(799a) Nickel Aluminate Spinel Reinforced Porous Ceramic for Membrane Application

Ceramic is a suitable
material for membranes because of their high thermal and chemical stability. 
Ceramic membranes could operate without fouling, melting or swelling under
extreme conditions such as high temperature and highly acidic and basic
environment, which could not be achieved by polymeric membranes.  The brittle
nature of ceramic material, however, remains a limitation for the production
and application of ceramic membranes in an industry scale.  Brittleness is an
even more significant held back for extending the use of ceramic hollow fibre
membranes which are very small in size and have thin walls, yet have the
highest compactness among all current membrane geometries.

The reinforcement of
alumina (Al₂O₃), a common and economical ceramic used in
the industry, by nickel aluminate spinel (NiAl₂O₄) is
reported here.  This is done by the formation of NiAl₂O₄
as a new phase in between the Al₂O₃, through the addition
of nickel (II) oxide (NiO) powder in the Al₂O₃ ceramic
precursor.  The reaction between NiO and Al₂O₃ during the
sintering of precursors in air formed the new NiAl₂O₄ phase.

The porous ceramic was
produced using alumina powder and NiO powder as starting materials.  The powder
was mixed in ratios of 0 to 15wt% NiO and dry ball-milled for a homogeneous
mixture.  The powder was then pressed into bar shape and sintered at 1600^oC
in air.

X-ray Diffraction (XRD)
was used to confirm the complete transformation of NiO into NiAl₂O₄
during the sintering process.  By stoichiometry, the content of NiAl₂O₄
in the final product ranged from 0 to 42.2wt%.  The three-point-bending test
showed that as the content of NiAl₂O₄ increased, the
flexural strength of the porous ceramics increased up to a maximum of 146MPa at
14.7 wt% of of NiAl₂O₄, which is roughly four times of
that of the pure alumina sample.  The flexural strength then decreased when the
NiAl₂O₄  content is increased further.  Porosity and pore
size distribution was measured by mercury intrusion.  All the ceramics tested
had a porosity of over 30% which is appreciable for ceramic membranes.  The
sample that contained 14.7wt% of NiAl₂O₄ has the minimum
porosity out of all the sample tested, which was only 6% below that of pure
alumina.  Samples with higher amount of NiAl₂O₄ than this
have porosity higher than pure alumina.  The pore size distribution of pure
alumina, 14.7wt% of NiAl₂O₄ and 42.2wt% of NiAl₂O₄
all fell between the range 0.1-1.6 µm which is within the microfiltration
range.  The SEM images of the cross sections of the sample shows that as the
content of NiAl₂O₄ went over 14.7wt%, loose particles
appeared.  These loose particles hindered the close packing of particles as
well as shrinkage during sintering, which lead to a drop in flexural strength
and increase in porosity.  A nitrogen permeance test was carried out to further
confirm the feasibility of using NiAl₂O₄ reinforced
porous alumina as a membrane.  For all the samples tested, the nitrogen
permeance increased with increasing applied pressure.  This indicated the
macroporous nature of this ceramic, which supported the pore size distributions
obtained by mercury intrusion.  The nitrogen permeance of the 14.7wt% NiAl₂O₄
sample was found to be 75% of that of pure alumina.

As a conclusion, the
formation of NiAl₂O₄ in porous alumina successfully
increased the flexural of alumina, while the porosity and gas permeance was
still satisfying.  NiAl₂O₄ reinforcement is therefore a
feasible method for enhancing the flexural strength of ceramic membranes. 
Future work will be carried out to extend this reinforcement of porous ceramic
into ceramic hollow fibre membrane fabrication.