Fabrication of Nanostructured Materials for Energy Applications

Detta är en avhandling från Stockholm : KTH

Författare: Shanghua Li; Kth.; [2008]

Nyckelord: TEKNIKVETENSKAP; TECHNOLOGY; Materials Chemistry;

Sammanfattning: World energy crisis has triggered more attention to energy saving and energy conversion systems with high efficiency. There is a growing awareness that nanoscience and nanotechnology can have a profound impact on energy generation, conversion, and recovery. Nanotechnology-based solutions are being developed for a wide range of energy problems such as, solar electricity, hydrogen generation and storage, batteries, fuel cells, heat pumps and thermoelectrics. This thesis deals with the design and fabrication of novel functional materials/architectures for energy-related applications. The study includes two parts: Nanostructured thermoelectric (TE) materials for energy conversion and nanostructured metallic surfaces for energy heat transfer.In the first part, the focus is given to the fabrication of novel nanostructured TE materials and architectures. TE materials are very important functional materials that can convert heat to electrical energy and vice versa. Recently, nanostructuring TE materials showed very promising potential to improve their TE figure of merit which opens a new venue for the TE world. As a result, some advanced nanostructured TE architectures are proposed as the state-of-the-art TE materials/structures. Among these advanced TE architectures, bismuth telluride nanowires/thick films and skutterudite nanocomposites with nanoinclusions have been successfully fabricated and some of their advantageous TE performance has been demonstrated. For example, an improvement of 11% on the figure of merit, ZT, was achieved in the CoSb3 nanocomposite with 5 mole% ZrO2 as nanoinclusion. Comprehensive physico-chemical characterization techniques have been used for the synthesized TE materials. The potential-Seebeck microprobe, 4-point probe and laser flash apparatus have been used for the measurement of TE parameters on the TE materials.In the second part of the thesis, we developed a nanostructured macro-porous (NMp) surface for enhancing heat transfer in boiling process. Enhanced surfaces for boiling improve the energy efficiency of heat pumping equipment such as air conditioners, refrigerators, etc. Conventional techniques currently used for fabricating enhanced surfaces are often based on the use of complicated mechanical machine tools and require a large consumption of materials and give only limited enhancement of the boiling heat transfer. In this thesis, we present a new approach to fabricate enhanced surfaces by using electrodeposition under specific conditions forming in-situ dynamic gas bubble templates. As a result, the NMp metallic surface layer comprising of dendritically ordered copper branches is obtained. Since the structure is formed during the evolution of the dynamic bubbles, it is ideal for the bubble generation applications such as boiling. The efficiency of the NMp surfaces for boiling heat transfer was evaluated in pool boiling experiments. At the heat flux of 1 W/cm2, the heat transfer coefficient for the NMp surface is found to be more than 17 times higher than the reference surface. It's estimated that such an effective boiling surface would improve the energy efficiency of many heat pumping machines with 10 - 30 %. The extraordinary enhancement of boiling performance is explained by the structure characteristics, which assist in enhancing nucleation of the gas bubbles, subsequent coalescence, and facilitated departure from the surfaces.

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