In this thesis inorganic porous hollow fibers are studied for various applications. Because of their high surface-area-to-volume ratio, they can be used as: membranes, catalysts, electrodes, or a combiation of these.
‘Special in this PhD work is that we covered the full cycle of: fabrication, characterization and application aspects,’ says Patrick de Wit. ‘This approach came about gradually during the project, in collaboration with Rheinisch-Westfälische Technische Hochschule Aachen and various industrial partners, as members of the joint STW-DFG project program.’
As one of the highlights Patrick mentions the microgel coated SiC-C membranes. ‘These are heatable, due to their electrical conductivity,’ he says. ‘This allows tuning of their permeability and selectivity. The tuning behavior was proven to be stable and reproducible.
In this thesis 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 spun fibers undergo, to remove the polymeric binder, and to sinter the inorganic particles together.
Patrick: ‘Only at extreme temperatures of 2275 °C, the SiC particles sinter sufficiently together to obtain mechanically robust silicon carbide fibers. We performed these process steps, using special oven facilities at the Danish company LiqTech.’
Furthermore, the thesis describes a sustainable production method for inorganic porous hollow fibers that circumvents the use of organic solvents, such as N-methylpyrrolidone and dimethyl sulfoxide.
‘This method is based on ionic cross-linking of a sodium alginate polymer, in order to arrest the inorganic particles,’ Patrick says. ‘This cross-linking is carried out using multivalent cations such as Ca2+, Mg2+, Cu2+ and Al3+ that are supplied from the gelation bath. We named the method bio-ionic gelation because the sodium alginate is harvested from algae, and we used ions to form a cross-linked gel.’
Patrick: ‘Proof of concept for this method was already shown in our group, but only for a limited amount of very short fibers. Already in an early stage we showed production of several meters long fibers was feasible. Implementation of the BIG derived fibers in application devices will still take some time, I assume, as the ceramics used are brittle in nature. However BIG-I based fibers have great potential with respect to their mechanical strength. At this moment, their strength is still limited by deformations that occur during production.’
As a ‘side project’ Patrick was happy to have worked on 3D porous hollow fiber copper electrodes, for efficient and high-rate electrochemical CO2 reduction.
‘Using my fibers, research led to a publication in Nature Communications,’ he says. ‘The electrodes can be prepared via a facile method, compatible with existing large-scale production processes. New types of microtubular electrodes could be developed from there, for electrochemical processes. It would be a great result if these fibers one day could be used for applications.’
At the University of Twente, three PhD researchers associated with the Films in Fluids (FiF) and PhotoCatalytic Synthesis (PCS) groups. will continue on this work. ‘By the way, the abbreviation of the Group FiF inspired me to the title of my thesis: Funky Inorganic Fibers,’ Patrick adds.
Patrick enjoyed the freedom of organising his own research. ‘You are given the room to fill in your own research, and can get any help you need,’ he says. ‘Of course, results must be achieved. I could maintain my own way of working on the project, allowing myself some degree of organised chaos. In the end, it is the result that does count.’
From origin a chemical process engineer, during his PhD Patrick learnt a lot on the material science aspects of his work. ‘In an experimental project like this, all depends on details, making the real difference,’ he says. ‘Fabricating high quality hollow fibers, for example, sometimes depends on surprising parameters one cannot always control in advance, such as the exact humidity and temperature in the lab that very day, or some small variations in spinning mixtures. Step by step, one builds up a nice intuition in the working field.’
Patrick is attracted to a research position, either in a company or research institution. ‘During the four years of PhD research, I’ve proven to find my way in a complex, scientific subject. Therefore I now feel confident and open towards various kinds of research, knowing to be able to find my way into novel topics, building up new skills along the way.’