Preparation and microwave-absorbing properties of NiFe2O4-polystyrene composites

NiFe₂O₄ nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite-polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2-12.4 GHz) frequency range. All the parameters, ε′, ε″, μ′ and μ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3-13 GHz for an absorber thickness of 2 mm.     

共掺杂的二氧化锆纳米材料中Yb3+和Tm3+上转换发光

用共沉淀法制备了二氧化锆掺Yb3 和 0 2 (0 4 ,1)mol%Tm3 的纳米材料 ;用发射波长为 980nm的激光激发样品 ,测量了掺 0 2mol%Tm3 的纳米材料在不同退火温度下的上转换光谱 ;测量了在相同退火温度、不同泵浦电流下的上转换发射光谱 ;并研究了蓝色上转换发射的过程。     

Nanocrystalline aerogel VOx/MgO as a catalyst for oxidative dehydrogenation of propane

Nanocrystalline aerogel VO_x/MgO catalysts for the oxidative dehydrogenation of propane with high surface area and uniform vanadium distribution were synthesized by co-gelation followed by supercritical drying. The catalysts were shown to have superior performance compared to nanocrystalline VO_x/MgO catalysts prepared by impregnation.     

Synthesis of terbium doped calcium phosphate nanocrystalline powders by citric acid sol–gel combustion method

Terbium doped calcium phosphate (Tb-doped CaP) nanocrystalline powders were synthesized by the citric acid sol–gel combustion method. The phase composition, morphology and luminescent property of Tb-doped CaP nanocrystalline powders were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrophotometer and fluorescence microscopy. At 700 °C, Tb-doped CaP nanocrystalline powders are composed of HAP (main phase) and β-TCP (minor phase) with Tb doping content of 0.5–4%. SEM and TEM observations show that the 4% Tb-doped CaP nanocrystalline powders are about 50–150 nm spherical particles. The 4% Tb-doped CaP nanocrystalline powders exhibit the strongest emission at 548 nm (λ_excitation = 240 nm) and show strong green fluorescence under fluorescence microscopy.     

Indirect evidence of supramolecular changes within cellulose microfibrils of chemical pulp fibers upon drying

Dried and never-dried chemical pulps were subjected to strong sulfuric acid hydrolysis and the dimensions of the resulting cellulose nanocrystals (CNCs) were characterized by AFM image analysis. Although the average length of CNCs was fairly similar in all samples (55–65 nm), the length distribution histograms revealed that a higher number of longer crystals and a lower number of shorter crystals were present in the CNC suspensions prepared from never-dried pulps. The distinction was hypothetically ascribed to tensions building in individual cellulose microfibrils upon drying, resulting in irreversible supramolecular changes in the amorphous regions. The amorphous regions shaped by tensions were deemed as more susceptible to acid hydrolysis.     

A technique for production of nanocrystalline cellulose with a narrow size distribution

Nanocrystalline cellulose (NCC) was prepared by sulfuric acid hydrolysis of microcrystalline cellulose. A differential centrifugation technique was studied to obtain NCC whiskers with a narrow size distribution. It was shown that the volume of NCC in different fractions had an inverse relationship with relative centrifugal force (RCF). The length of NCC whiskers was also fractionized by differential RCF. The aspect ratio of NCC in different fractions had a relatively narrow range. This technique provides an easy way of producing NCC whiskers with a narrow size distribution.     

MCM-41分子筛担载纳米TiO2复合材料光催化降解罗丹明B

采用溶胶-凝胶法将TiO2担载在介孔MCM-41分子筛上, 制备了不同TiO2含量的系列TiO2/MCM-41复合材料, 利用X射线衍射、N2吸附、紫外-可见光谱和透射电镜等方法对其进行表征. TiO2的晶型为锐钛矿相, 复合材料的比表面积和孔体积随其中TiO2担载量(复合材料中TiO2与MCM-41的质量比)的增加而减小, TiO2的平均粒径随其担载量的增加而增大. 以罗丹明B的光催化降解为探针反应, 评价了TiO2/MCM-41复合材料的光催化降解活性. 结果表明, 在紫外光照射下, 罗丹明B在该复合材料上的光催化降解反应遵循一级反应动力学, 复合材料对罗丹明B的光催化降解活性明显高于商用TiO2 (P-25), 复合材料的光催化降解活性由复合材料的吸附能力和所含TiO2的光催化活性共同决定.     

An X-ray scattering study of hydrogenated nanocrystalline silicon with varying crystalline fraction

Using X-ray diffraction (XRD) and small angle X-ray scattering (SAXS), we probed the nanostructural features of several PECVD grown nc-Si:H thin films with varying crystalline volume fraction. XRD results of a mixed phase film, 70% a-Si:H and 30% c-Si:H, show these crystallites have a preferred [220] orientation in the growth direction. Another film with approximately 90% c-Si also shows elongated grains, but with a preferred [111] orientation. The SAXS results also show an increase in scattering intensity when compared to the mixed phase material. In the mixed phase material, models show that the electron density fluctuations between the amorphous and crystalline phases are not enough to explain the measured SAXS scattering. Hydrogen clustered at the crystallite boundaries and in void regions of the a-Si phase must be included as well.     

Control of the nanocrystalline zirconia structure through a colloidal sol-gel process

A simple method to synthesize tetragonal zirconia stabilized at ambient temperature is developed and allows the monitoring of the tetragonal-monoclinic transition via a colloidal sol-gel process. By increasing the pH of an aqueous solution consisted of a zirconium precursor and a complexing agent (acetylacetone), a colloidal sol and then a gel can be formed under slightly acidic condition. After a drying step, tetragonal zirconia is easily obtained with an adequate thermal treatment at low temperature. The tetragonal-monoclinic transition occurs when the calcination temperature is increased. The relationship between the crystallite size, the crystallographic structure and the thermal treatment has been investigated by X-Ray Diffraction and the behaviour of the system from the gel state to the final powder has been studied by using Small Angle X-Ray Scattering and thermal analysis techniques. We demonstrate that compared to a chemical precipitation route, this colloidal sol-gel process allows the nanostructure of the material to be controlled due to the formation of primary nanoparticles. The presence of these nanoparticles makes possible the specific determination of the zirconia crystallographic phase through an accurate control of the nanostructure during the thermal treatment.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.