RU2846419C2 - Method of producing powder with core-shell structure - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to powder metallurgy, particularly, to production of powder materials with core-shell particle structure. It can be used for production of powders with combination of various metals of core and shell. Powder of core material is supplied by means of dosing disc rotating at constant speed and moving the above particles of core material to the zone of outlet nozzle, in which the above particles of core material powder are mixed with inert gas flow so that gas-powder mixture is obtained. Obtained mix is fed via pipeline into plasmatron working zone via annular nozzle to make annular powder flow along the perimeter of annular nozzle wall. Plasma arc is used to heat precursor end surface of shell material to produce liquid ply at translational displacement of said shell precursor along its axis. Core particles are then spheroidised, the shell material is deposited on the core particles and cooled in an inert gas to form particles with a core-shell structure.

EFFECT: higher efficiency and quality of produced powder particles.

6 cl, 1 dwg

Description

Field of technology to which the invention relates

The invention relates to methods for producing powder materials with a core-shell structure using plasma technology.

State of the art

A method for producing core/shell powder materials of nanoparticle size using plasma technology is known (RU2620318, publication date 24.05.2017), which includes a process causing the formation of plasma in a solution to extract from the solution salts of two metals that are dissolved in this solution by successive precipitation of the first metal, which forms the core, and the second metal, which forms the shell. By applying voltage between the electrodes in the solution, it is locally heated between the electrodes, and in the resulting finely dispersed bubbles, insulation breakdown occurs, and a plasma discharge begins. In this case, at the first stage, a plasma of the first power is generated to generate plasma in order to selectively cause the deposition of the first metal and to form nanoparticles as a core, and at the second stage, a second power is applied, which is greater than the said first power, in order to cause the generation of plasma in order to deposit the second metal, which has a lower oxidation-reduction potential than the first metal, on the surface of the said cores in order to form shells that consist of the second metal. The evaluation criterion for switching the power is the termination of the deposition of the first metal and the beginning of the deposition of the second metal, when the core, which is formed by the first metal, increases to a size large enough to begin the formation of the shell by the second metal. In order to begin the formation of the shell from the second metal, the size of the core formed by the first metal is preferably in the range of more than several nm.

The disadvantage of the known method is the difficulty of determining a sufficiently accurate moment of switching the power depending on the size of the core particles. The proposed method of switching depending on the value of the solution transmission coefficient, which reaches a given value (recommended in the range of 3-5%), requires UV-Vis analysis using ultraviolet and visible light spectrometry, which is a complex process. In addition, before applying the shell, the obtained core particles are extracted from the solution, washed and impurities are removed from the surface, which is technologically complex and requires execution in a vacuum or inert gas environment to avoid oxidation of the core surface.

A method for producing core-shell powder materials using plasma technology is known (CN105328182, publication date 09/29/2016). In the known method, a structured copper nanopowder "core-shell" material with a silver coating is obtained. In this case, copper powder and silver powder with a mass fraction of purity of 99.9% are weighed and uniformly mixed, and then pressed into a block body as an anode material of a plasma-arc furnace. One block contains 16 g of copper powder and 4 g of silver powder. Metallic tungsten or graphite are used as a cathode material of the plasma-arc furnace, and gaseous argon and gaseous hydrogen are used as a working gas, while the volume ratio of gaseous argon is from 50% to 100%. First, cooling water is passed through, the working chamber is evacuated, and used repeatedly. Cleaning and washing with high-purity argon are performed. A DC power source is used to create an arc between the anode and cathode, with the operating current being 60-100 A, and the operating current and voltage are kept relatively stable during

arc discharge time, maintaining a constant value for 0.5 ~ 1 hour. Through arc discharge, rapid heating, melting and evaporation of the anode metal are realized to form metal vapor; the melting point of copper is 1083 °C, and the melting point of silver is 960.7 °C. During the process, copper vapor with a higher melting point first condenses into copper nanoparticles, and silver vapor with a lower melting point is easily adsorbed on the surface of copper particles. When the temperature drops below the melting point of silver, a silver-coated copper nanocore with a core-shell structure is formed. Then the power is turned off and argon gas is introduced for passivation treatment. The passivation time is up to 2 hours.

A method for producing core-shell powder materials using plasma technology (CN105127437, publication date 09.12.2015) is known, similar to the previous method, in which iron particles are coated with silver. In this case, various proportions of metallic iron powder and silver powder are pressed into a block, which will be used as the anode material of a plasma-arc furnace. (The mass fraction of silver in the anode material is from 10 to 25%). Metallic tungsten or graphite is used as the cathode material, and argon and hydrogen are used as the working gas. Passivation is performed after a certain period of time to obtain a nanopowder iron material coated with silver, the particle diameter of which is 30-70 nm and has a core-shell structure.

The common disadvantage of the two above methods for obtaining core-shell powder materials using plasma technology is the low productivity of the process, due to the small mass of the initial block of pressed initial powders and the need to clean the working chamber and collect the obtained nanopowder from the side wall and the top cover of the plasma-arc furnace after several cycles of obtaining the powder. In addition, when obtaining the powder, the required sphericity may not be ensured due to uneven condensation of the shell material on the surface of the core.

The closest to the claimed technical solution in terms of the set of essential features, adopted as a prototype, is the known method for producing powder with a core-shell structure using, in particular, plasma technology (CN111872378, publication date 03.11.2020), in which the powder for the spherical core is transported from the storage device to the spray chamber through an annular powder channel using a pump and a ring mechanism to form an annular powder flow along the perimeter of the annular channel wall. Metal rods made of the required material with a diameter of 10 to 200 mm are used as a precursor for forming the shell. The end surface of the rod is heated with a plasma, electric arc or electron beam to form a liquid layer on it. The rod is rotated around the axis at a frequency of 1000 to 100,000 rpm using the first drive mechanism and is progressively moved along its axis using the second drive mechanism as the rod melts and is consumed. When the rod rotates around the axis, the liquid layer is dispersed, melt droplets are formed, which, due to centrifugal forces, rush to the core powder particles located on the periphery of the annular channel, settle on them and cover their surface due to surface tension forces. Before performing the powder production operation, the spray chamber is evacuated to vacuum it and then inert protective gas is supplied to the chamber until the specified pressure is reached in the spray chamber. The manufactured powder with the core-shell structure is moved to the powder collection unit, which includes at least two powder collection tanks communicating with the lower part of the spray chamber.

The disadvantage of the known method is the need for high-precision balancing of the precursor, since at a high frequency of its rotation around the axis using the first drive mechanism with a significant imbalance, large centrifugal forces and vibrations will arise, which will negatively affect the process of dispersion of the drops of the precursor material. High-quality coating of the core of a powder particle of a given thickness with a drop of molten precursor during mechanical dispersion of the melt from the end surface of the precursor requires a high-precision assignment of the rotation frequency of the first drive mechanism, which can only be determined empirically, which is also a disadvantage of the known method.

The disadvantage of the known method is also the liquid drop deposition of the melt of the precursor material on the surface of the core powder particle, since the uniformity of the coating application will be significantly affected by the gravity of the drop and, possibly, during the deposition process, its partial flow to one side of the surface of the core particle.

The problem solved by the present invention consists in increasing the productivity of manufacturing powder materials with a core/shell structure and improving the quality of the manufactured powder particles using plasma technology.

The technical result is achieved by loading the initial core powder into the hopper of the gas-powder mixture dispenser in a volume sufficient for operation over a long period of time (up to several hours). From the dispenser hopper, the initial core powder flows by gravity onto the dosing disk, which rotates at a given adjustable constant speed and moves the powder into the outlet nozzle zone, where the inert flow captures it from the disk and carries it away along the pipeline towards the plasma torch. The speed of rotation of the disk, with constant parameters of the hopper window and the nozzle cross-section, determines the volume of powder entering the working zone of the plasma torch per unit of time. As a precursor for the manufacture of the shell

a wire of the required material is used. The wire is fed from a coil using a controlled drive into the working area of the plasma torch through a bushing. A small gap is made between the wire and the bushing, through which cooled inert gas is fed under pressure to prevent air from entering the working area of the plasma torch through the precursor feed and precursor cooling channel. The incoming gas-powder mixture is directed into the annular nozzle of the plasma torch and then into the area where an arc discharge occurs between the end surface of the precursor, which is the cathode, and the surface of the inner part of the nozzle, which is the anode. In this area, melting and spheronization of the particles of the original powder and application of the precursor material, i.e. the outer shell, to the surface occurs. The annular nozzle surrounds the precursor, the area of plasmoid formation and precursor material vapors. In this case, the inner insert of the annular nozzle functions as the anode in the gas discharge and the outer shell of the plasma torch nozzle.

List of drawing figures

Fig. 1 shows a device for implementing the method.

Implementation of the invention

Core-shell particles are a new class of composite materials that feature a set of unique characteristics provided by the particle structure. Each granule is a sphere consisting of two dissimilar or multifunctional materials - a core and a shell.

Fig. 1 shows a device illustrating a simplified diagram of the implementation of the proposed method for producing powder materials with a core-shell structure using plasma technology, on which the following is designated: 1 - powder feeder hopper, 2 - dosing disk of the dosing unit with a drive and a nozzle, 3 - adjustable drive for rotating the dosing disk; 4 - sealed body of the dosing unit; 5 - pipeline for feeding the gas-powder mixture of the initial core material; 6 - wire precursor, 7 - plasma torch anode; 8 - ignition electrode with a nozzle and a guide sleeve for the precursor; 9 - feed drum with a coil of wire precursor; 10 - drive rollers of the feed adjustable drive for moving the precursor; 11 - guide sleeve; 12 - cooler; 13 - particles of core material powder; 14 - particles of the powder of the obtained core-shell material; 15 - cooler manifold; 16 - cooler chamber, 17 - inert gas supply to cooler chamber 16; 18 - inert gas supply to sealed body of dispenser 4; 19 - inert gas supply to gap between precursor 6 and guide sleeve 11; 20 - flow of produced cooled particles.

The initial powder of the core 13 is loaded into the hopper of the gas-powder mixture dispenser 1 in a volume sufficient for operation over a long period of time (up to several hours). From the hopper 1 of the dispenser, the initial powder of the core flows by gravity onto the dosing disk 2, which rotates at a given adjustable constant speed and moves the powder into the zone of the outlet nozzle, where the flow 18 of the inert gas captures it from the disk 2 and carries it away along the pipeline 5 towards the plasma torch. The speed of rotation of the disk 2, adjusted by the rotation drive 3 with constant parameters of the window of the hopper 1 and the cross-section of the nozzle, determines the volume of powder entering the working zone of the plasma torch per unit of time.

An arc discharge is excited between the precursor 6 and the inner surface of the nozzle. The precursor 6, which functions as a cathode, is subjected to intensive spraying in the cathode spot, the precursor material vapors are carried away by the inert gas supplied together with the precursor and which is a plasma-forming gas flow. The plasmoid formed in this case melts and spheronizes the initial powder, which is constantly supplied in the form of a gas-powder mixture of core material granules and inert gas through an annular nozzle in the form of a thin-walled funnel 7, which has a slope toward the center, where the precursor 6 is located. The design of the nozzle parts assumes the possibility of changing the width and angle of inclination of the slit in the process of selecting the optimal values of the process parameters. By changing the width and angle of inclination of the slit, it is possible to change the speed of movement of the initial powder particles in the working area of the plasma torch and, consequently, the thickness of the shell without changing the flow rate of the transporting inert gas for different materials and sizes of the core and shell parts.

In front of the gap formed by the nozzle and the anode, a volume in the form of a ring is organized, the cross-section of which significantly exceeds the cross-section of the nozzle itself. In this volume, tangentially, through two holes located exactly opposite each other, a gas-powder mixture is introduced, which is fed from the feeder through the pipeline 5 and the channel in the plasma torch body. Due to the tangential arrangement of the holes, the gas-powder mixture makes a circular motion in the volume and is gradually sucked into the gap, forming a uniform and axially symmetric flow. Getting into the plasmoid, the particles of the original powder 5 are heated, partially or completely melted, acquiring a spherical shape due to the forces of surface tension, at the same time, vapors or particles of nanopowder of the precursor material 6 are deposited on their surface, forming a shell. The particles then leave the plasmoid region and are cooled in inert gas flows 17 supplied to the cooling chamber 16, crystallizing and forming granules of particles with a core-shell structure, which are directed to the collector 15 and then by flow 20, for example, for separation.

The proposed process does not require high intensity of evaporation of the precursor material with the simultaneous need to obtain high temperature, density and linear dimensions of the plasmoid, sufficient for the particles that have entered it to completely or partially melt and acquire a spherical shape. The magnitude of the electric discharge power required for this should not cause explosive evaporation of the precursor and especially its partial melting, which will interfere with the process, provided that the precursor is the only cathode in the electrode system.

The use of a dosing disk for feeding the initial core powder, the use of a precursor shell in the form of a wire, the use of an inert gas to prevent air from entering the plasma torch working area through the precursor feed channel and cooling the precursor, the application of a shell to the core in a controlled gas environment using the gas-dynamic dispersion of the melt, and additional spheronization of the initial powder particles in the plasma torch working chamber make it possible to increase the productivity of manufacturing powder materials with a core/shell structure and improve the quality of the manufactured powder particles using plasma technology.

Claims (6)

1. A method for producing a powder with a core-shell structure, including feeding particles of core material powder into the working area of a plasma torch through an annular nozzle with the formation of an annular powder flow along the perimeter of the annular nozzle wall, heating the end surface of a precursor made of a shell material with a plasma arc with the formation of a liquid layer during the progressive movement of said shell precursor along its axis as it melts and is consumed, spheroidization of the core material powder particles, deposition of the shell material on the core material powder particles and cooling in an inert gas with the formation of particles with a core-shell structure, characterized in that the core material powder is fed by means of a metering disk rotating at a constant speed and moving said particles of the core material into the area of the outlet nozzle, in which said particles of the core material powder are mixed with a flow of inert gas with the formation of a gas-powder mixture, after which the resulting mixture is fed through a pipeline into the working area of the plasma torch. 2. The method according to paragraph 1, characterized in that the precursor made from the shell material is a wire fed into the working area of the plasma torch from a coil using drive rollers of an adjustable precursor movement drive. 3. The method according to paragraph 2, characterized in that a gap is created between the wire and the bushing guiding its movement, through which cooled inert gas is supplied under pressure to prevent air from entering the working area of the plasma torch through the precursor supply and precursor cooling channel. 4. The method according to any one of paragraphs 1-3, characterized in that the gas-powder mixture is fed into the working zone of the plasma torch, in which an arc discharge is formed between the end surface of the precursor, which functions as a cathode, and the surface of the inner part of the annular nozzle, which functions as an anode, with the formation of a plasmoid. 5. The method according to any one of paragraphs 1-4, characterized in that the gas-powder mixture is fed through an annular nozzle made in the form of a thin-walled funnel with the possibility of changing the width and angle of the slit, ensuring a change in the speed of movement of the core material powder particles in the working area of the plasma torch and the thickness of the shell without changing the flow rate of the transporting inert gas and the dimensions of the core part and the shell. 6. The method according to paragraph 4 or 5, characterized in that the gas-powder mixture is fed into the plasmoid, providing heating, partial or complete melting and spheroidization of the core material powder particles, and at the same time the shell material is deposited on their surface in the form of vapors or nanopowder particles by gas-dynamic dispersion of the melt.