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PhD defence Patrick de Wit

Funky inorganic fibers 

Inorganic porous hollow fibers are interesting for various applications that could benefit from a high surface-area-to-volume ratio, such as membranes, catalysts, electrodes, or a combination of these.

The introduction starts with an overview of conceivable materials and applications for inorganic porous hollow fibers, followed by a brief account of the major methods that are currently used to fabricate such fibers. Particular emphasis is given to the dry-wet spinning of polymer/solvent/particle mixtures into a coagulation bath. Next, it discusses the intricacies of the thermal treatment that the spun fibers undergo to remove the polymeric binder and to sinter the inorganic particles together. Finally, the chapter provides the scope and outline of the thesis. 

The thesis describes a production method for the fabrication of silicon carbide (SiC) hollow fibers by non-solvent induced phase separation. This method produces fibers with sufficient mechanical strength after thermal treatment at temperatures of 1500 °C  in argon. The fibers still contain a substantial amount of residual carbon that can be removed with additional thermal treatment at temperatures in the range of 1790-2075 °C. Removal of the residual carbon results in a loss of mechanical strength. Only at extreme temperatures of 2075 °C, the SiC particles sinter sufficiently together to obtain a mechanically robust silicon carbide fiber. The fibers showed a 4-point bending strength of 30-40 MPa, together with extremely high clean water fluxes of 50000 l m-1h-1bar-1. These silicon carbide fibers can be used directly as a microfiltration membrane, or as a membrane support.

Furthermore, it describes a production method for inorganic porous hollow fibers that circumvents the use of organic solvents, such as N-methylpyrrolidone or dimethyl sulfoxide. The method is based on ionic cross-linking of a sodium alginate polymer in order to arrest the inorganic particles. This cross-linking is carried out using multivalent cations such as Ca2+, Mg2+, Cu2+ and Al3+ that are supplied from the gelation bath. In contrast to non-solvent induced phase separation, ionic cross-linking circumvents the formation of a polymer-lean phase and the associated large macrovoids in the fiber wall. In addition, the introduced multivalent ion persists in the fiber after thermal treatment, allowing the facile incorporation of functional metal oxides on the pore surface of the fiber.

Chapter 4 presents a modification of the ionic cross-linking that is discussed earlier. Here, the multivalent cations are added directly to the spinning mixture in the form of an insoluble carbonate salt. This mixture is then spun into an acidic gelation bath, where the low pH triggers the dissociation of the carbonate into multivalent cations and carbon dioxide. The multivalent ions cross-link the alginate, thereby consolidating the 3D structure. Adequate gelation requires a sufficiently low pH of the acid bath and a sufficient buffering capacity of the acid. In order to facilitate proper cross-linking, it is crucial that the acid has a conjugated base with limited propensity for complexing the cations. Lactic and acidic acid are shown to be suitable acids for this method. The fibers prepared via this method show outstanding properties, such as high mechanical strength, a homogeneous morphology, and a sharp distribution of narrow pores.

A part of the thesis is dedicated to the effect of different measurement geometries on the measured mechanical strength of Al2O3 porous hollow fibers. The value obtained for the mechanical strength depends strongly on the measurement method; values from  3-point bending tests are systematically lower as compared to values from 4-point bending tests. The specimen size also influences the measured value; a larger span size systematically results in lower strength values. A statistical analysis of the strength data has been conducted to attain the failure probability of the fibers. It is found that fibers prepared using phase inversion do not necessarily follow the Weibull model and other models (e.g., normal or log-normal) have to be considered. In particular for systems design it is important that the statistical representation of the strength distribution is accurate. An inappropriate distribution may predict the wrong design strength, potentially resulting in premature failure.

The thesis continues on the statistics associated with the mechanical strength of inorganic porous hollow fibers. It investigates the effect of production methods, and the resultant micro structures, on the mechanical strength using a standardized 4-point bending test. Fibers were prepared using non-solvent induced phase separation (NIPS), internal, and external bio-ionic gelation (BIG-I and BIG-E). Fibers prepared using BIG-I seem to have a larger bending strength compared to fibers prepared using NIPS or BIG-E, yet have a larger scatter in their strength data. This greater strength originates from better stacking of the inorganic particles, caused by the low pH used during their fabrication. The low pH results in a surface charge of the particles facilitating a more homogenous stacking. To predict failure behavior, statistical models are fitted to the measured strength data. All production methods result in fibers of which the strength distribution appears to follows a Weibull model, in which failure occurs at the weakest-link. The BIG-I fibers have a large scatter in their strength data, which is likely due to surface deformations present in the fiber wall that act as a weak link. If the strength data is re-analyzed with the surface-deformed fibers excluded, the BIG-I fibers no longer follow the Weibull model but start to follow a normal distribution. This shows that BIG-I based fibers have great potential with respect to their mechanical strength. At this moment, their strength is limited by deformations that occur during production, contrary to NIPS fibers where inherent macrovoids and less ideal stacking of the particles cause the weakness.

Finally, the use of the fibers is discussed. First, the use of electrically conductive silicon carbide-carbon fibers to adjust membrane selectivity and permeability. On the surface of this fiber, thermo-responsive poly(N-vinylcaprolactam) (P-VCL) microgels have been immobilized. The permeability and selectivity of the membrane can be adjusted by controlling the applied electrical power to the membrane. The thermo-responsiveness is reversible and stable in all the conducted experiments. No change in permeability over time is observed, indicating inconsiderable microgel loss. Also during backwash the permeability remains constant. The hydraulic resistance of the membrane is affected by the hydrodynamic radius of the microgel. Electrical heating of the membrane is found to be 14 % more energy efficient compared to heating of the whole feed stream, when operating in crossflow conditions.

The prerequisites in order to use inorganic porous hollow fibers as a support material for thin films prepared by interfacial polymerization are discussed. By modification of the surface with multiple inorganic repair layers, a (poly)amide layer is prepared on the outside of the fiber. Defect free films are only obtained when the fiber is coated with γ-alumina, which increases the amount of hydroxyl-groups on the surface and provides a large volume of small pores for the aqueous phase. The hydroxyl groups allow for covalent attachment of the film to the ceramic substrate. In the fabrication process, the vertical drying step after immersion in the aqueous phase is identified to be critical for obtaining a high quality layer. Inadequate drying (locally) results in excess of the aqueous phase on the outer wall of the fiber, causing film formation to occur at a distance from the ceramic fiber and preventing the hydroxyl groups to participate in the polymerization. The prepared fibers showed acceptable clean water fluxes (2-4 l m-1h-1bar-1) and good retention of Rose Bengal dye (1017 g mol-1).

The thesis finishes with reflections on the main findings and attempts to put the results in perspective. Also, suggestions are given for possible routes for further research, focusing on functional fibers and the application thereof.