Xilong Zhou

Temperature-dependent mechanical depolarization of ferroelectric ceramics

Mechanical depolarization of poled lead titanate zirconate (PZT) ceramics was conducted at a series of temperatures from 25 to 180??C in a temperature-controlled silicon oil tank. Both the longitudinal strain and polarization of the bar-shaped PZT samples were measured during uni-axial compression up to 400?MPa. It is found that both the stress-induced switchable polarization and the switchable strain of the poled PZT samples decrease steadily with increasing temperature. Unpoled PZT samples were also tested and the switchable strains follow a rule similar to the poled ones but of smaller magnitude. By measuring the P?E hysteresis loops, polarization variations and strain variations of a compression-free, poled PZT sample during a full cycle of heating?cooling, it is found that at elevated temperatures, the reduced switchable polarization by stress is caused by the pyroelectric effect and the reduced switchable strain is mainly due to the decreasing tetragonality (c/a). Furthermore, it was found that both the poled and unpoled PZT ceramics show a recoverable thermal shrinkage effect within the measurement temperature range.

Contact resonance force microscopy for nanomechanical characterization: Accuracy and sensitivity

Contact Resonance Force Microscopy (CRFM), based on dynamic force microscopy, is a new promising technique for quantitative nanoscale mechanical characterization of various materials. In this work, we systematically investigated the measurement accuracy and sensitivity of CRFM both experimentally and numerically. For the accuracy study, we first evaluated the validity of the Euler-Bernoulli beam model used in CRFM and found that it is accurate enough for practical testing. Then, the influence of the tip location was also analyzed and results show that it can significantly affect the obtained indentation modulus. The measurement accuracy of CRFM was then compared with that of nanoindentation and it shows that CRFM has less relative testing errors than nanoindentation for modulus mapping but a larger data scattering for single-point measurements. As to the sensitivity study, we first conducted the cantilever-stiffness dependent sensitivity analysis using both numerical and experimental approaches, and sugge...

More ferroelectrics discovered by switching spectroscopy piezoresponse force microscopy

The local hysteresis loop obtained by switching spectroscopy piezoresponse force microscopy (SS-PFM) is usually regarded as a typical signature of ferroelectric switching. However, such hysteresis loops were also observed in a broad variety of non-ferroelectric materials in the past several years, which casts doubts on the viewpoint that the local hysteresis loops in SS-PFM originate from ferroelectricity. Therefore, it is crucial to explore the mechanism of local hysteresis loops obtained in SS-PFM testing. Here we proposed that non-ferroelectric materials can also exhibit amplitude butterfly loops and phase hysteresis loops in SS-PFM testing due to the Maxwell force as long as the material can show macroscopic D-E hysteresis loops under cyclic electric field loading, no matter what the inherent physical mechanism is. To verify our viewpoint, both the macroscopic D-E and microscopic SS-PFM testing are conducted on a soda-lime glass and a non-ferroelectric dielectric material Ba0.4Sr0.6TiO3. Results show that both materials can exhibit D-E hysteresis loops and SS-PFM phase hysteresis loops, which can well support our viewpoint.

Piezoelectricity and ferroelectricity of cellular polypropylene electrets films characterized by piezoresponse force microscopy

Cellular electrets polymer is a new ferroelectret material exhibiting large piezoelectricity and has attracted considerable attentions in researches and industries. Property characterization is very important for this material and current investigations are mostly on macroscopic properties. In this work, we conduct nanoscale piezoelectric and ferroelectric characterizations of cellular polypropylene (PP) films using piezoresponse force microscopy (PFM). First, both the single-frequency PFM and dual-frequency resonance-tracking PFM testings were conducted on the cellular PP film. The localized piezoelectric constant d33 is estimated to be 7–11pC/N by correcting the resonance magnification with quality factor and it is about one order lower than the macroscopic value. Next, using the switching spectroscopy PFM (SS-PFM), we studied polarization switching behavior of the cellular PP films. Results show that it exhibits the typical ferroelectric-like phase hysteresis loops and butterfly-shaped amplitude loops,...

An optimization-based “phase field” model for polycrystalline ferroelectrics

An optimization-based computational model is proposed to study domain evolution in polycrystalline ferroelectrics composed of numerous grains, each of which consists of multiple domains. Domain switching is realized by an optimization process to minimize the free energy of each grain. Similar to phase field modeling, no priori domain-switching criterion is imposed in the proposed model. Moreover, by focusing on the volume fractions of domains only, the computational complexity of this model becomes much smaller and the domain textures evolution can be captured. Simulation results on both tetragonal and rhombohedral lead titanate zirconate ceramics illustrate the efficiency of this model.

Mechanism of crystal-symmetry dependent deformation in ferroelectric ceramics: Experiments versus model

In this paper, we investigate the mechanism of crystal-symmetry dependent deformation in ferroelectrics both experimentally and theoretically. We fabricated three types of Pb(ZrxTi1-x)O3 ceramics including the tetragonal (PZT45/55), the rhombohedral (PZT60/40), and the morphotropic (PZT52/48), where the tetragonal and rhombohedral phases coexist. X-ray diffraction and piezoresponse force microscopy were performed to characterize the crystal structures and domain patterns. Deformation of both poled and unpoled PZT ceramics was tested under bipolar electric fields and uniaxial compression, respectively. It is found that in both loading cases, the deformation of the morphotropic PZT is obviously larger than that of the tetragonal and rhombohedral PZT. As to the latter two, the electric field induced strain in the tetragonal PZT is smaller than that in the rhombohedral PZT, while the compression induced strains show the opposite tendency. To explore the observed crystal-symmetry deformation mechanism, we empl...

Contact resonance force microscopy with higher-eigenmode for nanoscale viscoelasticity measurements

Nanoscale viscoelastic properties are essential for polymeric materials in their wide applications in nanotechnology. Here we proposed a contact resonance force microscopy (CRFM) method for viscoelasticity measurements by utilizing a cantilevers higher-eigenmode (n > 3). Numerical analysis results show that, compared to its lower eigenmodes, a cantilevers higher eigenmode is more sensitive to contact damping and less affected by contact stiffness variations. This tendency is then verified by nanoscale viscoelasticity mapping on a polystyrene (PS)/polymethyl methacrylate (PMMA) copolymer thin film using a compliant cantilevers different eigenmodes. Results show that higher-eigenmode CRFM can provide better imaging contrast and is thus suggested for viscoelasticity measurements.

Simulations of domain evolution in morphotropic ferroelectric ceramics under electromechanical loading using an optimization-based model

In this paper, an optimization-based computational model is proposed to study the nonlinear performance and domain texture evolution in morphotropic ferroelectric ceramics composed of numerous random oriented grains, each of which contains fourteen types of domains (six are tetragonal and eight are rhombohedral). Field induced phase transformations between the tetragonal and rhombohedral domains are permissible; thus, both the in-phase and inter-phase domain switching can occur in this model. The charge screening effect in real ceramics is taken into account and thus the electric depolarization field caused by incompatible polarization vanishes. The mechanical constraint from the neighboring grains, however, cannot be neglected and is considered using the Eshelby inclusion approach. Under any prescribed electromechanical loading, the volume fraction of each domain in a grain is obtained by minimizing the sum of the potential energy and switching the related dissipation energy of the whole grain using the ...

Ferroelastic domain switching in lead titanate zirconate ceramics: Temperature dependence and fracture toughness variations

Ferroelastic domain switching in electrically-poled lead titanate zirconate (PZT) ceramics were investigated during uni-axial compression up to 400MPa, at a series of temperatures from 25°C to 180°C. It is found that both the stress-induced switchable polarization and switchable strain in the electrically-poled PZT samples decrease steadily with the increasing temperature. By measuring the polarization variations and strain variations of unpoled, prepoled and depoled PZT samples during a full cycle of heating-cooling, it is found that the reduced switchable polarization by stress is caused by the pyroelectric effect and the reduced switchable strain is due to the lattice distortion and thermal shrinkage effect. The fracture toughness of electrically poled and mechanically depoled PZT ceramics were both measured by Vickers indentation and it is found that the fracture toughness anisotropy can even reach 3.63 in the later, significantly larger than that of 2.62 in the former.

Local Contact Stiffness Detection for Nondestructive Testing Based on the Contact Resonance of a Piezoelectric Cantilever

In the field of nondestructive testing (NDT), a suitable defect identification parameter plays an important role in evaluating the reliability of structures or materials. In this work, we proposed a NDT method which detects the samples local contact stiffness (LCS) based on the contact resonance of a piezoelectric cantilever. First, through finite element analysis (FEA) we showed that LCS is quite sensitive to typical defects including debonding, voids, cracks, and inclusions, indicating that LCS could be a good identification parameter. Then, a homemade NDT system containing a piezoelectric cantilever was assembled to detect the samples LCS by tracking the contact resonance frequency (CRF) of the cantilever-sample system. Testing results indicated that the proposed NDT method could detect the above mentioned defects efficiently and precisely. The cantilever-stiffness dependent detection sensitivity was specially investigated and the stiffer cantilevers were found to be more sensitive to small defects, while the softer cantilevers were more suitable for large defects detecting with smaller pressing force. Finally, the detection limit of this NDT method is investigated both experimentally and computationally. The proposed LCS-based NDT method could be very promising for defect detecting in noncontinuous structures and composite materials.