Defect and Diffusion Forum Vols. 258-260

Abstract: The effect of the SiO2/Si interface on oxygen self-diffusion in SiO2 during thermal oxidation was investigated using oxygen isotopes. A Si18O2 layer was first grown in 18O2 and then the sample was reoxidized in 16O2 at 900 ~ 1100 °C. The O diffusion in SiO2 during the 16O2 oxidation was investigated by secondary ion mass spectrometry (SIMS) measurements. Near the SiO2/Si interface, a significant broadening of the 18O profile toward the newly grown Si16O2 was observed. This 18O diffusion became slower with oxidation time and hence with increasing distance between 18O diffusion region and the interface. This distance-dependent 18O self-diffusion was simulated taking into account the effect of SiO generated at the interface upon oxidation and diffusing into SiO2 to enhance O self-diffusion. The simulation fits the SIMS profiles and shows that the SiO diffusion is greatly retarded by the oxidation with O2 from the oxygen-containing atmosphere and that the O self-diffusion therefore becomes distance-dependent. In addition, near the SiO2 surface, 16O diffusion profiles develop with the 16O2 oxidation time from the surface into the initially grown Si18O2. The integrated surface 16O concentration increases with oxidation time and the SiO from the interface affects the O self-diffusion near the surface as well.

Abstract: Drying kinetics of carrot cubes were carried out at 1 m/s air velocity at different air drying temperatures (30, 40, 50, 60 and 70±0.1 °C) (AIR experiments), and also at the same experimental conditions but applying high power ultrasound (US experiments). Two kind of diffusion models were used to simulate the drying kinetics, according to external resistance to mass transfer being considered (ER model) or neglected (NER model) for solving the diffusion equation. Diffusion ER model was solved using a finite difference method. Drying rate increased as air temperature was higher. Ultrasound also increased drying rate at the different temperatures, but the improvement on drying rate decreased at high temperatures, and almost disappeared at 70 °C. Effective moisture diffusivities only showed an Arrhenius type relationship with temperature for AIR experiments. The NER diffusion model was not accurate to simulate the drying kinetics at any experimental conditions tested. However, diffusion ER model provided a high closeness between experimental and calculated drying data (VAR>99.80). Through the parameters identified of the ER diffusion model, effective moisture diffusivity and mass transfer coefficient, the influence of the power ultrasound application on internal and external resistance to mass transfer was shown to be significant (p<0.05).

Abstract: Chesnut and pumpkin fruits were dehydrated with osmotic solutions of sucrose and NaCl at 25°C. These food materials have different structure, composition and porosity. Water loss and solids gain kinetics were experimentally determined and modeled using a diffusional model. In spite of the several mass transfer mechanisms taking place along with diffusion during osmotic dehydration, the modeling was satisfactory and involved effective coefficients of diffusion useful to quantify the different mass transfer fluxes. Water and sucrose transfer rates during osmotic dehydration with sucrose solutions are independent on the initial food material characteristics; however they seem to be related with the permeability of these components to a sucrose layer formed in the surface of the samples. In the case of osmotic dehydration with sodium chloride solutions, the coefficients of diffusion show a dependence on food material characteristic and higher values of these coefficients for pumpkin (more porous material) were found.

Abstract: In this work the application of delay differential equations to the modelling of mass transport in porous media, where the convective transport of mass, is presented and discussed. The differences and advantages when compared with the Dispersion Model are highlighted. Using simplified models of the local structure of a porous media, in particular a network model made up by combining two different types of network elements, channels and chambers, the mass transport under transient conditions is described and related to the local geometrical characteristics. The delay differential equations system that describe the flow, arise from the combination of the mass balance equations for both the network elements, and after taking into account their flow characteristics. The solution is obtained using a time marching method, and the results show that the model is capable of describing the qualitative behaviour observed experimentally, allowing the analysis of the influence of the local geometrical and flow field characteristics on the mass transport.

Abstract: This work describes the process of mass transfer which takes place when a fluid flows past a soluble surface buried in a packed bed of small inert spherical particles of uniform voidage. The fluid is assumed to have uniform velocity far from the buried surface and different surface geometries are considered; namely, cylinder in cross flow and in flow aligned with the axis, flat surface aligned with the flow and sphere. The differential equations describing fluid flow and mass transfer by advection and diffusion in the interstices of the bed are presented and the method for obtaining their numerical solution is indicated. From the near surface concentration fields, given by the numerical solution, rates of mass transfer from the surface are computed and expressed in the form of a Sherwood number (Sh). The dependence between Sh and the Peclet number for flow past the surface is then established for each of the flow geometries. Finally, equations are derived for the concentration contour surfaces at a large distance from the soluble solids, by substituting the information obtained on mass transfer rates in the equation describing solute spreading in uniform flow past a point (or line) source.

Abstract: Ultrasounds are mechanical waves that produce different effects when travelling through a medium, some related to mass transfer (i.e. microstirring at the interface, the so called "sponge effect" and cavitations). Thus, ultrasound appears to be a way to reduce both the internal and external resistances in osmotic food drying processes. In this study, the influence of the ultrasounds on water and solute transports during osmotic processes of drying is evaluated. Two different systems have been studied, apple slabs immersed in 30ºBrix sucrose solution, and pork loin slabs in sodium chloride saturated brine. The mathematical modelling of the mass transfers has been carried out by assuming diffusional mechanism and considering the mutual effect between the two mass transfers, the water losses and solute gains. The mass transfer curves in the osmotic process of apple drying in sucrose solution were satisfactorily simulated by using a diffusional model considering independent mass fluxes. Nevertheless, this model did not allow for the accurate simulation of the water losses in the system constituted by pork-loin in saline solution. When the mass fluxes were considered mutually affected, the simulation was accurate for both cases water and solute transfer.