N. Vitorino

Solutions for Heat or Cold Discharge from Encapsulated Phase-Change Materials

Stable, convergent, and fast computing codes have been developed to describe heat or cold discharge from encapsulated phase-change materials (PCMs). Numerical algorithms are based on previous transformations of relevant variables to yield immobilization of the moving boundary and also a flat-like condition to enhance convergence of the computing codes. These algorithms are used to analyze heat transfer limitations imposed by the low conductivity of the solidifying PCM layer and the mixed control related to the thermal resistance of the encapsulated wall and/or heat transfer limitations to the external fluid. The stability and the convergence of the codes are analyzed by varying the mesh size, and the relative time step, and are also compared with a space grid method solution. Quasi-steady-state solutions are also proposed and their applicability examined.

Thermal properties of compacted pharmaceutical excipients

Thermal properties of powders are critical material attributes that control temperature rise during tableting and roll compaction. In this study, various analytical methods were used to measure the thermal properties of widely used pharmaceutical excipients including microcrystalline cellulose (MCC) of three different grades (Avicel PH 101; Avicel PH 102 and Avicel DG), lactose and mannitol. The effect of relative density on the measured thermal properties was investigated by compressing the powders into specimen of different relative densities. Differential thermal analysis (DTA) was employed to explore endothermic or exothermic events in the temperature range endured during typical pharmaceutical manufacturing processes, such as tabletting and roll compaction. Thermogravimetric analysis (TGA) was performed to analyse the water/solvent content, either in the form as solvates or as loosely bound molecules on the particle surface. Thermal conductivity analysis (TCA) was conducted to measure thermal conductivity and volumetric heat capacity. It is shown that, for the MCC powders, almost no changes in morphology or structural changes were observed during heating to temperatures up to 200°C. An increase in relative density or temperature leads to a high thermal conductivity and the volumetric heat capacity. Among all MCC powders considered, Avicel DG showed the highest increase in thermal conductivity and the volumetric heat capacity, but this heat capacity was not sensitive to the measurement temperature. For lactose and mannitol, some endothermic events occurred during heating. The thermal conductivity increased with the increase in temperature and relative density. A model was also developed to describe the variation of the thermal conductivity and the volumetric heat capacity with the relative density and the temperature. It was shown that the empirical model can well predict the dependency of the thermal conductivity and the volumetric heat capacity on the relative density and the temperature.

Rheological Behavior of Paraffin-Alumina Emulsions and their Microstructural Effects

Cellular ceramics were prepared by emulsification of aqueous alumina suspensions with liquid paraffin, using collagen as a shape stabilizer and a dispersant to adjust viscosity. Rheological measurements of emulsions and microstructural analysis of the sintered samples were performed. The results show that the concentration of collagen, dispersant content and stirring rate affect directly the emulsion viscosity which causes changes in porosity and average cell size.