Synthesis of nanocrystalline magnesium titanate by an auto-igniting combustion technique and its structural, spectroscopic and dielectric properties

Nanocrystalline magnesium titanate was synthesized through an auto-ignited combustion method. The phase purity of the powder was examined using X-ray diffraction, thermo gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy and Raman spectroscopy. The transmission electron microscopy study showed that the particle size of the as-prepared powder was in between 20 and 40 nm. The nanopowder could be sintered to 98% of the theoretical density at 1200 °C for 3 h. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant (?_r) of 16.7 and loss factor (tan δ) of the order of 10⁻⁴ were obtained at 5 MHz when measured using LCR meter. The quality factor (Q_u × f) 73,700 and temperature coefficient of resonant frequency (τ_f) −44.3 ppm/°C, at 6.5 GHz are the best reported values for sintered pellets obtained from phase pure nanocrystalline MgTiO₃ powder.     

A novel approach for the synthesis of nanocrystalline zinc oxide powders by room temperature co-precipitation method

A single step co-precipitation route has been employed for the first time in the preparation of ZnO nanoparticles using ammonium hydroxide and zinc nitrate tetrahydrate. The X-ray diffraction analysis revealed that the synthesized powder has the hexagonal (wurtzite) structure. The as-prepared ZnO powder was well crystalline, without any calcination. This is a promising result compared to those mentioned in the literature, in which crystallization of ZnO nanoparticles was detected at > 300 °C. The average crystallite size of the as-prepared ZnO nanopowder is 20-40 nm. The nanocrystalline ZnO could be sintered to ~ 95% of the theoretical density at 1300 °C in 4 h.     

FT-Raman and FT-IR vibrational spectroscopic studies of nanocrystalline Ba2RESbO6 (RE = Sm, Gd, Dy and Y) perovskites

The nanoparticles of Ba₂RESbO₆ (RE = Sm, Gd, Dy and Y) were synthesized using auto ignition combustion process. The structure and phase purity of the as-prepared nanopowders were examined by X-ray diffraction pattern. A systematic analysis of the structure of the four compounds was carried out for the first time by recording Raman and IR spectra. The four Raman active modes A_1g, E_g and 2F_2g were observed as strong or medium intense bands in the Raman spectra and the IR active F_1u(1) mode is obtained as a strong absorption band around 630 cm⁻¹ in all the four compounds. Hence it is confirmed that the above compounds were crystallized in the cubic symmetry with space group.     

Percolation Behavior of the Normal-State Resistivity and Superconductivity of the YBa2Cu3O7−δ-Ba2GdNbO6 Composite System

The percolation behavior of the normal-state resistivity and superconductivity of the YBa₂Cu₃O_7?δBa₂GdNbO₆ composite system were studied by X-ray diffraction and temperature-resistivity measurements. No detectable chemical reaction was observed between the YBa₂Cu₃O_7?δ superconductor and the ceramic insulator Ba₂GdNbO₆, even after severe heat treatment above 950°C. The normal-state and superconducting percolation threshold values were found to be 17 vol.% and 30 vol.% of YBa₂Cu₃O_7?δ respectively in the YBa₂Cu₃O_7?δ-Ba₂GdNbO₆ composite system. The values obtained for the critical exponents describing the normal-state pecolation behavior of the system matched fairly well with the theoretically expected values for an ideal metal-insulator composite system.     

Nanocrystals of a new complex perovskite dielectric Ba2TmSbO6

Nanocrystals of a new complex perovskites ceramic oxide, barium thulium antimony oxide - Ba₂TmSbO₆, were synthesized using a single step auto-ignition combustion process. The combustion product was single phase and composed of aggregates of nanocrystals of sizes in the range 20-50 nm. Ba₂TmSbO₆ crystallized in cubic perovskite structure with lattice parameter, a = 8.4101 Å. The polycrystalline fluffy combustion product was sintered to high density (∼97%) at ∼1450 °C for 4 h. Resistivity of the sintered specimen was ∼5 MΩ/cm. The Ba₂TmSbO₆ has dielectric constant (?′) and dielectric loss (tan δ) of 17 and ∼10⁻⁴ at 5 MHz; the new material would probably be developed as a low-loss dielectric material.     

Combustion synthesis and characterization of Ba<Subscript>2</Subscript>NdSbO<Subscript>6</Subscript> nanocrystals

Nanocrystalline Ba₂NdSbO₆, a complex cubic perovskite metal oxide, powders were synthesized by a self-sustained combustion method employing citric acid. The product was characterized by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, transmission electron microscopy and scanning electron microscopy. The as-prepared powders were single phase Ba₂NdSbO₆ and a mixture of polycrystalline spheroidal particles and single crystalline nanorods. The Ba₂NdSbO₆ sample sintered at 1500°C for 4 h has high density (∼ 95% of theoretical density). Sintered nanocrystalline Ba₂NdSbO₆ had a dielectric constant of ∼ 21; and dielectric loss = 8 × 10⁻³ at 5 MHz.     

Microwave and photoluminesent characterizations of (Ca<Subscript>2</Subscript>Mg<Subscript>3</Subscript>)(X<Subscript>1.75</Subscript>Sb<Subscript>0.25</Subscript>)TiO<Subscript>12</Subscript> [X = Nb and Ta] ceramics

(Ca₂Mg₃)(X_1.75Sb_0.25)TiO₁₂ [X = Nb and Ta] ceramics are prepared through the conventional solid-state route. The samples are calcined at 1,100 and 1,180 °C, and are sintered at 1,250 and 1,375 °C. The substitution of Sb decreases the calcination and sintering temperatures of pure (Ca₂Mg₃)(Nb/Ta)₂TiO₁₂. The structure of the samples is analyzed using X-ray diffraction method. The microstructure of the sintered pellet is studied using scanning electron microscopy. The dielectric properties such as dielectric constant (ε_r), quality factor (Q_uxf) and temperature coefficient of resonant frequency (τ_f) are measured in the microwave frequency region. By Sb substitution, thermal stability is achieved, with the increase in dielectric constant, without much change in the quality factor. The materials have intense emission lines in the wavelength region 500–700 nm. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Percolation Behavior of the Normal-State Resistivity and Superconductivity of the YBa2Cu3O7?δ-Ba2GdNbO6 Composite System

The percolation behavior of the normal-state resistivity and superconductivity of the YBa₂Cu₃O_7–Ba₂GdNbO₆ composite system were studied by X-ray diffraction and temperature-resistivity measurements. No detectable chemical reaction was observed between the YBa₂Cu₃O_7– superconductor and the ceramic insulator Ba₂GdNbO₆, even after severe heat treatment above 950°C. The normal-state and superconducting percolation threshold values were found to be 17 vol.% and 30 vol.% of YBa₂Cu₃O_7– respectively in the YBa₂Cu₃O_7–-Ba₂GdNbO₆ composite system. The values obtained for the critical exponents describing the normal-state pecolation behavior of the system matched fairly well with the theoretically expected values for an ideal metal-insulator composite system.     

Characterization,sintering and dielectric properties of nanocrystalline barium titanate synthesized through a modified combustion process

Nanocrystalline barium titanate has been synthesized through a modified combustion process in a single step for the first time. The as-prepared barium titanate powder is cubic perovskite with lattice constant a = 4.018 Å. The phase purity of the nanopowder was examined using thermo gravimetric analysis, differential thermal analysis and Fourier transform infrared spectroscopy. Transmission electron microscopic investigations have shown that the particle size of the as-prepared powder is in the range 20–40 nm. The agglomerate size distribution of the as-prepared powder was studied using atomic force microscopy. The nanoparticles of barium titanate were sintered to 97% of the theoretical density at a temperature of 1350 °C for 3 h. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant and loss factor of the sintered pellets at 1 MHz measured at room temperature were 1223 and 3.5 × 10^? 3 respectively.     

Synthesis,structural analysis and dielectric properties of Ba8(Mg1−xZnx)Nb6O24 hexagonal perovskites

Ba₈(Mg_1−xZn_x)Nb₆O₂₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered at 1375–1425 °C. The structure of the system was analyzed using X-ray diffraction and vibrational spectroscopic studies. The microstructure of the sintered pellet was analyzed using scanning electron microscopy. The dielectric constant (ε_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) were measured in the microwave frequency region. The τ_f values of the compositions were reduced by varying the value of x from 0 to 1. The dielectric responses to frequency were also studied in the radio frequency region. The compositions have good microwave dielectric properties and hence are suitable for dielectric resonator applications.