Study On Structure And Properties Of （Li, Na, K）_0.5Bi_2.5Nb₂O₉Bismuth Layer-structured Lead-free Piezoelectric Geramics

Bismuth layer-structured ferroelectrics （BLSFs） materials receivedsignificant attention for their potential use in high-temperature devices field,owing to their low temperature coefficients of dielectric （ε_r）, low aging rate,low dielectric loss （tanδ）, strong anisotropic electromechanical couplingfactors （Q_m）, high Curie temperature （T_c） and good high temperaturefrequency stability. However, the piezoelectric property of BLSFs ceramics islow because the rotation of the spontaneous polarization is restricted to thea-b plane. In this thesis, we engaged in the improvement of the piezoelectricactivity without producing a large decrease in their high T_c. Moreover, thethermal stabilities of piezoelectric properties for BLSFs materials wereinvestigated. The systems0.9Na_0.5Bi_2.5Nb₂O₉-0.1LiNbO₃and Na_0.25K_0.25Bi_2.5-Nb₂O₉were chosen in the thesis due to their high T_c（⁷88oC and⁶50oC,respectively）. Good performance for obtaining relatively high T_cand excellentelectrical properties by using a conventional mixed oxide processing routeprocedure （at the pressure of <20MPa）.Fistly, Bismuth layer-structured ferroelectric ceramics of Ce-doped0.9Na_0.5Bi_2.5Nb₂O₉-0.1LiNbO₃（NBN-LN+xwt%CeO₂,0≤x≤1.25） were synthesiz-ed by traditional solid state reaction. The results showed that all the ceramicsamples were single-phase ferroelectrics with high Curie points （T_c≥780℃）.With the increasing of the CeO₂dosage, the best sintering temperature of theceramic samples increasingly went up and grain size becomes larger. With theaddition of CeO₂, the piezoelectric activity and temperature stability ofNBN-LN-based ceramics were significantly improved, with the increasingamount of CeO₂, piezoelectric properties d₃₃increase and then decrease.Besides, the NBN-LN+0.75wt%CeO₂ceramic exhibited the optimumelectrical properties: d₃₃=28pC/N, k_p=12.11%, tanδ=0.10%, P_r=9.50μC/cm²; And after annealing at700℃, this component ceramic sample’s d₃₃remained at over22pC/N, which indicates that the ceramic possess excellentthermal stabiliy.Secondly, the effects of Na_0.25K_0.25Bi_2.5Nb₂O₉-xmol%MnCO₃（NKBN-Mn,0≤x≤8.0） ceramics have been investigated in detail by Mn addtion. It wasfound that the Curie temperature T_cgradually decreases from653°C to624°C with increasing the Mn modification. The electrical properties ofNKBN–based ceramics are improved significantly by the addition of Mn. The piezoelectric constant d₃₃, dielectric loss tan δ, mechanical quality factor Q_mand remanent polarization P_rfor the NKBN ceramics with3.0mol%MnCO₃modification were found to be25pC/N,0.28%,2817,14.50μC/cm²,respectively, together with the high T_c（⁶43°C） and stable piezoelectricproperties.Lastly, the effects of Co₂O₃addition on the microstructure and electricalproperties of Na_0.25K_0.25Bi_2.5Nb₂O₉（NKBN-Co,0.00≤x≤0.40） ceramics havebeen investigated carefully. It was found that the Curie temperature T_cgradually increases from653°C to662°C with increasing the Co₂O₃modification. The electrical properties of NKBN-based ceramics are improvedsignificantly by the addition of Co₂O₃. The piezoelectric constant d₃₃,dielectric loss tan δ, mechanical quality factor Q_mand remanent polarizationP_rfor the NKBN ceramics with0.20wt%Co₂O₃modification were found tobe23pC/N,0.35%,3028,12.03μC/cm², respectively. Thermal annealingstudies indicated that Na_0.25K_0.25Bi_2.5Nb₂O₉+xwt%Co₂O₃ceramics systempossesses stable piezoelectric properties, demonstrating that the Co₂O₃-modified NKBN–based ceramics are the promising candidates for high-temperature devices field applications.