A and B Site Substitutions of BaTiO3 Based Ferroelectrics for Piezoelectric, Electrocaloric and High-Power Energy Storage Applications

Abstract

Ferroelectric materials are widely used in many applications such as, sensors, actuators, electrocaloric devices as well high-power energy storage capacitors. Conventional Pb-based ABO₃ ferroelectrics with perovskite crystal structure exhibit large polarization and high ferroelectric transition temperature (T_c), which make them attractive for these applications. However, due to the health hazards and environmental concerns regarding lead (Pb), the development of Pb free ceramics has attracted much attention among the scientific community in recent years. In this work, BaTiO₃-based solid solutions are explored as potential Pb-free ferroelectrics for specific applications by employing different doping strategies. An underlying theme is to effectively examine the possibility co-substituting Sn²⁺ and Ca²⁺ at the A-site of BaTiO₃-based perovskites to enhance off-centered polar displacements of the A-site ions, while facilitating formation of polar nanoregions and relaxor behavior through effective B-site dopants. In order to take into account the disproportion of Sn among Sn²⁺ and Sn⁴⁺, a co-doping strategy of Sn at both A and B sites were implemented.

In the first part, Sn²⁺ and Ca²⁺ substituted (1-x)(Ba)TiO₃-xBiScO₃ ceramics are presented as material for high temperature capacitor applications. In recent years, solid-solutions of the form (1-x)BaTiO₃-xBiMeO₃, where Me represents one or more metal ions, were shown to exhibit promising dielectric properties for high-energy-density, high-temperature capacitor applications. The attractiveness of these materials is due to a relatively flat region in their high-temperature dielectric spectrum, which ensures minimum fluctuations in capacitor performance as a function of temperature. Nevertheless, they face several shortcomings, such as a large frequency dispersion of dielectric properties near room temperature and relatively high concentration of Bi that could react with metal electrodes.

In this work, the incorporation of Sn²⁺ and Ca²⁺ at the A-site is demonstrated as an effective strategy for partial replacement of Bi, which produced frequency independent (200 Hz ≤ f ≤ 1 MHz) dielectric response and less than ±15% variation in dielectric permittivity in the temperature range of RT-200 C. Such behavior could be explained as a result of stabilization of the cubic structure, which is deduced from Rietveld refinement analysis of temperature-dependent X-ray diffraction data. Sn²⁺ and Ca²⁺ substituted (1-x)(Ba)TiO₃-xBiScO₃ ceramics exhibited attractive properties for high temperature capacitor applications, such as extremely low loss, large energy discharge density of 0.55 J/cm³ and high-energy storage efficiency of ~ 91%.

In the second part, co-substitution at A and B sites of BaTiO₃ with ions of different radii and covalency is explored as a possible means to induce relaxor behavior and large electrothermal properties. Specifically, Sn²⁺ (A-site) and Nb⁺⁵ (B-site) doped (Ba,Ca)(Ti)O₃ ceramics are developed, which exhibited relaxor behavior, together with showed large pyroelectric and electrocaloric properties. Specific heat measurements and Rietveld analysis of high-resolution X-ray diffraction patterns revealed a broad dielectric permittivity peak near Tm ~ 300 K, which marks transition from from cubic to tetragonal phase. Interestingly, a nonlinear increase in tetragonality (c/a) of the crystal structure is also observed below 270 K, which further broadens the dielectric peaks and can be likely attributed to a growth of polar nanodomains. The broad dielectric permittivity peak of the material can be selectively used for pyroelectric energy harvesting or electrocaloric cooling. A peak value of pyroelectric coefficient (∂P/∂T)~ 4000 μCm^-2K^-1 is observed near 270 K, followed by a relatively temperature independent (∂P/∂T)~ 916 μCm^-2K^-1 in the range of 275-325 K. The two broad peaks for pyroelectric coefficient observed in the range of 270–325 K can be potentially useful for energy harvesting by utilizing the induced pyroelectric current from large thermal fluctuations. The electrocaloric temperature changes were deduced from temperature dependent polarization values using Maxwell’s relations, as well as from direct measurements. The material exhibited a maximum electrocaloric temperature change of 0.5 K and refrigeration capacity of ~11 J/kg near room temperature, which are comparable to some of the more well-known

In the third part, the incorporation of Sn²⁺ at the A-site of (Ba,Ca)(Ti,Zr)O₃ or BCZT ceramics is presented as a possible Pb-free piezoceramic material for actuator applications with extended operational temperature range. In recent years, Pb-free (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BCZT) piezoceramics have shown extraordinary electromechanical properties. However, one purported shortcoming for BCZT ceramics is their low Tc (50-80 C), which limit the maximum operational temperature to below 100 C. Here, doping with Sn²⁺ is used a potential strategy to extend the temperature range of operation for BCZT ceramics. Solid solution of (Ba_0.95Ca_0.043Sn_0.005)(Ti_0.875Zr_0.12Sn_0.005)O₃ was successfully synthesized using conventional solid state method, followed by annealing under low pO₂ environment. The co-existence of Sn²⁺ and Sn⁴⁺ was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The phase transition behavior of the synthesized compound was examined using temperature dependent dielectric spectroscopy and X-ray diffraction measurements. Examination of temperature dependent electromechanical properties showed that the new material exhibited a maximum d*₃₃ ~ 500 pm/V at room temperature and d*₃₃ ~ 300 pm/V at 120 C (for applied field of 3 kV/mm), which are higher than those of undoped BCZT ceramics. A nearly linear electric field-induced strain response under low electric fields (< 1 kV/mm) at room temperature can be attributed to an orthorhombic-tetragonal phase transition. In comparison, a quadratic electrostrictive response at high temperatures can be attributed to the presence of local polar distortions.

Overall, the present work demonstrates that a co-doping strategy of Sn₂₊ and Sn⁴⁺ at A- and B-sites, in combination with appropriate B-site substitutions, in BaTiO3 could be a potential route for designing new Pb-free ferroelectrics for electromechanical, electrocaloric, and energy storage applications.

Date of Award	22 Jul 2021
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Abhijit PRAMANICK (Supervisor) & S. Pang (Supervisor)