Fabrication of silicon-germanium thermoelectric materials for terrestrial general purpose applications

Si-Ge thermoelectric materials (TEM) have been used for special purpose applications particularly in radioisotope thermoelectric generators (RTG) for a few decades. Despite many of their desirable properties they have not been used as widely for day to day direct energy conversion purposes. Considering the recently developed technologies in materials science and TEM processing, this study investigated the possibility of making and using Si-Ge based TEM for direct heat to electricity power conversion processes in general purpose applications. Firstly the benefit of using a thermoelectric generator (TEG) based on Si-Ge TEM was analytically assessed in the context of a solar thermal power system. The aim was to identify one of the several possible non-conventional applications of a Si-Ge TEG where the cost of its utilisation would be the major constraint. Appropriate analysis may reveal similar type of benefit in other applications or other ways of utilisation. Using parameters obtained from the state of the art technique it was seen that a substantial improvement in total system efficiency could be achieved thereby reducing the cost of generating the electricity. The second phase of the work was focussed on the practical aspects of realising the above potential. Various ways of making Si-Ge materials were reviewed. Considering all potential preparation methods a method based on room temperature alloying and pressure assisted sintering process was selected for further experimental investigation. The selection made was based on the possibility of easier automation of the processes involved so that a cost effective mass production would be possible for an wide scale utilisation. Experimental studies were performed to identify the actual preparation procedures. Mechanical alloying method using special controlled milling technique was used to produce homogeneous solid solution alloys of very small grain sizes. A pressing equipment was designed to press the powders in vacuum. A pressure assisted sintering furnace was designed and built to sinter the pellet at temperatures up to 1300 deg. C while maintaining a pressure of about 100 kg/cm2. Using the pressure assisted sintering furnace green-pressed samples could be compacted to a high density pellet. SEM/EDXS, X-ray and transmission electron microscopy techniques were used to characterise the materials at the various stages during processing. kamal Aryal 4 In the third stage, the prepared samples were assessed regarding their physical and electrical parameters. The density measured was greater than 90% of the theoretical. The pellets constituted homogeneous solution of Si and Ge. A special electrical resistivity measuring apparatus (ERMA) was designed and built for an accurate measurement of electrical resistivity from room temperature to about 400 °C. The ERMA was designed with a travelling probe mechanism such that it could be used to measure the variation in electrical resistivity within a sample to identify anisotropy in the sample, if any. The mechanism can also be used to measure the contact resistivity in case of a pellet with an appropriate electrical contact. Hall effect techniques were also used to measure the electrical resistivity, the charge carrier concentration and the mobility of the charge carriers. The proposed method and the developed technique could result in highly doped, homogeneous and fine-grained solid solution alloys of high density as required. Electrical resistivities are, however, relatively high showing that additional improvements are necessary in this process of preparing low cost Si-Ge alloys for terrestrial applications. Further experiments showed that the oxygen in the powdered sample is relatively high which could have degraded the electrical conductivity. Consequently this work suggests that further investigations are necessary to determine the exact procedure to reduce oxygen during powder processing to an acceptable level. Also several experiments were performed to make an electrical contact to the SiGe alloy pellets. The attempts were partially successful as a good mechanical contact of Mo powder and Mo foil could be made using the solid state diffusion bonding process. The electrical contact resistance, however, is not low. One of the possible causes of this may be the high level of oxygen observed in the sample. So in parallel with the above, the work on contacts could be continued to prepare an appropriate electrical contact to the TEM developed.