Heberto Balmori-Ramírez

Mechanical activation of the synthesis reaction of BaTiO3 from a mixture of BaCO3 and TiO2 powders

Abstract The influence of long term milling in an attritor of a mixture of BaCO3 and TiO2 powders on the reaction synthesis of BaTiO3 was studied. Thermal analysis (TG and DTA) of the unmilled and milled powders and X-ray diffraction of powders calcined at different temperatures were undertaken. Milling does not change the reaction sequence between BaCO3 and TiO2. BaTiO3 and Ba2TiO4 are produced in both kinds of powders. However, milling reduces the formation temperature and accelerates the formation rate of these two phases. The production of final BaTiO3 is incomplete in the milled powder, probably because of the formation of big crystals of Ba2TiO4. As a consequence, the milled powder contains large amounts of Ba2TiO4 after calcination at 1200°C. Milling produces allotropic transformations in TiO2 from anatase phase, to α-PbO2-like phase and finally to rutile structure.

Colloidal processing of BaTiO3 using ammonium polyacrylate as dispersant

Abstract The variation of the ζ potential of BaTiO3 particles in aqueous suspension as a consequence of changing the pH and the concentration of ammonium polyacrylate (NH4PA) has been studied. An isoelectric point at pH ∼2.5 was found. The ζ potential decreases upon addition of NH4PA up to 0.1 g of dispersant/g of powder. Using sedimentation tests, it was found that the stabilization of suspensions is attained at pH greater than or equal to 8 and concentrations greater than or equal to 1.5×10−3 g of dispersant/g of powder. By rheological measurements, a pseudoplastic behavior and little thixotropy was observed in the suspensions. The maximum solid content attained during this work was 48 vol%. Infrared spectroscopy of the sedimented and dried powder was carried out. It was observed the presence of BaCO3 and the change of ionization of the NH4PA molecules when the pH changes. High green density values (up to ∼60% of the theoretical density) in BaTiO3 cakes were achieved at pH of 11.3, at 2.5×10−3 g of dispersant/g of powder and with a solid content between 40 and 48 vol%.

Fractional crystallization of oil droplets in O/W emulsions dispersed by Synperonic F127.

The aim of this works is to study an oil-in-water emulsion stabilized with a triblock copolymer Synperonic F127 which presents a double size distribution of oil droplets. The emulsions were studied experimentally by means of differential scanning calorimetry (DSC) and dynamic light scattering (DLS). The DSC analysis was carried out focusing on the cooling behavior of the emulsion. The cooling thermograms of the oil-in-water emulsion revealed two crystallization peaks with Gaussian profile; the interesting characteristic is that both peaks are separated in temperature. In accordance to previous works for a single oil dispersed within an aqueous phase, the DSC technique must show a single Gaussian peak of crystallization attributable to a size distribution of droplets. In the present case of emulsions stabilized with 1 g/L of Synperonic F127, the aggregation behavior of triblock as a function of temperature allows to produce an emulsion with a double size droplet distribution. Comparison with emulsions stabilized with 2 and 4 wt% of non-ionic Tween 20 are also presented.

Biological Compatibility of a Polylactic Acid Composite Reinforced with Natural Chitosan Obtained from Shrimp Waste

The aim of this work is to evaluate the effect of chitosan content (1, 3 and 5 wt %) dispersed in polylactic acid (PLA) on the structure and properties of composites. Also, the hydrolytic degradation, and the cell viability and adhesion of human MG-63 osteoblasts are analyzed to determine the composites’ suitability for use in tissue engineering. For the manufacture of the materials, natural chitosan was extracted chemically from shrimp exoskeleton. The composites were fabricated by extrusion, because it is a low-cost process, it is reproducible, and it does not compromise the biocompatibility of the materials. FT-IR and XRD show that the chitosan does not change the polymer structure, and interactions between the composite components are discarded. In vitro degradation tests show that the composites do not induce significant pH changes in phosphate buffer solution due to their low susceptibility to hydrolytic degradation. The adhesion and morphological characteristics of the osteoblasts are evaluated using confocal microscopy and scanning electron microscopy. The cell viability is determined by the MTT assay. Osteoblasts adhesion is observed on the surface of PLA and composites. A higher amount of chitosan, higher number of cells with osteoblastic morphology, and mineralized nodules are observed on the composite surface. The highest metabolic activity is evidenced at 21 days. The results suggest that the Polylactic acid/chitosan composites are potentially suitable for use as a biomaterial.

The Transformation of Co-Rich Alloys Produced by Mechanical Alloying

Elemental powder mixtures of Co and Ti were subjected to high-energy ball milling in order to produce mechanically alloyed powders with nominal compositions Co64Ti36, Co67Ti33, Co70Ti30, Co73Ti27, Co76Ti24 and Co85Ti15. The mechanically alloyed powders were treated during 30 minutes in inert atmosphere at temperatures in the range 300 – 700 °C. Both the as-milled powders as well as those subjected to heat treatments have been characterized by x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectrometry and differential thermal analysis. As-milled products consist mostly of agglomerated powders with a size between 10 and 80 µm which give an amorphous-like diffraction pattern, except for the Co85Ti15 sample whose pattern presents the characteristic peaks of the Co3Ti intermetallic phase. The transformation of the asmilled powders occurs at temperatures in the range of about 530 – 670 °C with clearly observed exothermic events. The Co3Ti phase is found in all heat treated samples, together with fcc-Co (in Co76Ti24 and Co85Ti15) or the hexagonal Co2Ti intermetallic phase (in Co64Ti36, Co67Ti33 and Co70Ti30); the Co73Ti27 sample was essentially single-phase Co3Ti after heating to 700 °C. Our results suggest the occurrence of crystallization of an amorphous phase in two overlapping stages during heating of the mechanically alloyed powders.

Thermal Analysis and Evolution of Phases during the Synthesis of Cordierite with Kyanite and Talc

Cordierite has been synthesized by heating mixtures of kyanite and talc, using kyanite powders of two different sizes. The reaction is studied by thermogravimetric and differential thermal analysis. Phase changes are followed by X-Ray diffraction of samples fired between 750 °C and 1400 °C. The evolution of the reaction starts with the dehydration of talc at approximately 880 °C. The kyanite decomposes at approximately 1140 °C. The first traces of α-cordierite appear at around 1200 °C. The influence of the kyanite particle size on the kinetics and microstructure is analyzed.