Dielectric Barrier Discharges in Micrometer Sized Voids
1 Introduction: Ferroelectrets
Figure 1: Top: Cross-sectional view ofa cellular polypropylene
film. Bottom:Schematic view of the charged voids.
Piezo- and pyroelectricity of mechanically and
electricallyheterogeneous polymers were first described three
decadesago [1], but at that time, the experimentally observed
ef-fects were too small to be of use in most applications.
Thischanged with the advent of electrically charged cellular
poly-mers or polymer foams [2] (now frequently called
ferroelec-trets or piezoelectrets) in the mid-1990s. Since then,
the fieldhas expanded rapidly, as documented in several review
arti-cles [3–6]. Ferroelectret films have been produced in a
varietyof ways, including blow-extrusion of charge-storing
polymers(e. g. polypropylene (PP)), voiding of various polymers
withsupercritical carbon dioxide [7, 8]), and most recently as
astemplate-based regular structures [9–12]. Typical void heightsare
of the order of 1. . . 30 µm.
The voided structure is then electrically charged, either in
acorona discharge [15], or in direct contact with metallic
thin-film electrodes [16]. As the field in the gas-field voids
exceeds the threshold for Paschen break-down, a dielectric barrier
discharge results, in which pairs of electrical charges are created
anddeposited on opposite surfaces of the internal void (Fig. 1). It
is these “engineered” electricaldipoles that give the material its
piezoelectric properties.
Ferroelectrets have a variety of applications such as sensors
(intrusion detection, smart packag-ing), actuators (wall-sized loud
speakers, active noise canceling) and energy harvesting
(proposed
sv1
sv2
sv2
sv2
sv1
sv1
n1
n2
svNnN
+σ1−σ1+σ1−σ1
+σ1−σ1−σ2
−σ0
+σ0
+σ2
svN
svN
polymer
void
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0 1 2 3 4 5 6
pie
zo
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ctr
ic c
oe
ffic
ien
t d
33 [
pC
/N]
peak charging voltage [kV]
ExperimentalCalc, standard Paschen coeff.Calc., modified Paschen
coeff.
Figure 2: Left: Multilayer electromechanical model of
ferroelectret films, showing the abstraction process from avoided
film to a linear stack of void height classes; the number of voids
in each class is determined from scanningelectron micrographs of
cellular polypropylene samples. (From [13].) Right: experimental
and calculated piezoelectricd33 coefficients. Good agreement is
only achieved with modified Paschen coefficients. (Adapted from
[14].)
1
applications such as shoe inserts and tire implants). Some
applications of ferroelectrets have re-cently appeared on the
market, most notably bed sensors for monitoring the movement of
hospitalpatients [17].
While there has been considerable research to widen the
materials base of ferroelectrets, signifi-cant work remains to be
done in order to understand the charging process, and recent
research hasopened up significant questions. Investigation of the
light emission during the charging of cellu-lar PP films [6, 18,
19], confirmed the hypothesis that charges are generated in
dielectric barrierdischarges. The quasi-polarization of the
internal voids (positive charge on one surface, negativecharge on
the opposite surface) can be reversed by sufficiently high electric
fields and the result-ing “effective charge density versus field”
curves exhibit hysteresis behavior [18] It was shownthat
back-discharges (initiated by the electric field of the deposited
space charges) destroy approx.75% of the deposited space charge
[18]. Minimizing the back discharges could thus increase
thepiezoelectric d33 coefficient by a factor of up to 4.
Very recently, earlier electromechanical models [20–22] were
extended by taking into account arealistic distribution of void
heights in cellular polypropylene, using a “thick stack” of voids
withdifferent height classes (Fig. 2 (left)). While the calculated
space charge hysteresis curves were invery good agreement with
experimental data, the onset of piezoelectric activity was observed
atsignificantly higher electric fields than predicted by Townsend’s
model of Paschen breakdown [14].Using modified Paschen constants,
however, good agreement between observed and calculated
d33coefficients as a function of the applied charging voltage was
reached, as shown in Fig. 2 (right). Itis evident that the commonly
accepted Paschen curve for electric breakdown in air poorly
describesthe critical electric field for dielectric barrier
discharges in micrometer-size cavities. Interestingly,the observed
departure (higher Ec at small gap sizes) is opposite to the results
of recent particle-in-cell simulations [23].
When applied to charging conditions at various pressures in a
dry nitrogen atmosphere, the modelpredicted that the piezoelectric
charge density could be more than doubled by optimizing thepressure
[13,24]; Fig. 3 (left) shows that this result is in excellent
agreement with the experimentaldata. Moreover, the calculations
yielded the amount of space charge deposited on the internal
void
5
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40
45
50 100 150 200 250 300 350 400
pie
zo
ele
ctr
ic c
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ffic
ien
t d
33 [
pC
/N]
pressure [kPa]
ExperimentalComputational
50 100 150 200 250 300 350 400
pressure [kPa]
0
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vo
id h
eig
ht
[µm
]
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sp
ace
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arg
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ace
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nsity [
mC
/m2]
50 100 150 200 250 300 350 400
pressure [kPa]
0
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ht
[µm
]
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0.9sp
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/m2]
Figure 3: Left: Maximization of the piezoelectric coefficient
through charging at elevated pressure. Right: spacecharge density
deposited in the voids of a ferroelectret as a function of gas
pressure and void height. Note that smallvoids do not get charged,
while large voids retain only a relatively low charge density at
atmospheric pressure. This isdue to back discharges that occur when
the external charging voltage is turned off. (From [13].)
2
surfaces, as shown in Fig. 3 (right). Two features stand
out:
• Small voids (height < 7 µm) do not get charged at all.
Since these voids exist in largenumbers in most voided materials,
being able to electrically charge them promises largegains in the
piezoelectric activity.
• Large voids can potentially be charged to a high surface
charge density; however, when theexternal electric field is turned
off at the end of the charging cycle, back discharges limit
theamount of residual charge density to approximately 0.3 mC/m2 at
atmospheric pressure.
Hence, improving charge deposition in micrometer-sized voids is
the primary goal of this project.
2 Proposed Work: Microplasma Discharges and Optimized
Charging
Recent research has indicated that conventional charging methods
have only a limited effi-ciency [13, 18], since (a) only voids
larger then 6-8 µm are charged, and (b) back dischargesdestroy a
substantial amount of the space charge in the larger voids.
Improving the performanceof ferroelectrets requires careful study
of the microplasma discharges in the closed voids using
acombination of electromechanical techniques with optical
spectroscopy and imaging. The goals ofour project are:
• Measure breakdown fields in µm-size cavities as a function of
void height and gas composi-tion
• Use these measured breakdown fields (instead of the currently
used values that are based ona modification of Paschen’s law) to
verify our recently developed model [13].
• Using the model to predict (and experimentally verify)
approaches to maximize the chargingefficiency by optimizing
pressure, gas composition and the applied voltage vs. time.
Figure 4: Experimental setup for studying the charg-ing of model
cavities.
A “model cavity” consisting of a single void withan adjustable
µm-size air gap will be built and sub-jected to high-voltage
profiles while the light emis-sion is imaged and spectroscopically
analyzed withan electron-multiplying CCD camera (Andor IXONDU-987),
featuring a readout noise of less than1 electron per pixel (Fig.
4). Simultaneously, thebreakdown currents due to barrier discharges
willbe amplified with a FEMTO DLCPA-200 currentamplifier and
subsequently recorded with a NI PCI-6221 data acquisition board.
These experimentswill yield breakdown fields for a variety of
gases,indicating to what extent the Townsend model forPaschen
breakdown is applicable to dielectric bar-rier discharges in small
gaps. The breakdown data will be added to our model describing
thehysteresis behavior the effective space charge density σeff [13,
14]. This model will then allowpredictions about the
electromechanical behavior of other polymer foams, leading to
ferroelec-trets with higher performance. The model cavity will be
built using one of the recently publishedtemplating techniques
[9–12], where patterned spacer films are sandwiched or laminated
between
3
patterned spacerelectret
electret
electret
patterned electret
Figure 5: Model cavities. Left: 3-layer sandwich, consisting of
two electret polymer films and a spacer. Right:Sandwich of a flat
electret film with an electret film into which a structure was
imprinted with a template.
non-patterned electret films (typically, biaxially oriented
polypropylene, cycloolefin copolymersor various fluoropolymers, cf.
Fig. 5).
Based on the predictions of the charging model, parameters such
as gas pressure and gas compo-sition will be varied to maximize the
deposited space charge. In addition to dc charging voltages,short
pulses generated by a high-voltage switch (Willamette High Voltage
PHVSW-015V, rise-time10 ns) will be used, as recent research on
pulsed microplasma discharges has indicated an
improvedplasma-generation efficiency [25,26], and it is likely that
this approach will also increase the spacecharge yield.
3 Resources
All instrumentation listed is available in Dr. Mellinger’s
research laboratory at the Department ofPhysics, Central Michigan
University. In addition, the applicant will be able to use
resources andinstrumentation in the Department of Chemistry and the
School of Engineering, which are bothparticipating in the
interdepartmental “Science of Advanced Materials” Ph.D.
program.
References
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http://www.emfit.com/en/care/products_care/movement-monitor/
Introduction: FerroelectretsProposed Work: Microplasma
Discharges and Optimized ChargingResources