Graduate Thesis Or Dissertation
 

Small sample, low-temperature calorimetry

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mw22v785z

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  • A calorimeter capable of measuring the heat capacity of 1 mg size samples from 4.2 to greater than 100 K has been designed, constructed, and tested. The sample is bonded to the end of a 0.002 inch diameter, 0.5 cm long chromel-constantan thermocouple (type E) and heated optically with a laser and fiber optic. An advantage of this calorimeter is the low addenda heat capacity of the thermocouple. The thermocouple, which serves not only as the temperature sensor of the sample but also as the thermal link between the sample and a constant temperature reservoir, is anchored to a copper block, which acts as the constant temperature reservoir. Heat capacity is determined from the temperature rate of decay of the sample using a sweep method. The sample is heated to an initial temperature above the block temperature by the laser. The laser is then turned off and the sample temperature is allowed to decay to the block temperature. By measuring the temperature of the sample as a function of time and relating it to the thermal conductivity of the thermocouple in a separate experiment, the sample's heat capacity can be determined. The thermal conductivity of the thermocouple is determined by performing an experiment with a sample of known heat capacity. A design model created with a spreadsheet helped to determine what size thermocouple should be used as well as the best materials and dimensions of the components that make up the calorimeter. The model was also useful in determining the nature of a calorimetry experiment and helped determine how high above the block temperature the sample should be heated, how low the pressure inside the calorimeter should be, and how much time a calorimetry experiment would require. Experiments using copper samples have confirmed the validity of the design. The results of an experiment using a 1.1 mg copper sample agree (within expected uncertainty) with the accepted heat capacity of copper from 7 to 100 K. One factor in the uncertainty is the large heat capacity of the grease (Apiezon N) used to bond the sample to the tip of the thermocouple, especially below 15 K.
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