Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si₃N₄–SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄–SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10⁻³ at a resonant frequency of 14 GHz.     

EPR and SQUID studies on magnetic properties of SiO2-doped Ni–Zn ferrite nanocomposites

Effects of the silica content and temperature on the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites have been studied by electron paramagnetic resonance (EPR) and superconducting quantum interference device (SQUID) technique. The content of TEOS in the starting solution affects the interaction between NiZn ferrite and silica, and then determines the particle size and the EPR properties. In addition, the SQUID properties of the composites were sensitive to particle size. It was observed that the saturation magnetization (M_s), remnant magnetization (M_r) and initial susceptibility (χ_i) decreased with increasing SiO₂ content.     

Preparation and application of magnetic cobalt/SiO2 core/shell nanospheres

Homogeneous SiO₂-coated cobalt nanospheres with tunable silica shell thickness from 21.7 nm to 4.5 nm were synthesized by using modified Stöber method. These nanocomposites were used as source materials to prepare SiO₂ semi-hollow and hollow nanospheres by partially and completely etching cobalt cores, respectively. A proposed formation mechanism of these Co/SiO₂ nanospheres with a core/shell structure was presented in this paper, which is also important for the rational design and synthesis of other monodisperse core/shell nanoarchitectures with uniform size and shape. Furthermore, these Co/SiO₂ nanospheres were also used as a substrate for the deposition of CdS nanocrystals to prepare magnetic luminescent Co/SiO₂/CdS nanocomposites.     

Enhanced ferroelectric properties of Fe-doped BaTiO3 thin film deposited on LaNiO3/Si substrate by sol–gel technique

The ferroelectric thin films of Fe-doped BaTiO₃ and undoped BaTiO₃ were prepared on LaNiO₃ coating Si substrates by sol–gel technique. It was found that a small amount of Fe dopant could significantly enhance the ferroelectric properties of the BaTiO₃ thin film. The remnant polarization of Fe-doped BaTiO₃ thin film at room temperature reached to 14.9 μC/cm². The loss tangent, compared to the undoped BaTiO₃ film, was increased with frequency increasing and the dielectric constant was decreased. The possible mechanism of enhanced ferroelectric properties of Fe-doped BaTiO₃ thin film was discussed. The results show the potential role of Fe dopant in improving the ferroelectric properties of BaTiO₃ thin film.     

Atomic force microscopy study of TiO2 sol–gel films thermally treated under NH3 atmosphere

Multilayered TiO₂ films were obtained by sol–gel and dipping deposition on quartz substrate followed by thermal treatment under NH₃ atmosphere. In an attempt to understand the close relationship between microstructural characteristics and the synthesis parameters, a systematic research of the structure and the morphology of NH₃ modified TiO₂ sol–gel films by XRD and Atomic Force Microscopy is reported. The surface morphology has been evaluated in terms of grains size, fractal dimension and surface roughness. For each surface, it was found a self-similar behavior (with mean fractal dimension in the range of 2.67–3.00) related to an optimum morphology favorable to maintain a nano-size distribution of the grains. The root mean square (RMS) roughness of the samples was found to be in the range of 0.72–6.02 nm.     

Synthesis of Fe/SiO2 composite particles and their superior electromagnetic properties in microwave band

Fe/SiO₂ composite particles were synthesized by hydrogen reduction of Fe₂O₃/SiO₂ precursor, which was prepared by sol-gel method. A reduction temperature higher than 600 °C is required for the complete conversion of Fe₂O₃ to Fe. Fe/SiO₂ composite particles exhibit superior complex permittivity and permeability in the microwave band. A reflection loss higher than − 70 dB as well as a broad absorption band can be simultaneously obtained for Fe/SiO₂-based coatings about 2 mm in thickness, suggesting that the Fe/SiO₂ composite particles are a promising candidate for high performance electromagnetic absorption materials.     

Preparation and photoluminescence properties of ZnO/amorphous-BaTiO3 thin-films by sol–gel process

ZnO/amorphous-BaTiO₃ thin-films were prepared on glass substrates by a sol–gel process. DSC/TGA, XRD and AFM were used to analyze the thermolysis of the precursor and the crystal growth characteristics of ZnO, and IR spectroscopy was used to determine the presence of OH groups and CO impurities containing in the films. Violet emission centering at 418 and 438 nm was observed in room temperature PL spectra of the films annealed at 400–500 °C. The strongest violet emission was observed in the film annealed at 400 °C. The emission intensity reduces as the annealing temperature increases, and vanishes in the film annealed at 550 °C. All of the films contain OH and CO groups, and the films annealed at the temperature over 450 °C contain CO₂ impurities, indicating the enhancing oxidation of the films. The vanishing of the violet emission is probably due to the improvement of the oxygen deficient of ZnO crystals.     

Rietveld refinement of amorphous SiO2 prepared via sol–gel method

Using the Rietveld refinement, we analyze the structural evolution under thermal treatment of silica xerogel samples prepared by the sol–gel method with molar ratio (R) of water to TEOS of R = 5 and R = 11.66. We refine the structure of compounds using the Maud program and we found the unit cell parameters and atomic positions of the refined silica amorphous atoms for whole samples. The results show us that the amorphous structure is quartz-like or low-cristobalite-like, depending on the molar ratio values and the heat-treatment of the samples. For R = 5 different quartz structures are obtained, whereas for R = 11.66 we obtain close structures in which a transformation of quartz-like amorphous to low-cristobalite amorphous phase occurs at about 600 °C. These results give some scopes to explain the partial crystallization of the silica at relatively low temperatures obtained when metallic species are present.     

Structure and magnetic properties of FeCo-SiO2 nanocomposite synthesized by a novel wet chemical method

A novel soft magnetic nanocomposite with FeCo particles encapsulated by amorphous SiO₂ was synthesized using a co-precipitation combined H₂ reduction method. The saturation magnetization of the (Fe₇₀Co₃₀)₉₀/(SiO₂)₁₀ nanocomposite is as high as 200 emu/g, which is 4-5 times larger than that of traditional spinel ferrites. The frequency dependence of the complex initial permeability is intensely dependent upon the content of SiO₂ insulating phase. With increasing the content of SiO₂ to 10 wt.%, the cut-off frequency is drastically increased to over 1 GHz. The results show that a new high-frequency soft magnetic material with high saturation magnetization (M_s) can be achieved by introducing FeCo/SiO₂ nanocomposite.     

Natural and persistent superhydrophilicity of SiO2/TiO2 and TiO2/SiO2 bi-layer films

Sol-gel SiO₂/TiO₂ and TiO₂/SiO₂ bi-layer films have been deposited from a polymeric SiO₂ solution and either a polymeric TiO₂ mother solution (MS) or a derived TiO₂ crystalline suspension (CS). The chemical and structural properties of MS and CS bi-layer films heat-treated at 500 °C have been investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscospy. Water contact angle measurements show that MS SiO₂/TiO₂ and CS TiO₂/SiO₂ bi-layer films exhibit a natural superhydrophilicity, but cannot maintain a zero contact angle for a long time over film aging. In contrast, CS SiO₂/TiO₂ bi-layer films exhibit a natural, persistent, and regenerable superhydrophilicity without the need of UV light. Superhydrophilic properties of bi-layer films are discussed with respect to the nature of the TiO₂ single-layer component and arrangement of the bi-layer structure, i.e. TiO₂ underlayer or overlayer.     

Structure and up-conversion luminescence in sol–gel derived Er–Yb co-doped SiO2:PbF2 nano-glass–ceramics

Transparent oxyfluoride nano-glass–ceramics 90(SiO₂)10(PbF₂) co-doped with 0.3 Yb³⁺ and 0.1 Er³⁺ (mol%) have been prepared by thermal treatment of precursor sol–gel glasses. X-ray diffraction and high resolution transmission electron microscopy analysis pointed out a precipitation of cubic β-PbF₂ nanocrystals of certain diameter in nano-glass–ceramics varying from 10 to 20 nm depending on heat treatment conditions. The incorporation of Yb³⁺ and Er³⁺ dopants in these nanocrystals has been confirmed by signatures of luminescence spectroscopy. Up-conversion luminescence pumped at 980 nm has been detected. Colour tuneability of up-conversion luminescence varying pump power has been analyzed in terms of standard chromaticity diagram. This tuneability opens applications for up-conversion phosphors and three-dimensional optical recording.     

Preparation and characterization of NaSm9(SiO4)6O2 powders with infrared absorptive properties

NaSm₉(SiO₄)₆O₂ powders were synthesized by mild hydrothermal method at 180 °C for 24 h. The infrared optical properties and structure of the obtained powders were characterized. There existed two narrow and sharp absorptive bands near 943 cm^− 1 (10.6 μm). The band at 938 cm^− 1 was assigned to the stretching vibrations of SiOSm groups connecting to Q¹ species and the band at 989 cm^− 1 was attributed to the stretching vibrations of SiOSm groups linking with Q⁰ species. The reflectivity was lower than 1% from 900 to 1200 nm and reached the minimum of 0.46% at 1073 nm. The prepared powders exhibit potential to act as a new kind of absorptive material for the infrared light of 10.6 μm and 1.06 μm.     

Simulation of Si and SiO2 etching in CF4 plasma

The reactive ion etching (RIE) of Si and SiO₂ in CF₄ plasma is considered. The dependences of RIE rates of Si and SiO₂ on pressure have maxima due to the presence of single-atom vacancies. The RIE rates approach the maximum values at different pressures but at the same concentration of SiF and SiOF molecules in the adsorbed layer. Using the obtained results Si/SiO₂ etching selectivity is investigated.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Nano-crystalline LaFeO3 powders synthesized by the citrate–gel method

A novel sol–gel process was developed for preparing nano-sized, perovskite-type LaFeO₃ powder by the thermal decomposition of the gel-complex of LaFe–(C₆H₈O₇·H₂O). The structural evolution has been systematically investigated by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Perovskite powder of  25 nm size could be obtained at a temperature of  600 °C without formation of any secondary phases of La₂O₃ and Fe₂O₃ single oxides and no requirements of high temperature/vacuum/pH control etc. Analysis of the X-ray powder diffraction data showed a decrease in the value of lattice strains with increasing decomposition temperature, whereas the particle size increases with increasing decomposition temperature.     

N2 adsorption curve of mesoporous SiO2: A neglected characterization technique

Mesoporous SiO₂ sieve and SiO₂ nanoparticles were synthesized by a traditional method in the presence and absence of tri-block copolymer surfactant P123 as structure directed agent respectively. The characterization results show that the mesoporous SiO₂ sieve has larger specific area (789 m²/g) than SiO₂ nanoparticles (373 m²/g), and there exists a substantial difference in the N₂ adsorption curves between the two samples. A speculative scheme shows that the formation of bottleneck during the N₂ adsorption process of mesoporous SiO₂ sieve should be responsible for the phenomenon.     

Chemical deposition of Al2O3 thin films on Si substrates

Uniform Al₂O₃ films were deposited on silicon substrates by the sol–gel process from stable coating solutions. The technological procedure includes spin coating deposition and investigating the influence of the annealing temperature on the dielectric properties. The layers were studied by Fourier transform infrared spectroscopy and Scanning Electron Spectroscopy. The electrical measurements have been carried out on metal–insulator–semiconductor (MIS) structures. The C–V curves show a negative fixed charge at the interface and density of the interface state, Dit, 3.7 × 10¹¹ eV^− 1cm^− 2 for annealing temperature at 750 °C.     

SiO2 layer thickness effects on the (001) texture of FePtCu:SiO2 nanocomposite films

The influence of SiO₂ layer thickness of (Fe₅₂Pt₄₈)₈₈Cu₁₂:SiO₂ multilayer nanocomposite films on their structural and magnetic properties were investigated. The films were deposited on (001) textured FePt films, and then annealed at 873 K. The crystalline texture of (Fe₅₂Pt₄₈)₈₈Cu₁₂:SiO₂ films changes drastically with respect to the thickness of the SiO₂ layers. In the film with 50-Å thick SiO₂ layers, the (111) peak was strong although the (001) orientation is dominant, and self-organized spherical FePtCu particles were formed in the SiO₂ matrix. However, in the film with 19-Å thick SiO₂ layers, flat FePt grains with perfect (001) orientation were obtained. In addition, twins with different crystalline orientations were seen in the above films with different thicknesses of the SiO₂ layers. Accordingly, different perpendicular hysteresis loops were obtained.     

Optical bistability for ZnO–Nb2O5–TeO2 glasses

We examined the optical nonlinear properties of ZnO–Nb₂O₅–TeO₂. The absorption and Raman spectra were measured, the third-order nonlinear susceptibility was determined by degenerated four wave mixing technique. The magnitude of χ⁽³⁾ is about 1.0 × 10⁻¹² esu, larger than that of silica glasses, and the optical bistability was observed in a Fabry–Perot cavity.     

聚四氟乙烯/二氧化硅杂化材料的性能研究

为解决纳米粒子在聚四氟乙烯(PTFE)树脂中难分散均匀的问题,使用PTFE乳液通过原位溶胶-凝胶法(Sol-Gel)制备了聚四氟乙烯/二氧化硅(SiO2)杂化材料,并对其性能进行了表征与研究.研究表明:杂化材料的拉伸强度在SiO2含量为1.05%(记为FS-2)时达到最大值20.96 MPa,为纯PTFE的两倍;断裂伸长率随着SiO2含量的增加而降低,硬度逐渐升高,接触角由121°降至79°.由SEM结果发现,试样FS-2中SiO2的粒径仅为100 nm左右,但随着SiO2含量的增加,粒径变大.杂化材料的耐热性较PTFE有一定提高,当杂化材料中SiO2含量达到5.00%时,热分解温度由纯PTFE的503℃上升至528℃.