bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of The ge Content on The Radiation Hardness of Hetero-Junction Diodes in Sige Strained Layers
Authors: Ohyama H. , Simoen E. , Claeys C. , Takami Y. , Hayama K. , Hakata T. , Tokuyama J. , Kobayashi K. , Sunaga H. , Poortmans J. , Caymax M.
Published: April 2011

The degradation of the electrical performance of strained Si1-xGex epitaxial diodes by 220-MeV carbon particles is reported and compared with the effect of 20-MeV alpha rays and 20-MeV protons. The macroscopic damage is studied in a broad fluence range and for different Ge contents, ranging from 8 to 16 %. It is shown that the radiation damage of carbon irradiated diodes is about one order of magnitude larger than that for alpha ray irradiation, which can be explained by considering the difference of the nonionizing energy loss (NIEL). It is observed that the reverse current at a fixed bias increases with increasing fluence, while the rate of increase decreases with increasing fluence and/or Ge content. The fact that a close to square root dependence exists between the boron deactivation in the diode depletion region, derived from capacitance-voltage measurements and the reverse current increase suggests that the device degradation is dominated by radiation induced deep levels associated with interstitial boron complexes.
Growth of Ain on Si(111) by Plasma-Assisted Molecular Beam Epitaxy: Application to Surface Acoustic Wave Devices
Authors: Schenk H. P. D. , Kipshidze G. D. , Weihnacht M. , Weihnach M. , Kaiser U. , Schulze J. , Richter Wo.
Published: April 2011

Epitaxial wurtzite aluminum nitride (AIN) films have been grown on (111) oriented silicon substrates by plasma-assisted solid source molecular beam epitaxy (MBE). The growth has been performed in the two-dimensional (2D) growth mode. Occuring surface reconstructions have been monitored by reflection of high-energy electron diffraction (RHEED). The films have been characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). 2D grown films exhibit atomically smooth surfaces, are singlecrystalline and void of oriented domains. High-resolution (HR) TEM micrographs of the heterointerface show the presence of an interfacial atomic arrangement where (5:4) unit cells coincide along the AIN[2110]/Si[110] direction.

Surface acoustic wave (SAW) properties of MBE-grown AIN(0001)/Si(ll1) have been studied for the first time. The AIN film exhibits very good piezoelectricity. The SAW time response indicates low scattering of the waves during their propagation. An electromechanical coupling coefficient (EMC) of 0.07% has been measured for the present interdigital transducer (IDT) geometry.
Si/SiGe and III-V Integrated Circuit Technology for Next Generation High-Speed Systems: Comparisons and Tradeoffs
Authors: Larson Lawrence E.
Published: April 2011

This paper will summarize the technology tradeoffs that are involved in the implementation of high-speed integrated circuit technology for communications applications. The advantages of Si/SiGe and III-V technology with respect to CMOS and Si bipolar technologies are discussed.
Improved process window using low-carbon Gexsil-x-yCyEpitaxial layers
Authors: Mocuta A. , Greve D.W. , Strong R.M.
Published: April 2011

Processing of silicon-based heterojunction devices is severely constrained by the relaxation of strained epitaxial layers. Generally the equilbrium critical thickness cannot be exceeded if high-temperature process steps such as oxidation and diffusion are performed. In this paper, we report on the beneficial effects of small amounts of carbon ( 0.2%) added to germanium-silicon epitaxial layers. We will show that such low concentrations result in a substantial decrease of boron diffusivity and strain relaxation. We will also report on the fabrication of GexSil-x-yCy heterostructure MOS capacitors with a channel thickness of 300 A° and a maximum germanium fraction of 50% A thermal oxidation at 800 ‘C was performed resulting in good C(VG) characteristics along with improved hole confinement.
Heteroepitaxy of GaAs on CaF2/Si(111) By Surface Free Energy Modulation Method
Authors: Kawasaki K. , Tsutsui K.
Published: April 2011

An epitaxial GaAs layer was grown on a CaF2 (111) by means of surface free energy modulation of CaF2 by one-monolayer-height island formation. An additional low-temperature growth at a final stage of growth of the CaF2 layer on Si(111) produced high density islands whose height was one monolayer of CaF2. This surface structure contributed to enhance wettability of overgrown GaAs. Two-step growth sequence was also examined in order to grow GaAs layer on the modified CaF2/Si(111) with good crystallinity. The electron mobility of 2,000cm2/Vs in a Si doped GaAs layer (6×1017 cm−3) was obtained by the combination of the surface energy modulation method and the two-step growth method.
Computer Simulation of Defect Production and Behaviour in Displacement Cascades in Metals
Authors: Bacon D.J. , Gao F. , Barashev A.V. , Osetsky Yu.N.
Published: April 2011

Recent research using molecular dynamics to simulate radiation damage due to displacement cascades in metals is reviewed. It includes results dealing with the effect on defect formation of primary knock-on atom energy and irradiation temperature. Clear dependencies and trends have emerged in these areas. In terms of the development of models to describe the evolution of radiation damage microstructure, the important parameters are not only the total number of Frenkel defects but also the distribution of their population in clusters and the form and mobility of these clusters. Results on these aspects are reviewed and it is shown that computer simulation is providing detailed information that paves the way for successful development of models of the evolution of damage beyond the stage of the cascade process.
Computation of Microsegregation and Microstructure in Solidification with Fluid Convection
Authors: Juric D.
Published: April 2011

The effects of fluid flow on the dendritic solidification morphology of pure materials is studied using a computational methodology based on a two-dimensional front tracking/finite difference method. A general single-field formulation is presented for the full coupling of phase change, fluid flow, and heat transport in both the solid and liquid phases. This formulation accounts for interfacial rejection/absorption of latent heat, interfacial anisotropies, discontinuities in material properties between the liquid and solid phases, shrinkage/expansion upon solidification and motion and deformation of the solid. Numerical results are presented for the dendritic solidification of pure succinonitrile in a shear flow. Comparison with solidification into a quiescent liquid indicates that fluid convection increases the overall rate of solidification while the growth rate of the leading dendrite tip is unchanged.
Atomistic Simulation of Mobile Defect Clusters in Metals
Authors: Osetsky Yu.N. , Bacon D.J. , Serra A.
Published: April 2011

The structure, stability and thermally-activated motion of interstitial and vacancy clusters in Fe and Cu have been studied using atomic scale computer simulation. All studied interstitial clusters and perfect interstitial loops (PILs) in Fe are mobile whereas their mobility in Cu can be suppressed at large sizes (bigger than 49-61 self-interstitials depending on the temperature) due to dissociation. A comparative study of relaxed configurations has shown that the structure of small perfect dislocation loops of vacancy and self-interstitial nature is very similar. Molecular dynamics simulation has demonstrated that small perfect vacancy loops (PVLs) in Fe consisting of more than 37 vacancies are stable over a wide temperature range and produce atomic displacements by a thermally-activated movement in the direction of the Burgers vector. The mechanism is qualitatively similar to that of SIA clusters studied earlier. Motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking fault energy.
Molecular Dynamics Simulations of Crystal Growth from Melted silicon: Defect Formation Processes
Authors: Ishimaru Manabu , Motooka Teruaki
Published: April 2011

Molecular dynamics calculations have been performed to simulate crystal growth from melted silicon (Si) and defect formation processes based on the ordinary Langevin equation employing the Tersoff interatomic potential. The findings of this investigation are as follows: (i) The [110] bonds at the solid-liquid interface induce the eclipsed configurations or hexagonal Si structures which stabilize microfacets composed of the {111} planes. (ii) Defect formation during crystal growth processes is due to misorientations at the {111} interfaces which result in an “elementary” grown-in defect structure including five- and seven-member rings. (iii) The “elementary” grown-in defect migrates in c-Si by bond-switching motions during further crystal pulling or annealing.
Modeling of Failure in Metallic Thin Films Induced by Stress and Electromigration: A Multiscale Computational Analysis
Authors: Gungor M. R. , Gray L. J. , Zhou S. J. , Maroudas D.
Published: April 2011

A common failure mechanism in metallic thin-film interconnects is void propagation driven by electric fields and thermomechanical stresses. In this paper, a multiscale computational analysis is presented for predictive modeling of transgranular void dynamics. The modeling approach is hierarchical and involves atomistic simulations for property database development, molecular-dynamics simulations for understanding of void-tip mechanisms, and self-consistent mesoscopic simulations based on boundary-element methods and techniques for moving boundary propagation. An extremely rich void dynamical behavior is predicted, which includes faceting, facet selection, propagation of slits from the void surface, as well as formation of fine-scale crack-like features on the void surface, in agreement with recent experimental data.
Vacancy Migration Barrier Energetics and Pathways in Silica
Authors: Corrales L. R. , Ginhoven R.M. Van , Song J. , Jónsson H.
Published: April 2011

A study of vacancy defect migration pathways and energetics in a-quartz is carried out using an empirical simulation methodology that is coupled with the nudged elastic band method. Results from this study indicate that the migration pathway for migration is between adjacent sites. We anticipate the results will guide modifications to empirical potentials for use in the study of point defect formation of more complex systems.
Primary and Secondary Grain Boundary Dislocations in Symmetric Tilt Grain Boundaries of Finite Length
Authors: Lakshmanan Annamalai , King Alexander H.
Published: April 2011

Secondary grain boundary dislocations in large-angle grain boundaries (also called dsc-dislocations) can be described as local variations in the density of the primary (or lattice) dislocations that make up the boundary. We present a simple meso-scale simulation in which the interactions of primary dislocations with each other and with the crystal lattice produce secondary dislocations with smaller Burgers vectors and larger spacing. We use the model to explore the interactions of the primary and secondary defects with a free surface terminating the grain boundary, and demonstrate cases in which the primary dislocations dominate the interactions, forcing the secondary dislocations to increase their energy. Other cases are found, for which all of the dislocations can respond in such a way that their energy decreases. The creation of interfacial disclinations is also demonstrated.
Digital Material: A Framework for Multiscale Modeling of Defects in Solids
Authors: Myers C. R. , Arwade S.R. , Iesulauro E. , Wawrzynek P.A. , Grigoriu M. , Ingraffea A.R. , Dawson P.R. , Miller M.P. , Sethna J.P.
Published: April 2011

We are designing and developing a novel software environment - the Digital Material -to support materials modeling across many length and time scales, in order to develop improved descriptions of material structure and evolution. Software support is required for high-performance numerical kernels, lightweight infrastructures for prototyping, steering and analysis, information transfer across scales, coupling of disparate simulation modules, and collaboration between a multidisciplinary collection of researchers.
Modeling of Inelastic Deformation of f.c.c, Single- and Polycrystalline Materials with Low Stacking Fault Energies
Authors: Staroselsky A. , Anand L.
Published: April 2011

A new rate-independent constitutive model for plastic deformation of crystalline materials deforming by slip and twinning has been formulated, and implemented in a finite-element program. We have simulated three different structural levels by choosing representative volume elements (RVEs) as (i) a small part of a single crystal for the analysis of the heterogeneity of plastic deformation in single crystals, (ii) a whole single crystal for polycrystal simulations, and (iii) a group of crystals for a Taylor-type model of polycrystals. We show that the predictions for the texture and stress-strain response from the model are in reasonably good agreement with experiments in plane-strain compression for a different single crystal and polycrystalline f.c.c, materials.
AB Initio Study of Changes in the Magnetism of Iron During the BCC-HCP Phase Transformation
Authors: Šob M. , Friák M. , Wang L.G. , Vitek V.
Published: April 2011

The pressure-induced bcc-hcp transformation in iron is studied ab initio by following a constant-volume one-parameter transformation path. The calculations are performed in the generalized gradient approximation (GGA). It is shown that the analysis of both spin-polarized and non-spin-polarized states is essential in order to correctly obtain the energetics of the phase transformation. Our calculations make it possible to locate, with high precision, the transition configuration at which the bcc ferromagnetic structure transforms into the hcp nonmagnetic structure.
Dynamics of Dissociated Dislocations in SI: A Micro-Meso Simulation Methodology
Authors: Cai W. , Bulatov V.V. , Justo J.F. , Yip S. , Argon A.S.
Published: April 2011

The theory of dislocation motion in materials with high Peierls stress relates dislocation mobility to the underlying kink mechanisms. While one has been able to describe certain qualitative features of dislocation behavior, important details of the atomic core mechanisms are lacking. We present a hybrid micro-meso approach to modeling the mobility of a single dislocation in Si in which the energetics of defect cores and kink mechanisms are treated by atomistic simulation, while dislocation motion under applied stress and at finite temperature is described through kinetic Monte Carlo. Three important aspects pertaining to treating the details of local structure and dynamics of kinks are incorporated in our approach: (1) Realistic complexity of (multiple) kink mechanisms in the dislocation core. (2) Full Peach-Koehler formalism for treatment of curved dislocation. (3) Detailed investigation of interaction between partials. This simulation methodology is used to calculate micron-scale dislocation mobility, with no adjustable parameters; specifically we obtain temperature and stress dependent velocity results that can be compared with experimental measurements.
Three-Dimensional Numerical Simulation of Interacting Dislocations in a Strained Epitaxial Surface Layer
Authors: Kuktat R. V. , Freundt L. B.
Published: April 2011

In the context of elastic dislocation theory, the glide of arbitrarily shaped dislocations near a free surface is simulated numerically. The simulation is made possible by utilizing a compact line-integral representation of the elastic field produced by a piecewise planar dislocation loop near a free surface. The interaction between a threading dislocation and a misfit dislocation in its path is simulated and the blocking effect is shown to be much more significant than found in simulations which neglect free surface effects.
Molecular Dynamics Simulation of Neutron Damage in β-SIC
Authors: Perlado J.M. , Malerba L. , Rubia T. Diaz De La
Published: April 2011

Molecular Dynamics (MD) simulations of neutron damage in β-SiC have been performed using a modified version of the Tersoff potential. The Threshold Displacement Energy (TDE) for Si and C atoms at 300 K has been determined along directions [001], [110], [111] and [ 1 1 1 ]. The existence of recombination barriers, which allow the formation of metastable, temperature-sensitive defects even below the threshold, has been observed. Displacement cascades produced by both C- and Si-recoils of energies spanning from 0.5 keV up to, respectively, 5 keV and 8 keV have also been simulated at 300 K and 1300 K. Their analysis, together with the analysis of damage accumulation (∼3.4×10-3 DPA) at 1300 K, reveals that the two sub-lattices exhibit opposite responses to irradiation: whereas only a little damage is produced on the “ductile” Si sub-lattice, many point-defects accumulate on the much more “fragile” C sub-lattice. A preliminary study of the nature and clustering tendency of these defects is performed. The possibility of disorder-induced amorphization is considered and the preliminary result is that no amorphization takes place at the dose and temperature simulated.
Damage Response to Irradiation Temperature and Ion Fluence In C+-Irradiated 6H-SiC
Authors: Jiang W. , Weber W.J. , Thevuthasan S.
Published: April 2011

Irradiation experiments have been performed 60° off the surface normal for 6H-SiC single crystals at various temperatures (185-870 K) using 550 keV C+ ions over a fluence range from 1×1018 to 5×109 ions/m2. Atomic disorder on the Si sublattice, as determined by in-situ RBS/channeling analysis, ranged from dilute defects to complete amorphization. The critical amorphization dose of ∼0.23 dpa (on the Si sublattice) at 185 K has been determined. Asymmetric shapes in angular yield profiles across the crystallographic axis <0001> emerged above 1.5×1019 C−/m2 (∼0.05 dpa in the near-surface region), which might be associated with the lattice disturbance in the crystal structure. A gradual decrease in half-angular width was observed with the increase of ion fluence in the experiment. The minimum yield exhibits a rather linear relationship with ion dose at the surface. Post-irradiation annealing at the irradiation temperature did not result in measurable recovery for fluences ranging from 4 × 1018 to 2×1019 C+/m2 at 300, 470 and 670 K. Results also show that low fluence (<8×1018 C+/m2) irradiation at 185 K followed by thermal annealing results in similar defect concentrations to irradiation at that same temperature to the same ion fluence. Thus, at low fluences, the accumulated defects are in thermal equilibrium with the structure.
Microstructural Evolution of Radiation Induced Defects In ZnO During Isochronal Annealing
Authors: Brunner S. , Puff W. , Mascher P. , Balogh A.G.
Published: April 2011

In this study we discuss the microstructural changes after electron and proton irradiation and the thermal evolution of the radiation induced defects during isochronal annealing. The nominally undoped samples were irradiated either with 3 MeV protons to a fluence of 1.2× 1018 p/cm2 or with 1 MeV electrons to a fluence of 1×1018 e/cm2. The investigation was performed with positron lifetime and Doppler-broadening measurements. The measurements were done at room temperature and in some cases down to 10 K to investigate the thermal dependence of the trapping characteristics of the positrons.
Ion-Implantation/Annealing-Induced Precipitation of Nanophase Ferromagnetic Particles In Yttrium-Stabilized ZrO2
Authors: Honda S. , Modine F.A. , Meldrum A. , Budai J.D. , Haynes T.E. , Boatner L.A. , Gea L.A.
Published: April 2011

Ion implantation and thermal processing techniques have been used to form embedded ferromagnetic nanophase precipitates and thereby create magneto-optically active near-surface regions on otherwise inactive materials. Ferromagnetic precipitates were formed by first implanting Fe+ or Ni− into Y0.15Zr0.85O1.93(YSZ) with an implant energy of 140 keV, a fluence of 8.0 × 1016 ions/cm2, and at a temperature of-189°C. After implantation, the specimens were annealed at temperatures ranging from 500 to 1100°C in several types of reducing atmospheres. X-ray diffraction and TEM analysis of the Fe- or Ni-implanted/annealed specimens revealed that crystallographically coherent precipitates of metallic α-Fe, magnetite (Fe3O4), or Ni could be formed in YSZ depending on the annealing conditions. In particular, the cooling rate was established as the critical factor that determined whether Fe or Fe3O4 precipitates were created. Magneto-optical effects arising from ferromagnetic precipitates of Fe, Fe3O4, and Ni in the near-surface region of YSZ were observed and characterized using magnetic circular dichroism (MCD). The magneto-optical response of the α-Fe, Fe3O4, and Ni precipitates was markedly different as indicated by the MCD-detected hysteresis curves. The precipitation mechanism, the chemical nature of the precipitates, and the particle-size distributions resulting from different annealing conditions were investigated and correlated with the precipitate magneto-optical properties.
Low Temperature Ripple Formation: Ion-Induced Effective Surface Diffusion in Ion Sputtering
Authors: Makeev M. A. , Barabási A.-L.
Published: April 2011

Surfaces bombarded with energetic ions may develop a rough or a ripple surface morphology. The ripple formation has been successfully described by the instability caused by preferential erosion, the ripple wavelength being determined by the competition between surface erosion and thermally activated diffusion. However, as recent experiments and computer simulations have shown, ripple formation takes place even at the low temperatures, when thermally activated processes are suppressed. In this paper we propose a theory to explain low-temperature ripple formation based on the ion-induced effective surface diffusion.
Ion Beam Irradiation—An Efficient Method to Modify The Subnanometer Scale Microstructure of Polymers In A Controlled Way
Authors: Xu Xinglong , Coleman M. R. , Myler U. , Simpson P. J.
Published: April 2011

The microstructural evolution of polymers induced by ion beam irradiation was investigated using gas permeation measurements with different molecule size gases and positron annihilation spectroscopy (PAS) using variable-energy positron. Simultaneous large increases in gas permeability and permselectivity of polymer-ceramic composite membranes modified by 180 keV H+ ion irradiation indicated that ion irradiation of polymers can modify the microstructure of polymer at sub-nanometer level in a controlled way. PAS results were consistent with the gas permeation results. The results of this work demonstrated ion beam irradiation has a promising application potential in the separation industry.
Light Ion Irradiation Creep of SCS-6 Silicon Carbide Fibers in the Temperature Range 450 – 1100°C
Authors: Scholz R. , Pasic H.
Published: April 2011

Creep tests were conducted in torsion on Textron SCS-6™ silicon carbide (SiC) fibers during irradiation with light ions in the temperature range 450-1100°C up to doses of 0.06 dpa. The fibers, produced by chemical vapor deposition (CVD), should be representative of a SiC/SiC composite matrix produced by chemical vapor infiltration (CVI).

For temperatures between 450 and 600°C, the irradiation creep curves were characterised by long lasting strain transients during which the creep rate slowed down before reaching approximately steady state values. On a decrease in temperature the creep rate increased.

For temperatures between 900 and 1100°C, the transient creep regime was negligible. The creep rates reached constant values shortly after starting the irradiation and increased with temperature. The activation energy was E = 0.55±0.15 eV.

The results are discussed in terms of concentration and mobility of point defects and the change of these quantities with temperature.
Effect of Low Temperature ion Irradiation on the Microstructure of Nitride Ceramics
Authors: Zinkle S.J. , Snead L.L. , Eatherly W. S. , Jones J.W. , Hensley D.K.
Published: April 2011

Cross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si3N4) and aluminum nitride (AIN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×1020ions/m2(maximum damage of ∼ 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si3N4in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si3N4specimens irradiated at different fluences, it is concluded that the amorphization is primarily controlled by the implanted Si concentration rather than the displacement damage level. Si3N4 amorphization did not occur in regions well-separated from the implanted ions for doses up to at least 3 dpa at 80 K, whereas amorphization occurred in the ion implanted region (calculated Si concentration >0.01 at.%) for damage levels as low as ∼0.6 dpa. The volumetric swelling associated with the amorphization of Si3N4, is <10%. Amorphization was not observed in any of the irradiated AIN specimens. A moderate density of small (∼3 nm) defect clusters were observed in the crystalline damaged regions of both the Si3N4and AIN specimens at both irradiation temperatures. Aligned network dislocations were also observed in the AIN specimen irradiated to high dose at 80 K.
Bubble formation and growth in glasses
Authors: Chen Xidong , Birtcher Robert C. , Donnelly S.E.
Published: April 2011

In this study, we simulated gas bubble formation in glasses by in-situ ion implantation. Alkali silicate glass and Na-borosilicate glass were implanted in situ with 50 keV Xe ions at temperatures at 200°C in a Hitachi-9000 electron microscope. Bubble formation was studied by transmission electron microscopy images taken during interruptions of the ion beam after discrete implanted-ion dose steps. We present a possible mechanism of bubble formation and growth based on amorphous network structures.
Molecular Dynamics Simulation of Recoil Nucleus Displacement Cascade in Zircon
Authors: Crocombette Jean-Paul , Ghaleb Dominique
Published: April 2011

The results of molecular simulations of a-recoil nucleus displacement cascades in zircon (ZrSiO4) are presented. The local temperature and pressure are found to increase dramatically during the cascade. The structure of the cascade tracks is amorphous. Its shape has been analyzed in terms of disordered and distorted cations. SiO2 nanophase are found to exist in the tracks consistently with what is observed in the experiments.
Electron Irradiation of Zeolites
Authors: Wang S. X. , Wang L. M. , Ewing R. C.
Published: April 2011

Three different zeolites (analcime, natrolite, and zeolite-Y) were irradiated with 200 keV and 400 keV electrons. All zeolites amorphized under a relatively low electron fluence. The transformation from the crystalline-to-amorphous state was continuous and homogeneous. The electron fluences for amorphization of the three zeolites at room temperature were: 7.0× 1019 e/cm2 (analcime), 1.8×1020 e/cm2 (natrolite), and 3.4×1020 e/cm2 (zeolite-Y). The different susceptibilities to amorphization are attributed to the different channel sizes in the structures which are the pathways for the release of water molecules and Na+. Natrolite formed bubbles under electron irradiation, even before complete amorphization. Analcime formed bubbles after amorphization. Zeolite-Y did not form bubbles under irradiation. The differences in bubble formation are attributed to the different channel sizes of the three zeolites. The amorphization dose was also measured at different temperatures. An inverse temperature dependence of amorphization dose was observed for all three zeolites: electron dose for amorphization decreased with increasing temperature. This unique temperature effect is attributed to the fact that zeolites are thermally unstable. A semi-empirical model was derived to describe the temperature effect of amorphization in these zeolites.
The Effect of Temperature and Damage Energy on Amorphization in Zircon
Authors: Weber W. J. , Devanathan R. , Meldrum A. , Boatner L. A. , Ewing R. C. , Wang L. M.
Published: April 2011

Several groups have irradiated single crystals of synthetic and natural zircon (ZrSiO4) with a wide range of ions (from He to Bi) over a wide range of temperatures. The results of these studies show that amorphization in zircon is controlled by a variety of parameters and is a more complex process than previously thought. The critical dose for amorphization increases with temperature, similar to other materials. However, the critical displacement dose (in dpa) for amorphization is significantly higher at lower temperatures (below 500 K) for very heavy ions, such as Pb and Bi. This unusual dependence on the damage energy density has not been observed previously in other materials. Possible explanations for this behavior in terms of amorphization mechanisms are discussed. In addition, there is a significant difference in the temperature dependence of the critical dose in synthetic and natural zircons, which suggests that the impurities in natural zircons may affect the kinetics of recovery processes.
Tin E' Centers in X-Ray Irradiated Sn-Doped Silica
Authors: Chiodini N. , Meinardi F. , Morazzoni F. , Paleari A. , Scotti R. , Spinolo G.
Published: April 2011

Sn centers are point defects easily formed by X-ray irradiation of Sn-doped silica. Point defects are a tool for assessing the substitutional character of the dopant in Sn-doped SiO2 glasses with different Sn content (SnO2/SiO2 ratio from 0.01 to 10.0 % w/w) and prepared by two sol-gel methods differing in tin precursors (tin tetraterbutoxide and tin dibutyldiacetate). The former method produced transparent crack-free monolithic glasses of Sn-doped silica with maximum tin content of 0.1 % (SnO2/SiO2 % w/w). The latter method is more efficient and produced transparent glasses with a tin content of 1%.
Modeling of Microstructure Evolution in Austenitic Stainless Steels Irradiated Under Light Water Reactor Conditions
Authors: Gan J. , Was G.S. , Stoller R.E.
Published: April 2011

A model for the development of microstructure during irradiation in fast reactors has been adapted for light water reactor (LWR) irradiation conditions (275 ∼ 325 °C, up to ∼ 10 dpa). The original model was based on the rate-theory, and included descriptions of the evolution of both dislocation loops and cavities. The model was modified by introducing in-cascade interstitial clustering, a term to account for the dose dependence of this clustering, and mobility of interstitial clusters. The purpose of this work was to understand microstructural development under LWR irradiation with a focus on loop nucleation and saturation of loop density. It was demonstrated that in-cascade interstitial clustering dominates loop nucleation in neutron irradiation in LWRs. Furthermore it was shown that the dose dependence of in-cascade interstitial clustering is needed to account for saturation behavior as commonly observed. Both quasi-steady-state (QSS) and non-steady-state (NSS) solutions to the rate equations were obtained. The difference between QSS and NSS treatments in the calculation of defect concentration is reduced at LWR temperature when in-cascade interstitial clustering dominates loop nucleation. The mobility of interstitial clusters was also investigated and its impact on loop density is to reduce the nucleation term. The ultimate goal of this study is to combine the evolution of microstructure and microchemistry together to account for the radiation damage in austenitic stainless steels.
Cr-Vacancy Elastic and Chemical Interactions in Irradiated Stainless Steels
Authors: Simonen E. P. , Bruemmer S. M.
Published: April 2011

Interactions between point defects and major solute strongly influence grain boundary concentrations during heat treatment, irradiation and annealing of austenitic stainless steels. Previous approaches to nonequilibrium segregation emphasize only elastic defect-solute interactions. The present evaluation of nonequilibrium concentrations at grain boundaries indicates chemical interactions unique to solution annealing and cooling during thermal nonequilibrium segregation (TNES). Subsequent to TNES, radiation-induced segregation and post-irradiation annealing are modeled and compared with measured changes in grain boundary composition. The latter two mechanisms are controlled by exchanges between vacancies and major solute such as Cr.
Evidence for Thermal Spike Effects in Ion-Irradiated Ni3Al
Authors: Jenkins M. L. , Mavani P. , Müller S. , Abromeit C.
Published: April 2011

The influence of the irradiation temperature Tirr on the development of disordered zones produced at displacement cascades in Ni3A1 by heavy-ion irradiation with 50 keV Ta+ and 300 keV Ni+ ions has been investigated. The normalised number density (yield) of disordered zones for 300 keV Ni+ irradiation showed a sharp fall between Tirr= 373 K and 573 K. For 50 keV Ni+ irradiation there was a similar fall between 573 K and 673 K. The mean diameters of the disordered zones produced by 300 keV Ni+ ions decreased by about 2 nm between room temperature and 573 K, and there was a tendency for larger zones to become more regular in shape. For 50 keV Ta+ ions, a similar trend was observed between 573 K and 873 K. An annealing experiment confirmed that disordered zones produced at lower temperatures were stable up to a temperature of about 673 K, showing that these trends cannot be due to thermal annealing of disordered zones. The experimental results are consistent with an increased tendency for reordering at the peripheries of disordered zones, due to the increased lifetimes of thermal spikes at higher irradiation temperatures.
Sink Strength Evolution of Heavy Ion Irradiated Nickel
Authors: Fielitz P. , Macht M.-P. , Naundorf V. , Wollenberger H.
Published: April 2011

Atom transport under irradiation is determined by the concentration of freely migrating defects, which depends on the dynamical equilibrium between production and annihilation rates. In order to determine effective values of both of these quantities for the case of ion irradiation, spatially resolved self-diffusion measurements were performed on single crystals of nickel which contained several thin tracer layers at different depths.

For fixed depth the radiation-enhanced diffusion coefficient (DK) was determined as function of displacement rate (K0) and fluence (Φ). The DK essentially representing the ratio of the rates of production and annihilation was found to be proportional to K0 for 800 K irradiation temperature and to K00.4for Ni and K00.4for Kr irradiation at 950 K. It is independent of Φ for 800 K and decreases with increasing Φ for 950 K.
Craters Produced on Metals by Single Ion Impacts
Authors: Birtcher R. C. , Donnelly S. E.
Published: April 2011

summary

The surface is a major perturbation on displacement cascades resulting from ion impacts. Ion impacts on dense materials result in nanometer sized craters on materials with densities greater than 7.3 g/cm3. Although stable when the irradiation is interrupted, single ion impacts can modify or annihilate existing craters. Single ion impacts can extrude material from the specimen. Flow from the melt zone of cascades is responsible for the observed changes.
Effects of Neutron Irradiation on Tensile Properties of V-Ti-Cr-Si Type Alloys with Pre-Existing Helium
Authors: Satou M. , Chuto T. , Koide H. , Hasegawa A. , Abe K.
Published: April 2011

Helium effect on loss of uniform elongation after low-temperature neutron irradiation of the V-Ti-Cr-Si-Al-Y alloy was studied. Helium implantation to about 30 atomic ppm was carried out before neutron irradiation to 50 dpa at 406°C. The yield stress of the irradiated specimen with helium pre-implantation was slightly smaller than that of irradiated specimens without helium. The uniform elongation and the increase in yield stress of the irradiated specimens were not affected by helium pre-implantation at ambient temperature. It might be possible that the helium effect appears after lower temperature irradiation such as 300°C or lower, which corresponds to the temperature range where the loss of uniform elongation of the alloy is particularly pronounced.
Effect of Formation and Growth of Dislocation Loops and Cavities on Low-Temperature Swelling of Irradiated Uranium-Molybdenum Alloys
Authors: Rest J. , Hofman G. L. , Konovalov I. I. , Maslov A. A.
Published: April 2011

Scanning electron photomicrographs of U–10 wt.% Mo irradiated at low temperature in the Advanced Test Reactor (ATR) to about 40 at.% burnup show the presence of cavities. We have used a rate-theory-based model to investigate the nucleation and growth of cavities during low-temperature irradiation of uranium-molybdenum alloys in the presence of irradiation-induced interstitial-loop formation and growth. Our calculations indicate that the swelling mechanism in the U–10 wt.% Mo alloy at low irradiation temperatures is fission-gas driven. The calculations also indicate that the observed bubbles must be associated with a subgrain structure. Calculated bubble-size-distributions are compared with irradiation data.
Kinetic Monte Carlo Simulations of Fecu Alloys
Authors: Domain C. , Becquart C.S. , Van Duysen J.C.
Published: April 2011

The steel vessels of pressurized water reactors are embrittled by neutron irradiation. It is well known that copper atoms play an important role in the embrittlement and that different Cu- containing defects such as Cu-rich clusters (sometimes called atmospheres), Cu precipitates and Cu-vacancy complexes have been identified experimentally. It is still difficult to link the formation of these defects to the primary damage resulting from the neutron inducing displacement cascades. Therefore, we investigate the evolution of the primary damage in FeCu alloys using kinetic Monte Carlo simulations based on a vacancy diffusion mechanism. The calculations rely on adapted, phenomenological, n-body potentials that satisfactorily reproduce properties of FeCu. At room temperature, experimentally identified defects such as Cu-vacancy complexes (one Cu atom bound to three or four vacancies) form in the course of our simulations. Furthermore, it appears that complex defects such as a Cu atom linked to two vacancies are very mobile and are responsible for the formation of small Cu clusters.
Orientation and Temperature Dependence of Electron-Induced Crystallization in Si and Ge
Authors: Hollar E. P. , Robertson I. M. , Jeničič I.
Published: April 2011

Si and Ge samples of different substrate orientations were implanted with 50 keV Xe+ ions to a dose around 1011 ions/cm2 where the amorphous zones, created by individual ions, remained spatially isolated. The samples were subsequently irradiated at either 90 or 300K with electrons having energies from 25 to 300 keV in a transmission electron microscope (TEM). At all electron energies and at both temperatures a significant fraction of amorphous zones crystallized, showing that this effect is not due to a temperature increase and occurs at energies below the threshold displacement energy. Preliminary results show that in Ge the crystallization rate depends on the substrate orientation, while in Si this effect was not observed. The results are discussed in terms of possible explanations for epitaxial growth.
Point Defect Cluster Formation in Iron Displacement Cascades UP to 50 keV
Authors: Stoller Roger E.
Published: April 2011

The results of molecular dynamics displacement cascade simulations in iron at energies up to 50 keV and temperatures of 100, 600, and 900K are summarized, with a focus on the characterization of interstitial and vacancy clusters that are formed directly within the cascade. The fraction of the surviving point defects contained in clusters, and the size distributions of these in-cascade clusters have been determined. Although the formation of true vacancy clusters appears to be inhibited in iron, a significant degree of vacancy site correlation was observed. These well correlated arrangements of vacancies can be considered nascent clusters, and they have been observed to coalesce during longer term Monte Carlo simulations which permit short range vacancy diffusion. Extensive interstitial clustering was observed. The temperature and cascade energy dependence of the cluster size distributions are discussed in terms of their relevance to microstructural evolution and mechanical property changes in irradiated iron-based alloys.
Kinetic Monte Carlo Annealing Simulation of Damage Produced by Cascades in Alpha-Iron
Authors: Gao F. , Bacon D. J. , Barashev A. V. , Heinisch H. L.
Published: April 2011

The mobile defects created in displacement cascades can either interact within the cascade region or undergo long-range diffusion in the crystal. The kinetic Monte Carlo code ALSOME has been used in the present work to carry out annealing simulations of electron irradiation and single cascades with energy in the range of 2 to 40 keV in α-Fe as a function of temperature. Isochronal annealing of electron irradiation shows a temperature-dependence of the recovery stages that is reasonably close to experiment, but Stage I is controlled by the rotation energy of the <110> dumbbell to the <111> crowdion. The annealing of single cascades has demonstrated that nearly 60% of SIAs formed in the primary state of cascade damage escape from the cascade at temperatures above stage I. Most of the escaping SlAs are in clusters, for only 10% of them are mono-interstitials. Although the number of escaping defects increases with increasing cascade energy, the relative fraction is almost constant for the recoil energies considered. The results are compared with those for copper obtained using the same code [1].
Local Structural Changes of Ion Damaged InGaAs
Authors: Yu Kin Man
Published: April 2011

Ion damage and amorphization behavior in InGaAs with InAs mole fractions in the range of 0 to 50%are studied. We found that the degree of dynamic annealing increases as the InAs mole fraction increases in the InGaAs when the implantation is carried out at room temperature (RIT). Extended x-ray absorption fine structure measurements reveal that in the amorphous state the InAs nearest neighbor distance, RIn-As is very different from that in the crystalline InGaAs and is ∼0.01 Å longer than that in pure crystalline InAs. For RT implanted materials, before a complete amorphous layer is formed, the RIn-As remains close to its crystalline value even when the layer is heavily damaged. A sudden increase of the RIn-As is observed when a complete amorphous layer is formed. The behavior of the measured values of RIn-As for InGaAs implanted with various doses, indicates that at RT the formation of amorphous InGaAs occurs by the simultaneous nucleation of the amorphous phase when the critical free energy in the damage layer is exceeded. At liquid nitrogen temperature, when dynamic annealing is negligible, the RIn-As value increases as the damage in the layer increases, suggesting that the amorphous InGaAs is formed by the accumulation and overlapping of amorphous zones created along the individual ion tracks.
Fabrication of Piezoelectric Ceramic / Polymer Composite Transducers using Fused Deposition of Ceramics
Authors: Lous G. M. , Cornejo I. A. , McNulty T. F. , Safari A. , Danfortha S. C.
Published: April 2011

The Fused Deposition of Ceramics (FDC) technique was used to fabricate ceramic skeletons for development of piezoelectric composite transducers for medical imaging. The green parts were designed in order to have 30 vol% of PZT-5H ceramic in the final composites. Physical characterization of the sintered samples revealed that 96% of the theoretical density was achieved. Optical microscopy showed that defects, such as small roads and bubbles were eliminated due to powder processing improvements. The electromechanical properties of the final composites were found to be similar to properties obtained for conventionally made composites.
Effects of Additives on Thermomechanical Properties of Composites Formed from Thermoset Polymers
Authors: Radisic A. , Valimbe P. S. , Malhotra V. M.
Published: April 2011

Automotive and heavy-duty friction materials are complex multimaterial composites that contain, besides polymers and fibers, a number of constituents whose exact role is not well delineated. Since the performance of these composites requires not only uniform frictional properties but also complex shapes, advanced formation techniques are required. However, before these techniques can be harnessed, we attempted to understand how nitrile rubber-modified phenolic polymer's structural and thermal behaviors were modified by the incorporation of materials such as BaSO4, graphite, coal tar pitch, slag fibers, alumina, and silicon carbide, typical friction composite ingredients. The formed composites were examined by FTIR, DSC, and DMA techniques at 300 K < T < 600 K. Our results suggested that the incorporation of ingredients, singly and additively, strongly affected the thermomechanical properties of the formed
Gradient and Discrete Region Composites Produced by Solid Free Form Fabrication Techniques
Authors: Crockett R. S. , Gervasi V. R.
Published: April 2011

Stereolithography patterns have been used to create composites with gradient properties. Patterns are produced with an open cellular or columnar structure inside a surface shell. This serves as a host for filler materials, generally epoxies or polyurethane matrices loaded with various fibers or microspheres. Regions within a single object may be separated by thin barriers, allowing filling with different matrix materials to create regions of differing local properties.
The internal structure can also be continually gradated in dimension to produce composites with properties ranging from that of the filler material to that of the SFF material, currently epoxy or ABS plastic. This is a general set of processes that can be applied to a variety of existing Solid Freeform Fabrication systems.
Synthesis and Thermoelectric Properties of Cs2Bi7.33Se12, A2Bi8Se13 (A = Rb, Cs), Ba4-xBi6+2/3xSe13, and Ba3±xPb3±xBi6Se15
Authors: Iordanidis Lykourgos , Brazis Paul W. , Kannewurf Carl R. , Kanatzidis Mercouri G.
Published: April 2011

We are continuing our synthetic investigations in the ternary A/Bi/Se systems and also expanded our interests into the quaternary A/M/Bi/Se (A = Rb, Cs, Sr and Ba; M = lanthanide or Pb) systems. We have synthesized several new ternary and quaternary bismuth selenides with band gaps <0.6eV such as Cs2Bi7.33Se12, A2Bi8Se13 (A = Rb, Cs), Ba4-xBi6+2/3xSe13, and Ba3±xPb3±xBi6Se15. The synthesis, crystal structures and charge transports properties of these new compounds are presented.
Solid State Chemistry Approach to Advanced Thermoelectrics. Ternary and Quaternary Alkali Metal Bismuth Chalcogenides as Thermoelectric Materials
Authors: Kanatzidis Mercouri G. , Chung Duck-Young , Iordanidis Lykourgos , Choi Kyoung-Shin , Brazis Paul , Rocci Melissa , Hogan Tim , Kannewurt Carl
Published: April 2011

Our exploratory research to identify new promising candidates for next generation thermoelectric applications has produced several interesting new materials which are briefly described here. We present their compositions, solid state structures, properties and charge transport behavior. The compounds CsBi4Te6, β-K2Bi8Se13, Ba4Bi6Se13, Eu2Pb2Bi6Se13, KBi6.33S10, Eu2Pb2Bi4Se10, Ba2Pb2Bi6S13 and K1.25 Pb3.5Bi7.25Se15 are particularly noteworthy.