National Institute Of Materials Physics - Romania

Publications

838 articles found

651. BST thin films obtained by PLD for applications in electronics

Authors: Scarisoreanu, N; Filipescu, M; Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Dutu, A; Buda, M; Alexandru, HV; Dinescu, M

Published: JUL 31 2007, APPLIED SURFACE SCIENCE, 253, 8257, DOI: 10.1016/j.apsusc.2007.02.111

Barium strontium titanate (BST) bulk ceramic with composition Ba0.5Sr0.5TiO3 was produced by solid-state reaction and was used as target for PLD thin film deposition. A Nd-YAG laser, working at 5-10 Hz and different wavelengths has been used. Thin films of stoichiometric BST were deposited on alumina substrate with the thickness between 400 and 500 nm. An aditional annealing was carried out at 800 degrees C for 6 h. XRD and SEM were used for sample characterization. Capacity measurements were performed versus temperature at 100 kHz in a large temperature range. A diffuse ferroelectric-paraelectric phase transition with T-C = -72 degrees C for the BST thin film was determined. A 5% tunability was measured in the transition region. (c) 2007 Elsevier B.V. All rights reserved.

652. Transitions of barium strontium titanate ferroelectric ceramics for different strontium content

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Dutu, A; Antohe, S; Berbecaru, C; Georgescu, L; Stoica, G; Alexandru, HV

Published: JUN 4 2007, THIN SOLID FILMS, 515, 6293, DOI: 10.1016/j.tsf.2006.11.097

Ceramic based barium strontium titanate (BST) solid solutions with the formula Ba1-xSrxTiO3 are very important candidates for a wide range device applications. Several doped (Mn and Mg) and undoped samples were prepared by standard solid-state reaction. Special emphasis was put on compositions with x=0.35 and 0.60, with high potential for applications. The samples were sintered at temperatures in the 1200 divided by 1260 degrees C range. Structural X-ray diffraction analysis preformed confirms the perovskite structure. The dielectric parameters were investigated in a wide temperature range between - 150 and 150 degrees C. The temperature was cyclically changed in both directions, up and down, at a rate of less than 2 degrees C/ min. Both permittivity and dielectric loss were measured at low frequencies, 1 kHz. The peak values of the permittivity are increasing from 2000 to 4000 with the sintering temperature increase. Moreover the dielectric parameters were measured at room temperature in microwave domain (1 divided by 2 GHz). The Curie temperature of BST samples with x=0.35 and x=0.6 is in agreement with the Curie point dependence on Sr content, as we have previously reported. (C) 2007 Published by Elsevier B.V.

653. Effects of porosity on ferroelectric properties of Pb(Zr0.2Ti0.8)O-3 flims

Authors: Stancu, V; Lisca, M; Boerasu, I; Pintilie, L; Kosec, M

Published: JUN 4 2007, THIN SOLID FILMS, 515, 6561, DOI: 10.1016/j.tsf.2006.11.165

The sol-gel deposition method has been successfully applied to obtain Pb(Zr0.2Ti0.8)03 thin films on platinized silicon wafers. Addition of different amounts (7-15 wt.%) of organic macromolecular polyvinylpyrrolidone in the precursor solution prior to spin coating proves to be an excellent method for obtaining porous films. The crystal structure of as deposited films was analyzed by X-ray diffraction. The porous films show perovskite phase after annealing at 650 degrees C. The surface morphology has been studied by Atomic Force Microscopy and Scanning Electron Microscopy. The surface profile indicates a roughness of the film of 5 nm and no microcracks on the surface. The ferroelectric behavior was proved for each film, by hysteresis loops and by the "butterfly" shape of the capacitance-voltage characteristics. The remnant polarization and the coercive field decrease while the amount of added PVP increases. (C) 2006 Elsevier B.V. All rights reserved.

654. Magnesium doped (Zr-0.8, Sn-0.2)TiO4 ceramics for microwave devices

Authors: Nedelcu, L; Ioachim, A; Toacsan, MI; Banciu, MG; Dutu, CA; Lifei, F; Vasiliu, F; Stoica, G

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1729, DOI:

Magnesium doped (Zr0.8Sn0.2)TiO4 material has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1270 divided by 1315 degrees C for 2 hours. The effect of sintering temperature on structural and dielectric properties was investigated. The 0.2 wt.% MgO addition improves the sintering process and well sintered samples with a high value of bulk density were achieved. The material exhibits a dielectric constant epsilon(r) similar to 36.6 and high values of the Qxf product, greater than 61000 at microwave frequencies. The dielectric properties make the magnesium doped (Zr0.8Sn0.2)TiO4 material very attractive for such microwave applications as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc.

655. Extrinsic contributions to the apparent thickness dependence of the dielectric constant in epitaxial Pb(Zr,Ti)O-3 thin films

Authors: Pintilie, L

Published: JUN 2007, PHYSICAL REVIEW B, 75, DOI: 10.1103/PhysRevB.75.224113

The problem of the thickness dependence of the dielectric constant, as well as the extrinsic contributions to its value, is analyzed for the case of epitaxial Pb(Zr,Ti)O-3 (PZT) thin films. It is shown that the frequency dependence of the measured capacitance is best simulated by an equivalent circuit incorporating the trap-containing capacitance of a Schottky contact. The thickness dependence of the dielectric constant, calculated using the formula of a plane capacitor, appears to be an extrinsic effect due to the interface phenomena in the metal-ferroelectric-metal structure. The intrinsic dielectric constant of the PZT material seems to be thickness independent and of low value of about 30-40. This is closer to the values estimated from Raman measurements or from quantum theories of ferroelectricity. The thickness independence is also proven by piezoresponse force microscopy measurements. The presence of traps is evidenced by the presence of a photovoltaic effect at subgap wavelengths.

656. Dielectric properties of BZT ceramics for microwave applications

Authors: Ioachim, A; Toacsan, MI; Nedelcu, L; Banciu, MG; Dutu, CA; Andronescu, E; Jinga, S; Nita, P; Alexandru, HV

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1838, DOI:

High permittivity dielectric materials (epsilon(r) > 20) are used in mobile communications due to low dielectric loss in microwaves and millimeter waves, specific temperature dependence of the dielectric properties and high dielectric permittivity, which allows device miniaturization. Preparation of Ba(Zn1/3Ta2/3)O-3 ceramic materials (BZT) with optimal properties for applications requires special thermal treatments due to the difficult control of the cationic ordering. The samples were prepared by using solid-state reaction, doped with Eu2O3, ZrO2, Nb2O5, or Al2O3-Y2O3 and sintered at the following temperatures: 1400, 1500, 1550 degrees C for 2 or 3 hours. In order to improve the microwave properties, annealing treatments at 1410 degrees C for 10h or 30h were performed on ceramic samples. XRD was used for compositional and structural characterization. The XRD data allowed the study of the transition from the pseudo-cubic cell to the hexagonal cell. The dielectric properties were measured in the microwave range and were correlated with additives and structural properties. Sintering temperatures greater than 1500 degrees C are required for doped BZT compositions, in order to obtain high dielectric constant (epsilon(r) similar to 28) and high quality factor (Q = 10000 at 10 GHz).

657. A PZT type piezoceramic material with high planar coupling factor used for a miniature bimorph ventilator

Authors: Miclea, C; Iuga, A; Tanasoiu, C; Amarande, L; Cioangher, M; Miclea, CT

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1515, DOI:

New PZT type ceramic materials, doped with Ni, were prepared by the usual solid state reaction. The best composition proved to be the one containing 6% atomic Ni and having the following chemical formula: Pb0.95Bi0.03Nb0.02Zr0.51 Ti0.43Ni0.06O3. Rectangular samples of this powder were pressed and sintered at 1200 OC for 4 hours and then it was cut in thin slices used to construct an efficient bimorph type ventilator to be used in miniaturized devices for cooling. Miniature ventilators made of PZT ceramics are preferred due to their smallness, high electromechanical efficiency and lack of rotating parts and last but not least of contiguous electrical contacts. We made such a device by using two thin ceramic PZT plates of the material with the best piezoelectric properties, assembled in a flexural oscillator through a proper bimorph association. The emerging doublet behaves itself very much like a bimetal system and possesses a good developed flexural mode. Attaching a metallic elastic plate to the bimorph produces a highly efficient miniature ventilator. A theoretical quantitative study of this flexural bimorph system with elastic plate attached was proposed.

658. Microwave properties of Ba1-xPbxNd2Ti5O14 dielectric resonators

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Dutu, CA; Stoica, G

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1575, DOI:

The Ba1-xPbxNd2Ti5O14 (BNT: Pb) ceramics are appropriate for microwave devices due to their high dielectric constant and low loss in the microwave domain. Samples with x Pb content in the range 0 + 0.5 were prepared by solid-state reaction and sintered at temperatures T-s = 1230 + 1260 degrees C. XRD and SEM analysis indicate very good crystalline structure and polyhedral well formed grains. Controlled temperature coefficient of resonant frequency alpha is essential for dielectric resonators. The temperature coefficient of the resonant frequency alpha takes values from - 15 ppm/degrees C to + 70 ppm/degrees C. Highdensity values and adequate microwave parameters were obtained for x = 0.5 Pb content. The BNT: Pb ceramics sintered at 1250 degrees C and 1260 degrees C for 2 hours are suitable for dielectric resonators for frequencies up to 5 GHz. epsilon r = 76 + 87 and Q x f similar to 5000 GHz).

659. Dielectric and morphological studies of BST ferroelectric ceramics

Authors: Ioachim, A; Vasiliu, F; Toacsan, MI; Nedelcu, L; Banciu, MG; Alexandru, HV; Stoica, G

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1581, DOI:

Barium strontium titanate with molar formula Ba0.75Sr0.25 TiO3 (BST) was prepared from raw materials by solid-state reaction. Samples were sintered at 1230 degrees C and 1260 degrees C for 2 h. In order to improve the sintering process, 1 wt. % MgO and 1 wt. % MnO2 were used as sintering additives. The sinterability, structural, morphological and dielectric properties of pure and doped BST samples were investigated. Perovskite structure of polycrystalline BST ceramics was determined by Xray diffraction. SEM investigations revealed a grain and pore size distribution, which is shifted to higher values by a slight increase of the sintering temperature from 1230 degrees C to 1260 degrees C. In the presence of additives, a better sintering and grain refining could be observed, whereas their lack induce a strong effect of exaggerated grain growth, by forming of large faceted grains. The presence of Mn ions in the doped BST ceramics was revealed by the EDX analysis. On the other hand, Mg ions are not evidenced. Temperature dependence of dielectric constant measured at 1 kHz shows a diffuse ferroelectric-paraelectric transition, which can be attributed to the large size dispersion of grains. The increase of sintering temperature leads to a shift of the Curie temperature to higher values.

660. The ferroectric-electric characterization of PZT-PbS composites

Authors: Stancu, V; Buda, M; Pintilie, L; Popescu, M; Sava, F

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1520, DOI:

A new method for the preparation of PbS/PZT composite materials is proposed. The PZT matrix has been obtained using the sol-gel method on Pt-coated Si substrates. The PbS (nano) crystals were subsequently obtained by dipping the porous PZT matrix in Pb(NO3)(2) and Na2S solutions at room temperature. The XRD spectra show the presence of the tetragonal phase of PZT. The microstructure of the film was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The size of the PbS (nano)-particles at the sample's surface is about 25 nm, estimated from the AFM analysis. The roughness of the sample's surface is about 5 nm. The porosity of the composite, estimated from the SEM analysis in cross-section is smaller than for the reference PZT porous matrix. The equivalent dielectric constant of the composite increases by a factor of 4-5 in comparison with the porous PZT reference and does not depend on the thickness of the surface PbS (nano)-crystalline layer. The saturated polarization (Ps) value for the PZT-PbS composite is smaller than that corresponding to the bulk PZT but higher than for the reference porous PZT. We attribute these effects to the partial filling of the pores in the PZT matrix with PbS.