Melt Mixing
It is possible to form or arrest the nanoparticles in glass. Structurally, glass is an
amorphous solid, lacking large range periodic arrangement as well as symmetry
arrangement of molecules. When a liquid is cooled below certain temperature
, it forms either a crystalline or amorphous solid
. Besides temperature, rate of cooling and tendency to nucleate decide
whether the melt can be cooled as a glass or crystalline solid with long range order.
Nuclei are formed spontaneously with homogeneous (in the melt) or inhomoge-
neous nucleation which can grow to form ordered,
crystalline solid. Metals usually form crystalline solids but if cooled at very high rate , they can form amorphous solids. Such solids are known as metallic
glasses. Even in such cases the atoms try to reorganize themselves into crystalline
solids. Addition of elements like B, P, Si etc. helps to keep the metallic glasses in
amorphous state. It is indeed possible to form nanocrystals within metallic glasses
by controlled heating.
It is also possible to form some nanoparticles by mixing the molten streams of
metals at high velocity with turbulence. On mixing thoroughly, nanoparticles are
found to have been formed. For example a molten stream of Cu-B and molten stream
of Ti form nanoparticles of TiB2.
Methods Based on Evaporation
There is a variety of methods to form nanostructures by evaporating the materials
on some substrates. The nanostructured materials can be in the form of thin films,
multilayer films or nanoparticulate thin films (thin films composed of nanoparticles).
There are several methods in which material of interest is brought in the gaseous
phase atoms or molecules which can form clusters and then deposit on appropriate
substrates. It is also possible to obtain very thin even atomic layers, known as
monolayers layers or multilayers (multilayers are layers of two or more materials
stacked over each other) forming nanomaterials of wide interest Evaporation can be achieved by various methods like resistive heating, electron
beam heating, laser heating, sputtering. It should be remembered that all the
synthesis processes need to be carried out in a properly designed vacuum system, so as to avoid uncontrolled oxidation of
source materials and final product as well as that of components of the synthesis
system. Mean free path of the particles also increases in vacuum system, which is
often desired. Even if some reactive gases are used in certain cases of depositions,
it is useful first to evacuate the system to very low pressure so that the materials
to be synthesized do not get contaminated by undesired atoms and then pressurize
the system to desired value by introducing the high purity gases in the synthesis
chamber.
Materials to be evaporated are usually heated from some suitable filament,
crucible, boat collectively called as ‘evaporation source’ or ‘crucible’.
Usually the sources are electrically heated so that enough vapours of the material
to be deposited are generated. If the material to be deposited wets the filament
material without forming any alloy or compound, the filament is considered to be
suitable. Otherwise one needs to melt the material in a basket, canoe-like container.
However this type of heating has the disadvantage that the crucible itself and
surrounding parts also get heated and become the source of unwanted contamination
or impurities. Therefore evaporation by electron beam heating method is desired.
Electron beam focuses on the material to be deposited, kept in the crucible as
it is generated from a filament that is not in the proximity of the evaporating
material. It melts only some central portion of the material in crucible avoiding
any contamination from crucible. Thus high purity vapours of materials can be
obtained.
Now, let us discuss how the deposition occurs by evaporation. At any given
temperature there is some vapour pressure of the material. In evaporation, the
number of atoms leaving the surface of solid or liquid material should exceed the If none of the evaporated molecules or atoms return to the surface of the
evaporation source, p D 0. The coefficient of evaporation arises as some of the
atoms returning to the surface get reflected back into vapour phase and
also change the pressure due to evaporant. also suggests that at a
given temperature, there would be some specific rate of deposition. It may be noted
that evaporation rate equations would in general be more complicated than given by
a simple HertzKnudsen equation when evaporation from solids, compounds, alloys
etc. are taken into account.
It is necessary that the material to be evaporated creates a pressure, so as to achieve adequate vapour pressure for synthesis. Some materials
like Ti, Mo, Fe and Si have large vapour pressure at a temperature, much below
their melting points and can be easily evaporated or sublimated from their solids.
On the other hand metals like Au and Ag have very low vapour pressures even close
to their melting points. Therefore, they need to be melted (for evaporation to occur)
to achieve adequate vapour pressure required for deposition.
The constituents of the alloys evaporate at different rates depending upon their
pure metal forms. Therefore, the deposited films may have different stoichiometry.
Physical Vapour Deposition with Consolidation
This technique basically involves use of materials of interest as sources of evaporation, an inert gas or reactive gas for collisions with material vapour, a cold finger on
which clusters or nanoparticles and piston-anvil an arrangement in which nanoparticle powder can be compacted.
All the processes are carried out in a vacuum chamber so that the desired purity
of the end product can be obtained.
Usually metals or high vapour pressure metal oxides are evaporated or sublimated from filaments or boats of refractory metals like W, Ta and Mo in which the
materials to be evaporated are held. The density of the evaporated material close
to the source is quite high and particle size is small. Such particles would
prefer to acquire a stable lower surface energy state. Due to small particle or clustercluster interaction bigger particles get formed. Therefore, they should be removed
away as fast as possible from the source. This is done by forcing an inert gas near
the source, which removes the particles from the vicinity of the source. In general
the rate of evaporation and the pressure of gases inside the chamber determine the
particle size and their distribution. Distance of the source from the cold finger is
also important. Evaporated atoms and clusters tend to collide with gas molecules
and make bigger particles, which condense on cold finger. While moving away
from the source to cold finger the clusters grow. If clusters have been formed on
inert gas molecules or atoms, on reaching the cold finger, gas atoms or molecules
may leave the particles there and then escape to the gas phase. If reactive gases like
O2, H2 and NH3 are used in the system, evaporated material can interact with these
gases forming oxide, nitride or hydride particles. Alternatively one can first make
metal nanoparticles and later make appropriate post-treatment to achieve desired
metal compound etc. Size, shape and even the phase of the evaporated material can depend upon the gas pressure in deposition chamber. For example using gas
pressure of H2 more than 500 kPa, TiH2 particles of 12 nm size were produced.
By annealing them in O2 atmosphere, they could be converted into titania
having rutile phase. However if titanium nanoparticles were produced in H2 gas
pressure less than 500 kPa, they could not be converted into any crystalline oxide
phase of titanium but always remained amorphous.
Clusters or nanoparticles condensed on the cold finger can be scraped off inside the vacuum system. The process of evaporation
and condensation can be repeated several times until enough quantity of the material
falls through a funnel in which a piston-anvil arrangement has been provided. One
can even have separate low and high pressure presses. A pressure of few mega pascal
to giga pascal is usually applied depending upon the material. Low
porosity pellets are easily obtained. Density of the material thus can be 70 to 90 percentage
of the bulk material.