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Zirconia (ZrO2) membranes experienced rapid progress in applications demanding high-stability membranes own to their higher chemical resistance and hydrophilicity compared to silica and alumina. Moreover, ZrO2 membranes have increased fouling resistance, high permeability, and a long lifetime making them broadly applied in drinking water production, wastewater treatment, petrochemical, food, and beverages industries. However, fabricating ZrO2 membranes for Nanofiltration and Gas Separation is still challenging. This paper reviews the progress in fabricating ZrO2 membranes, focusing on strategies for achieving smaller pores without losing their high permeability and selectivity. The current state of the art in commercial ZrO2 membranes and the recent innovations in academia are critically reviewed. A comprehensive revision of sol-gel technique's critical synthesis and process parameters is presented along with the most recent molecular layer deposition method. This work aims to provide a guide for both starting and established researchers, thus filling a gap in the present literature.

The fabrication of B4C ultrafiltration membranes is described. Firstly, a semi-dilute B4C slurry was environmentally-friendly prepared by aqueous colloidal processing, optimizing its dispersion by sonication, and used to deposit B4C membranes onto SiC macro-porous supports by dip-coating. Secondly, the resulting green membranes were characterised microstructurally by scanning electron microscopy (SEM), and pressureless sintered within the intermediate sintering regime. Thirdly, the sintered membranes were calcined in air to clean them from possible free carbon in the smallest pores, with the optimal calcination conditions having been identified by thermogravimetry coupled with mass spectrometry. Next, the calcined, sintered membranes were characterised microstructurally by SEM, tested mechanically against scratching, and characterised texturally by capillary flow porometry, thus identifying the optimal among them. Lastly, as a complement to the fabrication study, the filtration permeability of the optimal membrane was evaluated using deionized water. This work thus paves the way towards the fabrication of ceramic membranes based on B4C, lighter and potentially more durable than others, for filtration applications.

Concentrated solar power (CSP) is an important option as a competitive, secure, and sustainable energy system. At the moment, cost-effective solutions are required for a wider-scale deployment of the CSP technology: in particular, the industrial exploitation of CSP has been so far hindered by limitations in the materials used for the central receiver—a key component in the system. In this context, the H2020 NEXTOWER project is focused on next-generation CSP technologies, particularly on advanced materials for high temperatures (e.g., &gt, 900 °C) and extreme applications environments (e.g., corrosive). The research activity described in this paper is focused on two industrial solutions for new SiC ceramic receivers for high thermal gradient continued operations: porous SiC and silicon-infiltrated silicon carbide ceramics (SiSiC). The new receivers should be mechanically tough and highly thermally conductive. This paper presents the activity related to the manufacturing of these components, their joining, and characterization.

Water scarcity and water pollution are major issues in today’s world. Several industries including oil and gas, water treatment, wastewater treatment, and food and beverage processing have turned their attention to silicon carbide membranes to solve these issues, due to the increased water flux and enhanced mechanical, chemical, and thermal stability observed for this material. To shift the material dominance on the membrane market from polymeric and ceramic oxide to silicon carbide, a deep understanding of the fabrication of silicon carbide membranes is important. Therefore, this review focuses on the development of silicon carbide membranes from an industrial standpoint. The discussed approaches include the deposition of colloidal suspensions and polymeric precursors. In addition to describing the significant steps in the fabrication of silicon carbide membranes, the advantages, disadvantages, and remaining key challenges related to these approaches are emphasized. Furthermore, key aspects in the preparation of silicon carbide supports are presented to provide an understanding of how membrane support influences membrane performance. The wide range of silicon carbide membrane applications in water and wastewater treatment and other applications are then reviewed. Cleaning methods for silicon carbide membranes are also described to address fouling issues during filtration processes along with commercialization of these membranes. Finally, future perspectives on silicon carbide membrane material development and potential applications in other industrial sectors are discussed. Overall, this review paper aims to provide a roadmap for potential applications and further development of silicon carbide membranes in the field of liquid filtration.

The development of a low-cost and environmentally-friendly procedure for the fabrication of silicon carbide (SiC) membranes while achieving good membrane performance is an important goal, but still a big challenge. To address this challenge, herein, a colloidal coating suspension of sub-micron SiC powders was prepared in aqueous media by employing aluminum nitrate nonahydrate as a sintering additive and was used for the deposition of a novel SiC membrane layer onto a SiC tubular support by dip-coating. The sintering temperature influence on the structural morphology was studied. Adding aluminum nitrate nonahydrate reduced the sintering temperature of the as-prepared membrane compared to conventional SiC membrane synthesis. Surface morphology, pore size distribution, crystalline structure, and chemical and mechanical stability of the membrane were characterized. The membrane showed excellent corrosion resistance in acidic and basic medium for 30 days with no significant changes in membrane properties. The pure water permeance of the membrane was measured as 2252 L h−1 m−2 bar−1. Lastly, the final membrane with 0.35 µm mean pore size showed high removal of oil droplets (99.7%) in emulsified oil-in-water with outstanding permeability. Hence, the new SiC membrane is promising for several industrial applications in the field of wastewater treatment.

This paper presents the results of the lab-scale thermophysical characterization of materials for the next generation of solar receivers employed in CSP plants. The receiver material plays a crucial role in absorbing sunlight, minimising heat losses and transferring heat to the working fluid. Therefore, new receivers should be mechanically tough and highly thermally conductive. Porous SiC and SiSiC were the two industrial grade solutions investigated for this purpose. The Laser Flash Method was employed to analyse the transient thermal behaviour of the materials. The results of the measurements in terms of thermal diffusivity, specific heat and thermal conductivity are provided and compared with the literature data available.

A photocatalytic Ce-Y-ZrO2/TiO2 ultrafiltration membrane was successfully prepared on a ZrO2/SiC support by a modified sol-gel process. The top active layer is 2 μm thick, uniform and defect-free. The membrane presented a molecular weight cut-off (MWCO) of 19 kDa (equivalent to a pore size of 6 nm) and a high pure water permeability, 160 Lm-2h-1bar-1. The high hydrophilicity and negative surface charge of the membrane favoured its great retention of proteins (bovine serum albumin, whey protein, and hemoglobin), indigo dye, and humic acid. The membrane was effective in photodegrading phenol and humic acid under simulated sun light irradiation. During humic acid filtration tests, the membrane presented better anti-fouling properties (smaller flux decline) and higher permeate flux under irradiation compared to the filtration in the dark. Moreover, self-cleaning properties were observed by irradiating the membrane, which enabled recovering up to 97% of the original flux. Consequently, a longer operation without chemical cleaning is possible, reducing costs and the process footprint. Further investigation would allow the development of innovative treatments for dinking and wastewaters by combining filtration and advanced oxidation processes for the abatement of contaminants of emerging concern in the presence of natural organic matter.

The comminution of a typical submicrometre B4C powder with a high-energy mill (i.e., a shaker mill) operated in air in either a dry or a wet environment was investigated. It was found that dry shaker milling (i.e., high-energy ball-milling) is able to progressively refine the B4C particles to the nanoscale. While this is accompanied by oxidation and aggregation, these are not serious drawbacks. Wet shaker milling in methanol (i.e., conventional ball-milling) resulted only in a moderate B4C particle refinement with greater contamination by the milling tools, which limits its usefulness. It was also found that both dry and wet milling modify the B4C crystal structure, attributable to carbon enrichment. Consequently, dry shaker milling was found to be more recommendable than wet shaker milling to provide B4C starting powders with superior sinterability. A comparative densification study by spark-plasma sintering confirmed this recommendation, and also showed the usefulness of dry shaker milling to obtain refined B4C microstructures for structural applications.

The appropriate conditions for liquid-phase assisted spark-plasma sintering (SPS) were identified for the fabrication of both SiC nanoceramics and their nanocomposites with carbon nanotubes (CNTs). A parametric study of the nanoceramics and nanocomposites with a given type of CNTs showed that the SPS temperature (as measured by the radial optical pyrometer) optimizing their densification, nanograin size, and mechanical properties is 1700 °C (soaking for a few minutes), below which there is incomplete densification, and above which there is obvious grain growth with no benefit in hardness or toughness in the case of the nanoceramics, and prejudicial to both properties in the case of the nanocomposites due to the CNT degradation. It was also shown that the nanocomposites have smaller nanograins than their nanoceramic counterparts, and are softer but tougher. Extension to nanocomposites with different types of CNTs confirmed these trends, and showed that the CNT features do not condition the densification, microstructure or mechanical properties of these nanocomposites.

An environmental-friendly procedure has been developed for the fabrication of pure silicon carbide membranes on macroporous SiC support via ceramic processing. Water dispersions of α-SiC powders were used for deposition of membrane layers by dip-coating. The influence of the fine/coarse powder mixing ratio, solid loading, use of α-SiC powders with different particle sizes as well as sintering temperature on the structural morphology of the membranes were investigated in order to obtain uniform, homogeneous, and defect-free SiC membrane layers. The optimized protocol was up-scaled on industrial SiC tubular supports with high reproducibility, reducing the sintering temperature compared to conventional SiC membrane synthesis. The new SiC membranes were used for the treatment of a secondary effluent from Biofos wastewater treatment plant in Avedore, Denmark, and their performances evaluated in terms of removal of the suspended solids, colloidal particles and reduction of chemical oxygen demand. According to filtration results, the new SiC membranes showed high removal of suspended solids (99.4%) and colloidal particles (96%) along with significant reduction of chemical oxygen demand (83%). The pure SiC membranes developed in this study have a potential to be applied in the wastewater treatment since they combine the robustness of SiC with high selectivity.

SiC nanostructured ceramics without and with carbon nanotubes (CNTs) were prepared by aqueous colloidal processing plus liquid-phase assisted spark-plasma sintering, and their sliding wear behaviour compared. It was found that the SiC nanoceramic is highly resistant to wear, with specific wear rates in the order of 10−8 mm3/N m or lower under the tribological conditions used, but also that the SiC/CNT nanocomposite is even more so. Qualitatively, the nanocomposite displayed a simple wear curve with a steady-state wear rate, over the wear curve of the nanoceramic with two wear rates, the second being one order of magnitude greater than the first. Quantitatively, the nanocomposite wore less overall with smaller worn volume, and especially had a lower wear rate (particularly marked for moderate and prolonged sliding distances). It was also found that the superior wear resistance of the SiC/CNT nanocomposite is due to combined effects of lubrication and toughening provided by the CNTs, the former reducing the severity of the sliding-contact conditions and the latter hindering the propagation and coalescence of the short cracks responsible for nanograin pull-out. CNT addition thus emerges as a hopeful line for processing ceramic nanocomposites that are both toughened and highly wear resistant for use in contact-mechanical and tribological applications.

Carbon nanotubes (CNTs) have occasionally been observed to benefit the aqueous colloidal processing of nano-SiC with its nano-Y3Al5O12 liquid-phase-sintering additives. Experimental evidence is here presented for a broad set of CNTs with different morphology and/or surface functionalization confirming that CNTs (7 vol.% addition), regardless of their features, prevent the coagulation of these nanoceramic suspensions, whence it is inferred that aqueous colloidal processing is well-suited for the environmentally friendly preparation of the homogeneous mixtures of nanoceramic particles and CNTs required for the fabrication of CNT-reinforced ceramic matrix nanocomposites. Furthermore, it is shown that surface-functionalized CNTs seem to work better than deflocculated CNTs for the preparation of stable concentrated colloidal suspensions, whose rheological properties are in general very close, but with thinner CNTs being nonetheless preferable. Finally, the feasibility is demonstrated of fabricating SiC/CNT nanocomposites by aqueous colloidal processing followed by liquid-phase assisted spark-plasma sintering.

The sliding-wear resistance of pure near fully-dense B4C is investigated, and the wear mode/mechanisms identified, under lubrication with water, diesel fuel, and paraffin oil. It is found that the wear is mild in the three cases, with specific wear rates (SWRs) of 10−16–10−17 m3/N m. Nonetheless, the wear resistance of the B4C ceramic is one order of magnitude greater under oil lubrication (1016 N m/m3) than under water lubrication (1015 N m/m3), and twice as great for the specific case of paraffin oil than diesel fuel, attributable to the lubricant’s viscosity. It is also found that the wear mode is always abrasion, and that the wear mechanisms are plastic deformation and localized fracture with grain pullout. However, in agreement with the macro-wear data, the severity of the wear damage is lower under lubrication with paraffin oil, followed by diesel fuel, and lastly water. Finally, microstructural considerations are discussed with a view to enhancing the sliding-wear resistance of B4C triboceramics.

A route based on aqueous colloidal processing followed by liquid-phase assisted spark-plasma-sintering (SPS) is described for fabricating eco-friendly nanocomposites of SiC with nano-carbonaceous phases (nanotubes, nanoplatelets, or nanoparticles). To this end, the conditions optimizing the aqueous colloidal co-dispersion of SiC nanoparticles, Y3Al5O12 nanoparticles (acting as sintering additives), and carbon nanotubes (CNTs), graphene oxide (GO) nanoplatelets, or carbon black (CB) nanoparticles were first identified. Next, homogeneous powder mixtures were prepared by freeze-drying, and densified by liquid-phase assisted SPS, thus obtaining nanocomposites of SiC with CNTs, reduced GO (rGO) nanoplatelets, or pyrolized + graphitized CB (p + gCB) nanoparticles. It is also shown that these nanocomposites are dense and have a high hardness of ∼20 GPa regardless of the nano-carbonaceous phase chosen, but are markedly tougher with CNTs and rGO (i.e., with high aspect ratio nano-carbonaceous phases). Finally, arguments are provided for the appropriate choice of nano-carbonaceous phases for engineering ceramic nanocomposites.

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.

Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviour when dispersed colloidally in aqueous media, whereas shear-thickening is the expectation for large aspect ratio phases such as, for example, carbon nanotubes (CNTs). Here, contrary experimental evidence is presented demonstrating the occurrence of severe coagulation at high shear rates in colloidally stable, semi-concentrated, aqueous suspensions of equiaxed SiC nanoparticles (major phase) mixed with equiaxed Y 3 Al 5 O 12 nanoparticles (liquid-phase sintering additive), and how CNT addition prevents this coagulation if sufficient sonication is applied. It is also shown that although shear-thinning is the natural behaviour of the ceramic suspension up to moderate shear rates, coagulation is eventually a phenomenon inherent to the aqueous colloidal processing of these suspensions, with the critical shear rate for coagulation increasing and the rheopexy decreasing the better is the initial dispersion state achieved with the sonication. It is also shown that the critical shear rate for coagulation depends on the exact condition of shear rate increase, and that the re-sheared suspensions coagulate more significantly and at lower shear rates than the fresh suspensions. The mechanisms by which this coagulation occurs and is impeded by the CNTs are discussed, together with broader implications of these phenomena for the environmentally friendly processing of nanostructured ceramics and ceramic composites.

Aqueous colloidal processing was used for the environmentally friendly preparation of well-dispersed concentrated suspensions and powder mixtures of submicrometric SiC powders with submicrometric Y3Al5O12 as sintering additive plus diamond nanoparticles as reinforcing phase. It is shown that the addition of nano-diamond markedly increases the viscosity and thixotropy of the SiC + Y3Al5O12 suspensions, and also that, by adjusting the pH, deflocculant content, and sonication time it is possible to co-disperse these three rheologically different ceramic phases (i.e., non-oxide, oxide, and hydrophobic compounds) in aqueous media, thereby avoiding the otherwise irremediable severe hetero-aggregation. Moreover, the microstructural characterization of the powder mixtures obtained by freeze-drying the suspensions confirmed the homogeneous dispersion of the diamond nanoparticles among the submicrometric SiC and Y3Al5O12 particles in the form of isolated or adhered nanoclusters and nanodeposits. Implications for engineering the microstructure of non-oxide ceramics with diamond nanodispersoids are discussed.

The aqueous colloidal processing of SiC with Y3Al5O12 liquid-phase sintering additives was investigated for two different additive systems, one the mixture of Y2O3 and Al2O3 in a 3:5 molar ratio and the other directly Y3Al5O12. The investigation involved the study of the colloidal stability of the different components, and the comparison of the rheological behaviour of concentrated suspensions of SiC, SiC + 3Y2O3:5Al2O3, and SiC + Y3Al5O12 as a function of the sonication condition, dispersant content, and solid loading. This allowed appropriate conditions for the preparation of well-dispersed, single-phase, and multi-component concentrated suspensions of SiC to be identified. It was found that the multi-component suspensions have better rheological behaviour than the single-phase ones, and that in terms of rheology and slip casting the Y3Al5O12 additives are more functional than the conventional 3Y2O3 + 5Al2O3 additives. It was also demonstrated that the Y3Al5O12 additive is as effective as the 3Y2O3 + 5Al2O3 additive in densifying SiC via liquid-phase sintering, with there existing no differences either in the microstructure or in room-temperature mechanical properties (hardness, toughness, and fracture mode). Implications of interest for the wet-shaping of complex SiC parts are discussed.

This work focused on the fabrication of a ZrO2/SiC ultrafiltration membrane by dip coating a high porous SiC support with a ZrO2 slurry prepared by ceramic processing. The membranes were sintered in different temperatures (1000−1300 °C). With the optimal temperature, it was obtained a mechanically strong, homogenous, and defect free separation layer with 45 μm of thickness and average pore size of 60 nm. A pure water permeability of 360 L.m−2 h−1 bar-1 and high retentions of humic acid, indigo dye, and hemoglobin were observed. In a pilot test with an olive oil/water emulsion, 99.91 % of oil was removed without fouling. Long-term corrosion tests at basic and acid baths did not cause change in pore size and morphology. In conclusion, the ZrO2/SiC membrane has potential to operate in harsh conditions (e.g. heavily contaminated industrial effluents or urban wastewaters) and when severe membrane cleaning and disinfection are required, such as food and pharmaceutical industries.