Photopyroelectric study of specific heat, thermal conductivity, and thermal diffusivity of Cr2O3 at the Néel transition

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature-dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂ Se, Cu₂ S, Ag₂ S, and Ag₂ Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.

Generalizing the flash technique in the front-face configuration to measure the thermal diffusivity of semitransparent solids

In this work, we have extended the front-face flash method to retrieve simultaneously the thermal diffusivity and the optical absorption coefficient of semitransparent plates. A complete theoretical model that allows calculating the front surface temperature rise of the sample has been developed. It takes into consideration additional effects, such as multiple reflections of the heating light beam inside the sample, heat losses by convection and radiation, transparency of the sample to infrared wavelengths, and heating pulse duration. Measurements performed on calibrated solids, covering a wide range of absorption coefficients (from transparent to opaque) and thermal diffusivities, validate the proposed method.

3D-XY critical behavior of CsMnF₃ from static and dynamic thermal properties

Static and dynamic critical behavior of the easy-plane antiferromagnet CsMnF3 have been studied by means of a high-resolution ac photopyroelectric calorimeter. Thermal diffusivity, thermal conductivity and specific heat have been carefully measured in the near vicinity of the antiferromagnetic to paramagnetic transition (51.1 K). Specific heat and thermal diffusivity show singularities at the Néel temperature while thermal conductivity does not. Both the static and dynamic critical parameters agree with the standard 3D-XY universality class (α = -0.014, A⁺/A⁻ = 1.06): for specific heat α = -0.016, A⁺/A⁻ = 1.09 and for thermal diffusivity b = -0.010, U⁺/U⁻ = 1.09. As the dynamic critical behavior of thermal diffusivity has not yet been theoretically established for the 3D-XY universality class, an approximate equation relating static and dynamic critical parameters has been obtained for it, leading to b ≈ α and A⁺/A⁻ ≈ U⁺/U⁻ by studying the asymptotic behavior of the functions. This equation has also been experimentally verified for another XY antiferromagnet (SmMnO₃). As an easy-plane antiferromagnet with a hexagonal structure, CsMnF₃ could have been expected to comply with the 3D-XY chiral class (α = +0.34, A⁺/A⁻ = 0.36) (as is the case of CsMnBr₃), but the experimental results rule out that possibility. This is attributed to the presence of a small in-plane anisotropy of the spins in CsMnF₃, which breaks the chiral degeneracy of the 120° spin structure.

Invited review article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

The study of thermophysical properties is of great importance in several scientific fields. Among them, the heat capacity, for example, is related to the microscopic structure of condensed matter and plays an important role in monitoring the changes in the energy content of a system. Calorimetric techniques are thus of fundamental importance for characterizing physical systems, particularly in the vicinity of phase transitions where energy fluctuations can play an important role. In this work, the ability of the Photopyroelctric calorimetry to study the versus temperature behaviour of the specific heat and of the other thermal parameters in the vicinity of phase transitions is outlined. The working principle, the theoretical basis, the experimental configurations, and the advantages of this technique, with respect to the more conventional ones, have been described and discussed in detail. The integrations in the calorimetric setup giving the possibility to perform, simultaneously with the calorimetric studies, complementary kind of characterizations of optical, structural, and electrical properties are also described. A review of the results obtained with this technique, in all its possible configurations, for the high temperature resolution studies of the thermal parameters over several kinds of phase transitions occurring in different systems is presented and discussed.

Critical behavior near the Lifshitz point in Sn(2)P(2)(S(1 - x)Se(x))(6) ferroelectric semiconductors from thermal diffusivity measurements

The thermal diffusivity of the ferroelectric family Sn(2)P(2)(Se(x)S(1 - x))(6) (0 ≤ x ≤ 1) has been measured by a high-resolution ac photopyroelectric technique, using single crystals, with the aim of studying the evolution of the ferroelectric transition with Se doping. Its change from second order character to first order while passing the Lifshitz point (x approximately 0.28) has been evaluated, as well as the splitting of the transition at high Se concentrations. The critical behavior of the ferroelectric transition in terms of the different universality classes and their underlying physical dominant effects (tricriticality, long-range dipole interactions, Lifshitz point) has been discussed using thermal diffusivity measurements in the very close vicinity of the critical temperature. This study reveals that for Se concentrations around the Lifshitz point, long-range dipole interactions do not play a significant role and that the critical parameters are close to those predicted for the Lifshitz universality class.