| Ferroelectrics with perovskite structure, especially those Ti-based ferroelectrics, have attracted considerable attention due to their contributions both in intrinsic mechanism exploration and practical applications. The movement of Ti ion within the crystal lattice and its interaction with the surrounding ions will exert an influence on the dielectric, ferroelectric, and piezoelectric properties of the materials. It was already reported that some interesting anomalous dielectric behaviors emerged in the incipient ferroelectrics, such as SrTiO3and its doped systems, which were considered to be related with the defetcs induced by the donor/acceptor type. These studies can help to clarify the origin of the ferroelectricity and eliminate the extrinsic contributions. Typical perovskite ferroelectric material, BaTiO3, is adopted both for basic investigation to understand the ferroelectricity and practical applications for MLCC capacitors. A solid solution between SrTiO3and BaTiO3—(Ba,Sr)TiO3(BST), whose dielectric permittivity decreases with increasing the dc bias electric field within a certain temperature range, dielectric nonlinearity namely. Such characteristics can be used for phase shifters. It is suggested that the origin of the dielectric nonlinearity is a combined effect of the intrinsic and extrinsic contributions. The extrinsic contribution is mainly related with the polarized nano regions (PNRs) and their coupling effect with the external electric field. Considering the requirements of the phase shifter, it is better for BST working at its paraelectric phase. But the dielectric constant and loss for pure BST is too high for practical applications. In addition, its dielectric temperature stability also needs improvement. Therefore, BST system should be modified via combining some microwave dielectrics for improving its integrated dielectric properties.A considerable amount of piezoelectrics are derived from ferroelectrics. Traditional piezoelectrics with perovskite structure, such as Pb(Zr,Ti)O3(PZT) and Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT), have been widely applied in piezoelectric actuators, ultrasonic motors, and surface acoustic wave devices. However, the toxicity of lead has raised severe issues both to environment and human health. Thus more environmental friendly lead-free piezoelectric materials are in urgent demand. Among the lead-free piezoelectric families, NKN-based ceramics are especially interesting due to higher values of Curie temperature, piezoelectric coefficient, and planar coupling factor. However, pure NKN ceramics are difficult to densify via conventional routes due to the high volatility of alkaline elements at high sintering temperatures in air atmospheres. In addition, there still exists a wide performance gap between the traditional lead-based system such as PZT and NKN based systems. Therefore, it is necessary to modify NKN based systems not only via additives but also improved processing solutions.Based on the study background discussed above, we came to the following main conclusions from experimental results and theoretical analysis:1. We investigated the interesting anomalous dielecrtric behaviors observed in doped SrTiO3ceramics and tried to discuss the complicated doping mechanism. Sr1-xPr,TiO3(0.00≤x≤0.07) ceramics were fabricated via a traditional solid state method. Anomalous dielectric peaks were observed in the temperature dependent dielectric spectra for the specimens with x>0.04. A colossal dielectric constant (CDC) was obtained in Sri-xPrxTiO3ceramics with a low doping level (x=0.01) for the first time. Based on our experimental results, it is suggested that the CDC behavior is related with the localized charge carriers stimulated during a high temperature and the effect of interfacial layers. The anomalous dielectric peaks appearing in the samples with x≥0.04were ascribed to a thermal stimulated electron transportation process in stead of a ferroelectric phase transition.2. We improved the temperature stability of (Ba,Sr)TiO3(BST) based ceramics via a ferroelectric diffuse phase transition (DPT) characteristic combined with a second phase.100(1-y)wt%(Ba0.6Sr0.4)(Ti0.8Zr0.2)03-100>wt%MgO (y=0.4,0.5,0.6,0.7) composite ceramics were fabricated via the traditional solid state method. Based on our experiemental results, Zr substitution can help to reduce the grain size, broaden the phase transition peak, and lower the dielectric loss at low frequency. The addition of the secondary phase MgO is also conducive to the refined grains, reduced dielectric loss, and the shifted Curie point toward lower temperatures. Among all specimens, the sample with y=0.6exhibited better dielectric performance:low εr (88) and tanδ (0.00027), moderate tunability (6.2%at3kV/mm), higher FOM (230), notably low TCP value (-0.003℃-1) and high Qf value (780). 3. High-engergy Ball-milling (HEBM) was adopted for studying the effects of milling time, rotating speed, and firing conditions on the microstructure and dielectric properties of BST system. According to the experimental results, HEBM is effective in lowering the calcination temperature about200℃lower than the BST powders derived from the traditional solid state method. HEBM can also help to reduce the particle size to nano and sub-micro level. BST ceramics derived from HEBM possess lower sintering temperatures with nano and sub-micro sized grains, which can be modulated with the sintering temperature. The secondary phase BaWO4is favorable for reducing the dielectric loss without deteriorating its performance. In general, the sample sintered at1100℃for2h after milled at400/800rpm for10h possesses better dielectric properties:lower permittivity and loss (εr=332, tan8=0.0035); relatively high tunability (37.5%at2kV/mm and20℃); higher FOM (107.4); and low TCP value (-0.016℃-1). Therefore, HEBM is an effective way to lower the sintering temperature, reduce the grain size, and improve the temperature stability, which promotes BST system for practical applications.4. We combined BST with another A(B’B")O3type perovskite—0.4La(Mg0.5Ti0.5)O3-0.6(La0.5Na0.5)TiO3. The new mixed system exhibited an anomalous dielectric nonlinearity (ADN), i.e., the permittivity increased with dc bias electric field (.E-field), which was totally different from the traditional dielectric nonlinearity that was decreased with dc E-field. In addition, low temperature dielectric relaxation (LTDR) behaviors were also observed. The experimental data were fitted through the Arrhenius law. It was suggested that the LTDR was originated from a charge-associated process between electron-oxygen vacancy pairs during a thermal stimulation, while the ADN was related with a metastable state induced by the interaction of the polarized nano-regions (PNRs) and the oxygen vacancies generated by the introduction of the heterovalent ions.5. As pure (Na,K)NbO3(NKN) ceramics are easily to deliquesce and difficult to be fully densified via the conventional solid state reaction due to the high volatility of alkaline elements at high sintering temperatures in air atmospheres, we modified this system with a NaF based additive. Based on our experimental facts, the conventional route prepared x(NaF-0.5Nb205)-(1-x)[(Na0.5K0.5)(Nb0.8Ta0.2)O3](1OOxNN-NKNT, x=0.02,0.04,0.06) ceramics possess dense structure and enhanced piezoelectric properties. Corresponding single layered ceramics with Ni inner electrodes were fabricated via a tape casting method followed by a reduced atmosphere sintering. For the specimens fired in air, their density was enhanced while the grain size was reduced. The piezoelectric performance was also improved and stabilized with the NaF based additive. For the single layered ceramics cofired with Ni inner electrodes in the reduced atmosphere, no chemical reaction and diffusion occurred between the nickel electrodes and the NKN-based matrix. High d33values were gained in the single-layered samples. When Emax=50kV/cm, the high fiel d33values for2NN-NKNT,4NN-NKNT, and6NN-NKNT are397pm/V,347pm/V, and274pm/V, respectively. |
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