Article Screening of Different Carbon Nanotubes in Melt-Mixed Polymer Composites with Different Polymer Matrices for Their Thermoelectrical Properties

Beate Krause , Carine Barbier, Juhasz Levente, Maxim Klaus and Petra Pötschke *

Leibniz-Institut für Polymerforschung Dresden e.V. (IPF), Hohe Str. 6, 01069 Dresden, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-351-4658-395



Received: 14 November 2019; Accepted: 5 December 2019; Published: 7 December 2019


Abstract: The aim of this study is to reveal the influences of carbon nanotube (CNT) and polymer type as well as CNT content on electrical conductivity, Seebeck coefficient (S), and the resulting power factor (PF) and figure of merit (ZT). Different commercially available and laboratory made CNTs were used to prepare melt-mixed composites on a small scale. CNTs typically lead to p-type composites with positive S-values. This was found for the two types of multi-walled CNTs (MWCNT) whereby higher Seebeck coefficient in the corresponding buckypapers resulted in higher values also in the composites. Nitrogen doped MWCNTs resulted in negative S-values in the buckypapers as well as in the polymer composites. When using single-walled CNTs (SWCNTs) with a positive S-value in the buckypapers, positive (polypropylene (PP), polycarbonate (PC), poly (vinylidene fluoride) (PVDF), and poly(butylene terephthalate) (PBT)) or negative (polyamide 66 (PA66), polyamide 6 (PA6), partially aromatic polyamide (PARA), acrylonitrile butadiene styrene (ABS)) S-values were obtained depending on the matrix polymer and SWCNT type. The study shows that the direct production of n-type melt-mixed polymer composites from p-type commercial SWCNTs with relatively high Seebeck coefficients is possible. The highest Seebeck coefficients obtained in this study were 66.4 µV/K (PBT/7 wt % SWCNT Tuball) and 57.1 µV/K (ABS/0.5 wt % SWCNT Tuball) for p- and n-type composites, − respectively. The highest power factor and ZT of 0.28 µW/m K2 and 3.1 10 4, respectively, were · × − achieved in PBT with 4 wt % SWCNT Tuball.

Keywords: thermoelectric; carbon nanotubes; polymer composites; nitrogen-doping

1. Introduction Electrically conductive polymer composites (CPCs) based on insulating thermoplastic polymer matrices filled with carbon based materials like carbon nanotubes (CNTs), graphite (G), expanded graphite (EG) or graphene nanoplatelets (GNP) can also be used as potential thermoelectric (TE) materials for converting waste heat into electrical energy [1,2]. The thermoelectric effect, also known as Seebeck effect after its discoverer Thomas Johann Seebeck, describes the direct conversion of temperature differences on an electrically conductive material into an electrical voltage, the thermovoltage ∆U. The advantages of polymer-based TE materials over typically used metal oxides such as bismuth tellurides are not only their better availability and cost efficiency, but also their ease of processing, mechanical flexibility, low density and low thermal conductivity. Some studies on thermoelectric properties of thermoplastic polymer/CNT composites have already been published. Thereby the characteristic value of the Seebeck coefficient (S) representing the ratio between the generated thermovoltage and the temperature difference applied is used from which together with the electrical conductivity σ of the samples the power factor (PF) is calculated using

J. Compos. Sci. 2019, 3, 106; doi:10.3390/jcs3040106 www.mdpi.com/journal/jcs J. Compos. Sci. 2019, 3, 106 2 of 18 the equation PF = S2 σ. When the thermal conductivity к of the materials in the direction of the · thermovoltage generation is known, also the figure of merit (ZT) can be calculated using the relation ZT = PF T/к (with T as absolute temperature). A negative Seebeck coefficient indicates an n-type · (electron-donating) material and a positive Seebeck coefficient a p-type (electron-withdrawing) material. In order to achieve high TE performance, high Seebeck coefficient S, high electrical conductivity σ and low thermal conductivity к are desirable. However, these parameters are heavily interrelated and optimization is a challenge. In one of the first studies on the TE performance of melt-processed polymer nanocomposites, Antar et al. [3] reported poly(lactic acid) (PLA)/multi-walled CNT (MWCNT)/expanded graphite (EG) materials, in which the highest Seebeck coefficient was 17 µV/K at 32 vol % EG and the highest power factor 9.6 10 2 µW/(m K2) at 18 vol % MWCNTs. Sun et al. [4] reported on poly (vinylidene × − · fluoride) (PVDF) composites filled with MWCNTs or graphite nanoplatelets (GNPs) as solid sample as well as foam. The Seebeck coefficients for solids were found to be 10 µV/K (PVDF/2 or 5 wt % MWCNT) and around 25 µV/K (PVDF/5–15 wt % GNP). Hewitt et al. [5] found values of 10 µV/K for solution cast composites based on PVDF with 20 wt % MWCNTs. For solution cast single-walled CNT (SWCNT)/PVDF films, Hewitt et al. [6] reported Seebeck coefficients between 32 µV/K (5 wt % SWCNT, type HiPco™) and 16 µV/K (100 wt % SWCNT) at room temperature. Melt-mixed composites of polycarbonate (PC)/MWCNT were studied by Liebscher et al. [7]. When comparing the effect of different surface functionalization of MWCNTs on the TE properties of PC composites, at a constant loading of 2.5 wt %, the highest Seebeck coefficient was found to be 11.3 1.2 µV/K for PC/carboxyl-functionalized MWCNTs (MWCNT-COOH, Nanocyl NC3153), ± whereas PC with unfunctionalized MWCNT (Nanocyl NC3150) resulted in an S-value of 7.5 0.9 ± µV/K. PF values varied in dependence on the MWCNT functionalization between 10 3–10 6 µW/m K2. − − · Furthermore, a cyclic butylene terephthalate (CBT) oligomer was added to successfully improve both the processing behavior and the thermoelectric properties of PC/MWCNTs [8]. An increase of ZT from 10 8 10− (PC/MWCNT-COOH) to 10− (PC/MWCNT-COOH/CBT) could be reached. More detailed studies on TE properties of melt-mixed composites based on polypropylene (PP) with SWCNTs were presented by Luo et al. [9–12]. For PP composites with the SWCNT type Tuball (OCSiAl), Seebeck coefficients between 25 and 34 µV/K (0.8–6.0 wt % Tuball) were reported. The highest power factor was found at 0.066 µW/m K2 (PP/4 wt % Tuball) [10]. A Seebeck coefficient of 36.8 µV/K · and a power factor of 0.02 µW/m K2 were described in [9] for PP composites with 2 wt % Tuball in · which copper oxide was added as a second filler with a high S-value. The addition of 2 wt % ionic liquid (IL) to PP/2 wt % SWCNT composites leads to an increase of Seebeck coefficients from 43.2 2.2 µV/K ± to 63.4 2.8 µV/K and power factors of 0.12 µW/m K2 (without IL) and 0.26 µW/m K2 (with IL) [11]. ± · · Furthermore, Luo et al. reported ways to generate stable n-type PP composites by adding polyethylene glycol [9,12] or polyoxyethylene 20 cetyl ether [12]. Recently, Tzounis et al. [13] described Seebeck values between 16 µV/K and 55 µV/K for polyetherimide/SWCNT HiPco nanocomposites depending on the molecular structure of the polyetherimide. No literature about thermoelectric investigation on melt-prepared polyamide (PA), poly (butylene terephthalate) (PBT) or acrylonitrile butadiene styrene (ABS) based composites filled with conductive fillers could found up to now. Regarding the published results about thermoplastic polymer/CNT composites, it is noticeable that always positive Seebeck coefficients were determined. Furthermore, for pristine SWCNT films, positive S-values are reported, e.g., by Nonoguchi et al. [14] with 49 µV/K and Piao et al. [15] with 39 µV/K (SWCNT from Thomas Swan Co. LTD, Consett, UK). Only recently a study by Paleo et al. [16] reported a negative Seebeck coefficient for melt mixed composites based on PP in which the filler was an industrial high temperature treated carbon nanofiber (CNF) and no additives were added. Seebeck coefficients up to 8.5 µV/K (at 2.4 vol % loading) were reported. − It is a hypothesis that the incorporation of n-type CNTs in thermoplastic polymer matrix leads to n-type polymer composites. The thermoelectric properties of CNT filled composites are interplay of J. Compos. Sci. 2019, 3, 106 3 of 18 the properties of polymers and CNTs. The n-type doping of SWCNTs is a challenge in terms of doping stability. The literature [14,15,17–22] describes different ways of doping, such as polymer wrapping, charge transfer from polymer chains to CNTs, encapsulation of organometallic materials within the nanotubes, doping by salt anions with counter cations or nitrogen incorporation. The stability of the nanotube doping during further processing plays an important role. Especially, the replacement of C atoms in the hexagonal lattice of nanotube walls by, e.g., nitrogen is a suitable way to reach long-term stable doped nanotubes. Dörling et al. [22] reported that the addition of n-type nitrogen-doped CNTs in p-type poly(3-hexylthiophene) (P3HT) matrix leads to n-type composites. This behavior was explained by polymer wrapping along or around the CNTs due to van der Waals interactions, especially if the polymer contains aromatic groups. Depending on the polymer type, the formation of specific π–π interactions between aromatic groups of polymers and nanotubes is possible, leading to improved electronic transport properties in the composites [23]. From the doping point of view, PVDF, PC, poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA) and polystyrene (PS) are formed from polymer chains that possess neither electron-withdrawing groups nor electron-donating groups. As reported in a study by Piao et al. [15], for SWCNT buckypapers having p-type character which were infiltrated with polymer solutions of the mentioned polymers, the Seebeck coefficient increased from 38 µV/K up to around 50 µV/K due to polymer penetration by sacrificing the electrical conductivity. In contrast, if the SWCNT buckypapers were infiltrated with solutions of poly(vinyl pyrrolidone) (PVP), poly(ethylene imine) (PEI), or N,N,N0,N0-tetramethyl-p-phenylenediamine (TMPD) the S-values switched to negative values up to 40 µV/K[15]. This behavior was explained by a shift of the − Fermi level. Based on our previous work on PC with MWCNTs and PP with SWCNTs, the aim of this study was to show the effects of CNT type, polymer type, and CNT content on electrical conductivity, Seebeck coefficient (S), and the resulting power factor and figure of merit ZT. Different commercially available CNTs as well as N-doped MWCNTs synthesized in different laboratories were used to produce melt-mixed composites with various thermoplastic polymers on a small scale. Both p-type CNTs (electron-withdrawing) and n-type CNTs (electron-donating) were used to produce composites with positive or negative Seebeck coefficient. Relationships between CNT content, electrical conductivity and the Seebeck coefficient were evaluated. The main question was whether there are design rules for producing not only p-type composites, which have been the major part of all the literature reports so far, but also tailor-made stable n-type composites without additives by simple variation of matrix and CNT type.

2. Materials and Methods As polymer matrices eight different thermoplastic polymers were applied (Table1). The polymer matrices differ in polarity, crystallinity, and molecular structure.

Table 1. Polymer matrices used in this study.

Polymer Matrix Trade Name Producer Polyamide 6 (PA6) Ultramid® B27E BASF, Ludwigshafen, Germany Partially aromatic polyamide IXET-BXT2000 Solvay, Freiburg, Germany (PARA) Polyamide 66 (PA66) Noryl GTX 979 Sabic, Riyadh, Kingdom of Saudi Arabia Poly(butylene terephthalate) (PBT) Vestodur 1000 Evonik Industries, Marl, Germany Poly (vinylidene fluoride) (PVDF) Kynar® 720 Arkema S.A., Colombes, France Polypropylene (PP) Moplen HP400R Basell, Frankfurt, Germany Bayer MaterialScience, Leverkusen, Polycarbonate (PC) Makrolon2600 Germany Acrylonitrile butadiene CycolacTM S570 Sabic, Riyadh, Kingdom of Saudi Arabia styrene (ABS) J. Compos. Sci. 2019, 3, 106 4 of 18

The carbon nanotubes selected for this study (Table2) di ffer in their number of walls (SWCNT or MWCNT), structure (branched MWCNT or linear), and doping (nitrogen-doped or undoped) to identify influencing factors for thermoelectric applications. Their properties can be seen from the references given in Table2[ 24–30]. Among the SWCNT materials, according to the datasheets TuballTM SWCNTs have a mean diameter of 1.6 nm [24], whereas HiPco® SWCNTs of 0.8–1.2 nm [27]. Furthermore, the length is different, Tuball has a length >5 µm, while HiPco lengths are between 0.1 µm and 1 µm. This leads to a three times higher aspect ratio of Tuball compared to HiPco. The carbon content in Tuball is >85 wt %, that in HiPco >95 wt %.

Table 2. Carbon nanotubes used in this study.

Nanotube Kind Trade Name Company Reference Multi-walled carbon NC7000 Nanocyl, S.A., Sambreville, Belgium [25,26] nanotube (MWCNT) Applied NanoStructured Solutions LLC, Branched MWCNT CNS-PEG [26] Baltimore, MD, USA Single-walled CNT OCSiAl S.a.r.l., Luxembourg, TuballTM [24,26] (SWCNT) Luxembourg SWCNT HiPco®, super pure Unidym Inc., Sunnyvale, CA, USA [27] Nitrogen-doped Self-synthesized in a Chemical Vapor N-A11 [28] MWCNT Deposition System Nitrogen-doped Self-synthesized in an Aerosol Assisted N-IFW2 [29] MWCNT Chemical Vapor Deposition System Nitrogen-doped Nanostructured & Amorphous Materials, N400 [30] MWCNT Inc., Katy, TX, USA 1 Synthesized with iron as catalyst, a gas ratio of 50/50/50 and synthesis time of 2 h [28] at the University of Calgary, Canada. 2 Synthesized at the Leibniz Institute for Solid State and Material Research (IFW), Dresden, Germany.

Carbon nanotube buckypapers were produced by dispersing 125 mg CNTs in 100 mL chloroform for 10 min using ultrasonic treatment (UP400S, Hielscher Ultrasonics GmbH, Teltow, Germany; 60% amplitude, cycle 1, sonotrode H3). The dispersion was placed on a polytetrafluoroethylene (PTFE) filter and the solvent is sucked off. The buckypaper was then dried at 40 ◦C in an oven and lifted from the filter paper to obtain a free-standing film. Melt mixing of composites was performed in a small-scale conical twin-screw micro compounder DSM15 (Xplore Instruments BV, Sittard, The Netherlands) having a volume of 15 ccm. The melt mixing conditions are summarized in Table3.

Table 3. Melt mixing and compression molding conditions for composite preparation.

Polymer Mixing Mixing Time Rotation Pressing Pressing Time Matrix Temperature (◦C) (min) Speed (rpm) Temperature (◦C) (min) PA6 260 5 250 260 1.5 PARA 270 10 100 270 1.5 PA66 280 5 50 320 2 PBT 265 5 200 265 1 PVDF 210 10 200 200 2.5 PP 210 5 250 210 2 PC 280 5 250 280 2.5 ABS 260 5 250 260 2

The composites were compressing molded to plates having a diameter of 60 mm and a thickness of 0.3 mm using the hot press PW40EH (Paul-Otto Weber GmbH, Remshalden, Germany). The pressing conditions are summarized in Table3. Strips cut from such plates were used for measurements of electrical resistivity as well as thermoelectric properties. The Seebeck coefficient (S) and electrical volume resistivity were determined using the self-constructed equipment TEG at Leibniz-IPF [31]. The measurements were performed at 40 ◦C with temperature differences between the two copper electrodes up to 8 K. For the measurements on J. Compos. Sci. 2019, 3, 106 5 of 18 composites and buckypapers, the strips were painted with conductive silver at their ends. Since it was not possible to produce stable free-standing buckypapers for all CNT types, TE measurements were also carried out on CNT powders. For the measurement on powders, the CNTs were filled in a PVDF tube (inner diameter 3.8 mm, length 16 mm) which is closed with copper plugs (see Figure1). TheJ. Compos. final Sci. CNT 2019, package3, x has a length of 6 mm. The length of the entire double-T-test specimen5 of 18 is 30 mm. This double T-shaped specimen was clamped between the copper electrodes so that the copper plugsplugs have have direct direct planar planar contact contact with thewith copper the electrodescopper electrodes of measurement of measurement device. Depending device. Dependingon the kind ofon CNTsthe kind and of their CNTs bulk and density, their 0.01–0.04bulk density, g powder 0.01–0.04 was used.g powder The volumewas used. resistivity The volume of all resistivitysamples was of all measured samples using was measured a 4-wires using technique. a 4-wires The measurementtechnique. The of measurement voltage and resistanceof voltage wasand performedresistance usingwas performed the Keithley using multimeter the Keithley DMM2001 multimeter (Keithley Instruments,DMM2001 (Keithley Cleveland, Instruments, OH, USA). TheCleveland, values OH, given USA). represent The values mean values given ofrepresent three measurements. mean values of The three figure measurements. of merit ZT was The calculated figure of meritusing aZT value was ofcalculated thermal conductivity using a value of theof thermal composites conductivity of 0.28 W /of(m theK) whichcomposites was obtained of 0.28 W/(m·K) in many · pre-investigationswhich was obtained as in a propermany pre-investigations mean value of thermoplastic as a proper polymers mean value filled of withthermoplastic a low CNT polymers content (betweenfilled with 0 a and low 5 CNT wt %) content [32,33]. (between 0 and 5 wt %) [32,33].

Figure 1. Tube and plugs for thermoelectric measurements on powder samples (pencil for size Figure 1. Tube and plugs for thermoelectric measurements on powder samples (pencil for size comparison). comparison). 3. Results and Discussion Table 3. Melt mixing and compression molding conditions for composite preparation. 3.1. Thermoelectrical Properties of Nanotube Buckypapers and Powder Packages Polymer Mixing Mixing Time Rotation Pressing Pressing MatrixThe firstTemperature step was to (°C) characterize (min) the thermoelectricSpeed (rpm) properties Temperature of carbon nanotubes (°C) Time themselves (min) (TablePA64 ). It is assumed260 that the di fferences 5 in the characteristics 250 of the CNTs 260 will certainly be reflected1.5 inPARA the TE performance270 of the composites. 10 It should be 100 determined whether 270 the CNTs are n- or 1.5 p-type materialsPA66 indicated280 by a negative or positive 5 Seebeck value, 50 respectively. 320 2 PBT 265 5 200 265 1 PVDF Table 4. Thermoelectrical210 properties 10 of carbon nanotube 200 buckypapers (BP) 200 and powders (P). 2.5

PP 210 5 Volume Conductivity 250 Seebeck 210 Power Factor 2 Type of Nanotube Trade Name PC 280 5 (S/m) 250 Coefficient (280µV/ K) (µW/m K 2.52) · ABS 260 NC7000 (BP) 5 3125 250 8.0 0.0 260 0.2015 2 MWCNT ± NC7000 (P) 417 6.3 0.0 0.0165 ± 3. Results and DiscussionCNS-PEG (BP) 9622 15.3 0.0 2.2569 Branched MWCNT ± CNS-PEG (P) 933 10.1 0.0 0.0949 ± 3.1. Thermoelectrical PropertiesTuball of (BP) Nanotube Buckypapers 42,227 and Powder Packages 37.4 0.9 59.1933 SWCNT ± Tuball (P) 1790 39.6 0.2 2.8094 The first step was to characterize the thermoelectric properties of ±carbon nanotubes themselves HiPco (BP) 10,208 28.6 1.4 8.3590 (Table SWCNT4). It is assumed that the differences in the characteristics of± the CNTs will certainly be HiPco (P) 753 35.7 0.1 0.9597 reflected in the TE performance of the composites. It should be determined± whether the CNTs are n- N-doped MWCNT N400 (BP) 157 7.3 0.0 0.0084 or p-type materials indicated by a negative or positive Seebeck value,− respectively.± N-doped MWCNT N-A1 (P) 86 11.6 0.1 0.0117 For buckypapers (BP) of NC7000, CNS-PEG, Tuball, and HiPco,− a± positive Seebeck coefficient N-IFW (BP) 2196 10.3 0.0 0.2348 couldN-doped be measured MWCNT and the values were between 8.0 and 37.4 µV/K.− These± values are slightly lower N-IFW (P) 506 12.7 0.1 0.0812 than those reported by Nonoguchi et al. [14] (49 µV/K) and Piao et −al. [15]± (39 µV/K) for thin SWCNT (Thomas Swan & Co. LTD, Crockhall, Consett, UK) films. The Seebeck coefficient of the HiPco buckypaper (28.6 µV/K) is higher than the value of ca. 16 µV/K reported by Hewitt et al. [6] for a buckypaper prepared from HiPco (raw material, purified by the authors). Measurements of the Seebeck coefficients on nanotube powders (P) resulted in slightly different values but of the same magnitude. This difference is assigned to a different CNT packing density of the nanotubes in the buckypaper or in the compressed nanotube packing. As expected, after the incorporation of electron-donating nitrogen in the CNT structure [34], a negative Seebeck coefficient of buckypapers as well as powders was determined for the laboratory

J. Compos. Sci. 2019, 3, 106 6 of 18

For buckypapers (BP) of NC7000, CNS-PEG, Tuball, and HiPco, a positive Seebeck coefficient could be measured and the values were between 8.0 and 37.4 µV/K. These values are slightly lower than those reported by Nonoguchi et al. [14] (49 µV/K) and Piao et al. [15] (39 µV/K) for thin SWCNT (Thomas Swan & Co. LTD, Crockhall, Consett, UK) films. The Seebeck coefficient of the HiPco buckypaper (28.6 µV/K) is higher than the value of ca. 16 µV/K reported by Hewitt et al. [6] for a buckypaper prepared from HiPco (raw material, purified by the authors). Measurements of the Seebeck coefficients on nanotube powders (P) resulted in slightly different values but of the same magnitude. This difference is assigned to a different CNT packing density of the nanotubes in the buckypaper or in the compressed nanotube packing. As expected, after the incorporation of electron-donating nitrogen in the CNT structure [34], a negative Seebeck coefficient of buckypapers as well as powders was determined for the laboratory synthesized nitrogen-doped MWCNTs N-A1 and N-IFW. In order to verify these results, a buckypaper made of commercially available nitrogen-doped MWCNT N400 was measured, also resulting in a negative S coefficient. The measurements made on nanotube powders resulted in negative S-values, whereby the one on MWCNTs N-IFW led to the highest negative number amount of 12.9 µV/K. − When comparing the values of the different buckypapers, SWCNTs show higher values of the Seebeck coefficients than MWCNTs. This is in accordance with the discussion in the literature and given by the semimetallic nature of MWCNTs versus the combination of chiralities in SWCNTs ranging from semiconducting to metallic [21]. Among the SWCNTs, the buckypaper consisting of SWCNT Tuball showed a higher Seebeck coefficient of 37.4 µV/K as well as power factor of 59.2 µW/m K2. · The difference is expected to result from the different structural characteristics of both SWCNTs. All thermoelectric values of SWCNT Tuball are much higher than the values of the other buckypapers. This order is also reflected when comparing the values measured on nanotube powders. Next to the Seebeck coefficients, the electrical conductivity also behaves differently between the different CNT types. The buckypapers result in higher values than the CNT powder packages, SWCNTs show higher values than MWCNTs, and the highest value of more than 40,000 S/m was measured for the SWCNT Tuball buckypaper.

3.2. Thermoelectric Properties of Polymer Composites Filled with Different Kinds of Nanotubes The thermoelectrical properties of CNT filled composites can be regarded as interplay of the thermoelectric properties and the CNT network formed in a given matrix and a possible charge transfer from the polymer matrix to the CNTs. Thus, it can be expected that the same nanotubes forming networks in different matrices can have different charge carrier behavior. The results for composites based on different polymer matrices with 2 wt % of different CNTs are summarized in Figures2–5 and Table5. For ABS and PA6, additional results are shown for di fferent CNT contents. It was found that all composites containing MWCNTs NC7000 or CNS-PEG exhibit a positive Seebeck coefficient whereby the S-values for polymer/CNS-PEG composites are always higher than those for composites with NC7000. This result correlates with the higher S-value of buckypaper based on CNS-PEG than the one made of NC7000. J. Compos. Sci. 2019, 3, x 8 of 18 J. Compos. Sci. 2019, 3, x 8 of 18 J. Compos. Sci. 2019, 3, x 8 of 18 J. Compos. Sci. 2019, 3, 106 3 3 3 3 3 3 7 of 18 70 10 10 10 70 10 10 10 J. Compos. 40°C,Sci. 2019PBT , 3, x 40°C, PVDF 2 2 28 of 18 60 2 2 2 60 3 3 3 70 103 103 103 70 10 10 10 10 10 10 50 40°C, PVDF 1 1 1 50 40°C, PBT 31 31 31 1023 1023 1023 6070 102 102 102 6070 10 10 10

40 40°C, PBT 0 0 )] 0 40 40°C, PVDF 0 0 0 )] 12 12 2 12 5060 1012 1012 1012 5060 10 10 10 3 3 2 3 70 103 103 103 70 10 10 10 30 10-1 10-1 10-1 30 10-1 10-1 10-1 4050 40°C, PBT 01 01 )] 01 4050 40°C, PVDF 1001 1001 1001 10 10 10 2 2 )] 2 2 2 2 2 60 10 10 10 60 10 10 10 20 10 10 2 10 20 10-2 10-2 10-2 30 -2 -2 -2 3040 -10 -10 )] -10 40 10-10 10-10 10-10 )]

10 10 2 10 1 1 1 50 101 101 101 50 10 10 10 10 10 10 2 10 10 10-3 10-3 10-3 10-3 10-3 10-3 2030 -2-1 -2-1 -2-1 2030 10-2-1 10-2-1 10-2-1

10 10 )] 10 40 0 0 0 40 0 0 0 10 10 10 10 10 )] 10

2 0 10 10 10 ZT [-] 0 10 10 10 2 1020 10-4 10-4 10-4 1020 -4 -4 -4 -3-2 -3-2 -3-2 30 10-3-2 10-3-2 10-3-2 [-]ZT -1030 10-1 10-1 10-1 -10 10-1 10-1 10-1

10 10 10

10 10 10 0 10 10 10 100 -5 -5 -5 ZT [-] 10 -5 -5 -5 20 10-4-3 10-4-3 10-4-3 20 10-4-3 10-4-3 10-4-3 -20 10-2 10-2 10-2 -20 10-2 10-2 10-2 [-]ZT

-100 10 10 10 10 10 10 ZT [-] -100 -6 -6 -6 -6 -6 -6 10 -5-4 -5-4 -5-4 10 10-5-4 10-5-4 10-5-4 Seebeck coefficient [µV/K] coefficient Seebeck -30 10-3 10-3 10-3 -30 10-3 10-3 10-3 [-]ZT Seebeck coefficient [µV/K] coefficient Seebeck -20-10 10 10 10 -20-10 10 10 10 Power factor [µW/(m·K Power factor -7 -7 -7 -7 -7 -7 0 ZT [-] 0 10-6-5 10-6-5 10-6-5 -40 10-6-5 10-6-5 [µW/(m·K factor Power 10-6-5 -40 -4 -4 -4 -4 [S/m] conductivity Electrical -4 -4 10 10 10 10 10 10 [-]ZT 10 10 10 [µV/K] coefficient Seebeck -30-20 10 10 10 -30-20 10-8 Electrical conductivity [S/m] 10-8 10-8 Seebeck coefficient [µV/K] coefficient Seebeck -10 -8 -8 -8 -10 -6 -6 -6 -50 10-7-6 10-7-6 [µW/(m·K Power factor 10-7-6 -50 10-5-7 10-5-7 10-5-7 -5 -5 -5 10 10 [µW/(m·K factor Power 10 -40-30 10 10 10 [µV/K] coefficient Seebeck -40-30 10 10 10 10 [S/m] conductivity Electrical 10 10 10 10 10

Seebeck coefficient [µV/K] coefficient Seebeck -9 -9 -9 -9 -9 -9 -20 -20 Electrical conductivity [S/m] -60 10-8-7 10-8-7 [µW/(m·K Power factor 10-8-7 -60 10-8-7 10-8-7 10-8-7

-6 -6 -6 -6 -6 [µW/(m·K factor Power -6 NC7000 -50 Tuball 10 10 10

-50 -40 CNS-PEG -40 N-IFW 10 10 10 10 10 10 Tuball NC7000 Electrical conductivity [S/m] conductivity Electrical CNS-PEG 10 10 10 10 10 10 [µV/K] coefficient Seebeck -30 -10 -10 -10 -30-70 -10 -10 -10 -70 Electrical conductivity [S/m] Seebeck coefficient [µV/K] coefficient Seebeck 10-9-8 10-9-8 10-9-8 10-9-8 10-9-8 10-9-8 -60-50 10-7 10-7 [µW/(m·K Power factor 10-7 -60-50 10-7 10-7 10-7 10 10 [µW/(m·K factor Power 10 NC7000 10 10 10 -40 Tuball

-40 CNS-PEG N-IFW Electrical conductivity [S/m] conductivity Electrical Tuball NC7000 CNS-PEG -9 -9 -9 -70-60 -8-10-9 -8-10-9 -8-10-9 -70-60 -8-10 Electrical conductivity [S/m] -8-10 -8-10 10 10 10 -50 10 10 10 NC7000 -50 Tuball 10 10 10

10 10 10 CNS-PEG N-IFW Tuball NC7000 CNS-PEG -10 -10 -10 -70 -9-10 -9-10 -9-10 -70 10-9 10-9 10-9 Figure-60 2. Composites filled with10 2 10wt %10 different-60 carbon nanotubes (CNTs): 10poly 10(butylene10 NC7000 Tuball CNS-PEG N-IFW Tuball NC7000 -70 CNS-PEG 10-10 10-10 10-10 -70 10-10 10-10 10-10 Figureterephthalate) 2. Composites (PBT) (left filled), poly(vinylidene with 2 wt % fluoride) different (PVDF) carbon (right nanotubes). (CNTs): poly (butylene Figure 2. Composites filled with 2 wt % different carbon nanotubes (CNTs): poly (butylene Figureterephthalate) 2. Composites (PBT) ( filledleft), withpoly(vinylidene 2 wt % different fluoride) carbon (PVDF) nanotubes (right (CNTs):). poly (butylene terephthalate) Figureterephthalate) 2. Composites (PBT) (left filled), poly(vinylidene with 2 wt % fluoride) different (PVDF) carbon (right nanotubes). (CNTs): poly (butylene 70 103 103 103 70 103 103 103 (PBT)40°C, (left PP ), poly(vinylidene fluoride) (PVDF) (right). 40°C, PC terephthalate)60 (PBT) (left), poly(vinylidene2 2 fluoride)2 60(PVDF) (right). 2 2 2 70 103 103 103 70 103 103 103 50 40°C, PP 1 1 1 50 40°C, PC 1 1 1 6070 1023 1023 1023 6070 1023 1023 1023 40 40°C, PP 0 0 0 40 40°C, PC 0 0 0 12 12 12 12 12 )] 12 50 5060 10 10 2 10 60 103 103 103 70 103 103 103 7030 10 10 )] 10 30 10 10 10 -1 -1 2 -1 -1 -1 -1 4050 40°C, PP 01 01 01 4050 40°C, PC 01 01 01 102 102 102 60 102 102 )] 102 6020 10 10 10 20 10 10 2 10 30 -2 -2 )] -2 3040 -2 -2 -2 40 -10 -10 2 -10 -10 -10 -10 50 101 101 101 50 101 101 )] 101 2 10 10 10 10 10 10 10 10 10 10 10 10 10 10 -3 -3 )] -3 -3 -3 -3 2030 2030 40 100-2-1 100-2-1 2 100-2-1 40 100-2-1 100-2-1 100-2-1

)] 0 10 10 10 0 10 10 10 ZT [-]

10 10 2 10

10 10 10

-4 -4 -4 -4 -4 -4

1020 ZT [-] 1020 -3-2 -3-2 )] -3-2 -3-2 -3-2 -3-2

30 10 10 10 30 10 10 10 -10 10-1 10-1 2 10-1 -10 10-1 10-1 10-1

10 10 10 10 10 10 ZT [-] 100 -5 -5 -5 100 -5 -5 -5 -4-3 -4-3 -4-3 -4-3 -4-3 -4-3

20 20 -20 10-2 10-2 10-2 ZT [-] -20 10-2 10-2 10-2

-100 10-6 10-6 10-6 -100 10-6 10-6 10-6 ZT [-]

10 -5-4 -5-4 -5-4 10 -4 -4 -4 -30 10-3 10-3 10-3 ZT [-] 10-3-5 10-3-5 10-3-5

Seebeck coefficient [µV/K] -30 10 10 10 10 10 10 Seebeck coefficient [µV/K] coefficient Seebeck -20-10 10 10 10 -20-10 10 10 10

-7 -7 -7 -7 -7 -7 0 ZT [-]

0 [µW/(m·K factor Power -40 10-4-6-5 10-4-6-5 10-4-6-5 -40 10-4-6-5 10-4-6-5 10-4-6-5 -30 10 10 10 ZT [-] 10 10 10

Seebeck coefficient [µV/K] -30 Electrical conductivity [S/m] -20 10 10 Power factor [µW/(m*K 10 -20 10 10 10 Seebeck coefficient [µV/K] coefficient Seebeck -8 Electrical conductivity [S/m] -8 -8 -8 -8 -8 -10-50 10-7-6 10-7-6 10-7-6 -10-50 10-7-6 10-7-6 10-7-6 -40-30 10-5 10-5 10-5 -40 10-5 10-5 [µW/(m·K factor Power 10-5 10 10 10 Seebeck coefficient [µV/K] -30 10 10 10 Seebeck coefficient [µV/K] coefficient Seebeck -20 -9 -9 -9 -20 -9 -9 -9 Electrical conductivity [S/m] -7 -7 Power factor [µW/(m*K -7 -60 10-8 Electrical conductivity [S/m] 10-8 10-8 -60 -8-7 -8-7 -8-7 -6 -6 -6 10 10 [µW/(m·K factor Power 10 Tuball

N-IFW -6 -6 -6 N-A1 NC7000

-50 10 10 10 CNS-PEG Tuball 10 10 10 -40 CNS-PEG 10 10 10 -50-40 10 10 10 -30 NC7000 10 10 10 10 10 10

-10 -10 -10 Seebeck coefficient [µV/K] -30 Electrical conductivity [S/m] Power factor [µW/(m*K -10 -10 -10

Seebeck coefficient [µV/K] coefficient Seebeck -70 -9-8 Electrical conductivity [S/m] -9-8 -9-8 -70 -9-8 -9-8 -9-8 -60-50 10-7 10-7 10-7 -60-50 10-7 10-7 10-7 10 10 [µW/(m·K factor Power 10 Tuball N-IFW 10 10 10 N-A1 -40 NC7000 10 10 10 CNS-PEG -40 Tuball CNS-PEG NC7000 -9 -9 -9 Electrical conductivity [S/m] -10 -10 Power factor [µW/(m*K -10 -10-9 -10-9 -10-9 -70-60 10-8 Electrical conductivity [S/m] 10-8 10-8 -70-60 10-8 10-8 10-8 Tuball -50 N-IFW N-A1 NC7000

10 10 10 -50 CNS-PEG Tuball

CNS-PEG 10 10 10

NC7000 -70 -9-10 -9-10 -9-10 -70 -10 -10 -10 -60Figure 3. Composites filled with10 2 wt10 % different10 CNTs:-60 polypropylene (PP) (left),10 polycarbonate-9 10-9 10-9 Tuball N-IFW

N-A1 NC7000 CNS-PEG Tuball CNS-PEG NC7000 -10 -10 -10 -10 -10 -10 Figure-70(PC) (right 3. Composites). filled with10 2 wt10 % different10 CNTs:-70 polypropylene (PP) (left),10 polycarbonate10 10 Figure 3.3. CompositesComposites filledfilled with with 2 wt2 wt % % di ffdifferenterent CNTs: CNTs: polypropylene polypropylene (PP) (PP) (left ),(left polycarbonate), polycarbonate (PC) (PC) (right). (Figureright). 3. Composites filled with 2 wt % different CNTs: polypropylene (PP) (left), polycarbonate (PC) (right). 3 3 3 70 10 10 10 70 103 103 103 40 °C, ABS 2 2 2 40 °C, PA6 60(PC) (right). 103 103 103 60 2 2 2 70 10 10 10 70 103 103 103 40 °C, ABS 1 1 1 10 10 10 50 1023 1023 1023 50 40 °C, PA6 1 1 1 6070 10 10 10 6070 1023 1023 1023 4 wt% 4 wt% 4 wt% HiPco 10 10 10 40 40 °C, ABS 0 0 0

4 wt% 4 40 40 °C, PA6 0 0 0 1012 1012 1012 )] 5060 3 3 ] 3 12 12 12 10 10 10 5060 10 10 2 10

10 10 10 5 wt% Tuball 2 wt% HiPco 70 10 10 2 10 70 103 103 103 1 wt%

30 4 wt% 4 wt% -1 -1 -1 10 10 10 4 wt% HiPco 1 wt% 1 30 -1 -1 -1 4050 40 °C, ABS 1001 1001 1001 5 wt% NC7000 1 1 1 4 wt% 4 2 2 2 4050 40 °C, PA6 0 0 0 10 10 10 10 10 )] 10

60 ] 2 2 2

10 10 10 60 10 10 2 10 5 wt% Tuball 2 wt% HiPco 2

20 4 wt% 5 wt% CNS-PEG 4 wt% -2 -2 -2 20 4 wt% HiPco 10-2 10-2 10-2 1 wt% 3040 wt% 1 -10 -10 -10 1 wt% 1 10 10 10 30 4 wt% 4 40 -10 -10 -10

1 1 1 )] 5 wt% NC7000 4 wt% 4 10 10 10 10 10 10 50 10 10 ] 10 50 1 1 1 10 10 10 10 10 2 10 5 wt% Tuball 2 wt% HiPco 10 -3 -3 2 -3 10 10 10 10

1 wt% -3 -3 -3 4 wt%

20 4 wt% 4 wt% HiPco

30 5 wt% CNS-PEG 2 wt% -1 -1 -1 1 wt% 1 10-2 10-2 10-2 2030 -2-1 -2-1 -2-1 5 wt% NC7000 40 wt% 1 100 100 100 10 10 10

4 wt% 4 10 10 10 40 0 0 0

10 10 )] 10

0 wt% 4 10 10 10 10 10 10

] 0

-4 -4 -4 10 10 2 10 5 wt% Tuball 2 wt% HiPco 1020 2 10 -4 -4 -4 5 wt% CNS-PEG 10-3-2 10-3-2 10-3-2 20 ZT [-] 1 wt% -3-2 -3-2 -3-2 30 wt% 1

-1 -1 -1 ZT 10 10 10 1 wt% 1 2 wt% 10 10 10 30 -1 -1 -1

-10 5 wt% NC7000 4 wt% 4 10 10 10 -10 10 10 10

100 -5 -5 -5 100 10-5 10-5 10-5 20 10-4-3 10-4-3 10-4-3 -4-3 -4-3 -4-3 20 5 wt% CNS-PEG 2 wt% 10-2 10-2 10-2 10-2 10-2 10-2 ZT [-] -20 wt% 1 -20 10 10 10 10 10 10 ZT 10 10 10

-100 wt% 4 -6 -6 -6 -100 10-6 10-6 10-6 10 NC7000 10-5-4 10-5-4 10-5-4 10 -5-4 -5-4 -5-4 -30 10-3 10-3 10-3 -30 10-3 10-3 10-3 ZT [-]

10 10 10 [µV/K] Seebeck coefficient CNS-PEG 2 wt% ZT 10 10 10 -20 10 10 [µW/m·K Power factor 10 -20 Seebeck coefficient-10 [µV/K] -7 -7 -7 -10 10-7 10-7 10-7

0 -5 -5 -5 0 Power factor [µW/(m·K 10-6 [S/m] conductivity Electrical 10-6 10-6 -6-5 -6-5 -6-5 -40 NC7000 10-4 10-4 10-4 -40 10-4 10-4 10-4 10 10 10 ZT [-]

HiPco 10 10 10 -30 10 10 10 -30-20 [S/m] conductivity Electrical -20 [µV/K] Seebeck coefficient CNS-PEG

ZT 10 10 10 -10 -8 -8 [µW/m·K Power factor -8 -10 -8 -8 -8 Seebeck coefficient [µV/K] 10-7-6 10-7-6 10-7-6 -7-6 -7-6 -7-6 -50 NC7000 -5 -5 -5 -50 10 10 10 10 10 10 -5 -5 Power factor [µW/(m·K -5 -40-30 10 [S/m] conductivity Electrical 10 10 -40-30 10 10 10 Seebeck coefficient [µV/K] Seebeck coefficient CNS-PEG 10 10 10 Power factor [µW/m·K Power factor HiPco -9 -9 -9 -20 -9 -9 -9 -20 [S/m] conductivity Electrical Seebeck coefficient-60 [µV/K] 10-8-7 10-8-7 10-8-7 -60 10-8-7 10-8-7 10-8-7

-6 -6 -6 Power factor [µW/(m·K 10 [S/m] conductivity Electrical 10 10 -6 -6 -6

NC7000 10 10 10 -50-40 Tuball 10 10 10 -50-40 10 10 10 HiPco -30 -10 -10 -10 -30 -10 [S/m] conductivity Electrical -10 -10 Seebeck coefficient [µV/K] Seebeck coefficient CNS-PEG -70 10-9-8 10-9-8 [µW/m·K Power factor 10-9-8 -70 -9-8 -9-8 -9-8 Seebeck coefficient [µV/K]-60-50 10-7 10-7 10-7 -60-50 10-7 10-7 10-7 Power factor [µW/(m·K 10 [S/m] conductivity Electrical 10 10 10 10 10 -40 Tuball -40 HiPco -9 -9 -9 -10-9 [S/m] conductivity Electrical -10-9 -10-9 -70 10-8-10 10-8-10 10-8-10 -70-60 -8 -8 -8 -60 10 10 10 -50 10 10 10 -50 Tuball 10 10 10 -10 -10 -10 -10 -10 -10 -70 -9 -9 -9 -70 -9 -9 -9 -60Figure 4. Composites filled with different10 10CNTs10 and-60 CNT contents: acrylonitrile butadiene10 10 styrene10 Tuball -70Figure 4. Composites filled with different10-10 10 CNTs-10 10 and-10 -70 CNT contents: acrylonitrile butadiene10-10 10 styrene-10 10-10 Figure(ABS) ( left4. Composites), polyamide filled 6 (PA6) with (right different). CNTs and CNT contents: acrylonitrile butadiene styrene (ABS)Figure ( left4. Composites), polyamide filled 6 (PA6) with (right different). CNTs and CNT contents: acrylonitrile butadiene styrene (ABS) (left), polyamide 6 (PA6) (right). Figure 4. Composites filled with different CNTs and CNT contents: acrylonitrile butadiene styrene (ABS) (left), polyamide 6 (PA6) (right). 3 3 3 70 103 103 103 70 10 10 10 40 °C, PARA 40 °C, PA66 (ABS) (left), polyamide 6 (PA6) (right). 2 2 2 60 2 102 2 60 103 103 103 70 103 103 103 70 10 10 10 1 1 1 1 40 °C, PA66 1 1 50 40 °C, PARA 3 1023 3 50 1023 1023 1023 6070 102 10 102 6070 10 10 10 40 °C, PARA 0 40 40 °C, PA66 0 0 0 0 0 10 )] 40 2 1012 2 1012 12 1012 101 101 5060 3 3 2 3 5060 103 103 )] 103 10 10 10 70 10 10-1 2 10 70 10 10-1 10 30 -1 10 -1 30 -1 1001 -1 40 °C, PARA 1 01 1 4050 40 °C, PA66 1001 1001 4050 1020 102 1020 102 102 )] 102 10 10 60 10-2 2 60 10 10-2 )] 10 20 10 10 20 10 2 -2 10-10 -2 -2 -10 -2 3040 -10 1 -10 0 0 10 10 )] 10 3040 101-1 101 101-1 101 101 10 10 50 10 10-3 2 10 50 10 10-3 )] 10 10 10 10 10 10-2 2 -3 10-2-1 -3 20 -3 -1 -3 2030 -2-1 100 -2-1 30 10-2-1 100 10-2-1 40 100 -4 100 40 100 10-4 100 0 10 10 )] 10 0 10 10 10-3-2 2 -4 10-3-2 )] -4 -4 -4 1020 2 1020 10-1 10-3-2 10-1 10-3-2 30 10-1-3-2 10-5 10-1-3-2 30 10-1 10-5 10-1 [-] ZT -10 10 10 -10 10 10-4-3 10 10 10-4-3 10 [-] ZT 100 -5 10-2 -5 100 -3-5 10-2 -3-5 20 10-4-3 10-6 10-4-3 20 10-2-4 10-6 10-2-4 -20 10-2 10 10-2 -20 10 10-5-4 10 10 -5-4 10 [-] ZT -10 10 10 -100 -6 10-3 -6 0 -6 10-3 -6 [-] ZT 10 10-5-4 10-7 10-5-4 10 10-5-4 10-7 10-5-4 -30 10-3 10 10-3 -30 10-3 10-6-5 10-3

Seebeck coefficient [µV/K]Seebeck coefficient -6 10 -5 10 [-] ZT -20-10 10 10 -20-10 10-4 [-] ZT -7 10-4 [µW/(m·K Power factor -7 -7 -7 Power factor [µW/(m·K factor Power

2 wt% NC7000 0 -5 -5 0 -5 -8 -5 Seebeck coefficient [µV/K] -6 10-8 -6 10-4-6 10 10-4-6 -40 10-4 10-4 -40 10 10-7-6 10 10 Electrical conductivity [S/m] 10-7-6 10 10 [S/m] conductivity Electrical 10 -30-20 10 10-5 10 -30-20 wt% 2 HiPco 10 10 10-5 [-] ZT Seebeck coefficient [µV/K]Seebeck coefficient -8 -8 -10 -8 -9 -8 -10 10-9 -6 10 -6 [-] ZT 10-7-6 [µW/(m·K Power factor 10-7-6 -50 10-5-7 10-5-7 Power factor [µW/(m·K factor Power

-50 2 wt% NC7000 10 10-5 10-8-7 10-5 Seebeck coefficient [µV/K] -40-30 10 -8-7 10 -40-30 10 10-6 10 Electrical conductivity [S/m] 10-6 Seebeck coefficient [µV/K]Seebeck coefficient

Electrical conductivity [S/m] conductivity Electrical -9 -9 -20 wt% 4 HiPco -9 -9 -20 10-10 2 wt% 2 HiPco 10-10 10-8-7 [µW/(m·K Power factor 10-8-7 -60 10-8-7 10-8-7

-60 10 [µW/(m·K factor Power

2 wt% NC7000 -9 10-6 10-9-8 10-6 Seebeck coefficient [µV/K] -50 10-6 -8 10-6 -40 Tuball NC7000

CNS-PEG -7 2 wt% CNS-PEG 10 -50-40 10 10 Electrical conductivity [S/m] 10 10

4 wt% Tuball 10 10-7 10 2 wt% 2 Tuball -30 -10 [S/m] conductivity Electrical -11 -10 -30 -10 10-11 -10

2 wt% 2 HiPco 10 Seebeck coefficient [µV/K]Seebeck coefficient -9-8 -9-8 -70 wt% 4 HiPco 10-9-8 10-10-9 10-9-8 -70 10 10-10-9 10 10-7 [µW/(m·K Power factor 10-7 -60 10-7 10-7 10 10 -50 [µW/(m·K factor Power 2 wt% NC7000 -60-50 10 10-8 10 Seebeck coefficient [µV/K] -40 10 10-8 10 Tuball Electrical conductivity [S/m] NC7000 10

-40 CNS-PEG 2 wt% CNS-PEG 10 4 wt% Tuball 2 wt% 2 Tuball Electrical conductivity [S/m] conductivity Electrical -9 -9 4 wt% 4 HiPco -9 -9 -10 -11-10 -10 2 wt% 2 HiPco -8-10 -11-10 -8-10 -70 10-8 10-8 -70-60 10 10-9 10 -50-60 10 10-9 10

-50 Tuball 10 NC7000 CNS-PEG

2 wt% CNS-PEG 10 4 wt% Tuball 2 wt% 2 Tuball -10 -11 -10 -9-10 -11 -9-10 -70Figure 5. PARA composites filled withwt% 4 HiPco di-9 fferent CNTs-9 and-70 different CNT contents (left),10 polyamide10-10 10 66 -60Figure 5. PARA composites filled with10 different10-10 10 CNTs-60 and different CNT contents (left10), polyamide10 10 Tuball NC7000 CNS-PEG 2 wt% CNS-PEG 4 wt% Tuball 2 wt% 2 Tuball -10 -11 -10 -70 -10 -11 -10 -70(PA66)Figure66 (PA66) composites5. PARA composites composites filled filled with withfilled 2 wt 2 with %wt di 10% ffdifferent erentdifferent10 CNTs10 CNTs CNTs (right and (right). different). CNT contents (left10), polyamide10 10 Figure 5. PARA composites filled with different CNTs and different CNT contents (left), polyamide 66 (PA66) composites filled with 2 wt % different CNTs (right). Figure66 (PA66) 5. PARA composites composites filled withfilled 2 withwt % different different CNTs CNTs and (right different). CNT contents (left), polyamide

66 (PA66) composites filled with 2 wt % different CNTs (right).

J. Compos. Sci. 2019, 3, 106 8 of 18

Table 5. Thermoelectrical parameters of the composites shown in Figures2–5.

Volume Conductivity Seebeck Power Factor Figure of Composite (S/m) Coefficient (µV/K) (µW/m K2) Merit ZT · PBT + 2 wt % NC7000 5.1 6.8 0.0 0.0002 2.7 10 7 ± × − PBT + 2 wt % CNS-PEG 13.5 15.7 0.1 0.0033 3.7 10 6 ± × − PBT + 2 wt % Tuball 18.6 58.2 0.2 0.0630 7.0 10 5 ± × − PBT + 2 wt % N-MWCNT IFW 1.0 8.2 0.1 7.0 10 5 7.8 10 8 − ± × − × − PVDF + 2 wt % NC7000 3.2 14.3 0.3 0.0007 7.4 10 7 ± × − PVDF + 2 wt % CNS-PEG 194.5 23.4 0.2 0.1063 1.2 10 4 ± × − PVDF + 2 wt % Tuball 139.3 24.0 0.3 0.0803 9.0 10 5 ± × − PP + 2 wt % NC7000 1.6 9.5 0.1 0.0001 1.7 10 7 ± × − PP + 2 wt % CNS-PEG 95.6 17.5 0.0 0.0291 3.3 10 5 ± × − PP + 2 wt % Tuball 12.1 47.2 1.9 0.0270 3.0 10 5 ± × − PP + 2 wt % N-MWCNT A1 0.1 17.3 0.0 2.5 10 5 2.8 10 8 − ± × − × − PP + 2 wt % N-MWCNT IFW 10.5 10.3 0.1 0.0011 1.2 10 6 − ± × − PC + 2 wt % NC7000 8.5 8.5 0.0 0.0006 6.8 10 7 ± × − PC + 2 wt % CNS-PEG 72.8 15.8 0.2 0.0181 2.0 10 5 ± × − PC + 2 wt % Tuball 1.0 34.8 0.3 0.0012 1.4 10 6 ± × − ABS + 1 wt % NC7000 0.3 3.0 1.0 2.2 10 6 2.9 10 9 ± × − × − ABS + 4 wt % NC7000 25.7 3.6 0.4 0.0003 3.8 10 7 ± × − ABS + 1 wt % CNS-PEG 4.0 3.1 1.2 3.9 10 5 4.4 10 8 ± × − × − ABS + 4 wt % CNS-PEG 91.7 6.1 1.2 0.0035 3.9 10 6 ± × − ABS + 1 wt % Tuball 4.2 50.9 3.0 0.0109 1.2 10 5 − ± × − ABS + 4 wt % Tuball 66.2 32.7 6.4 0.0706 7.9 10 5 − ± × − ABS + 2 wt % HiPco 4.8 10 4 16.8 4.2 1.4 10 7 1.5 10 10 × − − ± × − × − ABS + 4 wt % HiPco 0.1 15.2 1.8 2.7 10 5 3.0 10 8 − ± × − × − PA6 + 5 wt % NC7000 2.0 10 2 6.1 1.2 7.7 10 7 8.6 10 10 × − ± × − × − PA6 + 5 wt % CNS-PEG 2.9 10 3 12.6 0.0 4.5 10 7 5.1 10 10 × − ± × − × − PA6 + 5 wt % Tuball 64.6 47.0 2.9 0.1425 1.6 10 4 − ± × − PA6 + 2 wt % HiPco 4.6 10 2 27.5 1.0 3.5 10 5 3.9 10 8 × − ± × − × − PA6 + 4 wt % HiPco 0.4 9.1 0.0 3.2 10 5 3.5 10 8 ± × − × − PARA + 2 wt % NC7000 6.5 1.5 0.2 1.5 10 5 1.7 10 8 ± × − × − PARA + 2 wt % CNS-PEG 40.6 9.1 0.0 0.0033 3.7 10 6 ± × − PARA + 2 wt % Tuball 2.4 46.6 1.3 0.0052 5.8 10 6 − ± × − PARA + 4 wt % Tuball 27.3 26.2 0.1 0.0188 2.1 10 5 − ± × − PARA + 2 wt % HiPco 2.4 10 4 0.7 4.0 1.1 10 10 1.2 10 13 × − ± × − × − PARA + 4 wt % HiPco 0.3 33.6 0.4 0.0003 3.5 10 7 ± × − PA66 + 2 wt % NC7000 5.0 6.3 0.0 0.0002 2.2 10 7 ± × − PA66 + 2 wt % CNS-PEG 32.7 4.2 0.0 0.0006 6.5 10 7 ± × − PA66 + 2 wt % Tuball 28.2 29.5 1.4 0.0246 2.7 10 5 − ± × −

At the same CNT content of 2 wt % in the composites, the addition of SWCNT Tuball resulted in either positive or negative Seebeck coefficients, depending on the matrix polymer. In PBT, PVDF, PP and PC, positive Seebeck coefficient values were found which exceed the values for the corresponding composites with both MWCNTs (Figures2 and3). The higher magnitude of the Seebeck coe fficients in the SWCNT Tuball based composites is in accordance with its highest value among all buckypapers. In ABS, PA6, PARA and PA66, negative Seebeck coefficients of relatively high magnitude ( 30 to − 51 µV/K) were reached. Variation of the SWCNT Tuball content, as performed in ABS and PARA, − resulted in higher (less negative) values with increasing Tuball loading. Based on the positive Seebeck coefficient of the Tuball buckypaper and the previously published values for composites containing SWCNTs (in matrices different than those used here), this behavior was unexpected and not yet been described in the literature. The Tuball material is characterized by the highest electrical conductivity and the highest Seebeck coefficient of the buckypaper (equivalent to 100% CNTs). Based on that, the power factor PF of the SWCNT Tuball buckypaper showed by far the highest value. The addition of SWCNT HiPco, only studied in ABS, PA6 and PARA resulted in negative values in ABS, but positive values in PA6 and PARA. This was proven at two different SWCNT contents, namely 2 and 4 wt %. Thereby, higher SWCNT HiPco content decreased the (positive) Seebeck coefficient in J. Compos. Sci. 2019, 3, 106 9 of 18

PA6 but increased it in PARA. In ABS the (negative) Seebeck coefficient gets higher (less negative) when the SWCNT content is increased. If n-type N-MWCNTs are included in the comparison, as studied for PBT and PP, according to the n-type of those buckypapers all composites show negative Seebeck coefficients. This indicates that the n-type character of the N-MWCNTs dominates the composite TE properties even though the three polymers were p-type materials. Thereby, the achieved values are in the range between 8.2 µV/K (PBT) and 17.3 µV/K (PP) (Table5). A direct comparison was made between 2 wt % N-A1 − − and N-IFW in PP based composites whereby the tubes N-A1 resulted in more negative values (Figure3 left, Table5). In addition, the electrical network structure of the CNTs in the composite is important for the TE properties. The comparison of the same CNT kind in different matrices shows that the TE properties are also significantly dependent on the polymer type. For example, the Seebeck for composites filled with 2 wt % NC7000 varied between 1.5 µV/K (PARA, 6.3 µV/K (PA66), 6.8 µV/K (PBT), 8.5 µV/K (PC), 9.5 µV/K (PP) up to 14.3 µV/K (PVDF) (see Table5). It has to be considered that the di fferent composites may have different states of the conductive network, as the processing conditions were different which are known to significantly influence the state of the CNT dispersion and network formation [35,36]. In addition, the interactions between the polymer types and the CNTs are different. This results in different nanotube dispersibility and dispersion. The relationship between the amount of non-dispersed CNTs (determined using transmission light microscopy at 1 vol % loading of MWCNTs Baytubes C150 HP, Bayer Material Science, Leverkusen, Germany) and the calculated interfacial energy between CNTs and some polymers is described by Kasaliwal et al. [36]. The comparison showed very low interfacial energy and good macrodispersion for PA66 and PC, but worse dispersion and higher interfacial energy for PP. One also may argue that the distance between the actual used CNT concentration of 2 wt % and the electrical percolation threshold concentration are different when using the various polymer matrices, so that the composites are compared at different network states. The electrical percolation threshold reflects the state of the dispersion of conductive fillers and is generally lower with better dispersion. The electrical percolation thresholds of the studied composites with NC7000, CNS-PEG and SWCNT Tuball are summarized in Table6.

Table 6. Electrical percolation threshold of polymer/CNT composites.

Polymer +NC7000 +CNS-PEG +Tuball PBT 0.1–0.2 wt % [37] <0.25 wt % 0.1–0.2 wt % [37] PVDF 1 wt % [26] 0.1–0.2 wt % [26] 0.1–0.2 wt % [26] PP 0.5–0.75 wt % [26] <0.1 wt % [26] 0.075–0.1 wt % [24] PC 0.25–0.5 wt % [26] 0.05–0.075 wt % [26] 0.1–0.2 wt % [26] ABS 0.25–0.5 wt % [38] <0.25 wt % [38] <0.25 wt % [39] PA6 2 wt % 1 wt % <0.1 wt % [39] PARA <2 wt % 1 <2 wt % 1 <2 wt % 1 PA66 0.1–0.25 wt % [40] <2 wt % 1 <2 wt % 1 1 Composites with CNT concentrations < 2 wt % were not prepared.

The trend is that the electrical percolation thresholds for composites with NC7000 are typically higher than for the other two CNT types (CNS-PEG, Tuball). Even if the percolation thresholds between the different MWCNTs in the same polymer or the same CNTs in different polymers differ strongly, it can be seen that the TE investigations presented were carried out on composite materials whose CNT concentrations were clearly above the respective electrical percolation threshold. The influence of CNT content on TE properties is discussed for selected matrices (PBT and ABS) in more detail in Section 3.3. Interestingly, for the composites filled with SWCNTs Tuball and HiPco, which showed positive Seebeck coefficients in the form of buckypaper, positive and negative Seebeck coefficients were measured in the composite materials depending on the polymer type. For Tuball, a positive S-value was determined if PBT, PVDF, PP or PC were the polymer matrix. For composites based on ABS, J. Compos. Sci. 2019, 3, 106 10 of 18

PA6, PARA or PA66 a negative value was detected. For HiPco negative values were only achieved in ABS, but positive in PA6 and PARA. It can be assumed that electron doping occurs in the interaction between SWCNTs and these polymers, but to a different extent when comparing both SWCNT types. An explanation for this doping effect could lie in the molecular structure of the polymers that is shown for PA and ABS in Figure6. For the di fferent polyamide types, the amide group which has one free electron pair at nitrogen and two free electron pairs at oxygen is considered for electron donation. In ABS, the nitrile group consisting of carbon and triple-bound nitrogen could serve as an electron supplier. Both the amide group and the nitrile group have very electron-rich regions due to the multiple bonds between the atoms and the free electron pairs. The planar spatial arrangement of the groups can also contribute to the high electron density leading to doping of the CNTs. The assumption that the amide group can play a key role in the doping of CNTs by the polymer is also supported by results from Piao et al. [15]. The authors showed that the Seebeck coefficient of SWCNT (product of Thomas Swan Co. LTD, Consett, UK) films infiltrated with poly(vinyl pyrrolidone) (PVP) or poly(ethylene imine) (PEI) show negative Seebeck coefficients (around 40 µV/K) even though the pristine SWCNT buckypaper − had a positive Seebeck coefficient (+39 µV/K). Both PVP and PEI contain molecular elements like amid groups, which also appear in PA in each repeat unit (Figure6). The treatment of the SWCNT network with N,N,N0,N0-tetramethyl-p-phenylenediamine (TMPD) (structure see Figure6) leads also to a negative S-value with a lower magnitude of 3 µV/K[15]. Thus, the switching from p-type − buckypapers to n-type infiltrated buckypapers was achieved only with molecules containing amide or amine groups. This indicates that the SWCNT doping is connected with the electron-donating effect of these nitrogen containing groups. All other infiltrated buckypapers studied by Piao et al. [15] led to positive S-values, whereby the molecular structures of the used polymers did not contain nitrogen groups. From this, a clear correlation can be shown with the results of the present study. Positive S-values were obtained for all polymers which do not contain an amide group in their molecular structure, namely PBT, PP, PC and PVDF. Only for the polyamides (PA6, PA66, PARA) and ABS the switching from p-type to n-type was observed using SWCNT Tuball. The different kinds of PA contain amid groups. ABS contains a nitrile group instead of an amide group. Interestingly, for SWCNT HiPco such a switching only took place in ABS, which indicates a different degree of interaction with or electron donation through the PA matrices compared to SWCNT Tuball. More investigations are needed to study the influence of the structural parameters of SWCNTs on this behavior. To our knowledge, it has not yet been researched which parameters are decisive for thermoelectric properties inJ. Compos. general Sci. and 2019 which, 3, x for the doping efficiency of polymers. 11 of 18

Figure 6. Molecular structure of polymers resulting in negative Seebeck coefficients: PA, ABSFigure (as 6. used Molecular in this structure study) and of polymers poly(vinyl resulting pyrrolidone) in negative (PVP), Seebeck poly(ethylene coefficients: imine) PA, (PEI) ABS and (as used in this study) and poly(vinyl pyrrolidone) (PVP), poly(ethylene imine) (PEI) and N,N,N0,N0-tetramethyl-p-phenylenediamine (TMPD) (as applied in [15]). N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) (as applied in [15]).

Such doping behavior of nitrogen-containing polymer matrices on CNTs was only observed in the case of SWCNTs. It is assumed that the electron donating is only effective if only one wall is involved. For MWCNTs with typically eight to 12 walls, electron donation is much less efficient and there is no conversion to n-type composites. In addition to the Seebeck coefficient S, the power factor PF and figure of merit ZT are important for the evaluation of the TE properties of composites. The highest power factors in this screening were found for PVDF/2 wt % CNS-PEG at 0.1063 µW/m·K2 and for PA6/5 wt % Tuball at 0.1425 µW/m·K2. Such high PF values for melt compounded CNT polymer composites were only published by Luo et al. for a PP/4 wt % SWCNT composite with 0.066 µW/m·K2 (PP/4 wt % SWCNT) [10] or 0.12 µW/m·K2 (PP/2 wt % SWCNT) [11]. ZT values of 1.2E-4 (PVDF/2 wt % CNS-PEG) and 1.6E-4 (PA6/5 wt % Tuball) can be calculated from these PF values. For composites based on PP, PC, and PVDF, a comparison with TE properties reported in the literature is possible. Comparing the values of PP/2 wt % Tuball composites to the reported values, in our study a slightly higher Seebeck coefficient (47.2 µV/K vs. 43 µV/K [11]) and lower power factor (0.027 µW/m·K2 vs. 0.12 µW/m·K2 [11]) were found. In addition, Luo et al. [11] described the influence of the functionalization of Tuball on the TE properties of PP/2 wt % SWCNT composites by comparing batches of unfunctionalized Tuball materials (batches A and B) with a batch which was plasma oxidized (batch mA). The Seebeck coeffiecient varied between 23.2 µV/K (for CNTs batch A), 43 µV/K (for CNTs batch B) and 35.6 µV/K (for CNTs batch mA) and PF between 0.009 µW/m·K2 (batch A), 0.12 µW/m·K2 (batch B) and 0.01 µW/m·K2 (batch mA). These TE results show that both batch and functionalization have an impact on TE results. For PC/2 wt % CNT composites, in our study Seebeck coefficients between 8.5 and 34.8 µV/K were determined (Figure 3 right, Table 5). Compared to PC/2.5 wt % unfunctionalized MWCNT composites as reported by Liebscher et al. [7] to have an S-value of 7.5 ± 0.9 µV/K, our results are higher. Furthermore, the PF values of our composites are higher compared to the reported one in reference [7]. Comparing the S-values of CNT buckypapers and PC/CNT composites, for all three CNT types the S-values were only slightly different indicating that each composite well reflects the value of the buckypaper. This finding correlates with the results of Piao et al. [15] on SWCNT buckypapers infiltrated with PC where the value of the buckypaper was hardly changed. The TE results for PVDF/CNT composites can be compared with some values from the literature. Sun et al. [4] used the same type of PVDF and MWCNTs and the same preparation method for composites, namely melt compounding using a microcompounder DSM15 but with different melt mixing conditions than in our study. The measured S-values with 10 µV/K are slightly lower than in our investigation with 14.3 µV/K. It can be concluded that when using the same raw materials (PVDF, MWCNT), sample preparation plays also a role in the TE properties. It has been

J. Compos. Sci. 2019, 3, 106 11 of 18

Such doping behavior of nitrogen-containing polymer matrices on CNTs was only observed in the case of SWCNTs. It is assumed that the electron donating is only effective if only one wall is involved. For MWCNTs with typically eight to 12 walls, electron donation is much less efficient and there is no conversion to n-type composites. In addition to the Seebeck coefficient S, the power factor PF and figure of merit ZT are important for the evaluation of the TE properties of composites. The highest power factors in this screening were found for PVDF/2 wt % CNS-PEG at 0.1063 µW/m K2 and for PA6/5 wt % Tuball at 0.1425 µW/m K2. · · Such high PF values for melt compounded CNT polymer composites were only published by Luo et al. for a PP/4 wt % SWCNT composite with 0.066 µW/m K2 (PP/4 wt % SWCNT) [10] or 0.12 µW/m K2 · · (PP/2 wt % SWCNT) [11]. ZT values of 1.2E-4 (PVDF/2 wt % CNS-PEG) and 1.6E-4 (PA6/5 wt % Tuball) can be calculated from these PF values. For composites based on PP, PC, and PVDF, a comparison with TE properties reported in the literature is possible. Comparing the values of PP/2 wt % Tuball composites to the reported values, in our study a slightly higher Seebeck coefficient (47.2 µV/K vs. 43 µV/K[11]) and lower power factor (0.027 µW/m K2 vs. 0.12 µW/m K2 [11]) were found. In addition, Luo et al. [11] described the influence · · of the functionalization of Tuball on the TE properties of PP/2 wt % SWCNT composites by comparing batches of unfunctionalized Tuball materials (batches A and B) with a batch which was plasma oxidized (batch mA). The Seebeck coeffiecient varied between 23.2 µV/K (for CNTs batch A), 43 µV/K (for CNTs batch B) and 35.6 µV/K (for CNTs batch mA) and PF between 0.009 µW/m K2 (batch A), 0.12 µW/m K2 · · (batch B) and 0.01 µW/m K2 (batch mA). These TE results show that both batch and functionalization · have an impact on TE results. For PC/2 wt % CNT composites, in our study Seebeck coefficients between 8.5 and 34.8 µV/K were determined (Figure3 right, Table5). Compared to PC /2.5 wt % unfunctionalized MWCNT composites as reported by Liebscher et al. [7] to have an S-value of 7.5 0.9 µV/K, our results are ± higher. Furthermore, the PF values of our composites are higher compared to the reported one in reference [7]. Comparing the S-values of CNT buckypapers and PC/CNT composites, for all three CNT types the S-values were only slightly different indicating that each composite well reflects the value of the buckypaper. This finding correlates with the results of Piao et al. [15] on SWCNT buckypapers infiltrated with PC where the value of the buckypaper was hardly changed. The TE results for PVDF/CNT composites can be compared with some values from the literature. Sun et al. [4] used the same type of PVDF and MWCNTs and the same preparation method for composites, namely melt compounding using a microcompounder DSM15 but with different melt mixing conditions than in our study. The measured S-values with 10 µV/K are slightly lower than in our investigation with 14.3 µV/K. It can be concluded that when using the same raw materials (PVDF, MWCNT), sample preparation plays also a role in the TE properties. It has been known for some time that both the melt mixing conditions and the compression molding conditions strongly influence the formation of the conductive network [35,41,42]. Hewitt et al. [5] described multilayered fabrics in which the p- and n-type layers were fabricated by solution mixing of CNTs with PVDF. The authors name a similar room temperature Seebeck coefficient of 10.05 µV/K for PVDF/95 wt % p-type CNTs and of about 30 µV/K for films with 20 wt % CNT loading. When using n-type CNTs, a room temperature value of 5.04 µV/K is reported for the n-type film. In [6], the same authors − report for solution processed PVDF with SWCNT HiPco (raw material, purified by the authors), room temperature values of about 32 µV/K at 5 wt % loading and lower values at increasing SWCNT content. Comparing the S-values of the CNT buckypapers and PVDF/CNT composites, for both MWCNT types the S-value increases with incorporation in the PVDF matrix. This finding correlates with the results of polymer infiltrated SWCNT buckypaper as reported in [15]. However, for SWCNT Tuball, in our study, the PVDF/2 wt % composite shows with 24.0 µV/K a lower Seebeck coefficient than the buckypaper (37.4 µV/K) which does not follow this line. In order to see relations between the Seebeck coefficient and electrical conductivity, both depending on the CNT network structure, Figure7 plots all values measured for polymer /MWCNT composites J. Compos. Sci. 2019, 3, x 12 of 18

known for some time that both the melt mixing conditions and the compression molding conditions strongly influence the formation of the conductive network [35,41,42]. Hewitt et al. [5] described multilayered fabrics in which the p- and n-type layers were fabricated by solution mixing of CNTs with PVDF. The authors name a similar room temperature Seebeck coefficient of 10.05 µV/K for PVDF/95 wt % p-type CNTs and of about 30 µV/K for films with 20 wt % CNT loading. When using n-type CNTs, a room temperature value of −5.04 µV/K is reported for the n-type film. In [6], the same authors report for solution processed PVDF with SWCNT HiPco (raw material, purified by the authors), room temperature values of about 32 µV/K at 5 wt % loading and lower values at increasing SWCNT content. Comparing the S-values of the CNT buckypapers and PVDF/CNT composites, for both MWCNT types the S-value increases with incorporation in the PVDF matrix. This finding correlates with the results of polymer infiltrated SWCNT buckypaper as reported in [15]. However, for SWCNT Tuball, in our study, the PVDF/2 wt % composite shows with 24.0 µV/K a lower Seebeck coefficient than the buckypaper (37.4 µV/K) which does not follow this line. In order to see relations between the Seebeck coefficient and electrical conductivity, both J. Compos. Sci. 2019, 3, 106 12 of 18 depending on the CNT network structure, Figure 7 plots all values measured for polymer/MWCNT composites and their buckypapers. SWCNTs and N-MWCNTs are not included in this overview. It andwas their found buckypapers. that over a SWCNTs wide range and N-MWCNTsof conductivity are notvalues included the Seebeck in this overview.coefficient It increase was found slightly that overwith a wideconductivity range of conductivity(including variation values the of SeebeckMWCNT coe ffitypecient and increase content slightly (ABS)). with However, conductivity this (includingrelationship variation does ofnot MWCNT apply to type PA6 and and content PA66, (ABS)). for which However, decreasing this relationship Seebeck does coefficients not apply with to PA6increasing and PA66, conductivity for which decreasing (only variation Seebeck of coe MWCNfficientsT with type) increasing is seen. conductivity This may (onlybe related variation to ofthe MWCNTpreviously type) discussed is seen. This effect may of be the related amine to thegroup previously electron discussed donation, eff whichect of the may amine also group be present electron in donation,MWCNTs, which but mayto a lesser also be extent. present It inis MWCNTs,obvious that but the to avalues lesser of extent. all MWCN It is obviousT containing that the composites values of allare MWCNT lower or containing in the range composites of the corresponding are lower or in thebuckypapers range of the with corresponding only PVDF buckypaperscomposites having with onlyslightly PVDF higher composites Seebeck having values slightly in the higher composites Seebeck. valuesThis indicates in the composites. that the Thisconductivity indicates of that the theMWCNTs conductivity themselves of the MWCNTs significantly themselves influences significantly the behavior influences of the the composites. behavior of Conversely, the composites. this Conversely,means that this CNTs means with that significantly CNTs with higher significantly Seebec higherk coefficient Seebeck are coe requiredfficient are to requiredproduce tocomposites produce compositeswith higher with S-values. higher This S-values. assumption This is assumption confirmed if is the confirmed composite if materials the composite with SWCNTs, materials which with SWCNTs,lead to positive which lead Seebeck to positive coefficients, Seebeck are coe takenfficients, into are account. taken intoThe account.S-values The of SWCNT S-values buckypapers of SWCNT buckypapersand powder and packages powder packagesare significantly are significantly higher highercompared compared to those to those of MWCNTs. of MWCNTs. The The values values of ofSeebeck Seebeck coefficients coefficients of of SWCNT SWCNT based based composites composites ar aree quite quite higher higher than than thos thosee based based on on MWCNTs. MWCNTs. In Incontrast contrast to to MWCNTs, MWCNTs, the the S-values of SWCNT-containingSWCNT-containing compositecomposite materialsmaterials werewere sometimes sometimes higherhigher than than those those of of the the corresponding corresponding buckypapers, buckypapers, with with the the e ffeffectect being being most most pronounced pronounced with with SWCNTSWCNT Tuball. Tuball.

10

PARA PA66 PA6 ABS PC

Seebeck [µV/K] coefficient PP PVDF PBT polymer/MWCNT composites MWCNT 1 10-2 10-1 100 101 102 103 104 Electrical volume conductivity [S/m]

Figure 7. Seebeck coefficient versus electrical volume conductivity for all polymer/multi-walled CNTs (MWCNT)Figure 7. compositesSeebeck coefficient of Table5 versusand MWCNT electrical buckypapers volume conductivity (dashed lines for are all guides polymer/multi-walled for the eyes). CNTs (MWCNT) composites of Table 5 and MWCNT buckypapers (dashed lines are guides for the 3.3. Influenceeyes). of Single-Walled Carbon Nanotube (SWCNT) content on Thermoelectric (TE) Properties Exemplarily, for three polymer matrices the influence of SWCNT content on TE properties was studied. On the one hand, p-type PBT/SWCNT Tuball composites (Figure8, Table7) are considered as p-type composites, and on the other hand, ABS/SWCNT Tuball (Figure9, Table8) and PA6 /SWCNT Tuball (Figure 10, Table9) as n-type composites. For the PBT/SWCNT system, it was found that the Seebeck coefficient, power factor and figure of merit increase with CNT content. The S-value increases slightly from 54.3 µV/K at 0.2 wt % Tuball up to 66.4 µV/K at 5 wt % Tuball. The increasing trend implies that no maximum of the Seebeck coefficient may be obtained up to 5 wt %. In accordance with the also increasing electrical conductivity (only very slightly lower values at 5 wt % compared to 4 wt %), PF and ZT increase up to values of 0.28 µW/m K2 · and 3.1 10 4, respectively. The values for both parameters are the highest values described so far × − for melt prepared polymer/CNT composites in the literature, when excluding reported values for composites containing other additives or prepared by solvent processes. J. Compos. Sci. 2019, 3, x 13 of 18

3.3. Influence of Single-Walled Carbon Nanotube (SWCNT) content on Thermoelectric (TE) properties Exemplarily, for three polymer matrices the influence of SWCNT content on TE properties was studied. On the one hand, p-type PBT/SWCNT Tuball composites (Figure 8, Table 7) are considered as p-type composites, and on the other hand, ABS/SWCNT Tuball (Figure 9, Table 8) and J.PA6/SWCNT Compos. Sci. 2019 Tuball, 3, 106 (Figure 10, Table 9) as n-type composites. 13 of 18

70 103 103 103 40°C, PBT/Tuball 60 102 102 102 50 101 101 101 40 100 100 100 30 10-1 10-1 10-1 )]

20 2 10-2 10-2 10-2 10 10-3 10-3 10-3 0 10-4 10-4 10-4 -10 10-5 10-5 10-5 ZT [-] -20 [µW/(m·K PF -6 -6 -6 -30 10 10 10

-7 Electrical conductivity [S/m] -7 -7 -40 10 10 10 Seebeck coefficient [µV/K] coefficient Seebeck -50 10-8 10-8 10-8 -60 10-9 10-9 10-9 -70 10-10 10-10 10-10 012345 SWCNT content [wt%]

Figure 8. Thermoelectric (TE) parameters of of PBT/single-walled PBT/single-walled CNTs CNTs (SWCNT) Tuball composites, numeric values are given in Table7 7.. Table 7. Thermoelectrical parameters of PBT/single-walled CNT (SWCNT) Tuball composites shown in FigureTable 87.. Thermoelectrical parameters of PBT/single-walled CNT (SWCNT) Tuball composites shown in Figure 8. Volume Conductivity Seebeck Power Factor Composite Figure of Merit ZT Volume(S/m) CoeSeebeckfficient (µ V/K) (PowerµW/m K Factor2) Figure of · 2 6 PBTComposite+ 0.2 wt % Tuball Conductivity 0.5 Coefficient 54.3 (µV/K)0.6 (µW/m·K 0.0013 ) 1.5Merit10− ZT ± × 6 PBT + 0.25 wt % Tuball(S/m) 0.7 54.7 0.1 0.0022 2.4 10− ± × 6 PBT + 0.5 wt % Tuball 0.9 53.2 0.2 0.0027 3.0 10− −6 PBT + 0.2 wt % Tuball 0.5 54.3 ±± 0.6 0.0013 1.5× × 105 PBT + 1.0 wt % Tuball 8.4 56.4 0.1 0.0268 3.0 10− ± × 5 −6 PBTPBT + 0.25+ 2.0 w wtt % % Tuball 0.7 18.6 54.7 58.2 ± 0.11.5 0.0630 0.0022 7.0 2.410 ×− 10 ± × 4 PBTPBT + 0.5+ 3.0 wt wt % % Tuball Tuball 0.9 49.1 53.2 61.8 ± 0.21.7 0.1877 0.0027 2.1 3.010 ×− 10−6 ± × 4 PBT + 4.0 wt % Tuball 72.1 62.3 0.2 0.2797 3.1 10− −5 J.PBT Compos. + 1.0 Sci. wt 2019 %, 3Tuball, x 8.4 56.4 ±± 0.1 0.0268 3.0× ×14 104 of 18 PBT + 5.0 wt % Tuball 45.0 66.4 1.0 0.1983 2.2 10− PBT + 2.0 wt % Tuball 18.6 58.2 ±± 1.5 0.0630 7.0× × 10−5 PBT + 3.0 wt % Tuball 49.1 61.8 ± 1.7 0.1877 2.1 × 10−4 70 103 103 103 PBT + 4.0 wt % Tuball40 °C, ABS/Tuball 72.1 62.3 ± 0.2 0.2797 3.1 × 10−4 60 102 102 102 PBT + 5.0 wt % Tuball 45.0 66.4 ± 1.0 0.1983 2.2 × 10−4 50 101 101 101 )] For the PBT/SWCNT40 system, it was found that the Seebeck coefficient,100 power100 2 factor100 and figure 30 of merit increase with CNT content. The S-value increases slightly from10 -154.3 µV/K10-1 at10 0.2-1 wt % Tuball 20 up to 66.4 µV/K at 5 wt % Tuball. The increasing trend implies that10 no-2 maximum10-2 10 of-2 the Seebeck 10 coefficient may be obtained up to 5 wt %. In accordance with 10the-3 also10 -3increasing10-3 electrical 0 -4 -4 -4 conductivity (only very slightly lower values at 5 wt % compared to 410 wt %),10 PF and10 ZT increase [-] ZT up -10 2 −4 to values of 0.28 µW/m·K and 3.1 × 10 , respectively. The values for bo10th-5 parameters10-5 10 are-5 the highest -20 values described so far for melt prepared polymer/CNT composites in the-6 literature,-6 when-6 excluding -30 10 10 Power factor [µW/(m·K 10

reported values for composites containing other additives or prepared by solventElectrical conductivity [S/m] processes. 10-7 10-7 10-7 Seebeck coefficient [µV/K] -40 Seebeck coefficient -50 10-8 10-8 10-8 -60 10-9 10-9 10-9 -70 10-10 10-10 10-10 01234567 SWCNT content [wt%]

Figure 9. TE parameters of ABS/SWCNT Tuball composites, numeric values are given in Table8. Figure 9. TE parameters of ABS/SWCNT Tuball composites, numeric values are given in Table 8.

For the n-type ABS/Tuball composites, a maximum negative Seebeck coefficient of −57.1 µV/K was found at 0.5 wt % Tuball. Especially at higher CNT content, the S-value increases up to −29.7 µV/K at 7 wt % SWCNT. However, due to the increasing electrical conductivity, the power factor and ZT value increase with the CNT content to values up to 0.14 µW/m·K2 and 1.6 × 10−4, respectively. Thereby, mainly due to the significantly lower values of conductivity in ABS/SWCNT as compared to PBT/SWCNT, the PF and ZT values were lower compared to the values of PBT/SWCNT composites.

Table 8. Thermoelectrical parameters of ABS/SWCNT Tuball composites shown in Figure 9.

Volume Seebeck Power Factor Figure of Composite Conductivity Coefficient (µV/K) (µW/m·K2) Merit ZT (S/m) ABS + 0.3 wt % Tuball 0.1 −51.0 ± 3.8 0.0001 1.5 × 10−7 ABS + 0.4 wt % Tuball 0.1 −51.5 ± 0.3 0.0004 4.0 × 10−7 ABS + 0.5 wt % Tuball 0.2 −57.1 ± 4.1 0.0005 5.6 × 10−7 ABS + 0.75 wt % Tuball 1.1 −41.6 ± 0.4 0.0019 2.2 × 10−6 ABS + 1.0 wt % Tuball 4.2 −50.9 ± 3.0 0.0109 1.2 × 10−5 ABS + 2.0 wt % Tuball 14.8 −34.2 ± 0.7 0.0173 1.9 × 10−5 ABS + 4.0 wt % Tuball 66.2 −32.7 ± 6.4 0.0706 7.9 × 10−5 ABS + 5.0 wt % Tuball 137.6 −32.0 ± 0.4 0.1409 1.6 × 10−4 ABS + 6.0 wt % Tuball 130.1 −30.5 ± 0.5 0.1207 1.3 × 10−4 ABS + 7.0 wt % Tuball 111.2 −29.7 ± 0.5 0.0980 1.1 × 10−4

J. Compos. Sci. 2019, 3, 106 14 of 18

Table 8. Thermoelectrical parameters of ABS/SWCNT Tuball composites shown in Figure9.

Volume Conductivity Seebeck Power Factor Composite Figure of Merit ZT (S/m) Coefficient (µV/K) (µW/m K2) · ABS + 0.3 wt % Tuball 0.1 51.0 3.8 0.0001 1.5 10 7 − ± × − ABS + 0.4 wt % Tuball 0.1 51.5 0.3 0.0004 4.0 10 7 − ± × − ABS + 0.5 wt % Tuball 0.2 57.1 4.1 0.0005 5.6 10 7 − ± × − ABS + 0.75 wt % Tuball 1.1 41.6 0.4 0.0019 2.2 10 6 − ± × − ABS + 1.0 wt % Tuball 4.2 50.9 3.0 0.0109 1.2 10 5 − ± × − ABS + 2.0 wt % Tuball 14.8 34.2 0.7 0.0173 1.9 10 5 − ± × − ABS + 4.0 wt % Tuball 66.2 32.7 6.4 0.0706 7.9 10 5 − ± × − ABS + 5.0 wt % Tuball 137.6 32.0 0.4 0.1409 1.6 10 4 − ± × − J. Compos.ABS + Sci.6.0 2019 wt %, 3 Tuball, x 130.1 30.5 0.5 0.1207 1.3 10154 of 18 − ± × − ABS + 7.0 wt % Tuball 111.2 29.7 0.5 0.0980 1.1 10 4 − ± × −

70 103 103 103 40 °C, PA6/Tuball 60 102 102 102 50 101 101 101 )] 40 100 100 2 100 30 10-1 10-1 10-1 20 10-2 10-2 10-2 10 10-3 10-3 10-3 0 10-4 10-4 10-4 ZT [-] -10 10-5 10-5 10-5 -20 -6 -6 -6

10 10 [µW/(m·K factor Power 10

-30 Electrical conductivity [S/m] -7 -7 -7 -40 10 10 10 Seebeck coefficient [µV/K] Seebeck coefficient -50 10-8 10-8 10-8 -60 10-9 10-9 10-9 -70 10-10 10-10 10-10 012345 SWCNT content [wt%]

Figure 10. TE parameters of PA6/ SWCNT Tuball composites, numeric values are given in Table9. Figure 10. TE parameters of PA6/ SWCNT Tuball composites, numeric values are given in Table 9. Table 9. Thermoelectrical parameter of PA6/SWCNT Tuball composites shown in Figure 10. Table 9. Thermoelectrical parameter of PA6/SWCNT Tuball composites shown in Figure 10. Volume Conductivity Seebeck Coeff. Power Factor Composite Figure of Merit ZT (S/m) (µV/K) (µW/m K2) Volume · Seebeck Coeff. Power Factor Figure7 of PA6Composite+ 0.3 wt % Tuball Conductivity 0.1 51.9 5.7 0.0001 1.4 10− − ± 2 × 6 PA6 + 0.4 wt % Tuball 0.5 (µV/K)50.3 2.5(µW/m·K 0.0011 ) 1.3Merit10− ZT (S/m) − ± × 6 PA6 + 0.5 wt % Tuball 0.6 59.0 4.6 0.0021 2.4 10− − ± × 6 −7 PA6PA6 + +0.30.75 wt wt % % Tuball Tuball 0.1 0.3 −51.958.0 ± 5.75.3 0.00010.0009 1.0 1.4 10× −10 − ± × 6 PA6 + 1.0 wt % Tuball 0.4 59.8 3.5 0.0015 1.6 10 −6 PA6 + 0.4 wt % Tuball 0.5 −50.3− ± 2.5± 0.0011 1.3× × −10 PA6 + 2.0 wt % Tuball 3.8 56.3 2.9 0.0122 1.4 10 6 − ± × − −6 PA6PA6 + 0.5+ 4.0 wt wt % % Tuball 0.6 32.3 −59.046.2 ± 4.60.5 0.00210.0689 7.7 2.4 10× 106 − ± × − PA6PA6 + 0.75+ 5.0 wt wt % TuballTuball 0.3 64.6 −58.047.0 ± 5.32.9 0.00090.1425 1.6 1.0 10× 104 −6 − ± × − PA6 + 1.0 wt % Tuball 0.4 −59.8 ± 3.5 0.0015 1.6 × 10−6 −6 PA6For + 2.0 the wt n-type % Tuball ABS /Tuball composites, 3.8 a maximum−56.3 ± 2.9 negative Seebeck 0.0122 coe fficient of 1.457.1 × 10µV /K − −6 wasPA6 found + 4.0 wt at 0.5 % wtTuball % Tuball. Especially 32.3 at higher− CNT46.2 ± content, 0.5 the S-value 0.0689 increases up to 7.729.7 × 10µV /K − −4 atPA6 7 wt + 5.0 % wt SWCNT. % Tuball However, due 64.6 to the increasing−47.0 ± electrical 2.9 conductivity, 0.1425 the power 1.6 factor × 10 and ZT value increase with the CNT content to values up to 0.14 µW/m K2 and 1.6 10 4, respectively. For PA6 composites filled with 0.3–5 wt % SWCNT Tuball, the ·thermoelectrical× − measurements Thereby,shows n-type mainly behavior. due to the The significantly highest Seebeck lower coefficient values of conductivity was found at in − ABS59.0/ SWCNTµV/K for as PA6/0.5 compared wt to% PBTSWCNT./SWCNT, As thewith PF ABS/Tuball and ZT values composites, were lower the compared S-value toincreases the values with of PBT the/SWCNT SWCNT composites. content to −46 µV/KFor PA6 to − composites47 µV/K (4–5 filled wt % with SWCNT). 0.3–5 wt It has % SWCNT been observed Tuball, in the both thermoelectrical ABS and PA6 measurementsmatrices that a shows n-type behavior. The highest Seebeck coefficient was found at 59.0 µV/K for PA6/0.5 wt % lower amount of SWCNTs can obviously be doped more strongly by− the surrounding polymer SWCNT. As with ABS/Tuball composites, the S-value increases with the SWCNT content to 46 µV/K chains, leading to a higher absolute value of the S-value than at a higher SWCNT contents.− In the p-type composites based on PBT no such doping occurs and the S-values are more dependent on the conductive network formation. For the three SWCNT composites based on PBT, ABS or PA6, maximum values of the TE results (PF, ZT) are achieved in the CNT concentration range between 4 wt % and 5 wt %. A maximum of these values at similar SWCNT concentrations was also found in PP based composites, as reported by Luo et al. [10]. In this SWCNT concentration range, which is far above the percolation threshold concentration (compared to Table 6), a very dense carbon nanotube network can be assumed. Electrical conductivity can be used as a measure of the density and quality of the conductive network, which approaches a limit as the CNT content increases. If the maximum conductivity is reached, then further network paths do not lead to a further increase in conductivity. It is assumed that the charge transport, which cannot be further increased by the addition of CNT, is reflected in almost constant PF and ZT values.

J. Compos. Sci. 2019, 3, 106 15 of 18 to 47 µV/K (4–5 wt % SWCNT). It has been observed in both ABS and PA6 matrices that a lower − amount of SWCNTs can obviously be doped more strongly by the surrounding polymer chains, leading to a higher absolute value of the S-value than at a higher SWCNT contents. In the p-type composites based on PBT no such doping occurs and the S-values are more dependent on the conductive network formation. For the three SWCNT composites based on PBT, ABS or PA6, maximum values of the TE results (PF, ZT) are achieved in the CNT concentration range between 4 wt % and 5 wt %. A maximum of these values at similar SWCNT concentrations was also found in PP based composites, as reported by Luo et al. [10]. In this SWCNT concentration range, which is far above the percolation threshold concentration (compared to Table6), a very dense carbon nanotube network can be assumed. Electrical conductivity can be used as a measure of the density and quality of the conductive network, which approaches a limit as the CNT content increases. If the maximum conductivity is reached, then further network paths do not lead to a further increase in conductivity. It is assumed that the charge transport, which cannot be further increased by the addition of CNT, is reflected in almost constant PF and ZT values.

4. Conclusions The thermoelectrical performance of different kinds of polymer/CNT composites was studied. A variety of combinations of different polymers, such as PP, PC, PVDF, PBT, PA6, PA66, PARA and ABS with different CNT types, such as SWCNT and MWCNT in varied contents, were investigated. In addition, the Seebeck coefficients of buckypapers and powder packages of the CNT materials were measured. For MWCNTs, the order of the Seebeck coefficients of the composite materials followed the order of the corresponding buckypapers. CNS-PEG with higher Seebeck coefficients in buckypaper also resulted in higher composite values compared to NC7000. The highest Seebeck coefficient was achieved for polymer composites filled with SWCNT Tuball whereas both positive and negative S-values were obtained. While the incorporation of Tuball in PP, PC, PVDF and PBT leads to a positive Seebeck coefficient, the use in polyamides (PA6, PA66, PARA) and ABS resulted in negative S-values. This doping was assigned to the electron-donating effect of nitrogen containing amide or amine groups present in these polymers. In further investigations, the doping of CNTs by polymers must be investigated more closely. Nitrogen doped MWCNTs with negative Seebeck value in the buckypapers resulted in negative values of the composites for all studied polymers. The magnitude of the Seebeck coefficient was lower as compared to the application of SWCNT Tuball in the polyamides and ABS. Not only the type of carbon nanotubes influences the TE properties of polymer/CNT composites, but also the polymer type. At the same MWCNT content, the S-values of PVDF composites were higher than those of the other composites. For composite materials filled with 2 wt % SWCNT Tuball, the highest Seebeck coefficient of 58.2 µV/K was determined in the PBT matrix. The highest Seebeck coefficients obtained in this study were 66.4 µV/K (PBT/7 wt % SWCNT Tuball) and 59.0 µV/K − (PA6/0.5 wt % SWCNT Tuball) for p- and n-type composites, respectively. The highest power factor and ZT of 0.28 µW/m K2 and 3.1 10 4, respectively, were achieved in PBT with 4 wt % SWCNT Tuball. · × − In summary, this study presents for first time the Seebeck coefficients of melt-mixed CNT composites with polyamides, poly(butylene terephthalate) and poly(acrylonitrile butadiene styrene). The major novelty is the direct preparation of n-type melt-mixed polymer composites from p-type commercial SWCNTs with relatively high Seebeck coefficients up to 57 µV/K. Thereby, the selection of − SWCNTs combined with matrix polymers containing nitrogen groups seems to be favorable to achieve n-type composites. Thus, with a purposeful selection of the CNTs and the polymer matrix, both n-type and p-type elements can be produced for the assembly of a TE generator.

Author Contributions: Conceptualization, B.K. and P.P.; methodology, B.K., P.P.; investigation, B.K., C.B., J.L., M.K.; data curation, B.K.; writing—original draft preparation, B.K.; writing—review and editing, P.P.; visualization, B.K.; supervision, P.P.; project administration, P.P.; funding acquisition, P.P. J. Compos. Sci. 2019, 3, 106 16 of 18

Funding: This research received no external funding. Juhasz Levente and Carine Barbier were financially supported by the ERASMUS program of the German Academic Exchange Service (DAAD) during their traineeships at IPF. Acknowledgments: The authors thank ANS (Applied NanoStructured Solutions LLC, Baltimore, MD, USA) for supplying the branched MWCNT CNS-PEG material. Thanks to Uttandaraman Sundararaj (University of Calgary, Canada) and Mohammad Arjmand (now University of British Columbia, Kelowna, Canada) for supplying the N-doped MWCNTs N-A1 and Robert Fuge and Silke Hampel (Leibniz Institute for Solid State and Materials Research Dresden, Germany) for production of N-doped MWCNTs N-IFW. Conflicts of Interest: The authors declare no conflict of interest

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