Study On Structures And Properties Of Rare-earth Ions Doped CaBi₈Ti₇O₂₇ Intergrowth Bismuth Layer-structured Piezoelectric Ceramics

Bismuth layer-structured ferroelectrics have a lot of potential applications in high temperature piezoelectric sensors,energy converters,filters and ferroelectric memories owing to their high Curie temperature,low dielectric loss,low aging rate and other advantages.Moreover,these materials possess relative low phonon energy,broad band gap and excellent physical and chemical stablility,which is expected as the great promising host to optical materials.However,because the rotation of the spontaneous polarization is restricted to a-b planar plane,the piezoelectric property of the ferroelectrics is so low that it is difficult to be practical application.Compared to B-site substitution,A-site substitution is a more effective approach for improving electrical properties,because cations in the A-site easily distort the structure and dominate the BLSF polarization process.It is well known that rear-earth ions are widely used not only as dopants to improve the properties of piezo/ferroelectric materials,but also as activators for the preparation of high efficient luminescence materials due to its unique electronic configuration.Based on this,CaBi₈Ti₇O₂₇7 intergrowth ceramic was selected as matrix materials.The effects of rear-earth doping,such as Ce³⁺,Eu³⁺and Pr³⁺on the structure,electrical and optical properties of CaBi₈Ti₇O₂₇7 ceramics were systemically investigated in this work.The research contents of this dissertation mainly include following three parts:Firstly,CaBi₈Ti₇O₂₇-x Ce³⁺(CBT-BIT-x Ce³⁺,x=0.00,0.02,0.04,0.06,0.08 and 0.10)intergrowth ceramics were prepared via a solid-state reaction method.The structural and electrical properties of CBT-BIT-x Ce³⁺ceramics were systematically studied by XRD,HR-TEM,Raman spectra,dielectric spectra and impedance spectra.Ce element mainly exists in form of Ce³⁺chemical valence states in ceramics and occupied the A site in the perovskite-like layers,the rest few in form of Ce⁴⁺occupied the B site.The increasing Ce³⁺contents induced a larger degree of distortion and a consequent increase in Curie temperature.Grain interior oxygen vacancies played a major role in the electrical conduction process at high temperature.The increased activation energies for conductivity evidenced the reduction of oxygen vacancy concentration after Ce³⁺substitution,which contributes to the decrease of dielectric loss and the increase of piezoelectric property.The sample with x=0.06 exhibited the optimum electrical properties,T_c=746℃and d₃₃=22 pC/N.Secondly,CaBi₈Ti₇O₂₇-x Eu³⁺(CBT-BIT-x Eu³⁺,x=0.00,0.05,0.10,0.15and 0.20)intergrowth ceramics were synthesized.The structure,electric and photoluminescence properties of CBT-BIT-x Eu³⁺ceramics were investigated.Eu³⁺did not enter into the（Bi₂O₂）²⁺layers but into the A sites of pseudo-perovskite layers.The introduction of Eu³⁺did not significantly influence the characteristic plate-like morphology.These results revealed that an appropriate doping of Eu³⁺improved the Curie temperature,piezoelectric constant and decrease of dielectric loss effectively.Under a 465 nm excitation,strong emission peaks located at 596（orange）and 617 nm（red）of all doped ceramics.The CBT-BIT-0.1Eu³⁺ceramics presents the strongest red-orange emission and the optimum electrical properties,i.e.,T_c=758℃,d₃₃=12 pC/N,tanδ<1%.Finally,CaBi₈Ti₇O₂₇-x Pr³⁺(CBT-BIT-x Pr³⁺,x=0.00,0.005,0.010,0.015and 0.020)intergrowth ceramics were synthesized.The structure,electric and photoluminescence properties of CBT-BIT-x Pr³⁺ceramics were investigated.These results revealed that an appropriate doping of Pr³⁺did not noteworthy change the Curie temperature,but decreased the dielectric loss and increased the piezoelectric constant from7 pC/N to 11 pC/N.The PL spectra excited by450 nm light exhibited a strongest emission peak at 611 nm（red）and a relatively weak emission peak at 653 nm（red）.The PL emission spectra showed that the relative intensity of red emissions increased with Pr³⁺substitution.The Commission Internationale de I’éclairage（CIE）chromaticity coordinates for CBT-BIT-0.020Pr³⁺ceramics were located in the red-orange color region and the coordinates were calculated to be（0.587,0.390）.