STRUCTURE - PROPERTY - COMPOSITIONAL RELATIONSHIPS IN THE BaTiO3 - BiScO3 CERAMIC SYSTEM

Open Access
Author:
Ogihara, Hideki
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 16, 2008
Committee Members:
  • Clive A Randall, Thesis Advisor
  • Susan E Trolier Mckinstry, Thesis Advisor
Keywords:
  • non-lead materials
  • tolerance factor
  • perovskite structure
  • relaxors
  • dielectrics
  • ferroelectrics
  • bismuth
  • scandium
  • barium titanate
  • permittivity
  • high energy density capacitor materials
  • dielectric relaxation
Abstract:
The structural and dielectric properties of bulk (1-x) BaTiO3 - x BiScO3 (x = 0 to 0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the phase diagram, displacive phase transitions, the associated dielectric properties, and the differences in the roles of Bi2O3 and BiScO3 doping in BaTiO3. The solubility limit for BiScO3 into the BaTiO3 perovskite structure was determined to be about x = 0.4, which is much higher than the solubility of Bi alone. A structural change from tetragonal to pseudocubic was observed at about x = 0.05 - 0.075 at room temperature. The solubility was investigated through the systematic trends in the dielectric data, lattice parameter, and microstructure. Dielectric measurements revealed a gradual change from normal ferroelectric behavior in pure BaTiO3 to a highly diffusive and dispersive relaxor behavior from 10 to 40 mol% BiScO3. Several of the compositions showed high permittivities (approximately 1000) with low temperature coefficients of capacitance over a wide range of temperature. This result confirmed the result from the earlier work in thin films that the broad, frequency-dependent-permittivity maximum is an intrinsic material property, neither a consequence of defects or stresses associated with the thin film preparation, nor a space charge contribution. It was found that the temperature for the maximum dielectric constant, Tmax, increases with increasing BiScO3 concentration. The Vogel-Fulcher model gave an activation energy of 0.2-0.3 eV for the relaxor behavior range, which was high compared with other relaxors. The attempt frequency was on the order of 1013 Hz and the freezing temperature, Tf, ranged from -177 to -93 oC (96 to 160 K) as a function of composition. The dielectric constant is high and stable from 0 E300 oC for the 0.7 BaTiO3 - 0.3 BiScO3 ceramics, making this composition attractive for high energy density capacitor applications. A single dielectric layer capacitor was prepared to confirm the feasibility of BaTiO3 - BiScO3 as a capacitor material. The 0.7 BaTiO3 - 0.3 BiScO3 capacitors exhibited an energy density of about 6.1 J/cm3 at a field of 73 kV/mm at room temperature, which is superior to some commercial X7R capacitors. Moreover, this system may be very useful at high temperatures, as the permittivity of commercial BaTiO3 Ebased capacitors drops rapidly above the transition temperature (around 130 oC). In this study, relatively high, temperature-stable energy density values were confirmed from room temperature to 300 oC. Another application possibility is as a base composition for lead-free piezoelectrics. Undoped BaTiO3 - BiScO3 ceramics are electrostrictors at room temperature over a wide composition range, but the highly polarizable material may be useful as a piezoelectric matrix if a normal ferroelectric phase could be developed. Possible off-center ions, Li and Na were doped into the system in an attempt to stabilize a normal ferroelectric phase. LiNbO3 could not achieve a solid solution. For the case of Na, solid solution was confirmed up to 15 mol% NaNbO3, but there was no indication of the stabilization of a normal ferroelectric. Instead, it was found that Tmax and Tf decrease with increasing NaNbO3 concentration. This result suggests that NaNbO3 doping breaks up the polar ordering developed in the BaTiO3 - BiScO3 system.