86,660 research outputs found
Compositional pathways and anisotropic thermal expansion of high-entropy transition metal diborides
No full textThe recent discovery of high entropy transition metal diborides (HEBs) has sparked renewed interest in ultra-high temperature ceramics (UHTCs). Presently, transition metal (Me) oxides based boro-carbo/thermal reduction (BCTR) syntheses show great promise as relatively cheap production methods, but also may present limits to attain single phase pure HEBs. Herein, by selectively tuning the concentration of boron and carbon, the reducing agents of Me oxide mixture (Me = Ti, Ta, Nb, Zr and Hf), and exploiting high-resolution synchrotron X-ray powder diffraction, we first identified and quantified the formation of intermediate phases during the BCTR synthesis, with the ultimate intent to achieve a full dense (Ti,Ta,Nb,Zr,Hf)B2 solid solution (SS). Additional insight was obtained by temperature dependent diffraction, which highlighted, for the first time in this class of materials, anisotropic thermal expansion, most likely at the origin of the SS micro-cracking, as was also observed by electron microscopy
Compositional disorder and sintering of entropy stabilized (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders
No full textEntropy-stabilized (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders produced by a carbo/boro-thermal reduction fol
lowed by solid solution formation were first analysed by synchrotron radiation x-ray diffraction, and their long
range periodicity (i.e. lattice parameters) as well as the micro-strain intended as lattice disorder were quanti
tatively determined. A model to describe the micro-strain was proposed. The as-synthesized (Hf,Nb,Ta,Ti,Zr)B2
solid solution powders were then hot-pressed at 2200K and 50MPa until near full densification was achieved.
The hot-pressed material had a residual micro-porosity of 1.3vol.% and consisted of a (Hf,Nb,Ta,Ti,Zr)B2
ceramic matrix, 0.3-1 μm grain size range, and of a residual 10vol.% B4C particulate component, grain size in
the range 0.2-2 μm. B4C was a side product of the former synthesis and, after hot-pressing, remained trapped
along the grain boundaries of the primary (Hf,Nb,Ta,Ti,Zr)B2 solid solution ceramic matrix. Micro-hardness
HV0.2=22.7 ± 1.9GPa for 1.96N applied force was measured
Off-axis damage tolerance of fiber-reinforced composites for aerospace systems
No full textOff-axis strength retention of continuous carbon fiber-reinforced dense ZrB2-based ceramics (Cf/ZrB2) after thermal or indentation damage was evaluated. Thermal damage was in-situ induced and characterized by cyclic dilatometric analysis. Indentation damage was induced through Vickers indentation and then characterized by digital microscopy. The investigation of Vickers imprints suggested that residual stresses promoted the material pileup onto the fibers’ plane and the appearance of out-of-plane freed fibers (OFF). On the other hand, thermal damage reduced the residual stresses and left inner freed fibers (IFF) that enhanced the elastic response. Finally, the flexural tests on damaged specimens unexpectedly revealed that Cf/ZrB2 kept its load bearing capability either after thermal or indentation damage (in both cases) and showed damage insensitivity although tested in fully matrix-dominated loading configuration (off-axis configuration)
La capacité de la femme mariée (France, Roumanie) : thèse pour le doctorat / par N. Saraga... ; Université de Paris, Faculté de droit
No full textContient une table des matièresAvec mode text
Compositionally complex catalytic oxide beds free of noble metals for H2O2 fuelled monopropellant thrusters
No full textQuantitative Inspection of Grain-Scale Chemical Inhomogeneities in High-Entropy AlB₂-Type Transition Metal Diborides
No full textThe chemical homogeneity of single phase high-entropy AlB2-type Ti-Zr-Hf-Ta-TM diboride (TM = Cr, V, W, Mo), as well as Ti-Zr-Hf-Mo-W solid solutions was investigated using a new method based on the comparative examination of information provided by electron microscopy and structural parameters. The study of the densification behavior was accomplished, and strong correlations among densification rate-grain coarsening-long range chemical randomization were found. High-resolution synchrotron radiation X-ray diffraction supported by grain-scale chemical analyses by energy dispersive spectroscopy indicated that homogenization of the metals was incomplete, with direct impact on the refined lattice μ-strain. The chemical inhomogeneity was on the same length scale as the grain size, which makes it hardly detectable by typical chemical mapping using energy dispersive spectroscopy. Based on this analysis, the resulting μ-strain broadening is not an intrinsic property of the material, but strongly depends on its processing history
Anisotropic thermal expansion in high-entropy multicomponent AlB2-type diboride solid solutions
No full textHigh-entropy (HE) ultra-high temperature ceramics have the chance to pave the way for future applications propelling technology advantages in the fields of energy conversion and extreme environmental shielding. Among others, HE diborides stand out owing to their intrinsic anisotropic layered structure and ability to withstand ultra-high temperatures. Herein, we employed in-situ high-resolution synchrotron diffraction over a plethora of multicomponent compositions, with four to seven transition metals, with the intent of understanding the thermal lattice expansion following different composition or synthesis process. As a result, we were able to control the average thermal expansion (TE) from 1.3 × 10−6 to 6.9 × 10−6 K−1 depending on the combination of metals, with a variation of in-plane to out-of-plane TE ratio ranging from 1.5 to 2.8
Local structure in high-entropy transition metal diborides
No full textStudies on high-entropy materials often speculate about the effects of lattice distortion and disorder on characteristics such as hardness, thermal expansion, and electronic properties. Notwithstanding the ongoing race to discover new compositions, investigations of the local structure at the atomic level remain sparse at best. Additionally, assessments of the homogeneity of the distribution of metals within the lattice sites are often restricted to techniques such as energy dispersive spectroscopy which might lead to an inaccurate picture of the bulk material. Herein, we report an extensive and systematic study of a class of emerging high-entropy ceramics that uses a combination of high-resolution synchrotron powder diffraction and extended X-ray absorption fine structure analysis. Our data are consistent with a random distribution of atoms with local strain around the d-metals sites, which describes the bulk structure of these materials. Moreover, a linear trend is observed between the average structure and the first-neighbour distances, regardless the number (from 3 to 5) and type (Ti, Zr, Nb, Hf, Ta, Mo, W) of metals that constitute the high-entropy ceramic, which suggests that any description of properties for such materials need to go beyond the simple dichotomy of long-range order and local structure
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
[Newspaper Clipping: Author Claims Evidence of Second JFK Assassin #1]
No full textNewspaper article titled "Author Claims Evidence of Second JFK Assassin." The article states that author Richard J. Whalen concluded "that there is circumstantial evidence to support the theory of a second assassin in the shooting of President John F. Kennedy.
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