Research On Multifunctional Bismuth Layered Lead-Free Piezoelectric Ceramics Based On Molecular Simulation

With the rapid development of industries such as aerospace,petrochemical and nuclear power generation,the demand for high-temperature piezoelectric devices and multifunctional sensors is becoming more and more urgent.Based on this,the bismuth layered high-temperature piezoelectric ceramics are used as the research object of this thesis.Their microstructure change law are explored by means of rare-earth ion doping combined with molecular simulation techniques in this thesis.And the polarization microprocesses and multifunctional mechanisms of ceramics are studied to lay the theoretical and practical foundation for the development and application of bismuth layered piezoelectric ceramics in high temperature and multifunctional sensors.The following are the details and results of the main study:（1）Three bismuth layered multifunctional ceramic systems,Sr1-x Smx Bi2Nb2O9（x=0.000,0.002,0.004,0.006,abbreviated as SBN-x Sm）,Sr Bi_2-xEr_xNb₂O₉（x=0.00,0.02,0.04,0.06,abbreviated as SBN-x Er）and Sr Bi_2-0.06-xEr_0.06Yb_xNb₂O₉（x=0.000,0.002,0.004,0.006,abbreviated as SBEN-x Yb）were successfully prepared by solid-state synthesis method.Although all specimens show typical bismuth layered microscopic morphology,the effects of different rare earth ion doping on their grain size and macroscopic properties were different.The grain size of SBN ceramics doped with Sm³⁺decreases and then increases.At x=0.004,SBN-x Sm specimens possess increased residual polarization intensity（2P_r=10.8μC/cm²）,piezoelectric coefficient(d₃₃=17 p C/N)and excellent thermal stability.Photoluminescence spectra reveal that the specimens exhibit intense red emission at 601nm,which derives from the ⁴G_5/2→⁶H_7/2 transition under 406 nm laser excitation.The Curie temperature（Tc）and piezoelectric temperature stability of SBN are enhanced by the doping of Er³⁺.For specimen with x=0.06,d₃₃ at 400 ^oC retains 80%of its room temperature（RT）d₃₃ value.Considering the luminescence characteristic of Er³⁺ions,specimens possess bright up-conversion photoluminescence performance with the strong green emission band at 550 nm and the weak red emission band at 670 nm under 980nm laser source.The effect of introducing Yb³⁺into Sr Bi_2-0.06Er_0.06Nb₂O₉ ceramics was investigated.The outstanding comprehensive characteristics with low dielectric loss（tanδ=0.094%）,high remnant polarization（2P_r=10.73μC/cm²）,large piezoelectric responses(d₃₃=15p C/N)are simultaneously observed by Yb doping with a concentration of x=0.004.The Upconversion Photoluminescence（UC-PL）spectrum excited by 980 nm laser shows double bands at 533 nm and 554 nm in the green region,and one band at 672 nm in the red region.The cross-relaxation phenomenon occurred between Yb³⁺and Er³⁺,which modulated the Upconversion（UC）luminescence color of Er³⁺ion.Based on the above experiments,the theoretical calculations were performed for the SBN-x Sm,SBN-x Er and SBEN-x Yb systems using the first principles in combination with molecular simulation techniques.The obtained energy band and density of states maps show that the band gap value（E_g）decreases,while the f-state contribution of rare earth ions is intensified with increasing x,revealing the physical origin of piezoelectric stability,conductivity,and photoluminescence intensity in respect of molecular dynamics.（2）Sr Bi_3.992-xHo_0.008Yb_xTi₄O₁₅（x=0.000,0.002,0.004,0.006,abbreviated as SBHT-x Yb）multifunctional ceramics were fabricated by solid-phase reaction method.The XRD patterns show that the crystal structure of SBHT-x Yb belongs to the orthorhombic system with A2₁am space groups and no other heterogeneous phases exist.At x=0.002,the specimens with dense microstructures exhibit optimal electrical performance(T_c=516 ^oC,d₃₃=19 p C/N,2P_r=8.28μC/cm²)and excellent luminous performance.Photoluminescence spectra reveal that the specimens exhibit intense red emission at 548 nm,which derives from the ⁵S₂→⁵I₈ transition under 980 nm laser excitation.The energy band structure reveals the existence of impurity energy levels and further demonstrates the mechanism of impurity conductivity at RT.The total and partial density of states show that the contributions of 4f orbitals of（Ho,Yb）³⁺ions are strengthened by doping of Yb³⁺,which is in good agreement with luminous performance.（3）Na0.5Bi4.5-x（Eu0.5Ce0.5）x Ti4O15（x=0.000,0.004,0.008,0.012,abbreviated as NBT-x（Eu,Ce））multifunctional ceramics were synthesized by solid-phase reaction method.It is observed that all specimens formed a single phase with dense microstructure.At x=0.008,specimen shows the highest Tc（630 ^oC）,2Pr（15.36μC/cm²）and piezoelectric temperature stability.The d₃₃ value of NBT-0.008（Eu,Ce）maintains quite stable（only decrease by 6%）in the temperature range of 500 ^oC.With the enhancement of（Eu,Ce）³⁺concentration,the narrowing band gap and the intensified f-state component elucidate the microscopic mechanism of the luminescence intensity variation.