| Lead-containing piezoceramics,e.g.,Pb?Zr,Ti?O3,have been the canonical piezoelectric materials for over half a century,due to the superior piezoelectricity and stabilities against alternating conditions.However,the toxicity of lead has raised intense health and environmental concerns;thus,the last decade has witnessed the surging dedication to develop viable lead-free alternatives.High-performance?K,Na?Nb O3-based lead-free perovskite has been considered as one of the most promising candidates to substitute for the ones rich in lead.Aimed at practical applications,this study focuses on characterizing and enhancing electrical properties and stabilities of?K,Na?Nb O3-based piezoceramics.Firstly,the piezoelectricity of?K,Na?Nb O3-based ceramics was enhanced by means of manganese doping and optimizing poling temperature.It was found that moderate incorporation of Mn O2 into?K,Na?Nb O3 can immensely improve its performance,whose piezoelectric coefficient d33 was raised from 214 up to 351 p C/N.And the occupancy of manganese ions in ABO3 perovskites was also studied.Meanwhile,optimizing poling temperature was demonstrated to be an effective approach to increase the piezoelectricity of?K,Na?Nb O3-based ceramics by about 15%.Based on in-depth analysis of electrical hysteresis at varied poling temperatures,a poling modelbased on the competition between domain reorientationand space charge accumulation was proposed to describe thetemperature-dependent poling behavior of piezoceramics.Secondly,a facile method based on the field-dependent piezoelectriccoefficient d33?E?measurement was verified to characterize in situ temperature dependence ofpiezoelectric coefficientd33,as an alternative for the conventional ex situroute.The temperature stability of d33 for Li/Ta/Sb co-doped?K,Na?Nb O3 ceramics was investigated using the method ofd33?E?measurement.It was revealed that d33 decreases dramatically with temperature,closely associated with the loss of polymorphic phase transition effect at elevated temperatures.To strengthen the temperature stability of?K,Na?Nb O3-based ceramics,Ca Zr O3 was introduced into the?K,Na?Nb O3 matrix to smoothen the transition between tetragonal and orthorhombic phases.It was found that?K,Na?Nb O3-based ceramics modified with 5%mol Ca Zr O3 not only possess decent piezoelectricity at room temperature?d33=357 p C/N?,but also show enhanced temperature stability of unipolar strain behavior,being comparable to that of commercial Pb?Zr,Ti?O3 ceramics.In situ synchrotron X-ray diffraction experiments combined with first-principles calculations disclose the occurrence of a new phase transformation under an electrical field,which extends the transition range between tetragonal and orthorhombic phases,consequently contributes to the boosted thermal stability of?K,Na?Nb O3-based lead-free piezoceramics with high piezoelectricity.Finally,the unipolar,bipolar,and sesquipolar fatigue behaviors of Ca Zr O3-modified?K,Na?Nb O3-based piezoceramics were studied systematically,where excellent fatigue resistance was observed in all scenarios.It was uncovered that the exceptionally good unipolar fatigue resistanceis identified due to the presence of additional process,assigned as a “softening” effect thatcompetes against the usual fatigue effect.Reduced lattice distortion?c/a ratio?of?K,Na?Nb O3-based piezoceramics upon introducing Ca Zr O3 is beneficial for enhancing the bipolar fatigue resistance.And the samples exhibit good sesquipolar fatigue characteristics,as sesquipolar cycling lies in between the unipolar and bipolar ones.Additionally,influence of both temperature and electricfield on the unipolar fatigue behavior of Ca Zr O3-modified?K,Na?Nb O3-based piezoceramics was investigated,revealing the overwhelming role of temperature over electric field in determining unipolar fatigue behaviors of?K,Na?Nb O3-based piezoceramics. |
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