O. Alvarez-Fregoso

ac impedance analysis on crystalline layered and polycrystalline bismuth titanate

A comparison is made on the dielectric properties of the ferroelectric perovskite bismuth titanate for both crystalline (along different directions) and polycrystalline (with and without flux) samples, using the complex ac impedance technique. Results for the impedance, conductivity activation energies, bulk dielectric constant, and temperature behavior of the low‐frequency dielectric constant are presented. We discuss these results in terms of the conduction mechanisms depending on the nature of the sample, and compare results for crystalline and polycrystalline samples. Also, we show the important influence of the flux on the dielectric properties of the different samples, which must be taken into account in the use of these materials as substrates for the growth of superconducting thin‐film ceramic oxides.

Characterization of europium doped zinc aluminate luminescent coatings synthesized by ultrasonic spray pyrolysis process

Abstract Europium doped zinc aluminate (ZnAl 2 O 4 ) photoluminescent films have been deposited by ultrasonic spray pyrolysis deposition process. Different substrate temperatures and doping concentrations in the start spraying solution were studied. It is observed that the crystalline structure of this material depends on the substrate temperature during deposition of the films. For low substrate temperatures, the deposited films are amorphous. When the substrate temperature is increased at 500 °C some peaks corresponding to hexagonal phase of ZnO (zincite) appears. At substrate temperatures of 550 °C, the crystalline structure of the ZnAl 2 O 4 :Eu films presents the close-packed face centered cubic phase. The excitation and emission spectra were obtained; for an excitation wavelength of 260 nm, all the photoluminescence (PL) spectra show peaks located at 589, 615, 652 and 700 nm. Concentration quenching of the PL occurs at activator concentrations greater than 0.85 at.% inside the deposited films. The PL intensity increases as the substrate temperature rises. In addition, the surface morphology features of the films, as a function of the deposition temperature, are shown.

Preparation and characterization of Eu doped zirconia luminescent films synthesized by the pyrosol technique

The pyrosol method is a well-known process for de- positing films [l, 21. The main advantages of this tech- nique are its low cost, a high deposition rate, the pos- sibility to coat large areas, its ease of operation and the quality of the coatings obtained. This technique has been used to deposit successfully a variety of films such as oxides, sulfides and metals [I]. Zirconia has attracted much attention in science and technology be- cause its high melting temperature, high refractive in- dex, low thermal conductivity, hardness and corrosion barrier properties [3]. Furthermore these films are use- ful in corrosion resistance applications [4], as insulat- ing layers in very large scale integrated (VLSI) cir- cuits [5], for optical coatings [6], in buffer layers for high Tc oxide superconductors [7], etc. There are also some studies on rare earth doped luminescent zirco- nia single crystals and powders [8-101. The present contribution is devoted to the preparation and char- acterization of Zr02 : Eu photoluminescent (PL) and cathodoluminescent (CL) films synthesized by the py- rosol technique. To our best knowledge, there has been no report on luminescent coatings of Zr02

Characterization of ZrO2:Mn, Cl luminescent coatings synthesized by the Pyrosol technique

Abstract Manganese doped zirconia luminescent films have been deposited at temperatures ranging from 250 to 500 °C using the Pyrosol technique. The material obtained is in an amorphous state up 400 °C. For higher temperatures a polycrystalline material is obtained with a cubic and/or tetragonal crystalline zirconia phase. The photoluminescence and cathodoluminescence spectra show the bands associated with the electronic transitions 4 T 1 ( 4 G )→ 6 A 1 ( 6 S ) of the Mn2+ ions. A decrease of the luminescence, as a function of the doping concentration, substrate temperature and electron accelerating voltage is observed. The presence of chlorine seems necessary for the red luminescence. The surface morphology of the films is also presented.

The growth and structure of Cd0.95Fe0.05Te thin films grown by radio‐frequency sputtering

The growth and structure properties of Cd1−xFexTe thin films prepared by rf sputtering were studied. Films with thicknesses in the range 0.75–2.45 μm were grown onto Corning glass substrates at temperatures between 50 and 300 °C in an argon atmosphere at a pressure of 3 mTorr. The deposition rate was determined by film thickness profile analysis. Transmission electron microscopy and x‐ray diffractometry techniques were used to evaluate the structure, grain size, and orientation of the films. These properties were found to be related to the substrate deposition temperature.

Characterization of Al2O3:Eu3+ Luminescent Coatings Prepared by Spray Pyrolysis Technique

Europium doped Al2O3 photoluminescent coatings have been prepared by an ultrasonic spray pyrolysis process, using a solution of 0.05M of AlCl3 · 6H2O as starting material and EuCl3 as doping. These films were deposited on Pyrex glass substrates at atmospheric pressure using purified air as a carrier gas. All films were amorphous in structure. The photoluminescence spectra were measured at room temperature as a function of substrate temperature and doping concentration. For an excitation wavelength of 395 nm, all the photoluminescent spectra show bands located at 587, 600, 612 and 648 nm, characteristics of the trivalent europium ion. We observed a concentration quenching of the photoluminescence at values of activator concentration above 10 at% in the starting solution.

X-Ray Microanalysis of Human Cementum

An energy dispersive x-ray microanalysis study was performed throughout the total length of cementum on five impacted human teeth. Mineral content of calcium, phosphorous, and magnesium were determined with an electron probe from the cemento-enamel junction to the root apex on the external surface of the cementum. The concentration profiles for calcium, phosphorous, and magnesium were compared by using Ca/P and Mg/Ca atomic percent ratio. Our findings demonstrated that the Ca/P ratio at the cemento-enamel junction showed the highest values (1.8-2.2). However, the area corresponding to the acellular extrinsic fiber cementum (AEFC) usually located on the coronal one-third of the root surface showed a Ca/P media value of 1.65. Nevertheless, on the area representing the fulcrum of the root there is an abrupt change in the Ca/P ratio, which decreases to 1.3. Our results revealed that Mg(2+) distribution throughout the length of human cementum reached its maximum Mg/Ca ratio value of 1.3-1.4 at.% around the fulcrum of the root and an average value of 0.03%. A remarkable finding was that the Mg/Ca ratio pattern distribution showed that in the region where the Ca/P ratio showed a decreasing tendency, the Mg/Ca ratio reached its maximum value, showing a negative correlation. In conclusion, this study has established that clear compositional differences exist between AEFC and cellular mixed stratified cementum varieties and adds new knowledge about Mg(2+) distribution and suggests its provocative role regulating human cementum metabolism.

Structure and electrical properties of CdNiTe nanostructured thin films

CdNiTe nanostructured thin films were prepared by radio frequency sputtering from a target of CdTe and nickel compressed powders. The structural and electrical film properties were studied as a function of the atomic nickel concentration in the films (x=0.05, 0.10, and 0.15). X‐ray diffraction patterns showed a cubic CdTe parent structure with a (111) preferential orientation. The microcrystalline grain size in the films showed a systematic decrease with the increase of Ni content, starting with grain sizes of around 35 nm for x=0.05, down to an average of 26 nm for x=0.15. From scanning electron microscopy micrographs, a fine granular morphology with a random distribution of grain sizes in the films was observed. The film electrical resistivity was measured as a function of the temperature in the range T: 26–473 K. The temperature dependence of the dark resistivity over this wide temperature range showed a clear deviation from a simple thermally activated carrier transport mechanism.

Microcrystalline grain size growth in Cd0.95Fe0.05Te thin films

Cd0.95Fe0.05Te thin films have been grown on glass slides using RF sputtering and various substrate temperatures. Films grown at 100°C, which showed a mixture of amorphous and polycrystalline phases, were thermally annealed in an inert gas atmosphere at temperatures Ta in the range 100–450°C. The growth of the microcrystalline grain size was studied using X-ray diffraction and transmission electron microscopy techniques. The growth rate showed an energy-activated behavior characterized by two activation energies (Et); for Ta 300°C, Et= 0.165 eV. We have associated t he temperature (300°C) at which the change in the activation energy takes place with the crystallization of the film amorphous phase. Room-temperature resistivity measurements are intrepreted in terms of a grain boundary-controlled conductivity. The activation energy for the conductivity process shows a linear increase with annealing temperature in the range 100–350°C. The activation energy saturates at approximately 0.6 eV as a result of Fe-related deep donor levels.

Cu/Mo Nanostructured Alloy Thin Films

The synthesis of nanostructured materials was pioneered by the inert gas condensation and compaction technique. We investigate magnetron sputtering as an alternative processing route in comparison with thermal evaporation. We have studied the elaboration process and alloy formation by annealing as a function of molybdenum content. The sputtered material was investigated by means of X-ray diffraction, scanning electron microscopy, electron dispersion spectroscopy, atomic force microscopy and Vickers hardness tester. The results indicate that Cu/Mo alloys are semi-hard nanostructures with colors that change from black to yellow-golden when the Mo content increases. Hard nanostructured composite alloys were obtained at 500 °C only at high Mo content. The yellow-golden alloys can be considered as molybdenum bronzes and can be used for integrated circuits (IC), electrical contacts, metallic resistors and high voltage sockets.