Reducing Uncertainty in MDSC Arising from Simplifications in the Measurement of Heat Flow Rate

摘要

The measured heat flow rate in DSC is generally taken to be the temperature differencernmeasured between the sample and reference calorimeters multiplied by a temperature dependentrnfactor that accounts for variation of the thermal resistance of the calorimeter. Although thernmethod is almost universally applied, it is an approximation to the actual sample heat flow raternand strictly applies only under very restricted conditions. They include the assumptions that thernsample and reference calorimeters are identical and that the heating rates of both calorimeters arernidentical. During MDSC experiments, those assumptions are violated, leading to systematicrnerrors in the measurement of heat capacity and all associated heat flow rate data. The errors arerndependent upon the modulation period and result in the well-known decrease of measured heatrncapacity with decreasing modulation period. An improved MDSC method is described thatrngreatly reduces the period dependence by including the effects of imbalances and differences inrnheating rates between the sample and reference calorimeters. It includes three components: 1) arnnew DSC cell that has independent sample and reference calorimeters and includes tworndifferential temperature measurements, 2) a new DSC heat flow rate measurement method basedrnupon independently measured sample and reference heat flow rates and 3) a new MDSCrncalculation method that uses the independent sample and reference heat flow rate measurements.rnModulated DSC experiments using this new technique display greatly reduced periodrndependence, allowing modulation periods as short as 20 seconds to be used. Higher averagernheating rates may be used and productivity of the DSC is increased as much as threefold or more.