The effect of sintering temperature on the permeation
properties of the substrate was evaluated using nitrogen
permeation.
As shown in Fig. 5, the nitrogen permeance increases
linearly with pressure and is not significantly affected
by the sintering temperatures of the substrates.
For instance, the nitrogen permeance of the substrate sintered at 1400 C is
only 5e10% lower than that sintered at 1300 C, although there
is obvious changes in micro-structures (Fig. 3).
Moreover, nitrogen permeances presented in this study are about 8e9
times greater than for an asymmetric Al2O3 hollow fibre substrate
of a different structure (with thicker sponge-like layer(s)
and shorter micro-voids) and sintered at 1450 C .
Although the sintering temperatures applied in this study are
lower, the enhanced permeation properties of the substrates
should be mainly an outcome of the improved substrate
micro-structures (thinner sponge-like layer).
Nevertheless, the permeation properties of the substrate can also be highly
improved by enlarging substrate pore size.
However, the minimum thickness required to ensure the formation of a
dense defect-free Pd membrane would be much higher .
Therefore, the current substrate structure combines the benefits
of reducing mass transfer resistance and allowing the
formation of thin and integrated Pd membranes.