Stability and Physical Properties of Metastable Ba-Si Clathrates.

In the present study, we have investigated the stability of different Ba-Si clathrates with pressure and temperature using DFT calculations and studied the stability of type I Ba8Si46 and type IX Ba24Si100 clathrates using high pressure─high temperature synthesis technique, calorimetry, and diffraction experiments. When increasing pressure, the type I Ba8Si46 clathrate and BaSi6 become more stable. In good qualitative agreement with experiments, the type IX Ba24Si100 clathrate becomes stable at a pressure of 1-2 GPa thanks to the pressure and thermal effect of both electronic and vibrational contributions. One can notice that the presence of Ba in the cages of type IX clathrate increases significantly the stability and the mechanical properties of type IX clathrate. We have determined the P-T existence domain of type IX Ba24Si100 clathrate from ex situ experiments, which was confirmed by in situ synchrotron X-ray experiments. At room pressure and under an oxidizing atmosphere, the type I Ba8Si46 and the type IX Ba24Si100 clathrates are stable up to about 560 °C and up to about 600 °C, respectively. The thermoelectric properties of type IX Ba24Si100 are also reported.

In Situ Vibrational Probes of Epoxy Gelation.

This paper presents an efficient way to measure the curing kinetics and gel point, αgel, in epoxy resins from one single experiment. The epoxy curing reaction is herein monitored using in situ and time-resolved near-infrared absorption spectroscopy (NIR). The curing profiles over different isothermal conditions are in good agreement with DSC. Furthermore, the increase of the NIR absorption bands of aromatic rings (unreactive throughout curing) probe the cure shrinkage, as more and more aromatic rings are localized within the fixed sample volume. Therefore, the gel point is determined using the onset of the aromatic absorption increase. The results are in good agreement with the theoretical gel point, as well as DMA results. This innovative approach enables gelation measurements on epoxy neat resins and film composites with an easy-to-perform, accurate, robust, and versatile method.

Ab initio study of the nature and stability of the defects and multi-vacancies in TaN. Comparison with TiN.

First principles calculations have been carried out to study the native single-defects and multi-vacancies in TaN and TiN with a cubic rocksalt structure mainly used as diffusion barriers. Our results indicate that vacancies are the most stable single-defects in both compounds and that nitrogen interstitial defects in tetrahedral interstitial site are significantly more stable in TaN than in TiN. The interactions between vacancies are attractive in TaN in contrast to the case of TiN. The vacancies show a much larger tendency to cluster and to form bi- and tri- vacancies in TaN than in TiN. We suggest that the number of d electrons might explain this difference in the defect stability. These results will have impact on the use of these materials as diffusion barrier.

Optimization of the electrodeposition process of high-performance bismuth antimony telluride compounds for thermoelectric applications.

High-quality films of bismuth antimony telluride were synthesized by electrodeposition from nitric acid electroplating baths. The influence of a surfactant, sodium ligninsulfonate, on the structure, morphology, stoichiometry, and homogeneity of the deposited films has been investigated. It was found that addition of this particular surfactant significantly improved the microstructural properties as well as homogeneity of the films with a significant improvement in the thermoelectric properties over those deposited in the absence of surfactant. A detailed microprobe analysis of the deposited films yielded a stoichiometric composition of Bi(0.35)Sb(1.33)Te(3) for the films electrodeposited in the absence of surfactant and a stoichiometry of Bi(0.32)Sb(1.33)Te(3) for films deposited in the presence of surfactant.