| Piezoelectric and dielectric materials are two important classes of electronic materials. Piezoelectric materials are a class of function materials that can realize the conversion between electrical energy and mechanical, and thus can play an important role in actuators, sensors, transducers and other electronic devices. Piezoelectric ceramics are widely used because of their low cost and the easiness of being fabricated into different shapes. Owing to their excellent piezoelectric properties and temperature stability, Pb(Zr,Ti)O3 (PZT) ceramics have become the largest share of global piezoelectric materials. However, due to the toxicity of lead oxide of PZT ceramics, fabricating high-performance lead-free candidates to replace PZT becomes a significant and urgent issue. Dielectric materials are used to fabricate capacitors due to its dielectricity. Miniaturization, high performance and reliability become the development trends of capacitor, since the capacitor can be used as the basic circuit elements and energy storage devices. Fabricating dielectric material with large permittivity, low dielectric loss, high dielectric breakdown strength and good temperature stability is extremely challenging task. Under these circumstances, this thesis concentrates on the studies of material preparations, physical properties and the related mechanisms for BaTiO3-based piezoelectric ceramics and (In, Nb) co-doped TiO2 giant-dielectric ceramics.BaTiO3 is a fundamental ferrolectric material from the viewpoints of both fundamental research and industrial applications and had been once widely used as a piezoelectric material at the early stage. The discovery of high-performance PZT ceramics made BaTiO3 ceramics drop out the predominated position in the piezoelectric material market. It is noteworthy that some important progresses had been made on fabricating high performance BaTiO3 piezoelectric ceramics in recent years. More importantly, we have recently succeeded in obtaining BaTiO3 ceramics with high d33 value of 574 pC/N prepared from hydrothermally synthesized fine BaTiO3 powders by hot-press sintering. These breakthroughs indicate that BaTiO3-based ceramics possess a great potential as lead-free piezoelectric materials. On the other hand, in order to obtain high-performance BaTiO3 ceramics, lots of researches had been made on the direct piezoelectric effect. Although converse piezoelectric effect is considered to be of critical importance to the applications such as actuators, systematic investigation concerning the influence of microstructure on converse piezoelectric effect has been rarely reported for BaTiO3 ceramics so far. In this thesis, direct and converse piezoelectric grain size effects are studied in detail for BaTiO3 ceramics, which can not only obtain the factors for enhancing the electric field-induced strain but also investigate the relationship between electric field-induced and domain structure of BaTiO3 ceramics.TiO2 ceramics are a kind of dielectric material with permittivity approximately 120, low dielectric loss, low conductivity and high dielectric breakdown strength. It has also attracted considerable attentions, due to the important research value on the application of dielectric capacitor with high energy density. More recently, interesting dielectric properties with extremely large ε’(≥104), low tan δ (≤5%), and weak temperature dependence over a broad temperature range from 80 K to 450 K are reported for (In, Nb) co-doped TiO2 ceramics by Liu et al. This result may open up a new way of investigating material with excellent dielectric properties. Subsequently, some experimental results about (In, Nb) co-doped TiO2 ceramics (INTO) were reported. However, a significant controversy concerning the mechanism of the giant dielectric properties still exists, and further in-depth studies are essential to clarify the relevant mechanism. Moreover, the INTO ceramics can’t meet the practical needs since its low electric field strength values. Therefore, how to improve the electric field strength is also an important issue that must be solved.This thesis mainly carries out research on the BaTiO3-based ceramics and INTO ceramics that prepared by the conventional solid-state reaction. For the BaTiO3-based ceramics, the direct and converse piezoelectric effects are investigated, the related mechanisms are discussed, and we have prepared BaTiO3-based ceramics with better temperature stability. For the INTO ceramics, the microstructure, dielectric property, impedance spectroscopy, electric field strength and the related electrical property are investigated, and the related giant-dielectric mechanism is discussed. In addition, we have successfully prepared ZrO2 modified INTO ceramics with higher electric field strength.1. A series of BaTiO3 ceramics with different grain sizes prepared by solid-state reaction are investigated from the aspects of piezoelectric and ferroelectric properties. It has been revealed that the dielectric and piezoelectric properties of BaTiO3 ceramics exhibit a significant dependence on grain size. The dielectric grain size effect is consistent with previous reports. The piezoelectric coefficient d33 value changes with the change of average grain size. The piezoelectric constant d33 increases with decreasing the average grain size, reaches a maximum value of 413 pC/N at 1.2 μm, and then decreases with further reduction of the average grain size. Converse piezoelectric effect was characterized by measurement of unipolar S-E curve. The converse piezoelectric coefficient d33 was quantitatively calculated from the slope of S-E curve at relatively large E. It is interesting that the d33* shows a quite similar trend of grain size dependence with d33.The remnant polarization Pr value increase with decreasing of grain size, reaches a maximum value at 3.3 μm, and then decrease with the further decreasing of grain size. Moreover, the domain structure of BaTiO3 ceramics changes with the variation of grain size. In general, it is found that the different domain configuration has a great impact on the piezoelectric property of BaTiO3 ceramics. The BaTiO3 ceramics with small grain sizes have stable and parallel 90° domain structure, exhibiting high piezoelectric performance and d33 value of 413 pC/N. Moreover, systematic investigation concerning the influence of microstructure on converse piezoelectric effect indicated that non-1800 domain reversal is an important factor that impacting the converse piezoelectric effect of BaTiO3 ceramics. The remnant polarization is an important indicator that evaluating the reversal degree of non-1800 domain, which has an important effect on the BaTiO3 ceramics. Moreover, the experiment results indicate that the remnant polarization, the piezoelectric coefficient, the converse piezoelectric coefficient d33* show similar grain size dependence. Thus, we consider that large remnant polarization is a requirement of obtaining BaTiO3 ceramics with high piezoelectric activity. Besides, the defects such as oxygen vacancies, which tend to produce pinning effects on the movement of domain wall, can also affect the contribution to the piezoelectricity of BaTiO3 ceramics. In a word, it can be concluded that the domain structure, remnant polarization and defects are considered to be important factors which significantly influence the direct and converse piezoelectric grain size effect.2. Physical properties such as piezoelectric, dielectric, ferroelectric and microstructure of Ba(Ti0.96Sn0.04)O3 ceramics that prepared through conventional solid-state reaction are investigated. It can be found that raw material and preparation technology have a great impact on the piezoelectric property. Compared with the pure BaTiO3 ceramics, the Ba(Ti0.96Sn0.04)O3 ceramics show better temperature stabilities, since the TO-T has been shifted up to 36.9℃, and the thermal hysteresis beside TO-T is only 1.8℃. Moreover, the P-E hysteresis loop exhibits ideal square-like saturation curve, the remnant polarization Pr is 18.9 μC/cm2 and the coercive field Ec is 2.5 kV/cm. Besides, it can be concluded that the non-1800 domain reversal is the major factor for the large converse piezoelectric coefficient d33* of Ba(Ti0.96Sn0.04)O3 ceramics.3. Some factors affecting the giant-dielectric property of INTO ceramics are investigated. It is indicated that the bulk samples formed by isostatic cool pressing are more uniform. Compared with the as-sintered INTO sample, the annealed ones are quite different from them in dielectric property. The annealed samples have lower dielectric constant and loss, the Debye-like relaxation peak around several MHz shift to higher frequency and the dielectric spectrum platform become broaden. Moreover, due to the different thickness of the oxide layer, the sample thickness has a greater impact on the dielectric property. For the thinner annealed sample, the ratio of oxide layer is larger, the insulation is stronger, the loss is lower, the resistance value is large and the dielectric frequency spectrum appears flat region over a broad frequency range.4. In order to clarify the mechanism of giant-dielectric property of the INTO ceramics, this thesis investigates two kinds of ceramics (as-sintered and annealed) in a comparison study. Compared with the as-sintered ceramic, the annealed one shows particularly lower dielectric loss and large dielectric constant about 1.1 X 104 with weak frequency dependence over a broad range, presenting a large dielectric spectrum platform. Particularly, the dielectric loss is smaller than 2% over a broader frequency spectrum range and a relatively high dielectric breakdown strength value of 750 V/cm. The two kinds of ceramics are qualitatively similar. An unusually large dielectric response is observed for both kinds of INTO ceramics in the low frequency range. Besides, a Debye-like dielectric relaxation appears around 1 MHz. Furthermore, they all show three Cole-Cole semicircles over the complex impedance, which represent three electrical mechanisms respectively. Besides, an equivalent electrical circuit, which contains three RC elements in series connection, can well explain the observed dielectric properties and electrical behaviors of the INTO ceramics. Thus, it can be concluded that the large dielectric response in the low frequency of the INTO ceramics is ascribed to the contributions from the non-ohmic contact and the Debye-like dielectric relaxation around 1 MHz is caused by the internal barrier-layer effect.5. In order to obtain high dielectric breakdown strength, some ZrO2 modified INTO ceramics are prepared. The microstructure, dielectric property and complex impedance are systematically investigated. It is indicated that the ZrO2 modified INTO ceramics present similar giant-dielectric property to the INTO ceramics. Specially, the dielectric constant of annealed In0.005Nb0.005Zr0.04Ti0.95O2 ceramics is weak frequency dependence over a broad frequency band from 3 mHz-10 MHz. Moreover, Both the permittivity and dielectric loss present smooth upward trend, and no large dielectric response is found in the low frequency range. Besides, it can be observed from the I-V characteristic curve that the electric field is about 1600 V/cm when the current density reaches 0.1 mA/cm2, and the dielectric breakdown strength can be as large as above 2000 V/cm. In summary, it is considered that the large grain boundaries resistivity is the main factor on enhancing the electric field strength of the In0.005Nb0.005ZrxTi0.99-xO2 (x=0,0.01,0.02,0.04,0.08) ceramics. |
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