Abstract
Three-dimensional (3D) printing of thermoelectric polymer nanocomposites is reported for the first time employing flexible, stretchable and electrically conductive 3D printable thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWCNT) filaments. TPU/MWCNT conductive polymer composites (CPC) have been initially developed employing melt-mixing and extrusion processes. TPU pellets and two different types of MWCNTs, namely the NC-7000 MWCNTs (NC-MWCNT) and Long MWCNTs (L-MWCNT) were used to manufacture TPU/MWCNT nanocomposite filaments with 1.0, 2.5 and 5.0 wt.%. 3D printed thermoelectric TPU/MWCNT nanocomposites were fabricated through a fused deposition modelling (FDM) process. Raman and scanning electron microscopy (SEM) revealed the graphitic nature and morphological characteristics of CNTs. SEM and transmission electron microscopy (TEM) exhibited an excellent CNT nanodispersion in the TPU matrix. Tensile tests showed no significant deterioration of the moduli and strengths for the 3D printed samples compared to the nanocomposites prepared by compression moulding, indicating an excellent interlayer adhesion and mechanical performance of the 3D printed nanocomposites. Electrical and thermoelectric investigations showed that L-MWCNT exhibits 19.8 ± 0.2 µV/K Seebeck coefficient (S) and 8.4 × 103 S/m electrical conductivity (σ), while TPU/L-MWCNT CPCs at 5.0 wt.% exhibited the highest thermoelectric performance (σ = 133.1 S/m, S = 19.8 ± 0.2 µV/K and PF = 0.04 μW/mK2) among TPU/CNT CPCs in the literature. All 3D printed samples exhibited an anisotropic electrical conductivity and the same Seebeck coefficient in the through- and cross-layer printing directions. TPU/MWCNT could act as excellent organic thermoelectric material towards 3D printed thermoelectric generators (TEGs) for potential large-scale energy harvesting applications.
Highlights
Three-dimensional (3D) printing has dramatically expanded over the last 20 years, while currently being a cutting-edge and one of the most widely used additive manufacturing (AM) technologies amongst others [1]. 3D printed parts and components due to variable mechanical and physicochemicalMaterials 2020, 13, 2879; doi:10.3390/ma13122879 www.mdpi.com/journal/materialsMaterials 2020, 13, 2879 properties that can be achieved could be employed in a wide range of applications [2]
thermoplastic polyurethane (TPU)/MWCNT could act as excellent organic thermoelectric material towards 3D printed thermoelectric generators (TEGs) for potential large-scale energy harvesting applications
Previous research from our group investigating the thermoelectric properties of melt-mixed polycarbonate (PC)/MWCNT conductive polymer composites (CPC) at different MWCNT loadings, showed that higher carbon nanotubes (CNTs) contents resulted in increased power factors (PF = σ × S2 in W/mK2 ), which has been mainly explained by the increase of the electrical conductivity [20]
Summary
Three-dimensional (3D) printing has dramatically expanded over the last 20 years, while currently being a cutting-edge and one of the most widely used additive manufacturing (AM) technologies amongst others [1]. 3D printed parts and components due to variable mechanical and physicochemical. Previous research from our group investigating the thermoelectric properties of melt-mixed polycarbonate (PC)/MWCNT CPCs at different MWCNT loadings, showed that higher CNT contents resulted in increased power factors (PF = σ × S2 in W/mK2 ), which has been mainly explained by the increase of the electrical conductivity [20]. High-performance polyetherimide–SWCNT thermoplastic CPCs have been reported capable of operation up to 200 ◦ C with electrical conductivity reaching 20 S/m and Seebeck coefficients +55 μV/K at 10 wt.% SWCNT loading [21]. TPU/MWCNT 3D printable, flexible, stretchable and thermoelectric filaments have been developed with two different types of commercially available MWCNTs, while 3D printed thermoelements were fabricated and fully characterised. All 3D printed samples exhibited an anisotropic electrical conductivity and same Seebeck coefficient since it is an inherent material property arising from the CNT filler. The 3D printed TPU/MWCNT thermoelements printed can be the building blocks for the fabrication of flexible and stretchable organic thermoelectric generators (TEGs)
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