Thickness-dependent thermoelectric power factor of polymer-functionalized semiconducting carbon nanotube thin films

The effects of polymer structures on the thermoelectric properties of polymer-wrapped semiconducting carbon nanotubes have yet to be clarified for elucidating intrinsic transport properties. We systematically investigate thickness dependence of thermoelectric transport in thin films containing networks of conjugated polymer-wrapped semiconducting carbon nanotubes. Well-controlled doping experiments suggest that the doping homogeneity and then in-plane electrical conductivity significantly depend on film thickness and polymer species. This understanding leads to achieving thermoelectric power factors as high as 412 μW m^?1 K^?2 in thin carbon nanotube films. This work presents a standard platform for investigating the thermoelectric properties of nanotubes.     

Fabrication and characterization of thermoelectric thick film prepared from p-type bismuth telluride nanopowders

In this study, bismuth telluride (Bi2Te3)-based nanopowders with particle size ranging from 100 to 300 nm are prepared by high-energy ball milling. Then, the prepared nanopowders are homogeneously mixed with organic binders to form a paste; this paste is used as the raw material to prepare thick-film thermoelectric modules. The thick film prepared by screen printing followed by hot pressing of p-type pastes show reproducible thermoelectric properties, exhibiting an electrical resistivity of 2.0 m Omega cm and a Seebeck coefficient of 298 muVK-1. The prepared p-type Bi2Te3 thick film has a high power factor because its Seebeck coefficient is significantly higher than that of Bi2Te3 based-bulk materials. These results indicate that a thick film prepared from bismuth telluride nanopowders has potential for use as high-performance thermoelectric modules in practical applications such as power generation and cooling system in electronic devices.     

A micromachined carbon nanotube film cantilever-based energy cell

This paper reports a new type of energy cell based on micromachined carbon nanotube film (CNF)-lead zirconate titanate cantilevers that is fabricated on silicon substrates. Measurements found that this type of micro-energy cell generates both AC voltages due to the self-reciprocation of the microcantilevers and DC voltages due to the thermoelectric effect upon exposure to light and thermal radiation, resulting from the unique optical and thermal properties of the CNF. Typically the measured power density of the micro-energy cell can be from 4 to 300?μW?cm(-2) when it is exposed to sunlight under different operational conditions. It is anticipated that hundreds of integrated micro-energy cells can generate power in the range of milliwatts, paving the way for the construction of self-powered micro-?or nanosystems.     

Multilayered carbon nanotube/polymer composite based thermoelectric fabrics

Thermoelectrics are materials capable of the solid-state conversion between thermal and electrical energy. Carbon nanotube/polymer composite thin films are known to exhibit thermoelectric effects, however, have a low figure of merit (ZT) of 0.02. In this work, we demonstrate individual composite films of multiwalled carbon nanotubes (MWNT)/polyvinylidene fluoride (PVDF) that are layered into multiple element modules that resemble a felt fabric. The thermoelectric voltage generated by these fabrics is the sum of contributions from each layer, resulting in increased power output. Since these fabrics have the potential to be cheaper, lighter, and more easily processed than the commonly used thermoelectric bismuth telluride, the overall performance of the fabric shows promise as a realistic alternative in a number of applications such as portable lightweight electronics.     

Light-induced EMF in silver-palladium film resistors

We have studied the generation of nanosecond emf pulses in silver-palladium film resistors under the action of radiation of a Q-switched laser. The samples were fabricated using the technology of thick film resistors on dielectric substrates, based on fusing a resistive paste that contains palladium, silver oxide, glass, and an organic binder into the substrate at 880 K. The amplitude of detected pulses exhibits linear growth with the power of incident laser radiation, depends on the angle of light incidence (vanishes at the normal incidence) onto the film and the angle of film rotation about the normal to its surface, and changes its sign with that of the incidence angle. The duration of the generated emf pulses is several times that of the incident laser pulses. The signal is not of a thermoelectric nature and can be related to the current generation by means of the surface photogalvanic effect and the photon quasi-momentum transfer to charge carriers during light absorption by the film material.     

Carbon‐Nanotube‐Based Thermoelectric Materials and Devices

Conversion of waste heat to voltage has the potential to significantly reduce the carbon footprint of a number of critical energy sectors, such as the transportation and electricity‐generation sectors, and manufacturing processes. Thermal energy is also an abundant low‐flux source that can be harnessed to power portable/wearable electronic devices and critical components in remote off‐grid locations. As such, a number of different inorganic and organic materials are being explored for their potential in thermoelectric‐energy‐harvesting devices. Carbon‐based thermoelectric materials are particularly attractive due to their use of nontoxic, abundant source‐materials, their amenability to high‐throughput solution‐phase fabrication routes, and the high specific energy (i.e., W g^?1) enabled by their low mass. Single‐walled carbon nanotubes (SWCNTs) represent a unique 1D carbon allotrope with structural, electrical, and thermal properties that enable efficient thermoelectric‐energy conversion. Here, the progress made toward understanding the fundamental thermoelectric properties of SWCNTs, nanotube‐based composites, and thermoelectric devices prepared from these materials is reviewed in detail. This progress illuminates the tremendous potential that carbon‐nanotube‐based materials and composites have for producing high‐performance next‐generation devices for thermoelectric‐energy harvesting.     

Thermoelectric measurement of multi-walled carbon nanotube bundles by using nano-probes

We report here a method for measurement of thermoelectric power of quasi-one dimensional nano-materials with a simple platform, where individual nanomaterial is assembled with nano-probes in a scanning electron microscope. This approach allows repeated manipulation and thermoelectric measurement of the same loaded nanosample with adjustable number of individual nanotubes or nanowires. It also allows assembly of multiple samples on one measurement stage. For multi-walled carbon nanotube bundles, we have observed a weak trend that, when the number of individual tubes in a bundle varies from ten millions to around a hundred thousand, the thermoelectric power almost remains at around 10 microV/K. When the tube number in the bundle is further reduced, the up-limit of the thermoelectric power gradually increases to a value near 20 microV/K.     

碲纳米线柔性薄膜的制备及其热电性能

柔性热电器件能够直接将人体热量转化为电能, 因而受到广泛的关注。本研究采用水热法合成了碲纳米线, 探究了水热温度和反应溶液的还原性强弱(添加抗坏血酸与否)对碲纳米线形貌及热电性能的影响。与强还原性反应液(添加抗坏血酸)中制备的碲纳米线相比, 弱还原性反应液(未添加抗坏血酸)中制备的碲纳米线具有较高的长径比, 最高可达200, 其组装的薄膜具有更高的电导率, 达到26 S·m^-1。进一步研究了成膜工艺对碲纳米线薄膜热电性能的影响, 发现湿压法可提升薄膜的微观致密度, 使薄膜中碲纳米线之间的微观连接更为紧密, 从而改善了薄膜的载流子迁移率和载流子浓度, 使薄膜的电导率提升了18.3倍, 达到476 S?m^-1, 塞贝克系数为282.9 μV?K^-1, 功率因子达到38 μW?m^-1?K^-2。     

Measurement of metal/carbon nanotube contact resistance by adjusting contact length using laser ablation

A technique of measuring contact resistance between an individual nanotube and a deposited metallic film is described. Using laser ablation to sequentially shorten the contact length between a nanotube and the evaporated metallic film, the linear resistivity of the nanotube as well as the specific contact resistivity between the nanotube and metallic film can be determined. This technique can be generally used to measure the specific contact resistance that develops between a metallic film and a variety of different nanowires and nanotubes.     

Preparation and characterization of the Ag2Se flexible films tuned by PVP for wearable thermoelectric generator

In recent years, the demand for wearable devices has promoted the research of flexible thermoelectric generators. Herein, this work reports a facile method to prepare the flexible Ag₂Se/polyvinyl pyrrolidone (PVP) composite films. The Ag₂Se nanorods are synthesized by the template method and then mixed with PVP solution to form the composite film optimized by the content of Ag₂Se. The maximum power factor of the film is 16.18?µW m^?1 K^?2 at 320?K, and the thermoelectric generator (TEG) has an output voltage and power of 5.49?mV and 55.57 nW at a temperature difference of 40?K, respectively. Moreover, the Ag₂Se/PVP film has good flexibility and can withstand multiple bending, the electrical conductivity reaching 89.8% and 76?% at 500 and 1000, respectively. PVP has an excellent protective capabilities effect so that the TE performance can remain stable in 40?days, and then the electrical conductivity only drops to 89?% of the initial value at 50?days. This work provides an effective method for preparing wearable thermoelectric devices, which can be extended to other composite materials.

     

The effect of physical vapor deposition parameters on the thermoelectric power of thin film molybdenum-nickel junctions

The effect of various physical vapor deposition parameters on the thermoelectric power of thin film (10 000 Å) Mo-Ni junctions deposited by electron beam evaporation was investigated in this study. The deposition parameters of interest were the substrate temperature, the deposition rate and the vapor source purity. The thermoelectric power of the thin film Mo-Ni junctions was dependent on the structure of the Mo thermoelectric element of the couple, the characteristics of which were significantly altered by varying the deposition parameters. Varying the deposition parameters caused a change in lattice imperfections in the metal, which changed the mean free path of the conduction electrons and the thermoelectric power of the couple. The parameters having the greatest effect on the thermoelectric power were the deposition rate and the substrate temperature. Couples deposited at a high deposition rate (10 000 Å min^-1) and a high substrate temperature (? 300 °C) demonstrated e.m.f. characteristics closest to bulk values.     

低维Bi_2Te_3基热电材料的研究进展

Bi_2Te_3及其固溶体金金是目前室温附近发展最为成熟、性能最好的一类热电材料,在热电制冷及温差发电方面具有广阔的应用前景.如何最大限度地提高材料的热电优值是当今热电材料研究的主要问题.传统块体Bi_2Te_3基热电材料的最高ZT值只能达到1.0左右,而低维化、纳米化的Bi_2Te_3基热电材料可使电子和声子的传输得到合理调控,从而大幅提升材料的热电性能.综述了二维纳米薄膜、一维纳米线(管、棒)和准零维纳米颗粒等低维Bi_2Te_3基热电材料的最新研究进展,并结合目前的研究状况展望了今后的研究重点及发展方向.     

Phase transitions and transport properties of Cu<Subscript>3</Subscript>Co<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> crystals

We have synthesized a solid solution with the composition Cu₃Co_0.5Se₂ and investigated its structural phase transitions by high-temperature X-ray diffraction on a D8 Advance diffractometer. The results demonstrate that, in the range 100–773 K, the solid solution undergoes two phase transitions: at 560 K, the low-temperature, orthorhombic phase (α) transforms into an intermediate, primitive cubic phase (β); at 765 K, the β-phase transforms into a high-temperature phase (γ, sp. gr. Fm3m). The thermoelectric power, electrical conductivity, and Hall coefficient of the solid solution were measured in the temperature range 80–350 K. The observed temperature variation of its thermoelectric power and conductivity can be accounted for under the assumption that its conduction band has an additional subband due to the cobalt atoms.     

Single-walled carbon nanotube thermopile for broadband light detection

We designed a thermopile based on a PN doping profile engineered in a suspended film of single-walled carbon nanotubes (SWNTs). Using estimates of the film local Seebeck coefficients, the SWNT thermopile was optimized in situ through depositions of potassium dopants. The overall performances of the thermopile were found to be comparable to state-of-the-art SWNT bolometers. The device is characterized at room temperature by a time response of 36 ms, typical of thermal detectors, and an optimum spectral detectivity of 2 × 10(6) cm Hz(1/2)/W in the visible and near-infrared. This paper presents the first thermopile made of a suspended SWNT film and paves the way to new applications such as broadband light (including THz) detection and thermoelectric power generation.     

Electrical properties of Bi2S3 thin films prepared by the dip-dry method

Bi₂S₃ thin films have been prepared by the dip-dry method and their electrical properties such as conductivity, thermoelectric power and spectral response of photoconductivity were investigated. Some uncommon findings, such as a simultaneous increase in conductivity and thermoelectric power with thickness of film, are explained by Seto's model for polycrystalline silicon film duly modified for in homogeneities.     

A Solution Processable High‐Performance Thermoelectric Copper Selenide Thin Film

A solid‐state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low‐cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Herein, a flexible thermoelectric copper selenide (Cu₂Se) thin film, consisting of earth‐abundant elements, is reported. The thin film is fabricated by a low‐cost and scalable spin coating process using ink solution with a truly soluble precursor. The Cu₂Se thin film exhibits a power factor of 0.62 mW/(m K²) at 684 K on rigid Al₂O₃ substrate and 0.46 mW/(m K²) at 664 K on flexible polyimide substrate, which is much higher than the values obtained from other solution processed Cu₂Se thin films (<0.1 mW/(m K²)) and among the highest values reported in all flexible thermoelectric films to date (≈0.5 mW/(m K²)). Additionally, the fabricated thin film shows great promise to be integrated with the flexible electronic devices, with negligible performance change after 1000 bending cycles. Together, the study demonstrates a low‐cost and scalable pathway to high‐performance flexible thin film thermoelectric devices from relatively earth‐abundant elements.     

The resistivity, temperature coefficient of resistivity and thermoelectric power of thin continuous metal films I: A survey and critical appraisal of the application of processing methods to experimental data

In this paper the results are reported of an investigation into the accuracy and applicability of the methods which have been used to extract parameter values from sets of experimental data points. Theoretical models describing resistivity, temperature coefficient of resistivity and thermoelectric power of thin continuous metal films and experimental processing methods are identified and categorized. A critical appraisal is given of experimental work reported in papers published between 1961 and 1984. This is followed by details of investigations into the methods commonly used for fitting data to exact and thick film relationships. These investigations are presented within the framework of the Fuchs-Sondheimer relationships. However, the major problem areas that are identified are of a general nature and do not appear to be limited to this model. The problems with the methods investigated, especially those associated with thick film approximations, are shown to be so great as to cast doubt on many published values.     

Synthesis and thermoelectric power of nitrogen-doped carbon nanotubes

We have previously shown that high-purity multiwalled carbon nanotubes (pristine MWNTs) can be prepared from a mixture of xylene-ferrocene (99 at% C:1 at% Fe) inside a quartz tube reactor operating at approximately 700 degrees C. In a similar process, approximately 3 g of melamine (C3H6N6) was introduced during the growth of MWNTs to prepare nitrogen-doped nanotubes. The structural and electronic properties of nitrogen-doped MWNTs were determined by scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and thermopower measurements. The individual nitrogen-doped nanotube exhibits a bamboo-like structure and comprises 6-16 tube walls, as evidenced by HRTEM studies. The EELS measurements yielded an average nitrogen content of approximately 5 at% in the doped tubes. The thermoelectric power data of nitrogen-doped MWNTs remained negative even after exposure to oxygen for an extended period of time, suggesting that nitrogen doping of MWNTs renders them n-type, consistent with scanning tunneling spectroscopic studies on similar nanotubes.     

Extraordinary Thermoelectric Performance Realized in n‐Type PbTe through Multiphase Nanostructure Engineering

Lead telluride has long been realized as an ideal p‐type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n‐type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record‐high ZT value of ≈1.83 can be achieved at 773 K in n‐type PbTe‐4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state‐of‐the‐art thermoelectric systems.     

Formation of free-standing carbon nanotube array on super-aligned carbon nanotube film and its field emission properties

Flexible and free-standing well-aligned carbon nanotube arrays have been synthesized on super-aligned carbon nanotube films. The combined structure of the carbon nanotube array and carbon nanotube film was formed during chemical vapor deposition on a quartz substrate which had previously been covered with a super-aligned carbon nanotube film. It was found that the growing carbon nanotube array could support up the super-aligned carbon nanotube film entirely, and the top of the array became densely entangled with the super-aligned carbon nanotube film. The carbon nanotube array with the super-aligned carbon nanotube film could be easily peeled off from the quartz substrate as a whole, giving a flexible and free-standing structure with good mechanical properties. The bottom of the array was also exposed after being peeled off and was used as a field emitter. The combined structure of the carbon nanotube array with the carbon nanotube film allowed adsorbent-free field emission by passing a heating current through it. Furthermore, due to the fast thermal response of the structure and the long time needed for re-adsorption of adsorbates in vacuum, it was found that pulsed heating with a 10% duty ratio was sufficient for adsorbent-free field emission. The heating power necessary to sustain the adsorbent-free state can be lowered in this way.

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