JP2856970B2 - Gas detection element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detection element, and more particularly to a gas detection element made of fiber-shaped polycrystalline tin oxide.

[0002]

2. Description of the Related Art Conventionally, semiconductor type gas sensors have been widely used as sensors for detecting gas components in the atmosphere. In this sensor, a metal oxide semiconductor such as tin oxide, zinc oxide, or iron oxide is used as a detection element which is a gas detection unit. That is, such a gas sensor detects a gas by connecting a pair of electrodes to a gas detection element and measuring a change in resistance that occurs when the gas detection element adsorbs the gas.

However, these materials are used in the form of a sintered body, and have a problem that detection sensitivity is low or response speed is low. For this reason, attempts have been made to improve sensitivity and response speed by adding a catalyst such as a noble metal to an oxide semiconductor. However, the improvement was not satisfactory, and there was a natural limitation in extending the conventional method. For this reason, the use of a thin film element instead of a sintered body as a new method has been energetically studied. However, despite the many studies, a serious problem such as low durability has not been solved yet, and it has not been replaced with a sintered mold.
Further, the conventional fiber type detection element has a metal wire such as a platinum wire as a core and an oxide attached around the core. The detection element manufactured by this method generally has a problem that the diameter is large and the sensitivity and the response speed are low.

On the other hand, it has been difficult to produce a polycrystalline tin oxide fiber by the conventional solid-state reaction method. Japanese Patent Application Laid-Open No. 60-54 discloses a method for producing tin oxide fiber.
No. 997, JP-A-60-161337 and JP-A-62-158199 only propose a method of producing by a melt deposition method. However, these methods require a high temperature of 1000 ° C. or more and a reaction time of many days, and are practically impossible to obtain. In addition, the shape of the obtained tin oxide fiber is limited to 1 μm or less in diameter and 3 mm in length. In addition, the obtained fiber is often a whisker, and is generally sensitive as a semiconductor material for detecting gas concentration by resistance change. Tend to be low. In order to improve sensitivity and the like, it is conceivable to add a catalyst such as a noble metal. However, it is difficult to finely adjust the amount of a catalyst having a melting point different from that of tin oxide in the production method. Was. This is a fatal drawback for a semiconductor gas sensor in which gas selectivity greatly changes depending on a delicate amount of catalyst added.

[0005]

Accordingly, the present inventors have conducted intensive research on the creation of a gas detection element having high sensitivity and high response speed.

[0006]

As a result, it has been found that the gas sensitivity and the response speed are improved by using fiber-shaped polycrystalline tin oxide instead of the sintered body used so far. Here, the present invention has been completed.

That is, the present invention is a gas detecting element comprising a polycrystalline tin oxide fiber.

The present invention will be described more specifically.

The detection element of the present invention is required to be made of fiber-shaped polycrystalline tin oxide. Detection of a gas using an oxide is performed in the form of a change in resistance of the oxide due to adsorption of the gas. It is said that this resistance change is greatly contributed by the grain boundaries of the oxide. Therefore, if it is close to a single crystal like a whisker, there is no grain boundary,
Alternatively, the sensitivity is low because the number is small, and the tin oxide needs to be polycrystalline.

The shape of the fiber is 1 mm in diameter.
Or less, preferably 500 μm or less, particularly preferably 80 μm or less.
μm or less is used. When the diameter is larger than 1 mm, only performance equivalent to that of the sintered body can be obtained. That is, the smaller the diameter of the fiber, the higher the sensitivity and the higher the response speed. The lower limit of the fiber diameter is not particularly limited, but is preferably 0.1 μm or more from the viewpoint of ease of production. The length of the fiber is not particularly limited, but is preferably 2 cm or less in view of the size of the gas detection element. The lower limit is L / D of 20 or more, where D is the diameter of the fiber and L is the length. It is preferable to select as follows.

In the gas detecting element of the present invention, in order to increase the mechanical strength of the polycrystalline tin oxide fiber, a method of coating a part of the fiber with another material may be used.

A gas detecting element comprising a polycrystalline tin oxide fiber according to the present invention is generally formed of an insulating substrate by connecting lead wires to both ends of the fiber with a conductive paste as shown in FIGS. Used in a structure that is fixed on a support base.

As the gas detecting element, one polycrystalline tin oxide fiber may be used alone as shown in FIG. 1, or a plurality of polycrystalline tin oxide fibers may be used so as not to be in contact with each other except at both ends as shown in FIG. Alternatively, a plurality may be bundled and used as shown in FIG. Such a choice depends on the resistance value or shape of the fiber used. That is, if a fiber having a large resistance value or a fiber having a small diameter and a large length is used alone, the resistance becomes extremely large, and matching with the detection circuit becomes difficult. In this case, a plurality of fibers are used. Good to do. Also, the smaller the diameter of the fiber, the higher the sensitivity tends to be. However, the smaller the diameter, the higher the resistance. Therefore, it is preferable to use a large number of fibers. Further, as shown in FIG. 4, a plurality of single fibers may be arranged and signal processing may be performed by independent detection circuits. In FIGS. 2, 3 and 4, it is also effective to use various catalysts having different gas detection characteristics by selecting various catalysts shown below.

In the above-mentioned polycrystalline tin oxide fiber, platinum, palladium, gold, silver, rhodium, iridium, antimony,
It is also effective to include a metal such as vanadium, tantalum, niobium, bismuth, or a metal oxide (catalyst) alone or in combination. The type and amount of the catalyst may be appropriately determined according to the type of gas to be detected, and usually, 0.1 to 30 mol is contained in the obtained tin oxide.

Further, it is preferable to include silica and / or alumina to increase the mechanical strength of the fiber.

Next, a typical method for producing a polycrystalline tin oxide fiber used in the present invention will be described.

An alcohol represented by the general formula ROH (wherein R represents an unsubstituted or substituted alkyl group, an unsubstituted or substituted alkenyl group, or an unsubstituted or substituted aryl group) is added to an alcohol represented by the general formula SnXa. bH ₂ O (wherein X is Cl atom, Br atom, I atom, F atom, OH group, SO ₄
Group, an NO ₃ group or CH ₃ COO radical, a represents an integer of 1 to 4, b is spun spinning dope tin compound is formed by dissolving represented by an integer from 0 to 6), followed Obtained by heat treatment.

In the alcohol represented by the general formula ROH, R represents an unsubstituted alkyl group such as methyl group, ethyl group, propyl group, butyl group, octyl group, 2-methoxyethyl group, 2-ethoxyethyl group, 2-hydroxyethyl group. 1-methoxy-2-propyl group, methoxyethoxyethyl group, 2-phenylethyl group, substituted alkyl group such as phenylmethyl group, unsubstituted alkenyl group such as allyl group,
A substituted alkenyl group such as a 2-methyl-2-propenyl group or a 3-methyl-3-butenyl group; an unsubstituted aryl group such as a phenyl group; or a substituted aryl group such as a methoxyphenyl group, an ethoxyphenyl group, a methylphenyl group or an ethylphenyl group. Show.

Specific examples of the substituent in the above-mentioned substituted alkyl group, substituted alkenyl group or substituted aryl group include alkoxyl groups such as methoxy group and ethoxy group, hydroxyl group, phenyl group, etc. And an alkyl group such as an aryl group, a methyl group, and an ethyl group, as well as a halogen atom such as an amino group, a cyano group, a Cl atom, a Br atom, an I atom, and an F atom.

Specific examples of these alcohols include methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, octyl alcohol, 2-methoxyethanol, 2-ethoxyethanol, ethylene glycol and 1-methoxy-2-propyl. Alcohol, methoxyethoxyethanol, 2-phenylethyl alcohol,
Benzyl alcohol, allyl alcohol, 2-methyl-2-propen-1-ol, 3-methyl-3-buten-1-ol, phenol, methoxyphenol, ethoxyphenol, cresol, ethylphenol and the like can be mentioned. In particular, methyl alcohol and ethyl alcohol are preferable because of high solubility of the tin compound. The above alcohol is usually used alone, but a mixture of two or more alcohols may be used to control the reactivity with the tin compound or the solubility of the tin compound.

In order to improve the stability of the spinning solution,
Compounds having two or more carbonyl groups, such as acetylacetone, ethyl acetoacetate and diethyl malonate, can also be used as an auxiliary.

In the tin compound represented by the general formula SnXa.bH ₂ O, X represents Cl atom, Br atom, I atom,
F atom, OH group, SO ₄ group, NO ₃ group or CH ₃ COO
A represents an integer of 1 to 4; b represents an integer of 0 to 6; Of these, tin chloride and tin bromide are preferred in terms of price and stability. Specifically, SnCl ₂ , SnCl _2.
2H ₂₀ , SnBr ₂ , SnI ₂ , SnF ₂ , SnSO ₄ ,
_{Sn (CH3COO) 2, Sn (} NO 3) 2 and the like, in _{particular, SnBr 2, SnCl 2 · 2H} 2 O, SnC
l ₂ is preferably used. Further, those obtained by modifying the above-mentioned tin compound with an organic compound, for example, Sn (CH ₃ ) ₂ Cl _{2 and the} like can also be used.

The mixing ratio of the above-mentioned tin compound and alcohol is not particularly limited as long as the tin compound is uniformly dissolved in the alcohol. However, when the ratio of the tin compound is too low, the spinning property is not exhibited, so that it is necessary to concentrate the alcohol, and the alcohol is wasted. On the other hand, if the concentration of the tin compound is too high, precipitation occurs, and a uniform spinning solution cannot be obtained. Therefore, the mixing ratio varies depending on the type of the tin compound and the alcohol used. Generally, however, the molar ratio of the tin compound to the alcohol is 0.02.
~ 0.5 is preferred.

Improved sensitivity, gas selectivity and response speed
The catalyst to be contained in the spinning solution is a catalyst described below in advance.
Precursors can be added in the form of a solution to ensure uniform dispersion and
Preferred for inclusion. The catalyst precursor is heated after spinning.
Changes into metal or metal oxide by catalysis and shows catalytic action
Become like Polycrystalline tin oxide by the above spinning method
The bar manufacturing method allows for the delicate control of the content of these catalysts.
Very advantageous because it can be added uniformly in the form of a solution
It is a way. As a specific catalyst precursor, VB
r_Three, VCl_Two, VCl_Three, VCl_Four, VOBr_Two, VOB
r_Three, VOCl_Three, VF_Three, VF_Four, VF_Five, VI_Three6H
_TwoO, vanadium alkoxide, NbCl_Five, NbBr
_Five, NbF_Five, NbOCl_Three, Niobium alkoxide, T
aBr_Five, TaCl _Five, Alkoxide of tantalum, SbC
l_Three, SbCl_Five, SbBr_Three, Antimony oxychloride,
Alkoxide of antimony, BiCl_Three, BiI_Two,Screw
Mass alkoxide, chloroplatinic acid, palladium chloride, salt
Auric acid, indium chloride, silver nitrate, ruthenium chloride, salt
Iridium chloride, rhodium chloride, etc.
Are added to the spinning solution.

In order to increase the mechanical strength of the fiber,
It is also preferable to add a silica and / or alumina precursor to the spinning solution in advance. These precursors also become silica and / or alumina by heat treatment after spinning. Examples of the precursor include silicon alkoxide, aluminum alkoxide, aluminum chloride, and aluminum nitrate.

The silicon alkoxide is represented by the general formula S
_{i (OR A) 4, R} B Si (OR A) 3, R B R C (OR A) 2
Is used. Here, R _A , R _B , and R _C represent an alkyl group, an alkenyl group, and an aryl group. Examples of the alkyl group include linear and branched alkyl groups such as a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group.Examples of the alkenyl group include an ethenyl group, a propenyl group, a butenyl group, and a pentenyl group. It is a linear or branched alkenyl group.

More specifically, a part thereof is exemplified by tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, phenyltrimethoxysilane, propyltrimethoxysilane, n -Butyltrimethoxysilane, isobutyltrimethoxysilane, n
-Hexyltrimethoxysilane, n-octadecyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, amyltriethoxysilane, phenyltriethoxysilane, n-octyltriethoxysilane, n-octadecyltriethoxysilane, n-dodecyl Triethoxysilane, methyltributoxysilane, ethyltributoxysilane, ethyltripropoxysilane, vinyltriethoxysilane, allyltriethoxysilane, diphenyldimethoxysilane, dimethyldiethoxysilane, vinylmethyldiethoxysilane, diethyldimethoxysilane, ethylmethyl Diethoxysilane and the like can be mentioned.

As the aluminum alkoxide, alkoxides represented by the general formulas Al (OR _A ) ₃ and R _B Al (OR _A ) ₂ represented by R _A and R _B similar to the above silicon alkoxides can be used.

The addition amount of the silica and / or alumina precursor is 0.01 to 3 with respect to the tin compound.
0% by weight is preferred. If the amount is less than 0.01 wt%, sufficient effects can not be obtained, also when Ru 20 weight% Yue <br/> example, silica in tin oxide fibers obtainable, alumina component increases Therefore, the sensitivity and the like are undesirably reduced.

The method for dissolving the tin compound, the catalyst precursor, and the silicon alkoxide and / or aluminum compound and the alcohol is not particularly limited, and the tin compound, the catalyst precursor, and the silicon alkoxide and / or aluminum compound are stirred. Or a method of sequentially or simultaneously dissolving a tin compound, a catalyst precursor, a silicon alkoxide and / or an aluminum compound in an alcohol with stirring.

An acid such as hydrochloric acid, nitric acid or acetic acid, a compound having a carbonyl group such as acetylacetone, or ammonia or the like may be appropriately used as a complexing agent.

The spinning method is not particularly limited, and a conventional spinning method can be used. For example, there is a method of extruding a spinning solution from a spinning nozzle. The fiber length, diameter, and the like of the obtained fiber can be arbitrarily controlled by adjusting the viscosity of the spinning solution or the speed of extruding the spinning solution from the spinning nozzle.

The heat treatment of the gel fiber obtained by spinning is performed at a temperature at which the gel fiber can be converted into polycrystalline tin oxide. The gel fiber as spun from the spinning solution contains an organic substance such as alcohol as it is, and is an insulator. The semiconductivity is manifested by heating the gel fiber. The heat treatment temperature is not particularly limited as long as it is within a temperature range where semiconductivity can be imparted to the obtained fiber. In general, when the heat treatment temperature is low, since organic substances such as alcohol, water, etc. remain in the gel fiber, the components such as the catalyst precursor do not become a metal or an oxide form and sufficiently cooperate with tin oxide. Sensitivity cannot be improved because of no solid solution or dispersion. On the other hand, if the heat treatment temperature is too high, the catalyst component in the fiber will volatilize and the performance will decrease, the decomposition of tin oxide will proceed, and the crystal grains in the fiber will grow too much and the strength will decrease. Therefore, the heat treatment temperature is 250 ° C. to 1550
A temperature range of ° C. is preferred. More preferably, 300 ° C.
It is desirable to perform the heat treatment at a temperature of 1500 ° C. The heat treatment is usually performed in air, but when it is desired to obtain a highly conductive fiber, the heat treatment is performed in a reducing atmosphere such as nitrogen, argon, hydrogen, a mixed gas of argon and hydrogen, or in a vacuum. be able to.

In the heat treatment, it is desirable to remove volatile components such as water and alcohol present in the gel fiber by drying to obtain a good conductive fiber. Such drying may be performed simultaneously with the heat treatment, but it is more preferable to perform the drying before the heat treatment in order to obtain a good conductive fiber. In these cases, the drying temperature is preferably performed at a temperature as low as possible in order to prevent occurrence of cracks in the obtained fiber.However, when an alcohol having a high boiling point is used as the solvent, it is too low. It takes a long time to dry and is not effective. A general drying temperature is preferably in the range of room temperature to 300 ° C.

In the present invention, a known structure of the gas detector using the gas detecting element can be employed without any particular limitation. FIG. 5 shows a representative embodiment of a gas detector using the gas detection element of the present invention. That is, the gas detection element 5 is installed on a support base made of the insulating substrate 6 so that a part thereof is exposed,
A pair of electrodes 7 are connected to the detection element. Further, a heater 8 is installed in the vicinity to control the temperature of the detection element. The insulating substrate 6 supports and fixes the gas detection element 5, the heater 8, and the electrode 7. Insulating substrate 6
Any material may be used as long as it has an insulating property and has heat resistance to the heating temperature of the heater 8. As such a material, an alumina-based material is generally preferred. The heater 8 heats the gas detection element 5 to control gas selectivity and response speed. The heater 8 is preferably installed near the gas detection element 5 so that the gas detection element 5 can be heated via an insulating substrate. The material of the heater is not particularly limited as long as it can be heated to a predetermined temperature by energization and has durability. Some examples include platinum-based, tungsten, ruthenium oxide, and silicon carbide.

FIG. 6 shows a typical circuit diagram of a gas measuring device incorporating a gas detector. That is, the gas detector 10 is connected in series with the circuit power supply 13 and the voltmeter 12 via the electrodes. Further, a load resistor 11 is connected to the voltmeter 12 in parallel. The heater 8 is connected to a heater power supply 9.

The gas concentration can be measured by the above gas measuring device as follows. The heater 8 is operated, and the gas detection element 5 is placed in the gas to be measured with the gas detection element 5 held at a predetermined temperature between room temperature and 600 ° C., and the voltage at that time is measured by the voltmeter 12. . At this time, a relational expression such as the following expression is established between the voltage V _C of the circuit power supply 13, the resistance R _L of the load resistor 11, the output voltage V _out measured by the voltmeter 12, and the resistance R _{S of the} detection element. And R _S can be calculated.

R _S = R _L (V _C −V _out ) / V _out The gas detection element of the present invention exhibits a certain resistance according to the gas concentration in the atmosphere when the element temperature is constant. By using R _S , the gas concentration can be measured using a calibration curve created in advance.

[0039]

The present invention provides a novel fiber type gas detection element different from the conventional sintered body or thin film type. This gas detection element is an extremely excellent element having higher sensitivity and faster response speed than the conventional sintered type.

[0040]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

Example 1 Stannous chloride (SnCl ₂ ) 10 g (0.05 mol)
Chloroplatinic acid _{(H 2 PtCl 6 · 6H 2} O) 1.554g
(0.003 mol) in 100 ml of methanol (2.47)
Mol) to obtain a homogeneous solution. This solution is at 40 ° C
The solution was concentrated by keeping the solution in a dryer kept at a pH of 2 to obtain a highly viscous sol. The sol was spin-spun from a nozzle having a diameter of 0.5 mm to spin a gel fiber having a length of about 1 m. After leaving the obtained fiber at room temperature for 1 day, 2 ° C
The temperature was raised to 120 ° C. at a rate of / min and maintained at that temperature for 30 minutes. Thereafter, the temperature was raised to 500 ° C. at a rate of 10 ° C./min, and the temperature was maintained for 30 minutes to perform a heat treatment. The resulting fiber has a diameter of about 15 μm and X
As a result of line diffraction, it was confirmed to be polycrystalline SnO ₂ . 200 obtained fibers are piled up and the length is 7m
m and baking gold paste on both ends to reduce the length between the poles to 5
mm. A gas detector shown in FIG. 5 was manufactured using this gas detection element, and was incorporated in a gas measurement device circuit shown in FIG. Next, the temperature of the detection element was kept at 25 ° C., and the gas detector portion was put in the box. Then, carbon monoxide was injected so that the concentration in the box became 100 ppm, and the change in resistance was examined. As a result, when carbon monoxide was injected, the resistance was reduced to 1/110. Also, if the resistance is 99
The time it took to decrease by a factor of 1 (90% change) was 2
5 seconds.

Comparative Example 1 6 mol of chloroplatinic acid was added to tin oxide particles having an average particle diameter of 1 μm.
%, And mixed in a mortar with ethanol as a dispersion medium. The above slurry was applied to a base having a length of 5 mm and an outer diameter of 1 mm on a base in which a gold paste was baked so that the distance between the electrodes was 2 mm.
m, and heat-treated at 500 ° C. for 30 minutes. Next, as a result of evaluating the performance in the same manner as in Example 1,
When carbon monoxide was injected, the resistance was reduced by a factor of 20. The time required for the resistance to decrease by a factor of 18 (90% change) was 180 seconds.

[Brief description of the drawings]

FIG. 1 is a schematic view of a representative gas detection element of the present invention.

FIG. 2 is a schematic view of another gas detection element of the present invention.

FIG. 3 is a schematic view of another gas detection element of the present invention.

FIG. 4 is a schematic view of another gas detection element of the present invention.

FIG. 5 is a schematic view showing a typical embodiment of a gas detector using the gas detection element of the present invention.

FIG. 6 is a typical circuit diagram of a gas measuring device incorporating a gas detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Crystalline tin oxide fiber 2 Conductive paste 3 Support 4 Lead wire 5 Gas detection element 6 Insulating substrate 7 Electrode 8 Heater 9 Heater power supply 10 Gas detector 11 Load resistance 12 Voltmeter 13 Circuit power supply

Claims (1)

(57) [Claims] 1. A gas detecting element comprising a polycrystalline tin oxide fiber.