Computational and Experimental Study of Thermodynamics of the Reaction of Titania and Water at High Temperatures

Abstract

Gaseous titanium hydroxide and oxyhydroxide species were studied with quantum chemical methods. The results are used in conjunction with an experimental transpiration study of titanium dioxide (TiO) in water vapor-containing environments at elevated temperatures to provide a thermodynamic description of the Ti(OH)(g) and TiO(OH)(g) species. The geometry and harmonic vibrational frequencies of these species were computed using the coupled-cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)]. For the OH bending and rotation, the B3LYP density functional theory was used to compute corrections to the harmonic approximations. These results were combined to determine the enthalpy of formation. Experimentally, the transpiration method was used with water contents from 0 to 76 mol % in oxygen or argon carrier gases for 20-250 h exposure times at 1473-1673 K. Results indicate that oxygen is not a key contributor to volatilization, and the primary reaction for volatilization in this temperature range is TiO(s) + HO(g) = TiO(OH)(g). Data were analyzed with both the second and third law methods using the thermal functions derived from the theoretical calculations. The third law enthalpy of formation at 298.15 K for TiO(OH)(g) at 298 K was -838.9 ± 6.5 kJ/mol, which compares favorably to the theoretical calculation of -838.7 ± 25 kJ/mol. We recommend the experimentally derived third law enthalpy of formation at 298.15 K for TiO(OH), the computed entropy of 320.67 J/mol·K, and the computed heat capacity [149.192 + (-0.02539)T + (8.28697 × 10)T + (-15614.05)/T + (-5.2182 × 10)/T] J/mol-K, where T is the temperature in K.

Environmental resistance of a Ti2AlC-type MAX phase in a high pressure burner rig

“…Furthermore, only one experiment (preoxidized sample) directly lends itself to this discussion since the other tests exhibit only weight gains. Volatility rates of oxides can be calculated from the equilibrium vapor pressure of the hydroxide species and the gas conditions in the burner using the mass transport equation for laminar (or turbulent) flow in a moving boundary layer [32][33][34]1,4]. Using published values for the thermodynamic energy of compound formation, these pressures and fluxes were calculated (i.e., Jacobson) and listed in Table 3 [32, 37,38].…”

“…Some uncertainty has been raised regarding the thermodynamic data used for TiO(OH) 2 due in part to analysis difficulties for the low yields of condensed species in the transpiration experiment [4,32]. Thus it appears that experimental rates are higher than those predicted for Ti-oxide losses in water vapor.…”

Matrix Development for Water Vapor Resistant SiC-Based Ceramic Matrix Composites

“…Weight 111 , however they are significantly higher than the flux calculated for these test conditions using thermodynamic data of Nguyen et al 110 .…”

“…A transpiration study by Nguyen et al 110 concluded that TiO2 (rutile) reacts with water vapor by Equation (3.1),…”

“…The gas concentrations,  and ', are calculated using the ideal gas law. Partial pressures of Si(OH)4 (g), TiO(OH)2 (g) and Y(OH)3 (g) were calculated using data from Plyasunov 109 , Nguyen et al 110 and Krikorian 113 respectively. The gas velocity was modeled using ANSYS CFX as described in Section 2.3.…”