Piezoelectric Properties Of BaTiO₃-based Ceramics And Dielectric Properties Of CaCu₃Ti₄O₁₂ Ceramics

Piezoelectric and dielectric materials are two important classes of electronic materials. Piezoelectric materials are a class of functional materials that realize the conversion between mechanical energy and electrical energy and thus are popularly utilized to fabricate sensors, actuators, transducers and other electronic devices. Currently, Pb(Zr,Ti)O3 (PZT)-based piezoelectric ceramics take the predominated position in the market of practical piezoelectric materials because of their excellent electrical properties. However, due to the toxicity of lead oxide that is largely used during the production process, there is an increasing demand to replace PZT with the environment-benign lead-free alternatives. Dielectric materials are widely used to form capacitive devices such as capacitance, resonators and filters. Those dielectric materials with high dielectric permittivity have attracted considerable interest in recent years since they might offer the opportunity to enhance the performance or shrink the dimensional sizes of the microelectronic device. Under these circumstances, this thesis concentrates on the studies of material preparations, physical properties and the related mechanisms for BaTiO3-based piezoelectric ceramics and CaCu3Ti4O12 (CCTO) high-dielectric ceramics.BaTiO3 ceramics is historically the first polycrystalline piezoelectric material and had been once widely used as a piezoelectric material before the discovery of PZT. Nowadays, however, its main technical applications are no longer as a piezoelectric but as a dielectric material, largely because of its poor piezoelectric properties (usually, d33≤190 pC/N) compared with PZT. Nevertheless, surprisingly high d33 values (350, 460 and 788 pC/N, respectively) were reported recently for those BaTiO3 ceramics that were prepared from hydrothermally synthesized fine BaTiO3 powders by some special fabrication techniques like microwave sintering, two-step sintering and templated grain growth (TGG). More importantly, we have recently succeeded in obtaining BaTiO3 ceramics with high piezoelectric properties through conventional solid-state reaction route with starting raw materials of ordinary BaCO3 and TiO2 powders. These results indicate that BaTiO3-based ceramics possess a high possibility to become a good lead-free piezoelectric material. However, related mechanism for the excellent piezoelectric properties remains unclear. Furthermore, considering BaTiO3 undergoes three polymorphic phase transitions in a relatively narrow temperature region, the piezoelectric temperature dependence of the recently obtained high piezoelectric constant BaTiO3 ceramics another concern.CCTO is an oxide that has a cubic perovoskite-related crystal structure and exhibits an enormously large dielectric permittivity (ε') at low frequencies in both forms of single crystals and ceramics. The dielectric permittivity keeps almost constant in the low frequency range below 100 kHz at room temperature and is nearly independent of temperature over the wide temperature region. So far, several models have been proposed to explain the dielectric behavior but are quite controversial, including both intrinsic and extrinsic mechanism explanations from the viewpoint such as crystal structure, internal barrier layer capacitance (IBLC) effect and contact-electrode depletion effect. Besides, though it is widely accepted that the grains of CCTO ceramics are semiconductive, the origin of the semiconductive and relevant conduction behavior are still disputable. This situation is extremely unfavorable for a full understanding of the unusual dielectric property and its related mechanism of CCTO and further research needs to be done.This thesis takes the BaTiO3-based piezoelectric ceramics and CCTO high-dielectric ceramics prepared by the conventional solid-state reaction as research objects. For BaTiO3-based piezoelectric ceramics, the grain size effect on the piezoelectric properties of BaTiO3 ceramics is investigated. The piezoelectric temperature dependence of BaTiO3 ceramics is discussed and ceramics with high piezoelcectric activities and stable temperature dependence are successfully obtained. For CCTO high-dielectric ceramics, the effects of microstructure and electrode on the dielectric and electrical properties are investigated and the high temperature conduction behavior is discussed.1. BaTiO3 ceramics with high piezoelectric coefficient (d33) have been successfully obtained through the conventional solid-state reaction route starting from ordinary BaCO3 and TiO2 powders. The BaTiO3 ceramic with an average grain size about 0.94μm is found to have the excellent piezoelectric properties of d33= 340 pC/N andε'= 4700. This result suggests that it is possible to obtain very high piezoelectric activities and permittivity by the grain size control. By carefully analyzing the variations of permittivity and piezoelectric activities with the changing of grain sizes, it is found that the high piezoelectric acitivities and the high permittivity have the same physical origins. XRD analyzing results show that changes of crystal structure in fine grain BaTiO3 ceramics can not account for the high permittivity and high piezoelectric constant. Some extrinsic contributions must exist in BaTiO3 ceramics with high dielectric permittivity and piezoelectric acitivities. The effect of internal stress on the dielectric permittivity and piezoelectric constant can not be ignored in BaTiO3 ceramics. Internal stress in fine grain BaTiO3 ceramics can lead to phase transition temperature shifts, but great differences of room-temperature dielectric permittivity and piezoelectric constant can not be fully ascribed to the shift of phase transition temperature. However, it is found that high dielectric permittivity and piezoelectric constant are closely related to domain configuration. The d33 values firstly increases and then decreases with the decrease of average domain width. A possible mechanism that results in the piezoelectric properties grain size effect in the present BaTiO3 ceramics is discussed. It is suggested that both density and effective mass of the 90°-domain wall in the BaTiO3 ceramics are considered to be important factors which significantly influence the d33 value.2. The piezoelectric temperature dependence of BaTiO3 ceramics is discussed. Though BaTiO3 ceramics with high piezoelectric activities can be obtained by choosing appropriate preparing parameters, the temperature unstability is a great obstacle to the application of BaTiO3 ceramics. However, by comparing the piezoelectric properties in tetragonal phase with that in orthorhombic phase, it is found that BaTiO3 ceramics exhibit more stable piezoelectric properties in the orthorhombic phase than in the tetragonal phase. This is a very important clue for us to gain the good piezoelectric temperature stability in BaTiO3-based ceramics. Partially substituting Ti with Zr can shift the phase transition temperature upward and is effective in reducing the piezoelectric temperature dependence. However, it is found that the piezoelectric activities decrease with the substituting Ti with Zr. Beside the effect of phase shift, more herringbone pattern domains in the large grain Ba(Ti,Zr)O3 ceramics is an important factor that lead to the decrease of piezoelectric activities. It is found that this could be overcome by incorporating a small amount of CuO additive. Furthermore, the CuO-modified BZT ceramics exhibit weaker long-time degradation and better temperature stability. CuO-modified Ba(Tio.9625Zro.o375)03 ceramics possess piezoelectric properties of d33= 300 pC/N, kp= 0.493, and k33= 0.651 with tanδ= 0.011, and its k33 remains larger than 0.50 in the broad temperature range from -60 to 85℃and is almost constant between -30 and 55℃. The results indicate that CuO-modified Ba(Ti,Zr)O3 ceramics are a promising low-cost lead-free material for practical applications.3. A series of CCTO ceramics are prepared by the conventional solid-state reaction method under various sintering conditions. Dielectric properties and complex impedance spectra are investigated within the frequency range of 40 Hz-110 MHz at room temperature and within the frequency range of 40 Hz-4 MHz at higher temperatures up to 350℃. The high dielectric constant is found to closely relate to the microstructure. A Debye-like relaxation appears above 75℃in the frequency range of 100 Hz-100 kHz, which shows the larger dielectric dispersion strength than that existing in the frequency region higher than 100 kHz. High-temperature dielectric dispersion exhibits a large low-frequency response and two Debye-like relaxations. Their characteristic frequencies follow the Arrhenius-law with the activation energy values of 0.086 eV and 0.632 eV, respectively. Furthermore, the existence of three semicircles in the complex impedance plane is disclosed in the present study, which differs in the reported number of two in literature. These semicircles are considered to represent different electrical mechanisms. The contact-electrode depletion effect is examined. From the analysis, we attribute the impedance semicircle in the high frequency region to the contribution of semiconducting grains and the other two to the contributions of the grain boundaries and electrode depletion effect respectively. An equivalent electrical circuit model is suggested to explain the dielectric and electrical properties, in which a frequency-dependent ZUDR term is included in parallel to one of three in-series connected RC elements. The model well fits simultaneously the data of dielectric dispersion and complex impedance. The activation energy values of the three resistances are calculated to be 0.471 eV,0.627 eV and 0.107 eV, respectively.4. The electrical property of CaCu3Ti4O12 ceramics was studied over the high temperature range of 300-800℃. The Seebeck coefficient S is negative with a large absolute value of～560μV/K at 300℃. The measuredⅠ-Ⅴresponses are highly linear, which indicates that CCTO ceramics as a whole are ohmic at high temperatures and that the measured p reflects essentially the electrical conduction property in the grains of ceramic polycrystalline structure. The change of p with T follows the rule of adiabatic hopping conduction of small-polaron rather than the one of thermally activated conduction. Possible mechanism for the small-polaron formation and transport is discussed. Oxygen vacancy is not the reason for the grain semiconductivity. Possible conduction mechanism may come from the aliovalence of Cu or Ti. A model that the small polaron originates from the aliovalence of Ti4+/Ti3+ is proposed.