Dicalcium Phosphate Dihydrate Precipitation: Characterization and Crystal Growth

The precipitation of dicalcium phosphate dihydrate, brushite, by mixing a calcium hydroxide suspension and an orthophosphoric acid solution in equimolar quantities, has been investigated in a batch system at 25°C. The concentration of calcium hydroxide and orthophosphoric acid, before mixing, ranged from 50 to 400 mmol dm⁻³. The precipitation process occurs in five stages, being the hydroxyapatite, Ca₁₀(OH)₂(PO₄)₆, the first phase to be precipitated, as shown in a previous work. This present work deals with the phenomena occurring during the last stage, according to the model presented: brushite grows due to direct consumption of calcium in solution, and due to the transformation of HAP into brushite. An equilibrium and mass balance equations system is formulated describing the chemical species present in solution and the change of their concentrations along each run. This system was validated through conductivity measurements. From this system, the type and composition of precipitate is determined, and the evolution of particle size is estimated and compared with experimental results. The kinetics of brushite growth due to calcium consumption (determined from the decrease of calcium concentration) was studied. The results obtained in these studies allow validating the proposed model concerning the transformation of HAP and brushite growth.     

Influence of zirconium on microstructure and toughness of low-alloy steel weld metals

The influence of zirconium on microstructure and toughness of low-alloy steel weld metal was studied. Weld metals with different zirconium contents were obtained adding iron-zirconium alloy in the welding flux formulation. Weld metal chemical composition proved that zirconium was able to be transferred from the flux to the weld metal. The addition of zirconium refined the weld metal microstructure, increasing the acicular ferrite content. Weld metal toughness, determined by means of impact Charpy-V tests, showed that the zirconium addition is beneficial up to a content of 0.005 wt.%. Above this level, zirconium was not able to produce further microstructure refinement, although the toughness was reduced, possibly due to the formation of microconstituent such as the martensite-austenite constituent (M-A), which is considered to be deleterious to the weld metal toughness.     

GC/MS characterization of mate tea leaves extracts obtained from high-pressure CO2 extraction

This work presents a study concerning the chemical characteristics and analytical separation process of the essential oil obtained from high-pressure carbon dioxide extraction of Ilex paraguariensis. The experiments were performed in a laboratory-scale unit in the temperature range of 20–40 °C, from 100 to 250 bar. A blend of the I. paraguariensis extracts was percolated through a preparative chromatographic column, containing silica gel, and successively eluted with 150 mL of each of the following solvents: hexane, toluene, dichloromethane, ethyl acetate, acetone and methanol. The raw extract and its fractions were analyzed by gas chromatography coupled to mass spectrometry detection (GC/MS). The fractionation procedure showed to be a good clean up technique due to the isolation of different classes of compounds in each fraction. Chromatographic analyses allowed the identification of caffeine, fatty acids and esters, phytol, squalene, Vitamin E, stigmasterol derivatives and saturated hydrocarbons.     

Relationship between crystalline structure and mechanical behavior in isotropic and oriented polyamide 6

Polyamide 6 (PA6) isotropic films and oriented cables were prepared by compression molding or by consecutive extrusion and cold‐drawing. These samples were isothermally annealed in the 120–200°C range and were then subjected to tensile tests at room temperature. Synchrotron wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) patterns were obtained before and after mechanical failure. These data were related with the mechanical properties of the respective PA6 samples. The annealing of isotropic PA6 resulted in an increase in the Young's modulus (E) and yield stress (σ_y) values, which was attributed to the observed proportional reduction of the d‐spacings of the intersheet distances in both the α‐PA6 and γ‐PA6 polymorphs. Analysis of the WAXS and SAXS patterns of isotropic PA6 after break allowed the supposition of structural changes in the amorphous phase, with these being better pronounced with increasing annealing temperature; this made the samples less ductile. In oriented PA6 samples, annealing resulted in a drastic increase in the E and σ_y values accompanied by a phase transition from γ‐PA6 to α‐PA6 and a well‐pronounced reduction in the intersheet distances of both polymorphs. The stretching of the oriented samples led to an additional γ‐to‐α transition, whose extent was also related to structural changes in the amorphous phase. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2242–2252, 2007     

Study of the thermomechanical and electrical properties of conducting composites containing natural rubber and carbon black

This work describes the preparation and characterization of composite materials obtained by the combination of natural rubber (NR) and carbon black (CB) in different percentages, aiming to improve their mechanical properties, processability, and electrical conductivity, aiming future applications as transducer in pressure sensors. The composites NR/CB were characterized through optical microscopy (OM), DC conductivity, thermal analysis using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMA), thermogravimetry (TGA), and stress–strain test. The electrical conductivity varied between 10^?9 and 10 S m^?1, depending on the percentage of CB in the composite. Furthermore, a linear (and reversible) dependence of the conductivity on the applied pressure between 0 and 1.6 MPa was observed for the sample with containing 80 wt % of NR and 20% of CB. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Poly(vinyl alcohol)/Clay‐Based Nanocomposite Hydrogels: Swelling Behavior and Characterization

Polymer/clay composite hydrogels were prepared based on PVA hydrogels containing 3–10 wt.‐% MOM. Their microstructure and morphology were studied by FT‐IR, WAXS and SEM, whereas the interactions between MOM and PVA were evaluated by thermal analyses. The swelling ratios for the PVA/MOM hydrogels decrease with increasing MOM content. WAXS results indicate that MOM was intercalates, and DSC results show a strong interaction between PVA and MOM. This interaction results in a stable network, which is confirmed by the elastic modulus and the thermal decomposition behavior of the hydrogels. Therefore, MOM acts as a co‐crosslinker, improving the stability of the network.

     

Properties of Triaxial Porcelain Bodies: Interpretation of Statistical Modeling

The effect of raw materials type and content on the properties of clay–feldspar–quartz compositions was studied using the statistical design of mixture experiments. Based on wall and floor tile industrial practice, 10 mixtures of three raw materials were selected and processed. Characterization results were used to calculate statistically significant and valid regression equations, relating fired body properties with clay, feldspar, and quartz contents in the unfired mixture. Such statistical modeling is discussed against quantitative X-ray diffraction and scanning electron microscopy results. The glassy phase present was found to control the microstructure and the mechanical strength of the fired ceramic body.     

Corrosion resistance of anodic oxides on the Ti-50Zr and Ti-13Nb-13Zr alloys

Electrochemical analyses on the biocompatible alloys Ti-50Zr and Ti-13Nb-13Zr, as-cast and heat-treated, in distinct electrolytes (simulating or not physiological media: pH 7, Ringer, PBS, phosphate buffer and Na₂SO₄ solutions; pH 1, H₂SO₄, HNO₃, CH₃SO₃H, HClO₄ and H₃PO₄ solutions) are reported. Analyses of the open-circuit potential values indicated that either as-cast or heat-treated samples of both alloys present the same tendency to spontaneously form an oxide film on their surfaces and that the oxide grown in the Na₂SO₄ solution presents better protection characteristics than those formed in the phosphate buffer, Ringer and PBS solutions, in this order. Cyclic voltammograms obtained in the Ringer and PBS solutions showed that the Ti-13Nb-13Zr alloy does not transpassivate up to potentials of 8 V (versus SCE), while the Ti-50Zr alloy presents corrosion at potentials lower than 2 V (versus SCE), indicating that this alloy is less resistant to corrosion in solutions simulating physiological conditions. The obtained pit potential values showed that the heat treatment has very little influence on the corrosion behavior of the alloys. Finally, both alloys did not present transpassivation at potentials up to 8 V (versus SCE) in the H₂SO₄, HNO₃, CH₃SO₃H and H₃PO₄ solutions; however, when immersed in the HClO₄ solution the Ti-50Zr alloy underwent pit corrosion while the Ti-13Nb-13Zr alloy remained corrosion resistant in this potential range.     

Novel all-solid-state copper(II) microelectrode based on a dithiomacrocycle as a neutral carrier

A novel dithiomacrocycle (4-phenyl-11-decanoyl-1,7-dithia-11-azacyclotetradecane-4-sulfide) has been synthesized and used as a new ionophore in order to develop a plasticized poly(vinyl chloride) membrane for copper ion detection. The performance of these novel planar copper(II)-selective potentiometric microelectrodes was investigated using potentiometric measurements. The developed microelectrodes exhibits a good linear response of 29.5 ± 1 mV per decade within the concentration range of 1.0 × 10⁻⁶ to 1.0 × 10⁻² M (r = 0.9995) of Cu²⁺. The detection limit was determined as 5.62 × 10⁻⁷ M and the selectivity coefficients for possible interfering cations were evaluated. The microelectrodes are suitable for use with aqueous solutions of pH 3.5-6.0 and were found to be insensitive to the nature of the anions used in the sample.     

The effect of cooling rate on the impact performance and dynamic mechanical properties of rotationally molded metallocene catalyzed linear low density polyethylene

This article examines changes to the morphology of rotationally molded metallocene catalyzed linear low density polyethylene brought about by varying the cooling rate during processing. These changes in morphology lead to variations in the impact performance, which is reflected in the dynamic mechanical characteristics of the materials. Various analytical techniques are used in an attempt to explain the differences in impact behavior. Slow cooling is shown to result in high crystallinity, and in the formation of large spherulites, which in turn is detrimental to the impact performance of the material, particularly at low temperatures. The high crystallinity corresponds with a shift in the β transition of the material to a higher temperature, and is shown to result in a higher brittle–ductile transition. A case study was also carried out on samples from a finished part provided by an industrial molder, one section of which failed in a brittle manner when impact tested while the other failed in a ductile manner. Microscopy results showed that the brittle material had large spherulites at the inside surface, while the ductile material showed incipient degradation at this surface, which has previously been shown to be of benefit to impact strength in rotationally molded parts. Dynamic mechanical studies again showed a β transition at a higher temperature in the brittle samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1963–1971, 2006