Fig. 4
From: Optically-controlled long-term storage and release of thermal energy in phase-change materials
Influence of measurement conditions and dopant structures on thermal storage properties. a Impact of cooling rate on ∆T c at varied doping levels. ∆T c generally increases with higher cooling rates, and was measured by DSC through heating and cooling between 10 °C and 70 °C. b Impact of functionalization of azobenzene dopants on ∆T c. Compound 2 is functionalized with a long alkyl chain (decyloxy group), and compound 3 is decorated with a bulky substitution (tert-butyl group). Error bars indicate standard deviations of temperature measured at least 5 times on each type of composite. c DFT-calculated binding energy between two neighboring molecules (either as trans/cis isomer or PCM) varying with the functional group on the para-position of the azobenzene core. Dotted lines represent the head-to-head interaction between acid groups of the PCM (green) or the side-by-side interaction between alkyl chains of the PCM (blue). d DFT-simulated configurations of two π–π stacking dopants (compound 1 and 3) as trans or cis isomer. e DFT-simulated configurations of a dopant (compound 3, cis or trans) and a neighboring PCM molecule interacting around N=N group of azobenzene and –COOH group of PCM. f Stability of heat storage in a UV-charged liquid composite (30 mol% of compound 1) in the dark at 36 °C. The thermal reverse conversion of azobenzene dopants (cis → trans) was monitored by 1H NMR (half-life of 24 h). For 10 h (with more than 65% cis fraction), the composite remains a liquid, conserving heat (shaded area). After 10 h, PCM molecules gradually solidify, and the composite loses the stored heat over time under isothermal conditions
