Title of Invention	PROCESS AND APPARATUS FOR SYNTHESIZING A NANOPOWDER
Abstract	A process and apparatus for synthesizing a nanopowder is presented. In particular, a process for the synthesis of nanopowders of various materials such as metals, alloys, ceramics and composites by induction plasma technology, using organometallic compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates and carbonates as precursors is disclosed. The process comprises feeding a reactant material into a plasma torch in which is generated a plasma flow having a temperature sufficiently high to yield a superheated vapour of the material; transporting said vapour by means of the plasma flow into a quenching zone; injecting a cold quench gas into the plasma flow in the quenching zone to form a renewable gaseous cold front; and forming a nanopowder at the interface between the renewable gaseous cold front and the plasma flow.

Title of Invention

PROCESS AND APPARATUS FOR SYNTHESIZING A NANOPOWDER

Abstract

A process and apparatus for synthesizing a nanopowder is presented. In particular, a process for the synthesis of nanopowders of various materials such as metals, alloys, ceramics and composites by induction plasma technology, using organometallic compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates and carbonates as precursors is disclosed. The process comprises feeding a reactant material into a plasma torch in which is generated a plasma flow having a temperature sufficiently high to yield a superheated vapour of the material; transporting said vapour by means of the plasma flow into a quenching zone; injecting a cold quench gas into the plasma flow in the quenching zone to form a renewable gaseous cold front; and forming a nanopowder at the interface between the renewable gaseous cold front and the plasma flow.

Full Text	WO 2006/079213 PCT/CA2006/000110 1 TITLE OF THE INVENTION INDUCTION PLASMA SYNTHESIS OF NANOPOWDERS FIELD OF THE INVENTION [0001] The present invention relates to the plasma synthesis of nanopowders using induction plasma technology. More specifically, but not exclusively, the present invention relates to a process for the synthesis of nanopowders of various materials such as metals, alloys, ceramics and composites by induction plasma technology, using organometallic compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates and carbonates as precursors. BACKGROUND OF THE INVENTION [0002] The plasma synthesis of nanopowders has attracted ncreasing attention over the past few years. Numerous processes have been developed for preparing metal, alloy and ceramic-based nanopowders using a wide variety of technologies including plasma discharge, arc discharge, electro-explosion, self propagating high temperature synthesis, combustion synthesis, electric discharge, spray pyrolysis, sol-gel, and mechanical attrition. [0003] High temperature plasma processing routes are based on the concept of heating the reactant precursors (in solid, liquid or vapor/gaseous form), to relatively high temperatures followed by rapid cooling of the reaction products through their mixing with a cold gas stream as in the "high intensity turbulent quench technique" or through their contacting with a cold surface on which the nanoparticles form and deposit. The use of a "highly turbulent gas quench zone" has been previously described by Boulos etal. in U.S. 20050217421 and U.S. 20030143153 as filed on March 25, 2005 WO 2006/079213 PCT/CA2006/000110 2 and December 6, 2002 respectively. A common objective to all of these processes is the desire to closely control the particle morphology, the particle size distribution, and the agglomeration of the powders obtained. However, a drawback of the use of traditional "cold-surface" condensation techniques is that the nature and the temperature of the condensation surface changes with the build-up of the condensed nanopowder layer. [0004] U.S. Patent 6,379,419 issued to Celik et al. on April 30, 2002 discloses a transferred arc thermal plasma based vapor condensation method for the production of fine and ultra fine powders. The method calls upon a condensation procedure involving an indirect cooling step and a direct cooling step. The indirect cooling step involves a cooling surface whereas the direct cooling step involves the injection of a cooling gas directly onto the vapor. The use of a cooling surface suffers from the drawback of particle build-up on the condensation surface. [0005] It has been shown theoretically that by controlling the initial vapor concentration and temperature, residence time of particle nucleation and growth, and cooling profile, one may have some control on the particle size distribution and crystallinity. This is shown by Okuyama et al. in AlChE Journal, 1986, 32 (12), 2010-2019 and Girshick et al. in Plasma Chem. and Plasma Processing, 1989, 9 (3), 355-369. However, these references remain silent as to an efficient method for producing nanopowders of well defined particle size distribution and morphology. [0006] There remains a need for an improved process for the preparation of nanopowders in which the particle morphology, the particle size distribution, and the agglomeration of particles is readily controlled and that it easily scalable. WO 2006/079213 PCT/CA2006/000110 3 [0007] The present invention seeks to meet these and other needs. [0008] The present invention refers to a number of documents, the content of which is herein incorporated by reference in their entirety. SUMMARY OF THE INVENTION [0009] The present invention relates to the plasma synthesis of powders calling upon an induction plasma apparatus comprising an induction plasma torch and a quenching chamber in which a renewable "gaseous cold front" is generated, through the injection of a quench gas, and on which the gaseous reactants/reaction products nucleate. The nucleation generates a nanopowder which is rapidly transported to a collection chamber by the moving cold front. It was surprisingly discovered that by generating a renewable gaseous cold front for nucleating (i.e. condensing) the reactants/reaction products present in the plasma flow, excellent control of the morphology and particle size distribution of the resulting nanopowder could be achieved. Moreover, the use of the renewable gaseous cold front offers a close control over particle agglomeration. [0010] More specifically, as broadly claimed, the present invention relates to a process for synthesizing a nanopowder comprising feeding a reactant material into a plasma torch in which is generated a plasma flow having a temperature sufficiently high to yield a superheated vapour of the material; transporting the vapour by means of the plasma flow into a quenching zone; injecting a cold quench gas into the plasma flow in the quenching zone to form a renewable gaseous cold front; and forming a nanopowder at the interface between the renewable gaseous cold front and the plasma flow. WO 2006/079213 PCT/CA2006/000110 4 [0011] The present invention also relates to an apparatus for synthesizing nanopowders comprising a plasma torch to generate a plasma flow and to produce a superheated vapour from reactant material supplied to the plasma torch in the plasma flow; and a quenching chamber mounted to the plasma torch downstream therefrom and in fluid communication with said plasma torch to receive the superheated vapour from the plasma torch, the quenching chamber being structured to receive a quench gas and to generate from said quench gas a renewable gaseous cold front to rapidly cool the superheated vapour, yielding nanopowder. [0012] The foregoing and other objects, advantages and features of the present invention will become more apparent upon reading of the following non restrictive description of illustrative embodiments thereof, given by way of example only with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0013] In the appended drawings: [0014] FIG. 1 is a schematic cross-sectional elevation view of an induction plasma assembly in accordance with the present invention; [0015] FIG. 2 is a schematic cross-sectional elevation view of an induction plasma assembly of FIG. 1 without reactor component; [0016] FIG. 3 is an illustration of the temperature isocontours in the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz] for (3A) a quench gas (Ar) flow rate of 400 slpm and (3B) a quench gas (Ar) flow rate of 800 slpm, respectively; WO 2006/079213 PCT/CA2006/000110 5 [0017] FIG. 4 is an illustration of the stream lines in the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz] for (4A) a quench gas (Ar) flow rate of 400 slpm and (4B) a quench gas (Ar) flow rate of 800 slpm, respectively; [0018] FIG. 5 is graph illustrating temperature profiles along the centerline of the induction plasma torch/reactor sections (5A, 5B) and in the radial direction at different axial locations within the reactor/quench sections of the induction plasma assembly of FIG.1 (5C, 5D) using an Ar/H2 plasma gas [65kW; 3MHz] for a quench gas (Ar) flow rate of 400 slpm (5A, 5C) and a quench gas (Ar) flow rate of 800 slpm, respectively (5B, 5D); [0019] FIG. 6 is an illustration of the "gaseous cold front" showing the quench gas concentration isocontours in the quench section of the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz] for (6A) a quench gas (Ar) flow rate of 400 slpm and (6B) a quench gas (Ar) flow rate of 800 slpm, respectively; [0020] FIG. 7 is an illustration of the reaction product cooling rate isocontours (darker areas are representative of cooling rates in the range of 105 to 106 K/s) in the quench section of the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz] for (7A) a quench gas (Ar) flow rate of 400 slpm and (7B) a quench gas (Ar) flow rate of 800 slpm, respectively; [0021] FIG. 8 is a graph showing the particle size distribution for an aluminum nanopowder as obtained using the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz]; WO 2006/079213 PCT/CA2006/000110 6 [0022] FIG. 9 is a graph showing the particle size distribution for a nickel nanopowder as obtained using the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz]; [0023] FIG. 10 is a graph showing the particle size distribution for a tungsten nanopowder as obtained using the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz]; [0024] FIG. 11 is an illustration of the quench gas flow within the quench section of the induction plasma assembly of FIG.1; [0025] FIG. 12 is a graph showing the mean particle size (12A) and the specific surface area (12B) of a nanometric nickel powder as produced by a process according to the present invention as a function of the quench gas (Ar) flow rate; [0026] FIG. 13 is a graph showing the mean particle size (13A) and the specific surface area (13B) of a nanometric cupric oxide (CuO) powder as produced by the process of the present invention (starting with micrometric-sized copper powder) as a function of the quench gas (Ar/02) flow rate; [0027] FIG. 14 is a graph showing the particle size distribution of a cupric oxide nanopowder (BET = 23.08 m2/g (14A) and 22.11 m2/g (14B) respectively) as produced by the process of the present invention (starting with micrometric-sized copper powder); [0028] FIG. 15: is a graph showing the mean particle size (15A) and the specific surface area (15B) of a nanometric germanium dioxide WO 2006/079213 PCT/CA2006/000110 7 (GeO2) powder as produced by the process of the present invention (starting with liquid GeCI4) as a function of the quench gas (Ar/O2) flow rate. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS [0029] In order to provide a clear and consistent understanding of the terms used in the present specification, a number of definitions are provided below. Moreover, unless defined otherwise, all technical and scientific terms as used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention pertains. [0030] The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and/or the specification may mean "one", but it is also consistent with the meaning of "one or more", "at least one", and "one or more than one". Similarly, the word "another" may mean at least a second or more. [0031] As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "include" and "includes") or "containing" (and any form of containing, such as "contain" and "contains"), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps. [0032] The term "about" is used to indicate that a value includes an inherent variation of error for the device or the method being employed to determine the value. WO 2006/079213 PCT/CA2006/000110 8 [0033] The present invention relates to a novel process for the plasma synthesis of nanopowders using induction plasma technology and calling upon a renewable "gaseous cold front", for example a laminar "gaseous cold front" which serves to rapidly quench the reactants/reaction products present in the plasma flow. According to a first alternative, the gaseous cold front may be generated in a quenching chamber comprising a porous metal or ceramic wall through which is uniformly injected a cold quench gas. According to a second alternative, the gaseous cold front may be generated in a quenching chamber comprising a perforated refractory wall. [0034] FIG. 1 shows an illustrative embodiment of the induction plasma assembly, generally identified by the reference 10. The induction plasma assembly 10 of FIG. 1 comprises an upper section consisting of an inductively coupled radio frequency (rf) plasma torch 12 in which the reactants are substantially axially introduced at the feed upper end of the torch 12 and dispersed into the center of the plasma flow using techniques well known to those of ordinary skill in the art. A reactor 14, mounted to a quenching chamber 16, is affixed to the lower end of the plasma torch 12, generally coaxial with the induction plasma assembly 10 between the plasma torch 12 and the quenching chamber 16. The induction plasma assembly 10 further comprises a collection chamber 18 coaxially mounted to the lower end of the quenching chamber 16. Of course, the plasma torch 12, the reactor 14, the quenching chamber 16 and the collection chamber are in fluid communication with each other. [0035] The plasma is generated using any suitable gas that will ionize when subject to a high frequency electromagnetic field, such as a radio frequency field. It is believed to be within the skill of an expert in the art to select a suitable gas. Suitable inductively coupled radio frequency (rf) plasma torches are disclosed in U.S. Patent No. 6,919,527 issued to Boulos et al. on WO 2006/079213 PCT/CA2006/000110 9 July 19, 2005. Other suitable plasma torches as contemplated by the process of the present invention include high performance induction plasma torches such as the one disclosed in U.S. Patent No. 5,200,595 issued to Boulos et a/, on April 6,1993. [0036] The reactant feed may be in the form of fine solid particles, liquid droplets or in the vapor/gaseous phase. In the case of a solid feed, the reactant, upon entering the plasma flow, is melted and vaporized forming a vapor cloud which is superheated to a reaction temperature. In the case of a liquid feed, the heat load on the plasma is substantially limited to that required to heat up the liquid droplets to the vaporization temperature and superheating the vapor to a reaction temperature. In the case of a gaseous feed, the heat load on the plasma is substantially limited to that required to superheat the gaseous feed to the reaction temperature. The plasma flow axially delivers the vapor cloud to the reactor 14 in which it may be further mixed with other components present in the vapor/gaseous phase. In an embodiment of the present invention, a further component may be an oxidizing agent such as oxygen, a carburizing agent such as methane or acetylene, or a nitrating agent such as ammonia. Of course, the other components may be concomitantly introduced into the plasma torch 12 with the reactant feed, or introduced in the reactor section 14, or in the quench section 16, using techniques well known to those of ordinary skill in the art. [0037] The plasma flow carries the reactants/reaction products emerging from the plasma torch 12 into a reactor 14 in fluid communication therewith. In an embodiment of the present invention, the reactor 14 may be a high temperature graphite/refractory lined reactor. It is believed to be within the skill of an expert in the art to select other suitable reactors and reactor configurations. In the event that the feed was concomitantly introduced into the plasma torch 12 mixed with other components, the reactor 14 allows for WO 2006/079213 PCT/CA2006/000110 10 any reaction to be completed. Alternatively, the superheated feed exits the plasma torch 12 at a reaction temperature and is carried by the plasma flow into the reactor 14 where it is mixed and reacted with other components. [0038] Upon exiting the reactor 14, the reactants/reaction products are carried into a quenching chamber 16. In an embodiment of the present invention, the quenching chamber 16 may comprise a porous metal or ceramic wall section 17 through which a cold quench gas and/or reactants are injected. Alternatively, the quenching chamber 16 may comprise a perforated or slotted refractory wall section through which the cold quench gas and/or reactants may be injected. It is believed to be within the skill of an expert in the art to determine and select other suitable quenching chamber configurations. The injection of the cold quench gas, mixed with or without other reactants, generates a mobile and continuously renewable laminar "gaseous cold front" on which the gaseous reactants/reaction products condense. Such a condensation results in a nucleation of reactants/reaction products generating a nanopowder. The "gaseous cold front" is renewable since it is in movement; the movement being imparted by the continuous injection of cold quench gas and by the movement of the plasma flow. The physical separation of the reaction section (i.e. the plasma torch 12 and/or the reactor 14) and the quench section (quenching chamber 16) provides for a better means of controlling the location of the condensation front over which nanopowder formation occurs. The gaseous cold front is further illustrated in FIGS. 3, 4, 6 and 7. In an embodiment of the present invention, argon was used as the quench gas. It is believed to be within the skill of an expert in the art to determine and select other suitable quench gases. [0039] The quench rate to which the gaseous reactants/reaction products are exposed will depend on the temperature of the quench gas and its flow rate. The flow rate of the quench gas also affects the position of the WO 2006/079213 PCT/CA2006/000110 11 cold front within the quenching chamber 16 and the manner in which the cold front interacts with the hot flow of plasma gases emerging from the reactor 14 and comprising the reactants/reaction products. The nanopowder is carried away on the mobile cold front into the collection chamber 18. Of course, the plasma gases will also be at significantly lower temperatures, due to their interaction with the cold front, upon reaching the collection chamber 18. [0040] The process according to the present invention involves the concept of a renewable "gaseous cold front" as an effective means of controlling the uniformity and particle size distribution of the produced nanopowder. Furthermore, the possibility for condensed nanoparticle agglomeration is significantly reduced since the nanopowder is rapidly evacuated, substantially upon its formation, from the quenching chamber 16 to the collection chamber 18 by the gaseous cold front at present comprised of plasma gases and the quench gas. Moreover, by using a high volume quench gas flow rate, it is possible to maintain the generated nanopowder in dilute suspension while it is being evacuated from the quenching chamber 16 into the collection chamber 18. Additionally, by maintaining a dilute suspension, both the deposition of the nanopowder on the walls of the quenching chamber 16 and nanoparticle agglomeration through inter-particle collision are substantially eliminated. Typical temperature and flow fields as observed in the plasma torch 12, the reactor 14 and the quenching chamber 16 respectively, are illustrated in FIGS. 3 and 4 for quench gas flow rates of 400 and 800 standard liters per minute (slpm). These figures clearly demonstrate the constricting effect of the "gaseous cold front" on the hot plasma gas stream. The more intense the gaseous cold front, the greater the constricting effect. Moreover, as may be observed from FIGS. 3 and 6, an increase in the quench gas flow rate results in a significant increase in the thickness of the cold boundary layer and a gradual displacement of the cold front toward the center of the plasma flow. Furthermore, the increase in the WO 2006/079213 PCT/CA2006/000110 12 thickness of the cold boundary layer is accompanied by the development of steep temperature gradients at the interface of the cold front and the plasma flow, where the nucleation of the reactants/reaction products takes place. While not shown in these figures, changing the length of the reactor, combined with a close control of the quench gas flow rate offers tangible means of controlling the position of the cold front in the reactor assembly and accordingly the exact moment at which the reactants/reaction products are subjected to rapid cooling. In the case wherein the reactor 14 is absent, the reactants/reaction products are substantially immediately exposed to the cold front following their vaporization in the plasma torch 12. In the illustrative embodiment as shown in FIGS. 3 and 4, the plasma gas was composed of an argon/hydrogen mixture (80%vol /20%vol) whereas the gaseous cold front was created using argon which was injected through the porous metal wall of the quenching chamber 16. [0041] A typical "gaseous cold front" and the associated quench gas concentration isocontours as observed in the quenching chamber 16 is illustrated in FIG. 6. Furthermore, typical isocontours of reaction product cooling rate, as produced when the hot plasma gases comprising the reaction products collide with the gaseous cold front, are illustrated in FIG. 7. These figures illustrate a substantially uniform, renewable and mobile cooling gas front across which the reaction products are exposed to high cooling rates in the order of about 105to about 106 K/s. Such a gaseous cold front allows for particle nucleation to occur at the interface with the hot plasma gases and the formation of a nanopowder having a substantially uniform particle size distribution, representative examples of which are illustrated in FIGS. 8-10. [0042] The process of the present invention offers the additional advantages of being compact, scalable and simple to operate. Moreover, the reactor assembly 10 can be readily modified depending on the needs and WO 2006/079213 PCT/CA2006/000110 13 type of nanopowder to be produced. Vapor, liquid or solid precursors may be used in the process according to the present invention with the produced nanopowder being either of identical or different chemical composition as that of the precursor. In the case wherein the produced nanopowder is of identical chemical composition, the process is limited to the vaporization and condensation of the feed generating nanoparticles thereof. In the case wherein the produced nanopowder is of different chemical composition, the feed was reacted with a second reactant which can be either injected in the plasma torch 12 or introduced in the reactor 14. Alternatively, the quench gas may react with the vaporized feed in which case it serves a dual function. As illustrated in FIG. 2, the reactor 14 can be removed such that the plasma feed comprising the gaseous reactants or alternatively the gaseous reactants/reaction products, are brought directly into the quenching chamber 16. If the feed material is to be chemically modified, a non-limiting example of which is oxidation, the presence of the reactor might be required. It is believed to be within the skill of an expert to select an appropriate reactor assembly depending on the type of nanopowder to be produced. WO 2006/079213 PCT/CA2006/000110 14 EXPERIMENTAL [0043] A number of examples are provided hereinbelow, illustrating the efficiency of a renewable laminar gaseous cold front in the synthesis of a number of metallic and ceramic nanopowders using induction plasma technology. [0044] Synthesis of nanopowders using solid precursors. [0045] An argon/hydrogen induction plasma flow is generated using a 50 mm internal diameter induction plasma torch with an oscillator frequency of 3 MHz, a plasma plate power of 65 kW, and a reactor pressure of approximately 500 Torr. Different metals, in the form of a micron-sized metallic powder, were axially injected into the center of the plasma discharge and vaporized. As the hot plasma gases comprising the vaporized metal emerge from the reactor, they are intercepted by the gaseous cold front which is created by means of injecting argon through the porous walls of the quenching chamber. The interaction of the hot plasma gases with the gaseous cold front gives rise to a nucleation phenomenon at the interface hot plasma gases/gaseous cold front, leading to the formation of a nanopowder having a well defined particle size distribution. The continuously moving gaseous cold front, now also made-up of cooled plasma gases, rapidly evacuates the nanopowder to a collection chamber where it can be collected on standard sintered metal or cloth filter elements. The experimental results as obtained for aluminum, nickel and tungsten powders are summarized hereinbelow in Table 1. Data regarding plasma plate power, precursor feed rates, quench gas rates, and physical characteristics of the collected nanopowder such as their Specific Surface Area (as measured using the BET method) and particle mean diameter are also provided. The Specific Surface Area is expressed in terms of m2/g of collected powder. The mean particle WO 2006/079213 PCT/CA2006/000110 15 diameter of the collected powder can be calculated assuming a spherical particle shape having an equivalent surface area to volume ratio. The particle size distributions, obtained by light scattering analysis using a Malvern Mastersizer™ instrument are illustrated in FIGS. 8-10 respectively. [0046] Table 1: Examples of metallic nanopowders as obtained using the process of the present invention. [0047] Synthesis of nanopowders using liquid precursors. [0048] As illustrated in FIG. 15 and in Table 2 below, the process according to the present invention lends itself to the synthesis of nanopowders using liquid feeds. Liquid germanium tetrachloride (GeCU4, b.p. = 83 °C) was sent by a Masterflex™ pump into an oxygen plasma. The liquid was vaporized and oxidized. The produced germanium dioxide (GeO2) vapor was condensed to nanometric powders by injection of cold quench gas flow generating the "gaseous cold front". The process as occurring in Fig. 15 can be illustrated by the following reaction: WO 2006/079213 PCT/CA2006/000110 16 [0049] Table 2: Examples of process parameters* for producing GeO2 (g) starting with GeCI4 (I). *Plasma power: 65 kW; Plasma sheath: 18 Ar + 82 O2 (slpm); Plasma central: 30Ar (slpm); Injection probe: SA953; Atomization gas: 8O2 (slpm); Reactor pressure: 80 kPa. [0050] Synthesis of nanopowders using gaseous precursors. [0051] The process of the present invention also lends itself to the synthesis of nanopowders using gaseous feeds as illustrated by the following reaction: 4BCI3(g) + CH4(g) + 4H2 >- B4C(s)+12 HCI (g) [0052] It is to be understood that the invention is not limited in its application to the details of construction and parts as described hereinabove. The invention is capable of other embodiments and of being practiced in various ways. It is also understood that the phraseology or terminology used herein is for the purpose of description and not limitation. Hence, although the present invention has been described hereinabove by way of illustrative embodiments thereof, it can be modified, without departing from the spirit, scope and nature of the subject invention as defined in the appended claims. I7 WHAT IS CLAIMED IS: 1. A process for synthesizing a nanopowder comprising: a) feeding a reactant material into a plasma torch in which is generated a plasma flow having a temperature sufficiently high to yield a superheated vapour of said material; b) transporting said vapour by means of said plasma flow into a quenching zone; c) injecting a cold quench gas into the plasma flow in the quenching zone to form a renewable laminar gaseous cold front; and d) forming a nanopowder at the interface between the renewable laminar gaseous cold front and the plasma flow. 2. The process of claim 1 further comprising collecting the nanopowder in a collection zone. 3. The process of claim 2, wherein the gaseous cold front exerts a constricting effect on the plasma flow. 4. The process of claim 3, wherein the constricting effect is proportional to the quench gas flow rate. 5. The process of claim 4, comprising injecting the cold quench gas in the quenching zone by means of a plurality of openings in a wall section of said quenching zone. 6. The process of claim 5, wherein the plurality of openings define a porous wall section. 7. The process of claim 5, wherein the plurality of openings define a slotted wall section. 8. The process of claim 5, wherein the plurality of openings define a perforated wall section. 9. The process of any one of claims 5, 6, 7 or 8, wherein the quenching zone is a quenching chamber. 10. The process of claim 1, wherein the reactant material is selected from the group consisting of solid, liquid and gaseous feeds. 11. The process of claim 1, wherein said superheated vapour is at a reaction temperature capable of reacting with said plasma flow and/or said quench gas. 12. The process of claim 10, wherein the reactant material is selected from the group consisting of metals, alloys, organometallic compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates, carbonates, oxides and composites. 13. The process of claim 1, further comprising: e) feeding a second reactant in the plasma flow; and f) reacting the second reactant with the reactant material to produce a nanopowder of chemical composition different from the reactant material. 14. The process of claim 13, comprising injecting the second reactant into the plasma torch. 15. The process of claim 13, comprising injecting the second reactant into a reactor between the plasma torch and the quenching zone. 16. The process of claim 13, wherein the second reactant is the quench gas. 17. An apparatus for synthesizing nanopowders comprising: a) a plasma torch to generate a plasma flow and to produce a superheated vapour from reactant material supplied to the plasma torch in the plasma flow; and b) a quenching chamber mounted to the plasma torch downstream therefrom and in fluid communication with said plasma torch to receive the superheated vapour from the plasma torch, said quenching chamber being structured to receive a quench gas and to generate from said quench gas a renewable laminar gaseous cold front to rapidly cool the superheated vapour, yielding nanopowder. 18. The apparatus of claim 17, further comprising a collection chamber to collect the nanopowder. 19. The apparatus of claim 17, wherein the gaseous cold front exerts a constricting effect on the plasma flow. 20. The apparatus of claim 19, wherein the constricting effect is proportional to the quench gas flow rate. 21. The apparatus claim 17, wherein the quenching chamber comprises a wall section with a plurality of openings for injecting the quench gas in the quenching chamber. 22. The apparatus of claim 21, wherein the wall section is a porous wall section. 23. The apparatus of claim 21, wherein the wall section is a slotted wall section. 24. The apparatus of claim 21, wherein the wall section is a perforated wall section. 25. The apparatus of claim 17, wherein the reactant material is selected from the group consisting of solid, liquid and gaseous feeds. 26. The apparatus of claim 17, wherein said superheated vapour is at a reaction temperature capable of reacting with said plasma flow and/or said quench gas. 27. The apparatus of claim 17, wherein the reactant material is selected from the group consisting of metals, alloys, organometallic compounds, 20 chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxaiates, carbonates, oxides and composites. 28. The apparatus of claim 17, further comprising: means for feeding a second reactant in the plasma flow: and means for reacting the second reactant with the reactant material to produce a nanopowder of chemical composition different from the reactant material. 29. The apparatus of claim 28, comprising means for injecting the second reactant into the plasma torch. 30. The apparatus of claim 28, comprising means for injecting the second reactant into a reactor between the plasma torch and the quenching zone. 31. The apparatus of claim 28, wherein the second reactant is the quench gas. 32. The apparatus of claim 17, further comprising a reactor to react the reactant material with a second reactant, said reactor being in fluid communication with the plasma torch and the quenching chamber, and said reactor being disposed between the plasma torch and the quenching chamber. A process and apparatus for synthesizing a nanopowder is presented. In particular, a process for the synthesis of nanopowders of various materials such as metals, alloys, ceramics and composites by induction plasma technology, using organometallic compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates and carbonates as precursors is disclosed. The process comprises feeding a reactant material into a plasma torch in which is generated a plasma flow having a temperature sufficiently high to yield a superheated vapour of the material; transporting said vapour by means of the plasma flow into a quenching zone; injecting a cold quench gas into the plasma flow in the quenching zone to form a renewable gaseous cold front; and forming a nanopowder at the interface between the renewable gaseous cold front and the plasma flow.

Full Text

WO 2006/079213 PCT/CA2006/000110
1
TITLE OF THE INVENTION
INDUCTION PLASMA SYNTHESIS OF NANOPOWDERS
FIELD OF THE INVENTION
[0001] The present invention relates to the plasma synthesis of
nanopowders using induction plasma technology. More specifically, but not
exclusively, the present invention relates to a process for the synthesis of
nanopowders of various materials such as metals, alloys, ceramics and
composites by induction plasma technology, using organometallic
compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates
and carbonates as precursors.
BACKGROUND OF THE INVENTION
[0002] The plasma synthesis of nanopowders has attracted
ncreasing attention over the past few years. Numerous processes have
been developed for preparing metal, alloy and ceramic-based nanopowders
using a wide variety of technologies including plasma discharge, arc
discharge, electro-explosion, self propagating high temperature synthesis,
combustion synthesis, electric discharge, spray pyrolysis, sol-gel, and
mechanical attrition.
[0003] High temperature plasma processing routes are based on
the concept of heating the reactant precursors (in solid, liquid or
vapor/gaseous form), to relatively high temperatures followed by rapid cooling
of the reaction products through their mixing with a cold gas stream as in the
"high intensity turbulent quench technique" or through their contacting with a
cold surface on which the nanoparticles form and deposit. The use of a
"highly turbulent gas quench zone" has been previously described by Boulos
etal. in U.S. 20050217421 and U.S. 20030143153 as filed on March 25, 2005

WO 2006/079213 PCT/CA2006/000110
2
and December 6, 2002 respectively. A common objective to all of these
processes is the desire to closely control the particle morphology, the particle
size distribution, and the agglomeration of the powders obtained. However, a
drawback of the use of traditional "cold-surface" condensation techniques is
that the nature and the temperature of the condensation surface changes with
the build-up of the condensed nanopowder layer.
[0004] U.S. Patent 6,379,419 issued to Celik et al. on April 30,
2002 discloses a transferred arc thermal plasma based vapor condensation
method for the production of fine and ultra fine powders. The method calls
upon a condensation procedure involving an indirect cooling step and a direct
cooling step. The indirect cooling step involves a cooling surface whereas the
direct cooling step involves the injection of a cooling gas directly onto the
vapor. The use of a cooling surface suffers from the drawback of particle
build-up on the condensation surface.
[0005] It has been shown theoretically that by controlling the initial
vapor concentration and temperature, residence time of particle nucleation
and growth, and cooling profile, one may have some control on the particle
size distribution and crystallinity. This is shown by Okuyama et al. in AlChE
Journal, 1986, 32 (12), 2010-2019 and Girshick et al. in Plasma Chem. and
Plasma Processing, 1989, 9 (3), 355-369. However, these references remain
silent as to an efficient method for producing nanopowders of well defined
particle size distribution and morphology.
[0006] There remains a need for an improved process for the
preparation of nanopowders in which the particle morphology, the particle
size distribution, and the agglomeration of particles is readily controlled and
that it easily scalable.

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[0007] The present invention seeks to meet these and other
needs.
[0008] The present invention refers to a number of documents,
the content of which is herein incorporated by reference in their entirety.
SUMMARY OF THE INVENTION
[0009] The present invention relates to the plasma synthesis of
powders calling upon an induction plasma apparatus comprising an induction
plasma torch and a quenching chamber in which a renewable "gaseous cold
front" is generated, through the injection of a quench gas, and on which the
gaseous reactants/reaction products nucleate. The nucleation generates a
nanopowder which is rapidly transported to a collection chamber by the
moving cold front. It was surprisingly discovered that by generating a
renewable gaseous cold front for nucleating (i.e. condensing) the
reactants/reaction products present in the plasma flow, excellent control of
the morphology and particle size distribution of the resulting nanopowder
could be achieved. Moreover, the use of the renewable gaseous cold front
offers a close control over particle agglomeration.
[0010] More specifically, as broadly claimed, the present invention
relates to a process for synthesizing a nanopowder comprising feeding a
reactant material into a plasma torch in which is generated a plasma flow
having a temperature sufficiently high to yield a superheated vapour of the
material; transporting the vapour by means of the plasma flow into a
quenching zone; injecting a cold quench gas into the plasma flow in the
quenching zone to form a renewable gaseous cold front; and forming a
nanopowder at the interface between the renewable gaseous cold front and
the plasma flow.

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[0011] The present invention also relates to an apparatus for
synthesizing nanopowders comprising a plasma torch to generate a plasma
flow and to produce a superheated vapour from reactant material supplied to
the plasma torch in the plasma flow; and a quenching chamber mounted to
the plasma torch downstream therefrom and in fluid communication with said
plasma torch to receive the superheated vapour from the plasma torch, the
quenching chamber being structured to receive a quench gas and to generate
from said quench gas a renewable gaseous cold front to rapidly cool the
superheated vapour, yielding nanopowder.
[0012] The foregoing and other objects, advantages and features
of the present invention will become more apparent upon reading of the
following non restrictive description of illustrative embodiments thereof, given
by way of example only with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the appended drawings:
[0014] FIG. 1 is a schematic cross-sectional elevation view of an
induction plasma assembly in accordance with the present invention;
[0015] FIG. 2 is a schematic cross-sectional elevation view of an
induction plasma assembly of FIG. 1 without reactor component;
[0016] FIG. 3 is an illustration of the temperature isocontours in
the induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW;
3MHz] for (3A) a quench gas (Ar) flow rate of 400 slpm and (3B) a quench
gas (Ar) flow rate of 800 slpm, respectively;

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[0017] FIG. 4 is an illustration of the stream lines in the induction
plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz] for (4A)
a quench gas (Ar) flow rate of 400 slpm and (4B) a quench gas (Ar) flow rate
of 800 slpm, respectively;
[0018] FIG. 5 is graph illustrating temperature profiles along the
centerline of the induction plasma torch/reactor sections (5A, 5B) and in the
radial direction at different axial locations within the reactor/quench sections
of the induction plasma assembly of FIG.1 (5C, 5D) using an Ar/H2 plasma
gas [65kW; 3MHz] for a quench gas (Ar) flow rate of 400 slpm (5A, 5C) and a
quench gas (Ar) flow rate of 800 slpm, respectively (5B, 5D);
[0019] FIG. 6 is an illustration of the "gaseous cold front" showing
the quench gas concentration isocontours in the quench section of the
induction plasma assembly of FIG.1 using an Ar/H2 plasma gas [65kW;
3MHz] for (6A) a quench gas (Ar) flow rate of 400 slpm and (6B) a quench
gas (Ar) flow rate of 800 slpm, respectively;
[0020] FIG. 7 is an illustration of the reaction product cooling rate
isocontours (darker areas are representative of cooling rates in the range of
105 to 106 K/s) in the quench section of the induction plasma assembly of
FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz] for (7A) a quench gas (Ar)
flow rate of 400 slpm and (7B) a quench gas (Ar) flow rate of 800 slpm,
respectively;
[0021] FIG. 8 is a graph showing the particle size distribution for
an aluminum nanopowder as obtained using the induction plasma assembly
of FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz];

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[0022] FIG. 9 is a graph showing the particle size distribution for a
nickel nanopowder as obtained using the induction plasma assembly of FIG.1
using an Ar/H2 plasma gas [65kW; 3MHz];
[0023] FIG. 10 is a graph showing the particle size distribution for
a tungsten nanopowder as obtained using the induction plasma assembly of
FIG.1 using an Ar/H2 plasma gas [65kW; 3MHz];
[0024] FIG. 11 is an illustration of the quench gas flow within the
quench section of the induction plasma assembly of FIG.1;
[0025] FIG. 12 is a graph showing the mean particle size (12A)
and the specific surface area (12B) of a nanometric nickel powder as
produced by a process according to the present invention as a function of the
quench gas (Ar) flow rate;
[0026] FIG. 13 is a graph showing the mean particle size (13A)
and the specific surface area (13B) of a nanometric cupric oxide (CuO)
powder as produced by the process of the present invention (starting with
micrometric-sized copper powder) as a function of the quench gas (Ar/02)
flow rate;
[0027] FIG. 14 is a graph showing the particle size distribution of a
cupric oxide nanopowder (BET = 23.08 m2/g (14A) and 22.11 m2/g (14B)
respectively) as produced by the process of the present invention (starting
with micrometric-sized copper powder);
[0028] FIG. 15: is a graph showing the mean particle size (15A)
and the specific surface area (15B) of a nanometric germanium dioxide

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(GeO2) powder as produced by the process of the present invention (starting
with liquid GeCI4) as a function of the quench gas (Ar/O2) flow rate.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0029] In order to provide a clear and consistent understanding of
the terms used in the present specification, a number of definitions are
provided below. Moreover, unless defined otherwise, all technical and
scientific terms as used herein have the same meaning as commonly
understood to one of ordinary skill in the art to which this invention pertains.
[0030] The use of the word "a" or "an" when used in conjunction
with the term "comprising" in the claims and/or the specification may mean
"one", but it is also consistent with the meaning of "one or more", "at least
one", and "one or more than one". Similarly, the word "another" may mean at
least a second or more.
[0031] As used in this specification and claim(s), the words
"comprising" (and any form of comprising, such as "comprise" and
"comprises"), "having" (and any form of having, such as "have" and "has"),
"including" (and any form of including, such as "include" and "includes") or
"containing" (and any form of containing, such as "contain" and "contains"),
are inclusive or open-ended and do not exclude additional, unrecited
elements or process steps.
[0032] The term "about" is used to indicate that a value includes
an inherent variation of error for the device or the method being employed to
determine the value.

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[0033] The present invention relates to a novel process for the
plasma synthesis of nanopowders using induction plasma technology and
calling upon a renewable "gaseous cold front", for example a laminar
"gaseous cold front" which serves to rapidly quench the reactants/reaction
products present in the plasma flow. According to a first alternative, the
gaseous cold front may be generated in a quenching chamber comprising a
porous metal or ceramic wall through which is uniformly injected a cold
quench gas. According to a second alternative, the gaseous cold front may
be generated in a quenching chamber comprising a perforated refractory wall.
[0034] FIG. 1 shows an illustrative embodiment of the induction
plasma assembly, generally identified by the reference 10. The induction
plasma assembly 10 of FIG. 1 comprises an upper section consisting of an
inductively coupled radio frequency (rf) plasma torch 12 in which the
reactants are substantially axially introduced at the feed upper end of the
torch 12 and dispersed into the center of the plasma flow using techniques
well known to those of ordinary skill in the art. A reactor 14, mounted to a
quenching chamber 16, is affixed to the lower end of the plasma torch 12,
generally coaxial with the induction plasma assembly 10 between the plasma
torch 12 and the quenching chamber 16. The induction plasma assembly 10
further comprises a collection chamber 18 coaxially mounted to the lower end
of the quenching chamber 16. Of course, the plasma torch 12, the reactor 14,
the quenching chamber 16 and the collection chamber are in fluid
communication with each other.
[0035] The plasma is generated using any suitable gas that will
ionize when subject to a high frequency electromagnetic field, such as a radio
frequency field. It is believed to be within the skill of an expert in the art to
select a suitable gas. Suitable inductively coupled radio frequency (rf) plasma
torches are disclosed in U.S. Patent No. 6,919,527 issued to Boulos et al. on

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July 19, 2005. Other suitable plasma torches as contemplated by the process
of the present invention include high performance induction plasma torches
such as the one disclosed in U.S. Patent No. 5,200,595 issued to Boulos et
a/, on April 6,1993.
[0036] The reactant feed may be in the form of fine solid particles,
liquid droplets or in the vapor/gaseous phase. In the case of a solid feed, the
reactant, upon entering the plasma flow, is melted and vaporized forming a
vapor cloud which is superheated to a reaction temperature. In the case of a
liquid feed, the heat load on the plasma is substantially limited to that required
to heat up the liquid droplets to the vaporization temperature and
superheating the vapor to a reaction temperature. In the case of a gaseous
feed, the heat load on the plasma is substantially limited to that required to
superheat the gaseous feed to the reaction temperature. The plasma flow
axially delivers the vapor cloud to the reactor 14 in which it may be further
mixed with other components present in the vapor/gaseous phase. In an
embodiment of the present invention, a further component may be an
oxidizing agent such as oxygen, a carburizing agent such as methane or
acetylene, or a nitrating agent such as ammonia. Of course, the other
components may be concomitantly introduced into the plasma torch 12 with
the reactant feed, or introduced in the reactor section 14, or in the quench
section 16, using techniques well known to those of ordinary skill in the art.
[0037] The plasma flow carries the reactants/reaction products
emerging from the plasma torch 12 into a reactor 14 in fluid communication
therewith. In an embodiment of the present invention, the reactor 14 may be
a high temperature graphite/refractory lined reactor. It is believed to be within
the skill of an expert in the art to select other suitable reactors and reactor
configurations. In the event that the feed was concomitantly introduced into
the plasma torch 12 mixed with other components, the reactor 14 allows for

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any reaction to be completed. Alternatively, the superheated feed exits the
plasma torch 12 at a reaction temperature and is carried by the plasma flow
into the reactor 14 where it is mixed and reacted with other components.
[0038] Upon exiting the reactor 14, the reactants/reaction
products are carried into a quenching chamber 16. In an embodiment of the
present invention, the quenching chamber 16 may comprise a porous metal
or ceramic wall section 17 through which a cold quench gas and/or reactants
are injected. Alternatively, the quenching chamber 16 may comprise a
perforated or slotted refractory wall section through which the cold quench
gas and/or reactants may be injected. It is believed to be within the skill of an
expert in the art to determine and select other suitable quenching chamber
configurations. The injection of the cold quench gas, mixed with or without
other reactants, generates a mobile and continuously renewable laminar
"gaseous cold front" on which the gaseous reactants/reaction products
condense. Such a condensation results in a nucleation of reactants/reaction
products generating a nanopowder. The "gaseous cold front" is renewable
since it is in movement; the movement being imparted by the continuous
injection of cold quench gas and by the movement of the plasma flow. The
physical separation of the reaction section (i.e. the plasma torch 12 and/or the
reactor 14) and the quench section (quenching chamber 16) provides for a
better means of controlling the location of the condensation front over which
nanopowder formation occurs. The gaseous cold front is further illustrated in
FIGS. 3, 4, 6 and 7. In an embodiment of the present invention, argon was
used as the quench gas. It is believed to be within the skill of an expert in the
art to determine and select other suitable quench gases.
[0039] The quench rate to which the gaseous reactants/reaction
products are exposed will depend on the temperature of the quench gas and
its flow rate. The flow rate of the quench gas also affects the position of the

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cold front within the quenching chamber 16 and the manner in which the cold
front interacts with the hot flow of plasma gases emerging from the reactor 14
and comprising the reactants/reaction products. The nanopowder is carried
away on the mobile cold front into the collection chamber 18. Of course, the
plasma gases will also be at significantly lower temperatures, due to their
interaction with the cold front, upon reaching the collection chamber 18.
[0040] The process according to the present invention involves
the concept of a renewable "gaseous cold front" as an effective means of
controlling the uniformity and particle size distribution of the produced
nanopowder. Furthermore, the possibility for condensed nanoparticle
agglomeration is significantly reduced since the nanopowder is rapidly
evacuated, substantially upon its formation, from the quenching chamber 16
to the collection chamber 18 by the gaseous cold front at present comprised
of plasma gases and the quench gas. Moreover, by using a high volume
quench gas flow rate, it is possible to maintain the generated nanopowder in
dilute suspension while it is being evacuated from the quenching chamber 16
into the collection chamber 18. Additionally, by maintaining a dilute
suspension, both the deposition of the nanopowder on the walls of the
quenching chamber 16 and nanoparticle agglomeration through inter-particle
collision are substantially eliminated. Typical temperature and flow fields as
observed in the plasma torch 12, the reactor 14 and the quenching chamber
16 respectively, are illustrated in FIGS. 3 and 4 for quench gas flow rates of
400 and 800 standard liters per minute (slpm). These figures clearly
demonstrate the constricting effect of the "gaseous cold front" on the hot
plasma gas stream. The more intense the gaseous cold front, the greater the
constricting effect. Moreover, as may be observed from FIGS. 3 and 6, an
increase in the quench gas flow rate results in a significant increase in the
thickness of the cold boundary layer and a gradual displacement of the cold
front toward the center of the plasma flow. Furthermore, the increase in the

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thickness of the cold boundary layer is accompanied by the development of
steep temperature gradients at the interface of the cold front and the plasma
flow, where the nucleation of the reactants/reaction products takes place.
While not shown in these figures, changing the length of the reactor,
combined with a close control of the quench gas flow rate offers tangible
means of controlling the position of the cold front in the reactor assembly and
accordingly the exact moment at which the reactants/reaction products are
subjected to rapid cooling. In the case wherein the reactor 14 is absent, the
reactants/reaction products are substantially immediately exposed to the cold
front following their vaporization in the plasma torch 12. In the illustrative
embodiment as shown in FIGS. 3 and 4, the plasma gas was composed of an
argon/hydrogen mixture (80%vol /20%vol) whereas the gaseous cold front
was created using argon which was injected through the porous metal wall of
the quenching chamber 16.
[0041] A typical "gaseous cold front" and the associated quench
gas concentration isocontours as observed in the quenching chamber 16 is
illustrated in FIG. 6. Furthermore, typical isocontours of reaction product
cooling rate, as produced when the hot plasma gases comprising the reaction
products collide with the gaseous cold front, are illustrated in FIG. 7. These
figures illustrate a substantially uniform, renewable and mobile cooling gas
front across which the reaction products are exposed to high cooling rates in
the order of about 105to about 106 K/s. Such a gaseous cold front allows for
particle nucleation to occur at the interface with the hot plasma gases and the
formation of a nanopowder having a substantially uniform particle size
distribution, representative examples of which are illustrated in FIGS. 8-10.
[0042] The process of the present invention offers the additional
advantages of being compact, scalable and simple to operate. Moreover, the
reactor assembly 10 can be readily modified depending on the needs and

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type of nanopowder to be produced. Vapor, liquid or solid precursors may be
used in the process according to the present invention with the produced
nanopowder being either of identical or different chemical composition as that
of the precursor. In the case wherein the produced nanopowder is of identical
chemical composition, the process is limited to the vaporization and
condensation of the feed generating nanoparticles thereof. In the case
wherein the produced nanopowder is of different chemical composition, the
feed was reacted with a second reactant which can be either injected in the
plasma torch 12 or introduced in the reactor 14. Alternatively, the quench gas
may react with the vaporized feed in which case it serves a dual function. As
illustrated in FIG. 2, the reactor 14 can be removed such that the plasma feed
comprising the gaseous reactants or alternatively the gaseous
reactants/reaction products, are brought directly into the quenching chamber
16. If the feed material is to be chemically modified, a non-limiting example of
which is oxidation, the presence of the reactor might be required. It is
believed to be within the skill of an expert to select an appropriate reactor
assembly depending on the type of nanopowder to be produced.

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EXPERIMENTAL
[0043] A number of examples are provided hereinbelow,
illustrating the efficiency of a renewable laminar gaseous cold front in the
synthesis of a number of metallic and ceramic nanopowders using induction
plasma technology.
[0044] Synthesis of nanopowders using solid precursors.
[0045] An argon/hydrogen induction plasma flow is generated
using a 50 mm internal diameter induction plasma torch with an oscillator
frequency of 3 MHz, a plasma plate power of 65 kW, and a reactor pressure
of approximately 500 Torr. Different metals, in the form of a micron-sized
metallic powder, were axially injected into the center of the plasma discharge
and vaporized. As the hot plasma gases comprising the vaporized metal
emerge from the reactor, they are intercepted by the gaseous cold front which
is created by means of injecting argon through the porous walls of the
quenching chamber. The interaction of the hot plasma gases with the
gaseous cold front gives rise to a nucleation phenomenon at the interface hot
plasma gases/gaseous cold front, leading to the formation of a nanopowder
having a well defined particle size distribution. The continuously moving
gaseous cold front, now also made-up of cooled plasma gases, rapidly
evacuates the nanopowder to a collection chamber where it can be collected
on standard sintered metal or cloth filter elements. The experimental results
as obtained for aluminum, nickel and tungsten powders are summarized
hereinbelow in Table 1. Data regarding plasma plate power, precursor feed
rates, quench gas rates, and physical characteristics of the collected
nanopowder such as their Specific Surface Area (as measured using the BET
method) and particle mean diameter are also provided. The Specific Surface
Area is expressed in terms of m2/g of collected powder. The mean particle

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diameter of the collected powder can be calculated assuming a spherical
particle shape having an equivalent surface area to volume ratio. The particle
size distributions, obtained by light scattering analysis using a Malvern
Mastersizer™ instrument are illustrated in FIGS. 8-10 respectively.
[0046] Table 1: Examples of metallic nanopowders as obtained
using the process of the present invention.

[0047] Synthesis of nanopowders using liquid precursors.
[0048] As illustrated in FIG. 15 and in Table 2 below, the process
according to the present invention lends itself to the synthesis of
nanopowders using liquid feeds. Liquid germanium tetrachloride (GeCU4, b.p.
= 83 °C) was sent by a Masterflex™ pump into an oxygen plasma. The liquid
was vaporized and oxidized. The produced germanium dioxide (GeO2) vapor
was condensed to nanometric powders by injection of cold quench gas flow
generating the "gaseous cold front". The process as occurring in Fig. 15 can
be illustrated by the following reaction:

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[0049] Table 2: Examples of process parameters* for producing
GeO2 (g) starting with GeCI4 (I).

*Plasma power: 65 kW; Plasma sheath: 18 Ar + 82 O2 (slpm); Plasma central: 30Ar
(slpm); Injection probe: SA953; Atomization gas: 8O2 (slpm); Reactor pressure: 80 kPa.
[0050] Synthesis of nanopowders using gaseous precursors.
[0051] The process of the present invention also lends itself to the
synthesis of nanopowders using gaseous feeds as illustrated by the following
reaction:
4BCI3(g) + CH4(g) + 4H2 >- B4C(s)+12 HCI (g)
[0052] It is to be understood that the invention is not limited in its
application to the details of construction and parts as described hereinabove.
The invention is capable of other embodiments and of being practiced in
various ways. It is also understood that the phraseology or terminology used
herein is for the purpose of description and not limitation. Hence, although
the present invention has been described hereinabove by way of illustrative
embodiments thereof, it can be modified, without departing from the spirit,
scope and nature of the subject invention as defined in the appended claims.

I7
WHAT IS CLAIMED IS:
1. A process for synthesizing a nanopowder comprising:
a) feeding a reactant material into a plasma torch in which is
generated a plasma flow having a temperature sufficiently high to yield a superheated
vapour of said material;
b) transporting said vapour by means of said plasma flow into a
quenching zone;
c) injecting a cold quench gas into the plasma flow in the quenching
zone to form a renewable laminar gaseous cold front; and
d) forming a nanopowder at the interface between the renewable
laminar gaseous cold front and the plasma flow.

2. The process of claim 1 further comprising collecting the
nanopowder in a collection zone.
3. The process of claim 2, wherein the gaseous cold front exerts a
constricting effect on the plasma flow.
4. The process of claim 3, wherein the constricting effect is
proportional to the quench gas flow rate.
5. The process of claim 4, comprising injecting the cold quench gas in
the quenching zone by means of a plurality of openings in a wall section of said
quenching zone.
6. The process of claim 5, wherein the plurality of openings define a
porous wall section.
7. The process of claim 5, wherein the plurality of openings define a
slotted wall section.
8. The process of claim 5, wherein the plurality of openings define a
perforated wall section.

9. The process of any one of claims 5, 6, 7 or 8, wherein the
quenching zone is a quenching chamber.
10. The process of claim 1, wherein the reactant material is selected
from the group consisting of solid, liquid and gaseous feeds.
11. The process of claim 1, wherein said superheated vapour is at a
reaction temperature capable of reacting with said plasma flow and/or said quench gas.
12. The process of claim 10, wherein the reactant material is selected
from the group consisting of metals, alloys, organometallic compounds, chlorides,
bromides, fluorides, iodides, nitrites, nitrates, oxalates, carbonates, oxides and
composites.
13. The process of claim 1, further comprising:

e) feeding a second reactant in the plasma flow; and
f) reacting the second reactant with the reactant material to produce
a nanopowder of chemical composition different from the reactant material.

14. The process of claim 13, comprising injecting the second reactant
into the plasma torch.
15. The process of claim 13, comprising injecting the second reactant
into a reactor between the plasma torch and the quenching zone.
16. The process of claim 13, wherein the second reactant is the
quench gas.
17. An apparatus for synthesizing nanopowders comprising:

a) a plasma torch to generate a plasma flow and to produce a
superheated vapour from reactant material supplied to the plasma torch in the plasma
flow; and
b) a quenching chamber mounted to the plasma torch downstream
therefrom and in fluid communication with said plasma torch to receive the superheated

vapour from the plasma torch, said quenching chamber being structured to receive a
quench gas and to generate from said quench gas a renewable laminar gaseous cold
front to rapidly cool the superheated vapour, yielding nanopowder.
18. The apparatus of claim 17, further comprising a collection chamber
to collect the nanopowder.
19. The apparatus of claim 17, wherein the gaseous cold front exerts a
constricting effect on the plasma flow.
20. The apparatus of claim 19, wherein the constricting effect is
proportional to the quench gas flow rate.
21. The apparatus claim 17, wherein the quenching chamber
comprises a wall section with a plurality of openings for injecting the quench gas in the
quenching chamber.
22. The apparatus of claim 21, wherein the wall section is a porous
wall section.
23. The apparatus of claim 21, wherein the wall section is a slotted wall
section.
24. The apparatus of claim 21, wherein the wall section is a perforated
wall section.
25. The apparatus of claim 17, wherein the reactant material is
selected from the group consisting of solid, liquid and gaseous feeds.
26. The apparatus of claim 17, wherein said superheated vapour is at a
reaction temperature capable of reacting with said plasma flow and/or said quench gas.
27. The apparatus of claim 17, wherein the reactant material is
selected from the group consisting of metals, alloys, organometallic compounds,

20
chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxaiates, carbonates, oxides and
composites.
28. The apparatus of claim 17, further comprising:
means for feeding a second reactant in the plasma flow: and
means for reacting the second reactant with the reactant material
to produce a nanopowder of chemical composition different from the reactant material.
29. The apparatus of claim 28, comprising means for injecting the
second reactant into the plasma torch.
30. The apparatus of claim 28, comprising means for injecting the
second reactant into a reactor between the plasma torch and the quenching zone.
31. The apparatus of claim 28, wherein the second reactant is the
quench gas.
32. The apparatus of claim 17, further comprising a reactor to react the
reactant material with a second reactant, said reactor being in fluid communication with
the plasma torch and the quenching chamber, and said reactor being disposed between
the plasma torch and the quenching chamber.

A process and apparatus for synthesizing a nanopowder is presented. In particular, a process for the synthesis
of nanopowders of various materials such as metals, alloys, ceramics and composites by induction plasma technology, using
organometallic compounds, chlorides, bromides, fluorides, iodides, nitrites, nitrates, oxalates and carbonates as precursors is
disclosed. The process comprises feeding a reactant material into a plasma torch in which is generated a plasma flow having a
temperature sufficiently high to yield a superheated vapour of the material; transporting said vapour by means of the plasma flow
into a quenching zone; injecting a cold quench gas into the plasma flow in the quenching zone to form a renewable gaseous cold
front; and forming a nanopowder at the interface between the renewable gaseous cold front and the plasma flow.

PROCESS AND APPARATUS FOR SYNTHESIZING A NANOPOWDER

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