Polyurethane-Based Electrostrictive Nanocomposites as High Strain

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C: Energy Conversion and Storage; Energy and Charge Transport

Polyurethane-Based Electrostrictive Nanocomposites as High Strain -Low Frequency Mechanical Energy Harvesters Fabio Invernizzi, Maddalena Patrini, Keti Vezzù, Vito Di Noto, and Piercarlo Mustarelli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04002 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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The Journal of Physical Chemistry

Polyurethane-based Electrostrictive Nanocomposites as High Strain Low Frequency Mechanical Energy Harvesters F. Invernizzia, M. Patrinib, K. Vezzùc, V. Di Notoc, P. Mustarellia,* a

Department of Chemistry of University of Pavia, and INSTM, Via Taramelli 16, 27100 Pavia, Italy Department of Physics of University of Pavia, Via Bassi 6, 27100 Pavia, Italy c Section of Chemistry for Technology (ChemTec), Department of Industrial Engineering, University of Padova and INSTM, Via Marzolo 1, Padova, Italy b

* Corresponding Author ([email protected]) Abstract Harvesting of wasted mechanical energy is increasingly important for powering wearable electronics in Internet-of-Things world. Here, we reported on innovative nanocomposites made of thermoplastic polyurethane (TPU) and a high-dielectric constant ceramic nano-filler (CaCu3Ti4O12), which offer good results in recovering energy by human gait. Power densities of the order of 300 µW cm-3 at 12% strain were obtained with 50 vol% of filler. The film was strained more than 105 times without losing its properties. By means of careful broadband electric spectroscopy coupled with microstructure analysis, we were able to address the mechanisms underlying energy recovery. Our model allows optimal tailoring of electrostrictive nano-composite harvesters.

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1. Introduction The term energy harvesting (EH) refers to processes able to convert any environmentallywasted energy into electrical energy that can be directly used or stored 1. In the case of direct powering of small electronic devices, this concept can be readily linked to the Internet of Things (IoT) concept, where different types of sensors and actuators collect data from the ambient and communicate each other to provide real-time information molded on the reality. At present, one of the main limitations towards massive IoT applications is the dependence of these devices from storage systems (e.g. batteries), which need to be recharged and substituted, or from electrical connections, that result very difficult and expensive to be supported. Local energy harvesters can provide a solution to this problem. As an example, vibrational energy harvesters (VEHs) of kinetic energy developed during human gait can feed sensors or other devices placed in a shoe, e.g. to collect physiological data or to monitor shoe performances and degradation 2. Many approaches have been proposed during years for VEHs, by exploiting different physical phenomena like electromagnetic induction 3, (nano)triboelectricity piezoelectricity

8-11

4-7

, and

of polymers or inorganic materials. Unfortunately, these approaches do not

match completely the features of human kinetic energy production (high amplitudes and low frequency), and/or the needs of electronic devices in terms of generated power 2. Recently, however, piezoelectric

12

and triboelectric nanogenerators (TENG)

13

have been proposed for

integration in shoes. Electrostrictive polymers (EPs)

14

, and related organic-inorganic composites, seem suitable for

human gait EH, since they can work in high amplitude/low frequency conditions. EPs belong to the wider family of electroactive polymers (EAPs), which are materials able to change their shape or size under an applied voltage 15. These materials can work as actuators, e.g. when used to make artificial muscles, or as sensors 16 or generators. Ren et al. studied EPs to perform mechanical-to2 ACS Paragon Plus Environment

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electrical energy harvesting. They showed that EPs have better performances than electromagnetic devices as energy harvesters from human motion

17

. Guyomar and Cottinet

investigated the main variables that affect the energy generation in the pseudo-piezoelectric mode 18, 19

. They found that the harvesting capabilities may be improved by increasing the dielectric

permittivity, εr, through the incorporation of dielectric and conductive fillers 20. Whereas analogies exist, it is important to stress that piezoelectricity must not be confused with electrostriction. The former effect, in fact, takes place only in crystals without centre of symmetry 2, 21. Piezoelectric materials, including ceramics, polymers and some biological systems, are able to convert a mechanical signal into an electric signal (so called direct piezoelectricity), and vice-versa (so called inverse piezoelectricity). Electrostriction is a property of all dielectric materials, which is caused by the displacement of ions in the lattice upon being exposed to an external electric field. This displacement will accumulate throughout the bulk material, resulting in an overall strain (elongation) in the direction of the field. All insulating materials consisting of more than one type of atoms will exhibit electrostriction.

2

Unlike piezoelectricity, electrostriction is a “one-way” effect, since mechanical

stresses do not generate, per se, any electric field (as in direct piezoelectricity). Therefore, from a conceptual point of view, insulating piezoelectric materials are electrostrictive, but not necessarily vice-versa. Regarding to EH applications, this leads to a fundamental difference: unlike piezoelectric materials, the electrostrictive ones need an external bias field to generate energy when subjected to external stresses - by changing their capacity 2. Whereas this could be considered as a drawback, indeed it allows using a much wider spectrum of materials with welltailored properties. In particular, thermoplastic polyurethane (TPU) – which is not piezoelectric in nature – is a standard component of shoes, therefore it is particularly well suited for



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electrostriction. In this case, the bias electric field can be obtained by an ancillary piezoelectric element, by an electret, by a backup battery or even by a TENG. In this work, we discussed new nano-composites based on the incorporation of the high dielectric constant (high-k) filler calcium copper titanate (CaCu3Ti4O12, hereafter CCTO), at different volume ratios, in a commercial thermoplastic polyurethane elastomer (TPU, Estane 58887 (polyester polyether)). The interest of CCTO as filler for EH devices is due to its large permittivity (up to 105) that is almost constant in a wide temperature range

22

. The TPU-CCTO

nano-composites showed improved energy harvesting capabilities with increasing the filler loadings (up to 50 vol%), and very high strain, without compromising the elasticity and the electrical properties of the final product. Full material characterization was performed by imposing an external bias electric field, which could be varied in a large interval. After this stage, a hybrid device integrating an electret was tested to show the actual possibility to use the composite in practical applications in shoe soles.

2. Materials and Methods 2.1 CaCu3Ti4O12 nanopowder synthesis CaCu3Ti4O12 (CCTO) was obtained by a sol-gel synthesis according to the literature

23

.

Appropriate amounts (of the order of 1 g) of Ca(NO3)2·4H2O (1 eq.) and Cu(NO3)2·2.5H2O (3 eq.) were dissolved in 50 ml of distilled water. In a separate beaker Ti(iPrO)4 (4 eq.) were placed with acetylacetone (acac) and stirred for 10 minutes. After this, an aqueous solution of citric acid (4 eq., 0.6M) was poured into the Ti(iPrO)4-acac mixture and stirred for 30 minutes. Subsequently, the water solution containing Ca and Cu salts was dropped into the mixture in about 10 minutes. The final solution was heated until complete solvent evaporation and burning of the blue gel, so 4 ACS Paragon Plus Environment

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obtaining a dark brown-black powder that was treated at 300°C for 3h, grounded and heated at 800°C for 30 minutes. Finally, the powder was ball-milled with ethanol for 4h at 200 rpm, to obtain the final nano-powder. The dielectric constant of CCTO was found to be in agreement with the literature 24.

2.2 Composite fabrication The TPU-CCTO composites were prepared by the solvent-casting method. In a typical lab-scale preparation procedure, 1.5 g of TPU (Estane 58887, Lubrizol, OH, USA) granules were dissolved into 15ml of dimethyl acetamide under stirring at 50°C. After the dissolution was complete, CCTO was added and sonicated for 3h to break the aggregates and homogeneously disperse the filler into the TPU solution. After the sonication step, the suspension was poured onto a glass sheet and heated at 80°C in an oven to evaporate the solvent. Finally, the resulting film was peeled off from the glass sheet and heated again at 80°C under vacuum to remove solvent traces. The actual films thickness was in the range 40-100 μm.

2.3 Characterization X-rays diffraction patterns (XRD) of pure CCTO and the composite films were acquired in BraggBrentano geometry on a Bruker D8 Advance diffractometer with Cu Kα radiation in the 20-80° 2θ range with step size of 0.2° and fixed counting time of 20 seconds per step. High Resolution SEM analysis was carried out by means of a Tescan Mira 3 microscope. Broadband Electric Spectroscopy (BES) spectra were collected in the frequency range from 10-3 to 107 Hz at environmental temperature of 25°C, with an accuracy higher than 0.2°C, using a Novocontrol Alpha-A analyzer. The geometrical cell constant was determined by measuring the electrode–electrolyte contact surface and the distance between the electrodes. Measurements 5 ACS Paragon Plus Environment


were carried out by sandwiching the films of the samples between two Pt blocking electrodes with a diameter of 20 mm.


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2.4 Energy harvesting tests: experimental setup To test the nanocomposites (5x2 cm samples), we used a homemade apparatus consisting of a mechanical actuator to submit the film to stress/relax cycles, and of an electrical circuitry to apply the bias electric field and to measure the sample electrical response (see Figure S1). The mechanical part consists of a PC-controlled, vertically placed pneumatic piston, and two jaws, one linked to piston, and the other fixed to the instrument base. The electrical circuit consists of a power supply to induce a polarization field (bias) across the film. The film is then connected in series with a 1 MΩ resistor. The change in the voltage drop across the resistor, due to the ACcurrent generation, is recorded with a digital oscilloscope and used to calculate the generated energy. Beside pure Estane 58887, we tested nanocomposite strips up to x(CCTO) = 50 vol%. Nanocomposites containing more than 50 vol% CCTO were too fragile to sustain the imparted mechanical stresses.

3. Results and Discussion 3.1 Nanocomposite structure Calcium copper titanate (CCTO) is an inorganic compound that crystallizes in the body-centered cubic lattice (s.g.: Im3, No. 204). Figure S2 (part a) shows the recorded X-Rays diffraction pattern for the calcined powder, with the relative Miller indices. Figure S2 (part b) reports the XRD pattern of the CCTO-polymer nanocomposite. By direct comparison, it is clear the crystalline peaks of the two materials fully overlap, which demonstrates the formation of the composite. The amorphous bump due to TPU is also clearly visible around 20o. From the XRD pattern of the TPU-CCTO composite, the average size of the CCTO crystallites could be determined by using the Scherrer equation: 7 ACS Paragon Plus Environment


߬=

‫ߣܭ‬ ߚܿ‫ߠݏ݋‬

where ߬ is the mean size of the crystallites, K is a shape factor close to unity, λ is the radiation wavelength, β is the line broadening at half-maximum intensity (FWHM) and θ is the Bragg angle. In case of our CCTO sample we found a crystallite mean size of 47 nm.

3.2 Nanocomposite microstructure The morphology of the samples and the homogeneity of the filler dispersion in the TPU matrix were investigated by scanning electron microscopy (SEM) as reported in Figure 1. The filler distribution was investigated by considering both the cross-sectional and the surface views. Figure 1a, first panel, shows the cross-sectional image of the TPU matrix without filler, which was taken as a reference to evaluate the filler dispersion in the composite samples. Noteworthy, in the pure TPU matrix, two zones with different morphology can be observed: a denser lower zone and a less dense upper one, whose formation is due to the solvent casting procedure. The cross-sectional morphology was investigated for all the loaded samples, and the obtained images are reported in Figure 1a. In the SEM images, the upper side corresponds to the lower side of the film obtained from solvent casting.


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Pure TPU (I)

(a)

30 vol% (II)

(b)

10 vol% ( I )

40 vol% (II)

50 vol% (III)

Pure TPU (I)

30%vol (II)

30%vol (II)

20 vol% (II)

70 vol% (III)

10%vol (I)

50%vol (III)

50%vol (III)

70 vol% (III)

70 vol% (III)

Figure 1 (a) Cross-sectional SEM images of the pure TPU matrix and of the TPU-CCTO nanocomposites, scale: 20 µm, but for 40 vol% (10 µm); (b) Surface SEM images of the pure TPU matrix and of some TPU-CCTO samples, scale: 2 µm. The marks I, II and III refer to the composition regions identified by BES analysis (see following). 9 ACS Paragon Plus Environment


In the case of the composite with 10 vol% loading, the cross-sectional distribution of the filler particles is quite homogeneous (Figure 1a). We stress here that the observed particles are agglomerates (in the micrometer range) of primary particles whose diameter was estimated by the Scherrer equation (see above). In contrast, the surface distribution of the filler particles is not homogeneous on the reported 10 µm length scale, including agglomerates ranging from hundreds of nanometers to about five microns. On the other hand, better homogeneity can be appreciated on a larger scale (see Figure S3). As far as concerns the samples with higher loadings, SEM images evidenced a CCTO concentration gradient through the cross section, with CCTO increasing from upper to lower surfaces, due to its sedimentation during the solvent evaporation step. This inhomogeneous dispersion of the filler in the composite, with enrichment of the lower side, is a typical effect of solvent casting as also reported for a different polymeric matrix as P(VDF-TrFE) 25. The CCTO enrichment is more evident for 40 and 50 vol% samples. The increase in filler loading also gives origin to the formation of bigger agglomerates. Additional information is reported in Figures S4-6, which show SEM images of the samples at lower magnifications. Figure 1b reports the SEM images of the samples surfaces. The surface filler density in the 10 vol% composite is lower with respect to the 30 vol% one, whereas there is not a clear difference between 30 and 50 vol%. The micrograph of 70 vol% sample is reported for the sake of comparison.

3. 3 Dielectric properties and polymer-filler interactions The dielectric properties of TPU-CCTO composites were investigated by Broadband Electrical Spectroscopy (BES) at room temperature (25°C) and in the frequency range from 10-3 to 107 Hz. The profiles of complex components of permittivity (ε*(ω) = ε’ (ω) – i ε’’ (ω)) and conductivity (σ(ω) = σ’(ω)+iσ’’(ω)) with σ*(ω)=i ωε0ε*(ω) are shown in Figure 2. 10 ACS Paragon Plus Environment

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Figure 2. Real and imaginary components of complex permittivity (ε*(ω) =ε’(ω) - iε’’(ω)) and conductivity (σ*(ω) = iωε0ε*(ω) = σ’’(ω) + iσ’ (ω)) spectra as function of frequency (f in Hz) and x (v/v %). P and D indicate the regions dominated by polarization and dielectric phenomena, respectively.

A careful analysis of the real and imaginary components of σ*(ω) and ε*(ω) allowed us to identify two electric response regions named P and D. In agreement with other dielectric studies 26 carried out on conventional polymers, the polarization processes are peaked in P region, while the dielectric relaxation is observed in D. In P region, an electrode (σEP) and an inter-domain polarization event (σIP) are measured at low and high frequencies, respectively. σIP corresponds to 11 ACS Paragon Plus Environment


the Maxwell-Wagner-Sillar process.

26

In details, σEP corresponds to the conductivity of the faster

percolation pathway, which acts to quickly accumulate charge at the interfaces between the electrodes and the sample. On the other hand, σIP is associated to the conductivity pathways which are originated when the accumulation of charges at the interfaces between nanodomains with a different permittivity are present in the membrane. Indeed, the presence of domains with different permittivity triggers the accumulation of charge at the interfaces between them, which thus generate percolation pathways able to facilitate the charge migration events. The σEP and σIP conductivity values are easily measured on the plateaus of σ’(ω) profiles at low and high frequencies of P region, respectively observed

27-30

27

. In D, the typical α-mode of polyether chains is also

. It is associated to the diffusion of conformational states, which occurs along

polyether chains of soft polymer component, and corresponds to the dynamic glass transition temperature of the material. The relationship existing between relaxation rate of α-mode and the assignment of this event to the diffusion of conformational states of polymer repeat units, which corresponds to the segmental motion of polymer chains, can be easily shown by analyzing the dependence on 1/T of the frequency of the α-mode maximum, fα. This last profile shows a Vogel-Fulcher-TammannHesse (VFTH) behavior

26-28

(data not shown) and accounts of the cooperativity of underlying

molecular motions in polymer domains 26-28.


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Figure 3. Behaviour of some relevant physical quantities vs. the volume fraction x (v/v %): (a) contribution of the different polarization processes to σi’; (b) ε’r at f = 10-3, 1 and 100 kHz; (c) fα and ∆εα (see text); (d) η = ∆εα⋅τα-1 (see text).

The dependence of σEP and σIP values on the volumetric fraction of filler (x) is reported in Figure 3a. It is observed that on x: 1) three different conductivity regions (named I, II and III) may be identified; 2) for all materials, σEP 40 v/v%) regions, while showing the opposite behaviour in II (20 ≤ x ≤ 40 v/v %). These results demonstrate that in the investigated materials the long-range charge migration phenomena mainly occur along conductivity pathways, 13 ACS Paragon Plus Environment


which are formed at the interfaces between high-k (the CCTO nanofiller) and low-k (the polymer matrix) domains. Actually, the overall conductivity of these materials is σ0 ≅ σEP + σIP ≈ σIP. On this basis, we should admit that in region I, polymer domains are larger in size and the density of charge “free to move” decreases as the filler concentration increases. In region II, most of the polymer matrix is involved in the formation of a polymer “shell” wrapping the filler nanoparticles (NPs). This interstitial NPs “shell” forms a “soft” and more efficient conductivity pathway. At x > 40 v/v % (region III) the polymer matrix starts being minority, whereas the CCTO phase is the main component. In this case, the conductivity of composite materials decreases as the amount of polymer decreases. Taking all together, BES studies demonstrate that the electric response of these nanocomposite material is significantly correlated to the ion and dipole dynamics taking place within the thin interstitial layer of the polymer “shells” which are wrapping the high k-CCTO nanoparticles. This interpretation is confirmed by the morphological analyses shown in Figure 1, which are carried out by SEM measurements. The dependence on x of the real component of permittivity at 10-2, 1 and 100 kHz (Figure 3b) is in perfect agreement with the trend above discussed for σEP and σIP values. The highest permittivity is observed for samples in region II. Indeed, in this case the inter-domains polarization process is significantly influenced by the properties of interfaces between the polymer and the CCTO nanofiller. It results that the high k-component acts to polarize the “soft” material by increasing the intensity of dipole moments per unit of volume of polymer matrix, and thus concurrently increasing the overall permittivity of materials. This suggests that the polymer dipole moments are significantly coupled to the external applied field. This latter coupling phenomenon gets its maximum for samples with a concentration of nanofiller in the range 20 ≤ x ≤ 40 v/v %. Further confirmation is obtained if we consider that, with respect to other CCTO-based polymer composites, the effect of the CCTO component on the permittivity values of the composites 14 ACS Paragon Plus Environment

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materials is very limited. This indicates that the accumulation of charge at the interface between the nanofiller and polymers is significantly dependent on the secondary structure and interactions characterizing the polymer domains. Indeed, a comparison of the results here reported with those of the literature shows that at room temperature and 1 kHz, the permittivity rises in the order: PU88 31 < TPU-CCTO < PVDF-CCTO 32 < CCTO-P(VDF-TrFE) 33 with ε values of respectively 5 < 8 < 53 < 110. PU88 is a segmented polyurethane, where the “hard” segments fill 45 vol% of the system; in TPU-CCTO, PVDF-CCTO and CCTO-P(VDF-TrFE), CCTO fills 40 vol% of each material. This indicates that the effect of the nanofiller in increasing the permittivity of highly flexible polyurethane-based polymers mixing “hard” and “soft” segments is less pronounced, and confirms that accumulation of charge at the interfaces nanofiller/polymer domains plays a crucial role in the modulation of the electric properties of the composite materials. Further information on the electric response of materials is obtained by analysing the dependence vs. x of the frequency maximum, fα, and dielectric strength, ∆εα, parameters of the α-relaxation mode (Figure 3c). Here ∆εα is defined as the peak height. In regions I and III, both fα and ∆εα are decreasing vs. x. This can be easily explained by considering that the reorientation rate and the intensity of the overall dipole moment of polymer matrix is decreasing on x. Therefore, we can summarize that the dynamics and the intensity of the overall dipole moment per unit volume of the “soft” material: 1) is lower when one of the two phases composing the material is predominant in the sample; 2) is higher when x is ranging from 20 to 40 v/v %, i.e. when, in agreement with the SEM images, the size of polymer and inorganic CCTO domains are comparable. Thus, strong interactions take place at the interface between high-k (filler) and low-k (polymer) domains, which are responsible of the secondary structure of the polymer matrix and of the reorientation phenomena modulating the dynamics of polymer component. These results are



confirmed by analyzing vs. x (Figure 3d) the behavior of reorientation energy dissipation, η (i.e. the energy dissipated in heat owing to the dipole moment reorientation of polymer domains), which is determined using the equation:

ߟ=ఛ

∆ఌ ೘ೌೣ

=

ଵ଺గ ாబమ

ߦ

where E0 is the amplitude of the applied electric field, τmax is the relaxation time of the peak maximum of the α-mode, and ξ is the energy heat-dissipated per second and per unit volume of the sample. Figure 3d shows also that, in I and III regions, η decreases with x. In region II, η increases with x, in agreement with the above-described results. This confirms that the interactions occurring between the low and high-k domains are crucial to modulate and improve the mechanical energy harvesting effects exhibited by these materials. Finally, the correlations between σEP, σIP, σ0 and fα values were investigated, with the aim to discriminate how polarization and the α-dielectric relaxation are related to the electromechanical coupling processes, which are at the basis of the energy harvesting capabilities of these materials.


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(a)

(b)

Figure 4 (a) Correlation between σ0 (top), σIP (middle) and σEP (bottom) and the frequency of dielectric α-mode (fα). I, II and III regions are indicated; (b) naïve picture of the three regions vs. x parameter.

The analysis of correlations shown in Figure 4 confirms the presence of two correlation domains (C1 and C2). It is observed that σEP, σIP and σ0 are strongly correlated to the α-mode and all behave similarly. Indeed: 1) in C1, they are decreasing and increasing monotonically on x and fα, respectively; 2) in C2, they are increasing on both x and fα. 17 ACS Paragon Plus Environment


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In addition, in C1, σEP, σIP and σ0 of region I are better modulated by α-mode with respect to same quantities of region III. On this basis, we can conclude that, with respect to III, in region I segmental motion is better modulating the long-range charge migration events. C2 included only region II, which shows conductivity values better modulated by the α-mode, with respect to III. Taken all together, if we consider a Smoluchowsky-Einstein process

34

for the long-range charge

migration event, σi ∝ fα, a smaller slope is observed in II with respect to I and III. This suggests that the charge migration distance in II is smaller. In summary, the polarization phenomena occurring at the polymer-nanofiller interfaces (polymer “shells” directly interacting with high-k nanofillers) are strongly modulated by the segmental motions characterizing the polyether fragments of polymer domains. A naïve picture of the three regions (I, II and III) is finally reported in Figure 4b.

Energy harvesting properties Using the setup described in the experimental section (see Figure S1a) we investigated the capability of these composites to perform mechanical-to-electrical energy conversion when subjected to low frequency stress/relax cycles under a bias electric field, E (pseudo-piezoelectric conditions

18, 19

, see also Figures S7 and S8). The Young modulus of the nanocomposite films vs.

CCTO content is reported in Figure S9. In a typical EH test, a rectangular composite film strip (5x2 cm), provided with gold-sputtered electrodes on both faces to form a parallel plates capacitor (A=6.7 cm2), is clamped at its ends with a mobile jaw, connected to the rod of the piston, and with a fixed jaw. Inside the jaws, two electrical contacts connecting the film electrodes to the circuit, as the plates of a capacitor, are placed. Once the sample has been correctly clamped, the pneumatic piston movement allows the user to set the frequency of activation (stress/relax cycle frequency). A bias electric field is induced across the strip by a power supply. Since the film is not an ideal 18 ACS Paragon Plus Environment

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insulator, the equivalent electric circuit must consider a loss resistance, RL, in parallel to the variable film capacitance (see Figure S1b). A leakage current, IL, will then exist, which will be detectable with the oscilloscope placed across the experimental resistor (Re). The current generated during the stress cycles, IG, must be higher than IL to have a positive energy balance. In our case, actual film resistance is higher than 109 Ω (see Table S.I. 1), which gives origin to IL ≤ 10-7 A, in comparison with strain-induced currents, IG, of the order of 10-5-10-6 A. Therefore, even in case of bias electric field provided by a backup battery, the leakage is of the order of 1%. In order to fully rule out that the electric pulses were due to current leakage, we performed careful simulations of the equivalent electric circuit using LSpiceTM software. The results are reported in Supplementary Information (see Figure S10 and related discussion). Within this experimental arrangement, in principle, the user can control the applied electric field (E), the frequency (f) and the amplitude of strain (γ). Here we imposed a fixed frequency, f = 1 Hz. Figure 5 shows the typical result of a EH experiment in terms of the voltage vs. time (a) and power vs. time (b) responses.



(a)

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(b)

Figure 5 a) voltage vs. time graph recorded from EH experiment (IL = 291 mV); b) calculated power vs. time graphs from figure 5a. Both graphs refer to 50 vol% TPU-CCTO composite at E = 8.75 MV/m; f = 1 Hz; γ = 12%.

In Figure 5a, the voltage peaks correspond to the stress (positive peak) and relax (negative peak) states of the film during the EH experiment, respectively. The voltage variations reflect the capacity of the electrostrictive strip to let the stored charges flow through the circuit. The stress/relax cycles allow generating an electric current by moving the charges stored on the strip 2. The electrical power vs. time plot of Figure 5b is obtained using the usual relation P=V2Re, with Re = 1 MΩ. The comparison of our data with similar VEHs reported in the literature is not easy, because of the many parameters involved in (bias electric field, frequency, strain, load resistance value, etc.), as well as of the different metrics (e.g. surface vs. volume energy and power density) considered to estimate the quality of the harvester. It is even more difficult to compare VEHs based on different physical principles, as piezoelectricity, of triboelectricity. Table 1 shows a comparison among VEHs based on different physical principles, working at frequency comparable


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with that of the human gait (e.g. 1 Hz). The attention is focused on volume power density as the metric of interest.

Table 1: comparison of VEHs for human gait based on different physical principles. * = estimated. Material

Type

TPUCCTO

Electrostrictive

P(VDFTrFECTFE)

Electrostrictive

PVDF

Piezoelectric

PZT

Piezoelectric

PDMSPET

TENG

ABS-PTFE

Hybrid TENGElectromagnetic

Energy (µJ)

0.26

Power (µW)

15

∼1000

∼1000

Energy density -3 (µJcm )

Power density

5.2

Frequency (Hz)

Pros

300

1

Low cost, easiness, compatibility

240

10

∼500*

1

410

1

∼50*

4-8

Cons

Ref.

-3

(µWcm )

0.22 (TENG) 5000

This work

High frequency

High power

High efficiency

35

36 Weight (105 g)

37

High voltage, low current

13

Difficult to insert into a shoe

38

(Electr.)

We obtained high density power values of the order of 300 µW cm-3 at E = 8.75 MV/m; f = 1 Hz; γ = 12%, compared with the value of 240 µW cm-3 at E = 10 MV/m, 10 Hz and γ = 5%, which was recently reported by Yin et al. on a plasticizer-modified P(VDF-TrFE-CTFE) terpolymer operating in electrostriction conditions 35. On the other hand, the current scales down by a factor ∼5 passing from 10 Hz to 1 Hz, which means our device is about 6 times more efficient than that of Yin et al. This indeed was made possible thanks to the very good elastic properties of our nanocomposite films, which allowed large strains values. From Table 1 it is also clear that piezoelectric and TENG 21 ACS Paragon Plus Environment


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HEVs can offer very good performances in terms of power density. However, it should be considered that TENGs generally work at high voltage and low currents, which is not the optimal condition to power low-impedance portable electronics. The group of Wang proposed the triboelectric couple polydimethylsiloxane (PDMS) / polyethylene terephthalate (PET) to harvest energy from human walking 13. The reported output power was very high, reaching 1.5 mW on a 100 MOhm load resistance. However, this high power was obtained at very high voltage (about 200V) and low current, in the range 0.1-1 µA/cm2, which is not well suited for powering electronic devices. In contrast, our device can sustain current density higher than 10 µA/cm2 at 1 MOhm load. Indeed, our electrostrictive approach is better suited to power up low impedance devices. The problem of lower voltage may be simply overcome by parallelizing several thin films (work in progress). Moreover, concerning the piezoelectric VEHs, we recall that PZT is a brittle ceramic, whereas PVdF is not a typical constituent of shoes. In contrast, thermoplastic polyurethane (TPU), the base polymer of our devices, is a standard industrial component of shoes insoles, and can be easily integrated by melting, extrusion and solvent casting. Another widely adopted metric is efficiency, ηe, defined as the ratio between the electric power density, Pel, obtained from a strain/relax cycle on the films, and the mechanical power density, Pm, which is needed to operate the device. Despite of diffused agreement on the efficiency definition, however, the reported efficiency values in the literature may vary considerably because of the different ways of calculating the input and output energy of inertial energy harvesters

39

. By

extrapolating the data of Yin et al. 35 at 10 MV/m and 1 Hz, to be congruent with our experimental conditions, we estimated ηe ≅ 0.02%, against ηe ≅ 1% of the present work. A further comparison could be made in terms of the figure-of-merit, FOM = P/(V2.f.A) recently introduced in the


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literature 40. Here P is the power produced per cycle, V is the applied voltage, f is frequency and A the active area. We obtained FOM = 3.29, against FOM = 0.4 for Yin et al. 35. From the relationship dE = W(t).dt, the amount of energy generated during the cycles is obtained by integrating the power signal over time. Figure 6 shows the energy values, as a function of the filler loading and the electric field, E, for the investigated nanocomposites with acceptable mechanical properties (x ≤ 50 vol% CCTO).

Figure 6 Generated energy values per cycle as function of CCTO loading (left panel) and applied electric field (right panel). The inset shows the energy generation at low electric fields. Conditions: f = 1 Hz; γ = 12%.

Figure 6 (left) shows that a quadratic relation holds between the applied bias electric field and the energy generated during stress/relax cycles, in good agreement with data reported by Cottinet et al.

20

. Energy produced by the electrostrictive films increases monotonically with the filler

content, as it is particularly evident from the inset relative to the lower electric field values. Figure 6 (right) shows the behavior of the harvested energy vs. filler loading (up to 50 vol%) at three selected bias fields. The increase of filler loading leads to the increase of the produced energy. The effect of the bias field is more evident for filler contents higher than 20 vol%. For example, at 23 ACS Paragon Plus Environment


E = 5.5 MV/m the composite with 10 vol% of CCTO generates about 10 times the energy produced at 1 MV/m. The ratio increases to more than a factor of 30 for the sample with 40 vol% CCTO. In agreement with the results obtained by BES studies, the best performances of proposed composite materials are obtained for samples with x in region II. Here, the overall electric response of the materials is modulated by inter-domain polarization (σIP). Moreover, in this region, the SEM images are in accordance with BES results, showing that the polymer is mostly wrapping the CCTO nanoparticles, so maximizing the amount of the polymer directly interacting with the high-k nanofiller. This allows us to conclude that a high EH is observed for those samples exhibiting the highest electric energy absorption (Figure 3d). This phenomenon, as previously described, easily occurs when an efficient coupling takes place between the dipole moment per unit of volume of polymer matrix and the external electrical field or mechanical stimulation. As expected, this phenomenon occurs efficiently when in polymer domains both high energy dissipation (η) and high dielectric relaxation rate (fα) are observed owing to a significant polarization phenomenon of soft domains by the high k-CCTO nanofiller, see Fig. 3c and d. Figure 7 shows the behavior of the generated energy vs. the induced strain. All the measurements were performed without applying a pre-strain to the films. The very good elastic properties of thermoplastic polyurethane matrix allow imparting very large strains, also in presence of significant filler amounts.


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Figure 7 Energy generation as a function of the amount of strain applied to the film strip. Conditions: E = 4.28 MV/m; f = 1 Hz. For all the samples, there is an almost linear relationship between the applied strain and the generated energy. Except for 10 vol% composite, which constitutes an outlier, the higher the loadings the higher the slope is. In the case of the 30 vol% composite, at large strains the generated energy increase is less than that expected from a linear behavior. This could be caused by an inhomogeneous dispersion of CCTO in the sample, with the formation of percolating pathways through its section. Noteworthy, the 40 and 50 vol% composites show higher slopes than the sample with lower loadings. The 50 vol% generates an energy value about one order of magnitude higher than the pure one. The film was strained more than 105 times without losing its elastic and EH properties. In order to fully demonstrate the possibility of practical use of the TPU-CCTO composite to harvest energy from human gait, we also prepared a hybrid harvester (HEH) by coupling the composite strip with a commercial electret with the following experimental conditions: E = 0.7 MV/m, strain = 2%, f = 1Hz. The obtained energy was 1nJ/cycle, in reasonable agreement with the value 25 ACS Paragon Plus Environment


obtained using the bias external field. Figure S11 shows a scheme of the device including the electret film (left), and the voltage-time characteristic of the test (right).

Conclusions We prepared nanocomposite films made of thermoplastic polyurethane and CaCu3Ti4O12, to obtain electrostrictive layers for energy harvesting at low frequency (e.g. from human gait). The combined use of SEM and BES analyses allowed us obtaining a clear and detailed insight into the relationships among samples microstructure, polarization effects of the microdomains, and functional properties. The excellent mechanical properties of TPU allowed up to 50 vol% of filler into the composite without losing film elasticity even for high strains. BES studies allowed us to detect that the electric response of investigated materials is modulated by two polarization phenomena (σEP and σIP) and by a α-dielectric relaxation event of polymer moiety. Results show that the overall conductivity, σ0=σEP+σIP ≅ σIP, is mostly modulated by the inter-domain migration processes. σIP occurs when accumulation of charges at the interfaces between nanodomains with high-k (CCTO) and low-k permittivity (TPU) are formed. In materials here proposed it results that, in the compositional region II, where 20 ≤ x ≤ 40 (v/v%), both σ0 ≅ σIP and fα, the relaxation frequency of the α-mode, show the highest values. In II, also the parameters η (the dissipation energy) and ε’r show the maximum values. On this basis, it is easy to propose that in II, dipole moments of repeat units of the polymer “shell” wrapping CCTO nanoparticles are significantly oriented and that their dynamics and associated energy absorption-dissipation phenomena are highly facilitated. Finally, this also supports the fact that for samples in the compositional region 20 ≤ x ≤ 40 (v/v%), the polymer dipole moments of the “shell” wrapping the high-k CCTO NPs are efficiently coupled to the external applied electric field. 26 ACS Paragon Plus Environment

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CCTO increases by more than one order of magnitude the resistance of the film harvesters with respect to pure TPU, without lowering the dielectric constant and the breakdown voltage. The combined effect of high dielectric strength, high resistance and allowable strain make these composites very promising as energy harvesting actuators. With respect to piezoelectric and TENG EH, our approach allows working under lower voltage and higher current conditions, which is well suited for low frequency, mechanical energy harvesting e.g. from human gait.

Acknowledgements F.I. gratefully acknowledges support for his PhD grant by Atom S.p.a. (Italy).

Supporting Information Available: Scheme of the instrumentation for VEH test. Additional physicochemical characterization of the materials (X-rays, SEM, dielectric properties). LSpiceTM equivalent circuit for simulation. Scheme of the hybrid harvester including electret. This material is available free of charge via the Internet at http://pubs.acs.org.



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TOC graphic


Polyurethane-Based Electrostrictive Nanocomposites as High Strain

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