Riya Chakraborty

Regeneration of complex Bragg gratings

Complex regenerated Bragg gratings, seeded by complex type-I gratings in H2 loaded germanosilicate optical fibre is reported. By this means, dual channel grating filters which are stable beyond 1000°C are produced. These high temperature stable co-located dual gratings have potential application in sensing and multi-wavelength high power lasers.

Thermal preparation of highly stable glass periodic changes with nano-scale resolution using a laser-inscribed hydrogen loaded seed template

Strong regenerated gratings with a maximum grating strength exceeding (40-50) dB are fabricated inside an optical fibre by bulk macro thermal processing ~900degC using a UV-laser seeded Bragg grating. Further annealing between 1000 and 1100degC leads to a stabilised grating ~18 dB in strength. This suffers no further degradation at 1100degC for the period monitored, over 4 hrs. To determine the potential resolution of this process, two regenerated complex profiles with no phase difference between seed and processed structures is reported. This opens the way for nano-engineering of materials using thermal processing and seed templates.

Improvement in hardness of soda-lime-silica glass

Hardness is a key design parameter for structural application of brittle solids like glass. Here we report for the first time the significant improvement of about 10% in Vickers hardness of a soda-lime-silica glass with loading rate in the range of 0.1-10 N.s−1. Corroborative dark field optical and scanning electron microscopy provided clue to this improvement through evidence of variations in spatial density of shear deformation band formation as a function of loading rate.

Comparative Study of Indentation Size Effects in As-Sintered Alumina and Alumina Shock Deformed at 6.5 and 12 GPa

Nanohardness of alumina ceramics determines its performance in all contact-related applications because the issue of structural integrity gets determined at the nanoscale of contact. In spite of the wealth of the literature, however, it is not yet known in significant details how the high-strain rate flyer-plate impact at different pressure affects the nanohardness of dense, coarse grain alumina ceramics. Thus, the load controlled nanoindentation experiments were performed with a Berkovich indenter on an as-received coarse grain (~10 μm), high density (~3.98 gm·cc−1) alumina, and shock recovered tiny fragments of the same alumina obtained from gas gun experiments conducted at 6.5 GPa and 12 GPa shock pressures with stainless steel flyer plates. The nanohardness of the as-received alumina was much higher than that of the 6.5 GPa and 12 GPa shock-recovered alumina. The indentation size effect (ISE) was the strongest in alumina shocked at 12 GPa and strong in alumina shocked at 6.5 GPa, but it was mild in the as-received alumina sample. These results were rationalized by analysis of the experimental load depth data and evidences obtained from field emission scanning electron microscopy. In addition, a rational picture of the nanoindentation responses of the as-received and shocked alumina ceramics was provided by a qualitative model.

A New Mechanism for ISE in Alumina Shock Deformed at 6.5 GPa

With a view to understand how high strain rate flyer plate impact affects the nanohardness of a coarse (10 μm) grain high density (3.978 gm.cc−1) alumina, load controlled nanoindentation experiments were conducted with a Berkovich indenter on as received alumina (ARA) and shocked alumina fragments (SA) of obtained from a flyer plate shock impact study at 6.5 GPa. The results showed that the nanohardness of the shocked alumina (SA) samples was much lower than that of the as received alumina (ARA) samples and that the indentation size effect (ISE) was mild in the ARA but quite severe in the SA samples. Extensive additional characterization by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and a physical model based analysis of the experimental load depth data were utilized to provide a new explanation for the presence of strong indentation size effect in the shock recovered alumina.

Shock deformation of coarse grain alumina above HEL

Asymmetric shock recovery experiments with a two stage gas gun were conducted on a 10 μm grain size alumina at 6.5 and 12 GPa shock pressures levels which were more than three to six times as high as the Hugoniot Elastic Limit (HEL) of the same alumina and the shock recovered alumina fragments were characterized by X-ray diffraction (XRD), nanoindentation, scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Based on these results a new qualitative damage model was developed to explain the deformation mechanisms of shock loaded alumina.

Damage Evolution Mechanisms in Plate Impact of Alumina

Asymmetric shock recovery experiments with a two stage gas gun were conducted on a 10 μm grain size alumina at 6.5 GPa shock pressures levels which was more than three times as high as the Hugoniot Elastic Limit (HEL) of the same alumina and the shock recovered alumina fragments were characterized by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Based on these results a new qualitative damage model was developed to explain the deformation mechanisms of shock loaded alumina.