Hitesh Borkar

Room temperature lead-free relaxor–antiferroelectric electroceramics for energy storage applications

Round the globe, scientific communities have been searching for new materials for “green” energy, producing efficiently both high power as well as high energy density. Relaxor ferroelectrics (RFEs) have shown immense potential to achieve this goal. We report fabrication of [Na0.42Bi0.44Al0.06Ba0.08)TiO3 (NBAT–BT)], a lead-free-relaxor antiferroelectric ceramic, via a conventional solid-state reaction method. A small fraction of trivalent cations (Al3+) doping at Na1+/Bi3+ sites develop anti-polar phase in the ferroelectric matrix which in turn changes its functional properties. Rietveld refinement suggests the existence of both tetragonal and rhombohedral phases which is well supported by d-spacing values obtained in high resolution transmission electron microscopy (HRTEM) studies. Elemental analysis confirms the stoichiometry of the system and matches the starting composition well within the experimental uncertainty (±10%) of secondary electron microscopy (SEM) and HRTEM data. Raman spectra suggest the substitution of Al3+ cation at an A-site sublattice. Temperature-dependent dielectric spectra show frequency dependent dielectric dispersion near 80–110 °C, high dielectric loss at high probing frequency, and a non-linear Vogel–Fulcher relation, substantiating the relaxor–antiferroelectric (r-AFE) nature of NBAT–BT. A second order diffuse anti/ferro-electric to paraelectric phase transition near 230–240 °C was observed which follows a modified Curie–Weiss law. The energy density was calculated from polarization–electric field (P–E) loops and dielectric–electric field (e–E) plot. The values were in the range of 0.4–0.6 J cm−3, which is reasonably good for bulk polar material. NBAT–BT shows a much thinner AFE hysteresis above its relaxor FE phase transition; that favors the enhanced energy storage capacity at elevated temperature in the depolarized paraelectric region.

Anomalous change in leakage and displacement currents after electrical poling on lead-free ferroelectric ceramics

We report the polarization, displacement current, and leakage current behavior of a trivalent nonpolar cation (Al3+) substituted lead free ferroelectric (Na0.46Bi0.46−xAlxBa0.08)TiO3 (NBAT−BT) (x = 0, 0.05, 0.07 and 0.10) electroceramics with tetragonal phase and P4 mm space group symmetry. Almost, three orders of magnitude decrease in leakage current were observed under electrical poling, which significantly improves microstructure, polarization, and displacement current. Effective poling neutralizes the domain pinning, traps charges at grain boundaries and fills oxygen vacancies with free charge carriers in matrix, thus saturated macroscopic polarization in contrast to that in unpoled samples. E-poling changes “bananas” type polarization loops to real ferroelectric loops.

Electroforming free high resistance resistive switching of graphene oxide modified polar-PVDF

Future nanoelectronics for nonvolatile memory elements require novel materials and devices that can switch logic states with a low power consumption, minimum heat dissipation, high-circuit density, fast switching speed, large endurance and long charge retention period. Herein, we report novel high resistance resistive switching in a polar beta-polyvinylidene fluoride (β-PVDF) and graphene oxide (GO) composite. A high resistance switching ratio was achieved without the realization of the essential current-filament forming condition mainly responsible for switching the device from high to low resistance states. β-PVDF is a well known ferroelectric/piezoelectric material which changes shape and size after application of an external electric field. We propose a model which describes how the present β-PVDF–GO composite changes shape after application of an external electric field (E) which provides a favorable environment for the formation of the current linkage path of GO in the PVDF matrix. The applied positive SET electric fields (+E) switch the composite from a high to a low resistance state, which further re-switches from a low to a high resistance state under negative RE-SET electric fields (−E). The positive and negative E-fields are responsible for the contraction and expansion of β-PVDF, respectively, redox reactions between GO and adsorbed water, oxygen migrations, and/or metal diffusion from the electrode to the β-PVDF–GO matrix. The above mentioned characteristics of the composite allows switching from one high resistance state to another high resistance state. The switching current lies below the range of 10–100 μA with an exceptionally high switching ratio, which meets one of the pre-requisite criteria of low power nanoelectronics memristors.

Light assisted irreversible resistive switching in ultra thin hafnium oxide

An ultra thin film (∼5 nm) high-k Hafnium oxide dielectric, grown on a doped p-Si(100) substrate by the atomic layer deposition technique has been investigated for resistive and capacitive switching with and without illumination of light. As grown samples illustrate small non-switching leakage current under high applied electric fields and probe frequencies and trap charge assisted counter-clockwise capacitance–voltage behavior. A unique resistance switching was observed under illumination of 15–60 mW light. In the first cycle, the light assisted switching provide a 104 : 1 resistance ratio, which diminishes in the next cycle onward, which may be due to irreversible charge injection in the oxide layers. The band offset and band match-up energy diagram for the charge carriers responsible for resistive switching and charge trapping near the interface have been demonstrated under the application of a bias electric field and light.

Photoconductivity and photo-detection response of multiferroic bismuth iron oxide

We report visible light detection with in-plane BiFeO3 (BFO) thin films grown on pre-patterned inter-digital electrodes. In-plane configured BFO film displayed photocurrents with a 40:1 photo-to-dark-current ratio and improved photo-sensing ability for >15 000 s (4 h) under small bias voltage (42 V). Nearly, sixty percent of the photo-induced charge carriers decay in 1.0 s and follow a double-exponential decay model. At 373 K, the effect of light does not significantly increase the dark current, probably due to reduced mobility. Sub-bandgap weak monochromatic light (1 mw/cm2) shows one fold increase in photo-charge carriers.

Ferroelectric polymer-ceramic composite thick films for energy storage applications

We have successfully fabricated large area free standing polyvinylidene fluoride -Pb(Zr0.52Ti0.48)O3 (PVDF-PZT) ferroelectric polymer-ceramic composite (wt% 80–20, respectively) thick films with an average diameter (d) ∼0.1 meter and thickness (t) ∼50 μm. Inclusion of PZT in PVDF matrix significantly enhanced dielectric constant (from 10 to 25 at 5 kHz) and energy storage capacity (from 11 to 14 J/cm3, using polarization loops), respectively, and almost similar leakage current and mechanical strength. Microstructural analysis revealed the presence of α and β crystalline phases and homogeneous distribution of PZT crystals in PVDF matrix. It was also found that apart from the microcrystals, well defined naturally developed PZT nanocrystals were embedded in PVDF matrix. The observed energy density indicates immense potential in PVDF-PZT composites for possible applications as green energy and power density electronic elements.

Experimental evidence of electronic polarization in a family of photo-ferroelectrics

We report a family of ferroelectric materials which exhibit optical control of dielectric constant, polarization, and conductivity at ambient conditions. The evolution of photo-induced charge carriers under illumination of weak monochromatic light of wavelength λ = 405 nm developed electronic polarization that reorients or enhances the ionic polarization in the direction of the applied electric field, hence significantly increasing the net polarization. The presence of electronic polarization in perovskites, especially ABO3 oxides, and the contribution to the ionic polarization, is a matter of debate among ferroelectric community; however, recent experimental evidence proved that electronic polarization contributes to overall polarization in different ways: if it is additive, it will act along the direction of ionic polarization; and if it is subtractive, it will act in the opposite direction. We report a newly developed lead-based ferroelectric family Pb1−x(Li0.5Bi0.5)x(Zr0.2Ti0.8)O3 (PLBZT) for x = 0.3, 0.4, 0.5, which have been successfully synthesized by a conventional solid state reaction route. The X-ray diffraction patterns were recorded at room temperature and refined with Rietveld parameters which confirm its tetragonal structure with P4mm space group symmetry. The Raman spectra, dielectric properties and leakage currents are analyzed. The current conduction mechanisms obey bulk limited Poole–Frenkel model and provide optical dielectric constant similar to theoretical value to lead zirconate titanate.

Novel optically active lead-free relaxor ferroelectric (Ba0.6Bi0.2Li0.2)TiO3.

We discovered a near-room-temperature lead-free relaxor-ferroelectric (Ba0.6Bi0.2Li0.2)TiO3 (BBLT) having A-site compositionally disordered ABO3 perovskite structure. Microstructure-property relations revealed that the chemical inhomogeneities and development of local polar nano-regions (PNRs) are responsible for dielectric dispersion as a function of probe frequencies and temperatures. Rietveld analysis indicates mixed crystal structure with 80% tetragonal structure (space group P4mm) and 20% orthorhombic structure (space group Amm2), which is confirmed by the high resolution transmission electron diffraction (HRTEM). Dielectric constant and tangent loss dispersion with and without illumination of light obey nonlinear Vogel-Fulcher (VF) relations. The material shows slim polarization-hysteresis (P-E) loops and excellent displacement coefficients (d 33 ~ 233 pm V(-1)) near room temperature, which gradually diminish near the maximum dielectric dispersion temperature (T m ). The underlying physics for light-sensitive dielectric dispersion was probed by x-ray photon spectroscopy (XPS), which strongly suggests that mixed valence of bismuth ions, especially Bi(5+) ions, comprise most of the optically active centers. Ultraviolet photoemission measurements showed most of the Ti ions are in 4 +  states and sit at the centers of the TiO6 octahedra; along with asymmetric hybridization between O 2p and Bi 6s orbitals, this appears to be the main driving force for net polarization. This BBLT material may open a new path for environmental friendly lead-free relaxor-ferroelectric research.

Studies of ferroelectric properties and leakage current behaviour of microwave sintered ferroelectric Na0.5Bi0.5TiO3 ceramic

ABSTRACT Single phase lead-free ferroelectric ceramic Na0.5Bi0.5TiO3 (NBT) is synthesized using sol-gel method and sintered using microwave sintering technique. The structural, microstructural, and electrical properties are investigated in detail. Rietveld refinement technique is used to analyse XRD pattern. SEM micrograph shows the densely packed micrometre sized grains. Ferroelectric hysteresis loops for unpoled and poled samples confirm its ferroelectric nature. The characteristic ferroelectric properties like remanent polarization and coercive field are found to be 84.8 µC/cm2, 71.8 µC/cm2 and 58.8 kV/cm, 58.6 kV/cm respectively for the unpoled and poled samples. The current versus voltage curve of the NBT follow the bulk-limited space-charge conduction mechanism.

Polaron-electron assisted giant dielectric dispersion in SrZrO3 high-k dielectric

The SrZrO3 is a well known high-k dielectric constant (∼22) and high optical bandgap (∼5.8 eV) material and one of the potential candidates for future generation nanoelectronic logic elements (8 nm node technology) beyond silicon. Its dielectric behavior is fairly robust and frequency independent till 470 K; however, it suffers a strong small-polaron based electronic phase transition (Te) linking 650 to 750 K. The impedance spectroscopy measurements revealed the presence of conducting grains and grain boundaries at elevated temperature which provide energetic mobile charge carriers with activation energy in the range of 0.7 to 1.2 eV supporting the oxygen ions and proton conduction. X-ray photoemission spectroscopy measurements suggest the presence of weak non-stoichiometric O2− anions and hydroxyl species bound to different sites at the surface and bulk. These thermally activated charge carriers at elevated temperature significantly contribute to the polaronic based dielectric anomaly and conductivity. O...