Thermodynamics and the intrinsic stability of lead halide perovskites CH3NH3PbX3

The role of thermodynamics in assessing the intrinsic instability of the CH3NH3PbX3 perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH3NH3X(s) and PbX2(s) is first analysed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH3NH3PbI3 the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH3(g)+CH3X(g), CH3NH2(g)+HX(g) or CH3NH3X(g) is analysed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized

Screen printed Pb₃O₄ films and their application to photoresponsive and photoelectrochemical devices

A new and simple procedure for the deposition of lead (II, IV) oxide films by screen printing was developed. In contrast to conventional electrochemical methods, films can be also deposited on non-conductive substrates without any specific dimensional restriction, being the only requirement the thermal stability of the substrate in air up to 500 °C to allow for the calcination of the screen printing paste and sintering of the film. In this study, films were exploited for the preparation of both photoresponsive devices and photoelectrochemical cell photoanodes. In both cases, screen printing was performed on FTO (Fluorine-Tin Oxide glass) substrates. The photoresponsive devices were tested with I-V curves in dark and under simulated solar light with different irradiation levels. Responses were evaluated at different voltage biases and under light pulses of different durations. Photoelectrochemical cells were tested by current density⁻voltage (J-V) curves under air mass (AM) 1.5 G illumination, incident photon-to-current efficiency (IPCE) measurements, and electrochemical impedance spectroscopy

Quantum Klein Space and Superspace

We give an algebraic quantization, in the sense of quantum groups, of the complex Minkowski space, and we examine the real forms corresponding to the signatures $(3,1)$, $(2,2)$, $(4,0)$, constructing the corresponding quantum metrics and providing an explicit presentation of the quantized coordinate algebras. In particular, we focus on the Kleinian signature $(2,2)$. The quantizations of the complex and real spaces come together with a coaction of the quantizations of the respective symmetry groups. We also extend such quantizations to the $\mathcal{N}=1$ supersetting

Spinning particles and higher spin fields on (A)dS backgrounds

Spinning particle models can be used to describe higher spin fields in first quantization. In this paper we discuss how spinning particles with gauged O(N) supersymmetries on the worldline can be consistently coupled to conformally flat spacetimes, both at the classical and at the quantum level. In particular, we consider canonical quantization on flat and on (A)dS backgrounds, and discuss in detail how the constraints due to the worldline gauge symmetries produce geometrical equations for higher spin fields, i.e. equations written in terms of generalized curvatures. On flat space the algebra of constraints is linear, and one can integrate part of the constraints by introducing gauge potentials. This way the equivalence of the geometrical formulation with the standard formulation in terms of gauge potentials is made manifest. On (A)dS backgrounds the algebra of constraints becomes quadratic, nevertheless one can use it to extend much of the previous analysis to this case. In particular, we derive general formulas for expressing the curvatures in terms of gauge potentials and discuss explicitly the cases of spin 2, 3 and 4.Comment: 35 pages, added reference

Physics of reshock and mixing in single-mode Richtmyer-Meshkov instability

The ninth-order weighted essentially nonoscillatory (WENO) shock-capturing method is used to investigate the physics of reshock and mixing in two-dimensional single-mode Richtmyer-Meshkov instability to late times. The initial conditions and computational domain were adapted from the Mach 1.21 air (acetone)/SF6 shock tube experiment of Collins and Jacobs [J. Fluid Mech. 464, 113 (2002)]: the growth of the bubble and spike amplitudes from fifth- and ninth-order WENO simulations of this experiment were compared to the predictions of linear and nonlinear amplitude growth models, and were shown to be in very good agreement with the experimental data prior to reshock by Latini, Schilling, and Don [Phys. Fluids 19, 024104 (2007)]. In the present investigation, the density, vorticity, baroclinic vorticity production, and simulated density Schlieren fields are first presented to qualitatively describe the reshock process. The baroclinic circulation deposition on the interface is shown to agree with the predictions of the Samtaney-Zabusky model and with linear instability theory. The time evolution of the positive and negative circulation on the interface is considered before and after reshock: it is shown that the magnitudes of the circulations are equal before as well as after reshock, until the interaction of the reflected rarefaction with the layer induces flow symmetry breaking and different evolutions of the magnitude of the positive and negative circulation. The post-reshock mixing layer growth is shown to be in generally good agreement with three models predicting linear growth for a short time following reshock. Next, a comprehensive investigation of local and global mixing properties as a function of time is performed. The distribution and amount of mixed fluid along the shock propagation direction is characterized using averaged mole fraction profiles, a fast kinetic reaction model, and mixing fractions. The modal distribution of energy in the mixing layer is quantified using the spectra of the fluctuating kinetic energy, fluctuating enstrophy, pressure variance, density variance, and baroclinic vorticity production variance. It is shown that a broad range of scales already exists prior to reshock, indicating that the single-mode Richtmyer-Meshkov instability develops nontrivial spectral content from its inception. The comparison of the spectra to the predictions of classical inertial subrange scalings in two-dimensional turbulence shows that the post-reshock spectra may be consistent with many of these scalings over wave number ranges less than a decade. At reshock, fluctuations in all fields (except for the density) are amplified across all scales. Reshock strongly amplifies the circulation, profiles, and mixing fractions, as well as the energy spectra and statistics, leading to enhanced mixing followed by a decay. The mole and mixing fraction profiles become nearly self-similar at late times following reshock; the mixing fraction exhibits an approach toward unity across the layer at the latest time, signifying nearly complete mixing of the gases. To directly quantify the amplification of fluctuations by reshock, the previously considered quantities are compared immediately after and before reshock. Finally, to investigate the decay of fluctuations in the absence of additional waves interacting with the mixing layer following reshock, the boundary condition at the end of the computational domain is changed from reflecting to outflow to allow the reflected rarefaction wave to exit the domain. It is demonstrated that the reflected rarefaction has an important role in breaking symmetry and achieving late-time statistical isotropy of the velocity field

Test of different sensitizing dyes in dye-sensitized solar cells based on Nb2O5 photoanodes

High-performance dyes routinely employed in TiO2-based dye-sensitized solar cells (DSSCs) were tested in cells assembled using Nb2O5 nanostructure-based photoanodes. The sensitizers were chosen among both metal-complex (two Ru-based, N749 and C106, and one Zn-based dye, DNF12) and metal-free organic dyes (DNF01, DNF11 and DNF15). Two different sensitization processes were performed: the one commonly used for TiO2 photoanodes, and a new process relying on high pressure by autoclavation. The assembled cells were characterized by current density–voltage (J–V) curves under air mass (AM) 1.5 G illumination and in the dark, incident photon-to-current efficiency (IPCE) measurements, and electrochemical impedance spectroscopy. The tested cells show different proportional efficiencies of the dyes under investigation for Nb2O5- and TiO2-based devices. Furthermore, the results were compared with those obtained in our previous work using N719 anchored on Nb2O5. A remarkable efficiency value of 4.4% under 1 sun illumination was achieved by coupling the C106 dye with a nonvolatile electrolyte. This value is higher than the one attained under the same conditions by using N719

Massive and massless higher spinning particles in odd dimensions

We study actions for massive bosonic particles of higher spins by dimensionally reducing an action for massless particles. For the latter we take a model with a SO(N) extended local supersymmetry on the worldline, that is known to describe massless (conformal) particles of higher spins in flat spacetimes of even dimensions. Dimensional reduction produces an action for massive spinning particles in odd dimensions. The field equations that emerge in a quantization a la Dirac are shown to be equivalent to the Fierz-Pauli ones. The massless limit generates a multiplet of massless states with higher spins, whose first quantized field equations have a geometric form with fields belonging to various types of Young tableaux. These geometric equations can be partially integrated to show their equivalence with the standard Fronsdal-Labastida equations. We covariantize our model to check whether an extension to curved spacetimes can be achieved. Restricting to (A)dS spaces, we find that the worldline gauge algebra becomes nonlinear, but remains first class. This guarantees consistency on such backgrounds. A light cone analysis confirms the presence of the expected propagating degrees of freedom. A covariant analysis is worked out explicitly for the massive case, which is seen to give rise to the Fierz-Pauli equations extended to (A)dS spaces. It is worth noting that in D=3 the massless limit of our model when N goes to infinity has the same field content of the Vasiliev's theory that accommodates each spin exactly once.Comment: 31 page

Estintione e violenza : il carattere distruttivo nell'opera di Thomas Bernhard

In tutte le sue diverse declinazioni, il dissenso nei riguardi della propria patria costituisce un motivo centrale e ricorrente nella cultura austriaca del Novecento. Come in una sorta di contagio il carattere distruttivo, soprattutto nel Secondo dopoguerra, si è diffuso – e continua a diffondersi – ad ampio raggio: esempi emblematici sono offerti da Peter Handke, con i suoi "insulti all'Austria" urlati dall'esilio francese, dalla prosa sofferta di Ingeborg Bachmann, che ha riparato a Roma, dalla scrittura caustica del Premio Nobel Elfriede Jelinek, o da quella estrema di Werner Schwab. Ma anche da autori meno noti, come Anna Mitgutsch, che, dagli Stati Uniti, non manca di esternare la sua avversione nei confronti del paese d'origine, o dai toni aspri dello scrittore Robert Menasse. Lo steso vale per il campo cinematografi co, con il cinema a tinte forti di Ulrich Seidl. Una passion durevole, quella degli austriaci per i toni antipatriottici, una tradizione che vanta come anticipatore il sacro nome del grande polemista Karl Kraus, o forse, ancor prima, affonda le sue radici nel teatro di Raimund Nestroy. Se l'ostilità per l'Austria sembra un Leitmotiv della grande retorica austriaca, Thomas Bernhard (1931-1989) può essere a giusto titolo considerate non solo come il legittimo erede di questa grande tradizione, ma anche come il suo massimo rappresentante. Ereditare signifi ca infatti fare i conti in modo cinico e rischioso con l'orizzonte vitale e culturale a cui si appartiene e che ci appartiene. Autore diffi cile, maniacale, irriverente ed eccessivo, Bernhard si è imposto all'attenzione del grande pubblico per la sua appassionata denuncia dell'universo austriaco, e della sua atmosfera piccolo-borghese, conformista, ottusamente fi loclericale. I suoi frequenti interventi pubblici – ora raccolti e pubblicati con il titolo Meine Preise – presentano come unico obiettivo la provocazione, e di fatto hanno suscitato scandali, polemiche. Un esempio per tutti è il discorso tenuto nel 1967 per il Conferimento del "Premio Nazionale Austriaco per la Letteratura": "Noi siamo austriaci, siamo apatici, siamo la vita intesa come ignobile disinteresse nei confronti della vita, siamo, nel processo naturale, la megalomania intesa come futuro". Un testo scritto con un inchiostro che è succo di nervi. Ma questa è solo una delle tante puntate che vedono Bernhard in polemica con i suoi connazionali, e che hanno contribuito a designarlo come un Nestbeschmutzer. È questa una visione decisamente riduttiva. Lo humour nero bernhardiano, che di certo ci ha regalato pagine irresistibili, nasce da un sentimento di amore e odio nei confronti dell'Austria. Per dirla in altri termini, è sintomo non di una nevrastenica insofferenza (di cui poco ci importerebbe), quanto piuttosto di una Unheimlichkeit, di un sofferto legame con l'origine. Su questo punto, per ora solo accennato, avrò occasione di tornare