Enhanced thermoelectric properties of tungsten disulfide-multiwalled carbon nanotube composites

Abstract

We investigated WS 2 -multiwalled carbon nanotube composites prepared by powder metallurgy. The inclusion of a small amount of nanotubes (0.75 wt%) dramatically increased electrical conductivity (by 12 300%) with a moderate decrease in the Seebeck coefficient (by 22%) and thermal conductivity (by 43%) enhancing both power factor and thermoelectric figure of merit at 300 K.Thermoelectric materials, which allow direct conversion between thermal energy and electricity, have received considerable attention recently as an eco-friendly energy transformation source. 1 The conversion efficiency is described by the dimensionless thermoelectric figure of merit zT ¼ (a 2 s/k)T where a, k, s, and T are the Seebeck coefficient, thermal conductivity, electrical conductivity, and absolute temperature, respectively. 2 Materials with superior thermoelectric properties typically contain rare and potentially toxic elements such as Te, Sb, Pb, Bi and Se. 3-5 The hybrid composite structures containing these elements have also been actively investigated to enhance thermoelectric performance. 6-8 Therefore, new thermoelectric materials are of interest due to the toxicity and abundance issues. Tungsten disulfide (WS 2 ), a layered transition metal dichalcogenide, is a promising candidate since it is an environmentally friendly and abundant material. 5,9-12 It also has a high a ($375 to 1000 mV K À1 ) 10,13,14 and low k ($1.1 to $2.25 W m À1 K À1 ). 14,15 However, s is nearly 3-6 orders of magnitude lower ($10 À1 to $10 1 S m À1 ) than those of binary tellurides resulting in a very low power factor (sa 2 ) and zT. 8,10,13-16 Here we have investigated WS 2 -multiwalled carbon nanotube (MWNT) composites, prepared by powder metallurgy, over a wide range of temperatures (300-800 K). The inclusion of a small amount of nanotubes with high aspect ratios (0.75 wt%) dramatically increased both carrier concentration (by 8670%) and electrical conductivity (by 12 300%) at room temperature. There was a moderate decrease in the Seebeck coefficient by 22%. Interestingly, the thermal conductivity was reduced by 43% probably due to the increase in the grain boundary and interface scattering of phonons. This increased both power factor (68.4 mW m À1 K À2 ) and thermoelectric figure of merit (0.0063) at 300 K by 7330 and 13 100%. The thermoelectric performance increased with increasing temperature. The maximum power factor and thermoelectric figure of merit were 390 mW m À1 K À2 and 0.22 at 700-800 K respectively.Recently, substantial amounts of single-walled carbon nanotubes (SWNTs) (10-100 wt%) were mixed with exfoliated WS 2 to synthesize freestanding WS 2 -SWNT hybrid films by a vacuum filtration method. 15 The s could be increased in the hybrid films ($10 4 S m À1 ). However, a was significantly reduced (60-80 mV K À1 ) 15 and k was increased compared with the pure WS 2 specimens. 14,15 The addition of nanotubes increased the power factor faster than k leading to an increase in zT. However, the maximum power factor ($120 mW m À1 K À2 ) and zT ($0.001)...

Ultrahigh Thermal Conductivity of Interface Materials by Silver‐Functionalized Carbon Nanotube Phonon Conduits

“…This could be due to the higher intrinsic n (5.85 × 10 22 cm −3 ) of pure Ag compared with pure MWNTs (n = 1.94 × 10 19 cm −3 ). [5,26] Interestingly, microscale Ag flakes and Ag NWs provided smaller n than Ag NPs at the identical secondary filler concentration (2.3 vol%). This could be due to the defects and impurities although the precise mechanism needs to be investigated further.…”

“…[26,42] The specimen density (ρ) was measured by the Archimedes method (Alfa Mirage, MD-300S), and the relative density (ρ measured /ρ theory ) was calculated using a previously published protocol assuming complete removal of solvents. [43][44][45] The out-of-plane κ was then calculated from the formula, κ = ραC p .…”

Enhanced figure of merit in Mg2Si0.877Ge0.1Bi0.023/multi wall carbon nanotube nanocomposites

“…One of the strategies to overcome this issue is through embedding nanomaterials into the bulk matrix, which is known as "nano-inclusion". [11][12][13][14][15] Even though CNTs had a positive effect on the thermoelectric properties of Bi 2 Te 3 16 and Bi 2 (Se,Te) 3 17 by achieving zT ~ 1.5 at 350 K as compared to zT ~ 1 for the sample without nanotubes, the presence of rare and toxic tellurium would not fulfill the criteria for large scale industrial applications, further the temperature at which these materials are most efficient are well below those of most targeted waste heat recovery applications. [8][9][10] Carbon nanotubes, due to their spectacular electronic properties, can be considered as potential candidates for nanoinclusions in thermoelectric materials.…”

Heterostructured Approaches to Efficient Thermoelectric Materials

“…These hybrid structures include structural polymer--CNTs, [69][70][71][72][73][74][75][76][77] conductive polymer--CNTs, 78-81 metal decorated CNTs [82][83][84] and solid--state compound--CNTs. [85][86][87] The main function of CNTs in the polymer is to facilitate the electron transport by creating electron transport pathways and retain the low thermal conductivity as illustrated in Figure 13. Substantial progress in understanding and predictively designing the properties of carbon nanotube (CNT)--based hybrid structures is being made.…”