Note: High resolution alternating current/direct current Harman technique

This note describes the construction and engineering of a high precision Harman set-up for metrology of the thermoelectric figure of merit (ZT) of modules and materials based on steady state AC and DC measurements. The Harman technique presented in this article has a resolution of milli-ZT and it does not employ lock-in amplifiers or AC bridges; rather, the technique is developed to avoid typical complications experienced in AC Harman systems. By one-time reference measurements the best operation point for the system is chosen, minimizing the effects of capacitive loads due to AC signals.

A new thermoelectric concept using large area PN junctions

A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines thermally induced charge generation and separation in a space charge region with the conventional Seebeck effect by applying a temperature gradient parallel to the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. The investigated PN junction devices are produced by stacking n-type and p-type nanopowder prior to a densification process. The nanoparticulate nature of the densified PN junction lowers thermal conductivity and increases the intraband traps density which we propose is beneficial for transport across the PN junction thus enhancing the thermoelectric properties. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator.

Influence of the annealing atmosphere on solution based zinc oxide thin film transistors

Zinc oxide layers with a thickness of less than 10 nanometers have been synthesized from an aqueous solution for the application as active layer in thin film transistors. They have been conditioned by applying different oxidizing and reducing atmospheres during an annealing process at a temperature of 125°C. It is shown that the charge carrier mobility and threshold voltage is strongly influenced by the annealing atmosphere. Samples annealed in 10% forming gas (H2 in N2 - reducing atmosphere) show the highest field-effect-mobility of 0.6 cm2V-1s-1, but no saturation of the drain current, due to a high free carrier concentration. Samples treated under oxygen (strongest oxidizing atmosphere) show significantly lower mobilities. Subsequently, the samples have been exposed to synthetic air, with varying exposure times. Samples which have been annealed under hydrogen atmospheres show a pronounced decay of the drain current if exposed to synthetic air, whereas all samples conditioned under hydrogen-free atmospheres are significantly more stable under synthetic air. This enhanced sensitivity against oxygen after hydrogen treatment is attributed to residual hydrogen content in the sample that supports the formation of OH-groups which act as electron acceptors.

From nanoparticles to nanocrystalline bulk: percolation effects in field assisted sintering of silicon nanoparticles

Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom-up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocrystallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet. In this combined experimental and theoretical work, we show how the developing microstructure during sintering correlates with the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks.