CC BY
23XV International Symposium on Self-Propagating High-Temperature Synthesis
FILTRATION COMBUSTION OF SiF4 AND CaH2 IN VERTICAL FLOW REACTOR
A. Yu. Lashkov*a, A. D. Bulanovab, and O. Yu. Troshin
aG.G. Devyatykh Institute of Chemistry of High-Purity Substances, Nizhny Novgorod, 603951 Russia
bNational Research Lobachevsky State University, Nizhny Novgorod, 603950 Russia *e-mail: lashkov@ihps.nnov.ru
DOI: 10.24411/9999-0014A-2019-10079
Interaction of silicium tetrafluoride and calcium hydride for preparation of monosilane is one of the stages of fluoride-hydride technology for production of high-purity silicon, including monoisotopic silicon. Silicon is the main material of nanotechnologies [1] and quantum electronics [2]. De Pape showed [3] that formation of SiH4 occurs in the reaction of SiF4 and CaH2 at the temperature lower than 300°C. If the temperature is higher than 300°C then thermal decomposition of silane occurs. First experiments of interaction of these reagents took place in the vertical silica flow reactor (Fig. 1) and the initial temperature was 180-200°C [4]. We have defined that at this temperature the conversion of silicon tetrafluoride is 100%. We observed the planar reaction front propagating and change of solid phase color from light grey to brown after SiF4 flowing through the reactor with CaH2. The temperature increased up to ~ 250°C when the darkening zone of chromel-copel thermocouple, located inside reactor, was achieved. The temperature decreased when the reaction front passed through thermocouple. While comparing the literature date with the results of experiments we hypothesized that the reaction of silicium tetrafluoride with calcium hydride is the mode of filtration combustion. Next we replaced the silica flow reactor with stainless steel reactor (steel grade 12Kh18N10T) for filtration combustion (Fig. 2) due to starting of "Avogadro" project.
Fig. 1. Vertical silica flow reactor for preparing silane and temperature profile of process. 1 solid reaction products (brown); 2 reaction front (change colors); 3 CaH2 (light grey); 4 thermocouple.
Fig. 2. Stainless steel reactor for filtration combustion of SiF4 and CaH2. a input gases (SiF4 + H2); b output gaseous products (SiH4 + H2); c reactor heating; d resistive furnace; e stainless steel tube with thermocouples (1-6); f CaH2 powder.
Geometric parameters of stainless steel reactor are: the height (h) is 120 cm, the diameter (d) is 5 cm, the volume (V) is 2356 cm3, the cross section (S) is 19 cm2. The technique of experiment is as following. Hydrogen gas flows bottom-up with the rate of 120 ml/min. Then we loaded
iSHS 2019
Moscow, Russia
into reactor CaH2 powder (the fraction is 0.6 mm, the bulk density is 1.3 g/sm3). Reactor and resistive furnace (Figs. 2c, 2d) preheating up 100 and 150°C, respectively. Then the gas mixture of SiF4 + H2 is input. The gases flow rates: 200 ml/min is the reagent gas (SiF4) and 120 ml/min is the carrier gas (H2). Approximately after 10 min the temperature increase is observed it the location of resistive furnace (Fig. 3) indicating the starting of reaction. The counter-flow reaction is starting. Resistive furnace is "off' when temperature is the maximum on thermocouple 3. Counter wave motion is uniformly accelerated and the average rate is 1.7 x 10-4 m/s. Reflection of combustion wave occurs when the counter front succeed to bottom flange. Then the co-flow reaction wave starts propagating. Co-flow motion is uniform with the front rate of 6.7 x 10-5 m/s. The process is completed when the temperature maximum is attained on thermocouple 6. We have determined the basic kinetic parameters of this reaction in a counter wave in the vertical flow reactor: the reaction order is n = 1, the rate constant is k = 0.38±0.05 s-1, the activation energy is Ea = 17±2 kJ/mole [5].
The thermogram in Fig. 3 shows the steady-state combustion at the above-mentioned temperatures of reactor and resistive furnace. There is a split reaction front if the reactor is heated above 110°C and the furnace is heated above 170°C. When the CaH2 powder of different dispersion is filled into the reactor, a standing combustion wave occurs [6].
Fig. 3. Thermogram of filtration combustion of SiF4 and CaH2 in vertical flow reactor. a counter wave; b co-flow wave; 1-6 thermocouples.
1. O. Moutanabbir, S. Senz, Z. Zhang, U. Gosele, Synthesis of isotopically controlled metal-catalyzed silicon nanowires, Nanotoday, 2009, vol. 4, no. 5, pp. 393-398.
2. K. M. Itoh, An all-silicon linear chain NMR quantum computer, Solid State Commun., 2005, vol. 133, no. 11, pp. 747-752.
3. R. De Pape, Reduction of silicon tetrafluoride and boron trifluoride by calcium hydride, Ann. Chim., 1963, vol. 8, nos. 3-4, pp. 185-196.
4. G.G. Devyatykh , A.D. Bulanov, O.Yu. Troshin, V.V. Balabanov, D A. Pryakhin, E.M. Dianov, Preparation of High-Purity Monoisotopic Silane: 28SiH 4, 29SiH4, and 30SiH4, Dokl. Chem., 2003, vol. 391, nos. 4-6, pp. 204-205.
5. A.Yu. Lashkov, A.D. Bulanov, O.Yu. Troshin, Filtration combustion of silicon tetrafluoride and calcium hydride for the preparation of monosilane, Inorg. Mater., 2016, vol. 52, no. 9, pp. 915-918.
6. A.Yu. Lashkov, A.D. Bulanov, O.Yu. Troshin, Influence of technological parameters of SiF4 and CaH2 interaction on the nature of filtration combustion, Russ. J. Appl. Chem., 2019, vol. 92, no. 4, (in print).
