Heat Capacity and Thermal Damping Properties of Spin‐Crossover Molecules: A New Look at an Old Topic

Abstract

The thermally induced spin‐crossover (SCO) phenomenon in transition metal complexes is an entropy‐driven process, which has been extensively studied through calorimetric methods. Yet, the excess heat capacity associated with the molecular spin‐state switching has never been explored for practical applications. Herein, the thermal damping effect of an SCO film is experimentally assessed by monitoring the transient heating response of SCO‐coated metallic microwires, Joule‐heated by current pulses. A damping of the wire temperature, up to 10%, is evidenced on a time scale of tens of microseconds due to the spin‐state switching of the molecular film. Fast heat‐charging dynamics and negligible fatigability are demonstrated, which, together with the solid‐solid nature of the spin transition, appear as promising features for achieving thermal energy management applications in functional devices.

Unprecedented switching endurance affords for high-resolution surface temperature mapping using a spin-crossover film

“…Obviously, under non-thermal-equilibrium conditions, the temperature difference between the heating element and the thin-film surface is expected to be larger. In particular, in addition to the “normal” heat capacity of the SCO film, an excess heat capacity should be also considered due to the endothermic character of the LS-to-HS transition (Δ H = 40 J g −1 in 1 ) 35 , which can give rise to a transient thermal damping effect on microsecond time scales in sufficient thick (µm range) films 46 . However, this effect, which becomes undetectable in films as thin as 200 nm, can be reasonably neglected in practical thermometry applications.…”

Advances and obstacles in pressure-driven solid-state cooling: A review of barocaloric materials

“…In the SCO complex [CrI(depe) 2 ] [58], the transition entropy change is mostly contributed by phonons, whereas the large change in the local magnetic moment ∆S mag = k B ln[(2S HS + 1)/(2S LS + 1)] is secondary. A shortcoming for these compounds is the very low thermal conductivity [60] due to the porous structure, H bridges and disorder. Actually, single crystals can reach moderate (κ ~ 0.1-0.4 W m −1 K −1 [61,62]) or even notable values (κ ~ 1.3 W m −1 K −1 [63]).…”

Iron/2,6‐Di(pyrazol‐1‐yl)pyridine Complexes with a Discotic Pattern of Alkyl or Alkynyl Substituents

“…An increasing number of applications are being proposed to harness SCO phenomena [21] . Recent examples include SCO materials as switching centers in nanoelectronics; [22] solid state refrigeration; [23] protective heat sinks for electronic circuitry; [24] thermochromic printing materials; [25] and contact pressure sensors [26] …”