Статьи на перевод PVDF_P(VDF-TrFE) / 2012-Dodds,J-Thesis (Development of Piezoelectric Zinc Oxide Nanoparticle-Poly(Vinylidene Fluoride))

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Figure 1: Piezoelectric Effect Schematic

The piezoelectric effect is demonstrated here at a domain level. Domains are represented here by arrows within the light-blue specimen with positive and negative symbols indicating each domain’s polarity. When a force is applied, a net charge is generated across it to, for example, power a light bulb.

is seen in Figure 1. This separation creates a differential voltage across the material, which in turn creates a current in a closed circuit. A piezoelectric material under changing stress is having its electric dipoles constantly stretched or compressed, which generates a corresponding charge in that material. This is known as the direct piezoelectric effect. It is important to note that due to this principle, piezoelectric responses only occur with varying applied stresses and not with a constant stress.

Another way of looking at this phenomenon is from the perspective of Curie’s equilibrium principle. This principle states that the symmetry of the effect (in this case the piezoelectric effect) is greater than or equal to the symmetry of the cause (in this case the asymmetry in the crystal) [26]. This principle means that the material will be forced to expand or contract to compensate for any electrical imbalance. Of course, this symmetry works in both ways, leading to the converse piezoelectric effect.

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The converse piezoelectric effect is defined by a change in voltage across a piezoelectric material generating a mechanical deformation. A high-enough electric field induces increased electric separation within the dipoles in the material, thereby causing it to experience either elongation or shrinkage. When the electric field across a material remains steady for a sufficient time period, the dipoles within the material have changed their strength and direction to accommodate the new electric field, and this effect ends. Only continually changing electric fields continue to create mechanical displacement of the material, in the same way that for the direct effect, only changing strains create charge output from the material.

Piezoelectric polymers experience the piezoelectric effect in a slightly different manner. Broadhurst and Davis [27] have provided a list of criteria for a polymer to be piezoelectric, which includes the presence of permanent molecular dipoles in that material, the ability to align these dipoles and sustain this alignment, and material flexibility. Piezoelectric semi-crystalline polymers contain an amorphous region, which gives the material mechanical properties (of flexibility, etc.), and a crystalline portion which provides its piezoelectricity [28]. The crystalline region acts like a crystalline piezoelectric material as described before, but because of its location inside an amorphous polymer matrix, the entire material can simultaneously have flexibility and piezoelectricity.

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Figure 2: Material Classification Breakdown

A chart is provided here to visually delineate different material categories.

Dielectric materials can fall into a number of categories, including paraelectric, piezoelectric, pyroelectric, and ferroelectric, as shown in Figure 2. First, a dielectric material is either a paraelectric or a piezoelectric material. Paraelectric materials tend to have symmetric crystal structures and do not change their shape in response to an electric field. Naturally, piezoelectric materials are those that exhibit the direct and converse piezoelectric effects. Pyroelectric materials, a specific subset of piezoelectric materials, make up 10 out of 20 of the possible piezoelectric crystal classes and have an inherent off-center distribution of charges. This off-center distribution allows these materials to respond to a change in heat by lengthening or shrinking, causing the asymmetric dipoles within the material to also shrink or lengthen and release a charge based on this change. Ferroelectric materials, a subset of five of the 10 pyroelectric material crystal classes, have the additional ability to reverse their overall polarization direction upon the application of a high electric field. For ferroelectric materials, the

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coercive field is the strength of electric field required to reverse its internal dipole and the remnant polarization is the polarization of the material under no electric field.

So far, these concepts of piezoelectricity have been discussed only in onedimension. In reality, piezoelectricity takes place in three dimensions, and there are matrix-based equations to link stress, strain, temperature, and electric field in all three dimensions. Note that, while the piezoelectric effect for the primary direction (i.e., in the 1-direction) may be positive (where positive force causes a positive electric charge), the piezoelectric coefficients for the off-center directions may be negative as a result of the material’s inherent crystal structure.

Formally, these concepts of piezoelectricity are represented in Equation 2.1:

Because a constant temperature is typical, the more important equations are 2.1b and 2.1d. The superscripts in Equation 2.1 refer to states that are held constant for that variable measurement, including constant for temperature, T for stress, and D for electric displacement field; the subscripts refer to dimensions. For Equation 2.1b, in metric units, S is the material strain [unitless], s is the compliance coefficient [m2-N-1]

(inverse of Young’s modulus), T is the stress on the material [N-m-2], g is the piezoelectric voltage coefficient [m2-C], and D is the electric displacement field [C-m-2]. For equation 2.1d, E is electric field [V-m-1], is dielectric impermittivity (inverse of permittivity) [m-F-1], and other variables are as before.

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As different piezoelectric coefficients vary, different SHM applications become more appropriate. High piezoelectric strain coefficients (called d and not represented in Equation 2.1 but represented in an equivalent equation set) give these materials the ability to actuate at high power (and thus have a lower signal-to-noise ratio (SNR)) or sense at a level of high sensitivity. However, those materials with high piezoelectric strain coefficients often have limited flexibility and limited area coverage. Piezoelectric materials with higher flexibility are especially suited for sensing [29]. The key parameter for sensors is g, the piezoelectric stress constant, which is higher for piezopolymers than for piezoceramics [28]. Also, polymers can often operate under higher voltages than ceramics and have the ability to be selectively poled, which can lead to some interesting applications [28].

2.4 Piezoelectric Material Applications

Instrumented piezoelectric sensors and actuators as part of an SHM system are able to provide quantitative measurements of structural performance and can even detect damage initiation and propagation. Reviews of SHM have included piezoelectric transducer fabrication and use in many SHM applications [25, 30-32]. This section provides an overview of select studies which use piezoelectric materials for SHM as are most appropriate for the purposes of this research.

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2.4.1 Piezoelectric Sensors

One of the simplest applications of piezoelectric materials for SHM is to use them as piezoelectric sensors. Piezoelectric materials can be used directly as sensors in structures, not having to generate any of their own power and requiring little external circuitry. These piezoelectric sensors can measure signals from vibrations in the structure to indicate the onset of damage (i.e., acoustic emissions). Sensors can also be used in concert with actuators to perform more complicated SHM assessments, as will be discussed in subsequent sections.

Piezoelectric sensors are widely used for measuring acoustic emissions. Acoustic emission monitoring involves using a piezoelectric sensor to passively monitor the propagation of cracks in a structure based on the acoustic signal that cracking produces. Polla et al. [33] have demonstrated the use of PZT sensors in an acoustic emission setup to form a high-frequency “listening” device to detect crack initiation in an aircraft-type structure. Using these devices, light banging with a hammer has detected voltages as low as 50 V without amplification. Additionally, an electrospun PZT matrix has been employed for basic acoustic sensing on a steel table, being able to detect vibrations induced with the impact of a steel bar [34]. Also, a PZT sensor matrix has been employed to locate damage in a beam using acoustic emissions [35]. This damage has been located by measuring the change in acoustic measurements at various sensor nodes in the sensor matrix.

PVDF and its copolymer PVDF-TrFE are used as sensors for many applications, often as part of a sensor and actuator network for SHM and damage detection. PVDF, whose piezoelectricity was first observed in 1969 by Kawai [36], has a polymeric nature that allows it to conform to complex structural surfaces and cover large areas. In

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general, it has consistent properties in all dimensions, has high mechanical resistance, possesses dimensional and temperature-related stability, has a high dielectric constant, is chemically inert, is easy to handle, and is relatively inexpensive [37]. The PVDF-TrFE copolymer has been employed for easier material processing; it does not require stretching in order to enter a phase that is more piezoelectrically susceptible (i.e., the alltrans phase) [28]. PVDF-TrFE also has a higher remnant polarization and requires a lower coercive field, meaning that it is easier to polarize and thus have its piezoelectric features carefully controlled. Unfortunately, the TrFE copolymer has a lower piezoelectric coefficient than PVDF, but the previously-mentioned advantages generally outweigh this limitation. PVDF and its copolymer also have several other drawbacks, mainly being its lower piezoelectricity. Its piezoelectric coefficients are unfortunately on the order of 7 pC-N-1, as compared to 160 pC-N-1 for PZT [38]. This has limited its applications for SHM despite its favorable mechanical flexibility and conformability to structural surfaces. Composites able to compensate for these deficiencies are discussed in Section 2.4.3.

2.4.2 Piezoceramic Actuators

Piezoelectric materials, using their converse piezoelectric properties as described in Section 2.3, can be used as actuators. They have the unique property of being able to impact mechanical excitations onto a structure (e.g., guided waves) when these transducers are driven using a tuned AC electrical signal. Literature reviews and books available on this subject can provide a more general overview than this thesis,

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which provides a specific outline tailored for understanding the context of this research [31, 39-41].

Generally, piezoelectric actuators operate in one of two modes, namely though its thickness or longitudinally along the horizontal plane of the transducer. Piezoelectric actuators that operate through the thickness are generally piezoceramic PZTs. Here, the device is excited across its thinnest direction (i.e., perpendicular to the structural surface) and a mechanical wave is propagated through the structure parallel to the thickness of the film. This actuator is placed at various angles to the structural surface such that the wave travels in the desired direction through the structure to which the device is attached. Piezoelectric actuators that operate longitudinally are generally made from piezopolymer films, such as PVDF, and will be discussed in Chapter 5.

PZT has been used for many SHM applications, including for generating guided waves in composite beams and structures for damage detection [30]. Using PZTs, the amount of structural damage can be quantified using techniques such as root-mean- square deviation (RMSD) [42] or by measuring impedance and capacitance changes [43], among others. Park et al. [44] have used PZT transducers to assess the health of steel bridge components, specifically related to finding artificial cracks and bolt deficiencies. RMSD and wavelet coefficients have been employed for signal analysis to design a detection system that effectively assesses the presence and change in severity of damages, but unfortunately would not be sensitive to changes in boundary conditions.

PZT has many inherent advantages for actuation purposes. It possesses high piezoelectric coefficients, including a d33 of 593 pm-V-1, derived from its perovskite ceramic structure [25]. This high coefficient gives it good electromechanical coupling properties, which means it requires less power to drive a given structure as compared to

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a transducer with a lower coefficient. Because PZT actuators require such low power, lower cost and smaller equipment can be used for driving them. A larger piezoelectric coefficient also enables the generation of higher amplitude acoustic signals, resolution measurements, and SNR. In addition, PZTs offer the advantage of being able to harvest energy from ambient structural vibrations and can be used for recharging batteries onboard wireless sensor nodes [45].

While PZTs possess high piezoelectricity, they are brittle and can be difficult to use on complex structural surfaces. They can generate large-amplitude surface waves but have a high profile on the surface of a material. These larger form factors can provide instrumentation difficulties on various structural surfaces or when embedding within structural materials such as fiber-reinforced polymer (FRP) composites. PZTs also receive and send guided waves in all directions (unless specially designed and shaped), making crack location more difficult than if guided waves are primarily sent in one direction [46]. Also, the resonant frequency of PZT discs is limited and determined by the geometry of the disc, which means a different PZT disc is often required in different monitoring situations [47].

There are, however, a number of additional options available for selecting piezoelectric transducers. It has been shown that patterned PVDF can be used to actuate and receive guided waves in a manner similar to PZT transducers [48]. A piezopolymer transducer has a low profile but generates less power and has a lower SNR ratio than its PZT counterpart. A composite material needs to be provided which will take advantage of the best properties of both flexible piezoelectric materials and ceramic piezoelectric materials.

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2.4.3 Piezoelectric Composites

One goal of piezoelectric composites is to achieve the best attributes offered by both piezopolymers and piezoceramics. One example of this is to preserve the high piezoelectricity of piezoceramics while achieving the favorable mechanical attributes of piezopolymers. Much research has been conducted on composite piezoelectric materials and an overview of some important efforts will follow.

Akdogan et al. [49] review many of the piezoelectric composites available for sensors and actuators, categorizing them by connectivity and materials employed, and using computer-aided drafting techniques to model complex composite structures. Newnham et al. [50] present a connectivity scheme that describes different types of composites with two numbers, the first representing the dimensional connectivity of the distributed particle, and the second representing the matrix. The composite to be used for this research is 0-3, indicating small particles (0) randomly distributed within a threedimensional matrix (3).

There are many examples of piezoelectric composites being investigated for possible use in SHM and already in use for SHM applications. Pb(Mg1/3Nb2/3)O3-PbTiO3

(PMN-PT) composites, for example, have been successfully employed to monitor delamination in glass FRP composite plates [51]. These materials have been able to withstand the forces involved in creating the delamination and yet still take successful measurements afterwards. PZT/PVDF-TrFE films have been successfully screen-printed [52] and spin coated [53] and their dielectric properties investigated for future work in SHM [52]. PMN-PT/PVDF and PZT/PVDF-TrFE composites have had their piezoelectric properties compared and PMN-PT/PVDF films have been found to be superior because of their properties which allow enhanced ability to be polarized [54]. PVDF/PZT