JP3521204B2 - Acid mist sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acid mist sensor which can be used for detecting, for example, acid rain. 2. Description of the Related Art Recently, a sensor utilizing the electric field effect of a semiconductor has been proposed as a sensor for measuring a hydrogen ion concentration index (pH) in a liquid.
It is listed in the Sensor Handbook issued by Baifukan in May. The structure shown in the figure is an example.
Its structure is similar to that of a so-called insulated gate field effect transistor. In this sensor, an electrolyte to be measured is brought into contact with the gate, and the interfacial potential with the electrolyte on the surface of the gate insulating film changes depending on the activity of specific ions in the electrolyte. If the potential of the electrolyte is kept constant using the reference electrode, the conductivity of the semiconductor surface (channel) under the gate insulating film changes depending on the interface potential. When silicon nitride or the like is used on the surface, a response to pH appears. However, the following technical problems have been pointed out in the conventional semiconductor sensor having such a structure. That is, the conventional semiconductor sensor described above has a transistor structure, requires a plurality of battery power sources, and uses a reference electrode. Since the structure is complicated and the potential of the electrolytic solution to be measured must be kept constant by the reference electrode, it is impossible to measure the mist. Further, the semiconductor sensor shown in FIG. 6 has a problem that a large current flows and power consumption is large because the internal resistance is small. For example, the semiconductor sensor is used outdoors for detecting acid rain. In this case, this point was also an important issue to be solved. The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a highly sensitive acid mist sensor having a simple structure. Another object is to provide an acid mist sensor that consumes very little power. [0007] [Means for Solving the Problems] To achieve the above object, acid mist sensor of the present invention is formed on a substrate not
A diamond thin film without adding pure things, acid from this on the diamond thin film Ri Do a pair of electrodes formed at predetermined intervals, a decrease in the electrical resistance of the diamond thin film
Mist is detected . According to the acid mist sensor having the above structure, when a mist containing H ⁺ ions comes into contact with the diamond thin film,
A conductive layer is formed on the surface of the diamond thin film in a short time, and the formed conductive layer greatly reduces the electric resistance between the pair of electrodes. Therefore, by detecting the change in the electric resistance, it is possible to function as an acid mist sensor. Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows an embodiment of the acid mist sensor according to the present invention. The acid mist sensor shown in FIG. 1 includes a substrate a formed of silicon or the like, a diamond thin film b deposited and formed on the substrate a by a CVD method, and a predetermined interval formed on the diamond thin film b. And a pair of electrodes c. The diamond thin film b is a so-called i-type diamond thin film to which no impurity is added, and has a pair of electrodes c.
Is formed, for example, by depositing a metal such as gold on the diamond thin film b. An example of a method for manufacturing an acid mist sensor having such a structure will be described with reference to FIG. FIG. 2 shows a C film for depositing and forming an i-type diamond thin film.
1 shows an example of a VD device. The CVD apparatus shown in FIG. 1 is of a so-called hot filament type, in which a sealable reaction vessel 10, a raw material container 14 containing a raw material 12 for forming a diamond thin film b, a carrier gas (hydrogen gas) 16 The reaction vessel 10 and the raw material vessel 14 are connected by a gas introduction pipe 20. The gas container 18 is connected to a gas introduction pipe 20 through a gas supply pipe 22 and is connected to the raw material container 1.
4 communicates with the carrier gas supply pipe 24, and the carrier gas supply pipe 24 extends to near the bottom of the raw material container 14. A heater 26 made of tungsten is provided in the reaction vessel 10.
Is installed, and a holder 28 is provided below the
A substrate a for depositing and forming a diamond thin film b is placed on the holder 28. The outer periphery of the reaction vessel 10 is
An electric heater 30 for heating the raw material gas and the substrate a introduced therein is provided, and an exhaust port 32 for discharging an unreacted gas from the inside of the reaction vessel 10 is provided at one end of the reaction vessel 10. Have been. Note that reference numeral 34 in FIG.
Is a sensor for measuring the temperature of the substrate a mounted on the holder 28. As the raw material 12, only a liquid organic substance such as acetone, which is a raw material of the diamond thin film, is used. The carrier gas 16 is blown into the raw material 12 without adding any impurities. The raw material 12 is introduced into the reaction vessel 10 via the gas introduction pipe 20. When depositing the diamond thin film a, the gasified raw material 12 is introduced into the reaction vessel 10 with the diagram substrate a placed on the holder 28. The raw material 12 introduced into the reaction vessel 10
The heater 26 is heated to, for example, about 2240 ° C., and by this heating, carbon in the organic substance serving as the raw material of the diamond thin film becomes a radical, and the carbon thus radicalized is deposited on the substrate a. ,
A diamond thin film b is deposited and formed on a substrate a. At this time, the substrate a is heated to about 800 ° C. When a pair of electrodes c is formed on the diamond thin film b thus obtained by vapor deposition or the like, the acid mist sensor of the present invention is obtained. Here, the present inventors have experimentally manufactured an acid mist sensor having the above-described structure and confirmed the operation and effect thereof. At this time, the prototype acid mist sensor was manufactured by the hot filament CVD method, the pressure in the reaction vessel 10 was reduced to 100 Torr without adding impurities, a silicon substrate was used as the substrate a, and the diamond thin film raw material 12 was used.
, Acetone was used, and the concentration was 1 Vol%. The diamond thin film b of the prototype acid mist sensor had a size of 5 mm × 5 mm and a thickness of 15 μm. Gold was used for the electrode c, and the size was set to 100 μmφ. In this trial production process, when the diamond thin film b was deposited and formed on the substrate a and the electric resistance of the diamond thin film b was measured in a vacuum, a large value exceeding 10 ¹⁰ was observed, and no decrease was observed. The measured respectively in an atmosphere of _{N 2, O 2, CO 2} , H 2 which is the main component of the atmosphere and dried in air resistance of the diamond film b, similarly to a decrease in electrical resistance observed Did not. FIG. 3 shows the measurement results of the electrical resistance of the diamond thin film b in dry air and in air. From this figure, the i-type diamond thin film b containing no impurities is
It can be seen that the electric resistance decreases with time when exposed to the atmosphere. The present inventors believe that this phenomenon is due to ions in the atmosphere,
In particular, the inventors inferred that H ⁺ ions adhere to the surface of the diamond thin film b to form a conductive layer and reduce its electrical resistance, and tried to prove it. FIG. 4 shows the results of this demonstration experiment. In this demonstration experiment, as shown in FIG. 5, a predetermined amount of pure water was put into a beaker, the beaker was sealed with a lid, and the inside was filled with nitrogen gas. Then, a pipette was inserted through a hole formed in the lid, and the hydrochloric acid was stored in the tip of the pipette so as to prevent hydrochloric acid from dropping, and the pipette was evaporated into nitrogen gas. Then, the prototype acid mist sensor was suspended in an atmosphere filled with hydrochloric acid mist by a lead wire, and a battery, a voltmeter and an ammeter were connected to the lead wire. The change with time of the electric resistance was measured. As is apparent from FIG. 4, when the acid mist sensor is exposed to an atmosphere filled with hydrochloric acid mist, the electric resistance of the diamond thin film b is reduced by 4 to 5 digits or more in about several minutes. By measuring the electric resistance of the thin film b, acid mist can be detected, and in this case, the change rate is large, so that the detection sensitivity is improved. The mechanism for detecting such acid mist is as follows.
Although not always clear, the present inventors presume as follows. That is, according to the acid mist sensor having the above structure, when a mist containing H ⁺ ions comes into contact with the diamond thin film b, a conductive layer is formed on the surface of the diamond thin film b in a short time, and the formed conductive layer forms a pair. The electric resistance between the electrodes c greatly decreases. Therefore, by detecting the change in the electric resistance, it is possible to function as an acid mist sensor. Further, as is clear from FIG. 4, in the acid mist sensor of the present invention, since the diamond thin film b has a very large resistance value, for example, a current is applied to the diamond thin film b via the electrode c. Even if it flows, the size becomes very small, and the power consumption in the sensor part becomes very small. FIGS. 6 to 11 show experimental results in which the effect of the acid mist sensor according to the present invention was confirmed under conditions different from those of the above-mentioned demonstration experiment. In this experiment,
As shown in FIG. 12, the beaker was closed with a lid, and the inside was filled with nitrogen gas. Then, a pipette was inserted through a hole formed in the lid, and the hydrochloric acid aqueous solution was dropped from the tip of the pipette, and 1 ml of hydrochloric acid aqueous solution was sequentially supplied to evaporate into nitrogen gas. Then, the prototype acid mist sensor was suspended in an atmosphere filled with hydrochloric acid mist by a lead wire, and a battery, a voltmeter and an ammeter were connected to the lead wire. The change with time of the electric resistance was measured. The measurement results shown in FIG. 6 indicate that hydrochloric acid: pure water = 1:
This is a case where a hydrochloric acid aqueous solution of 1000 is used. The hydrogen ion concentration index of this aqueous hydrochloric acid solution was 2 as measured with a glass electrode type pH meter. The measurement results shown in FIG. 7 indicate that hydrochloric acid: pure water =
In this case, a 1: 100 hydrochloric acid aqueous solution was used. In addition,
The hydrogen ion concentration index of this aqueous hydrochloric acid solution was 0.95 as measured with a glass electrode type pH meter. The measurement results shown in FIG. 8 are obtained when an aqueous hydrochloric acid solution of hydrochloric acid: pure water = 1: 10 is used. Incidentally, the hydrogen ion concentration index of this aqueous hydrochloric acid solution was measured by a glass electrode type pH meter.
0.1. The measurement results shown in FIG. 9 indicate that hydrochloric acid: pure water =
In this case, a 1: 3 hydrochloric acid aqueous solution was used. The measurement results shown in FIG. 10 are obtained when a hydrochloric acid: pure water = 1: 1 hydrochloric acid aqueous solution was used. The measurement results shown in FIG. 11 are obtained when a stock solution of hydrochloric acid was used. The hydrochloric acid used in the above test was a commercially available hydrochloric acid (hydrogen chloride content: 36%). As is apparent from the results shown in FIGS. 6 to 11, in the case of a hydrochloric acid aqueous solution having a low concentration, the amount of evaporation of the acid is small, so that the resistance reduction rate of the diamond thin film b is small, and the concentration of the aqueous solution is small. It can be seen that the evaporation amount increases as becomes closer to the stock solution, and the resistance reduction rate of the diamond thin film b increases. Therefore, for example, the diamond thin film b
The concentration of the acid mist can also be measured by determining the time-dependent decrease ratio of the electric resistance of the mist. FIG. 13 shows the results of an experiment in which the acid mist sensor was exposed to an ammonia atmosphere after being exposed to a hydrochloric acid atmosphere. The electric resistance of the thin film b returns to almost the original state, and can be used again for detecting the acid mist. Means for restoring the function of the mist sensor is not limited to exposure to the above-described ammonia atmosphere, but also, for example, by wiping the surface of the diamond thin film b with pure water or an organic solvent (for example, acetone). Has also been confirmed to recover. FIGS. 14 and 15 show experimental results when a plurality of acids are mixed. FIG. 14 shows a case where nitric acid and sulfuric acid are mixed. FIG. 15 shows a case where hydrochloric acid and hydrogen peroxide are mixed. This is the case. As is clear from the results shown in these figures, in the acid mist sensor of the present invention, the electrical resistance of the diamond thin film b was significantly reduced in all cases. Therefore, for example, it can be effectively used as a sensor for acid rain in which a plurality of acids are mixed. The acid mist sensor according to the present invention is not only used for detecting the acid shown in the above experimental examples, but also used for various acid mist such as nitric acid, acetic acid, hydrofluoric acid, hydrogen peroxide, sulfuric acid and phosphoric acid. As a result, the same experimental result as that of the hydrochloric acid mist was obtained. As described above in detail, the acid mist sensor according to the present invention has a simple structure and can detect the acid mist with high sensitivity, as described in detail in the experiments. Moreover,
This acid mist sensor has very low power consumption.
You.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing an embodiment of an acid mist sensor according to the present invention. FIG. 2 is an explanatory view of a CVD apparatus used for forming a diamond thin film of the acid mist sensor of FIG. FIG. 3 is a graph showing measurement results of electric resistance of a diamond thin film formed by the CVD apparatus of FIG. 2 in dry air and in air. FIG. 4 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put in a hydrochloric acid mist. FIG. 5 is an explanatory diagram showing a state of a test in which the result of FIG. 4 was obtained. FIG. 6 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put into a hydrochloric acid aqueous solution mist. FIG. 7 is a graph showing measurement results of electric resistance when the acid mist sensor of FIG. 1 is put into a hydrochloric acid aqueous solution mist. FIG. 8 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put into a hydrochloric acid aqueous solution mist. FIG. 9 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put in a hydrochloric acid aqueous solution mist. FIG. 10 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put into a hydrochloric acid aqueous solution mist. FIG. 11 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put in a hydrochloric acid mist. FIG. 12 is an explanatory diagram showing a state of a test in which the results of FIGS. 6 to 11 are obtained. FIG. 13 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put into an ammonia atmosphere from a hydrochloric acid mist. FIG. 14 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put in a mixed atmosphere of nitric acid and sulfuric acid. FIG. 15 is a graph showing a measurement result of electric resistance when the acid mist sensor of FIG. 1 is put in a mixed atmosphere of hydrochloric acid and hydrogen peroxide. FIG. 16 is a structural diagram showing an example of a conventional semiconductor pH sensor. [Description of Signs] a substrate b diamond thin film c electrode

────────────────────────────────────────────────── ─── Continuing from the front page (56) References JP-A-5-72163 (JP, A) Ken Okano, Tateo Kurosu, Masamori Iida, Film forming technology, evaluation and application of diamond thin film Electronic device of CVD diamond thin film Application to Surface Technology, 1991, Vol. 42, No. 12, 1230-1234 (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 27/12 G01N 27/04 JICST file (JOIS)

Claims (1)

(57) Claims: 1. A diamond thin film formed on a substrate without adding impurities, and a pair of electrodes formed on the diamond thin film at predetermined intervals. Ri Do not because the, the die
An acid mist sensor for detecting an acid mist from a decrease in electric resistance of a thin film of a diamond .