Synthesis of nanocrystalline gadolinium doped ceria via sol–gel combustion and sol–gel synthesis routes

Gadolinium doped ceria (GDC) has been investigated as a promising material for application as an electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFC). In this work, 10GDC powders (Gd_0.1Ce_0.9O_1.95) were synthesized by sol–gel combustion and sol–gel synthesis routes using the same complexing agents in both procedures. The thermal behavior of Gd–Ce–O precursor gels was investigated by TG–DSC measurements. X-ray diffraction (XRD) analysis was used for the characterization of phase purity and crystallinity of synthesized samples. Scanning electron microscopy (SEM) was employed for the estimation of surface morphological features. Nitrogen adsorption–desorption (BET model) was used for evaluation of specific surface area. The surface composition was determined by X-ray photoelectron spectroscopy (XPS). Electrical properties of synthesized ceramic samples were studied by means of impedance spectroscopy.     

Synthesis and characterisation of self-assembled SiC nanowires and nanoribbons by using sol–gel carbothermal reduction

ABSTRACT

Hexagonal-shaped 3C-SiC nanowires were grafted onto SiC nanoribbons by a sol–gel technique using ferrocene as catalyst. The nanowire diameter (～200?nm) and the nanoribbon width–thickness ratio (20:1) are uniform along their entire length. Their length is about several tens to several hundreds of micrometres. Meanwhile, single SiC nanostructure (nanowire or nanoribbon) was obtained by adjusting temperature field. A novel cooperative growth mechanism of vapour–liquid–solid and vapour–solid was proposed for the self-assembled SiC nanostructure. The self-assembled SiC nanowires and nanoribbons exhibit two strong broad photoluminescence peaks at wavelengths of about 373 and 471?nm, which are significantly shifted to the blue compared with the reported luminescence of SiC nanowires. This study will pave a way for the controllable synthesis of SiC nanowires and nanoribbons, and provide a simple method to connect them together firmly as potential applications for nanodevices in future.     

Synthesis and microwave absorption properties of Fe–C nanofibers by electrospinning with disperse Fe nanoparticles parceled by carbon

The Fe–C nanofibers were achieved using electrospinning technique. The microstructure was characterized by field emission scanning electron microscope and high resolution transmission electron microscopy equipped with energy-dispersive X-ray analysis. The results indicated that magnetic Fe nanoparticles uniformly dispersed along nanofibers and were parceled by carbon matrix. For the Fe–C nanofibers/paraffin composite, a minimum reflection loss (RL) value of −44 dB was observed at 4.2 GHz. Moreover, the frequency range with RL peak value below −10 dB was achieved in a wide frequency range from 2.2 to 13.2 GHz. The excellent microwave absorption properties were due to the combination of complex permeability and permittivity resulting from magnetic Fe particles and lightweight carbon.     

Synthesis and electrochemical properties of LiV3O8 via an improved sol–gel process

LiV₃O₈ cathode material was synthesized via a hydrothermal improved sol–gel process using LiOH, NH₄VO₃ and oxalic acid as raw materials. The thermal decomposition process of the as-prepared LiV₃O₈ precursor was investigated by thermogravimetric (TG) and differential scanning calorimetry (DSC). The structure, morphology and electrochemical performance of the as-synthesized LiV₃O₈ samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) and the galvanostatic charge–discharge test. The effects of synthesis conditions on phases, structure and electrochemical performance of the LiV₃O₈ samples were particularly discussed. Result shows that pure LiV₃O₈ sample can be obtained at 300 °C, which is much lower than that of normal citric assisted sol–gel method. The sample synthesized at 350 °C exhibits the best electrochemical performance, which can present an initial discharge capacity of 301.1 mAh/g at a current density of 50 mA/g and maintain 271.6 mA/g (about 90.2% of its initial value) after 10 cycles.     

High-temperature properties of 2.5D SiO2f/SiO2 composites by sol–gel

2.5D SiO_2f/SiO₂ composites were fabricated by sol–gel process. The mechanical and fracture behavior of SiO_2f/SiO₂ composites under higher temperature were discussed. The oxidation behavior at 1200 °C and 1500 °C was investigated. The results showed that SiO_2f/SiO₂ composites had high flexural strength, and the fracture mechanism was a combination of brittle and ductile fracture. After higher temperature oxidation, the fracture mechanism changed to typical brittle/sudden fracture. For long time usage at higher temperature, it was necessary to stabilize SiO₂ fibers and SiO₂ matrix of SiO_2f/SiO₂ composites.     

Preparation and properties of sol–gel synthesized Mg-substituted Ni2Y hexagonal ferrites

A series of single phased Y-type hexagonal ferrites Sr₂Ni_2?xMg_xFe₁₂O₂₂ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by the sol–gel auto combustion method. The effects on structural, magnetic and electrical properties have been investigated by substituting Mg²⁺ at Ni²⁺ sites. The X-ray diffraction (XRD) patterns confirm single phase Y-type hexaferrite and various parameters such as lattice constants, cell volume, X-ray density, bulk density and porosity have been calculated from XRD data. The Fourier transform infrared (FTIR) spectra show the characteristics absorption ferrite peaks of the sintered sample. The microstructure was studied by scanning electron microscopy (SEM). All the ferrites show a hexagonal platelet-like shape which is a most suitable shape for microwave absorption. The dielectric constant followed the Maxwell–Wagner interfacial polarization and relaxation peaks were observed in the dielectric loss properties. The room temperature dc electrical resistivity and activation energy were found to decrease for samples x=0.1, 0.2 and increase for the rest of samples hence making these materials suitable for multilayer chip inductors (MLCIs). A soft magnetic behavior was revealed by M–H loops. Saturation magnetization (M_s), retentivity (M_r), coercivity (H_c) and magnetic moment (n_B) were found to decrease as the Mg²⁺ contents increased.     

Phase evolution and electrical properties of lead zirconate titanate thin films grown by using a triol sol–gel route

Lead zirconate titanate (PZT) precursor sols were prepared using a triol based sol–gel route. Inorganics salts metal alkoxides lead acetate trihydrate [Pb(OOCCH₃)₂·3H₂O], titanium (IV) isopropoxide [Ti(OCH(CH₃)₂)₄], and zirconium n-propoxide [ZrOC₃H7)₄] were used as starting materials. Thin films were deposited by spin coating onto Pt/Ti/SiO₂/Si substrates. The samples were pre-heated (pyrolysis) on a calibrated hotplate over the temperature range of 200–400 °C for 10 min then firing at a temperature of 600 °C for 30 min. Randomly-oriented PZT thin films pre-heated at 400 °C for 10 min and annealed at 600 °C for 30 min showed well-defined ferroelectric hysteresis loops with a remanent polarization of 27 μC/cm² and a coercive field of 115 kV/cm. The dielectric constant and dielectric loss of the PZT films were 621 and 0.040, respectively. The microstructures of the thin films are dense, crack-free and homogeneous with fine grains about 15–20 nm in size.     

Preparation,characterization and photocatalytic activity of a novel nanostructure composite film derived from nanopowder TiO2 and sol–gel process using organic dispersant

A novel sol–gel-derived titanium dioxide nanostructure composite has been prepared by spin-coating and investigated for the purpose of producing films. The processing of the composite sol–gel photocatalysts involved utilizing of precalcinated nanopowder titanium dioxide as filler mixed with sol and heat treated. The sol solution was prepared by adding titanium tetra isopropoxide (Ti(OPr)₄ or TTP) to a mixture of ethanol and HCl 35.5% (mole ratio TTP:HCl:EtOH:H₂O = 1:1.1:10:10), then a solution of 2 wt% methylcellulose was added and stirred at room temperature. Precalcinated TiO₂ nanopowder was dispersed in the sol and the prepared mixture was deposited on the microscope glass slide by spin-coating. The inhomogeneity problem in preparation of composite film which causes peeling off and cracking after calcination due to the shrinkage of the films with thermal treatment were overcome by using methylcellulose (MC) as a dispersant. The composite heat treated at approximately 500 °C has the greatest hardness value. Surface morphology of composite deposits by scanning electron microscopy (SEM) showed remarkable increase in the composite surface area. Evaluation of the adhesion and bonding strength between the coating and substrate was carried out by the scratch test technique. The minimum load which caused the complete coating removal, for composite thick film was 200 g/mm² which indicates a strong bond to the substrate. Photocatalytic activity of the composite film was evaluated through the degradation of a textile dye, Light Yellow X6G (C.I. Reactive Yellow 2) as a model pollutant and were compared with those of similar composite thick film without MC, thin film of TiO₂ and TiO₂ nanopowder. The results show that the photocatalytic activity and stability of the composite films are higher than those of nanopowder TiO₂. However, a remarkable increase in the composite surface and good mechanical integrity make this composite film a viable alternative for commercial applications.     

Synthesis and modification of silica-based epoxy nanocomposites with different sol–gel process enhanced thermal and mechanical properties

This research article describes the results of nano-silica composites filled with different epoxy contents containing nano-SiO₂ particles from (5–25 wt%). Reinforcing hybrid composites enhance thermal and mechanical properties to achieve vital and sustainable products. Silica-based nanocomposites with high purity were prepared and used for the surface modification of nanosized silica particles. The surface structure's composition and physical properties of modified nano-SiO₂ particles were characterized through Fourier transferred infrared spectrometer, X-ray photoelectron spectroscopy, thermogravimetric analyzer, and scanning electron microscopic. Silica-based nanocomposites were prepared by incorporating of modified nano-SiO₂ as an enhancing filler. The morphology of fracture surface and dynamic mechanical properties were investigated. Results showed that the silica-based epoxy nanocomposites are bearing a long chain structure that could improve the compatibility of silica nanocomposites with epoxy resin and contribute to a better dispersion state in the matrix, which enhanced the overall performance of epoxy-cured products.     

Influence of sol–gel derived ZrO2 and ZrC additions on microstructure and properties of ZrB2 composites

ZrB₂ powder was coated with 5% ZrOC sol–gel precursor and sintered by SPS. Relative densities >98% were achieved at 1800 °C with minimal grain growth and an intergranular phase of ZrC. Carbon content in the precursor determined the type of reinforcing phase and porosity of the sintered composites. XRD, SEM and EDS studies indicated that carbon deficiency resulted in ZrO₂ retention, improving ZrB₂ densification with oxide particle reinforcement. Excess carbon resulted in ZrC formation as the reinforcing phase, but could yield porosity and residual carbon at grain boundaries. These two types of ZrB₂ composites displayed different densification and microstructural evolution that explain their contrasting properties. In the extreme oxidative environment of oxyacetylene ablation, the composites with ZrC-C maintained superior leading edge geometry; whereas for mechanical strength, a bias towards the residual ZrO₂ content was beneficial. This highlighted the sensitivity of processing carbon-precursors in the initial sol–gel process and the carbon content in ZrB₂-based composite systems.     

Synthesis of mesoporous SiO2–ZrO2 mixed oxides by sol–gel method

A series of SiO₂–ZrO₂ mixed oxides were prepared by sol–gel method in the presence of directing agent, with variable amounts of ZrO₂ between pure silica and pure zirconia, with the aim to obtain catalytic materials suitable as solid acid catalysts. SiO₂–ZrO₂ mixed oxides differ from the two pure starting oxides. While SiO₂ has a low OH density without peculiar acid character, the introduction of increasing amounts of Zr increases the density of the acid sites in the materials. Furthermore both SiO₂/ZrO₂ molar ratio and drying procedure are able to influence the physico-chemical characteristics (textural properties, acid sites distribution, etc.) of these mixed oxides.     

Entrap-immobilization of invertase on composite gel fiber of cellulose acetate and zirconium alkoxide by sol–gel process

We prepared a composite gel fiber by the gel formation of cellulose acetate and zirconium tetra-n-butoxide. Gel fiber is stable in common solvents, phosphate solution, and electrolyte solution. Invertase was entrap-immobilized on the gel fiber. The immobilization was easily performed under the mild conditions. The apparent Michaelis constant (K_m) and maximum reaction velocity (V_max) were estimated from Eadie–Hofstee plot for immobilized invertase. The K_m of immobilized invertase was larger than that of native invertase, while the opposite tendency was observed for the V_max. The activity for the immobilized invertase became higher with increasing fiber diameter. It indicates that the hydrolysis of sucrose occurs in the neighborhood of the fiber surface. The thermal stability of the immobilized invertase was higher than those of its native counterpart. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2084–2088, 2001     

Synthesis and characterization of mesoporous ZnTiO3 rods via a polyvinylpyrrolidone assisted sol–gel method

Mesoporous ZnTiO₃ rods were fabricated via a polyvinylpyrrolidone assisted sol–gel method. In this method, the control of nanostructure growth was achieved by the cooperative assembly among precursors and polyvinylpyrrolidone, through which well-designed one-dimensional morphology and mesoporosity could be obtained. The regularity of rod-like morphologies was sensitive to cooperative assembly temperature. Furthermore, the mesoporous ZnTiO₃ rods were used for photodegradation of organic dyes and proved to be useful photocatalysts with excellent reusability thanks to the well-designed nanostructure and one-dimensional structure. Hence mesoporous ZnTiO₃ rods with good photocatalytic activity and low cost could offer broad opportunities for environmental remediation.     

Preparation of nano-AlN powder by sol–gel foaming and its sintering properties

Uniformly dispersed nano-sized aluminum nitride powders were prepared by the sol–gel foaming method using aluminum nitrate as the aluminum source, sucrose as the carbon source, and ammonium chloride as the foaming agent. The effects of ammonium chloride content on the particle size and the sintering properties of aluminum nitride were investigated. The results showed that when the molar ratio of ammonium chloride to aluminum nitrate was .5, the colloidal foams were uniform, large, and fluffy, and amorphous alumina precursors with uniform particles could be prepared. Aluminum nitride powder with a particle size of 22–27 nm can be obtained by calcining these precursors in nitrogen atmosphere at 1400°C for 2 h. At the same time, aluminum nitride bulk material with a relative density of 95% can be obtained by sintering the compact samples in nitrogen atmosphere at 1700°C for 2 h.     

In-situ sol–gel synthesis of zirconia networks in flexible and conductive composite films

The conductive and stretchable films with improved hardness are suitable for the fabrication of flexible electronic devices. This study describes the sol–gel technique to prepare a novel conductive and flexible film consisting of epoxidized natural rubber (ENR), doped polyaniline (PD) and zirconia. The zirconia networks were directly synthesized in-situ in ENR/PD solution and flexible conductive composite film with improved hardness was obtained. The morphology study revealed the size of PD decreased significantly from 77.89 ± 43.95 nm to 4.32 ± 1.13 nm and highest electrical conductivity of 1.9 × 10⁻³ S/cm was achieved with 10 wt.% percolation threshold of zirconia precursor. The binding energy of Zr3d_5/2 and Zr3d_3/2 decreased, suggesting that zirconia was converted to the lower oxidation state. Furthermore, the shape of PD changed from spherical to rod-like structure with root mean square value of 2 nm, while the hardness and reduced modulus improved to 1.72 MPa and 36.7 MPa, respectively.     

Synthesis of ultra-fine tantalum carbide powders by a combinational method of sol–gel and spark plasma sintering

Tantalum carbide (TaC) nanopowders were synthesized by a novel method combining the sol–gel and spark plasma sintering (SPS) processes using tantalum pentachloride (TaCl₅) and phenolic resin as the sources of tantalum (Ta) and carbon (C), respectively. Gels of Ta-containing chelate with good uniformity and high stability were prepared by solution-based processing. The products with the structure of carbon-coated tantalum pentoxide (Ta₂O₅) were obtained after pyrolysis at 800?°C. Further heat treatment by SPS resulted in the fast formation of TaC at a relatively low temperature. The effects of the C/Ta molar ratio in the raw materials and the heat treatment temperature on the prepared powders were investigated. With increase in the C/Ta molar ratio from 3.75 to 4.25, the synthesis temperature, oxygen content and average crystallite size of the TaC powders decreased. Furthermore, the oxygen content of the powders prepared at the C/Ta molar ratio of 4.25 could be reduce by increasing the heat treatment temperature from 1400° to 1600°C, which unfortunately also induced a mean crystallite size increase from 30 to 100?nm. The TaC powders obtained at a comparatively low C/Ta molar ratios of 4.25 at 1500?°C had an average particle size of about 50?nm and a low oxygen content of about 0.43?wt%.     

Processing and properties of sol gel derived alumina–carbon nano tube composites

High purity alumina–carbon nano tube (CNT) composites were prepared by an aqueous sol–gel processing route. CNTs were dispersed in alumina sol containing appropriate amount of MgO precursor. Aqueous slurry of alumina was seeded into the sol followed by gelation, drying and calcination at 1000 °C for 1 h. The calcined powder consisting of alumina-coated CNTs and alumina was milled, sieved, dried, pressed and pressureless sintered at 1400–1600 °C for 1 h in nitrogen atmosphere. Sintered samples were further isostatically hot pressed at 1300 °C and the properties were compared with the pressureless sintered samples. Phase formation was followed by XRD study, CNT retention was confirmed by Raman studies and the samples were further characterized for mechanical and microstructural properties.     

Preparation and characterization of ultrafine ZrB2–SiC composite powders by a combined sol–gel and microwave boro/carbothermal reduction method

A combined sol–gel and microwave boro/carbothermal reduction technique was investigated and used to synthesize ultrafine ZrB₂–SiC composite powders from raw starting materials of zirconium oxychloride, boric acid, tetraethoxysilane and glucose. The effects of reaction temperature, molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) on the synthesis of ultrafine ZrB₂–SiC composite powders were studied. The results showed that the optimum molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) for the preparation of phase pure ultrafine ZrB₂–SiC composite powders were 2.5 and 8.0, respectively, and the firing temperature required was 1300 °C. This temperature was 200 °C lower than that require by using the conventional boro/carbothermal reduction method. Microstructures and phase morphologies of as-prepared ultrafine ZrB₂–SiC composite powders were examined by field emission-scanning electron microscopy (FE-SEM) and transmission electron microscope (TEM), showing that SiC grains were formed evenly among the ZrB₂ grains, and the grain sizes of ZrB₂ in the samples prepared at 1300 °C for 3 h were about 1–2 μm. The average crystalline sizes of these two phases in the as-prepared samples were calculated by using the Scherrer equation as about 58 and 27 nm, respectively.     

Synthesis and characterization of boehmitic alumina coated graphite by sol–gel method

Non-wettability property makes graphite a good protecting material against the molten metal and/or slag. Properties like high oxidation potential between 600 and 1200 °C and non-wettability with water at room temperatures limits the usage of graphite in castable refractory applications. In this study, sol–gel method, which is a relatively cheaper process, was used. Boehmitic sol was obtained by hydrolyzing and peptiziting the alkoxide AIP (aluminum isopropoxide) used as alumina source. Then natural flake graphite was mixed with the boehmitic solution for coating of graphite. At 120 °C boehmitic sol coated graphite was dried and gelled. Then heat threaded at 550 °C for γ-Al₂O₃ transformation of boehmite. Products that obtained from the studies were characterized with FTIR and XRD tests. Alumina coated graphite samples were made by repeating the same steps and TG analysis were made to investigate the oxidation behaviour of the samples. Finally, SEM–EDS analyses were carried out to investigate the microscopic properties of the alumina coated graphite powders.     

Synthesis and microwave dielectric properties of CaSiO3 nanopowder by the sol–gel process

The synthesis and microwave dielectric properties of CaSiO₃ nanopowder by sol–gel method have been investigated in this paper. CaSiO₃ nanoparticles with an average grain size of 50–60 nm were obtained by calcining the CaO–SiO₂ xerogel that was prepared from Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and tetraethylortho silicate (TEOS). Calcining the CaO–SiO₂ xerogel at 1150 °C, the pseudowollastonite-CaSiO₃ phase was completely formed. However, the main phase is not CaSiO₃ or CaSi₂O₄ but SiO₂ when calcining the mixture of SiO₂ and CaCO₃ at 1150 °C. Comparing with CaO–SiO₂ ceramics prepared by solid-state process, the CaSiO₃ ceramics made from nanopowders calcined at 1000 °C achieved more compact structure at the sintering temperature of 1320 °C, and then had excellent microwave dielectric properties: ?_r = 6.69, Qf = 25398 GHz.