102,045 research outputs found
Il dono inatteso della disperazione. Intorno alla presenza di Cioran nell’opera poetica e filosofica di Roberto Carifi
No full textIl presente articolo descrive il serrato confronto tra Carifi e Cioran, mettendo in luce il fatto che, sebbene Cioran non appartenga apertamente alla costellazione genealogica del pensiero di Carifi, la sua presenza è comunque attestabile in vari luoghi della sua opera poetica e filosofica. Carifi, in fondo, intende oltrepassare la cosiddetta concezione nichilistica dell’esperienza umana di Cioran, elaborando una concezione poetico-filosofica che interpreta la poesia come atto di riparazione di fronte alla dimensione fondamentalmente tragica dell’esistenza, perché soltanto la poesia, apparentandosi celanianamente alla preghiera, riesce segretamente a “riscrivere la ferita” del soggetto, in quanto evento e dono inatteso
Cancrinite-group minerals ([CAN]-framework type) at non-ambient conditions
No full textAll the isotypic minerals of the cancrinite-group share the [CAN]-framework type, built up by layers of single six-membered rings of tetrahedra centered in an “A” or “B” position, according to the ABAB stacking sequence. The resulting framework has the following secondary building units: 12-membered ring channels parallel to the [0001] axis, bound by columns of base-sharing cages, and the so-called can units. A large chemical variability is shown by both natural and non-natural isotypic compounds. Among the natural species, the majority shows an alumino-silicate framework (Al6Si6O24), and two subgroups can be identified according to the extraframework content of the can units: the cancrinite- and the davyne-subgroups, showing Na-H2O and Ca-Cl chains, respectively. Several cations, anionic and/or molecular groups lie in the channels.
The description of the phase-stability fields and the of the thermo-elastic behavior of the cancrinite-group minerals play a key role in the study of the natural and industrial processes where these compounds are primary constituents (a short summary of which is in [1,2]).
We aim to model the thermo-elastic behavior and (P,T)-induced structure evolution of these isotypic compounds, with a focus on the influence played by the different extraframework constituents on the structure deformation mechanisms. The study is restricted to the chemical compositions commonly occurring in Nature, delimited by the (CO3)-rich and (SO4)-rich end-members within the two aforementioned subgroup: cancrinite {[(Na,Ca)6(CO3)1.2-1.7][Na2(H2O)2][Al6Si6O24]} and vishnevite {[(Na,Ca,K)6(SO4)][Na2(H2O)2][Al6Si6O24]}, balliranoite {[(Na,Ca)6(CO3)1.2-1.7][Ca2Cl2][Al6Si6O24]} and davyne {[(Na,Ca,K)6((SO4),Cl)][Ca2Cl2][Al6Si6O24]}, respectively. The high-pressure and low-temperature (T < 293 K) studies of the carbonate end-members (i.e. cancrinite and balliranoite) have been performed by means of in situ single-crystal X-ray diffraction. The results [1-4] show that, though sharing a similar volume compressibility and thermal expansivity, these minerals have a different thermo-elastic anisotropy, being more pronounced in cancrinite. This is due to different (P,T)-induced structure deformation mechanisms, likely governed by the different coordination environment of the cage population. An in situ high-temperature (293 ≤ T(K) ≤ 823(7)) single-crystal X-ray diffraction study of cancrinite, allowed the description of thermo-elastic behavior and anisotropy. An irreversible dehydration process takes place at 748(7) K. Preliminary results of the high-pressure studies of the sulfatic end-members (i.e. vishnevite and davyne) are available. A clear change of the elastic behavior of vishnevite, with an increase of compressibility, is shown between 2.47(2)-3.83(2) GPa. A similar increase of compressibility was also reported for cancrinite at 4.62-5.00(2) GPa.
References. [1] Lotti, P., Gatta, G.D., Rotiroti, N., Cámara, F. (2012): Am. Mineral., 97, 872-882; [2] Gatta, G.D., Lotti, P., Kahlenberg, V. (2013): Micropor. Mesopor. Mater., 174, 44-53; [3] Gatta, G.D., Lotti, P., Kahlenberg, V., Haefeker, U. (2012): Miner. Mag., 76, 933-948; [4] Lotti, P., Gatta, G.D., Rotiroti, N., Cámara, F., Harlow, G.E. (2013): Z. Kristallogr., in press
Rhinitis in children: common clinical presentations and differential diagnoses
No full textRhinitis is a common presentation in childhood. Acute virally induced rhinitis is generally self-limiting and usually does not require medical attention. Whilst allergic rhinitis is the focus of the paediatric allergist, the presentation of other diseases or comorbidities that can complicate or mimic allergic rhinitis needs to be considered. Effects on the child's quality of life also need to be addressed. Rhinitis can be associated with asthma and other significant comorbidities: importantly, non-allergic rhinitis can sometimes be a consequence of systemic immune impairment. The diagnosis of rhinitis is based on clinical findings with directed investigations. Nasal nitric oxide measurement is an emerging diagnostic tool and helpful particularly in relation to evaluating the differential diagnosis in more difficult rhinitis. Successfully identifying the cause of rhinitis in childhood and associated comorbidities can ensure that the patient is successfully treated as described in the recently published EAACI Pediatric Rhinitis Position Paper
„Când soarele ajunge la solstițiu”: Urmuz și scriitorii de avangardă din exil
No full textLa Urmuz, poate pentru prima dată în România, scrisul e o practică impersonală și neutră, e suplimentul unui cuvânt oral cufundat în tăcere și cu o origine care n-a fost nicicând prezentă: se sustrage oricărui sistem de control, iar în literă devine loc material al diseminării limbajului, al fugii lui care nu ajunge nicăieri. Tristan Tzara, Eugène Ionesco, Jacques G. Costin, Paul Celan, Gherasim Luca, Dumitru Țepeneag au încercat să recupereze moștenirea scripturală a lui Urmuz, să și-o însușească, subiectivând – mai ales în străinătate, în Occident – datoria contractată față de el. Nu s-au limitat să îl repete așa cum fusese efectiv, ci fiecare i-a reluat în felul său moștenirea, conferindu-i o semnificație nouă și lăsând o amprentă profundă pe prezentul și viitorul literaturii europene din secolul X
Structural study of nano-sized gahnite (Znal2o4): From the average to the local scale
No full textSpinel gahnite (ZnAl2O4) has been obtained through a hydrothermal synthesis method with a grain size of about 2 nm. The sample was calcined for a few hours at two different temperatures (800 and 900 °C) in order to obtain larger grain sizes to be analyzed by means of powder diffraction with the Rietveld method, and by means of total scattering with the Pair Distribution Function (PDF) method. The idea is to compare the average to the local structure, as a function of increasing grain size. The total scattering data were collected at the European Synchrotron Radiation Facility (ESRF), Grenoble. The samples have been also characterised by means of high resolution Transmission Electron Microscopy (TEM), showing an increasing grain size up to about 9 nm. The average structure presented variations in the inversion degree and an increase in grain size. TEM observations demonstrated that the small crystals are well crystallised: The high resolution images neatly showed the atomic planes, even in the smallest particles. However, the average structure did not properly fit the PDF data in the local region, owing to a slightly different coordination among the octahedra. A new structural model is proposed for the local region of the PDF, that helped our understanding of the differences between a real nanostructured sample and that of a microcrystalline one. The oxygen disorder, due to the inversion grade of the spinel, is demonstrates to be at the basis of the local deviation. No signals of interstitial Zn atoms were detected
Stability at high pressure, elastic behavior and pressure-induced structural evolution of "Al(5)BO(9)", a mullite-type ceramic material
No full textElastic behavior and pressure-induced structural evolution of synthetic boron-mullite "Al5BO9" (a = 5.678(2) Å, b = 15.015(4) Å and c = 7.700(3) Å, space group Cmc21, Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P-V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V0 = 656.4(3) Å3 and KT0 = 165(7) GPa (βV0 = 0.0061(3) GPa-1). The evolution of the Eulerian finite strain versus normalized stress (fE-FE plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of "Al5BO9" within the investigated P range. The weighted linear regression through the data points gives: FE(0) = 159(11) GPa. Axial compressibility coefficients yielded: βa = 1.4(2) × 10-3 GPa-1, βb = 3.4(4) × 10-3 GPa-1, and βc = 1.7(3) × 10-3 GPa-1 (βa:βb:βc = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1-Al3-Al4-Al1-Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as "pillars", making the structure less compressible along the a-axis than along the b- and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al-O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out
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