J Nanopart Res (2009) 11:1729–1738 DOI 10.1007/s11051-008-9541-6
RESEARCH PAPER
Synthesis and thermoelectric characterisation of bismuth nanoparticles Gianfranco Carotenuto Æ Cornelia L. Hison Æ Filomena Capezzuto Æ Mariano Palomba Æ Pietro Perlo Æ Pellegrino Conte
Received: 13 February 2008 / Accepted: 12 October 2008 / Published online: 31 October 2008 Springer Science+Business Media B.V. 2008
Abstract An effective method of preparation of bismuth nanopowders by thermal decomposition of bismuth dodecyl-mercaptide Bi(SC12H25)3 and preliminary results on their thermoelectric properties are reported. The thermolysis process leads to Bi nanoparticles due to the efficient capping agent effect of the dodecyl-disulfide by-product, which strongly bonds the surface of the Bi clusters, preventing their aggregation and significantly reducing their growth rate. The structure and morphology of the thermolysis products were investigated by differential scanning calorimetry, thermogravimetry, X-ray diffractometry, 1 H nuclear magnetic resonance spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. It has been shown that the prepared Bi nanopowder consists of spherical shape nanoparticles, with the average diameter depending on the
G. Carotenuto (&) C. L. Hison F. Capezzuto M. Palomba Istituto dei Materiali Compositi e Biomedici, Consiglio Nazionale delle Ricerche, Piazzale Tecchio 80, 80125 Napoli, Italy e-mail:
[email protected] P. Perlo Centro Ricerche Fiat, Strada Torino 50, 10043 Orbassano (TO), Italy P. Conte Dipartimento di Ingegneria e Tecnologie Agro-Forestali (DITAF), Universita` degli Studi di Palermo, Viale delle Scienze 13, 90128 Palermo, Italy
thermolysis temperature. The first results on the thermoelectric characterization of the prepared Bi nanopowders reveal a peculiar behavior characterized by a semimetal–semiconductor transition, and a significant increase in the Seebeck coefficient when compared to bulk Bi in the case of the lowest grain size (170 nm). Keywords Bismuth nanoparticles Mercaptide thermolysis Semimetal–semiconductor transition Thermoelectric characteristics Nanopowder
Introduction Thermoelectric materials generate electrical power from a temperature gradient through Seebeck effect and use electricity to work as heat pumps through Peltier effect, providing active cooling (in the absence of refrigerants) or heating without the need of moving parts, but based on carrier conduction (Nolas et al. 2001). These materials, with long life and maintenance-free, have important economical applications in systems where the waste heat generated can be harvested to provide useful power, such as in microelectronics–microprocessor cooling, optoelectronics, etc. (Heremans 2005). The high simplicity and environmental-friendly (no noise, no pollution) characteristics of the thermoelectric energy
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conversion principle have cost, until now, lower efficiency. Consequently, the use of thermoelectric devices, consisting of a sequence of thermoelectric materials connected electrically in series and thermally in parallel, has been limited to niche applications for which the high reliability, compactness, and user-friendly performance overcome the lack of efficiency. As well known, the measure of the thermoelectric efficiency of a material is given by the dimensionless figure of merit ZT defined as (Goldsmid 1964): ZT ¼
S2 r T ke þ kL
ð1Þ
where S is the Seebeck coefficient, defined as the thermoelectric voltage induced by a temperature gradient across the material, S ¼ DV DT , r is the electrical conductivity, T is the temperature (in Kelvin degrees), ke and kL are the electronic and lattice (phononic) thermal conductivities, respectively. An efficient thermoelectric energy conversion requires large ZT values, which means large electrical conductivity, high Seebeck coefficient (large voltage in power generation and large Peltier coefficient in cooling) and low thermal conductivity (to allow large temperature differences and, consequently, large voltage in power generation or to reduce the heat leakage between the hot and cold side of the device when used as refrigerator) (Nolas et al. 2001). In the recent years, the identification of new materials with high ZT value has proved to be an extremely challenging task due to the interdependence among the Seebeck effect, thermal, and electrical conductivity. The tailoring of the three parameters in view of large ZT values is difficult in conventional bulk crystalline solids because the modification of one of them adversely affects the other (e.g., an increase in the electrical conductivity leads to an additional enhancement in the electronic contribution to the thermal conductivity) (Ashcroft and Mermin 1976; Hicks and Dresselhaus 1993a, b). The recent approach for the thermoelectric efficiency improvement is based on nanoscale structuring to benefit from the phonon boundary scattering and quantum size effects (i.e., charge carriers confinement at nanoscale in one (quantum wires), two (quantum wells, superlattices) or three (quantum dots) dimensions), which determine the decoupling
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of ZT parameters (Hicks and Dresselhaus 1993a, b); Dresselhaus et al. 1999; Chen et al. 2000, 2003). In this way, the thermoelectric energy conversion in low-dimensional structures could reach the kind of performances needed for the widespread application of the thermoelectric technology. The peculiar electronic transport characteristics of bulk semimetal bismuth Bi, such as small band overlap of the conduction and valence bands (Gallo et al. 1963), very small electron effective masses (Isaacson et al. 1969), very long carrier mean-free path (several orders of magnitude greater than for most metals) (Rogacheva et al. 2003), low carrier concentration, and highly anisotropic carrier effective masses (varying as much as 2009) (Gallo et al. 1963; Issi 1979) make it particularly interesting for thermoelectric applications when the size reduction of the building blocks induces quantum confinement effects (e.g., semimetal–semiconductor transition) and phonon boundary scattering (Gallo et al. 1963; Heremans et al. 2000; Black et al. 2002). In fact, the nanostructured Bi exhibits significantly enhanced thermoelectric efficiency when compared to bulk Bi. Most of the previous works on nanostructured bismuth show a long-standing interest in nanotubes, nanowires and nanowire arrays (Hicks and Dresselhaus 1993a, b); Zhang et al. 1998; Heremans and Thrush 1999; Li et al. 2001; Huber et al. 2003; Heremans 2005), and thin films structures/quantum well superlattices (Hoffman et al. 1993; Lu et al. 1996; Cho et al. 1997; Rogacheva et al. 2003). Although larger enhancements in the thermoelectric performance are predicted in quantum dot structures (Heremans et al. 2002; Lin et al. 2003), essentially confined in all three dimensions, much less studies on Bi nanoparticles production and thermoelectric characterization are available in the literature (Zhao et al. 2004; Balan et al. 2004; Grass and Stark 2006; Hostler et al. 2007) with respect to the researches directed toward thin films and nanowire/nanorod arrays. It has been reported that Bi nanoparticles are often contaminated during the production process (e.g., by oxidation, from remaining surfactants or solvents, etc.) (Balan et al. 2004; Zhao et al. 2004; Fu et al. 2005; Hostler et al. 2007) and their large-scale preparation is limited by loweffectiveness production rates and complicated procedures (Wegner et al. 2002; Hostler et al. 2007).
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In this context of just a few studies in the field of thermoelectric Bi nanoparticles and of their difficult preparation, the present work reports an effective synthesis method of zero-valent Bi nanopowder by thermal decomposition of bismuth dodecyl-mercaptide and the first experimental results on the thermoelectric characterisation of Bi pills sintered by powder uniaxial compression. The structural and morphological characterisation of Bi nanopowder prepared at different thermolysis temperatures is also provided.
Experimental The precursor bismuth dodecyl-mercaptide Bi(SC12H25)3 is not a commercially available product. Therefore it was synthesised in the laboratory by reacting stoichiometric amounts of dodecanethiol C12H25SH (Aldrich, 98.5%) with bismuth (III) chloride BiCl3, (Aldrich, 99.999%) according to a simple chemical route already described in a previous article (Nicolais and Carotenuto 2008). The thermal decomposition of Bi(SC12H25)3 is expected to give zerovalent bismuth Bi(0) and dodecyl-disulfide (SH25C12)2, as organic by-product. The mercaptide thermolysis was performed in a glass tube by immersion in an oil thermostatic bath with the temperature in the range 140–180 C, for 3 min under vacuum in order to prevent bismuth oxidation. The thermolysis product was dispersed in chloroform, separating the organic and non-organic products by centrifugation at 8,000 rpm, for 10 min. The purified non-organic product Bi(0) was isolated as a gray powder, while the organic by-product (SH25C12)2 was obtained as a white solid layer after chloroform evaporation. Bulk samples in the shape of pills with the diameter of 13 mm and about 0.5 mm in thickness were obtained by synthesising the as-prepared Bi powder at 590 MPa for 15 min at room temperature by means of an uniaxial hydraulic press (Retsch PP 25). The thermal decomposition of bismuth dodecylmercaptide was studied by differential scanning calorimetry (DSC, TA INSTRUMENTS 2920) and thermogravimetric analysis (TGA, TA INSTRUMENTS 2950). Two consecutive DSC runs were performed from 0 to 300 C, at a rate of 10 C/min under fluxing nitrogen and using sealed aluminum capsules to avoid changes in the thermogram baseline
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due to organic by-product evaporation. The TGA thermograms were acquired by heating the samples from room temperature up to 850 C, at a rate of 10 C/min, under fluxing nitrogen. The identification of the organic by-product of Bimercaptide thermal decomposition was performed by solution-state 1H nuclear magnetic resonance (1H-NMR) spectroscopy, carried out on a Bruker Avance 400 MHz instrument, operating at a proton frequency of 400.13 MHz. The spectrometer was equipped with a 5-mm Bruker inverse broadband probe with an actively shielded z-gradient coil. The spectra were acquired and elaborated by Bruker Topspin 1.3 software. 1H NMR spectra were referenced to the chemical shift of the solvent (deuterated chloroform), resonating at 7.26 ppm. Two-dimensional correlation spectroscopy (2DCOSY) experiments were acquired with a 16:12:40 gradient ratio (duration, 1 ms), 44 scans, 2,000 points in F2, and 256 points in F1. COSY spectra were transformed with a sine-bell weighting function in both dimensions applying a sine-bell shift (SSB) of 0. The composition and crystal structure of the asprepared gray powder were investigated by X-ray powder diffractometry (XRD, Rigaku DMAX-IIIC), using CuKa radiation (k = 0.154056 nm), in a standard Bragg-Brentano geometry. The detection range was 2h = 5–80 in steps of 0.02 and with a counting rate of 8 s/step. The morphology of both as-prepared powder and sintered pills was examined by scanning electron microscopy (SEM, Cambridge-S360). The SEM specimens were obtained by placing the powder/pill fragment(s) onto an aluminum stab, on a biadhesive graphite tape, and performing successively a graphitisation process. The powder composition was further examined by means of an energy dispersive spectrometer coupled with the used scanning electron microscope (SEM–EDS). The thermoelectric characterization of the synthesized Bi pills was accomplished by DC electrical resistance and Seebeck coefficient measurements, as well as by investigating the electrical resistance behavior with the temperature. The resistance was measured by conventional two-point probe DC measurements performed at room temperature and at 77 K, in a probe station, with an error not exceeding ±5%. Practically, the pills resistance was determined from current measurements over a range
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of applied voltage (?/–10 V) using a high resolution picoamperometer (Keithley 6487). The resistance versus temperature dependence was obtained by cooling the bismuth pill from 280 to 80 K under vacuum at 1 V constant voltage using a cryogenic probe station. The Seebeck coefficient S ¼ DV DT was determined by means of a modified Z-Meter (Gromov et al. 2001), as first screening parameter for the improvement in the thermoelectric efficiency. A small AC voltage was applied to the pill under test, inducing a temperature gradient across its thickness, through the Peltier effect. The current was periodically disconnected to measure the induced temperature difference across the pill and the generated Seebeck voltage simultaneously. The effective Seebeck coefficient was evaluated by fitting the Seebeck voltage linearly as a function of the temperature difference.
Results and discussion The typical DSC thermogram of pure bismuth dodecyl-mercaptide presented in Fig. 1a shows: a quite intensive endothermic peak at 66 C corresponding to the melting point of the mercaptide; a much less intensive endothermic peak at 105 C determined by the mercaptide thermal decomposition; a broad exothermic peak of low intensity in the range 180–200 C related to the clustering of the Bi atoms generated from the mercaptide thermal decomposition; and another endothermic peak at 271 C,
corresponding to Bi melting point. The second DSC run (Fig. 1b), performed on the same sample, exhibits two endothermic peaks: the first, at 27 C, is determined by the melting of the organic by-products mixture resulted from the thermal decomposition reaction and the second, at 271 C, corresponds to the Bi melting point. A complementary method employed to investigate the product of the mercaptide thermolysis was the TGA, which gives information about the reaction stoichiometry. The TGA thermogram of pure Bi(SC12H25)3 (Fig. 2) reveals one distinct weight loss in the range 175–320 C and a residual weight equal to 26 wt.% at temperatures over 320 C. Since the organic by-product of the Bi dodecyl-mercaptide thermolysis is completely removed by evaporation at temperatures close to 300 C, the residual weight corresponds to the synthetised inorganic phase. It was verified that the experimentally found residual weight corresponds perfectly to the theoretically calculated Bi percentage in the precursor mercaptide. This result confirms, therefore, that the mercaptide thermolysis gives only zero-valent bismuth (apart the organic byproduct), as theoretically predicted. The 2DCOSY spectrum (Fig. 3), with monodimensional 1H-NMR experiments on the two F1 and F2 axes, of the organic by-product of Bi(SC12H25)3 thermal decomposition shows only one spin system, attributable to (SH25C12)2. The cross peak A (Fig. 3) was assigned to carbon 1 C1 resonating at 2.69 ppm correlated with C2 at 1.69 ppm. The signal at 1.69 ppm (C3) was, in turn, 0,5
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Fig. 4 Typical XRD spectrum of the gray powder resulting from the bismuth dodecyl-mercaptide thermolysis
measurements. The typical XRD spectrum of the gray powder resulted after the thermolysis process is shown in Fig. 4. All the observed diffraction peaks can be indexed to the rhombohedral crystal structure of bismuth, according to the standard ICDD PDF (Card. No. 05–0519), apart only one peak corresponding to Bi2S3 (240). No other foreign phases can be seen within the apparatus detection limits. The good purity of the Bi(0) product was confirmed by SEM–EDS measurements. The EDS pattern (Fig. 5) of the gray powder resulted from the mercaptide thermolysis shows only peaks related to Bi (apart one peak corresponding to carbon C, which comes from the graphite substrate of the sample or from the graphitization process). The obtained structural data allow us to assume that the thermolysis process of Bi(SC12H25)3 is based on the homolytic dissociation of the Bi–S bonds with Fig. 3 2DCOSY spectrum, with mono-dimensional 1H-NMR experiments on the two F1 and F2 axes, of the organic byproduct of bismuth dodecyl-mercaptide thermal decomposition
correlated to carbon 3 at 1.38 ppm (cross peak B), whereas C3 and C4–11 (1.33 ppm) were correlated through cross peak C. Finally, carbons 4–11 appeared to be correlated to the methyl group at 0.91 ppm through cross peak D (Fig. 3). The results of the NMR experiments reveal that the thermal decomposition of Bi(SC12H25)3 produces disulfide as organic by-product. The DSC and TGA results in what concern the formation of only Bi(0) as non-organic product of the Bi mercaptide thermolysis are confirmed by XRD
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the formation of Bi atoms and H25C12S• radicals, which combine together leading to disulfide molecules (SC12H25)2. The Bi phase separation and atomic clustering take place following a temperature-dependent mechanism that is further described below. The Bi clusters are an electrophilic species due to the presence of 6p empty orbitals, while the disulfide molecules are strongly nucleophilic owing to the high polarizable lone-pair electrons on the sulfur atoms. Consequently, the nucleophilic disulfide molecule bonds the electrophilic surface of the Bi clusters, leading to the formation of an effective efficient steric barrier which prevent the particles from aggregation and limit their growth (Larsen et al. 2003). Therefore, the Bi(SC12H25)3 thermolysis leads to nanosized Bi powder due to the disulfide capping layer on the metallic particles’ surface. The temperature of the mercaptide thermolysis was varied in the range 140–180 C, in order to investigate the temperature influence on the morphology of the resulting Bi(0) particles. The reproducibility of the samples morphology as a function of the processing temperature has been verified by several sample preparations for each Fig. 6 Representative SEM micrographs of the Bi nanopowder obtained by thermal decomposition of bismuth dodecylmercaptide at 140 C (a), 160 C (b) and 180 C, (c) and of the Bi pills (d) prepared by uniaxial compression at 590 MPa, for 15 min of the Bi nanopowder obtained at 180 C. The related size histograms of the Bi nanoparticles are presented in the insets
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decomposition temperature. The representative SEM images of the Bi(0) powders prepared at 140, 160 and 180 C (Fig. 6a–c, respectively) reveal the formation of well defined, regular spherical shape nanoparticles for all performed thermolyses. The Bi particles’ size and the size distribution (Fig. 6 insets) were evaluated examining the SEM micrographs by means of an image analysis software. The determined average diameter D and the related standard deviation r of the Bi particles obtained from the mercaptide thermal decomposition at 140, 160 and 180 C are D = 601 nm and r = 202 nm, D = 202 nm and r = 32 nm, and D = 170 nm and r = 31 nm, respectively. On increasing the thermal decomposition temperature, a decrease in the particles size can be seen together with an increase in shape regularity and a narrower size distribution. The mechanism governing the nucleation and growth of the Bi particles in the used thermolysis conditions is the following: during the thermal treatment, the mercaptide decomposes producing a large amount of Bi atoms and the phase separation takes place at a high supersaturation level for the whole duration of the thermal treatment. The metal nuclei are continuously
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generated by atomic clustering, at a nucleation rate increasing with the thermolysis temperature, but with a low growth rate due to the short treatment time and limited Bi atomic diffusion into the mercaptide/disulfide mixture. The SEM analysis of the as-synthesised Bi pills morphology (Fig. 6d) shows the preservation of the nanostructure, with no essential grains deformation from their spherical shape, but with the increase in the average particles size (e.g., D = 272 nm, r = 54 nm for the pills obtained by the compression of Bi nanopowder with the average size of 170 nm prepared from 180 C mercaptide thermolysis) with respect to the precursor Bi nanopowder, due to grain aggregation events induced by the compression process. The first thermoelectric investigation of the assynthesised Bi pills was focused on the measurement of the current–voltage curves I = f(V) at both room temperature and %77 K. In the case of the pill prepared by pressing the 170 nm Bi nanopowder, a non-linear current dependence on the applied voltage and a higher electrical resistance at %77 K with respect to room temperature for the same values of applied voltage can be seen in Fig. 7a. These results indicate a semiconductive behavior. A better visualized evidence of the resistance non-linearity is furnished by the non-linear conductance (1/R) dependence on the applied voltage V at room temperature and at %77 K (Fig. 7b). The current–voltage curves for the pills obtained by pressing the 601 and 202 nm Bi nanopowders exhibit a typical metallic behavior, similar to bulk Bi. Until now, semimetal–semiconductor transition in Bi nanostructures, induced by quantum confinement effects, has been reported for nanoparticles with diameter less than 40 nm (Wang et al. 2006), for nanowire arrays and thin films below
where R0, Eg, kB, and T are pre-exponential constant, the energy gap, the Boltzmann constant (kB = 8.617 9 10-5 eV), and the temperature, respectively. Therefore, the energy gap of the investigated Bi pill can be found from a non-linear curve fit of R ¼ f T1 in two temperature ranges, 280–117 K and 100–82 K, from the relation: Eg 1 ¼ t1 2kB
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where t1 was determined to be t1 = 0.00102 ± 0.00006 for 100–82 K and t1 = 0.00294 ± 0.00003 for 280–117 K. The energy gap evaluated using Eq. 3 is Eg % 0.06 eV in 280–117 K temperature range and Eg % 0.17 eV for 100–82 K.
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Fig. 7 DC current intensity I versus applied voltage V at room temperature (d) and at 77 K (s) (a) and conductance versus applied voltage (b) for the Bi pill synthesised by uniaxial pressing the Bi powder obtained from 180 C mercaptide thermolysis
50 nm (Lin et al. 2000; Heremans 2005; Wang et al. 2006), and for nanowires in the range 100–60 nm (Yonghui and Jingying 2005). A further evidence for the semiconductor behavior of the pills prepared by pressing the 170 nm Bi nanopowder was obtained from the temperaturedependent resistance measurements performed by cooling the Bi pill from 280 to 80 K at 1 V constant voltage (Fig. 8a). As shown in the figure, the electrical resistance decreases with increase in temperature. This inverse trend with respect to the bulk Bi semimetal behavior is typical of a semiconductor. The measured resistance R values were further plotted versus (1/T) (Fig. 8b) in order to evaluate the energy gap Eg. It is well known that the resistance of a semiconductive material is strongly dependent on the width of the band gap, following the equation (Seeger 1985): Eg R ¼ R0 exp ð2Þ 2kB T
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The effective Seebeck coefficient S ¼ DV DT of the prepared nanostructured Bi pills was estimated from the linear fitting of the Seebeck voltage, as a function of the temperature difference in the range 318–373 K. Figure 9 reports the effective Seebeck coefficient as a function of temperature for the Bi pill prepared by pressing the 170 nm Bi nanopowder. It can be seen that the average absolute value of the Seebeck coefficient S % -146 lV K-1 is higher than that of bulk Bi [-72 lV K-1 (Hostler et al. 2007)] over the whole range of temperatures investigated. The negative sign of the determined Seebeck coefficients shows an n-type semiconductive behavior. The pills obtained by the consolidation of the higher size Bi powders obtained after 140 C (601 nm) and 160 C (202 nm) mercaptide thermolysis exhibit no improvement in
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Seebeck coefficient, which has similar values with bulk Bi. These results could be interpreted in the frame of the theory which predicts the largest enhancement in the Seebeck coefficient in comparison to bulk Bi in the case of quantum dots structures, with respect to nanowires and thin film systems (Sun et al. 1999). It is well known that the S value for bulk Bi is quite small due to the equal concentrations of electrons and holes, which lead to a nearly complete cancelation between the positive and negative contribution to the Seebeck coefficient, while the increase in the S value of nanostructures is determined by the quantum confinement effects on the electrical charge that result in an enhanced electronic density of states near the Fermi energy. In any case, until now, none of the investigated Bi nanostructures (nanowires and thin films) with the dimension of the constituent units similar to the size of the prepared semiconductive nanopowder (D = 272 nm, r = 54 nm) have shown such improvement in the Seebeck coefficient value. For example, literature reports S values similar to bulk Bi, in the range 70–80 lV K-1 at room temperature, for Bi nanowires with the diameter of 240 and 480 nm (Nikolaeva et al. 2008; Lin et al. 2000).
Conclusions The thermal decomposition of Bi(SC12H25)3 represents an effective preparation route of Bi nanoparticles, with spherical shape and rhombohedral crystal structure, offering the possibility to control the grains size through: (1) the formation of an efficient
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steric barrier made of (SC12H25)2 molecules (capping agent), which prevents the aggregation of particles and reduce their growth (ii) the control of the thermolysis temperature. The investigation of the transport properties of the Bi pills made of nanoparticles with the average diameter of 270 nm shows an n-type semiconductor behavior, with a direct band gap energy Eg % 0.06 eV in 280–117 K temperature range and Eg % 0.17 eV in 100–82 K, while the pills sintered from the higher dimension nanoparticles exhibit a semimetal behavior typical to bulk Bi. To the best of our knowledge, the observed semimetal–semiconductor transition for Bi grains size of about 270 nm is the first report on quantum-like confinement effects in Bi nanosystems with constituent units higher than 100 nm. Consistent with these results, the preliminary study on the thermoelectric characteristics of the prepared Bi nanopowders shows: (1) larger value for the effective Seebeck coefficient in the investigated temperature range with respect to bulk Bi in the case of the Bi pills made of 270 nm nanoparticles; (2) no enhancement in S magnitude with respect to bulk Bi for higher dimension systems. The obtained improvement in the thermoelectric characteristics with respect to bulk Bi even at grain dimensions of hundreds of nanometres shows a great potentiality of the Bi nanoparticles in thermoelectricity, but much research is still needed before their full potential is realized. Further research on tailoring the process parameters for decreasing the nanoparticles size and on the control of uniform morphologies formation is currently underway. Further investigation on the thermoelectric characteristics, on the origin of the semimetal–semiconductor transition and of the Seebeck coefficient increment in absolute value will be done to gain insight and understanding about the real possibilities of Bi nanopowders in reaching, by itself and in compounds, the efficiency level required for thermoelectric applications. Acknowledgment The technical assistance of Dr. Manlio Colella for SEM investigations is gratefully acknowledged.
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