S. Vidya

Optical properties of nanocrystalline HfO2 synthesized by an auto-igniting combustion synthesis

Abstract The optical properties of nanocrystalline HfO2 synthesized using a single-step auto-igniting combustion technique is reported. Nanocrystalline hafnium oxide having particle size of the order 10–15 nm were obtained in the present method. The nanopowder was characterized using X-ray diffraction, Fourier transform infrared and Fourier transform Raman spectroscopic studies. All these studies confirm that the phase formation is complete in the combustion synthesis and monoclinic phase [P21/c(14)] of HfO2 is obtained without the presence of any impurities or additional phases. The powder morphology of the as-prepared sample was studied using transmission electron microscopy and the results were in good agreement with that of the X-ray diffraction studies. The optical constants such as refractive index, extinction coefficient, optical conductivity and the band gap were estimated from UV–vis spectroscopic techniques. The band gap of nanocrystalline HfO2 was found to be 5.1 eV and the sample shows a broad PL emission at 628 nm. It is concluded that the transitions between intermediate energy levels in the band gap are responsible for the interesting photoluminescent properties of nanocrystalline HfO2.

Structural, Optical, and Compactness Characteristics of Nanocrystalline Synthesized through an Autoigniting Combustion Method

Nanoparticles of calcium metaniobate compound are prepared by an autoigniting combustion technique and its structural, optical, and dielectric properties are investigated. The X-ray diffraction, Fourier-transform Raman, and infrared studies reveal that calcium metaniobate possesses phase pure orthorhombic columbite structure with space group of Pbcn. The average particle size of the as-prepared nanoparticles obtained from both the Scherrer formula and transmission electron microscopy is ~37 nm. The optical band gap calculated from Taucs Plot is 3.25 eV. Photoluminescence studies reveal that Calcium metaniobate can be used as an idealphotoluminarmaterial. The powders are pelletised and sintered at an optimized temperature of in a short duration of two hours, yielding a high density. The morphology of the sintered pellet is further examined using scanning electron microscopy. The dielectric constant and loss factor values measured at 5 MHz for a well-sintered Calcium metaniobate pellet are found to be 27.6 and respectively, at room temperature.

Synthesis, Characterization, and Low Temperature Sintering of Nanostructured BaWO4 for Optical and LTCC Applications

Synthesis of nano-BaWO4 by a modified combustion technique and its suitability for various applications are reported. The structure and phase purity of the sample analyzed by X-ray diffraction, Fourier transform Raman, and infrared spectroscopy show that the sample is phase pure with tetragonal structure. The particle size from the transmission electron microscopy is 22 nm. The basic optical properties and optical constants of the nano BaWO4 are studied using UV-visible absorption spectroscopy which showed that the material is a wide band gap semiconductor with band gap of 4.1 eV. The sample shows poor transmittance in ultraviolet region while maximum in visible-near infrared regions. The photoluminescence spectra show intense emission in blue region. The sample is sintered at low temperature of 810°C, without any sintering aid. Surface morphology of the sample is analyzed by scanning electron microscopy. The dielectric constant and loss factor measured at 5 MHz are 9 and . The temperature coefficient of dielectric constant is −22 ppm/°C. The experimental results obtained in the present work claim the potential use of nano BaWO4 as UV filters, transparent films for window layers on solar cells, antireflection coatings, scintillators, detectors, and for LTCC applications.

Structural, dielectric and optical characterization of BaMoO4 nano powder synthesized through an auto-igniting combustion technique

Nanocrystalline BaMoO4 was synthesized through auto-ignited combustion technique. The X-ray diffraction studies of BaMoO4 nanoparticles reveals that the as-prepared powder is single phase, crystalline, and has tetragonal structure. The average particle size of the as-prepared powder from transmission electron microscopic is found to be 22nm. The thermal stability of the nano powder was examined using thermo gravimetric analysis and differential thermal analysis. The band gap determined from absorption spectra is 3.20eV.Photoluminescence spectra of the samples shows green emission peak. The dielectric constant and loss factor of the sample at 1 MHz is found to be 9.75 and 1.38×10−2 at room temperature.

Enhancement in the transport critical current density Jc in YBa2Cu3O7−δ added with an insulating nano crystalline YBa2HfO5.5 perovskite

When a magnetic field is applied to type II superconductors, such as YBa2Cu307−δ (YBCO), the flux quanta penetrate the material as a regular array of vortices. However when transport currents are applied, they act to move these vortices, thus lowers the critical current density (Jc) as well as destroying superconductivity. The development of microstructures made of YBCO materials has enabled engineers to increase the critical current density, within Type II materials by introducing flux pinning centres into the material. The microstructure and flux pinning properties of YBa2Cu3O7−δ system with varying levels (0-5 wt. %) of a nano perovskite ceramic insulator; YBa2HfO5.5 addition was studied in detail. Orthorhombic YBa2Cu3O7−δ powder was prepared through conventional solid state route and a modified combustion method was used for synthesizing nanocrystalline YBa2HfO5.5. The structure and microstructure of the samples examined by X-ray diffraction and scanning electron microscopy showed that YBa2HfO5.5 and ...

SmBa2NbO6 Nanopowders, an Effective Percolation Network Medium for YBCO Superconductors

The percolation behavior of superconductor-insulator composite, YBa2Cu3O7–δ, and nano SmBa2NbO2 synthesized by modified combustion technique was studied. Particle size of nano SmBa2NBO6 was determined using transmission electron microscopy. The chemical nonreactivity of nano SmBa2NbO6 with YBCO is evident from the X-Ray diffraction study which makes it a suitable nanoceramic substrate material for high temperature superconducting films. A systematic increase in the sintered density, approaching the optimum value of the insulating nanophase is clearly observed, as the vol.% of YBCO in the composite decreases. SEM micrograph showed uniform distribution of nanopowder among the large clusters of YBCO. The obtained percolation threshold is ~26 vol% of YBCO in the composite. All the composites below the threshold value showed K even though the room resistivity increases with increase in vol.% of nano SmBa2NbO6. The values of critical exponents obtained matches well with the theoretically expected ones for an ideal superconductor-insulator system.

A study of structural, optical and dielectric properties of crystalline Sr2Nb2O7 nanoparticles synthesized by a modified combustion technique

Nanocrystalline Strontium Pyroniobate is synthesized by a novel auto-igniting combustion technique. The X-Ray diffraction studies reveal that Strontium Niobate possesses orthorhombic structure. Phase purity and structure of the nanopowder is further examined using Fourier-Transform Infrared and Raman spectroscopy. The average particle size of the as prepared nanoparticles from the Transmission Electron Microscopy is 30 nm. Sr2Nb2O7 is a photoluminescent material and the optical band gap determined from the UV-DRS spectrum is 2.7eV. The sample is sintered at an optimized temperature of 1350°C for 2 hours and obtained maximum density. The dielectric constant and loss factor values obtained at 5MHz for a well-sintered Strontium Niobate pellet is found to be 40 and 3.9×10−3 respectively, at room temperature.

Electrical Transport and Lowered Percolation Threshold in YBa2Cu3O7−δ-Nano-YBa2ZrO5.5 Composites

We investigated the chemical reactivity and percolation characteristics of insulating nanocrystalline YBa2ZrO5.5 prepared by modified combustion route and the YBa2Cu3O7−δ superconductor composite system. Structural analysis was done by using X-ray diffraction technique, surface morphology of the samples was studied using scanning electron microscopy, and electrical transport measurements like critical transition temperatures () and self-field transport critical current () were done by using standard four-probe technique. It is found that, in YBa2Cu3O7−δ-nano-YBa2ZrO5.5 composite system, the superconductor and insulator materials coexist as separate phases without any noticeable chemical reaction even after sintering at high temperatures. Furthermore, percolation threshold and critical exponent are found to be , and . And the analysis of the current flow in the polycrystalline samples reveals weak link behavior in the majority of grain connections.