Phase diagram for freeze-dried persimmon

Phase transitions of freeze-dried persimmon in a large range of moisture content were determined by differential scanning calorimetry (DSC). In order to study this transitions at low and intermediate moisture content domains, samples were conditioned by adsorption at various water activities (a(w) = 0.11-0.90) at 25 degreesC. For the high moisture content region, samples were obtained by water addition. At a(w) less than or equal to 0.75 two glass transitions were visible, with T(g) decreasing with increasing water activity due to water plasticizing effect. The first T(g) is due to the matrix formed by sugars and water, the second one, less visible and less plasticized by water, is probably due to macromolecules of the fruit pulp. At a(w) between 0.80 and 0.90 a devitrification peak appeared after T(g) and before T(m). At this moisture content range, the Gordon-Taylor model represented satisfactorily the matrix glass transition curve. At the higher moisture content range (a(w) > 0.90), the more visible phenomenon was the ice melting. T(g) appeared less visible because the enthalpy change involved in glass transition is practically negligible in comparison with the latent heat of melting. In the high moisture content domain T(g) remained practically constant around T(g)' (-56.6 degreesC). (C) 2001 Elsevier B.V. B.V. All rights reserved.

Quantitative structural analysis of the transition from LT-LixCoO2 to HT-LixCoO2 using the rietveld method: correlation between structure and electrochemical performance

A quantitative phase analysis was made of LiXCoO2 powders obtained by two distinct chemical methodologies at different temperatures (from 400 to 700degreesC). A phase analysis was made using Rietveld refinements based on X-ray diffraction data, considering the LiXCoO2 powders as a multiphase system that simultaneously contained two main phases with distinct, layered and spinel-type structures. The sults showed the coexistence of both structures in LiXCoO2 obtained at low temperature (400 and 500degreesC), although only the layered structure was detected at higher temperatures (600 and 700degreesC, regardless of the chemical powder process employed. The electrochemical performance, evaluated mainly by the cycling reversibility of LiXCoO2 in the form of cathode insertion electrodes, revealed that there is a close correlation between structural features and the electrochemical response, with one of the redox processes (3.3 v/3.9 v) associated only with the presence of the spinel-type structure. (C) 2003 Elsevier B.V. All rights reserved.