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.

Study of Ti-modified columbite route and the effect of lithium doping in 0.9PMN-0.1PT ceramics

The influence of lithium on the structural characteristics of PMN-PT ceramic was studied. The synthesis of PMN-PT powders using this precursor leads to the formation of high amount of perovskite phase. The insertion of Li(+) ions in B-site affects the microstructure because the rise in mass transport changes the mechanical characteristics of sintered ceramic. Higher values of K(m) and T(m) were gotten when lithium is inserted into perovskite phase. Secondary phase was found when lithium content increase beyond 1 mol%, besides the occurrence of transgranular fractures in sintered ceramic. Also, the additive acts increasing the relaxor behavior.

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.

Electrical properties of YSZ/NiO composites prepared by a liquid mixture technique

We report the preparation and characterization of yttria-stabilized zirconia/nickel oxide composites (YSZ/NiO). This composite is the precursor material of the cermet YSZ/Ni, which is used as solid oxide fuel cell anode material. The performance of the anode is strongly dependent on the microstructural properties of the cermet. Therefore, the control of the microstructure of the YSZ/NiO composite is a key step for the fabrication of high-performance anodes. In this study, the composites were prepared by a modified liquid mixture technique. Scanning electron microscopy analysis evidenced the good dispersion of the phases and that NiO nanoparticles are spread over the YSZ surface. Sintered pellets were studied by X-ray diffraction and impedance spectroscopy. The main results show that the composite is comprised of a well-dispersed mixture of the two phases. The electrical conductivity data show that there is a strong dependence of the transport mechanism on the relative composition of phases. (c) 2005 Elsevier Ltd. All rights reserved.

Analysis of secondary phases segregated and precipitated in SnO2-based varistors

Transmission and scanning electron microscopy techniques were used to study the heterogeneities found in the microstructure of (SnO2Co3O4Nb2O5Fe2O3)-Co-.-Nb-.-Fe-. and (SnO2ZnONb2O5FC2O3)-Zn-.-Nb-.-F-. varistors. Second phases encountered both inside the grains and ingrain boundary regions were identified using energy dispersive spectrometry and electron diffraction patterns. Through the electrical characterisation, the presence of iron oxide among the additives was determined to highlight the non-linear properties of the specimens. A discussion on the influence of second phases on the non-linear features of these systems is also addressed. (C) 2004 Elsevier Ltd. All rights reserved.

The effect of ceramic surface treatment on bonding to densely sintered alumina ceramic

Statement of problem. Ceramic surface treatment is crucial for bonding to resin. High crystalline ceramics are poorly conditioned using traditional procedures.Purpose. The purpose of this study was to evaluate the effect of silica coating on a densely sintered alumina ceramic relative to its bond strength to composite, using a resin luting agent.Material and methods. Blocks (6 X 6 X 5 mm) of ceramic and composite were made. The ceramic (Procera AllCeram) surfaces were polished, and the blocks were divided into 3 groups (n = 5): SB, airborne-particle abrasion with 110-mu m Al(2)O(3); RS, silica coating using Rocatec System; and CS, silica coating using CoJet System. The treated ceramic blocks were luted to the composite (W3D Master) blocks using a resin luting agent (Panavia F). Specimens were stored in distilled water at 37 degrees C for 7 days and then Cut in 2 axes, x and y, to obtain specimens with a bonding area of approximately 0.6 mm(2) (n = 30). The specimens were loaded to failure in tension in a universal testing machine, and data were statistically analyzed using a randomized complete block design analysis of variance and Tukey's test (alpha=.05). Fractured surfaces were examined using light microscopy and scanning electron microscopy to determine the type of failure. Energy-dispersive spectroscopy was used for surface compositional analysis.Results. Mean bond strength values (MPa) of Groups RS (17.1 +/- 3.9) (P = .00015) and CS (18.5 +/- 4.7) (P=.00012) were significantly higher than the values of Group SB (12.7 +/- 2.6). There was no statistical difference between Groups RS and CS. All failures occurred at the adhesive zone.Conclusion. Tribochemical silica coating systems increased the tensile bond strength values between Panavia F and Procera AllCeram ceramic.

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.

Preparation of ceramic membranes from surface modified tin oxide nanoparticles

The preparation of crack-free SnO2 supported membranes requires the development of new strategies of synthesis capable to allow controlled changes of surface chemistry and to improve the processability of supported layers. In this way, the controlled modification of the SnO2 nanoparticle surface by adding capping molecules like Tiron(R) ((OH)(2)C6H2(SO3Na)(2)) during the sol-gel process was studied, aiming to obtain high performance membranes. Colloidal suspensions were prepared by hydrolyzing SnCl4.5H(2)O aqueous solution with NH4OH in presence of Tiron(R). The effect of the amount of Tiro(R) (from I to 20 wt.%) on the structural features of nanoparticles, powder redispersability and particle-solution interface properties was investigated by X-ray powder diffraction (XRPD), extended X-ray absorption fine structure (EXAFS), quasi-elastic light scattering and electrophoretic mobility measurements. XRPD and EXAFS results showed that the addition of Tiron(R) up to 20 wt.% to colloidal suspensions does not affect the crystallite size of SnO2 primary particles, determined around 2-3 nm. This value is comparable to the hydrodynamic size measured after redispersion of powder prepared with amount of Tiro(R) higher than 7.5 wt.%, indicating the absence of condensation reactions between primary particles after the initial precipitation step. As a consequence the powder with amount of Tiron(R) > 7.5 wt.%, can be fully redispersed in aqueous solution at pH greater than or equal to I I until a nanoparticle concentration of 6 vol.%. The electrophoresis measurements showed a decrease of the isoelectric point by increasing the amount of grafted Tiron(R) at the SnO2 nanoparticle surface, resulting in negatively charged particle-solution interface in all the studied pH range (2-11). These features govern the gelation process favoring the preparation of crack-free SnO2 supported membranes. The control exercised by Tiron(R) modifying agent in the aggregation process allows the fine-tuning of the porosity, from 0.124 to 0.065 cm(3) g(-1), and mean pore size, from 6.4 to 1.9 nm, as the amount of grafted molecules increases from 0 to 10 wt.%. In consequence, the membrane cut-off determined by filtration of polyethylene glycol standard solutions can be screened from 1500 to 3500 g mol(-1). (C) 2002 Elsevier B.V. B.V. All rights reserved.

Negative temperature coefficient thermistor based on Bi3Zn2Sb3O14 ceramic: An oxide semiconductor at high temperature

Bi1.5ZnSb1.5O7 dielectric ceramic with pyrochlore structure was investigated by impedance spectroscopy from 400 to 750 degreesC. Pyrochlore was synthesized by the polymeric precursor method, a chemical synthesis route derived from Pechini's method. The grain or bulk resistance exhibits a sensor temperature characteristic, being a thermistor with a negative temperature coefficient (NTC). Only a single region was identified on the resistance curve investigated. The NTC thermistor characteristic parameter (beta) is equal to 7140 degreesC, in the temperature range investigated. The temperature coefficient of the resistance (alpha) was derived, being equal to -4.46x10(-2) degreesC(-1) at 400 degreesC. The conduction mechanism and relaxation are discussed. (C) 2003 American Institute of Physics.

Mechanochemical synthesis of PZT powders

PZT ceramic powders were successfully prepared from the mixture of PbO, ZrO2 and TiO2 by mechanochemical synthesis in a planetary ball mill, under different milling conditions. Phase evolution during synthesis was monitored by X-ray diffraction analysis. Intensive milling resulted in formation of the nanocrystalline, perovskite PZT powders after 1 h of milling. This is a significant improvement in comparison to milling conditions reported by other authors. Depending on milling parameters the presence of some other phases, such as unreacted ZrO2, was also detected in some samples. The changes in powder size and morphology due to intensive milling, were determined by SEM and TEM, while BET analysis was used to determine specific surface area of the powders. Conclusions about processes taking place during mechanochemical synthesis of PZT powders were made based on the results of characterization. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Low temperature crystallization of SrBi2Ta2O9 thin films using microwave oven

SrBi2Ta2O9 thin films, produced by the polymeric precursor method, were crystallized at low temperature using a domestic microwave oven. A SiC susceptor were used to absorb the microwave energy and rapidly transfer the heat to the film. Low microwave power and short time have been used. The films thus obtained are crack-free, well-adhered, and fully crystallized, even when treated at 600 degreesC for 10 min. The microstructure displayed a polycrystalline nature with an elongate grain size comparable to the films obtained by the conventional treatment. The dielectric constant values are 240, 159 and 67, for the films treated at 600 degreesC, 650 degreesC and 700 degreesC, respectively, when the films are placed directly on the SiC susceptor. Electrical measurements revealed that the increase of the temperature treatment to 700 degreesC causes a complete loss of ferroelectricity due to degradation of the bottom interface. A 4 nun-ceramic wool put between the susceptor and the substrate minimizes the interface degradation leading to a dielectric constant, a dielectric loss, and a remnant polarization (2P(r)) of 181 muC/cm(2), 0.032 muC/cm(2), and 12.8 muC/cm(2), respectively, for a film treated at 750 degreesC for 20 min. (C) 2004 Elsevier B.V. All rights reserved.

Theoretical and experimental study of the relation between photoluminescence and structural disorder in barium and strontium titanate thin films

Thin films of barium and strontium titanate (BST), synthesized by the polymeric precursor solution and spin coated on [Pt (140nm)/Ti (10 nM)/SiO2(1000 nm)/Si] substrates were found to be photoluminescent at room temperature when heat treated below 973 K, i.e. before their crystallization. First principles quantum mechanical techniques, based on density functional theory (DFT) were employed to study the electronic structure of two periodic models: one is standing for the crystalline BST thin film and the other one for the structurally disordered thin film. The aim is to compare the photoluminescence (PL) spectra of the crystalline and disordered thin films with their UV-vis spectra and with their computed electronic structures. The calculations show that new localized states are created inside the band gap of the crystalline model, as predicted by the UV-vis spectra. The study of the charge repartition in the structure before and after deformation of the periodic model shows that a charge gradient appears among the titanate clusters. This charge gradient, together with the new localized levels, gives favorable conditions for the trapping of holes and electrons in the structure, and thus to a radiative recombination process. Our models are not only consistent with the experimental data, they also allow to explain the relations between structural disorder and photoluminescence at room temperature. (c) 2005 Elsevier Ltd. All rights reserved.

Barium strontium titanate nanocrystalline thin films prepared by soft chemical method

Barium strontium titanate (Ba0.65Sr0.35TiO3) nanocrystalline thin films, which were produced by the soft chemical method, were crystallized at low temperature using a domestic microwave oven. A SiC susceptor were used to absorb the microwave energy and rapidly transfer the heat to the film. Low microwave power and short time have been used. The films obtained are crack-free, well-adhered, and fully crystallized. The microstructure displayed a polycrystalline nature with nanograin size. The metal-BST-metal structure of the thin films treated at 700 degrees C show food electric properties. The ferroelectric nature of the BST35 thin film was indicated by buttertly- shaped C-V curves. The capacitance-frequency curves reveal that the dielectric constant may reach a value up to 800 at 100kHz. The dissipation factor was 0.01 at 100kHz. The charge storage density as function of applied voltage graph showed that the charge storage densities are suitable for use in trench type 64 Mb ( 1-5 mu C/cm(2) and 265 Mb (2-11 mu C/cm(2)) DRAMs. (C) 2007 Elsevier Ltd. All rights reserved.