OXIDATION OF SIC/SIC COMPOSITES IN GAS FAST REACTORS IN
OPERATING CONDITIONS:
THERMODYNAMIC AND EXPERIMENTAL APPROACHES
Nicolas HUN1,2, Francis REBILLAT2, Laurent BRISSONNEAU1
1
CEA, DEN/DTN/STPA/LPC, F-13108 Saint-Paul-lez-Durance, France,
nicolas.hun@cea.fr, laurent.brissonneau@cea.fr
2
Université de Bordeaux, Laboratoire des Composites Thermostructuraux (LCTS), France,
rebillat@lcts.u-bordeaux1.fr
Abstract:
Gas Fast Reactor (GFR) is a promising Generation IV concept for energy production. The
hard neutron spectrum allows waste burning and a better use of fuel resource. The helium
coolant enables easy in-service inspection, low corrosion and high core outlet temperature
(850°C) for high efficiency or versatile applications. SiC/SiC composites are candidates of
primary interest for a GFR (Gas Fast Reactor) fuel cladding use, from stability under neutron
irradiation, high temperature mechanical properties and corrosion resistance. The helium
coolant has to be slightly oxidant to passivate heat exchangers in the hot leg of the reactor.
The amount of oxidizing species in the gas, expected to be very small (impurity partial
pressure as H2O or O2 below a few dozens of Pascal), is reached through a purification loop.
In standard operating conditions and such composition specifications, SiC/SiC can present
two different oxidation processes, depending on the nature and concentration of impurities.
Under oxygen atmospheres and high partial pressures, SiC tends to form a protective silica
layer at the material surfaces (passive oxidation), but the pyrocarboned fibre/matrix
interphase is very reactive. Under moist atmospheres, higher temperatures and low partial
pressures, SiC is volatilised (active oxidation) but the interphase is less reactive regarding its
behaviour under O2. The mechanical protection of the cladding is highly dependant on the
interphase integrity, and it is important to know which oxidation mechanism is the worse.
Therefore, to guarantee the protection of materials by purification processes, the active to
passive oxidation transition of SiC has to be known, in order to target the less damaging
range of oxidizing species composition.
The active to passive transition is studied using a thermodynamic approach providing useful
information on possible mechanisms. It shows that major volatile species are CO and SiO, as
hydroxide silicon species form in negligible amounts. Experimental tests are performed by
thermogravimetric analysis at 1100°C in Ar/O2, Ar/H2O, Ar/H2O/O2, Ar/H2O/H2, corresponding
to active and passive domains. The provided kinetics data will help in the modelling of the
composite oxidation, following an approach developed at LCTS.
Keywords:
SiC/SiC composites, GFR, operating conditions, Oxidation transitions, Determination
-1-
Gas Fast Reactor (GFR) is a promising Generation IV concept for energy production. The
hard neutron spectrum allows waste burning and a better use of fuel resource. SiC/SiC
composites are candidates of primary interest for a GFR fuel cladding use, from stability
under neutron irradiation, high temperature mechanical properties and corrosion resistance. A
SiC/SiC composite is made of woven SiC fibers, wrapped in a SiC matrix. The attractive
mechanical properties (impact resistance, creep resistance, Young modulus…) of the
composite are mainly due to the fibers [1], and to the pyrocarbon fiber/matrix interphase,
which are able to deviate the crack propagation [2].
The helium coolant enables easy in-service inspection, low corrosion and high core outlet
temperature (850°C) for high efficiency or versatile applications. The coolant has to be
slightly oxidant to passivate heat exchangers in the hot leg of the reactor. The amount of
oxidizing species in the gas, expected to be very small (impurity partial pressure as H2O or O2
below a few dozens of Pascal), is reached through a purification loop. In standard operating
conditions and such composition specifications, SiC/SiC can present two different oxidation
processes, depending on the nature and concentration of impurities. The major oxidizing
species are O2 and H2O (as CO2 oxidation rate is negligible compared to O2 and H2O) [3]. O2
and H2O have different roles towards SiC and PyC oxidation:
Under oxygen atmospheres and high enough partial pressures, SiC tends to form a protective
silica layer at the material surface (passive oxidation), but the pyrocarbon at fiber/matrix
interphase is very reactive. On the other hand, low O2 partial pressure might lead to SiC
volatilization in SiO (active oxidation). Likewise, H2O lowers the protective role of silica, and
favours under low pressure the active oxidation of SiC. However, in this case, the interphase
is less reactive regarding its behaviour under O2 [4], especially in the presence of H2 [5].
Passive oxidation kinetics of SiC under H2O atmospheres are up to ten times faster than under
O2 [4].
An intermediate rate of volatilisation-oxidation, described by Tedmon [6], also has to be taken
into account. SiC oxidizes, actively and passively at the same time. This oxidation rate could
be the worse towards SiC consumption.
In this paper, a study of the active to passive transitions is studied using a thermodynamic
approach providing useful information on the possible mechanisms. Experimental tests are
performed by thermogravimetric analysis (TGA) in order to identify the active and passive
domains. The experimental data will be used as input in the modeling of composites oxidation
through a matrix crack.
1 Transition domains determination by thermodynamic
approach
The calculations have been performed with the HSC software, and the Allendorf’s [7] Si-O-H
database for Sandia National Laboratories.
1.1 Update of the HSC software with the Sandia database
Opila [8, 9] observed hydroxygenated species when silica reacts with H2O, which are not
described in HSC classical database for Si-O-H species. Allendorf [7] provides a larger Si-OH database than HSC. There are, in particular, data on Si(OH)4 and SiO(OH)2 determined by
Opila [8]. As the format of the models used by HSC and Sandia are different (cf. Equation 1
and Equation 2), it has been necessary to convert the data of Allendorf to the HSC format.
-2-
HSC is able to convert data automatically and to get a value for A, B C and D coefficients.
These coefficients and H° and S° values have then to be computed into HSC database.
The Cp values calculated with HSC are exactly the same as Allendorf’s, but G values are
diverging, especially at high temperatures (cf. Figure 1). A possible explanation is that the
HSC system is using only 4 coefficients to calculate G evolutions, versus 7 for Allendorf’s
system. The lack of precision implied by the low number of coefficients can explain the
growing gap between the values of the two equations.
Cp(T )  A  B  10 3 T  C  10 5 T 2  D  10 6 T 2
Cp(T ) / R  a1  a 2T  a 3T 2  a 4T 3  a 5T 4
a T a T 2 a 4 T 3 a 5T 4
H (T )
 a1  2  3


RT
2
3
4
5
2
3
a
T
a T4
a
T
S (T )
 a1 ln( T )  a 2T  3
 4
 5
 a7
R
2
3
4
Equation 1 : Allendorf’s data format
Equation 2 : HSC data format
It is possible to reduce the gap by changing H° and S° values in HSC. The initial and
corrected values of H° and S° are shown in Table 1.
Si(OH)4
H° (kcal/mol)
S° (cal/(mol.K))
SiO(OH)2
H° (kcal/mol)
S° (cal/(mol.K))
Initial value
Corrected value
Correction
-428.005
46.058
-420.005
64.058
+8
+18
-192.194
69.931
-187.194
81.931
+5
+12
Table 1: Modification of Allendorf data in order to use them in HSC
The correction values have been chosen in order to minimize an error function defined as
2
error   HSC  Allendorf  .
Comparison of Allendorf and HSC G values for Si(OH)4
and SiO(OH)2
-200
G (kcal/mol)
-250
0
500
1000
1500
Si(OH)4 Allendorf
Si(OH)4 modified HSC
values
SiO(OH)2 Allendorf
-300
-350
-400
SiO(OH)2 modified HSC
values
SiO(OH)2 non modified
HSC values
Si(OH)4 non modified
HSC values
-450
-500
-550
T (°C)
Figure 1 : Comparison of DG with Allendorf and HSC modified values for Si(OH)4 and
SiO(OH)2
-3-
The use of the modified data was validated by comparing calculations performed with another
software (GEMINI) at LCTS. The study of a Ar-Si-O-H system at atmospheric pressure and
constant temperature with the two different software showed comparable results for
equilibrium composition. The transition partial pressure of oxidation can therefore be
calculated with the HSC software and its updated database.
1.2 Results
The aim of this study is to delimitate the passive/active transition domains of oxidation under
O2, H2O, and different O2/H2O and H2/H2O ratios. The calculations have been done with 1
mole of silicon, at constant temperature and pressure. The total amount of gaseous species
(helium + oxidizing species) is constant and equals to 1 mole, but the amount of oxidizing
species is changing. The transition criterions are reached when SiO2 appears (low transition)
and when the amount of SiO equals the amount of SiO2 (high transition). The gap between
these two criterions may correspond to the volatilization-oxidation domain described by
Tedmon [6].
1.2.1 Main species
In the operating conditions of GFR, the following oxidation reactions of SiC with O2 or H2O
are the most likely to occur [8, 10, 11]:
SiC + O2(g) = SiO(g) + CO(g)
SiC + 3/2O2(g) = SiO2 + CO(g)
SiC + 2H2O(g) = SiO(g) + CO(g) +2H2(g)
SiC + 3H2O = SiO2 + CO(g) + 3H2(g)
SiO2 + 2H2O(g) = Si(OH)4(g)
SiO2 + H2O(g) = SiO(OH)2(g)
2H2O(g) = 2H2(g) + O2(g)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
A typical result of the calculated amounts of reaction products, in mixed atmosphere,
depending on the amount of O2 or H2O is shown in Figure 2. Calculations show that the more
stable species are the reaction products of (1), (2), (3) and (4). Reaction (3) produces 2 moles
of gas, and reactions (4)+(5) neither produce, nor consume any. At low total or partial
pressure of H2O, reaction (3) is favored, while (5) is most likely to occur at high pressures.
This remark is confirmed by thermodynamics, which show that the higher the partial pressure
of water, the higher the Si(OH)4 amount and the lower the SiO amount (cf Table 2).
Partial pressure of water (Bar)
SiO (mol)
Si(OH)4 (mol)
0,1
1,38E-04
1,42E-18
1
6,93E-05
4,47E-16
10
4,90E-05
1,10E-13
50
4,81E-05
2,48E-12
100
4,54E-05
8,97E-12
Table 2: SiC volatilization: compared amount of SiO and Si(OH)4 depending on the partial
pressure of water at total atmospheric pressure.
Opila observed the volatilization of SiO2 in Si(OH)4 by a transpiration method [9] (up to 10-6
bar at 1500°C in moist saturated oxygen). This difference is due to the fact that Opila worked
on pure silica. Calculations done with the HSC software on silica give that Si(OH)4 is favored
at very high pressures, but SiO is in GFR operating condition the main volatilized specie.
-4-
Partial pressure of water (Bar)
SiO (mol)
Si(OH)4 (mol)
0,1
2,16E-08
1,30E-12
1
9,18E-09
2,20E-10
10
4,63E-09
1,34E-08
50
2,70E-09
3,35E-07
100
2,16E-09
1,31E-06
Table 3: SiO2 volatilization: compared amount of SiO and Si(OH)4 depending on the partial
pressure of water
These calculations show that SiO is the main volatile specie for SiC oxidation and allow us to
use transition criterions independent of Si(OH)4 and SiO(OH)2. It is interesting to note that the
same chemical species are formed under O2 or H2O, which will simplify the modeling work.
Thermodynamic equilibrium depending on the water amount in the system at 1100°C
0
Log(kmol)
SiC(C)
H2(g)
CO(g)
SiO2(V)
SiO(g)
-5
High transition
Low transition
H2O(g)
CO2(g)
CH4(g)
-10
SiO2(g)
-15
SiO(OH)2(g)
-20
O2(g)
-25
-30
0.00000
Si(OH)4(g)
0.00005
0.00010
0.00015
0.00020
0.00025
H2O(g)
0.00030 kmol
Figure 2 : Thermodynamic equilibrium depending on the water amount in the system at
1100°C
The transition points are circled in red on
Figure 2. The low transition is determined when SiO2 starts to form, and the high transition
when as many SiO as SiO2 is formed.
1.2.2 Influence of the oxygen amount, equivalent partial pressure
Under O2, H2O or mixed atmospheres, the higher the temperature is, the larger the passive
domain of oxidation. It means that high temperatures help SiC volatilization under higher
oxidizing partial pressures. Transition pressures are following a Poxt  A  exp(  B / T ) type
law, therefore Log(POx)=f(1/T) diagrams can be drawn to visualize the transition frontiers.
As hydroxide silicon species compounds form in negligible amount (cf § 1.2.1), transitions
can be determined as a function of an equivalent amount of oxygen. Water mainly reacts by
reaction (3) and (4). Neglecting the other reaction products, reactions (3) and (4) (for H2O)
and (1) and (2) (for O2) are equivalent. The amount of H2O just needs to be twice the amount
-5-
of O2 to form as many SiO as SiO2. As reported in the literature, the active oxidation domain
is larger in presence of water than oxygen [10, 12]. The calculation of transition domains
under O2, H2O, O2/H2O, H2/H2O are in agreement with this assumption. The results of these
calculations are presented in Figure 3 and Table 4.
Equivalent transition partial pressure of oxygen (Pa)
Transtion pressure (Pa)
1,00E+04
1,00E+03
Passive oxidation domain
1,00E+02
1,00E+01
Active oxidation domain
1,00E+00
1,00E-01
5,50E+00
6,00E+00
6,50E+00
7,00E+00
7,50E+00
O2 high
O2 low
H2O high
H2O low
H2O/O2 (35/65) high
H2O/O2 (35/65) low
H2O/H2 (50/50) high
H2O/H2 (50/50) low
8,00E+00
4
10 /T
Figure 3 : Comparison of equivalent transition partial pressure under O2, H2O, O2/H2O,
H2/H2O as a function of temperature
Table 4 gives A and B values, at equivalent transition partial pressure under different
atmospheres.
Limite haute
Limite basse
O2
A
B
1,05E+14
42600
4,45E+13
42403
H2O/O2 (35-65)
A
B
9,40E+13
42443
4,46E+13
42408
H2O
A
B
1,11E+14
42673
4,73E+13
42489
H2O/H2 (50-50)
A
B
1,10E+14
42662
5,09E+13
42612
Table 4: Exponential and pre-exponential coefficients for transition pressure evolution
Using an average of these values, we get the following relations:
  (42478  97) 
tO 2 eq
Plow
 (4,68  0,3)  1013 exp 

T


  (42595  105) 
tO 2 eq
Phigh
 (10,5  0,7)  1013 exp 

T


The standard deviations (values given in the measurement uncertainties) are rather small
(about 1% for A and 0,2% for B). This validates the assumption of an equivalent oxygen
amount. This hypothesis would enable us to easily determine theoretical transition frontiers
for mixed atmospheres.
1.2.3 Comparison with literature
The results can be synthesized in a log(PO2)=f(1/T) graph (cf Figure 4). Vaughn [13] and
Schneider [14] showed that an increase in the gaseous flow increases the passive oxidation of
SiC, by lowering the transition partial pressure. Many authors [13-16] observed that
experimentally determined transitions are lower than calculated ones. It is interesting to note
that the slope of the experimental domains is in each case nearly equivalent, and aligned. This
-6-
means that different experimental systems allow us to determine the same mechanisms. The
gap between experimental and calculated values can be explained by the difficulty to take the
nature and the impurities of SiC into account, and also by the fact that the calculations are
based on a only 2 equation system.
Active/passive transition diagram, comparison between experimental and calculated transition frontiers
10000,00
1700
1600
1500
1400
1300
1200
Schneider high frontier
Passive oxydation
1000,00
Thermodynamic study
100,00
PO2 (Pa)
Goto and
Homma
Vaughn and
Maahs
10,00
Eck et al. (Wagner model)
Theoretical
calculations
Schneider low frontier
1,00
Experimental points
Gulbransen et al.
0,10
Experimental tests
Active oxidation
0,01
5
5,2
5,4
5,6
5,8
6
6,2
6,4
6,6
6,8
7
104/T
Figure 4 : Active/passive transition diagram comparison between experimental (Goto and
Homma [16], Vaughn and Maahs [13] and Gulbransen et al. [15]) and calculated (Eck et al.
[10], Schneider [14] and our study) transition frontiers
In the experimental conditions of the thermobalance (atmospheric pressure and low gas
speed), the high frontier calculated in this study with HSC is very close to the frontier
calculated by Schneider [14] and Eck [10] by using Wagner model and GEMINI calculations.
Schneider high frontier
PO2  3 1013  exp( 42233 / T )
PO2  5 1013  exp( 43292 / T )

O2
Eck (Wagner model)
P  9,89 10  exp( 43644 / T )
Eck (GEMINI calculations)
  (42595  105) 
tO 2 eq
Phigh
 (10,5  0,7)  1013 exp 

T


Our calculations
13
At equal oxidizing species amount, the experimental transition temperatures (Vaughn and
Maahs [13], Goto and Homma [16] and Gulbransen [15]) are higher than the calculated ones.
We were therefore expecting our experimental results to be between theoretical and
experimental values.
-7-
1.3 Experimental approach
1.3.1 Experimental installation
The identification of active/passive transitions is made using thermogravimetric analysis
(TGA). The TGA machine is placed into a glove box. This glove box is swept out by argon, to
avoid oxygen contamination in the TGA. The reactive gases are mixed in the mixing bay and
are sent in the TGA. The moist amount is controlled at the inlet or at the outlet of the glove
box with a chilled mirror hygrometer (dew point meter). The reaction products are controlled
with a micro gas chromatograph (µGC). A diagram of this installation is shown in Figure 5.
O2
H2
O
Glove box
Mixing bay
TGA
Purification bay
Ar
Water trap
Hygrometer
Oxygen trap
µGC
Figure 5 : Diagram of the experimental installation
Data of the installation are given below:
Gas properties:
- Glove box gas :
o Purified argon (purification bay): below 10 ppm O2 and 40 ppm H2O
O2
Ar
Compressed tank purity
Flowmeter precision
Maximal flow allowed
Total flow during test
H2 O
pure
<100 ppb O2 744 ± 15 ppm 326 ± 16 ppm
<500 ppb H2O
1 ml/min
200 ml/min
0,01ml/min
10ml/min
150 ml/min
Table 5: Gas characteristics
TGA Mettler Toledo TGA/SDTA851:
- Detection limit: 0,1µg/s
- Maximal reachable temperature: 1600°C
- Maximal temperature for extended periods: 1450°C
-8-
0,01ml/min
10ml/min
Chilled mirror Mitchell S4000:
- method: The mirror surface is cooled by Peltier effect. When temperature is slow
enough, water in the gas condensate at the mirror surface. A beam control system
enables to know the gas dew point as the first droplets of water deflect the beam.
Knowing the dew point of the gas, the amount of water is calculated.
- Detection limit: -85°C ie 0,01 ± 0,01 ppm
µGC Agilent S973 quad serie:
- Detected species:
o He, H2, O2, Ar, N2, CH4, CO, CO2, C2H6, C2H4, H2S, H2O, C3H8, SO2
- Detection limit: 1 ± 1 ppm
1.3.2 Experimental procedure
Two materials were tested in this study: Tyranno SA SiC fibers and Hi Nicalon S fibers. Both
fibers are stoechiometric fibers with 1% O and 2% free C remaining [17]. Test specimens
were fibers cut in 20 pieces of 1 cm (about 30g of material) and put in a 10x9 mm in diameter
alumina crucible. Fibers were deseizing following a method explained below. The specimens
were tested in an atmospheric flowing argon glove box using a high temperature TGA to
determine the active to passive transition. Argon and reactive gases are supplied to the TGA
at a constant flow rate (150 ml/min), via the mixing bay. The amounts of O2 and H2O injected
are respectively measured with the µGC and the cooled mirror. The amount of water in the
system is not completely monitored: some water adsorbed in the gas pipes desorbs during the
experiment. The residual amount of water in the pipes is measured to be around 16 ppm.
However, this value can vary depending on the room. After the desired flow rate and the
composition are established in the TGA, the temperature is raised at the rate of 10°C/min to
1300°C, then the temperature is raised at 1°C/min through the transition temperature up to
1450°C. The temperature is held at 1450°C for 1 hour in order to volatilize the SiO2 formed.
The temperature is then decreased at the rate of 1°C/min through the transition temperature
down to 1300°C. During a run, the recorded mass change is due to four separate effects:
instrument thermal drift, flow impingement, SiC volatilization (-40g/mol) and SiO2 growth
(+20g/mol). First the balance exhibits some drift in mass with temperature, at high
temperature the offset is consistent and reversible from run to run. Second, the temperature
variations are changing the gas velocity and density. The change of the air flow through the
sample chamber strikes the sample and the balance mechanism, causing a wrong mass
reading. Blanks tests are realized to get rid of these effects. These blanks are subtracted to the
test curves in order to visualize the active and passive oxidation only.
1.3.3 Fiber deseizing
Samples are deseized by a four hours oxidation run at 100°C under 40 ppm O2 and around 16
ppm H2O. The amount of oxygen is the maximal amount reachable with this experimental
setup (cf Table 5 § 1.3.1). Four hours is the needed time to observe a mass gain after the loss
due to the lubrication. The mass gain corresponds to the beginning of passive oxidation of the
fibers.
The deseizing procedure is described below:
- Raise up of temperature from 25 to 120°C at 10°C/min under pure Ar
- Desorption of potentially adsorbed humidity at the sample surface by a one hour stage
at 120°C under pure Ar
-9-
-
Raise up from 120 to 1000°C at 10°C/min under pure Ar
30 minutes stage at 1000°C under pure Ar to stabilize the temperature in the balance
Injection of reactive gas (40 ppm O2 around 16 ppm H2O) for 4 hours
1.3.4 Performed experiments
The experimental procedure described in § 1.3.2 was adopted after non conclusive tests based
on calculated thermodynamic transition temperatures. The samples did not oxidize actively
for temperatures below 1300°C at 8 ppm O2 and around 16 ppm H2O. An experiment with
temperature steps between 1450°C (maximal step temperature for the TGA) and 1200°C was
performed to determine if the transitions could be observable within the operating range of the
TGA. A mass loss at 1450°C was observed followed by a mass gain for temperatures under
1400°C (cf Figure 6). This experiment shows that transition occurs between 1450 and 1400°C
under 8 ppm O2 and around 16 ppm H2O at atmospheric pressure.
1600
17,4
17,38
1400
17,36
1200
17,34
17,32
800
17,3
17,28
Sample mass (mg)
température (°C)
1000
600
17,26
400
17,24
200
17,22
0
17,2
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
7,0
time (h)
Tr [ｰ C]
Value [mg]
Figure 6 : Mass change and temperature versus time
The transition tests have been defined § 1.3.2 using Vaughn and Maahs [13] procedure as a
starting point. The difference between their method and ours is that we also observe the
transition during the decrease of the temperature, which enables us to observe SiC behavior
whether it is oxidized or not. Indeed, a one hour plateau at 1450°C is imposed to volatilize all
the silica formed at the SiC surface during the deseizing and the temperature raise up.
On the whole, Hi Nicalon S and Tyranno SA fibers have the same behavior. (cf Figure 7 and
Figure 8). We can observe:
- Weight mass gain from 1300 to about 1340°C (point 1 Figure 8)
- Weight mass loss from 1340 to about 1380°C (point 2 Figure 8)
- Weight mass gain, or slow weight mass loss from 1380 to about 1410°C (point 3
Figure 8)
- 10 -
-
Linear weight mass loss during the 1450°C plateau
Diminution of the weight mass loss between 1430 and 1400°C (while decreasing
temperature) (point 4 Figure 8)
Weight mass gain for temperatures under 1400°C (point 5 Figure 8)
1500
17
1480
1460
16,95
1440
1420
16,9
1380
1360
16,85
1340
1320
1300
sample mass (mg)
température (°C)
1400
16,8
1280
1260
16,75
1240
1220
1200
16,7
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
7,0
7,5
8,0
8,5
9,0
9,5
10,0
time (h)
Tr [ｰ C]
Value [mg]
Figure 7 : Typical shape of a transition curve, Tyranno SA fiber under 8 ppm O2 and around
16 ppm H2O
Table 6 below compares temperature values for different performed experiments under 8 ppm
O2 and around 16 ppm H2O (cf Figure 8).
- TSA et HNS are reference names for Tyranno SA and Hi Nicalon S fibers
- 09-032 like notations are reference numbers of each experiment
- 1, 2, 3, 4, and 5 are noteworthy points (cf Figure 8)
- 09-042 experiment was realized with slower increasing and decreasing rates of
temperature (0,5°C/min) to observe the heat rate influence.
TSA
TSA
HNS
HNS
HNS
09-032
09-034
09-035
09-036
09-042
1
1350
1335
1338
1338
1330
2
X
1368
1370
1380
X
3
X
1410
1410
1425
1408
4
1430
1430
1430
1435
1425
5
1390
1400
1400
1400
1400
Amount of water measured at the outlet of the TGA for each test:
13 ppm
9 ppm
9 ppm
20 ppm
12 ppm
Table 6: Temperature of noteworthy points on transition curves
- 11 -
moyenne
1338
1373
1413
1430
1398
ecart type
7
6
8
4
4
Forme
typiqueshape
d'une courbe
lors d'uncurves,
essai denoteworthy
transitions 1300Typical
of transition
1450-1300°C
8ppmon
O2the
et ~15ppm
pointssous
plotted
curve H2O
masse de l'échantillon (mg)
20,55
1
20,5
3
20,45
2
20,4
20,35
4
5
20,3
2,0
3,0
4,0
5,0
6,0
7,0
8,0
temps (h)
Time (h)
Value [mg]
Figure 8 : Noteworthy points on transition curves
It is worth noting that the temperature difference between the 5 tests is very small (8°C max).
That means that the behavior of Hi Nicalon S and Tyranno SA fibers towards oxidation
transition is similar. The values measured during the cooling period are very close, while they
are more dispersed during the heating period. A possible explanation is that the fibers are
more or less oxidized during the increase of temperature, while they are identically
deoxidized during the decrease of temperature (following the 1 hour at 1450°C temperature
stage which volatilizes silica). Indeed, the mass loss between point 3 and 1450°C is more
important than the masse gain between 1300°C and point 1. All the silica formed should be
volatilized at this point. What’s more, the linear mass loss at 1450°C can mean that only SiC
is volatilized.
The determination of the nature of points 1, 2 and 3 is still under study. The most possible
explanation would be that, during the raise up of temperature, residual silica remains from
deseizing and from oxidation before point 1. The change in the silica nature (amorphous silica
crystallizes in cristobalite near 1400°C [18]) could explain the shape of these curves.
Anyway, the nature of the mechanisms corresponding to these points are not fully understood,
and this point is still under study.
The slope change between point 4 and 5 is due to silica creation at the sample surface. The
following weight mass gain (after point 5) is due to the faster creation of silica than
volatilization of SiO.
These first results are in accordance with Vaughn and Maahs (cf Figure 4) experimental
points. This shows that experimental results can be compared when using different
experimental installations. Other tests are done to end this study on passive to active
transitions. Kinetic studies of oxidation and volatilisation will further be realized.
- 12 -
1.4 Modeling approach
The experimental tests are performed in order to feed the model. By now, there are not
enough data to completely run the program. The composites considered in the model are in
fact micro-composites: One SiC fiber is coated with pyrocarbon and infiltrated with a SiC
matrix. A crack in the matrix allows the access of oxygen to the fiber/matrix interphase. The
oxygen diffusion through the crack and its reaction with the SiC matrix, the PyC interface and
the SiC fiber is modeled. This model is based on Rebillat [19], Filipuzzi [20] and Lamouroux
[21] work. It allows the modelling of active and passive oxidation, and volatilizationoxidation type oxidation under O2 or H2O. Oxidation under mixed O2/H2O atmospheres is not
calculated as not enough information on this kind of oxidation was available. The difference
between CVD matrix SiC and SiC fibers oxidation kinetics is done. In such environmental
conditions, the reactions taken into account are:
2C  O2  2CO
SiC  3 2 O2  SiO2  CO
SiC  O2  SiO  CO
C  H 2O  CO  H 2
SiC  3H 2O  SiO2  CO  3H 2
SiC  2H 2O  SiO  CO  2H 2
(1)
(2)
(3)
(4)
(5)
(6)
The modelling approach is described below:
(i)
calculation of the oxidant concentration profile along the matrix crack,
(ii)
determination of the end of pore concentration,
(iii) calculation of the carbon consummated amount,
(iv)
determination of the weight mass change as a function of time.
The concentration evolution profile, along the matrix crack and the interphase is given below:
2. oxyde  k p C ( z ) Np k l .C ( z ) Nl 
nO 2
d   Deff .e( z ).C0 dC ( z ) 

0




dz  C0  C ( z )1    dz  noxyde. formé M oxyde  2 ( z ) Cr Np
Cr Nl 
Equation 3
Where Deff is the effective diffusion coefficient (m2.s-1),
e(z) is the crack width at z depth (m),
C(z) is the oxygen concentration at z depth (mol.m-3),
 is the ratio of oxidized gas amount versus the oxidizing gas amount for
stoechiometric reactions,
X0 is the oxygen molar fraction (-),
SiO2 is the volumetric mass of silica (g.m-3),
Moxyde is the molar mass of silica (MSiO2 = 60 g.mol-1),
kp is the parabolic oxidation rate constant of SiC for a 1 atm pure oxygen pressure
( m2.s-1),
kl is the linear volatilisation rate constant of SiC for a pure 1 atm pure oxygen pressure
(g.s-1)
(z) is the silica layer thickness along the crack (m)
Cr is the oxygen concentration for a 1 atm pure oxygen pressure (mol.m-3),
Np is the reaction partial order for passive oxidation (-),
Nl is the reaction partial order for active oxidation (-),
- 13 -
Boundary conditions are used at depth z=lr (lr being the carbon interphase consumed length).
At steady state, the oxygen diffusing at the bottom of the cracks is totally consumed by the
carbon oxidation at interphase.
Nc
 Deff .S f ( z ) dC ( z )
 C ( z) 
 K C .S i ( z ).

1  1   X 0 dz
 Cr 
Equation 4
Where KC is the linear volatilisation rate constant of C for a pure 1 atm pure oxygen pressure
(g.s-1),
NC is the reaction partial order for carbon oxidation (-),
Sf(z) is the crack section at z depth (m)
Si(z) is the carbon interphase section (m)
The outputs of the model are:
- Oxidant concentration as a function of time and depth
- Silica layer thickness as a function of time and depth
- Depth of the crack as a function of time
- Width of the crack as a function of time
- Mass variation as a function of time
2 Conclusion
Thermodynamic calculations highlighted the fact that in GFR functioning environment, the
oxidation products under oxygen or moist atmospheres are the same, as hydroxygenated
species form in negligible amount. Active to passive transitions can thus be expressed as a
function of equivalent oxygen partial pressure.
The complete study on transitions will allow to confirm this information. The first results give
us experimental points very close to other authors’. This shows that experimental results can
be comparable, despite the different setups. Other tests are currently done to end this
oxidation transition study.
Kinetic studies will be performed, in order to complete the modelling of a SiC/C/SiC
composite oxidation through a crack in the matrix.
- 14 -
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