NASA Technical Reports Server (NTRS) 20150010199: Silicon Carbide Nanotube Oxidation at High Temperatures

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by  NASA Technical Reports Server (NTRS)

Silicon Carbide Nanotube Oxidation at High
· Temperatures
Nadia Ahlborg
The Ohio State University, Columbus, Ohio 43210
Dongming Zhu
NASA Glenn Research Center, Cleveland, Ohio 44135
36th International Conference on Advanced Ceramics and Composites
Daytona Beach, Florida, USA
January 22-27, 2012
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Abstract
Silicon Carbide Nanotubes (SiCNTs) have high mechanical
strength and also have many potential functional applications. In
this study, SiCNTs were investigated for use in strengthening high
temperature silicate and oxide materials for high performance
ceramic nanocomposites and environmental barrier coating bond
coats. The high · temperature oxidation behavior of the nanotubes
was of particular interest.
The SiCNTs were synthesized by a direct reactive conversion
process of multiwall carbon nanotubes and silicon at high
temperature. Thermogravimetric analysis (TGA) was used to study
the oxidation kinetics of SiCNTs at temperatures ranging from 800°C
to1300°C. The specific oxidation mechanisms were also
investigated.
Experimental: SiCNT Nanotube an
Coating Processir:1g
• SiCNTs were synthesized in the furnace in a Ar+H reducing
2
environment. Carbon nanotube and Si powders were mixed and
heat treated in two process cycles for 4 hours each at 1400°C.
...... ----······-···· ....... ------------------------------------------------------------------------------- ...... ... ················-········· -········· ---.... ········· ........................................... ;··· ................. -.
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Experimental: Thermog
• Schematic of TGA experiment setup. 60-1 00 mg of SiCNTs were
tested for 100 hours in dry 02 at 800°C, 900°C, 1 000°C, 11 00°C,
1200°C, and 1300°C.
• The Specimen weight changes were recorded vs. time.
Cahn 1 000 balance
• Gas ·in
/-- Sampl'e
!Furnace ·· Thermocouple
To vacuum pump
----.-
TGA
• Previous work1 2 3 predicted that formation of Si0 on the
, ,
2
nanotube surface follows a parabolic rate law:
l:l.w = change in weight
A = initial surface area = 134500cm2/g
KP = parabolic rate constant
t =time
C =constant
• The rate constant (Kp) are related to temperature by the
Arrhenius equation:
KP = Aexp( - Q/ RT)
(2)
Q = activation energy
R = gas constant
T = temperature
Specific Weight Gain vs.
• (Specific weight change)2 vs. time is plotted for each temperature. The
soooc
oxidation behavior appears pseudo-parabolic at up to 1200°C for the
ooooc
fiber-like SiCNTs, and Kp was calculated using equation 1. For 1 to
1300°C, initial shrinking fiber surface area causes the reaction to slow
down over time. Only the initial, parabolic portion of each curve was used
to calculate Kp-
- 800C - 900C -lOOOC --llOOC - 1200C - 1300C
6.E-06
N
Qj S.E-06
bO
s:::
co
0
4.E-06
.+.c-:'
-~ 3.E-06
sQJ
-~ 2.E-06
·-
"'-
u
QJ
-
~ l.E-06
0 20 40 60 80 100
Time (hours)
SiCNT Oxidation Activation Energy
Tests
• Equation 2 was used to calculate the activation energy (Q) of 155 kJ/mol.
The graph of Kp vs. 1/T gives a straight line with little deviation,
indicating that Q is valid for the 800°C - 1300°C temperature range.
-12 -
-13 -
; -14
t:
"'
t; -15
t: ., ""'
8 ...........,. ....-------. Activation Energy
....
-16
=155.4 kJ/mol
CIJ
10 ....
-17 ..... ....
D: ................ ~
.........,
....
'
-t: -18
-19
-20 --·--,-- - I -- I -- - I
6.0E-04 7.0E-04 8.0E-04 9.0E-04 l.OE-03
1/T (K-1
)
............................ -·-·-·-···---·-----------·····. ------·-····-···-·····-------·-·······---········----···-····--··-·-·•-·-······--·---········-----·-········-·····--··-········--·---·········-···---------·-···-·····-···-··-···-··········-···-···································•···c·--c·••''. .., . • ,• • ,.,• •.•. ,
SEM Observation of SiCNTs
• SEM images of SiCNTs after 100 hours of testing. The SiC nanotube
structure appears degraded after oxidation at 11 00°C and above.
• Box and whisker plot of nanotube diameters vs. TGA temperature. The
SiCNTs grew in diameter during each test due to formation of a Si0 scale.
2
Nanotube Diameter vs. TGA Temperature
1.0
0 '
s
~
2-
0.8
~
~
~
s
~
0.6
cj
·~
Q
0
] 0.4
0
0 0
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§ 0
a 8
0
o.2 • + 0
z
-
.
X-Ray Diffraction Results of Oxjd
• X-ray diffraction spectra for
SiC, cubic
SiC, hexagonal
SiCNTs after TGA tests at
I cl
I I
different The lack
te~peratures. I i
I
10 20 30 40 50 60 70
of Si0 in the 800°C, 900°C, and
2 SiCNT, TGA 1000°C, llOOh
SiC
10 00°C spectra suggests that the
; I
Si0 scale is and
a~orphous
2
20 30 40 50 60 70
very thin.
100°C, lOOh SiC
Si02, hexagonal
Si02, tetragonal
oooc,
• The 11 1200°C, and 1300°C
L ,
spectra show the presence of s·cNr TGA 12oooc 1ooh SiC
Si02, hexagonal
crystalline Si0 The 1200°C
2• I Si02, tetragonal
sa~ple contains al~ost no SiC, 1L0~~~· ~~~: ~~=·~~~~~~~~~~
20 30 40 50 60 70
and the 300°C does
spectru~ SiCNTTGA 13oooc, lOOh sio2, hexagona:l
Si02, tetragonal
not contain SiC, suggesting that
all the SiC has been converted to .1 l 1
10 20 30 40 50 60 70
Si0
•
2