Mass transfer in polycrystalline alumina under oxygen potential gradients at high temperatures

Abstract

The mass-transfer mechanisms of polycrystalline alumina with and without oxygen reactive elements (REs) such as Ln (Y and Lu) and Hf were investigated by evaluation of the oxygen permeability through alumina wafers, which served as a model for alumina scale, at accelerated temperatures up to 1923 K. Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher oxygen partial pressure [P O2 (hi)] surface side to the lower P O2 [P O2 (lo)] surface side, along with the simultaneous GB diffusion of aluminum in the opposite direction, maintaining the GibbsDuhem relationship. The chemical potentials, GB diffusion coefficients, and fluxes of oxygen and aluminum in alumina wafers with applied oxygen potential gradients (d® O ) were calculated from the oxygen permeability constants. The fluxes of oxygen and aluminum at the outflow side of the wafer were significantly larger than those at the inflow side. Ln and Hf segregation at the GBs selectively reduced the diffusivity of oxygen and aluminum, respectively. Thus, the mesoscopic dopant arrangements, which were selected by taking into consideration the behavior of the diffusion species and the role of dopants, enabled the alumina layers to have enhanced oxygen shielding capability at high temperatures. Furthermore, the GB diffusion data derived from the oxygen permeation experiments were compared to those for alumina scale formed by the so-called two-stage oxidation of alumina-forming alloys.

Grain Boundary Engineering of Alumina Ceramics

“…Therefore, it is extremely difficult to quantitatively determine the degree of influence for individual factors that influence the movement of each diffusion species. The oxygen permeability technique with polycrystalline α-alumina wafer, which served as a model scale, is thus expected to be very useful to accurately evaluate mass-transfer through the wafers because the dµ O applied to the wafers and the diffusion length are constant [16][17][18][19][20][21][22][23][24].…”

“…Polycrystalline alumina wafer specimens with or without REs such as Ln (Lu, Y) and Hf, which were cut from the sintered bodies and polished to a mirror-like finish, served as a model scale for the measurement of oxygen permeability constants using a technique described in detail elsewhere [16][17][18][19][20][21][22][23][24]. Ln doping was expected to effectively retard mass-transfer in alumina under application of a dµ O because Ln can significantly improve high-temperature GB creep resistance in polycrystalline alumina [25][26][27].…”

“…Furthermore, mass-transfer along single GBs in two types of non-doped alumina bicrystal wafers was also evaluated by the oxygen permeation technique to clarify the correlation between the mass-transfer along each GB and the GB structural characteristics [18]. Figure 11.1 shows a schematic diagram of the oxygen permeability apparatus [23]. Each wafer specimen was placed between two alumina tubes under an Ar gas flow in a furnace, with Pt gaskets to create a seal between the wafer and the tubes.…”

“…Each wafer specimen was placed between two alumina tubes under an Ar gas flow in a furnace, with Pt gaskets to create a seal between the wafer and the tubes. [23] The P O2 , included as an impurity in the Ar gas, was monitored at the outlets of the upper and lower chambers that enclosed the wafer and the alumina tubes using a zirconia oxygen sensor at 973 K. The partial pressure of water vapor (P H2O ), another impurity in the Ar gas, was measured at room temperature using an optical dew point sensor. A gas-tight seal was achieved in both chambers by heating to 1893-1923 K, after which the wafer was kept at temperatures above 1773 K for 3 h in Ar at a flow rate of 1.67 × 10 −6 m 3 /s 1 for measurement of the oxygen permeability constants.…”

“…In the present work, all β values were larger than 100 and thus met the requirement. Figure 11.2 shows the effect of the steady-state P O2 in the upper chamber on the oxygen permeability constants of non-doped and RE-doped samples [16,17,19,23]. P O2 in the lower chamber was held constant at approximately 1 Pa.…”