Phase Boundary Tailoring And Electrical Properties Of Silver Niobate-based Lead-free Ceramics

Phase boundaries play an important role in ferroelectric and antiferroelectric material fields.Ferroelectric and antiferroelectric materials at the phase boundary usually have complex phase structures and show rich physical effects under the external fields（electric field,temperature,stress）.It is more important that the development of ferroelectric and antiferroelectric materials with excellent properties is often dependent on the control and construction of phase boundaries.Currently,silver niobate（Ag Nb O₃）is attracting progressively more attention and is considered as one of the promising lead-free alternatives to lead-containing antiferroelectric ceramics.The present researches about Ag Nb O₃-based materials mainly focus on their applications in dielectric energy storage,but the ferrielectric behavior and hysteresis caused by the antiferroelectric-ferroelectric phase transition greatly limit their applications in energy storage,these problems are urgent to be solved.Besides,Ag Nb O₃ is one of few lead-free antiferroelectric materials,it has high scientific potential and value to construct ferroelectric/antiferroelectric phase boundary by combining it with rich lead-free ferroelectric components,which can greatly expand the applications of silver niobate based materials.Based on the above background,this thesis selected silver niobate ceramics as the research object,firstly,the intrinsic M₁-M₂ and M₃-O₁ phase transitions of silver niobate materials were tailored,therefore silver niobate-based antiferroelectric materials with excellent energy storage performance were developed,and the corresponding mechanisms of enhancement of energy storage properties were specially studied.After these,This thesis combined silver niobate with lead-free ferroelectric components,and successfully constructed ferroelectric/antiferroelectric phase boundaries in the silver niobate-based system,these materials at ferroelectric/antiferroelectric phase boundary showed high potential in infrared detectors and energy harvesting.Finally,for the ferroelectric-ferroelectric phase boundary in silver niobate-based materials,This thesis focused on the variations of phase structure and piezoelectric properties near the phase boundary,and made some attempts to tailor the phase boundary,these works laid a foundation for the development of high-performance silver niobate-based lead-free piezoelectric ceramic.The main research contents are as follows:（1）Yb³⁺ions doping was employed to construct chemical pressure in silver niobate lattice,thus enhancing the antiferroelectricity of silver niobate-based materials.The enhanced antiferroelectricity was evidenced by X-ray diffraction patterns and Raman spectroscopy.Moreover,the decreasing M₁-M₂ phase transition temperature and freezing temperature,as well as the increasing switching field with the increment of Yb content,also confirmed the enhanced antiferroelectricity of Yb-doped Ag Nb O₃ceramics,but the solid solution limit of Yb content in silver niobate was less than 4%mol.At the same time,Yb³⁺ions could increase the density and effectively boost the dielectric breakdown strength.The enhanced antiferroelectricity and dielectric breakdown strength led to a high energy storage performance in Ag_1-3xYb_xNb O₃ceramics and the recoverable energy storage density could reach up to 4.56 J cm^-3,the recoverable energy storage density is at a high level among that of lead-free energy storage ceramics.（2）This thesis has carried out researches on La³⁺doped silver niobate ceramics.La³⁺ions are one of few ions with high solubility in Ag Nb O₃,the La content could effectively enhance the antiferroelectricity,reduce the grain size,and boost the dielectric breakdown strength of silver niobate based ceramics.The most important is that the long-range antiferroelectric order in Ag Nb O₃ could be interrupted by the incorporation of La,resulting in the transformation of micron-scale antiferroelectric domains into antiferroelectric nanodomains.The corresponding macro performance of this process is the decreasing M₃-O₁ phase transition temperature and the more linear hysteresis loop of silver niobate-based ceramics.The Ag_0.76La_0.08Nb O₃ ceramics possessed both antiferroelectric microdomain and nanodomain,the polarization hysteresis loops of Ag_0.76La_0.08Nb O₃ ceramics had higher polarization and lower hysteresis,therefore Ag_0.76La_0.08Nb O₃ ceramics possessed excellent energy-storage performance(W_re=7.01 J cm^-3,η=77%).The comprehensive energy-storage performance of Ag_0.76La_0.08Nb O₃ ceramics is the highest in the silver niobate system,and better than that of the most lead-free energy-storage ceramics,indicating very high potential in dielectric energy storage.（3）This thesis has combined Ag Nb O₃with Li Ta O₃,therefore constructing an antiferroelectric/ferroelectric phase boundary.The ferroelectric（1-x）Ag Nb O₃-x Li Ta O₃ceramics could transform into an antiferroelectric phase with the increment of temperature,and the ferroelectric-antiferroelectric phase transition temperature could be tuned by the Li Ta O₃ content.The（1-x）Ag Nb O₃-x Li Ta O₃ceramics have excellent pyroelectric properties,especially at the antiferroelectric/ferroelectric phase boundary.It was found that the x=0.05 composition at the antiferroelectric/ferroelectric phase boundary had a large room-temperature pyroelectric coefficient(3.68 10^-8 C cm^-2 K^-1)and exhibited excellent figures of merit for infrared detectors due to its low relative permittivity（252 at 1 k Hz）.Moreover,an ultrahigh pyroelectric energy density(1.4 J cm^-3)was harvested using an Olsen cycle for the x=0.5 composition,which was several times larger than that of other ceramic systems reported recently.These results suggest the（1-x）Ag Nb O₃-x Li Ta O₃ ceramic would be an attractive multifunctional material for applications in infrared detectors and energy harvesting.（4）The temperature-dependent phase structures and electrical properties of the0.9Ag Nb O₃-0.1KNb O₃ceramics were investigated in detail.Based on our results,a ferroelectric orthorhombic-tetragonal phase boundary was confirmed for the first time in Ag Nb O₃-based materials.At this orthorhombic-tetragonal phase boundary,an enhanced piezoelectric constant of 147 p C N^-1 was obtained in the0.9Ag Nb O₃-0.1KNb O₃ceramics.Furthermore,the 0.9Ag Nb O₃-0.1KNb O₃ceramics can keep high piezoelectric performance in the temperature range of the overall tetragonal phase.Besides,we also investigated the effect of Li content on the ferroelectric orthorhombic-tetragonal and ferroelectric tetragonal-paraelectric cubic phase transition temperatures of 0.935Ag Nb O₃-0.065KNb O₃.These results provide an effective route and robust foundation to develop Ag Nb O₃-based piezoelectric materials with high piezoelectric properties.