Synthesis of ultrafine magnesium oxide and titanium oxide powders

The formation of magnesium and titanium hydroxides (differing in the rate of primary particle formation) and conversion of the hydroxides to ultrafine oxide powders of controlled size have been studied using scanning electron microscopy, X-ray diffraction, differential thermal analysis, and analysis of the particle size distributions of solid materials from scattered laser light intensity. The particle size of the synthesis products has been shown to be influenced by the initial concentration and nature of precursors and precipitants, ultrasonic processing, additions of low-molecular-weight organic compounds, and dehydration temperature and time.     

红外上转换纳米发光材料的水热合成

利用稀土离子Y~(3+)、Yb~(3+)、Er~(3+)与乙二胺四乙酸形成的配合物,加入可溶性氟化物.控制溶液的pH值,利用配位-沉淀平衡体系制备了复合氟化物纳米粒子前驱物,在240℃下水热晶化前驱物,合成了红外上转换纳米发光材料.     

The production of ultrafine zirconium oxide powders by spray pyrolysis

Twin-fluid atomisation spray-pyrolysis has been investigated for the production of ZrO₂ powders. The atomiser used in this study has a novel internal arrangement that can produce a spray with a mean diameter (SMD) of less than 5 m. Spray pyrolysis tests with zirconium nitrate as a precursor salt were performed and the formation of ZrO₂ powder was studied under substantially different heating rates and initial solution concentrations. A mean particle diameter, d(0.5), of 0.67 m and 0.77 m was achieved for 0.05 M and 0.5 M solutions, respectively. It was concluded that the new nozzle design performed well and was successful in producing ultra-fine ZrO₂ powder with a principally tetragonal structure when the correct process conditions of heating rate and residence time were applied.     

Oxidation of ultrafine (Si-) SiC powders

The increasing usage of ultrafine ceramic powders in the fabrication of highly reliable ceramics results in a growing interest in appropriate processing conditions for these powders. During processing the extremely high surface areas might lead to significant absorbtion of oxygen even at low temperatures. But especially in this temperature regime, oxidation data of powders are rarely available; as far as the authors know, no investigations have been published in the case of ultrafine powders with particle sizes below 100 nm. In this study the oxidation kinetics of ultrafine (Si-) SiC powders ( 20 nm) in the temperature range between room temperature and 1000 °C in air were investigated. Thermobalance experiments showed that at least three different oxidation mechanisms are operating. At temperature above 650 °C the fraction of completion R is proportional to the square root of time, indicating a diffusion-controlled mechanism (activation energy -1.8 eV). At lower temperatures the best data fit is obtained by a Cabrera-Mott-like equation. At room temperature and for thin silica-layer thicknesses a third oxidation mechanism was determined. The formation of the first monolayer of silicon oxide obeys the kinetics of a first-order reaction, namely an exponential one with a time constant of 1.25× 10^–4 s^–1. An investigation of the influence of oxygen pressure on the oxidation of ultrafine Si-SiC powders revealed a low pressure influence at 500 °C. An approximately linear relation between pressure and oxidation rate constant is observed between 30 and 1000 mbar air pressure at 800°C. The kinetic data were used to construct an oxidation map for ultrafine SiC powders, as a help to determine appropriate processing conditions.     

Microwave-assisted hydrothermal synthesis of nanocrystalline SnO powders

Tin oxide (SnO) powders were obtained by the microwave-assisted hydrothermal synthesis technique using SnCl₂·2H₂O as a precursor. By changing the hydrothermal processing time, temperature, the type of mineralizing agent (NaOH, KOH or NH₄OH) and its concentration, SnO crystals having different sizes and morphologies could be achieved. The powders were characterized by X-ray diffraction (X-ray), Field Emission Scanning Electron Microscopy (FE-SEM), High Resolution Transmission Electron Microscopy (HR-TEM) and Selected Area Electron Diffraction (SAED). The results showed that plate-like form is the characteristic morphology of growth and the TEM analyses indicate the growth direction as (200).     

低温水热合成制备Ce_(0.8)Sm_(0.2)O_(1.9)电解质粉体

采用水热合成方法制备了Ce_(0.8)Sm_(0.2)O_(1.9)电解质材料,水热合成的温度范围为120~180℃。使用X射线衍射、扫描电镜及交流阻抗谱技术,分别表征了电解质粉体及电解质粉体的烧结样品。研究发现,电解质粉体的晶粒尺寸在13 nm左右,在140℃合成的Ce_(0.8)Sm_(0.2)O_(1.9)粉体具有更好的烧结性能和更高的离子电导率。     

纳米羟基磷灰石粉体的水热合成

采用Ca(NO3)2·4H2O和(NH4)3PO4·3H2O作为反应前驱物,通过水热合成颗粒尺寸在100nm以下的短棒状或针状HA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.实验结果表明升高反应温度和延长反应时间有利于HA的生成;表面活性剂有助于改善粉体的分散性能;烧结能提高晶体的结晶程度,但粉体易团聚,当温度高于800℃时HA发生分解.     

Carbothermal synthesis of titanium carbide using ultrafine titania powders

The synthesis of titanium carbide (TiC) by the carbothermal reduction of carbon coated titanium dioxide (TiO2), a novel synthesis process, and titanium dioxide (TiO2) mixed with carbon black was investigated. A high surface area (64 m2g-1) TiO2 powder consisting of anatase and rutile phases was used for starting powders. The carbon coated method is a two-step process that utilizes a precursor derived from decomposing propylene (C3H6) and depositing carbon on the TiO2 particles. TiO2 powders were also mechanically mixed with carbon black for comparison. Both starting precursors and mixtures were reacted in a tube furnace for 2 and 4 h at temperatures of 1100°C to 1550°C under 1 l min-1 flowing argon. The TiC powders were characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analyser, chemical analysis (oxygen and carbon) and transmission electron microscopy (TEM). The carbon coating process provides high contact area between the reactants which results in a TiC product with lower oxygen content (0.6 wt%), finer particle size (0.1 m), and uniform shape when synthesized at 1550°C for 4 h.     

Microemulsion-mediated hydrothermal synthesis of photocatalytic TiO2 powders

A reverse microemulsion-mediated hydrothermal route has been employed to synthesize photocatalytic titanium dioxide (TiO2) powders. Nano-crystalline monophasic anatase TiO2 powders were successfully prepared when the microemulsion-derived precursors were hydrothermally treated. The advantage of using this microemulsion mediated hydrothermal route is the significant reduction in reaction time and temperatures as compared with the conventional hydrothermal process. The oil/water emulsion ratio significantly affected the particle sizes of the obtained TiO2 powders. The specific surface area of TiO2 powders was increased with the oil/water ratio, thereby leading to enhanced photocatalytic activity of TiO2 powders. As the hydrothermal temperature was elevated, the morphology of the TiO2 particles changed from a rod-like shape into a polyhedral shape. The variation in microstructures decreased the specific surface area of the TiO2 powders and lowered the photocatalytic activity.     

Hydrothermal synthesis of fine oxide powders

This is a review and an overview on hydrothermal synthesis of fine oxide powders. The term hydrothermal today includes methods which involve water at pressures (from 1 atm-several kilobars) and high temperatures from 100–10,000°C. Hydrothermal is one of the best methods to produce pure fine oxide powders. The authors describe (i) hydrothermal decomposition, (ii) hydrothermal metal oxidation, (iii) hydrothermal reaction, (iv) hydrothermal precipitation and hydrothermal hydrolysis, (v) hydrothermal electrochemical, (vi) reactive electrode submerged arc, (vii) hydrothermal microwave, (viii) hydrothermal sonochemical, etc and also ideal and real powders     

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.     

Isopropanol-assisted hydrothermal synthesis of (K, Na)NbO3 piezoelectric ceramic powders

(K, Na)NbO₃ particles were synthesized using the isopropanol-assisted hydrothermal method. The addition of isopropanol made it possible to prepare (K, Na)NbO₃ solid solutions under a mild hydrothermal condition of 2 mol/L alkali solution. The effects of the K/(K + Na) molar ratio, (K + Na)/Nb molar ratio in the starting suspensions, and annealing process on the phase evolutions of the (K, Na)NbO₃ solid solutions were investigated. The phases of the as-prepared particles were investigated by X-ray diffraction technique. The microstructure and composition were analyzed by scanning electron microscope and energy dispersive X-ray analysis. (K, Na)NbO₃ solid solutions were successfully synthesized by hydrothermal method with the addition of isopropanol through adjusting the K/(K + Na) molar ratio and (K + Na)/Nb molar ratio in the starting suspensions. The change of morphology is also in agreement with the phase evolution. The phase evolutions were discussed and the formation mechanism of the (K, Na)NbO₃ solid solutions was proposed.     

Detonation synthesis of zinc oxide nanometer powders

Novel zinc oxide nanometer powders have been synthesized via detonation reaction with respect to the presence of an energetic precursor, such as lithium nitrate and zinc nitrate. The detonation products of emulsion explosives were characterized by powder X-ray diffraction. Transmission electron microscopy was used to observe the structures. Zinc oxide with primary particles of diameters from 20 to 50 nm and a distribution with proportional spacing of spherical morphologies were found. The oxides produced by this cheap method affirmed the validity of detonation synthesis of nanosize powders. It is concluded that unconventional emulsion explosives are designed uniquely for synthesis of the nanometer zinc oxide.     

Preparation and characterisation of ultrafine lead titanate (PbTiO3) powders

Ultrafine lead titanate (PbTiO₃) powders in tetragonal form have been successfully prepared via three processing routes, namely, conventional co-precipitation, microemulsion-refined freeze drying, and microemulsion-refined co-precipitation. The formation process of lead titanate from the resulting precursors was monitored using techniques such as thermal analyses and X-ray diffraction for phase identification. It was found that the two microemulsion-refined processing routes led to a lower formation temperature for lead titanate than that observed in the conventional co-precipitation route. The three lead titanate powders have also been compared for particle and agglomerate size distributions and specific surface area. It appears that the microemulsion-refined co-precipitation is the technique which results in the formation of the finest lead titanate powder amongst the three processing routes investigated in the present work. This revised version was published online in September 2006 with corrections to the Cover Date.     

Low-temperature acetone-assisted hydrothermal synthesis and characterization of BiFeO3 powders

Well-crystallized pure perovskite bismuth ferrite (BiFeO₃) powders have been synthesized by a facile hydrothermal route at the temperature as low as 130 °C with the aid of acetone. In the synthesis, acetone played important roles in the low-temperature synthesis of pure BiFeO₃. The as-prepared BiFeO₃ powders mainly consisted of cubic particles with the size range from 50 to 200 nm. zero-field-cooled and field-cooled magnetization measurements indicated that pure BiFeO₃ powders showed a spin-glass transition below the freezing temperature. The as-prepared pure BiFeO₃ powders showed weak ferromagnetism and ferroelectricity simultaneously at room temperature. Moreover, the bismuth ferrite BiFeO₃ exhibit efficient photocatalytic activity under visible light irradiation.     

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t ^1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.     

Reaction sequences and influence factors during carbothermal synthesis of ultrafine TiN powders

Ultrafine TiN powders were synthesized by Carbothermal Reduction-Nitridation (CRN) method using nano titania and nano carbon black as raw materials. Phase transition sequences during reaction and influence of main technological factors were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). The results indicate that phase transition sequences were TiO₂→Ti₁₀O₁₉→Ti₃O₅→TiO _x N _y →TiN. At the same time, CRN reaction consists of three continuous stages. In the first stage, nano TiO₂ powders were reduced to Ti₃O₅. In the second stage, Ti₃O₅ was totally converted into TiO _x N _y . TiN solid solution formed completely by the end of the third stage. The rapidest reaction rate was observed in the second stage and the lowest was in the third stage. Higher reaction temperature, longer isothermal time, or larger nitrogen pressure was in favor of synthesizing TiN powders.     

Novel synthesis of MoSi2 and MoSi2-Al203 ultrafine powders

Abstract

Molybdenum disilicide (MoSi₂) is a promising high temperature material with a high melting point (2032°C), low density (6.24 Mg m^-3 ), excellent high temperature oxidation resistance (up to about 1900°C), and metal like thermal and electrical conductivities. For structural applications, the formation of composites can be benejicial. Alumina (Al₂O₃) has a thermal expansion coefficient close to that of MoSi₂ in a wide temperature range from 200°C to 1400°C. Improvement in elevated temperature strength has been reported by the formation of a composite between MoSi₂ and A1₂0₃. However, the brittle nature of MoSi₂ based materials at medium to low temperatures remains an obstacle for their applications. One possible solution is to develop microstructures with ultrajine grains so that ductility can be improved through the grain boundary sliding mechanism. The present work gives preliminary results related to the synthesis ofultrajine MoSi₂-Al₂0₃ composite powders. The MoSi₂ powders (0.1–4 μm) were produced by an electric arc discharge process under varying voltage and capacitance. Three powder generating mechanisms have been identified. The influence of processing parameters (voltage, capacitance, and energy input) on the powder characteristics is discussed. Finally, the synthesis of MoSi₂-A1₂0₃ composite powders by coating individual MoSi₂ particles with an alumina gel through a sol-gel process is also demonstrated.