Inert Gas Condensation 626k1w

INERT GAS CONDENSATION METHOD Inert Gas Condensation Method: The inert gas evaporation–condensation (IGC) technique, in which nanoparticles are formed via the evaporation of a metallic source in an inert gas, has been widely used in the synthesis of ultrafine metal particles since the 1930s. A similar method has been used in the manufacture of carbon black, an ink pigment, since ancient times. The technique employed now for the formation of nanopowders, in reality, differs from that used to produce carbon

and

lampblack

primarily

in

the

choice

of

atmospheric

composition and pressure and in the use of a chemically reactive source. Thus, although the technology is old, the application to the production of truly nanoscaled powders is relatively recent. In its basic form, the method consists of evaporating a metallic source, using resistive heating (although radio frequency heating or use of an electron or laser beam as the heating source are all equally effective methods) inside a chamber which has been previously evacuated to about 10 power -7 torr and backfilled with inert gas to a low pressure. The metal vapour migrates from the hot source into the cooler inert gas by a combination of convective flow and diffusion and the evaporated atoms collide with the gas atoms within the chamber, thus losing kinetic energy. Ultimately, the particles are collected for subsequent consolidation, usually by deposition on a cold surface.

Most applications of the inert gas condensation technique carry this approach to extremes by cooling the substrate with liquid nitrogen to enhance the deposition efficiency. Particles collected in this manner are highly concentrated on the deposition substrate. While the particles deposited on the substrate have complex aggregate morphology, the structure tends to be classified in of the size of the crystallites that make up these larger structures. The scraping and compaction processes take place within the clean environment to ensure powder surface cleanliness (i.e., to reduce oxide formation) and to minimise problems associated with trapped gas.

Sol-Gel methods The sol-gel process may be described as: ”Formation of an oxide network through polycondensation reactions of a molecular precursor in a liquid.” A sol is a stable dispersion of colloidal particles or polymers in a solvent. The particles may be amorphous or crystalline. An aerosol is particles in a gas phase, while a sol is particles in a liquid, A gel consists of a three dimensional continuous network, which encloses a liquid phase, In a colloidal gel, the network is built from agglomeration of colloidal particles. In a polymer gel the particles have a polymeric sub-structure made by aggregates of sub-colloidal particles. Generally, the sol particles may interact by van der Waals forces or hydrogen bonds. A gel may also be formed from linking polymer chains. In most gel systems used for materials synthesis, the interactions are of a covalent nature and the gel process is irreversible. The gelation process may be reversible if other interactions are involved.  The idea behind sol-gel synthesis is to “dissolve” the compound in a liquid in order to bring it back as a solid in a controlled manner.  Multi component compounds may be prepared with a controlled stoichiometry by mixing sols of different compounds.  The sol-gel method prevents the problems with co-precipitation, which may be inhomogeneous, be a gelation reaction.  Enables mixing at an atomic level. •Results in small particles, which are easily sinterable. The sol-gel method was developed in the 1960s mainly due to the need of new synthesis methods in the nuclear industry. A method was needed where dust was reduced (compared to the ceramic method) and which needed a lower sintering temperature. In addition, it should be possible to do the synthesis by remote control. One of the well known examples of a sol-gel system often cited is quick clay. Clay may form a sol (quick clay) if it is washed sufficiently to remove the counter ions. Quick clay may be gelled if enough counter ions are added, so that the colloidal particles aggregate. (exfoliation/restacking may be involved) Sol-gel synthesis may be used to prepare materials with a variety of shapes, such as porous structures, thin fibers, dense powders and thin films.

Approach 1: Electrochemical deposition of nanomaterials Electrochemical deposition is a deposition process in which metal ions in a solution are transported by an electric field to coat the surface of a substrate. The deposition process can be either cathodic or anodic reaction depending on the work piece to be coated (cathode or anode). Electrodeposition of nanostructures, eg. nanocrystallines,

nanocomposites, amorphous film or layered materials can be obtained by controlling the electrolysis parameters. The most commonly practiced techniques are (a) pulse current deposition to manipulate the growth of deposits, (b) deployment of additives and surfactants to alter the grain size of deposits and (c) nanoparticles inclusion into deposits to form nanocomposites. Pulse current electrodeposition of nanostructured coating

Direct current is the most commonly deployed technique to deposit a metal coating. Recent years have seen the use of pulsating the current

to

achieve

nanostructure

coating.

The

pulse

regime

parameters include pulse duty, pulse cycle, frequency and its amplitude, cathodic or anodic current, zero current at open-circuit, etc. Pulsating the deposition current can affects the diffusion layer next to the electrode surface which is in with the liquid solution. This will influence the deposition mechanisms of metal deposits such as altering the nucleation process and the subsequent growth of the deposit. Pulsed current can enable the incorporation of nanoparticles to a high content in the coating as well as producing a wider range of alloys, deposit composition and material properties

2.

Nanoparticles in a metal coating to form

nanocomposites

Nanosized particles can be incorporated into metallic coating to form nanocomposite coating. Two common processes involved in the incorporation of particles into metallic coatings are (a) physical dispersion of particles in the electrolyte and (b) electrophoresis migration of particles to the work piece ed by surface charged particles. The metal coating can be plain metal, eg. nickel, copper, tin, gold, or alloys and multilayered coatings. Nanoparticles may include metals, alloys, ceramics, metal oxides, nitrides, carbides, etc. The inclusion of nanoparticles into a metal coating is dependent on many electrolysis parameters such as characteristics of the nanoparticle (particle concentration, surface charge, type, shape, size), electrolyte composition (electrolyte concentration, additives, temperature, pH, surfactant type and concentration), current density (direct current, pulsed current, potentiostatic control) and flow hydrodynamics (laminar, turbulent regimes), electrode geometry and electrodeposition reactor, eg. rotating disk electrode, rotating cylinder electrode, parallel plate electrodes, etc. Figures below show electrodeposited nickel

coatings containing nanoparticles of silicon carbide (SiC) and titanium dioxide nanotubes (TiO ), for wear and corrosion resistance. 2

3.

Deployment

of

electrolyte

additives

and

surfactant technology Electrolyte additives and surfactant technology are keys to the development of nanostructured materials and coatings. Surfactants can be categorised into groups such as: cationic, anionic, non-ionic or amphoteric. Surfactants can be hydrocarbon or fluorocarbon based. In the surface metal finishing industry, electrolyte additives are commonly grouped by names such as brighteners (provide surface finish as matte, semi-matte or bright appearance), surface wetters (reduce surface tension between, reduce coating porosity or liberation of gas bubbles) and stress relievers (relieve compressive or tensile stress of the coating). Additives and surfactants are deployed to affect the growth mechanisms.

of metal deposits, via adsorption or desorption

Many metallic coatings are conventionally designed on the macroscale. By reducing the macro-scale to the nano-scale could provide enhanced surface properties, leading to a longer lasting, lighter weight and more protective coatings. Electrolyte additives and surfactants are used to affect the grain size of coating. The figure shows a polycrystalline vs. nanocrystalline coating. A nanocrystalline coating has nm grain size, with enhanced coating performance against an external load.

Pulsed Laser Deposition An introduction to Pulsed Laser Deposition The technique of PLD has been used to deposit high quality films of materials for more than a decade.The technique uses high power laser pulses (typically ~108 Wcm-2) to melt, evaporate and ionize material from the surface of a target.This "ablation" event produces a transient, highly luminous plasma plume that expands rapidly away from the target surface.The ablated material is collected on an appropriately placed substrate upon which it condenses and the thin film grows. Applications of the technique range from the production of superconducting and insulating circuit components to improved wear and biocompatibility for medical applications.In spite of this widespread usage, the fundamental processes occurring during the transfer of material from target to substrate are not fully understood and are consequently the focus of much research. In principle PLD is an extremely simple technique, which uses pulses of laser energy to remove material from the surface of a target, as shown schematically on the right. The vaporized material, containing neutrals, ions, electrons etc., is known as a laser-produced plasma plume and expands rapidly away from the target surface (velocities typically ~106 cms-1 in vacuum).Film growth occurs on a substrate upon which some of the plume material recondenses.In practice, however, the situation is not so simple, with a large number of variables affecting the properties of the film, such as laser fluence, background gas pressure and substrate temperature.These variables allow the film properties to be manipulated somewhat, to suit individual applications. However, optimization can require a considerable amount of time and effort.Indeed, much of the early research into PLD concentrated on the empirical optimization of deposition conditions for individual materials and applications, without attempting to understand the processes occurring as the material is transported from target to substrate.

The PLD process can be crudely split into two sections, i.e. the plasma creation and expansion, followed by film growth at the substrate. In the current article only data relating to the initial stage will be presented. The temporal evolution of densities, temperatures and velocities within laser-produced plasmas can only be determined using fast diagnostics (~ns time-scales), due to the high luminosity and transient nature of the plumes. A variety of techniques, including interferometry, optical spectroscopy and Laser-Induced Fluorescence (LIF), are used to investigate different stages of the plasma creation and expansion.At short delay times after the ablation pulse (<100ns) Mach-Zehnder interferometry has been used to study the free electron component within the plume expanding into vacuum. The time-varying electron density was calculated from a series of interferograms of the plume, captured using an Andor ICCD camera with a 2ns gate width.A typical image is shown in the on the left.