Elsevier

Journal of Membrane Science

Volume 483, 1 June 2015, Pages 1-14
Journal of Membrane Science

Formation of micro-channels in ceramic membranes – Spatial structure, simulation, and potential use in water treatment

https://doi.org/10.1016/j.memsci.2015.02.023Get rights and content

Highlights

  • Alumina disc membranes with different micro-channels were fabricated.

  • They can be cylindrical and tightly packed, or pear shaped and conical.

  • They display a regular and periodic distribution with a hierarchical structure in cross section.

  • Their characteristics were mimicked using the Rayleigh-Taylor theory with reasonable accuracy.

  • The presence of micro-channels reduce the mass transfer resistance.

Abstract

In this study, alumina membranes with distinct types of micro-channels have been developed via a fingering induced phase inversion and sintering technique. The designed membrane morphologies were achieved by using five different solvents DMSO, NMP, DMAc, DMF and TEP, which led to unique changes in the macro- and microstructures of the membranes. The micro-channel shapes vary from long, straight, cylindrical and densely packed (DMSO) to pear-shaped conical structures (NMP and DMAc). When DMF and TEP were used, symmetric membranes with sponge-like structure were formed. These micro-channels display a regular and periodic distribution and also have a hierarchical spatial structure with a distribution in number, length and width along the depth of the membranes. Dead end water permeation tests reveal that the micro-channels can greatly reduce the resistance to water permeation. Furthermore, the microstructures also vary with a change in solvent, and different membrane pore sizes were observed. The initialisation of the micro-channels was interpreted using the Rayleigh–Taylor Instability, driven by acceleration on the interface and facilitated by a difference in density between the suspension and the coagulant. The spacing between the micro-channels and their hierarchical structure was quantitatively mimicked using R–T Instability theory and the simulation results matched the experimental results reasonably well.

Introduction

The use of porous ceramic membranes in conventional separation technologies for water treatment such as micro- and ultrafiltration processes has shown improved long-term reliability over polymeric membranes [1]. Their many advantages such as superior chemical and thermal stabilities and mechanical robustness allow them to be cleaned easily and operated over extended periods of time. They can therefore be a reliable alternative to use in membrane bioreactors, a fast growing market in water treatment technologies [2], whereby frequent and strenuous cleaning regimes are needed. They can also be combined with ion exchange, UV disinfection or activated carbon, etc., to form other hybrid systems that are gaining attention as cost and space saving alternatives to the long conventional treatment trains for water treatments [3], [4], [5], [6].

Currently, the main deterrent for the use of ceramic membranes in large-scale applications is their high fabrication costs when produced via the conventional multi-step fabrication method [7], [8]. The conventional multi-step method is highly time and energy consuming and contributes generously to the high capital cost of commercial ceramic membranes. Furthermore, these commercially available ceramic membranes are most often found in the tubular or flat sheet configurations, which offer much lower packing densities compared to hollow fibre or spiral-wound flat sheet polymeric membranes [7]. The fingering induced phase inversion and sintering method is a relatively new method for producing asymmetric hollow fibre ceramic membranes and considerably reduces the number of steps in the fabrication process. More details on this method can be found in the literature [7], [9]. The steps are greatly reduced by eliminating the need to deposit additional layers on a substrate to achieve micro/ultrafiltration selectivity. This method allows facile fabrication of ceramic hollow fibre membranes, which significantly improves the packing density. Hence fabrication costs can be potentially reduced and productivity improved. Also the ability to produce a wide range of different membrane microstructures means that they can be tailored according to the application [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26].

The morphology of the membranes formed via the fingering induced phase inversion and sintering method is unique and advantageous over commercial ceramic membranes. Not only do they consist of the sponge-like structures formed by the packing of the ceramic particles, much larger finger-like structures can also be formed in the same membrane in one step, similar to those in Loeb–Sourirajan polymeric membranes [27]. For polymeric membranes, these fingers are undesired as they are generally in the form of macrovoids and can reduce the membrane performance as well as mechanical stability. However, the finger-like structures observed in ceramic membranes can resemble micro-channels, which can be long, straight, cylindrical and highly packed. In terms of filtration, these micro-channels can reduce the trans-membrane resistance to permeate flow and the sponge-like structures act as separation layers to offer the desired selectivity. Due to the advantages offered by these ordered micro-channels, they have widened the application of ceramic membranes, with examples such as hollow fibre membrane micro-reactors, solid-oxide fuel cells, membrane contactors, gas separation, etc. [16], [19], [21], [24], [28]. Considering the different characteristics and roles of these finger-like voids in ceramic and polymeric membranes, the term ‘micro-channels’ is used for ceramic membranes in this article, and ‘finger-like voids’ is used for polymeric membranes, even if the origin of this structure is the same for both cases.

For the ceramic membranes made via the phase inversion and sintering method, much literature is available investigating the effects of different fabrication parameters on the range of cross-section morphologies achievable for various inorganic materials and applications [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. However, an improved understanding of the formation mechanism behind these different macro- and microstructures is still needed in order to be able to accurately control and tailor the membrane characteristics according to the application. This is important in order to improve the reproducibility of the fabrication process before large-scale production of these unique asymmetric membranes can be achieved. Currently there is little consensus on the mechanisms responsible for the formation of the micro-channels.

For polymeric systems, there is a rich history of researches carried out with various different theories used to postulate and predict the formation mechanism as well as characteristics of the finger-like voids. For example, one popular hypothesis is the skin rupture theory, whereby large mechanical stresses experienced by the thin skin during immersion precipitation can lead to points of rupture in the skin from which fingers can be initiated and propagate [29]. Another popular theory attributes the fingers to the growth of nucleated polymer-lean phases in polymer solutions under the circumstance of instantaneous demixing. The theories were built upon from with more observations and characterisations [30], [31]. However, these theories do not explain the ordered and periodic spacing between the finger-like structures as well as their hierarchical distribution along the depth of the membrane. Furthermore, with polymeric membranes, there is only very limited quantitative data available on the periodic distance between the fingers, despite the unanimous observation of this characteristic from many researchers, due to practical constraints. Therefore other theories put forward to explain the mechanisms behind finger formation also underwent the same bottlenecks with limited supporting experimental data.

Possible origins of the micro-channels are interfacial instabilities, which can arise from many different sources, such as interfacial tension gradients (Marangoni instability) [32], viscosity gradients (viscous fingering) [33], or density gradients (Rayleigh–Taylor Instability (RTI)) [34], [35], [36], and so on. Interfacial instabilities caused by periodic perturbations experienced by the surface of the suspension solution during immersion into another fluid medium can explain the periodic and hierarchical structure of the micro-channels. One possible type of interfacial instability that may cause the formation of the micro-channels is the Rayleigh–Taylor Instability (RTI). It is a naturally occurring phenomenon whereby periodic perturbations at the interface of two fluids with opposite pressure and density gradients destabilise the interface due to acceleration on the interface and can form finger-like structures. In later parts of the article, it can be seen that on the suspension/coagulant phase-inversing interface the requirements for RTI to occur are fulfilled and micro-channels can be initialised due to the instability.

This study aims to gain more fundamental information regarding the formation of the micro-channels in ceramic membranes by reducing the amount of parameters affecting the phase inversion process. This was achieved by fabricating flat disc ceramic membranes and investigating the effects of using five different solvents as the suspension medium on the final membrane macro- and microstructures, and in particular, the properties of the micro-channels. For the first time, the micro-channels in ceramic membranes have been quantified with their stratified pore densities and periodic distances obtained. An indication of the effect of differences in morphology on membrane performance in possible water treatment was thus investigated by looking at the water permeation fluxes of the 5 membranes. Then a possible mechanism incorporating RTI is proposed to explain the formation of the different membrane microstructures and preliminary experimental values for the characteristic wavelengths of the different membranes were obtained. These values were mimicked using simulations based on RTI with the conditions of the suspension/solvent/non-solvent system that can be characterised. A possible direction for controlling and engineering the different microstructures is thus suggested.

Section snippets

Materials

Aluminium oxide powders of 1 μm (alpha, 99.9% metals basis, surface area 6–8 m2/g) were purchased from Alfa Aesar (a Johnson Matthey company) and used as supplied. Polyethersulfone (PESf, Radal A300, Ameco Performance, USA) and Arlacel P135 (polyethyleneglycol 30–dipolyhydroxystearate, Uniqema) were used as binder and dispersant, respectively. Dimethyl Sulphoxide (HPLC grade, VWR International), N-Methyl-2-pyyrolidone (HPLC grade, VWR International), Dimethylacetamide (HPLC grade, Sigma Aldrich),

Rheology of ceramic suspensions

Table 1 shows the viscosity values of the 5 suspensions consisting of different solvents at a shear rate of 1.45 s−1. Suspension-TEP has a viscosity over the range at which the viscometer was capable of measuring. During the preparation process difficulty was encountered when suspending the alumina particles in TEP, and solvation of the PESf in TEP was only achieved at 80 °C.

It can be seen clearly that the viscosity varies significantly when different solvents are used to disperse the alumina

The hierarchical micro-channel structure and linear stability theory

From the description in Section 3.2, it is very clear that the micro-channels formed during the phase-inversion process have the following two features:

  • The micro-channels distribute uniformly on the planes parallel to the surface and the channels form regular patterns.

  • The micro-channels can be distinguished into multiple levels in terms of length, diameter and density.

The first feature has long been noticed in polymer phase-inversion membranes, and some researchers have related it to the

Conclusions and remarks

The influence of the solvent on the structure of phase-inversion ceramic disc membranes and the formation of micro-channels in them has been investigated. Directly from the experimental results, we may summarise the effects of the solvents as follows:

  • 1.

    The choice of the solvent determines the macrostructure of the membranes, cylindrical micro-channels were formed when DMSO is the solvent, and pear or tear-like micro-channels were formed when NMP or DMAc are the solvent. Symmetric

Acknowledgements

The authors gratefully acknowledges the research funding provided by EPSRC in the United Kingdom (Grant no. EP/J014974/1) and B. Wang gratefully acknowledges the Marie Curie International Incoming Fellowships (Grant no. 627591).

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