Morphological, mechanical and gas transport properties of stainless steel and composite hollow fibres

Abstract

This work focuses on the development of stainless steel (SS) and SS composite hollow fibres (with carbon (CSS) and alumina (CASS)) as novel, robust inorganic membranes prepared via dry-wet phase inversion from a spinning dope containing a mixture of SS and/or alumina particles, polymeric binders and solvents. The morphological features and mechanical properties of the hollow fibres were evaluated by varying the spinning dope composition, including: different binders; SS-to-alumina volume, binder-to-solvent, and binder-to-SS particle ratios; the spinning dope viscosity; and the SS particle size (6-45m); as well as sintering parameters such as temperature, atmosphere and time. It was found that the addition of larger particles to the spinning dope favoured the creation of large macrovoids in both the inner and outer shells of the hollow fibre. In contrast, small particles delayed de-mixing, forming sponge-like regions at the outer shell, although the finger-like macrovoids were retained at the lumen. Polyvinylpyrrolidone (PVP) was used as a viscosity enhancer, altering the kinetics of the phase inversion process, leading to an increase in finger-like macrovoids with increasing PVP addition. Polyetherimide (PEI) was preferred as the polymeric binder due to its favourable phase inversion kinetics. Conversely, polyethersulfone promoted faster de-mixing resulting in finger-like macrovoids at both surfaces. The morphological structure of the sintered SS hollow fibres did mimic the morphology of the green fibres. Sintering was controlled by SS mass diffusion limitations, lower densification was achieved at 950°C with necks forming between particles in close contact. At 1000°C, surface diffusion became important to densification. At 1050 and 1100°C, smaller pores in the sponge-like region started closing whilst larger finger-like pores and macrovoids remained open as the inter-particle space was too wide to be filled by surface diffusion. Densification, as function of mass diffusion, was accelerated for smaller particles and retarded for larger particles, showing an inverse impact in the total surface area available for diffusion. Fibres with SS particle loadings below 50wt% showed irregular geometries. However at 70wt% particle loading the required particle packing condition was achieved to form inter-particle necks during sintering. The mechanical resistance of the hollow fibres was closely related to morphology and densification. Samples with high porosity showed low mechanical strength, especially for porosity related to finger-like macrovoids, whereas smaller pores (sponge-like region) resulted in higher mechanical strengths. Higher densification led to stronger hollow fibres, due to bold necks that could withstand higher loads. CSS hollow fibres were created by pyrolysing the polymeric binder during the sintering process, resulting in inter-particle pore filling with the degraded and retained carbon. The morphology of the CSS hollow fibres resembles the green fibres after the sintering/pyrolysis process, though some differences appeared. The finger-like structure in the lumen disappeared due to carbon filling the pores and/or densification. However, round macrovoids in the middle wall of the CSS hollow fibres remained as they were too large to close. CSS hollow fibres made from the 6µm SS particles demonstrated a quasi-bi-modal pore size distribution dominated by sponge-like structures at both surfaces. Increasing the particle size resulted in a multi-modal pore size distribution, with 45µm SS particles yielding a high porosity and larger macrovoids. Incorporating alumina particles into the spinning dope resulted in the formation of a bi-modal pore size distribution, independent of the alumina/SS particle ratio. The large pores were predominantly finger-like structures at the lumen, indicating that the smaller alumina particles (0.5µm) densified less than the CSS fibres. Additionally, increasing the amount of alumina allowed the formation of a closely packed carbon-alumina region predominantly within the sponge-like region of the hollow fibre. The mechanical properties of the CSS and CASS were affected by the fact that carbon and carbon/alumina inhibited sintering, thus a reduction in both strength and flexibility of the CSS and CASS were obtained when compared to SS hollow fibres. Finally, the hollow fibres were tested for single gas permeation (20-100°C), and binary gas mixtures (75-150°C). Adsorption of nitrogen and carbon dioxide was investigated to understand the synergistic effect of the composite material on the transport of gases. This work shows that CSS hollow fibres did not adsorb N2. The isosteric heat of adsorption for CO2 was much higher for the hollow fibres containing only SS as opposed to alumina particles. It was observed that the CSS hollow fibres separated N2 from CO2 at low feed concentrations (less than 20/80 CO2 to N2). This was attributed to the strong interaction of CO2 with the surface of the CSS hollow fibre, whilst the non-adsorbable N2 flowed unimpeded through the pores. This behaviour is anomalous for mesoporous or macroporous materials such as the CSS hollow fibres, and only possible due to the synergistic effect of the SS and CO2 materials. This separation effect was not observed in the composite fibres containing alumina, as N2 adsorption was noticeable resulting in high heat of adsorption.