Plasma Chemistry and Plasma Processing (v.30, #6)

A modeling study is conducted to investigate the effect of hydrogen content in propellants on the plasma flow, heat transfer and energy conversion characteristics of low-power (kW class) arc-heated hydrogen/nitrogen thrusters (arcjets). 1:0 (pure hydrogen), 3:1 (to simulate decomposed ammonia), 2:1 (to simulate decomposed hydrazine) and 0:1 (pure nitrogen) hydrogen/nitrogen mixtures are chosen as the propellants. Both the gas flow region inside the thruster nozzle and the anode-nozzle wall are included in the computational domain in order to better treat the conjugate heat transfer between the gas flow region and the solid wall region. The axial variations of the enthalpy flux, kinetic energy flux, directed kinetic-energy flux, and momentum flux, all normalized to the mass flow rate of the propellant, are used to investigate the energy conversion process inside the thruster nozzle. The modeling results show that the values of the arc voltage, the gas axial-velocity at the thruster exit, and the specific impulse of the arcjet thruster all increase with increasing hydrogen content in the propellant, but the gas temperature at the nitrogen thruster exit is significantly higher than that for other three propellants. The flow, heat transfer, and energy conversion processes taking place in the thruster nozzle have some common features for all the four propellants. The propellant is heated mainly in the near-cathode and constrictor region, accompanied with a rapid increase of the enthalpy flux, and after achieving its maximum value, the enthalpy flux decreases appreciably due to the conversion of gas internal energy into its kinetic energy in the divergent segment of the thruster nozzle. The kinetic energy flux, directed kinetic energy flux and momentum flux also increase at first due to the arc heating and the thermodynamic expansion, assume their maximum inside the nozzle and then decrease gradually as the propellant flows toward the thruster exit. It is found that a large energy loss (31–52%) occurs in the thruster nozzle due to the heat transfer to the nozzle wall and too long nozzle is not necessary. Modeling results for the NASA 1-kW class arcjet thruster with hydrogen or decomposed hydrazine as the propellant are found to compare favorably with available experimental data.
Keywords: Low-power arcjet; Plasma flow and heat transfer; Numerical modeling; Hydrogen/nitrogen mixtures

Numerical Modeling in Radio Frequency Suspension Plasma Spray of Zirconia Powders by Lijuan Qian; Jianzhong Lin; Hongbin Xiong (733-760).
A comprehensive model was developed to investigate the suspension spraying for a radio frequency (RF) inductively coupled plasma torch. Firstly, the electromagnetic field is solved with the Maxwell equations and validated by the analytical solutions. Secondly, the plasma field with different power inputs is simulated by solving the governing equations of the fluid flow coupled with the RF heating. Then, the suspension droplets embedded with nano particles are modeled in a Lagrangian manner, considering feeding, collision, heating and evaporation of the suspension droplets, as well as tracking, acceleration, melting and evaporation of the nano or agglomerate particles. The non-continuum effects and the influence of the evaporation on the heat transfer are considered. This particle model predicts the trajectory, velocity, temperature and size of the in-flight nano- or agglomerate particles. The effects of operating conditions and intial inputs on the particle characteristics are investigated. The statistical distributions of multiple particles’ size, velocity, temperature are also discussed for the cases with and without consideration of suspension droplets collision.
Keywords: Suspension injection; RF plasma spray; Numerical modeling

Powder Loading Effects of Yttria-Stabilized Zirconia in Atmospheric dc Plasma Spraying by Kentaro Shinoda; Yang Tan; Sanjay Sampath (761-778).
Powder loading effects have been reexamined for various yttria-stabilized zirconia powders under atmospheric dc plasma spraying. A laser illumination method was utilized to observe powder injection into the plasma jet, while single particle and ensemble methods to measure particle state parameters. Statistical temperature distributions of in-flight particles suggested a rapid increase in the number of semi-molten particles above a certain powder loading rate. Despite drops in the particle temperature and velocity due to the powder loading effect, the deposition efficiency tends to have increased in some cases. Reliability of the single particle and ensemble methods has also been examined at various powder feed rates. Particle temperature measurement by the ensemble method at low powder feed rates could cause a significant error, which may affect powder injection optimization. Particle plume trajectory was not affected as much by the powder loading, which hence had only a limited effect on the particle diagnostics.
Keywords: Plasma spraying; Loading effects; Diagnostics; Particle state parameters; Temperature distribution

Titanium carbonitride nanoparticles have been produced in an inductively coupled thermal plasma and subsequently modified using a surfactant that has been deposited in situ on their surface in-flight. The surfactant was injected in the reactor while the nanoparticles are still dispersed in the gas phase, allowing the coating of primary particles instead of the corresponding agglomerates. In contrast to naked TiCN nanoparticles, the surfactant coated particles could be readily dispersed in water with a short ultrasonic treatment and built up no large agglomerates as proved by Photon Correlation Spectroscopy measurements. The investigated surfactants seem, however, to undergo a chemical modification and/or a thermal degradation at the surface of the TiCN nanoparticles.
Keywords: Nanoparticles; Surface modification; Thermal plasma; Dispersion; Safety

Direct Synthesis of Yttrium Aluminum Garnet Particles in an Inductively Coupled Radio-Frequency Plasma System by Ricky Jain; Steven L. Girshick; Joachim V. Heberlein; Rajesh Mukherjee; Bin Zhang; Toshitaka Nakamura; Amane Mochizuki (795-811).
Advanced phosphor materials such as cerium-doped yttrium aluminum garnet (YAG) are of interest for a variety of applications, including light-emitting diodes. Previous studies have shown that it is difficult to produce the desired YAG phase without ex-situ annealing irrespective of the synthesis technique used. This study focuses on direct synthesis of YAG phosphor particles using an inductively coupled thermal plasma system with a ceramic tube inserted coaxially into the chamber. Numerical modeling indicates that the tube provides a more uniform high-temperature region, without flow recirculation. This is hypothesized to aid in size and phase control through selective particle collection and in-flight annealing. Experiments conducted with the tube-insertion setup indicate that phase and size control of the particles is possible to a certain extent, depending on the size of the tube. Characterization results of the synthesized particles showed that submicron-sized YAG particles are synthesized as the majority phase through the tube-insertion setup.
Keywords: Yttrium aluminum garnet (YAG); RF plasma; Phosphors; In-flight annealing

Decomposition mechanism of HFC-134a, HFC-32, and CF4 in water plasmas at atmospheric pressure has been investigated. The decomposition efficiency of 99.9% can be obtained up to 3.17 mol kWh−1 of the ratio of hydrofluorocarbon (HFC) feed rate to the arc power and 1.89 mol kWh−1 of the ratio of perfluorocarbon (PFC) feed rate to the arc power. The species such as H2, CO, CO2, CH4, and CF4 were detected from the effluent gas of both PFC and HFC decomposition. However, CH2F2 and CHF3 were observed only in the case of HFC decomposition. The HFC and PFC decomposition generate CH2F, CHFx (x:1–2), and CFy (y:1–3) radicals, then those radicals were subsequently oxidized by oxygen, leading to CO and CO2 generation in the excess oxygen condition. However, when there is insufficient oxygen available, those radicals were easily recombined with fluorine to form by-product such as CH2F2, CHF3, and CF4.
Keywords: Hydrofluorocarbons; Perfluorocarbons; Water plasmas; Thermal plasmas

Killing Effects of Hydroxyl Radical on Algae and Bacteria in Ship’s Ballast Water and on Their Cell Morphology by Mindong Bai; Zhitao Zhang; Xiaohong Xue; Xianli Yang; Liusan Hua; Dan Fan (831-840).
So far there is no an effective method for fast killing of algae and bacteria in the course of conveying ship’s ballast water. With a physical method of strong electric-field discharge, O2 in air and H2O in seawater are ionized and dissociated and then dissolved into seawater to generate •OH radicals. In the experimental system of 10 t/h, the algae and bacteria are rapidly killed using •OH radicals with the low •OH concentration of 0.7 mg/L within only 6 s, and D-2 ballast water discharge standard of IMO is realized. Under fluorescence microscope, cell morphological changes such as the disruption of cell membrane, sometimes to the point of cell lysis, has taken place when cells are exposed to a killing dose of •OH.
Keywords: Strong electric-field discharge; •OH radicals; Ship’s ballast water; Algae and bacteria; Cell morphology

Hydroxyl Radicals Formation in Dielectric Barrier Discharge During Decomposition of Toluene by Xiao-bin Liao; Yu-fang Guo; Jian-hua He; Wei-jian Ou; Dai-qi Ye (841-853).
A method based on high performance liquid chromatography (HPLC), has been developed to measure hydroxyl radical (·OH) in plasma reactors. The determination was performed indirectly by detecting the products of the reaction of ·OH with salicylic acid (SAL). The applicability, and effect of time, specific input energy (SIE), relative humidity (RH), catalyst were investigated. It was found that 3 h was the optimal trapping time; concentration of ·OH was (5.9–23.6) × 1013 radicals/cm3 at SIE range. The highest ·OH yield and toluene removal efficiency (η) were achieved with a RH of 20%. With MnO x , η was two times that without catalyst, while ·OH yield in gas stream was one-sixth that without catalyst. However, if summed with ·OH adsorbed on catalyst surface, the total ·OH yield was the same as without catalyst. Experiments performed with/without toluene allowed to determine the role of ·OH on decomposition of toluene in air plasma.
Keywords: Hydroxyl radical; HPLC; Salicylic acid; DBD; Toluene removal

Degradation of Acid Orange 7 by Gliding Arc Discharge Plasma in Combination with Advanced Fenton Catalysis by ChangMing Du; LuLu Zhang; Jing Wang; ChuangRong Zhang; HongXia Li; Ya Xiong (855-871).
A new plasma–catalysis process of gliding arc discharge (GAD) plasma with zero–valent iron (ZVI) was examined. Because GAD plasma creates an acidic environment, solid iron releases ferrous ions which act as a catalyst for the decomposition of the hydrogen peroxide. A comparative study of the catalytic effects between Fe2+ and Fe0 in GAD plasma was investigated. The decolorization reactions of Acid Orange 7 (AO7) followed pseudo–first–order kinetics. And the rate constants for the process of GAD with ZVI was increased by 30% and by 19%, respectively, compared with the process of GAD alone and the process of GAD with ferrous. The investigations of solution pH and hydrogen peroxide both demonstrated that the GAD plasma induced conditions are much suitable for advanced Fenton reactions. The corrosion of ZVI in GAD plasma can give continuous ferrous ions to sustain Fenton reaction. Also, ZVI was demonstrated to have favorable reusable feature.
Keywords: Gliding arc discharge; Plasma; Fenton reaction; Acid Orange 7; Degradation

Crosslinking graft polymerization of poly acrylic/montmorillonite superabsorbent composite in aqueous solution was prepared by using glow-discharge electrolysis plasma, in which acrylic acid and acid-activated montmorillonite were used as starting materials, N,N-methylenebisacrylamide as a crosslinking agent. To optimize the synthetic conditions, seven important parameters of the graft-polymerization were examined in detail, such as the discharge voltage, discharge time, the neutralization of acrylic acid, post polymerization temperature, amounts of crosslinking agent, montmorillonite and acrylic acid used in this study. The structure, thermal stability and morphology of product were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, respectively. The absorbency of poly acrylic/montmorillonite superabsorbent was examined to yield the following results: 1,834 g/g for distilled water, 116 g/g for 0.05 mol/L and 75 g/g for 0.15 mol/L sodium chloride solution. Such excellent character could be important to use in many fields, for example, in agricultural and horticultural applications.
Keywords: Glow-discharge electrolysis plasma (GDEP); Acrylic acid; Montmorillonite; Superabsorbent composite

Low-temperature plasma has been studied for the textile industry as an environmentally friendly alternative to conventional finishing treatments but its biomedical applications are limited. The present work is devoted to analyze how plasma treatment modifies the wetting properties of polypropylene and polyamide 6.6 fabrics and its influence on the drug release from these textile materials. Drug release studies from textiles pre-treated by plasma are unexplored and constitute an innovative research that could provide a new way to develop controlled drug delivery systems. Methods of surface analysis for polypropylene and polyamide 6.6, including contact angle measurements, drop test, SEM, as well as drug release studies, using ketoprofen as a model drug, are considered in this research. Results obtained have shown that the increase of the hydrophilic properties of polypropylene and polyamide fabrics with plasma treatment reverts in a higher release of the model drug studied from plasma-treated fabrics.
Keywords: Biomedical textiles; Drug delivery; Plasma; Surface modification

Cutting Of Carbon Nanotubes Via Solution Plasma Processing by D. G. Tong; Y. Y. Luo; W. Chu; Y. C. Guo; W. Tian (897-905).
A simple solution plasma process was developed for cutting multi-wall carbon nanotubes (MWCNTs). During the solution plasma processing, defects and oxygen-containing functional groups (hydroxyl or carboxylic acid groups) were introduced onto the sidewalls of the MWCNTs via hydroxyl radicals. After being cut for 30 min, short MWCNTs of 100–400 nm in length were obtained.
Keywords: Carbon nanotubes; Cutting; Solution plasma processing

Simulation of excimer ultraviolet (EUV) emission from a coaxial xenon excimer ultraviolet lamp driven by distorted bipolar square voltages is presented in this study. A self-consistent radial one-dimensional fluid model, considering local mean energy approximation (LMEA), along with a set of simplified xenon plasma chemistry was employed to simulate the discharge physics and chemistry. Emitted powers of EUV light and deposited powers to the charged species were simulated by varying the values of four key parameters, which include the driving frequency, gas pressure, gap distance and number of dielectric layers. Results show that there are three distinct periods that include pre-discharge, discharge and post-discharge ones. It is found that intensive EUV (172 nm) emission occurs during the early part of the discharge period, which correlates very well in time with the power deposition through electrons. In addition, power deposition through $$ { ext{Xe}}^{ + } $$ and $$ { ext{Xe}}_{2}^{ + } $$ occurs mainly in the discharge period and later part of discharge period, respectively. Surprisingly, the emission efficiency of 172 nm increases slightly with increasing driving frequency of power source, while it increases dramatically with increasing gap distance. In addition, the maximal emission efficiency is found to take place at gas pressure of 600 torr. The emission efficiency of one-dielectric case is found to be better than that of two-dielectric one. The underlying mechanisms in the above observations are discussed in detail in the paper.
Keywords: Coaxial xenon excimer lamp; Excimer ultraviolet; Fluid modeling; Local mean energy approximation