Preparation,microstructure, and improved dielectric and nonlinear electrical properties of Na1/2La1/2Cu3Ti4O12 ceramics by sol–gel method

Na_1/2La_1/2Cu₃Ti₄O₁₂ (NLCTO) ceramics were prepared by the sol–gel method (SG) and solid-state method (SS), and the effect of sintered temperature on microstructure and dielectric and nonlinear electrical properties has investigated. The experimental results indicated that NLCTO-SG ceramics sintered at 1090 °C for 10 h showed larger grain size, higher density and more homogeneous microstructure, especially higher dielectric constant up to ca 1.3–1.8 × 10⁴, lower dielectric loss of about 0.057 and better temperature stability than NLCTO-SS counterparts. However, the good low-voltage varistor behavior and the third kind of dielectric relaxation behavior could not be observed from the NLCTO-SS ceramics. The higher dielectric constant of the NLCTO-SG ceramics may be due to the stronger internal barrier layer capacitor (IBLC) effect. Two values of grain activation energy were obtained above room temperature for the first time. The transition temperature of two values was basically coincident with that of sudden changes of the dielectric properties.     

Preparation process,microstructure and dielectric properties of Na0.5La0.5Cu3Ti4O12 ceramics by a sol–gel method

Effect of concentration, water content (molar ratio of the water and titanium) and pH value of the sol, and sintering temperatures and holding time on microstructure and dielectric properties of Na_0.5La_0.5Cu₃Ti₄O₁₂ (NLCTO) ceramics by a sol–gel method were investigated in detail, respectively. It is found that the optimum concentration, the molar ratio of the water and titanium, and pH value of the sol were 1.00 mol/L, 11.0, and 0.3, respectively. The NLCTO ceramics sintered at 1,080 °C for 10 h exhibited more homogeneous microstructure, higher dielectric constant (about 1.1–1.8 × 10⁴) and lower dielectric loss (about 0.051–0.064 at 1–10 kHz). The higher dielectric constant of the NLCTO ceramics might be due to the internal barrier layer capacitor effect. The NLCTO ceramics prepared by the sol–gel method showed two kinds of dielectric relaxation at higher temperature by electric modulus analysis, and two relaxation activation energy values were obtained.     

Synthesis and characterization of NiO nanoparticles by sol–gel method

Nickel oxide nanoparticles have been synthesized in the presence of agarose polysaccharide by sol–gel method. The structure, morphology, optical and magnetic properties of the product was examined by X-ray diffraction, transmission electron microscopy, UV–visible spectrophotometer and superconducting quantum interference device magnetometer. The result of thermogravimetric analysis of the precursor product showed that the proper calcination temperature was 400 °C. X-ray diffraction result revealed that the obtained product was nickel oxide with face-centered cubic structure. TEM image demonstrated that the nickel oxide nanoparticles have spherical shape with size around 3 nm. Analysis of FTIR spectra confirmed the composition of product. The optical absorption band gap of the NiO nanoparticles was estimated to be 3.51 eV. Magnetic measurement showed that the nickel oxide nanoparticles exhibit superparamagnetic behavior at 300 K. Moreover, the nanoparticles show ferromagnetic interactions at 4.2 K owing to the existence of uncompensated moments on the surface of the nanoparticles.     

Microstructure and ferroelectric properties of compositionally graded Nd-doped Bi4Ti3O12 thin films prepared by sol–gel method

The compositionally graded Bi_4?xNd_xTi₃O₁₂ (BNT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by sol–gel method. Their microstructure, ferroelectric and dielectric properties were investigated. The single-phase upgraded and downgraded BNT films were obtained with (117) preferred orientation. Compared to the homogeneous BNT films prepared by the same conditions, the remanent polarization (P _r) and permittivity (ε _r) of compositionally graded BNT films were significantly enhanced. The upgraded BNT film showed larger 2P _r (34.9?μC/cm²) and ε _r (509), and those of downgraded BNT film were 29.4?μC/cm² and 505. Bi element in the downgraded BNT film accumulated near the interface of film/Pt bottom electrode, which deteriorated the compositional gradient and resulted in decreasing 2P _r and ε _r compared to the upgraded BNT film.     

Synthesis, characterization and electrical properties of nanostructured LaAlO3 by sol–gel auto combustion method

Semiconductive nanostructured LaAlO₃ was synthesized by sol–gel auto combustion method using inorganic reagents as La (NO₃)₃·6H₂O, Al (NO₃)₃·9H₂O and glycine as a fuel. Structural characterization of prepared sample was characterized by X-ray diffraction technique (XRD), and Fourier transforms infrared spectroscopy (FTIR). The morphology of the particle was found by scanning electron microscopy while elemental composition by energy dispersive X-ray spectroscopy and transmission electron microscopy (TEM) was used to study the nanostructure of the material. XRD pattern reveals the formation of single phase orthorhombic structure without any other phase formation. The system LaAlO₃ show porous morphology. Particle size obtained from TEM analysis was found to be ~40 nm. The FTIR spectra showed the characteristic of two strong absorption bands at 656 and 442 cm^?1 corresponding to metal–oxygen bond vibrations for the perovskite structure compound. Superhydrophilic nature of LaAlO₃ was investigated at room temperature by sessile drop technique. The electrical properties of the compound showed that LaAlO₃ exhibits conducting behavior.     

Structural characterization and frequency response of sol–gel derived Bi3/2MgNb3/2O7 thin films

The Bi_3/2MgNb_3/2O₇ (BMN) thin films were prepared via a modified sol–gel process on glass substrates at various post-annealing temperatures. The crystalline structure, morphology and frequency response have been investigated systematically. The X-ray diffraction results indicated that the BMN thin films had different orientations depending on post-annealing temperature. Thin films annealed above 650 °C presented well crystallized cubic pyrochlore structure with (222) orientation, and (400) preferentially oriented were observed when they were annealed below 600 °C. The surface morphology images of the BMN thin films revealed different grain size and grain size distribution, and the average grain size increased from 28.3 to 37.0 nm as the post-annealing temperature increasing. The low frequency dielectric properties of the BMN thin films were closely correlated with the (222) orientation, which was favorable to enhanced dielectric constant and tunability. The high-frequency optical measurements revealed an average transmittance (T _av ) varying between 76.6 and 82.2 % and band gap energy (E _g ) ranging from 3.40 to 3.44 as a function of the temperature and the crystallite size. Thin film annealed at 700 °C possessed the best crystallinity and highest (222) orientation, and showed the best electrical properties, with a dielectric constant of 105 at 1 MHz, dielectric tunability of 25.8 %, and an average optical transmittance of 82.2 % in the visible range (400–800 nm), making it promising for optical/electronic tunable devices applications.     

Synthesis and characterization of pyrochlore-type yttrium titanate nanoparticles by modified sol–gel method

Pyrochlore-type yttrium titanate (Y₂Ti₂O₇) nanoparticles were successfully synthesized by a simple soft-chemistry technique viz. citric acid sol–gel method (CAM). The preparation process was monitored by X-ray diffraction, thermogravimetric–differential thermal analysis and Fourier transform–infrared experiments and the microstructures and average size of as-prepared products were characterized by transmission electron microscopy and high resolution transmission electron microscopy images. It was found that compared with traditional solid state reaction (SSR), Y₂Ti₂O₇ nanopowders were synthesized at a relatively low temperature (750°C) for shortened reaction time. Detailed analysis showed that the as-prepared Y₂Ti₂O₇ with good dispersibility and narrow size distribution were quasi-spherical; the average size was about 20–30 nm, also, the obtained products had higher BET surface area (50 m²/g). These properties are very helpful for a photocatalyst to achieve excellent activity and may result in better behaviour in hydrogen storage.     

Nonlinear current–voltage behavior of CaCu3Ti4O12 thin films derived from sol–gel method

CaCu₃Ti₄O₁₂ (CCTO) thin films were synthesized by a sol–gel method followed with spin-coating and sintering process in order to investigate the effects of sintering temperature and thickness of CCTO thin films on the electrical properties as a varistor. The phase identification and morphology of the films have been characterized using X-ray diffractometer and field emission scanning electron microscope. A semiconductor parameter analyzer was used to determine the non-Ohmic (I–V) behaviors. The results showed that the breakdown voltage firstly decreased and then increased with the rising of sintering temperature and was directly proportional to the thickness of the film. Furthermore, the threshold voltage of each sample exhibited nearly twofold relationship as functions of two different measuring mode, surface-to-surface and surface-to-substrate. The lowest leakage current (I _L) and the highest nonlinear coefficient (α) of CCTO thin films were found in the sample sintered at 700 °C. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was employed to explained the non-Ohmic behaviors. A linear relationship between ln(J) and E ^1/2 indicated that a Schottky barrier should exist at the grain boundary.     

Rheological and micro-Raman time-series characterization of enzyme sol–gel solution toward morphological control of electrospun fibers

Rheological and micro-Raman time-series characterizations were used to investigate the chemical evolutionary changes of silica sol–gel mixtures for electrospinning fibers to immobilize an enzyme (tyrosinase). Results of dynamic rheological measurements agreed with the expected structural transitions associated with reacting sol–gel systems. The electrospinning sols exhibited shear-thinning behavior typical of a power law model. Ultrafine (200–300 nm diameter) fibers were produced at early and late times within the reaction window of approximately one hour from initial mixing of sol solutions with and without enzyme; diameter distributions of these fibers showed much smaller deviations than expected. The enzyme markedly increased magnitudes of both elastic and viscous moduli but had no significant impact on final fiber diameters, suggesting that the shear-thinning behavior of both sol–gel mixtures is dominant in the fiber elongation process. The time course and scale for the electrospinning batch fabrication show strong correlations between the magnitudes in rheological property changes over time and the chemical functional group evolution obtained from micro-Raman time-series analysis of the reacting sol–gel systems.