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Effective medium theories (EMT) are powerful tools to calculate sample averaged thermoelectric material properties of composite materials. However, averaging over the heterogeneous spatial distribution of the phases can lead to incorrect estimates of the thermoelectric transport properties and the figure of merit ZT in compositions close to the percolation threshold. This is particularly true when the phases’ electronic properties are rather distinct leading to pronounced percolation effects. The authors propose an alternative model to calculate the thermoelectric properties of multi‐phased materials that are based on an expanded nodal analysis of random resistor networks (RRN). This method conserves the information about the morphology of the individual phases, allowing the study of the current paths through the phases and the influence of heterogeneous charge transport and cluster formation on the effective material properties of the composite. The authors show that in composites with strongly differing phases close to the percolation threshold the thermoelectric properties and the ZT value are always dominated exclusively by one phase or the other and never by an average of both. For these compositions, the individual samples display properties vastly different from EMT predictions and can be exploited for an increased thermoelectric performance.
High-performance Ag–Se-based n-type printed thermoelectric (TE) materials suitable for room-temperature applications have been developed through a new and facile synthesis approach. A high magnitude of the Seebeck coefficient up to 220 μV K–1 and a TE power factor larger than 500 μW m–1 K–2 for an n-type printed film are achieved. A high figure-of-merit ZT ∼0.6 for a printed material has been found in the film with a low in-plane thermal conductivity κF of ∼0.30 W m–1 K–1. Using this material for n-type legs, a flexible folded TE generator (flexTEG) of 13 thermocouples has been fabricated. The open-circuit voltage of the flexTEG for temperature differences of ΔT = 30 and 110 K is found to be 71.1 and 181.4 mV, respectively. Consequently, very high maximum output power densities pmax of 6.6 and 321 μW cm–2 are estimated for the temperature difference of ΔT = 30 K and ΔT = 110 K, respectively. The flexTEG has been demonstrated by wearing it on the lower wrist, which resulted in an output voltage of ∼72.2 mV for ΔT ≈ 30 K. Our results pave the way for widespread use in wearable devices.
Radio frequency identification (RFID) antennas are popular for high frequency (HF) RFID, energy transfer and near field communication (NFC) applications. Particularly for wireless measurement systems the RFID/NFC technology is a good option to implement a wireless communication interface. In this context, the design of corresponding reader and transmitter antennas plays a major role for achieving suitable transmission quality. This work proves the feasibility of the rapid prototyping of a RFID/NFC antenna, which is used for the wireless communication and energy harvesting at the required frequency of 13.56 MHz. A novel and low-cost direct ink writing (DIW) technology utilizing highly viscous silver nanoparticle ink is used for this process. This paper describes the development and analysis of low-cost printed flexible RFID/NFC antennas on cost-effective substrates for a microelectronic vital parameter measurement system. Furthermore, we compare the measured technical parameters with existing copper-based counterparts on a FR4 substrate.