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Graphene nanosheets were produced by graphite exfoliation through underwater electrical discharge, also known as plasma underwater. Graphite particles were dispersed in different solvents with different ratios between polar and dispersive surface tension components (pure water and an aqueous solution of isopropyl alcohol (IPA) at different concentrations (1:1 and 7:3 IPA:Water)) under constant stirring and low voltage pulses (about 1.5 kV). Even under relative low voltage, the production of graphene was observed in all solvents with the most of the graphene sheets containing less than 10 monolayers. Results show that further affinity between the surface tensions of the graphite and the solvents influenced in the characteristic of the graphene obtained. The 7:3 IPA/water solution has a similar ratio between polar and dispersive surface tension components to the graphite and the mixture. Exfoliation product obtained with this mixture presented an intense peak at ∼284.3 eV, which is corresponding to sp2 binding, characteristic of the graphene. However, in this solution, it was observed further break of C-C bonds and a significant reduction of its surface area. Smaller graphene flakes sizes present more C-O contents due to the carboxyl groups be more likely to form at graphene edges than at defect sites within the basal plane. [ABSTRACT FROM AUTHOR]

Power supplies have played crucial role to improve and extend plasma applications. Several dc, pulsed, and radio frequency (RF) stabilized power supplies have been developed over a wide range of frequencies and voltage waveforms. However, the influence of voltage waveform on the plasma densification, mainly due to the secondary electrons emission from the electrodes, has not been quite explored. This paper proposes an asymmetric pulsed bipolar power supply, adjustable in frequency, amplitude, pulsewidth, and number of pulses for generation of cold plasmas. Short period positive pulses, between longer negative pulses, increase greatly the secondary electron emission from the electrodes and, consequently, the plasma ionization rate. Short pulses (ns) are obtained by the phase shift between each carrier, and the high-intensity pulses are obtained by cascading the cell submodules in a half-bridge cascade topology, using a modular half-bridge converter configuration. The intensity of the positive and negative pulses can be varied independently. There is no unintended dc voltage level between the electrodes, which appear in RF plasmas. The plasma generated in this way is free of electric arc and can be kept stable within a broader range of operating parameters, mainly the voltage and the working gas pressure. [ABSTRACT FROM AUTHOR]

This paper presents experimental results that indicate the possibility of selecting the crystalline phase obtained by nitriding pulsed plasma, allowing the choice among the ε-Fe2-3N and γ'-Fe4N single-phase layers formed under the sample surface, in the white layer. AISI 1015 low carbon steel samples were treated with pressures between 100 Pa and 1400 Pa at 500°C. Short bipolar pulses of voltage were applied at 125kHz to perform the plasma nitriding. The XRD and SEM results show that the increase in the pressure favors the selection of γ'-Fe4N. [ABSTRACT FROM AUTHOR]