Nonlinear thermopower behaviour of n-type carbon nanofibres and their melt mixed polypropylene composites

• Autor(a) principal: Vieito, António Jose Paleo
• Data de Publicação: 2022
• Outros Autores: Krause, Beate, Cerqueira, M. F., Muñoz, Enrique, Pötschke, Petra, Rocha, A. M.
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: https://hdl.handle.net/1822/75889
• topic: Polypropylene; Carbon nanofibers; Thermoelectric properties; N-type polymer composites; Variable range hopping; Science & Technology

The temperature dependent electrical conductivity <i>σ</i> (<i>T</i>) and thermopower (Seebeck coefficient) <i>S</i> (<i>T</i>) from 303.15 K (30 °C) to 373.15 K (100 °C) of an as-received commercial n-type vapour grown carbon nanofibre (CNF) powder and its melt-mixed polypropylene (PP) composite with 5 wt.% of CNFs have been analysed. At 30 °C, the <i>σ</i> and <i>S</i> of the CNF powder are ~136 S m⁻¹ and −5.1 μV K⁻¹, respectively, whereas its PP/CNF composite showed lower conductivities and less negative S-values of ~15 S m⁻¹ and −3.4 μV K⁻¹, respectively. The <i>σ</i> (<i>T</i>) of both samples presents a d<i>σ</i>/d<i>T</i> < 0 character described by the 3D variable range hopping (VRH) model. In contrast, their <i>S</i> (<i>T</i>) shows a d<i>S</i>/d<i>T</i> > 0 character, also observed in some doped multiwall carbon nanotube (MWCNT) mats with nonlinear thermopower behaviour, and explained here from the contribution of impurities in the CNF structure such as oxygen and sulphur, which cause sharply varying and localized states at approximately 0.09 eV above their Fermi energy level (<i>E_F</i>).