Growth And Properties Of Perovskite Layered Niobate Piezoelectric Single Crystals

Piezoelectric materials have become one of the most valuable information functional materials in industry because of their unique characteristics of mechanical energy and electrical energy conversion.Whether it is the precise navigation and control system in the aerospace field,the ultrasound detection technology in medical imaging,the complex monitoring system in nuclear power plants,or the sensors and braking systems in the automotive industry,piezoelectric materials play an indispensable role.Among numerous piezoelectric materials,lead-based piezoelectric materials such as Pb（Zr,Ti）O₃ have long dominated the market due to their excellent piezoelectric and ferroelectric properties.However,with the increasing awareness of environmental protection,the potential harm of lead elements in lead-based piezoelectric materials to the environment and human health has attracted widespread attention.At the same time,with the continuous advancement of industrial technology,the demand for electronic devices that can work stably in extreme environments is also increasing.In this context,lead-free high-temperature piezoelectric materials have come into the vision of researchers.Perovskite layered structure（PLS）materials have become the focus of research due to their excellent characteristics such as ultra-high Curie temperature and high resistivity.For PLS materials,most of the research focuses on ceramic samples.The preparation process of ceramic samples is relatively simple and cost-effective,but their microstructure is complex,density and orientation are relatively poor,leading to difficulties in polarization and low piezoelectric activity.In contrast,the single crystal samples have good orientation,which is beneficial to the study of inherent properties and physical mechanism of PLS materials.Therefore,this article systematically studies the crystal growth,crystal structure,electrical properties,piezoelectric mechanism,and the influence of temperature on the structure and properties of Ca2Nb2O7,Ca2-xBix Nb2O7 and Sr2Nb2O7.This provides a reference for a deeper understanding of the performance of PLS materials and also provides possibilities for the development and application of lead-free high-temperature piezoelectric materials.The main research contents include:1.The Ca₂Nb₂O₇ single crystals were grown using the optical floating zone method.X-ray diffraction results indicate that its space group is Pna2₁,and it is observed that the single crystal exhibits natural cleavage characteristics along the a-axis,spontaneous polarization along the b-axis,and superstructure phenomena along the c-axis.Subsequently,the dielectric,piezoelectric,and ferroelectric properties of Ca₂Nb₂O₇ crystals with different orientations were studied.The study of dielectric properties suggests that both superstructure and domain wall displacement can lead to increased dielectric loss.Notably,the piezoelectric and ferroelectric properties of Ca₂Nb₂O₇ crystals are most prominent along the b-axis.The piezoelectric constant(d₃₃)of the b-axis orientation can reach 10.4 p C/N at room temperature,and this value gradually increases as the temperature rises.When the temperature increases to 750 K,d₃₃ improves to 15.2 p C/N.To further reveal the underlying mechanisms of this phenomenon,this article conducted a systematic study from the aspects of microstructure,lattice structure,and vibration.The results indicate that the origin of the piezoelectric effect,the orientation dependence of d₃₃,and the physical mechanism of thermally enhanced d₃₃ are all the result of the combined effect of the eccentric displacement of Nb atoms in the Nb O₆ octahedron and the motion of Ca²⁺.2.The effect of A-site doping on the structure and properties of Ca₂Nb₂O₇ single crystal was studied by adjusting the chemical composition.Ca_2-xBi_xNb₂O₇ single crystals were grown using the optical floating zone method.The results of X-ray diffraction and Raman spectroscopy revealed that pure-phase samples were obtained when x<0.14,while Ca Nb₂O₆ impurities were generated when x＜0.14.Subsequently,the dielectric,ferroelectric,and piezoelectric properties were conducted along the b-axis orientation of the crystals.The results demonstrate that Bi doping can effectively enhance the properties of Ca₂Nb₂O₇.Notably,the Ca_1.94Bi_0.06Nb₂O₇ single crystals exhibits the best piezoelectric performance,with a d₃₃ value of 12.8 p C/N,which gradually increases as the temperature rises.Additionally,Ca_1.94Bi_0.06Nb₂O₇ also exhibits excellent high-temperature stability,with no phase transition occurring even at temperatures up to 1673 K.To further reveal the mechanism of Bi doping improving performance,spectral and microstructural analyses were conducted.The results indicate that Bi³⁺doping causes distortion of Nb O₆ octahedron and changes in the domain structure.These structural changes play a crucial role in improving the piezoelectric properties of the material.Meanwhile,the high polarizability of Bi³⁺itself also contributes significantly to the improved performance.3.The effect of the defect dipole on the structure and properties of Sr₂Nb₂O₇ single crystals was investigated by adjusting the growth atmosphere.Sr₂Nb₂O₇ single crystals were grown in argon,air,and oxygen atmospheres,respectively.X-ray diffraction analysis revealed that the introduction of defect dipoles does not change the crystal structure.The dielectric,ferroelectric,and piezoelectric properties of crystals grown in different atmospheres were studied.The results indicate that the single crystals grown in oxygen exhibits the highest d₃₃ value of 24.7 p C/N.The single crystals grown in an argon atmosphere exhibit high residual polarization due to the larger polarization of defect dipoles.Additionally,through temperature-dependent X-ray diffraction,temperature-dependent Raman spectroscopy,and temperature-dependent dielectric spectroscopy,it was confirmed that Sr₂Nb₂O₇ exhibits three phase transition temperatures of 113 K,480 K,and 1573 K,and defect dipoles do not affect these phase transition temperatures.Within the range of 298 K-500 K,the d₃₃ first increases and then decreases as the temperature rises.The phase transition occurring around 480 K leads to a change in the direction of spontaneous polarization,resulting in corresponding changes in performance.These findings provide valuable references for the development and research of other PLS materials.