Studies On Giant Dielectric-permittivity Properties Of CCTO-type Ceramics And Dielectric Spectra Of Two Types Of Perovskite-structured Lead-free Ceramics

Dielectric materials are widely used to fabricate capacitive devices such as capacitance, resonators and filters. High dielectric-permittivity materials have attracted considerable interest in recent years because they may offer an opportunity to enhance the performance and reduce the sizes of microelectronic devices. Under the circumstances, this thesis work concentrates mainly on the studies of material preparations, physical properties and the related mechanisms for CaCusTiO2and CaCu3Ti4O12-type ceramics, and the investigation on dielectric dispersion spectra of BaTiO3and a sort of KNN-based ceramics.CCTO is the oxide that has a cubic perovskite-related crystal structure and exhibits an enormously large dielectric permittivity at low frequencies in both forms of single crystals and ceramics. The dielectric permittivity is almost a constant in the low frequency range below100kHz at room temperature and is nearly independent of temperature over a wide temperature region. In order to interpret the observed giant dielectric-permittivity (giant-ε') phenomenon for CCTO, several possible mechanisms have been proposed so far from both intrinsic and extrinsic viewpoints and still remain controversial. On the other hand, that giant-ε' phenomenon in CCTO ceramics originating from an internal barrier layer capacitance (IBLC) effect seems to be the most widely accepted explanation at present. In connection with it, oxygen vacancies, CuO segregation as well as aliovalences of Ti and Cu ions were suggested to contribute greatly to the IBLC formation. By this line of reasoning, giant-ε' phenomena are then anticipated to occur popularly in the large family of compositionally and structurally CCTO-like oxides. Nevertheless, only few members were reported to show similarly the giant-ε' behaviors besides CCTO, which is incomprehensible. Therefore, it is essential to quest the physical properties and interrelated mechanisms of CCTO and CCTO-like ceramics, and understand fully the various above issues.The high dielectric properties of BaTiO3were discovered in1943. Since then, BaTiO3-based ceramics have been widely used in manufacturing various electronic components. Initially, the value reported for the dielectric permittivity of BaTiO3ceramics was only about1200～1500. In1954, H. Kniekamp and W. Heywang first found that the dielectric permittivity in BaTiO3ceramics with an average grain size of about1μm is over3000, marking the start of reserch on dielectric grain-size effect in BaTiO3ceramics. In the next decades, many scholars observed high dielectric permittivity of3500～4000in fine-grained BaTiO3ceramics (with the average grain size of～1μm); more surprisingly, R. J. Brandmayr obtained a hot-pressed BaTiO ceramic that has an average grain size is about1μm and the dielectric permittivity of about6000. However, the dielectric permittivity sharply decreases with further decreasing the grain size. A lot of work have been done on the dielectric grain size effect in BaTiO3ceramics, and it is considered that the density of90°-domain has the great influence on dielectric properties. However, no studies have been carried out about the influence of domain configuration change due to the tetragonal-orthohombic phase transition on their dielectric dispersion spectra.As typical lead-free piezoelectric materials,(K,Na)NbO3-based (abbreviated as KNN-based) ceramics have attracted much attention in past few years. In2004, Saito et al. reported that excellent piezoelectric properties are found in some KNN-based ceramics, and the ceramic with chemical composition of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3prepared by a reactive template grain growth technique has a d33value of416pC/N. Recently, we have succeeded in obtaining high piezoelectric properties (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3ceramic with d33value of413pC/N through the conventional solid-state reaction method. It is consdiered that the enhancement of piezoelectric activity in KNN-based ceramics with the substitution of Li1+for K1+or Na1+and the substitution of Ta5+or Sb5+for Nb5+should be mainly ascribed to the shifting the tetragonal-orthohombic phase transition temperature to room temperature. Further, it has been found that the substitution of Sb5+for Nb5+is most effective in the enhancing the piezoelectric activity for KNN-based ceramics. On the other hand, domain structure should have also played some important roles in the observed high piezoelectric activity in KNN-based ceramics, studies on domain structure are still few. In particular, systematical studies on domain-related dielectric relaxation have scarcely been reported. In this work, dielectric spectra of (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics were thus systematically investigated.This thesis treats the CCTO, CCTO-type, BaTiO3and some KNN-based ceramics prepared by the conventional solid-state reaction as the study objects. For CCTO-type ceramics, dielectric properties as well as the effects of sintering condition on their microstructure, crystalline structure, dielectric and electric properties are investigated, and the related high-dielectric mechanisms in CCTO and CCTO-type ceramics are discussed. For the BaTiO3ceramics, attention was paid to the influences of the grain size and the tetragonal-orthohombic phase transition on the dielectric spectra. For the (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics, attention was focused on the influences of the Sb-substitution and and the tetragonal-orthohombic phase transition on the dielectric spectra.1. SrCu3Ti4O12, La2/3Cu3Ti4O12, Bi2/3Cu3Ti4O12and NaCu3Ti3SbO12ceramics were investigated to obtain a systematical understanding about the dielectric behaviors of the compositionally and structurally CaCu3Ti4O12-like oxides and the underlying related mechanism. In opposite to the literature results, giant dielectric-permittivity phenomena with low-frequency ε' larger than104have been observed in all of these ceramics. Within the measuring frequency range of40Hz～100MHz, a single dielectric relaxation with the characteristic frequency around1MHz is seen at room temperature an additional one in low frequency region is also observed at high temperatures. Furthermore, the existence of CuO secondary phase has been confirmed in them. In general, the results can be explained by the similar mechanism of internal barrier layer capacitance effect formerly proposed for CaCu3Ti4O12ceramics and indicate that the giant-ε' phenomena should be quite common in the large family of compositionally and structurally CaCu3Ti4O12-like oxide ceramics. 2. By comparing the dielectric dispersion spectra of CaCu3TiO4, SrCu3Ti4Oi2, and NaCu3Ti3SbOi2ceramics coated with different metal electrodes, the electrode influence on the giant dielectric-permittivity properties in CCTO-type ceramics was studied. Within the measuring frequency range of40Hz-100MHz, a single dielectric relaxation with the characteristic frequency around1MHz is seen at room temperature or below whereas an additional one in low frequency region is also observed at high temperatures. On the basis of the experimental results, the two dielectric relaxations are ascribed to an internal barrier layer capacitance effect and an electrode polarization effect, respectively.3. An attempt to preparing high-density CaCu3Ti4O12ceramics was made by the hot-press sintering technique under ambient atmosphere. Microstructure, crystalline structure and dielectric property of the obtained CaCu3Ti4O12ceramics were investigated and compared with those of the conventionally sintered CaCu3Ti4O12ceramics. While the conventionally sintered ceramics show generally their theoretical relative density values lower than94.5%, the hot-pressed one with a short sintering duration reaches the theoretical relative density value of98.3%and exhibits the microstructure of bimodal grain distribution. X-ray diffraction analysis showed that CU2O minor phase exists in the hot-pressed ceramic and only CuO secondary phase is present after a further thermal treatment in ambience. Both the hot-pressed ceramics and its further thermally treated one exhibit two dielectric relaxations at room temperature, whereas the conventionally sintered ceramics have only one dielectric relaxation. Moreover, the hot-pressed CCTO ceramics has an extremely large low-frequency permittivity of2×105, which further increases to nearly1×106through the ambient thermal treatment. The relation between microstructure and dielectric property is discussed.4. The dielectric dispersion spectra and their evolution with temperature in BaTiO3and (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics were studied.(1) for BaTiO3ceramics, the influence of tetragonal-orthohombic phase transition on dielectric dispersion spectra was examined On the cooling process from a high temeprarure, the characteristic relaxational frequency of the dielectric relaxation related to the motion of domain walls decreases gradually at first with decreasing the testing temperature, drops drastically to a minimum in the vicinity of To-T and increases conversely with further decreasing the testing temperature. Besides, difference in the characteristic relaxational frequency is observed between the heating process and the cooling process. In particular, the values measured during the cooling process are signicantly higher than those measured during the heating process in the tetragonal phase, about250MHz.(2) For (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics, the influences of Sb5+-substitution for Nb5+ions and tetragonal-orthohombic phase transition on dielectric dispersion spectra were investigated. The result shows that the Sb5+-substitution decreases largely the characteristic relaxational frequency of the dielectric relaxation from several GHz to several tens of MHz at room temperature. Furthermore, the characteristic frequency increases monotonically with temperature and shows the thermally-activated behavior with slightly larger activation energy values in orthohombic phase than those in tetragonal phase. Thus, distinctly different behaviors in the changes of dielectric characteristic relaxational frequency with temperature have been observed between (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics and BaTiO3ceramics around TO-T.