Preparation And Photoelectric Properties Of KNN-based Lead-free Piezoelectric Ceramics

Environmentally friendly potassium sodium niobate(K_0.5Na_0.5NbO₃;abbreviated as KNN)lead-free piezoelectric materials were considered as a promising piezoelectric transducer material due to its low dielectric constant and high electromechanical coupling coefficient.In the last decade,the piezoelectric performance of KNN-based ceramics had gained tremendous improvement,and their piezoelectric performance was already comparable to that of commercial lead zirconate titanate（PZT）piezoelectric materials.However,high-performance KNN-based ceramics were usually fabricated based on a coexistence phase structure,and the material was usually"soft",with poor temperature stability and low fatigue resistance,which was not conducive to the application of the material in electronic components such as drivers and transducers.Therefore,high-performance soft KNN-based ceramics required further studied.It was found that rare earth ions as a class of modifiers could effectively modify the electrical properties of various bulk piezoelectric materials.Also,since rare earth ions were recognized as luminescent active ions with abundant electronic energy levels and long-lived excited states,when rare earth ions were introduced into the piezoelectric material matrix,the new components exhibited rich absorption and fluorescence emission spectra,which were valuable for the development of multifunctional properties of piezoelectric functional materials.In this study,multifunctional KNN-based lead-free piezoelectric ceramics were prepared by a conventional solid-phase synthesis process with rare earth elements as dopants,and the effects of earth elements types and contents on the phase structure,microscopic morphology,electrical properties and luminescence properties of KNN-based ceramic materials were investigated.The main studies were as follows:1.The effects of Sm doping on the phase structure and electrical/luminescent properties of KNN-based ceramics were investigated.The based KNN-based compositionswere 0.96(K_0.48Na_0.52)(Nb_0.95Sb_0.05)–0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃（KNNS-4BNKZ） and 0.965(K_0.48Na_0.52)(Nb_0.95Sb_0.05)–0.035Bi_0.5(Na_0.82K_0.18)_0.5HfO₃（KNNS-3.5BNKH）.For Sm-doped KNNS-BNKZ system,it was found Sm doping increased the crystal structure symmetry of KNNS-BNKZ,and a single solid solution phase with pseudo-cubic perovskite structure was formed in all samples.Thus,the Sm doped samples exhibited slim hysteresis loops and broadened dielectric peaks,which were typical for relaxor ferroelectrics.Accordingly,the temperature stability and fatigue behavior of the modified ceramics were significantly improved.It was found that the KNSN-0.04BNKZ ceramics with 0.002 mol Sm addition exhibited nearly temperature independent properties and fatigue-free behavior.Moreover,Sm-modified KNSN-0.04BNKZ exhibited a bright photoluminescence with a strong orange emission under visible light irradiation.For Sm-doped KNNS-3.5BNKH system,the phase structure of the ceramics transformed from coexisting orthogonal and tetragonal ferroelectric phases to a relaxor pseudocubic phase along with the strong destruction of the long-range ferroelectric order and difficult domain switching behavior.For 1.0 mol%Sm-modified samples,pure electrostrictive effect with an electrostrictive coefficient of0.022 m⁴/C²was obtained.Moreover,Sm-modified KNNS-4BNKZ ceramics had a good temperature stability and cyclic electric field stability.In terms of luminescence performance,ceramics exhibited strong orange luminescence when excited by visible light at 407 nm.The luminescence intensity increased with the increase of Sm doping concentration and showed the highest value when the content of Sm³⁺was 0.008 mol.The emission peak was a strong red emission peak near 597 nm,which corresponded to the⁴G_5/2→⁶H_7/2 energy level transition.2.The effects of Er doping on the phase structure and electrical/luminescent properties of KNNS-4BNKH ceramics were investigated.The results showed that the phase structure of the ceramics underwent a transition from the orthorhombic-tetragonal coexisting phase to the pseudocubic phase with the increase of Er doping.From the TEM images,it was seen that two kinds of ferroelectric domains existed within the Er₂O₃-modified ceramics,one was a regularly arranged strip domain and the other was an irregular nanodomain.The PFM images showed that the ceramics modified by Er had good domain switching behavior.The electrical performance results showed that the ceramic had a good stability.After the electric field was cycled for 10⁵ times at a high temperature of120 ℃,the performance of the material showed very slight changes,and its field-induced strain and remanent polarization could still maintain 90%of room-temperature values.In terms of photoluminescence,the ceramics exhibited green photoluminescence under 980nm visible light irradiation;meanwhile,the ceramics exhibited high temperature sensing performance.The sensitivity coefficient was 0.00608 K^-1 at x=0.0005 mol.In addition,ceramics exhibited thermoluminescence properties.It was found that the thermoluminescence occurred near the orthogonal-tetragonal phase transition.The presence of a large number of vacancy defects near the phase transition might be the reason for the generation of pyrophoric light.3.The effects of Ho doping on the phase structure and electrical/luminescent properties of KNLNS-2.5BNZ ceramics were investigated.The doping with Ho³⁺ions induced the phase transition from the coexisting orthorhombic-tetragonal phase into pure tetragonal symmetry along with a dispersion dielectric peak.Therefore,Ho₂O₃ doped materials showed very excellent temperature stability:the large-signal piezoelectric constant,d^*₃₃ was almost constant in the temperature range from 30 to 120 ℃.In-situ dielectric permittivity measurement indicated that the O-T phase transition became more diffuse in the electrical field,contributing to improved thermal stability.Furthermore,the Ho-doped ceramics exhibited a fatigue-free behavior regardless of the temperature,which could be linked to the dense microstructure.In addition,the Ho-modified ceramics showed excellent photoluminescence（PL）properties with strong green light emission upon453 nm visible light excitation.Polarization-and temperature-induced PL quenching behaviors were observed.These changes might originate from the electric field-or temperature-induced phase transitions from the mixed O-T to pure T crystal symmetry.These excellent and reliable electrical and photoluminescent properties reflected the great potential of the synthesized ceramics for applications in multifunctional devices.