Bibian Hoyos

Electrodeposición de Níquel Duro con Ondas de Corriente Pulsante Triangulares

Resumen Se estudia el efecto de la frecuencia y la densidad de corriente sobre la dureza en electrodepositos de niquel obtenidos mediante corrientes pulsantes triangulares y rectangulares y su comparacion con los obtenidos empleando corriente directa. Las mediciones de dureza se realizaron segun la norma ASTM E384. La descripcion de la morfologia de los depositos se realizo mediante un microscopio de barrido electronico (SEM) segun las normas ASTM E112, E3 y E93. Los resultados experimentales muestran que a 4 A/dm 2 la dureza de los depositos esta influida por la forma de onda, produciendose durezas en el orden rampa descendente > rectangular > rampa ascendente. Para las condiciones exploradas, la mayor dureza se obtiene con onda rectangular a 20 A/dm 2 y 60 Hz. Para esta onda de corriente, la dureza disminuye al aumentar el ciclo de trabajo. La dureza tiene un aumento de hasta un 300% cuando se emplea corriente pulsante frente a los valores alcanzados con corriente directa.

Ánodos de Pt-Ru y Pt-Ir para Celdas de Combustible Alimentadas con Metano y Propano Directo

In this paper, the effect of temperature in the performance of proton exchange membrane fuel cells feed with methane and propane, using oxygen as feed to the cathode, is presented. For the fuel oxidation in the anodes, five carbon supported catalysts were tested: Pt, Pt85/Ru15, Pt50/Ru50, Pt90/Ir10, and Pt50/Ir50. Carbon-supported pure platinum was used as catalysts in the cathode side. The performance of the fuel cells was evaluated by polarization curves obtained from the current-potential data. Results indicate that methane oxidation is favoured at high temperatures on the Pt90/Ir10, Pt50/Ir50 and Pt50/Ru50 catalysts. At low temperatures the best catalysts were Pt and Pt85/Ru15. The Pt85/Ru15 bimetallic mixture showed the best performance to carry out propane oxidation at 30 °C.

Modelo Matemático de la Nucleación Electroquímica con Ondas de Corriente Pulsante

A mathematical model has been established for the description of concentration overpotential, nucleation rate and nucleus size formed on metallic substrates when four types of pulse current waveforms are used: rectangular, ramp-down, ramp-up and triangular. The mathematical model was developed considering that the species diffusion in the limit layer is the rate determining step of the process. The model predicts that when using wave current with the same current average and the same time of application, the ramp-down current produces the shortest transition time, with the highest rise of overpotential concentration, this drives to high nucleation rates with small nucleus size. For the case of current wave forms with the same pick and average current, the rectangular and ramp-down waves show practically the same transition time, with equivalent nucleation rate and deposit size.

Viscosity of heptane-toluene mixtures. Comparison of molecular dynamics and group contribution methods

Three methods of molecular dynamics simulation [Green–Kubo (G–K), non-equilibrium molecular dynamics (NEMD) and reversed non-equilibrium molecular dynamics (RNEMD)], and two group contribution methods [UNIFAC–VISCO and Grunberg–Nissan (G–N)] were used to calculate the viscosity of mixtures of n-heptane and toluene (known as heptol). The results obtained for the viscosity and density of heptol were compared with reported experimental data, and the advantages and disadvantages of each method are discussed. Overall, the five methods showed good agreement between calculated and experimental viscosities. In all cases, the deviation was lower than 9%. It was found that, as the concentration of toluene increases, the deviation of the density of the mixture (as calculated with molecular dynamics methods) also increases, which directly affects the viscosity result obtained. Among the molecular simulation techniques evaluated here, G–K produced the best results, and represents the optimal balance between quality of result and time required for simulation. The NEMD method produced acceptable results for the viscosity of the system but required more simulation time as well as the determination of an appropriate shear rate. The RNEMD method was fast and eliminated the need to determine a set of values for shear rate, but introduced large fluctuations in measurements of shear rate and viscosity. The two group contribution methods were accurate and fast when used to calculate viscosity, but require knowledge of the viscosity of the pure compounds, which is a serious limitation for applications in complex multicomponent systems.