| The piezoelectric ceramics are widely used in modern industry, which are usually used to achieve the transformation between the electrical signal and the mechanical strain. The lead-based piezoelectric ceramics, represented by Pb(Zr, Ti)O3 (referred to as PZT) ceramics, are widely used in a variety of piezoelectric devices because of their excellent dielectric and piezoelectric properties. However, lead is toxic and volatile. In order to protect the ecological environment, lead-free piezoelectric ceramics is getting more and more attention. (K, Na)NbO3-based piezoelectric ceramic is one kind of promising lead-free piezoelectric materials with great prospects to replace the traditional lead-based piezoelectric materials.(K, Na)NbO3 (referred to as KNN) ceramics pocess high Curie temperature, small temperature dependence and excellent thermal stability. However, the piezoelectric properties of conventionally prepared KNN ceramics are not very good, the piezoelectric constant J33 at room temperature is about 125 pC/N, and the planar electromechanical coupling coefficient kp is about 40%. Therefore, the KNN ceramics are insufficient for practical application as alternatives of PZT ceramics in most cases. In recent years, in order to further improve the performance of KNN-based piezoelectric ceramics, a lot of researches focused on optimizing the sintering conditions or doping KNN ceramics by LiSbO3, LiTaO3, CaTiO3, etc. In the matter of doping, the author’s group has improved the d33 of KNN-based piezoelectric ceramic to 413 pC/N by using conventional sintering methods and Li, Ta, Sb as doping alternatives. However, it is worth to note that most of these studies focused on composition optimization. The excellent piezoelectric properties near room temperature were believed to associate with the shift of the phase transition temperature to the room temperature, and some undesirable behaviors, such as thermal instability, time-aging instability and depolarization of the piezoelectric properties, were reported to exist in KNN-based ceramics.On the other hand, the origin of the piezoelectric and dielectric properties of ferroelectric ceramics are attributed to the intrinsic and extrinsic contributions in general. The intrinsic contribution is related to the lattice distortion, whereas the extrinsic one is ascribed mainly to the domain wall motion. Therefore, the domain structure and the feature of domain switching are of important significance for the piezoelectric and dielectric properties of ferroelectric ceramic materials. In addition, the stability of ferroelectric domain structure is also believed to attribute to the thermal stability of piezoelectric and dielectric properties in ferroelectric materials. BaTiO3 ceramics and PZT ceramics are two types of ceramics that found very early and used widely, and the domain researches on BaTiO3-based and PZT-based ceramics were mainly focused on the tetragonal and rhombohedral phases. It is reported that in these two types of polycrystalline materials, the extrinsic contribution to the piezoelectric properties from domain switching and domain wall motion is between 45%and 80%. Compared with the BaTiO3-based and PZT-based ceramics, the studies on domain structure of KNN-based ceramics are still limited. In [100] and [110] orientated pure (or Mn-doped) (K0.50Na0.50)NbO3 single crystals, the domain patterns with 60°-and 90°-domain walls have been observed by polarizing optical microscope and the domain width was found to be around few tens of micrometers; while only 90°-and 180°-domain walls were reported in unpoled (K0.50Na0.50)Nb03 ceramics by Scanning Electron Microscope (SEM). As expected, the simple lamellar twinning was observed in grains with sizes of 3-15 μm by Piezoelectric Force Microscope in poled Li-doped KNN ceramics, where the domain widths were about 275-300 run. Analogous to PZTs, the domain size in pure KNN ceramics is closely related to the grain size, where the domain size was found to decrease from 110 to 40 nm with decreasing grain size (from 1 to 0.2 μm). Additionally, domain texture of Li-doped KNN ceramics with coexistent orthorhombic and tetragonal phases was observed by in situ TEM, where the herringbone domains change to lamellar domains during poling and the field-induced monoclinic phase was responsible for the enhanced piezoelectric properties. However, the correlation between domain structure and piezoelectric properties in KNN-based ceramics are still far from being well understood.Under the above background, this thesis mainly carried out research on the piezoelectric properties and ferroelectric domain structure of potassium sodium niobate based lead-free piezoelectric ceramics, and studied the relationship between the piezoelectric properties and the domain structure in-depth. The main contents, results and conclusions obtained in this thesis are as follows:1. The domain structure of poled (K, Na)NbO3 ceramics were studied, two models were proposed to explain the two types of domain configurations for poled KNN ceramics. First, the author observed the domain patterns in poled KNN ceramics by acids etching. It was found the domain patterns show usually one single sets of parallel domain stripes or a few sets of parallel domain stripes in polycrystalline grains. The average domain widths in different sets of parallel domain stripes vary largely from 150 nm to 750 nm. The intersection angles between two adjacent sets of domain stripes are around 45° or 135°. The domain configuration in poled KNN ceramics can be classified as two types according to the intersection angles between adjacent sets of parallel domain stripes:domain configuration I with acute angle and domain configuration II with obtuse angle, respectively. Two models were proposed to explain the observed domain patterns. Domain configuration type I is composed of 90°-,60°-, and 120°-domain walls; type II is composed of 180°-,90°-, and 120°-domain walls. The mathematical analysis of the intersection angle between adjacent sets of parallel domain stripes in different observation plane for the two domain configuration models coincide well with the intersection angles observed in experiments.2. The domain structure of hot-pressed and large-grained (K, Na)NbO3 ceramics before and after poling, and the impact of domain switching and domain wall motion during poling process on the domain structure and piezoelectric properties, were studied. The domain configuration of hot-pressed KNN ceramics became relatively simple after poling, comparing with the complicated domain configuration before poling. Besides, the average domain size increased during the poling. The irreversible extrinsic contribution to the piezoelectric properties from domain wall motion in KNN ceramics is comparable with that in PZT and BaTiO3 ceramics. The domain structure of KNN ceramics before poling is very complicated with many Watermarks, Herringbones and Zigzags. The Herringbone configuration is composed of 90°-and 120°-domain walls, the Zigzags structure is composed of 90°-,120°-and 180°-domain walls, while the Watermarks is formed by 180°-domain wall. The domain configuration of hot-pressed KNN ceramics after poling is very similar as the domain configuration in poled KNN ceramics prepared by conventional sintering method. The relatively simple domain configuration in poled KNN ceramics with only single or more than two sets of parallel domain stripes, is ascribed to the domain switching and domain wall motion during the poling process. In addition, by the Rayleigh analysis method, the author found the irreversible extrinsic contribution to the piezoelectric properties from the domain switching and domain wall motion in hot-pressed KNN ceramics is 71%; and in conventional method sintered KNN ceramics is 68%.3. The piezoelectric properties, domain structure and domain reversal behavior of (K0.50Na0.50)1-xLix(Nb0.80Ta0.20)03 (abbreviated as KNNT-Lix) ceramics in orthorhombic phase, tetragonal phase and orthorhombic- tetragonal coexisting phase were studied, and it was found that the excellent piezoelectric properties in KNNT-Lix ceramics with orthorhombic-tetragonal coexisting phase were ascribed to the-easier domain switching, compared with the other two compositions. The domain configurations in KNNT-Lix ceramics with different crystalline phase shew different characteristics:the KNNT-Lix ceramics with orthorhombic-tetragonal coexisting phase shew larger domain size and curved domain stripes. In addition, studied the domain reversal behavior in different crystalline phase by analyzing the hysteresis loops and field-induced strain curves, and found KNNT-Lix ceramics with orthorhombic-tetragonal coexisting phase shew a high strain nonlinearity and high piezoelectric constant in high electric field.4. The (1-x)(K0.48Na0.52)(Nb1-ySby)03-xBi0.5(Na0.82K0.18)0.5ZrO3 (abbreviated as KNNS-BNKZ) ceramics were prepared, and samples with good temperature stability, good thermal aging performance, and J33 of up to 460 pC/N were obtained. Then the piezoelectric properties, dielectric properties, temperature stability, ferroelectric properties, strain nonlinearity with the changing external electric field, irreversible extrinsic contribution from domain motion, and the effect of sample thickness to the domain reversal and piezoelectric properties, were studied in KNNS-BNKZ ceramics. By controlling the incorporation of Sb5+ and BNKZ, obtained KNNS-BNKZ ceramics with rhombohedral-tetragonal coexisting phase at room temperature; in addition, by using cold isostatic pressing (abbreviated as CIP), obtained KNNS-BNKZ ceramic samples with with up to 460 pC/N at (x=0.0375, y=0.04) composition. Besides, the temperature stability of d33 and kp, and thermal aging performance of KNNS-BNKZ ceramic with (x=0.0375, y=0.04) was also excellent:d33 maintained higher than 350 pC/N at temperature from -30℃ to 70℃; kp maintained higher than 40% at temperature from -50℃ to 75℃; d33 value maintained higher than 400 pC/N after samples was thermally aged at temperature from 25℃ to 150℃. The piezoelectric properties of KNNS-BNKZ ceramics decreased when the sintering temperature increased. The incorporation of BNKZ decreased the orthorhombic-tetragonal phase transition temperature of KNNS-BNKZ ceramics, and increased rhombohedral-orthorhombic phase transition temperature. The filed-induced strain curves of KNNS-BNKZ ceramics at different electric field shew an "S" type nonlinearity. The irreversible extrinsic contribution to the piezoelectric properties from the domain switching and domain wall motion in KNNS-BNKZ (x=0.0375, y=0.05) ceramic composition is 68%. In addition, for the effects of the sample thickness, it was found in KNNS-BNKZ ceramic with (x=0.0375, y=0.04) and (x=0.0375, y=0.05), the thickness vibration electromechanical coupling factor, dielectric constant and piezoelectric constant under high electric field remained unchanged when the sample thickness decreased. With the gradual decrease of sample thickness of KNNS-BNKZ ceramic with (x=0.0375, y=0.05), the coercive electric remained almost unchanged, the remnant polarization decreased and the internal bias electric field increased.5. The piezoelectric properties and domain structure of two representative ceramics in the (Na, Bi)TiO3-based ceramics, which is another type of lead-free piezoelectric ceramic materials were studied. The two representative ceramics were (Na, Bi)TiO3-(K, Bi)TiO3-BaTiO3 (abbreviated as BNBK) ceramics and (Na, Bi)TiO3-(K, Bi)TiO3-(Li, Bi)TiO3-BaTiO3 (abbreviated as BNKLBT) ceramics. The piezoelectric properties, ferroelectric properties and microscopic structure were studied, in order to understand the impact of the hard doping on the piezoelectric properties and the domain structure, and the relationship between the piezoelectric properties and the domain structure. It was found after the hard doping, the domain structure changed, coercive electric field and internal bias electric field increased, mechanical quality factor increased, and the piezoelectric constant under low and high electric field decreased. The domain structure of BNBK79 ceramic and BNKLBT83 ceramic changed into relatively simple domain configuration after poling with sets of parallel domain stripes, compared with complicated domain configuration before poling with many irregular stripes. The average domain size of BNBK79 ceramic and BNKLBT83 ceramic was smaller than that of KNN ceramics. The author successfully observed domain stripes with average size of only about 50 nm in BNBK79 ceramic and BNKLBT83 ceramic by acid etching method. |
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