Abstract:
The invention concerns a process for the spheroidisation, densification and purification of powders through the combined action of plasma processing, and ultra-sound treatment of the plasma-processed powder. The ultra-sound treatment allows for the separation of the nanosized condensed powder, referred to as ‘soot’, from the plasma melted and partially vaporized powder. The process can also be used for the synthesis of nanopowders through the partial vaporization of the feed material, followed by the rapid condensation of the formed vapour cloud giving rise to the formation of a fine aerosol of nanopowder. In the latter case, the ultra-sound treatment step serves for the separation of the formed nanopowder form the partially vaporized feed material.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to processes for the synthesis, separation and purification of powder materials. More specifically, the present invention is concerned with processes involving materials transformation under plasma conditions  
       BACKGROUND OF THE INVENTION  
       [0002]     The processing of powder materials through the in-flight melting of the individual particles under plasma conditions followed by the solidification of the formed droplets has been known for some time and is attracting increasing attention as a means of densification and spheroidisation materials in powder form. The process, generally known as powder spheroidisation, results in a significant improvement of the flow properties of the powders, and the increase of their resistance to attrition during their handling and transport.  
         [0003]     The powder spheroidisation process has also been recognized as an effective means for the proper control of the chemical composition of the powder materials as well as for the synthesis of new materials and composite mixtures.  
         [0004]     Through the use of inductively coupled, radio frequency (r.f.) electrodless discharges, as a heat source for the process, it has also been observed that the process can be used for the significant purification of the powder being treated through the partial loss of some of the impurities either as a result of a simple volatilization step from the molten droplets, or the reactive volatilization of the impurities. In the former case, the impurities of lower boiling point compared to that of the particle matrix are preferentially vaporized; the gaseous impurities can escape from the particle matrix. In the latter case, the impurity is chemically transformed at the surface of the molten droplet through its contact with the processing environment, followed by the volatilization of the formed compound. The chemical reaction involved can be, though not limited to, for example, the oxidation of the impurities through their contact with oxygen in the plasma flow. The process results in a net reduction of the level of impurities in the powder and subsequently its purification.  
         [0005]     The problem that arises in such circumstances, however, is that the formed vapour cloud of the impurities, whether they are in their elemental form, or as a compound, remains mixed with the plasma gas transporting the purified powder. As the overall plasma stream with its powder content is cooled down, the impurities also condenses in the form of a very fine soot that deposits on all available surfaces in the reactor including the surface of the processed/purified powders which are then contaminated again with the same impurities that were eliminated in the first place. In the case of metal powder, this soot is composed of very fine metallic particle. These fine particles are, in turn, very sensitive to oxidation when they come in contact with the ambient air, with which they react, resulting in the significant increase of the oxygen content of the powder.  
         [0006]     In a different context, the induction plasma processing of powders has also been successfully used for the synthesis of metallic and ceramic nanopowders through the in-flight heating, melting and vaporization of the feed precursor followed by the rapid quench of the formed vapours in order form a fine aerosol of nanopowder thorough the homogenous condensation of the vapour cloud. In such a case, however, the formed aerosol of nanopowder is mixed with residual fraction of the feed material, which is only partially vaporized, resulting in a mixed powder with a broad particle size distribution. Depending on the operating conditions, the collected powder can often have a bimodal particle size distribution, which represents a major limitation to the acceptance of such a powder for most nanopowder applications.  
       OBJECTS OF THE INVENTION  
       [0007]     An object of the present invention is therefore to provide an improved process for the synthesis of powder materials.  
         [0008]     Another object of the invention is to provide an improved process for the separation and/or purification of powder materials.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention concerns a process for the spheroidisation, densification and purification of powders through the combined action of plasma processing, and ultra-sound treatment of the plasma-processed powder. The ultra-sound treatment allows for the separation of the nanosized condensed powder, referred to as ‘soot’, from the plasma melted and partially vaporized powder. The process can also be used for the synthesis of nanopowders through the partial vaporization of the feed material, followed by the rapid condensation of the formed vapour cloud giving rise to the formation of a fine aerosol of nanopowder. In the latter case, the ultrasound treatment step serves in this case for the separation of the formed nanopowder form the partially vaporized feed material.  
         [0010]     More specifically, in accordance with a first aspect of the present invention, there is provided a process for the purification of a material comprising:  
         [0011]     providing powder particles of the material including impurities;  
         [0012]     plasma heating and melting of the powder particles of the material and release of the impurities in vapour phase through a plasma stream, yielding molten particle droplets of the material mixed in the plasma stream and vaporized impurities;  
         [0013]     cooling of the molten particle droplets of the material mixed in the plasma stream with the vaporized impurities, yielding a mixture of purified powder particles of the material and soot;  
         [0014]     exposing the mixture of purified powder particles of the material and soot material to ultrasound vibrations in a sonification medium, yielding separated purified powder particles of the material and soot in the sonification medium; and  
         [0015]     recovering the purified powder particles of the material from the sonification medium and the soot.  
         [0016]     According to a second aspect of the present invention, there is provided a process for the separation of nanopowder mixed with a coarse powder by exposing the nanopowder mixed with the coarse powder to ultrasound vibrations in a sonification medium.  
         [0017]     According to a third aspect of the present invention, there is also provided a process for the synthesis of a material nanopowder comprising: 
        i) providing the material in powder form;     ii) plasma heating, melting and vaporization of the powder of the material through a plasma stream, yielding the material in vapour form mixed with partially vaporized particles in the plasma stream;     iii) running the material in vapour form mixed partially vaporized particles in the plasma stream through a quench stream, yielding a mixture of formed material nanopowder and residual coarse material powder; and     iv) exposing the mixture of formed material nanopowder and residual coarse material powder to ultrasound vibrations in a sonification medium, yielding separated nanopowder particles of the material and coarse powder of the material.        
 
         [0022]     Processes for the synthesis or purification of material according to the present invention allows for the purification of powder material for the manufacture high purity materials such as solar cells and sputtering target for example.  
         [0023]     A process for the synthesis of nanopowders according to the present invention allows for the separation of the synthesized nanopowder from the remaining partially vaporized precursor material through intense ultrasound action identified in the present invention as a sonification process.  
         [0024]     Processes according to the present invention allows to purify, synthesize and separate powders of a wide range of materials including , but not limited to ceramics, alloys, composites, and pure metals including, but not limited to, silicon, chromium, molybdenum, tungsten, tantalum and ruthenium.  
         [0025]     Other objects, advantages and features of the present invention will become more apparent upon reading the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     In the appended drawings:  
         [0027]      FIG. 1  is a flowchart illustrating a process for the purification of powder materials according to an illustrated embodiment of the present invention;  
         [0028]      FIG. 2  is a schematic view of a plasma reactor for performing a first part of the process from  FIG. 1 ;  
         [0029]      FIGS. 3A and 3B  are electron micrographs of respectively plasma spheroidised silicon and ruthenium powder particles following steps  104 - 106  of the process from  FIG. 1 , illustrating webs of agglomerated nanopowders soot condensed on the powder particles;  
         [0030]      FIG. 4  is a schematic view of a sonification assembly for performing a second part of the process from  FIG. 1 ;  
         [0031]      FIGS. 5A, 5B  and  5 C are electron micrographs of respectively raw WC powder particles, and two examples of spheroidised WC powder particles obtained through the process from  FIG. 1 ;  
         [0032]      FIGS. 6A-6D  are electron micrographs of silicon powder following plasma treatment according to the first steps of the process from  FIG. 1 , but before the sonification step of the process from  FIG. 1 ;  
         [0033]      FIGS. 7A-7D  are electron micrographs of silicon powder corresponding respectively to  FIGS. 6A-6D  after the sonification step of the process from  FIG. 1 ;  
         [0034]      FIGS. 8A-8E  are electron micrographs of plasma processed ruthenium powder obtained trough the process from  FIG. 1 , after increasing period of sonification time;  
         [0035]      FIG. 9  is a graph showing the residual oxygen concentration of the ruthenium powder illustrated in  FIGS. 8A-8E ;  
         [0036]      FIGS. 10A-10C  are electron micrographs of tungsten powder after plasma treatment and respectively prior to sonification ( FIG. 10A ) and after sonification where coarse particle fraction ( FIG. 10B ) and fine particle fraction ( FIG. 10C ) are obtained; and  
         [0037]      FIGS. 11A-11C  are graphs illustrating the particle size distribution of the tungsten powder respectively illustrated in  FIGS. 10A-10C . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]     A process  100  for the purification of a material according to an illustrative embodiment of the present invention will now be described with reference to  FIG. 1 .  
         [0039]     In step  102 , the material is provided in the form of raw powder. The powder particles are then inserted axially into the center of an inductively coupled, radio frequency plasma stream.  
         [0040]     In step  104 , the powder particles of the material are then heated and melted as they are injected into the center of an inductively coupled radio frequency (r.f.) plasma reactor  10 , which is illustrated in  FIG. 2 .  
         [0041]     Indeed, as the individual powder particles come in contact with the plasma stream, they are heated and melted in a relatively short time, of the order of milliseconds, yielding molten particle droplets of the material mixed in the plasma stream. In addition to the melting of the particle of material, step  102  also causes the partial vaporization of the particle material itself and/or of any impurities in them. Encapsulated impurities in the particles can also find their way during the melting step to the surface of the particle under the influence of surface tension effects.  
         [0042]     Concerning the plasma reactor operation, the plasma gas composition is an inert, an oxidizing or a reducing atmosphere depending on the chemistry of the materials processed and the impurities present.  
         [0043]     The operating pressure is atmospheric, low pressure, ‘soft vacuum’, or above atmospheric pressure. The evaporation can be the result of a simple volatilization of the particle material, or the separation of impurities from the particle in a vapor phase without involving any chemical transformation. A reactive evaporation involving a chemical transformation of the particle material, or the impurities present, is also possible through their interaction with the plasma gas, followed by the evaporation of the formed chemical compounds.  
         [0044]     Since such a r.f. plasma reactor is believed to be well known in the art, it will not be described herein in more detail. It is to be noted that other types of plasma reactor such as direct current (d.c.) plasma jets or a capacitive coupled r.f. plasma, or a microwave plasma can be also used to heat and melt the powder particles.  
         [0045]     In step  106 , the resulting molten particle droplets of the material mixed the plasma stream are then cooled, resulting in the solidification and spheroidisation of the molten particle droplets of purified material, and the condensation of the transported vapours in the form of a nanosized aerosol which deposits on all available surfaces of the plasma reactor  10  and the surface of the transported solidified particle droplets. The latter case results in a soot-like material being mixed with the purified powder.  
         [0046]      FIGS. 3A and 3B  show two example of agglomerated nanopowder soot condensed respectively on silicon and ruthenium solidified particle droplet following steps  102 - 106  of the process  100 .  
         [0047]     As illustrated in  FIGS. 3A-3B , step  106  results in a loss of the purification action achieved during the plasma step.  
         [0048]     To achieve the separation of the soot-like nanosized particles from the solidified particles droplets and therefore achieving the purification thereof, the resulting mixture of powder particles of the material and soot material are exposed to intense ultrasound vibrations in a sonification medium (step  108 ). Depending on the volume of the sonification medium and its powder loading, the required intensity of sonification can be as low as a hundred Watts, and as high as a few kilowatts. The separation is achieved through the Faraday wave pattern composed of standing waves setup in response to intense coherent vibration Since the Faraday wave principal is believed to be well known in the art, it will not be described herein in more detail.  
         [0049]     An example of an ultrasound assembly  20  that can be used to carry out step  108  is illustrated in  FIG. 4 . The assembly  20  comprises a small, water-cooled, glass beaker  22 , which is filled with the mixture of powder particles of the material and soot material resulting from step  102 - 106  in suspension in an appropriate sonification liquid such as, though not limited to, water, acetone or alcohol (generally referred to in  FIG. 4  with numeral reference  24 ).  
         [0050]     The assembly  20  further comprises an ultrasound generation probe  26 . The tip  28  of the ultrasound generation probe  26  is immersed into the suspension  24  and energized to expose the powder to intense vibration and cause the dislodging of the nanosized ‘soot’ particles from the surface of the larger purified or partially vaporized powder particles.  
         [0051]     Even though step  108  is illustrated as being carried out in a sonification liquid, it can also be carried out in other sonification medium such as air.  
         [0052]     Of course, other type of container can be used to carry the suspension  24 . The sonification medium can be already provided in the plasma treated collection chamber of the plasma reactor  10  (see  FIG. 2 ). Also, the assembly  20  may take many other forms allowing exposing the mixture of soot-like nanosized particles and solidified particles droplets to ultrasound.  
         [0053]     Since ultrasound probes are believed to be well known in the art, they will not be described herein in more detail.  
         [0054]     The next step ( 110 ) is the recovering of the purified powder particles of the material from the sonification medium.  
         [0055]     Step  110  first includes the separation of the two particle fractions (the separated powder and the nanosized soot), for example, by wet sieving or differential sedimentation under normal gravitational forces, or by intense centrifugation under multiple values of gravitational forces. Then, the separated powder and/or nanosized soot are recovered from the sonification medium by filtration followed by a final evaporation/drying step in cases when step  108  is carried out in a liquid medium and, if necessary, vacuum packing.  
         [0056]     Other purified powder material recovering process may alternatively be used.  
         [0057]     The powder treatment process  100  allows improving the flow properties of the powder. Indeed, Hall tests have been performed on spheroidised powder particles following the process  100 . For example, it has been measured that raw WC (tungsten carbide) powder, which is illustrated in  FIG. 5A  has a Hall flow value of 54.3 s/20 cm 3 , while WC powder spheroidised following the process  100 , which is illustrated in  FIG. 5B and 5C , shows a Hall flow value of 32.5 and 34.3 s/20 cm 3  respectively.  
         [0058]     In the following, specific examples of applications of the process  100  using the apparatuses  10  and  20  will now be described. The specific examples will highlight additional features and advantages of a purification process from the present invention.  
         [0000]     Purification of Silicon Powder for Solar Grade Silicon Applications  
         [0059]     The first example relates to the purification of silicon powder for solar grade silicon applications. According to this first example, medium purity silicon powder is melted through its exposure to an argon/hydrogen inductively coupled plasma discharge operating at near atmospheric pressure according following steps  102 - 106  from the process  100 .  
         [0060]     The collected powder is composed of individual spheroidal particles mixed with a network of agglomerated nanosized soot particles condensed on its surface.  
         [0061]     Following step  108  of the process  100 , the soot is separated from the purified silicon particles through intense sonification in an acetone bath followed by differential sedimentation, filtration and drying, this latter steps corresponding to step  110  of the process  100 .  
         [0062]     Electron micrographs of the silicon particles after the plasma treatment, prior and after the intense sonification step  108  are shown in  FIGS. 6A-6D  and  7 A- 7 D respectively. The corresponding values of BET (Brunauer Emmett Teller) specific surface area analysis of the powder are given in Table 1.  
                                                       TABLE 1                           BET specific surface area analysis of the plasma       treated powder before and after the sonification       step 108 at power levels of 17 W and 100 W            BET m 2 /g   BET m 2 /g   Wt of powder (g)       17 W Sonification   100 W Sonification   100 W Sonification            Before   After   Before   After   Before   After               0.332   0.302   0.332   0.11   14.86   13.59                  
 
         [0063]     The results given for two levels of ultrasound intensity (17 W and 100 W) for the same period of exposure time, show that better results are obtained by using at least a minimum level of power for the sonification step, which is more precisely within the range of about 50 to 100 W. The results illustrated in Table 1 show a visible reduction of the soot deposition level on the surface of the powder particles as demonstrated by the considerable drop of its specific surface area.  
         [0000]     Purification of Ruthenium for Electronic Applications  
         [0064]     The second example concerns the purification of ruthenium for electronic applications. According to this example, ruthenium powder is exposed to an argon/helium inductively coupled plasma at near atmospheric pressure where the individual particles are heated melted and spheroidised according to steps  104 - 106  of the process  100 . Steps  104 - 106   106  also cause the vaporization of present impurities from the surface of the particles. The processed powders are then exposed to intense ultrasound vibration using a 100 W ultrasound horn generator  26  according to step  108 . The tip  28  of the generator  26  is immersed in a 100 ml suspension of 250 g of the processed powder in acetone.  
         [0065]     Electron micrographs of the plasma-processed ruthenium powder, at the onset of the sonification step  108  (t=0), and following different periods of sonification treatment (30, 60, 90 and 120 min), are shown in  FIGS. 8A-8E .  FIGS. 8A-8E  show a gradual and systematic purification of the powder through the dislodging of the soot particles from the surface of the ruthenium particles. The purification effect is also confirmed by oxygen level analysis of the powder given in  FIG. 9  as function of the sonification time (step  108 ). The results clearly indicate a significant drop in the residual oxygen level of the powder with the sonification time beyond the first 60 minutes of treatment for a sonification power level of 100 W used in the experiments.  
         [0000]     Synthesis of Nanosized Tungsten Powders  
         [0066]     The third example relates to the synthesis of nanosized powders using the process  100 . According to this example, the process  100  is used for the synthesis of nanopowders of a refractory metal such as tungsten through the partial vaporization of a fine metallic tungsten powder in an argon/hydrogen inductively coupled plasma at near atmospheric pressure, followed by the rapid quench of the plasma gases and the generated metallic vapors (steps  104 - 106 ). The rapid quench is achieved through the injection of a cold gas stream. Rapid quenching can also be achieved through an atomized liquid stream or by contact of the plasma gases with a cold surface.  
         [0067]     The collected mixture of formed tungsten nanopowders and residual partially-vaporized tungsten powder is subjected to an intense sonification step  108  in order to separate the nanopowder from the larger tungsten particles.  FIG. 10A  shows an electron micrograph of the mixed coarse and nanosized tungsten powders as collected at the exit of the plasma reactor and quench section (see  FIG. 2 ).  
         [0068]     Electron micrographs of the corresponding coarse and fine powder fractions obtained through intense sonification with acetone as the sonification fluid are given respectively in  FIGS. 10B and 10C . The corresponding particle size distribution of the mixed powder and each of the separated coarse and fine powder fractions after sonification are given in  FIGS. 11A-11C .  
         [0069]     Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention, as defined in the appended claims.