Thermal applied in the fields of environmental remediation, magnetic

Thermal plasma synthesis
of nanopowders is a method which is gaining importance due to its unique
features in recent years. It has been employed for the formation of nanoparticles
and nanostructures over decades. Its high enthalpy and high density of the ions
makes it advantageous for material processing. The plasma temperature,
approximately 1000 – 20000 K provides immense thermal energy to evaporate the
precursors which condense on the chamber walls to form nanoparticles. The size
and shape of the resultant nanoparticles are tuned as a result of which its
magnetic, optical and electronic properties change. Fe3O4
(Fe2+Fe3+2O2-4) is also
commonly known as rust. It has an inverse spinel structure where oxygen forms a
close-packed cubic lattice with metal ions
located at the intermediate sites of the oxygen ions. Fe3+ ions
occupy the tetrahedral and half of the octahedral sites and Fe2+ ions
occupy the other half of the octahedral sites. Magnetite nanoparticles are
applied in the fields of environmental remediation, magnetic fluids, catalysis,
magnetic resonance imaging, magnetic data storage, drug delivery, gas sensors
and magnetic refrigeration 1-8.  There
are many chemical and physical routes available to synthesize Fe3O4
nanoparticles which include flow injection synthesis 9, electro-thermal
synthesis 10, co-precipitation 11,12, sol-gel 13, hydrothermal synthesis 14,
mechanical ultra-sonication synthesis 15 and thermal plasma synthesis 16-19.
Among these methods, thermal plasma
technique is a single step process involving simultaneous melting, evaporation,
nucleation and condensation. It has the advantages of avoiding chemical effluents,
providing continuous production of powders, rapid processing and is a clean and
dry process which is better than most chemical processes which involve multiple steps 20. It has higher
plasma volume and hence higher rate of heat transfer than RF and
non-transferred torches. Unlike non-transferred torches,
the plasma arc is generated between the workpiece and the cathode enabling it
to produce higher heat fluxes and minimise heat loss due to radiation. Another
important advantage of DC transferred arc plasma is that any conducting
material can be synthesized in any ambient atmosphere. The electrodes are made
up of refractory materials commonly graphite, molybdenum or tungsten in order
to avoid mixing with the source material. Thus synthesis through transferred
arc plasma is highly pure and quick. On applying plasma processing to
industries, it will be a novel method for mass production, chemical-free
synthesis and the best way to recycle
metal scrap.

Several
researchers involved in plasma synthesis techniques have used commercial iron
block, ferrocene (Fe(C5H5)2) and iron
pentacarbonyl (Fe(CO)5).
A summary of structural and magnetic properties of iron oxide nanoparticles
processed through plasma techniques are presented in table 1.

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The
main objective of the present work is to synthesize iron oxide nanoparticles by
recycling mild steel scrap by DC transferred arc plasma. The
steep thermal gradients in the plasma reaction chamber permit the evaporated
species to quench at very fast rates, producing a possibility of meta-stable
states and non-equilibrium compositions. Plasma can be used for the treatment
of a wide range of wastes including liquids, solids and gases 20, 21. Mild
steel scrap has been chosen as the source material to be processed. Mild
steel is a rich source of iron with 0.05% to 0.15% of carbon and is easily
prone to corrosion. Mild steel is easy to form into structures and residual
pieces become scrap.  The rusted mild steel scrap can be processed
into nanoparticles in a novel way using DC transferred arc plasma.

            Magnetite (Fe3O4)
nanoparticles were synthesized using various operating current from 50A to 80A.
The phase, size, morphology and magnetic properties of the product were
characterized.

 

 

1.    Materials and methods       

            The schematic figure of DC transferred arc plasma
setup is shown in fig. 1. It consists of a double-walled
stainless steel chamber with an immovable anode
and a movable cathode which is connected
to a constant current power supply. A graphite crucible is fixed to the stainless
steel rod in which the workpiece is
placed. A mild steel piece was taken from scrap having dimensions of 8cm length
and 0.5cm diameter was fixed as the cathode. The mild steel scrap pieces were
placed inside the graphite crucible. De-moisturized
air was used as the ambient gas.

            An
arc was generated by breaking down the air between the electrodes. The de-moisturized
air inside the chamber acted as a source of oxygen for the experiment. Experiments
were performed by varying the operating currents from 50A to 80A at an
operating pressure of 1atm.

            The
electrode (anode) was melted and evaporated simultaneously due to the high heat
transfer from the cathode during the experiment.
The metal vapours experienced a temperature gradient ?T during its path from
the arc to the walls of the chamber. During its trajectory,
the metal vapours react with the oxygen and form a metal oxide. The metal oxide vapours form clusters during their
dwelling time in order to minimise the absorbed energy and condense
heterogeneously on the walls of the chamber. The detailed description of the
process is also shown in fig. 1. The processed nanopowders were collected by
removing the layer deposited on the inner walls of the chamber, few hours after
completion of the experiment.

            The
phase and crystallite size were analysed by X-ray diffraction analysis (XRD)
and elemental composition was identified through energy dispersive spectroscopy
analysis (EDS). The powder morphology, particle size and particle size
distributions were analysed using field emission scanning electron microscope
(FE-SEM). The M-H curves, zero field cooling (ZFC) – field cooling (FC) and M-T
curve of the as-synthesized powders were studied using superconducting quantum
interface device (SQUID) magnetometer and by high temperature vibrating sample
magnetometer (HTVSM).