Effect of Dopants on the Distribution of Aluminum and Oxygen Fluxes in Polycrystalline Alumina Under Oxygen Potential Gradients at High Temperatures

The oxygen permeability of Lu‐, Y‐, and Hf‐doped polycrystalline alumina wafers under steep oxygen potential gradients was evaluated at temperatures above 1773 K. Oxygen permeation occurred by the grain‐boundary (GB) diffusion of oxygen from the higher oxygen partial pressure (P_O2) surface to the lower P_O2 surface, and was coincident with GB diffusion of aluminum in the reverse direction. Lu‐ and Y‐doping both suppressed oxygen permeation to the same extent, owing to the decrease in oxygen mobility, but neither had any significant effect on aluminum mobility. Hf‐doping had the opposite effect. The fluxes of oxygen and aluminum at the inflow side in all wafers were significantly smaller than those at the outflow side, regardless of whether or not these dopants were added. Consequently, the intersection of the fluxes shifted to the lower P_O2 side upon Lu‐ and Y‐doping, and to the higher P_O2 side upon Hf‐doping. Furthermore, the effect of dopants on the mass transfer in scales formed by oxidation of FeCrAl‐based alloys at 1300–1500 K was analyzed through predictions of the flux distributions of oxygen and aluminum in the scales.     

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

Oxygen diffusion mechanisms during oxidation of ZrB₂‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an ¹⁸O tracer technique. Double oxidation experiments in ¹⁶O₂ and ¹⁸O₂ were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO₂ oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO₂ was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.     

Synthesis and characterization of fluorene‐based polybenzimidazole copolymer for gas separation

The purpose of this study is to develop new cardo‐polybenzimidazole (CPBI) copolymers containing cardo fluorene, with improved gas permeability, specifically oxygen permeability. The characteristics of these copolymers are investigated by Fourier transform infrared spectroscopy, nuclear magnetic resonance, ¹H and ¹³C), thermo‐gravimetric analysis, and wide‐angle X‐ray diffraction. These membranes fabricated from copolymers have relatively high oxygen diffusion coefficients, determined using gas permeation. In particular, the CPBI‐co91 (synthesized by using the monomers ratio containing dibenzoic acid = 9 : 1) membrane have oxygen permeability coefficient (P_O2) of 10.69 Barrer, theoretical selectivity of 5.4 for oxygen to nitrogen, and O₂ diffusion coefficient of 9.64 × 10^?8 cm²/s. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40521.     

Oxygen Ion Conduction in Bulk and Grain Boundaries of Nominally Donor‐Doped Lead Zirconate Titanate (PZT): A Combined Impedance and Tracer Diffusion Study

Oxygen ion conduction in Nd³⁺‐doped Pb(Zr_xTi_1?x)O₃ (PZT) was investigated by impedance spectroscopy and ¹⁸O‐tracer diffusion with subsequent secondary ion mass spectrometry (SIMS) analysis. Ion blocking electrodes lead to a second relaxation feature in impedance spectra at temperatures above 600°C. This allowed analysis of ionic and electronic partial conductivities. Between 600°C and 700°C those are in the same order of magnitude (10^?5–10^?4 S/cm) though very differently activated (2.4 eV vs. 1.2 eV for ions and electron holes, respectively). Oxygen tracer experiments showed that ion transport mainly takes place along grain boundaries with partly very high local ionic conductivities. Numerical analysis of the tracer profiles, including a near‐surface space charge zone, revealed bulk and grain‐boundary diffusion coefficients. Calculation of an effective ionic conductivity from these diffusion coefficients showed good agreement with conductivity values determined from impedance measurements. Based on these data oxygen vacancy concentrations in grain boundary and bulk could be estimated. Annealing at high temperatures caused a decrease in the grain‐boundary ionic conductivity and onset of additional defect chemical processes near the surface, most probably due to cation diffusion.     

Oxygen transport in crosslinked,silicon-containing copolymers of methyl methacrylate

Copolymers of methyl methacrylate and 1,3-bis(methacryloxy methyl)-1,1,3,3-tetramethyl disiloxane were prepared by chemically induced copolymerization/crosslinking at 60°C and 49 mm Hg. Crosslinked, glassy copolymers were obtained with copolymer mole fraction of the silicon-containing monomer varying from 0.09 to 0.55. Oxygen transport studies were performed with thin films as prepared and after sub-T_g annealing. The results of this study indicated that an enhancement of both the steady state oxygen permeation rate and the oxygen diffusion coefficient was achieved through copolymerization. The oxygen diffusion coefficients through the copolymers were found, within experimental error, to be independent of silicon content and ranged from 0.80 × 10^?7 to 1.90 × 10^?7 cm²/s vs. oxygen diffusion coefficient for pure poly(methyl methacrylate) of 1.5 × 10^?8 cm²/s. Sub-T_g annealing effected a reduction of approximately equal magnitude in both the oxygen diffusion coefficient and the steady state oxygen flux for the copolymers. In addition, the normalized oxygen flux data were predicted with Fick's law, assuming constant boundry conditions and diffusion coefficient. These results were explained in terms of the free volume theory and the combined effects of increased crosslinking density, chain mobility, and oxygen solubility with increased copolymer silicon content.     

Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

In this study, La_0.4Sr_0.6CoO_3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (D_v) and surface exchange coefficient (K_ex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La_0.4Sr_0.6CoO_3‐δ membrane was ～4.0 × 10^?7 mol cm^?2 s^?1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10^?5 mol cm^?2 s^?1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Bonding strength and thermal shock resistance of a novel bilayer (c-AlPO4-SiCw-mullite)/SiC coated carbon fiber reinforced CMCs

Mullite coating, SiC whiskers toughened mullite coating (SiC_w-mullite), and cristobalite aluminum phosphate (c-AlPO₄) particle modified SiC_w-mullite coating (c-AlPO₄-SiC_w-mullite) were prepared on SiC coated C/SiC composites using a novel sol-gel method combined with air spraying. Results show that molten SiO₂ formed by the oxidation of SiC whiskers and molten c-AlPO₄ improved the bonding strength between mullite outer coating and SiC–C/SiC composites due to their high-temperature bonding properties. The bonding strength between mullite, SiC_w-mullite, c-AlPO₄-SiC_w-mullite outer coatings and SiC–C/SiC composites were 2.41, 4.31, and 7.38 MPa, respectively. After 48 thermal cycles between 1773 K and room temperature, the weight loss of mullite/SiC coating coated C/SiC composites was up to 11.61%, while the weight losses of SiC_w-mullite/SiC and c-AlPO₄-SiC_w-mullite/SiC coatings coated C/SiC composites were reduced to 7.40% and 5.12%, respectively. The addition of appropriate SiC whiskers can considerably improve the thermal shock resistance of mullite coating owing to their excellent mechanical properties at high temperature. In addition, c-AlPO₄ particles can further improve the thermal shock resistance of SiC_w-mullite coating due to their high-temperature bonding and sealing properties. No obvious micro-pores and cracks were observed on the surface of c-AlPO₄-SiC_w-mullite coating after 48 thermal cycles due to timely healing effect by formation of secondary mullite.     

Study of oxygen exchange and transport in mixed conducting electroceramics

Oxygen diffusion is treated in a dense electronically conducting perovskite pellet blocked ionically on one surface, electronically on the other, and sealed on the periphery. Oxygen exchange at the electronically blocked surface is assigned first order reaction kinetics. An equivalent circuit model is suggested for the cell impedance by the Laplace transform of Fick’s second law. Asymptotic expressions are employed to extract the slope of the coulometric titration curve and the chemical diffusion coefficient from electrochemical-impedance-spectroscopy (EIS) data. NLLS fit with the theoretical model is performed to evaluate the surface oxygen exchange coefficient at the interface of the electrochemical cell. The methodology is applied to determine the chemical diffusion and surface exchange coefficients of oxygen in SrCo_0·5Fe_0·5O_3−δ, interfaced with a YSZ electrolyte. The experimental results are used to link the processes corresponding to the diffusion of oxygen vacancies to the ionic conductivity of the material. The data is applicable to solid oxide fuel cell cathodes, oxygen permeation membranes and related systems.     

Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

Oxygen Isotope Transport Properties of Yttria‐Stabilized Zirconia (YSZ) in O2‐ and H2O‐Containing Atmospheres

Oxygen isotope exchange experiments, H₂¹⁸O/H₂¹⁶O (”wet” anneals) and ¹⁸O₂/¹⁶O₂ (”dry” anneals), were performed on single crystal samples of yttria‐stabilized zirconia (YSZ) at a temperature of T = 1073 K with subsequent determination of the oxygen isotope profiles in the solid by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Such experiments yielded oxygen tracer diffusion coefficients (D*) and oxygen tracer surface exchange coefficients (k*), from both the polished (smooth) and unpolished (rough) sides of single crystal samples, as a function of water partial pressure pH₂O and oxygen partial pressure pO₂. Isothermal values of D* were found to depend on neither pO₂ nor pH₂O (nor surface roughness). Isothermal values of k*, in contrast, displayed a strong dependence on pO₂ or pH₂O; k*_wet was, in addition, 2–3 orders of magnitude higher than k*_dry. Surprisingly, surface roughness had little effect on k*_wet, whereas rough surfaces exhibited much higher k*_dry values than smooth surfaces. Data for k*_wet obtained as a function of temperature at pH₂O = 18 mbar show a change in activation enthalpy at T ≈ 973 K. The behavior of k* is discussed in terms of surface composition, surface area and surface reaction mechanisms.     

In-situ synthesis and textural evolution of the novel carbonaceous SiC/mullite aerogel via polymer-derived ceramics route

A novel carbonaceous SiC/mullite composite aerogel is derived from catechol-formaldehyde/silica/alumina hybrid aerogel (CF/SiO₂/AlOOH) via polymer-derived ceramics route (PDCR). The effects of the reactants concentrations on the physicochemical properties of the carbonaceous SiO₂/Al₂O₃ aerogel and SiC/mullite aerogel are investigated. The mechanism of the textural and structural evolution for the novel carbonaceous SiC/mullite is further discussed based on the experimental results. Smaller reactants concentration is favorable to formation of mullite. Reactants concentration of 25% is selected as the optimal condition in considering of the mullite formation and bulk densities of the preceramic aerogels. Spherical large silica particles are also produced during heat treatment, and amorphous silica is remained after this reaction. With further heat treatment at 1400 °C, silicon carbide and mullite coexist in the aerogel matrix. The mullite addition decreases the temperature of SiC formation, when compared with the conventional methods. However, after heat treatment at 1450 °C, the amount of mullite begins to decrease due to the further reaction between carbon and mullite, forming more silicon carbide and alumina. The carbonaceous SiC/mullite can be transferred to SiC/mullite binary aerogel after carbon combustion under air atmosphere. The carbonaceous SiC/mullite has a composition of SiC (31%), mullite (19.1%), SiO₂ (14.4%), and carbon (35%). It also possesses a 6.531 nm average pore diameter, high surface area (69.61 m²/g), and BJH desorption pore volume (0.1744 cm³/g). The oxidation resistance of the carbonaceous SiC/mullite is improved for 85 °C when compared with the carbon based aerogel.     

Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite

Nine elastic stiffness coefficients, c_ij, of a mullite single crystal (2Al₂O₃·SiO₂) are measured using acoustic resonance spectroscopy. The obtained values are similar to those of the structurally related aluminosilicate phase sillimanite (Al₂O₃·SiO₂). Characteristic elastic properties of the two minerals are interpreted with the help of their crystal structures and atomic force constants for sillimanite. The high longitudinal stiffness coefficients, c₃₃, of mullite (∼352 GPa) and sillimanite (∼388 GPa) are caused by continuous “stiff” load-bearing tetrahedral chains parallel to c-axis, while the “soft” octahedral chains have minor direct influence. They stabilize the tetrahedal chains against tilting. The lower c₃₃ value of mullite in comparison to the sillimanite value may be caused by a weakening of the load-bearing tetrahedral chains which are parallel to c-axis because of partial replacement of silicon by the weaker-bonded aluminum. The longitudinal stiffness coefficients perpendicular to c-axis are significantly lower, because of sequences of alternating “soft” octahedral and “stiff” tetrahedral units. Within the plane (001), the compliant octahedra exhibit stiffness-controlling influence with coefficients parallel to b-axis (c₂₂∼ 233 GPa) being somewhat lower than parallel to a-axis (c₁₁∼ 291 GPa). This is explained with the occurrence of compliant octahedral Al(1)–O(D) bonds, which are more effective parallel to b-axis rather than to a-axis. Because octahedra are unaffected by the aluminum to silicon substitution, c₁₁ and c₂₂ coefficients of mullite and sillimanite are very similar. Shear stiffness coefficients of mullite increase from c₅₅ (∼77 GPa) to c₆₆ (∼80 GPa) to c₄₄ (∼110 GPa), indicating increasing resistance against shear deformation within the planes (010), (001), and (100). The lattice plane of the highest shear stiffness (100) is built up of an oxygen-oxygen network, diagonally braced along 〈011〉 (“Jägerzaun”). This network with short oxygen–oxygen distances can be sheared by compression and elongation along oxygen–oxygen interaction lines only which is rather unlikely. Because of the lack of such networks in the planes (010) and (001), bending and deformation of structural units become easier, and consequently c₅₅ and c₆₆ are <c₄₄. All three shear stiffness coefficients of mullite are slightly lower than those of sillimanite because of the reduction of the mean tetrahedral bond strength in mullite caused by partial substitution of silicon by aluminum.     

A core-shell structured diffusion-bubbling membrane for efficient oxygen separation: Formation and transport properties

The newly developed core-shell structured molten oxide membranes with fast combined diffusion-bubbling oxygen mass transfer and theoretically infinite selectivity are of technological interest because of their high separation efficiency. In this article, a core-shell structured molten V₂O₅–Cu₂O- based diffusion-bubbling membrane was prepared by one-step thermal treatment of initial CuO–25 wt.% Cu₅V₂O₁₀ ceramic composite in a chemical field (under an oxygen partial pressure difference across the composite) above copper vanadate peritectic transformation temperature (816°C). Oxygen fluxes through the membrane were measured at 830°C, using either gas mixtures (O₂ + N₂) with different oxygen concentrations or air as feed gas at the shell of the membrane and helium (He) as sweep gas. Oxygen flux through the membrane with a shell thickness of 0.15–0.61 mm was 3.8·10^–8–1.4·10^–7 mol/cm²/s under an oxygen partial pressure difference of 0.1 –0.75 atm, respectively. The effect of oxygen partial pressure on the thickness of the membrane shell is found. The relationship between membrane shell thickness, oxygen partial pressure difference across the membrane, and oxygen permeation flux through the membrane is established. Oxygen permeation flux through the dual-phase MIEC membrane shell is described in terms of the diffusion model. Oxygen permeation flux through the membrane core is described both within the framework of the stationary model and nonstationary model for uniform (the membrane thickness is much larger than the characteristic distance of bubble dynamic relaxation) and accelerated (the membrane thickness is comparable to the characteristic distance of bubble dynamic relaxation) bubble motion in a viscous oxide melt, respectively.     

Oxygen Diffusivities and Surface Exchange Coefficients in Porous Mullite/Zirconia Composites Measured by the Conductivity Relaxation Method

Oxygen diffusivities and surface exchange coefficients in various porous mullite/zirconia composites were measured at oxygen partial pressures ranging from 20.2 to 2.02 kPa using the conductivity relaxation method. However, the oxygen diffusivities in porous high-zirconia composites could not be determined because of the predominant surface exchange reaction. Oxygen diffusivities and surface exchange coefficients in low-zirconia composites increased with the zirconia content, while the surface exchange coefficients in high-zirconia composites were approximately constant. A percolation threshold of the surface exchange coefficients occured at ∼40 vol% zirconia for porous zirconia/mullite composites. The oxygen diffusivities in porous low-zirconia composites were independent of the oxygen partial pressure, implying that oxygen diffusion in these composites was related to the migration of oxygen vacancies, whose concentration was independent of the oxygen partial pressure. The surface exchange coefficients of high-zirconia composites decreased with increasing oxygen partial pressure. Finally, it was inferred that the rate-limiting step for oxygen surface exchange could be the charge-transfer process.     

Oxygen transport kinetics of MIEC membranes coated with different catalysts

Oxygen permeation through mixed ionic‐electronic conducting membrane may be controlled by oxygen bulk diffusion and/or oxygen interfacial exchange kinetics. In this article, we chose BaCe_0.05Fe_0.95O_3‐δ (BCF) as a representative to study the oxygen transport resistances of the membrane coated with different porous catalysts, including BCF itself, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and Sm_0.5Sr_0.5CoO_3‐δ (SSC). The oxygen transport resistances of bulk, gas‐solid interfaces of feed‐side and sweep‐side of the catalyst‐coated membranes can be separately obtained through a linear regression of experimental data according to an oxygen permeation model. The three resistances of the membrane coated with BCF catalyst are smaller than those of the membrane coated with BSCF and SSC catalysts, although BSCF catalyst itself has the fastest bulk diffusion and interfacial exchange kinetics. The catalytic activities of BSCF and SSC catalysts on BCF membranes are impacted by the transport kinetics of catalysts, microstructure of catalyst layers, and cationic inter‐diffusion between the membrane and catalysts. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2803–2812, 2016     

Wet-oxygen corrosion resistance and mechanism of bi-layer Mullite/SiC coating for Cf/SiC composites

A bi-layer oxidation-resistant coating consisting of a mullite outer coating, and a SiC inner coating on the surface of C_f/SiC composites was prepared by the chemical vapour deposition and an air spray sol-gel process, and its corrosion behavior was evaluated in a wet-oxygen coupling environment. Results show that the formation of SiO₂ glass layer and its reaction with mullite particles to form aluminosilicate glass layer, leading to an increase in the density of the mullite outer coating, so that the weight loss of bi-layer Mullite/SiC coating coated C/SiC sample was only 1.11 × 10^?3 g·cm^?2 in the first 100 h of oxidation. However, the weight loss of the coated sample reached 26.82 × 10^?3 g·cm^?2 after 200 h of oxidation due to a part of the mullite outer coating was detached. The SiO₂ glass phase reacted with water vapour to generate gaseous Si(OH)_x, which created distinct holes on the surface of the SiO₂ glass layer or inside the molten aluminosilicate glass layer. Eventually, the mullite outer coating was blistered and detached from the surface of the sample due to the combination and growth of holes.     

Control of Oxygen Permeability in Alumina under Oxygen Potential Gradients at High Temperature by Dopant Configurations

The oxygen permeability of polycrystalline α‐alumina wafers, which served as models for alumina scales on alumina‐forming alloys, under steep oxygen potential gradients () was evaluated at 1873 K. Oxygen permeation occurred by the grain‐boundary (GB) diffusion of oxygen from the higher‐oxygen‐partial‐pressure () surface to the lower‐ surface, along with the simultaneous GB diffusion of aluminum in the opposite direction. The fluxes of oxygen and aluminum at the outflow side of the wafer were significantly larger than at the inflow side. Furthermore, Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. A wafer with a bilayer structure, in which a Lu‐doped layer was exposed to a lower and an Hf‐doped layer was exposed to a higher , decreased the oxygen permeability. When the sign of was reversed, however, the oxygen permeability of the wafer was comparable to that of a nondoped wafer. Co‐doping with both Lu and Hf markedly increased the oxygen permeation, presumably because the Lu‐stabilized HfO₂ particles that were segregated at the GBs acted as extremely fast diffusion paths for oxygen through the large number of oxygen vacancies introduced by the solid solution of Lu in the particles.     

Permeation model and experimental investigation of mixed conducting membranes

A simple oxygen permeation model was developed based on the theoretical analysis of the role of interfaces of mixed conducting membranes. The developed model equations contain three resistance constants, which can be determined by correlating oxygen permeation flux to oxygen partial pressure on each side. A series of experimental measurements of oxygen fluxes of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ membranes over a wide range of temperature and oxygen partial pressures were tested for the regression of three resistance constants with good correlation (R > 0.997). With this model, the interfacial exchange resistances of each side can be well distinguished from the bulk‐diffusion resistance under a wide‐temperature range. The kinetics parameters, including interfacial exchange coefficients on each side and ionic diffusion coefficient, can be obtained through the three resistance constants. Parametric studies can predict the influences of membrane thickness, oxygen partial pressures on oxygen flux, distribution of permeation resistances, and characteristic thickness. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1744–1754, 2012     

Sorption and diffusion of water into melamine–formaldehyde‐incorporated poly(vinyl acetate)–polyester nonwoven fabric composites

A series of composites were fabricated by impregnating a polyester nonwoven fabric with melamine–formol (MF)‐incorporated poly(vinyl acetate) (PVAc) latex. The effect of different weight ratios of MF/PVAc, i.e. 0/100, 5/100, 10, 100, 15/100 and 20/100 (dry, wt/wt), on the water sorption and diffusion into the composites was evaluated. Water sorption studies were carried out at different temperatures, i.e. 30, 50 and 70 °C, based on the immersion weight gain method. From the sorption results, the diffusion (D) and permeation (P) coefficients of water penetrant were calculated. A significant increase in the diffusion and permeation coefficients was observed with an increase in the temperature of sorption. Drastic reductions in diffusion and permeation coefficients were noticed with increasing MF content in the composites. Attempts were made to estimate the empirical parameters like n, which suggests the mode of transport, and K, a constant which depends on the structural characteristics of the composite in addition to its interaction with water. The temperature dependence of the transport coefficients was used to estimate the activation energy parameters for diffusion (E_D) and permeation (E_p) processes from Arrhenius plots. Copyright © 2006 Society of Chemical Industry     

Preparation of self‐standing polyaniline‐based membranes: Doping effect on the selective ion separation and reverse osmosis properties

The selective ion transport of aqueous salt solutions including mono‐, di‐, trivalent cations across both doped and undoped polyaniline (PAni) membranes was described. PAni‐based asymmetric membranes were prepared by the phase inversion method from the casting solution containing N‐methyl pyrrolidone. It was found that the permeation rates (P_R) decline in the sequence of P_R(NaCl) > P_R(MgCl₂) > P_R(LaCl₃). HCl‐doped PAni membrane exhibited higher permeation rates for the salts than undoped one due to its more hydrophilic nature. In reverse osmosis (RO) applications, it was observed no water permeation through undoped PAni due to less permeable and hydrophobic nature, under even at 40 bar pressure. Concerning HCl‐doped PAni, this membrane showed very low water flux (J_w) and it was found as 1.01 L m^?2 h^?1 under 40 bar pressure. On the other hand, the J increased linearly with the applied pressure. Furthermore, typical separation factor (α)values calculated from permeation rate ratios were found to be as 6.3 and 39 for Na⁺/Mg²⁺ and Na⁺/La³⁺ for HCl‐doped PAni, respectively. Especially, HCl‐doped PAni membrane can be used for removing rare‐earth metal salts due to its high separation efficiency in high temperature applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007