Antoine de Padoue Shyikira

• Shyikira, Antoine de Padoue; Akhtar, Naureen; Skomedal, Gunstein; Sætre, Tor Oskar & Middleton, Peter Hugh (2021). High temperature oxidation of higher manganese silicides. Corrosion Science. ISSN 0010-938X. 185. doi: 10.1016/j.corsci.2021.109327. Fulltekst i vitenarkiv
• Shyikira, Antoine de Padoue; Akhtar, Naureen; Skomedal, Gunstein & Middleton, Peter Hugh (2021). The effect of Mo and Ge reactive elements on high-temperature oxidation of higher manganese silicide. Corrosion Science. ISSN 0010-938X. 194. doi: 10.1016/j.corsci.2021.109920. Fulltekst i vitenarkiv Vis sammendrag

  Higher manganese silicide (HMS) alloys (Mnx-αMoαSiy-βGeβ (x = 0.99–1.011, α = 0.005–0.02, y = 1.75, β = 0.005–0.01)) were studied to elucidate the effect of Mo and Ge pertaining to oxidation. Oxidation experiments were conducted using thermogravimetry and characterized using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Isoconversion experiments below 450 ◦C, shows that doping (up to 2 at%) raises the oxidation potential of HMS. Isothermally, the oxidation rate reduces buy one order of magnitude by doping on Mn and/or Si sites from 0.5 to 2 at%, revealing that the dopants-based oxides do not lessen the robustness of SiO2 oxide.

• Shyikira, Antoine de Padoue; Skomedal, Gunstein & Middleton, Peter Hugh (2020). Performance evaluation and stability of silicide-based thermoelectric modules. Materials Today: Proceedings. 44(4), s. 3467–3474. doi: 10.1016/j.matpr.2020.05.193. Fulltekst i vitenarkiv Vis sammendrag

  Long-term studies on thermoelectric generators based on N-type magnesium silicide (Mg2.01Si0.49Sn0.5Sb0.01) and P-type higher manganese silicide (Mn0.98Mo0.02Si1.73Ge0.02) materials are presented, in the operating temperature range of 200 °C–400 °C. Emphasis is put on the performance and reliability of the current collector configuration, especially on the hot side of the module, and on the thermomechanical stresses that are created during operation and lifetime testing as a result of large temperature gradients experienced across the thermoelectric legs. With silver (Ag) paste as contact material, the long term-stability of the uni-couples was carried out on non-metalized legs and gold metalized legs under ambient conditions. Under isothermal and thermocycling tests, the non-metalized legs showed a gradual decrease in open circuit voltage (after a period of 200 h) and increase in internal resistance. Conversely, the module made of metalized legs was robust and stable for the same isothermal period. However, after 300cycles the n-type material showed mechanical failure (cracks) but the p-type resisted. Post-operation analysis by SEM/EDS and mechanical testing revealed that oxidation, adherence of the contact material and diffusion of the bonding material were the cause of performance degradation of the unicouples.

• Nkurikiyimfura, Innocent & Shyikira, Antoine de Padoue (2015). Photothermal characteristics of magnetic nanofluids for solar thermal applications. International Journal of Engineering Research & Technology. ISSN 2278-0181. 4(1). Fulltekst i vitenarkiv Vis sammendrag

  Photothermal characteristics of nanofluids are of special importance in the design and performance analysis of solar thermal collectors with nanofluids as heat transfer media. The present work investigated the photothermal characteristics of the magnetite (Fe3O4) based nanofluids in zero and applied magnetic fields. Stable kerosene and water based magnetite nanofluids were prepared via a coprecipitation method and their photothermal characteristics were measured under direct sun. Effects of magnetite particle volume fraction and magnetic field on the temperature enhancement as function of irradiation time were analyzed. The results showed that the photothermal characteristics could be enhanced with magnetite nanoparticles used in nanofluids at lower particle volume fraction. In addition the, the photothermal characteristics of magnetic nanofluids (MNFs) were found to be related to magnetic field.

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• Shyikira, Antoine de Padoue; Skomedal, Gunstein & Middleton, Peter Hugh (2019). Performance and stability of silicide based thermoelectric modules. Fulltekst i vitenarkiv Vis sammendrag

  Long-term studies on thermoelectric generator uni-couple based on N-type magnesium silicide (Mg2.01Si0.49Sn0.5Sb0.01) and P-type higher manganese silicide (Mn0.98Mo0.02Si1.73Ge0.02) materials are presented, in the operating temperature range of 2000C-4000C. Emphasis is put on the performance and reliability of the current collector configuration, especially, on the hot side of the module and on the thermomechanical stresses that are created during operation and lifetime testing, as a result of large temperature gradients experienced across the thermoelectric legs. With silver (Ag) paste as contact material, the long term-stability of the uni-couples were carried out on non-metalized legs and gold metalized legs under ambient conditions. Under isothermal and thermocycling tests, the non-metalized legs showed a gradual decrease in open circuit voltage (after a period of 200hours) and increase in internal resistance. In contrary, the metalized leg-based module was robust and stable for the same isothermal period, however, after 300cycles the n-type material showed mechanical failure (cracks) but the p-type resisted. Post-operation analysis by SEM/EDS and mechanical testing revealed that oxidation, adherence of the contact material and diffusion of the bonding material were the cause of performance degradation of the unicouples.

• Shyikira, Antoine de Padoue; Skomedal, Gunstein; Middleton, Hugh P. & Sætre, Tor Oskar (2018). High Temperature Oxidation of Higher Manganese silicide and Alloys. Fulltekst i vitenarkiv
• Shyikira, Antoine de Padoue (2023). Long term stability of silicide based thermoelectric materials and modules. Universitetet i Agder. ISSN 9788284271279. Fulltekst i vitenarkiv Vis sammendrag

  Silicide-based thermoelectric generators are potential candidates for waste heat recovery at temperatures below 500  C. For the last two decades, the conversion efficiency of modules based on n-type magnesium silicides and p-type higher manganese silicide has improved significantly. However, the conditions in which thermoelectric generators operate (for example, remote areas in the oil, gas, and telecommunication industries, in automobiles, etc.) are harsh (corrosive, for example) and hostile (due to thermal instability). In this project, there was much focus on the stability of the thermoelectric modules, with special interest given to oxidation of the thermoelectric materials and module stability. The thermal oxidation studies were conducted on higher manganese silicide alloys; the studies mainly investigated the effect of the alloys’ composition, consolidation techniques and the operational atmosphere’s effect on their oxidation potential. Moreover, the choice of matching electrodes and good bonding technology for the module assembly was the ultimate step before finally testing the actual performance and stability of the module over an extended period. The thorough oxidation studies conducted in this thesis revealed the importance of different production processes for the higher manganese silicide thermoelectric materials on the oxidation robustness of the alloys. The study showed that the purity (fewer impurities) of the raw elements and optimal doping level are among the key factors for the alloys to resist oxidation by growing a protective SiO2 protective oxide layer. Moreover, it was also shown that powder consolidation by spark plasma sintering produced stronger bulk pellets, and mechanical strength played a key role in passive oxidation. During the module’s contacts design, silver electrodes and solid liquid interdiffusion bonding technology were used. The contact resistance of the assembled modules were measured using an automated point contact measurement test rig. On the magnesium silicide the specific contact resistance was on average 0.17 m cm2 with 2.1% standard deviation. The higher manganese silicide’s contact interface, on the other hand, the results were dispersed along the bond, where 0.07 m cm2 was the lowest value and 1.12 m cm2 the highest (81.3% standard deviation). Finally, the module stability was investigated by testing the performance of the assembled modules. The tested modules produced up to 7.4mW/cm2 power density at 400  C and sustained more than 300 thermal cycles. The gradual degradation was found to mainly originate from the mechanical failure of the contact interfaces and oxidation of the n-type magnesium silicide relative to the p-type material. Applying a high-temperature coating did not reduce the degradation rate, which showed that it would be better to encapsulate the modules to count-act the effect of oxidation.

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