Publications

Lead titanate (PT) is well known as a good piezoelectric material with a high Curie temperature (similar to 490 degreesC) and a low dielectric constant (similar to 200). But pure lead titanate cannot be sintered as ceramic bodies, for practical purposes, due to its large crystal anisotropy (tetragonality c/a=1.064). However, dense PT ceramic bodies can be obtained if small additions of other elements are used. Ca and Sm modified PT were succesfully tried, but such dopings proved rather expensive for mass production. Therefore some other doping elements must be tried. In this regard we investigated the effect of Bi and Y additives on the piezoelectric properties of PT ceramics. The general formula of our Bi, Y doped lead titanate ceramics was: (Pb1-3x/2Mex)(Ti0,98Mn0,02)O-3, with Me=Bi,Y and x=0.04, 0.06, 0.08. The modified PT ceramics were prepared by conventional ceramic technique, using p.a.purity raw materials. The mixed powders were sintered at temperatures between 1000-1300 degreesC. Poling was done in fields of about 70kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degreesC. It was found that Bi,Y modified lead titanate ceramics have a very high Curie point (T-c>450 degreesC), a low dielectric constant (<140) and a large electromechanical anisotropy (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4), which make them competitive to the rare-earth doped PT ceramics, for high temperature and high frequency transducers applications.

Piezoelectric properties of four tungsten doped samples of PZT type were determined as a function of composition. The samples were prepared by the usual ceramic technique from raw oxide of 99.9% purity. All compositions exhibited maximum values for the density at a sintering temperature of 1180 degrees C with a soaking time of 4h, so that the piezoelectric properties were determined on the samples sintered at this temperature only. The results showed that the piezoelectric properties were not drastically modified by the doping level, but dopings higher than 5% at. sightly alter the properties. The best values obtained for the piezoelectric parameters were: density rho= 7.7 g/cm(3); dielectric constants, epsilon(T)=1410; electromechanical coupling factors, k(p) = 0.63, and k(T) = 0.55; mechanical quality factor Q(m) approximate to 90; piezoelectric voltage constant d(33) = 470.10(-12) m/V; piezoelectric charge constant g33 = 25.10(-3) Vm/N. These values show that tungsten doped materials are piezoelectric soft type ceramics of high quality that can be successfully used for manufacturing ultrasonic transducers.

Ce-doped BaTiO3 thin films prepared on silicon-platinium by r.f. sputtering has been investigated. BaTiO3 doped with 5.5 mol.%CeO2 thin film was deposited at 550 degreesC substrate temperature in Ar atmosfere. The crystal structure and shape were examined by X-ray diffraction and scanning electron microscopy with EDAX. Analysis by X-ray diffraction patterns show that the crystalline film with a cubic structure of BaTiO3, was obtained. The surface morphology (roughness, the grain size and the droplet size) of the thin film surface was examined by atomic force microscopy (AFM). The grain size is about 160 nm, the droplet size is about 0.675 mum and the roughness is 36.88 nm. EDAX analyse established a composition of the film identicaly with that of target (BaTiO3 doped with 5.5 mol.%CeO2). The broad peak in the capacitance versus temperature curve at Curie point indicate that the r.f. sputtered Ce-doped BaTiO3 film is ferroelectric. The values of the capacitance of the thin film at 1 KWz were found to be 86 pF and the loss dielectric was tan delta =0.0875. The film have presented dielectric anomaly peak at 23 degreesC showing ferroelectric to paraelectric phase transition.

The formation of quasicrystalline (QC) single-phases at compositions close to Ti53Zr27Ni20 was investigated using transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and high resolution powder diffraction using synchrotron radiation. The primary Ti-Zr-Ni alloys were melt-spun in protective Ar atmosphere using peripheral wheel speeds omega = 20 m/s and omega = 25 m/s. TEM and SAED investigations revealed that significant microstructural differences occur even for rather small changes of the rapid quenching rate. Grain refinement and improved material homogeneity were found to be responsible for the observed gain in the ductility of the as-quenched ribbons.

The processing of electrical contacts of low electrical resistance constitutes a serious problem in the case of high temperature superconducting ceramics. Bad electrical contacts generate an important heat flux if the current overcomes several ampere. Moreover, the contacts tend to detach during the experiment. To avoid this difficulty, we made a special device, based on the elastic steel blades at low temperatures (external adjustment permits total surmount of the detach tendency). If the sample has a convenient geometry (bar bells) and the electrical contacts are made inside of the little silver paste island, the resistance of the current contacts is below 0.1 Omega.

There was a wide range of structures which are neither monocrystalline nor amorphous. Setting up a general model for such structures is very difficult because lattice distortions have different reasons and configurations. Therefore only approximations can be simulated. One of the most useful approximation is the model of paracrystals mainly developed by R. Hosemann. His idea was to postulate the distance to the neighbour atom or particle by a certain statistics. Starting with the linear "paracrystal" it is very easy to generate a statistical distribution of atoms or panicles. But even for two dimensional paracrystals the problem arises that the diagonal statistics does not obey the general idea of the paracrystal. Several attempts have been made to model the two or three dimensional paracrystal but only the present authors potential field model offers the possibility to simulate those structures. The basic idea is very simple. To each of the atoms (or particles) positive center Gaussian functions are attributed together with a certain number of negative Gaussian traps; there interaction leads to the position of the neighbour atoms. Using the Monte Carte method each of the neighbours can occupy a statistically generated position. So the three dimensional paracrystal can be generated. Several possibilities of additional parameters enable to adapt the model to the wanted conditions.

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu= 1.8:0,4:1.8:0.2:2.2:3.0, the precursor powder for synthesis of Bi-2223 superconducting phase, was obtained by spray-frozen, freeze drying technique. Samples of the nitrate precursor powder were placed in a heated furnace and extracted in air when temperature of the powder has attained values of 640, 660, 710, 750, 760, 778, 790, 815 and 850 degrees C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

Metal/ Ferroelectric/ Semiconductor (MFS) heterostructures have been of interest in the last decade due to possibility of their potential integration in classical silicon technology and because their potential use in memory devices. Research in this field has lead to new generations of sensors and actuators, laser modulators and deflectors, non-volatile random access memories (NRAM's). Unfortunately many at these types of devices remain in the prototype phase because of the impossibility to know and control phenomena which appear at the ferroelectric/ semiconductor interface. The present paper presents some results on the investigation of a PbTiO3/ Si heterostructure. A study of the film occurring at the interface and how it influences the ferroelectric properties of the heterostructure can clarify some problems of the reproducibility of the parameters of a MFS heterostructure.

Anodic film oxide was deposited in a alcohol-glycol-water (AGW) solution on n-GaAs for passivation purpose in stripe technology for laser diodes. The characteristics of anodic oxide were measured by scanning electron microscopy (SEM), Rutherford backscattering (RBS) analysis and elastic recoil detection (ERD) techniques. The result indicates a complex oxide structure in the phase base Ga2O3:As2O3 (1:1) joint together with carbon bonds. Due to the presence of carbon in anodic oxide, laser diodes are exposed to rapid degradation during operation.

In this paper the quasi-energies and the reduced Floquet vectors corresponding to a particle in 2-dimensions interacting with a time-dependent, monochromatic, linearly polarized electro-magnetic field, and in the presence of a weak, static, spatial-periodic field, are calculated. The later is treated using the stationary perturbation theory in the extended Hilbert space adapted for the eigenvalue equation of the Floquet Hamiltonian. The results have similarities to those of the quasi-free electron model.