Scielo RSS <![CDATA[Materials Research]]> vol. 7 num. 2 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Editorial</b>]]> <![CDATA[<b>Effect of porosity on the tensile properties of low ductility aluminum alloys</b>]]> The literature contains reports of several studies correlating the porosity and mechanical properties of aluminum alloys. Most of these studies determine this correlation based on the parameter of global volumetric porosity. These reports, however, fail to separate the effects of microstructural features and porosity on alloys, though recognizing the influence of the latter on their mechanical properties. Thus, when the decrease in tensile strength due to the porosity effect is taken into account, the findings are highly contradictory. An analysis was made of the correlation between mechanical properties and global volumetric porosity and volumetric porosity in the fracture, as well as of the beta-Al5FeSi phase present in 380 aluminum alloy. Our findings indicate that mechanical properties in tension relating to global volumetric porosity lead to overestimations of the porosity effect in detriment to the mechanical properties. Moreover, the proposed models that take into account the effects of particles, both Si and beta-Al5FeSi, are unapplicable to low ductility alloys. <![CDATA[<b>Avaliação <i>in vitro</i> do potencial antimicrobiano de diferentes materiais restauradores</b>]]> The aim of this study was to evaluate the antimicrobial activity of restorative materials - Fuji II LC, Ketac-Fil, Ketac-Molar, Vidrion R, Fuji IX, Ariston pHc, Degufill Mineral, Z100 and Compoglass. For this purpose the agar diffusion test was used determining the bacterial growth inhibition halo over Streptococcus mutans, Streptococcus sobrinus, Staphylococcus aureus and Micrococcus luteus. The materials were prepared and manipulated under aseptic conditions, following the manufacturer instructions. The specimens were immersed in media (Mha and BHIa) that presented the microorganisms. After a 2 h pre-incubation period, at room temperature the plaques were incubated at 37 °C for 48 h. After that, the presence or absence of bacterial growth inhibition halo was verified. Vidrion R and the liquid component of all glass ionomers tested showed antimicrobial activity. However, the resin composite and the "compomer" did not show any antimicrobial activity. In conclusion, only the glass ionomer cement Vidrion R showed antimicrobial activity. <![CDATA[<b>Demineralization around restorations with different restorative materials containing fluoride</b>]]> The aim of this study was to evaluate in vitro the demineralization on tooth/restoration interface of eight restorative materials after demineralization/remineralization cycling. Eighty class V cavities were prepared with margins at enamel and dentin/cementum, and were restored with Fuji II LC, Fuji IX, Ketac-fil, Ketac Molar, Ariston pHc, Compoglass, Degufill Mineral and Z100. After the restorative procedures, the restorations were submitted to demineralization/ remineralization cycling during 14 days. Specimens were embedded in acrylic resin and submitted to serial sectioning. The sections were examined by optical microscope, and demineralization around restoration was measured on cervical and occlusal margins. The data were analyzed using the ANOVA and Tukey test (p<0.05). Glass ionomer cements showed less demineralization on enamel and dentin/restoration interfaces when compared to the tested composite resins (Z100 and Degufill Mineral). In conclusion, glass ionomer cements suffered less demineralization but did not protect completely the tooth/restoration interface. <![CDATA[<b>Comparison of the compressive strength of impregnated and nonimpregnated eucalyptus subjected to two different pressures and impregnation times</b>]]> The durability of wood is affected by several factors. For this reason, much research has been done on a variety of chemical compounds for impregnating wood, aimed at preserving it while simultaneously improving its properties. Recent studies of the properties of impregnated wood have demonstrated the possibility of substantially improving its mechanical characteristics. Thus, the purpose of this work was to compare the strength to parallel compression of wooden fibers (Eucalyptus grandis), both nonimpregnated and impregnated with a monocomponent resin, from the standpoint of pressure and impregnation time, aiming at its structural utilization. The results demonstrate that the compressive strength of impregnated test specimens is greater than that of nonimpregnated ones, indicating that monocomponent polyurethane resin can be considered suitable for impregnating wood, since it increases the compressive strength of eucalyptus. <![CDATA[<b>Influence of deformation on the kinetics of phase transformation in a forging steel during warm working</b>]]> Dilatometric techniques were used to determine the start and finish transformation heating temperatures for a carbon steel (0.30% C - 1.5% Mn). The mechanical behavior of the steel was measured by torsion testing in the temperature range of 700 to 820 °C with holding times ranging from 1 to 30 min. The flow stress curves presented different shapes and stress levels. These differences were attributed to the ferrite and pearlite, ferrite and austenite, and austenite strained structures. When ferrite and pearlite were deformed together, the flow stress presented a hump with little straining; when the austenitic structure was deformed the shape of the flow stress curve was typical of materials having low stacking fault energy. The microstructural evolution observed by optical and scanning electron microscopy revealed that the evolution of the phase transformation was dependent on the testing temperatures, holding times and amount of straining. Comparisons were made on the kinetics of phase transformation with and without the application of plastic deformation, and evidence of strain-induced dynamic transformation was investigated. <![CDATA[<b>Mechanical behavior of silver alloys used as cast post and core in maxillary incisors</b>]]> This experimental "in vitro" investigation is mainly concerned with the mechanical behavior of twenty plastic analogous simulating endodonticaly treated maxillary central incisor roots, divided in two groups, which were restored with: (i) silver (10 specimens, Ag - 80%); and (ii) silver - palladium (10 specimens, Ag - 58.5% / Pd - 27.4%) cast alloys. All the restored analogous were clamped inside stainless steel cylinders filled with acrylic resin, and were tested to failure in an INSTRON machine. The static compressive loads were applied with an inclination of 45° relatively to the longitudinal direction of the teeth. On average, the specimens restored with Ag- Pd presented higher values of final failure load (i.e. fracture or excessive plastic bending) and stiffness. On the other hand, those restored with Ag presented average proportional limit loads which were, in comparison, about 5% higher. <![CDATA[<b>Low carbon content NiTi shape memory alloy produced by electron beam melting</b>]]> Earlier works showed that the use of electron beam melting is a viable process to produce NiTi shape memory alloy. In those works a static and a semi-dynamic processes were used producing small shell-shaped and cylindrical ingots respectively. The main characteristics of those samples were low carbon concentration and good composition homogeneity throughout the samples. This paper presents the results of scaling up the ingot size and processing procedure using continuous charge feeding and continuous casting. The composition homogeneity was very good demonstrated by small variation in martensitic transformation temperatures with carbon content around 0.013wt% compared to 0.04 to 0.06wt% of commercial products. <![CDATA[<b>Plasma sintering of unalloyed iron</b>: <b>a study of surface porosity</b>]]> Samples of unalloyed iron powder were compacted and sintered in an abnormal glow discharge, generated in a gas mixture of 80% Ar + 20% H2 by using a pulsed power source. The samples were placed on a holder, acting as the discharge cathode, and were heated by the bombardment of ions, strongly accelerated in the cathode sheath. Sintering was performed at temperatures of 1173, 1273 and 1373 K for 30 min, varying the voltage applied to the cathode from 400 to 700 V and pressure ranging from 470 to 2650 Pa. It is shown that the kinetic energy of ions striking the sample surface increased approximately three times, when the voltage changed from 400 to 700 V, with a corresponding reduction of surface porosity. The surface sealing is related to the ion bombardment, which produced a high mobility of surface atoms and consequent enhanced diffusion as well as sputtering and condensation on the concave surface, resulting in an activation of surface sintering. <![CDATA[<b>Evaluation of the biocompatibility of a new biomembrane</b>]]> Biocompatibility has been considered one of the most important items to validate a biomaterial for its application in human organisms. The present work evaluates the biocompatibility of a new biomembrane using in vivo assay in different animal species. The experiments to evaluate the cellular reaction were carried out through the implantation of the material into the subcutaneous tissue of animals and the results showed a good reaction of the host tissue without any signal of fibrosis or rejection. The cell adhesion experiments were done by means of the measure of the DNA content on the material surface after its implantation into the subcutaneous tissue of animals and the results showed a growing number of DNA that was proportional to the time of implantation. The healing process was evaluated using a dermal ulcer model and the results showed a good tissue repair resembling a physiologic process. The overall results presented here lead to the conclusion that this new biomembrane is a biocompatible material but more research must be done, as it is a new material desired for medical use. <![CDATA[<b>Characterization of growth sectors in synthetic quartz grown from cylindrical seeds parallel to [0001] direction</b>]]> In the present study, the morphology and the impurity distribution were investigated in growth sectors formed around the [0001] axis of synthetic quartz crystals. Plates containing cylindrical holes and cylindrical bars parallel to [0001] were prepared by ultrasonic machining and further used as seed-crystals. The hydrothermal growth of synthetic quartz was carried out in a commercial autoclave under NaOH solution during 50 days. The morphologies of crystals grown from cylindrical seeds were characterized by X-ray diffraction topography. For both types of crystals, +X- and X- growth sectors were distinctly observed. Infrared spectroscopy and ionizing radiation were adopted to reveal the distribution of point defects related to Si-Al substitution and OH-species. It was found a different distribution of Al-related centers in relation to the crystals grown from conventional Y-bar and Z-plate seeds. <![CDATA[<b>Evaluation of the adhesion strength of diamond films brazed on K-10 type hard metal</b>]]> The coating of cutting tools with diamond films considerably increases the tool performance due to the combination of the unique tribological properties of diamond with the bulk properties of the substrate (toughness). The tool performance, however, is strongly related to the adhesion strength between the film and the substrate. In this work our main goal was to propose and to test a procedure, based on a tensile strength test, to evaluate the adhesion strength of a diamond wafer brazed on a hard metal substrate, taking into account the effect of the brazing temperature and time. The temperature range studied was from 800 to 980 °C and the brazing time ranged from 3 to 40 min. The obtained results could be used to optimize the costs and time required to the production of high performance cutting tools with brazed diamond wafers. <![CDATA[<b>Calcium phosphate formation on alkali-treated titanium alloy and stainless steel</b>]]> Alternatives to the plasma spraying method have been developed to obtain calcium phosphate coatings, like the biomimetic method. This process is a physicochemical method in which a substrate is soaked in a solution that simulates the physiological conditions, for a period of time enough to form a desirable layer of the calcium phosphate on the substrate. The titanium substrate usually investigated in the literature is subjected to an alkali treatment to induce the calcium phosphate formation and improve adhesion of the coating. The goals of this work are to compare the effect of alkaline treatments on two substrates titanium alloy and stainless steel, usually used for implants and orthopedic prostheses. The metallic substrates were treated with NaOH 5N at 60 °C for 24 h and NaOH 20N at 90 °C for 30 min. The samples were immersed in simulated body fluid for 3 days and in a solution with a higher calcium concentration for another three days. The modified substrates and coatings were characterized using profilometry, scanning electron microscopy and X-ray diffraction analysis. The alkaline treatment modified the characteristics of both substrates and allowed the nucleation a calcium phosphate film. <![CDATA[<b>Coupled stress-strain and electrical resistivity measurements on copper based shape memory single crystals</b>]]> Recently, electrical resistivity (ER) measurements have been done during some thermomechanical tests in copper based shape memory alloys (SMA's). In this work, single crystals of Cu-based SMA's have been studied at different temperatures to analyse the relationship between stress (s) and ER changes as a function of the strain (e). A good consistency between ER change values is observed in different experiments: thermal martensitic transformation, stress induced martensitic transformation and stress induced reorientation of martensite variants. During stress induced martensitic transformation (superelastic behaviour) and stress induced reorientation of martensite variants, a linear relationship is obtained between ER and strain as well as the absence of hys teresis. In conclusion, the present results show a direct evidence of martensite electrical resistivity anisotropy. <![CDATA[<b>Effect of light intensity and irradiation time on the polymerization process of a dental composite resin</b>]]> Polymerization shrinkage is a critical factor affecting the longevity and acceptability of dental composite resins. The aim of this work was to evaluate the effect of light intensity and irradiation time on the polymerization process of a photo cured dental composite resin by measuring the Vickers hardness number (VHN) and the volumetric polymerization shrinkage. Samples were prepared using a dental manual light-curing unit. The samples were submitted to irradiation times of 5, 10, 20 and 40 s, using 200 and 400 light intensities. Vickers hardness number was obtained at four different moments after photoactivation (immediate, 1 h, 24 h and 168 h). After this, volumetric polymerization shrinkage values were obtained through a specific density method. The values were analyzed by ANOVA and Duncan's (p = 0.05). Results showed increase in hardness values from the immediate reading to 1 h and 24 h readings. After 24 h no changes were observed regardless the light intensities or activation times. The hardness values were always smaller for the 200 light intensity, except for the 40 s irradiation time. No significant differences were detected in volumetric polymerization shrinkage considering the light intensity (p = 0.539) and the activation time (p = 0.637) factors. In conclusion the polymerization of the material does not terminate immediately after photoactivation and the increase of irradiation time can compensate a lower light intensity. Different combinations between light intensity and irradiation time, i.e., different amounts of energy given to the system, have not affected the polymerization shrinkage. <![CDATA[<b>Comparison of two different synthesis methods of perovskites, SrCo<sub>0.5</sub>FeO<sub>3</sub> type, aiming at evaluating their use as membranes for partial oxidation of methane</b>]]> In this work two different synthesis methods of perovskites, SrCo0.5FeO3, were compared: combustion synthesis and oxides mixture aiming at evaluating their use as membranes for partial oxidation of methane. The combustion synthesis method explores an exothermic, generally very fast and self-sustaining chemical reaction between the desired metal salts and a suitable organic fuel, which is ignited at a temperature much lower than the actual phase formation temperature. The oxides mixture are based on a physical mixture of the powder oxides followed by calcination to obtain the desired phase. In order to obtain the membranes, we studied the conformation of bodies and the temperatures of sintering in the two powders synthesized. The powders were analyzed by density and grain size distribution and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). After conformation, in cylindrical form, the green bodies were analyzed by density. After sintering at 1150 °C, the membranes were analyzed by density and they were characterized by XRD and SEM. The powder obtained by combustion synthesis shows lower density and fine grains than the other obtained by oxides mixture. The membranes obtained present very different morphology depending on the precursor powder synthesis. The sintered membranes obtained by combustion method also present a very uniform morphology without segregation. <![CDATA[<b>High frequency dielectric relaxation in lanthanum modified PbTiO<sub>3</sub> ferroelectric ceramics</b>]]> Dielectric measurements of lanthanum modified lead titanate ceramics were carried out in a frequency and temperature range of 70 ´ 10(6)Hz - 2 ´ 10(9)Hz and 300 K - 420 K, respectively. Dielectric relaxation appears around 7 ´ 10(8)Hz at room temperature, and the maximum of the dielectric relaxation was around 393 K, the transition temperature (Tc). The relaxation frequency passes through a minimum at Tc and the dielectric relaxation exists in both paraelectric and ferroelectric phases. A fitting of the experimental data with the Debye relations gives some keys for understanding the dielectric behavior of such materials. <![CDATA[<b>Synthesis, characterization and determination of the metal ions adsorption capacity of cellulose modified with p-aminobenzoic groups</b>]]> This paper describes the synthesis and characterization of p-aminobenzoic modified cellulose (Cell-PAB), as well as the separation and preconcentration of Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) ions from an aqueous medium using a Cell-PAB bath and the column technique. The quantity of p-aminobenzoic groups attached to the cellulose surface, i.e., 1.81±0.04 mmol/g, was determined by elemental analysis. The FT-IR spectra of Cell-PAB revealed characteristic bands of p-aminobenzoic groups, indicating the desired functionalization of the cellulose surface. The adsorptive capacities for the metals (Ns) Cd, Cu, Ni, Pb and Zn (Ns) were, respectively, 1.72 ± 0.02, 1.96 ± 0.02, 1.88 ± 0.04, 2.01 ± 0.03 and 1.93 ± 0.03 mmol/g. Metal separations performed in flow experiments using a column packed with 1 g of Cell-PAB and 5 mL of 1.5 mol/L HCl solution as eluent achieved an almost 100% metal ions recovery rate. <![CDATA[<b>Influence of high sintering pressure on the microhardness and wear resistance of diamond powder and silicon carbide-based composites</b>]]> The work reported on here involved the development of several samples of "diamond-SiC" composite produced under sintering pressures of up to 9.0 GPa at temperatures of up to 1973 7K. The average size of the diamond micropowder crystals used was 40/28 µm. The sintering process was carried out in a 2500-ton hydraulic press equipped with an anvil-type high-pressure device having a toroidal work surface and a central concavity diameter of 20 mm. The microhardness and wear resistance of the samples were found to be dependent on the sintering pressure. The experimental results indicated that the maximum microhardness and minimum wear resistance coefficients of each compact were attained when the pressure applied during sintering exceeded 6.5 GPa. Based on the established values of pressure, this study served to identify the types of devices applicable for the manufacture of composite material inserts for a variety of rock drilling applications. <![CDATA[<b>The role of carbon contamination in metallic nanowires</b>]]> Metallic nanowires have attracted much attention in the last years due to new phenomena such as quantum conductance and the existence of unexpected long interatomic distances attaining 0.3-0.5 nm. These large distances represented a challenge for physical interpretation. In this work we present experimental data from high-resolution transmission electron microscopy and results from ab initio calculations for suspended gold chains and show that these large distances can be easily explained by the presence of carbon atoms as contaminants. In principle the present conclusions can be also applied to other metallic nanowires (such as Ag and Pt) whose structures also present large interatomic distances. <![CDATA[<b>The use of niobium based catalysts for liquid fuel production</b>]]> The catalytic properties of niobium based catalysts were investigated in the conversion of oleic acid to liquid fuels at atmospheric pressure and at 623 K. The catalytic tests were performed in a fixed bed and continuous flow reactor using an acid to catalyst ratio equal to 4 and N2 as carrier gas. The reaction products were analyzed by gas chromatography and acidity measurements. NH3 temperature programmed desorption, N2 adsorption-desorption (BET method) and Xray diffraction were also performed in order to determine the structural and acidic properties of the catalysts. From the catalytic tests, it was detected the formation of compounds in the range of gasoline, diesel and lubricant oils. Higher catalytic activity and selectivity for diesel fuel were observed for the catalysts NbOPO4 and H3PO4/Nb2O5 that possesses higher acidities and surface areas. <![CDATA[<b>Mathematical modeling and experimental analysis of the hardened zone in laser treatment of a 1045 AISI steel</b>]]> The aim of this work is to develop a mathematical model to predict the depth of laser treated zone in the LTH process. The Fourier equation of heat conduction is solved by using the Finite Difference Method in cylindrical coordinates in order to study the temperature distribution produced in a workpiece and hence to obtain the depth to which hardening occurs. The theoretical simulations are compared with results produced experimentally by a CO2 laser operating in continuous wave, showing good agreement. <![CDATA[<b>Reentrant spin glass behavior in polycrystalline La<sub>0.7</sub>Sr<sub>0.3</sub>Mn<sub>1-X</sub>Fe<sub>X</sub>O<sub> 3</sub></b>]]> The magnetic and transport properties of the compound La0.7Sr0.3Mn1-xFe xO 3 (0.1 < x <0.4) have been studied by means of electrical resistivity, AC magnetic susceptibility, and DC magnetization. At low concentrations (x <0.1), the system displays essentially para-to-ferromagnetic transitions as the temperature is decreased, although a decrease in the magnetic moment has been observed in previous studies at temperatures a little below T C. This ferromagnetism is explained by double exchange theory in terms of the formation of Mn+3/Mn+4 ions pairs in the system. At concentrations in the range 0.1 < x <0.4 the system is more complex. Increased Fe doping not only weakens the ferromagnetic (FM) order and augments the resistivity of the samples, but also induces the appearance of a reentrant spin glass phase at low temperatures (T < 60 K). Irreversibility of the magnetization measured with zero field cooling and with field cooling has been observed. In addition, the AC susceptibility peak position varies with frequency. All these effects are characteristic of spin glass behavior. The results have been interpreted based in an increase of frustration due to increasing competition between FM Mn+3/Mn+4 interactions and antiferromagnetic interactions between ions at the boundaries of Fe clusters. <![CDATA[<b>Thermal decomposition of illite</b>]]> The effect of heat treatment on illite in air at temperatures ranging from 750 to 1150 °C was studied using the Mössbauer effect in 57Fe. The dependence of the Mössbauer parameters and relative percentage of the radiation absorption area was measured as a function of the firing temperature. The onset of thermal structural decomposition occurred at 800 °C. With rising temperature, the formation of hematite (Fe2O3) increased at the expense of the silicate mineral. <![CDATA[<b>Influence of the crystallization thermal treatment on the structural and electrical properties of PZT thin films</b>]]> Different methods have been proposed in the literature to deposit PZT thin films. Among these, chemical based processes have been revealed as a promising preparation route due to their low cost processing and the facility for controlling the stoichiometry of complex systems. The objective of this work was to investigate the influence of the time and temperature of crystallization on the physical properties of the Pb(Zr0.53Ti0.47)O3 thin films prepared by a hybrid method. The structural and microstructural dependence on the crystallization conditions of the PZT films were investigated by X-ray diffraction and atomic force microscopy. Dielectric and ferroelectric properties were also characterized and their behaviors were related to the structural evolution of films. <![CDATA[<b>Temperature coefficient of piezoelectric constants in Pb(Mg<sub>1/3</sub> Nb<sub>2/3</sub>)O<sub>3</sub> - PbTiO<sub>3</sub> ceramics</b>]]> In this work, the thermal stability of piezoelectric constants of PMN-PT ceramics in the tetragonal and rhombohedral phases were investigated in a wide range of temperatures. The results showed that the tetragonal PMN-PT presented higher thermal stability and, consequently, the temperature coefficients for the piezoelectric constants were approximately zero. This result revealed to be much better than that commonly found for PZT ceramics. Although the rhombohedral PMN-PT presented a slight lower thermal stability, the values found for the coupling factor were significantly higher than the tetragonal composition. <![CDATA[<b>Determination of composition of pozzolanic waste mixtures with optimized compressive strength</b>]]> The utilization of ceramic wastes with pozzolanic properties along with other compounds for obtaining new materials with cementating properties is an alternative for reducing the environmental pollution. The acceptance of these new products in the market demands minimal changes in mechanical properties according to its utilization. For a variable range of compositional intervals, attempts were made to establish limiting incorporation proportions that assure the achievement of minimum pre-established mechanical strength values in the final product. In this case minimum compressive strength value is 3,000 kPa. A simultaneous association of other properties is also possible. <![CDATA[<b>Erratum</b>]]> The utilization of ceramic wastes with pozzolanic properties along with other compounds for obtaining new materials with cementating properties is an alternative for reducing the environmental pollution. The acceptance of these new products in the market demands minimal changes in mechanical properties according to its utilization. For a variable range of compositional intervals, attempts were made to establish limiting incorporation proportions that assure the achievement of minimum pre-established mechanical strength values in the final product. In this case minimum compressive strength value is 3,000 kPa. A simultaneous association of other properties is also possible.