Short Review on Porous Metal Membranes—Fabrication, Commercial Products, and Applications

While there are many widespread suppliers and uses of inorganic membranes made from ceramics, there are limited suppliers and applications of metal membranes despite their unique advantages over all membrane materials, including ceramics. Research on metal membranes is currently being intensified due to their higher mechanical strength, high temperature stability, easier sealing, and integrated processing when compared to their ceramic and polymeric counterparts [ 4 ]. Most metal membranes are characterised by a gradient composite structure consisting primarily of a metal base and a metal, metal oxide, or metal alloy separation layer. Metal membranes are generally categorised into dense membranes (such as hydrogen soluble palladium membranes) and porous membranes. This review, however, focuses on porous metal membranes. Porous metal membranes have been studied for numerous applications related to liquid and slurry food preparation and filtration, and for products such as dairy, fruit juice, and alcohol [ 5 8 ]. Metal membranes offer some advantages over the polymeric membranes for these applications because the modules can be cleaned by high pressure back-flushing, thus reducing the use and influence of cleaning chemicals [ 9 ]. Another advantage of using metal membranes in food processing is their stability during steam sterilisation [ 9 ]. Porous metal membranes have also been applied for water applications such as MF, membrane reactors and bio-reactors, electrolysers, and membrane evaporators [ 10 21 ]. However, no review has been carried out on the status of application of porous metal membranes. This work aims to highlight the various fabrication techniques that have been used to successfully fabricate porous metal membranes and to critically review the applications of porous metal membranes. Novel fabrication techniques that were recently investigated for potentially producing nanoscale porous metal membranes and future research directions will also be discussed.

Inorganic membranes prepared from materials such as carbon, silica, zeolite, and various oxides (e.g., Al, TiO, ZrO) and metals (e.g., Pd, Ag, and their alloys, and steel) are very attractive in membrane separation due to their higher chemical, thermal, and mechanical resistances, exceptional separation features, and long operational lifetime [ 1 ]. Although inorganic membrane materials cost more than organic polymeric membrane materials, they offer some advantages such as temperature stability, resistance towards solvents, narrow pore size distribution, and the opportunity for more sterilisation options [ 2 ]. Therefore, they can be widely used for the chemical and pharmaceutical industry and for many water applications, particularly for high temperature, extreme pH, abrasive environments, and high-pressure processes that preclude the use of existing polymeric membranes. Although important progress has been achieved in the fabrication and use of inorganic membranes over the last two decades, extensive application has not yet been achieved when compared to polymeric membranes that currently dominate the industry. High capital costs and low surface area packing are generally regarded as the core hindrance for inorganic membranes [ 3 ].

Membrane-based processes have become increasingly attractive due to their ability to reliably remove contaminants, their affordability, and the increasing prominence of energy and water issues necessitating efficient treatment of waste effluents. The application of membrane-based processes is well established for municipal water treatment, with cost effective polymeric microfiltration (MF)/ultrafiltration (UF) and reverse osmosis (RO)/nanofiltration (NF) membranes available. Municipal water and wastewater applications are characterised by low inorganic particulate loads and organic based water contaminants of low to moderate concentration, enabling efficient processing. However, many emerging membrane applications involve the separation and/or the filtration of higher strength contaminant loads or abrasive particles in waste concentrate streams or slurries, or processing of gas and vapours, and require porous or dense membranes with robust mechanical properties, and high thermal and chemical stability.

2. Fabrication of Porous Metal Membranes

Porous metal membranes can be generally classified into two main types: unsupported metal membranes (metal membrane filters) and porous metal supported membranes. Unsupported metal membranes are made from pure or alloyed metals, while supported metal membranes utilise porous metals for the primary structure, and metal, metal oxide, or alloys are used as the membrane selective layer. The preparation of porous metal membranes mainly utilises the techniques used for fabrication of ceramic membranes and dense metal membranes. The morphology of porous metal membranes such as pore size, pore depth and porosity strongly depend on the fabrication technique used for their porous metal frameworks. There is a range of approaches for preparation of porous metal frameworks [ 9 ]. Table 1 summaries a number of processes that are used for the fabrication of porous metal frameworks with an achievable pore size range, and their advantages and disadvantages.

28,29,30,31,32,28,29,30,31,32,®) from sinter bonded metal powders [® metal membrane filters with 2 µm pores [

Although a range of techniques as listed above can be potentially used for the fabrication of porous metal membranes, one of the main approaches that has been used for fabrication of unsupported porous metal membranes is particle sintering. Sintering of particles is derived from traditional powder metallurgy technology, which involves the hot-compression of micron-sized metal particles or fibres at the softening temperature of the metal to produce a semi-porous network. The major principle of sintering is to bring the particles together forming necks arising from slow coalescence of the soften metals at high temperatures [ 72 ]. Stainless steel (SS) is the most commonly used for producing porous metal membranes by particle sintering. Other metals such as gold, silver, copper, nickel, aluminum, magnesium, titanium, chromium, tungsten, and molybdenum can also be used [ 4 ]. A range of conditions (e.g., particle size, sintering atmosphere) has a significant impact on the products from the sintering process [ 4 ]. Larger particles not only limit neck formation due to the slow rate of mass transport, but also lead to a mechanically weak membrane due to the favourable formation of finger-like macrovoids [ 73 ]. The properties of the metal powders (e.g., size, compressibility and reactivity) used for sintering must be well known to determine the most appropriate heating technique and avoid densification by over-heating [ 74 75 ]. The pore size formed using this technique is controlled by the average particle deformation induced by the process and the remaining distance between the particles after sintering [ 9 ]. Changing the sintering atmosphere in the sintering process can also result in different morphological features [ 73 ]. This technique typically leads to large pores (e.g., >1 µm) [ 27 76 ]. Sintering process has been widely used to produce commercial metal filters/membranes [ 27 76 ]. For example, Porvair Filtration manufactures porous SS membrane filters (Sinterflo) from sinter bonded metal powders [ 29 ]. Pall Corporation’s AccuSep™ sintered SS filters have a removal rating of 2–5 µm [ 31 ], and PMMmetal membrane filters with 2 µm pores [ 32 ]. Most recently, a rolling-sintering process was also reported by Park and co-workers [ 77 ] to fabricate porous metal membranes with a pore size of ~4.5 µm.

78,80,81,82,83,83,2 showed a bending strength of 384 MPa, which was greater than that of most of ceramic hollow fibres reported in the literature [86,87,2 (YSZ) (83,

The sintering technique has great potential to fabricate hollow fibre metal membranes [ 73 78 ]. The first metallic hollow fibre membrane was fabricated by Liu et al. [ 79 ] by extruding a polymer solution with suspended Ni particles to green hollow fibres, followed by sintering at elevated temperature under an argon atmosphere. Several research groups [ 73 84 ] have recently fabricated SS hollow fibres by micron-sized SS particles. SS hollow fibres can be fabricated by extruding a suspension of SS powders in a polymer solution through a spinneret, followed by coagulating in a phase-inversion process. Once the phase-inversion process is complete, the hollow fibre precursors are sintered at a high temperature. By combining the phase-inversion process with the sintering technique, large amounts of hollow fibres can be continuously produced in one step, and the microstructure of the membranes can be adjusted for targeted applications. Some studies [ 73 84 ] have been carried out to investigate the effects of sintering conditions on the properties of SS hollow fibres. Schmeda-Lopeza et al. [ 73 ] reported that during stainless steel hollow fibre membrane preparation by the sintering process, sintering of fibres in a nitrogen atmosphere resulted in large pores >200 μm, and small pores of 3–5 μm and 0.01–0.1 μm, whilst hollow fibres prepared in an argon atmosphere showed distinct pores in the range of 50–70 μm. Sintering in an argon atmosphere led to the development of some large pores that propagated cracks resulting in lower flexural strain. Sintering with inert gases led to the mass transfer of residual carbon from the binder to the stainless steel powders, resulting in the formation of carbide-rich regions. The sample sintered in argon contained more carbide-rich regions (by area) than the hollow fibres prepared under nitrogen. However, the chemical changes mentioned here did not have an impact on the mechanical properties of the final materials [ 73 ]. Rui et al. [ 84 ] fabricated SS hollow fibres in a variety of sintering atmospheres (air, carbon dioxide, nitrogen, helium, hydrogen). The prepared SS hollow fibres ( Figure 1 (1)) showed different exterior colours, which was attributed to the formation of different composition of membrane materials under various sintering atmospheres, and the hollow fibres prepared under hydrogen displayed the largest shrinkage among the fabricated hollow fibres ( Figure 1 (2)). They found that sintering with hydrogen can assist in removing the polymer binder and could eliminate oxidation on the metal surface. The sample (mean pore size 2.1 µm) sintered at 1100 °C in Hshowed a bending strength of 384 MPa, which was greater than that of most of ceramic hollow fibres reported in the literature [ 85 88 ]. Wang et al. [ 83 ] developed SS hollow fibre membranes with a three-channel structure ( Figure 2 ) and demonstrated a further improvement in the mechanical properties of the membrane. Most recently, Li’s research group used this technique to develop dual-layer composite hollow fibres [ 89 ]. The fabricated dual-layer SS/SS-ceramic composite hollow fibres have an inner SS layer and an outer layer composed of a mixture of SS and yttria-stablilised ZrO(YSZ) ( Figure 3 ), with a mean pore size of ~0.3 µm and an enhanced bending strength (>400 MPa). Although the mechanical strength has been much enhanced, the SS hollow fibres still display relatively large pores (in the µm range) [ 78 84 ], which has restrained their practical applications such as MF. In addition, very limited studies [ 89 ] have been carried out to develop SS hollow fibres with smaller pore sizes. The influence of sintering conditions including temperature, time, and atmosphere on SS hollow fibres is still not well known [ 73 ]. Further work is necessary to precisely control sintering conditions by carefully considering the properties of the materials used (e.g., size and size distribution of particles, binder thermal stability), coupled with the atmosphere, temperature, and time used for the sintering process [ 73 ]. Using precursor materials with smaller particle size (e.g., nano-particles) is also necessary to possibly achieve smaller pore (e.g., sub-micron) metal membranes for MF applications [ 78 ].

2, Al2O3, or ZrO2 onto a porous metal substrate (generally porous stainless steel) followed by a sintering process. The Sol–gel process is one of the conventional methods used to coat the thin layer onto the porous SS substrate [2O3 intermediate layer onto a porous SS substrate, and then they fabricated a top layer of TiO2, SiO2, or TiO2-SiO2 on the α-Al2O3 intermediate layer by the same technique. By using this 2-step approach, they successfully reduced the pore size from 1.5 µm (porous SS substrate) to 0.7 µm (α-Al2O3 intermediate layer), and further down to ~0.3 µm (top layer of TiO2, SiO2, or TiO2-SiO2).

Supported metal membranes are commonly fabricated by coating a thin layer of ultrafine metal oxides such as TiO, Al, or ZrOonto a porous metal substrate (generally porous stainless steel) followed by a sintering process. The Sol–gel process is one of the conventional methods used to coat the thin layer onto the porous SS substrate [ 90 ]. Li et al. [ 90 ] used this technique to introduce an α-Alintermediate layer onto a porous SS substrate, and then they fabricated a top layer of TiO, SiO, or TiO-SiOon the α-Alintermediate layer by the same technique. By using this 2-step approach, they successfully reduced the pore size from 1.5 µm (porous SS substrate) to 0.7 µm (α-Alintermediate layer), and further down to ~0.3 µm (top layer of TiO, SiO, or TiO-SiO).

92,93,−2·h−1. Zhou et al. [2 membrane supported on a planar porous Ti–Al alloy by combined electrophoretic deposition and dip-coating process followed by sintering in argon atmosphere at 1050 °C. The fabricated TiO2/Ti–Al showed a defect-free surface and an average pore size of 0.28 µm, and achieved a pure water flux of 3037 L·m−2·h−1. Yang and co-workers [

Metal membranes supported on porous metal alloys substrates such as Ti–Al or Fe–Al have also been studied [ 91 94 ] as potential functional materials for molecular separation at high temperature or in corrosive environments. Wang [ 91 ] fabricated a porous Ti–Al-supported Ni using a dip-coating process followed by sintering. The prepared Ni/Ti–Al membrane showed an average pore size of 0.83 µm and a pure water flux of 6782 L·m·h. Zhou et al. [ 92 ] prepared a MF TiOmembrane supported on a planar porous Ti–Al alloy by combined electrophoretic deposition and dip-coating process followed by sintering in argon atmosphere at 1050 °C. The fabricated TiO/Ti–Al showed a defect-free surface and an average pore size of 0.28 µm, and achieved a pure water flux of 3037 L·m·h. Yang and co-workers [ 94 ] developed a Ti–48Al–6Nb porous MF coating ( Figure 4 ) on a high Nb–TiAl porous alloy support using cold gas spraying followed by reactive sintering. The porous high Nb–TiAl alloy support used in their study had the same composition as the coating and was prepared using the same procedures as described by Wang et al. [ 95 ]. The prepared porous Ti–48Al–6Nb coating had an average pore size of 1.8 μm, and showed high permeability and sufficient strength for potential MF applications in extreme environments. Shen et al. [ 93 ] fabricated Fe–Al alloy supported membranes with graded pores ( Figure 5 ) by Fe and Al elemental reactive synthesis. An Fe–Al alloy with large connecting open pores and permeability used as a support was formed using powder metallurgy techniques. The coating was achieved by spraying slurries containing mixtures of Fe particles and Al particles (both 3–5 μm in diameter) onto Fe-Al alloy support followed by sintering at 1100 °C. The prepared membranes showed a mean pore size of coating is 2.5 μm (maximum pore size 6 μm, minimum pore 1.7 μm).