Techniques for Synthesis and Consolidation of NSM (Nano Structured Material)

 

       Synthetically produced nanoparticles play an important role in nanotechnology. They are the basis for many applications currently being used on a large scale, and they have a great potential in the development of new materials.

        The diversity of synthetic nanoparticles is considerable. They are distinct in their properties and applications. In addition to their size, synthetic nanoparticles vary in chemical composition, shape, surface characteristics and mode of production.

Introduction 

      Nanoparticles are not solely a product of modern technology, but are also created by natural      processes such as volcano eruptions or forest fires. Naturally occurring nanoparticles also include ultrafine sand grains of mineral origin (e.g. oxides, carbonates).

In addition to commercially produced nanomaterials, many nanoparticles are unintentionally created by the combustion of diesel fuel (ultrafine particles) or during barbecuing.

Synthetic nanoparticles find use in many applications. This includes dispersions in gases (e.g. as aerosols), as ultrafine powder, for films, distributed in fluids (dispersed, for example ferrofluids) or embedded in a solid body (nanocomposites). The present dossier focuses on those nanoparticles present in a solid state.

There are two general approaches to the synthesis of nanomaterials and the fabrication of nanostructures: one is the bottom-up approach and the top-down approach of the self-assembly of molecular components, where each nanostructured component becomes part of a superstructure.

Approaches of Nanotechnology         (growth methods ):
 Bottom-up or top-down?

      Bottom-up, or self-assembly, approaches to nanofabrication use chemical or physical forces operating at the nanoscale to assemble basic units into larger structures, while top-down approaches seek to create nanoscale devices by using larger, externally controlled ones to direct their assembly.

       The top-down approach often uses the traditional workshop or microfabrication methods where externally controlled tools are used to cut, mill, and shape materials into the desired shape and order.

     Micropatterning techniques, such as photolithography and inkjet printing belong to this category.

    Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to

(a) self-organize or self-assemble into some useful conformation, or

(b) rely on positional assembly. 

Simple Difference between Top Down and Bottom Up

      Nanoparticles can be synthesized in many different ways, both in chemical reactions nd physical processes. Most common methods used for the commercial or industrial manufacture of nanoparticles may be divided into four main groups:

Synthetic Techniques for Nano-particles

   

        Gas phase processes including vapor deposition, flame pyrolysis, high temperature evaporation and plasma synthesis.

• Liquid phase methods in which chemical reactions in solvents lead to the formation

of colloids, aerosols.

• Sol-gel technique.

• Solid phase mechanical processes including grinding, milling and alloying.

Even for the same material different methods are often used in order to optimize

specific properties of nanoparticles such as size, size distribution, symmetry, purity and

others.

          Even for the same material different methods are often used in order to optimize specific properties of nanoparticles such as size, size distribution, symmetry, purity and others.

Vapor – phase synthesis

             Vapor phase deposition can be used to fabricate thin films, multilayers, nanotubes, nanofilaments or nanometer-sized particles. The general techniques can be classified broadly as either physical vapor deposition (PVD) or chemical vapor deposition (CVD). PVD involves the conversion of solid material into a gaseous phase by physical processes; this material is then cooled and re-deposited on a substrate with perhaps some modification, such as reaction with a gas. Examples of PVD conversion processes include thermal evaporation (such as resistive or electron beam heating or even flame synthesis), laser ablation or pulsed laser deposition (where a short nanosecond pulse from a laser is focused onto the surface of a bulk target), spark erosion and sputtering (the removal of a target material by bombardment with atoms or ions).

                Most nanoparticle synthesis methods in the gas phase are based on homogeneous nucleation of a supersaturated vapor and subsequent particle growth by condensation, coagulation and capture.

                     In general, vapor forms within an aerosol reactor at elevated temperatures. The precursor material in the form of as solid, liquid or gas is introduced into the reactor where it is heated and mixed with a carrier gas. The supersaturated vapor is produced by cooling or by chemical decomposition reaction or by some combination of these. The most straightforward method of achieving super saturation is to evaporate a solid into a background gas. By including a reactive gas such as oxygen, oxides or other compounds of the evaporated material can be produced.

                             The nucleation process is initiated by the formation of very small nucleus from the molecular phase. These nuclei subsequently grow by surface growth mechanisms (heterogeneous condensation, surface reaction) and by collision and coagulation.

                           Further collisions can result in the formation of loosely bound agglomerates or chain like, dendritic forms.

                          The most common heating or evaporation process are: the flame pyrolysis, furnace flow reactors, laser induced pyrolysis, laser vaporization, thermal plasma, microwave plasma, sputtering, laser ablation.

Gas-Vapor deposition

                       Chemical Vapor Deposition (CVD) methods are well known in a semiconductors industry. In CVD process, vapor is formed in a reaction chamber by pyrolysis, reduction, oxidation or nitridation, and then deposited on the surface. Areas of growth are controlled by patterning processes like photolithography or photomasking (deposition patterns are etched on to the surface layers of the wafers). The most important application of CVD methods is the synthesis of carbon nanotubes where CVD is considered to offer one of the most effective routes for scaling up to industrial production. Many other nanoparticles are synthesized by CVD as well.

Chemical Vapor Deposition (CVD)

Plasma – based synthesis

Plasma spraying of materials onto substrates to form protective coatings is widely used industrial practice. The use of plasma (i.e., ionized gas) during vapor deposition allows accessing to different chemical and physical processes and obtaining final materials of high-purity. There are several different types of plasma deposition reactor for plasmaassisted PVD and CVD.

In plasma reactors temperatures of the order of 10,000°C can be achieved, causing evaporation or initiating chemical reactions. The main types of plasma used are Direct Current (DC) plasma jet, DC art plasma and Radio-Frequency (RF) induction plasma.

DC glow discharge involves the ionization of gas atoms by electrons emitted from a heated filament. The gas ions in the plasma are then accelerated to produce a directed ion beam. If the gas is a reactive precursor gas, this ion beams used to deposit directly onto a substrate. If an inert gas is used, the ion beam strikes a target material which sputters neutral atoms onto a neighboring substrate.

Another modification is a magnetron sputtering. In sputtering methods material is vaporized from a solid surface by bombardment with ions of inert gas from sputter sources like an ion gun or hollow cathode plasma sputter. Plasma is created by the application of a large DC potential between two parallel plates. A static magnetic field is applied near a sputtering target and confines the plasma to the vicinity of the target. Ions from the high-density plasma sputter material, predominantly in the form of neutral atoms, from the target onto a substrate. One of the greatest benefits of the magnetrons is high deposition rate (about 1 μm/min) that makes the method to be industrially viable. Moreover, multiple targets can be rotated so as to produce a multilayered coating on the substrate.

Nowadays, vacuum arc deposition is well-established process for producing of thin films and nanoparticles. This technique involves the initiation of an arc by contacting a cathode made of a target material. An igniter is attached to an anode in order to generate a low-voltage, high-current self-sustaining arc. The arc ejects ions and material droplets from a small area on the cathode. Further, the ions are accelerated towards a substrate while any large droplets are filtered out before deposition.

Vacuum arc deposition technique

     developed process of the vapor condensation. The principle of this method is illustrated in Figure. The precursor material is put into the working chamber with a stable arc. The chamber is filled by reactive gas that becomes ionized; then molecular clusters are formed and cooled to produce nanoparticles. In the plasma-assisted PVD processes the vapor phases originate from a solid target. Instead, plasma-enhanced CVD employs gas phase precursors that are dissociated to form molecular fragments which condense to form thin films or nanoparticles. The dissociation temperatures required for CVD tend to be much lower than for conventional CVD processes due to the high energy of the plasma, and this may be of importance for deposition on sensitive substrates such as semiconductors and polymers.

Principle of the vapor condensation process