GB2279942A - Method for production of SiO-containing microfibres - Google Patents

Abstract

A method for producing a condensed fibrous, solid product comprising silicon monoxide by reacting a silicon containing material selected from silicon, silica sand, quartz and mineral silicates at sufficiently elevated temperatures above 1500 DEG C in a first zone to produce silicon monoxide gas and condensing the gas in a second zone by means of a condensing gas inert to the silicon monoxide gas, to produce a solid, substantially fibrous product. The inert condensing gas contains from 0.5 to 5% by volume of C2H2 gas or from 1 to 20% by volume of AlCl3 vapour.

Description

METHOD FOR PRODUCTION OF SiO-CONTAINING MICROFIBRES The present invention relates to a method for producing SiO-containing microfibres by the condensation of SiO vapour.

Production of microfibres by condensation of Sio vapour is known from US Patents Nos. 2,807,600, 2,823,979. In these patents a process is disclosed for producing fibres where silicon monoxide is prepared by heating a mixture of sand and carbon in an arc furnace at a temperature of from about 1500 to about 2000"C to form gaseous Sio which is then condensed in a vacuum or in an inert atmosphere.

The resulting material is substantially fibrous, having an average fibre length of from 50 to 600 nm (nanometers), an aspect ratio (length to width ratio) of up to 50:1 and a surface area of 60 to 200 m2/g. However, a part of the product is in the form of spherical and/or horn-like particles. The product is descr ted to be a disproportionation product of SiO having the formula (Six (Sio2)y), where x and y are whole numbers. The fibres produced are called "monox" and can be used as a reinforcing filler for rubber.

From US Patent No. 2,823,980 it is known that the amount of fibres in the above described product can be increased by adding 0.5 to 9% by volume of ammonia or a gaseous amine to the inert condensing gas. By this method the percentage of fibrous product is increased, but the length and the aspect ratio of the fibres are not substantially effected. Thus in the case of the product from the method described in US Patent No. 2,823,980, the fibre length is still in the range from about 50 to about 600 nm.

In these methods, it is important in order to produce fibres, that the flow of sio gas is substantially laminar. If the condensation of Sio is carried out with a turbulent flow of Sio, the product will consist substantially of spherical particles.

It has now surprisingly been found that by adding certain other gaseous compounds to the inert condensing gas,-the length and the aspect ratio of the produced fibres can be substantially increased and in addition, the amount of spherical particles in the product is substantially eliminated.

Thus, the present invention relates to a method for producing a condensed fibrous, solid product comprising silicon monoxide by reacting a silicon-containing material selected from silicon, silica sand, quartz and mineral silicates at sufficiently elevated temperatures above 1500"C in a first zone to product silicon monoxide gas and condensing the gas in a second zone by means of a condensing gas inert to the silicon monoxide gas, to produce a solid, substantially fibrous product, which the inert condensing gas contains from 0.5 to 5% by volume of C2H2 gas or from 1 to 20% by volume of AlCl3 vapour.

It has surprisingly been found that by addition of the above amounts of acetylene or A1C13 to the inert condensing gas, solid products are obtained containing fibres having a significantly higher aspect ratio than the fibrous products produced according to the known methods.

According to a preferred embodiment of the present invention, the amount of C2H2 in the inert condensing gas is between 0.5 and 2% by volume.

According to another preferred embodiment of the present invention, the amount of AlCl3 in the inert condensing gas is between 3 and 10% by volume.

The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of an apparatus for producing SiO-containing fibres according to the present invention; Figure 2 is a scanning electron microscope (SEM) micrograph of fibres produced according to a first aspect of the present invention; Figure 3 is a SEM micrograph of fibres produced according to a second aspect of the present invention; and Figures 4 and 5 are SEM micrographs of fibres produced according to the prior art.

The apparatus shown in Figure 1 comprises a quartz tube 1 with a diameter of 60 mm, and a graphite crucible 2 arranged inside the quartz tube 1. The crucible 2 is intended to contain a charge 3 for the production of SiO gas. The crucible 2 can be heated by means of an rf coil 4 arranged about the outside of the quartz tube 1. A pipe 5 is arranged for passing an inert gas into the crucible 2 to drive out the sio vapour produced in the crucible. The quartz tube 1 is sealed against the atmosphere at its lower end by an oil seal.

Above the crucible 2 in the quartz tube 1 there is a Pyrex tube 6 with a diameter of 45 mm. The tube 6 is open at its lower end and the annulus between the quartz tube 1 and the Pyrex tube 6 is sealed by suitable sealing means at the upper end of the quartz tube 1. The lower end of the tube 6 is arranged a distance x of 15-2t mm above the upper end of the crucible 2. The quartz tube 1 has a first inlet opening 7 near its upper end for the supply of quench gas, and a second, lower inlet opening 8 for the supply of quench gas additives. The products leaving the Pyrex tube 6 at its top 9, are conveyed to a filter (not shown) for the recovery of the fibres produced.Graphite felt 10 is preferably arranged between the outside of the crucible 2 and the inside of the tube 1 in order to prevent a downward flow of vapour and also to insulate the crucible thermally.+ The invention will now be further illustrated in the following Examples.

EXAMPLE 1 The apparatus shown in Figure 1 was used to produce Siofibres according to the method of the present invention.

About 8 grams of an equimolar mixture of silicon powder (99.5% purity) and precipitated silica (99% pure) was ball-milled, pelleted and dried at 350"C and packed in the graphite crucible 2. The charge was heated inductively at 450 kHz by the rf coil 4 under an N2 or Ar atmosphere with a power input of about 3.8 kW. When the crucible temperature was above 1800"C and the first wisps of Sio vapour were seen emerging from the top of the crucible, N2 or Ar gas was passed through the crucible from the pipe 5 to drive out the SiO vapour. Heating was continued for a further 5 minutes resulting in about half the charge being converted to sio gas. The production rate was fairly constant at about 18 mmol/min.

Condensation of the SiO took place in a region where the rising hot gas from the crucible was joined by a rising column of cooler "quench gas" as shown in Figure 1. This quench gas was N2 or Ar, mixed with other gases or vapours to react with the sio. Condensation of the Sio vapour in N2 or Ar gave identical products and N2 was used for most reactions. Hydrogen could also have been used as the inert carrier and quench gas for the preparation of -fibrous SiO. When tried, it behaved very similarly to N2 or Ar, but was not preferred for safety reasons.

1% by volume of C2H2 was added to the inert quench gas through the inlet opening 8. The produced sio containing fibres had an average length of above 1 ssm and an average aspect ratio of at least 100:1. Figure 2 shows a scanning electron microscope micrograph of the SiOcontaining fibres produced. When the addition of C2H2 to the quench gas was reduced to below 0,5t by volume of the quench gas, the fibres produced were similar to the state of art fibres produced using only inert gas for quenching (see Figure 4). When the addition of C2H2 was increased to above 5% by volume of the quench gas, a diminution in fibre concentration and an increase in spherical particles was found.

EXAMPLE 2 SiO-vapour was produced in the same way as described in Example 1. 4% by volume of AlCl3 vapour was added to the inert quench gas through the opening 8. The produced Sio containing fibres had an average length of above 1 ssm and an average aspect ratio of at least 100:1. Figure 3 shows a scanning electron microscope micrograph of the SiO-containing fibre produced.

EXAMPLE 3 (Comparisons) - In order to compare the method of the present invention with the prior art of US Patents Nos. 2,800,600 and 2,823,979, SiO vapour was produced in the same way as described in Example 1 but the vapour was condensed using nitrogen as quench gas. Figure 4 shows a scanning electron microscope picture of the Sio-containing product formed. Similarly, in order to compare the method of the present invention with the prior art of US Patent 2,823,980, Sio vapour was produced in the same way described in Example 1 but the vapour was condensed using nitrogen to which had been added different volume percentages of ammonia. The highest proportion of fibres to spherical particles was obtained by the addition of 5% of NH3 to the inert quench gas. Figure 5 shows a scanning electron micrograph of the SiO-containing product formed.

By comparing the micrographs of Figures 2 and 3 with 4 and 5, it can clearly be seen that in the products formed by the method according to the present invention, both the proportion of fibres relative to spherical particles and the average aspect ratios of the fibres are significantly higher than for the products made in Example 3.

Claims (4)

1. A method for producing a condensed fibrous, solid product comprising silicon monoxide by reacting a silicon containing material selected from silicon, silica sand, quartz and mineral silicates at a temperature above 1500"C in a first zone to produce silicon monoxide gas and condensing the gas in a second zone by means of a condensing gas inert to the silicon monoxide gas7 to produce a solid, substantially fibrous product, in which the inert condensing gas contains from 0.5 to 5% by volume of C2H2 gas or from 1 to 20% by volume of AlCl3 vapour.

2. A method as claimed in Claim 1, in which the amount of C2H2 in the inert condensing gas is between 0.5 and 2% by volume.

3. A method as claimed in Claim 1, in which the amount of AlCl3 in the inert condensing gas is between 3 and 10% by volume.

4. A method for producing a condensed fibrous solid product comprising silicon monoxide substantially as herein specifically described in Example 1 or Example 2.