Key Engineering Materials Vols. 512-515

Abstract: Solution chemical reduction method is a means which used reducing agent in aqueous or organic systems to deoxidize silver ions. Its essence is an integrated process of the electrochemistry, thermodynamics, kinetics and fluid dynamics and so on. The process determines the physical and chemical properties such as the size distribution of powder, purity and reunion situations, etc. Nano-silver powder has high surface activity and catalytic properties, and is widely used to a lot of fields. So it has very important realistic significance to study the preparation method. In this paper, nano-silver powder was prepared with industrial AgNO3 as raw materials, sodium borohydride or hydrazine hydrate as reducing agent, polyvinyl alcohol, SDBS or polyvinylpyrrolidone(PVP) as the surface protective agent by solution chemical reduction method. The influences of different reducing agents and protective agents on the preparation of nano-silver were analyzed. The results from the XRD diffraction analysis and TEM show that it can be obtained high purity flake nano-silver in the reaction temperature range of 50-60°C.

Abstract: This paper presents the process of preparing spherical silver-coated copper powder with a silver content of 30-50%, using chemical reduction and by adjusting the silver-coating process. By means of SEM, XRD, grain size analyser, digital ohmmeter and differential thermal analyser, the surface topology, structure and conductivity of silver-coated copper powder and raw copper powder are characterised. The results show that the spherical silver-coated copper powder has superior compact surface, complete coverage, a coated layer reaching a thickness of 336nm and excellent conductivity and anti-oxidation property.

Abstract: Nanocrystalline lead zirconate titanate (PZT) powders with composition at the morphotropic phase boundary (MPB) were synthesized by a simple aqueous based sol-gel method, using lead nitrate, zirconium nitrate and tetrabutyl titanate as the starting materials. The sol could be easily transformed into gel, firstly heated at 120°C for 10h, then at 180°C for 24h. The thermal decomposition process of the gel was investigated by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The effect of citrate addition amount on the calcining temperature was discussed. The results reveal that pure perovskite phase PZT powders can be obtained at a calcining temperature as low as 600°C. The average grain size of the powders was determined by transmission electron microscope and X-ray diffraction. The influences of calcining temperature and the pH value of the solution on the grain size were investigated. The sintering temperature and electrical properties of the ceramics derived by nano-powders were compared with those prepared by the conventional ceramic processing. The result shows that using the nanopowder, the sintering temperature could be reduced by about 100°C and the ferroelectric properties were enhanced.

Abstract: Dispersion plays an important role in aqueous chemical coating process for the preparation of BaTiO3 based next generation BME MLCC powder, since it affects coating coverage and layer morphology of the BaTiO3 particles as well as the microstructure and dielectric properties of the ceramics. In this paper, Poly (acrylic acid-co-maleic acid) (PAM) is used as dispersant, its content influences are carefully investigated. It is known that low content of dispersant leads to not only poor dispersion, low coverage and irregular coating layer of BaTiO3 particles, but also bad temperature coefficient of capacitances and inhomogeneous microstructure of the resultant BaTiO3 ceramics, while excessive dispersant deteriorates the microstructure and performance by reducing the density of ceramic. With proper dispersant content, the particles are dispersed and coated perfectly, finally the dense X7R-satisfied ceramic are obtained with average grain size 103nm and fine microstructure, meeting the requirement of next generation MLCCs.

Abstract: As a new candidate material for the ceramic layer in thermal barrier coatings (TBCs) system, La₃NbO₇ was synthesized with La₂O₃ powder and Nb₂O₅ powder by solid state reaction. The stating powders with a mole ratio of La to Nb of 3:1 were mixed and then the mixture was calcined under the different temperatures(800°C, 1000°C, 1200°C) and dwell times(2h, 6h, 10h). The phase structure of the powder was observed by X–ray diffraction(XRD), and the microstructure of the sample was observed by scanning electron microscope(SEM). The effect of calcination temperature and dwell Time on the phase formation were examined. The results indicate that the La₃NbO₇ powder with single phase can be synthesized successfully at 1200°C for 10h in air, and the La₃NbOsub>7 powders synthesized have an ultra-fine particle size of 0.5˜1µm with a granular particle shape. With the temperature increasing, LaNbO_{4/sub> was synthesized firstly and then La3}NbO₇ was synthesized with a mole ratio of La₂O₃ to LaNbO₄ of 1:1.

Abstract: Nanostructured cobalt sulfide(CoS) can be widely used as high energy density batteries, supercapacitors, solar photovoltaic materials and catalysts due to its excellent electronical, optical, magnetic and catalytic performance. In order to synthesize CoS crystallites in a efficient route, a facile microwave hydrothermal process was developed by using cobalt nitrate hexahydrate and thioacetamide(TAA) as source materials. The phase compositions and morphologies of the crystallites were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The influences of microwave hydrothermal temperature and time on the phase, grain size and morphology of the CoS nanocrystallites were particularly investigated. Results show that CoS crystallites can be obtained at the hydrothermal temperature of 180 °C. The as-synthesized CoS nanocrystallites are hexagonal phase that with the grain size of 10~30 nm. With the increase of hydrothermal time, the increase in particle size and the (102)-oriented growth are obviously.

Abstract: In this work, cobalt oxide nanodiscs were successfully fabricated through a simple solvothermal synthesis method. The obtained cobalt hydroxide nanodiscs are of single-crystalline hexagonal structure with well-defed nanodiscs were found to have very weak magnetic properties, much weaker than its bulk cobalt oxide counterparts, which might find interesting application in special electromagnetism devices.

Abstract: Monodisperse Fe₃O₄ Superparamagnetic Nanoparticles Were Synthesized in N-Hexadecylphosphonic Acid/ Cyclohexane/ Water/ Ethanol Microemulsion under Solvothermal Conditions at 100°C. the Crystal Structure and Particle Size of Synthesized Fe₃O₄ Were Observed by X-Ray Diffraction (XRD) as Well as Transmission Electron Microscopy (TEM). the Results Show that the Nanoparticles Have a Cubic Crystal System and a Average Particle Size of about 10nm. each Nanoparticle Has a Single Crystal Structure. the Surface Chemistry of Synthesized Fe₃O₄ Nanoparticles Was Characterized by Fourier Transform Infrared Spectroscopy (FTIR), Indicating that the Nanoparticles Were Covered by a Layer of N-Hexadecylphosphonic Acid, which Made the Nanoparticles Totally Lipophilic. Magnetic Properties of the Nanoparticles Were Investigated by Using Vibrating Sample Magnetometer (VSM). the Result Reveals that the Saturation Magnetization (Ms) of the Nanoparticles Is Higher than 40 Emu/G and the Coercive Force Is near to 0. the Monodisperse Fe₃O₄ Nanoparticles Have Superparamagnetic Property and May Find Potential Applications in many Fields, such as Ferrofluids, Drug Loading and Release, Selective Biomolecular Separation and MRI.

Abstract: Eu³⁺ doped terbium hydroxide nanosheets were successfully synthesized from layer compound of Tb₂(OH)₅NO₃·1.5H₂O doped by Eu³⁺. Firstly layer compound of Tb₂(OH)₅NO₃·1.5H₂O doped by Eu³⁺ was obtained by hydrothermal reaction at 120⁰ . After ion exchange reaction with SDS under the microwave condition, the layer space of the compound increases from 0.907 nm to 2.34 nm. Furthermore stable nanosheet sol was obtained after exfoliation under ultrasonic condition. At the mean time we discovered that microwave and ultrasonic method can greatly shorten the preparation time. The structure of layer compound and the morphology of nanosheets were determined by XRD and TEM. The luminescence properties of terbium hydroxide nanosheets doped with Eu³⁺ were examined; the relationship between structure and spectra was discussed

Abstract: MgLaLiSi₂O₇:Eu³⁺ phosphors have been prepared through the sol-gel process. X-ray diffraction (XRD), thermogravimetric and ddifferential thermal analysis (TG-DTA), FT-IR spectra and photoluminescence spectra were used to characterize the resulting phosphors. FT-IR spectra suggested that crystallized silicates have formed in the powders annealed at 1050°C. The results of XRD indicated that the phosphors crystallized completely at 1050°C. In MgLaLiSi₂O₇:Eu³⁺ phosphors, the Eu³⁺ shows its characteristic red (613nm, ⁵D₀–⁷F₂) emissions.