A number of materials have been explored for their use as artificial muscles Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied Materials combining very high energy densities, strains, and efficiencies have been known for some time To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications

Advances in dielectric elastomers for actuators and artificial muscles.

After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

A hybrid-fiber nanogenerator comprising a ZnO nanowire array, PVDF polymer and two electrodes is presented. Depending on the bending or spreading action of the human arm, at an angle of ∼90°, the hybrid fiber reaches electrical outputs of ∼0.1 V and ∼10 nA cm(-2) . The unique structure of the hybrid fiber may inspire future research in wearable energy-harvesting technology.

https://www.nanoscience.gatech.edu/paper/2012/12_AM_04.pdf

A Hybrid Piezoelectric Structure for Wearable Nanogenerators

The development of a method for integrating highly efficient energy conversion materials onto stretchable, biocompatible rubbers could yield breakthroughs in implantable or wearable energy harvesting systems. Being electromechanically coupled, piezoelectric crystals represent a particularly interesting subset of smart materials that function as sensors/actuators, bioMEMS devices, and energy converters. Yet, the crystallization of these materials generally requires high temperatures for maximally efficient performance, rendering them incompatible with temperature-sensitive plastics and rubbers. Here, we overcome these limitations by presenting a scalable and parallel process for transferring crystalline piezoelectric nanothick ribbons of lead zirconate titanate from host substrates onto flexible rubbers over macroscopic areas. Fundamental characterization of the ribbons by piezo-force microscopy indicates that their electromechanical energy conversion metrics are among the highest reported on a flexible medium. The excellent performance of the piezo-ribbon assemblies coupled with stretchable, biocompatible rubber may enable a host of exciting avenues in fundamental research and novel applications.

Piezoelectric Ribbons Printed onto Rubber for Flexible Energy Conversion

Films of semicrystalline poly(vinylidene fluoride) (PVDF) in the β-phase were studied by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The main goal of this study was to improve the understanding of the structural changes that occur in β-PVDF during a mechanical deformation process. FTIR spectroscopy was used to examine the structural variations as a function of strain. DSC data allowed measurement of the melting temperatures and enthalpies of the material before and after deformation, providing information about the changes in the crystalline fraction. After the molecular vibrations were assigned to the corresponding vibrational modes, we investigated the energy and intensity variations of these vibrations at different deformations. A reorientation of the chains from perpendicular to parallel to the stress direction was observed to occur in the plastic region. During the deformation, a decrease in the degree of crystallinity of the material was observed, but ...

/pdf/ftir-and-dsc-studies-of-mechanically-deformed-b-pvdf-films-30lsgnipww.pdf

FTIR AND DSC STUDIES OF MECHANICALLY DEFORMED β-PVDF FILMS

A consistent experimental program has been developed to investigate the mechanical and electrical properties of polyvinylidene fluoride (PVDF). The results of the study indicate that this piezoelectric polymer can be characterized as an orthotropic, thermorheologically simple material with constant piezoelectric strain coefficients over the experimental range of stresses, frequencies and temperatures. The mechanical properties of PVDF are time-dependent and, under certain loading and temperature conditions, can be treated using the principles of linear hereditary viscoelasticity.

Electro-mechanical properties of the piezoelectric polymer PVDF

Damping and electromechanical energy losses in the piezoelectric polymer PVDF

The diverse group of “smart” piezoelectric materials is distinguished by their ability to react actively to changing stimuli as a result of converting mechanical to electrical energy and vice versa. Synthetic piezoelectric polymers, an integral part of the “smart” materials group, exhibit a type of behavior that is often compared with biological reactions involving transformations of the sensed information into the desired response. Due to such special qualities, piezoelectric polymers have been increasingly used in a rapidly expanding range of applications. At present, these materials continue to offer unprecedented design opportunities, leading to the belief that the industry is on the verge of major technological breakthroughs.



The diverse group of piezoelectric materials includes a variety of synthetic polymers such as polypropylene, polystyrene, and poly(methyl methacrylate); semicrystalline polyamides such as nylon-11; and amorphous polymers such as vinyl acetate. However, piezoelectric effects in these materials are relatively weak, often unstable, and are considered of limited practical significance. Strong piezoelectricity has been observed only in the synthetic polymer poly(vinylidene fluoride) and PVDF copolymers.


Keywords:

Overview;
Synthetic piezoelectric polymers;
Poly(vinylidene fluoride);
Electromechanical properties;
Nonlinear;
Time-dependent effects;
Applications

Piezoelectricity in Polymers

The paper presents a review in the field of electroactive polymers. It outlines the main classes of electroactive polymers, their properties and applications. Current efforts to synthesize electroactive polymers with novel or improved characteristics along with the challenges, opportunities and future research directions are discussed.

State-of-the-Art Developments in the Field of Electroactive Polymers

The paper concerns the dynamic response of polyvinylidene fluoride (PVDF), a commercially available piezoelectric polymer currently used in numerous applications. The paper consists of two parts. The first part presents a summary review of recent experimental results regarding the time-dependent behavior of PVDF tested under combined static and cyclic loading conditions. It has been observed that the polymer has demonstrated accelerated creep rates due to cyclic loading effects. Cyclic creep acceleration in PVDF has been significant even in the range of stresses well below the viscoelastic linearity limit. This phenomenon, defined as "vibrocreep", appears to be essentially nonlinear, since the material response under combined static and cyclic loads did not represent a simple superposition of the responses to static and fully reversed cyclic loads applied separately. The second part of the paper provides a formulation of a nonlinear constitutive model, which describes the synergistic interaction between creep and cyclic damage evolution. The proposed constitutive model combines the concepts of linear hereditary viscoelasticity with damage characterization through a cyclic damage function. The latter is formulated using the basic principles of continuum damage mechanics. The potentials and limitations of the proposed constitutive model are discussed, and experimental results providing model validation are presented.

Aleksandra M. Vinogradov

Papers

Electro-mechanical properties of the piezoelectric polymer PVDF

Damping and electromechanical energy losses in the piezoelectric polymer PVDF

Piezoelectricity in Polymers

State-of-the-Art Developments in the Field of Electroactive Polymers

Dynamic response of the piezoelectric polymer PVDF