bluuebunny/crossref_metadata_2025 huggingface dataset:
Wafer scale production of carbon nanotube scanning probe tips for atomic force microscopy
Authors: Yenilmez Erhan , Wang Qian , Chen Robert J. , Wang Dunwei , Dai Hongjie
Published: July 2002

A methodology is developed to enable wafer scale fabrication of single-walled carbon nanotube (SWNT) tips for atomic force microscopy. Catalyst selectively placed onto 375 prefabricated Si tips on a wafer is made possible by a simple patterning technique. Chemical vapor deposition on the wafer scale leads to the growth of SWNTs protruding from more than 90% of the Si tips. This represents an important step towards the scale up of nanotube probe tips for advanced nanoscale imaging of solid-state and soft biological systems and for scanning probe lithography.
Terahertz pulse propagation in a plastic photonic crystal fiber
Authors: Han H. , Park H. , Cho M. , Kim J.
Published: July 2002

Guided-wave single-mode propagation of subpicosecond terahertz (THz) pulses in a plastic photonic crystal fiber has been experimentally demonstrated. The plastic photonic crystal fiber is fabricated from high-density polyethylene tubes and filaments. The fabricated fiber exhibits the low loss and relatively low dispersive propagation of THz pulses within the experimental bandwidth of 0.1–3 THz.
Optical and luminescence properties of Sm3+-doped cadmium–aluminum–silicate glasses
Authors: Lin H. , Pun E. Y. B. , Huang L. H. , Liu X. R.
Published: July 2002

Sm 3+ -doped cadmium–aluminum–silicate glasses (CdO–Al2O3–SiO2) have been fabricated and characterized optically. Intense visible lights and near-infrared emissions were observed under ultraviolet light and 488 nm laser excitation. The density, refractive index, optical absorption, Judd–Ofelt parameters, and spontaneous transition probabilities have been measured and calculated. Intense fluorescences at 1.2 and 1.29 μm wavelengths were observed, indicating that the glasses are promising materials for laser and optical amplifier applications operating in the 1.2–1.3 μm low-loss window of telecommunication fiber.
Short wavelength temperature gradient driven modes in tokamaks
Authors: Hirose A. , Elia M. , Smolyakov A. I. , Yagi M.
Published: July 2002

Stability analysis of tokamaks based on a fully kinetic, electromagnetic integral equation code indicates the existence of a temperature gradient driven instability in the short wavelength regime (k⊥ρi)2>1. The mode propagates in the ion diamagnetic direction (ion mode with ωr<0) and requires that both ηi and ηe exceed thresholds. Circulating (untrapped) electrons are not adiabatic and parallel electron dynamics provides destabilization. Trapped electrons are not essential for the mode. Toroidicity has a stabilizing influence and the growth rate does not vanish in the slab limit. The growth rate is approximately proportional to |s| where s is the magnetic shear parameter. The mode is subject to finite α (ballooning parameter) stabilization as the conventional ηi mode in the long wavelength regime (k⊥ρi)2<1.
Effects of insertion of strain-mediating layers on luminescence properties of 1.3-μm GaInNAs/GaNAs/GaAs quantum-well structures
Authors: Pavelescu E.-M. , Peng C. S. , Jouhti T. , Konttinen J. , Li W. , Pessa M. , Dumitrescu M. , Spânulescu S.
Published: July 2002

We present a 1.3-μm GaInNAs/GaAs quantum-well heterostructure, which consists of a strain-mediating GaInNAs layer grown between a compressive-strained quantum well and a tensile-strained GaNAs layer. Compared to a similar sample with no strain-mediating layer, this heterostructure exhibits improved material properties and remarkable redshift of emission with enhanced light intensity. The observations are based on photoluminescence spectra and x-ray diffraction data measured for the active region of the samples.
Compartmentalized CNx nanotubes: Chemistry, morphology, and growth
Authors: Trasobares S. , Stéphan O. , Colliex C. , Hsu W. K. , Kroto H. W. , Walton D. R. M.
Published: July 2002

A systematic study of the effect of different synthesis parameters on N incorporation into C nanotubes is presented. CNx nanotubes prepared by catalyzed pyrolysis of melamine exhibit a highly compartmentalized morphology with a remarkable periodicity structure along the nanotube axis. Spatially resolved electron energy loss spectroscopy (spectrum-imaging mode) indicates that the nanotubes are made of carbon and nitrogen, inhomogeneously distributed with an enrichment of carbon on the external surfaces. The evolution of the C-K-edge shape across the nanotube reveals a transition from a graphitic stacking on the outside to a disorganized-mixed type in the core of the nanotube. For the N-K edge, the situation is more complex. The fine structure of the N-K edge differs depending on the used catalyst, which indicates differences in the bonding configuration. When Ni is used as a catalyst, N replaces C in the graphitic structure whereas C–N pyridinic-like bonds are formed when the catalyst is Fe. The compartmentalized periodic morphology is the result of a systematic catalytic particle movement from the root of the nanotube to the tip. This displacement is defined by the nature of the catalytic particle, diffusion, and supersaturation (C/N) in the liquid particle and precipitation process.
Structure and properties of spinel Fe3N4 and comparison to zinc blende FeN
Authors: Ching W. Y. , Xu Yong-Nian , Rulis Paul
Published: July 2002

The structure and properties of iron nitride with a cubic spinel structure are predicted. Spin-polarized calculations show it to be weakly ferromagnetic with a total magnetic moment of 3.26 μB per formula unit. Total energy versus volume calculations yield a large bulk modulus of 304 GPa, which is attributed to the strong covalent bonding between Fe and N. These results differ significantly from those of stoichiometric FeN in a zinc blende structure.
Anthracene derivatives for stable blue-emitting organic electroluminescence devices
Authors: Shi Jianmin , Tang Ching W.
Published: July 2002

A class of anthracene derivative which is suitable used as emitting materials for producing efficient and stable blue emission for full color organic electroluminescence (EL) devices has been developed. Multilayer organic EL devices using these fluorescent materials as an emitting layer produced blue emissions with good chromaticity and luminous efficiency as high as 3.5 cd/A. The half life of 4000 h of blue emission EL device with initial light output 700 cd/m2 has been achieved.
Strain and composition distributions in wurtzite InGaN/GaN layers extracted from x-ray reciprocal space mapping
Authors: Pereira S. , Correia M. R. , Pereira E. , O’Donnell K. P. , Alves E. , Sequeira A. D. , Franco N. , Watson I. M. , Deatcher C. J.
Published: July 2002

Strain and composition distributions within wurtzite InGaN/GaN layers are investigated by high-resolution reciprocal space mapping (RSM). We illustrate the potential of RSM to detect composition and strain gradients independently. This information is extracted from the elongation of broadened reciprocal lattice points (RLP) in asymmetric x-ray reflections. Three InxGa1−xN/GaN (nominal x=0.25) samples with layer thickness of 60, 120, and 240 nm, were grown in a commercial metal-organic chemical vapor deposition reactor. The RSMs around the (105) reflection show that the strain profile is nonuniform over depth in InGaN. The directions of “pure” strain relaxation in the reciprocal space, for a given In content (isocomposition lines), are calculated based on elastic theory. Comparison between these directions and measured distributions of the RLP shows that the relaxation process does not follow a specific isocomposition line. The In mole fraction (x) increases as the films relax. At the start of growth all the films have x∼0.2 and are coherent to GaN. As they relax, x progressively increases towards the nominal value (0.25). Compositional gradients along the growth direction extracted from the RSM analysis are confirmed by complementary Rutherford backscattering measurements.
X-ray photoemission spectroscopy study of fluorinated single-walled carbon nanotubes
Authors: An Kay Hyeok , Heo Jeong Goo , Jeon Kwan Goo , Bae Dong Jae , Jo Chulsu , Yang Cheol Woong , Park Chong-Yun , Lee Young Hee , Lee Young Seak , Chung Young Su
Published: July 2002

We have investigated the change of atomic and electronic structures of fluorinated single-walled carbon nanotubes (SWCNTs) using x-ray photoemission spectroscopy (XPS), electrical resistivity measurements, and transmission electron microscopy (TEM). The fluorine content increases with increasing reaction temperature up to 300 °C. XPS indicated that the fluorinated SWCNT reveals an ionic-bonding character at low concentration and covalent-bonding character at high concentration. The resistivity increases with reaction temperatures, resulting from the band gap enlargement at high fluorine concentration. It is also observed from TEM that the fluorination at reaction temperature above 250 °C leads to the disintegration of the CNT structures and formation of various phases such as multiwall-like and turbostratic morphologies.
Band filling and depletion through the doping of polyaniline thin films
Authors: Xu B. , Choi Jaewu , Caruso A. N. , Dowben P. A.
Published: July 2002

The position of the molecular orbitals of polyaniline, relative to the Fermi level, shifts with the doping of polyaniline by both charge donor and charge acceptor species. Sodium as an electron donor, when added to the polyaniline system, results in an increase in electron population in the polyaniline bands but a decrease in density of states near EF. This is evident from the consistent shifts toward the greater binding energy of the occupied as well as unoccupied molecular orbitals, the valence bands and conduction band edges and characteristic core levels. In the case of iodine doping, an electron acceptor, all the states were shifted toward smaller binding energy. These results provide compelling evidence that the molecular orbitals of vapor deposited polyaniline thin films are delocalized.
Steady vortices in plasmas and geophysical flows
Authors: Nycander Jonas
Published: July 2002

A number of two-dimensional fluid models in geophysical fluid dynamics and plasma physics are examined to find out whether they have steady and localized monopole vortex solutions. A simple and general method that consists of two steps is used. First the dispersion relation is calculated, to find all possible values of the phase velocity of the linear waves. Then an integral relation that determines the center-of-mass velocity of localized structures must be found. The existence condition is that this velocity should be outside the region of linear phase velocities. After a presentation of the method, previous work on the plasma drift wave model and the shallow-water equations is reviewed. In both cases it is found that the center-of-mass velocity is larger than the maximum phase velocity of the linear waves if the amplitude is large enough, and steady localized vortices can therefore exist. New results are then obtained for a number of two-field models. For the coupled ion acoustic-drift modes in plasmas, it is found that the center-of-mass velocity depends on the ratio between the parallel ion velocity component and the electrostatic potential in the vortex. If this ratio is large enough, the vortex can be steady. For the drift-Alfvén mode the ‘‘center-of-charge’’ velocity is proportional to the ratio between the parallel current and the total charge in the vortex. It can therefore be steady if this ratio satisfies the appropriate conditions. For the quasigeostrophic two-layer equations, describing stratified flow on a rotating planet, it is found that the center-of-mass velocity is determined by the ratio between the baroclinic and the barotropic components in the vortex. If a baroclinic component with an appropriate sign is added to a barotropic vortex, it propagates faster than the barotropic Rossby waves, and can be steady. Finally, the existence conditions for a vortex in an external zonal flow are examined. It is found that the center-of-mass velocity acquires an additional westward contribution in an anticyclonic shear zone in the framework of the shallow-water equations, and also that an easterly jet south of this shear zone partly shields a vortex situated in the shear zone from the dispersive influence of the fast Rossby waves on the equatorward side.
Using electrical noise to enhance the ability of humans to detect subthreshold mechanical cutaneous stimuli
Authors: Richardson Kristen A. , Imhoff Thomas T. , Grigg Peter , Collins James J.
Published: July 2002

Stochastic resonance (SR) is a phenomenon wherein the response of a nonlinear system to a weak input signal is optimized by the presence of a particular, nonzero level of noise. Our objective was to demonstrate cross-modality SR in human sensory perception. Specifically, we were interested in testing the hypothesis that the ability of an individual to detect a subthreshold mechanical cutaneous stimulus can be significantly enhanced by introducing a particular level of electrical noise. Psychophysical experiments were performed on 11 healthy subjects. The protocol consisted of the presentation of: (a) a subthreshold mechanical stimulus plus electrical noise, or (b) no mechanical stimulus plus electrical noise. The intensity of the electrical noise was varied between trials. Each subject’s ability to identify correctly the presence of the mechanical stimulus was determined as a function of the noise intensity. In 9 of the 11 subjects, the introduction of a particular level of electrical noise significantly enhanced the subject’s ability to detect the subthreshold mechanical cutaneous stimulus. In 2 of the 11 subjects, the introduction of electrical noise did not significantly change the subject’s ability to detect the mechanical stimulus. These findings indicate that input electrical noise can serve as a negative masker for subthreshold mechanical tactile stimuli, i.e., electrical noise can increase the detectability of weak mechanical signals. Thus, for SR-type effects to be observed in human sensory perception, the noise and stimulus need not be of the same modality. From a bioengineering and clinical standpoint, this work suggests that an electrical noise-based technique could be used to improve tactile sensation in humans when the mechanical stimulus is around or below threshold.
Axial segregation of granular materials
Authors: Levine Dov
Published: July 2002

When mixtures of granular materials are rotated, it is often found that they segregate into bands, along the axis of rotation, which are rich in the various components. This effect is discussed experimentally and theoretically, with emphasis on a mechanism based on surface flow. The complimentary phenomenon of radial segregation is reviewed, and a mechanism is proposed. Finally, we consider the long-time behavior of rotating mixtures, particularly their anomolous coarsening.
Effect of mismatch strain on band gap in III-V semiconductors
Authors: Kuo C. P. , Vong S. K. , Cohen R. M. , Stringfellow G. B.
Published: July 2002

Interfacial elastic strain induced by the lattice parameter mismatch between epilayer and substrate results in significant energy–band-gap shifts for III-V alloys. The epilayers used in this study are GaxIn1−xAs on (100) InP and GaxIn1−xP on (100) GaAs prepared by organometallic vapor phase epitaxy. For layer thicknesses between 1 and 1.5 μm, and Δas.f./a0≤3.5×10−3 the misfit strain is assumed to be accommodated elastically. The energy–band-gap shifts are determined by comparing the photoluminescence peak energies of the epilayers with the best experimental relation of band gap versus composition for unstrained layers. A calculation of the energy–band-gap shift due to biaxial stress made for GaxIn1−xAs is found to agree with the photoluminescence measurements. In addition, a comparison of the energy–band-gap shift for GaxIn1−xP shows a clearly different dependency for tensile and compressive strain, in good agreement with calculated results.
Ionic conductivity and mobility in network polymers from poly(propylene oxide) containing lithium perchlorate
Authors: Watanabe M. , Sanui K. , Ogata N. , Kobayashi T. , Ohtaki Z.
Published: July 2002

Ionic conductivity σ and mobility μ in the amorphous network polymers from poly(propylene oxide) (PPO) containing lithium perchlorate (LiClO4) at the concentration of [LiClO4]/[PO unit]=0.042 and 0.076 were investigated by means of complex impedance and time-of-flight methods. The σ values of the PPO–LiClO4 complexes reached 10−5 S cm−1 at 70 °C. The temperature dependence of σ deviated from a single Arrhenius behavior above a critical temperature (−1 °C and 11 °C) which approximately corresponded to the glass transition temperature Tg. The μ values were relatively high and changed from 10−6 to 10−5 cm2 V−1 s−1 in the temperature range of 40–100 °C. The Nernst–Einstein equation correlated μ with the ionic diffusion coefficient D. The Williams–Landel–Ferry equation with C1≂5 and C2≂30–50 held with a temperature dependence of D in the order of 10−8–10−7 cm2 s−1. The change in the number of ionic carriers n with temperature obeyed the Arrhenius equation with the activation energy of 0.26 and 0.34 eV. The degree of dissociation for LiClO4 in the PPO networks was 1–6%, and the dissociation was facilitated in the low LiClO4 concentration complex. The temperature dependence of σ above Tg was interpreted quantitatively in terms of n and μ.
Growth and properties of GaAs/AlGaAs on nonpolar substrates using molecular beam epitaxy
Authors: Fischer R. , Masselink W. T. , Klem J. , Henderson T. , McGlinn T. C. , Klein M. V. , Morkoç H. , Mazur J. H. , Washburn J.
Published: July 2002

Using molecular beam epitaxy, we have successfully grown device quality GaAs/AlGaAs on (100)-oriented Ge and Si substrates. Modulation doped field effect transistors have been fabricated from these layers which exhibit room-temperature transconductances as high as 160 and 170 mS/mm for layers on Ge and Si, respectively, and showed no looping in either case. At 77 K, these values rose to 345 and 275 mS/mm for Ge and Si, respectively. Analysis by transmission electron microscopy of layers grown on Ge showed that the antiphase disorder was contained within the 250-Å-thick initial layer which was grown at a 0.1-μ/h growth rate at a substrate temperature of 500 °C. For both the layers grown on Si and Ge specular surface morphologies were obtained. The photoluminescence of GaAs/AlGaAs quantum wells grown on Si and Ge was similar in intensity to the same quantum well structures grown on GaAs. In quantum wells grown on Ge, the luminescence was dominated by a donor-acceptor recombination at 1.479 eV, which appears to be Ge0Ga -Ge0As. These results show that high-quality GaAs/AlGaAs is obtainable on nonpolar substrates, which has important implications for the monolithic integration of III-V’s with Si.
Ion-beam mixing of Ni-Mo and of Ni-Ti binary systems
Authors: Bhattacharya R. S. , Rai A. K.
Published: July 2002

Cross-sectional transmission electron microscopy and Rutherford backscattering techniques were employed to study the mixing rates of NiMo and NiTi binary systems under high-energy ion bombardment. Mixing was carried out by using 1-MeV Au+ ions at various doses. Results show that the amount of mixing of NiTi is about three times higher than NiMo. Cascade collision theory can only account for a maximum of 20% difference in the amount of mixing of these two systems. However, a model based on thermodynamics has been found to predict correctly the amount of mixing of these two systems.
Formation of the A15 phase in thin-film Nb-Al diffusion couples
Authors: Gavaler J. R. , Braginski A. I. , Greggi J. , Schulze K.
Published: July 2002

Single-crystal and polycrystalline thin-film Nb-Al diffusion couples have been prepared by evaporation at pressures of 10−11–10−10 Torr. Sapphire, single-crystal MgO, polycrystalline Al2O3, and polycrystalline Nb were used as substrates. These couples were annealed in situ at temperatures between T=780 and 950 °C for 30 min, in order to study the formation of the A15 phase. Based on resistivity data, between 800 and 880 °C, the A15 phase was formed only on oxide substrates. The relative A15 content was higher in polycrystalline couples than in single-crystal couples. Between 880 and 950 °C the A15 phase was also obtained in couples deposited on Nb substrates. X-ray data confirmed that the temperature of the A2 (bcc) to A15 structure transformation was lower in couples where the Nb-Al solid solution was in contact with an oxide substrate. X-ray photoelectron spectroscopy indicated the occurrence of surface reactions between the oxide substrate and Nb or Al. Auger depth profiles of the Nb/Al ratio were similar in couples on all substrate types studied and could not be correlated with A15 phase formation. The A15 phase having Tc’s corresponding to the upper (Al-rich) boundary of the A15 phase stability field appeared to form locally at defects, grain boundaries, and other imperfections. Alternative explanations of the experimental findings are discussed.
A new charge pumping method of measuring Si-SiO2 interface states
Authors: Tseng Wesley L.
Published: July 2002

A new method of measuring the density of band-gap interface states at the Si-SiO2 interface has been developed. This method is based on a charge pumping phenomenon and can be performed on a metal-oxide-semiconductor transistor structure without the need for Shockley–Hall–Read statistics. Although the method requires knowledge of the gate voltage-surface potential relationship, the measured density of states is not as sensitive to either errors or fluctuations in this relationship as other more common techniques. Other advantages of this new method are that it can measure the density of interface states over most of the band gap, it is relatively simple to implement, and has a resolution comparable to either the conductance or deep level transient capacitance techniques. Applying the new method to a transistor with a channel length of 50 μm and a width of 25 μm, the density of interface states has been measured to within 80 meV of the conduction band.
Thermal conductivity of heavily doped low-pressure chemical vapor deposited polycrystalline silicon films
Authors: Tai Y. C. , Mastrangelo C. H. , Muller R. S.
Published: July 2002

The lateral thermal conductivity of heavily doped low-pressure chemical vapor deposited polycrystalline silicon films is measured using polycrystalline silicon microbridges elevated three micrometers above a silicon substrate. The bridges, lightly doped in their central regions and heavily doped elsewhere, are fabricated using a sacrificial silicon-dioxide layer and phosphorus out-diffusion from doped oxide. Voltage-current characteristics measured on the bridges both under high vacuum and in silicone oil are used to calculate lateral thermal conductivity in the polycrystalline silicon. The experimental values for the thermal conductivity of heavily doped polycrystalline silicon range from 0.29 to 0.34 W cm−1 K−1 and average 0.32 W cm−1 K−1. These values agree closely with results obtained by a second method that employs uniformly doped polycrystalline silicon bridges. In the second method, high-vacuum, voltage-current characteristics are measured and the indicated thermal conductivities for two samples are 0.29 and 0.30 W cm−1 K−1, respectively.
Microstructural characterization of AlGaAs-GaAs modulation-doped field-effect transistor ohmic contacts formed by transient annealing
Authors: Rai A. K. , Ezis A. , Graham R. J. , Sharma R. , Langer D. W.
Published: July 2002

Transient annealing of a sequentially deposited metallization scheme, Au/Ni/Au/Ge/Ni, was used to obtain low resistivity ohmic contacts to GaAs-AlGaAs based modulation-doped field-effect transistors. Cross-sectional transmission electron microscopy and energy dispersive x-ray analysis techniques were employed to determine the type and distribution of various phases formed. Three different phases, namely, Au rich, Ni-Ge, and Ni-As(Ge), were observed. The Ni-As(Ge) phase was found to be in contact with GaAs. The Ni-Ge phase was present in localized regions in the Au-rich phase. Also, the Ni-Ge phase was found to be in direct contact with either Ni-As(Ge) or with GaAs. The Au-rich region manifested itself as a layered structure and was observed above the Ni-As(Ge) phase. As a consequence of the anneal cycle, movement of contact material both parallel and perpendicular to the device surface occurred at the metallization edges.
Elongated single-domain ferromagnetic particles
Authors: Aharoni Amikam
Published: July 2002

The ‘‘fundamental theorem’’ of Brown on spherical particles is extended to prolate spheroids, whose long axis is also an easy axis for a cubic or uniaxial anisotropy. It is rigorously proven that for such a particle, with a minor semiaxis a, the lowest free-energy state is that of a uniform magnetization, if a<ac 0, and an expression is given for this ac 0. Typical recording particles are thus shown to be single domains in their remanent state, with their magnetization reversing by the curling mode.
Strain distribution in thin aluminum films using x-ray depth profiling
Authors: Doerner M. F. , Brennan S.
Published: July 2002

Thin polycrystalline Al films deposited on oxidized Si(100) substrates have been studied using grazing incidence x-ray scattering techniques. The films become strained after high-temperature annealing due to the differential thermal expansion of the substrate and film. By varying the grazing angle of incidence on the surface, the penetration depth of the incident x rays can be varied from a minimum of roughly 30 Å to a maximum which is deeper than the film thickness. By measuring the change in the Al in-plane peak position with penetration depth, the strain distribution within the film can be determined. For thin films (less than 3000 Å) the strain is uniform throughout much of the film, decreasing only very near the surface. Strain relaxation with time occurs uniformly throughout the film over the first 4000 min.
On the location of ytterbium in GaP and GaAs lattices
Authors: Kozanecki A. , Gröetzschel R.
Published: July 2002

Yb-implanted layers in GaP and GaAs were analyzed by Rutherford backscattering and channeling of 1.7-MeV 4He+ ions. It is shown that Yb diffused to the surface and formed precipitates in both crystals, and the nonsubstitutional fraction of Yb in furnace annealed samples exceeded 95%. Pronounced segregation of Yb to the surface in laser annealed GaAs was also observed. Nonsubstitutional location of rare earth ions and their out-diffusion from the crystals seem to be the main difficulties in obtaining III–V semiconductors doped with rare-earth elements.
Magnetization reversal in nucleation controlled magnets. I. Theory
Authors: Ramesh R. , Srikrishna K.
Published: July 2002

A statistical model, based upon earlier models of Brown [J. Appl. Phys. 33, 3022 (1962)] and McIntyre [J. Phys. D 3, 1430 (1970)] has been developed to examine the magnetization reversal of domain-wall nucleation controlled permanent magnets such as sintered Fe-Nd-B and SmCo5. Using a Poisson distribution of the defects on the surface of the grains, a ‘‘weakest link statistics’’ type model has been developed. The model has been used to calculate hysteresis loops for sintered Fe-Nd-B-type polycrystalline magnets. It is shown that the intrinsic coercivity measured for a bulk magnet should vary inversely as the logarithm of the surface area of the grain. The effect of demagnetizing field has been incorporated by a mean-field-type approximation, to calculate the overall nucleation field from the intrinsic coercivity. The hysteresis loops theoretically calculated are in excellent agreement with the overall form of those experimentally determined for similar nucleation controlled magnets. The model also predicts that for an inhomogeneous grain size distribution, such as a bimodal distribution, kinks will be observed in the second quadrant of the hysteresis loops.
Commercial scale production of Fe-6.5 wt. % Si sheet and its magnetic properties
Authors: Takada Y. , Abe M. , Masuda S. , Inagaki J.
Published: July 2002

Commercial scale production of a Fe-6.5 wt. % Si sheet has been successfully developed. Presently manufactured sheets are in coil form, whose thickness ranges from 0.1 to 0.5 mm with a maximum width of 400 mm. Magnetic properties of the manufactured sheet have been investigated. The permeability of Fe-6.5 wt. % Si sheet is about 10 times higher than the conventional nonoriented silicon steel sheet. The core losses are less than half the conventional, and even less than that of the grain-oriented silicon steel sheet at frequencies over 400 Hz. Superior soft magnetic properties are attributed to the low magnetostriction and high electric resistivity of this alloy. It is well known that the Fe-6.5 wt. % Si alloy has poor ductility in conventional mechanical work. But investigation of the forming conditions has enabled the stamping and bending of alloy sheets. Low core losses and high permeability make Fe-6.5 wt. % Si sheet adequate for motor cores, transformer cores operating at high frequencies, and magnetic shielding. Application to the micromotor core shows that Fe-6.5 wt. % Si sheet reduces the consumption of no-load electric current by 25% in comparison with the conventional silicon steel.
Dopant-dependent formation and annealing of the dominant native deep-level defect in liquid-phase epitaxial AlGaAs
Authors: Krispin P.
Published: July 2002

In liquid-phase epitaxial GaAs the hole traps with levels at EV+0.40 eV and EV+0.70 eV (labeled A and B) are commonly observed. Here, AlGaAs LPE layers doped with Mg, Zn, Si, Ge, Sn, or Te are investigated in order to assess the solubility and the annealing characteristic of both hole traps. It is shown that the concentration of the traps is strongly dependent on the incorporated impurity. The formation of the underlying native defect is determined by the type of conductivity, the amount of the dopant, as well as the lattice site occupied by the impurity atom. The mechanism of dopant-induced solubility is clearly demonstrated for a particular deep-level defect in a semiconductor. Experimental evidence is given that the formation of the native defect in n-type material is enhanced by Ga-site dopants. The solubility characteristic unambiguously reveals that the native defect is mobile at temperatures above 550 °C and that the deep levels are of acceptorlike character. By incorporating high concentrations of As-site dopants the formation of the native defect is significantly suppressed in n-type as well as in p-type material. Therefore, it is suggested that the native defect studied here is linked with the As lattice site. The GaAs antisite defect model is corroborated with regard to the charge character, the defect site, and the two coupled charge states. Usually, the native defect is stable against thermal treatment. Annealing, most likely due to a defect reaction, is observed only in the presence of Ge or Si.
Preparation and microstructure of La-containing R-Fe-B permanent magnets
Authors: Tang Weizhong , Zhou Shouzeng , Wang Run
Published: July 2002

The unfavorable effect of La on the magnetic properties of R-Fe-B permanent magnets is shown to be related closely to remarkable changes in microstructure due to excessive oxidation of La-containing R-rich phases in the alloys. Ce and Al are found to be effective in promoting the sintering process of La-containing alloys, resulting in improvements in both microstructure and magnetic properties. Al segregates into the fcc R-rich phase and improves the wetting ability of its liquid at high temperatures. It is suggested that the influence of Al on coercivities of R-Fe-B magnets is connected with a depression of magnetic interactions between hard magnetic grains.
Structural characteristics of Sr1−xLaxTi3+δ as a function of oxygen partial pressure at 1400 °C
Authors: Howard S. A. , Yau J. K. , Anderson H. U.
Published: July 2002

The structural characteristics of Sr1−x Lax TiO3+δ (0≤x≤0.4) at 1400 °C have been investigated as a function of ambient oxygen partial pressure. A modified Rietveld pattern-fitting structure-refinement program [H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969)] was used to determine the nature of the distortions of the fundamental perovskite unit cell, the degree of lattice perfection, and the cation vacancy concentrations. Specimens equilibrated in forming gas displayed a linear relation between x, the lattice parameters, and the degree of lattice perfection while those samples annealed in air and oxygen deviated significantly from linearity. A monoclinic distortion of the perovskite structure was seen in the samples at low oxygen partial pressures while a second phase or layer type of distortion appeared in samples with x>0.2 under oxidizing conditions.
A high-pressure cell for electrical resistance measurements at hydrostatic pressures up to 8 GPa: Results for Bi, Ba, Ni, and Si
Authors: Andersson G. , Sundqvist B. , Bäckström G.
Published: July 2002

A modified Bridgman anvil high-pressure device, capable of producing hydrostatic pressures up to 8 GPa (80 kbar), was designed and built. The size of the pressure chamber (10 mm in diameter) allows the use of large specimens and simple experimental procedures. Experimental results show that hydrostatic conditions are necessary if accurate quantitative information is desired about the electrical properties of materials under pressure. Accurate data on resistance (and resistivity) versus pressure at 294 K are given for Bi, Ba, Ni, and Si. The initial pressure coefficients of R were d(ln R)dP=0.13, −7.6×10−2, −2.0×10−2, and −0.26 GPa−1, respectively. Barium has a resistance minimum near 0.9 GPa. For Bi we observe sharp transitions at 2.55, 2.7, and 7.7 GPa, and for Ba at 5.55 GPa, but we cannot verify the existence of a transition in Ba near 7 GPa. Neither do we confirm the phase transformation in Ni recently reported to occur above 2.5 GPa. For Si, R(P) agrees very well with a theoretical function calculated from the change in band gap and electron mobility with pressure.
Solution procedure for the Laplace equation in bispherical coordinates for two spheres in a uniform external field: Perpendicular orientation
Authors: Stoy Richard D.
Published: July 2002

A solution procedure is developed for the Laplace equation in bispherical coordinates for two spheres which are embedded in an infinite medium and which are aligned perpendicular to an external field which is uniform at infinity. The boundary conditions, continuity of the potential and continuity of the normal component of the flux density at the surfaces of the two spheres, are often encountered in several branches of the macroscopic field theory of linear media: electrostatics, magnetostatics, steady-state diffusion, electric current, and flow of heat.
Direct and efficient solutions of integral equations for scattering from strips and slots
Authors: Tsalamengas John L. , Fikioris John G. , Babili Barbara Th.
Published: July 2002

Singular integral or integro-differential equations (SIE or SIDE) are often used for the analytical formulation of two-dimensional boundary-value problems. The methods for solving them depend primarily on the complexity of their kernel and on the kind (first or second) of the SIE itself. First-kind SIEs with a Laplacian kernel are characteristic in electrostatics. A successful method for solving them is a regularization approach based on the transformation of the SIE to an equivalent Fredholm regular integral equation of the second kind. Well-known inversion formulas are essential to this approach. In electromagnetics, a Hankel-type kernel complicates matters considerably; inversion formulas and regularization techniques end up as cumbersome indirect procedures making necessary the recourse to a more direct method. Such a method is developed in this paper in combination with a very suitable expansion of the Bessel function, that multiplies the logarithmic singularity of the Hankel kernel, into a series of Chebyshev polynomials of the first or second kind. It is essentially a direct analytical approach that requires fewer expansion functions per wavelength than the method of moments and whose matrix elements are not numerical integrals of singular functions, but quite concise and rapidly convergent series expansions. The efficiency of the method is shown by applying it to scattering of E-polarized waves from a strip conductor right on the interface between two different dielectric half-spaces and of E- or H-polarized waves from a slot in the presence of a uniaxially gyrotropic half-space. Asymptotic expressions for the far-scattered field are given in all these cases and numerical results are plotted and compared with existing similar ones in certain special situations.
Deposition of aluminum thin films by photochemical surface reaction
Authors: Hanabusa Mitsugu , Oikawa Akira , Cai Peng Ying
Published: July 2002

Aluminum thin films have been deposited from dimethylaluminum hydride (DMAlH) on silicon substrates illuminated with a deuterium lamp or an ArF laser. DMAlH was found to be useful as a new source gas for photodeposition of aluminum films at a low carbon level if it was used with photons with wavelengths below 200 nm. Illumination is effective not only to produce films at a substrate temperature lower than required by thermal decomposition, but also to reduce the electrical resistivity of the deposited films. To emphasize surface, rather than gas-phase, reactions, the vapor pressure in a reaction cell was lowered (typically at 6.7×10−3 Pa). Evidence for the photochemical surface reaction has been provided by area selectivity and the time needed to renew the adlayer on the surface. In spite of low vapor pressure, the deposition rate was typically 19 nm/min for the lamp and 0.06 nm/pulse for the laser at a substrate temperature of 200 °C. A rate equation, which included both photodissociation and photoinduced desorption, accounted for the observed dependence of deposition rate on substrate temperature, light intensity, vapor pressure, and laser pulse repetition rate. The role of secondary reactions that follow initial photodissociation was discussed briefly. The electrical resistivity for the film produced at 270 °C under illumination was 6.2 μΩ cm, while a much higher resistivity of 140 μΩ cm was observed for a thermally grown film. The carbon level was low (2–3%) in agreement with the low film resistivity. The surface was rough, leaving room for improvement.
Fifth-order corrected electromagnetic field components for a fundamental Gaussian beam
Authors: Barton J. P. , Alexander D. R.
Published: July 2002

Fifth-order corrected expressions for the electromagnetic field components of a monochromatic fundamental Gaussian beam (i.e., a focused TEM00 mode laser beam) propagating within a homogeneous dielectric media are derived and presented. Calculations of relative error indicate that the fifth-order Gaussian beam description provides a significantly improved solution to Maxwell’s equations in comparison with commonly used paraxial (zeroth-order) and first-order Gaussian beam descriptions.
Radiation effects on ferroelectric thin-film memories: Retention failure mechanisms
Authors: Scott J. F. , Araujo C. A. , Meadows H. Brett , McMillan L. D. , Shawabkeh A.
Published: July 2002

Ferroelectric thin-film (200–350 nm) memories were fabricated and tested. Two materials were used as memory cells: potassium nitrate (KNO3) and lead zirconate titanate (PbZr0.54Ti0.46O3, usually abbreviated as PZT). These devices were tested as arrays deposited either by thermal evaporation (in the case of KNO3) or sputtered films (PZT). Fully packaged devices were tested to determine switching speed and polarization (switched charge) as functions of temperature and applied voltage. Radiation hardness was also tested for both dose rate and total dose. The switching kinetics were investigated in considerable detail and found to confirm the theory of Ishibashi. The dimensionality of domain growth for the switching process in KNO3 lowers from 2.3 before irradiation to 1.6 after 0.5 Mrad. For PZT rad hardness exceeds 5 Mrad total dose and 2×1011 rad/s. A surprising result was that the hysteresis curves for all of the PZT samples became more symmetric after 5 Mrad irradiation; this is interpreted as a destruction of internal biasing fields. All PZT memories continued to function after 5 Mrad, but 2 out of 8 of those stored under short-circuited conditions failed to retain information. An explanation of the retention failure of the short-circuited cells is given in terms of space-charge and well-known electret effects.
Inelastic neutron scattering studies of II-VI diluted magnetic semiconductors (invited)
Authors: Giebultowicz T. M. , Rhyne J. J. , Furdyna J. K. , Klosowski P.
Published: July 2002

Inelastic neutron scattering studies of AII1−xMxBVI diluted magnetic semiconductor (DMS) alloys with low and high concentations of the magnetic component (M) are reviewed. Measurements of inelastic scattering from isolated pairs of magnetic ions makes possible a direct determination of the ion–ion exchange parameters. This method has been used for JNN determination in Zn1−xMnxB (B=S, Se, Te) and Zn1−xCoxB DMS alloy families. Inelastic scattering experiments on DMS alloys with high magnetic ion concentrations (Zn1−xMnxTe and Cd1−xMnxTe with x=0.4–0.7) reveal that at low T the magnetic excitation spectra of these systems are dominated by magnonlike modes despite the absence of long-range magnetic order. A theoretical description of these excitations can be obtained using the numerical method developed by Ching and Huber. The model results reproduce remarkably well the dispersion and the line-shape characteristics of the excitation modes seen in the experiments.
Solid-state sensors for trace hydrogen gas detection
Authors: Christofides Constantinos , Mandelis Andreas
Published: July 2002

This paper reviews the development, history, theoretical basis, and experimental performance of solid-state hydrogen detectors under flow-through conditions available to date such as pyroelectric, piezoelectric, fiber optic, and electrochemical devices. Semiconductor hydrogen detectors will only be reviewed briefly, as excellent reviews on this subject already exist. In view of the fact that almost all the devices that will be discussed later in this paper use Pd as a hydrogen trap, we devote a subsection to examining the role of palladium as a catalyst as well as some of the characteristics of the Pd-H2 system. Non-solid-state hydrogen sensors, such as the flame ionization detector are not the object of this review. A useful feature of this review is a comparison of operating characteristics of each device in a general table in Sec. VII. In that section a general discussion is presented, including a critical comparison of the capabilities and parameters of various solid-state hydrogen sensors in the form of a table showing data collected from the literature. The Pd-fiber optic sensor is the most sensitive optical device operating at room temperature. The Pd-photopyroelectric sensor appears to be most economical and second best in sensitivity at room temperature; it has the best potential for high signal-to-noise operation at the widest temperature range, down to cryogenic temperatures. The Pd-field effect transistor devices exhibit the second highest sensitivity at elevated temperatures.
Analytical solution for minority-carrier transport in heavily doped semiconductors with position-dependent band structures
Authors: Mohammad S. Noor
Published: July 2002

An analytical solution for minority-carrier current in a heavily doped semiconductor region with a position-dependent band structure has been presented. The solution is general enough to be valid for both degenerate and nondegenerate semiconductors, and for any kind of doping profile. The accuracy of the solution depends on the accuracy of the minority-carrier lifetime model. Such a model developed in the present investigation agrees closely with available experiments.
Light polarization effect in measurement of thin wire diameter by laser diffraction and its explanation with boundary diffraction wave
Authors: Xie Jianping , Qiu Yanhui , Ming Hai , Li Chuanqi
Published: July 2002

The polarization effect in the measurement of thin wire diameter by laser diffraction has been studied experimentally and theoretically in this paper. The experimental values of a diameter vary sinusoidally with the vibration direction of incident polarized light. The difference between measured diameter values with S and P component of incident light is nearly proportional to the imaginary part of the complex index which represents the optical property of metal wire. Taken into account the phase change of boundary diffraction wave originating from the edges of metal wire, the physical cause of this polarization effect, and its experimental results have been clearly explained.
Transient pressure effects in a pinch-compressed cylindrical metallic conductor
Authors: Krehl P.
Published: July 2002

For a cylindrical aluminum conductor, 0.2 cm in diameter and loaded by a current pulse from an overdamped discharge with an initial current rate of 490 kA/μs, dynamic pinch compression and temperature rise have been calculated. The numerical solution for the azimuthal distribution of the pinch pressure, obtained by applying Picard’s method of successive iteration, consists of the superposition of a quasi-stationary and a transient term. In the very initial phase of pinching a steep, narrow pressure pulse is created below the surface of the wire which converges at the center, thereby broadening in width, growing in strength, and increasing in propagation velocity. Already in the early stage of current flow at about 0.4 μs, well before reaching maximum current and pinch pressure, the quasi-stationary term becomes dominant. Then the conductor is compressed quite homogeneously with the exception of a narrow shell below the surface where maximum pressure gradients exist. For applications on dynamic materials research various experimental aspects of dynamic pinch compression and possible high-speed methods of sample diagnostics are reviewed.
Structural and hard magnetic properties of rapidly solidified Sm–Fe–N
Authors: Katter M. , Wecker J. , Schultz L.
Published: July 2002

Sm–Fe alloys have been produced by rapid quenching and the resulting phases have been investigated in the as-quenched state and after nitriding. Besides the well-known equilibrium phases of the binary Sm–Fe system (Fe, Sm2Fe17, SmFe3, SmFe2 and Sm), a hexagonal TbCu7-type phase shows up in melt spun ribbons (a=4.88 Å, c=4.23 Å). Its stoichiometry is about Sm1Fe9 and it is formed only at wheel velocities above 15 m/s. The Curie temperature and the saturation polarization of this new phase is 210 °C and 1.25 T, respectively. At higher Sm concentrations or lower quenching rates the structure changes to the Th2Zn17-type. The Th2Zn17-type ribbons are magnetically soft whereas the TbCu7-type samples show moderate coercivities of up to 1.7 kA/cm. Nitrogenation leads to an expansion of the lattice and to an overall improvement of the hard magnetic properties for both phases. Their Curie temperatures are increased to 470 °C and the saturation polarizations are raised to 1.40 and 1.51 T for the TbCu7- and the Th2Zn17-type phases, respectively. The best hard magnetic properties for isotropic TbCu7-type material are obtained for quenched, annealed, and nitrided Sm10.6Fe89.4 which shows a coercivity, Hci, of 4.9 kA/cm, a remanence, Jr, of 0.86 T and an energy product, (BH)max, of 69.6 kJ/m3. For similarly treated Sm12Fe88, which crystallizes in the Th2Zn17 structure, a coercivity of 16.7 kA/cm, a remanence of 0.73 T and an energy product of 65.6 kJ/m3 are achieved.