bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Crystallographic Orientation on Film Morphological Evolution
Authors: Sanchez John E.
Published: April 2011

The factors that determine the thermal strains, stresses and plastic yielding during the annealing of deposited Al thin films on Si substrates are reviewed. The effect of film texture orientation on dislocation glide is described and is shown to lead to variations in film strain energy with texture orientation. The strain energy difference between adjacent grains of different texture is proposed to account for several morphological changes during film annealing. If grain boundaries are mobile (i.e., with small initial diameter) film stresses and strain energies may induce the (secondary) growth of weakly oriented (110), (112), etc., grains at the expense of the normally favored (111) strongly oriented grains. If grain boundaries are stagnant, the strain energy differences between grains may induce the hillock growth of weakly oriented (110) grains in order to reduce film compressive stresses, and may also induce the depletion “sinking” of strongly oriented (111) grains in order to relieve film tensile stresses. This model generally accounts for these experimental findings of others.
Mechanical Behavior of Single Crystal A1 (111) and Bicrystal Al (110) Films on Silicon Substrates
Authors: Besser Paul R. , Sanchez John E. , Brennan S. , Bravman John C. , Takaoka G. , Yamada I.
Published: April 2011

Single crystal Al (111) films and bicrystal Al (110) films have been deposited on bare Si (111) and (100) wafers using ion-cluster beam deposition. The stress in the films was determined using X-rays diffraction as the films were thermally cycled from room temperature to 400C. The (111) film exhibited nearly ideal elastic behavior as the stress was essentially linear with temperature. The (110) Al film yielded in compression at a lower temperature and stress than the (111) film and exhibited broad hillocks not found in the Al (111) film. During the second thermal cycle, both films behaved in a nearly ideal elastic fashion. Measurement of the strain relaxation in the films during the second thermal cycle showed that the (110) film relaxed significantly while the (111) did not relax.
Crystallization of Hydrogenated Amorphous Silicon Thick Films on Molybdenum Substrates
Authors: Sridhar Nagarajan , Chung D. D. L. , Anderson W. A.
Published: April 2011

Crystallization of hydrogenated amorphous silicon thick films deposited by dc glow discharge on molybdenum substrates was studied by Raman scattering and x-ray diffraction. Investigation was made as a function of amorphous silicon film deposition temperature. On heating the films at a rate of 5 °C/min to 650 °C for various times, it was observed that the film deposited at 300 °C started crystallization faster than the film deposited at 150 °C. The degree of cirystallinity increased with increasing annealing time for all the films. However, at all annealing times, the degree of crystallinity for the annealed film deposited at 150 °C was higher than that of the annealed film deposited at 300 °C, indicating that the crystallization growth rate was higher for the film deposited at a lower temperature. These results were consistent with the dark conductivity measurements. The film deposited at 150 °C showed a photoresponse which increased with increasing annealing time whereas no photoresponse was observed for the film deposited at 300 °C. This was probably due to the degree of crystallinity and grain size being much larger for the film deposited at 150 °C than the film deposited at 300 °C.
XRD Texture and Morphology Analysis of Polycrystalline LPCVD Germanium-Silicon
Authors: Salm Cora , Klappe Jos G.E. , Holleman Jisk , Bart Rem Jan , Woerlee Pierre H.
Published: April 2011

The morphology and texture of polycrystalline LPCVD GexSi1-x alloys have been studied for various deposition temperatures and for x = 0, x = 1 and x ≈ 0.3. The transition temperature of amorphous to polycrystalline growth is much lower for Ge and GexSi1-x than for Si. Just above this transition temperature, we observed a (220) orientation for all polycrystaline layers grown. At elevated temperatures a change to the (004) orientation for poly-GexSi1-x and (331) for poly-Si and poly-Ge is noted.
Piezoresistive Properties of Boron-Doped PECVD Micro- and Polycrystalline Silicon films
Authors: Le Berre M. , Lemiti M. , Barbier D. , Pinard P. , Cali J. , Bustarret E. , Bruyère J.-C. , Sicart J. , Robert J. L.
Published: April 2011

The electrical and piezoresistive properties of in-situ doped PECVD silicon films deposited on oxided silicon wafers have been investigated. One series of films was deposited in the so-called microcrystalline state at 450°C. The other set of samples was deposited in the amorphous state at 320°C and subjected to rapid thermal annealing. Structural properties (grain size, texture, residual stress) were evaluated experimentally through TEM and grazing angle X ray diffraction and related to the measured gauge factor. A maximum longitudinal gauge factor of 28 is measured in the case of advantageously textured microcrystalline material, the magnitude of the gauge factor decreasing sharply for randomly oriented material. For the amorphous deposited and subsequently annealed material, the longitudinal gauge factor is in the range 22–27 depending on dopant concentration. These experimental features are compared to the results of a theoretical approach of piezoresistance in polysilicon. We derive various expressions of the gauge factor according to the assumptions of either constant stress or constant strain within the aggregate. In the case of untextured films, analytical Voigt-Reuss-Hill averages for the elements of piezoresistive and elastoresistive tensors lead to greatly simplified expressions. Theoretical estimates are shown to be in reasonable agreement with the experimental measurements. This confirms the great potential of PECVD microcrystalline and polycrystalline silicon for strain gauges.
Pulsed Laser Deposition of NbNx (0 ≤ x ≤ 1.4) Thin Films
Authors: Treece Randolph E. , Horwitz James S. , Chrisey Douglas B.
Published: April 2011

The structure, morphology, and electrical properties of pulsed laser deposited NbNx (0 ≤ x ≤ 1.4) thin films have been investigated. Films were deposited from Nb metal targets on oriented MgO (100) and amorphous fused silica substrates as a function of substrate temperature and ambient pressure. A reducing atmosphere (N2 with 10% H2) was used to prevent oxide formation. At elevated temperatures, the N/Nb ratios of films deposited on MgO, as determined from Rutherford Backscattering Spectroscopy, increased from 0 −1.4 as the ambient pressure was varied from vacuum to 200 mTorr, respectively. NbNx films (x=l) were deposited at 600 °C and 60 mTorr. Both the structure and electrical properties of the deposited films varied strongly with the substrate type. On fused silica, NbN films were poorly crystalline with low critical temperatures (Tc ∼ 8 – 11 K) and low critical currents (Jc (4.2 K) ∼ 2 MA/cm2), whereas on MgO the NbN films were oriented and had better transport properties (Tc ∼ 16.4 K and Jc (4.2 K) = 7.1 MA/cm2).
Evolution of the Properties of Reactively Sputtered Zirconium Nitrides on Silicon Substrates
Authors: Brande Pierre Vanden , Lucas Stephane , Winand Rene , Renard Lucien , Weymeersch Alain
Published: April 2011

Zirconium nitride films on silicon substrates have been prepared by DC magnetron enhanced reactive sputtering in a N2/Ar gas mixture under various experimental conditions. The films properties (chemical composition, structure, morphology and optical response) were investigated and related to the experimental conditions. It is shown that these properties are strongly related to the target current density which governs the deposition rate, to the deposit thickness, to the nitrogen partial pressure and to the residual gas contamination. The objective of this paper is to detect the minimum film thickness threshold under which a zirconium nitride coating cannot be used in decorative applications.
Improved Abruptness of Si/Sin Interface by ArF Excimer-Laser Pre-Annealing to Sin
Authors: Uchida Yasutaka , Matsumura Masakiyo
Published: April 2011

XPS measurement showed that undesirable SiNH component was reduced drastically from the low-temperature deposited SiN surface by intense ArF excimer-laser irradiation. Although the improved layer was as thin as 15nm, it was very effective to stop diffusion of N atoms from the bottom SiN layer to the top Si layer during the excimer-laser recrystallization step. N-diffused Si layer at the Si/SiN interface was less than the XPS resolution limit for the pre-annealed SiN structure, but about 5nm thick. As a result, the field-effect mobility of the poly-Si/SiN TFT was increased drastically by laser-irradiation to SiN film. Annealing characteristics are also presented for the various SiN film thicknesses and for both the ArF and KrF excimer-laser lights.
A New Offset Gated Poly-Si TFT Using a Wet Oxidation
Authors: Min B.H. , Oh J.M. , Han M.K.
Published: April 2011

Several off-set gated TFTs have been proposed to reduce the off current. However, those structures may require an additional mask and photolithography process step. We propose a novel off-set gated poly-Si TFT which reduces the off current considerably without any additional mask.

We have simulated the device characteristics by a numerical simulator in order to verify the proposed TFT. The simulation results show that the off current of the new device has been reduced to 0.ipA. The reduction of on current is much less than the off current reduction and the on/off current ratios exceeding 107 have been accomplished.
Structural, Optical And Electronic Properties Of Semi-Insulating TIN Oxide Thin Films
Authors: Acutis A. De , Rossi M. C. , Barbetti M.
Published: April 2011

Bismuth doped SnO2 films with reproducible optical and electronic properties were deposited by spray-pyrolysis on quartz and glass substrates.Deposition temperatures in the range 300°C–360°C were selected and resistivity values in the range 104−1011Ω/square were obtained. Scanning Electron Microscope (SEM) analysis and X-ray diffractometry showed that samples grown at low temperature are amorphous, while at higher deposition temperature the material structure becomes microcrystalline. The spectral transmittance of the films was measured in the UV-VIS range. The absorption coefficients were interpreted to give values of both direct and indirect band gap. The slope of Tauc's plot, which is commonly related to the degree of disorder in the material, confirmed the SEM observations. Conduction mechanisms were also studied through resistivity measurements as a function of deposition temperature. Results obtained for undoped tin oxide were also investigated for comparison.
Transparent Conducting Films Grown By Pulsed Laser Deposition
Authors: Phillips Julia M. , Cava R. J. , Hou S. Y. , Krajewski J. J. , Kwo J. , Marshall J. H. , Peck W. F. , Rapkine D. H. , Thomas G. A. , Dover R. B. Van
Published: April 2011

We have studied the properties of two types of transparent conducting oxides grown by pulsed laser deposition (PLD) on quartz substrates. We have grown films of ITO with resistivity as low as 250 μΩ-cm and with absorption coefficient < 600 cm−1 throughout the visible spectrum. Even films deposited at room temperature can have ρ < 500 μΩ-cm, although the optical transmission characteristics are worse than those of commercially available ITO. Important parameters governing film quality include the oxygen partial pressure during film deposition and substrate temperature. GaInO3 is a recently identified transparent conducting material which is structurally distinct from ITO. Films have been grown with no intentional dopants and with either Ge substitution for Ga or Sn substitution for In. Doping concentrations as high as 10 at. % have been studied. There is no evidence for dopant segregation. Films with resistivities as low as 3 μΩ-cm have been achieved, and the absorption coefficient can be less than 500 cm−1 throughout the entire visible spectrum.
Novel Thin Film Light Emitting Diode Display Made Of Amorphous Silicon-Based Semiconductors
Authors: Kruangam Dusit , Boonkosum Wirote , Panyakeow Somsak , Delong Bancherd
Published: April 2011

A novel Thin Film Light Emitting Diode (TFLED) flat panel display was developed. The TFLED is a carrier injection-type electroluminescence and made of hydrogenated amorphous silicon-based semiconductor p-i-n junctions. The amorphous layers employed in this work are for example, a-Si1−xCx:H and a-Si1−xNx:H. The TFLED has two basic structures; 1) glass substrate/ITO/amorphous p-i-n layers/Al and 2) metal sheet substrate amorphous n-i-p layers/ITO. The typical thicknesses of the amorphous p-i-n layers are 150 Å, 500 Å and 500 Å, respectively. The color of the emission can be changed from red to white-blue by increasing the optical energy gap (2.5−3.5 eV), that is the atomic fraction x, in the i-layer. The brightness of the TFLEDs are of the order of 1–10 cd/m2 with injection current density of 100–1000 mA/cm2 and applied voltage of 8–15 V.
Optical Properties Of Silicon Clusters Deposited On The Basal Plane Of Graphite
Authors: Dinh L. N. , Chase L. L. , Balooch M. , Tenninello L.J. , Tench R. J. , Wooten F.
Published: April 2011

Laser ablation and post annealing was employed for the synthesis of silicon (Si) clusters on highly oriented pyrolytic graphite surfaces in an ultra high-vacuum environment. The size distribution of the clusters was determined as a function of annealing time and temperature using an in situ scanning tunneling microscope (STM). Pure Si clusters with sizes ranging from 1 to 10 nm showed no detectable photoluminescence (PL) in the visible range. Exposure of these clusters to oxygen at 10−6 Torr and for up to 8 hours showed adsorption of oxygen on the surface of the clusters without Si oxide formation and no detectable PL. Hydrogen termination of these clusters was accomplished by exposing them to atomic hydrogen beam but did not result in any detection of the PL spectra either. Prolonged exposure of these clusters to ambient air, however, resulted in strong PL spectra with color ranging from red to greenish-blue depending on average cluster size. Auger electron spectra (AES) revealed the existence of partially oxidized Si clusters for these samples. This PL could be due to either an oxide phase or to quantum confined Si inner cores.
Non Destructive Thermometry by Means of Microwave Radiometry
Authors: Bocquet B. , Van De Velde J. C. , Mamouni A. , Leroy Y.
Published: April 2011

The measurement of the thermal noise transmitted by lossy materials in the microwave frequency range is, at the present time, the only one thermometric method able to bring information about the temperature distribution inside a material without insertion of any sensor. In this paper, we first briefly recall the principles of the method, and the present state of the experimental set-ups. This technique interest quite different domains of applications such as industrial thermometry, biomedicai engineering, domotics and microelectronics. In other words, the presentation is related to different situations:

∼ material at uniform or non uniform temperatures

∼ temperatures over or beneath the room, or reference temperature

∼ different materials: high or low losses, homogeneous, multilayered or heterogeneous

∼ measurement of a thermal noise temperature or of a correlation temperature.

The paper analyses also the reasons why these method have not still much penetrated the industrial market and explains the arguments for a future extension of these kinds of applications.
Microwave On-Line Measurements on Conveyed Products Application to Paper and Wood Industries
Authors: Bolomey J.Ch. , Cottard G. , Berthaud P. , Lemaitre A. , Portala J. F.
Published: April 2011

Microwave multiport sensors have been shown to provide some unique capabilities to achieve real-time testing of products conveyed at high speed. In many applications, quantitative measurements of physical quantities such as moisture content, density, etc… are required, either to guarantee reliable production or to optimally control a fabrication/transformation process. In this paper, different ways of extracting such physical quantities from microwave measurements performed by multiport sensors are presented. Model approaches are used, based on polynomial expansions of the physical quantities to be measured as a function of the microwave amplitude and phase data. Calibration procedures have been investigated for both paper and wood material samples. Comparisons between in-situ, microwave and conventional, measurements are analysed.
Evaluation of the Bonded Silicon-on-Insulator Wafer with Lifetime Measurement Using a Non-Contact Laser-Microwave Method
Authors: Usami Akira , Yamaguchi Yuji , Ichimura Masaya , Ishigami Shun-ichiro , Matsuki Kazunori , Takeuchi Tsutomu , Wada Takao
Published: April 2011

In this study, we evaluated the electrical properties of the bonded silicon-on-insulator (SOI) wafers with lifetime measurements using a non-contact laser-microwave method. We prepared one group that consisted of bonded SOI wafers with different active layer thicknesses (I0,30,100μm) and another group consisting of bonded SOI wafers with different buried oxide layer thicknesses(0,0.01,0.1,0.75μm). Primary mode lifetime (τ1) was measured by the photoconductivity decay (PCP) method using the laser diode (λ= 774nm) as a carrier-injected light source. Steady-state change in the conductivity was measured by the photoconductivity modulation (PCM) method using a He-Ne laser (λ = 633nm) as a carrier-injected light source. τ1 decreases as the active layer thickness decreases. The PCM intensity also decreases with decreasing active layer thickness. Surface and interface recombination rates of the SOI are increased with decreasing layer thickness. The PCM intensity also decreases as the buried layer thickness decreases.
Contactless Characterization of the Surface Property of the Si+-Implanted GaAs
Authors: Usami Akira , Yoshida Hideaki , Ichimura Masaya , Wada Takao
Published: April 2011

The surface recombination of GaAs which has a high carrier concentration layer (HCCL) formed by Si+ implantation has been investigated using the reflectance microwave probe (RMP) method. The RMP method enables us to evaluate the surface property of GaAs contactlessly and easily. The experimental results of the samples which were implanted with doses ranging from 1.0×1011 to 3.9×1012cm-2 at an energy of lOOke V indicate that the effective surface recombination velocity decreases with dosage because of HCCL formed after the annealing. On the other hand, the results of the samples which were implanted with a dose of 3.9 × 1012cm-2 at energies ranging from 30 to 180keV indicate that the effective surface recombination velocity increases with energy for energies larger than 50keV. We understood the reason by comparing with the numerical calculation results of an effective surface recombination velocity at a high-low junction interface.
Contactless Mass Determination of Arbitrarily Shaped Objects by Microwave Resonator Measurements
Authors: Kraszewski Andrzej W. , Nelson Stuart O.
Published: April 2011

The basis is presented for using a microwave resonant cavity as an effective “contactless balance”, providing information about the mass of dielectric objects inserted into the cavity. An uncertainty analysis presented in the paper confirms that the mass of small dielectric objects can be determined by this technique with an accuracy better than 4%.
Simulation of Hybrid Heating
Authors: Skamser Daniel J. , Johnson D. Lynn
Published: April 2011

A hybrid microwave heating arrangement used to heat alumina fiber preforms has been modeled to determine what conditions are needed to create significant differences in temperature across the specimen while still maintaining control of the temperature. The model assumes a uniform external microwave field and dielectric properties for the specimen as a function of temperature. The results of the model show that significant differences in temperature can be attained while still having control of the temperature at the center. Experiments show that the temperature at the center can be held at 1000°C and still allow a difference in temperature of 50°C between the center and the surface of the specimen to be present.
Growth and Stabilization of Hot Spots in Microwave Heated Ceramic Fibers
Authors: Kriegsmann Gregory A.
Published: April 2011

Recently the heating of a thin ceramic cylinder in a single mode applicator was modeled and analyzed assuming a small Biot number and a known uniform electric field through out the sample. The resulting simplified mathematical equations explained the mechanism for the generation and growth of localized regions of high temperature. The results predicted that a hot-spot, once formed, will grow until it consumes the entire sample. Although this phenomenon is seen in some experiments, others show that the hot-spot stabilizes and moves no further.

A new model is proposed which incorporates the dependence of the thermal conductivity and the effective heat transfer coefficient upon temperature, and the nonuniformity of the electric field along the fiber axis. The resulting simplified mathematical equations indicate that these effects may stabilize the growth of hot-spots.
Microwave Heating of Carbon Coated Ceramic Fibers
Authors: Kriegsmann G. A. , Wagner B. A.
Published: April 2011

The microwave heating of a thin carbon coated ceramic fiber is modeled and analyzed in the small Biot number regime. The electric field is assumed known and uniform throughout the cross-section, and does not vary along the axis of the cylinder. We present a model which gives a functional relationship between the coating thickness and the final temperature of the fiber. It gives a plausible explanation of the experimentally observed time evolution of temperature for a tow of fibers [1].
Microwave Sintering of Baryum Hexaferrite
Authors: Rousselle D. , Autissier D. , Guillou B. , Leclerc J.
Published: April 2011

Conventional sintering of baryum hexaferrite Co2Z leads to a mixture of phases with a low density. In this paper we try to obtain a pure densified CO2Z hexaferrite by microwave heating. The design of the applicator is computed from both the electromagnetic physical characteristics, the dimensions and the shape of the hexaferrite sample. The travelling microwave mode is preferred to the resonant one. Microwave process consists in a debinding step followed by the high temperature sintering cycle. Sintering temperature evaluated by infrared sensors seems to be over-valued. A skin effect is observed which can be attributed to electronic conductivity due to high temperature formation of Fe2+ ions.
Improvements on Electrical Properties of Ultra-Thin Silicon Oxides Grown by Microwave Afterglow Oxygen Plasma
Authors: Chen P. C. , Lin J. Y. , Hwang H. L.
Published: April 2011

Fundamental characteristics such as the oxide breakdown fields, oxide charges and interface state density of various ultra-thin silicon oxides (≤ 8 nm) grown by microwave plasma afterglow oxidation at low temperatures (400 °C and 600 °C) were investigated. The effective Oxide charge density of 600 °C as-grown oxide was as low as 6×1010 cm-2. The breakdown fields of the oxides were further enhanced and the interface state densities were reduced by employing fluorination (HF soaked) and low temperature N2O plasma annealing. The breakdown field of the thin oxide grown at 600 °C with 15 min N2O plasma annealing was 12 MV/cm. The reduction of interface state density was about 35% for 600 °C fluorinated oxide. When integrated with poly-gate process, the interface state density was as low as 5×1010 cm-2eV-1.
Alternative Cavity Design for Microwave Joining of Long Ceramic Tubes
Authors: Sa'adaldin Hussamaldin S. , Black W. Murray , Ahmad Iftikhar , Tian Yong-Lai , Silberglitt Richard
Published: April 2011

A new single mode applicator has been designed and fabricated to provide effective and nearly uniform heating of long ceramic tubes or rods at 2.45 GHz. The applicator is made of two double mitered waveguide corners. A comparison between the new cavity and a rectangular cavity has been carried out to heat various ceramic samples. Results show a significant improvement in the heating rate as well as the reduction of the power needed to attain a given temperature. This paper describes the preliminary test results of the new cavity and the potential to achieve joining of long tubes or rods.
Polymer Curing Using Variable Frequency Microwave Processing
Authors: Surret April D. , Lauf Robert J. , Paulauskas Felix L. , Johnson Arvid C.
Published: April 2011

Epoxy resin was cured by microwave heating using a variable frequency furnace. Curing behavior and uniformity were characterized by visual inspection, hardness, and differential scanning calorimetry. Frequency sweeping was used as a means of mode stirring to minimize power nonuniformities in the furnace cavity. Cure times were reduced by half and with the appropriate post-cure treatment (also done in the microwave furnace) the resin had a glass transition temperature of 165–175°C. Small amounts of additives such as carbon black and various oxide pigments were tested to determine if they would improve microwave coupling and further reduce cure time. There was little or no effect at a standard concentration of 1 wt% pigment.
Status of Cerium Fluoride Performances
Authors: Auffray E. , Dafinei I. , Lecoq P. , Schneegans M.
Published: April 2011

Cerium fluoride offers a reasonable compromise between parameters like the density, the light yield, the scintillation characteristics (particularly the decay time) and the radiation hardness, and is considered today as the best candidate for large electromagnetic calorimeters in future High Energy Physics experiments. Details on the performances of large crystals produced by different manufacturers all over the world and measured by the Crystal Clear collaboration will be shown and the usefulness of a good collaboration between the industry and the users will be highlighted by some examples on the light yield and radiation hardness improvement.
The Role of Core Levels in Scintillation Processes
Authors: Belsky Andrey N. , Kamenskikh Irina A. , Vasil'ev Andrey N. , Gektin Alexander V. , Pedrini Christian , Martin Patrick
Published: April 2011

The Auger decay of a core hole results in appearance of several strongly correlated excitations. This excited region strongly polarizes the lattice and thus the defect creation is possible. In all cases the core hole causes the strong local perturbation of electronic and lattice subsystems. The creation of such excited region with mutual relaxation of correlated electrons and holes can result in the increase of the efficiency of energy transfer to activators, acceleration of the luminescence kinetics, and the appearance of radiation-induced luminescence centers.
Dependence of Scintillators Optical Properties on Intrinsic Structural Defects
Authors: Kilassen N.V.
Published: April 2011

The studies of the dependence of the optical properties of various scintillators on intrinsic structural defects have been reviewed. The greater part of the review is devoted to the defects introduced by plastic deformation. A wide range of variations in the light output, spectral distribution, kinetics and other properties has been observed. These defects can be induced during crystal growth, annealing, processing, etc. The proper regulation of the superstructure of intrinsic defects can ensure the production of high quality scintillators having required properties.
Comparative Studies of Optical Spectra in PbF2 at Different Excitations
Authors: Egorov V. K. , Klassen N.V. , Negrii V.D. , Prokopenko V.M. , Shmurak S.Z. , Sinitzin V.V. , Solov'ev A.V.
Published: April 2011

The spectral characteristics for cubic and orthorhombic lead fluoride excited by light with energies of 5–2.5 eV, X-ray, and α-particles were studied. It is shown that the room temperature luminescence in orthorhombic lead fluoride is connected with resonance excitations of some luminescence centers. Possible models of the centers responsible for this phenomenon are proposed.
Suppression of BaF2 Slow Component of X-RAY Luminescence in Non-Stoichiometric Bao.9R0.1F2.1 Crystals (R=Rare Earth Element)
Authors: Sobolev B.P. , Krivandina E.A. , Derenzo S.E. , Moses W.W. , West A.C.
Published: April 2011

A series of BaF2 crystals doped with 10% mole fraction of each rare earth element has been grown, and the effect of these dopants on the slow luminescence (due to anionic self trapped excitons) and fast luminescence (due to core-valence transitions) has been studied. While significant suppression was observed for both components, the best dopants for suppressing the slow component of barium fluoride (up to 25-50%) while maintaining the luminosity of the fast component are La, Y, and Lu. The luminescence of Ba0.9Eu0.1F2.1 is almost entirely fast (>90%), but with low light output. For two rare earth dopants (La and Gd), the effect of slow and fast component suppression was studied as a function of concentration (Ba1-xRxF2+x with x≦0.5 for R=La and x≦0.3 for R=Gd). The suppression is non-linear with dopant concentration, with the relative degree of slow component suppression correlated with the melting point of these samples.
The Scintillation Mechanism in PbWO4 Crystals
Authors: Korzhik M.V. , Pavlenko V.B. , Katchanov V.A. , Peigneux Jean-Pierre , Poulet Maurice
Published: April 2011

A mechanism of scintillations based on sensitisation of radiating centres luminescence by Pb2+ ions is proposed for PbWO4 crystals.
Monte-Carlo Study of Energy Losses in Hot Stage of Electronic Excitation Relaxation in Scintillators
Authors: Glukhov Roman A. , Vasil'ev Andrey N.
Published: April 2011

The results of computer simulation of the fast stages of energy relaxation in insulators after the VUV or XUV photon absorption are presented. The simulation involves two stages:the inelastic scattering of excitations with production of secondary excitations and thermalization through phonon emission. The main attention is focused on the spatial distribution functions of excitations, i.e. one-particle and two-particle distribution functions.The latter Function determines the energy transfer at final stages of energy relaxation and is important for different quenching processes and the acceleration of the luminescence decay. The Monte-Carlo simulation was carried out for BaF2 crystal for photon energies from 20 eV to 100 eV. The simulation shows that the two-particle distribution functions and thus the kinetics depend on the energy of excitation.
The Mechanism of Slow Component Suppression in Lanthanum Doped Barium Fluoride Crystal
Authors: Mu Gu , Lingyan Chen , Qing Li , Liming Wang , Kaihua Xiang
Published: April 2011

The electronic structures of pure BaF2 crystal and lanthanum doped BaF2 crystal have been calculated in a self-consistent molecular-cluster model. The cluster is embedded in the crystal lattice and the entire system treatediteratively in the Hartree-Fock-Slater local-density theory. As lanthanum doped BaF2 is concerned, the obtained results revealed that the F1–i which is introduced by the lanthanum may contribute to the suppression ofthe slow component in the scintillation light of BaF2 crystal.
Low-Voltage Cathodoluminescent Properties of Europium-Activated Anion-Deficient Fluorites
Authors: Anderson Mark T. , Phillips Mark L. F. , Walko Robert J.
Published: April 2011

We have examined the optical bandgap, particle size distribution, photoluminescence spectra, and cathodoluminescent response of fifteen phosphors that contain transition-metal and main-group sensitizers. We determined luminance versus applied voltage curves for electron energies from 5 to 1000 eV. Seven phosphors exhibit threshold voltages less than 110 V. Y1.96Eu0.04O3 exhibits a threshold voltage of 13 V and, at 300 V, displays a luminance of 25 fL and a luminous efficiency of 0.02 lm/W.
Particle Size Effects in YAG:CR Phosphors
Authors: Mckityrick J. , Hoghooghi B. , Dubbelday W. , Kavanagh K. , Kinsman K. , Shea L. , Sluzky E.
Published: April 2011

Chromium doped yttrium aluminum garnet (Y3Al5O12:Cr or YAG:Cr) phosphors were synthesized by hydrolysis of aqueous nitrate solutions. The resulting as-synthesized powder was amorphous and crystallized into the YAG structure at ~1000°C. The light output as a function of annealing temperature was found to increase with increasing temperature. Particle size, adsorbed surface species or residual impurities, lattice stability and chromium site occupation were investigated as possible explanations. It was found that there was a small decrease in lattice parameter upon annealing, the particle size increased roughly five times, and residual or adsorbed surface species were present for the low temperature annealed powders. There was no evidence that kinetic barriers to diffusion of Cr ions into the luminescent sites occurred.
Research and Development Works on BaF2 Crystals in Shanghai Institute of Ceramics
Authors: Yin Z. W.
Published: April 2011

A summary of some experimental results and achievements on BaF2 scintillating crystals research and development done in Shanghai Institute of Ceramics (SIC).
The Effect of Elevated Temperature Exposure of Composites on the Strength Distribution of the Reinforcing Fibers
Authors: Gambone M. L. , Wawner F. E.
Published: April 2011

Unidirectionally-reinforced Timetal® 21S composite specimens were subjected to elevated temperature heat treatments. The SiC fibers were then chemically extracted from the matrix, and their tensile strengths were measured at room temperature. A Weibull statistical analysis of fiber strength distribution was performed to compare the Weibull parameters of fibers from the as-consolidated and heat-treated composites. Fractographic analysis of the tested fibers was used to identify the flaws which caused failure in each condition. Surface flaws were found to initiate low strength failures in all conditions, and the number of surface initiated failures increased with an increase in severity of heat-treatment. A relationship between the fiber/matrix chemical reaction and surface flaw development is demonstrated. A fracture mechanics analysis that explains the relationship between surface flaw size, fiber fracture toughness, and the measured tensile strengths is suggested.
Mechanical Behavior of Pressure-Cast Al2O3/NiAlFe Composites
Authors: Liu Cyrus , Jeng S. M. , Yang J.-M. , Slate E. , Cook A.
Published: April 2011

The microstructure and mechanical behavior of Al2O3 fiber-reinforced NiAlFe composites fabricated using the pressure casting technique were investigated. The mechanical response and fracture behavior of these unidirectional composites under tensile and three-point flexural loading were characterized. The results were compared to those measured for Al2O,/NiAl-based composites fabricated by powder metallurgy techniques. Preliminary results indicate that pressure casting appears to be a viable processing route for fabricating continuous fiber-reinforced NiAl-based composites.
Synthesis and Mechanical Properties of Nano-Scale NiAl and NiAl/Al2O3 Composites
Authors: Smith Timothy R.
Published: April 2011

This work reports on the synthesis, characterization and preliminary mechanical properties of nanocrystalline NiAl/Al2O3 composite materials. Nano-scale crystallites of NiAl were formed by high energy milling of pre-alloyed powder with and without Al2O3 under an Ar atmosphere. Consolidation of the nano-scale powders was done by sinter-forging at < 0.4 Tm in air. Characterization of the powders and consolidated microstructures and hardness data are reported.
Fatigue and Fracture of Nickel Aluminide Composites
Authors: Ramasundaram Padu , Bowman Randy , Soboyejo Wole
Published: April 2011

The results of preliminary investigations of the room-temperature fatigue and fracture mechanisms in model NiAl composites are presented. The composite systems studied include: NiAl reinforced with ductile second phase particles (Mo); ductile fibers (Mo); brittle second phase particles (zirconia partially stabilized with yttria) and brittle fibers+particles (Al2O3 + zirconia). Mechanisms of fatigue crack growth in heat treated specimens of the fiber-reinforced composites are also elucidated. The investigations indicate that both ductile and brittle reinforcements can enhance the toughness of NiAl significantly, and the ductile phase particulate reinforcement may even contribute to room-temperature ductility in the composite.
Atomistic Simulation of Dislocation Motion as Determined by Core Structure
Authors: Ternes Kevin , Farkas Diana , Xie Zhao-Yang
Published: April 2011

Two different interatomic potentials of the embedded atom type were used to study the relationships between dislocation core structure and mobility. Core structures were computed for a variety of dislocations inB2 NiAl. Several non-planar cores were studied as they reacted to applied stress and moved. The results show that in some cases, the dislocation core transforms to a planar structure before the dislocation glides, whereas in some other cases the core retains the non-planar structure at stresses sufficient to sustain glide. The effects of stoichiometry deviations on the core structure and motion were also studied.
Bio-Mimetic Composites
Authors: Burdon Jeremy , Szmania Jeffrey , Calvert Paul
Published: April 2011

The properties of bone, as a polymer reinforced with nanometer-sized ribbon-shaped crystals of mineral, are compared with the properties of synthetic polymer composites. Bone does show some superiority to existing composites. The improvements can be attributed to the microstructure. Methods for reproducing this structure in a synthetic material are discussed.
Frequency-Dependent Second Harmonic Generation in Acentric Chromophoric Self-Assembled NLO Materials
Authors: Yitzchaik Shlomo , Lundquist Paul M. , Lin Weiping , Kanis David R. , Ratner Mark A. , Marks Tobin J. , Wong George K.
Published: April 2011

An attractive and challenging approach to the construction of robust, thin film materials with large second-order optical nonlinearities is the covalent self-assembly of aligned arrays of high-β molecular chromophores into multilayer superlattices. In this paper, we describe the dispersion of second harmonic generation (SHG) in a self-assembled (SA) monolayer containing a stilbazolium chromophore. The frequency-dependent measurements were performed on 25 Å thick monolayers on glass using a tunable (0.4–2 μm) light source based on optical parametric amplification (OPA). The SHG spectrum contains a clear two-photon resonance at hω = 1.3eV. The maximum in the second-order susceptibility coincides with a low energy chromophore-centered charge-transfer excitation at 480 nm. The experimental SHG dispersion values compare favorably with theoretical results computed using a sum-over-states (SOS) formalism. However, the measured values exhibit a somewhat broader band response than the theoretical curve, and the origin of this behavior is discussed.
Optical Studies of Silicon Nanocrystals in Colloidal and Sol-Gel Matrices
Authors: Lee Howard W. H. , Davis Jeffrey E. , Olsen Margaret L. , Kauzlarich Susan M. , Bley Richard A. , Risbud Subhash H. , Duval David J.
Published: April 2011

We studied the optical properties of silicon nanocrystals incorporated into colloidal and solgel matrices. The silicon nanocrystals are produced by ultrasonic dispersion of porous silicon layers. We report results on the dependence of the photoluminescence (PL) spectra with excitation intensity. The PL shows a blue peak (at ∼ 415-460 nm.) and a red peak (at ∼ 680 nm). This PL spectrum shows a remarkable dependence on the excitation intensity. As the intensity is increased, the blue peak grows at the expense of the red. A model is suggested for this behavior. We also report on the excitation intensity dependence and the emission wavelength dependence of the PL decay at low (1 kHz) and high (82 MHz) repetition rates of optical excitation. When low repetition rate excitation is used, the PL decay times are all exponential, short (ns), and appear to vary little with emission wavelength. This sharply contrasts with what is observed in porous silicon. With high repetition rate excitation, both red and blue peaks show long (100's ns) and short (ps-ns) lifetime components. We contrast the different optical properties of these silicon nanocrystals with that observed in porous silicon.
Relations Between Structure and Texture in Nano-Sized Aluminas
Authors: Coster Dominique J. , Levitz Pierre , Fripiat Jose J.
Published: April 2011

Alumina xerogels were prepared by a two step hydrolysis in non-aqueous media. This synthesis produced high surface area materials with controlled amount of pentacoordinated aluminum. The effect of heat treatment and of the preparation procedure on the physical characteristics of the powder was determined by nitrogen adsorption and X-Ray diffraction, whereas the aluminum coordination was studied by 27Al MAS NMR. The pore size distribution and the pentahedral aluminum content (up to 700°C) are closely related to the degree of condensation of the prehydrolysed oligomer species.
Synthesis and Characterization of Star Polypeptide Nonlinear Optical Materials
Authors: Cooper Thomas M. , Su Weijie , Tokarski Zbigniew , Adams W. Wade
Published: April 2011

To develop novel polypeptide-based thin films, a series of star polypeptides modified with nonlinear optical chromophores has been synthesized. Using amino-substituted tetraphenyl porphyrin as an initiator, the N-carboxy anhydride of γ-benzyl L-glutamic acid was polymerized onto the porphyrin at a monomer to initiator ratio 20:1. The resulting four-branch star was modified with a selection of dyes. Dyes that modified both the N-terminus and benzyl side chain were used. We demonstrated feasibility of insertion of a metal ion into the polypeptide porphyrin core. The polypeptide series was characterized by UV/VIS, FTIR, and CD. The UV/VIS data suggested ease of modification of both the N-terminus and side chains. The FTLR and CD data show the resulting polypeptides were ∝-helical. The results demonstrate the feasibility of modifying the optical properties of a porphyrin by three approaches: insertion of metal, attachment of dye to N-terminus or modification of γ-benzyl L-glutamate side chain.
Molecular Sieve Based Chemical Sensors
Authors: Sottile Laura J. , Balkus Kenneth J. , Riley Scott J. , Gnade Bruce E.
Published: April 2011

By virtue of their shape selectivity and stability, molecular sieves are ideal components for discriminating chemical sensors. In this paper we report the preparation of capacitance type sensors based on aluminum phosphate molecular sieves. Thin films of the molecular sieves AlPO4-5, A1PO 4-H3 and AlPO4-H1, which cover a range of pore dimensions, were deposited on titanium nitride coated silicon wafers by laser ablation. A subsequent hydrothermal treatment followed by a Pd/Au coating and the application of standard photoresist/masking techniques were used to generate the capacitors. The molecular sieves exhibit significant changes in capacitance upon exposure to target molecules, including CO2, CO, N2, H2O and toluene.
Self-Assembly of Oriented Metal Bisphosphonate Multilayers with Potential Nonlinear Optical Properties
Authors: Neff Grace Ann , Zeppenfeld Astrid C. , Klopfenstein Bethany , Page Catherine J.
Published: April 2011

We are investigating a method for synthesizing oriented layered thin film structures that exhibit nonlinear optical properties. This method involves self-assembly of metal bisphosphonate multilayers on gold and silicon substrates. Multilayers are characterized via optical ellipsometry, diffuse reflectance and attenuated total reflectance FTIR, and grazing angle X-ray diffraction.

To incorporate nonlinear optical activity into self-assembled films, we use a,w-bisphosphonates containing aromatic π systems sandwiched between donor and acceptor groups. In order to maximize the overall polarizability of the film, the polarizable bisphosphonate molecules must preferentially bind in only one orientation. To accomplish this, one of the terminal phosphonate groups of the molecule is protected (in ester form). After binding the free phosphonic acid end, the terminal ester is converted to a phosphonic acid group by hydrolysis with bromotrimethylsilane (BTMS). Layering is continued by binding a metal layer and repeating the cycle. Films prepared via this route do exhibit second harmonic generation, but apparently incomplete deprotection of each layer leads to increasing disorder with increasing number of layers.
Characterization of Silicon Nanoparticles Prepared from Porous Silicon
Authors: Bley Richard A. , Kauzlarich Susan M. , Lee Howard W. H. , Davis Jeffrey E.
Published: April 2011

Nanometer sized silicon particles have been produced by ultrasonic dispersion of thin sections of porous silicon in organic solvents. High resolution TEM and FTIR have been used to establish the size range and surface structure/composition of these particles. The larger particles, which range in size from 20 to 50 nm, are made up of a conglomeration of smaller particles with a diameter of a few nanometers. The HRTEM shows an amorphous layer on the surface of many of the clusters. FTIR data suggest this amorphous layer is silicon-dioxide which may also have organic constituents.
A Low Temperature, Solution Phase Synthesis of III-V Semiconductor Nanocrystals
Authors: Kher Shreyas S. , Wells Richard L.
Published: April 2011

Nanocrystalline materials have been intensely investigated in the recent past due to the novel properties associated with size-quantized particles. We have developed a new method for high yield, solution phase synthesis of nanocrystalline III-V semiconductors which eliminates the use of substituted or unsubstituted Group V hydrides and Group III alkyls. Our approach consists of in situ syntheses of (Na/K)3E (E = P, As, Sb) in aromatic solvents and subsequent reactions of these pnictides with Group III halide solutions in coordinating solvents. The nanocrystalline III-V semiconductors GaP, GaAs, GaSb, InP, InAs and InSb are readily prepared in a wide range of particle sizes (4–36 nm) and in high yields. The resultant Ill-V materials have been characterized by XRD, EDXA, TEM and elemental analyses.
Molecular Precursors to Functional Materials
Authors: Herron Norman , Thorn David L. , Harlow Richard L.
Published: April 2011

The design of molecular precursors to materials for catalysis will be described. Aluminum fluoride is an important catalytic material for fluorocarbon transformations which are key to the production of CFC alternatives. Catalytic activity is closely related to the crystalline phase of the bulk AIF3 and we show how this phase chemistry can be precisely controlled using molecular precursors to produce pure known phases and also to extend to previously unknown phases. A similar approach to vanadyl phosphate oxidation catalysts will be described where cluster chemistry can be invoked to generate isolated fragments of a catalyst's structure and then used either to explore mechanism or be used as precursors to functional catalyst materials. The underlying themes of controlled molecular precursor synthesis, resultant processability and eventual easy conversion to useful materials are emphasized throughout.