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.     

High-flux CO2-stable oxygen transport hollow fiber membranes through surface engineering

The influences of bulk diffusion and surface exchange on oxygen transport of (La_0.6Ca_0.4)(Co_0.8Fe_0.2)O_3-δ (LCCF) hollow fiber membranes were investigated. As an outcome, two strategies for increasing the oxygen permeation were pursued. First, porous LCCF hollow fibers as support were coated with a 22 μm dense LCCF separation layer through dip coating and co-sintering. The oxygen permeation of the porous fiber with dense layer reached up to 5.10 mL min^?1 cm^-2 at 1000 °C in a 50 % CO₂ atmosphere. Second, surface etching of dense LCCF hollow fibers with H₂SO₄ was applied. The surface etching of both inner and outer surfaces leads to a permeation improvement up to 86.0 %. This finding implies that the surface exchange reaction plays a key role in oxygen transport through LCCF hollow fibers. A good long-term (>250 h) stability of the asymmetric hollow fiber in a 50 % CO₂ atmosphere was found at 900 °C.     

Preparation and characterization of poly(vinyl chloride)/polystyrene/poly(ethylene glycol) hollow‐fiber ultrafiltration membranes

Hollow‐fiber ultrafiltration (UF) membranes were prepared from blends of poly(vinyl chloride) (PVC) and polystyrene (PS) with a dry/wet phase inversion method. Poly(ethylene glycol) (PEG) and N,N‐dimethylacetamide were used as the additive and solvent, respectively. The effects of the PEG concentration in the dope solution as an additive on the cross sections and inner and outer surface morphologies, permeability, and separation performance of the hollow fibers were examined. The mean pore size, pore size distribution, and mean roughness of both the inner and outer surfaces of the produced hollow fibers were determined by atomic force microscopy. Also, the mechanical properties of the hollow‐fiber membranes were investigated. UF experiments were conducted with aqueous solutions of poly(vinyl pyrrolidone) (PVP; K‐90, M_w = 360 kDa). From the results, we found that the PVC/PS hollow‐fiber membranes had two layers with a fingerlike structure. These two layers were changed from a wide and long to a thin and short morphology with increasing PEG concentration. A novel and until now undescribed shape of the nodules in the outer surfaces, which was denoted as a sea‐waves shape, was observed. The outer and inner pore sizes both increased with increasing PEG concentration. The water permeation flux of the hollow fibers increased from 104 to 367 L m^?2 h^?1 bar^?1) at higher PEG concentrations. The PVP rejection reached the highest value at a PEG concentration of 4 wt %, whereas at higher values (from 4 to 9 wt %), the rejection decreased. The same trend was found also for the tensile stress at break, Young's modulus, and elongation at break of the hollow fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 989‐1004, 2013     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Thermo-responsive mixed-matrix hollow fiber membranes

Up to date, preparation of thermo-responsive mixed-matrix membranes (MMM) has only be described as small scale flat membranes or multi-step processes for hollow fiber membranes. In this work, the development of thermo-responsive MMM hollow fibers composed of polyethersulfone as membrane polymer and poly(N-isopropylacrylamide) (PNIPAM) microgel particles via the wet spinning process is presented. PNIPAM particles are synthesized with (NP-S, z_{avg 20°C} = 105 nm) and without (NP-L, z_{avg 20°C} = 250 nm) sodium dodecyl sulfate and their thermo-responsive behavior is characterized by dynamic light scattering. Particle size (NP-S, NP-L), particle content (10%, 15%) and the extrusion pressure in the wet spinning process (1.0–3.0 bar) are investigated as experimental parameters. Reversible thermo-responsive behavior of the hollow fibers is demonstrated by water permeability measurements at different temperatures (20 and 50°C). The largest switching factors (R) are observed for the hollow fibers containing NP-L. For 15% NP-L and 1 bar extrusion pressure, water permeances between 0.5 and 6.0 L m⁻² h⁻¹ bar⁻¹ are observed, corresponding to R = 12 and a dextran (500 kDa) rejection of 91% at 25°C.     

Ni4Nb2O9 ceramics prepared by the reaction-sintering process

Fabrication of Ni₄Nb₂O₉ ceramics via a reaction-sintering process was investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. Ni₄Nb₂O₉ phase appeared at 1300 °C and increased with increasing soak time. Ni₄Nb₂O₉ content was found >96% in 1350 °C/2 h sintering pellets. A density of 5.71 g/cm³ was obtained for pellets sintered at 1350 °C for 2 h. This reaches 96.5% of the theoretical density. As the sintering temperature increased to 1350 °C, an abnormal grain growth occurred and grains >100 μm could be found. ?_r of 15.4–16.9 are found in pellets sintered at 1200–1300 °C. Q × f increased from 9380 GHz in pellets sintered at 1200 °C to 14,650 GHz in pellets sintered at 1250 °C.     

Preparation and oxygen permeation of U‐shaped perovskite hollow‐fiber membranes

The oxygen permeation of dense U‐shaped perovskite hollow‐fiber membranes based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ prepared by a phase inversion spinning process is reported. The perovskite hollow fibers with totally dense wall were obtained with the outer diameter of 1.147 mm and the inner diameter of 0.691 mm. The dependences of the oxygen permeation on the air flow rate on the shell side, the helium flow rate on the core side, the oxygen partial pressures, and the operating temperatures were experimentally investigated. According to the Wagner theory, it follows that the oxygen transport through the U‐shaped hollow‐fiber membrane is controlled by both surface reaction and bulk diffusion at the temperature ranges of 750–950°C. High oxygen permeation flux of 3.0 ml/(min cm²) was kept for about 250 h at 950°C under the conditions of the air feed flow rate of 150 ml/min and the helium flow rate of 50 ml/min. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Thermodynamic Study of Nonstoichiometric Niobium Pentoxide

The partial pressure of oxygen in equilibrium with a niobium dioxide-niobium pentoxide mixture was determined from electrochemical cell measurements over the range 800° to 1070°C and from thermogravimetric gas-equilibrium measurements at 1000° and 1200°C. These measurements compare favorably with those of previous investigators. An isopiestic technique was used to determine the composition at 100°C intervals for nonstoichiometric Nb₂O₅ in equilibrium with NbO₂ between 900° and 1100°C. The departure from stoichiometry for Nb₂O₅ decreased from Nb₂O₄.₈₁ at 1090°C to Nb₂0_4.887 at 889°C. The partial pressure of oxygen in equilibrium with different compositions of nonstoichiometric Nb₂O₅ was determined from electrochemical cell measurements over the range 800° to 1070°C and from thermogravimetric measurements by equilibrating the nonstoichiometric Nb₂O₅ specimen with an H₂/H₂O gaseous mixture at 900° to 1200°C. Th_e relative partial molar enthalpy of oxygen, ΔII O₂ for the Nb₂O₅ -χ phase (calculated from the cell measurements) was independent, within experimental error, of both temperature and composition, the average value being -164 kcal/ mole. These equilibrium measurements were compared with the therrnogravimetric study of nonstoichiometric Nb₂O₅ by Kofstad and Anderson and a departure from their relation χ (Nb₂O₅-χ) ∝pO₂-¹/6 was observed at the low oxygen end of the homogeneous phase field.     

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.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.     

Synthesis of Zn3Nb2O8 ceramics using a simple and effective process

Synthesis of Zn₃Nb₂O₈ ceramics using a simple and effective reaction-sintering process was investigated. The mixture of ZnO and Nb₂O₅ was pressed and sintered directly without any prior calcination. Single-phase Zn₃Nb₂O₈ ceramics could be obtained. Density of these ceramics increased with soaking time and sintering temperature. A maximum density 5.72 g/cm³ (99.7% of the theoretical density) was found for pellets sintered at 1170 °C for 2 h. Pores were not found and grain sizes >20 μm were observed in pellets sintered at 1170 °C. Abnormal grain growth occurred and grains >50 μm could be seen in Zn₃Nb₂O₈ ceramics sintered at 1200 °C for 2 h and 1200 °C for 4 h. Reaction-sintering process is then a simple and effective method to produce Zn₃Nb₂O₈ ceramics for applications in microwave dielectric resonators.     

Microstructure and mechanical behavior of TiO2-MnO-doped alumina/alumina laminates

Tapes of TiO₂-MnO-doped alumina (d-Al₂O₃) and pure alumina (Al₂O₃) were shaped via tape casting. Laminates with three different layer numbers and respective thicknesses were produced and sintered at 1200°C. The microstructure and mechanical behavior of laminates were investigated and compared to the respective monolithic references (d-Al₂O₃ and Al₂O₃). The use of dopants in alumina decreased the initial sintering temperature, leading to higher densification at 1200°C (~98% theoretical density (TD)) when compared to Al₂O₃ (~73% TD). The higher density was reflected in a higher Young's modulus and hardness for doped alumina. A region of diffusion of dopants in pure alumina layers was observed along the interface with doped layers. The mechanical strength of d-Al₂O₃ samples sintered at 1200°C was not statistically different from Al₂O₃ samples sintered at 1350°C. The strength of laminates composed of doped layers with undoped, porous interlayers did not change. Nevertheless, as the thickness of these porous interlayers increases, a loss of strength was observed. Monolithic references showed constant values of fracture toughness (K_IC), ~2 MPa·m^1/2, and linear crack path. On the other hand, K_IC of laminates increases when the crack propagates from weak Al₂O₃ layers to dense d-Al₂O₃ layers.     

Large-scale synthesis of hollow carbon fibers with ultra-large diameter by thermally controlled pyrolysis

Hollow carbon fibers (HCF) with ultra-large diameter have been synthesized and the versatility to convert them into the corresponding carbon-based composites has been demonstrated. The hollow carbon fibers were fabricated by thermal controlled carbonization of electrospun polyacrylonitrile fibers. For the existence of inorganic silica shell during pyrolysis, heat release will be blocked at the boundary, driving the polyacrylonitrile precursor fiber to form hollow structure. The diameter of the as-prepared hollow carbon fibers can exceed 150 nm. Sol-gel-derived Fe₃O₄ nanoparticles can grow on the outer-surface and the inner-surface of hollow carbon fibers. The microwave absorption performance of ternary HCF@Fe₃O₄@PPy composite is testified and the values of reflection loss exceeding −10 dB can be obtained in the frequency of 3.3-11.3 GHz. The large diameter of hollow carbon fibers can have inner and outer interfaces in the corresponding composites, which make them great potential for a variety of applications in future.     

Microstructure and dielectric properties of strontium barium niobate ceramics synthesized by partial coprecipitation

Strontium Barium Niobate, Sr_0.5Ba_0.5Nb₂O₆ (SBN50), has been synthesized, for the first time, by partial coprecipitation of SrCl₂ and BaCl₂ on Nb₂O₅. Powder X-ray diffraction study shows tetragonal tungsten bronze phase formation at 1200 °C. Particle morphology and size of calcined powder has been examined using Transmission Electron Microscopy. The particle size of calcined powder ranges between 250 and 300 nm. The green compacts have been sintered at 1250, 1300 and 1350 °C and at each temperature for 6, 12 and 24 h, respectively. Effect of sintering time and temperature on dielectric properties has been investigated. Scanning electron microscopy has been used for grain morphology studies. Grains have been found to be tetragonal in shape and show variation in size for different sintering conditions. Highest dielectric constant (ε) has been observed for the pellet sintered at 1350 °C for 6 h. A variation in T_c from 93 to 119 °C has also been observed.     

Dielectric temperature stability of Nb-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

In this study, the phase structure, microstructure and dielectric properties of Bi_0.5(Na_0.78K_0.22)_0.5(Ti_1-xNb_x)O₃ lead-free ceramics prepared by traditional solid phase sintering method were studied. The second phase pyrochlore bismuth titanate (Bi₂Ti₂O₇) was produced in the system after introduction of Nb⁵⁺. The dielectric constant of the sample (x = 0.03) sintered at 1130 °C at room temperature reached a maximum of 1841, and the dielectric loss was 0.045 minimum. It had been found that the K⁺ and Nb⁵⁺ co-doped Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics exhibited outstanding dielectric-temperature stability within 100–400 °C with T_cc ≤±15%. Result of this research provides a valuable reference for application of BNT based capacitors in high temperature field.     

Structure and dielectric properties of perovskite ceramics in the system Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3

Ceramics in the system Ba(Ni_1/3Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ (BNN–BZN) were prepared by the mixed oxide route. Powders were mixed and milled, calcined at 1100–1200 °C then pressed and sintered at temperatures in the range 1400–1500 °C for 4 h. Selected samples were annealed or slowly cooled after sintering. Most products were in excess of 96% theoretical density. X-ray diffraction confirmed that all specimens were ordered to some degree and could be indexed to hexagonal geometry. Microstructural analysis confirmed the presence of phases related to Ba₅Nb₄O₁₅ and Ba₈Zn₁Nb₆O₂₄ at the surfaces of the samples. The end members BNN and BZN exhibited good dielectric properties with quality factor (Qf) values in excess of 25,000 and 50,000 GHz, respectively, after rapid cooling at 240 °C h⁻¹. In contrast, mid-range compositions had poor Qf values, less than 10,000 GHz. However, after sintering at 1450 °C for 4 h and annealing at 1300 °C for 72 h, specimens of 0.35(Ba(Ni_1/3Nb_2/3)O₃)–0.65(Ba(Zn_1/3Nb_2/3)O₃) exhibit good dielectric properties: τ_f of +0.6 ppm °C⁻¹, relative permittivity of 35 and quality factor in excess of 25,000 GHz. The improvement in properties after annealing is primarily due to an increase in homogeneity.     

Multichannel mixed‐conducting hollow fiber membranes for oxygen separation

A multichannel mixed‐conducting hollow fiber (MMCHF) membrane, 0.5 wt % Nb₂O₅‐doped SrCo_0.8 Fe_0.2O_3‐δ (SCFNb), has been successfully prepared by phase inversion and sintering technique. The crystalline structure, morphology, sintering behavior, breaking load, and oxygen permeability of the MMCHF membrane were studied systematically. The MMCHF membrane with porous‐dense asymmetrical microstructure was obtained with the outer diameter of 2.46 mm and inner tetra‐bore diameter of 0.80 mm. The breaking load of the MMCHF membrane was 3–6 times that of conventional single‐channel mixed‐conducting hollow fiber membrane. The MMCHF membrane showed a high oxygen flux which was about two times that of symmetric capillary membrane at similar conditions as well as a good long‐term stability under low oxygen partial pressure atmosphere. This work proposed a new configuration for the mixed‐conducting membranes, combining advantages of multichannel tubular membrane technology and conventional hollow fibers. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1969–1976, 2014     

Pushing the limits of high performance dual‐layer hollow fiber fabricated via I2PS process in dehydration of ethanol

The immiscibility induced phase separation (I²PS) process was introduced as a novel method to fabricate hollow fibers with exceptionally high water permeance and reasonably high water/ethanol selectivity in dehydration of ethanol by pervaporation. As a continuation of the previous work, this study discloses the mechanisms to enhance the performance of hollow fibers spun via I²PS by elucidating the material selection at the inner‐layer. Moreover, it revealed the methods to reduce mass‐transport resistance by enhancing surface porosity for both inner and outer surfaces to further improve the permeation flux of the membranes. The continuous performance test demonstrates that the fibers spun from the I²PS possess a stable dehydration performance throughout the monitored period of 300 h. A comparison with pervaporation membranes in the literatures verifies the superiority of the membranes spun via I²PS process with the highest water permeation flux of 9.5 kg/m² h and the permeate water purity of 95.8 wt % at 80°C. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3006–3018, 2013     

High strength,low thermal conductivity and collapsible of Y2O3-stablized HfO2 crystalline fibrous membranes

HfO₂ is an important high temperature resistant material, which has a high melting point (2810 °C), good chemical stability and high thermal radiation. Fibrous material has some advantages for heavy refractory matter, such as large aspect ratio, light weight, more energy saving, flexibility and further can be processed into a variety of product forms. In this paper, high strength HfO₂ crystalline nanofibers were prepared through self-synthesis polyacetylcatonahafnium (PAHf) with electrospinning method. Phase variation process was discussed in detail, and the HfO₂ fibers that use Y₂O₃ as stabilizer with phase stability were prepared finally. The fibers were composed of nanocrystals with good flexibility even folded in half, tensile strength of 1.6 MPa @ 1000 °C and can lifting up 1288 times its own weight, low density around 35 mg/cm³. The thermal conductivity was stable around 26 mW/(m·K) @ 1200 °C and the temperature can be cooled from 1200 °C to 396 °C after 180 s through 7 mm fibrous membranes. And after treated at 1200 °C for 5 h, the fibrous membranes also maintain high strength and good flexibility. By this kind of precursor method, high strength oxide fibers can be obtained, which has a wide application prospect in field of high temperature thermal protection.     

Hydrothermal synthesis and sintering of electroceramics

The hydrothermal processing of ultrafine Pb_1·88(Zn_0·3Nb_1·25)O_5·305 and Ba(Mg_0·33Nb_0·67)O₃ powders as well as their sintering behavior at 1000°C were investigated. The morphological analyses on nanometric powders indicated the presence of mesopores in micrometric agglomerates with open cylindrical pores. High dielectric constants were observed, showing normal frequency dispersion and an intrinsic ac relaxation, relatively masked by low frequency conductivity. The sintered ceramics presented lower dielectric permittivities than the hydrothermal powders, and a classical Cole–Cole plot could be obtained for the Pb_1·88(Zn_0·3Nb_1·25)O_5·305 electroceramic for frequencies above 10³ Hz.