This review summarizes the phase stability in the group IVB (Ti‐C; Zr‐C; Hf‐C) and group VB (V‐C; Nb‐C; Ta‐C) transition‐metal carbides. The order parameter functional (OPF) method and density ...functional theory (DFT) method have been used to predict phase equilibria in these systems. Extensive experimental investigations have attempted to determine both phase stability as a function of composition as well the crystal structures present using X‐ray diffraction, neutron diffraction, electron backscatter detection, and selected area electron diffraction. These investigations have demonstrated that the structures that form are based on the close‐packing of the metal atoms and the arrangement of the carbon atoms in the octahedral interstices. In general, the rocksalt B1 phase is stable for all of the transition‐metal carbides, with their substoichiometry tolerance increasing with temperature; vanadium carbide is the exception due to its negative vacancy formation energy. Vacancy‐ordered M6C5 phases have been predicted and experimentally confirmed in both groups of carbides; however, kinetic limitations often inhibit the formation of vacancy‐ordered phases, which has contributed to controversy in phase identification. The vacancy‐ordered M4C3 phase has been predicted for select carbides and has only been observed in zirconium carbide. In contrast, the stacking fault phase ζ‐M4C3−x has been readily reported in the group VB carbides (but not in the group IVB carbides). The vacancy‐ordered M3C2 has been predicted by DFT for the group IVB carbides but not in the group VB carbides, whereas OPF predicts its stability in both carbides. Vacancy‐ordered M3C2 phases have been experimentally observed in the Ti‐C and Hf‐C systems. Finally, the M2C phase has been predicted in both group carbides, except for hafnium carbide, with an order‐disorder transition with temperature. These factors result in phase diagrams that are similar among all the carbides, but each phase diagram is unique due to subtle differences in bonding that result in slight differences in thermodynamically stable phases.
The aim of this guideline is to present current and comprehensive recommendations for the diagnosis and treatment of aneurysmal subarachnoid hemorrhage (aSAH).
A formal literature search of MEDLINE ...(November 1, 2006, through May 1, 2010) was performed. Data were synthesized with the use of evidence tables. Writing group members met by teleconference to discuss data-derived recommendations. The American Heart Association Stroke Council's Levels of Evidence grading algorithm was used to grade each recommendation. The guideline draft was reviewed by 7 expert peer reviewers and by the members of the Stroke Council Leadership and Manuscript Oversight Committees. It is intended that this guideline be fully updated every 3 years.
Evidence-based guidelines are presented for the care of patients presenting with aSAH. The focus of the guideline was subdivided into incidence, risk factors, prevention, natural history and outcome, diagnosis, prevention of rebleeding, surgical and endovascular repair of ruptured aneurysms, systems of care, anesthetic management during repair, management of vasospasm and delayed cerebral ischemia, management of hydrocephalus, management of seizures, and management of medical complications.
aSAH is a serious medical condition in which outcome can be dramatically impacted by early, aggressive, expert care. The guidelines offer a framework for goal-directed treatment of the patient with aSAH.
Intracerebral hemorrhage (ICH) is the most common hemorrhagic stroke subtype, and rates are increasing with an aging population. Despite an increase in research and trials of therapies for ICH, ...mortality remains high and no interventional therapy has been demonstrated to improve outcomes. We review known mechanisms of injury, recent clinical trial results, and newly discovered signaling pathways involved in hematoma clearance. Enthusiasm remains high for methods of minimally invasive clot removal as well as pharmacologic strategies to improve recovery after ICH, both of which are currently being evaluated in clinical trials.
This article is part of the Special Issue entitled ‘Cerebral Ischemia’.
•Intracerebral hemorrhage remains a deadly hemorrhagic stroke subtype.•Trials of interventional strategies have failed to show a beneficial effect.•Multiple trials of minimally invasive clot removal strategies are underway.•Studies of hematoma clearance mechanisms may reveal pharmacologic targets.
•Initial coatings improve the microstructure of surrogate fuel elements.•Initial coating thickness is crucial in tuning the microstructure.•Potential for fuel elements with greater than 60 vol.% ...fissile fuel exists.•Higher fuel fractions would result in improved efficiency and decreased weight.
Nuclear thermal propulsion (NTP), which may utilize cermet fuels, shows promise in improving the feasibility of long-distance space travel. These cermet fuels are composed of a ceramic nuclear fuel phase, such as UO2 or UN, and a protective refractory metal (W or Mo) phase. In this work, a material system consisting of tungsten-yttria stabilized zirconia (W-YSZ) surrogate fuel elements were fabricated via spark plasma sintering. Prior to consolidation, magnetron sputtering was utilized to provide conformal W coatings to the YSZ particles, such coatings were 80 to 550 nm in thickness. These coatings, when excess W powder was added, were found to reduce the ceramic-ceramic contacts as well as improve the densification of the cermet over powders that were not coated. Furthermore, when the coating thickness was at least 350 nm, and additional W powder added to yield a metallic fraction of 32 vol.%, the ceramic-ceramic contacts remained nearly identical to higher W fractions. This suggests that conformal coated ceramic fuel particles would reduce the necessary amount of refractory metal matrix, which in turn would reduce neutron poisoning, lower fuel element weight, and increase fissile fuel content from the 60 vol.% fuel in conventional NTP cermets.
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•Hydrogen atoms have the strongest binding, or trapping, at the octahedral interstices of the near stoichiometric group IV-VB carbides.•The group IVB carbides have the strongest ...trapping or binding energies of around −1 eV, relatively independent of carbon concentration.•As carbon loss occurs, the tetrahedral interstices become more favorable, with it being the most favorable trap in some hemicarbides.•The group IVB hemicarbides have the largest trapping capacity of approximately 4 hydrogen atoms per carbon vacancy.•Ti2C and Zr2C are the most effective carbides for hydrogen trapping and storage having strong traps and large capacity.
The transition metal carbides have been known to act as traps for hydrogen in steels as well as potential materials for hydrogen storage. Despite numerous experimental and a few theoretical studies, what impacts hydrogen trapping and storage in the transition metal carbides is not well understood. In this work, we use density functional theory to systematically investigate the bulk trapping and storage capabilities of the transition metal carbides. We specifically examine how trapping and storage changes with the transition metal, from the group IVB to group VIB, carbon concentration, and structure. Our results demonstrate a strong correlation between the trap energy and the number of valence electrons in the transition metal, suggesting that the group IVB transition metal carbides are the best carbides for trapping and storage. Hydrogen preferentially sits at octahedral interstices, e.g., the carbon vacancies, in the structure except in certain cases near the Me2C concentration when tetrahedral interstices, devoid of nearest neighbor carbon, can become more favorable. Our results further demonstrate that the lower the carbon concentration, the more hydrogen the transition metal carbide can store. These results demonstrate which carbides will act as the best traps for hydrogen.
Aneurysmal subarachnoid hemorrhage (aSAH) continues to be associated with significant morbidity and mortality despite advances in care and aneurysm treatment strategies. Cerebral vasospasm continues ...to be a major source of clinical worsening in patients. We intended to review the clinical and experimental aspects of aSAH and identify strategies that are being evaluated for the treatment of vasospasm. A literature review on aSAH and cerebral vasospasm was performed. Available treatments for aSAH continue to expand as research continues to identify new therapeutic targets. Oral nimodipine is the primary medication used in practice given its neuroprotective properties. Transluminal balloon angioplasty is widely utilized in patients with symptomatic vasospasm and ischemia. Prophylactic “triple‐H” therapy, clazosentan, and intraarterial papaverine have fallen out of practice. Trials have not shown strong evidence supporting magnesium or statins. Other calcium channel blockers, milrinone, tirilazad, fasudil, cilostazol, albumin, eicosapentaenoic acid, erythropoietin, corticosteroids, minocycline, deferoxamine, intrathecal thrombolytics, need to be further investigated. Many of the current experimental drugs may have significant roles in the treatment algorithm, and further clinical trials are needed. There is growing evidence supporting that early brain injury in aSAH may lead to significant morbidity and mortality, and this needs to be explored further.
A cross-correlative precession electron diffraction - atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be ...dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.
Using a variable-composition ab initio evolutionary algorithm, the stability of various tantalum carbide compounds at ambient pressure and at 0K was investigated. The results revealed that TaC, Ta6C5 ...and Ta2C are the lowest energy configurations, with Ta4C3 and Ta3C2 having slightly higher energies. The vacancy ordered Ta6C5 phase had three energetically degenerate structures. A competition between the vacancy ordered and stacking fault variation of the phases was seen, with the latter becoming more favorable with lower carbon content. The close formation enthalpy of each stable and metastable phase appears to "frustrate" the carbide in the co-precipitation of multiple phases for substoichiometric compositions. Density functional theory calculations also provided the elastic constants for each of the stable and metastable phases. As the carbon content increased, the elastic constant values increased. This was associated with the change in metallic to more covalent bonding of the carbide from the density of states. The collective results of this computational work provide insight into why specific tantalum carbide phases form and the consequences they have on microstructure and properties.
•Corrosion behavior of cold spray deposited AA7075 was characterized by optical and scanning electron microscopy.•Helium and Nitrogen were used as carrier gas and the resultant coatings were corroded ...in a salt water environment.•Electrochemical and immersion testing was utilized.•The Helium processed coatings corrosion resistance was superior to that of the Nitrogen processed coating.
Salt water corrosion behavior of cold sprayed AA7075 is examined. Coatings on AA7075-T651 substrates were prepared by cold spraying AA7075 powder using He and N2 as carrier gases. Severe plastic deformation is only observed in the helium sprayed samples. Electrochemical experiments performed on the coatings reveal inferior corrosion resistance of the nitrogen sprayed samples, although both coatings exhibit lesser corrosion resistance than the AA7075-T651 substrate. The localized corrosion response is qualitatively different between the three materials with large scale pitting in the plate material, limited pitting in the helium sprayed material, and microscale crevice corrosion in the nitrogen sprayed material.
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