) = 100 μA. FIG. 2 shows the results. In the figure, ◯ marks are for sulfuric acid, ● marks are for hydrochloric acid, and □ marks are for nitric acid. From the figure, regardless of the type of acid, the relationship between the logarithm of the hydrogen ion concentration and the voltage shows a linear relationship in a wide concentration range, and the sensitivity (voltage change when the concentration changes 10 times) is about 60 mV. It was Further, it was shown that the time required for the potential of the ion-selective field effect transistor according to the present invention to stabilize is extremely short, within about 5 seconds, and the response speed is fast.

[Embodiment 2] Under the same conditions as in Embodiment 1, FIG.
On the metal surface of the tip of the electrode of the shape shown in cross section,
A hydrogen ion electrode was obtained by forming a niobium oxide (Nb ₂ O ₅ ) thin film. In the figure, 6 is a niobium oxide (Nb ₂ O ₅ ) thin film, 7 is a titanium disk, 8 is a lead wire, 9 is a polycarbonate tube, and 10 is a soldered portion.

[Embodiment 3] Next, the ion-selective field-effect transistor described in Embodiment 1 and a silver-silver chloride electrode as a reference electrode were combined and immersed in the electrolyte of a lead acid battery for automobiles. Then, the voltage between the ion selective field effect transistor and the reference electrode was measured according to the charge / discharge state of the battery. FIG. 4 shows how electrodes are attached to a lead storage battery. In the figure, 11 is an ion-selective field effect transistor, 12 is a reference electrode, 13 is a positive electrode plate, and 1 is a positive electrode plate.
Reference numeral 4 is a negative electrode plate, 15 is a separator, 16 is an insulating film for preventing contact between the ion selective field effect transistor and the positive electrode plate, and 17 is an electrolytic solution. The measurement conditions of the ion-selective field effect transistor were the same as in the case of Example 1. At the same time, the specific gravity of the electrolytic solution was measured with a float type hydrometer.

FIG. 5 shows the relationship between the voltage and the specific gravity of the electrolytic solution. When the specific gravity of the electrolytic solution is 1.28 (fully charged state), the voltage is about 200 mV and the specific gravity of the electrolytic solution is 1.05 (fully discharged state).
The voltage at the time of was about 140 mV, and the logarithm of voltage and concentration showed a linear relationship in this concentration range. As can be seen from FIG. 5, the specific gravity of the electrolytic solution can be known by knowing the voltage between the ion selective field effect transistor and the reference electrode. In lead acid batteries, the specific gravity of the electrolyte is known to directly indicate the charge / discharge state of the battery, so from the voltage between the ion selective field effect transistor and the reference electrode,
It is possible to know the charge / discharge status of the battery.

[0025]

According to the present invention, a hydrogen ion concentration detecting element in which a niobium oxide thin film is formed on a gate insulating film or a metal surface of a field effect transistor and is combined with an appropriate reference electrode is used in a wide range of hydrogen. The ion concentration can be displayed in the form of voltage. Moreover, the handling is simple, the response speed is fast, and continuous measurement is possible while being immersed in the liquid to be measured.

The manufacturing method of the niobium oxide thin film is not limited to the plasma CVD method shown in the embodiment, and various other methods such as a method of oxidizing a metal niobium thin film in a high temperature oxygen atmosphere can be used. it can.

Further, as niobium oxide, niobium pentoxide (Nb ₂ O ₅ ) which is a typical compound in the examples is used.
However, other stoichiometric ratios of niobium oxide, such as niobium monoxide (NbO) and diniobium trioxide (Nb ₂ O
Needless to say, the same effect can be obtained when ₃ ) or niobium dioxide (NbO ₂ ) is used.

Further, since the niobium oxide film is chemically very stable, it can be used for a long period of time and has extremely high reliability. Further, even if the film portion is small, the obtained voltage characteristics are the same, so it is possible to make the size as small as possible by devising the manufacturing method.

Therefore, the hydrogen ion detecting element according to the present invention is not only effective for the measurement of the thin hydrogen ion concentration which has been conventionally measured by using the glass electrode, but is also attached to the lead storage battery. In addition, it has an advantage that the charge / discharge state of the lead storage battery can be always known, and its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of an ion selective field effect transistor according to the present invention.

FIG. 2 shows the voltage between the ion selective field effect transistor according to the present invention and the reference electrode in various acid aqueous solutions,
It is the figure which showed the relationship with the logarithm of hydrogen ion concentration.

FIG. 3 is a cross-sectional view of a hydrogen ion electrode having a metal surface provided with a niobium oxide thin film according to the present invention.

FIG. 4 is a diagram showing a state in which an ion selective field effect transistor according to the present invention is attached to a lead acid battery.

FIG. 5 is a diagram showing the relationship between the voltage between the ion-selective field-effect transistor and the reference electrode according to the present invention, the electrolytic solution specific gravity of the lead storage battery, and the charge / discharge state of the battery.

[Explanation of symbols]

3 gates 5 Niobium oxide thin film formed on the gate 6 Niobium oxide thin film 7 Titanium disc 11 Ion-selective field effect transistor 12 Reference electrode

Claims (2)

[Claims] 1. A hydrogen ion detection element comprising a hydrogen ion electrode having a niobium oxide thin film formed on the surface of a gate insulating film or a metal surface of a field effect transistor, and a reference electrode. 2. A lead acid battery provided with the hydrogen ion detection element according to claim 1.