Thermal transport crossover from crystalline to partial-crystalline partial-liquid state

Abstract

Phase-change materials (crystalline at low temperatures and partial-crystalline partial-liquid state at high temperatures) are widely used as thermoelectric converters and battery electrodes. Here, we report the underlying mechanisms driving the thermal transport of the liquid component, and the thermal conductivity contributions from phonons, vibrations with extremely short mean free path, liquid and lattice-liquid interactions in phase-changed Li2S. In the crystalline state (T ≤ 1000 K), the temperature dependent thermal conductivity manifests two different behaviors, i.e., a typical trend of 1/T below 800 K and an even faster decrease between 800 and 1000 K. For the partial-crystalline partial-liquid Li2S when T ≥ 1100 K, the contributions of liquid and lattice-liquid interactions increase significantly due to the fluidization of Li ions, and the vibrations with extremely short mean free path, presumably assimilated to diffusons, can contribute up to 46% of the total thermal conductivity at T = 1300 K.

Thermal Transport in Conductive Polymer–Based Materials

“…Lithium‐ion (Li‐ion) batteries are another example for the application of conductive polymers with high thermal conductivity. High‐energy‐capacity, safe, and low‐cost Li‐ion batteries put forward new battery electrode materials and new composites . To maintain the high charging and discharging efficiency, it is crucial to maintain the work temperature for the Li‐ion batteries to be between 18 and 45 °C .…”

Thermal transfer in amorphous superionic Li2S

“…This description was first proposed by Allen and Feldman [6] and Cahill [12], then extended by Larkin and MacGauhey [8] and Lv and Henry [13]. The similar classification is also applied to quantify the thermal excitations in systems which are in the superionic state at high temperatures such as crystal Li 2 S [14], Ag 2 Te [15], AgCrSe 2 [16], and Cu 3 SbSe 3 [17], but with an extra contribution from convection heat transfer as proved by Zhou et al [14]. In liquids where convection heat transfer predominates, acoustic and optical phonons are experimentally observed such as in liquid Ga [18], water [19], and liquid helium [20].…”

Thermal transport in amorphous small organic materials: a mechanistic study

“…Although there is no well-defined wave vector in amorphous materials, the low frequency modes can still travel as acoustic waves with fairly long mean free paths, i.e., behaving as phonons. Previous studies in amorphous inorganic materials reveal that the total TC can be divided into the contributions by propagating modes at low frequencies and "diffusons" at high frequencies [21][22][23] . This conclusion is also valid in amorphous organic materials as will be demonstrated by our mode relaxation time analysis.…”

“…Diffusons, while not propagating as phonons in crystals, are spatially delocalized and can carry heat through a hopping mechanism among modes with similar frequency. This mechanism contributes the most to heat transport in disordered systems 18,23,24 . The delocalization of vibrational modes over neighboring molecules can be evidenced by the participation ratio as shown in Fig.…”