A high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described. Emphasis is given to a new version of the relaxation technique in which the heater power is ramped linearly in time. This improvement results in superior performance and convenience in studying both first- and second-order phase transitions and allows quantitative evaluation of latent heats as well as pretransitional heat capacity variations. Examples are given for the use of this calorimeter in the study of liquid crystal phase transitions.

Nonadiabatic scanning calorimeter

Modulation calorimetry and related techniques

Modern low-temperature calorimetry

An ac calorimetric technique has been used to study the nematic---smectic-$A$ ($N\ensuremath{-}\mathrm{Sm}A$) and nematic-isotropic ($N\ensuremath{-}I$) transitions in octyloxycyanobiphenyl (8OCB) along a series of isobars between 1 atm and 2 kbar. The excess heat capacity $\ensuremath{\Delta}{C}_{p}(\mathrm{NA})$ associated with the essentially second-order $N\ensuremath{-}\mathrm{Sm}A$ transition is found to diminish rapidly with pressure and is not observed for $pg1.5$ kbar. The variation in $\ensuremath{\Delta}{C}_{p}(\mathrm{NA})$ with reduced temperature is not consistent with a logarithmic singularity but can be characterized by a critical exponent $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=0.25\ifmmode\pm\else\textpm\fi{}0.05$ for all the pressures studied. The excess heat capacity at the $N\ensuremath{-}I$ transition is almost independent of pressure; these data suggest but do not clearly establish quasitricritical behavior. Details of the experimental method and the fitting procedures are given.

Critical heat capacity near the nematic-smectic-A transition in octyloxycyanobiphenyl in the range 1-2000 bar

Photothermal techniques offer a nice tool for the determination of changes in thermal parameters of different types of samples. Using a very thin pyroelectric transducer, a very simple and versatile ac calorimeter can be built. The described inverse pyroelectric technique allows high-resolution ac-calorimetric measurements of the temperature dependence of the specific heat capacity. Since measuring frequencies up to a few hertz can be used, which are higher than in most alternative ac-calorimetric setups, noise can be reduced and measuring times shortened. With a small modification of the measurement cell, the determination of absolute specific heat capacity and thermal conductivity values of liquid samples was achieved. Comparison of the frequency response of the system with and without a solid thermal load in the neighborhood of the sample yields data with an accuracy of about 5% and 10% for specific heat capacity and thermal conductivity, respectively. If necessary, absolute values of specific heat cap...

Absolute values of specific heat capacity and thermal conductivity of liquids from different modes of operation of a simple photopyroelectric setup

The ac calorimetry technique has been adapted for use at hydrostatic gas pressures up to 3 kilobar. The theory of this method has been extended to take into account high gas densities and low sample thermal conductivities, and the resulting equations have been tested on samples of Invar and ammonium chloride. The method is especially attractive for studying heat capacity variations in samples that undergo second‐ or higher order phase transitions at high pressures.

J. D. Baloga

Papers

ac calorimetry at high pressure