Fang-Zhou Yao

Fatigue-free unipolar strain behavior in CaZrO3 and MnO2 co-modified (K,Na)NbO3-based lead-free piezoceramics

The unipolar fatigue behavior of CaZrO3 and MnO2 co-modified (K,Na,Li)(Nb,Ta)O3 lead-free piezoceramics was investigated systematically. The well-known charge agglomeration model is shown to explain the overall changes observed during unipolar fatigue, such as the development of bias field as well as the anisotropy in bipolar strain hysteresis and field-dependent dielectric permittivity. In addition, it is found that the unipolar strain exhibits only small degradation within 3% at the field amplitude of 2 kV/mm up to 107 cycles. This exceptionally good fatigue resistance is identified due to the presence of additional process, assigned as a “softening” effect that competes against the usual fatigue effect.

Enhanced bipolar fatigue resistance in CaZrO3-modified (K,Na)NbO3 lead-free piezoceramics

The bipolar fatigue behavior of (K,Na)NbO3 (KNN)-based lead-free piezoceramics was investigated. A comparative analysis demonstrated that CaZrO3-modified KNN ceramics exhibited highly enhanced bipolar fatigue resistance due to the reduced lattice distortion (c/a ratio) and coexistence of orthorhombic and tetragonal phases. The hypothesis was verified by systematical studies of cycle-dependent large and small signal parameters and micro-morphologies. It was identified that mechanical stress induced microstructure damage beneath the electrodes renders the KNN ceramics to be vulnerable to bipolar cycling; while the mild fatigue behavior for CaZrO3-modified ones mainly originates from a thermally reversible domain wall pinning.

Ferroelectric domain morphology and temperature-dependent piezoelectricity of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics

Domain morphology and temperature-dependent piezoelectricity in terms of piezoelectric coefficient d33 and normalized strain d33* of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics at the polymorphic phase boundary were investigated. Transmission electron microscopy (TEM) and piezoresponse force microscopy (PFM) studies revealed a characteristic domain morphology comprising strip-like domains and featureless domains. Moreover, a facile method based on the field-dependent piezoelectric coefficient d33(E) measurement was verified to characterize in situ temperature dependence of piezoelectric coefficient d33, as an alternative for the conventional ex situ route. It was demonstrated that the normalized strain d33* exhibits superior thermal resistance to piezoelectric coefficient d33, though both parameters are susceptible to temperature variation.

Multi-scale thermal stability of niobate-based lead-free piezoceramics with large piezoelectricity

Growing environmental concerns are pushing the development of lead-free piezoceramics with both outstanding piezoelectric properties and reasonable thermal stability. Herein, we realized a large piezoelectric coefficient d33 of 430 pC N−1 in 0.96(K0.4Na0.6)(Nb0.96Sb0.04)O3–0.04Bi0.5K0.5Zr0.85Sn0.15O3 (KNNS–BKZS) polycrystals by constructing a rhombohedral–tetragonal (R–T) phase boundary. Investigations of the in situ thermal stability of the piezoelectric properties on multiple scales reveal that the micro-scale piezoelectric response is much more stable compared to the macro-scale response, indicating the significant role of extrinsic contributions from domain wall movements. These findings demonstrate the relationship between multi-scale properties and domain structures, revealing that the high piezoelectricity is attributed to nano-domains at the R–T phase boundary.

Comprehensive investigation of elastic and electrical properties of Li/Ta-modified (K,Na)NbO3 lead-free piezoceramics

A full set of elastic and electrical coefficients of (K,Na)NbO3-based lead-free piezoceramics with a nominal composition of Li0.03(K0.48Na0.52)0.97(Nb0.8Ta0.2)O3 (abbreviated as KNNLT) was evaluated by the standard resonance method, and compared to those of K4CuNb8O23 doped (K0.45Na0.55)NbO3 (KNN-KCN) as well as typical Pb(Zr,Ti)O3-based piezoceramics PZT5A. The measurement of intermediate elastic stiffness and compliance coefficients of KNNLT indicated that KNNLT is elastically “softer” than KNN-KCN but “harder” than PZT5A. An extremely high piezoelectric stiffness coefficient h33 = 68.8 × 108 V/m was obtained, due to an especially low clamped dielectric constant e33S of 361. And a large longitudinal electromechanical coupling factor k33 = 57% was observed in the KNNLT system. Furthermore, the piezoelectric coefficient d33 of KNNLT was characterized via three distinctive methods: the resonance method, the Berlincourt method, and the converse piezoelectric strain measurement; while these three techniques ...

Effect of poling temperature on piezoelectricity of CaZrO3-modified (K, Na)NbO3-based lead-free ceramics

Electrical poling is indispensable for endowing isotropic ferroelectric polycrystals with a net macroscopic polarization and hence piezoelectricity. However, little attention has been paid to the optimization of poling conditions in (K, Na)NbO3-based ceramics with a polymorphic phase transition. This study investigated the electrical properties of CaZrO3-modified (K, Na, Li)(Nb, Ta)O3 lead-free piezoceramics as a function of the poling temperature. Peak piezoelectric coefficient d33 of 352 ± 7 pC/N and planar electromechanical coupling factor kp of 0.47 were obtained at the optimized poling temperature of 120 °C, which crosses the polymorphic phase transition regime. In-depth analysis of the asymmetric polarization hysteresis loops and bipolar strain curves uncovered striking analogy between electrical poling and unipolar cycling in the current system, which is attributed to a competition between domain reorientation and space charge accumulation.

Robust CaZrO3-modified (K, Na)NbO3-based lead-free piezoceramics: High fatigue resistance insensitive to temperature and electric field

Robust resistivity against electrical cycling at not only ambient conditions but also enhanced temperatures is an essential requirement for high-end actuator applications. In this study, the temperature and electric field dependence of unipolar fatigue behaviors of CaZrO3-modfied (K, Na)NbO3 lead-free piezoceramics were investigated. The space charge accumulation during unipolar cycling is responsible for the build-up of internal bias field, the dynamics of which with respect to temperature and driving field can be described using a model based on Maxwell-Wagner relaxation process. Besides, clamping of domain walls can be inferred by comparing the large and small signal permittivity. Most intriguingly, the unipolar strain exhibits a fatigue-free behavior even at elevated temperatures, rendering the material exceptionally suitable for actuator applications.

Identifying phase transition behavior in Bi1/2Na1/2TiO3-BaTiO3 single crystals by piezoresponse force microscopy

Using piezoresponse force microscopy (PFM) and Raman spectroscopy, we studied the local temperature-dependent piezoelectric properties and phase structures of 0.95(Bi0.5Na0.5)TiO3-0.05BaTiO3 (BNT-BT) single crystals. Local-area PFM revealed non-ergodic relaxor behavior around 160 °C. Switching spectroscopy-PFM (SS-PFM) results also supported the transition around 160 °C, with a gradual decrease in hysteresis width and nucleation bias. Moreover, Raman spectroscopy provided structural evidence of a phase transition in the same temperature region. These results are consistent with other theories of phase transitions in BNT-BT-based materials and verify the existence of a phase transition from a non-ergodic relaxor to ergodic relaxor of BNT-5.0%BT near 160 °C.

Temperature dependence of the local piezoresponse in (K,Na)NbO3-based ceramics with large electromechanical strain

We report on temperature dependence of local electromechanical properties of lead-free (K,Na)NbO3-based ceramics that macroscopically manifests a large temperature-insensitive strain. Piezoresponse force microscopy reveals the particular role of the orthorhombic-tetragonal phase transition, where a reconstruction of the domain structure occurs and local piezoelectric response shows a peak value. A good quantitative agreement between temperature dependences of the local and previously reported macroscopic small-signal piezoelectric coefficients is observed. An influence of the polymorphic phase transition on polarization switching kinetics was revealed.

Refreshing Piezoelectrics: Distinctive Role of Manganese in Lead-Free Perovskites

Driven by an ever-growing demand for environmentally compatible materials, the past two decades have witnessed the booming development in the field of piezoelectrics. To maximally explore the potential of lead-free piezoelectrics, chemical doping could be an effective approach, referenced from tactics adopted in lead-based piezoelectrics. Herein, we reveal the distinct role of manganese in a promising lead-free perovskite (K, Na)NbO3 (denoted by KNN) in comparison to that in market-dominating lead-based counterparts [Pb(Zr, Ti)O3, PZT]. In contrast to the scenario in PZT, manganese doping in KNN results in tremendously improved piezoelectric coefficient d33 by nearly 200%, whereas the same doping species in PZT deteriorates the d33 down to less than 30% of its original value. The result is rationalized from macroscopic and local electrical characterizations down to atomic-scale visualization. This study demonstrates that there is enormous space to further enhance piezoelectricity in lead-free systems because the chemical doping effect may completely differ in lead-containing and lead-free perovskites.