Synthesis of high purity Tl1212 superconductor with ultrafine grains from coprecipitated Tl-free precursor route

In this study, high purity and fine-grained Tl1212 superconductor samples were successfully synthesized from Tl_0.8Bi_0.2Sr₂Ca_0.8Y_0.2Cu₂O₇ starting composition by a two-step solid-state reaction method using Tl-free precursor powder prepared from coprecipitation method. XRD pattern of the sample, which was sintered at 1,000 °C for 6 min essentially showed formation of high purity 1212 phase. The sample’s zero resistance critical temperature, T _{c zero} was 91 K and bulk critical current density, J _c measured at 40 K in zero magnetic field was 11.2 A/cm². SEM investigation on the sample revealed ultrafine homogeneous grains that are randomly orientated with grain sizes of approximately 0.5–1 μm. Resintering the sample did not affect its superconducting properties but induced clustering of grains as observed by SEM. It is suggested that the sample be used in specialized measurement techniques such as in ultrasonic studies where high purity and ultrafine-grained samples are required.     

Crystallization of a coprecipitated mullite precursor during heat treatment

Powder of mullite composition (3Al₂O₃·2SiO₂) has been made by a coprecipitation method. The evolution of mullite in this precursor powder during heat treatment has been studied using differential thermal analysis, electron microscopy and X-ray diffraction techniques. It is shown that during calcination below 1100°C the coprecipitate develops -Al₂O₃ and perhaps cristobalite crystallites within the basic grains, whose morphology is otherwise invariant with temperature. Mullite forms above 1100°C by reaction of these -Al₂O₃ and SiO₂ crystallites, and the grain morphology changes markedly. Small exothermic events occur at 1000 and 1250 °C. The former is associated with the decomposition of a small content of aluminosilicate or perhaps with the conversion of - to -Al₂O₃, and the latter with mullite formation. For comparison, the behaviour of a polymeric mullite precursor during calcination is also examined. This material showed a large exothermic event at 1000°C which could be associated with the decomposition of the (amorphous) aluminosilicate to crystalline -Al₂O₃ and SiO₂, and a small exothermic event at 1250° C due to mullite formation.     

Synthesis of relatively monodisperse ZnO nanocrystals from a precursor zinc 2,4-pentanedionate

Monodisperse ZnO nanoparticles were successfully prepared through the decomposition of zinc acetylacetonate precursor in oleylamine. The samples were characterized by transmission electron microscope, X-ray diffraction and infrared spectroscopy. The particle size and morphology could be modified through tuning the reaction temperature, reaction period and concentration of zinc acetylacetonate. The grain size of monodisperse ZnO could be obtained at 205 and 300 °C for 1 h with 1:100 molar ratio of acetylacetonate and oleylamine, respectively. The average grain size of ZnO nanoparticles increased and the shape of ZnO particles varied with the reaction period prolonging or reaction temperature increasing. Some ZnO particles had self-aggregated into a belt using a solution of 1:10 molar ratio of zinc acetylacetonate and oleylamine at 250 °C for 1 h. The oleylamine plays an important role in preventing aggregation of ZnO nanoparticles.     

Synthesis of cerium hydroxycarbonate powders via a hydrothermal technique

CeOHCO₃ powders have been directly synthesized using a hydrothermal process at temperatures as low as 160°C. The well-dispersed powders are obtained in a short period of reaction time during hydrothermal reaction via the hydrolysis of urea. For synthesizing CeOHCO₃, the concentration of urea is found to be a crucial determinant, which has significant effects on the morphology of the derived powders. When low urea concentrations are provided, the formed particles are rhomboidal platelets. On the other hand, the high urea concentrations cause the shape of the powders to become prismatic. Increasing the concentration of urea tends to increase the particle size as well as the aspect ratio of CeOHCO₃ powders. After further heating at 500°C, a phase transformation from orthorhombic CeOHCO₃ to cubic CeO₂ takes place. The crystallinity and size of CeO₂ strongly depend on the particle size of CeOHCO₃.     

Synthesis and features of binary cobaltite spinels

The main developments in the synthesis of cobaltites are surveyed. Solid state reactions between oxides, thermal decomposition of mechanical mixtures of transition metal salts, the use of complex compounds as precursors, mechanochemical synthesis, deposition of cobaltites on supports and cobaltite synthesis by thermal treatment of coprecipitated compounds are discussed and the specific advantages and disadvantages of each method pointed out. The binary spinel cobaltites find applications as materials in many fields, their main importance being as catalysts. In order to achieve high specific surface area, cobaltite synthesis by thermal treatment of coprecipitated precursors proves to be the most promising method. Different kinds of precursors are regarded. Transition metal basic salts are promising for the synthesis of binary mixed oxides and, in particular, of cobaltites. The conditions suited by the coprecipitated precursors for the synthesis of cobaltites with preset properties, such as high-dispersity, homogeneity, a definite stoichiometry and a low impurity content, are indicated. Special attention is paid to hydroxidecarbonates. Their use as precursors ensures these demands for the final product and also evolution of toxic gases during the thermal decomposition is avoided.     

Synthesis of chemically coprecipitated hexagonal strontium-ferrite and its characterization

Highly uniform submicrometre size particles of hexagonal strontium ferrite (SrFe₁₂O₁₉) have been synthesized by chemical coprecipitation technique at pH ⋍ 13. Chemical coprecipitation technique has helped in bringing down the ferritization temperature from 1300 to 925° C which is revealed by DTA-TG and XRD studies. Reproducible uniform single domain particle size and its distribution has been observed by scanning electron microscopy. X-ray and Mossbauer studies have identified single phase ferrite with Fe³⁺ ions occupying the proper crystallographic sites. The performance parameters of the sintered isotropic strontium ferrite magnets have proved to be superior by about 20% over the ferrites prepared by conventional ceramic technique.     

Microwave-assisted hydrothermal synthesis of zinc oxide particles starting from chloride precursor

Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn²⁺ ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)₆·H₂O), and the conversion from Zn₄CO₃(OH)₆·H₂O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 °C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved.     

Synthesis of silicon oxynitride from a polymeric precursor

In this final part of our investigation, the copolymers prepared (reported in Part III) have been pyrolysed to yield silicon oxynitride. Thermogravimetric studies performed on several copolymers in nitrogen, ammonia/nitrogen mixtures, air and oxygen are discussed in the present paper. One copolymer was used for further investigation and its pyrolysis conditions (variation of pyrolysis atmosphere according to temperature) were optimized to produce the maximum yield of white amorphous residue which was crystallized by heating to 1600°C to produce silicon oxynitride powder. The powder particles were equiaxed in shape and in the size range 50–150 nm but in agglomerates of size 0.3–0.1 m. Crystallization at higher temperatures resulted in a mixture of a silicon nitride-silicon oxynitride.     

Synthesis of silicon oxynitride from a polymeric precursor

In the present work, homopolymerization and copolymerization of methylcyclosiloxanes and methylcyclosilazanes are investigated. Homopolymers and copolymers produced are characterized using ¹H NMR and Fourier transform infrared spectroscopy. It is found that the reactivity of methylcyclosiloxanes is much lower than that of methylcyclosilazanes. The structures of the homopolymers and copolymers produced are discussed and it is suggested that the latter consists of a block structure containing siloxanes and silazanes in which the siloxane units are the smaller.     

Synthesis of silicon oxynitride from a polymeric precursor

Previous studies suggest that silicon oxynitride can be synthesized by pyrolysing polymers which are produced by the copolymerization of cyclic polysiloxanes and cyclic polysilazanes. The cyclic polysiloxanes required are [CH₃Si(H)O]_m with m=4, 5 and 6, and these can be produced by the hydrolysis of dichloromethylsilane. In the present work, a significantly improved combined yield of these cyclic polysiloxanes is reported. Three main reasons are attributed to this achievement: an improved hydrolysis procedure; optimization of the initial acidity of the reaction medium; and changes in the total reaction time. Thus a mixture containing > 95 mol% cyclic tetramer, pentamer and hexamer has been prepared and this represents a 20 mol% improvement over previously reported results. Investigations of the preparation and characterization of the cyclic polysilazanes, copolymers and silicon oxynitride will be described in Parts II–IV, respectively.     

Synthesis of silicon oxynitride from a polymeric precursor

Proton nuclear magnetic resonance (¹H NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and gas chromatography (GC) have been used to study the products of ammonolysis of dichloromethylsilane in diethyl ether at 0 °C. Results indicate that the major products are trimethylcyclotrisilazane and tetramethylcyclotetrasilazane. Hydrolysis of the cyclosilazanes has also been qualitatively investigated. A simple and useful method for the identification of cyclosilazanes has been developed using the ¹H NMR technique.     

Synthesis of zinc and zinc oxide nanoparticles from zinc electrode using plasma in liquid

Nanoparticles are synthesized efficiently from zinc electrode by microwave plasma in liquid. The nanoparticles synthesized from alcohol resulted in pure zinc particles in the shape of spheres or hexagonal cylinders with a production rate of 3.3 g/h, and energy consumption of 267 J/mg for 1 mg. Whereas the nanoparticles synthesized in pure water are composed of Zn and ZnO. The Zn reacts with water through heat or the passage of time to become ZnO, releasing hydrogen gas. An upper disk placed 1 mm away from the electrode along with the bubbles generated simultaneously with the plasma ignition plays a key role in the synthesis of nanoparticles.     

Synthesis and characterization of a novel precursor for thin films of zinc oxide by SSCVD

A novel solid organic zinc fountain as a precursor for thin films of zinc oxide by single source chemical vapour deposition (SSCVD) had been prepared through simple procedures. To synthesize the precursor, zinc oxide and acetic acid of the molar ratio 2:3 were used to react with ethanol as solution in this experiment. The reaction products as the precursor obtained after reflux of the mixture solution were characterized by Fourier transform infrared spectroscopy analysis and thermogravimetric analysis, and the thin films on silicon substrate by SSCVD using the precursor were investigated by X-ray photoelectron spectroscopy analysis. All these provide evidence that the precursor has volatility and thermolysis properties suitable for SSCVD of ZnO thin films. It is interesting to note that the films are found to have a small amount of excess O, whereas ZnO films obtained by other techniques are often O deficient.     

Nanocrystalline copper doped zinc oxide produced from copper doped zinc hydroxide nitrate as a layered precursor

Undoped and copper doped nanostructured zinc oxides were synthesized by using a series of synthetic layered material, undoped and copper doped zinc hydroxide nitrates at various molar percentages of copper (2–10) within the layers as precursors. The layered materials were heat-treated at 500 °C to produce zinc oxide nanostructures with crystallite sizes in the range of 23–35 nm. Optical studies of the nanostructured copper doped zinc oxides showed the decrease in band gap with increasing content of the doping agent, copper.     

Preparation of LaNiO3 powder from coprecipitated lanthanum-nickel oxalates

Coprecipitation of lanthanum and nickel oxalates in a water-alcohol mixed solution of an oxalic acid resulted in a simultaneous and homogeneous deposition of the respective oxalate particles with a desired cation ratio. The coprecipitated oxalate could be readily converted to a fine powder (5 to 6 m² g^–1) of the desired LaNiO₃ by heating at 800 to 850° C. Detailed examination of some precipitation conditions established an optimum procedure needed for the powder synthesis of LaNiO₃. Thermal analysis showed that La₂NiO₄ (high temperature form) is transiently produced prior to the formation of LaNiO₃. Mixed valency of the nickel ion in the synthesized powders was quantitatively determined by means of the oxidation-reduction titration, suggesting that the chemical formula of the powders might be LaNiO_{2.85 to 2.90}.     

非晶CIGS前驱膜无硒退火的相变历程

采用一步射频磁控溅射法在室温获得了CIGS薄膜,研究了不同的真空无硒退火温度(150～350℃)对CIGS薄膜相变历程的影响。薄膜相变历程中的结构和性能采用XRD、SEM、EDS、紫外-可见光吸收和四探针等测试手段进行测试表征。结果表明,室温下制备的CIGS薄膜为非晶态,随退火温度升高发生非晶CIGS→CuSe→CIGS的相变。150℃退火形成的CuSe薄膜的电阻率最低,光透过性能最差。退火温度超过200℃便生成CIGS相,CIGS相的结晶质量随退火温度升高而改善,薄膜的电阻率和光透过率也随退火温度的提高而增加。