bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect Of Orientation On Electrically Conducting Polymer Composite Properties
Authors: Genetti W. B. , Grady B. P. , O'Rear E. A.
Published: April 2011

Conductive polymer matrix composites have been a focus of research for the past 30 years, due to applications such as electromagnetic shielding. The electrical resistance of these materials can be broken into three major categories: the intrinsic resistance of the filler and matrix, the particle‐particle contact resistance, and the tunneling resistance. Both the particle‐particle contact and tunneling resistance in nickel filled low density polyethylene (LDPE) composites can be reduced by the addition of an ultrathin polypyrrole (PPy) coating on the nickel surface. Coating the nickel particles with PPy leads to increases in conductivity as much as three order of magnitude at concentrations well above the percolation threshold without significantly changing the mechanical properties. We believe at concentrations above the critical region PPy forms “molecular wires” reducing the loss of conductivity associated with the uniaxial orientation of the polymer matrix.
A Conversion-Electron Mssbauer Study of Tin-Based Compounds Formed During Pads Treatment (Plasma-Assisted Dry Soldering) of a Solder Foil
Authors: Reno Robert C. , Piekarski Brett
Published: April 2011

We have used conversionelectron Mössbauer spectroscopy (CEMS) to examine the surfaces of solder foils treated in a plasma assisted dry soldering (PADS) system which has been developed1 to convert surface oxides to more desirable fluorides in order to protect the solder surface prior to assembly and to eliminate the need for flux during the soldering process. CEMS samples only the outer 1 μm of a solder foil and this, coupled with the fortuitous fact2 that Mössbauer spectroscopy is 10–20 times more sensitive to oxides and fluorides than to the metallic form of solder, means that surface layers of oxides and fluorides as thin as 10 nm can be detected and chemically identified. We have followed the evolution of surface layers produced in PADS treatments ranging from 7 minutes to several hours. Our results show that fully treated foils contain surface layers of SnF2 and Sn2F6. Partially treated foils also contain some SnO2 and perhaps a small amount of SnO. No Sn2OF2 or SnF4 was produced during any portion of the PADS treatment cycle.
Electromagnetic Interference Shielding Reaching 130 Db Using Flexible Graphite
Authors: Xiangcheng Luo D.D.L. Chung
Published: April 2011

Electromagnetic interference (EMI) shielding effectiveness reaching 130 dB at 1 GHz was achieved by using flexible graphite (Grafoil). Due to the resilience of flexible graphite, it is particularly attractive as an EMI gasket. The high shielding effectiveness is due to the large specific surface area and large skin depth. In contrast, the shielding effectiveness was only 90 dB for polycrystalline graphite of low specific surface area.
Multilayer Composite Diamond Heat Spreaders for Electronic Packaging
Authors: Jagannadham K. , Fan W. D. , Dinwidde R. B. , Narayan J.
Published: April 2011

Heat spreader characteristics of single layer and multilayer diamond composite substrates are determined by bonding to electronic device wafers. Preparation of diamond substrates, metallization and bonding procedures to device wafers are described. Infrared microscopy imaging of the bonded device wafers is used to determine the heat spreading characteristics. Advantages associated with multilayer diamond composite heat spreaders are discussed.
Mcm Development Using Fine Pitch Ball Bump
Authors: Dohya Akihiro , Morisaki Ikushi , Bonkohara Manabu , Nakamura Hirofumi
Published: April 2011

A fine-pitch ( less than 100 micro-meter pads pitch ) aluminum pad flip chip assembly technology for use with the built-up Printed Wiring Boards (PWB) has been developed and named “Lead-Less-Chip Assembly (LLC Assembly)” technology. Our newly developed technology consists of three points: Forming gold-ball-bumps on the aluminum pads of the bare LSI chip by a wire ball bonding method. Solder-coating on the copper pads of the built-up PWB using Micro Punching technology or the Super-Juffit technology. Flux-less solder bonding the bare chip onto the PWB with a nitrogen atmosphere.

A reliability evaluation performed on a single chip and multi chip modules assembled using this LLC Assembly technology with test chips and real C-MOS devices showed a high level of endurance. We have applied this technology to Multichip modules (MCMs) for Note Personal Computers and portable terminals.
Pulsed Excimer Laser Processing of AlN/GaN Thin Films
Authors: Wong W. S. , Schloss L. F. , Sudhir G.S. , Linder B. P. , Yu K-M. , Weber E. R. , Sands T. , Cheung N. W.
Published: April 2011

A KrF (248 nm) excimer laser with a 38 ns pulse width was used to study pulsed laser annealing of AIN/GaN bi-layers and dopant activation of Mg-implanted GaN thin films. For the AIN/GaN bi-layers, cathodoluminescence (CL) showed an increase in the intensity of the GaN band-edge peak at 3.47 eV after pulsed laser annealing at an energy density of 2000 mJ/cm2. Rutherford backscattering spectrometry of a Mg-implanted A1N (75 nm thick)/GaN (1.0 μm thick) thin-film heterostructure showed a 20% reduction of the 4He+ backscattering yield after laser annealing at an energy density of 400 mJ/cm2. CL measurements revealed a 410 nm emission peak indicating the incorporation of Mg after laser processing.
Thermal Stability of Pt, Pd, and Ni on GaN
Authors: Duxstad K. J. , Haller E. E. , Yu K.M. , Hirsch M. T. , Imler W. R. , Steigerwald D. A. , Ponce F. A. , Romano L. T.
Published: April 2011

The development of reliable Ohmic and Schottky contacts on GaN will require an understanding of the thermal stability and metallurgy of metal-GaN contact structures. We investigated the behavior of Pt, Pd, and Ni on GaN as a function of annealing temperature. Rutherford backscattering spectrometry, x-ray diffraction, and scanning electron microscopy were used to characterize the interface and surface behavior. 800Å thin metal films were deposited by sputtering on 2μm GaN films grown by metal organic chemical vapor deposition on sapphire substrates. No reactions occur in the Pt, Pd, and Ni systems with annealing up to 800°C. The Pt film begins to form submicron spheres and islands after annealing above 600°C. The Pd and Ni films begin to island with annealing above 700°C. Below these temperatures no structural changes were observed.
Cr/Ni/Au ohmic contacts to the moderately doped p- and n-GaN
Authors: Kim Taek , Yoo Myung C. , Kim Taeil
Published: April 2011

We report new Cr/Ni/Au and Ni/Cr/Au tri-layer metallization schemes for achieving low resistance ohmic contacts to moderately doped p- (∼1 × 1017/cm3), and n-GaN (∼1 × 1018/cm3) respectively. The metallizations were thermally evaporated on 2 μm-thick GaN layers grown on c-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD). Comparisons with bi-layer metallizations such as Ni/Au and Cr/Au were also made. The Cr/Ni/Au contacts showed a low specific contact resistivity of 9.1 × 10−5 Ω⋅cm2 to n-GaN while that of Ni/Cr/Au to p-GaN was 8.3 × 10−2 Ω⋅cm2. The Ni/Cr/Au contacts also showed a low specific contact resistivity of 2.6 × 10−4 Ω⋅cm2 to n-GaN. The Ni/Cr/Au metallization could made reasonable ohmic contacts to p- and n-GaN simultaneously
Structural and Optical Properties of Nitride-Based Heterostructure and Quantum-Well Structure
Authors: Amano H. , Takeuchi T. , Sota S. , Sakai H. , Akasaki I.
Published: April 2011

Structural and optical properties of nitride based heterostructure and quantum well structure were investigated. Both AIGaN and GaInN ternary alloys are found to grow coherently on the underlying GaN layer. Compressive strain of GaInN is found to cause quantum confined Stark effect, thus affects the luminescence properties of nitride-based quantum wells.
GaN-Based MSM uv Photodetectors
Authors: Liang S. , Cai W. , Li Y. , Liu Y. , Lu Y. , Tran C.A. , Karlicek R.F. , Ferguson I.
Published: April 2011

Results obtained from interdigital metal-semiconductor-metal (MSM) type of GaN based UV photodetectors are presented. MSM devices were fabricated using two types of GaN; high-resistive GaN and Mg doped GaN. For the high-resistive GaN detector, the lowest dark current is ∼0.1 nA and the UV responsivity of the device was about 460 AAV at a DC bias of 30 V. The Mg doped GaN exhibited large gains, 1150 A/W at 2.0 V, but at much higher dark currents, 400 nA. The high gain in this device is not well understood but was attributed to an ‘avalanche’ effect and is under investigation. It was found that the surface plasma treatment plays an important role in the device performance. After an appropriate plasma treatment the detectors showed higher responsivity with lower dark current.
On Surface Cracking of Ammonia for MBE Growth of GaN
Authors: Kamp M. , Mayer M. , Pelzmann A. , Ebeling K. J.
Published: April 2011

With the use of ammonia as nitrogen precursor for Group III-Nitrides in an on surface cracking (OSC) approach, MBE becomes a serious competitor to MOVPE. Homoepitaxial GaN exhibits record linewidths of 0.5 meV in PL (4K), whereas GaN grown on c-plane sapphire also reveals reasonable material properties (PL linewidth ≈ 5 meV, n ≈ 1017 cm-3 , μ 220 cm2/Vs). Beside results on hetero- and homoepitaxial growth of GaN, insights into the growth mechanisms are presented. The growth of ternary nitrides is discussed, p- and n-doping as well as first LED results are reported.
TOF-LEIS Characterization and Growth of GaN Thin Films Grown with ECR and NH3
Authors: Kim E. , Bensaoula A. , Rusakova I. , Shultz A. , Waters K.
Published: April 2011

GaN thin films were grown on various substrates by GSMBE using ECR nitrogen and ammonia. The growth of GaN was monitored by real time analysis, time of flight low energy ion scattering (TOF-LEIS) and RHEED.

Growth of GaN on GaAs, ZnO, Ge and Al2O3 was investigated. The substrates’ surfaces were analyzed during pre-growth annealing and during GaN growth. The removal of surface contaminants and the modification of the surface stoichiometry from these surfaces are presented.

GaN films on sapphire (0001) grown under different conditions were examined real time by low energy ion scattering mass spectroscopy of recoiled ion (MSRI) and the relationship between the in-situ surface composition with ex-situ photoluminescence measurement results are discussed.
Deposition of AlN on WS2 (0001) Substrate by Atomic Layer Growth Process
Authors: Chung J-W. , Ohuchi F.S.
Published: April 2011

Close proximity of the lattice constant for tungsten disulfide and aluminum nitride has lead to an investigation to use WS2 as a potential substrate for the growth of AIN. Metal organic chemical vapor deposition(MOCVD) has been develop to fabricated WS2 thin films on Si(001) with their basal planes parallel to the substrate. AIN thin film was subsequently grown by atomic layer growth (ALG) process using dimethylamine-alane (DMEAA) and ammonia (NH3). Deposition conditions for WS2 thin films by MOCVD, and AIN growth on WS2 by ALG are described.
Spatial distribution of electron concentration and strain in bulk GaN single crystals - relation to growth mechanism
Authors: Perlin Piotr , Suski Tadeusz , Polian Alain , Chervin Jean Claude , Litwin-Staszewska Elzbieta , Grzegory Izabella , Porowski Sylwester , Erickson J. W.
Published: April 2011

Micro-infrared reflectivity and micro-Raman scattering have been used to determine free electron density and residual strain distribution in bulk GaN crystals. As-grown samples exhibit significant variation of electron concentration and strain along their surfaces. Increase of electron concentration correlates with the growth direction. The observed inhomogeneities in the properties of bulk GaN crystals can be eliminated by mechanical polishing of the crystal and removing the surface layer or growth figures formed during the sample cooling, which follows its high-pressure, high-temperature growth. Properties of that surface layer can differ from the bulk due to different growth mechanisms and impurity distribution. We suggest that oxygen is a likely candidate for the donor impurity supplying high amount of electrons to the conduction band of GaN.
Electron-Phonon Scattering in Si-Doped GaN
Authors: Wetzel C. , Walukiewicz W. , Ager J. W.
Published: April 2011

Phonon-plasmon scattering in non-resonant Raman spectroscopy is used to determine the free electron concentration in Si doped GaN films. For various doping concentration and variable temperature the correlation with magneto-transport data is established. The freeze-out of the carrier concentration at low temperature is thus observed in a purely optical detection scheme. We observe a very long transient time of several hours for the carrier concentration as a reaction to temperature variation. This indicates an indirect capture and emission process with a very small cross section. The value of the Faust-Henry coefficient is determined.
Effect of the Nitridation of the Sapphire (0001) Substrate on the GaN Growth
Authors: Grandjean N. , Massies J. , Vennègues P. , Laugt M. , Leroux M.
Published: April 2011

The analysis of the sapphire surface nitridation by in situ reflection high-energy electron diffraction evidences the formation of a relaxed AIN layer. Its role on the early stage of the GaN growth is investigated by transmission electron microscopy (TEM). GaN crystallites of high structural quality, with the c axis perpendicular to the sapphire basal plane, are observed when the starting surface is nitridated. On the other hand, the growth of GaN on a bare substrate involves the formation of larger islands with numerous defects. TEM study reveals that the c axis of these latter crystallites is systematically tilted by about 19° with respect to the sapphire basal plane. Actually, this orientation corresponds to a particular epitaxial relationship between GaN and sapphire (0001) substrates. Finally, the optical properties of GaN thin layers are shown to be strongly dependent on the nitridation state of the sapphire surface.
Mapping of Donor Impurities in Gan By Raman Imaging
Authors: Ponce F. A. , Steeds J. W. , Dyer C. D. , Pitt G. D.
Published: April 2011

Raman scattering experiments on silicon-doped GaN show that donor impurities quench the Al(LO) Raman line at 735 cm−1. This is due to interaction between lattice vibrations and the free carrier plasma. The spatial variation of the Al(LO) signal has been imaged directly using newly developed instrumentation. Features with dimension under on micron are observed in faceted GaN crystallites. The variation in free carrier concentration is attributed to preferential incorporation of donor impurities during growth.
Optical Transitions and Recombination Lifetimes in GaN and InGaN Epilayers, and InGaN/GaN and GaN/AlGaN Multiple Quantum Wells
Authors: Smith M. , Lin J. Y. , Jiang H. X. , Khan A. , Chen Q. , Salvador A. , Botchkarev A. , Morkoc H.
Published: April 2011

Time-resolved photoluminescence (PL) has been employed to study the optical transitions and their dynamical processes in GaN and InxGa1-xN epilayers, and GaN/GaN and GaN/ALxGa1-xN multiple quantum wells (MQW). We compare the results from both metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) grown samples. In addition, results are also compared with GaAs/ALxGa1-xAs MQW. It was found for all samples that the low temperature emission lines were dominated by radiative recombination transitions of either localized or free excitons, which demonstrates the high quality and purity of these III-nitride materials.
Bound Exciton Energies, Biaxial Strains, and Defect Microstructures in GaN/AlN/6H-SiC(0001) Heterostructures
Authors: Perry W. G. , Zheleva T. , Linthicum K. J. , Bremser M. D. , Davis R. F. , Shan W. , Song J. J.
Published: April 2011

Biaxial strains resulting from mismatches in thermal expansion coefficients and lattice parameters in 22 GaN films grown on A1N buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured via changes in the c-axis lattice parameter (c). Six of the films were in compression, indicating the residual strain due to lattice mismatch was not relieved. A Poisson's ratio of v=0.18 was calculated. The bound exciton energy (EBx) was a linear function of these strains. The shift in EBx with film stress was 23 meV/GPa. The role of the SiC off-axis tilt was investigated for GaN films grown concurrently on the vicinal and on-axis 6H-SiC substrates. Marked variations in EBx and c were observed, with a maximum shift of ΔEBx = 15 meV and Δc = 0.0042 Å. Threading dislocations densities of ~1010/cm2 and ~108/cm2 were determined for GaN films grown on vicinal and on-axis SiC, respectively. A 0.9% residual compressive strain at the GaN/AIN interface was observed by high resolution transmission electron microscopy (HRTEM). It is proposed that the on-axis SiC substrate does not offer a sufficient density of steps for defect formation to relieve the lattice mismatch between GaN and A1N and A1N and SiC.
The Composition Pulling Effect in InGaN Growth on the GaN and AlGaN Epitaxial Layers Grown by MOVPE
Authors: Kawaguchi Yasutoshi , Shimizu Masaya , Hiramatsu Kazumasa , Sawaki Nobuhiko
Published: April 2011

InGaN has been grown on GaN and AlGaN epitaxial layers by metalorganic vapor phase epitaxy (MOVPE) and ‘the composition pulling effect’ at the initial growth stage of InGaN has been studied in relation to the lattice mismatch between InGaN and the bottom epitaxial layers. Crystalline quality of InGaN is good near the interface of InGaN/GaN and the composition of InGaN is close to that of GaN. With increasing growth thickness, the crystalline quality becomes worse and the indium mole fraction is increased. The composition pulling effect becomes stronger with increasing lattice mismatch.
Improvement of the Luminescing Behaviour of Simmplanted SiO2 Films
Authors: Schuster T. , Dittrich T. , Porteanu H. E. , Fischer T. , Hechtl E. , Petrova-Koch V. , Koch F.
Published: April 2011

In previous work we reported on the observation of continuously tunable photoluminescence in Si+-implanted SiO2-films with moderate intensities. In this paper we demonstrate improved performance of such samples. The photoluminescence intensity increases abruptly up to two orders of magnitude when the anneal temperature is elevated to values higher than 1000…1100°C. This strong photoluminescence degrades less than that of porous silicon. Very fine tunability in the spectral range from 2.1 eV to 1.3 eV is achieved in samples implanted with a graded dose. In the analysis of the results we try to distinguish between the contributions of the Si-nanocrystals and of the oxide related defects.
Self-Assembling Formation of Silicon Quantum Dots by Low Pressure Chemical Vapor Deposition
Authors: Nakagawa K. , Fukuda M. , Miyazaki S. , Hirose M.
Published: April 2011

The growth of Si dots on thermally-grown SiO2/c-Si from a thermal decomposition of pure silane has been systematically studied in the temperature range from 500 to 650°C. It has been suggested mat the Si dot height and dot diameter on as-grown SiO2 are rate-limited by the cohesive action of adsorbed precursors and the thermal decomposition of silane on Si nucleation sites, respectively. The nucleation site on as-grown SiO2 is likely to be generated by the thermal dissociation of surface Si-O bonds. It has been also found that in Si dot formation on OH-terminated SiO2 surface the nucleation density is dramatically enhanced and consequently the dot size and its distribution become small. This implies that surface OH bonds can provide the nucleation sites.
Structural Investigations of Colloidal Semiconductor Nanocrystal Heterostructures: Faceting and Epitaxy
Authors: Kadavanich A. V. , Mews A. , Tolbert S. H. , Peng X. , Schlamp M. C. , Lee J. C. , Alivisatos A. P.
Published: April 2011

We use High Resolution Transmission Microscopy (HRTEM) to study CdSe, CdS/HgS/CdS quantum-dot quantum well (QDQW), and CdSe/CdS core-shell nanocrystals, grown by wet-chemical techniques. The nanocrystals have faceted Wulff polyhedron shapes.

In addition to HRTEM, we employ multi-wavelength anomalous dispersion (MAD) x-ray diffraction. We use computer modeling to help interpret the experimental data.

Growth of a heterogeneous phase proceeds epitaxially preserving the overall shape and point-group symmetry of the original seed nanocrystals, both for wurtzite (CdSe/CdS) and zincblende (CdS/HgS/CdS) type structures.

Recently prepared InAs nanocrystals also show evidence of faceting as observed by HRTEM and may lend themselves equally well to epitaxy.
Synthesis and Optical Properties of MoS2Nanoclusters
Authors: Wilcoxon J. P. , Newcomer P. P. , Samara G. A.
Published: April 2011

Highly crystalline nanoclusters of MoS2were synthesized and their optical absorption and photoluminescence spectra were investigated. Key results include: (1) strong quantum confinement effects with decreasing size; (2) preservation of the quasiparticle (or excitonic) Nature of the optical response for clusters down to ∼2.5 nm in size which are only two unit cells thick; (3) demonstration that 3-D confinement produces energy shifts which are over an order of magnitude larger than those due to 1-D confinement; (4) observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size; and (5) observation of photoluminescence due to both direct and surface recombination.
Optical Properties of Deuterium Terminated Porous Silicon
Authors: Matsumoto Takahiro , Masumoto Yasuaki , Koshida Nobuyoshi
Published: April 2011

We have studied the optical properties of deuterium-terminated porous silicon. The photoluminescence spectrum was different from that of usual hydrogen-terminated porous Si despite porous Si showing both the same structure and the same absorption spectrum. These results indicate that the surface vibration of terminated atoms couples to the quantum confined states.
Effects of Nanocrystalline Structure and Passivation on the Photoluminescent Properties of Porous Silicon Carbide
Authors: Spanier Jonathan E. , Cargill G. S. , Herman Irving P. , Kim Sangsig , Goldstein David R. , Kurtz Anthony D. , Weiss Ben Z.
Published: April 2011

We present the results of an investigation of the dependence of the photoluminescence (PL) spectra on preparation conditions, the resulting microstructure, and post-anodization treatment of porous silicon carbide films which were formed from both p and n-type 6H-SiC substrates. Porous samples were prepared by anodic dissolution under different galvanostatic conditions, resulting in different porosities and crystallite sizes. Selected-area electron diffraction patterns taken on similarly prepared porous silicon carbide (PSC) samples confirmed that the films were monocrystalline. Transmission electron microscopy of as-anodized films revealed an isotropie porous network; a dependence of porosity and nanocrystallite size on porous layer formation current density was established. Some PSC samples were passivated using a short, thermal oxidation treatment. The effects of porosity and crystallite size, and of oxide passivation in these PSC films, on PL spectra and intensity were studied using a 365 nm Kr-ion laser as excitation. Under certain conditions, the spectrally integrated PL intensity of a passivated film is more than 450x that for the original bulk SiC substrate. PL spectra are presented, and possible mechanisms are discussed.
Photoluminescence from Porous Silicon Anodized with Monochromatic Light Illumination
Authors: Matsumoto Takahiro , Masumoto Yasuaki , Arata Go , Mimura Hidenori
Published: April 2011

The effect of monochromatic light during the anodization of porous silicon is investigated from the points of view of structural characterization and photoluminescence. A clear correlation is observed between the photoluminescence peak energy and the bandgap energy determined by the illuminating photon energy. The difference of these two energies is analyzed by a size-dependent Stokes shift.
Auger Ionization of Silicon Nanocrystals
Authors: Averboukh B. , Kovalev D. , Ben Chorin M. , Koch F. , Efros Al.L. , Rosen M.
Published: April 2011

Photoluminescence saturation under intense CW optical excitation and optical degradation of photoluminescence from porous Si are studied. The anisotropy of the luminescence is observed under intense linearly polarized illumination at room temperature and after polarized light induced degradation at low temperature. The Auger process is shown to be responsible for these observations.
Silicon as an Active Biomaterial
Authors: Canham L. T. , Reeves C. L. , Wallis D. J. , Newey J. P. , Houlton M. R. , Sapsford G. J. , Godfrey R. E. , Loni A. , Simons A. J. , Cox T. I. , Ward M. C. L.
Published: April 2011

The response of a range of porous Si and poly Si films to storage in acellular simulated body fluids is summarised and its implications discussed. It is suggested that the combination of VLSI technology, micromachining and surface microstructuring achievable with silicon, could establish this prominent semiconductor as a very useful biomaterial by the next century. The ‘biocompatibility’ of a variety of silicon microstructures, and even bulk silicon has received surprisingly little study, but now warrants detailed in-vitro and in-vivo assessment.
Electroluminescent Devices Made from Silicon Nanocrystals Embedded in Various Host Matrices
Authors: Delgado Gildardo R. , Lee Howard W. H. , Pakbaz Khashayar
Published: April 2011

Si nanocrystals produced by ultrasonic fracturing of porous silicon (PSi) were used to fabricate electroluminescent (EL) devices. The active EL material consists of Si nanocrystals embedded in various host matrices such as polyvinylcarbazole (PVK), polymethylmethacrylate (PMMA), and silica sol-gels. Several device configurations were used to induce EL processes that rely on radiative electron-hole recombination via carrier injection or impact excitation of the nanocrystals. We report on the optical and electrical properties of these devices. We discuss relevant physics pertaining to the Si nanocrystals/host and discuss advantages and disadvantages among the different host matrices.
Growth and Structure of Microcrystalline Silicon Prepared with Glow Discharge at Various Plasma Excitation Frequencies
Authors: Finger F. , Carius R. , Hapke P. , Houben L. , Luysberg M. , Tzolov M.
Published: April 2011

Microcrystalline silicon was prepared with glow discharge deposition from silane/hydrogen mixtures at plasma excitation frequencies in the range 13.56 MHz - 116 MHz. The influence of the plasma excitation frequency on the growth and the structural properties of the material is investigated. At high excitation frequencies, higher growth and etching rates, larger grain sizes with less disorder within the grains, higher crystalline volume fractions, a reduced amorphous but more porous interface layer on glass and quartz substrates, and faster nucleation on amorphous silicon substrates are obtained. The results are discussed within a schematical growth model.
Charge Carrier Absorption in Doped Microcrystalline Silicon Films
Authors: Heintze M. , Lotter E. , Abel C.-D. , Schubert M. B.
Published: April 2011

The infrared absorption in doped microcrystalline silicon thin films is analyzed by modelling the complex permittivity as the sum of the contributions resulting from interband transitions and from absorption by charge carriers. Their density and the drift mobility within grains is described by free carrier motion according to the Drude theory. However, for a good fit to experimental data a small trapping energy, reflecting the effect of grain boundaries, is included in the model. By comparing results obtained from the analysis of infrared data with conductivity and Hall measurements in films grown in a very high frequency (VHF) plasma and by hot-wire chemical vapor deposition (CVD), we show that infrared transmission measurements provide a simple access to transport parameters in these films.
Application of MeV Ion Implantation in the Formation of Nano-Metallic Clusters in Silica
Authors: Ila D. , Wu Z. , Zimmerman R. L. , Sarkisov S. , Smith C. C. , Poker D. B. , Hensley D. K.
Published: April 2011

The implantation of metal ions into photorefractive materials followed by thermal annealing leads to an increase in resonance optical absorption as well as an enhancement of the nonlinear optical properties. We have implanted ions of Au (3.6 MeV), Ag (1.5 MeV) and Cu (2.0 MeV) into pure silica followed by careful heat treatment. Using optical absorption spectrophotometry and rutherford backscattering spectrometry we have measured the cluster size for each heat treatment temperature and determined the activation energies for their formation. The third order electric susceptibility for silica with 2 nm gold clusters has been determined by Z-scan to be 65×10−8 esu.
Self Organized Growth and Ultrafast Electron Dynamics in Metallic Nanoparticles
Authors: Stella A. , Cheyssac P. , De Silvestri S. , Kofman R. , Lanzani G. , Nisoli M. , Tognini P.
Published: April 2011

We present ultrafast transient reflectivity measurements performed on metallic tin nanoparticles with an average radius between 20 and 60 Å in amorphous matrix. The samples, grown using an evaporation-condensation technique, were characterized by a relatively low nanocrystals size dispersion and a negligible clusters-matrix interaction. The excitation decays exhibited a size-dependent behaviour, which is interpreted in terms of the important role played by the electron-surface interactions.
Characterization of Nanosized Silicon Prepared by Mechanical Attrition for High Refractive Index Nanocomposites
Authors: Bhagwagar Dorab E. , Wisnfficki Peter , Papadimtrakopoulos Fotios
Published: April 2011

High pressure nanomilling provides an inexpensive, environmentally conscious method to fabricate large quantities of nanoparticles. The presence of large particle sizes, inherent in mechanical attrition processes pose obstacles in identifying the optical properties of these nanosized particles. The high refractive index and relatively small absorption coefficient of silicon (Si) directed our research efforts towards Si nanoparticles. We presently report simple separation procedure which allows us to utilize a range of tools to characterize and exploit properties in the nano size range. Employing these Si nanoparticles, high refractive index nanocomposites in gelatin were fabricated with values as high as 3.2.
Direct Observation of Fracture Mechanisms in Polymer-Layered Silicate Nanocomposites
Authors: Manias Evangelos , Han Wook Jin , Jandt Klaus D. , Kramer Edward J. , Giannelis Emmanuel P.
Published: April 2011

Conventional three point bending and TEM techniques are employed to determine the fracture toughness and identify the failure mechanisms in model layered-silicate polymer nanocomposites.
Nanophase Copper Ferrite Using an Organic Gelation Technique
Authors: Sriram D. , Snyder R. L. , Amarakoon V. R. W.
Published: April 2011

Nanocrystalline copper ferrite (Cu0.5Fe2.5O4) was synthesized using a forward strike gelation method with polyacrylic acid (PAA) as a gelating agent. The dried gel was calcined at a low temperature of 400 °C to get the final powder. The effect of pH and the ratio of the cation to the carboxylic group in the initial gel were studied with respect to both the phases and the crystallite size of the final powders synthesized. Phase and crystallite size analysis were done using x-ray diffraction and TEM. Saturation magnetization results were obtained using a SQUID magnetometer. The reactions occurring in the nano-size copper ferrite, in air as a function of temperature, were tracked using adynamic high temperature x-ray diffraction (HTXRD) system.
Coulometric Titration Studies of Nonstoichiometric Nanocrystalline Ceria
Authors: Porat O. , Tuller H. L. , Lavik E. B. , Chiang Y.-M.
Published: April 2011

Oxygen nonstoichiometry measurements in nanocrystalline ceria, x in CeO2-x, were performed using coulometric titration. The measurements reveal large apparent deviations from stoichiometry, of the order of 10−3 − 10−4 at T = 405 − 455 °C and Po2 = 0.21 − 10−5 atm, as compared to levels of ∼10−9 for coarsened materials under the same conditions. The level of nonstoichiometry is, however, larger then expected from previous electrical conductivity data of nanocrystalline ceria. In addition, x ∝ Po2−½ while Σ ∝po2−1/6. The observed dependence of x(Po2, T) can be explained by either the formation of neutral oxygen vacancies at or near the interface, or by surface adsorption.
Formation of Fine γ Grain Structure Through Fine α2 / γ Lamellar Structure in Ti-Rich TiAl Alloy
Authors: Kumagai T. , Abe E. , Nakamura M.
Published: April 2011

Microstructural development of an extremely fine α2-Ti3Al / γ-TiAl lamellar structure, which was formed by ice water quenching after solution-treatment in a high-temperature α-Ti phase field for a long period of time, was examined during isothermal aging treatment. In an as-quenched Ti-48at.%Al alloy, the massively transformed γ (γm) and untransformed (meaning massively untransformed) fine α2/γ lamellar regions were observed. Fine γ grains, which were similar to γm, were generated both within the fine α2/γ lamellae and at the boundary area between the γm and the fine α2/γ lamellar regions by aging at a low-temperature (1173K) for a short time (180s). Further aging (1.8ks) caused the coarsening of these newly generated fine γ grains. On the other hand, the coarsening of the γ grains occurred by a high-temperature (1323K) aging treatment even for 180s. Fine α2 plates and particles, which were aligned to a particular direction, were observed in the γ grain interiors, indicating that the newly generated γ grains grew at the expense of the fine α2/γ lamellae. It can be considered that the γ grain formation through the fine α2/γ lamellae is closely related to the α2→γ reaction of the α2 plates sandwiched by the γ plates, and needs the fast heating rate enough to overcome the α2/γ → γ/γ lamellae reaction.
Tensile and Compression Testing of Single-Crystal Gamma Ti-55.5 Al
Authors: Zupan Marc , LaVan David , Hemker K. J.
Published: April 2011

The orientation and temperature dependence of the flow strength of γ-TiAl is being measured to promote a fundamental understanding of the deformation mechanisms that are active in this alloy. High quality single crystals of γ- Ti-55.5 Al have been grown using an optical float zone furnace, which allows for crystal seeding and provides a containerless growth environment. These crystals have been oriented using back reflection Laue and TEM and cut into microsample tensile specimens by electric discharge machining. The microsample testing technique developed at Johns Hopkins [1] is being utilized to measure the orientation, temperature and tension/compression dependence of the flow strength of TiAl. An outline of the microsample testing techniques that have been developed for this study and preliminary results follow in this paper.
Ultrafine Fully-Lamellar Structures in Two-Phase γ-TiAl Alloys
Authors: Maziasz P. J. , Liu C. T.
Published: April 2011

Special ultrafine fully-lamellar microstructures have been found recently in γ-TiAl alloys with 46–48 at.% Al, when they are processed or heat-treated above the α-transus temperature (Tα). Hot-extrusion above Tα also produces a refined colony or grain size. Refined-colony/ultrafine-lamellar (RC/UL) microstructures produce an excellent combination of room-temperature ductility and high-temperature strength in Ti-47Al-2Cr-2Nb (at.%) alloys. UL structures generally have an average interlamellar spacing of 100–200 nm, and have regularly alternating γ and α2 lamellea, such that they are dominated by γ/α2 interfaces with relatively few γ/γ twin boundaries. The focus of this study is how variations in processing parameters or alloy composition affect formation of the UL structure, particularly the α2 component.
Comparison of the Creep Properties of Cast and Powder Metallurgy-Extruded Binary NiAl
Authors: Raj S. V. , Garg Anita , Bieter Thomas R.
Published: April 2011

A comparison of published creep data on binary NiAl showed that there is a discrepancy in the reported magnitudes of the stress exponents, n, which usually vary between about 4.5 and 6.5. In general, a close examination of the data suggested that n ≈ 4.5 for cast materials arid 6.5 for powder-metallurgy extruded NiAl. Constant load compression creep tests were conducted on a cast and extruded binary NiAl between 800 and 1200 K over a wide range of initial applied stresses varying between 4.0 and 200 MPa. The microstructures were characterized by transmission electron microscopy. The observed variations in the creep behavior of the extruded cast and powder-metallurgy NiAl appeared to be due to a grain size effect. Despite similarities in the values of n, no significant substructure was observed in most of the grains in the cast and extruded specimens at 1100 and 1200 K in contrast to the PM-extruded alloy, which revealed a wide range of substructural features in the power-law creep region. However, extensive subgrain formation and dislocations were widely observed at lower temperatures and higher stresses in the cast and extruded material.
Strain Rate Sensitivity of Mechanical Properties and Related Thermal Activation Process in a Two-Phase γ Titanium Aluminide
Authors: Lin Dongliang , Lin T. L. , Wang Yu , Lin Yun , Kim Young-Won
Published: April 2011

Tensile properties of a two-phase γ titanium aluminide with duplex microstructure are tested under different strain rates from 5×10-5 to 5×10-3S-1 at temperature from 1123K to 1273K. It is found that there exists approximate linear relationship between the flow stresses and the logarithm of the strain rate at different temperatures. The strain rate sensitivity can be explained by thermal activation theory, and dislocation climbing is identified as the rate controlling mechanism
Yield Point Phenomenon and Serrated Yielding in Nb-40Ti-15Al
Authors: Perungulam S. , Wheeler R. , Hou D. H. , Grylls R. J. , Banerjee S. , Fraser H. L.
Published: April 2011

The ternary Nb-40Ti-15Al (at%) intermetaltic has an ordered B2-type crystal structure and shows appreciable ductility. Deformation is mainly accommodated in slip bands by the glide of b=l/2 dislocations on {110} planes. The tensile behavior of this material has been studied at room temperature and at intermediate temperatures. Static and dynamic strain aging have been observed in certain temperature regimes. A model based on dislocation pinning by precipitates is proposed to explain the serrations observed during dynamic strain aging.
Platinum Group Metals Base Refractory Superalloys
Authors: Yamabe-Mitarai Y. , Koizumi Y. , Murakami H. , Ro Y. , Maruko T. , Harada H.
Published: April 2011

Ir- and Rh-base refractory superalloys with an fee and Lb two phase structure similar to Ni-base superalloys, yet with considerably higher melting temperatures have been proposed. Fee and Ll2two phases were observed in these alloys by transmission electron microscopy and X-ray powder diffractometry. The compression tests of these alloys showed that the strengths of several alloys were about 200 MPa at 1800 °C and these alloys have potential to become ultra-high temperature materials for use in power engineering field.
The Role of Copolymer Architecture on the Interfacial Structure of a Ternary Polymer Blend
Authors: Dadmun M. D.
Published: April 2011

The role of copolymer sequence distribution on the interfacial characteristics of a ternary polymer blend containing 2 homopoiymers and a copolymer in the phase separated state are examined using Monte Carlo Simulation. The copolymer does migrate to the biphasic interface in the phase separated regime while the configuration and expansion of the copolymer at the interface is a function of sequence distribution within the copolymer. This effect is interpreted in terms of the efficiency of the copolymer to strengthen the biphasic interface. These results suggest that block, alternating, and block-ran structures show promise as interfacial modifiers, while the purely random and alt-ran copolymers will be less efficient as an interfacial strengthener. It is also found that a variation of the sequence distribution away from a purely random structure can dramatically effect the ability of the copolymer to modify the interface. As most polymers which are not block nor alternating are termed ‘random’, this differentiation may have an effect on experimental studies of ‘random’ copolymers as compatibilizers in polymer blends.
Morphology Characterization in Multicomponent Polymer Systems using Scanning Probe Microscopy
Authors: Viswanathan Ravi , Tian Jing , Marr David W.M.
Published: April 2011

A newly available scanning probe microscopy, phase imaging, has been used to image polyethylene (PE) blends and composites. By providing not only a topographical map of the surface but also information on material properties, this technique has allowed ready identification of individual components within the mixtures investigated.
3D Micro-Tomographic imaging and Quantitative Morphometry for the Nondestructive Evaluation of Porous Biomaterials
Authors: Müller R. , Matter S. , Neuenschwander P. , Suter U. W. , Rüegsegger P.
Published: April 2011

Micro-computed tomography (μCT) is a new and emerging technique for the nondestructive assessment and analysis of the three-dimensional trabecular bone architecture. The applications of μCT with respect to the analysis of bone are manyfold. Nevertheless, it also holds high promise for the microstructural measurement and analysis of porous biomaterials. For the purpose of the study, a desk-top μCT providing a nominal isotropie resolution of 14 μm was used. Since the polymeric material has a very low X-ray absorption coefficient, the scaffolds were stained prior to measurement using a commercial X-ray contrast agent. This allowed not only to acquire important microstructural features of the diree-dimensional scaffold but also to compute standard structural indices such as BV/TV, BS/BV, Tb.N, Tb.Th, Tb.Sp and the degree of anisotropy (DA) using mean intercept length measurements. The preliminary results show that different types of scaffolds can be distinguished both qualitatively (visualization) and quantitatively (morphometry) provided an adequate X-ray staining technique is used. It can be concluded that, in the future, μCT may be of considerable help in basic as well as in applied research and development.
Molecular and Textural Ordering of Thermotropic Polymers in Shear Flow
Authors: Romo-Uribe A , Mathers P. T. , Chaffee K. P. , Han C.D
Published: April 2011

The texture and microstructural order present in mesomorphic polymers and their relation to their macroscopic behavior has been investigated using rheological, optical and dynamic scattering (WAXS and SALS) experiments. Shear orientation is observed under constant rate-of-deformation conditions where this orientation is always parallel to the flow direction. However, the high degree of orientation suggested by optical and SALS measurements is not reflected in the degree of molecular order observed in WAXS experiments. After cessation of flow, a rapid relaxation of stress is observed, while only little microstructural relaxation is found; i.e., the state of orientation is very stable
Investigation of the Altered Layer on Ancient Chinese Bronze Mirrors and Model High-Tin Bronzes
Authors: Taube Michelle , King Alexander H. , Thomas Chase W.
Published: April 2011

Many ancient Chinese bronze mirrors have a smooth patina. An ingress of corrosion to a depth of approximately 100 μm is found beneath the patina. The corrosion selectively replaces the Cu-rich α phase leaving the Sn-rich σ phase intact. Previous work by x-ray diffraction has shown that the a-phase replacement product is poorly crystallized or nanocrystalline SnO2. Transmission electron microscopy was employed to further characterize the replacement product in both ancient mirror and replication samples. Nanocrystalline SnO, in the form of small spheroids has been found. Remnants of an original alloy phase appear to be interspersed with the tin oxide.
An Investigation of the Antimony-Containing Minerals used by the Romans to Prepare Opaque Colored Glasses
Authors: Mass Jennifer L. , Stone Richard E. , Wypyski Mark T.
Published: April 2011

Roman colored opaque vessel glasses and mosaic tesserae were examined using energy dispersive X-ray analysis, wavelength dispersive X-ray analysis, and scanning electron microscopy in order to identify the origins of the antimony-based glass opacifying agents used in the Roman period. Bindheimite and stibnite were considered as mineralogical sources of antimony, and antimonial litharge was investigated as a metallurgical source of antimony. The refining of antimonial silver ores was discussed as a source for antimonial litharge in the Roman period. The morphologies of the antimonate crystallites, their distributions, and the observed correlations of lead to antimony in the glasses indicated that roasted stibnite was the antimony source for the white and blue opaque glasses and antimonial litharge was the antimony source for the yellow and green opaque glasses. Opaque yellow Roman glasses were found to contain a mixture of clastic, subhedral, and euhedral lead pyroantimonate (Pb2Sb2O7) particles. The euhedral crystallites are a rhombohedral modification of Pb2Sb2O7that is formed above 900 °C.
Use of Coupled Passivants and Consolidants on Calcite Mineral Surfaces
Authors: Nagy Kathryn L. , Cygan Randall T. , Scotto Cathy S. , Jeffrey Brinker C. , Ashley Carol S.
Published: April 2011

Deterioration of monuments, buildings, and works of art constructed of carbonate-based stone potentially can be arrested by applying a combination of chemical passivants and consolidants that prevent hydrolytic attack and mechanical weakening. We used molecular modeling and laboratory synthesis to develop an improved passivating agent for the calcite mineral surface based on binding strength and molecular packing density. The effectiveness of the passivating agent with and without a linked outer layer of consolidant against chemical weathering was determined through leaching tests conducted with a pH-stat apparatus at pH 5 and 25°C. For the range of molecules considered, modeling results indicate that the strongest-binding passivant is the trimethoxy dianionic form of silylalkylaminocarboxylate (SAAC). The same form of silylal-kylphosphonate (SAP) is the second strongest binder and the trisilanol neutral form of aminoethylaminopropylsilane (AEAPS) is ranked third. Short-term leaching tests on calcite powders coated with the trisilanol derivative of SAAC, the triethoxy neutral form of SAP, and the trimethoxy neutral form of AEAPS show that the passivant alone does not significantly slow the dissolution rate. However, all passivants when linked to the sol consolidant result in decreased rates. Combined AEAPS plus consolidant results in a coating that performs better than the commercial product Conservare® OH and at least as well as Conservare® H. The modeling results indicate that there may be a threshold binding energy for the passivant above which the dissolution rate of calcite is actually enhanced. More strongly-binding passivants may aid in the dissolution mechanism or dissociate in aqueous solution exposing the calcite surface to water.
A Study of the Role of Heterogeneities in the Initial Stages of Corrosion of Glazes Using Dynamic Imaging Microellipsometry (DIM)
Authors: McCarthy B.E. , Vandiver P.B. , Kruger J.
Published: April 2011

Common processes used in glazed ceramic production often result in a glaze with a heterogeneous microstructure. Heterogeneities may be due to residual batch materials, intentionally added colorants and opacifiers and/or the products of devitrification and phase separation. To study the effect of heterogeneities in the corrosion process, dynamic imaging microellipsometry was used in-situ in aqueous solutions to measure spatially and temporally resolved changes in the surface of glasses (model glazes). The measurements showed increased durability near inclusions. Residual stress fields surrounding the heterogeneities influenced the results. Decoupling of chemical and mechanical factors causing this increased durability was not possible.