Tuning the shape and thermoelectric property of PbTe nanocrystals by bismuth doping
Qian ZhangTing SunFeng CaoMing LiMinghui HongJikang YuanQingyu YanHuey Hoon HngNianqiang WuXiaogang Liu
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Abstract:
We report the synthesis of a series of monodispersed Bi-doped PbTe nanocrystals with tunable morphologies by using a doping precursor of bismuth(III) 2-ethylhexanoate. The as-synthesized Pb1−xBixTe (x = 0.005, 0.010, 0.015, 0.020) nanocrystals are characterized by X-ray diffraction, X-ray photoelectron spectroscopy and Hall measurements. The nanocrystals with controlled spherical, cuboctahedral, and cubic shapes were readily prepared by varying the Bi doping concentration. Thermoelectric investigation of these nanocrystals shows that the Bi3+ doping increases electrical conductivity from 350 to 650 K and changes the Seebeck coefficient sign from positive to negative.Keywords:
Bismuth
Thermoelectric materials are electronic materials that can exhibit noticeable voltage under temperature gradient and high electrical conductivity. The conventional thermoelectric materials are inorganic semiconductors or semimetals. Recently, flexible thermoelectric materials including conducting polymers and polymer composites gained great attention. The thermoelectric performance is usually characterized by the dimensionless figure of merit ( ZT ), ZT = S 2 σT / κ , where S being the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. S 2 σ is called the power factor. To achieve high thermoelectric performance, it is important to possess the fundamental knowledge of thermoelectric materials, particularly the physics of the Seebeck coefficient, Peltier coefficient, electrical conductivity, and thermal conductivity. The Seebeck coefficient is related to the dependence of charge carrier density on temperature. The Seebeck coefficient of conducting polymers is usually lower than that of their inorganic counterparts by about one to two orders of magnitude. The electrical conductivity depends on the charge carrier density and charge carrier mobility. But the Seebeck coefficient and electrical conductivity are interdependent. Decreasing the charge carrier density can increase the Seebeck coefficient while decreasing the electrical conductivity. Thus, there is an optimal power factor in terms of the charge carrier density. Low thermal conductivity is required for high thermoelectric performance. The thermal conductivity includes the contributions by phonons and charge carriers. The thermal conductivity of conducting polymers is usually significantly lower than the inorganic thermoelectric materials. The thermoelectric materials have important application in harvesting heat, cooling, or sensing. The efficiency of thermoelectric generators depends not only on the ZT values of the thermoelectric materials but also their electrical and thermal contact resistances. The operation mechanism of thermoelectric coolers is basically the opposite process of thermoelectric generators. The temperature-sensitive voltage due to the Seebeck effect of thermoelectric materials is the basic principle of their application in sensors.
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The measurement of the thermoelectric properties of thin film thermoelectric materials has been an issue due to the difficulty and inaccuracy. In this work, we present a new model to simultaneously extract the Seebeck coefficient and thermal conductivity in the cross-sectional direction of thin film thermoelectric material. The proposed method uses a sandwich structure composed of a metal electrode/TE film/metal electrode and measures the external Seebeck coefficient at two different intervals on the metal electrode. A theoretical model enables us to extract the Seebeck coefficient and thermal conductivity of the thermoelectric material from the two external Seebeck coefficient measurement values. The proposed method is applied to screen-printed ZnSb film with copper electrodes and the measurement results were found to lie in a reasonable range. Given that this method is simple to use, it will contribute to the development of thin film thermoelectric devices.
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To improve the thermoelectric performance of molecular junctions formed by polyaromatic hydrocarbon (PAH) cores, we present a new strategy for enhancing their Seebeck coefficient by utilizing connectivities with destructive quantum interference combined with heteroatom substitution. Starting from the parent PAH, with a vanishing mid-gap Seebeck coefficient, we demonstrate that the corresponding daughter molecule obtained after heteroatom substitution possesses a non-zero, mid-gap Seebeck coefficient. For the first time, we demonstrate a "bi-thermoelectric" property, where for a given heteroatom and parent PAH, the sign of the mid-gap Seebeck coefficient depends on connectivity and therefore the daughter can exhibit both positive and negative Seebeck coefficients. This bi-thermoelectric property is important for the design of tandem thermoelectric devices, where materials with both positive and negative Seebeck coefficients are utilized to boost the thermovoltage. Simple parameter-free rules for predicting the Seebeck coefficient of such molecules are presented, which form a powerful tool for designing efficient molecular thermoelectric devices.
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Thin film thermoelectric materials (TF TEMs) based on organic semiconductors or organic/inorganic composites exhibit unique properties such as low-temperature processability, mechanical flexibility, great freedom of material design, etc. Thus they have attracted a growing research interest. Similar to inorganic bulk thermoelectric materials (IB TEMs), the Seebeck coefficient combined with electrical conductivity and thermal conductivity is a fundamental property to influence the performance of TF TEMs. However, due to the differences in material and sample geometries, the well-established characterization devices for IB TEMs are no longer applicable to TF TEMs. And until now, a universal standard of measuring the Seebeck coefficient of TF TEMs is still lacking. This mini-review presents the development of instruments designed for measuring the Seebeck coefficient of TF TEMs in the last decade. Primary measurement methods and typical apparatus designs will be reviewed, followed by an error analysis induced by instrumentation. Hopefully this mini-review will facilitate better designs for a more accurate characterization of the Seebeck coefficient of thin film thermoelectric materials.
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The measurement of thermoelectric current is a new and effective method for inline wear detection in sheet metal forming. The measuring principle is based on the Seebeck effect, whose characteristic value, the Seebeck coefficient depends on the material composition. In the previous research of the authors, a boundary value of the thermoelectric value that separates the mild and severe wear was identified. Due to the large deviation of the Seebeck coefficient of the material used in sheet metal forming, it is worth discussing the influence of the Seebeck coefficient of the sheet metal material on the effectiveness and boundary value of the thermoelectric current for wear detection. In this paper, the measuring principle is first illustrated using an equation based on thermoelectricity. The Seebeck coefficients of the tools and sheet metals are then determined by a specifically designed device. At the same time, the wear tests for different materials are used to determine the boundary values for different tribological systems. Finally, the obtained Seebeck coefficient and boundary values are compared. From the results it can be concluded that the value of the measured Seebeck coefficients have a discernible effect on the boundary values, which provides a useful insight for inline wear diagnosis for practical applications.
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We show that carbon-based nanostructured materials are a novel testbed for controlling thermoelectricity and have the potential to underpin the development of new cost-effective environmentally-friendly thermoelectric materials. In single-molecule junctions, it is known that transport resonances associated with the discrete molecular levels play a key role in the thermoelectric performance, but such resonances have not been exploited in carbon nanotubes (CNTs). Here we study junctions formed from tapered CNTs and demonstrate that such structures possess transport resonances near the Fermi level, whose energetic location can be varied by applying strain, resulting in an ability to tune the sign of their Seebeck coefficient. These results reveal that tapered CNTs form a new class of bi-thermoelectric materials, exhibiting both positive and negative thermopower. This ability to change the sign of the Seebeck coefficient allows the thermovoltage in carbon-based thermoelectric devices to be boosted by placing CNTs with alternating-sign Seebeck coefficients in tandem.
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The Seebeck coefficient of plain cement paste is usually less than 1 μV/°C. To enhance the thermoelectric performance of cement composites, a kind of MnO2 powder was synthesized and used as a thermoelectric component in this work. The synthesized MnO2 powder was proven to be a kind of effective thermoelectric component in cement paste. The synthesized MnO2 powder possesses a nanorod structure with a diameter of about 50 nm and a length up to 1.4 μm. The measured Seebeck coefficient of the compacted MnO2 powders was about −5,490 μV/°C. The nanorod structured design was proven to be an effective method to increase the Seebeck coefficient of the thermoelectric component in this research. Then the MnO2 nanorods were incorporated into the cement matrix to enhance the thermoelectric effect of cement composites. A relatively high Seebeck coefficient of about −3,085 μV/°C could be obtained when the content of MnO2 powder was 5.0% by weight of cement, which is more than 1,000 times higher than that of the cement paste without MnO2 powder. And at this content, the electrical conductivity, thermal conductivity, and ZT of cement composites were 1.88×10−4 S/m, 0.72 W/mK, and 7.596×10−7, respectively. The thermoelectric effect of the cement composites is enhanced mainly due to the enhanced Seebeck coefficient, while the influence of electrical conductivity and thermal conductivity caused by MnO2 powders is not as obvious as Seebeck coefficient.
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The authors report measurements of the electrical resistivity, Seebeck coefficient, and thermal conductivity for a series of misfit-layered oxides Ca3Co4−xFexO9+δ (x=0, 0.05, 0.1, 0.15, 0.2) prepared by solid state reaction. Structural parameters are refined with superspace group of X2∕m(0b0)s0 using powder x-ray diffraction data. With partial substitution of Fe+2 for Co+3, the resistivity decreases, while the thermopower increases simultaneously. The x=0.05 sample exhibits a higher figure of merit (Z=3.01×10−4K−1) than that of Ca3Co4O9+δ (0.33×10−4K−1) at 300K, indicating much improvement of thermoelectric characteristics via partial substitution of Fe for Co.
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We report the strongly correlated, electrical transport, magnetic, and thermoelectric properties of a series of Fe, Mn, and Cu doped Ca3Co4O9. The results indicate that Fe/Mn substitutes for Co in CoO2 layers whereas Cu substitutes for Co in Ca2CoO3 layers. Because of the different doping sites, the electronic correlations increase remarkably in Fe and Mn doped series while remaining unchanged in Cu doped series. Correspondingly, the transport mechanism, magnetic properties, and some characteristic parameters along with transition temperatures all exhibit two distinct evolutions for Fe/Mn doping and Cu doping. The thermoelectric characteristics are improved in each series. Nevertheless, the improvement of thermoelectric performance is most significant in Fe doped samples due to the unexpected changes in thermopower and resistivity. The unusual thermopower behavior can be well described by the variations of electronic correlation. Possible approaches for further improvement of the thermoelectric performance in Ca3Co4O9 and other relevant strongly correlated systems are also proposed at the end.
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