Induction plasma spheroidization of tungsten and molybdenum powders

The melting, evaporation and oxidation behaviors as well as the solidification phenomena of tungsten and molybdenum in induction plasma were studied. Scanning electron microscopy was used to examine the morphology and the cross section of plasma-processed powders. X-ray diffraction was used to analyze the oxides formed on the particle surface of these two metals. The influence of spray chamber pressure on the spheroidization and oxidation phenomena was discussed. The results show that fewer Mo particles than W particles are spheroidized at the same powder feed rate under the same plasma spray condition although molybdenum has a lower melting point. A small fraction of tungsten is evaporized and condensed either on the surface of tungsten particles nearby or on the wall of spray chamber. Tungsten oxides were found in tungsten powder processed under soft vacuum condition. Extremely large grains form inside some spheroidized particles of tungsten powder.

Plasma spray forming of tungsten coatings on copper electrodes

Both direct current dc plasma and radio frequency induction plasma were used to deposit tungsten coatings on copper electrodes. Fine tungsten powder with mean particle size of 5 μm and coarse tungsten powder with particle size in the range from 45 μm to 75 μm were used as plasma spray feedstock. It is found that dc plasma is only applicable to spray the fine tungsten powder and induction plasma can be used to spray both the coarse powder and the fine powder. The tungsten coating deposited by the induction plasma spraying of the coarse powder is extremely dense. Such a coating with an interlocking structure and an integral interface with the copper substrate demonstrates high cohesion strength and adhesion strength.

Radio Frequency Induction Plasma Spraying of Molybdenum

Radio frequency (RF) induction plasma was used to make free-standing deposition of molybdenum (Mo). The phenomena of particle melting, flattening, and stacking were investigated. The effect of process parameters such as plasma power, chamber pressure, and spray distance on the phenomena mentioned above was studied. Scanning electron microscopy (SEM) was used to analyze the plasma-processed powder, splats formed, and deposits obtained. Experimental results show that less Mo particles are spheroidized when compared to the number of spheroidized tungsten (W) particles at the same powder feed rate under the same plasma spray condition. Molten Mo particles can be sufficiently flattened on substrate. The influence of the process parameters on the flattening behavior is not significant. Mo deposit is not as dense as W deposit, due to the splash and low impact of molten Mo particles. Oxidation of the Mo powder with a large particle size is not evident under the low pressure plasma spray.

Relation between etching profile and voltage-current shape of sintered SiC etching by atmospheric pressure plasma

Sintered silicon carbide(SiC)was etched by a dielectric barrier discharge source.A high voltage bipolar pulse was used with helium gas for the plasma generation.One stable filament plasma was generated and could be used for SiC etching.As the processing gas(NF3)mixing rate increased,the width and depth of the etching profile became narrower and deeper.The differentiated V-Q Lissajous method was used for measuring the capacitances(Ceq)of the electrode after the plasma turned on.The width of the etching profile was proportional to Ceq.As the current peak value/smx of the substrate current in creased,the volume removal rate of SiC increased.The etch depth was proportional to the ratio of/smx to Ceq.Additionally,because of the different characteristics of the plasma disks on SiC substrate by the voltage polarity,the etching profile was unstable.However,in high NF3 mixing process,the etching profile became stable and deeper.

Synthesis of titanium carbide by induction plasma reactive spray

A novel method capable of sufficient mixing of titanium powder and methane of carbon source was developed in the synthesis of titanium carbide by induction plasma reactive spray. X ray diffraction analysis, optical microscopy, scanning electron microscopy, and microhardness test were used to characterize the spray formed deposit. The experimental results show that both primary carburization of the titanium particles inside the plasma flame and secondary carburization of the growing deposit on high temperature substrate contribute to the forming of titanium carbide. The transitional phase of TiC 1- x has the same crystal structure as TiC, but has a slightly low lattice constant. The deposit consists of fine grain structure and large grain structure. The fine grain structure, harder than large grain structure, shows grain boundary fracture.

Estimation of plasma density perturbation from dusty plasma injection by laser irradiation on tungsten target in DiPS

To investigate the interaction of dusty plasma with magnetized plasmas at divertor plasma simulator,radial profiles of plasma density(ne)and electron temperature were measured in terms of plasma discharge currents and magnetic flux intensity by using a fast scanning probes system with triple tips.Dusty plasma with dusts(a generation rate of 3μg s-1 and a size of 1–10μm)was produced via interactions between a high-power laser beam and a full tungsten target.As ne increases,the scale of the effects of dusty plasma injection on magnetized plasmas was decreased.Also,the duration of transient fluctuation was reduced.For numerical estimation of plasma density perturbation due to dusty plasma injection,the result was 10%at a core region of the magnetized plasma with ne of(2–5)×10^11 cm^-3 at steady state condition.

Hydrogen Generation from the Dissociation of Water Using Microwave Plasmas

Hydrogen is produced by direct dissociation of water vapor,i.e.,splitting water molecules by the electrons in water plasma at low pressure(<10–50 Torr)using microwave plasma discharge.This condition generates a high electron temperature,which facilitates the direct dissociation of water molecules.A microwave plasma source is developed,utilizing the magnetron of a microwave oven and a TE10 rectangular waveguide.The quantity of the generated hydrogen is measured using a residual gas analyzer.The electron density and temperature are measured by a Langmuir probe,and the neutral temperature is calculated from the OH line intensity.

Heat Transfer During Radio Frequency Inductively Coupled Plasma Deposition of Tungsten

Particle melting and substrate temperature are important in controlling deposited density and residual stress in thermal plasma deposition of refractory materials. In this paper, both the heating and cooling behaviours of tungsten particles inside a radio frequency inductively coupled plasma （ICP） and the plasma heat flux to the substrate were investigated. The distribution of the plasma-generated heat on device, powder injection probe, deposition chamber, and substrate was determined by measuring the water flow rate and the flow-in and flow-out water temperatures in the four parts. Substrate temperature was measured by a two-colour pyrometer during the ICP deposition of tungsten. Experimental results show that the heat flux to the substrate accounts for about 20% of the total plasma energy, the substrate temperature can reach as high as 2100 K, and the heat loss by radiation is significant in the plasma deposition of tungsten.

Impact of microsecond-pulsed plasma-activated water on papaya seed germination and seedling growth

The seed of Carica papaya consists of a hard shell-like testa with inhibitors in vivo causing slow,erratic and asynchronous germination.In this work,plasma-activated water prepared by microsecond-pulsed plasma jets(μPAW)was applied to treat papaya seeds.TheμPAW after plasma activation of 30 min was about 40℃.The reactive species such as NO_(2),NO_(3),and H_(2)O_(2)in theμPAW activated from deionized water were measured and correlated to the seed germination rate and the seedling growth performance.TheμPAW-treated papaya seed achieved a higher germination rate of 90%,which is 26%higher than the control group using deionized water.Comparing the results with a hot water(40℃)reference group showed that the reactive species inμPAW played primary roles in germination improvement,with little effect caused by the heat shock.TheμPAW also sterilized the treated seeds,reducing the germination stress.The morphological change in the seeds was observed by SEM,showing an effect of physical etching after treatment promoting seed imbibition.The biochemical mechanism of the seed germination was deduced with reference to the evolution of surface chemistry,functional groups,and ABA content.The accelerated seed metabolism observed was corresponded to the chemical modification pathway.Besides,early seedlings developed from treated seeds were observed to be healthy,grow more leaves,and have better root structures.The content of MDA in the treated papaya seedlings decreased along with increased SOD and higher ion concentration.TheμPAW that can be prepared at atmospheric pressure for bulk production offers a low-risk and cost-effective seed priming technology that may significantly increase the production of agricultural crops.

Variation of Radiation Emission with Argon Gas Pressure in UM Plasma Focus with the Hollow Anode

A Plasma Focus device (2.2 kJ, 12 kV) is studied as a pulsed X-ray source, operated with Argon at a filling pressure in the range of 0.7 to 2.5 mbar. The time resolved X-ray signals are measured with an array of PIN diode detectors. The X-ray emission produced by the plasma focus discharge at various pressures is investigated and compared. It is found that at the high pressure regime of more than 1.5 mbar, very consistent and high output of X-ray radiation is obtained, at the peak of the discharge current. A remarkable increase of about five times of the average X-ray yield is achieved at optimum pressure 1.7 mbar compared to that obtained at other pressures. An indirect method to determine the electron temperature of the plasma is achieved by using the array of 5 channel PIN diode detector coupled with Al foil of different thicknesses. The result shows that the electron temperature of the plasma is 7 keV, when the operating pressure is at 1.7 mbar. The maximum total X-ray yield is about 2.53 mJ per shot at optimum pressure, equivalent to the efficiency of 0.00012%.

Causality and Collisionless Damping in Plasma

We derive the collisionless Landau damping in a plasma by satisfying the causal requirement that the susceptibility function of the plasma for time t should be nil. The causality condition should be satisfied by the susceptibility function of a plasma no matter what equations we employ to describe the plasma. Thus we conclude that the fundamental reason of the collisionless damping can be traced to the causality. As an example, we derive the collisionless damping of ion acoustic wave in a plasma by employing fluid equations.

Synthesis of SiOx Nano-Powders Using a Microwave Plasma Torch at Atmospheric Pressure

The silicon oxide nano-powders (SiOx-NPs) were obtained in an atmospheric microwave plasma torch using a gas-phase silicon tetrachloride (SiCl4) with N2 and H2. The gas-phase SiCl4 was injected with H2 gas into the microwave plasma torch generated by N2 and air swirl gas, and then the dark brown powders were deposited on the inner wall of a quartz tube. The sample was analyzed by an X-ray photoelectron spectroscopy (XPS), a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), and an X-ray diffraction (XRD). The average size and oxidation x values of synthesized SiOx-NPs were approximately 230 nm and 0.91, respectively. Furthermore, the volumetric charge capacity is 1127 mAh/g and has 89.2% retention after 100 cycles.

Thermal Spray Coating of Tungsten for Tokamak Device

Thermal spray, such as direct current （d.c.） plasma spray or radio frequency induced plasma spray, was used to deposit tungsten coatings on the copper electrodes of a tokamak device. The tungsten coating on the outer surface of one copper electrode was formed directly through d.c. plasma spraying of fine tungsten powder. The tungsten coating/lining on the inner surface of another copper electrode could be formed indirectly through induced plasma spraying of coarse tungsten powder. Scanning electron microscopy （SEM） was used to examine the cross section and the interface of the tungsten coating. Energy Dispersive Analysis of X-ray （EDAX） was used to analyze the metallic elements attached to a separated interface. The influence of the particle size of the tungsten powder on the density, cracking behavior and adhesion of the coating is discussed. It is found that the coarse tungsten powder with the particle size of 45-75μm can be melted and the coating can be formed only by using induced plasma. The coating deposited from the coarse powder has much higher cohesive strength, adhesive strength and crack resistance than the coating made from the fine powder with a particle size of 5μm.

Anisotropic transport of microalgae Chlorella vulgaris in microfluidic channel

In this work, we study the regional dependence of transport behavior of microalgae Chlorella vulgaris inside microflu- idic channel on applied fluid flow rate. The microalgae are treated as spherical naturally buoyant particles. Deviation from the normal diffusion or Brownian transport is characterized based on the scaling behavior of the mean square displacement （MSD） of the particle trajectories by resolving the displacements in the streamwise （flow） and perpendicular directions. The channel is divided into three different flow regions, namely center region of the channel and two near-wall boundaries and the particle motions are analyzed at different flow rates. We use the scaled Brownian motion to model the transitional characteristics in the scaling behavior of the MSDs. We find that there exist anisotropic anomalous transports in all the three flow regions with mixed sub-diffusive, normal and super-diffusive behavior in both longitudinal and transverse directions.

Development of a dimensionless parameter for characterization of dielectric barrier discharge devices with respect to geometrical features

Non-thermal plasma（NTP） devices produce excited and radical species that have higher energy levels than their ground state and are utilized for various applications. There are various types of NTP devices, with dielectric barrier discharge（DBD） reactors being widely used. These DBD devices vary in geometrical configuration and operating parameters, making a comparison of their performance in terms of discharge power characteristics difficult. Therefore, this study proposes a dimensionless parameter that is related to the geometrical features, and is a function of the discharge power with respect to the frequency, voltage, and capacitance of a DBD. The dimensionless parameter, in the form of a ratio of the discharge energy per cycle to the gap capacitive energy, will be useful for engineers and designers to compare the energy characteristics of devices systematically, and could also be used for scaling up DBD devices.From the results in this experiment and from the literature, different DBD devices are categorized into three separate groups according to different levels of the energy ratio. The larger DBD devices have lower energy ratios due to their lower estimated surface discharge areas and capacitive reactance. Therefore, the devices can be categorized according to the energy ratio due to the effects of the geometrical features of the DBD devices, since it affects the surface discharge area and capacitance of the DBD. The DBD devices are also categorized into three separate groups using the Kriegseis factor, but the categorization is different from that of the energy ratio.

A Crystalline Material Made from Oxidation of Vanadium Nitride via Foam Route

Facile method of synthesizing organic-inorganic solid foam in complex structure is presented. The synthesis method is based on the use of neutral surfactant (1-hexadecylamine, C16H33NH2) as structure directing agent and inorganic salt (vanadium nitride, VN) as precursor. The foam containing C16H33NH2 has been synthesized by evolution of oxygen gas produced spontaneously from hydrogen peroxide through a viscous VN gel. The foam from the precursor of 0.5 gVN nanopowder grows gradually, reaching about 0.3 liters 10 minutes after excess addition of H2O2 and stirring. The ultralight, crystalline material made via foam processing was observed by XRD, SEM, TEM, and FTIR.

Discrete element method study of shear-driven granular segregation in a slowly rotating horizontal drum

Segregation and mixing of granular materials are complex processes and are not fully understood. Motivated by industrial need, we performed a simulation using the discrete element method to study size segregation of a binary mixture of granular particles in a horizontal rotating drum. Particles of two dif- ferent sizes were poured into the drum until it was 50% full. Shear-driven segregation was induced by rotating the side-plates of the drum in the opposite direction to that of the cylindrical wall. We found that radial segregation diminished in these systems but did not completely vanish. In an ordinary rotating drum, a radial core of smaller particles is formed in the center of the drum, surrounded by larger revolving particles. In our system, however, the smaller particles were found to migrate toward the side-plates. The shear from anti-spinning side-plates reduces the voidage and increases the bulk density. As such, smaller particles in the mixer tend to move to denser regions. We varied the shear by changing the coefficient of friction on the side-plates to study the influence of shear rate on this migration. We also compared the extent of radial segregation with stationary side-plates and with side-plates moving in different angular directions.