WO2002043905A2

WO2002043905A2 - A method and apparatus for the production of metal powder granules by electric discharge

Info

Publication number: WO2002043905A2
Application number: PCT/IL2001/001102
Authority: WO
Inventors: Moisei Esterlis; Leonid Avrukh
Original assignee: M.P.I. Metal Powders Industries Ltd.
Priority date: 2000-12-01
Filing date: 2001-11-29
Publication date: 2002-06-06
Also published as: WO2002043905A3; AU2002221014A1; IL140034A0

Abstract

Method for the manufacture of metal granules, in which a metal source (14) is placed in a vacuum chamber (1). A process anode (17) and a means for initiating discharge from the metal source are provided in the chamber. An electric discharge is generated between the discharge initiation means and the metal source as the cathode, in order to raise the surface temperature of the metal source above its boiling point and to generate metal vapor and metallic plasma. A flow of current is generated between the process anode and the metal source through the metallic plasma, whereby to continue to generate metal vapor from the cathode. The vapor is allowed to condense into spherical molten drops and the molten drops to solidify to spherical particles, which are then withdrawn from the vacuum chamber.

Description

AMETHOD ANDAPPARATUS FOR THE PRODUCTION OF METAL POWDER GRANULES

Field of the Invention

The present invention relates to the field of powder metallurgy. More particularly, the invention relates to a method and apparatus for producing spherically shaped non-oxidized metal granules having a large percentage of small particles.

Background of the Invention

There are three major methods for producing a metallic object with a desired shape: metal cutting, casting and sintering.

Metal cutting such as turning, drilling and milling is the more conventional method for achieving a desired shape, as well known to those skilled in the art. Generally speaking, six-sevenths of the workpiece material is lost during metal cutting.

Casting is the method in which molten metal, at 1200 °C for copper, is poured into a mold to take its shape. Approximately 20% of the material is lost when poured. An additional drawback of casting is due to the oxidation resulting from the contact between metal and air.

Sintering is the process in which metal powder, after having been placed within a mold and pressed to assume the shape of the mold and to achieve a desired density and cohesion, is heat treated so that the particles will fuse together to form a metallic element having desired properties. In this process, a heat treatment below the melting point of the metal, e.g. of only 1000 °C for copper, is needed to provide the necessary time and electron activity for establishing bonded strength between particles, and a material loss of only 5% occurs. Sintering is therefore a desirable metal working processsince it reduces material loss and energy consumption.

Metal powder is a term that denotes a collection of small metal particles having varying shapes and sizes. The particle sizes range from tenths of a micron to a millimeter. The material produced from metal powder is typified in that the unique characteristics of each particle mutually influence those of the other particles and also the collective characteristics of the produced material.

The density, solidity, and other physical and mechanical properties of the end product are dependent upon the properties of the powder and upon the manufacturing conditions, i.e. the temperature, process time and environment. An increase in temperature will result in an increase in density and solidity. A correct choice of the protective atmosphere existing during sintering will prevent poor results, such as oxidation of the metal. The resistivity of the end product will be lower if the grain size of the powder is lower and if the base material is chemically purer. Porosity of the powder will cause the development of cracks and corrosion of the end product. In .addition, the porosity of the base material from which the powder was produced will make any required metal cutting of the end product difficult.

As mentioned, the properties of the metal powder will directly influence the properties of the end product, and therefore the method of producing the metal powder has much importance. Several prior art methods of producing metal powder are well known to those skilled in the art: mechanical, chemical and electrolysis. There are various mechanical methods utilized for solid or liquid metals. The most widely used methods for solids are hammer or ball milling. If the metal is brittle, it will break into many particles. These methods are characterized by having high productivity and low energy consumption. Alloy powders containing several constituents may be produced in this way. However, the grains that are produced are relatively large and non-spherical. Furthermore, if the metal is ductile, as is the case of copper and iron as well as their alloys, it can be readily deformed and will not break as a brittle metal will.

Liquid metal may be transformed into metal powder by atomization. A stream of liquid metal is transformed by a blast of compressed gas into metal droplets, which in turn form metal powder after solidification. Powder made in this form is generally pure and homogeneous; however, it is subject to oxidation due to contact with air and cooling fluid.

The chemical method can be used with a large number of metals. A chemical agent reduces a compound of the metal into particles of the desired metal and a by-product which must be removed. This process is highly productive, but the by-products, if solidified around and between the metal powder, require additional stages of processing for removal and also may be ecologically harmful.

Metal powder may also be produced through electrolysis of a liquid solution which is mixed with the desired metal. An electric current is passed through the solution, and the metal is then deposited on the cathode. The coating is further disintegrated to produce the powder. This method is also ecologically harmful and is limited to the number of metals that may be processed. U.S. Patent 4,474,604 discloses a method of producing high-grade metal or alloy powder in which a solid electrode material is heated and molten in a vacuum chamber. The molten metal or alloy in the form of droplets collide with the surface of a rotating cylinder. The main disadvantage of this method is that powder with a very small grain size may not be produced since the temperature of the molten droplets is no higher than their melting point. Other drawbacks relate to the large size of the production unit, the utilization of an expensive inert gas, large energy expenditure in heating the gas, and difficulty in producing powder from metals having a high melting point.

U.S. Patent 4,762,975 discloses a process for producing ultrafine particles from a consumable electrode by melting the tip of the electrode by means of electron bombardment and applying an intense electric field to the molten tip to generate a beam of charged droplets. Production is limited with this method since dense vapor, generated as a result of the high rate of metal fragmentation, would obstruct the electron beam. Furthermore utilization of an electron beam requires a large amount of energy consumption. In addition, some of the powder will be contaminated when it is collected at the bottom of the vessel.

An object of the present invention is therefore a method for the production of metal powder that is free of the drawbacks of prior art methods.

It is another object of the invention to provide a method for the production of spherical metal powder having desired dimensions.

It is a further object of the invention to provide such a method which does not cause oxidation of the metal particles. It is a still further object of the invention to provide such a method which does not produce by-products and does not require further processing stages.

It is a still further object of the invention to provide such a method which is not ecologically harmful.

It is a still further object of the invention to provide such a method which is applicable to all electrically conducting metals

It is a still further object of the invention to provide such a method which has a low energy consumption.

It is a still further object of the invention is to produce powders coated with one or more shells of metal or alloy.

It is a still further object of the invention is to produce hollow shells of metal or alloy with or without perforations. The shells may be composed of one or more layers each of a different metal or alloy.

It is a still further object of the invention is to produce semiconductor powders.

It is a still further object of the invention is to provide a method for the production of high quality spherical metal powder by fragmenting the metal droplets emanating from the electrode during their passage to the collecting plate.

Other objects and advantages of the invention will become apparent as the description proceeds. Summary of the Invention

The method of the invention comprises the following steps: a) Placing a piece of the metal to be formed into particles — hereinafter

"the metal source" - in a vacuum chamber. b) Providing in said chamber an anode — hereinafter "the process anode" — and a means for initiating discharge from said metal source. c) Generating an electric discharge between said discharge initiation means and said metal source as the cathode, whereby to raise the surface temperature of metal source above its boiling point, whereby to ^"generate metal vapor and metallic plasma. d) Generating a flow of current between said process anode and said metal source through said metallic plasma, whereby to continue to generate metal vapor. e) Allowing said vapor to condense into spherical molten drops and said molten drops to solidify to spherical particles. f) Withdrawing said spherical particles from said vacuum chamber.

Preferably, the vacuum in the chamber is at least lOE-l torr. The preferable range is from lOE-l to 10E-4 torr. This level of vacuum is desirable since it lowers the boiling point of the metal, and further, reduces and generally prevents the collision of the metal vapor particles with air molecules and therefore prevents any change in the chemical composition of the metal.

Preferably, the metal source is placed at the top of the vacuum chamber and the metal drops and spherical particles are allowed to move to the bottom of said chamber by gravity and are withdrawn from said chamber at the bottom thereof. The metal droplets and the spherical particles have a considerable amount of kinetic energy resulting from their ejection from the metal source surface and descend to the bottom of the vacuum chamber. It is desirable that, during the passage of the larger droplets from the metal source surface to the collecting plate, they be divided into smaller droplets. This can be achieved either by increasing the time during which particles are in the plasma stream or by passing the droplets between an additional pair of electrodes, or by other methods as enumerated below.

Preferably the discharge initiation means consists of an ignition anode which is placed in the vicinity of the metal source cathode to make instantaneous contact therewith. After a spark is produced, initiating discharge between the process anode and cathode, the ignition anode is disconnected from its circuit. A resistor, e.g. of between 10 and 50 ohms, is included in the circuit to prevent discharge between the ignition anode and the cathode.

The voltage between the process anode and the metal source cathode is preferably from 12 to 100 Volts, but not more than twice the ionization potential of the cathode material. The two anodes are made of any metal with a high melting temperature, e.g. steel.

As the electric discharge is generated between the ignition anode and the metal source cathode, the metal begins to melt, e.g. at a surface temperature of 4000-5000 °C, then it explodes producing plasma comprised of positively charged ions and negatively charged electrons, in addition to metal vapor and clusters of 5-10 atoms. The plasma fills the space between the electrodes and forms a conductive connection between the process anode and the metal source cathode. The resulting electric current between the process anode and the metal source cathode causes the surface temperature of the latter, at the loci of highest discharge density, to be above its boiling point, e.g. 4000-5000 °C. The regions of highest discharge density radiate to the entire surface area of the metal source cathode. The surface of the metal source cathode continues to explosively boil, resulting in a range of droplet sizes from a single atom to relatively large drops, e.g. with a diameter of 1 mm. The droplets continue to move downwards away from the metal source cathode, become spherical due to surface tension, and finally reach a zone of the vacuum chamber the temperature of which is below the melting point of the metal and solidify into spherical particles within a bath of vacuum oil located on the collecting plate to prevent the agglomeration of the particles.

The diameter of the drops is dependent on the metal of the cathode and the energy density on the cathode. The electrode surface begins melting when heated rapidly, and the intensity of the explosive boiling is a function of the energy intensity. When the energy density is 10E6 to 10E7 amp/cm² large drops at high velocity (several hundred meters per second) result from explosions on the surface of the electrode. When the energy density is between 10E3 and 10E5 amp/cm², drops are produced by explosive boiling on the surface of the electrode but, as the energy density is lower, the intensity of the explosions is less and thus the velocity of the drops is much lower, i.e. 1-2 m/sec. This is a region of glow discharge or anomalic glow discharge. The electrode may be fragmented in any regime of electrical discharge (electric arc to glow discharge), but for optimal fragmentation of larger droplets anomalic glow discharge is preferred.

Glow discharge differs from other forms of electric discharge insofar as volumetric ionization occurs, in contrast with electric arc discharge by which ionization occurs at only a close proximity to the cathode. Ionic collisions are practically non-existent between the cathode and anode, and consequently ionization is negligible in that region. The discharge voltage is a multiple of the ionization potential of the gas in which the discharge occurs. Glow discharge occurs at a current density of 10E2 to 10E5 amp/cm², and electric arc at above 10E6 amp/cm². Anomalic glow discharge exhibiting characteristics of both electric arc and glow discharge occurs during the transitional stage of 10E4 to 10E6 amp/cm².

The substantially lower velocity increases the time that the plasma affects the drop. This results in instantaneous heating of the drop and its explosion into smaller droplets. The result is an increase in the fraction of smaller drops. Larger drops of copper, for example, are in the range of 10 to 100 microns and smaller drops, under 10 microns. The drop size is also dependent on other parameters such as the material and the intensity of the discharge. For example an alloy of lead and tin can produce drops with a diameter of 1mm to 2mm. Larger drops exit at larger angles (30 to 90 degrees relative to the vertical axis of the cathode).

This invention offers several ways of decreasing the percentage of larger particles within the produced powder.

1) The larger drops pass between two additional electrodes where there is already a glow discharge or a voltage differential of up to 100V. At this voltage a passing drop causes a discharge between the additional electrodes. The discharge passes through the drop, heats it rapidly and causes it to explode. These effects were observed and filmed during experimentation.

The metal particles produced as hereinbefore described have diameters from 0.01 μm to 1 mm. They may be further processed to produce granules of an even smaller size, e.g. from O.OOlμm to 0.1 mm. For this purpose, the process of the invention optionally comprises the additional step of passing the larger droplets and/or granules through at least one second set of electrodes between which an electric discharge is generated. The larger droplets and/or granules are heated as a result of this second electric discharge and then explode. Additional stages of electric discharge processing, similar to the above ones, may be implemented to produce even smaller granules. By "larger droplets and/or granules" are meant the droplets and/or granules having a diameter not smaller than 0.01 μm. The physical process of electrical discharge between electrodes in a vacuum are described in "Impulsny Electrichesky Razniad V Vacuumy", Mesatz G.A., Proskorosky, D.I., and Nauka, Novosibirsk, 1984. It is mentioned that discharge can be caused by a particle impinging on the electrode because there will be an ignition of discharge between the particle and the electrode. If the particles passing between the electrodes are liquid and are at a high temperature (above 1000 deg c), greater electron emission from the surface of the drop results and therefore the discharge voltage is lowered. Experiments show that 100V is sufficient for discharge.

The book mentions that electric discharge between two electrodes can occur when a fragmented droplet approaches one electrode and sparking occurs. As the droplet travels towards the cathode it evaporates because of auto emission from the cathode, i.e. the emission of electrons from a surface at a temperature of at least 800 °C. In the inventive method in which an electric field is added to enhance the emission, thermo-auto emission occurs. The droplets are dispersed between electrodes at a high temperature, and therefore discharge at a lower voltage may be used. The reason for a lower voltage requirement is because the charge on the drop resulting from the thermo-auto emission will be higher. Formulae for the calculation of the discharge voltage as a function of the drop size and intensity of the electric charge in the vicinity of the drop are disclosed in "Electritchesky Proboy Y Razriad V Vacuumy" , Slivkov I.N. et al, Moscow Atomisdat, 1966. According to the authors, as the drop size is larger and the charge is higher, the discharge voltage between the drop and cathode will be lower. This voltage causes the discharge between the anode and cathode. Experimentation has shown that the development of the discharge is such that a major portion of the energy is absorbed by the drop, increasing its temperature and causing it to evaporate explosively into finer droplets.

2) The time during which the particle is in the plasma stream is increased. Low particle velocity is conducive to increased drop explosion. Experiments show that the time needed for exploding the drop is approximately 10E-3 seconds. To achieve the time conducive to drop explosion the length of the electrode may be increased and in this region the plasma density will be maintained. While in the plasma region, the drop is affected by the plasma ions which increases its temperature rapidly and causes explosive evaporation.

3) It is possible to rotate the cathode electrode with or against the direction of rotation of the magnetic field caused by the plasma. The centrifugal force assists in the creation of the stream of droplets.

The powder produced according to methods 1 to 3 is collected in a collection plate on which a layer of vacuum oil is provided and/or from the surface of the electrodes. The material deposited on the electrodes is removed by scraping. ) The stream of particles can be collected on a rotating drum or a belt. The particles are removed from the drum or belt by a brush or scraper and then fall into the collecting tray below. A voltage of 12V to 100V is applied to the drum or belt. As a result of this voltage, the particles are easily removed by the brush or scraper. An application of voltage is unnecessary for certain metals having a low melting temperature. Experiments show that the particles collected in this way are soft and that slight abrasion by the scraper is sufficient to create fine particles.

5) The granules may be pulverized. The collecting tray is filled with metal balls having a diameter between 5mm to 20mm made of a hard metal with a high melting point. Droplets resulting from the fragmentation of a metal sample deposit onto these balls. The rotating motion of the tray causes the balls to move with respect to one another. This action grinds the deposit creating a fine powder which is collected at the bottom of the tray. The tray is conical with an angle of 1 to 10 degrees in the center of which a hole is bored for the extraction of the powder.

Most of the said larger droplets and/or particles separate from those having a smaller diameter as they move downwards through the vacuum chamber. The smaller droplets and/or particles follow downwardly trajectories enclosed in a cone having an aperture from 0 to 40 degrees about the vertical - as it will be better explained hereinafter — and therefore fall directly on the vacuum chamber bottom, whereas the larger droplets tend to follow trajectories outside said cone. The separation between larger and smaller droplets and/or particles, however, is not complete, and therefore some larger ones may escape the additional stage or stages of electric discharge processing. In a separate process, droplets may be deposited on metallic or non-metallic granules previously introduced onto an oil-free collection plate thereby producing encapsulated powder. The consumable cathode producing the shells may be a metal or alloy. The thickness of the shell may be controlled by varying the time during which the consumable cathode is deposited on the existent granules or by varying the distance between the consumable cathode and the collection plate.

Multi-layered shells may be produced by fragmenting, sequentially, electrodes of varying metals or alloys whereby the droplets emanating from these electrodes are deposited on the granules disposed on the collection plate.

Hollow shells may be produced from metals or alloys, either as single or multi-layered shells. The base powder is chosen such that it is easily dissolved in an acid or alkali which does not affect the material of the shell. After the shells have been produced, the granules are placed in a bath of acid or alkali and the base granule is dissolved, thereby resulting in hollow shells.

If a voltage of between — 2V to -6V is applied to the collection plate, perforated shells may be produced.

Semiconductor powder may be produced with the aforementioned methods. To achieve this powder, the semiconductor material has to be heated to a temperature at which it becomes fully conducting and thereby may be used as a cathode. The ensuing processes are similar to those mentioned above.

The invention also provides an apparatus for the production of spherical metal particles, which comprises: a) a vacuum chamber, provided with a source of vacuum; b) a support for a metal source at or about the top of said chamber; c) a process anode, preferably of substantially annular shape; d) a first electrical circuit comprising a source of direct voltage and joining said process anode and said metal source as cathode; e) a means for initiating discharge from said metal source, said discharge initiation means being disposed in the vicinity of said metal source; f) a second electrical circuit comprising a source of direct voltage and joining said ignition anode and said metal source as cathode; and g) a collector at or about the bottom of said chamber for collecting granules of the metal.

The collector preferably comprises a substantially cup -like plate having a preferably central discharge opening and a vessel or vessels for gathering and retaining the granules of metal. Said cup-like plate is preferably cooled. For the production of one-layer metal powder the collector is filled with vacuum oil. For shell production a brush is mounted on the collector which continuously mixes the powder.

In one embodiment, a plurality of metal sources may be disposed at or about the top of the vacuum chamber, and spherical metal particles may be simultaneously or consecutively produced from said plurality of metal sources to increase the production rate.

An additional embodiment of the apparatus consists of a plurality of cathode metal sources for producing encapsulated metal particles.

In a further embodiment of the invention, the apparatus comprises a means for reducing the size of the larger metal droplets and/or granules. In a preferred embodiment at least one additional pair of secondary electrodes, preferably of frusto-conical shape, and voltage sources for creating a voltage difference between said secondary electrodes are provided being so structured and positioned that at least a substantial portion of the larger metal droplets and/or granules pass between them thereby being fragmented into smaller metal droplets and/or granules as they descend in the vacuum chamber .

Brief Description of the Drawings

In the drawings:

Fig. 1 is a schematic diagram of the inventive system required for the production of fragmented metal powder with a cross-sectional view of the electrode unit.

Fig. 2 is a wiring diagram of the electrical system with the use of one set of discharge electrodes. Fig. 3 is a partial schematic diagram, partial side view representing the generation of metal droplets in a two-stage discharge unit.

Fig. 4 is a graph of droplet distribution as a function of droplet size showing the improvement with the use of the inventive method. Fig. 5(a) is a schematic diagram of a one-stage discharge unit with an elongated anode. Fig. 5(b) is a schematic diagram of a one-stage discharge unit with an elongated conical anode. Fig. 6 is a schematic diagram of a one-stage discharge unit with a rotating drum to pulverize the granules.

Fig. 7 is a schematic diagram of a one-stage discharge unit with a conveyor belt to pulverize the granules.

Fig. 8 is a schematic diagram showing the cooling system of the collection plate.

Fig. 9 is a schematic diagram of a method to produce smaller granules by means of metal balls. Fig.10 is a block diagram representing the various inventive methods of reducing the percentage of larger granules within the produced powder.

Fig. 11 is a schematic diagram representing the method of coating granules with a succession of different materials. Fig. 12(a) is a front view with a partial schematic diagram of an apparatus for producing powder from semiconductors. Fig. 12(b) is a side view with a partial schematic diagram of an apparatus for producing powder from semiconductors.

Detailed Description of Preferred Embodiments

Figure 1 shows vacuum chamber 1 in which the metal powder is formed, according to an embodiment of the invention. Vacuum chamber 1 is cylindrical, in this embodiment, and made e.g. from stainless steel. Electrode unit 10 passes through upper cover 7 and collection unit 30 passes through lower cover 8 of said chamber. The vacuum is regulated by auxiliary pump 27 for low vacuum applications and by diffusion pump 13 for high vacuum applications. Auxiliary pump 27 is isolated from the rest of the system by means of valve 29. The vacuum may be adjusted by means of valve 28. Vacuum chamber 1 is provided with windows 2,3 for viewing the fragmentation process and with orifice 5 for the inclusion of vacuum gauge 6.

Electrode unit 10 comprises cylindrical metallic source 14 to be fragmented, constituting the cathode, and annular process anode 17. The vertical axis of process anode 17 is situated vertically below that of metal source 14. The inner diameter of process anode 17 is greater than the outer diameter of metal source 14, so that the particles emitted from said metal source will pass through the anode 17. The outer diameter of process anode 17 is greater than that of metal source 14, e.g. at least l.ltimes greater. The vertical length of process anode 17 is e.g. 2.0-150.0 cm. Rings 16, electrically insulated from the cathode, are mounted around metal source 14 to ensure that all emission will take place from the lower extremity of said source and not from its side. Upper cover 7 is constructed from a preferably dielectric material, to prevent short-circuiting. Metal source 14 is supported by mechanical means 15 at the top of vacuum chamber 1.

Igniting electrode 19, situated in close proximity to metal source 14, which constitute the cathode in the process of the invention, initiates the electrical discharge. The electrical connections 18 to electrode unit 10 are located outside of vacuum chamber 1. In order to prevent cathode 14 and process anode 17 from reaching their melting point, each electrode is provided with cooling liquid. Only the discharge surface reaches the melting point- the remaining portion of each electrode is cooled.

Figure 2 illustrates the electrical system relating to the electrodes situated within vacuum chamber 1. Cathode 14 is connected to the negative pole of DC power supply 25 via varistor 24 and switch 22. The resistance of varistor 24 is changed to adjust the intensity of the electric discharge. Process anode 17 is connected to the positive pole of DC power supply 25 via switch 23. Ignition electrode 19 is connected to the positive pole of DC power supply 25 via varistor 28 and switch 21. The electrical discharge is initiated by a quick one-time closing of the circuit between electrodes 19 and 14. Switch 21 is closed and immediately reopened. Alternatively the electrical discharge is initiated by a short circuit between cathode 14 and process anode 17, by means of a pulse of voltage (not shown), or by any other method known to those skilled in the art. While auxiliary pump 27 and diffusion pump 13 are in operation providing a vacuum of at least lOE-l torr, switches 21,22 and 23 are closed. Switch 23 is then instantaneously re-opened inducing a spark. Electrical discharge continues by means of cathode 14 and process anode 17. Metal source 14 first melts and then it vaporizes, producing vapor and plasma, schematically indicated by 41 in Fig. 3, that fills the volume between electrodes 14 and 17, as shown in Fig. 3. Electrical discharge therefore continues between said electrodes through said plasma. The material sprayed from the electrode forms metal droplets 46, and they assume a spherical shape and finally produce solid, spherical granules when being cooled during their descent through vacuum chamber 1 and precipitate on collection plate 32. Vaporization may be intensified if metallic sample 14 is rotated, by means not shown.

The granules may be further fragmented by providing a second set of electrodes, consisting of secondary cathode 47 and secondary anode 48, whose electrical wiring (not shown) is similar to that of the first set of electrodes 14 and 17. Said secondary electrodes, preferably located vertically below the first set of electrodes 14 and 17, are preferably hollow, inverted frusto-cones having a vertical axis and an aperture angle between 60 to 140 degrees, and flaring out towards the bottom. The smaller, topmost inner diameter of cathode 47 is larger than the smaller, topmost outer diameter of anode 48, thereby enabling free passage of droplets and/or granules between said two secondary electrodes.

During the first stage of fragmentation resulting from the glow discharge between electrodes 14 and 17, metal source 14 is fragmented into droplets 46 having varying diameters. Droplets 46 that are dispersed at a small angle, e.g. smaller than 30 degrees, from the vertical axis of the apparatus, fall through the interior of secondary anode 48 onto collection plate 32 and do not undergo a second stage of fragmentation. Droplets 49 that are dispersed at an angle e.g. between 30 and 70 degrees from the vertical axis of the apparatus pass through the gap between secondary electrodes 47 and 48. The glow discharge resulting from secondary electrodes 47 and 48 selectively causes the larger droplets 49 to explode producing smaller droplets 50. Droplets 50 acquire a spherical shape upon their descent through vacuum chamber 1. They then harden and form granules 53, and these latter precipitate onto collection plate 32. They are discharged from said plate through discharge 54 and therefrom to storage vessels 55.

The graph shown in Figure 4 represents a Gaussian curve of the distribution of droplets relative to their size. Curve 56 is a typical distribution of prior art granules, whereas curve 57 represents the distribution of droplets obtained by the process of the invention. Region 58 indicates the change in distribution of droplet size resulting from the second stage of fragmentation.

Figure 5 (a) shows a detail of another embodiment in which only one stage of fragmentation is employed. Process anode 60 is much longer than process anode 17 of the first embodiment, e. g. by about 100 cm. The time during which the droplets produced by the condensation of vapor from metal source 14 pass through the process anode is increased thereby, enabling the explosion of the larger droplets.

A scraper (not shown) may be used to remove any particles that deposited on the electrodes. To prevent droplets from adhering to the inner wall of the process anode, such as 61 in Fig. 5(a), said process anode may be made with a frusto-conical shape, flaring out towards the bottom, such as process anode 63 of Fig. 5(b). Figure 6 shows another embodiment of one-stage fragmentation in which the droplets precipitate on and adhere to a rotating drum 65. As drum 65 rotates, the granules formed from the precipitated droplets come in contact with rotating metallic brush 66. Metallic brush 66 rubs against the granules and pulverizes them. The pulverized particles will not oxidize, as they do in the prior art mechanical method, since the brushing action takes place within vacuum chamber 1 in which a negligible amount of oxygen exists. The smaller particles ultimately fall onto collection plate 67.

Fig. 7 shows another embodiment, in which the droplets precipitate on conveyor belt 68, driven by two pulley 69 actuated by an electric motor (not shown) located externally to the vacuum chamber. Two metallic brushes 66 pulverize the granules that are formed until they fall onto collection plate 70. Drum 65, in the embodiment of Fig. 6, and conveyor belt 68 are cooled by a cooling e.g. by a cooling fluid.

In the embodiments of Figures 6 and 7, the droplets may be collected on rotating drum 65 or belt 68. The granules produced from the precipitated droplets are removed from the drum or belt by metallic brush 66, or by means of a scraper (not shown), and then fall into the collection plate below. A voltage of 12V to 100V, with a current of no more than 10 amperes, may be applied to drum 65 or to belt 68, and the granules are thereby easily removed by the brush or scraper. An application of voltage is unnecessary for certain metals having a low melting temperature. In this configuration the drum or belt act as an anode either alone or in conjunction with at least one process anode. When additional anodes are utilized the full discharge current will not flow through the drum or belt. Collection unit 30 includes collection plate 32 for collecting precipitated granules 53, metallic brush 36 for preventing their agglomeration, motor 33 with reduction gearing 34 for rotation of collection plate 32, and vibrator 37 for providing a vibrating action to collection plate 32 (see Fig. 1). Metallic brush 36 is also used to mix the particles during the encapsulation process (Fig. 11) Hollow shaft 54 is driven by motor 33 and serves to rotate collection plate 32. In addition to agitating granules 53, hollow shaft 54 also serves as the means through which granules 53 are transported from vacuum chamber 1 to collection vessel 38 (Fig 3). Alternatively the vibrating action is accomplished by means of forked conduit 40 (Fig. 3) End product 55 is stored in collection vessel 38.

The collection plate may be is constructed from three similarly shaped circular shells, having an inclined profile with gap 71 between the shells, as shown in Figure 8, where the plate is generally indicated at 74. Cooling liquid passes through gap 71 to cool the granules that have fallen on collection plate 74. Collection plate 74 is provided with a layer of oil 73 covering the granules for additional cooling. Without layer of oil 73 the granules would coagulate. If the cooling liquid were not provided, layer of oil 73 would evaporate within the vacuum.

An additional embodiment of the invention is shown in Figure 9. The collection plate is filled with metal balls 75 having a diameter between 5mm to 20mm made of a hard metal with a high melting point. Alternatively, non-metallic balls may be utilized. Droplets resulting from the fragmentation process deposit onto these balls forming coated balls 76. The rotating motion of the collection plate by means of motor 33 (Fig.l) and metallic brush 36 causes one coated ball 76 to move relative to another. This action grinds the deposit from the surface of coated balls 76 creating a fine powder which is collected at the bottom of the collection plate. The collection plate is conical with an angle of 1 to 10 degrees. A hole (not shown) is bored in the center of the collection plate for the extraction of the powder.

A block diagram representing the various inventive methods of decreasing the fraction of larger granules is shown in Figure 10. The metal source constituting the cathode is represented in step 80, and is alternatively fragmented by low density plasma in step 81 or by high density plasma in step 82. In a preferred embodiment a molten droplet passes through an additional set of electrodes in step 83 and explodes, or even through a third (or more) set of electrodes in step 84. The larger and smaller droplets precipitate on the collection plate in step 85, and the solidified granules are removed for powder collection in step 86. A layer of oil is provided on this collection plate to prevent agglomeration of the precipitated particles.

Alternatively the fragmented particles are exposed to an elongated process anode in step 90, thereby increasing their time in the plasma stream. The plasma stream in this alternative is of high density, whereas in the other alternatives high density plasma is dispersed in the vacuum chamber, due to the configuration of the apparatus, to thereby form low density plasma. A third alternative consists of rotating the cathode in step 91 with or against the rotation direction of the magnetic field induced by the plasma to increase the rate of fragmentation. In both of these cases the droplets precipitate on the collection plate in step 85, and the solidified granules are removed for powder collection in step 86.

In a fourth alternative the droplets precipitate on a belt or on a rotating drum in step 92. The granules are then pulverized by a scraper or brush in step 93 and then removed for powder collection in step 94. In a fifth alternative the droplets precipitate on metal balls in step 95. The deposit is pulverized by a mutual grinding action of the coated metal balls by means of a scraper or metallic brush in step 96. The pulverized deposit is removed for powder collection in step 94.

The inventive method may also be used to encapsulate previously produced granules (up to 50 microns), metallic or non-metallic. The thickness of the shell may be controlled to be between 0.1-150.0 microns. The granules are encapsulated in a metallic shell whereby the shell is of the cathode material used for fragmentation. Initially the granules to be encapsulated are placed on the collection plate. The stream of fine particles arising from the cathode is deposited on these granules. The spherical particles, being molten upon impact, flatten to coat the existing granules. The encapsulated granules are constantly agitated and mixed by rotating collection plate 32 with motor 33 and by means of vibrator 38 (Fig.l) and metallic brush 36. As a result of the agitation, the molten particles uniformly impact these particles, and an even shell is therefore built up on each granule. For a given cathode material the thickness of the shell is dependent only on the encapsulation time, the longer the time the thicker the shell.

It is possible therefore to build up multiple shells on the given powder grains by depositing shells of different metals consecutively on top of the other. This can be achieved for example as shown in Fig. 11 where each of the three electrodes 97, 98 and 99 consists of a different metal and each serves as the cathode in turn in the order that the shells are required. The thickness of each respective shell may be accurately controlled by varying the time during which the consumable cathode is deposited on the existent granules or by varying the distance between the cathode and the collection plate. The collection plate is mechanically positioned before the commencement of the fragmentation process.

The output of the end product is a function of the discharge power, i.e., the current and voltage applied to the electrodes, and of the number of electrodes mounted in vacuum chamber l.The production rate may be increased by fragmenting several consumable electrodes simultaneously.

It is also possible to create hollow shells. It has been found that hollow metallic shells may be used as efficient catalysts due to the larger surface area and resulting reaction time. Hollow shells are achieved by encapsulating a powder which dissolves in a given acid or alkali with a shell of a desired metal. After removal of the shelled granules from the vacuum chamber they are immersed in a bath of the applicable acid or alkali (which is chosen such that it reacts with the granule material and not with the shell metal). In this way the inner granule is dissolved leaving only the empty shell. The reaction between the acid or the alkali and the inner granule occurs via pinhole porosity in the shell surface. The pinholes are created by applying a voltage of between -2V and -6V, which is less than the ionization potential of the metal source that generated the shell. This voltage is applied only for a short time after the shell has been produced. The length of time that the voltage is applied is determined experimentally for each material. The voltage raises the kinetic energy of the ions and the positively charged droplets, causing collisions and sparking between the ions and the newly produced shell and thereby resulting in perforations. Voltage above the ionization potential would induce further discharge of the material of the shell. It is possible to improve the mechanical properties of the material of the shell by further heat treatment. Semiconductor powder may be produced with the inventive method. In order for semiconductors (eg. silicon) to be used as a cathode material for powder production, the sample material must exhibit the conductivity of metals for plasma generation in conjunction with the process anode. This is achieved by heating the sample material to a temperature unique for each given element. The apparatus for semiconductor powder production is shown in Fig. 12. Heating element 101 is wrapped around the upper periphery of semiconductor material sample 100 constituting the cathode. Heating element is provided with current source 102. Metal cap 104 is fastened to semiconductor sample 100 by means of a friction fit, and through direct contact, current 105 is applied to semiconductor sample 100 via connectors (not shown) attached to metal cap 104. The connectors cannot be attached directly to semiconductor sample 100 due to the nature of the material. Anti-discharge shield 106 is used to prevent any discharge from heating element 101.

This effect in which a metal sample may be fragmented into minute spherical particles through various stages of electric discharge processing may be accomplished most effectively using only anomalic or glow discharge, at the current density mentioned above. Hot spots appear on the electrodes, and electrode material is evaporated in the form of droplets or atoms. These droplets or atoms are partially ionized by bombardment from electrons present between the electrodes. With glow discharge, in which light is emitted when electrons absorb a low level of thermal energy and reach an excited state, metal droplets reach velocities of 1-2 m/sec on average. Under these conditions an optimal number of droplets will explode.

The following table compares the different methods of producing metal powder: Table 1

Comparison of granule size (microns) relative to metal powder production method employed

As shown with usage of the method of this invention the granule size of the metal powder is greatly lowered. An application of this method may produce nanomaterials, viz. materials the linear dimensions of which are in the order of nanometers. At these dimensions, a variety of confinement effects change the property of most materials. The granules produced by this inventive method are spherical whereas in other prior art methods the particles are of random shapes.

The following tables show that employment of the inventive method will result in an apparatus being more compact in size and having a longer longevity.

Table 2

Comparison of space required (m²) for apparatus relative to metal powder production method employed

Table 3

Comparison of longevity (years) for apparatus relative to metal powder production method employed

The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention.

Claims

1. Method for the manufacture of metal granules, which comprises: a) Placing a metal source in a vacuum chamber. b) Providing in said chamber a process anode and a means for initiating discharge from said metal source. c) Generating an electric discharge between said discharge initiation means and said metal source as the cathode, whereby to raise the surface temperature of the metal source above its boiling point and to generate metal vapor and metallic plasma. d) Generating a flow of current between said process anode and said metal source through said metallic plasma, whereby to continue to generate metal vapor from the cathode. e) Allowing said vapor to condense into spherical molten drops and said molten drops to solidify to spherical particles. f) Withdrawing said spherical particles from said vacuum chamber.

2. Method according to claim 1, wherein the vacuum is at least 10 E-l torr.

3. Method according to claim 1, wherein the metal source is placed at the top of the vacuum chamber and the metal drops and spherical particles are allowed to move down to the bottom of said chamber by gravity and are withdrawn from said chamber at the bottom thereof.

4. Method according to claim 1, wherein said discharge initiation means is an ignition anode, said ignition anode being in momentary contact with the metal source as an ignition voltage of 10 to 100 volts is applied between them, thereby creating an ignition spark.

5. Method according to claim 1, wherein said discharge initiation means is a short circuit produced between said process anode and metal source.

6. Method according to claim 1, wherein said discharge initiation means is a pulse of voltage.

7. Method according to claim 1, wherein the voltage between the process anode and the metal source cathode is of 10 to 100 volts.

8. Method according to claim 1, wherein the metal particles produced have diameters of 0.001 μm to 1 mm.

9. Method according to claim 1, wherein said molten drops and/or particles are further subdivided.

10. Method according to claim 9, further comprising passing at least a part of the larger drops and/or granules formed through at least one second set of electrodes between which an electric discharge is generated, whereby to heat said larger drops and/or granules and cause them to explode.

11. Method according to claim 10, wherein most of the smaller drops and/or particles follow, as they move downwards through the vacuum chamber, trajectories enclosed in a frusto-cone having an aperture from 0 to 30 degrees about the vertical, whereas the larger drops and/or particles tend to follow trajectories outside said frusto-cone.

12. Method according to claim 9, wherein the time during which said generated metal vapor is in communication with said metallic plasma is increased.

13. Method according to claim 12, wherein said process anode is annular, the length of which being at least 2 cm.

14. Method according to claim 13, wherein said process anode has a length of between 2 and 150 cm.

15. Method according to claim 9, wherein said metal source is rotated.

16. Method according to claim 9, wherein said molten drops precipitate on a rotating drum to which is applied a voltage of between 10 to 100 volts, said solidified particles being subdivided by a scraping means.

17. Method according to claim 9, wherein said molten drops precipitate on a belt to which is applied a voltage of between 10 to 100 volts, said solidified particles being subdivided by a scraping means.

18. Method according to claim 1, wherein said molten drops precipitate on a collection means including a cooled metallic surface and a means for uninterrupted scraping of said solidified spherical particles.

19. Method according to claim 18, wherein said collection means includes a container containing cooling liquid.

20. Method according to claim 19, wherein said cooling liquid is vacuum oil.

21. Method for the manufacture of encapsulated objects, which comprises: a) Placing a metal source in a vacuum chamber. b)Providing in said chamber a process anode and a means for initiating discharge from said metal source. c) Introducing objects to be encapsulated onto said collection means. d) Generating an electric discharge between said discharge initiation means and said metal source as the cathode, whereby to raise the surface temperature of the metal source above its boiling point and to generate metal vapor and metallic plasma. e) Generating a ^' flow of current between said process anode and said metal source through said metallic plasma, whereby to continue to generate metal vapor from the cathode. f) Allowing said vapor to condense into spherical molten drops. g) Allowing said molten drops to be flattened as they impinge said objects to be encapsulated, thereby coating said objects, h) Allowing coating to solidify, i) Withdrawing encapsulated objects from said vacuum chamber.

22. Method according to claim 21, wherein the objects to be encapsulated consist of metallic or non-metallic granules.

23. Method for the manufacture of metal shells in accordance with claim 22 whereby a plurality of metal sources are fragmented to produce coatings in succession of each metal source, each coating being produced with a controlled coating thickness and composition.

24. Method according to claim 23, further comprising applying a negative voltage to said collection means of less than the ionization potential of the metal source that generated the shell, whereby to produce porous shells.

25. Method according to claim 24, further comprising: a) Immersing said encapsulated granules in a solvent of the granules which is not a solvent of the coating, whereby said solvent dissolves the granules and leaves the coating unaffected. b) Removing the hollow shells from said solvent.

26. Method according to claim 21, wherein the objects to be encapsulated consist of metallic balls.

27. Method according to claim 21, wherein the objects to be encapsulated consist of non-metallic balls.

28. Method according to claims 26 and 27, further comprising: a) Rotating said collection means. b) Allowing the encapsulated balls to move with respect to one another, whereby to produce a grinding action. c) Allowing the coating to be pulverized thereby producing powder. d) Withdrawing said powder from the collection means.

29. Method for the manufacture of semiconductor granules, which comprises: a) Placing the semiconductor source in a vacuum chamber. b) Heating the semiconductor source with a heating element thereby acquiring the characteristics of an electrical conductor. c) Providing in said chamber a process anode and a means for initiating discharge from said semiconductor source. d) Generating an electric discharge between said discharge initiation means and said semiconductor source as the cathode, whereby to raise the surface temperature of the semiconductor source above its boiling point and to generate semiconductor vapor and semiconductor plasma. e) Generating a flow of current between said process anode and said semiconductor source through said semiconductor plasma, whereby to continue to generate semiconductor vapor. f) Allowing said vapor to condense into spherical molten drops and said molten drops to solidify to spherical particles. g) Withdrawing said spherical particles from said vacuum chamber. .

30. Apparatus for the production of metal particles, which comprises: a) a vacuum chamber, provided with a source of vacuum; b) a support for a metal source at or about the top of said chamber; c) a process anode; d) a first electrical circuit comprising a source of direct voltage and joining said process anode and said metal source as cathode; e) a means for initiating discharge, said discharge initiation means being disposed in the vicinity of said metal source; f) a second electrical circuit comprising a source of direct voltage and joining said discharge initiation means as anode and said metal source as cathode; g) a means for cooling said cathode and said process anode; and h) a collector at or about the bottom of said chamber for collecting granules of the metal.

31. Apparatus according to claim 30, wherein said discharge initiation means is an ignition electrode.

32. Apparatus according to claim 30, wherein the process anode is of substantially annular shape.

33. Apparatus according to claim 32, wherein the length of said process anode is between 2 and 150 cm.

34. Apparatus according to claim 30, wherein said collector comprises a substantially cup-like plate having a discharge opening and a vessel for gathering and retaining the granules of metal.

35. Apparatus according to claim 34, comprising a means for cooling the cup -like plate.

36. Apparatus according to claim 30, further comprising a means for subdividing the droplets and/or granules as they descend in the vacuum chamber.

37. Apparatus according to claim 36, further comprising at least one additional pair of secondary electrodes and voltage sources for creating a voltage difference.

38. Apparatus according to claim 37, wherein the secondary electrodes being so structured and positioned at least a substantial portion of the larger metal droplets and/or granules between said electrodes pass between them as they descend in the vacuum chamber.

39. Apparatus according to claim 38, wherein the secondary electrodes are of frusto-conical shape.

40. Apparatus according to claim 36, wherein said container includes: a) two shafts at least one of which being driven by an electric motor situated outside of said vacuum chamber; b) an endless and cooled metal belt means wrapped around said shafts; and c) a metallic brush in contact with the underside of said metal belt means which pulverizes the granules precipitating on said metal belt means.

41. Apparatus according to claim 40, further including a source of direct voltage, said direct voltage source being applied to said metal belt means.

42. Apparatus according to claim 40, wherein said metallic brush rotates in a reverse orientation to that of said metal belt means.

43. Apparatus according to claim 36, wherein said container includes a rotating and cooled drum and a metallic brush which pulverizes the granules precipitating on said drum.

44. Apparatus according to claim 43, further including a source of direct voltage, said direct voltage source being applied to said rotating and cooled drum.

45. Apparatus of claim 36, further including a means for uninterrupted agitation of said granules.