S N Sahasrabudhe

In Situ Optical Emission Spectroscopic Investigations During Arc Plasma Synthesis of Iron Oxide Nanoparticles by Thermal Plasma

Investigations using in situ precursor spectroscopy during the growth of nanoparticles of iron oxide by thermal plasma induced gas phase condensation method have been shown to be useful for correlating the size of nanoparticles with existing plasma parameters. The relative abundance of ionized Fe species inside the plasma plume is seen to directly establish the relation between particle size, arc current, arc length, and ambient pressure of the reacting oxygen gas. The argon plasma from a transferred arc reactor is made to impinge on the anode that is allowed to vaporize and react with oxygen. The spectral line profiles of both Ar and Fe along the plasma column during the synthesis of nanoparticles have been proved to be useful in understanding the growth mechanism. Band intensities of FeO molecular states indicated the inverse relation with particle sizes that have been correlated to the two competitive processes in which energy is released, namely: 1) one involving the radiative transition and 2) the other that of the growth by coagulation. Atomic Boltzmann plots are used for estimating the temperatures of the zones, whereas particle sizes have been inferred using transmission electron microscopic measurements

Dynamic characteristics of a hollow copper electrode plasma torch through measurement and analysis of acoustic, optical, and voltage fluctuations

The basic nature of arc root fluctuation in a plasma torch is an extremely important factor from the point of view of improving and optimizing performance of a plasma torch in present-day technology. In spite of a number of ingenious attempts, this particular phenomenon is still not fully explored. In this paper, voltage, acoustic, and optical signals generated from a hollow cathode plasma torch are analyzed using various tools of dynamical analysis such as real-time behavior, phase portraits, power spectra, Lyapunov exponent, dimension, etc. Origin of each of the signal in relation to arc-root fluctuation and mutual correspondence among themselves are described in detail. For the first time, all the signals are found to exhibit clear evidence of chaotic behavior in all respect. A dimensional analysis reveals all the three signals to be originated from the same chaotic phenomenon, i.e., fluctuation of are root.

Synthesis and characterization of Nd2O3 nanoparticles in a radiofrequency thermal plasma reactor

The synthesis of nanocrystalline Nd2O3 through an inductively coupled radiofrequency thermal plasma route is reported. Unlike in conventional synthesis processes, plasma-synthesized nanoparticles are directly obtained in a stable hexagonal crystal structure with a faceted morphology. The synthesized nanoparticles are highly uniform with an average size around 20 nm. The nanoparticles are characterized in terms of phase formation, crystallinity, morphology, size distribution, nature of chemical bonds and post-synthesis environmental effects using standard characterization techniques. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy are used for structural and morphological studies. The thermo-gravimetric technique, using a differential scanning calorimeter, is used to investigate the purity of phase. Fourier transform infrared spectroscopy is used to investigate the nature of existing bonds. The optical response of the nanoparticles is investigated through the electronic transition of Nd(3+) ions in its crystalline structure via UV-visible spectroscopy. The presence of defect states and corresponding activation energies in the nanocrystalline Nd2O3 compared to those of the precursors are studied using thermoluminescence.

Theory of dynamic behavior in atmospheric pressure arc plasma devices: part-II: validation of theory with experimental data

The results of an application of the developed nonlinear theory for atmospheric pressure arc plasma instability to various experiments are presented in this paper. Most of the important experiments on atmospheric pressure arc plasma instability reported in literature are addressed. In all cases, a good match has been observed between experiment and theory. General nature of the theory to explain observed instability features is brought out in this paper.

In Situ Studies of Emission Characteristics of the DC Thermal Arc Plasma Column During Synthesis of Nano-AlN Particles

The growth process of nanoparticles and nanowires of AlN by thermal-plasma-assisted gas phase condensation reaction has been investigated by optical emission spectroscopy. The concentrations of the reacting precursors in the plasma have been correlated to the crystalline phases of nanoparticles of AlN found from X-ray diffraction analysis. The size and morphology of the nanoparticles have been studied by transmission electron microscope investigations of as-synthesized powder at a set of reactor parameters, which included arc current, reactor pressure, and standoffs of the arc column. An attempt has been made to correlate the growth of AlN to that of the precursor density present in the plasma reaction zone

Role of arc plasma instability on nanosynthesis

Recent studies have shown that use of direct current arc plasma jet is a promising technique for bulk generation of nanostructures. This paper presents a dynamical study on such systems during the synthesis and attempts to correlate the role of inherent arc fluctuations on the properties of nanostructures produced. Different fluctuations detected during the process of nanosynthesis have been characterized in terms of behavior in real time, phase space, frequency space, fractal dimension, Lyapunov exponent of evolution and diagnosed to be chaotic. For the first time, it has been shown that size of nanoparticles are strongly related to the Lyapunov exponent of inherent voltage fluctuations realized during the synthesis. Interesting features of such dependences under different process conditions have been brought out. Since chaotic systems are controllable, the study brings out the possibility of development of new size control strategies for nanosynthesis.

Direct probing of anode arc root dynamics and voltage instability in a dc non-transferred arc plasma jet

The transient dynamics of the anode arc root in a dc non-transferred arc plasma torch is captured through fast photography and directly correlated with the associated voltage instability for the first time. The coexistence of multiple arc roots, the transition to a single arc root, root formation and extinction are investigated for the steady, takeover and re-strike modes of the arc. Contrary to the usual concept, the emerging plasma jet of a dc non-transferred arc plasma torch is found to carry current. An unusually long self-propelled arc plasma jet, a consequence of the phenomenon, is demonstrated.

Current transfer in dc non-transferred arc plasma torches

Fundamentals of current transfer to the anodes in dc non-transferred arc plasma torches are investigated. Specially designed anodes made of three mutually isolated sections and external dc axial magnetic fields of various strengths are utilized to explore the conditions for different diffused and constricted attachments of the arc with the anode. A number of new facts are revealed in the exercise. Under constricted attachment, formation of arc root takes place. Spontaneous and magnetically induced movements of the arc root, their dependence on the arc current and the strength of the external magnetic field, most probable arc root velocity, variation of the root velocity with strength of the applied magnetic field, the effect of swirl on the rotational speed of the arc root are some of the important features investigated. Two new techniques are introduced: one for measurement of the arc root diameter and the other for determination of the negative electric field in the boundary layer over the anode. While the first one exploits the rigid column behaviour of the arcs, the second one utilizes the shooting back of the residual electrons over an arc spot. Sample calculations are provided.

Synthesis of nanocrystalline Y2O3 in a specially designed atmospheric pressure radio frequency thermal plasma reactor

Synthesis of yttrium oxide nanoparticles in a specially designed radio frequency thermal plasma reactor is reported. Good crystallinity, narrow size distribution, low defect state concentration, high purity, good production rate, single-step synthesis, and simultaneous formation of nanocrystalline monoclinic and cubic phases are some of the interesting features observed. Synthesized particles are characterized through X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, thermo-luminescence (TL), and Brunauer–Emmett–Teller surface area analysis. Polymorphism of the nanocrystalline yttria is addressed in detail. Synthesis mechanism is explored through in-situ emission spectroscopy. Post-synthesis environmental effects and possible methods to eliminate the undesired phases are probed. Defect states are investigated through the study of TL spectra.

Characteristics of Synthesized Alumina Nanoparticles in a High-Pressure Radio Frequency Thermal Plasma Reactor

Nanophase alumina is synthesized in an atmospheric pressure radio frequency (RF) plasma reactor through melting, evaporation, and vapor phase nucleation technique. A specially designed high-pressure RF plasma reactor fitted with an indigenously built RF plasma torch converts commercially available micrometer size alumina chunks into spherical nanophase alumina in a single step under ambient quenching conditions without use of any additional quenching gas. Obtained powder contains mixed phases of α and δ -alumina as revealed by X-ray diffraction studies. Transmission Electron Microscopy analyses exhibit very small particle size (peak at 15 nm), narrow size distribution (half width ~ 24 nm), zero agglomeration, and good crystallinity. Obtained particle characteristics together with the high purity owing to inherent electrode-less feature of the RF discharge are suitable for important technological applications including fabrication of high-power ceramic laser gain media like Y3Al5O12 (YAG) from composites of Al2O3 and Y2O3. Characteristics of the synthesized alumina are compared with that of nanoalumina synthesized in atmospheric arcs.