Effects Of Rare Earth Elements Multidoped On Properties Of Bismuth Layered Structural CaBi₂Nb₂O₉ High-temperature Piezoceramics

The aerospace,aircraft,and nuclear power industries have a desperate need for sensors,which can operate at 500°C or higher temperature with a long mean time between failures,because sensors need to be as close as possible to engine component of interest for adequate sensitivity to monitor engine health and enable safer operation.Bismuth layered structural ferroelectrics?BLSFs?possess some unique properties such as high Curie temperature and fatigue-free properties.These shining properties make Aurivillius compounds as promising candidates for high-temperature piezoelectric sensors operating in some hazardous environments.However,piezoelectric constant of BLSFs is relatively poor,hindering their practical applications.CaBi₂Nb₂O₉?CBN?is one of Bismuth layered structural ferroelectrics,which possesses high Curie temperature about 940?.In this paper,?Li,Ce,Sm?,?Li,Ce,Nd?and?Li,Ce,La?-multidoped CBN Aurivillius phase ceramics were prepared via conventional solid-state sintering route.This study works over the relationship of the co-substitution with two or more types of rare-earth elements,the proportion of Ce/La and sintering temperature and time on the crystal structure,microstructure and electric properties such as dielectric constant,DC resistivity and piezoelectric properties.The reason of different piezoelectric properties between?Li,Ce,Sm?,?Li,Ce,Nd?and?Li,Ce,La?-multidoped CBN was explored.Rietveld-refinement showed that multiple rare earth elements entered into lattice point and formed solid solutions.The crystal structure had a trend toward moving from orthorhombic system to tetragonal system and Bi³⁺in the A-site with 6s2 long pair electrons suppresses this change,especially when doping concentration was low.Ce element dominates translation of crystal structure from orthorhombic system to pseudo-tetragonal system,which may be related to outer shell electron distribution of Ce³⁺.What is more,increasing sintering temperature and extending sintering time logically decreased orthorhombic distortion and transformed crystal structure to pseudo-tetragonal system.Rare earth elements have high melting point,which delays the diffusion mechanism during the sintering process and restrains the grain growth in the direction perpendicular to the c-axis.Grain size can also be enlarged by increasing sintering temperature and extending sintering time.Crystal structure and grain size have important effect on electric properties including dielectric constant,DC resistivity and piezoelectric properties.The value of T_c was found to be varied with development of orthorhombic distortion and electronic configuration.Decreasing orthorhombic distortion and concentration of Bi³⁺in the A-site would lead to reducing of T_c.With reducing of orthorhombic distortion,the degree of NbO₆ octahedral sloping along a axis and rotating along c axis decreased and the obstruction of carrier movement reduced,leading to decrease of value of DC resistivity and activation energy.Meanwhile,the grain size decreased and grain-boundary conductivity become outstanding which contribute to its lower DC resistivity and activation energy,owing to space charges and defects mainly concentrating on grain boundaries.The morphotropic phase boundary between orthorhombic system and pseudo-tetragonal system leaded to vanishing of a glide plane,which made spontaneous polarization along a and b axis,resulting in increase of piezoelectric properties.What is more,small grain size had better piezoelectric properties,attributed to the decrease in the electric domain width and increase in the domain wall per unit volume,leading to more reversible domains.However,with increase of?Li,Ce,Nd?dopants,defect concentration increased,leading to decrease of depolarization properties.On the other hand,the internal stresses increase with decreasing grain size,providing an additional driving force leading to the obvious d₃₃degradation against temperature.Eventually,LCLM-10 ceramics obtained optimal piezoelectric properties(d₃₃¹7.5pC/N,T_c⁹25?C)and exhibited excellent thermal stability that remained 90%of their initial d₃₃3 values after annealing at 850?C for 2 h,which are a promising candidate for ultrahigh temperature applications.