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Coarsening induced reverse phase transformation of hafnia in polymer-derived Si-Hf-C-N-O ceramics was studied experimentally. X-ray diffraction and high resolution transmission electron microscopy were used to obtain information regarding nanocrystalline structure. Hafnia crystallites precipitated out from the amorphous ceramic matrix on heat treatment were observed to remain as tetragonal hafnia below a critical crystallite radius of 4 nm. Prolonged heat treatment has resulted in coarsening, as explained by Lifshitz-Slyozov-Wagner model, resulting in reverse phase transformation from tetragonal to monoclinic hafnia.S. R. Boddapati-contributing editor Manuscript No. 29543.

Photocure kinetics of acrylated epoxidized soybean oil (AESO) was studied via photocalorimetry without adding any diluent/comonomer, in the presence of two different photoinitiators, namely, 2,2-dimethoxy phenylacetophenone and 1-hydroxycyclohexyl phenyl ketone. The effect of varying photoinitiator concentration, light intensity, and temperature on the extent of crosslinking was calculated from the ratio of experimentally measured reaction enthalpy to the theoretical enthalpy of reaction (ΔHtheoretical). Photocuring of AESO was observed to be a second-order reaction exhibiting autocatalytic behavior. Nevertheless, due to the occurrence of vitrification, incomplete crosslinking (α ≠ 1) was observed in most curing conditions. Rate constants and activation energies were determined using both nonlinear model-fitting and model-free isoconversional methods. Activation energy, as determined from the model-free isoconversional method, was observed to increase as the reaction proceeded, indicating the shift in cure mechanism from kinetic-controlled to diffusion-controlled. Finally, the reaction termination mechanism was observed to be a combination of second-order and primary radical termination mechanisms.

Polyorganoborosilazane ((B[C 2 H 4 -Si(CH 3 )NH] 3 ) n ) was synthesized via monomer route from a single-source precursor and thermolyzed at 1300 ℃ in argon atmosphere. The as-thermolyzed Si-B-C-N ceramic was characterized using X-ray diffraction (XRD) and Raman spectroscopy. The crystallization behavior of silicon carbide in the as-thermolyzed amorphous Si-B-C-N matrix was understood by XRD studies, and the crystallite size calculated using Scherrer equation was found to increase from 2 nm to 8 nm with increase in dwelling time. Concomitantly, Raman spectroscopy was used to characterize the free carbon present in the as-thermolyzed ceramic. The peak positions, intensities and full width at half maximum (FWHM) of D and G bands in the Raman spectra were used to study and understand the structural disorder of the free carbon. The G peak shift towards 1600 cm 1 indicated the decrease in cluster size of the free carbon. The cluster diameter of the free carbon calculated using TK (Tuinstra and Koenl) equation was found to decrease from 6.2 nm to 5.4 nm with increase in dwelling time, indicating increase in structural disorder.