Embedded Layers Research Articles

Damping Effect of Multi-Layer Beams with Embedded Electro-Rheological Fluid

The damping effect of multi-layered composite beams with embedded ER fluid layers is investigated. A three-layer beam was composed of the aluminum layers bonded together by silicone rubber with an internal cavity filled with ER fluid. The damping factor of the beam can be controlled by the voltage between the elastic layers. To increase the damping effect, an inner layer was inserted into the cavity so that the composite beam consists of three elastic layers and two ER fluid layers. The frequency characteristics of the three-layer beam and the five-layer beam were investigated experimentally and numerically. The results show that the damping factor of the composite beams can be considerably raised by increasing the electric field. The natural frequency of the beams increases with the field due to the variation of stiffness of the ER fluid layers. The influence of size of inner layer on the damping effect of the five-layer beam was also investigated. It is shown that the damping factor of a five-layer beam could be larger than that of a three-layer beam if the size of the inner beam is appropriately designed.

Accurate viscoelastic modeling by frequency‐domain finite differences using rotated operators

The viscoelastic wave equation is an integro‐differential equation that requires special methods when using time‐domain numerical finite‐difference methods. In the frequency domain, the integral terms are easily represented by complex valued elastic media properties. There are further significant advantages to using the frequency domain if the forward or the inverse problem requires modeling or inverting a large number of prestack source gathers. Numerical modeling is expensive for seismic data because of the large number of wavelenghths typically separating sources from receivers, which results in a need for a large number of grid points. A major obstacle to using frequency‐domain methods is the consequent storage requirements. To reduce these, we maximize the accuracy and simultaneously minimize the spatial extent of the numerical operators. We achieve this by extending earlier published methods introduced for the viscoacoustic case to the viscoelastic case. This requires the formulation of two new numerical operators: a differencing operator in a rotated coordinate frame and a lumped mass term. The new operators are combined with ordinary second‐ order, finite‐difference operators in an optimal manner to minimize numerical errors without increasing the size of the numerical operator. For a fixed number of grid points, the resulting second‐ order differencing scheme is no more expensive than an ordinary second‐order differencing scheme, but a numerical dispersion analysis shows that the number of grid points required per smallest wavelength is reduced from approximately 15 to approximately 4. The new scheme is also capable of handling embedded fluid layers without instability. We demonstrate that no further improvement in performance can be achieved using higher order spatial operators because of the associated computational overheads associated with the larger differencing operators. The new viscoelastic modeling scheme is used to study a crosshole data set in which the exact nature of the seismic coda is unclear. The results of the modeling study indicate this coda is likely related to the generation of mode‐converted shear waves within the complicated, finely layered sediments at the site.

Carbon self-organization in the ternary Si1−x−yGexCy alloy

This article demonstrates for the first time the possible self-ordering of carbon in Si1−x−yGexCy thin films pseudomorphically grown on silicon. Germanium and carbon atomic distributions have been studied for a C-rich Si0.9−yGe0.1Cy heterostructure using high-resolution transmission electron microscopy (HRTEM), high-resolution x-ray diffraction, Raman spectrometry, and secondary ion mass spectrometry (SIMS). HRTEM images show the spontaneous formation of carbon-rich tilted sublattices and local germanium fluctuations, despite constant growth parameters. X-ray diffraction confirms this thin sublayers formation. A complementary insight into local ordering effects around C is obtained by Raman spectroscopy. A new model for perpendicular lattice parameter reduction is proposed. It involves C atoms mostly in third-nearest-neighbor positions and the local formation of a distorted CSi3 graphitic arrangement. In these C-rich sublayers, the perpendicular lattice mismatch to silicon is as low as −0.014. This aperiodic structure remains highly distorted and a statistical description of these strain fluctuations is detailed. The atomistic configuration of these δ layers indicates the likely contribution of surface steps during the growth, while SIMS measurements hint at the probable involvement of carbon interstitials to explain this ordering. For technological applications, this self-organization of carbon is promising for the ultrashallow junction challenge. These carbon-rich embedded layers can be considered as quantum wells, etch stops or very thin barriers against transient enhanced diffusion.

Damping of Flexural Waves With Imbedded Viscoelastic Materials

This paper considers a passive damping method that can be applied to beam-like structures such as machine tool bases and columns. The method uses viscoelastic materials to dissipate energy in the manner of classic constrained-layer damping; however, the layers are embedded within the structure as opposed to being applied externally. This provides a robust means of incorporating damping without encountering several of the common disadvantages associated with external damping treatments. An analytical solution to the amount of damping that can be achieved using embedded layers is available, but is known to be inaccurate when the viscoelastic stiffness approaches that of the structural components. Therefore, a new prediction of the maximum damping level that can be expected in a structure is developed and presented here. This prediction gives good results in a wide variety of applications, and offers insight into the relationship between key design parameters. Finite element and experimental verification of the maximum damping predictor are also presented.

High-resolution depth profiling of InxGa1−xAs/GaAs multiple quantum well structures by combination of secondary ion mass spectrometry and x-ray diffraction techniques

In this work we investigate multiple quantum well semiconductor structures by secondary ion mass spectrometry (SIMS) and high-resolution x-ray diffraction measurements. The combined use of these techniques turns out to be a very powerful tool for an accurate investigation of both structural and chemical characteristics of complex structures consisting of thin alternating layers. We show that the x-ray data allow us to increase the accuracy of the SIMS analyses providing internal standards, for both depth scale and concentration calibration. This procedure allows us: (i) to determine accurate quantitative SIMS concentration profiles which lead to the determination of the elemental concentration in the quantum wells, (ii) the mole fraction and layer thickness of embedded layers, and (iii) also to investigate eventual segregation and diffusion phenomena occurring at the interfaces. Our procedure is demonstrated on a set of InxGa1−xAs/GaAs multiple quantum well structures grown by metalorganic chemical vapor phase epitaxy. The experiments, performed by secondary ion mass spectrometry and high-resolution x-ray diffraction measurements, as well as the methodology of the data analyses are discussed in detail. As the main result of our investigation we obtained the precise indium distribution in each of the investigated samples with an accuracy below 5% and the individual barrier and well layer thickness.

Strain-stabilized highly concentrated pseudomorphic Si1-xCx layers in Si.

We present evidence that ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$ layers with x\ensuremath{\approxeq}0.20 can be grown pseudomorphically on a Si (001) substrate despite the large difference of the C and Si lattice constants. Calculations based on density-functional theory and a Keating model predict that embedded layers of certain structures with stoichiometry ${\mathrm{Si}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$C where n=5,6,. . . are considerably more stable than isolated C impurities. The common feature of these structures is that the carbon atoms tend to arrange as third-nearest neighbors. Multilayer structures grown by molecular beam epitaxy strongly suggest that defect-free heterostructures with such high C concentrations can be fabricated.

Structural calculations of oxide surfaces at finite temperature

Abstract An Einstein-like approximation for phonons is presented and used to estimate the free energy of ionic crystals at finite temperatures. The long-range Coulomb forces in these materials correlate the motion of the ions so that the local modes correspond to the vibrations of embedded layers. The thermal expansion of bulk oxides can be efficiently calculated using these modes and is found to be in good agreement with lengthier calculations that use the full phonon spectrum. The model can also be applied to defect structures with two-dimensional symmetry which are normally inaccessible to the harmonic approximation due to computational limitations. Results of calculations on a (100) rocksalt surface are presented. The local modes reflect the symmetry of the surface and are sensitive to variations of the ionic locations in the surface region. This model provides a reasonably accurate and efficient method of studying surface relaxation with temperature.

Additional comments on buckling analyses of embedded layers

Recently the author showed that for an axially compressed embedded layer the bifurcation force, P cr , is not necessarily the buckling force. This analysis was based on a simple model able to undergo large deformations, and the commonly used assumption that the vertical bedding resistance is linear. However, in an actual situation, the bedding resistance is non-linear; especially for large vertical displacements. Also, the base of the layer undergoes a permanent sag, due to the weight of the layer and the overburden. The purpose of the present note is to analyze these effects. It is shown that they profoundly influence the buckling force of an embedded layer. Of interest is also the finding that for a perfectly straight layer model, P cr may not exist. The effects of geometrical imperfections are shown.

Fold propagation in single embedded layers

This paper investigates the folding of single competent layers embedded in a less competent matrix, where the competence contrast is about 10: 1. Folds result from buckling during layer-parallel compression. A geometrical study of natural examples shows that individual folds tend to be grouped into fold complexes. The amplitude varies from a maximum at the centre of a complex to a minimum at each end. Each complex is often centred about a sedimentary lens or nodule which may have triggered the folding and localized the complex. The formation of folds of this kind has been simulated experimentally by deformation of models made from paraffin waxes of known rheological properties. Early in the deformation of a model, buckling starts at a localized site of disturbance, producing only one fold. With further deformation, new folds appear at either side of the initial one. The buckling then propagates along the layering, further folds appearing serially in time and distance. The end result is a complex with many individual folds and a regularly periodic shape. With a competence contrast of 10: 1, the rate of fold propagation is slow, and formation of a periodic complex requires an overall shortening of at least 15%. The shapes of folds formed experimentally are similar to those formed naturally.

Dosisverteilung in inhomogenen Medien bei Bestrahlungen mit schnellen Elektronen

Es werden systematische experimentelle Untersuchungen darüber durchgeführt, wie sich die Tiefendosiskurve energiereicher Elektronen (25 MeV) in Gewebe (Wasser) durch eingelagerte Schichten aus Knochen (angenähert durch Graphit) wie auch Lungengewebe (angenähert durch Weichholz) verändert. Für die Verschiebung des abfallenden Teils der Tiefendosiskurve zur Haut (durch Knochen) und in die Tiefe (durch Lungengewebe) wird ein außerordentlich einfaches Berechnungsverfahren angegeben, so daß der Dosisverlauf auch bei Berücksichtigung dieser Gewebearten für jeden in der Praxis vorkommenden Fall ermittelt werden kann.

The Globigerina-marls and Basal Reef-rocks of Barbados

Mention was made of the Globigerina -marls of Bissex Hill in a paper on the Geology of Barbados written by one of us in collaboration with Mr. Jukes-Browne, and the section along the road ascending the hill from the south-western side was there described. The ordinary succession of the Oceanic deposits occurs in the lower part of this section for 70 feet vertical, up to an elevation of about 865 feet. Here the hill rises somewhat abruptly from a small plateau of the siliceous earths to a knoll at an elevation of 966 feet, falling away to a spur on the east, the ridge of which, to the point where it terminates abruptly, has a level of about 900 feet. On the north the knoll slopes steeply down to a level of 820 feet. On the west, below the road, the land falls away rapidly to Dark Hole and the Vale, the lowest exposure of the Oceanic Beds being north of Park's Windmill, at the height of 670 feet. The whole of the crown of Bissex Hill, instead of showing the usual passage from siliceous earths through white chalks into red clays, was found to consist of a yellowish or buff-coloured marl, which proved to be crowded with Globigerinæ , and to include occasional large blocks and several embedded layers of hard Globigerina -limestone. In the first instance no change in the character of the deposit was noticed from where it first occurred to the summit of the hill, and it was supposed