Microstructure of organometallic derived PLZT ceramics

The PLZT powders with the formula Pb0.905La0.095(Zr0.65Ti0.35)(0.976)O-3+3.5 wt.% PbO were prepared by the organometallic precursor method (Pechini and partial oxalate processes). The microstructure of sintered 9.5/65/35 PLZT ceramics obtained from a partial oxalate procedure shows that the outstanding feature of this microstructure is its fairly uniform grains of about 1.8 mum. The microstructure of sintered PLZT ceramics obtained by the Pechini process consists of uniform small randomly- oriented grains tightly bonded together in the central part of the sample with,a grain size of about 1.2 mum. Cubic and elongated grains are formed at the sample's border. The microstructures of hot pressed PLZT ceramics obtained from both processes are dense and rather uniform. After a double stage of hot pressing (2 plus 20 h) the microstructure of PLZT is fully dense, uniform and homogeneous with a grain size of approximately 2.5 mum. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Effects of powder preparation and sintering procedure on microstructure and dielectric properties of PLZT ceramics

PLZT ceramics belong to one of the very important groups of functional materials that make a basis for the production of a large range of electronic devices. The microstructure and properties of ceramics depend on the powder preparation and thermal processing conditions. Various techniques have been used to obtain chemically homogeneous and fine starting powders. PLZT powders have been prepared by two different production routes: by a modified Pechini method, using a polymeric precursor method (PMM) and by a partial oxalate method. A two-step sintering process, including a hot pressing, was carried out at 1100 and 1200degreesC Distinct phases obtained during the sintering process have been investigated by SEM and EDS techniques and dielectric properties such as permittivity and dielectric loss were measured in a frequency range from 1 to 20 kHz.. A significant difference in microstructure and dielectric properties, depending on powder origin and sintering procedure, has been noticed.

EFFECT OF THE ADDITION OF CR AND NB ON THE MICROSTRUCTURE AND ELECTROCHEMICAL CORROSION OF HEAT-TREATABLE AL-ZN-MG ALLOYS

The effect of the addition of Cr and Nb on the microstructure and the electrochemical corrosion of the weldable, high-strength and stress corrosion cracking (SCC) resistant Al-5%Zn-1.67%Mg-0.23%Cu alloy (H) has been studied. Combined additions of the alloying elements, J (with Nb), L (with Cr) and O (with Cr and Nb) and different heat treatments, ST (cold-rolled), A (annealed), F (quenched), B (quenched and aged) and C (quenched in two steps and aged), to obtain different microstructures and hardness have been performed. To correlate the electrochemical corrosion with the microstructure of the specimens, corrosion potential (E(cor)) measurements in different chloride solutions were performed and optical microscopy, SEM, TEM and EDX were applied. In chloride solutions containing dissolved O-2 or H2O2, the present alloys were polarized up to the pitting attack. It was shown that the E(cor) measurements were very sensitive to the alloy composition and heat treatment, increasing in the order H < J < L < O < Al (for a given heat treatment) and F < A approximate to ST < B < C (for a given alloy). The MgZn2 precipitates of the annealed (A) and cold-rolled (ST) specimens were dissolved in chloride solutions containing oxidizing agents and pitting attack was shown to develop in the cavities where the precipitates were present. In the specimens B and C, the compositions of the precipitate free zones was found to be equal to that of the matrix solid solution and preferential intergranular attack was not evident, this being in agreement with their SCC resistance. The addition of Cr and Nb increased the pitting corrosion resistance. The effects of Cr and Nb were additive, that of Cr being predominant, either, in the E(cor) shift or in the increase in the pitting corrosion resistance.

Effect of La2O3 doping on the microstructure and electrical properties of a SnO2-based varistor

The effect of La2O3 addition on the densification and electrical properties of the (0.9895 - x) SnO2 + 0.01 CoO + 0.0005 Nb2O5 + x La2O5 system, where x = 0.0005 or 0.00075, was considered in this study. The samples were sintered at 1300 degreesC for 2 and 4 h and a single SnO2 phase was identified by X-ray diffraction. Microstructure analysis by scanning electron microscopy showed that the affect of La2O3 addition is to decrease the SnO2 grain size. J versus E curves indicated that the system exhibits a varistor behavior and the effect of La2O3 is to increase both the non-linear coefficient (alpha) and the breakdown voltage (E-2). Considering the Schottky thermionic emission model the potential height and the width were estimated. The addition of small amounts of La2O3 to the basic system increases the potential barrier height and decreases both grain size and potential barrier width. (C) 2001 Kluwer Academic Publishers.

Microstructure and dielectric properties of (Ba,Sr)TiO3 thin film produced by the polymeric precursor method

BaxSr1-xTiO3 (x = 0.6) (BST) thin films were successfully prepared on a Pt(111)/TiO2/SiO2/Si(100) substrate by spin coating, using the polymeric precursor method. BST films with a perovskite single phase were obtained after heat treatment at 700 degrees C. The multilayer BST thin films had a granular structure will a grain size of approximately 60 nm. A 480-nm-thick film was obtained by carrying out five cycles of the spin-coating/heating process. Scanning electron microscopy and atomic force microscopy analyses showed that the thin films had a smooth, dense, crack-free surface with low surface roughness (3.6 nm). At room temperature and at a frequency of 100 kHz, the dielectric constant and the dissipation factor were, respectively, 748 and 0.042. The high dielectric constant value was due to the high microstructural quality and chemical homogeneity of the thin films obtained by the polymeric precursor method.

Macro- and microstructure of the superior cervical ganglion in dogs, cats and horses during maturation

The superior cervical ganglion (SCG) provides sympathetic input to the head and neck, its relation with mandible, submandibular glands, eyes (second and third order control) and pineal gland being demonstrated in laboratory animals. In addition, the SCG's role in some neuropathies can be clearly seen in Horner's syndrome. In spite of several studies published involving rats and mice, there is little morphological descriptive and comparative data of SCG from large mammals. Thus, we investigated the SCG's macro- and microstructural organization in medium (dogs and cats) and large animals (horses) during a very specific period of the post-natal development, namely maturation (from young to adults). The SCG of dogs, cats and horses were spindle shaped and located deeply into the bifurcation of the common carotid artery, close to the distal vagus ganglion and more related to the internal carotid artery in dogs and horses, and to the occipital artery in cats. As to macromorphometrical data, that is ganglion length, there was a 23.6% increase from young to adult dogs, a 1.8% increase from young to adult cats and finally a 34% increase from young to adult horses. Histologically, the SCG's microstructure was quite similar between young and adult animals and among the 3 species. The SCG was divided into distinct compartments (ganglion units) by capsular septa of connective tissue. Inside each ganglion unit the most prominent cellular elements were ganglion neurons, glial cells and small intensely fluorescent cells, comprising the ganglion's morphological triad. Given this morphological arrangement, that is a summation of all ganglion units, SCG from dogs, cats and horses are better characterized as a ganglion complex rather than following the classical ganglion concept. During maturation (from young to adults) there was a 32.7% increase in the SCG's connective capsule in dogs, a 25.8% increase in cats and a 33.2% increase in horses. There was an age-related increase in the neuronal profile size in the SCG from young to adult animals, that is a 1.6-fold, 1.9-fold and 1.6-fold increase in dogs, cats and horses, respectively. on the other hand, there was an age-related decrease in the nuclear profile size of SCG neurons from young to adult animals (0.9-fold, 0.7-fold and 0.8-fold in dogs, cats and horses, respectively). Ganglion connective capsule is composed of 2 or 3 layers of collagen fibres in juxtaposition and, as observed in light microscopy and independently of the animal's age, ganglion neurons were organised in ganglionic units containing the same morphological triad seen in light microscopy. Copyright (c) 2007 S. Karger AG, Basel.

Estudo Comparativo de Juntas Soldadas a TIG e Laser Empregando-se Ti com Pureza Comercial Utilizado em Prótese sobre Implantes

Welding in the dentistry has been used for great part of the specialized dentist-surgeons in the implants area to solve prosthesis supported by implant adaptation problems. The development of new equipments Laser and TIG allowed a larger use of these processes in the prosthesis production. In this work, it was studied welded joints made by Laser and TIG, using commercial purity titanium, cpTi, applied in prosthesis supported by implants. The weld characterizations were carried out by light microscopy, EDS_elementary mapping, microhardness and tensile test. Through metallographic characterization, the weld bead presented a martensitic microstructure in the Laser welding process, originated from shear provoked by deformations in the lattice. This caused structural changes of the transformed area, which determines a fine plate-like morphology. In the weld bead from TIG, besides presenting higher hardness, was observed formation of Widmansttaten structure, which is characteristic of a geometric model, resulted of new phase formation along of the crystallographic plans. The martensitic structure is more refined than Widmansttaten structure, due to the high-speed cooling (10(3)degrees C/s) imposed by the Laser process.

The role of oxygen vacancies on the microstructure development and on the electrical properties of SnO2-based varistors

Variations on the microstructure development and on the electrical properties of SnO2-based varistors are discussed on the basis of the oxygen vacancies created or annihilated by the presence of different additives. Electron paramagnetic resonance (EPR) analysis of sintered samples evidenced a substantial increase in the paramagnetic oxygen vacancies concentration when Nb2O5 is added to the SnO2 center dot Co3O4 system. on the other hand, the observed diminution in the concentration of such species after the addition of Fe2O3 indicates solid solution formation. The quantification of paramagnetic oxygen vacancies allowed to confirm the proposed substitutions taking place in the lattice during sintering. These findings are supported by scanning electron microscopy, by density measurements and by current density versus electric field curves. The characterization of secondary phases through EDS assisted SEM and TEM is also reported in this work.

How Cr2O3 influences the microstructure and nonohmic features of the SnO2(COx, Mn1-x)O-based varistor system

An investigation was made to discover how the addition of Cr2O3 affects the microstructural heterogeneity and nonohmic features of the SnO2(Co-x, Mn1-x)O-based varistor system, with x varying from 0 to 1. The presence of Cr2O3 was found to strongly increase the nonohmic features when x = 1. However, the nonohmic features of the system decrease when x drops from 1 to 0, a behavior explained by the increase of the junction heterogeneity within the system's microstructure. accompanied by ail excess of precipitates at the triple point in the grain boundary region due to modified MnO sintering. The presence of these precipitates causes the leakage current to increase in response to the creation of ail ineffective barrier. The effect produced by heat-treating these systems in oxygen- and nitrogen-rich atmospheres suggests that, according to mechanisms previously discussed in the literature, Cr2O3 is more susceptible to oxygen, so that increasing the amount of oxygen in the grain boundary region may improve the system's nonohmic properties. (c) 2005 Elsevier Ltd. All rights reserved.

The effect of isostructural seeding on the microstructure and piezoelectric properties of PZT ceramics

Pechini's method was used to prepare lead titanate zirconate with Zr/Ti ratio equal to 53/47. X-ray diffraction data revealed the presence of a rhombohedral phase, rich in zirconium, due to difference in carbonate stabilities, in PZT ceramics calcined at 600 degrees C. Infrared spectroscopy presented COO- bonds in the 1400 cm(-1) region, which disappeared after calcining at 700 degrees C. Seeds with rhombohedral (PZT 57/43) or tetragonal structure (PZT 45/55) were added to the precursor. The microstructure was differentially influenced by the nature of seed particles. Rhombohedral nuclei promoted preferential crystallization of lead zirconate. This heterogeneity directly reflected on values of k(p) and d(33). (C) 1999 Elsevier B.V. Limited and Techna S.r.l. All rights reserved.

Laser weld: microstructure and corrosion study of Ag-Pd-Au-Cu alloy of the dental application

The laser Welding process was introduced into dentistry by the end of the 1980s, resulting on a great impulse to that area with the development of cheaper and smaller equipment, using simpler technique. This allowed greater use of that process on the confection of prostheses compared to the brazing process since the heat source for that process is a concentrated light beam of high power, which minimizes distortion problems on the prosthetic pieces. Ag-Pd-Au-Cu alloy used on the confection of dental implant prostheses was observed before and after subjection to the laser welding process. The microstructure was analyzed with the. use of optic microscopy and the corrosion resistance was studied by the traditional electrochemical techniques and by electrochemical impedance, under environmental conditions simulating the aggressiveness found in the mouth cavity. A structural change was detected on the weld area, which presented a refined microstructure deriving from the high-speed cooling. The base metal out of the weld area presented a fusion coarse microstructure. The electrochemical essays showed differences on the potentiodynamic polarization behavior in both weld and metal base areas, indicating superior corrosion resistance in the weld area. The impedance spectra were characterized by capacitive distorted components, presenting linear impedance in the low frequencies area. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Microstructure and corrosion resistance of inorganic-organic (ZrO2-PMMA) hybrid coating on stainless steel

Zirconia-polymethylmetacrylate hybrids prepared by a sol-gel method were deposited by dip-coating on stainless steel to improve the resistance against wet corrosion. The effect of the concentration of polymethylmetacrylate and the number of coating applications on the microstructure and corrosion performance of coated samples was investigated. The microstructural properties of samples was analyzed by scanning electron and atomic force microscopy, adhesion tests and profilemeter measurements. The electrochemical corrosion was evaluated through potentiodynamic polarization curves at room temperature. Results show that the sample prepared with 17 vol.% of polymethylmethacrylate has a maximum corrosion resistance, smaller roughness, are hermetic and adherent to the substrate. This film increases the life time of the stainless steel by a factor 30. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Microstructure and electric properties of a SnO2 based varistor

High non-linear J x E electrical characteristic (alpha=41) were obtained in the Nb2O5 and Cr2O3 doped CoO highly densified SnO2 ceramics. X-ray diffraction analysis showed that these ceramics are apparently single phase. Electrical properties and microstructure are highly dependent on the Cr2O3 concentration and on the sintering temperature. Excess of Cr2O3 leads to porous ceramics destroying the material's electrical characteristics probably due to precipitation of second phase of CoCr2O4 Dopant segregation and/or solid solution formation at the grain boundaries can be responsible for the formation of the electrical barriers which originate the varistor behaviour. (C) 1998 Elsevier B.V. Limited and Techna S.r.l. All rights reserved.