Effect of Sr, La, Zr and Mn dopants on the microstructure and dielectric properties of BaTiO3-based electroceramics

Abstract

Electroceramics are necessary component in modern technologies of many kinds. In this regard, barium meta-titanate (BaTiO3 or BT) based materials with ABO3 perovskite structure are potential candidates for applications in electronic devices because of their piezoelectric, ferroelectric and optical properties. Barium titanate based materials have attracted significant interest; due to their wide spread applications in wireless communication, space and defense. Barium titanate based ferroelectric materials are suitable for these applications due to their electric field dependent permittivity (εr) and low dielectric loss (tan δ) above Curie temperature (Tc). Ferroelectric materials generally have high dielectric losses, which is due to piezoelectric grain resonance and domain wall motion. It is known that Tc and dielectric properties of BaTiO3 can be systematically changed by chemical substitution at A or B-site by a wide variety of isovalent and aliovalent dopants. The effect of small additives and dopants on the dielectric properties of BaTiO3 based ceramics fabricated by conventional solid state sintering route have been investigated in detail; however, little attention have been given to sol-gel processing of ceramics for dielectric applications. In the present study, the microstructure and dielectric properties of BaTiO3- based ceramics doped with Sr, La, Zr and Mn were investigated in detail. Various compositions [BaTiO3, Ba1-xSrxTiO3 (x = 0.25, 0.35, 0.45, 0.55, 0.65), Ba1-xLaxTi1- x/4O3 (x = 0.00, 0.015, 0.025, 0.035, 0.045, 0.055), Ba0.7Sr0.3Ti1-xZrxO3 (x = 0.02, 0.04, 0.06, 0.08, 0.1), (Ba0.5Sr0.5)Mnx(Ti0.95Fe0.05)1-xO3 (x = 0.00, 0.004, 0.006, 0.008, 0.01, 0.02)] with different stoichiometric ratios were synthesized by using modified sol-gel technique. The dielectric properties of the BaTiO3-based ceramics were studied by making different compositions by substituting the above elements to form solid solutions. Thermal analysis (TG/DTA) was used to trace the synthesis temperature. The temperature range from 750 oC‒900 oC was found suitable for single phase formation of the BaTiO3-based ceramics processed via sol-gel route. The phase and microstructure of the final ceramics were investigated using X- ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Fourier transform infrared spectroscopy (FTIR) was used to investigate the water content and organic functional groups in calcined samples. The dielectric properties were investigated using LCR-meter and impedance analyzer. The tetragonal symmetry (P4mm) of the BaTiO3 ceramics was confirmed by the XRD studies. XRD studies confirmed the tetragonal crystal structure of the x = 0.00 – 0.035 compositions at room temperature of the Ba1-xLaxTi1-x/4O3 ceramic samples. A cubic symmetry (Pm3m) was observed for the x > 0.035 compositions around room temperature. Analysis of the XRD data confirmed the formation of tetragonal (P4mm) phase for all the Ba0.7Sr0.3Ti1-xZrxO3 samples at room temperature. Ceramic samples of the (Ba0.5Sr0.5)Mnx(Ti0.95Fe0.05)1-xO3 showed tetragonal (P4mm) perovskite structure. Microstructure analysis of the thermally etched surface showed a heterogeneous grain growth of the BaTiO3 ceramics. The microstructure investigation revealed that La and Mn substitution acted as growth inhibitor for the BaTiO3- ceramics. Samples doped with Sr and Zr resulted in large grains with a heterogeneous microstructure. Dielectric investigation confirmed that Tc was shifted to low temperatures for the samples doped with Sr, La and Zr, while no obvious changes in the Tc was detected for the samples doped with Mn. Moreover, samples doped with Mn and Zr revealed a diffused phase transition (DPT). The impedance studies confirmed both the grain and grain boundary contributions for the BaTiO3 ceramics. For Sr-doped samples, compositions with x = 0.25 and x = 0.45 showed one semicircle in the capacitance range (≈ 3.4178 × 10-10 F and 5.2016 × 10-10 F, respectively), which was associated with the grain boundary contribution. The samples with x = 0.35, 0.55 and 0.65 showed two semicircles in low and high frequency regimes, which confirmed the bulk and grain boundary contributions to the conduction mechanism. The samples doped with La showed the grain boundary contributions, only. Similarly, Zr and Mn-doped compositions showed both grain boundary and bulk contributions in the conduction mechanisms for all the samples. The total resistance (RT) was observed to decrease with increasing temperature for the pure and Sr, La, Zr and Mn doped BaTiO3 based compositions showing a typical ceramic behaviour.