Practical remarks concerning phase diagrams determination on the basis of differential scanning calorimetry measurements

Abstract

Phase diagrams of binary systems at constant pressure are representations of one-and two-phase regions with their boundaries being functions of temperature and concentration. The most popular techniques used in determination of phase diagrams are thermal analysis (TA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The first of them, based on recording of cooling curves, has no significant meaning nowadays; however, it is still used, especially in didactics. Actually DTA and DSC are widely used in phase diagrams determination. DSC has an advantage over DTA, because in addition to temperature it gives precise value of enthalpy of thermal effect. Two types of DSCs must be distinguished: the heat flux DSC and the power compensation DSC.The characteristic feature of all DSC measuring systems is the twin-type design and the direct in-difference connection of the two measuring systems which are of the same kind. It is the decisive advantage of the differential principle that, in first approximation, disturbances such as temperature variations in the environment of the measuring system and the like, affect the two measuring systems in the same way and are compensated when the difference between the individual signals is formed [1]. The differential signal is the essential characteristic of each DSC. Another characteristic-which distinguishes it from most classic calorimeters-is the dynamic mode of operation. The DSC can be heated or cooled at a preset heating or cooling rate. A characteristic common to both types of DSC is that the measured signal is proportional to a heat flow rate (in opposition to classical calorimeters where heat flow is measured). This fact-directly measured heat flow rates-enables the DSC to solve problems arising in many fields of application [1]. In the heat flux DSC a defined exchange of the heat to be measured takes place via a thermal resistance. The measurement signal is the temperature difference; it describes the intensity of the exchange and is proportional to the heat flow rate. There are two main types of the heat flux DSC: the disc-type measuring system with solid sample support (disc) and the cylinder-type measuring system with integrated sample cavities. Heat flux DSCs with a disctype measuring system are available for temperatures between -190 and 1,500°C [1]. In the heat flux DSC with a cylindertype measuring system, the outer surfaces of each sample container are in contact with a great number of thermocouples connected in a series between the container and furnace cavity. The thermocouples bands or wires are the dominating heat conduction path from the furnace to samples. Both sample containers are thermally decoupled; heat exchange takes place only with parts of the massive furnace. These apparatuses are available for temperature range between -190 and 1,500°C [1]. The power compensation DSC belongs to the class of heat-compensating calorimeters. The heat to be measured is compensated with electric energy, by increasing or decreasing an adjusta...

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