WO2011042742A1

WO2011042742A1 - Process for the preparation of nano-scale particulate silicon

Info

Publication number: WO2011042742A1
Application number: PCT/GB2010/051676
Authority: WO
Inventors: Daniel Robert Johnson
Original assignee: Intrinsiq Materials Global Ltd
Priority date: 2009-10-08
Filing date: 2010-10-07
Publication date: 2011-04-14
Also published as: GB0917635D0

Abstract

A process for the preparation of porous or thin- walled particulate silicon, comprising: (i) reacting a porous or thin-walled particulate silicon compound that can be reduced to elemental silicon with a reducing agent in an inert atmosphere and the presence of an inert particulate moderator to control the temperature of the reaction to a temperature at which the silicon product is porous or thin walled, and (ii) dissolving out from the silicon product any other reaction products and the moderator.

Description

Process for the preparation of nano-scale particulate silicon

This application relates to nano-scale silicon, especially nano-scale particulate silicon that is porous or is in the form of very thin platelets or the like. Porous silicon is in demand as a delivery medium for products such as pharmaceuticals and nutrients, for use externally or internally, and especially for the controlled release of such products. Examples of such products are active ingredients in foodstuffs such as vitamins or other nutritional supplements, and dental care ingredients for toothpaste. Patent Application WO 2007/012847 relates to the use of porous silicon to deliver neutriceuticals.

Although other nano-scale products are known for the delivery of active substances, porous silicon has some key benefits in that it can provide controlled release over time rather than releasing the active substances very quickly. Also, unlike nano-scale silicon, most competitive materials are non-biodegradable. Being biodegradable means that porous silicon is broken down in the body and consequently releases the product absorbed within it over a period controlled and determined by the wall thickness of the porous silicon.

The rate at which nano-scale porous silicon particles deliver the active ingredients can be modified by controlling the wall thickness of their pores and the pore size. It is already known, e.g. from published patent application US 2006 867520P, to prepare finely-divided silicon by reducing silica particles with magnesium, and to control this very exothermic reaction by incorporating an inert moderator such as sodium chloride. However, the particles of silica produced are non-porous. We have found that in order to obtain porous silicon particles from a porous silica starting material, it is necessary to control the temperature attained during the reaction very carefully. This temperature must not exceed 850°C for any significant length of time, otherwise the resulting silica particles become sintered and the porous structure is substantially lost.

According to a first aspect of the invention, we provide a process for the preparation of porous particulate silicon, comprising:-

(i) reacting a nano-scale porous particulate silicon compound that can be reduced to elemental silicon with a vaporisable reducing agent in an inert atmosphere and in the presence of an inert moderator to control the temperature of the reaction to a temperature at which the silicon product is porous, and

(ii) dissolving out from the silicon product any other reaction products and the moderator.

As mentioned above, in order to avoid sintering and loss of porosity of the silicon product the reaction temperature should not exceed 800°C. Preferably, the temperature should be maintained in the range 550-800°C, more preferably 700- 750°C, for from 1 minute to 1 hour, more preferably 1-15 minutes.

The moderator must be inert to the extent that it remains unreacted throughout the process. Preferred moderators are salts that are soluble in water or organic solvents so that they can be dissolved out easily from the silicon product once the reaction has been completed.

The choice of the moderator is important. Ideally, it should have a relatively- high specific heat and also a relatively-high latent heat of fusion, and it should preferably melt at a temperature close to or just above the temperature to which the reaction is to be controlled. If the reaction reaches a temperature where the moderator begins to melt it will absorb a great deal of heat from the reaction mixture, thus keeping the temperature of the reaction under control. Thus, the moderator acts as a heat sink, both by virtue of its specific heat and also its latent heat of fusion. A preferred moderator is sodium chloride which melts at 801°C (just above the most preferred temperature range for the reaction) and has a relatively-high latent heat of fusion of 2,600 J/mole. The meaning of the term "inert atmosphere" would be well-known to the person skilled in the art as including, for instance, noble gas atmospheres such as argon. In addition, as used herein the term "inert atmosphere" is intended to include processes in which the porous particulate silicon is prepared in reducing atmospheres. A reducing atmosphere would include atmospheres in which some reaction is possible, but in which oxygen has been removed substantially or entirely. For instance, nitrogen, argon or hydrogen containing atmospheres may be used, most often these would be argon or hydrogen containing atmospheres, in some instances the inert atmosphere would comprise substantially entirely argon and/or hydrogen, with other gases present only as impurities.

In some examples, the atmosphere may be at reduced pressure (i.e. less than atmospheric pressure). It is specifically envisaged that that reduced pressures may be used and that the inert atmosphere may be at a pressure in the range 10 - 80% atmospheric, 20 - 60% atmospheric, 30 - 50% atmospheric.

A preferred silicon compound that can be reduced to elemental silicon is silica, Si0₂. A preferred reducing agent for silica is metallic magnesium. Magnesium is a particularly preferred reducing agent because it is very reactive towards silica and, in contrast to aluminium for example, it develops a high vapour pressure at temperatures at which the reaction is carried out. The magnesium vapour will percolate into the porous structure of the silica where it reacts in the vapour phase.

For the reaction between silica and magnesium using sodium chloride as moderator the preferred weight ratio NaCl : Si product is from about 1: 1 to 10: 1.

For the reaction between magnesium and silica another example of a suitable moderator is calcium chloride, which melts at 772°C. The preferred weight ratio of silica to calcium chloride is from about 1: 1 to 10: 1. Another possible reducing agent is metallic sodium. Apart from the porous particles of elemental silicon, the other product of this reaction is magnesium oxide. This can be dissolved out of the silicon product, together with the moderator, by an aqueous solution of an acid that does not react with the silicon product, e.g. hydrochloric acid. A wetting agent may be added to assist dissolution, for example an alcohol such as propan-2-ol.

The reaction between silica and magnesium generally takes less than one hour, and is usually complete within 5 minutes. Careful control of the temperature at which the reaction takes place is essential. Otherwise, owing to its exothermic nature, the temperature would increase to a level far above the maximum that will allow a porous silicon product. A preferred method of carrying out the reaction in a controlled manner is by means of a belt furnace, in which the reactants are conveyed through a series of zones on a continuously-moving belt.

In a first zone, a mixture of porous silica particles, magnesium, and moderator (preferably sodium chloride) is pre-heated, preferably to a temperature just below that at which reaction between the magnesium and the silica would initiate.

The reactants are then conveyed to a second zone in which the temperature is raised sufficiently to initiate the reaction (about 520-550°C), in which zone the temperature is controlled, preferably so that it remains within the range 700-750°C. Should the temperature rise to 801°C the sodium chloride will begin to melt, absorbing much heat from the reactants as a result of its latent heat of fusion, inhibiting any further temperature rise. The speed of the belt is adjusted so that when the reaction has been completed the reactants leave the second zone and pass into a third, cooling, zone. There, the temperature is reduced to a much lower temperature, for example, ambient temperature or below.

Cooling gases, e.g. hydrogen or helium, may be introduced into the third zone, if required. In fact, as the magnesium should essentially all have reacted by this stage nitrogen or air could be used for cooling. The materials resulting from the third zone are then treated with an acidified aqueous solution (e.g. where the acid is HC1) in order to dissolve out both the sodium chloride moderator and the magnesium oxide resulting from oxidation of the magnesium.

Graded silica/silicon structures can be obtained by varying the amount of magnesium, or by controlling the extent of the reaction by control of the temperature or the time the reaction is allowed to proceed. If desired, the silica starting material may be reacted with an amount of magnesium insufficient to reduce the particles completely to elemental silicon. In that case, the product will comprise particles with pores where only the pore surface layers comprise silicon, the remaining parts of the particles being unreacted silica. Such particles would be useful in protecting a product absorbed by them from degradation by UV light, because elemental silicon is an effective filter for UV light. The surface layers of silicon would shield the product absorbed within the pores of the particles.

As mentioned above, the preferred reducing agent is magnesium, because this will reduce silica at a relatively-low temperature (less than 800°C). In contrast, a carbothermal reduction, for example, would require a temperature of about 1700°C as it would require the formation of carbon vapour. Thus, the energy requirement when magnesium is utilised is significantly reduced. Also, relatively-little energy needs to be supplied because, once the reaction between silica and magnesium has begun, it is self-sustaining.

Where the process is carried out in a belt furnace as is preferred, magnesium vapour is evolved in the first zone provided that the temperature is at least about 450°C. The vapour is able to enter the pores of the silica starting material without difficulty, especially if the structure of the silica is a scaffold. As there will tend to be a dynamic equilibrium between gaseous and liquid magnesium, reaction between liquid magnesium and the silica may also take place. The reaction must take place under an inert gas (for example argon) and air must be excluded as both oxygen and nitrogen would interfere with the reaction. Because the reaction takes place at a temperature below which sintering can occur, the three-dimensional structure of the initial silica particles (e.g. a scaffold structure) can be maintained in the final silicon product. The internal surface area of the silica particles can be maintained or even increased as a result of the loss of oxygen from the silica.

Another advantage of conducting the reaction below sintering temperatures is that any silica present in the amorphous form will not be converted into crystalline silica. For some applications, only amorphous silica is acceptable. A further advantage of conducting the reaction below sintering or melting temperatures of the silica or silicon is that any impurities present in the moderator or in the magnesium will not be able substantially to diffuse into the silica or silicon product. Thus, the purity of the original silica starting material will be retained in the product. In cases where the reaction takes place between magnesium vapour and silica, any impurities in the solid magnesium will remain there and thus not contaminate the product. The moderator will also act as a diffusion barrier between the solid and molten magnesium phase and the silica, thus inhibiting contaminants reaching the silica and silicon product from this source. As the moderator is a potential source of impurities it should be supplied in as pure a form as possible and advantageously recycled from the end products of the reaction, where it can be re- purified. In a pure state, the moderator salt may also act as a sink for impurities emanating from the silica, further increasing the purity of the end product.

Yet another advantage of a moderated reaction in accordance with the invention is that the reaction is more uniform throughout the reaction mixture so that the likelihood of non-stoichiometric or unwanted by-products such as unreacted silica, magnesium silicate or magnesium silicide is reduced. Any magnesium silicide that were produced could lead to the formation of silane gas which would be an explosion hazard.

If desired, the temperature of the reaction can be adjusted so that the individual porous particles of silicon, whilst retaining their porous nature, would be sintered together to form a mesoporous mass, provided that the silica particles are small in origin.

It is envisaged that the process may include an additional step wherein the inert particle is reacted with the porous particulate silicon in the absence of the reducing agent so that the particulate moderator restricts the reduction process to the surface of the particles resulting in a shell of silicon which has reacted surrounding an un-reacted core. This may be achieved, for instance, by dissolving the particulate moderator and absorbing it onto the surface of the silicon. Once absorbed the solvent may be removed and the porous particulate silicane compound- inert particulate moderator adduct reacted with the reducing agent as described above.

A large range of porous silica feedstocks having different structures is available, and as the present invention allows the silicon product to retain the original porous structure, a corresponding range of porous structures for the product is also available.

In addition to the preparation of a porous silicon material starting from a porous reducible silicon compound, the invention also includes within its scope the preparation of a very thin-walled silicon product, such as platelets or other structures having a wall thickness of up to 200 nm, and preferably up to 100 nm. In that case the reducible silicon compound would be in a similar form and have a similar wall thickness, and by carrying out the reaction in the presence of a moderator as described above at the temperatures described above the initial form and wall thickness of the starting material would be retained in the silicon product, without the risk of sintering.

Thus, according to a second aspect of the invention, we provide a process for the preparation of particulate silicon comprising :- (i) reacting a particulate silicon compound having a mean wall thickness in the range 50 to 200 nm with a vaporisable reducing agent in an inert atmosphere and in the presence of an inert moderator to control the temperature of the reaction to a temperature at which the silicon product retains a mean wall thickness within said range, and

Example

Tests were carried out on porous silica particulates by heating them to 700°C, 800°C and 900°C, and monitoring any change in their porosity.

The silica particles tested were Syloid 74FP from W.R. Grace. This is a food grade silica having a well-defined pore structure and <1 impurities. It is mesoporous with a pore size of about 17nm. The silica particles were heated in a tube furnace purged with argon at 1 litre per minute. The samples were pre-dried at 120°C under vacuum then transferred to an open-ended stainless steel ¾ inch pipe where they were flash heated to the final treatment temperature. Samples were taken at various time intervals and BET Area / BJH pore volume measurements were taken using a Micromeritcs Tristar 3000 nitrogen physisorption analyser.

Figure 1 is a graph showing the surface area of the particles when heated to 700°C, 800°C and 900°C over different periods of time. It is clear from this that little loss of porosity occurs, even on prolonged heating, at 700°C (reduction in porosity from about 370 to about 320 m /gram). At 800°C, whilst there is increased loss of porosity demonstrated by the further reduction in surface area to about 240m , the particles still remain significantly porous. At 900°C, on the other hand, loss of porosity is almost complete.

Figure 2 is a graph showing differences in pore volume after heating for 4 hours at 700°C, 800°C and 900°C. It is clear that after heating to 900°C the pore volume is greatly reduced, i.e. from about 4.0 cm /g for the starting material, to only about 0.9 cm /g. At 700°C, on the other hand, there is little change in pore size, but only some closing of the very small pores and some size reduction in the larger ones. At 800°C, whilst there is a further loss in porosity, the silica remains usefully porous.

Claims

1. A process for the preparation of porous particulate silicon, comprising: (i) reacting a porous particulate silicon compound that can be reduced to elemental silicon with a vaporisable reducing agent in an inert atmosphere and the presence of an inert particulate moderator to control the temperature of the reaction to a temperature at which the silicon product is porous, and

2. A process for the preparation of particulate silicon comprising:-

(i) reacting a particulate silicon compound having a mean wall thickness of up to 200 nm with a vaporisable reducing agent in an inert atmosphere and in the presence of an inert moderator to control the temperature of the reaction to a temperature at which the silicon product retains a mean wall thickness within said range, and

3. A process according to claim 1 or 2, wherein the reducible silicon compound is silica.

4. A process according to claim 2 or 3, wherein the reducing agent is magnesium.

5. A process according to claim 2, 3 or 4 wherein the reducing agent is coated onto the silica particles.

6. A process according to any preceding claim, wherein the moderator is a salt.

7. A process according to claim 1, comprising:- (i) mixing porous particulate silica with magnesium and an inert particulate moderator,

(ϋ) heating the resulting mixture to vapourise at least part of the magnesium and to initiate a reaction in which the silica is reduced to silicon and the magnesium is oxidised to magnesium oxide, wherein the reaction is controlled to a temperature not exceeding 800°C by selecting the amount of moderator relative to the amount of silica, and whereas the moderator has a melting point close to the temperature to which the reaction is to be controlled,

(iii) dissolving out the moderator and the magnesium oxide from the silicon product.

8. A process according to claim 6 or 7, wherein the moderator is sodium chloride or calcium chloride.

9. A process according to claim 8, wherein the moderator is sodium chloride and the ratio by weight of the sodium chloride to silicon product is about 10: 1.

10. A process according to claim 8, wherein the moderator is calcium chloride and the ratio by weight of calcium chloride to silicon product is from about 2: 1 to about 3: 1.

11. A process according to any of claims 7 to 10 at which the temperature is controlled within the range 550°C - 800°C, for from 1 minute to 1 hour.

12. A process according to claim 11 at which the temperature is controlled within the range 700 - 750°C, for from 1 minute to 15 minutes.

13. A process according to any one of claims 7 to 12, wherein the moderator and magnesium oxide are dissolved out together by means of an acidic aqueous solution using an acid that will not react with the silicon product.

14. A process according to claim 13, wherein the solution contains a wetting agent.

15. A process according to any preceding claim comprising the additional step of combining the inert particulate moderator with the porous particulate silicon prior to reaction with the vapourisable reducing agent.

16. A process according to claim 15 wherein the inert particulate moderator is absorbed onto the porous particulate silicon compound by removal of solvent prior to reaction with vapourisable reducing agent.

17. A process according to any of claims 7 to 16, carried out in a belt furnace including :-

(i) a first zone in which the mixture of magnesium, silica, and moderator are pre-heated;

(ii) a second zone in which the mixture is heated further to the temperature at which the reaction is to be controlled, so that the reaction between the silica and the magnesium takes place.

(iii) a third zone in which the materials from the second zone are cooled.

18. A process according to any of claims 7 to 17, wherein the amount of magnesium is less than the stoichiometric amount required to reduce all of the silica to silicon, so that the reaction product comprises particles containing both silicon and unreacted silica.

19. A process according to any of claims 7 to 17, wherein moderator is incorporated into the silica particles so that only the outer layers of those particles become reduced, resulting in particles having a silica core and a silicon outer layer.

20. A process according to any of claims 7 to 19, wherein in the pores of the reaction product only their surfaces comprise silicon.

21. Porous particulate silicon prepared by the process of any of claims 1 and 3 to 20.

22. Use of porous particulate silicon according to claim 20 as a delivery medium for pharmaceuticals or nutrients.

23. Particulate silicon prepared by the process of any of claims 2 to 19 having a mean wall thickness in the range 5 to 200 nm.