Fabrication of La2Zr2O7 ceramic fibers via electrospinning method using different La2O3 precursors

Despite La₂Zr₂O₇ ceramic fiber has been fabricated by electrospinning method, the effects of thermal pyrolysis process on the microstructures and properties of the fibers have seldom been considered. Three La₂O₃ precursors were used to prepare La₂Zr₂O₇ ceramic fibers. Phase transformation of La₂Zr₂O₇ ceramic fibers were characterized by XRD and Raman spectra. The influence of the decomposition process on the microstructure of the fibers was studied by TG/DSC, XPS and SEM. The results show that slow weight loss leads to smaller grain size which could obtain higher strength fibers while fast weight loss could develop pores which are benefit to the decrease of the thermal conductivity.     

Freestanding hematite nanofiber membrane for visible-light-responsive photocatalyst

Freestanding mesoporous hematite (α-Fe₂O₃) nanofiber membranes were successfully fabricated by sol–gel electrospinning process using ferratrane precursor for use as a high-performance material for visible-light-responsive photocatalyst. Non-porous nanofiber membranes spun on the heated collector at 300 °C were crystalline α-Fe₂O₃ phase. Upon calcination, pure mesoporous nanofiber membranes were obtained even at a low temperature of 400 °C. The photocatalytic membrane calcined at 400 °C showed the highest efficiency for methylene blue (MB) degradation under visible-light irradiation. The synergetic effects of higher surface area, pore volume and pore diameter promoted the photocatalytic efficiency for MB degradation under visible light. The utilization of photocatalyst in the form of membrane could not only solve the problems of catalyst separation and recovery, but also produce high photodegradation efficiency for both systems without and with hydrogen peroxide even at a catalyst loading as low as 0.04 g/L. No appreciable loss in photocatalytic activity was observed and structural integrity was retained, even after five cycles of photodegradation, which predicted the stability and reusability of these nanofiber membranes for practical use in environmental applications.     

Synthesis of mullite nanofibres by electrospinning of solutions containing different proportions of polyvinyl butyral

In this paper, the synthesis of continuous mullite (3Al₂O₃·2SiO₂) nanofibres by combination of the sol–gel and electrospinning technique is reported. To find out the optimum viscosity of the electrospinning solution for obtaining the high quality mullite nanofibers, solutions containing different amounts of polyvinyl butyral (PVB, 0–8 wt%) and the precursor sol were prepared for the electrospinning process. The precursor sol was made by using proper amounts of aluminium isopropoxide (AIP), hydrated aluminium nitrate (AN) and tetraethylorthosilicate (TEOS). Crystal phase, microstructure and thermal decomposition behaviour of the electrospun mullite nanofibres were investigated by conventional methods of analysis. The optimal amount of PVB in the electrospinning polymeric solutions was found to be between 4 and 6 wt% and the mullite nanofibres obtained as such were pure, smooth and uniform with diameter sizes of 85–130 nm after calcination at 1200 °C.     

New solvent for polyamides and its application to the electrospinning of polyamides 11 and 12

Polyamides with long hydrocarbon chains, e.g. PA11 and PA12, are generally dissolved in phenolic or fluoric solvents that prevent these polymers from being electrospun and used in many applications because of their high boiling point and/or prohibitive cost. We demonstrate that a mixture of formic acid and dichloromethane can lead to the dissolution of various polyamides enabling their subsequent electrospinning. Nanofibers and nanoribbons of 130 nm and greater in average diameter were obtained and characterized using scanning electron microscopy and Raman spectroscopy.     

A facile method to prepare ZrC nanofibers by electrospinning and pyrolysis of polymeric precursors

Zirconium carbide (ZrC) is one of the most attractive ultra-high temperature ceramics due to its excellent properties. ZrC nanofibers were fabricated via electrospinning and pyrolysis of a novel polymeric precursor, Polyzirconosaal (PZSA), with the addition of polyvinylpyrrolidone (PVP) as the spinning aid. The polymer PZSA was prepared from the chemical reaction between Polyzirconoxane (PZO) and Salicyl alcohol. The as-spun PZSA/PVP fibers were converted to ZrC nanofibers with a diameter ~200 nm after carbothermal reduction at 1300 °C in argon. The obtained ZrC nanofibers maintained its excellent fibrous morphology. The microstructures exhibited that nanoscale ZrC particles dispersed in the fibers containing free carbon. The average crystallite size of ZrC particles using Scherrer method was 42 nm. The obtained ZrC nanofibers were characterized by XRD, SEM and TEM. The current material would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems.     

Electrical properties of Co-doped CaCu3Ti4O12

Co-doped CaCu₃Ti₄O₁₂ samples were synthesized by solid-phase reaction. Electrical properties were studied by impedance spectroscopy in wide temperature (25–450 °C) and frequency (10 Hz–10 MHz) intervals. It was shown that the presence of the copper oxide interlayer significantly reduces the value of the dielectric constant. The amount of impurity copper in the CaCu₃Ti_4-хCo_хO_12-δ samples (x = 0.06; 0.12; 0.24) rise with an increase in the cobalt content. The samples are characterized by a granular microstructure, with an average grain size ranging from 2 to 10 μm. The impedance of the samples was simulated at a temperature of 25 °C and in the range of 100–450 °C. It was found that the samples are characterized by low- and high-frequency polarization. The conductivity activation energy varied from 0.94 to 0.87 eV depending on the cobalt content. The CaCu₃Ti_3.94Co_0.06O_12-δ sample are characterized by the best values of the dielectric permittivity and the dielectric loss tangent, ε = 400 and tanδ = 0.2 (at 1 MHz and room temperature), respectively.     

Improved dielectric,nonlinear and magnetic properties of cobalt-doped CaCu3Ti4O12 ceramics

The dielectric properties and voltage–current nonlinearity of the pure and various cobalt-doped CaCu₃Ti₄O₁₂ (Co-doped CCTO) prepared by solid-state reactions were investigated. The improved dielectric properties in the Co-doped CCTO, with a dielectric constant $\text{ε'}$ ≈7.4?×?10⁴ and dielectric loss $\text{tan}\text{δ}$?≈?0.034, were observed in the sample with a Co doping of 5% (CCTO05) at room temperature and 1?kHz. The related multi-relaxations, RII (?20 to 40?°C) and RIII (100–150?°C), were demonstrated to be a Debye-like relaxation and a Maxwell–Wagner relaxation related to oxygen vacancies. The low dielectric loss of CCTO05 was associated with the high grain boundary resistance and the increase in cation vacancies. The improved nonlinear electrical properties (CCTO05, with nonlinear coefficients $\text{α}$?≈?5.22 and breakdown electric field ${\text{E}}_{\text{b}}$?≈?300.46?V/cm) and the ferromagnetism in Co-doped CCTO were also discussed.     

Dielectric properties of Sm-doped CaCu3Ti4O12 ceramics

The effects of Sm substitution on structure, dielectric properties and conductivity of CaCu₃Ti₄O₁₂ ceramics were investigated. Ca_1?xSm_xCu₃Ti₄O₁₂ (x=0.0%, 0.5%, 1.0%) ceramics were synthesized by the solid-state reaction method. Single phase crystal of the ceramics with space group Im3 was obtained. With increasing Sm content, the dielectric loss of Ca_1?xSm_xCu₃Ti₄O₁₂ ceramics improved but the dielectric constant also decreased significantly, with both the low- and high-temperature dielectric relaxations suppressed.     

CaCu3Ti4O12多孔陶瓷的介电性能

采用固相烧结法在较低温度(＜1000℃)制备了CaCu3 Ti4 O12 (CCTO)多孔陶瓷.研究了淀粉对CCTO陶瓷的气孔率、显微结构、介电性能的影响.随淀粉含量的增加,CCTO陶瓷的气孔率从32.4％大幅度上升到淀粉含量为5％和10％时的41.3％和48.6％;阻抗明显增大,介电常数和介电损耗明显降低.通过这些结构和性能的变化,分析讨论了CCTO的巨介电机理.     

Effect of Y doping on transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

In this study, La_0.8-xY_xSr_0.2MnO₃ (LYSMO) polycrystalline ceramics were prepared by means of sol-gel technique using methanol as solvent. X-ray diffraction (XRD) showed all samples to possess standard perovskite structure. Scanning electron microscopy (SEM) revealed samples with high compactness and grain size from 27.80 to 29.73 μm. Resistivity–temperature tests indicated sharp metal-insulator transition behavior of all samples accompanied by rapid transformation from ferromagnetism to paramagnetism (FM-PM). As Y³⁺ doping amounts rose, radius of A-site ions decreased, metal-insulator transition temperature (T_p) of polycrystalline samples shifted to lower temperatures, and resistivity increased. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by introduction of Y³⁺. At x = 0.06, peak TCR and peak MR reached 4.85% K⁻¹ and 52.34%, respectively. Using double exchange (DE) interaction mechanism, electric transport performances of as-prepared ceramics were explained. These findings look promising for future applications of LYSMO materials in magnetic devices and infrared detectors.     

Electrospinning synthesis and negative thermal expansion of one-dimensional Sc2Mo3O12 nanofibers

Orthorhombic Sc₂(MoO₄)₃ nanofibers have been prepared by ethylene glycol assisted electrospinning method. The effects of annealing temperature, precursor concentration, spinning distance and solvent on the preparation of Sc₂(MoO₄)₃ nanofibers were characterized by XRD, SEM, HRTEM, EDX and high-temperature XRD. XRD analysis shows as-prepared nanofibers are amorphous. Orthorhombic Sc₂(MoO₄)₃ nanofibers can be fabricated after annealing at different temperatures in 500–800 °C for 2 h. The crystallinity of Sc₂(MoO₄)₃ nanofibers improves and the nanofiber diameter decreases gradually as the annealing temperature increases. However, the nanofiber structure was destroyed at the annealing temperature above 700 °C. Higher precursor concentration results in a slight increase of diameter and decrease in destroying temperature of Sc₂(MoO₄)₃ nanofibers. Spinning distance also affects the diameter of nanofibers, and the nanofiber diameter decreases as the distance increases. One-dimensional orthorhombic Sc₂(MoO₄)₃ nanofibers exhibit anisotropic negative thermal expansion. In 25–700 °C, the coefficients of thermal expansion (CTE) of α_a, α_b and α_c are ?5.81 × 10^?6 °C^?1, 4.80 × 10^?6 °C^?1 and -4.33 × 10^?6 °C^?1, and the α_l of Sc₂(MoO₄)₃ nanofibers is ?1.83 × 10^?6 °C^?1.     

Effect of calcium stoichiometry on the dielectric response of CaCu3Ti4O12 ceramics

Ca_xCu₃Ti₄O₁₂ (x = 0.90, 0.97, 1.0, 1.1 and 1.15) polycrystalline powders with variation in calcium content were prepared via the oxalate precursor route. The structural, morphological and dielectric properties of the ceramics fabricated using these powders were studied using X-ray diffraction, scanning electron microscope along with energy dispersive X-ray analysis, transmission electron microscopy, electron spin resonance (ESR) spectroscopy and impedance analyzer. The X-ray diffraction patterns obtained for the x = 0.97, 1.0 and 1.1 powdered ceramics could be indexed to a body-centered cubic perovskite related structure associated with the space group Im3. The ESR studies confirmed the absence of oxygen vacancies in the ceramics that were prepared using the oxalate precursor route. The dielectric properties of these suggest that the calcium deficient sample (x = 0.97) has a reduced dielectric loss while retaining the high dielectric constant which is of significant industrial relevance.     

Effects of parameters on nanofiber diameter determined from electrospinning model

In this paper the effects of 13 material and operating parameters on electrospun fiber diameters are determined by varying the parameter values in an electrospinning theoretical model. The complexity of the electrospinning process makes empirical determination of the effects of parameters very difficult. The results show that the five parameters (volumetric charge density, distance from nozzle to collector, initial jet/orifice radius, relaxation time, and viscosity) have the most significant effect on the jet radius. The other parameters (initial polymer concentration, solution density, electric potential, perturbation frequency, and solvent vapor pressure) have moderate effects on the jet radius. Parameters relative humidity, surface tension, and vapor diffusivity have minor effects on the jet radius. Knowing the relative effects of parameters on jet radius should be useful for process control and prediction of electrospun fiber production.     

Electrospinning,optical properties and white LED applications of one-dimensional CaAl12O19:Mn4+ nanofiber phosphors

Non-rare-earth, red-emitting CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphors have been successfully prepared by an electrospinning technique followed by an annealing process. The as-prepared precursor fibers have smooth surfaces with an average diameter of 5 µm. After annealing at high temperature, the diameter of the fibers gradually reduces due to the decomposition of the organic polymers. The photoluminescence and crystalline properties of the fibers were investigated as a function of Mn⁴⁺ concentration and the annealing temperature. Under ultraviolet and blue light excitation, CaAl₁₂O₁₉:Mn⁴⁺ exhibits a characteristic red emission at 655 nm with three satellite peaks due to the ²E→⁴A₂ transition of Mn⁴⁺. The highest PL intensity is achieved at a 0.5% Mn⁴⁺ concentration and a firing temperature of 1400 °C. In comparison to CaAl₁₂O₁₉:Mn⁴⁺ prepared by a usual solid-state reaction, the luminescence of the as-prepared nanofiber phosphors in the present work has been strongly enhanced by optimizing the morphology and improving the crystallinity and phase purity. The absorption band in the blue region and a bright emission in the red region make the CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphor a candidate for achieving high color rendering in YAG:Ce-based WLEDs. A warm WLED with a high CRI of 88.5 at a CCT of 4553 K has been successfully achieved by coating YAG:Ce with CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphors on blue InGaN chips.     

Effect of tethering chemistry of cationic surfactants on clay exfoliation, electrospinning and diameter of PMMA/clay nanocomposite fibers

We examine the influence of tethering chemistry of cationic surfactants on exfoliation of montmorillonite (MMT) clay dispersed in methyl methacrylate (MMA) followed by in-situ polymerization to form poly(methyl methacrylate) (PMMA) nanocomposites, the effect of exfoliation and clay loading on the rheology of polymer/clay dispersions in dimethyl formamide, and the diameters of nanocomposite fibers formed from these dispersions by electrospinning. Incorporation of an additional reactive tethering group of methacryl functionality significantly improves the intercalation and exfoliation of clays in both in-situ polymerized PMMA nanocomposites and the corresponding electrospun fibers. The proper surfactant chemistry also increases the dispersion stability, extensional viscosity, extent of strain hardening and thus the electrospinnablity of the nanocomposite dispersions, especially at low nanocomposite concentrations. The degree of the enhancement in electrospinnability by clays with proper tethering chemistry is at least the same as or greater than that obtained with three times higher loading level of clay particles without proper tethering chemistry in the nanocomposites. These results suggest a new strategy to produce smaller diameter fibers from very dilute polymer solutions, which are otherwise not electrospinnable, by incorporating a small amount of well-exfoliated clays.     

Dielectric properties of CaCu3Ti4O12 based multiphased ceramics

A “soft chemistry” method, the coprecipitation, has been used to synthesize the perovskite CaCu₃Ti₄O₁₂ (CCT). Three main types of materials were obtained for both powders and sintered ceramics: a monophased consisting of the pure CCT phase, a biphased (CCT + CaTiO₃), and a three-phased (CCT + CaTiO₃ + copper oxide (CuO or Cu₂O)). These ceramics, sintered at low temperature, 1050 °C, present original dielectric properties. The relative permittivity determined in the temperature range (−150 < T < 250 °C) is significantly higher than the one reported in the literature. Internal barrier layer capacitor is the probable mechanism to explain the particular behaviour. Moreover, the presence of a copper oxide phase beside the perovksite CCT plays an important role for enhancing the dielectric properties.     

Effect of cesium and cerium substitution on the dielectric properties of CaCu3Ti4O12 ceramics

In this study, CaCu₃Ti₄O₁₂ (CCTO) ceramics were doped with cesium and cerium atoms to possibly improve the electrical properties of these widely used ceramics. In all cases, pure phase perovskites were produced where cesium doping enhanced the grain growth and cerium doping produced grain growth inhibition. The cesium doping showed an improvement in loss tangent performance, in contrast to the cerium doping which showed a negative result. A high dielectric constant >15,000 with a dielectric loss lower than 0.06 was observed for cesium 2.0 mol% doped at high frequencies. These results were related to the change in microstructure and the properties of grain boundary after doping.     

Photoluminescence and energy transfer of Lu3Al5O12:Ce3+, Pr3+ phosphors

Ce³⁺ and Pr³⁺ co?doped Lu₃Al₅O₁₂ phosphors were synthesized by the sol–gel process, and their crystal structure, photoluminescence (PL) properties, and energy transfer (ET) from the Ce³⁺ to Pr³⁺ were studied. The Lu_2.94?yAl₅O₁₂:0.06Ce³⁺, yPr³⁺ phosphors (0.002 ≤ y ≤ 0.008) showed the green?yellow emission from the ²D_3/2 → ²F_{5/2, 7/2} transition of Ce³⁺ and the red emission at 610 and 637 nm, which were caused by the ¹D₂→³H₄ and ³P₀→³H₅ transitions of Pr³⁺, respectively. The optimal concentration of Pr³⁺ for efficient ET was found to be x = 0.006. The electric quadrupole?quadrupole interaction was responsible for the concentration quenching in the Lu_2.94?yAl₅O₁₂:0.06Ce³⁺, yPr³⁺ phosphors, based on Dexter's theory. The incorporation of Pr³⁺ for Lu³⁺ enhanced the red PL intensity in the Lu_2.94Al₅O₁₂:0.06Ce³⁺ phosphor.     

Enhancement of giant dielectric response in Ga-doped CaCu3Ti4O12 ceramics

The influences of Ga³⁺ doping ions on the microstructure, dielectric and electrical properties of CaCu₃Ti₄O₁₂ ceramics were investigated systematically. Addition of Ga³⁺ ions can cause a great increase in the mean grain size of CaCu₃Ti₄O₁₂ ceramics. This is ascribed to the ability of Ga³⁺ doping to enhance grain boundary mobility. Doping CaCu₃Ti₄O₁₂ with 0.25 mol% of Ga³⁺ caused a large increase in its dielectric constant from 5439 to 31,331. The loss tangent decreased from 0.153 to 0.044. The giant dielectric response and dielectric relaxation behavior can be well described by the internal barrier layer capacitor model based on Maxwell?Wagner polarization at grain boundaries. The nonlinear coefficient, breakdown field, and electrostatic potential barrier at grain boundaries decreased with increasing Ga³⁺ content. Our results demonstrated the importance of ceramic microstructure and electrical responses of grain and grain boundaries in controlling the giant dielectric response and dielectric relaxation behavior of CaCu₃Ti₄O₁₂ ceramics.     

溶胶-凝胶法制备巨介电常数材料CaCu3Ti4O12

通过溶胶-凝胶法制备CaCu3Ti4O12干凝胶,再经700～900℃,6～10h预烧和950～1 100℃,16～20h烧结,成功制备了CaCu3Ti4O12粉体和CaCu3Ti4O12巨介电常数陶瓷材料.用X射线衍射、扫描电镜分别确定了样品的结晶性能和形貌.用阻抗分析仪在10～106Hz范围内测试了陶瓷样品的介电性能.结果表明:粉体的结晶性能与煅烧温度有关,陶瓷介电性能与其晶粒大小有关.相对于传统固相反应合成法制备的粉体和陶瓷,粉体的预烧和陶瓷的烧结温度都有明显降低,烧成温度至少降低100℃.在800℃预烧的CaCu3Ti4O12粉体并在1 100℃温度下烧结制备的陶瓷,其介电常数可达194753.