U.S. patent application number 16/745175 was filed with the patent office on 2020-07-23 for methods and compositions for highly purified boron nitride nanotubes.
This patent application is currently assigned to College of William & Mary. The applicant listed for this patent is College of William & Mary. Invention is credited to Mahmoud S. Amin, David E. Kranbuehl, Hannes C. Schniepp.
Application Number | 20200231439 16/745175 |
Document ID | / |
Family ID | 71608549 |
Filed Date | 2020-07-23 |
United States Patent
Application |
20200231439 |
Kind Code |
A1 |
Amin; Mahmoud S. ; et
al. |
July 23, 2020 |
METHODS AND COMPOSITIONS FOR HIGHLY PURIFIED BORON NITRIDE
NANOTUBES
Abstract
Herein we describe purified boron nitride nanotube compositions
substantially free from hexagonal boron nitride. The compositions
have a mass ratio of boron nitride nanotubes to hexagonal boron
nitride of at least 100. Methods are provided for producing said
purified boron nitride nanotube compositions wherein impure
compositions are subjected to heating with a C.sub.5 to C.sub.11
hydrocarbon solvent under specified conditions.
Inventors: |
Amin; Mahmoud S.;
(Williamsburg, VA) ; Kranbuehl; David E.;
(Williamsburg, VA) ; Schniepp; Hannes C.;
(Williamsburg, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
College of William & Mary |
Williamsburg |
VA |
US |
|
|
Assignee: |
College of William &
Mary
Williamsburg
VA
|
Family ID: |
71608549 |
Appl. No.: |
16/745175 |
Filed: |
January 16, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62793621 |
Jan 17, 2019 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 21/0648 20130101;
B82Y 30/00 20130101; B82Y 40/00 20130101; C01P 2002/82 20130101;
C01P 2004/03 20130101; C01P 2004/13 20130101; C01P 2002/74
20130101 |
International
Class: |
C01B 21/064 20060101
C01B021/064 |
Claims
1. Purified boron nitride nanotubes having a mass ratio of boron
nitride nanotubes to hexagonal boron nitride exceeding 100.
2. The purified boron nitride nanotubes of claim 1, having a
BNNT/h-BN X-ray diffraction spectral peak ratio of at least
100.
3. The purified boron nitride nanotubes of claim 1, having a
BNNT/h-BN X-ray diffraction spectral peak ratio of at least 100 and
lacking a significant Raman spectrum peak in the 1366 cm.sup.-1
region.
4. A method for producing purified boron nitride nanotube
compositions comprising the steps of: mixing an impure BNNT sample
with a hydrocarbon solvent; heating said mixture of said impure
BNNT sample and said hydrocarbon solvent for a period of time; and
separating said hydrocarbon solvent from said BNNT sample; wherein
said step of heating is performed at a temperature within
50.degree. C. of the boiling point of said hydrocarbon solvent,
wherein said heating is performed for a period exceeding five
minutes, and wherein said hydrocarbon solvent comprises one or more
hydrocarbons possessing at least five carbon atoms and no more than
eleven carbon atoms.
5. The method of claim 4, wherein said hydrocarbon is heated such
that its vapor pressure is at least 300 mm Hg, and less than 1,150
mm Hg.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn. 119(e) to U.S. Provisional Patent Application No.
62/793,621, filed Jan. 17, 2019, the entire disclosure of which is
incorporated by reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] The field of the invention relates to methods of preparing
purified boron nitride nanotubes (BNNT or BNNTs) and purified
compositions of BNNT.
Description of the Related Art
[0004] Due to their unique mechanical and thermal properties,
interest in BNNT has grown dramatically over the past two decades.
Like carbon nanotubes, BNNTs display exceptional strength. Despite
being electrically insulating semiconductors, BNNTs possess high
thermal conductivity. They are promising materials for many
applications.
[0005] Realization of these promising applications has been
hindered by significant challenges in BNNT synthesis and
purification which has proven far more difficult, for example, than
carbon nanotubes (CNTs). For example, in WO20181024231 A1,
Dushatinski describes problems with prior art synthesis and
purification approaches for BNNT. Depending on the synthesis
conditions, as-synthesized BNNTs have substantial amounts of small
boron-containing particles in the form of boron, amorphous boron
nitride (a-BN), and hexagonal boron nitride (h-BN). Depending on
the synthesis conditions, these small boron-containing particles
can account for 5% to 95% of the mass of the as-synthesized
materials.
[0006] These impurities are problematic, as, for example, they can
reduce the BNNT surface area, reduce strength, and/or reduce
thermal conductivity of the BNNT materials. These impurities can
further compromise the interface of BNNTs with other materials and
thus diminish their ability to be dispersed, and/or to transfer
mechanical load and heat across such interfaces in nanocomposites,
with the consequence of reduced structural and thermal performance
of the BNNT composites.
[0007] Accordingly, methods have been sought for purification of
as-synthesized BNNTs. Methods for removing boron and boron oxides
are known in the prior art and have been reasonably effective. In
contrast, prior art methods have been deficient in removing h-BN,
which is challenging to remove from impure BNNTs (due to its
chemical similarity with BNNTs), without damaging the BNNTs. These
prior art methods can be costly, time-intensive, can reduce yields
of BNNTs, and can damage BNNTs.
[0008] There is a need in the art for a method that virtually
completely removes h-BN impurities without damaging the BNNTs, and
there is a need in the art for high-purity BNNT compositions.
BRIEF SUMMARY OF THE INVENTION
[0009] The present disclosure relates to novel compositions of
boron nitride nanotubes, and methods to produce said compositions.
More specifically, the present disclosure relates to a method for
the removal of boron nitride impurities and h-BN sheets from BNNT
compositions using a low-temperature, non-destructive, hydrocarbon
solvent-based method for the purification of impure BNNTs. The
present disclosure relates to purified boron nitride nanotubes
having a mass ratio of boron nitride nanotube to hexagonal boron
nitride exceeding 100. A process is provided for producing said
purified BNNT compositions comprising: (a) mixing an impure BNNT
sample with a hydrocarbon solvent; (b) heating said mixture of said
impure BNNT sample and said hydrocarbon solvent for a period of
time; and (c) separating said hydrocarbon solvent from said BNNT
sample; wherein said step of heating is performed at a temperature
within 50.degree. C. of the boiling point of said hydrocarbon
solvent, wherein said heating is performed for a period exceeding
five minutes, and wherein said hydrocarbon solvent comprises one or
more hydrocarbons possessing at least 5 carbon atoms and no more
than 11 carbon atoms.
[0010] Suitable hydrocarbon solvents include C.sub.5 to C.sub.11
alkanes, C.sub.5 to C.sub.11 cycloalkanes, C.sub.5 to C.sub.11
alkenes, C.sub.5 to C.sub.11 cycloalkenes, C.sub.5 to C.sub.11
isoalkanes, and mixtures thereof. For example, suitable C.sub.5 to
C.sub.11 alkane hydrocarbon solvents include pentane, hexane,
heptane, octane, nonane, and decane. In some embodiments, heptane
is the hydrocarbon solvent. While other compounds can be included
in the C.sub.5 to C.sub.11 hydrocarbon solvent, whether
intentionally or unintentionally, the concentration of C.sub.5 to
C.sub.11 hydrocarbons in the C.sub.5 to C.sub.11 hydrocarbon
solvent must be at least 90% by weight.
[0011] In some preferred embodiments, during the step of heating
the hydrocarbon solvent with the BNNT material, the heating
temperature is the boiling point of the hydrocarbon solvent. In
other embodiments, the mixture must be heated to at least
70.degree. C. In some embodiments, the heating temperature is
within 50.degree. C. of the boiling point of the hydrocarbon
solvent, or within 25.degree. C., or within 10.degree. C. In all
suitable embodiments, the temperature is below 300.degree. C., and
preferably below 200.degree. C. In some embodiments, the
temperature is below 175.degree. C., or below 150.degree. C., or
below 125.degree. C., or below 100.degree. C.
[0012] In some embodiments, the step of heating is performed in a
pressure vessel. In other embodiments, the purification is
performed in a Soxhlet apparatus, or a suitable flask with a
condenser, or simply by exposing the BNNT to the hydrocarbon vapor
in any suitable container. Use of agitation, including sonication,
can be used to enhance the process.
[0013] In some embodiments, the BNNT can be separated from the
hydrocarbon solvent simply by pouring off, decanting, or filtering
the solvent.
[0014] The purified BNNT compositions described herein are
substantially free of h-BN impurities. Such h-BN impurities are
known in the art to be very difficult to remove from impure BNNT
samples. More specifically, the purified boron nitride nanotubes,
as described herein, have a mass ratio of boron nitride nanotube to
hexagonal boron nitride exceeding 100. In other words, any boron
atom in the purified BNNT is roughly 100 times more likely to be
incorporated into a boron nitride nanotube than in hexagonal boron
nitride. In some embodiments, the purified boron nitride nanotubes
have a molar ratio of boron nitride nanotube to hexagonal boron
nitride exceeding 200, or 300, or 400, or 500, or 1,000. In typical
embodiments, the purified boron nitride nanotubes have a BNNT/h-BN
X-ray diffraction spectral ratio of at least 100.
[0015] The purified BNNT compositions described herein can be used,
for example, to enhance the mechanical and thermal conductivity of
polymer composites. BNNT is known through theoretical and
experimental reports to be 60 times stronger than steel. Thus, the
addition of small quantities of BNNT to a high performance polymer
such as an epoxy or polyimide has the potential to significantly
increase the mechanical performance properties of the part. BNNT
has six times the thermal conductivity of copper but, unlike most
highly thermally conductive materials, it is an electrical
insulator. Removal of heat is the major factor limiting the
development of smaller and more powerful electronic devices.
Addition of BNNT to polymers used to support high power diodes
would make it possible to conduct heat away from the diode. Thus,
electronic components can potentially be made much more powerful
and much smaller.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The summary above, and the following detailed description,
will be better understood in view of the drawings which depict
details of preferred embodiments.
[0017] FIG. 1A shows a field emission scanning electron microscopy
(FESEM) image of as-synthesized BNNT prior to purification. FIG. 1B
shows an FESEM image of BNNT after purification according to the
methods described herein. FIG. 1C shows an FESEM image of heptane
purification residue removed from as-synthesized BNNT, and FIG. 1D
shows a higher magnification FESEM image of heptane purification
residue removed from as-synthesized BNNT.
[0018] FIGS. 2A and 2B show overlaid X-ray diffraction (XRD)
patterns of as-produced BNNT and purified BNNT. FIG. 2A compares an
XRD image of as-produced BNNT with literature XRD values of h-BN,
showing that the as-produced BNNT has a sharp shoulder peak around
26.69.degree. which is consistent with the literature peak of h-BN
at 26.63.degree.. FIG. 2B compares XRD images of as-produced BNNT
with purified BNNT, and demonstrates the absence of the sharp
shoulder peak at 26.69.degree. in the purified BNNT.
[0019] FIG. 3A shows overlaid Raman spectra of purified BNNT and
as-produced BNNT, demonstrating the lack of a pronounced peak
around 1366 cm.sup.-1 in the purified BNNT. FIG. 3B shows overlaid
Raman spectra of h-BN powder and residue material removed from
as-produced BNNT during the purification process, both of which
have a strong peak in the 1366 cm.sup.-1 region.
[0020] FIG. 4A shows an FESEM image of a BNNT sample prior to a
higher temperature heptane purification procedure. FIG. 4B shows an
FESEM image of the BNNT sample after the higher temperature heptane
procedure which damaged the BNNTs.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present disclosure is directed to purified boron nitride
nanotube compositions and methods for producing such
compositions.
[0022] The term "boron nitride nanotube(s)", abbreviated BNNT or
alternatively BNNTs, refers to cylindrical boron nitride structures
with sub-micrometer diameters and lengths exceeding 1 micrometer.
They are a polymorph of boron nitride, and have a theoretical boron
to nitrogen atomic ratio of one. Boron nitride nanotube
compositions are produced with significant levels of impurities,
including boron, amorphous boron nitride, and hexagonal boron
nitride. The term "hexagonal boron nitride", abbreviated as h-BN,
includes both h-BN nanocages and h-BN nanosheets.
[0023] The term "as-produced boron nitride nanotubes", also
referred to as "as-synthesized boron nitride nanotubes" and
abbreviated as "as-produced BNNT", means compositions of BNNT that
have been produced but have not been substantially freed from all
impurities.
[0024] The term "purified boron nitride nanotube compositions",
abbreviated as "purified BNNT", means BNNT compositions purified
such that any boron atom in the purified BNNT composition is 100
times more likely to be incorporated into a boron nitride nanotube
than into hexagonal boron nitride.
[0025] The purification ratio can be determined by analyzing X-ray
diffraction spectra. More specifically, the BNNT/h-BN X-ray
diffraction spectral peak ratio refers to the ratio of the peak
height (a.u.) at the main peak of the XRD spectrum for BNNT (around
25.4.degree.) to the peak height (a.u.) at the shoulder peak around
26.69.degree..
[0026] Any BNNT compositions can be used as starting materials.
Such BNNTs can be produced using any methods suitable for producing
BNNTs, including but not limited to high-temperature/pressure (HTP)
methods, hydrogen-assisted BNNT synthesis (HABS), extended-pressure
inductively-coupled plasma (EPIC), and ball milling/annealing.
[0027] The present disclosure relates to purified boron nitride
nanotubes having a mass ratio of boron nitride nanotube to
hexagonal boron nitride exceeding 100. A process is provided for
producing said purified BNNT compositions comprising: (a) mixing an
impure BNNT sample with a hydrocarbon solvent; (b) heating said
mixture of said impure BNNT sample and said hydrocarbon solvent for
a period of time; and (c) separating said hydrocarbon solvent from
said BNNT sample; wherein said step of heating is performed at a
temperature within 50.degree. C. of the boiling point of said
hydrocarbon solvent, wherein said heating is performed for a period
exceeding five minutes, and wherein said hydrocarbon solvent
comprises one or more hydrocarbons possessing at least 5 carbon
atoms and no more than 11 carbon atoms.
[0028] In some embodiments, said step of heating is performed in a
pressure vessel that can permit pressures many times atmospheric
pressure. Alternatively, the purification can be performed in a
Soxhlet apparatus, or a suitable flask with a condenser, or simply
by exposing the BNNT to the hydrocarbon vapor in any suitable
container.
[0029] Suitable hydrocarbon solvents include C.sub.5 to C.sub.11
alkanes, C.sub.5 to C.sub.11 cycloalkanes, C.sub.5 to C.sub.11
alkenes, C.sub.5 to C.sub.11 cycloalkenes, C.sub.5 to C.sub.11
isoalkanes, and mixtures thereof. For example, suitable C.sub.5 to
C.sub.11 alkane hydrocarbon solvents include pentane, hexane,
heptane, octane, nonane, and decane. In some embodiments, heptane
is the hydrocarbon solvent. While other compounds can be included
in the C.sub.5 to C.sub.11 hydrocarbon solvent, whether
intentionally or unintentionally, the concentration of C.sub.5 to
C.sub.11 hydrocarbons in the C.sub.5 to C.sub.11 hydrocarbon
solvent must be at least 90% by weight.
[0030] For example, pentane is an effective solvent for BNNT
purification when used according to the methods described herein.
Hydrocarbons shorter than pentane are gases at room
temperature.
[0031] The C.sub.12 hydrocarbon dodecane was ineffective when used
in an attempt to purify BNNTs. Raman spectroscopy showed that h-BN
was not removed from a BNNT sample treated with dodecane according
to the methods described herein. More polar solvents such as
isopropanol were also found to be ineffective.
[0032] Without wishing to be bound by theory, it is believed that
the vapor pressure of the heated solvent is suitable to liberate
the impurities from the BNNT structure, leaving purified BNNT. This
process is particularly effective for purifying away h-BN. For
example, hydrocarbon molecules can diffuse into cleavage spaces
between planar h-BN and curved BNNT, and the trapped hydrocarbon
can exert pressure to drive apart the h-BN and the BNNT.
Alternatively, hydrocarbon molecules can diffuse into the interior
of a BNNT, and increased pressure can induce small changes in the
tube conformation, potentially releasing impurities in the
process.
[0033] Accordingly, it is important to have significant vapor
pressure. In some embodiments, the solvent is heated such that the
vapor pressure of the solvent exceeds 300 mm Hg, or exceeds 400 mm
Hg. In some preferred embodiments, during the step of heating the
hydrocarbon solvent with the BNNT material, the heating temperature
is the boiling point of the hydrocarbon solvent. In other
embodiments, the mixture must be heated to at least 70.degree. C.
In some embodiments, the heating temperature is within 50.degree.
C. of the boiling point of the hydrocarbon solvent, or within
25.degree. C., or within 10.degree. C.
[0034] If the pressure is too high, the BNNT can be destroyed, as
is described later in Example 2 in the specification. Accordingly,
there is a vapor pressure range for which the purification process
is particularly effective. That vapor pressure range when the
solvent is heptane is between about 300 mm Hg and 1,150 mm Hg,
preferably between about 350 mm Hg and 1,000 mm Hg, or between
about 400 mm Hg and 900 mm Hg. The pressure can be cycled such that
BNNTs are subjected to fluctuations in pressure, which can
facilitate sequential contraction and relaxation of the BNNTs.
[0035] The duration of heating at the requisite temperature is at
least five minutes, and can be any longer duration that provides
the desired purification without substantial damage to the BNNTs.
For example, the heating can be performed for at least five
minutes, or at least ten minutes, or at least 30 minutes. The
heating can be performed in one event, or can be performed in a
cyclic heating pattern comprising alternate heating and cooling.
Use of agitation such as sonication during heating can be used to
enhance the process.
EXAMPLES
[0036] The examples that follow are intended in no way to limit the
scope of this invention but are provided to illustrate
representative embodiments of the present invention. Many other
embodiments of this invention will be apparent to one skilled in
the art.
Example 1
[0037] As-produced BNNT material was synthesized using a high
temperature pressure method coupled with a procedure to remove
boron impurities. The as-produced BNNTs were further purified as
described below.
[0038] Removal of boron nitride and purification of BNNT was
accomplished using the low temperature hydrocarbon procedure
described herein. This procedure involved mixing 50 mg of
as-produced BNNT with 15 mL of heptane (Sigma Aldrich, Raleigh,
N.C., USA) in a 21 mL Ace pressure tube (Ace Glass Incorporation,
Vineland, N.J., USA). The system was heated and maintained at
90.degree. C. for 5 hours in an oil bath. The system was cooled to
room temperature before the pressure tube was opened. Heptane was
separated from the BNNT sample by decantation. The purified BNNT
was dried in a vacuum oven at -1 atm, 250.degree. C. overnight. The
decanted heptane solution containing the BN impurities was
concentrated in the vacuum oven at 60.degree. C. for further
investigation.
[0039] Field emission scanning electron microscopy (FESEM) data was
captured using Hitachi S-4700 field emission scanning electron
microscope. The samples were then dispersed in isopropanol,
pipetted onto graphite tape, spin dried, and sputter coated.
[0040] Raman data was collected using a Renishaw inVia dispersive
Raman spectrometer using 514 nm with excitation power 10-20 mW and
a 100.times. objective with an N.A. of 0.65. The sample preparation
for the residual impurities was done by drop casting the
concentrated heptane solution on a cleaned silicon wafer. The Raman
spectrum had a broad background peak over the range 500-7000
cm.sup.-1 that was due to fluorescence. Using MATLAB software, this
background data was fitted using second order polynomial and
subtracted. To get rid of noise, the data was subsequently smoothed
using 10-point averaging. The purified BNNT signal noise was
calculated using root mean square method (RMS) to define the
detection limit of the technique.
[0041] X-ray diffraction (XRD) analysis was performed using a
Bruker SMART APEX II diffractometer equipped with an APEX II CCD
Detector and a copper K.alpha. source (wavelength .lamda.=1.54
.ANG.). The samples were prepared by compressing 20 mg of material
into 3 mm discs. Due to the inhomogeneity of the BNNT samples,
different locations on the BNNT disc were measured. The XRD pattern
signal noise was calculated using the root mean square method (RMS)
which enabled us to define the detection limit of the technique to
be impressively low, between 0.2-0.4%.
[0042] Field emission scanning electron microscopy (FESEM) was
conducted before and after the purification. As seen in FIG. 1A,
the as-produced BNNT consists of multiple nanoscale tube networks
that entangle and extend to microns in length. This structure is
generally understood to be result of the BNNT growth process which
has been hypothesized to originate from droplets of pure boron
contained in fullerene-like BN cages ("nano-cocoons"). The larger
impurities seen entangled within BNNTs may be the remnants of these
BN nano-cocoons and BN nanoparticles, or they could be other
impurities. As seen in FIG. 1A, these impurities (which appear as
bumps along the long tubes) form aggregates on the tube surface and
nodes. FIG. 1B shows the BNNTs after purification according to the
methods described above. Visual impurities on the surface of the
BNNTs are nearly absent, in stark contrast to FIG. 1A. Note that
the BNNTs also appear to be thinner after the purification
treatment.
[0043] Analyzing the decanted cleaning residue collected after
drying the heptane filtrate yielded important insight into the
composition of the material removed from the nanotubes via the
purification method. FESEM analysis suggests that the impurities
consisted of an abundance of small particles. FIG. 1C shows a
representative FESEM image of a micron-sized particle hinting at a
stacked surface structure. A higher-resolution FESEM image of the
material is shown in FIG. 1D.
[0044] To further understand the cleaning process, we carried out
X-ray diffraction (XRD) of the BNNT material before and after
cleaning, which is shown in FIG. 2. The as-produced BNNT X-ray
pattern shown in both FIG. 2A and FIG. 2B has a pronounced series
of peaks in the range 2.theta.=20.degree.-30.degree., corresponding
to d-spacings in the range 4.43 .ANG.-2.97 .ANG.. This range
overlaps well with literature values for inter-layer spacings
within multi-layer, 2D materials such as graphite or h-BN. The
spectrum features a second series in the range
2.theta.=40.degree.-55.degree., corresponding to d-spacings in the
range 2.25 .ANG.-1.67 .ANG., likely representing the intra-layer
spacings between the atoms of the in-plane honeycomb lattice.
[0045] The first peak series was analyzed using Lorentzian peak
fitting, with the main peak at 25.33.degree. and a sharp sub-peak
on the right (26.69.degree.) with FWHMs of 2.30.degree. and
0.18.degree., respectively. To help interpret the measured BNNT
peak positions, we overlaid h-BN peaks based on literature values
as shown in FIG. 2A. There is a remarkable agreement between the
position of the sharp 26.69.degree. BNNT sub-peak on the right of
the as-produced BNNT and the h-BN peak at 2.theta.=26.63.degree..
The presence of this h-BN peak suggests that the as-produced BNNT
contained a significant amount of h-BN.
[0046] When XRD was carried out on the purified BNNT compositions
(purified using heptane as a solvent in accordance with the methods
of the disclosure), we found that the main peak retained virtually
the same position, shifting slightly to 25.46.degree. (with a
narrower FWHM 1.82.degree.) as shown in FIG. 2B, which overlays the
spectra of purified BNNT and as-produced BNNT. Importantly, the
sharp sub-peak at 26.69.degree. was absent in the purified BNNT
spectrum, suggesting that the methods described herein removed
virtually all of the h-BN from the as-produced BNNT. In the
2.theta.=40.degree.-55.degree. range, the spectra of the
as-produced and purified materials look very similar, indicating
that the in-plane lattice of the purified BNNT is not significantly
altered by the purification procedure described herein.
[0047] To assess the presence of remaining h-BN and/or quantify the
degree of h-BN removal, a Gaussian was fitted to the dominating
main peak of the purified sample and subtracted from the purified
XRD spectrum in FIG. 2. This procedure allowed us to inspect all
remaining features in the spectrum at greater amplification.
Nevertheless, the XRD peak at 26.69.degree. was not detectable
above the general noise level of approximately 0.5 intensity units
(RMS noise). Having verified the absence of this h-BN peak, more
investigation was conducted to quantify the sensitivity of the XRD
measurement and to define the detection limit of the XRD technique.
First, the peak heights were determined using Gaussian fitting and
are shown in Table 1. Then, the noise of the purified BNNT pattern
was determined using the root mean square method (RMS). The height
of the second Gaussian peak of the as-produced BNNT was compared to
the noise RMS. Using the ratio of the noise to the peak height
indicates that the h-BN detection limit is 0.002. The absence of
this sub-peak around 26.69.degree. in the XRD spectrum of the
purified BNNT shows that the purification method described herein
succeeded in removing at least 99.8 percent of the h-BN
impurities.
TABLE-US-00001 TABLE 1 XRD fitting parameters Peak FWHM Height
Sample ID Peak position [.degree.] [.degree.] [a.u.] R.sup.2
As-produced Main 25.33 2.30 1000.0 0.94 BNNT h-BN 26.69 0.18 284.1
Purified BNNT Main 25.42 1.82 1000.0 0.98
[0048] In this example, the as-produced BNNT had a BNNT/h-BN X-ray
diffraction spectral peak ratio of 3.52 (i.e., the ratio of the
heights of the main peak to the h-BN peak, corresponding to 1,000
divided by 284.1). The purified BNNT has a BNNT/h-BN X-ray
diffraction spectral peak ratio of at least 1,000 (i.e., the peak
height of 1,000 a.u. at the main BNNT peak of 25.42.degree. divided
by the h-BN peak height which was not distinguishable above the
general noise level of 0.5 a.u.).
[0049] We further analyzed the effectiveness of our purification
technique to remove h-BN using non-resonant Raman spectroscopy with
an excitation wavelength .lamda.=514 nm (the expected bandgap of
BNNTs is 5.5 eV, corresponding to a photon wavelength of 225 nm,
while for h-BN the corresponding wavelength is 215 nm). According
to literature (Nemanich, R J et al., "Light scattering study of
boron nitride microcrystals". Physical Review B 23, 6348-6356
(1981)), h-BN is expected to feature a pronounced peak at 1365
cm.sup.-1, whereas BNNTs have been shown not to feature this peak
in non-resonant Raman spectroscopy.
[0050] FIG. 3A shows overlaid Raman spectra of (i) as-produced BNNT
and (ii) purified BNNT. The spectrum of the as-produced BNNT has
one pronounced peak, located at 1368.5 cm.sup.-1, while the
purified BNNT featured no peak in this spectral region. FIG. 3B
shows overlaid spectra of (i) h-BN and (ii) the purification
residue removed from as-produced BNNT when purified using the
methods disclosed herein. Each spectrum features one pronounced
peak, located at 1364.6 cm.sup.-1 and 1367.4 cm.sup.-1,
respectively. Table 2 lists the corresponding peak positions and
full widths at half maximum (FWHM) from the peak fitting parameters
of the Raman peaks. The observed h-BN peak at 1364.6 cm.sup.-1 is
in excellent agreement with the literature value.
TABLE-US-00002 TABLE 2 Raman FWHM and peak position based on peak
fitting FWHM Peak Sample [cm.sup.-1] position [cm.sup.-1] R.sup.2
h-BN 12.5 1364.6 .+-. 0.1 0.99 Residue material 26 1367.4 .+-. 0.2
0.97 As-Produced BNNT 36 1368.5 .+-. 0.2 0.98
[0051] The Raman data for the as-produced BNNT (FIG. 3A) shows a
symmetric peak similar to the peak in the h-BN and residue
material. The BNNT has no peak in the non-resonance Raman,
suggesting that the as-produced BNNT contains h-BN impurities,
which is further support for the XRD findings described above.
Furthermore, the peak position of the residue peak is very close to
the h-BN peak, which suggests that the collected purification
residue contains the removed h-BN impurities. Interestingly, the
peak position did not change much between the as-produced material
(1368.5 cm.sup.-1) and the residue (1367.4 cm.sup.-1), suggesting
that the size of the h-BN particles was not altered much during the
purification procedure. Thus, we can conclude from our Raman
analysis that the as-produced BNNTs contain h-BN, and that our
purification procedure removes substantially all of the h-BN, which
is in full agreement with the XRD results.
Example 2
[0052] As-produced BNNT material was synthesized using a high
temperature pressure method coupled with a procedure to remove
boron impurities. The as-produced BNNTs were further purified as
described below.
[0053] To test the range of the purification procedure described
herein, as-produced BNNT was mixed with heptane (Sigma Aldrich,
Raleigh, N.C., USA) in an Ace pressure tube (Ace Glass
Incorporation, Vineland, N.J., USA). The system was heated and
maintained at 120.degree. C. for 5 hours in an oil bath,
corresponding to a projected pressure of 1187 Torr (1.56 atm). The
system was cooled to room temperature before the pressure tube was
opened. Heptane was separated from the BNNT sample by decantation.
FIG. 4A shows an FESEM image of the BNNT sample before the heptane
purification procedure, while FIG. 4B shows an FESEM image of the
BNNT sample after the heptane procedure. FIG. 4B shows that the
BNNTs shown in FIG. 4A were damaged by the relatively high pressure
and temperature heptane procedure used in this example.
[0054] Incorporation by Reference
[0055] All publications, patents, and patent applications cited
herein are hereby expressly incorporated by reference in their
entirety and for all purposes to the same extent as if each was so
individually denoted.
[0056] Equivalents
[0057] While specific embodiments of the subject invention have
been discussed, the above specification is illustrative and not
restrictive. Many variations of the invention will become apparent
to those skilled in the art upon review of this specification. The
full scope of the invention should be determined by reference to
the claims, along with their full scope of equivalents, and the
specification, along with such variations.
[0058] The articles "a" and "an" are used herein to refer to one or
to more than one (i.e. to at least one) of the grammatical object
of the article. By way of example, "a boron nitride nanotube" means
one boron nitride nanotube or more than one boron nitride
nanotube.
* * * * *