P.S.R. Prasad

Direct formation of the γ-CaSO4 phase in dehydration process of gypsum: In situ FTIR study

Abstract The dehydration mechanism of natural single crystals of gypsum was investigated in the temperature range 300-430 K by in situ infrared (FTIR) spectroscopy. The thermal evolution of the second-order modes of H2O and SO4 groups in gypsum, in the wavenumber range 4850.5450 cm-1 and 2050.2300 cm-1 respectively, were used to probe the dehydration and rehydration sequence. A total disappearance of the combination modes of H2O and the replacement of four SO-24 bands (2245, 2200, 2133, and 2117 cm-1) observed at room temperature by three bands (2236, 2163, and 2131 cm-1) observed at 390 K indicates the direct formation of γ-CaSO4 upon heating. Upon cooling water re-enters into the γ-CaSO4 structure at around 363 K to form bassanite. This observation, that the dehydration of gypsum directly yields γ-CaSO4 (anhydrite) without the intermediate formation of hemi-hydrate (bassanite), is further corroborated by the dehydration behavior of bassanite. The second-order SO4 modes of bassanite observed around 2218, 2136, and 2096 cm-1 were replaced with the bands of γ-CaSO4 at about 378 K upon heating.

Dehydration of natural stilbite: An in situ FTIR study

Abstract The dehydration behavior of a natural stilbite sample from Poona (India) was investigated by in situ FTIR. The thermal induced variations of the water molecule bending (ν2) mode around 1653 cm-1, the stretching (ν3 and ν1) modes around 3587 and 3426 cm-1, and the corresponding second-order modes in the wavenumber region 4000.8000 cm-1 were followed as indicative of the dehydration process. The observed spectral variations indicate that stilbite undergoes a transformation at about 448 K due to the loss of half of the original content of water molecules. The rehydration of stilbite is partial in samples heated up to 630 K. Concerning both the dehydration and rehydration behaviors of stilbite, our results are in concert with those proposed in the literature. In addition, the growth of a new mode around 4550 cm-1 is observed in the temperature range 430-650 K and may indicate the presence of hydroxyl groups created by the breaking of the T-O-T linkages.

Inscription and characterization of micro-structures in silicate, Foturan and tellurite glasses by femtosecond laser direct writing

We present some of our results on the femtosecond laser direct writing and characterization of micro-structures in silicate, FoturanTM, and tellurite glasses. Structures with different sizes were fabricated with varying input energy and spatially modified pulse using a slit. Various characterization techniques including fluorescence spectroscopy, micro- Raman spectroscopy, and laser confocal microscopy were employed to analyze the structural and physical modifications at focal volume resulting in the change of refractive index (RI). The RI change due to material modification was estimated using diffraction from a continuous wave laser beam and is presented in this work. The results obtained are analyzed vis-a-vis the recent work in similar glasses and the applications of such structures in the fields of photonics.

In-situ FTIR study of dehydration of natural Fe-stilbite

Thermally induced dehydration behavior of a naturally occurring iron bearing stilbite around Poona region, India has been studied by comparing with the one depleted in iron. In-situ FTIR spectroscopy in the wavenumber range 4000–10,000 cm −1 has been used to monitor the dehydration behavior in the temperature range 300–850 K. The thermal induced variations in the integrated peak areas of water modes indicate two step dehydration behaviors. During dehydration (in phase B) process growth of Si-OH bonds (4550 cm −1 ) formed by virtue of the breakage of T-O-T linkages is observed. A peak around 8650 cm −1 indicates the presence of iron ion in stilbites as Fe 2+ . Further, abrupt disappearance of this mode indicates Fe 2+ ion in extra framework sites. The presence of iron in stilbites increased the formation of Si-OH bonds in phase B and grossly did not effect the dehydration behavior of stilbites.