Energy harvesting textiles for a rainy day: woven piezoelectrics. based on melt-spun PVDF microfibres with a conducting core

Transcription

1 Supplementary Information Energy harvesting textiles for a rainy day: woven piezoelectrics based on melt-spun PVDF microfibres with a conducting core Anja Lund 1,2, Karin Rundqvist 2, Erik Nilsson 3, Liyang Yu 1, Bengt Hagström 3,4 and Christian Müller 1 1 Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg, Sweden 2 The Swedish School of Textiles, University of Borås, Borås, Sweden 3 Department of Materials, Swerea IVF, Box 104, Mölndal, Sweden 4 Department of Industrial and Materials Science, Chalmers University of Technology, Göteborg, Sweden Correspondence: anja.lund@chalmers.se, christian.muller@chalmers.se 1

2 Fig. S1 Photograph and WAXS results for the melt spun yarns and its components. (a) Wide-angle X-ray scattering (WAXS) image for the bicomponent fibres. Using the (110/200) reflection for β-phase PVDF, Herman s orientation factor fx was calculated as 1 3 / / (S1) where δ is the angle from the position of maximum intensity and I(δ) is the intensity at δ. (b) Diffractograms obtained by integration of WAXS images along the azimuthal axis for ( ) pristine PVDF (granules), ( ) bicomponent fibres and ( ) the core compound of 10% carbon black in polyethylene. After fibre spinning, the peak positions for PVDF are largely shifted from the positions representative for α phase to the positions representative for β phase. (c) A roll of melt spun PVDF bicomponent yarn. 2

3 Fig. S2 Photograph and schematic illustration of the contact poling procedure. (a) Schematic illustration of a piezoelectric fibre and its electrical connections during poling and electromechanical characterization. The axis denoted 1 is the mechanical axis and the axis denoted 3 is the electrical or the polarization axis. The piezoyarn has 24 fibres electrically connected in parallel with a common outer silver paste-electrode. As the electric field during poling is between the perimeters of the fibres and their centre, it follows that for a high piezoelectric effect the applied deformation is ideally a symmetrical radial compression. This translates to tensile strain. (b) Photo of a piezoyarn in the setup for contact poling. During poling, the core electrode is connected to ground and the outer electrode is connected to the positive node of a high voltage supply. The general principle for poling is as follows: after melt spinning and cold drawing, dipoles constituted by polar crystals are (c) randomly oriented in the amorphous phase of the semi-crystalline PVDF. By applying a high electric field the dipoles are (d) aligned resulting in macroscopic polarization. In a coaxial fibre, the dipole orientation (e) is expected to follow a rotational symmetry about the core. In this schematic cross-section of a polar PVDF crystallite (e bottom part), dipoles are stabilized by trapped charges. The piezoelectricity in PVDF is due to the presence of a net dipole in the crystal unit cell, possibly combined with some effect from trapped charges 1,2. 3

4 Fig. S3 Modelled frequency behaviour for a single piezoyarn (L = 10 mm) as a function of frequency and load. (a) Peak voltage Vout over a load equivalent to the (b) oscilloscope, for frequencies of 0.1 Hz to 1 khz. (c) Generated power for different values of Rload (no Cload) and as a function of frequency up to 10 Hz. Rload was ( ) 10 MΩ, ( ) 100 MΩ and ( ) 468 MΩ. The real (or active) power output P was calculated as: P = 0.5VpIpcosΘ (= VrmsIrmscosΘ) (S2) where Vp is the peak voltage, Ip is the peak current and Θ is the phase angle. 4

5 Fig. S4 Schematic illustration and photographs of the corona poling procedure. (a) Schematic illustration of corona poling of bicomponent fibres. Corona poling can be used for continuous poling of piezoyarns 3, in-line with or separate from melt spinning. Here, corona poling was used for the woven textiles, as the method is less sensitive (compared to contact poling) to the macroscale inhomogeneities of the textile geometry. (b) A setup for corona poling was devised as an open box with two needle boards, facing the top and bottom of the piezoelectric textile. Each needle board had 45 stainless steel needles evenly distributed over an area of 30 mm x 100 mm. The textile was suspended in air between the needle boards, with the cores of the piezoyarns and the needles connected to the respective nodes of a high voltage supply. The conducting yarn constituting the outer electrode, is expected to distribute the electric field through all parts of the fabric, resulting in a radial polarization similar to that in contact poling (Fig. S2). (c) Close-up of the needle tips during corona poling, showing local high voltage discharge. 5

6 Fig. S5 Schematic illustrations and photographs of woven bands. (a) In the woven bands, the piezoyarns core electrodes were accessed by cutting the fibre ends with a razor blade and subsequently melt pressing the fibre ends sandwiched between two films of a 10 wt% carbon black/polyethylene composite at 135 C. For characterisation, the black film accessing the core electrodes was connected to electrical ground, and the conducting yarn constituting the outer electrode was connected by a hook probe. (b) In a weft rib weave construction the weft yarn floats over two warp yarns. It results in a construction where the warp yarns run relatively straight, without crimp, through the weave. Both surfaces are dominated by the weft yarn. (c) Twill construction; the back side of the fabric is dominated by the weft yarn. (d) Photo of three of the woven bands, with the three different weft yarns. The width of the bands is on average 25 mm. 6

7 Fig. S6 Tensile tests of woven bands. Tensile test results for the bands of (a) plain weave and (b) twill construction, carried out on 3 specimen of each textile at a pre-load of 0.5 N. (c) When a yarn is woven into a textile, its effective length L will decrease to a crimped length x. When stretched, the initial deformation is related to removal of the crimp and requires little force. Once the crimp is removed, the stiffness of the textile increases. Tensile tests of the bands were carried out on an Instron 5966 with a gauge length of 100 mm, with a pre-load of 0.5 N. The strain was εmax = 5% with a strain rate of 360 mm/min. 7

8 Fig. S7 Piezoelectric voltage generated by the woven bands of weft rib construction. Electromechanical characterization was carried out at εmax = 0.25% and f = 4 Hz, for 3 samples each with (a) PAsilver and (b) PAcarbon black based weft yarns. The pre-load was 30 N. 8

9 Fig. S8 Piezoelectric performance of several samples of the woven bands. Piezoelectric voltage generated by woven bands at ε = 0.25% and f = 4 Hz, for 3 samples each of the (a) twill/pasteel, (b) plain weave/pasteel, (c) twill/pasilver, (d) plain weave/pasilver, (e) twill/pacarbon black and 2 samples of (f) plain weave/pacarbon black. The pre-load was 30 N. 9

10 Fig. S9 Frequency characteristics of woven bands. Generated voltage from woven bands of twill construction with PAsteel and PAsilver based weft yarns, at ε = 0.1% and f = Hz. The pre-load was 30 N. Fig. S10 Open-circuit voltage of model circuits. Modelled generated voltage Vout from equivalent circuits (see Fig. 4a, b) for woven bands of twill construction with (a) PAsteel, (b) PAsilver and (c) PAsilver + water based weft yarns, at f = 2 Hz and a load equivalent to a measurement device (NI-DAQ: Rinput = 100 GΩ, Cinput = 100 pf). 10

11 Fig. S11 Schematic illustrations of the electrical contact points in the piezoelectric textile in the case of (a) dry state, (b) with the addition of a liquid, (c) schematic illustration of the plain-weave fabric and (d) close-up of a cross-section depicting all 24 fibres of the piezoyarn and with an added liquid. Consider the simplified case of fibres as rigid rods with one-on-one contact points as in (a). Here, the piezoelectric fibre constitutes a cylindrical capacitor with one of its electrodes limited to the small point of contact with the conducting yarn. If the liquid added to this system (b) is a conductor, it will act as an expanded outer electrode for the cylindrical capacitor, in the water volume where there is overlap of the piezoelectric and conducting yarn. The increase in contact area will result in an increase of the circuit capacitance (cf eq. 3). Moreover, the liquid will increase the contact area between the conducting yarns (grey fibres in (c)) thereby lowering the contact resistance and contributing to a decrease in the resistance of the outer electrode. If instead, the liquid is a dielectric, the 11

12 result will be the formation of a double-layer capacitor in the overlapping volume. This does not affect the circuit resistance, but contributes to a decrease in capacitance Ctot according to: 1/Ctot = 1/C1 + 1/C2 (S3) Note that this case also results in an increase in the electrode area for the cylindrical capacitors. Considering that tap water can be regarded as a leaky dielectric 4, and further considering the complex geometry of the piezoyarns in the textile (d), we expect that both of the described cases are in play to some extent. Based on our observation that the capacitance of the textile system increases considerably in the wet state, we conclude that the most important role that the water plays is that it increases the contact surface area between the yarns of the textile. 12

13 Fig. S12 Model and photograph of the energy harvesting circuit. (a) Screen-shot from the simulation software LTspice, modelling the piezoelectric shoulder strap connected to an energy harvesting circuit (EHC) LTC and its related components, as supplied on a (b) demo-board from Linear Technology. Notably, in the schematic, C1 is the 22µF capacitor for energy storage and Rload is the 1MΩ-resistor connected to the output of the EHC. The piezoelectric circuit was modelled according to the measured impedance of the twill-woven band with the PAsilver + water based outer electrode, and its inherent voltage Vi was set to an amplitude of 8 V and a frequency of 3 Hz, to emulate the piezoelectric voltage generated by a handheld case as recorded (c) by an oscilloscope. 13

14 Fig. S13 Piezoelectric voltage generated by the modified shoulder strap. Stair-walking with the case over the shoulder generated a peak voltage > 4 V (a) for the dry strap with the twill/pasilver band. (b) The voltage amplitude doubled when water was added to the surface of the strap. When stair-walking with the dry strap (c) the voltage of the storage capacitor reached 0.9 V after 1 minute, equivalent to a stored energy W = 10 µj. The energy W stored in a capacitor is calculated as W = 0.5CV 2 (S4) where C is the capacitance and V is the voltage across the capacitor. 14

15 Table S1. Electrical properties of piezoyarns and woven bands. Capacitance C (nf) Resistance R (kω) Piezoyarn, L = 10 mm Plain weave, PAcarbon black Twill, PAcarbon black Woven bands Plain weave, PAsteel Twill, PAsteel Plain weave, PAsilver Twill, PAsilver Twill, PAsilver + water The capacitance and resistance were measured using a Keysight U17331C LCR-meter set to series-coupled RC-circuit and f = 100 Hz. We have previously carried out capacitance measurements in the frequency range Hz on the polarized bicomponent yarns, and found that the capacitance varies very little at frequencies below 100 Hz 5. 15

16 Table S2. Voltage and electric field during corona poling of the textile bands. Voltage (kv) Electrical field E Sample Needle tips Fabric surface (MV/m) VN VF Plain weave PAsilver Plain weave PAsteel Plain weave PAcarbon black Plain weave insulating weft Weft rib PAsilver During corona poling, the electrical voltage on the tips of the needles and on the surface of the textile bands was measured using a high voltage probe, FLUKE 80K-40 HV. The electrical field E over the piezoyarns was calculated as E = VF/t where t is the thickness of the PVDFlayer in the yarns ( m). A control sample woven with pure PVDF yarns as (insulating) weft was also included. 16

17 Table S3. Density and cover factor of the weft yarns for the different constructions. Construction Weft yarn Weft density (No. of threads per cm) Weft cover factor (%) Plain weave PAsilver Plain weave PAsteel Plain weave PAcarbon black Twill PAsilver Twill PAsteel Twill PAcarbon black Weft rib PAsilver Weft rib PAcarbon black

18 Supplementary References 1 Naegele, D. & Yoon, D. Y. Orientation of crystalline dipoles in poly(vinylidene fluoride) films under electric field. Appl. Phys. Lett. 33, (1978). 2 Rollik, D., Bauer, S. & Gerhard-Multhaupt, R. Separate contributions to the pyroelectricity in poly(vinylidene fluoride) from the amorphous and crystalline phases, as well as from their interface. J. Appl. Phys. 85, (1999). 3 Hagström, B., Lund, A. & Nilsson, E. Method of producing a piezoelectric and pyroelectric fiber. WO 2014/ A1 (2014). 4 Wang, D.-W., Du, A., Taran, E., Lu, G. Q. & Gentle, I. R. A water-dielectric capacitor using hydrated graphene oxide film. J. Mater. Chem. 22 (2012). 5 Nilsson, E., Lund, A., Jonasson, C., Johansson, C. & Hagström, B. Poling and characterization of piezoelectric polymer fibers for use in textile sensors. Sens. Act. A Phys. 201, (2013). 18