swapan das - Academia.edu (original) (raw)
Uploads
Papers by swapan das
Journal of Asian Earth Sciences, 2013
ABSTRACT Theories related to the precipitation mechanism of the metallic elements in marine manga... more ABSTRACT Theories related to the precipitation mechanism of the metallic elements in marine manganese nodules have remained controversial between two schools of thoughts (1) chemical oxidation (abiotic origin) and (2) deposition of the metals through microbial enzymatic processes (biogenic origin). One of the most important evidence in support of the biogenic origin is the occurrence of fossilized microbes. However, well-documented literature in this regard is either lacking or very scanty in case of Indian Ocean nodules. Using high resolution FEG-SEM we have recorded various biogenic signatures and ultra microfossils in the ferromanganese nodule samples from Central Indian Ocean Basin (CIOB) that are presented in this paper. The microfossils are mostly protozoans belonging to varieties of bacteria, diatoms and foraminifera. Some of the features recorded in this study have perhaps never been reported before from any manganese nodules. The chemical compositions of these ultra microfossils indicate a high-level of manganese precipitation in and around them in comparison to the distant surrounding areas. While clumpy microbes are enriched with nickel, the rod shaped bacteria are rich in copper. Up to 4.70 wt.% nickel and 5.31 wt.% Cu have been recorded in the fossilized microbe bodies. The high abundance of biogenic features as well as microfossils in the ferromanganese nodules and their chemical compositions support arguments in favor of a dominant role of the microorganisms in the construction of the nodules of the CIOB.
Engineering Failure Analysis, 2002
The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed.... more The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed. One tube fractured with a fish mouth opening and another thinned down considerably. A significant amount of oxide layer was observed in the inner side of the failed tube. In the other tube the inner oxide layer was very small and the tube was thinned down due to fireside corrosion. The unexposed tube from the same lot revealed a ferritic bainitic microstructure. The failed tube also showed a ferrite bainite microstructure but most of the bainitic carbide had transformed to globular form. Considerable carbide precipitation both in grain boundary (GB) and inside grains were observed. A large number of cavities and micro-cracks were observed along the grain boundaries. A similar microstructure was observed on the same tube 10 cm away from the failed region. The micrograph at the fracture location showed a similar microstructure but the grains are very much elongated. The formation of globular oxide on the other tube was compared to that of the failed tube. Due to breakage of oxide scale of the failed tube, the metal surface of the failed tube was exposed to steam at a higher temperature and this accelerated the oxide growth. This led to creep damage of the material. Ultimately, failure occurred due to rapid overheating of the tube material. #
Journal of Asian Earth Sciences, 2013
ABSTRACT Theories related to the precipitation mechanism of the metallic elements in marine manga... more ABSTRACT Theories related to the precipitation mechanism of the metallic elements in marine manganese nodules have remained controversial between two schools of thoughts (1) chemical oxidation (abiotic origin) and (2) deposition of the metals through microbial enzymatic processes (biogenic origin). One of the most important evidence in support of the biogenic origin is the occurrence of fossilized microbes. However, well-documented literature in this regard is either lacking or very scanty in case of Indian Ocean nodules. Using high resolution FEG-SEM we have recorded various biogenic signatures and ultra microfossils in the ferromanganese nodule samples from Central Indian Ocean Basin (CIOB) that are presented in this paper. The microfossils are mostly protozoans belonging to varieties of bacteria, diatoms and foraminifera. Some of the features recorded in this study have perhaps never been reported before from any manganese nodules. The chemical compositions of these ultra microfossils indicate a high-level of manganese precipitation in and around them in comparison to the distant surrounding areas. While clumpy microbes are enriched with nickel, the rod shaped bacteria are rich in copper. Up to 4.70 wt.% nickel and 5.31 wt.% Cu have been recorded in the fossilized microbe bodies. The high abundance of biogenic features as well as microfossils in the ferromanganese nodules and their chemical compositions support arguments in favor of a dominant role of the microorganisms in the construction of the nodules of the CIOB.
Engineering Failure Analysis, 2002
The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed.... more The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed. One tube fractured with a fish mouth opening and another thinned down considerably. A significant amount of oxide layer was observed in the inner side of the failed tube. In the other tube the inner oxide layer was very small and the tube was thinned down due to fireside corrosion. The unexposed tube from the same lot revealed a ferritic bainitic microstructure. The failed tube also showed a ferrite bainite microstructure but most of the bainitic carbide had transformed to globular form. Considerable carbide precipitation both in grain boundary (GB) and inside grains were observed. A large number of cavities and micro-cracks were observed along the grain boundaries. A similar microstructure was observed on the same tube 10 cm away from the failed region. The micrograph at the fracture location showed a similar microstructure but the grains are very much elongated. The formation of globular oxide on the other tube was compared to that of the failed tube. Due to breakage of oxide scale of the failed tube, the metal surface of the failed tube was exposed to steam at a higher temperature and this accelerated the oxide growth. This led to creep damage of the material. Ultimately, failure occurred due to rapid overheating of the tube material. #