bluuebunny/crossref_metadata_2025 huggingface dataset:
A thermophotonic heat pump/heat engine
Authors: Oksanen Jani , Tulkki Jukka
Published: April 2011

We propose a solid state heat pump based on photon assisted heat transfer between two large-area light emitting diodes coupled by the electromagnetic field and enclosed in a semiconductor structure with a nearly homogeneous refractive index. Ideally the thermophotonic heat pump (THP) allows heat transfer and electricity generation at the Carnot efficiency, but in reality there are several factors that limit the efficiency. We present a numerical model that accounts for the most important losses of the thermophotonic heat pump to study the operating regimes and the fundamental limitations of the THP structure. The results show that the thermophotonic heat pump has potential to outperform heat pumps based on the thermoelectric effect especially for heat transfer across large temperature differences. In energy harvesting applications the performance of the THP is good for small power densities, but drops at high power densities unless the losses in the structure can be efficiently minimized.
Constrained Deformation of TiAl PST Crystals
Authors: Goto Kengo , Kishida Kyosuke , Inui Haruyuki
Published: April 2011

TiAl polysynthetically twinned (PST) crystals were deformed under plane strain condition, in which the anisotropic macroscopic deformation of PST crystals is restricted with a channel die, in order to clarify the deformation behavior of TiAl/Ti3Al lamellar structure under constraint conditions. TEM analysis of deformation modes together with the Taylor analysis reveals that all TiAl orientation variants deform to yield the relaxed-constraint-type plastic strain, where three shear strain components are not zero for each TiAl variant but are macroscopically compensated to zero by the existence of twin-related TiAl lamellae at the early stage of deformation. The Taylor analysis assuming the relaxed constraint conditions is found to be adaptable for predicting the operative deformation modes in TiAl at the early stage of deformation and also for correlating quantitatively the stress-strain behavior of PST crystals under external constraint with those under the unconstraint condition.
Laser-Assisted Atom Probe Analysis of Bulk Insulating Ceramics
Authors: Ohkubo Tadakatsu , Chen Yimeng M. , Kodzuka Masaya , Li Fang , Oh-ishi Keiichiro , Hono Kazuhiro
Published: April 2011

A Yb:KGW femtosecond laser (400 fs) with 3rd and 4th harmonic generators (wavelength = 343 and 258 nm) was adopted to a locally built 3DAP instrument to assist field evaporation from ceramics tips that were bonded on tungsten wire. Using this setting, we have demonstrated that quantitative atom probe tomography is possible from Y2O3-ZrO2-MgAl2O4, (Ce,Y)O2, Li(Co,Ni,Mg,Al)O2 sintered bulk ceramics, which are all insulators.
Characterization and Evaluation of the Integrity of the Onshore Pipelines
Authors: Hernández S. L. , Adame M. , Gonzalez J. , Padilla D. , Contreras A.
Published: April 2011

The pipelines are the main transport and distribution system of production in the oil industry, which are subject to environmental and operational conditions which are not favorable for the operation of the pipelines and sometimes represent the risk of accidents and high economic losses. The evolution of risk begins with a systematic search of possible threats to the integrity of the pipeline. The identification of potential threats should not be limited to known risk categories reviewed, but must complete the steps to find new and unique expressions of risk and the study of particular cases. Thus, the importance of a comprehensive risk assessment of the transmission pipeline is crucial. In this paper an integrity assessment for corrosion damages in pipelines was developed through a methodology based on risk analysis to estimate the propagation rate on the time, the size, location and number of damages. To perform this study a data obtained from smart pig runs and an artificial neural network (ANN) model with retro propagation was used. From the data obtained from launching smart pigs on the pipeline it was carried out the training of the neural network, later on it was applied the network previously trained to get the predictions of damages on the pipeline, considering that pipeline did not have any maintenance.
A Study of the Kinetics of Polylactic Acid Crystallization by Image Processing
Authors: Hernández-Sánchez F. , Carrillo-Escalante H.
Published: April 2011

The polylactic acid (PLLA) is commonly used in the biomedical application. The physical and mechanical properties of PLLA depend on the molecular weight, crystallinity, and synthesis. In this work, we study the PLLA crystallization kinetics. One of the main attractive of these polymers is that its degree of crystallinity can easily be modified, from amorphous state to a high crystallinity degree. In this study a new technique is applied to follow the crystallization kinetic. An optical microscope with cross polarizers, warming plates and video camera were used. A series of images were obtained under isothermal conditions to observe the growth spherulite. Since an image is composed by pixels and each pixel has a value from 0 to 255, which represents gray intensity, being zero for black and 255 are white. Then, the pixels average versus time was plotting and from the plots crystallization kinetic and secondary crystallization are easily obtained.
Phase Stability of the Al-Cu-Co Decagonal Phase Under Conventional Solidification
Authors: Téllez-Vázquez J.O. , Patiño-Carachure C. , Bedolla-Jacuinde A. , García-De León E. , Pérez R. , Rosas G.
Published: April 2011

Al65Cu15Co20and Al67Cu13Co20(% at.) alloys with composition near to the quasicrystalline decagonal phase was produced by melting in an induction furnace and solidified at room temperature. The structural characterization was carried out by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). For the structural model of decagonal quasicrsytals, it is important to know which crystalline phases have a structural relationship on the formation and decomposition of this type of phases. In the present investigation, the decagonal phase usually coexist with small amounts of the Al(Cu,Co) cubic phase of B2 type. Then, the quasicrystalline (QC) phase is outside of a single-phase region under equilibrium conditions at room temperature. DSC and TGA techniques showed the thermal stability of the alloy system up to 1000 °C.
Structural Characterization of Aluminum Foams Obtained by Powder Metallurgy
Authors: León-Patiño C. A. , Monje-García M. A. , Aguilar-Reyes E. A. , Bedolla-Becerril E.
Published: April 2011

The porous structure of aluminum foams was quantitatively monitored in terms of pore density, pore area, and shape and size distribution using image analysis; then, related to density and expansion profiles by interrupted experiments. This practice offers important information in the control and reproducibility of foams. The aluminum foams were produced by the powder compact melting method at 800°C. The foamable precursors consisted in uniaxial cold pressed Al-TiH2 mixtures compacted at 387 MPa; the pressure applied and particle size distribution of the mixture originated preforms with 95.9% densification. This procedure eliminated the traditional hot-compaction step; besides, the amount of foaming agent was kept to a minimum of 0.5 wt.% TiH2. A volume expansion of 215 to 236% and densities from 0.7730 to 0.8206 g/cm3were obtained in a time window of 420 to 570 s. The calculated shape factors and Feret diameters defined how the roundness of pores varies with size all along the foaming process.
Biocompatibility and Anti-microbial Properties of Silver Modified Amorphous Carbon Films
Authors: Almaguer-Flores Argelia , Olivares-Navarrete René , Ximénez-Fyvie Laurie A. , García-Zarco Oscar , Rodil Sandra E.
Published: April 2011

Infection due microbes on implant surfaces has a strong influence on healing and long term viability of dental implants. The prevention and control of biofilms can be achieved by reducing the bacterial adhesion on the surface. The coating of medical devices with silver, or the addition of silver nanoparticles, are two possible ways to prevent device-associated infections. On the other hand, amorphous carbon films, in its different forms and compositions, have been studied as beneficial surface modification for implant materials. However, the bacterial adhesion on these films by oral bacteria in comparison to standard surfaces (Ti and SS) has been seen to be relatively high. In the oral cavity, the microbial ecology is complex and consists of hundreds of bacterial species, and therefore it is recommendable to study bacteria adhesion using various strains. In this work, we tested the biocompatibility and the anti-microbial properties of amorphous carbon films with the addition of silver nanoparticles. The a-C:Ag films were deposited by co-sputtering in an Argon plasma using a target made of graphite with a small piece of pure silver. Biocompatibility tests were performed using osteoblast-like cells (MG63) and included: cell proliferation, alkaline phosphatase specific activity and OPG. The bacterial adhesion test was evaluated after 1, 3 and 7 days of incubation. We used nine oral bacteria strains: Aggregatibacter actinomycetemcomitans serotype b, Actinomyces israelii, Campylobacter rectus, Eikenella corrodens, Fusobacterium nucleatum ss nucleatum, Parvimonas micra, Porphyromonas gingivalis, Prevotella intermedia and Streptococcus sanguinis. The effect of including silver in the a-C films was studied by X-ray Diffraction, Energy Dispersive spectroscopy, Scanning Electron Microscopy. The results showed that the films had silver nanoparticles (40-60 nm) uniformly distributed in the carbon matrix. The silver was crystalline with a maximum content of around 6 at%. The biological tests showed that a-C:Ag films had good biocompatibility properties, allowing the osteoblast to proliferate and produced osteogenic local factors. Concerning the antimicrobial properties of the a-C:Ag films, we did not observe an effect of the silver particles on bacterial adherence after 1 and 3 days of incubation; however, a significant reduction was observed after 7 days, compared to the a-C, Ti films or the bare SS substrate, suggesting that silver nanoparticles have a time-dependent antimicrobial effect.
Graphene Devices Processing Using Direct Writing Laser Lithography and Electrical Properties Characteristics
Authors: Leon Jorge Augusto , Alves Elmo Salomão , Elias Daniel Cunha , Moreira Leandro Malard , Barbosa Tiago Campolina , Pimenta Marcos Asunção , Plentz Flavio Orlando
Published: April 2011

In this work, we establish the use of lithography technique by laser direct writing for fabricating bilayer graphene devices. This technique, which is based on direct laser writing on graphene coated with a photoresist is simple to implement, versatile, and capable of achieving good throughput. Double-layer graphene flakes were obtained by micromechanical cleavage of graphite producing large graphene samples up to 40μm in size. The presence of a bilayer of graphene on SiO2/Si substrate was verified by optical microscopy and resonant Raman spectroscopy. We have measured the four-terminal resistance as a function on the back-gate voltage and found initially p-type doping in graphene, but annealing inside cryostat at 127C° in He atmosphere, the samples become n-type. Our measurements show electron mobility reached values around ˜1,900 cm2/V.s at high electron concentration.
Microstructural and Mechanical Characterization of a TRIP-800 Steel Welded By Laser-CO2 Process
Authors: Perez-Medina G. Y. , Zambrano P. , López H. F. , Reyes-Valdés F. A. , López-Cortés V. H.
Published: April 2011

This paper presents results on the impact of Laser CO2 process variables on the weldability, phase transformations and tensile properties of a TRIP800 Steel. The microstructure of this steel is comprised of ferrite, bainite and retained austenite phases. This is obtained by controlled cooling from the intercritical annealing temperature to the isothermal bainitic holding temperature. These steels have been increasingly used in the last 10 years in the automotive industry and for these materials to be used effectively; the influence of material and the CO2 laser welding process condition must be clearly understood. Hence, in this work the effect of the welding process on the resultant microstructures and on the exhibited mechanical properties is investigated. It is found that the tensile strength of welded specimens falls below 800 MPa and that the elongation becomes 15 % or lower. In turn, this clearly indicates that the implemented laser welding process leads to a reduction in the TRIP800 steel toughness.
Evolution of microstructure of 304 stainless steel joined by brazing process
Authors: García-Vázquez F. , Guzmán-Flores I. , Garza A. , Acevedo J.
Published: April 2011

Brazing is a unique method to permanently join a wide range of materials without oxidation. It has wide commercial application in fabricating components. This paper discusses results regarding the brazing process of 304 stainless steel. The experimental brazing is carried out using a nickel-based (Ni-11Cr-3.5Si-2.25B-3.5Fe) filler alloy. In this process, boron and silicon are incorporated to reduce the melting point, however they form hard and brittle intermetallic compounds with nickel (eutectic phases) which are detrimental to the mechanical properties of brazed joints. This investigation deals with the effects of holding time and brazing temperature on the microstructure of joint and base metal, intermetallic phases formation within the brazed joint as well as measurement of the tensile strength. The results show that a maximum tensile strength of 464 MPa is obtained at 1120°C and 4 h holding time. The shortest holding times will make boron diffuse insufficiently and generate a great deal of brittle boride components.
Strengthening phases in the production of Al2024-CNTs composites by a milling process
Authors: Pérez-Bustamante R. , Pérez-Bustamante F. , Herrera-Ramíreza J. M. , Estrada-Guel I. , Amézaga-Madrid P. , Miki-Yoshida M. , Martínez-Sánchez R.
Published: April 2011

Carbon nanotubes (CNTs) synthesized by a chemical vapor deposition (CVD) method and the 2024 aluminum alloy (Al2024) are used in the production of Al2024-CNTs composites. An homogeneous dispersion of the CNTs into the aluminum matrix is achieved by a mechanical milling processing. CNTs keet their morphology after milling and sintering processes. Formation of aluminum carbide as a function of CNTs contents is observed. Formation of equilibrium phases during sintering is observed by electron microscopy. CNTs and aluminum carbide in the composites are characterized by transmission electron microscopy. Hardness results of sintered products show an increment of up to 285% over the unreinforced alloy prepared by the same route.
Mathematical Modeling of Impingement of an Air Jet in a Liquid Bath
Authors: Solórzano-López J. , Zenit R. , Ramírez-Argáez M. A.
Published: April 2011

Physical and mathematical modeling of jet-bath interactions in electric arc furnaces represent valuable tools to obtain a better fundamental understanding of oxygen gas injection into the furnace. In this work, a 3D mathematical model is developed based on the two phase approach called Volume of Fluid (VOF), which is able to predict free surface deformations and it is coded in the commercial fluid dynamics software FLUENTTM. Validation of the mathematical model is achieved by measurements on a transparent water physical model. Measurements of free surface depressions through a high velocity camera and velocity patterns are recorded through a Particle Image Velocimetry (PIV) Technique. Flow patterns and depression geometry are identified and characterized as function of process parameters like distance from nozzle to bath, gas flow rate and impingement angle of the gas jet into the bath. A reasonable agreement is found between simulated and experimental results.
Magnetic properties of bulk composite FeBSiM (M=Cr,Zr) alloys with high microhardness
Authors: Báez S. , Betancourt I. , Hernandez-Rojas M. E. , Figueroa I. A.
Published: April 2011

The synthesis, mechanical and magnetic properties of bulk composite Fe72B19.2Si4.8M4 (M=Cr, Zr) alloys obtained by a copper mould injection casting technique under a protective helium atmosphere are reported and discussed. The resultant microstructure of the composite alloys consists of crystalline Fe92Si8 and Fe2B phases embedded in a glassy matrix. The values of microhardness (Hv) show maxima for the alloy containing Cr with 10.24±0.95 GPa. The maximum value of saturation magnetization average (μ0Ms) is 1.25±0.02 T for Cr-containing alloy. The Curie temperatures (Tc) of amorphous phases are higher than 390 K for both alloys. However, the bulk composite alloys presents values for crystallization temperature (Tx) of 1289±10 K and 1140±10 K for Cr and Zr-containing alloys, respectively. These results are interpreted on the basis of the interplay between the crystalline and amorphous phases.
Stress concentration on artificial pitting holes and fatigue life for aluminum alloy 6061-T6, undergoing rotating bending fatigue tests
Authors: Lemus Víctor H. M. , Almaraz Gonzalo M. D. , V. Lopez J. Jesús
Published: April 2011

This work deals with rotating bending fatigue tests on aluminum alloy 6061-T6, under loading condition close to the elastic limit of the material. Results have been obtained for three types of specimens: without artificial pitting, specimens with one artificial pitting hole and specimens with two neighboring artificial pitting holes. Results show that fatigue endurance is reduced in the case of one pitting hole and considerably for two neighboring pitting holes. In order to explain this behavior, numerical analysis by FE are carried out to determine the stress concentrations for the three types of specimens. It is found that the stress concentration for two neighboring pitting holes is an exponential function of the separation between the two holes, under uniaxial loading. The probability to find two or more neighboring pitting holes in real industrial materials, such as cast iron, corroded or pitting metallic alloys is high; then, the stress concentration for two or more neighboring pitting holes needs to be considered for the fatigue prediction life under fatigue loading and corrosion attack applications.
Effect of the deposition rate on thin films of CuZnAl obtained by thermal evaporation
Authors: López-Pavón L. , López-Cuellar E. , Torres-Castro A. , Ballesteros C. , de Araújo C. José
Published: April 2011

Thermal evaporation is used to deposit thin films of CuZnAl on silicon substrates. For this purpose, a CuZnAl shape memory alloy is used as evaporation source. The chemical composition and the phases present in the films are evaluated at two different deposition rates: 7 and 0.2 Å/s. The thin films are heat treated to promote the diffusion of the elements and characterized by X-ray Diffraction, Energy Dispersive X-ray Spectroscopy and Scanning Transmission Electron Microscopy (STEM). It is shown that the chemical composition of the thin films is significantly different to that of the CuZnAl alloy used as evaporation source. Moreover, the films produced at 7 Å/s show a significant loss of Zn, contrary to the results obtained using a deposition rate of 0.2 Å/s. It is also observed that the composition varies across the thickness of the film, suggesting that the various alloying elements are evaporated at different rates during the deposition process. Finally the predominant phases present in the films belong to the AlxCuy family.
Development of a New Nickel-base Superalloy for High Temperature Applications
Authors: Covarrubias Octavio
Published: April 2011

Since their appearance during in the 1940 decade, nickel-base alloys are appreciated for their superior mechanical properties and microstructural stability at elevated temperatures and high stresses. They are typically used in jet-engines and land-based turbines for energy generation. Such materials, known as superalloys are in constant evolution as designers are encouraged to propose more efficient and powerful systems of propulsion and energy generation. This evolution leads to conceive and manufacture new superalloys capable to fulfill higher requirements. Alloy 718Plus® is emerging as an alternative material for the design and construction of components to be used in jet-engines and land-based turbines for energy generation. 718Plus® is a precipitation hardened nickel-base alloy designed to have the stability of superalloys similar to Waspaloy and the good processing characteristics of other materials as the 718 alloy. Since the early 2000 decade, ATI Allvac has lead a complete program in order to validate capabilities and properties of the 718Plus® alloy. Objectives for this effort include a characterization and its introduction as a viable material for the design and manufacture of components to be installed technologically. As part of this project, contoured rings made of 718Plus® are rolled considering industrial conditions. Several heat treatments, involving solution and precipitation processes are performed on segments extracted from involved contoured rings. Effects of such hot-working conditions and heat treatment procedures on properties as forgeability, tensile, hardness and stress-rupture characteristics are evaluated. Optical and electron microscopy are performed to evaluate microstructural properties as grain size and promotion of precipitates, in order to complement reported results.
Application of clay as a means of support to biomaterial with anti-inflammatory agents
Authors: Domínguez-Patiño M.L. , Chávez-Castillo M , Rodríguez-Martínez A. , Alemán R.M. Melgoza , Jiménez-Ferrer E , Herrera-Ruíz M.
Published: April 2011

In the last fifteen years, there have been significant changes in the production of medicines, mainly in the addition of new components to the formulation of solid dosage forms. The current trend of “back to nature” to lead to a healthier life has led those who are engaged in the pharmaceutical field to develop new formulations that allow the use of natural products of plant origin

For example, excipient ingredients, used as carriers for a drug's active ingredients, are now being used. These are incorporated into a drug in order to facilitate the drug's preparation, maintenance, or administration. The excipient is beneficial to the patient because it allows the drug to be easily administered and absorbed by the human body [3]

There is evidence that the clays have great potential for both absorption and adsorption due to its tiny particles. In addition, it has been reported that some clays have an effective antiseptic and healing ability [5–8]

In the present work, a clay called bentonite was tested as a support vehicle of an inflammatory agent derived from a Mexican native plant called Distictis buccinatoria, commonly named “Tonacaxóchitl”. Studies carried out by Rojas et al. have shown that the organic extract of this plant has important antibacterial, antifungal, cytotoxic and anti-inflammatory properties [9]
Thermoluminescence Properties of Novel La2O3 Phosphor Obtained by Solution Combustion Synthesis
Authors: Orante-Barrón V. R. , Cruz-Vázquez C. , Bernal R. , Denis G. , Yukihara E. G.
Published: April 2011

Thermoluminescence (TL) of La2O3 is reported for the first time. Novel La2O3 phosphor was obtained by solution combustion synthesis (SCS) in which a redox combustion process between lanthanum nitrate and urea at 500 °C is accomplished. The powder samples obtained were annealed at 900 °C during 2 h in air. X-Ray Diffraction (XRD) results showed the hexagonal phase of La2O3 for annealed powder samples. The TL glow curve obtained after exposure to beta radiation of these samples, displayed two maxima located at ˜ 101 °C and ˜ 200 °C, and a shoulder at ˜ 247 °C. Results from experiments such as dose response and fading showed that annealed La2O3 powder obtained by SCS is a promising material for radiation dosimetry applications.
Hydrogen Production by Catalytic Steam Reforming of Model Compounds of Biomass Fast Pyrolysis Oil
Authors: Lan P. , Xu Q. L. , Lan L. H. , Yan Y. J. , Wang J. A.
Published: April 2011

A Ni/MgO-La2O3-Al2O3 catalyst with Ni as active component, Al2O3 as support, MgO and La2O3 as additives was prepared and its catalytic activity was evaluated in the process of hydrogen production from catalytic steam reforming of bio-oil. In the catalytic evaluation, some typical components present in bio-oil such as acetic acid, butanol, furfural, cyclopentanone and m-cresol were mixed following a certain proportion as model compounds. Reaction parameters like temperature, steam to carbon molar ratio and liquid hourly space velocity were studied with hydrogen yield as index. The optimal reaction conditions were obtained as follows: temperature 750-850 °C, steam to carbon molar ratio 5-9, liquid hourly space velocity 1.5-2.5 h-1. The maximum hydrogen yield was 88.14%. The carbon deposits were formed on the catalyst surface but its content decreased as reaction temperature increased in the bio-oil steam reforming process.
A Comparative Study on the Sorption of BTEX and Cr (VI) in Drinking Water by Anionic and Cationic Sorbents
Authors: Flores-Valle S. O , Carrera S. P. Paredes , Valenzuela-Zapata M. A.
Published: April 2011

A comparative study of different sorbent materials was performed in order to propose an integrated system to eliminate both, anionic and cationic contaminant ions in drinking water, such as Cr (VI) and BTEX (benzene, toluene, ethylbenzene and xylenes). The adsorption process was studied using several adsorbents: activated carbon, cationic clays (bentonite) and natural zeolite as well as anionic clays (Al-Mg/Nitrate hydrotalcites). The activated carbon and natural zeolites were commercial samples, while hydrotalcite-like compounds were synthesized by an ultrasound-assisted method. It was found that although activated carbon showed a good performance in the cationic sorption, the calcined hydrotalcites presented the highest sorption capacity of chromates compared with activated carbon which had a good performance only up to 100 ppm. For higher concentrations (>100 ppm) activated carbon is saturated rapidly and its sorptive capacity is practically null.
Variable Temperature Study of Au and Au-Pt Nanoparticles on Selected Oxide Supports
Authors: Barrett D. , Franklyn P. , Scurrell M.
Published: April 2011

We report on the size relationship of Au and Au-Pt nanoparticles that were synthesised on silica and anatase phase titania supports. Deposition-precipitation (DP) of metal chlorides with the addition of urea and ammonium hydroxide was used to produce the nanoparticles. The relative particle size relationship of the Au and Au-Pt nano particles (NP's) was investigated, relating the Pt concentration and the support polymorph over a temperature range. It was found, with the use of in-situ variable temperature powder X-ray diffraction (VT-PXRD) and transmission electron microscopy (TEM), that the addition of Pt to the Au system corresponded to a reduction in particle size over a broad temperature range.
Photocatalytic Deposition of Nickel Nanoparticles on Titanium Dioxide
Authors: Rodríguez Julia L. , Pola Francisco , Valenzuela Miguel A. , Poznyak Tatiana
Published: April 2011

Metallic nanoparticles may be tailored to obtain specific size and morphology. In this work we present the synthesis of Ni/TiO2 catalysts by a photodeposition method. Our investigation included the photochemical and photocatalytic reduction of the nickel organometallic precursor (Ni (II) acetylacetonate) over titania (Degussa P-25) support. The photo-reduction kinetics was followed by UV-Vis and the catalysts were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to the results, the photochemical reduction of the Ni precursor was only 30% at λ= 254 nm, whereas, the photocatalytic route was approximately 90 % (λ= 365 nm) yielding Ni nanoparticles with diameter ranging from 10 to 40 nm.
Characterization of Growth Defects in ZnTe Single Crystals
Authors: Zhou W. , Wu J. , Dudley M. , Su C.H. , Volz M.P. , Gillies D.C. , Szofran F.R. , Lehoczky S.L.
Published: April 2011

(111) wafers sliced from a boule of ZnTe grown by horizontal physical vapor transport (PVT) have been characterized using synchrotron white beam X-ray topography. The growth axis was about 6° off [311]. The presence of dislocation slip bands, subgrain structures and [111] axis 180° rotational twins were revealed. The slip bands were observed to break up the ordered dislocation cell structures comprising the subgrain boundaries. The initiation of slip at regions of stress concentration at the junctions of subgrain boundaries and twin boundaries was observed. The asymmetric distribution of slip bands either side of the twinned region of crystal suggests that twin boundaries can act as barriers for slip. Several types of detailed twin boundary configuration were determined from the topographs. Using a combination of white beam X-ray topography and Nomarski interference microscopy, the three dimensional shapes of the twin boundaries were determined. Approximate atomic structures at these boundaries are presented.
InAs/InxGa1-xSb Type II Strained Layer Superlattices for Long Wavelength Infrared Detection Applications
Authors: Talwar D. N. , Loehr John P. , Jogai B.
Published: April 2011

Short period InAs/InxGa1−xSb superlattices (SLs) may allow strong optical transitions in the long wavelength infrared (> 10 μm) spectral region. Absorption calculations can be difficult, however, because of the strongly type - II interface and because of the large lattice mismatch. We propose that a long wavelength response can be achieved for substantially thinner layers of SLs if the In composition in InxGa1−xSb is properly chosen. This will misalign the bands through strain effects and further reduce the superlattice bandgap. Band structure calculations are reported for InAs/InxGa1−xSb type - II SLs grown on GaSb substrate by using an empirical tight-binding model (ETBM). All of the structures considered here are assumed to be well within the critical strain thickness. Particular care is taken to incorporate the strain effects accurately in the ETBM formalism by modifying the overlap integrals according to the bond lengths and bond angles. We compute the band structure and the cutoff wavelengths of InAs/InxGa1−xSb (001) SLs and compare the results with the existing magnetooptical and photo conductivity data. In addition, we compare the ETBM with the k.p and effective bond orbital models.
Effects of the Substrate on the Response of P(VDF-TrFE) Pyroelectric Detector
Authors: Mahrane A. , Djafari-Rouhani M. , Najmi A. , Esteve D.
Published: April 2011

The thermal analysis on the response of pyroelectric detector revealed that an optimal configuration structure could be found for a certain frequency range. Thus, it appeared that the pyroelectric layer should be thick at low frequencies but it should be very thin at high frequencies. The introduction of a thermal insulator is done trying to find out a trade off between sensitivity and response time. It has been found that its influence is restricted to a certain frequency range. The optimal thickness is related to the thermal diffusion length. We noticed also that it depends very little on the configuration structure.
Bulk Homogeneity of Iron Doped InP
Authors: Jimenez J. , Avella M. , Gonzalez M. A. , Martin P. , Sanz L. F. , Chafai M.
Published: April 2011

Fe- doped InP is studied by a spatially resolved photocurrent technique, probing electronic transitions related to the iron impurity levels. This scanning technique allows to get information on bulk homogeneity regarding to the electrically active iron impurity distribution. The presence of doping growth striations is revealed with a high sensitivity. Also results about the local photocurrent at the Cottrell atmospheres of the dislocations are shown. The role played by other defect levels in the photocurrent contrast is also discussed.
Modeling of MBE Growth with Interacting Fluxes
Authors: Tomich David H. , Eyink K. G. , Haas T. W. , Capano M. A. , Kaspi R. , Cooley W. T.
Published: April 2011

Ternary and quaternary III-V alloys are important for many optical device applications, and a precise control of the composition is required. Molecular beam epitaxy (MBE) is generally considered a non-equilibrium or kinetically controlled process but most of these models are too computationally intensive for real time control. We report on using a precursor state growth model 1,2 for the growth of GaAsSb to control the growth conditions and hence the film composition. The activation energies and the parameters appearing in the relationship are determined by fitting the calculated compositions to experimental ones as determined by x-ray diffraction. The effect of substrate temperature, growth rate and flux intensities on composition is discussed.
More Insight into the VHF-Glow-Discharge by Plasma Impedance Measurements
Authors: Kroll U. , Ziegler Y. , Meier J. , Keppner H. , Shah A.
Published: April 2011

We performed plasma impedance measurements at room temperature for a hydrogen plasma using an impedance analyser. The plasma excitation frequency range spans from 40 to 70 MHz. Both the real and imaginary part of the impedance decrease monotonously with increasing frequency. These measurements are in agreement with prior experimental observations [1], that the required peak-to-peak voltage between the electrodes is reduced at higher excitation frequencies. Using a simple equivalent circuit for the plasma this effect can be mainly attributed to the increased sheath capacitance. Furthermore, by modelling the sheath with a simple parallel plate capacitor, its thickness could be estimated: it decreases from 2 MM at 40 MHz to about 1.4 MM at 70 MHz plasma excitation frequency. Finally, a possible link between the decreasing sheath thickness on the increase of deposition rate is discussed.
Characterization of Amorphous Silicon Deposited at high rate by Helium Dilution PECVD and used for Applications in Radiation Detection
Authors: Ilie A. , Pochet T. , Foulon F. , Equer B.
Published: April 2011

PECVD hydrogenated Amorphous silicon (a-Si:H) layers have been deposited at high rates using helium dilution of the reacting precursors. This allows the fabrication of very thick layers (a few tens of microns) within reasonable processing times. This technique can be applied to the fabrication of thick p-i-n detector devices to be used for direct X-ray detection. Several p-i-n devices have been processed at deposition rates as high as 15 A/sec without significant changes in the optoelectronic properties of the a-Si:H films. Capacitance-Voltage measurements have been performed in order to measure the residual space charge density in the intrinsic region of the devices. The results demonstrate a moderate increase of the N*DoS concentration with the deposition rate. DC bias photoconductivity measurements have also been performed. The experimental data have been fitted with a multiple trapping transport model for holes. The model parameters have been studied and related to the electronic properties of the films.
Defect-Related Photoluminescence of a-Si:H
Authors: Ulber Isabell , Barth Axel , Fuhs Walther , Mell Helmut
Published: April 2011

The defect-related photoluminescence (0.8–0.9eV) of a-Si:H is studied as a function of the defect density ND and the temperature using subgap excitation of Ex= 1.16eV. Electron bombardment, light exposure and phosphorus doping were employed to create defects. The defect density ND was infered from CPM- and PDS-spectroscopy. We conclude from the measured density-of-states distributions of doped films that the radiative process is tunneling of bandtail carriers (Majority carriers) into the defect states. The dependence of the emission intensity on ND suggests that only a subgroup of the defects acts as radiative center. We propose that the local structure of the defect determines whether it acts as radiative or nonradiative center.
On Modeling Trivalent Dangling Bonds with Bivalent Levels
Authors: Suntharalingam Vyshnavi , Branz Howard M.
Published: April 2011

We examine the treatment of the hydrogenated Amorphous silicon (a-Si:H) dangling-bond defect used in the Analysis of microelectronic and Photonic Structures (AMPS) computer model and in other models of a-Si:H semiconductor devices. The dangling bond defect (D) is trivalent, with two correlated electronic levels in the gap. However, Modelers typically employ two uncorrelated bivalent levels to represent D; this introduces fictitious neutral dangling bond (D°) levels whenever D is charged. We find that the bivalent (AMPS) representation captures the important aspects of D physics and introduces only small errors into simulations. To reach this conclusion, we examine charge density and recombination and compare the trivalent D defect and its bivalent representation in both thermal equilibrium and non-equilibrium (illuminated) cases. The charge density is identical, implying that band bending computed by the simulation is accurate. We find errors in the recombination rate for the bivalent representation are normally less than or equal to σ0/σch, where σ° is the capture cross section of D0 and σch is the capture cross section of a charged state of D. Typically Modelers use σ0/σch of 1:10 to 1:100 yielding insignificant errors in the recombination rate with the uncorrelated bivalent representation.
Molecular Dynamics Simulation of Hydrogenated Amorphous Silicon with Tersoff Potential
Authors: Ohira Tatsuya , Inamuro Takaji , Adachi Takeshi
Published: April 2011

A Molecular dynamics method with a Many-body Tersoff-type interatomic potential has been being applied to analyses of hydrogenated Amorphous silicon (a-Si:H) thin-film growth processes. As a first step toward film growth simulations, Molecular dynamics simulations of SiH3 radical, which would be a significant precursor for the a-Si:H thin-film growth processes, and a-Si:H formation with a rapid quenching method have been performed by developing new Tersoff-type interatomic potential between Si and H in this study. Visualization of SiH3 radical dynamics by computer graphics has made it possible to observe the inversion and rotation of SiH3 radical, which had been predicted by infrared diode-laser spectroscopie measurement in other group. In addition, visualization of the a-Si:H sample has helped us to find that there are some microcavities in the sample and that there are two kinds of hydrogen in the sample, gathering closely together while lying scattered, which had been predicted in IR absorption experimental results.
Non-Radiative Recombination in a-Si:H
Authors: Schubert M. , Stachowitz R. , Saleh R. , Fuhs W.
Published: April 2011

Frequency-resolved photoluminescence spectroscopy (FRS) is used to study non-radiative recombination in a-Si:H using generation rates sufficiently small to garantee geminate recombination at low temperature. The quenching of the photoluminescence by a higher defect density ND and an increase of temperature influences the QFRS spectra differently: Whereas for increasing ND the quenching of the signal is more pronounced on the low frequency side raising temperature leads to a uniform decrease in the entire frequency range. The dependence of the lifetime distribution on ND is quantitatively explained in a model where radiative recombination competes with non-radiative tunneling into defect states.
Progress in large area Selective Silicon Deposition for TFT/LCD Applications
Authors: Souk Jun H. , Parsons Gregory N.
Published: April 2011

We have previously demonstrated selective area deposition of n+ Macrocrystalline silicon at 250°C using time modulated silane flow into a hydrogen plasma, and applied the technique to form high performance top-gate Amorphous silicon TFT's with two mask sets. In this paper, we discuss issues related to process scale-up, including the effect of deposition rate on selectivity loss and non-uniformity. Uniformity can be achieved with higher growth rates by expanding the window for selectivity, and using conditions well within the process limits. We show that lower pressure and higher rf power can enlarge the window by enhancing the hydrogen-Mediated silicon etching.
Monte Carlo Modelling of Electrophotographic Dark Discharge
Authors: Elmer S. J. , Marshall J. M. , Van Swaaij R. A. C. M. M. , Bezemer J. , Hepburn A. R.
Published: April 2011

In an electrophotographic experiment, surface voltage is measured for a-Si1-xCx:H films of different thicknesses. It is observed that the thickness normalised dark decay rate (TNDDR) at longer times is smaller for thicker than for thinner specimens. This phenomenon has also been reported for other materials. In order to get a better understanding of the dark discharge process, the monte Carlo technique is used to model electrophotographic dark discharge in materials of which a-Si1-xCx:H is typical. The present study considers every carrier within the model after each increment of a short time step. This then allows bimolecular effects and the presence of space charge to be accounted for.

The study concentrates on two different discharge mechanisms and the effects they have on the TNDDR for varying specimen thicknesses. For the first of these, a negative charge is deposited on the surface of the sample and drifts through the bulk under the influence of the local internal field (Surface Injection model). In the second, electron/hole pairs are generated within the bulk with both carriers being mobile through the specimen (Bulk Generation model). The subsequent surface decay is due to the recombination of bulk generated holes and trapped surface electrons. Results from these models are compared with those found experimentally.
Exploration of Defect Relaxation Dynamics in Hydrogenated Amorphous Silicon using Temperature Switching Experiments
Authors: Gardner Adam , David Cohen J.
Published: April 2011

We have carried out a series of charge transient measurements on a-Si:H in which we insert a double temperature step during the period when electrons are being emitted from deep defects. The behavior of this emitted defect charge is completely inconsistent with any density of states that remains static during the emission; that is, defect relaxation must be invoked. Such measurements allow us to separate the temperature dependence of relaxation from that of thermal emission. In particular, we demonstrate that the emission itself exhibits thermally activated behavior in spite of the ongoing relaxation processes.
Explanation of Light-Enhanced Annealing of Defects in Amorphous Silicon
Authors: Redfield David , Bube Richard
Published: April 2011

Several recent measurements have shown that annealing of metastable defects in a- Si:H can be accelerated by the presence of light. This is the opposite of the usual light-induced defect generation, and no existing rate equation explains it while maintaining the necessary symmetry of generation and recovery processes, and consistency with the stretched-exponential transients that best describe observed generation and anneal behavior. This paper shows that this light-enhanced annealing (LEA) can be explained readily by the usual rate equation leading to stretched exponentials with no other terms by allowing a variation of coefficients with temperature or light intensity. This equation then leads to good simulations of observed LEA. Interpretation of these results in terms of distributional changes is presented, and an experimental test is proposed.
A Quantitative Model for the Metastable Defect Creation in Amorphous Semiconductors by Kev-Electron Irradiation
Authors: Scholz A. , Schröder B. , Oechsner H.
Published: April 2011

The interaction mechanisms of keV-electrons with the hydrogenated Amorphous semiconductor are briefly discussed and the differences to the metastable defect creation by photons are set out. Based on the knowlegde of the energy dissipation mechanisms of keV-electrons in the hydrogenated Amorphous semiconductor, a model for the creation of metastable defects by keV-electron irradiation is developed and its quantitative agreement with the experimental results is shown.
Hydrogen Bonding and Microvoid Stability in a-Si:H
Authors: Van Den Boogaard M. J. , Van Der Steege A. C. , Van Sark W. G. J. H. M. , Van Der Weg W. F.
Published: April 2011

We have annealed PECVD a-Si:H films at 250, 300, and 350°C and measured the evolution of the infrared absorption spectrum. We observe that, during the initial stage of such a heat treatment, atomic hydrogen migrates from the isolated state to the clustered state. Thus diffusion of atomic hydrogen must occur around 300°C. Microvoids with internal surfaces covered with SiH bonds appear to be more stable than voids lined with SiH2 bonds and (SiH2)n polymers.
Metastable Defect Studies in Hydrogen Modulated Multilayer Amorphous Silicon
Authors: Kwon Daewon , David Cohen J.
Published: April 2011

Junction capacitance methods were employed to investigate the metastable states in a-Si:H with hydrogen modulated Multilayer sample as well as companion homogeneous samples. Each layer in the multilayer sample was grown from the same gas mixture (SiH4+Ar or SiH4+H2) as that of homogeneous samples. We observed that in state A and metastable states produced by a short time light soaking the defect density of each layer in the multilayer sample track well the defect density of the corresponding homogeneous sample. This implies that “local” aspects of metastable defect creation dominate in the earlier light soaked states, and also the stable defects in state A are due to local properties of each layer as introduced during growth. However, for longer time light soaked states and succeeding partial annealed states the defect level of the multilayer sample is higher than either of homogeneous samples. However, the spatial variations of the stable defects are finally recovered when the sample is annealed at the same temperature as the initial state A.
Hydrogen Diffusion in Undoped and B-Doped a-Si1-xCx:H
Authors: Shinar R. , Shinar J. , Subramania G. , Jia H. , Sankaranarayanan S. , Leonard M. , Dalai V. L.
Published: April 2011

A deuterium secondary ion mass spectrometry (SIMS) study of hydrogen diffusion in undoped and boron-doped a-Si0.86C0.4:H deposited by an electron cyclotron resonance plasma is described. The undoped films deposited at 250°C clearly indicated deuterium-hydrogen interdiffusion at T ≥ 350°C. The dispersion parameter a of the power-law time dependent diffusion constant D = D00(ωt)−α decreased from ∼0.3 at T = 350 and 400°C to ∼0.1 at 450°C, and the activation energy for a diffusion length of 1000 Å was ∼1.0 eV. These results are discussed in relation to previous studies of a-Si:H. The diffusion in ∼0.2 and ∼0.6 at.% B-doped a-Si0.86C0.14:H sharply differs from that in B-doped a-Si:H, where an enhancement of up to ∼103 was previously observed. In doped a-Si0.86C0.14:H, the diffusion of most of the H atoms is strongly suppressed, but a small fraction undergoes fast diffusion. IR Measurements indicate that the B-doping reduces the bulk-like Si-H stretch vibration at ∼2000 cm1. Upon annealing, the Si-CHn and C-H wag modes at ∼780 and ∼1000 cm−1, resp., increase, while the 640 and ∼2000 cm1 Si-H wag and stretch modes, resp., weaken, indicating transfer of hydrogen from Si- to C-bonds, in which the H atoms are apparently deeply trapped. As in a-Si:H, the fast diffusion component is apparently due to carrier recombination-enhanced weak Si-Si bond breaking. The results suggest that B-doping also induces microvoids and enhances the rate of breaking of weak Si-C bonds, leading to enhanced trapping of H.