P- and n-type thermoelectric cement composites with CVD grown p- and n-doped carbon nanotubes: Demonstration of a structural thermoelectric generator

Abstract

The thermoelectric (TE) behavior and microstructure of p- and n-doped multi-walled carbon nanotube (CNT) filled cement nanocomposites over 3, 7, 14 and 28 days age are reported for the first time. Pristine CNTs with inherent p-type semiconductor characteristics (p-CNT) and n-type Nitrogen-doped CNTs (n-CNT) were incorporated by shear mixing in a cement matrix at 1% by weight of cement and 0.5 water to cement ratio (w/c). CNTs were dispersed initially by ultrasonication using a surfactant and mixed further with the cement powder. Raman, high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), electric and thermoelectric measurements were employed to characterize the p-CNT and n-CNT, as well as the corresponding cement/CNT nanocomposites. The Seebeck coefficient (S) and electrical conductivity (σ) of nanocomposites for 3, 7, 14 and 28 days of sample's age were examined. All the measurements were performed on as received samples after mould casting at each specific age (samples denoted as before drying) and after drying at 80 °C overnight in order to elucidate the significant effect of moisture. The cement/p-CNT nanocomposite exhibited n-type TE behavior before drying due to the water content and the anions stemming from the cementitious matrix. A p-type behavior was observed after drying and elimination of water, due to the inherent p-type characteristics of CNTs. This is realised by plausible electron withdrawing and p-doping mechanism that adsorbed and/ or chemisorbed oxygen species facilitate to the CNT backbone. The 14 days cement/n-CNT exhibited the highest performance in terms of σ and power factor (PF) reaching 1.44 µWm−1K−2 before drying, and 7.64 × 10−3 µW m−1K−2 after drying. The cement/CNT nanocomposites with p- and n-type TE characteristics allowed the construction of a thermoelectric generator (TEG) for plausible large-scale thermal energy harvesting via cement-based structural thermoelectric generators (TEGs).