Study On (Bi_0.5Na_0.5TiO₃-based And High Temperature Aurivillius Phase Lead-free Piezoceramics

Piezoelectric materials are such a functional materials which can be polarized to generate the surface electric charge by exerting a mechanical stress on them, or can be deformed by exerting an electric field on them. The phenomenon of generating electrical charge by applying a stress on an object is called direct piezoelectric effect, which was first discovered by the brothers Curie in 1880. The piezoelectric effect is an electromechanical coupling effect, the direct piezoelectric effect transforms mechanical energy into electric energy. Otherwise, the converse piezoelectric effect describes the strain developed in a piezoelectric material due to the displacement of the positive and negative electric charge center (the polarization displacement) when an electric field is applied. The direct piezoelectric effect and the converse piezoelectric effect are both called the piezoelectric effect.The Aurivillius phase BLSF materials, possessing unique structure characteristic and high Curie temperature, have been given more attention. The bismuth layer-structured piezoelectrics have been widely used in filter, energy transform , high temperature and high frequency situations. The Aurivillius phase bismuth layer-structured compounds are comprised of planar pseudo-perovskite layers interleaved with (Bi₂O₂)²⁺ layers along the c axis of the BLSFs.In this thesis, (1-y)[(Na_0.80K_0.16Li_0.04)_0.5Bi_0.5]TiO₃-yBa(Zr_0.055Ti_0.945)O₃ lead-free piezoelectric ceramics were syetematically investigated. The 0.94 [(Na_0.80K_0.16Li_0.04)_0.5Bi_0.5]TiO₃-0.06Ba(Zr_0.055Ti_0.945)O₃ ceramics having a piezoelectric strain constant d₃₃ as high as 185(pC/N) was obtained.The A-site substitution or/and B-site substitution have been shown to be effective approaches in modifying the structure and polarization process of the BLSF ceramics, and it has been demonstrated that doping Ce into the BLSF is an effective way to increase their resistivity and improve their piezoactivity. In this thesis, the Ce doped ultrahigh Curie temperature bismuth layer-structured ferroelectrics bismuth titanate niobate (Bi₃TiNbO₉) compound is investigated, the (LiCe) co-substituted sodium potassium bismuth titanate [(N,K)_0.5Bi_4.5Ti₄O₁₅] and sodium potassium bismuth niobate [(Na,K)_0.5Bi_2.5Nb₂O₉] materials are investigated. We analyzed the origin of low piezoelectric activity of the reported BLSFs, and put forward a new idea to improve the piezoelectric activity for bismuth layer-structured compound.The effect of (Li,Ce) substitution for A-site on the properties of (Na,K)_0.5Bi_2.5Nb₂O₉-based ceramics with A-site vacancy was investigated. The piezoelectric activity of (Na,K)_0.5Bi_2.5Nb₂O₉-based ceramics is significantly improved by the modification of lithium and cerium. The piezoelectric coefficient d₃₃ of the (Na,K)_0.5Bi_2.5Nb₂O₉-based ceramic was found to be 28 pC/N, 50% higher than the reported d₃₃ values of other BLSF systems (-5-19pC/N).(Na,K)_0.5Bi_4.5Ti₄O₁₅-based materials with A-site vacancy were synthesized using conventional solid state processing. The (Li,Ce) modification of (Na, K)_0.5Bi_4.5Ti₄O₁₅-based materials resulted in the obvious improvement of the piezoelectric activity and dielectric permittivity. The piezoelectric coefficient d₃₃ of the (Na,K)_0.5Bi_4.5Ti₄O₁₅-based ceramic was found to be 26 pC/N, more than 2.5 times as large as the d₃₃ value of the pure Na_0.5Bi_4.5Ti₄O₁₅ ceramics (-10pC/N).Among the ScTa co-substituted BTNO, fabricated Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ ceramics has a quite high piezoelectric constant d₃₃ of 12 pC/N and an ultrahigh T_C of 905℃.The result has broken the former record that the T_C can reach to 900℃but the piezoactivity is quite low( d₃₃-6 pC/N ).To increase the resistivity and further improve the piezoactivity of ScTa co-substituted BTNO, the effects doping CeO₂ into ScTa co-substituted BTNO on its resistivity and piezoactivity were systematically investigated. The d-(33) of Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉+xwt.%CeO₂(x=0.35) was found to be 18 pC/N, which is the highest value among the Bi₃TiNbO₉-based ceramics, and 3 times as large as the d₃₃ values of the former not modified BTNO ceramics (-6pC/N).The thickness fundamental resonance is liable to coupling with the higher overtone of low-frequency-resonant modes, which leads to a bad measurement accuracy of the thickness series resonance fundamental frequencies or a failure of the resonance fundamental frequency measurement, resulting in deteriorating the accuracy and the identity of the electromechanical coupling factor determined by the traditional overtone ratio method. To overcome the shortcoming of the traditional overtone ratio method, a higher overtone ratio method to determine the thickness vibration electromechanical coupling factors was advanced in the paper. The higher overtone ratio method determines the thickness vibration electromechanical coupling factors by measuring third-tone and higher-tone series resonance frequencies. Experiments showed that the higher overtone ratio method has a distinct advantage over the measurement accuracy and the identity of the electromechanical coupling factor in comparison with the traditional overtone ratio method. To facilitate the application of the higher overtone ratio method, a table of the higher overtone ratio with the corresponding electromechanical coupling factors was provided.