Synthesis of functional nanocrystallites through reactive thermal plasma processing

A method of synthesizing functional nanostructured powders through reactive thermal plasma processing has been developed. The synthesis of nanosized titanium oxide powders was performed by the oxidation of solid and liquid precursors. Quench gases, either injected from the shoulder of the reactor or injected counter to the plasma plume from the bottom of the reactor, were used to vary the quench rate, and therefore the particle size, of the resultant powders. The experimental results are well supported by numerical analysis on the effects of the quench gas on the flow pattern and temperature field of the thermal plasma as well as on the trajectory and temperature history of the particles. The plasma-synthesized TiO₂ nanoparticles showed phase preferences different from those synthesized by conventional wet-chemical processes. Nanosized particles of high crystallinity and nonequilibrium chemical composition were formed in one step via reactive thermal plasma processing.     

Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor

The present study reveals some of the important parameters which control the in-flight nitridation of molybdenum disilicide (MoSi₂) powders when carried out in an induction thermal plasma reactor. Initially, gradients of temperature, velocity and concentration were evaluated, using an enthalpy probe system, for the plasma flow without injection of MoSi₂ powders. Radial profiles were then measured at the torch exit to examine the mass and energy transfer mechanisms occurring under different nitridation conditions. These measurements were performed using an induction plasma torch connected to a 50 kW radio-frequency (r.f.) power supply, the torch being attached to a water cooled cylindrical reactor. The process operating conditions studied were plasma plate power, chamber pressure, sheath gas composition, composition and flow rate of quench gas. The effect of last named parameter on the nitridation of the powders was found to be the most important parameter in the nitridation process. The results show that there is an optimum flow rate value for each type of quench gas and the temperature and concentration mapping demonstrates that the combination of high temperatures and high concentrations of N₂ are necessary to reach maximum nitridation levels in MoSi₂.     

Controlled synthesis of alumina nanoparticles using inductively coupled thermal plasma with enhanced quenching

Synthesis of alumina nanoparticles in an Ar-O₂ inductively coupled radio frequency (rf) plasma controlled by axial quench gas injection was investigated. The flow and temperature fields for various quench gas flow rates in the plasma reactor, as well as the corresponding quench rates, were predicted using a renormalization group (RNG) k-ε turbulence model. Nanosize alumina particles were synthesized from the vapor phase by oxidation of aluminum in the plasma. The collected products were characterized by means of field emission scanning electron microscopy (FE-SEM), dynamic light scattering (DLS), and BET surface area analysis. The dependences of particle size and shape on the injection position and the flow rate of the quench gas were discussed.     

Nano-crystalline titanium dioxide formed by reactive plasma synthesis

Nano-crystalline titanium dioxide finds extensive applications in photocatalytic degradation of harmful organic compounds pollutants in air and water. Synthesis and characterization of nano-crystalline TiO₂ powder by reactive thermal plasma synthesis is presented here. Processing was carried out in a plasma reactor designed and developed in our laboratory. TiH₂ powder was used as the precursor, which was injected into the thermal plasma jet along with air as the reactive gas. TiH₂ particles, as they traverse the plasma jet, react with oxygen to form nano-sized TiO₂ powder. Nano-sized TiO₂ was also synthesized using Ti powder as the precursor. X-ray diffraction results showed complete conversion of the precursor into oxide. More than 75% of TiO₂ formed was present as anatase. TEM showed well-resolved nano-sized particles with more than 70% lying in the range of a few nanometer to 20 nm. The powder has been used to study photocatalytic degradation of dye.     

Characterization and comparison between APS coatings prepared from ball milled and plasma processed nickel–aluminium powders

Plasma spraying has wide range of applications which include corrosion, thermal and abrasion resistance coatings. In the present work, nickel and aluminium powders were ball milled and the same were thermal plasma processed to produce spherical nickel alumindes particles. Both ball milled and plasma processed powders were spray deposited on stainless steel (SS 304) substrate using atmospheric plasma spray technique (APS). The experiments were carried out for different plasma input power levels, torch to base distances and coating thicknesses. Microstructure, micro hardness, adhesive strength, and porosity of the coatings are reported and discussed. Effect of plasma processing parameters and plasma spheroidization of powders on coating properties has been evaluated and reported. High plasma power, low torch to base distance lead to high temperature supplied to in-flight particles which correspond to high hardness, low porosity and high adhesion. Spherical morphology and formation of nickel aluminide intermetallic were achieved by plasma spheroidization. Coatings prepared from plasma processed powders enhance the coating properties positively.     

Mechanochemical effects on hydrogen absorption in Mg<Subscript>2</Subscript>Ni alloys under mechanical processing conditions

The present investigation focuses on the kinetics of hydrogen absorption in Mg₂Ni powders subjected to thermal or mechanochemical activation. The process was initially carried out under isobaric-isothermal conditions in a mechanochemical reactor at rest. Once the static hydrogen absorption process approached completion, the system was subjected to mechanical activation at constant hydrogen pressure. The mechanical treatment of powders induced further hydrogen absorption at rates depending on the processing intensity. The observed mechanochemical effect is related to the generation of reactive surfaces when mechanical loads apply on powders. The reactive surface area involved in the mechanochemical hydrogen absorption and the duration of the absorption process are roughly estimated.     

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC₂H₅)₅, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.     

Direct synthesis of nano-sized glass powders with spherical shape by RF (radio frequency) thermal plasma

A new route for obtaining very small, spheroid glass powders is demonstrated using an RF (radio frequency) thermal plasma system. During the process, four kinds of chemicals, here SiO₂, B₂O₃, BaCO₃, and K₂CO₃, were mixed at pre-set weight ratios, spray-dried, calcined at 250 °C for 3 h, and crushed into fragments. Then, they were successfully reformed into nano-sized amorphous powders (< 200 nm) with spherical shape by injecting them along the centerline of an RF thermal plasma reactor at ~ 24 kW. The as-synthesized powders show negligible (< 1%) composition changes when compared with the injected precursors of raw material compounds.     

Efficiency of particle acceleration,heating, and melting in high-enthalpy plasma jets

Local and average values across the cross section of a plasma jet of the velocity and temperature of Al₂O₃, Ti, and Mo particles heated by plasma are determined as a result of data processing on high-speed registration of particle tracks; the velocity changes of these particles along the axis of a plasma jet at different modes of operation of the experimental setup of plasma spraying are also determined. These data made it possible to choose the optimal spraying mode, wherein the particles collide with the surface of the substrate in a molten state, where the fields of velocities and temperature are homogeneous. Calculation of all the parameters of particles mentioned is carried out with regard to their dynamic and thermal delay relative to carrying flow, nonisothermality of particles of low-heat-conducting oxides, as well as matter evaporation from the particles’ surfaces. Calculation results are in satisfactory agreement with the experimental data.     

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC₂H₅)₅, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.     

Novel 3D Wollastonite‐Based Scaffolds from Preceramic Polymers Containing Micro‐ and Nano‐Sized Reactive Particles

Wollastonite (CaSiO₃) ceramics are well known biomaterials which can be produced using many different techniques. The present paper illustrates an innovative processing method employing preceramic polymers (silicone resins) containing CaCO₃ micro‐ and nano‐sized particles, which act as reactive fillers. Silica from the decomposition of the silicone resins reacts at low temperature with the CaO deriving from the fillers, yielding wollastonite ceramics. Hydroxyapatite powders can also be added, to modify the biological response of the material. This approach enables the fabrication of 3D scaffolds via fused deposition or via conventional hot extrusion.     

Preparation of silica coated iron oxide nanoparticles using non-transferred arc plasma

Silica coated iron oxide nanoparticles were prepared using non-transferred arc plasma. The plasma was discharged with argon. Vapors of iron pentacarbonyl (Fe(CO)₅) and tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄) were injected into a plasma torch with carrier gas and reacted in the plasma chamber. In addition, two types of reaction chambers that are a hot wall reactor and a cold wall reactor were used to investigate the effect of temperature gradient on the synthesis of silica coated iron oxide nanoparticles. The synthesized nanoparticles were collected on the chamber wall and bottom. Phase compositions of the obtained nanoparticles were characterized by X-ray diffractometer (XRD) and the morphologies and the size distributions of the synthesized particles were analyzed by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Additionally, elements mapping of the coated particles was performed by energy dispersive spectroscopy (EDS). The phase composition of the prepared particles was mainly composed of amorphous silica and polycrystalline Fe₃O₄. It was confirmed that the silica was adsorbed on iron oxide particles or encapsulated iron oxide particles. Furthermore, the mechanism of the formation of silica coated iron oxide in the plasma chamber was predicted.     

Characterization of nano-crystalline ZrO2 synthesized via reactive plasma processing

Nano-crystalline ZrO₂ powder has been synthesized via reactive plasma processing. The synthesized ZrO₂ powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and FTIR spectroscopy. The synthesized powder consists of a mixture of tetragonal and monoclinic phases of zirconia. Average crystallite size calculated from the XRD pattern shows that particles with crystallite size 20 nm or less than 20 nm are in tetragonal phase, whereas particles greater than 20 nm are in the monoclinic phase. TEM results show that particles have spherical morphology with maximum percentage of particles distributed in a narrow size from about 15 nm to 30 nm.     

Parametric study of the plasma synthesis of ultrafine silicon nitride powders

The high-temperature plasma synthesis of ultrafine silicon nitride (Si₃N₄) powders through the vapour-phase reaction between SiCl₄ and NH₃ in an Ar/H₂ radio frequency (r.f.) inductively coupled plasma was investigated. The experiments were carried out at a 25–39 kW plate power level and at atmospheric pressure. Special attention was paid to the influence of the reactor wall temperature and plasma operating conditions on the quality of the powder. With a cold-wall reactor, the powders obtained were white to light brown in colour and were composed of crystalline, amorphous and Si₃N₄ whisker phases. Both and -Si₃N₄ were present in these products. The NH₄Cl, formed as a by-product of the reaction, could be eliminated from the Si₃N₄ by thermal treatment. The BET specific surface area of the powder after thermal treatment was about 60 m²g^–1. The use of the hot-wall reactor resulted in a considerable reduction in the amount of NH₄Cl remaining in the powder (less than 1 wt%) and a considerable increase in the fraction of the powder obtained in crystalline form. These powders were composed of a mixture of amorphous phase and 30 wt% or more of the and -Si₃N₄ crystalline phases. The BET specific surface area of the powder after thermal treatment was found to be 40 m²g^–1. The experimental results are discussed in relation to their use for optimizing reactor design for the vapour-phase synthesis of ultrafine ceramic powders.     

Plasma spheroidization of nickel powders in a plasma reactor

Thermal spray coatings of surfaces with metal, alloy and ceramic materials for protection against corrosion, erosion and wear is an intense field of research. The technique involves injection of the powder into a plasma flame, melting, acceleration of the powder particles, impact and bonding with the substrate. Feedstock powders of metals, alloys and ceramics for thermal spray applications have to meet several requirements. Particle shape, size and its distribution, powder flow characteristics and density are the important factors to be considered in order to ensure high spray efficiency and better coating properties. For smooth and uniform feeding of powders into plasma jet, the powder particles have to be spherical in shape. High temperatures and steep temperatures present in thermal plasma is exploited to spheroidize particles in the present investigation. Nickel powder particles in the size range from 40–100 μm were spheroidized using plasma processing. SEM and optical micrographs showed spherical shape of processed particles.     

Effect of Y(NO3)3 additive on morphologies and size of metallic W particles produced by hydrogen reduction

Tungsten, one of the refractory metals, has lots of excellent properties, making tungsten powders become a choice of raw material for many applications. This work reported a simple method (the salt-assisted hydrogen reduction of WO₃ powders) for preparing tungsten powders with desired morphologies and size. By adjusting the amount of additive (Y(NO₃)₃) and reduction temperature, metallic tungsten particles with different morphologies and size were obtained. It was found that both temperature and additive had apparent effects on the morphology and size of metallic W particles, and the large parallelepiped-shaped powders disappeared and large irregular blocky-shaped powders were degraded under the influence of additive. With the increase of temperature and amount of salt, particles became more dispersed and the size increased at higher temperatures, and the shape of particles was changed from spherical to polyhedral as well. On the contrary, the size of sub-particles became smaller as increasing amount of Y(NO₃)₃ at lower temperatures. Based on the results, the reaction and refining mechanisms for preparing tungsten were proposed as well.     

Synthesis and microstructure of nanocrystalline Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

Nanocrystalline ytterbia powders have been synthesized using different precursors prepared by precipitation from nitrate solutions: ytterbium carbonates, oxalates, and hydroxides. The powders have been characterized by X-ray diffraction and scanning electron microscopy. The nature of the precursor has no effect on the crystallization temperature of ytterbia but influences its microstructure. The particles range in shape from spherical to platelike. The average crystallite size of the Yb₂O₃ powders is 20–25 nm. Raising the heat-treatment temperature from 600 to 1000°C increases the crystallite size to 33–46 nm. The highest thermal stability is offered by the ytterbia powders prepared through carbonate decomposition.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Theoretical Investigations on Sprayed Particle-Plasma Interactions to Optimize Processing Parameters in D.C. Thermal Plasma Spraying

A numerical model is proposed to simulate the behavior of particles injected into a dc spraying plasma in conjunction with the code that calculates the plasma jet characteristics. The sprayed particle-plasma interaction, particle loading effect and the particle internal heat transfer including phase transformations are considered together in the model to present a realistic prediction for the characteristic features of plasma spraying.

The temperature and velocity fields in a plasma jet expanding region outside the nozzle are found by a pre-developed code based on the arc-gas interactions inside the nozzle depending on the operating conditions of the torch. The influence of particle injection on the plasma jet characteristics is treated as the source in the fluid equations governing the plasma jet in semi-three dimensional calculations caused by non-axisymmetric injection of powders. The calculated velocity profiles of powder particles along the axial distance are compared with those obtained from experiments, and the determination of optimal spraying conditions are discussed from the results calculated from the present model.

This model gives more realistic information for plasma spraying than earlier works which have not included the loading effects and non-axisymmetric injection of powders in dc thermal plasma spraying. The theoretical predictions of the sprayed particle characteristics using this model can be helpful for determining preliminary range and optimal values of operating parameters for plasma spray processes.     

Gas-phase synthesis of AlN powders from AlCl3-NH3-N2

Aluminium nitride powders were synthesized by the gas-phase reaction of AlCl₃-NH₃-N₂ system. The yield of AlN powders from AlCl₃ and their crystallinity increased as the reaction temperature and the mole ratio of NH₃/AlCl₃ were increased. AlN powders were also formed outside the reactor and their crystallinity was amorphous. The yield of AlN powders was as high as 80% when the reactor temperature was 1000 °C and the mole ratio of NH₃/AlCl₃ was 8. Most of the HCl by-product was recovered as NH₄Cl outside the reactor. Powder characteristics, such as shape, size distribution and crystallinity, were also studied.