The Fatigue Characteristics And Fatigue Mechanisms Of Hard Type Lead Based Piezoelectric Ceramics

Piezoelectric ceramics can quickly convert mechanical energy into electrical energy,and vice versa.Due to the excellent electromechanical coupling performance and fast response characteristics,piezoelectric ceramics have been widely used in various intelligent devices,such as piezoelectric sensors,piezoelectric transducers,piezoelectric actuators,and so on.In practical applications,piezoelectric ceramics will not only be stimulated by strong electric fields,but also often be affected by stress,elevated temperature,leading to the decline of electrical properties and even fracture failure,breakdown.Up to now,lots of works focus on the fatigue behavior of soft type PZT ceramics and the evolution of electric properties under the application of combined electric field-temperature,electric field-mechanical loading.However,there are still less works concerning the the fatigue behavior of hard type PZT ceramics,especially the evolution of electric properteis and fatigue behavior under combined electric field-mechnical loading.The hard type PZT piezoelectric ceramics usually have the characteristics of high electromechanical coupling coefficient,high mechanical quality factor and low loss,which are particularly suitable for manufacturing high-power devices such as piezoelectric motors,emitting piezoelectric transducers,and piezoelectric transformers.Hard type Mn-doped PMS-PZT piezoelectric ceramics with internal bias fields were chosen as the main research object.The fatigue resistance and pressure dependent electrical properties of Mn-doped PMS-PZT piezoelectric ceramics were systematically studied.Firstly,the aging characteristics of Mn-doped PMS-PZT piezoelectric ceramics were systematically analyzed.It was found that the dielectric constant,piezoelectric coefficient d₃₃ and electromechanical coupling factor k_p gradually decrease with increasing time.The ferroelectric hysteresis loops P-E and strain curves of unaged and aged poled Mn-doped PMS-PZT piezoelectric ceramics were compared.Displaced P-E loops with apparanet internal bias fields and asymmetric strain curves S-E were observed for poled samples after aging treatment.The valence state of Mn ions and defects were analyzed by means of XPS,EPR.The interaction between defect dipoles and spontaneous polarization were thought to be responsible for the aging effect of Mn-doped PMS-PZT piezoelectric ceramicsSecondly,the influences of electric field frquencies,electric field strength,electric field loading methods?unipolar,bipolar,sesquipolar?and temperature on the cyclic stability of Mn-doped PMS-PZT piezoelectric ceramics were systematically studied.It was found that the introduce of oxyge vacancies,defect dipoles making the evolution of properties under cyclic fields more complicated.On the one hand,the electrical properties of Mn-doped PMS-PZT piezoelectric ceramics increases with increasing cyclic number due to the"soften effect".However,on the other hand,the"fatigue effect"decreases its electrical performance.These features make the fatigue behavior of Mn-doped PMS-PZT ceramics different from that of soft type PZT-based ceramics.The remanent polarization P_r shows a tendency of first increasing and then decreasing,and the internal bias field E_biasias first decreases and then increases.Furthermore,the competition between softening"and"fatigue"effect are highly influenced by electric field frquencies,electric field strength,electric field loading methods and temperature.The ferroelectric domain configuration of Mn-doped PMS-PZT piezoelectric ceramics were observed through SEM and TEM test methods.Compared with unfatigued samples,the domain configuration of fatigued samples is quite complicated.Apart from stripe-like domain configurations,lots of herringbone type and zig-zig type domains were observed for fatigued samples,which may due to that these type domains may facilitate the relaxiation of internal stress.In addition,increased domain wall density,decreased domain size along with nanodomains were observed after fatigue treatment.The HRTEM images show the existence of local lattice distortions in Mn-doped PMS-PZT piezoelectric ceramics,which may be due to the accumulation of defects such as oxygen vacancies.The lattice distortion has an inhibitory effect on the nucleation of ferroelectric domains and thus hinders the response of ferroelectric domains to the external fields.Furthermore,asymmetric strain curves with strain memory effect were observed in poled and aged Mn-doped PMS-PZT piezoelectric ceramics.The value of strain memory and its cyclic stability were both improved through sesquipolar loading owing to more random distributed ferroelectric domain state and higher stable defect dipoles.Maximum value of 0.32%,which is more than twice the unipolar strain in the same electric field,was achieved when the applied negative field around negative coercive field.And this strain memory shows very good cycling stability,varying less than 5%up to 10⁴ cycles,while almost 40%degradation is found under bipolar signals.Morover,the efffects of uniaxial compressive stress and hydrostatic pressure on the electric properties of Mn-doped PMS-PZT piezoelectric ceramics were investigated,which are partically important for the piezoelectric ceramics and devices used under external mechanical conditions.It was found that the interaction between defect dipoles and ferroelectric domains can be modified through the application of external pressure.Pressure-induced depolarization and domain clamping effect significantly reduce the ferroelectric properties of Mn-doped PMS-PZT piezoelectric ceramics.However,after removing pressure,the dielectric and ferroelectric properties increase,and the internal bias fields E_bias decrease.These features mean that pressure is capable of modifying the orientation of defect dipoles and weakening the pinning effect.In addition,bipolar field cycling tests show that the application of a moderate uniaxial stress can improve the fatigue resistance of Mn-doped PMS-PZT piezoelectric ceramics.Finally,spark plasma sintering?SPS?were used to improve the fatigue resistance of PMN-PZT piezoelectric ceramics.It was found that 2P_r of SPS samples increased slightly up to 10⁶ cycles.However,2P_r of conventional sintering samples?CS?decreased by 80%just up to 4×10⁵ cycles.The severe fatigue degradation of CS samples can be attributed to crack propagation,field-induced phase transition and strong domain wall pinning effect.