bluuebunny/crossref_metadata_2025 huggingface dataset:
Ligand-Receptor Contact Interactions Using Self-Assembled Bilayer Lipid Membranes
Authors: Ottova A. Leitmannova , Tien H. Ti
Published: April 2011

Biological membranes play a crucial role in signal transduction and information processing as well as in energy conversion. This is owing to the fact that most physiological activities involve some kind of lipid bilayer-based receptor-ligand contact interactions. There are many outstanding examples such as ion sensing, antigen-antibody binding, and ligand/voltage--gated channels, to name a few. One approach to study these interactions in vitro is facilitated by employing artificial bilayer lipid membranes (BLMs). We have focused the efforts on ion and/or molecular selectivity and specificity using newly available self-assembled BLMs on solid support (i.e., s-BLMs), whose enhanced stability greatly aids in research areas of membrane biochemistry, biophysics and cell biology as well as in biosensor designs and molecular devices development. In this paper, our current work along with the experiments carried out in close collaboration with others on s-BLMs will be presented.
A Theoretical Examination of Mems Microactuator Responses with an Emphasis on Materials and Fabrication
Authors: Glumac D.E. , Cooney T.G. , Francis L.F. , Robbins W.P.
Published: April 2011

A free standing cantilever beam consisting of a support structural material (polysilicon/silicon nitride), a piezoelectric PZT ceramic layer, and metal electrode layers has been analyzed. Beam theory and finite element analysis were used to model the electric field induced deflections of this structure, and provided information as to how material choices influenced actuator function. Both support material and PZT thicknesses varied from 0-1.0 gim, and bulk piezoelectric coefficients and elastic moduli were assumed. The beam theory uses known (or assumed) material properties to predict actuator responses. Conversely, if device responses can be measured, material properties may be inferred from the theory. For a PZT thickness of 0.3 μm, a core layer thickness of 0.13 μm was found to maximize displacement. Also, the force output was found to be more dependent on the core thickness than that of the PZT. This information can then be used to predict the response of a more complex microactuator.
Fabrication and Characterization of Barium Strontium Titanate and Non-Ferroelectric Oxide Composites
Authors: Ngo E. , Stowell S. , Sengupta L.C. , O'day M.E. , Lancto R
Published: April 2011

Various composites of BSTO combined with other nonelectrically active oxide ceramics have been formulated. In general, the composites have adjustable electronic properties which can be tailored for use in various electronic devices. Two applications, which are of interest in regard to the composites being discussed here, are phased array antennas and capacitor-varistor monolithic devices. For the phased array antennas, these properties include reduced dielectric constants, ε′, where ε = ε′ - iε″, and loss tangents, tan δ. The reduction of the dielectric constant and lowering of the loss tangents reditce the overall impedance mismatch and the insertion loss of the device. In addition, the overall tunability, change in the dielectric constant with applied voltage, is maintained at a sufficiently high level for all dielectric constants of interest. Another application (for one of the composite materials) is a combined capacitorvaristor device. In general, this would be used to protect against spurious voltage surges and voltage transients in electronic micro-circuits. The microstructures, including grain size and phase analysis, have been examined using SEM and X-ray diffraction.
Some Durability Issues Associated with the Use of Shapememory Alloy 'Smart' Composites
Authors: Friend Clifford M. , Morgan Neil B. , Wise Vivienne D.
Published: April 2011

This paper presents the results from an experimental study on the durability of embedded Shape-Memory Alloy (SMA) hybrid composites. It shows that the cyclic actuation of such composites can be degraded due to the accumulation of internal damage within the host composite structure. This damage results from thermal effects within the host composite, and from mismatch in the strain capacities of the actuators and structural composite. The implications of this data for the design of embedded SMA actuator 'smart' composites is discussed.
Oxidation Resistance of TiAl Intermetallics Improved by Vacuum Aluminizing
Authors: Hsu I-Chung , Wu Shyi-Kaan , Yang Lin Ray
Published: April 2011

TiAl binary intermetallics were aluminized in a vacuum system in order to improve their oxidation resistance. From the observations of optical microscopy and EPMA line scan, several different reaction layers were observed to be formed on respective substrates: pure Ti, Ti-32at.%Al, Ti-50at.%Al. From the results of the X-ray diffraction, the primary reaction layers were identified as TiAl3 structure in these three substrates This reacted layer grows with preferred orientation having [002] axis of TiAl3 perpendicular to the substrate surface from XRD experiments. High temperature oxidation tests revealed that vacuum aluminized layer was responsible for remarkable improvement on the oxidation resistance.
Effect of Forging Temperature on Microstructure and Tensile Properties in Fe3Al-Based Alloy
Authors: Yang Y. , Yan W. , Liu J. N. , Hanada S.
Published: April 2011

Forging processes at two different temperatures are performed to examine the relation between the microstructure and room temperature tensile properties in a Ce doped Fe3Al-based alloy. Results show that the microstructure and the ductility are sensitive to the forging temperature before annealing treatment. Higher yield strength and ductility can be obtained through forging at a relatively low temperature of 750°C followed by annealing at 800°C and 500°C. It is suggested that the formation of non-equilibrium grain boundaries and banded subgrains within carbide-free areas along grain boundaries enhances the local plastic deformation and results in the improvement of ductility. During the initial deformation at room temperature <111> slip is predominant for both microstructures.
Solid-Solution Strengthening Effect of Vanadium Addition to Iron-Modified L12 Titanium Trialuminides
Authors: Takahashi Tohru , Hasegawa Tadashi
Published: April 2011

Two types of aluminum–titanium–iron–vanadium ( Al–Ti–Fe–V ) quarternary intermetallic compounds have been prepared by arc melting under argon atmosphere. Their compositions were nominally Al66Ti25Fe6V3 and Al66Ti25Fe3V6. These alloys are based on the iron–modified titanium trialuminide with L12 cubic structure. Vanadium addition up to about 6 mol% did not destroy the cubic symmetry, and L12 solid solution compounds were produced in these two Al–Ti–Fe–V quarternary alloys. Microstructure and mechanical properties have been investigated. It has been demonstrated that vanadium addition to iron–modified L12 titanium trialuminides can enhance their strength.
High Temperature Deformation of a Nb-Ti-Al Alloy With σ+γ Microstructure
Authors: Ebrahimi Fereshteh , DeAragao Bernardo J. G.
Published: April 2011

An alloy with composition 27Nb-33Ti-40Al (at. %) was heat treated to produce a fine two-phase microstructure of σ (Nb2Al) and γ (TiAl) phases. This microstructure exhibited a peak in the flow curves obtained in the strain rate range 5 × 10−4 to 5 × 10−6s−1 at 1000°C, with a stress exponent of 3. The deformation occurred by dislocation activity and twinning accompanied by dynamic recrystallization of the γ-phase and extensive grain boundary sliding of the σ-phase, which resulted in the separation and fragmentation of the latter phase.
The Effect of Various Oxide Dispersions on the Oxidation Resistance of Fe3Al
Authors: Pint B. A. , Alexander K. B. , Tortorelli P. F.
Published: April 2011

Oxide-dispersed Fe-28at.%Al-2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. Yttria and ceria were added to the base alloy to assess the effect of these dopants on the oxidation behavior. The amount of dopant was varied from 0.05–0.5 at.% Y in a series of Y2O2-dispersed alloys. Isothermal and cyclic oxidation testing was conducted at temperatures from 800˚ to 1300˚C. A Ce02 addition was detrimental to the oxidation behavior. The Y2O3 improved the (α-Al2O3 scale adhesion relative to an undoped alloy, but was not as effective as similar additions to an oxide-dispersed FeCrAl alloy.
Fatigue Crack Growth In Niobium Aluminide Intermetallics
Authors: Dipasquale Jeff , Gahutu David , Konitzer Doug , Soboyejo Wole
Published: April 2011

The results of a recent study of the fatigue crack growth in a new class of niobium aluminide (Nb3Al) intermetallics alloyed with various levels of Ti are presented. The alloys studied have the following nominal compositions (note that all compositions are quoted in atomic &): Nb-15Al-10Ti(10Ti alloy), Nb-15Al-25Ti(25Ti alloy), Nb-15Al-40Ti(40Ti alloy) and Nb-12.5A1–41Ti-1.5Mo(41Ti alloy). Fatigue crack growth behavior was studied at room-temperature in all the alloys. Fracture mechanisms were determined by inspection of SEM images of fracture surfaces. Faceted cleavage fracture modes were apparent on the lOTi and 25Ti alloys. The 40Ti and 41Ti alloys exhibited clear evidence of ductile dimpled fracture. The ductile 40Ti and 41Ti alloys are shown to have comparable or better fatigue crack growth resistance than mill annealed Ti-6A1–4V. The effects of crack orientation (relative to elongated grains) on the fatigue crack growth behavior of the 41Ti alloy are also discussed.
An Atomistic Study of Dislocations Controlling the Deformation Behavior of TiAl
Authors: Girschick A. , Vitek V
Published: April 2011

Atomistic studies of dislocation cores in a model TiAl compound (L10) carried out using Finnis-Sinclair type many body potentials are presented. Two different core configurations, one planar and the other non-planar, were found for both 1/2〈110] ordinary dislocations and 〈101] superdislocations while a core containing a microtwin was found in the case of the l/2[112] dislocation. The impact of these core structures upon the plastic behavior or PST and single-phase TiAl crystals is discussed.
The Creep Deformation and Elevated Temperature Microstructural Stability of a Two-Phase TiAl/Ti3Al Lamellar Alloy.
Authors: Bartholomeusz M. F. , Wert J. A.
Published: April 2011

Enhanced work hardening of the phases in the lamellar microstructure has been cited as an explanation for the lower minimum creep rates of a two-phase TiAl/Ti3Al lamellar alloy compared with the minimum creep rates of the individual TiAl and Ti3Al single-phase alloys tested between 980 K and 1130 K. This proposition is confirmed by TEM observations. Thermal and thermomechanical exposure result in the microstructural evolution, which increases the minimum creep rate (εmin) of the lamellar alloy. The effect of microstructural evolution on εmin will be discussed in the present paper.
In Situ Deformation in the T.E.M. on the two Phase Alloy Ti-24A1-9Nb First Results at Room Temperature
Authors: Legros Marc , Caillard Daniel , Couret Alain
Published: April 2011

In situ deformation experiments have been performed on a two-phase Ti-23.7Al-9.4Nb (%at) alloy at room temperature. Dislocation glide processes in each phase are studied and discussed. An exemple of dislocation transmission through an interface is analysed in detail.
Atomistic Simulation of Grain Boundaries of the Twin Limited Structure in Ni3Al
Authors: Turi Maria-Lynn , Zugic R. , Szpunar B. , Erb U. , Palumbo G. , Krstic V.
Published: April 2011

Embedded atom method molecular dynamics simulations of low Σ grain boundaries in Ni3Al are presented. The results show that the grain boundary plane has a larger effect on grain boundary energy than the Σ value, rigid body translations and stoichiometry. Assessment of the energies of Σ3n (n ≥ 1) grain boundaries in Ni3Al for various grain boundary planes indicates that only the Σ3 grain boundary is energetically preferred. The implications of this result for the development of the twin limited structure based on energetic considerations are discussed.
Diffusion Bonding Of Ti-48A1-2Mn-2Nb (at. %)
Authors: Godfrey S. P. , Threadgill P. L. , Strangwood M.
Published: April 2011

The diffusion bonding behaviour of Ti-48at.%Al-2at.%Mn-2at.%Nb has been studied as a function of temperature (in the range 1200–1350°C), time (15–45 minutes) and starting microstructure (lamellar, duplex and near γ) at constant bonding pressure of 10 MPa. It was found, that under the above conditions, small twin related γ grains, approximately 10–20 μm in size, nucleated at the original interface and grew into the matrix forming a double necklace grain structure. Particles of α2 were observed around the interface, the formation of α2 particles was believed to be related to oxygen partitioning and stabilisation effects from dissolved oxide films during the bonding process. Evidence for this mechanism was obtained from parallel electron energy loss spectroscopy (PEELS), which identified oxygen partitioning in the α2 particles. For the fully lamellar structure bonded at 1250°C for 45 minutes the failure strength of the bond was found to be 250 MPa, approximately 50 MPa lower than the failure strength of the base material.
Soft and Hard Modes of Deformation in Mosi2 Single Crystals
Authors: Ito K. , Yano T. , Inui H. , Yamaguchi M.
Published: April 2011

The deformation behavior of MoSi2 single crystals has been studied in the temperature range of -196<1500°C. While [001]-oriented crystals can be plastically deformed only above 1300°C, plastic flow is possible from temperatures as low as room temperature for single crystals with orientations other than [001]. Five slip systems, {110)<111], {011)<100], {010)<100], {023)<100] and {013)<331], are identified to be operative, depending on crystal orientation. An anomalous increase in CRSS is observed in the intermediate temperature range for the former three slip systems. Schmid’s law is generally valid for the soft modes, {110)<111], {011)<100] and {023)<100]. In contrast, the CRSS for the hard mode, {013)<331], strongly depends on crystal orientation with the higher values for orientations closer to [001].
Pip Processing, Microstructure and Properties of Si34N Fiber and Al2O3 Fiber Reinforced Silicon Nitride
Authors: Schwab Stuart T. , Page Richard A. , Davidson David L. , Graef Renee C.
Published: April 2011

Polymer infiltration/pyrolysis (PIP) processing has the potential to become an affordable means of manufacturing continuous fiber-reinforced ceramic-matrix components. The PIP method is very similar to the well-known polymer-matrix and carbon-carbon composite manufacturing techniques, the major difference being the use of a preceramic polymer in place of the organic polymer or carbon precursor. To date, the majority of research in the field of preceramic polymers has centered on precursors to silicon carbide (SiC). The Southwest Research Institute (SwRI) has focused on the development of polymeric precursors to silicon nitride (Si3N4) because its high-temperature strength, resistance to oxidation, and other properties make it an attractive candidate for many advanced high-temperature structural applications. PIP Si3N4 composites with NICALON SiC fiber reinforcement have exhibited good fracture toughness (KIC ∼ 16MPa·m1/ 2). We report here processing, microstructure and preliminary mechanical properties of two new PIP Si3N4 composites. One is reinforced with Tonen Si3N4 fiber (plain weave) while the other is reinforced with ALMAX Al2O3 fiber (8 Harness satin weave).
Processing and Properties of Some Alumina-Boride Composites
Authors: Edirisinghe Mohan J.
Published: April 2011

Alumina (Al2O3) test bars containing a small (5-10%) volume of titanium diboride (TiB2) or zirconium diboride (ZrB2) particles have been pressed and sintered (pressureless) in an argon atmosphere. The microstructure of the sintered bodies was characterized by X-ray diffraction and a range of microscopical techniques and shows that 3ppm (by volume) of oxygen present in the argon caused the boride particles in the surface regions of the test bars to oxidize during sintering, to a greater extent in the Al2O3-TiB2 composites. Mechanisms of oxidation are discussed. The boride particles retarded the densification of the composites, to a greater extent in the Al2O3-ZrB2 bodies. However, densification in the Al2O3-ZrB2 system was enhanced by sintering in an Ar-4% H2 atmosphere. The decrease in flexural strength due to the retardation of sintering has been overcome in both types of composite.
High Temperature Mechanical Properties of Al2O3/ZrO2(Y2O3)Fibers
Authors: Sayir A. , Farmer S. C. , Dickerson P. O. , Yun H. M.
Published: April 2011

In-situ composite fibers produced by directional solidification of two phase oxide eutectics are one means of producing fibers with good strength and higher creep resistance than single crystal fibers. In this work, directionally solidified alumina-yttria stabilized zirconia eutectic fibers have been grown by the laser heated float zone (LHFZ) method at NASA Lewis. The average tensile strength of the alumina-zirconia (60.8 m/o Al2O3; 39.2 m/o ZrO2 (9.5 m/o Y2O3)) eutectic fibers was 1.2 GPa at room temperature. The high temperature tensile strength and creep resistance of the eutectic fiber were determined and compared to single crystal Al2O3.
Matrix Cracking In Brittle-Matrix Composites with Tailored Interfaces
Authors: Danchaivijit Sawai , Chao L-Y. , Shetfty D. K.
Published: April 2011

Matrix cracking from controlled through cracks with bridging filaments was studied in a model unidirectional composite of SiC filaments in an epoxy-bonded alumina matrix. An unbonded, frictional interface was produced by moderating the curing shrinkage of the epoxy with the alumina filler and coating the filaments with a releasing agent. Uniaxial tension test specimens (2.5 × 25 × 125 mm) with filament-bridged through cracks were fabricated by a novel two-step casting technique involving casting, precracking and joining of cracked and uncracked sections. Distinct matrix-cracking stresses, corresponding to the extension of the filamentbridged cracks, were measured in uniaxial tension tests using a high-sensitivity extensometer. The crack-length dependence of the matrix-cracking stress was found to be in good agreement with the prediction of a fracture-mechanics analysis that employed a new crack-closure force - crack-opening displacement relation in the calculation of the stress intensity for fiber-bridged cracks. The prediction was based on independent experimental measurements of the matrix fracture toughness (Kcm), the interfacial sliding friction stress (τ) and the residual stress in the matrix (σmI). The matrix-cracking stress for crack lengths (2a) greater than 3 mm was independent of the crack length and agreed with the prediction of the steady-state theory of Budiansky, Hutchinson and Evans[2]. Tests on specimens without the deliberately introduced cracks indicated a matrix-cracking stress significantly higher than the steady-state stress.
Strong Interface in CMCs, a Condition for Efficient Multilayered Interphases
Authors: Droillard Christine , Lamon Jacques , Bourrat Xavier
Published: April 2011

A fiber treatment was used to change the bonding strength of the Nicalon NLM 202 SiC fiber from weak to strong, in a series of 2D-SiC/SiC composites with multilayered interphases. The materials with the pre-treated fibers were compared to the same materials but reinforced with as received fibers. The stress-strain behavior and the fracture toughness were examined as a function of crack patterns identified by TEM. All the materials could be grouped into two distinct families: (i) materials reinforced with untreated fibers have a weak fiber bonding and are characterized by a low strength and a low toughness and (2) materials with the pre-treated fibers have a strong fiber bonding and are characterized by a high strength and a high toughness. This latter behavior is identified by TEM. It corresponds to a new interfacial behavior with a cohesive mode of interfacial cracking, involving branching and deflection by the successive interfaces. In the former family, the adhesive interfacial failure mode corresponds to the classical debond/sliding mechanism.
Interface Degradation in CAS/Nicalon During Elevated Temperature Aging
Authors: Plucknett Kevin P. , Cain Rebecca L. , Lewis M.H.
Published: April 2011

A CaO-Al203-SiO2 (CAS)/Nicalon glass-ceramic matrix composite has been subjected to elevated temperature oxidation heat-treatments between 375 and 1200°C, for up to 100 hours. Micro- and macro-mechanical properties have been determined by fiber push-down, using a mechanical properties microprobe, and flexure testing, respectively. Aging between 450 and 800°C results in significant property degradation, with reduced bending modulus and flexure strength, increased fiber sliding stress, and a transition to a purely brittle failure mode. Aging degradation is due to oxidative removal of the carbon interlayer, with the subsequent formation of a silica bond between fiber and matrix. At higher temperatures, carbon is retained due to the formation of a protective silica plug at exposed fiber ends, with the subsequent retention of composite properties. Short duration pre-treatment schedules, at 1000 or 1100°C, were developed to prevent intermediate temperature property degradation.
Use of CVD Ain Films as Protective Coatings in MoSi2 Composites
Authors: Cook Mark J. , Hudson John B. , Hillig William B.
Published: April 2011

In many fiber reinforced composite systems, protective coatings on the fibers are required in order to protect the fiber from reaction with the matrix, to modify the mechanical properties of the fiber-matrix interface, or to protect the fiber during processing steps such as chemical vapor infiltration. We have developed techniques for coating single crystal sapphire fibers with thin AIN layers, using CVD from an organometallic precursor in a cold wall reactor. Side by side tests of coated and uncoated fibers introduced into a MoSi2 matrix, either by hot isostatic pressing or by reactive vapor infiltration (RVI), indicate that the coating inhibits both fibermatrix interdiffusion and volatilization of the fibers by HC1 produced in the RVI process.
Deformation Mechanisms in a Proton-Irradiated Austenitic Stainless Steel
Authors: Carter R.D. , Atzmon M. , Was G.S. , Bruemmer S.M.
Published: April 2011

Samples of austenitic 304L stainless steel have been irradiated with 3.4 MeV protons to atotal dose of 1 dpa. The microstructure of the irradiated stainless steel has been quantified by transmission electron microscopy and shown to be similar to that found in neutron-irradiated core components. Constant extension rate tensile tests have been performed at strain rates of 3x10−7 s−1 and 3x10−8 s−1 to strains of up to 27% at 23°C and 288°C. The resulting microstructures were characterized using electron and optical microscopy. Deformation of the unirradiated material is similar to that reported by others in previous work on austenitic steels, consisting of dislocation source activation and formation of dense dislocation networks with increasing strain. In the irradiated samples tested at 288°C, deformation consists of dislocation source activation at grain boundaries punching dislocations through the grain interior to the opposing grain boundaries. The dislocations create channels that are free of radiation-produced defects and on which dislocation motion is concentrated. Dislocations in the channels pile-up at the boundaries, creating regions of highly localized stress concentration at the grain boundary. The mechanism by which this stress is relieved is still unknown. Deformation at 23°C consists of the nucleation and propagation of microtwins across the width of the grains.
Crystalline-To-Amorphous Transformation of Intermetallic Compounds in the Zr-Fe-M System Induced by Irradiation
Authors: Motta Arthur T. , Howe Lawrence M. , Okamoto Paul R.
Published: April 2011

The binary and ternary intermetallic compounds Zr3Fe, Zr2 Fe, (Zr0.5,Nb0.5)3Fe, Zr3(Fe0.9,Ni0.1) and Zr3(Fe0.5,Ni0.5) were subjected to 900 keV electron irradiation until amorphous to study the change in the dose-to-amorphization with temperature. The critical temperatures were observed to vary with dose rate, and with the type of compound. Hexagonal (Zr0.5,Nb0.5)3Fe had an appreciably lower critical temperature and higher dose to amorphization at low temperature than orthorombic Zr3Fe, whereas other orthorombic Zr3(Fex,NiI-x) compounds were essentially identical in behavior to Zr3Fe. The electron energy dependence of the dose-to-amorphization was studied in Zr3Fe between 250 and 900 keV. The analysis of the results gives displacement energies of EZrd = 26 eV, EFed = 18 eV in the Zr3Fe compound.
Ion Implantation of Iron into an Immiscible Copper Matrix
Authors: Murzin Ivan H. , Potter Donald I.
Published: April 2011

Fluences from 1×1017 to 9.6×1017 cm−2 of single and doubly charged iron ions accelerated through 170 kV were implanted into polycrystalline copper substrates at room temperature. An order of magnitude decrease in reflectivity was observed during implantation as a result of sputter-induced changes in surface topography and subsurface accumulation of iron. Both scanning electron microscopy and optical microscopy showed that the bombarded surfaces consisted of (110) and (100) grains having an intragranular, ridge-like structure. Significant height differences formed between grains as a result of their different sputtering yields. Transmission electron microscopy reveals that the implanted iron forms a face-centered-cubic metastable solid solution with the copper. Auger electron spectroscopy confirmed iron accumulation beneath the surface with maximum concentrations approaching in excess of 20 atomic percent.
Defect Production Mechanisms During keV Ion Irradiation: Results of Computer Simulations
Authors: Averback R.S. , Ghaly Mai , Zhu Huilong
Published: April 2011

MD simulations have been employed to investigate damage processes during keV bombardment of metal targets. For self-ion irradiations of Au, Cu, and Pt in the range of 5-20 keV, we have found that both the amount and the character of the damage created in the surface depends sensitively on the details of the energy deposition along individual ion trajectories. In all of these cases, significantly more damage is produced and more atomic mixing takes place relative to corresponding recoil events in the crystal interior. In some cases, enormous craters are formed in an explosive event, while in others a convective flow of atoms to the surface leaves dislocations behind. The results of these simulations will be summarized and their significance for damage studies of ion irradiated materials, discussed.
Temperature and Dose Dependence of Metal Colloid Production in Alpha-Irradiated CaF2 Single Crystals
Authors: Weber W. J. , Exarhos G. J. , Wang L. M.
Published: April 2011

Single crystals of CaF2 were irradiated with 5.8 MeV alpha particles from a 244Cm source, and the absorbance spectra were recorded as a function of dose and temperature. Absorbance from the metal colloid band increases with dose at all temperatures. The colloid band exhibits a sharp increase in rate of growth over a narrow temperature range with a peak at 150°C. After a dose of 1.4 x 1016 α/cm2 at 150°C, the intensity of the colloid band is a factor of 7 to 8 higher than the intensity produced at 100 and 200°C. The optimum temperature (150°C) for colloid formation in CaF2 under the alpha-irradiation conditions (σ5 x 108 rad/h) of this study is significantly higher than the peak temperature (60°C) reported for colloid production in electron irradiated CaF2 at slightly higher dose rates (σ109 rad/h). Cryogenic transmission electron microscopy of the irradiated crystals reveals dislocation loops and small dark clusters as the dominant microstructural features.
Characterization of Collision Cascade Damage in Ca2La8(SiO4)6O2 by Hrtem
Authors: Wang L.M. , Webert W.J.
Published: April 2011

Ca2La8(SiO4)6O2 thin crystals become amorphous under ion beam irradiation. The ion dose required for complete amorphization of the thin crystal (critical amorphization dose, Dc) increased with the increasing irradiation temperature and decreased with ion mass at elevated temperatures. Samples irradiated with 1-1.5 MeV Ar+, Kr+ and Xe+ ions to doses much lower than Dc, in the temperature range from 20 to 498 K were used for a detailed HRTEM investigation to study the amorphization process. The residual collision cascade damage after irradiation appeared as nanometer scale amorphous domains. The images of these domains are extremely sensitive to the sample thickness. Small domains of cascade size were found only at the very thin edge of the sample. In thicker regions, amorphous domains appear after higher doses as the result of cascade overlap in projection. At higher temperatures, the observed amorphous domains are smaller indicating thermal recovery at the amorphous/crystalline interface. The amorphous domains are also larger in size after irradiation with ions of higher mass at a fixed ion dose. These results are consistent with the Dc-temperature curves determined by in situ TEM with the HVEM-Tandem Facility at Argonne National Laboratory. The width of the amorphous rim along the edge of the specimen grew with increasing ion dose suggesting that amorphization also proceeds from the sample surface. Images of the collision cascade damage were compared to the cascade sizes calculated with the TRIM code. Some digitally acquired HRTEM images of the cascade damage were processed to reveal more detailed information.
Cation Disorder Determined by MAS 27Al NMR in High Dose Neutron Irradiated Spinel
Authors: Cooper Elizabeth A. , Hughes Craig D. , Earl William L. , Sickafus Kurt E. , Hollenberg Glenn W. , Garner Frank A. , Bradt Richard C.
Published: April 2011

Spinel (MgAl2O4) single crystals which had been neutron irradiated at (nominally) 400 and 750°C to high doses (53-250 dpa) were examined using 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). The sensitivity of this procedure to a specific cation (Al) residing in different crystallographic environments allowed us to determine the distribution of the Al between the tetrahedral and octahedral cation sites in the spinel structure. Our results indicate that the Al was distributed nearly equally over both cation sites in the spinel, resulting in a nearly inverse structure.
Amorphization Threshold in Si-Implanted Strained Sige Alloy Layers
Authors: Simpson T.W. , Love D. , Endisch D. , Goldberg R.D. , Mitchell I.V. , Haynes T.E. , Baribeau J.-M.
Published: April 2011

We have examined the damage produced by Si-ion implantation into strained Si1-xGex epilayers. Damage accumulation in the implanted layers was monitored in situ by time-resolved reflectivity and measured by ion channelling techniques to determine the amorphization threshold in strained Si1-xGex, (x = 0.16 and 0.29) over the temperature range 30-110°C. The results are compared with previously reported measurements on unstrained Si1-xGex, and with the simple model used to describe those results. We report here data which lend support to this model and which indicate that pre-existing strain does not enhance damage accumulation in the alloy layer.
Point Defects in Gallium Arsenide Characterized by Positron Annihilation Spectroscopy and Deep Level Transient Spectroscopy
Authors: Mih R. , Gronsky R.
Published: April 2011

Positron annihilation lifetime spectroscopy (PALS) is a unique technique for detection of vacancy related defects in both as-grown and irradiated materials. We present a systematic study of vacancy defects in stoichiometrically controlled p-type Gallium Arsenide grown by the Hot- Wall Czochralski method. Microstructural information based on PALS, was correlated to crystallographic data and electrical measurements. Vacancies were detected and compared to electrical levels detected by deep level transient spectroscopy and stoichiometry based on crystallographic data.
Monte Carlo Simulations for Bcc Alloys Under Irradiation: Phase Stability and Microstructural Evolutions
Authors: Soisson F. , Bellon P. , Barbu A. , Martin G.
Published: April 2011

When a compound is maintained in far for equilibrium configurations by nuclear collisionsunder irradiation, the steady-state properties of the system can no longer be predicted from equilibrium thermodynamics. Here we propose Monte Carlo simulations for addressing the question of phase stability under irradiation. They are based on a kinetic model with two dynamics acting in parallel: thermally activated jumps of vacancies and ballistic events induced by nuclear collisions. Two transformations are studied: the A2-B2 order-disorder transition and the precipitation of copper in iron. In the former case a shift from second to first order of the A2-B2 transition, predicted by the model, has been experimentally checked by I MeV electron irradiations of a FeA1 alloy. In the latter case, the precipitation kinetics are determined by Monte Carlo simulations and are found to be in very good agreement with available experimental data.
Radiation-Induced Segregation: A Microchemical Gauge to Quantify Fundamental Defect Parameters
Authors: Simonen E. P. , Bruemmer S. M.
Published: April 2011

Defect kinetic parameters for radiation-induced grain boundary segregation in austenitic stainless alloys are evaluated by comparing model predictions to measured responses. Heavy-ions, neutrons, and proton irradiations having substantial statistical bases are examined. The combined modeling and measurement approach is useful for quantifying fundamental defect parameters. The mechanism evaluation indicates that vacancy migration energies were 1.15 eV or less and the vacancy formation energy at grain boundaries was 1.5 eV. Damage efficiencies of heavy ions and light-water reactor neutrons were about 0.03. Inferred proton damage efficiencies were about 0.15. Segregation measured in an advanced gas-cooled reactor component was much greater than predicted from those parameters alone.
Optimization of Reverse Saturable Absorber Limiters: Material Requirements and Design Considerations
Authors: Hagan D. J. , Xia T. , Dogariu A. , Said A. A. , Van Stryland E. W.
Published: April 2011

We present numerical beam-propagation simulations of optimized reverse-saturable absorption (RSA) based optical limiters where the depth of focus of the input beam is much smaller than the thickness of the nonlinear material. The optimization is achieved by allowing the molecular concentration to vary along the propagation path, allowing the entire length of the limiter to reach the maximum possible nonlinear absorption before eventual damage to the limiter. We review in detail the analytic model originally derived by Miles [1] to determine the design and performance of such limiters. This model requires the usual 5-level model used in the numerical solution to be approximated by a quasi-three-level system. We show that this effective 3-level excited-state cross section is both pulsewidth and fluence dependent. The numerical propagation output shows that there is considerable diffractive beam distortion, which cannot be accounted for in the analytic model. The end result is that while there is qualitative agreement with numerical results, the magnitude of the limited output can be an order-of-magnitude underestimated. We determine that the fluence level at all parts of the limiter must be at least ten times the saturation fluence to efficiently utilize the nonlinear absorption. We further describe how the optimized distribution of molecular density is the limit of the multi-element tandem limiter for an infinite number of elements. By carefully accounting for saturation over the entire length of each individual element, we show how a multi-element limiter may be designed to closely approach the performance of the optimized distribution for as few as four elements. With current materials technology the damage threshold of solid hosts needed to vary the molecular density is much lower than that of glass cuvettes used for liquid based limiters. Therefore, a multi-element liquid based tandem limiter can be used in full saturation so that better limiter performance should be obtained. Ultimately, however, the operation of all RSA-based limiters involves a strict trade-off between performance and linear transmittance.
Broadband Excited State Dynamics of Common Reverse Saturable Absorbing Dye Media.
Authors: McEwan Kenneth J. , Hollins Richard C.
Published: April 2011

In this paper we describe a technique by which the broadband excited state absorption/emission and dynamics can be measured for materials with either linear or two photon absorption. The technique is applied to two different dyes in solution and to a cyano-biphenyl organic liquid. The ratio of σex/σgr for HITCI in methanol is greater than 1 from 400 to 570nm and at 532nn is determined to be 25; the excited state lifetime is 800ps. For CAP in methanol σex/σgr is greater than 1 from 400–650nm and is 14 at 532nm. Dimer absorption and emission are found to contribute to the excited state response in this material. In the organic liquid broadband (400–570nm) excited state absorption is measured following two photon excitation at 588nm.
Plasma Deposition and Characterization of Stable a-Si:H (Cl) From a Silane-Dichlorosilane Mixture
Authors: Osborne I. S. , Hata N. , Matsuda A.
Published: April 2011

Hydrogenated amorphous silicon containing chlorine (a-Si:H (Cl)) films have been grown by plasma enhanced chemical vapor deposition from a mixture of silane and dichlorosilane with a dichlorosilane concentration up to 60%. We report on the film properties in the as-deposited state and the behavior of the films under both high intensity pulsed laser illumination and long-term AMI illumination. With increasing dichlorosilane concentration the films show an increased resilience to the creation of light induced defects, as determined from the constant photocurrent method. After 900 hours under AMI illumination, the defect density shows a minimum (< 1016 cnr−3) for a 10 % mixing ratio.
Characterization of Compositional Gradients in Amorphous Semiconductor thin Films By Real Time Spectroscopic Ellipsometry
Authors: Kim Sangbo , Lu Yiwei , Collins R. W.
Published: April 2011

A new approach has been developed for the analysis of real time spectroscopie ellipsometry (RTSE) data that has important applications in characterizing compositionally-graded layers in amorphous semiconductor devices. The new RTSE data analysis approach determines (i) the instantaneous deposition rate, (ii) the surface roughness layer thickness, and (iii) the dielectric function of the near-surface layer (top 20 Å) of the growing film, all without any knowledge of the deposition history. We tested this new approach by characterizing a-Si1-xCx:H (x ∼ 0.09) alloy films with continuously graded void volume fractions. The results reported here are unique in that they represent the first successful optical analysis of amorphous semiconductor structures having continuously varying properties with depth into the film.
The Influence of Frequency and Pressure on the Material Quality of PECVD A-SI:H
Authors: Van Sark W.G.J.H.M. , Bezemer J. , Heller E. M. B. , Kars M. , Van Der Weg W. F.
Published: April 2011

A systematic study of material quality has been performed for a-Si:H layers deposited by plasma enhanced chemical vapour deposition at frequencies between 30–80 MHz. The effect of frequency variation was studied in combination with the variation of pressure and power density. The process conditions were optimised not only for ‘device quality’ opto-electronic properties but also for a uniformity in layer thickness better than 5 %. For every frequency an optimum pressure exists for which the properties of the deposited layer satisfy the ‘device quality’ requirements. A clear correlation is observed between the transition from the so-called α- to the γ-regime of the plasma and the dependence of the deposition rate γd with pressure pand frequency ƒ: γ d ∝ pƒ2/3.
Equilibration in Amorphous Silicon Nitride Alloys
Authors: Dunnett Bruce , Cooper Christopher H. , Murley Darren T. , Gibson Roderick A. G. , Jones David I. , Powell Martin J. , Deane Steven C. , French Ian D.
Published: April 2011

Several series of amorphous silicon nitride thin films have been grown by plasma-enhanced chemical vapour deposition, where the ratio of ammonia and silane feed gases was held constant for each series while the deposition temperature was varied from 160 °C to 550 °C, and all other deposition conditions were held constant. Photothermal Deflection Spectroscopy measurements were used to determine the Urbach slope E0 and the defect density ND. It is found that ND is determined by E0 for most of these samples, suggesting that defect equilibration occurs in a-SiNx:H for x up to at least 0.6. The growth temperature at which the disorder is minimised increases to higher values with increasing x, which is explained in terms of a hydrogen-mediated bond equilibration reaction. Fourier Transform Infra Red spectroscopy measurements were performed to determine the changes in hydrogen bonding with growth temperature. The results suggest that a second bond equilibration reaction also occurs at the growing surface, but that equilibrium cannot be reached at higher temperatures because of hydrogen evolution from Si-H bonds.
Defects and Microstructure in Optimized a-Si:Ge:H - New Results from Electron Spin Resonance Experiments
Authors: Malten C. , Finger F. , Fölsch J. , Kulessa T. , Wagner H. , Ray S. , Middya A. R. , Hazra S.
Published: April 2011

Amorphous silicon germanium alloys with improved properties for photovoltaic applications are investigated by electron spin resonance. Spin densities in this material are strongly reduced with optimized deposition conditions. Still the Ge dangling bond remains the dominating deep defect and the ratio of Ge over Si dangling bond defects is not much changed. The spin resonance spectra of these samples show distinctive differences from conventional material. The superposition signal can not be deconvoluted into modified Si- and Ge- dangling bond resonances as done previously. Instead a lineshape asymmetry or additional resonances are needed to deconvolute the spectra. The development of g-values with alloy composition indicate a more homogeneous chemical ordering in optimized material in agreement with other experimental data.
Band Discontinuity Effect on a-Si:H and a-SiGe:H Solar Cells
Authors: Xu X. , Banerjee A. , Yang J. , Guha S. , Vasanth K. , Wagner S.
Published: April 2011

The electrical bandgap of microcrystalline silicon (μc-Si:H) p type layers used in a-Si:H alloy solar cells and the band edge discontinuities between μc-Si:H and a-Si:H alloys have been determined by internal photoemission measurements. The bandgap of μc-Si:H is found to be in the range of 1.50 to 1.57 eV, and the discontinuities at the conduction and the valence band edges are 0 to 0.07 and 0.26 to 0.35 eV, respectively. Use of these parameters in the numerical simulation of single-junction a-Si:H and a-SiGe:H alloy solar cells is found to predict experimental results of solar cell performance.
A Semi-Insulating Layer for Novel High Voltage Polysilicon Thin Film Transistors
Authors: Clough F. J. , Chen Y. , Brown A. O. , Sankara Narayanan E. M. , Milne W. I.
Published: April 2011

This work describes the deposition and characterisation of semi-insulating oxygen-doped silicon films for the development of high voltage polycrystalline silicon (poly-Si) circuitry on glass. The performance of a novel poly-Si High Voltage Thin Film Transistor (HVTFT) structure, incorporating a layer of semi-insulating material, has been investigated using a two dimensional device simulator. The semi-insulating layer increases the operating voltage of the HVTFT structure by linearising the potential distribution in the device offset region.

A glass compatible semi-insulating layer, suitable for HVTFT applications, has been deposited by the Plasma Enhanced Chemical Vapour Deposition (PECVD) technique from silane (SiH4), nitrous oxide (N2O) and helium (He) gas mixtures. The as-deposited films are furnace annealed at 600°C which is the maximum process temperature. By varying the N2O/SiH4 ratio the conductivity of the annealed films can be accurately controlled up to a maximum of around 10−7 Ω−1.cm−1 Helium dilution of the reactant gases improves both film uniformity and reproducibility. Raman analysis shows the as-deposited and annealed films to be completely amorphous. A model for the microstructure of these Semi-Insulating Amorphous Oxygen-Doped Silicon (SIAOS) films is proposed to explain the observed physical and electrical properties.
Realization of an Amorphous N-I-P-I-N Three Color Sensor
Authors: Eberhardt K. , Neidlinger T. , Schubert M.
Published: April 2011

Utilizing the high optical absorption of amorphous silicon in conjunction with advanced signal processing capabilities of modern ASICs offers the possibility of simply realizing smart color sensors with random pixel access and without any losses in active area due to the signal processing circuitry. Thin-film photodiodes have to be incorporated into a layer sequence of ASIC/insulator/metal/a-Si:H based multilayer/TCO for this so-called Thin Film on ASIC (TFA) concept[l]. Amorphous silicon and silicon carbide photodiodes have been studied, and discrimination of the fundamental colors (red, green and blue) has been obtained from 2-terminal n-i-p-i-n devices at small bias voltages within ±2 V. Changing of the bias voltage applied to the two back-to-back p-i-n junctions allows for adjusting the location of predominant photo-carrier collection, and therefore enables color readout according to the energy dependent absorption profile of the incident light. Film thickness and optical bandgap of the individual layers have been optimized to achieve optimum color sensitivity. Switching experiments in the voltage mode give some information about conceivable frame rates these TFA sensors could be operated at. Light-induced degradation does not hamper the performance of the sensors thanks to low light intensity, reverse bias conditions, and small i-layer thickness.
Hydrogenated Amorphous Silicon Speed Sensor Based on the Flying Spot Technique
Authors: Vieira M. , Fantoni A. , Maçarico A. , Soares F. , Evans G. , Martins R.
Published: April 2011

In the past we have developed a transient technique, called the Flying Spot Technique (FST). FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known.

In this paper, we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in the direction of a light source. The light reflected back from the object is analyzed through a p.i.n structure being the transient transverse photovoltage dependent on the movement of the object (position and velocity). Assuming that the transport properties of the material and the geometry of the device are known and using a triangulation method we show that it is possible to map the movement of the object. Details concerning material characterization, simulation and device geometry are presented.