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Nanosize La‑filled ­CoSb3 skutterudite fabricated by electrospinning Ana C. Ferreira1,2   · Joaquim B. Branco1,2 · António P. Gonçalves1 Received: 26 April 2020 / Accepted: 12 August 2020 © Springer Nature Switzerland AG 2020

Abstract Nanostructured binary skutterudites represented by M ­ X3 are potential thermoelectric materials for high efficiency thermoelectric. In this work, we synthesized for the first-time high purity nanofibers of C ­ oSb3 and rare earth filled L­ aCo4Sb12 with external diameters  0.8. Generally, the filler forms weak bonds with Sb, and their delocalization is responsible for the decrease of thermal conductivity. [46, 57–59]. So far, the effects of partially filled skutterudites with lanthanum, cerium, and ytterbium have been reported to possess lower thermal conductivities and better electrical transport properties than ­CoSb3 [23, 46]. The purpose of the present work was to use nanostructure engineering in the preparation of ­CoSb3-based skutterudites through the employment of the electrospinning technique. We fabricated for the first time C ­ oSb3 and lanthanum filled ­La0.5Co4Sb12 nanofibers by applying the electrospinning followed by specific heat treatments. The structure and microstructure of the materials obtained were characterized by powder X-ray diffraction (XRD) and Vol:.(1234567890)

| https://doi.org/10.1007/s42452-020-03334-5

scanning electron microscopy with energy dispersive X-ray detection (SEM/EDS). BET measurements and temperature-programmed reduction under oxygen ­(O2-TPO) and hydrogen ­(H2-TPR) were used to characterize this preparation route.

2 Experimental 2.1 Synthesis Nanofibers of C ­ oSb3-based materials were obtained by a three-step methodology: (i) electrospinning of the appropriate solution containing a mixture of Co(NO3)2∙6H2O (Sigma, purity 99.9%) and Sb(CH3COO)3 (Aldrich, purity 99.9%), followed by (ii) calcination and (iii) reduction steps. All the reagents were used without further purification. Solutions were prepared by mixing the starting metal salts (molar ratio Co:Sb, 1:3 or 1:5) with 42 wt.% PVP40 (AlfaAsaer, average mol wt. 40,000) in a solution of absolute ethanol (Fischer-Scientific, purity > 99.9%). To filled ­CoSb3 with lanthanum, a third solution was prepared using La(NO3)3.6H2O (molar ratio La:Co:Sb, 0.125:1:5). The solutions were stirred at 50 °C for 15 min to dissolve the metal salts, cooled down to room temperature and collected in a syringe with a ~ 0.9 mm interior diameter stainless steel flat tip needle. To start the electrospinning experiments, the solution was pumped continuously using a syringe pump (KW scientific) at a rate of 1 mL h−1, with an electric field of 17 kV applied between the syringe tip needle and a grounded aluminum plate placed 10 cm from the needle tip and used as a collector. The electrospun materials were subsequently calcined at 600 °C for 2 h in air atmosphere and then reduced under pure hydrogen flow (2 L ­h−1) at 500 °C for 2 h, both at 1 °C ­min−1 heating rate (Scheme 1).

2.2 Characterization Nitrogen ga