A K Das

A novel approach towards selective bulk synthesis of few-layer graphenes in an electric arc

The paper demonstrates the selective bulk synthesis of few-layer graphenes by optimizing an external magnetic field assisted electric arc. An ultra-high purity glassy graphite anode was sublimated in an argon atmosphere, and carbon nanotubes (CNTs), along with graphene sheets, were found inside the deposit formed on the cathode. Both the high purity CNTs and the graphene sheets, with minimal structural defects, were synthesized separately by varying the strength and orientation of the external magnetic field. The as-synthesized carbonaceous samples were characterized with the help of transmission electron microscopy, selected area electron diffraction (SAED), Raman spectroscopy and thermogravimetry with the objective of optimizing the highest selective production of 2D graphene structures. The as-synthesized graphene sheets exhibited a relatively high degree of graphitization and low structural defect density as confirmed by Raman spectroscopy. They were found to exhibit higher oxidation temperature (767 °C) than that of the carbon nanocrystalline particles (690 °C), as inferred from the thermogravimatric analysis. Moreover, they were found to roll up at their edges on account of their surface energy minimization. This was confirmed by the SAED analysis. With this new technique, we could successfully synthesize 2D graphene structures at the rate of a few g h−1.

Low temperature magnetic structure of MnSe

In this paper we report low temperature neutron diffraction studies on MnSe in order to understand the anomalous behaviour of their magnetic and transport properties. Our study indicates that at low temperatures MnSe has two coexisting crystal structures, high temperature NaCl and hexagonal NiAs. NiAs phase appears below 266 K and is antiferromagnetically ordered at all temperatures while the NaCl phase orders antiferromagnetically at 130 K.

Experimental evidence of chaotic behavior in atmospheric pressure arc discharge

In free burning as well as in stabilized arc columns, the inherent movement of arc root results in fluctuation in arc voltage. A full knowledge and control over the arc root dynamics can effectively lengthen the life time, drastically improve performance and reliability in arc plasma devices. In this paper, we experimentally investigate the fluctuating voltage signals generated from an atmospheric pressure arc discharge produced in a hollow electrode plasma torch. For the first time, analysis of these signals reveal them to exhibit chaotic behavior. The present analysis is supported with real time behavior, phase portraits, power spectra and Lyapunov exponents. Dependence of system behavior on various control parameters is also investigated. This approach is interesting in the sense that it can lead to better understanding of physics for future researches on arc plasma jets and related devices.

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

A new approach towards improving the quality and yield of arc-generated carbon nanotubes

The paper reports the deterministic effects of a focusing electric field in improving the purity and yield of the arc-generated carbon nanotubes (CNTs). The method utilizes a focusing electrostatic field, which was superimposed on the arc symmetrically. The focusing voltage was varied from 0 to 1200 V at steps of 200 V and a number of cathode deposits, thus generated, were collected and thoroughly analysed in their totality with the help of weight balance, Raman spectroscopy, transmission electron microscopy and thermogravimetry. With the optimally configured focusing electric field, the arc generator is found to utilize nearly 85% of the consumed anode material, for converting into cathode deposit consisting of CNTs, as compared with about 35% in the conventional arc plasma method. The sample prepared under optimized conditions exhibited high oxidation temperature (851 °C) in the thermogravimetric analysis, negligible D band intensity along with a reduced G band line-width (14 cm−1) in the Raman spectrum, confirming the presence of high purity CNTs with a high relative yield.

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.

Collision integrals for charged-charged interaction in two-temperature non-equilibrium plasma

Choice of an appropriate form of shielding distance in the estimation of collision integrals under screened coulomb potential for two-temperature non-equilibrium plasma is addressed. Simple expressions for collision integrals for charged-charged interactions are derived. It is shown that while some of the formalisms used earlier completely ignore the presence of ions, the others incorporating it may result in negative collision integrals for the interactions involving particles at higher charged states. The parametric regimes of concern and impact of different formalisms on the computed transport properties are investigated with specific reference to nitrogen plasma. A revised definition of the shielding distance is proposed, which incorporates both electrons and ions, avoids the problem of negative collision integrals in all practical regimes of interest and results in calculated property values in close agreement with experimentally observed results.

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.

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.