Microwave-assisted synthesis and characterization of SrMoO4 and SrWO4 nanocrystals

Journal of Nanoparticle Research - SrMoO4 and SrWO4 nanocrystals have been synthesized using Sr(NO3)2 and Na2MeO4 (Me = Mo and W) in ethylene glycol by a microwave irradiation...     

Characterization of PbS with different morphologies produced using a cyclic microwave radiation

PbS was produced from different lead (Pb(CH₃COO)₂·H₂O, PbCl₂·2.5H₂O, Pb(NO₃)₂) and sulfur (CH₃CSNH₂, CH₅N₃S, NH₂CSNH₂) sources in propylene glycol using a cyclic microwave radiation at different powers and prolonged times. PbS (cubic) was detected using X-ray diffraction (XRD) and selected area electron diffraction (SAED) techniques. The interpreted and simulated patterns are in good accord. Raman spectrometer revealed the presence of vibrations at 138, 273 and 439 cm⁻¹. Different morphologies (nano-sized particles, hexapods, cubes, ferns and magic squares) were characterized using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The product morphologies were influenced by the starting agents, microwave powers and prolonged times.     

Preparation and structural characterization of nanocrystalline SnO2 powders

Nanocrystalline SnO₂ powders have been prepared by solid–liquid reaction and solid-state thermal oxidizing techniques. The microstructures and phase compositions of the product were characterized by thermogravimetry analysis, X-ray diffraction, and the Raman spectrum. It is shown that at least two phases, SnO₂ and SnO_x, coexist at 450 °C. However, only the tetragonal rutile structure SnO₂ phase is detected after the Sn powders were annealed at 550 °C. The Raman peaks of the nanocrystalline SnO₂ powders reveal remarkable red shift and broadening, which could be attributed to the phonon confinement effect, oxygen vacancies, and the stress effect. PACS 81.07.Wx; 81.10.Jt; 78.30.-j     

具有广阔应用前景的纳米金刚石膜

纳米金刚石膜的制备、表征和应用研究已经成为CVD金刚石膜研究领域的一个新的热点．文章重点介绍了Gruen等人在贫氢和无氢环境中制备纳米金刚石膜的开创性工作,并和在富氢气氛中制备的纳米金刚石膜进行了对比评述．对纳米金刚石膜的表征技术和应用前景进行了讨论．     

Thermal characterization of nanocrystalline porous CePO4 ceramics

A laser-induced photoacoustic technique was employed to investigate thermal transport through nanocrystalline CePO₄ samples prepared via the sol–gel route. Evaluation of thermal diffusivity was carried out using the one-dimensional model of Rosencwaig and Gersho for the reflection configuration of the photoacoustic method. Structural analyses of samples revealed that they are nanoporous in nature, possessing micron-sized grains. Analysis of results shows that thermal diffusivity value varies with sintering temperature. Results are explained in terms of the variation in porosity with sintering temperature and the effects of various scattering mechanisms on the propagation of phonons through the nanoporous ceramic matrix. Further analyses confirm that apart from porosity, grain boundary resistance and interface thermal resistance influence the effective value of thermal diffusivity of the samples under investigation.     

Preparation and characterization of nanocrystalline ZnO particles from a hydrothermal process

Rod-like ZnO nanoparticles were prepared by the hydrolysis of zinc acetate under heating in diethylene glycol (DEG). Structural characterization of the synthesized powders was investigated by XRD, FT-IR, electron paramagnetic resonance (EPR) and transmission electron microscopy (TEM). The size of the particles increased as the amount of H₂O added increased in the nano size range. The average crystallite size calculated from the XRD patterns varied from 6 to 64 nm corresponding to the amount of H₂O added. The ZnO nanopartilces possess the wurtzite type crystallographic structure. It was found that these ZnO nanoparticles had singly ionized oxygen vacancy defect () and superoxide ions from the EPR investigations. A strong near UV emission of the ZnO nanoparticles at about 380 nm was observed and its intensity decreased as the amount of H₂O increased. This emission of ZnO nanoparticles is found to be particles size dependent due to the confinement effect. A green emission at about 540 nm due to the recombination of electrons trapped at singly ionized oxygen vacancies defect () appeared when the amount of H₂O increased. The intensity of the green emission increases as the concentration of increases.     

Preparation and microwave characterization of submicrometer-sized hollow nickel spheres

The hollow structural submicrometer-sized nickel spheres were successfully fabricated by the autocatalytic reduction method. Because of the metallic and ferromagnetic behaviour of the nickel spheres, the low-density microspheres could obtain high dielectric constant and magnetic loss in microwave frequencies. The abrupt variation of the real part and the sharp peaks of the imaginary part of permittivity and permeability were observed for the micrometer-sized and nanometer-sized nickel hollow spheres. Reflection loss less than −25 dB were predicted over 11 GHz with a thickness of 1.5–2.0 mm.     

Selective removal of metallic SWNTs using microwave radiation

We report new method for selectively removing the metallic CNTs from semiconducting CNTs in a powder using high-power microwave radiation in the infrared and radio frequency range of the electromagnetic spectrum. SWNTs in a powder film were heated in a 2.5 GHz microwave oven for a few minutes, and the metallic nanotubes burned more rapidly than the semiconducting nanotubes. Raman data showed that the ratio of metallic to semiconducting nanotubes decreased dramatically after exposure to microwave radiation. Using their more rapid absorption of the radiation energy of the microwaves, we achieved the selective removal of metallic SWNTs from semiconducting SWNTs. This method results in the high-purity of semiconducting SWNTs necessary for sensor and electronic applications.     

外腔式SrWO4拉曼激光器的输出特性研究

研究了外腔式SrWO4拉曼激光器在ns脉冲抽运下的输出特性。利用主动调Q的Nd∶YAG激光器产生的脉冲宽度为11.7 ns,输出能量为80 mJ的激光作为抽运源,拉曼激光谐振腔采用平平腔,实验采用4片对一阶斯托克斯脉冲和二阶斯托克斯脉冲不同反射率的输出耦合镜,测量了输出能量与抽运能量的关系,计算了转换效率与抽运能量的关系。当输出耦合镜对一阶斯托克斯脉冲的反射率为39.9%时,实验得到一阶斯托克斯脉冲的最大能量和转换效率分别为23.9 mJ和36.2%,当输出耦合镜对一阶斯托克斯脉冲和二阶斯托克斯脉冲的反射率分别为80.5%和12.4%时,得到二阶斯托克斯脉冲的最大输出能量和转换效率分别为16.4 mJ和25.4%,典型的一阶斯托克斯脉冲和二阶斯托克斯脉冲的脉冲宽度分别为6.1 ns和5.8 ns。     

Picosecond Stimulated Raman Scattering of SrWO4 Crystal

Stimulated Raman scattering （SRS） of picosecond pulses is investigated in a new crystal SrWO4. The second harmonic generation of a mode-locked Nd：YAG laser system is used as the pump source. In an external singlepass configuration, the SRS thresholds for the first to the fourth Stokes lines are measured. For the first Stokes line, the steady-state gain coefficient of the SrWO4 crystal is calculated to be 15.96cm//GW. In our experiment, as many as five Stokes lines （559.23 nm, 589.61 nm, 623.49 nm, 661.50 nm, 704.44 nm） and three anti-Stokes lines （506.97nm, 484.34 nm, 463.65nm） are observed, and the total conversion efficiency is as high as 62%.     

Preparation,surface modification and microwave characterization of magnetic iron fibers

In this paper, magnetic iron fibers of 3–10 μm diameter and an adjustable aspect ratio were synthesized successfully by a method involving pyrolysis of carbonyl under a magnetic field. A surface modification technology was also investigated. The electromagnetic parameters of the iron-fiber–wax composites were measured using the transmission/reflection coaxial line method in the microwave frequency range of 2–18 GHz. The results show that the prepared iron-fiber–wax composites exhibit high magnetic loss that can be further improved after phosphating. On the other hand, the complex permittivity was significantly decreased after phosphating. As a result, this kind of iron fiber may be useful for thin and lightweight radar-absorbing materials.     

Optical properties of Pr3+-doped SrWO4 crystal

The polarized absorption and emission spectra of Pr³⁺ ions in SrWO₄ single crystals were investigated at room temperature. The standard and modified Judd–Ofelt theories have been applied to analyze the polarized absorption spectra to determine the spectroscopic parameters, including the Judd–Ofelt intensity parameters Ω_t (t=2,4,6), radiative transition probabilities, radiative lifetimes and branching ratios. The stimulated emission cross sections and fluorescence lifetimes of the promising laser level were obtained. PACS 78.55.Hx; 42.70.Hj; 78.20.-e     

Preparation and characterization of GIAR-films for monochromatization of synchrotron radiation

The use of GIAR-films for nuclear resonant filtering of synchrotron radiation has been proposed. A deposition setup was developed to produce suitable layer systems. Various alloy coatings containing⁵⁷Fe were prepared. The hyperfine splittings have been investigated by CEMS. The layer systems have been analyzed in grazing incidence geometry with respect to thickness, density and boundary roughness using Mo K_α radiation.     

喷雾热解法制备的SrWO4膜的阴极射线发光

利用超声喷雾热解法制备了钨酸锶SrWO₄多晶发光膜,并研究了制备条件及掺杂对其阴极射线发光特性的影响.生成的发光膜在300℃以上退火后具有白钨矿结构,其阴极射线发光为一宽带的蓝光,包括一个位于448nm的蓝色发光带和一个位于488.6nm的蓝绿色发光带,是由阴离子络合物WO₄^2-的电荷转移跃迁引起的.发光强度随着退火温度的升高而增强,而退火气氛对其影响不大.在SrWO₄膜中掺入银离子Ag⁺和镧离子La³⁺后,不影响其发光特性,但铕离子Eu³⁺的掺入对发光特性有影响.     

Synthesis and characterization of nanocrystalline TaN

Tantalum nitride (TaN) nanocrystals have been successfully synthesized at 650 °C through a solid-state reaction in an autoclave. The X-ray powder diffraction pattern indicates that the product is a mixture of hexagonal and metastable cubic TaN. Transmission electron microscopy images and selected area electron diffraction patterns show that the hexagonal TaN crystallites consist of nanorod with a typical size of about 50×1000 nm and the cubic TaN crystallites are composed of uniform particles with an average size of about 30 nm.     

Preparation, characterization and microwave absorption properties of electroless Ni-Co-P-coated SiC powder

Silicon carbide particles reinforced nickel-cobalt-phosphorus matrix composite coatings were prepared by two-step electroless plating process (pre-treatment of sensitizing and subsequent plating) for the application to lightweight microwave absorbers, which were characterized by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), vibrating sample magnetometer (VSM) and vector network analyzer, respectively. The results show that Ni-Co-P deposits are uniform and mixture crystalline of α-Co and Ni₃P and exhibit low-specific saturation magnetization and low coercivity. Due to the conductive and ferromagnetic behavior of the Ni-Co thin films, high dielectric constant and magnetic loss can be obtained in the microwave frequencies. The maximum microwave loss of the composite powder less than −32 dB was found at the frequency of 6.30 GHz with a thickness of 2.5 mm when the initial atomic ratio of Ni-Co in the plating bath is 1.5.     

Theoretical and experimental study on intracavity pumped SrWO4 anti-Stokes Raman laser

An intracavity pumped SrWO₄ anti-Stokes Raman laser is realized by placing an inclined SrWO₄ Raman cavity in a Q-switched Nd:YAG fundamental cavity. This structure is used to achieve non-collinear phase matching between the fundamental, the first-order Stokes and the first-order anti-Stokes waves. The maximum forward and backward first anti-Stokes outputs are 0.683 and 0.667 mJ, respectively, and the corresponding first anti-Stokes pulse widths are both 3.3 ns. A rate equation model is set up to simulate the output energies and temporal characteristics of these pulses. The stimulated results are in agreement with the experimental ones on the whole.     

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe₂O₄) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe₂O₃ powders. The structural and microstructural evolution of NiFe₂O₄ caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe₂O₃ phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe₂O₄ is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe₂O₄ phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.