Shear driven planar Couette and Taylor-like instabilities for a class of compressible isotropic elastic solids

We study the possibility for an isotropic elastic body to support forms of instability induced by shear stress states which are reminiscent of the planar Couette and the Taylor–Couette patterns observed in the flow of viscous fluids. Here, we investigate the emergence of bifurcating periodic deformations for an infinitely long compressible elastic block confined between and attached to parallel plates which are subject to a relative shear displacement. We specialize our analysis by considering a generalized form of the Blatz–Ko strain energy function and show through numerical representative examples that planar Couette modes are always preferred with respect to the twisting Taylor–Couette modes. Finally, we introduce a suitably restricted form of the strong ellipticity condition for the incremental elasticity tensor and discuss its significance in this bifurcation problem.     

Heavy fragments emission in the 84Kr on 12C reaction at 35 MeV/nucleon

Heavy residues of collisions of ⁸⁴Kr + ¹²C at 35 MeV/amu were measured using time of flight ΔE?E telescopes. An important part of the total cross section leads to events that have all the characteristics of an equilibrated composite system decaying by heavy fragment emission. The experimental yields are well reproduced using a statistical model.     

Flow through geometrically irregular media with lattice gas automata

Summary The determination of the permeability is an interesting problem of fluid dynamics of wide interdisciplinary concern. Many authors approached this subject by developing numerical models of flows through porous media at either macro-scale and micro-scale. According to the latter point of view, we present in this paper a verification of Darcy's law and a first determination of the permeability starting by the knowledge of the microstructure of a three-dimensional random medium. The flow is here reproduced by using a particular class of cellular automata with the Boltzmann approximation.

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Sommario La determinazione del coefficiente di permeabilità rappresenta un interessante problema comune a molte discipline. Tale determinazione viene affrontata dai diversi autori o tramite la conoscenza di grandezze macroscopiche — come la resistività o la porositá — ovvero partendo dalla conoscenza della microstruttura del mezzo poroso e cercando di simulare il processo di moto fluido che si svolge lungo i canalicoli del mezzo stesso. Seguendo quest'ultimo punto di vista, viene qui di seguito presentata una tecnica alternativa per la verifica della legge di Darcy e, quindi, per una prima valutazione del coefficiente di permeabilità per un mezzo tridimensionale complesso. La simulazione dei moti fluidi viene effettuata tramite una particolare classe di gas reticolari che evolvono secondo le regole degli automi cellulari.

     

Dark material by fast protons on frozen methane

Summary We describe in detail techniques and qualitative results of experiments conduced in our laboratory at Catania (Italy) on the production of stable residing dark (polymerlike) material obtained by bombarding methane frosts with fast (100÷1500) keV proton beams. We give, in particular, results on the cross-section of the process and show that this occurs along the intere ion path so that thick residues are obtained. The thickness of the residues has been measured by bothin situ and remote analysis that also give information on the surface roughness and on the chemical composition. The purpose of the experiments is to get some insight into the possibility that the interaction between energetic ions and hydrocarbon frosts is responsible of the build-up of dark carbonaceous materials widely thought to be present in many astrophysical scenarii. Some astrophysical applications are, in fact, discussed in a correlated paper.

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Riassunto Si descrivono in dettaglio tecniche e risultati quantitativi di esperimenti condotti nel nostro laboratorio di Catania e riguardanti la produzione di materiali scuri (di tipo polimerico) resistenti a temperatura ambiente ed ottenuti dal bombardamento di metano congelato tramite fasci di protoni veloci ((100÷1500) keV). In particolare, si presentano risultati sulla sezione d'urto del processo e si mostra che questo avviene lungo l'interocammino dello ione cosicchè si ottengono residui spessi. Lo spessore di essi è stato misurato siain situ che in un'analisi remota e ciò consente di ottenere informazioni sulla topografia superficiale e sulla composizione chimica. Lo scopo degli esperimenti è di far luce sulla possibilità che l'interazione tra ioni energetici e idrocarburi congelati sia responsabile della formazione di materiale scuro carbonioso che si ritiene ampiamente presente in molti contesti astrofisici. Alcune applicazioni astrofisiche sono infatti discusse in un lavoro connesso al presente.

     

On the 18O positive-parity rotational band

Deuteron-alpha angular correlations have been measured for the reaction ${}^{14}\text{C}\text{(}{}^6\text{Li,d}\text{)}{}^{18}\text{O}{}^1\text{→α}{}_0\text{+}{}^{14}\text{C at}\text{E(}{}^6\text{Li}\text{)=34}\text{MeV}$ and θ_d^lab=10°. Transitions involving the 11.69 MeV (6⁺) and the 17.6±0.2 MeV ¹⁸O states have been analyzed. Spin and parity are confirmed for the known 11.69 MeV (6⁺) state and assigned to be 8⁺ for the 17.6 MeV level. This last is suggested to be the fifth member of the positive-parity ¹⁸O rotational band built on the 3.63 MeV (0⁺) level.     

Determination of picogram amounts of technetium in environmental samples by neutron activation analysis

A procedure for the determination of trace amounts of ⁹⁹Tc in filter paper and vegetation samples by neutron activation analysis has been developed. The procedure consists of the following major steps: (a) pre-irradiation separation of technetium from the sample and purification of the technetium fraction; (b) thermal neutron irradiation of the ⁹⁹Tc fraction to produce ¹⁰⁰Tc ; (c) post-irradiation separation and purification of ¹⁰⁰Tc from other activated nuclides: (d) counting of the 16-sec ¹⁰⁰Tc in a low-background β-counter.

The estimated detection limits for ⁹⁹Tc by this procedure with irradiations at a thermal neutron flux of about 5.10¹³ n cm^-2 sec^-1 are: 5.lO^-12g ⁹⁹Tc in filter paper samples, and 9. 10^-12 g ⁹⁹Tc in vegetation samples.     

The influence of the boat-to-chair conversion on the demetallation of the nickel(II) complex of an open-chain tetramine containing a piperazine fragment

The nickel(II) complex with an open chain tetramine containing a piperazine fragment (1) displays an unusual resistance to demetallation in acidic solution and exhibits a lifetime of about five minutes in a solution 0.1 M in HClO4 and 7.0 M in NaClO4. Sluggishness with respect to the demetallation is ascribed to the occurrence of the boat-to-chair conformational conversion of the piperazine fragment, which implies the passage through the highly energetic half-boat transition state. The use of a high concentration of the inert electrolyte induces a 'salting out' effect on both thermodynamics (stability of metal complexes is enhanced) and kinetics (resistance to demetallation is increased).     

Solubility and thermal stability of some amino-mellitate compounds

By reaction between the anion of mellitic acid (benzenehexacarboxylic acid) and some protonated linear polyamines (diethylenetriamine, triethylenetetramine, tetraethylene-pentamine, pentaethylenehexamine, spermidine, and spermine), fairly insoluble complexes have been obtained, with the general formula (amine)(x)(mellitate)H(6) (diethylenetriamine and spermidine, x=1; triethylenetetramine and spermine, x=0.75; tetraethylenepentamine, x=0.6 and 0.8; pentaethylenehexamine, x=0.5). K(s0) values for these complexes have been determined at I=0 mol dm(-3) and T=25 degrees C (logK(s0) ranges between -48.2 and -56.6). The solubility has been studied as a function of pH and of ionic strength. The thermal analysis, performed using air or argon flow, showed that all the solids behave in a similar way. In the range 20-120 degrees C the loss of hydration water occurs, and in the range 150-350 degrees C the first step of non oxidative decomposition takes place, with complete decomposition at 650 degrees C in air flow, whilst in argon flow the decomposition is still incomplete at 900 degrees C. Preliminary results of a parallel diffractometric study are also reported.     

Effect of metal oxides on the evolution of aromatic hydrocarbons in the thermal decomposition of PVC

The thermal decomposition of poly(vinyl chloride) (PVC) mixed with several metal oxides was investigated by direct pyrolysis in a mass spectrometer (MS) and flash pyrolysis–gas chromatography. Our results show that the thermal decomposition of PVC occurs in two stages. Unsubstituted aromatic hydrocarbons (benzene, naphthalene, and anthracene) are evolved mainly in the first stage, alkyl-aromatics (e.g., toluene) in the second. Although the addition of some metal oxides results in an overall suppression of aromatic hydrocarbons, the unsubstituted aromatics are much more suppressed with respect to alkyl-aromatics. Furthermore, the formation of ZnCl₂ and SnCl₄ was revealed by the mass spectra of PVC–metal oxide pyrolysates. This suggests that, at least in these two cases, metal chlorides are responsible for aromatic hydrocarbon suppression. With this information a detailed reaction mechanism could be formulated for the thermal degradation of PVC.     

Dark material by fast protons on frozen methane

Summary In the light of the experimental results presented in a correlated paper on the production of organic residues by bombarding hydrocarbon frosts with fastions, we show that substantial amount of these materials can by synthesized on insterstellar grains and objects in the solar system by cosmic-ray fluence. The IR spectra of laboratory residues are shown. The main features we find at 2.9, 3.4, 6.2 and 6.8 μm resemble those observed on interstellar sources and on some meteoritic extracts. Some implications these results have on grain models and evolution and on the possible similarity between interstellar grains and objects in the solar system are also discussed.

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Riassunto Alla luce di risultati sperimentali, presentati in un precedente lavoro di questa serie e concernenti la produzione di residui organici del bombardamento di idrocarburi congelati tramite ioni veloci, noi dimostriamo che apprezzabili quantit? di questi materiali possono essere sintetizzati sui grani interstellari e su oggetti nel sistema solare a causa del bombardamento di raggi cosmici. Sono mostrati gli spettri IR dei residui di laboratorio le cui bande prineipali a 2.9, 3.4, 6.2 e 6.8 μm ricordano quelle osservate su sorgenti astrofisiche e su alcuni estratti da meteoriti. Si discutono anche alcune implicazioni che questi risultati hanno sui modelli e sull'evoluzione dei grani interstellari e sulle possibili similarità tra questi grani ed oggetti nel sistema solare.

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Резюме йспользуя экспериментальные резуьтаты, полученные ранее, по образованию органических остатков при бомбардировке замороженных углеводородов быстрыми ионами, мы показываем, что затемненные материалы могут быть синтезированы в заметных количествах на межзвездных зернах и обБектах в солнечной системе под действием космических лучей. Приводятся инфракрасные спектры лабораторных остатков. Главные особенности этих спектров при 2.9, 3.4, 6.2 и 6.8 мкм похожи на особенности, обнаруженные на межзвездных источниках и на некоторых метеорных экстрактах. Обсуждается применение полученных результатов к моделям зерен и эволюции. Также рассматривается возможное сходство между межзвездными зернами и обБектами в солнечной системе.