U.S. patent application number 16/366640 was filed with the patent office on 2019-08-22 for low thermal conductivity carbon-containing materials and methods of producing the same.
The applicant listed for this patent is North Carolina Agricultural and Technical State University, Triad Growth Partners. Invention is credited to Alexis Wells Carpenter, Charlie Boyd Gause, Spero Gbewonyo, Lifeng Zhang.
Application Number | 20190257005 16/366640 |
Document ID | / |
Family ID | 61763063 |
Filed Date | 2019-08-22 |
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United States Patent
Application |
20190257005 |
Kind Code |
A1 |
Zhang; Lifeng ; et
al. |
August 22, 2019 |
LOW THERMAL CONDUCTIVITY CARBON-CONTAINING MATERIALS AND METHODS OF
PRODUCING THE SAME
Abstract
The presently disclosed subject matter relates generally to low
thermal conductivity carbon materials and methods of producing the
same. In some embodiments, the carbon materials are doped with low
thermally conductive nanoparticles. In some embodiments, carbon
fibers are prepared by electrospinning a mixture of polymers;
and/or incorporating a low thermal conductivity additive, such as
nanoparticles.
Inventors: |
Zhang; Lifeng; (Oak Ridge,
NC) ; Carpenter; Alexis Wells; (Carrboro, NC)
; Gause; Charlie Boyd; (Providence, NC) ;
Gbewonyo; Spero; (Greensboro, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
North Carolina Agricultural and Technical State University
Triad Growth Partners |
Greensboro
Greensboro |
NC
NC |
US
US |
|
|
Family ID: |
61763063 |
Appl. No.: |
16/366640 |
Filed: |
March 27, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/US2017/053762 |
Sep 27, 2017 |
|
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16366640 |
|
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62400410 |
Sep 27, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 2235/9607 20130101;
D01F 6/54 20130101; D10B 2505/00 20130101; C04B 38/0615 20130101;
C04B 35/524 20130101; C04B 35/83 20130101; D01D 5/247 20130101;
D10B 2321/10 20130101; C04B 38/0615 20130101; C04B 2111/28
20130101; D01D 5/0007 20130101; D01D 5/003 20130101; C04B 2235/3418
20130101; C04B 35/524 20130101; C04B 38/0054 20130101; D01F 9/22
20130101; C04B 2235/5454 20130101; D10B 2101/122 20130101; D01F
9/14 20130101; D10B 2401/10 20130101; C04B 2235/3454 20130101 |
International
Class: |
D01D 5/247 20060101
D01D005/247; D01D 5/00 20060101 D01D005/00; D01F 9/22 20060101
D01F009/22; D01F 6/54 20060101 D01F006/54 |
Claims
1. A method of preparing a multi-scale porous carbon-containing
material, having a thermal conductivity of less than about 5 W/m K,
the method comprising a. electrospinning a spin dope comprising a
first polymer that is a carbon precursor; and further comprising i.
a second polymer having a decomposition temperature lower than
about 600.degree. C.; and/or ii. nanoparticles having a boiling
point above about 1400.degree. C. and a thermal conductivity of
less than about 10 W/m K; wherein said electrospinning yields a
polymer nanofiber; b. stabilizing said polymer nanofiber; and c.
carbonizing said stabilized nanofiber at no more than about
1000.degree. C.
2. The method of claim 1, wherein the multi-scale porous
carbon-containing material comprises an electrospun carbon
nanofiber and has: a. pores having an average pore width between
about 1 .mu.m and about 10 .mu.m; b. pores having an average pore
width between about 100 nm and about 1000 nm; and/or c. pores
having an average pore width between about 1 nm and about 100
nm.
3. The method of claim 1, wherein said multi-scale porous
carbon-containing material has a thermal conductivity of less than
about 3 W/m K.
4. The method of claim 1, wherein said first polymer is
polyacrylonitrile.
5. (canceled)
6. (canceled)
7. The method of claim 1, wherein said second polymer is
poly(methyl methacrylate).
8. The method of claim 1, wherein said first polymer is
polyacrylonitrile, said second polymer is poly(methyl methacrylate)
and the ratio of polyacrylonitrile to poly(methyl methacrylate) is
between about 70:30 and about 50:50.
9. The method of claim 1, wherein said nanoparticles comprise
silicon dioxide.
10. The method of claim 4, wherein said nanoparticles in the spin
dope comprise at least about 2.5 wt % relative to the weight of
polyacrylonitrile.
11. (canceled)
12. The method of claim 1, comprising: a. electrospinning a spin
dope comprising polyacrylonitrile and i. a second polymer having a
decomposition temperature lower than about 600.degree. C.; and ii.
nanoparticles comprising silicon dioxide or calcium silicate;
wherein said electrospinning yields a polymer nanofiber; b.
stabilizing said polymer nanofiber; and c. carbonizing said
stabilized nanofiber at no more than about 1000.degree. C.
13. The method of claim 1, comprising electrospinning a spin dope
comprising polyacrylonitrile, poly(methyl methacrylate), and
nanoparticles comprising silicon dioxide.
14. The method of claim 1, wherein said stabilizing comprises
heating said polymer nanofiber to between about 220.degree. C. and
about 300.degree. C. and holding said temperature for sufficient
time to yield a stabilized intermediate; and wherein said
carbonizing comprises heating said stabilized intermediate to no
more than about 900.degree. C. and holding at said carbonization
temperature for at least about 30 minutes.
15. (canceled)
16. A nanofibrous carbon product comprising nanoparticles wherein
said nanoparticles have a boiling point above about 1400.degree. C.
and a thermal conductivity of less than about 10 W/m K, wherein
said nanofibrous carbon product has a thermal conductivity of no
more than about 5 W/m K.
17. The nanofibrous carbon product of claim 16, wherein said
product comprises a. pores having an average pore width between
about 1 .mu.m and about 10 .mu.m; b. pores having an average pore
width between about 100 nm and about 1000 nm; and/or c. pores
having an average pore width between about 1 nm and about 100
nm.
18. The nanofibrous carbon product of claim 16, wherein said
nanofibrous carbon product comprises electrospun carbon nanofibers,
optionally wherein said nanofibers comprise a carbon nanofiber
yarn.
19. The nanofibrous carbon product of claim 16, wherein said
nanoparticles comprise silicon dioxide.
20. (canceled)
21. A thermal insulating material comprising the nanofibrous carbon
product of claim 16.
22. A thermal insulating material comprising a multi-scale porous
carbon-containing material prepared according to the method of
claim 1, wherein said carbon-containing material has a thermal
conductivity of no more than about 4 W/m K.
23. A multi-scale porous carbon-containing structure comprising
carbon nanofibers having an average nanofiber diameter of between
about 300 nm and about 700 nm, the structure having a. a thermal
conductivity below about 4 W/m K; b. a specific surface area of at
least about 30 m.sup.2/g as measured by BET isotherm; and/or c. a
total pore volume of at least about 0.13 cm.sup.3/g as measured by
N.sub.2 gas sorption.
24. The structure of claim 23, wherein the structure has a thermal
conductivity below about 2 W/m K and an average nanofiber diameter
of between about 400 nm and about 600 nm.
25. A thermal insulating material comprising the multi-scale porous
structure of claim 23.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of PCT International
Patent Application Serial No. PCT/US2017/053762, filed Sep. 27,
2017, which is hereby incorporated by reference in its entirety and
which itself claims the benefit of and priority to U.S. Provisional
Patent Application Ser. No. 62/400,410, filed on Sep. 27, 2016,
which is hereby incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] The presently disclosed subject matter relates generally to
low thermal conductivity carbon materials having a multiscale
porous structure and methods of producing the same. In some
embodiments, the carbon materials are doped with low thermal
conductivity particles. In some embodiments, carbon nanofibers are
prepared by electrospinning a mixture of polymers and carbonizing
the electrospun nanofibers, which introduces pores, and/or
incorporating a low thermal conductivity additive, such as
nanoparticles. In other embodiments, carbon films are prepared by
casting a mixture of polymers, carbonizing the cast films to
introduce pores, and/or incorporating a low thermal conductivity
additive, such as nanoparticles.
BACKGROUND
[0003] Carbon-containing materials, in particular, carbon
nanofibers (`CNF`), are of great technological and industrial
importance because of properties such as high strength to weight
ratio, excellent chemical resistance, and superior electrical
conductivity. Polyacrylonitrile (`PAN`) is the precursor for nearly
90% of carbon fibers manufactured today. PAN-based carbon fibers
are normally high thermal conductivity material as influenced by
their carbonization temperature and resulting graphitic structure.
Generally the thermal conductivity of typical PAN-based carbon
fibers is .about.20-180 W/m K, but can sometimes reach .about.13
W/m K. CNF have been used to develop high-performance
fiber-reinforced composites with thermal conductivity
enhancement.
[0004] Low thermal conductivity carbon materials have a variety of
uses such as thermal insulation, particularly in ablative thermal
protection materials of reentry vehicles and rocket engine
components (motor nozzles and exit cones). Ablative applications
require low fiber thermal conductivity to minimize composite char
depth and temperature rise at the backface of the composite.
Rayon-based carbon fibers with thermal conductivity of .about.4 W/m
K (watts per meter Kelvin) have been used as key components for
ablative insulator composite materials. However, due to source
availability and manufacturing processes, replacements for
rayon-based carbon fibers are needed. Thus there is a need to
produce carbon-containing materials with low thermal
conductivity.
SUMMARY
[0005] Disclosed herein are low thermal conductivity carbon
materials and methods of producing the same. In some embodiments,
the carbon materials are prepared by electrospinning precursor
polymers doped with nanoparticles having low thermal conductivity
and carbonizing the product. In other embodiments, carbon fibers
are prepared by electrospinning spin dopes comprising a carbon
precursor and a second polymer which decomposes in the process of
carbonization, optionally in combination with low thermally
conductivity nanoparticles. In some embodiments as disclosed
herein, low thermal conductivity electrospun carbon nanofiber
(ECNF) mats having a unique multi-scale (micro-, submicro, and
nano-) porous structure result from electrospinning a spin dope
comprising a bicomponent polymer system and/or insulating
nanoparticles. In other embodiments, carbon films are prepared by
casting a spin dope comprising a bicomponent polymer system and/or
insulating nanoparticles, and carbonizing the cast films.
[0006] In one aspect, the present application discloses a method of
generating a multi-scale porous carbon-containing material, having
a thermal conductivity of less than about 5 W/m K, the method
comprising: (a) electrospinning a spin dope comprising a first
polymer that is a carbon precursor; and further comprising: (i) a
second polymer having a decomposition temperature lower than about
600.degree. C.; and/or (ii) nanoparticles having a boiling point
above about 1400.degree. C. and a thermal conductivity of less than
about 10 W/m K; wherein said electrospinning yields a polymer
nanofiber; and (b) stabilizing the polymer nanofiber and (c)
carbonizing said stabilized nanofiber at no more than about
1000.degree. C.
[0007] In another aspect, the present application discloses a
nanofibrous carbon product wherein said carbon product has a
thermal conductivity of no more than about 5 W/m K and comprises
nanoparticles with a boiling point above about 1400.degree. C. and
a thermal conductivity of less than about 10 W/m K.
[0008] In yet another aspect, the present application discloses a
thermal insulating material comprising a nanofibrous carbon product
disclosed herein.
[0009] In one aspect, the present application discloses a
multi-scale porous structure comprising carbon nanofibers, the
structure having: (a) a thermal conductivity below about 4 W/m K;
(b) an average nanofiber diameter of about 300-700 nm; (c) a
specific surface area of at least about 30 m.sup.2/g as measured by
BET isotherm and/or (d) a total pore volume of at least about 0.13
cm.sup.3/g as measured by N.sub.2 gas sorption.
[0010] Accordingly, it is an object of the presently disclosed
subject matter to provide low thermal conductivity carbon materials
and methods of producing the same.
[0011] An object of the presently disclosed subject matter having
been stated hereinabove, and which is achieved in whole or in part
by the presently disclosed subject matter, other objects will
become evident as the description proceeds when taken in connection
with the accompanying drawings as best described herein below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A better understanding of the features and advantages of the
present application will be obtained by reference to the following
detailed description that sets forth illustrative embodiments, in
which the principles of the application are utilized, and the
accompanying drawings of which:
[0013] FIG. 1 is a schematic diagram showing thermal conductivity
test using a Hot Disk TPS 2500.
[0014] FIG. 2A is a scanning electron microscopy (`SEM`) image of
ECNFs prepared from polyacrylonitrile (`PAN`) carbonized at
1200.degree. C.;
[0015] FIG. 2B is an SEM image of ECNFs prepared from PAN
carbonized at 900.degree. C.;
[0016] FIG. 2C is an SEM image of ECNFs prepared from a spin dope
comprising PAN with 5 wt. % SiO.sub.2 nanoparticles (`SiO.sub.2
NPs`) carbonized at 900.degree. C.;
[0017] FIG. 2D is an SEM image of ECNFs prepared from a spin dope
comprising PAN with 10 wt % SiO.sub.2 NPs carbonized at 900.degree.
C.;
[0018] FIG. 2E is an SEM image of ECNFs prepared from a spin dope
comprising a 70/30 mixture of PAN and poly(methyl methacrylate)
(`PMMA`): PAN/PMMA (70/30) carbonized at 900.degree. C.;
[0019] FIG. 2F is an SEM image of ECNFs prepared from a spin dope
comprising PAN/PMMA (50/50) carbonized at 900.degree. C.
[0020] FIG. 3A is an SEM image of a cross-section of ECNFs
carbonized at 900.degree. C.;
[0021] FIG. 3B is an SEM image of a cross-section of ECNFs with 10
wt % SiO.sub.2 NPs carbonized at 900.degree. C.
[0022] FIG. 4A is a transmission electron microscopy (`TEM`) image
of ECNFs prepared from PAN and carbonized at 900.degree. C.;
[0023] FIG. 4B is a TEM image of ECNFs prepared from PAN with 10
wt. % SiO.sub.2 NPs and carbonized at 900.degree. C.;
[0024] FIG. 4C is a TEM image of ECNFs prepared from a spin dope
comprising PAN/PMMA (70/30) and carbonized at 900.degree. C.;
[0025] FIG. 4D is a TEM image of ECNFs prepared from a spin dope
comprising PAN/PMMA (70/30) with 10 wt. % SiO.sub.2 NPs and
carbonized at 900.degree. C.;
[0026] FIG. 4E is a TEM image of ECNFs prepared from a spin dope
comprising PAN/PMMA (50/50) and carbonized at 900.degree. C.;
[0027] FIG. 4F is a TEM image of ECNFs prepared from a spin dope
comprising PAN/PMMA (50/50) with 10 wt. % SiO.sub.2 NPs and
carbonized at 900.degree. C.
[0028] FIG. 5A is an SEM image of PAN nanofiber yarns as collected
after electrospinning into a water bath;
[0029] FIG. 5B is an SEM image of PAN nanofiber stretched according
to the methods disclosed herein;
[0030] FIG. 5C is an SEM image of nanofibers stabilized according
to the methods disclosed herein; and
[0031] FIG. 5D is an SEM image of nanofibers carbonized according
to the methods disclosed herein.
DETAILED DESCRIPTION
[0032] The presently disclosed subject matter will now be described
more fully. The presently disclosed subject matter can, however, be
embodied in different forms and should not be construed as limited
to the embodiments set forth herein below and in the accompanying
Examples. Rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope of the embodiments to those skilled in the art.
[0033] All references listed herein, including but not limited to
all patents, patent applications and publications thereof, and
scientific journal articles, are incorporated herein by reference
in their entireties to the extent that they supplement, explain,
provide a background for, or teach methodology, techniques, and/or
compositions employed herein.
Definitions
[0034] While the following terms are believed to be well understood
by one of ordinary skill in the art, the following definitions are
set forth to facilitate explanation of the presently disclosed
subject matter.
[0035] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood to one of
ordinary skill in the art to which the presently disclosed subject
matter belongs.
[0036] Following long-standing patent law convention, the terms
"a", "an", and "the" refer to "one or more" when used in this
application, including the claims.
[0037] The term "and/or" when used in describing two or more items
or conditions, refers to situations where all named items or
conditions are present or applicable, or to situations wherein only
one (or less than all) of the items or conditions is present or
applicable.
[0038] The use of the term "or" in the claims is used to mean
"and/or" unless explicitly indicated to refer to alternatives only
or the alternatives are mutually exclusive, although the disclosure
supports a definition that refers to only alternatives and
"and/or."
[0039] As used herein "another" can mean at least a second or
more.
[0040] As used herein, the term "about", when referring to a value
is meant to encompass variations of in one example .+-.20% or
.+-.10%, in another example .+-.5%, in another example .+-.1%, and
in still another example .+-.0.1% from the specified amount, as
such variations are appropriate to perform the disclosed methods.
Accordingly, unless indicated to the contrary, the numerical
parameters set forth in this specification and attached claims are
approximations that can vary depending upon the desired properties
sought to be obtained by the presently disclosed subject
matter.
[0041] As used herein, "spin dope" refers to a fluid to be
electrospun as disclosed herein. When the spin dope comprises a
first polymer and a solvent, electrospinning is referred to as
solution electrospinning. Typically the spin dope solvent is
dimethylformamide (`DMF`), but other appropriate solvents are
well-known to those of skill in the art. In some examples, the spin
dope does not include a solvent and is used in a process referred
to as "melt electrospinning."
[0042] Generally, the spin dope comprises a first polymer which is
a carbon precursor, (e.g. a polymer that upon carbonization yields
a substantially pure carbon material). Such carbon precursors
include, but are not limited to polyacrylonitrile (PAN), polyvinyl
alcohol (PVA), and pitch. Often, the spin dope comprises a second
polymer, which decomposes significantly (e.g. more than about 50%,
more than about 75%, more than about 90% or more than about 95%)
during the disclosed carbonization process. The most commonly
employed second polymer is poly(methyl methacrylate) (PMMA), but
other polymers can have the disclosed characteristics of the second
polymer.
[0043] As disclosed herein, the ratio of the first
carbon-containing polymer to the second polymer in the spin dope
can vary. Using PAN as representative of the first polymer and PMMA
as the second polymer: the ratio of PAN to PMMA by weight
("PAN/PMMA") in the spin dope can vary. In one variation, the spin
dope comprises 100% PAN. Alternately, the ratio is 80/20 PAN/PMMA
or 70/30 PAN/PMMA. In another variation, the ratio is 50/50
PAN/PMMA. Usually the ratio comprises at least 50% PAN, for example
a ratio of 55/45, 60/40, 65/35, 70/30, 75/25, 80/20, 85/15, 90/10
or 95/5 PAN/PMMA. The pore structure, including the degree of
porosity, in the electrospun carbon nanofiber can vary based on the
proportion of the second polymer. Larger relative amounts of the
second (decomposing) polymer typically lead to larger pores.
[0044] As used herein "inert atmosphere" refers generally to an
inert gas, such as nitrogen, argon, helium, neon, or the like, or a
combination thereof, or may refer to any other gas, as long as the
gas is not chemically reactive with the electrospun fibers or the
nanofibers described herein.
[0045] As used herein, "stabilization" refers to heat treatment of
the polymer nanofibers between about 100.degree. C. and about
400.degree. C., typically in the range of about 200.degree. C. to
about 300.degree. C., with a heating rate of up to about 2.degree.
C./min, or between about 0.5.degree. C./min and about 2.degree.
C./min, in an oxygen-containing atmosphere (e.g. air). Without
being bound by theory, the heat stabilization is generally thought
to crosslink the carbon-containing molecules so that they can
survive higher temperature carbonization without melting and/or
decomposing
[0046] As used herein, the term "carbonization," "carbonized," or
"carbonize" refers to heat treatment of the stabilized nanofibers
described above, wherein carbonization temperature is typically no
higher than about 1000.degree. C., or generally between about
600.degree. C. to about 1000.degree. C., alternately from about
700.degree. C. to about 900.degree. C. In some embodiments, the
carbonization temperature is reached via a heating rate of up to
about 10.degree. C./min, generally in an inert atmosphere, such as
for example, argon, or under a vacuum. Alternately, the
carbonization temperature can be reached using a heating rate of up
to 5.degree. C./min or at a rate between about 5.degree. C./min and
10.degree. C./min. In some variations, the carbonization
temperature is no more than 600.degree. C., no more than
700.degree. C., no more than about 800.degree. C., no more than
about 900.degree. C., or no more than about 1000.degree. C.
[0047] As used herein, "carbon nanofibers" refers to carbon fibers
with diameters from about 10 nm to about 1000 nm. The electrospun
carbon nanofibers (`ECNF`) described herein result from heat
treatment of polymer nanofibers, wherein the resulting carbon
nanofibers have a cylindrical shape and relatively high specific
surface area. When a spin dope comprises a combination of two
polymers, such as a first polymer, PAN, and a second polymer, PMMA,
the PMMA component can be removed via thermal decomposition during
the carbonization processes and the resulting carbon nanofiber will
have particular properties as outlined herein. When a spin dope
comprises nanoparticles and the spin dope is electrospun into
nanofibers, the nanoparticles are not displaced by the heat
treatment nor thermally decomposed during the carbonization
process, but as shown herein, remain a component part of the
resulting modified ECNFs. The nanoparticles can be found on the
surface of the nanofibers, as well as incorporated into the
nanofibers themselves.
[0048] As used herein, "nanoparticles" refers to mostly spherical
particles having an average diameter of less than about 1000 nm.
Generally, nanoparticles have an average diameter of less than
about 750 nm, less than about 500 nm, less than about 250 nm, less
than about 100 nm, less than about 25 nm, less than about 12 nm, or
even less than about 7 nm. Alternately, the nanoparticles can have
an average diameter of between about 5 and about 15 nm, between
about 15 nm and 50 nm, between about 100 nm and 300 nm, or between
about 10 nm and 500 nm. "Low thermal conductivity" when used to
describe nanoparticles generally refers to nanoparticles having
thermal conductivity lower than 10 W/m K. Generally low thermal
conductivity nanoparticles have a thermal conductivity lower than
about 7.5 W/m K, lower than about 5 W/m K, lower than about 2.5 W/m
K or lower than about 2 W/m K, or even lower than about 1 W/m K.
Nanoparticles used in the methods and compositions disclosed herein
generally includes nanoparticles that are stable during
carbonization (that is, have a decomposition temperature well above
the carbonization temperature) and have low thermal conductivity,
as disclosed above. In some embodiments, the nanoparticles having a
boiling point above about 1400.degree. C. Such nanoparticles can
include, but are not limited to silica (SiO.sub.2) nanoparticles,
calcium silicate (Ca.sub.2SiO.sub.4) nanoparticles, and clay
nanoparticles.
[0049] "Multi-scale porous" such as when used in combination with a
material, e.g. "multiscale porous material," or "multiscale porous
nanofiber mat," refers to a material or mat having pores having
average pore widths of different scales, such as two or more of the
following: nanometer-sized, sub-micrometer-sized, and
micrometer-sized pores. Micrometer-sized pores generally arise from
inter-fiber pores (pores formed between nanofibers) and have an
average pore width above 1 micron (.mu.m). In some embodiments, the
micrometer-sized pores can have an average pore width of between
about 1 .mu.m and about 25 .mu.m; alternately, between about 1
.mu.m and about 10 .mu.m, between about 2 .mu.m and about 8 .mu.m,
or between about 1 .mu.m and about 5 .mu.m. Sub-micrometer pores
generally have an average pore width between about 100 nm and 1000
nm, or between about 100 nm and about 750 nm, or between about 100
nm and about 500 nm. Pores of nanometer scale are typically less
than about 100 nm, generally between about 1 nm and 100 nm, or
between about 1 nm and about 75 nm, or between about 1 nm and about
50 nm, or between about 1 nm and about 25 nm, or between about 1 nm
and about 15 nm. In some variations, the micrometer-sized pores can
have an average width of no more than about 16 .mu.m, no more than
about 14 .mu.m, no more than about 12 .mu.m, no more than about 10
.mu.m, no more than about 9 .mu.m, no more than about 8 .mu.m, no
more than about 7 .mu.m, no more than about 6 .mu.m, no more than
about 5 .mu.m, no more than about 4 .mu.m, no more than about 3
.mu.m no more than about 2 .mu.m, or no more than about 1 .mu.m. In
some variations, the sub-micrometer-sized pores can have an average
width of no more than about 1000 nm, or no more than about 900 nm,
or no more than about 800 nm, or no more than about 700 nm, or no
more than about 600 nm, or no more than about 500 nm, or no more
than about 400 nm, or no more than about 300 nm, or no more than
about 200 nm. In some variations, the nanometer-sized pores can
have an average width of no more than about 100 nm, or no more than
about 90 nm, or no more than about 80 nm, or no more than about 70
nm, or no more than about 60 nm, or no more than about 50 nm, or no
more than about 40 nm, or no more than about 30 nm, or no more than
about 20 nm.
[0050] As used herein "silica nanoparticles" or "SiO.sub.2 NPs"
refer to particles comprising or consisting essentially of
SiO.sub.2 (MW 60.08). Silica nanoparticles can be found in a
variety of particle sizes (and corresponding surface areas).
Representative sizes include, but are not limited to particle size:
average particle size: 7 nm, surface area: 370-420 m.sup.2/g, (ii)
average particle size: 12 nm, surface area: 175-225 m.sup.2/g, and
(iii) 5-15 nm, surface area: 590-690 m.sup.2/g. Alternate sizes of
nanoparticles or combinations of the nanoparticles disclosed herein
can similarly be used in the methods disclosed herein.
[0051] As used herein "thermal conductivity" refers to the property
of a material to conduct heat. It is commonly measured through
transient plane source (TPS) method (see e.g. Log, T., and S. E.
Gustafsson. "Transient plane source (TPS) technique for measuring
thermal transport properties of building materials." Fire and
materials 19.1 (1995): 43-49). "Low" thermal conductivity generally
refers to compositions having thermal conductivity lower than 10
W/m K, lower than about 8 W/m K, lower than about 5 W/m K, lower
than about 2.5 W/m K, or lower than about 2 W/m K, or lower than
about 1 W/m K, or even lower than about 0.5 W/m K.
[0052] Thermal conductivity measurements of carbon fibers disclosed
herein generally show that an increase in the heat treatment
temperature corresponds to an increase in the thermal conductivity
of the resulting carbon nanofibers. Due to structural
characteristics at both the microscale and nanoscale, the thermal
conductivity of a material can be affected by its porosity, at
least in part due to phonon scattering, a mechanism of heat
conduction in carbonaceous materials. Thermal conductivity
generally decreases with increasing porosity.
[0053] As shown herein, low thermal conductivity carbon materials
have been prepared from electrospinning. Electrospinning a
polymer-containing spin dope yields a non-woven nanofibrous mat
with micrometer scale inter-fiber pores. Sub-micrometer and
nanometer pores can be introduced, according to the methods of the
present application, with the addition to the spin dope of a second
polymer and/or nanoparticles. Multiscale porous structures are
disclosed herein; the presence of only one set (micrometer scale,
sub-micrometer scale, or nanometer scale) of pores do not lead to
the same improved properties reported herein. According to the
methods disclosed herein, produced products have a distribution of
pores across two or more scales, e.g. micrometer and
sub-micrometer, micrometer and nanometer, sub-micrometer and
nanometer, or have a distribution of pores across all three scales:
micrometer, sub-micrometer and nanometer.
[0054] One factor influencing the size of the multiscale pores
prepared according to the methods disclosed herein is the diameter
of the electrospun nanofibers themselves, both before and after
carbonization. In particular, the sub-micrometer and nanometer pore
domains, the sources of which are typically the removal of the
second polymer during carbonization and the presence of
nanoparticles incorporated into the nanofibers or onto the
nanofiber surfaces, are constrained by the size of the nanofibers
themselves. Generally, carbon nanofibers prepared from a mixed
polymer system have diameters of between about 400 nm and about 600
nm and have sub-micrometer pores of between about 100 nm and about
500 nm. As disclosed herein, a sub-micrometer scale carbon porous
structure was prepared by addition of varying amounts of the second
polymer. Smaller diameter carbon nanofibers typically have smaller
sub-micrometer pores, while carbon nanofibers with larger diameters
will typically have sub-micrometer pores that are larger.
[0055] The methods disclosed herein include, but are not limited
to: (a) electrospinning a spin dope comprising a carbon precursor
and a second polymer that decomposes during carbonization; and
controlled carbonization of to yield a micrometer scale inter-fiber
porous structure of electrospun carbon nanofiber; optionally
nanoparticles are included in the spin dope. Exemplifying of such
methods, varying ratios of poly(methyl methacrylate) (PMMA) were
added to a PAN-containing spin dope. SiO.sub.2 NPs (thermal
conductivity .about.1.5 W/m K) were used as a representative
nanoparticulate insulator additive. According to the methods
disclosed herein, a carbon nano-fibrous material having a
multi-scale porous structure with thermal conductivity as low as
0.15 W/m K was prepared.
[0056] In one aspect, the present application discloses a method of
preparing a multi-scale porous carbon-containing material, having a
thermal conductivity of less than about 5 W/m K, the method
comprising: (a) electrospinning a spin dope comprising a first
polymer that is a carbon precursor; and further comprising: (i) a
second polymer having a decomposition temperature lower than about
600.degree. C.; and/or (ii) nanoparticles having a boiling point
above about 1400.degree. C. and a thermal conductivity of less than
about 10 W/m K; wherein said electrospinning yields a polymer
nanofiber; and (b) carbonizing a stabilized nanofiber at no more
than about 1000.degree. C. In another aspect, the present
application discloses a method of preparing a multi-scale porous
carbon-containing material, having a thermal conductivity of less
than about 4 W/m K, the method comprising: (a) electrospinning a
spin dope comprising a first polymer that is a carbon precursor; a
second polymer having a decomposition temperature lower than about
600.degree. C.; and nanoparticles having a boiling point above
about 1400.degree. C. and a thermal conductivity of less than about
5 W/m K; wherein said electrospinning yields a polymer nanofiber;
(b) stabilizing said polymer nanofiber; and (c) carbonizing said
stabilized nanofiber at no more than about 1000.degree. C.
[0057] In one embodiment, the multi-scale porous carbon-containing
material comprises an electrospun carbon nanofiber and has: (a)
pores having an average pore width between about 1 .mu.m and about
10 .mu.m; (b) pores having an average pore width between about 100
nm and about 1000 nm; and/or (c) pores having an average pore width
between about 1 nm and about 100 nm. In another embodiment, the
material comprises an electrospun carbon nanofiber and has: (a)
pores having an average pore width between about 1 .mu.m and about
10 .mu.m; (b) pores having an average pore width between about 100
nm and about 1000 nm; and (c) pores having an average pore width
between about 1 nm and about 100 nm. In yet another embodiment, the
fibrous material has (a) pores having an average pore width between
about 1 .mu.m and about 5 .mu.m; (b) pores having an average pore
width between about 100 nm and about 500 nm; and (c) pores having
an average pore width between about 1 nm and about 75 nm. In yet
another embodiment, the fibrous material has (a) pores having an
average pore width between about 1 .mu.m and about 5 .mu.m; (b)
pores having an average pore width between about 100 nm and about
200 nm; and (c) pores having an average pore width between about 1
nm and about 25 nm.
[0058] In one variation of any aspect or embodiment, the
multi-scale porous carbon-containing material disclosed herein has
a thermal conductivity of less than about 10 W/m K. In another
variation of any aspect or embodiment, the multi-scale porous
carbon-containing material disclosed herein has a thermal
conductivity of less than about 5 W/m K; less than about 4 W/m K,
less than about 3 W/m K, less than about 2 W/m K, less than about 1
W/m K, less than about 0.5 W/m K, or less than about 0.25 W/m K, or
even less than about 0.2 W/m K.
[0059] In one variation of any aspect or embodiment disclosed
herein, the first polymer is polyacrylonitrile (PAN). In another
variation, the first polymer is polyvinyl alcohol (PVA). In one
embodiment, the second polymer has a decomposition temperature
below about 500.degree. C. In one variation of any aspect or
embodiment, the second polymer in the bicomponent polymer system is
poly(methyl methacrylate).
[0060] In one embodiment, the ratio of the first polymer to the
second polymer is 90:10 or 80:20 or 70/30 or 60:40 or 50:50.
Usually the ratio comprises at least 50% of the first polymer, for
example a ratio of 55:45, 60:40, 65:35, 70:30, 75:25, 80:20, 85:15,
90:10 or 95:5 first polymer:second polymer. In one variation, the
ratio of polyacrylonitrile to poly(methyl methacrylate) is between
about 80:20 and about 50:50; alternately the ratio of PAN to PMMA
is between about 70:30 and about 50:50. In another variation, the
ratio of polyvinyl alcohol to poly(methyl methacrylate) is between
about 80:20 and about 50:50; alternately the ratio of PVA to PMMA
is between about 70:30 and about 50:50.
[0061] In one variation of any aspect or embodiment disclosed
herein, nanoparticles disclosed in the methods and compositions
herein have a boiling point above about 1400.degree. C. and a
thermal conductivity of less than about 7.5 W/m K. In another
variation, the nanoparticles have a thermal conductivity of less
than about 5 W/m K or less than about 2.5 W/m K. In one variation,
the nanoparticles comprise silicon dioxide. In another variation,
the nanoparticles comprise calcium silicate. In another variation,
the nanoparticles comprise nanoclay.
[0062] In one variation of any aspect or embodiment disclosed
herein, nanoparticles disclosed in the methods comprise at least
about 2.5 wt % of the spin dope relative to the weight of the first
polymer. In another variation, the nanoparticles comprise at least
about 5 wt % of the spin dope relative to the weight of first
polymer; alternately, the nanoparticles comprise at least about 10
wt % of the spin dope relative to the weight of the first polymer
or at least about 20 wt % of the spin dope relative to the weight
of the first polymer. In another variation, nanoparticles in the
fibrous compositions disclosed herein comprise at least about 5 wt
% of the carbonized nanofiber. In another variation, nanoparticles
comprise at least about 10 wt % of the carbonized nanofiber, or at
least about 20 wt % of the carbonized nanofiber, or even at least
about 40 wt % of the carbonized nanofiber. In one embodiment of the
methods disclosed herein, heat treating the electrospun polymer
nanofibers comprises heating to a stabilization temperature between
about 220.degree. C. and about 300.degree. C. and holding at the
stabilization temperature for sufficient time to yield a stabilized
nanofiber; and then heating the stabilized nanofiber to a
carbonization temperature of no more than about 1000.degree. C. and
holding at said carbonization temperature for at least about 30
minutes. In one variation, the stabilizing temperature is between
about 250.degree. C. and about 290.degree. C. and the sufficient
time is at least about 30 minutes, at least about 1 hour, at least
about 2 hours, at least about 3 hours, or at least about 4 hours.
In another variation, the stabilizing temperature is about
280.degree. C. and the sufficient time is at least about 30
minutes, at least about 1 hour, at least about 2 hours, at least
about 3 hours, or at least about 4 hours. In one variation, the
carbonization temperature is no more than about 950.degree. C., or
no more than about 900.degree. C., no more than about 850.degree.
C., no more than about 800.degree. C., no more than about
750.degree. C. Alternately, the carbonization temperature is at
least about 600.degree. C., at least about 650.degree. C., at least
about 700.degree. C., at least about 750.degree. C., at least about
800.degree. C., at least about 850.degree. C., at least about
900.degree. C., at least about 950.degree. C., or at least about
1000.degree. C. In one variation, the carbonization temperature is
held for at least about 30 minutes, at least about 1 hour, at least
about 2 hours, at least about 3 hours, at least about 4 hours, at
least about 5 hours, at least about 6 hours, at least about 7
hours, at least about 8 hours, at least about 9 hours, or at least
about 10 hours.
[0063] In one aspect disclosed herein is a nanofibrous carbon
product having a thermal conductivity of no more than about 5 W/m
K, comprising nanoparticles wherein the nanoparticles have a
boiling point above about 1400.degree. C. and a thermal
conductivity of less than about 10 W/m K. In one embodiment, the
nanofibrous carbon product comprises (a) pores having an average
pore width between about 1 .mu.m and about 10 .mu.m; (b) pores
having an average pore width between about 100 nm and about 1000
nm; and/or (c) pores having an average pore width between about 1
nm and about 100 nm. In another embodiment, the nanofibrous carbon
product comprises electrospun carbon nanofibers and has: (a) pores
having an average pore width between about 1 .mu.m and about 10
.mu.m; (b) pores having an average pore width between about 100 nm
and about 1000 nm; and (c) pores having an average pore width
between about 1 nm and about 100 nm. In yet another embodiment, the
nanofibrous carbon product has (a) pores having an average pore
width between about 1 .mu.m and about 5 .mu.m; (b) pores having an
average pore width between about 100 nm and about 500 nm; and (c)
pores having an average pore width between about 1 nm and about 75
nm. In yet another embodiment, the nanofibrous carbon product has
(a) pores having an average pore width between about 1 .mu.m and
about 5 .mu.m; (b) pores having an average pore width between about
100 nm and about 200 nm; and (c) pores having an average pore width
between about 1 nm and about 50 nm. In one variation, the
nanoparticles comprise silicon dioxide. In another variation, the
nanoparticles comprise calcium silicate. In one variation, the
nanofibrous carbon product comprises electrospun carbon nanofibers.
In one variation, the nanofibrous carbon product has a thermal
conductivity of no more than about 2 W/m K, no more than about 1
W/m K, no more than about 0.5 W/m K, no more than about 0.25 W/m K
or no more than about 0.20 W/m K.
[0064] In another aspect, the present application discloses a
thermal insulating material comprising a nanofibrous carbon product
described herein.
[0065] In some embodiments, the polymer nanofibers are prepared by
electrospinning a spin dope. In some embodiments, the
electrospinning is solution electrospinning. In some embodiments,
the electrospinning is melt electrospinning.
[0066] As disclosed herein, the electrospun polymer nanofibers are
stabilized, or heated in an oxygen-containing atmosphere using a
heating rate of no more than about 5.degree. C. per minute, or
alternately between about 0.5.degree. C. per minute and 2.0.degree.
C. per minute to a stabilization temperature of at least about
200.degree. C. In one variation the stabilization temperature is
held for at least about 1 hour to yield stabilized nanofibers. In
some embodiments, the heating rate to the stabilization temperature
is about 0.75.degree. C./min to about 1.75.degree. C./min, or about
1.degree. C./min to about 1.5.degree. C./min. In some embodiments,
the stabilization temperature is at least about 210.degree. C., at
least about 220.degree. C., at least about 230.degree. C., at least
about 240.degree. C., at least about 250.degree. C., at least about
260.degree. C., at least about 270.degree. C., at least about
280.degree. C., at least about 290.degree. C., or at least about
300.degree. C. In some embodiments, the stabilization temperature
is between about 220.degree. C. and about 300.degree. C. or between
about 250.degree. C. and about 290.degree. C., or the stabilization
temperature is about 280.degree. C. The polymer nanofibers are held
at the stabilization temperature for at least about 30 minutes, at
least about 1 hour, at least about 2 hours, at least about 3 hours,
at least about 4 hours, at least about 5 hours, or at least about 6
hours. Alternately the nanofibers are held at the stabilization
temperature for between about 1 hour and about 10 hours, or between
about 2 and about 9 hours, or between about 4 and about 7 hours, or
held for about 5 hours to yield a stabilized nanofiber.
[0067] As disclosed herein, the stabilized nanofibers can be cooled
to room temperature. Alternately, the stabilized nanofibers can be
directly subjected to heating for carbonization.
[0068] As disclosed herein, the stabilized nanofibers are
carbonized, or heated in an inert atmosphere using a heating rate
of between about 1.degree. C. per minute and 15.degree. C. per
minute to a carbonization temperature of no more than about
1000.degree. C. and held there for at least about 0.5 hours to
yield carbonized nanofibers. In some embodiments, the heating rate
to the carbonization temperature is about 2.degree. C./min to about
15.degree. C./min, or about 2.degree. C./min-12.5.degree. C./min,
or about 2.degree. C./min-10.degree. C./min, or about 5.degree.
C./min-10.degree. C./min, or is about 10.degree. C./min, or about
9.degree. C./min, or about 8.degree. C./min, or about 7.degree.
C./min, or about 6.degree. C./min, or about 5.degree. C./min, or
about 4.degree. C./min. In some embodiments, the carbonization
temperature is about 700.degree. C-1000.degree. C. or about
750.degree. C-950.degree. C., or is about 800.degree. C-900.degree.
C. In some embodiments, the carbonization temperature is at least
about 600.degree. C., or at least about 700.degree. C., or at least
about 800.degree. C., or at least about 900.degree. C. In some
embodiments, the carbonization time is at least about 30 minutes;
at least about 1 hour, at least about 2 hours, at least about 3
hours, at least about 4 hours, at least about 5 hours, or at least
about 6 hours. In some embodiments the carbonization time is about
0.5-5 hrs, or about 1-4 hrs, or about 1-3 hrs.
[0069] In some embodiments, a carbon nanofiber mat of the present
application has a multi-scale porous structure, a thermal
conductivity below about 4 W/m K, an average nanofiber diameter of
between about 200 nm and about 900 nm; and (a) a specific surface
area of at least about 30 m.sup.2/g as measured by BET
(Brunauer-Emmett-Teller) isotherm and/or (b) a total pore volume of
at least about 0.13 cm.sup.3/g as measured by N.sub.2 gas sorption.
In other embodiments, a carbon nanofiber mat of the present
application has a multi-scale porous structure, a thermal
conductivity below about 4 W/m K, an average nanofiber diameter of
between about 300 nm and about 700 nm; and (a) a specific surface
area of at least about 45 m.sup.2/g as measured by BET isotherm
and/or (b) a total pore volume of at least about 0.17 cm.sup.3/g as
measured by N.sub.2 gas sorption. In other embodiments a carbon
nanofiber mat of the present application has a multi-scale porous
structure, a thermal conductivity below about 2 W/m K, an average
nanofiber diameter of between about 400 nm and about 600 nm; and
(a) a specific surface area of at least about 50 m.sup.2/g as
measured by BET isotherm and/or (b) a total pore volume of at least
about 0.17 cm.sup.3/g as measured by N.sub.2 gas sorption. In some
embodiments, the nanoparticles comprise silicon dioxide; in other
embodiments, the nanoparticles comprise calcium silicate. In
another embodiment, the present application discloses a thermal
insulating material comprising a carbon nanofiber mat having a
multi-scale porous structure as disclosed herein.
EXAMPLES
[0070] The following Examples have been included to provide
guidance to one of ordinary skill in the art for practicing
representative embodiments of the presently disclosed subject
matter. In light of the present disclosure and the general level of
skill in the art, those of skill can appreciate that the following
Examples are intended to be exemplary only and that numerous
changes, modifications, and alterations can be employed without
departing from the scope of the presently disclosed subject
matter.
Materials
[0071] Each of the following were purchased from Sigma-Aldrich (St.
Louis, Mo.): Polyacrylonitrile (PAN, average molecular weight
Mw=150,000, catalog #181315), Poly(methyl methacrylate) (PMMA,
average molecular weight Mw=120,000, catalog #182230), three
different types of silicon dioxide nanoparticles (SiO.sub.2,
molecular weight Mw=60.08): (i) particle size: 7 nm, surface area:
370-420 m.sup.2/g, catalog #S5130; (ii) average particle size: 12
nm, surface area: 175-225 m.sup.2/g, catalog #718483; and (iii)
particle size: 5-15 nm, surface area: 590-690 m.sup.2/g, catalog
#637246, and N,N-Dimethylformamide (DMF, catalog #227056). The
chemicals were used as received without further purification.
Example 1
CF and ECNF optionally with SiO.sub.2 NP
Spin Dope Preparation
[0072] The targeted polymers were dissolved in DMF to prepare 9 wt.
% solutions for spinning. For those spin dopes containing SiO.sub.2
nanoparticles (SiO.sub.2 NPs, average particle size 12 nm),
SiO.sub.2 NPs were first added to DMF and the suspension was
sonicated for 20 min; then the solution containing SiO.sub.2 NPs
was mixed with the disclosed polymer solution under constant
stirring followed by sonication for another 10 minutes to break up
any aggregates of the SiO.sub.2 NPs and ensure even distribution of
SiO.sub.2 NPs within the solution before electrospinning (Refer to
Table 1). The amount of SiO.sub.2 NPs was determined relative to
the mass of PAN used in the spin dope. As shown in Table 1,
examples of 5 wt % SiO.sub.2 NP relative to the amount of PAN and
10 wt % SiO.sub.2 NP relative to the amount of PAN were used.
TABLE-US-00001 TABLE 1 Preparative spin dopes Product name
Preparative spin dope ECNFs 9 wt % PAN in DMF ECNFs with 5 wt %
SiO.sub.2 9 wt % PAN + NPs 2.5 (wt % of PAN) SiO.sub.2 in DMF ECNFs
with 10 wt % SiO.sub.2 9 wt % PAN + NPs 5 (wt % of PAN) SiO.sub.2
in DMF ECNFs from electrospinning 9 wt % PAN/PMMA (mass ratio:
70/30) PAN/PMMA (70/30) in DMF ECNFs from electrospinning 9 wt %
PAN/PMMA (mass ratio: 50/50) PAN/PMMA (50/50) in DMF ECNFs with 5
wt % SiO.sub.2 9 wt % PAN/PMMA (mass ratio: 70/30) + NPs from
electrospinning 2.5 (wt % of PAN) SiO.sub.2 in DMF PAN/PMMA (70/30)
ECNFs with 5 wt % SiO.sub.2 9 wt % PAN/PMMA (mass ratio: 50/50) +
NPs from electrospinning 2.5 (wt % of PAN) SiO.sub.2 in DMF
PAN/PMMA (50/50) ECNFs with 10 wt % SiO.sub.2 9 wt % PAN/PMMA (mass
ratio: 70/30) + NPs from electrospinning 5 (wt % of PAN) SiO.sub.2
in DMF PAN/PMMA (70/30) ECNFs with 10 wt % SiO.sub.2 9 wt %
PAN/PMMA (mass ratio: 50/50) + NPs from electrospinning 5 (wt % of
PAN) SiO.sub.2 in DMF PAN/PMMA (50/50)
Electrospinning Nanofibers
[0073] The spinning solution was transferred to a 30 ml syringe
fitted with a blunt 18-gauge stainless steel needle. Ultrafine
fibers were electrospun from each spin dope at 15 kV and collected
on a grounded aluminum foil 20 cm from the tip of the
electrospinning syringe at a flow rate of 1 mL/hr maintained by a
digital syringe pump. The resulting polymer fibrous mats were
detached from the collector, dried at room temperature for at least
24 hr, and kept in a fume hood until needed for further
analysis.
Polymer Film Preparation
[0074] Each of the polymer solutions identified in Table 1 was cast
on Teflon plates at room temperature followed by drying in fume
hood for at least 24 hr, to enable formation of a polymer film.
Stabilization and Carbonization
[0075] The nanofibrous polymer mats, as well as corresponding
polymer films, were stacked between 6.times.6 inch graphite plates
from Graphitestore.com and placed in a furnace (Carbolite HTF 18/8,
Watertown, Wis.) for stabilization and carbonization. All samples
were stabilized in air from room temperature to 280.degree. C. at a
heating rate of 1.degree. C./min, then the temperature was held at
280.degree. C. for 6 hr. As an optional step, the stabilized
samples were cooled to room temperature. After stabilization, the
nanofibers were heated in a nitrogen atmosphere at a heating rate
of 5.degree. C./min to a carbonization temperature of 900.degree.
C. or 1200.degree. C.; the samples were allowed to dwell at the
carbonization temperature for about an hour before the samples were
cooled down to room temperature.
Characterization
[0076] The structure and morphology of the nanofibrous materials
were examined under a scanning electron microscope (`SEM`) with an
attached energy-dispersive X-ray spectrometer (`EDX`) (Carl Zeiss
Auriga-BU FIB FESEM, Oberkochen, Germany) and a transmission
electron microscope (`TEM,` Carl Zeiss Libra 120 Plus TEM,
Oberkochen, Germany).
[0077] The pore volume and specific surface area of the prepared
samples were each characterized by nitrogen adsorption using a
surface area and porosity analyzer (Micromeritics, ASAP 2020,
Norcross, Ga.). The orientation of graphitic planes in the
carbonized nanofibers was characterized by an Agilent Oxford Gemini
X-Ray Diffractometer (XRD, Oxfordshire. UK) using Cu K.alpha.
(.lamda.=0.15418 nm) radiation over the 2.theta. range
20.degree.-40.degree.. The pore structure and structural
conversions resulting from stabilization and carbonization and
effect of porous nature were investigated by a HORIBA LabRAM ARAMIS
Raman Spectrometer (Kyoto, Japan).
[0078] In-plane thermal conductivity of ECNFs nanofibrous materials
and carbonized cast films was measured at room temperature using a
HotDisk TPS 2500 (ThermTest Inc., Fredericton, Canada), shown
schematically in FIG. 1. To measure thermal conductivity, square
pieces S-CF1 and S-CF2 (30 mm.times.30 mm.times.0.3 mm) were cut
from each electrospun carbon nanofiber (ECNF) mat or carbon film
sample. A round Hotdisk Kapton sensor 7577 (ThermTest Inc.,
Fredericton, Canada) HDS with radius 2.001 mm was put in between
two square pieces S-CF1 and S-CF2 with a radial probing depth of 10
mm. The two carbon pieces S-CF1 and S-CF2 with sensor HDS were then
sandwiched between two Styrofoam films with just enough external
force to secure but not damage the carbon pieces S-CF1 and S-CF2.
The thermal conductivity was measured three times for each carbon
sample S-CF1 and S-CF2.
Results and Discussion
[0079] SEM images of ECNFs nanofibrous mats clearly showed a
microporous structure with randomly deposited carbon nanofibers at
-400-500 nm scale and inter-fiber pores at micrometer scale at both
carbonization temperatures (FIGS. 2A and 2B). These ECNFs have
relatively uniform size and smooth surface without any
microscopically identifiable particles and are basically solid
fibers based on their cross-sectional SEM image (FIG. 3A) and TEM
image (FIG. 4A).
[0080] Surface morphology, as well as the structure of ECNFs, was
affected by addition of either SiO.sub.2 NPs or PMMA to the PAN
spin dopes.
[0081] ECNFs prepared from SiO.sub.2 NP-doped PAN solution showed
surface nanoparticle clusters (FIGS. 2C and 2D), confirming
SiO.sub.2 NPs integration. The cross-sectional SEM image of a
randomly identified nanofiber from this type ECNFs clearly showed
that SiO.sub.2 NPs and/or their clusters exist on the nanofiber
surface as well as inside the nanofiber. Nano-pores of between
about 10 nm and about 90 nm were also observed on the nanofiber
surface and inside ECNFs (FIG. 3B). The surface nanoparticle
clusters, as well as nanoporous structure of the ECNF itself, was
confirmed by TEM image (FIG. 4B). The density of SiO.sub.2 NPs on
ECNFs surface can be controlled by adjusting the amount of
SiO.sub.2 NPs that are added to the electrospinning solution.
[0082] The ECNFs resulting from PAN/PMMA bicomponent spin dope
demonstrated a coarse surface morphology with narrow surface
indents (FIGS. 2E and 2F), which may have been caused by PMMA
burn-off during heat treatment and were confirmed by elongated
light domains (voids) in TEM images (FIGS. 4C and 4E). The light
domains in TEM indicated voids in individual ECNF and confirmed its
nanoporous structure. These voids also indicate the location of
previous PMMA domains in the PAN/PMMA bicomponent nanofibers before
heat treatment. These voids became larger and merge into continuous
channels along the length of nanofibers upon increasing PMMA
content from 30% (PAN/PMMA=70/30) to 50% (PAN/PMMA=50/50).
Phase-separated PMMA domains in the PAN/PMMA bicomponent precursor
nanofibers decomposed completely at .about.370.degree. C. (during
stabilization/carbonization), generating the submicroporous
structure observed in the resultant ECNFs. As shown (FIGS. 4D and
4F), introducing SiO.sub.2 NPs to PAN/PMMA spin dopes also
contributed to the nanoporous structure in the resultant ECNFs.
Carbonization of PAN is a volume-shrinking process, as non-carbon
elements are removed; this is especially true in the bicomponent
system, from which PMMA is removed during
stabilization/carbonization. Inclusion of SiO.sub.2 NPs in both the
PAN and the PAN/PMMA systems resulted in nanoscale voids/pores in
the final ECNFs, as the individual nanofiber volume shrinkage
changed the location of SiO.sub.2 NPs.
[0083] XRD was employed to investigate crystalline structure of the
prepared ECNFs. ECNFs showed diffraction peaks centered at 2.theta.
angle 26-27.degree., attributed to the (002) crystallographic plane
of graphite crystallites. An increase in intensity and sharpness of
(002) peak was observed as the carbonization temperature increased
from 900.degree. C. to 1200.degree. C. The XRD curves, in
conjunction with the calculated results in Table 2 using the
Scherrer equation, demonstrates that an increase in carbonization
temperature (from 900.degree. C. to 1200.degree. C.) correlates
with a growth of ordered carbon structure in both proportion and
size, as well as a reduction of interplanar spacing. Stated another
way, decreasing the carbonization temperature led to a growth of
disordered carbon structure and a decrease in the size of graphite
crystallites as well as an increase in the interplanar spacing.
Based on the data summarized herein, addition of SiO.sub.2 NPs and
introduction of PMMA to the PAN spin dope reduced graphite
crystallite sizes and led to larger inter-planar distances in the
product ECNF.
TABLE-US-00002 TABLE 2 The average interplanar spacing
"d.sub.(002)" and lateral dimension of crystallites "L.sub.c" of
ECNFs calculated from XRD results (all ECNF samples were carbonized
at 900.degree. C. unless stated otherwise) 2.theta..sub.(002)
.beta. d.sub.(002) L.sub.c Sample (.degree.) (radian) (.ANG.) (nm)
ECNFs (1200.degree. C.) 26.5 0.0979 3.36 1.45 ECNFs (900.degree.
C.) 26.4 0.1169 3.38 1.22 ECNFs with 10% SiO.sub.2 NPs 26.4 0.1392
3.38 1.02 ECNFs from electrospinning 26.4 0.7861 3.38 0.18 PAN/PMMA
(70/30) ECNFs with 10% SiO.sub.2 NPs 26.3 0.7861 3.39 0.18 from
electrospinning PAN/PMMA (70/30)
[0084] BET surface area results of the ECNF nanofibrous mats (Table
3) indicated that higher carbonization temperature led to smaller
pore size and lower pore volume as well as lower BET specific
surface area. Integration of SiO.sub.2 NPs led to larger pore size,
higher pore volume and higher BET specific surface area. Increasing
the SiO.sub.2 NPs content led to higher pore volume and higher BET
specific surface area.
[0085] Inclusion of PMMA in the spin dopes yielded ECNFs with
greater pore volume and a higher BET specific surface area while
each of pore size, pore volume and BET specific surface area of
ECNFs increased with increasing PMMA content. Without being bound
by theory, the structural changes appear to be due to burning off
PMMA during carbonization (PMMA decomposes to gaseous monomers at
.about.370.degree. C.). Larger proportions of PMMA occupied more
volume and formed larger phase domains in bicomponent PAN/PMMA
nanofibers. Greater pore volume, larger pore size, and higher
specific surface area were thus generated upon PMMA removal during
carbonization. Integration of SiO.sub.2 NPs gave rise to greater
pore volume and higher BET specific surface area. The pore size,
however, is independent of amount of SiO.sub.2 NPs even at 10 wt %
loading, consistent with TEM observations. The movement of
SiO.sub.2 NPs due to fiber shrinkage in the process of
carbonization resulted in voids whose sizes are dependent on
SiO.sub.2 NP sizes regardless of SiO.sub.2 NP proportion in the
ECNFs. Combination of PMMA and SiO.sub.2 NPs in spinning solution
further increased pore volume and BET specific surface area of
resultant ECNFs. The largest BET surface area of 55.14 m.sup.2/g
was observed from the ECNFs containing 10 wt % SiO.sub.2 NPs that
were electrospun from 50/50 PAN/PMMA spinning solution.
TABLE-US-00003 TABLE 3 Average pore width, pore volume and BET
surface area of ECNFs (All samples were carbonized at 900.degree.
C. unless otherwise stated) Average Pore BET Surface Pore Width
Volume Area Sample (nm) (cm.sup.3/g) (m.sup.2/g) ECNFs
(1200.degree. C.) 12.48 0.02 7.66 ECNFs (900.degree. C.) 16.11 0.03
10.65 ECNFs with 5 wt. % SiO.sub.2 18.88 0.07 15.61 NPs ECNFs with
10 wt. % SiO.sub.2 18.78 0.13 39.18 NPs ECNFs from electrospinning
16.11 0.13 31.81 PAN/PMMA (70:30) ECNFs from electrospinning 20.66
0.16 33.90 PAN/PMMA (50:50) ECNFs with 5 wt. % SiO.sub.2 15.63 0.17
45.15 NPs from electrospinning PAN/PMMA (70:30) ECNFs with 5 wt. %
SiO.sub.2 18.58 0.18 49.02 NPs from electrospinning PAN/PMMA
(50:50) ECNFs with 10 wt. % SiO.sub.2 17.57 0.18 54.21 NPs from
electrospinning PAN/PMMA (70:30) ECNFs with 10 wt. % SiO.sub.2
20.09 0.19 55.14 NPs from electrospinning PAN/PMMA (50:50)
[0086] Raman spectra of ECNFs show two characteristic bands:
"D-band" and "G-band." "D-band" is centered at .about.1340
cm.sup.-1 and is related to disordered turbostatic carbon structure
which is attributed to breakdown of lattice symmetry of the
graphitic cell. "D-band" is usually correlated to small crystal
size and structural disorder that are visible in poorly graphitized
fibers with a tendency to disappear at higher carbonization
temperatures. "G-band" is centered at .about.1580 cm.sup..about.1
and is related to ordered graphitic structures. The positions of
these two bands are independent of the carbonization temperature
and the intensity ratio of the "D-band" to the "G-band"
((I.sub.D/I.sub.G, known as the "R-value") generally indicates the
amount of structurally ordered graphite crystallites in the ECNFs.
The R-values of the two bands were calculated (Table 4) and showed
a decrease with increasing final carbonization temperature,
suggesting that the disordered carbonaceous components were
converted into more ordered graphite crystallites. Increasing the
amount of SiO.sub.2 NPs and/or PMMA in spin dopes led to increased
R-values of the product ECNFs, signifying an increase in the
disorder of carbonaceous structure in the ECNFs, which is
consistent with XRD results--crystallization of the component
carbon is obstructed by PMMA removal and the presence of SiO.sub.2
NPs.
TABLE-US-00004 TABLE 4 I.sub.D/I.sub.G and R-value of ECNFs
(Samples were carbonized at 900.degree. C. unless otherwise stated)
I.sub.D/I.sub.G Sample I.sub.D I.sub.G (R-value) ECNFs
(1200.degree. C.) 271.30 307.12 0.88 ECNFs (900.degree. C.) 961.05
1037.99 0.92 ECNFs with 10 wt. % SiO.sub.2 NPs 410.35 431.38 0.95
ECNFs from electrospinning 1681.52 1760.79 0.95 PAN/PMMA (70/30)
ECNFs from electrospinning 1675.83 1722.41 0.97 PAN/PMMA (50/50)
ECNFs with 10 wt. % SiO.sub.2 NPs 614.02 624.37 0.98 from
electrospinning PAN/PMMA (70/30) ECNFs with 10 wt. % SiO.sub.2 NPs
828.96 835.61 0.99 from electrospinning PAN/PMMA (50/50)
[0087] Thermal conductivity results (Tables 5A and 5B) indicated
that lower carbonization temperature corresponded to a decrease in
the thermal conductivity of ECNFs. An ECNF mat prepared from
carbonization at 1200.degree. C. possessed a thermal conductivity
of --33 W/m K, even larger than regular carbon fibers. Reducing the
carbonization temperature from 1200.degree. C. to 900.degree. C.
reduced the corresponding thermal conductivity by more than a
magnitude, to 2.5 W/m K, a value even lower than rayon-based carbon
fibers (typically .about.4 W/m K).
[0088] To investigate the effect of inter-fiber microporosity on
thermal conductivity of the ECNF mats, carbon films were cast from
the spinning solutions and carbonized under the same condition as
the ECNF mats (at 900.degree. C.). The carbon films showed similar
trends in porosity and specific surface area as ECNFs after
inclusion of PMMA and SiO.sub.2 NPs (Table 6). The porosity and
specific surface area results confirmed smaller pore volume and
lower specific surface area in carbon films compared to
corresponding ECNF samples; higher thermal conductivities were
observed for the carbon films. Compared to ECNF nanofibrous mats,
the carbon films showed up to 18-fold higher thermal
conductivities, suggesting that inter-fiber porosity of ECNF
nanofibrous mats play an important role in lowering thermal
conductivity. See Table 5A and 5B. The microporous carbon structure
achieved through electrospinning nanofibers reduced thermal
conductivity from 7.64 W/m K measured for a carbon film carbonized
at 900.degree. C. down to 2.48 W/m K, a 67.5% reduction, for ECNF
(900.degree. C.).
TABLE-US-00005 TABLE 5A Thermal conductivity of ECNFs (Samples were
carbonized at 900.degree. C. unless otherwise stated) Thermal
Conductivity Sample (W/m K) ECNFs (1200.degree. C.) 33.11 .+-. 0.57
ECNFs (900.degree. C.) 2.48 .+-. 0.08 ECNFs with 5 wt. % SiO.sub.2
NPs 1.70 .+-. 0.19 ECNFs with 10 wt. % SiO.sub.2 NPs 1.25 .+-. 0.05
ECNFs from electrospinning PAN/PMMA (70/30) 1.49 .+-. 0.14 ECNFs
from electrospinning PAN/PMMA (50/50) 1.28 .+-. 0.08 ECNFs with 5
wt. % SiO.sub.2 NPs from electrospinning 1.15 .+-. 0.08 PAN/PMMA
(70/30) ECNFs with 5 wt. % SiO.sub.2 NPs from electrospinning 0.87
.+-. 0.04 PAN/PMMA (50/50) ECNFs with 10 wt. % SiO.sub.2 NPs from
electrospinning 0.38 .+-. 0.02 PAN/PMMA (70/30) ECNFs with 10 wt. %
SiO.sub.2 NPs from electrospinning 0.15 .+-. 0.02 PAN/PMMA
(50/50)
TABLE-US-00006 TABLE 5B Thermal conductivity of carbon films (`CF`)
(Samples were carbonized at 900.degree. C.) Thermal Conductivity
Sample (W/m K) CF (900.degree. C.) 7.64 .+-. 0.33 CF with 10 wt. %
SiO.sub.2 NPs 4.41 .+-. 0.16 CF from PAN/PMMA (70/30) 5.05 .+-.
0.03 CF from PAN/PMMA (50/50) 4.87 .+-. 0.46 CF with 10 wt. %
SiO.sub.2 NPs from PAN/PMMA (70/30) 2.89 .+-. 0.31 CF with 10 wt. %
SiO.sub.2 NPs from PAN/PMMA (50/50) 2.72 .+-. 0.02
TABLE-US-00007 TABLE 6 Average pore width, pore volume and BET
specific surface area of carbon films (CF) that were cast from spin
dopes and carbonized at 900.degree. C. Average Pore BET Surface
Pore Width Volume Area Sample (nm) (cm.sup.3/g) (m.sup.2/g) CF
15.09 0.02 4.43 CF with 10 wt. % SiO.sub.2 NPs 18.96 0.05 17.84 CF
from PAN/PMMA (70/30) 15.62 0.08 10.04 CF from PAN/PMMA (50:50)
19.74 0.09 17.35 CF with 10 wt. % SiO.sub.2 NPs 16.26 0.11 18.20
from PAN/PMMA (70:30) CF with 10 wt. % SiO.sub.2 NPs 19.01 0.13
24.84 from PAN/PMMA (50:50)
[0089] The low thermal conductivity of the ECNF nanofibrous mats
prepared according to the methods disclosed herein is attributed to
the unique multi-scale (micro- and nano-) porous structures derived
from integration of a secondary polymer, such as PMMA, and an
insulating nanoparticle, such as SiO.sub.2 NPs, in the polymer spin
dopes. Phonons are a key mechanism of heat conduction in
carbonaceous materials. At the micro- and nano-scale, the thermal
conductivity of a material can be affected by its porosity due to
phonon scattering. Thermal conductivity of the ECNF mats decreased
with the increase of PMMA in proportion to PAN in the polymer
nanofibers. ECNF nanofibrous mats have inter-fiber micrometer scale
pores, typically between about 1 .mu.m and about 10 .mu.m, while
the submicron porous structure of individual ECNFs is derived from
PAN/PMMA phase separation. Generally polymer-polymer phase
separation generates micrometer scale domains but herein the
PAN-PMMA phase separation is confined in electrospun nanofibers.
PMMA decomposition during carbonization eventually removes PMMA
domains completely from the nanofibers and leaves submicrometer
scale pores/channels behind. The submicroporous carbon structure
achieved by including PMMA in the spin dope reduced the thermal
conductivity from 2.48 W/m K for ECNF prepared from pure PAN-based
spin dope to 1.28 W/m K, a 48.4% reduction, for ECNF prepared from
a 50/50 PAN/PMMA spin dope.
[0090] Integration of SiO.sub.2 NPs in the PAN electrospinning
solution also reduced thermal conductivity of the resultant ECNFs.
The presence of SiO.sub.2 NPs, as demonstrated herein, led to
nano-scale porous structure (pores/voids) in ECNF. This effect is
in part due to nanoparticle translocation during the volumetric
shrinkage of the fibers during the carbonization process. To
clarify the role of SiO.sub.2 NPs in reducing thermal conductivity
of resultant ECNFs, different types of SiO.sub.2 NPs were employed
and compared to the ECNF samples with SiO.sub.2 NP-1 (12 nm) as
described above: one with a smaller diameter 7 nm (SiO.sub.2 NP-2)
and the other with a broader diameter range 5-15 nm (SiO.sub.2
NP-3). Thermal conductivity results (Table 7) indicated that the
ECNF nanofibrous mat containing smaller size SiO.sub.2 NPs, i.e.
SiO.sub.2 NP-2 herein, had even lower thermal conductivity at the
same loading. To achieve the same weight percent, a larger number
of smaller SiO.sub.2 NPs were added to the spin dope, thereby
increasing the loading of insulating nanoparticles in the ECNFs.
This increase of nanoparticle volume in the ECNF reduced the mean
free path of phonons, and thus increased phonon scattering and
yielded a lower thermal conductivity.
TABLE-US-00008 TABLE 7 Thermal conductivity of ECNFs with SiO.sub.2
NPs at different sizes (carbonization temperature: 900.degree. C.)
Thermal Conductivity Sample (W/m K) ECNFs with 5 wt. % SiO.sub.2
NP-2 1.44 .+-. 0.02 ECNFs with 5 wt. % SiO.sub.2 NP-3 1.78 .+-.
0.09 ECNFs with 10 wt. % SiO.sub.2 NP-2 1.19 .+-. 0.06 ECNFs with
10 wt. % SiO.sub.2 NP-3 1.27 .+-. 0.04
[0091] The larger pores from PAN/PMMA phase separation and smaller
pores from SiO.sub.2 NPs had a synergistic effect on thermal
conductivity reduction. In particular, the combination of PMMA
removal and SiO.sub.2 NP addition resulted in the least thermally
conductive ECNF nanofibrous mat. The ECNF showed a reduction in
thermal conductivity from 1.28 W/m K for ECNF prepared from 50/50
PAN/PMMA down to 0.15 W/m K, an 88.3% reduction, for an ECNF
prepared from 50/50 PAN/PMMA+10 wt % SiO.sub.2 NPs (average
particle size 12 nm). This new ECNF has a thermal conductivity that
is .about.6% of the original ECNFs and is .about.3.8% of
rayon-based low thermal conductivity carbon fibers (generally
.about.4 W/m K).
[0092] An increase in carbonization temperature corresponds to
decreasing pore sizes and pore volume. Polymer-polymer phase
separation typically generate micrometer scale domains. PMMA
decomposition during carbonization removes PMMA domains from the
nanofibers and leaves submicrometer scale pores/holes behind.
SiO.sub.2 nanoparticles contribute to nano-voids/pores in the final
ECNFs due to nanoparticle location change during volumetric
shrinkage of the fibers in carbonization process. Under normal
condition, these pores are filled with low thermal conductivity air
(0.026 W/m K) and thus result in low thermal conductivity ECNF
nanofibrous mat.
[0093] A decrease in carbonization temperature corresponded to less
densely packed graphite crystallites with more voids/defects. At
the same time, graphite crystallites became not only smaller but
also less perfect and arranged in a more disordered way. Without
being bound by theory, smaller graphite crystallite in more
disordered arrangement as well as voids/defects increased random
phonon scattering per unit length of heat path and led to lower
thermal conductivity. Coupled with poor thermal conductivity of the
imperfect graphite crystallites, significant reduction of thermal
conductivity was thus observed in the resultant ECNF samples. In a
polycrystalline material, solid phase thermal conductivity
typically depends on intrinsic thermal conductivity of crystal
grains as well as thermal resistance of grain boundaries. The poor
graphite crystalline structure resulting from a low carbonization
temperature and integration of impurities such as PMMA and
SiO.sub.2 NPs led to a low thermal conductivity of the graphite
grains in resultant ECNFs. In the meantime, the micro- and
nano-porous structure provides large thermal resistance at grain
boundaries. Furthermore smaller graphite crystallite (Table 2)
increased the number of crystal grains per unit length of heat path
thus decreasing the thermal conductivity of the solid phase.
Electrospinning provided inter-fiber pores at micrometer scale for
ECNF nanofibrous mats while introduction of PMMA and SiO.sub.2 NPs
further reduced graphite crystallite size and generated
submicroporous and nanoporous carbon structure in each individual
ECNF. These submicrometer and nanometer pores as well as smaller
crystallite resulted in more random photon scattering, further
reduced the phonon mean free path and facilitated lower thermal
conductivity.
[0094] Compared to carbon, SiO.sub.2 is a low thermal conductivity
material (1.3-1.5 W/m K) with a high surface to volume ratio.
Addition of silica nanoparticles contributed to lower thermal
conductivity of ECNFs as shown herein. SiO.sub.2 NPs were dispersed
in a continuous phase of carbon in the ECNF. There was a
significant reduction in thermal conductivity upon incorporation of
SiO.sub.2 NPs due to decreased phonon mean free path and increased
random scattering of phonons. As shown herein, SiO.sub.2 NPs were
distributed homogeneously in the continuous phase of carbon in
ECNFs without significant agglomeration, as evidenced by EDX
mapping of Si in the final ECNF mats with SiO.sub.2 NPs. Without
being bound by theory, with lower thermally conductive SiO.sub.2
NPs spread within the continuous phase of a relatively higher
thermal conductive carbon, the flow of heat through the ECNFs would
move around, not through, these SiO.sub.2 NPs as much as possible.
There is thus a significant reduction in thermal conductivity upon
incorporation of SiO.sub.2 NPs due to decreased phonon mean free
path and increased random scattering of phonons. Additionally
SiO.sub.2 NPs on surface of ECNFs increased nanofibers' surface
roughness, which also decreased the phonon mean free path and in
turn reduced the thermal conductivity.
[0095] Overall, low thermal conductivity ECNFs mats were
successfully prepared by integration of a second polymer as well as
nanoparticles with low thermal conductivity into a spin dope
comprising a carbon-based polymer, followed by electrospinning,
stabilization, and carbonization. Without being bound by theory,
the low thermal conductivity is attributed to a synergistic effect
from unique multi-scale (micro-, submicro-, and nano-) porous
structures, which is a result from both larger pores from
bicomponent polymer phase separation and smaller pores from
nanoparticles during carbonization, and low thermal conductivity of
the nanoparticles themselves. Compared to rayon-based low thermal
conductivity carbon fibers with thermal conductivity of .about.4
W/m K, the lowest thermal conductivity of ECNFs disclosed herein is
.about.0.15 W/m K, .about.3.8% that of the rayon-based carbon
fibers.
[0096] The multi-scale porous structure of the ECNF nanofibrous
mats disclosed herein is filled with air having a thermal
conductivity of 0.024 W/m K. The thermal conductivity of air
further decreases when the size of pore/cavity drops to nanometer
scale (generally <40 nm) which is comparable to the mean free
path of air molecules. The disclosed ECNF samples have an average
pore width that is below 40 nm and are modulated by this effect,
although, without being bound by theory, the properties of ECNF
samples with comparable average pore width are expected to be more
influenced by the phonon scattering mechanism.
Example 2
ECNFs with Ca.sub.2SiO.sub.4 NP
Spin Dope Preparation
[0097] The targeted polymers are dissolved in DMF to prepare 9 wt.
% solutions for spinning. For those spin dopes containing calcium
silicate (Ca.sub.2SiO.sub.4) nanoparticles (CS NPs), CS NPs are
added to DMF and the suspension sonicated for 20 min; then the
solution containing CS NPs is mixed with the disclosed polymer
solution under constant stirring followed by sonication for another
10 minutes to break up any aggregates of the CS NPs and ensure even
distribution of CS NPs within the solution before electrospinning
(Refer to Table 8). The amount of CS NPs was determined relative to
the mass of PAN used in the spin dope. As shown in Table 8,
examples of 5 wt % CS NP relative to the amount of PAN and 10 wt %
CS NP relative to the amount of PAN were used.
TABLE-US-00009 TABLE 8 Preparative spin dopes Product name
Preparative spin dope ECNFs 9 wt % PAN in DMF ECNFs with 5 wt % CS
NPs 9 wt % PAN + 2.5 (wt % of PAN) Ca.sub.2SiO.sub.4NP in DMF ECNFs
with 10 wt % CS NPs 9 wt % PAN + 5 (wt % of PAN) Ca.sub.2SiO.sub.4
NP in DMF ECNFs from electrospinning 9 wt % PAN/PMMA (mass ratio:
70/30) PAN/PMMA (70/30) in DMF ECNFs from electrospinning 9 wt %
PAN/PMMA (mass ratio: 50/50) PAN/PMMA (50/50) in DMF ECNFs with 5
wt % CS NPs 9 wt % PAN/PMMA (mass ratio: 70/30) + from
electrospinning 2.5 (wt % of PAN) Ca.sub.2SiO.sub.4NP in DMF
PAN/PMMA (70/30) ECNFs with 5 wt % CS NPs 9 wt % PAN/PMMA (mass
ratio: 50/50) + from electrospinning 2.5 (wt % of PAN)
Ca.sub.2SiO.sub.4 NP in DMF PAN/PMMA (50/50) ECNFs with 10 wt % CS
NPs 9 wt % PAN/PMMA (mass ratio: 70/30) + from electrospinning 5
(wt % of PAN) Ca.sub.2SiO.sub.4 NP in DMF PAN/PMMA (70/30) ECNFs
with 10 wt % CS NPs 9 wt % PAN/PMMA (mass ratio: 50/50) + from
electrospinning 5 (wt % of PAN) Ca.sub.2SiO.sub.4 NP in DMF
PAN/PMMA (50/50)
Electrospinninq Nanofibers
[0098] The spinning solution is transferred to a 30 ml syringe
fitted with a blunt 18-gauge stainless steel needle. Ultrafine
fibers are electrospun from each spin dope at 15 kV and collected
on a grounded aluminum foil 20 cm from the tip of the
electrospinning syringe at a flow rate of .about.1 mL/hr maintained
by a digital syringe pump. The resulting polymer fibrous mats are
detached from the collector, dried at room temperature for at least
24 hr, and kept in a fume hood until needed for further
analysis.
Stabilization and Carbonization
[0099] The nanofibrous polymer mats prepared above are stacked
between 6.times.6 inch graphite plates from Graphitestore.com and
placed in a furnace (Carbolite HTF 18/8, Watertown, Wis.) for
stabilization and carbonization. All samples are stabilized in air
from room temperature to 280.degree. C. at a heating rate of
1.degree. C./min, then the temperature is held at 280.degree. C.
for about 6 hr. After stabilization, the nanofibers are heated in a
nitrogen atmosphere at a heating rate of 5.degree. C./min to a
carbonization temperature of 900.degree. C. or 1000.degree. C.; the
samples are allowed to dwell at the carbonization temperature for
about an hour before the samples are cooled down to room
temperature.
Characterization
[0100] The structure and morphology of the nanofibrous materials
are examined under a scanning electron microscope (`SEM`) with an
attached energy-dispersive X-ray spectrometer (`EDX`) (Carl Zeiss
Auriga-BU FIB FESEM, Oberkochen, Germany) and a transmission
electron microscope (`TEM,` Carl Zeiss Libra 120 Plus TEM,
Oberkochen, Germany).
[0101] The pore volume and specific surface area of the prepared
samples are each characterized by nitrogen adsorption using a
surface area and porosity analyzer (Micromeritics, ASAP 2020,
Norcross, Ga.). The orientation of graphitic planes in the
carbonized nanofibers is characterized by an Agilent Oxford Gemini
X-Ray Diffractometer (XRD, Oxfordshire. UK) using Cu K.alpha.
(.lamda.=0.15418 nm) radiation over the 2.theta. range
20.degree.-40.degree.. The pore structure and structural
conversions resulting from stabilization and carbonization and
effect of porous nature are investigated by a HORIBA LabRAM ARAMIS
Raman Spectrometer (Kyoto, Japan).
[0102] In-plane thermal conductivity of ECNFs nanofibrous materials
is measured at room temperature using a HotDisk TPS 2500 (ThermTest
Inc., Fredericton, Canada). To measure thermal conductivity, square
pieces (30 mm.times.30 mm.times.0.3 mm) are cut from each
electrospun carbon nanofiber (ECNF) mat or carbon film sample. A
round Hotdisk Kapton sensor 7577 (ThermTest Inc., Fredericton,
Canada) with radius 2.001 mm is put in between two square pieces
with a radial probing depth of 10 mm. The two carbon pieces with
sensor are sandwiched between two Styrofoam films with just enough
external force to secure but not damage the carbon pieces. The
thermal conductivity is measured three times for each carbon
sample.
[0103] The ECNF resulting from a bicomponent PAN/PMMA spin dope has
a thermal conductivity that is expected to be lower than ECNF
produced from a PAN-only spin dope. The addition of CS NPs to the
PAN/PMMA spin dope yields a product with a lower thermal
conductivity. The use of a bicomponent polymer spin dope and
inclusion of NPs is expected to yield a multi-scale porous
carbon-containing product that has even lower thermal
conductivity.
Example 3A
Carbon Nanofiber Yarn (CNFY)
Spin Dope Preparation
[0104] In preparation of CNFY, the concentration of PAN relates to
the integrity of nanofibers. In particular, the concentration of
PAN in solution influences (1) nanofiber formation; (2) nanofiber
size; and (3) nanofiber strength. Generally, lower PAN
concentrations lead to beaded fibers, smaller fiber sizes and lower
fiber strengths while higher PAN concentrations result in less
beaded fibers, thicker fibers and higher fiber strengths. However,
too high a PAN concentration leads to more viscous solutions and
adversely affects processing. As described herein, PAN was
dissolved in DMF to prepare 10-15 wt. % solutions, generally about
13 wt. % solution, for electrospinning. Additional experimental
details can be found in Liu, Jie, et al. "Structure and
thermo-chemical properties of continuous bundles of aligned and
stretched electrospun polyacrylonitrile precursor nanofibers
collected in a flowing water bath." Carbon 50.3 (2012):
1262-1270.
[0105] Alternately, various targeted polymers, as identified in
Table 1 can be dissolved in DMF to prepare 9 wt. % solutions for
spinning.
Electrospinning Nanofibers
[0106] Ultrafine fibers were electrospun from the PAN spin dope at
15 kV into a flowing water bath about 20 cm from the needle and
collected on a ceramic tube held out of the bath. Manual collection
of the electrospun fibers enabled continuous processing of up to
0.5 hour before breakage creating multiple meter draw lengths
greater than 50 m.
PAN Nanofiber Yarns Drawing
[0107] Drawing pulls the molecular chains in fibers and orients
them along the fiber axis, creating a considerably stronger yarn.
Nanofiber yarn drawing was carried out in hot water. As the fibers
were heated they lengthen and density of each fiber increased. The
drawing of PAN nanofiber yarns was done by hanging the yarns in a
water bath (in a glass container) with weights attached to one end,
while the PAN nanofiber yarns were completely submersed in the
water. In particular, the drawing was done in three steps: (1)
initial drawing in 50-60.degree. C. water to up to 50% elongation;
(2) drawing in 90-100.degree. C. water to up to 200% elongation and
(3) drying in 100.degree. C. air.
Stabilization/Carbonization
[0108] Stabilization of PAN nanofiber yarns was done by hanging the
yarns in an oven with weights attached to one end. The weight was
used to provide tension during stabilization. In the stabilization
process, PAN nanofiber yarns were heated at a rate of 1.degree.
C./min from 30.degree. C. to 280.degree. C. and held there for 6
hours in a constant flow of air. Carbonization was carried out by
heating the stabilized PAN nanofiber mat in a constant flow of
nitrogen gas at a rate of 5.degree. C./min to 1000.degree. C. and
keeping this temperature maintained for 1 hour.
[0109] As-collected PAN nanofiber yarns from the flowing water bath
showed good fiber alignment (FIG. 5A). Over 60% nanofibers aligned
with respect to the yarn axis. After stretching, the alignment
reached over 90% of the nanofibers, which became elongated and
densely packed (FIG. 5B). Nanofiber yarns retained their morphology
through the process of stabilization and carbonization, and shrunk
significantly during the carbonization process (FIGS. 5C and
5D).
Example 3B
Carbon Nanofiber Yarn with SiO.sub.2 (CNFY-SiO.sub.2)
Spin Dope Preparation
[0110] Various targeted polymers, as identified in Table 1 can be
dissolved in DMF to prepare 9 wt. % solutions for electrospinning.
For those spin dopes containing SiO.sub.2 nanoparticles (SiO.sub.2
NPs), SiO.sub.2 NPs are first added to DMF and sonicated for 20
min; then the solution containing SiO.sub.2 NPs is mixed with
corresponding polymer solution under constant stirring followed by
sonication for another 10 minutes to break up any aggregates of the
SiO.sub.2 NPs and ensure even distribution of SiO.sub.2 NPs within
the solution before spinning.
Electrospinning Nanofibers
[0111] Ultrafine fibers are electrospun from each of the spin dopes
at 15 kV into a flowing water bath and collected on a ceramic tube
held out of the bath as disclosed above.
Nanofiber Yarns Drawing
[0112] Nanofiber yarn drawing is carried out in hot water as
described above. In particular, the drawing is done in three steps:
(1) initial drawing in 50-60.degree. C. water to up to 50%
elongation; (2) drawing in 90-100.degree. C. water to up to 200%
elongation; and (3) drying in 100.degree. C. air.
Stabilization and Carbonization
[0113] Stabilization of nanofiber yarns is done by hanging the
yarns in an oven with weights attached to one end. In the
stabilization process, the nanofiber yarns are heated at a rate of
1.degree. C./min from 30.degree. C. to 280.degree. C. and held at
280.degree. C. for 6 hours in a constant flow of air. Carbonization
is carried out by heating the stabilized PAN nanofiber mat in a
constant flow of nitrogen gas at a rate of 5.degree. C./min to
1000.degree. C. and maintaining this temperature for 1 hour.
Characterization
[0114] Structure and morphology of the nanofibrous yarns are
examined under a scanning electron microscope (`SEM`) with an
attached energy-dispersive X-ray spectrometer (`EDX`) (Carl Zeiss
Auriga-BU FIB FESEM, Oberkochen, Germany) and a transmission
electron microscope (`TEM,` Carl Zeiss Libra 120 Plus TEM,
Oberkochen, Germany).
[0115] Pore volume and specific surface area of the prepared
samples are characterized by nitrogen adsorption using a surface
area and porosity analyzer (Micromeritics, ASAP 2020, Norcross,
Ga.). The orientation of graphitic planes in the carbonized
nanofiber yarns are characterized by an Agilent Oxford Gemini X-Ray
Diffractometer (XRD, Oxfordshire. UK) using Cu K.alpha.
(.lamda.=0.15418 nm) radiation over the 2.theta. range of
20.degree.-40.degree.. Structural conversions resulting from
stabilization and carbonization and effect of porous nature are
investigated by a HORIBA LabRAM ARAMIS Raman Spectrometer (Kyoto,
Japan).
[0116] In-plane thermal conductivity of manually woven fabric
prepared from CNFY and CNFY-SiO.sub.2 are measured at room
temperature using a HotDisk TPS 2500 (ThermTest Inc., Fredericton,
Canada). To measure thermal conductivity, pieces (.about.30
mm.times..about.30 mm.times..about.0.3 mm) are cut from each CNF
weave. A round Hotdisk Kapton sensor 7577 (ThermTest Inc.,
Fredericton, Canada) with radius 2.001 mm is put between two square
pieces with a radial probe depth of 10 mm. The two pieces carbon
yarn fabric with sensor are then sandwiched between two Styrofoam
films with enough external force to secure, but not damage, the
yarn fabric. The thermal conductivity is measured three times for
each carbon sample.
[0117] The CNFY resulting from a bicomponent PAN/PMMA spin dope is
expected to have a thermal conductivity that is lower than CNFY
produced from a PAN spin dope. The addition of SiO.sub.2 NPs to the
spin dope, to yield CNFY-SiO.sub.2, yields a product that is
expected to have a lower thermal conductivity. The use of a
bicomponent polymer spin dope and inclusion of low thermal
conductivity nanoparticles is expected to yield a CNF yarn that has
even lower thermal conductivity.
[0118] It will be understood that various details of the presently
disclosed subject matter may be changed without departing from the
scope of the presently disclosed subject matter. Furthermore, the
foregoing description is for the purpose of illustration only, and
not for the purpose of limitation.
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