U.S. patent application number 12/806981 was filed with the patent office on 2011-09-01 for piezoelectric composite nanofibers, nanotubes, nanojunctions and nanotrees.
Invention is credited to Yong Shi, Shiyou Xu.
Application Number | 20110209820 12/806981 |
Document ID | / |
Family ID | 40405212 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110209820 |
Kind Code |
A1 |
Shi; Yong ; et al. |
September 1, 2011 |
Piezoelectric composite nanofibers, nanotubes, nanojunctions and
nanotrees
Abstract
Piezoelectric nanostructures, including nanofibers, nanotubes,
nanojunctions and nanotrees, may be made of piezoelectric materials
alone, or as composites of piezoelectric materials and
electrically-conductive materials. Homogeneous or composite
nanofibers and nanotubes may be fabricated by electrospinning.
Homogeneous or composite nanotubes, nanojunctions and nanotrees may
be fabricated by template-assisted processes in which colloidal
suspensions and/or modified sol-gels of the desired materials are
deposited sequentially into the pores of a template. The
electrospinning or template-assisted fabrication methods may employ
a modified sol-gel process for obtaining a perovskite phase in the
piezoelectric material at a low annealing temperature.
Inventors: |
Shi; Yong; (Nutley, NJ)
; Xu; Shiyou; (Harrison, NJ) |
Family ID: |
40405212 |
Appl. No.: |
12/806981 |
Filed: |
August 25, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12229192 |
Aug 20, 2008 |
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12806981 |
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60957034 |
Aug 21, 2007 |
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Current U.S.
Class: |
156/244.17 ;
977/762; 977/840 |
Current CPC
Class: |
Y10T 29/42 20150115;
H01L 41/1876 20130101; H01L 41/39 20130101; H01L 41/082
20130101 |
Class at
Publication: |
156/244.17 ;
977/762; 977/840 |
International
Class: |
B32B 37/24 20060101
B32B037/24; B32B 37/02 20060101 B32B037/02; B32B 37/06 20060101
B32B037/06 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Some of the research performed in the development of the
disclosed subject matter was supported by grant CMMI #0826418 from
the National Science Foundation. The U.S. government may have
certain rights with respect to this application.
Claims
1-18. (canceled)
19. A method of fabricating an article that includes piezoelectric
fibers having nanoscale diameters using an electrospinninq
apparatus having a hollow, electrically-conductive needle, said
method comprising the steps of: selecting a precursor material
comprising lead zirconate titanate, poly(vinyl pyrrolidine) and an
alcohol, the precursor material being a sol-gel of the lead
zirconate titanate; providing the precursor material to an inlet
opening in the needle at a controlled rate, the precursor material
flowing from the inlet opening to an outlet opening in the needle,
the outlet opening having a micron-scale diameter; applying a high
electrical voltage across the hollow needle and a substrate
proximate to the outlet opening, so as to spin a fiber from the
needle; collecting the fiber on the substrate; and heating the
fiber at a temperature of about 80.degree. C. for about 5 minutes,
then heating the fiber at a temperature of about 380.degree. C. for
about 5 minutes, then maintaining the fiber at a temperature of
about 650.degree. C. for about 1 hour, thereby transforming at
least a portion of the lead zirconate titanate into crystals having
a perovskite structure.
20. (canceled)
21. The method of claim 19, wherein the substrate has a high
electrical resistance.
22. A method of fabricating an article that includes piezoelectric
fibers having nanoscale diameters using an electrospinning
apparatus having a hollow, electrically-conductive needle, said
method comprising the steps of: selecting a precursor material that
is transformed into a piezoelectric material upon heating and has a
sufficiently low viscosity such that the precursor material flows
through the hollow needle; providing the precursor material to an
inlet opening in the needle at a controlled rate, the precursor
material flowing from the inlet opening to an outlet opening in the
needle, the outlet opening having a micron-scale diameter; applying
a high electrical voltage across the needle and a substrate
proximate to the outlet opening, so as to spin a fiber from the
needle; collecting the fiber on the substrate; and heating the
fiber, whereby the fiber is transformed into a piezoelectric
fiber.
23. The method of claim 22, wherein the substrate has a high
electrical resistance.
24. The method of claim 22, wherein the precursor material includes
lead zirconate titanate, and said heating step includes the step of
maintaining the material at a temperature of about 650.degree. C.
until at least a portion of the precursor material is transformed
into crystals having a perovskite structure.
25. The method of claim 22, wherein the precursor material is a
sol-gel of lead zirconate titanate and said heating step includes
the steps of heating the fiber at a temperature of about 80.degree.
C. for about 5 minutes, then heating the fiber at a temperature of
about 380.degree. C. for about 5 minutes, then maintaining the
fiber at a temperature of about 650.degree. C. for about 1 hour,
thereby transforming at least a portion of the lead zirconate
titanate into crystals having a perovskite structure.
26. The method of claim 25, wherein the sol gel includes poly(vinyl
pyrrolidine) and an alcohol.
27. The method of claim 22, comprising the further step of
depositing an electrically-conductive material on at least a
portion of the piezoelectric fiber.
28. The method of claim 22, wherein the substrate has a trench
therein, and said collecting step includes the step of collecting
the fiber such that it extends across said trench.
29. The method of claim 22, wherein the substrate has an electrode
thereupon and said collecting step includes the step of applying an
electrical voltage at said electrode so as to control the position
of the fiber.
30. The method of claim 22, comprising the further step of
depositing a dielectric material on at least a portion of the
piezoelectric fiber.
31. The method of claim 22, wherein the electrospinning device
includes a dielectric tube that is within the hollow needle, the
dielectric tube having an inlet opening that is hydraulically
isolated from the inlet opening and the outlet opening of the
hollow needle, whereby said fiber is spun as a hollow fiber.
32. The method of claim 22, wherein the electrospinning device
includes a dielectric tube that is within the hollow needle, the
dielectric tube having an inlet opening that is hydraulically
isolated from the inlet opening and outlet opening of the hollow
needle, the dielectric tube having an outlet opening proximate to
the outlet opening of the hollow needle and having a micron-scale
diameter, said method comprising the further steps of selecting
another precursor material including a substance that is
electrically-conductive in a solid state and has a viscosity such
that the another precursor material flows through the dielectric
tube, providing the another precursor material to the inlet opening
in the dielectric tube at a controlled rate during the step of
providing the precursor material to the inlet opening in the hollow
needle such that the another precursor material moves from the
inlet opening in the dielectric tube to the outlet opening in the
dielectric tube, and said heating step converting the substance to
a solid state, whereby the piezoelectric fiber has a solid
electrically-conductive core.
33. The method of claim 32, wherein the substance includes indium
titanium oxide.
34. The method of claim 32, comprising the further step of
depositing an electrically-conductive material on at least a
portion of the piezoelectric fiber.
35. The method of claim 22, wherein the hollow needle is one of a
plurality of hollow needles, each hollow needle of the plurality of
hollow needles having an outlet opening having a micron-scale
diameter and an inlet opening, the plurality of hollow needles
being substantially parallel to one another, wherein the precursor
material is provided to the inlet opening of each hollow needle of
the plurality of hollow needles at a controlled rate during said
providing step, so as to spin a plurality of fibers from the
plurality of hollow needles.
36. The method of claim 35, wherein the substrate has an electrode
thereupon and said collecting step includes the further step of
applying another electrical voltage at the electrode so as to align
fibers from the plurality of fibers such that they are
substantially parallel with one another.
37. The method of claim 22, comprising the further step of applying
an electrode to the piezoelectric fiber.
38. The method of claim 37, comprising the further step of applying
a dielectric material to the piezoelectric fiber and the
electrode.
39. The method of claim 22, wherein said providing and collecting
steps are repeated so as to collect a plurality of fibers on the
substrate, and said heating step transforming the plurality of
fibers to a plurality of piezoelectric fibers, said method
comprising the further steps of forming an electrode across the
plurality of piezoelectric fibers and applying a dielectric
material to the electrode and the plurality of piezoelectric
fibers, thereby encapsulating the electrode and the plurality of
piezoelectric fibers with the dielectric material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. Provisional Patent
Application No. 60/957,034, filed Aug. 21, 2007, the entirety of
which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to piezoelectric
nanostructures, including highly-branched nanostructures
("nanotrees"), and variations of the aforesaid having multiple
layers and/or electrically-conductive features, and methods for
fabrication of such nanostructures.
BACKGROUND OF THE INVENTION
[0004] Piezoelectric materials have been utilized widely in sensors
and actuators. Compared to commonly used piezoelectric structures,
such as those based on bulk and thin films, piezoelectric fibers
have attracted more attention because they allow greater
flexibility in the design and application of various structures.
Such fibers can be made of a number of materials, such as zinc
oxide (ZnO), barium titanate (BaTiO.sub.3), lead zirconate titanate
(PbZr.sub.1-xTi.sub.xO.sub.3, PZT) or a piezoelectric polymer such
as polyvinylidine fluoride (PVDF). In particular, fibers made of
PZT have provided the basis for devices having high bandwidth, fast
response, and high sensitivity.
[0005] While there are many methods for fabricating piezoelectric
fibers having microscale dimensions, there are few methods for
fabricating piezoelectric nanofibers (i.e., fibers having
dimensions on the order of nanometers), including hydrothermal
synthesis, sol electrophoresis and metallo-organic decomposition
(MOD) electrospinning. Fibers fabricated by the hydrothermal and
electrophoretic methods are discontinuous, which limits their
usefulness as components of working devices. In contrast, the
electrospinning method can fabricate continuous fibers having
diameters from tens to hundreds of nanometers. Further, aligned
fibers can be fabricated using simple auxiliary methods.
[0006] Piezoelectric fibers, in general, have been used in active
fiber composites (AFC) as sensors and actuators. AFC typically
comprise piezoelectric fibers in a polymer matrix, and are more
flexible and robust than monolithic piezoelectric devices because
they combine the physical properties of the fibers and the matrix.
Devices known in the prior art have used fibers with diameters as
small as 30 microns, but such fibers are too large to be embedded
in active structures or micro or nanoscale devices. Further, AFC
typically incorporate interdigitated electrodes to simplify
fabrication and take advantage of the non-isotropic character of
the piezoelectric properties of the fiber.
SUMMARY OF THE INVENTION
[0007] In one aspect, the subject matter disclosed herein is
directed to the fabrication of piezoelectric structures having
nanoscale dimensions, and the characteristics and uses of the
nanostructures themselves. Fibrous structures ("nanofibers"),
tubular structures ("nanotubes"), simple branched structures
("nanojunctions") and highly-branched structures ("nanotrees") are
disclosed. The disclosed nanostructures include structures that are
fabricated entirely from piezoelectric materials. The disclosed
nanostructures further include composite nanostructures which
comprise adjacent layers of piezoelectric materials and
electrically-conductive materials. Such composite nanostructures
may act as mechanical-electrical energy transducers and as
electrical conductors or electrodes.
[0008] In another aspect, the subject matter disclosed herein is
directed to methods for fabricating homogeneous or composite
nanofibers and nanotubes by electrospinning. The disclosed methods
may employ a modified sol-gel process for obtaining a perovskite
phase at a low annealing temperature, which is also disclosed
herein. The disclosed methods also present methods and devices for
aligning the nanofibers and nanotubes as they are collected.
Devices are disclosed for fabricating single homogeneous nanofibers
and for fabricating multiple nanofibers at high rates. Further, a
co-axial device for electrospinning composite nanofibers and
nanotubes, or homogeneous nanotubes, is disclosed.
[0009] In yet another aspect, the subject matter disclosed herein
is directed to template-assisted methods for fabricating nanotubes,
nanotubes and nanotrees, as homogeneous structures of piezoelectric
materials or composite structures of piezoelectric materials and
electrically-conductive materials. In the disclosed methods, a
template having pores of the desired configuration (i.e., straight
pores to fabricate nanotubes, simple branched pores to fabricate
nanojunctions, or highly-branched pores to fabricate nanotrees) are
selected or fabricated, and modified sol-gels or colloidal
suspensions of piezoelectric materials or electrically-conductive
materials are deposited, sequentially, into the pores to build-up
the desired nanostructure. The nanostructures may be solidified or
annealed during or after the build-up process. The disclosed
template-assisted fabrication methods may also employ the modified
sol-gel process that was discussed with respect to the disclosure
of electrospinning methods.
[0010] In a further aspect, the subject matter disclosed herein is
directed to micron-scale active fiber composite devices comprising
piezoelectric nanostructures (NAFC), and methods for fabricating
such devices. Such devices comprise piezoelectric nanostructures
that are in direct contact with electrodes and encased in a
dielectric matrix material. Such devices may, in an alternative,
include composite piezoelectric nanostructures, which may eliminate
the need to provide separate electrodes in the NAFC.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a better understanding of the present invention,
reference is made to the following detailed description of the
exemplary embodiments considered in conjunction with the
accompanying drawings, in which:
[0012] FIG. 1 is a schematic diagram of a single-needle
electrospinning apparatus used to fabricate homogeneous
piezoelectric nanofibers.
[0013] FIG. 2 is a scanning electron microscopy (SEM) image of
randomly distributed piezoelectric nanofibers collected on a
substrate.
[0014] FIG. 3 is a transmission electron microscopy (TEM) image of
an annealed piezoelectric nanofiber.
[0015] FIG. 4 is an x-ray diffraction pattern of an annealed
nanofiber fabricated from a modified lead zirconate titanate (PZT)
sol-gel.
[0016] FIG. 5 is a TEM image of an annealed PZT nanofiber showing
crystalline structures.
[0017] FIG. 6 is a SEM image of a PZT nanofiber collected across a
trench etched into a silicon substrate.
[0018] FIG. 7 is a schematic diagram of a three-point bending test
performed on the PZT nanofiber of FIG. 6 using an atomic force
microscope (AFM).
[0019] FIG. 8 is a schematic diagram of a three-point bending test
performed on a collection of annealed PZT nanofibers using a
dynamic mechanical analyzer (DMA).
[0020] FIG. 9 is a reproduction of a screen-capture of the
graphical output obtained during the test of FIG. 8.
[0021] FIG. 10 is a schematic drawing of a multi-needle device for
high-rate fabrication of multiple homogenous nanofibers.
[0022] FIGS. 11a-11h are schematic drawings representing steps in a
process for fabricating the device of FIG. 10.
[0023] FIG. 12 is a schematic drawing of a coaxial electrospinning
device for fabrication of core-shell nanofibers.
[0024] FIGS. 13a-13c are schematic drawings representing steps in a
template-assisted fabrication of a composite nanotube.
[0025] FIG. 14 is a schematic drawing of a set-up for
vacuum-assisted fabrication of a nanotube.
[0026] FIG. 15 is a SEM image of a cross-section of an anodic
aluminum oxide template having PZT nanotubes within its pores.
[0027] FIG. 16 is a SEM image of a collection of annealed PZT
nanotubes that have been recovered from their template.
[0028] FIG. 17 is a SEM image of a transverse cross-section of the
PZT nanotubes of FIG. 16.
[0029] FIG. 18 is an x-ray diffraction pattern of a PZT nanofiber
of FIGS. 16 and 17.
[0030] FIG. 19 is a schematic drawing of a nugget drop test
performed on nanotubes within their template.
[0031] FIG. 20 is a graph of the voltage measured across a group of
nanotubes during the test of FIG. 19.
[0032] FIG. 21 is graph of the voltages measured across a group of
nanotubes during a series of nugget drop tests.
[0033] FIG. 22 is a SEM image of a simple Y-branched piezoelectric
nanostructure.
[0034] FIG. 23 is a schematic drawing of a highly-branched
piezoelectric nanostructure.
[0035] FIG. 24 is a graph of resonant frequencies modeled for the
vibrational modes of a highly-branched piezoelectric
nanostructure.
[0036] FIG. 25 is a schematic drawing of a template for the
fabrication of highly-branched piezoelectric nanostructure.
[0037] FIG. 26 is a schematic drawing of a conventional active
fiber composite device.
[0038] FIG. 27 is a schematic cross-sectional drawing of the active
fiber composite device.
[0039] FIG. 28 is a schematic cross-sectional drawing of an active
fiber composite device incorporating electrodes in direct contact
with piezoelectric nanofibers.
[0040] FIG. 29 is a schematic drawing of aligned nanofibers
collected on interdigitated electrodes over a dielectric
substrate.
[0041] FIG. 30 is a SEM image of aligned nanofibers collected on a
substrate.
[0042] FIG. 31 is a SEM image of a nanofiber active fiber composite
beam.
DETAILED DESCRIPTION OF THE INVENTION
Fabrication of PZT Nanofibers by Electrospinninq Sol-Gel
Precursors
[0043] As discussed herein, the fabrication of piezoelectric
nanofibers (e.g. fibers having diameters with range of tens to
hundreds of nanometers) by electrospinning sol-gel precursors and
annealing the collected fibers at low temperatures is a promising
technique for fabricating ceramic nanofibers, and other ceramic
nanostructures, having excellent mechanical and piezoelectric
properties. Nanofibers fabricated by such methods may be actuated
in either transverse mode or longitudinal mode, which provides more
flexibility in designing the devices and systems in which they are
used. Such nanofibers may be used directly as sensors or as
actuators in microscale or nanoscale devices. With appropriate
surface functionalization, nanofiber resonators could be used as
biosensors. Further, such nanofibers may be used as active
structures in applications for which thin films could not be used.
Nanotubes (i.e., hollow tubes having diameters in the same range as
nanofibers) having properties and uses comparable to the aforesaid
nanofibers may be fabricated by a modified electrospinning process
using a specially-designed device discussed separately herein.
[0044] As an example of nanofiber fabrication, lead zirconate
titanate (PZT) nanofibers (specifically, nanofibers formed from
PbZr.sub.52Ti.sub.48O.sub.3) were prepared by an electrospinning
process and tested to determine their structures and properties.
The primary component of the precursor mixture was a commercial PZT
sol-gel (MMC Electronics America Inc.). Several materials were used
to modify the viscosity and conductivity of the PZT sol-gel, from
which poly(vinyl pyrrolidine) (PVP) was selected for further use.
The PZT sol-gel was mixed with a solution of PVP in alcohol, and
acetic acid was added to stabilize the solution and to control
hydrolysis of the sol-gel. The amount of PVP was varied to control
the diameter of the nanofibers. The compositions of the precursor
mixtures tested were as follows:
[0045] PZT sol-gel: 3 ml
[0046] PVP: variable, but on the order of 0.1 to several grams
[0047] Alcohol: 5 ml
[0048] Acetic acid: 2 ml
[0049] It was observed that the diameters of nanofibers fabricated
by electrospinning could be controlled by adjusting the amount of
PVP in the precursor mixture. Continuous nanofibers having an
average diameter of about 50 nm were obtained at a PVP
concentration of roughly 0.007 g PVP/ml of precursor mixture. The
average diameter of the continuous nanofibers was increased to
about 150 nm by adjusting the PVP concentration upward to roughly
0.028 g PVP/ml of precursor mixture. At PVP concentrations above
0.028 g/ml, the diameters of the nanofibers decreased gradually
until continuous fibers could no longer be collected, probably due
to the loss of PVP during the annealing process.
[0050] FIG. 1 is a schematic drawing of the electrospinning
apparatus 10 used to fabricate nanofibers for testing. The
apparatus 10 includes a stainless steel needle 12, which has an
opening 14 with a diameter of about 200 microns, and which is
mounted on a frame 16 over a collecting substrate 18. It is
preferable to use a wafer of high-resistance silicon or a
high-dielectric material such as alumina or silica as the substrate
18. Parallel electrodes or interdigitated electrodes (not shown)
may be deposited on the substrate by any of a number of known
methods to aid in aligning the nanofibers. Nanofibers collected for
mechanical testing in the present example were collected across 5
micron trenches etched into the substrate.
[0051] In the electrospinning process, a feeding system 20, which
includes a high-pressure pump (not shown) and syringe (not shown)
containing the precursor mixture, was used to provide a continuous
supply of precursor mixture to the needle 12. A high-voltage power
source 22 was used to apply a voltage across the needle 12 and the
substrate 18. The high-pressure pump applied pressure to the
syringe to maintain a flow range of about 0.5 .mu.l/min, which was
sufficient to maintain a small drop of the precursor mixture at the
needle tip in the absence of a voltage across the needle 12 and the
substrate 18. A high voltage (in this instance, 10,000 V) was then
applied across the needle 12 and substrate 18. The high voltage
overcame the surface tension of drop at the tip of the needle,
producing a highly-charged jet of the precursor mixture. The jet
underwent a stretching and whipping process during which the
solvent in the precursor mixture evaporated and nanoscale fibers
were deposited on the collecting substrate.
[0052] The collected as-spun fibers (also, "green fibers") were
cured in a three-step process. First, the fibers were solidified at
80.degree. C. for 5 minutes, then held at 380.degree. C. for
minutes to drive off solvent. Finally, the nanofibers were heated
at 650.degree. C. for 1 hour to create a pure perovskite phase, in
which phase PZT and other ceramic compounds have piezoelectric
properties. This temperature is lower than that needed to anneal
nanofibers made from metal-organic precursors, possibly because of
the precursor mixture that was used.
[0053] Annealed nanofibers fabricated as described above may be
released from the substrate by any of a number of known dry etching
methods, depending on the substrate which is used. In instances
where a nanofiber is laid across a trench in a substrate, such
release may not be required.
Test Results for PZT Nanofibers Fabricated by Electrospinning
[0054] Characterization: Nanofibers fabricated as described above
were characterized by scanning electron microscopy (SEM),
transmission electron microscopy (TEM) and X-ray diffraction. FIG.
2 is a SEM image of a collection of randomly distributed PZT
nanofibers 24 on a silicon substrate 26. Aligned nanofibers were
obtained using pre-patterned electrodes, such as those described
above. FIG. 3 is a TEM image of a PZT nanofiber 28 having a
diameter of about 150 nm.
[0055] FIG. 4 shows the x-ray diffraction pattern of the annealed
PZT nanofibers. The pattern indicates that the nanofibers have a
pure perovskite crystalline phase, which provides the piezoelectric
properties of the nanofibers. FIG. 5 is a TEM image of an annealed
PZT nanofiber and shows that the crystalline grain sizes are about
10 nm.
Mechanical properties: The Young's modulus of a single nanofiber
was determined using a three-point bending test using atomic force
microscopy (AFM). FIG. 6 is a SEM image of a nanofiber 30 across a
5 micron trench 32 etched into a silicon substrate 34, such as were
used in the test. FIG. 7 presents a schematic drawing of the test,
in which the nanofiber 30 was deflected using the AFM tip 36. The
tip 36 was used to apply a force at the midpoint 38 of the
suspended nanofiber 30. The AFM was operated in contact mode, and a
force plot was obtained to determine the displacement of the
suspended nanofiber and the applied force. The Young's modulus of
the nanofiber was calculated to be 42.99 GPa. Piezoelectric
properties: The piezoelectric response of the nanofibers to strains
in the transverse direction was evaluated using a three-point
bending test similar to that described above. FIG. 8 presents a
schematic of the test. Aligned nanofibers 40 were collected on a
substrate comprising a titanium (Ti) strip 42 with a layer of
zirconium oxide 44 (ZrO.sub.2) as an insulator. Conductive adhesive
was used to attach the nanofibers to the substrate and also as a
pair of electrodes 46. The distance between the electrodes 46 was 1
mm. The three-point bending test was conducted using a dynamic
mechanical analyzer (DMA) comprising two fixed points 48, 50 in
contact with the insulator 44 and a block 52 to provide pressure to
the opposite side 54 of the substrate 42. Different strains were
applied to the substrate and the changes in voltage across the
electrodes were measured. A voltage of about 0.17 V was generated
by applying 0.5% strain on the substrate. The piezoelectric
coefficient, g.sub.33, was calculated to be 0.079 Vm/N. FIG. 9 is a
reproduction of a screen-capture from a graphical display of the
voltage changes measured during the test. The higher peaks 56 of
the graph correspond to the application of the strain, while the
smaller peaks 58 correspond to the vibration of the substrate when
the strain was released.
Multi-Needle Device for Electrospinninq Nanofibers
[0056] The single needle electrospinning process discussed above is
not efficient in fabricating large numbers of nanofibers. FIG. 10
illustrates a multi-needle spinning device 56 that has been
designed for high rate fabrication of nanofibers.
[0057] Referring to the cross-section of the device 56 shown in
FIG. 10, the device 56 comprises an element 58 having a low
electrical resistance, such as a wafer of doped silicon or a metal
plate, and a dielectric element 60 that is joined to the
low-resistance element 58. The dielectric element has a concave
recess 62 that faces the low-resistance element 58 so as to form a
chamber 64. The chamber 64 is hydraulically connected to a
high-pressure pump (not shown) through tube 66 by means of fitting
68, which communicates with chamber 64 through port 70. The
low-resistance element 58 has a number of hollow needles 72, each
having a low electrical resistance, that penetrate the
low-resistance element 58 through holes 74 so that they are
hydraulically connected to the chamber 64.
[0058] The device is intended to replace the needle 12 of the
electrospinning apparatus 10 illustrated by FIG. 1. Referring again
to FIG. 10, a feeding system (not shown) comprising a syringe and a
high-pressure pump would be provided to continuously supply the
precursor mixture to all of the needles 72 through the chamber 64.
A high voltage would be applied across the low-resistance element
and a collecting substrate (not shown), causing nanofibers to be
spun from all needles 72 simultaneously.
[0059] A fabrication process for the multi-needle device is
illustrated in cross-section in FIGS. 11a-11h. A low-resistance
wafer or plate (subsequently, "the low-resistance wafer" 76),
corresponding to low-resistance element 58 of FIG. 10, is provided
(FIG. 11a), and a silicon-dioxide mask 78 is laid on its surface 80
(FIG. 11b). Openings 82 in the mask correspond to the outer
diameters and desired locations of the holes 84 corresponding to
holes 74 in the completed device 56 (see FIG. 10). Referring again
to FIGS. 11a-11h, the low-resistance wafer 76 is then etched by
deep reactive ion etching (RIE) to create holes 84 that penetrate
through it in the positions of the openings 82 in the silicon
dioxide mask 78 (FIG. 11c). The mask 78 is then removed from the
low-resistance wafer 76 by buffered-oxide etching (BOE) (FIG.
11d).
[0060] A dielectric wafer (subsequently, "the dielectric wafer" 86)
is provided (FIG. 11e) and a silicon dioxide mask (not shown) is
laid on its surface 88 to protect an area 90 (shown in
cross-sectional side view), extending along its perimeter. A
concave recess 92 is then etched into the dielectric wafer 86 by
RIE, but not through the wafer 86, so as to leave a thickness 94 of
the wafer 86 intact (FIG. 11f). A port 96 is then etched through
the remaining thickness 94 of the dielectric wafer 86 to receive a
fitting (not shown) that will provide a hydraulic connection
between the recess 92 and a high-pressure pump (not shown) (FIG.
11g). The dielectric wafer 86 is then wafer-bonded to the
low-resistance wafer 76 with the recess 92 facing the
low-resistance wafer 76 so as to form a cavity 98 (FIG. 11h).
Needles (not shown), corresponding to needles 72 of FIG. 10, are
then fitted to the low-resistance wafer 76 through holes 84, and a
fitting (not shown) corresponding to fitting 68 of FIG. 10 is
inserted into the dielectric wafer 86 through port 96 to produce a
device such as device 56 shown in FIG. 10.
Core-Shell Composite Nanofibers
[0061] Core-shell type composite nanofibers or nanotubes,
comprising discrete layers of piezoelectric materials and
electrically-conductive materials, may be formed by electrospinning
or by templating methods. Structures of composite nanofibers may
include, for example, those in which a solid fiber of an
electrically-conductive material is provided with an outer layer of
a piezoelectric material, or in which a layer of electrically
conductive material overlies a layer of piezoelectric material,
which may also overlie an electrically-conductive core. Structures
of composite nanofibers may include, for example, those in which a
tube of piezoelectric material has an inner layer of an
electrically-conductive material, or both inner and outer layers of
an electrically-conductive material. Such composite nanofibers or
nanotubes may be used in place of homogenous nanofibers in the
applications discussed above, and, because of their
electrically-conductive properties, may eliminate the need for the
interdigitated electrodes used in active fiber composites (AFC),
which are discussed separately.
[0062] The piezoelectric material discussed herein is PZT, but
other piezoelectric materials, such as ZnO, BaTO.sub.3, or a
piezoelectric polymer like PVDF, may be used. The
electrically-conductive material discussed herein is indium
titanium oxide (ITO), but other electrically-conductive materials,
such as other electrically-conductive metallic compounds (e.g.,
cadmium sulfide (CdS)), noble metals (e.g., gold or platinum), or
electrically-conductive polymers, may be used.
Fabrication of Composite Nanofibers by Electrospinning
[0063] Electrospinning can be used to fabricate core-shell
composite nanofibers using any piezoelectric and
electrically-conductive materials that can be prepared in sol-gel
forms or nanoparticle colloid forms. The electrospinning methods
used to prepare composite nanofibers are similar to those described
above with respect to fabrication of homogenous nanofibers, except
as modified to use two precursor mixtures, rather than one.
Further, piezoelectric nanotubes without electrically-conductive
layers (i.e., homogeneous nanotubes) may also be made using a
further modification of the method described herein.
[0064] FIG. 12 is a schematic drawing of a coaxial electrospinning
device 100 for the fabrication of core-shell nanofibers, such as
those described above. The device 100 comprises a tube housing 102
for receiving first and second Teflon.RTM. tubes 104, 106, which
are hydraulically connected, respectively, to first and second
syringes (not shown). A stainless steel tube 108 is hydraulically
connected to the first syringe through first tube 104. A fused
silica tube 110 is situated concentrically within stainless steel
tube 108, and is sized so that there is a concentric gap (not
shown) between stainless steel tube 108 and fused silica tube 110.
The fused silica tube 110 is hydraulically connected to the second
syringe through second tube 106. The stainless steel tube 108 and
fused silica tube 110 have respective open tips 112, 114 outside of
the tube housing 102. In one embodiment of the device 100, the
stainless steel tube 108 has an inner diameter of 460 microns and
the fused silica tube 110 has an outer diameter of 360 microns and
an inner diameter of 75 microns. A first Teflon.RTM. sleeve 116
holds stainless steel tube 108 in place and acts as a seal between
the stainless steel tube 108 and the tube housing 102. A second
Teflon.RTM. seal 118 holds fused silica tube 110 in place and acts
as a seal between first tube 104 and second tube 106. Persons
skilled in the relevant arts will recognize that materials other
than stainless steel and fused silica may be used for tubes 108,
110, and that materials other than Teflon.RTM. may be used for the
other tubes 104, 106 and seals 116, 118. Persons skilled in the
relevant arts will also recognize that the design of the device 100
can be modified to accommodate three or more syringes.
[0065] In one embodiment of the electrospinning process, the first
syringe is filled with a PZT precursor mixture and the second
syringe is filled with an ITO precursor mixture. It will be
recognized from the configuration of the device 100 shown in FIG.
12 that the contents of the first syringe will be extruded through
the gap between stainless steel tube 108 and fused silica tube 110
to form an outer layer of the nanofiber, and that the contents of
the second syringe will be extruded through the open tip 114 of
fused silica tube 110, to form an inner core of the nanofiber. It
will be further recognized that providing a PZT precursor material
through first tube 104 without providing any material through
second tube 106 will result in the fabrication of a PZT
nanotube.
[0066] The electrospinning apparatus for producing core-shell
nanofibers may be configured according to the schematic in FIG. 1,
with the device 100 of FIG. 12 replacing needle 12 of FIG. 1.
Further, separate high-pressure pumps (not shown) should be
provided to apply pressure to the respective syringes and maintain
a continuous flow of the precursor materials to tips 112 and 114.
In other respects, the electrospinning process for producing
core-shell nanofibers may be the same as the process previously
described for producing homogenous nanofibers. The annealing and
release processes may also be the same as described for the
fabrication of homogenous nanofibers. If a third, outer layer of
material is desired, it may be applied by coating, chemical vapor
deposition, or other known processes suitable for the material to
be used, or it may be co-extruded through a tip adapted from that
illustrated in FIG. 12.
Fabrication of Composite Nanofibers and Composite or Homogenous
Nanotubes By Template-Assisted Processes
[0067] Composite nanofibers and nanotubes may be produced by
depositing layers of precursor mixtures for piezoelectric materials
and precursors for electrically-conductive materials within the
pores of a dielectric template and annealing the nanofibers or
nanotubes within the template. Homogeneous nanotubes may also be
prepared by a similar deposition method, where all of the deposited
layers comprise a precursor mixture for a piezoelectric material.
Template-assisted methods can be used to fabricate nanofibers and
nanotubes, or other nanoscale piezoelectric structures, such as
those discussed elsewhere in this specification, using any pairs of
piezoelectric and electrically-conductive materials which can be
prepared as solutions, sol-gels or nanoparticle colloids, or as
vapors such as those used in chemical vapor deposition. Further,
although the exemplary methods discussed herein use anodic aluminum
oxides (AAO), persons skilled in the relevant arts will recognize
that other materials, including other ceramic materials or silicon,
can be used to form useful templates.
[0068] FIGS. 13a-13c illustrate a generalized procedure for
template-assisted formation of composite, nanofibers or composite
or hetrogeneous nanotubes. First, an appropriate template 120 is
selected that has pores 122 having sizes commensurate with the
desired outer diameter of the nanostructure to be fabricated. These
pores 122 will extend through the entire thickness of the template
120. AAO templates having substantially-aligned pores of known
sizes can prepared on aluminum foil by known methods using the foil
as a support layer for the template. Suitable template materials
are also available from commercial sources, or templates having
desired thicknesses, pore diameters and pore structures may be
custom-made. In the example shown in FIG. 13a, precursor for a
piezoelectric material, in liquid or vapor form, is deposited,
coated or grown on the interior surfaces of the pore 122 to form a
shell layer 124. In a variation of the general process, a precursor
for an electrically-conductive material may be used in place of the
precursor for the piezoelectric material. The thickness of the
shell 124 can be controlled by progressively adding layers of the
desired material.
[0069] Turning to FIG. 13b, when a shell having the desired
thickness has been formed, a second shell 126, or a core (not
shown), of a precursor mixture for a second material can be formed
on the interior of the first shell 124 by adding layers of the
appropriate precursor until a second shell 126 of a desired
thickness, or a core, has been formed. This process can be adapted
to produce at least a third shell (not shown), if desired. The
resulting nanostructure is then annealed to produce nanofibers or
nanotubes having the desired piezoelectric and conductive
properties. Turning to FIG. 13c, the template may then be etched
away to recover the nanofibers or nanotubes (e.g., composite
nanotube 128). Because the pores in the templates will be
substantially aligned, the recovered nanostructures will also be
substantially aligned.
Fabrication and Testing of PZT Nanotubes
[0070] In an example of the generalized procedure described above
with respect to FIGS. 13a-13c, nanotubes comprising a shell of PZT
were formed in the pores of an AAO template using a vacuum-assisted
deposition process. FIG. 14 is a schematic illustration of the
set-up for the vacuum-assisted deposition process. An AAO template,
130, having pores 132, was placed on a filter holder 134 connected
to a vacuum pump (not shown) by a tube 136. A PZT precursor mixture
138 was then placed on the template 130, and the vacuum pump was
allowed to run until about half of the precursor mixture 138 had
been pulled into the pores 132. Excess precursor mixture 138 was
removed from the outside of the template 130 using acetic acid, and
the template 130, with precursor mixture 138 within its pores 132,
was dried at 80.degree. C. to remove solvent and solidify the
precursor mixture 138 to form nanotubes (not shown). The nanotubes
were then annealed by heating at 380.degree. C. for 5 minutes, then
at 650.degree. C. for 1 hour to create a pure perovskite phase of
the PZT.
[0071] Characterization of PZT nanotubes: Nanotubes fabricated as
described above were characterized by scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and X-ray
diffraction. FIG. 15 is a SEM image of a cross-section of an AAO
template 140 before the nanotubes 142 are collected. It can be seen
that the nanotubes 142 (dark strips) are substantially aligned.
FIG. 16 is a SEM image of a collection of substantially-aligned PZT
nanotubes 144 that have been recovered from the AAO template (not
shown) in which they were formed. These tubes have diameters of
about 190 nm to 210 nm and wall thicknesses of about 20 nm. They
were formed with five layers, by consecutive deposition steps. FIG.
17 is an end-view of the PZT nanotubes 144 of FIG. 16. The
templates used to form the nanotubes 144 were nominally 60 microns
thick, resulting in nanotubes that were nominally 60 microns
long.
[0072] FIG. 18 shows the x-ray diffraction pattern of the annealed
PZT nanotubes 144. The pattern indicates that the nanotubes 144
have a pure perovskite crystalline phase, which provides the
piezoelectric properties of the nanotubes 144.
Electromechanical coupling tests: Nugget drop tests were conducted
on PZT nanotubes within an AAO template to demonstrate the
piezoelectric properties of the nanotubes. AFM and DMA tests,
similar to those discussed above with respect to PZT nanofibers,
can also be performed on PZT nanotubes with little to no
modification of the procedures previously discussed. Other tests
typically used to evaluate mechanical and piezoelectric properties
of materials, such as dynamic vibration tests, may also be
performed.
[0073] FIG. 19 is a schematic illustration of the nugget drop test.
Upper and lower electrodes 146, 148 were formed on the opposite
faces 150, 152 of the AAO template 154 using a conventional
technique, so as to contact the ends 156, 158 of the nanotubes 160
within the template. Nuggets 162 were dropped from different
heights onto the upper electrode 146. The impact force of the
nugget 162 onto the electrode 146 was transferred to the nanotubes
160, resulting in their deformation and an accumulation of charge
upon them.
[0074] FIG. 20 is a graph of the voltage measured during a typical
drop. As can be seen, the impact of the nugget causes a sudden
spike in the voltage measured across the template. A sudden
reversal of voltage also occurs immediately after the spike,
probably resulting from rebound of the deformed material.
[0075] FIG. 21 is a graph of the measured voltage as a function of
the height from which the nugget was dropped. The measured voltage
increases in a roughly linear relationship with the drop
height.
Branched Piezoelectric Structures
[0076] FIG. 22 is a SEM image of a simple Y-branched peizoelectric
structure 164 ("nanojunction") in a nanofiber formed by a
template-assisted method. Such structures 164 occur when the pores
in the template are themselves branched. Branched pores are often
unintended artifacts of the template formation process. However,
branched pores, including highly-branched pores, can intentionally
be formed in templates by a controlled process described herein,
and used to form highly-branched piezoelectric nanostructures
("nanotrees"). An example of such a nanotree 166 is illustrated
schematically in FIG. 23. As can be seen, a nanotree has a primary
stem 168 and one or more generations of branches 170, 172 174
connected to the stem 168. The nanotree 166 may be attached through
its stem 168 to an insulating oxide layer 176 or other substrate
178. Such nanotrees may be fabricated with layers of piezoelectric
and electrically-conductive materials, or may be homogenously
formed from a piezoelectric material.
[0077] Because of their highly-branched structures, piezoelectric
nanotrees, such as nanotree 166, have potential applications in
energy scavenging or as high-frequency wide-band energy filters. In
general, a piezoelectric structure can convert mechanical
vibrational energy into electrical energy, with the conversion
efficiency peaking at the structure's resonant frequency. Finite
element analysis of a nanotree structure showed that it has a
number of complex vibrational modes, resulting in a series of
closely-spaced resonant frequencies (FIG. 24). As a result, a
nanotree structure can be used to scavenge energy from a wide range
of ambient mechanical vibrations having the same frequencies. A
nanotree structure can also be used as a high-frequency wide-band
energy filter that passes signals at frequencies other than its
resonant frequencies. Because a nanotree structure has numerous
closely-spaced resonant frequencies, the filter is "wide-band" in
nature.
Fabrication of Templates Having Highly-Branched Pores
[0078] The fabrication of templates having highly-branched pores is
discussed herein with respect to AAO templates. Methods for
fabricating such templates from other materials, such as other
ceramic materials, will be recognized by persons having skill in
the relevant arts.
[0079] Typically, a porous AAO template may be formed by an
electrolytic process wherein a direct current is passed through an
acidic electrolyte, using an aluminum foil as the anode on which
the AAO is formed. Pore size is controlled by balancing the rate at
which AAO is formed with the rate at which it is etched by the
acid.
[0080] FIG. 25 is a schematic representation of an AAO template 180
having highly-branched pores 182 for fabricating piezoelectric
nanotrees. In a method for fabricating templates, such as template
180, the anodization process is started according to the typical
method for producing AAO templates having pores without branches.
When primary pores 184 have formed, the anodizing voltage will be
reduced to (1/2).sup.1/2 of its initial value causing the primary
pore 184 to branch in a Y-formation, forming secondary pores 186.
Additional generations of Y-branched pores 188, 190 can be formed
by further sequential reduction of the anodizing voltage. In fact,
highly-branched pore structures can be obtained by adjusting the
anodizing voltage to (1/n).sup.1/2 of its initial value, where n is
the number of generations of pores. For example, branches 190 would
represent the fourth generation of pores, and would be formed by
adjusting the anodizing voltage to (1/4).sup.1/4 of its initial
value.
Fabrication of Composite or Homogeneous Nanotrees
[0081] Composite nanotrees may be produced using adaptations of the
template-assisted methods discussed with respect to composite or
homogenous nanofibers and nanotubes. That is, composite nanotrees
may be formed in templates having highly-branched pores by forming
layers of precursor mixtures for piezoelectric materials and
precursor mixtures for electrically-conductive materials within the
pores and annealing the nanotrees within the template. Homogeneous
nanotrees may also be prepared by a similar layer formation
process, wherein all of the deposited layers comprise a precursor
mixture for a piezoelectric material. Composite nanotrees can be
fabricated from any pairs of piezoelectric and
electrically-conductive materials which can be prepared as
solutions, sol-gels, or nanoparticle colloids, or as vapors, such
as those used in chemical vapor deposition.
Characterization and Testing of Nanotrees
[0082] Nanotrees may be characterized and tested by the same
methods used to characterize and test nanofibers or nanotubes. For
example, nanotrees may be characterized using SEM, TEM and X-ray
diffraction. Mechanical and electromechanical coupling tests that
may be used include AFM and DMA tests, dynamic vibration tests, and
nugget drop tests.
Nanoscale Active Fiber Composites (NAFC)
[0083] FIGS. 26 and 27 are schematic illustrations, in orthogonal
and cross-sectional views, respectively, of a typical design for an
active fiber composite (AFC) device 192, wherein piezoelectric
fibers 194 are encased in a dielectric matrix 196, such as a resin
epoxy, onto which electrodes 198, 200 are deposited. As a result,
there are electrically-insulated gaps 202, 204 between the
electrodes 198, 200 and the fibers 194. When a voltage is applied
across the electrodes 198, 200 of such an AFC device 192, most of
the voltage drop will occur across the insulated gap 202, 204,
making it necessary to supply a high voltage, sometimes in the
thousands of volts, to actuate the AFC device 192. At such
voltages, the dielectric matrix 196 may break down, diminishing the
efficiency of the AFC device 192. FIG. 28 is a cross-sectional
schematic view of a NAFC device 206 fabricated using piezoelectric
nanofibers 208. The nanofibers 208 are coated with dielectric
matrix material 210, with electrodes 212, 214 deposited directly on
the nanofibers 208. Because the electrodes 212, 224 are in direct
contact with the nanofibers 208, the NAFC 206 can be actuated at
voltages as low as a few volts to a few tens of volts.
Fabrication of NAFC Devices
[0084] In a method of fabricating NAFC devices, aligned
piezoelectric nanofibers are collected on a substrate during an
electrospinning process. As discussed elsewhere with respect to
electrospinning, short nanofibers may be collected on dielectric
layers over a grounded, uniformly-deposited electrode or doped
silicon wafer, or a trench can be made over the electrode, and the
short fibers collected across the trench. Referring to FIG. 29,
interdigited electrodes 216, 218 can be formed on a silicon wafer
220 and electrically grounded, and long fibers 222 collected across
the electrode fingers 224, 226. The alignment of the nanofibers is
controlled by the electric field applied to the substrate during
the electrospinning process. These methods can be readily adapted
for collection of aligned composite nanofibers. A collection of
aligned nanofibers 228 is shown in FIG. 30.
[0085] After the aligned nanofibers have been collected, electrodes
(not shown) may be deposited directly on top of the nanofibers by
methods such as sputtering or e-beam deposition. Then a matrix
material, such a resin epoxy or silicone in solvent, may be
deposited on the electrodes by methods such as spin-coating, and
the solvent baked off in a curing process. By adjusting the ratio
of solvent to silicone, matrix membranes having thicknesses as
small as about 1.0 micron to about 1.2 microns have been
obtained.
Testing of NAFC Devices
[0086] An example of a NAFC beam 230 made by a fabrication process
such as that described is shown in FIG. 31. The beam is about 3 to
4 microns wide and extends over a 20 micron trench 232 in a silicon
substrate. The ends of the beam are anchored to the substrate.
[0087] The mechanical properties of the beam were tested using an
AFM method similar to the three-point deflection test discussed
above with respect to testing a homogenous nanofiber. The stiffness
of the beam was calculated to be 0.148 N/m.
[0088] Other methods of testing piezoelectric structures, such as
the DMA tests, dynamic vibration tests, and nugget drop tests
discussed elsewhere in this specification, may be adapted for
testing the properties of NAFC devices. Appropriate adaptations
will be recognized by persons skilled in the relevant arts.
[0089] It should be understood that the embodiments described
herein are merely exemplary and that a person skilled in the art
may make many variations and modifications thereto without
departing from the spirit and scope of the present invention. All
such variations and modifications, including those discussed above,
are intended to be included within the scope of the invention,
which is described, in part, in the claims presented below.
* * * * *