Gabi Schierning

Plasma synthesis of nanostructures for improved thermoelectric properties

The utilization of silicon-based materials for thermoelectrics is studied with respect to the synthesis and processing of doped silicon nanoparticles from gas phase plasma synthesis. It is found that plasma synthesis enables the formation of spherical, highly crystalline and soft-agglomerated materials. We discuss the requirements for the formation of dense sintered bodies, while keeping the crystallite size small. Small particles a few tens of nanometres and below that are easily achievable from plasma synthesis, and a weak surface oxidation, both lead to a pronounced sinter activity about 350 K below the temperature usually needed for the successful densification of silicon. The thermoelectric properties of our sintered materials are comparable to the best results found for nanocrystalline silicon prepared by methods other than plasma synthesis.

Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites

Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95% and 96% compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribu...

Microcrystalline silicon formation by silicon nanoparticles

Thin silicon films are of great importance for large-area electronic applications, for example, as the basis for switching electronics in flat-panel display devices or as the active layer of solar cells. In this paper, we show that silicon nanoparticles have the potential to be used as raw material for further processing toward a microcrystalline silicon film. This can be done by thermal treatment with a reduced thermal budget because the melting point of the nanoparticles is much lower with only 60% of the equilibrium melting temperature of silicon. Coagulation processes of liquid droplets then lead to the growth of microcrystalline silicon in agglomerated nanoparticles. We demonstrate by in situ transmission electron microscopy (TEM) and differential thermal analysis that silicon nanoparticles with a size of approximately 20nm start melting at around 1000K; furthermore, the TEM observations directly demonstrate the details of the coagulation process leading to microcrystalline silicon.

Concepts for medium-high to high temperature thermoelectric heat-to-electricity conversion: a review of selected materials and basic considerations of module design

Within the last decade, novel materials concepts and nanotechnology have resulted in a great increase of the conversion efficiency of thermoelectric materials. Despite this, a mass market for thermoelectric heat-to-electricity conversion is yet to be opened up. One reason for this is that the transfer of the lab records into fabrication techniques which enable thermoelectric generator modules is very challenging. By closing the gap between record lab values and modules, broad industrial applications may become feasible. In this review, we compare three classes of materials, all designed for medium-high to high temperature applications in the field of waste heat recovery: skutterudites, half-Heusler compounds, and silicon-based materials. Common to all three classes of thermoelectric materials is that they are built from elements which are neither scarce (e.g. tellurium) nor toxic (e.g. lead) and therefore may be the foundation of a sustainable technology. Further, these materials can provide both, n-type and p-type materials with similar performance and thermomechanical properties, such that the fabrication of thermoelectric generator modules has already been successfully demonstrated. The fabrication processes of the presented materials are scalable or have already been scaled up. The availability of thermoelectric materials is only one important aspect for the development of thermoelectric generator modules and heat conversion systems based on this technology. The design and configuration of the thermoelectric generator modules is similarly important. Hence, basic considerations of module configuration and different fundamental layouts of the thermoelectric heat-to-electricity conversion system are discussed within an additional chapter of this review.

Formation of metallic indium-tin phase from indium-tin-oxide nanoparticles under reducing conditions and its influence on the electrical properties

The correlation between defect structure, metal segregation, and electrical resistivity of indium-tin-oxide nanopowder upon treatment in reducing atmosphere was investigated. Morphology and defect structure have been investigated by in situ synchrotron x-ray diffraction and transmission electron microscopy, while traces of metallic indium have been detected by susceptibility measurements utilizing the superconducting properties of indium. With increasing treatment temperature under reforming gas the film resistivity decreases down to ρ=1.6×10−2 Ω cm at 330 °C annealing temperature. For even higher treatment temperatures, the resistivity increases further. This is accompanied by extractions of metallic indium. Under forming gas, grain growth could be observed at 350 °C, while in air grain growth starts at 650 °C. Furthermore forming gas causes a lattice expansion of ITO which persists in oxygen, at least for several hours. The results are discussed with respect to results published in the literature.

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.

Thermoelectric properties of pulsed current sintered nanocrystalline Al-doped ZnO by chemical vapour synthesis

ZnO is a promising n-type oxide thermoelectric material, which is stable in air at elevated temperatures. In the present study, we report the bottom-up approach to create Al-doped ZnO nanocomposites from nanopowders, which are prepared by chemical vapour synthesis. With our synthesis route, we are able to create highly doped Al-containing ZnO nanocomposites that exhibit bulk-like electrical conductivity. Moreover, the impact of the microstructure of the nanocomposites on their thermal conductivity is enormous, with a value of 1.0 W m−1 K−1 for 1% Al–ZnO at room temperature, which is one of the lowest values reported, to date, on ZnO nanocomposites. The optimization of the Al-doping and microstructure with respect to the transport properties of bulk Al–ZnO nanocomposites leads to a zT value of about 0.24 at 950 K, underlining the potential of our technique.

Laser-sintered thin films of doped SiGe nanoparticles

We present a study of the morphology and the thermoelectric properties of short-pulse laser-sintered (LS) nanoparticle (NP) thin films, consisting of SiGe alloy NPs or composites of Si and Ge NPs. Laser-sintering of spin-coated NP films in vacuum results in a macroporous percolating network with a typical thickness of 300 nm. The Seebeck coefficient is independent of the sintering process and typical for degenerate doping. The electrical conductivity of LS films rises with increasing temperature, best described by a power-law and influenced by two-dimensional percolation effects.

The effect of Peltier heat during current activated densification

It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample.It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample.

Effects of impurities on the lattice dynamics of nanocrystalline silicon for thermoelectric application

Doped silicon nanoparticles were exposed to air and sintered to form nanocrystalline silicon. The composition, microstructure, and structural defects were investigated with TEM, XRD, and PDF and the lattice dynamics was evaluated with measurements of the heat capacity, of the elastic constants with resonant ultrasound spectroscopy and of the density of phonon states (DPS) with inelastic neutron scattering. The results were combined and reveal that the samples contain a large amount of silicon dioxide and exhibit properties that deviate from bulk silicon. Both in the reduced DPS and in the heat capacity a Boson peak at low energies, characteristic of amorphous SiO2, is observed. The thermal conductivity is strongly reduced due to nanostructuration and the incorporation of impurities.