The effect of pre-annealing on the microstructure of (K,Na)NbO3 ceramics

The effects of pre-annealing on the microstructure development and piezoelectric properties for 0.95(K_0.5Na_0.5)NbO₃–0.05LiSbO₃ (0.95KNN–0.05LS) ceramics were investigated. The pre-annealing suppressed the abnormal grain growth in both the undoped and Mn-doped 0.95KNN–0.05LS ceramics. The pre-annealed samples possessed smaller abnormal grains, larger matrix grains, and a broader grain size distribution compared to the samples sintered without a pre-annealing step. The pre-annealed samples presented better dielectric and piezoelectric properties, a larger dielectric constant (ε_r) and electromechanical coupling factor (k_p), and a smaller dielectric loss factor (tan δ).     

Internal bias field relaxation in poled Mn-doped Pb(Mn1/3Sb2/3)O3–Pb(Zr,Ti)O3 ceramics

The frequency, electric field cycling and temperature dependences of the polarization–electric field (P–E), strain–electric field (S–E) loops in poled Mn-doped 0.05Pb(Mn_1/3Sb_2/3)O₃–0.50PbZrO₃–0.45PbTiO₃ ceramics have been investigated. The P–E and S–E loops are strongly asymmetric corresponding to the presence of an internal bias field E_i after poling and aging, indicating that the domain walls are strongly pinned by preferentially oriented defect dipoles formed by the acceptor dopant ions (Mn²⁺/Mn³⁺) and O²⁻ vacancies. Whereas, the loops exhibit a tendency of changing from asymmetric shapes to normal symmetric ones with increasing electric field amplitude or decreasing frequency. Repeated electric field cycling as well as high temperature results in a similar effect. Meanwhile, the E_i reduces consequently, providing evidence of domain depinning or internal bias field relaxation. It is suggested that the reorientation of the defect dipoles and depinning of domain walls arising from high temperature or electric field cycling are responsible for this extrinsic internal bias field relaxation process.     

Investigation of dielectric and piezoelectric properties in Pb(Ni1/3Nb2/3)O3–PbHfO3–PbTiO3 ternary system

The dielectric and piezoelectric properties were investigated in the (1 ? x)Pb(Hf_1?yTi_y)O₃–xPb(Ni_1/3Nb_2/3)O₃ (PNN–PHT, x = 0.05–0.50, y = 0.55–0.70) ternary system. The morphotropic phase boundary (MPB) was determined by X-ray powder diffraction analysis. Isothermal map of Curie temperature (T_C) related to the compositions in the phase diagram was obtained. The optimum dielectric and piezoelectric properties were achieved in ceramics with the MPB compositions, with the maxima values being on the order of 6000 and 970pC/N, respectively. Rayleigh analysis was used to study the extrinsic contribution (domain wall motion) in PNN–PHT system, where the extrinsic contribution was found to be ～30% for composition 0.49PNN–0.51PHT(30/70), showing a high nonlinearity.     

PMN-PZT压电陶瓷的凝胶注模研究

对三元系PMN—PZT压电陶瓷的凝胶注模成形工艺进行了研究,重点讨论了分散剂加入量、pH值、球磨时间与注模浆料流变之间的关系,当分散剂用量为0 .95 %～1 .0 5 % ,pH值为10 .5～11 .5 ,球磨时间为8h时,可以制备出固相含量为5 5 % ,粘度小于1Pa·s的PMN -PZT压电陶瓷悬浮体。将凝胶注模与传统模压成形工艺制备的样品进行了比较,通过扫描电镜(SEM )显微形貌分析和电学性能测定。结果表明:凝胶注模工艺制备的样品与模压工艺制备的样品相比较,其显微结构均匀致密、晶粒大小与晶界分布均匀,气孔率更小,电学性能更均匀。     

Fabrication of piezoelectric Ca-P-Si-doped PVDF scaffold by phase-separation-hydration: Material characterization,in vitro biocompatibility and osteoblast redifferentiation

Native bone is piezoelectric in nature and generates abundant surface charges under mechanical compression, which regulate osteoblast proliferation, differentiation, adhesion, and so on. Poly (vinylidene fluoride) (PVDF) is becoming one of the most popular piezoelectric polymers because of its easy processability and good biocompatibility. Unfortunately, because only the β and γ crystal phases of PVDF have piezoelectricity, post-treatments, for example, polarizing at high temperature, are required to enhance the piezoelectricity of PVDF scaffolds after fabrication. In this study, we reported a phase-separation-hydration method to fabricate a calcium phosphate silicate (CPS)-doped PVDF scaffold. Our method fabricated a better piezoelectric scaffold than native bone without further processing (～ 3 pC/N vs. 0.7 pC/N). In addition, the scaffold was mechanically compatible (～ 7 MPa) with the cancellous bone with sufficient porosity (～ 45%) to facilitate osteoblast infiltration and bone ingrowth. The in vitro biocompatibility analyses proved that the prepared CPS-PVDF scaffold was biocompatible with osteoblast cells and encouraged osteoblast redifferentiation. In conclusion, our results suggest that this CPS-PVDF scaffold is a promising candidate for bone tissue engineering applications.     

Direct and controlled device integration of graphene oxide on Quartz Crystal Microbalance via electrospray deposition for stable humidity sensing

Integration of advanced and functional materials onto conventional sensing platforms can improve the device performances and even discover new applications. For piezoelectric resonant sensors, an addition of sensing materials can induce damping and hinder a stable device operation. Hence, the development of efficient method for materials integration is important to ensure high-performance and reliable sensor operation. This work presents a direct and precisely controlled integration of graphene oxide (GO) using the electrospray deposition (ESD) onto a 10 MHz Quartz Crystal Microbalance (QCM) for humidity sensing. The proposed ESD method achieves a high mass resolution of a few nanograms. Moreover, the GO uniformly coats across the sensing electrode region as it acts as a ground electrode during ESD. The proposed ESD method also works for a wide range of nanomaterials, such as carbon nanotubes, tin oxide, and silicon carbide micro-and nano-powders. Compared to the conventional drop-casting and dip coating approaches, our method ensures minimal GO agglomeration, resulting in a stable QCM-oscillator operation in a wide range of relative humidity from 11% to 97%. The measurement sensitivity increases with an amount of GO, but less GO results in better noise and detection limit performances. The results shed light on the importance of selecting an optimal amount of sensing materials for stable sensor operations.     

Tetragonal tungsten bronze phase potential in increasing the piezoelectricity of sol-gel synthesized (K0.5Na0.5)1-xLixNbO3 ceramics

(K,Na)NbO₃ (KNN)-based ceramics have been proven to be formidable candidates among lead-free piezoelectric materials, yet poor reproducibility always hinders their progress. In the present study, the effects of low lithium substitution on the electrical properties and microstructure of (K_0.5Na_0.5)_1-xLi_xNbO₃ (KNLN) ceramics were investigated. All samples were synthesized by the sol-gel method. The Curie temperature (T_C) of the ceramics shifted to higher temperature and gradually decreased the monoclinic-tetragonal (T_M-T) phase transition. Li⁺ substitution had a prominent effect on the ferroelectric properties and improved the piezoelectric coefficient (d₃₃) up to 181 pC/N. X-Ray Diffraction (XRD) studies and Field Emission Scanning Electron Microscopy (FESEM) images revealed an inevitable tetragonal tungsten bronze (TTB) secondary phase, which was formed during the preparation process. It was demonstrated that the volatilization of Li⁺ cations facilitated TTB growth. The coexistence of two different phase structures proved to enhance the KNN piezoelectric performance.     

Large piezoelectricity and potentially activated polarization reorientation around relaxor MPB in complex perovskite

Improving piezoelectric performance is always favorable to further enhance the sensitivity and accuracy of piezoelectric devices. Here, a complex piezoelectric system of Pb(Ni_1/3Nb_2/3)O₃-Pb(Yb_1/2Nb_1/2)O₃-Pb(Hf_0.1Ti_0.9)O₃ is designed and investigated in detail. Optimized piezoelectric response of ～ 880 pC/N is achieved at the composition of 0.51PNN-0.09PYN-0.40PHT. The characterization of TEM and In-situ high-energy synchrotron diffraction indicate that nanodomain growth and microdomain switching occurs in succession at around coercive electric field. Most interestingly, the coexisted tetragonal and rhombohedral-like phase transforms into multiple monoclinic-like phases with polarization vectors aligned as close to the electric field direction as possible under the strong electric field. The enhanced polarization instability in this complex morphotropic phase boundary sample should be ascribed to the strong local heterogeneity. The novel polarization rotation behavior found in this work would be important guidance for designing high-performance piezoceramics.     

Cold sintering of the ceramic potassium sodium niobate, (K0.5Na0.5)NbO3, and influences on piezoelectric properties

Cold Sintering was applied to densify a Potassium-Sodium Niobate solid solution composition, 0.5KNbO₃-0.5NaNbO₃ (KNN); the process uses a transient chemical sintering aid, moderate pressure (400 MPa), and temperatures between 230–300 °C to obtain ceramics of ～92 to 96 % theoretical density. Typically, sintering temperatures between ～1000?1050 °C are required to density KNN using conventional methods. In this paper, the densification was investigated during heating, particularly the shrinkage in the first 60 min of the cold sintering process. The low-field dielectric and electrical properties of the resulting ceramics were found to be comparable to conventionally sintered KNN. Electric fields up to 80 kV/cm could be applied, however the ceramics showed pinched hysteresis loops, even after poling over a wide range of temperatures and electric fields. A Rayleigh analysis was used to investigate domain dynamics and high reversible permittivity. The irreversible behavior was an order of magnitude lower than in conventionally sintered KNN, likely associated with defect pinning of ferroelectric domains. A Transmission Electron Microscopy (TEM) study revealed a high density of line defects in most grains; dislocations in the grains limit poling and domain wall movement, thus suppressing both the piezoelectric properties and the hysteresis. Furthermore, TEM observations indicated crystalline grain boundaries that were faceted with terrace kink ledges. These observations point to the importance of the initial powder optimization and grain boundary diffusion when using cold sintering to prepare ceramics that are intended to show bulk cooperative properties such as ferroelectricity. The impact of limited high temperature homogenization of bulk diffusional processes is discussed.     

SnO2 modified PNN-PZT ceramics with ultra-high piezoelectric and dielectric properties

In this work, a two-step solid-state reaction method is used to prepare the 0.55 Pb(Ni_1/3Nb_2/3)O₃-0.135PbZrO₃-0.315PbTiO₃/xSnO₂ (PNN-PZT/xSnO₂) ceramics. The influences of SnO₂ on the crystalline structure, electromechanical properties, and temperature stability of PNN-PZT ceramics were studied in detail. The results demonstrate that the Sn⁴⁺ ions are successfully introduced into the PNN-PZT crystalline lattice and substitute B-site Ni²⁺ and Zr⁴⁺. The x = 0.0025 ceramic with the coexistence of rhombohedral, tetragonal, and pseudocubic phases exhibits the optimized comprehensive properties: the quasi-static piezoelectric constant d₃₃, large-signal d₃₃*, electromechanical coupling coefficients k_p and k_t, free dielectric constant ε_r, and mechanical quality factor Q_m are 1123 pC/N, 1250 p.m./V, 0.63, 0.54, 9529, and 57, respectively. Meanwhile, the Curie temperature for this composition is 103 °C, almost maintaining the same level as the PNN-PZT matrix. After annealing at 75 °C, the retained d₃₃ of x = 0.0025 ceramic is as high as 975 pC/N, superior to the PNN-PZT matrix (retained d₃₃ ≈ 873 pC/N). Our results provide a promising piezoelectric material for board bandwidth, high sensitivity, and miniaturized medical ultrasonic transducers applications.