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An analytical study of the small amplitude electron acoustic double layers in a magnetized plasma consisting of superthermal electrons and ions along with cold fluid electrons is discussed. The dispersion relation allows electron acoustic waves with the frequency within electron and ion gyro-frequency in the modelled plasma. In the process of study of the nonlinear structures, the Sagdeev pseudo-potential method for small amplitude regions is employed. The existence domains for the double layers are investigated in terms of the Mach numbers of the structures and the temperature ratios of the species for different ratios of their concentration. The effects of the compositional parameters on the nature and size of the double layers are also explored and it is observed that the plasma can support both compressive and rarefactive double layers depending on the values of those parameters and the Mach numbers.
For scalar conservation laws in one space dimension with a flux function discontinuous in
space, there exist infinitely many classes of solutions which are L1 contractive.
Each class is characterized by a connection (A,B) which determines the interface entropy. For
solutions corresponding to a connection (A,B), there exists convergent numerical schemes
based on Godunov or Engquist−Osher schemes. The natural question is how to obtain schemes,
corresponding to computationally less expensive monotone schemes like Lax−Friedrichs etc., used
widely in applications. In this paper we completely answer this question for more general
stable monotone schemes using a novel construction of interface flux function. Then from
the singular mapping technique of Temple and chain estimate of Adimurthi and Gowda, we
prove the convergence of the schemes.
Four series of samples, prepared at 250° C by decomposition of a mixture of silane and argon in a radio frequency powered deposition systems (rf-PECVD), have been studied. The dilution rates were 1 %, 1.5 %, 5 % and 10 % of silane in argon and the total pressure was 0.5 Torr for the first series and 0.2 Torr for the others. Structural and transport properties of the materials have been studied as function of power density. Structural studies show the transition from purely amorphous material towards microcrystalline material with increasing rf power density. The transport parameters were measured in the as-deposited, light-soaked and annealed states and compared to those obtained on state of the art material. The best material obtained is clearly device grade material. This study shows that argon dilution allows to tailor the material for a given application.
Thin a-Si:H films, with a thickness of 1 µm, with different hydrogen concentrations, prepared by hot wire deposition were crystallized by 514.5 nm cw Ar ion laser radiation, with a power density between 150 and 270 kW/cm2. The crystallization was continuously monitored by Raman spectroscopy for exposures up to hours. The analysis of crystallization process using Johnson-Mehl phenomenological equations showed an apparent crystallization energy of around 0.5 eV and low dimensional crystal growth. The mean value of the crystal size decreases with increasing irradiation energy and initial hydrogen content and varies between 3 and 6 nm.
Recent developments in our laboratory related to polymer-based light sensors are reviewed. The inherent processibility of the active polymer medium is utilized in the implementation of different designs for the opto-electronic applications. The utility of these devices as sensitive photodetectors, image sensors and position sensitive detectors is demonstrated. The schottky-type layer formation at interfaces of polymers such as polyalkylthiophenes and aluminum accompanied by the enhanced photo-induced charge separation due to high local electric field is tapped for some of these device structures. The sensitivity of polymer-based field effect transistors to light also provides a convenient lateral geometry for efficient optical-coupling and control of the transistor state. The range of these polymer-detectors available with the option of operating in the diode and transistor modes should be an attractive feature for many potential applications.
A novel approach for a single step lapping and chemical-mechanical polishing of antimonide-based III-V compounds using agglomerate-free alumina slurries is presented. Relatively high removal rates, minimal scratching, and low surface roughness have been obtained. The effects of slurry preparation cycle on the slurry properties and chemomechanical polishing results are discussed.
Raman spectroscopy of the solution, solid and gel phases present during crystallization of zeolite X was investigated. The vibrational data indicate that an amorphous aluminosilicate solid, composed primarily of four membered aluminosilicate rings is in contact with monomeric silicate ions during the prenucleation stages of the zeolite formation. No intermediate building blocks specific to zeolite X could be discerned from the vibrational spectra. The influence of a series of monovalent cations on the crystallization process was also examined, and a model of zeolite formation has been proposed.
The fabrication and performance characteristics of an integrated distributed feedback (DFB) laser and optical amplifier structure are described. The structure utilizes semi-insulating Fe doped InP layers for current confinement to the active region, electrical isolation between the two sections and for lateral index guiding. The amplified output has a slope of 1 mW/mA of laser current with the amplifier biased at 150 mA which is a factor of 5 larger than that for a typical laser. The laser emits near 1.55 μm and the spectral width under modulation of the amplified output is considerably smaller than that for a DFB laser for the same on/off ratio.
Microelectronic solder joints are exposed to aggressive thermo-mechanical cycling (TMC) during service, resulting in strain localization near solder / bond-pad interfaces, which eventually leads to low-cycle fatigue (LCF) failure of the joint. In order to mitigate these strain concentrations and thereby improve LCF life, a ‘smart solder’ reinforced with a martensitic NiTi based shape memory alloy (SMA) is being developed. This paper presents an overview of processing, characterization and modeling of these composite solders, and articulates the role of NiTi particles on strain evolution in composite solders. Based on finite element modeling and experiments on model single fiber composites, it is shown that NiTi pariculate reinforcements can reduce inelastic strain levels in the solder via shape recovery associated with the B19′→B2 transformation. In situ TMC studies in the SEM, in conjunction with strain analysis via digital image correlation, show evidence of reverse deformation in the solder commensurate with the NiTi phase transformation, demonstrating the conceptual viability of the smart solder approach. Details of processing and joint formation, and the resultant microstructures of smart solder are discussed. Finally, results of TMC experiments on monolithic solder and NiTi/solder composite joints are reported, highlighting the beneficial effect of shape-memory transformation in reducing inelastic strain range, and hence enhancing the LCF life, of solders.
Chromophoric multilayer thin films exhibiting efficient second harmonic generation have been constructed on inorganic substrates via a stepwise layer-by-layer process using molecular self-assembly techniques. In each step, chemical species bearing appropriate functional groups form covalent bonds with functional groups deposited in the previous step. Bulk acentricity is achieved by the orientation of chromophore-containing layers outward from the substrate surface. A new chromophore having comparable hyperpolarizability but different steric and transparency characteristics than the stilbazolium chromophore used previously has been incorporated into self-assembled films. The large effects of octachlorotrisiloxane capping on the structure of these films have been investigated by second harmonic generation and X-ray reflectivity measurements. Novel in situ measurements of second harmonic generation efficiency as a function of chromophore layer growth are described and provide information useful for optimizing deposition conditions and understanding the film growth process.
Photoreflectance (PR) has been performed on a series of undoped and n-type, InGaAs and InAlAs molecular beam epitaxy (MBE) grown layers with different In mole fractions, and epilayer thicknesses on Fe-doped semi-insulating (SI)-InP substrates. From investigations of the temperature dependence, time constant dependence and an additional cw light beam intensity dependence, three substrate peaks are identified as an excitonic transition from the substrate, a free electron transition near the interface which gives a Franz-Keldysh oscillation (KFO), and a transition from the spin-orbit split-off valence band. The results are indicative of a redistribution of charge near the substrate interface in the process of MBE growth; the associated PR signal (phase) could be used for in-situ monitoring of epilayer growth on SI-InP wafers.
In order to produce a three-dimensional interconected graphitic network, foams were produced from carbon fiber precursor pitch and processed similarly to high modulus carbon fibers. Uniform size distributions of open spherical cell graphitic carbon foams were produced by microcellular foam blowing of anisotropic pitch using homogeneous and heterogeneous nucleation. The widths of cell walls and ligaments were in the range of the diameters of pitch-based carbon fibers (7-10 μm) and possessed significant alignment of anisotropic pitch crystallites.
Quantitative tests for interfacial adhesion between films and substrates are of criticalimportance in micro-electronic applications. However, many of the available tests are applicable to a limited array of materials systems, or are experimentally complex. In thispaper, a constant depth scratch test, which has been designed to circumvent the limitations associated with currently available tests, is presented along with theoretical developments for the quantification of interfacial shear strength. Because of a fixed scratchgeometry, the test is amenable to straightforward analytical formulations unlike other versions of scratch tests. It is unique in its experimental simplicity, and allows evaluation of interface strength as function of position on the sample. Sample outputs from the test based on Cr films on glass substrates are presented.
Absorption, photoluminescence, photoreflectance and transmission electron microscopy measurements have been performed on 2 and 3-monolayer thin InAs quantum wells(QW) (with 250Å wide GaAs barriers) grown on (511) GaAs substrate. For comparison, similar samples grown on (100) substrates have also been studied. (511)-grown thin quantum wells may have possible quantum wire configuration. However, polarization studies show a small anisotropic absorption from the (511) sample, which indicates that the optical property of the (511)-quantum well is different than either that of a quantum wire or that of a (l00)-grown quantum well structures. A theoretical calculation, making a unitary transformation of the valence band k•P Hamiltonian matrix to the (511) base, and using a perturbation method to determine the new wavefunctions, yield an anisotropic absorption comparable with the experimental result. We have also compared the transition energies from PL data with the calculated one using the conventional effective mass approximation.
The nucleation morphologies of LPE grown GaSb, AlGaSb and AlGaAsSb layers on GaSb substrates are presented. The morphology of the GaSb layers grown from Sb-rich melts showed facets on highly terraced surface, whereas those grown from Ga-rich melts exhibited fine terraces without facets. An optimum temperature in the range of 500 – 550°C was found to be suitable for the growth of mirror smooth layers from Ga-melts. The surface morphology of the AlxGa1-xSb layers degrades drastically with increase in Al content beyond x = 0.5. The surface morphology of AlGaAsSb epilayers has been found to depend strongly on the pre-growth melt dissolution sequence.
The effect of hydrogen plasma treatment on the optical and electrical properties of Gallium Antimonide bulk single crystals is presented. Plasma exposure gives rise to a layer of defects on the surface. These defects introduce multiple trap levels in the band gap from which a slow emission of carriers is observed during the capacitance - voltage measurements. On removal of the defect layer by controlled etching, the effects of hydrogen passivation are seen. The results of optical measurements indicate that passivation of shallow acceptors is more efficient than that of the donors and in general the passivation efficiency depends on the doping level. Passivation of deep levels and extended defects like grain boundaries and dislocations has also been observed. The thermal stability of the passivated deep level and extended defects is higher than that of the shallow level.
Lignins, a truly abundant group of biopolymers exhibiting some significant diversity, are usually thought to be constituted by a random proportionate distribution of ten different linkages between p-hydroxphenylpropane units. Over 20 million tons of kraft lignin derivatives are produced annually in the United States by the pulping industry, but 99.9% of these aromatic polymeric materials are consumed as fuel. Such industrial byproducts are generally viewed as being almost hopelessly complicated mixtures of partially degraded and condensed chemical species. However, a very different picture has begun to emerge from a more coherent understanding of the physicochemical behavior exhibited by kraft lignin preparations. Noncovalent interactions between the individual molecular components under a variety of solution conditions orchestrate pronounced associative processes that are characterized by a remarkable degree of specificity. Their consequences may be readily observed both size-exclusion chromatographically and electron microscopically, and are reflected in an anomalous variation of glass transition temperature, Tg, with molecular weight of paucidisperse kraft lignin fractions. How these effects may influence the mechanical properties of lignin-based polymeric materials is presently being scrutinized at the University of Minnesota.
We have deposited unhydrogenated diamond-like carbon (DLC) films with 100 femtosecond laser pulses, at intensities in the 3x1014 - 6.5x1015 W/cm2 range. Film surface topography, optical property, and bonding structure were examined, respectively, with atomic force microscopy (AFM), spectroscopie ellipsometry (SE) and Raman spectrometry. The femtosecond pulse generated plasma was studied through time-of-flight (TOF) experiment. The most probable kinetic energy of carbon ions was estimated to be in the 300 – 2000 eV range, increasing with laser intensity. In addition, a unique ‘suprathermal’ component with kinetic energy ranging from 4 to 40 keV was observed in the TOF spectrum. This high energy peak is believed to originate from fast ions in a solid density plasma created during the absorption of each femtosecond laser pulse.