RU2412784C2 - Method of producing composite nanopowders - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to electron-beam production of ultra dispersed materials. Proposed method comprises heating the substance by relativistic electron beam at atmospheric pressure to vapor-phase state, condensing vapors by cooling them in gas flow and separating the formed two-phase system. Two single-elements substances are subjected to heating to form a homogeneous melt the vapors of which form the particles of solid composite nanopowders of the core-shell type. Note here that condensation temperature of one substance is lower than that of the second substance and higher than condensation temperature of both substances. Note also that heating is carried out stage-by-stage, first, to produce homogeneous melt, and, second, to reach vapor-phase state on increasing electron beam power.

EFFECT: nanoparticles coated by thin shell from different substance, reduced sintering.

1 tbl, 7 dwg, 1 ex

Description

The present invention relates to electron beam technology for producing nanodispersed materials.

The resulting product is a nanopowder consisting of nanoparticles of one single-element substance, each of which is coated with a thin shell of another single-element substance, can be widely used in the development of functional elements in electronics, electrical engineering, in nonlinear optics, to obtain metal-containing materials, thin-film composite materials, metal polymers and to develop efficient catalytic systems.

Obtaining and studying the properties of nanopowders of various substances is an important section of modern science and technology. Firstly, this is due to the practical need to create new materials, which in some cases is possible only with the use of nanoparticles. Secondly, the problem of studying very small particles, especially those having sizes less than 100 nm, is an integral part of a more general fundamental field of knowledge, collectively called "nanotechnology". Particular attention is paid to the development of high-performance methods for the production of nanopowders. However, the productivity of the bulk of the known methods, especially for producing nanopowders of metals, nitrides and carbides, is small.

It is also known that particles smaller than 200 nm are highly agglomerated and can ignite spontaneously, so it is important to exclude such possibilities. One way to reduce spontaneous combustion and the degree of agglomeration is to coat nanoparticles with a thin shell of a substance that is not prone to agglomeration, that is, in other words describing, it is necessary to produce composite nanoparticles.

Currently, special attention is directed to the preparation and study of nanoparticles. In the field of developing new materials, composite metal-containing materials with nanostructured morphology of individual elements are of great interest. For example, it is known / 1 / that two-component composite layered materials consisting of nanoparticles containing metals, one of which is ferromagnetic, such as copper-cobalt, are the basis for the creation of new highly sensitive read heads of magnetic disks. The giant magnetoresistance effect observed for such materials determines their use.

Known methods for producing metal nanopowders, such as soft hydrothermal treatment / 2 /, electric explosion of the conductor / 3 /, mechanical grinding / 4 /. These methods have low productivity, which is an obstacle to their practical use.

A known method of producing nanopowders of complex compounds and mixed compositions and a device for its implementation. (Patent RU No. 2185931 C1, IPC B22F 9/02. A method for producing nanopowders of complex compounds and mixed compositions and a device for its implementation. 07.27.2002). In this way, nanopowders of pure chemicals, various complex compounds and homogeneous mixed formulations are obtained. In the known method, the evaporation of substances is carried out by the action of laser radiation in a pulse-periodic mode, followed by condensation of the vaporized substance in a gas stream, while the surface of the vaporized substance is moved in the focal plane relative to the focal point of the laser radiation at a constant speed. This method has disadvantages due to its low productivity, due to the fact that laser radiation does not penetrate into the substance and its surface is heated.

A known method of producing ultrafine silica. (Patent RU No. 2254292 C1, IPC C01B 33/18. A method for producing ultrafine silicon dioxide. 06/20/2005). The invention relates to the field of electron beam technology and is used to produce ultrafine materials, in particular ultrafine silica. The known method includes heating a silica-containing substance with a relativistic electron beam at atmospheric pressure to a vapor phase state, exposure to the melt with ultrasound at a frequency of 0.15 to 5 MHz, cooling in a gas stream, coagulation and separation of the resulting two-phase system. The implementation of the known method is carried out in a device where an electron beam of an accelerator located above the evaporation chamber is used as a heater, and the media are separated after cooling in a vortex dust collector made in the form of a cone with channels, one of which is used for powder output, the other with an integrated fan - for gas outlet.

The technical result is to increase the productivity of obtaining ultrafine silica. The disadvantage is that in the production of ultrafine particles, their spontaneous combustion and oxidation can occur.

A known method of producing ultrafine silica, a device for its implementation and ultrafine silica (RF Patent No. 2067077, IPC 7, C01B 33/18, published September 27, 1996, Bulletin No. 27). This method is the closest to the proposed technical solution and adopted as a prototype. This invention relates to the field of electron beam technology and can be used to obtain ultrafine materials, in particular ultrafine silica. A method of producing ultrafine silica involves heating a silica-containing substance by a relativistic electron beam at atmospheric pressure to a vapor phase state, cooling in a gas stream, coagulation and separation of the resulting two-phase system.

The technical result is to increase the productivity of obtaining ultrafine silica. Compared with known methods, the method of producing ultrafine silica, adopted as a prototype, has advantages, since energy due to the penetration of electrons into the substance is released inside the substance. This contributes to the efficient transfer of energy by electrons to matter and ensures high productivity of the method. However, the disadvantages of this method is the inability to obtain ultrafine composite powders. Since it is known that particles smaller than 200 nm are highly agglomerated and can spontaneously ignite, it is therefore important to exclude such possibilities. One way to reduce spontaneous combustion and the degree of agglomeration is to coat nanoparticles with a thin shell of a substance that is not prone to agglomeration, that is, in other words describing, it is necessary to produce composite nanoparticles.

The main task posed by the authors is to obtain new composite nanopowders consisting of core-shell nanoparticles.

The technical result of the invention is to reduce the degree of agglomeration of nanoparticles, preventing the oxidation of susceptible nanoparticles. The specified technical result is achieved in that in the method for producing a composite nanopowder, comprising heating a substance with a relativistic electron beam at atmospheric pressure to a vapor phase state, condensation by cooling the vapor in a gas stream and separating the resulting two-phase system, according to the invention, two single-element substances are formed that are heated during heating homogeneous melt, upon condensation of the vapor of which particles of a solid composite nanopowder of the core-shell type are formed, eratura one substance below the condensation temperature of the second condensation agent and higher than the maximum melting temperature of both substances, wherein heat produced in stages previously - to obtain a homogeneous melt, and then - by increasing the power of the electron beam to the vapor state.

A significant difference between the proposed method and the known one is that in order to obtain composite nanopowders, its constituent parts are preheated until a homogeneous melt is obtained, the substances being selected with certain ratios of their condensation and melting temperatures. Further, the power of the electron beam increases and the melt evaporates. Due to the difference in the condensation temperatures of the two substances, their vapors will condense in different places of the chamber (temperature zones). First, the vapors of a substance with a higher condensation temperature will condense, and the vapors of the second substance present at this location will be inside the droplet of the first substance formed. With further movement and cooling of the vapor, the second substance condenses into a liquid state, and then into a solid state. The first substance also turns into a solid state at its melting point. As a result, a solid composite nanoparticle of the core-shell type is formed, the core of which is from a substance with a lower condensation temperature, and the shell with a higher condensation temperature. The combination of such particles forms a composite nanopowder. To obtain a composite core-shell type nanoparticle, it is necessary not only to select solids with different condensation temperatures, but also to fulfill the condition that the condensation temperature of the substance forming the core be higher than the maximum melting temperature of both substances. Otherwise, one of the substances will evaporate during the melting of solids in the process of obtaining a uniform melt. Thus, in the proposed method revealed a new property, i.e. the interaction of two substances with different melting and condensation temperatures, under the proposed method conditions, leads to the formation of a composite nanopowder consisting of core-shell nanoparticles, which reduces the degree of agglomeration of nanoparticles and prevents oxidation of the susceptible nanoparticles. In the claimed method used equipment, techniques and substances that are approved for use in technology.

To implement the proposed method using the following device (figure 1).

A device for producing a composite nanopowder contains an electron accelerator 1 of high specific power, mounted coaxially above the evaporation chamber 2, in which a refractory crucible 3 is located, connected to a feeder 4 for supplying solids and containing a set of slot holes 5 in the upper part of the side walls to enter the evaporative chamber 2 transport gas and in the lower part, to create and exit a directed flow of an aerosol mixture of vaporized substances.

The evaporation chamber 2 through the axial channel 6 is connected to the intermediate chamber 7, the dust-gas mixture enters the intermediate chamber 7 for cooling the dust-gas mixture (aerosol) and partial precipitation of the powder. The intermediate chamber 7 is connected via a channel 8 to a filter chamber 9, in which a filter 10 is located for deposition of the composite material and a gas outlet channel 11.

The method is as follows.

Two solid initial single-element substances with a certain ratio of their condensation and melting temperatures through the feeder 4 are placed in the refractory crucible 3 of the evaporation chamber 2 and heated to a liquid state by a concentrated relativistic electron beam of high specific power generated by the ELV-6 electron accelerator. Heating is carried out at atmospheric pressure. Next, the power of the electron beam is increased and the melt evaporates. The energy of the irradiating electrons is 1.4 MeV, the power density of the electron beam on the surface of substances varies from 1 kW / cm ² to 60 kW / cm ² .

Due to a gas stream, for example argon, supplied through a slit hole 5 in the wall of the evaporation chamber 2, the vaporized substance with small particles of the starting materials trapped in the gas stream is quickly removed from the hot zone through channel 6 to the intermediate (expansion) chamber 7 and then to the filter chamber 9. In these cells, the temperature drops to room temperature. Since the condensation temperature of one of the substances is higher than the second, liquid droplets of the first substance begin to form first in the hotter zone compared to the temperature of the zone of formation of the second substance. The process of formation of liquid droplets begins in the evaporation chamber, then, due to the supply of argon through the slit openings in the chamber wall, with the flow of argon, vapors of two substances move into the expansion chamber 7, in which the composite particles of the substances also precipitate. However, the vapor pressure of the first substance (created in this method due to its evaporation) and a relatively small concentration of the second substance do not allow the formation of pure droplets of the second substance, and droplets of the first substance are created, filled with vapor of the second substance. Composite nanoparticles created in this way form a structure in the form of a core of the first substance coated with a thin hard shell of the second substance.

As mentioned above, to obtain a composite core-shell nanoparticle, it is necessary to select solids with certain ratios of condensation and melting temperatures. So, for example, the table shows the condensation and melting temperatures of several substances and the core-shell type nanoparticles created from them, in which the core has a lower condensation temperature and the condition that the condensation temperature of the core substance is higher than the maximum melting temperature of both substances is fulfilled.

Table. The substance is the core The condensation temperature of the substance, ° C The melting point of the substance, ° C Possible combinations of core-shell material Zinc 907 419 Zinc lead Lead 1750 327.4 Silicon Lead, Nickel Lead, Cobalt Lead Copper 2590 1083 Copper-silicon dioxide, copper-nickel, copper-cobalt Nickel 2800 1453 Nickel-silicon dioxide, Nickel-cobalt Cobalt 2960 1494 Cobalt silicon Silicon 3249 1420 No combinations for this table of substances Tungsten 5900-6000 3380 No combinations for this table of substances

The expansion of technological capabilities of the method lies in the fact that two substances are introduced into the crucible 3 of the evaporation chamber of the substance through the feeder 4 with certain ratios of the values of their condensation and melting temperatures.

An example of a specific implementation of the proposed method and obtaining a new composite nanoparticle containing copper and silicon dioxide.

When using the proposed method, implemented on the device shown in Fig. 1, of inert gas and air as a cooling gas, and as a starting material - copper and silicon, copper-containing non-agglomerated copper nanoparticles in a thin shell of silicon dioxide were obtained (Fig. 2 ) The melting point of silicon is 1420 ° С, copper - 1083 ° С, silicon condensation temperature 3249 ° С, copper condensation temperature - 2590 ° С. From these temperature data it follows, firstly, the possibility of obtaining core-shell type nanoparticles from copper and silicon, since the condensation temperature of copper is higher than the melting temperature of silicon. Secondly, the core will be copper, and the shell will be silicon, since the condensation temperature of copper is lower than the condensation temperature of silicon.

10 mass% of silicon and 90 mass% of copper are laid in the crucible. After the end of the technological process, composite copper-containing nanoparticles are obtained. The particle size distribution (Fig. 3) shows that the nanoparticles consist of two groups: large submicron particles with predominant sizes from 50 to 200 nm and small nanoparticles with predominant sizes from 50 to 70 nm. The size of composite nanoparticles is within the same range as for particles of other powders, for example, Mo and Ni, obtained on the same equipment as in the prototype. However, in the prototype obtained materials containing nanoparticles of one substance. Silicon dioxide is formed as a result of oxidation of silicon due to oxygen in the air, which is present in small quantities in the evaporation, expansion chambers and filter chamber. In the absence of oxygen in the air in the chambers, as a result of the present production method, a silicon shell is created in this process.

The thickness of the coating of silicon dioxide according to transmission electron microscopy is not more than 12% of the diameter of the composite nanoparticles. 4, the diameter of the nanoparticle is 100 nm, and the shell thickness is from 10 nm to 12 nm.

For example, let us estimate the electron beam power density (P _eff ) for melting a silicon ingot weighing 50 g, a cross-sectional area of 40 cm ² and a copper ingot weighing 450 g, the same area for 10 s. P _eff ≈ (P ₁ + P ₂ ) · (K1 + k2 + k3 + s) / t,

where P ₁ = c ₁ · m ₁ · (t _L1 · t _R ) + λ ₁ · m ₁ ;

P ₂ = c ₂ · m ₂ · (t _L2 · t _R ) + λ ₂ · m ₂ ;

c ₁ is the specific heat of copper;

c ₂ is the specific heat of silicon;

m ₁ is the mass of the copper ingot;

m ₂ is the mass of the silicon ingot;

t _L1 is the melting temperature of copper;

t _R is room temperature;

λ ₁ - specific heat of fusion of copper;

t _L2 is the melting temperature of silicon;

t _R is room temperature;

t = 10 s is the exposure time;

λ ₂ - specific heat of fusion of silicon;

k1≈0.03 (energy loss of electrons in air);

k2≈0.02 (electron energy loss due to radiation);

k3≈0, l (energy loss due to electron reflection);

s≈0.3 (thermal loss of matter due to infrared radiation),

then P _eff ≈15 kW / cm ² .

Further, the beam power is increased so that copper and silicon evaporate from the melt. High heating rate (above 1000 deg / s) provides simultaneous evaporation of copper and silicon. As an example, we estimate the required electron beam power density P _ieff for the evaporation of a uniform melt of copper (450 g) and silicon (50 g) with a total mass of 500 g over a period of 100 s.

P _i = r ₁ · m ₁ + r ₂ · m ₂ ,

where r ₁ is the specific heat of vaporization of silicon;

r ₂ - specific heat of vaporization of copper.

Then P _iefi = P _i · (k1 + k2 + k3 + s) / 100≈25 kW / cm ² .

Installation parameters at the stage of evaporation of substances:

Accelerating voltage, MeV 1.4 Beam current, mA thirty Power in an electron beam, kW 40

The power density of electrons on the melt surface of two substances during their evaporation is 25 kW / cm ² .

In the proposed method, the following mechanism of formation of composite nanoparticles occurs. After the evaporation of copper and silicon from the liquid surface of the melt, mixed pairs of copper and silicon fall in the proposed method into a stream of gaseous argon. Since the condensation temperature of silicon (3249 ° C) is higher than that of copper (2567 ° C), liquid silicon droplets begin to form in the colder zone, with the temperature of the zone of formation of copper droplets. However, a higher vapor pressure of copper and a relatively low concentration of silicon do not allow the formation of pure silicon droplets, but silicon droplets filled with copper vapor are created. When droplets arrive in a colder zone, where the temperature is equal to the melting point of silicon dioxide, the composite particles, mixed with the oxygen in the air, which is present in small quantities in this zone, will have the structure of liquid copper coated with a solid shell of silicon dioxide. Liquid copper hardens in a colder zone corresponding to its melting point. According to the results of atomic emission spectroscopy with induced plasma, copper-containing composite nanoparticles consist of 6.34 wt.% Si and 88.9 wt.% Cu and contain a small amount of impurities (Ca less than 0.04, Fe less than 0.04, Na less than 0.03, Ti less than 0.02 wt.%).

The obtained powder from composite nanoparticles is nanodispersed and has a red-brown color characteristic of copper, which indicates the transparency of the shell of silicon dioxide. This powder in air under normal conditions does not change this color for an arbitrarily long time. This fact means that there is no oxidation of the nuclei of copper nanoparticles as a result of interaction with the environment.

Thus, when implementing the proposed method, a composite nanopowder was obtained, consisting of weakly agglomerated copper nanoparticles coated with a thin transparent shell of silicon dioxide. Positive effect - new spherical-shaped composite slightly agglomerated nanoparticles with a thin transparent shell of silicon dioxide were obtained.

Justification of the obtained new substance and its properties.

From a photomicrograph of transmission electron microscopy (Fig. 2) it follows that composite copper-containing nanoparticles having the shape of copper balls (nuclei) in a thin shell of silicon dioxide are obtained, this follows from the EDS spectrum (Energy Dispersive X-ray Spectroscopy) in Fig. 5 and smaller particles of silicon dioxide. The contrast in the color of the nuclei and the shell of the composite nanoparticles in the micrograph is due to the difference in the atomic numbers of copper and silicon dioxide. Smaller particles are the same in color of the shell, i.e. they are particles of silicon dioxide. We at the same facility produced pure copper ultrafine particles by melting a copper ingot with an electron beam and subsequent evaporation. This is confirmed by the EDS spectrum of these particles in Fig.6. From a comparison of two micrographs of pure copper and composite copper particles with a shell of silicon dioxide (Fig.2,7) it is seen that the degree of agglomeration of composite particles is much less than pure copper, that is, a shell of silicon dioxide really reduces the agglomeration of copper nanoparticles. The phase structure of the copper core of the nanoparticle is mixed — the presence of a defective crystalline structure and an amorphous phase is visible in the photomicrograph fragment (Fig. 4). The phase structure of the nanoparticle shell is amorphous: this is evidenced by the absence of lines of silicon or silicon dioxide in the spectrum of x-ray phase analysis of the composite nanopowder.

Thus, the result of the research confirms that when implementing the proposed method, a new nanodispersed powder was obtained, consisting of copper slightly agglomerated nanoparticles with a thin transparent shell of amorphous silicon dioxide.

The present invention can be used to obtain metal-containing materials, thin-film composite materials, metal polymers for the development of functional elements in electronics, electrical engineering, optics and for the development of effective catalytic systems.

Bibliography

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Claims (1)

A method of producing a composite nanopowder, comprising heating a substance with a relativistic electron beam at atmospheric pressure to a vapor phase state, condensation by cooling the vapor in a gas stream and separating the resulting two-phase system, characterized in that two single-element substances are heated, forming a uniform melt upon heating, upon vapor condensation which forms particles of a solid composite nanopowder of the core-shell type, and the condensation temperature of one substance is lower than ndensatsii second material and higher than the maximum melting temperature of both substances, wherein heat produced in stages previously - to obtain a homogeneous melt, and then - by increasing the power of the electron beam to the vapor state.

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