We propose a kind of general framework for the design of a perfect linear polarization converter that works in the transmission mode. Using an intuitive picture that is based on the method of bi-directional polarization mode decomposition, it is shown that when the device under consideration simultaneously possesses two complementary symmetry planes, with one being equivalent to a perfect electric conducting surface and the other being equivalent to a perfect magnetic conducting surface, linear polarization conversion can occur with an efficiency of 100% in the absence of absorptive losses. The proposed framework is validated by two design examples that operate near 10 GHz, where the numerical, experimental and analytic results are in good agreements.
High-throughput roll-to-roll processes are desirable to scale up the manufacture of flexible thermoelectric generators. Whilst vacuum deposition onto a heated dynamic substrate presents a considerable engineering challenge, viable post-deposition in-line annealing processes are considered as an alternative to improve the functional performance of as-deposited films. The effect of infrared and electron-beam irradiations of 1-𝜇m thick bismuth telluride thin films, produced by a vacuum roll-to-roll process for use as thermoelectric materials, were examined. Static vacuum oven and pulsed high-energy electron beam were also studied as control groups. All annealing strategies increased the crystallite size, and decreased the Te content. Only the static vacuum oven treatment was shown to significantly improve the film's crystallinity. After 1-hour annealing, the power factor improved by 400% (from 2.8 to 14 × 10 -4 W/mK 2 ), which, to the knowledge of the authors, is the highest reported thermoelectric performance of post-annealed or hot-deposited Bi-Te films. As for inline annealing, infrared and electron-beam post treatments improved the power factor by 146% (from 2.8 to 6.9 × 10 -4 W/mK 2 ) and 64% (from 2.8 to 4.6 × 10 -4 W/mK 2 ), respectively.
High-throughput roll-to-roll processing could be used to scale up the manufacture of flexible thermoelectric generators. Very thin thermoelectric layers can be manufactured at high throughput speed and low cost and, most importantly, are predicted to possess better thermoelectric properties than thicker layers. Here we present a study on a series of bismuth telluride films of different thickness (few nm to 370 nm), deposited on polymer substrates at room temperature using DC magnetron sputtering. Unlike previous studies of deposition of bismuth telluride films onto heated substrates, an island-growth mode, indicated by AFM, was observed for Bi-Te films grown at room temperature. A period of growth in which the layer only partially coats the substrate, with only imperfect connections between islands, was observed. In this partially coated region, the coating exhibited an extremely high Seebeck coefficient. An energy barrier mechanism, similar to the interface effect in nanomaterials, is proposed to explain this phenomenon, along with a possible quantum confinement effect. We found that a thinner Bi-Te film could generate a greater power factor because of a quasidecoupling of Seebeck coefficient and electrical resistivity. In addition, ensuring that the
The optimization of flexible thin-film thermoelectric generator suitable for large-area roll-toroll processing is investigated. The selection of suitable contact materials, in-line patterning of connections, and dimension of the thermoelectric strip are investigated. As a result, copper is selected for contacts because it possesses a similar performance to gold while being cheaper.Both in-series and in-parallel connected devices are found to work well and provide a voltagedominant and current-dominant power source, respectively. The Seebeck coefficient and internal resistance of a device are extracted from fits to the measured power data. The inparallel connected thermoelectric generator has a much smaller internal resistance and is thus suitable for wearable/portable devices with the small load resistance. A shorter and wider thermoelectric strip generates more power. To the authors' knowledge, this is the first study that experimentally proves a downward trend of power output with increasing the strip length.In addition, an industrially feasible/continuous process is proposed for large-scale manufacture of flexible thermoelectric generators, by roll-to-roll sputtering thermoelectric materials on polymer web, inkjet printing contacts, and segmenting using a laser. A segmented
In this work, we investigated the use of in-line linear electron beam irradiation (LEB) surface treatment integrated into a commercially compatible roll-to-roll (R2R) processing line, as a single fluorocarbon cleaning step, following flexography oil masking used to pattern layers for devices. Thermoelectric generators (TEGs) were selected as the flexible electronic device demonstrator; a green renewable energy harvester ideal for powering wearable technologies. BiTe/BiSbTe-based flexible TEGs (f-TEGs) were fabricated using in-line oil patterned aluminium electrodes, followed by a 600 W LEB cleaning step, in which the duration was optimised. A BiTe/BiSbTe f-TEG using an oil-patterned electrode and a 15 min LEB clean (to remove oil prior to BiTe/BiSbTe deposition) showed similar Seebeck and output power (S ~ 0.19 mV K−1 and p = 0.02 nW at ΔT = 20 K) compared to that of an oil-free reference f-TEG, demonstrating the success of using the LEB as a cleaning step to prevent any remaining oil interfering with the subsequent active material deposition. Device lifetimes were investigated, with electrode/thermoelectric interface degradation attributed to an aluminium/fluorine reaction, originating from the fluorine-rich masking oil. A BiTe/GeTe f-TEG using an oil-patterned/LEB clean, exceeded the lifetime of the comparable BiTe/BiSbTe f-TEG, highlighting the importance of deposited material reactivities with the additives from the masking oil, in this case fluorine. This work therefore demonstrates (i) full device architectures within a R2R system using vacuum flexography oil patterned electrodes; (ii) an enabling Electron beam cleansing step for removal of oil remnants; and (iii) that careful selection of masking oils is needed for the materials used when flexographic patterning during R2R.
pollution, a tradeoff between size and storage, inconvenience of disassembly and replacement for wearers [2] ) for such applications. As a green/clean and self-powered source, wearable TEGs can locally/continuously convert thermal energy (i.e., the temperature difference, ΔT, between the human body and the surroundings) into electrical energy according to the Seebeck effect using thermoelectric (TE) materials. Flexible/wearable TEGs are commonly developed as either fully organic [3][4][5][6] or inorganic/organic hybrids. [7][8][9][10] Inorganic TE thin-film deposition on flexible polymer sheet is a typical route to inorganic/organic hybrids, which has attracted significant attention recently because a thin-film configuration has the potential compatibility with low-cost fabrication technologies (e.g., roll-to-roll, R2R [11] ), low material consumption/cost, [12] minimum size/weight, [13] large-area manufacturability, [14] and a wide range of structural designs of TEGs (e.g., planar, [15] cylindrical, [16] Y-type, [17] corrugated-structure [18] or folded-mode, [14] slope-type, [19] coil-up coin-shape, [20] and oxide-based transversal [21] ). In laboratory research, various techniques have been investigated to fabricate TE thin films, e.g., sputtering, [22] evaporation, [23] inkjet [24] / screen [25] printing, pulsed laser deposition, [26] molecular beam epitaxy, [27] and electrodeposition. [28] Among them, only sputtering shows the most promise for scale-up manufacture of TEGs like R2R processing. [29] Hence, sputtering is employed in this study.Flexible thin-film TEGs can be assembled in both crossplane (CP-TEG) and in-plane (IP-TEG) structural designs, which allow heat flows/TE legs perpendicular and parallel, respectively, to the substrate. [30] CP-TEG has already been commercialized in bulk TEGs and some μTEGs, however, to further decrease the size to nanorange, the crossplane configuration is not practical since maintaining a large ΔT across a nanothick TE leg is an almost impossible challenge [14,31] and the power output is very poor (e.g., refs. [32-39]). Although the development of nanosize CP-TEG is restricted, nanomaterials still attract significant interest for scientists, and the research on IP-TEG is moving toward the use of nanomaterials (e.g., thin film, [40] quantum well, [41] and nanowire [42] ), A stacked thermoelectric generator on a flexible polymer sheet is investigated that can utilize a low-cost high throughput roll-to-roll process, employing a metal-insulator-semiconductor structure of <100 nm thick Cu and bismuth telluride films with a ≈1 µm thick acrylate insulating coating. Thermoelectric strips can be stacked and connected in the out-of-plane direction, which significantly decreases the size required in the substrate plane and also gives rise to the opportunity for greatly extending power output by stacking thousands of layers. A smooth surface of stacked layers is confirmed due to the nature of the acrylate layer. Room-temperature sputtering can produce good quality/crystalline films, ...
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