1,721,156 research outputs found
Catastrophe theory and thermodynamic instability to predict congruent melting temperature of crystals
Full text linkMelting temperature (Tm) is a crucial physical property of solids and plays an important role in the characterization of materials. Therefore, the capacity to predict Tm is a relevant issue for solid state sciences. This investigation aims i) to provide a theoretical basis for the link between catastrophe theory and thermodynamic instability; ii) to estimate Tm through the notion of “degenerate critical temperature” (Td), related to (Pd,Vd,Td), where KT → 0 and the Gibbs function shows a non-Morse behaviour; iii) to compare predictions of (Pm,Tm) with observations for three crystalline pure substances that undergo congruent melting and exhibit different bonding and stability ranges: NaCl (halite), SiO2,st (stishovite), and MgSiO3 (perovskite). The P-T locus of KT = 0 associated with melting is identified using the maximum values of Td and ΔH/ΔV at a given pressure. We observed an average absolute discrepancy ranging between 0.2 % (halite) and 5.8 % (stishovite), and an agreement between theoretical and experimental T(P)melting-points from better than 1 to approximately 14 %
Aluminium distribution in an Earth's non–primitive lower mantle
Full text linkThe aluminium incorporation mechanism of perovskite was explored by means of quantum mechanics in combination with equilibrium/off-equilibrium thermodynamics under the pressure-temperature conditions of the Earth’s lower mantle (from 24 to 80 GPa). Earth’s lower mantle was modelled as a geochemically non-primitive object because of an enrichment by 3 wt% of recycled crustal material (MORB component). The compositional modelling takes into account both chondrite and pyrolite reference models.
The capacity of perovskite to host Al was modelled through an Al2O3 exchange process in an unconstrained Mg-perovskite+Mg-Al-perovskite+free-Al2O3(corundum) system. Aluminium is globally incorporated principally via an increase in the amount of Al bearing perovskite [Mg-Al-pv(80 GPa)/Mg-Al-pv(24 GPa)1.17], rather than by an increase in the Al2O3 content of the average chemical composition which changes little (0.11-0.13, mole fraction of Al2O3) and tends to decrease in Al. The Al2O3 distribution in the lower mantle was described through the probability of the occurrence of given compositions of Al bearing perovskite. The probability of finding Mg-Al-perovskite is comparable to Mg-perovskites. Perovskite with Al2O3 mole fraction up to 0.15 has an occurrence probability of ~28% at 24 GPa, increasing up to ~43% at 80 GPa; on the contrary, perovskite compositions in the range 0.19-0.30 Al2O3 mole fraction drop their occurrence probability from 9.8 to 2.0%, over the same P-range. In light of this, the distribution of Al in the lower mantle shows that, among the possible Al bearing perovskite phases, the (Mg0.89Al0.11)(Si0.89Al0.11)O3 composition is the likeliest, providing from 5 to 8% of the bulk perovskite in the pressure range from 24 to 80 GPa. The occurrence of the most Al rich composition, i.e. (Mg0.71Al0.29)(Si0.71Al0.29)O3, is a rare event (probability of occurrence < 1.7%). This study predicts that perovskite may globally host Al2O3 in terms of 4.3 and 4.8 wt% (with respect to the non-primitive lower mantle mass), thus accounting for ~ 90% and 100% of the bulk Al2O3 estimated in the framework of pyrolite and chondrite reference models, respectively. A calcium-ferrite type phase (on the MgAl2O4-NaAlSiO4 join) seems to be the only candidate that can compensate for the 10% gap of the perovskite Al incorporation capacity, in the case of a pyrolite non-primitive lower mantle model
Electron-density critical points analysis and catastrophe theory to forecast structure instability in periodic solids
Full text linkThe critical points analysis of electron density, i.e. ρ(x), from ab initio calculations is used in combination with the catastrophe theory to show a correlation between ρ(x) topology and the appearance of instability that may lead to transformations of crystal structures, as a function of pressure/temperature. In particular, this study focuses on the evolution of coalescing non-degenerate critical points, i.e. such that ρ(xc) = 0 and λ1, λ2, λ3≠ 0 [λ being the eigenvalues of the Hessian of ρ(x) at xc], towards degenerate critical points, i.e. ρ(xc) = 0 and at least one λ equal to zero. The catastrophe theory formalism provides a mathematical tool to model ρ(x) in the neighbourhood of xcand allows one to rationalize the occurrence of instability in terms of electron-density topology and Gibbs energy. The phase/state transitions that TiO2(rutile structure), MgO (periclase structure) and Al2O3(corundum structure) undergo because of pressure and/or temperature are here discussed. An agreement of 3-5% is observed between the theoretical model and experimental pressure/temperature of transformation.Electron-density topology is used to detect instability in periodic solids
Influence of Al, C, N and H on the iron redox state in the Earth’s lower mantle: A geochemical quantum model
Full text linkIron is the most abundant element by weight in our planet, the dominant component of the core and the only major transition metal in the mantle. Its speciation in the present-day lower mantle remains one of the most controversial aspects to deal with in modelling the deep interiors of the Earth. Here, we present an unconventional approach that relies upon quantum mechanics and the bonding Bader theory to predict the iron oxidation state at lower mantle conditions (24/1900–90/2700 GPa/K) in bridgmanite, the major mineral phase. This approach provides insights into the lower mantle geochemistry on a global scale and a unified viewpoint. The chemical species that on account of their electronic/steric features and mobility expectably induce redox effects on iron in bridgmanite are Al, N, C and H. Hydrogen causes reduction, whereas the other species promote oxidation. The combination of the probability of occurrence of the Al-N-C-H driven reactions with the availability of the involved species points to iron never achieving full oxidation; instead, it reaches a maximum average oxidation number of ∼ 2.4. This is equivalent to a Fe3+/Fetot ratio that varies with depth from 15.9 to 12.1 % (if Al-N-C-H are accounted for), and from 19.3 to 29.0 % (if only Al is considered). Iron in the lower mantle is therefore more reduced than previously expected, in terms of ferric fraction, because of the important reducing action of H. If we assume that Fe3+ is always associated with iron disproportionation (3Fe2+→2Fe3++Fe0), then the Al-N-C–H atom exchange reactions yield an estimate of metallic iron fraction in the lower mantle as large as ∼ 0.4 wt%. This figure increases up to ∼ 0.8 wt% when neglecting N-C–H effects on ferric iron formation and is fully comparable to the latest experimental result (0.7 wt%) using aluminium only
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“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
Effects of particle size on properties and thermal inertization of bottom ashes (MSW of Turin’s incinerator)
Full text linkThe aim of this study is twofold: (i) characterization of the bottom ashes from the Incinerator plant of the city of Turin (northern Italy), in terms of their chemical/phase compositions and capacity to release heavy metals in leachates, as a function of particle size; (ii) investigation of thermal treatments’ efficacy to promote inertization of the same bottom ashes, exploring time-temperature ranges with t ≤ 6 h and T ≤ 1000 °C. Special attention is paid to macro-sampling techniques in order to have samples that are representative of the average bottom ashes production. Micro-XRF, ICP-OES, SEM-EDS, Ion Chromatography and X-ray powder diffraction were used to investigate bottom ashes and leachates. Bottom ashes are mainly constituted by an amorphous phase, ∼66–97 wt%, regardless of particle size; the remaining phases are quartz, calcite, Fe-oxides, melilite and other minor crystalline materials. The amorphous phase exhibits a relevant dependence on particle size, and undergoes dissolution in water up to 20 wt%, thus being the most important component in affecting chemical species release. The smaller the bottom ashes’ particle size, the more the heavy metals (major species: Zn, Cu, Ti, Pb) and calcium contents increase, whereas silicon’s decreases. Electrolytic current observations in combination with phase/chemical composition and metals release as a function of particle size, suggest that bottom ashes partition into two classes, i.e. ≥1 and <1 mm, for inertization purposes. Thermal treatments exhibit partial efficacy to curb heavy metals mobility: whilst they reduce Cu release, they lead to a inverse effect in the case of Cr
Evaluations of the reuse of municipal solid waste incinerator bottom ashes as aggregated materials in civil applications
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