Simultaneous Transition of Temperature Coefficient and p-n in Carbon Nanotube Thermoelectric Composites.

Abstract

Elucidating the mechanism of electrical carrier transport and thermoelectrics is crucial for wide applications in circuit protection, waste heat harvesting, flexible and wearable electronics, etc. This study reports an unusual dual-transition behavior in carbon nanotube (CNT)/sodium lignosulfonate (LS) composite materials, wherein a distinctive transition from a positive temperature coefficient to a negative temperature coefficient occurs simultaneously with a p-type to n-type transition during vacuum heating. The transition temperature demonstrates a strong dependence on the interfacial interaction. The evolution of electronic states and their interfacial interactions are monitored and studied by in situ Raman spectroscopy and XPS and UPS techniques. Through Ab Initio molecular dynamics simulations of the heating process combined with density functional theory calculations analyzing electron transfer pathways, we find that the intrinsic mechanism driving this simultaneous transition is the temperature-induced oxygen desorption and LS decomposition, which induce a switch in carrier transport from hole-dominated to electron-dominated behavior. These findings not only shed light on fundamental insights into the CNT electronic properties but also offer valuable guidance for the applications of thermoelectric materials.