Scielo RSS <![CDATA[Materials Research]]> vol. 10 num. 3 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<B>Materials research - Ibero-American Journal of Materials</B>: <B>judicious, fair and educative!</B>]]> <![CDATA[<B>Preparation and characterization of thermoplastic starch/zein blends</B>]]> Blends of starch and zein plasticized with glycerol were prepared by melting processing in an intensive batch mixer connected to a torque rheometer at 160 &deg;C. The resulting mixtures were compression molded and then characterized by scanning electron microscopy, differential scanning calorimetry, wide-angle X ray diffraction and water-absorption experiments. The blends were immiscible, showing two distinct phases of starch and zein. The water uptake at equilibrium and its diffusion coefficient were determined. The water uptake at equilibrium decreased with increasing zein content. The diffusion coefficient fell sharply on addition of 20% zein and remained constant as zein content was increased. No appreciable effect of zein on starch crystallization was observed by X ray diffraction. The use of zein in thermoplastic starch compositions causes a decrease in the water sensitivity of these materials and lower its melt viscosity during processing making zein a suitable and very promising component in TPS compositions. <![CDATA[<B>Assessement of tensile strength of graphites by the Iosipescu coupon test</B>]]> Polycrystalline graphites are widely used in the metallurgical, nuclear and aerospace industries. Graphites are particulated composites manufactured with a mixture of coke with pitch, and changes in relative proportions of these materials cause modifications in their mechanical properties. Uniaxial tension tests must be avoided for mechanical characterization in this kind of brittle material, due to difficulties in making the relatively long specimens and premature damages caused during testing set-up. On other types of tests, e.g. bending tests, the specimens are submitted to combined stress states (normal and transverse shear stresses). The Iosipescu shear test, is performed in a beam with two 90&deg; opposite notches machined at the mid-length of the specimens, by applying two forces couples, so that a pure and uniform shear stress state is generated at the cross section between the two notches. When a material is isotropic and brittle, a failure at 45&deg; in relation to the beam long axis can take place, i.e., the tensile normal stress acts parallel to the lateral surface of the notches, controls the failure and the result of the shear test is numerically equivalent to the tensile strength. This work has evaluated a graphite of the type used in rocket nozzles by the Iosipescu test and the resulted stress, ~11 MPa, was found to be equal to the tensile strength. Thus, the tensile strength can be evaluated just by a single and simple experiment, thus avoiding complicated machining of specimen and testing set-up. <![CDATA[<B>Thermal characterization of commercially pure titanium for dental applications</B>]]> Thermal characterization of commercially pure titanium was carried out in dry air to investigate the oxidation kinetics, the oxide structures and their properties. Oxidation kinetics were performed by thermogravimetry in isothermal conditions between 300 and 750 &deg;C for 48 hours and the oxide structures were studied by differential thermal analyses and X ray diffraction between room temperature and 1000 &deg;C. The oxidation kinetic increases with temperature and is very fast in the initial period of oxidation, decreasing rapidly with time, especially up to 600 &deg;C. Kinetic laws varied between the inverse logarithmic for the lower temperatures (300 and 400 &deg;C) and the parabolic for the higher temperatures (650, 700 and 750 &deg;C). Evidences from X ray diffraction and differential thermal analyses showed that crystallization of the passive oxide film, formed at room temperature, into anatase occurs at about 276 &deg;C. The crystallized oxide structure is composed of anatase between 276 and 457 &deg;C, anatase and rutile sublayers between 457 and 718 &deg;C, and a pure layer of rutile after 718 &deg;C. Rockwell-C adhesion tests reveled that the oxide films formed up to 600 &deg;C have a good adhesion. Vickers indentations on the oxidized surfaces showed that the hardness of the oxide film, measured at 600 and 650 &deg;C, is approximately 9500 MPa. At these temperatures the surface roughness varied between 0.90 and 1.30 mm. <![CDATA[<B>Characterization of PVDF/HAP composites for medical applications</B>]]> Biomaterials (composites and blends) play a major role in the health of modern society. This paper reports on the preparation and characterization of polyvinylidene fluoride (PVDF) and hydroxyapatite (HAP) composites, analyzing the incorporation of HAP in PVDF and investigating their mechanical properties and cytotoxicity (biocompatibility) for use in bone restoration and filling. The material was prepared in film form by the casting method. PVDF pellets were dissolved in dimethylacetamide (DMA), a HAP/DMA emulsion was prepared. The materials were mixed in proportions of 100/00, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70 in weight and left to dry in an oven, resulting in homogeneous, flexible films which were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X ray diffractometry (XRD), contact angle measurement, and by mechanical and cytotoxicity tests. <![CDATA[<B>Influence of different light-curing units on the surface roughness of restorative materials</B>: <B>in situ study</B>]]> The aim of this study was to evaluate the influence of different light sources (LED and Halogen lamp) on the roughness (superficial) of composite resin (Filtek Z250, Filtek P60, Charisma and Durafill) varying post-irradiation times, in an in situ experiment. For this purpose, 80 specimens were made in polyurethane moulds. Ten volunteers without medicament use and good oral condition were selected and from them study moulds were obtained. A palatal intra-oral acrylic resin appliance was made for each of the subjects of the experiment. In each appliance, two specimens of each material were fixed (LED/Halogen lamp - control group). Roughness tests were performed immediately and 30 days after initial light-curing. Statistical analysis was performed using ANOVA. Statistically significant difference was observed only between post-irradiation times, where the 30th day showed the highest roughness values. It be concluded that roughness was influenced only by post-irradiation times, presenting the 30- days period inferior behavior. <![CDATA[<B>Synthesis and characterization of polyesters derived from glycerol and phthalic acid</B>]]> The production of polyester via polycondensation between glycerol and phthalic acid using dibutyltin dilaurate is reported. Three glycerol:phthalic acid molar ratio used for the bulk polymerization were: 2:2; 2:3 and 2:4. FTIR confirmed the esterification of glycerol by the acid for all the polymers. DSC indicated no crystallinity, although the XRD plots indicate a very incipient crystallinity for the polymers containing higher amounts of phthalic anhydride. Scanning electron microscopy results indicates high homogeneity for all the polymers prepared. <![CDATA[<B>Synthesis and characterization of bismuth zinc niobate pyrochlore nanopowders</B>]]> Bismuth zinc niobate pyrochlores Bi1.5ZnNb1.5O7 (alpha-BZN), and Bi2(Zn1/3Nb2/3)2O 7 (beta-BZN) have been synthesized by chemical method based on the polymeric precursors. The pyrochlore phase was investigated by differential scanning calorimetry, infrared spectroscopy, and X ray diffraction. Powder and sintered pellets morphology was examined by scanning electron microscopy. The study of alpha-BZN phase formation reveals that, at 500 &deg;C, the pyrochlore phase was already present while a single-phased nanopowder was obtained after calcination at 700 &deg;C. The crystallization mechanism of the beta-BZN is quite different, occurring through the crystallization of alpha-BZN and BiNbO4 intermediary phases. Both compositions yielded soft agglomerated powders. alpha-BZN pellets, sintered at 800 &deg;C for 2 hours, presented a relative density of 97.3% while those of beta-BZN, sintered at 900 &deg;C for 2 hours, reached only 91.8%. Dielectric constant and dielectric loss, measured at 1 MHz, were 150 and 4 x/10-4 for a-BZN, and 97 and 8 x 10-4 for beta-BZN. <![CDATA[<B>Adhesiveness of cold rolled steels for car body parts</B>]]> The aim of this work was to evaluate the adhesiveness of uncoated and zinc-electrogalvanized steel sheets used in the automotive industry. Three types of adhesives, one acrylic and two epoxy resins, were employed to join low carbon cold rolled steels, one uncoated and another electrogalvanized, both previously degreased or chemically pickled. Mechanical strength of the joints was evaluated by the T-peel and tensile strength tests. Steel grade, surface condition and heating below the cure temperatures did not influence the joints' mechanical strength. However, their shear strength decreased drastically as the test temperature increased. The exposure of the joints to an atmosphere with 90% relative humidity at 40 &deg;C caused reduction of their shear strength. Epoxy adhesives showed higher mechanical strength, but exhibited higher degradation by humidity. <![CDATA[<B>Magnetic properties of Pr-Fe-Co-B bonded HDDR magnets with alloying additions</B>]]> Microstructures and magnetic properties of Pr-Fe-Co-B bonded magnets were investigated. The magnets can be represented by the formulae, Pr14Fe63.9Co16B6M 0.1 (M = Ti, V, Cr, Ni, Zr, Nb or Mo), Pr14Fe63.8Co16B6Nb 0.1T0.1 (T/= Al, Si, P, Cu or Ga) and Pr14Fe63.6Co16B6Nb 0.1R0.3 (R = Gd, Tb or Dy). The effects of additions on the magnetic properties of PrFeCoB-based magnets have been studied. Magnetically hard powders have been produced from homogenised alloys using the hydrogenation, disproportionation, desorption and recombination (HDDR) process. The HDDR powders were isostatically pressed and bonded with cyanoacrylate adhesive to form permanent magnets. <![CDATA[<B>High temperature oxidation resistance of rare earth chromite coated Fe-20Cr and Fe-20Cr-4Al alloys</B>]]> Doped lanthanum chromite has been used in solid oxide fuel cell (SOFC) interconnects. The high costs involved in obtaining dense lanthanum chromite have increased efforts to find suitable metallic materials for interconnects. In this context, the oxidation behavior of lanthanum chromite coated Fe-20Cr and Fe-20Cr-4Al alloys at SOFC operation temperature was studied. Isothermal oxidation tests were carried out at 1000 &deg;C for 20, 50 and 200 hours. Cyclic oxidation tests were also carried out and each oxidation cycle consisted of 7 hours at 1000/&deg;C followed by cooling to room temperature. The oxidation measurements and the results of SEM/EDS as well as XRD analyses indicated that lanthanum chromite coated Fe-20Cr and Fe-20Cr-4Al alloys were significantly more resistant to oxidation compared with the uncoated alloys. <![CDATA[<B>Chemical changes of titanium and titanium dioxide under electron bombardment</B>]]> The electron induced effect on the first stages of the titanium (Ti0) oxidation and titanium dioxide (Ti4+) chemical reduction processes has been studied by means of Auger electron spectroscopy. Using factor analysis we found that both processes are characterized by the appearance of an intermediate Ti oxidation state, Ti2O3 (Ti3+). <![CDATA[<B>Age-hardening of Ti-35Nb-7Zr-5Ta alloy for orthopaedic implants</B>]]> Bone injuries and failures often require the inception of implant biomaterial. Research in this field has received increasing attention recently. In particular, porous metals are attractive due to its unique physical, mechanical, and new bone tissue ingrowth properties. The aim of this work is to investigate age hardening behavior of Ti-35Nb-7Zr-5Ta alloy produced by powder metallurgy. Samples of Ti-35Nb-7Zr-5Ta were produced from a mixture of hydrided powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering at 1500 &deg;C, in vacuum. After that, the samples were heat treated in three steps; solution treatment (1100/&deg;C), air cooling and aging (600 &deg;C for 4 and 6 hours). Sintered samples were characterized for microstructure and microhardness by scanning electron microscopy and Vickers indentation, respectively. In the homogenized condition alloy exhibited a microstructure consisting primarily of a beta Ti matrix with grain boundary alpha precipitates and a low volume fraction of primary alpha precipitates. In the ageing condition the alloy exhibited the precipitation of refined scale secondary alpha precipitates distributed homogeneously in the beta matrix. <![CDATA[<B>The effect of polarisation on the electrochemical behavior of Ti-13Nb-13Zr alloy</B>]]> The effect of potentiostatic polarisation on the electrochemical behavior of the Ti-13Nb-13Zr alloy was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation curves in Hanks' solution at 37 &deg;C. Potentiodynamic polarisation curves show a passive behavior with a slight current increase as potentials around 1300 mV(SCE). Based on these curves, different potentials were chosen to perform potentiostatic EIS experiments. EIS experimental data were interpreted using different equivalent circuits associated with the duplex nature of the oxide layer. The fitting procedure evidenced the thickening of a defective oxide layer with the applied potentials, corresponding to key points in the potentiodynamic polarisation curves. <![CDATA[<B>Polyaniline and mineral clay-based conductive composites</B>]]> Composite materials have attracted the attention of numerous researchers due to the distinct properties shown by this class of materials and the mineral clay used in their synthesis. In this study, conductive composites were obtained by mixing polyaniline (PAni) with clay (kaolinite and montmorillonite). The aniline was polymerized in a medium with clay and the powder was characterized by X ray diffraction, electrical conductivity and morphology. The results suggest PAni chain linearization in a kaolinite medium. The addition of montmorillonite resulted in PAni chain linearization and intercalation of mineral clay, although the montmorillonite was not pretreated. The PAni-clay composites showed an electrical conductivity of 0.01, which appeared not to be influenced by the amount of clay used. <![CDATA[<B>Evaluation of the thermal comfort of ceramic floor tiles</B>]]> In places where people are bare feet, the thermal sensation of cold or hot depends on the environmental conditions and material properties including its microstructure and crustiness surface. The uncomforting can be characterized by heated floor surfaces in external environments which are exposed to sun radiation (swimming polls areas) or by cold floor surfaces in internal environments (bed rooms, path rooms). The property named thermal effusivity which defines the interface temperature when two semi-infinite solids are putted in perfect contact. The introduction of the crustiness surface on the ceramic tiles interferes in the contact temperature and also it can be a strategy to obtain ceramic tiles more comfortable. Materials with low conductivities and densities can be obtained by porous inclusion are due particularly to the processing conditions usually employed. However, the presence of pores generally involves low mechanical strength. This work has the objective to evaluate the thermal comfort of ceramics floor obtained by incorporation of refractory raw materials (residue of the polishing of the porcelanato) in industrial atomized ceramic powder, through the thermal and mechanical properties. The theoretical and experimental results show that the porosity and crustiness surface increases; there is sensitive improvement in the comfort by contact. <![CDATA[<B>A study of the consolidation method with albumin to obtain porous <FONT FACE=Symbol>b</FONT>-TCP ceramics</B>]]> In many 'in-vivo' and 'in-vitro' studies, the behavior of calcium phosphate ceramics like beta - tricalcium phosphate in biological environments has been reported to be predictive and positive. In terms of bone tissue growth, these ceramics are attractive biomaterials due to their porous microstructures. To obtain biomaterial quality ceramics, in this investigationbeta- TCP porous ceramics were prepared by a special consolidation method with albumin as a foam generating agent. This technique enables a variety of formats with complex geometries to be prepared. To obtain porous samples using albumin, heat had to be introduced into the system during the consolidation stage. After consolidation, the samples were sintered at 1250 &deg;C for 30 minutes and characterized using X ray diffractometry, scanning electron microscopy and mercury porosimetry. The foams that were obtained by this method exhibited spherical and interconnected pores, characteristics desirable in biomedical implants. <![CDATA[<B>A study of high-energy milling for the production of sintered PrFeB magnets</B>]]> Sintered Pr16Fe76B8 magnets have been produced using the hydrogen decrepitation (HD) process and high-energy planetary ball milling. Investigations have been carried out to evaluate the influence of the milling speed and time. The best magnetic properties obtained were Br = (1020 &plusmn; 20) mT, ยต0iHc = (1420 &plusmn; 30) mT and (BH)max/= (200 &plusmn; 4) kJm-3, for a magnet prepared with the alloy milled at 200 rpm for 4.5 ks. Magnets prepared from this powder exhibited a superior intrinsic coercivity compared to that of magnets produced using low-energy ball milling. However, the remanence and energy product of the latter were somewhat lower. An important feature was the dramatic reduction in the processing time (about 90%). Microstructural observation have shown that increasing the milling time and keeping constant the rotational mill speed caused an exponential grain size reduction in the sintered magnet. Increasing the milling speed also reduced the grain size and influenced both remanence and intrinsic coercivity. <![CDATA[<B>Thermodynamic simulation as an assistant tool in the development of iron based hardfacing materials</B>]]> A thermodynamic model was used to investigate the solidification of a hardfacing material in Fe-Cr-C-Nb system designated CNO, and developed for cladding components subjected to severe abrasive wear by welding. Microstructural characterization of the alloy showed that the theoretical simulations carried out to predict the amount and volume fraction of the phases using the module Scheil of MT-DATA Software gave very realistic results in the case of the rapid solidification of the hardfacing alloy CNO. The results obtained in this study indicate that the utilization of this tool may provide a good understanding of alloys solidification. As a consequence, it is possible to enable the refinement of alloy composition during the early stages of hardfacing materials development, leading to a decreasing of overall cost and time of manufacturing of hardfacing electrodes. <![CDATA[<B>Cyclic oxidation resistance of rare earth oxide gel coated Fe-20Cr alloys</B>]]> Rare earths (RE) have been used to increase high temperature oxidation resistance of chromium dioxide and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as an oxide coating to the alloy surface. RE oxide gels, obtained by the sol-gel technique, were used to coat Fe-20Cr specimens prior to cyclic oxidation tests in the temperature range RT-900 &deg;C. The cyclic oxidation resistance of the alloy increased with time required to reach a specific chromium dioxide layer thickness, and this was influenced by RE ion radius, the coating morphology and coverage. Extended cyclic oxidation tests from peak temperatures of 900, 1000 and 1100 &deg;C at cooling rates of 330 and 1000 &deg;C/s revealed the marked influence of La2O3 coatings. The role of RE in increasing overall oxidation resistance of chromium dioxide forming alloys is discussed. <![CDATA[<B>Evaluation of radiopacity and microhardness of composites submitted to artificial aging</B>]]> The purpose of this study was to assess the radiopacity and microhardness of different types of resin-based composites (RBC - hybrid; microhybrid; flowable; cement and polyacid modified) before and after being submitted to artificial accelerated aging. Fifty specimens (7 mm in diameter and 2 mm thick) were fabricated, 10 for each material. The specimens were light-cured and submitted to radiopacity and microhardness tests. After obtaining initial radiopacity and microhardness values, the specimens were taken to the artificial accelerated aging, and new measurement of radiopacity and microhardness of the samples was performed. Data were submitted to statistical analysis (Student's t-test - p < 0.05). None of the materials studied showed changes in radiopacity after artificial accelerated aging. There was a significant decrease in microhardness for the microhybrid and polyacid-modified RBC&acute;s. This study suggests that radiopacity remains unaltered after materials are submitted to artificial accelerated aging. Microhardness, however, may vary due to plasticization of the aged RBC matrix.