Process for producing tin oxide fibres

Abstract: A tin oxide fiber expressed by the general formula Sn (1-y) Z y O₂ (I) wherein Z is a metal element of the Group V of the periodic table, and y is 0 to 0.3. having a diameter which is greater than 1 µm or having a length which is greater than 3 mm and having an aspect ratio which is greater than 10. Process for producing a tin oxide fiber comprising spinning an alcohol solution of a tin compound soluble in alcohol, or spinning an alcohol solution of said tin compound and a compound of a metal of the Group V of the periodic table that is soluble in alcohol, followed by heating.

Description: The present invention relates to a process for producing a tin oxide fiber, particularly a tin oxide fiber having a diameter which is greater than 1 µm or having a length which is greater than 3 mm and having an aspect ratio which is greater than 10. Tin oxide fibers have been widely known. Being limited by the process of production, however, the tin oxide fibers that have been known so far all have diameters which are smaller than 1 µm. It has been demanded to develop a tin oxide fiber having a large diameter and an increased length to meet a variety of applications. However, a technology that satisfies the above demand has not yet been developed. In the field of gas sensors, it has been strongly desired to provide the tin oxide in the form of a fiber from the standpoint of improving sensitivity and response time. It has further been attempted to add, for example, a carbon fiber in order to give electric conductivity to the high molecular weight materials. When carbon is used, however, there arise a problem in that the black color of carbon makes it difficult to obtain materials in bright colors. Therefore, it has been attempted to add a metal fiber or the powder of a metal oxide. However, the metal fiber which has a high electric conductivity permits the fiber surfaces to be oxidized or corroded after long periods of time and decreases electric conductivity. Moreover, a conventional metal oxide powder does not have electric conductivity that is as high as that of the metal fibres, and must be added in relatively large amounts to give electric conductivity to the high molecular weight materials, causing the properties inherent in the high molecular weight materials to be deteriorated. Attempts have further been made to add the tin oxide having excellent resistance against the chemicals and heat in the form of a powder. It has been known that the effect of giving electric conductivity increases with an increase in the aspect ratio of the electric conductivity-imparting material. It has therefore been demanded to provide the tin oxide having high electric conductivity in the form of a fibre. Tin oxide fibres have heretofore been obtained exclusively by the so-called melt-precipitation method according to which the starting material of a tin compound is melted at high temperatures in order to precipitate the tin oxide fibre. Japanese Patent Laid-Open Nos. 54997/1985 and 161337/1985 teach tin oxide fibres having a diameter of 0.5 µm and a maximum length of 3 mm obtained by the above melt-precipitation method. The above methods, however, require a reaction temperature which is higher than 1000°C and a reaction time of several days. In addition, the obtained fibre has a diameter that is so small that the handling becomes difficult. When used as a composite material, therefore, the properties are not sufficiently exhibited and the applications to which the fibres can be put are restricted. Moreover, the length is so small that paper-like materials cannot be prepared therefrom. DE-A-1,918,754 describes a process for producing inorganic fibres such as tin oxide fibres by dry-spinning a solution of, for example, a tetravalent tin compound. The present invention seeks to provide a process for producing a tin oxide fibre which may have a large diameter and an increased length. The present invention also seeks to provide a process for producing a tin oxide fibre from a starting material that is industrially cheaply available. In order to accomplish the above objects, the present inventors have conducted keen study, and have astonishingly discovered the fact that a tin oxide fibre having any aspect ratio can be obtained easily and very stably by using divalent tin compounds instead of using the traditionally employed starting material of tin alkoxide that is expensive, unstable and cumbersome to handle. Thus the present invention provides a process for producing a oxide fibre of formula: Sn (1-y) Z y O 2 wherein Z is a metal element of Group V of the periodic table and y is 0 to 0.3, comprising spinning an alcohol solution of a divalent tin compound soluble in alcohol, or spinning an alcohol solution of the divalent tin compound and a compound of a metal of Group V of the periodic table that is soluble in alcohol, followed by heating. The tin oxide fibre desirably has a diameter which is greater than 1 µm, or has a length which is greater than 3 mm, and has an aspect ratio which is greater than 10. The metal element of Group V of the periodic table denoted by Z in formula (I) is a component that is added to increase the electric conductivity of the fiber of SnO 2 . The component Z is contained in the SnO 2 in the form of a solid solution and contributes to improving the electric conductivity. The component Z that is contained in an increased amount does not all exist in the form of a solid solution but is partly mixed with the SnO 2 . Generally, the component Z is contained in the SnO 2 in the form of a solid solution in an amount of up to about y=0.15. As the fraction y becomes 0.20 or greater, however, the component Z is partly contained in the form of a mixture in addition to the component that is contained in the SnO 2 in the form of a solid solution. Even when the component Z is contained partly as a mixture, the object of improving the electric conductivity can be sufficiently achieved when the fraction y of the component Z is equal to a less than 0.3. The fact that component Z is contained in the tin oxide fiber in the form of a solid solution can be confirmed by fluorescent X-ray analysis and by the fact that there is no peak of component Z in X-ray diffraction. The composition of the tin oxide fiber can be sufficiently confirmed by fluorescent X-ray analysis and X-ray diffraction. When a more accurate composition is required, elemental analysis should be carried out. Even in the tin oxide fiber produced by the present invention, however, there are oxygen deficiencies as often happen in other oxides. The number of deficiencies varies to some extent as a matter of course. The spinning method of the present invention can provide a tin oxide fibre having any diameter, length and aspect ratio. Generally, the tin oxide fiber has a diameter greater than 1 µm, preferably 5 to 500 µm, a length longer than 3 mm, preferably longer than 1 cm, and an aspect ratio of greater than 10, preferably greater than 50, and more preferably greater than 100. It is, however, also allowable to produce fibers having a diameter smaller than 1 µm and a length greater than 3 mm or fibers having a length of 0.5 mm to 3 mm and a diameter of greater than 1 µm, depending upon the applications. By changing the spinning conditions, in particular, a fiber can be obtained having a diameter of 1 µm or greater. Moreover, the length of the fiber can be increased to as long as desired by continuously winding it. The tin oxide fiber has an electrically conductive property. When the component denoted by Z is not present, the electric conductivity ranges from about 1 x 10 3 ohm-cm to about 1 x 10 5 ohm-cm. When component Z is contained in the form of a solid solution, the electric conductivity can be improved to 1 x 10 -2 ohm-cm or further to 1 x 10 -3 ohm-cm. In order to enhance the electric conductivity, therefore, component Z should preferably be added in an amount such that y=0.3 at the greatest. The greatest effect is exhibited when component Z is added in an amount such that y=0.15 to about Y=0.20, in which it is contained in the SnO 2 in the form of a solid solution. There is no particular limitation on the metal elements of the Group V of the periodic table denoted by Z in formula (I). And any of vanadium, niobium, tantalum, antimony or bismuth can be used. From the standpoint of improving the electric conductivity, however, antimony (Sb), is most desired and, then, tantalum (Ta) or niobium (Nb). Therefore, the compound Sn (1-y) Sb y O 2 is most desired and, then, the compounds Sn (1-y) Ta y O 2 and Sn (1-y) Nb y O 2 (y is 0 to 0.3 in all cases). Though there is no particular limitation on the alcohol that is used in the process of the present invention, an alcohol of formula ROH is preferably used, in which R is an unsubstituted or substituted alkyl group such as a methyl group, ethyl group, propyl group, butyl group or octyl group, or an unsubstituted or substituted alkenyl group such as a vinyl group, allyl group or butenyl group. Examples of the substituent group in the above substituted alkyl group and alkenyl group include an alkoxyl group such as a methoxy group or ethoxy group; a hydroxyl group; a hydroxyalkyl group; an aryl group such as a phenyl group; an alkyl group such as a methyl group or ethyl group; an amino group; a cyano group; and a halogen atom such as a chlorine atom, bromine atom, iodine atom or fluorine atom. Examples of the alcohol include methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, octyl alcohol, 2-methoxyethanol, 2-ethoxyethanol, ethylene glycol, 1-methoxy-2-propyl alcohol, ethoxymethoxyethanol, 2-phenylethyl alcohol, benzyl alcohol, allyl alcohol, 2-methyl-2-proprene-1-ol, 3-methyl-3-butene-1-ol, phenol, methoxyphenol, ethoxyphenol, cresol and ethylphenol. In particular, methyl alcohol and ethyl alcohol are desired because of their high capabilities of solubilizing the tin compound. The alcohol is usually used alone, but it is also allowable to use two or more alcohols mixed together to control the reactivity to the tin compound or to control the solubility of the tin compound. Any divalent tin compound can be used as the starting material without any particular limitation provided it is soluble in the above-mentioned alcohols. The alcohol-soluble tin compound which is available most cheaply is a halogenated tin compound. Particularly suited examples include tin chloride and tin bromide such as SnCl 2 , SnBr 2 and SnCl 2 ·2H 2 O. There is no particular limitation on the ratio of blending the tin compound and alcohol together provided the tin compound is uniformly dissolved in the alcohol. When the ratio of the tin compound is too small, however, no spinnability is exhibited and the mixture must be condensed resulting in the waste of alcohol. When the concentration of tin compound is too high, on the other hand, precipitation takes place making it difficult to obtain a homogeneous spinning solution. Therefore, though the blending ratio varies depending upon the kinds of the tin compound and alcohol that are used, it is generally preferred that the tin compound is used in a molar ratio of 0.02 to 0.5 with respect to the alcohol. In order to improve the stability of the spinning solution it is allowable to additionally use a compound having two or more carbonyl groups such as acetylacetone, ethyl acetoacetate or diethyl malonate. In order to adjust the viscosity an organic high molecular weight substance can be added, such as polyethylene oxide or polyvinyl alcohol. There is no particular limitation on the compound of a metal of Group V of the periodic table that is added to improve the electric conductivity of the SnO 2 fiber; any known metal compound can be used. Examples of the compound of the metal of Group V are a vanadium compound, niobium compound, tantalumn compound, antimony compound and bismuth compound. Among them, the antimony compound is most preferred and, then, the tantalum compound and niobium compound are preferred. There is no particular limitation on these compounds provided they are soluble in the alcohol. The most preferred examples include halides such as chlorides and bromides, and alkoxy compounds such as methoxy, ethoxy and propoxy. When it is desired to give electric conductivity to the tin oxide fiber, the compound of Group V should be added at a ratio such that y is less than y=0.3, preferably, at a ratio of y=0.001 to y=0.25 with respect to the tin oxide reckoned as oxide. When the above ratio is too small, the tin oxide fiber that is obtained exhibits low electric conductivity. Even when the above ratio is increased, on the other hand, the electric conductivity is not given too much. There is no particular limitation on the method of dissolving the tin compound and the compound of Group V in the alcohol; e.g., the alcohol may be dropped on the tin compound and the compound of Group V, or the tin compound and the compound of Group V may be dissolved in the alcohol with stirring. It is further allowable to suitably use an acid such as hydrochloric acid, nitric acid or acetic acid, a compound having a carbonyl group such as acetylacetone, and ammonia as a catalyst and a complexing agent. There is no particular limitation on the method of spinning; any conventional spinning method can be employed. For instance, there can be employed a method in which a spinning solution is extruded into the open air from the spinning nozzle. The length and diameter of the obtained fiber can be arbitrarily controlled by adjusting the viscosity of the spinning solution or the velocity at which the spinning solution is extruded from the spinning nozzle. The relative humidity at the time of spinning plays an important role in determining the spinnability. For example, when the atmosphere in which the spun fiber comes in contact has a high relative humidity, it becomes difficult to obtain a fiber having a large diameter and to spin a fiber having increased length. In order to stably spin a fiber having any aspect ratio and, particularly, having a length of greater than 2 m, therefore, it is desired to maintain the relative humidity of the atmosphere in which the fiber comes in contact to be less than 40%. Although it is possible to spin the fiber even when the relative humidity is greater than 40%, it becomes difficult to stably spin the fiber having a length greater than 2 m and, particularly, to stably spin continuous fibers. The present inventors are not sure about the reasons but presume that when the atmosphere has a high relative humidity, the alcohol and water contained in the spun gel fiber vaporize sparingly, and the gel fiber is not quickly solidified and does not acquired stabilized shape. Moreover, because of its high water absorbing property and deliquescent property, the gel fiber becomes soft as soon as it comes in contact with the atmosphere of a high relative humidity. It is therefore important to maintain the relative humidity to be less than 40%. The gel fiber obtained without heat-treatment does not exhibit high electric conductivity. High electric conductivity is attained as the gel fiber is heat-treated. There is no particular limitation on the temperature of the heat treatment provided it lies within a range to give electric conductivity to the fiber. When the temperature of the heat treatment is low, in general, organic material such as the alcohol and water remain in the gel fiber, and the compound of Group V is not converted into an oxide and is not sufficiently contained in the tin oxide in the form of a solid solution. Therefore, high electric conductivity is not exhibited. When the temperature of the heat treatment is too high, on the other hand, the compound of Group V escapes from the fiber causing the electric conductivity to decrease, decomposition of the tin oxide is promoted, and crystalline particles grow excessively in the fiber causing the strength to decrease. In general, the temperature of the heat treatment ranges from 150°C to 1550°C, and tin oxide fiber is crystalized preferably over a temperature range of 250°C to 1550°C. More preferably, the heat treatment should be carried out at a temperature of 300°C to 1500°C. The heat treatment is usually carried out in the air. When it is desired to obtained a fiber having high electric conductivity, in particular, the heat treatment should be carried out in a reducing atmosphere such as of nitrogen, argon, hydrogen, or a mixture of argon and hydrogen, or in vacuum. In order to obtain a fiber having good electric conductivity, furthermore, it is desired to remove volatile components such as water and alcohol present in the gel fiber by drying. The drying may be carried out simultaneously with the heat treatment. To obtain a fiber having good electric conductivity, however, it is desired to dry the fiber prior to the heat treatment. In the case of the continuously spinning method, the drying step is placed preceding the step of heat treatment. It is desired that the drying is carried out at a temperature as low as possible so that cracks will not develop in the fiber that is obtained. When an alcohol having a high boiling point is used as a solvent, however, an extended period of time is required for the drying if the drying temperature is too low. In general, the drying should desirably be carried out at a temperature over a range of room temperature to 300°C. The tin oxide fiber assumes either the crystalline form or the non-crystalline form depending upon the temperature of the heat treatment. When it is desired to obtain a tin oxide fiber having good electric conductivity, the heat treatment should be carried out at a high temperature to obtain the tin oxide fiber in the crystalline form. According to the present invention as described above, a tin oxide fiber having a controlled diameter and a controlled length can be obtained. The fiber obtained by the above-mentioned method can be used directly as a raw material, for example for gas sensors or as a filler of plastics. As required the tin oxide fiber obtained by the above method can be processed into fibers of suitable sizes by such means as cutting or pulverizing to use them in a variety of applications. The invention is now further described in the following Examples. Example 1 10 Grams (0.05 mols) of stannous chloride (SnCl 2 ) was dissolved in 100 ml (2.47 mols) of methanol to obtain a homogeneous solution thereof. The solution was held in a dryer maintained at 40°C and was concentrated to obtain a sol having a high viscosity. The tip of a glass rod was immersed in the sol and was pulled up in an atmosphere of a relative humidity of 55% while manually changing the pull-up speed from about 1 cm/s to 10 cm/s to spin many gel fibers having lengths of about one meter. The obtained fibers were left to stand at room temperature for one day, heated to 120°C at a rate of 2°C/min. and held at that temperature for 30 minutes. The fibers were then heated to 500°C at a rate of 10°C/min. and held at that temperature for 30 minutes. The obtained fibers possessed diameters over a range of from 1 µm to 2 mm depending upon the pull-up speed, and were confirmed to comprise crystalline tin oxide by X-ray analysis. Example 2 13.9 Grams (0.05 mols) of stannous bromide (SnBr 2 ) was dissolved in 100 ml of ethanol to obtain a homogeneous solution thereof. The solution was held in a dryer maintained at 40°C and was concentrated to obtain a sol having a high viscosity. The tip of a glass rod was immersed in the sol and was pulled in an atmosphere of a relative humidity of 55% while variously changing the pull-up speed in the same manner as in Example 1 to spin many gel fibers having lengths of about one meter. The obtained fibers were left to stand at room temperature for one day, heated to 120°C at a rate of 2°C/min. and were held at that temperature for 30 minutes. The fibers were then heated to 500°C at a rate of 10°C/min. and were held at that temperature for 30 minutes to effect the heat treatment. The obtained fibers possessed diameters over a range of from 1 µm to 2 mm depending upon the pull-up speed, and were confirmed to comprise crystalline tin oxide by X-ray analysis. Example 3 11.3 Grams (0.05 mols) of stanous chloride dihydrate (SnCl 2 ·2H 2 O) was dissolved in 100 ml (2.47 moles) of methanol to obtain a homogeneous solution thereof. The solution was held in a dryer maintained at 40°C and was concentrated to obtain a sol having a high viscosity. The tip of a glass rod was immersed in the sol and was pulled up in an atmosphere of a relative humidity of 55% while variously changing the pull-up speed in the same manner as in Example 1 to spin many gel fibers having lengths of about one meter. The obtained fibers were left to stand at room temperature for one day, heated to 120°C at a rate of 2°C/min. and were held at that temperature for 30 minutes. Then, the fibers were heated to 500°C at a rate of 10°C/min. and were held at that temperature for 30 minutes to effect the heat treatment. The obtained fibers possessed diameters over a range of from 1 µm to 2 mm depending upon the pull-up speed, and were confirmed to comprise of crystalline tin oxide by X-ray analysis. Example 4 to 10 Procedures were carried out in the same manner as in Example 1 with the exception of using a variety of alcohols instead of methanol. A rotary evaporator was used to concentrate the solution. The results are shown in Table 1. Examples 11 and 12 Procedures were carried out in the same manner as in Example 1 with the exception of setting the relative humidity in the spinning atmosphere to 40% and 10%. The results are shown in Table 1. Example 13 The procedure was carried out in the same manner as in Example 1 with the exception of changing the temperature of the heat treatment from 500°C to 180°C. The result is shown in Table 1. Example 14 10 grams (0.05 mols) of stannous chloride (SnCl 2 ) and 1 g (0.004 mols) of antimony trichloride (SbCl 3 ) were dissolved in 100 ml (2.47 mols) of methanol to obtain a homogeneous solution thereof. The solution was held in a dryer maintained at 40°C and was concentrated to obtain a sol having a high viscosity. The tip of a glass rod was immersed in the sol and was pulled up in an atmosphere of a relative humidity of 55% while variously changing the pull-up speed to spin many gel fibers having lengths of about one meter. The obtained fibers were left to stand at room temperature for one day, heated to 120°C at a rate of 2°C/min. and were held at that temperature for 30 minutes. The fibers were then heated to 500°C at a rate of 10°C/min. and were held at that temperature for 30 minutes to effect the heat treatment. The obtained fibers possessed diameters over a range of from 1 µm to 2 mm depending upon the pull-up speed, and were confirmed by fluorescent X-ray analysis to contain antimony in the fibers in amounts in compliance with the feeding composition. By X-ray diffraction, no peaks were found except those that corresponded to the tin oxide, from which it was confirmed that the antimony had been contained in the tin oxide in the form of a solid solution. The fibers exhibited resistivity of about 8 x 10 -1 ohm-cm. Example 15 13.9 Grams (0.05 mols) of stannous bromide (SnBr 2 ) and 1 g (0.004 mols) of antimony trichloride (SbCl 3 ) were dissolved in 100 ml (2.47 moles) of methanol to obtain a homogeneous solution thereof. The solution-was held in a dryer maintained at 40°C and was concentrated to obtain a sol having a high viscosity. The tip of a glass rod was immersed in the sol and was pulled up in an atmosphere of a relative humidity of 55% while variously changing the pull-up speed to spin many gel fibers having lengths of about one meter. The obtained fibers were left to stand at room temperature for one day, heated to 120°C at a rate of 2°C/min. and were held at that temperature for 30 minutes. The fibers were then heated to 500°C at a rate of 10°C/min. and were held at that temperature for 30 minutes to effect the heat treatment. The obtained fibers possessed diameters of from 1 µm to 2 mm depending upon the pull-up speed, and were confirmed to comprise crystalline tin oxide in which antimony had been contained in the form of a solid solution by fluorescent X-ray analysis and X-ray diffraction. The fibers exhibited a resistivity of 8 x 10 -1 ohm-cm. Examples 16 to 44 Procedures were carried out by changing the tin compound, alcohol, the compound of Group V of the periodic table, humidity at the time of spinning and the temperature of the heat treatment. The results are shown in Table 2. When alcohols other than methanol and ethanol were used, however, the solutions were concentrated using a rotary evaporator. Example 45 The procedure was carried out in the same manner as in Example 14 with the exception of rotation-spinning the spinning solution from an orifice 0.5 mm in diameter at a relative humidity of 55% at a speed of 2000 rpm. The obtained gel fiber of a length of about one meter was heat-treated. The diameter was about 15 µm. It was confirmed by fluorescent X-ray analysis that antimony was contained in the fiber in an amount in compliance with the feeding composition. By X-ray diffraction, furthermore, no peak was found except the peak that corresponded to the tin oxide, from which it was confirmed that the antimony had been contained in the tin oxide in the form of a solid solution. The obtained fiber exhibited a resistivity of about 8 x 10 -1 ohm-cm.

Claims: A process for producing a tin oxide fibre of formula: Sn (1-y) Z y O 2 wherein Z is a metal element of Group V of the periodic table and y is 0 to 0.3, comprising spinning an alcohol solution of a divalent tin compound soluble in alcohol, or spinning an alcohol solution of the divalent tin compound and a compound of a metal of Group V of the periodic table that is soluble in alcohol, followed by heating. A process for producing a tin oxide fibre according to claim 1, wherein the spinning atmosphere has a relative humidity of lower than 40%. A process for producing a tin oxide fibre according to claim 1 or 2 wherein the divalent tin compound is a halogen compound. A process for producing a tin oxide fibre according to any one of claims 1 to 3 wherein the compound of a metal of the Group V of the periodic table is an antimony compound that is soluble in alcohol. A process for producing a tin oxide fibre according to any one of claims 1 to 4 wherein the alcohol is a saturated or an unsaturated alcohol. A process for producing a tin oxide fibre according to any one of claims 1 to 5, wherein the alcohol is a lower alcohol having 1 to 5 carbon atoms. A process for producing a tin oxide fibre according to any one of claims 1 to 6, wherein the heating is effected at a temperature of 150°C to 1550°C. A process for producing a tin oxide fibre according to claim 7, wherein the heating is effected at a temperature of 250°C to 1550°C. A process for producing a tin oxide fibre according to any one of claims 1 to 8 wherein y is 0.001 to 0.25 and the fibres have (i) a diameter of 5 to 500 µm, (ii) a length which is greater than 1 cm, (iii) an aspect ratio greater than 50 and (iv) an electric resistivity of 1 x 10 -3 Ohm.cm to 1 x 10 5 Ohm.cm. A process for producing a tin oxide fibre according to any one of claims 1 to 9 wherein the tin oxide is a crystalline substance.

Inventors: KEIKO SEKI, SAKKA SUMIO, TOSHINOBU YOKO, YAMASHITA HIROYA, KEIKO, SEKI, SAKKA, SUMIO, TOSHINOBU, YOKO, YAMASHITA, HIROYA, Keiko, Seki, c/o Tokuyama Soda K.K., Yamashita, Hiroya, c/o Tokuyama Soda K.K.
Applicants: TOKUYAMA CORP, TOKUYAMA CORPORATION
Application year: 1992
Main classification: C04B35
IPC codes: C04B35, D01F9, C01G19