Non-porous Layer Research Articles

A possible method to avoid skin effect in polymeric scaffold produced through thermally induced phase separation

Scaffold's morphology and in particular pore architecture is a key parameter for cell viability and tissue regeneration. Usually, morphology is managed through Thermally Induced Phase Separation (TIPS) consisting of controlled quenching and freeze-drying to remove ice crystals to create porosity. Nevertheless, the so-called skin effect, a less-/non-porous layer, usually occurring at scaffold's air–liquid or material-mold interface. Skin effect reduces scaffold's performance then the layer have to cut out with consequent loss of material and damage risks. Here, it is presented a possible method to avoid skin effect at mold-polymer interface in biopolymer-based scaffolds. It is based on producing scaffolds not directly in a mold but on a previously frozen distilled water surface. SEM analysis showed the absence of skinned surfaces and a uniform pore pattern in shape and size.

Fabrication of Hierarchical Multilayer Poly(Glycerol Sebacate urethane) Scaffolds Based on Ice-Templating

In this study, it was demonstrated that ice-templating via freeze drying with custom-made moulds, in combination with air brushing, allows for the fabrication of poly(glycerol sebacate urethane) (PGSU) scaffolds with hierarchical multilayer microstructures to replicate various native soft tissues. The PGSU scaffolds were either monolayered but exhibited an anisotropic microstructure, or bilayered and trilayered, with each layer showing different microstructures. By using freeze drying with custom-made moulds, the ice crystals of the solvent were grown unidirectionally, and after freeze-drying, the scaffolds had an anisotropic microstructure, mimicking tissues such as tendon and skeletal muscle. The anisotropic PGSU scaffolds were also examined for their tensile strength, and a range of mechanical properties were obtained by altering the reactants’ molar ratio and polymer concentration. This is of importance, since soft tissues exhibit different mechanical properties depending on their native location and functionality. By combining freeze drying with airbrushing, scaffolds were fabricated with a thin, non-porous layer on top of the porous layers to allow three-dimensional cell co-culture for tissues such as skin and oral mucosa. These results show that fabrication techniques can be combined to produce PGSU scaffolds with tailored hierarchical microstructures and mechanical properties for multiple tissue engineering applications.

A highly selective porous composite membrane with bromine capturing ability for a bromine-based flow battery

Bromine-based flow batteries (Br-FBs) are receiving more and more attention because of the high potential, high solubility, and low cost of the Br2/Br− redox couple. However, the self-discharge reaction caused by the bromine diffusion leads to serious and irreversible capacity decay of Br-FBs. Herein, a porous composite membrane is designed for Br-FBs, upon which a highly selective separation layer is constructed by introducing a bromine complexing agent. Apart from the intrinsically high selectivity on bromine, the complexing agent in the non-porous selective layer can complex with diffused bromine. Therefore, diffused bromine is captured by the separation layer, consequently suppressing the self-discharging of the battery. The zinc–bromine single flow battery (ZBSFB) assembled with the prepared porous composite membrane demonstrates a Coulombic efficiency of 97.42% and an energy efficiency of 85.31% at 40 mA/cm2. Moreover, this battery can operate stably at a high current density of 140 mA/cm2, much higher than that of the pristine membrane matrix (only 40 mA/cm2), enabling a higher-power density ZBSFB. Most importantly, this ZBSFB exhibits a much higher capacity retention rate after resting for 24 h than that based on a pristine membrane matrix (69.3% vs. 35.7%), suggesting a dramatically improved membrane selectivity. Therefore, the designed porous composite membranes can be a very promising prospect for Br-FB applications.

Preparation and characterization of novel cellular/nonporous foam structures derived from tannin furanic resin

Recently, tannin furanic resin-based foams exhibiting excellent properties have been developed. However, its friability and thermal insulation must be improved prior to industrialization. Therefore, the tannin preconcentrate from wood bark obtained via liquefaction was mixed with furfuryl alcohol in the presence of a catalyst, surfactant, and solvent. This process includes instant in situ formation of a resin matrix, followed by spontaneous molding and curing to yield a distinct cellular/nonporous structure. Further, the cell structure, mechanical strength, friability, thermal insulation, and fire resistance properties of this material were investigated. Results indicated that the developed material displayed a distinguished bilayer structure in the form of a cellular layer crossing the interface into another nonporous layer, as revealed via scanning electron microscopy imaging. In addition, the porosity of the cellular layer was considerably lower than that observed in case of standard tannin foam. Mechanical testing demonstrated a compression strength of 0.183 and > 50 MPa (there is no yield point for the nonporous layer), and internal bond strength of 0.014 and 0.61 MPa for the cellular and nonporous layers, respectively. Rupture was always observed to occur in the cellular layer rather than at the interface. The limiting oxygen index of the cellular layer was approximately 24.51 %, which is comparable to that of the nonporous layer (28.32 %). Based on thermogravimetric analysis (TGA), the overall foam structure can easily decompose into a final residual mass of 2.8 %. Regardless, compared with other tannin foams prepared in the traditional manner by adding a foaming agent, this material exhibited superior friability resistance and thermal insulation, as corroborated from the thermal conductivity measurement (0.0239 W/m K along with an abrasion fraction of 16.49 % after friction, indicating strong durability against abrasion.

Synthesis and Characterization of TiC-reinforced Metallic Glass Composite Coatings

In this paper, the Co-based metallic glass coatings deposited on soft metal substrates by pneumatic squeezing method were investigated. Also, the effects of TiC additions (%0-10) on the mechanical properties of the coatings were discussed in terms of hardness measurements. Microstructural observations have shown that the coatings with a thickness range of about 20-30 µm can be produced as non-porous layers in either entirely amorphous (0-6% TiC) or composite (8-10% TiC) structures. The glassy and TiC-reinforced glass composite coatings exhibit good adhesion characteristics and form a harder layer with the average hardness values between 1000 and 1600 Hv respectively. The hardness measurements evaluated together with XRD and EDX analyses showed that (Ti,Ta)C crystals precipitated in the amorphous matrix are responsible for further increase of the coatings hardness value up to 1600 Hv. The results also provide clues for mass manufacturing route in one step synthesis of the metallic glass and their composite coatings.

Characterization of the Pore Structure of Well Cement under Carbon Capture and Storage Conditions by an Image-Based Method with a Combination of Metal Intrusion.

To more quantitatively and subtly analyze effects of carbonation on the pore structure of well cement by supercritical CO2 under carbon capture and storage (CCS) conditions, a digital scanning electron microscopy-backscattered electron (SEM-BSE) image analysis with a combination of nontoxic low-melting point metal intrusion is used to characterize the exposed cements to humid supercritical CO2 for 10 and 20 days. The porous area fraction (PAF) and pore size distribution (PSD) profiles obtained by slicing operation are used to describe the pore structure variation along the corrosion direction in a two-dimensional (2D) plane. The results show that the image-based method with the combination of metal intrusion is an effective method for characterizing the layer structure of exposed cement and getting quantitative information about the pore structure. From the surface to the core, the main altered layers in exposed cement for 10 days include the partially leached layer, the carbonated layer, and the calcium hydroxide (CH)-dissolved layer. For the exposed cement for 20 days, the main altered layers include the porous leached layer, the partially leached layer, the carbonated layer, and the carbonated transition layer. The nonporous carbonated layer can effectively block the flow parallel to the corrosion direction, while the porous leached layer can facilitate the flow perpendicular to the corrosion direction. Findings from this study will provide valuable information for understanding the effects of carbonation on the pore structure of well cement under CCS conditions.

Mixed matrix membranes containing fluorescent coordination polymers for detecting Cr2O72- with high sensitivity, stability and recyclability.

Three coordination polymers (CPs) were synthesized by using CdII, fluorescent 9,10-di(4-pyridyl)anthracene (dpa), and cyclohexane-1,4-dicarboxylic acid (H2cda), and they are [Cd2(dpa)2(cda)Cl2]n (1), [Cd(dpa)2(cda)]n (2) and [Cd(dpa)(cda)(H2O)]n (3). Both 1 and 2 are fluorescent and contain nonporous layers. 3 is an isomer of 2 and contains a porous diamondoid network. Fluorescent mixed matrix membranes were prepared by dispersing the particles of 1 or 2 within the matrix of polymethyl methacrylate, and showed high sensitivity and selectivity for detecting Cr2O72- in water. Both stability and recyclability of the MMMs were remarkably higher than those of the CP powders.

Solar-thermal membrane for dewatering aqueous organic-acid solutions

A thermally conductive porous membrane has been developed to directly absorb solar energy and conduct heat that can effectively evaporate liquid water at the interface between the membrane and the bulk feed solution. Black, porous, thermally conductive graphite foam support is employed as an effective photothermal-energy absorber and heat conductor that heats up an aqueous feed solution to produce vapor. Graphite nanoparticle-slurry coating is applied onto the foam surface to reduce the pore size and generate a microporous membrane. Then, application of a dense graphene oxide membrane coating as an active separation layer on the microporous nanoparticle surface of internal foam channels allows vapor permeation of volatile molecules. Acetic acid/water solution was studied as a model feed system to experimentally demonstrate effective separation. This model vapor permeation system demonstrates excellent acetic acid separation, with a separation factor of 8.3, under simulated 0.7 sun irradiation. The membrane hydrophobicity and nanostructure, including pore size and surface chemistry, played a significant role in enabling high permselectivity of water vapor over acetic acid molecules, as well as liquid solution. Pore size reduction from open pores to a nonporous dense layer in the membrane enables effective separation of water vapor from the organic vapor, while it reduces the permeation flux. Also, the hydrophilicity of the membrane surface shows ∼3 times higher permeation flux with higher permselectivity, compared with the hydrophobic membrane surface. This work introduces a process of directly using renewable energy instead of conventional heating to drive selective separation of water from organic acids.

Multilayered film for the controlled formation of freestanding lipid bilayers

A freestanding lipid bilayer or black lipid membrane is a powerful tool for studying ion channels and for biophysical studies of other membrane proteins under controlled chemical and physical conditions. Even though the lipid bilayer has been considered an excellent sensing platform to detect diverse single molecules from nucleotides to cells, it is not yet widely used, mainly due to its low stability and the expertise needed for membrane formation. To ameliorate the issues of conventional membrane formation techniques, we report a novel layered film that consists of a nonporous layer sandwiched between two porous layers to facilitate bilayer formation. Moreover, the absorption of excess solvent present in the membrane precursor solution can be achieved by the film, enabling control over the membrane formation process. Through this layered design, we could obtain an ideal film that has a reduced and controlled membrane formation time (<30 min) and a sufficient bilayer lifetime (3 h) for ion channel studies and biosensing.

CO&lt;sub&gt;2&lt;/sub&gt; Absorption/Desorption on Gas-Liquid Membrane Contactors Using Monoethanolamine Solvent: Comparison of Porous and Composite Hollow Fibers

The operating efficiency of asymmetric porous and composite membranes with a thin non-porous selective layer was compared in the processes of CO2 absorption and desorption in gas-liquid membrane contactors using aqueous solutions of monoethanolamine (MEA) with low concentration (&lt;14 %). Composite membranes were prepared by direct deposition of poly (1-trimethylsilyl-1-propyne) (PTMSP) in a hollow fiber membrane module. The effects of gas flow rate and MEA solvent linear velocity on the CO2 mass transfer were evaluated. Porous membranes were shown to be more effective in the process of CO2 absorption, because they allow to remove more than 90 % of CO2 from the gas mixture during one pass of the solvent through the contactor. Composite membranes were more promising for CO2 desorption, since they provide half as much of the solvent vapor losses with comparable desorbed CO2 fluxes (0.12-(STP)/(m2·h)). The contributions of membrane and liquid phase to the overall mass transfer resistance during the CO2 absorption process were estimated. It was demonstrated that deposition of a thin selective layer from a highly permeable PTMSP with a thickness of only 3 μm increases the membrane contribution to the total mass transfer resistance from 10-20 % to 60-80 %.

Breath figures makes porous the “so-called” skin layer obtained in polymer foams prepared by supercritical CO2 treatments

In this article, the preparation of porous polymeric scaffolds of the synthetic poly (isopropylideneglycerol methacrylate) (PIPGMA) is described combining two complementary techniques. Supercritical CO2 (SCCO2), has been used as foaming agent to obtain internal porosity by varying parameters such as pressure and depressurization time. However, the disadvantage of SCCO2 technology is the formation of a non-porous skin layer described in the majority of the samples. This skin layer was modified by the breath figures technique (BF), providing superficial porosity to the polymeric sample. The result of using both techniques subsequently showed the formation of a porous network throughout the samples with inner and outer pores interconnected, characterized by scanning electron microscopy (SEM), micro X-ray computerized tomography scanning (micro CT), and processing those images with Image J. Finally, preliminary biological evaluation of the porous samples has been carried out in order to evaluate the biocompatibility and cellular viability of these scaffolds.

Strengthening of the mechanical and tribological properties of composite oxide film formed on aluminum alloy with the addition of graphite

Composite oxide films containing graphite were fabricated using the anodizing technique on AA2017- T4 aluminum alloy in diluted sulfuric acid (20 wt%). This study aims to evaluate the effect of graphite on the film growth and investigate its impact on the self-lubricating ability and durability of the composite films. The growth mechanism was determined by manipulating the anodizing time while the mechanical and tribological properties of the films were studied with different graphite content. The surface morphology, topography, and chemical compositions were examined with scanning electron microscopy (SEM), 3D optical profiler, and energy- dispersive spectroscopy (EDS), respectively. The microhardness was characterized at the surface and cross-section of the films by Vickers microhardness tester. The structure of the oxide films and graphite were studied by X-ray diffractometer (XRD) and Raman spectroscopy. The tribological characterization was investigated by using a ball-on-disk sliding test in dry condition. The growth of the film was initiated by the formation of the non-porous barrier layer at 5–10 min. The pores started to grow at 20 min and caused the break of the barrier layer. For this process, the complete development of porous composite oxide film can be achieved at 60 min with the pore dimension (width: 23.74 ± 8.91 μm; depth: 27.9 ± 9.09 μm). The thickness of 1 g/L oxide film at 60 min was about 32.45 ± 4.92 μm, approximately. The incorporation of graphite reduces the surface porosity of oxide films and improves the mechanical and tribological properties significantly. The service life of the composite oxide film is affected by the hardness and graphite-self-lubricated layer.

Improved accuracy of U-series and radiocarbon dating of ostrich eggshell using a sample preparation method based on microstructure and geochemistry: A study from the Middle Stone Age of Northwestern Ethiopia

The Middle Stone Age (MSA) is important to studies of human evolution because it witnessed the origin of modern Homo sapiens and its dispersal out of Africa and across the rest of the Old World. Obtaining accurate ages for these and other events that occurred during this time interval is of critical importance. Ostrich eggshell (OES) is commonly found in MSA sites, and OES calcite can be dated by radiocarbon and U-series geochronologic techniques. It is important to understand how both techniques are potentially compromised when the eggshell’s pore clusters and intercrystalline microporosity are infilled with younger calcite and/or soil-derived detrital materials. We investigated this question by using high-resolution X-ray computed tomography (HRXCT) to reveal the extent and three-dimensional structure of the pore clusters, and element mapping to evaluate concentrations of detrital elements in modern versus ancient OES within the pore clusters and the crystal (outer) and cone (inner) layers relative to the palisade (central) layer. HRXCT results support earlier findings that OES pores are generally composed of one to several openings on the cone surface that develop into a complex three-dimensional cluster of anastomosing and branching pores that extend through the palisade layer to exit on the crystal surface. These hollow pore clusters provide a considerable total volume for the potential ingress of detrital materials. Element mapping revealed high concentrations of detrital mineral indicator elements (Al, Mg) in ancient relative to modern OES, and in the cone and crystal layers relative to the palisade layer. Within the palisade layer of ancient OES, there are relatively high detrital mineral indicator element concentrations within filled pore clusters. Extracted pore cluster infill from the palisade layer yields significantly higher and more variable concentrations of 232Th (2,200-10,000 ppt) and 238U (35–54 ppb) relative to the non-porous palisade layer (150–3,100 ppt 232Th, and 19–25 ppb 238U). Based on these results, we developed a mechanical preparation method that combines the removal of the pore cluster infill with the more common removal of the cone and crystal layers. Using this method, U–Th analyses of four OES from an MSA archaeological site in NW Ethiopia yielded a mean age of 75.7±4.7 ka (2 SD). The inclusion of the pore cluster infill with the palisade in a different OES from the same site produced a much younger age of 53±1.2 ka (1 SE). Splits of several of these same OES samples prepared by this new method yield AMS 14C ages that are beyond the range of 14C dating (∼50 ka), while ∼60% of the OES analyses of the palisade layer that retained the pore cluster infill yielded much younger ages (27–38 ka). Our results demonstrate that dating OES that retains the pore cluster infill shifts both U–Th and 14C analyses to younger ages. In order to obtain more precise and accurate ages, we recommend that preparation of OES for both U-series and 14C dating include the mechanical removal of both the pore cluster infill and the cone and crystal layers in order to more fully account for the incorporation of Th, U, and C from detrital and authigenic minerals within the OES structure.

2024 ALUMINUM OXIDE FILMS PREPARED BY THE INNOVATIVE AND ENVIRONMENT-FRIENDLY OXIDATION TECHNOLOGY

A kind of environment-friendly anodic oxidation technology was used to oxidize 2024 aluminum alloys from mixed acid solutions to effectively improve the anticorrosion and mechanical performances. The influences of anode oxidation method on surface morphology, microstructure, composition, electrochemistry parameter, anticorrosion property were studied. Aluminum alloy oxidation is actually a dynamic equilibrium process of the formation and dissolution of oxide film which is composed of porous and non-porous layer. With the treatment of anodic oxidation, [Formula: see text]-Al2O3 and [Formula: see text]-Al2O3 structures were obtained on the surface of aluminum alloys, which contributed directly to the increase of anticorrosion performance. The potassium dichromate solution was used to seal the surface of oxide films to further improve the anticorrosion property. The oxide films sealed with potassium dichromate were covered with leaf-like structures resulting in larger corrosion resistance that attributed directly to the decrease of corrosion current.

Non-porous thin dense layer coating: Key to achieving ultrahigh peak capacities using narrow open tubular columns

Chromatographic theory has predicted that narrow open tubular (OT) columns are essential toward achieving ultrahigh-efficiency liquid chromatographic separations. It is also well-recognized that low loadability is a major issue for capitalizing the benefits of open tubular liquid chromatography (OTLC). To mitigate this issue, researchers have tried to increase surface area of or create a thick porous coating on the column wall. In this work, we hypothesize that a thin dense non-porous coating is well-suited for us to achieve ultrahigh efficiency or ultrahigh peak capacity if the inner diameter of the OT column is small (e.g., 1−2 µm). We first discuss the rationale of this hypothesis. To provide evidence to further support the hypothesis, we prepare, optimize and characterize the coating for high-resolution separations. We utilize columns prepared under the optimized conditions to separate a peptide sample form E. coli lysate digested first by pepsin and then by trypsin, and we show ultrahigh-peak-capacity results (770 within 47 min using a 45-cm-long column and 2000 within 155 min using a 155-cm-long column). The coating is stable, and the coating method is reproducible. Ultrahigh-resolution separations and ultra-sensitive analyses are critically important and currently challenging for analyses of complex proteomics samples; employing narrow OT columns can alleviate this problem effectively.

In vivo evaluation of bilayer ORC/PCL composites in a rabbit model for using as a dural substitute

ABSTRACT Objective After a neurosurgical procedure, dural closure is commonly needed to prevent cerebrospinal fluids (CSF) leakage and to reduce the risk of complications, including infections and chronic inflammatory reactions. Although several dural substitutes have been developed, their manufacturing processes are complicated and costly and that many of them have been implicated in causing postoperative complications. This study aimed to assess the effectiveness and safety of new bilayer ORC/PCL composites in a rabbit model. Methods Two formulations of bilayer oxidized regenerated cellulose (ORC)/poly ε-caprolactone (PCL) knitted fabric-reinforced composites and an autologous graft (pericranium) were employed for dural closure in forty-five male rabbits. Systemic reaction and the local reaction of the samples were assessed and compared at one-, three- and six-months post-implantation by blood chemistry and gross, and microscopic assessment using hematoxylin-eosin and Masson’s trichrome stains. Results No signs of CSF leakage or systemic infection were seen for all samples. All samples demonstrated minimal adhesion to adjacent tissues. The degree of host fibrous connective tissue ingrowth into both composites was comparable to that of the autologous group, but bone formation and osteoclast activities were significantly greater. Both composites progressively degraded over times and the residual thickness of the nonporous layer was 50% of the initial thickness at six months post-implantation. Discussion Bilayer ORC/PCL composites were successfully employed for dural closure in the rabbit model. They were biocompatible and could support dural regeneration comparable to that of the autologous group, but induced greater osteogenesis.

Utilization of cross-linked polybutadiene nanoparticles for asymmetric membrane synthesis

This research synthesized polybutadiene nanoparticles and investigated the induction time for their nucleation under different supersaturations and their cross-linking to be utilized in an asymmetric membrane synthesis. Purchased (bulk) polybutadiene was dissolved in toluene and then ethanol was added dropwise to the solution as an antisolvent. The induction time, i.e., the time interval between adding the last drop of ethanol till appearance of a turbidity due to polybutadiene nucleation, was determined. The nucleation mechanism was investigated. Dependence of the induction time on supersaturation suggests that the mechanism of nucleation is primary. The correlation coefficient was obtained as 0.986. After formation, the nanoparticles were cross-linked with sodium persulfate as initiator and divinylbenzene as cross-linking agent. The SEM photos showed the formation of nanoparticles as well as their cross-linking. Such a finely porous layer can act as an effective support for active non-porous layers in applications such as gas separation. Also, based on the practical results, it was determined that for pressure equal 7 bar, temperature of 70 °C, membrane thickness of 1.15 mm and polybutadiene to divinyl benzene ratio of 1.5, the relative separation will be 93.58.

Superparamagnetic Iron Oxide Nanoparticles Modified with Silica Layers as Potential Agents for Lung Cancer Treatment.

Superparamagnetic iron oxide nanoparticles (SPIONs) are promising drug delivery carriers and hyperthermia agents for the treatment of cancer. However, to ensure their safety in vivo, SPIONs must be modified in order to prevent unwanted iron release. Thus, SPIONs were coated with silica layers of different morphologies: non-porous (@SiO2), mesoporous (@mSiO2) or with a combination of non-porous and mesoporous layers (@SiO2@mSiO2) deposited via a sol–gel method. The presence of SiO2 drastically changed the surface properties of the nanoparticles. The zeta potential changed from 19.6 ± 0.8 mV for SPIONs to −26.1 ± 0.1 mV for SPION@mSiO2. The Brunauer–Emmett–Teller (BET) surface area increased from 7.54 ± 0.02 m2/g for SPIONs to 101.3 ± 2.8 m2/g for SPION@mSiO2. All types of coatings significantly decreased iron release (at least 10 fold as compared to unmodified SPIONs). SPIONs and SPION@mSiO2 were tested in vitro in contact with human lung epithelial cells (A549 and BEAS-2B). Both nanoparticle types were cytocompatible, although some delay in proliferation was observed for BEAS-2B cells as compared to A549 cells, which was correlated with increased cell velocity and nanoparticles uptake.

One-step preparation of bilayered films from kraft lignin and cellulose acetate to mimic tree bark

This contribution presents the development of a dry-cast method for the one-step preparation of bio-based films from wood polymers that mimic the bilayered structure of tree bark, the natural protective layer of the tree. In a simplified view, natural bark can be considered as the superposition of an external homogeneous and non-porous layer (outer bark) and a porous substructure layer (inner bark). This work is a first step for the future development of bio-based biomimetic wood coatings. The film had a bark-like appearance and its total density, bulk density and porosity were similar to values measured in natural bark. Furthermore, the structural characteristics of the studied film, namely specific surface area (BET) and pore size distribution, as well as the performance of the water adsorption ability were investigated and discussed.

The effect of Ta additions on the oxidation resistance of SPS-produced TiAl alloys

The influence of the environment on the properties of TiAl alloys is a concern for elevated temperature applications. One element that stands out in improving the resistance of TiAl alloys against high-temperature oxidation is Nb. Nb promotes the activities of Al and favors the formation of a more protective Al2O3 surface oxide which subsequently stymies oxygen diffusion to the alloy. Ta is an equal replacement of Nb. Thus, this study aims to evaluate the effect of 0.8, 4, and 8 at. % Ta additions on the oxidation resistance of spark plasma sintered Ti-46.5Al (at. %) alloy. Isothermal oxidation tests were carried out for 360 and 100 h at 1123 and 1273 K respectively. Surface oxides examination include scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray diffractometry. The results show that 4 and 8 at. % Ta additions significantly support the formation of an interconnected, non-porous Al2O3 layer at the metal-oxide interface. The continuous non-porous Al2O3 layer serves as a diffusion barrier which leads to superior oxidation resistance of the TiAl alloys.