Electrochemical synthesis of nanocrystalline SrNb2O6 powders and characterization of their photocatalytic property

Nanocrystalline SrNb₂O₆ powders were successfully prepared by a simple electrochemical method for the first time, and the influences of electrolytic solution ingredients, electrolyte concentration, and applied electric current intensity on electrochemical process and products were systemically studied. It was found that the formation of strontium niobates strongly relied on the basic or acidic condition of electrolytic solution. When Sr(OH)₂·8H₂O or mixed Sr(OH)₂·8H₂O with SrCl₂·6H₂O was used as electrolyte, Sr₅Nb₄O₁₅ and SrNb₂O₆ phases could simultaneously form, and the relative ratio of Sr₅Nb₄O₁₅ phase to SrNb₂O₆ declined with the increase of SrCl₂·6H₂O content in electrolytic solution. The higher the basicity of electrolytic solution, the more favored the crystallization and development of Sr₅Nb₄O₁₅ phase. Only monoclinic phase SrNb₂O₆ was obtained in neutral SrCl₂·6H₂O solution or acidic solution of SrCl₂·6H₂O with adding hydrochloric acid solution, and a lower concentration of SrCl₂·6H₂O (≤0.50 M) was beneficial to the formation of SrNb₂O₆ phase and stable anodic sparks could be observed. Moreover, the crystallization and development of the nanocrystalline SrNb₂O₆ powders seem to be not sensitive to the applied electric current intensity, their similar particle size and morphology ultimately result in the resemblance in UV-vis absorption and photodegradation. They show a slight red shift of light absorption onset and better photocatalytic performance compared with that prepared by the solid-state reaction.     

Eutectic assisted synthesis of nanocrystalline NiO through chemical precipitation

Cubic NiO nanocrystallites with the particle size of 15-20 nm were successfully prepared through the decomposition of the precursor and eutectic as the assistant. The precursor was obtained by precipitating Ni(NO₃)₂·6H₂O with NaOH in a water solvent. The compared experiments were carried out either with the presence or absence of eutectic. It was indicated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis that the eutectic played an important part in overcoming the agglomerations and the prepared NiO nanocrystallites have better morphology and uniform granularity. The mechanism of eutectic during the synthesis of nanocrystalline NiO was also discussed.     

水热法合成PZT粉体的研究

以硝酸铅、氧氯化锆、钛酸四丁酯为反应物前躯体,氢氧化钾为矿化剂,采用水热法合成PZT粉体.系统研究了反应温度、反应时间、矿化剂浓度和引入表面活性剂聚乙烯醇(PVA)对PZT晶体生长的影响.通过XRD和SEM对粉体进行了表征,探索出制备PZT粉体的最佳条件,研究结果表明:在KOH浓度为2 mol/L、180℃反应12 h的条件下可制备得到粒径约为1μm,粒径均匀,晶体发育完整的钙钛矿结构的PZT粉体;在该反应体系中加入适量PVA时得到了棒状纤维结构的粉体.     

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).     

Size effect studies on nanocrystalline Pb(Zr0.53Ti0.47)O3 synthesized by mechanical activation route

The nanocrystalline lead zirconate titanate (PZT) with the composition Pb(Zr_0.53Ti_0.47)O₃ has been synthesized by mechanical activation, without calcination at intermediate temperature. The PZT powder that has been mechanically activated for 10, 20 and 30 h shows crystallite size of 15, 12 and 8 nm, respectively. Size effect on PZT formation, tetragonality factor and dielectric constant has been investigated in detail. TEM and AFM studies have been carried out to confirm the formation of nanocrystalline PZT. The temperature dependence of dielectric constant at different frequencies was also measured, to study the size effects in PZT ferroelectrics. The nanocrystalline PZT obtained by this route has high dielectric constant at Curie temperature in comparison to that prepared conventionally.     

前驱体溶液配比及pH值对凝胶-燃烧合成纳米MgO颗粒的影响

以硝酸镁、柠檬酸及氨水为原料,用凝胶-燃烧合成法制备了MgO纳米颗粒.系统研究了前驱体溶液中硝酸镁与柠檬酸的配比及其pH值对凝胶的形成、凝胶的形态、凝胶的燃烧过程以及最终合成产物特性的影响,并在优选的工艺条件下,获得了平均尺寸约为10nm且粒径分布窄的MgO颗粒.     

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.     

Mechanochemical synthesis of sodium tungsten bronze nanocrystalline powders

Nanocrystalline powders of sodium tungsten bronze Na_xWO₃ (x∼0.88) have been prepared by mechanochemical process using starting materials of Na pieces and WO₃ powders in a planetary ball mill. The synthesis reactions proceed with increasing milling time and are almost completed after 44 h. Phase-pure nanoparticles of Na_xWO₃ with average size of 17 nm were directly obtained after a simple washing process to remove the by-product of Na₂WO₄. The resistivity was measured in the temperature range from 77 to 300 K. The sample displays semiconducting behavior and can be characterized by three-dimensional variable-range hopping. The mechanochemical process seems to be an attractive route to fabricate tungsten bronzes because of several advantages such as easy preparation, less cost, operating at low temperature and suitability for a large-scale production.     

Polymeric precursor synthesis of Ba2Ti9O20

Ba₂Ti₉O₂₀ was synthesized by Pechini method using citric acid as a chelating agent and ethylene glycol as an esterification agent. The effects, of the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration on the formation of Ba₂Ti₉O₂₀, were investigated. Increasing the pH of the starting solutions enhances the formation of Ba₂Ti₉O₂₀, but the increase of the molar ratio of citric acid to total metal cations concentration retards the formation of Ba₂Ti₉O₂₀ due to increased diffusion distances. Single-phase Ba₂Ti₉O₂₀ was obtained at 1200 °C for 4 h when the molar ratio of citric acid to total metal cations concentration was unity, regardless of the pH of the starting solutions. Increasing the molar ratio of citric acid to total metal cations concentration up to 2, single-phase Ba₂Ti₉O₂ was synthesized at 1200 °C for 6 h for the precursors with pH 3.5 and 6, but not for the precursors with pH 2.0. DTA, TG, XRD, FT–IR spectroscopy and Raman spectroscopy were used to characterize the precursors and the derived oxide powders. Details of the synthesis and characterizations of the resultant products were given.     

Low-temperature synthesis of nano-sized metal molybdate powders

Nanocrystalline metal molybdate [i.e., AMoO₄ where A=Ca(II), Co(II),Cu(II), Ni(II), and Zn(II)] powders have been prepared from the complete evaporation of a polymer-based metal-complex precursor solution. The precursor solution was constituted of the metal ions that were in aqueous solution through complexation with ethylene diamine tetraacetic acid (EDTA) in the presence of diethanolamine (DEA), and a polymeric reagent, which was an aqueous solution mixture of sucrose and polyvinyl alcohol (PVA). Complete dehydration of the precursor solution generated a voluminous mesoporous carbon-rich precursor mass, heat treatment (at temperatures ≤500°C) of which resulted in the respective metal molybdate phase. The average diameter of the particles from transmission electron microscopy and X-ray diffraction studies ranged between 15 and 40 nm.     

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     

Hydrothermal synthesis of fibrous lead titanate powders

A fibrous lead titanate (PbTiO₃) powder with light-yellow colour has been prepared by hydrothermal synthesis. The influences of Pb/Ti ratio (0.3 to 1.0) in the mixture and reaction time on the formation of fibrous PbTiO₃ under hydrothermal conditions have been investigated. The preferable conditions for preparing fibrous perovskite-type PbTiO₃ from fibrous potassium titanate are that the Pb/Ti ratio is 1.0, reaction temperature 150 °C and time 72 h. The particles of fibrous powder of perovskite type are usually less than 2 m in diameter and more than 50 m in length. The fibrous morphology is essentially unchanged up to about 650 °C, but it disappeared after heating to 1000 °C.     

Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors

Zirconium carbide (ZrC) and hafnium carbide (HfC) powders were produced by the carbothermal reduction reaction of carbon and the corresponding metal oxide (ZrO₂ and HfO₂, respectively). Solution-based processing was used to achieve a fine-scale (i.e., nanometer-level) mixing of the reactants. The reactions were substantially completed at relatively low temperatures (<1500°C) and the resulting products had small average crystallite sizes (50–130 nm). However, these products contained some dissolved oxygen in the metal carbide lattice and higher temperatures were required to complete the carbothermal reduction reactions. Dry-pressed compacts prepared using ZrC-based powders with 100 nm crystallite size could be pressurelessly sintered to 99% relative density at 1950°C.     

不同Sol-Gel前躯体体系制备纳米ZnO粉体的研究

为研究不同溶胶前躯体体系对制备纳米ZnO粉体粒径大小的影响,采用溶胶-凝胶法分别研究了硬脂酸体系、草酸体系、高分子网络体系制备纳米ZnO粉体,并利用TG-DTA、XRD、TEM对其结构、形貌和粒径进行了表征.在3种体系中,高分子网络体系所得粉体粒径最小,粒径在30～40nm,粒径大小均匀,团聚较少.硬脂酸体系得到ZnO粉末粒径较小,范围分布在30～50nm,分散性较好.草酸体系制得ZnO粉末粒径相对较大,在40～60nm,有轻微团聚.     

Low temperature synthesis of nanocrystalline calcium aluminate compounds with surfactant-assisted precipitation method

Nanocrystalline calcium aluminates with different CaO:Al₂O₃ and surfactant/metal ion molar ratios were prepared by wet chemical synthesis method using Poly (ethylene glycol)-block-poly(propylene glycol)-block poly(ethylene glycol) (PEG–PPG–PEG, MW:5800) as surfactant. X-ray diffraction (XRD) and N₂ adsorption–desorption results showed that the increase in CaO:Al₂O₃ ratio decreased the specific surface area and increased the particle sizes of prepared samples while the surfactant/metal ion molar ratios were kept constant. These analyses also declared that for the sample with CaO:Al₂O₃ = 1:2 (CA2) addition of polymeric surfactant increased the specific surface area and decreased the crystallite size. Scanning electron microscopy (SEM) results confirmed that size of particles for CaO:Al₂O₃ = 1:6 (CA6) sample are smaller than CA2. Transmission electron microscopy (TEM) revealed no particular particle shape for the CA2 sample but it showed the high degree of crystallinity and single phase for the prepared sample at 1100 °C.     

Structural and microstructural phase evolution during mechano-synthesis of nanocrystalline/amorphous CuAlMn alloy powders

The formation mechanism of Cu–11.5Al–4Mn alloys by mechanical alloying (MA) of pure elemental powders was investigated. During milling, the powder sampling was conducted at predetermined intervals from 1 h to 96 h. The quantitative phase analyses were done by X-ray diffraction and the particles size and morphology were studied by scanning electron microscopy. Furthermore, the microstructure investigation and phase identification were done by transmission electron microscopy. Concerning the results, the nanocrystalline Cu solid solution were formed at short milling times and, by milling evolution, the austenite-to-martensite (2H) phase transformation occurred. Moreover, the formation of considerable amount of amorphous phase and its partial transformation to crystalline phases during the milling process were revealed. It was also found that, by milling development, the powder morphology changes from lamellar to semi-spherical and their size initially increases, then reduces and afterward re-increases.     

Electrodeposition of nanocrystalline nickel by using rotating cylindrical electrodes

In this research, nanocrystalline nickel (14–25 nm) was electrodeposited on rotating cylindrical electrodes in a modified Watts bath. Saccharin was used as a grain refiner. The effect of cathode rotation speed and saccharin concentration on the grain size was studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The preferred orientation of deposits progressively changed from a (2 2 0), (2 0 0), and (1 1 1) fiber texture for a saccharin free bath to a (1 1 1) and (2 0 0) double fiber texture for a bath containing 5 g l⁻¹ saccharin. Cathode rotation enhanced the intensity of (1 1 1) peak relative to (1 0 0). The effect of cathode rotation speed, current density, and saccharin concentration on the coating microhardness was investigated. The maximum recorded hardness was 620 HV for 14 nm grain size. The effect of current density and saccharin concentration on morphology was observed by scanning electron microscopy (SEM). The current efficiency changes were studied as a result of saccharin concentration.     

Chemical synthesis of nanosized oxides

Fine powder of single and binary mixed oxides can be produced by decomposition of the respective metal nitrates and polyvinyl alcohol (PVA) or, a mixture of PVA and polyacrylic acids. These mixtures, after spray drying, yield a brown fluffy mass, which is spontaneously combustible and the heat liberated is sufficient for the crystallization of the desired oxide phase. The rate of combustion controls the growth of the particles. This can be manifested by combustion of the mixture in controlled atmosphere. The nanoparticles of the oxide system studied are: spinels [MFe₂O₄ where M = Ni(II), Co(II), Zn(II), Mg(II)]; orthoferrites [MFeO₃ where M = Gd(III), Sm(III)]; LaAlO₃, NdGaO₃, CaO/MgO/Y₂O₃ stabilized zirconia (ZrO₂); lead zirconate titanate (PZT), lanthanum modified lead zirconate titanate (PLZT) and BaTiO₃.     

Facile synthesis of nanocrystalline zinc ferrite via a self-propagating combustion method

Macroporous nanocrystalline zinc ferrite with single spinel-phase was prepared by a facile self-propagating combustion method using zinc nitrate, iron nitrate and glycine. The as-prepared ZnFe₂O₄ were characterized by X-ray diffraction (XRD) analysis, N₂ adsorption, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectrum (EDS). The magnetic properties of the prepared ZnFe₂O₄ were also studied.     

Chemical solution approach to SrTiO3 synthesis using a new precursor

We have synthesized SrTiO₃ (STO) using a newly developed precursor route by chemical solution approach. The new STO precursor solution was prepared in ambient atmosphere. Various characteristic methods, including thermal analyses, infrared spectroscopy, and transmission electron microscopic analyses techniques, were applied to study the thermal decomposition and crystallization behavior of STO precursor gel. The acquirement of single-phase STO powders at the different annealing atmospheres demonstrates that STO with perovskite structure is prone to tolerate the oxygen vacancies defect. Epitaxially grown STO film on textured Ni–W substrate by a seeded nucleation method shows a high c-axis orientation and a good out-of-plane texture. Scanning electron microscopy and atomic force microscopy investigations of STO film reveal the continuous, crack-free, and smooth surface morphology. The results suggest that STO film fabricated by the newly developed precursor route may be suitable to be used as buffer layer for subsequent growth of YBCO in coated conductors.