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91.
In this paper we report some of the important results of experimental investigations of the flicker noise near the metal-insulator
(MI) transition in doped silicon single crystals. This is the first comprehensive work to study low-frequency noise in heavily
doped Si over an extensive temperature range (2 K<T<500 K). The measurements of conductance fluctuations (flicker noise) were carried out in the frequency range 10−2<f<4 × 101 Hz in single crystalline Si across the MI transition by doping with phosphorous and boron. The magnitude of noise in heavily
doped Si is much larger than that seen in lightly doped Si over the whole temperature range. The extensive temperature range
covered allowed us to detect two distinct noise mechanisms. At low temperatures (T<100 K) universal conductance fluctuations (UCF) dominate and the spectral dependence of the noise is determined by dephasing
the electron from defects with two-levels (TLS). At higher temperatures (T>200 K) the noise arises from activated defect dynamics. As the MI transition is approached, the 1/f spectral power, typical of the metallic regime, gets modified by the presence of discrete Lorentzians which arise from generation-recombination
process which is the characteristic of a semiconductor. 相似文献
92.
H. Sakurai 《The European Physical Journal A - Hadrons and Nuclei》2002,13(1-2):49-53
Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator
(RIPS) are introduced. Special emphasis is given to two selected experiments from recent programs that highlight studies on
the magicity loss observed for very neutron-rich nuclei beyond N = 20 in the “island-of-inversion” region; the particle stability of 31F, and the low-lying excited states of 34Mg.
Received: 1 May 2001 / Accepted: 4 December 2001 相似文献
93.
S. Kalavathi J. Janaki Awadhesh Mani V. Sankara Sastry 《Solid State Communications》2006,139(7):334-338
Some new members of a ruthenocuprate(2212) series have been synthesized by Mn substitution for Ru in Gd1.4Ce0.6Sr2RuCu2O10. Characterization by x-ray diffraction (XRD) phase analysis has been carried out. Changes in structural features on substitution, including a significant change in lattice parameter for a very low substitution level, have been observed. Four-probe resistivity studies indicate the coexistence of superconductivity and magnetism for the pristine compound and a semiconductor-like upturn in resistivity and the absence of superconductivity even for very low levels of Mn substitution. AC susceptibility measurements show a progressive suppression of the magnetic transition temperature as well as a smearing of the magnetic transition as a function of Mn substitution. Possible reasons for the absence of superconductivity have been discussed. 相似文献
94.
Hyeong Taek Ham Yeong Suk Choi Mu Guen Chee In Jae Chung 《Journal of polymer science. Part A, Polymer chemistry》2006,44(1):573-584
This work is to make carbon nanotubes dispersible in both water and organic solvents without oxidation and cutting nanotube threads. Polystyrene‐singlewall carbon nanotube (PS‐SWNT) composites were prepared with three different methods: miniemulsion polymerization, conventional emulsion polymerization, and mixing SWNT with PS latex. The two factors, crosslinking and surface coverage of PS are important factors for the mechanical and electrical properties, including dispersion states of SWNT in various solvents. The PS‐SWNT composite prepared via a conventional emulsion polymerization showed SWNT bundles entirely covered with PS, whereas the PS‐SWNT composite prepared via a miniemulsion polymerization showed SWNT partially covered with crosslinked PS nanoparticles. The method of mixing SWNTs with PS latex did not show the well dispersed state of carbon nanotubes because PS was not crosslinked and was dissolved in a solvent, and nanotubes separated from PS precipitated. So the PS nanoparticle‐SWNT composite had lower electrical resistance, and higher mechanical strength than the other composites made by the latter two methods. As the amount of SWNT increases, the bare surface area of SWNT increases and the electrical conductivity increases in the composite made by the miniemulsion polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 573–584, 2006 相似文献
95.
Rachel K. O'Reilly Maisie J. Joralemon Craig J. Hawker Karen L. Wooley 《Journal of polymer science. Part A, Polymer chemistry》2006,44(17):5203-5217
Block copolymer micelles and shell cross-linked nanoparticles (SCKs) presenting Click-reactive functional groups on their surfaces were prepared using two separate synthetic strategies, each employing functionalized initiators for the controlled radical polymerization of acrylate and styrenic monomers to afford amphiphilic block copolymers bearing an alkynyl or azido group at the α-terminus. The first route for the synthesis of the azide-functionalized nanostructures was achieved via sequential nitroxide-mediated radical polymerization (NMP) of tert-butyl acrylate and styrene, originating from a benzylic chloride-functionalized initiator, followed by deprotection of the acrylic acids, supramolecular assembly of the block copolymer in water and conversion of the benzylic chloride to a benzylic azide. In contrast, the second strategy utilized an alkynyl-functionalized reversible addition fragmentation transfer (RAFT) agent directly for the RAFT-based sequential polymerization of tetrahydropyran acrylate and styrene, followed by selective cleavage of the tetrahydropyran esters to give the α-alkynyl-functionalized block copolymers. These Click-functionalized polymers, with the functionality located at the hydrophilic polymer termini, were then self-assembled using a mixed-micelle methodology to afford surface-functionalized “Clickable” micelles in aqueous solutions. The optimum degree of incorporation of the Click-functionalized polymers was investigated and determined to be ca. 25%, which allowed for the synthesis of well-defined surface-functionalized nanoparticles after cross-linking selectively throughout the shell layer using established amidation chemistry. Functionalization of the chain ends was shown to be an efficient process under standard Click conditions and the resulting functional groups revealed a more “solution-like” environment when compared to the functional group randomly inserted into the hydrophilic shell layer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5203–5217, 2006 相似文献
96.
Kozo Matsumoto Junichi Nakashita Hideki Matsuoka 《Journal of polymer science. Part A, Polymer chemistry》2006,44(15):4696-4707
Diblock copolymer poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane)‐block‐polystyrene (polyVSA‐b‐polySt) and triblock copolymer poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane)‐block‐polystyrene‐block‐poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane) (polyVSA‐b‐polySt‐b‐polyVSA), consisting of silazane and nonsilazane segments, were prepared by the living anionic polymerization of 1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane and styrene. PolyVSA‐b‐polySt formed micelles having a poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane) (polyVSA) core in N,N‐dimethylformamide, whereas polyVSA‐b‐polySt and polyVSA‐b‐polySt‐b‐polyVSA formed micelles having a polyVSA shell in n‐heptane. The micelles with a polyVSA core were core‐crosslinked by UV irradiation in the presence of diethoxyacetophenone as a photosensitizer, and the micelles with a polyVSA shell were shell‐crosslinked by UV irradiation in the presence of diethoxyacetophenone and 1,6‐hexanedithiol. These crosslinked micelles were pyrolyzed at 600 °C in N2 to give spherical ceramic particles. The pyrolysis process was examined by thermogravimetry and thermogravimetry/mass spectrometry. The morphologies of the particles were analyzed by atomic force microscopy and transmission electron microscopy. The chemical composition of the pyrolysis products was analyzed by X‐ray fluorescence spectroscopy and Raman scattering spectroscopy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4696–4707, 2006 相似文献
97.
Dilyana Paneva Laetitia Mespouille Nevena Manolova Philippe Dege Iliya Rashkov Philippe Dubois 《Journal of polymer science. Part A, Polymer chemistry》2006,44(19):5468-5479
Polyelectrolyte complexes (PECs) have been prepared from well‐defined (quaternized) poly[2‐(dimethylamino)ethyl methacrylate] (PDMAEMA) and high molecular weight poly(2‐acrylamido‐2‐methylpropane sodium sulfonate) (PAMPSNa) after a thorough study of their viscometric properties. The effect of pH and quaternization degree of PDMAEMA on PECs stoichiometry has been examined. PEC‐based materials have been characterized in terms of thermal stability, equilibrium swelling degree, and free/bound water composition. The stoichiometry and swellability of the physically crosslinked hydrogels obtained from fully quaternized PDMAEMA/PAMPSNa complexes do not depend on pH. In contrast, PECs made of non quaternized PDMAEMA and PAMPSNa are highly affected by pH, and could reversibly disintegrate at pH ≥ 9. Partially quaternized PDMAEMA/PAMPSNa PECs exhibit intermediate properties and form stable loose structures in the whole investigated pH range. Finally, stable dispersions of PECs nanoparticles have been successfully produced from dilute solutions of the complementary polyelectrolytes. The nanoparticle average diameter as determined by dynamic light scattering proved to depend on the molar fraction of DMAEMA‐based subunits and on the initial polyelectrolyte concentration. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5468–5479, 2006 相似文献
98.
Zhicheng Xiao Ying Li Dongling Ma Linda S. Schadler Yvonne A. Akpalu 《Journal of Polymer Science.Polymer Physics》2006,44(7):1084-1095
Small‐angle light scattering (SALS) measurements were used to study the structure of titanium dioxide (TiO2)/low‐density polyethylene (LDPE) nanocomposites. The results showed that the scattering from LDPE crystalline structures and the scattering from TiO2 nanoparticles can be resolved and separated. It is shown that the independent effects of crystallization conditions and the presence of nanoparticle aggregates on the spherulitic structure of the LDPE matrix can be determined by analyzing the scattering patterns using the methods proposed. From the SALS results, we conclude that the nanoparticle surface chemistry affects both nucleation of spherulites and their structure particularly under rapid cooling conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1084–1095, 2006 相似文献
99.
X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) have been used to investigate the effect of reactive ion etching (RIE) on poly(methylhydrogensiloxane-co-dimethylsiloxane) surface in fluorine-based plasmas. Polysiloxane layers supported on the standard silicon wafers were etched using SF6 + O2 or CF4 + O2 plasmas. SEM studies show that the polysiloxane morphology depends on plasma chemical composition strongly. Presence of a columnar layer likely covered with a fluorine rich compound was found on the elastomer surface after the CF4 + O2 plasma exposure. After the SF6 + O2 or CF4 + O2 plasma treatment the polysiloxane surface enriches with fluorine or with fluorine and aluminum, respectively. Different morphologies and surface chemical compositions of the silicone elastomer etched in both plasmas indicate different etching mechanisms. 相似文献
100.
Hynd Remita Prem Felix Siril Israel-Martyr Mbomekalle Bineta Keita Louis Nadjo 《Journal of Solid State Electrochemistry》2006,10(7):506-511
Three Pt-based catalysts prepared in different radiolytic conditions and supported on graphite powder were packed into a carbon paste electrode configuration. They were compared to each other, to the commercial (Pt) deposited on activated carbon powder (Johnson Matthey) and to pure Vulcan XC-72 for their respective abilities toward the hydrogen evolution reaction (HER). The Tafel parameters were determined for all these electrodes. From the I–V curves and their quantitative treatment, the following order of activity emerged unambiguously and reads: (PtCO)2 (fcc structure) > (PtCO)1 (Chini cluster) > (Pt)neat > (Pt)JM (Johnson Matthey) ≫ (Vulcan XC-72). As expected, all the Pt-loaded electrodes were more efficient than Vulcan XC-72. The classification appears to be linked with the mean nanoparticle size, and for comparable sizes, with the surface morphology of the materials. The results and the stability of the electrodes suggest that the small particle sizes and the good dispersity on the carbon support were maintained during the HER. 相似文献