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ELECTROACTIVE MATERIALS
FOR ENERGY HARVESTING
Maurizio Mattarelli
Noise in Physical System Laboratory
Department of physics and geology
University of Perugia
IESRES teaching activity -PITESTI,ROMANIA 27/06-01/07/2016
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SUMMARY
Introduction
Materials and devices
Piezoelectrics
Physical properties/fabrication
ZnO microrods
Electrets
Physical properties/fabrication
SiO2 micro particles as electrets
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TRANSDUCTION MECHANISMS AND MATERIALS
Electrets
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Electromagnetic
InductionElectrostaticStrain conversion
Piezoelectrics
Electroactive polymers
Magnetostrictive mat.
Magnetoelectrics
Magnets
Mechanical action
Motion («free» inertial mass,)Strain (cantilever, etc)
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ELECTRIC BEHAVIOR OF MATERIALS
Depending on the external conditions (electric field intensity or
frequency, temperature, shape, strain) the behaviour of real
materials can move between these extrema
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Perfect
Conductor
(superconductors?)
Perfect
Insulator
(vacuum?)
Piezoelectrics
Dielectrics
Low k high k
Semiconductors
semimetalsConductors
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ELECTRIC POLARIZATION
It can have different origins
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In an external electric field, materials acquire an induced
dipole moment or POLARIZATIONEP
4
1
+
-
--
-
+
+
+-
-
+
+
-
-
+
+
+
-
--
- +
++ --
+
+-
-
+
+
𝑬
Orientation (of pre-existing dipoles)
Deformation (of electron or ions)
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FREQUENCY DEPENDENCE
The response of the electroactive materials isstrongly frequencydependent.
They work best atresonance when the trasferred power ismaximum.
They still react at lowerfrequency, while at higherfrequency they cannotrearrange following the external field
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MATERIALS IN MAGNETIC OR ELECTRIC FIELDS
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Diamagnetic
Paramagnetic
Ferromagnetic
Its atoms have no permanent magnetic
moment. The induced moment opposes
the external field.
Its atoms have permanent magnetic
moment, which aligns with the external field
Its atoms have permanent magnetic
moment and strong exchange energy
connects them into domains
Paraelectric
Ferroelectric
Also the induced dipole
in dielctrics is aligned
with the external field
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HYSTERESIS
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In Ferroelectrics (Ferromagnetics) materials, the thermal agitation
cannot overcome the alignement of the domains. A coercive field is
needed to reverse polarization (magnetization).
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THE MICROSCOPIC ORIGIN
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Simmetry of the unit cell
is a necessary, but not
sufficient condition.
Different charge
distributions can cancel
or strongly decrease the
piezoelectric effect
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PIEZOELECTRICITY
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Non Centro-symmetricCentro-symmetric
In centrosymmetric crystals the strain does not move the center of
charge of the positive or negative charges. On the other hand, in in
non centro-symmetric crystals, if the atoms have different charges
because of the strain provokes the formation of an electric dipole.
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PIEZOELECTRIC COEFFICIENTS
𝑃 = 𝑑 𝜎
𝜀 = 𝑑𝑡 𝐸
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The direct piezoelectric effect is used as the basis for ENERGY HARVESTING
(and force, pressure, vibration and acceleration sensors) while converse
effect is used as a basis for actuator and displacement devices.
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PIEZOELECTRIC COEFFICIENTS
d is third-rank tensor (dijk 3x3x3) , as it links the effect of strain/stress (second order tensor) to the induced Polarization/Electric field (vector).
However, it is often written in a contract matrix form (3x6), where 4,5,6 index are used to express shear stress/strain
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Axis «3» is usually the anomalous
axis of uniaxial piezoelectric
crystals and the one where the
effect is stronger
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FERROELECTRICITY
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In BaTiO3 at room temperature the stable phase
has the Ti4+ ion displaced form the center of the
cell. It has two stable positions: above and below
the 4 central oxygens.
Depending on its position, the dipole will be
reversed.
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POLING
Under a strong external field it is possible to induce the poling of the ferroelectric material(i.e. polarize)
IESRES teaching activity -PITESTI,ROMANIA 27/06-01/07/2016
If the material is poled, the stress acting on it can generate an intense changeof the electricdipole (usuallystronger than in common piezolectric materials)
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PYROELECTRICS
Crystal with a permanent dipole, not reversible.
They have to be grown as single crystals.
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Wurtzite structure crystals,
such as ZnO, are
asymmetric along the
[001] axis, ([001] is
different from [00-1])
Because of thermal
dilatation, the electric
dipole increases
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ZINC-OXIDE MICRORODS
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Wang 2008
Objective: exploiting the difference of
potential at the base of the pillar induced
by the bending
Growth of ZnO
pillars on IDE
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17
FABRICATION
Hydrothermal synthesisSonochemical synthesis
Reagenti:
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Reagents:
• HMTA e Acetato di Zinco (Rapporto
molare 4:1)
• Temperatura 85°C (5h)
• Substrato IDE
18
HYDROTHERMAL SYNTHESIS ON IDE
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SPECTROSCOPIC CHARACTERIZATION
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Raman and Brillouin
spectroscopy on single crystals
Raman
Shift
(cm-1)
E2low A1(E2) A1(E1,E2) A1 E2
high
Ref. Damen
et al.
101 208 332 380 437
Risultati 101 205 332 379 438
Elementi Matrice C
(GPa)
C11 C33 C44 C66
Ref. Bhat et al. 209 210 42 44
Risultati 209 198 42 43
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SEM CHARACTERIZATION
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Electrical measurements on single crystals in the SEM environment.
It is possible to observe the normal mode of vibrations of the crystal
inducing electrical signa at the borders
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Dielectrics with unbalanced charge (permanent oriented
electric dipoles or a net charge.)
ELECTRETS
Old applications: microphone
New ones : energy
harvesting devices
MATERIALS: dielctrics (polymers,
oxides) with high dielectric
strength and low conducibility
S Boisseau et al. Smart Materials and Structures 20, 105013, 2011
Electrical analogue of a magnet: able to generate an electric field
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FORMATION OF ELECTRETS
Two types of electrical charges in an electret :
monocharges (also called real charges)
dipolar charges (such as in ferroelectrets)
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Several fabrication techniques:
• Thermo-Electrical Method (dipolar)
• Electromagnetic Radiation Method (dipolar)
• Liquid-Contact Method (real charges)
• Corona Discharge Method (real charges)
• Electron-Beam Method (real charges)
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THERMO-ELECTRICAL METHOD
Dipolar molecules are randomly arranged but they will
actively orient under an electric field at a temperature
higher than the glass transition temperature, Tg
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Typycal materials is the Carnauba wax–beeswax
(first electret, made by Eguchi in 1919)
Drawback: stability
Ferroelectrets can be considered electrets obtained by thermoelectrical
method (the poling) but with, possibly, much higher Tg
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ELECTROMAGNETIC RADIATION METHOD
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Displacement of the charge carriers generated by
penetrating radiation ( x-rays or ultraviolet light), under an
externally applied electric field.
These carriers can be trapped near the electrodes to
create a space charge polarization.
The polarization remains after the external field removal
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(LIQUID) CONTACT METHOD
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Transfer of real charges into the material, by a conductive
contact. This can be made at large scale down to the
nanoscale (AFM)
The advantage of the conductive liquid is in the possibility to
move the metallic contact all over a large surface
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CORONA DISCHARGE
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The bottom electrode is a vacuum-
deposited metallic film on the material
surface, and the top forming metallic
electrode is usually made of a metallic
wire
Around the point electrode it is possible to exceed
the breakdown strength of the air in a region of a few millimeters.
The so formed ions/free electrons can be accelerated toward the grid
and so be implanted in the target dielectric material
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ELECTRON BEAM METHOD
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It is possible to inject/extract real charges
(electrons) into the electret by SEM
The energy of the electron beam (<50
keV) should be controlled according to
the structure and thickness of the
material specimens to be used for
forming electret
A similar mechanism can be used with ion
implantation instrument
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ELECTROACTIVE POLYMERS(ARTIFICIAL MUSCLES)
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Dipolar «soft» electrets have interesting
properties similar to piezolectric materials .
They can react to external field changing
shape or , viceversa, change their own
dipolar field because of a change in shape.
More complex structures can be engineered. Polymer are especially intersting.
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But also dynamic applications
Electro-Mechanical resonators
Miniaturization of devices (EH,
MP)
The charge provides a further
way to functionalize the
nano/micro- material
SIZE REDUCTION OF ELECTRETS
Stability of charging (surface vs space)
Control of charging
Drawbacks
Bianchi et al, Nano Lett. 2014, 14, 3412 − 3418Tofail, Biological Interactions with Surface Charge in Biomaterials (RSC Publ. )
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SiO2 particles (0.5 e 1 μm) deposited by drop casting
on a polished copper substrate
SAMPLES
Fabrication by SEM
1) High lateral resolution (5 nm
at 20 keV)
2) Energy dependent
penetration
R = (76/ρ)E0 1.67
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CHARGING MECHANISM
LB IIEt
Q
)(1 0
1) Crossover energies (E1 e E2)
2) Three chraging region: (σ>1 e σ<1)
Emission curve Charge effect:
EL =E0 – eVs
Small size material:
1) Leakage current IL can be
significant
2) Surface potential VS up to 200-
300 V
Total Yield Approach
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DUANE-HUNT LIMIT SHIFT
EL =E0 – eVs
Bremsstrahlung x-ray spectrume
EEV
offcut
S
0
η( π/2)→1
eQVdrRVS
510350..2IESRES teaching activity -PITESTI,ROMANIA 27/06-01/07/2016
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Voltage Contrast
Output depends on gridpotential
ELECTRONIC SPECTROSCOPY
4
0 )(
E
E
E
k
dE
dN
Chung-Everhart
Everhart-Thornley
detector (E-T)
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ELECTRONIC SPECTROSCOPY
)/())sec(( 321 ddfffS extSE
Strong background from the SEM
chamber
Increasing
Increasing Working Distance WD
E0=1.8 keV
WD=6.8mm
Mag=150KX
Vs=(121±19)V
Vsbrems=(130 ±50)V
4
0 )(
E
E
E
k
dE
dN
Derivative from the fitNumerical derivative
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CHARGING VS ENERGY
Initial linear increase
Potential saturation
0.5 µm 1.0 µm
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CHARGE TEMPORAL EVOLUTION
Samples in air
Monitoring charge by non penetratingelectrons (0.5-1.5 keV)
Observations by In Lens detector
More electrons detectedfrom charged particles
13 days 56 days
111 days
Two
characteristic
lifetimes:
1) fast (surface)
2) Slow (space)
neutralsedchs NVNVN )()( arg
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A «MIXED» DEVICE
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Microantenna
:
EM wave
V V
Electret particle
ZnO Microrod
A useful commistion of the properties of electrets particles and
piezoelectric rod
The particle works as a
(em)force collector.
The force impulse is
trasnferred into electrical
form by means of the piezo
transduction
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FINAL COMMENTS
Device and materials are strongly correlated
Electro active mterials are effective ways to
harvest mechanical energy (noise vibration
and direct forces)
Piezoelectric properties depend on the
asymmetric structure of the crystal
Electrets are artificial materials which provide
significant external electric field
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