National Institute Of Materials Physics - Romania

Soft ferrites in the CuZnTi system, having the chemical composition Cu1-x-yZnxTiyFe2O4 with 0.5 <= x <= 0.7 and 0.00 <= y <= 0.05 were investigated as a function of composition, sintering temperature and cooling speed, in order to obtain materials with different Curie temperatures, between 30 and 180 degrees C and very high change rate of permeability with temperature around their Curie points. Such materials are well suited to use as high sensitive magnetic temperature sensors and transducers for temperature control. Zn and Ti additions to copper ferrite change the Curie temperature in a controllable manner, thus being possible to produce ferrite material with a fine control of the Curie point at any desired temperature. Most important was the behavior of magnetic permeability with temperature around the Curie point, where it may change with about 60 %/degrees C by a proper choice of the cooling speed of samples from the sintering temperature to room temperature. This makes such materials extremely attractive as magnetic temperature sensors of high sensitivity. Two applications Of Such materials as temperature sensors, namely an ultrathermostat and an on-off switch type relay were designed. Their functionality and performances are presented and discussed.

We present the preparation, structural, and magnetic properties of a series of Fe-containing borosilicate glasses as a function of the ratio SiO2/Fe2O3 which is ranging from 1.49 to 2.68. The role of nucleators (Cr2O3 and P2O5) was also investigated. X-Ray diffraction has revealed the formation of magnetite as the major or unique crystalline phase. As SEM micrographs have revealed, the addition of P2O5 give rise to a finer structure as compared with Cr2O3. In addition, the same oxide decreases the temperature of structural transition with almost 100 degrees C. The magnetization data reveal a two step transition at low temperatures: a high temperature transition at T-v = 128 K, which we attribute to the Verwey transition, and a low temperature transition at T-s = 48 K which, most likely, is the result of change in the dynamic of the domain motion.

This work is focusing on the investigation of those radiation induced defects causing degradation effects of Silicon detector performance. Comparative studies of the defects induced by irradiation with Co-60- gamma rays, 23 GeV protons and I MeV equivalent reactor neutrons revealed the existence of some point defects and cluster related centers having a strong impact to damage properties of Si diodes. The detailed relation between the "microscopic" reasons as based on defect analysis and their "macroscopic" consequences for detector performance are presented. In particular, it is shown that the changes in the Si device properties (depletion voltage and leakage current) after exposing to high levels of Co-60- gamma irradiation can be completely understood by the microscopically investigated formation of two point defects: i) a defect formed via a second order process (I-p) that can be associated with the long searched for V2O complex or with a Carbon related center and is the cause for the observed type inversion effect in Oxygen lean material; ii) a bistable donor (BD) created during irradiation that is strongly generated in Oxygen rich material, associated with one of the earlier thermal donors in Si. It is the cause for the observed positive space charge induced by irradiation in oxygenated Si diodes. Specific for hadron irradiation are the annealing effects which decrease resp. increase the originally observed damage effects as seen by the changes of the depletion voltage (effects known as "beneficial" and "reverse" annealing, respectively). A group of four cluster related defects proved to be responsible for these annealing effects. Their formation is not affected by the Oxygen content or Si growth procedure suggesting that they are complexes of multivacancies located inside extended disordered regions.

Electrical properties, i.e., dielectric hysteresis and leakage current, of epitaxial Pb(Zr0.2Ti0.8)O-3 films with bottom SrRuO3 electrode and different metals as top contact were investigated. The leakage current is largely insensitive to the work function of the top metal but increases with decreasing electronegativity as well as with decreasing number of electrons on the d-shell of the top metal. The best rectifying properties are obtained for metals with complete d-shell (Cu, Au, Ag, Pd), while the metals with few electrons on the d-shell (Ta, Cr) form Ohmic-like contacts. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3021293]

An optimized electroless deposition procedure to form Pd membranes onto ceramic substrates involving the thermal decomposition of a Pd complex compound ([Pd (NH3)(4)] [PdCl4]) is discussed. This allowed a reaction efficiency of about 90-95%. The proposed method significantly diminishes the Pd wastes as compared with the classical procedure involving hydrazine based reduction step. A Pd layer of about 3-5 mu m has been uniformly deposited on the inner surface of the alpha-Al2O3 based substrate, being characterized by a very good adherence and light grey metallic appearance. Homogeneous films of Pd were obtained by annealing the deposited Pd layers at temperatures higher than 500 degrees C in a hydrogen atmosphere. XRD and laser confocal microscopy techniques have been proved to be suitable to characterize the composition, thickness and uniformity of the formed Pd membranes.

We study the phase segregation in magnetite ferrofluids under the influence of an external magnetic field. A phase with lower nanoparticle density and corresponding higher optical transmission is formed in the bottom of a glass cell in the presence of only a very modest magnetic field gradient (smaller than 25 T/m). The flux density in our magnetic configuration is simulated using finite element methods. Upon switching off the external magnetic field, the low-density phase develops into a 'bubble'-like feature. The kinetics of this 'bubble' in the absence and presence of a magnetic field are described and analyzed in terms of a simple model, which takes into account buoyancy and drag forces. (C) 2008 Elsevier Inc. All rights reserved.

Pb(Zr0.2Ti0.8)O-3-BiFeO3 (PZT-BFO) multilayer structures were realized by sol-gel on Pt/Si substrate and their electrical properties were investigated using top Pt, Au and Cu electrodes. All multilayers start and end with PZT films. The polarization-voltage and current-voltage hysteresis loops resemble to those encountered in case of antiferroelectric (AFE) films, although component materials are both ferroelectric (FE). The AFE-like behavior is more clearly visible in the case of three-layer structures of PZT-BFO-PZT type, and it is not dependent of the metal used as top electrode. After fatigue cycling the polarization hysteresis apparently changes from characteristic AFE double-loop to FE single loop, although the four switching current peaks characteristic for an AFE behavior are still present in the current hysteresis. These results suggest the presence of an AFE interfacial coupling between the PZT and BFO layers.

SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr0.6Ba0.4Nb2O6 and ceramic Sr0.5Ba0.5Nb2O6 stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31 degrees in-plane orientation relative to the MgO (100) axis is identified.

PZT type films were prepared using the thermionic vacuum arc (TVA) method; a crucible filled PZT type powder (PbO/ZrO/TiO) was heated by a thermo-electron beam emitted by the circular cathode of the TVA gun. Applying high voltage potential (3000 +/- 200V) on the anode from a regulated HEINZINGER high voltage power supply the thermo-electrons were accelerated toward the anode crucible, evaporating the anode materials and initiating a plasma discharge. The morphology and the structure of the prepared films analyzed by transmission electron microscopy, (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy, showed nanostructured, smooth thin films with specific PZT stoichiometry.

Ba0.6Sr0.4TiO3 (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance-voltage (C-V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.