Project Topics
Get Complete Project Material File(s) Now! »
Piezoelectric materials and application in energy harvesting
Mainly, the piezoelectric materials are classified into four different groups: single crystals, ceramics, polymers and polymer composites or nanocomposites100. Figure 1.9 shows the schematic representation of the main groups. The choice of a piezoelectric material will be based on several factors and applications. Such a choice is not only based on the piezoelectric properties but there are other factors also associated with the material selection. That includes the design flexibility, frequency range, functionality of the application fields etc.101.
Piezoelectric single crystals found applications mainly in the fields of sensors and actuators due to their higher piezoelectric coefficients100 but the most used are ceramics. The widely used piezoelectric material in the field of energy harvesting is lead zirconium titanate (PZT) which has by far the best piezoelectric coefficients64. Nevertheless, the lead-based materials are toxic in nature and lead-free materials such as BaTiO3, AIN and ZnO, although less efficient, are sometimes used as an alternative to PZT materials3. Despite the high piezoelectric coefficient and high electromechanical coupling of the piezoelectric ceramic materials, they present some drawbacks, which include rigidity, high density, high-temperature processing needed for thin-film fabrication, high cost for manufacturing, low design flexibility. All these limits their application towards energy harvesting devices37,38. Furthermore, and this is a crucial point in our case, they can’t endure large deformation due to their brittleness. Moreover, the piezoceramic based devices required an additional nonpiezoelectric substrate layer for the fabrication of the energy harvester. This affects the degradation of the electromechanical coupling coefficient of the device97,102. As an example, a comparison between PZT and P(VDF-TrFE) is presented in Table 1.1.
As we are more focussing on energy harvesting devices to power small scale electronic devices, many of the applications, such as in microelectromechanical systems (MEMS), wireless sensors, biomedical implants, wearable and bendable electronic devices demand specific properties103,104. Therefore, the choice of the material plays an important role as it need to provide the design flexibility (bending, twisting, fold). It should also maintain the original electronic and structural properties. In this regards, piezoelectric polymers found more attention in the field of energy harvesting because they can satisfy most of the above-mentioned characteristics. They exhibit good mechanical flexibility, biocompatibility, ease of processing and fabrication and are more resistant to mechanical shock39,40.
There are several piezoelectric polymers reported and studied in literature for energy harvesting applications which include PVDF and its copolymers, cellulose and its derivatives, polyurethanes (PU), polyimides (PI), polylactic acid (PLA)3,41,42. PVDF and its copolymers are well established and most expansively studied materials among all other polymers due to their high piezoelectric coefficient, higher elastic compliances, electromechanical coupling, structural flexibility, good chemical resistance, biocompatibility. Additionally, the off-resonance figure of merit (dij gij ) of PVDF is comparable to PZT ceramics, which is beneficial for energy scavenging3,110. Depending on the synthesis methods and the high voltage polarization, which is necessary to obtain the piezoelectric properties after shaping the polymer usually in thin layers, the piezoelectric coefficient of P(VDF-TrFE) is the highest (30 pC/N) among all piezoelectric polymers. Due to their other characteristics such as, low density, lightweight, low refractive index, low cost, PVDF and P(VDF-TrFE) are good candidates for different energy harvesting systems37,48. Another important property is that they can be fabricated with different design configuration and pattern electrodes on their surface. The polarisation can be done in the required portions according to the applications48.
Introduction to PVDF and its copolymer for piezoelectric energy harvesting
For the reasons outlined above, PVDF and P(VDF-TrFE) are the most frequently utilized and quoted piezopolymer in electromechanical devices and energy harvesting devices. Considering the other existing ferroelectric semicrystalline polymers, they are vastly used and find more attractive research interest, because of their unique characteristics including fast-electro-mechanical response, high mechanical stability, low acoustic impedance and excellent piezo and ferroelectric response111,112. The other copolymers belong to PVDF family including poly (vinylidene fluoride-trifluoroethylene) (P[VDF-TrFE]), poly (vinylidene fluoride- tetrafluoroethylene) (P[VDF-TeFE]), and poly (vinylidene cyanide-vinyl acetate) (P[VDCN-VAC]) and poly(vinylidene fluoride-hexafluoropropylene) (P[VDF-HFP]). They also exhibit all these characteristics113,114.
PVDF is a semi-crystalline polymer that is synthesized by the polymerization of the H2C = CF2 monomer ( Figure 1.10). Various crystalline polymorphs namely α, β, γ, δ and ε are associated with PVDF, in which α, β, γ are the most studied polymorphs ( Figure 1.11)115. The α and ε phases are non-polar because of the antiparallel alignment of the dipoles. The α phase is the most stable conformation with a semi-helical TGTG’ (trans-gauche-trans-gauche) conformation. The β phase can be obtained by mechanical deformation and electrical poling of PVDF and it is polar. Each unit cell has two polymer chains with all trans (TTT) conformation the dipoles point to the same direction and net zero dipole moment. The γ and δ phases are also polar and have a conformation of TTTTGTTG’ and TGTG’ respectively. These two phases are also responsible for the piezo, pyroelectric characteristics of PVDF and its copolymers along with β phase. The same case with γ (T3GT3GT’) conformation and shows polarity. Compared to other crystalline phases, the β phase shows a higher dipole moment thereby a higher electroactive (piezo, pyro and ferroelectric) properties114,116,117. Hence the improvement of β phase content in PVDF polymers with different methods is discussed widely in the literature116,118,119. Among the different copolymers of PVDF, (PVDF-TrFE) is able to form directly the polar phase by introducing the TrFE units to the PVDF which improves the piezoelectric effect. The additional fluorine atoms induce steric hindrance and avoid the formation of the non-polar α phase. Therefore (PVDF-TrFE) don’t need any additional stretching or drawing of the polymer chain, which is a strong experimental advantage120, even though poling is necessary to increase the piezoelectric effect. Correspondingly, PVDF and its copolymer solution can be prepared in a different liquid solvent. Therefore it offered various desired shapes and designs by extrusion, moulding or can easily coat on a substrate121,122.
There are various methods used to attain the electroactive phase of PVDF. Especially different synthesis methods have been developed in order to improve the crystal orientation which includes, as said, the addition of trifluoroethylene (TrFE) units122,124. Similarly, the addition of certain fillers, in particular nanofillers, enhances the structural, electrical, and thermal properties of the material. The β phase being responsible for the piezoelectric behavior of PVDF polymer, utmost of the literature studies showed that the β phase content in the polymer can be increased by the addition of certain fillers such as graphene, carbon nanotubes, metal oxides, clay, ceramic fillers, etc125–127. The substantial increase in the piezoresponse of the polymer mainly depends on their percentage and on the processing of the polymer. Wan et al. 2017128 summarized the different groups and their improved piezoresponse . Also, the other important processing techniques are stretching and poling of the polymer.
Similar to other fluorocarbon polymers PVDF is also chemically inert. It is highly resistive to hydrolysis and a low degradation rate provides the biocompatibility of the polymer. Therefore, it found great application in biomedical implementation, biosensing, tissue engineering, and drug delivery. In addition, a PVDF based device can convert the small mechanical response in the body into readable electronic signals. As follows, it can be employed for in vivo and in vitro studies.
The drawbacks of PVDF based polymers include poor adhesion to other materials and the low thermal stability of piezoelectric properties. Indeed, it shows a decrease in piezoelectric coefficient above 80°C and losing the piezoelectric effect.
The stretching and poling mechanism plays an important role in this work. Thus, the following section gives a brief description of the two mechanisms.
Stretching
Crystalline phase transformation is essential to obtain the piezoelectricity in semicrystalline polymers like PVDF. The crystalline phase transformation can be effectually induced by thermal annealing, mechanical orientation, and the application of high voltage129. Stretching allows the polymer to fundamentally align the amorphous strands in the planar direction and thereby it’s help for the uniform rotation of the crystallites by the application of an electric field in further steps130. The electrical and mechanical characteristics depend principally on whether the stretching is uniaxial or biaxial. Normally, a PVDF film has high α phase content with zero net dipole moment. While stretching the α phase is transformed into more β phase, where the fluorine atom is located on one side and the hydrogen on the other side. Therefore, it possesses a net dipole moment in a stacked direction. Temperature is also a factor during stretching, most of the studies showed that the transformation happens between 50°C and 140°C of temperature drawing131,132.
Poling
A ferroelectric material comprises many individual units’ cell and each with a corresponding electrical dipole. In which the regions where the unit cells possess equal polarization directions are named as ferroelectric domains. When a ferroelectric material is cooled below the curie temperature Tc, the domain polarizations are randomly oriented in the material and there is no net polarization128. In order to exhibit piezoelectricity in a material, a net polarization will achieve by reorienting the electrical dipoles in one direction. This process is usually known as poling. Thus, the poling or electrical alignment is very essential to convert an inactive polymer into a useful electromechanically active material for many applications. Poling can be obtained by the application of a high electric field E at an elevated temperature below TC. Thereby the molecular dipoles of the polymer sustain and permanently align in the direction of the electric field93,133.
Figure 1.12 demonstrates the schematic diagram of the poling process and the hysteresis curve of a piezoelectric polymer. Figure 1.12(a) showed the initial state of the material where the molecular dipoles are randomly oriented through the volume of the material. When the application of a high electric field causes spontaneous polarization within the material, the dipoles start to align in one direction (Figure 1.12b). After complete removal of the electric field, most of the dipoles are locked in a configuration of near alignment. The polymer now obtains a remnant polarization (Pr). The remnant polarization is retained after the complete removal of the electric field. The schematic representation of the orientation of the dipoles after poling process is indicated in Figure 1.12(c). The maximum possible value of the remnant polarization is known as saturation polarization (Ps)93. Figure 1.12(d) represents the polarization- electric field (P-E) loop of typical piezoelectric material. The electric field is applied until the maximum saturation polarization is achieved and it is denoted as Ps. Reducing the field to zero governs the remnant polarization (Pr), and from there, reversing the field reaches a negative maximum (saturation) polarization and negative remnant polarization. The coercive electric field Ec represents the electric field where there is no net polarization due to the mutual compensation of the polarization of different domains. A high value of Pr is an important parameter to get high piezoelectric activity for energy harvesting application. The applied electric field is significantly influencing the final piezoelectric constant. After a certain value of an electric field, the piezoelectric coefficient saturates. Therefore, the poling time and temperature are important factors affecting the piezoelectric constant93,95.
To pole a material the most used methods are the direct or contact method and the non-contact method called corona poling. In the direct method, poling is done by depositing conductive electrodes on each side of the polymer and a high voltage is applied across the sample ( Figure 1.13a). The sample is placed inside a vacuum chamber or immersed in an electrically insulating fluid to avoid its dielectric breakdown. The applied voltage ought to be DC, AD sinusoidal or triangular low-frequency waveforms134. In the case of piezoelectric polymers, the applied electric field is in the range of 5 to 100 MV/m. The final quality of the crystallite’s alignment and subsequently the piezoelectric coefficient ‘d’ of the polymer significantly depends on certain parameters which include, the strength and time of the applied electric field , the value and degree of uniformity of the applied temperature, the degree of impurity or voids between the electrodes and the uniformity of the polymer surface135. In some materials like PVDF, the crystalline orientation can be improved by the mechanical stretching performed prior to the poling process. Only this contact method allows to obtain the hysteresis curve of the piezoelectric material, such as the one shown in Figure 1.12(d), which is a valuable data to characterize the piezoelectric and ferroelectrical properties of our material.
Corona poling method offers some advantages, as large area samples can be poled without depositing any electrodes which help to reduce the risk of electrical breakdown of the sample. A typical corona poling system is demonstrated in Figure 1.13(b). A high voltage (8-20 kV) is applied through a conductive needle. Which is placed on the top of the grid with a specific distance(gap) and the sample is placed below on a ground plate. The grid voltage serves for the uniform distribution of the high voltage for a large area of the sample. The medium of the chamber should be vacuum or argon, or dry air. The gas molecules around the needle tip become ionized and are accelerated towards the sample surface which causes an electric field across the surface. The position of the grid and the applied voltage is controlling the amount of charge deposited on the polymer surface. The heat can be introduced through the hot plate which helps for better control of the poling system37,136.
PVDF for mechanical energy harvesting
PVDF and its copolymers are widely used for various mechanical energy harvesting such as vibration, stretching, raindrop energies. Here we are however focusing on the bendable, stretchable, and flexible PVDF devices and their different fabrication techniques to generate electricity to power small scale electronic devices. Therefore, to fabricate a hybrid flexible composite device, PVDF and its copolymers suit well because they can combine with different substrates and nanofillers. Nowadays wide studies are focused on the enhanced energy harvesting performance of PVDF based polymers. The following section presents some of the recent literature studies on the flexible PVDF and its copolymers-based generators.
Solvent casting is a simple process for the fabrication of PVDF and P(VDF-TrFE) thin films by mixing and stirring at the desired temperature. Dimethylformamide (DMF), N-methyl pyrrolidone (NMP), dimethyl sulfoxide (DMSO), or dimethylacetamide (DMAc) are utilized as polar solvents to dissolve P(VDF-TrFE) and the other copolymers116,137. The spin coating technique is an appropriate way to attain large-area uniform films and performance like mechanical stretching and improvement of the β phase crystal orientation. The processing conditions such as solution concentration, spin rotation speed (rpm) and annealing temperature also considered because these parameters contribute to the percentage of β-phase content in PVDF film116. Electrospinning is the most popular method for fabricating PVDF fibres, many research groups are actively working on electrospun PVDF fibres due to their various applications and better dielectric and piezoelectric constants138. The comparison of the output performance of PVDF energy harvesters in recent years is shown in Table 1.2.
Table of contents :
General introduction
1.1 Energy harvesting
1.2 Feasibility of energy harvesting via SMA/Piezoelectric polymer composite
1.3 State of the art of SMA/piezoelectric composite system
1.4 Introduction to piezoelectricity and principle
1.5 Piezoelectric materials and application in energy harvesting
1.6 Introduction to PVDF and its copolymer for piezoelectric energy harvesting
1.6.1 Stretching
1.6.2 Poling
1.6.3 PVDF for mechanical energy harvesting
1.7. Modeling of the ferroelectric and ferroelastic behaviors of piezoelectric materials
1.8 Introduction to shape memory alloys
1.8.1 Shape memory effect
1.8.2 Superelasticity
1.8.3 Modeling of the SMA thermomechanical behavior
1.9 Conclusion
2.1 Processing of SMA/P(VDF-TrFE) composite
2.1.1 Materials and methods
2.1.2 Synthesis of P(VDF-TrFE) solution
2.1.3 Spin coating technique
2.1.4 SMA/(PVDF-TrFE) composite
2.1.5 Multi-layered SMA/(PVDF-TrFE) composite
2.1.6 Polarisation hysteresis
2.2. SMA/P(VDF-TrFE) composite with interface layer
2.2.1 PEN/P(VDF-TrFE) device
2.2.2 Polarisation hysteresis
2.2.3 SMA/Epoxy/PEN/P(VDF-TrFE) composite
2.2.5 Summary
2.3 Experimental characterization for energy harvesting
2.3.1 Mechanical energy harvesting performance
2.3.2 Output voltage
2.3.3 Output power
2.3.4 SMA/ P(VDF-TrFE) composite
2.3.7 Output voltage
2.4 Thermomechanical loading test in tensile machine for SMA/P(VDF-TrFE) device
2.4.1 Stress-strain -temperature
2.4.2 Voltage-strain-temperature
2.4.3 Summary
2.5 Micro energy harvester characterisation system
2.5.1 Support system and frame
2.5.2 Cooling and heating source
2.5.3 Optical lens system
2.5.4 Summary
2.6 Conclusion
3.1 Modeling of SMA behavior
3.2 Modeling of piezoelectric behavior
3.2.1 Linear model
3.3 Finite element simulation of the SMA/(PVDF-TrFE) composite
3.3.1 SMA thermomechanical properties
3.3.2 Piezoelectric properties
3.4 Finite element (FE) model of the device
3.4.1 Tensile and bending tests
3.5 SMA behaviour simulation with Chemisky-Duval’s model
3.5.1 Superelastic effect
3.5.2 One-way shape memory effect
3.5.3 Assisted Two-way shape memory effect
3.6 Simulation of the two-layer composite material
3.6.1 Mesh and element type
3.7 Simulation results and discussion
3.7.1 One-way shape memory effect
3.7.2 Two-way shape memory effect
3.7.3. SMA/Piezoelectric polymer composite
3.8 Comparison between simulation and experimental analysis of the composite
3.9 Prototype geometry
3.10 Conclusion
4.1. Shaker test bench for piezoelectric energy recovery
4.1.1 Components of the shaker test bench
4.1.2 Study with the shaker test bench
4.2 Mechanical system for bending test
4.2.1 Principle of operation
4.2.2 Study with the mechanical bending system
4.3 Electronic circuits for piezoelectric energy harvesting: state of the art
4.3.1 Electrical modelling of the piezoelectric generator
4.3.2 Piezoelectric energy recovery standard circuits
4.3.3 Optimal recovery techniques
4.4 Practical application to our system
4.4.1 Equivalent electrical circuit of a piezoelectric generator
4.4.2 Choice of energy harvesting electronics
4.4.3 Simulation of the system with the LTC 3108 board
4.4.4 Simulation of the system with the LTC 3588-1 board
4.5 Conclusion
5. General conclusions and perspective
