Boron nitride nanotubes-reinforced glass composites

Article abstract of DOI:10.1111/j.1551-2916.2005.00701.x

Boron nitride nanotubes (BNNT) of significant lengths were synthesized by reaction of boron with nitrogen. Barium calcium aluminosilicate glass composites reinforced with～4 wt% of BNNT were fabricated by hot pressing. Ambient-temperature flexure strength

Full text of DOI:10.1111/j.1551-2916.2005.00701.x

J. Am. Ceram. Soc., 89 [1] 388–390 (2006)
DOI: 10.1111/j.1551-2916.2005.00701.x
r 2005 The American Ceramic Society
No claim to original US government works
ournal
J
Boron Nitride Nanotubes-Reinforced Glass Composites
Narottam P. Bansal,**,w Janet B. Hurst,* and Sung R. Choi*,z
NASA Glenn Research Center, Cleveland, Ohio 44135
Boron nitride nanotubes (BNNT) of significant lengths were
synthesized by reaction of boron with nitrogen. Barium calcium
aluminosilicate glass composites reinforced withB4 wt% of
BNNT were fabricated by hot pressing. Ambient-temperature
flexure strength and fracture toughness of the glass-BNNT
composites were determined. The strength and fracture tough-
ness of the composite were higher by as much as 90% and 35%,
respectively, than those of the unreinforced glass. Microscopic
examination of the composite fracture surfaces showed pullout
of the BNNT. The preliminary results on the processing and im-
provement in mechanical properties of BNNT-reinforced glass
matrix composites are being reported here for the first time.
ite were determined. The results on the processing and mechan-
ical properties of BNNTs-reinforced glass matrix composites are
being reported here for the first time.
II. Experimental Procedures
BNNTs were synthesized in-house at NASA Glenn Research
Center. Amorphous boron powder was mixed with several
weight percent of fine iron catalyst particles in a hydrocarbon
solvent and ball milled in a polythene bottle using ceramic
grinding media. The milled material was applied to various
high-temperature substrates such as alumina, silicon carbide,
platinum, and molybdenum and reacted in a flowing atmos-
phere of nitrogen containing a small amount of ammonia.
Nanotubes of significant length and abundance were formed
on heat treatments at temperatures from 11001 to 14001C for 20
min to 2 h. The resulting product consisted of a mixture of NTs
and particulates of BN. Batch sizes of typically 2 g were
produced, but the process should be easily scalable to larger
amounts.
For composite processing, a barium calcium aluminosili-
cate (BCAS)7 glass powder of composition (mol%) 35BaO–
15CaO–5Al2O3–10B2O3–35SiO2 or 56.4BaO–8.8CaO–5.4Al2O3
–7.3B2O3–22.1SiO2 (wt%) with an average particle size of 14.2
mm was used. This glass composition labeled as G-18 has a glass
transition temperature of 6191C, dilatometric softening point of
6821C, and coefficient of thermal expansion 10.5 ꢀ 10ꢁ61Cꢁ1
(from room temperature to 5001C) and 11.8ꢀ 10ꢁ61Cꢁ1 (201–
8001C). BCAS glass composites reinforced with B4 wt% of as-
synthesized BNNTs were fabricated.8 Appropriate quantities of
BCAS glass and BNNT were mixed in acetone, and ball milled
for B24 h using zirconia milling media. Acetone was evaporated
and the powder dried in an electric oven. The resulting mixed
powder was loaded into a graphite die and hot pressed in vac-
uum at 6301C under 10 MPa for 15 min into 50 mm ꢀ 25 mm
plates using a mini hot press. The applied pressure was released
before onset of cooling. Glass powder without any BNNT was
also hot pressed at 6301C. Grafoil was used as spacers between
the specimen and the punches.
I. Introduction
ARBON nanotubes (CNT) show many properties that are
superior to those of graphite. Boron nitride nanotubes
C
(BNNT) have similar structure as CNT and exhibit many sim-
ilar properties. Pure BNNTs were first synthesized using a plas-
ma arc discharge method yielding mainly double-walled BNNTs
or by a CNT substitution reaction giving multiwalled BNNTs.
Many other approaches, such as pyrolysis over cobalt, chemical
vapor deposition, laser ablation, and reactive milling, have been
tried for the synthesis of BNNTs with varying degrees of suc-
cess. BNNTs show interesting mechanical, electrical, and ther-
mal properties. Pure CNTs show metallic or semiconducting
behavior depending on composition, diameter, and chirality. On
the other hand, BNNTs are semiconductors with a band gap of
4–5 eV. BNNTs are potential candidate materials for a large
variety of nanosized electronic and photonic devices. CNTs ox-
idize in air at 4001–6001C and burn completely by 7001C, lim-
iting their high-temperature applications. On the other hand,
BNNTs show stability1 in air at 7001C. Some nanotubes with
perfect cylindrical structure may be stable1 up to 9001C. BNNTs
are, therefore, more suitable as reinforcement for composite
materials for applications at elevated temperatures in oxidizing
environment.
A number of studies are currently available on metal, ceram-
ic, and polymer matrix composites reinforced with CNTs as seen
from the recent review articles.2–6 However, to the best of our
knowledge, until now, no results have been published on the
subject of BNNTs-reinforced composites. The objective of this
work was to improve the strength and fracture toughness of a
solid oxide fuel cell (SOFC) seal glass7 by reinforcing with
BNNT. Ambient-temperature mechanical properties including
flexure strength and fracture toughness of glass-BNNT compos-
The hot-pressed plates were machined into flexure bar test
specimens with nominal depth, width, and length of 2.0
mm ꢀ 3.0 mm ꢀ 25 mm, respectively. The final finishing was
completed with a #500 diamond grinding wheel under the spec-
ified conditions in accordance with ASTM standard C1161.9
Machining direction was longitudinal along the 25 mm length
direction. The sharp edges of test specimens were chamfered to
reduce any spurious premature failure emanating from those
sharp edges.
F. Zok—contributing editor
Strength testing was conducted in four-point flexure at am-
bient temperature in air. A test fixture with 10 mm inner and 20
mm outer spans was used in conjunction with an electrome-
chanical test frame (Model 8562, Instron, Canton, MA). A
stress rate of 50 MPa/s was applied in load control using the
test frame. A total of 10 test specimens were tested. One of two 3
mm wide sides of each test specimen was subjected to maximum
tension or compression. Although the specimen configuration
Manuscript No. 20788. Received July 19, 2005; approved July 27, 2005.
This work was supported by Low Emission Alternative Power (LEAP) and Alternate
Energy Foundation Technology (AEFT) Programs, NASA Glenn Research Center, Cleve-
land, OH.
*Member, American Ceramic Society.
**Fellow, American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: narottam.p.bansal@
nasa.gov
zPresent address: University of Toledo, Toledo, OH.
388

January 2006
Communications of the American Ceramic Society
389
Table I. Strength and Fracture Toughness of G18 Glass and
Composite Reinforced with 4 wt% BNNTs
Property
Flexure strength
(MPa)
Fracture toughness
(MPa ꢂ m1/2
Materials
)
Glass G18
Glass G18-BNNT
composite
4877
0.5170.03
92717
0.6970.09
BNNT, boron nitride nanotube.
Hitachi S4700 (Hitachi High Technologies America, Inc., Plea-
santon, CA) field emission scanning electron microscope (FE-
SEM) equipped with a super-thin window EDAX Genesis
System (EDAX, Inc., Mahwah, NJ) energy-dispersive spectro-
meter (EDS).
Fig. 1. Field emission scanning electron microscope micrographs of
boron nitride nanotubes.
was different, in general, testing was carried out in accordance
with the ASTM test standards C 1161.9
Fracture toughness was determined using the flexure bar test
specimens at ambient temperature in air using the single-edge
v-notched beam (SEVNB) method.10 The same technique was
previously used to measure fracture toughness of G18 glass and
G18 glass composites reinforced with alumina or zirconia.8 This
method utilizes a razor blade with diamond paste, with grain
size of 9 mm, to introduce a final sharp notch with a root radius
ranging from 10 to 20 mm by tapering a saw notch. The sharp v-
notched specimens with a notch depth of about 1.0 mm along
the 3 mm wide side of test specimens were fractured in a four-
point flexure fixture with 10 mm inner and 20 mm outer spans
using the Instron test frame (Model 8562) at an actuator speed
of 0.5 mm/min. Three specimens were tested. Fracture toughness
was calculated based on the formula by Srawley and Gross.11
Morphology of the BNNTs, microstructures of polished
cross-sections of hot-pressed composites, and fracture surfaces
of specimens after strength measurements were observed in a
III. Results and Discussion
FESEM micrographs of the BNNTs synthesized at NASA
Glenn are shown in Fig. 1. BNNTs having diameter of 10–40
nm and of significant lengths (tens of micrometers) are observed.
The NTs were found to consist of nearly stoichiometric BN. BN
particles were also seen to be present in some areas.
Typical FESEM micrographs showing polished cross-sec-
tions of the G18 glass reinforced with BNNTs are shown in
Fig. 2. The NTs are dispersed homogeneously in the matrix.
From EDS analysis, it was found that the black regions consist
of BN and the white particles were zirconia, most probably in-
troduced as impurities during ball milling. Results of flexure
strength testing for the G18 glass-BNNT composite are present-
ed in Table I. Strength of G18 glass8 is also shown for compar-
ison. The addition of just 4 wt% of BNNT increases the glass
strength from 4877 to 92717 MPa. This 90% increase in
strength of the glass with BNNT reinforcement is notable, com-
pared with a moderate strength increase (40%–60%) for G18
glass reinforced with 5 mol% alumina platelets or zirconia par-
ticulates.8 Weibull strength distributions of the glass and
BNNT-reinforced glass composite are presented in Fig. 3 de-
spite insufficient number of test specimens used. Weibull mod-
ulus (m) ranged from 7 to 8, indicating the same degree of scatter
in strength (or flaw population) for both the glass and the
BNNT-glass composite. FESEM micrographs showing typical
fracture surfaces of flexure-strength-tested specimens of G18
glass reinforced with 4 wt% BNNTs are shown in Fig. 4. Lim-
ited amount of pullout of the NTs is observed as indicated by
arrows. Troughs resulting from pullout of the NTs can also be
seen in Fig. 4.
2
G18 glass-BNNT
G18 glass
1
0
–1
–2
–3
–4
σθ = 98 MPa
m = 6.5
σθ = 51 MPa
m = 8.1
2.6
3.0
3.4
3.8
4.2
4.6
5.0
5.4
ln[strength], ln [MPa]
Fig. 3. Weibull strength distribution of G18 glass reinforced with 4
wt% boron nitride nanotubes. The Weibull data for G18 glass are also
included for comparison8; m is the Weibull modulus and sy is the char-
acteristic strength.
Fig. 2. Field emission scanning electron microscope micrographs of
polished cross-section of G18 glass composite reinforced with 4 wt%
boron nitride nanotubes.

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Vol. 89, No. 1
Fig. 4. Field emission scanning electron microscope micrographs showing typical fracture surfaces of flexure-strength-tested specimens of G18 glass
reinforced with 4 wt% boron nitride nanotubes. Pullout of nanotubes is indicated by arrows.
The results of fracture toughness testing by the SEVNB meth-
od are given in Table I. Fracture toughness shows a similar
trend as strength. However, the increase in fracture toughness
was less significant than strength. The addition of just 4 wt% of
BNNTs increases the fracture toughness (KIc) of glass from
0.5170.03 to 0.6970.09 MPa ꢂ m1/2. This 35% increase in frac-
ture toughness for the glass-BNNTs composite is comparable
with that for the G18 glass composites reinforced with similar
amounts of alumina or zirconia.8
glass. Microscopic examination of the fracture surfaces showed
pull out of the NTs. The results on the processing and improved
mechanical properties of BNNTs-reinforced glass matrix com-
posites are being reported here for the first time.
Acknowledgments
The authors are grateful to Dan Gorican for assistance during processing of
BNNTs, Ralph Pawlik for mechanical testing, and John Setlock for composite
fabrication.
Limited availability of the BNNTs-reinforced glass composite
did not allow the evaluation of R-curve behavior or crack
growth resistance of the composite material in the current study.
However, it should be mentioned that the G18 glass reinforced
with alumina platelets or zirconia particulates has exhibited a
rising R-curve, with its degree being increased with increasing
content of the reinforcement.8 Crack bridging and/or crack de-
fection12 was considered to be a major strengthening or tough-
ening mechanism operative for those composites.8 A similar
mechanism may also be attributed to a possible R-curve and the
increase in strength and fracture toughness of the glass com-
posite reinforced with BNNTs, as seen from Fig. 4.
Because of their small size and large aspect ratio, CNTs are
considered to be highly promising reinforcement materials as the
energy dissipation during crack propagation can be greatly en-
hanced because of pullout. A few studies are available on the
SiC,13 alumina,14–16 and glass matrix17 composites reinforced
with CNTs. Improvements in strength and fracture toughness of
silica glass17 and alumina14 by the addition of 5%–10% of
CNTs have been reported earlier. However, Peigney et al.15and
Flahaut et al.16 failed to observe any beneficial effect on the
mechanical properties of alumina from the addition of long
CNT bundles.
The work presented in this paper has been done primarily for
the purposes of the fabrication of glass-BNNT composites and
the determination of their ambient-temperature mechanical
properties. However, SOFCs operate in a typical temperature
range of 7001–10001C, so that thermal and mechanical proper-
ties and environmental durability of the glass composites need to
be evaluated at elevated temperatures, as done previously for
10-YSZ composites.18 Needed work includes constitutive rela-
tion (viscosity)/sealability,19 coefficient of thermal expansion,
intermediate-temperature strength, thermal fatigue, life-limiting
factors, and stability of material including crystallization,7 etc.
Results of detailed investigations of this glass-BNNT composite
system will be reported in the near future.
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A BCAS glass composite containing 4 wt% BNNTs was fabri-
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both the flexure strength and fracture toughness of the glass.
The strength of the composite was higher by as much as 90%
and fracture toughness by as much as 35% than those of the
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