Smart elastomers a touch of robotics

Transcription

1 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

2 Introduction 1. Fundamentals of ferroelectricity, piezoelectricity and pyroelectricity 2. Ferroelectric materials 3. Sensing and energy harvesting performance 4. Dielectric EAPs and actuation

3 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, This is the sensing/generator mode of operation. g ij = d ij / ε ij FOM = d ij. g ij

4 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 ~ % AS Karapuzha et al. Ferroelectrics, 2016

5 Dielectric constant Ferroelectric materials Ceramics, perovskite ferroelectrics: BaTiO 3 Ahn et al. Science (2004) 303: 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

6 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

7 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.

8 Ferroelectric materials Polymers: dipolar Poly(vinylidene-fluoride) and copolymers 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] g 33 [mvm/n] T c [C]

9 Ferroelectric materials Polymers: Nylons (polyamides) Closely packed H-bonded sheets O H T g = C T m = C N C P Applied external electric field P Takase, Macromolecules 1991,24,

10 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 pC/N d 33 (PVDF) <30pC/N d 33 (PZT) ~ 500pC/N

11 d 33 [pc/n] Ferroelectric materials Polymer composites, connectivity d d 33 structured Electroceramic d 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

12 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] Flexible and highly sensitive 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

13 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.

14 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 = Mpa Strain at break = %

15 Mechanical energy harvesting d 31.g 31 Brittle Low thermal stability Battery-and wire-less tire pressure measurement systems (TPMS) sensor Noaman Makki Remon Pop-Iliev, Microsyst Technol (2012) 18:

16 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) , Direct strain energy harvesting in automobile tires using piezoelectric PZT polymer composites, D A van den Ende et al.

17 Pyroelectric effect Thermoelectric temperature gradients Pyroelectrics temperature fluctuations Sidney Lang, Physics Today.

18 Thermal sensing PyzoFlex: a printed piezoelectric pressure and temperature sensing foil for human machine interfaces Pyzoflex

19 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)

20 Harvested current and voltage 88% coverage 63% % 70% 88% 100% 45% 28% Temperature [C] time [sec.] 45% coverage: open circuit voltage by 380% closed circuit current by 420% Current [na] % 70% 88% 100% Volt [V] % 70% 88% 100% time [sec.] time [sec.]

21 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)

22 Adding conductors to increase ε r. E b 2 dielectric constant frequency (Hz) Weight perc ac conductivity (Siemens/m) weight perc frequency (Hz) theta (degree) weight perc frequency (Hz) ε r Increases permittivity at the expense of E b E b Graphene oxide (GO) polymer composite

23 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, Calame et al, Electr. Insul. Mag. 2008, 24, 5 10 Low breakdown field

24 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)