K. B. R. Varma

Tuning of dielectric, pyroelectric and ferroelectric properties of 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ceramic by internal clamping

This study systematically investigates the phenomenon of internal clamping in ferroelectric materials through the formation of glass-ceramic composites. Lead-free 0.715Bi(0.5)Na(0.5)TiO(3)-0.065BaTiO(3)-0.22SrTiO(3) (BNT-BT-ST) bulk ferroelectric ceramic was selected for the course of investigation. 3BaO - 3TiO(2) - B2O3 (BTBO) glass was then incorporated systematically to create sintered samples containing 0%, 2%, 4% and 6% glass (by weight). Upon glass induction features like remnant polarization, saturation polarization, hysteresis losses and coercive field could be varied as a function of glass content. Such effects were observed to benefit derived applications like enhanced energy storage density similar to 174 k J/m(3) to similar to 203 k J/m(3) and pyroelectric coefficient 5.7x10(-4) Cm-2K-1 to 6.8x10(-4) Cm-2K-1 by incorporation of 4% glass. Additionally, BNT-BT-ST depolarization temperature decreased from 457K to 431K by addition of 4% glass content. Glass incorporation could systematically increases diffuse phase transition and relaxor behavior temperature range from 70 K to 81K and 20K to 34 K, respectively when 6% and 4% glass content is added which indicates addition of glass provides better temperature stability. The most promising feature was observed to be that of dielectric response tuning. It can be also used to control (to an extent) the dielectric behavior of the host ceramic. Dielectric permittivity and losses decreased from 1278 to 705 and 0.109 to 0.107 for 6% glass, at room temperature. However this reduction in dielectric constant and loss increases pyroelectric figures of merit (FOMs) for high voltage responsivity (F-v) high detectivity (F-d) and energy harvesting (F-e) from 0.018 to 0.037 m(2)C(-1), 5.89 to 8.85 mu Pa-1/2 and 28.71 to 61.55 Jm(-3)K(-2), respectively for 4% added ceramic-glass at room temperature. Such findings can have huge implications in the field of tailoring ferroelectric response for application specific requirements

Relaxor Ferroelectric Behavior and Structural Aspects of SrNaBi2Nb3O12 Ceramics

The SrNaBi2Nb3O12 (SNBN) powder was prepared via the conventional solid-state reaction method. X-ray structural studies confirmed the phase to be a three-layered member of the Aurivillius family of oxides. The SNBN ceramics exhibited the typical characteristics of relaxor ferroelectrics, associated with broad and dispersive dielectric maxima. The variation of temperature of dielectric maxima (T-m) with frequency obeyed the Vogel-Fulcher relationship. Relaxor behavior was believed to be arising from the cationic disorder at A-site. Pinched ferroelectric hysteresis loops were observed well above T-m.

Incipient ferroelectric to a possible ferroelectric transition in Te4+ doped calcium copper titanate (CaCu3Ti4O12) ceramics at low temperature as evidenced by Raman and dielectric spectroscopy

Partial replacement of Ti4+ by Te4+ ions in calcium copper titanate lattice improved its dielectric behaviour mostly due to cubic-to-tetragonal structural transformation and associated distortion in TiO6 octahedra. The relative permittivity values (23–30 x 103) of Te4+ doped ceramics is more than thrice that of un-doped ceramics (8 x 103) at 1 kHz. A decreasing trend in relative permittivity with increasing temperature (50–300 K) is observed for all the samples. Barrett’s formula, as a signature of incipient ferroelectricity, is invoked to rationalize the relative permittivity variation as a function of temperature. A systematic investigation supported by temperature dependent Raman studies reveal a possible ferroelectric transition in Te4+ doped ceramic samples below 120 K. The possible ferroelectric transition is attributed to the interactions between quasi-local vibrations associated with the micro-clusters comprising TiO6 and TeO6 structural units and indirect dipole-dipole interactions of off-center ...

Effect of nano- and micron-sized K0.5Na0.5NbO3 fillers on the dielectric and piezoelectric properties of PVDF composites

Polymer nanocrystal composites were fabricated by embedding polyvinylidene fluoride (PVDF) with K0.5Na0.5NbO3 (KNN) nanocrystallites of different volume fraction using the hot-pressing technique. For comparison, PVDF–KNN microcrystal composites of the same compositions were also fabricated which facilitated the studies of the crystallite size (wide range) effect on the dielectric and piezoelectric properties. The structural, morphological, dielectric, and piezoelectric properties of these nano and micro crystal composites were investigated. The incorporation of KNN fillers in PVDF at both nanometer and micron scales above 10 vol% resulted in the formation of polar β-form of PVDF. The room temperature dielectric constant as high as 3273 at 100 Hz was obtained for the PVDF comprising 40 vol% KNN nanocrystallites due to dipole–dipole interactions (as the presence of β-PVDF is prominent), whereas it was only 236 for the PVDF containing the same amount (40 vol%) of micron-sized crystallites of KNN at the same frequency. Various theoretical models were employed to predict the dielectric constants of the PVDF–KNN nano and micro crystal composites. The PVDF comprising 70 vol% micron-sized crystallites of KNN exhibited a d33 value of 35 pC/N, while the nanocrystal composites of PVDF–KNN did not exhibit any piezoelectric response perhaps due to the unrelieved internal stress within each grain, besides the fact that they have less domain walls.

Enhancement in the physical properties of K0.5Na0.5NbO3 ceramics by the addition of 0.5 Li2O – 0.5K2O – 2B2O3 glass

ABSTRACT The addition of powdered 0.5 Li2O–0.5K2O–2B2O3 (LKBO) glass (0.5 to 2 wt%) to potassium sodium niobate, K0.5Na0.5NbO3 (KNN) powder facilitated higher densification which resulted in improved physical properties that include dielectric, piezoelectric and ferroelectric. The required polycrystalline powders of KNN were synthesized through solid-state reaction route, while LKBO glass was obtained via the conventional melt-quenching technique. Pulverized glass was added to KNN powders in different wt% and compacted at room temperature and these were sintered around 1100°C. Indeed the addition of optimum amount (1 wt %) of LKBO glass to KNN ceramics facilitated lowering of sintering temperature accompanied by larger grains (8 µm) with improved density. The dielectric constant (ϵr) measured at room temperature was 475 (at 10 kHz), whereas it was only 199 for the LKBO glass free KNN. The piezoelectric coefficient (d33) was found to be 130 pC/N for 1 wt% LKBO added glass, which was much higher than that of pure KNN ceramics (85 pC/N). Indeed, the LKBO glass added samples did exhibit well saturated P versus E hysteresis loops at room temperature. Though there was no particular trend observed in the variation of Pr with the increase in glass content, the Pr values were higher than that obtained for KNN ceramics. The improved physical properties of KNN ceramics encountered in these studies were primarily attributed to enhancement in density and grain size.

Nonlinear dielectric response of BaBi4Ti4O15ceramics

Nonlinear dielectric response of BaBi4Ti4O15 ceramics synthesized via the conventional solid-state reaction route has been monitored over a wide range of electric field strengths (E0 = 0.5-5 kV/cm). Dielectric permittivity was found to increase linearly within the range of applied field. Rayleigh relations were employed to interpret the nonlinear dielectric response and the contribution of irreversible domain wall motion to the macroscopic permittivity was separated. The values of room temperature Rayleigh dielectric coefficient (α) and relative initial permittivity (e′init) were found to be 2.28±0.02 cm/kV and 146.10±0.07, respectively. A reasonable agreement between the simulated and measured polarization-electric field (P-E) hysteresis loops was observed at an applied electric field of 5 kV/cm.

Nonlinear dielectric response of BaBi[sub 4]Ti[sub 4]O[sub 15] ceramics

Nonlinear dielectric response of BaBi4Ti4O15 ceramics synthesized via the conventional solid-state reaction route has been monitored over a wide range of electric field strengths (E0 = 0.5-5 kV/cm). Dielectric permittivity was found to increase linearly within the range of applied field. Rayleigh relations were employed to interpret the nonlinear dielectric response and the contribution of irreversible domain wall motion to the macroscopic permittivity was separated. The values of room temperature Rayleigh dielectric coefficient (α) and relative initial permittivity (e′init) were found to be 2.28±0.02 cm/kV and 146.10±0.07, respectively. A reasonable agreement between the simulated and measured polarization-electric field (P-E) hysteresis loops was observed at an applied electric field of 5 kV/cm.