JP2639190B2 - Carbon dioxide concentration measurement sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a sensor for measuring carbon dioxide concentration.

(Prior art) Global warming due to the greenhouse effect has become a major social problem. In particular, carbon dioxide generated by the burning of fossil fuels accounts for a large proportion of the gases that cause various greenhouse effects. I have.

At present, expensive and large analytical instruments are used to measure such carbon dioxide concentration, but there are problems such as the need to remove in advance the coexisting gas that adversely affects the detection performance, and it is not always satisfactory. It's not cool.

(Problems to be Solved by the Invention) Accordingly, an object of the present invention is to provide a sensor having high selectivity to carbon dioxide gas and capable of measuring carbon dioxide gas concentration quickly and continuously.

(Means for Solving the Problems) The sensor for measuring carbon dioxide concentration of the present invention is a concentration cell type of carbon dioxide using a lithium ion conductive solid electrolyte. Thus, the above object can be achieved. That is, since the solid electrolyte itself has only one kind of mobile ion,
The selectivity to carbon dioxide is significantly improved.

According to the present invention, a measurement comprising a lithium ion conductive solid electrolyte comprising a mixture of LiTi ₂ (PO ₄ ) ₃ and Li ₃ PO ₄ and lithium carbonate provided in contact with one surface of the solid electrolyte There is provided a carbon dioxide concentration cell type sensor for measuring the concentration of carbon dioxide, comprising a gas side electrode and a reference gas side electrode made of lithium carbonate provided in contact with the other surface of the solid electrolyte.

Solid Electrolyte The lithium ion conductive solid electrolyte used in the present invention is composed of a mixture of LiTi ₂ (PO ₄ ) ₃ and Li ₃ PO ₄ and is usually used as a sintered body. In the mixture, Li
The quantitative ratio of Ti ₂ (PO ₄ ) ₃ to Li ₃ PO ₄ is LiTi ₂
(PO ₄ ) ₃ : Li ₃ PO ₄ = 1: 0.05 to 1: 0.4, preferably 1: 0.1 to
1: 0.3, most preferably 1: 0.2.

LiTi ₂ (PO ₄ ) ₃ used here is, for example, titanium oxide (TiO ₂ ), lithium salt (Li ₂ CO ₃ , Li ₂ O, etc.), phosphate [(NH ₄ ) ₂ HPO ₄ , (NH ₄ ) H ₂ PO ₄ etc.], followed by a solid phase reaction by heating. In this case, the raw materials titanium oxide and lithium salt are:
Although a commercially available product can be used as it is, it is generally preferable to use it after purification and vacuum drying. The means for mixing these raw materials is not particularly limited. For example, a predetermined amount is weighed and then mixed in a powder form using a mortar or the like. The use ratio of each of these raw materials is, on a molar basis, TiO ₂ / Li ₂ CO ₃ / (NH ₄ ) ₂ HPO ₄ = 36-36.6 / 8.8-9.4 / 54.3
~ 54.9, preferably 36.2 ~ 36.4 / 9 ~ 9.2 / 54.5 ~ 54.7,
Most preferably, it is 36.3 / 9.1 / 54.6.

The solid-phase reaction by heating each of the raw material mixtures can be performed, for example, by reacting in an electric furnace in air or an inert gas at a temperature of 900 to 1200 ° C. for 1 to 10 hours. After the solid-phase reaction, the solid-phase reaction product is usually pulverized. In this case, it is preferable to carry out the pulverization using a ball mill or the like for about 5 to 15 hours. The solid-phase reaction by heating and the pulverization after the solid-phase reaction may be performed once, but it is preferable to repeat the solid-phase reaction several times in order to sufficiently advance the solid-phase reaction.

In the present invention, the lithium ion conductive solid electrolyte is
The pulverized product obtained after the solid phase reaction obtained above is mixed with a predetermined amount of Li ₃ PO ₄ , and is generally obtained by sintering after pressure molding.

The method of pressure molding is not particularly limited, and it can be performed, for example, using a press machine at a pressure of 1 ton / cm ² or more. At this time, it is also possible to add a binder such as polyvinyl alcohol or a molding aid in order to enhance the moldability during pressure molding.

The method of sintering the obtained pressure-molded product is not particularly limited. For example, in an electric furnace in air or in an inert gas, 850 to 1
It is performed by heating at a temperature of 300 ° C. for about 1 to 5 hours.

Structure of Sensor By forming a carbon dioxide gas concentration battery using the above-described lithium ion conductive solid electrolyte, an all-solid-state and small-sized sensor for measuring carbon dioxide gas concentration can be obtained. FIG. 1 shows a partially enlarged sectional view of an example of such a sensor of the present invention.

In this sensor, a lithium carbonate plate 2,2 'having a thickness of usually 0.6 to 0.6 mm is provided in close contact on both sides of a lithium ion conductive solid electrolyte 1 usually formed to a thickness of 0.7 to 1.5 mm as electrodes. These are, by inorganic adhesive,
It is fixed to the mullite tube 3.

Here, platinum networks 4, 4 'are provided on the lithium carbonate plates 2, 2' as electrode terminals, and platinum wires 5, 5 'as lead wires extend from the platinum networks 4, 4', respectively. The wires 5, 5 'are connected to a potentiometer 6.

A constant carbon dioxide gas concentration is maintained inside the lithium carbonate plate 2 'constituting the inner electrode. For example, in the embodiment shown in FIG.
A solid electrode 8 is joined via an alumina plate 7 of mm, and this solid electrode is fixed to an inner mullite tube 9 as described above.

In this case, the solid electrode 8 is made of calcium carbonate (Ca
CO ₃ ), and a constant reference carbon dioxide concentration is maintained inside the lithium carbonate plate 2 ′ by the carbon dioxide partial pressure generated from the equilibrium of CaCO ₃ ⇔CaO + CO ₂ .

A thermocouple 11 is provided inside the mullite tube 9 if necessary.
Are inserted so that the temperature in the interior 10 of the mullite tube 9 can be measured.

In the above-described sensor, various settings can be changed. For example, as long as a constant reference carbon dioxide concentration is maintained inside the lithium carbonate plate 2 ', the mullite tube 3
It is not necessary to use a double wall structure using the mullite tube 9 and a single wall structure without using the mullite tube 9. The tubes 3 and 9 need not be made of mullite as long as they maintain gas-tightness and electrical insulation. For example, they may be made of quartz, hard glass, zirconia, alumina, Pyrex, or the like.

The alumina plate 7 is provided to prevent a reaction due to the contact between the solid electrode 8 and the solid electrolyte 1, and is made of any material as long as the reaction due to the contact between the two is effectively prevented. For example, in addition to alumina, a plate made of quartz, Pyrex, mullite, zirconia, or the like can be used.

Measurement of Carbon Dioxide Concentration According to the present invention, a carbon dioxide gas concentration cell is formed by placing the above-described sensor in a desired atmosphere, and the sensor is provided according to the difference between the carbon dioxide concentration in the atmosphere and the reference carbon dioxide concentration. An electromotive force is generated, and lithium ions are exchanged between the lithium carbonate electrodes 2, 2 'via the solid electrolyte 1 by the electromotive force. Therefore, by measuring the generated electromotive force, the concentration of carbon dioxide in the atmosphere is calculated from a calibration curve prepared in advance by the Nernst equation.

(Example) TiO ₂ , Li ₂ CO ₃ and (NH ₄ ) ₂ HPO ₄ were mixed at a ratio of 0.363: 0.091: 0.545 on a molar basis, and about 10 g of the mixture was placed in a platinum crucible at 900 ° C. × The mixture was heated, pulverized and mixed for 2 hours. Then, the mixture was again heated and pulverized and mixed under the same conditions to obtain LiTi ₂ (PO ₄ ).
₃ was obtained.

This was mixed with Li ₃ PO ₄ so that LiTi ₂ (PO ₄ ) ₃ : Li ₃ PO ₄ = 1: 0.2 (molar ratio) and molded into pellets.
By sintering in a dry air at 1050 ° C. for 2 hours on a platinum board, a disc-shaped sintered body having a diameter of 13 mm and a thickness of 1 mm was obtained.

Using this sintered body as a solid electrolyte, a sensor for measuring carbon dioxide concentration as shown in FIG. 1 was produced.

The lithium carbonate plate is about 0.7 mm thick and 13 mm in diameter, and the alumina plate is about 0.5 mm thick and 8 x 4 mm in size.
The reference gas chamber was 6 mm in diameter × 300 mm in length, and 0.2 g of calcium carbonate was used as a solid electrode.

This sensor for measuring the concentration of carbon dioxide was inserted into an atmosphere diluted and adjusted to various concentrations of carbon dioxide with air, and the change in electromotive force was measured. The measurement was performed at a temperature of 650 ° C.

The electromotive force value in a variety of carbon dioxide concentration (if if carbon dioxide gas is present alone and SO ₂ is 100ppm coexist),
The results plotted against the logarithm of the carbon dioxide concentration are shown in FIG. In FIG. 2, a solid line indicates a calculated electromotive force value obtained from the Nernst equation. When the test gas was carbon dioxide gas alone, the electromotive force values for carbon dioxide gas concentrations of 1000 ppm to 1% were in good agreement with the calculated values, and a linear relationship close to the solid line was obtained.

FIG. 3 shows the results when the electromotive force values when the carbon dioxide concentration was kept constant at 100 ppm and the SO ₂ concentration was varied were plotted against the logarithm of the SO ₂ concentration. As shown in the results of FIG. 2, when the SO ₂ coexists at 100 ppm, the electromotive force value is lower than the calculated value, but as shown in FIG. 3, the coexisting SO ₂ concentration is 20 ppm or less. In this case, an electromotive force value substantially equal to the calculated value is obtained. From this result, it is understood that when the coexisting SO ₂ concentration is 20 ppm or less, the carbon dioxide gas detection ability is not adversely affected.

Similarly, the influence of the coexistence of NO _{2 on} the ability to detect carbon dioxide was examined, but it was found that the effect was not affected at 1000 ppm or less.

(Effects of the Invention) In the present invention, since a solid electrolyte having high lithium ion conductivity is used, the selectivity is remarkably high, and it is hard to be affected by the coexisting gas. No cumbersome processing is required. Such a solid electrolyte is stable in air,
Moreover, it has the advantage of being inexpensive.

When the sensor of the present invention is used, the electromotive force can be obtained by the Nernst's equation, so that the carbon dioxide concentration can be measured quickly and continuously.

[Brief description of the drawings]

FIG. 1 is a partially enlarged cross-sectional view showing an example of a sensor for measuring carbon dioxide concentration according to the present invention, FIG. 2 is a diagram showing an electromotive force response to a change in carbon dioxide concentration in the embodiment, and FIG. FIG. 4 is a diagram showing the dependency of the measured electromotive force on the SO ₂ concentration when the carbon dioxide gas concentration is fixed. 1 ... solid electrolyte, 2,2 '... lithium carbonate 7 ... alumina plate, 8 ... solid electrode

Claims (1)

(57) [Claims] 1. A lithium ion conductive solid electrolyte comprising a mixture of LiTi ₂ (PO ₄ ) ₃ and Li ₃ PO _4, and a measurement gas comprising lithium carbonate provided in contact with one surface of the solid electrolyte. A carbon dioxide concentration cell type sensor for measuring carbon dioxide concentration, comprising: a side electrode; and a reference gas side electrode made of lithium carbonate provided in contact with the other surface of the solid electrolyte.