National Institute Of Materials Physics - Romania

A barium titanate precursor with barium: titanium ratio of 2: 9 was prepared by coprecipitation of mixed barium and titanium chlorides with ammonium oxalate aqueous solution at pH 7-7.5. Single-phase of Ba2Ti9O20 crystallised in the triclinic system and dense ceramic are obtained by sintering at 1375 degrees C for 4-8.5 h. The grain size of the sintered ceramics is 3 to 5 mu m. Dielectric properties of the sintered ceramics in the microwave frequency region were measured using the TE0(11) mode. Good microwave characteristics for Ba2Ti9O20 Were found. At 5 GHz, the dielectric constant and the dielectric loss (tg sigma) were 36 respectively, 0.6x10(-3). The quality factor due to dielectric loss at 5 GHz was similar to 1700.

Transport and thermodynamic properties were studied for the complex metallic compound beta-Al3Mg2, composed of 925 atoms per unit cell. beta-Al3Mg2 exhibits bulk superconductivity below T-c=0.87 K. An exponential temperature dependence of the specific heat well below T-c indicates BCS-like behavior with a nodeless gap of width Delta(BCS)(0)approximate to 1.5 K. The coherence length is derived as xi(0)=4.85x10(-8) m and the Ginzburg Landau parameter kappa(GL)approximate to 13, characterizing beta-Al3Mg2 as a type II superconductor. Superconductivity in beta-Al3Mg2 occurs in the absence of inversion symmetry of the crystal. Surprisingly, in spite of the rather complex crystal structure of beta-Al3Mg2, physical properties turn out to be quite simple.

X-ray optics is a relatively new instrument for the configuration X-ray beams. Presently this method is mainly used for diffraction and material tests. Although there are problems for medical use because of the larger wavelength (lower energy), for some applications it might be suitable.

The dielectric and optical properties of a series of aromatic polyimides synthesized by solution polycondensation of equimolar amounts of 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexeiiyl-1,2-dicarboxylic acid anhydride (epiclon) and diamines, in N-methylpyrrolidinone as solvent, under inert atmosphere were investigated. The effect of diamine type, which contains phenilenic units in the vicinity of methylenic groups or ether linkages, on the refractive index and dielectric constant was examined from theoretical and experimental data. Minimizing polarizability by modification of number of polarizable groups per unit volume in polyimide backbone determines lower values for dielectric constant. Thus, optical dielectric constants obtained from the refractive indices are low and are less influenced by the Ar groups. Higher values of the dielectric constant corresponding to the dipolar polarization are observed by decreasing frequency from 100 KHz to 1 KHz due to the increase of the polarization of induced dipoles, depending on the chemical structures.

Polystyrene-CrO2 composite films with extrinsic tunneling magnetoresistance have been obtained. The electric conduction is 2D Mott variable range hopping in the temperature range 77-300 K. The magnetoresistance is negative when the field is aligned parallel to the current (in-plane geometry) and is due to the spin dependent intergrain tunneling of the charge carriers. It reaches a value MR = -10.6 % at room temperature and a magnetic field of 1T. When the field is applied in-plane but perpendicular to the current the magnetoresistivity is positive and has the average value MR = 3.56 %.

A problem of electronic correlations is considered for two specific mesoscopic systems: the quantum point contact (QPC) and the double quantum dot (2QD) system. The systems are described using a generalized Anderson Hamiltonian. We show that charge fluctuations are relevant for electronic transport. In the QPC a local accumulation of charge and the dynamical Coulomb blockade effect lead to the 0.7 structure in the conductance characteristics. The evolution of the conductance with a magnetic field and in non-equilibrium situations is presented as well. The double quantum dot is studied in the approach, in which correlations within the 2QD are treated exactly, whereas the coupling of the 2QD to the leads is considered in the approximation valid at temperatures above the Kondo temperature. We analyse the evolution of the gate voltage dependence of the spin correlation functions and the conductance with the change of the interdot hopping. For the hopping parameter greater than a threshold value of the on-dot repulsion the physics of the device is dominated by the ground state eigenstates of the 2QD and antiferromagnetic correlations in the case of the doubly occupied 2QD. With a decrease of the interdot hopping repulsion below the threshold we observe a significant reduction of the antiferromagnetic coupling between the dots together with an enhanced occupation of the triplet states.

The electrical properties of two types of imidic polymers, i.e. copolymaleimides containing azobenzene groups and epiclon-based polyimides with their corresponding poly( amic acid)s, have been investigated. The dielectric constant and electrical conductance were experimentally measured over the 1-100 KHz frequency range. Also, the dielectric constants for all samples were determined from the experimentally and theoretically evaluated refractive indices at optical frequencies, when Maxwell's relationship can be applied. From the current-voltage characteristic I( U), electrical resistance can be determined, providing information on the relationship between the nature of the material and its electric properties. The obtained values show that epiclon-based polyimides and copolymaleimides present higher electrical resistivity than the poly( amic acid) s. The dissipation factor was evaluated, revealing low energy loss under the form of heat in the studied dielectrics. These results show that these polymeric materials are good insulators, which recommends them for microelectronic applications.

The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma(0) in detail gives some hint that sigma(0)(T) has also an unusual saddlelike form.

Amorphous SnSe, films were prepared by pulsed laser deposition (PLD) from solid polycrystalline targets. The atomic scale structure has been revealed by X-ray diffraction. Hardness and electrical properties were measured. Strong gas sensing properties have been evidenced for the first time in heat treated films. (c) 2007 Elsevier B.V. All rights reserved.

xTiO(2)-(1- x)alpha-Fe2O3 (x = 0.0-1.0) mixed oxide nanoparticle, system ha, been synthesized by a hydothermal route, X-ray diffraction, Mossbauer spectroscopy and electron microscopy have been used to study the structure, morphology and substitution in the nanosized samples. Experimental evidence for the solubility limits of Ti4+, in alpha-Fe2O3 lattice and of Fe3+ in TiO2 structure is given. (c) 2007 Elsevier B.V. All rights reserved.