Synthesis of nano material

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.