EP2456718A1

EP2456718A1 - Production of silanes from silicon alloys and alkaline earth metals or alkaline earth metal silicides

Info

Publication number: EP2456718A1
Application number: EP10745671A
Authority: EP
Inventors: Dominique Belot
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2009-07-22
Filing date: 2010-06-28
Publication date: 2012-05-30
Also published as: KR20120047959A; WO2011010032A1; CN102471073A; FR2948354B1; JP2012533511A; FR2948354A1; US20120128568A1

Abstract

The invention relates to a method for preparing a compound or mixture of compounds of the formula Si_nH_2n+2, where n is an integer greater than or equal to 1 and less than or equal to 3, said method including a step a) of reacting at least one silicide or silicon alloy in powder form and having the formula M¹ _xM² _ySi_z, where M¹ is a reducing metal, M² is an alkaline metal or alkaline earth metal, and x, y, and z vary from 0 to 1, z being different from 0 and the sum x+y being different from 0, with chlorhydric acid being pre-dissolved in an ether aprotic solvent.

Description

Production of silanes from silicon alloys and alkaline earth metals or alkaline earth metal silicides.

The present invention relates to the production of silicon hydrides or silanes from silicon alloys or silicides.

Certain silanes and more particularly monosilane, or silicon tetrahydride (SiH ₄ ) are used as a silicon vector in deposition techniques of amorphous silicon, polycrystalline silicon, nanocrystalline silicon or microcrystalline also called nano or micromorph, silica, nitride silicon, or other silicon compound for example in vapor deposition techniques.

Thin film deposition of amorphous silicon, and microcrystalline silicon obtained from silane, make it possible to manufacture solar cells.

It is also possible to obtain coatings resistant to acid corrosion, by cracking silane and to manufacture compounds such as silicon carbide.

Finally, the silane is capable of adding to the single or multiple bonds of the unsaturated hydrocarbons to give organosilanes.

The monosilane market will experience a very strong expansion both for the manufacture of integrated semiconductors and the manufacture of thin or crystalline solar (photovoltaic) cells, semiconductor components and the manufacture of flat screens.

Several types of processes described below have been used until now.

First of all, the reduction of SiCl ₄ by LiH in a KCl / LiCl bath at temperatures between 450 ^° C. and 550 ^° C. is known. The efficiency of the reaction is interesting, but the process is based, on the one hand, on the availability of LiH while the lithium resources are very limited and on the other hand on the possibility of recycling the lithium metal by electrolysis. The reaction medium is very corrosive and uses particular materials. This process has been used to produce small amounts of silane.

The reduction of SiF ₄ by NaAIH ₄ in organic solvent medium is another example. This process is industrially viable only when there is SiF ₄ , by-product of another chemical production and sodium to make sodium aluminum hydride. This method is not easily usable, especially for these two reasons.

Another known reaction is the acid attack in a liquid NH ₃ medium of a stoichiometric alloy Mg ₂ Si. The reaction is as follows:

Mg ₂ Si + 4 HCI - * SiH ₄ + 2 MgCl ₂

NH3 Mq.

This process is carried out at a temperature close to ambient temperature at atmospheric pressure. Magnesium silicide (Mg ₂ Si) has been extensively tested in aqueous media and ammonia. Although sufficiently acceptable to lead to industrial production units, the process has the following major drawbacks:

• Industrial magnesium silicide due to the volatility of magnesium contains only 70% to 80% of the stoichiometric compound. The conditions of manufacture of the stoichiometric compound make it a product too expensive for this industry.

In parallel with the production of monosilane by this route, many higher silanes including polychlorosilanes, siloxanes and silicone gums are manufactured, making the balance of monosilane material unattractive and inducing significant difficulties in the management of the process.

This process is not satisfactory because of the difficulty of controlling the process and the implementation of highly regulated liquid ammonia.

Another known reaction is the disproportionation of SiHCb on resins having grafted amino groups or the like. The complete process is described as follows:

a) 4 Si Metai + 12 HCl -> 4 SiHCb + 4 H ₂ (temperature between about 300 ^° C. and about 1000 ^° C.)

b) 4 SiHCI ₃ ^^ SiH ₄ + 3 SiCl ₄ (temperature close to ambient)

c) 3 SiCl ₄ + 3H ₂ -> 3 SiHCI ₃ + 3HCl (temperature of about 1000 ° C.),

the following reaction report:

4 Si Metal + 9 HCI -> SiH ₄ + 3 SiHCI ₃ + H ₂

A variant of the above reaction is described as follows: a) 4 Si Metai + 16 HCl -> 4 SiCl ₄ + 8 H ₂ (temperature between about 1000 ^° C. and about 1100 ^° C.)

b) 4 SiCl ₄ + 4H ₂ -> 4 SiHCI ₃ + 4HCl (temperature around 1000 ° C)

4 SiHCI ₃ -> SiH ₄ + 3 SiCl ₄ ,

the following reaction report:

4 Si metal + 12 HCl ^ SiH ₄ + 3 SiCl ₄ + 4 H ₂

This process requires high temperatures in an extremely corrosive environment and consumes a lot of energy (about 50 kWh / kg for step b)). To achieve the maximum yield, step b) requires many recirculation loops of chlorosilane mixtures. In addition to the use of highly corrosive, toxic and flammable products, such a type of process is very energy-intensive and presents a lot of industrial risks.

The generation of monosilane and higher silanes has been described in

Handbook of Inorganic Chemistry Gmelin Si-Silicon, by reacting silicon silicides and alloys in an acidic or basic medium in the aqueous phase. In the patent applications EP146456 and WO2006 / 041272, the synthesis of monosilane in aqueous phase by dropping a powder of Alx Siy Caz, x, y and z representing the percentages respectively of aluminum, silicon and calcium, in a solution of HCI, is described. The composition of the gases produced was about 80% monosilane, 10% disilane and 5% trisilane as well as traces of disiloxane. This type of process has the disadvantage of the handling and storage of pure or highly concentrated HCl. By-products resulting from such a reaction are produced in large quantities and are harmful to the environment (in particular chlorides). Another disadvantage of such a process is the formation in abundance of a foam in the reaction medium, which decreases the yield of the reaction and requires the presence of an anti-foaming agent. Such a reaction is very exothermic and temperatures above 100 ^° C. are reached fairly quickly if the rate of introduction of the alloy powder is not considerably reduced.

All these works described above do not guarantee the conditions necessary for the realization of a profitable process for an industrial development. The development of processes involving reaction conditions less difficult and or allowing to be used for small and medium units in almost all environments and close to the use of monosilane is a major issue for the industries mentioned above.

It has been found a simple process, using inexpensive raw materials and producing silicon hydrides with an industrial yield and not having all the disadvantages seen above.

An object of the present invention is to alleviate all or part of the disadvantages of the prior art noted above. The object of the invention described hereinafter is to propose additional optimizations of the process starting from the alloys AISiCa while minimizing the disadvantages mentioned above.

To this end, the subject of the invention is a process for preparing a compound or a mixture of compounds of formula Si _n H _2n + 2 in which n is an integer greater than or equal to 1 and less than or equal to 3 , comprising a step a) of reacting at least one silicide or silicon alloy in the form of a powder of formula

M ¹ χM ² _y Si _z in which M ¹ is a reducing metal, M ² an alkali or alkaline earth metal, x, y and z vary from 0 to 1, z being different from 0 and the sum x + y different from 0, with hydrochloric acid previously dissolved in an aprotic solvent of the ether type. The reducing metals are, for example, Al, B, Ga, In. The alkali metals are, for example, Li, Na, K, Cs. The alkaline earth metals are, for example, Mg, Ca, Sr, Ba.

Furthermore, embodiments of the invention may include one or more of the following features:

In the process as defined above, said aprotic solvent is tetrahydrofuran (THF).

The above process is characterized in that M ¹ is aluminum and M ² is calcium or magnesium.

The above process is characterized in that the silicon alloy has the formula CaAl ₂ Si ₂ .

The above process is characterized in that the solution of tetrahydrofuran containing hydrochloric acid is injected so as to permanently remove from the surface of the silicide or silicon alloy, the chlorides formed during the reaction of the step a). - The above method is characterized in that step a) is conducted at a temperature between 20 ⁰ C and 130 ⁰ C and at a pressure between 1 bar and 10 bar.

The above process is characterized in that the particle size of the silicon alloy is between 0.2 mm and 0.9 mm and preferably between 0.2 mm and 0.2 mm.

0.5 mm.

- The above method further comprises a step of recycling said aprotic solvent not used in step a), by heating the latter to a temperature above 170 ° C.

- The process above includes the steps:

a ') Mixture of hydrochloric acid with a crown ether such as tetrahydrofuran;

b ') mixing between said silicon alloy and the mixture resulting from step a'); c ') fractional distillation at near ambient pressure to separate monosilane from higher silanes and other volatile compounds.

The present invention also relates to an on-site unit for implementing the silane manufacturing process as defined above, comprising:

at least one reactor equipped with means for introducing the powdered silicon alloy and means for introducing a solution of the crown ether, such as tetrahydrofuran, containing hydrochloric acid;

a purification circuit comprising a fractionation column for separating the silanes and a double distillation column for recovering the pure monosilane and / or a mixture of silane and disilane;

at least one recycling means for recycling to the reactor the crown ether such as tetrahydrofuran not used during the reaction between said silicon alloy and hydrochloric acid previously dissolved in crown ether, such as tetrahydrofuran; .

The silicon alloy is selected from CaAl ₂ Si 2, Sio.sMg, Sio.sCa, AISiCa, CaSi, Cao.sSi, MgSi, AISiNa, AISiMg, SiNa, AISiLi, SiK, Ca _o, 5AISi _o, 33 _o and Ca , 5AISi _o, 75, or a mixture thereof, preferably Sio.sMg, AISiNa, SiNa, Si _o, 25Li, Si _o, 25Na, Si _o, 2SK or SiK. Other silicon alloys that are suitable for the present invention are ferosilicon-type alloys, for example FeSi, FeSiMg, FeSiCa. Preferably, the alloy used in the process that is the subject of the present invention is the CaAl ₂ Si ₂ composition, which is the most active phase giving the best yields.

By higher silanes are understood compounds of formulas Si _n H2 _{n +} 2, n> 2 including disilane, trisilane or tetrasilane.

The alloys or silicides used in the implementation of the process according to the invention are alloys or silicides also used to control the foaming and deoxygenation of slags in steel foundries. These are low-cost, easily produced industrial products. One of the advantages of the process that is the subject of the invention is that it can conduct the reactions under conditions close to ambient conditions (temperature and pressure) in materials that are common in the mineral chemical industry, such as glass-reinforced reactors, for example. Processes involving these alloys or silicides can make it possible to produce silane in small and medium size units closest to the markets. Regardless of the alloys and silicides available and the operating and environmental constraints, the same unit can be used by adjusting the operating parameters. In all cases, by-products are valuable or reusable mineral products.

It has also been found that the particle size of the alloy powder has an influence on the kinetics of the reaction and consequently on the reaction efficiency. The kinetics increases when the particle size decreases. Foaming during the reaction is the limiting factor for particle size. All other conditions being equal, when the particle size is divided by 10, the amount of silane produced at the same time is multiplied by about 15.

The process according to the invention also has the advantage that the proportion of monosilane formed relative to the silanes produced during the reaction is at least equal to 60%, which is important in view of the fact that the silane sought for the targeted applications by the present invention is especially monosilane.

According to a preferred embodiment of the present invention, the alloy used is CaAl ₂ Si ₂ . The inventors have found that surprisingly and unexpectedly, it was the alloy providing the best results. The theoretical equation of monosilane production is written as follows:

CaAI ₂ Si ₂ + 8 HCI -> 2 SiH ₄ + 2 AICI ₃ + CaCl ₂

For a yield of 100%:

SiH ₄ 1 kg

HCI 4.56 kg

AICI ₃ 4.17 kg

CaCI ₂ 1, 74 kg

For comparison, the equation from Mg ₂ Si gives:

Mg ₂ Si + 4 HCI - * SiH ₄ + 2 MgCl ₂

For a yield of 100%:

SiH ₄ 1 kg

HCI 4.56 kg

MgCl ₂ 5.94 kg

We see that the two paths lead to scales of equivalent materials. In a particular embodiment, the present invention relates to an aluminum alloy reaction containing at least 90% CaAl ₂ Si ₂ and less than 10% CaSi ₂ by weight.

The process which is the subject of the present invention uses an aprotic solvent of the ether type and preferably tetrahydrofuran (THF). Other solvents are conceivable, for example, diethyl ether, dimethyl ether, diglyme (di (2-methoxyethyl) ether), or triglyme.

Such a solvent has the advantage of avoiding the formation of siloxanes and hydroxides of silicon.

In one embodiment of the process according to the invention, hydrochloric acid (HCl) is first dissolved in a solvent such as THF in order to form a homogeneous reaction medium.

THF dissolves the chlorides of the alkaline earth metals and aluminum in large quantities thanks to its crown ether function and the HCl acid dissolved in THF reacts very rapidly with the metals to form the corresponding chlorides which dissolve at the same time in the THF. . Thanks to this enhanced "activity" of the HCl acid for metals, the reaction will preferentially focus on the production of metal chlorides (such as AlCl ₃ ) instead of competing production of chlorosilanes. According to a preferred embodiment of the process according to the invention, the THF solution containing the HCl will be injected so as to continuously "wash" the surface of the alloy in order to achieve the almost complete reaction of the solid medium and in proportion such that there will be no excess of unreacted HCl present in the medium.

It is known that during the production of silane from Mg ₂ Si, as described above, the reaction of HCl in aqueous medium or liquid ammonia is accompanied by the formation of higher silanes up to gums of highly viscous and flammable silicones. This is because the link

Si-H is very labile with a pronounced ionic character (Si ⁺ - H ^" ) The chlorides of which MgCl ₂ , CaCl ₂ , AlCl 3 readily catalyze linear or branched polymerization to give H- (SiH ₂ ) type chains _n -H. during implementation of a particular mode of the method of the present invention, the solution

HCI in THF washes the alloy powder so as to dissolve the chlorides permanently. Thus the desired silanes are separated from the chlorides immediately after their formation.

In addition, while remaining at a temperature well below 200 ⁰ C, the risks of formation of chlorosilanes by reaction between the light silanes and HCl are very low.

Other particularities and advantages will appear on reading the following description, made with reference to FIG.

- Figure 1 shows a diagram of an installation used to implement the method according to the invention.

The production unit 1 comprises at least three parts containing a reactor 2, a purification system 3 and a system 4 for recycling the solvent from the reactor 2.

The silane production reaction takes place in a reactor 2 equipped with a mixing means 40 such as a scraper or a kneader. The reactor 2 is filled with a silicon alloy, such as CaAl ₂ Si ₂ , originating from a source 5 on the one hand and, on the other hand, from a source 6, with a solution containing an acid, such as HCl for example, previously mixed with an aprotic solvent of the ether type, such as THF. The proportions of the mixture being selected prior to the reaction, by the user in order to obtain the best possible yield given the problems to be solved identified above. The reactor 2 may for example be secured to a removable cover at the top fixed by arches. The lid has a sealed opening for connecting a sealed hopper 7. Preferably, the reactor is surrounded by a thermal insulation sheath.

In the vicinity of the reactor 2, a means 7 for the flow of the silicon alloy is present. Such a flow means is, for example, a hopper 7 initially filled with a silicon alloy in the form of a powder of formula

M ¹ χM ² _y Si _z in which M ¹ is a reducing metal, M ² an alkali or alkaline earth metal, x, y and z ranging from 0 to 1, z being different from 0. Preferably, the alloy is CaAI ₂ Si2. For example, the hopper 7 comprises a worm and a constriction hose for isolating the hopper 7 of the reactor 2. The design of the hopper 7 is for example similar to the hoppers used to pour the calcium carbide into the reaction vessels. 'acetylene. The alloy is fed into drums similar to the drums used to transport the calcium carbide in the acetylene production units.

According to a particular embodiment of the invention, above the reactor lid 2, there are arranged two sealed valves, gas, series isolation between the two, a lateral tapping to purge the reactor 2 before disconnection . An analogous device 8 equips the outlet of the bottom of the reactor 2 to evacuate the liquids. The bottom of the reactor 2 is closed by means 9, for example a dome, preventing the liquid products from stagnating in the bottom channel. This dome is raised by actuating the bottom valve 8. The liquids are sent to a crystallizer system 10 via line 11.

In order to recover the desired silanes, the products from the reactor 2 that are not sent to the crystallizer system 10 are directed to a purification system 3 via a pipe 12. Said purification system 3 comprises at least one fractionation column present to separate the silanes from the other products present and finally a double distillation column used to recover the pure monosilane, which is then used for the desired applications. It is also provided, a fractionation system capable of delivering a silane / disilane mixture.

Indeed, the use of a mixture containing about 80% of silane and about 20% of disilane can be envisaged in silicon deposition techniques.

Thus, according to one particular embodiment, the subject of the invention is a process and a production unit making it possible to continuously produce mixtures of gaseous silanes, including monosilane / disilane mixtures. These mixtures are usable directly for the manufacture of thin-film solar cells. The process makes it possible to produce mixtures with a composition of 80% monosilane and 20% disilane by volume.

The powder of the compound (the silicon alloy, for example CaAl ₂ Si ₂ ) is introduced into the reactor 2 continuously through the hopper 7. The reactor 2 consists of a first slightly frustoconical bottom 13 consisting of a metallic filter media To which is added an organic filter medium 14, for example a filter paper, the purpose of which is to retain the unreacted particles. The frustoconical bottom 13 contains in its center a hole closed by a bung removable mechanically from the outside. The reactor 2 also contains a scraper 40 whose function is to stir the powder of the compound.

Acid, for example hydrochloric acid (HCl), gaseous is injected by diffusers 16 above the maximum level 17 provided for the level of solid content in the frustoconical bottom. The aprotic solvent, such as liquid THF, comes from the condenser (s) 18 surmounting the reactor 2. The gaseous THF from the crystallizer (s) 10 is injected into the reactor 2, under the condensers 18. In a particular implementation example of the present invention, by the device described here, the powder of the alloy compound injected by the hopper 7, first meets an atmosphere composed of gaseous THF, gaseous HCl, then a liquid containing the THF-HCl mixture. The alloy compound therefore begins, before encountering the frustoconical bottom, in contact with gaseous HCl and THF, by forming silanes discharged to the purification system 3 via line 19. Chlorides, such as AlCl 3 or CaCl ₂ resulting from the reaction between the alloy and the HCI dissolved in the solvent, are themselves dissolved by the THF present in the frustoconical bottom 13, then discharged via the pipe 11 and the liquid discharge system 8. These liquid products (CaCl ₂ , AICI ₃ , THF ....) then reach the crystallizer system (s) 10. Once the THF solution containing the chlorides (CaCl ₂ and AlCl 3) has passed through the layer of alloy compound resting on the frustoconical bottom 13, said solution is directed to one or more crystallizers 10 operating at more than 170 ⁰ C to evaporate the THF and precipitate the solid chlorides. These chlorides can be either extracted in the state for recycling or dissolved in water to make a brine transported to recycling units. Thanks to the liquid flow flowing with THF from the condensers 18, the unreacted HCI vapors and the dusts of the alloy compound are trapped and entrained towards the frustoconical bottom 13 where there is a layer of the alloy compound not yet reacted. The THF-HCl liquid stream passes through this layer reacting with the alloy compound while the THF dissolves the previously formed chlorides. Preferably, the process is regulated so that the flow rate of THF circulating in the reactor 2 is 4 to 5 times the mass HCI flow rate. The general conditions of the reaction are preferably, for the pressure between 1 bar and 10 bar, and for the temperature between 50 ⁰ C and 130 ⁰ C. An advantage of THF is to dissolve the silicone polymers that may be formed during the reaction. Thus, THF is recycled by heating the solution after reaction to at least 170 ° C.

In addition, THF can dissolve silicone polymers that could be deposited on the cold walls of the system.

Claims

A process for preparing a compound or a mixture of compounds of formula Si _n H _2n + 2 wherein n is an integer greater than or equal to 1 and less than or equal to 3, comprising a reaction step a) at least one silicide or silicon alloy in the form of a powder of formula M ¹ _× M ² _y Si _z in which M ¹ is a reducing metal, M ² an alkali metal or alkaline earth metal, x, y and z vary from 0 at 1, z being different from 0 and the sum x + y different from 0, with hydrochloric acid previously dissolved in an aprotic solvent of the ether type.

2. Method according to claim 1, characterized in that said aprotic solvent is tetrahydrofuran (THF).

3. Method according to one of claims 1 and 2, characterized in that M ¹ is aluminum and M ² is calcium or magnesium.

4. Method according to claim 3, characterized in that the silicon alloy has the formula CaAl ₂ Si ₂ .

5. Method according to one of claims 1 to 4, characterized in that the tetrahydrofuran solution containing hydrochloric acid is injected so as to permanently remove from the surface of the silicide or silicon alloy, the chlorides formed during the reaction of step a).

6. Method according to one of claims 1 to 5, characterized in that step a) is conducted at a temperature between 20 ⁰ C and 130 ⁰ C and at a pressure between 1 bar and 10 bar.

7. Method according to one of claims 1 to 6, characterized in that the particle size of the silicon alloy is between 0.2 mm and 0.9 mm and preferably between 0.2 mm and 0.5 mm .

8. Method according to one of claims 1 to 7, further comprising a step of recycling said aprotic solvent not used in step a), by heating the latter at a temperature above 170 ⁰ C.

9. Method according to one of claims 1 to 8, comprising the steps:

a ') Mixture of hydrochloric acid with a crown ether such as tetrahydrofuran;

10. On-site unit (1) for implementing the silane manufacturing method according to one of claims 1 to 9, comprising:

at least one reactor (2) equipped with means (5, 7) for introducing the powdered silicon alloy and means (6, 16) for introducing a solution of the crown ether, such as the tetrahydrofuran, containing hydrochloric acid;

a purification circuit (3) comprising a fractionation column for separating the silanes and a double distillation column for recovering the pure monosilane and / or a mixture of silane and disilane;

at least one recycling means (4) for recycling in the reactor (2), tetrahydrofuran not used in the reaction between said silicon alloy and hydrochloric acid previously dissolved in crown ether, such as tetrahydrofuran; .