RU2489232C1 - Method of producing metal nano-sized powders - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to power metallurgy, particularly, to production of metallic nano-sized powders.Initial powder of metal oxide compounds with particle size not exceeding 50 mcm is fed by carrier gas into reactor of gas discharge plasma. Initial material is heated to temperature exceeding that of oxides sublimation to evaporate metal and to reduce metal oxides in hydrogen flow or its mix with nitrogen or by argon. Metallic powder is isolated on cooling metal vapors by pulsating inert gas flow at gas flow rate of 1·10^-6-1·10^-3 m³/s.

EFFECT: ruled out or minimised agglomeration of condensed nano-sized pareticles.

7 cl, 3 ex

Description

The invention relates to technologies for the production of metal nanoscale powders, and can be used for the production of powders of pure metals, as well as metal alloys in the chemical industry, powder metallurgy, mechanical engineering and other industries.

There are two main methods for producing metal nanosized powders: chemical and physical. The chemical method involves precipitation in aqueous solutions and reduction of the powder from metal oxides and hydroxides. Physical - this is the high-temperature evaporation of the metal and its subsequent condensation, or a method for producing powders by mechanical grinding (mechanical activation). The latter method, due to its low productivity, is used exclusively in special production of powders.

A known method of producing nickel powder / 1 / by reducing basic nickel carbonate in an aqueous medium at a temperature of 80-90 ° C, in accordance with which the reduction is carried out with an aqueous solution of hydrazine hydrate with a hydrazine concentration of at least 4 mol / l with a molar ratio of hydrazine to nickel of at least 1.3.

A known method of producing nanodispersed copper powder / 2 / reduction, which includes mixing a copper salt with a glucose solution, dissolving the salt when heated, introducing sodium hydroxide, holding in isothermal mode and the subsequent selection of metallic copper in the form of nanodispersed powder. Copper sulfate is used as the copper salt. The inventors believe that this method simplifies and reduces the cost of technology for producing nanoscale copper powders by reducing the number of technological operations of synthesis.

The method of producing nanosized metal powder described in the patent / 3 / consists in grinding a difficultly deformed material with an amorphous structure in a high-speed disintegrator due to the relative movement of the shock elements with a regulated speed and frequency of impacts. A material with an initial particle size of not more than 80 μm is preliminarily selected, it is subjected to heat treatment, in which nanocrystalline inclusions are formed in an amorphous matrix. Grind at speeds of relative movement of the shock elements 410-450 m / s and a shock frequency of 5000-8000 beats / s. The technical result is to obtain a finer powder.

A known method of producing nanosized powders of iron, copper, nickel, cobalt, tungsten, molybdenum and metal alloys, which includes chemical precipitation of at least one metal hydroxide with an alkali solution to form a suspension, diafiltration of the resulting suspension with separation of the solution of at least one metal hydroxide , its dehydration, preheating of at least one metal hydroxide and its reduction to obtain a metal powder and the subsequent passivation of the specified powder, When this diafiltration is carried out simultaneously with the purification sorption slurry and reduction of the metal hydroxide and passivation of the metal powder is carried out with the active material stirring / 4 /. The specified method allows to obtain a nanoscale metal powder with a structure of particles having a low level of distortion and the absence of extended defects, as well as a high-purity metal powder consisting of particles of a monodisperse state while maintaining a narrow fractional composition and a given morphology, and provides the ability to control dispersion at all stages of the technological process .

The methods of evaporation (condensation), or gas-phase synthesis of obtaining nanopowders of metals, are based on the evaporation of metals, alloys or oxides with their subsequent condensation in a reactor with controlled temperature and atmosphere. The phase transitions of vapor — liquid — solid — or vapor — solid — occur in the reactor volume or on the surface of the cooled substrate or walls. The essence of the method lies in the fact that the starting material is evaporated by intensive heating, with the help of a carrier gas it is fed into the reaction space, where it is rapidly cooled. The evaporated substance is heated using plasma, a laser, an electric arc, resistance furnaces, by an induction method, by passing an electric current through a wire.

Depending on the type of starting materials and the resulting product, evaporation and condensation is carried out in vacuum, in an inert gas, in a gas or plasma stream. The size and shape of the particles depend on the process temperature, the composition of the atmosphere and the pressure in the reaction space. In an atmosphere of helium, the particles will have a smaller size than in an atmosphere of argon, a denser gas. This method produces powders of Ni, Mo, Fe, Ti, Al. The particle size in this case is tens of nanometers. At one time, a method for producing nanomaterials by means of an electric explosion of wires (conductors) appeared and later became established. In this case, metal wires are placed between the electrodes in the reactor, from which it is planned to obtain nanopowder with a diameter of 0.1-1.0 mm. A large current pulse (104-106 A / mm ² ) is supplied to the electrodes. In this case, instant heating and evaporation of the wires occurs. Metal vapors fly apart, cool and condense. The process takes place in an atmosphere of helium or argon. Nanoparticles settle in the reactor. In this way, metal (Ti, Co, W, Fe, Mo) and oxide (TiO ₂ , Al ₂ O ₃ , ZrO ₂ ) nanopowders with particle sizes up to 100 nm are obtained.

A known method of producing metal nanoscale powders / 5 /, which includes heating the source metal in an inert gas stream to the evaporation temperature of the source metal with the formation of metal vapor in the inert gas stream and the selection of the inert gas stream of metal powder at a temperature below the melting temperature of the source metal . The source metal is heated by an electron beam having an energy of 0.4-3 MeV and a power of not more than 200 kW, at a pressure close to atmospheric and an inert gas flow rate of 0.5-25000 l / min. The inert gas is argon, or helium, or neon, or krypton, or carbon dioxide, or a mixture thereof. The starting metal can be almost any known metal or two metals that are heated together.

In the invention / 6 /, metal nanopowders are obtained by decomposing metal carbonyl using an induction plasma torch at a temperature of 3000–11000 K. Metal carbonyls have a low decomposition and sublimation temperature of metals, as a result of which finely dispersed spherical metal particles are formed upon cooling. A metal carbonyl is at least one carbonyl selected from the group consisting of nickel carbonyl, iron carbonyl, copper carbonyl, cobalt carbonyl, chromium carbonyl, molybdenum carbonyl, tungsten carbonyl and ruthenium carbonyl. Nanosized particles are rapidly cooled in a reactor located on the downstream side of the plasma torch. The carrier gas is at least one gas selected from the group consisting of helium, argon, nitrogen, hydrogen and carbon monoxide. At least one gas selected from the group consisting of helium, argon, nitrogen and hydrogen is used to form the plasma. The screening gas may be one of the gases: helium, argon, nitrogen or hydrogen. Gas for rapid cooling can be one of the gases: argon, nitrogen, oxygen, ammonia or methane. The time spent by carbonyl in the plasma torch is 0.001-10 seconds. Particles of metal nanopowders have a typical average size of from 1 to 100 nm.

A method of producing nanopowders is described in US patents / 7 /. The plasma installation is designed in such a way that the direction of flow of the plasma-forming gas, reaction gas and gas, which slows down the speed of the metal particles, varies depending on the required residence time of the material in the reaction zone and the cooling time of the metal particles. Alloying components can be introduced into the plasma reactor.

A method of obtaining a nanoscale tungsten powder is described in the patent / 8 /. As a result of sublimation of the tungsten containing compound, tungsten is released in an inert gas atmosphere at a pressure below atmospheric. The starting material used is tungsten ethylate, tungsten chloride and tungsten hexacarbonyl. In a similar way, it is possible to obtain tungsten carbide nanopowders, as follows from the patent / 9 /.

A known method of producing powders based on tungsten carbide, proposed in the patent / 10 /. In a plasma reactor, oxygen-containing compounds are reduced with hydrocarbons using an electric discharge plasma to obtain a mixture of nanopowders consisting of WC, WC ₂ , tungsten and free carbon, with a total carbon content of 5.5-7.0 wt.% And a specific surface area of 15-60 m ² / g After averaging, the resulting powder mixture is subjected to heat treatment in the presence of hydrogen at a temperature of 850-1300 ° C to obtain a WC-based powder. The resulting powder has a hexagonal structure and a wide range of average particle size from 0.03 to 1 μm. In addition, metal compounds selected from the groups V, Cr, Nb, and Ta are added to the reactor in an amount of 0.1-3.0 wt.% To obtain the corresponding carbides, which are grain growth inhibitors when compacting tungsten carbide-based hard alloys.

The authors of the patent proposed a device for producing nanopowders of metal oxides / 11 /. The device is characterized by the possibility of varying the directions of gas flows, which allows you to change the characteristics of the obtained nanopowders of metal oxides.

The closest in technical essence to the claimed invention is a patent (prototype), which describes the preparation of nanosized powders in a plasma installation, the design of which allows you to remove the deposited nanopowder of metals from various surfaces of the reactor / 12 /. The reactor has a certain ratio of geometric dimensions, connecting the output diameter of the plasma torch nozzle, the diameter and length of the reactor. The reactor surfaces on which the resulting nanopowder is deposited have special cleaners to remove the nanopowder. The reactor allows you to increase the temperature of the plasma for the implementation of the processes, while preventing sintering of the resulting nanopowders, to obtain a nanopowder without contamination with coarse inclusions of cakes.

The disadvantages of the prototype, as well as other cited inventions, is the inability to prevent the formation of cakes and agglomerates of condensed powders and the complexity of the design of removing deposits of nanopowder from different surfaces of the reactor.

The technical result of the invention is the production of metal nanoscale powders with a minimum content of cakes and agglomerates.

The technical result is achieved in that in a method for producing nanosized metal powders, comprising supplying an initial powder of metal oxide compounds with particle sizes of not more than 50 μm to a gas-discharge plasma reactor with a conveying gas, heating the starting oxide material above the sublimation temperature of the starting metal oxides, evaporating the metal oxide and reducing metal oxide compounds in a stream of hydrogen or its mixture with nitrogen or argon, the allocation of metal powder when cooling metal vapor tnym gas to a temperature below the melting point of the metal while the inert gas serves to cool the nozzle to a pulsating gas input.

Inert gas is pulsed by an injector using an impeller. The use of larger starting powders of metal oxide compounds is undesirable since however, complete processing of the feedstock in the plasma stream will not be ensured.

A common property of all small metal particles is their agglomeration. Metal nanopowders have a high reactivity, in particular, they sinter at relatively low temperatures. The high chemical activity of nanopowders is associated with their metastability and energy saturation, because their formation occurs under highly nonequilibrium conditions. The average particle cooling rate is 10 ⁹ K / s. Sintering of metal nanopowders leads to the formation of bulk agglomerates in nanopowders of strongly bonded particles - fractal clusters. Nanoparticles combine with each other, forming agglomerates or chain structures. As a rule, chain structures of small particles are grouped around large particles. According to the theory of mutual charge / 13 /, large particles are charged oppositely to small particles. This phenomenon adversely affects the formation of products from nanosized powders. Therefore, the solution to the problem of obtaining metal powders with a minimum content of agglomerates is very relevant.

Among the numerous methods of combating the formation of agglomerates and cakes from nanopowders, methods are used such as reducing contact between particles by coating (encapsulation), which is then removed before being compacted. This method leads to additional technological operations and, therefore, is ineffective.

Mechanical methods of vibration of the surface of the receiving chambers, where the condensation of nanosized powders occurs, are ineffective, because do not affect the behavior of particles in the chamber volume. The use of ultrasonic energy to prevent the agglomeration of nanoparticles is also inefficient, since This method is effective for continuous media.

The proposed method is as follows. The initial powder of metal oxide compounds, with particle sizes of not more than 50 microns, is fed into the gas-discharge plasma reactor by a transporting gas - nitrogen or argon. The plasma-forming gas is hydrogen or a mixture thereof with nitrogen or argon. At a temperature above the sublimation temperature of the starting metal oxide, the metal oxide evaporates and the metal oxide compounds are reduced in a stream of hydrogen or its mixture with nitrogen or argon, with the formation of metal vapors in the gas phase, which at the outlet of the plasma reactor are picked up by a stream of cooled inert gas and precipitate in a water-cooled receiver at a temperature below the melting point of the metal and are partially carried out by the gas stream into the filter. To prevent or minimize agglomeration of condensed nanosized particles of metal powder, a stream of cooled inert gas is supplied by a nozzle with a pulsating gas inlet. Thus, instead of a uniform laminar flow, turbulence is created. The laminar flow of the cooled gas in contact with the walls of the water-cooled receiver forms the so-called “boundary layer”, which contributes to the agglomeration of nanosized metal powder. A pulsating turbulent flow avoids the formation of a “boundary layer”. In addition, the vibration of a dense gas stream creates additional vibrations of metal nanoparticles, which also reduces the likelihood of agglomerates of condensed metal powder. The pulsed input of the cooled inert gas is carried out by a nozzle with a gas feed chopper made in the form of an impeller. By changing the flow rate of the inert cooled gas (inert gas flow), the speed of rotation of the impeller, i.e. pulse frequency of the gas stream. The inert gas flow rate varied in the range 1 · 10 ^-6 -1 · 10 ^-3 m ² / s. Such an inert gas flow rate provided the impeller rotation speed at which the pulsed gas flow made it possible to obtain metal powders with typical sizes of 0.5-100 nm and a specific surface area of 10-55 m ² / g.

The method is implemented in the following way.

Example 1

Copper oxide powder with a particle size <50 μm is introduced into the stream of thermal plasma obtained by heating a mixture of hydrogen (70 vol.%) And nitrogen (30 vol.%) In a plasma generator with a nitrogen transporting gas. Sublimation of the initial copper oxide and reduction with hydrogen in the gas phase results in the formation of metallic copper, which is picked up at the exit from the plasma reactor by a pulsating stream of inert argon gas at a flow rate of 5 · 10 ^-4 m ³ / s. The copper powder condensed in the water-cooled receiving chamber had an average particle size of 30 nm and a specific surface area of 35 m ² / g.

Example 2

Tungsten trioxide powder with a particle size <50 μm is introduced into the stream of thermal plasma obtained by heating in a plasma hydrogen generator (100 vol.%) Using argon transporting gas. As a result of the sublimation of the initial tungsten trioxide and reduction with hydrogen in the gas phase, metallic tungsten is formed, which is picked up at the outlet of the plasma reactor by a pulsating stream of inert gas of krypton at a flow rate of 2 · 10 ^-5 m ³ / s. The tungsten powder condensed in the water-cooled receiving chamber had an average particle size of 20 nm and a specific surface area of 45 m ² / g.

Example 3

An alumina powder with a particle size <50 μm is introduced into the stream of thermal plasma obtained by heating a mixture of hydrogen (50 vol.%) And argon (50 vol.%) In a plasma generator by transporting nitrogen gas. As a result of the sublimation of the starting alumina and reduction with hydrogen in the gas phase, metallic aluminum is formed, which at the exit from the plasma reactor is picked up by a pulsating stream of inert helium gas at a flow rate of 1 · 10 ^-3 m ² / s. The aluminum powder condensed in the water-cooled receiving chamber had an average particle size of 40 nm and a specific surface area of 30 m ² / g.

Granulometric analysis of nanoscale metal powders obtained in the above examples showed that metal powders had a homogeneous structure without agglomerates. The proposed method for producing nanosized metal powders is highly effective and reliable in its implementation and can be used for the production of powders of pure metals, as well as metal alloys in the chemical industry, powder metallurgy, mechanical engineering and other industries.

List of cited sources:

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2. Simenyuk G.Yu., Obraztsova II, Eremenko N.K. The method of obtaining nanodispersed powder of copper. RF patent 2426805, 12/21/2009.

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Claims (6)

1. A method for producing nanosized metal powders, comprising supplying an initial powder of metal oxide compounds with particle sizes of not more than 50 μm to a gas-discharge plasma reactor with a conveying gas, heating the starting oxide material above the sublimation temperature of the starting metal oxides, evaporating the metal oxide, reducing the metal oxide compounds in the stream hydrogen or its mixture with nitrogen or argon, the allocation of a metal powder when the metal vapor is cooled with an inert gas to a temperature below the temperature ION metal, characterized in that the cooling of the metal vapor carried pulsating inert gas stream at a flow rate of 1 × 10 ^-6 -1 × 10 ^-3 m ³ / s. 2. The method according to claim 1, characterized in that the starting oxide material can be one of the oxides of the following metals: Ti, Zr, Hf, Cr, Cu, Mo, W, V, Ta, Nb, Al, Si, Pb, Sn , Na, K, Mg, Ca, Zn, Fe, Co, Ni. 3. The method according to claim 1, characterized in that argon or nitrogen is used as the transporting gas. 4. The method according to claim 1, characterized in that the plasma-forming gas is hydrogen or a mixture thereof with nitrogen or argon. 5. The method according to claim 1, characterized in that one of the following gases is used as an inert gas for cooling: helium, neon, argon, krypton, xenon. 6. The method according to claim 1, characterized in that the pulsed input of an inert gas for cooling is carried out using a nozzle with an impeller.