Ravi Kumar N V

Porous ceria for high temperature catalytic applications demands structural integrity concomitant with sinter resistance and improved gas permeability. The current state of the art hinges on complex synthesis methodologies which are not only expensive but also lack flexibility in pore tailorability. Hence, the development of porous scaffolds through low-cost processes without compromising on the functionality is in order. Herein, we demonstrate porous ceria with an open porosity of 88% developed through camphene assisted freeze casting for the first time. Microstructural evolution with different building blocks–micrometre-sized particles and short fibres were also studied. Preliminary catalytic activity obtained via temperature programmed reduction exemplified similar profiles showing no effect of the initial building blocks on the activity.

In this work, heterojunctioned TiO2/ZrO2 – zirconium titanate phases have been designed through a precursor derived approach. The X-ray powder diffractograms (XRD)revealed that the synthesized powder contained phase pure rutile, a mixture of rutile/zirconia and zirconium titanate or monoclinic zirconia (m-ZrO2)as we vary the amount of initial precursors. The scanning electron micrographs (SEM)and transmission electron micrographs (TEM)showed a decrease in crystallite size with increase in the amount of Zr to Ti precursor and formation of heterojunctions, respectively. An anomalous reduction in band gap was observed for 70T-30Z (2.54 eV)and 60T-40Z (2.71 eV)samples when compared to their parent oxides, TiO2 (2.86 eV)and ZrO2 (4.5 eV)and this could be attributed to the formation of heterojunctions between the two phases.

In situ age hardening phenomenon observed in as-sprayed Al-Sc binary alloys is reported. This, along with substantial grain refinement in the as-sprayed alloys exhibited hardness in the range of 605 MPa (Al-0.3 wt% Sc) to 923 MPa (Al-0.8 wt% Sc) that are comparable to the optimum hardness values typically obtained by age hardening in similar as-cast alloys. Transmission electron microscopy of the spray deposits revealed densely spaced Al3Sc precipitates of about 10 nm suggesting in-situ age hardening of these alloys during spray forming. This behavior is attributed to the inherent heating cycle associated with the spray forming process and the rapid kinetics of Al3Sc precipitation.

Friction stir welding (FSW) is a promising technique to join HSLA steels without the problems encountered during fusion based welding processes. In the present work, 3 mm thick HSLA plates were successfully welded using FSW. A tool made of tungsten-rhenium alloy was used in this work. The relationship between microstructure and tensile strength was studied under various welding conditions i.e. change in traverse speed (57–97 mm/min). The microstructure of the weld nugget revealed the presence of upper bainite and fine ferrite phases. The amount of upper bainite reduced with increase in traverse speed. EBSD images showed a reducing trend for grain size. The details of hardness, tensile strength and bending test were reported.

Polymer derived Si-Hf-C-N(O) foams were produced by the pyrolysis of polysilazane containing varying amounts of hafnium tetra n-butoxide. X-ray tomography was used to determine the cell size distribution. The ablation resistance of the foams was studied by subjecting it to different oxyacetylene flame temperatures. After ablation, fragmentation was not observed and the mass ablation rate was found to be minimal for the foams produced from highest vol% of hafnium tetra n-butoxide. The foams remained X-ray amorphous after ablation and the presence of [Formula presented] and [Formula presented] bonds were confirmed using ATR spectroscopy. For comparison, a reasonably dense spark plasma sintered pellet was also ablated and the evolution of monoclinic hafnia was confirmed using X-ray diffraction. The microstructural characterization exhibited three different zones on the ablated surface for sintered sample and the ablation mechanisms were understood using thermodynamic calculations.

The Al-4.5 wt.% Cu powder was compacted by spark plasma sintering at three compaction pressures namely 30, 40, and 50 MPa maintaining temperature constant at 500°C. Porosity seems to be closed in all the 3 cases. Relative density of S-50 is decreased due to increase in electrical erosion holes. Transmission electron microscopy studies reveal the presence of dislocations in all the compacts and shear bands observed only in the compact sintered at 50 MPa. A power law creep mechanism involving dislocations is found to be observed in all the compacts sintered at all pressures, which is evident from the TEM micrographs as well. The dissolution of Al2Cu precipitates in the aluminum matrix, and the increase in electric erosion holes were noticed with increase in pressure from 30 to 50 MPa. The decrease in the volume fraction of Al2Cu phase and the increase in the formation of electric erosion holes resulted in reduced hardness and compression strengths of the compacts.