There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.

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Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites

A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

Processing technologies for poly(lactic acid)

Investigations in the development of lead-free piezoelectric ceramics have recently claimed comparable properties to the lead-based ferroelectric perovskites, represented by Pb(Zr,Ti)O3, or PZT In this work, the scientific and technical impact of these materials is contrasted with the various families of “soft” and “hard” PZTs On the scientific front, the intrinsic nature of the dielectric and piezoelectric properties are presented in relation to their respective Curie temperatures (T
 C) and the existence of a morphotropic phase boundary (MPB) Analogous to PZT, enhanced properties are noted for MPB compositions in the (Na,Bi)TiO3-BaTiO3 and ternary system with (K,Bi)TiO3, but offer properties significantly lower The consequences of a ferroelectric to antiferroelectric transition well below T
 C further limits their usefulness Though comparable with respect to T
 C, the high levels of piezoelectricity reported in the (K,Na)NbO3 family are the result of enhanced polarizability associated with the orthorhombic-tetragonal polymorphic phase transition being compositionally shifted downward As expected, the properties are strongly temperature dependent, while degradation occurs through the thermal cycling between the two distinct ferroelectric domain states Extrinsic contributions arising from domains and domain wall mobility were determined using high field strain and polarization measurements The concept of “soft” and “hard” lead-free piezoelectrics were discussed in relation to donor and acceptor modified PZTs, respectively Technologically, the lead-free materials are discussed in relation to general applications, including sensors, actuators and ultrasound transducers

Lead-free piezoelectric ceramics: Alternatives for PZT?

Thermal Conductivity of Polymer-Based Composites: Fundamentals and Applications

Structure, electrical properties and depolarization temperature of (Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoelectric ceramics

In this letter the authors report the observation of double hysteresis loops in Cu-doped K0.5Na0.5NbO3 (KNN) ceramics. Unlike other ferroelectric titanates (e.g., BaTiO3), aging is not required for the ceramic to exhibit the double-loop-like characteristics. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions-Cu2+ and O2− vacancies along the polarization direction after the diffuse tetragonal-orthorhombic phase transition of the ceramic. Because of the low migration rates of defects, the defect dipoles remain in the original orientation during the P-E loop measurement, providing a restoring force to reverse the switched polarization. The defect dipoles also provide “pinning” effects in the normal piezoelectric activities. As a result, the ceramic becomes “hardened,” exhibiting an extraordinarily high mechanical quality factor (2500), while the other piezoelectric properties remain reasonably good: electromechanical couplin...

Double hysteresis loop in Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

Lead-free piezoelectricceramics ( 1 − x ) K 05 Na 05 Nb O 3 – x Li Sb O 3 have been fabricated by a conventional ceramicsintering technique The results of x-ray diffraction suggest that Li + and Sb 5 + diffuse into the K 05 Na 05 Nb O 3 lattices to form a solid solution with a perovskite structure The ceramics can be well sintered at 1070 – 1110 ° C  The introduction of Li Sb O 3 into the Na 05 K 05 Nb O 3 solid solution decreases slightly the paraelectric cubic-ferroelectric tetragonal phase transition temperature ( T c ) , but greatly shifts the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ( T O – F ) to room temperature Coexistence of the orthorhombic and tetragonal phases is formed at 005 < x < 007 at room temperature, leading to a significant enhancement of the piezoelectric properties For the ceramic with x = 006 , the piezoelectric properties become optimum: piezoelectric constant d 33 = 212 pC ∕ N , planar and thickness electromechanical coupling factors k P = 46 % and k t = 47 % , respectively, remanent polarization P r = 150 μ C ∕ cm 2 , coercive field E c = 174 kV ∕ mm , and Curie temperature T C = 358 ° C 

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Structure and electrical properties of K0.5Na0.5NbO3-LiSbO3 lead-free piezoelectric ceramics

The elastic, dielectric and piezoelectric constants of four piezoelectric materials, including polyvinylidene fluoride, vinylidene fluoride-trifluoroethylene copolymer, PZT/epoxy 1-3 composite, and lead metaniobate ceramic, have been evaluated from the impedance data using five different methods. A method described in ANSI/IEEE Std. 176-1987, though based on formulae derived for loss-less materials, is found to be applicable to materials with moderate loss. However, for high-loss materials such as polyvinylidene fluoride, the electromechanical coupling constant (/spl kappa//sub t/) obtained by the method of Std. 176 is substantially higher than the actual value. Calculations based on a piezoelectric resonance analysis program (PRAP) combine the best features of two earlier methods. In addition to the impedance at the parallel resonance frequency, impedances at two other frequencies are required for calculation. The PRAP method gives quite accurate material parameters regardless of the magnitude of the loss, but the parameters (including /spl kappa//sub t/) vary by as much as 15% depending on the choice of data. In the nonlinear regression method described in the present work, all the impedance data points around the resonance are least-squares fitted to the theoretical expression for the impedance. Besides the advantage of requiring no arbitrary choice of data, the nonlinear regression method can readily take account of the frequency dependence of the dielectric constant.

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Evaluation of the material parameters of piezoelectric materials by various methods

Biodegradable poly(L-lactic acid) (PLLA) fibers were processed by a two-step melt-spinning method (melt extrusion and hot draw) from PLLA with three different viscosity-average molecular weights (494,600, 304,700, and 262,800). Before spinning, the polymer flakes were first milled into powders and dried under vacuum. Viscosity-average molecular weight of PLLA following the fabrication process was monitored. Tensile properties of as-spun and hot-drawn fibers were investigated. Morphology of the PLLA fibers was viewed under a scanning electron microscope. Crystallinity of these fibers was assessed by thermogram analysis of differential scanning calorimetry. Results showed that the extent of decrease in the viscosity-average molecular weight of PLLA dropped sharply by 13.1–19.5% during pulverization and by 39.0–69.0% during melt-extrusion. The hot-draw process in this study had a little effect on the viscosity-average molecular weight of PLLA. Smoother fibers could be obtained for the die temperature at least 230°C for raw materials with higher crystallinity (more than 75%) and at least 220°C for raw materials with lower crystallinity (about 60%). The as-spun fibers showed crystallinity of 16.5–22.8% and the value increased to 50.3–63.7% after hot draw. Tensile moduli of the as-spun fibers were in the range of 1.2–2.4 GPa, which were raised to 3.6–5.4 GPa after hot draw. The final PLLA fibers with 110–160 μm diameters showed tensile strengths of 300–600 MPa. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 251–260, 2001

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Kin Wing Kwok

Papers

Structure, electrical properties and depolarization temperature of (Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoelectric ceramics

Double hysteresis loop in Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

Structure and electrical properties of K0.5Na0.5NbO3-LiSbO3 lead-free piezoelectric ceramics

Evaluation of the material parameters of piezoelectric materials by various methods

Characterization of poly(L‐lactic acid) fibers produced by melt spinning