Diffusion Mullite

FIGURE 13. Diffuse reflectance infrared Fourier transform spectra of OOPS mullite precursor heated to selected temperatures (air/10°C/min/l h dwell)... 

The abundance of mullite in the mullite/ZTA system increased with increasing infiltration time [Low et al., 1993]. The density (p) and mullite content of the sintered sample as a function of infiltration time are shown in Table 5.2. The results suggest that the infiltration process was time (t) dependent and diffusion-controlled with the infiltration front travelled as a function of t112. The content of mullite was greatest near the surface and decreased sharply towards the core of the sample. 

The underline beneath Si02 indicates that it can exist at subunit activities as either mullite (discussed in the next section) or an aluminosilicate liquid. While reaction (4) is written in terms of 02(g) and CO(g), this does not mean that they exist as gaseous species at the reaction site or that oxygen diffuses as a... 

At temperatures of about 1300 °C and higher, only mullite and the melt are equilibrium phases. In fact, however, porcelain usually contains the non-equilibrium phase Si02 in the form of quartz. This is due to the very high melt viscosity and the low diffusion coefficient for S1O2 dissolution of quartz particles, which is controlled by diffusion, is therefore very slow, even though, according to the equilibria, all the Si02 should dissolve. 

The dehydration of kaolinite has been the subject of several kinetic studies and Brett et al. [1] summarize the salient features of the mechanisms proposed for the sequence of reactions by which kaolinite is converted to mullite (920 to 1370 K). The first step, water loss, is most satisfactorily described by a two-dimensional diffusion equation. Brindley et al. [57] proposed this model from isothermal kinetic measurements (670 to 810 K) and reported a marked increase of in a maintained pressure of water vapour. Anthony and Gam [58] concluded that random nucleation is rate limiting at low pressures of water vapour and that this accounts for reports of first-order kinetic behaviour. Increase in the rate of nucleation, as the (HjO) is increased, is ascribed to a proton transfer mechanism, and acceleration of the growth process may result from contributions due to the onset of the reverse reaction. 

Given that mullite is a defect structure, one would expect high ionic conductivity. Rommerskirchen et al. have found that mullite has ionic conductivity superior to that of the usual CaO-stabilized Zr02 solid electrolytes at temperatures from 1,400 to 1,600°C [52], The oxygen self diffusion coefficient in the range 1,100 single crystal of 3 2 mullite has been given by [53] ... 

The activation energy for silicon diffusion during the formation of mullite from fused couples at 1,600 < T < 1,800°C [55] is in the range of 730 diffusion coefficients are much higher than those of silicon at temperatures above the mullite-silica eutectic [56],... 

P. Fielitz, G. Borchardt, M. Schmuecker, H. Schneider, and P. Willich, Measurement of oxygen grain boundary diffusion in mullite ceramics by SIMS depth profiling, Appl. Surf. Sci. 203-204, 639-643 (2003). 

The thermal diffusivity of mullite with 0 to 30% SiC whiskers was detamined from room temperature to 1500°C. Data are presented in Fig. 4 for measurements made perpendicular to the hot-pressing direction. It is seen that the SiC whiskers increase the thermal diffusivity at room temperature, but values decrease for all compositions with increasing temperature. Similar trends are seen for values measured parallel to the hot-pressing direction, but values were lower. Lower values were expected because the whiskers orient perpendicularly to the hot-pressing direction during hot pressing. 

The thermal diffusivity of mulhte-ZrOa with and without 30% SiC whiskers was determined atroom and elevated temperatures, as sear in Fig. 5. At room temperature thermal diffusivity values for the matrix material show little anisotrophy. These values generally increase with increasing monoclinic Zr02 phase, consistent with higher values measured for the monoclinic phase. Thermal diffusivity values for mullite fall midway in the range of values for the mullite-ZrOa matrices used in this study. 

The addition of SiC whiskas generally results in a significant increase in thermal diffusivity values over those measured for the unremfoiced matrix. Anisotrophy in thennal diffusivity values also increases with increasing SiC whiska" content. The same general trends were found for SiC whiska" additions to mullite. ... 

FIGURE 5. Thermal diffusivity parallel and perpendicular to the hot-pressing direction for mullite-Zr02-SiC whisker composites, as a function of temperature. Published with permission of the J. Am. Ceram. Soc. (26)... 

---