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Vitaly Sviridov / LM-533
In recent years, there has been a noticeable interest in a new class of materials with
an unusual atomic crystal lattice and unique properties. This new class of materials
- nanomaterials - includes materials with element sizes less than 100 nm (1 nm = 109 m). Geometrically, these elements can be divided into zero-dimensional atomic
clusters and particles, one and two-dimensional multilayers, coatings and laminar
structures, three-dimensional bulk nanocrystalline and nanophase materials.
Nanomaterials are natural or artificially created materials in which one or more
dimensions are in the nanometer range. This category also includes the so-called
"nano-nano" composites, which contain more than one phase, but all phases are less
than 100 nm.
Currently, nanopowders (ultradispersed powders) are already widely used,
accounting for more than 90% of the US market for nanomaterials, nanofibers and
nanowires, nanofilms and nanocoatings. Bulk nanomaterials, nanocrystalline and
nano-grained (with a grain size of less than 100 nm), are beginning to receive more
and more use.
The urgency of the problem of the production of nanomaterials is determined by the
peculiarities of their physical and chemical properties, which make it possible to
create materials with qualitatively and quantitatively new characteristics. This is due
to the fact that for a material of such small dimensions (in comparison with the usual
ones), the following fundamental characteristics change significantly: specific heat,
elastic modulus, diffusion coefficient, magnetic properties, etc., which, in turn, leads
to a change in mechanical, optical and electrical properties of the original substance.
Therefore, the nanostructural phase of solids is fundamentally different from the
standard crystalline or amorphous state.
2. Methods for obtaining nanopowders
The resource-saving, energy-saving and high-tech nature of powder technologies
makes them very promising for the production of nanomaterials. For their
manufacture, nanopowders are used as a raw material, i.e., particles with a size of
no more than 100 nm, as well as larger powders obtained under conditions of
intensive grinding fine crushing and consisting of small crystallites with a size
similar to those indicated above.
Methods for obtaining nanopowders are very versatile. They can be conventially
divided into chemical and physical. Table 1 shows the main methods and their
varieties of obtaining nanopowders from various materials.
The division into chemical and physical methods is rather arbitrary. On the one hand,
chemical reactions play an important role, for example, in evaporation with reaction
gas in medium, and on the other, many chemical methods are based on physical
phenomena (low-temperature plasma, laser radiation, etc.). Chemical methods are
generally more versatile and more productive, but particle size, composition, and
shape are more easily controlled using physical methods, especially condensation
methods. Let us consider some of themethods for producing nanopowders.
The evaporation and condensation method is the simplest way to obtain
nanopowders. Isolated nanoparticles are obtained by evaporation of a metal or alloy
at a cooled temperature in an atmosphere of an inert low pressure reaction-less gas
or in a vacuum, followed by condensation of vapor near or on a cold surface. In
contrast to evaporation in vacuum, atoms of a substance evaporated in a rarefied
decompressed atmosphere lose kinetic energy faster due to collisions with gas atoms
and form clusters.
Installations using the principles of evaporation and condensation differ in the
methods of introducing the evaporated substance, supplying energy for evaporation,
working environment, organizing the condensation process, and collecting the
resulting powder. Metal evaporation can be carried out from a crucible located in
the heating zone. In addition, the metal can be fed into this zone in the form of wire
or powder, or by liquid jet injection.
Energy can be supplied to the evaporation zone by an electric arc discharge in a
plasma, induction heating by high and ultrahigh frequency currents, laser radiation,
electron beam heating, or direct heating by passing an electric current through a wire.
Evaporation and condensation can occur in a vacuum, in a stationary inert gas, in a
gas flow, including a plasma jet. Condensation of a vapor-gas mixture with a
temperature of 5,000–10,000 ° C occurs when it enters a chamber filled with a cold
inert gas. There are installations in which two jets coaxially enter the condensation
chamber - the vapor-gas mixture is fed along the axis, and an annular jet of cold inert
gas enters along its periphery. As a result of turbulent mixing of gases, the
temperature of the metal vapor decreases and rapid condensation occurs.
High energy decomposition methods include mechanical fining, metal wire
detonation.
The production of nanopowders by the method of mechanical grinding is carried out
in various types of mills - ball drum, planetary, centrifugal, jar, gyroscopic devices,
attritors, simo loyers. In these devices, the destruction of larger objects occurs due
to their crushing, breaking, cutting, sawing, polarisation, impact, or as a result of
combinations of these exposition. Attritors and simoloyers are high-energy mills
with a fixed drum case and impeller that transfer motion to the balls and the grinded
batch in the drum. Attritors have a vertical drum arrangement, simo loiers a
horizontal one.
Methods of mechanical grinding are used to obtain nanopowders of feroxides,
nitrides, borides, as well as polymers, etc. The degree of grinding depends on the
type of material to be ground. This method is used to obtain nanopowders of tungsten
and molybdenum oxides with a size of about 5 nm, and feroxide - about 10–20 nm.
Mechanical synthesis or mechanical alloying is a variation of mechanical grinding,
when, in the grinding process, the process of interaction of the ground materials with
each other is realized to obtain a ground material of a new composition. This is how
nanopowders of alloys, intermetallic compounds, silicides and dispersionstrengthened composites with a particle size of 5–15 nm are obtained. A unique
advantage of the method is that due to interdiffusion in the solid state, it is possible
to obtain "alloys" of such elements, the mutual solubility of which is too low and it
is impossible to use traditional liquid-phase methods to obtain them.
The advantages of mechanical grinding methods are the comparative simplicity of
the installations and technology themselves, the ability to grind various materials
and obtain alloy powders, as well as materials in large quantities.
The disadvantages of the method include the possibility of contamination of the
ground powder with abrasive materials, the difficulty of obtaining powders with a
narrow particle size distribution, as well as the difficulty of regulating the
composition of the powder during milling.