2.2 reacción. Cinética de reacción.

1.1         2.2 reacción. Cinética de reacción.

En investigaciones anteriores, El proceso ha sido empleado exitosamente en la síntesis de SiC usando como elementos de entrada dióxido de silicio (cuarzo) y negro de humo, carbono amorfo. Los estudios termogravimétricos han demostrado que la reacción de formación de SiC y liberación de CO comienza a temperaturas tan bajas como 800 grados centígrados.

MET_54_3_579_582 SiO2plusC - SiC

1.1.1        2.2.1 reaccion en la superficie (SiO2 + C)

Carbothermal reduction is the primary chemical process practiced commercially for the synthesis of many ceramic materials, including SiC, The overall reduction reaction can be written as:

SiO2 (s) + 3C (s) = SiC (s) + 2CO (g)                                                           (1)

DG° = 598.18 - 0.3278 T (kJ)

The standard Gibbs free energy of the above reaction decreases with increasing temperature and becomes zero at 1525 °C. However, the reaction can take place at lower temperatures when the CO partial pressure is significantly lower than atmospheric pressure. It is generally accepted that SiC is synthesized through intermediate silicon monoxide (SiO) [2]:

C (s) + SiO2 (s) = SiO (g) + CO (g)                                                                  (2)

DG° = 668.07 - 0.3288 T (kJ)

SiO (g) + 2C (s) = SiC (s) + CO (g)                                                                  (3)

DG° = -78.89 + 0.0010 T (kJ)

The standard Gibbs free energy of Reaction (3) is negative in the examined temperature range (1000 to 2000 oC), therefore theoverall reaction of SiC formation is defined by the reaction of SiO formation. It is well established that the kinetics of carbothermal reduction of SiO2 is affected by temperature [3], molar SiO2/C ratio [4], carbon and SiO2 particle size [5], and initial bulk density [6]. The effect of gas atmosphere on the carbothermal reduction of silica has been studied to a lesser extent. The SiC formation will stop when the CO partial pressure in Reaction (1) reaches  the equilibrium value. CO has to diffuse out of the reactant mixture for further reaction to take place.

Carbothermal reduction of silica involves gaseous intermediate SiO. When the reduction of quartz is carried out in Ar, SiO is initially formed at the contact points of graphite and quartz particles following the solid-solid Reaction (2), and then SiO is further reacts with carbon to form SiC following Reaction (3). SiO can also be generated by Reaction (6) which, with the decrease in the contact area between quartz and graphite, becomes a major reaction for the SiO formation. CO2 formed in Reaction (6) is converted back to CO by Boudouard Reaction (7) which is highly favoured thermodynamically within the temperature range of isothermal reduction experi ments of this investigation.

SiO2 (s) + CO (g) = SiO (g) +CO2 (g)              (6)

C (s) + CO2 (g) = 2CO (g)                       (7)

Figure 7: CO evolution rate in the temperature programmed reduction of quartz in different gas atmospheres.

In the carbothermal reduction of SiO 2 in Ar, the conversion of quartz to SiC started at 1300 °C, and
was incomplete after 270 minutes at 1500 °C.

SYNTHESIS OF SILICON CARBIDE BY CARBOTHERMAL REDUCTION OF QUARTZ IN H2-Ar GAS MIXTURES
Xiang Li1, Guangqing Zhang1, Kai Tang2, Oleg Ostrovski3 and Ragnar Tronstad

The Fourteenth International Ferroalloys Congress Energy efficiency and environmental friendliness are the future of the global Ferroalloy industry May 31-June 4, 2015 Kiev, Ukraine pag.548

2 Lee, J.G., Miller, P.D., Cutler, I.B., Reactivity of solids. Plenum Press, 1976.

3 Moshtaghioun, B.M., Poyato, R., Cumbrera, F.L., Bernardi-Martin, de S., Monshi, A., Abbasi, M.H., Karimza-deh, F., Dominguez-Rodriguez, A.,Rapid carbothermic synthesis of silicon carbide nano powders by using mi-crowave heating. J. Eur. Ceram. Soc. 32(2012), pp. 1787–1794.

4 Agarwal, A., Pal,U., Influence of pellet composition and structure on carbothermic reduction of silica. Metall. Mater. Trans. B 30(1999), pp. 295–306.

5 Martello,D. E., Tranell, G. , Gaal, S., Raaness, O.S., Tang, K., Arnberg, L.,Study of pellets and lumps as raw materials in silicon production from quartz and silicon carbide. Metall. Mater. Trans. B 42( 2011), pp. 939–950.

6 Yao,J., Wang,H., Zhang,X., Zhu,W., Wei,J., Cheng, Y.B., Role of pores in the carbothermal reduction of carbonsilica nanocomposites into silicon carbide nanostructures. J. Phys. Chem. C 111(2007), pp. 636–641.

Dado que se escogio la ruta de carboreduccion a través de monóxido de silicio se tiene que tomar en cuenta que hay una segunda reacción que descompone el SiC en monóxido de silicio y CO.

SiO₂(s) + C(s) → SiO(g) + CO(g);
SiO(g) + 2C(s) → SiC(s) + CO(g).

The second stage consists of reactions between silica with silicon carbide which form silicon monoxide and, where conditions permit, may also form silicon metal.
The major reaction during this stage is
2SiO₂(g) + SiC(s) → 3SiO(g) + CO(g).

Biernacki, J. J. and Wotzak, G. P. (1989), Stoichiometry of the C + SiO₂ Reaction. Journal of the American Ceramic Society, 72: 122-129. doi:10.1111/j.1151-2916.1989.tb05964.x

Como ambos productos son gases se espera que no permanezcan en el film obtenido. Sino que se difundan y se liberen a la atmosfera.

Hay que detener el proceso de anneado antes de que todo el SiC obtenido se volatilice.

1.1.2        2.2.2REACCION en el sustrato Si+SiO2

En la interfaz del sustrato y el SiO2 sucede una reaccion que genera vapor de SiO

By generating SiO vapor from the silicon reduction of silica (SiO2 ) [28, 29, 32] as illustrated by equation (1.3).
Si (solid) + SiO2 (solid) → 2SiO (vapor) (1.3)

Moreover, some amount of silica (SiO2 ) was also found in the sample. It was
suggested that SiO vapor decomposed into SiO2 , according the following equation:

2 SiO (vapor)→ SiO2 (solid) + Si (2.2)

*SYNTHESIS AND CHARACTERIZATION OF a-SILICON CARBIDE NANOSTRUCTURES
Enagnon Thymour Legba  University of Kentucky, etleg0@engr.uky.edu
2.2 SYNTHESIS AND CHARACTERIZATION OF a-SILICON CARBIDE NANOSTRUCTURE.pdf

Recommended Citation

Legba, Enagnon Thymour, "SYNTHESIS AND CHARACTERIZATION OF a-SILICON CARBIDE NANOSTRUCTURES".

(2007). University of Kentucky Master's Theses. 494.

http://uknowledge.uky.edu/gradschool_theses/494