Powders In Plasma Research Articles

Effect of the solid precursors on the formation of nanosized TiBx powders in RF thermal plasma

Continuous synthesis of TiBx (x≈0.5–2) nanoparticles from various low cost solid precursors such as titanium and titanium dioxide admixed with boron and/or carbon in radiofrequency thermal plasma was studied. Feasibility of TiB2 formation was predicted by thermodynamic equilibrium calculations in the 500–5000K temperature range. In all the investigated system high temperature reactions resulted in nanometer-sized TiBx powders with a mean size varying between 13 and 83nm. The yield of particular runs ranged from 38% to 97%. Among the synthesized products in addition to TiBx, oxidized precursor residues were also found in smaller quantities. Although addition of carbon to the precursors could not completely prevent surface oxidation of boride particles, it contributed to the reduction of the mean particle size of the formed TiB2.

Production and modification of hollow powders in plasma under controlled pressure

Conducting plasma processes under high or low pressure is an efficient way to affect the heat, mass and momentum exchange in a two-phase flow and this technique is widely used in such well-developed technologies as low-pressure plasma spraying (LPPS) and high pressure plasma-chemical processes. In addition operating pressure is a key parameter in novel plasma process for modification of hollow powders properties. Plasma processing of porous ceramic powders is an effective method of producing hollow spheres (HOSP) with predefined properties. Regardless the method hollow powders were produced their geometric and structural properties can be adjusted by re-melting in plasma of certain pressure: low pressure processing will expand hollow spheres and high pressure – contract it. Regulating the outer diameter of hollow sphere allows adjusting its shell thickness, apparent density, gas pressure in the cavity etc. Preliminary experiments with zirconia hollow powders demonstrated good agreement with theoretical estimations of HOSP properties. The same technique can be used for adjusting properties of ceramic hollow powders produced by different methods, including cost effective fly-ash particles (cenospheres).

The Preparation Technology of Spherical Molybdenum Powder in Continous Induction Plasma

In this paper, the preparation technology of spherical molybdenum powder has been studied by using of continuous induction plasma powder synthesis system, and the flow-ability, particle size , the impurity content of powder before and after plasma treatment have been measured and compared. The result shows that, by applying for induction plasma preparation technology ,the powder’s flow-ability is improved, oxygen and carbon content is decreased markedly. the continuous induction plasma powder synthesis system shows high performance on particle size control.

Corrosion resistance of phosphorus-clad iron powders in biological and inorganic media

The production and use of clad iron-based powder materials is a promising area of powder metallurgy that allows variation in their process and magnetic properties in wide ranges. The corrosion properties of phosphorus-clad iron powders and their interaction with biological media of the living organisms are studied. The PZhRV 3.200.26 (Ukraine), AHC 100.29 (Sweden), and PZhV 200 (Russia) iron powders with different particle sizes are clad with phosphorous by the method of thermochemical decomposition of phosphorous compounds and by the method of thermochemical synthesis in a vibrating bed. Corrosion tests of the iron powders clad with phosphorous are performed in a 3% NaCl solution. Calculations of the corrosion depth index show that the clad powders are much more resistant to corrosion in aggressive media (3–4 points according to the ISO 11130:2010 standard) than the starting powders (1 point). The low values of the corrosion depth index of phosphorus-clad powders testify that surface corrosion proceeds on powder particles. It is shown that the interaction of the starting PZhRV 3.342.28 and AHC 100.29 iron powders with human blood plasma is 5.8 and 7.2 times more intensive, respectively, than that for PZhRV 3.342.28 and AHC 100.29 powders clad with phosphorous. On the surface of iron powders, phosphorus interacts with blood plasma proteins to form a protective colloidal biocomplex, which increases substantially the resistance of clad powders in blood plasma. Thus, the cladding of iron powders with phosphorus enhances significantly their chemical stability both in human blood plasma and in air.

Accumulation of hydrogen by silicon powders in HF inductive discharge plasma

The possibility of saturation with hydrogen of micro- and nanostructured silicon powders in electrodeless plasma is demonstrated. It is established that the hydrogen concentration in the bulk of the powder is determined by the size of its particles, the hydrogen pressure in the reaction chamber, and the HF power. The mechanism of plasma saturation with hydrogen is based on the chemical interaction of hydrogen atoms and active radicals with the surface of particles of the silicon powder, which is a surface with a distorted order of stacking of silicon atoms.

Synthesis of Pure, Crystalline (Ba,Sr)TiO3 Nanosized Powders in Radio Frequency Induction Thermal Plasma

Barium strontium titanate ((Ba,Sr)TiO3) nanosized powders were synthesized via spray pyrolysis of a liquid precursor mist in Ar–O2 radio frequency induction thermal plasma. To adjust the cation stoichiometry of the complex oxide, liquid precursors were prepared by mixing titanium butoxide (Ti(OBu)4) stabilized with diethanolamine and aqueous solutions of barium nitrate (Ba(NO3)2) and strontium nitrate (Sr(NO3)2) stabilized with citric acid. The molar ratios of titanium, barium, and strontium ions (Ba/(Ba+Sr) and (Ba+Sr)/Ti ratios) in the liquid precursor were varied. Pure, high-crystalline (Ba,Sr)TiO3 (Ba/(Ba+Sr)=0.0–1.0) powders of ∼45 nm in size were synthesized in this one-step process.

Stability of iron-based powders with different phase compositions in biological media

The interaction of iron-based powders with different phase compositions with model biological media is examined in experiments in vitro. The powders are synthesized from iron oxalates over a range from 450 to 830°C in a carbon-containing atmosphere and from citrates at 390 and 450°C with prolonged holding in a protective hydrogen atmosphere. The phase composition of the powders is determined using a DRON 3.0 x-ray diffractometer in Co-K α radiation. Human blood plasma is used as a biological medium and distilled water and a 0.9% NaCl solution as inorganic media. It is shown that different processes occur in blood plasma and inorganic media: corrosion in water media and interaction between iron and proteins to form iron–protein complexes in plasma-containing media. The dissolution rate of the powders in blood plasma depends on their α-iron content: the higher the latter, the more active the interaction of the powder with plasma proteins such as ferritin and transferrin, basic components of γ-globulin fraction. The powders synthesized by the decomposition–reduction of citrates at 450 and 390°C with prolonged holding in a protective hydrogen atmosphere interact with model solutions of digestive juice far more intensely, which makes them promising for the development of iron-containing food supplements.

Crystalline islands of semiconductor films

Silicon carbide (SiC) nucleation in the form of powder in a discharge plasma and the formation of thin film islands on a Si(100) substrate in the course of gas-phase epitaxy are simulated numerically. Models of plasma-like media and nonequilibrium processes accompanying phase transitions of the first kind (such as condensation and crystallization) in the initial fast (fluctuation) stage are described. The nonstationary evolution of nuclei size distribution functions is modeled by solving kinetic equations in partial derivatives and stochastic Ito-Stratonovich analog equations. This makes it possible to refine the formation mechanisms of microcrystalline state polytypes and calculate the nucleation rate and the initial roughness of a SiC coating.

Decay kinetics of luminescence and electron emission from MgO crystal powders in ac plasma display panels

Decay kinetics of luminescence at 5.2 eV and those of sustained electron emission from MgO crystal powders were investigated in order to understand the mechanism of discharge delay in ac plasma display panels and sustained electron emission from insulator. The decay of UV-luminescence was nonexponential and the decay was enhanced by increasing the temperature after showing negligible temperature dependence at low temperature. Existence of carrier traps was indicated by thermoluminescence measurements. As a possible mechanism, luminescence from a donor-acceptor pair in combination with thermal activation from carrier traps was introduced. The electron emission properties were evaluated by the measurement of statistical discharge delay time in ac plasma display panels as an indicator. By coating MgO crystal powders on the MgO protective film, the intensity of electron emission increased and its time and temperature dependence was reduced. A comparison under the same excitation revealed that the decay of UV-luminescence was steeper than that of electron emission. To explain the experimental data, we proposed the involvement of direct electron emission from electron traps synchronized with measuring voltages in addition to luminescence-associated electron emission.

Magnetic Characterization of Radio Frequency Heat Affected Micron Size Fe3O4 Powders: A Bio-Application Perspective

Micron size Fe3O4 powders were chemically prepared and processed by radio frequency (13.56 MHz) oxygen plasma irradiation technique at different elevated temperatures using low radio frequency (RF) power level. Low magnetic field RF superconducting quantum interference device (SQUID) magnetization studies were performed up to a maximum magnetic field of 100 Oe, which was well below the magnetic field tolerance factor of human beings and at different temperatures (down to 5 K). Heat-treated powders in RF oxygen plasma showed significant changes in blocking temperature, magnetization and susceptibility, which are important parameters for bio-applications. It is observed that blocking temperature is decreased under identical RF heat treatment in oxygen plasma and noted to be dependent on average particle size. Microscopic rise in electron temperature during RF heating may likely to enhance the electron-hopping rate between Fe(+2) and Fe(+3) in the octahedral site of Fe3O4 molecular crystal structure, which in turn exhibit changes in blocking temperature including low field magnetization and susceptibility. These properties of Fe3O4 fine powder are likely to play important role in generating and processing biocompatible Ferro-fluid down to nanoscopic size for biomaterials applications.

Thermophysical processes in vacuum plasma electric furnaces for heating powdered materials

Vacuum hollow-cathode plasmatrons permit heating, melting, and refining of fine powders in plasma during the flight of particles from the introduction zone to a receiver. A powder in plasma is heated through its interaction with electrons and ions. Such a plasmatron exhibits the following performance characteristics: the total heat flow per particle is 10 6 ‐10 7 W/m 2 , the operation pressure range is 0.1‐100 Pa, the current is 7.5‐10 kA, the voltage is 80‐100 V, and the working gas is argon. As a result of the electrodynamic interaction of every powder particle with the magnetic field of the plasma column, the powder concentrates at the plasma-column axis and moves only within the plasma column. All of the charged powder is treated. The initial content of impurities in the powder can reach 3% [1]. As the probe measurements of the column structure established, the plasma density decreases when going from the cathode cut toward the anode [2]. In this case, the plasma density is maximal in the active zone in the cathode hollow. To intensify the heat flow to a powder particle, we developed a special ring cathode, in which the inner cylinder is shorter than the external cylinder by (0.5‐1) the inner cylinder diameter. The external cylinder has a diaphragm and serves as a guard ring, and it prevents the plasma from sharp decomposition. As a result, the zone of the interaction of the powder with the dense plasma increases. Figure 1 presents a photograph of the ring hollow cathode providing effective heating of powder materials in a plasma column. The new cathode unit design allowed us to increase the temperature of the treated powdered material to 3500 ° C. When producing a compacted tantalum ingot from a powder of sodium-thermic reduction in a commercial plant, we achieved a decrease in the metallicimpurity content by a factor of 10‐40 and in the gaseousimpurity content by a factor of 5‐50. The quality of the ingots fabricated using the vacuum plasmatron is significantly higher than that of similar ingots produced in vacuum arc furnaces and is close to the quality of the metal produced by electron-beam remelting [1].

Formation of carbon nanotubes from a silicon carbide/carbon composite

The reaction of a SiC/C composite powder in an arcing plasma forms carbon nanotubes in good yield. Besides carbon nanotubes, a Si/C composite composed of β SiC covered with a shell of graphite is formed. The graphitic carbon surface layers of the carbon shell of this composite reacts further to form carbon nanotubes when heated to 600 °C. This process seems highly effective since only a small overall low weight loss, indicative for a complete carbon shell oxidation is observed by thermal analysis. The formation of the carbon nanotubes from SiC is unlikely since no SiO 2 has been found when heating the SiC/C core shell composite to its reaction temperature of 600 °C under O 2. The CNTs formed are of good quality with 3 to 6 concentric walls and high aspect ratio. Occasionally even single walled carbon naotubes have been observed.

Influence of metal powder shape on drag coefficient in a spray jet

In plasma spraying, particle shape, size, distribution and density are the important factors to be considered in order to ensure high spray efficiency and better coating properties. In the present work, nickel, iron and aluminium irregular powders in the size range from 50 to 63 μm were spheroidized using thermal plasma processing. The spheroidization experiments have been carried out at different gas flow rates and plasma torch power levels. The sphericity was analyzed using shape factor. Drag coefficients of the powders were estimated using Reynolds number and sphericity of the powders in plasma. For irregular particles, the drag coefficient is higher than that of the spherical because of its large area of contact with plasma. The temperature dependent on drag coefficient is also discussed. Increasing temperature increases the drag coefficient of the powder particles injected in to the plasma jet. Increasing plasma jet temperature changes the density and viscosity of the plasma which affects the particle’s drag coefficient in the plasma. The results are reported and discussed.

Splat Formation of Plasma Sprayed Functionally Graded YSZ/NiCrCoAlY Thermal Barrier Coatings

Splat morphologies of YSZ/NiCrCoAlY TBCs by plasma spray were characterized using SEM. Based on some assumptions, plasma jet temperature field was established by numerical simulation and the effects of spray parameters are individually studied to optimize the spray process parameters. Furthermore, the interaction wetting ability was taken into account in order to investigate the interaction effect between ZrO2 and NiCrCoAlY droplets. It shows that T enhances markedly with increasing in I and decreasing in FAr; while in despite of the insignificant effect of FHe on T, the temperature of powder in plasma jet enhances with increasing in FHe. As a result of higher thermal conductivity of powder in He plasma gas. NiCrCoAlY droplet impacting solidified NiCrCoAlY splat wets well and spreads fluently while that impacts solidified YSZ splat spreads restrictedly for the poor wetting on the YSZ splat surface.

熱プラズマプロセスによる高機能ナノ粒子の合成

A method of synthesizing functional nanostructured powders through thermal plasma processing has been developed. Nano-submicron-size metal and carbide particles were synthesized through evaporation and subsequent condensation of powders in thermal plasma. The synthesis of nano-size titanium oxide powders was performed by the oxidation of solid and liquid precursors. The plasma-synthesized TiO2 nanoparticles showed phase preferences different from those synthesized by the conventional wetchemical processes. Nanosized particles of high crystallinity and nonequilibrium chemical composition were formed in one step via the reactive thermal plasma processing.

高周波熱プラズマによる球状銅系サブミクロン粒子の作製

Spherical submicron copper particles were synthesized through evaporation and subsequent condensation of copper powders in Ar-H2 thermal plasma. Copper powders of ∼40μm in size injected into the plasma, evaporated instantly and fine particles were formed through homogeneous nucleation and subsequent heterogeneous condensation. Particle size and the distribution in the synthesized powders were changed depending on the powder feed rate. Formation of submicron size spherical powders attributed to a significantly high degree of supersaturation in a vapor phase. Particle sizes of the as-produced powders range from submicron to several tens of micrometers. A simple sedimentation treatment was successfully applied to separate the as-formed powders with alcohol solvent to give uniform spherical submicron-size particles with monomodal size distributions. Cu-W composite powders were synthesized through the Ar-H2 plasma treatment of Cu-WO3 composite powders. The starting temperature of shrinkage was made significantly higher than pure Cu powders by mixing with several wt% of W.

Antioxidant Effects of the Mixture of Mulberry Leaves and Silkworm Powder on the Plasma and Liver in Streptozotocin-Induced Diabetic Rats

This study was carried out to examine the antioxidant effects of a mixture of mulberry leaves and silkworm powder in plasma and liver of streptozotocin-induced diabetic rats. Sprague-Dawley male rats weighing 100±10 g were used and their diets were supplemented with 0.4% (4 g/kg) of the mixtures. Experimental groups were diabetic rats without supplements (DM group) or with a combination of the supplements: 100% mulberry leaves (M group), 25% silkworm powder mixed with mulberry leaves (25SM group), 50% silkworm powder mixed with mulberry leaves (50SM group), 75% silkworm powder mixed with mulberry leaves (75SM group) or 100% silkworm powder (100S group). The rats were fed experimental diets and water ad libitum. All animals were injected with streptozotocin at the 3^(rd) week for inducing diabetes and were sacrificed on 9^(th) day thereafter. Hepatic xanthine oxidase (XOD) activity significantly decreased in the mixture supplemented groups compared to the DM group. Hepatic superoxide dismutase (SOD) activity was not significantly different among any of the experimental groups, but glutathione peroxidase (GSH-px) activity increased in the mixture supplemented groups compared to the DM group. In particular, it was the highest in the 50SM group. The hepatic TBARS values were lower in all the mixture supplemented groups than in the DM group, and it was as lowest when ratio of mulberry leaves to silkworm powder was highest. Hepatic lipofuscin contents were similar with the TBARS value. In conclusion, the mixtures containing silkworm powder reduced oxidative damage by strengthening the antioxidative system and suppressing oxidative stress in the STZ-induced diabetic rat. The 1:1 blend of silkworm powder and mulberry leaves was the most effective combination for antioxidant activity.

Development of coating by thermal plasma spraying under very low-pressure condition

Very Low-Pressure Plasma Spray (VLPPS under 1 mbar) has been developed and used for deposition of copper powder (10–63 μm) coating on stainless steel substrate. In order to improve the understanding of this technique and to study the volatilization of the injected powder in supersonic plasma, optical emission spectroscopy study has been carried out. Preliminary results of the properties and the structure of VLPPS copper coating are presented. It is shown that the plasma expansion with decreasing chamber pressure increases the volatilization of particles and allows to obtain a very dense coating with crystalline structures and avoids oxygen, due to the preheating of the substrate with plasma (length about 1.50 m).

Microwave Plasma Synthesis of Nanostructured γ‐Al2O3 Powders

Nanostructured Al2O3 powders have been synthesized by combustion of aluminum powder in a microwave oxygen plasma, and characterized by X‐ray diffraction and electron microscopy. The main phase is γ‐Al2O3, with a small amount of δ‐Al2O3. The particles are truncated octahedral in shape, with mean particle sizes of 21–24 nm. The effect of reaction chamber pressure on the phase composition and the particle size was studied. The γ‐alumina content increases and the mean particle size decreases with decreasing pressure. No α‐Al2O3 appears in the final particles. Electron microscopy studies find that a particle may contain more than one phase.

Modeling and Optimization of the Synthesis of Fine-Particle Powders in High-Frequency Plasma

The synthesis of SiO2, Al2O3, BaO, PbO, B2O3, ZnO, and SiO2–Al2O3–BaO fine-particle powders was studied experimentally and optimized. The metal oxides, silicon, and an SiO2–Al2O3–BaO mixture were treated in an argon–oxygen or air high-frequency induction plasma, whereupon amorphous layers were deposited onto silicon substrates. Heat treatment of these layers at 1473 K for 15 min yielded continuous amorphous dielectric films with desired properties.