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.