WO2010041077A2

WO2010041077A2 - Nanoparticle purification

Info

Publication number: WO2010041077A2
Application number: PCT/GB2009/051344
Authority: WO
Inventors: Anthony Kynaston-Pearson; Daniel Johnson; Alistair Godfrey; Jonathan Tunbridge
Original assignee: Intrinsiq Materials Limited
Priority date: 2008-10-08
Filing date: 2009-10-08
Publication date: 2010-04-15
Also published as: GB2464384A; GB0917623D0; WO2010041077A3; GB0818414D0

Abstract

Apparatus for the purification of an impure element in particulate form, comprising: a sample preparation section enabling mixing of the particulate with a carrier gas; a coating section comprising apparatus for creating a vapour of gettering agent and a mixing vessel; and an injection assembly which effectively mixes the prepared sample of carrier gas and the particulate in the coating section so as to enable coating of the particulate by the vaporised gettering agent thereby to enable reduction of impurity level in the particulate itself.

Description

Nanoparticle purification

Technical field

The present invention relates to a method of production of high purity nanoparticles, in particular nano-silicon particles. In particular, but not exclusively, the invention relates to a cost effective method of producing high purity silicon (99.99%, 4N, or above) from reasonable (99%) purity silicon or preferably silica (SiO₂) by using a gettering agent, such as magnesium, to reduce the oxide and remove the impurities. The process could equally be applied to other materials that require purification and/or reduction.

Background to the invention

It is known to use high quality silicon in photovoltaic applications such as solar cells. The typical level of purity required for the use photovoltaic applications is 99.9999% (known as 6N purity), which is lower than the grade required for electronic grade silicon (9N or higher).

The production of 6N silicon in industry is typically a two-stage process. Firstly, silica is thermally reduced to produce a low cost, metallurgical grade silicon. This metallurgical grade silicon is cheap and easy to produce. However, the purity is typically low, between 99.5 to 99.9%. In order to use the silicon in electronics or photovoltaic applications the purity will further need to be increased. It is known to purify metallurgical grade silicon via the Siemens process, a chemical vapour deposition method.

The second purification stage is expensive, considerably adding to the production costs of the silicon. Further costs in the production of silicon may also be incurred if the process requires a dopant to be included into the high purity silicon.

A further disadvantage of the industrial scale production of silicon is there is currently no specific production process to create photovoltaic grade silicon i.e. 6N purity. The Siemens process produces electronic grade silicon, 9N or above, therefore requiring the use of, expensive to produce 9N silicon, in photovoltaic applications. Application WO 2008/062204 teaches a method of production for purifying metallurgical grade silicon (typically 3N or 99.9% pure) to 4N silicon (99.99% pure), by removing the impurities in the silicon by coating the metallurgical grade silicon with a gettering agent; causing the impurities to leach onto and into the gettering agent and removing the gettering agent to obtain high grade silicon. Such a method is cheaper than the Siemens process and produces 4N grade silicon which is adequate for use in photovoltaic applications (providing certain doping materials are present in lower concentrations).

However, the methods taught in WO 2008/062204 are only suitable for small-scale, laboratory sized, production of silicon. Issues such as: the contamination of the silicon powder from the machinery involved; controlling the temperatures through the system; the mixing zone for the silica and gettering agent; controlling the condensation of the gettering agent; air flows within the system and their effect on the purification process; optimisation of the yields; preventing loss of the product during the process; quality of the purified silicon obtained etc. are not addressed.

It is therefore desirable to be able to produce 4N grade silicon or above, especially suitable for the use in photovoltaic applications, on an industrial scale, using a method which is considerably cheaper than the currently known methods and is able to maximise the yields at a relatively low cost.

To solve one or more of these problems and other problems with the prior art, there is disclosed a method of reducing and purifying nanoparticle sized silica using readily available materials, which can be applied to an industrial scaled process. This solution is based partly on the readily available commercial supply of inexpensive and relatively pure fumed silica, which is also in suitably fine form.

The said silica to be purified may be in the form of solid particles or mesoporous silica, to be converted into solid or mesoporous silicon respectively, for which there is also a commercial demand. Where particulate silica is referred in this document, it is intended to include either of these forms, and non- fumed silica as well as that made by a fuming process.

Whilst the above review of the prior art has outlined the problems with the purification of silicon, the same problems are equally true for the purification for other nanosized materials. The same problems as discussed in relation to WO 2008/062204 exist and are also addressed by the following disclosure. Therefore, it will be obvious to one skilled in the art that this process may be applied to materials other than silica, and gettering agents other than Magnesium, if the costs and benefits render it commercially viable.

According to an aspect of the invention there is provided an apparatus according to claim 1 herewith appended.

Further aspects, features and advantages of the present invention will be apparent from the following description and appended claims.

Brief description of the drawings

An embodiment of the invention will now be described by way of example only, with reference to the following drawings, in which:

Figure 1 is a schematic of the apparatus;

Figure 2 is a schematic of the sample preparation apparatus;

Figure 3 is a schematic of the coating apparatus;

Figure 4 is an overview of an annealing oven;

Figure 5 is a schematic of the filtering unit;

Figure 6 is a plot of experimental data showing surface area at various temperatures for Syloid 74 (RTM); and Figure 7 is a plot of experimental data showing pore volume at various temperatures for Syloid 74 (RTM).

Detailed description Figure 1 shows a schematic overview of the entire apparatus used to create high quality (4N or above) silicon from silica.

There is shown the four separate sections of the apparatus 98 comprising; a sample preparation, or silica feed section 100, a coating section apparatus 200, a heat exchanger, or annealing oven 300 and a collector 400.

Each section of the apparatus will be discussed in greater detail with respect to Figures 2 to 5. The material to be coated and purified (in the preferred embodiment, readily available fumed particulate silica with a purity of around -99.9%, i.e. ~3N, and around 40nm in diameter) is fed into the silica feed section 100 into a feeder and is carried along in a gas flow (usually argon) into the feed tube that enters the coating section apparatus 200. Additional gas may be added to the prepared mixture of gas and particulate silica before the prepared mixture enters the coating section apparatus 200. The prepared gas mixture is fed into the coating section apparatus 200 via the injection assembly.

The prepared mixture is fed into the atmosphere above a molten gettering material (here magnesium) through a vent. A gettering agent is a material that is added in small amounts during a chemical or metallurgical process to absorb impurities. Gettering agents are commonly metals which are more electropositive than the matrix from which the impurities are to be removed, but are not exclusively so. In the preferred embodiment the getter agent is Mg which favourably reacts with the O₂ in the SiO₂ to reduce it, and being of higher reactivity and higher purity will by diffusion and reaction absorb the impurities in the silicon or silica. The injection of the mixture causes the various currents with the vessel encouraging mixing of the gas and entrained particles with the magnesium vapour. The walls of the coating section apparatus 200 are insulated to maintain the temperature and help prevent magnesium from condensing on the chamber walls. The gases are mixed and the particulate silica becomes coated by the vaporised gettering agent. The gas entraining the coated particulate silica is passed onto a heat exchanger, preferably an annealing oven 300, where the temperature and the residence time of the coated particulate silica and gas are controlled. The initial mixing and the subsequent annealing stage ensures that the impurities that were present in the original fumed particulate silica are disassociated from the silica and diffuse from the core to react with the gettering agent, leaving a purer silicon core. A similar process also occurs for the purification of particles other than silica. Once the annealing process has terminated the annealed matter is passed to the collector 400 where it is collected so that unwanted reactants may be removed from the surface of purified silicon cores.

Figure 2 is a detailed view of the sample preparation apparatus, in the preferred embodiment a silica feeder 100.

The silica feed section 100 comprises, an input gas pipe 102, fumed silica 104, a vibratory feeder 106, connected to a motor 108, an exhaust gas pipe 110, an extra gas input 112 and a coating section gas input pipe 114. The silica feed section 100 is connected to the coating section apparatus 200 via a gas input pipe 114.

The silica feed section 100 contains the raw material from which the high purity silicon is created. In the preferred embodiment the raw material is nano silica of a reasonable purity (at least -99%) such as in the form of fumed silica. This raw material is relatively inexpensive, easily obtainable, and comes in particulate sizes of ~ 40nm or less. A key aspect of the invention is the cost-effective purification of the inexpensive, and easily obtained, fumed silica. Preferably, the silica can be in a glassy (vitreous) form or a crystalline form.

In use, the fumed silica 104, is fed into the vibratory feeder 106 via a hopper (not shown). Preferably the silica feed section 100, is pressurised, with an inert gas preferably argon, to exclude air which may be carried into the reactor, causing oxidation of the product or of the gettering agent. A gas is introduced into the sample preparation apparatus, or silica feed section 100 via a known inlet valve. The gas in the preferred embodiment is inert, usually argon, though other gases, inert or reactive, such as hydrogen may be used. The vibratory feeder 106 is powered by a motor 108 which, together with gas flow, causes the fumed silica 104 to become dispersed and entrained by the argon gas.

In the preferred embodiment, as a bench-top apparatus, argon is passed through the vibratory feeder 106 at a rate of about 10-20 litres per minute, with the vibratory feeder 106 powered to disperse between 0.5-1 kg/hr of fumed silica 104 into the gas, and the sample preparation taking place at room temperature. Those skilled in the art will appreciate that the rates used may be adjusted dependent on the scale of the production of silicon desired. Such values will typically produce 0.25 to 0.5 kg/hour. If the production rate were to increase, the gas flow rate and sample dispersion rate would also have to increase accordingly, and each of the sections could be scaled accordingly to handle the required production volumes.

The carrier argon gas entrains the fumed particulate silica in its flow, and exits the silica feed via the exhaust gas pipe 110.

The speed of the gas is optionally also monitored at the exhaust gas pipe 110 using a turbine meter (not shown) though those skilled in the art will appreciate that other suitable types of meters may be used. If the speed of the gas measured is too low, below 10 metres/second, there is a risk of the fumed silica that is entrained in the argon settling in the exhaust pipe 110. This is undesirable as it reduces the yield, and may block the pipe 110. Extra inert gas may be introduced at the extra input 112 to maintain the desired gas flow as the gas enters the coating apparatus 200.

Figure 3 shows the detailed view of the coating section apparatus 200. The coating section apparatus 200 comprises a chamber 201, insulation 202, heater elements 206 which heat a vessel 204, the vessel contains a gettering material 210, (which in the diagram additionally contains a SiO₂ sludge 208 though this is not a feature of the vessel), the heated vessel 204 is at the base of the chamber 201 and creates a magnesium vapour 212. There is also shown the gas input nozzle 214, the gas output pipe 220 and output nozzle 222. The output nozzle 222 has insulation 224 and connects the coating section apparatus 200 to the heat exchanger which in the preferred embodiment is an annealing oven 300.

The prepared gas is injected into the chamber 201 by the input nozzle 214 or injection assembly. The nozzle 214 is designed to introduce the gas turbulently into the chamber. The nozzle 214 creates instabilities in the gas flow, via known means such as pinching of the flow and the introduction of a kink in the nozzle near the exit to create a turbulent boundary. The introduction of the cold (room temperature) gas into the chamber 201, which will typically have a temperature of greater than the boiling point of the gettering medium at the pressure in the system (e.g. for Magnesium at 1 Atmosphere this is 1,38O⁰K), will also create turbulence due to the temperature differences. The gas is also, preferably, introduced at a high velocity of approximately 100 metres/second allowing the particulate fumed silica mixture to enter the chamber 201 with a substantial momentum in the direction of the main gas flow. To introduce gas at the preferred velocities, the gas may need to be accelerated by know means, such as the reduction of the cross-section of the pipes, addition of further high speed carrier gas (preferably at the extra input 112, or at further inputs, not shown in Figure 3) etc. The turbulent flows of the inert gas containing the particulate silica and the Mg vapour will ensure substantial mixing between the two gases.

The velocity of the gas from the nozzle 214 will also cause the flow to act as an abrasive cleaning material, due to the fumed silica. It is found that some level of "cleaning" from the gas occurs on the wall and beneficially removes condensed Mg on the walls of the chamber 201.

The coating section apparatus 200 is fully insulated 202 to maintain the high temperatures introduced by the heating elements 206. The heating elements 206 are typically positioned around a vessel 204, which contains a gettering agent 210, in the preferred embodiment, the gettering agent is magnesium (Mg), though other appropriate gettering agents may be used. The gettering agent 210 is heated to around the boiling point (~l,380°K for pure Mg at 1 bar) which creates a hot magnesium vapour 212 in the chamber 201. The rate of the evaporation of the gettering agent is typically, determined ideally by the stoichiometry of the reduction sought. Again, the rates may be scaled dependent on the yield that is required. As an example of how stoichiometry defines ideal evaporation rate, for silica reduction by magnesium it is that 2 moles of magnesium is needed for every one mole of silica introduced (or produced if 100% yield), based on: SiO₂ + 2Mg = 2MgO + Si.

This can readily be converted to mass ratio, giving the ratio 2 moles Mg = 2 x 24 = 48, 1 mole SiO₂ = 28 + 2 x 16 = 60. So Mg to SiO₂ ratio is 48:60 (mass unit independent, typically kg)

The purity of the Mg placed in the vessel 204 need not be very high as by controlling the temperature of the heating elements 206, it is possible to maintain the vessel 204 at a temperature that will boil the Mg but not the impurities (at or above 1,38O⁰K depending on impurities in the Mg and the pressure in the vessel), therefore the Mg vapour 212 is formed by substantially pure Mg. This further reduces the cost of the production process. Additionally, Mg is particularly preferable as it has a high vapour pressure above its melting point so it readily coats the silica particulates in the gas. The optimisation of the coating process is found to increase the yields of silicon obtained of 4N purity or above .

The thermal insulation 202 maintains the high temperature from the heating elements 206 and Mg vapour 212 preventing the Mg from condensing when it comes into the walls of the chamber 201. Additional heating elements (not shown) may be placed around the walls of the chamber 201 to maintain the temperature. The insulating material is preferably rockwool or silicate fibre, though other insulating materials may be used.

The hot atmosphere in the chamber (~l,380°K or above) is substantially hotter than the input gas from the nozzle 214 that is typically at room temperature. The nozzle 214 turbulent Iy introduces the gas into the hot atmosphere of the chamber 201 which will also rise and sink due to the buoyancy of the gas, causing the Mg vapour 212 to further mix with the introduced cool gas. As the fumed silica is entrained by the gas it will also be moved by the various currents present within the chamber 201, ensuring that silica in the argon gas mixes repeatedly with the Mg vapour 212 and that the silica, Mg and argon form a suspension. This process will also coat the fumed particulate silica with the vaporised Magnesium gettering agent. It is found that impurities from contact with the walls of the chamber 201 may also be present on the coating of the silica.

As the gas descends (either from the turbulent current or via convective current) some of the gas will come into contact with the hot liquid Mg 210 and the particles of fumed silica will settle into the vessel 204 creating a layer of SiO₂ sludge 208 typically found in the base of the heated vessel 204. This is undesirable as it reduces the yield of the process. To mitigate this loss further vents of inert gas are also introduced into the chamber (the vents are not shown in Figure 3) which helps speed up the gas stream through into the chamber and helps prevent the silica from sinking into the vessel 204. These additional vents introduce the inert gas upwards towards the top of the chamber thereby imparting an upward momentum to the descending cold gas and particulates, preventing them from contacting the vessel 204. The vents are positioned below the nozzle 214, but above the vessel 204, forcing the gas upwards.

However, it is found that some silica is deposited into the vessel 204, and therefore the vessel 204 in the preferred embodiment also has a sink and a refill (not shown) to remove the SiO₂ sludge 208 and replenish the gettering agent. Those skilled in the art will understand that this may also be achieved by introducing a vessel 204 or vessels that are easily replaceable.

Eventually the gas suspension (argon, silica and Mg) becomes sufficiently heated that it rises and exits the chamber 201 via the output pipe 220 which is positioned at the top of the chamber 201. The output pipe 220 leads to an output nozzle 222, which is preferably has a smaller cross-section than the pipe 220, to help maintain a high velocity flow through to the annealing oven 300. In the preferred embodiment, the residence time of the silica in the magnesium vapour is sufficient for the silica to become adequately coated. Once the silica has exited the chamber 201 it preferably moves in a plug flow, i.e. laminar. The stable laminar flow helps prevent the sedimentation of the particles, and back- flow of the gas, through the annealing oven 300. The nozzle 222 is also insulated 224 to prevent heat losses as the gas suspension passes from the coating section apparatus 200 to the annealing oven 300.

Figure 4 shows an embodiment of the preferred heat exchanger, an annealing oven 300 used to control the annealing process of the gas coated silica suspension. There is shown the output nozzle 222 from the coating section apparatus (not shown in Figure 4) and the anneal oven 300. The oven 300 comprising: insulation 302, heating elements 304, 306 , 308 the tubing 310 and the output pipe 312.

Due to the natural temperature gradient in the chamber 201, the temperature at the bottom of the chamber is close to the boiling point of Mg (~l,380°K) and the temperature of the exit gas, entraining the Mg-coated silica is ~l,200°K. The gas enters the oven 300, via the tubing 310. The heating elements 304, 306, 308 are enabled to be selectively turned on and may be at different temperatures. The arrangement of the tubing 310 and heating elements 304, 306 , 308 in Figure 4 is illustrative and those skilled in the art will appreciate that the elements may be placed at different positions along the tubing 310. By selectively controlling the heating elements 304, 306, 308 it is possible to create a heating gradient across the annealing oven 300.

The tubing 310 is typically of an adequate diameter to maintain a gas flow rate of >10metres/second. This gas flow rate is preferred as it is found to maintain a laminar flow, which reduces the eddies in gas stream which cause the particles to settle in the pipes. In another embodiment the oven 300 contains multiple tubing 310, which can selectively be opened or closed using known valve technology. This allows for further control of the residence time, by increasing the length of tubing and also allows tubing 310 to be selectively closed allowing for ease of replacement and cleaning and maintenance of the pipes.

In the preferred embodiment the gas temperature as it enters the oven is ~l,000°K and the temperature is gradually lowered as the gas traverses the tubing to ~800°K. In a further embodiment the temperature of the gas is increased by the heating elements as the gas flows through the tubing, therefore making the exit gas hotter than the input gas. In yet another embodiment the gas is kept at a constant temperature as it remains in the oven 300. The residence times in the oven 300 are typically a few minutes.

Experimental data collected by the applicant has constrained the temperature ranges used during the annealing process. In particular it was found that excessive temperatures results in sintering, and the agglomeration of particles. Additionally, excessive temperatures result in the collapse of the nanoporous or mesoporous structures that are formed as result of the process (see below).

It has been found that temperatures in excess of ~800°C, 1075K, can result in thermite runaway reactions causing the silicon to sinter. Therefore, such temperatures are undesirable for the production of porous silicon or of silicon powder, as it results in a sintered mass that is no longer a powder and from which removal of the gettering agent is far more difficult. It has also been found that when the silica material is kept at temperatures in excess of -750⁰C, -1025K, the porous silicon structures can collapse and fuse, resulting in the loss of the mesoporous structure.

Figure 6 shows a plot of experimental data of BET surface area versus time for various temperatures for a starting material of Syloid 74 (RTM). As can be clearly seen at 700⁰C, the BET surface area remains approximately constant, indicating that the structure of the silica remains substantially unchanged. At temperatures of 800⁰C, and above, there is a marked decrease in the surface area, indicating that the silica has collapsed or fused.

Figure 7 shows a plot of experimental data of pore volume versus pore diameter various temperatures for a starting material of Syloid 74 (RTM), having been heated for a period of 4 hours. As can be seen, the volume decreases between 700⁰C to 800⁰C and decreases rapidly between 800⁰C and 900⁰C. Again, this is an indication of the collapse and fusing of the starting silica at temperatures between 700-800⁰C, therefore in order to maintain the mesoporous or nanoporous structure of the silicon the preferred temperature range is -750⁰C, or below. During the coating stage the surface of the nano-silica particles have been coated with the a layer of the vaporised gettering agent, in this case Mg. The Mg coating creates a diffusion interface between the Mg and silica from which impurities are diffused from the core of the silica. In the annealing oven 300, the heat treatment causes the structure of the silica to alter and the oxygen and impurities to diffuse into the surface coating. Due to the small size of the particles, ~40nm or less, the diffusion lengths are similar or greater than the radius of the particles (typical diffusion lengths for impurities are found experimentally to be ~20 to 50nm depending on the species, time, temperature, matrix, etc.) and it is found that the impurities from the core of the silica are removed. The annealing process creates a pure silicon core with a outer shell of the MgO and other impurities. As a result the SiO₂ has lost its bound O₂ which in turn affects the structure of the remaining Si.

It is important to note that because the diffusion lengths are typically longer than the silica particle size impurities from the core to the surface are removed, thereby creating silicon. Additionally, any impurities that have collected on the surface of the coating from any oxygen present in the chamber 201 or from sample preparation apparatus or silica feed section 100, or from the chamber walls or various tubing in the invention are all also removed when the gettering agent is stripped from the purified product. The majority of the impurities will be bound onto or in the coating and those that are contained in the silica will be removed during annealing as their diffusion lengths' are typically greater than the silica particulate size (Diffusion lengths of typical contaminants ~20-50nm to a particle size of ~40nm). Additionally, it follows that any impurities that are found in the Mg will not affect the purity of the silicon after the annealing process as they are either not vaporised and therefore do not coat the silica particulates or are annealed from the silica in the annealing oven 300.

The annealed material in gas stream consists of a pure silicon core with a Mg coating and contaminants e.g. MgO. The gas is cooled to 35O⁰K by virtue of passing through known double-jacketed pipe work (not shown), and passed to a collection apparatus 400. In a further embodiment the gas is not cooled but passed from the annealing oven 300 to the collection apparatus 400 whereupon it is cooled to <350°K. In yet another embodiment the gas is cooled using other known gas coolers, such as those found in commercially available furnaces.

The gas entraining the annealed silicon and coating with impurities enters the collector via the annealing oven 300 output pipe 312. The gas is passed onto a known sintered filter 408. The filter (typically containing one or many sintered metal or ceramic tubes, which give very fine pores) 408 allows the gas and the particulates to be separated and the particulate to settle at the bottom of the filter. As the gas is inert it may then be recycled as it exits via the exhaust valve 404 and pipe 406. Inert gas is introduced at the "blow-back" pipes 420 at controlled intervals to pulse-off the accumulated particulates.

The unwanted reactants can be removed chemically from the surface of the silicon particles. The particles of the coated silicon are placed in a vessel 416 containing any acid group reacting with the magnesium oxide and yet unable to react with silicon.

Examples include carboxylic acids such as acetic acid, citric acid, halogenated acids such as hydrochloric acid, sulphamic acid or sulphuric acid etc. In the preferred embodiment the collector has two valves 412, 414 so that the vessel 416 may be removed and replaced when sufficient coated silicon has been collected.

The reaction product of the acid and magnesium oxide is a soluble salt in the chosen solvent for example water. The salt is separated by leaching the solid silicon from the material by filtration and rinsing with more pure solvent.

It is preferable that the silicon at this point is unable to form dispersions to aid the separation process. For example, if the silicon is hydrogenated during the course of reaction by using a flow gas containing molecular or atomic hydrogen, then it will float on water and can be separated by flotation. If the silicon has its surface affected by chemical species able to cause flocculation or sedimentation then separation can occur by decanting and filtration. This occurs in the presence of hydrochloric acid. A further method of separation is by solvent extraction, whereby the solid silicon in a first solvent is able to form dispersion in a second solvent, which is immiscible with the first solvent. However, the dissolved magnesium salt in the first solvent is unable to dissolve in the second solvent, thus separating solid silicon from the soluble magnesium salt by this process.

The collector 400 may be replaced with any suitable method for removing particles from a gas and removing the coating from the silicon. The sintered filter 408 and acid removal of the coating are preferred due to their relatively low costs and suitability to a continuous production method. Those skilled in the art will appreciate that any suitable method may be used at this stage. Equally the particles may be kept in argon or in any inert and degassed liquid for subsequent handling until it is most prudent to remove from them the protective gettering agent and safe to expose the purified product without harmful contamination.

It is found that the above method of production of silicon results in the production of mesoporous and/or nanoporous silicon. The removal of the oxygen from the silicon is found to leave cavities in the silicon cores were the oxygen was bound. These cavities are particularly evident in larger particles (> 10 nm), though it is also found also to depend on the temperature and residence times in the annealing oven 300. Mesoporous or nanoporous silicon particles that are formed by this method are readily identifiable by SEM.

The preferred embodiment of such a nanoparticle purification is one that involves the purification of nanosized particles of fumed silica, using a Mg gettering agent.

However, those skilled in the art will realise that this method of the purifying nanoparticles is not limited to the production of silicon with a Mg gettering agent. It is found, that whilst the method is particularly valuable in the production of silicon, it also is able to purify other elements. It is found that gettering agents such as sodium chloride are also particularly effective and that a range of materials may also be purified using the method and apparatus described above. This method is particularly appropriate for reactions where the nanoparticles when coated with the gettering agent form an effective diffusion interface. Such an interface is required to allow the impurities to diffuse from the core of the nanoparticles and onto the coating, creating a pure material surrounded by a gettering agent containing the impurities. The gettering agent may then be removed thereby leaving a nanoparticle core from which the impurities have been removed.

A significant consideration is the effect of the diffusion of oxygen from the core of the SiO₂ and the changes in the structure of the particles as a result of such diffusion. The removal of the oxygen atoms from the core will result in a change in the structural form of the silica. A further effect will be a change in the chemical bonds present in the material, and depending on the silica form may result in spaces or pores in the material. Additionally, such changes may result in changes in the crystalline structure of the silica.

An example of such changes would be seen in the change of density. For example, cristabolite has a density of 2.6 gem^"3, whereas pure silicon has a density of 2.33 gem^"3 and the intermediate silicon monoxide has a density of 2.13 gem^"3.

Furthermore, the purification of the starting silica to the purified silicon will also result in a change in physical form. Silica in the quartz form has a hexagonal close packed structure (HCP), whereas silicon in its crystalline form is a face-centred diamond cubic (FCC). The change from silica to silicon therefore results in changes in the crystalline structure. Such changes are measurable by known techniques such as X-ray diffraction.

Therefore, the changes in the silica are both chemical and physical. It has also been found by the applicant that the diffusion of the oxygen and other impurities result in further chemical processes which affect the purification process. Preliminary experimental data suggests that the chemical process involves the SiO₂ and the magnesium creating a diffusion interface. The reaction results in the diffusion on an oxygen atom yielding magnesium oxide (MgO) and silicon monoxide (SiO). The SiO reacts with a further Mg thereby resulting in Si, thus: SiO₂ + Mg > SiO + MgO

SiO + Mg > Si + MgO

Therefore, the diffusion interface will potential yield a Si and at least the two MgO particles (and possibly other Mg and impurities). This results in a chemical gradient across the diffusion interface.

Whilst the SiO₂ is the main reaction other impurities will also be present in the starting material, such as other elements and salts, and these, depending on the nature of the impurity, may also react with the Mg. Depending on the nature of the impurity, the reaction with the Mg may be reduction, dissolving or combination with the Mg particles.

Data suggests that oxygen atoms bond hop across the diffusion interface to the Mg. It is believed that such bond hopping will in effect 'entrain' impurities and carry them to the outer reductant shell, particularly as the whole structure is undergoing the physical transition. Some of the other impurities present in the starting material are more reactive than the silicon and will preferably bond and/or react with the freed oxygen and Mg. Preliminary data suggests that the entrainment process aids and accelerates the purification process compared to non-reduction processes.

This process, along with the change in the structure and the removal of the oxygen results in the formation of nanoporous or mesoporous silicon.

Further experimental data has indicated that increases in the production of the silicon may be achieved with a relatively cold (lower than room temperature) starting silica material and a supersaturated Mg vapour in the coating zone.

There are several benefits associated with these conditions. In particular, the cold silica minimises the length of time the silica is heated and therefore reduces the potential for the changes in structure which result in the loss of the porous structure. As can be seen in Figure 6, the longer the silica is heated the greater the change in structure.

The introduction of the silica particles into a supersaturated vapour, in effect introduces nucleation zones into the supersaturated vapour which will spontaneously condense around the nucleation zones (i.e. feedstock material) thereby efficiently coating the material. Furthermore, the latent heat released by the Mg, as result of the transition from gas to liquid, will heat the silica particles. This heating of the silica particles is thought to represent the starting of the purification/reduction process, providing for improved control over this stage of the preparation process.

Similarly, the inert gas used to entrain the feedstock is preferably introduced at lower than room temperature. As discussed, a consideration is the prevention of the condensation of the Mg and feedstock material on the walls of the chambers and pipes. Such condensation reduces the yield and may potentially lead to blockages in the pipe-work. By injecting the lower than room temperature gas into the middle of the pipe, the hot gases present in the pipe prevent the cold gas condensing on the walls of the pipe.

Claims

1. Apparatus for the purification of an impure element in particulate form, comprising: a sample preparation section enabling mixing of the particulate with a carrier gas; a coating section comprising apparatus for creating a vapour of gettering agent and a mixing vessel; and an injection assembly which effectively mixes the prepared sample of carrier gas and the particulate in the coating section so as to enable coating of the particulate by the vaporised gettering agent thereby to enable reduction of impurity level in the particulate itself.

2 The apparatus of claim 1 where the feedstock is silica, or silicon, and the gettering agent reduces and/or purifies the particulates.

3. The apparatus of claims 1 or 2 where the gettering agent is Magnesium.

4. The apparatus of claims 1,2 or 3 further comprising a heat exchanger suitable for annealing the coated particulate, and preferably further comprising a collector suitable for the collection of the annealed particulate from the heat exchanger.

5. Apparatus of any preceding claim wherein the injection assembly comprises a nozzle enabled to create a turbulent current in the gas flow.

6. Apparatus of claim 5 where a further nozzle is introduced to impart upward momentum onto the gas in the coating section.

7. Apparatus of claims 5 and 6 where the nozzle is tapered to impart a rotational force to the carrier gas containing the particulate matter.

8. Apparatus of any preceding claim where the sample preparation section comprises a gas passing through a container of particulate matter, preferably where the container is a vibrating hopper.

9. Apparatus of any preceding claim where the apparatus for creating the vapour of the gettering agent is removable and/or replaceable.

10. Apparatus of claims 4 to 9 where the heat exchanger for annealing the coated particulate is enabled to control the temperature and residence time of coated particulate.

11. Apparatus of claims 4 to 9 where the collector uses a sintered element filter.

12. Apparatus of any preceding claim wherein the feeder is a vibratory feeder.

13. Apparatus of any preceding claim wherein the gettering agent is Magnesium.

14. Apparatus of any preceding claim wherein a turbulent current is created in the first vessel.

15. Apparatus of any preceding claim wherein the inert gas and nanosized particulate suspension is at substantially room temperature.

16. Apparatus of any preceding claim wherein the gettering agent vapour is created by heating a liquid of the gettering agent in the first vessel.

17. Apparatus of any preceding claim wherein the filter is a sintered element filter.

18. Apparatus of any preceding claim wherein the gettering agents and/or impurities are removed using any acid suitable for reacting the gettering agent but does not react with the element being purified.

19. Apparatus of claims 2 to 18 where the silicon produced is mesoporous and/or nanoporous silicon.

20. Apparatus for the purification of nanosized material, the apparatus comprising: a sample preparation section adapted to create a gas suspension which comprises an inert gas and nanosized particles of the material to be purified; a first vessel into which the gas is introduced, the vessel containing a vapour of a gettering agent, wherein the vapour and gas suspension are mixed in the vessel, and part or all of the nanosized material is coated by the gettering agent vapour; a further vessel which is suitable for annealing the gas suspension by controlling the temperature of the suspension, thereby ensuring that part or all of the gettering agent that coats the nanosized material particles reacts with the impurities in the said particles; and a collector suitable for removing the gettering agent and impurities from the surface of the said particles.

21. A method for purifying nanosized silicon, the method comprising; creating a suspension comprising an inert gas and nanosized silicon particles; mixing the inert gas and nanosized silicon particle suspension with a vapour of a gettering agent, thereby coating part or all of the nanosized particles with the gettering agent; varying the temperature of the gas containing the coated nanosized particles to anneal the mixture, thereby removing impurities from the core of the nanosized silicon; removing the gettering agents and impurities from the surface of the nanosized silicon.

22. A method of claim 21 characterised in that the purified nanosized silicon is mesoporous and/or nanoporous silicon.

23. Apparatus or method of claims of any preceding claim wherein the annealing temperature is less then 800⁰C and more preferably approximately 700⁰C.

24. Apparatus or method of claims of any preceding claim wherein the silicon produced is characterised by a porous crystalline structure.

25. Mesoporous and/or nanoporous silicon formed by the method of any of claims 1 to 22.