Additional Diffusion Barrier Research Articles

Novel Cr/Si-Slurry Diffusion Coatings for High Temperatures

Surface enrichment in Al, Si, and Cr can greatly improve high temperature oxidation resistance of many alloys. Al, Si, and Cr coatings are commonly applied via simple slurries or more complex pack cementation processes. Due to the high melting point of Cr, the deposition of Cr-based diffusion coatings by the slurry technique has proved challenging, and to date, Cr has mostly been applied by pack cementation. Here, a novel Cr-Si coating process via the slurry technique is described which has been developed and then demonstrated on two Ni-based superalloys, Rene 80 and Inconel 740H. The addition of Si to the slurry lowers the melting point via a Cr-Si eutectic and enables the formation of a liquid phase during heat treatment. Through this Cr-Si slurry coating process diffusion layers enriched by Cr and Si of about 150 µm were achieved. Oxidation behavior was studied through isothermal exposures at 900 °C for 1000 h in lab air. Uncoated Rene 80 and IN740H both showed formation of a Ti-containing Cr2O3 scale below a thin TiO2 top layer. Underneath the external scale a zone of internally oxidized Al grew over the exposure time and reduced the load-bearing cross-section progressively. In comparison, the Cr/Si-coated samples did not show internal Al oxidation, but a slow-growing Si-rich oxide film underneath the external Cr2O3 scale. This subscale represents an additional oxygen diffusion barrier. Thus, the weight gain during exposure for the coated samples was significantly lower than for the uncoated materials.

The Role of Extracellular Loops in the Folding of Outer Membrane Protein X (OmpX) of Escherichia coli.

The outer membrane of Gram-negative bacteria acts as an additional diffusion barrier for solutes and nutrients. It is perforated by outer membrane proteins (OMPs) that function most often as diffusion pores, but sometimes also as parts of larger cellular transport complexes, structural components of the cell wall, or even as enzymes. These OMPs often have large loops that protrude into the extracellular environment, which have promise for biotechnological applications and as therapeutic targets. Thus, understanding how modifications to these loops affect OMP stability and folding is critical for their efficient application. In this work, the small outer membrane protein OmpX was used as a model system to quantify the effects of loop insertions on OMP folding and stability. The insertions were varied according to both hydrophobicity and size, and their effects were determined by assaying folding into detergent micelles in vitro by SDS-PAGE and in vivo by isolating the outer membrane of cells expressing the constructs. The different insertions were also examined in molecular dynamics simulations to resolve how they affect OmpX dynamics in its native outer membrane. The results indicate that folding of OMPs is affected by both the insert length and by its hydrophobic character. Small insertions sometimes even improved the folding efficiency of OmpX, while large hydrophilic inserts reduced it. All the constructs that were found to fold in vitro could also do so in their native environment. One construct that could not fold in vitro was transported to the OM in vivo, but remained unfolded. Our results will help to improve the design and efficiency of recombinant OMPs used for surface display.

Inertia in the response of radon monitors introduced by diffusion anti-thoron barriers

The reduction of thoron (220Rn) influence on radon (222Rn) monitors by diffusion barriers may cause some deterioration of the quality of the radon measurements. Therefore, the best compromise has to be found between ensured anti-thoron protection and deterioration of the quality of the radon measurements. In this report, the focus is on the additional inertia in the response introduced by passive diffusion barriers against thoron. The characteristic inertia time introduced by diffusion barriers is theoretically modeled for the levels of thoron interference down to 1%. Experiments were carried out with an active monitor working in diffusion mode, using its built-in diffusion barrier and with an additional diffusion barrier added. The experimental results showed very good correspondence with the estimates based on the theoretical model. In summary, when using passive diffusion barriers against thoron, the greater is the reduction in the thoron interference, the larger is the inertia time in the response of active monitors that are introduced.

High Interfacial Thermal Stability of Flexible Flake-Structured Aluminum Thin-Film Electrodes for Bi2Te3-Based Thermoelectric Devices.

Environmental thermal energy harvesting based on thermoelectric devices is greatly significant to the advancement of next-generation self-powered wearable electronic devices. However, the rigid electrodes and interface diffusion of electrodes/thermoelectric materials would lead to the wearable discomfort and performance degradation of the thermoelectric device. Here, a flake-structured Al thin-film electrode with high conductivity and excellent reliability is prepared by regulating the microstructure and crystallinity of the films. The as-prepared Al thin film not only maintains its robustness after 1000 bending cycles but also does not delaminate from the substrate when subjected to the 3M tape test, exhibiting excellent flexibility and adhesion to substrate. By comparing with the annealed interface of the double-layer Cu/Bi2Te3 film, the interface of the heat-treated Al/Bi2Te3 film has almost no element diffusion, demonstrating high interfacial thermal stability. Moreover, a thermoelectric temperature sensor based on the Al thin-film electrode is prepared. The sensitivity of the annealed sensor is still linear, and it can stably monitor the temperature variation, showing high reliability. This discovery could provide a facile and effective strategy to achieving highly reliable thermoelectric devices and flexible electronic devices without any additional diffusion barriers.

Designing a high-reflectivity normal-incidence Ge/Si multilayer X-ray mirror for the 44–50 nm wavelength range

For many years, the Mo/Si multilayer mirror has been the standard choice for normal incidence optics in the EUV/X-ray range below 20 nm. As one goes to longer wavelengths, such as the 46.9 nm Ne-like Ar X-ray laser, the absorption in Mo increases substantially, which is why the Sc/Si mirror was developed to operate in this region. To solve the problem of the metastable interface of Sc/Si mirrors due to diffusion between the two materials, this paper looks at an alternative Ge/Si multilayer mirror that replaces Sc with Ge, which offers potential advantages for fabricating a multilayer structure without the additional diffusion barriers needed with Sc. Ge can also be used to produce a narrow band filter in this wavelength regime.

CVD synthesis and characterization of thin Mo2C crystals

AbstractIn this study, we present an investigation on the growth of thin Mo2C crystals via chemical vapor deposition using CH4. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy(AFM), and Raman spectroscopy studies show that the morphology and the thickness of Mo2C crystals are strongly affected by the impurities in the system, the thickness of the copper substrate, and the graphene presence on Cu surface prior to Mo2C formation. Our studies show that during the CVD process, orthorhombic Mo2C crystals grow along the [100] direction on two different regions: directly on Cu surface or on graphene covered regions. Mo2C crystals that form on graphene are found to be thinner and less defective compared to the ones formed on the Cu surface. This is attributed to graphene acting as an additional diffusion barrier for Mo atoms diffusing through the copper. In addition to the graphene beneath the Mo2C crystal, Raman studies indicate that graphene may grow also on top of the Mo2C crystal, forming a graphene/Mo2C/graphene sandwich structure which may offer interesting properties for electronic applications.

Surface related differences between uncoated versus carbon-coated silicon nanowire electrodes on performance in lithium ion batteries

Abstract We present electrode assemblies of Si nanowires (NW) partially coated with a highly conductive nano-graphitic carbon layer. The surface modifications and overall electrode performance of these electrodes are investigated at high charge and discharge rates. The possibility to directly compare carbon-coated with uncoated SiNW electrodes allows to systematically separate the influences of Si and C on the battery performance and stability from a typically mixed system in electrodes. Consequently, a nanometer thin carbon layer is found to be efficient in withstanding mechanical stress and cracking during battery cycling. Moreover, the protective layer reduces the surface reactivity towards the electrolyte and enhances the electrode conductivity. On the other hand, we conclude that thicker carbon coatings reduce the performance of the Si nanowire network at high currents by introducing an additional diffusion barrier for Li ions.

Microstructure change and deuterium permeation behavior of ceramic-metal multi-layer coatings after immersion in liquid lithium-lead alloy

For the establishment of liquid tritium breeding concepts, static lithium-lead corrosion tests for single-layer erbium oxide coatings and erbium oxide-metal multi-layer coatings were carried out, followed by deuterium permeation measurements. Grain boundary corrosion of erbium oxide coatings was confirmed by the static immersion tests at 550 and 600 °C. An erbium oxide-iron two-layer coating sustained its structure after lithium-lead immersion at 600 °C for up to 3000 h. The results of gas-driven deuterium permeation measurements for multi-layer coatings immersed at 550 and 600 °C indicated that crystallization and grain growth of erbium oxide would sufficiently occur during the immersion at 600 °C, and then the sample showed lower permeabilities in the first measurements at lower temperature. On the other hand, permeation reduction factors of the sample immersed at 550 °C were estimated to be 100‒200 in the temperature range of 400‒550 °C. A corrosion layer formed on the coating surface might work as an additional diffusion barrier after the permeation test at 600 °C.

Effects of zeolite particle size and internal grain boundaries on Pt/Beta catalyzed isomerization of n-pentane

The impact of particle size and internal grain boundaries of Beta zeolites was investigated in n-pentane isomerization over bifunctional Pt/Beta catalysts, by comparing the catalytic performance of four as-synthesized Pt/Beta samples that possess an identical Pt loading (0.5 wt%), but use four distinct Beta zeolites. Three of them contain polycrystalline zeolites, consisting of nano-sized crystals with a similar size of 9–13 nm, but having different average particle sizes (i.e., 1340, 830, and 250 nm) and numerous internal grain boundaries, as found via high-resolution transmission electron microscopy. The last catalyst contains single-crystalline zeolite, with an average particle size of 225 nm, and no observed internal grain boundaries. At low reaction temperature (<578 K), the particle size and internal grain boundaries do not change the apparent activity, because activity is controlled by reaction on the acid sites of the zeolite. At high reaction temperature (>614 K), a large particle size and the presence of internal grain boundaries significantly reduce the apparent activity, because of the extended diffusion path and additional diffusion barriers, which are probably caused by a mismatch in micropore alignment and gas-zeolite interfaces at these grain boundaries. Due to the small particle size and absence of internal grain boundaries, the observed activity for single-crystalline Beta can be 60–212% higher than for polycrystalline counterparts, even though it possesses a much weaker intrinsic acidity. This shows, remarkably, that single-crystalline zeolites with less internal grain boundaries can achieve a much higher catalytic activity.

Markov-state model for CO2 binding with carbonic anhydrase under confinement.

Enzyme immobilization with a nanostructure material can enhance its stability and facilitate reusability. However, the apparent activity is often compromised due to additional diffusion barriers and complex interactions with the substrates and solvent molecules. The present study elucidates the effects of the surface hydrophobicity of nano-confinement on CO2 diffusion to the active site of human carbonic anhydrase II (CA), an enzyme that is able to catalyze CO2 hydration at extremely high turnover rates. Using the Markov-state model in combination with coarse-grained molecular dynamics simulations, we demonstrate that a hydrophobic cage increases CO2 local density but hinders its diffusion towards the active site of CA under confinement. By contrast, a hydrophilic cage hinders CO2 adsorption but promotes its binding with CA. An optimal surface hydrophobicity can be identified to maximize both the CO2 occupation probability and the diffusion rate. The simulation results offer insight into understanding enzyme performance under nano-confinement and help us to advance broader applications of CA for CO2 absorption and recovery.

Codelivery of anti-cancer agents via double-walled polymeric microparticles/injectable hydrogel: A promising approach for treatment of triple negative breast cancer.

Triple negative breast cancer (TNBC) is an aggressive sub-type of breast cancer that rarely responds to conventional chemotherapy. Therefore, novel agents or new routes need to be developed to improve treatment efficacy and diminish severe side-effects of anti-cancer agents in TNBC patients. This study explores a novel localized co-delivery platform with potential application against TNBC. Uniform core-shell microparticles encapsulating cisplatin (Cis-DDP) and paclitaxel (PTX) are fabricated using coaxial electrohydrodynamic atomization technique and subsequently are embedded into an injectable hydrogel. The hydrogel provides an additional diffusion barrier against Cis-DDP and confines premature release of drugs. In addition, the hydrogel can provide a versatile tool for retaining particles in the tumor resected cavity during the injection following debulking surgery and prevent surgical site infection due to its inherent antibacterial properties. The combination of Cis-DDP and PTX demonstrates a synergistic effect against MDA-MB-231 cell line assigned to three different mechanisms of action, including denaturation of DNA strands, stabilization of microtubules, and amplification of intracellular reactive oxygen species (ROS) and activation of caspase-3 pathways. The results show a significant accumulation of mitochondrial ROS insults in cells upon treatment that consequently causes programmed cells death. The performance of microparticles/hydrogel carrier is evaluated against three-dimensional MDA-MB-231 (breast cancer) 3D spheroids, where a superior efficacy and a greater reduction in spheroid growth are observed over 14 days, as compared with free-drug treatment. Overall, drug-loaded core-shell microparticles embedded into injectable hydrogel provides a promising strategy to treat aggressive cancers and a modular platform for a broad range of localized multidrug therapies customizable to the cancer type.

Double-Walled Microparticles-Embedded Self-Cross-Linked, Injectable, and Antibacterial Hydrogel for Controlled and Sustained Release of Chemotherapeutic Agents.

First-line cancer chemotherapy has been prescribed for patients suffered from cancers for many years. However, conventional chemotherapy provides a high parenteral dosage of anticancer drugs over a short period, which may cause serious toxicities and detrimental side effects in healthy tissues. This study aims to develop a new drug delivery system (DDS) composed of double-walled microparticles and an injectable hydrogel for localized dual-agent drug delivery to tumors. The uniform double-walled microparticles loaded with cisplatin (Cis-DDP) and paclitaxel (PTX) were fabricated via coaxial electrohydrodynamic atomization (CEHDA) technique and subsequently were embedded into injectable alginate-branched polyethylenimine. The findings show the uniqueness of CEHDA technique for simply swapping the place of drugs to achieve a parallel or a sequential release profile. This study also presents the simulation of CEHDA technique using computational fluid dynamics (CFD) that will help in the optimization of CEHDA's operating conditions prior to large-scale production of microparticles. The new synthetic hydrogel provides an additional diffusion barrier against Cis-DDP and confines premature release of drugs. In addition, the hydrogel can provide a versatile tool for retaining particles in the tumor resected cavity during the injection after debulking surgery and preventing surgical site infection due to its inherent antibacterial properties. Three-dimensional MDA-MB-231 (breast cancer) spheroid studies demonstrate a superior efficacy and a greater reduction in spheroid growth for drugs released from the proposed composite formulation over a prolonged period, as compared with free drug treatment. Overall, the new core-shell microparticles embedded into injectable hydrogel can serve as a flexible controlled release platform for modulating the release profiles of anticancer drugs and subsequently providing a superior anticancer response.

Peculiarities of the influence of nanomodification on the properties of the Cu–Fe–Co–Ni binder for a diamond tool

The peculiarities of the influence of nanodimensional additives of WC, ZrO2, and micrometer particles of hexagonal BN on the compression ratio under cold pressing of briquettes, shrinkage under the solid-phase sintering, and mechanical properties (bending strength limit and hardness) of powder alloys in the Cu–Fe–Co–Ni system are investigated. It is shown that the introduction of alloying additives of WC, ZrO2, and hexagonal BN insignificantly affect the charge resistance to compaction under cold pressing, while they are an additional diffusion barrier in the course of sintering increasing the activation energy. An increase in hardness to 7 HRB and strength to 200 MPa are attained due to the implementation of the effect of dispersed reinforcement in a compact composite.

Peculiarities of Nano Modification Influence on Properties of Bond Cu-Fe-Co-Ni for Diamond Tool.

There were investigated peculiarities of nano sized additions’ influence WC, ZrO 2 , as well as micrometric particles of hexagonal BN on compatibility un- der cold pressing of briquettes, shrinkage under solid-phase sintering, as well as on mechanical properties (bending strength, hardness) of powder alloys in the system Cu-Fe-Co-Ni. There was showed that introduction of alloy additives WC, ZrO 2 and hexagonal BN has negligible influence on resistance of charging material to packing under cold pressing, and under sintering these additives are additional diffusion barrier, increasing energy of activation. At the expense of realization of dispersion hardening effect in compact composite was reach increase of hardness up 7 HRB and strength up 200 MPa.

The influence of porous transport layer modifications on the water management in polymer electrolyte membrane fuel cells

In this study, the influence of modifications of the porous transport layer (PTL) on the water management in polymer electrolyte membrane fuel cells is investigated. Laser perforation and milling are used to locally remove either the whole PTL or only the micro porous layer in the PTL. The changed liquid water distribution is visualized using synchrotron radiography and ESEM liquid water imaging. The observed effects are correlated with in-situ performance characteristics, pre-assembly and post-mortem analysis to examine on the beneficial and obstructive effects of the modifications.The analysis reveals that laser-perforation results in PTFE loss and hydrophilic regions which results in a good performance at dry conditions but serious flooding at high humidification conditions.Machined perforations show beneficial effects in the in-situ performance characteristics at high humidification conditions. A draining of the vicinity of the perforations is observed which results in an improved oxygen diffusivity. Water balancing and synchrotron visualization indicate that under the analyzed operating conditions the MPL increases the humidification of the ionomer on the one hand but creates an additional diffusion barrier by liquid water in the interface between the cathode catalyst layer and the MPL on the other hand.

Effect of sodium on recrystallization and photovoltaic properties of CdTe solar cells

During processing of CdTe solar cells, sodium can diffuse from the glass substrate into the active layers and can considerably affect CdTe solar cell properties. In this paper, the effects of sodium on structural and photovoltaic properties of low temperature grown CdTe solar cells are investigated in detail. Presence of sodium during the CdCl2 treatment enhances CdTe grain growth and causes widening of grain boundaries, creating pinholes and possible shunt paths in the solar cell. Furthermore, even small concentrations of sodium strongly enhance CdS–CdTe intermixing and a recrystallization leading to CdS island formation, which results in direct contacts between absorber and transparent front contact. Due to sodium present during the CdCl2 treatment the space charge region width strongly decreased, indicating the possibility of increasing acceptor density in CdTe. In spite of the doping effect, cells, which were CdCl2-treated in the presence of sodium, showed deterioration of photovoltaic performance due to excessive CdS–CdTe intermixing and electrical shunting. A sufficiently thick and large grained TCO layer of ZnO:Al can effectively suppress sodium diffusion in low temperature grown CdTe solar cells on soda-lime glass preventing the deterioration of solar cell properties due to sodium. This enabled the growth of 13.5% efficiency (VOC=853mV, FF=74.4%, JSC=21.3mAcm−2, without anti-reflection coating) CdTe solar cells on low cost soda-lime glass without additional diffusion barrier layer in spite of large sodium content of the substrate.

Key steps influencing the formation of ZSM-5 films on aluminum substrates

Zeolite ZSM-5 films were synthesized on various supports, including aluminum alloys widely used in aerospace industry. The different key steps influencing the formation of the MFI films, from substrates cleaning to in situ crystallization time were investigated. A complete characterization of the hybrid material is proposed using various techniques, such as X-ray diffraction, scanning electron microscopy, X-ray fluorescence, scratch test and nitrogen/ n-hexane sorption measurements. A minimum 9 μm thick, dense, continuous and highly crystallized film of ZSM-5 zeolite has been obtained. Depending on the substrate nature, the ZSM-5 film molar Si/Al ratio can vary between 15 and 30. The microporous coating is well adhered to the aluminum substrates. Sorption capacities were identical on ZSM-5 films compared to corresponding powders, revealing the absence of additional diffusion barriers. The mechanical stability and sorption capacity of the ZSM-5 films are promising for technological uses in space decontamination and corrosion protection of aluminum substrates.

Hydrogel–electrospun fiber composite materials for hydrophilic protein release

Although hydrogels are widely used in controlled-release systems, obtaining extended, uniform drug release with little initial burst has been challenging. However, recently researchers have shown that combining hydrogels with another drug delivery material can dramatically improve release kinetics. Here we describe a novel hydrogel-based composite material that exhibits stable, near-linear, sustained release of a model hydrophilic protein (e.g., bovine albumin serum, BSA) for over two months with a significant reduction in initial burst release (7% vs. 20%). The composite is comprised of poly(ε-caprolactone) (PCL) electrospun fiber mats coupled with poly(ethylene glycol)-poly(ε-caprolactone) diacrylate (PEGPCL) hydrogels through photo-polymerization. It is believed that the additional diffusion barrier provided by hydrophobic electrospun fiber mats reduces hydrogel swelling and water penetration rates and increases the diffusion path length, resulting in delayed, more uniform drug release. Further, released proteins remain bioactive as demonstrated by PC12 cell neurite extension in response to released nerve growth factor (NGF). The use of electrospun fiber mats to modulate hydrogel drug release provides a new method to control release kinetics of hydrophilic proteins, reducing burst release and extending the release duration.

The diffusion parameters of the extracellular space are altered in focal cortical dysplasias

Most hypotheses concerning the mechanisms underlying seizure activity in focal cortical dysplasia (FCD) are based on alterations in synaptic transmission and glial dysfunction. However, neurons may also communicate by extrasynaptic transmission, which was recently found to affect epileptiform activity under experimental conditions and which is mediated by the diffusion of neuroactive substances in the extracellular space (ECS). The ECS diffusion parameters were therefore determined using the real-time iontophoretic method in human neocortical tissue samples obtained from surgically treated epileptic patients. The obtained values of the extracellular space volume fraction and tortuosity were then correlated with the histologicaly assessed type of cortical malformation (FCD type I or II). While the extracellular volume remained unchanged (FCD I) or larger (FCD II) than in normal/control tissue, tortuosity was significantly increased in both types of dysplasia, indicating the presence of additional diffusion barriers and compromised diffusion, which might be another factor contributing to the epileptogenicity of FCD.

Effect of Hydrophobicity on the Interaction between Antimicrobial Peptides and Poly(acrylic acid) Microgels

The influence of peptide hydrophobicity on the interaction between antimicrobial peptides and poly(acrylic acid) microgels was studied by end-tagging the kininogen-derived antimicrobial peptide GKHKNKGKKNGKHNGWK (GKH17) and its truncated variant KNKGKKNGKH (KNK10) with oligotryptophan groups of different lengths. Microgel deswelling and reswelling in response to peptide binding and release was studied by micromanipulator-assisted light- and fluorescence microscopy, peptide uptake in microgels was determined from solution depletion measurements, and peptide oligomerization was monitored by fluorescence spectroscopy. Results showed that oligomerization/aggregation of the hydrophobically end-tagged peptides is either absent or characterized by exposure of the tryptophan residues to the aqueous ambient, the latter suggesting small aggregation numbers. In addition, peptide uptake and affinity to the poly(acrylic acid) microgels increase with the number of tryptophan residues in the hydrophobic end tag, whereas peptide-induced microgel deswelling kinetics did not display this tag-length dependence to the same extent. Instead, long end tags resulted in anomalous shell formation, opposing further peptide-induced network deswelling. Theoretical modeling suggested that the deswelling kinetics in response to peptide binding is largely controlled by stagnant layer diffusion, but also that for peptides with sufficiently long hydrophobic tags, the shell constitutes an additional diffusion barrier, thus resulting in slower microgel deswelling. In addition, GKH17 and KNK10 peptides lacking the tryptophan end tags were substantially released on reducing peptide-microgel electrostatic interactions through addition of salt, an effect more pronounced for the shorter KNK10 peptide, whereas the hydrophobically end-tagged peptides remained bound to the microgels also at high ionic strength.