22 research outputs found
Magnetic Properties of a-Si films doped with rare-earth elements
Amorphous silicon films doped with Y, La, Gd, Er, and Lu rare-earth elements
(a-Si:RE) have been prepared by co-sputtering and studied by means of electron
spin resonance (ESR), dc-magnetization, ion beam analysis, optical
transmission, and Raman spectroscopy. For comparison the magnetic properties of
laser-crystallized and hydrogenated a-Si:RE films were also studied. It was
found that the rare-earth species are incorporated in the a-Si:RE films in the
RE3+ form and that the RE-doping depletes the neutral dangling bonds (D0)
density. The reduction of D0 density is significantly larger for the magnetic
REs (Gd3+ and Er3+) than for the non-magnetic ones (Y3+, La3+, Lu3+). These
results are interpreted in terms of a strong exchange-like interaction, J RE-DB
SRE SDB, between the spin of the magnetic REs and that of the D0. All our
Gd-doped Si films showed basically the same broad ESR Gd3+ resonance (DHpp ~
850 Oe) at g ~ 2.01, suggesting the formation of a rather stable RE-Si complex
in these films.Comment: 15 pages, 7 figure
Evidence for internal field in graphite: A conduction electron spin resonance study
We report conduction electron spin resonance measurements performed on highly
oriented pyrolitic graphite samples between 10 K and 300 K using S (f = 4 GHz),
X (f = 9.4 GHz), and Q (f = 34.4 GHz) microwave bands for the external
dc-magnetic field applied parallel (H || c) and perpendicular (H perp c) to the
sample hexagonal c-axis. The results obtained in the H || c geometry are
interpreted in terms of the presence of an effective internal
ferromagnetic-like field Heff-int(T,H) that increases as the temperature
decreases and the applied dc-magnetic field increases. We associate the
occurrence of the Heff-int(T,H) with the field-induced metal-insulator
transition in graphite and discuss its origin in the light of relevant
theoretical models.Comment: 10 pages (tex), 5 figures (ps
Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit
We report on the magnetic field (0TT) dependence of the
longitudinal thermal conductivity of highly oriented pyrolytic
graphite in the temperature range 5 K 20 K for fields parallel to
the axis. We show that shows large oscillations in the
high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are
observed in the Hall resistance. With the measured longitudinal electrical
resistivity we show that the Wiedemann-Franz law is violated in the high-field
regime.Comment: 4 Figures, to be published in Physical Review B (2003
Ferromagnetism in Oriented Graphite Samples
We have studied the magnetization of various, well characterized samples of
highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite
to investigate the recently reported ferromagnetic-like signal and its possible
relation to ferromagnetic impurities. The magnetization results obtained for
HOPG samples for applied fields parallel to the graphene layers - to minimize
the diamagnetic background - show no correlation with the magnetic impurity
concentration. Our overall results suggest an intrinsic origin for the
ferromagnetism found in graphite. We discuss possible origins of the
ferromagnetic signal.Comment: 11 figure
Magnetic field driven metal-insulator phase transition in planar systems
A theory of the magnetic field driven (semi-)metal-insulator phase transition
is developed for planar systems with a low density of carriers and a linear
(i.e., relativistic like) dispersion relation for low energy quasiparticles.
The general structure of the phase diagram of the theory with respect to the
coupling constant, the chemical potential and temperature is derived in two
cases, with and without an external magnetic field. The conductivity and
resistivity as functions of temperature and magnetic field are studied in
detail. An exact relation for the value of the "offset" magnetic field ,
determining the threshold for the realization of the phase transition at zero
temperature, is established. The theory is applied to the description of a
recently observed phase transition induced by a magnetic field in highly
oriented pyrolytic graphite.Comment: 22 pages, REVTeX, 16 figures. The version corresponding to that
published in Phys.Rev.
Alkylguanidine-catalyzed heterogeneous transesterification of soybean oil
Transesterification of soybean oil with methanol has been carried out in the presence of heterogenized alkylguanidines as catalysts. The alkylguanidines were anchored to modified polystyrene or siliceous MCM-41, encapsulated in the supercages of zeolite Y, or entrapped in SiO2 sol-gel matrices. The catalytic activity of these catalysts was compared with that of their homogeneous counterparts, showing that the yields of methyl esters obtained in the homogeneous phase can be obtained with the guanidines anchored to the supports after longer reaction times. The catalysts prepared by immobilization of alkylguanidines in microporous systems showed diffusion restrictions for the vegetable oil as well as the low stability of the inorganic framework.76101207121
Guanidines encapsulated in zeolite Y and anchored to MCM-41: synthesis and catalytic activity
The preparation and characterisation of guanidines encapsulated in zeolite Y and anchored to MCM-41 are described. The catalytic activity of the immobilised and free guanidines were tested in the aldol reaction of benzaldehyde with acetone. In homogeneous phase, the guanidines give in 90-94% yield the condensation product alone. When anchored to MCM-41, the total yields range from 31% to 89% and the addition product is also formed depending on the solvent used. However, for the guanidine encapsulated in zeolite Y, the rate is strongly reduced and the addition product is preferentially formed in only 48% yield after 6 days. The reduced activity of this catalyst is explained by diffusional restrictions of the reactants and products inside the microporous system. (C) 1999 Elsevier Science B.V. All rights reserved.1484167117318
Transition-metal catalyzed autoxidation of cis- and trans-pinane to a mixture of diastereoisomeric pinanols
Autoxidations of the pinanes, obtained after hydrogenation of naturally occurring Pinus elliottii oil, were performed with or Without solvent, using the catalytic system Co(OAc)(2)/Mn(OAc)(2)/NH4Br in a 9:1:5 molar ratio, and dioxygen as the oxidant. The best selectivity for the pinanols was 71% (cis:trans ratio, 3:1) with 17% conversion. Autoxidations were also carried out in the absence of catalyst. The hydroperoxides formed with 17% conversion were decomposed with Na2SO3 and PPh3, resulting in 62% pinanols (cis:trans ratio, 5:1). The pyrolysis of the pinanols at 600 degrees C and a contact time of 1.15 x 10(-2) s/mol yielded 54% of linalool. The side products were mainly due to an 'ene' reaction, giving diastereoisomeric 1,2-dimethyl-3-isopropenylcyclopentanols.4541361136
Encapsulation of N,N',N''-tricyclohexylguanidine in hydrophobic zeolite Y: Synthesis and catalytic activity
N,N',N ''-Tricyclohexylguanidine was encapsulated in hydrophobic zeolite Y by reacting N,N'-dicyclohexylcarbodiimide with cyclohexylamine within the supercage. The encapsulated guanidine catalyzes the addition reaction of acetone to benzaldehyde forming 4-phenyl-4-hydroxybutan-2-one as the principal product, as opposed to the homogeneous system which produces 4-phenyl-3-buten-2-one via condensation reaction. (C) 1997 Elsevier Science Ltd.3881325132