Determination of Accurate Temperature and Enthalpy Measurements using Differential Scanning Calorimetry

Abstract

This thesis shows that the equilibrium transition temperatures of materials can be accurately determined using two developed methodologies designed to be used with differential scanning calorimeters. These methods are the extrapolation to zero heating rate and a stepwise isothermal method. The use of these methods allows for the instrument, which is typically scanning in nature to produce accurate temperature values under near equilibrium conditions. By employing the DSC methodologies for accurate temperature measurements it has been possible to determine a range of temperatures of transition and fusions under near equilibrium conditions for inorganic potassium salts (nitrate, perchlorate, and dinitramide), and rubidium salts (nitrate, and dinitramide) all of which have transitions of pyrotechnic interest. In cases where decomposition occurs at slower heating rates, the modification of the extrapolation to zero heating method to incorporate the use of faster heating rates above 10°C min-1 has enabled accurate temperatures of transition to be determined. In addition, this thesis also explores the use of materials with solid-solid transitions susceptible to thermal history such as potassium nitrate and re-evaluates the suitability for use in DSC calibration. The close proximity in temperature of the LGC Limited DSC standards indium and diphenylacetic acid, has enabled a direct assessment to be made of any resulting from the use of a metal or an organic substance in the calibration of DSC equipment. These measurements clearly established that indium may be used as a calibrant when making accurate equilibrium temperature and enthalpy measurements on organic materials in the same temperature range. Accurate enthalpy measurements have also been made on the above inorganic nitrates and perchlorates and measurements on the transitions of rubidium nitrate have established that equivalent values are given by heat flux and power compensated instruments. Enthalpy measurements on organic DSC calibration standards in the range 42-147 °C using a power compensated instrument demonstrated that there was no significant error in using indium as a single point calibrant for measurements on the fusion of organic materials at lower temperatures