High Speed Thermal Analysis with Thin-Foil Calorimeter

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Abstract
A method is described for performing adiabatic scanning calorimetry on thin samples heated at high speeds. A heater is fabricated by soldering copper bus bars to opposite ends of a 45×2.54 cm strip of 0.001065 cm Advance foil. The heater is folded at 1.77 cm intervals to form an accordionlike array. Two 2.54×3.80 cm sheets of sample film are inserted between each fold in the heater, so there are twice as many sample sheets as heater sections. A 0.001 cm thick copper‐constantan foil thermocouple is inserted between two sample sheets near the median plane of the array. The experiment consists of bringing the sample array to equilibrium, turning on the heater current, and recording the temperature of the median plane as a function of time. During an initial period, when heating at the thermocouple location is adiabatic, the sample specific heat is calculated on an absolute basis from the slope of the recorded curve. Scanning rates of the order of 500 K/sec are obtained with 0.005 cm thick polymer samples when the heater is run at 40 A or 1.6 kW. With a 2 mm thick array, the adiabatic period lasts about 1 sec; thus, the recording contains continuous data on specific heat vs. temperature over a 500 K temperature range. Polytetrafluoroethylene and polyethylene terephthalate, heated at rates of about 75 and 500 K/sec, respectively, yield apparent specific heat results which agree with values shown in the literature. When heated at about 15 K/sec, polyvinyl chloride exhibits an unusually sharp peak in apparent specific heat at the glass transition temperature; during a 0.2 sec period the temperature of the sample actually falls even though being heated by the foil heater at a rate of about 30 W/cm3.