U.S. patent application number 14/506670 was filed with the patent office on 2015-07-30 for method of making ceramic nanofibers.
This patent application is currently assigned to eM-TECH, Inc.. The applicant listed for this patent is Pawel Czubarow. Invention is credited to Pawel Czubarow.
Application Number | 20150211152 14/506670 |
Document ID | / |
Family ID | 40468695 |
Filed Date | 2015-07-30 |
United States Patent
Application |
20150211152 |
Kind Code |
A1 |
Czubarow; Pawel |
July 30, 2015 |
METHOD OF MAKING CERAMIC NANOFIBERS
Abstract
Continuous ceramic (e.g., silicon carbide) nanofibers (502, 602,
604, 606, 608, 702, 704, 1102, 1104) which are optionally p or n
type doped are manufactured by electrospinning a polymeric ceramic
precursor to produce fine strands of polymeric ceramic precursor
which are then pyrolized. The ceramic nanofibers may be used in a
variety of applications not limited to reinforced composite
materials (400), thermoelectric generators (600, 700) and high
temperature particulate filters (1200).
Inventors: |
Czubarow; Pawel; (Wellesley,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Czubarow; Pawel |
Wellesley |
MA |
US |
|
|
Assignee: |
eM-TECH, Inc.
Wellesley
MA
|
Family ID: |
40468695 |
Appl. No.: |
14/506670 |
Filed: |
October 5, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12678396 |
Mar 16, 2010 |
8865996 |
|
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PCT/US08/10900 |
Sep 19, 2008 |
|
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14506670 |
|
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60994326 |
Sep 19, 2007 |
|
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Current U.S.
Class: |
264/10 |
Current CPC
Class: |
D01D 10/02 20130101;
C01B 32/977 20170801; C04B 35/62876 20130101; C04B 35/62281
20130101; C22C 49/14 20130101; C01B 32/956 20170801; C04B 2235/526
20130101; D10B 2101/14 20130101; C04B 2235/441 20130101; D01F 9/08
20130101; D01D 5/003 20130101; C04B 35/62259 20130101; C01B 32/991
20170801; C22C 47/08 20130101; H01L 35/22 20130101 |
International
Class: |
D01F 9/08 20060101
D01F009/08 |
Claims
1. A method of manufacturing continuous ceramic nanofibers
comprising: obtaining a polymeric ceramic precursor selected from
the group consisting of non-oxide polymeric ceramic precursor and
alcoxide polymeric ceramic precursor; dissolving the polymeric
ceramic precursor in a solvent to obtain a solution of polymeric
ceramic precursor; electrospinning the solution of polymeric
ceramic precursor to produce fine strands of continuous polymeric
ceramic precursor; and pyrolizing the fine strands of polymeric
ceramic precursor.
2. The method according to claim 1 wherein: obtaining the polymeric
ceramic precursor comprises obtaining a polymeric precursor
selected from the group consisting of a silicon carbide precursor
and a boron carbide precursor.
3. The method according to claim 2 wherein: obtaining the polymeric
ceramic precursor comprises obtaining a polysilane.
4. The method according to claim 2 wherein obtaining the polymeric
ceramic precursor comprises obtaining a polyborane.
5. The method according to claim 2 further comprising: obtaining a
dopant bearing compound; and combining the dopant bearing compound
with the solution prior to electrospinning.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is divisional U.S. patent application Ser.
No. 12/678,396 filed Mar. 16, 2010 which is a National Stage
Application of PCT International Application No. PCT/US2008/010900
filed Sep. 19, 2008 which was based on U.S. Provisional Patent
Application 60/994,326 filed Sep. 19, 2007.
FIELD OF THE INVENTION
[0002] The present invention relates in general to nanotechnology.
More particularly, the present invention relates to continuous
silicon carbide nanofibers.
BACKGROUND
[0003] During the last 10 years there has been heightened interest
in nanotechnology. Nanotechnology is concerned with properties of
materials that arise only when at least one dimension of a material
is reduced to a very minute scale. At such scales quantum
mechanical effects arise leading to altered properties of the
materials that is distinct from that of the bulk materials.
[0004] Ceramics are known to have many favorable characteristics
for demanding engineering applications. It would be desirable to
mass produce continuous ceramic nanofibers so that properties of
ceramic materials including those that arise at nanoscales could be
exploited.
[0005] Silicon carbide in particular is a material of extreme
hardness (9 on Mohs' scale on which diamond is 10) and high modulus
that is mainly used as an abrasive but has also been used as a
semiconductor material in electronic devices. Although silicon
carbide nanofibers have been produced by Chemical Vapor Deposition,
the process is slow and costly and produces fibers of limited
length. It would be desirable to be able to mass produce silicon
carbide nanofibers.
BRIEF DESCRIPTION OF THE FIGURES
[0006] The present invention will be described by way of exemplary
embodiments, but not limitations, illustrated in the accompanying
drawings in which like references denote similar elements, and in
which:
[0007] FIG. 1 is a flowchart of a method of producing ceramic
nanofibers;
[0008] FIG. 2 is a flowchart of a method of preparing a silicon
carbide precursor;
[0009] FIG. 3 is a schematic diagram of an apparatus for producing
ceramic nanofibers;
[0010] FIG. 4 shows a bar of composite material that has ceramic
nanofibers embedded in a matrix of another material;
[0011] FIG. 5 is a magnified view of a portion of the bar shown in
FIG. 3;
[0012] FIG. 6 is a schematic of a thermoelectric generator made
with n and p type doped silicon carbide nanofibers;
[0013] FIG. 7 is an explode view of a thermoelectric generator made
with n and p type doped silicon carbide nanofibers according to
another embodiment of the invention;
[0014] FIGS. 8 and 9 show top and bottom view of arrangement of
trapezoidal wave shaped p and n type doped strips of silicon
carbide nanofibers used to make a relatively high power density
thermoelectric generator;
[0015] FIG. 10 shows a perspective view of the arrangement shown in
FIGS. 8 and 9;
[0016] FIG. 11 shows an alternative arrangement of trapezoidal
strips of n and p type doped silicon carbide nanofibers.
[0017] FIG. 12 is a particular filter that uses silicon carbide
nanofiber filter media and is capable of operation at high
temperature, e.g., for filtering diesel particulate motor
exhaust.
DETAILED DESCRIPTION
[0018] As required, detailed embodiments of the present invention
are disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention, which
can be embodied in various forms. Therefore, specific structural
and functional details disclosed herein are not to be interpreted
as limiting, but merely as a basis for the claims and as a
representative basis for teaching one skilled in the art to
variously employ the present invention in virtually any
appropriately detailed structure. Further, the terms and phrases
used herein are not intended to be limiting; but rather, to provide
an understandable description of the invention.
[0019] The terms a or an, as used herein, are defined as one or
more than one. The term plurality, as used herein, is defined as
two or more than two. The term another, as used herein, is defined
as at least a second or more. The terms including and/or having, as
used herein, are defined as comprising (i.e., open language). The
term coupled, as used herein, is defined as connected, although not
necessarily directly, and not necessarily mechanically.
[0020] FIG. 1 is a flowchart of a method of producing continuous
ceramic nanofibers 100, such as for example silicon carbide
nanofibers. In block 102 a polymeric ceramic precursor is obtained.
For example in order to produce silicon carbide one type of
precursor that may be used is polycarbosilane made. Another type of
silicon carbide precursor that may be used is polysilane. Both are
available from by Starfire.RTM. Systems, Inc of Malta, N.Y.
Polycarbosilanes are also available from Nippon Carbon, Co of
Japan. Clariant's Kion Specialty Polymers of Charlotte, N.C. makes
polysilazanes which are precursors to silicon nitride.
[0021] Another polymeric silicon carbide precursor that may be used
can be synthesized by according to the teachings of U.S. Pat. No.
6,020,447. In brief '447 patent teaches a process that involves
reductive coupling of chlorosilane to form polysilane in the
presence of ultrasonification.
[0022] Polysilazanes precursors can be used to produce silicon
nitride (Si3N4) nanofibers. Polysilazanes are made by amonolysis of
chlorosilanes. Polyborazine precursors can be used to produce boron
nitride (BN) nanofibers. Polyborazine can be made by amonolysis of
chloroboranes. Polyborane precursors can be used to produce boron
carbide (B4C). The foregoing can be pyrolized in argon or
nitrogen.
[0023] Oxide ceramic nanofibers can also be made by the methods
described herein. For example sol gels, or hydrolyzed alkoxides
(e.g., titanium isopropoxide) can be pyrolized in air to obtain
oxide type ceramic nano fibers. Some oxides can also be used to
make thermoelectric devices.
[0024] In block 104 the polymeric ceramic precursor is dissolved in
a solvent to produce a solution of polymeric ceramic precursor. A
solvent such as toluene, tetrahydrofuran or mixtures thereof may be
used.
[0025] For some applications such as the thermoelectric generators
500, 600, 700 shown in FIGS. 5, 6, 7 and described below it is
desirable to obtain p and n type doped silicon carbide continuous
nanofibers. In block 106 a dopant precursor is added to the
solvent. A suitable p type dopant precursor is a phosphorous (III)
organometallic compound e.g., _diphenylphosphino ethylene. A
suitable polymeric precursor for making n type doped silicon
carbide nanofibers can be made by dissolving SiC precursor in a
suitable solvent in which is dissolved a small amount of dopant in
a form of nitrogen containing species such as primary, secondary,
or tertiary organic amines (e.g., melamine, ganidine), inorganic
amines, organometallic silazanes.
[0026] Other dopants may also be made of boron, aluminum, and
carbon containing organometallic compounds.
[0027] Referring again to FIG. 1 in block 108 the polymeric ceramic
precursor is electrospun to produce fine continuous strands of
polymeric ceramic precursor. An apparatus for elecrospinning the
polymeric ceramic precursor is shown in FIG. 2 and described below.
In block 110 of the method 100 the fine continuous strands of
polymeric ceramic precursor are pyrolized in order to convert the
fine continuous strands of polymeric ceramic precursor into
continuous ceramic nanofibers. The polymeric ceramic precursors can
be pyrolized by heating to a temperature of 1000 C or more in an
inert atmosphere of argon or nitrogen.
[0028] FIG. 2 is a flowchart of a method of preparing a silicon
carbide precursor. In block 202 a solution of monomers (e.g.,
chlorosilane) in a solvent (e.g., toluene) is prepared. In block
204 an alkali metal (e.g., sodium) or an alloy including at least
one alkali metal is introduced into the solution. In block 206 the
solution is ultrasonicated or heated (e.g., to a temperature of
.sup..about.80 C) to synthesize a polymeric precursor (e.g.,
polysilane) in the solution. In block 208 the polymeric precursor
is extracted from the solution by centrifuging and subsequent
washing.
[0029] FIG. 3 is a schematic diagram of an apparatus 300 for
producing ceramic nanofibers, e.g., silicon carbide nanofibers.
Referring to FIG. 3 it is seen that a reservoir of polymeric
ceramic precursor solution 302 is connected to a pump 304. The pump
304 pumps the polymeric ceramic precursor solution to an
electrospinning spinneret 306. A high voltage power supply 308 is
connected to the spinneret 306 and to a drum 310. A high voltage
potential is established between the spinneret 306 and the drum
310. The high voltage potential helps eject the polymeric ceramic
precursor solution from the spinneret 306 and produce the fine
stands of polymeric ceramic precursor. The fine continuous strands
of polymeric ceramic precursor are collected on the drum 310.
Periodically the drum can be stopped and the fine strands of
polymeric ceramic precursor that have collected on the drum
removed. A drum motor 312 turns the drum 310 as the apparatus 300
operates. A master controller 314, e.g., a desk top computer
configured with instrument control software, is coupled to the drum
motor 312, the high voltage power supply 308 and the pump 304 in
order to control the operation of the apparatus 300. Continuous
silicon carbide nanofibers that are produced by the apparatus shown
in FIG. 3 and in accordance with the methods described with
reference to FIG. 1 and FIG. 2 are distinguished by their length
from silicon carbide nanofibers that are produced by CVD which are
less than 1 millimeter in length.
[0030] Alternatively rather than using the drum 310 to collect the
fine strands of polymeric ceramic precursor a flat plate, a
conveyor belt or a continuous web of material may be used.
According to another alternative, rather than using the drum 310,
an object that has a shaped surface (formed by machining or another
process) is used. The shaped surface acts as a template to
determine the shape of a matt of fine continuous strands of
polymeric ceramic nanofibers that is deposited thereon. For example
for forming silicon carbide strips used in the thermoelectric
generator described below and shown in FIGS. 7 and 8 a template
that has a square wave surface profile may be used. In other cases
templates may be used to form structural parts having a other
shapes determined by the templates. The final shape of the nonwaven
fiber strands can also be formed by embossing.
[0031] FIG. 4 shows a bar of composite material 400 that has
ceramic nanofibers 502 embedded in a matrix of another material 504
and FIG. 5 is a magnified view of a portion of the bar 400 shown in
FIG. 4. The other material that forms the matrix may for example be
a polymeric material such as imidized polyamic acid (polyimide),
polyester, or polyetheretherketone (PEEK), or a metal such as
copper, tungsten, indium, gallium, or aluminum. In the case of the
polymeric matrix the ceramic nanofibers may be added by
infiltration of matrix in to the fibers in a molten state or in the
case of polyimides, polyamic acid can be infiltrated and
subsequently imidized to poyimide at >200 C. In the case of a
metal matrix the ceramic nanofibers may be introduces by forcing
them into the metal when the metal is in a molten state. The
ceramic nanofibers enhance the mechanical properties, e.g.,
strength, modulus of the composite material. Optionally the ceramic
nanofibers can be chopped before introducing them into the matrix
material.
[0032] FIG. 6 is a schematic of a thermoelectric generator 600 made
with strips of continuous n type doped silicon carbide nanofibers
602, 604 and strips of continuous p type doped silicon carbide
nanofibers 606, 608. The strips are cut from mats of randomly
aligned silicon carbide nanofibers. Silicon carbide nanofibers work
well in the thermoelectric generator because their nanoscale causes
phonons to scatter thereby enhancing their thermal resistance. As
shown in FIG. 6 the n type doped silicon carbide nanofibers 602,
604 and the p type doped silicon carbide nanofiber 606, 608 are
arranged in an alternating arrangement between a heat source
contact 610 and a heat sink 612. The heat source contact can be put
in contact with a source of waste heat such as the exhaust system
of an internal combustion engine. A first terminal electrode 614 is
positioned on the heat sink 612 in contact with a first strip 602
of n type doped silicon carbide nanofibers. A first coupling
electrode 616 is positioned on the heat source contact 610 in
contact with the first strip of n type doped silicon carbide
nanofibers 602 and the first strip of p type doped silicon carbide
nanofibers 606. A second coupling electrode 618 is positioned on
the heat sink 612 in contact with the first strip of p type doped
silicon carbide nanofibers 606 and the second strip of n type doped
silicon carbide nanofibers 604. A third coupling electrode 620 is
positioned on the heat source contact 610 in contact with the
second strip of n type doped silicon carbide nanofibers 604 and the
second strip of p type doped silicon carbide nanofibers 608. A
second terminal electrode 622 is positioned on the heat sink 612 in
contact with the second strip of p type doped silicon carbide
nanofibers 608. The coupling electrodes 616, 618, 620 connect the
silicon carbide nanofibers in a series circuit. The coupling
electrodes 616, 618, 620 and the terminal electrodes 614, 622 also
thermally couple the silicon carbide nanofibers 602, 604, 606, 608
to the heat source contact 610 and the heat sink 612. It is noted
that the pattern of repeating n and p type doped silicon carbide
nanofibers and coupling electrodes shown in FIG. 6 can be extended
to provide higher voltage output. The strips of n and p type doped
nanofibers extend perpendicularly into the plane of the drawing
sheet, so that they have a relatively high area and so that the
thermoelectric generator 600 generates a relatively high electrical
current. At least portions of the silicon carbide nanofibers 602,
604, 606, 608 that are in contact with the coupling electrodes 616,
618, 620 can be metalized to enhance electrical coupling.
Metallization may be accomplished by electroplating or
electrolessly plating the silicon carbide nanofibers 602, 604, 606,
608 with a metal such as Cu, Ag, Ni. The plated portions can then
be soldered or brazed to the coupling electrodes 602, 604, 606,
608. Alternatively, silver filled adhesives or glasses may be used
in lieu of soldering or brazing in which case plating would also
not be necessary.
[0033] FIG. 7 is an exploded view of a thermoelectric generator 700
according to another embodiment of the invention. The
thermoelectric generator 700 has a set of five rectilinear
serpentine shaped strips of n type doped silicon carbide nanofibers
702 extending in a Y-axis direction (indicated in the figure).
Arranged perpendicularly crossing the n type doped strips 702 are a
set of five rectilinear serpentine shaped strips of p type doped
silicon carbide nanofibers 704 extending in an X-axis direction
(indicated in the figure). Although five strips of each type are
shown in practice fewer or more of each type may be used. The
strips also have the character of non-woven mats in that their
thickness greatly exceeds the diameter of individual nanofibers and
there is a relatively large volume of empty space separating the
fibers. The strips 702, 704 are located between a heat source
contact 706 and a heat sink contact 708. (Rather than using a heat
sink contact a heat sink itself may be substituted.) A bottom
surface 710 of the heat source contact 706 that faces the strips
702, 704 is visible in FIG. 7 along with a top surface 712 of the
heat sink contact 708 that also faces the strips 702, 704. A
plurality of cold-side coupling electrodes 714 (only a couple of
which are numbered to avoid crowding the figure) are disposed on
the top surface 712 of the heat sink contact 708, and a plurality
of hot-side coupling electrodes 716 are disposed on the bottom
surface 710 of the heat source contact 706. Applying, by analogy,
the terms used to describe waves, it can be said that the strips
702, 704 have crests that are aligned, and make contact with, the
hot side coupling electrodes 716 and troughs that are aligned, and
make contact with, the cold side coupling electrodes 714. The
hot-side coupling electrodes 716 extend in the Y-axis direction and
couple pairs of crests of adjacent n type doped strips 702 and p
type doped strips 704. The cold-side coupling electrodes 714 extend
in X-axis direction and couple pairs of adjacent troughs of
adjacent n type doped strips 702 and p type doped strips 704. The
coupling electrodes 714, 716 and the strips 702, 704 together form
a plurality of electrical circuit pathways through the
thermoelectric generator 700. Arrows 1, 2, 3, 4 shown in FIG. 7
show a basic part of an electrical circuit pathway which repeats
periodically in the X-direction forming a single electrical pathway
through the thermoelectric generator 700. Arrow 1 goes through a
hot-side coupling electrode 716 from an n type doped strip 702 to a
p type doped strip 704. Arrow 2 goes down a sloped portion from a
peak to a trough of the p type doped strip 704. Arrow 3 goes
through a cold side coupling electrode 714 from the p type doped
strip 704 to an n type doped strip 702 and arrow 4 goes up a sloped
portion of the n type doped strip 702 from a trough to a peak. This
pattern is repeated several times in traversing the generator 700
generally in the X-direction. Multiple such pathways through the
generator can be connected either in series or in parallel
depending on the impedance characteristic of the load that is to be
powered by the thermoelectric generator. The rectilinear serpentine
configuration provides for large contact areas, to increase the
current generation capacity, while limiting parasitic thermal
conductivity from the heat source contact 706 to the heat sink
contact 708.
[0034] By the term "rectilinear serpentine" used above to describe
the strips it is meant that the contour is generally similar to a
serpentine contour but is made up of linear segments as opposed to
being continuously curved. Another way to describe the shape is
"trapezoidal wave" shaped. Alternatively, in lieu of a trapezoidal
wave shape the strips may be shaped as a saw tooth wave, triangular
wave or square wave. More generally any periodic shape that
successively makes contact with cold-side coupling electrodes
714.
[0035] In the case of the thermoelectric generator shown in FIG. 7,
in order to make electrical contact with the coupling electrodes
714, 716, it will be sufficient to metallize electrolessly or by
electroplating the crest and trough portions of strips and solder
or braze these to the coupling electrodes 714, 716. Selected
coupling electrodes may be used as terminal electrodes.
[0036] In order to increase the utilization of the areas of the
heat source contact 706 and the heat sink contact 708, two
arrangements of the strips 702 704 shown in FIG. 7 can be nested
together. In particular between n type doped strips 702 shown in
FIG. 7 there will be placed additional n type doped strips 702 that
also extend in the Y-axis direction and additional p type doped
strips 704 will be placed between those shown in FIG. 7 and
arranged extending in the X-axis direction-as those shown in FIG.
7. Considering that the strips 702, 704 are periodic in shape, it
can be said that the additional strips will be "phase shifted" by
one-half the wavelength. In other words, a trough of each strip of
each dopant type will be aligned with a crest of its neighbor of
the same dopant type. The resulting nested structure will manifest
a checker-board pattern of n type doped and p type doped areas when
viewed from the top and bottom. Top and bottom views of the
resulting structure are shown in FIGS. 8 and 9. In FIGS. 8 and 9
positive slope cross-hatched squares represents p type doped areas
of the p type doped strips 704 and negative slope cross-hatched
squares represent n type doped areas of the n type doped strips.
FIG. 10 shows a perspective view of the arrangement shown in FIGS.
8 and 9. This design provides for increased electric power
generation per unit volume of thermoelectric generator.
[0037] FIG. 11 shows an alternative arrangement of p type doped
1102 and n type doped 1104 trapezoidal wave shaped strips of
silicon carbide nanofibers, that can be used in a thermoelectric
generator such as shown in FIG. 6. In this case the strips 1102,
1104 extend in a common direction (e.g., into the plane of page,
for FIG. 6.) This arrangement also offers the advantage of
increased contact area (at the crests and troughs of the strips),
for increased current generation while reducing cross section area
of the material in gap of the thermoelectric generator so as to
control parasitic thermal conduction between the hot side and the
cold side of the thermoelectric generator.
[0038] FIG. 12 is a particular filter 1200 that uses ceramic, e.g.,
silicon carbide nanofiber filter media 1202 and is capable of
operation at high temperature, e.g., for filtering diesel
particulate motor exhaust. The ceramic nanofiber filter media 1202
is disposed in a housing 1204 that includes an inlet 1206 and an
outlet 1208 to allow gas to flow through.
[0039] While the preferred and other embodiments of the invention
have been illustrated and described, it will be clear that the
invention is not so limited. Numerous modifications, changes,
variations, substitutions, and equivalents will occur to those of
ordinary skill in the art without departing from the spirit and
scope of the present invention as defined by the following
claims.
* * * * *