The electromotive force (e. m. f.) of mullite ceramics with a density of 97.9% T. D. as a solid electrolyte for oxygen concentration cell was measured between 873 and 1573K in the oxygen pressure range of 1.0•~10.5 to 1.0•~10-2Pa. The measured e. m. f. indicates that the ionic transport number (ti) of oxygen is higher than 0.8 above 1273K at oxygen pressures above 1.0•~102.5Pa. The ti decreased with decreasing oxygen pressure and was not sensitive to oxygen pressure below 1073K. Above results were discussed in relation to the elec tronic conduction due to thermal decomposition of lat tice oxygen of mullite in a reduced atmosphere.
The electromotive force (EMF) of an oxygen concentra tion cell using Si2-A23 ceramics (x=1, 2, 3 and 4 in xAl232Si2)as a solid electrolyte was measured at 1073-1573K in the oxygen pressure range of 1.0 10-1.0 to 1.0103.5Paat the anode side. The Si2-A23 ceramics (96.2-98.5% of theoretical density) sintered at 1873-1903K in air were annealed at 1673K for 8h to produce equilibrium phases before EMF measurement. Air as a reference gas and Ar-O2 mixed gas were in troduced to cathode and anode sides, respectively. The measured EMF indicates that ionic transport number (ti) of oxygen is higher in the following order: mullit SiO2 composite (x=1, 2)>mullite (x=3)>mullit A23 composite (x=4). At lower oxygen pressures, t decreased with decreasing oxygen pressure due to for mation of electrons available for conduction. The criti cal oxygen pressure for t1 was 101.8 to 102.7Pa for x=1, 2. The concentration of electrons formed in creased in proportional to P-1/4 at 1473-1573K, and P-1/6 at 1073K.
The interfaces of Si-TiC -O fiber/mullite matrix com posite hot-pressed at 1500•Ž in N2 atmosphere were characterized by analytical transmission electron microscopy. Partial decomposition of Si-TiC -O fibers produced graphite layers 30-60 nm thick on the sur face of fibers and crystalline TiC -N particles of about 800 nm in size at the triple points among fibers. Diffu sion of Al and O from mullite matrix into Si-TiC -O fibers was interpreted as being due to the reaction be tween mullite and C or SiC segregated on the surface of fibers.
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