bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electrical Characterization of Blue Light Emitting Diodes as a Function of Temperature
Authors: Madhu Murthy , Kamto Tegueu Alphonse Marie , Awaah Michael , Wang Dake , Park Minseo , Walker Frederick J. , Kumar Das Kalyan 
Published: April 2011

AbstractBlue light emitting diodes (LEDs) based on an AlGaN/GaN/AlGaN double heterojunction structure were electrically characterized as a function of temperature. Current-voltage (I-V), capacitance-voltage (C-V) and reverse recovery storage time measurements were conducted at temperatures in the range between -8° and 75° C. Capacitance-voltage measurements as a function frequency (20 Hz – 1 MHz) and electroluminescence study at room temperature were also performed. It was observed that the diode turn-on voltage decreased with increasing temperature, however, reverse leakage currents monitored at -1, -5 and -10 V showed only a slight increase with increasing temperature. The concentration of deep states and their position in the bandgap, as extracted from logarithmic plots of the forward characteristics, were not influenced by the measurement temperature. Recombination lifetimes, as obtained from experimentally determined reverse recovery storage times, remained constant over the range of temperature considered. A higher value of diode capacitance was observed at low measurement frequencies (20 Hz – 1 kHz), gradually dropping to a lower value over a frequency range between (1 kHz – 100 kHz) and remained constant from 100 kHz to 1 MHz. A loss peak centered about 10 kHz was observed in the corresponding plot of gm/ω as a function of frequency, f. The position of the peak in the gm/ω - frequency (f) plot and dC/dω(for f in the range 1 kHz – 100 kHz ), yielded a concentration of deep-states of approximately 2.2 × 1015/cm3, located at 0.39 eV above the valence bandedge.
Reduction of Base Access Resistance in AlGaN/GaN Heterojunction Bipolar Transistors using GaInN Base Cap Layer and Selective Epitaxial Growth
Authors: Shah Jay M , Gessmann Thomas , Luo Hong , Xi Yangang , Chen Kaixuan , Kim Jong Kyu , Schubert E. Fred 
Published: April 2011

AbstractOne of the major challenges affecting the performance of Npn AlGaN/GaN heterojunction bipolar transistors (HBTs) is the high base access resistance, which is comprised of the base contact resistance and the base bulk resistance. A novel concept is proposed to reduce the base access resistance in Npn AlGaN/GaN HBTs by employing polarization-enhanced contacts and selective epitaxial growth of the base and emitter. In addition, this technique reduces the exposed base surface area, which results in a lower surface recombination current. Such a structure would enable better performance of AlGaN/GaN HBTs in terms of higher current gain and a lower offset voltage. Theoretical calculations on polarization-enhanced contacts predict p-type specific contact resistance lower than 10-5 Ωcm2. Experimental results using transmission line measurement (TLM) technique yield specific contact resistances of 5.6×10-4 Ωcm2 for polarization-enhanced p-type contacts and 7.8×10-2 Ωcm2 for conventional p-type contacts.
W2B based High Thermal Stability Ohmic Contacts to n-GaN
Authors: Khanna Rohit , Pearton S J , Kao C J , Kravchenko I I , Ren F , Chi G C , Dabiran A , Osinsky A 
Published: April 2011

AbstractA novel metallization scheme for Ohmic contact (Ti/Al/ W2B /Ti/Au) to n-GaN using high temperature boride was studied using contact resistance, scanning electron microscopy and Auger Electron Spectroscopy measurements. A minimum contact resistance of 7×10-6 Ω.cm2 was achieved for W2B based scheme at an annealing temperature of 800 °C. Contact resistances were found to be essentially independent of measurement temperature, indicating that tunneling plays a dominant role in the current transport. The outdiffusion of Ti to the surface at temperatures of ∼500°C, and at 800°C the onset of intermixing of Al within the contact was found to occur. By 1000°C, the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization. The reliability measurements for the contact resistance of W2B based contact showed excellent stability for extended periods at 200°C, which simulates the type of device operating temperature that might be expected for operation of GaN-based power electronic devices.
Highly p-Type a-GaN Grown on r-Plane Sapphire Substrate
Authors: Tsuchiya Yosuke , Okadome Yoshizane , Furukawa Hiroko , Honshio Akira , Miyake Yasuto , Kawashima Takeshi , Iwaya Motoaki , Kamiyama Satoshi , Amano Hiroshi , Akasaki Isamu 
Published: April 2011

AbstractMg-doped p-type a-plane GaN films were grown on unintentionally doped a-plane GaN templates by metalorganic vapor phase epitaxy (MOVPE). The Mg concentration in a-plane GaN increased with increasing Mg source gas flow rate. A maximum hole concentration of 2.0 × 1018 cm-3 with a hole mobility of 4.5 cm2/Vs and resistivity of 0.7 Ω·cm were achieved. The activation ratio was 5.0 × 10-2. It was found that a maximum hole concentration in p-type a-plane GaN was higher than that in p-type c-plane GaN. The activation energy of Mg acceptors in p-type a-plane GaN with the maximum hole concentration was found to be 118 meV by temperature-dependent Hall-effect measurement.
Structural analysis of ELO-GaN grown on sapphire using the x-ray micro-beam of an 8-GeV storage ring
Authors: Miyajima Takao , Takeda Shingo , Kurihara Hideaki , Watanabe Kyoko , Kato Madomi , Hara Nobuhide , Tsusaka Yoshiyuki , Matsui Junji , Kudo Yoshihiro , Tomiya Shigetaka , Goto Shu , Ikeda Masao , Narui Hironobu 
Published: April 2011

AbstractWe investigated the micrometer-scale structure of epitaxially laterally overgrown GaN (ELO-GaN) on a sapphire without a SiO2 mask using the 2×4 μm2 micro-beam x-ray of an 8-GeV storage ring. The GaN (0 0 0 12) rocking curve of a wing region had a sharp peak with a FWHM of 46 arcsec, while that of a seed region had several broad peaks. This peak-narrowing indicates that the density of threading dislocations (TD) is reduced and the grain size is extended in the wing region by the lateral growth. The observed grain size in the wing region was 10 μm × 5 μm. The mapping of the rocking curves shows that the wing regions hang down from the seed region at a tilting angle of 81.5 arcsec, although there is no SiO2 mask which is a cause of c-axis tilting in ELO-GaN. After comparing the results of ELO-GaN with and without coalescence, we conclude that the small c-axis tilting is caused by the compressive stress induced by the difference in the thermal expansion coefficient of the GaN and the sapphire substrate. Because a shrinkage of grain size was observed after the coalescence of the wing regions, it is thought the coalescence is one of the causes of the generation of the few threading dislocations in the wing regions. We believe that the reduction of the threading dislocations and the expansion of gain size in the wing regions are important for the further improvement of the device characteristics of GaN-based LDs grown on ELO-GaN.
Misfit dislocations in green-emitting InGaN/GaN quantum well structures
Authors: Costa Pedro MFJ , Datta Ranjan , Kappers Menno J , Vickers Mary E , Humphreys Colin J 
Published: April 2011

AbstractMisfit dislocations (MDs) have been observed using transmission electron microscopy (TEM) in InGaN/GaN quantum well (MQW) structures grown under different metal-organic vapour phase epitaxy (MOVPE) regimes and with In-contents equal to or higher than 20%. These dislocations are even observed in a single quantum well 3 nm thick with an In-content of 22%. Conversely, no MDs were observed in QW structures with an In-content of 16%. The presence of MDs in the QW stack leads to strain relaxation which has been confirmed in the indium-rich structures by high resolution X-ray diffraction (HRXRD).
Life cycle analysis of mortars and its environmental impact
Authors: Moropoulou Antonia , Koroneos Christopher , Karoglou Maria , Aggelakopoulou Eleni , Bakolas Asterios , Dompros Aris 
Published: April 2011

AbstractOver the years considerable research has been conducted on masonry mortars regarding their compatibility with under restoration structures. The environmental dimension of these materials may sometimes be a prohibitive factor in the selection of these materials. Life Cycle Assessment (LCA) is a tool that can be used to assess the environmental impact of the materials. LCA can be a very useful tool in the decision making for the selection of appropriate restoration structural material. In this work, a comparison between traditional type of mortars and modern ones (cement-based) is attempted. Two mortars of traditional type are investigated: with aerial lime binder, with aerial lime and artificial pozzolanic additive and one with cement binder. The LCA results indicate that the traditional types of mortars are more sustainable compared to cementbased mortars. For the impact assessment, the method used is Eco-indicator 95
The Application of Energetic SHS Reactions in the Synthesis of Multi-functional Bone Tissue Engineering and Drug Delivery Systems
Authors: Ayers Reed , Burkes Doug , Gottoli Guglielmo , Yi H.C. , Guigné Jaque , Moore John 
Published: April 2011

AbstractThe term combustion synthesis, or self-propagating high temperature synthesis (SHS), refers to an exothermic chemical reaction process that utilizes the heat generated by the exothermic reaction to ignite and sustain a propagating combustion wave through the reactants to produce the desired product(s). The products of combustion synthesis normally are extremely porous: typically 50 percent of theoretical densityAdvantages of combustion synthesis over traditional processing routes, e.g., sintering, in the production of advanced materials such as ceramics, intermetallic compounds and composites include process economics, simplicity of operation, and low energy requirements. However, the high exothermicity and rapid combustion propagation rates necessitate a high degree of control of these reactions.One research area being conducted in the Institute for Space Resources (ISR) at the Colorado School of Mines (CSM) is the application of combustion synthesis (SHS) to synthesize advanced, engineered porous multiphase/heterogeneous calcium phosphate (HCaP), NiTi, NiTi-TiC, TiB-Ti, TiC-Ti for bone tissue engineering and drug delivery systems. Such material systems require a complex combination of properties that can be truly classified as multi-functional materials. The range of properties includes: an overall porosity of 40-60% with a pore size of 200-500 μm; mechanical properties (compression strength and Young’s modulus) that match those of natural bone to avoid ‘stress shielding’; and a surface chemistry that is capable of facilitating bone growth and mineralization.The paper will discuss the synthesis of porous multiphase/heterogeneous calcium phosphate (HCaP), NiTi, NiTi-TiC, TiB-Ti, TiC-Ti for bone tissue engineering and drug delivery systems.
Ultrafast Dynamics of Nanotechnology Energetic Materials
Authors: Yu Hyunung , Hambir Selezion A. , Dlott Dana D. 
Published: April 2011

AbstractOur work involves understanding the chemical reaction dynamics of nanotechnology energetic materials on the time and length scales of individual molecules or nanoparticles. These types of measurements provide insights into fundamental mechanisms and make a close connection to modern atomistic simulation methods. We are especially interested in the relationships between performance and nanostructure. We have developed a number of diagnostic instruments in our laboratory that can be used to probe chemical reaction dynamics, reaction propagation over short length scales, and explosive performance. Some recent results on energetic materials containing Al nanoparticles and either nitrocellulose (NC) or Teflon oxidizers are presented.
Understanding and Tuning the Reactivity of Nano-Energetic Materials
Authors: Rai A. , Zhou L. , Prakash A. , McCormick A. , Zachariah M. R. 
Published: April 2011

AbstractMixtures of fuel and oxidizers with particle sizes in the nanometer range have been widely used for energy intensive applications like propellants and explosives. Nano- Al is invariably used as fuel, while a host of metal oxide nanoparticles are used as oxidizers. This article aims at understanding and tuning the reactivity of these nano-energetic materials. The first part of this article discusses the oxidative reactivity of aluminum nanoparticles as measured experimentally using single-particle mass-spectrometer (SPMS) and microscopy and then modeled. Experimental evidence suggests that outward diffusion of aluminum is an important phenomenon in the oxidation of aluminum nanoparticle. Also melting of the aluminum core is necessary for the reaction to take place vigorously. In the second part of the paper we discuss the formation of novel oxidizers. A super-reactive formulation of Al/KMnO4 has been developed which is shown to be orders of magnitude more reactive than the traditional formulations of Al/Fe2O3, Al/MoO3 and Al/CuO. We demonstrate the formation of novel composite oxidizers to tune the reactivity of the Al/Metal oxide system.
The Inelastic Neutron Scattering spectrum of 2,4-Dinitroimidazole and the Reproduction of Its Solid-State Features by Periodic DFT methods
Authors: Ciezak Jennifer , Trevino Samuel F. 
Published: April 2011

AbstractThe inelastic neutron scattering spectra of 2, 4 –Dinitroimidazole (24DNI) are presented from 25 to 1200 cm-1. Direct comparisons were made to the experimental spectra using solid-state calculation methods at the BLYP/dnd, BP/dnd, and PWC/dnd theory levels. Comparison of the observed and calculated INS spectra revealed that the BLYP/dnd calculations provide the best description of the experimental INS spectrum below 1000 cm-1. The PWC/dnd calculations were found to give the best vibrational agreement with the experimental frequencies above 1000 cm-1. In addition, the six experimental lattice vibrations were assigned. The first overtone of the lattice vibration with the highest vibrational energy is nearly degenerate with one of the fundamental vibrations, suggesting the possibility of a “doorway mode”.
Theoretical Chemical Characterization of Energetic Materials
Authors: Rice Betsy Mavity , Byrd Edward F. C. 
Published: April 2011

AbstractOur research is focused on developing computational capabilities for the prediction of properties of energetic materials associated with performance and sensitivity. Additionally, we want to identify and characterize the dynamic processes that influence conversion of an energetic material to products upon initiation. We are attempting to achieve these goals through the use of standard atomistic simulation methods. In this paper various theoretical chemistry methods and applications to energetic materials will be described. Current capabilities in predicting structures, thermodynamic properties, and dynamic behavior of these materials will be demonstrated. These are presented to exemplify how information generated from atomistic simulations can be used in the design, development, and testing of new energetic materials. In addition to illustrating current capabilities, we will discuss limitations of the methodologies and needs for advancing the state of the art in this area.
Characterising the Response of Energetic materials and Polymer-Bonded Explosives (PBXs) to High-Rate Loading.
Authors: Proud William , Greenaway M.W. , Siviour C.R. , Czerski H. , Field J.E. , Porter D. , Gould P. , Church P.D. , Cullis I.G. 
Published: April 2011

AbstractPolymer-bonded explosives (PBXs) are being increasingly used as energetic fillings and components in many systems. They are perceived as more chemically and mechanically stable than traditional fillings such as RDX/TNT. They are castable into predetermined shapes, machinable and can be used as structural components. However, along with all these undeniable advantages, as a class, these materials are now undergoing extensive characterisation to ensure they comply with both the legal and technical requirements in energetic systems.It is well-known that polymers display non-linear behaviour and are much more complex than, for example, simple metal systems at any rate of strain. The understanding of PBX systems involves areas as diverse as polymer chemistry, chemical compatibility, mechanical properties, impact tests, and thermal stability. In this paper, aspects of energetic material response are outlined which are relevant to the understanding of PBX sensitivity.
Mechanical Properties of Biological Nanocomposite Nacre: Multiscale Modeling and Experiments on Nacre from Red Abalone
Authors: Ghosh Pijush , Verma Devendra , Mohanty Bedabibhas , Katti Kalpana S , Katti Dinesh R 
Published: April 2011

AbstractNacre, the inner iridescent layer of mollusks shell is a bio-nanocomposite with the mineral aragonite as a major constituent and 2-5% of organics mainly in the form of proteins. Our multiscale modeling and experimental studies reveal that the microstructure and the small weight percent of organics are the key parameters attributed to the extreme toughness of nacre. We report that the presence of platelet interlocks nacre have a significant role in the enhancement of mechanical properties. Molecular simulation study is conducted to understand the behavior of aragonite-organic interface. The mechanical behavior of organics and inorganics in presence of each other is described using steered molecular dynamics simulations. This provides some understanding on the deformation mechanisms of the protein present between the aragonite layers. Our nanoindentation results indicate that the elastic modulus and hardness of nacre decreases as it is exposed to a denaturing temperature for proteins. The changes in the organic inorganic interaction have been experimentally described using Fourier Transform Infrared Spectroscopy. This work gives insight into the contribution of the various factors existing at different length scales on the overall mechanical behavior of nacre.
Rheological and Morphological Characterization of Triblock Copolymer (PEO-PPO-PEO)-Clay Gel in Aqueous Solution
Authors: Jiang Jun , Li Chunhua , Rafailovich Miriam , Sokolov Jonathan 
Published: April 2011

AbstractThermal gelation and structure properties of high concentration triblock copolymer poly (ethylene oxide)99-poly(propylene oxide)69-poly(ethylene oxide)99 (Pluronic F127)-clay aqueous solutions were characterized by rheological measurements and scanning electron microscopy (SEM). The sol-gel transition temperature, as well as the viscosity of the F127 solution was found to depend both on the concentration of polymer and clay filler. In all cases, Pluronic F127 gels are pseudoplastic, showing the shear-thinning behavior. Above the gel transition, the viscosity of the solutions increased with increasing the clay concentration. The corresponding macrostructure of the gels was studied by lyophilizing thick samples and scanning them with a scanning electron microscope (SEM). A well-ordered network of porosity was observed as a function of polymer concentration and orientation.
A Model for Nonlinear Viscoelastic Mechanical Responses of Collagenous Soft Tissues
Authors: Oyen Michelle 
Published: April 2011

AbstractExperimental observations of the time-dependent mechanical responses of collagenous tissues have demonstrated behavior that deviates from standard treatments of linear or quasi-linear viscoelasticity. In particular, time-dependent deformation can be strongly coupled to strain level, and strain-rate independence can be observed under monotonic loading, even for a tissue with dramatic stress relaxation. It was postulated that this nonlinearity is fundamentally associated with gradual recruitment of individual collagen fibrils during applied mechanical loading. Based on previously observed experimental results for the time-dependent response of collagenous soft tissues, a model is developed to describe the mechanical behavior of these tissues under uniaxial loading. Tissue stresses, under applied strain-controlled loading, are assumed to be a sum of elastic and viscoelastic stress contributions. The relative contributions of elastic and viscoelastic stresses is assumed to vary with strain level, leading to strain- and time-dependent mechanical behavior. The model formulation is examined under conditions of monotonic loading at varying constant strain rates and stress-relaxation at different applied strain levels. The model is compared with experimental data for a membranous biological soft tissue, the amniotic sac, and is found to agree well with experimental results. The limiting behavior of the novel model, at large strains relative to the collagen recruitment, is consistent with the quasi-linear viscoelastic approach.
Bio-mimetic study of novel materials for joint replacements
Authors: Ribeiro Rahul , Ganguly Poulomi , Darensbourg Donald , Usta Meitin , Ucisik A. Hikmet , Liang Hong 
Published: April 2011

AbstractPolytrimethylene carbonate (PTMC) and poly ε-caprolactone are conventional biodegradable, biocompatible polymers. Their friction response against cartilage and surface attraction forces were studied toward using them as artificial cartilage materials. Their behavior was compared to that of natural cartilage and a conventional joint replacement material, ultrahigh molecular weight polyethylene. It was possible to polymerize these materials using a calcium based catalyst. Simulated body fluid (SBF) was used as lubricant between surfaces in the friction tests. It was found that higher surface attractive forces on a silicon tip AFM related to lower friction coefficients. This confirs the fact that hydrophilic surfaces enhance the effectiveness of boundary lubrication of simulated body fluid. PTMC and PTMC-ε-caprolactone co-polymer showed lower hydrophilicity and higher friction coefficients and need to be modified in order to bring them closer in behavior to natural cartilage.
Polypeptide-Solvent Interactions Measured by Single Molecule Force Spectroscopy
Authors: Valiaev Alexei , Lim Dong Woo , Chilkoti Ashutosh , Schmidler Scott , Zauscher Stefan 
Published: April 2011

AbstractStimulus-responsive biomolecules have attracted a large research interest because of their potential application in various areas such as drug delivery, actuators and sensing devices at the nanoscale. Using single-molecule force spectroscopy (SMFS) we studied elastin-like polypeptides (ELPs). These stimulus-responsive polypeptides undergo an inverse temperature transition, accompanied by a large conformational change, when the solvent quality is changed by increasing the temperature or by addition of salt. Understanding the relationship between peptide sequence and mechanisms of force generation can provide a route to engineer ELPs with desirable mechano-chemical properties. Here we studied the effect of solvent quality and type of guest residue on the mechanical properties of ELPs on the single-molecule level. We used a statistical approach to estimate polymer elasticity parameters from model fits to the data. With this approach we were able to resolve small changes in the Kuhn segment length distributions associated with different molecular architectures. We then show that these mechanical differences likely arise from differences in the hydrophobic hydration of sidegoups, in line with recent predictions from molecular dynamics simulations.
Biosynthesis and Electron Microscopy Characterization of Diatom Nanocomposites
Authors: Gutu Timothy , Jiao Jun , Jeffryes Clayton , Qin Tian , Chang Chih-hung , Rorrer Gregory L. 
Published: April 2011

AbstractThe fabrication of Si-Ge oxide composites in a two-stage photobioreactor cultivation process was systematically optimized by increasing the amount of germanium assimilated into the diatom cells. In this optimization process of the synthesis of Si-Ge oxides that maintain the original morphology of the diatoms, high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) both equipped with an energy dispersive X-ray spectrometer were extensively used to characterize the evaluation of the chemical composition and structural properties of the processed diatoms.