bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase Transformations in the Fe-Al System Studied by “In-Situ” Tem Heating
Authors: Korner A.
Published: April 2011

The domain structure and the evolution of antiphase boundaries (APBs) have been investigated in Fe-Al by means of “in-situ” transmission electron microscopy (TEM) heating experiments. Single crystals with composition Fe22.1at%Al and Fe25.6at%Al have been used.

The grown-in structure of the Fe22.1at%al single crystal is composed of DO3 ordered particles embedded in the disorderd ±-matrix. A bimodal distribution of the particles was found. Small ordered particles are in between the large precipitates which are surrounded by particle-free zones. Numerous of this large ordered precipitates contain APBs. Crossing the transition temperature to the disordered phase, the small particles dissolve into the ±-matrix and the large particles start to shrink by dissolving.

The single crystal with composition Fe25.6at%Al was found to be completely DO3 ordered. The grown-in domains are separated by APBs of type a′0/2〈100〉. At temperatures far below the transition temperature to the B2 phase no significant change in the APB and domain structure has been detected. In contrast, a remarkable evolution in the APB structure has been observed approaching the transition temperature. Coarsening of the domains has been found. Furthermore, APBs of B2-type (a′0/4〈lll〉 shear) are dragged out by dislocation motion. B2- and DC3-type APBs react and junctions are formed. With increasing annealing time, the density of B2-type boundaries increases. The TEM image is dominated by B2-type boundaries linked by the D03-type boundaries. The DO3 superlattice spots are clearly excited approaching the transition temperature to B2. Above the transition temperature, the DO3 spots disappear completely and the diffraction pattern reveals B2 long range order.
Möbius Inversion and Mechanical Properties of Ni3Al
Authors: Chen Nan-Xian , Li Mi , Liu Shao-Jun , Chen Zhao-Duo
Published: April 2011

In this work, a very abstract formula from number theory has been applied to solving the practical problems related to the important industrial material Ni3Al from the microscopic point of view.
The Influence of Short Range Order on the Energetics of Ni-Rich Nial Alloys
Authors: Wang Yang , Stocks G. M. , Nicholson D.M.C. , Shelton W. A. , Hines E. L. , Szotek Z. , Temmerman W. M.
Published: April 2011

Total energy calculations for Ni-rich ß′-phase NiAl have been performed using the large system multiple scattering (LSMS) method. The large samples used to model the alloys involved up to 128 atoms per cell, and were constructed to have the experimental short range order (SRO) parameters. Both short range ordering and charge transfer effects are automatically taken into account in the calculation. The calculated formation energies of both stoichiometric and non-stoichiometric compounds are in excellent agreement with experiment.
Atomistic Studies of Grain Boundaries in NiAl
Authors: Yan M. , Chen S. P. , Vitek V.
Published: April 2011

This paper presents the results of atomistic studies of grain boundaries in NiAl B2 alloy. The interatomic forces are described by Finnis-Sinclair type N-body potentials, and are fitted to properties of NiAl. The results show that the structure, energy and cohesive strength of a grain boundary depend strongly on its chemistry, and a grain boundary possessing more Al is the weakest. Energies of antisite defects at the grain boundary ∑5 {210} are also calculated, and the results suggest that Al has much larger tendency to segregate at a grain boundary than Ni does.
Subboundary Characterization and Creep Deformation in Investment Cast Ti-48Al-2Nb-2Cr
Authors: Jin Zhe , Bieler Thomas R.
Published: April 2011

Subgrain boundaries (subboundaries) in a creep deformed investment cast Ti-48Al-2Nb-2Cr specimen were characterized. A multi-stress drop test was performed at temperature 765 °C and stresses from 276 MPa to 103 MPa. The stress exponent n = 7. Subboundaries were observed both in equiaxed γ grains and within γ laths in lamellar grains. Dislocations within the subboundaries are characterized to be ordinary 1/2<110] dislocations. Subboundaries are found to occur on many crystallographic planes and no preferred crystallographic planes are found for subboundary formation in the crept TiAl specimen. Misorientations across the subboundaries are less than 1°. Analysis of subboundary formation mode indicates that the creep deformation in TiAl is pure metal type.
Creep of Particle-Reinforced NiAl Intermetallics: New Materials For Up to 1400°C
Authors: Arzt E. , Grahle P.
Published: April 2011

The intermetallic compound NiAl is a promising high temperature material as it possesses a favorable combination of advantageous properties (high melting point, low density, high thermal conductivity, good oxidation resistance). However its creep resistance is far inferior to that of advanced superalloys. We have developed new powder-metallurgical NiAl alloys, where the creep strength is raised by incorporating fine yttria dispersoids. The materials are produced by mechanical alloying and subsequent coarse-grain recrystallization at temperatures close to the solidus temperature. The resulting alloys show exceptional creep strength up to 1400 °C, thereby exeeding the temperature capability of superalloys significantly. The creep properties are in agreement with our recently developed model for “detachment-controlled creep”, in which the release of dislocations from incoherent particles determines the creep rate. In addition, the brittleness problem at room temperature can be moderately alleviated by incorporating coarse ductile phases such as Mo. While the particles raise the critical stress intensity factor, they do not improve the room temperature tensile ductility.
Wear of Ductile-Phase Toughened NiAl
Authors: Kennedy F.E. , Baker I. , Johnson B.J.
Published: April 2011

The purpose of this work was to study the effect of ductile phases on the wear of brittle intermetallic compounds. The material system of interest was NiAl with an incorporated ductile Fe phase. Three different compositions were studied: Fe10Ni45Al45, Fe30Ni35Al35, and Fe44Ni28Al28, each after two different heat treatments. Their wear behavior in unlubricated sliding against zirconia was investigated using pin-on-disk tests at room temperature in air. Hardness and fracture toughness tests were run for the same materials to clarify the relationship between wear and deformation. Results for the ductile-phase toughened materials were compared with results of similar tests for unalloyed NiAl.
NiAl-Base Composite Containing High Volume Fraction of Aln Particulate for Advanced Engines
Authors: Hebsur Mohan G. , Whittenberger J. D. , Lowell C. E. , Garg A.
Published: April 2011

Cryomilling of prealloyed NiAl containing 53 at. % AJ was carried out to achieve high nitrogen levels. The consolidation of cryomilled powder by extrusion or hot pressing/hot isostatic pressing resulted in a fully dense NiAl-base composite containing 30 vol. % of inhomogeneously distributed, nanosized AlN particulate. The NiAl-30AlN composite exhibited the highest compression yield strengths at all temperatures between 300 and 1300 K as compared with other compositions of NiAl-AlN composite. The NiAl-30AlN specimens tested under compressive creep loading between 1300 and 1500 K also exhibited the highest creep resistance with very little surface oxidation indicating also their superior elevated temperature oxidation resistance. In the high stress exponent regime, the strength is proportional to the square root of the AlN content and in the low stress exponent regime, the influence of AlN content on strength appears to be less dramatic. The specific creep strength of this material at 1300 K is superior to a first generation Ni-base single crystal superalloy. The improvements in elevated temperature creep strength and oxidation resistance have been achieved without sacrificing the room temperature fracture toughness of the NiAl-base material. Based on its attractive combination of properties, the NiAl-30AlN composite is a potential candidate for advanced engine applications.
Development of Microstructure in Mechanically Alloyed NiAl
Authors: Pyo Sung G. , Kim Nack J.
Published: April 2011

NiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmosphere. The mechanically alloyed (MA) powders were consolidated by vacuum hot pressing (VHP), resulting in a material which was fully dense. Analytical transmission electron microscopy has been used to understand the development of microstructure with processing. The dispersoid particles present are mostly A12O3 with small amount of A1N. It has been shown that morphology of the dispersoid particles are not uniform and the larger dispersoid particles are located mostly along the grain boundaries with the dispersoid-free zones near them. Coarsening behavior of the dispersoid particles is different depending on their locations, i.e., within the grain or along the grain boundaries. The accelerated coarsening of dispersoids along the grain boundaries occurs concurrently with that of the grains and the formation of dispersoid-free zones (DFZs) are observed along the grain boundaries. It is suggested that this abnormal dispersoid coarsening and the formation of DFZ are due to the coupled migration of dispersoids and grain boundaries.
Interfacial Structure and Lattice Mismatch in Lamellar TiAl Alloys
Authors: Dimiduk Dennis M. , Sun Yongqian , Hazzledine Peter M.
Published: April 2011

The gamma lamellae in TiAI alloys are rotated relative to each other approximately by multiples of 60˚ so that close packed planes and directions nearly align. However, the deviations caused by the tetragonality of the lattice are important in determining the structure of the interfaces and the internal stresses in the lamellae. These, in turn, strongly affect the mechanical properties of the alloys. In this paper two new electron optical techniques are used to quantify the deviations from pure twist boundaries (1) selected area diffraction from two adjacent twin-related lamellae permits c/a to be measured with high precision, (2) convergent beam diffraction is used to measure tilt components of lamellar boundaries and also rotations of vectors within the lamellar boundaries. The results are discussed in connection with the weak beam observations of interface dislocation structures.
Complex Anti Phase Domain Boundaries In A Gamma TiAl Alloy
Authors: Zhang X. D. , Loretto M.H.
Published: April 2011

Recent work has shown that antiphase domains are observed in alloys based around TiAl if they are quenched from the alpha field. The present work has shown that the anti phase domain boundaries (APDBs) are in fact not simple APDBs but are composed of very thin (about 2 nm) 90° domains. Although the contrast is dominated by the fact there is a 1/2<101> displacement between the domains either side of this thin 90° domain, there is somewhat more complex contrast associated with the APDBs than the hard sphere model would predict.
Annealing studies of the complex APDBs in a Ti49Al alloy have shown that the originally irregular APDBs in the quenched material facet on {111} planes. The significance of these observations is discussed in terms of mechanisms by which the APDBs are formed in TiAl and in terms of the planar fault energies in the Llo structure.
Phase Stability, Microstructure and Mechanical Properties in the Multi-Phse Alloys Based on the L12-Ni3(Al,Be)
Authors: Matsuo Takahiro , Hosoda Hideki , Miura Seiji , Mishima Yoshinao
Published: April 2011

Intermetallic alloys based on the Ll2 Ni3(Al,Be) phase in the ternary Ni-Al-Bc system are prepared so that the alloys are multi-phase with the B2 intermetallic compound NiBe and a Ni primary solid solution denoted as (Ni). Such three-phase alloys, Ni-16 to 20 at%Al-10 at%Be, exhibit good room temperature ductility as measured by four-point bending. In order to examine the phase stabilities and relations among constituent phases, a vertical section of the ternary system is constructed at a constant 10 at%Bc mainly by differential thermal analysis. It is found that improvement in room temperature ductility can be achieved by the formation of a fine mixture of constituent phases during invariant reactions during solidification, which is further enhanced by the co-existence of the Ll2 phase formed as the primary solidification phase.
Creep and Fracture Mechanisms in an Oxide-Dispersion Strengthened Ni3Al-Based Alloy Between 649°C and 982°C
Authors: Mason Ralph P. , Grant Nicholas J.
Published: April 2011

An oxide-dispersion strengthened (ODS) Ni3Al-based alloy has been fabricated and creep tested. Previously reported data for minimum creep rate as a function of stress indicated that two creep mechanisms operate at intermediate temperatures of 732 and 816°C [1]. This paper reports the results of recent interrupted creep tests and fractographic studies which serve to identify the two creep mechanisms. Creep at low stresses or low creep-rates occurs by constrained growth of cavities on transverse grain boundaries. In this low stress region an apparent stress exponent of 5.1 is observed. Creep at high stresses or high creep-rates results from the bulk deformation of grains by power law creep with a much smaller contribution due to grain boundary cavitation. The stress exponents of 13 and 22 observed in this high stress region are typical of ODS alloys. In both regions fracture is observed to be mixed mode with a large transgranular component due to the high grain aspect ratio developed in this material. Limited data at 982°C indicate the occurrence of only one mechanism which can be described by a stress exponent of 9.1. It was not possible, based on fractographic studies, to associate the creep mechanism at 982°C with either of those observed at the intermediate temperatures. No fractographic studies were performed at 649°C due to lack of valid specimens; however, the stress exponent of 13.5 observed at 649°C suggests that creep occurs by deformation of the grains.
Effect of Zirconium On Mechanical Properties and Grain Boundary Chemistry in Ni3Al Alloys
Authors: Lin Dongliang , Gu Yuefeng , Guo Jianting
Published: April 2011

The effects of zirconium addition on tensile properties and grain boundary (GB) chemistry in the Ni3Al alloys were studied. The results showed that with the increasing amount of Zr, the yield and ultimate strength increase, the elongation increases below 0.7 at. % Zr content and decreases above that content. Zirconium segregation to grain boundaries (GBs) is found by Auger electron spectroscopy (AES) study and the amount of Zr at GB is three times greater than that in the bulk. In situ transmission electron microscopy (TEM)/ scanning electron microscopy (SEM) investigation showed that slip can transfer across the GB to the adjacent grain in Zr-doped Ni3Al alloys. These results indicate that the effect of Zr on improving ductility of Ni3Al alloys had a strong relationship with the segregation of Zr on the GBs.
The Cyclic Oxidation behavior of Oxide-Dispersed β-NiAl
Authors: Pint B. A. , Hobbs L. W.
Published: April 2011

Short-term cyclic oxidation tests were performed at 1200°C and 1500°C on ²-NiAl with 1 vol% oxide dispersions of Y2O3, La2O3 and Al2O3. The Y2O3 addition resulted in a very adherent ±-Al2O3 scale at 1200°C, with some improvement in the scale adhesion at 1500°C, compared to an undoped ²-NiAl alloy. The La2O3 and Al2O3 additions resulted in more severe oxidation than the undoped ²-NiAl at 1200°C. Both Y and La ions are found to segregate to ±-Al2O3 scale grain boundaries using scanning transmission electron microscopy/energy dispersive x-ray analysis. However, at both 1200°C and 1500°C, the La addition appears to cause a rapid increase in the isothermal oxidation rate, leading to excessive scale spallation.
Improved Dispersion of Silicon Nitride Whiskers
Authors: Shih Wei-Heng , Buchta Michael
Published: April 2011

To improve the dispersion of silicon nitride whiskers in aqueous suspensions, a standard dispersant used in glass fiber industry, 3-aminopropyltriethoxysilane (APS) is added. It was found that the viscosity of the whisker suspensions is lowered and the centrifuged density of the suspensions is increased with the addition of the APS. The pH values of suspensions before and after the addition of APS indicate that APS extracts H+ ions from the solutions and the adsorption of APS on silicon nitride is saturated in our experiment. Our results indicate a colloidal route to the processing of ceramic composites with silicon nitride whiskers as reinforcements.
Enhanced Thermal Conductivity of CVD SiC via Beryllium and Boron Dopings
Authors: Kowbel W. , Gao F. , Withers J. C.
Published: April 2011

SiC-SiC composites exhibit excellent mechanical and corrosion properties, which are desired for a variety of advanced applications including fusion reactors, heat exchangers and advanced turbine engines. However, SiC-SiC composites fabricated by the CVI methods have unacceptably low through-the-thickness thermal conductivity, which limits their use in these applications. CVD SiC co-deposition with either beryllium or boron compounds significantly enhanced the thermal conductivity of CVD SiC at temperatures up to 1000°C. Results pertinent to the doping processes and the measured thermal conductivity of doped CVD SiC are reported. Mechanisms for the enhanced thermal conductivity of the CVD SiC are discussed based on the microstructural characteristics of the doped specimens.
Interfacial Properties of As-Received and Coated SiC (SCS-6) Fiber Reinforced Reaction-Formed SiC Matrix Composites
Authors: Gaeta Paula J. , Sisson Richard D. , Singh M. , Eldridge Jeffrey I.
Published: April 2011

The possibility of improving the interfacial shear strength of the SiC fiber reinforced SiC matrix composite system was examined. A ceramic fiber coating was chosen based on availability and chemical stability with the fiber and matrix. Fiber push-out tests conducted on as-received and coated fiber reinforced composite samples allowed characterization of the interfacial shear strength. Average debond shear and frictional sliding stresses were calculated. The effects of sample thickness and second phase addition in the matrix were also evaluated. Tested samples were examined by SEM to determine the location of the interfacial failure and to determine if any interface reactions had occurred. The coating was then evaluated based on the resulting interfacial shear strength, failure location, and integrity of the interface as compared to those properties of samples reinforced with as-received fibers.
“Chemically Bonded Ceramic Matrix Composites: Densification and Conversion to Diffusion Bonding”
Authors: Johnson Bradley R. , Gülgün Mehmet A. , Kriven Waltraud M.
Published: April 2011

Chemically bonded ceramics appear to be a promising alternative route for near-net shape fabrication of multi-phase ceramic matrix composites (CMC's). The hydraulic (and refractory) properties of fine mono-calcium aluminate (CaAl2O4) powders were used as the chemically bonding matrix phase, while calcia stabilized zirconia powders were the second phase material. Samples containing up to 70 wt% (55 vol%) zirconia have been successfully compacted and sintered.

Various processing techniques were evaluated. Processing was optimized based on material properties, dilatometry and simultaneous thermal analysis (DTA'TGA ). The physical characteristics of this novel CMC were characterized by hardness, density, and fracture toughness testing. Microstructures were evaluated by SEM and phase identification was verified using XRD.
The Effect of Migration Energies on Reducing Overprediction in Radiation-Induced Segregation Models
Authors: Allen T. R. , Was G. S.
Published: April 2011

Models for calculating radiation-induced segregation (RIS) in concentrated alloys based solely on differences in the magnitude of the vacancy driven flux typically overpredict the amount of grain boundary segregation. A simple sensitivity analysis is used to show that the overprediction may be linked to the choice of input parameters. By varying the Cr-vacancy migration energy from 1.300eV to 1.320 eV and the Fe-vacancy migration energy from1.300eV to 1.305 eV, well within their experimental uncertainty, RIS model calculations are brought into better agreement with AES and STEM-EDS measurements of grain boundary segregation in Fe-Cr-Ni alloys irradiated with protons at 7x 10−6 dpa/s at various temperatures and doses.
Microstructural and Mechanical Property Changes in Model Fe-Cu Alloys
Authors: Rice Philip M. , Stoller Roger E. , Lucas Barry N. , Oliver Warren C.
Published: April 2011

This paper describes a technique developed to determine values for the dislocation barrier strength of the defects believed to be responsible for the embrittlement of light water reactor (LWR) pressure vessel steels. Microstructures consisting of a single defect type were introduced by ion irradiation or thermal annealing, and the defect distributions were determined by TEM. Hardness changes were measured using a nano indenter and the dislocation barrier strengths for the defects involved were computed based on a dispersed barrier hardening model.
Modelling the Structure of Ion Bombarded Binary Alloys
Authors: Ossi Paolo M.
Published: April 2011

An atomistic model of phase formation in ion bombarded thin binary metallic films is discussed. Compositional changes occur at the interface between collision cascades and crystal matrix. Relaxation arises via elementary charge transfer reactions, with formation of dimers of an effective compound. Comparing surface and thermochemical properties of initial and effective alloys, a set of conditions specific to vitrification and respectively tocrystal formation under ion bombardment is obtained.
Microstructure of A12O3 Irradiated with an Applied Electric Field
Authors: Zinkle S. J. , Hunn J.D. , Stoller R.E.
Published: April 2011

A thin amorphous film of alumina was irradiated with 2-MeV He+ ions at ~400°C up to a damage level of about 0.01 displacements per atom (dpa). The alumina films were sufficiently thin (~1.8 μm) to allow the ion beam to be completely transmitted through the specimen. An electric field of μ280 V/;mm (dc) was applied continuously during the irradiation. Radiation induced electrical degradation (RIED), i.e. a permanent increase in the conductance of the film, was observed in specimens irradiated at temperatures near 400 to 450°C but did not occur in a specimen irradiated above 500°C. An investigation by transmission electron microscopy found no evidence for colloid formation. The observed increase in the conductance of the alumina film may be due to radiation-induced microcracking.
Photoplastic Effect on Ice
Authors: Khusnatdinov Niyaz N. , Petrenko Victor F.
Published: April 2011

The study of the photoplastic effect (PPE) on ice is essential for both fundamental and applied reasons. It is important for an understanding of dislocation motion as well as the flow of glaciers in cold regions that occurs under intensive solar radiation. It was found that the illumination of ice with UV light (λ < 300 nm) leads to its irreversible hardening1. A prolonged irradiation with a total light exposure of about 8.10−5 J/cm2 at λ= 260 nm can change the creep rate up to 60 percent. Even more pronounced PPE was found in HCI-doped ice with the concentration, n= 1018 cm−3. It is suggested that PPE is caused by the excitation of “autoionization” reaction which was found responsible for the photoconductivity of ice.
Ion-Beam and Electron-Beam Induced Amorphization of Berlinite (AIPO4)
Authors: Bordes N. , Ewing R.C.
Published: April 2011

Berlinite (AIPO4) is isostructural with α-quartz. Like α-quartz, berlinite undergoes a pressure-induced amorphization at 15 ±3 GPa; however, upon release of the pressure, unlike α-quartz which remains amorphous, berlinite returns to the original crystalline structure of the single crystal. Berlinite was irradiated with 1.5 MeV Kr+ at temperatures ranging from 20 to 600K. The onset of amorphization was examined by monitoring the electron diffraction pattern by in situ transmission electron microscopy (TEM) at the HVEM-Tandem Facility at Argonne National Laboratory. The berlinite was easily amorphized at 20K at a relatively low dose of 4x1013 ions/cm2 or 0.05 dpa (displacements per atom). The critical amorphization dose increases with the sample temperature. These experiments also showed that the focused electron beam can locally amorphize the berlinite. After these irradiations, berlinite remained amorphous. At 500 °C, berlinite began to recrystallize: small areas of crystalline material appear in the aperiodic matrix. These results suggest that pressure-induced amorphization and ion-beam induced amorphization, in the case of berlinite, are different processes that result in two different aperiodic structural states.
Ion Damage Production in GaAs/Al0.6Ga0.4As Heterostructures
Authors: Turkot B.A. , Robertson I.M. , Kirk M.A. , Rehn L.E. , Baldo P.M. , Forbes D.V. , Coleman J.J.
Published: April 2011

The non-uniform distribution of damage with depth and the onset of amorphization produced in Al0.6Ga0.4As/GaAs heterostructures by 1.5 MeV Kr+ ion implantation at 77K have been investigated by using a combination of RBS channeling and cross-sectional high-resolution TEM techniques. The extent of damage increases with increasing depth in the Al0.6Ga0.4As layer. This depth dependence of the damage can be correlated with an increase in the number of recoil events with an energy between 30 and 50 keV. We propose that the amorphization of Al0.6Ga0.4As requires the superposition of cascade-type damage on a background population of point defects which is created by implantation with high energy ions.
Radiation-Resistant Properties of Inp-Related Materials
Authors: Yamaguchi Masafumi , Ando Koshi , Kamada Hidehiko
Published: April 2011

Irradiation effects of high-energy electrons and protons, and 60Co gamma-rays on InP-related materials have been examined in comparison with those of GaAs and Si. Superior radiation-resistance of InP-related materials and their devices compared to GaAs and Si has been found by using deep-level transient spectroscopy (DLTS), photoluminescence (PL) and properties of devices such as solar cells and lightemitting devices. Moreover, minority-carrier injection enhanced annealing phenomena of radiation-induced defects in InP-related materials have also been observed even at low temperature of around 150K.
Two-Photon Absorption in Polybenzidine
Authors: Aranda F. J. , Cheng C. F. , Rao D. V. G. L. N. , Akkara J. A. , Kaplan D. L. , Roach J. F.
Published: April 2011

Polymers of Benzidine were synthesized by hydrogen peroxide reaction catalyzed by horseradish peroxidase enzyme. The polymerization reaction was carried out at room temperature in a monophasic organic solvent with a small amount of water at pH 7.5.

The technique of degenerate four-wave mixing (DFWM) with nanosecond and picosecond pulses was employed to measure the third-order nonlinear optical susceptibility χ. The samples were studied in solution in Dimethyl sulfoxide:Methanol in volume ratio 4:1. The observed values are of order 10−9 to 10−8esu. Measurements on a thin film agree approximately with the extrapolated values fron solution measurements. Picosecond time resolved measurements indicate a pulse-width limited response followed by a small slow component. Investigation of the total energy transmission as a function of the incident intensity and fluence at 532 nm for pico- and nanosecond pulses indicates reverse saturable absorption. As we observe the nanosecond and picosecond curves to be superimposed for the intensity plot but not for fluence, we conclude that the nonlinearity is predominantly due to two-photon absorption. Numerical analysis of the data yields a value of 12.25 cm/GW for the two-photon absorption coefficient β. The imaginary component of χ obtained is 5 × 10−9esu. The material appears to be a good candidate for applications in optical power limiting and switching.
Optical Limiting in Solutions of Polycyclic Aromatic Hydrocarbon Diones.
Authors: Natarajan L. V. , Sutherland R. L. , McLean D. , Brant M. , Brandelik D. M. , Cooper T. M. , Crane R. L.
Published: April 2011

Optical limiting in toluene solutions of substituted benzanthracene- 3,4 diones and three other quinones, namely, benzpyrene-7,8-dione, benzphenanthrene-3,4-dione and chrysene-5,6-dione was investigated at laser wavelengths 532 nm and 694 nm. Low limiting threshold values of the order of 0.01J/cm2 were obtained for some of the quinones at 532nm. Strong triplet-triplet absorption at the excitation laser wavelengths may explain the optical limiting behavior.
Complex Formations in C60 Solutions
Authors: Brandelik D. M. , McLean D. G. , Brant M. C. , Sutherland R. L. , Frock L. R. , Crane R. L.
Published: April 2011

Complex formations in C60 solutions have previously been observed in diethylaniline. In a wide survey of solvents conducted in our laboratory, several additional solvents exhibited complexing behavior with C60 when analyzed with UV-Vis and fluorescence spectroscopy. All solvents with complexing behavior contained nitrogen. They are benzonitrile, N-methylpyrrolidone, and triphenylamine. A careful study was done to characterize the formation of the complexes formed in C60/toluene solutions when each complexing agent was added. Diethylaniline was added to the study as a reference complexing system. Formation of the complexes was followed with UV-Vis and fluorescense spectroscopy. Nash plots of the complexes were done to determine the equilibrium constant and the isosbestic point for each system. Nonlinear optical characterization of the complexing solvents was analyzed.
Quantum Chemical Methods for the Design of Molecular Non-Linear Optical Materials
Authors: Till Stephen , Till Jennifer
Published: April 2011

The quantum chemical design of new molecular materials for non-linear applications requires a fundamental understanding of electronic structure and properties. Targeted synthesis of candidates greatly reduces the costs and timescales of an empirical search and this is aided by prior calculation of excited state energies, energy relaxation and transfer rates, molecule-environment interactions and excited state chemistry.

Essentially, the problems encountered in the routine application of standard quantum chemical methods are caused by the large size of the molecules of interest. This necessitates either the design of ultra-fast computers or numerical methods which facilitate the application of ‘exact’ techniques or the development of less resource intensive approximate methods with proven accuracy.

We shall outline the theories used in the calculation of optical properties and review their computational implementation. Calculations on annellated tetraazaporphyrazines will be presented in illustration.
Correlation of Film Quality and Hydrogen NMR Resonance Shifts in a-Si:D,H; a-Ge:D,H; and a-SiGe:D,H
Authors: Norberg R. E. , Kim Y. W. , Chan P. H. , Bodart J. R. , Kernan M. J. , Leopold D. J. , Fedders P. A. , Turner W. A. , Paul W.
Published: April 2011

A systematic study of both proton and deuteron NMR in hydrogenated amorphous silicon films has revealed significant resonance shifts among various resolved components. The shifts include both paramagnetic and diamagnetic displacements of resolved spectral features from trapped molecular hydrogens. The shifts depend on film quality and deposition conditions. Some of the shifts vary as 1/T and reflect Curie susceptibilities characteristic of local regions of differing dangling bond densities. Spectra from relatively immobile hydrogen molecules trapped in nanovoids are shifted diamagnetically and broadened as temperature decreases. Hydrogens tightly bound to Si do not show similar changes and thus are more remote from dangling bonds and other magnetic defects. Similar spectrally-resol ved shifts have been observed in a-Ge and a-SiGe films and are correlated with film photovoltaic quality as measured by mobility-lifetime products.
Light-Induced Degradation of a-Si:H - A Comparison of Short-Laser-Pulse and Steady Light Degradation
Authors: Tzanetakis P. , Kopidakis N. , Androulidakp M. , Kalpouzos C. , Stradins P. , Fritzsche H.
Published: April 2011

An undoped and a compensated a-Si:H sample have been degraded by 17–34 ns laser pulses and by steady light (CW) at 78K and 300K. The light-induced defect concentration N is monitored by the increase in subgap absorption a. For the same change in a pulses degrade the photoconductivity σp more than CW light and more strongly for exposures at 78K than at 300K. The lack of correlation between σp and N suggests that light soaking causes additional damage besides an increase in N. This additional effect is more pronounced for pulse and low temperature exposures.
Examination of the Defect Pool Model by An Improved Analysis of the Constant Photocurrent Method
Authors: Stiebig Helmut , Siebke Frank
Published: April 2011

We have developed an improved analysis of constant photocurrent method (CPM) data. It is based on a numerical simulation of CPM spectra taking into account the full set of optical transitions between localized and extended states, capture and emission processes as well as the position of the Fermi level. Comparing measured and simulated CPM spectra provides information about the density of localized states in a-Si:H, i.e. the valence band tail, the integrated defect density, the energy distribution and the charge state of defect states. Based on these results we examine the predictions of the defect-pool model. The defect distribution in undoped and doped a-Si:H can be described by the defect-pool model taking into account the doping level dependence of principal parameters including the valence band tail, the equilibration temperature, and the width of the defect-pool.
Evidence for Exchange Between Free and Deep Hydrogen (Deuterium) During Diffusion
Authors: Branz Howard M. , Asher Sally , Xu Yueqin , Kemp Mathieu
Published: April 2011

We do not observe any immobile deuterium in secondary ion mass spectrometry D profiles taken after long anneals of hydrogenated amorphous silicon sandwich structures with a thin deuterated interior layer. This suggests that a single deep H level (∼1.4 eV deep) controls H diffusion. On its face, our result contradicts nuclear magnetic resonance and H effusion measurements that show about 30% of H in a-Si:H is “isolated” and deeply bound (∼ 2.1 eV deep). We reconcile our experimental results with the existence of isolated deep H by assuming there is a low-barrier (<< 1.4 eV) exchange process between free H and deep D. In tracer experiments, exchange has the effect of increasing the apparent emission rate of the deep D to nearly that of the shallowest trapped H. We solve for the D profiles and confirm that a deep-trapped D component is consistent with our D tracer profiles if and only if exchange processes are important. We also find that the mean distance D travels before retrapping (100–200Å) is determined by an exchange process of free D with trapped H.
High Electric Field Forming of a-Si:H P-I-N Diodes
Authors: Ilie A. , Equer B. , Pochet T.
Published: April 2011

Amorphous hydrogenated p-i-n diodes submitted to a high reverse bias for a long period of time undergo a metastable evolution which tends to improve their properties. A forming procedure based on this effect has been developed and leads to a significant decrease of the reverse current and increase of the breakdown voltage. In this paper, the mechanisms underlying this forming process have been investigated using the Constant Photocurrent Method (CPM), (I-V) characteristics and Spectral Response under reverse bias. The effects of annealing and current induced defect creation have been studied. The forming process is found to be most consistent with a metastable increase in the dopant activation of the p-layer and a small decrease of the density of states in the i-layer.
Transient Photocurrent Measurements on Current-Stressed a-Si:H Schottky Diodes
Authors: Buykx Andreas A. , Zeman M. , Spiekerman A. J. G. , Metselaar J. W.
Published: April 2011

The characteristics of a-Si:H based switches, as used in liquid crystal displays, degrade as a result of current injection. To assess this degradation we investigated the effects of current injection on the electron mobility, lifetime and the built-in electric field profile in a-Si:H Schottky diodes.

We present steady-state current-voltage and transient photocurrent (TP) measurements on Mo and Pd a-Si:H Schottky diodes. The mobility, lifetime and electric field profile were determined from TP measurements. The electric field profile was calculated by fitting simulations from a non-linear model of the measurement to the measured TP currents.

The electron drift mobility is not affected by current stressing, the electron lifetime reduces slightly, the built-in voltage decreases significantly. The widths of the built-in fields of Schottky and ohmic back contact are reduced with approximately the same factor, corresponding with a spatially homogeneous increase of the defect density.
AC Theory of Constant Photocurrent Measurement
Authors: Conte G. , Eray A. , Nobile G. , Palma F.
Published: April 2011

In this paper we present a study of ac the Constant Photocurrent Measurement (CPM) with the aim to assess the precision of this method compared to the de CPM. In particular, we introduce a small signal theory of the ac photoconductivity, we relate the sub-bandgap absorption to the density of defects, and we assume a combination of a constant plus a sinusoidal generation rate. The method allows to calculate the variation occurring in defect occupation as a function of the wavelength and intensity of the light, and of the ac test frequency. We show that, in ac condition, carriers lifetime is a complicated function of the chosen parameter, and it is affected by the delay due to charge accumulation of free and trapped carrier densities. Large influence on the lifetime is played not only by recombination, which takes place all along the gap defects included between the two quasi-Fermi levels, but also by the process of carrier trapping and emission by defects located close to the quasi-Fermi levels. Both processes introduce large phase delay. Further information can thus be obtained from CPM by evaluating the phase behavior. Our analysis leads to a new method of evaluation of the dangling bond capture cross section and of their charged-to-neutral capture cross section ratio.
VHF-Deposited a-SiC:H Alloys for High-Bandgap Solar Cells: Combining High Voc and Reasonable Stability
Authors: Platz R. , Fischer D. , Shah A.
Published: April 2011

The material properties of a-SiC:H alloys deposited by VHFGD are studied, with a special emphasis on the effect of hydrogen dilution of the plasma on layer quality. By incorporating these layers into p-i-n solar cells the authors compare layer properties and cell performance. Special attention is paid to the stability of the solar cells against light soaking. Furthermore, the authors show that the insertion of a buffer layer can, also in the case of entirely a-SiC:H cells, lead to a substantial increase of Voc. A reasonable stability of these cells is maintained by an appropriate doping of the buffer layers.
Organic Thin-Film-Transistors with High on/off Ratios
Authors: Torsi L. , Dodabalapur A. , Katz H. E. , Lovinger A. J. , Ruel R.
Published: April 2011

In this article a new procedure to obtain alpha-hexathienylene (α-6T) thin-film-transistors (TFTs) with on/off ratios in excess of one million is reported. This procedure involves subjecting the TFTs to rapid thermal annealing. Previously, high on/off ratios have been achieved with improved device design and better chemical synthesis of α-6T oligomers. High on/off ratios, along with a switching time of ∼ 10 μs, render α-6T TFTs potential candidates as switching devices in active matrix displays. The experimental current-voltage (I-V) characteristics of oc-6T TFTs with channel length L = 4μm are also presented and a measured field effect mobility of 0.02 cm2/V-s is extracted from these characteristics using an analytical model which we have developed for short-channel α-6T TFTs.
Passivation of InGaAs/GaAs Vertical-Cavity Surface-Emitting Lasers by Amorphous GaAs Deposition
Authors: Park Hyo-Hoon , Yoo Byueng-Su , Park Min Soo , Lee Jaejin , Lee Hae Gwon , Lee El-Hang , Shin Jae-Heon , Lee Yong Hee
Published: April 2011

We report for the first time a successful application of semi-insulating amorphous GaAs layer for surface passivation of index-guided vertical-cavity surface-emitting lasers. The amorphous GaAs layers on ion-beam-etched active region and mirror layers provide a significant improvement, more than 20%, in the threshold current density and differential quantum efficiency. The improvement of these performances is attributed to low defect density at the surface of active layers induced by amorphous GaAs.
Extraction of Elastic Parameters of Heavily Boron-Doped Silicon Layer by Elimination of Misfit Dislocations
Authors: Lee HO-JUN , Han Chul-Hi , Kim Choong-Ki
Published: April 2011

In this paper, various elastic parameters of heavily boron-doped silicon layer have been extracted by eliminating the misfit dislocations in the layer. The dislocation-free silicon membranes doped with the boron concentration of 1.3 × 1020 atoms/cm3 have been fabricated and the Young’s modulus of 1.45 × 1012 dyn/cm2 and residual tensile stress of 2.7 × 109 dyn/cm2 have been extracted by blister method. From the Young’s modulus and residual stress, the residual tensile strain of 1.34 × 10−3, lattice constant of 5.424 Å, and misfit coefficient of 1.03 × 10−23 cm3/atom have been calculated. These parameters are very similar to those obtained from X-ray diffraction analysis and theory.
Surfaces and Crystal Defects of Silicon
Authors: Wagner P. , Brohl M. , Gräf D. , Lambert U.
Published: April 2011

Bulk crystal defects are accessible for investigation when silicon crystals are sliced and the defects occur close to or at the surface of wafers. Such near-surface defects can then be delineated by modifying some processes used for preparing clean, polished wafers. The delineated defects usually occur as pits the shape of which depends on the delineation process used. The different shapes of the pits has consequences for their detection by light scattering techniques (laser scanners or surface inspection systems). The density of the such generated surface defects is related to the defect density in the crystal bulk and is influenced by the growth parameters. These surface defects therefore provide a means for studying and for characterizing the bulk defect density.
Magnetic Circular Dichroism of Low-Temperature-Grown AlxGa1-xAs
Authors: Prasad A. , Liu X. , Stallinga P. , Weber E. R. , Verma A. K. , Smith J. S.
Published: April 2011

We study magnetic circular dichroism of absorption (MCDA) of LT AlxGa1−xAs as a function of aluminum content. The MCDA spectrum of LT AlxGa1−xAs is distinctly different from the MCDA spectrum of LT GaAs, which has one paramagnetic and one diamagnetic peak at 0.95 eV and 1.18 eV, respectively. As the aluminum content increases, the spectrum of LT AlxGa1−xAs is dominated by a diamagnetic peak similar to the 1.18eV peak of the EL20-like defects in LT GaAs. However, the peak shifts to higher energies as x increases. The photoquenching and temperature dependence of this peak indicates an association with the EL2 defect. The paramagnetic peak observed in LT GaAs also shifts to higher energies but faster and eventually merges with the diamagnetic peak as the Al content increases. The study of the MCDA spectrum of LT AlxGa1−xAs as a function of aluminum content allows a better understanding of the MCDA phenomena of LT AlxGa1−xAs and LT GaAs, as well as the EL2-related transitions in bulk semi-insulating GaAs.
Uniformity of Arsenic Dimer and Tetramer Fluxes from a Valved Arsenic Cracking Source in MBE
Authors: Patterson George A. , Chang James S.C. , Kellert Forrest G.
Published: April 2011

We have studied the uniformity of the arsenic flux from a commercially available valved arsenic cracker source for MBE. There are many epitaxial structures that require a uniform arsenic flux distribution for rotated epitaxial growth and, in the non-rotated case, for RHEED. For non-stoichiometric GaAs growth at low substrate temperatures, which results in arsenic precipitates being formed during the in-situ anneal process, a specific As/Ga ratio must be used to maintain high crystalline quality and to achieve the desired electrical properties. Also, in the growth of InGaAsP the As/P ratio is equally important for lattice match and quality device results. We report on the uniformity of amorphous arsenic (a-As) films condensed on the surface of 3" diameter sapphire wafers slightly below room temperature. Sapphire substrates were used in place of GaAs wafers to facilitate the use of x-ray fluorescence for the measurement of the arsenic film uniformity. Uniformity maps will be presented for rotated and non-rotated a-As depositions of As2 and As4. Results indicate the uniformity of a-As, As4, films to have a standard deviation of < 2.5% across a 3" diameter substrate for the rotated case. Characteristics of the non-rotated case will be presented.
Dopant-Defect Interactions in II-VI Semiconductors Studied by PAC
Authors: Wichert Thomas
Published: April 2011

The application of the perturbed γγ angular correlation technique (PAC) as an analytical tool for the characterisation of atomic defect configurations is discussed, using recent results on donor-acceptor pairs, which were observed in CdTe and other II-VI semiconductors by the radioactive donor 111In. For bulk CdTe crystals and MOCVD grown CdTe films, the role of the cation vacancy, group I and group V elements is discussed.
Hydrogen Passivation of Defects in Electron Irradiated Polycrystalline Silicon Solar Cells
Authors: Bilyalov R.R. , Abdurakhmanov B.M.
Published: April 2011

The effect of hydrogen passivation on photovoltaic performance of 1 MeV electron irradiated polycrystalline cast silicon solar cells is described. These cells were processed on cast p-type boron doped polycrystalline silicon substrates using standard technology. Passivation was made by low-energy hydrogen ion implantation on the front side. Cells performance was measured as a function of fluence, and it was found that the hydrogenated cell had the higher radiation resistance.

Defect behavior were studied using deep level transient spectroscopy and infra-red spectroscopy. It was shown that the concentration of vacancies (Ec −0,09 eV), divacancies (Ec −0,23 eV) and A-centers (Ec −0,18 eV) is significantly lower in hydrogenated samples. This consistency strengthens the belief that hydrogen interacts with vacancy-type defects to prevent formation of the secondary radiation defects. It is confirmed by IR-measurements.
Hydrogen Diffusion in n-GaAs:Si Under Hydrostatic Pressure
Authors: Machayekhi B. , Chevallier J. , Theys B. , Besson J. M. , Weill G. , Syfosse G.
Published: April 2011

It has recently been shown that deuterium diffusion experiments can provide information on the deepening of the hydrogen acceptor level in the band gap of AlxGa1-xAs alloys with increasing x. In the present work, we report on the influence of hydrostatic pressure on deuterium diffusion in n-GaAs:Si. SIMS analysis reveals that the deuterium profiles in n-GaAs:Si are sensitive to hydrostatic pressure: the diffusion depth decreases and a plateau appears in the diffusion profile as the pressure is applied. The results are interpreted in terms of an increasing amount of the H- species as pressure is applied. This increase is mainly attributed to a deepening of the H acceptor level with respect to the bottom of the Γ conduction band of GaAs. Qualitatively, this effect is similar to the deepening of the H acceptor level in AlxGa1-xAs alloys as x increases.