JPH05232059A - Gas detecting element - Google Patents

Abstract

PURPOSE:To improve a gas sensitivity and a response time by using a polycrystalline tin oxide in the fiber state for a gas detecting element. CONSTITUTION:A gas detecting element constituted by a polycrystalline tin oxide fiber 1 is used in the structure that it is fixed to a supporting table 3 made of an insulating base by connecting both ends of the fiber 1 with lead wires 4. In order to further improve sensitivity, metal such as platinum, gold, silver, etc., or its metal oxide (catalyst) is contained. The content is usually 0.1 to 30Mol relative to tin oxide. Tin oxide is obtained by spinning a spinning solution in which a tin compound expressed by a general formula, SnXa.bH2O (X is a Cl atom, OH group, etc. 'a' and 'b' are integrals) is dissolved in an alcohol expressed by a general formula, ROH and by giving heat treatment to it. The catalyst contained to improve a sensitivity and a response speed is added to the spinning solution in advance. There arc many catalytic forerunners such as VCl 2, SbBr3 silver nitrate, etc., and one or several of them are added to the spinning solution.

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, a semiconductor type gas sensor has been widely used as a sensor for detecting gas components in the atmosphere. In this sensor, a metal oxide semiconductor such as tin oxide, zinc oxide or iron oxide is actually 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 the gas detection element and measuring a change in resistance that occurs when the gas detection element adsorbs the gas.

However, since these materials are used in the form of a sintered body, there is a problem that the detection sensitivity is low or the response speed is slow. Therefore, attempts have been made to improve sensitivity and response speed by adding a catalyst such as a noble metal to the oxide semiconductor. However, the improvement is not satisfactory, and extension of the conventional method naturally has a limit. Therefore, as a new method, use of a thin film type element instead of the sintered body has been vigorously studied. However, in spite of many studies, a serious problem such as low durability cannot be solved yet, and the sintering type has not been replaced.
Further, the conventional fiber-type detecting element has a metal wire such as a platinum wire as a core and an oxide attached to the periphery thereof. The detection element manufactured by this method generally has a large diameter, and has a problem of low sensitivity and low response speed.

On the other hand, it has been difficult to produce a polycrystalline tin oxide fiber by the conventional solid-phase reaction method. A method for producing tin oxide fiber is disclosed in JP-A-60-54.
No. 997, JP-A-60-161337, and JP-A-62-158199 only propose a method of producing by a melt precipitation method. However, these methods require a high temperature of 1000 ° C. or higher and a reaction time of many days, and are practically impossible to obtain. Moreover, the shape of the obtained tin oxide fiber is limited to 1 μm or less in diameter and 3 mm in length, and the obtained fiber is often a whisker, which is generally sensitive as a semiconductor material for detecting gas concentration by resistance change. Is low. Further, it is possible to add a catalyst such as a noble metal in order to improve the sensitivity and the like, but it is difficult to finely adjust the addition amount of the catalyst having a different melting point from that of tin oxide in the manufacturing method. It was This is a fatal drawback for the semiconductor type gas sensor in which the gas selectivity largely changes depending on the delicate addition amount of the catalyst.

[0005]

Therefore, the inventors of the present invention have conducted extensive studies on the creation of a gas detection element having high sensitivity and high response speed.

[0006]

As a result, they have found that gas sensitivity and response speed are improved by using fiber-shaped polycrystalline tin oxide in place of the sintered body that has been used so far. The present invention has been completed here.

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

The present invention will be described more specifically.

The detection element of the present invention essentially comprises fiber-shaped polycrystalline tin oxide. The detection of a gas using an oxide is performed in the form of a resistance change of the oxide due to the adsorption of the gas. It is said that the grain boundary of the oxide largely contributes to this resistance change. Therefore, if there is a grain boundary in a near single crystal such as whiskers,
Alternatively, even if there are few, the sensitivity is low because the number is small, and tin oxide must be polycrystalline.

The fiber has a diameter of 1 mm.
Or less, preferably 500 μm or less, particularly preferably 80
Those having a size of μm or less are used. If the diameter is larger than 1 mm, only the performance equivalent to that of the sintered body can be obtained. That is, the smaller the fiber diameter, the higher the sensitivity and the faster the response speed. The lower limit of the fiber diameter is not particularly limited, but 0.1 μm or more is preferable from the viewpoint of easy production. The length of the fiber is not particularly limited, but considering the size of the gas detection element, it is preferably 2 cm or less, and the lower limit is L / D is 20 or more, where D is the diameter of the fiber and L is the length. It is preferable to choose

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.

The gas detecting element made of polycrystalline tin oxide fiber of the present invention is generally made of an insulating substrate with lead wires connected to both ends of the fiber with a conductive paste as shown in FIGS. Used in a structure that is fixed on a support.

As the gas detecting element, one polycrystal tin oxide fiber may be used alone as shown in FIG. 1, or a plurality of polycrystal tin oxide fibers may be used as shown in FIG. Alternatively, a plurality of them may be bundled and used as shown in FIG. Such selection depends on the resistance value or the shape of the fiber used. In other words, if a fiber with a large resistance value or a fiber with a small diameter and a long length is used alone, the resistance will become very large and matching with the detection circuit will be difficult, so in this case use multiple fibers. Good to do. Further, the smaller the diameter of the fiber, the higher the sensitivity tends to be. However, since the resistance increases as the diameter becomes smaller, 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 each independent detection circuit may perform signal processing. Further, in FIGS. 2, 3 and 4, it is also effective to use a combination of the 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, in order to enhance gas selectivity and sensitivity,
It is also effective to contain a metal such as vanadium, tantalum, niobium, or bismuth or a metal oxide (catalyst) thereof alone or in combination. The type and amount of the catalyst to be added may be appropriately determined depending on the type of gas to be detected, and usually 0.1 to 30 mol is contained with respect to the tin oxide obtained.

Further, it is preferable to contain silica and / or alumina in order to enhance the mechanical strength of the fiber.

Next, a typical method for producing the 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) has the general formula SnXa. bH ₂ O (In the formula, X is Cl atom, Br atom, I atom, F atom, OH group, SO ₄
Group, a NO ₃ group or a CH ₃ COO group, a is an integer of 1 to 4, and b is an integer of 0 to 6) is spun to form a spinning solution, and Obtained by heat treatment.

In the alcohol represented by the general formula ROH, R is 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, 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 alkoxy groups such as methoxy group and ethoxy group, hydroxyl group, phenyl group and the like found in the above-mentioned specific examples of R. In addition to the aryl group, the alkyl group such as a methyl group and an ethyl group, 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, 1-methoxy-2-propyl. Alcohol, methoxyethoxy ethanol, 2-phenylethyl alcohol,
Examples thereof include benzyl alcohol, allyl alcohol, 2-methyl-2-propen-1-ol, 3-methyl-3-buten-1-ol, phenol, methoxyphenol, ethoxyphenol, cresol and ethylphenol. In particular, methyl alcohol and ethyl alcohol are preferred because of their high solubility of tin compounds. The above alcohols are usually used alone, but a mixture of two or more kinds of alcohols can be used in order 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,
A compound having two or more carbonyl groups such as acetylacetone, ethyl acetoacetate and diethyl malonate can also be used supplementarily.

In the tin compound represented by the general formula SnXa.bH ₂ O, X is Cl atom, Br atom, I atom,
F atom, OH group, SO ₄ group, NO ₃ group or CH ₃ COO
Shows a group, a shows the integer of 1-4, b shows the integer of 0-6. Among these, tin chloride and tin bromide are preferable in terms of price and stability. Specifically, SnCl ₂ , SnCl ₂ ·
2H ₂ 0, 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
₁₂ is preferably used. Also those modified with an organic compound of the above tin compounds, may also be used, for example Sn (CH _{3) 2} Cl ₂ and the like.

The mixing ratio of the tin compound and the alcohol is not particularly limited as long as the tin compound can be dissolved uniformly in the alcohol. However, if the proportion of the tin compound is too low, it does not show spinnability, so it is necessary to concentrate it, and the alcohol is wasted. Further, if the tin compound concentration is too high, precipitation occurs and a uniform spinning solution cannot be obtained. Therefore, the compounding ratio of the tin compound and the alcohol used varies depending on the type of the alcohol, but generally, the ratio of the tin compound to the alcohol is 0.02 in terms of molar ratio.
~ 0.5 is preferred.

Improved sensitivity, gas selectivity and response speed
The catalyst to be contained for this purpose is the catalyst described below in the spinning solution in advance.
Adding the precursor in the form of a solution ensures uniform dispersion and
Preferred for inclusion. The catalyst precursor is heated after spinning.
It changes to metal or metal oxide by reason and shows catalytic action
Like Polycrystalline tin oxide powder prepared by the above spinning method
The manufacturing method of the bar is to control the content of these catalysts delicately.
It is very advantageous because it can be added uniformly in the form of a solution.
That's the method. Specific catalyst precursors include VB
r₃, VCl₂, VCl₃, VCl_Four, VOBr₂, VOB
r₃, VOCl₃, VF₃, VF_Four, VF_Five, VI₃6H
₂O, vanadium alkoxide, NbCl_Five, NbBr
_Five, NbF_Five, NbOCl₃, Niobium alkoxide, T
aBr_Five, TaCl _Five, Tantalum alkoxide, SbC
l₃, SbCl_Five, SbBr₃, Antimony oxychloride,
Antimony alkoxide, BiCl₃, BiI₂,Screw
Trout 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 silica and / or alumina precursors to the spinning solution in advance. These precursors also become silica and / or alumina by the heat treatment after spinning. Examples of the precursor include silicon alkoxide, aluminum alkoxide, aluminum chloride, aluminum nitrate and the like.

The silicon alkoxide has the general formula S
i (OR _A ) ₄ , R _B Si (OR _A ) ₃ , R _B R _C (OR _A ) ₂
A silicon alkoxide represented by Here, R _A , R _B and R _C represent an alkyl group, an alkenyl group and an aryl group. The alkyl group is a linear or branched alkyl group such as a methyl group, an ethyl group, a propyl group, a butyl group or a pentyl group, and the alkenyl group is an ethenyl group, a propenyl group, a butenyl group, a pentenyl group or the like. It is a linear or branched alkenyl group.

More specific examples are tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, phenyltrimethoxysilane, propyltrimethoxysilane, and 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 Examples include diethoxysilane.

As the aluminum alkoxide, the 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 alkoxide can be used.

The amount of the silica and / or alumina precursor added is 0.01 to 2 with respect to the tin compound.
0% by weight is preferred. If the addition amount is less than 0.01% by weight, a sufficient effect cannot be obtained, and even if it exceeds 20% by weight, the effect is saturated and the silica and alumina components in the obtained tin oxide fiber increase. Then, the sensitivity and the like may decrease, which is not preferable.

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 can be used under stirring. A method in which alcohol is added dropwise to the mixture, or a method in which a tin compound, a catalyst precursor, a silicon alkoxide and / or an aluminum compound are dissolved in alcohol under agitation, sequentially or simultaneously, is adopted.

Acids such as hydrochloric acid, nitric acid and acetic acid, compounds having a carbonyl group such as acetylacetone, or ammonia may be appropriately used as the complexing agent.

The spinning method is not particularly limited, and a conventional spinning method can be used. For example, a method of extruding a spinning solution from a spinning nozzle may be used. 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 at which the spinning solution is extruded from the spinning nozzle.

The heat treatment of the gel fiber obtained by spinning is carried out at a temperature at which the gel fiber can be converted into polycrystalline tin oxide. The gel fiber as it is spun from the spinning solution contains an organic substance such as alcohol as it is, is an insulator, and semiconductivity is exhibited by heating the gel fiber. The heat treatment temperature is not particularly limited as long as it is within a temperature range capable of imparting semiconductivity to the obtained fiber. Generally, when the heat treatment temperature is low, organic substances such as alcohol, or water remains in the gel fiber, and the components such as the catalyst precursor are not in the form of metal or oxide, and tin oxide is sufficient. Since it does not form a solid solution or disperse, the sensitivity cannot be improved. On the other hand, if the heat treatment temperature is too high, the catalyst component in the fiber is volatilized to deteriorate the performance, the decomposition of tin oxide proceeds, the crystal grains in the fiber grow excessively, and the strength decreases. Therefore, the heat treatment temperature is 250 ° C. to 1550.
A temperature range of ° C is preferred. More preferably, from 300 ° C
It is desirable to perform heat treatment at a temperature of 1500 ° C. The heat treatment is usually performed in air, but when a fiber having particularly high conductivity is desired, the heat treatment is performed in a reducing atmosphere such as nitrogen, argon, hydrogen, a mixed gas of argon and hydrogen, or in vacuum. be able to.

In the heat treatment, it is desirable to remove the volatile components such as water and alcohol existing in the gel fiber by drying in order to obtain a good conductive fiber. Such drying may be performed at the same time as the heat treatment, but it is 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 cracks from being generated in the obtained fiber, but 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. The general drying temperature is preferably in the range of room temperature to 300 ° C.

In the present invention, the gas detector using the gas detecting element may have a known structure without particular limitation. FIG. 5 shows a typical embodiment of a gas detector using the gas detecting element of the present invention. That is, the gas detection element 5 is installed on the 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 in order to control the temperature of the detection element. The insulating substrate 6 is for supporting and fixing the gas detection element 5, the heater 8 and the electrode 7. Insulating substrate 6
Any material can be used without particular limitation as long as it has insulation properties and heat resistance to the heating temperature of the heater 8. As such a material, an alumina-based material is generally preferable. The heater 8 heats the gas detection element 5 to control gas selectivity and response speed. The heater 8 is preferably installed in the vicinity of the gas detection element 5 so that the gas detection element 5 can be heated via the insulating substrate. Further, 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 are 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, the load resistance 11 is connected in parallel with the voltmeter 12. The heater 8 is connected to the heater power supply 9.

The gas concentration can be measured by the above gas measuring device as follows. The gas detecting element is placed in the gas to be measured while the heater 8 is operated and the gas detecting element 15 is maintained 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, the following relational expression is given among the voltage V _C of the circuit power supply 13, the resistance R _L of the load resistance 11, the output voltage V _out measured by the voltmeter 12, and the resistance R _{S of the} detection element. There is R _S
Can be calculated.

[0038] When R _S = R _L (V _C over V _out) / V _out gas detection element of the present invention the element temperature is constant, it indicates constant resistance which is subject to the gas concentration in the atmosphere, the With R _S , the gas concentration can be measured using a calibration curve prepared in advance.

[0039]

The present invention provides a novel fiber type gas detecting element, which is different from the conventional sintered body or thin film type. This gas detection element is highly excellent in sensitivity and response speed as compared with 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 10 g (0.05 mol) of stannous chloride (SnCl ₂ ),
Chloroplatinic acid (H ₂ PtCl ₆ · 6H ₂ O) 1.554 g
(0.003 mol) was added to 100 ml of methanol (2.47 mol).
To obtain a uniform solution. This solution at 40 ℃
The solution was concentrated by keeping it in a drier kept at 1, to obtain a highly viscous sol. The sol was spin-spun through 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 for 1 day at room temperature, 2 ℃
The temperature was raised to 120 ° C. at a speed of / min and held at that temperature for 30 minutes. Then, the temperature was raised to 500 ° C. at a rate of 10 ° C./min, and the temperature was maintained for 30 minutes to perform heat treatment. The resulting fiber has a diameter of about 15 μm and has X
As a result of line diffraction, it was confirmed to be polycrystalline SnO ₂ . 200 fibers are piled up and the length is 7m.
Cut to m and bake gold paste on both ends to make the length between both electrodes 5
mm. A gas detector shown in FIG. 5 was produced using this gas detecting element and incorporated in the circuit of the gas measuring device shown in FIG. Then, the temperature of the detection element was kept at 25 ° C., the gas detector portion was put in the box, and 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 decreased to 1/110. Also, the resistance is 99
The time required to reduce to 1/90 (90% change) is 2
It was 5 seconds.

Comparative Example 1 6 mol of chloroplatinic acid was added to tin oxide particles having an average particle size of 1 μm.
% So that the mixture was mixed in a mortar with ethanol as a dispersion medium. The thickness of the slurry is about 0.5 m on a substrate in which a gold paste is baked around an alumina tube having a length of 5 mm and an outer diameter of 1 mm so that the distance between the electrodes is 2 mm.
It was applied so as to have a thickness of 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 dropped to 1/20. The time required for the resistance to decrease to 1/18 (90% change) was 180 seconds.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a typical gas detection element of the present invention.

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

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

FIG. 4 is a schematic diagram 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]

 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)

[Claims] 1. A gas detection element comprising a polycrystalline tin oxide fiber.