The effect of ultrathin ALD films on the oxidation kinetics of SiC in high-temperature steam
Introduction
Silicon Carbide (SiC) is an attractive material for high-temperature applications due to its high thermal stability, high thermal conductivity, and resistance to thermal shock (Munro, 1997). It is a promising containment material for solar thermochemical hydrogen (STCH) applications (Agrafiotis, 2007), such as in the fabrication of solar reactors and steam/steam heat exchangers required for redox-driven water splitting. These containment materials must be robust and stable in the high-temperature chemical processing environment (Ehrhart, 2016). SiC has ideal thermal properties and is inert to reactive materials at high temperatures; however, it is reactive with high-temperature steam (Eq. (1)). As SiC is oxidized, silicon contained in silica (SiO2) scales on the surface, and volatilizes (Eq. (2)), resulting in the degradation of the overall structure and eventual failure of the component (Lee, 2014, Eaton and Linsey, 2002).
A common approach to reduce oxidation and recession of SiC is to apply an oxidation resistant environmental barrier coating (EBC) on SiC (Lee, 2014, Feng, 2016, Zemskova, 2001, Haynes, 2000, Krishnamurthy et al., 2005, Hoskins, 2018, Lee et al., 1995, Lee et al., 2005, Richards, 2016, Xu et al., 2017). To reduce the reaction of SiC with steam at high temperatures, these coatings must be composed of materials that are chemically inert and structurally stable at the high operating temperatures. Thermal expansion coefficients for the EBCs should approximately match those of the underlying SiC. EBCs are commonly deposited using plasma spraying and chemical vapor deposition (CVD) with thicknesses of hundreds of microns (Lee, 2014). These deposition techniques pose a variety of challenges such as inherently forming cracks and pores upon deposition, poor control over composition, and geometry limitations (Lee, 2014). While CVD is not line-of-sight limited, it cannot be used to coat high aspect ratio micrometer scale pores or tubes without non-uniform film characteristics, which in extreme cases blocks the internal passage due to the non-self-limiting nature of the process (George, 2010). This becomes important in the application of EBCs for high aspect ratio micrometer scale structures such as microchannel heat exchangers used in solar thermochemical applications.
Previously, we showed that the deposition of EBC films using atomic layer deposition (ALD) can effectively reduce the oxidation of SiC in high-temperature steam (1000 °C) environments by over 60% using a 10 nm film (Hoskins, 2018). ALD is a gas phase deposition method that is performed using repeated cycles of alternating exposures of different precursors separated by purges of the unreacted precursor (George, 2010). Precursors do not self-react but only react with functional groups present on the initial surface or the functionalized surface produced by reaction with the complementary precursor. Consequently, the deposition produced by each half-reaction only proceeds until no active sites accessible to the precursor remain on the substrate surface, making the deposition self-limiting. The ALD process controls the coating thickness to within several angstroms depending on the specific ALD chemistry, and specifically to within 1 or 2 Å for alumina ALD (Puurunen, 2003). ALD inherently deposits conformal, pin-hole free films on the surface (Weimer, 2019). Furthermore, the composition of ALD coatings can be controlled by varying the number of cycles performed during a super-cycle that is used to deposit multivalent materials with specifically desired stoichiometric ratios. The super-cycle approach requires compatible precursors and deposition conditions to successfully deposit the desired material. ALD, like CVD, is not line-of-sight limited; but, as it deposits only on the complementary functionalized surface, is self-limiting and thus will not block high aspect ratio micrometer scale pores or microchannel structures (George, 2010, Elam, 2003). Here we probe the oxidation reaction kinetics and develop a rate expression for ALD deposited ultra-thin EBCs. This is useful for the design of SiC containment materials for heat exchangers or solarthermal reactors where steam is present at up to 1150 °C.
Previous studies have investigated the oxidation kinetics of uncoated SiC in both dry oxygen and water vapor environments. These studies demonstrate that the oxidation of SiC is diffusion controlled and can be accurately modeled (Takayuki et al., 1989, Opila and Hann, 1997, Opila, 1994, Ramberg, 1996, Opila, 1999, Zheng et al., 1990, Maeda et al., 1988, Jorgensen et al., 1959, Li et al., 1990, Singhal, 1976, Takayuki, 1990, Jorgensen et al., 1961, Jacobson, 1993, Ogbuji and Opila, 1995, Tortorelli and More, 2003, Park, 2014, Costello and Tressler, 1981). Various studies performed in wet environments have concluded that the presence of water vapor increases the oxidation rate by an order of magnitude (Maeda et al., 1988, Takayuki, 1990, Jorgensen et al., 1961, Tortorelli and More, 2003). This work is the first study to investigate the steam oxidation (Eq. (1)) kinetics of SiC particles coated with alumina deposited by ALD. The kinetic analysis in this work is based on previous studies which investigated the theory behind model-free and model-fit methods as well as the errors associated with these analyses (Khawam and Flanagan, 2006, Vyazovkin and Wight, 1999, Vyazovkin, 2011).
Section snippets
Experimental methods
For this study, a particulate system with high surface area was chosen for investigation as opposed to coating only low surface area flat samples. SiC α-phase particles were obtained from Alfa Aesar. These particles were analyzed to have an average diameter of 2 μm, and a specific surface area of 10 m2/g. The particles were coated with alumina by ALD in a vibrating fluidized bed reactor (Wank et al., 2004, Hakim et al., 2005, Hakim, 2005, Ferguson et al., 2000). The reactor system with in-situ
Kinetic analysis
The resulting TG data and the extracted conversion values for each sample are presented in Fig. 1, Fig. 2, Fig. 3. The instrumental error associated with the NETZSCH TGA was determined to be ±0.32% in mass gain from triplicate experiments conducted at three temperatures. This error was taken into account within the error presented in the final kinetic parameters. For these results, mass gains are commonly on the order of 10–40% which correlates to an absolute mass gain on the order of tens of
Conclusions
Solid-state kinetic analysis was performed on three samples of SiC powders. The isothermal isoconversional method determined that the D1, D4 and F1 models accurately describe the oxidation of SiC in water vapor, although the D4 model was chosen for further analysis because it was derived to describe 3-dimensional diffusion in spherical particles. The D4 kinetic model was used to calculate kinetic parameters for an uncoated SiC powder, the SiC powder coated with Al2O3 deposited by 50 ALD cycles
Acknowledgements
This work was funded through DOE EERE grant DE-EE0006671 “Flowing Particle Bed Solarthermal Redox Process to Split Water”.
Declaration
A.W. Weimer has a significant financial interest in ALD NanoSolutions, Inc.
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