U.S. patent application number 13/840115 was filed with the patent office on 2013-11-28 for aluminum metallic nanoparticle-polymer nanocomposites for energy storage.
The applicant listed for this patent is Massimiliano Delferro, Lisa A. Fredin, Michael T. Lanagan, Zhong Li, Tobin J. Marks, Mark A. Ratner. Invention is credited to Massimiliano Delferro, Lisa A. Fredin, Michael T. Lanagan, Zhong Li, Tobin J. Marks, Mark A. Ratner.
Application Number | 20130317170 13/840115 |
Document ID | / |
Family ID | 49622093 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130317170 |
Kind Code |
A1 |
Marks; Tobin J. ; et
al. |
November 28, 2013 |
ALUMINUM METALLIC NANOPARTICLE-POLYMER NANOCOMPOSITES FOR ENERGY
STORAGE
Abstract
A nanoparticle composition comprising a substrate comprising
aluminum nanoparticles, an Al.sub.2O.sub.3 component coating said
aluminum nanoparticles, and a metallocene catalyst component
coupled to the Al.sub.2O.sub.3 component; and a polyolefin
component coupled to said substrate.
Inventors: |
Marks; Tobin J.; (Evanston,
IL) ; Lanagan; Michael T.; (State College, PA)
; Ratner; Mark A.; (Glencoe, IL) ; Delferro;
Massimiliano; (Chicago, IL) ; Fredin; Lisa A.;
(Austin, TX) ; Li; Zhong; (New Albany,
OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Marks; Tobin J.
Lanagan; Michael T.
Ratner; Mark A.
Delferro; Massimiliano
Fredin; Lisa A.
Li; Zhong |
Evanston
State College
Glencoe
Chicago
Austin
New Albany |
IL
PA
IL
IL
TX
OH |
US
US
US
US
US
US |
|
|
Family ID: |
49622093 |
Appl. No.: |
13/840115 |
Filed: |
March 15, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13449750 |
Apr 18, 2012 |
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13840115 |
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11985930 |
Nov 19, 2007 |
8163347 |
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13449750 |
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60859873 |
Nov 17, 2006 |
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Current U.S.
Class: |
524/783 ;
502/152; 502/155; 524/786 |
Current CPC
Class: |
C08K 2003/2241 20130101;
C08K 9/02 20130101; H01G 4/10 20130101; C08K 2003/2227 20130101;
H01B 3/10 20130101; C08F 4/6592 20130101; H01G 4/206 20130101; C08F
10/00 20130101; C08F 110/06 20130101; C08K 2003/0812 20130101; C08F
110/06 20130101; C01P 2002/86 20130101; C08F 4/025 20130101; C08F
2500/15 20130101; C08L 23/12 20130101; C08F 4/65912 20130101; C08F
10/00 20130101; H01B 3/307 20130101; C08K 2201/011 20130101; C08K
9/02 20130101; C09C 1/648 20130101; B82Y 30/00 20130101; H01G 4/18
20130101; C01P 2004/51 20130101; C01P 2006/40 20130101; C08F
4/65927 20130101; C08F 10/00 20130101; C08F 4/65916 20130101 |
Class at
Publication: |
524/783 ;
502/152; 502/155; 524/786 |
International
Class: |
H01B 3/10 20060101
H01B003/10; H01B 3/30 20060101 H01B003/30 |
Goverment Interests
[0002] This invention was made with government support under
N00014-05-1-0766 awarded by the Office of Naval Research;
DE-FG02-86ER13511 awarded by the Department of Energy; and
DMR1121262 awarded by the National Science Foundation. The
government has certain rights in the invention.
Claims
1. A solid nanoparticle composition comprising a substrate
comprising a metal nanoparticle component and a metal oxide
component; and a metallocene olefin polymerization catalyst
component coupled to said metal oxide component of said substrate,
wherein said metal oxide component is homogenously dispersed
throughout said nanocomposite composition.
2. The composition of claim 1 wherein said metallocene component is
EBIZrCl.sub.2.
3. The composition of claim 1 in a polyolefin matrix.
4. The composition of claim 3 wherein the polyolefin is
polypropylene.
5. The composition of claim 4 wherein the metal nanoparticles are
aluminum nanoparticles.
6. The composition of claim 5 wherein the metal oxide component is
Al.sub.2O.sub.3.
7. The composition of claim 6 wherein the metal nanoparticles are
coated by the metal oxide component.
8. A composite comprising a nano-dimensioned substrate comprising
aluminum nanoparticles homogenously dispersed throughout said
substrate, an Al.sub.2O.sub.3 component coating said aluminum
nanoparticles, and a metallocene catalyst component; and a
polyolefin component coupled to said substrate.
9. The composite of claim 8 wherein said polyolefin component is
selected from C.sub.2 to about C.sub.12 polyalkylenes, substituted
C.sub.2 to about C.sub.12 polyalkylenes, and copolymers
thereof.
10. The composite of claim 8 wherein said metallocene component is
EBIZrCl.sub.2.
11. The composite of claim 10 wherein said polyolefin component is
isotactic polypropylene.
12. The composite of claim 10 having an energy density of about 14
J/cm.sup.3.
13. The composite of claim 10 wherein the Al nanoparticle volume
fraction is from between 0.100 and 0.125.
14. The composite of claim 13 wherein the composite maintains a
permittivity of at least about 6 in the 1 MHz-7 GHz frequency
range.
15. The composite of claim 8 wherein the Al nanoparticles are from
50-150 nm in diameter.
16. A commodity material composition comprising a polyolefin
component and a nano-dimensioned substrate component dispersed
therein, said substrate component comprising a metal nanoparticle
component, an Al.sub.2O.sub.3 component, and a metallocene catalyst
component.
17. The composition of claim 16 wherein said substrate dispersion
is substantially homogenous on a nanoscale dimension.
18. The composition of claim 16 wherein said polyolefin component
is selected from C.sub.2 to about C.sub.12 polyalkylenes,
substituted C.sub.2 to about C.sub.12 polyalkylenes, and copolymers
thereof.
19. The composition of claim 16 wherein the metal nanoparticle
component is an aluminum nanoparticle component.
20. The composition of claim 19 as a thin film in an insulator
device.
Description
[0001] This application is a continuation-in-part of and claims
priority benefit from co-pending application Ser. No. 13/449,750
filed Apr. 18, 2012, a divisional application of application Ser.
No. 11/985,930 filed Nov. 19, 2007, now issued as U.S. Pat. No.
8,163,347, which claims priority benefit from application Ser. No.
60/859,873 filed on Nov. 17, 2006, the entirety of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] Future pulsed-power and power electronic capacitors will
require dielectric materials ultimately having energy storage
densities >30 J/cm.sup.3, with operating voltages >10 kV, and
msec/.mu.sec charge/discharge times with reliable operation near
the dielectric breakdown limit. Importantly, at 2 J/cm.sup.3 and
0.2 J/cm.sup.3, respectively, the operating characteristics of
current state-of-the-art pulsed power and power electronic
capacitors, which utilize either ceramics or polymers as dielectric
materials, remain significantly short of this goal. An order of
magnitude improvement in energy density will require development of
revolutionary new materials that substantially increase intrinsic
dielectric energy densities while operating reliably near the
dielectric breakdown limit. For simple linear response dielectric
materials, energy density is defined in eq. 1, where .di-elect
cons..sub.r is relative dielectric permittivity, E is the
dielectric breakdown strength, and .di-elect cons..sub.0 is the
vacuum permittivity. Generally, inorganic metal oxides exhibit high
permittivities, however, they suffer from low breakdown fields.
While organic materials (e.g., polymers) can provide high breakdown
strengths, their generally low permittivities have limited their
application.
U.sub.e=1/2.di-elect cons..sub.r.di-elect cons..sub.0E.sup.2
(1)
[0004] Recently, inorganic-polymer nanocomposite materials have
attracted great interest due to their potential for high energy
density. By integrating the complementary properties of their
constituents, such materials can simultaneously derive high
permittivity from the inorganic inclusions and high breakdown
strength, mechanical flexibility, facile processability, light
weight, and properties tunability (molecular weight, comonomer
incorporation, thermal properties, etc.) from the polymer host
matrix. Additionally, there are good reasons to believe that the
large inclusion-matrix interfacial areas will afford higher
polarization levels, dielectric response, and breakdown
strength.
[0005] Although inorganic-polymer nanocomposites can be prepared
via mechanical blending, solution mixing, in situ radical
polymerization, and in situ nanoparticle synthesis, host-guest
incompatibilities frequently result in nanoparticle aggregation and
phase separation, detrimental to the electrical properties.
Covalently grafting polymer chains to inorganic nanoparticle
surfaces has also proven promising, leading to more effective
dispersion and enhanced properties, however, such processes may not
be cost-effective and nor easily scaled up.
[0006] Illustrating another approach, in the large-scale
heterogeneous or slurry olefin polymerizations practiced on a huge
industrial scale, SiO.sub.2 is generally used as the catalyst
support. However, very large local hydraulic pressures arising from
the growing polyolefin chains are known to effect extensive
SiO.sub.2 particle fracture and lead to SiO.sub.2-polyolefin
composites. As a result, there remains an on-going search in the
art for an alternate route to inorganic-polymer nanocomposites, to
better utilize the benefits and advantages afforded by such
materials.
SUMMARY OF THE INVENTION
[0007] In light of the foregoing, it is an object of the present
invention to provide various high energy nanocomposites, related
components and devices, and/or methods for their preparation and/or
assembly, thereby overcoming various deficiencies and shortcomings
of the prior art, including those outlined above. It will be
understood by those skilled in the art that one or more aspects of
this invention can meet certain objectives, while one or more other
aspects can meet certain other objectives. Each objective may not
apply equally, in all of its respects, to every aspect of this
invention. As such, the following objects can be viewed in the
alternative with respect to any one aspect of this invention.
[0008] It can be an object of the present invention to provide one
or more methods for nanocomposite preparation to prevent
nanoparticle agglomeration problems associated with the prior
art.
[0009] It can be another object of the present invention to provide
an in situ polymerization technique using one or more metallocene
catalyst components supported on a nanoparticle, with a range of
available olefin monomers.
[0010] It can be another object of the present invention, alone or
in conjunction with one of the preceding objectives, to provide a
nanocomposite comprising a nanoparticle component homogeneously
dispersed within a matrix of a high-strength, high-energy commodity
polymer material of the sort used in the art with energy storage
capacitors and insulators.
[0011] Other objects, features, benefits and advantages of the
present invention will be apparent from this summary and the
following descriptions of certain embodiments, and will be readily
apparent to those skilled in the art having knowledge of various
high energy nanocomposites and assembly/production techniques. Such
objects, features, benefits and advantages will be apparent from
the above as taken into conjunction with the accompanying examples,
data, figures and all reasonable inferences to be drawn therefrom,
alone or with consideration of the references incorporated
herein.
[0012] In part, the present application can be directed to a
particulate composition comprising a substrate comprising a metal
oxide component and an aluminum oxide component; and a metallocene
olefin polymerization catalyst component coupled to such a
substrate. Without limitation, such a substrate and/or particulate
can be nano-dimensioned. In certain other embodiments, such a
substrate and/or particulate composition can be
micro-dimensioned.
[0013] In certain embodiments, a metal oxide can be but is not
limited to binary and ternary metal oxides, such oxides as can
comprise a dopant, and combinations thereof. In certain such
embodiments, a metal oxide component can be selected from
Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, BaTiO.sub.3
BaZrO.sub.3, PbO.sub.3, together with Pb(TiZr)O.sub.3 and other
such oxides comprising a dopant. Regardless, a metallocene
component can be selected from any such polymerization catalyst
known to those skilled in the art, made aware of this invention.
Without limitation, such a metallocene component can be selected
from EBIZrCl.sub.2, CGCTiCl.sub.2 and CpTiCl.sub.3, as described
more fully below, and structural variations thereof. Without
limitation as to metal oxide and metallocene identity, such a
composition can be provided in a polyolefin matrix.
[0014] In part, the present invention can also be directed to a
composite comprising a nano-dimensioned substrate comprising a
metal oxide component, an aluminum oxide component and a
metallocene catalyst component; and a polyolefin component coupled
thereto. In certain embodiments, a polyolfin component can be
selected from C.sub.2 to about C.sub.12 polyalkylenes, substituted
C.sub.2 to about C.sub.12 polyalkylenes, and copolymers thereof,
such polyolefin components limited only by alkylene monomer (s)
reactive with such a metallocene catalyst component under
polymerization conditions of the sort described herein. In certain
such embodiments, metallocene and metal oxide components can be as
described above or illustrated elsewhere herein. Accordingly, with
choice of alkylene monomer(s), such a polyolefin component can be
select from isotactic polypropylene, a linear polyethylene, and a
polystyrene and copolymers thereof.
[0015] In part, the present invention can also be directed to a
commodity or bulk material composition comprising a polyolefin
component and a nano-dimensioned or micro-dimensioned substrate
component dispersed therein, with such a substrate component
comprising a metal oxide component, an aluminum oxide component and
metallocene catalyst component. Such metal oxide, aluminum oxide
and metallocene components can be as described above. Without
limitation as to substrate identity, volume fractions or
percentages can range from about 0.05 percent to about 15 percent.
Likewise, substrate dispersion can be substantially homogeneous on
a nano- or microscale dimension. In certain such embodiments, a
metal oxide component of such a substrate can have a shape about or
substantially spherical or a shape about or substantially rod-like,
as demonstrated below.
[0016] A polyolefin component of such a composition is limited only
by monomer polymerization in the presence in such a metallocene
catalyst. For instance, in certain embodiments, a polyolefin can be
selected from C.sub.2 to about C.sub.12 polyalkylenes, substituted
C.sub.2 to about C.sub.12 polyalkylenes, and copolymers thereof.
Regardless, depending upon polyolefin and/or substrate component
identity, such a composition can be present as a thin film and/or
incorporated into a range of device structures, including but not
limited to insulator devices. Alternatively, depending upon a
particular composition, such materials can find utility in the
context of cable insulation.
[0017] In part, the present invention can also be directed to a
method of preparing a metal oxide-polyolefin nanocomposite. Such a
method can comprise providing a substrate comprising a metal oxide
component and a metallocene olefin polymerization catalyst
component coupled thereto; and contacting such a substrate with an
olefin component, such contact for a time and/or an amount
sufficient to at least partially polymerize an olefin on such a
substrate, to provide a nanocomposite. Without limitation, metal
oxide, metallocene and/or olefin/alkylene components can be
selected as described above. Depending upon olefin content and
degree of polymerization, such a substrate component can have a
volume percentage ranging from about 0.05 percent to about 15
percent. In certain, embodiments, increasing volume percent can be
used to affect melt temperature, leak current density and/or
relative permittivity of a resulting nanocomposite. In certain
other embodiments, choice of metal oxide shape can be used to
affect one or more composite physical characteristics. Without
limitation, the relative permittivity of such a nanocomposite can
be increased using a rod-shaped metal oxide component.
[0018] Illustrating yet another aspect thereof, the present
invention can be directed to a method of using an aluminoxane
component to moderate phase energy densities of a metal
oxide-polyolefin composite. Such a method can comprise providing a
metal oxide component as can be selected from binary and ternary
metal oxides and such oxides comprising a dopant; contacting such a
metal oxide component with an aluminoxane component for a time at
least partially sufficient to provide an aluminum oxide coating on
the metal oxide component; contacting such a coated metal oxide
with a metallocene olefin polymerization catalyst component, to
provide a nano- or micro-dimensioned substrate of the sort
described above; and contacting such substrate with one or more
olefin components, such contact for a time and/or an amount
sufficient to at least partially polymerize the olefin(s) on such a
substrate.
DETAILED DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1. .sup.13C NMR spectrum of an isotactic-polypropylene
nanocomposite (100 MHz, C.sub.2D.sub.2Cl.sub.4, 130.degree.
C.).
[0020] FIG. 2. .sup.13C NMR spectrum of a
poly(ethylene-co-1-octene) nanocomposite (100 MHz,
C.sub.2D.sub.2Cl.sub.4, 130.degree. C.).
[0021] FIG. 3. .sup.13C NMR spectrum of a syndiotactic-polystyrene
nanocomposite (100 MHz, C.sub.2D.sub.2Cl.sub.4, 130.degree.
C.).
[0022] FIGS. 4A-B. Electron microscopic characterization of: (A)
as-received pristine ZrO.sub.2 (SEM) and (B) 7.4 vol %
ZrO.sub.2-.sup.isoPP nanocomposite (TEM).
[0023] FIGS. 5A-B. Electron microscopic characterization of: (A)
as-received pristine TZ3Y (SEM) and (B) 31.1 wt % TZ3Y-.sup.isoPP
nanocomposite (TEM).
[0024] FIGS. 6A-B. Electron microscopic characterization of: (A)
as-received pristine TZ8Y (SEM) and (B) 39.2 wt % TZ8Y-.sup.isoPP
nanocomposite (TEM).
[0025] FIG. 7. Representative C (capacitance) vs. A (electrode
area) plot for a 2.6 vol % BaTiO.sub.3-.sup.isoPP
nanocomposite.
[0026] FIGS. 8A-C. Leakage current density vs. field measurement
results for the nanocomposite MIS or MIM devices (legends are for
the volume fraction of the inorganic particles): (A)
n.sup.+--Si/BaTiO.sub.3-polypropylene/Au; (B)
n.sup.+--Si/sphere-TiO.sub.2-polypropylene/Au; (C)
Al/rod-TiO.sub.2-polypropylene/Au.
[0027] FIGS. 9A-C. Leakage current density vs. field measurement
results for the nanocomposite MIS or MIM devices (legends are for
the volume fraction of the inorganic particles):
(A)Al/ZrO.sub.2-polypropylene/Au; (B) Al/TZ3Y-polypropylene/Au; (C)
Al/TZ8Y-polypropylene/Au.
[0028] FIG. 10. Normalized effective permittivity (.di-elect
cons..sub.eff-.di-elect cons..sub.b/.di-elect cons..sub.a.di-elect
cons..sub.b) for composite dielectrics of polypropylene with
spherical inclusions (eq. 6), and with ellipsoidal inclusions (eq.
7).
[0029] FIG. 11. Comparison of effective permittivities for
spherical- and rod-shaped TiO.sub.2 nanoparticle-polypropylene
nanocomposites.
[0030] FIG. 12. Real (.di-elect cons.') and imaginary (.di-elect
cons.'') parts of the complex permittivity for a material having
interfacial, orientational, ionic, and electronic polarization.
[0031] FIG. 13. Depiction of possible orientations of an aligned
two-particle aggregate compared to a single isolate particle when
an applied voltage induces charges on each of the electrodes; the
corresponding polarization of the particles within the matrix
results in charge accumulation at the particle surface, wherein the
response of these charges to the oscillating field is the
Maxwell-Wagner-Sillars (MWS) polarization.
[0032] FIG. 14. (A) Photograph of a thick film prepared in a PET
washer, and TEM of (B) Al nanoparticles after washing, (C) as
fabricated composite powder and (D) melt-processed composite powder
of the 0.104 v.sub.f composite where the dark spots are
nanoparticles; as well as, SEM characterization of a 0.104 v.sub.f
composite (E) thick film surface and (F) thin film torn edge.
[0033] FIG. 15. Permittivity of Al-.sup.iso nanocomposites from 100
MHz to 7 GHz as a function of Al nanoparticle volume fraction.
[0034] FIG. 16. Graph showing tan .delta. of Al-.sup.iso
nanocomposites as a function of nanoparticle volume fraction from
100 MHz-7 GHz.
[0035] FIG. 17. Representation of Al nanoparticle-polypropylene
composite synthesis.
[0036] FIG. 18. Examples of metallocene polymerization
catalysts.
[0037] FIG. 19. Scheme 1. Synthesis of Isotactic
Polypropylene-Metal Oxide Nanocomposites.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
[0038] Illustrating certain embodiments of this invention, high
energy density BaTiO.sub.3- and TiO.sub.2-isotactic polypropylene
nanocomposites were prepared via in situ metallocene
polymerization. The resulting nanocomposites were found to have
effective nanoparticle dispersion and to possess energy densities
as high as 9.4 J/cm.sup.3, as determined from relative
permittivities and dielectric breakdown measurements. To
demonstrate various other aspects of this invention, the scope of
inorganic inclusion can be extended to include a broad variety of
nanoparticles, with corresponding effects of nanoparticle identity
and shape on the electrical/dielectric properties of the resulting
nanocomposites. Likewise, the scope of metallocene polymerization
catalysts and olefinic monomers can be extended (e.g., Chart 1,
FIG. 18) to enhance nanoparticle processability and thermal
stability. Representative of a range such of embodiments,
nanoparticle coating with methylaluminoxane (MAO) and subsequent in
situ polymerization can be used effectively for effective
dispersion, to realize high breakdown strengths, permittivities and
energy storage densities.
[0039] Accordingly, a series of 0-3 metal oxide-polyolefin
nanocomposites was synthesized via in situ olefin polymerization
using the metallocene catalysts C.sub.2-symmetric
dichloro[rac-ethylenebisindenyl]zirconium(IV) (EBIZrCl.sub.2),
Me.sub.2Si(.sup.tBuN)(.eta..sup.5-C.sub.5Me.sub.4)TiCl.sub.2
(CGCTiCl.sub.2), and (.eta..sup.5-C.sub.5Me.sub.5)TiCl.sub.3
(Cp*TiCl.sub.3) immobilized on methylaluminoxane (MAO)-treated
barium titanate (BaTiO.sub.3), zirconium dioxide (ZrO.sub.2), 3 mol
% yttria-stabilized zirconia (TZ3Y), 8 mol % yttria-stabilized
zirconia (TZ8Y), sphere-shaped titanium dioxide (TiO.sub.2), and
rod-shaped TiO.sub.2 nanoparticles. The resulting composite
materials were characterized by X-ray diffraction (XRD), scanning
electron microscopy (SEM), transmission electron microscopy (TEM),
.sup.13C nuclear magnetic resonance (NMR) spectroscopy, and
differential scanning calorimetry (DSC). It was shown by TEM that
the nanoparticles are well-dispersed in the polymer matrix and each
individual nanoparticle is surrounded by polymer. Electrical
measurements reveal that most of the nanocomposites have leakage
current densities 10.sup.-8-10.sup.-6 A/cm.sup.2, and the relative
permittivities of the nanocomposites increase as the nanoparticle
volume fraction increases, with measured values as high as 6.1. At
the same volume fraction, rod-shaped TiO.sub.2
nanoparticle-polypropylene nanocomposites exhibit greater relative
permittivities than the corresponding sphere-shaped TiO.sub.2
nanoparticle-polypropylene nanocomposites. The energy densities of
these nanocomposites are estimated to be as high as 9.4
J/cm.sup.3.
[0040] In another embodiment, metal nanoparticle-polyolefin
composites were prepared by chemisorbing a metallocene precatalyst,
for example [rac-ethylenebisindenyl]zirconium dichloride
(EBIZrCl.sub.2), onto a native oxide of Al nanoparticles. Addition
of a methylaluminoxane (MAO) co-catalyst then activated the
adsorbed EBIZrCl.sub.2 for in situ synthesis of isotatic
polypropylene. Capacitors were then fabricated with films of these
materials for dielectric characterization. As seen in other ceramic
composites fabricated using the in situ polymerization method, the
present composite films have no discernible voids and the
transmission electron microscopy (TEM) and scanning electron
microscopy (SEM) images of the films indicate uniform morphologies
(FIG. 14). The native Al.sub.2O.sub.3 coating on the particles is
.about.2 nm thick as confirmed by TEM. It is noted that this oxide
thickness is comparable to that of a single Al.sub.2O.sub.3 layer
derived from exposing methylaluminoxide (MAO) to air, and is thin
enough that the volume fraction of Al metal in the samples is
equivalent to the volume fraction of nanofiller.
[0041] In yet another embodiment, and depending upon polyolefin
and/or substrate component identity, a composition according to the
invention can be present as a thin film. Thin film capacitors as
produced by the instant invention can be used in diverse high
frequency electronic applications ranging from signal coupling,
filtering, and impedance matching to advanced packaging
applications.
[0042] As one of the most commonly used polymers in large-scale
power capacitors, isotactic polypropylene offers greater stiffness,
lower shrinkage, and less deterioration of the dielectric
properties at higher temperatures than other grade polypropylenes.
Therefore, the C.sub.2-symmetric metallocene catalyst
dichloro[rac-ethylenebisindenyl]zirconium(IV) (EBIZrCl.sub.2),
known for highly isospecific olefin polymerization, was selected to
demonstrate immobilization on the surfaces of MAO-treated metal
oxide nanoparticles, to synthesize metal oxide-isotactic
polypropylene nanocomposites. X-ray diffraction (XRD) linewidth
analyses using the Scherrer equation indicate that the
microstructures and coherence lengths of the individual
nanoparticles remain largely unchanged upon deagglomerization
(Table 1). (See, Jenkins, R.; Snyder, R. L. InIntroduction to X-ray
Powder Diffractometry; Winefordner, J. D., Ed.; Wiley: New York,
1996; pp 89-91; and Scherrer, P. Gott. Nachr. 1918, 2, 98-100.)
.sup.13C NMR spectroscopy (FIG. 1) shows that the present
polypropylenes are highly isotactic, as evidenced by the
isotacticity index ([mmmm]=83%). (See, Busico, V.; Cipullo, R.;
Monaco, G.; Vacatello, M. Macromolecules 1997, 30, 6251-6263;
Busico, V.; Cipullo, R.; Corradini, P.; Landriani, L.; Vacatello,
M.; Segre, A. L. Macromolecules 1995, 28, 1887-1892; and Zambelli,
A.; Dorman, D. E.; Brewster, A. I. R.; Bovey, F. A. Macromolecules
1973, 6, 925-926.) DSC confirms the absence of extensive amorphous
regions in the composites since only isotactic polypropylene
melting features (142-147.degree. C.) are detected. XRD data for
the nanocomposites also reveal the presence of monoclinic a phase
crystalline isotactic polypropylene (2.theta.=14.2, 17.0, 18.6, and
21.8.degree.). It is found that the melting temperatures of the
nanocomposites generally increase as the nanoparticle loading
increases (Table 2), possibly due to attractive interactions
between the nanoparticles and the crystalline regions of the
isotactic polypropylene.
TABLE-US-00001 TABLE 1 XRD Linewidth Analysis Results of the
Nanocomposites 2.theta. FWHM Crystallite Powder (deg) (deg) Size
(nm) BaTiO.sub.3-polypropylene 31.649 0.271 32.8 BaTiO.sub.3 31.412
0.254 35.6 TiO.sub.2-polypropylene 25.358 0.361 23.5 TiO.sub.2
25.360 0.317 27.1 Crystallite size (L) is calculated using the
Scherrer equation L = 0.9.lamda./Bcos.theta..sub.B (.lamda. = x-ray
wavelength, B = full-width-at-half maximum (FWHM) of the
diffraction peak, and .theta..sub.B = Bragg angle).
[0043] Linear low-density polyethylene (LLDPE) is another polymer
that is widely used in power capacitors. Compared to isotactic
polypropylene, the chain branching in the LLDPE affords better
processability. Therefore, the sterically open constrained geometry
catalyst
Me.sub.2Si(.sup.tBuN)(.eta..sup.5-C.sub.5Me.sub.4)TiCl.sub.2
(CGCTiCl.sub.2) was utilized to synthesize BaTiO.sub.3-LLDPE
nanocomposites via in situ ethylene+1-octene copolymerization. FIG.
2 presents a representative .sup.13C NMR spectrum of the
nanocomposite, with the 1-octene incorporation level calculated to
be 25.0 mol %. (See, Qiu, X.; Redwine, D.; Gobbi, G.; Nuamthanom,
A.; Rinaldi, P. L. Macromolecules 2007, 40, ASAP.) DSC measurements
also confirms the formation of LLDPE, which has a typical melting
temperature of 125.3.degree. C.
[0044] Syndiotactic polystyrene has greater heat resistance than
isotactic polypropylene, which can only operate below 85.degree. C.
when incorporated into film capacitors. Employing the same protocol
as EBIZrCl.sub.2, the half-metallocene catalyst
Cp*TiCl.sub.3.sup.25 was immobilized on MAO-treated ZrO.sub.2
nanoparticles. Subsequent in situ styrene polymerization affords
ZrO.sub.2-syndiotactic polystyrene nanocomposites. A representative
.sup.13C NMR spectrum is shown in FIG. 3. The characteristic single
resonance near .delta.=145.6 ppm for the ipso phenyl carbon atom
confirms the production of syndiotactic polystyrene, which is
further substantiated by the melting temperature (267.0.degree. C.)
as measured by DSC.
[0045] During the course of in situ metallocene polymerization, the
polymer chains propagating at the nanoparticle-immobilized
metallocene catalytic centers may be expected to create large local
hydrostatic pressures and thus help to disrupt the nanoparticle
agglomeration. Such results are confirmed by the comparative
electron microscopic characterization of the as-received pristine
nanoparticles and the resulting nanocomposites. As can be seen from
FIGS. 4, 5, and 6, the as-received pristine nanoparticles evidence
very high levels of agglomeration, however, for the polyolefin
nanocomposites, the agglomeration of the nanoparticles is shown to
be disrupted and each individual nanoparticle is surrounded by a
layer of matrix polymer.
[0046] To assess nanocomposite permittivity properties,
metal-insulator-metal (MIM) or metal-insulator-semiconductor (MIS)
devices for nanocomposite electrical measurements were fabricated
by first doctor-blading nanocomposite films onto aluminum or
n.sup.+-Si substrates, followed by vacuum-depositing top gold
electrodes through shadow masks. The capacitances were measured at
1 kHz, a sufficiently high frequency to avoid the complications
arising from conduction and interfacial polarization effects. After
the capacitance was measured at multiple locations on the
nanocomposite film surface using different electrode areas, the
relative permittivity (.di-elect cons..sub.r) of the nanocomposite
was derived using eq. 2, where C is the capacitance, A is the
electrode area,
C = 0 r A d ( 2 ) ##EQU00001##
.di-elect cons..sub.0 is the vacuum permittivity
(8.8542.times.10.sup.-12 F/m), .di-elect cons..sub.r is relative
permittivity, and d is the nanocomposite film thickness. FIG. 7
shows a representative capacitance vs. electrode area plot, the
linearity of which indicates the good dielectric uniformity of the
nanocomposite film.
[0047] Table 2 summarizes the relative permittivity measurement
results for the present nanocomposites. As the nanoparticle loading
increases, the relative permittivity of the nanocomposites also
increases as predicted by the effective medium approximation. At
the same volume fraction, rod-shaped TiO.sub.2-polypropylene
nanocomposites exhibit significantly greater relative
permittivities than those prepared with sphere-shaped TiO.sub.2
nanoparticles (compare entries 1-4 versus 11-13) under identical
reaction conditions. Without limitation, this shape effect is
thought to arise from the different depolarization factors for
different inclusion particle geometries.
TABLE-US-00002 TABLE 2 Electrical Characterization Results for
Metal Oxide-polypropylene Nanocomposites.sup.a Film Nanoparticle
T.sub.m.sup.c Breakdown Thickness.sup.e Energy Density.sup.f Entry
Composite vol %.sup.b (.degree. C.) Permittivity.sup.d Field (kV)
(.mu.m) (J/cm.sup.3) 1 .sup.isoPP-.sup.sTiO.sub.2 0.1% 135.2 2.2
.+-. 0.1 >10.0 36 >0.8 .+-. 0.1 2 .sup.isoPP-.sup.sTiO.sub.2
1.6% 142.4 2.8 .+-. 0.2 9.5 23 2.1 .+-. 0.2 3
.sup.isoPP-.sup.sTiO.sub.2 3.1% 142.6 2.8 .+-. 0.1 7.5 27 1.0 .+-.
0.1 4 .sup.isoPP-.sup.sTiO.sub.2 6.2% 144.8 3.0 .+-. 0.2 9.3 20 2.8
.+-. 0.2 5 .sup.isoPP-BaTiO.sub.3 0.5% 136.8 2.7 .+-. 0.1 8.8 28
1.2 .+-. 0.1 6 .sup.isoPP-BaTiO.sub.3 0.9% 142.8 3.1 .+-. 1.2
>10.0 21 >4.0 .+-. 0.6 7 .sup.isoPP-BaTiO.sub.3 2.6% 142.1
2.7 .+-. 0.2 9.8 25 1.8 .+-. 0.2 8 .sup.isoPP-BaTiO.sub.3 5.2%
145.6 2.9 .+-. 1.0 8.2 30 1.0 .+-. 0.3 9 .sup.isoPP-BaTiO.sub.3
6.7% 144.8 5.1 .+-. 1.7 9.0 22 3.7 .+-. 1.2 10
.sup.isoPP-BaTiO.sub.3 13.6% 144.8 6.1 .+-. 0.9 >10.0 17 >9.4
.+-. 1.3 11 .sup.isoPP-.sup.rTiO.sub.2 1.4% 139.7 3.4 .+-. 0.3 12
.sup.isoPP-.sup.rTiO.sub.2 3.2% 142.4 4.1 .+-. 0.7 13
.sup.isoPP-.sup.rTiO.sub.2 5.1% 143.7 4.9 .+-. 0.4 14
.sup.isoPP-ZrO.sub.2 1.6% 142.9 1.7 .+-. 0.3 15
.sup.isoPP-ZrO.sub.2 3.9% 145.2 2.0 .+-. 0.4 16
.sup.isoPP-ZrO.sub.2 7.5% 144.9 4.8 .+-. 1.1 17
.sup.isoPP-ZrO.sub.2 9.4% 144.4 5.1 .+-. 1.3 18 .sup.isoPP-TZ3Y
1.1% 142.9 1.1 .+-. 0.1 19 .sup.isoPP-TZ3Y 3.1% 143.5 1.8 .+-. 0.2
20 .sup.isoPP-TZ3Y 4.3% 143.8 2.0 .+-. 0.2 21 .sup.isoPP-TZ3Y 6.7%
144.9 2.7 .+-. 0.2 22 .sup.isoPP-TZ8Y 0.9% 142.9 1.4 .+-. 0.1 23
.sup.isoPP-TZ8Y 2.9% 143.2 1.8 .+-. 0.1 24 .sup.isoPP-TZ8Y 3.8%
143.2 2.0 .+-. 0.2 25 .sup.isoPP-TZ8Y 6.6% 146.2 2.4 .+-. 0.4
.sup.aPolymerizations carried out in 50 mL of toluene under 1.0 atm
of propylene at 20.degree. C. .sup.bFrom elemental analysis.
.sup.cFrom differential scanning calorimetry. .sup.dDerived from
capacitance measurement. .sup.eFilm thicknesses measured using
profilometry. .sup.fEnergy density (U) calculated from U =
0.5.epsilon..sub.0.epsilon..sub.rE.sub.b.sup.2 (.epsilon..sub.0,
vacuum permittivity; .epsilon..sub.r, relative permittivity; and
E.sub.b, breakdown field (MV/cm) calculated by dividing breakdown
voltage by film thickness).
[0048] The leakage current densities of all the nanocomposite films
prepared in this investigation (FIGS. 8 and 9) are mostly within
the range 10.sup.-8-10.sup.-6 A/cm.sup.2 at 100 V, indicating that
the aforementioned nanocomposites are all excellent insulators. As
the nanoparticle loading increases, most of the nanocomposites
exhibit lower leakage current densities, presumably a result of
modified charge transport and interruption of the crystalline
conduction pathways within the composite structure. However, at the
highest nanoparticle loadings, the nanocomposites have the largest
leakage current densities, simply because the weight percentages of
the nanoparticles have reached the respective percolation
thresholds. Increasing the relative permittivity of the
nanocomposite by changing the shape of the inclusion does not
appear to compromise the good insulating properties of these
composites.
[0049] The present measured breakdown strengths for some of the
nanocomposites are invariably .about.3-6 MV/cm, indicating that
metal oxide nanoparticle inclusion does not significantly depress
the polymer dielectric breakdown strength. Without limitation, in a
well-dispersed nanoparticle composite, interfaces between the
ceramic nanoparticles and polymer phases can create effective
electron scatterers and trapping centers, thus reducing the
breakdown probability. Moreover, well-dispersed ceramic
nanoparticles may block degradation tree growth and can increase
the long-term breakdown strength. Energy densities of the present
nanocomposites are estimated to be as high as 9.4 J/cm.sup.3, which
rivals or exceeds those reported for conventional ceramic, polymer,
and composite dielectrics.
[0050] A challenge in the preparation of inorganic metal
oxide-polyolefin nanocomposites is the general phase
incompatibility between inorganic polar metal oxide inclusions and
the non-polar organic host materials. For example, ferroelectric
metal oxides are highly hydrophilic, while isotactic polypropylene
is highly hydrophobic. Simple admixing of the two constituents
negligibly disrupts the extensive nanoparticle agglomeration nor
affects the um-scale or larger phase separation, which can lead to
local dielectric breakdown and degrade the nanocomposite electrical
properties. In contrast, the present in situ supported metallocene
polymerization approach minimizes these deficiencies by achieving
homogeneous nanoscale dispersion of the metal oxide phase: each
individual nanoparticle is surrounded by polymer chains propagating
in situ from the surface-immobilized metallocenium catalyst
centers, and thus offers improved dielectric properties (energy
densities as high as 9.4 J/cm.sup.3).
[0051] However, such nanocomposites can have very large contrasts
in relative permittivities between host and guest materials,
leading to a large disparity in the electric fields within the
constituent phases, thus preventing the realization of maximum
energy densities for both constituents simultaneously. For
representative BaTiO.sub.3-polypropylene nanocomposites, however,
the achieved energy density is as high as 9.4 J/cm.sup.3 although
the materials permittivity ratio approaches .about.1000:1. Without
limitation to any one theory or mode of operation, the
Al.sub.2O.sub.3 (.di-elect cons..sub.r.apprxeq.10) layer (thickness
1 nm, estimated from ICP-OES analysis) evolving from ambient
exposure of the MAO coating, can act as a dielectric buffer layer
between the high permittivity BaTiO.sub.3 nanoparticles (.di-elect
cons..sub.r.apprxeq.2000) and low permittivity polypropylene
(E.sub.r z 2.2).
[0052] As discussed above, the aforementioned in situ synthetic
approach was also applied to metallic nanoparticles having a native
metal oxide coating. As a non-limiting example thereof, metallic
aluminum nanoparticles are coated with a metal oxide coating, such
as, for example, Al.sub.2O.sub.3, wherein the oxide coating is, for
example, 2 nm thick (see FIG. 17). It was found that the complex
permittivity of the metallic Al particles affords composites with
high permittivities, up to .about.15 at 100 Hz, and significant
recoverable energy storage of up to .about.14 J/cm.sup.3. These
composites maintain permittivities greater than 10 up to 1 MHz,
with only the highest Al volume fraction (0.12) material exhibiting
significant relaxation in the 100 Hz to 1 MHz range. While Al is
specifically mentioned, it will be understood that other metal
nanoparticles other than aluminum can be employed to produce
similar results.
[0053] With regard to characterizing the aluminum nanoparticle
materials prepared according to the invention, the frequency
dependence of permittivities is conventionally measured using
dielectric relaxation spectroscopy (DRS) which probes the
interaction of the sample with a time-dependent electric field. The
resulting polarization, expressed by the frequency-dependent
complex permittivity (here by the real permittivity and tan
.delta.), characterizes the amplitude and timescale of charge
density fluctuations across the sample. Such fluctuations generally
arise from electronic polarization, or more significantly, the
reorientation of permanent molecular dipole moments, of
nanoparticles or of dipolar moieties appended to polymers. Other
possible mechanisms include ion transport or the reorganization of
interfacial charge in heterogeneous systems, with the timescale of
the fluctuations depending on the material and the relevant
relaxation mechanism. Relaxation timescales range from psec in
low-viscosity liquids to hours in glasses, with the corresponding
frequencies encompassing 0.1 mHz-1 THz, (FIG. 12). In FIG. 12, the
frequencies shown are typical of homogenous materials reported in
the literature, with the type of relaxation and approximate
characteristic frequency indicated.
[0054] Since in all polarization mechanisms (except those at
optical frequencies arising from electronic polarization) the
dipolar response to an oscillating field involves displacement of
masses, inertia constrains arbitrarily rapid movements. Two
physical parameters describe the movement of the charged masses in
response to alternating fields, polarization response and
relaxation. Response can be modeled kinematically and relaxation
describes the decay of polarization from excited states to the
ground state. For every response mechanism operative in a material,
the polarization decays below certain limiting frequencies above
which the dipole can no longer reorient with the speed of the
fluctuating field. As noted above, each response type has a
characteristic relaxation frequency as shown in FIG. 12. While
relaxation frequencies are typically in the GHz region, electronic
polarization responds to very high frequencies (optical,
.about.1000 THz) and limits the index of refraction (n.apprxeq.
{square root over (.epsilon..sub.r)}). In contrast, interfacial
polarization often decays at low frequencies (sometimes <1 MHz)
and ionic polarization has resonances between GHz and optical
frequencies. Maxwell-Wagner-Sillars polarization is the interfacial
polarization between the internal dielectric boundary layers in a
material, and generally occurs between the (slower) macroscopic
interfacial relaxation at the electrode-dielectric layer interface
and the (faster) orientational relaxation in the GHz range. While
both Maxwell-Wagner-Sillars (MWS) polarization and macroscopic
interfacial polarization are due to the reorganization of charges
at surfaces, MWS polarization contributes orders of magnitude less
to the permittivity than does the electrode interface polarization
due to the microscopic nature of the internal dielectric surfaces.
However, MWS also exhibits polarization response until much higher
frequencies because there are far fewer charges that must
reorganize in the oscillating field, resulting in lower
reorganization energies and potential faster reorganization
times.
[0055] As described herein, the frequency response of the
aforementioned Al-polypropylene nanocomposites are analyzed between
200 MHz and 7 GHz to understand the types of dielectric relaxation
operative in the present metallic nanocomposites. From
Maxwell-Wagner-Sillars modeling, it is appears that conductive
particle aggregation leads to strong dielectric relaxation, where
increasing aggregate size depresses the relaxation frequencies.
Mixing approaches such as percolation theory, which accurately
predict permittivities for typical nanocomposites at low
frequencies, argue that higher volume fractions of extremely high
permittivity nanoparticles (.di-elect cons..sub.r>2000) lead to
aggregates (e.g., chains) of particles that behave like single
particles (at least for transport). For ferroelectric materials,
such particle chains are thought to exhibit a combined dipole
moment which responds to the field, resulting in dipolar
polarization. In contrast, conductive particle surfaces instead
accumulate charge at interfaces with the matrix, which effectively
thin the dielectric layer and cause additional interfacial MWS
polarization (FIG. 13). These internal interfacial polarizations
can be a major component of the dielectric response of the material
and are highly sensitive to the orientation and alignment of the
charge accumulation surfaces. MWS modeling offers a means to
quantify the loss and frequency dependence of this polarization.
Using geometrical arguments based on the conductive particle shape
and orientation, MWS theory predicts that at high metallic
nanoparticle volume fractions, above the percolation threshold
(v.sub.f=0.16), particle aggregation will significantly lower the
relaxation frequency. Accordingly, it is shown that above
.about.0.10 volume fraction and to about 0.125 volume fraction,
Al-polypropylene nanocomposites have relatively high permittivities
that are sustainable up to at least 5 GHz, and that composites with
high Al volume fractions undergo relaxation at lower frequencies
than their lower volume fraction counterparts.
[0056] The complex reflection (both magnitude, .GAMMA., and phase,
.theta.) for aluminum nanocomposite capacitors was measured using
lumped impedance methods. From the magnitude and phase of the
complex reflection, the dielectric permittivities (eq. 3) of the
thick films can be calculated. Here .omega. is the radial
frequency, C.sub.o=A.sub..di-elect cons.o/d, wherein A is the area,
d is the thickness of the sample, and Z.sub.o the characteristic
impedance of a lossless transmission line (50.OMEGA.).
r '' = 2 .GAMMA. sin .theta. .omega. C o Z o ( .GAMMA. 2 + 2
.GAMMA. cos .theta. + 1 ) ( 3 ) ##EQU00002##
[0057] FIG. 15 summarizes the frequency-dependent permittivity of
Al-.sup.isoPP nanocomposites as a function of composition from 200
MHz to 7 GHz. For the lowest volume fraction Al nanocomposites
(0.007-0.029), the high frequency permittivities are statistically
indistinguishable and .about.2. This is consistent both with the
low measured permittivities at lower frequencies and the fact that
these samples have very little MWS polarization but instead are
expected to have dielectric relaxation dominated by optical
relaxations which occur at frequencies higher than 7 GHz. The 0.104
volume fraction composite has a permittivity .about.10 at 1 MHz but
experiences a relaxation between 1 and 200 MHz, and the dielectric
permittivity falls to .about.6 by 200 MHz, and falls further as 7
GHz is approached. The permittivity of the 0.124 composite begins
to undergo a dielectric relaxation before 1 M decreases by
.about.50% between 1 and 200 MHz and appears to have another
relaxation near 5 GHz. Nevertheless, it maintains a permittivity
>5 at frequencies between 200 MHz and 5 GHz. The observed
relaxations in the hundreds of MHz are most likely a result of MWS
interfacial relaxations while the relaxations in the GHz range may
also be orientational polarization relaxations from the polymer
matrix. In polymer films, typical orientational relaxations arise
from dipolar groups attached to the backbone and small oscillations
of the chain geometries, especially reorientation of chain
ends.
[0058] It is seen that while the 0.104 and 0.124 volume fraction Al
nanocomposites undergo a dielectric relaxation and permittivity
decrease of almost 50% between 1 and 200 MHz, they maintain
relatively large permittivities (.about.6) in the 1 MHz-7 Ghz
range, and preferably in the 200 MHz-7 GHz range. It is also noted
that these materials appear to undergo another relaxation around
5-7 GHz. Relatively frequency-insensitive, high permittivities in
the GHz range make these composites ideal candidates for high
frequency dielectric applications. The ceramic particle
counterparts to these composites all have permittivities below 2 by
100 MHz and common radio frequency dielectrics have permittivities
on the order of 3-4.
[0059] The value of tan .delta. is the imaginary part divided by
the real part of the permittivity (tan .delta.=.di-elect
cons.''/.di-elect cons.').
r '' = 1 - .GAMMA. 2 .omega. C o Z o ( .GAMMA. 2 + 2 .GAMMA. cos
.theta. + 1 ) ( 4 ) ##EQU00003##
[0060] By calculating both the real part (eq 3) and the imaginary
part (eq 4) of the permittivity from the measured complex
reflection, tan .delta. is obtained (FIG. 16). Because the present
lumped impedance technique measures the permittivity via a
reflection method, there is some noise in the derived imaginary
part of the permittivity, which introduces tan .delta. noise due to
the very high measured magnitude of the reflection, .GAMMA.. The
loss, tan .delta., is proportional to the difference between the
reflection magnitude of the sample and perfect reflection (100%).
Lumped impedance measurements are typically limited to higher loss
systems, however, since the measured tan .delta. here is greater
than 0.02, this is not a major concern. However, overall these
composites have relatively low loss, since up to 7 GHz the loss
remains below 0.20.
[0061] As expected from the trends in permittivity, the tan .delta.
of the 0.124 volume fraction Al nanocomposite begins to rise
dramatically around 1 GHz, resulting in the permittivity fall
evident in FIG. 15. For both the 0.104 and 0.124 volume fraction
materials, the permittivity data suggest a dielectric relaxation in
the MHz range, but since the accuracy of the tan .delta. data in
this region does not allow extraction of the exact frequency of
this relaxation, the maximum in tan .delta. is not well-determined.
Such GHz frequency relaxations seen in the high volume fraction
polymer composites are most often attributed to orientational
relaxation such as rotation of dipolar groups around bonds, chain
twisting or libration. In ordered polymers lacking dipolar groups
as in isotatic polypropylene, such polarizations have been assigned
to chain end rotation, and therefore this relaxation is a fairly
small fraction of the total response. Because the polypropylene is
grown in situ on the nanoparticle surfaces, the higher volume
fraction nanocomposites should have greater surface areas, hence
higher chain end densities, and hence greater contributions from
reorientational polarization processes, likely enhancing the
relaxation processes observed at >3 GHz. Nevertheless, the 0.10
of Al-.sup.isoPP nanocomposite is the most useful material for GHz
range capacitor applications since the permittivity of .about.6 is
maintained above 5 GHz.
[0062] The most common effective medium models for permittivity are
derived for the simple case of a spherical dielectric inclusion
embedded in a sphere of the host material. However, most materials
do not occur naturally as spheres, and therefore effective medium
models for other shapes have also been developed. (See, e.g.,
Brosseau, C.; Beroual, A.; Boudida, A. J. Appl. Phys. 2000, 88,
7278-7288; Green, N. G.; Jones, T. B. J. Phys. D: Appl. Phys. 2007,
40, 78-85.) Simple analytical solutions for the effective
permittivity (.di-elect cons..sub.eff) can be derived only for
ellipsoids, whereas all other shapes require numerical solutions.
Depolarization factors along each semi-axis of the ellipsoid
(N.sub.x, N.sub.y, N.sub.z), where N.sub.x=N.sub.y=N.sub.z=1, can
be used to estimate geometrical effects. The depolarization factors
are calculated from integrals, e.g., eq. 5, where a.sub.x, a.sub.y,
a.sub.z are the semi-axes of the ellipsoid. For spheres, all
three
N x = a x a y a z 2 .intg. 0 .infin. 1 ( s + a x 2 ) ( s + a x 2 )
( s + a y 2 ) ( s + a z 2 ) s ( 5 ) ##EQU00004##
depolarization factors are equal (1/3, 1/3, 1/3), however, for
ellipsoids the depolarization factors are, 0, 1/2, 1/2,
respectively and for discs, 1, 0, 0, respectively. Since the
dielectric energy is a stationary functional of the electric field,
the result is that permittivities arising from spherical inclusions
are the lowest and any deviation from the spherical shape results
in an increase in the effective permittivity of the mixture at the
same volume fraction. These observations prompted study of
TiO.sub.2-isotactic polypropylene nanocomposites with different
inclusion shapes.
[0063] In FIG. 10, the calculated effective permittivities of the
nanocomposites containing spherical inclusions to the
nanocomposites are compared with ellipsoidal inclusions. For the
case of spherical inclusions, the effective permittivities are
calculated using the Maxwell-Garnett effective medium theory (eq.
6), and for the case of ellipsoidal inclusions,
eff = b a + 2 b + 2 f a ( a - b ) a + 2 b - 2 f a ( a - b ) ( 6 )
eff = b + f a 3 ( a - b ) j = x , y , z eff eff + N j ( a - eff ) (
7 ) ##EQU00005##
the effective permittivities are calculated using the Polder-Van
Santen formalism (eq. 7), where .di-elect cons..sub.a is the
relative permittivity of the TiO.sub.2 inclusions, .di-elect
cons..sub.b is the relative permittivity of isotactic
polypropylene, f.sub.a is the volume fraction of TiO.sub.2 in the
polymer, and N.sub.j is for the depolarization factors. (See, e,g.,
Busico, V.; Cipullo, R.; Monaco, G.; Vacatello, M. Macromolecules
1997, 30, 6251-6263.) As expected, the effective medium theory
predicts that composites containing ellipsoidal inclusions will
have larger effective permittivities at low volume loadings than
composites containing spherical inclusions.
[0064] The experimental results are plotted in FIG. 11. Remarkably,
the effective permittivities for spherical inclusions remain
constant over a small range of volume fractions, exactly as the
Maxwell-Garnett equation predicts (FIG. 10). In marked contrast,
the effective permittivity of composites having inclusions with
ellipsoidal shapes increases rapidly with increasing inclusion
volume fraction, which is again similar to trend predicted for
ellipsoidal inclusions using eq. 7 (FIG. 10).
Examples of the Invention
[0065] Materials and Methods. All manipulations of air-sensitive
materials were performed with rigorous exclusion of oxygen and
moisture in flamed Schlenk-type glassware on a dual-manifold
Schlenk line or interfaced to a high-vacuum line (10.sup.-5 Torr),
or in a dinitrogen-filled Vacuum Atmospheres glove box with a high
capacity recirculator (<1 ppm O.sub.2 and H.sub.2O). Propylene
(Matheson, polymerization grade) was purified by passage through a
supported MnO oxygen-removal column and an activated Davison 4
.ANG. molecular sieve column. Toluene was dried using an activated
alumina column and Q-5 columns according to the method described in
literature, and was additionally vacuum-transferred from Na/K alloy
and stored in Teflon-valve sealed bulbs for polymerization
manipulations. BaTiO.sub.3 and TiO.sub.2 nanoparticles were kindly
provided by Prof. Fatih Dogan (University of Missouri, Rolla) and
Prof. Thomas Shrout (Penn State University), respectively.
ZrO.sub.2 nanoparticles were purchased from Aldrich. The reagents 3
mol % yttria-stabilized zirconia (TZ3Y) and 8 mol %
yttria-stabilized zirconia (TZ8Y) nanoparticles were purchased from
Tosoh, Inc. TiO.sub.2 nanorods were purchased from Reade Advanced
Materials, Riverside, R.I. All of the nanoparticles were dried on a
high vacuum line (10.sup.-5 Torr) at 80.degree. C. overnight to
remove the surface-bound water, known to adversely affect the
dielectric breakdown performance. The deuterated solvent
1,1,2,2-tetrachloroethane-d.sub.2 was purchased from Cambridge
Isotope Laboratories (.gtoreq.99 atom % D) and used as received.
Methylaluminoxane (MAO; Aldrich) was purified by removing all the
volatiles in vacuo from a 1.0 M solution in toluene. The reagent
dichloro[rac-ethylenebisindenyl]zirconium (IV) (EBIZrCl.sub.2) was
purchased from Aldrich and used as received. n.sup.+--Si wafers
(rms roughness.apprxeq.0.5 nm) were obtained from Montco Silicon
Tech (Spring City, Pa.) and cleaned according to standard
procedures. Aluminum substrates were purchased from McMaster-Carr
(Chicago, Ill.) and cleaned according to standard procedures.
[0066] Physical and Analytical Measurements. NMR spectra were
recorded on a Varian Innova 400 (FT 400 MHz, .sup.1H; 100 MHz,
.sup.13C) spectrometer. Chemical shifts (.delta.) for .sup.13C
spectra were referenced using internal solvent resonances and are
reported relative to tetramethylsilane. .sup.13C NMR assays of
polymer microstructure were conducted in
1,1,2,2-tetrachloroethane-d.sub.2 containing 0.05 M Cr(acac).sub.3
at 130.degree. C. Resonances were assigned according to the
literature for stereoregular polypropylenes. Elemental analyses
were performed by Midwest Microlabs, LLC, Indianapolis, Ind.
Inductively coupled plasma-optical emission spectroscopy (ICP-OES)
analyses were performed by Galbraith Laboratories, Inc., Knoxyille,
Tenn. The thickness of the dielectric film was measured with a
Tencor P-10 step profilometer and used to calculate the dielectric
constant and breakdown strength of the sample. X-ray powder
diffraction patterns were recorded on a Rigaku DMAX-A
diffractometer with Ni-filtered Cu K.alpha. radiation (1.54184
.ANG.). Pristine ceramic nanoparticles and composite
microstructures were examined with a FEI Quanta sFEG environmental
scanning electron microscope with an accelerating voltage of 30 kV.
Transmission electron microscopy was performed on a Hitachi H-8100
TEM with an accelerating voltage of 200 kV. Composite melting
temperatures were measured on a TA Instruments 2920 temperature
modulated differential scanning calorimeter. Typically, ca. 10 mg
samples were examined, and a ramp rate of 10.degree. C./min was
used to measure the melting point. To erase thermal history
effects, all samples were subjected to two melt-freeze cycles. The
data from the second melt-freeze cycle are presented here.
[0067] Electrical Measurements.
[0068] Gold electrodes for metal-insulator-semiconductor (MIS)
devices were vacuum-deposited through shadow masks at (3-4) .delta.
10.sup.-6 Torr (500 .ANG., 0.2-0.5 .ANG./s). Direct current MIS
leakage current measurements were performed using Keithley 6430
sub-femtoamp remote source meter and a Keithley 2400 source meter
using a locally written LABVIEW program and general purpose
interface bus communication. A digital capacitance meter (Model
3000, GLK Instruments, San Diego) was used for capacitance
measurements. All measurements were performed under ambient
conditions. Dielectric breakdown strength measurements were carried
out with a high-voltage amplifier (TREK 30/20A, TREK, Inc., Medina,
New York), and the experimental parameters were: ramp rate, 1,000
V/S; peak voltage, 30,000 V; ext. amplifier, 3,000; temperature,
room temperature.
[0069] With reference to the following representative examples, any
metal oxide component, metallocene catalyst component, aluminoxane
component and olefin monomer component of the sort described herein
can be used interchangeably with any one of the other. Accordingly,
the general procedures of examples 1-2 were used to prepare the
range of nanocomposites referenced in conjunction with the
corresponding figures, such procedures as can be modified by those
skilled in the art made aware of this invention. As such, the
embodiments of example 3 were also prepared using such procedures
(corresponding figures and data not shown).
Example 1
Representative immobilization of Metallocene Catalysts on Metal
Oxide Nanoparticles
[0070] In the glovebox, 2.0 g nanoparticles, 200 mg MAO, and 50 mL
dry toluene were loaded into a predried 100 mL Schlenk flask. Upon
stirring, the mixture turned into a very fine slurry. The slurry
was next subjected to alternating sonication and vigorous stirring
for 2 days with constant removal of evolving CH.sub.4. Next, the
nanoparticles were collected by filtration and washed with fresh
toluene (50 mL.times.4) to remove any residual MAO. Then, 200 mg
metallocene catalyst was loaded in the flask with 50 mL toluene.
The color of the nanoparticles immediately turned purple. The
slurry mixture was again subjected to alternating sonication and
vigorous stirring overnight. The nanoparticles were then collected
by filtration and washed with fresh toluene until the color of the
toluene remained colorless. The nanoparticles were dried on the
high-vacuum line overnight and stored in the glovebox at
-40.degree. C.
Example 2
Representative Synthesis of Nanocomposites via In Situ Propylene
Polymerization
[0071] In the glovebox, a 250 mL round-bottom three-neck Morton
flask, which had been dried at 160.degree. C. overnight and
equipped with a large magnetic stirring bar, was charged with 50 mL
dry toluene, 200 mg functionalized nanoparticles, and 50 mg MAO.
The assembled flask was removed from the glovebox and the contents
were subjected to sonication for 30 min with vigorous stirring. The
flask was then attached to a high vacuum line (10.sup.-5 Torr),
freeze-pump-thaw degassed, equilibrated at the desired reaction
temperature using an external bath, and saturated with 1.0 atm
(pressure control using a mercury bubbler) of rigorously purified
propylene while vigorously stirring. After a measured time
interval, the polymerization was quenched by the addition of 5 mL
methanol, and the reaction mixture was then poured into 800 mL of
methanol. The composite was allowed to fully precipitate overnight
and was then collected by filtration, washed with fresh methanol,
and dried on the high vacuum line overnight to constant weight.
Example 3
[0072] More specifically, with reference to the preceeding,
BaTiO.sub.3 and TiO.sub.2 40 nm nanoparticles were dried on a
high-vacuum line to remove surface-bound water, known to adversely
affect dielectric breakdown performance. Nanocomposites were then
synthesized via sequential nanoparticle MAO functionalization,
catalyst immobilization/activation, and in situ isotactic propylene
polymerization (Schemes 1 and 2, FIG. 19). The first step is the
anchoring MAO onto the nanoparticle surfaces via surface hydroxyl
group reaction to form covalent Al--O bonds. Anchored MAO functions
as a cocatalyst to activate the metallocene, and in addition, the
hydrophobic MAO helps disrupt, in combination with ultrasonication,
hydrophilic nanoparticle agglomeration in the hydrophobic reaction
medium. After washing away unbound MAO, the MAO-coated
nanoparticles are subjected to reaction with the C.sub.2-symmetric
polymerization catalyst EBIZrCl.sub.2 to afford surface-anchored,
polymerization-active species. EBIZrCl.sub.2 is known to produce
highly isotactic polypropylene, which, in conventional capacitors,
affords enhanced mechanical and dielectric properties at elevated
operating temperatures. Subsequent in situ polymerization yields
isotactic polypropylene-BaTiO.sub.3/TiO.sub.2 nanocomposites, the
compositions of which can be tuned by the polymerization
conditions.
Example 4
[0073] Al (d=100 nm) nanoparticles with 2 nm native Al.sub.2O.sub.3
were purchased from Sigma-Aldrich. From the TEM it is clear that
the particles range from about 50 nm to 150 nm in diameter. The
nanoparticles were dried on a high vacuum line (10.sup.-5 Torr) at
80.degree. C. overnight to remove the surface-bound water. The
reagent [rac-ethylenebisindenyl]zirconium dichloride was purchased
from Sigma-Aldrich and used as received. MAO, 10% solution in
toluene, was also purchased from Sigma-Aldrich and purified by
removing the volatiles in vacuo. All manipulations of air-sensitive
materials were performed with rigorous exclusion of O.sub.2 and
moisture using Schlenk techniques, or a high-vacuum line (10.sup.-6
Torr), or a N.sub.2-filled MBraun glove box with a high capacity
recirculator (<1 ppm O.sub.2 and H.sub.2O). Propylene (Matheson,
polymerization grade) was purified by passage through a supported
MnO O.sub.2-removal column and an activated Davison 4 A molecular
sieve column. Toluene was dried using an activated alumina column
and Q-5 columns, and is then vacuum-transferred from Na/K alloy and
stored in Teflon-valve sealed bulbs.
[0074] In the glovebox, 2.0 g of nanoparticles, 200 mg of the
metallocene precatalyst EBIZrCl.sub.2 and 50 mL of toluene were
loaded into a predried 200 mL flip-fit flask. The color of the
particle suspension turned to light orange. The slurry mixture was
subjected to alternating sonication and vigorous stirring
overnight. The particles were then collected by filtration and
washed with fresh toluene until the color of the toluene remained
colorless. The particles were dried on the high-vacuum line
overnight and stored in the glovebox at -40.degree. C. in the
dark.
[0075] In the glovebox, a 250 mL round-bottom three-neck Morton
flask, equipped with a large magnetic stirring bar, was charged
with 50 mL of dry toluene, 200 mg of the above
catalyst-functionalized nanoparticles, and 50 mg of MAO. The
assembled flask was removed from the glovebox and the mixture was
subjected to sonication and vigorous stirring for 30 min. The flask
was then attached to a high vacuum line (10.sup.-5 Torr), the
catalyst slurry was degassed, equilibrated at the desired reaction
temperature using an external water bath, and saturated with 1.0
atm (pressure control using a mercury bubbler) of rigorously
purified propylene while vigorously stirring. After a measured time
interval (changing the interval results in different particle
loadings), the polymerization was quenched by the addition of 5 mL
of methanol, and the reaction mixture was then poured into 800 mL
of methanol. The composite was allowed to fully precipitate
overnight and was then collected by filtration, washed with fresh
methanol, and dried on the high vacuum line at 80.degree. C.
overnight to constant weight.
[0076] Elemental analyses were performed by Midwest Microlabs, LLC,
Indianapolis, Ind. Inductively coupled plasma-optical emission
spectroscopy (ICP-OES) analyses were performed by Galbraith
Laboratories, Inc., Knoxyille, Tenn.
Example 5
[0077] Films with diameters between 3 and 7 mm are required. To
obtain films that are robust to tearing at these diameters,
relatively thick films were fabricated. The films were fabricated
by slowly pressing the nanocomposite samples, that were heated
slowly in a crucible until the composite powder was viscous
(maximum surface temperature of crucible and composite was
100.degree. C.), into the openings of small metal or PET washers, 3
mm in diameter and 1 mm thick. The thick films were then pressed
with additional composite powder using a hot press at 100.degree.
C. and 500-800 psi pressure. The pressing helps create the
smoothest electrode-dielectric interface possible. Postpressing
vacuum treatment at 80.degree. C. was then performed overnight to
remove any residual moisture and trapped air bubbles. Next,
parallel-plate capacitors were fabricated by vapor-depositing gold
electrodes on the dielectric nanocomposite films. Gold electrodes
for metal insulator metal (MIM) devices were vacuum deposited
through shadow masks at (3-4).times.10.sup.-7 Torr (500 .ANG.,
0.2-1.0 .ANG./s). The films were then removed from the washers by
either cutting away the PET washer or boring the sample out of the
metal washer.
[0078] The thicknesses of the films were measured with calipers and
used to calculate the dielectric permittivity and tan .delta.. Film
topography and RMS roughnesses were imaged using a JEOL SPM atomic
force microscope. The thick films had rms roughnesses of 3-4 nm.
Low frequency (1 MHz) capacitance was measured on an HP 4384A
precision meter.
[0079] High frequency capacitance was measured using a lumped
impedance method on an HP 8510 network analyzer whose sample holder
terminated in an APC7 connector. This connector was fitted with the
"shorted coaxial cable" sample holder. Each 3 mm thick film sample
was placed directly on the center of the APC7 and then enclosed in
the sample holder. The sample holder was fitted with an electrode
at the end of a movable plunger which is brought into contact with
the upper film surface by tightening the feed screw. In this
configuration the sample holder inductance is minimized. To measure
the high frequency capacitance, a lumped impedance method of
measuring the complex reflection coefficient (both magnitude,
.GAMMA., and phase, .theta.) is utilized. By placing a thick sample
(.about.1 mm) at the end of the coaxial line, the reflection
coefficient of the impedance transported down the line can be
measured. Before measuring an unknown capacitor, a calibration was
performed by attaching known standards (short, open, and load) to
the end of the coaxial line.
[0080] As demonstrated, the present invention provides a range of
well-dispersed metal oxide-polyolefin nanocomposites via a
scalable, in situ supported metallocene olefin polymerization
process. Leakage current densities .about.10.sup.-8-10.sup.-6
A/cm.sup.2 suggest that the nanocomposites are excellent
insulators. The relative permittivity of the nanocomposites
increases as the nanoparticle fraction increases. At the same
inclusion loading, rod-shaped TiO.sub.2 nanoparticle-polypropylene
nanocomposites exhibit significantly greater relative
permittivities than sphere-shaped TiO.sub.2
nanoparticle-polypropylene nanocomposites. Energy densities of the
BaTiO.sub.3-polypropylene nanocomposites are found to be as high as
9.4 J/cm.sup.3. Energy densities of the
Al.sub.2O.sub.3-polypropylene nanocomposites are found to be as
high as about 14 J/cm.sup.3. This versatile approach offers
effective control over composite composition and ready scalability.
That is, simply by varying nanoparticle identity as well as their
sizes, shapes, and the metallocene catalysts used, a wide array of
nanocomposites with desired dielectric and mechanical properties
can thus be catalytically synthesized in situ.
* * * * *