Intermediate Bilayer Research Articles

Investigating strengthening and softening mechanisms in Al/Ni multilayers via molecular dynamics simulations of uniaxial compression

Due to their nanoscale features, nanometric multilayers can have a large variation in properties for varying bilayer heights. While the hardening at small feature sizes and the consequent softening at even smaller feature sizes have been observed for decades, the underlying mechanisms are still under debate. In this study, molecular dynamics uniaxial compression simulations are employed to study the mechanical properties of Al/Ni multilayers for bilayer heights h from 100 nm down to 5 nm. The effect of the microstructure on Young’s modulus and the yield strength was investigated. Furthermore, the mechanical properties of equiatomic and equivolumetric multilayers were compared. A comparison with experimental results from the literature showed good agreement. Both the hardening at intermediate bilayer heights as well as the softening at very small bilayer heights were observed. The results are discussed in the context of possible hardening and softening mechanisms. While the Hall–Petch effect with a h−1/2 scaling is not contradicted, it is shown that, although the underlying mechanisms are different, both the hardening as well as the softening are based on a general size effect with a scaling of ln(h)/h.

Evaluation of mechanical properties and tribological behavior of DLC/WC/WCN/W multilayer coatings deposited by HiPIMS

In the present research, thin films of diamond-like carbon (DLC), tungsten carbide (WC), tungsten carbonitride (WCN) and tungsten (W) were deposited onto the surface of an AISI 52100 steel using the PVD process by HiPIMS. A (DLC/WC/W), B (DLC/WCN/W) and C (DLC/WC/WCN/W) coating systems were evaluated through mechanical and tribological tests to establish the optimal system for its application in the bearing industry. The physicochemical characterization was carried out using SEM and EDS. On the other hand, the mechanical properties were obtained by Berkovich instrumented indentation tests, where it was observed that the synergy between the WC/WCN intermediate bilayer in Coating C increased the hardness and elastic modulus concerning the WC and WCN intermediate monolayers. Finally, the coating systems were evaluated through the linear reciprocating wear test. Coating C presented the highest wear resistance and the lowest values of coefficient of friction, due to the positive synergy between the WC/WCN intermediate bilayer.

Multifaceted membrane binding head of the SARS-CoV-2 spike protein.

The SARS-CoV-2 spike protein presents a surface with enormous membrane binding potential to host tissues and organelles of infected cells. Its exposed trimeric head binds not only the angiotensin-converting enzyme 2 (ACE2), but also host phospholipids which are missing from all existing structures. Hence, the membrane interaction surfaces that mediate viral fusion, entry, assembly and egress remain unclear. Here the spike:membrane docking sites are identified based on membrane optimal docking area (MODA) analysis of 3D structures of spike proteins in closed and open conformations at endocytic and neutral pH levels as well as ligand complexes. This reveals multiple membrane binding sites in the closed spike head that together prefer convex membranes and are modulated by pH, fatty acids and post-translational modifications including glycosylation. The exposure of the various membrane interaction sites adjusts upon domain repositioning within the trimer, allowing formation of intermediate bilayer complexes that lead to the prefusion state while also enabling ACE2 receptor recognition. In contrast, all antibodies that target the spike head would block the membrane docking process that precedes ACE2 recognition. Together this illuminates the engagements of the spike protein with plasma, endocytic, ER or exocytic vesicle membranes that help to drive the cycle of viral infection, and offers novel sites for intervention.

Structural optical and electrical properties of a transparent conductive ITO/Al-Ag/ITO multilayer contact.

Indium tin oxide (ITO) is a widely used material for transparent conductive oxide (TCO) films due to its good optical and electrical properties. Improving the optoelectronic properties of ITO films with reduced thickness is crucial and quite challenging. ITO-based multilayer films with an aluminium–silver (Al–Ag) interlayer (ITO/Al–Ag/ITO) and a pure ITO layer (as reference) were prepared by RF and DC sputtering. The microstructural, optical and electrical properties of the ITO/Al–Ag/ITO (IAAI) films were investigated before and after annealing at 400 °C. X-ray diffraction measurements show that the insertion of the Al–Ag intermediate bilayer led to the crystallization of an Ag interlayer even at the as-deposited stage. Peaks attributed to ITO(222), Ag(111) and Al(200) were observed after annealing, indicating an enhancement in crystallinity of the multilayer films. The annealed IAAI film exhibited a remarkable improvement in optical transmittance (86.1%) with a very low sheet resistance of 2.93 Ω/sq. The carrier concentration increased more than twice when the Al–Ag layer was inserted between the ITO layers. The figure of merit of the IAAI multilayer contact has been found to be high at 76.4 × 10−3 Ω−1 compared to a pure ITO contact (69.4 × 10−3 Ω−1). These highly conductive and transparent ITO films with Al–Ag interlayer can be a promising contact for low-resistance optoelectronics devices.

Elastic deformations mediate interaction of the raft boundary with membrane inclusions leading to their effective lateral sorting

Liquid-ordered lipid domains represent a lateral inhomogeneity in cellular membranes. These domains have elastic and physicochemical properties different from those of the surrounding membrane. In particular, their thickness exceeds that of the disordered membrane. Thus, elastic deformations arise at the domain boundary in order to compensate for the thickness mismatch. In equilibrium, the deformations lead to an incomplete register of monolayer ordered domains: the elastic energy is minimal if domains in opposing monolayers lie on the top of each other, and their boundaries are laterally shifted by about 3 nm. This configuration introduces a region, composed of one ordered and one disordered monolayers, with an intermediate bilayer thickness. Besides, a jump in a local monolayer curvature takes place in this intermediate region, concentrating here most of the elastic stress. This region can participate in a lateral sorting of membrane inclusions by offering them an optimal bilayer thickness and local curvature conditions. In the present study, we consider the sorting of deformable lipid inclusions, undeformable peripheral and deeply incorporated peptide inclusions, and undeformable transmembrane inclusions of different molecular geometry. With rare exceptions, all types of inclusions have an affinity to the ordered domain boundary as compared to the bulk phases. The optimal lateral distribution of inclusions allows relaxing the elastic stress at the boundary of domains.

High-Strength Diffusion-Bonded Joints of 17-4 Stainless Steel and T64 Alloy Using Nickel and Copper Bilayer

The vacuum-assisted diffusion-bonded joints (DBJs) between 17-4 PH grade stainless steel (SS) and Ti-6Al-4V (T64) using Ni and Cu as transitional bilayer materials were prepared using 3.5 MPa bonding pressure at 750-850 °C for 45 min. Interfaces were characterized using optical and scanning electron microscopes. Atomic interdiffusion across the joining interfaces was assessed using electron probe microanalyzer. For the DBJs processed at 750 °C, layer-wise intermetallic of Cu-Ti type was found at the Cu/T64 interface; however, both the SS/Ni and Ni/Cu interfaces were free from intermetallics. Till 800 °C, Ni and Cu intermediate bilayer materials restricted the movement from T64 to SS, and vice versa. At 825 °C and higher bonding temperatures, both Ni and Cu layers did not impede the atomic movement from T64 to SS; so layer-wise reaction products (σ phase, χ phase, and λ + FeTi) were observed along the SS side. High-strength diffusion-bonded joint was achieved at 775 °C processing temperature, that is, maximum tensile strength of ~ 643 MPa along with ~ 9.3% elongation. The tensile strength of the DBJs gradually decreased with the increase in temperature due to the presence of Ni + Cu + T64 diffusion reaction zone (800 °C) and the evolution and broadening of SS + Ni + Cu + T64 diffusion reaction zone at and above 825 °C.

Development of multilayer Hydroxyapatite - Ag/TiN-Ti coatings deposited by radio frequency magnetron sputtering with potential application in the biomedical field

The use of composite coatings is emerging as a great alternative to conventional coatings, allowing the combination of different superficial properties that are widely desired in surgical implants, such as osteointegration and bactericidal character, and cannot be provided by one material alone. In the present investigation the effect of the incorporation of a TiN-Ti intermediate bilayer on the chemical composition, structure, morphology, roughness, residual stresses and adhesion of a multi-layer Hydroxyapatite (HA)-Ag coating deposited on Ti-6Al-4V by magnetron sputtering was evaluated. Additionally, the cytotoxicity of the developed system was evaluated by in vitro tests. According to the results obtained, a decrease in the Ca/P ratio from 1.85 to 1.74 was obtained through the deposition of an HA-Ag system on the intermediate bilayer, and the crystallinity of the developed coating was favored. The multi-layer structure was effectively observed by field emission scanning electron microscopy, where it was possible to identify each of the HA, Ag, TiN and Ti layers. Meanwhile, an increase of 7% in crystallite size, a decrease of 36% in residual stresses and an increase of 32% in adhesion were registered for this composite coating compared to the free intermediate bilayer system. Finally, biological evaluation allowed the non-cytotoxic character of the deposited coatings to be confirmed.

Site-specific variations in cutaneous autofluorescence revealed by excitation-emission matrix spectroscopy.

Although cutaneous autofluorescence has been utilized for evaluation of skin conditions, there is a paucity of data on normal human skin autofluorescence and its dependence on anatomical site. The objective of this study is to use excitation-emission matrix spectroscopy to quantify and characterize skin autofluorescence at different body sites. Ten anatomical sites from 30 healthy volunteers were measured with a double-grating excitation-emission matrix spectrofluorometer. For the 10 body sites evaluated, there were four overall patterns of autofluorescence: skin from the head and neck exhibits high superficial and low bilayer fluorescence; the dorsal forearm and dorsal hand have both low superficial and bilayer fluorescence; the upper inner arm and back have high superficial and intermediate bilayer fluorescence; while the palm and thumbnail have both high superficial and bilayer fluorescence. The corresponding fluorescence excitation-emission peaks for these patterns were as follows: head and neck, 3 peaks at 290-300/330-350, 360-380/460-485, and 380-420/610-630nm; dorsal forearm and dorsal hand, 2 peaks around 295-300/345-360 and 385-395/460-485nm; upper inner arm and back, 3 peaks around 295-300/335-355, 335-340/390-410, and 375-390/455-480nm; palm and thumbnail, 3 peaks around 285-300/345-355, 335-345/390-410, and 365-390/450-480nm. Cutaneous fluorescence varies in distinct patterns according to anatomical site, due to the component fluorophores present, skin thickness, and the degree of melanization and long term sun exposure. These EEM patterns for normal skin should be accounted for when interpreting fluorescence signals from disease states and can also be used to guide the selection of optimal wavebands when applying this optical modality.

Effect of Nanocrystalline Perovskite-Based Cathode Compositions on SOFC Performance: An Aspect of Cell Stability with Composite Interlayer

Multicomponent nanocrystalline Sr-doped lanthanum manganite (La0.65Sr0.3MnO3, LSM) and a composite cathode comprising of Sr-doped lanthanum ferrite (La0.54Sr0.4Co0.2Fe0.8O3, LS-1) and cobaltite (La0.54Sr0.4Fe0.2Co0.8O3, LS-2) were synthesized. Powder was characterized thoroughly. Sintering characteristics of all the cathode compositions were correlated with densification of the bulk cathode material and the electrical characterization of the sintered cathode compositions showed that the composite cathode exhibits higher electrical conductivity compared to LSM. Another composite (L80-C20) material has been synthesized to be used as a part of the intermediate bilayer containing 80% (50%LS-1+50%LS-2) cathode composite and 20% Co-doped CGO. Symmetric cell study showed that the electrode polarization of this composite as low as 0.012 ohm-cm2. Systematic stability studies of the electrochemical performance using single cells with different configurations revealed that use of intermediate bi-layer, one layer of CoCGO and another interlayer containing L80-C20 increases the cell stability in conjunction with doped lanthanum ferrite and cobaltite based composite electrodes.

Myelin swelling and measurement of forces between myelin membranes

MYELIN has been observed by many workers to swell to a limited extent in water1,2; a swelling thought to result from electrostatic repulsion between the myelin membranes1. The extracellular space between membrane pairs increases by approximately 80 A on immersion in water, whereas the intracellular space changes by only a few angstroms1,2. On the other hand, in phospholipid bilayer systems, very low surface charge densities (approximately 2 mol per cent charged phospholipid or 1 charge per 3,500 A2) cause otherwise neutral phospholipid bilayers, which normally swell to a very limited extent, to separate indefinitely in water as a result of electrostatic repulsion3,4. At even lower surface charge densities, intermediate bilayer separations between that of neutral bilayers (about 27 A) and charged bilayers (indefinite swelling) have rarely, if ever, been observed. Why, then, does nerve myelin swell to only a limited extent in water? We describe here our attempts to answer this question, and report that myelin is prevented from swelling indefinitely by mechanical forces. We have also succeeded in measuring the net repulsive force between myelin membrane pairs.