Melting vs. Solidification of a Pure Metal Analysed by DSC
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quantity. However, current measurement of melting/solidification by DSC is performed in the continuous change of temperature regime. Direct measurement of the temperature of both melting of an overheated solid and solidification of an undercooled melt implies calibration of the equipment. Such calibration is attempted in the present paper under isothermal regime. Then, the rate of the melting/solidification transformation of the overheated solid/undercooled liquid is obtained and analysed under isothermal regime. Measurements were conducted in a Perkin Elmer DSC 7 equipment for pure In and Pb. MELTING UNDER ISOTHERMAL REGIME The sample (5-10 mg) in the DSC equipment is held in a crucible and its temperature is adjusted to the programmed time evolution by use of a heater resistor (Fig. 1). To conduct isothermal measurements of melting, the sample is heated at a constant rate P up to a temperature T close to Tm and then kept at that temperature. As shown in Fig. 2c continuous heating proceeds up to the isothermal temperature. Then, the sample is held at this constant temperature for a given time. The general form of the calorimetric signal is depicted in Fig. 2a and 2b. If both the heating rate and the isothermal temperature are correctly selected, melting is hindered during the heating and starts under isothermal regime, as depicted in Fig. 2a. If the temperature is close but lower than Tm no melting endotherm appears in the calorimetric signal (as in Fig. 2b). When changing regime from continuous heating to isothermal one there is a transient until the sample temperature is re-established to the programmed one. This transitory state is the only signal detected in Fig. 2b. In a very sensitive DSC such transient lasts typically less than 20 s. 33 Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
Some of the results obtained under isothermal regime are shown in Fig. 3 in the sample form of the heat flow versus time for In at several values of T and two different values sensor of P3.At very low superheating the signal heater detected is very low and it is hard to differentiate from the base line (see curve for Tm + 0.01 K). The increase of drastically enlarges the superheating Fig. 1. Schematic view of a DSC cell. calorimetric signal and reduces the time, At, needed for the sample to transform. From the kind of measurements depicted in Fig. 3 it can be concluded that the melting behaviour is insensitive to the heating rate, j0,used to reach the isothermal regime. Further, the accuracy in the temperature calibration of the equipment may reach less than ± 0.05 K. ndothermic
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