| Piezoelectric materials is a kind of functional materials which can realize the energy transformation between mechanical energy and the electrical energy, and have been widely used in filters, motors, vibrators, sensors, buzzers and ultrasonic transducers. The common piezoelectric materials mainly contain piezoelectric single crystals and piezoelectric poly cry stals(also called piezoelectric ceramics). In particular, piezoelectric ceramics play an important role in application because of their relatively simple fabrication process and relatively low cost. Traditional piezoelectric ceramics are referred to as ferroelectric ceramics. The common ceramics are isotropic because of their spherical symmetry, and then does not exhibit piezoelectricity. However, for the ceramics containing ferroelectric phases, it can exhibit piezoelectricity by reorienting the spontaneous polarization along the applied electric field because the isotropic macroscopic symmetry was broken after a poling process. Ferroelectric ceramics can present hysteresis loop under some condition because of the reversion of spontaneous polarization.Up to now, lead zirconate titanate(PZT) is the most widely used of all piezoelectric material due to their excellent piezoelectric properties and their adjusting ingredients, and dominate the market for piezoelectric materials. However, these Pb-based material using plenty of PbO as raw material in fabrication process will bring hazard to the environment in their fabrication, application and disposal process. In recent year, owning to the improvement of environmental consciousness and the demands of sustainable development, environment-friendly piezoelectric materials have caught much attention in many countries.Overall, there are two routes to settle the problem of Pb hazard of PZT. The first route is to replace PZT with the lead-free or lead less piezoelectric materials, and the second one is to explore new piezoelectric materials. For the first route, doping,substitution and improving the fabrication technique will be used to improve the piezoelectric properties of the candidate piezoelectric materials, such as BaTiO3,(K,Na)NbO3, sodium bismuth titanate, in the hope that the improved properties can meet the requirement in particular field where PZT always had been used. Unfortunately, there is still a large gap in the piezoelectric properties between the candidate piezoelectric materials and PZT. For the second route, the exploration of new piezoelectric materials mainly contains exploration of new piezoelectric materials and fabrication of the flexoelectric-based piezoelectric materials. In particular, the flexoelectric-based piezoelectric materials has become the focus in recent years, and the amount of the published articles related to this field has increased gradually year by year.Recently, we reported a new kind of piezoelectric ceramics named as non-ferroelectric piezoelectric ceramics(also named as flexoelectric-type polar ceramics in our published articles). These ceramics were fabricated by traditional solid state reaction method and could present piezoelectricity after sintered in furnace without any electric field poling process. The macroscopic symmetry of these ceramics should only be described as triclinic system, which is freshly different from that of ferroelectric ceramics (described as6mm point group). Moreover, ferroelectric phases would be not the essential element for fabrication of these ceramics. Even if these ceramics contain ferroelectric phases, piezoelectric response will be detected far above the Curie temperature of the ferroelectric phases. From the perspective of constituents, these ceramics are made up of heterogeneous phases, and then large amount of heterogeneous interface will exist. Base on our knowledge about the research on flexoelectric effect, we proposed that the unusual piezoelectricity in non-ferroelectric piezoelectric ceramics may rely on the flexoelectric effect at heterogeneous interface. Moreover, the flexoelectric effect in non-ferroelectric piezoelectric ceramics would be similar to that originating from the temperature gradient in quasi-amorphous films reported by lubomirsky, which are different from that reported by Cross. In addition, compared with those reported flexoelectric-related low-dimensional films, non-ferroelectric piezoelectric ceramics are bulk polar materials.Since we considered the importance of the flexoelectric effect in determining the unusual piezoelectricity in non-ferroelectric piezoelectric ceramics, an overview of the representative flexoelectric effect research in recent years was carried out in the introduction of this dissertation. Meanwhile, to give a distinct comparison, an overview of traditional piezoelectric materials was also conducted where represented the features of piezoelectric single crystals and polycrystals, piezoelectric films and polar glass-ceramics. Based on these reviews, the introduction gave an general overview of the discovery of non-ferroelectric piezoelectric ceramics and the achievement at present.In the text of this dissertation, a more detailed review of the discovery, fabrication and research achievement was undertaken. Currently, the samples containing perovskite phase and Bi12TiO20phase predominate the research of non-ferroelectric piezoelectric ceramics. In this dissertation, Na0.5Bi0.5TiO3-based, SrTiO3-based and Sr2Bi4Ti5O18-based non-ferroelectric piezoelectric ceramics were chosen as the study subjects, and their properties were presented in different chapters respectively. Besides, another kind of non-ferroelectric piezoelectric ceramics was introduced in this dissertation. These ceramics did not contain Bi12TiO20phase, and the dissertation only gave a brief review of their piezoelectric properties.The discovery of non-ferroelectric piezoelectric materials was attribute to a mistake made in the process of fabricating traditional Na0.5Bi0.5TiO3-based lead-free piezoelectric materials, and the initial research on this field had been set focus on Na2O-Bi2O3-TiO2system to explore new non-ferroelectric piezoelectric materials. Chapter3gave details of properties of Na0.5Bi0.5TiO3-based non-ferroelectric piezoelectric materials prepared by traditional solid state reaction method. Overall, this chapter can be divided into two components. The first component was set focus on the Nao.5Bio.5Ti03+xBi2O3(x=0.05,0.1,0.15,0.25,0.4,0.5,0.75,1)(abbreviated as NBT-xB,x=0.05,0.1,0.15,0.25,0.4,0.5,0.75,1) system. XRD analysis indicated that all of as-sintered ceramics contained two main crystalline phases:Nao.5Bio.5Ti03phase and Bi12Ti02o phase. The piezoelectric response was studied by an Agilent4294A impedance analyzer and a d33meter. It had been found that the piezoelectric response became stronger by increasing x values; for the samples with x values above0.5, the piezoelectric coefficient could reached7～8pC/N. This suggested that one could improve the piezoelectricity by optimizing constituent. Besides, Chapter3dealt with the XPS analysis of the NBT-xB samples, and discussed the underlying mechanism of the origin of piezoelectricity in these ceramics. Additionally, the influence of sintering temperature was also evaluated in virtue of the XPS analysis of NBT-0.4B compositions. On the basis of the analysis of experimental results and the inspiration of lubomirsky’s research, we tried to give our probably plausible interpretations to illustrate the different origin of piezoelectricity in NBT-xB samples. For those compositions with smaller x values, their piezoelectricity might mainly originate from the partial alignment of distorted TiO6octahedra in amorphous phase at interface. Moreover, relatively low-level content of excess Bi2O3, would result in comparatively low-level content of amorphous phase. Combining these two points, then it was not surprised for the relatively weak piezoelectric response for those samples. For those compositions with larger x values, their piezoelectricity might mainly originate from the partial alignment of distorted TiO6octahedra and BiOs polyhedra in amorphous phase at interface. Besides, relatively high-level content of excess Bi2O3would result in comparatively high-level content of amorphous phase. Then, on the contrary, these samples exhibited relatively strong piezoelectric response. The second component mainly referred to the exploration of non-ferroelectric piezoelectric materials and the12(Nao.5Bio.5)1-x(Sr,Ba,Ca)xTi03-Bi12Ti02o (x=0.06,0.2,0.3,0.5,0.8,1) compositions were chosen as study subject. This study confirmed that the anomalous piezoelectricity found in Na0.5Bi0.5TiO3-based non-ferroelectric piezoelectric materials would be not an isolated case.Although the study of Na0.5Bi0.5TiO3-based non-ferroelectric piezoelectric ceramics suggested that the anomalous piezoelectricity did not originate from ferroelectric phases, whether the ferroelectric phase was the essential element or the key factor to fabricate these unusual ceramics was still a question. Moreover, the ferroelectric phase sensitive to ambient environment would bring interferences to the study of virtual piezoelectricity coming from the amorphous. In chapter4, we chose12SrTiO3-Bi12TiO20composition as the main study subject to solve this problem. In chapter4, the compositions chosen as study subject were as follow:12(Nao.5Bio.5)1-xSrxTi03-Bi12Ti02o(x=0.2,0.3,0.5,1). We identified their main crystalline phases as a perovskite phase and a Bi12TiO20phase and studied their piezoelectric properties. Then, we chosen the12SrTi03-Bi12Ti02o (abbreviated as ST-BT) composition as the main study subject. The study of ST-BT ceramics indicated that the ferroelectric phase was not the essential element to fabricate these unusual ceramics and confirmed that ST-BT ceramics would be an appropriate candidate in the study of non-ferroelectric piezoelectricity. By combination of XPS, raman spectroscopy and positron annihilation, we proposed that the origin of polarity in non-ferroelectric piezoelectric ceramics had a close relationship with the large amount of defects generating in the sintering process. By electric poling study and DSC analysis, we proposed that Bi12TiO210could have an important role in these non-ferroelectric piezoelectric ceramics. Inspired the research achievement of quasi-amorphous films, we proposed that the piezoelectricity in ST-BT ceramics originate from the partial alignment of distorted TiO6octahedra and BiO5polyhedra in amorphous phase at interface. At last, a probable flexoelectric-related polarization mechanism was briefly discussed where the comparison between quasi-amorphous films and non-ferroelectric piezoelectric ceramics was mentioned.In chapter5, a kind of non-ferroelectric piezoelectric ceramics containing Aurivillius compounds was mentioned. Sr2Bi4Ti5O18-based non-ferroelectric piezoelectric ceramics prepared by solid state reaction method was chosen as the main study subject. These ceramics fabricated by using a nominal composition formula: Sr2Bi4Ti5O18+2Bi2O3were studied by XRD, SEM, XPS, dielectric and piezoelectric measurement. Based on these analysis, we suggested that the origin of Sr2Bi4Ti5O18-based non-ferroelectric piezoelectric ceramics was similar to that of Nao.5Bio.5Ti03-based and SrTiO3-based non-ferroelectric piezoelectric ceramics, which correlated with the generation of large amount of defects. In addition, Bi12TiO20might play a more important role in generating the anomalous piezoelectricity in Sr2Bi4Ti5O18-based non-ferroelectric piezoelectric ceramicsIn chapter5another kind of piezoelectric ceramics were also mentioned. The main crystalline phase of these ceramics did not contain Bi12TiO20phase, but these ceramics also could exhibit piezoelectricity before applying an electric poling process. Chapter5only gave an overview of the piezoelectricity of Bi4Ti3012+BiA103composition. It was found that the macroscopic symmetry of these ceramics should be described as triclinic system, which is similar to that of Na0.5Bi0.5TiO3-based and SrTiO3-based non-ferroelectric piezoelectric ceramics. The mechanism of origin of the piezoelectricity in these ceramics was still unclear, and more works needed to be done.The basic purpose of this dissertation is to present our works in this new field, and expound our acquaintance and understanding of these non-ferroelectric piezoelectric ceramics. Moreover, we wish people who take interest in these ceramics can participate in the research of this field. After all, these unusual ceramics provide us a freshly new way to review the development of piezoelectric materials either from the perspective of pure scientific research(for example, rethinking the intrinsic origin of polarity) or from the perspective of potential utilization value(for example, the potential value in high temperature application). |
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