ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 3, march 2006
631
Single Crystal PMN-PT/Epoxy 1-3 Composite
for Energy-Harvesting Application
Kailiang Ren, Yiming Liu, Student Member, IEEE, Xuecang Geng, Heath F. Hofmann, Member, IEEE,
and Qiming M. Zhang, Senior Member, IEEE
Abstract—One key parameter in using electroactive materials to harvest electric energy from mechanical sources
is the energy conversion efficiency. Recently, it was shown
that, in the relaxor ferroelectric PMN-PT single crystals,
a very high longitudinal electromechanical coupling factor
( 90%) can be obtained. This paper investigates energy
harvesting using 1-3 composites of PMN-PT single crystals in a soft epoxy matrix. It is shown that 1-3 composites enable the single crystals operating in the longitudinal mode to achieve high efficiency for energy harvesting,
and the soft-polymer, matrix-supported single-crystal rods
maintain high mechanical integrity under different external
loads. For comparison, 1-3 composites with piezoceramic
PZT also are investigated in energy-harvesting applications,
and the results show that the high coupling factor of single crystal PMN-PT 1-3 composites leads to much higher
electric energy output for similar mechanical energy input.
The harvested energy density of 1-3 composite with single
crystal (22.1 mW/cm3 under a stress of 40.4 MPa) is about
twice of that harvested with PZT ceramic 1-3 composite
(12 mW/cm3 under a stress of 39 MPa). At a higher stress
level, the harvested-energy density of 1-3 PMN-PT single
crystal composite can reach 96 mW/cm3 .
>
I. Introduction
sing electroactive materials to harvest electric energy
from various parasitic mechanical sources is attractive
for a broad range of applications, especially in view of the
fact that some of these materials can be made into flexible forms and in very small volumes. In a typical energyharvesting process, an electroactive material absorbs the
external mechanical energy, and through a proper electromechanical conversion process converts that energy into
electric form. This converted electric power, through a
power electronic interface, is delivered to the electronic
load. Therefore, several steps must be taken in designing an energy-harvesting system to effectively convert the
mechanical energy available in an external medium into
the electric energy to be delivered to an electric load: a
mechanical impedance match of the electroactive material
system to the environment to realize maximum mechanical energy transfer into the electroactive material system;
high electromechanical conversion efficiency of the electroactive material system; a proper power electronics to
U
Manuscript received July 5, 2005; accepted September 23, 2005.
K. Ren, Y. Liu, H. F. Hofmann, and Q. M. Zhang are with the
Department of Electrical Engineering, Pennsylvania State University,
University Park, PA (e-mail: kxr233@psu.edu).
X. Geng is with Blatek, Inc., State College, PA.
efficiently transfer converted electric power into the electric
load. Here we focus on the electroactive material system,
which includes possible modification to the pure material
by other mechanical or electrical parts. These additions or
modifications may improve the electromechanical conversion efficiency and/or the mechanical impedance match to
the external mechanical environment [1].
Recently, it was reported that, in relaxor ferroelectric
single crystals of PZN-PT and PMN-PT, a very high longitudinal electromechanical coupling factor (>0.9) can be
achieved [2]–[4]. Such a high coupling factor is attractive
for energy harvesting using electroactive materials. This
paper investigates the energy harvesting using 1-3 composites with relaxor ferroelectric single crystal PMN-PT
in a soft epoxy matrix. The single crystal at the composition of 0.67PMN-0.33PT is chosen for this investigation.
This composition is in the rhombohedra side near the morphotrophic phase boundary between the tetragonal and
rhombohedra phases. When oriented and poled along the
001 direction, a piezoelectric d33 coefficient higher than
2000 pC/N and a longitudinal coupling factor k33 of 0.94
have been reported. In contrast, the thickness coupling
factor kt of the same crystals is not very high (in fact,
it is nearly the same as that of the piezoelectric PZT ceramics) [2]–[4]. In this investigation, 1-3 single crystal/soft
epoxy composites are used to take advantage of the high
k33 coefficient and avoid the low kt of the single crystal. As
has been shown both theoretically and experimentally for
a properly designed 1-3 piezoceramic-polymer composite,
the thickness coupling factor can approach the longitudinal coupling factor of the piezoelectric phase [5]–[8]. In
addition, the soft polymer matrix also provides support to
the single crystals, which improves the mechanical reliability against mechanical loads in energy harvesting experiments. The use of 1-3 composites also reduces the elastic
impedance of the electroactive material system (defined as
ρ∗ Va , where ρ is the density and Va is the acoustic velocity). When harvesting energy from a person during walking or from many other mechanical sources such as ocean
waves, a lower mechanical impedance of the electroactive
material system, compared with the piezoceramic and single crystals, is highly desirable.
It is noted that, from the engineering perspective, the
single crystal PMN-PT is much more expensive than the
PZT. Hence, it can be prohibitive for large-scale applications. However, there are applications such as microelectromechanical systems and microsensors for which only a
small amount of materials is needed.
c 2006 IEEE
0885–3010/$20.00 632
ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 3, march 2006
(a)
(b)
Fig. 1. (a) Optical image of 1-3 composite: dark area, PMN-33PT;
white area, epoxy. (b) Diphase 1-3 composite arranged in parallel
configuration along the z-direction, Hatched area, PMN-PT; white
area, epoxy; Z, the direction of spontaneous polarization and the
applied electric field).
II. 1-3 Composite Fabrication and
Characterization
Single crystals of 0.67PMN-0.33PT oriented along 001
direction are used in this investigation. The single crystals
are poled at room temperature under a field of 10 kV/cm.
The poled single crystals exhibit a d33 of 2060 pC/N measured using a d33 meter. In the 1-3 composite fabrication
process, extra precaution has to be taken not to damage
the single crystals. The poled single crystals of 1-mm thick
(001 oriented) are first diced along the 100 direction
using the Automatic Dicing Saw K&S 982-6 (supplied by
Kulicke & Soffa Industries, Willow Grove, PA) with a kerf
width of 100 µm, then diced in the perpendicular direction
to produce an array of posts with a rectangular cross section [Fig. 1(a)]. The width (0.1 and 0.3 mm along the two
perpendicular directions, respectively) of the single crys-
tal rods is much smaller than the thickness of the crystal
(or length of the rods, along the z-direction [Fig. 1(b)]),
required in order to realize the longitudinal coupling factor of the crystal in the composite. The cutting process
is followed by filling of Mereco 1650 series epoxy (Mereco
Technology Group Inc., West Warwick, RI) to form a 1-3
composite [Fig. 1(a)]. The Young’s modulus of the epoxy
is 14 MPa, much smaller than that of single crystal. The
1-3 composites are polished to a thickness of about 0.7 mm
and sputtered Au films of 20-nm thick are used as the electrodes. The finished composites are poled again along the
thickness direction in silicone oil for 5 minutes under a
direct current (DC) field of 10 kV/cm. We find that this
second poling process is necessary to have high piezoelectric coefficient and electromechanical coupling factor of the
composites.
The dielectric, piezoelectric, and electromechanical
properties are characterized for the 1-3 composites. For
a 1-3 composite, because the two constituents are electrically in parallel [Fig. 1(b)], the dielectric constant can be
expressed as ε = 1 vε1 + 2 vε2 , if the fabrication process
does not affect the properties of the single crystals [9]. In
this equation, 1 ν and 2 ν denote the volume fraction of
phase 1 and phase 2, ε1 and ε2 are the dielectric permittivity of phase 1 and phase 2, respectively. Fig. 2 presents
data of the dielectric constant as a function of temperature
(measured during cooling) for a 1-3 single crystal composite with 44.4% volume fraction of PMN-PT single crystal.
For comparison, the data for the single crystal of the same
composition and orientation is shown in Fig. 2(b). The dielectric peak at 130◦ C for the composite is the same as that
in the single crystal. At room temperature, the dielectric
constant of the crystal is 3390 and the 1-3 composite is
1536, which is close to 44.4% of 3390 (=1505). The result
indicates that the 1-3 composite fabrication process used
in this investigation does not have a marked effect on the
single crystal piezoelectric properties.
For 1-3 composites with a small aspect ratio (the width
of the crystal rods and width of the polymer phase kerf
versus the thickness), the piezoelectric coefficient d33 of
1-3 composites can be expressed as [10]:
1 1
d33 =
v d33 2 s33 + 2 v 2 d33 1 s33
,
1 v2 s
2 1
33 + v s33
(1)
where 1 d33 and 2 d33 are the piezoelectric coefficient and
1
s33 and 2 s33 are the elastic compliance of phase 1 and
phase 2, respectively. For the PMN-PT/epoxy composite,
the piezoelectric coefficient of the epoxy 2 d33 is 0 and the
elastic compliance of epoxy 2 s33 is much larger than 1 s33
(7.1 × 10−8 1.2 × 10−10 ), which yield d33 ≈ 1 d33 . Directly measuring the d33 coefficient of 1-3 composites using
a d33 meter shows that this is indeed the case. As has been
shown by earlier works [11], [12] on piezoelectric 1-3 composites, the thickness electromechanical coupling factor kt
of 1-3 composites, which is the coefficient related to the
energy harvesting experiment in this paper, can be very
close to the longitudinal coupling factor k33 of the single
crystal.
ren et al.: energy harvesting single crystals and soft epoxy material
633
(a)
Fig. 3. Electrical impedance Z and phase angle θ for a
PMN-33PT/Epoxy composite with 0.56 volume fraction, thickness/width = 1.77.
(b)
Fig. 2. The dielectric constant as a function of temperature for (a) 1-3
PMN-33PT/Epoxy composite with a 44.4% volume fraction of PMNPT; (b) PMN-33PT single crystal (measured at cooling at 1 kHz).
The coupling factor kt of the 1-3 composites is characterized using an HP Impedance Analyzer (HP Model
4284A, Agilent, Palo Alto, CA), and the data is shown
in Fig. 3. From the series resonance frequency fs and
the parallel resonance frequency fp , kt can be determined
from [13]:
kt2
π fs
=
tan
2 fp
π fp − fs
2 fp
Fig. 4. Thickness coupling coefficient kt versus volume fraction for
PMN-33PT/Epoxy composites. Open circles, experimental data and
solid curve, a fitting to the data using the relation in [7] and [12].
The elastic stiffness cD
33 of the 1-3 composite can be
deduced from the resonance frequency [12]:
2
.
(2)
The coupling factors of composites with different volume fraction of PMN-PT single crystals are shown in
Fig. 4. In the composition with PMN-PT single crystals
near 50%, the coupling factor kt reaches 0.855, close to the
single crystal k33 value of about 0.9, which is higher than
the values reported by others on single crystal-polymer
composites [12]. The difference might be due to the polymer matrix used by different groups because the matrix
used here possesses a very low elastic modulus. For comparison, the coupling factor kt of the same PMN-PT single
crystals also is measured, and it is 0.59.
cD
33 = ρ (2tfp ) ,
(3)
where ρ is the density of the composite and t is its thickness. Using the relationship [13]:
D
2
cE
(4)
33 = 1 − kt c33 ,
the elastic stiffness under constant electric field also can be
E
obtained. The elastic stiffness cD
33 and c33 as a function of
the volume fraction of PMN-PT single crystals are shown
D
in Fig. 5. Both cE
33 and c33 increase with the volume content of the single crystal, and the elastic stiffness of 1-3
composites whose composition is about 50% single crystal
is much smaller than that of the crystal itself, which are
expected.
634
ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 3, march 2006
(a)
Fig. 6. Electrical impedance Z and phase angle θ for a PZT/Epoxy
composite with 0.4 volume fraction.
Fig. 7. Schematics for the energy-harvesting experiment setup in this
study.
(b)
Fig. 5. (a) Elastic stiffness constant cD
33 versus volume fraction.
(b) Elastic compliance constant cE
33 versus volume fractions for 13 PMN-33PT/Epoxy composite. Data points are shown and solid
curves are drawn to guide eyes.
For comparison, 1-3 composites of piezoceramic PZT in
a polymer matrix also are used in an energy-harvesting
experiment. The 1-3 piezocomposites are purchased from
Smart Material Corp., Osprey, FL. The volume fraction
of the PZT ceramic is 40%. The electric impedance curve
for the PZT-piezoceramic/polymer composite is shown in
Fig. 6. The coupling factor deduced from the resonance
D
data is 62.8%. The elastic stiffness cE
33 and c33 of the PZT
10
2
composite are 3.32 × 10 N/m and 2.1 × 1010 N/m2 ,
respectively.
III. Energy Harvesting Using 1-3 Composites
The schematic of the energy-harvesting experiment
setup is shown in Fig. 7 in which the harvested electric
energy is delivered to a resistive load. In the experiment, a
Universal Test Machine (MTS 810, Supplied by Material
Test System Corp., Eden Prairie, MN) is used to provide
the mechanical energy input to the electroactive materi-
als [an alternating current (AC) compressive stress of 4 to
10 Hz]. The compressive stress is applied along the poling
direction (the 3- or z-direction). The energy-harvesting experiment also is intended to evaluate how high of a stress
field can be applied to the composites to generate a highelectric, energy-density output without damaging or depoling the composite. For PMN-PT single crystals, it has
been shown that a very large strain can be induced by an
external applied electric field for the crystals at the composition investigated here when oriented along the 001
direction [2]–[4]. Hence, it is expected that, in poled composites, a high stress can be applied to induce large strain
and consequently a large mechanical energy density in the
composites. It also has been shown by earlier investigations
[14] that a high compressive mechanical stress applied to a
piezoelectric material, as in the energy-harvesting experiment presented here, can depole the piezoelectric material.
In order to prevent possible depoling at high stress level,
a DC electric bias field is applied to the composites to
stabilize the polarization against high-compressive stress.
A very large resistor R1 is used to isolate the DC power
source from the AC energy-harvesting circuit (the R1 Cs
time constant is much larger than 1/f , where f is the AC
mechanical signal frequency, in the 4 to 10 Hz range in
the present experiment, and Cs is the capacitance of the
electroactive composites).
ren et al.: energy harvesting single crystals and soft epoxy material
For the piezo-composites in the energy-harvesting experiment, assuming that the nonzero strain component is
S3 (the strain S along the applied stress direction due
to the fact that the lateral dimensions are much larger
than the thickness of the composites), which is close to
the experimental situation, the piezoelectric constitutive
relations are:
T = cE S − eE,
D = eS + εs E,
(5)
where cE is the elastic stiffness under constant field, e is the
piezoelectric constant, and εs is the dielectric permittivity
under constant stress, T is the stress component T3 , E is
the electric field E3 , and D is the displacement component
D3 , respectively.
When a sinusoidal mechanical force F = F eiωt is applied on the sample and a sinusoid voltage and strain are
generated, the phase of which are determined by:
Fe
iωt
E
= c l0 e
Q = el0 e
i(ωt+α)
i(ωt+α)
− eV0 e
i(ωt+θ)
+ C0 V0 e
,
i(ωt+θ)
,
The generated voltage is given by:
V0 eiθ = RI = R iωel0 eiα + iωC0 V0 eiθ ,
iRωel0 eiα
,
1 − iωRC0
V02
Re2 l02 ω 2
=
,
2R
2 (1 + ω 2 R2 C02 )
1
,
ωC0
2
2
e2 l02 2ω
,
4C0
In addition:
cE
e2
=1+ E .
D
c
c C0
(14)
Substituting (13) and (14) into (10) yields the harvested
power in the load resistor:
Re2 ω 2 F 2
2
2 cE
1 + (ωRC0 )2
1
1 − kt2
2 .
1 − kt2
,
ωC0
(8)
(9)
(10)
(11)
(12)
S ω
or the power density is e 4ε
s ; (12) shows that the output power is proportional to the excitation frequency of
(15)
Hence, the maximum power delivered to the load resistor
is achieved when:
and the maximum power in this case is given by:
F 2 e2 ω 1 − kt2
F 2 kt2 ω
Powerpeak =
=
,
E 2
4cE
4 c
C0
and the peak power is:
Powerpeak =
(13)
(7)
which shows that, when the load resistor is equal to the
impedance of the sample voltage source, the maximum
power dissipated in the resistor can be reached, i.e., when:
R=
F
.
iRωe2
cE −
1 − iωRC0
R=
and the power dissipation in the load resistor is:
Power =
l0 eiα =
Power =
where R is the total external load resistor (∼ R4 in our
experiment). Solving these equations yields the generated
voltage:
V0 eiθ =
the mechanical force, the square of the displacement, and
the piezoelectric constant of the material. This means
that, if a material with a high-piezoelectric constant and a
high-strain capability is used, the harvested-electric energy
would improve greatly. For the single crystal PMN-33PT,
the strain can be more than 1%, which is much larger than
the strain of the piezoceramic [2]–[4].
The relationship between the output power and input
mechanical F also can be derived. From (6) and (9), l0 is:
(6)
where A is the cross section and t0 is the thickness, l0 =
St0 is the displacement, F = T A is the applied force,
Q = DA is the charge, V = Et0 is the voltage, cE = cE tA0 ,
s
e = e tA0 and C0 = εt0A , respectively.
The current generated in an external load, therefore, is:
∂Q
I=
= iωel0 ei(ωt+α) + iωC0 V0 ei(ωt+θ) .
∂t
635
T 2 k2 ω
(16)
(17)
or the peak power density is 4cEt . The result indicates
that the output power density is proportional to the square
of the stress, excitation frequency of the mechanical force,
and the coupling factor of the material. It is also inversely
proportional to the elastic stiffness of the material. Hence,
a soft material with a high-electromechanical coupling factor will deliver higher output electrical energy. The 1-3
composites with PMN-PT single crystals meet these conditions.
It should be pointed out that the equations derived
are based on the assumption that all the coefficients do
not change with the applied mechanical stress and electric
field.
The schematics of the energy-harvesting experiment using a resistive load are shown in Fig. 7. The energy harvested is measured directly from the voltage across R2 . In
the experiment, two different types of 1-3 composites are
used. One is the 1-3 PMN-33PT/Epoxy composite. Another one is PZT/Epoxy 1-3 composite, which is used for
comparison.
To determine the maximum output power under a constant stress, a variable resistor is connected to the circuit
636
ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 3, march 2006
Fig. 8. Harvested power density versus the load resistor in
the circuit for the 1-3 PMN-PT/Epoxy composite with volume fraction 56.3%. Applied AC stress amplitude is 88.9 MPa,
DC bias field is 4.35 kV/cm, and the sample dimension is
10 mm × 10 mm × 0.69 mm.
and an 88.9 MPa AC stress (4 Hz) is applied to the 1-3
composite (56% of PMN-PT crystal). The result is shown
in Fig. 8, and the maximum power density is 96 mW/cc
when the load resistor is 9.5 Mohm. Under high compressive stress, both the capacitance and the coupling factor will decrease compared with their stress-free values.
Hence, the load resistor value at high stress for the maximum power output is higher than for the stress-free case
(∼4 MPa). Such a nonlinear material response is a common feature of piezomaterials. In this experiment, a DC
bias field of 4.65 kV/cm is applied to prevent depoling due
to stress. After energy-harvesting experiments, the coupling factor kt of the 1-3 composite was measured, and
the results showed that the coupling factor of the composite did not change after the energy-harvesting experiments,
indicating that no depoling occurs under a high-stress level
when the composites are subject to the DC bias field.
For comparison, the dependence of the power harvested on applied stress level is measured using a 1-3
PZT/Epoxy composite (purchased from Smart Material
Corp.) in which soft PZT (PZT-5H) is used. As shown in
the data of Fig. 9(a), when no DC bias is applied to the
composites, a partial depolarization occurs that results in
a much smaller harvested power as the stress amplitude
is reduced (reduction of the coupling factor and piezoelectric coefficient). When a high DC bias field is applied, from
the data in Fig. 9(b), the harvested energy is enhanced because the DC bias prevents the partial depoling of the 1-3
composite during the measurement. The maximum power
density is 12 mW/cc from the 1-3 PZT/Epoxy composite
under a 40 MPa AC stress of 4 Hz.
The results from these studies are summarized in Table I. Due to a higher coupling factor, the harvested-energy
density of 1-3 composite with single crystal (22.1 mW/cc
under 40.4 MPa) is about twice of that harvested with
(a)
(b)
Fig. 9. Harvested power density versus applied mechanical stress
for a PZT/Epoxy composite with 0.4 volume fraction. Sample diameter, 2 cm and thickness, 1.1 mm. (a) DC bias = 0 V. (b) DC
bias = 5.45 kV/cm. The direction of the stress cycle is indicated in
the figure.
PZT ceramic 1-3 composite (12 mW/cc under 39 MPa).
At a higher stress level, the harvested-energy density of
1-3 PMN-PT single crystal composite can be 96 mW/cc.
From Table I, when the applied mechanical stress is increased by 88.9/40.4 = 2.2 times, the square dependence
of the output power on the applied stress indicates that
the harvested-energy density should be increased by a factor of 4.8. The result in Table I shows an increase in the
power density of a factor of (96.2/22.1) 4.4, close to 4.8 as
predicted.
It should be pointed out that, because the composites
are insulators, there is very little energy consumption of
the DC power source used to provide DC bias voltage.
In fact, such a DC voltage source can be provided by the
harvested-electric energy through properly designed power
electronics.
ren et al.: energy harvesting single crystals and soft epoxy material
TABLE I
Summary of the Experimental Results.
Volume
fraction
E
cD
33 c33
(GPa)
Stress
(MPa)
Power
density
(mW/cc)
1-3 PZT/Epoxy
composite
40%
34.8/21
39
12
1-3 single crystal
PMN-33PT/
Epoxy composite
37.5%
56%
49/12.8
60/15.6
40.4
88.9
22.1
96.2
Material
IV. Conclusions
In this paper, 1-3 PMN-PT single-crystal/polymer composites are investigated for energy-harvesting applications.
To make use of the high-strain capability, and hence
the high-energy density, while maintaining mechanical integrity, 1-3 composites are a preferred form for the single
crystals to be utilized for energy harvesting. The single
crystal PMN-PT/Epoxy composites with different volume
fraction of single crystals are fabricated and characterized.
The results show that, for a properly designed and fabricated 1-3 single crystal/polymer composite, the thickness
coupling factor kt , which is used in this energy-harvesting
experiment, can be very close to the longitudinal coupling
factor k33 of the single crystal. For the experiments conducted in this paper, kt of 1-3 single crystal composite is
0.855, close to k33 = 0.9 of the single crystals used in this
study.
In the energy-harvesting experiments, a resistive load
circuit is used to measure the energy harvested. Based on
the piezoelectric constitutive equations, the relations between the input mechanical strain, stress and harvested
electrical energy density are derived. For comparison,
the energy-harvesting experiment using 1-3 PZT/Epoxy
composites also is performed. Due to the higher coupling factor, the harvested energy density from 1-3 PMNPT/Epoxy composite is about twice of that from the 1-3
PZT/Epoxy. Under a mechanical stress of 88.9 MPa at
4 Hz, a harvested-energy density of near 0.1 W/cc can be
achieved. The experimental results also show that a high
stress (more than 40 MPa) can be applied to 1-3 single
crystal composites without damaging the composites, and
a DC bias field applied to the composites is very effective in preventing the stress depoling in the composites.
No degradation is observed after many energy harvesting
cycles under high stress using these 1-3 composites.
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Kailiang Ren was born in Shaanxi, China, in
1974. He received the B.S. degree in electrical
engineering from Tianjin University, Tian’jin,
China, in 1997 and the M.S. degree in electrical engineering from Pennsylvania State University, University Park, PA, in 2005. He is
currently a Ph.D. student in Pennsylvania
State University.
His research interests are in preparation
and characterization of ferroelectric materials (electroactive polymer and composite) and
designing some devices for applications, including energy harvesting, transducer, actuator, and microfluidic devices.
Yiming Liu (S’03) was born in Lanzhong,
China, in 1972. He received his B.S. degree
in physics from Nanjing University, Nanjing,
China, in 1996. Between 1996 and 1999 he
worked in The National Laboratory of Solid
State Microstructures, Nanjing, China, as a
research assistant on ferroelectric thin films.
He was admitted to the Ph.D. program of the
Department of Physics at the Pennsylvania
State University, University Park, PA, in fall
1999. After finishing core courses and passing
candidacy examination, he changed his major
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ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 3, march 2006
to electrical engineering and continues his Ph.D. study. He worked
as a research assistant for the Material Research Institute at Penn
State University on loss mechanism of piezoelectric and ferroelectric
materials between 2001 and 2002. At the end of 2002 he joined the
Power Electronic Group at Penn State University led by Dr. Heath
Hofmann. He has dedicated himself to research on energy harvesting
since March 2003. His research interests are power electronics and
piezoelectric devices, analytic and numerical modeling of dynamic
systems, and control theory of switched mode power electronics.
Xuecang Geng received a B.S. degree in physics from Xi’an Jiaotong University, Xi’an, China, in 1983, a M.S. degree in acoustics
from the Institute of Acoustics, Chinese Academy of Sciences, Beijing, China, in 1989, and a Ph.D. degree in material engineering from
Pennsylvania State University, University Park, PA, in 1997.
From 1983 to 1986 he was an engineer at Center of Microelectronics, Chinese Academy of Sciences, Beijing, China. This work focused
on the computer-aided design of integrated circuit modeling of the
integrated circuit process. From 1990 to 1993 as a research assistant
at the ultrasonic lab of the Institute of Acoustics, Beijing, China,
he worked on non-destructive testing of materials, acoustic wave logging, design, fabrication, and characterization of 1-3 piezocomposites
for ultrasonic transducer applications. Since 1998, he has worked as
a senior scientist at Blatek, Inc., State College, PA, focusing on ultrasonic transducer design for medical and industrial applications.
Heath F. Hofmann (M’89) is an associate professor at Pennsylvania State University, University Park, PA. Dr. Hofmann’s
research area is power electronics, specializing in the design and control of electromechanical systems. Specific interests are sensorless control techniques, low-power electronic
design, and finite-element analysis. Ongoing
projects include the design of a high-speed
motor/generator for flywheel energy storage
systems, optimization of ultrahigh power density electric machines, design of an ultra-
capacitor energy storage system for hybrid electric vehicles, energyharvesting circuitry for vibrating piezoelectric devices, sensorless
control of interior permanent magnet machines, induction machines,
and linear machines, design of a unity-power factor bi-directional
AC-DC converter, and stability analysis of DC distribution systems.
Dr. Hofmann was awarded a Prize Paper Award (First Prize) by
the Electric Machines Committee at an IEEE Industry Applications
Society Annual Meeting in 1998.
Qiming M. Zhang (M’96–SM’00) is a professor of electrical engineering. He obtained a Ph.D. degree in 1986 from Pennsylvania State
University, University Park, PA. The research areas in his group include fundamentals and applications of novel electronic and electroactive materials. Research activities in his group include actuators and sensors, transducers, dielectrics and charge storage devices,
polymer thin film devices, polymer microelectromechanical systems
(MEMS), and electro-optic and photonic devices. He has over 230
publications and 8 patents in these areas. His group has discovered
and developed a ferroelectric relaxor polymer that possesses room
temperature dielectric constant higher than 50, an electrostrictive
strain higher than 7%, and electric energy density near 10 J/cm3 .
His group also proposed and developed nano-polymer composites
based on delocalized electron systems to raise the nano-polymeric
composites dielectric constant near 1,000.
He is the recipient of the 1999 Penn State Engineering Society
Outstanding Research Award. The research works in his group have
been funded by The Defense Advanced Research Projects Agency
(DARPA), the Office of Naval Research (ONR), National Institutes
of Health (NIH), National Science Foundation (NSF), and many companies.