ENERGY STORAGE CERAMICS: SYNTHESIS AND CHARACTERIZATION

Abstract

In today’s era, where the world is moving towards miniaturization, light weight, cheaper electronics, easy integration etc., high energy density dielectric capacitors are essential to increase its volumetric efficiency. If the energy density of dielectric capacitors come at par with electrochemical capacitors or even batteries, the diversity of energy storage applications will increase dramatically. Lead zirconate titanate (Pb(Zr1−xTix)O3 or PZT) ferroelectric ceramics exhibit high dielectric and piezoelectric properties and are widely used in many applications such as sensors, actuators, transducers, ceramic capacitors, FRAM chips etc. The sintering of PZT ceramics has been challenging due to the volatile nature of PbO at temperatures >600℃ which is detrimental to dielectric and energy storage performance. Spark plasma sintering (SPS) technique has been successfully used for sintering ceramics to high density at lower sintering temperature than required for conventional sintering process. The tetragonal, rhombohedral, orthorhombic and MPB phase composition of PZT ceramics are synthesized by using SPS technique. Each sintered specimen showed density greater than 98% of theoretical density. The effect of SPS temperature (800, 850, 900, 950℃) on phase composition, phase constitution, grain size and permittivity was investigated for MPB phase composition of PZT ceramics. It was observed that the rapid sintering technique causes compositional fluctuation and optimization of SPS temperature is essential. Rietveld refinement was carried out to identify the phases in the sintered specimens. The PZT specimens sintered at or above 900℃ showed presence of multiple phases instead of single tetragonal phase. It was proposed that the highest permittivity observed for 900℃ sintered specimen was due to the presence of monoclinic phase resulting in enhanced polarization in PZT. The impedance and electric modulus studies were conducted on the PZT samples at room temperature over the frequency range 0.01 Hz to 1 MHz to investigate the different electrical relaxation mechanisms. The analysis revealed approximated Debye type behaviour for low frequency relaxation process for all the samples. The separation of grain and grain boundary relaxation processes was observed in the frequency dependent imaginary electrical modulus M plots and complex modulus spectra rather than in Bode plots and Nyquist plots. Such contrast was attributed to high resistance and low capacitance of the bulk contribution. PZT sintered at 900C compared to samples sintered at other temperatures, has shown unique combination of tetragonal and monoclinic crystal structures of constituent phases and the material exhibits highest impedance, electrical modulus and energy storage performance among all the samples sintered at different temperatures. The transition of micron-sized grains to nano-sized grains in these ceramics significantly influences its phase transition, ferroelectric properties and energy storage performance. Generally, the low permittivity grain boundary layers were reported as the key factor in influencing the dielectric properties in nano ceramics. PbZr0.52Ti0.48O3 compound was synthesized via solid state reaction and sintered by both SPS and conventional sintering (CS) techniques. The sintering time was greatly reduced in SPS samples with time period of the order of minutes relative to time requirement of hours for CS samples. The SPS specimen exhibited dense microstructure with grains in nano-scale region whereas for CS specimen, the microstructure shows broad grain size distribution with micro-scale grains. The Rietveld refinement data revealed that the SPS specimen has the desired phase composition and constitution which is in agreement to the distinct narrow peaks observed at each phase transition in the permittivity curves. In addition, the energy storage efficiency of SPS sample is two times that of CS sample. The underlying reason for such behaviour is due to the suppression of hysteresis behaviour in fine-grained SPS samples due to the increased clamping of domain wall motion that leads to lower remnant polarization. In spite of high dielectric and electromechanical properties of PZT, even for compositions close to the MPB, they are rarely used in commercial devices in pristine form. The effect of 2 mol% La3+ doping on phase composition, microstructure, dielectric permittivity and energy storage performance of PZT(52/48) ceramics has been studied. The spark plasma sintering profile adopted was similar to undoped PZT ceramics and the relative density of the sintered specimen becomes greater than 99%. The analysis of microstructure revealed decrease in grain size of PLZT ceramics over PZT ceramics due to smaller ionic radii of La3+. The room temperature dielectric constant of SPS ceramics was significantly higher than the conventionally sintered counterparts reported which is attributed to enhanced domain wall motion. The temperature dependent permittivity curves are more broadened compared to that of undoped ceramics suggesting increased diffusivity of La3+ ions. The impedance and modulus studies confirm the presence of grain and grain boundary relaxation mechanisms in the PLZT ceramics where the bulk resistance of grain boundaries are higher. Low frequency conductivity behaviour suggests hopping of charge carriers whereas long range conductivity is revealed at high frequency region. The saturation polarization has increased for PLZT ceramics, however, the energy efficiency is decreased by 20% due to large hysteresis in PLZT ceramics. Nejezchleb et al. 1980 reported that the oxygen partial pressure during sintering influences the dielectric properties of the material. The chromium ion has been known to exist in multiple valence states and is doped in PZT to increase the mechanical quality factor when Cr exhibits +3 oxidation state. Therefore, it is interesting to see how SPS technique influences the oxidation state of Cr to affect the dielectric properties and energy storage performance of Cr doped PZT ceramics. The microstructure, phase analysis, impedance and modulus spectroscopy, dielectric and energy storage characteristics have been investigated on Cr doped (0.2, 0.4, 0.6, 0.8 wt %) PZT (52/48) ceramics. The particle size lies between 200 and 500 nm and the particles mostly exhibit faceted morphology. The perovskite structure is not distorted by introduction of Cr ions and no separate peaks related to Cr has been detected which indicates solubility of Cr ions in the PZT ceramics. The Cr doped specimens exhibit ≥ 99.3 % relative density with average grain size lower than the average grain size of undoped PZT specimens. The dielectric permittivity of the materials was found to increase up to 0.6 wt% Cr doping which is explained on the basis of multivalent nature of chromium ion. The energy storage studies reveal increase in remnant polarization and coercive field up to 0.6 wt% Cr content and the results are in agreement with the dielectric behavior. The impedance and modulus spectroscopic studies reveal the contributions from grain relaxation in the material and confirm the non-debye type of relaxation in the material. The DC conductivity curves show a typical Arrhenius type behaviour of electrical conductivity.