National Institute Of Materials Physics - Romania

Publications

849 articles found

771. Second-order generation of point defects in gamma-irradiated float-zone silicon, an explanation for "type inversion"

Authors: Pintilie, I; Fretwurst, E; Lindstrom, G; Stahl, J

Published: MAR 31 2003, APPLIED PHYSICS LETTERS, 82, 2171, DOI: 10.1063/1.1564869

Radiation-induced defects in silicon diodes were investigated after exposure to high doses of Co-60-gamma irradiation using the thermally stimulated current method. We have found that, for high irradiation doses, a second-order defect can be detected. This defect is largely suppressed in oxygen-enriched material while it is the main cause for the space charge sign inversion effect observed in standard float-zone material. (C) 2003 American Institute of Physics.

772. Microstrip patch antenna with dielectric substrate

Authors: Sandu, DD; Avadanei, O; Ioachim, A; Banciu, G; Gasner, P

Published: 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1387, DOI:

The paper refers to the study of patch antennas with a high permittivity dielectric substrate ((Zr-0.8 Sn-0.2)TiO4) obtained at National Institute of Materials Physics Bucharest. Because of the high value of the permittivity of the substrate material it was necessary to perform a comparative investigation of different prototypes and arrays. It was considered two substrate thickness h(1)=1mm and h(2)=2mm and two frequencies f(1)=2.4 GHz and f(2)=1.8 GHz. For each case we presented the radiation pattern in the two principal planes (E and H) and input impedance. Also, some radiation pattern for rectangular array with n=4, 16, 36 patches are presented. In the Conclusion section a discussion on the suitability of patch antennas with high permittivity substrate is done.

773. BNT ceramics in paraelectric phase: Dielectric properties

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Plapcianu, C; Alexandru, H; Berbecaru, C; Stoica, G

Published: 2003, 2003 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, PROCEEDINGS, 318, DOI: 10.1109/SMICND.2003.1252443

The paper describes the investigations of Nd doped Barium titanate (BNT) ferroelectric ceramic materials prepared by solid state reaction techniques. The decrease of the Curie temperature with the increase of the Nd2O3 concentration from 0 to 28 wt % was observed. High density values of BNT ceramics were obtained by adding 0.5 mol % PbO substitutional for BaO. The samples were sintered at temperatures T-s=1200divided by12600degreesC. At room temperature, the materials are in paraclectric phase, suitable for high frequency applications.

774. ZST type material for dielectric resonators and substrates for hybrid integrated circuits

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Stoica, G; Annino, G; Cassettari, M; Martinelli, M; Ramer, R

Published: 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1398, DOI:

(Zr0.8Sn0.2)TiO4 ceramic (ZST) has been prepared and characterized. The effects of sintering parameters such as sintering temperature, sintering time and NiO addition on structural and dielectric properties were investigated. The material has a dielectric constant epsilon(r) similar to 36.0 and high values of the Q . f product from 32,170 to 50,000 at microwave frequencies. The tan delta values are decreased by low level doping, of NiO, while the temperature coefficient of the resonance frequency tau(f) takes values in the range (-2divided by+4) ppm/degreesC. Investigations on whispering gallery modes revealed low dielectric losses in millimeter wave domain. An intrinsic quality factor of 480 was measured at 136.3 GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter wave applications such as filters, hybrid microwave integrated circuits, etc.

775. Frequency agile BST materials for microwave applications

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Plapcianu, C; Alexandru, H; Berbecaru, C; Ghetu, D; Stoica, G; Ramer, R

Published: 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1393, DOI:

(Ba1-xSrx)TiO3 solid solution (BST), with different molar compositions (x = 0.25, 0.5, 0.75, 0.9) Were prepared by conventional solid-state reaction from raw materials. Structural parameters, crystallite sizes and bulk densities were determined. Perovskite type polycrystalline structure of the BST ceramics was revealed by X-ray diffraction (XRD) data. The dependence of permittivity and losses at low frequency (1 kHz) was analyzed. The microwave investigations at room temperature revealed dielectric constant around 1000 and loss smaller than 1% at 1.1 GHz. The results indicate that some BST dielectric ceramics are suitable in paraelectric phase for microwave devices. The dielectric constant can be adjusted by using a DC-bias field and such materials are appropriate for manufacturing electric field controlled components such as resonators, phase shifters etc.

776. Formation of the Z(1,2) deep-level defects in 4H-SiC epitaxial layers: Evidence for nitrogen participation

Authors: Pintilie, I; Pintilie, L; Irmscher, K; Thomas, B

Published: DEC 16 2002, APPLIED PHYSICS LETTERS, 81, 4843, DOI: 10.1063/1.1529314

As-grown 4H-SiC epitaxial layers were investigated by deep-level transient spectroscopy to study the formation of the well-known Z(1,2) defect with energy levels normally detected at about E-C-0.7 eV. Chemical vapor deposition, applying various nitrogen-doping concentrations and C/Si ratios (1.2-3) in the gas phase, was used to prepare the samples. The Z(1,2) defect concentration was observed to increase with the incorporated nitrogen concentration. The dependence was linear for medium C/Si ratios (1.5-2.5). The highest and lowest applied C/Si ratios (3 and 1.2) enhanced and suppressed the Z(1,2) defect formation, respectively. This behavior tentatively suggests a complex of nitrogen with interstitial carbon atoms or, less probably, silicon vacancies. In particular, the correlation between the Z(1,2) defect formation and the nitrogen incorporation was clearly shown in the present investigation, in contradiction to conclusions of other authors. Previously reported negative-U properties of the Z(1,2) deep-level defects could be confirmed. A 1:1 relation between the concentrations of Z(1) and Z(2) was obtained for the present as-grown epitaxial layers. (C) 2002 American Institute of Physics.

777. Nondestructive testing techniques and piezoelectric ultrasonics transducers for wood and built in wooden structures

Authors: Tanasoiu, V; Miclea, C; Tanasoiu, C

Published: DEC 2002, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 4, 957, DOI:

A survey of the main NDT methods used to evaluate the quality and properties of wood and wooden structures is given. Among the most important ones there arc visual inspection, sonic and ultrasonic stress wave. Some other methods include: deflection, electrical, isotope and Xray methods. The ultrasonic methods seem to be the most suitable ones in many cases especially for quality control purposes, but these methods depend on the quality of the piezoelectric transducers. Consequently, the construction and characteristics of a new type of piezoelectric transducer are reported. The new transducer is a composite type transducer, consisting of a sandwich type piezoactive element and a steel cylinder glued together. The sandwich piezoactive element was made of five thinner discs of 20 mm diameter and 2 mm thickness. The whole construction has the resonant frequency at 60 kHz, proper to wood investigation. The transducer can be successfully used for NDT of wood, either in the trough transmission or pulse-echo arrangements. The transducers were tested for sound velocity measurements in different wood samples.

778. 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.

779. Effect of excess PbO on the structure and piezoelectric properties of Bi-modified PbTiO3 ceramics

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

Published: AUG 2002, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 22, 1275, DOI: 10.1016/S0955-2219(01)00442-3

Effect of 1-4 wt.% excess PbO, added to the calcined powder, on the structure and dielectric and piezoelectric properties of Bi-doped lead titanate ceramics was investigated. The ceramics containing 4, 6 and 8 mol% Bi were prepared by the usual ceramic technique, using high purity oxides. The calcined powder of each composition was next intimately mixed with 1, 2, 3 and 4 wt.% PbO and then sintered at 1200-1300 degreesC for 3 h. The dielectric piezoelectric and electromechanical properties were determined as a function of excess PbO. Addition of PbO up to 2 wt.% was found to improve the densification, by the presence of the PbO liquid phase during sintering. For all Bi doping levels, the dielectric constant and the thickness coupling factor reach maxima (representing an increase of 2.5-8% for dielectric constant and 7-8% for thickness coupling factor) at about 2 wt.% PbO addition. The planar coupling factor is very low, less than 0.01, regardless the PbO content. The disipation factor slightly decreases with increasing PbO content and the mechanical quality factor of the thickness mode was about 300. The experimental results were discussed in terms of the mechanisms of densification in the presence of the liquid phase and of lead vacancy concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.

780. Close to midgap trapping level in Co-60 gamma irradiated silicon detectors

Authors: Pintilie, I; Fretwurst, E; Lindstrom, G; Stahl, J

Published: JUL 1 2002, APPLIED PHYSICS LETTERS, 81, 167, DOI: 10.1063/1.1490397

The deep level transient spectroscopy method was applied on standard and oxygenated float-zone silicon detectors exposed to high doses of Co-60-gamma irradiation. We have detected and characterized a close to midgap trapping level having an ionization energy of E-C-(0.545+/-0.005) eV and electron/hole capture cross sections of sigma(n)=(1.7+/-0.2)x10(-15) cm(2)/sigma(p)=(9+/-1)x10(-14) cm(2) respectively. This level has a strong impact on the detector performance being responsible for more than 90% of the change in the effective doping concentration. The defect is strongly oxygen related and a possible connection with the V2O complex is discussed. (C) 2002 American Institute of Physics.