National Institute Of Materials Physics - Romania

A general method for mapping the equipotential profile surrounding a conductive cylindrically symmetric high aspect ratio structure, such as a carbon nanotube or a Spindt tip, is devised. The surface of the object is replaced by a discrete set of charges located on the symmetry axis. The overall electrostatic potential must satisfy a set of boundary conditions imposed on the original surface. The optimum number of charges is determined through an iterative self-validating process such that the obtained equipotential mimics the surface of the object. The method is exemplified by calculating the electric field enhancement factor for rounded cones and cylinders resembling Spindt tips and carbon nanotubes, respectively. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3582141]

Phase transformation behavior of Ti50Ni30Cu20 shape memory alloys prepared by powder metallurgy is analyzed with respect to the duration of mechanical alloying. The processed blends were studied by differential scanning calorimetry and room temperature X-ray diffraction. The martensitic transformations evidenced by thermal scans are discussed in correlation with the relative phase content obtained from the refinement of the X-ray diffraction patterns. (c) 2011 Elsevier B.V. All rights reserved.

The paper presents the influence of pulsed laser deposition (PLD) parameters on the structural and optical properties of PZT thin films grown on platinum substrate. X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) are used to determine the thin film properties. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are employed to get additional information. By changing the distance between target and substrate, different crystalline orientations of PZT are obtained. The thin film thickness and its roughness, as well as the refractive index are also influenced by the chosen distance. (C) 2011 Elsevier B.V. All rights reserved.

The paper presents the influence of pulsed laser deposition (PLD) parameters on the structural and optical properties of PZT thin films grown on platinum substrate. X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) are used to determine the thin film properties. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are employed to get additional information. By changing the distance between target and substrate, different crystalline orientations of PZT are obtained. The thin film thickness and its roughness, as well as the refractive index are also influenced by the chosen distance. (C) 2011 Elsevier B.V. All rights reserved.

The electronic properties of two-dimensional toroidal surfaces of nanometer size have been investigated by approximately solving the time-independent Schrodinger equation. The effects of the quantum confinement on the electron population of these structures were shown by studying the average electron density on the surface of the torus. The unique electron spread that resulted from such computations encouraged further study on the field emission properties of this two-dimensional manifold. The Wentzel-Kramers-Brillouin approximate approach was used for a preliminary study of the field emission properties of the nanotori for various geometrical parameters. (c) 2011 American Vacuum Society. [DOI: 10.1116/1.3531935]

Ba(Zn1/3Ta2/3)O-3 (BZT) dielectric resonators were prepared by solid-state reaction. The starting materials were BaCO3, ZnO, and Ta2O5 powders with high purity. The double calcined BZT pellets were sintered in air at temperatures of 1575, 1600, 1625, and 1650 degrees C for 4 h. The X-ray diffraction data allowed the study of the unit cell distortion degree and the presence of the secondary phases. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed with the increase of the sintering temperature. The dielectric constant of BZT resonators measured around 6 GHz was between 26 and 28. High values of Q x f product (120 THz) were obtained for BZT resonators sintered at 1650 degrees C/4 h. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/degrees C. The achieved dielectric parameters recommend BZT dielectric resonators for microwave and millimeter wave applications. (C) 2010 Elsevier B.V. All rights reserved.

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO2 matrix. GeSiO thin films are prepared by two methods, sol-gel and radio frequency magnetron sputtering. After the deposition, the sol-gel films are annealed in either N-2 (at 1 atm and 800 A degrees C) or H-2 (at 2 atm and 500 A degrees C), and the sputtered films in H-2 (at 2 atm and 500 A degrees C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3-7 nm size) are observed in sol-gel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10-50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol-gel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.

The Bi4Ge3O12 (BGO) glass materials were sintered from a mixture of oxides powders, rapidly heated and melted at 1323 K and poured on the graphite plates heated at different temperatures. Their optical properties are compared with BGO single crystals being strongly dependent by the experimental conditions, which influence the further stability of the samples. The dielectric functions of the samples reveal a clear difference between the two phases (amorphous and crystalline). The refractive index of the amorphous phase has lower values which indicate that the density of the material is diminishes. Also, a sign of disorder is seen in optical absorption (extinction coefficient). From the optical band gap determinations of those two phases, it is confirmed the disorder structure and red-shifts of the band gap value compared with crystalline phase. The absorption spectra are in good agreement with the ellipsometric measurements, regarding the transparency and the edge of the band gap. (C) 2010 Elsevier Masson SAS. All rights reserved.

The rad-hard EPI diode was characterized envisaging its application in on-line 1.5 MeV electron beam dosimetry in radiation processing. As expected from a current sensitivity loss (13.5%) to the accumulated dose up to 750 kGy, the EPI diode exhibited good short-term repeatability (CV = 1.9%) which is better than that required for CTA dosimeters, routinely applied in radiation processing. The dose-response curve of the EPI diode was fitted by a second order polynomial function for doses up to 775 kGy. In addition, some preliminary studies on the radiation damage effects induced in this diode using a Capacitance Deep Level Transient Fourier Spectroscopy system are presented. The results showed that up to almost 1.5 MGy only 3 defects (VO, V-2 and CiOi) can influence the carrier lifetime and current generated. Further radiation damage studies are under way.

Three ceramic magnetic compositions within the CuZnTi ferrite system, having the Curie temperatures centered around 60 degrees C, 80 degrees C and 100 degrees C, respectively were investigated in order to determine the time-temperature stability of their main magnetic properties. Rates of changes of 33, 66 and 100 ppm/h of the maximum slopes of permeability were found for the samples aged at 60 degrees C, 80 degrees C and 100 degrees C, respectively. The samples aged at temperatures higher than 200 degrees C show a rather sudden decrease of the maximum slope of permeability, but no shift of the Curie temperature and the working point, corresponding to the temperature where the slope has the maximum value. The results are discussed in terms of the migration processes of the cations, especially Cu2+, from metastable positions on which they were frozen during the rapid cooling of the samples from the sintering temperature, to the more stable ones, namely the octahedral sites into the spinel lattice.