Layers Of Equal Thickness Research Articles

Size-dependent plastic deformation characteristics in He-irradiated nanostructured Cu/Mo multilayers: Competition between dislocation-boundary and dislocation-bubble interactions

Nanoindentation methodology was used to investigate the plastic deformation characteristics, including the hardness (H), strain rate sensitivity (SRS, m) and activation volume (V*), of Cu/Mo nanostructured metallic multilayers (NMMs) with equal layer thickness (h) spanning from 10 to 200nm before and after He-implantation at room temperature. Compared with the as-deposited Cu/Mo NMMs, the irradiated Cu/Mo samples exhibited the enhanced hardness particularly at great h, which is caused by the bubble-hardening effect. Unlike the as-deposited Cu/Mo NMMs displayed a monotonic increase in SRS (or a monotonic decrease in activation volume) with reducing h, the irradiated Cu/Mo samples manifested an unexpected non-monotonic variation in SRS as well as in activation volume. It was clearly unveiled that the SRS of irradiated Cu/Mo firstly decreased with reducing h down to a critical size of ~50nm and subsequently increased with further reducing h, leaving a minimum value at the critical h. These phenomena are rationalized by considering a competition between dislocation-boundary and dislocation-bubble interactions. A thermally activated model based on the depinning process of bowed-out partial dislocations was employed to quantitatively account for the size-dependent SRS of Cu/Mo NMMs before and after irradiation. Our findings not only provide fundamental understanding of the effects of radiation-induced defects on plastic characteristics of NMMs, but also offer guidance for their microstructure sensitive design for performance optimization at extremes.

Interrelationship of in situ growth stress evolution and phase transformations in Ti/W multilayered thin films

This paper addresses the in situ growth stress evolution and phase transformation of bcc to hcp Ti in Ti/W multilayered thin films. A series of equal layer thicknesses from 20 nm to 1 nm were deposited. As the bilayer thickness reduced, the overall film stress became less compressive until the Ti transformed from hcp (at the larger layer thicknesses) to bcc in the 1 nm/1 nm multilayer. The pseudomorphic bcc stabilization resulted in a recovery of the compressive stress to values near that for the bulk phase stabilized for the 5 nm/5 nm multilayer. A discernable change in stress slope was noted for the bcc to hcp Ti transition as a function of Ti layer thickness. The stress states for each film, during film growth, are rationalized by the lattice matching of the phase with the growth surface. These results are coupled to a molecular dynamics deposition simulation which revealed good agreement with the experimentally observed transformation thickness.

Length-scale-dependent deformation mechanism of Cu/X (X=Ru, W) multilayer thin films

The microstructure and deformation/strengthening behavior of Cu/Ru (face-centered cubic (fcc)/hexagonal close-packed) and Cu/Cr (fcc/body-centered cubic) multilayered films with equal individual layer thickness h ranging from 0.8 to 200nm were investigated. Based on systematic X-ray diffraction and transmission electron microscopy analyses, as h was varied, interface structure transitions from semi-coherent to fully coherent structures were observed in Cu/Ru rather than Cu/W. These structure transitions were found to play crucial roles in length-scale dependent strengthening mechanisms. Specifically, the multilayer strength derived for relatively large h (≥50nm) followed the classic Hall–Petch relation, while the load-bearing effect was proposed to be the dominant mechanism for relatively small h (50nm≥h≥4nm). More importantly, as h was further reduced to below 4nm in Cu/Ru, a strengthening mechanism based on dislocation loop crossing several coherent interfaces was proposed and quantitatively evaluated, The number of layers needed to be crossed by the dislocation loop was found to determine the strength.

Growth and characterisation of n- and p-type ZnTe thin films for applications in electronic devices

The growth of n- and p-type ZnTe thin films have been achieved intrinsically by potentiostatic electrodeposition method using a 2-electrode system. Cyclic voltammogram have been used to obtain range of growth voltages required to form stoichiometric thin films of ZnTe. The ZnTe thin films have been electrodeposited (ED) on glass/fluorine-doped tin oxide (FTO) conducting substrates in aqueous solutions of ZnSO4·7H2O and TeO2. The films have been characterised for their structural, electrical, morphological, compositional and optical properties by using X-ray diffraction (XRD), Raman spectroscopy, Photoelectrochemical (PEC) cell measurements, DC conductivity measurements, Scanning electron microscopy (SEM), Atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDX) and Optical absorption techniques. The XRD results reveal that the electroplated films are polycrystalline and have hexagonal crystal structure with the preferred orientation along (002) plane. UV–Visible spectrophotometer has been used for the bandgap determination of as-deposited and heat-treated ZnTe layers. The bandgap of the heat-treated ZnTe films are in the range (1.90–2.60) eV depending on the deposition potential. PEC cell measurements show that the ED-ZnTe films have both n- and p-type electrical conductivity. The DC conductivity measurements revealed that the average resistivity of n-ZnTe and p-ZnTe layers of equal thickness is of the order of 104 Ωcm; the magnitude of the electrical resistivity of p-ZnTe is almost five times greater than that of the n-ZnTe layer. Using the n- and p-type ZnTe layers, p-n homo-junction diodes with device structure of glass/FTO/n-ZnTe/p-ZnTe/Au were fabricated. The fabricated diodes showed rectification factor of 102, reverse saturation current of ∼10.0 nA and potential barrier height greater than 0.77 eV indicating electronic device quality of these layers.

Resonant magnetoelectric response of composite cantilevers: Theory of short vs. open circuit operation and layer sequence effects

The magnetoelectric effect in layered composite cantilevers consisting of strain coupled layers of magnetostrictive (MS), piezoelectric (PE), and substrate materials is investigated for magnetic field excitation at bending resonance. Analytic theories are derived for the transverse magnetoelectric (ME) response in short and open circuit operation for three different layer sequences and results presented and discussed for the FeCoBSi-AlN-Si and the FeCoBSi-PZT-Si composite systems. Response optimized PE-MS layer thickness ratios are found to greatly change with operation mode shifting from near equal MS and PE layer thicknesses in the open circuit mode to near vanishing PE layer thicknesses in short circuit operation for all layer sequences. In addition the substrate layer thickness is found to differently affect the open and short circuit ME response producing shifts and reversal between ME response maxima depending on layer sequence. The observed rich ME response behavior for different layer thicknesses, sequences, operating modes, and PE materials can be explained by common neutral plane effects and different elastic compliance effects in short and open circuit operation.

Ballistic Test of Multilayered Armor with Intermediate Epoxy Composite Reinforced with Jute Fabric

Multilayered armors with a front ceramic followed by aramid fabric (Kevlar™) are currently used against high velocity ammunition. In these armors, a front ceramic layer that shatters and spalls the bullet is followed by an intermediate layer, usually plies of aramid fabric, which dissipates both the bullet and ceramic fragments energy. In the present work, the intermediate aramid fabric layer was replaced by an equal thickness layer of 30 vol% jute fabric reinforced epoxy composite. Ballistic impact test with 7.62 caliber ammunition revealed that both the plain epoxy and the jute fabric composite have a relatively similar performance of the Kevlar™ and also attended the NIJ standard for body protection. The energy dissipation mechanisms of jute fabric composite were analyzed by scanning electron microscopy and found to be the rupture of the brittle epoxy matrix as well as the interaction of the jute fibers with the post-impact fragments. This latter is the same mechanism recently disclosed for aramid fabric. However, the lightness and lower cost of the jute fabric composite are additional advantages that favor its substitution for the aramid fabric.

Chemical vapor deposition of Si/SiC nano-multilayer thin films

Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311).

Natural Curaua Fiber-Reinforced Composites in Multilayered Ballistic Armor

The performance of a novel multilayered armor in which the commonly used plies of aramid fabric layer were replaced by an equal thickness layer of distinct curaua fiber-reinforced composites with epoxy or polyester matrices was assessed. The investigated armor, in addition to its polymeric layer (aramid fabric or curaua composite), was also composed of a front Al2O3 ceramic tile and backed by an aluminum alloy sheet. Ballistic impact tests were performed with actual 7.62 caliber ammunitions. Indentation in a clay witness, simulating human body behind the back layer, attested the efficacy of the curaua-reinforced composite as an armor component. The conventional aramid fabric display a similar indentation as the curaua/polyester composite but was less efficient (deeper indentation) than the curaua/epoxy composite. This advantage is shown to be significant, especially in favor of the lighter and cheaper epoxy composite reinforced with 30 vol pct of curaua fiber, as possible substitute for aramid fabric in multilayered ballistic armor for individual protection. Scanning electron microscopy revealed the mechanism associated with the curaua composite ballistic performance.

Detection of early cartilage deterioration associated with meniscal tear using T1ρ mapping magnetic resonance imaging.

BackgroundIn patients with degenerative meniscal tears, subclinical cartilage degeneration may be present even if gross morphological changes are not evident. The aim of this study was to detect occult cartilage degeneration using T1ρ MRI mapping in patients with meniscal tears without obvious radiographic osteoarthritis (OA).MethodsA total of 22 subjects with degenerative meniscal tears in the early stages of osteoarthritis [Kellgren-Lawrence (KL) grade of 0–2] and 19 healthy subjects as the control group were examined. The femoral condyle was divided into four 30° wedges (−30°–0° anteriorly, 0°–30°, 30°–60° and 60°–90° posteriorly), and each area of cartilage was further divided into superficial and deep layers of equal thickness. The tibial side was divided into anterior and posterior areas with superficial and deep layers in each. The mean T1ρ values (ms) in each area were calculated.ResultsOn the femoral side, T1ρ values of the superficial and deep regions (−30°–0°, 0°–30° and 30°–60°) in the meniscal tear group were significantly higher than those in the control group [superficial (−30°–0°): 49.0 ± 4.0 (meniscal tear group) vs 45.1 ± 2.1 (control group), deep (−30°–0°): 45.2 ± 3.3 vs 39.5 ± 5.0, superficial (0°–30°): 54.5 ± 5.3 vs 47.4 ± 5.7, deep (0°–30°): 46.8 ± 4.0 vs 40.7 ± 6.3, superficial (30°–60°): 50.5 ± 3.1 vs 47.1 ± 5.7]. On the tibial side, the meniscal tear group had significantly higher T1ρ values superficially in both anterior and posterior regions compared with the control group [superficial (anterior): 52.0 ± 4.3 vs 46.7 ± 5.4, superficial (posterior): 53.1 ± 5.1 vs 46.0 ± 4.9]. Moreover, these significant differences were observed when comparing patients in the meniscal tear group with KL grades of 0 or 1 and the control group.ConclusionsOur study suggested that early biochemical changes in cartilage associated with degenerative meniscal tears occur first in the superficial zones in areas of contact during slight flexion. Characterising the early relationship between cartilage degeneration and degenerative meniscal tears using T1ρ MRI mapping may be of clinical benefit and provide further evidence linking meniscal injury to OA.

Studies on ballistic impact of the composite panels

The ballistic impact of the composite materials is studied using the numerical models. Individual impact studies are conducted on the composite plate made-up of woven fabric CFRP, E-glass/epoxy and the Kevlar/epoxy composites. The plate is fabricated with 8 layers of equal thickness arranged in different orientations. A spherical steel projectile is considered for the high velocity impact. The projectile is placed very close to the plate, at the center and impacted with a velocity of 100m/s. The displacement and the stress distribution in each layer are studied for the layup sequence [+45/−45/+45/−45/−45/+45/−45/+45]. The variation of the kinetic energy, the increase in the internal energy of the laminate and the decrease in velocity of the projectile with time are also studied. Based on the results, the best layup sequence for the ballistic impact of each material is suggested.

Strain rate sensitivity of nanolayered Cu/X (X=Cr, Zr) micropillars: Effects of heterophase interface/twin boundary

Manipulation of internal features, e.g., layer/twin interfaces, is standard practice in engineering structural materials to tailor their properties. In the present work, we fabricated nanotwinned (NT)- and nanocrystalline (NC)–Cu/X (X=Cr, Zr) nanolayered micropillars (NLs) with equal layer thickness (h) spanning from 5 to 125nm. The microcompression methodology was employed to investigate the layer/twin interfaces effects on plastic characteristics of nanolayered materials at different strain rates. Experimental results clearly unveil that the Cu/X NTNLs exhibit a significant increase in strength and in strain rate sensitivity (SRS) by introducing nanotwins, in comparison with the Cu/X NCNLs. The non-monotonic evolution of SRS with h observed in the Cu/X NTNLs is explained by a competition between the monotonically increased interfaces density and the decreased twin boundaries density with reduction in feature size h. Unlike the monotonically enhanced SRS of Cu/X NCNLs, the SRS of Cu/X NTNLs first decreases and subsequently increases with reducing h. A phenomenological model is proposed to rationalize these experimental findings and highlight the microstructural feature size effects on the rate-limiting processes of metallic materials.

Achieving optimum mechanical performance in metallic nanolayered Cu/X (X = Zr, Cr) micropillars

The selection and design of modern high-performance structural engineering materials such as nanostructured metallic multilayers (NMMs) is driven by optimizing combinations of mechanical properties and requirements for predictable and noncatastrophic failure in service. Here, the Cu/X (X = Zr, Cr) nanolayered micropillars with equal layer thickness (h) spanning from 5–125 nm are uniaxially compressed and it is found that these NMMs exhibit a maximum strain hardening capability and simultaneously display a transition from bulk-like to small-volume materials behavior associated with the strength at a critical intrinsic size h ~ 20 nm. We develop a deformation mode-map to bridge the gap between the interface characteristics of NMMs and their failure phenomena, which, as shrinking the intrinsic size, transit from localized interface debonding/extrusion to interface shearing. Our findings demonstrate that the optimum robust performance can be achieved in NMMs and provide guidance for their microstructure sensitive design for performance optimization.

The geological CO2 storage capacity of the Jeju Basin, offshore southern Korea, estimated using the storage efficiency

We estimated the storage capacity for geological CO2 sequestration of the Jeju Basin off southern Korea, northern East China Sea using the storage efficiency and a deterministic, volumetric method. We analyzed seismic and well-log data to get the geological parameters and used the storage efficiency parameters for deep saline, nonmarine sediments in open systems. The time interval of the basin fill from 1.2 to 2.8s two-way traveltime was selected for the depth zone for CO2 storage. This interval corresponds to the depth range of about 1200m to about 3900m and occurs mostly below the Late-Miocene unconformity which largely separates the thin nonmarine to shallow-marine sediments above and the thicker nonmarine sediments below. The time interval was divided into eight layers of equal time thickness of 0.2s two-way traveltime to determine the representative porosity and the in situ pressure, temperature, and density of CO2 for different depth ranges. The estimated geological CO2 storage capacities for the rock volume in the eight-layer interval range from about 23.5×1012kg (23.5Gt) to about 687.0×1012kg (687Gt) with an average of about 196.0×1012kg (196.0Gt).

Design and optimization of antireflecting coatings from nanostructured porous silicon dielectric multilayers

We report the design and fabrication of an optimized antireflecting structure with the maximum transmittance (Tmax) and minimum reflectivity (Rmin), based on a porous silicon dielectric multilayer (PS-DM). The structures consist of 50 layers of equal thicknesses with an increasing refractive index profile, n(z)~zk. Numerical results based on the transfer matrix method, along with an average spectral analysis, were used to compute a certain range of thicknesses (with the optimal thickness of 235nm) to obtain similar optical response (Tmax and Rmin). The average reflectivity spectrum of a PS-DM structure in the proposed optimal range was experimentally measured to be 1.3% from 300 to 1100nm. Such structures can be used to enhance the efficiency of silicon solar cells and optoelectronic devices.

Structure and lattice dynamics of heterostructures based on bismuth ferrite and barium strontium titanate

The surface morphology, lattice parameters, and Raman spectra have been investigated at the stages of successive formation of a three-layer heterostructure consisting of epitaxial layers Ba0.8Sr0.2TiO3 (BST) and (Bi0.98Nd0.02)FeO3 (BNFO). The structural distortions arising from sequential deposition of BST/BNFO/BST layers of equal thickness have been determined using X-ray diffraction. It has been found that the degree of tetragonal distortion of the BST film on MgO increases after deposition of the BNFO film on the BST film surface, indicating the appearance of compressive stresses in BST. Based on the analysis of polarized Raman spectra, it has been shown that, in the BNFO layer, there is a new phase state which is not observed in the bulk samples. It has been demonstrated that the degree of tetragonality of the BST film grown on the BNFO surface is higher than that of the BST film grown directly on MgO.

The microstructure and mechanical behavior of Mg/Ti multilayers as a function of individual layer thickness

We have used magnetron sputtering to deposit magnesium and titanium layers alternately onto a single-crystal silicon substrate with equal individual layer thickness (h, from 2.5 to 200nm) to form multilayers. We have investigated the mechanical behavior of the multilayers and its dependence on h. Transmission electron microscopy and X-ray diffraction analyses suggest that the multilayers exhibit strong texture with respect to Mg (0002) and Ti (0002) with an epitaxial growth pattern. Two primary orientation relationships between Ti and Mg have been identified, depending on h. Instrumented nanoindentation and microcompression have been used to examine the hardness/strength and the strain rate sensitivity of the multilayers. Based on nanoindentation, we have found that the strength of these multilayers generally increases as h is decreased. The microcompression measured strength is remarkably higher than that derived from indentation. The Hall–Petch law can be used to interpret the increase in strength at relatively large h (>50nm), while the confined layer slip model provides a better explanation for the relationship between strength and h at smaller h. We have also attempted to present an in-depth discussion about the applicability of relevant strengthening mechanisms on these hexagonal close-packed/hexagonal close-packed multilayers.

Phase transitions in ferroelectric-paraelectric superlattices: Stability of single domain state

We studied stability of the single-domain state with respect to domain formation within Landau-Ginzburg-Devonshire theory for ferroelectric-paraelectric superlattices having equal layer thickness. Single-domain state is possible if dielectric constant of the paraelectric is larger than that of the ferroelectric for non-polar directions as in the BaTiO3/SrTiO3 system, which was taken as an example. Stability limit of the single-domain state is found as a function of temperature and layer thickness where we show a strong dependence of this limit on character of near-electrode regions, a point often overlooked. We also show that transition between single- and multi-domain states is discontinuous.

Crystallization-aided extraordinary plastic deformation in nanolayered crystalline Cu/amorphous Cu-Zr micropillars

Metallic glasses are lucrative engineering materials owing to their superior mechanical properties such as high strength and great elastic strain. However, the Achilles' heel of metallic amorphous materials — low plasticity caused by instantaneous catastrophic shear banding, significantly undercut their structural applications. Here, the nanolayered crystalline Cu/amorphous Cu-Zr micropillars with equal layer thickness spanning from 20–100 nm are uniaxially compressed and it is found that the Cu/Cu-Zr micropillars exhibit superhigh homogeneous deformation (≥ 30% strain) rather than localized shear banding at room temperature. This extraordinary plasticity is aided by the deformation-induced devitrification via absorption/annihilation of abundant dislocations, triggering the cooperative shearing of shear transformation zones in glassy layers, which simultaneously renders the work-softening. The synthesis of such heterogeneous nanolayered structure not only hampers shear band generation but also provides a viable route to enhance the controllability of plastic deformation in metallic glassy composites via deformation-induced devitrification mechanism.

An efficient cost function for the optimization of an n-layered isotropic cloaked cylinder

In this paper, we present an efficient cost function for optimizing n-layered isotropic cloaked cylinders. Cost function efficiency is achieved by extracting the expression for the angle independent scatterer contribution of an associated Green's function. Therefore, since this cost function is not a function of angle, accounting for every bistatic angle is not necessary and thus more efficient than other cost functions. With this general and efficient cost function, isotropic cloaked cylinders can be optimized for many layers and material parameters. To demonstrate this, optimized cloaked cylinders made of 10, 20 and 30 equal thickness layers are presented for TE and TM incidence. Furthermore, we study the effect layer thickness has on optimized cloaks by optimizing a 10 layer cloaked cylinder over the material parameters and individual layer thicknesses.The optimized material parameters in this effort do not exhibit the dual nature that is evident in the ideal transformation optics design. This indicates that the inevitable field penetration and subsequent PEC boundary condition at the cylinder must be taken into account for an optimal cloaked cylinder design. Furthermore, a more effective cloaked cylinder can be designed by optimizing both layer thickness and material parameters than by additional layers alone.

Nonlinear Convective Oscillations in Two-Layer Systems under the Action of the Horizontal Component of the Temperature Gradient

The influence of the horizontal component of the temperature gradient on nonlinear oscillatory convective flows, developed under the joint action of buoyant and thermocapillary effects in the 47 v2 silicone oil - water system, is investigated. The layers of equal thicknesses are considered. Transitions between nonlinear regimes of convection, have been studied. It is shown that under the action of the horizontal component of the temperature gradient, the asymmetric oscillatory flow takes place in the system.