National Institute Of Materials Physics - Romania

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5781 articles found

4361. Compact microstrip resonators for 900 MHz frequency band

Authors: Banciu, MG; Ramer, R; Ioachim, A

Published: MAY 2003, IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, 13, 177, DOI: 10.1109/LMWC.2003.811673

A technique to reduce the size of planar resonators is investigated using a transmission line model and it finite-difference time-domain (FDTD) method. New miniaturized square microstrip resonators are proposed for 900 MHz band. T he resonators offer low impedance sections leading to a better handling of power. The feasibility of compact filters using the proposed resonators is demonstrated. The input/output coupling lines of a two-pole. filter have been positioned in a way that produces a filter response with transmission zeros on the both sides of the pass-band. Sharper skirts and higher out-of-band rejection are obtained for four-pole filters with cross-coupled resonators.

4362. Electron-doped superconductivity in Sr1-xCaxCuO2-delta infinite-layer thin films

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

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 442, DOI: 10.1016/S0921-4534(02)02568-6

Thin films of infinite-layer compound Sr1-xCaxCuO2-delta have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed (T-c (onset) = 42 and 50 K, T-c(rho=0) = 11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity. (C) 2003 Elsevier Science B.V. All rights reserved.

4363. Long-term damage induced by hadrons in silicon detectors for uses at the LHC-accelerator and in space missions

Authors: Lazanu, I; Lazanu, S

Published: MAY 2003, PHYSICA SCRIPTA, 67, 394, DOI: 10.1238/Physica.Regular.067a00388

In the present paper. the phenomenological model developed by the authors in previous papers has been used to evaluate the degradation induced by hadron irradiation at future accelerator facilities or by cosmic protons in high resistivity silicon detectors. The damage has been analysed at the microscopic (defects production and their evolution toward equilibrium) and at the macroscopic level (changes in the leakage current of the p-n junction). The rates of production of primary defects. its well as their evolution toward equilibrium have been evaluated considering explicitly the type of projectile particle and its energy. Vacancy-interstitial annihilation. interstitial migration to sink. divacancy and complex (VP, VO, V2O, CiOi, CiCs) formation are taken into account for differently doped initial silicon material. The influence of these defects on the leakage current density has been compared with experimental data from the literature, and predictions for the LHC (Large Hadron Collider) radiation fields. as well as for space missions in the near Earth orbits have been done. in the frame of the Schokley-Read-Hall model.

4364. Role of fluorite phase formation in the texture selection of sol-gel-prepared Pb(Zr1-x,Ti-x)O-3 films on Pt electrode layers

Authors: Norga, GJ; Vasiliu, F; Fe, L; Wouters, DJ; Van der Biest, O

Published: MAY 2003, JOURNAL OF MATERIALS RESEARCH, 18, 1238, DOI: 10.1557/JMR.2003.0169

Transmission electron microscopy-selected area electron diffraction studies were performed on as-pyrolyzed sol-gel-derived Pb(Zr1-xTix)O-3 films deposited on Pt electrode layers to elucidate the structural cause behind the large effects of pyrolysis conditions on orientation selection [L. F, G.J. Norga, H.E. Maes, and G. Maes, J. Mater. Res.. 16, 2499 (2001)]. The crystallinity of the intermediate pyrochlore phase, which forms during pyrolysis, was found to strongly depend oil pyrolysis conditions. Specifically, pyrolysis for 10 s at 350 degreesC was seen to result in conversion of the film to a well-crystallized, oxygen-deficient pyrochlore phase with the fluorite crystal structure (disordered pyrochlore). We speculate that formation of the metastable fluorite phase is favored by the reduced oxygen partial pressure, caused by burnoff of residual acetates. Longer pyrolysis times and/or higher pyrolysis temperatures result in a quasi-amorphous intermediate phase. The presence of the well-crystallized fluorite phase in the pyrolyzed film is seen to result in (111)-oriented films after crystallization, while pyrolyzed films consisting of a quasi-amorphous intermediate phase turn out (100) or mixed (100)/(111) oriented after crystallization. An explanation for the observed orientation effects, on the basis of the different surface energetics of fluorite versus perovskite structure oxides, is proposed.

4365. Preparation of Tl-2212 and -1223 superconductor thin films and their microwave properties

Authors: Sundaresan, A; Asada, H; Crisan, A; Nie, JC; Kito, H; Iyo, A; Tanaka, Y; Kusunoki, M; Ohshima, S

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 474, DOI: 10.1016/S0921-4534(02)02585-6

Tl2Ba2CaCu2Oy (Tl-2212) and Tl(Ba,Sr)(2)Ca2Cu3Oy (Tl-1223) superconductor thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process namely, amorphous phase epitaxy (APE) method. In the case of Tl-1223, an amorphous layer of TlSr2CaCu2Oy (TlSr-1212) was deposited on top Of CeO2 layer to avoid chemical reaction between Tl-1223, containing Ba, and CeO2 layers. All films are epitaxial with smooth surface and excellent crystallinity. Superconducting transition temperatures (T-c) of Tl-2212 and 1223 are around 95 and 104 K, respectively. Critical current density (J(c)) at 77 K measured on 5 x 5 mm. films by inductive technique is as high as 2 MA/cm(2). Surface resistance R-s of Tl-2212 films measured by a dielectric resonator technique at 38 GHz is comparable to the best YBCO films. (C) 2003 Elsevier Science B.V. All rights reserved.

4366. Coupled confinement effect on the photoluminescence and electrical transport in porous silicon

Authors: Ciurea, ML; Draghici, M; Iancu, V; Reshotko, M; Balberg, I

Published: MAY 2003, JOURNAL OF LUMINESCENCE, 102, 497, DOI: 10.1016/S2313(02)00589-6

In this paper we report correlations between the structure, the photoluminescence and the transport properties of luminescent porous silicon. These correlations combined with the observed temperature dependence of tunneling characteristics yield quite a wholesome (pea-pod-like) model for this system. (C) 2002 Elsevier Science B.V. All rights reserved.

4367. LiF addition to (Cu,C) Ba2Ca3Cu4Oy superconductor

Authors: Badica, P; Crisan, A; Hirai, M; Iyo, A; Kito, H; Aldica, G; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 396, DOI: 10.1016/S0921-4534(02)02535-2

Samples of (Cu0.6C0.4)Ba2Ca3Cu4Oy ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes' the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba2Can-1CunOy series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T-c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J(c) has shown a fish-tail effect. (C) 2003 Elsevier Science B.V. All rights reserved.

4368. Annealing effect of the irreversibility fields in (Cu,C)Ba2Can-1CunOy (n=3 and 4)

Authors: Hirai, M; Iyo, A; Kito, H; Crisan, A; Tokiwa, K; Watanabe, T; Arai, J; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 428, DOI: 10.1016/S0921-4534(02)02552-2

Annealing effect of the irreversibility fields (H-irr) in (Cu,C)Ba2Ca2Cu3Oy ((Cu,C)-1223) and (Cu,C)Ba2Ca3Cu4Oy ((Cu,C)-1234) have been investigated. The samples prepared under high pressure were post-annealed in nitrogen gas in the temperature range of 400-500degreesC. H-irr of (Cu,C)-1234 was improved after annealing at 450degreesC while T-c remained almost constant. This suggests that carrier density of five-coordinated CuO2 planes or conductivity of charge reservoir layers is changed by annealing while keeping that of four-coordinated CuO2 planes almost constant. Both samples show H-irr similar or equal to 4 T at 90 K by the post-annealing, which is quite attractive for practical application. (C) 2003 Elsevier Science B.V. All rights reserved.

4369. Heavy-ion irradiation dependence of the superconducting properties of (Cu,C)Ba2Ca3Cu4O10.5-delta

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

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 712, DOI: 10.1016/S0921-4534(02)02488-7

To enhance or improve critical currents density (J(c)) and irreversibility field (H-irr) of (Cu,C)Ba2Ca3Cu4O10.5-delta ((Cu,C)-1234), pinning centers were introduced by heavy-ion irradiation. The polycrystalline samples were irradiated with An(15+) ions (240 MeV energy) at various fluence of 1 x 10(11), 2.5 x 10(11) and 5 x 10(11) ions/cm(2). J(c) and H-irr were determined for the irradiated samples as well as unirradiated samples. J(c) (77 K, 1 T) increase from 3.9 x 10(4) A/cm(2) to 4.1 x 10(6) A/cm(2) for at fluence of 1 x 10(11) ions/cm(2) of heavy-ion irradiated sample and decreases with further increase of fluence. These results indicate the possibility of further enhancement of Jc and of achieving a very high H-irr of (Cu,C)-1234 below a fluence of 1 x 10(11) ions/cm(2). (C) 2003 Elsevier Science B.V. All rights reserved.

4370. Complex semiconducting ceramic

Authors: Leonovici, M; Steflea, M; Popescu, MA

Published: MAY 2003, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 23, 886, DOI: 10.1016/S0955-2219(02)00228-5

Complex semiconducting ceramic compositions were prepared. Structure, crystallite morphology, hardness and electrical conduction were investigated. After annealing at 150 degreesC a distinct crystalline phase was obtained and the ceramic became compact, hard, of higher resistivity, while preserving the basic semiconducting properties (C) 2002 Elsevier Science Ltd. All rights reserved.