Secondary Ion Mass Spectroscopy Study of Oxygen‐18 Tracer Diffusion in 2/1‐Mullite Single Crystals

Abstract

Oxygen 18 O tracer diffusion in Czochralski-grown mullite single crystals is investigated along [010] and [001]. Oxygen diffusion coefficients range between ϳ5 ؋ 10 ؊20 m 2 /s (1250°C) and ϳ9 ؋ 10 ؊18 m 2 /s (1525°C). The data does not show any significant anisotropy. The values of the activation enthalpy (4.5 eV) and of the activation entropy ((3.4 ؎ 1.6)k B , where k B is the Boltzmann constant) suggest that the atomic transport occurs via thermally activated vacancies.

Creep of mullite zirconia composites

“…The values obtained hereespecially for the reference mullite -are then consistent with the lower range reported for mullites [7,16]. This lower value is very similar to the activation energy for oxygen diffusion in single crystal mullite [12], which has been measured as 397 kJ mol −1 [22] and 433 ± 21 kJ mol −1 [23] by two independent methods. The majority of data given in the literature is taken from creep under atmospheric conditions, whereas these experiments were done under vacuum in the presence of graphite.…”

“…In this work the samples were coated in boron nitride to reduce contamination and thus from the graphite push rods, minimizing the reaction toward the alumina rich region of mullite. Any small movement toward the alumina rich region would give minimal change to the creep rate as shown by Fielitz et al [23]. XRD data (not shown) confirms that the samples are unchanged after creep tests.…”

Structure and properties of mullite—A review

“…21 at temperatures above 1200 • C. [49][50][51][52] If the structural oxygen vacancies in mullite are part of the path ways for Al, Si, and O migration, the diffusion coefficients parallel c-axis should be higher than the corresponding ones parallel to b-axis. This follows from the fact that the density of structural oxygen vacancies occurring along [0 0 1] is higher than perpendicular to it.…”

Kinetics of alumina segregation in mullite ceramics

“…Tracer diffusivity studies in single crystalline mullite [2][3][4][5] show that silicon is the slowest species compared to oxygen and aluminium. Therefore, we can neglect Si 4+ ion fluxes and suppose that Al 2 O 3 is transported via coupled Al 3+ and O 2− ion fluxes through the single crystalline mullite grains.…”

“…Thus, for a deeper insight into diffusion-related processes we have carried out comprehensive tracer diffusion experiments ( 18 O, 30 Si, 26 Al) within the last few years. [2][3][4] Recently, we have presented a consistent reaction model for the solid state formation of mullite basing upon the tracer diffusivity data. 5 The aim of the present paper is to analyse compositional variations of mullite crystals in the light of the diffusivity of the involved atomic species.…”