High-resolution ac calorimetry and critical behavior at phase transitions

• Published in: Thermochimica acta 1985-06, Vol.88 (1), p.127-142
• Main Author: Garland, Carl W.
• Format: Article
• Language: English
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/0040-6031(85)85420-4

An AC calorimeter technique operating at very low frequencies has been developed for the study of second-order phase transitions. Only a small amount of sample is required (50 to 100 mg), and samples with low thermal conductivity, such as insulator crystals, fluids and liquid crystals, can be investigated at I atm and at pressures up to ∼3000 bar. Two versions of this AC calorimeter will be described: a manually operated high-precision (±0.05%) calorimeter with a computerized data-acquisition system, and a fully computerized calorimeter with good precision (±0.2%) which can be operated in a scanning mode with linear drift rates in the range 0.01 K/hour to 1 K/hour. Experimental results and a discussion of their analysis will be presented for order-disorder transitions in ionic crystals (ammonium halides), consolute-point phase separation in binary liquids (3-methylpentane + nitroethane), orientational ordering in aqueous micelle solutions (cesium perfluoro-octanoate), and a variety of liquid crystal systems (with emphasis on transitions involving nematic and various smectic phases).