title: High-power properties of lead-based and lead-free ferroelectric ceramics
Creators: name: Slabki, Mihail
Publisher: name: UNSPECIFIED
Year: 2022

description: Hard-type ferroelectric ceramics are increasingly demanded as indispensable parts in numerous high-power applications, ranging from ultrasonic welding, over voltage transformers, to miniaturized ultrasonic motors in robotics. In these devices, the ferroelectric resonator is driven at or near its piezoelectric resonance frequency, which is a unique constitution that enables the generation of large oscillating displacement/strain at comparably small driving electric fields. The resonance amplification is thereby determined by the energy dissipation and induced hysteretic loss, usually represented by the resonators quality factor, i.e., the strain generation does not require exceptionally large piezoelectric coefficients but relies on the combination of moderate electromechanical coupling and minimal loss generation. However, state-of-the-art lead-based ferroelectrics are hitting their operational limits and are restricted to low output power densities due to rapidly evolving loss. This sets a natural boundary to the maximum achievable vibration velocities and terminates into overheating, depolarization, fracture, and ultimately device failure. Recently-emerged lead-free ferroelectrics demonstrated promising high-power properties and are discussed as potential alternatives, enabling to push the vibration limits to higher velocities. However, outperforming their lead-based counterparts in terms of reduced loss generation, they generally fall short on the poor electromechanical coupling and the narrow operational temperature window. In both cases, a consistent rationalization of the underlying resonance mechanisms and a systematic study of the decisive impact parameters are missing. This hampers the development of future high-power ferroelectric. The present study investigates and compares the high-power properties and mechanistic processes of several Pb(Zr,Ti)O3-based and (Na1/2Bi1/2)TiO3 BaTiO3 based ferroelectric compositions in piezoelectric resonance. Pulse drive measurements with burst excitation were utilized to determine piezoelectric, mechanical, and dielectric coefficients, as well as quality factors in a broad vibration velocity, temperature, and frequency range and under various vibration modes. The resonance performance is thereby best expressed by accumulating several of the coefficients to a high-power figure of merit; however, the properties are predominantly dictated by the values and the relative stability of the piezoelectric coefficients and the quality factors which are subjected to significant variation. The largest combination of both was determined in acceptor-doped Pb(Zr,Ti)O3 compositions. However, while the piezoelectric coefficients slightly increase with increasing vibration velocity, the quality factors reveal a rapid decrease by more than 80 % already in the range below 1 m/s, which is the detrimental limitation for the vibration velocity generation. The massive decrease appears qualitatively equivalent in all Pb(Zr,Ti)O3 compositions, irrespective of the doping element and concentration (acceptor/donor), crystal structure (Zr/Ti ratio), or grain size (domain size), i.e., an increase of the quality factor values does not result in improved relative stability. Moreover, normalizing the vibration velocity dependence of the quality factor to the small-field values revealed a clustering of all compositions, which manifests that the poor stability is primarily determined by the inherent properties of the ferroelectric matrix and mostly independent of chemical doping or other modifications. (Na1/2Bi1/2)TiO3 BaTiO3 compositions possess, in general, inferior piezoelectric coefficients and quality factors, but can compete with Pb(Zr,Ti)O3 if morphotropic phase boundary compositions (large piezoelectric coefficients) are ferroelectrically hardened (increasing quality factor) by Zn2+ acceptor-doping or composite formation with ZnO inclusions. The compositions reveal inherently superior stability, i.e., the piezoelectric coefficients are almost constant up to considerably large vibration velocities, while the quality factors exhibit only a moderate decrease. The pronounced stability in combination with profound fracture toughness and heat conductivity transforms into excellent high-power performance and enables the generation of large vibration velocities beyond 4 m/s with an essentially reduced self-heating, while state-of-the-art Pb(Zr,Ti)O3 already fail below 2.6 m/s. The mechanistic origin of the stability difference was identified by synchronizing the pulse drive measurements with in situ synchrotron x-ray diffraction. The quality factor decrease in Pb(Zr,Ti)O3 was directly associated with increased non-180° domain wall motion, which is a loss-afflicted strain-generating process driven by the evolving dynamic mechanical stress. A general relation between the microstructural strain contributions and macroscopic electromechanical behavior was established, which is suggested to predict the inherent high-power stability of ferroelectric systems. Moreover, the stability of (Na1/2Bi1/2)TiO3 BaTiO3 was found to correlate with a significantly lower domain wall contribution to the generated strain. The strain has a predominantly intrinsic nature and results from lattice distortion, which is the consequence of a significant coercive stress in these materials. However, the yet smaller quality factors indicate a substantial intrinsic loss in addition to the usually-dominant extrinsic loss. The intrinsic loss was rationalized to originate from a pronounced lattice polarization rotation compared to the polarization extension, exhibiting only weak vibration velocity dependence. The results confirmed that the modifications and hardening mechanisms influence the absolute values while the high-power stability is predominantly related to the basic ferroelectric material. Furthermore, the above-mentioned relative stability and clustering were determined to be temperature independent. On the other hand, the values reveal a pronounced temperature dependence. Especially acceptor-doped compositions exhibit a severe discontinuous temperature alternation, which was found to originate from evolving dielectric loss associated with the generated oxygen vacancies and the thermally-activated ionic hopping conductivity. This leads to the ambiguity that heavily acceptor-doped compositions reach exceptionally large quality factors, but only in a narrow temperature window and are prone to substantial temperature variance. A promising alternative is the second phase hardening approach demonstrated upon ZnO inclusions in (Na1/2Bi1/2)TiO3 BaTiO3. These ceramic-ceramic composites possess enhanced quality factors, delayed thermal depolarization, and almost temperature-independent properties, since the introduced mismatch stress does not contribute to conductivity. In combination with the inherent relative stability, the composites retain a broader temperature-velocity operation window and are suggested to be better suitable for high-power applications under thermal load and/or pronounced self-heating. A further approach based on ferroelectric property manipulation through DC bias superposition was suggested and examined. To this end, a piezoelectric resonance impedance spectrometer capable of superimposing excitation AC voltages with high-voltage DC bias has been designed. Intriguing effects were determined in acceptor-doped Pb(Zr,Ti)O3, where a six-fold increase of the quality factors at DC bias fields of 2 kV/mm was found to overcompensate the decrease of the piezoelectric coefficient, resulting in a substantial increase of the high-power figure of merit. The results suggest that the replacement of acceptor-doping with DC bias hardening has the potential to enhance high-power properties and temperature stability simultaneously, and reduce parasitic conductivity in extrinsic loss dominated ferroelectrics.