Effect of Oxygen Partial Pressure and Temperature on the Oxidation Behavior of SiB 6
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, JACOB S. YOUNG, and RAMANA G. REDDY
The oxidation kinetics of silicon hexaboride (SiB6) was studied at different partial pressures of oxygen. The specific weight gain was measured at 1173 K, 1223 K, and 1273 K for PO2 = 0.1, 0.23, and 0.33 atm using thermogravimetric analysis. The conventional empirical expressions for oxidation were observed at all selected oxygen partial pressures and temperatures. The structural characterization of the oxidation product was characterized using XRD and FT-IR, with SiB6, SiO2, B, and amorphous B2O3 observed after oxidation for 25 hours. The oxidation surface morphology was also characterized to obtain the oxidation product size, ranging from 4.54 to 24.69 lm with increasing PO2 and temperature. The diffusional activation energy for the oxidation process was also calculated from the empirical constant, obtained from the mathematical fitting of the specific weight gain with time. The oxidation activation energies for SiB6 are 250.72, 235.64, and 232.65 kJ/mol at PO2 = 0.1, 0.23, and 0.33 atm, respectively. https://doi.org/10.1007/s11663-019-01749-z The Minerals, Metals & Materials Society and ASM International 2019
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INTRODUCTION
WITHIN the Si-B system, boron-rich compositions have drawn recent attention due to their high-temperature performance as thermoelectric (TE) materials and thermal barrier coatings (TBC).[1] The thermodynamic properties and phase diagram of the Si-B system, as well as other boride systems, have been thoroughly studied previously.[2–14] Experimental data indicate the presence of various binary phases, namely SiB3, SiB6, and SiBn (n = 14 to 50).[15] Among these phases, SiB6 is the most thermodynamically stable in an inert atmosphere.[3,6,7,11] As a result, SiB6 is one of the prominent thermal emittance agents used for protective ceramic coatings of space vehicles and combustion chambers to re-radiate thermal energy.[16,17] Oxidation of silicon borides is known to reduce critical thermoelectric and transport properties.[18] The effect of partial pressure of oxygen and temperature on the oxidation kinetics of Si-B materials is an essential factor toward better optimization of TE and TBC performance. Boron and silicon oxidation has been well studied in the literature.[19–24] Pure boron particles generally react readily with oxygen, forming a thick, vitreous, molten layer of B2O3 on the surface, as boron oxide melts at a low temperature (~ 723 K) and has a wide liquidus
MUHAMMAD A. IMAM, JACOB S. YOUNG, and RAMANA G. REDDY are with the Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. Contact e-mail: [email protected]
METALLURGICAL AND MATERIALS TRANSACTIONS B
range before boiling at 2340 K.[25] The literature suggests that oxidation of B is very slow and leads to only partial oxidation of B due to scale formation of B2O3.[26] The oxidation behavior of Si is dependent upon the formation of a uniform, amorphous, thin layer of SiO2, inhibiting oxygen diffusion through it as the layer grows in thickness. This limitation of
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