Reaction of silicon tetrafluoride with calcium hydride as a propagating wave

Article abstract of DOI:10.1134/S0036023608010026

The synthesis of silane by reacting silicon tetrafluoride and calcium hydride is shown to proceed as a propagating wave. The main parameters of the process, namely, the propagation velocity, the reaction front temperature, and calcium conversion, are determined. The mathematical model of wave propagation for an exothermic reaction in a porous medium with purging a gaseous oxidant describes qualitatively the major laws of the process.

Full text of DOI:10.1134/S0036023608010026

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 1, pp. 6–10. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.D. Bulanov, V.S. Mikheev, O.Yu. Troshin, A.Yu. Lashkov, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 1, pp. 11–15.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Reaction of Silicon Tetrafluoride with Calcium Hydride
as a Propagating Wave
A. D. Bulanov, V. S. Mikheev, O. Yu. Troshin, and A. Yu. Lashkov
Institute of High-Purity Substances, Russian Academy of Sciences, ul. Tropinina 49, Nizhni Novgorod, 603600 Russia
Received December 15, 2006
Abstract—The synthesis of silane by reacting silicon tetrafluoride and calcium hydride is shown to proceed as
a propagating wave. The main parameters of the process, namely, the propagation velocity, the reaction front
temperature, and calcium conversion, are determined. The mathematical model of wave propagation for an exo-
thermic reaction in a porous medium with purging a gaseous oxidant describes qualitatively the major laws of
the process.
DOI: 10.1134/S0036023608010026
When isotope-enriched silicon 28Si was obtained by
the hydride method, silane 28SiH4 was synthesized by
reacting silicon tetrafluoride 28SiF4 with calcium
hydride ë‡ç2 occurring with a significant heat [1, 2]. It
is known that similar processes can proceed in the form
of a propagating reaction wave [3], and they can be car-
ried out continuously [4].
A mixture of gaseous products was analyzed gravi-
metrically by weighing evacuated ampules filled with
the gaseous mixture on a VL E134 balance. The accu-
racy of gas-weight determination was ≤0.5%. The
effective molecular weight of a mixture of gaseous
components was determined using the ideal gas law.
Gas-chromatographic analysis was carried out on a
Tsvet 100 gas chromatograph equipped with a heat-
This work studies the macroscopic reaction of sili-
con tetrafluoride with calcium hydride and determines
the main parameters of the process.
Outlet of gaseous
reaction products
EXPERIMENTAL
Experiments were carried out on a flow-through
cylindrical reactor of stainless steel 1.2 m long with an
internal diameter of 52 mm (Fig. 1). A tube with a
diameter of 14 mm was inserted coaxially to the reac-
tor. Six Chromel–Copel thermoelements (CCTs) were
placed inside the tube ~20 cm apart from each other.
The signal from each CCT was processed and detected
on a personal computer. The temperature of the outer
wall of the reactor was set using a resistor furnace con-
trolled by a high-frequency temperature regulator.
Pipes for sampling gaseous reactants were mounted at
the inlet and outlet of the reactor. The process was ini-
tiated using an ignition resistor furnace 10 cm long,
which can move along the length of the reactor.
V6
V5
CCT
junctions
V4
V3
Ignition furnace
V2
The experimental procedure was as follows. The
reactor was filled with disperse calcium hydride (frac-
tion <0.6 mm), and purged hydrogen. Then, the reactor
and ignition furnace were heated to the initial tempera-
tures, and a gaseous reactant flow was admitted. The
flow amount was maintained using gas-flow regulators.
The gaseous reaction products were passed through
traps cooled with liquid nitrogen to condense (freeze
out) silane that formed. The skip of the gaseous reac-
tion products did not exceed 0.1 wt %.
Heater
V1
Inlet of initial
gas mixture
Fig. 1. Scheme of the reactor for the synthesis of silane
by the reaction of silicon tetrafluoride with calcium
hydride.
6

REACTION OF SILICON TETRAFLUORIDE WITH CALCIUM HYDRIDE
7
T, °C
300
T, °C
300
CCT
2
1
4
τ1
3
τ2
τ3
τ4
250
200
150
100
τ5
250
200
150
100
0
50 100 150 200 250 300 350 400 450 500
Time, min
1
2
3
4
5
CCT no.
Fig. 3. Spatial profile of the reaction wave in different time
moments (τ < τ < τ < τ < τ ). The distance between
Fig. 2. Thermoanalytical curves for the reaction of silicon
tetrafluoride with calcium hydride (cocurrent wave):
1−4 are the numbers the CCTs.
1
2
3
4
5
CCTs is 5 cm.
conductivity detector on a column packed with the
Polisorb PS-1 polymeric sorbent using helium as a car-
rier gas and a thermostat temperature of 70°ë.
profile. This corresponds to the representation of the
regime of the reaction in the form of a propagating local
zone.
The chromatographic analysis of the gas phase at
the outlet of the reactor showed a considerable amount
of silane. In addition, the chromatogram contained
peaks presumably attributed to fluorosilanes. Gravi-
metric analysis showed a considerably decreased effec-
tive formula weight of the gaseous products (FW =
30.6), which was, however, somewhat larger than cal-
culated for the complete conversion of silicon tetraflu-
oride to silane (FW = 26).
RESULTS AND DISCUSSION
In a control experiment, the initial temperature of
the reactor was chosen to be 100°ë, a temperature at
which, according to [5], no reaction occurred. After a
mixture of 80 mol % SiF4 + 20 mol % ç2 was fed, the
temperature in the reactor remained unchanged and the
qualitative and quantitative composition of the gas
phase at the outlet of the reactor corresponded to the
initial composition.
The generation and propagation of the reaction
wave cocurrent with the gas flow were observed in the
experiments described. To check the possibility of the
counterflow propagation of the reaction zone, the fur-
nace for process initiation was placed at the end of the
reaction opposite to the inlet of the gaseous reactant.
The temperature on CCT 4 (Fig. 4) increased some time
after the reactor and initiating ovenfurnace were heated
to the corresponding temperatures and the flow of a
mixture of silicon tetrafluoride and dihydrogen was fed
and then on CCT 3, CCT 2, and CCT 1. Thus, in this
experiment the reaction wave propagated in the coun-
tercurrent flow with the gaseous reactant.
In the next experiment, the ignition furnace was
placed in the region of the reactor near the gas inlet
(Fig. 1) and the local region of the reactor was heated to
180°ë. At this temperature, silicon tetrafluoride con-
verts to silane within a short time (~10 s) [5]. Therefore,
it was assumed that the reaction is initiated in the heated
region. Indeed, after the reaction mixture was fed, a con-
siderable temperature (to about 280°C) increase was first
recorded by CCT 1, then by CCT 2, CCT 3, etc. (Fig. 2).
After achieving the maximum, the temperature gradu-
ally decreased in all cases. The temperature wave prop-
agation observed along the reactor can be explained
only by the appearance and propagation of a local
region of the exothermic reaction between silicon tet-
rafluoride and calcium hydride.
It is of interest that the processes did not stop after
the reaction wave reached the lowest end of the reactor.
After some time (~10 min), the temperature on CCT 1
increased again. Then, the reaction wave propagated up
the reactor in the concurrent flow with the gaseous reac-
tant, which was detected by the successive temperature
increase on CCT 1, CCT 2, CCT 3, CCT 4, CCT 5, and
The same experiment was carried out using a block
of five CCTs arranged at 5 cm from each other to mea-
sure temperature. This made it possible to record the
spatial profile of the temperature wave. The results of
the measurements are presented in Fig. 3. At least in a
short segment of the reactor length, the wave propa- CCT 6. The observed phenomenon is related to the so-
gates in the steady-state regime retaining the geometric called reflection of the wave of the self-propagating
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

8
BULANOV et al.
T, °C
300
CCT
5
2
1
6
4
3
250
200
150
100
50
0
100
200
300
400
500
600
Time, min
Fig. 4. Thermoanalytical curves for the reaction of silicon tetrafluoride with calcium hydride (countercurrent and cocurrent waves):
1–6 are the numbers of the CCTs.
exothermic reaction from the reactor top, which was
mentioned [6] as an effect of solid flames.
Thus, the results of our experiments showed that the
hydrogenation of silicon tetrafluoride by calcium
hydride can proceed via the propagation of a local zone
of the exothermic reaction.
The main experiments were carried out during the
synthesis of isotope-enriched silane 28SiH4. In these
entries, the reaction was always initiated from the top.
Representative thermoanalytical curves are shown in
Fig. 4.
The calcium hydride conversion was determined as
the ratio of the amount of reacted ë‡ç2 to the initial
amount of CaH2.
The amount of reacted calcium hydride upon the
movement of the reaction front by 1 cm m(ë‡ç2, react.,
“1 cm”) was estimated from the consumption of silicon
tetrafluoride in this region, taking into account the sto-
ichiometry of the reaction:
0.808G(SiF4)
---------------------------------
m(CaH2, react., “1cm”) =
,
(1)
u × τ
The major parameters of the process, namely, the
propagation rate of the reaction wave in the cocurrent
(ucc) and countercurrent (uct) flows, CaH2 conversion
(η), and maximum temperature in the reaction zone (t),
were determined from the experimental data. The
degree of hydrogenation of silicon tetrafluoride was
close to 100% in all entries and, hence, in the quantita-
tive estimations, SiF4 was considered to convert com-
pletely to silane.
where G(SiF4) is the consumption of silicon tetrafluo-
ride in the entry, and τ is the time of the entry.
The initial amount of calcium hydride per centime-
ter of the reactor length m(ë‡ç2, init., “1 cm”) is deter-
mined from the overall weight of the load and geomet-
ric characteristics of the reactor:
0.976m(CaH2, init)
-----------------------------------------------
m(CaH2, init, “1 cm”) =
, (2)
h(CaH2, init)
The temperature in the reaction zone was estimated
by the readings of CCTs arranged along the reactor
axis. Since the propagation rates of the reaction wave
front were rather low, it was assumed that the error in
temperature determination did not exceed 10 K.
where m(ë‡ç2, init) is the weight of the calcium
hydride load, and h(ë‡ç2, init.) is the height of the cal-
cium hydride load. The numerical coefficient appears
due to the presence of the dead volume in the reactor.
The ratio of (1) to (2) gives the conversion η:
The propagation rate of the reaction front (u) was
calculated as the ratio of the distance between neigh-
boring CCT to the difference between the times of
achieving the temperature maximum on these CCT.
0.828G(SiF4) × h(CaH2, init)
-------------------------------------------------------------------------
η =
.
u × t × m(CaH2, init)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

REACTION OF SILICON TETRAFLUORIDE WITH CALCIUM HYDRIDE
Main parameters of the process
9
CCT no.
Tcc, °C ucc × 105, m/s
ηcc
Tct, °C uct × 105, m/s
ηct
ηΣ
〈ηΣ〉
EF, %
9.6
1–2
2–3
3–4
1–2
2–3
3–4
1–2
2–3
3–4
1–2
2–3
3–4
1–2
2–3
3–4
241
265
263
243
253
254
237
248
258
257
262
259
232
230
238
5.7
8.2
7.0
8.5
8.2
7.0
6.5
8.2
6.3
6.2
5.2
4.7
6.2
5.5
7.2
0.098
0.068
0.079
0.073
0.077
0.089
0.096
0.076
0.097
0.091
0.107
0.120
0.099
0.111
0.087
182
173
155
199
203
200
182
189
194
161
172
188
181
202
204
24.5
30.5
38.8
15.3
14.7
13.5
13.7
12.7
14.0
18.3
13.7
14.7
15.2
11.8
13.0
0.023
0.018
0.014
0.041
0.043
0.046
0.046
0.049
0.044
0.030
0.041
0.038
0.041
0.052
0.048
0.121
0.084
0.093
0.114
0.120
0.135
0.142
0.125
0.141
0.121
0.148
0.158
0.140
0.163
0.135
0.100
0.123
0.136
0.143
0.145
10.3
10.9
12.2
11.7
Note: T is the maximum temperature in the reaction zone, u is the reaction wave propagation velocity, η is the conversion of CaH , cc
2
designates the cocurrent flow, ct designates the countercurrent flow, η is the total conversion of CaH , 〈η 〉 is the average conversion
Σ
2
Σ
of CaH , and EF is the efficiency factor.
2
The estimates of the propagation rates of the reac- cium hydride necessary for processing silicon tetraflu-
tion front and the calcium hydride conversion in the oride is determined from the stoichiometric equation of
countercurrent and cocurrent flows are given in the the reaction. In accordance with the physical meaning
table. The discussion was based on the concepts devel- of these quantities, the total conversion of calcium
oped in [3].
hydride should be close to EF but always slightly
higher than it, because the reactor contains volumes
filled with calcium hydride that are not involved in the
reaction. As can be seen from the tabulated data, this
holds in all the cases considered. Interestingly, small
variations in EF and total conversion repeat each other.
The steady-state propagation of the zone of exother-
mic reaction of porous particles with the oxidant con-
tained in the gas slowing in the direction of front move-
ment (cocurrent flow) or against it (countercurrent
flow) were studied in [3]. According to the results
obtained, the propagation rate of the reaction front in
the countercurrent flow is equal to the normal combus-
tion rate. The reaction wave velocity in the cocurrent
flow at relatively small consumptions of the gaseous
reactant should be lower than that in the countercurrent
flow. The data presented show that this condition is ful-
filled in all the cases.
Thus, the mathematical model of propagation of an
exothermic reaction wave through a porous medium
with purging a gaseous oxidant describes qualitatively
the main laws of the process under consideration.
As for the instability of the process parameters, we
can say the following. The current values depend on
various characteristics of the solid-phase reactant
(ë‡ç2 bed), which are randomly form in each region of
the reactor: dispersion composition in the local zone,
geometric characteristics of particle packing, sinter-
ability of between particles to form a intricate topolog-
ical structure on the path of the flowing gaseous reac-
The calcium hydride conversions and the tempera-
tures of the reaction front in the cocurrent flow are
always higher than those for the countercurrent flow
(table). These ratios correspond qualitatively to pub-
lished results [3].
The table contains average total conversions of cal- tant, etc. These factors, as well as the non-steady-state
cium hydride after the reaction wave passed the same character of heterogeneous processes, most likely
region of the reactor in the countercurrent or cocurrent determine the scatter of the process parameters. As a
flows. The efficiency-factors (EF), determined as the whole, the hydrogenation of silicon tetrafluoride by cal-
ratio of the amount of calcium hydride necessary for cium hydride in the self-propagating wave mode is
processing silicon tetrafluoride consumed in the entry characterized by high stability. None failure of the pro-
to the total amount of calcium hydride loaded to the cess was observed during the whole overall time of
reactor, are also given in the table. The amount of cal- experiments (~400 h). Thus, our study determined that
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

10
BULANOV et al.
the reaction of silicon tetrafluoride with calcium
hydride can proceed in the form of the propagating
reaction wave. The major macroscopic parameters of
the process were determined: the movement velocity
and temperature of the reaction front and the conver-
sion and EF of calcium hydride. The process consid-
ered is promising for the synthesis of silane.
REFERENCES
1. G. G. Devyatykh, A. D. Bulanov, A. V. Gusev, et al.,
Dokl. Akad. Nauk 376 (4), 492 (2001).
2. R. De Pape, Ann. Chim. 8 (3–4), 185 (1963).
3. A. P. Aldushin and B. S. Seplyarskii, Dokl. Akad. Nauk
SSSR 241 (1), 72 (1978).
4. The Concept of Combustion and Explosion Propagation
as a Field of Research and Engineering Progress (Terri-
toriya, Chernogolovka, 2001), p. 176 [in Russian].
ACKNOWLEDGMENTS
5. A. D. Bulanov, Doctoral Dissertation in Chemistry
(IKhVV RAN, Nizhnii Novgorod, 2005).
This work was supported by the President of the
Russian Federation in the framework of the Program
for State Support of Leading Scientific Schools of Rus-
sia (grant no. NSh 9614.2006.3).
6. A. G. Merzhanov, Combustion and Materials Synthesis
(ISMAN, Chernogolovka, 1998) [in Russian].
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008