A new interpretation for the degradation phenomenon of ZnO varistors

The electrical degradation phenomena of zinc oxide-based varistors were studied using a high-energy current pulse and a.c. polarization at different temperatures. Activation energy measurements during the degradation process showed that these phenomena are associated with diffusion and that the diffusion-controlling species are slower than Zn., For degradation promoted by current pulses of 8×20 μs, the Schottky potential barrier deformation was measured. A decrease in height and width of the potential barrier due to the reduction of surface states density, N s, without a significant change in donor density, N d, was observed. To explain these results, a modification of the unstable components model is proposed for the potential barrier in which the degradation is due to oxi-reduction reactions between atomic defects. These reactions promote the elimination of zinc vacancies and/or adsorbed oxygen on the grain boundaries. © 1992 Chapman & Hall.

An AM1 semiempirical study of the mechanism of sintering for ZnO in the presence of water and carbon dioxide

AM1 calculations were performed for the absorption of H2O and CO2 molecules on the surface of model ZnO crystals. The absorption of isolated molecules of each species and the co-absorption of both compounds simultaneously were considered. It was found that the absorption of H2O near a site where CO; is already absorbed favors the process of sintering, in agreement with the experimental findings. This is explained by the formation of Zn(OH)CO3H bound to the surface, a more mobile species than the ZnO unit itself. The roundening of the grains observed in atmospheres containing dry CO2 but suppressed when H2O is present, is also explained by these calculations. After absorption of CO2, the rupture of one bond - so that diffusion of the ZnCO3 species on the surface is allowed - requires much less energy than the breaking of two bonds, necessary for ZnO migration. These facts explain why the speed of surface transport does not decrease in CO2 atmospheres while sintering is indeed slowed down. © 1994.

Microstructural development of ZnO varistor during reactive liquid phase sintering

The microstructural evolution, grain growth and densification for the varistor systems ZnO-Bi2O3 (ZB), ZnO-Bi2O3-Sb2O3 (ZBS), ZnO-Bi2O3-Sb2O3-MnO-Cr 2O3-CoO (ZBSCCM) were studied using constant heating rate sintering, scanning electron microscopy (SEM) and in situ phase formation measurement by high temperature X-ray diffraction (HT-XRD). The results showed that the densifying process is controlled by the formation and decomposition of the Zn2Bi3Sb3O14 pyrochlore (PY) phase for the ZBS and ZBSCCM systems. The addition of transition metals (ZBSCCM system) alters the formation and decomposition reaction temperatures of the pyrochlore phase and the morphology of the Zn7Sb2O12 spinel phase. Thus, the spinel grains act as inclusions and decrease the ZnO grain growth rate. Spinel grain growth kinetics in the ZBSCCM system showed an n value of 2.6, and SEM and HT-XRD results indicate two grain growth mechanisms based on coalescence and Ostwald ripening. © 1996 Chapman & Hall.

Effect of in concentration in the starting solution on the structural and electrical properties of ZnO films prepared by the pyrosol process at 450°C

Undoped and indium-doped Zinc oxide (ZnO) solid films were deposited by the pyrosol process at 450°C on glass substrates from solutions where In/Zn ratio was 2, 5, and 10 at.%. Electrical measurements performed at room temperature show that the addition of indium changes the resistance of the films. The resistivities of doped films are less than non-doped ZnO films by one to two orders of magnitude depending on the dopant concentration in the solution. Preferential orientation of the films with the c-axis perpendicular to the substrate was detected by X-ray diffraction and polarized extended X-ray absorption fine structures measurements at the Zn K edge. This orientation depends on the indium concentration in the starting solution. The most textured films were obtained for solutions where In/Zn ratio was 2 and 5 at.%. When In/Zn = 10 at.%, the films had a nearly random orientation of crystallites. Evidence of the incorporation of indium in the ZnO lattice was obtained from extended X-ray absorption fine structures at the In and Zn K edges. The structural analysis of the least resistive film (Zn/In = 5 at.%) shows that In substitutes Zn in the wurtzite structure. © 2000 Elsevier Science B.V. All rights reserved.

Nanoparticles of La1-xSrxMnO3 compounds

A nanoparticles of La1-xSrxMnO3 were synthesized using homogenous coprecipitation method. The precipate was washed with water and dried at 80 °C. The samples were characterized by X-ray powder diffraction, transmission electron microscopy (TEM) and electrical resistivity as a function of temperature. The TEM results show that the particle size is in the nanometer scale.

Inorganic nanoparticles in organic-inorganic hybrid hosts for planar waveguides

Planar waveguides have been prepared on the ZrO2-(3-glycidiloxypropyl)trimethoxysilane (GPTS) system. Stable sols containing ZrO2 nanoparticles have been prepared and characterized by Photon Correlation Spectroscopy. The nanosized sol was embedded in (3-glycidoxipropyl)trimethoxisilane (GPTS) used as a hybrid host for posterior deposition. The opticalparameters of the waveguides such as refractive index, thickness and propagating modes and attenuation coefficient were measured at 632.8. 543.5 and 1550 nm by the prism coupling technique as a function of the Zr02 content. The planar waveguides present thickness of a few microns and support well confined propagating modes. Er doped samples display weak and broad (δλ≈96nm) emission at 1.5 μm.

Rare earth doped HfO2 nanoparticles embedded in SiO2-HfO2 planar waveguides. Preparation, optical, structural and spectroscopic characterization

In this work we present results on the preparation of planar waveguides based on HfO2 and HfO2-SiO2. Stable sols containing europium and erbium doped HfO2 nanoparticles have been prepared and characterized. The nanosized sol was either deposited (spin-coating) on quartz substrates or embedded in (3-glycidoxipropil)trimethoxisilane (GPTS) used as a hybrid host for posterior deposition. The refractive index dispersion and luminescence characteristics were determined for the resulting HfO2 films. The optical parameters of the waveguides such as refractive index, thickness and propagation losses were measured for the hybrid composite. The planar waveguides present thickness of a few micra and support well confined propagating modes.

Nucleation and crystallization of titania nanoparticles in silica titania planar waveguides: A study by low frequency Raman scattering

SiO2 (1-x) - TiO2 (x) waveguides, with the mole fraction x in the range 0.07 - 0.20 and thickness of about 0.4 μm, were deposited on silica substrates by a dip-coating technique. The thermal treatments at 700-900°C, used to fully densify the xerogels, produce nucleation of TiO2 nanocrystals even for the lowest TiO2 content. The nucleation of TiO2 nanocrystals and their growth by thermal annealing up to 1300°C were studied by waveguide Raman spectroscopy, for the SiO2 (0.8) - TiO2 (0.2) composition. By increasing the annealing temperature, the Raman spectrum evolves from that typical of the silica-titania glass to that of anatase, but brookite phase is dominant at intermediate temperatures. In the low. frequency region (5-50 cm-1) of the Raman spectra, acoustic vibrations of the nanocrystals are observed. From the measured line shapes, we can deduce the size distribution of the particles. The results are compared with those obtained from the line widths in the X-ray diffraction patterns. Nanocrystals with a mean size in the range 4-20 nm are obtained, by thermal annealing in a corresponding range of 800-1300°C.

Corrosion protection of fluorzirconate glasses coated by a layer of surface modified tin oxide nanoparticles

The protection efficiency against water corrosion of fluorozirconate glass, ZBLAN, dip-coated by nanocrystalline tin oxide film containing the organic molecule Tiron® was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The chemical bonding structure of the surface region and morphology were studied before and after two water exposure periods of 5 and 30 min. The results of the analysis for the as-grown sample revealed a SnO1.6 phase containing carbon and sulfur, related to Tiron®, and traces of elements related to ZBLAN (Zr, F, Ba). This fact and the clear evidence of the presence of tin oxifluoride specie (SnOxF y) indicates a diffusion of the glass components into the porous coating. After water exposure, the increase of the oxygen concentration accompanied by a strong increase of Zr, F, Ba and Na content is interpreted as filling of the nanopores of the film by glass compounds. The formation of a compact protective layer is supported by the morphological changes observed by AFM. © 2005 Elsevier B.V. All rights reserved.

Liposomes and micro/nanoparticles as colloidal carriers for nasal drug delivery

The use of the nasal route for drug delivery has attracted much interest in recent years in the pharmaceutical field. Local and principally systemic drug delivery can be achieved by this route of administration. But the nasal route of delivery is not applicable to all drugs. Polar drugs and some macromolecules are not absorbed in sufficient concentration due to poor membrane permeability, rapid clearance and enzymatic degradation into the nasal cavity. Thus, alternative means that help overcome these nasal barriers are currently in development. Absorption enhancers such as phospholipids and surfactants are constantly used, but care must be taken in relation to their concentration. Drug delivery systems including liposomes, cyclodextrins, micro- and nanoparticles are being investigated to increase the bioavailability of drugs delivered intranasally. This review article discusses recent progress and specific development issues relating to colloidal drug delivery systems in nasal drug delivery. © 2006 Bentham Science Publishers Ltd.

Degradación y recuperación de varistores de ZnO

In the present work it was studied the main aspect that influences on degradation and physical-chemical properties at grain-boundary region of metal oxide varistors and its consequences on microstructure and nonohmic electric properties. Based on the comprehension of the degradation aspects it was proposed some methodologies to recover the varistors nonohmic properties after being failed with long (2000 μs) and short current pulses (8/20 μs). Our analysis shown that one of the cause of degradation process is related to the lowering of oxygen species amount at grain-boundary region. Therefore, it is possible to re-promote oxygen enrichment of such regions by specific thermal treatments in rich oxygen atmospheres (the best condition found in the present work was temperatures around 900°C for 2 h at an oxygen flux of 15 1/h). The proposed nonohmic properties recovering procedure appear to be valid for all kind metal oxide varistors studied and is very important from technological point of view.

Thermoanalytical study of praziquantel-loaded PLGA nanoparticles

Polymeric nanoparticles have received great attention as potential controlled drug delivery systems. Biodegradable polymers has been extensively used in the development of these drug carriers, and the polyesters such as polylactic acid, polyglycolic acid and their copolymers as poly-lactide-co- glycolide are the most used, considering its biocompatibility and biodegradability. Thermal analysis techniques have been used for pharmaceutical substances for more than 30 years and are routine methods for screening drug-excipient interactions. The aim of this work is to use thermal analysis to characterize PLGA nanoparticles containing a hydrophobic drug, praziquantel. The results show that the drug is in an amorphous state or in disordered crystalline phase of molecular dispersion in the PLGA polymeric matrix and that the microencapsulation process did not interfere with the chemical structure of the polymer, mantaining the structural drug integrity.

Chitosan nanoparticles for non-viral gene therapy

The cationic polysaccharide chitosan has been widely used for non-viral transfection in vitro and in vivo and has many advantages over other polycations. Chitosan is biocompatible and biodegradable and protects DNA against DNase degradation. However following administration the ChitosanDNA polyplexes must overcome a series of barriers before DNA is delivered to the cell nucleus. This paper describes the most important parameters involved in the chitosan-DNA interaction and their effects of on the condensation, shape, size and protection of DNA. Strategies developed for chitosanDNA polyplexes to avoid non-specific interaction with blood components and to overcome intracellular obstacles as the crossing of die cell membrane, endosomal escape and nuclear import are presented. © 2006 American Chemical Society.

Study of Er3+ fluorescence on tellurite glasses containing Ag nanoparticles

Optical characteristics of tellurite glasses containing silver nanoparticles (NPs) and the influence on the emission spectrum of Er 3+ ions were studied. The transitions 4f ↔ 4f from erbium ions, mainly the 4I13/2 → 4I15/2 transition that involve upconversion energy process, have a strongly dependence with the chemical structure of the rare earth ion. In the present work, silver nanparticles (NPs) embedded in the host vitreous material, show a significant enhance (or quenching) on the erbium fluorescence due the long-range electromagnetic interaction between the plasmon surface energy of the Ag NPs (Localized Surface Plasmon Resonance -LSPR) and the Er3+ ions.

Effect of disinfection and accelerated ageing on dimensional stability and detail reproduction of a facial silicone with nanoparticles

The aim of the present study was to evaluate the effect of disinfection and accelerated ageing on the dimensional stability and detail reproduction of a facial silicone with different types of nanoparticle. A total of 60 specimens were fabricated with Silastic MDX 4-4210 silicone and they were divided into three groups: colourless and pigmented with nanoparticles (make-up powder and ceramic powder). Half of the specimens of each group were disinfected with Efferdent tablets and half with neutral soap for 60 days. Afterwards, all specimens were subjected to accelerated ageing. Both dimensional stability and detail reproduction tests were performed after specimen fabrication (initial period), after chemical disinfection, and after accelerated ageing periods (252, 504 and 1008 hours). The dimensional stability test was conducted using AutoCAD software, while detail reproduction was analysed using a stereoscope magnifying glass. Dimensional stability values were statistically evaluated by analysis of variance (ANOVA) followed by Tukey's test (p < 0.01). Detail reproduction results were compared using a score. Chemical disinfection and also accelerated ageing affected the dimensional stability of the facial silicone with statistically significant results. The silicone's detail reproduction was not affected by these two factors regardless of nanoparticle type, disinfection and accelerated ageing. © 2012 Informa UK, Ltd.