5,711 research outputs found
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
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
A multiple scales approach to maximal superintegrability
In this paper we present a simple, algorithmic test to establish if a
Hamiltonian system is maximally superintegrable or not. This test is based on a
very simple corollary of a theorem due to Nekhoroshev and on a perturbative
technique called multiple scales method. If the outcome is positive, this test
can be used to suggest maximal superintegrability, whereas when the outcome is
negative it can be used to disprove it. This method can be regarded as a finite
dimensional analog of the multiple scales method as a way to produce soliton
equations. We use this technique to show that the real counterpart of a
mechanical system found by Jules Drach in 1935 is, in general, not maximally
superintegrable. We give some hints on how this approach could be applied to
classify maximally superintegrable systems by presenting a direct proof of the
well-known Bertrand's theorem.Comment: 30 pages, 4 figur
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 , , , constructing the corresponding quantum
metrics and providing an explicit presentation of the quantized coordinate
algebras. In particular, we focus on the Kleinian signature . 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 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
Patterns of co-occurrence of rare and threatened species in winter arable plant communities of Italy
Detecting patterns of species co-occurrence is among the main tasks of plant community ecology. Arable plant communities are important elements of agroecosystems, because they support plant and animal biodiversity and provide ecosystem services. These plant communities are shaped by both agricultural and environmental drivers. The pressure of intensive agriculture worldwide has caused the decline of many characteristic arable species and communities. Italy is the European country where arable plant biodiversity is the best preserved. In this study, we assessed the patterns of co-occurrence of rare and threatened arable plants in 106 plots of winter arable vegetation located from Piedmont to Calabria, in the mainland part of the country. For this purpose, we based our
investigation on the analysis of a recently acquired dataset and on the European list of rare and threatened arable plants. We highlight how dierent species of conservation interest tend to occur in the same community. On the other hand, generalist and more competitive taxa show similar patterns of co-occurrence. We suggest that single species of conservation value could be suitable indicators of
a well-preserved community. On the other hand, to be eective, conservation strategies should target the whole community, rather than single species
Group Momentum Space and Hopf Algebra Symmetries of Point Particles Coupled to 2+1 Gravity
We present an in-depth investigation of the momentum
space describing point particles coupled to Einstein gravity in three
space-time dimensions. We introduce different sets of coordinates on the group
manifold and discuss their properties under Lorentz transformations. In
particular we show how a certain set of coordinates exhibits an upper bound on
the energy under deformed Lorentz boosts which saturate at the Planck energy.
We discuss how this deformed symmetry framework is generally described by a
quantum deformation of the Poincar\'e group: the quantum double of . We then illustrate how the space of functions on the group
manifold momentum space has a dual representation on a non-commutative space of
coordinates via a (quantum) group Fourier transform. In this context we explore
the connection between Weyl maps and different notions of (quantum) group
Fourier transform appeared in the literature in the past years and establish
relations between them
Poincare-Einstein Holography for Forms via Conformal Geometry in the Bulk
We study higher form Proca equations on Einstein manifolds with boundary data
along conformal infinity. We solve these Laplace-type boundary problems
formally, and to all orders, by constructing an operator which projects
arbitrary forms to solutions. We also develop a product formula for solving
these asymptotic problems in general. The central tools of our approach are (i)
the conformal geometry of differential forms and the associated exterior
tractor calculus, and (ii) a generalised notion of scale which encodes the
connection between the underlying geometry and its boundary. The latter also
controls the breaking of conformal invariance in a very strict way by coupling
conformally invariant equations to the scale tractor associated with the
generalised scale. From this, we obtain a map from existing solutions to new
ones that exchanges Dirichlet and Neumann boundary conditions. Together, the
scale tractor and exterior structure extend the solution generating algebra of
[31] to a conformally invariant, Poincare--Einstein calculus on (tractor)
differential forms. This calculus leads to explicit holographic formulae for
all the higher order conformal operators on weighted differential forms,
differential complexes, and Q-operators of [9]. This complements the results of
Aubry and Guillarmou [3] where associated conformal harmonic spaces parametrise
smooth solutions.Comment: 85 pages, LaTeX, typos corrected, references added, to appear in
Memoirs of the AM
- …