1,721,013 research outputs found
Crystal-fluid interaction in MFI zeolites at high pressure
No full textIn the last decades, several studies have been dedicated to investigate the Pressure-induced intrusion of molecules in nanoporous compounds, as this type of phenomenon may lead to new routes for tailoring functional materials. The zeolites with the MFI-type structure topology are used as catalysts in some olefins-production processes, representing an appealing alternative to the high-energy demanding Steam Cracking process, which presently accounts for ~95% of the total worldwide olefins production [1]. At ambient conditions, only the surfaces of the zeolite crystallites are supposed to be active in the methanol-to-olefins process. Pressure could significantly improve the efficiency of the catalytic process, driving the injection and diffusion of the methanol molecules through the zeolitic channels. In the present work, already reported in [2] six MFI-type zeolites with different chemical composition, characterized Fe-, Al- and B-doped siliceous frameworks, balanced by Na or H as extra-framework cations, have been synthesized. Their compressional behavior has been investigated up to 2 GPa by in-situ powder synchrotron X-ray diffraction using two different hydrostatic pressure transmitting fluids: methanol (capable of entering the structural voids of the MFI zeolite) and silicone-oil (a polymeric fluid with a kinetic diameter of the molecules larger than the diameters of the structural channels). For each sample, the compressibility in silicone-oil has been found to be considerably higher than that in methanol. This difference in the bulk elasticity is due to the injection of methanol within the structural voids of the zeolites and the magnitude of the intrusion process seems to be influenced by the sample crystal chemistry. Due to the high number of experimental pressure points, a phase transition from the monoclinic polymorph (P21/n, stable at room pressure) to the orthorhombic polymorph (Pnma) (hereafter MOPT) has been identified at about 0.4 GPa for all the monoclinic zeolites compressed in silicone oil. Conversely, only two zeolites compressed in methanol (i.e., a penetrant fluid) show the MOPT and, in these cases, the phase transition shifted at greater pressure with respect to what observed with in the silicone oil experiments. A comparative analysis of the effect of pressure on the methanol adsorption by the MFI zeolites with different chemical composition may provide useful information on their application as catalysts in the methanol-to-olefins conversion processes
Posttraumatic enophthalmos: Etiology, principles of reconstruction, and correction
No full textEnophthalmos is defined as a backward, and usually downward, displacement of the globe into the bony orbit. In posttraumatic enophthalmos, the mechanisms that determine globe position are: 1) the enlargement of the orbital cavity; 2) the herniation of orbital fat into the maxillary sinus; and 3) fat atrophy, loss of ligament support, and scar contracture. The aim of this article is to analyze the strategies to prevent enophthalmos and to correct late posttraumatic enophthalmos. In this study, 80 patients (52 cases of orbitozygomatic fractures and 28 late posttraumatic enophthalmos) were treated between January 1998 and January 2005. Fracture reduction in primary enophthalmos was performed. In enophthalmos as sequelae, the treatment consisted of orbital reconstruction in combination with bone grafts harvested from calvaria, iliac crest, and/or orbital osteotomies. In some cases, biomaterials were also used. All these techniques may also be combined depending on the severity of enophthalmos. The results were satisfactory in all cases. It is evident that a perfect correction of the deformity is difficult to achieve. Often soft tissue changes limit the aesthetic and morphologic results, despite adequate bony reconstruction
Thermodynamics, elasticity and phase stability of grossite (CaAl4O7) at high pressure and temperature
No full textAlthough grossite (CaAl4O7) is an important constituent phase of high alumina cements (HAC) and a common refractory phase in calcium-rich inclusions (CAIs) found in primitive chondritic
meteorites, its thermophysical properties are poorly constrained and its thermodynamic behaviour mostly unknown. In particular, the knowledge of phase stability relations up to high pressure and temperature conditions (HP-HT) is concealed by the lack of informations on elasticity and P-V-T equation of state parameters. Thermodynamics, equation of state and elasticity of grossite (space group C2/c) have been investigated in this work by first principles theory (using a WC1LYP hybrid functional and CRYSTAL program) and experimental methods (synchrotron radiation high-pressure single
crystal X-ray diffraction, using an ETH-type Diamond Anvil Cell and M.E.W. as pressuretransmitting fluid, at Xpress beamline at Elettra, Trieste, λ = 0.4957 Å) in a broad range of P-T conditions (i.e. 0-10 GPa and 0-2000 K). HP experiments reveal a displacive first order phase transition, which has been observed between 6.1 and 7 GPa by ca. a 3% change in lattice volume and the violation of C-centring extinction conditions. In fact, the symmetry changes by losing the centring of the lattice and the centre of symmetry, passing from C2/c to Pc space group. The phase transition is marked by a dramatic change of the isothermal bulk modulus (Ko) from 123(4) GPa in the low-pressure regime to 35(8) GPa in the high-P phase, which shows a very soft and anisotropic behaviour. First principles calculations provide static EoS parameters [e.g. K0 = 128.8(1) GPa and K′0 = 4.0(1) for the C2/c phase], which are in remarkable agreement with experiments. Furthermore, ab initio calculation of the full elastic tensor (with 13 independent components) permits to define the shear modulus (i.e. GVRH = 52.1 GPa) and seismic anisotropy of the C2/c phase, for which no experimental data exist so far. Seismic anisotropy of P-, S1- and S2-waves turns out to be equal to AP=16.0%, AS1=24.7% and AS2=36.3%. AP and AS1 both decrease with pressure, while AS2 increase up to a huge value (ca. 58.8%) at 9 GPa. Thermodynamic properties have been computed in the framework of quasi-harmonic approximation (QHA) by phonon dispersion calculations on 2×2×2 supercells to reach numerical convergence. The calculated heat capacity and standard-state entropy values shows a good agreement with the available calorimetric data. P-V-T relations have been obtained by a first-principles Mie-Gruneisen EoS [1] and permitted to calculate Gibbs free energy in the invetigated P-T range. Due to high computational cost, thermodynamic properties of the Pc phase have been defined by a modified Kieffer’s model approach splitting acoustic and optic contributions and defining the former from the ab initio elastic constant tensor, the latter from vibrational frequencies calculated at Γ-point [2]. A temptative phase transition boundary for the C2/c → Pc reaction is thus predicted by Gibbs free energy minimization. Finally, ab initio
thermodynamic properties of grossite have been used to constrain its phase stability field in the CaO-Al2O3-SiO2 (CAS) ternary system at HP-HT along the guidelines defined in a previous
study [3].
[1] D. Belmonte Minerals. 2017, 7, 183.
[2] D. Belmonte, C. Gatti, G. Ottonello, P. Richet J. Phys. Chem. A. 2016, 120, 8881.
[3] G. Ottonello, M. Attene, D. Ameglio, D. Belmonte, M. Vetuschi Zuccolini, M. Natali Chem. Geol.
2013, 346, 81
Hydrated borates at non ambient conditions: pivotal experiments in the production of neutron shielding concretes.
Full text linkHydrated borates (e.g., colemanite, kernite, ulexite, borax, tincalconite) are the most common ore minerals of
boron, an important geochemical marker, in pegmatitic and granitic systems, for petrogenetic processes and a
strategic element in a series of technological applications. Hydrated borates have been listed as critical raw
materials by the EU [1], and they could be used as aggregate in neutron-shielding Sorel or Portland concretes,
enhancing the adsorption of concrete towards thermal neutrons. The main structural units in hydrated borates
are Bφx units (fundamental building blocks, i.e., tetrahedra and planar trigonal group where φ is an anion, O2-
or OH-
), connected in such a way to form clusters of polyions connected to alkaline/Earth alkaline (mainly
Na+
, K+
, Ca2+, Mg2+) polyhedra. In these structures, H2O molecules and OHform a complex and pervasive
hydrogen-bond network, which reinforce the connection between the polyions clusters and the cationspolyhedrons, playing a paramount role in the stability of the crystalline edifice [2, 3]. In the last 4 years, a
number of studies have been performed at high temperature and pressure unveiling phase transition driving
deformation mechanisms’ that lead to the formation of their high-pressure polymorphs. Critically, the pressure
at which hydrated borates undergo a phase transition is related to the water content of the mineral itself. The
aim of this contribution is to provide insides on the high-pressure behavior and structure evolution of selected
hydrate borate minerals. These studies at non ambient conditions are pivotal to produce neutron shielding tiles
of Sorel concretes
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
Pressure-mediated crystal-fluid interactions in natural erionite-K
No full textInvestigating the behaviour at high pressure of crystalline compounds with a microporous structure, e.g. zeolites, has experienced a boosted interest in the last two decades, especially due to the P-induced intrusion of molecules and ions into the structural nano-cavities from the P-transmitting fluids [1]. Zeolites have a consolidated history of technological and industrial applications, but the understanding of these P-induced phenomena may further expand their utilizations, opening the way for new routes for tailoring functional materials. In this study, we have investigated the behaviour of the natural zeolite erionite when compressed in non-penetrating and potentially penetrating fluids: i.e. those fluids made by molecules having a kinetic diameter that may allow their P-mediated adsorption into the zeolite structural cavities.
Erionite is a zeolite with a wide chemical variability in Nature, expressed as solid solutions among three end-members: erionite-Ca, erionite-Na and erionite-K.
Our sample, classified as erionite-K, has an average chemical formula: K2.31Na0.02Ca2.15Mg0.69Ba0.04Sr0.02Al9.00Si27.19O72·18.66(H2O). Erionite crystal structure is characterized by the presence of large cages (23-hedron, called “erionite-cage”), superposed along the c axis, hosting most of the extra-framework population.
We have conducted experiments by single-crystal X-ray diffraction under in-situ high-pressure conditions at the Xpress beamline of the Elettra Synchrotron, using an ETH-type diamond anvil cell (DAC) and ruby as P-calibrant. We have performed two P-ramps using different P-transmitting media: the first one using the non-penetrating silicone oil, up to 2.60(5) GPa, and the second one with the potentially penetrating methanol:ethanol:H2O = 16:3:1 (hereafter mew) mixture, up to 4.97(5) GPa. Using the EoSFit7c software, the P-V data obtained by the silicone oil ramp were fitted by a II order Birch-Murnaghan equation of state, yielding the following refined isothermal bulk modulus KV0 = 44(1) GPa (βV0 = KV0-1 = 0.0227(5), where βV0 is the bulk volume compressibility).
P-V data from the mew ramp (Fig. 1) show a significant decrease in compressibility, which unambiguously suggests the (irreversible) P-induced intrusion of H2O (and possibly alcohols) molecules. The adsorption seems to occur in three different steps, approximately around 0.2, 1.2 and 2 GPa. This behaviour is somehow surprising if we consider that the magnitude of the intrusion process is comparable with that of synthetic SiO2-ferrierite [2] and AlPO4-5 [3] zeolites, but in this case has been observed in a natural sample of erionite, with structural cavities filled by extraframework population.
Further experiments with different classes of potentially penetrating fluids will allow a full understanding and constraints of the P-induced adsorption phenomena in erionite
Inderborite : a comprehensive reinvestigation of its technological features
Full text linkInderborite is a hydrated borate (ideal formula:...) often found in lower fractions alongside with the
five most important boron commodities (i.e., colemanite, kernite, ulexite, borax, tincalconite).
Nowadays, Turkish mines produce more than 70% of the worldwide B-minerals (e.g., Sarikaya borate
deposits, Baysal, 1973). Hydrated borates have been listed as critical raw materials by the EU (EU
Commission, 2017), and, because of the high neutron cross-section of B-10, they could be used as
aggregates in neutron-shielding Sorel or Portland concretes, enhancing their adsorption towards
thermal neutrons. In the forthcoming decades, with the advent of fusion power plants, it is predicted
that substantial quantities of neutron-activated elements (e.g., beryllium or tungsten), will be
produced (Gonzalez de Vincente et al. 2022). The main goal of this projects was to: i) re-investigate,
by means of a multi-methodological approach, the crystal chemistry (with a focus on the B isotopic
composition and trace elements) and structure of inderborite (even based on a single-crystal neutron
diffraction experiment), ii) assess the stability range of inderborite with respect to pressure and
temperature even for potential industrial utilization of this borates, iii) describe the structural
evolution of inderborite, at the atomic scale, with increasing pressure and temperature
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
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