High pressure annealing of CVD diamond films

5 p. : il.CVD diamond films were annealed from 600 to 1900 8C at 7.7 GPa in a toroidal high pressure (HP) apparatus, always inside the diamond-phase stability region. The annealed films were analyzed by Raman and infrared (IR) spectroscopy and the results showed that the diamond grains remained stable while the non-diamond carbon phases and impurities, responsible for the intricate film structure, changed after processing. For the HP annealing from 600 to 1300 8C, there were no major changes in the Raman spectra of the film, however, the film became easily broken and the IR spectra indicated a high reactivity of carbon with chemical elements from the environment. After annealing at 1500 8C and 7.7 GPa, the formation of diamond-like (DLC) and graphitic structures in-between the diamond grains were observed, while the reaction with the environment elements decreased. For higher temperatures, the DLC and graphitic structures persisted up to 1700 8C and the film incorporated OH in large amounts. The results showed that the non-diamond carbon species are susceptible to the HP annealing, and structural modifications in between the diamond grains are significant for temperatures above 1300 8C at 7.7 GPa

High-pressure Raman and infrared spectroscopy of polyacetylene

5 p. : il.A systematic study of the high-pressure (HP) effect on the Raman and infrared (IR) spectra of the cis and trans isomers of polyacetylene (PA) is presented. The experiments were performed in a diamond anvil cell (DAC) up to 9.6 GPa. The pressure-dependent Raman resonance effect observed for cis-PA was used to estimate the pressure dependence of the energy gap (Eg) for this isomer. The calculated pressure derivative at zero pressure, dEg/dP = −0.12 eV GPa−1, is the same as that for the trans isomer, estimated from previous results for optical absorption measurements. This is consistent with the idea that the relevant effect of moderated pressures on the PA structure should be the reduction of the interchain distance, which leads to increased interchain coupling that would affect both isomers in a similar way. Furthermore, the IR spectra indicated the onset of structural modifications on both isomers at pressures above 5 GPa. These structural transitions are reversible in the case of trans-PA and irreversible for cis-PA

Fluorescent compacts prepared by the entrapment of benzoxazole type dyes into a silica matrix at high pressure

5 p. : il.We have produced silica-gel compacts doped with 2,5-Bis(benzoxazol-2´-yl)-4-methoxyphenol dye using high-pressure processing of powders synthesized by the sol–gel technique. The high-pressure compaction of powders with three different dye concentrations was done at 4.5 GPa and room temperature. We have measured optical and mechanical properties of the obtained compacts. They were very stable, transparent, crack free, hard (3.56 ± 0.07 GPa) and dense (1.95 ± 0.03 g/cm3), being resistant to polishing and leaching, which enables its use in optical applications. The Stokes shift observed was higher than 100 nm indicating that the intramolecular proton-transfer in the electronically excited state (ESIPT) of this dye is maintained, even in an OH rich environment like silica. A shift to higher wavelength in the fluorescence spectra of the compacts, attributed to the increasing in the conjugation of the pi system, was observed

Hard-skin development during binder removal from Al2O3-based green ceramic bodies

25 p. : il.A paraffin-based binder system was developed for low-pressure injection molding of very fine (0.4 mum) average particle size alumina ceramic bodies. Complex chemical reactions take place, during the process of binder removal, when the ceramic parts are fired in air. The effects of these chemical reactions become more pronounced at about 250C, giving rise to a hard-skin and promoting cracks in the bodies with large cross-sections. The surface hardening occurs only when the bodies are fired in an oxygen-rich atmosphere. The hardness at the surface of the ceramic bodies fired in air at 250C varies from about 0.4 to 1.2 GPa, depending on binder composition. Upon increasing temperature, the surface hardness decreases monotonically to more conventional values. Infrared absorption spectroscopy was used to study the debinding process and the chemical mechanism behind the hard-skin development in ceramic bodies fired in air. The experimental evidence suggests that cross-linkage between long carbon chains present in the binder and its interaction with the alumina powder surface is responsible for the high surface hardness observed in parts fired in air at 250C

Isothermal equation of state for the skutterudites CoSb3 and LaFe3CoSb12

4 p. : il.The thermoelectric materials CoSb3 and LaFe3CoSb12 with skutterudite structure were subjected to high pressures using a diamond anvil high-pressure cell up to 20 GPa. Energy-dispersive X-ray diffraction was used to determine the dependence of the lattice parameter on pressure. No major change in the X-ray diffraction spectra was observed for both compounds, constituting evidence that both compounds are stable within this pressure range, despite their relatively open structures. Three distinct isothermal equations of state for solids under high pressure were fitted to the experimental data to determine the bulk modulus for both compounds. The filled skutterudite showed a greater compressibility than the unfilled one, this difference can be understood in terms of the larger lattice parameter of the former