Metallic and semiconducting nanowires (NWs) are of interest in the field of thermoelectrics, because they act as model system to investigate the influence of surfaces on the thermoelectric transport properties. In single crystalline NWs, the grain boundary scattering is negligible and the surface‐to‐volume‐ratio is high. We present state‐of‐the‐art of the combination of the structural, chemical, and temperature‐dependent full thermoelectric characterization for individual single crystalline NWs, which is essential to conclude on surface effects. Temperature‐dependent measurements allow further conclusions on the scattering mechanisms. Simulations by the finite element method are performed on indented NWs to interpret the measurement results. Calculated surface temperature of a single‐indented and a multi‐indented NW.
This study introduces a novel experimental set-up to measure the pyroelectric coefficient of materials at variable frequencies of temperature change. In this method, temperature changes are periodically applied through a mechanical set-up moving the sample between a hot and cold thermal reservoir in order to measure the current obtained from the sample's pyroelectric conversion effect. For low frequencies of temperature change, an exponential equation is suggested for the temperature change in the sample based on a unidirectional heat flux through the sample. The pyroelectric coefficient can be determined from this model by fitting an exponential decay function to the pyroelectric current obtained from a single heating cycle. In comparison to other approaches for the measurement of the pyroelectric effect, the described method exhibits some advantages concerning flexibility, accuracy and simplicity, e.g. an almost deliberate adjustment of the rate of temperature change, the avoidance of electrical noise induced by continuously temperature-modulated heat stages and furnaces, a high accuracy of temperature control and the acceptance of samples made from different materials or with different sizes. The method is verified by pyroelectric coefficient measurements on a commercial PZT ceramic and on triglycine sulfate (TGS) single crystals, which have been grown by a temperature-lowering technique in our laboratory. Pyroelectric measurements were conducted at different temperature differences for samples with different thicknesses and contact areas. The measurement results for PZT M202 (427 ± 1 μC m–2 K–1) are close to the datasheet value, which is 430 μC m–2 K–1. An average pyroelectric coefficient of 306 ± 2 μC m–2 K–1) is measured for the single crystal of TGS over multiple trials. The measurement results depend on the quality of the crystal and the process for the preparation of the sample. The results show internal consistency between the measurements, which are also in agreement with literature values.
Pyroelectric generators (PEGs) can be used for thermal energy harvesting and present a potential alternative to thermoelectric generators. However, in contrary to thermoelectric generators, the PEG principle requires thermal transients to stimulate the conversion process. Such suitable thermal transients are rare in nature, hindering the deployment. In this paper, we present a micro thermomechanic-pyroelectric energy generator (μTMPG) that converts a stationary spatial thermal gradient into the required transient temperature profile across the PEG. The measured power output of the μTMPG is 3 μW from a temperature difference of 79.5 K. However, with an optimized design, a power output of 39.4 mW is estimated for the same temperature difference.
This paper reports the design, and testing of a water turbine generator system for typical flow rates in domestic applications, with an integrated power management and a Bluetooth low energy (BLE) based RF data transmission interface. It is based on a commercially available low cost hydro generator. The generator is built into a housing with optimized reduced fluidic resistance to enable operation with flow rates as low as 6 l/min. The power management combines rectification, buffering, defined start-up, and circuit protection. An MSP430FR5949 microcontroller is used for data acquisition and processing. The data are transmitted via RF, using a Bluegiga BLE112 module in advertisement mode, to a PC where the measured flow rate is stored and displayed. The transmission rate of the wireless sensor node (WSN) is set to 1 Hz if enough power is available, which is the case for flow rates above 5.5 l/min. The electronics power demand is calculated to be 340 μW in average, while the generator is capable of delivering more than 200 mW for flow rates above 15 l/min.
In previous research we have demonstrated a micro thermomechanical pyroelectric generator (mu TMPG) as an alternative to thermoelectric generators to harvest ambient heat energy. In such a device, a thermal mass oscillates between a hot and a cold side by virtue of the bistability of its mechanical mount, thus generating a temporal thermal gradient over a pyroelectric material in between. The operational frequency as a major factor deciding the power output of the mu TMPG is in turn dependent on the thermal contact resistance (TCR) present at the mating regions of thermal mass, hot and cold sides. Hence, we have investigated the incorporation of an array of Galinstan droplets at the mating interfaces to reduce the TCR. These arrays are fabricated by selective deposition of Galinstan on a laser-micromachined silicon substrate. After incorporating such an array the operational frequency of the mu TMPG increases by at least 50%.
Abstract Thermoelectric phenomena can be strongly modified in nanomaterials. The determination of the absolute Seebeck coefficient is a major challenge for metrology with respect to micro- and nanostructures due to the fact that the transport properties of the bulk material are no more valid. Here, we demonstrate a method to determine the absolute Seebeck coefficient S of individual metallic nanowires. For highly pure and single crystalline silver nanowires, we show the influence of nanopatterning on S in the temperature range between 16 K and 300 K. At room temperature, a nanowire diameter below 200 nm suppresses S by 50% compared to the bulk material to less than S = 1 μ VK −1 , which is attributed to the reduced electron mean free path. The temperature dependence of the absolute Seebeck coefficient depends on size effects. Thermodiffusion and phonon drag are reduced with respect to the bulk material and the ratio of electron-phonon to phonon-phonon interaction is significantly increased.
In order to study the thermoelectric properties of individual nanowires, a thermoelectric nanowire characterization platform (TNCP) has been previously developed and used in our chair. Here, we report on a redesigned platform aiming to optimize performance, mechanical stability and usability. We compare both platforms for electrical conductivity and the Seebeck coefficient for an individual Ag nanowire of the previously-used batch and for comparable measurement conditions. By this, the measurement performance of both designs can be investigated. As a result, whereas the electrical conductivity is comparable, the Seebeck coefficient shows a 50% deviation with respect to the previous studies. We discuss the possible effects of the platform design on the thermoelectric measurements. One reason for the deviation of the Seebeck coefficient is the design of the platform leading to temperature gradients along the bond pads. We further analyze the effect of bonding materials Au and Pt, as well as the effect of temperature distributions along the bond pads used for the thermovoltage acquisition. Another major reason for the variation of the measurement results is the non-homogeneous temperature distribution along the thermometer. We conclude that for the measurement of small Seebeck coefficients, an isothermal positioning of voltage-probing bond pads, as well as a constant temperature profile at the measurement zone are essential.
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