Comparative research of plasma-assisted milling and traditional milling in synthesizing AIN

In this paper,traditional milling and discharge plasma-assisted milling are employed to synthesize aluminum nitride (AlN) powder at nanometer scale by milling the mixture of aluminum and lithium hydroxide monohydrate.AlN powders can be generated in traditional milling and plasma-assisted milling in an hour milling time.Differential thermal analysis curves show that the reaction temperature of the powders treated by plasma-assisted milling is lower than that of traditional milling.These results indicate that plasma-assisted milling has higher efficiency in the synthesis of AlN,getting smaller crystallite size and activating powder.Moreover,an optical emission spectrum is employed to demonstrate the active species in plasma.The different formation process of AlN in the two-milling process,and the promotion effects of plasma in the milling process are discussed.     

Synthesis of Nano-Size AlN Powders by Carbothermal Reduction from Plasma-Assisted Ball Milling Precursor

Nano-size aluminum nitride(AlN) powders have been successfully synthesized with a high efficiency method through annealing from milling assisted by discharge plasma(p-milling)alumina(Al_2O_3) precursors. The characterization of the p-milling Al_2O_3 powders and the synthesized AlN are investigated. Compared to conventional ball milling(c-milling), it can be found that the precursors by p-milling have a finer grain size with a higher specific surface area,which lead to a faster reaction efficiency and higher conversion to Al N at lower temperatures. The activation energy of p-milling Al_2O_3 is found to be 371.5 kJ/mol, a value that is much less than the reported value of the unmilled and the conventional milled Al_2O_3. Meanwhile, the synthesized Al N powders have unique features, such as an irregular lamp-like morphology with uniform particle distribution and fine average particle size. The results are attributed to the unique synergistic effect of p-milling, which is the effect of deformation, fracture, and cold welding of Al_2O_3 powders resulting from ball milling, that will be enhanced due to the introduction of discharge plasma.     

Study on the RF inductively coupled plasma spheroidization of refractory W and W-Ta alloy powders

Spherical powders with good flowability and high stacking density are mandatory for powder bed additive manufacturing. Nevertheless, the preparation of spherical refractory tungsten and tungsten alloy powders is a formidable task. In this paper, spherical refractory metal powders processed by high-energy stir ball milling and RF inductively coupled plasma were investigated.By utilizing the technical route, pure spherical tungsten powders were prepared successfully, the flowability increased from 10.7 s/50 g to 5.5 s/50 g and apparent density increased from6.916 g cm~(-3) to 11.041 g cm~(-3). Alloying element tantalum can reduce the tendency to microcrack during tungsten laser melting and rapid solidification process. Spherical W-6 Ta(%wt)powders were prepared in this way, homogeneous dispersion of tantalum in a tungsten matrix occurred but a small amount of flake-like shape particles appeared after high-energy stir ball milling. The flake-like shape particles can hardly be spheroidized in subsequent RF inductively coupled plasma process, might result from the unique suspended state of flaky particles under complex electric and magnetic fields as well as plasma-particle heat exchange was different under various turbulence models. As a result, the flake-like shape particles cannot pass through the high-temperature area of thermal plasma torch and cannot be spheroidized properly.     

MOX燃料混料过程的优化

本文采用House holder变换法对MOX燃料混料过程中Pu同位素均一化问题进行优化计算,并用轨迹求解法对球磨中的转速问题进行了初步探讨。     

Synthesis and Characterization of Nano-sized Boron Powder Prepared by Plasma Torch

Hydrogen thermal plasma jet was employed to prepare nano-sized boron powder with hydrogen reduction of BCl3. The maximum yield of nano-sized boron powders was about 50% with the operational conditions of H2/BCl3 of 4.5:1, total feed of 4.9 m3/h, and plasma power of 25 kW. The samples were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and inductively coupled plasma - mass spectrometry (ICP-MS), inductively coupled plasma - atomic emission spectrometry (ICP-AES), inductive combustion infrared absorption (ICIA) and Infrared thermal conductivity of oxygen and nitrogen analyzer (ITCA). The results show that the boron powders have different crystal structures with higher dispersion and purity. The average diameter is about 50 nm, and the purity is 90.29% or so. This new technology can use simple process to produce high quality boron powders, and is feasible for industrial production.     

Fabrication and basic characterization of silicon nitride ceramics as an inert matrix

Silicon nitride ceramics containing cerium as a simulating element of americium were fabricated to clarify proper sintering conditions. Basic properties of sintered specimens were evaluated for utilization to an inert matrix. Commercial powders of silicon nitride and cerium dioxide (16 or 24.6 wt%), and a powder of aluminum oxide or zirconium oxide as a sintering additive (5 wt%) for some specimens were mixed by ball milling in ethanol. Small amounts of stearic acid as a lubricant were also added. The mixed powder was uniaxially pressed into cylindrical pellets. Then, the pellets were embedded in a packing powder composed of 50 wt%-Si₃N₄ and 50 wt%-BN, and sintered at 2023 or 2073 K for 2 h in a 0.1 MPa N₂ atmosphere. Most of the sintered specimens had high densities (>95% TD). Sintered bodies consisted of columnar silicon nitride grains and grain-boundary phase. XRD analysis clarified that the grain-boundary phase contained crystalline compounds of cerium. The thermal conductivities of sintered specimens except for specimens containing aluminum oxide were about 40 W/m K at room temperature.     

Supersonic induction plasma jet modeling

Numerical simulations have been applied to study the argon plasma flow downstream of the induction plasma torch. It is shown that by means of the convergent–divergent nozzle adjustment and chamber pressure reduction, a supersonic plasma jet can be obtained. We investigate the supersonic and a more traditional subsonic plasma jets impinging onto a normal substrate. Comparing to the subsonic jet, the supersonic one is narrower and much faster. Near-substrate velocity and temperature boundary layers are thinner, so the heat flux near the stagnation point is higher in the supersonic jet. The supersonic plasma jet is characterized by the electron overpopulation and the domination of the recombination over the dissociation, resulting into the heating of the electron gas. Because of these processes, the supersonic induction plasma permits to separate spatially different functions (dissociation and ionization, transport and deposition) and to optimize each of them. The considered configuration can be advantageous in some industrial applications, such as plasma-assisted chemical vapor deposition of diamond and polymer-like films and in plasma spraying of nanoscaled powders.     

射频感应等离子体制备球形Ti粉的工艺

采用射频(RF)感应等离子体球化颗粒形状不规则的Ti颗粒。研究了加料速率、物料分散方式、Ti颗粒大小等因素对球化率的影响。电子扫描显微镜(SEM)观察的球化效果以及对钛粉振实密度的测定表明:当钛原粉以极短暂时间快速穿越等离子体炬时,钛粉颗粒因受热而熔化成液滴,快速冷却,形成球形固态颗粒。     

Study on the Enhancement Effect of Dielectric Barrier Discharge on the Premixed Methane/Oxygen/Helium Flame Velocity

Recently,plasma-assisted combustion has become a potentially applicable technology in many combustion scenarios.In this paper,a dielectric barrier discharge(DBD) plasma generator is designed to explore the effect of plasma on the CH4 oxidation process,and several properties of combustion are considered.First,in the presence or absence of plasma discharge,physical appearance of the flame is examined and analyzed.Second,the flame propagation velocity is calculated by the flame front extracted from the imaging data with the Bunsen burner method.Finally,the main molecular components and their intensity variation in the flame and the plasma zones are identified with an emission spectrograph to analyze the effect of active species on the combustion process.We also discuss the possible kinetic regime of plasma-assisted combustion.Experimental results imply that plasma discharge applied to the premixed CH_4/O_2/He mixture significantly raises the flame speed with equivalence ratios ranging from 0.85 to 1.10,with the flame speed improved by 17%to 35%.It can be seen that plasma can improve methane oxidation efficiency in the premixed fuel/oxidizer,especially at a low equivalence ratio.     

FIBSIM – dynamic Monte Carlo simulation of compositional and topography changes caused by focused ion beam milling

A new simulation program is presented for focused ion beam (FIB) induced sputtering in two-dimensional targets. The model combines dynamic Monte Carlo simulation of the collision cascades with cell-based topography simulation. This approach takes the nonlocal nature of the sputtering process into account, and treats doping, damage formation and compositional changes self-consistently with the evolution of the surface. Two applications are presented: erosion of a sample edge, and milling of a hole into a multilayer target.     

Preparation of Ultra—fine Aluminum Nitride in Thermal Plasma

Ultra-fine aluminum nitride has been synthesized by the evaporation of aluminum powder at atmospheric-pressure nitrogen plasma in a hot-wall reactor.The average size of aluminum nitride particle is 0.11μm measured by scanning electric mirror(SEM),and the purity is at least over90% evaluated by X-Ray diffraction(XRD).The conversion of Al powder to aluminum nitride is strongly depended on the injection of NH3.Typical experimental parameters such as the feed rate of raw material,the flow rate of ammonia and the position of injecting aluminum powder into the reactor are given.     

D.C.Plasma—Sprayed Coatings of Nanostructured Alumina—Titania—Silica

Nanocrystalline powders of w(Al2O3)=95%,w(TiO2)=3%,and w(SiO2)=2%,were reprocessed into agglomerated particles for plasma spraying,by using consecutive steps of ball milling,slurry forming,spray drying,and heat treatent,D.C.Plasma was used to spray the agglomerated nanocrystalline powders,and resultant coatings were depostied on the substrate of stainless steel,Scanning electron microscopy(SEM) was used to examine the morphology of the agglomerated powders and the cross section of the alumina-titania-silica coatings.Experimental results show that the agglomerated nanocrystalline particles are spherical,with a size from(10-90)um,The flow ability of the nanocrystalline powders is greatly improved after the reprocessing.The coatings deposited by the plasma spraying are mainly of nanostructure.Unlike conventional plasma-sprayed coatings,no laminar layer could be found in the nanostructured coatings.Although the nanostructured coatings have a lower microhardness than conventional microstructured coatings,the toughness of the nanostructured ceramic coatings is significantly improved.     

Estimation of plasma parameters in the process of micro-scale powder plastic and characteristics of its products

The temperature and density of plasma jets were estimated with a Boltzmann plot and Stark broadening of Ar I (696.54 nm) lines by optical emission spectroscopy (OES) in the process of plasma plastic, and the morphology and microstructure of tungsten (W) powders were investigated by scanning electron microscope (SEM) and x-ray Diffraction (XRD), respectively. The results show that the assumption of local thermodynamic equilibrium (LTE) was invalid at the end of the plasma jets, and earlier than this after the injection of tungsten powder. The temperature and electron density of the plasma jets were up to about T=6797 K with Q_c=50 slpm and n_e=1.05×10¹⁶ cm⁻³ with Q_s=115 slpm at Z=60 mm, respectively, and both dropped rapidly with the injected tungsten powders of 20 μm. After the plasma plastic process, the spherical tungsten powders were prepared and there were some satellite particles on the surface of the spherical products. The tungsten powders were both composed of a single equilibrium α-W phase with a body centered cubic (bbc) crystal structure before and after plasma treatment.     

A Moessbauer study of iron/polytetrafluoroethylene nanocomposites prepared by high-energy ball milling

Iron/polytetrafluoroethylene （Fe/PTFE） nanocomposites were prepared by means of high-energy ball milling for different lengths of time. Three new components of FeF2, FeF3, and Fe3C in the resultants were mainly investigated using the Mossbauer spectroscopy （MS）. The components and average grain size of the nanocomposites were also measured using X-ray diffraction （XRD） and transmission electron microscopy （TEM）, respectively.     

A M(o)ssbauer study of iron/polytetrafluoroethylene nanocomposites prepared by high-energy ball milling

Iron/polytetrafluoroethylene (Fe/PrFE) nanocomposites were prepared by means of high-energy ball milling for different lengths of time. Three new components of FeF2, FeF3, and Fe3C in the resultants were mainly investigated using the M(o)ssbauer spectroscopy (MS). The components and average grain size of the nanocomposites were also measured using X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively.     

Formation of SiC Nanostruture Using Hexamethyldisiloxane During Plasma-Assisted Hot-Filament Chemical Vapor Deposition

Growth of SiC nanowires in plasma-assisted hot filament chemical-vapor-deposition by using hexamethyldisiloxane (HMDSO) as the gas source is reported. The SiC nanowires (SiCNWs) grew on Au-coated silicon substrate with Core-Shell structure, where the cores consisted of polycrystalline SiC grains and the shell exhibited amorphous structure. The featured structures such as cones, polyhedrons, ball-liked particles were observed in the case without plasma assistance. The underlying mechanism for the growth of nanostructures was also discussed. The high chemical activity induced by the plasma process plays an important role in using monomer to generate nanostructure.     

Surface Treatment of Polypropylene Powders Using a Plasma Reactor with a Stirrer

A radio frequency argon plasma reactor with a stirrer was employed for the surface treatment of polypropylene (PP) powders. The changes in the superficial contact angle and the superficial composition of the un-treated and treated PP powders were analyzed by means of water contact angle (WCA) measurement, X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The water contact angle changed from the original value of 130.2° before plasma treatment to the value of 73.6° after treatment for 5 minutes. With the increase in plasma treating time, there were a decrease in the water contact angle and an increase in the content of oxygen containing polar functional groups (i.e., C-O, C=O and O-C=O). Both XPS and ATR-FTIR results indicated that the plasma treatment led to the formation of oxygen containing polar functional groups due to oxidation on the surface of the PP powders, and the trend of variation of the water contact angle with plasma treating time was related to the concentration of oxygen atom on the treated PP powders surface. Furthermore, the aging of the plasma-treated PP powders was investigated.     

DeNOx Study in Diesel Engine Exhaust Using Barrier DischargeCorona Assisted by V2O5／TiO2 Catalyst

A plasma-assisted catalytic reactor was used to remove nitrogen oxides (NOx) from diesel engine exhaust operated under different load conditions. Initial studies were focused on plasma reactor (a dielectric barrier discharge reactor) treatment of diesel exhaust at various temperatures. The nitric oxide (NO) removal efficiency was lowered when high temperature exhaustwas treated using plasma reactor. Also, NO removal efficiency decreased when 45% load exhaust was treated. Studies were then made with plasma reactor combined with a catalytic reactor consisting of a selective catalytic reduction (SCR) catalyst, V2O5/TiO2. Ammonia was used as a reducing agent for SCR process in a ratio of 1:1 to NOx. The studies were focused on temperatures of the SCR catalytic reactor below 200℃. The plasma-assisted catalytic reactor was operated well to remove NOx under no-load and load conditions. For an energy input of 96 J/1, the NOx removal efficiencies obtained under no-load and load conditions were 90% and 72% respectively at an exhaust temperature of 100℃.     

Design and characteristics of a triple-cathode cascade plasma torch for spheroidization of metallic powders

Arc plasma torch is an effective tool for spheroidization of metallic powders. However, as most conventional plasma torches were not specifically designed for plasma spheroidization, they may exhibit the disadvantages of the radial injection of powders, large fluctuations in the arc voltage, large gas flow rate, and disequilibrium between multiple plasma jets during the spheroidization process. Therefore, this paper presents a triple-cathode cascade plasma torch (TCCPT) for plasma spheroidization. Its structural design, including three cathodes, a common anode, and three sets of inter-electrodes, are detailed to ensure that powders can be inserted into the plasma jet by axial injection, the arc voltage fluctuations are easily maintained at a low level, and the plasma torches can work at a relatively small gas flow rate. Experimental results showed that the proposed TCCPT exhibits the following characteristics: (1) a relatively small arc voltage fluctuation within 5.3%; (2) a relatively high arc voltage of 75 V and low gas flow rate range of 10–30 SLM; (3) easy to be maintained at the equilibrium state with the equilibrium index of the three plasma jets within 3.5 V. Furthermore, plasma spheroidization experiments of SUS304 stainless steel powers were carried out using the proposed TCCPT. Results verified that the proposed TCCPT is applicable and effective for the spheroidization of metallic powders with wide size distribution.