U.S. patent application number 14/531843 was filed with the patent office on 2015-11-26 for barium titanate nanowire their arrays and array based devices.
The applicant listed for this patent is UNIVERSITY OF FLORIDA RESEARCH FOUNDATION, INC.. Invention is credited to ANEESH KOKA VENKATA, HENRY A. SODANO.
Application Number | 20150336789 14/531843 |
Document ID | / |
Family ID | 54542753 |
Filed Date | 2015-11-26 |
United States Patent
Application |
20150336789 |
Kind Code |
A1 |
SODANO; HENRY A. ; et
al. |
November 26, 2015 |
BARIUM TITANATE NANOWIRE THEIR ARRAYS AND ARRAY BASED DEVICES
Abstract
A nano-electromechanical system comprises piezoelectric
vertically aligned BaTiO.sub.3 nanowire arrays for
energy-harvesting applications, sensors, and other applications.
The aligned piezoelectric nanowire arrays provide highly accurate
nano-electromechanical system-based dynamic sensor with a wide
operating bandwidth and unity coherence and energy harvesters at
low frequencies. The growth of vertically aligned (B45-mm long)
barium titanate nanowire arrays is realized through a hydrothermal
synthesis.
Inventors: |
SODANO; HENRY A.;
(GAINESVILLE, FL) ; KOKA VENKATA; ANEESH;
(GAINESVILLE, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UNIVERSITY OF FLORIDA RESEARCH FOUNDATION, INC. |
GAINESVILLE |
FL |
US |
|
|
Family ID: |
54542753 |
Appl. No.: |
14/531843 |
Filed: |
November 3, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61898825 |
Nov 1, 2013 |
|
|
|
Current U.S.
Class: |
257/415 ;
438/50 |
Current CPC
Class: |
B81B 3/0021 20130101;
H01L 41/31 20130101; B81B 2201/032 20130101; B81C 1/0019 20130101;
B81C 3/005 20130101; H02N 2/18 20130101; G01P 15/0922 20130101;
H01L 41/1136 20130101 |
International
Class: |
B81B 3/00 20060101
B81B003/00; B81C 3/00 20060101 B81C003/00; B81C 1/00 20060101
B81C001/00 |
Goverment Interests
[0002] This invention was made with government support under
FA9550-12-1-0132 awarded by the Air Force Office of Scientific
Research. The government has certain rights in the invention.
Claims
1. A nano electromechanical system (NEMS), comprising an aligned
piezoelectric BaTiO.sub.3 nanowire array.
2. The nano electromechanical system (NEMS) according to claim 1,
wherein said NEMS is a sensor having an operating bandwidth over a
frequency spectrum up to 10 kHz and unity coherence.
3. The nano electromechanical system (NEMS) according to claim 1,
wherein said NEMS sensor is an energy harvester having a resonant
frequency less than 200 Hz.
4. The nano electromechanical system (NEMS) according to claim 1,
wherein the nanowires are 0.5 to 50 .mu.m in length.
5. The nano electromechanical system (NEMS) according to claim 1,
wherein the nanowires are 0.5 to 40 .mu.m in length.
6. The nano electromechanical system (NEMS) according to claim 1,
wherein the nanowires are 0.5 to 1.5 .mu.m in length.
7. A method of preparing an aligned piezoelectric BaTiO.sub.3
nanowire array comprising: providing a sodium titanate nanowire
array; providing a solution comprising barium hydroxide; combining
the solution with the sodium titanate nanowire array; and heating
the combination, wherein the sodium titanate nanowire array is
converted to a barium titanate nanowire array with equivalent
length nanowires to the sodium titanate nanowires.
8. The method according to claim 7, wherein the sodium titanate
nanowires are 0.5 to 50 .mu.m in length.
9. The method according to claim 7, wherein the heating is to a
temperature between 150 and 250.degree. C.
10. A method of preparing an aligned piezoelectric BaTiO.sub.3
nanowire array comprising: providing a conductive substrate;
providing a TiO.sub.2 precursor; growing a TiO.sub.2 nanowire array
from the TiO.sub.2 precursor on the conductive substrate; providing
a solution comprising barium hydroxide; combining the solution with
the TiO.sub.2 nanowire array; and heating the combination, wherein
TiO.sub.2 nanowire array is converted to a barium titanate nanowire
array with equivalent length nanowires to the TiO.sub.2
nanowires.
11. The method according to claim 10, wherein the TiO.sub.2
precursor is titanium isopropoxide.
12. The method according to claim 10, wherein the TiO.sub.2
nanowires are 0.5 to 50 .mu.m in length.
13. The method according to claim 10, wherein the heating is to a
temperature between 150 and 250.degree. C.
14. The method according to claim 10, wherein the conductive
substrate is fluorine doped tin oxide (FTO) glass.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims the benefit of U.S.
Provisional Application Ser. No. 61/898,825, filed Nov. 1, 2013,
which is hereby incorporated by reference herein in its entirety,
including any figures, tables, or drawings.
BACKGROUND OF INVENTION
[0003] Piezoelectric materials offer coupling between mechanical
and electrical energy, allowing them to be effectively fabricated
as transducers that can be configured as either sensors or
actuators. Many sensing applications incorporate piezoelectric
materials, because they require minimal signal conditioning and
have wide bandwidth well into the MHz range. Furthermore,
piezoelectric materials have been found particularly well suited
for micro-electromechanical systems (MEMS), as the energy density
does not decrease with the dimensions, as is the case in
electromechanical or magneto-mechanical systems. When applied in
MEMS applications, piezoceramics are typically constrained to a
thin film, which places certain limits on the design of the device
and typically requires the use of the lower k.sub.31 piezoelectric
coefficient (1). An alternative configuration for sensors is
vertically aligned piezoelectric nanowire (NW) arrays that allow
for facile interfacing with electrical interconnects. Piezoelectric
nanowires have evoked tremendous curiosity in the field of
nanotechnology for energy applications primarily due to their
excellent electro-mechanical energy conversion capabilities which
are unchanged as the scale is reduced and, in addition to their
ability to be utilized in advanced sensors, they can function as
sufficient power sources for certain low power wireless electronic
devices and miniature autonomous systems [1,2]. Power generating
nano-electro-mechanical system (NEMs) fabricated using
piezoelectric nanowires have become renowned in the research
community, as they are able to convert several different sources of
mechanical energy into electric power, such as: sound waves [3];
ultrasonic waves [4-6]; vibrational energy [7,8]; atomic force
microscope tip induced stimuli [9,10]; and biomechanical energy
[11,12].
[0004] The power generating capacity of devices based on aligned
piezoelectric ZnO nanowire (NWs) arrays has been studied rigorously
and it has been reported that the energy conversion efficiency of
such a device is sufficiently high for production of electricity
that can potentially power nanosystems [9]. The direct
piezoelectric effect responsible for the energy-harvesting behavior
is identical to the response required for sensing. However, energy
harvesting represents a more simplistic operation, as the voltage
output can contain significant noise, requires little to no
correlation to the input energy and places no limits on the
bandwidth or stability of the response. On the contrary, a
functional sensor must produce an output that can be very
accurately correlated to the force (mechanical measurands) acting
on it and without noise that would limit the sensitivity and
measurement floor. Among the piezoelectric NEMs, those made of
ferroelectric perovskite nanostructures and thin films such as PZT
(PbZr.sub.xTi.sub.1-xO.sub.3) [8, 13-15], and Barium Titanate
(BaTiO.sub.3) [16-18] can produce greater energy due to their
higher electro-mechanical coupling coefficients and thereby,
provide an efficient means to harvest mechanical energy. The NW
form of these materials offers considerable advantages due to the
high aspect ratio, which leads to highly deformable structures [19,
20] and size effects [21] that act to enhance the piezoelectricity
of the ceramic. Consequently, piezoelectric NWs have tremendous
potential to be applied in the emerging field of
nano-electromechanical systems (NEMS).
[0005] However, environmental concerns over the use of lead based
piezoelectric materials have enhanced the need to develop and
utilize lead-free BaTiO.sub.3 nanostructures. Moreover, prior to
this study the synthesis of vertically aligned arrays of
BaTiO.sub.3 nanowires (NWs) had not been developed and thus this
high performance lead free composition has received little
attention. Previously, Wang et al. [19] performed a numerical
analysis to show that the BaTiO.sub.3 NWs have higher power
generating capability as compared to ZnO NWs for the same size.
Recently, Wang et al. [20] applied ZnO NWs as vibration sensors to
detect the resonance characteristics of a cantilever beam and
evaluated the voltage generating performance. Although ZnO NWs have
garnered significant interest for sensing and energy harvesting, a
low piezoelectric coupling coefficient and semiconductor behavior
are unlike many ferroelectric ceramics and, therefore, sensors
therefrom show low sensitivity and a high noise floor [22].
Although ZnO NWs have a low dielectric constant, which increases
its voltage output, the performance has been very limited and no
sensor has been demonstrated to produce a high coherence between
the input and output across the sensor's bandwidth [3, 23-25].
Ferroelectric perovskite nanostructures such as PZT
(PbZr.sub.xTi.sub.1-xO.sub.3) [13, 26, 27] NWs improve the
electromechanical coupling performance of NW-based devices;
however, they have only been applied for energy harvesting
applications as the environmental concerns with lead-based
piezoelectric materials encourages the use of lead-free
piezoelectric nanostructures for sensors [28]. Among lead-free
ceramics, barium titanate (BaTiO.sub.3) possesses one of the
highest coupling values. However, no synthesis method for the
growth of vertically aligned BaTiO.sub.3 NW arrays has been
demonstrated prior to the inventors' efforts, and thus this
high-performance lead-free material has received little attention
in the NW form. Herein the preparation of ultra-long and short
BaTiO.sub.3 nanowires by two methods and their use for the
preparation of sensors and energy harvesting devices
SUMMARY OF THE INVENTION
[0006] The preparations, piezoelectric behaviors, and fabrications
of device from ultra-long and short vertically aligned array of
BaTiO.sub.3 NWs are utilized to fabricate vibration sensors and
energy harvesting devices by modifying top electrode
configurations. High voltages were obtained at low frequencies from
a sensor followed by a flat band region in the FRF observed from
white noise excitation with Harming window, correlating well with
even sine wave excitation of the sensor and, therefore, frequency
preservation is validated. In one embodiment of the invention, a
cantilever Indium top electrode with low resonant frequency was
used to apply BaTiO.sub.3 NWs for energy harvesting applications.
The added tip mass on the beam resulted in resonant peak shift and
magnitude increase in the FRF to clearly indicate piezoelectric
behavior of the BaTiO.sub.3 NWs.
[0007] In another embodiment of the invention, a novel NEMS sensor
comprises vertically aligned array of ultra-long, BaTiO.sub.3 NWs
to utilize their piezoelectric behavior to detect acceleration from
mechanical vibration source. This piezoelectric NW-based sensor
shows excellent coherence, linearity and wide operating bandwidth
over a frequency spectrum that spans up to 10 kHz. The resonant
frequency from the NW sensor can be adjusted by varying the seismic
mass during the fabrication process and, thereby, provides control
over the sensitivity and operating frequency bandwidth of the
BaTiO.sub.3 NW-based sensor to suit different application
requirements.
[0008] In another embodiment of the invention, NEMS vibrational
energy harvesters are fabricated with resonant frequencies that are
less than 200 Hz using vertically aligned ferroelectric BaTiO.sub.3
NW arrays that are about 1 .mu.m long on a conductive FTO glass.
Superior vibrational energy harvesting capability of the
BaTiO.sub.3 NW arrays is achieved. This power density, peak open
circuit voltage, and peak short circuit current levels at resonant
frequency measured from these BaTiO.sub.3 NW based NEMS energy
harvester are significantly greater than the response recorded from
a ZnO based NEMS energy harvester, where the power density of the
BaTiO.sub.3 NEMS energy harvester is comparable to many meso-scale
and MEMS-scale resonant vibrational energy harvesters.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1. Characterization of BaTiO.sub.3 NW arrays: (a) A
cross-sectional SEM image of BaTiO3 NW arrays (scale bar, 20 mm);
(b) XRD spectrum from the BaTiO.sub.3 NW arrays (JCPDS number
5-0626) synthesized using sodium titanate NW arrays as precursor;
(c) An HRTEM image of the single-crystal sodium titanate NW
precursor (scale bar, 100 nm) with the inset showing the clear
crystal lattice fringes (scale bar, 25 nm); and (d) An HRTEM image
of the single BaTiO.sub.3 NW showing single-crystal structure after
ion transfer (scale bar, 100 nm), with the inset showing the clear
crystal lattice fringes (scale bar, 5 nm; arrows are used to
indicate the crystal lattice spacing distance of 4.12 .ANG.).
[0010] FIG. 2. Characterization of material using X-ray diffraction
(XRD): (a) Starting titanium (Ti) foil used for the synthesis
(JCPDS No. 65-3362); (b) Oxidized Rutile titanium dioxide
(TiO.sub.2) obtained after heating Ti foil at 750.degree. C. for 8
hours (JCPDS No. 65-0191); and (c) Sodium titanate nanowire arrays
used as precursors synthesized on oxidized Ti foil, H denotes the
Sodium Hexatitanate (Na.sub.2Ti.sub.6O.sub.13) peaks (JCPDS No.
73-1398) and T denotes the Sodium Trititanate peaks (JCPDS No.
31-1329).
[0011] FIG. 3. Shows a sensor device configuration where: (a)
schematic diagram of sensor device using BaTiO.sub.3 NW arrays; and
(b) schematic of piezoelectric voltage generation from NWs
Polarization direction (P) represents the alignment direction of
the dipoles; with application of dynamic stress (s(t)) on NW arrays
produces voltage (V(t)) generation.
[0012] FIG. 4. Dynamic response characterization of BaTiO.sub.3 NW
sensor: (a) FRF illustrated by magnitude in dB scale of Vg.sup.-1;
(b) phase angle in degree; (c) the coherence function
(.gamma..sup.2f) of BaTiO.sub.3 NW sensors with a heated solder as
the top electrode demonstrated flat band region with strong unity
coherence up to 10,000 Hz from white noise excitation; (d) FRF
illustrated by magnitude in dB scale of Vg.sup.-1; (e) phase angle
in degree; and (f) the coherence function (.gamma..sup.2f) of
BaTiO.sub.3 NW sensors with an added mass on the top electrode
produced a resonance frequency at 450 Hz and demonstrated enhanced
sensitivity (B50 mVg.sup.-1) in the .+-.3 dB flat band region of up
to 300 Hz.
[0013] FIG. 5. Sinusoidal excitation on BaTiO.sub.3 NW sensor: (a)
RMS sensitivity from frequencies of 100-1,000 Hz; (b) Voltage
measured at 100 Hz with (c) showing the detailed shape; (d) Voltage
measured at 200 Hz with (e) showing the detailed shape; (f) voltage
measured at 300 Hz with (g) showing the detailed shape where
voltage measured at 100 (b), 200 (d) and 300 Hz (f) from 1 g
amplitude acceleration input correlates well with the mean RMS
sensitivity of .about.50 mV up to 300 Hz; and (h) voltage measured
near resonant frequency (450 Hz) with (i) clearly showing the
detailed shape of a high Vpp of 342 mV produced at resonance from 1
g amplitude acceleration input.
[0014] FIG. 6. Influence of depoling and switchable polarization
direction: (a) FRF illustrated by magnitude in dB scale of
Vg.sub.--1; (b) phase angle in degree; (c) the coherence function
(.gamma..sup.2f) of BaTiO.sub.3 NW sensors under white noise
excitation when heated to 150.degree. C. for 3 h (Curie
temperature, TC=120.degree. C.), which shows the loss in
piezoelectric behavior from depoling as the electric dipoles in the
NWs have been relaxed from their oriented poled state to random
directions; (d) acceleration measured by reference accelerometer
(PCB 352C22) to low-frequency pulse input; (e) the output voltage
response (V) from the same BaTiO.sub.3 NW sensor to pulse input
measured after poling it with positive voltage (75 kVcm.sup.-1 DC
field); and (f) then poling it with negative voltage (-75
kVcm.sup.-1 DC field) to investigate the influence of reversing the
polarization direction (P) on the output, where the sensor is
connected with same terminals to voltage follower to demonstrate
the reversing output voltage signal (V) (e,f) from the same
mechanical deformation by switching the polarization direction (P)
to confirm that the measured voltage response from the sensor is
generated by ferroelectric BaTiO.sub.3 NW arrays.
[0015] FIG. 7. Characterization and comparison of ZnO NW NEMS
sensor: (a) FRF illustrated by magnitude in dB scale of Vg.sup.-1;
(b) phase angle in degree; (c) the coherence function
(.gamma..sup.2f) of ZnO NW sensors obtained from white noise
excitation; (d) RMS sensitivity from frequencies ranging from 100
to 1,000 Hz from sinusoidal excitation; (e) Voltage measured at 200
Hz from 1 g amplitude acceleration input with (f) showing detailed
shape with a Vpp of .about.3.8 mV; (g) voltage measured near
resonant frequency (500 Hz) showing a Vpp of .about.22 mV from 1 g
amplitude acceleration input with (h) showing the detailed shape;
and (i) comparison of magnitude of the FRF with frequency axis in
log scale to clearly depict the higher .+-.3 dB flat band
sensitivity of .about.50 mVg.sup.-1 from BaTiO.sub.3 NW sensors as
compared with .+-.3 dB sensitivity of .about.2.5 mVg.sup.-1 from
ZnO NW sensors up to .about.350 Hz.
[0016] FIG. 8. Characterization of piezoelectric nanowires: (a)
Cross-sectional SEM image of BaTiO3 NW arrays with the inset
showing the top view; (b) X-ray diffraction spectrum from BaTiO3 NW
arrays showing the majority of peaks to be BaTiO.sub.3 (JCPDS No.
5-0626); (c) HRTEM image of the BaTiO.sub.3 nanowire showing the
clear crystal lattice fringes of single crystal structure; and (d)
Cross-sectional SEM image of ZnO NW arrays with the inset showing
the top view.
[0017] FIG. 9. NEMS energy harvester configuration and
characterization: (a) schematic diagram of NEMS energy harvester
fabricated using piezoelectric NWs array; and (b) Schematic of
voltage generation from piezoelectric NEMS Energy Harvester (P
denotes the polarization direction, a denotes the stress and V is
the piezo-voltage generated by the nanowires) with the inset at the
bottom showing the electrical circuit representation of
piezoelectric energy harvester with the voltage (VL) measured
across the load resistor (RL) to characterize AC power delivered to
the load (Cp denotes capacitance of source and Rp is the leakage
resistance).
[0018] FIG. 10. Open circuit voltage and Short circuit current
analysis from BaTiO.sub.3 NW NEMS Energy Harvester: (a) open
circuit voltage (VOC) FRF showing the resonant peak at .about.160
Hz associated with 90.degree. phase shift; (b) VOC from 1 g RMS
sinusoidal acceleration input near resonant frequency (.about.160
Hz) shown in the top panel with the bottom panel showing the
detailed shape of voltage and input base acceleration; (c) Short
Circuit Current (ISC) FRF with a resonant peak near .about.160 Hz;
and (d) ISC from 1 g RMS acceleration input near resonant frequency
shown in the top panel with the bottom panel showing the detailed
shape of current and input acceleration.
[0019] FIG. 11. Open circuit voltage and short circuit current
analysis from ZnO NW NEMS energy harvester: (a) open circuit
voltage (VOC) FRF showing the resonant peak near .about.190 Hz
associated with 90.degree. phase shift; (b) Short circuit current
(ISC) FRF with resonance peak near .about.190 Hz; (c) VOC measured
near resonance from 1 g RMS acceleration shown in the top panel
with the bottom panel showing the detailed shape of voltage and
acceleration; and (d) ISC from 1 g RMS acceleration input near
resonant frequency shown in the top panel with the bottom panel
showing the detailed shape of current and acceleration.
[0020] FIG. 12. Power characterization of NEMS energy harvesters:
(a) AC Power and Power Density of BaTiO.sub.3 NW NEMS energy
harvester with varying load resistor (RL) showing peak power of
.about.125.5 pW and peak power density of .about.6.27 .mu.W/cc at
optimal RL of 120 M.OMEGA. from 1 g RMS acceleration; (b) AC Power
and Power density of ZnO NW NEMS energy harvester with varying load
resistor (RL) showing a lower peak power of .about.8 pW and peak
power density of .about.0.4 .mu.W/cc at optimal RL of 50 Ma from 1
g RMS sinusoidal base acceleration; (c) Voltage Magnitude of FRF
from BaTiO.sub.3 NW NEMS Energy Harvester measured for various load
resistors shows increasing magnitude peak as load resistance
increases; and (d) Voltage Magnitude of FRF from ZnO NW NEMS Energy
Harvester measured for various load resistors shows increasing
magnitude peak as load resistance increases with the maximum peak
from VOC FRF (1 T.OMEGA.).
[0021] FIG. 13. Frequency Response Function and Coherence Function
for BaTiO.sub.3 Sensor with melted Indium as top electrode.
[0022] FIG. 14. Sensitivity in term of acceleration at low
frequencies from 1 g amplitude sine wave excitation for BaTiO.sub.3
Sensor with melted Indium as top electrode.
[0023] FIG. 15. Barium Titanate Nanowires Characterization and
Sensor Device Fabrication: (a) Cross-sectional SEM image of
BaTiO.sub.3 NWs; (b) X-ray diffraction spectrum from the as grown
nanowire arrays; (c) Image of the BaTiO.sub.3 NWs based sensor
device; and (d) Schematic diagram of the sensor device using
BaTiO.sub.3 NWs array.
[0024] FIG. 16. Shows the: (a) experimental setup and (b)
arrangement inside Faraday cage.
[0025] FIG. 17. Shows a schematic of the circuit representation fir
a piezoelectric BaTiO.sub.3 NW Sensor modeled as a charge source
(q) in parallel with the source capacitance (C.sub.p) and
insulation resistance (R.sub.p) where the high input resistance
(R.sub.i=1 T.OMEGA.) of the unity gain voltage follower (LTC
6240CS8) reduces the loading effect and tracks and converts the
high input impedance voltage signal (V.sub.i) from the sensor into
a low output impedance voltage signal (V.sub.0) measured using the
data acquisition system (DAQ).
[0026] FIG. 18. Shows images of the sensor fabrication process and
the test setup where: (a) BaTiO.sub.3 nanowire arrays handled with
tweezers with the images on the right showing the SEM of the top
surface and the cross-section of the NW arrays; (b) fabrication
process starting with the silver epoxy layer applied on
borosilicate glass substrate (top left), followed by NW arrays
placed on the silver epoxy (bottom left) and then solder film
placed on top of the NW arrays and heated to enhance bonding with
NWs (right); (c) BaTiO.sub.3 NWs based acceleration sensing device;
with (d) the experimental setup; and (e) the experimental
arrangement inside Faraday cage.
[0027] FIG. 19. Shows plots of the voltage noise floor and
Acceleration spectral density where: (a) is the voltage noise floor
spectral density that corresponds to a mean of
430 nV Hz ##EQU00001##
from 500 Hz-10 kHz; and (b) is the input rms acceleration spectral
applied to the sensor for FRF characterization.
[0028] FIG. 20. Shows output piezoelectric voltage generated by NW
sensor from 1 g amplitude sinusoidal acceleration input where: (a)
is a detailed plot of the acceleration input and output voltage at
100 Hz; and (b) is a detailed plot showing the output voltage
obtained near resonance at 450 Hz that clearly shows the 90.degree.
phase lag between the output piezoelectric voltage and the input
acceleration applied to the sensor.
[0029] FIG. 21. Shows plots of the frequency response function
(FRF) with magnitude in the top panel, phase in the middle panel,
and the coherence function (.gamma..sup.2(f)) in the bottom panel
from BaTiO.sub.3 NW sensor fabricated from annealed NW arrays at
700 C for 1 hour.
[0030] FIG. 22. Shows an analysis of de-poled NW sensor with the
frequency response function (FRF) and coherence function
(.gamma..sup.2(f)) of BaTiO.sub.3 NWs sensor under white noise
excitation when heated to 150.degree. C. for 3 hours (Curie
temperature, T.sub.c=120.degree. C.) where the loss in
piezoelectric behavior as the electric dipoles in the NWs have been
relaxed from their oriented poled state to random directions.
[0031] FIG. 23. Shows a composite plot of poling and depoling from
the BaTiO.sub.3 NW NEMS sensor where the magnitude of frequency
response function (FRF) from white noise excitation from poled
state by supplying DC field of 75 kV/cm for 12 hours, depoled state
after heating at 150.degree. C. for 3 hours (Curie temperature,
T.sub.C=120.degree. C.) shows the loss in piezoelectric behavior as
the electric dipoles in the NWs have been relaxed from their
oriented poled state to random directions, and from re-poled state
of poling at 75 kV/cm for 30 minutes clearly demonstrating the
return of the resonant peak, which validates the piezoelectric
behavior from the ferroelectric BaTiO.sub.3 NW arrays.
[0032] FIG. 24. Shows: (a) a cross-sectional SEM image of short ZnO
NW arrays; output piezoelectric voltage generated by ZnO NW sensor
from 1 g amplitude sinusoidal acceleration input for an
acceleration input and output voltage at (b) 100 Hz and (c) 200 Hz
showing in-phase relationship; and (d) a plot showing the output
voltage obtained near resonant frequency at 500 Hz and an
associated 90.degree. phase lag between the output piezoelectric
voltage and the input acceleration applied to the ZnO sensor.
DETAILED DISCLOSURE
[0033] According to an embodiment of the invention, BaTiO.sub.3 is
prepared as an array of vertically aligned nanowires. In another
embodiment of the invention, the vertically aligned BaTiO.sub.3
nanowires are used to form NEM devices. The NEM devices can be a
vibration sensor with a wide operating frequency bandwidth or a
vibrational energy harvester when excited near resonant
frequency.
[0034] According to an embodiment of the invention, ultra-long (up
to 45 mm), vertically aligned BaTiO.sub.3 NW arrays are achieved
through a low-cost, two-step hydrothermal growth method. These NWs
piezoelectric function as energy-harvesting material and can be
used to fabricate NEMS accelerometers with high sensitivity, unity
coherence, and wide operating bandwidth. High sensitivity of
.about.50 m Vg.sup.-1 is demonstrated from the NEMS sensor composed
of vertically aligned BaTiO.sub.3 NW arrays, which is much higher
than the sensitivity of ZnO NW sensor (.about.2.5 Vg.sup.-1) under
the same conditions.
[0035] A hydrothermal process is employed to grow vertically
aligned BaTiO.sub.3 NW arrays permits surfaces to be tailored using
MEMS processing. For example, isolated patches of NWs can be
fabricated such that sensing arrays can be used to achieve
extremely high spatial resolution. The preparation of the
BaTiO.sub.3 NW arrays occurs through the conversion of vertically
aligned sodium titanate NW arrays that is grown on an oxidized
titanium substrate. The nanowires' microstructure was characterized
using scanning electron microscopy (SEM), as shown in FIG. 1a, and
high-resolution transmission electron microscope (HRTEM). The
BaTiO.sub.3 NW arrays span, for example, a length of 45 mm and the
diameter of the individual NWs are 600 nm. These dimensions were
chosen such that the NWs are sufficiently rigid to allow their use
in compression without buckling or wicking together during drying
[29]. The crystallographic structure of the NWs was analyzed using
X-ray diffraction (XRD), and the XRD pattern in FIG. 1b shows the
NWs are BaTiO.sub.3 (JCPDS number 5-0626). The HRTEM image of the
as-prepared sodium titanate NW precursors showed a single
crystalline structure, as shown in FIG. 1c, which is transforms
into a single crystalline BaTiO.sub.3 NW after a second
hydrothermal reaction containing barium ions as shown in FIG. 1d.
To retain the vertically aligned morphology of the precursor sodium
titanate NW arrays, the concentration of the barium (Ba.sup.2+)
ions used in the second hydrothermal treatment is very low.
Structural transformation from sodium titanate NW precursor to a
single crystal BaTiO.sub.3 NW results by the diffusion of the
barium (Ba.sup.2+) ions into the sodium titanate NWs from the
temperature assisted hydrothermal ion-exchange reaction [30,
31].
[0036] Sensor are fabricated from the vertically aligned
BaTiO.sub.3 NW arrays by first removing the NW arrays from the
oxidized titanium substrate on which they were grown to a
borosilicate glass substrate. The NW arrays are released from the
oxidized growth substrate by immersing in dilute HCl solution. The
NW arrays are bonded to the glass substrate using silver epoxy,
which acted as a bottom electrode, and a thin solder foil
(Sn60Pb40) was applied to the top surface of the NWs, which forms
the top electrode. The NW arrays with the solder foil were heated
to 150.degree. C. for 1 h to improve bonding with the NWs. The two
electrodes sandwiched the vertically aligned BaTiO.sub.3 NW arrays,
with the glass acting as the substrate for handling and mounting to
the excitation source as shown in the schematic in FIG. 3. After
fabrication, the BaTiO.sub.3 NW arrays were polled by applying a DC
field of .about.75 kVcm.sup.-1 across the two electrodes of the
sensor for 12 h. The denser layer of BaTiO.sub.3 NWs near the base
electrode assists to insulate the sandwich structure from breakdown
during the poling process [32, 33]. High-voltage poling is
performed to ensure the dipoles in the BaTiO.sub.3 NW arrays align
in the electric field direction, which is normal to the plane of
the two electrodes and along the orientation of the NWs. Poling is
not required for non-ferroelectric piezoceramic materials such as
ZnO; however, it is critical for the piezoelectric function of the
single-crystal BaTiO.sub.3 NWs.
[0037] Acceleration is determined by mounting the sensor to a
vibrating surface and then measuring the piezoelectric potential
formed due to the dynamic stress resulting from the inertia of the
solder on the NWs' top surface (FIG. 4b). When the stress applied
is constant or removed from the NWs, the accumulated charge is
dissipated and the resulting piezoelectric voltage diminishes, and
the piezoelectric material is limited to dynamic sensing
applications. The generated piezoelectric voltage depends on the
direction, amplitude, and frequency of the stress that is applied
on the sensor, having the dynamic characteristics of a linear
system. The sensor detects the inertia from the top electrode that
imposes the time varying compressive and tensile stress on the
vertically aligned BaTiO.sub.3 NW arrays that is needed for
sensing. The stress on the NWs is proportional to the acceleration
of the base and, therefore, the NWs produce a voltage proportional
to the acceleration of the device.
[0038] The open-circuit piezoelectric voltage output is of the
BaTiO.sub.4 devices can be measured using a high impedance voltage
follower (1 T.OMEGA.), where test measurements are advantageously
performed inside a grounded faraday cage to eliminate the effects
of extraneous noise on the sensor output voltage. A voltage
follower such as the LTC6240CS8 used for exemplary devices can use
its very low-input bias current (0.2 pA) to function as an
efficient voltage measuring the interface circuit for the
piezoelectric NW sensor. Placing the voltage follower is near the
sensor minimizes leakage current during measurement from a sensor
source, where high input impedance (low capacitance) counteracts
parasitic capacitance that can adversely affect the sensor's actual
sensitivity. The grounded faraday cage shields electromagnetic
interference to improve the signal-to-noise ratio of the sensor.
Therefore, there is little detrimental extrinsic noise at the input
and output points to affect the sensor's linear
characteristics.
[0039] The dynamic characteristics of the novel compression-type
BaTiO.sub.3 NW-based accelerometer are described using the
frequency response function (FRF). The FRF defines the relative
magnitude and phase between the reference sensor (PCB 352C22),
which produces an accurate measure of the input acceleration acting
on the device, and the output piezoelectric voltage of the NW
sensor. These characteristics and the sensitivity define the
ultimate performance of the sensor. Linearity and validity of the
sensor's measurement are evaluated using the coherence function,
which represents the degree of linearity between the input base
acceleration (instrumentation-grade accelerometer) and the output
piezoelectric voltage of the NW sensor [34, 35]. The coherence is a
nonlinear function with values from 0 to 1, with 1 defining a
perfect linear relationship between the two signals permitting an
accurate measure of the input [35]. The coherence drops below unity
in the presence of noise, non-linearity in the measured
oscillations, or spurious frequencies in the output. A stationary
Gaussian white noise signal is used for excitation of the base, due
to its spectral density being flat across the entire test frequency
that spans up to 10 kHz.
[0040] The thin film of solder that is attached to the top of the
NWs as a top electrode provides the mass (16 mg) for acceleration
sensing and is utilized in two different settings to analyze the
sensing behavior of the BaTiO.sub.3 NWs. The solder is heated to
form a stable rigid contact with the NWs, resulting in a flat band
magnitude response in FRF (FIG. 4a) of up to 10 kHz with only a
slight phase variation (FIG. 4b). The flat band response region is
essential to characterize the sensor's operating frequency range
and is observed due to the resonant frequency of the sensor device
falling above the bandwidth of the excitation. The coherence
function is very strong, reaching unity across the entire test
frequency range, and thereby demonstrates the accurate measurement
of acceleration from the NW arrays (FIG. 4c). A mean sensitivity of
800 mVg.sup.-1 is obtained in the .+-.3 dB flat band region in the
FRF magnitude that spans across the entire frequency range where
`g` represents the acceleration due to gravity. Here the
sensitivity is lower but the operating bandwidth of the sensor
spans up to 10 kHz.
[0041] Higher mass of the top electrode decreases the resonant
frequency permitting a measurable frequency within the shaker's
bandwidth, which confirms the existence of a high-frequency
resonance of the unloaded device. By considering the NW arrays as a
spring, increased mass decreases the resonant frequency (450 Hz) as
observed in the FRF magnitude in FIG. 3d associated with 90.degree.
FRF phase shift at the resonant frequency (FIG. 4e). Added mass on
the sensor's top electrode enhances the dynamic stress level on the
sensing NWs, resulting in improved piezoelectric voltage generation
from the sensor. The FRF from the loaded sensor substantiates this
behavior, as there is a considerable increase in the magnitude,
which equates to a higher mean sensitivity of 50 mVg.sup.-1 in the
.+-.3 dB bandwidth of the sensor that spans up to 300 Hz. The
coherence is observed to be unity, confirming the accuracy of the
FRF measurement made from the loaded sensor (FIG. 40. Although
sensitivity is higher, operating bandwidth is reduced, which is a
"trade-off" in the performance of the sensor. A sine wave
excitation is performed to confirm frequency preservation. Using
input sinusoidal acceleration amplitude of 1 g, root mean square
(RMS) sensitivity is calculated, showing a peak of .about.180
mVg.sup.-1 at the resonance frequency (450 Hz) of the loaded sensor
as shown in FIG. 5a. A high mean RMS sensitivity of .about.50
mVg.sup.-1 is obtained up to 300 Hz, which correlates well with the
FRF magnitude, indicating that the loaded sensor is well suited for
accurate low frequency acceleration measurements. Moreover, the
acceleration noise floor (minimum detectable signal) from the NW
accelerometer is low (.about.0.005 g) owing to the higher
sensitivity and lower-voltage noise floor (RMS B250 mV) in the
operating bandwidth. Peak-to-peak voltage (Vpp) of .about.80,
.about.85 and .about.130 mV is obtained at 100 (FIG. 5b), 200 (FIG.
5d) and 300 Hz (FIG. 5f), respectively, for a 1 g input
acceleration sinusoidal amplitude with the corresponding detailed
shapes shown in FIG. 5c,e,g. High Vpp, of .about.340 mV, is
obtained from only 1 g acceleration amplitude from the sensor near
resonance (FIG. 5h,i). Therefore, the results recorded from the
novel BaTiO.sub.3 NW accelerometer demonstrate the potential of the
NWs to have a dual role in sensing and in power harvesting
applications.
[0042] The piezoelectric behavior of the BaTiO.sub.3 NW
accelerometer was verified by heating the accelerometer above the
Curie temperature of the BaTiO.sub.3, 120.degree. C., to relax the
orientation of the electric dipoles, which eliminates the formation
of a net charge on the sensor under stress [36]. The depoled NW
sensor was tested under white noise excitation and produced no
measurable signal in FRF and showed a loss in coherence, which
confirms the transition from tetragonal phase to cubic phase of the
BaTiO.sub.3 NWs on heating above the Curie temperature (FIG. 6a-c)
[35]. In addition, poling/depoling/repoling analysis and switching
polarity test were performed for BaTiO.sub.3 NEMS sensor to
validate piezoelectricity from ferroelectric BaTiO.sub.3 NW arrays
[37, 38]. When the poling voltage (DC field 75 kV cm.sup.-1) is
reversed in polarity to switch the polarization direction (P) of
the electric dipoles within the ferroelectric BaTiO.sub.3 NW arrays
of the same sensor with the polarization direction (P) switched in
two opposite directions, a reversal in the peaks (polarity) of the
output voltage response (V) from the same NW sensor is observed for
identical mechanical deformation when connected with same terminals
to the voltage follower, as shown in FIG. 6d-f. This demonstrates
the switchable polarization direction (P) property of ferroelectric
BaTiO.sub.3 NW arrays and its influence on output voltage response
(V) which confirms that the presence of piezoelectric property in
the ferroelectric BaTiO.sub.3 NW arrays is responsible for high
sensitivity, unity coherence and excellent linearity in a wide
operating bandwidth from the NW accelerometer that was poled before
testing.
[0043] The superior sensing performance of the ultra-long,
vertically aligned BaTiO.sub.3 NW arrays is demonstrated in
comparison to conventional ZnO NW arrays, where testing is carried
using the same procedure. A ZnO NW sensor was fabricated from ZnO
NW arrays grown on Au/Si substrate, using a seedless hydrothermal
synthesis procedure [39, 40]. The ZnO sensor is compared to the
BaTiO.sub.3 sensor with the same electrode configuration with a
thin solder foil as the top electrode but with the Au layer on a Si
substrate acting as the bottom electrode. The ZnO sensor loaded
with the same proof mass as the loaded BaTiO.sub.3 sensor excited
with white Gaussian noise to characterize the FRF displays a
resonant magnitude peak at 500 Hz associated with the 90.degree.
phase shift (FIG. 7a,b). The coherence function determined to be
unity from 100 to 800 Hz is observed to drop below unity owing to
weak piezoelectric voltage response at frequencies above 800 Hz, as
shown in FIG. 7c. The RMS sensitivity calculated from 1 g amplitude
sinusoidal acceleration input at frequencies ranging from 100 to
1,000 Hz agrees well with the FRF magnitude as shown in FIG. 7d.
The mean sensitivity in the .+-.3 dB flat region that spanned up to
350 Hz was equivalent to .about.2.54 mVg.sup.-1. Vpp from the ZnO
sensor is only .about.3.8 mV, with a maximum Vpp of .about.22 mV at
the resonant frequency of 500 Hz as shown in FIG. 7e,g with
corresponding detailed shape in FIG. 7f,h, respectively. The
measured .+-.3 dB flat band sensitivity from ZnO sensor of
.about.2.54 mVg.sup.-1 is lower than the flat band sensitivity
recorded from the novel BaTiO.sub.3 sensor of .about.50 mVg.sup.-1,
thus demonstrating the superior sensing performance of the
ultra-long BaTiO.sub.3 NW arrays, as shown clearly in the magnitude
comparison plot of FRF with frequency axis in a log scale (FIG.
7i).
[0044] According to an embodiment of the invention, aligned
BaTiO.sub.3 NW arrays as vertically aligned BaTiO.sub.3 nanowire
(NW) arrays are formed directly on a conductive fluorine doped tin
oxide (FTO) glass with a NW lengths of .about.1 .mu.m and an aspect
ratio of .about.12. The aligned array formed by these nanowires
exhibit higher strains when compared to the bulk BaTiO.sub.3 and
the aligned 1 .mu.m BaTiO.sub.3 NW arrays display enhanced
piezoelectric energy conversion capabilities [20, 21]. The 1 .mu.m
BaTiO.sub.3 NW arrays have superior power harvesting performance
over conventional ZnO NW arrays which can be driven by local
variations in acceleration from a vibrating source. The NEMS energy
harvester has resonance below 1 kHz for efficient energy harvesting
of ambient mechanical vibrations, which typically reside in the 1
Hz to 1 kHz range. High performance NEMS energy harvesters using
aligned arrays of BaTiO.sub.3 NWs that efficiently harvest
mechanical vibrations when integrated with a suitable low frequency
resonating structure are achieved.
[0045] Vertically aligned BaTiO.sub.3 NW arrays are grown directly
on conductive FTO glass substrates using a two-step hydrothermal
process. BaTiO.sub.3 NW arrays are synthesized by reaction between
Ba.sup.2+ ions in solution with precursor single crystal vertically
aligned titanium dioxide (TiO.sub.2) NW arrays. X-ray diffraction
(XRD) analysis of precursor TiO.sub.2 NW arrays on FTO glass used
for conversion to BaTiO.sub.3 is observed to match a rutile phase.
For example, BaTiO.sub.3 NWs having a length of .about.1 .mu.m and
a diameter of .about.90 nm are formed by the ion exchange reaction
with preservation of the morphology of precursor TiO.sub.2 NW
arrays. A detailed analysis of the microstructure of the aligned
array of NWs by scanning electron microscope (SEM) is shown in FIG.
8a. The crystallographic structure of the nanowires by X-ray
diffraction (XRD) and the XRD pattern in FIG. 8b shows that the NWs
are BaTiO.sub.3 (JCPDS No. 5-0626). A high resolution transmission
electron microscopy (HRTEM) image of the as prepared single
crystalline BaTiO.sub.3 NWs with clear crystal lattice fringes is
shown in FIG. 8c. For power harvesting performance comparison,
aligned ZnO NW arrays with a length of .about.1 .mu.m and diameter
of .about.100 nm were grown on a conductive FTO glass using a low
temperature solution-growth approach as shown in the SEM image in
FIG. 8d [41, 42]. Both the BaTiO.sub.3 NW arrays and ZnO NW arrays
grown on conductive FTO glass were sputter coated with 1 nm gold
(Au) layer on the top surface prior to applying them as NEMS energy
harvesters.
[0046] The exemplary BaTiO.sub.3 based NEMS energy harvester has a
strip of indium foil bonded to the non-conductive edge of the FTO
glass substrate that is formed as a beam to make contact with the
top of the as-synthesized vertically aligned BaTiO.sub.3 NW arrays
where the indium foil serves as the top electrode to the conductive
FTO glass bottom electrode. This configuration allows the NEMS
energy harvesting device to achieve a low resonant frequency by
capitalizing upon the beam's resonance rather than the NW's
resonance. A ZnO NW NEMS energy harvester fabrication was formed to
use the indium beam technique to contact the ZnO NW arrays grown on
FTO glass, as shown in the schematic of the configuration in FIG.
9a. An Au layer (work function of .about.5.1-5.47 eV) sputter
coated on as-synthesized ZnO nanowires (electron affinity of
.about.4.1-4.3 eV) prior to device fabrication helped to form a
Schottky barrier between indium (top electrode) and ZnO NW arrays
[9]. It is important to form a Schottky barrier to efficiently
extract piezoelectric charge from the nanowire's tip and to block
electron flow through the interface from the metal side to the
semiconducting nanowires side. The surface area of the indium top
electrode has a dimension of .about.5.times.4 mm.sup.2 in the ZnO
NW and BaTiO.sub.3 NW NEMS energy harvesters to permit power
density comparison. The BaTiO.sub.3 based NEMS energy harvester is
poled with high DC electric field (.about.120 kV/cm) for 24 hours
to ensure the dipoles of the single crystal BaTiO.sub.3 NWs align
in the electric field direction normal to the plane of the two
electrodes and oriented with the NWs [43]. High voltage poling
between the two electrodes is essential for piezoelectric function
of ferroelectric BaTiO.sub.3 NW arrays, but is not required for ZnO
NW arrays as they possess intrinsic spontaneous polarization. The
exemplary NW energy harvesters were excited through base vibration
generated by a permanent magnet shaker while the input base
acceleration was accurately measured using a reference shear
accelerometer (PCB 352C22).
[0047] Compressive and tensile stress generated from the inertial
force of the vibrating indium beam on the BaTiO.sub.3 and ZnO NW
arrays result in charge generation from the direct piezoelectric
effect, which develops an alternating potential difference across
the two electrodes, as shown schematically in FIG. 9b. This is the
working principle of vibration-driven NEMS energy harvesters. The
electrical equivalent circuit for the NEMS energy harvester is
shown as an inset in FIG. 9 where the piezoelectric voltage, V, is
induced from the vibration acceleration in series with the inherent
capacitance of the source, C.sub.p, and piezoelectric leakage
resistance, R.sub.p connected in parallel. The voltage, VL, is
measured across the load resistor, RL, to calculate the AC power
dissipation. Here, the piezoelectric leakage resistance,
R.sub.p=X.sub.C.sup.2/R.sub.S, is not taken into account as it is
normally two orders of magnitude higher than the impedance (ZS) of
the source capacitance (in pF range) where RS is the series
resistance and X.sub.C=1/j.omega.Cp is the reactance of the
capacitor. As a result, the effect of leakage resistance on the
overall impedance is negligible. The source capacitance (Cp), which
is the capacitance measured between the two electrodes of the
exemplary NEMS energy harvester, was performed using an Agilent
E4980A 5 LCR meter. The impedance measurements showing the series
resistance (RS) and the reactance (XC=1/j.omega.Cp) of the
exemplary BaTiO.sub.3 NEMS energy harvester and ZnO NEMS energy 10
harvester were performed. The impedance contributed by the
capacitance (Z.sub.S=1/(.omega..sub.nC.sub.p)) of the piezoelectric
NWs at resonant frequency, .omega..sub.n, is matched using purely
resistive loads to determine the AC power [44].
[0048] All measurements on exemplary devices were performed inside
a grounded faraday cage to reduce the effects of extrinsic
power-line noise (60 Hz harmonic noise) on the NEMS output voltage.
The output voltage was measured using a high impedance (1 T.OMEGA.)
voltage follower with unity gain, and the short circuit current was
measured using a high speed electrometer (Keithley 6514). The
dynamic response analysis of the NEMS energy harvester was
performed using the frequency response function (FRF)
characterization that gives the relative magnitude and phase of the
ratio of the response signal from the NW arrays to the stimulus
input base acceleration. Firstly, the FRF between the open circuit
output voltage from the NEMS energy harvester and the input base
acceleration measured by the reference shear accelerometer is
examined to determine the open circuit resonant frequency when the
harvester is excited with burst chirp and white Gaussian noise
signals from shaker that have flat power spectral density in the
test frequency range of up to 1 kHz. The FRF between the short
circuit current from the NW arrays and the input base acceleration
was characterized using burst chirp and white noise excitation.
Piezoelectric open circuit voltage (VOC) and short circuit current
(NC) is at the maximum at the resonant frequency, which corresponds
to the frequency where the indium beam generates maximum strain on
the NW arrays of the NEMS energy harvester. At the resonant
frequency, root mean square (RMS) voltage (VL) measured across the
external resistive load (RL) can be used to determine the AC power
(PL) experimentally from the NEMS energy harvester as shown in the
Eqn. 1 [45] Peak AC power is dissipated when the external resistive
load (RL) is matched with the source impedance (ZS) as per maximum
power transfer theorem.
P L = I L ( RMS ) 2 R L = { V ( RMS ) Z S + R L ) 2 RL = V L 2 (
RMS ) R L ( 1 ) ##EQU00002##
[0049] The capacitance of the exemplary BaTiO.sub.3 NW energy
harvester measured by the LCR meter is 8.21 pF at 1 kHz. The open
circuit voltage VOC FRF characterized from burst chirp voltage
response after poling produced a resonant peak at .about.160 Hz, as
shown in FIG. 10a. The sinusoidal excitation at resonant frequency
yielded a high peak to peak voltage Vpp of .about.623 mV from 1 g
RMS base acceleration input as shown in FIG. 10b. The high voltage
response is due to the high dynamic strain on the NW arrays from
the beam at resonance inducing an alternating piezoelectric charge
accumulation at the two electrodes. It is well known that when
measuring the open circuit voltage with a voltage buffer amplifier
with high input impedance (1 T.OMEGA.), the current is at its
minimum (theoretically zero) so the AC power is virtually zero.
[0050] The short circuit current (ISC) FRF from the exemplary
BaTiO.sub.3 NW NEMS energy harvester was characterized by using
burst chirp excitation input with ISC magnitude peak at resonant
frequency of .about.160 Hz associated with a 90.degree. phase
change, as shown in FIG. 10c. The ISC response to chirp input was
recorded. High ISC from the NW arrays were observed by exciting
with a sine wave at resonant frequency (.about.160 Hz) with a peak
to peak current (Ipp) of .about.1.8 nA recorded from base
acceleration input of 1 g RMS (FIG. 10d). High ISC is directly
proportional to the piezoelectric charge production from the poled
ferroelectric BaTiO.sub.3 NW arrays when increased strain is
applied by the resonating indium beam structure. In short circuit
electrical boundary conditions, voltage is theoretically zero and
the AC power is zero.
[0051] Capacitance of a comparable ZnO NW NEMS energy harvester was
measured by the LCR meter to be 8.72 pF at 1 kHz. A direct
vibration excitation experiment was carried out on the as
fabricated ZnO based NEMS energy harvester to investigate its
performance compared with the BaTiO.sub.3 NW NEMS energy harvester.
The VOC FRF and ISC FRF of the ZnO NW energy harvester were
analyzed by triggering white noise and burst chirp excitation with
a resonant magnitude peak observed at .about.190 Hz as shown in
FIG. 11a-b. The ZnO NW NEMS energy harvester's peak to peak open
circuit voltage Vpp and peak to peak short circuit current Ipp from
sine wave excitation at resonance were measured 5 to be .about.85
mV and .about.0.316 nA from the 1 g RMS input acceleration as shown
in FIG. 11c-d. The voltage and current levels produced by the ZnO
NW NEMS energy harvester are much lower than the BaTiO.sub.3 NW
NEMS energy harvester due to ZnO's lower coupling coefficient.
[0052] The AC power from the energy harvester is calculated by
measuring the voltage, VL, across several load resistors, RL,
ranging from 1 M.OMEGA. to 500 M. The source impedance, ZS, of
BaTiO.sub.3 NW arrays with capacitance of .about.8.21 pF at natural
frequency (.omega.n=2*.pi.*fn where fn=.about.160 Hz) was evaluated
to be .about.121 M.OMEGA.. The AC power from BaTiO.sub.3 NW NEMS
energy harvester increased rapidly as RL increases up to 50
M.OMEGA. reaching a uniform peak value of .about.125.5 pW at the
optimal RL of 120 M.OMEGA. and then reduces as RL is traced up to
500 M.OMEGA. since voltage across the increasing load resistors
starts saturating towards the VOC. The peak power density across
the optimal RL was calculated for the exemplary BaTiO.sub.3 NW NEMS
energy harvester to be .about.6.27 .mu.W/cc from 1 g RMS base
acceleration (FIG. 12a). For the comparative ZnO NW NEMS energy
harvester, the source impedance, ZS, at resonant frequency
(.omega.n=2*.pi.*fn where fn=.about.190 Hz) was measured to be
.about.96 M.OMEGA. and the peak AC power dissipated across the
optimal RL of 50 M.OMEGA. is only .about.8 pW from the same input
base acceleration of 1 g RMS. The peak power density from the ZnO
based NEMS energy harvester was calculated to be .about.0.4
.mu.W/cc as shown clearly in FIG. 12b. This power density for the
BaTiO.sub.3 NW NEMS energy harvester is .about.16 times lower than
the peak power density (.about.6.27 .mu.W/cc) recorded from
BaTiO.sub.3 based NEMS energy harvester driven with the same base
acceleration of 1 g RMS and, hence, illustrates the superior
vibrational energy harvesting performance from BaTiO.sub.3 NW
arrays. The voltage magnitude of the FRF from the exemplary
BaTiO.sub.3 NEMS was characterized across several load resistors
and the magnitude peak at resonant frequency was found to increase
with the increase in the load resistors with the maximum peak being
that of the open circuit voltage FRF (1 T.OMEGA.) as shown in FIG.
12c. Moreover, the magnitude of the voltage FRF from the
comparative ZnO NEMS was characterized across several load
resistors (RL) to demonstrate the similar increase in magnitude
with the highest peak at resonance from the VOC FRF (FIG. 10d). The
voltage VL across the optimal RL with RMS value of .about.123 mV
provides the maximum peak power density from BaTiO.sub.3 energy
harvester. For the comparative ZnO NEMS, the voltage 5 VL across
optimal RL has a RMS value of .about.20.2 mV to provide the lower
peak power density as compared to the exemplary BaTiO.sub.3 NEMS
from the same base acceleration input. In addition, a switching
polarity test was performed that confirms that the measured signal
responses from the NEMS energy harvester were generated by the
nanowires. The power density observed for the exemplary BaTiO.sub.3
NEMS energy harvester (.about.6.27 .mu.W/cc) is comparable to
several meso-scale and MEMS-scale resonant cantilever based energy
harvesters driven by base vibration. {44-46}
[0053] An exemplary BaTiO.sub.3 NWs sensor with a melted Indium top
electrode outputs piezo-potential when subjected to vibration due
to stress induced on the BaTiO.sub.3 NWs arrays from the mass of
the top electrode. The exemplary BaTiO.sub.3 NWs sensor was excited
with white Gaussian noise to characterize its frequency response. A
Hanning window was used to reduce the power leakage at frequencies
adjacent to the correct frequencies. The reference accelerometer
measured the input white noise acceleration imposed on the
exemplary BaTiO.sub.3 NWs sensor and, since the piezoelectric
phenomenon is linear when subjected to stress levels below
threshold, the output voltage from the exemplary BaTiO.sub.3 NWs
sensor was observed as white noise. A frequency response function
(FRF) of the exemplary BaTiO.sub.3 NWs sensor shows higher voltage
generating performance at low frequencies in terms of the input
acceleration followed by a flat band region from 500 Hz to 2500 Hz
where the mean is 880 .mu.V/gas shown in FIG. 13. The ordinary
coherence function between the input acceleration measured from the
accelerometer and the output voltage from the BaTiO.sub.3 NWs
sensor is very strong, reaching unity over a wide frequency range.
The coherence plot validated an excellent linear relationship of
the sensor with minimal spectral leakage. The initial loss in
coherence at frequencies below 10 Hz is due to the contributions to
the output voltage from the inputs other than the measured input
acceleration from the reference accelerometer. This coherence loss
is also due to extraneous noise present in the input acceleration
measurement at low frequencies, as shown in FIG. 13.
[0054] Sine wave excitation of the BaTiO.sub.3 NWs sensor at low
frequencies, below 200 hz, resulted in high sensitivity in terms of
acceleration with the sensitivity at 100 Hz being the highest at
6.67 mV/g, as shown in FIG. 14. The reason for this higher
sensitivity at low frequencies is attributed to better contact
between the melted Indium top electrode and the array of vertically
aligned BaTiO.sub.3 NWs resulting in higher voltage generation with
1 g amplitude acceleration.
[0055] Frequency preservation was observed from sine wave
excitation over a wide frequency range from the exemplary
BaTiO.sub.3 NWs sensor, which illustrates the excellent performance
characteristic of the BaTiO.sub.3 NWs sensor. High sensitivities
obtained at low frequencies from the exemplary BaTiO.sub.3 NWs
sensor permits enhance voltage generation for energy harvesting
applications due to the low frequency resonance to the BaTiO.sub.3
NWs arrays.
Methods and Materials
[0056] Ultra-long aligned arrays of BaTiO.sub.3 nanowires (NWs)
were obtained and the detailed analysis of their structure was
performed using JEOL 6335F scanning electron microscope (SEM), as
shown in FIG. 15a. The ultra-long BaTiO.sub.3 NWs arrays span a
length of 40 .mu.m and diameter of the individual NWs are 600 nm,
respectively. The crystallographic structure of the nanowires was
analyzed using the X-ray diffractometer and the XRD pattern in FIG.
15b shows the BaTiO.sub.3 peaks (JCPDS No. 5-0626). Sensors were
fabricated using Indium as a top electrode and silver epoxy as the
bottom electrode, which sandwiched the vertically aligned array of
BaTiO.sub.3 NWs with SiO.sub.2 transparent glass as the
non-conductive substrate. The fabricated sensor image and the
schematic diagram of the voltage generating BaTiO.sub.3 NWs sensor
are shown in FIG. 15c and FIG. 15d, respectively. Indium, as the
top electrode, is a soft malleable metal with favorable properties,
as shown in Table 1, below.
TABLE-US-00001 TABLE 1 Properties of Indium Foil Melting Young's
Density Point Modulus Thickness g/cm3 .degree. C. GPa Mm 7.31
156.59 11 0.127
[0057] Two different Indium top electrode sensor configurations
were fabricated to investigate the sensing and energy harvesting
capability of the BaTiO.sub.3 NWs arrays. In the first
configuration, the BaTiO.sub.3 NWs sensor was developed with melted
Indium as the top electrode with an indium electrode area of
6.times.5 mm.sup.2 and seismic mass of 18.53 mg and a BaTiO.sub.3
area of 4.times.5 mm.sup.2. The melted Indium electrode also
provides the proof mass for vibration acceleration sensing,
applying the stress on the vertically aligned array of NWs to
generate voltage from the piezoelectric effect.
[0058] In a second configuration, the BaTiO.sub.3 NWs based device
was fabricated for energy harvesting applications with the Indium
top electrode acting as a cantilever beam contacting the
BaTiO.sub.3 NWs array, whose properties are shown in Table 2.
Capacitance of the sensor was measured to validate the NWs contact
by the two electrodes. Additional tip mass on the cantilever beam
was added to further modify the resonant frequency shift and to
increase the amount of stress induced on the BaTiO.sub.3 NWs for
realizing enhanced voltage generation.
TABLE-US-00002 TABLE 2 Properties of BaTiO.sub.3 Sensor with
cantilever Indium beam acting as top electrode Beam Dimension = 10
.times. 4 mm.sup.2 BaTiO.sub.3 NWs area located below beam = 4
.times. 5 mm.sup.2 Effective Mass = 0.23 * Mbeam = 8.54 mg
Stiffness `Keff` of beam = 223.82 N/m Natural Frequency of beam
`fn` = 223.82 Hz Additional Tip Mass = 150 mg
[0059] After completion of successful fabrication, conventional
poling of the BaTiO.sub.3 NWs based sensor was carried out by
supplying 6.25 KV/cm DC voltage across the two electrodes of the
sensor for 12 hours. High voltage poling is performed at room
temperature to ensure the dipoles of the BaTiO.sub.3 NWs arrays
align in the electric field direction which is normal to the plane
of the two electrode along the orientation of the NWs. Application
of compressive stress on the vertically aligned BaTiO.sub.3 NWs
array results in charge generation from direct piezoelectric effect
and thereby, a potential difference develops across the two
electrodes.
[0060] The experimental setup for piezoelectric BaTiO.sub.3 NWs
based sensor characterization is shown in FIG. 16. Acceleration
input for the BaTiO.sub.3 NWs based sensor was provided by a
miniature electromagnetic shaker (Labworks, Inc. ET-132) driven by
a power amplifier (Labworks, Inc.) from the output signal generated
using a DAQ board (NI USB 4431). The BaTiO.sub.3 NWs sensor was
mounted on the base of the shaker beside a reference shear
accelerometer (PCB 352C22) that can measure the input acceleration
applied to the sensor. The capacitance of the BaTiO.sub.3 NWs
sensor was measured using a LCR meter (Agilent E4980A). A voltage
follower/buffer amplifier with unity gain from Linear Technologies
(LTC6240CS8) provided high input resistance (1 T.OMEGA.) and low
noise (Voltage noise <10 nV/ Hz). To reduce leakage current
during measurement from the BaTiO.sub.3 NWs sensor source, which
has high input impedance (low capacitance: 3.60 pF) and to
counteract the effects of parasitic capacitance that would
adversely affect the sensor's actual sensitivity, the voltage
follower was placed in the near proximity of the sensor and
connected using short low noise cable to the sensor source. The
grounded faraday cage, which acts as a noise shield from EMI
surrounds the sensor and voltage follower setup and improves the
signal to noise ratio of the sensor, assures no extraneous noise at
input and output points that affect the sensor's linear
characteristics. All signals were generated and acquired through
DAQ board (NI USB 4431) operated using NI SignalExpress software
from PC. All signals were evaluated using an oscilloscope
(Tektronix, DPO 3014 Digital Phosphor Oscilloscope).
Synthesis of Ultra Long Vertically aligned BaTiO.sub.3 Nanowire
Arrays
[0061] The hydrothermal method for the synthesis of aligned
BaTiO.sub.3 nanowire (NW) arrays since it is low cost, scalable and
enables control over the resulting nanowire morphology by tuning
the reaction parameters. Hydrothermally a synthesis began with
providing ultra-long (.about.45 .mu.m) vertically aligned single
crystal sodium titanate NW array on an oxidized Ti foil and using
that array as precursor for conversion to BaTiO.sub.3 NWs while
preserving the NW form. The Ti foil (MTI Corporation; 99.9%, 100 mm
thick) was cleaned via sonication for 30 min in a bath with
acetone, 2-proponal and deionized water (1:1:1) solution. It was
then oxidized in a furnace at 750.degree. C. for 8 h. The resulting
oxidized substrate was immersed in a Teflon-lined autoclave filled
with 37.5 ml of 12M NaOH solution (Fill Factor: 50%, 97% Alfa
Aesar) and sealed in a high-pressure reactor. The reactor was
placed in an oven at 210.degree. C. for 8 h to result in the
controlled growth of the sodium titanate NW arrays. After cooling
the reactor, the resulting structure was washed four times using
deionized water and ethanol, and allowed to dry at room
temperature. The sodium titanate nanostructures have strong
ion-exchange properties due to an open structure with titanium
octahedra (TiO.sub.6) units. Reaction parameters for the synthesis
of sodium titanate NW arrays were optimized to obtain nanowires
that displayed a sufficient aspect ratio (.about.75) yet does not
wick together from capillary forces during drying. The single
crystal sodium titanate NW arrays were converted to BaTiO.sub.3 NW
arrays using a second hydrothermal reaction with aqueous barium
hydroxide solution in a high pressure reactor at temperatures
between 150-250.degree. C. The dried substrate with a sodium
titanate NW array was immersed in a Teflon-lined autoclave
containing a solution form from barium hydroxide octahydrate
(Ba(OH).sub.2 8H.sub.2O) (Fill Factor: 33%, Sigma-Aldrich), placed
under an argon atmosphere and sealed in the reactor. During this
second hydrothermal reaction, the Ba ions diffuse into sodium
titanate NWs with transformation of NaTiO.sub.3 NWs into
BaTiO.sub.3 NWs. The reactor was placed in an oven at temperatures
between 150 and 250.degree. C. After cooling, the reactor the
substrate and NWs were removed from the oven, the substrate and NWs
washed with dilute nitric acid, deionized water, and ethanol, and
dried to yield the BaTiO.sub.3 NW arrays. Step by step
characterization was performed using X-ray diffraction (XRD) to
identify the material's crystal structure, starting from a pure Ti
foil substrate to formed sodium titanate nanowire precursors, which
yielded the BaTiO.sub.3 NW arrays (FIG. 2). The XRD analysis on the
crystal structure of sodium titanate nanowire arrays showed a
majority of the peaks that matched with sodium hexatitanate
(Na.sub.2Ti.sub.6O.sub.13) (JCPDS 31-1329) and some peaks from
sodium trititanate (Na.sub.2Ti.sub.3O.sub.7) (JCPDS 73-1398). The
crystal structure and lattice parameters of individual nanowires
were examined using high resolution transmission electron
microscopy (HRTEM) where the transformation from single crystal
sodium titanate NW precursor to single crystal BaTiO.sub.3 NWs upon
Ba.sup.2+ ion exchange was observed
[0062] Characterization of BaTiO.sub.3 NW Arrays.
[0063] The morphological properties, which include the orientation,
dimension and crystalline structure of the as-prepared BaTiO.sub.3
NW arrays, were examined using a JEOL 6335F SEM and an XRD equipped
with a curved position-sensitive detector (CPS120, Inel) with Cu Ka
radiation. The crystal structure, lattice parameter and diffraction
pattern of individual NWs were studied using the FEI Tecnai F30
(Philips) HRTEM that operates at 300 kV accelerating voltage
provided by field-emission electron gun.
Experimental Characterization of BaTiO.sub.3 NW Accelerometer
[0064] An accelerometer composed of the as-synthesized and dried
ultra-long aligned BaTiO.sub.3 NW arrays was fabricated by
transferring the BaTiO.sub.3 NW arrays to a borosilicate glass
substrate and using a uniform layer of silver epoxy as the bottom
electrode followed by the application of a thin film of solder
(Sn60Pb40) to the nanowire array surface, which acted as the second
electrode. Borosilicate glass (1 mm thick) was used as the base
substrate in the fabrication of the sensor by cutting a square (1
cm.sup.2) using a laser ablator (Epilog Laser). The as-synthesized
and dried film of BaTiO.sub.3 NW arrays (6.times.6 mm.sup.2) was
removed from the oxidized Ti foil and bonded to the borosilicate
glass with a uniform thin layer of conductive silver epoxy (MG
Chemicals). The silver epoxy was cured at 70.degree. C. for 10 min
to enhance the adhesion and served as the bottom electrode. A thin
solder film (4.times.4 mm.sup.2) formed the top electrode being
overlaid on top of the NW arrays and heated to 150.degree. C. to
improve the bonding with the NWs' top surface. Signal wires were
attached to the bottom and top electrode using silver epoxy. After
fabrication, the capacitance (C.sub.p) of the NW sensor was
measured to be 2.94 pF using a precision LCR meter (Agilent
E4980A), which validates the electrical contact made with the
nanowires and the absence of resistive contact between the two
electrodes. The insulation resistance (Rn) of the NW sensor is
typically two orders of magnitude higher than the impedance of the
source capacitance well into the G.OMEGA. range influencing the
source time constant (.tau.=R.sub.p*C.sub.p). The high insulation
resistance (R.sub.p) reduces the low cut-off (corner) frequency
limit (f.sub.L=1/(2*.pi.*.tau.)) of the piezoelectric NW sensor.
The low cut-off frequency limit precludes the piezoelectric sensor
performance for true static measurements. The sensor was poled by
applying a high DC field of =75 kVcm.sup.-1 (320 V) to align the
dipoles along the orientation of the nanowires, ensuring that any
dynamic strain applied on the NW results in net charge generation.
High-voltage poling was performed by maintaining strong electric
field from a DC voltage supply (TREK, 677A Supply/Amplifier) across
the signal wires of NW accelerometer for 12 h. The poling process
establishes the direction of polarization along the vertical
orientation of the NW arrays, and that the piezoelectric coupling
property in the poled axis is responsible for the voltage response
from the NW accelerometer.
TABLE-US-00003 TABLE 3 Properties of BaTiO.sub.3 Sensor with Solder
acting as top electrode Solder Dimension = 4 .times. 4 (mm).sup.2
BaTiO.sub.3 NWs area located below Solder = 6 .times. 6 (mm).sup.2
Seismic mass (Solder) = 16 mg
TABLE-US-00004 TABLE 4 Properties of ZnO Sensor with Solder acting
as top electrode Solder Dimension = 4 .times. 4 (mm).sup.2 Seismic
mass (Solder) = 16 mg
[0065] A poled functional sensor was tested by inducing vibration
from a miniature permanent magnet shaker and the true input
acceleration supplied to the base of the sensor was measured using
a shear accelerometer (PCB352C22) that has a sensitivity of 8.81
mV/g. The open circuit voltage measurements were performed using a
unity gain voltage follower (LTC6240 CS8) having 1 T.OMEGA. input
impedance and capacitance of 3.5 pF. The cut-off frequency is lower
for the voltage follower than the NW accelerometer and, therefore,
does not attenuate the piezoelectric voltage response at low
frequencies. Moreover, a voltage follower was used rather than a
charge amplifier to offer a wider range of working frequency
without imposing a reduction in the resonant frequency since the
stiffness of the piezoelectric material is maximum when the
electrical boundary conditions are open (FIG. 17). Output voltage
from the voltage follower is susceptible to cable capacitance,
which may adversely affect the sensor's actual sensitivity.
Therefore, the sensor is connected to the voltage follower using
short low noise cables with a grounded faraday cage surrounding the
entire connection setup to reduce the noise floor (FIG. 18b-c). The
voltage noise floor of the sensor is estimated by the voltage
spectral density acquired using NI DAQ system from the voltage
follower connected to the undisturbed BaTiO.sub.3 NW sensor (FIG.
19a). By considering the true base acceleration measured by the
reference accelerometer as the input to the NW sensor and the open
circuit voltage measured by the unity gain voltage follower as the
output, the NW sensor was characterized as a nanoelectromechanical
system (NEMS) accelerometer.
Performance Evaluation and Validation of the Piezoelectric
BaTiO.sub.3 NW Accelerometer.
[0066] The frequency response function (FRF) and coherence function
(.gamma..sup.2(f)) were used to evaluate the sensitivity, linearity
and operating bandwidth of the NW sensor from white noise
excitation that has a flat spectral density in the frequency range
of 10 kHz (FIG. S4b). 10 kHz is chosen as the test frequency range
since it was the maximum operating bandwidth of the electromagnetic
shaker utilized as the vibration source in the experiments. A
loaded NW sensor that having an added mass on the top electrode
demonstrated a resonance at 450 Hz and a high sensitivity of 50
mV/g in the 3 dB flat band region that spanned up to 300 Hz under
both white noise excitation as well as sine wave excitation.
Comparison of the detailed shape of the input sinusoidal
acceleration of 1 g amplitude and the output piezoelectric voltage
generated by the NW sensor showed an in-phase relationship at 100
Hz and a 90.degree. out of phase relationship at 450 Hz which
agrees well with the phase curve in the FRF observed from white
noise excitation (FIG. 20a-b). Poled functional NEMS sensors
composed of annealed BaTiO.sub.3 nanowire arrays that were annealed
at 700.degree. C. for 1 hour to remove hydroxyl defects were also
tested under vibration excitation but the results recorded did not
show an increase in the performance of the sensor (FIG. 21).
[0067] Verification of the piezoelectric behavior from the loaded
NW accelerometer with a low resonant frequency was performed by
heating the accelerometer above the Curie temperature of the
BaTiO.sub.3 at 120.degree. C. to relax the orientation of the
electric dipoles, which eliminates the formation of a net charge on
the sensor under stress. The NW sensor was tested under white noise
excitation and demonstrated to produce no measurable signal with
loss in coherence which confirms the transition from tetragonal
phase to cubic phase of the BaTiO.sub.3 nanowires on heating above
Curie temperature (FIG. 22). In addition, de-poling and re-poling
frequency response function analysis from the BaTiO.sub.3 NW NEMS
sensor was performed to validate that voltage response generated by
the BaTiO.sub.3 NW arrays was due to their piezoelectric behavior
(FIG. 23), confirming that the piezoelectric property in the
ferroelectric BaTiO.sub.3 NW arrays was responsible for high
sensitivity, unity coherence, and excellent linearity in a wide
operating bandwidth from the NWs accelerometer that was poled
before testing.
Synthesis of Aligned ZnO Nanowire Arrays.
[0068] ZnO NW arrays were synthesized on Au coated Si substrate
(Exsil, Inc., .about.500 .mu.m thick). The Au/Si growth substrate
cleaned in ethanol, isopropyl alcohol and acetone (1:1:1) solution
by sonication for 10 min. The substrate was removed and rinsed in
DI water for 2 min followed by drying at 100.degree. C. for 5 min.
The Si substrate with a top Au layer was annealed at 500.degree. C.
for 5 min to enhance crystallinity. A growth solution (Fill Factor:
40%) was prepared using 20 mM zinc nitrate hexahydrate
(Zn(NO.sub.3).sub.2.6H.sub.2O, 99%, Sigma-Aldrich) and 4% vol.
ammonium hydroxide (NH.sub.3.H.sub.2O, 28-30% wt %, Ricca Chemical
Company). The Au/Si substrate was placed on top of the growth
solution to avoid precipitation of ZnO particles on the NW arrays.
Reaction was carried out at 95.degree. C. for 5 hours in a
convection oven. The resulting substrate with ZnO NW arrays on Au
surface of the Si substrate was rinsed in DI water and dried at
room temperature. The hydrothermal synthesis process was repeated
to enhance the length of ZnO NW arrays on Au/Si substrate. The
microstructure of the ZnO nanowires were characterized using
scanning electron microscopy (SEM) and a cross-sectional SEM image
of aligned ZnO NW arrays is shown in FIG. 24a.
Fabrication and Performance Evaluation of Piezoelectric ZnO NW
Accelerometer.
[0069] The NEMS sensor using the as-synthesized aligned ZnO NW
arrays on Au/Si substrate with the similar configuration were
evaluated in the manner indicated for BaTiO.sub.3 NW based
accelerometers with a thin solder film (4.times.4 mm.sup.2) mounted
on the NW arrays as top electrode. The Au layer on Si substrate,
upon which the NW arrays were synthesized, served as the bottom
electrode for the ZnO NW based NEMS sensor, avoiding any need to
transfer the NW arrays to another conductive substrate. To provide
sufficient insulation at the Au/Si substrate edge, a Dupont Kapton
polyimide film was deposited to ensure that no resistive contact
between the two electrodes occurred while attaching the signal
wires. The ZnO sensor was heated to improve the bonding between
solder and ZnO NW arrays. After fabrication, the capacitance of the
sensor was measured to be 4.38 pF using an Agilent LCR meter,
implying that there is no resistive contact between two electrodes.
The performance of the loaded ZnO NW based accelerometer was
evaluated under the same testing procedure used for BaTiO.sub.3
sensor indicated above. The resonant frequency from ZnO NW sensor
was observed near 500 Hz from FRF analysis under white noise
excitation. The coherence (.gamma..sup.2(f)) was observed to be
unity up to 800 Hz beyond which it dropped as the voltage response
got weaker. The 3 dB flat band sensitivity up to 350 Hz was
evaluated to be .about.2.5 mV/g and the RMS sensitivity correlates
well with the FRF magnitude from 1 g amplitude sinusoidal
acceleration input from 100 Hz to 1,000 Hz. The detailed plot of
the acceleration and voltage showed in-phase relationship at 100 Hz
and 300 Hz (FIG. 24b-c) and 90.degree. out of phase relationship at
the resonant frequency of 500 Hz (FIG. 24d) which agrees well with
the phase plot in the FRF analyzed from white noise excitation.
[0070] Synthesis of Aligned Short BaTiO.sub.3 NW Arrays
[0071] Synthesis of short vertically aligned BaTiO.sub.3 nanowire
(NW) arrays was performed on a conductive substrate using a
two-step hydrothermal reaction. First, the precursor TiO.sub.2
nanowire arrays were grown on conductive fluorine doped tin oxide
(FTO) glass (Pilkington, TEC7 coated, 2.2 mm thick, 7 .OMEGA./sq)
through an acidic hydrothermal reaction process [46]. Initially,
FTO glass was cut into a square dimension (10.times.10 mm.sup.2)
using a laser ablator (Epilog Laser) and was cleaned by sonication
for 30 minutes in a 1:1:1 volume ratio solution of deionized water,
acetone, and 2-propanol. After sonication, the FTO glass substrate
was rinsed with methanol and water, and placed vertically inside a
high pressure reactor containing 10 mL of deionized water, 10 mL of
hydrochloric acid (Fisher, 37%) and 1 mL of titanium isopropoxide
(Fisher, ACS). The reactor was then heated at 200.degree. C. for 3
hours. Following the first hydrothermal process, the reactor was
cooled to room temperature and the resultant FTO glass substrate
with an array of vertically aligned TiO.sub.2 nanowires was rinsed
with deionized water and dried in ambient air. The substrates were
placed into a solution containing Ba.sup.2+ ions and converted to
BaTiO.sub.3 by a second hydrothermal reaction which was carried out
at temperatures between 150.degree. C. and 240.degree. C. for 4 to
8 hours.{48-50} The Ba.sup.2+ ions from a barium hydroxide
comprising solution and temperature (150-240.degree. C.) of the ion
exchange procedure were optimized to enable shape retention of the
precursor TiO.sub.2 NW arrays during conversion resulting in an
aligned BaTiO.sub.3 NW arrays. Lastly, the samples were rinsed
again with deionized water and dried in ambient air to yield
BaTiO.sub.3 NW arrays on a conductive FTO glass substrate. The
as-synthesized BaTiO.sub.3 NW arrays were heat treated at
600.degree. C. for 30 minutes to remove any hydroxyl defects before
their use as NEMS energy harvester[47, 48].
[0072] Synthesis of Aligned ZnO NW Arrays
[0073] ZnO NW arrays were synthesized on a FTO glass substrate
(.about.10.times.10 mm.sup.2, 2.2 mm thick) using low temperature
solution growth approach for comparison to the BaTiO.sub.3 NW
arrays [41]. The FTO glass substrate was cleaned in an ethanol and
acetone (1:1) solution by sonication for 10 min, removed, and
ultrasonicated in DI water for 2 min followed by drying at
100.degree. C. for 5 min. The conductive side of the FTO glass
substrate was seeded with 2 mM zinc acetate
(Zn(O.sub.2CCH.sub.3).sub.2, Alfa) in ethanol by dip coating and
thermal decomposed at 300.degree. C. for 20 min. The growth
solution (Fill Factor: 40%) was prepared using 25 mM zinc nitrate
hexahydrate (Zn(NO.sub.3).sub.2.6H.sub.2O, 99%, Sigma-Aldrich), 25
mM hexamethylenetetramine (HMTA, Sigma-Aldrich) and 5-7 mM
polyethylenimine (PEI, Aldrich). The FTO glass was immersed on top
surface of the growth solution with the seeded conductive side
facing down so that ZnO particles did not precipitate on the NW
arrays. The reaction was carried out at 85.degree. C. for 3 hours
in a convection oven. The resulting substrate with the aligned ZnO
NW arrays on FTO glass substrate was rinsed in DI water and dried
at room temperature.
[0074] Characterization of BaTiO.sub.3 NW Arrays and ZnO NW
Arrays
[0075] The morphological properties which include the orientation
and dimensions of the BaTiO.sub.3 NW arrays and ZnO NW arrays were
examined using an ultra-high resolution field-emission scanning
electron microscope (FESEM) FEI Nova NanoSEM 430. The crystal
structure of the as-prepared BaTiO.sub.3 NWs and ZnO NWs were
examined using an X-ray diffractometer (XRD) equipped with a curved
position sensitive detector (CPS120, Inel) with Cu K.alpha.
radiation. The crystal structure and lattice parameter of
individual BaTiO.sub.3 nanowires were studied using the FEI
(Philips) Tecnai F30 high resolution transmission electron
microscope (HRTEM) that operates at 300 kV accelerating voltage
provided by field-emission electron gun (FEG).
[0076] Fabrication of NEMS Vibrational Energy Harvester
[0077] The NEMS energy harvester using aligned BaTiO.sub.3 NW
arrays was fabricated by sputtering a 1 nm Au 5 layer on top of the
as-prepared NW arrays grown on FTO glass substrate using a PELCO
SC-7 Auto Sputter Coater. A malleable indium (Alfa-Aesar, 99.9%,
0.127 mm thick) foil was then bonded to the base of the
non-conductive glass substrate and formed into a beam to make
contact with the top of the NW arrays to serve as the top
electrode. The Au layer (work function .about.5.1-5.47 eV) that was
initially coated on top of the BaTiO.sub.3 NW arrays improved the
contact with the indium top electrode and also assists to form a
barrier to minimize leakage as reported by McCormick et al. [48].
The indium beam served as the top electrode while the conductive
side of the FTO glass substrate served as the bottom electrode,
with the BaTiO.sub.3 NW arrays in between to form a sandwich
configuration. The FTO glass substrate's edge was insulated using
Kapton polyimide (Dupont) film to ensure there is no resistive
electrode contact to cause shorting. The above fabricated
BaTiO.sub.3 NW NEMS energy harvester was poled at room temperature
by supplying a high DC voltage of .about.120 KV/cm (TREK 477A
Supply/Amplifier) across the two electrodes for 24 hours to ensure
the dipoles align in the electric field direction.
[0078] The NEMS energy harvester using ZnO NW arrays was also
fabricated using a 1 nm Au layer sputtered on the as-synthesized
aligned ZnO NW arrays on FTO glass substrate with the same
procedure as discussed above with the indium beam to serve as the
top electrode. The Au layer assists to form a Schottky barrier
between the indium electrode and the semiconducting ZnO nanowires
[49]. Similarly, sufficient insulation at the FTO substrate edge
was needed so polyimide film was used to ensure there was no
shorting between the two electrodes.
[0079] Electrical Measurement
[0080] The capacitance and impedance measurements from the NEMS
energy harvesters were made using an Agilent E4980A high precision
LCR meter. Mechanical vibration was generated from a Miniature
Permanent Magnet shaker (Labworks, Inc. ET-132) and the voltage
measurements from the NEMS energy harvester under vibration
excitation was performed using a voltage follower/buffer amplifier
with unity gain constructed using Linear Technologies (LTC6240CS8
CMOS Op Amp) which was chosen for its high input resistance (1
T.OMEGA.), low input bias current (0.2 pA) and low noise (Voltage
noise <10 nV/ Hz).54 The short circuit current measurement from
the NEMS energy harvester was performed using a high-speed
electrometer (Keithley 6514, up to 1200 readings/sec). The grounded
faraday cage used as a noise shield from electromagnetic
interference (EMI) surrounded the NEMS energy harvester thus
attenuating the extrinsic noise and preserving the piezoelectric
NWs linear characteristics. The burst chirp signals for FRF
characterization were generated using Spectral Dynamics Siglab data
acquisition (DAQ) system (Model 50-21) from virtual function
generator (vfg) in the MATLAB environment. All other signals were
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[0220] All patents, patent applications, provisional applications,
and publications referred to or cited herein are incorporated by
reference in their entirety, including all figures and tables, to
the extent they are not inconsistent with the explicit teachings of
this specification.
[0221] It should be understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggested
to persons skilled in the art and are to be included within the
spirit and purview of this application.
* * * * *