Study Of The Composition-structure-property Relationship In Alkaline Niobate Based Lead-free Piezoelectric Ceramics

The development of lead-free ceramics with high piezoelectric and electromechanical properties has attracted much attention in recent years, due to the demand of environment protection.(Na,K)NbO3(NKN) based piezoelectric ceramics have been considered as one of the most promising candidates. However, there still exist some problems in NKN based materials. For example, the piezoelectric and electromechanical properties of NKN-based ceramics cannot be compared with the commercial Pb-based piezoelectric ceramics and their electrical properties show an obvious temperature instability. The understanding of the composition dependence of the piezoelectric properties in NKN based piezoelectric ceramics was still unclear. The previously-reported word was mainly focused on compositions near the orthorhombic-tetragonal polymorphic phase boundary. The mechanism of the high piezoelectric activity for phase coexisted NKN-based ceramics was still not well understood.In this thesis, NKN-based lead-free piezoelectric ceramics with different types of phase boundaries such as orthorhombic-tetragonal, orthorhombic-rhombohedral and rhombohedral-tetragonal phase boundaries, were achieved at room temperature by optimizing the compoisitional design and modulating the phase transition behavior. The influence of the composition, the phase transition characteristic, the domain structure and its response to the applied electric field on the electrical properties were investigated as well as the mechanism of enhancing piezoelectric properties near phase boundaries. In addition, the nature of the phase coexistence in NKN-based ceramics and the effect of the phase structure and domain structure on piezoelectric properties were also explored. The main contents can be outlined as below:(1) In chapter2, the correlation of the rhombohedral-orthorhombic polymorphic phase transition and electric properties was studied by substituting of Sb for Nb in NKN based ceramics. It was found that this lower-temperature phase transition temperature can be tuned to near room temperature, resulting in the formation of the rhombohedral-orthorhombic phase boundary, in which the optimum piezoelectric properties of d33=230pC/N,ε33T/ε0=1470and Qm=102can also be obtained. In addition, the temperature dependence of piezoelectric properties was also investigated considering the existence of successive polymorphic phase transition from rhombohedral, orthorhombic to tetragonal phases during heating.(2) In chapter3, a seires of NKN-based compositions with an orthorhombic-tetragonal phase coexistence were obtained by fixing ta few Sb contents but changing the LiTaO3(LT) content. It was found that the LT content added to form the orthorhombic-tetragonal phase boundary decreases gradually with increasing the Sb content. Excellent and tunable dielectric, piezoelectric and electromechanical properties of d33=240-400pCN, kp=36%-54%, ε33T/ε0=750-2500were achieved by changing Sb and LT contents in compositions near the phase boundary. These properties obviously show overall advantages compared to other NKN based materials reported in the literature.(3) In chapter4, a rhombohedral-tetragonal phase coexistence were for the first time obtained in (Na,K)(Nb,Sb)O3-LiTaO3-xBaZrO3(NKNS-LT-xBZ) by substituting BZ for LT. It was found that the rhombohedral-tetragonal phase boundary line was linked between a rhombohedral-orthorhombic-tetragonal triple point at lower temperature and a rhombohedral-cubic-tetragonal triple point at higher temperature. The enhanced piezoelectric properties (d33=365pC/N, kp=45%, d33*=508pm/V) were achieved in compositions near the phase boundary, making this material system become a promising lead-free piezoelectric material. Our work may provide a new method for designing high-performance lead-free NKN-based piezoelectric materials.(4) In chapter5, the characteristic of the phase boundary coming from the change of polymorphic phase transition temperatures were studied by using the (Na0.52K0.48-x)(Nb0.92-xSb0.08)O3-xLiTaO3(NKNS0.08-xLT) ceramics as a case study. It was found that the phase coexistence zone in NKN-based ceramics is very broad and diffuse compared with that in lead-based materials with a morphotropic phase boundary, which induces an obvious temperature dependence of piezoelectric properties in NKN based ceramics. The influence of the compositions, the phase content and the poling temperature on piezoelectric properties of NKNSo.os-xLT ceramics was also investigated in detail. To distinguish the morphotropic phase boundary in lead-based materials, this phase boundary in NKN-based ceramics was defined as the polymorphic phase boundary, accordingly.(5) In chapter6, several typical NKN-based lead-free piezoelectric ceramics were studied in terms of the domain type and structure and its reponse to the external electric field. Firstly, the polarization reversal behavior of the CuO doped NKN-based ceramics with double hysteresis-like loops were studied in detail. It was found that the evolution of the polarization vs electric field loops can be divided into three stages according to the relation betwwen the loop area<A>, the amplitude Eo and the frequency f of the electric field. The role of the domain wall motion,180°domains and non180°domains during the polarization reversal were clarified. Secondly, the dielectric and piezoelectric nonlinearity induced by domain wall motion in NKNSo.os-xLT and NKNS-LT-xBZ ceramics were studied in detail. It was found that the lowest dielectric and piezoelectric response were located at the tetragonal phase zone for both NKNSo.os-xLT and NKNS-LT-xBZ ceramics, owing to that the tetragonal phase exhibits a relatively large degree of the lattice distortion and internal stress compared with the orthorhombic, monoclinic and rhombohedral phases. In addition, it was found that the largest intrinsic dielectric and piezoelectric responses in NKNSo.os-xLT were obtained in compositions at the orthorhombic-rich side of the orthorhombic-tetragonal phase boundary, whereas, the largest extrinsic responses were obtained in compositions at the tetragonal-rich side of the phase boundary. By comparison, the tendency of composition dependent dielectric response was inconsistent with that of the piezoelectric response due to the existence of normal-relaxor transformation in NKNS-LT-xBZ ceramics. Thirdly, the domain structure evolution of both NKNSo.os-xLT and NKNS-LT-xBZ ceramics was investigated by using a transmission electron microscope. the orthorhombic-tetragonal phase coexistence in NKNS0.08-xLT and rhombohedral-tetragonal phase coexistence in NKNS-LT-xBZ ceramics, were confirmed by using the convergent beam electron diffraction. On the one hand, nanodomains were obviously obversed near the orhorhombic-tetragonal phase coexistence zone in NKNSo.os-Xlt cermics, which were believed to be closely related to the high piezoelectric response for compositions near the orthorhombic-tetragonal phase boundary. On the other hand, it was found that the tetragonal to rhombohedral phase transition in NKNS-LT-xBZ ceramics was accompanied by a normal to relaxor ferroelectric transition, during which the domain morphology will evolve from normal micron-sized lamellar domains to tweed-like domains and finally to polar nanodomains.(6) In chapter7, the electric field induced phase structural transition in NKN based ceramics with orthorhombic-rhombohedral, orthorhombic-tetragonal and rhombohedral-tetragonal phase coexistence were investigated by using synchrotron X-ray diffraction combined with other testing methods. Firstly, a structural change from orthorhombic phase to rhombohedral phase in NKNSy ceramics was confirmed owing to the addition of Sb. In addition, a monoclinic phase was induced irreversibly from rhombohedral phase after poling for NKNSy ceramics. Secondly, it was found that the orthorhombic phase can be irreversibly changed into a low-symmetry Mc phase after poling in NKNS0.08-xLT ceramics. The enhancement of piezoelectric properties was mainly attributed to the Mc-T phase coexistence and the phase instability owing to the reversible Mc-T phase transition. In addition, the mechanism of the orthorhombic-tetragonal phase transition induced by changing the composition in NKNSo.os-xLT ceramics was investigated by using Raman spectra and XAFS. The results indicated that the composition induced orthorhombic-tetragonal phase transition can be completed through a jump of polar axis from [101] in orthorhombic phase to [001] in tetragonal phase, in which the tetragonal phase should be dominantly induced by A-site smaller Li ions with a large off centering along [001] direction. However, the B-site Nb atoms are only localized near their average positions of the NbO6octahedra between orthorhombic and tetragonal phases. The Nb-O octahedral distortion changes over the phase transition only by adjusting their positions and orientations. Thirdly, an orthorhombic transient phase could be evolved from initially rhombohedral and tetragonal coexisted NKNS-LT-xBZ ceramics after poling, which was considered to be due to the enlargement of the orthorhombic zone at a low temperature zone. The electric-field induced phase transition and modified polarization rotation path from rhombohedral-tetragonal to rhombohedral-orthorhombic-tetragonal were ascribed to high piezoelectric properties in NKNS-LT-xBZ ceramics.(7) In chapter8, the universal polarization rotation mechanism summarized in NKN based lead-free ceramics with different types of phase boundary were studied. It was found that the irreversible phase transition under the electric field can be described as the morphotropic phase transition, while the reversible phase transition can be described as the polymorphic phase transition. In addition, a universal phase diagram of NKN based lead-free piezoelectric ceramics with different types of phase structures and phase boundaries was established.