|Abstract or Summary
- The aim of this research is to develop a fundamental understanding of the dominant defect species and the relevant defect equilibrium conditions for bismuth-containing perovskites to help guide the development of these materials for emerging applications. This is of paramount importance for many demanding applications, because ultimately the defect equilibria have a profound influence on phenomena such as piezoelectric fatigue, reliability, and leakage current. At the same time, they can be used to tailor properties to make these materials better suited for specific applications. Perovskite materials with Bi³⁺ on the A-site have been the focus of great technical interest over the last decade. A number of compositions based on Bi-perovskites (e.g. (Bi,Ba)(B²⁺,Ti)O₃) are being studied for high energy density (or high temperature) capacitor applications. In addition, a number of Bi-based perovskite materials have shown great promise as a replacement for Pb(Zr,Ti)O₃ (PZT) for piezoelectric applications. Compounds such as (Bi₁/₂K₁/₂)TiO₃ (BKT) and (Bi₁/₂Na₁/₂)TiO₃ (BNT), and their solid solutions with BaTiO₃ and other tetragonal perovskites exhibit useful piezoelectric properties and are considered to be promising candidates to replace Pb-based materials if the underlying defect chemistry can be controlled. The technological impact of these materials is expected to grow because research in this area is being driven by increased environmental regulations and energy efficiency considerations. While much of the current research and progress on Bi-perovskites have been focused on primary materials properties like the piezoelectric coefficient, relative permittivity, etc., there have been few studies on the underlying fundamental defect chemistry and they are not fully understood.
This research focuses mainly on two material systems to study their defect chemistry and transport properties. First is Bi(Zn₁/₂Ti₁/₂)O₃–BaTiO₃, for high performance capacitor applications. Conventional approaches to this technical challenge include utilizing ferroelectric or antiferroelectric materials with permittivities in excess of 1000. However, these non-linear materials derive their high permittivity from domain contributions that saturate at relatively low fields ultimately resulting in limited energy densities. However, solid solutions based on BiMO₃-BaTiO₃ that exhibit relaxor behavior can potentially demonstrate high energy densities. The second material system investigated was BNT-BKT- Bi(Mg₁/₂Ti₁/₂)O₃. This material features a field-induced relaxor-to-ferroelectric transition that is accompanied by a large piezoelectric strain values.
The first part of the thesis focuses on polycrystalline BaTiO₃-Bi(Zn₁/₂Ti₁/₂)O₃ (BT-BZT) ceramics, which have been shown to exhibit superior dielectric properties for high temperature and high energy density applications as compared to the existing materials. As miniaturization without compromising cost and performance is vital for several applications, chapter 9.1 shows the results on multilayer ceramic capacitors based on relaxor BT-BZT ceramics. In bulk ceramic embodiments, BT-BZT has been shown to exhibit relative permittivities greater than 1000, high resistivities (ρ > 1 GΩ-cm at 300°C), and negligible saturation up to fields as high as 150 kV/cm. The multilayer capacitor embodiments exhibited similar dielectric and resistivity properties. The energy density for the multilayer ceramics reached values of ~2.8 J/cm³ at room temperature at an applied electric field of ~330 kV/cm. This represents a significant improvement compared to commercially available multilayer capacitors. The dielectric properties were also found to be stable over a wide range of temperatures with a temperature coefficient of approximately -2000 ppm/K measured from 50 to 350 °C, an important criteria for high temperature applications. Finally, the compatibility of inexpensive Ag-Pd electrodes with these ceramics was also demonstrated, which can have implications on minimizing the device cost.
Having demonstrated that BT-BZT exhibits promising properties, the primary focus of this thesis research is developing a fundamental understanding of the transport properties and defect chemistry. A significant improvement in insulation properties was measured with the addition of BZT to BT. Both low-field AC impedance and high field direct DC measurements indicated an increase in resistivity of at least 2 orders of magnitude at 400 °C with the addition of just 3% BZT (~10⁷ Ω-cm) into the solid solution as compared to pure BT (~10⁵ Ω-cm). This effect was also evident in dielectric loss data, which remained low at higher temperatures as the BZT content increased. In conjunction with band gap measurements, it was also concluded that the conduction mechanism transitioned from extrinsic for pure BT to intrinsic-like for 7.5% BZT suggesting a change in the fundamental defect equilibrium conditions. It was also shown that this improvement in insulation properties was not limited to BT-BZT, but could also be observed in SrTiO₃-BZT system.
While pure BT exhibits extrinsic p-type conduction, it is reported that BT-BZT ceramics exhibit intrinsic-like n-type conduction using atmosphere dependent conductivity measurements. Annealing studies and Seebeck measurements were performed and confirmed this result. For BT, resistivity values were higher for samples annealed in nitrogen as compared to oxygen, while the opposite responses were observed for BZT-containing solid solutions. This suggested a possibile unintentional donor doping upon addition of BZT to the solid solution, which may also be linked to the improvement in resistivity in BT-BZT ceramics as compared to pure BT.
Impedance spectroscopy in conjunction with small DC-bias provided further proof of the p-type to n-type transition and also demonstrated the field-stable properties of BT-BZT ceramics. For p-type BaTiO₃, the ceramics deviated from Ohm’s law behavior at very low voltage levels along with a reversible drop in bulk resistivity by several orders of magnitude starting at bias fields as low as 0.1 kV/cm (~8 V). In contrast, n-type BT-BZT ceramics exhibited a small (i.e. less than one order of magnitude) increase in resistivity on application of small field levels. These data indicate a hole-generation mechanism which becomes active at a low voltage threshold. The bulk capacitance values calculated using AC impedance spectroscopy, however, were relatively unaffected (<15% change) by this application of a DC bias (up to ~0.25 kV/cm). These findings provide further insights into the electric transport mechanisms in BT-based ceramics.
To investigate the possible presence of Bi⁵⁺ in BT-BZT ceramics, which was postulated to be one of the possible mechanisms for n-type behavior in BT-BZT ceramics, some BT-BaBiO₃ solid solutions were fabricated. The BaBiO₃ ceramics were sintered in oxygen to obtain a single phase with monoclinic I2/m symmetry as suggested by high-resolution x-ray diffraction. X-ray photoelectron spectroscopy confirmed the presence of bismuth in two valence states – 3+ and 5+. Optical spectroscopy showed presence of a direct band gap at ~2.2 eV and a possible indirect band gap at ~0.9 eV. This combined with determination of the activation energy for conduction of 0.25 eV, as obtained from ac impedance spectroscopy, suggested that a polaron-mediated conduction mechanism was prevalent in BaBiO₃. These BaBiO₃ ceramics were crushed, mixed with BaTiO₃, and sintered to obtain BaTiO₃-BaBiO₃ solid solutions. All the ceramics had tetragonal symmetry and exhibited a normal ferroelectric-like dielectric response. Using ac impedance and optical spectroscopy, it was shown that resistivity values of BaTiO₃-BaBiO₃ were orders of magnitude higher than BaTiO₃ or BaBiO₃ alone, indicating a change in the fundamental defect equilibrium conditions. A shift in the site occupancy of Bi to the A-site is proposed to be the mechanism for the increased electrical resistivity.
To investigate the effect of A-site nonstoichiometry in BT-BiMO₃ ceramics, BaTiO₃-BiScO₃ (BT-BS) and SrTiO₃-Bi(Zn₁/₂Ti₁/₂)O₃ (ST-BZT) were fabricated. The effect of nonstoichiometry on the dielectric and transport properties was studied using temperature- and oxygen partial pressure-dependent AC impedance spectroscopy. For p-type BT-BS ceramics, the addition of excess Bi led to effective donor doping along with a significant improvement in insulation properties. A similar effect was observed on introducing Ba vacancies onto the A-sublattice. However, Bi deficiency had an opposite effect with effective acceptor doping and a deterioration in the bulk resistivity values. For n-type intrinsic ST-BZT ceramics, the addition of excess Sr onto the A-sublattice resulted in a decrease in resistivity values, as expected. Introduction of Sr vacancies or addition of excess Bi on A-site did not appear to affect the insulation properties in air. These results indicate that minor levels of non-stoichiometry can have an important impact on the material properties and furthermore it demonstrates the difficulties encountered in trying to establish a general model for the defect chemistry of Bi-containing perovskite systems.
Finally, the other prospective candidates for n-type behavior in BT-BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x-ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT-BZT ceramics as compared to BT alone. X-ray photoelectron spectroscopy and x-ray absorption spectroscopy did not suggest presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT-BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of conductivity-oxygen partial pressure curves, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. However, these defects in BT-BZT ceramics have an effect of shifting the conductivity minimum in conductivity-oxygen partial pressure curves to higher oxygen partial pressure values, resulting in significantly higher resistivity values in air atmosphere. This provides an important tool to tailor transport properties and defects in BT-BiMO₃ ceramics, to make them better suited for dielectric applications.
The second Bi-based ceramic system which was looked at was lead-free Bi(Mg₁/₂Ti₁/₂)O₃-(Bi₁/₂K₁/₂)TiO₃-(Bi₁/₂Na₁/₂)TiO₃ for sensors and actuator applications. There has been a huge drive to replace Pb from existing ceramics (e.g. lead zirconate titanate) due to health and environmental concerns. The dielectric spectra showed a T[subscript max] of more than 320 °C for all compositions and the transitions became increasingly diffuse as the Bi(Mg₁/₂Ti₁/₂)O₃ content increased. A lower temperature transition, indicating a transformation from an ergodic to a non-ergodic relaxor state, was also seen for all compositions and this transition temperature decreased as the mole fraction of Bi(Mg₁/₂Ti₁/₂)O₃ increased. The composition with 1% Bi(Mg₁/₂Ti₁/₂)O₃ showed characteristic ferroelectric-like polarization and strain hysteresis. However, compositions with increased Bi(Mg₁/₂Ti₁/₂)O₃ content became increasingly ergodic at room temperature with pinched polarization loops and no negative strain. Among these compositions, the magnitude of d₃₃* increased with Bi(Mg₁/₂Ti₁/₂)O₃ content and the composition with 10% Bi(Mg₁/₂Ti₁/₂)O₃ exhibited a d₃₃* of 422 pm/V . Fatigue measurements were conducted on all compositions and while the 1% Bi(Mg₁/₂Ti₁/₂)O₃ composition exhibited a measurable, but small loss in maximum strain after a million cycles; all the other compositions from 2.5% to 10% Bi(Mg₁/₂Ti₁/₂)O₃ were essentially fatigue-free. Lastly, optical and AC impedance measurements were employed to identify intrinsic conduction as the dominant conduction mechanism. These compositions were also highly insulating with high resistivities (~10⁷ Ω-cm) at high temperatures (440 °C).
To investigate the role of point defects on the fatigue characteristics, the composition 5%BMT-40%BKT-55%BNT was doped to incorporate acceptor and donor defects on the A and B sites by adjusting the Bi/Na and Ti/Mg stoichiometries. All samples had pseudo-cubic symmetries based on x-ray diffraction, typical of relaxors. Dielectric measurements showed that the high and low temperature phase transitions were largely unaffected by doping. Acceptor doping resulted in the observation of a typical ferroelectric-like polarization with a remnant polarization and strain hysteresis loops with significant negative strain. Donor-doped compositions exhibited characteristics that were indicative of an ergodic relaxor phase. Fatigue measurements were carried out on all of the compositions. While the A-site acceptor-doped composition showed a small degradation in maximum strain after 10⁶ cycles, the other compositions were essentially fatigue free. Impedance measurements were used to identify the important conduction mechanisms in these compositions. As expected, the presence of defects did not strongly influence the fatigue behavior in donor-doped compositions owing to the nature of their reversible field-induced phase transformation. Even for the acceptor-doped compositions, which had stable domains in the absence of an electric field at room temperature, there was negligible degradation in the maximum strain due to fatigue. This suggests that either the defects introduced through stoichiometric variations do not play a prominent role in fatigue in these systems or it is compensated by factors like decrease in coercive field, an increase in ergodicity, symmetry change, or other factors.
The results obtained for these ceramic systems have provided significant insights in the defect chemistry and transport properties and are expected to help improve performance of these emerging materials for energy and MEMS technologies.