Electromechanical Properties of Porous Piezoelectric Ceramics

A theoretical approach is forwarded to predict the electromechanical properties of porous piezoelectric ceramics. The analysis is able to account for the effects of porosity shape and concentration and is applicable to piezoelectric ceramics of arbitrary material symmetry. By coupling the exact solution for a single ellipsoidal pore embedded in an infinite piezoelectric matrix with an effective medium approximation, the theory considers, in an approximate manner, interaction effects at finite porosity concentrations. The theoretical estimates are developed using a matrix formulation which enables all elastic, dielectric, and piezoelectric moduli of the porous solid to be readily computed. Numerical results are presented to illustrate the effects of the shape and concentration of the porosity on the effective electroelastic moduli and transducer parameters of practical importance. Particular attention is devoted to assessing the sensitivity of the effective electromechanical properties to the accuracy of the input data. Finally, theoretical estimates are shown to be in good agreement with existing experimental results for porous piezoelectric ceramics with various microstructural geometries.     

蜂窝材料增强聚氨酯硬泡

介绍了蜂窝增强聚氨酯硬泡复合材料的制备工艺,并讨论了其影响因素,说明了这一材料的用途及发展前景。     

填充蜜胺泡沫芳纶蜂窝复合芯吸声性能研究

制备了蜜胺泡沫填充芳纶蜂窝复合芯,分析了复合芯的吸声机理,考察了复合芯的密度和厚度对吸声性能的影响。结果表明,在较高频率范围内,复合芯的吸声系数高于蜜胺泡沫;随着复合芯的密度及厚度的增大,复合芯在低频下的吸声性能逐步增大;当复合芯密度和厚度分别为78 kg/m3及20 mm,其平均吸声系数可达0.61,是一种优异的工程吸声材料。     

Large Electromechanical Response in Lead‐Free La‐Doped BNKT–BST Piezoelectric Ceramics

Piezoceramics 0.99[(Bi_0.5Na_0.4K_0.1)_1?xLa_xTiO₃]?0.01[Ba_0.7Sr_0.3TiO₃] (BNKT–BST–La_x, x = 0–0.030) were synthesized using a conventional solid‐state reaction method. X‐ray diffraction revealed a phase transition from a tetragonal to cubic phase at x ≥ 0.005. The maximum dielectric constant as well as the depolarization temperature (T_d) decreased with increasing La content. La addition interrupted the polarization and strain hysteresis loops and demonstrates that the ferroelectric order of the BNKT–BST ceramics lead to a reduction in the remnant polarization and coercive field. However, the destabilization of the ferroelectric order is accompanied by a significant increase in the unipolar strain which is highest at x = 0.020 with a value of ~0.39% and corresponding normalized strain, d^*₃₃ (= S_max/E_max) of 650 pm/V. It was observed that the unipolar strain of x = 0.020 is very temperature insensitive up to 125°C, even at 125°C the d^*₃₃ is as high as ~431 pm/V. Moreover, an electric‐field‐dependent XRD was conducted to identify the main source of the high strain and a recoverable transformation from cubic to a rhombohedral–tetragonal mixed phase was observed. The recoverable field‐induced phase transformation is suggested to be the main cause for the obtained large strain at x = 0.020 in the BNKT–BST–La_x ceramics.     

Strong Electromechanical Response in Lead Zirconate Titanate Metamaterials

As a class of technically important functional materials, Pb(Zr,Ti)O₃ (PZT)‐based materials have been widely used in different types of piezoelectric devices. These materials are also ferroelectrics, and the materials lose their piezoelectric properties above the Curie temperature, a drawback limiting applications of the materials at high temperatures. Designing piezoelectric metamaterials by exploiting the flexoelectricity of the PZT‐based materials is a possible solution to this issue. In this work, PZT‐based piezoelectric metamaterials can be designed by applying an asymmetric chemical reduction to PZT ceramics to produce a curvature. The reduced PZT ceramic wafers exhibit a high apparent piezoelectric response (>2900 pC/N), which can be sustained after a heat treatment at 550°C for 5 h, which is more than 250°C above their Curie temperature. A substantial piezoelectric response can also be directly measured well above the Curie temperature. The mechanism underlying this very high apparent piezoelectric response with high‐temperature stability is investigated. The experimental results suggest that the electromechanical response mainly originates from the flexoelectric effect when the reduction‐induced curvature is bent under stress. The formation of the curvature also causes strain gradient‐induced poling of the reduced materials, resulting in a weak piezoelectric response from partly oriented polarization, which partly contributes to the observed response.     

Electromechanical Properties of Acceptor‐Doped Lead‐Free Piezoelectric Ceramics

We have studied the processing and electromechanical properties of Mn and Fe‐doped 0.88[Bi_0.5Na_0.5TiO₃]–0.08[Bi_0.5K_0.5TiO₃]–0.04[Bi_0.5Li_0.5TiO₃] piezoelectric ceramics prepared by the mixed oxide route. Different amounts of Mn (0.01, 0.014, 0.015, 0.016, 0.017, 0.02, 0.022) or Fe (0.0125, 0.015, 0.0175) were doped to this lead‐free piezoelectric composition. Ceramics were sintered at different temperatures (1075°C–1150°C) to achieve the highest density and mechanical quality factor. Mn or Fe doping resulted in a considerable enhancement of Q_m in both planar and thickness resonance modes. In 1.5 mol% Mn‐doped ceramics sintered at 1100°C, a planar Q_m of about 970 and tanδ of 0.88% were obtained. In Fe‐doped ceramics, a planar Q_m as high as 900 was achieved. Acceptor dopants also resulted in decreasing the coupling coefficients, the piezoelectric charge coefficient, and the dielectric constant.     

Construction of Micro-Patterned Polymer Structures by Piezoelectric Inkjet Printing

It is not simple to accurately deposit minute quantities of polymeric materials by inkjet printing systems. High viscosity, nozzle clogging, agglomeration, precipitation, and uncontrollable drying patterns are serious problems which are frequently encountered in polymer inkjet printing. In this study, we investigated how inkjet printability of polymers correlates with the polymer ink formulations and inkjet process variables. After a systematic study with different variables, various patterns such as dots, cross stripes, and honeycombs were fabricated on flexible polyimide (PI) films and the pattern morphology and spatial distribution of the resulting polymer deposits after solvent drying were characterized.     

Electromechanical Properties of PMN–PZT Piezoelectric Single Crystals Near Morphotropic Phase Boundary Compositions

Pb(Mg_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMN–PZT) ferroelectric single crystals near morphotropic phase boundary compositions were fabricated by solid-state crystal growth. The Curie temperatures ( T _C) of the grown PMN–PZT crystals were found to be on the order of 210°C, with ferroelectric phase transition temperatures ( T _{R – T} ) in the range of 96°–165°C. The electromechanical coupling factors k ₃₃ and k ₃₂ were found to be >90% and >−87%, respectively. The coercive field E _C for all the compositions was on the order of 5 kV/cm, double the value of pure Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMNT) crystals. The temperature dependence of the piezoelectric and electromechanical properties and dc bias effect on the dielectric behavior were investigated. The temperature usage range under dc bias was found to be improved when compared with pure PMNT crystals with similar piezoelectric properties.     

Collapse Mechanism of Foam Cored Sandwich Structures Under Compressive Load

In this work the moisture absorption capability, compressive properties, collapse modes of various types of composite sandwich structures are reported. The tested sandwich structures were constructed with varieties of hybridized skin materials and different compositions of the core materials. The moisture absorption, Flatwise compression and Edgewise compression tests are conducted for core as well as sandwich structures. Comparisons of results have been between the hybridized and non-hybridized sandwich structures. Two modes of collapse were noticed in the Edgewise compressive test, one of which being progressive end-crushing of the sandwich structure featured by significant crash energy absorption. This feature was highly desired for the parts of transportation vehicles. Microscopic analysis has been carried out to know the nature of failure under compressive loads. It has been observed that with increasing the debonding strength of the core–face interface, the failure mode changes from unstable collapse mode stable progressive crushing.     

Effect of Uniaxial Stress Upon the Electromechanical Properties of Various Piezoelectric Ceramics and Single Crystals

A systematic investigation of the stress-dependent (σ) electromechanical properties of various ferroelectric ceramics and single crystals has been performed. Studies have been carried out on "hard" and "soft" piezoelectrics, electrostrictive ceramics, and various orientations of (1− x )Pb(Mg_1/3Nb_2/3)O₃–( x ) PbTiO₃ PMN– x %PT single crystals. The large signal piezoelectric constant, acoustic power density, and coupling coefficient have been determined by calculation. The results are compared, in order to develop an understanding of the relative merits of the different types of active acoustic materials.     

Piezoelectric Film Response Studied with Finite Element Method

It is difficult to measure accurately the piezoelectric constant, d ₃₃, of either a thin or thick film on a substrate, because piezoelectric deformation of a film is small. The measured value of d ₃₃ for the film is often smaller than the value of d ₃₃ for the bulk material. This is partly because of bending of the sample and side clamping of the film. However, we used finite element method (FEM) to simulate common experimental conditions and show that inner local deformation was more significant than either bending or side clamping. Using our FEM results, we propose optimum conditions for making unbiased d ₃₃ measurements.     

Electromechanical Response of Polycrystalline Barium Titanate Resolved at the Grain Scale

Ferroic materials are critical components in many modern devices. Polycrystalline states of these materials dominate the market due to their cost effectiveness and ease of production. Studying the coupling of ferroic properties across grain boundaries and within clusters of grains is therefore critical for understanding bulk polycrystalline ferroic behavior. Here, three‐dimensional X‐ray diffraction is used to reconstruct a 3D grain map (grain orientations and neighborhoods) of a polycrystalline barium titanate sample and track the grain‐scale non‐180° ferroelectric domain switching strains of 139 individual grains in situ under an applied electric field. The map shows that each grain is located in a very unique local environment in terms of intergranular misorientations, leading to local strain heterogeneity in the as‐processed state of the sample. While primarily dependent on the crystallographic orientation relative to the field directions, the response of individual grains is also heterogeneous. These unique experimental results are of critical importance both when building the starting conditions and considering the validity of grain‐scale modeling efforts, and provide additional considerations in the design of novel ferroic materials.     

粘结层对压电纤维复合材料机电响应行为的影响

本文从功能材料力-电耦合的角度分析了粘结层参数对压电纤维复合材料(macro fiber composites, MFC)机电响应行为的影响。通过有限元模拟计算发现,减少粘结层厚度及增大其介电常数有利于缓解MFC介电失配现象,提高有效电场加载,从而获得高压电性。试验制备了MFC并进行了驱动及传感性能表征,试验结果与模拟仿真一致。减少粘结层厚度和弹性模量,及增大其介电常数,能有效增大MFC尖端位移和输出电压,提高驱动和传感性能。该研究对驱动和传感用MFC的设计提供了指导。     

Energy-Absorbing TRIP-Steel/Mg-PSZ Composite Honeycomb Structures Based on Ceramic Extrusion at Room Temperature

Porous materials have received extensive attention for energy absorption in the last few years. In terms of this study, austenitic TRIP-steel/Mg-PSZ-composite honeycomb structures are formed with different mixing proportions due to ceramic extrusion at room temperature. Their specific energy absorption SEA as well as their compression strength have been registered as a function of the compressive strain. X-ray diffractometry (XRD), electron backscatter diffraction (EBSD) as well as electron dispersive X-ray (EDX) analysis support the microstructure characterization. The zirconia addition has mainly contributed as a hard phase in a ductile TRIP steel matrix and has reinforced the composite material up to a compressive strain of about 24%.     

Fabrication of Transparent Spinel Honeycomb Structures by Methyl Cellulose–Based Thermal Gelation Processing

Transparent MgAl₂O₄ spinel of flat as well as honeycomb structure was fabricated by employing thermally induced gel casting of the slurry with 56 wt% solid loading containing 0.2 wt% of methylcellulose. The green specimens were pressureless sintered to 98%–99% of theoretical density with no open porosity at an optimum temperature of 1700°C. Final densification by hot isostatic pressing of both the specimens at the optimum temperature of 1800°C and 195 MPa pressure enabled further elimination of residual porosities and full densification resulting in theoretical density of 3.58 g/cc. The design of the honeycomb was such that it exhibited a surface area to volume ratio of 0.65 cm²/cm³ and a relative density of 0.69. The hardness of the honeycomb specimens has been found to be 13 GPa, which is at par with the solid specimens processed under identical conditions. Solid specimens of around 4 mm thickness exhibited a transmission of >80% in the visible (0.4–0.8 μm) region. Specimens were also tested according to ASTM procedures and have shown a flexural strength (σ_f) of 195 MPa and plane‐strain fracture toughness (K_Ic) of 1.87 MPa·m^1/2 as reported in this study.     

Electromechanical Imaging and Spectroscopy of Ferroelectric and Piezoelectric Materials: State of the Art and Prospects for the Future

Piezoresponse force microscopy (PFM) has emerged as a powerful and versatile tool for probing nanoscale phenomena in ferroelectric materials on the nanometer and micrometer scales. In this review, we summarize the fundamentals and recent advances in PFM, and describe the nanoscale electromechanical properties of several important ferroelectric ceramic materials widely used in memory and microelectromechanical systems applications. Probing static and dynamic polarization behavior of individual grains in PZT films and ceramics is discussed. Switching spectroscopy PFM is introduced as a useful tool for studying defects and interfaces in ceramic materials. The results on local switching and domain pinning behavior, as well as nanoscale fatigue and imprint mapping are presented. Probing domain structures and polarization dynamics in polycrystalline relaxors (PMN-PT, PLZT, doped BaTiO₃) are briefly outlined. Finally, applications of PFM to dimensionally confined ferroelectrics are demonstrated. The potential of PFM for studying local electromechanical phenomena in polycrystalline ferroelectrics where defects and other inhomogeneities are essential for the interpretation of their macroscopic properties is illustrated.     

Piezoelectric Moduli of Piezoelectric Ceramics

The effective piezoelectric moduli of polycrystalline piezoelectric ceramics are calculated using a straightforward averaging method (a Voigt average) and a more rigorous effective-medium theory recently developed for piezocomposites. The effects of polarization orientation within each crystallographic domain and the shape of the domain on the effective piezoelectric moduli are presented. The effective-medium theory, which includes full coupling of the elastic and dielectric interactions, gives results in reasonable agreement with experimental ones.     

Fabrication of Silicone Rubber Foam with Tailored Porous Structures by Supercritical CO2

In spite of great concern on the wide application of silicone rubber foams, few works have been reported about easy‐operating foaming method. In this study, the effects of silica content and foaming process on the porous structure of high‐temperature‐vulcanized silicon rubber foams are evaluated, which are prepared by supercritical CO₂ at different conditions, with fumed silica used for reinforcement. Silicone rubber foams with cell size in 8–120 μm, cell density in 10⁵–10⁸ cm⁻³, and density between 0.45 and 0.9 g cm⁻³ are prepared under different saturation conditions. The results show that increasing silica content can decrease cell size. It is also found that cell density improves exponentially with increasing saturation pressure and decreasing saturation temperature. Besides, it demands less than 1 h for specimens to reach equilibrium on thickness around 3 mm. All the results indicate that the porous structures of silicone foams can be tailored by foaming process parameters facilely and are predictable with fitted equation.

     

蜂窝密封的应用

介绍了蜂窝密封结构、密封机理,梳齿密封改蜂窝密封的改造方法,国内外发展应用情况和在镇海炼化首台机组上的改造应用情况。蜂窝密封正从航空领域向工业领域推广应用,与传统梳齿密封相比极具优越性,特别是在高压差密封场合。蜂窝密封在电力、石化行业的汽轮机、压缩机领域里具有广阔的应用前景。     

Small Reduction of the Piezoelectric d33 Response in Potassium Sodium Niobate Thick Films

We have investigated the electromechanical response of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. The high‐field strain hysteresis loops and weak‐field converse piezoelectric d₃₃ coefficient of the films were measured and compared with those of KNN bulk ceramics under the same electric field conditions. The converse d₃₃ values of the thick films and bulk ceramics were equal to 82.5 and 138 pm/V, respectively, at 0.4 kV/mm. The fundamental difference between the piezoelectric response of the KNN films and the ceramics was studied in terms of the effective (“clamped”) piezoelectric d₃₃ coefficient. The reduction in the piezoelectric d₃₃ coefficient of the KNN films, resulting from the clamping by the substrate, was compared to lead‐based ferroelectric thick films, including Pb(Zr,Ti)O₃ (PZT) and (1 ? x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ (PMN‐PT). We propose a possible explanation, based on the particular elastic properties of KNN, for the small relative difference observed between the “clamped” and “unclamped” (“bulk”) d₃₃ of KNN, in comparison with lead‐based systems.