National Institute Of Materials Physics - Romania

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5840 articles found

4491. The influence of Tl and O content from the starting mixture on phase formation in (Cu,Tl)-1234 system

Authors: Badica, P; Crisan, A; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 687, DOI: 10.1016/S0921-4534(02)01521-6

Superconducting samples have been synthesized from starting compositions CU0.5TlxBa2Ca3Cu4O12+y, x(Tl) = 0.35-1 in the sealed Au-tubes. Starting oxygen content was y = 0 and/or y = +/-0.225. Samples contain non-superconducting phases and only one main superconducting phase, 1234. The energy dispersive spectroscopy grain composition was Tl0.87-1.3Ba1.9-2.05Ca2.55-3.1Cu4Ox (normalized to Cu = 4). The starting Tl and oxygen content do not influence the occurrence of the phases, grain size, morphology and composition. On the other hand the phase content strongly depends on Tl content and almost does not depend on oxygen content. Both Tl and oxygen content are important for the transport properties of the superconducting grains and it seems that also of the non-superconducting matrix. T-c of the samples was in the range 119.1-111.3 K. (C) 2002 Elsevier Science B.V. All rights reserved.

4492. Cum-1Bam(Sr,Ca)(n)Cun+1O2m+2n+1 superlattice thin film by layer-by-layer growth techniques

Authors: Nie, JC; Sundaresan, A; Hirai, M; Ishiura, Y; Badica, P; Crisan, A; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 1282, DOI: 10.1016/S0921-4534(02)01690-8

Thin films of Cum-1Bam(Sr,Ca)(n)Cun+1O2m+2n+1, i.e., [BaCuO2](m)/[(Sr,Ca)CuO2](n) superlattices have been grown by layer-by-layer growth method with rf magnetron sputtering on NdGaO3(1 1 0) substrate. Two targets with the compositions BaCuO2 and (Sr,Ca)CuO2, which form the charge reservoir and superconducting layers, respectively, were used. [BaCuO2](m)/[(Sr,Ca)CuO2](n) superlattice was grown at 460-520 degreesC in a gas mixture of Ar and N2O by alternatively depositing the charge reservoir layer and superconducting layer. The superlattice modulation is close to that of high pressure synthesized bulk materials. By means of the layer-by-layer growth method, a system of Cu-based artificial lattice structures is formed. Electrical transport properties of these superlattices were also investigated. (C) 2002 Elsevier Science B.V. All rights reserved.

4493. Superconducting properties of the heavy-ions and neutron irradiated (Cu,C)Ba2Can-1CunO2n+4-delta (n-3, 4 and 5)

Authors: Kito, H; Iyo, A; Hirai, M; Crisan, A; Tokumoto, M; Okayasu, S; Sasase, M; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 332, DOI: 10.1016/S0921-4534(02)01437-5

For further enhancement of critical currents density J(c) and irreversibility field H-irr of (Cu1-xCx) Ba2Can-1CunO2n+4-delta ((Cu,C)-(n - 1)n, n = 3, 4 and 5) family, pinning centers were introduced by ion and neutron irradiation in these compounds. The polycrystalline samples were irradiated with 3.6 x 10(17) neutrons/cm(2) or with Au15+ ions (240 MeV energy) at a fluence of 1 x 10(11) ions/cm(2). J(c) and H-irr were determined for the irradiated samples as well as unirradiated samples. The intragrain J(c) was evaluated from M-H curves using Bean's critical state model. In (Cu,C-12(n - 1) n, J(c) (77 K, 1 T) shows a remarkable increase from 3.9 x 10(4) to 9.1 x 10(5) A/cm(2) for neutron irradiated (Cu,C- 1234 and from 3.9 x 10(4) to 4.1 x 10(6) A/cm(2) for heavy-ion irradiated (Cu,C)-1234. These results indicate the possibility of further enhancement of J(c) and of achieving a very high H-irr of (Cu,C)-1234 in (Cu,C)-12(n - 1)n family. (C) 2002 Elsevier Science B.V. All rights reserved.

4494. Magnetic properties of alpha-iron(II) phthalocyanine

Authors: Evangelisti, M; Bartolome, J; de Jongh, LJ; Filoti, G

Published: OCT 1 2002, PHYSICAL REVIEW B, 66, DOI: 10.1103/PhysRevB.66.144410

We report on the magnetic properties of the supramolecular compound iron(II) phthalocyanine in its alpha form. dc- and ac-susceptometry measurements and Mossbauer experiments show that the iron atoms are strongly magnetically coupled into ferromagnetic Ising chains with very weak antiferromagnetic interchain coupling. The transition to three-dimensional (3D) magnetic ordering below 10 K is hindered by the presence of impurities or other defects, by which the domain-wall arrangements along individual chains become gradually blocked/frozen, leading to a disordered 3D distribution of ferromagnetic chain segments. Below 5 K, field-cooled and zero-field-cooled magnetization measurements show strong irreversible behavior, attributed to pinning of the domain walls by the randomly distributed defects in combination with the interchain coupling. High-field magnetization experiments reveal a canted arrangement of the moments in adjacent ferromagnetic chains.

4495. Iterative evaluation of the complex constants of piezoceramic resonators in the radial mode

Authors: Amarande, L; Miclea, C; Tanasoiu, C

Published: OCT 2002, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 22, 1881, DOI: 10.1016/S0955-2219(01)00502-7

An iterative method is presented for the characterization of lossy piezoelectric materials in the radial resonant mode, which provides a new formula to calculate the frequencies at which the electrical admittance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for calculus. The accuracies of the material constants for several soft and hard piezoceramic materials, covering a wide range of values of the planar coupling factor and mechanical quality factor, were tested and it was evidenced that they depended on the type of the piezoelectric material. The new method proved to be as accurate as, or in some cases even more accurate than, the other iterative methods. This new method can also be applied whenever the standard method does not allow the determination of the material constants. (C) 2002 Elsevier Science Ltd. All rights reserved.

4496. Temperature dependence of the density of calamitic and discotic nematic lyotropic liquid crystals containing sodium lauryl sulphate/decanol/water

Authors: Beica, T; Moldovan, R; Puica, MR; Frunza, S

Published: OCT 2002, LIQUID CRYSTALS, 29, 1278, DOI: 10.1080/713935619

Experimental results for the temperature dependence of density at normal pressure for two compositions of sodium lauryl sulphate/decanol/water solutions, exhibiting either a calamitic or a discotic lyotropic nematic phase at room temperature, are presented. Within the limits of experimental precision (+/-1 x 10(-5) g cm(-3)), the systems show no jump in density at the nematic to isotropic phase transition. Over the studied temperature range, the mean thermal expansion coefficients were also evaluated.

4497. Unusual relaxation behavior of water inside the sodalite cages of Faujasite-type molecular sieves

Authors: Frunza, L; Kosslick, H; Frunza, S; Schonhals, A

Published: SEP 12 2002, JOURNAL OF PHYSICAL CHEMISTRY B, 106, 9194, DOI: 10.1021/jp021281v

Broad-band dielectric spectroscopy (10(-2) to 10(9) Hz) is applied to investigate the molecular dynamics of NaY, a zeolite of faujasite structure. A do conductivity contribution and three relaxation processes were revealed in the dielectric spectra. The relaxation process in the high-frequency range ascribed to orientational fluctuations of water molecules inside the sodalite cages is studied in detail. As a main result it is found that its mean relaxation time has an unusual saddle-like temperature dependence, which obeys neither an Arrhenius nor a Vogel-Fulcher-Tammann law. A quantitative description was given on the basis of a model recently developed by Feldman et al. (J. Phys. Chem. B 2001, 105, 1845) to analyze the dynamics of water confined in nanoporous glasses. Reasonable values for the activation energies of orientational fluctuations and of defect formation as well as for the defect concentration were obtained.

4498. Nanocapsules of biodegradable polymers: preparation and characterization by direct high resolution electron microscopy

Authors: Guinebretiere, S; Briancon, S; Fessi, H; Teodorescu, VS; Blanchin, MG

Published: SEP 1 2002, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 21, 142, DOI: 10.1016/S0928-4931(02)00073-5

In the field of pharmaceutical applications relying on encapsulation of drug by polymer coating, capsules based on biodegradable polymers with mean size of about 500 nm have been obtained by a patented emulsion-diffusion method. The morphology, size and structure of the nanocapsules (NC) control their pharmaceutical properties, especially release of the drug. Here is reported a decisive contribution of transmission electron microscopy (TEM) in revealing the controlled form and size of the NC: mean size values measured by TEM do agree with data obtained by laser granulometry. Moreover, the TEM magnification allows an estimation of the membrane thickness. (C) 2002 Elsevier Science B.V. All rights reserved.

4499. Thin films of langasite (La3Ga5SiO14) prepared by pulsed laser deposition

Authors: Popescu, M; Sava, F; Georgescu, S; Gheorghe, L; Mihailescu, IN; Cristescu, R; Socol, G; Bradaczek, H

Published: SEP 2002, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 4, 818, DOI:

Thin films of langasite (La3Ga5SiO14) have been prepared from langasite crystal targets by pulsed laser deposition. The films deposited at room temperature on silicon wafers are amorphous. After annealing at 700 degreesC, in normal atmosphere, the films crystallize as polycrystalline langasite oriented with the plane (001) parallel to the surface of the silicon substrate.

4500. On the mobility of sodium ions and of confined liquid crystals in molecular sieves of faujasite type

Authors: Frunza, L; Kosslick, H; Frunza, S; Fricke, R; Schonhals, A

Published: SEP 2002, JOURNAL OF NON-CRYSTALLINE SOLIDS, 307, 509, DOI: 10.1016/S0022-3093(02)01472-2

Broadband dielectric spectroscopy (10(-2)-10(9) Hz) was employed to study in a rather large temperature interval the molecular dynamics of the composite systems containing a nematic liquid crystal (LC), 4-n-pentyl-4'-cyanobiphenyl, confined to NaY, a zeolite with faujasite structure. Thermal analysis was also performed in order to get information concerning the loading of the zeolite and the possible phase transitions. No phase transitions and no relaxation processes of the LC molecules inside the pores were observed. But remarkably, in the presence of these molecules the dielectric behavior of the faujasite host changes due to modifications appeared in the mobility of Na+ ions. (C) 2002 Elsevier Science B.V. All rights reserved.