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

Influence of morpho-structural parameters on the environmental stress cracking of polyethylene

The environmental stress cracking resistance (ESCR) of four polyethylenes in active medium (10% Tergitol solution) was investigated. The four materials were chosen to explore a broad range of performance and applications; they are listed according to increasing expected ESCR: - an injection moulding, low-molecular weight (MW) HDPE homopolymer - two rotomoulding LLDPE copolymers with a different comonomer (butene or hexene) - a blown film extrusion high MW HDPE copolymer (hexene) The fracture resistance of the hexene LLDPE copolymer was expected to be slightly higher than that of the butene one, because of the longer alkyl group. The two materials were chosen to challenge the ability of the different testing methods to discriminate between similar levels of ESCR. Several analytical techniques were employed to obtain relevant morpho-structural parameters: density, degree of crystallinity, MW distribution, short chain branch content and average lamellar thickness. ESCR was evaluated by employing three well-known but widely different approaches: - the Bell Telephone test (ASTMD1693), performed on notched specimens immersed in the active environment at 50°C - strain hardening modulus (SHM), obtained from tensile tests performed in air at 80°C - fracture mechanics (FM) tests on three- and four-point bending notched specimens, performed at varying temperatures both in air and in the active environment; two different loading histories were considered (creep and constant displacement rate) MW distribution seems to have a greater effect on ESCR behaviour with respect to other morpho-structural parameters. There was instead a strong consistency of the different ESCR testing methods, so that SHM could replace ASTMD1693 as an industrial test aimed at ranking ESCR of polyethylenes. FM, while being more complex to carry out, provides a wealth of additional information which could be used to actually predict the lifetime of products working in an ESC environment

Spin splitting in a polarized quasi-two-dimensional exciton gas

We have observed a large spin splitting between "spin" $+1$ and $-1$ 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.

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 $\sim$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