bluuebunny/crossref_metadata_2025 huggingface dataset:
The Phytosociology of Weeds in Agroforestry Systems in Different Types of Amazonian Forest Cover
Authors: Martins Jefferson dos Santos , Parreira Mariana Casari , Cunha Antonio Marcos Quadros , Vasconcelos Omar Machado de , Redig Meirevalda do Socorro Ferreira , Santos Elonha Rodrigues dos , Souza Clovis Maurilio de , Terra Thiago Gledson Rios , Lopes Elessandra Laura Nogueira , Gonçalves Marcos Augusto de Souza , Santos Antonio Carlos Martins dos , Leite Jakson , Andrade Lucélia Martins de , Silva Evaldo Morais da
Published: October 2022

Agroforestry systems are examples of soil exploration that is closer to the natural form of the forest, with intercropping of several species within an area, thus being sustainable alternatives. Therefore, the objective of this work was to evaluate the weed community in agroforestry systems in two ecosystems: solid ground and floodplain in the state of Pará. Such collections were made in eight areas, with about one hectare each, cultivated in agroforestry systems, four of which are on dry land and four on floodplains, in the rural area of Cametá-PA. Four plots of one square meter per area were randomly sampled, where the species were identified, counted, and taken to weigh to evaluate the weed community. Among the evaluated environments, the species that stood out the most in the solid ground environment was Kyllinga brevifolia, leading most of the evaluated indices. In the lowland ecosystem, the Brachiaria species purpuracens were the most relevant.
Factors Influencing Implementation of Family Witnessed Resuscitation Practice among Nurses Working in Medical-Surgical Units of Siaya County Referral Hospital, Kenya
Authors: Angute Andrew , Gachathi Daniel Muya , Ramani Ramalingam
Published: October 2022

Background: The family witnessed resuscitation is offering family members an option to be present while a loved one is being resuscitated to sustain life. Despite the recommendation by various professional bodies, it is not widely practiced among medical-surgical nurses. This study aimed to describe the influence of nursing-related factors on this practice, and identify its perceived benefits and risks among nurses.

Methods: This was a cross-sectional analytical survey. Quantitative and qualitative approaches to data collection were used A stratified random sampling design was used to obtain study participants. A sample of 75 registered nurses was generated using the Yamane formula. Data was collected using the Family Presence risk and benefit assessment scale designed and validated by Twibell et al. Analysis done using SPSS and NVIVO version 26 and 11 respectively. Chi-square, crosstabulation and frequencies were computed to establish frequencies, and relation between independent and dependent variables.

Results: There was a significant association between the implementation of family witnessed resuscitation practice and the following independent factors: advanced specialized training in resuscitation ( X 2 =4.125, df =1, p =.042), training on family witnessed resuscitation practice (X 2 =6.728, df =1, p =.009). Perceived benefits were; recognition of the efforts of healthcare professionals (81.7%), acceptance that everything possible was done (83.1%), and better process of grieving (71.8%). Perceived risks identified were; family panic (78.9%), family suffering long-term emotional effects (57.7%), and trauma to the family (63.4%).

Conclusion: Overall, 47.9% of respondents had a perception of more benefits & fewer risks. Training in family witnessed resuscitation practice and advanced specialized training in resuscitation increase the implementation of family witnessed resuscitation by 4.4 and 3.3 respectively.
Sequential phosphorylation of adjacent serine residues on the N‐terminal region of cardiac troponin‐I: structure‐activity implications of ordered phosphorylation
Authors: Quirk Philip G. , Patchell Valerie B. , Gao Yuan , Levine Barry A. , Victor Perry S.
Published: July 2002

We have used NMR spectroscopy to monitor the phosphorylation of a peptide corresponding to the N‐terminal region of human cardiac troponin‐I (residues 17–30), encompassing the two adjacent serine residues of the dual phosphorylation site. An ordered incorporation of phosphate catalysed by PKA was observed, with phosphorylation of Ser‐24 preceding that of Ser‐23. Diphosphorylation induced a conformational transition in this region, involving the specific association of the Arg‐22 and Ser‐24P side‐chains, and maximally stabilised when both phosphoserines were in the di‐anionic form. The results suggest that the second phosphorylation at Ser‐23 of cardiac troponin‐I is of particular significance in the mechanism by which adrenaline regulates the calcium sensitivity of the myofibrillar actomyosin Mg‐ATPase.
Therapeutic drug monitoring of tacrolimus in pediatric liver transplant patients
Authors: MacFarlane Gordon D. , Venkataramanan Raman , McDiarmid Sue V. , Pirsch John D. , Scheller Daniel G. , Ersfeld Diana L. , Fitzsimmons William E.
Published: July 2002

Abstract: The clinical utility of tacrolimus monitoring in adults has been well documented. The present study compared tacrolimus monitoring in a pediatric population of 34 liver transplant patients in four US centers with an adult population of 111 patients in six US centers. Subjects (adult and pediatric) were evaluated, at defined intervals over 12 weeks post‐transplantation (Tx), for tacrolimus trough concentrations and 12 additional laboratory chemistries. Pediatric patient and graft survival for the 12 weeks were 91% and 88%, respectively, as compared to 97% and 93%, respectively, for the adult population. The mean oral dosage of tacrolimus for pediatric patients was 0.13 ± 0.1 mg/kg/day at week 1, increased to 0.30 ± 0.3 mg/kg/day by week 3 and remained constant for the remainder of the study. These dosages were two‐ to three‐fold higher than the dosage used in the adult population. In contrast, the mean whole‐blood trough concentration, as determined by PRO‐Trac™ II enzyme‐linked immunosorbent assay (ELISA), decreased from 11.3 ± 5.1 ng/mL at week 1 to 6.3 ± 3.7 ng/mL by week 12 and was not significantly different from the trough concentration in adults. The incidence and distribution of the clinical end‐points for the pediatric subjects (rejection, nephrotoxicity, death, re‐Tx) were different from those observed in adults. The total percentage of pediatric subjects reaching any end‐point was 74%, as compared to 54% in the adult population. These data indicate several differences between the adult and pediatric populations in their response to tacrolimus.
Effect of oxygen in diamond deposition at low substrate temperatures
Authors: Liou Y. , Weimer R. , Knight D. , Messier R.
Published: July 2002

Thin diamond films were deposited on different substrates at temperatures below 500 °C by a microwave plasma-enhanced chemical vapor deposition system. The deposited films were amorphous carbon or diamond films depending on the different gas mixtures used. The addition of oxygen to the gas mixtures was found to be critical for diamond growth at low temperatures. Without oxygen, the deposited films were white soots and easily scratched off. Increasing the oxygen input improved the quality of the Raman peaks and increased the film transparency. The diamond films were also characterized by scanning electron microscopy.
Magnetic field dependence of the critical current in YBa2Cu3O7−δ bicrystal grain boundary junctions
Authors: Mayer B. , Schuster S. , Beck A. , Alff L. , Gross R.
Published: July 2002

We have performed a detailed study of the magnetic field dependence of the critical current, Ic(B), of YBa2Cu3O7−δ bicrystal grain boundary junctions (GBJs). GBJs with width W much larger than the Josephson penetration depth λJ show Ic(B) dependencies that are close to those of ideal large Josephson junctions with overlap geometry. The Ic(B) dependencies are symmetrical with respect to B=0 and Ic decreases linearly with increasing applied magnetic field for B≤BJ1, where BJ1 is the lower critical field of the GBJ. Furthermore, Ic(B=0) increases linearly with increasing width of the GBJs as expected for Josephson junctions with overlap geometry. From the measured Ic(B) dependencies the temperature dependence of BJ1 and the London penetration depth could be derived.
Gas-phase oxidation of copper during laser ablation of YBa2Cu3O7−δ in different oxidizing ambients
Authors: Otis C. E. , Gupta A. , Braren B.
Published: July 2002

The gas-phase production of CuO during ablation of a YBa2Cu3O7−δ target in the presence of three different oxidizer background gases: oxygen (O2), nitrous oxide (N2O), and nitrogen dioxide (NO2), has been monitored using laser-induced fluorescence. The oxides of nitrogen are found to be much more efficient than oxygen in the production of CuO, with the order of reactivity being O2<N2O<NO2. In general for all the reactant gases, and independent of the ablation laser wavelength (193 and 308 nm), the CuO concentration has been observed to initially increase with increasing background gas pressure to reach a maximum before decreasing again to negligible levels at higher pressures. The rate of increase in the CuO concentration and its subsequent decrease with pressure, while being dependent on the nature of the oxidizer gas, is also very sensitive to the laser fluence used for ablation. The results are discussed in terms of the reaction energetics and the known dynamics of other gas-phase metal oxidation reactions.
Smooth YBa2Cu3O7−x thin films prepared by pulsed laser deposition in O2/Ar atmosphere
Authors: Kühle A. , Skov J. L. , Hjorth S. , Rasmussen I. , Hansen J. Bindslev
Published: July 2002

We report on pulsed laser deposition of YBa2Cu3O7−x in a diluted O2/Ar gas resulting in thin epitaxial films which are almost outgrowth-free. Films were deposited on SrTiO3 or MgO substrates around 800 °C at a total chamber pressure of 1.0 mbar, varying the argon partial pressure from 0 to 0.6 mbar. The density of boulders and outgrowths usual for laser deposited films varies strongly with Ar pressure: the outgrowth density is reduced from 1.4×107 to 4.5×105 cm−2 with increasing Ar partial pressure, maintaining a critical temperature Tc,zero≊90 K and a transport critical current density Jc(77 K)≥106 A/cm2 by extended oxygenation time during cool down.
Chemomechanical polishing of silicon: Surface termination and mechanism of removal
Authors: Pietsch G. J. , Higashi G. S. , Chabal Y. J.
Published: July 2002

Infrared spectroscopy of Si(111) samples immediately after chemomechanical planarization with silica slurry (‘‘siton polishing’’) shows that the surfaces are predominantly terminated by hydrogen. This hydrogen termination is responsible for the observed strong hydrophobicity peak at a slurry pH of 11, at which point a monohydride termination prevails. At higher or lower pH, silanol groups replace some of the hydrogen species causing an increase in surface hydrophilicity. A removal mechanism is proposed which involves the interplay of oxidation by OH− and subsequent termination by H.
Measurement of saturation magnetization of thin magnetic films using a magnetic balance
Authors: Tejedor M. , Fernandez A. , Hernando B.
Published: July 2002

A simple and rapid determination of the saturation magnetization Ms of thin magnetic films is achieved by measuring with a precision balance the force exerted on the film when it is saturated by a field that lies on its plane and varies with a constant gradient along it. This field is obtained with Faraday pole tips for which the exciting coils are connected in opposition.
Fiber-optic liquid contact measurements in pool boiling
Authors: Neti S. , Butrie T. J. , Chen J. C.
Published: July 2002

Two optical techniques for use in detecting liquid contacts in film and transition boiling regimes were developed. The first optical method involves the measurements of liquid contacts on a high-temperature glass boiling surface in a transient experiment. The second technique utilizes a small fiber-optic probe mounted flush on a copper surface. Experiments have indicated the probe is sufficiently fast responding to provide accurate local measurements of liquid to solid contact in film and transition boiling regimes. The relationship between surface superheat and liquid contact time fraction was determined for both methods.
Improving measurement accuracies in cross-talk optical sensors
Authors: Grochowski Lucjan , Bock Wojciech J. , Lit John W.
Published: July 2002

A compensation method, applied to sensors based on cross talk to measure physical parameters such as temperature, strain, and pressure, can improve their accuracies by at least an order of magnitude. This method uses an additional coupler that compensates the effects produced in the measuring coupler. The balance is achieved by means of a closed optoelectric feedback loop to reduce the noise, resulting in higher measuring accuracies.
Assembly subroutine to optimize the multiplexed A/D throughput of the IBM PC Data Acquisition and Control Adapter
Authors: Ricci D. , Fiore L.
Published: July 2002

A basic-callable assembly-language subroutine for the IBM PC Data Acquisition and Control Adapter is described. It allows data acquisition by scanning the multiplexed analog inputs of the adapter with a throughput to memory of up to 10 000 samples/s.
Two opto-electronic processes for measuring plano-cylindrical lens curvature radii
Authors: Niay P. , Bernage P. , Bocquet H.
Published: July 2002

This paper describes two opto-electronic processes useful for measuring curvature radii of optical components. The two methods start from the fact that, within the geometrical optics approximation, a pencil of rays can be marked electronically and thus located during its propagation through the optical system. The two methods have proved to be useful for accurately measuring plano-cylindrical lens curvature radius.
Instrumentation for treatment of cancer using proton and light-ion beams
Authors: Chu W. T. , Ludewigt B. A. , Renner T. R.
Published: July 2002

Clinical trials using accelerated heavy charged-particle beams for treating cancer and other diseases have been performed for nearly four decades. Recently there have been worldwide efforts to construct hospital-based medically dedicated proton or light-ion accelerator facilities. To make such accelerated heavy charged-particle beams clinically useful, specialized instruments must be developed to modify the physical characteristics of the particle beams in order to optimize their biological and clinical effects. This article reviews the beam modifying devices and associated dosimetric equipment developed specifically for controlling and monitoring the clinical beams.
High temperature characteristics of AlGaN/GaN modulation doped field-effect transistors
Authors: Aktas Ozgur , Fan Z. F. , Mohammad S. N. , Botchkarev A. E. , Morkoç H.
Published: July 2002

Current–voltage characteristics of AlGaN/GaN modulation doped field-effect transistors at elevated temperatures are studied experimentally. The drain–source current and extrinsic transconductance are both found to decrease with increasing temperature. Decrease in mobility with increasing temperatures is considered to be one of the causes of the reduction in the current and transconductance. The capacitance–voltage characteristic reveals the absence of heat activated traps in the modulation doped layer. The physics underlying various high-temperature operations of current–voltage characteristics is discussed.
Critical currents and Josephson penetration depth in planar thin-film high-T
c Josephson junctions
Authors: Tolpygo Sergey K. , Gurvitch Michael
Published: July 2002

The temperature dependence of the critical current in planar high-Tc Josephson junctions fabricated in YBa2Cu3O7 thin films by focused electron irradiation has been studied. It is shown that in the range of critical current densities spanning more than five orders of magnitude and temperature range 0.1≤T/Tc≤1, the critical current density jc varies as (1−T/Tc)2. The T dependence of the critical current, however, is affected by the transition from the narrow junction to the wide junction limit as jc increases. An expression for the Josephson penetration depth in thin-film coplanar structures is derived, and magnetic field penetration depth in junction banks is extracted from the Ic(T) dependences.
Calculated natural band offsets of all II–VI and III–V semiconductors: Chemical trends and the role of cation d orbitals
Authors: Wei Su-Huai , Zunger Alex
Published: July 2002

Using first-principles all-electron band structure method, we have systematically calculated the natural band offsets ΔEv between all II–VI and separately between III–V semiconductor compounds. Fundamental regularities are uncovered: for common-cation systems ΔEv decreases when the cation atomic number increases, while for common-anion systems ΔEv decreases when the anion atomic number increases. We find that coupling between anion p and cation d states plays a decisive role in determining the absolute position of the valence band maximum and thus the observed chemical trends.
Cooling by adiabatic pressure application in Pr1−xLaxNiO3
Authors: Alex Müller K. , Fauth François , Fischer Stephan , Koch Max , Furrer Albert , Lacorre Philippe
Published: July 2002

A novel principle for cooling by adiabatic pressure application in the mixed crystalline compound Pr1−xLaxNiO3 is described and experimentally verified. Cooling occurs in the vicinity of the structural phase transition where the electronic ground state of the Pr3+ ions changes from a singlet to a doublet state. By properly choosing the La concentration x, the cooling effect can be achieved down to some 100 mK. Furthermore, Pr1−xLaxNiO3 can be used for second and third stage cooling down to the μK region by classical paramagnetic and nuclear demagnetization techniques, respectively.
Long-lasting phosphorescence in Ce3+-doped Ca2Al2SiO7 and CaYAl3O7 crystals
Authors: Kodama Nobuhiro , Takahashi Tomoko , Yamaga Mitsuo , Tanii Yoshifumi , Qiu Jianrong , Hirao Kazuyuki
Published: July 2002

Long-lasting phosphorescence is observed in Ce3+-doped Ca2Al2SiO7 (CASM) and CaYAl3O7 (CYAM) crystals at room temperature. UV irradiation at 365 nm produces broadband emissions with peaks at 417 nm for CASM and 425 nm for CYAM, which persist for not less than 1 h after the removal of the irradiating light. Based on the time dependence of the intensity of the phosphorescence, the long-lasting phosphorescence in these crystals is considered to be due to the electron–hole recombination at shallow traps, which occurs by a thermally activated process.
Work functions and valence band states of pristine and Cs-intercalated single-walled carbon nanotube bundles
Authors: Suzuki Satoru , Bower Chris , Watanabe Yoshio , Zhou Otto
Published: July 2002

The electronic structures and the work functions of pristine and Cs-intercalated single-walled carbon nanotube bundles were investigated using ultraviolet photoemission spectroscopy. The valence bands of the pristine bundles were considerably altered from those of graphite. A spectral shift to the higher binding energy side was observed in the Cs-intercalated sample. The work function of the pristine bundles was found to be 4.8 eV, which is 0.1–0.2 eV larger than that of graphite. A drastic decrease of the work function to about 2.4 eV was observed in the Cs-intercalated sample.
Electric-field assisted assembly and alignment of metallic nanowires
Authors: Smith Peter A. , Nordquist Christopher D. , Jackson Thomas N. , Mayer Theresa S. , Martin Benjamin R. , Mbindyo Jeremiah , Mallouk Thomas E.
Published: July 2002

This letter describes an electric-field assisted assembly technique used to position individual nanowires suspended in a dielectric medium between two electrodes defined lithographically on a SiO2 substrate. During the assembly process, the forces that induce alignment are a result of nanowire polarization in the applied alternating electric field. This alignment approach has facilitated rapid electrical characterization of 350- and 70-nm-diameter Au nanowires, which had room-temperature resistivities of approximately 2.9 and 4.5×10−6 Ω cm.
Quantum oscillations in a stack of superconducting cylinders in a magnetic field: crossover from the Aharonov–Bohm to the Little–Parks regime
Authors: Gvozdikov V. M.
Published: July 2002

The Aharonov–Bohm (AB) oscillations of the free energy, critical temperature Tc, magnetization M, and magnetic susceptibility χ as functions of the magnetic flux Φ through the hollow in a stack of mesoscopic superconducting cylinders are studied both analytically and numerically. The shape of these oscillations at low temperature T and small level broadening ν is generally nonsinusoidal and has singularities that depend on the superconducting order parameter Δ and stacking sequence. The period of the oscillations is equal to the normal flux quantum Φ0. The harmonic amplitudes of the AB oscillations decrease exponentially if the diameter 2R of the cylinders becomes greater than the coherence length. Further increase of R results in a complete suppression of the AB oscillations and the development of parabolic Little–Parks (LP) oscillations of Tc(Φ) with half the period, Φs=Φ0/2. Therefore a crossover from the AB to LP oscillations takes place as the diameter 2R is increased. It is shown that the temperature behavior of the magnetic susceptibility below the superconducting transition is χ∝exp(−T/T*), where T*=ℏv0/2π2R (v0 is the Fermi velocity, and ℏ is Planck’s constant). Such dependence of χ(T) has been observed recently in Ag wires coated with thin Nb layers in a weak external field [R. Frassanito et al., Czech. J. Phys. 46, 2317 (1996)].
Intermediate Hamiltonian Fock-space coupled-cluster method: Excitation energies of barium and radium
Authors: Landau Arie , Eliav Ephraim , Ishikawa Yasuyuki , Kaldor Uzi
Published: July 2002

An intermediate Hamiltonian Fock-space coupled cluster method is introduced, based on the formalism developed by Malrieu and co-workers in the context of perturbation theory. The method is designed to make possible the use of large P spaces while avoiding convergence problems traceable to intruder states, which often beset multireference coupled cluster schemes. The essence of the method is the partitioning of P into a main Pm and an intermediate Pi serving as buffer, with concomitant definition of two types of wave and excitation operators. Application to atomic barium and radium yields converged results for a large number of states not accessible by traditional Fock-space coupled cluster. Moreover, states calculated by both methods exhibit better accuracy (by a factor of 2–5) in the intermediate Hamiltonian approach. Energies are given for low-lying states of Ra which have not been observed experimentally.
Structure of the acetone liquid/vapor interface
Authors: Yeh Yuh Ling , Zhang Chun , Held Hermann , Mebel A. M. , Wei Xing , Lin S. H. , Shen Y. R.
Published: July 2002

The structure of the liquid/vapor interface of acetone is investigated theoretically by molecular dynamics simulation and experimentally by infrared-visible sum frequency vibrational spectroscopy. The simulation yields the molecular orientational distribution at the liquid surface. In conjunction with the ab initio calculation, the sum-frequency vibrational spectra for the interface is then computed, allowing a direct comparison to the measured spectra. As the calculation agrees well with the experiment, we can conclude that the molecules at the liquid surface are polar ordered, with one of the methyl groups pointing away from the bulk and the molecular plane perpendicular to the surface. This orientation is similar to that of molecules in the layer planes of crystalline acetone. Together with the appreciable surface tension of acetone, it suggests that the acetone liquid surface has a more ordered structure than the bulk and is more or less crystal-like.
Dynamics of excited-state proton transfer systems via time-resolved photoelectron spectroscopy
Authors: Lochbrunner S. , Schultz T. , Schmitt M. , Shaffer J. P. , Zgierski M. Z. , Stolow Albert
Published: July 2002

We investigate the applicability of time-resolved photoelectron spectroscopy to excited state intramolecular proton transfer (ESIPT) and internal conversion dynamics in the model system o-hydroxybenzaldehyde (OHBA) and related compounds. Photoelectron spectra of both the excited state enol and keto tautomers were obtained as a function of pump laser wavelength and pump-probe time delay. The ESIPT was found to occur in less than 50 fs over the whole absorption range of the S1(ππ*) state for both OHBA and its monodeuterated analog, suggestive of a small or nonexistent barrier. The subsequent keto internal conversion rate in OHBA varies from 0.63 to 0.17 ps−1 over the S1(ππ*) absorption band and the OD-deuterated analog shows no significant isotope effect. Based upon ab initio calculations and comparison with the two-ring analog, 1-hydroxy-2-acetonaphthone (HAN), we suggest that the internal conversion dynamics in OHBA is influenced by interactions with a close-lying nπ* state.
Low dark current quantum-dot infrared photodetectors with an AlGaAs current blocking layer
Authors: Wang S. Y. , Lin S. D. , Wu H. W. , Lee C. P.
Published: July 2002

Low dark current InAs/GaAs quantum-dot infrared photodetectors (QDIPs) are demonstrated. The dark current is reduced by over three orders of magnitude by using a thin AlGaAs current blocking layer. This thin AlGaAs layer reduces the dark current much more than the response signal. The responsivity at 0.5 V is 0.08 A/W with a peak detection wavelength at 6.5μm. The corresponding detectivity is 2.5×109 cm Hz1/2/W1/2, which is the highest detectivity reported for a QDIP at 77 K.
Effects of annealing conditions on optical and electrical characteristics of titanium dioxide films deposited by electron beam evaporation
Authors: Mikhelashvili V. , Eisenstein G.
Published: July 2002

We report measured evolutions of the optical band gap, refractive index, and relative dielectric constant of TiO2 film obtained by electron beam gun evaporation and annealed in an oxygen environment. A negative shift of the flat band voltage with increasing annealing temperatures, for any film thickness, is observed. A dramatic reduction of the leakage current by about four orders of magnitude to 5×10−6 A/cm2 (at 1 MV/cm) after 700 °C and 60 min annealing is found for films thinner than 15 nm. An equivalent SiO2 thickness of the order of 3–3.5 nm is demonstrated. An approach is presented to establish that at different ranges of applied voltage the hopping, space charge limited current, and Fowler–Nordheim are the basic mechanisms of carrier transport into the TiO2 film.
A “fast growth” method of computing free energy differences
Authors: Hendrix D. A. , Jarzynski C.
Published: July 2002

Let ΔF be the free energy difference between two equilibrium states of a system. An established method of numerically computing ΔF involves a single, long “switching simulation,” during which the system is driven reversibly from one state to the other (slow growth, or adiabatic switching). Here we study a method of obtaining the same result from numerous independent, irreversible simulations of a much shorter duration (fast growth). We illustrate the fast growth method, computing the excess chemical potential of a Lennard-Jones fluid as a test case, and we examine the performance of fast growth as a practical computational tool.
Dielectric studies of ZnSe1−xTex epilayers
Authors: Lin H. M. , Chen Y. F. , Shen J. L. , Chou C. W.
Published: July 2002

We present the results of experimental investigations on dielectric properties of ZnSe1−xTex epilayers by capacitance and dissipation factor at temperature 5 K<T<475 K and frequency 20 Hz<f<1 MHz. A Debye-like relaxation of dielectric behavior has been observed, which is found to be a thermally activated process. The activation energies obtained from capacitance and dissipation factor are in very good agreement. The activation energies decrease with the increase of Se content, and range from 662 to 819 meV. The results are described by means of the four-center model, in which the number of different atoms occupying the nearest-neighbor sites of defects results in a different activation energy.
Thermodynamic model of hydrogen-induced silicon surface layer cleavage
Authors: Han Weihua , Yu Jinzhong
Published: July 2002

A thermodynamic model of hydrogen-induced silicon surface layer splitting with the help of a bonded silicon wafer is proposed in this article. Wafer splitting is the result of lateral growth of hydrogen blisters in the entire hydrogen-implanted region during annealing. The blister growth rate depends on the effective activation energies of both hydrogen complex dissociation and hydrogen diffusion. The hydrogen blister radius was studied as a function of annealing time, annealing temperature, and implantation dose. The critical radius was obtained according to the Griffith energy condition. The time required for wafer splitting at the cut temperature was calculated in accordance with the growth of hydrogen blisters.
Dynamic mechanical behavior of filled polymers. I. Theoretical developments
Authors: Clements B. E. , Mas E. M.
Published: July 2002

A constitutive theory is developed for modeling the mechanical response of dynamically loaded filled-polymer composites. The basis for this work is Mori and Tanaka’s effective medium theory. Expressions derived by Weng and co-workers are used for the elastic stiffness tensor of the composite. The filler is a low volume concentration of randomly positioned elastic ellipsoidal particles. Random and aligned orientations of the ellipsoids are considered. The viscoelastic stress–strain behavior of the polymer matrix is modeled using the Boltzmann superposition principle with a Prony series representation for the stress relaxation functions. We argue that for rubbery polymers it is reasonable to express the composite stress relaxation functions as series expansions about the ratio of the polymer shear relaxation function to its bulk modulus. The smallness of this ratio allows accurate results to be obtained when truncating the expansion at first order. Inverse Laplace transformations required by the theory can then be done analytically. The result of these manipulations is again a Prony series for the stress relaxation functions of the composite, but with series coefficients that are now functions of the filler concentration, ellipsoidal aspect ratio, and moduli. In the limit of low filler concentration the theory reduces to known results derived in classical suspension theory.
Effect of dislocations on thermal conductivity of GaN layers
Authors: Kotchetkov D. , Zou J. , Balandin A. A. , Florescu D. I. , Pollak Fred H.
Published: July 2002

We report calculation of the lattice thermal conductivity in wurtzite GaN. The proposed model is material specific and explicitly includes phonon relaxation on threading dislocations and impurities typical for GaN. We have found that a decrease of the dislocation density by two orders of magnitude in GaN leads to a corresponding increase of the thermal conductivity from 1.31 to 1.97 W/cm K. This theoretical prediction is in very good agreement with experimental data obtained from scanning thermal microscopy. The developed model can be used for thermal budget calculations in high-power density GaN devices.
Soret and mass diffusion measurements and molecular dynamics simulations of n-pentane–n-decane mixtures
Authors: Perronace Andrea , Leppla Cindy , Leroy Frédéric , Rousseau Bernard , Wiegand Simone
Published: July 2002

Soret coefficients and mass diffusion coefficients of three states of the n-pentane–n-decane mixture have been measured by thermal diffusion forced Rayleigh scattering (TDFRS) and are compared with molecular dynamics simulations values. Both equilibrium (EMD), synthetic (S-NEMD), and boundary driven (BD-NEMD) nonequilibrium techniques have been applied to compute the phenomenological and the transport coefficients relevant to the Soret effect. It is found that statistical error on cross-coefficients using equilibrium and dynamical S-NEMD is too high to enable any comparison with experiments, whereas stationary S-NEMD and BD-NEMD methods have statistical error less than ≈35%. S-NEMD simulations have been carried out in the center-of-mass reference frame and the resulting transport coefficients transformed to the center-of-volume frame of reference. The mass diffusion coefficients are sensibly affected by this transformation and show the same weight fraction dependence as the experimental value, although a difference of roughly a factor of 1.4 is found. The Soret coefficients are, as expected, unaffected by the frame of reference transformation and a good agreement between experiment and simulations is found.
Formation of Fe3C surface layers by laser plasma cementation
Authors: Carpene E. , Schaaf P.
Published: July 2002

We report here a method of iron cementation. By irradiating pure iron substrates in a methane atmosphere with a pulsed excimer laser, a rather thick and polycrystalline cementite layer was obtained. The characterization of the treated samples, carried out by means of Mössbauer spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry and nanoindentation, indicates a layer thickness of about 1 μm. A possible mechanism for the cementite formation after laser irradiation in methane atmosphere is proposed.
Effects of pulse duration on self-focusing of ultra-short lasers in underdense plasmas
Authors: Faure J. , Malka V. , Marquès J.-R. , David P.-G. , Amiranoff F. , Ta Phuoc K. , Rousse A.
Published: July 2002

An experiment investigating laser self-focusing in underdense plasmas is presented. It was shown experimentally that the critical power for relativistic self-focusing Pc is not the only relevant parameter, in particular when the laser pulse duration is comparable to plasma particle motion times: ωp−1 for electrons and ωpi−1 for ions. Using time resolved shadowgraphy, it was demonstrated that: (i) a pulse does not relativistically self-focus if its duration is too short compared to ωp−1, even in the case where the power is greater than Pc. This is due to defocusing by the longitudinal wake which is generated by the laser pulse itself. (ii) For pulses longer than ωpi−1, self-focusing can occur even for powers lower than Pc. This is due to the radial expansion of ions, creating a channel whose effect combines with relativistic focusing and helps the pulse to self-focus.
A simple model for the molecular dynamics of condensed phases
Authors: Pawley G. S.
Published: July 2002

A two-dimensional model for molecular condensed matter phases is proposed, based on triatomic triangular molecules. The governing potential function is carefully chosen to give many different structures with the same crystal potential. Part of a molecular dynamics teaching project is presented, showing that a number of these structures occur naturally in a simulation. This form of computational project well suits parallel computation, and the present results were obtained on an AMT DAP computer. Further possibilities for this model are presented, some of which involve the ideas of quasicrystals.
A particle-in-cell fluid model for radio frequency glow discharges
Authors: Li Chihwen ‘‘Chris’’ , Wu Chwan-Hwa ‘‘John’’
Published: July 2002

A self-consistent particle-in-cell (PIC) simulation method for solving fluid equations, coupled with Poisson’s equation, has been developed for parallel plate radio-frequency (rf) glow discharges. Using the PIC transport method preserves the steep and changing gradients of moments in the sheath region; therefore, the PIC method solves fluid equations for rf discharges more accurately than the other methods with numerical diffusion. However, the particle method requires a sufficient number of particles to achieve an accurate result. A memory bank is designed to store the electron and ion particle information efficiently. Although the memory size is limited by the computer hardware, renormalization can resolve the memory consumption problem caused by the increase of the secondary electrons that flow into the reactor from the electrodes. Moreover, renormalization solves the discontinuous ion density distribution problem caused by the decrease of the sheath ions that flow out of the electrodes. Due to the use of a finite number of particles, fluctuation of the convergence ratio is another problem with the particle method. The simulation convergence ratio affected by the number of superparticles, the applied field, the secondary electron emission coefficient, and renormalization are discussed. The simulation results of the PIC/fluid model agree well with experiment data.
Effect of oxidation on the magnetic properties of unprotected TbFe thin films
Authors: van Dover R. B. , Gyorgy E. M. , Frankenthal R. P. , Hong M. , Siconolfi D. J.
Published: July 2002

Amorphous Tb-Fe thin films prepared by dual magnetron cosputtering were exposed to air at 200 °C in order to investigate the evolution of the films as they oxidize. Magnetic properties of the films were measured using a vibrating-sample magnetometer and torque magnetometer and are interpreted in light of the structure of the films as revealed by Auger electron spectroscopy and composition-depth profiling. This leads to a detailed and self-consistent description of the oxidation process. At first a uniform and homogeneous oxidation layer grows from the surface toward the substrate. This layer has a high magnetization and low intrinsic anisotropy and consists of an intimate mixture of oxidized Tb and metallic TbxFe(1−x). The initially high intrinsic anisotropy of the unoxidized region decreases relatively quickly, while the composition changes only slowly as this region shrinks. When the oxidation layer reaches the substrate, two oxide phases (Fe2O3 and Tb2O3) begin to grow at the surface exposed to air.
A model for pixellation of nonlinear interference filters
Authors: Abraham E. , Godsalve C. , Wherrett B. S.
Published: July 2002

We present a theoretical model for the effect of material pixellation on the operating characteristics of optical logic elements using nonlinear interference filters. From our calculations we predict pixel packing densities of 250×250 per cm2 (an increase of three orders of magnitude over optical pixellation) and power levels of 100 μW. We define a figure of merit which describes the effects of pixellation of thermo-optic logic elements. We also show numerically the effect on cooling rate.
A quantitative analysis of the volume fraction dependence of the resistivity of cermets using a general effective media equation
Authors: McLachlan D. S.
Published: July 2002

The results analyzed, all from previous work, are for W-Al2O3 (as prepared and annealed; 300 K) and Ni-SiO2 (4.2 and 291 K). These results, in which the resistivity changes from 10−5 to 107 Ω cm as the metal volume fraction is varied, can all be quantitatively fitted to a general effective media (GEM) equation. The GEM equation was developed as an interpolation between Bruggeman’s symmetric and asymmetric media equations for ellipsoidal grains. It has four parameters. The conductivities of the low [σ(lo)] and high [σ(hi)] conductivity components, fc=(1−φc), where φc is the critical (percolation) volume fraction of the σ(hi) component and t is an exponent. f is the volume fraction of the dielectric or σ(lo) component. As, when σ(lo)=0 or σ(hi)=∞ the GEM equation reduces to the percolation equations, it can also be regarded an interpolation formula between these equations. The W-Al2O3 (as prepared; 300 K) and Ni-SiO2 (291 K) results need a five-parameter fit as ρ(hi) [1/σ(lo)] is modeled as an intergranular tunneling process with ρ(hi)=ρ(0)exp{C[(1−fc)/(1−f )]1/3−1}, ρ(0) and C being variable parameters. The log rms deviations for fits are W-Al2O3 (as prepared) 3.3%, W-Al2O3 (annealed) 0.66%, Ni-SiO (4.2 K) 2.3%, and Ni-SiO2 (291 K) 1.3%.
Spraying of the brittle ceramic zirconium diboride by a wire explosion technique
Authors: Tamura Hideki , Ogura Takahiko , Nagahama Mutsuhisa , Tanabe Yasuhiro , Sawaoka Akira B.
Published: July 2002

A wire explosion technique was employed for the spraying of zirconium diboride, a high-melting-point, brittle ceramic. To study its spraying mechanism and the feasibility of high-density coating, the explosion of a ZrB2 ceramic wire was examined with time-resolved measurements of the apparent resistance of an exploding wire, expansion of a discharge channel, wire fragmentation, and gas flow around the wire. The explosion under argon gas of 0.1 MPa in base pressure was started with heating up to the melting point of ZrB2, and with sequential gas breakdown around the wire. The breakdown was followed by the formation of a cylindrical shock wave and an electrical discharge channel, and by their expansion. Wire breaks were found at a later period of the electrical discharge, and resulted in the fragmentation of the wire of less than 1 mm in size, and in the transformation of almost the whole wire into fine-molten particles. On the other hand, at the explosion under air of 13.3 Pa in base pressure no shock wave was found, but larger fragments of the wire and the transformation of about 50% of the wire volume into fine particles were observed. ZrB2 coated substrates were analyzed with x-ray diffraction and scanning electron microscopy. The spraying under the high-base-pressure condition provided a much denser coating than that under the low base pressure. However, a small number of cracks was found on the sprayed film surface, and it was assumed that their formation was induced due to the brittleness of ZrB2. These measurements have revealed the close relation of the wire explosion to the base pressure of surrounding gas, and, in particular, the result that the high-base-pressure explosion associated with a cylindrical shock wave could provide a high-density coating. In consideration of the existence of high-base-pressure gas and the structure of a shock wave and an associated discharge channel, it is suggested that adequate heating and high-temperature holding of the wire plays an important role in the spraying. The spraying of ZrB2 wires presented here has revealed the feasibility of high-density coating of refractory, brittle ceramics with the wire explosion technique.
Grain-boundary-limited transport in semiconducting SnO2 thin films: Model and experiments
Authors: Prins M. W. J. , Grosse-Holz K.-O. , Cillessen J. F. M. , Feiner L. F.
Published: July 2002

We present a model that describes grain-boundary-limited conduction in polycrystalline semiconductors, for thermally assisted ballistic as well as diffusive transport, both for degenerate and nondegenerate doping. In addition to bulk parameters (the carrier effective mass and mean free path) the model contains grain boundary parameters (barrier height and width) and a coefficient of current nonuniformity. Temperature-dependent conductivity and Hall measurements on polycrystalline SnO2 thin films with different Sb concentrations are consistently interpreted.
Hydrogen plasma enhanced crystallization of hydrogenated amorphous silicon films
Authors: Pangal K. , Sturm J. C. , Wagner S. , Büyüklimanli T. H.
Published: July 2002

We report that a room temperature hydrogen plasma exposure in a parallel plate diode type reactive ion etcher can reduce the time required for the subsequent thermal crystallization of amorphous silicon time by a factor of five. Exposure to hydrogen plasma reduces the incubation time, while the rate of crystallization itself is not greatly affected. This plasma enhanced crystallization can be spatially controlled by masking with patterned oxide, so that both amorphous and polycrystalline areas can be realized simultaneously at desired locations on a single substrate. The enhancement of crystallization rate is probably due to the creation of seed nuclei at the surface. The films have been characterized by UV reflectance, x-ray diffraction, plan view transmission electron microscopy, Fourier transform infrared absorption, secondary ion mass spectroscopy, and four-point probe measurement of electrical conductivity.
Analytical description of “athermal” nucleation and its relevance to rapidly quenched fluids
Authors: Shneidman Vitaly A.
Published: July 2002

Results of a matched asymptotic analysis of the nucleation equation are used to obtain the “athermal” flux due to variation of the critical size. The total critical flux is also evaluated. Results, which also include estimations of crystallization rates, are used to refine the nucleation-mechanism diagram for liquid silicon proposed by J. S. Im, Gupta, and Crowder [V. V. Gupta, and M. A. Crowder, Appl. Phys. Lett. 72, 662 (1998)]. The possibility of experimental detection of athermal effects on the background of other effects of time-dependent nucleation is discussed.
Reversible magnetization behavior in Sm2(Co, Fe, Cu, Zr)17
Authors: Crew D. C. , Woodward R. C. , Street R.
Published: July 2002

The magnetization of Sm2(Co, Fe, Cu, Zr)17 is determined by the motion of domain boundary walls that are subject to pinning. Measurements have been made of the relationship between the reversible magnetization Mrev and the irreversible magnetization Mirr components in Sm2(Co, Fe, Cu, Zr)17. The results are consistent with a model in which rapid nucleation of domain walls occurs at the beginning of magnetization reversal followed by gradual unpinning of the domain walls. For Sm2(Co, Fe, Cu, Zr)17 the parameter η, defined as η=(dMrev/dMirr)H is similar in form to previously observed variations of η in sintered NdFeB but very different from η variations due to Stoner–Wohlfarth coherent rotation of magnetization vectors.
Generalized analysis of quasi-steady-state and quasi-transient measurements of carrier lifetimes in semiconductors
Authors: Nagel Henning , Berge Christopher , Aberle Armin G.
Published: July 2002

Recently, a simple yet powerful carrier lifetime technique for semiconductor wafers has been introduced that is based on the simultaneous measurement of the light-induced photoconductance of the sample and the corresponding light intensity [Appl. Phys. Lett. 69, 2510 (1996)]. In combination with a light pulse from a flash lamp, this method allows the injection level dependent determination of the effective carrier lifetime in the quasi-steady-state mode as well as the quasi-transient mode. For both cases, approximate solutions (those for steady-state and transient conditions) of the underlying semiconductor equations have been used. However, depending on the actual lifetime value and the time dependence of the flash lamp, specific systematic errors in the effective carrier lifetime arise from the involved approximations. In this work, we present a generalized analysis that avoids these approximations and hence substantially extends the applicability of the quasi-steady-state and quasi-transient methods beyond their previous limits.
Nonlinear optical properties of CuxS and CuInS2 nanoparticles in sol–gel glasses
Authors: Malyarevich A. M. , Yumashev K. V. , Posnov N. N. , Mikhailov V. P. , Gurin V. S. , Prokopenko V. B. , Alexeenko A. A. , Melnichenko I. M.
Published: July 2002

Silica glasses with ultrafine CuxS and CuInS2 particles (average radius of 5±3 nm) were fabricated by means of a sol–gel technique. Linear and differential absorption spectra were studied. The glasses reveal absorption peaks in the 600–700 nm range associated with the first excitonic resonance in the quantum confined particles. Laser excitation by 540 nm pulses leads to the induced absorption effect in the 480–980 nm spectral range. The relaxation of the induced absorption has a time constant equal to about 500 ps for CuS-doped glasses and a biexponential type (with τ1≈30 ps and τ2>1 ns) for CuInS2-doped ones.