FR2984867A1 - PROCESS FOR THE PHYSICAL SYNTHESIS OF SILICON CARBIDE NANOPOUDERS FOR MAINTAINING THE PHYSICO-CHEMICAL CHARACTERISTICS OF SILICON CARBIDE DURING THE SYNTHESIS - Google Patents

Abstract

The invention relates to a method for synthesizing silicon carbide (SiC) by laser or plasma pyrolysis by means of a reaction mixture comprising acetylene, acetone and silane. The reaction mixture is controlled so that the proportions of acetylene, acetone, and silane of said reaction mixture are kept constant during synthesis.

Description

A method of physically synthesizing silicon carbide nanopowders to maintain the physicochemical characteristics of silicon carbide during synthesis.

The invention relates to the field of physical synthesis processes of SiC silicon carbide nanopowders and more particularly laser or plasma pyrolysis synthesis methods. Naturally occurring silicon Si nanoparticles have an S102 oxide layer on the surface. By quantum confinement, these particles comprising a silicon core whose size is smaller or of the order of 10 nm exhibit photoluminescence properties. The photoluminescence attributed to the phenomenon of quantum confinement is observed when the structuring of silicon is reduced to the nanoscale, for a particle size of less than 10 nm. The photoluminescence wavelength is shorter as the size of the silicon core is small. Thus, silicon in the form of nanopowder is photoluminescent and can be used in very diverse fields such as silicon lasers, biological markers or the detection of counterfeiting by means of optical bar code. We then understand the advantages of producing nanoparticles as small as possible. Unfortunately, the growth of devices based on nanoparticles has slowed down due to the lack of availability of this type of material. Several types of synthesis have been explored in which one can evoke different techniques for developing deposition of thin layers of silicon-containing materials such as the implantation of silicon in an SiO 2 matrix followed by annealing. Plasma assisted chemical vapor deposition (PECVD) techniques have been developed but also liquid phase syntheses. These are often long techniques to implement and whose yields are relatively low. Physical synthesis methods make it possible to obtain free nanoparticles. These methods include laser ablation, thermal pyrolysis, laser or plasma pyrolysis. This technique has given very encouraging results both in terms of the quantities produced and in terms of the quality and reproducibility of the nanoparticles produced. Since the 80s, laser pyrolysis is used to synthesize nanopowders, it is a very flexible and effective method. Among the interesting characteristics, it is possible to introduce the reactants in gaseous, liquid or liquid and gaseous form so that the chemical composition of the nanopowders thus obtained can be multi-element. Since the beginning of the use of laser or plasma pyrolysis, different types of nanoparticles have been produced such as: Si, SiC, Si3N4, Si / C / N, Si / C / O, SiO2, B4C, TiO2, fullerenes or carbonaceous soot.

Laser pyrolysis is based on the resonance between the emission spectrum of a CO2 laser and the absorption spectrum of a reagent present in the medium. By collisions of the molecules with each other, the energy of the laser is transmitted to the medium which sees its temperature rise very rapidly, this rise in temperature leads to the thermal decomposition of the precursors. There is an emission of a black body called "pyrolysis flame". In this flame, the nanoparticles are formed by a so-called coalescence-growth mechanism. At the end of the flame, the nanoparticles are cooled, which stops their growth. They are transported out of the reaction zone by a flow and collected on filter barriers. According to the work described in 'Photoluminescence properties of silicon nanocrystals as a function of their size', G. Ladoux, O. Guilloix, D. Porterat, C.Reynaud, F.Huisken, B. Kohn, V. Paillard, Phys. Rev. B, vol. 62 (23) p. 15942-51 (2000), a pyrolysis of SiH4 silane by a pulsed laser of high power and of CO2 laser source makes it possible to obtain nanoparticles between 2.8 nm and 4.8 nm emitting an intense photoluminescence in the field of wavelength between 610 and 900 nm after passivation in air. However, the low yields obtained make it difficult to move to the industrial production stage. A first improvement described in the document `Process for preparing macroscopic quantities of brightly photoluminescent silicon nanoparticles with omission spanning the visible Spectrum ', X. Li, Y. He, S.

Talukdar, M. Swihart, Langmuir 19, p. 8490-8496 (2003) makes it possible to increase the speed of production up to 200 mg / hour. A second improvement described in an international patent application WO2008 / 152272 makes it possible to obtain a larger quantity of nanoparticles with a size of less than or equal to 10 nm in a single step.

The document describes a method of synthesis by laser pyrolysis of silicon-containing nanoparticles employing several steps. In a first step, a precursor comprising the silicon element is conveyed by a transport fluid to a pyrolysis reactor. In a second step, laser radiation, controlled at least in power, is applied to the reactor to a mixture composed of the transport fluid and the precursor. In a third step, nanoparticles containing silicon are recovered at the outlet of the reactor. This method is intended to substantially increase the rate of production while controlling the size of the nanoparticles obtained by varying the power of the laser and the percentage of effective pulse duration. But, increasing the energy input of the laser makes it possible to increase the speed of production, which increases the size of the particles formed. It is therefore essential to control certain synthesis parameters by laser pyrolysis in order to increase the speed of production while maintaining the physico-chemical characteristics, especially a particle size of less than 10 nm. In WO2008 / 152272, the dilution of the precursor comprising the silicon element is controlled so as to regulate the number of impact between the silicon atoms. By increasing the dilution factor of the precursor, the flow rate of the transport stream and the rate of passage through the reactor are increased. This limits the reaction time and inhibits the growth of the particles formed. It is also interesting to define a geometrical zone of interaction between the laser beam and the smallest possible precursor so that the growth time of the particles is as small as possible. Another way of limiting the interaction time between the precursor and the laser beam may be to control the flow rate of the transport fluid which makes it possible to set a rate of passage through the reactor.

The pressure of the mixture, composed of the silicon precursor and the neutral gas, is kept below the atmospheric pressure so as to inhibit the germination and collisions of the molecules responsible for the growth of the nanoparticles. In the case of the synthesis of silicon carbide SiC, one of the most used mixtures is thus the mixture of silane SiH 4 and acetylene C2H 2. As used in the present invention, it is known, as shown in FIG. 1, according to the aforementioned document, to choose to use a laser capable of operating at high power, for example a CO2 laser source capable of delivering 5 kW in continuous mode or in pulsed mode. The laser source SL sends a large beam, which is focused or not by means of a FOC focuser so as to obtain a thin beam LAS and to limit the REAC reaction zone. The silicon precursor, preferably SiH.sub.4 silane, is mixed with acetylene C2H.sub.2. The mixture is controlled by two valves V1 and V2. The mixture thus formed is directed to a reaction chamber CR at a pressure of between 850 and 900 mbar. Finally, the nP SiC nanoparticles formed are then recovered in a COLL collector comprising FILT metal filters. The SiC nanoparticles so formed are then characterized using various conventional analytical methods such as plasma torch spectroscopy (ICP-AES), luminescent discharge spectroscopy (SDL), X-ray diffraction spectroscopy or microscopy observations. These analytical methods are used to determine the stoichiometry, size, compactness and color of the produced nanopowders, for example. The laser pyrolysis existing since the 1980s, the influence of certain synthesis parameters has been extensively studied. Numerous articles refer to the synthesis of SiC from different gaseous or liquid precursors as inputs of silicon (SiH4, SiH2Cl2, CH3SiCl3, HMDS, diethoxydimethylsilane) and carbon (CH4, C2H2, C2H4). In the state of the art, the best results having been obtained are those using the reagents SiH4 and C2H2. However, production rates are relatively low, between 2 and 100 g / h. According to the work described in the development of nanostructured ceramics in silicon carbide: from the synthesis of powder to sintered ceramic ", A. Réau, University Paris-Sud thesis defended on December 19, 2008, a pyrolysis of a mixture silane SiH4 and acetylene C2H2 by a high power pulsed CO2 laser makes it possible to obtain SIC nanoparticles of between 15 nm and 90 nm at production rates of greater than 1.1 kg / h. , 1 kg / h, the durations of the continuous productions do not exceed the half hour The limiting parameters evoked by A. Réau, page 69 to 71, are: the power density of the laser, the speed of injection and the This work has confirmed that all these parameters have a strong influence on the quality of the powders produced, other parameters are mentioned as having an importance on the result: the size of the laser beam , the injection pressure ion of the reagents, the amount of argon injected. Despite a very extensive bibliography and a large number of patents, there is no study that deals with the influence of the duration of syntheses on the quality of powders. At the stage of production on an industrial scale, the nanoparticles synthesized according to the prior art have different physico-chemical characteristics, in particular a change in the color of the SiC nanopowders between the beginning and the end of the synthesis. The color of the nanopowders is representative of a set of characteristics such as grain size, compactness, crystallinity and finally the chemical composition. An object of the invention is to maintain during nanopowder production of SiC on a large scale the physico-chemical characteristics of the nanopowders throughout the synthesis. According to one aspect of the invention, there is provided a process for synthesizing silicon carbide (SiC) by laser or plasma pyrolysis by means of a reaction mixture comprising acetylene, acetone and silane characterized by controlling said reaction mixture so that the proportions of acetylene, acetone and silane of said reaction mixture are kept constant during the synthesis. The constant maintenance of the proportions of the reaction mixture makes it possible to preserve the physico-chemical characteristics of the silicon carbide SiC during the synthesis.

The invention will be better understood in the study of some embodiments described by way of non-limiting examples, and illustrated by the accompanying drawings in which: - Figures 1 and 2 show a simplified block diagram of a pyrolysis type installation laser, according to the prior art - FIG. 3 represents the evolution of a colorimetric component X as a function of the synthesis time, according to the prior art - FIG. 4 represents a simplified block diagram of a laser pyrolysis type installation, according to one aspect of the invention, FIG. 5 shows the evolution of a colorimetric component X as a function of the synthesis time, according to one aspect of the invention. FIG. 2 illustrates the simplified general diagram of a laser pyrolysis type installation, according to the prior art.

For example, for the synthesis of silicon carbide SiC one of the most used mixtures is the silane mixture SiH 4, as a precursor of silicon mixed with a transport fluid, usually a neutral gas, in this case argon, and acetylene C2H2, as precursor of carbon atoms. The silane reactant SiH4 is introduced in gaseous form. The flow rate of the silane is regulated by a valve V1 and the silane SiH4 is conveyed to a reaction chamber CR, at a pressure P maintained below atmospheric pressure, by means of a feed duct C1. The acetylene reagent C2H2 is introduced in gaseous form, the flow rate acetylene C2H2 is regulated by a valve V2.

Acetylene C2H2 is conveyed by means of a conduit C2 to the reactor chamber CR at a controlled pressure P below atmospheric pressure.

Inside the reaction chamber CR, gaseous precursors, for example silane SiH4 and acetylene C2H2 interact with the laser beam. The emission spectrum of the laser resonates with the absorption spectrum of at least one of the precursors. Energy transfer is effected by excitation of the vibrational levels of the molecules that absorb the energy of the laser beam. The energy is then transmitted to the entire reaction medium. During this process, a flame appears in which SiC nanoparticles form. At the flame outlet, the nanoparticles undergo a quenching effect that inhibits their growth. The nanoparticles formed are directed towards a collector COLL of nanopowders. FIG. 3 represents a diagram of the evolution of a colorimetric parameter according to a component as a function of the production time of the nanopowder. The SiC nanopowders formed from the laser pyrolysis synthesis process described in the prior art are characterized by means of a SI-COLO-30-DIL colorimeter from Sensor Instrument (trade mark). The apparatus sends a white light to a surface of the powder to be studied, the reflected light is sensed and directed to a color sensitive receiver. The light received is separated into three colorimetric components, the red according to the component X, the green according to the component Y and the blue according to the component Z. The color of the nanopowders is representative of a degree of purity of the powder. The characterization of the SiC nanopowders is done according to the X component. FIG. 3 shows that the X component increases over the synthesis time, which means that the color of the powder is changing. Usually, the powders obtained by laser pyrolysis are analyzed after synthesis by standard analysis techniques such as plasma torch spectroscopy (ICP-AES), glow discharge spectroscopy (SDL), X-ray diffraction spectroscopy or observations. electron microscopy (MEB, MET). These different techniques have not made it possible to demonstrate an evolution of the physicochemical characteristics of the powders with respect to their stoichiometry, their compactness or their size, the variations being too weak to be detected by these techniques. Strangely, the traditional analyzes of the powders obtained in large quantities by laser pyrolysis do not show evolution of the physico-chemical characteristics of the SiC nanopowders as a function of time, yet the colorimetric analysis shows an evolution of the optical properties of the SiC nanopowders. during the duration of the production. After much work, it has been established that this color variation demonstrated using the colorimetric method is independent of standard parameters that usually affect the physicochemical characteristics of the powders such as the laser power, the flow rate of the transport fluid, the interaction time between the laser beam and the reagents. Research has shown the presence of acetone in the acetylene reagent C2H2. In fact, the conditioning of acetylene in a pressurized bottle requires the dilution of acetylene in a solvent, usually acetone. Moreover, analyzes have shown that the proportion of acetone with a constant flow rate of acetylene varies during the lifetime of the acetylene bottle. In other words, for a constant acetylene flow rate, the proportion of acetone at the end of use of the acetylene bottle is greater than the proportion of acetone at the beginning of use of acetylene, typically the proportion of acetone increases by 0.4% between the beginning and the end of the use of the bottle.

FIG. 4 illustrates the simplified general diagram of a laser pyrolysis type installation, according to one aspect of the invention. The silane reactant SiH4 is introduced in gaseous form. The flow rate of the silane is regulated by a valve VI and the silane is conveyed to a reaction chamber CR at a controlled pressure P below atmospheric pressure by means of a feed pipe C1. The acetylene reactant is introduced in the form gas. The acetylene C2I-12 is conveyed by means of a conduit C2 to the reactor chamber CR at a controlled pressure P below atmospheric pressure. A Pure Ac purifier from Air Liquide (registered trademark) is introduced into the acetylene supply pipe C2. The purifier must be positioned upstream of the control valve V2 and downstream of the acetylene cylinder C2H2. Acetone present in the mixture of acetylene and acetone is removed. Thus, the ratio of the proportion of acetone to acetylene is kept constant. Inside the reaction chamber, gaseous precursors, in this case silane SiH4 and acetylene C2H2, interact with the laser beam. The emission spectrum of the laser resonates with the absorption spectrum of at least one of the precursors. Energy transfer occurs by excitation of the vibrational levels of the molecules that absorb energy from the laser beam. The energy is then transmitted to the entire reaction medium. During this process, a flame appears in which nanoparticles form. At the flame outlet, the nanoparticles undergo a quenching effect that inhibits their growth. The formed nanoparticles are directed to a nanopowder collector 2.

FIG. 5 represents a diagram of the evolution of the X component of the light received by the colorimeter detector as a function of the production time of the nanopowders, according to one aspect of the invention. The nanopowders formed from the laser pyrolysis synthesis process according to one aspect of the invention are characterized by means of a SI-COLO-30-DIL colorimeter from Sensor Instrument (trade mark). The device sends a white light to the surface of the powder to be studied, the reflected light is sensed and directed to a color-sensitive receiver. The received light is separated into three color components red according to the component X, green according to the component Y and blue according to the component Z. The characterization of nanopowder SiC is done according to the component X. Figure 5 shows that the component X is constant during the synthesis time, which means that the color of the powder is constant. However, the color of the nanopowders is representative of a set of characteristics of a powder such as grain size, compactness, crystallinity and finally the chemical composition. The production on an industrial scale of the SiC powder has revealed variations in the physico-chemical properties and in particular the color, representative of the physico-chemical characteristics, of the nanopowders produced over time. It is the identification of the source of these color variations, following numerous investigations, which made it possible to produce SiC nanopowders on an industrial scale, while maintaining the proportions of acetylene and acetone in the reaction mixture during the synthesis.

Claims (5)

REVENDICATIONS1. A process for synthesizing silicon carbide (SiC) by laser or plasma pyrolysis using a reaction mixture comprising acetylene, acetone and silane characterized in that said reaction mixture is regulated so that the proportions of acetylene, acetone and silane of said reaction mixture are kept constant during the synthesis. 10 2. The process according to claim 1, wherein the proportion of acetylene is kept constant over time with respect to the proportion of silane by modifying respective acetylene and acetone flow rate instructions during the synthesis. 15 The process of claim 1 wherein the acetone is removed from said reaction mixture. 4. The method of claim 3 wherein the acetone is removed by means of a purifier. 20 5. System for synthesizing silicon carbide (SiC) by laser or plasma pyrolysis, comprising means for injecting a reaction mixture comprising acetylene, acetone and silane, characterized in that it comprises means for controlling said reaction mixture adapted to maintain the proportions of acetylene, acetone and silane of said reaction mixture constant during synthesis.