Gelatinization characteristics of laboratory-isolated and commercial corn starch were compared by differential scanning calorimetry (DSC) before and after being annealed at subgelatinization temperatures in excess water. Prior to annealing, commercial corn starch has a relatively narrow gelatinization range, with a peak temperature at 71°C. Starches isolated in the laboratory have wide gelatinization ranges and lower peak temperatures. After annealing, commercial starch showed little change in gelatinization characteristics, whereas laboratory starches all had narrowed gelatinization ranges, higher peak temperatures, and increased gelatinization enthalpy, indicating changes in the internal structure of the starch granules. This demonstrates that the wet-milling process anneals corn starch during the isolation procedure.

No abstract

A model is presented from which one can calculate the Hugoniot of solid and porous twocomponent mixtures up to moderate pressures using only static thermodynamic properties of the components. The model does not presuppose either the relative magnitude of the thermal and elastic energies or temperature equilibrium between the two components. It is shown that for a mixture, the conservation equations define a Hugoniot surface and that the ratio of the thermal energy of the components determines where the shocked state of the mixture lies on this surface. This ratio, which may strongly affect shock-initiated chemical reactions and the properties of consolidated powder mixtures, is found to have only a minor effect on the Hugoniot of a mixture. It is also found that the Hugoniot of solids and solid mixtures is sensitive to the pressure derivative of the isentropic bulk modulus at constant entropy.

Shock initiated chemical reaction experiments have been performed on a 1:l atomic ratio mixture of 20-to 45,um nickel and -325 mesh crystalline silicon powders. It has been observed that no detectable or only minor surface reactions occur between the constituents until a thermal energy threshold is reached, above which the reaction goes to completion. The experiments show the energy difference between virtually no and full reaction is on the order of 5 percent. Differential scanning calorimetery (DSC) of statically pressed powders shows an exothermic reaction beginning at a temperature which decreases with decreasing porosity. Powder, shock compressed to just below the threshold energy, starts to react in the DSC at 621 "C while powder statically pressed to 23% porosity starts to react at about 30 "C higher. Tap density powder starts to react at 891 "C. The DSC reaction initiation temperature of the shock compressed but unreacted powder corresponds to a thermal energy in the powder of 382 J/g which agrees well with the thermal energy produced by a shock wave with the threshold energy (between 384 and 396 J/g). (Thermal energies referenced to 20 "C.) A sharp energy threshold and a direct correlation with DSC results indicates that the mean thermal energy determines whether or not the reaction will propagate in the elemental Ni + Si powder mixture rather than local, particle level conditions. From this it may be concluded that the reaction occurs on a time scale greater than the time constant for thermal diffusion into the particle interiors.

No abstract