Ceramic membranes from reaction-sintered silicon nitride on nitride and oxide substrates

The gas permeability and pore sizes in membranes and porous materials from reaction-sintered silicon nitride (RSSN) are studied. The substrates for the RSSN membranes are highly porous (80% pores) silicon nitride with pores 50–80 μm in size and alumina-based ceramics with pores about 10 μm in diameter. Silicon powder with a specific surface of 1.4 g/cm² is deposited onto the substrates in the form of an aqueous dispersion by various methods including filtering and sintered in nitrogen at 1300–1400°C. The main difficulty of the process is in preventing the formation of defects at the places where the largest channel pores reach the surface of the substrate. Membranes with 0.5 -1 μm pores can be obtained on both types of substrate after a single deposition.     

Optimization of structure and properties of polyphenylene sulfide porous membrane by controlling the process of thermally induced phase separation

Polyphenylene sulfide (PPS) porous membranes were successfully prepared from miscible blends of PPS and polyethersulfone (PES) via thermally induced phase separation followed by subsequent extraction of the PES diluent. The morphologies, crystalline structures, mechanical properties, pore structures and permeate fluxes of the PPS porous membranes obtained from different phase separation processes were characterized and are discussed. During the phase separation in the heating process, PPS and PES mainly underwent liquid–liquid phase separation, and then a nonhomogeneous porous structure with a mean pore size of 100 μm and a honeycomb‐like internal structure formed on the membrane surface. The phase separation of PPS/PES occurring in the cooling process was easier to control and the related pore diameter distribution was more regular. In the process of direct annealing, as the phase separation temperature decreased, the pore size distribution became more homogeneous and the mean diameter of the pores also decreased gradually. When the phase separation temperature decreased to 200 °C, PPS membranes with a network structure and a uniform as well as well‐interconnected porous structure could be obtained. In addition, the maximum permeation flux reached 1718.03 L m^–2 h^–1 when the phase separation temperature was 230 °C. The most probable pore diameter was 6.665 nm, and the permeate flux of this membrane was 2.00 L m^–2 h^–1; its tensile strength was 17.07 MPa. Finally, these PPS porous membranes with controllable pore structure as well as size can be widely used in the chemical industry and energy field for liquid purification. © 2020 Society of Chemical Industry     

Thermo‐Responsive Membranes with Cross‐linked Poly(N‐Isopropyl‐acrylamide) Hydrogels inside Porous Substrates

Thermo‐responsive membranes were prepared by fabricating cross‐linked poly(N‐isopropylacrylamide) (PNIPAM) hydrogels inside the pores of porous Nylon‐6 (N6) membranes by the free radical polymerization method. SEM micrographs of the prepared membranes showed that PNIPAM hydrogels were filled uniformly throughout the entire thickness of the porous N6 membranes. Both PNIPAM‐filled N6 membranes prepared at 60 °C and at 25 °C exhibited significant reversible and reproducible thermo‐responsive diffusional permeability. When the environmental temperature remained constant, the diffusional coefficient of vitamin B₁₂ (VB₁₂) across the PNIPAM‐filled N6 membrane prepared at 25 °C was ca. twice the value of that prepared at 60 °C due to different filling yields. The thermo‐response factor of the membrane prepared at 25 °C was higher than that prepared at 60 °C. The 3‐dimensional interpenetrating network structure of the cross‐linked PNIPAM hydrogels inside the N6 porous substrates could effectively ensure a repeatable thermo‐responsive permeation performance.     

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10 µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5 µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9 MPa and 12.4 MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600 °C.     

Effects of calcination temperature on morphological and crystallographic properties of oyster shell as biocidal agent

To develop biocompatible antimicrobial agent, oyster shell wastes were thermally calcined at different temperatures ranging from 300 to 1000 °C. The chemical compositions and properties of oyster shells were characterized. As such, crystallographic analysis presented that oyster shells had a hexagonal crystalline shape, and calcination process reduced their crystalline size, volume (grain dimension), and bond length, which strongly affected antimicrobial efficacy. Results showed that the main components of uncalcined and calcined oyster shells were CaCO₃ and CaO, by which CaO was found to be the main antimicrobial component. Notably, calcined oyster shells showed antimicrobial potency against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus). Furthermore, cytotoxicity analysis proved that calcined oyster shells had good cell viability and low cytotoxicity. Results highlighted that calcined oyster shells, particularly those treated at 750°C, could be a biocompatible alternative to synthetic biocidal and antimicrobial agents using in food packaging, biomedical, and cosmetic industries.     

Cellulose acetate reverse osmosis membranes: Optimization of preparation parameters

Asymmetric cellulose acetate based membranes usually employed in reverse osmosis as well as in separations in aqueous systems can possibly be applied in the so‐called salinity process of energy generation. For these applications, membranes with a relatively high water permeability (sometimes also called water flux) and low salt permeability (or high salt rejection) are required. In this study the authors present the optimization of such membranes, which concerns the preparation parameters. The membranes studied were prepared from a solution whose composition were previously optimized.⁴ The authors concluded that the optimum preparation parameters are as follows: thickness of the liquid film of 100 μm; 30 s allowed for evaporation of the solvent; and temperature of coagulation bath of 0–4°C and 80–85°C as annealing temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 134–139, 2007     

Polyimide Ultrafiltration Membranes with High Thermal Stability and Chemical Durability

Abstract

Asymmetric ultrafiltration membranes based on poly[(4,4′-oxydiphenylene)pyromelliteimide] were produced by wet technique from prepolymer casting solution, followed by solid-phase conversion of the prepolymer membranes into polyimide insoluble form at 200°C. It was demonstrated that by adding benzimidazole to the casting solution and filling of prepolymer membrane pores with inert high-boiling oil prior to thermal treatment allow us to prepare asymmetric porous polyimide membranes. The main characteristics of the membranes obtained (permeability coefficients and molecular weight cut-off) match those typical to ultrafiltration membranes. It was found that the developed asymmetric ultrafiltration polyimide membranes have excellent thermal and chemical resistance. The membranes retain rigidity above Tg (360°C) and are chemically stable at temperatures up to 400°C. The developed membranes are resistant against swelling and dissolving in aggressive and organic media including amide solvents.     

Aligned continuous cylindrical pores derived from electrospun polymer fibers in titanium diboride

The use of electrospun polystyrene (PS) fibers to create continuous long range ordered multi-scale porous structures in titanium diboride (TiB₂) is investigated in this work. The introduction of electrospun PS fibers as a sacrificial filler into a colloidal suspension of TiB₂ allows for easy control over the pore size, porosity, and long range ordering of the porous structures of the sintered ceramics. Green bodies were formed by vacuum infiltrating an electrospun-fiber-filled mold with the colloidal TiB₂ suspension. The size, volume, distribution, and dispersion of the pores were optimized by carefully selecting the sacrificial polymer, the fiber diameter, the solvent, and the solid content of TiB₂. The green bodies were partially sintered at 2000°C to form a multi-scale porous structure via the removal of the PS fibers. Aligned continuous cylindrical pores homogeneously distributed across the samples were derived from the PS fibers in a range of ~5-20 μm and partial sintering produced inter-particle porosity with sizes of ~0.3-1 μm. TiB₂ near-net-shaped parts with the multi-scale porosities (~50-70%) were successfully cast and sintered. This low-cost processing technique can be used in the production of thermally and mechanically anisotropic structures into near-net shape parts, for extreme environment applications, such as high temperature insulation.     

Salt‐leaching technique for the synthesis of porous poly(2,5‐benzimidazole) (ABPBI) membranes for fuel cell application

The first instance of synthesizing porous poly(2,5‐benzimidazole) (ABPBI) membranes for high‐temperature polymer electrolyte membrane fuel cells (HT‐PEMFCs), using solvent evaporation/salt‐leaching technique, is reported herein. Various ratios of sodium chloride/ABPBI were dissolved in methanesulfonic acid and cast into membranes by solvent evaporation, followed by porogen (salt) leaching by water washing. The membranes were characterized using SEM, FTIR, TGA, and DSC. The proton conductivity, water and acid uptake of the membranes were measured and the chemical stability was determined by Fenton's test. SEM images revealed strong dependence of sizes and shapes of pores on the salt/polymer ratios. Surface porosities of membranes were estimated with Nis Elements‐D software; bulk porosities were measured by the fluid resaturation method. Thermogravimetric analysis showed enhanced dopant uptake with introduction of porosity, without the thermal stability of the membrane compromised. Incorporating pores enhanced solvent uptake and retention because of capillarity effects, enhancing proton conductivities of PEMs. Upon acid doping, a maximum conductivity of 0.0181 S/cm was achieved at 130 °C for a porous membrane compared with 0.0022 S/cm for the dense ABPBI membrane at the same temperature. Results indicated that with judicious optimization of porogen/polymer ratios, porous ABPBI membranes formed by salt‐leaching could be suitably used in HT‐PEMFCs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45773.     

Microstructures and properties of solid-state-sintered silicon carbide membrane supports

Porous solid-state-sintered SiC (S–SiC) membrane supports were successfully fabricated by pressureless sintering at 2150 °C in argon, using fine and coarse graded SiC powders as the main starting material. There were uniformly distributed and fully interconnected pores in as-acquired S–SiC membrane supports, accompanied with similar apparent porosities for all of them. When increasing the size of coarse SiC powder, their average pore sizes were distinctly enlarged from ∼1.6 μm to ∼2.3 μm, which significantly enhanced their nitrogen permeability from 0.9 × 10⁻¹³ m² to 2.6 × 10⁻¹³ m². Moreover, S–SiC membrane supports possessed outstanding flexure strengths of 134.1 ± 21.3 MPa at room temperature and 88.7 ± 8.4 MPa at 1000 °C owing to strong interface bonding between SiC grains. Compared with the traditional SiO₂ -bonded and mullite-bonded SiC supports, S–SiC membrane supports presented their great superiority in high-temperature flexure strength as well as acid and alkali corrosion resistance, which permitted them to be potentially applied in high-temperature and strongly corrosive environments.     

The isothermal degradation of wood

The results of a study of spruce (whitewood) and its organic components (cellulose, hemicellulose, and lignin) by isothermal thermogravimetric analysis in air and inert atmospheres are presented. Data on the thermal decomposition of fuel wood in a temperature range from 200 to 450°C were acquired. The porous structure of biocoal and the process of its evolution were examined by scanning electron microscopy. The porous structure of the whitewood thermally treated at 200 and 300°C had pore sizes from 4 to 15 μm. The stratification of tracheids occurred in the above temperature range. At higher temperatures of 350°C or above, thermal pores with sizes of about 100 nm appeared. As the temperature was increased to 400°C, the pore size increased to 200–300 nm.     

Formation of porous alumina with oriented pores

Adopting the theories describing the bubble forming in the metal–hydrogen solidification process, porous alumina with oriented pores was prepared by combining a foaming method with sol-gel technology. The bubble forming process in the sol-gel is different from that in the metal–hydrogen system. Samples were calcined at temperature from 800 to 1200C. The volume-shrinkage and compressive-strength increased with increasing calcination temperature. The porous alumina exhibited a bimodal pore structure when prepared below 1200°C.     

Preparation of macroporous poly(acrylamide) hydrogels in DMSO/water mixture at subzero temperatures

The copolymerization reactions of acrylamide and N,N’-methylenebis(acrylamide) were carried out in DMSO/water mixture (1:1 by volume) at various temperatures T_prep between -18 and 22 °C. Scanning electron microscopy analysis of the networks revealed the presence of porous morphologies. All the network samples formed at or below 0 °C have relatively small pores with sizes about 10⁰ μm. In this range of T_prep, the pore size only slightly increases with the temperature. As the temperature is increased above 0 °C, both the average pore size and the degree of polydispersity of the pores rapidly increase. Between T_prep=0 and 13 °C, the microstructure gradually changes from networks having relatively small pores to those exhibiting regular assembly of polyhedral large pores of about 10¹ μm in sizes. The formation of the porous structure at or below 0 °C is as a result of the cooling-induced phase separation mechanism, while the large polyhedral pores in the networks prepared at higher temperatures form during the freeze-dry process of the hydrogels after preparation.     

Controlling micro-porous size in TiO2 pellets processed by sol-gel and rapid liquid phase sintering

A fast and lower electric energy consumption process to synthesize TiO₂ pellets with interconnected micropores, is proposed. Pellets were prepared by rapid liquid-phase sintering (RLPS) at different temperatures (900, 1000 and 1100 °C) and times (2, 5, 7 and 10 min). The density of these samples increases when temperature rises and decreases for longer sintering times; the highest density, of 2.78 g/cm³ was obtained when sintering at 1100 °C/2min. The addition of PEG and the annealing at 450 °C/2 min produced pores of 38.51 ± 27.51 μm and 48.98 ± 32.34 μm when PEG3350 and PEG8000 respectively, were used. An additional RLPS at 1100 °C/2 min gives rise to TiO₂ pellets in a rutile phase, with pores of 76.82 ± 34.23 μm and 173.04 ± 68.03 μm for PEG3350 and PEG8000, respectively. Interconnectivity of pores is obtained in all samples. The elastic module of these pellets was 39.22 ± 0.16 GPa, for the sample prepared with PEG3350; and 121.30 ± 0.04 GPa for the one made with PEG8000. The achieved pore size and interconnectivity at 1100 °C/2 min are a result of the optimized sintering conditions and the better control of PEG vapor pressure released when the intermediate annealing at 450 °C/2 min is introduced.     

Mechanical strengths of epoxy resin composites reinforced by calcined pearl shell powders

A series of epoxy resin (EP) composites were prepared using ground pearl shell powders, which had been calcined at various temperatures. The EP composite containing ～ 3% weight content of the calcined pearl shell powder had the highest impact strength and the presence of silane agent was found to be essential for the composite formulation. The impact strengths of the resultant EP composites were highly influenced by the specific surface area, surface morphological structure, and chemical composition of the calcined pearl shell powder. The highest mechanical improvement was obtained for the EP composite prepared with the pearl shell powder calcined at 700°C for 3 h. The layered biopolymeric materials were completely degraded for the pearl shell powder calcined at 700°C, resulting in “sponge‐like” or “net‐like” porous calcium carbonate powder. However, the degradation of the layered biopolymeric materials was not complete for the calcinations at lower temperatures (<600°C), while calcium carbonate decomposed to form calcium oxide at higher temperatures (>800°C). The mechanical improvements of the processed EP composites have been discussed along with the chemical compositions and surface microstructures of the incorporated pearl shell powders. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

CO2 Capture Behavior of Shell during Calcination/Carbonation Cycles

The cyclic carbonation performances of shells as CO₂ sorbents were investigated during multiple calcination/carbonation cycles. The carbonation kinetics of the shell and limestone are similar since they both exhibit a fast kinetically controlled reaction regime and a diffusion controlled reaction regime, but their carbonation rates differ between these two regions. Shell achieves the maximum carbonation conversion for carbonation at 680–700 °C. The mactra veneriformis shell and mussel shell exhibit higher carbonation conversions than limestone after several cycles at the same reaction conditions. The carbonation conversion of scallop shell is slightly higher than that of limestone after a series of cycles. The calcined shell appears more porous than calcined limestone, and possesses more pores > 230 nm, which allow large CO₂ diffusion‐carbonation reaction rates and higher conversion due to the increased surface area of the shell. The pores of the shell that are greater than 230 nm do not sinter significantly. The shell has more sodium ions than limestone, which probably leads to an improvement in the cyclic carbonation performance during the multiple calcination/carbonation cycles.     

Advancing the fabrication of YSZ-inverse photonic glasses for broadband omnidirectional reflector films

A single-step and all-colloidal deposition method to fabricate yttrium-stabilized zirconia (YSZ)-inverse photonic glasses with 3 μm pores was developed. The process is based on electrostatic attraction and repulsion in suspension, controlled by surface charge of polystyrene (PS) microspheres and YSZ nanoparticles, used as pore templates and matrix material, respectively. The pH was used as a tool to change surface charges and particle-particle interactions. Photonic glass films with 3 μm pores yielded broadband omnidirectional reflection over the wavelengths of 1–5 μm, relevant for thermal radiation at temperatures around 1200 °C. These highly porous materials maintained their structural stability and reflectance after being annealed at 1200 °C for 120 h.     

Permeation of gases across the poly(chloro-p-xylylene) membrane

Gas permeabilities across poly(chloro-p-xylylene) (parylene C) films are measured with different thicknesses of 20.2, 10.0, 8.9, 4.6, 3.4, and 1.0 μm. Measurements were carried out below 1 atm and between 10 and 80°C, which are under the glass transition temperature. The temperature and pressure dependencies of the permeability and the apparent diffusion coefficients were measured. If the membrane thickness is larger than 8 μ, the gas-transport mechanism is solution–diffusion, which implies that it is pinhole-free, because the pressure dependency of the permeability cannot be found and the apparent activation energy of permeation and diffusion are observed. If the membrane thickness is less than 8 μ, the gas transport mechanism is pore flow combined with solution–diffusion flow because gas may penetrate both the porous area and the polymer matrix. The thinner the membrane, the higher is the permeability coefficient, since the diameter and number of pores increase with decrease of the membrane thickness. The gas permeability coefficient has different values at the same pressure or temperature. As this film is in the glassy state, it should be explained using the average ordering parameter (ξ), which is a function of temperature, pressure, gas concentration, and time. © 1994 John Wiley & Sons, Inc.     

Fabrication and performance of reticular ceramic fiber membranes by freeze casting using a gel network

It is of great practical significance to manufacture ceramic membranes with good reusability. We used a macromolecular gel network for in situ enhancement of a reticulated skeleton ceramic (RSC) membrane. The prepared RSC membranes were firstly characterized to determine their physical and chemical properties. The sample calcined at 1400 °C has a porosity of 41.66 %, a pore size of 0.34 μm, a mechanical strength of 4.53 MPa, and a permeability of 63.92 L m⁻¹ h⁻¹ bar⁻¹. This membrane can effectively reduce the content of total organic carbon (TOC) (80 %) and calcium ion (>85 %). In addition, the flux of RSC fouled by alginate, calcium ions, or their mixture is effectively recovered by simple chemical rinsing using sodium hydroxide and ethylenediaminetetraacetic acid.     

Porous chitin matrices for tissue engineering: Fabrication and in vitro cytotoxic assessment

A series of porous chitin matrices were fabricated by freezing and lyophilization of chitin gels cast from a 5% N,N-dimethylacetamide (DMAc)/lithium chloride (LiCl) solvent system. The porous chitin matrices were found to have uniform pore structure in the micron range. Scanning electron microscopy (SEM) revealed that the pore size of the porous chitin matrices varied according to the freezing method prior to lyophilization. By subjecting the chitin gels to dry-ice/acetone (−38 °C), the final porous chitin matrix gave pore dimensions measuring 200–500 μm with 69% porosity. A smaller pore dimension of 100–200 μm with 61% porosity was produced when the chitin gels were frozen by liquid nitrogen (−196 °C) and 10 μm pores with 54% porosity were produced when the gels were placed in a freezer (−20 °C) for 20 min. In comparison, lower porosity structures of ca. 10% porosity were obtained from both air-dried and critical point dried chitin gels. Furthermore, a low gel concentration (< 0.5%, w/w) also produced porous morphology by vacuum drying without any freezing and lyophilization. The resulting pore properties influenced the water uptake profile of the materials in water. These porous chitin matrices are found to be non-cytotoxic and to hold promise as potential structural scaffolds for cell growth and proliferation in vitro.