Lightweight materials with excellent thermal insulating properties are highly sought after for a variety of aerospace and aeronautic applications. (1) Silica based aerogels with their high surface area and low relative densities are ideal for applications in extreme environments such as insulators for the Mars Rover battery. (2) However, the fragile nature of aerogel monoliths prevents their widespread use in more down to earth applications. We have shown that the fragile aerogel network can be cross-linked with a di-isocyanate via amine decorated surfaces to form a conformal coating. (3) This coating reinforces the neck regions between secondary silica particles and significantly strengthens the aerogels with only a small effect on density or porosity. Scheme 1 depicts the cross-linking reaction with the di-isocyanate and exhibits the stages that result in polymer cross-linked aerogel monoliths

Capadona, Lynn A.

Clark, Nicholas

Ghosn, Louis J.

McCorkle, Linda

Meador, Mary Ann B.

Sullivan, Roy M.

Vivod, Stephanie L.

NASA Technical Reports Server

CARBON NANOFIBER INCORPORATED SILICA BASED 
AEROGELS WITH DI-ISOCYANATE CROSS-LINKING 
Stephanie L. Vivod, Mary Ann B. Meador, Lynn A. Capadona,  
Roy M. Sullivan, Louis J. Ghosn, Nicholas Clark, Linda McCorkle,  
and Derek J. Quade  
NASA Glenn Research Center 
21000 Brookpark Road 
Cleveland, OH  44135 
Introduction 
Lightweight materials with excellent thermal insulating properties are 
highly sought after for a variety of aerospace and aeronautic applications.1 
Silica based aerogels with their high surface area and low relative densities are 
ideal for applications in extreme environments such as insulators for the Mars 
Rover battery.2 However, the fragile nature of aerogel monoliths prevents their 
widespread use in more down to earth applications.  We have shown that the 
fragile aerogel network can be cross-linked with a di-isocyanate via amine 
decorated surfaces to form a conformal coating.3  This coating reinforces the 
neck regions between secondary silica particles and significantly strengthens 
the aerogels with only a small effect on density or porosity.  Scheme 1 depicts 
the cross-linking reaction with the di-isocyanate and exhibits the stages that 
result in polymer cross-linked aerogel monoliths.     
Si NH2
C
O
N
R
N
C
O
Si
H
N
C
O
N
R
H
N
C
O
+
C
O
N
R
N
C
O
chain extension
Si
H
N
H
N
R
H
N
C
O
C
O
N
R
H
N
C
O
n
H2O
CO2
adsorbed
Si HN
NH2
RHN
C
O
+Si
NHSi
H
N
H
N
R
H
N
C
O
C
O
NH
R
H
N
C
O
n
crosslinking
 
 
Scheme 1.  Mechanism of incorporating fiber into amine modified silica 
aerogels and cross-linking with di-isocyanate  
Composites of fiber and silica aerogels have been produced in a number 
of ways as an attempt to fortify the structure.  Aerogel composites have been 
made by introducing a sol into a fibrous batting reinforcement,4  incorporating 
non-woven polypropylene fibers,5 or with non-woven carbon or silica fiber 
preforms for enzyme encapsulation.6   Silica aerogels have been reinforced 
with 5 to 25 w/w% ceramic fibers, reporting increased hardness, compressive 
strength, and elastic modulus of the monoliths as well as less shrinkage on 
supercritical drying.7  Finally, Lu et al have incorporated carbon nanofibers 
into uncross-linked silica aerogels to produce aerogel composites which were 
pyrolyzed for transformation into SiC/silica nanocomposites.8 However, no 
mechanical characterization of the composites was reported to demonstrate if 
carbon nanofibers were an effective reinforcement for the silica aerogels 
themselves.   
In our current research, we examine the effect of integrating carbon 
nanofibers into the silica aerogel matrix for added structural integrity.  The 
carbon fibers used are Pyrograf®-III nanofibers, a highly graphitic, low-cost 
carbon nanofiber.  The base fibers were subjected to a proprietary oxidation 
process to give about 5 atom % singly bonded oxygen (combination of ether, 
epoxide, and alcohol) by XPS analysis.  An SEM of the nanofibers is shown 
in Figure 1. 
   
 Figure 1.  Scanning electron micrographs of Pyrograf III nanofibers. 
A statistical experimental design approach was utilized to produce and 
analyze a series of cross-linked aerogels using different concentrations of 
silica precursors, di-isocyanate crosslinker and 0 to 5 w/w % carbon 
nanofibers in the silica backbone before cross-linking.  In all, 19 separate 
formulations were prepared and characterized and the results analyzed by 
multiple linear regression. 
Experimental 
Materials. Tetramethorthosilicate (TMOS), 3-aminopropyl-
triethoxysilane (APTES) and anhydrous acetonitrile (CH3CN) were purchased 
from Aldrich Chemical Company and used without further purification. 
Desmodur N3200 (1,6-hexamethylene diisocyanate-based oligomer) was 
provided by Bayer Corporation.  Pyrograf®-III carbon nanofibers (PR-24-XT-
LHT) were provided by Applied Sciences, Inc (ASI).  
Procedure.  Aerogels were prepared as previously described3 using a 
sol-gel process except that carbon nanofibers, in an amount equal to 0 to 5% 
of the theoretical weight of the silica in the gel was added to the silane 
solution and sonicated for approximately one half hour.  To the stable 
suspension, water in acetonitrile was added to the silanes.  The resulting 
solution was vigorously mixed and immediately poured into molds. Gelation 
occurred usually within 15 to 30 minutes. The gels were aged for 24 hrs 
before extracting into clean solvent. After exchanging with clean solvent three 
more times, the wet gels were soaked for 24 hrs in a di-isocyanate/acetonitrile 
solution followed by heating at 75˚C to cross-link.  After four additional 
solvent exchanges, the gels were dried by supercritical CO2 fluid extraction. 
Results and Discussion   
SEM of a fracture surface of a typical low density aergoel monolith (no 
fiber) is shown in Figure 2a.  In comparison, a micrograph of a monolith of 
similar density produced under similar conditions but containing 5 w/w % 
fiber is shown in Figure 2b.  The structure of the aerogel is similar to the non-
fiber containing sample, but the fibers clearly stand out as the diameters of the 
fibers are about an order of magnitude larger than the fine structure of the 
aerogel.  In places, the aerogel appears to be intimately wrapped around the 
fibers, while in others the fiber is free of aerogel.  For comparison, higher 
density aerogels are shown in Figures 2c (without fiber) and 2d (with fiber).   
      
             
    
         
Figure 2.  Scanning electron micrographs of monoliths with no fiber (left) and 
with fiber (right).   
Supercritical 
fluid 
extraction 
Gel stage  
Addition of 
Polymer 
Cross-linked wet 
gel 
Silica/Carbon 
Nanofiber Sol 
Cross-linked 
aerogel 
d. d = 0.321 g/cm3 c. d = 0.394 g/cm3  
b. d = 0.088 g/cm3 a. d = 0.071 g/cm3  
https://ntrs.nasa.gov/search.jsp?R=20080006602 2019-08-30T03:18:32+00:00Z
Figure 3 illustrates the response surface model for density vs. percent di-
isocyanate and total silane.   As is evident from the graph and in agreement 
with previous studies, the most influential variable on density of the aerogels 
is total silane concentration (total molar amount of APTES and TMOS).  Also, 
density slightly decreases with increasing di-isocyanate concentration at low 
total silane, while at high total silane, increasing di-isocyanate increases 
density.  Most notably, there was no significant effect of fiber concentration 
on density over and above random error.   
0.01
0.1
1
0.4
0.8
1.2
1.6
2.0
5
10
15
20
25
30
D
en
si
ty
, g
/c
m
3
Tot
al s
ilan
e, m
ol/LDi-isocyanate, w/w %  
Figure 3. Empirically derived graphs showing density vs. total silane and fiber 
concentration.  
Graphs of response surface models for Young’s modulus which were 
extracted from compression tests are shown in Figure 4 (top).  Once again, the 
most dominant effect on the measured property is the total silane 
concentration followed by an effect of di-isocyanate concentration. Overall, 
maximum modulus values are achieved when density is at a maximum.   
0.01
0.1
1
10
100
1000
0.4
0.8
1.2
1.6
2.0
0
12
34
M
od
ul
us
, M
P
a
To
tal
 si
lan
e, 
mo
l/L
Fiber, w/w %
0.1
1
10
0.4
0.8
1.2
1.6
2.0
0
1
2
3
4
5
Te
ns
ile
 s
tre
ss
 a
t b
re
ak
, 
M
P
a
To
tal 
sila
ne
, m
ol/L
Fiber, w/w %    
6% di-isocyanate
34% di-isocyanate  
Figure 4.  Empirically derived graphs showing modulus from compression vs. 
total silane concentration and fiber concentration (top) and tensile stress at 
break vs. total silane and fiber concentration (bottom).  
However, the model indicates that increasing fiber concentration heightens 
modulus, especially at low total silane concentration.  Hence, it is possible to 
obtain a three-fold increase in modulus from 0.2 MPa to 0.6 MPa by going 
from 0 to 5% fiber at low total silane concentration while not increasing 
density.   
The response surface model for tensile stress at break (Figure 4, bottom) 
is also dominated by total silane concentration although both fiber and di-
isocyanate concentration are also significant terms in the model.  When di-
isocyanate concentration is high, tensile stress at break reaches a maximum at 
approximately 2 % fiber for all levels of total silane.  The maximum value for 
tensile stress at break across the whole study is predicted to be 9.1 MPa when 
silane and di-isocyanate are both at the highest values studied and 1.8 w/w % 
fiber is used. It may be that when di-isocyanate concentration is high and 
especially when total silane is also high; the effect of the fiber is smaller 
because the large amount of polymer cross-linking toughens the aerogel 
structure enough to mask any further improvement due to the fibers. When di-
isocyanate concentration is low, the effect of fiber is much more pronounced 
especially when total silane is also low.  In fact, at low total silane and di-
isocyanate concentration, stress at break is predicted to be five times higher 
when fiber concentration goes from 0% fiber (0.07 MPa) to 5% fiber (0.36 
MPa).  This improvement in tensile strength comes with no penalty in 
increased density or decreased porosity, as these responses are unaffected by 
the amount of carbon nanofiber present. 
Conclusions 
We have examined the effect of including up to 5 w/w % carbon 
nanofibers in the silica backbone of di-isocyanate cross-linked silica aerogels.  
Most notably, a three-fold increase in compressive modulus with 5% fiber was 
observed for aerogels made from the lowest total silane concentration with no 
increase in density.   Similarly, a five-fold increase of tensile stress at break is 
predicted by including 5% fiber when total silane and di-isocyanate 
concentration are at a low.  At higher levels of total silane and low di-
isocyanate concentrations, there is still an increase in tensile stress at break but 
it is not as pronounced.  When both total silane and di-isocyanate 
concentration are high, tensile stress at break reaches a maximum when 2 w/w 
% fiber is used.  Since some agglomeration of fiber was observed in the dried 
aerogels at higher fiber loading, it may also be that better dispersion would 
result in greater improvement of mechanical properties.  
No effect was seen on the density or porosity of the aerogels.  Hence, the 
improvements in modulus and tensile strength due to fiber do not compromise 
other important properties of the aerogels.  Perhaps the biggest impact of 
including carbon fiber in the aerogels may be an improvement in the strength 
of the initial hydrogels before cross-linking.  Extracting the wet gels from the 
molds resulted in no breakage of the fiber containing gels, while nearly half of 
the gels from the lowest density runs with no fiber broke on initial extraction.  
This may have implications in large scale manufacturing of the lower density 
polymer cross-linked aerogels.   
Acknowledgements We thank Demetrios Papodopolous of University 
of Akron for running compression tests and Dr. Brad Lerch for consultation in 
running tensile tests.  Financial support from NASA’s Subsonics Program and 
Advanced Extravehicular Activities Program (AEVA) is gratefully 
acknowledged.  We also thank Bayer Corporation for providing the di-
isocyanate oligomers and Applied Sciences, Inc. for the carbon nanofiber.  
References 
(1) Pierre, A. C., Pajonk, G. M., Chem. Rev. 2002, 102, 4243-4265. 
(2) Jones, S. M, Journal of Sol-Gel Science and Technology, 2006, 40, 351-
357 
(3) Meador, M. A. B.; Capadona, L. A; McCorkle, L.; Papadopoulos, D. S.; 
Leventis, N., Chem. Mater. 2007, 19; Capadona, L. A.; Meador, M. A. B.; 
Alunni, A.; Fabrizio, E. F.; Vassilaras, P.; Leventis, N. Polymer, 2006, 47, 
5754-5761. 
(4) For example see, Stepanian; C. J.; Gould; G. L.; Begag; R., United 
States Patent , 7,078,359, July 18, 2006. 
(5)  Zhang, Z.; Shen, J.; Ni, X.; Wu, G.; Zhou, B., Yang, M.; Gu, X.; Qian, 
M.; and Wu, Y. J., Macromolecular Sci., Part A, 2006, 1663-1670. 
(6) Karout, A.; Buisson, P.; Perrrard, A.; Pierre, A.C. J.  Sol-Gel Science 
and Technology, 2005, 36, 163-171.   
(7) Parmenter, K. E.; Milstein, F. J. Non-Crystalline Solids, 1998, 223, 179-
189. 
(8) Lu, W.; Steigerwalt, E. S.; Moore, J. T.; Sullivan, L. M.; Collins, W. E.; 
Lukehart, C. M., J. Nanosci. Nanotech. 2004, 4, 803-808. 

CARBON NANOFIBER INCORPORATED SILICA BASED AEROGELS WITH 
DI-ISOCYANATE CROSS-LINKING 
 
Lightweight materials with excellent thermal insulating properties such as silica based aerogels are 
used for a variety of aerospace and aeronautic applications. Although the fragile nature of aerogel 
monoliths prevents their widespread use in many applications, we have shown that reinforcement 
with a structural polymer cross-linked with amine decorated silica surfaces significantly strengthens 
the aerogels with minimal effect on density or porosity.  In our current research, we further enhance 
the desired structural characteristics of cross-linked aerogels by integrating carbon nanofibers into 
the silica aerogel matrix.  A statistical experimental design approach was utilized to analyze  series 
of fiber-containing cross-linked aerogels using different concentrations of the silica precursors 
tetramethoxysilane (TMOS) and aminopropyltriethoxysilane (APTES), polymer crosslinker 
(Desmodur N3200, a hexamethylene diisocyanate), and Pyrograf-III carbon nanofiber.  Results 
include a three-fold increase in compressive modulus and a five-fold increase in tensile strength in 
the lowest density aerogels (0 -5% fiber) with no impact on density. 
Introduction
Stephanie L. Vivod*, Mary Ann B. Meador*, Lynn A. Capadona*, Roy M. Sullivan*, Louis J. Ghosn*, Derek J. Quade*, Linda McCorkle†, 
and Nicholas Clark* 
*NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, †Ohio Aerospace Institute, 22800 Cedar Point Rd, Cleveland, Ohio 44142 
Acknowledgement:  We thank Dan Scheiman (QSS) for running thermal analysis, and Baochau Nguyen 
(OAI) for helpful suggestions.  We also thank Applied Sciences, Inc for supplying the Pyrograf fiber and 
Bayer Corporation for supplying Desmodur N3200.  We gratefully acknowledge funding from the NASA 
Glenn Alliance for Technology Exchange (GATE), the Fundamental Aeronautics Program and the 
Exploration Systems Mission Directorate Student Internship Program.
Pyrograf® III Carbon Nanofibers
Carbon Nanofiber Incorporated Silica Based Aerogels with 
Di-isocyanate Cross-linking
5
1 0
1 5
2 0
2 5
3 0
3 5
0 .5
1 .0
1 .5
2 .0
0
12
34
5
Po
ly
m
er
, w
/w
%
To
tal
 si
lan
e, 
mo
l/L
F ibe r, w /w %
E x p e rim e n ta l d e s ig n  ru n s
Pyrograf® III carbon nanofibers were incorporated into the 
sol consisting of acetonitrile, APTES and TMOS by sonication 
prior to hydrolysis. 
Silica based aerogels have been of significant interest to the 
aeronautics and  aerospace industries due to their light weight 
and low thermal conductivity. Unfortunately, being primarily 
made of silica, the native aerogel tends to be extremely fragile.  
By incorporating an amine functionalized silica precursor, we 
are able to chemically modify the surfaces of secondary 
particles of the aerogel. These surface amines accommodate a 
conformal polymer coating to structurally enhance the weak 
neck regions which form the bead-like assembly associated 
with aerogels. To further enhance the structural integrity of the 
cross-linked aerogel, we have added carbon nanofibers to the 
silica matrix. The fibers have been oxidized using a proprietary 
process to provide surface ethers, epoxides and alcohols to 
promote good interface between matrix and nanofiber.
Statistical experimental design 
approach was utilized to reduce 
the number of experiments to 
assess the effects of different 
concentrations of the silica 
precursors, di-isocyanate, and 
carbon nanofibers on properties 
of resulting composite aerogels
Design of Experiments
Analysis of Tensile Measurements
Non-ideal cylindrical specimens used for tensile.  Finite 
element analysis (FEA) used to calculate area of peak 
stress and stress concentration factor for each 
specimen.  Adjusted stress at break calculated from 
measured value x stress concentration factor. 
Strain, %
0 5 10 15 20 25 30
S
t
r
e
s
s
,
 
M
P
a
0
1
2
3
4
14: 20% d, 2% f 
16: 20% d, 0% f 
13: 20% d, 2% f 
  9: 20% d, 2% f  
  6: 20% d, 2% f 
11: 34% d, 2% f
15: 34% d, 5% f
18:   6% d, 5% f 
  3: 20% d, 2% f 
  8: 20% d, 2% f
  5:   6% d, 0% f 
  2:  6%  d, 2% f 
  4: 34% d, 2% f 
12:   6% d, 0% f 
17: 34% d, 5% f
Stress-strain plots from tensile tests with 
s = 2.091 mol/L (blue), s = 1.23 mol/L 
(red) and s = 0.369 mol/L (green)
Epoxy tabs
Stress 
concentration 
factor
35.6mm
2.143.92
17.8 mm diameter
27.9mm
FEA mesh Stress contour plots with (left) and 
without (right) epoxy tabs.
Empirical Models
Conclusions
•Three-fold increase in compressive modulus with 5% fiber - lowest density aerogels 
•Five-fold increase in tensile stress at break shown by including 5% fiber at low density
•Some agglomeration of fiber was observed in aerogels at higher fiber loading, better 
dispersion may give greater improvement of mechanicals  
•No effect of fiber was seen on density or porosity
•Improvements in modulus and tensile strength due to fiber do not compromise other 
important properties of the aerogels.  
•Strength of initial hydrogels before cross-linking also improved by fiber—may have 
implications in large scale manufacturing of the lower density aerogels. 
0.01
0.1
1
0.4
0.8
1.2
1.6
2.0
5
10
15
20
25
30
D
en
si
ty
, g
/c
m
3
Tot
al s
ilan
e, m
ol/LDi-isocyanate, w/w %
50
60
70
80
90
100
0.4
0.8
1.2
1.6
2.0
10 15 20 25 30
35
Po
ro
si
ty
, %
To
tal
 si
lan
e, 
mo
l/L
Di-isocyanate, w/w %
Fiber has no effect on density or porosity
0.01
0.1
1
10
100
1000
0.4
0.8
1.2
1.6
2.0
0
12
34
M
od
ul
us
, M
Pa
To
ta
l s
ila
ne
, m
ol/
L
Fiber, w/w %
6% di-isocyanate
34% di-isocyanate
0.1
1
10
0.4
0.8
1.2
1.6
2.0
0
1
2
3
4
5
Te
ns
ile
 s
tre
ss
 a
t b
re
ak
, 
M
Pa
To
tal
 si
lan
e, 
mo
l/L
Fiber, w/w %
6% di-isocyanate
34% di-isocyanate
Tensile Stress at break increases 
with increasing fiber by as much as 
a factor of five.
In low density samples, increasing 
fiber concentration from 0 to 5% 
increases modulus by factor of 
three
Silica/Carbon 
Nanofiber Sol
Syringe used as a 
mold to shape gel 
monoliths, aged for 
24 hrs and extracted 
into clean solvent
Includes silica 
precursors, Solvent 
(CH3CN) , & 
carbon nanofibers,
sonicated for  30 
minutes
Cross-linked 
aerogel
Cross-linked 
wet gel
Addition of 
Polymer
Supercritical 
Fluid 
Extraction
Gel stage
60
80
100
120
140
160
180
200
220
240
0.4
0.8
1.2
1.6
2.0
0
12
34
S
ur
fa
ce
 a
re
a,
 m
2 /
g
To
tal
 si
lan
e, 
mo
l/L
Fiber, w/w%
16
20
24
28
32
36
40
0.4
0.8
1.2
1.6
2.0
0
1
2
3
4
Po
re
 d
ia
m
et
er
, n
m
To
tal 
sila
ne
, m
ol/L
Fiber, w/w%
6% di-isocyanate
34% di-isocyanate
Sol-gel Chemistry
Surface areas show a small but 
significant decrease with increasing 
fiber concentration 
At low di-isocyanate concentrations, 
pore diameter has linear increase with 
increasing fiber
Low density aerogel High density aerogel
No fiber
5w/w% 
fiber
2w/w% 
fiber
Di-isocyanate Cross-linking Mechanism
Tetramethyl Orthosilicate 
(TMOS)
Silica Precursors
3-Aminopropyltriethoxysilane 
(APTES)
OO
O
Si
H2N
CH2
CH2
CH2
H3C CH3
CH3
OO
O
O
S i
H 3C
C H 3
C H 3
H 3C
Si
NH2
C
N
R
N
C
OO
Si
N
H
C
O
N
H
R
N C O
Si
N
H
C
O
N
H
R
NH2
C
N
R
N
C
OO
Si
NH2
chain extension
C ON
RN
H
C
O
N
H
RN
H
C
O
NH
Si
H2O
CO2
n
Si
N
H
C
O
N
H
RN
H
C
O
N
H
Si
Si O
R H
O H
Hydrolysis
Reesterification
Si OH R OH
Si OH Si OH
Si OH Si O
R
Water Condensation
Hydrolysis
Si O
Si H
O H
Si O
Si
R OH
Alcohol Condensation
Alcoholysis


Carbon Nanofiber Incorporated Silica Based Aerogels with Di-Isocyanate Cross-Linking

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