1,720,982 research outputs found
First-Principles Simulation—Nano-Theory
Full text linkFirst-principles (or ab initio) simulation is one of the most significant theoretical approaches to study and model systems at the atomistic level [...
Monitoring water and oxygen splitting at graphene edges and folds: Insights into the lubricity of graphitic materials
Full text linkThe functionality of graphene as lubricant material is affected by extrinsic factors, such as the film thickness and the environmental conditions. Graphite lubricating capability depends as well on air humidity. To accurately describe the tribochemistry mechanisms underlying these behaviours we adopt a Quantum Mechanics/Molecular Mechanics approach. We show that reactive edges are able to cause a huge friction increase, which is quantified for graphene flakes between sliding diamond surfaces. Moreover, folds spontaneously formed in single layer graphene under tribological conditions are shown to be highly reactive due to carbon re-hybridization. This observation offers a new hint for interpreting the dependence of graphene friction on the number of layers. Both water and oxygen molecules are found to be effective in quenching the reactivity of defects by dissociative chemisorption. However, peculiar mechanisms of water molecules makes humidity more effective than oxygen for enabling the lubricity of graphitic media. They include collective processes as Grotthus-like proton diffusion enhanced by confinement, and the strong change in hydrophilic character of the passivated media. This comprehensive study sheds a new light on debated issues of graphene and graphite tribology, and highlights the potentiality of these materials for metal-free catalysis, e.g., for H production by water splitting
Il virus dell’avvizzimento maculato del pomodoro in coltivazioni di Papaver nudicaule
No full textIl Papavero d’Islanda (Papaver nudicaule; Papaveraceae) costituisce una delle più importanti coltivazioni del distretto floricolo di Sanremo. Si tratta di una specie a propagazione gamica: la semina si effettua in giugno/luglio (direttamente a dimora seguita da diradamento, oppure in contenitori alveolari) per ottenere la fioritura in autunno/inverno. Per quanto riguarda le operazioni colturali e le specifiche esigenze di questo Papavero (natura del terreno, pH, concimazioni, irrigazioni, ecc.,) si rimanda all’articolo apparso su Clamer informa 7/8 del 2003, nel quale viene anche fornito un quadro completo delle malattie più pericolose e frequenti negli impianti liguri. In particolare, vengono indicati i patogeni fungini che attaccano prevalentemente le radici (Phytium spp., Phytophtora spp., Sclerotinia spp. e Rhizoctonia solani), l’apparato fogliare (Erysiphe polygoni, Peronospora arborescens, Botrytis cinerea) o l’intera pianta (Helminthosporium spp. e Verticillium dahliae). A questi patogeni fungini si aggiungono pericolosi insetti fitofagi fra cui il tripide Frankliniella occidentalis responsabile di arricciamenti e deformazioni ben visibili su germogli e petali. In questo quadro complessivo delle malattie parassitarie del Papavero d’Islanda non compaiono le virosi.
Da indagini eseguite nel 2006 in alcune aziende dell’estremo Ponente Ligure specializzate nella produzione di fiori recisi di Papavero è risultata invece la presenza, per la prima volta, del virus dell’avvizzimento maculato del pomodoro (tomato spotted wilt virus: TSWV), da solo od in associazione con quello del mosaico del cetriolo (cucumber mosaic virus: CMV).
Di TSWV ci siamo già occupati di recente descrivendo gravi malattie che hanno interessato il ruscus (Danae racemosa) e la calla bianca (Zantedeschia aethiopica) coltivati sempre nella zona sanremese. Come è noto, si tratta di un virus altamente polifago che sta attualmente compromettendo la produzione di alcune importanti specie ornamentali da fiore reciso di tutta la Liguria, come ad esempio il ranuncolo.
Alla luce dei primi ritrovamenti di TSWV e di CMV in Papavero d’Islanda, appare necessario ed urgente informare i coltivatori affinché adottino tutte le misure di profilassi più idonee che ne scongiurino l’ulteriore diffusione.
Materiali e metodi, Risultati
Reperimento dei campioni - I campioni di Papavero d’Islanda oggetto di studio sono stati reperiti nel corso di sopralluoghi eseguiti nei mesi di ottobre/novembre 2006. Gli impianti esaminati, in piena aria, erano costituiti da piantine ottenute da seme proveniente dall’azienda stessa oppure da materiale da ricoltivare acquistato da vivaisti locali. Alcune delle piante in produzione, della grandezza di circa 15-20 cm evidenziavano sintomi fogliari consistenti in macchie clorotiche e necrotiche (“spots”) e necrosi diffuse. Le lamine apparivano più strette del normale e vistosamente malformate. Nel complesso, la taglia delle piante era ridotta. Da un primo esame visivo, i sintomi, e soprattutto le ampie zone necrotiche, sono stati diagnosticati quali tipici dell’infezione da TSWV. Considerando però che in Letteratura alcune specie del genere Papaver sono state indicate quali ospiti naturali di altri virus, fra cui CMV (tabella 1), e che la diffusione di entrambi i virus (TSWV e CMV) nelle coltivazioni floricole ed orticole della Liguria è attualmente elevata, sono state effettuate specifiche analisi per rilevarne l’eventuale presenza nei campioni in esame.
Analisi virologiche: Nel succo fogliare estratto da Papavero d’Islanda sintomatico, ed osservato al microscopio elettronico Philips CM10 (tecnica del leaf-dip), non sono state individuate particelle virali allungate (filamentose e/o bastocelliformi). Al fine di individuare la presenza di TSWV e/o di CMV è stata applicata la tecnica ELISA nella versione indiretta (PAS-ELISA) utilizzando il siero policlonale anti-TSWV (PVAS-450) e quello anti-CMV (PVAS-30) (American Type Culture Col..
Superlubricity in phosphorene identified by means of ab initio calculations
Full text linkPhosphorene possesses a great potential for tribological applications due to its layered structure and for the capability of phosphorus to reduce friction and adhesion in steel-steel contacts. Here we present a comprehensive analysis of the static tribological properties of phosphorene based on first principles calculations. The most suitable exchange-correlation functional for describing the structural and electronic properties of multilayer phosphorene is carefully selected. The interlayer binding energy and shear strength are then calculated for two relative orientations of the layers. Layers stacked with the same orientation (armchair-armchair and zigzag-zigzag) are slippery as common solid lubricants, as MoS2 and graphite. While the armchair-zigzag orientation shows a remarkable superlubricity, with a reduction of one order of magnitude for the shear stress. We uncover the microscopic origin of such superlubric phase by analyzing the electronic charge at the layer interface
A transferable prediction model of molecular adsorption on metals based on adsorbate and substrate properties
Full text linkSurface adsorption is one of the fundamental processes in numerous fields, including catalysis, the environment, energy and medicine. The development of an adsorption model which provides an effective prediction of binding energy in minutes has been a long term goal in surface and interface science. The solution has been elusive as identifying the intrinsic determinants of the adsorption energy for various compositions, structures and environments is non-trivial. We introduce a new and flexible model for predicting adsorption energies to metal substrates. The model is based on easily computed, intrinsic properties of the substrate and adsorbate, which are the same for all the considered systems. It is parameterised using machine learning based on first-principles calculations of probe molecules (e.g., H2O, CO2, O2, N2) adsorbed to a range of pure metal substrates. The model predicts the computed dissociative adsorption energy to metal surfaces with a correlation coefficient of 0.93 and a mean absolute error of 0.77 eV for the large database of molecular adsorption energies provided by Catalysis-Hub.org which have a range of 15 eV. As the model is based on pre-computed quantities it provides near-instantaneous estimates of adsorption energies and it is sufficiently accurate to eliminate around 90% of candidates in screening study of new adsorbates. The model, therefore, significantly enhances current efforts to identify new molecular coatings in many applied research fields
Interfacial Charge Density and Its Connection to Adhesion and Frictional Forces
Full text linkWe derive a connection between the intrinsic tribological properties and the electronic properties of a solid interface. In particular, we show that the adhesion and frictional forces are dictated by the electronic charge redistribution occurring due to the relative displacements of the two surfaces in contact. We define a figure of merit to quantify such a charge redistribution and show that simple functional relations hold for a wide series of interactions including metallic, covalent, and physical bonds. This suggests unconventional ways of measuring friction by recording the evolution of the interfacial electronic charge during sliding. Finally, we explain that the key mechanism to reduce adhesive friction is to inhibit the charge flow at the interface and provide examples of this mechanism in common lubricant additives
Structural Ordering of Molybdenum Disulfide Studied via Reactive Molecular Dynamics Simulations
No full textMolybdenum disulfide (MoS2) is a well-known and effective lubricant that provides extremely low values of coefficient of friction. It is known that the sliding process may induce structural transformations of amorphous or disordered MoS2 to the crystalline phase with basal planes oriented parallel to the sliding direction, which is optimal for reducing friction. However, the key reaction parameters and conditions promoting this structural transformation are still largely unknown. We investigate, by employing reactive molecular dynamics simulations, the formation of MoS2 layers from an amorphous phase as a function of temperature, initial sample density, and sliding velocity. We show that the formation of ordered crystalline structures can be explained in the framework of classical nucleation theory as it predicts the conditions for their nucleation and growth. These results may have important implications in the fields of coating and thin-film deposition, tribology, and in all technological applications where a fast and effective structural transition to an ordered phase is needed
Recent findings on viruses infecting ranunculus hybrids in Liguria (Italy).
No full textRanunculus asiaticus L. is an ornamental species grown in many areas of the world for tubers, potted plants, and cut flowers production. In Italy, the crops of Ranunculus hybrids are mainly located in Liguria region, where virus diseases represent a serious problem in cut flower production, however the impact of virus infections on the crop was not deeply evaluated. For this reason, in 2005-2007, two hundred samples of Ranunculus hybrids collected in Sanremo area (Imperia province) were examined to verify the presence of viruses associated to severe symptoms on leaves (chlorosis, mosaic, “parsley-like” appearance, vein-yellowing, necrotic spots and rings on the lamina; petiol and stem necrosis) and flowers (colour breaking, malformations), stunting and/or prematurely death of plants. By applying virological tests (mechanical inoculations on herbaceous plants, electron microscopy, PAS-ELISA, IME “decoration”, and RT-PCR) several virus species of different lengths (from 700 to 800 nm) and sizes (from 30 to 90-100 nm) singly or in mixed infections were detected. CMV, in a few cases alone, more often in combination with TSWV was identified in plants showing leaf mosaic, necrosis, stunting and severe flowers symptoms. Both TSWV and INSV, in some cases with also PVY-Ranunculus strain, were detected in leaves with mosaic, stem and petiole necrosis, and “parsley-like” symptoms. TuMV and other unidentified potyvirus-like viruses were also found.
Data obtained clearly show not only the high diffusion of different species of viruses in Ranunculus crops in Sanremo area, but also that the correlation between symptom expression and virus presence is very difficult. Considering the increasing of economic damages to cut flower production recently observed in this area, control measures to prevent virus spreading are required, such as monitoring and virus-testing programs for a rapid removal of inoculum sources and large-scale indexing of propagation materials
Unraveling the mechanism to form MoS2 lubricant layers from MoDTC by ab initio simulations
Full text linkThe morphology of molybdenum disulfide (MoS2) is a crucial aspect to ensure the functionality of this remarkable 2D-material both in electronic and tribological applications. Indeed, molybdenum dithiocarbamates (MoDTCs) can be tribochemically transformed into MoS2, which is able to reduce the friction coefficient of metallic moving parts. However, this transformation is influenced by temperature, sulfur/oxygen ratio, normal and shear stresses, making the mechanism of this process particularly challenging to explain. Ab initio simulations based on density functional theory (DFT), including a quantum mechanics/molecular mechanics (QM/MM) approach, are used here to shed light on the crystallization of MoS2 promoted by mechanical stresses. Chemistry plays an important role during the reorganization of the units of MoSx obtained from MoDTC, because sulfur and oxygen atoms tend to move outside of the amorphous layer, surrounding the molybdenum atoms and creating a structure that can crystallize into MoS2. Normal load and sliding have a synergistic effect in rearranging the amorphous units into a crystalline structure, as the former helps overcoming the energy barriers associated to bonds breaking and forming, while the latter allows misplaced atoms to be pulled towards the crystalline sites. A crystalline MoS2 was obtained by ab initio calculations below 1000 K
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