Smart elastomers a touch of robotics Chris Bowen, Vince Coveney, Hamideh Khanbareh, Mengying Xie Department of Mechanical Engineering, University of Bath 30 June 2017 BRL
Introduction 1. Fundamentals of ferroelectricity, piezoelectricity and pyroelectricity 2. Ferroelectric materials 3. Sensing and energy harvesting performance 4. Dielectric EAPs and actuation
Direct piezoelectric effect Tensile or compressive force parallel to the poling direction (polar axis) generates a potential difference across opposing faces. D = d.t + ε.e Bowen et al. Energy Environ. Sci., 2014, 7, 25-44 This is the sensing/generator mode of operation. g ij = d ij / ε ij FOM = d ij. g ij
Converse piezoelectric effect Electric field parallel to the poling direction (polar axis) extends the material. Electric field opposite to the polar axis results in contraction. S = d.e + s.t Extension Contraction This is the actuator mode of operation. Strains are small ~0.1-0.3% AS Karapuzha et al. Ferroelectrics, 2016
Dielectric constant Ferroelectric materials Ceramics, perovskite ferroelectrics: BaTiO 3 Ahn et al. Science (2004) 303:488-491 Heat Cool P P Cubic, Symmetrical Tetragonal, non-symmetrical (above Curie T = 120 C) (below Curie T = 120 C) Below the Curie temperate (T c ) the crystal structure distorts to tetragonal structure. Ti 4+ ion displaced from the centre, creating an electric dipole. T c T
Piezoelectric coefficients Dielectric constant Ferroelectric materials Ceramics: PbZr x Ti (1-x) O 4 6 polarisation directions 8 polarisation directions P = 0 P P PbZrO3 PbTiO3 Tetragonal Rhombohedral High spontaneous polarization High dielectric constant (~500-15,000). High strain response to applied electrical field piezoelectricity Strong variation in polarization with temperature pyroelectricity Composition
Poling achieving piezo response P (C/cm 2 ) E (kv/cm) Domains are randomly orientated. To achieve net polarisation apply a high electric field at elevated temperatures. Cool to room temperature (with the electric field still applied). This freezes in the alignment of the domains, resulting in a net polarisation.
Ferroelectric materials Polymers: dipolar Poly(vinylidene-fluoride) and copolymers 7.10-30 C.m P pyzoflex Hu et al. Scientific Reports 4, Article number: 4772 (2014) P PVDF P(VDF-TrFE) ε [-] 12 8 d 33 [pc/n] -30-38 g 33 [mvm/n] -330-540 T c [C] 80 120
Ferroelectric materials Polymers: Nylons (polyamides) Closely packed H-bonded sheets O H T g = 25-190 C T m = 150-235 C N C P Applied external electric field P Takase, Macromolecules 1991,24, 6644-6652
Ferroelectric materials Polymers: Ferro-electrets PTFE Force Metal mesh Grounded Grid Corona Point PTFE FEP PTFE 45mm 45mm Metal mesh Sample A A Metal reference plate Gerard et al, Ferroelectrics, 2011, 422:59 64 Force Conducting substrate S. Bauer Lu et al, Nature Communications 8,15310 (2017) d 33 100-500pC/N d 33 (PVDF) <30pC/N d 33 (PZT) ~ 500pC/N
d 33 [pc/n] Ferroelectric materials Polymer composites, connectivity d 33 1-3 d 33 structured Electroceramic d 33 0-3 RE Newnham, DP Skinner, LE Cross, Mat. Res. Bull, 1978, PSU Polymer matrix φ [-] V AC Random, Sensors (0-3) Structured (Quasi 1-3) Fiber composites, Transducers (1-3) Khanbareh et al, Smart Mater. Struct., 2014
Ferroelectric materials Polymer composites, fibrous PZT ceramic-pu elastomer Structure Polymer Filler Cross sections of orientation fibres (PZT5A) in cured polyurethane. Applied field = 1kV/mm, 100Hz. DEP g33[ mv.m/n] 450 400 350 300 250 200 150 100 50 0 Flexible and highly sensitive 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 phi [-] R ~ 150 R = 150 R ~ 110 R = 110 R ~ 70 R ~ 70 R ~ 24 R = 24 R = 11 R = 11 PZT Van den Ende et al. J Appl. Phys 2012
DEP Foaming DEP Foaming Ferroelectric materials Polymer composites, PZT-porous PU elastomer Structure Polymer Filler DEP PZT foaming E = 2.6 Mpa Strain at break = 110% Khanbareh et al, Sens Actuators A Phys., 2017.
Ferroelectric materials Polymer composites, Self-healing ionomer composites Structure Polymer Filler Ethylene methacrylic acid 30 vol% PZT-ionomer 30 vol% PZT-ionomer 30 vol% PZT-ionomer James et al, Smart Mater. Struct 23 (5), 2014 Before healing After healing E = 30-90 Mpa Strain at break = 150-550 %
Mechanical energy harvesting d 31.g 31 Brittle Low thermal stability 1.3 4.9 Battery-and wire-less tire pressure measurement systems (TPMS) sensor Noaman Makki Remon Pop-Iliev, Microsyst Technol (2012) 18:1201 1212
Mechanical energy harvesting Bonded devices High strain, low frequency Foil type devices Higher bandwidth Ease of manufacturing Max temperature in tyre Smart Mater. Struct. 21 (2012) 015011, Direct strain energy harvesting in automobile tires using piezoelectric PZT polymer composites, D A van den Ende et al.
Pyroelectric effect Thermoelectric temperature gradients Pyroelectrics temperature fluctuations Sidney Lang, Physics Today.
Thermal sensing PyzoFlex: a printed piezoelectric pressure and temperature sensing foil for human machine interfaces Pyzoflex
Thermal energy harvesting Micro-patterning of PVDF: improves heat transfer D Zabek, J Taylor, EL Boulbar, CR Bowen, Micropatterning of Flexible and Free Standing Polyvinylidene Difluoride (PVDF) Films for Enhanced Pyroelectric Energy Transformation, Advanced Energy Materials (2015)
Harvested current and voltage 88% coverage 63% 40.5 40 39.5 39 45% 70% 88% 100% 45% 28% Temperature [C] 38.5 38 37.5 37 36.5 36 35.5 0 10 20 30 40 50 60 time [sec.] 45% coverage: open circuit voltage by 380% closed circuit current by 420% Current [na] 40 30 20 10 0-10 -20-30 45% 70% 88% 100% Volt [V] 60 40 20 0-20 -40 45% 70% 88% 100% -40-60 0 10 20 30 40 50 60 time [sec.] 0 10 20 30 40 50 60 time [sec.]
Electro-active actuators DE advantages: Simplicity of structure Low mass/inertia Robustness Noise free operation Similar to human actuation force density Maximise 1. Add conductor 2. Add high permittivity filler L.J. Romasanta et al. / Progress in Polymer Science 51 (2015) 188 211 193
Adding conductors to increase ε r. E b 2 dielectric constant 10 3 10 2 10 1 10 0 10 1 10 2 10 3 10 4 10 5 frequency (Hz) Weight perc. 0.42 0.31 0.26 0.21 0.16 0.01 0.05 0.026 0.010 0.005 0 ac conductivity (Siemens/m) 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 weight perc. 10 1 10 2 10 3 10 4 10 5 frequency (Hz) 0.42 0.31 0.26 0.21 0.16 0.10 0.05 0.026 0.010 0.005 0 theta (degree) 90 80 70 60 50 40 0.42 30 0.31 0.26 0.21 20 0.16 0.10 0.05 10 weight perc. 0.026 0.010 0.005 0 10 1 10 2 10 3 10 4 0 10 5 frequency (Hz) ε r Increases permittivity at the expense of E b E b Graphene oxide (GO) polymer composite
Adding high ε filler to increase ε r. E b 2 Challenges: Uniform dispersion Percolation Melt processability High ε Infinitely high ε Low ε Percolation E polymer several times E applied Percolation Park et al, Ferroelectr. Freq. Control 2008, 55, 1038 1042 Calame et al, Electr. Insul. Mag. 2008, 24, 5 10 Low breakdown field
Summary Ferroelectric polymers Low strain / piezoelectric activity Medium fields for actuation High sensitivity [low permittivity] High energy harvesting capability Ferroelectrets Higher piezoelectric activity Lifetime EAPs High strain Very high fields Lifetime (dielectric breakdown, challenges for new materials)