Phonon Dispersion Analysis as an Indispensable Tool for Predictions of Solid State Polymorphism and Dynamic Metastability: Case of Compressed Silane

This work is dedicated to Prof. Russell J. Hemley in recognition of his seminal contributions to high pressure sciences Diamond anvil cell experiments suggest that upon compression above 26.5 GPa silane (SiH 4 ) forms a polymeric phase VI, whose crystal structure has not yet been solved. Here we present DFT calculations showing how phonon-guided optimization leads to a polymeric F dd2 structure which is the lowest-enthalpy polymorph of SiH 4 above 26.8 GPa, and which most probably can be identified as the experimentally observed polymeric phase. The new algorithm of predicting the lowest-energy structures enables simultaneous inspection of the potential energy surface of a given system, calculation of its vibrational properties, and assessment of chances for obtaining a metastable ambient-pressure structure via decompression. Our calculations indicate that at room temperature the differences in the vibrational and entropy terms contributing to the Gibbs free energy of different polymorphs of silane are negligible in comparison with corresponding differences in the zero-point energy corrections, in contrast to earlier suggestions. We also show that the F dd2 polymorph should be metastable upon decompression up to 5 GPa, which suggests the possibility of obtaining a polymeric ambient-pressure form of SiH 4 . Polymeric silane should exhibit facile thermal decomposition with evolution of molecular hydrogen and thus constitute an efficient (12.5 wt%) material for hydrogen storage

Lone-pair interactions and photodissociation of compressed nitrogen trifluoride

High-pressure behavior of nitrogen trifluoride (NF3) was investigated by Raman and IR spectroscopy at pressures up to 55 GPa and room temperature, as well as by periodic calculations up to 100 GPa. Experimentally, we find three solid-solid phase transitions at 9, 18, and 39.5 GPa. Vibrational spectroscopy indicates that in all observed phases NF3 remains in the molecular form, in contrast to the behavior of compressed ammonia. This finding is confirmed by density functional theory calculations, which also indicate that the phase transitions of compressed NF3 are governed by the interplay between lone-pair interactions and efficient molecule packing. Although nitrogen trifluoride is molecular in the whole pressure range studied, we show that it can be photodissociated by mid-IR laser radiation. This finding paves the way for the use of NF3 as an oxidizing and fluorinating agent in high-pressure reactions. (C) 2014 AIP Publishing LLC

Microstructure Evolution and Texture Development in a Cu-8.5%AT. AL Material Subjected to Hydrostatic Extrusion

The aim of the present paper was to investigate microstructure and texture evolution of two single crystals and polycrystal of Cu-8.5%at.Al material. All of mentioned samples were deformed by HE to achieve true strain ε = 1.17. For microstructure analyzes observations by transmission electron microscope (STEM) were done. Crystalline size for samples after SPD were determine using XRD method. The global texture measurements were done using Bruker D8 Discover diffractometer equipped in Cr radiation. Microstructure investigations revealed nanocrystalline structure in single crystals with initial orientations and and polycrystalline Cu-8.5%at.Al material after SPD. The global texture measurements have shown the stability of initial orientation of Cu-8.5%at.Al single crystal after HE, whereas the same SPD process strongly brakes up the orientation Cu-8.5%at. Al single crystal

Microstructure Evolution and Texture Development in a Cu-8.5%AT. AL Material Subjected to Hydrostatic Extrusion

The aim of the present paper was to investigate microstructure and texture evolution of two single crystals and polycrystal of Cu-8.5%at.Al material. All of mentioned samples were deformed by HE to achieve true strain ε = 1.17. For microstructure analyzes observations by transmission electron microscope (STEM) were done. Crystalline size for samples after SPD were determine using XRD method. The global texture measurements were done using Bruker D8 Discover diffractometer equipped in Cr radiation. Microstructure investigations revealed nanocrystalline structure in single crystals with initial orientations and and polycrystalline Cu-8.5%at.Al material after SPD. The global texture measurements have shown the stability of initial orientation of Cu-8.5%at.Al single crystal after HE, whereas the same SPD process strongly brakes up the orientation Cu-8.5%at. Al single crystal

Nanocrystalline Cu-Al2O3 composites sintered the pulse plasma technique

score: 0collation: 227-23

Some operational features of coating obtained with PACVD method

The main aim of presented work was the commencing the tests and analysis of the DLC type a:C:H coatings obtained with PACVD technique on the elements from Co-Cr-Mo alloys., among others: monitoring of structure changes, hardness measurement, estimation of the surface geometrical structure and tribologic tests. The analysis of surface structure and its elementary composition were estimated with the application of scanning SEM microscope, as well as TEM transmission microscope. The hardness of the elements with and without the coating was measured with the Matsuzawa micro hardness testing tester. Tribologic test of materials wear resistance was done with using T-01M apparatus ball-on-disc type, within a set of Al2O3 ball - disk from Co-Cr-Mo alloys with DLC coating, in technologically dry friction condition as well as with Ringer solution lubrication. Friction tests were conducted with using tester T-17, working in association: polymer pin made of high-molecular polyethylene PE - UHMW - plate from Co-Cr-Mo alloys with DLC coating with Ringer solution lubrication. The results obtained during mentioned above tests showed that the application of DLC coatings greatly enhances the operating features of the coated surfaces

Description of a new marine diatom, Cocconeis caulerpacola sp. nov. (Bacillariophyceae), epiphytic on invasive Caulerpa species

A new species of Cocconeis has been found growing on thalli of the invasive green alga Caulerpa taxifolia collected from the Croatian Adriatic Sea (Bay of Stari Grad, the Island of Hvar, Central Adriatic, Croatia), the coasts of the Mediterranean (Saint Raphaël, west of Cannes, France) and the eastern coast of Australia (Moreton Bay, southeast Queensland). Additionally, it was observed on samples of Caulerpa racemosa, another invasive alga in the Mediterranean. Preserved thalli of Caulerpa and cleaned material of the new diatom were studied by light and electron microscopy (SEM and TEM). The morphology and fine structure of the new marine epiphytic diatom, for which we propose the name Cocconeis caulerpacola Witkowski, Car &amp; Dobosz, was determined, including the internal and external structure of the raphe and sternum valve, and the cingulum. Comparison between the new species and three closely related species, C. borbonica, C. diruptoides and C. pseudodiruptoides, was made using material from our samples, as well as material from Vis (Adriatic Sea) obtained from the Hustedt collection. Surprisingly, C. caulerpacola is able to colonize Caulerpa taxifolia in very high abundance, but its occurrence seems to be strongly patchy. Indeed, it seems that C. taxifolia is a suitable host for epiphytic diatoms, in particular this tiny Cocconeis, despite its reputation as a 'killer seaweed'. Cocconeis caulerpacola was observed on Caulerpa species in varying abundance over a wide geographical range.</p