Synthesis of molybdenum carbide nanoparticles using pulsed wire discharge in mixed atmosphere of kerosene and argon

Pulsed wire discharge was used to prepare nanoparticles of molybdenum and its carbides from Mo wires in a gas mixture of argon and kerosene at pressures of 100, 50, and 25 kPa. The different pressures affected the carburization process and particle formation. The most effective pressure was 25 kPa, where the volume fraction of MoC was identified by Rietveld refinement of X-ray diffraction data to be 98.4%. The particle size distribution was also obtained from transmission electron microscopy measurements, and the smallest geometric mean diameter was determined tobe 24.3 nm for the sample prepared at 25 kPa.     

Synthesis of nanosized powders and coatings of calcium phosphates

Hydroxyapatite powders with a particle size of nearly 100 nm, i.e., rather close to bone crystals shaped as 60 × 20 × 5-nm flattened prisms, were synthesized by the pyrolysis of a solution of calcium oleate in tributylphosphate. The possibility of synthesizing hydroxyapatite coatings on different ceramic materials was demonstrated. The pyrolysis of a solution of strontium oleate in tributylphosphate was established to produce the Sr10(PO₄)₆(OH)₂ compound isotypical to hydroxyapatite, which in turn creates prospects for synthesizing materials based on hydroxyapatite with some calcium atoms substituted by strontium.     

Synthesis of nanosized Nd:YAG powders via gel combustion

Nanosized Nd:YAG powders with different doping concentrations were synthesized at a significantly low temperature by a gel combustion method with citric acid as fuel and nitrate as oxidizer. It is found that the precursor is composed of hydroxycarbonate and dehydrates at below 500 °C to form carbonate. Mono-phase Nd:YAG crystallites can be formed without any intermediate phase at 850 °C. The value of crystallite size of the 850 °C calcined 1.0 at% Nd:YAG powder is 59 nm and the average particle size is 86 nm. The doping of neodymium into YAG garnets increases the lattice constant and the fluorescent intensity decreases drastically when the neodymium concentration is higher than 3 at% because of the fluorescent quenching effect.     

Synthesis of thin bundled single walled carbon nanotubes and nanohorn hybrids by arc discharge technique in open air atmosphere

Cost-effective synthesis of single walled carbon nanotube (SWCNT) and single walled carbon nanohorn (SWCNH) hybrids, in a single step, by electric arc discharge technique in open air, at lower current densities is reported. The rate of production of the hybrids is 3–5 g/h. The presence of SWCNTs and SWCNHs is confirmed by a transmission electron microscope (TEM). In addition to conventional larger Dahlia-like aggregates of nanohorns, unique nearly-spherical shaped and relatively smaller sized aggregates (mean size ~ 25 nm) of nanohorns are formed along with thin bundles (mean diameter ~ 5.7 nm) of SWCNTs.     

Arch-Free flow in aerated silo discharge of cohesive powders

Arching can occur during silo discharge of cohesive powders. In general this happens when the outlet size is not wide enough. Flow aid devices, such as aeration pads, are commonly used in the industry to achieve proper flow of cohesive materials. However, no design criteria are presently available for such kind of devices and, in particular, for the intensity of aeration to be used to avoid arching. Aim of this paper is the evaluation of the limiting aeration condition to produce the collapse of established arches and the minimum aeration rate necessary for no arching discharge flow. Experimental tests are carried out in an aerated flat bottom silo. The measured quantities are the aeration rate at arch collapse and the arch size. Powder permeability is characterized by fluidization experiments. A simplified model is proposed to assess on the prevailing physical phenomena and predictively evaluate the minimum aeration rate to determine no arching discharge flow.     

水热法制备氧化锆微粉的进展

二氧化锆纳米粉是制造高科技陶瓷的重要原料，水热法是目前生产纳米粉体最具发展潜力的工艺方法之一。综述了水热法（包括水热沉淀、水热氧化、水热电埋弧、微波水热等）在生产纳米二氧化锆系粉体中的应用，以及水热法的发展情况。     

Improving the synthesis of single-walled carbon nanotubes by pulsed arc discharge in air by preheating the catalysts

Single-walled carbon nanotubes (SWCNTs) were synthesized in reduced pressure air using pulsed arc discharge after preheating the catalyst. Our experimental results revealed that preheating the catalysts can assist the synthesis of SWCNTs in air under a pressure of 5-10 kPa. The SWCNTs have a diameter of 1.5-2 nm and length can reach several micrometers. The consumption rate of the anode and the production rate of CNTs and SWCNTs in air are lower than in helium atmosphere at the same pressure, respectively. Further experiment demonstrates that 600 °C is optimum temperature for preheating the catalysts to synthesize SWCNTs in air.     

Growth behavior of nanosized ceria powders prepared by coprecipitation routes

Nanosized ceria (CeO₂) powders were obtained by coprecipitation routes of cerium nitrate hexahydrate [Ce(NO₃)₃·6H₂O]. The growth behavior of the nanosized CeO₂ powders was investigated by X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The XRD results showed that the dried precursor powders contained a single crystalline phase of CeO₂, and only a single phase of CeO₂ appeared when the dried precursor powders were calcined at different temperatures for various durations. Moreover, the crystallite size of CeO₂ increased on increasing the calcination temperature and duration. The kinetics equation of the nanosized CeO₂ powders grown between 673 and 1273 K for various durations is described as     

Production of nanosized graphite powders from natural graphite by detonation

A detonation method for preparing nanosized graphite powders from natural graphite is described. The process initiated by a detonator in an explosion vessel features quick release of energy, decomposition of HNO₃ graphite intercalation compounds and the smashing action due to the detonation. The products were soot-like deposits on the inner walls of the vessel, and were studied by XRD, SEM, TEM, BET, energy dispersive X-ray spectroscopy, and Raman spectroscopy. The results show that the as-made graphite powders have a perfect sheet-like structure, and vary in the two-dimensional plane at the micron scale and in thickness at the nanoscale. The specific surface area of the nanosized graphite is six times higher than that of the raw graphite. This technique may offer a low-cost, energy saving and efficient approach to the preparation of nanosized graphite powders.     

Synthesis of nanostructured powders in an aerosol flow condenser

The inert gas condensation method (IGC) that has been routinely used for production of nanostructured powders has a number of experimental limitations, especially in scale-up. To overcome these difficulties an aerosol flow condenser (AFC) was constructed and its characteristics and performance were determined. The structure, morphology and the primary particle-size distribution of palladium powders produced under various gas flow conditions were investigated by transmission electron microscopy. The size of the Pd particles were found to be log-normally distributed with median diameters of primary particles between 2.52 and 10.6 nm and geometric standard deviations (GSD) between 1.48 and 1.89. Compared with IGC, the AFC process results in an 80% reduction in the primary particle median diameter and a 17% reduction in the GSD. The primary particle median diameter and the GSD were found to be strongly dependent on the prevalent flow conditions.     

Machinability analysis of multi walled carbon nanotubes filled alumina composites in wire electrical discharge machining process

The approach of machining ceramics with electrical discharge machining process is a great challenge till date due to its low electrical conductivity. Machining is made possible by reinforcing with a conductive phase which increases the overall electrical conductivity. Present work focuses on machining of multi walled carbon nanotubes filled alumina composites. Samples with concentrations ranging from 2.5 to 12.5 vol.% are considered for machining. At lower concentration of 2.5 vol.%, effective machining is not possible. Wire lag phenomena is observed during machining at 5 vol.% sample concentration. Proper machinability is observed with concentration of 7.5 vol.% or more. Also, long micro-cracks are obtained during machining that leads to the workpiece breakage. Spalling effect is observed as the most dominating material removal mechanism. A comparison between alumina composites and conducting metallic alloys were carried out for surface characteristics.     

Synthesis of nanosized (Pb,Sr)TiO3 perovskite powders by coprecipitation processing

Nanosized perovskite lead strontium titanate (Pb_xSr_1−x)TiO₃ (PST) powders were successfully prepared by a simple coprecipitation method. Lead-strontium titanyl oxalate (PSTO) precursor was first synthesized at room temperature, and the precursor was then calcined at 600 °C for 1 h to produce the single phase perovskite PST powders. Characterization studies were carried out on the as-dried precursor and the calcined PST powders by various techniques. The results showed a strong dependence of the chemical composition of final PST powders on pH value in the coprecipitation reaction. PST powders with desired composition could be synthesized by adding 25 mol% excess Sr. PST particles were found to be spherical in nature with an average size of 10 nm.     

Low temperature chemical synthesis of nanosized ceramic powders

Abstract

A variety of nanosized (particle dia. <100 nm) ceramic powders have been prepared from metal ion complex based precursor solutions. The precursor solution was prepared at room temperature from metal hydroxides, nitrates, or acetates mixed with suitable complexing agents such as amines, carboxylic acids, hydroxy carboxylic acids, or poly-hydroxy compounds. Calcination of the precursor mass at low external temperatures produces the nanosized ceramic powders. The examples discussed include perovskites, i.e. lead zirconate titanate PbZr_0·6 Ti_0·4O₃(PZT) and lead magnesium niobate PbMg_1/3 Nb_2/3 O₃ (PMN).     

水热合成纳米羟基磷灰石粉体的研究

采用硝酸钙和磷酸铵为反应前驱物，通过水热合成颗粒尺寸在100nm以下的短棒状或针状羟基磷灰石晶体。X射线衍射(XRD)、透射电镜(TEM)和红外光谱(FTIR)分析讨论水热温度、反应时间、表面活性剂和烧结与物相组成、晶粒尺寸和晶体形貌的关系。     

Compaction behaviour of agglomerated alumina powders

The effects of agglomerate properties, such as the binder type, binder content, moisture level, and agglomerate size, on a model compaction process was investigated by using green density-pressure interrelationships for a range of agglomerated alumina powders. The model compaction process involved single ended nominal uniaxial stress transmission in a cylindrical die. The influences of the sample aspect ratio, die wall lubrication, and compaction rate were also investigated. Two types of water soluble polymeric agents, a poly(vinyl alcohol) (PVA) and a poly(ethylene glycol) (PEG), were used. It was shown that certain agglomerate properties have a strong influence upon the compaction behaviour of these ceramic powders. The extent of the compaction is enhanced by using agglomerates with a low agglomerate yield point. In the PVA system, the agglomerate yield points decreased with increasing moisture content. The compaction behaviour of the agglomerates showed a rate dependency, that is, the compaction is retarded with increased pressing rate. The green densities of the compacts prepared in the unlubricated die were lower than those of the compacts prepared in the lubricated die due to the higher wall frictional forces operating in the unlubricated die.     

3D printing of fine alumina powders by binder jetting

Additive manufacturing of ceramics is still at an early-development stage; however, the huge interest in custom production of these materials has led to the development of different techniques that could provide highly performing devices. In this work, alumina (α-Al₂O₃) components were produced by binder jetting 3D printing (BJ), a powder-based technique that enables the ex-situ thermal treatment of the printed parts. The employment of fine particles has led to high green relative density values (>60 %), as predicted by Lubachevsky-Stillinger algorithm and DEM modelling. Then, extended sintering has been observed on samples treated at 1750 °C that have reached a final density of 75.4 %. Finally, the mechanical properties of the sintered material have been assessed through bending test for flexural resistance and micro-indentation for Vickers hardness evaluation.     

单针-板脉冲放电处理染料废水的实验研究

采用单针-板脉冲放电反应器进行了罗丹明B染料废水的脱色研究。考察了脉冲电压、脉冲频率、针板间距和鼓气量等对罗丹明B脱色效率的影响,并对脱色过程中溶液TOC的变化进行了测试,得出当脉冲电压为32.5 kV、脉冲频率为60 Hz、针板间距为15 mm、曝气量为12 L/h时,处理50 min后,罗丹明B脱色效率达99.68%,溶液中TOC由70 mg/L降到19.2 mg/L。在以上实验的基础上,用该反应器进行罗丹明B、亚甲基蓝和甲基橙3种染料混合溶液脱色,脱色效率分别是99.75%,98.64%和97%,表明高压脉冲放电等离子体可无选择的对各种染料废水进行脱色。     

脉冲放电NO脱除过程模拟

建立了数学模型,把所考虑的NO/N₂/O₂/H₂O脉冲放电体系中的主要化学反应,归结为各物种浓度的刚性常微分方程组的初值问题来求解,计算方法选用Rosenbrock法,得到了物种浓度随停留时间的演化规律。结果表明：NO还原和氧化在其脱除过程中同时共存,当NO,N₂,O₂,H₂O和CO₂的体积浓度分别为200×10^-6,80%, 5％,6％和9％时,NO氧化所占的比例比NO还原的大很多;脉冲频率增大导致NO还原率和氧化率均增大;H₂O浓度增大导致HNO₃浓度增大,表明NO氧化所占的比例随H₂O浓度增大而增大。     

Dispersion properties of alumina powders in silica sol

The dispersion of alumina powders in silica sol has been investigated by zeta potential, sedimentation, and rheological measurements. Zeta potential of alumina in silica sol changes significantly in comparison with that of alumina in deionized water. This is caused by the absorption of silica colloidal particles with negative charge on the surface of alumina particles. Sol-dispersed alumina slurry shows a minimum in sedimentation volume and viscosity around pH 10. The viscosity depends strongly on the silica sol concentration and reaches a minimum in 10–15 wt.% silica sols. It is proposed that the dispersion and stabilization of alumina particles in silica sol are attributed to the electrostatic and steric effects of the colloidal particles absorption. Effects of pH, solids content and silica sol concentration on the rheological behavior of sol-dispersed alumina slurries are discussed in detail.