Microstructure of TiO2 porous ceramics by freeze casting of nanoparticle suspensions

During freeze casting of TiO₂ porous ceramics, the porous architecture is strongly influenced by TiO₂ particle size, solids loading, and cooling temperature. This work investigates the influences of particle size, freezing substrate, and cooling temperature on the TiO₂ green bodies prepared by freeze casting. The results show that the lamellar channel width with 100 nm particles is larger than that of 25 nm particles, yet the ceramic wall thickness is noticeably decreased. The lamellar structure is more ordered when using a copper sheet than glass as its freezing substrate. A finer microstructure results when frozen at − 50 ℃ than − 30 ℃. Such porous materials have application potentials in a wide range of areas such as photocatalysis, solar cells, and pollutant removal and should be further studied.     

Preparation and photocatalytic properties of macroporous honeycomb alumina ceramics used for water purification

In this work, porous alumina ceramics with highly ordered capillaries were successfully fabricated by ionotropic gelation process of alginate/alumina suspensions. By varying the initial solid loading (10–30 wt%) of slurries, the porosity of alumina ceramics ranged from 60.4% to 79.5% with controlled pore size (180–315 μm). Due to the well-crosslinked macroporous structure and large specific surface areas, the porous ceramics were utilized as the photocatalyst supports of TiO₂ catalysts whose photocatalytic activity was characterized by degrading methyl blue under UV irradiation. TiO₂ coatings prepared by sol–gel method demonstrated excellent adhesion to the substrates. When the solid loading of supports reached 15 wt%, the TiO₂ coatings showed the highest photocatalytic efficiency of 79.52%. Besides, TiO₂ films possessed nearly the same photocatalytic activity as titania/water suspension. Thus, the honeycomb ceramic prepared by self-organization process holds promise for use as photocatalyst supports in water purification without recycling process of powders.     

Rheological behavior of carbon nanotube-alumina nanoparticle dispersion systems

In this work, Al₂O₃ nanoparticle and CNT-Al₂O₃ nanoparticle suspensions were studied. Both Al₂O₃ nanoparticle and CNT-Al₂O₃ nanoparticle systems exhibit shear-thinning behavior. The viscosities increase monotonically with the suspension solids loading. For the 40 vol.% solids loading suspension, CNT effect on the viscosity is not substantial until the content is ≥ 1.3 vol.%. The suspension yield stress to flow provides a measure of the particle-particle networking in the suspension. With the adsorbed poly(acrylic acid) (PAA) layer on the particle surface, substantial colloidal interactions are observed when the solids loading is > 35 vol.% and the CNT content is > 1.3 vol.%. Storage modulus and loss modulus can be used to understand the relative magnitude of the viscoplastic behavior and the elastic behavior of the suspension as well as the transition between the two. The relative magnitude of the dynamic modulii is a strong function of the solids loading and the CNT content.     

Gelcasting of concentrated aqueous silicon carbide suspension

Gelcasting of high concentrated aqueous silicon carbide suspension with 50 vol.% solids loading is discussed in this paper. It is found that SiC powder is suitable for being suspended in basic solution, provided that a proper pH value is chosen for it. The rheological property of SiC suspension is also affected by solids loading. When SiC slurry is cast at 70°C, reaction between free Si on the surface of SiC powder and an organic base used as dispersant gives out H₂ gas, which produces pores in green bodies. However, by vacuum pumping and controlling pH value of the suspensions, complicated shapes of uniform SiC green bodies without pores are prepared.     

Fabrication of Highly Porous Chromium Carbide with Multiple Pore Structure

A unique process, which consists of camphene‐based freeze‐casting and reactive sintering, has been developed to fabricate highly porous Cr₃C₂ ceramics, which were featured with a well‐defined multisize porous structure, consisting of large pore channels throughout the sample and small pores on the porous struts. The porous structure was found to be affected by the solid loading content and the casting temperature. Decreasing the solid loading content or increasing the freezing temperature increased the size of large pore channels. However, the size of small pore was not significantly influenced by the variation in the freeze‐casting parameters. Compared with other techniques reported for the fabrication of porous ceramics, high porosity can easily be achieved by this method. The sintering mechanism of the formation of the porous structure was also discussed in this study.     

A colloidal approach to prepare binder and crack-free TiO2 multilayer coatings from particulate suspensions: Application in DSSCs

A porous well-consolidated and crack-free TiO₂ multilayer coatings have been manufactured from a colloidal approach based on the preparation of particulate suspensions for DSSC. The study of the suspension parameters to optimize dispersion and stabilization of the TiO₂ nanoparticle in the liquid media as well as a thermal stabilization step between the layers have been defined as two key points in the processing method to obtain interconnected microstructures, free of defects and heterogeneities, that prevent the application of an additional scattering layer or any kind of specific or clean conditions during deposition. The sintering process at low temperature, 450⁰C, has allowed obtaining open microarchitectures avoiding the complete densification and favoring the dye adsorption. A thickness of 12.8 μm resulted in a successful dye loading of 4.52 × 10⁻¹⁰ mol·mm⁻² and a photoefficiency of 5.7%, both in the range of the others particulate systems. EIS measurements were also made to study the transfer charge phenomena.     

Theoretical analysis of colloidal interaction energy in nanoparticle suspensions

This study is focused on understanding interaction energies for Al₂O₃ nanoparticle suspensions at high solids loadings. Among the four interaction energies: van der Waals interaction energy, electrostatic interaction energy, steric interaction energy, and depletion interaction energy, the study found that the van der Waals attraction is the dominant mechanism in destabilizing the dispersion system; the steric repulsion is a more effective stabilization mechanism than the electrostatic repulsion. The fundamental cause for an unstable suspension due to the particle–particle close contact and thus attraction at high solids loading can be overcome by increasing the particle–particle repulsion. When a dispersant is used in a suspension, the polymer chain length must be carefully selected so that the dispersant provides enough stabilization but does not compromise the maximum achievable solids loading. If an appropriate dispersant is chosen, up to 45 vol% solids loading suspension can be obtained for the Al₂O₃ nanoparticles.     

Controlled freezing and freeze drying: a versatile route for porous and micro‐/nano‐structured materials

Freeze drying is a process whereby solutions are frozen in a cold bath and then the frozen solvents are removed via sublimation under vacuum, leading to formation of porous structures. Pore size, pore volume and pore morphology are dependent on variables such as freeze temperature, solution concentration, nature of solvent and solute, and the control of the freeze direction. Aqueous solutions, organic solutions, colloidal suspensions, and supercritical CO₂ solutions have been investigated to produce a wide range of porous and particulate structures. Emulsions have recently been employed in the freeze drying process, which can exert a systematic control on pore morphology and pore volume and can also lead to the preparation of organic micro‐ and nano‐particles. Spray freezing and directional freezing have been developed to form porous particles and aligned porous materials. This review describes the principles, latest progress and applications of materials prepared by controlled freezing and freeze drying. First of all the basics of freeze drying and the theory of freezing are discussed. Then the materials fabricated by controlled freezing and freeze drying are reviewed based on their morphologies: porous structures, microwires and nanowires, and microparticles and nanoparticles. The review concludes with new developments in this area and a brief look into the future. Copyright © 2010 Society of Chemical Industry     

Fabrication of porous magnesium spinel with directional pores by unidirectional solidification

A porous magnesium spinel (MgAl₂O₄) with directional pores was fabricated by unidirectional solidification in pressurized argon and hydrogen mixed gases. Two different kinds of pores with large directional and small facet shape were formed in the solidified samples. The former pores were dominant in the porous structure. A small amount of free corundum phase was formed in the solidified porous spinel as a secondary phase by vaporization of MgO component during the solidification process. With increasing total gas pressure, the pore size of the solidified samples decreased while no change in the porosity. The porosity and pore size of the samples increased with increasing hydrogen partial pressure. The porosities of the samples fabricated under 10%H₂–90%Ar and 1%H₂–99%Ar mixed gases were 30 and 10%, respectively, and that under Ar atmosphere was very low, non-porous.     

Enhancement of super-hydrophilic/underwater super-oleophobic performance of ceramic membrane with TiO2 nanowire array prepared via low temperature oxidation

A gradient porous ceramic membrane with surface super-hydrophilic and underwater super-oleophobic performance was prepared by combining hydrogel directional freezing method and low temperature oxidation process. The effects of solid contents and sintering temperature on the ceramic membrane matrix were examined. The reaction time and synthesis temperature on the TiO₂ nanowire array were also evaluated. In addition, the related effects on pore size distribution, permeation flux, contact angle, and oil-in-water emulsion separation were systematically investigated. The ceramic membrane matrix pore size changed from 0.5 μm to 25 μm gradually, indicating the gradient structure controlled by the growth of ice. The super-hydrophilic and underwater super-oleophobic performance of ceramic membrane surface was obtained with surface modification by TiO₂ nanowire array, and the surface water contact angle and underwater oil contact angle were less than 5° and over 158°, respectively. The bonding strength between TiO₂ nanowire and ceramic membrane matrix was high enough to withstand ultrasonic waves. The ceramic membrane modified with TiO₂ nanowire array was used for 1000 ppm diesel oil-in-water emulsion separation, and the stable separation efficiency and flux were about 97% and 100–200 L/(m² h bar) even after 10 filtration cycles.     

Tribological properties of porous PPS/PTFE composite filled with mesopore titanium oxide whisker

Nano‐micro hierarchical porous polyphenylene sulfide/polytetrafluoroethylene (PPS/PTFE) composites were prepared by mold‐leaching and vacuum melting process under high temperature condition. The tribological behaviors of porous PPS/PTFE composites and the synergism as a result of incorporation of both micro‐porogen (NaCl) and mesoporous TiO₂ whiskers were investigated. The effects of mesoporous TiO₂ whiskers and nonperforated TiO₂ whiskers on the friction and wear properties of PPS/PTFE composites were comparatively studied, respectively. Results indicated that the wear rate of porous PPS/PTFE composites with 30 wt % NaCl and 7 wt % mesoporous TiO₂ whiskers obtained the lowest values under the load of 100 N. Compared with pure PPS, the wear resistance of nano‐micro porous PPS/PTFE composite was enhanced by 6.45 × 10³ times, showing outstanding wear resistance. During sliding condition, grease could be squeezed through the nano‐micro pores under the coupling effect of load and friction heat, and formed a lubricanting layer on friction surface, providing self‐lubricating effect and high wear resistance. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Microstructure and properties of porous nanocomposite films: effects of composition and process parameters

Porous nanocomposite films based on polypropylene (PP) and titanium dioxide (TiO₂) nanoparticles were prepared by melt extrusion followed by uniaxial stretching. The effects of drawing temperature, extension rate, stretching ratio and composition of the base films on the final properties and microstructure of the stretched films were studied. Water vapor permeability (WVP) results showed a significant decrease in permeability of the films stretched at temperatures higher than 60 °C. Porosity, pore size and water vapor transmission rate in the porous nanocomposite films had a direct relation with nanoparticle content, extension rate and stretching ratio. Study of the morphology of the stretched films, using SEM, revealed that the pores form due to PP/TiO₂ interfacial debonding at low stretching ratios. Higher stretching ratios cause an enlargement of the pores and the formation of a PP fibril structure parallel to the stretching direction. Quantification of dye adsorption revealed that the quantity of adsorbed dye increased with porosity and surface area of the pores. © 2014 Society of Chemical Industry     

Fabrication of wood-like porous silicon carbide ceramics without templates

The porous silicon carbide ceramics with wood-like structure have been fabricated via high temperature recrystallization process by mimicking the formation mechanism of the cellular structure of woods. Silicon carbide decomposes to produce the gas mixture of Si, Si₂C and SiC₂ at high temperature, and silicon gas plays a role of a transport medium for carbon and silicon carbide. The directional flow of gas mixture in the porous green body induces the surface ablation, rearrangement and recrystallization of silicon carbide grains, which leads to the formation of the aligned columnar fibrous silicon carbide crystals and tubular pores in the axial direction. The orientation degree of silicon carbide crystals and pores in the axial direction strongly depends on the temperature and furnace pressure such as it increases with increasing temperature while it decreases with increasing furnace pressure.     

Colloidal Assembly and Functionalization of Pore Channels in Polymer Foils

The deposition of ceramic colloidal particles into the pores of polymeric ion track membranes was studied. By means of a low-temperature sol-gel process, aqueous dispersions of ceramic nanoparticles can adhere to virtually any surface of a porous polymer foil (substrate) including the interior walls of the pores without adversely affecting the flexibility or ease of handling of the starting material. The coating method consists in depositing of a TiO₂-photocatalyst onto the substrate by providing either a SiO₂ or Ag specific layer or both between the photocatalyst and the substrate. Silica and metallic silver act as both binder and barrier against deterioration due to the photocatalytic action derived from TiO₂. On the other hand, the incorporation of such nanoparticles inside the pores provides active centres on which gas exchange coupled to photo-catalytic reactions can occur. Because of their small pore size, porous substrates can be very useful in a high purity filter application, for food package, gas-separation- and ion-exchange membranes. The structure and function of these ceramic-polymer composites were conceived and engineered taking ostrich eggshell as natural model. This bionic approach is based on the concept of immobilizing solid particles on a porous surface using polymer groups as tether.     

Morphology evolution of immiscible polymer blends as directed by nanoparticle self-agglomeration

This work aims to clarify the effect of nanoparticle self-agglomeration structure on the morphology of polymer blends. The morphology development of polystyrene (PS)/polyamide (PA6) blends with titanium dioxide (TiO₂) nanoparticles preferentially localized in the PA6 domains was investigated by means of electron microscopy observation, viscoelastic analysis and selective extraction tests. It was shown that the preferential dispersion of TiO₂ leads to a significant reduction of the PA6 continuity in the PS/PA6 blend. The size of the PA6 domain increases gradually with further increasing the nanoparticle loading whereas the co-continuity of the PS/PA6/TiO₂ mixture is destroyed by isothermal post-treatments. These experimental results are completely different from those in carbon black, nanoclay or nano-silica-filled immiscible polymer blends. To elucidate the progression to the uneven morphology change, the dynamic process of microfibril break-up and droplet coalescence in the molten PS/PA6/TiO₂ mixture was traced in real-time through optical microscopy. It was confirmed that the self-agglomerating pattern of the nanoparticle in the polymer melts plays a key role in directing the morphology evolution of the immiscible polymer blend: unlike the self-agglomeration of carbon black to form three-dimensionally continuous network structure, the TiO₂ nanoparticles tend to form separate clusters in the PA6 phase. This prevents PA6 droplets from fusing together to form a continuous network during the coalescence and producing larger PA6 domains at higher TiO₂ loads.     

Influence of nanoparticles and antioxidants on mechanical properties of titania/polydicyclopentadiene polyHIPEs: A statistical approach

Open porous Pickering poly(high internal phase emulsion) composites were prepared by ring opening metathesis polymerization of surface modified TiO₂ nanoparticle stabilized dicyclopentadiene (DCPD) high internal phase emulsions. Oxidation of the double bonds in the polyDCPD chains of the resulting materials was prevented using antioxidants. Oxidation dependent variation of mechanical properties was demonstrated by applying compression tests to the resulting composite materials periodically. Periodical measurements revealed significantly reduced brittleness in the case of antioxidant containing polyHIPE composites. Furthermore, it was determined that the initial compression moduli and the compressive strengths of the resulting materials were significantly improved by using antioxidants as compared to antioxidant free samples. Moreover, increasing nanoparticle amount was found to have a beneficial effect with the presence of antioxidants. The relationship between the compression modulus with nanoparticle loading and different types of antioxidants was revealed by developing model regression equations and graphs. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46913.     

ZnO Submicrometer Rod Array by Soft Lithographic Micromolding with High Solid Loading Nanoparticle Suspension

Microfabrication by soft lithography has attracted increasing attention in recent years. Ceramic microfabrication by soft lithography normally utilizes ceramic precursor solutions or sol–gel process. However, the low solids loading of the starting materials compromises the fidelity of soft lithography, causing the aspect ratio of obtained submicrometer ceramic features to be very small. In this study, well‐dispersed, high solids loading ZnO nanoparticle suspensions are prepared by electrosterically stabilizing 20 nm ZnO nanoparticles. ZnO nanoparticle‐based submicrometer rod arrays with an aspect ratio around 1 are fabricated by micromolding the ZnO nanoparticle suspension into polydimethylsiloxane (PDMS) molds. Modified PDMS molds are exploited to optimize the filling depth of the suspension into the mold cavities, producing ZnO submicrometer rods with the same dimensions as the mold features. ZnO rods with various diameters and shapes are fabricated, and the effect of solids loading on soft lithography fidelity is discussed. A mechanism describing the effects of surface wetting on the fidelity of soft lithographic micromolding is proposed.     

A novel method for preparing Si3N4 ceramics with unidirectional oriented pores from silicon aqueous slurries

We describe a low-cost and simple method to prepare porous silicon nitride (Si₃N₄) ceramics with unidirectional oriented pores. Using Si powders as raw material, porous ceramics were fabricated via solidification of the aqueous slurries followed by the nitridation and post-sintering. The formation mechanism is similar to the ordered porous metals that are fabricated under high temperature and high pressure. Differently, the green bodies with oriented pores were obtained at room temperature and atmospheric pressure. The nucleation and growth of the H₂ bubbles generated from hydrolyzed Si powders contributed to the highly oriented pores along the solidification direction of the aqueous slurries. Additionally, the pore microstructure was closely associated with the viscosity of the slurries, which can be well controlled by varying the concentration of organic additive and solid loading.     

Photocatalytic activity of transparent porous glass supported TiO2

A porous glass tube with a composition of 96SiO₂·4B₂O₃ (wt%) supported TiO₂ shows high photooxidation activity due to its transparency and large surface area. The surface area of the porous glass tube supported TiO₂ is 10,000 times larger than that of conventional materials. TiO₂ crystals supported are anatase type. Transparency of the porous glass tube is very important. Herein, sol–gel and chemical vapor deposition (CVD) processes were employed as TiO₂ supporting processes. CVD process is more effective. For instance, an aqueous methylene blue solution with 1 ppm concentration almost thoroughly decomposes at a contact time of 300 s using porous glass tube supported TiO₂ prepared by CVD process under irradiating with 10 W low-pressure mercury lamp, on the other hand, opaque porous alumina tube supported TiO₂ was only 25%. The smaller the pore size of the porous glass tube, the larger the transparency and the permeation resistance through porous glass tube. Hence, porous glass tube with ca. 40 nm pore diameter is suitable from the standpoint of a practical use.     

Preparation of high solids content nano-titania suspensions to obtain spray-dried nanostructured powders for atmospheric plasma spraying

Nanosized TiO₂ powder with an average primary size of ∼20 nm and surface area of ∼50 m²/g (Aeroxide^® P25, Degussa-Evonik, Germany) was used as starting material. A colloidal titania suspension from the same supplier was also used (W740X). The dispersing conditions were studied as a function of pH, dispersant content, and solids loading. Well-dispersed TiO₂ nanosuspensions with solids contents up to 30 vol.% (62 wt%) were obtained by dispersing the powder with 4 wt% PAA. Suspensions with solids contents as high as 35 vol.% were prepared by adding the TiO₂ nanoparticles to the TiO₂ colloidal suspension under optimised dispersing conditions.TiO₂ powder reconstitution was performed by spray drying both types of nanosuspensions to obtain free-flowing micrometre-sized nanostructured granules. The spray-dried nanostructured TiO₂ granules were deposited on austenitic stainless steel coupons using atmospheric plasma spraying. Coating microstructure and phase composition were characterised using scanning electron microscopy and X-ray diffraction techniques.