Four steps are identified for all the powders on the thermogravimetric curves as shown in Figure III.3 for the VF powder:
v. Below 400 °C, a mass loss is observed which is related to desorption of adsorbed species on the powder.
vi. Between 400 and 1025°C, a mass gain is measured which is due to the formation of SiO2(s) according to reaction (R1), since the temperature is initially lower than the active to passive transition. The silica growth is then controlled by the surrounding water vapor pressure, the water solubility into the silica layer and its diffusion towards the silicon surface. According to the literature [DG65], the silica growth rate is enhanced under humidified atmospheres compared to dry atmospheres because the
0,01 0,1 1 10 100
0 1 2 3 4 5
4 nm 2 nm
1 nm
F C
M Silica reduction mechanism
(R5) full line and (R6) dashed line and
Expected mass loss (%)
Specific surface area, SSA
BET (m2g-1)
VF
0.5 nm
100 10 1 0,1
Equivalent spherical diameter, F and VF powders, 2a (µm)
eSiO
2
1000 100 10 1 0,1
Equivalent polyhedron diagonal, C and M powders (n=5), 2a (µm)
equilibrium concentration of water is higher than the equilibrium concentration of oxygen in silica.
vii. Above 1025 °C, the samples start to experience a massive weight loss, which is attributed to SiO(g) release during the decomposition of the silica layer.
viii. At higher temperatures, the rate of mass loss suddenly collapses. The constant mass loss rate observed (about 3×10-3 mg min-1) is related to the active oxidation of silicon according to reaction (R2), as the silica layer has been removed. Assuming that the reaction is controlled by the diffusion of water vapor to the powder bed, the rate of weight loss is simply proportional to the surrounding water partial pressure,
f 2
z O
PH . Considering a diffusion length, zf, between 30 and 50 mm (see section III.3.3 and Appendix B.3.4), the values of 1.8 and 1 Pa are respectively deduced for f
2
z O
PH .
Figure III.3: Thermogravimetric curves of VF as-received powder (full line) and previously etched (dashed line), heated up to 1350 °C with a heating rate of 5 °C min-1 and a holding time of 3 h under 2 l h-1 He-4mol.% H2 atmosphere.
The partial pressure of water impurities deduced is larger than the gas specifications (0.5 Pa).
However, under stagnant gas conditions, it should even be lower as water would be rapidly consumed by silicon. Actually, the water is probably supplied by a condensate film that is observed 70 mm underneath the sample position, on the furnace tube and on the thermocouple (Figure III.4, Appendix B.3.4). According to XRD measurements, this condensate is made of
0 60 120 180 240 300 360 420 480
-3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5
viii.
viii.
vi.
v.
m/ t = -2.6 mg min-1
Mass variation (mg)
Time (min)
m/ t = -3.4 mg min-1 vii.
0 250 500 750 1000 1250 1500
Temperature (°C)
silicon, cristobalite (crystallized silica) and an
assumed that SiO(g) crystallizes in the form of silicon and silica on t and on the tungsten thermocouple according to react
The onset of weight loss which corresponds to the m is observed at 1025 °C (Figure
for the passive to active transition can be determi
above (# 1.4 Pa). The prediction is consistent with the red reaction (R5) rather than the reduction of silica by hydrogen thr passive, TA*mP, and passive to active,
to passive transition calculated by
active transitions are controlled by the same mecha
Figure III.4 : Silicon monoxide condensate film on the furnace t thermocouple (right).
Another evidence of the previous statement is the r measured on etched powders (dashed line in grown during passive oxidation should equal the amo mass loss and mass gain, about 2.8, is close to wha
75 .
O
2
SiO
M
M
and is different from what is expected from reactio 87. 1 2 O
SiO2 M
M .
Eventually, the initial masses of native oxide,
Equation (III.9) or (III.10) assuming respectively reaction involved for the oxide reduction. The global mass l layer thickness as estimated in
silicon, cristobalite (crystallized silica) and an amorphous phase, probably silica
crystallizes in the form of silicon and silica on the cold wall of the tube and on the tungsten thermocouple according to reaction (R5).
The onset of weight loss which corresponds to the measured passive active transition,
Figure III.3). Referring to Figure III.1 (dot), the mechanism involved for the passive to active transition can be determined with the water partial pressure calculated Pa). The prediction is consistent with the reduction of silica by silicon through
rather than the reduction of silica by hydrogen through reaction
, and passive to active, TP*mA, transitions are equal and correspond to the activ to passive transition calculated by Wagner, TAWmP. Actually, active to passive and passive to active transitions are controlled by the same mechanism which is reaction (R
: Silicon monoxide condensate film on the furnace tube (left), on the
Another evidence of the previous statement is the ratio between mass gain and mass loss measured on etched powders (dashed line in Figure III.3), on which the amount of silica grown during passive oxidation should equal the amount of silica reduced further
mass loss and mass gain, about 2.8, is close to what is expected from reaction and is different from what is expected from reactio
Eventually, the initial masses of native oxide, 5
2
R
mSiO or 6
2
R
mSiO , are estimated according to assuming respectively reaction (R5) or (R6)
involved for the oxide reduction. The global mass loss, mloss, depends on the initial silica layer thickness as estimated in Figure III.2 but also on the silica grown during passive amorphous phase, probably silica glass. It is he cold wall of the tube
easured passive active transition, TP*mA, (dot), the mechanism involved ned with the water partial pressure calculated uction of silica by silicon through ough reaction (R6). Active to , transitions are equal and correspond to the active . Actually, active to passive and passive to
(R5).
ube (left), on the
atio between mass gain and mass loss ), on which the amount of silica unt of silica reduced further. The ratio of t is expected from reaction (R5) and is different from what is expected from reaction (R6)
, are estimated according to as the mechanisms , depends on the initial silica but also on the silica grown during passive
oxidation as seen on etched powders. The mass gain, mgain, occurring during passive oxidation according to reaction (R1) is thus taken into account in the calculation. IGA measurements (Chapter II.1.3) are consistent with estimations predicted from the mechanism (R5) as can be seen in Figure III.5.
gain O
loss SiO SiO
R SiO
SiO 2 2
2 5 2
2 m
M m M
M
m M (III.9)
gain O
loss SiO R
SiO 2
6 2
2 m
M m M
m (III.10)
Figure III.5: Measured silica mass percent from two distinct methods: TGA, assuming reaction (R5) or (R6) as the mechanisms involved during the reduction of the silica layer, and IGA.