FR2989075A1 - Preparing silane compound, comprises reacting silicide or silicon alloy with hydrochloric acid that is dissolved in solvent, where the silane compound is useful in the manufacture of e.g. solar cells and flat panel displays - Google Patents

Abstract

Preparing a silane compound or its mixture, comprises reacting silicide or silicon alloy with hydrochloric acid, which is dissolved in solvent. Preparing a silane compound of formula (Si nH 2 n + 2) (I) or its mixture, comprises reacting silicide or silicon alloy consisting of alloys having a structure of formula (CaMgSi and Ca 7Mg 6Si 1 4) (II) with hydrochloric acid, which is dissolved in solvent. n : 1-3.

Description

-1- Production of silanes from silicon alloys of formula Ca, MgySiz.

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 (SiH4) and disilane (Si2H6) are used as a silicon vector in deposition techniques of amorphous silicon, polycrystalline silicon, nanocrystalline silicon or microcrystalline silicon also called nano or micromorph, of silica, silicon nitride, or other silicon compound for example in vapor deposition techniques. Thin layer 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 market for monosilane and disilane will experience a very strong expansion for the manufacture of integrated semiconductors and the manufacture of solar cells (photovoltaic), as well as for semiconductor components and the manufacture of flat screens. Several types of methods described below have been used up to now. First, the reduction of SiCl 4 by LiH in a KCl / LiCl bath at temperatures between 450 ° C and 550 ° C is known. The yield of the reaction is interesting, but the method 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 SiF4 by NaAIH4 in organic solvent medium is another example. This process is industrially viable only when there is SiF4, a 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 liquid NH 3 medium of a stoichiometric Mg 2 Si alloy. The balance of the reaction is as follows: Mg 2 Si + 4 HCl SiH 4 + 2 MgCl 2 NH 3 Liq. This process is carried out at a temperature close to ambient temperature at atmospheric pressure. Magnesium silicide (Mg2Si) has been extensively tested in aqueous media and ammonia. Although sufficiently acceptable to lead to industrial production units, the process is unsatisfactory because of the difficulty of controlling the process and the implementation of highly regulated liquid ammonia. Another known reaction is the disproportionation of SiHCI3 on resins having grafted or other amino groups. The complete process is described as follows: a) 4 Si Metal. +12 HCl 4 SiHCI3 + 4H2 (temperature between about 300 ° C and about 1000 ° C) b) 4 SiHCI3 SiH4 + 3 SiCl4 (near ambient temperature) 20 c) 3 SiCl4 + 3H2 3 SiHCI3 + 3HCl (temperature of about 1000 ° C), or the following reaction: 4 Si Metal + 9 HCl SiH4 + 3 SiHCI3 + H2. An alternative of the above reaction is thus described: a) 4 Si Metal + 16 HCl 4 SiCl 4 + 8H 2 (temperature between about 1000 ° C and about 1100 ° C) b) 4 SiCl 4 + 4H 2 SiHCI 3 + 4 HCl (temperature of about 1000 ° C.) 4 SiHCI 3 SiH 4 + 3 SiCl 4, ie the following reaction balance: 4 Si metal + 12 HCl 3 SiH 4 + 3 SiCl 4 + 4H 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 maximum efficiency, step b) requires many recirculation loops of chlorosilane mixtures. In addition to the use of extremely corrosive, toxic and flammable products, such a type of process is very expensive in energy and has many industrial risks. The generation of monosilane, disilane and higher silanes has been described in the Handbook of Inorganic Chemistry Gmelin Si-Silicon, by reacting in the aqueous phase, silicon silicides and alloys in an acidic or basic medium. In 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 significantly reduced. All these works described above do not guarantee the conditions necessary for the realization of a profitable process for an industrial development. The ternary alloys (CaAl 2 Si 2) have shown interesting results in terms of silane production and make it possible to obtain high bound silicon levels (of the order of 35% to 40%). The production of silane is then accompanied by a release of hydrogen, but the reaction mechanism resulting in the gas ratios produced is still unknown. The development of processes involving less difficult reaction conditions and / or which can be used for small and medium units in almost all environments and close to the use of monosilane and disilane is a major challenge. for the industries mentioned above. A simple method has been found, using inexpensive raw materials and producing silicon hydrides with industrial efficiency and not having all the disadvantages seen above. An object of the present invention is to overcome 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 described above while minimizing the disadvantages mentioned above. The proposed solution is to combine the reaction potential of Mg2Si with a high bound silicon content by producing a CaxMgySiz ternary alloy.

The process for producing such a ternary alloy is similar to that already used to produce the CaAl 2 Si 2 alloy. Such a method has the advantage of being less expensive and well controlled by the alloy suppliers. To this end, the subject of the invention is a process for preparing a compound or a mixture of compounds of formula Sin1-12, + 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 of formula CaMgSi or Ca7Mg6Sii4, with hydrochloric acid previously dissolved in a solvent. The use of Ca7Mg6Sii4 made it possible to reach a bound silicon content close to 50% (48%).

Furthermore, embodiments of the invention may include one or more of the following features: - Process as defined above, characterized in that said solvent is tetrahydrofuran (TH F). - Process as defined above, characterized in that the silicon alloy has the formula CaMgSi. - Process as defined above, characterized in that the silicon alloy has the formula Ca7Mg6Sii4. - Process as defined above, characterized in that step a) is conducted at a temperature between 20 ° C and 100 ° C and at a pressure between 1 bar and 10 bar. - 5 - - Method as defined above, 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. - Method as defined above, further comprising a step of recycling said solvent not used in step a), by heating the latter to a temperature above 170 ° C. - Process as defined above, comprising 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 a pressure near ambient pressure for separating monosilane and disilane from higher silanes and other volatile compounds. - Process as defined above, for preparing a gaseous mixture comprising up to 90% monosilane and at least 10% disilane, preferably comprising 80% monosilane and 20% disilane. - Method as defined above, of disilane preparation. By higher silanes, it is understood compounds of formulas Sir, H2,2, n 3 including 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, the 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 therefore on the reaction efficiency. The kinetics increase 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 quantity of silane produced at the same time is multiplied by about 15. The process which is the subject of the present invention uses an aprotic solvent of the ether type and preferentially the 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 in large quantities the chlorides of alkaline earth metals and aluminum by virtue of 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 THF. . 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 very high silicone gums. viscous and flammable. This is explained by the fact that the Si-H bond is very labile with pronounced ionic character (Si + - H-). Chlorides including MgCl2, CaCl2, AlCl3, readily catalyze linear or branched polymerization to give H- (SiH2), - H chains. During the implementation of a particular mode of the process which is the subject of the present invention, the HCl solution in THF wipes off 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, remaining at a temperature well below 200 ° C, the risk of formation of chlorosilanes by reaction between light silanes and HCl are very low. Other particularities and advantages will appear on reading the following description, made with reference to FIG.

FIG. 1 represents 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 from a source 5 on the one hand and, from a source 6, an acid-containing solution, such as HCl for example, previously mixed with an aprotic solvent of the ether type, such as THF. The proportions of the mixture are chosen before 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 lid 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 powder. 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 are arranged two gas-tight valves, isolated in series, having in between a lateral tapping for purging the reactor 2 before. disconnection. A similar device 8 equips the outlet 5 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 a 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 by a liquidator. 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 pure monosilane, which is then used for the desired applications. There 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 a 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 can be used 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.

There is also provided the possibility according to one embodiment of the present invention to produce only disilane. The powder of the compound (the silicon alloy) 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 metal filter medium 15 on which 30 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), is injected by gas diffusers 16 above the maximum level 17 provided for the level of solid contained in the frustoconical bottom. The aprotic solvent, such as liquid THF, comes from the condenser (s) 18 overcoming the reactor 2. The THF gas from the crystallizer (s) 10 is injected into the reactor 2, under the condensers 18. In a particular embodiment 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 starts, before encountering the frustoconical bottom, in contact with the HCl and gaseous THF, by forming silanes discharged to the purification system 3 via the pipe 19. The chlorides, such as 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, and then discharged via the pipe 11 and the liquid discharge system 8. These liquid products (CaCl 2, THF, etc. .) then reach the crystallizer system (s) 10. Once the THF solution containing the chlorides (CaCl 2) has passed through the layer of (the) alloy compound resting on the frustoconical bottom 13, said solution is directed to one or several crystallizers operating at more than 170 ° C to evaporate THF and precipitate 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. Due to the liquid flow flowing with the THF from the condensers 18, the unreacted HCI vapors and the dust 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 ° C and 130 ° C. An advantage of THF is to dissolve the silicone polymers that may form 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 (10)

REVENDICATIONS1. A process for preparing a compound or a mixture of compounds of the formula SinH2, -, 2 wherein n is an integer greater than or equal to 1 and less than or equal to 3, comprising a reaction step a) of at least a silicide or silicon alloy selected from alloys of formula CaMgSi and Ca7Mg6Siu, with hydrochloric acid previously dissolved in a solvent. 2. Method according to claim 1, characterized in that said solvent is tetrahydrofuran (THF). 3. Method according to one of claims 1 and 2, characterized in that the silicon alloy has the formula CaMgSi. 4. Method according to one of claims 1 and 2, characterized in that the silicon alloy has the formula Ca7Mg6Sii4. 5. Method according to one of claims 1 to 4, characterized in that step a) is conducted at a temperature between 20 ° C and 100 ° C and at a pressure between 1 bar and 10 bar. 6. Method according to one of claims 1 to 5, 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. mm. 7. Method according to one of claims 1 to 6, further comprising a step of recycling said solvent not used in step a), by heating the latter to a temperature above 170 ° C. 30 8. Method according to one of claims 1 to 7, comprising the steps: a) mixing hydrochloric acid with a crown ether such as tetrahydrofuran; b) mixing between said silicon alloy and the mixture derived from from step a ') c') fractional distillation at a pressure near ambient pressure for separating monosilane and disilane from higher silanes and other volatile compounds. 9. Process according to one of the preceding claims for the preparation of a gaseous mixture comprising up to 90% of monosilane and at least 10% of disilane, preferably comprising 80% of monosilane and 20% of disilane. 10. Process according to one of the preceding claims for the preparation of disilane.