NANOPOWDERS OF COPPER AND COPPER ALLOYS:
FORMATION, PROPERTIES AND PROMISING WAY OF USE
B. R. Gelchinski1,2 , L.V. Zolotukhina 1, M. V. Vakhrushev 1
1
2
Fine Metal Powders Company, Yekaterinburg, Russia
Institute of Metallurgy of Ural Branch of the Russian Academy of Science,
Yekaterinburg, Russia
Abstract. Among different existing methods, condensation of the corresponding
metal vapors is the predominant technology for synthesis of metal nanoparticles [1].
The process is so-called inert-gas consolidation (IGC) method with which Gleiter and
coworkers [2] first generated the materials that demonstrated the exciting properties of
nanostructures materials [3]. In this method, a metal is typically vaporized into a low
density gas by Joule heating, although other ways of evaporation such as thermal
plasma [4] and laser ablation [5] are also being used. Vapors migrate from the hot
source into a cooler gas by a combination of convective flows and diffusion. The
decreasing temperature leads to a far more rapid decrease in the equilibrium vapor
pressure and correspondingly high supersaturating [3]. At high supersaturating, the
vapors rapidly nucleate, forming very large numbers of extremely small particles. The
particles then grow by Brownian coagulation [6]. The product particles are generally
collected by thermophoretic deposition. In order to enhance the deposition efficiency,
a substrate surface cooled with liquid nitrogen may be used.
1. FORMATION AND PROPERTIES OF COPPER AND COPPER ALLOY
NANOPOWDERS
These short report concerns the copper and copper alloy powders produced at
the Fine Metal Powders company plant using the in-house developed technique and
equipment. The production process includes melting and evaporation of a metal,
condensation of the vapor in the presence of an inert gas and accumulation of the
formed powder in the cold part of the apparatus. Stabilization of the powder can be
carried out when necessary by introducing a passivating agent in the process giving an
inert film of less than 10 angstrom thickness. The commercial powders have the
particle size in the range of 0.01 to 1.0 micron. A comprehensive study of formation
of copper, tin and Cu-Sn alloys powders and of their properties have revealed their
peculiarities and
allowed to determine the basic operational parameters of
evaporation and condensation processes ensuring production of powders with targeted
properties.
The main features of the production process:
 The condensation of the metal vapour is carried out in the volume of the
apparatus, as a homophase process;
 The vapor supersaturating degree is 108 to1040 and higher;
 The inert gas pressure is 0.01 to 100 kPa;
 A molecular-viscous or a viscous flow mode of the gas medium is used
(Knudsen number less than 0.01).
An apparatus of a modular type for production of submicron powders differs
by the features of design ensuring process operation.
The microphotograph of the stabilized copper powder on fig.1.
2-33
Fig.1. Microphotograph of the copper nanopowder.
The formation of a copper alloy nanopowder has some additional features:
•First of all the powder of an alloy is formed, when the vapour phase contains
several metals at a given stoichiometric ratio.
•In the alloy nanodispersions, melting and consolidation points deviations
considerably exceed the corresponding values for individual metal.
This phenomenon results from a more complex character of the formation
process of an alloy powder in comparison with that of an individual metal. In
particular, at formation of copper-tin alloy powders, presence of cluster groups of a
CunSnm type in the vapor phase essentially influences the process of condensation.
The process of formation of alloy powders displays peculiarities associated with
their phase diagram. The latter determines the composition and structure of the
surface layer of the evaporating melt and, therefore, the properties of the obtained
powders.
2. APPLICATION OF THE COPPER ALLOY NANOPOWDERS
The large specific surface of submicron and nanopowders, exceeding by the
order of magnitude the specific surface of micron size powders, is responsible for a
dramatic increase in their chemical activity. This promises much interest to
investigators.
It is known, that under influence of the heavy load and high temperature surface
layers of metal are fragmented, forming practically nonporous nanocrystalline
structure with the size of fragments from 3 to 700 nanometers. This can’t be achieved
by any other processing. It was found by our studies, that introducing a submicron
copper alloy powder between two mating steel surfaces
leads to nanostructural
transformations in the superficial layers of steel.
We have found also that a copper alloy powder became chemically unstable
when subjected to friction.
2-34
The alloy decomposes like:
CunSnm  aCu + Cun-aSnm,
releasing nano particles of copper with juvenile surface. These particles interact
actively with the metal substrate and become built in its superficial layers. A
micromodifying of the metal surface occurs resulting in formation of an especial
nanostructural coat which displays an unusual feature - a combination of both high
hardness and plasticity
The structure of the microcoat was studied by means of various analytical
methods, including those of metallography, X-Ray phase and structural analysis,
electronic microscopy, X-ray spectrum microanalysis, nuclear gamma resonance, etc.
Surface modification in the presence of a copper nano-alloy is accompanied by
forming of so-called “rotational” microstructures of up to 25 micron size.
On the fig.2 see the fragment of rotational structure in a steel rubbing surface.
Fig. 2. Fragment of rotation structure of steel rubbing surface (х2000)
The occurrence of similar deformed areas is an attribute of high plasticity
usually observed in the structures arising at high-temperature phase transformations.
No such microstructures were observed in the experiments at the same condition
without introduction of the copper alloy. This fact confirms the hypothesis that the
copper alloy nanopowder is responsible for the increase in the plasticity of the
superficial steel layer.
The nanocrystalline copper is detected (by the electron probe microanalysis
data) in the whole thickness of new superficial layers. An assumption is pertinent that
the grain-boundary diffusion processes cause this penetration of copper. It is known,
that the intensity of the grain-boundary diffusion in nanocrystalline structures exceeds
by 1 to 4 orders of magnitude the corresponding value for large-grain structures. It can
be assumed also that the copper having negligibly low solubility in iron is adsorbed at
the iron grain and sub-grain boundaries. As far as copper lattice differs by a high
mobility of dislocations, nanocrystalline interlayers of copper arising at the iron grain
boundaries promote an increase in plasticity of the superficial metal layers while
retaining their high hardness and strength. The arises secondary superficial structures
(micro-coats) display higher resistance against wear in comparison with the substrate
metal. Data of electronic microscopy confirm influence of the copper alloy
2-35
nanopowder on the microstructure of the superficial layer. In the initial steel
microstructure (a) perlite areas alternate with laminas of ferrite and cementite. The
associated electron diffraction pattern (b) demonstrates that the cross-sections of
ferrite and cementite inverse lattices are mutually oriented. A bright field image of
the steel microstructure at 5-10 mm deep from the contact surface demonstrate that
nanocrystallites of 50 to 150 nanometers are formed as a result of the impact. The
annular reflexes observed on the micro diffraction pattern indicate the nanocrystalline
structure of the formed layer.
SUMMARY
Theoretical concepts of the formation of metal and metal alloys nano
dispersions at the inert gas condensation were developed and made a basis for a
commercial production technique. Copper powder with medium particle size of 100300 nm and copper alloy powders of medium particle size of 100 nm are produced in
commercial scale. Peculiarities of the formation of copper alloy powders were found
and studied in the association with their phase transition diagram. The superficial steel
layer enriched in nanocrytalline copper behaves as a specific microcoat protecting the
mating steel surfaces against friction and wear. The described technique of
nanostructural modifying of the mating steel surfaces opens promising areas of usage
of submicron copper alloys i.e. as a method of prolongation of the service life of
machines and mechanisms. The necessary concentration of the nanopowder can be
calculated for a certain substrate metal and a certain design of the mechanism.
Further study of the nanostructural modifying of metal surfaces directly at
operation makes a key item of the research plans.
ACKNOWLEDGEMENTS
This work was financially supported by the Russian Foundation for Basic Research,
projects # 08-03-99077-р_офи and # 08-03-99073-р_офи.
REFERENCES
1. Wegner K, Walker B, Tsavros S, Pratsinis S.E.: Design of metal nanoparticle
synthesis by vapor flow condensation. Chem. Eng. Sci. 2002 57 1753-62.
2. Birringer R, Gleiter H, Klein HP, Marquardt P.:Noncrystalline materials an
approach to a novel solid structure with gas-like disorder? Phys.Lett. 1984 102A
365-69.
3. Flagan RC, Lunden MM.: Particle structure control in nanoparticle synthesis from
the vapor phase. Mater. Sci. Eng. 1995 A204 113-24.
4. Wahok K, Hihata T, Peng DL, Sumiyama K.: Compositional partition in Ag-Nb
alloy clusters produced by a plasma-gas-condensation cluster source. Nanostruct.
Mater. 1999 11 1245-51.
5. Houriet R, Vacassy R, Hofmann H.: Synthesis of powders and films using a new
ablation technique. Nanostruct. Mater. 1999 11 1155-63.
6. Granqvist CG, Buhrman RA.: Ultrafine metal particles. J Appl Phys 1976 47 (5)
2200-19.
2-36