Venkata Sreenivas Puli

Barium zirconate-titanate/barium calcium-titanate ceramics via sol–gel process: novel high-energy-density capacitors

Lead-free barium zirconate-titanate/barium calcium-titanate, [(BaZr0.2Ti0.80)O3]1?x?[(Ba0.70Ca0.30)TiO3]x (x = 0.10, 0.15, 0.20) (BZT?BCT) ceramics with high dielectric constant, low dielectric loss and moderate electric breakdown field were prepared by the sol?gel synthesis technique. X-ray diffraction patterns revealed tetragonal crystal structure and this was further confirmed by Raman spectra. Well-behaved ferroelectric hysteresis loops and moderate polarizations (spontaneous polarization, Ps ~ 3?6??C?cm?2) were obtained in these BZT?BCT ceramics. Frequency-dependent dielectric spectra confirmed that ferroelectric diffuse phase transition (DPT) exists near room temperature. Scanning electron microscope images revealed monolithic grain growth in samples sintered at 1280??C. 1000/? versus (T) plots revealed ferroelectric DPT behaviour with estimated ? values of ~1.52, 1.51 and 1.88, respectively, for the studied BZT?BCT compositions. All three compositions showed packing-limited breakdown fields of ~47?73?kV?cm?1 with an energy density of 0.05?0.6?J?cm?3 for thick ceramics (>1?mm). Therefore these compositions might be useful in Y5V-type capacitor applications.

Structural, morphological and piezoresponse studies of Pr and Sc co-substituted BiFeO3 ceramics

Pristine (BiFeO3), Pr and Sc co-substituted Bi0.9Pr0.1Fe1−xScxO3 (0.01 ≤ x ≤ 0.07) ceramics have been investigated for their structural (including x-ray diffraction and Raman), morphological and piezoresponse behaviour. Secondary phases observed in pristine BiFeO3 (BFO) ceramic were significantly suppressed in the co-substituted samples. Grain size decreased in the co-substituted samples. Raman study revealed ten active phonon modes which remained almost invariant with co-substitution. Out-of-plane piezoresponse (OPP-PFM) exhibited negative self-polarization effect in the virgin state (without any poling) which has been explained in terms of built-in electric field. The self-polarization effect is largest in 3% Sc co-substituted sample. Poling with ±30 V dc voltage demonstrated both positive and negative domains. Maximum difference in the peak values (from histograms) of such opposite domains was observed in the Bi0.9Pr0.1Fe0.99Sc0.01O3 sample. Information from local measurements (such as PFM) should be useful in deciding such multiferroic materials for device application.

Studies on structural, dielectric, and transport properties of Ni0.65Zn0.35Fe2O4

We report the crystal structure, dielectric, transport, and magnetic properties of Ni0.65Zn0.35Fe2O4. Rietveld refinement results of X-ray diffraction patterns confirm the phase formation of the material with cubic crystal structure ( Fd3¯m). The frequency dependent ac conductivity behavior obeys the Jonschers power law and is explained using the jump relaxation model. The observed behavior of temperature dependent bulk conductivity is attributed to the variable-range hopping of localized polarons. The correlation of polaron conduction and high permittivity behavior of NZFO is established on the basis of long range and short range conduction mechanisms. The complex impedance spectra clearly show the contribution of both grain and grain boundary effect on the electrical properties.

Photovoltaic effect in transition metal modified polycrystalline BiFeO3 thin films

We report photovoltaic (PV) effect in multiferroic Bi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin films. Transition metal modified polycrystalline BiFeO3 (BFO) thin films have been deposited on Pt/TiO2/SiO2/Si substrate successfully through pulsed laser deposition (PLD). PV response is observed under illumination both in sandwich and lateral electrode configurations. The open-circuit voltage (Voc) and the short-circuit current density (Jsc) of the films in sandwich electrode configuration under illumination are measured to be 0.9 V and −0.051 µA cm−2. Additionally, we report piezoresponse for BSFCO films, which confirms ferroelectric piezoelectric behaviour.

Studies of the switchable photovoltaic effect in co-substituted BiFeO3 thin films

We report the photovoltaic properties of doped ferroelectric [Bi0.9La0.1][Fe0.97Ti0.02Zr0.01]O3 (BLFTZO) thin films. Polycrystalline BLFTZO films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition technique. Al-doped ZnO transparent top electrodes complete the ZnO:Al/BLFTZO/Pt metal-ferroelectric-metal capacitor structures. BLFTZO showed switchable photoresponse in both polarities. The open circuit voltage (VOC) and short circuit current (JSC) were found to be ∼0.022 V and ∼650 μA/cm2, respectively after positive poling, whereas significant difference in VOC ∼ 0.018V and JSC ∼ 700 μA/cm2 was observed after negative poling. The observed switchable photocurrent and photovoltage responses are explained on the basis of polarization flipping in BLFTZO due to the applied poling field.

Room temperature multiferroic properties of Pb(Fe0.5Nb0.5)O3–Co0.65Zn0.35Fe2O4 composites

We report the crystal structure, magnetic, ferroelectric, dielectric, and magneto-dielectric properties of [Pb(Fe0.5Nb0.5)O3](1−x)[Co0.65Zn0.35Fe2O4]x: (x = 0.1, 0.2, 0.3, and 0.4) composites. Rietveld refinement results of X-ray diffraction patterns confirm the formation of these composites for all x values. All the composites show well-saturated ferroelectric and ferromagnetic hysteresis (multiferroic-composite behavior) at room temperature. With increase in Co0.65Zn0.35Fe2O4 (CZFO) content an increase in saturation magnetization, and decrease in saturation polarization, remanent polarization, and dielectric constant are observed. The ferroelectric phase transition temperature increases with increase in CZFO content. All of the compositions undergo second-order ferroelectric phase transitions, which can be explained by Landau-Devonshire theory. The recoverable energy density (∼0.20 to 0.04 J/cm3) and charge-curve energy density (∼0.84 to 0.11 J/cm3) decrease with increase in the CZFO content. The room-t...

Synthesis and characterization of lead-free ternary component BST–BCT–BZT ceramic capacitors

Polycrystalline sample of lead-free 1/3(Ba0.70Sr0.30TiO3) + 1/3(Ba0.70Ca0.30TiO3) + 1/3(BaZr0.20Ti0.80O3)(BST-BCT-BZT) ceramic was synthesized by solid state reaction method. Phase purity and crystal structure of as-synthesized materials was confirmed by X-ray diffraction (XRD). Temperature-dependent dielectric permittivity studies demonstrated frequency-independent behavior, indicating that the studied sample has typical diffuse phase transition behavior with partial thermal hysteresis. A ferroelectric phase transition between cubic and tetragonal phase was noticed near room temperature (~ 330 K). Bulk P–E hysteresis loop showed a saturation polarization of 20.4 μC/cm2 and a coercive field of ~ 12.78 kV/cm at a maximum electric field of ~ 115 kV/cm. High dielectric constant (e ~ 5773), low dielectric loss (tan δ ~ 0.03) were recorded at room temperature. Discharge energy density of 0.44 J/cm3 and charge energy density of 1.40 J/cm3 were calculated from nonlinear ferroelectric hysteresis loop at maximum electric field. Dielectric constant at variable temperatures and electric fields, ferroelectric to paraelectric phase transition and energy storage properties were thoroughly discussed.

Nanoscale polarisation switching and leakage currents in (Ba0.955Ca0.045)(Zr0.17Ti0.83)O3 epitaxial thin films

This paper investigates the crystal structure, ferroelectric, temperature-dependent leakage current conduction mechanism and the piezoresponse force microscopy (PFM) behaviours of (Ba0.955Ca0.045) (Zr0.17Ti0.83)O3–BZT–BCT () films at the vicinity of the morphotropic phase boundary (MPB) of the [(1 − x) BaZr0.20Ti0.80O3–xBa0.70Ca0.30TiO3] () composition. Epitaxial BZT–BCT thin films were grown on conductive La0.5Sr0.5CoO3 (LSCO) layers coated onto MgO (1 0 0) single-crystal substrates by pulsed laser deposition (PLD). High resolution x-ray diffraction (HR-XRD) reciprocal space maps (RSMs) confirmed the epitaxy with in-plane tetragonal symmetry (c < a), and Raman spectra also revealed a tetragonal perovskite crystalline lattice structure. Polarisation studies demonstrate that BZT–BCT films exhibit a high saturation polarisation of 148 µC cm−2 and a high recoverable (discharged) energy-storage density of 39.11 J cm−3 at 2.08 MV cm−1. Temperature-dependent P–E hysteresis loops resulted in a decrease in polarisation. Temperature-dependent leakage current behaviour was obtained and possible conduction mechanism is discussed. PPFM images revealed switchable ferroelectric piezoelectric contrasts after the application of a ±9 V dc voltage on the conductive tip of the piezoresponse force microscope.

Structure, Ferroelectric, Dielectric and Energy Storage Studies of Ba0.70Ca0.30TiO3, Ba(Zr0.20Ti0.80)O3 Ceramic Capacitors

Ba0.70Ca0.30TiO3-(BCT),Ba(Zr0.2Ti0.8)O3-(BZT) ceramics were fabricated by conventional mixed oxide route to develop inorganic dielectric materials suitable for use as an insulator with high dielectric constant and low energy loss for capacitor applications. The structural phase transition, ferroelectric, dielectric and energy storage properties of BCT, BZT ceramic capacitors were investigated. Room temperature X-ray diffraction (XRD) patterns revealed prominent peaks corresponding to tetragonal perovskite crystal structure for both BCT, BZT solid solutions. Slim ferroelectric hysteresis (P-E) loops were observed for BCT, BZT solid solutions. Temperature dependent dielectric property measurements of BCT, BZT solid solutions have shown a high dielectric constant and low dielectric loss. Room temperature (300K) breakdown field strength and energy densities were obtained from the integral area of P-E loops. For the BCT ceramics, the largest recoverable energy (unreleased energy) density is 1.41 J/cm3 with dielectric breakdown strength as high as 150 kV/cm. For the BZT ceramics, the largest recoverable energy (unreleased energy) density is 0.71 J/cm3 with dielectric breakdown strength as high as 150 kV/cm. Bulk BCT, BZT ceramics have shown interesting energy densities; these might be the strong candidate materials for capacitor applications.

Investigations on electrical and magnetic properties of multiferroic [(1−x)Pb(Fe0.5Nb0.5)O3−xNi0.65Zn0.35Fe2O4] composites

Here, we report the magnetic, ferroelectric, dielectric properties, and Raman spectroscopic studies of multiferroic [(1−x)Pb(Fe0.5Nb0.5)O3−xNi0.65Zn0.35Fe2O4] composites at room temperature. The phase formation of composites was confirmed independently from the X-ray diffraction and Raman studies. The room temperature magnetic studies reveal ferromagnetic like behavior of these composites in contrast to the paramagnetic nature of Pb(Fe0.5Nb0.5)O3. Furthermore, with increasing x, the saturation magnetization, remnant magnetization, and coercive fields are found to increase. The electrical characterizations of these composites reveal a decrease in remnant polarization and dielectric constant with increasing x. More importantly, the x = 0.2 composite is found to be a very good multiferroic material at room temperature among the composites and could be a potential candidate for future potential applications.