Materials Science Forum Vol. 755

Abstract: Pt-Pd bimetallic nanoparticles were characterized using aberration (Cs) corrected scanning transmission electron microscopy (STEM) along with molecular dynamics simulations. The nanoparticles were synthesized through a microwave-assisted process. This technique has been applied to synthesize metallic nanoparticles at relatively short times, allowing a good control of size distribution. The structure of the bimetallic nanoparticles is fcc-like with an average size of 5 nm. To understand the properties of the bimetallic nanoparticles, it is necessary to know the positions of all the atoms in the nanostructure. We have used a recent quantitative method to analyze HAADF STEM images which allowed us to measure the total intensity of the scattered electrons for each atomic column. Beside with the characterization of the nanoparticles, we have performed classical molecular dynamics simulation for the structural and dynamical analysis of the cuboctahedral Pt-Pd bimetallic nanoparticles.

Abstract: Highly ordered through-hole anodic porous alumina membranes were fabricated by electrochemical oxidation of aluminum in a controlled two-step process. A teflon dispositive was used to ensure single side anodization. Under the most appropriate condition for the fabrication of ideally ordered anodic aluminum oxide (AAO), the voltage used was 15 V during 24 h in a 15 % w/v sulfuric acid solution. SEM, TEM and FESEM characterization shows that the as-fabricated AAO film has a defect-free array of straight parallel channels perpendicular to the surface. The thickness of the porous membrane is 20 microns, approximately. The ordered channels are formed in a honey comb arrange with a pore diameter in the range 20-30 nm, wall thickness of 10-20 nm, interpore distance of 40 nm, and high aspect ratio of 850. The pore density, quantified by image analysis, is 5.4×10¹⁰ pore/cm²; perfect ordering was maintained in the full depth of the membrane. Dimensions of this porous structure provide a convenient way to precision engineer the nanoscale morphology.

Abstract: Structural and spectral characteristics of functionalized Au nanoparticles have been studied by electron microscopy, vibrational spectroscopy and quantum chemical methods. Gold nanoparticles were synthesized through a microwave induced reaction, starting with a two phase system using 1-dodecanethiol as passivating agent. The nanoparticles were structurally characterized using aberration-corrected scanning transmission electron microscopy (Cs-STEM) equipped with a high-angle annular dark field (HAADF) detector. The main structures of the nanoparticles were fcc-like and icosahedrons. Density functional theory (DFT) calculations, optimized geometries and vibrational spectra, have been carried out using 6-31G* basis sets and B3LYP functional. The experimental and theoretical infrared spectra of the functionalized Au nanoparticles show the main absorption peaks from 1-dodecanethiol, C-H stretch from CH₂ at 2917, 2848 and from CH₃ at 2954 and 2872 cm^-1, C-H from-CH₂ deformation at 1467 and from CH₃ at 1376 cm^-1. Infrared spectra of octanethiol and gold nanoparticles functionalized with octanethiol were also obtained. These nanostructures can be used in applications such as diagnostics, biosensing, therapeutics, and drug delivery.

Abstract: In this work, a spectroscopic and structural characterization of diatomaceous earth-montmorillonite clays after impregnated with silver nanoparticles were carried out. The silver nanoparticles were synthesized by chemical reduction with sodium borohydride starting from silver nitrate solution. The obtained nanoparticles were stabilized with polyvinyl-pyrrolidone as a surface agent. In order to perform the homogeneous nucleation process, Ag nanoparticles solutions at concentrations of 1, 2 and 4 parts per million were magnetically mixed in the porous material. Additionally, we assessed the porous material adsorption ability of silver by atomic absorption spectroscopy. The quantity of Ag nanoparticles adsorbed by the diatomaceous earth and the adsorption rate as function of the concentration of Ag nanoparticles were established. Other complementary techniques such as X-ray diffraction, infrared spectroscopy and transmission and scanning electron microscopy were used.

Abstract: In this study, a natural zeolite clinoptilolite-type was impregnated through homogeneous and heterogeneous nucleation with silver nanoparticles. The synthesis of Ag nanoparticles was carried out by chemical reduction of silver nitrate (AgNO₃) with sodium borohydride (NaBH₄). In the case of homogeneous nucleation, colloidal solution of Ag nanoparticles at concentrations of 1, 2 and 4 parts per million was added and magnetically mixed with the porous material. With respect to heterogeneous nucleation, a solution of clinoptilolite and silver nitrate (0.01 M) was prepared and stirred; subsequently, the reduction of Ag was possible due to the addition of an aqueous solution of sodium borohydride. For the structural characterization, transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared spectroscopy (IR) techniques were carried out. The results were compared and discussed in both types of nucleation.

Abstract: Fe₄₀Al₆₀ (at%) intermetallic alloy composition was obtained by conventional casting methods and subsequently subjected to high-energy mechanical milling under different conditions of humidity. All samples were characterized by X-ray diffraction patterns (XRD), transmission electron microcopy (TEM) and DSC-TGA thermogravimetric experiments. After the milling process, the amount of hydrogen generated was determined using thermogravimetric analysis and chemical reactions (stoichiometry). All techniques confirm the formation of bayerite phase which is attributed to the hydrogen embrittlement reaction between the intermetallic material and water to release hydrogen. It was observed that the hydrogen generation is increased as the ball milling time is increased. The quantity of hydrogen evaluated is similar to that obtained in previous reported experiments with pure aluminum and some of its alloys.

Abstract: The present research work is focused on the development and characterization of light weight microporous carbonaceous material, such as graphite (53150 micron) incorporated with either titanium n-butoxide, titanium diisopropoxide bis (2,4-pentanedionate), vanadium 2,4-pentane-dionate or zinc 2,4-pentanedionate having varying wt% of the metal (28%) using 2-propanol/ethanol as solvent at 4050 °C. The calcination has been carried out at 100, 150 and 200 °C, except the samples with titanium diisopropoxide bis (2,4-pentanedionate) which are calcined at 80 °C. FESEM along with atomic absorption studies revealed that the maximum incorporation of metals (Zn, V and Ti) in graphite has been observed with 4 wt% of zinc 2,4-pentanedionate calcined at 100 °C, 4 wt% of vanadium 2,4-pentanedionate calcined at 100 °C, 4 wt% of titanium n-butoxide calcined at 100 °C and 2 wt% of titanium diisopropoxide bis (2,4-pentanedionate) calcined at 80 °C, and equilibration time of 20-24 h has been used in each case. These samples may be used for hydrogen storage.

Abstract: This work presents the electrochemical corrosion results of the structural metals, aluminium (Al), brass and copper (Cu), immersed in coastal waters of Veracruz Port in Mexico at room temperature, atmospheric pressure and eight weeks of the exposition time. The electrochemical technique used was electrochemical noise (EN). A typical three-electrode electrochemical cell was used. Where the reference electrode was the silver/seawater (Ag/SW) and two nominally identical metallic samples were used as working electrodes (WE1 and WE2). The metallic samples of Al, brass and Cu were used as working electrode. The potential and current fluctuations were measured simultaneously between the two working electrodes (current measured) and the Ag/SW electrode (potential measured). The electrochemical noise measurements (ENM) were analysed by three different methods: Potential and current versus time (transients), Localization Index (LI) and Electrochemical Noise Resistance (R_n). The results shown a good correlation between the superficial analysis and the results obtained by the ENM. In addition, Cu presents the highest corrosion rate and, a corrosion attack was obtained by localization index; this behaviour was confirmed by superficial analysis.

Abstract: Crystallization behavior and properties of glass-ceramics with content (mol%) of: K₂O = 3-25, BaO = 3-27, B₂O₃ = 8-26, Al₂O₃ = 13-18, and TiO₂ = 31-47 were studied. The relation between the initial composition and the formation of the crystalline phases was analyzed. When increasing [Al₂O₃] and [BaO], was increased the temperature of crystallization; likewise, different barium potassium titanates appeared with [BaO] = 3-13 (Group 1), whereas, different barium titanates were obtained with [BaO] =13-30 (Group 2). The influence of chemical composition on the properties of density, mechanical strength, chemical durability and dielectric properties (dielectric constant and dielectric losses), was discussed. Glass-ceramics with high mechanical resistance in the range of 189-480 MPa and permittivity values between 4.03 and 25.31, and dielectric losses < 2 x 10^-4 at 1MHz were produced.

Abstract: In this work, we reported the results obtained by the structural characterization of the FeAl₃ intermetallic compound. This material was synthesized by conventional casting technique using Fe (99.97%) and Al (99.92%) elemental metals. Then, the as-cast alloy was subjected to high-energy ball-milling for different times (1, 2.5, 5, 7.5, 10 and 15 h). The characterization of the alloy was performed by X-ray diffraction patterns, scanning and transmission electron microscopy. The results show that FeAl₃ intermetallic was produced as a single-phase after conventional casting. The milled experiments show that the FeAl₃ (monoclinic) transforms to Fe₂Al₅ (orthorhombic) after 15h of milling. Therefore, this phase transformation is characterized by a change from low to high symmetry systems.