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 capacity and thermal conductivity can also be obtained by making contact between a (solid) thermal load and the liquid. A pure ac-calorimetric operation is then no longer possible, thus restricting this mode of operation to homogeneous or to properly aligned anisotropic liquids.

We have measured the temperature dependence of the enthalphy and the specific heat capacity near 260 K orientational order-disorder transition of three different C 60 samples, using high-resolution adiabatic scanning calorimetry. Extremely slow heating and cooling runs ͑at rates of the order of 100 mK/h͒ show that even at these low scanning rates the results are rate dependent. This indicates that even at such low rates the material is not in a thermodynamic equilibrium state and behaves as a system with a very long internal relaxation time ͑order of 10 h͒ in completing the first-order transition, as found in some binary systems.

The critical behaviour of the dynamic and static thermal properties are investigated in the neighbourhood of the Neel temperature of single crystal antiferromagnetic COO and Cr203. Photoacoustic front and pyroelectric rear detection of front illumination induced temperature oscillations are used to determine critical exponents and amplitudes of the specific heat capacity anomaly. Simultaneously measured thermal conductivity data are also presented.

Using a recently developed photopyroelectric ac calorimetric technique we investigate two binary liquid crystal mixtures with so-called injected smectic-A phases. Characteristic of these systems is the occurrence of nematic-to-smectic-A transition lines in the phase diagram of mixtures of pure compounds having only a nematic phase. The two binary systems are pentylcyanobiphenyl with either 4-n-propylcyclohexyl-carboxylate or 4-n-pentylphenyl 4(')-n-pentyloxybenzoate. Both these systems have domelike smectic-A ranges with narrow nematic ranges at the top. Near the top the N-SmA transitions are of first order and are crossing over to second order at a tricritical point on either side of the top with the increasing width of the nematic range. The obtained critical exponents are almost completely Fisher renormalized because of the strong concentration dependence of the nematic-to-smectic-A transition temperatures.

A photopyroelectric setup for the accurate absolute measurement of the thermal conductivity of dilute gases is presented. The technique results from a modification of our previously described [Rev. Sci. Instrum. 69, 2452 (1998)] technique for the simultaneous measurement of absolute values of specific heat capacity and thermal conductivity of liquids. Measurement results are presented for different gases at room temperature and at one atmosphere as well as for the dilute Knudsen gas regime at reduced pressure. It is also demonstrated that the setup can be operated as a sensitive pressure gauge with adaptable sensitivity range.