一步成型制备非对称多孔YSZ中空纤维陶瓷膜

采用有机相转化与固态粒子烧结法相结合,通过一步成型制备了非对称多孔氧化钇稳定氧化锆(YSZ)中空纤维陶瓷膜。SEM微观结构分析表明,制备的YSZ中空纤维陶瓷膜为非对称多孔结构,中间为多孔海绵状结构,内外两侧为指孔结构,且内外表层平均孔径分别为0.43μm和0.18μm。YSZ中空纤维陶瓷膜抗弯强度和纯净水通量分别为210.5MPa和4.02m~3·m~(-2)·h~(-1)·bar~(-1)。     

Chemical and Microstructural Changes at Alkali-Resistant Glass Fiber–Cement Interfaces

Portland cement matrices have been reinforced by 5 wt% of "CemFIL 1'alkali-resistant glass fiber and isothermally wet cured for up to 1 yr at 20° and 55°C. Degradation of mechanical properties occurs as a consequence of chemical attack on the fibers. Various stages of the degradative process are followed by electron microscopy and analysis. The original composite microstructure is described in terms of a solid fiber reinforcing system. However, the fibers gradually become hollow, leaving a concentric shell of cement-fiber reaction product which still gives a useful measure of reinforcement. These changes, together with fiber shortening arising from local impingement of Ca(OH)₂ crystals, give rise to what is termed a hollow cylinder reinforcement system. The hollow shell structures consist of gel and semicrystalline material; glass is absent or nearly so. The role of zirconia in stabilizing the hollow cylinder structures is described. It is considered that the microstructure developed at ∼20°C, while not as effective a reinforcement as solid fibers, still provides useful tensile reinforcement relative to plain paste matrices.     

Porous YSZ Ceramics Reinforced by Different Kinds of Fibers

Porous YSZ ceramics reinforced by different fibers were prepared by gel‐casting with 15% solid content and pressureless sintering. The four kinds of fibers (mullite, aluminosilicate, Al₂O₃, and YSZ fibers) were added into the YSZ ceramics with the same 10% vol content. After sintered at 1500°C for 2 h, aluminosilicate and mullite fibers could not be found in the samples of porous YSZ ceramics, which showed they reacted with YSZ ceramics at high temperature, while YSZ and Al₂O₃ fibers still kept perfect after sintering. Furthermore, the influences of fiber content, sintering temperature, porosity of matrix materials on compressive strength and porosity of the porous YSZ ceramics were studied. The results showed that Al₂O₃ fiber showed more obvious reinforcing effect than YSZ fiber on porous YSZ ceramics. The fiber‐reinforcing effects depend on fiber content, sintering temperature, and porosity of matrix materials. The fiber addition can improve the shrinkage behavior of porous ceramics during sintering and strengthen the skeleton of porous ceramics.     

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe₂O₃ powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe₂O₃-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe₂O₃. It is suggested that the Fe₂O₃ dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe₂O₃-YSZ hollow fiber membranes sintered at 1150 °C show a 3-fold increase of the permeate flux of pure water (F) (743 L m⁻² h⁻¹) along with comparable bending strength (152 MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.     

Macro-porous dolomite hollow fibers sintered at different temperatures toward widened applications

Ceramic hollow fibers from natural dolomite with different pore structures have been designed. The unique hollow fibers were produced by the phase inversion method and applying different sintering temperatures. The hollow fiber precursor presented coagulated polymers through the fiber thickness due to the high granulometric size of the used dolomite material (11.3–47.2 µm). The fiber sintered at 400 °C presented mechanical strength of 4.5 MPa and water permeability of 84.7 L h⁻¹ m⁻² kPa⁻¹. The increase in the sintering temperature up to 1250 °C resulted in fragile hollow fibers due to dolomite transformations that resulted in gas release and a significant mass loss of 33.7%. At 1350 °C, the liquid phase sintering mechanism occurred and the dolomite hollow fiber sintered at 1350 °C presented mechanical strength of 5.5 MPa and water permeability of 2219 L h⁻¹ m⁻² kPa⁻¹. Doloma dissolution in water was investigated and calcium concentration was increased from 0.72 (pure water) to 2.905 ppm for a contact time from 4 h between the fiber sintered at 1250 °C and pure water. However, this dissolution did not decrease the mechanical resistance of the fiber. These results suggest the potential of applying natural dolomite for producing low cost membranes or substrates. The hollow fiber sintered at 400 °C is suggested to be used as a proper separation medium, while the hollow fiber sintered at 1350 °C may be used as a substrate for the deposition of a separation layer to be used in gas separations. The high porosity of the fiber sintered at 1250 °C suggests its application as a support for the impregnation of functional materials. Thus, depending on the applied sintering temperature the dolomite membrane can be used in different applications.     

From Chelating Precursor to Perovskite Oxides and Hollow Fiber Membranes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) is a promising mixed-conducting ceramic membrane material in addition to being a good electrode catalyst for solid oxide fuel cells. In this study, BSCF powder was synthesized via a chelated water-soluble complex method at relatively low temperatures. The combined ethylenediaminetetraacetic acid and citric acid was used for the synthesis of a complex-based precursor, followed by thermal decomposition of the precursor at high temperatures. Thermal behavior, crystal phases, and structures of the prepared powders were characterized by thermogravimetric analysis/differential scanning calorimetry, XRD, and scanning electron microscopic (SEM) techniques, respectively. Pure and single-phase perovskite could be obtained after sintering at a temperature higher than 800°C for 5 h. The soft precursor powder synthesized at lower temperatures, i.e., 600°C, is water insoluble and more appropriate for use as a membrane material to prepare gas-tight tubular or hollow fiber ceramic membranes. By contrast, the hollow fibers prepared via the traditional techniques where the perovskite powder is used as the starting membrane materials display gas leakage. The fibers were characterized by SEM, XRD, and tested for air separation at ambient pressure and temperatures between 700° and 950°C. The oxygen flux measured in this work reached 3.90 mL·(min·cm²)⁻¹ and compares favorably with any experimental values reported in the open literature.     

Preparation of poly(4‐methyl‐1‐pentene) asymmetric or microporous hollow‐fiber membranes by melt‐spun and cold‐stretch method

Poly(4‐methyl‐1‐pentene) (PMP) hollow fibers were prepared and fabricated into gas separation or microporous membranes by the melt‐spun and cold‐stretched method. PMP resin was melt‐extruded into hollow fibers with cold air as the cooling medium. The effects of take‐up speed and thermotreatment on the mechanical behavior and morphology of the fibers were investigated. Scanning electronic microscope (SEM) photos were used to reveal the geometric structure of the section and surface of the hollow fibers. It was found that the original fiber had an asymmetric structure. A “sandwich” mode was used to describe the formation of this special fine structure. And a series of PMP hollow‐fiber membranes were prepared by subsequent drawing, and it was found that there was a “skin–core” structure on the cross section of these hollow‐fiber membranes. Asymmetric or microporous PMP hollow‐fiber membranes could be obtained by controlling posttreatment conditions. The morphology of these membranes were characterized by SEM, and the gas (oxygen, nitrogen, and carbon dioxide) permeation properties of the membranes was measured. The results indicate that the annealing time of the original fiber and the stretching ratio were the key factors influencing the structure of the resulting membrane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2131–2141, 2006     

Temperature‐sensitive PVDF hollow fiber membrane fabricated at different air gap lengths

This article describes preparation of temperature‐sensitive poly(vinylidene fluoride) hollow fiber membranes using the dry‐wet spinning technique and investigates effects of air gap length on the structures and performances. In spinning these hollow fibers, N,N‐dimethyl formamide and poly(ethylene glycol) (10,000) were used as the solvent and pore‐forming agent, respectively. The prepared fiber membranes were characterized by scanning electron microscopy, pore size measurement, filtration experiments of pure water flux, and solutes with different molecular weights. The fiber membranes exhibit a quite asymmetric structures consisting of double skin layers situated on the fiber walls, two finger‐like layers near skin layers as well as macrovoids and sponge‐like structures at the center of the fiber cross‐sections. Remarkable changes of pure water flux and retention of solute are observed around 32°C, indicating an excellent temperature‐sensitive permeability. As the air gap length increases, the pore size of fiber membrane decreases, which results in decrease of pure water flux and allows small molecules to permeate through the fiber membrane. POLYM. ENG. SCI., 53:2519–2526, 2013. © 2013 Society of Plastics Engineers     

Improved microstructure of asymmetric niobium pentoxide hollow fiber membranes

Asymmetric niobium pentoxide (Nb₂O₅) hollow fiber membranes were prepared by the phase inversion and sintering process at temperatures ranging from 1000 to 1350°C. The effects of extrusion parameters on the morphology and properties of the produced membranes were systematically explored. Asymmetric hollow fibers with regular inner contour were obtained at extrusion flow rates of 15 and 25 ml min⁻¹ of ceramic suspension and internal coagulant, respectively. Hollow fibers sintered at temperatures greater than 1200°C presented modifications in the morphology of Nb₂O₅ grains, which were also evidenced by X-ray diffraction and Raman analyses. Hollow fibers produced with an air gap of 50 mm presented a dense outer sponge-like layer and micro-voids formed from the inner surface. These hollow fibers sintered at 1200°C presented suitable bending resistance and water permeability (24.2 ± 0.60 MPa and 3.00 ± 0.01 L h^-1 m^-2 kPa^-1, respectively). The outer sponge like layer was mitigated when the fibers were produced without air-gap.     

Influence of sintering temperature on the morphology of ceramic hollow fibers prepared from niobium pentoxide

Ceramic hollow fibers were prepared by the phase inversion and sintering method using niobium pentoxide (Nb₂O₅) as an innovative starting material. X-ray diffraction and Raman analyses revealed the same monoclinic crystalline phase for the ceramic material, H-Nb₂O₅, at all the evaluated sintering temperatures. According to SEM images, the starting material was composed of polydisperse particles of irregular size and shape with sizes ranging from 12.5 to 89.7 μm. The increase in the sintering temperature caused particles agglomeration. In the hollow fiber precursor (without sintering), Nb₂O₅ grains were surrounded by the coagulated polymer. The polymeric phase was eliminated when the fibers were sintered at temperatures above 600°C. When sintered at 1350°C, the outer surface of the fiber presented elongated crystals of well-defined shape, while agglomerated round shape grains were observed at the inner surface of the fiber. Formation of these elongated crystals was probable due to the material sintering at high temperatures (up to 1350°C) for more than 300 minutes. This study demonstrated the potential for general applicability of niobium pentoxide to fabricate ceramic hollow fiber membranes.     

Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported micro-tubular solid oxide fuel cells

In this work NiO/3 mol% Y₂O₃–ZrO₂ (3YSZ) and NiO/8 mol% Y₂O₃–ZrO₂ (8YSZ) hollow fibers were prepared by phase-inversion. The effect of different kinds of YSZ (3YSZ and 8YSZ) on the porosity, electrical conductivity, shrinkage and flexural strength of the hollow fibers were systematically evaluated. When compared with Ni–8YSZ the porosity and shrinkage of Ni–3YSZ hollow fibers increases while the electrical conductivity decreases, while at the same time also exhibiting enhanced flexural strength. Single cells with Ni–3YSZ and Ni–8YSZ hollow fibers as the supported anode were successfully fabricated showing maximum power densities of 0.53 and 0.67 W cm⁻² at 800 °C, respectively. Furthermore, in order to improve the cell performance, a Ni–8YSZ anode functional layer was added between the electrolyte and Ni–YSZ hollow fiber. Here enhanced peak power densities of 0.79 and 0.73 W cm⁻² were achieved at 800 °C for single cells with Ni–3YSZ and Ni–8YSZ hollow fibers, respectively.     

Space Charge-Mediated Ionic Transport in Yttria-Stabilized Zirconia–Alumina Composite Membranes

This paper reports ionic conductivity of yttria-stabilized zirconia (YSZ)–Al₂O₃ composite membranes. The tape cast specimens were subjected to binder burnout (500°C) and sintering (1550°C) processes to obtain 200–300 μm thick membranes. The ionic conductivity and microstructure of the membranes were characterized and are discussed in this paper. The ionic conductivity of the composite specimens was enhanced and was correlated with the number of charge carrier and their mobility. The solubility of Al₂O₃ in YSZ was minimal and nanosize Al₂O₃ of the batch sintered into microsize and existed as a distinct phase. The scanning electron microscopy micrographs revealed that YSZ and Al₂O₃ grains were strained.     

Synthesis and Characterization of Suspension-Derived, Porous Ion-Conducting Ceramic Membranes

Porous, oxygen-ion-conducting ceramic membranes can have applications as supports for fuel cells, sensors, and thin membrane films, or as filters for membrane filtration. This paper reports on the preparation of unsupported and supported yttria-stabilized zirconia (YSZ) and yttria-doped bismuth oxide (BY) membranes with submicrometer pore sizes. Fluorite-structured BY powder that has been synthesized using the citrate method and commercial YSZ powder have been used to prepare stable aqueous suspensions. Unsupported and supported YSZ and BY membranes have been prepared from the stable suspensions of YSZ and BY. The supported BY membranes are crack free but contain small defects. Defect-free YSZ membranes that are supported on porous alumina have been prepared under controlled conditions. The average pore size is 100 nm, with a porosity of 57%, for an unsupported YSZ membrane (measured by mercury porosimetry), and 114 nm for a supported membrane (as estimated via helium permeation). The ionic conductivity of the YSZ membranes is 0.00044–0.01 S/cm in the temperature range of 600°–900°C, which is lower than that of dense YSZ disks.     

Ultra-thin skin carbon hollow fiber membranes for sustainable molecular separations

A transformative platform is reported to derive ultra-thin carbon molecular sieve (CMS) hollow fiber membranes from dual-layer precursor hollow fibers with independently tuned skin layer and substrate properties. These ultra-thin CMS hollow fiber membranes show attractive CO₂/CH₄ separation factors and excellent CO₂ permeances up to ~1,400% higher than state-of-the-art asymmetric CMS hollow fiber membranes. They provide a unique combination of permeance and selectivity competitive with zeolite membranes, but with much higher membrane packing density and potentially much lower costs.     

Nanocrystalline Yttria-Stabilized Zirconia Fibers from Plant Fibers and their Polymer Composites

Nanocrystalline zirconia–8-mol% yttria (yttria-stabilized zirconia (YSZ): ZrO₂–8-m% Y₂O₃) fibers have been prepared from aqueous poly vinyl alcohol (PVA)–zirconium oxy nitrate solution and jute (plant fiber). Soluble Zr and Y ions in PVA solution formed a uniform coating on the surface of jute once it dried completely. Slow hydrolysis of zirconium ion with ammonium hydroxide deposited zirconium hydroxide on the jute surface. Decomposition of the dried zirconium hydroxide-coated jute at high temperature (1200°C/2 h) resulted in the formation of single-phase, nanocrystalline cubic-YSZ with the corresponding average X-ray crystallite size 30–35 nm. Heat-treated fibers have been characterized by X-ray diffraction and scanning electron microscopy. We also prepared polymer composites by incorporating chopped, ground YSZ fibers into epoxy matrix and investigated polymer/fiber interface by transmission electron microscopy analysis.     

Modeling study of oxygen permeation through an electronically short‐circuited YSZ‐based asymmetric hollow fiber membrane

Here, oxygen fluxes through an electronically short‐circuited asymmetric Ag‐YSZ|YSZ|LSM‐YSZ hollow fiber prepared via a combined spinning and sintering route were tested and correlated to an explicit oxygen permeation model. The average oxygen permeation through such asymmetric hollow fiber with a 27 μm‐thick YSZ dense layer reached 0.52 mL (STP) cm^?2 min^?1 at 1173 K. From the model results, we can obtain the characteristic thickness, the effects of the temperature, and the effect of He sweep gas flow rate to the individual step contribution. The oxygen partial pressure variation in the permeate side, the local oxygen flux, and the three‐different resistance distribution along the axial direction of the asymmetric hollow fiber are theoretically studied; providing guidelines to further improve the membrane performance for oxygen separation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3491–3500, 2017     

Phase stability (at 1000°C) of hollow t-ZrO2 fibers costabilized by lanthana and yttria

Yttrium-stabilized ZrO₂ (YSZ) hollow fibers derived from a ceiba template present a 25%–53% reduction in thermal conductivity compared with traditional YSZ solid fibers. However, after prolonged preservation at 1000°C, tetragonal ZrO₂ (t-ZrO₂) can easily transform to monoclinic ZrO₂ (m-ZrO₂), which destroys the hollow structure of the YSZ fibers and results in loss of the structural advantages for heat insulation. To overcome this, in this study, biomorphic lanthana and yttrium costabilized zirconia (LaYSZ) fibers with a hollow structure are fabricated by doping appropriate amounts of lanthanum in the raw materials of YSZ fibers. X-ray diffraction, scanning electron microscopy, and thermal conductivity measurements are utilized to confirm the phase-stability superiority of LaYSZ fibers to that of YSZ fibers under harsh conditions. After preservation at 1000°C for 150 hours, the m-ZrO₂ content in the LaYSZ hollow fibers increases from 0 to 3.4 mol%, whereas that in the YSZ fibers increases from 0 to 10.25 mol%. Furthermore, owing to their better phase stability at 1000°C, the morphologies and heat-insulating properties of LaYSZ fibers are more improved in several aspects compared with YSZ fibers.     

Combined Effects of Exposure to Salt (NaCl) Water and Oxidation on the Strength of Uncoated and BN-Coated Nicalon™ Fibers

The environmental stability of uncoated and boron nitride-coated (BN-coated) Nicalon fiber has been investigated by studying the effect of annealing in air at 1000°C (2 h) on the strength of the fibers. The results imply that uncoated and BN-coated fibers both degrade in strength, with the BN-coated fiber suffering a higher strength loss. The degradation is significantly enhanced if the fibers are exposed to salt (NaCl) water prior to the air anneal, if the concentration of salt is >0.5 wt%. The BN-coated Nicalon fibers also have been studied at 800° and 900°C; the degradation in strength due to salt water exposure is greater at 800°C than that at 900° or 1000°C.     

Upscaling of asymmetric hollow fiber-supported thin film membranes for oxygen separation from air: Proof of concept

Hollow fiber membranes demonstrate various advantages for high performance oxygen separation. However, the small diameters of hollow fibers and the brittleness of ceramics limit their mechanical strength, imposing great difficulties on stack and module development. Gas-tight sealing is another challenge for upscaling of hollow fiber membrane technology. Low temperature sealant materials of epoxy resin or silicon are typically used for hollow fiber stacks, requiring that the sealing portions be located out of hot zone. Consequently, only partial length of hollow fibers participates in oxygen permeation. In this study, upscaling of our recently developed asymmetric hollow fiber-supported thin film membranes is conducted, where individual hollow fibers are assembled in parallel to form a stack. A reliable gas-tight sealing is obtained by combining ceramic paste with conductive adhesive ink cohesively. Comprehensive oxygen permeation test is conducted with the sealing portions being in hot zone and compared with a single hollow fiber membrane. Fundamental mechanism is discussed to understand the performances and their differences. An accelerated long-term test (∼320 h, 16 thermal cycles) demonstrates excellent stability and robustness of the stack and sealing. The characterization of post-test samples further confirms excellent stability and robustness of the phases and microstructures of the stack.     

Effect of Nickel Oxide/Yttria-Stabilized Zirconia Anode Precursor Sintering Temperature on the Properties of Solid Oxide Fuel Cells

An NiO/yttria-stabilized zirconia (YSZ) layer sintered at temperatures between 1100° and 1500°C onto dense YSZ electrolyte foils forms the precursor structure for a porous Ni/YSZ cermet anode for solid oxide fuel cells. Conflicting requirements for the electrochemical performance and mechanical strength of such cells are investigated. A minimum polarization resistance of 0.09 Ω^.cm²at 1000°C in moist hydrogen is obtained for sintering temperatures of 1300°–1400°C. The mechanical strength of the cells decreases with increased sintering temperature because of the formation of channel cracks in the electrode layers, originating in a thermal expansion coefficient mismatch between the layers.