Madhusudhan Kundrapu

Arc plasma synthesis of carbon nanostructures: where is the frontier?

In this perspective paper, we critically analyse the state-of-the-art of arc discharge technique of carbon nanoparticle synthesis. We discuss improving controllability of the arc discharge synthesis of carbon nanotubes, synthesis of graphene as well as general understanding of the synthesis process. Fundamental issues related to relationship between plasma parameters and carbon nanostructure characteristics are considered. Effects of electrical and magnetic fields applied during single-wall carbon nanotube synthesis in arc plasma are explored. Finally our personal opinion on what future trends will be in arc discharge synthesis is offered.

Modeling Radio Communication Blackout and Blackout Mitigation in Hypersonic Vehicles

A procedure for the modeling and analysis of radio communication blackout of hypersonic vehicles is presented. The weakly ionized plasma generated around the surface of a hypersonic reentry vehicle is simulated using full Navier–Stokes equations in multispecies single fluid form. A seven-species air chemistry model is used to compute the individual species densities in air including ionization: plasma densities are compared with the experiment. The electromagnetic wave’s interaction with the plasma layer is modeled using multifluid equations for fluid transport and full Maxwell’s equations for the electromagnetic fields. The multifluid solver is verified for a whistler wave propagating through a slab. First principles radio communication blackout over a hypersonic vehicle is demonstrated along with a simple blackout mitigation scheme using a magnetic window.

Numerical simulation of carbon arc discharge for nanoparticle synthesis

Arc discharge with catalyst-filled carbon anode in helium background was used for the synthesis of carbon nanoparticles. In this paper, we present the results of numerical simulation of carbon arc discharges with arc current varying from 10 A to 100 A in a background gas pressure of 68 kPa. Anode sublimation rate and current voltage characteristics are compared with experiments. Distribution of temperature and species density, which is important for the estimation of the growth of nanoparticles, is obtained. The probable location of nanoparticle growth region is identified based on the temperature range for the formation of catalyst clusters.

A model of carbon nanotube synthesis in arc discharge plasmas

Nanoparticle growth in arc discharges is analysed numerically. An analysis is carried out for the root growth method of nanotubes in plasmas. The existing models for estimating the growth of nanoparticles in stationary plasmas are extended to plasmas with variable properties. The distributions of velocity, species density and temperature from numerical simulations are used as input to the growth models. The nickel particle diameter obtained from the numerical model is 9.2?nm and the frequency of finding this size in the experiment is 26 on the larger side. The length of the single-walled carbon nanotube obtained from the model is 2.1??m, which falls in the upper 10% of the size distribution from experiment. Parametric studies are carried out varying the arc current, inter-electrode gap and background pressure. Results showed 40?95% increment in the nanotube length by increasing the background pressure and the inter-electrode gap. A hot-chamber arc discharge method is proposed to maximize the growth of nanoparticles subjected to the conditions identical to those existing in convectional arc discharges.

Laboratory Modeling of the Plasma Layer at Hypersonic Flight

A simple approach to modeling the plasma layer similar to that appearing in the vicinity of a hypersonic vehicle is demonstrated in a laboratory experiment. This approach is based on the use of a hypersonic jet from a cathodic arc plasma. Another critical element of this laboratory experiment is a blunt body made from a fairly thin foil of refractory material. In experiments, this blunt body is heated by the plasma jet to a temperature sufficiently high to ensure evaporation of surface deposits produced by the metallic plasma jet. This process mimics reflection of gas flow from the hypersonic vehicle in a real flight. Two-dimensional distributions of the hypersonic plasma flow around the blunt body were measured using electrostatic Langmuir probes. Measured plasma density was typically 1012  cm−3, which is close to the values measured for real hypersonic flight. The demonstrated laboratory experiment can be used to validate numerical codes for simulating hypersonic flight and to conduct ground-based tests...

Emission spectra analysis of arc plasma for synthesis of carbon nanostructures in various magnetic conditions

Arc discharge supported by the erosion of anode materials is one of the most practical and efficient methods to synthesize various high-quality carbon nanostructures. By introducing a non-uniform magnetic field in arc plasmas, high-purity single-walled carbon nanotubes (SWCNT) and large-scale graphene flakes can be obtained in a single step. In this paper, ultraviolet-visible emission spectra of arc in different spots under various magnetic conditions are analyzed to provide an in situ investigation for transformation processes of evaporated species and growth of carbon nanostructures in arc. Based on the arc spectra of carbon diatomic Swan bands, vibrational temperature in arc is determined. The vibrational temperature in arc center was measured around 6950 K, which is in good agreement with our simulation results. Experimental and simulation results suggest that SWCNT are formed in the arc periphery region. Transmission electronic microscope and Raman spectroscope are also employed to characterize the p...

Laser ablation of metallic targets with high fluences: Self-consistent approach

Ablation of metallic target using Gaussian profile laser pulse with duration of 30 ns and fluence extending up to 100 J/cm2 is studied. A self-consistent model that involves numerical simulation to obtain the thickness of a hydrodynamic layer is proposed. The influence of the hydrodynamic layer thickness on Knudsen layer properties, and hence the rate of evaporation, is demonstrated. Self-consistent model is compared with model based on assumption about sonic speed at the Knudsen layer outer edge and validated with experimental results for Al alloy and Cu. Finally, sample calculations are performed using the proposed model to obtain depth of evaporation of Al alloy for single pulse duration.

Nautilus: A Tool For Modeling Fluid Plasmas

Plasmas are important in many situations including space and solar physics, lightning, re-entry heating and nuclear fusion. It’s most often the case that the plasma is generated from a cold neutral fluid so there is a transition regime where the plasma is neither fully ionized nor neutral. Furthermore in most high temperature devices a significant amount of neutral fluid may exist (particularly near the wall). As such, is is important that even for high temperature plasmas the transition regime between fully ionized and non-ionized plasmas be modeled. In addition, given the wide variety of temperature and densities that can exist in plasmas, there are many, many different plasma models that are relevant in the different situation. Nautilus is a code that is a collection of these various plasma fluid models encompassing the regimes of neutral flow (important in aeronautics) to high temperature plasmas important in nuclear fusion. The fluid models can use either ideal gas laws or general equation of state. Radiation transport can also be modeled. This paper outlines the physics modeled in Nautilus along with the algorithms and various applications including plasma jet merging, FRC formation and collisionless reconnection.

Simultaneous synthesis of single-walled carbon nanotubes and graphene in a magnetically-enhanced arc plasma.

Carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene attract a deluge of interest of scholars nowadays due to their very promising application for molecular sensors, field effect transistor and super thin and flexible electronic devices(1-4). Anodic arc discharge supported by the erosion of the anode material is one of the most practical and efficient methods, which can provide specific non-equilibrium processes and a high influx of carbon material to the developing structures at relatively higher temperature, and consequently the as-synthesized products have few structural defects and better crystallinity. To further improve the controllability and flexibility of the synthesis of carbon nanostructures in arc discharge, magnetic fields can be applied during the synthesis process according to the strong magnetic responses of arc plasmas. It was demonstrated that the magnetically-enhanced arc discharge can increase the average length of SWCNT (5), narrow the diameter distribution of metallic catalyst particles and carbon nanotubes (6), and change the ratio of metallic and semiconducting carbon nanotubes (7), as well as lead to graphene synthesis (8). Furthermore, it is worthwhile to remark that when we introduce a non-uniform magnetic field with the component normal to the current in arc, the Lorentz force along the J×B direction can generate the plasmas jet and make effective delivery of carbon ion particles and heat flux to samples. As a result, large-scale graphene flakes and high-purity single-walled carbon nanotubes were simultaneously generated by such new magnetically-enhanced anodic arc method. Arc imaging, scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectroscopy were employed to analyze the characterization of carbon nanostructures. These findings indicate a wide spectrum of opportunities to manipulate with the properties of nanostructures produced in plasmas by means of controlling the arc conditions.

Modeling and Simulation of Weakly Ionized Plasmas Using Nautilus

Weakly ionized plasma on a blunt body in high speed flows is simulated using magneto– gasdynamic and electrostatic multi–fluid reactive flow models. Small scale laboratory experiments are designed to generate high speed plasma flow over a blunt body and the plasma density on the cone is measured in radial direction using Langmuir probes. The whole experiment is simulated using magneto–gasdynamic multi–fluid reactive flow model and the results are compared. Then weakly ionized plasma on RAMC type blunt body is simulated and the RF wave propagation through the plasma layer is demonstrated using electrostatic multi–fluid reactive flow model.