The present invention relates to tin oxide based fibres having a diameter of between 5 and 100 micrometres, and a method of preparing such fibres. These tin oxide fibres have interesting conductive properties.

US Patent 4 725 331 describes tin oxide fibres having a mean diameter of the order of 0.5 to 1 micrometre and a length of 3 mm. These fibres are prepared by a vaporisation/growth process from a solid solution of tin oxide, known as a solute, and a solvent. This process comprises the following steps: (1) vaporisation of the solid solution by heating to temperatures as high as 1250°C, (2) condensation of the solute vapours in a "low" temperature area, that is to say at a temperature less than the vaporisation temperature of the mixture and (3) growth of the tin oxide crystals in the area maintained at a low temperature (1000°C) for approximately twenty days so as to obtain fibres. This indicates the enormous expenditure of energy and the technical difficulty which the implementation of this process entails.

This method does not make it possible to obtain a broad range of diameters of fibres and the tin oxide fibres obtained have a resistivity of the order of 100 Ω.cm. This resistivity remains equal to 100 Ω.cm even when the tin oxide fibres are doped with antimony. In addition, this resistivity cannot be modified after the fibres are obtained.

On the other hand, the tin oxide based fibres according to the invention have a diameter of between 5 and 100 micrometres and a conductivity which does not depend on the method of obtaining the fibres. According to the invention, the conductivity of the fibres can be varied so

as to obtain a previously determined value.

The fibres of the invention can be formed solely from tin oxide (IV) or comprise, in addition to this oxide, one or more constituents. The mean size of the tin oxide particles making up the fibres is advantageously between 50 and 10 micrometres and these fibres can be hydrated, corresponding to the following preferred composition: Sn0 ₂ ,xH ₂ 0 where 0.5 < x < 2.5.

Figures l and 2 show electron photomicrographs of tin oxide fibres as prepared.

Figures 3 and 4 are electron photomicrographs of tin oxide fibres after heat treatment.

Figure 5 shows an X-ray diffraction pattern of the tin oxide fibres after heat treatment at different temperatures.

Figure 6 shows electron photomicrographs of tin oxide fibres containing antimony after heat treatment at 1200°C.

Figure 7 shows electron photomicrographs of tin oxide fibres containing silicium after heat treatment at 700°C.

Figure 8 shows an X-ray di fraction spectrum of tin oxide fibres containing different concentrations of silicium after heat treatment at 700°C.

Figures 9 and 10 show the variation in the resistance of the fibres as a function of the temperature of the heat treatment, as is explained further on in detail.

Figures 11 and 12 show the diameter distribution of the fibres obtained as a function of the mode and temperature

of cooling.

These fibres are moreover obtained by a specific method which makes it possible to obtain fibres of predetermined diameter and conductivity.

In fact, the method according to the invention is of the "sol-gel" type, that is to say it consists of transforming a solution (sol) of a metallic salt or organo-metallic compound into a gel, that is to say a medium in which the viscosity tends towards infinity. This transformation corresponds to the formation of a three-dimensional grid by the polymerisation of the molecular entities present.

An example of such a "sol-gel" process is given in US Patent 4 122 041, which describes the obtaining of amorphous porous fibres of silicium dioxide (Si0 ₂ ) . The technique used comprises first of all the preparation of an aqueous solution of silicic acid from a solution of sodium silicate treated with an H ⁺ ion exchange resin. The gel is then formed by raising the pH by means of ammonia solution. The Si0 ₂ fibres are then obtained at the end of a unidirectional congelation process. The unidirectional congelation process consists of preparing the gel directly in a cylinder which is immersed progressively and very slowly in dry ice (acetone/carbon dioxide snow) . When the congelation is complete, the cylinder is removed from the bath and is left to return to ambient temperature. The silicium dioxide fibres formed are easily isolated by decanting or filtering.

Subsequently, this unidirectional congelation technique was applied to other oxides, in particular titanium dioxide Ti0 ₂ and zirconium dioxide Zr0 ₂ , as described by T Maki, Y Teranishi, T Kokubo, S Sakka, Yoαvo-Kvokai-Shi 93, 387-393, 1985 and T Kokubo, Y Teranishi, T Maki, Journal J Non-

Crvst. Solids 56, 411-416, 1983. In these cases, however, the gels were prepared not by ion exchange but by dialysis of aqueous solutions of respectively titanium tetrachloride and zirconium oxychloride.

In these documents of the prior art, the fibres are obtained solely when the congelation of the gel takes place at a slow controlled rate. In addition, these documents do not mention the possibility of obtaining, by means of this method, fibres containing several chemical constituents.

The method of obtaining fibres according to the invention comprises the following steps:

(a) preparation of an aqueous solution containing at least one tin salt;

(b) dialysis of solution (a) through a porous membrane enabling the ions associated with the Sn cation to be eliminated so as to obtain a gel;

(c) cooling of the gel to a temperature less than 0°C; and

(d) decongelation at ambient temperature.

The tin oxide based fibres thus obtained can be used as they are or can undergo heat treatment, for example at between 50 and 1500°C, aimed at densifying these fibres and for example eliminating the remaining interstitial water or the structural water. Depending on the temperature of the heat treatment, the fibres obtained will have specific conductive properties. In fact, the resistance of these fibres can easily be varied by means of appropriate heat treatment. For example, with a temperature between 200 and 600 ⁶ C, fibres will be obtained with a conductivity equal to or greater than 2.10 ^-1 Ω^.cm ^""1 (Fig 9).

This method also makes it possible to obtain tin oxide fibres comprising, in addition to tin, one or more other constituents. These constituents are, for example, silicium or a metallic element chosen from amongst Sb ¹¹¹ , Sb ^v , Sn ¹¹ , In, Zr or V.

The tin salt used for the preparation of solution (a) can be chosen from amongst SnCl ₄ ,5H ₂ 0, SnCl ₂ ,2H ₂ 0, Na ₂ Sn0 ₃ ,3H ₂ 0, K ₂ Sn0 ₃ ,3H ₂ 0. They can be used alone or in a mixture. Tin tetrachloride will be used for preference. The molar concentration of tin in solution (a) is preferably between 0.5 and 4. In the case of the fibres containing at least one metallic constituent other than tin, such metallic constituent or constituents are introduced directly into solution (a) in the form of water-soluble compounds. Solution (a) can be heated before dialysis in order to obtain a homogeneous solution more rapidly.

The tin oxide Sn0 ₂ fibres comprising Sb will advantageously be obtained from a solution of SnCl ₄ ,5H 0 and SbCl ₃ or SnCl ₄ ,5H ₂ 0 and KSb(OH) ₆ , the Sn0 ₂ fibres containing Si from a solution of SnCl ₄ ,5H ₂ 0 and polysilicic acid and the Snθ ₂ fibres comprising Sn ¹¹ from a solution comprising a mixture of SnCl ₄ ,5H ₂ 0 and SnCl ₂ ,2H ₂ 0.

Solution (a) is then dialysed to eliminate the anions associated with the tin and the cations or anions associated with the other metallic constituents, when they are present in the solution. This dialysis makes it possible to obtain the gelation of solution (a) . In the case of tin tetrachloride, the chloride ions will be eliminated. It is important to obtain, after dialysis, as low a concentration of associated ions in solution (a) as possible. In fact, the presence of associated ions in solution (a) during the cooling stage may limit the length

of the fibres obtained and cause the formation of particles. According to the invention, the concentration of associated ions is less than or equal to 5% molar. This dialysis can be effected with cellulose membranes with molecular cutoff thresholds preferably between 1000 and 8000.

According to the present invention, the cooling stage (c) can be carried out in accordance with one of the following two methods:

(1) the gel is introduced into the cooling system at a linear velocity which allows the appearance of a congelation interface in the gel;

(2) the gel is introduced into the cooling system without transition. In this case, a congelation interface does not appear and the solidification is instantaneous, which is analogous to actual quenching.

According to method (1) the cooling temperature is preferably between -20 and -100°C and the linear velocity of the congelation interface is preferably between 2 and 10 cm/h.

According to method (2) , the cooling temperature is preferably equal to or less than -100 ^β C.

According to the present invention, the choice of the temperature and the cooling method determines the diameter of the fibres obtained. However, for a given temperature and cooling method, fibres which are uniform in diameter are also obtained. In fact. Figures 11 and 12 show the diameter of the fibres obtained and the uniformity of the fibre diameters for a given cooling temperature.

In the case of the present invention, the length of the fibres obtained is less than 10 cm, but this characteristic is not limitative since it depends on the cooling apparatus used.

The present invention will be understood better with reference to the following examples, which are not limitative.

EXAMPLES

The resistance of the tin oxide based fibres in the following examples was measured in accordance with two methods. Method (1) described in US patent 4 495 276 is implemented by mixing the fibres obtained with a KBr powder. The ratio by weight of the KBr/fibre mixture is 1:2. A pellet is formed from 1 g of the mixture (diameter 1.3 cm; thickness 2.1 mm) by applying a high pressure (5000 kg/cm ² ) . The resistance of these pellets is measured by means of steel electrodes at a pressure of 1000 kg/cm ² .

Measurements of resistance have also been carried out on pellets comprising solely fibres according to method (2) , known as the "four points" method, well known to experts. This method consists of forming pellets by applying pressure of the order of 1000 kg/cm ² to the fibres at ambient temperature. The resistance is measured by applying four electrodes separated by a few millimetres to the pellet consisting solely of tin fibres. The value obtained is expressed in Ω.

The composition of the fibres was determined by X-ray microanalysis and by chemical analysis.

The structure of the fibres was determined by X-ray diffraction.

EXAMPLE 1: Preparation of tin oxide fibres

Gels are prepared by dialysis from an aqueous solution of tin chloride pentahydrate SnCl ₄ ,5H ₂ 0 of commercial origin. 150 g of SnCl ₄ ,5H ₂ 0 is dissolved in 500 ml of deionised water at ambient temperature and under agitation so as to obtain a molar solution of tin. If the solution is heated to between 70 and 80°C, the dissolving of the tin derivative is accelerated. Then 500 ml of the solution is introduced into a dialysis tube, in this case a Spectra/Por ^R 6 dialysis tube with a molecular cutoff of 1000, 1200 or more. The dialysis tubes are approximately 30 cm long and are closed off by Spectra-Por ¹ closures. The volume of the buffer is 2 litres and is renewed each hour for the first four or five hours. The gel forms after approximately 20 hours of dialysis. It is then removed from the dialysis tube and is washed with water to eliminate any residual chloride ions. Then the gel is poured into a polyethylene tube, enough water is added to cover it and it is subjected to unidirectional congelation in a bath of acetone at -50 or -55°C, at a velocity of 5 cm/hour. In practice the unidirectional congelation is obtained by slowly adding acetone to the cold bath by means of a peristaltic pump. The congelation rate is controlled by the pumping rate of a peristaltic pump. Once the congelation stage has ended, the frozen gel is thawed at ambient temperature and thus insoluble fibres are obtained.

The general yield of the process, based on the tin content, is of the order of 85%.

Fibres have been obtained with a concentration of SnCl ₄ ,5H ₂ 0 in the starting solution equal to 4 mol/1.

X-ray microanalyses show that the fibres obtained do not

contain any chloride ions. They have a length of approximately 5 cm, and have a characteristic polygonal cross section. Their diameter is of the order of 30 to 70 micrometres (Figs la, lb and 2a) . Figure 2b is an electron photomicrograph of the surface of a tin oxide fibre as prepared.

These fibres, once heated to 700°C, show, by X-ray diffraction, well crystallised particles of Sn0 ₂ . They are densified by heating to 700-900°C, in relation to the crystallisation of the fibres and the enlargement of the particles. The size of the constituent particles, estimated according to the Scherrer formula, is of the order of 50 A for the raw fibres, 150 A for the fibres heated to 500°C and 500 A for the fibres heated to 900°C. At the latter temperature, the weight loss observed by thermal analysis is 15 to 20%.

Figures 3, 4 and 5 show the change in crystallisation versus the temperature.

The conductivity according to method (1) before heat treatment is 8.10~ ³ Ω~ ¹ .cm ^""1 .

Figure 9 shows the resistance of the fibres measured in accordance with method (1) and in accordance with method (2) as a function of the heat treatment temperature.

EXAMPLE 2: Preparation of tin oxide SnQ ₂ fibres doped with antimony from SbCl ₃

The process is carried out as described above in Example 1, introducing antimony into the tin chloride solution in the form of antimony chloride SbCl ₃ . The Sb/Sn atomic ratio is approximately 4%. The dissolving of the mixture of SnCl ₄ ,5H ₂ 0 and SbCl ₃ takes a relatively long time.

approximately 24 hours, and must be carried out at ambient temperature. The resulting solution is transparent and yellow-coloured.

The gel is then prepared by dialysis as described above and the unidirectional congelation is carried out as indicated in the previous example. In this way yellow-coloured fibres are obtained with a diameter of the order of 40 μm (Fig 6a) .

After heat treatment at 1200°C for two hours, it can be observed that the fibres are well-crystallised (Fig 6b) .

The conductivity is 0.8 Ω ^-1 .cm ^-1 .

Figure 10 shows the change in resistance measured in accordance with method (1) versus the heat treatment temperature.

EXAMPLE 3: Preparation of tin oxide SnQ ₂ fibres doped with antimony from KSbfOH) _c

The procedure described above in Example 2 is followed, using not antimony chloride but potassium antimonate KSb(OH) ₆ , in the same Sb/Sn atomic ratio of 4%. The dissolving of the mixture is easier and can be carried out either at ambient temperature or at 70-80°C. The resulting solution is colourless.

The desired fibres, which in this case are colourless, are obtained in the same way. After heating to 700°C, they prove to be well crystallised. The conductivity of these fibres is identical to Example 3.

EXAMPLE 4: Preparation of tin oxide Sn ^IV fibres doped with Sn ¹¹

The fibres are prepared from a gel obtained by dialysis of a mixed aqueous solution of SnCl ₄ ,5H ₂ 0 and SnCl ₂ ,2H ₂ 0. The initial Sn ¹¹ to Sn ^IV ratio is 10% molar. The resulting solution and gel are yellow coloured. The fibres obtained after unidirectional congelation and decongelation under the conditions described in Example 1 are yellow-coloured, which indicates that the two degrees of oxidation of the tin are present in the fibre.

Their conductivity is 5.10 ^""2 Ω ^""1 .cm~ ¹ .

These tin fibres have improved conductivity compared with the tin fibres which contain only Sn ^IV (Fig 10) .

EXAMPLE 5: Preparation of mixed indium and tin oxide fibres

The same procedure is followed as described in example 1, introducing the indium into the tin chloride solution in the form of indium chloride InCl ₃ . The In/Sn atomic ratio can be between 10 and 40%. A transparent solution is obtained. Dialysis of the solution enables a gel to be obtained. In this way colourless fibres are obtained having a mean diameter of the order of 50 μm. After heat treatment at 600°C for two hours, crystallised fibres of tin oxide containing indium are obtained. The only crystalline phase detected is Sn0 ₂ .

EXAMPLE 6: Preparation of mixed zirconium and tin oxide fibres

The same procedure is followed as described in Example 1, introducing the zirconium into the tin chloride solution in

the form of ZnOCl ₂ ,8H ₂ 0. The Zn/Sn atomic ratio is approximately 10%. The solution obtained is transparent. The Zr/Sn fibres are colourless, with a mean diameter of the order of 50 μm. At 600°C, the only crystalline phase detected is Sn0 ₂ .

EXAMPLE 7: Preparation of mixed vanadium and tin oxide fibres

The same procedure is followed as described above in

Example 1, introducing the vanadium into the tin chloride solution in the form of polyvanadic acid obtained by ion exchange from a molar solution of NaV0 ₃ . The V/Sn atomic ratio is approximately 50%.

Yellow fibres are obtained with a diameter of the order of 50 μm. At 600°C they are only a little crystallised and only Sn0 ₂ is detectable.

EXAMPLE 8: Preparation of tin oxide SnQ ₂ fibres containing SiQ ₂

The gel is prepared by dialysis of a mixture of an aqueous solution of silicic acid and an aqueous solution of SnCl ₄ ,5H ₂ 0. The polysilicic acid is obtained by passing a solution of sodium silicate through an Amberlite IR120 ion exchange column.

80 ml of the silicic acid solution is added to 20 ml of the tin chloride solution; then the dialysis is carried out, the 2 litre volume of buffer being renewed every hour for two hours. The gel forms in four hours.

After unidirectional congelation and decongelation as indicated in Example 1, colourless fibres are obtained with a mean diameter of the order of 30 micrometres and a length

of 1 to 4 cm (7a) .

The Sn:Si ratio can vary fairly widely, and fibres have been prepared in which the Sn0 ₂ :Si0 ₂ ratio is 30:70 and 90:10, which proved to be well crystallised after heating at 700°C (Fig 7b) . Their X-ray diffraction patterns are similar to those of pure Sn0 ₂ fibres, as are the thermal analysis results. It is observed that the higher the proportion of Si0 ₂ the less good is the crystallisation. In addition, below 700°C, these fibres are rather like nanophases (Fig 8) .

EXAMPLE 9: Preparation of tin oxide fibres by quenching

The same procedure is carried out as described in Example

1, but carrying out the unidirectional congelation not with a controlled velocity but by quenching.

Thus fibres are obtained with a mean diameter of between 30 and 80 micrometres at a cooling temperature of -100°C. If the congelation takes place for example at -200°C, fibres with a mean diameter of only 10 micrometres can be obtained (Fig 12) .