Relative Oxygen Permeability Research Articles

Preparation and oxidation behaviour of MoSi2-SiC-Si-ZrB2 composite coatings on carbon surface via laser cladding

ABSTRACT Anti-oxidation coatings of MoSi2-SiC-Si-ZrB2 were prepared on the graphite surface by laser cladding. The XRD, scratching bonding force, SEM and isothermal oxidation tests were used to investigate the microstructure and properties of the coatings. The coating with 40 wt% ZrB2 added (MoSi2:SiC:Si:ZrB2 = 16:4:5:16.6) has the lowest degree of oxidation, the best oxidation resistance and the highest bonding strength between coating and matrix. The scratching bonding force still maintained at about 9.9 N after laser ablation, and there were no obvious destructive defects but some pores and cracks on the coating. After a Si-enriched amorphous layer was formed, the relative oxygen permeability was significantly decreased and the oxidation of the coating was passivated, which prevented the matrix from further oxidation.

Preparation of MoSi2-modified HfB2-SiC ultra high temperature ceramic anti-oxidation coatings by liquid phase sintering

Liquid-phase sintering combining an in-situ reaction method with a slurry method was used to prepare HfB2-MoSi2-SiC coatings of controllable composition and thickness. The effect of the MoSi2 content on the oxidation protection of HfB2-MoSi2-SiC composite coatings in a dynamic aerobic environment from room temperature to 1500 °C and a static constant temperature at 1500 °C in air was investigated. The relative oxygen permeability was used to characterize the oxidation resistance of the coatings. The results of dynamic oxidation test at room temperature~1500 °C show that the initial oxidation weight loss temperature of the samples is increased from 775 to 821 °C, and the maximum weight loss rate is decreased from 0.9×10−3 to 0.2×10−3 mg·cm−2·s−1 with increasing MoSi2 content, the lowest relative oxygen permeability is reduced to 12.2% with the weight loss of the sample being decreased from 1.8% to 0.21%. The mechanism of MoSi2 improving the oxidation protection of the coatings is revealed. With an increase of the MoSi2 content, the amount of SiO2 glass phase in the coating is increased, and the dispersion of Hf-oxide on the surface is improved so that a Hf-Si-O glass layer with high stability is formed and the weight loss of the sample is reduced from 0.46% to 0.08% after 200 h oxidation at 1500 °C in air.

Influence of MoSi2 on oxidation protective ability of TaB2-SiC coating in oxygen-containing environments within a broad temperature range

TaB2-SiC coating modified by different content of MoSi2 was fabricated on graphite substrate with SiC inner coating by liquid phase sintering to elevate the anti-oxidation capability of the TaB2-SiC coatings. As compared to the sample with the TaB2—40wt%SiC coating, the coating sample modified with MoSi2 exhibited a weight gain trend at lower temperatures, the fastest weight loss rate went down by 76%, and the relative oxygen permeability value reduced from about 1% to near 0. More importantly, the large amount of SiO2 glass phase produced over the coating during oxidation was in contact with the modification of MoSi2, which was proved to be beneficial to the dispersion of Ta-oxides. A concomitantly formed continuous Ta-Si-O-B compound glass layer showed excellent capacity to prevent oxygen penetration. However, when the TaB2 content was sacrificed to increase the MoSi2 content, the relative oxygen permeability of the coating increased instead of decreased. Thus, on the basis of ample TaB2 content, increasing the MoSi2 content of the coating is conducive to reducing the relative oxygen permeability of the coatings in a broad temperature region.

Influence of the ZrB2 content on the anti-oxidation ability of ZrB2-SiC coatings in aerobic environments with broad temperature range

ZrB2-SiC coatings with different ZrB2 contents were prepared by liquid phase sintering. The oxidation processes of coatings were explained according to TG results of ZrB2-SiC coatings and powders tested from 298 K to 1773 K. Results show that, increasing ZrB2 content made the weight of the samples changed from weight-loss of 10.04% to weight-gain of 0.14%, while the fastest weight-loss regions were narrowed, whose inflection points reduced from 1310℃ to 1050℃. Increasing ZrB2 content made the relative oxygen permeability of the ZrB2-SiC/SiC coatings reduced from 40%–60% to -10%-5%. Increasing ZrB2 content enhanced high-temperature stability of coatings, making final weight of samples changed from weight-loss of 0.16% to weight-gain of 0.11% after oxidation at 1773 K for 200 h. The peeling and dispersion of Zr-oxides formed Zr-B-Si-O compound glass layer, presenting enhanced stability, dispersion strengthening and pinning effect of Zr-oxides, which were responsible for the excellent anti-oxidation protective effect of coatings in a broad temperature region.

Investigations of TaB2 on oxidation-inhibition property and mechanism of Si-based coatings in aerobic environment with broad temperature region for carbon materials

To investigate the formation mechanism of the dendrites glass-ceramics and the effect of TaB2 modifier on the oxidation-inhibition ability of Si-based coatings in aerobic environment with broad temperature region for carbon materials, the Si-based coatings modified with different TaB2 content were fabricated by the liquid phase sintering method. The fabricated coatings and its corresponding composite powders were investigated in dynamic TG aerobic environments up to 1500 ℃. The initial oxidation temperatures of Si-based coatings modified with TaB2 powders are almost completely suppressed to 800 ℃. With the increase of TaB2 phase content, the relative oxygen permeability of the Si-based coatings is significantly reduced, which can be interpreted as the generation of liquid boride, silicide and Ta-Si-B-O compound glass ceramics those can improve the stability of the coating. The formation mechanism of the dendrites glass-ceramics was illustrated.

Effect of matrix morphology on mechanical and barrier properties of polypropylene nanocomposite films containing preferentially aligned organoclay platelets

The objective of this research was to study and optimize the effect of matrix morphology on the desirable mechanical and barrier properties of polypropylene/organoclay (PP/clay) nanocomposite films. Nanocomposites with different organoclay contents and three different matrix morphologies were compared with neat PP under identical conditions. The degree of organoclay dispersion was controlled through the use of a compatibilizer, maleic anhydride grafted polypropylene (PP-g-MA). The nanocomposite films were first prepared by melt casting and then subjected to annealing and re-crystallization. Organoclay platelets in all films were found to align parallel to the film plane. Crystalline PP in as-prepared films was found to have a preferred orientation with the lamellae normally lying perpendicular to the machine direction. Upon annealing, the stacks of parallel lamellae, aligned perpendicular to the machine direction and the film surface, tend to be more orderly than in as-prepared films (see models). No crystal orientation was found in re-crystallized films. A clear increase in elastic modulus and tensile strength with increasing organoclay content was observed. Relative oxygen permeability (ROP) of nanocomposite films decreased with increasing amount of organoclay. The effect of matrix morphology is as follows: annealed films display the highest elastic modulus, tensile strength and ROP, followed by as-prepared and re-crystallized films.

Stereocomplexed PLA nanocomposites: From in situ polymerization to materials properties

From the ecofriendly point of view, renewable and biodegradable polylactide (PLA) is considered as one of the best biobased polymers for many applications. However the use of PLA particularly in long-lasting applications like automotive, electronics and food packaging is restricted by its low crystallization ability/degree, impeding its industrial implementation. In this work, we propose to combine two strategies, often used to overcome this drawback, that is, via addition of nanofillers such as nanoclays and PLA stereocomplexation in order to enhance the rigidity, thermal and gas-barrier properties of the polyester matrix. To enhance the dispersion of nanoclays, the “grafting from” approach was thereby employed via in situ intercalative and metal-free ring-opening polymerization of L-lactide and D-lactide from the surface of a hydroxyl-functionalized organomodified montmorrilonite (i.e., Cloisite 30B). These nanohybrids were subsequently melt-blended with commercial PLA in order to obtain stereocomplexed (or not) PLA-based nanocomposites upon the stereochemistry of the nanohybrids. The morphology and dispersion of the resulting materials were examined by Wide-Angle X-ray Diffraction (WAXD) and Transmission Electron Microscopy (TEM). Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA) were carried out to assess the thermo-mechanical performances of the nanocomposites. Moreover, their oxygen-barrier properties were also evaluated. The stereocomplex directly obtained by melt blending PLA and the bio-PDLA nanohybrid has both enhanced melting temperature (more than 40°C) and storage modulus (higher than 60%) than those of commercial PLA. Moreover, the nanocomposites obtained show a significant improvement in oxygen-barrier properties. With homo-grafted blend and stereo-grafted blend strategy, the relative oxygen permeability was again reduced by, i.e. 41% and 46% respectively in comparison to PLA matrix.

Microbiological Quality and the Inability of Proteolytic Clostridium botulinum To Produce Toxin in Film-Packaged Fresh-Cut Cabbage and Lettuce

The production of toxin by a 10-strain mixture of proteolytic Clostridium botulinum in fresh produce packaged in polyethylene films having high (7,000 cc/m2/24 h; HOTR) and low (3,000 cc/m2/24 h; LOTR) relative oxygen permeability was determined. Shredded cabbage and lettuce inoculated with ~102 spores/g were placed in bags composed of the two films (1.4 kg/bag), and the bags were then vacuum sealed. Produce was stored at 4, 13, and 21°C for up to 21 (cabbage) or 28 (lettuce) days and analyzed periodically. At each sampling time, the gas composition within the bags, pH of the produce, and microbial populations (total aerobic and anaerobic microorganisms, lactic acid bacteria, psychrotrophic bacteria, and yeasts and molds) were determined. In addition, the presence of botulinal toxin was determined using the standard U.S. Food and Drug Administration mouse bioassay protocol. Bags made of HOTR film prolonged sensory quality of cabbage and lettuce, especially at 13 and 4°C. Packaging material had an effect on the growth of various groups of microorganisms; however, there was not a general trend. For example, lettuce packaged in HOTR bags had higher aerobic microbial populations than that packed in LOTR, but no significant difference (P ≤ 0.05) was observed with cabbage. Growth of psychrotrophic bacteria was greater in vegetables packaged in HOTR film while growth of yeasts and molds was not affected by either packaging film. Most differences in microbial populations in produce packaged in LOTR and HOTR films were less than 1 log10 CFU/g. Botulinal toxin was not detected in cabbage or lettuce packaged in either film or stored under any test condition.

Effect of the interfacial surfactant layer on oxygen transfer through the oil/water phase boundary in perfluorocarbon emulsions

The effect of interfacial surfactant molecules on oxygen transfer through oil/water phase boundary has been studied in FlurO(2) (TM) emulsions, i.e., perfluorocarbon (PFC) emulsions developed as oxygen carriers in cell culture. Measurements of oxygen permeability were made with a polarographic oxygen electrode in pure PFCs and in emulsions with various PFC volume fractions. Comparison of the experimental results with the theoretically derived values of relative oxygen permeability clearly indicates that the mass transfer resistance caused by the interfacial surfactant layer in PFC emulsions is insignificant. Therefore, oxygen dissolved in the enclosed PFC phase is readily available to cells growing in the aqueous media and FlurO(2) emulsions with very fine emulsion particles (< 0.2 microm) can be used to effectively enhance gas/liquid interfacial oxygen transfer in bioreactors. The inadequacy in describing mass transfer in heterogeneous systems, such as the PFC emulsions, by conventional concentration-based oxygen diffusion coefficients has also been discussed.