Thermal Conductivity and Heat Capacity of Adamantane and Hexamethylenetetramine Under Pressure

Reversible colossal barocaloric effects near room temperature in 1-X-adamantane (X=Cl, Br) plastic crystals

“…This is further supported by pressure-dependent measurements of heat capacity for adamantane [43], which shows that for this compound is approximately independent or very weakly dependent on pressure in the pressure range under study. It is worth mentioning here that the linear − behavior is commonly observed among organic crystals [35].…”

“…1-Cl-ada and 1-Br-ada, as plastic crystals in general, suffer from low thermal conductivities due to orientational disorder [9], which in the case of the plastic phase of adamantane reaches ~0.18 W m -1 K -1 under normal conditions [43]. Here, several aspects should be pointed out.…”

“…Here, several aspects should be pointed out. On the one hand, in the ordered phases and/or higher pressures, the thermal conductivities experience a significant increase [43,52]. On the other hand, powdered materials display lower thermal conductivities than their single crystal counterparts because the free path is shortened due to phonon scattering at the grain boundary.…”

Dynamics in the ordered and disordered phases of barocaloric adamantane

“…Good thermal conductivity is a key requirement for a working barocaloric cooling device, and this presents a challenge unique to solid-state cooling. In particular plastic crystals have low thermal conductivity 46,47 , with adamantane having only ∼ 0.18 Wm −1 K −1 in the plastic phase 8,48 . Several engineering solutions have been proposed to mitigate this issue (such as mixing the barocaloric with highly conductive materials 8,46,[49][50][51][52][53], but many open questions remain regarding the quantitative relation between disorder and thermal conductivity, and the specific conditions that might amplify or suppress this relation 54 .…”

“…In particular plastic crystals have low thermal conductivity, 46,47 with adamantane having only B0.18 Wm À1 K À1 in the plastic phase. 8,48 Several engineering solutions have been proposed to mitigate this issue (such as mixing the barocaloric with highly conductive materials 8,46,[49][50][51][52][53], but many open questions remain regarding the quantitative relation between disorder and thermal conductivity, and the specific conditions that might amplify or suppress this relation. 54 By directly modelling the influence of disorder on phonon lifetimes via phonon broadening, the SCLD approach presents an opportunity to further the understanding of the interplay between disorder, configurational entropy, vibrational entropy (via the broadened density of states) and thermal conductivity, and can also perhaps inspire ways of tuning thermal conductivity 14 in barocalorics.…”