Regulation Of Pseudo Phase Boundary In CaBi₂Nb₂O₉ High-temperature Piezoelectric Ceramics And Study On Piezoelectric Enhancement Mechanism

As an important functional material,piezoelectric materials have been widely used in industrial production,nuclear energy,aerospace and oil exploration.With the development of technology,the requirements for the performance of piezoelectric materials in various fields and applications have become more demanding.For example,piezoelectric sensors are often required to continuously monitor the operating conditions of the internal components of nuclear reactors above 500°C in the nuclear energy field.The conventional PZT piezoelectric materials has been difficult to cope with the special needs of these fields,so it is urgently needed to research a new piezoelectric material that could withstand high temperatures and high frequencies.Bismuth layered structure（BLSFs）ferroelectrics have become one of the most promising high-temperature piezoelectric materials in the world due to their high dielectric breakdown field strength,low ageing rate,high Curie temperature and good fatigue property,and are also the preferred high-temperature piezoelectric materials for high-temperature piezoelectric sensors.CaBi₂Nb₂O₉（CBN）,as a member of the bismuth layered structure ferroelectrics,has an ultra-high Curie temperature（～940°C）and thus has received a lot of attention from researchers,but its spontaneous polarisation direction is restricted to the two-dimensional plane due to its own crystal structure,resulting in a low piezoelectric coefficient（～5p C/N）,which hinders its application in practice.For CBN high-temperature piezoelectric ceramics,the ferroelectric phase is corresponding to orthorhombic symmetry,while the tetragonal for paraelectric phase structure.Due to the absence of dense ferroelectric domains within them,they lack the typical morphotropic phase boundary（MPB）and cannot construct an MPB to enhance their performance.In recent years,researchers have discovered that pseudo phase boundaries can be modulated by adjusting chemical composition of CBN through special ionic substitutions,giving a new research strategy.This paper focuses on the idea of constructing pseudo-phase boundaries.We try to construct pseudo-phase boundaries by doping different rare-earth ions in the A-site and doping different metal cations in the B-site of CBN and investigate the mechanism of pseudo-phase boundary construction and its effect on the electrical properties of ceramics.1.In the thesis,CBN-Ce-Cr,CBN-Nd-Cr and CBN-Y-Cr piezoelectric ceramics were prepared by using A-site doping of Ce³⁺,Nd³⁺and Y³⁺ions and B-site doping of Cr³⁺ions.The effects of doping with rare earth ions of different radius sizes on the construction of pseudo phase boundaries and the effects of the construction of pseudo phase boundaries on the properties of CBN ceramics were researched,such as the microstructure,piezoelectric coefficient,dielectric properties and resistivity.The results show that the introduction of rare-earth Ce³⁺ions with larger ionic radii at the A-site can modulate the pseudo phase boundary and increase the piezoelectric coefficient and resistivity of the CBN ceramics to 17 p C/N and 1.3×10⁵Ω·cm at600°C,respectively,where the piezoelectric property is about 3.4 times that of pure CBN,which is at a high level in the current literature.However,the Curie temperature of CBN-Ce-Cr ceramics dropped to 934°C.Although no pseudo phase boundaries were constructed in the CBN-Nd-Cr and CBN-Y-Cr ceramics,tetragonal distortions in the crystal structure were also induced,and the piezoelectric properties were improved to over 15 p C/N.In addition,the resistivity continues to increase as the radius of the introduced rare earth ions decreases.The CBN-Y-Cr piezoelectric ceramics obtained an ultra-high resistivity of 1.8×10⁶Ω·cm（at 600°C）,the highest value available for CBN-based piezoelectric ceramics.2.Based on the above experiments,CBN-Ce-Mn,CBN-Ce-Ti and CBN-Ce-Ta piezoelectric ceramics were prepared by introducing rare-earth Ce³⁺ions at the A-site and replacing metal Mn²⁺,Ti⁴⁺and Ta⁵⁺ions with different valence states at the B-site for doping experiments.The results show that the CBN-Ce-Mn,CBN-Ce-Cr and CBN-Ce-Ti piezoceramics all modulate the pseudo phase boundaries,but due to the chemical formula ratios,the lower valence Mn²⁺ions synergise with the entry of more Ce³⁺ions,and the doping of too many Ce³⁺ions will negatively affect the resistivity and Curie temperature of the ceramics.Finally,in experiment with equal doping of Ce³⁺and Ta⁵⁺ions,a high piezoelectric coefficient of 17 p C/N and a resistivity of 1.36×10⁵Ω·cm at 600°C were achieved without lowering the CBN Curie temperature.