2,189 research outputs found
Studio della propagazione della frattura in polibutene per tubi
Il Polibutene-1 isotattico (i-PB1) è un materiale polimerico usato per la produzione di tubi per il trasporto di fluidi in pressione. In questo lavoro si sono studiati due tipi di i-PB1 prodotti da Basell che differiscono per grado di isotatticità. Si sono condotte prove di frattura a diverse temperature e velocità di spostamento imposte. Si è utilizzata una configurazione di flessione su provini con singolo intaglio (SENB) unitamente a quella di doppia trave a sbalzo (DCB), quest’ultima limitatamente allo studio della fase di propagazione. Al fine di individuare con precisione l’innesco della frattura e la velocità di propagazione della stessa si è fatto ricorso a metodi ottici. Dal punto di vista fenomenologico durante la propagazione si assiste alla formazione di zone in cui il materiale risulta fortemente stirato. La frattura in esse avanza con una lacerazione continua che si alterna a salti repentini in occasione del
brusco cedimento di queste zone, associato a conseguenti cadute del carico. Questa parziale instabilità è stata osservata sui due materiali per entrambe le configurazioni di prova. I risultati ottenuti sono stati interpretati seguendo l’approccio della meccanica della frattura e applicando uno schema di riduzione di tipo tempo-temperatura che ha permesso di descrivere il comportamento viscoelastico del
materiale su un intervallo temporale di diverse decadi. I risultati hanno permesso di applicare un modello analitico per la previsione della vita utile di tubi in pressione. Il modello si è mostrato in buon accordo con i dati sperimentali disponibili da prove condotte su tubi dello stesso materiale
Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties
The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on
the electronic and vibrational properties is investigated using surface
electron spectroscopy in combination with density functional theory (DFT)
calculations. For the surface mediated dissociation of H2 and the subsequent
adsorption of H, an energy barrier of 0.55 eV has to be overcome. The
calculated kinetic surface phase diagram indicates that the reaction is
kinetically hindered at low pressures and low temperatures. At higher
temperatures ab-initio thermodynamics show, that the H-free surface is
energetically favored. To validate these theoretical predictions experiments at
room temperature and under ultrahigh vacuum conditions were performed. They
reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100)
surface. Only activated atomic hydrogen atoms attach to the surface. At
temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed
hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface
atoms and result in an inversion of the Ga-N surface dimer buckling. The
signatures of the Ga-H and N-H vibrational modes on the H-covered surface have
experimentally been identified and are in good agreement with the DFT
calculations of the surface phonon modes. Both theory and experiment show that
H adsorption results in a removal of occupied and unoccupied intragap electron
states of the clean GaN(1-100) surface and a reduction of the surface upward
band bending by 0.4 eV. The latter mechanism largely reduces surface electron
depletion
Are Assumptions of Well-Known Statistical Techniques Checked, and Why (Not)?
A valid interpretation of most statistical techniques requires that one or more assumptions be met. In published articles, however, little information tends to be reported on whether the data satisfy the assumptions underlying the statistical techniques used. This could be due to self-selection: Only manuscripts with data fulfilling the assumptions are submitted. Another explanation could be that violations of assumptions are rarely checked for in the first place. We studied whether and how 30 researchers checked fictitious data for violations of assumptions in their own working environment. Participants were asked to analyze the data as they would their own data, for which often used and well-known techniques such as the t-procedure, ANOVA and regression (or non-parametric alternatives) were required. It was found that the assumptions of the techniques were rarely checked, and that if they were, it was regularly by means of a statistical test. Interviews afterward revealed a general lack of knowledge about assumptions, the robustness of the techniques with regards to the assumptions, and how (or whether) assumptions should be checked. These data suggest that checking for violations of assumptions is not a well-considered choice, and that the use of statistics can be described as opportunistic
Spin splitting in a polarized quasi-two-dimensional exciton gas
We have observed a large spin splitting between "spin" and
heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs
quantum wells in the absence of any external magnetic field. Time-resolved
photoluminescence spectroscopy, under excitation with circularly polarized
light, reveals that, for high excitonic density and short times after the
pulsed excitation, the emission from majority excitons lies above that of
minority ones. The amount of the splitting, which can be as large as 50% of the
binding energy, increases with excitonic density and presents a time evolution
closely connected with the degree of polarization of the luminescence. Our
results are interpreted on the light of a recently developed model, which shows
that, while intra-excitonic exchange interaction is responsible for the spin
relaxation processes, exciton-exciton interaction produces a breaking of the
spin degeneracy in two-dimensional semiconductors.Comment: Revtex, four pages; four figures, postscript file Accepted for
publication in Physical Review B (Rapid Commun.
Assembly of the Fungal SC3 Hydrophobin into Functional Amyloid Fibrils Depends on Its Concentration and Is Promoted by Cell Wall Polysaccharides
Spin Dynamics of Cavity Polaritons
We have studied polariton spin dynamics in a GaAs/AlGaAs microcavity by means
of polarization- and time-resolved photoluminescence spectroscopy as a function
of excitation density and normal mode splitting. The experiments reveal a novel
behavior of the degree of polarization of the emission, namely the existence of
a finite delay to reach its maximum value. We have also found that the
stimulated emission of the lower polariton branch has a strong influence on
spin dynamics: in an interval of 150 ps the polarization changes from
+100% to negative values as high as -60%. This strong modulation of the
polarization and its high speed may open new possibilities for spin-based
devices.Comment: 4 pages, 3 eps figures, RevTeX, Physical Review B Rapid (submitted
Spin controlled atom-ion inelastic collisions
The control of the ultracold collisions between neutral atoms is an extensive
and successful field of study. The tools developed allow for ultracold chemical
reactions to be managed using magnetic fields, light fields and spin-state
manipulation of the colliding particles among other methods. The control of
chemical reactions in ultracold atom-ion collisions is a young and growing
field of research. Recently, the collision energy and the ion electronic state
were used to control atom-ion interactions. Here, we demonstrate
spin-controlled atom-ion inelastic processes. In our experiment, both
spin-exchange and charge-exchange reactions are controlled in an ultracold
Rb-Sr mixture by the atomic spin state. We prepare a cloud of atoms in a
single hyperfine spin-state. Spin-exchange collisions between atoms and ion
subsequently polarize the ion spin. Electron transfer is only allowed for
(RbSr) colliding in the singlet manifold. Initializing the atoms in various
spin states affects the overlap of the collision wavefunction with the singlet
molecular manifold and therefore also the reaction rate. We experimentally show
that by preparing the atoms in different spin states one can vary the
charge-exchange rate in agreement with theoretical predictions
O-006. Computerized fast-motion video capture of mouse embryo development and hatching: a comparative analysis between normal and assisted hatching
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