EPR studies on donor doped BaTiO3 grain boundary layer ceramic dielectrics

Donor doped BaTiO₃ ceramics become insulating₅ under controlled conditions with effective dielectric constants >10. The changes in EPR signals indicate that a certain fraction of the donor doped BaTiO₃ is cubic even at room temperature and that the cubic fraction increases with the donor content. X-ray powder diffraction data support the EPR results. The coexistence of both the phases over a range of temperature is characteristic of diffused phase transition. The effect of grain size variation on EPR signal intensities indicate that the boundary layers surrounding the grains may constitute the cubic phase as a result of higher Ba-vacancies and donor contents at the grain boundary layer than in the bulk. Since the acceptor states arising from the Ba-vacancies and the impurities are activated in the cubic phase, they capture electrons from the conduction band, rendering the cubic phase electrically more insulating than the semiconductive tetragonal grain interiors. Thus, the cubic grain boundary layers act as effective dielectric media where the field tends to concentrate.     

Size effect and order–disorder phase transition in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: synthesized by co-precipitation method

We report on the size effect and order–disorder phase transitions in MgAl₂O₄ system synthesized by chemical co-precipitation method. The prepared samples were sintered at various temperatures (in steps of 200 °C). Initially, the order–disorder phase evolution of the cubic spinel aluminates were analyzed by powder X-ray diffraction and UV-absorbance spectral analysis. The optical band gap was calculated from UV–DRS absorbance spectra. Also, the grain size of the sintered aluminates was calculated by high resolution scanning electron microscopy through surface morphological image analysis and discussed. Moreover, the particle size was calculated by using transmission electron microscopy. The stretching and bending mode of tetrahedral and octahedral coordinates for vibration modes of cations were studied through Fourier transform infrared spectral analysis. In addition, the cation distributions in the prepared samples were carried out by solid-state nuclear magnetic resonance spectroscopic measurement and analyses. The frequency dispersive behavior of dielectric constant was analyzed at room temperature for the synthesized samples using impedance analyzer. The observed results are discussed and reported.     

ZnO nanorefrigerant in R152a refrigeration system for energy conservation and green environment

In this paper the reliability and performance of a vapour compression refrigeration system with ZnO nanoparticles in the working fluid was investigated experimentally. Nanorefrigerant was synthesized on the basis of the concept of the nanofluids, which was prepared by mixing ZnO nanoparticles with R152a refrigerant. The conventional refrigerant R134a has a global warming potential (GWP) of 1300 whereas R152a has a significant reduced value of GWP of 140 only. An experimental test rig is designed and fabricated indigenously in the laboratory to carry out the investigations. ZnO nanoparticles with refrigerant mixture were used in HFC R152a refrigeration system. The system performance with nanoparticles was then investigated. The concentration of nano ZnO ranges in the order of 0.1% v, 0.3% v and 0.5%v with particle size of 50 nm and 150 g of R152a was charged and tests were conducted. The compressor suction pressure, discharge pressure and evaporator temperature were measured. The results indicated that ZnO nanorefrigerant works normally and safely in the system. The ZnO nanoparticle concentration is an important factor considered for heat transfer enhancement in the refrigeration system. The performance of the system was significantly improved with 21% less energy consumption when 0.5%v ZnO-R152a refrigerant. Both the suction pressure and discharge pressure were lowered by 10.5% when nanorefrigerant was used. The evaporator temperature was reduced by 6% with the use of nanorefrigerant. Hence ZnO nanoparticles could be used in refrigeration system to considerably reduce energy consumption. The usage of R152a with zero ozone depleting potential (ODP) and very less GWP and thus provides a green and clean environment. The complete experimental results and their analysis are reported in the main paper.     

EPR evidence for activation of trap centers in PTCR BaTiO3 ceramics

Donor-doped BaTiO₃ ceramics, exhibiting PTCR, showed an EPR signal with g=1.997 which acquires high intensity above T_C. This is indicative of activation of the corresponding defect centers, possibly V′_Ba^.. The intensity of g=1.997 varies with Ba/Ti ratio, rate of cooling from the sintering temperature and the addition of TiO₂ as sintering agent. The signal intensity decreases with increase in grain size and is weak in La-doped BaTiO₃ single crystals. Therefore the concentration of V_Ba is more around the grain boundaries than that in the bulk of grains. Activated trap centers around T_C also arises from background impurities; those with change of electronic state: Mⁿ⁺¹+e′ = Mⁿ+(eg:Mn) and those associated with oxygen vacancies: M^p+V_O(e′) (eg:Fe³⁺). Activation of these trap centers are not dependent on the disappearance of spontaneous polarization but to the structural changes during phase transformation. The trapped charge carriers are not available for conduction and hence the increase in electrical resistivity.     

Zn interstitial defects induced room temperature ferromagnetism in Na<Superscript>+</Superscript> ions codoped Zn<Subscript>0.95</Subscript>Co<Subscript>0.05</Subscript>O powders

Sodium co-doped Zn_0.95Co_0.05O nanoparticles (0.01 ≤ X ≤ 0.05) were synthesized using simple wet chemical (co-precipitation) method. The structural, surface morphology and the optical properties of the prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscope, UV-diffuse reflectance spectroscopy (UV-DRS) and Photoluminescence (PL) spectra measurements. The magnetic measurements of the prepared samples with vibrating sample magnetometer exhibited room temperature ferromagnetism. The results obtained from the above studies shows no trend following on Na co-doping concentrations, within our arbitrary selection limit of Na ion. However, the XRD pattern confirms the single phase of Na co-doped samples without any secondary phases. The behaviors associated with intrinsic defects are explored by UV-DRS and PL emission spectra. Further, the origin of observed ferromagnetism and its lack of dependence on Na ion doping was analysed and reported.     

Bismuth effect in the structural, magnetic and dielectric properties of CoZn ferrite

Nano particles of Co_0.5Zn_0.5Bi_xFe_(2−x)O₄, with x varying from 0.0 to 0.3 in steps of 0.1 were synthesized using the chemical co-precipitation method. The powder X-Ray diffraction pattern confirms the formation of spinel phase for all prepared samples. The lattice parameters are calculated by powder X-Ray diffraction, and it is observed that the values of the lattice parameter are less than those of bulk materials. The saturation magnetization is found to decrease with the increase of concentration of Bi ion up to x = 0.2 and then increases for x = 0.3. The hysteresis loop for concentration x = 0.0, 0.1 and 0.3 shows almost zero coercivity and remanance at 300 K, implying that the samples behave as superparamagnetic at this temperature; whereas for the concentration at x = 0.2 the coercivity was found to be 32 Oe at 300 K. The dielectric measurements were carried out in the Co_0.5Zn_0.5Bi_xFe_(2−x)O₄ system over the temperature range from 300 to 700 K as a function of frequency, from 5 to 5 MHz. The variation of dielectric constant and dielectric loss factor for the prepared samples are explained on the basis of Maxwell–Wagner interfacial polarization.     

Structural,Magnetic, Optical and Electrical Properties of Ba Substituted BiFeO3

Ba²⁺ substituted bismuth ferrite (BFO) (Bi_1?x Ba _x FeO₃, x=0, 0.1, 0.2, and 0.3) was prepared by chemical route using nitrates as a precursor, and sintered at 830 °C for 1 hr. The structural, optical, magnetic, and electrical properties of the prepared samples were investigated to understand the effect of Ba²⁺ substitution in BFO. The samples were characterized by X-ray diffraction (XRD), and shows an impurity phase which reduces drastically on substitution. Magnetic measurements were carried out at room temperature up to a field of 20 kOe. Magnetic hysteresis loops revealed a significant change in magnetization on Ba²⁺ substitution in BFO with unsaturated nature. Further, the Fourier transformed Infrared (FTIR) and Ultra Violet Visible (U-V Vis) spectra of the samples at room temperature shows as a change in spectral behavior on substitution. The AC electrical conductivity analyzed through Impedance analyser at different temperatures and frequencies. The ac conductivity and its activation energies for the samples are found to be frequency and temperature dependent and also vary with Ba²⁺ substitution in BFO.     

Growth and characterization of large crystals of GdBa2Cu3O7 superconductor

We report here on the growth of large crystals of Gd-Ba-Cu-O superconductor by using the ‘K₂CO₃-method’ adopted earlier for preparing oriented ceramics and large crystals of Y-Ba-Cu-O superconductors. The temperature favourable for the growth of GdBaCuO crystals is found to be higher than that required for YBaCuO crystals. Also, the crystal perfection is better for these crystals and as a result, the oxygenation of the sample becomes more difficult. Resistivity measurements on these samples show a sharp drop in resistivity at 90 K which then tails off at lower temperatures and attains zero resistance around 35 K. Such behaviour is normally due to an inhomogeneous oxygen content in the sample. Chemical analysis shows that the deviation from ‘123’ composition in GdBaCuO-samples is much less as compared to YBaCuO-samples obtained by the same technique.