摘要:

Investigation on magnetocaloric effects, magnetic entropy, Curie temperature, and specific heat of nanocomposite binary gadolinium alloys Gd–Tb, Gd–Zn, and Gd–Y has been carried out with an applied magnetic field of 1 T and in a temperature range of 233–313 K. Compared with the respective bulk alloys, the as‐prepared nanocomposite alloys were found to have higher specific heat and lower Curie temperature. The nanocomposite Gd–Y alloy exhibited distinctive enhancement of both magnetocaloric effect and magnetic entropy over that of the bulk alloy. The finding is of importance for developing new materials for magnetic refrigeration at room temperature. A discussion concerning the enhanced magnetic entropy in nanometer superparamagnetic systems is presented in detail. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362773

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

X‐ray reciprocal space mapping has been used to investigate the strain status of microgun‐pumped blue and blue‐green laser structures. The devices exploit graded‐index, separate confinement Zn1−xCdxSe/ZnSe heterostructures grown on InGaAs or GaAs substrates by molecular‐beam epitaxy. The location of the reciprocal lattice point of the ZnSe buffer layer within a normally forbidden region of reciprocal space indicates that the ZnSe buffer layer is unusually strained, with an appreciable biaxial tensile strain despite the smaller lattice parameter of the III–V substrate relative to ZnSe. We associate such a phenomenon with the presence of the highly strained laser structure coupled with preferential strain relaxation at the II–VI/III–V heterointerface. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362775

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Grazing‐incidence x‐ray diffraction (GIXD) permits the direct measurement of in‐plane lattice parameters of SiGe films that are too thin to yield good results from normal‐geometry triple‐axis techniques. A unique ‘‘X’’‐shaped pattern has been seen inH–Kreciprocal space maps of diffracted x‐ray intensity from SiGe films that have relaxed via a modified Frank–Read mechanism. Contours of intensity are seen along the ⟨110⟩ directions from the (4¯00) reciprocal lattice peak with the introduction of the first dislocations. For higher dislocation densities the X‐shaped contours are anisotropically distorted and a satellite peak, corresponding to the lattice parameter for a partially relaxed film, becomes identifiable at lowerH. In contrast,H–Kreciprocal‐space contours from thin SiGe films that have relaxed by roughening and subsequent random nucleation of dislocations display broad, oval‐shaped contours centered at the (4¯00) reciprocal lattice point for the film. Numeric simulations of GIXD from a variety of dislocation arrangements were performed in order to understand the origin of the X pattern. We show that this pattern arises from an array of long misfit dislocations running in the ⟨110⟩ directions. The anisotropic distortion of the X pattern arises at higher dislocation densities from orthogonal intersections of dislocations with equal Burgers vector, which are characteristic of dislocation networks generated by the modified Frank–Read mechanism. We also verify that the measured values of the in‐plane lattice parameter, together with the out‐of‐plane lattice parameter determined from the symmetric (004) reflection, lead to accurate calculation of the composition and strain in these SiGe layers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362776

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In plasma‐depositeda‐Si:H films, the time dependence and saturation value of the metastable defect density produced by high‐intensity illumination are found to depend on the material properties. From a wide range of deposition techniques, we have observed a strong correlation between the steady‐state defect density Nssand themonohydrideSi–H bond concentration observed in infrared transmission, with [SiH]2000values between 2% and 16%. We propose a microscopic description of the light‐induced defect creation and light‐induced defect annealing phenomena ina‐Si:H, based on the trapping of mobile H at two specific sites: the Si–H bond and the Si dangling bond. This model predicts a steady‐state defect density proportional to themonohydride[SiH]. For the production of mobile H in three‐center (Si–H–Si) configurations, a low‐energy mechanism with no necessity for Si–H bond breaking is suggested from the observed changes in the infrared absorption of the various H‐bonding configurations during light soaking. With the assistance of an electron‐hole pair recombination, it consists of the excitation of two adjacent H atoms from a (Si–H H–Si) configuration along with a Si–Si bond reconstruction. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362777

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Carbon‐coated iron, cobalt, and nickel particles were produced by an arc discharge process modified in the geometry of the anode and the flow pattern of helium gas. Field emission scanning electron microscopy shows that the resulting material consists of only carbon‐coated metal particles without any nanotubes or other unwanted carbon formations present. The diameters of iron, cobalt, and nickel particles range predominantly from 32 to 81 nm, 22 to 64 nm, and 16 to 51 nm, respectively. X‐ray diffraction analysis confirmed that the as‐made particles are carbon‐coated elements rather than metal carbides. High resolution transmission electron microscopy reveals that the as‐made cobalt and nickel particles are covered by 1–2 graphitic layers, while iron particles are surrounded by amorphous carbon. When the samples were treated by annealing or immersion into nitric acid, particles completely coated by carbon resisted both postdeposition treatments. However, further graphitization of the carbon coating by either of the two treatments was observed. Particles only partially coated by carbon were not protected, but sintered by annealing or dissolved in the acid. The magnetic properties of the as‐made particles were measured by a vibrating sample magnetometer. The values of the saturation magnetic moment per gram of each type of metal particle are 56.21, 114.13, and 34.9 emu/g representing 26%, 71%, and 64% of the saturation moments of the bulk ferromagnetic elements iron, cobalt, and nickel, respectively. All the metal particles were shown to be ferromagnetic with a ratio of remnant to saturation magnetizationMR/MS∼0.3 at room temperature (25 °C). In this article the detailed preparation and the properties of these carbon‐coated metal particles will be discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362765

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Quantitative in‐depth distribution of the elements contained in silicon‐rich oxide thin films deposited on single‐crystal silicon by low temperature plasma‐assisted deposition has been performed by a combination of various MeV ion beam techniques. The quantity of oxygen and nitrogen has been measured by nuclear reactions, the silicon content has been determined by Rutherford backscattering, and elastic recoil detection was used for hydrogen. All the samples contain not only Si and O, but also N and H, which are residuals from the reactions involved in the deposition process. We did find that the MeV beam used in the nuclear techniques can induce a process of hydrogen desorption, which causes the measured H content to be a function of the He dose received by the sample. This phenomenon, not previously reported, must be taken into account to give the correct H content. The study of the kinetics of the He‐induced hydrogen desorption has been used to correct the experimental data and to determine the original hydrogen content. The correction factor is in most of the cases close to 2 and outside any experimental error. Moreover the studies of the kinetics give information on the kinds of hydrogen complexes contained in the films. The results suggest that, on the basis of the strength of the binding energies, hydrogen is present in at least two different configurations, weakly and strongly bonded. In the first configuration hydrogen is easily desorbed either under the action of the ion beam or of the heat treatment at 600 °C, in the second, hydrogen is lost only after treatment at 900 °C. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362766

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report on the photometric and colorimetric properties of surface and polymer network stabilized reflective cholesteric displays. Both diffuse and partially diffuse illumination are used, the latter being an experimental approach to emulating typical room light conditions. It is shown that addition of polymer increases the field of view while decreasing angular dependence of the color quantities: hue, chroma, and lightness. Total luminance and contrast ratio, however, are also decreased. Therefore, it is concluded that optimum polymer concentration is dependent on the viewing geometry. Luminance and contrast ratio of the surface stabilized cells exceeded that of polymer stabilized cell when viewed in a geometry void of specular reflection. Colorimetric quantities in surface stabilized cells are less sensitive to illumination geometry. This suggests that for displays in which specular reflection has been suppressed, surface treatment represents the best method of stabilization. If the viewing angle allows specular reflection, polymer stabilization yields the largest luminance and contrast ratio. This behavior is explained in terms of angular distribution of helical axes due to presence of the stabilizer. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362785

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Viscoelastic solids with high viscosity were experimentally studied with the intention of having them deformed under uniform shear stress. A kind of sandwich method was developed for applying a constant shear stress to a specimen, and its deformation was observed optically using heterodyne interferometry with a sensitivity of 10 nm in displacement measurement. Time‐dependent deformation data were analyzed on the basis of a mechanical model of anelasticity plus viscosity. Viscosity in the range of 108–1014Pa⋅s could be determined at temperatures of 20–200 °C. Through a simulation using the finite element method together with an experiment visualizing the deformation, the specimen deformation was shown to be of an almost uniform shear mode. Experiments were performed to determine the temperature dependence of viscosity for several kinds of glasses near their glass transitionTg, and the determined viscosity values were about 107Pa⋅s atTg. The viscosity values measured by a rotation disk viscometer were in good agreement with the present data. The viscosity of some glasses was also measured by the beam bending and penetration methods, and values of 109–1012Pa⋅s atTgwere obtained. The large discrepancy between the two kinds of data was considered. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362767

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Soft materials have long been sought after for use in devices such as actuators, artificial muscles, separators, switches, sensors, memories, and so forth. We developed a soft, optically transparent material using polymer gels that can not only be activated by visible light (switched on) but also deactivated (switched off) by altering the local environment using three different means:pH, temperature, and light. This copolymer gel is a covalently cross‐linked network ofN‐isopropylacrylamide, sodium acrylate, and a chromophore, which is found to undergo phase transitions exhibiting large hysteresis in the degree of swelling in response topH, temperature, and light. In each system, between the transitions for swelling and shrinking, the gel can show either a swollen or a collapsed state, which can be selected according to the history of the variables. It has been established that a thermoresponsive gel with chromophore exhibits a local volume phase transition upon illumination with visible light. By making use of this phenomenon, we have successfully controlled the phase in which a gel exists with visible light: Without light illumination the gel stays in the swollen state. Upon illumination beyond a threshold intensity, however, a volume transition is locally induced, thereby forming a material in which both phases coexist stably for at least several hours after the light source has been removed. The phenomenological stability of the material in the coexistence state is discussed on the basis of the Landau theory. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362768

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this article, the solidus temperatures of the Hg1−xZnxTe pseudobinary phase diagram for several compositions in the lowxregion were measured by differential thermal analysis and the HgTe–ZnTe pseudobinary phase diagram was constructed. The densities of two HgZnTe melts,x=0.10 and 0.16, were determined by aninsitupycnometric technique in a transparent furnace over, respectively, 110 and 50 °C ranges of temperature. The thermodynamic properties of the melts, such as the heat capacity and enthalpy of mixing, were calculated for temperatures between the liquidus and 1500 °C by assuming an associated solution model for the liquid phase. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362826

出版商:AIP    

年代:1996

数据来源: AIP