Total Number Of Layers Research Articles

Basic aspects of microindentation in multilayered poly(ethylene terephthalate)/polycarbonate films

The microhardness H of multilayered poly(ethylene terephthalate) (PET)/polycarbonate (PC) films, produced by continuous layer multiplying coextrusion has been determined. These materials present rather uniform laminates up to thousands of layers in the micrometre and submicrometre range. The micromechanical properties have been investigated as a function of layer thickness of the single polymer components, the total number of layers, the film thickness and the influence of heat treatment. The microhardness of the microlayered structure has also been determined across the profile in the parallel direction to the packing of the layers. The hardness in the vicinity of the PET/PC phases has been examined. Results reveal that the influence of the interphase on the H values for the samples with a large number of layers is rather small. The most important parameter in determining the final hardness of the multilayered films is the ratio of the penetration depth to the thickness of the layer. Upon heating, a microhardness increase is observed as a consequence of a double contribution: the crystallization of the PET layers, on the one hand, and the physical ageing of the PC zones on the other.

A-Si:H/SiO2 multilayer films fabricated by radio-frequency magnetron sputtering for optical filters.

We examined the optical properties of a-Si:H/SiO2 multilayer films fabricated by radio-frequency magnetron sputtering for optical bandpass filters (BPFs). Because of the high refractive-index contrast between a-Si:H and SiO2, the total number of layers of an a-Si:H/SiO2 multilayer can be relatively small. We obtained an a-Si:H refractive index of 3.6 at lambda = 1550 nm and its extinction coefficient k < 1 x 10(-4) and confirmed by Fourier-transform infrared spectroscopy that such small k is influenced by the Si-H bonding in the film. We fabricated a-Si:H/SiO2 BPFs by using in situ optical monitoring. Thermal tuning of a-Si:H/SiO2 BPF upon a silica substrate was also performed, and a thermal tunability coefficient of 0.07 nm/degree C was obtained.

Stacked Microchannel Heat Sinks for Liquid Cooling of Microelectronic Components

A novel heat sink based on a multilayer stack of liquid cooled microchannels is investigated. For a given pumping power and heat removal capability for the heat sink, the flow rate across a stack of microchannels is lower compared to a single layer of microchannels. Numerical simulations using a computationally efficient multigrid method [1] were carried out to investigate the detailed conjugate transport within the heat sink. The effects of the microchannel aspect ratio and total number of layers on thermal performance were studied for water as coolant. A heat sink of base area 10 mm by 10 mm with a height in the range 1.8 to 4.5 mm (2–5 layers) was considered with water flow rate in the range 0.83×10−6m3/s (50 ml/min) to 6.67×10−6m3/s (400 ml/min). The results of the computational simulations were also compared with a simplified thermal resistance network analysis.

Recursive surface impedance matrix methods for ultrasonic wave propagation in piezoelectric multilayers

Two recursive surface impedance methods are described for acoustic wave propagation in multilayered piezoelectric structures. Both methods have the advantage of conceptual simplicity and flexibility brought about by the transfer matrix method. Moreover they do not have a priori computational limitations with respect to the total number of layers of the stratified structure nor with respect to the thickness of individual layers; nor is the computational stability limited by the frequency range. For both methods, numerical simulations were carried out in order to illustrate their performances and robustness when combined with suitable recursive numerical algorithms.

The Emission of Terahertz Radiation from Doped Silicon Devices

ABSTRACTRecent interest in biological imaging, remote sensing, and biochemical spectroscopy has made nanoscale Terahertz (THz) emitters based on semiconductors become extremely attractive. THz lasers with good performance (emitting over 1 milliWatt CW at 10 K) have been fabricated from the group III-V compounds by molecular beam epitaxy, but such devices are complex with over 500 quantum wells and barriers and are incompatible with low cost Si processing. Similar devices from the SiGe system have had difficulties because the strain limits the total number of active layers, and have produced relatively poor performance (peak powers near 5 nW), even using strain symmetric active regions on virtual substrates of relaxed SiGe buffers. We report here on the characteristics of a new type of THz emitting device with higher powers (above 0.1 milliWatt) based on radiative impurity transitions in doped silicon, which can be simply fabricated without Ge alloying.The THz emitters were fabricated from both p-type and n-type doped silicon wafers with typical resistivities of 5 Ω-cm, using conventional photolithography and metal contact lift off. Samples were electrically pulsed and the electroluminescence was measured by Fourier Transform Infrared spectrometry. At 4.2 K, the emission spectra showed several peaks centered around 8.1 THz for the boron doped device, 7 to 13 THz for the gallium doped device, and 6.6 THz for the phosphorus doped device. The electroluminescence was attributed to radiative transitions from excited p-like to s-like hydrogenic dopant states, with emission energies in remarkable agreement with the known dopant absorption levels. Current-voltage measurement suggested an excitation mechanism based on impact ionization of neutral dopants by hot carriers. The net quantum efficiency (emitted photons per injected electron) was estimated to be up to 2 × 10-3. The temperature dependence will be reported with operation above 30 K.

Modal analysis of a truck tyre using FE tyre model

A nonlinear finite element (FE) tyre model incorporating the nearly incompressible material property of the rubber block and the anisotropy of the layers is used to conduct a modal analysis of a selected truck tyre. The three-dimensional deflection patterns and the associated natural frequencies of the tyre with different inflation pressures are derived incorporating the pre-stress effects and the nonlinearities. The influences of the individual anisotropy-related structural parameters and the inflation pressure on the deflection patterns and the natural frequencies of the selected truck tyre structure have been investigated. The structural parameters considered in the modal analysis are the cord angles in each layer, the total number of belt layers and the amount of the twisted cords per unit width of each individual layer. The modal analysis is performed using the commercial software program ANSYS®. The application of the model in conjunction with the adequately measured material properties of the layers in belt and carcass casing can yield a quantitative modal analysis of a specific tyre. It is concluded that a tyre's vibration property, such as its natural frequencies, can be modified by varying the anisotropy-related structural parameters so as to improve the dynamic behaviour of a tyre in service.

PH-Responsive Properties of Multilayered Poly(l-lysine)/Hyaluronic Acid Surfaces

Multilayer films have been prepared by the sequential electrostatic adsorption of poly(L-lysine) and hyaluronic acid onto charged silicon surfaces from dilute aqueous solutions under various pH conditions. Microelectrophoresis was used to examine the local acid-base equilibria of the polyelectrolytes within the films as a function of the total number of layers in the film and the assembly solution pH. The acid-base dissociation constants were observed to deviate significantly from dilute solution values upon adsorption, to be layer dependent only within the first 3-4 layers, and to show sensitivity to the assembly solution pH. As a result, some of the physicochemical properties of the films have also been found to exhibit pH-responsive behavior. For example, the thickest films result when at least one of the polyelectrolytes is only partially dissociated in solution. As well, the pH-dependent degree of dissociation of the surface functional groups can be used to vary the contact angle of a water droplet by as much as 25 degrees and the coefficient of friction by up to an order of magnitude. In addition, the extent to which PLL/HA films can be made to swell in solution can be varied by a factor of 7 depending on the assembly solution and swelling solution pH. The anomalies found in the dissociation constants account for the trends in these pH-dependent properties. Here, we demonstrate that knowledge of the acid-base dissociation behavior in multilayer films is key to understanding and controlling the physical properties of the films, particularly surface friction and wettability, which are fundamentally important factors for many biomaterials applications. For example, we outline a mechanism whereby biopolymer thin films can be electrostatically adsorbed under highly charged "sticky" conditions and then quickly transformed into stable low-friction films simply by altering the pH environment.

Simulating ground water-lake interactions: approaches and insights.

Approaches for modeling lake-ground water interactions have evolved significantly from early simulations that used fixed lake stages specified as constant head to sophisticated LAK packages for MODFLOW. Although model input can be complex, the LAK package capabilities and output are superior to methods that rely on a fixed lake stage and compare well to other simple methods where lake stage can be calculated. Regardless of the approach, guidelines presented here for model grid size, location of three-dimensional flow, and extent of vertical capture can facilitate the construction of appropriately detailed models that simulate important lake-ground water interactions without adding unnecessary complexity. In addition to MODFLOW approaches, lake simulation has been formulated in terms of analytic elements. The analytic element lake package had acceptable agreement with a published LAKI problem, even though there were differences in the total lake conductance and number of layers used in the two models. The grid size used in the original LAKI problem, however, violated a grid size guideline presented in this paper. Grid sensitivity analyses demonstrated that an appreciable discrepancy in the distribution of stream and lake flux was related to the large grid size used in the original LAKI problem. This artifact is expected regardless of MODFLOW LAK package used. When the grid size was reduced, a finite-difference formulation approached the analytic element results. These insights and guidelines can help ensure that the proper lake simulation tool is being selected and applied.

Synthesis and characterization of multilayered TiC/TiB 2 coatings deposited by ion beam assisted, electron beam–physical vapor deposition (EB–PVD)

Polycrystalline 4 μm and 14 μm thick multilayer TiB 2/TiC coatings with varying number of total layers (2–20) were deposited by ion beam assisted, electron beam–physical vapor deposition (IBA, EB–PVD) on WC–6wt.%Co–0.3wt.%TaC substrates. The average Vicker's hardness numbers were found to increase with increasing total number of layers and ranged from 3215 VHN 0.050 to 3726 VHN 0.050 and 3294 VHN 0.050 to 3991 VHN 0.050 for the 4 μm and 14 μm thick multilayer TiB 2/TiC coatings, respectively. The adhesion of the multilayer coatings was found to be greater than 50 N for a 4 μm thick film and less than 30 N for the thicker films. The degree of crystallographic texture was found to change with varying total number of layers, as well as the orientation of both materials within the multilayer system. The grain size and amount of residual compressive stress was found to decrease with increasing number of layers. In addition, the fracture toughness was found to decrease with increasing total number of layers. This paper discusses the effect of changing the individual layer thickness (i.e. total number of layers) for TiC/TiB 2 multilayer coatings deposited on WC–6wt.%Co–0.3wt.%TaC cutting inserts on the average Vicker's hardness number, adhesion, stress, fracture toughness, argon impurities, average crystallite size, crystallographic texture, surface morphology and microstructure using a variety of characterization techniques.

Transmission of Ultrasound from Layered Structures into a Liquid

The transmission of ultrasound through a layered structure into a liquid is controlled by the effective reflection coefficient of the layered structure. Under the conditions of resonant transmission, the effective reflection coefficient coincides with the reflection coefficient in the layer at the end of the layered structure that is in contact with the liquid. When the total number of layers increases, the resonant transmission disappears and the transmission rate of ultrasound oscillates rapidly with respect to the change of frequency. However, in the case of actual transmission measurements, this oscillation is difficult to observe. Furthermore, in the case where the substrate of the layered structure has lower acoustic impedance, the observed transmission rate has a large mismatch with the transmission rate given by the theoretical predictions.

Influence of structural and geometric parameters on the inter-ply shear stresses in a radial truck tyre

In this paper, a non-linear finite element model of a truck tyre is employed to conduct a parametric study on the shear stresses developed in different belt layers. The parametric study incorporates the geometric and anisotropic material properties of the individual layers in the multi-layered system and the orientations of the cords in different layers. The parameters considered concerning the geometry of the tyre include the aspect ratio, rim radius and tread depth. The parameters related to structural features and material properties of the individual layers in the belts, such as the cord angle, total number of belt layers under the crown and the number of twisted cords per unit width of an individual layer, are further considered. The influences of these parameters on the maximum shear stresses developed in individual belt layers are investigated for a non-rolling radial truck tyre with focus on the 100 psi (690.3 kPa) inflation pressure, and 20 and 30 mm normal deflections. The analysis is performed using the ANSYS® software and the results are used to derive a more desirable set of structural parameters so that the maximum shear stresses in the belt layers of a loaded radial truck tyre can be reduced. The maximum inter-ply shear stresses computed using the proposed set of parameters are compared with the stresses derived corresponding to the pre-estimated nominal parameters. Based on the analysis, it is concluded that a tyre design realised on the basis of the proposed set of parameters can yield considerably lower magnitudes of the maximum shear stresses in the multi-layered system of a radial truck tyre under a wide range of inflation pressures and normal loads.

Interference of opposing longitudinal acoustic waves in an isotropic absorbing plate and a periodic structure with defects

Interference of longitudinal acoustic waves propagating in opposite directions in a homogeneous isotropic absorbing plate and a periodic structure with a defect is considered theoretically. The periodic structure consists of alternating absorbing solid and transparent liquid layers. The defect is modeled by replacing a solid layer by a liquid layer of the same thickness. The dependences of the transmission spectrum of the energy flux on the amplitude ratio and phase difference of the interacting waves are studied. It is shown that, by varying the parameters of the opposite pressure wave, it is possible to change the transmission spectrum of the direct wave in a wide frequency range. An expression is obtained to determine the extremums of the wave amplitude transmitted through an absorbing plate depending on the amplitude ratio of the interacting waves. The results of studying a one-dimensional periodic structure demonstrate the possibility to considerably change the transmission spectrum of the pressure wave leaving the structure and also to eliminate the invariance of this spectrum under the interchange of the kth and (n−k+1)th layers (where n is the total number of layers in the structure).

Stable sensor layers self-assembled onto surfaces using azobenzene-containing polyelectrolytes.

Polyelectrolytes functionalized with photoisomerizable azobenzene chromophores were multi-layered onto inorganic and metal surfaces, by the repeated adsorption from dilute aqueous solution, alternating between oppositely charged polymers. These layer-by-layer ionically self-assembled thin films were investigated for their suitability as sensor host materials with respect to the criteria of control over physical layer properties, versatility to different substrates and adsorption geometries, and stability of the formed layers to heat, solvent, and sonication. Layer thickness was found to be controllable between 5 A and 500 nm by varying the total number of layers deposited, from a single monolayer to 1000 layers. Control over individual layer thickness was achieved by varying the pH of the adsorption solutions. This multi-layer self-assembly was demonstrated to be suitable for a wide range of metal and inorganic substrates, and achievable with surfaces of high curvature (r = 50 nm), and confined geometry. The deposited layers exhibited good stability to desorption in a range of organic solvents, aqueous temperatures to 100 degrees C, and cleaning protocols such as sonication. The laser-induced geometric isomerization of the azobenzene chromophores was shown to be strongly dependent on aqueous solution properties, demonstrating an application as a hydroxide ion sensor in highly alkaline media.

Measurements of Ballistic Impact Response of Novel Coextruded PC/PMMA Multilayered-Composites

Ballistic impact behavior of coextruded composites consisting of alternate layers of ductile polycarbonate (PC) and brittle poly(methyl methacrylate) (PMMA) were investigated. These PC/PMMA composites were 256, 1024, 2048 and 4096 layers; each consisted of various component compositions and their corresponding layer thickness was on the order of micron or sub-micron scale. Individual layer thickness of PMMA was determined to be critical for the impact response of these ductile/brittle hybrids. A brittle mode of failure was encountered as the thickness of PMMA layers reached 0.5 mm or higher, regardless of their composition or layer configuration. Damage zone size increased significantly as the PMMA layer thickness reduced to approximately 0.36 mm; a mixed mode of failure resulted in these PC/PMMA composites. With the PMMA layer thickness further reduced to approximately 0.14 mm, ductile deformation, which was predominant in the pure PC, occurred in these coextruded composites. Ballistic impact energy data showed that the extent of energy absorption or dissipation increased with an increase in either the PC content or the total number of layers. These results further revealed that the individual layer thickness of PMMA was the critical material parameter, and the determined impact energy values were consistently higher for the composites with PMMA layer thickness being 0.14 mm or thinner. Adhesion between the layers appeared to be very good since delamination did not occur whether these PC/PMMA composites failed in a brittle or a ductile mode.

Effects of ply-arrangement on compressive failure of layered structures

Using detailed finite element models, compressive failure mechanisms of composite structures consisting of many laminae are analyzed. It is assumed that the structures contain interlaminar delamination and their failure mode to be characterized by either buckling or delamination growth. Our primary goal is to identify the effects of delamination and ply-arrangement on the multi-layered structures. Up to 32 laminae are distinctly modeled in this investigation. Our study considers two most basic geometry; one is flat panels under compressive load and the other is cylindrical shells subjected to external pressure. In both cases, the energy release rate and mixed-mode stress intensity factors are computed to quantify the crack driving force. The results are used to determine dominant failure initiation mode, structural buckling or delamination growth. Regardless of the structural type and total number of layers, a significant reduction in the load carrying capacity may occur when interlaminar delamination exists. In the flat panels, interlaminar delamination can generate unstable post-buckling behavior and lower the steady-state post-buckling load. However, delamination growth does not likely to occur during the pre-buckling stage. For the cylindrical shells, delamination growth may initiate prior to structural buckling. The location of delamination also plays an important role in defining the critical crack initiation load. When the delamination is located within 25% to 40% of shell thickness measured from the outer surface, the crack initiation load can be as low as half of the buckling load. In both types of structures, as the total number of plies increases, the layer effect diminishes. The overall deformation and failure behaviors of panels with large number of layers approach those of the infinite-layer model with homogenized material properties.

Electron field emission from tetrahedral amorphous carbon films with multilayer structure

A multilayer structure with alternating metal and semiconductor layers is proposed to occur in tetrahedral amorphous carbon (ta-C) films prepared by using an intermittent layer-by-layer deposition method. In this model, the multilayers can be represented as A/B/A/B/…/A/B/A stacks, in which A is considered to be a semimetallic sp2-rich graphite-like layer with B being a semiconducting sp3-rich diamond-like layer. According to the proposed structural model, the electron field emission properties of the ta-C multilayers that could be modulated by adjusting the total number of layers, layer thickness and sp3 content of each layer have been predicted. Correspondingly, three kinds of ta-C multilayers were designed and deposited to confirm this model by enabling us to measure the electron field emission properties. Agreement between the prediction and the experimental results has been observed. It was found that field emission from ta-C multilayers can be optimized by changing the number of layers, layer thickness and sp3 content of each layer. In our experiments, a threshold electric field (Eth) as low as ∼5 V/μm has been obtained for field emission from ta-C multilayers with a total of 20 layers and with a 10 nm layer thickness.

Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.

Breathable polymer films produced by the microlayer coextrusion process

Fabrication of a breathable film by the microlayer coextrusion process is described. Poly(ethylene oxide) (PEO) was microlayered with a filled polyolefin, either CaCO3-filled polyethylene or CaCO3-filled polypropylene. The thickness of individual layers was varied by increasing the total number of layers in the microlayered film from 8 to 4096. The water vapor transport rate (WVTR) was measured for microlayer films that varied in composition and number of layers. Especially with the PP(CaCO3)/PEO system, systematic variation in composition and number of layers made it possible to obtain large changes in the WVTR. The results were related to the tortuousity of the pathway through the microlayer. The filled polyolefins acted as a barrier to water vapor transport through the hydrophilic PEO. As the individual layers were made thinner by increasing the total number of layers, the polyolefin layers changed from continuous to discontinuous. Tortuousity concepts were used to correlate the increase in WVTR with an effective aspect ratio of the discontinuous polyolefin layers. In addition to high WVTR values, the breathable films produced by microlayering PEO with a filled polyolefin exhibited excellent mechanical properties. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 816–828, 2000

Spin reorientation of the micromagnetic structure in Co/Au multilayer films with moderate perpendicular anisotropy

The evolution of the micromagnetic structure in Co/Au multilayer films with moderate perpendicular anisotropy with increasing the total stacking number of ferromagnetic layers, N, was investigated by using a Mössbauer-probe method ( 57Fe) and magnetic force microscopy. As a result, (1) smooth spin reorientation from in-plane to out-of-plane was found with increasing N, and (2) for larger N where the out-of-plane spin component exists, the micromagnetic structure was found to be stripe-type structure. These phenomena are understood in terms of dipole interactions among the ferromagnetic layers in the multilayers. A possible change of the kind of the micromagnetic structure as a parameter of N is also described.

Jute Sack Cloth Reinforced Polypropylene Composites: Mechanical and Sorption Studies

Composite materials were fabricated using jute and polypropylene sacks by compression molding. Different types of composites were fabricated by varying the number of jute layers, number of polypropylene layers and the mode of combination of jute and polypropylene layers keeping the total number of layers a constant. It was found that as the number of polypropylene layers increases, the tensile strength, tear strength and modulus increased, but the elongation at break and equilibrium water uptake were found to be decreased. Also, when the number of jute layers increases, by keeping the number of polypropylene layers constant, generally failure properties decreased but an increased water uptake was observed. The influence of various constructions on the properties was analysed by changing the mode of combination of jute and polypropylene layers. In these experiments, the total number of layers were fixed a con stant. It was found that the preferred combination is one with jute as core layer and polypropylene as skin layers. Scanning electron microscopy was used to analyse the failure surface morphology of the composites. Finally value added product was fabricated using selected composites.