Large Sheets Research Articles

Formation and Structure of Hydrolytic Methylaluminoxane Activators.

Methylaluminoxane (MAO) activators have sheet structures which form ion‐pairs on reaction of neutral donors such as octamethyltrisiloxane (OMTS). The ion‐pairs can be detected by electrospray ionization mass spectrometry (ESI‐MS) in polar media. The growth of these reactive precursors during hydrolysis of Me3Al can be monitored using ESI‐MS. Density functional theory, combined with numerical simulation of growth, indicates that this process involves rapid formation of low MW oligomers, followed by assembly of these species into low MW sheets. These can grow through further addition of low MW oligomers or by fusion into larger sheets. The mechanism of these growth processes leads to the prediction that even‐numbered sheets should be favored, and this surprising result is confirmed by ESI‐MS monitoring experiments of both activator growth and MAO aging.

Stress concentration factors around a circular hole in two fiber reinforced materials under large deformations

In this paper, we study the deformation and the stress concentration factor due to a small circular hole in a thin nonlinearly elastic large sheet reinforced by two families of fibers under a state of plane stress undergoing large deformations. We restrict our attention to the class of materials wherein the fibers are mechanically equivalent, rendering the body orthotropically symmetric with respect to the planes whose normals are along the angular bisectors of the angles between the fibers. Furthermore, we limit ourselves to the problems of equibiaxial, uniaxial and pure shear deformations of the body whose fibers are symmetrically oriented with respect to the Cartesian coordinate axes. These deformations indicate strong influence of the fiber angle on the stress concentration factors and a significant difference in the stress concentration factor distribution around the hole when using a constitutive relation based on a partial set of invariants (I4,I6) versus an extended constitutive relation that incorporates the full set of invariants appropriate for a body reinforced with fibers and reduces appropriately to the orthotropic linearized elastic case. We show how two constitutive relations that exhibit a similar biaxial behavior in the absence of discontinuities show noticeable differences under uniaxial and pure shear deformation with the dissimilarities magnified in the presence of discontinuities like a circular hole.

The tensile deformation behavior of AZ31B magnesium alloy sheet under intermittent pulse current

In this paper, to investigate the effect of loading method of pulse current on the tensile deformation behavior of AZ31B magnesium alloy sheet, the intermittent pulse current with short-time and high-frequency was introduced in uniaxial tensile tests and the influence of duty ratio and loading time of pulse current on the deformation behavior of AZ31B magnesium alloy sheet was discussed. The strain and temperature field distributions on the specimens were measured during the intermittent pulse electrically-assisted tension (IPEAT), and the microstructure and fracture morphology under different pulse current conditions were observed. Results shows appropriate pulse current parameters can effectively improve the elongation of AZ31B magnesium alloy sheet. The strain of the sample is closely related to temperature distribution. With the deformation of the sample, the temperature on the sample increases gradually and the temperature distribution is non-uniform along the tensile direction, resulting in an inhomogeneous strain distribution of the sample. In addition, grain growth and dynamic recrystallization were observed on the AZ31B magnesium alloy sheet in different degrees under intermittent pulse current. Fracture morphology analysis shows that the number of dimples and tearing edges increased on the fracture obtained under IPEAT. The microhardness analysis shows that when intermittent pulse current is applied in the tensile test, the hardness of the sheet may change. This research provides an effective idea for the forming process of magnesium alloy sheets, which can be used to form large size thin-walled sheet components, and can significantly improve the forming quality of the sheets.

Inert Mask Lithography of Edge Narrowed Graphene Nanoribbons Directly Contacted to Metallic Electrodes

AbstractPatterning graphene into strips with nanometer‐scale widths, i.e. graphene nanoribbons, is an effective approach to customize different properties of the material. Narrowing a large sheet of graphene via conventional top‐down approaches meets the intrinsic resolution limitations of lithography. Process residuals as a result of using sticky polymeric templates may also influence the performance of the eventual device, in addition to altering the chemistry of the ribbons. Here, a facile approach for pattering graphene nanoribbons via an inert metallic mask lithography is reported. The masks are prepared by the microtomy of metallic thin films, embedded within polymeric scaffolds and precisely deposited on top of a graphene monolayer. The inertness of the masks allows the precise narrowing of the graphene under different etching environments, permitting the control over the edge chemistry. Remarkably a mild ultrasonication is enough to remove the mask entirely at the end of the process, without leaving any residue. For electronic applications, however, partial removal of the metallic mask using a laser power, in situ realizes a pair of electrodes, hence avoiding the often difficult issue of electrode alignment. The method proposes a simple and direct approach towards the design of chemically tailored, scalable, and electrically connected graphene nanoribbons.

Multi-layered Barium and Strontium Carbonate Structures Induced by the Small Organic Dye Acid Orange 7.

The crystal growth behavior induced by small molecular additives is commonly assumed to be far less complex and rich in comparison to that obtained when using macromolecules. Herein, we demonstrate that the small organic molecule Acid Orange 7 can induce a large diversity of multi-layered barium carbonate structures. These multi-layered structures stem from the small molecule imperfectly blocking the fastest growing crystal face. By tuning the balance of growth and inhibition, we control the layer shape and thickness of the structures. Extending these strategies to strontium carbonate enables the precipitation of large quasi two-dimensional multi-layer sheets. Collectively, these findings highlight the unforeseen potential for using small organic molecules to induce the formation of complex inorganic structures.

Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation

AbstractFjords are glacially carved estuaries that profoundly influence ice-sheet stability by draining and ablating ice. Although abundant on modern high-latitude continental shelves, fjord-network morphologies have never been identified in Earth's pre-Cenozoic glacial epochs, hindering our ability to constrain ancient ice-sheet dynamics. We show that U-shaped valleys in northwestern Namibia cut during the late Paleozoic ice age (LPIA, ca. 300 Ma), Earth's penultimate icehouse, represent intact fjord-network morphologies. This preserved glacial morphology and its sedimentary fill permit a reconstruction of paleo-ice thicknesses, glacial dynamics, and resulting glacio-isostatic adjustment. Glaciation in this region was initially characterized by an acme phase, which saw an extensive ice sheet (1.7 km thick) covering the region, followed by a waning phase characterized by 100-m-thick, topographically constrained outlet glaciers that shrank, leading to glacial demise. Our findings demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia at different times during the LPIA. The fjords likely played a pivotal role in glacier dynamics and climate regulation, serving as hotspots for organic carbon sequestration. Aside from the present-day arid climate, northwestern Namibia exhibits a geomorphology virtually unchanged since the LPIA, permitting unique insight into this icehouse.

Ambient energy dispersion and long-term stabilisation of large graphene sheets from graphite using a surface energy matched ionic liquid†

Liquid phase exfoliation (LPE) is the most promising method for scalable graphene production, but conventionally requires significant energy input to break apart layers in the bulk material, introducing defects and limiting production of large, few-layer sheets. In this work, we exfoliated graphene under ambient conditions with minimal external energy input using the surface energy matched ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIm Ac), then isolated and characterised the exfoliated material. This method produced large and thin, graphene sheets with size up to 3 μm. Monolayer sheets were identified in samples that had been left undisturbed for 6 months prior to sampling, meaning the graphene sheets suspended in the ionic liquid are stabilized against restacking; to our knowledge, this is the longest period of time large (non-oxidised) graphene monolayers have been suspended in any liquid. Molecular dynamics simulations show the IL cation is enriched at the graphene surface. We postulate this leads to a positive charge on the graphene sheets in EMIM Ac leading to an electrostatic repulsion which contributes to colloidal stability. Strong adsorption of the EMIM cation to graphene is confirmed by energy dispersive X-ray spectroscopy images that show ionic liquid adsorbed to graphene sheets even after washing. Ambient energy exfoliation using ionic liquids offers simple, energy efficient method for producing large, high quality graphene sheets, with long-term stability in the ionic liquid. [Display omitted]

Human orbital muscle in adult cadavers and near-term fetuses: its bony attachments and individual variation identified by immunohistochemistry.

To compare fetal and adult morphologies of the orbital muscle (OM) and to describe the detailed topographical anatomy in adults. Using unilateral orbits from 15 near-term fetuses and 21 elderly cadavers, semiserial horizontal or sagittal paraffin sections were prepared at intervals of 20-100µm. In addition to routine histology, we performed immunohistochemistry for smooth muscle actin. At near term, the OM consistently extended widely from the zygomatic bone or the greater wing of the sphenoid to the maxilla or ethmoid. Thus, it was a large sheet covering the future inferior orbital fissure. In contrast, the adult OM was a thin and small muscle bundle connecting (1) the greater wing of the sphenoid to the maxilla (11/19 cadavers), (2) the lesser wing of the sphenoid to the maxilla (5/19) or the greater wing (3/19). The small OM was likely to be restricted within the greater wing (5/19 cadavers) or the maxilla (3/19). Two of these five types of OM coexisted in eight orbits. OM attachment to the lesser wing was not seen in fetuses, whereas ethmoid attachment was absent in adults. The lesser wing attachment of the OM seemed to establish after birth. A growing common origin of the three recti was likely involved in "stealing" the near-term OM attachment from the ethmoid. The strong immunoreactivity of remnant-like OM in the elderly suggests that OM contraction is still likely to occur against the increased flow through a thin vein. However, the contraction might have no clinical significance.

Tethered vagina syndrome

The key to understanding the pathogenesis leading to the “Tethered vagina syndrome” and its cure by a skin graft applied to the anterior vaginal wall is to understand the importance of the three oppositely-acting directional forces which close the urethral tube and on relaxation of the forward force, open it for micturition. Any scarring from vaginal excision during “native tissue repair”, application of large mesh sheets, or overstretching by a Burch colposuspension effectively “tethers” the more powerful posterior forces to the weaker anterior force; the posterior urethral wall is pulled open exactly as happens during micturition; the patient loses urine uncontrollably typically on getting out of bed in the morning, which is the classical diagnostic symptom.

Unusual stress and strain concentration behaviors at the circular hole of a large monodomain liquid crystal elastomer sheet

Liquid crystal elastomers combine the hyperelasticity of elastomers with the multi-functionality of liquid crystals and have emerged as an important class of soft active materials. Monodomain liquid crystal elastomers under loading exhibit the soft elastic behavior due to the stress induced director rotation and the resulting spontaneous strain. Here, we numerically study their stress and strain concentration behavior by considering the classical example, a large sheet with a small circular hole in the middle under uniaxial loading. The concentration behavior is found to be very different from regular elastomers. Firstly, the concentration factors are much bigger at both small and large strains. Secondly, the locations of the strain concentration may not coincide with that of the stress concentration. Detailed analysis of the director rotation and the resulting spontaneous strain around the free edge of the hole are shown to be the main causes for the unusual concentration behavior. Moreover, under a given strain, the stress level of the LCE sample, and therefore the free energy, is slightly lower than that of the neo-Hookean material, while the local free energy density on the hole edge is much bigger due to the severer concentrations. By considering material parameters obtained from various samples of polysiloxane and polyacrylate side-chain nematic elastomers, we find that their stress and strain concentration behaviors are qualitatively similar but quantitatively quite different. As a result, at a prescribed strain, the samples with a larger maximal spontaneous strain has severer stress and strain concentrations. The stress concentration factor difference scaled by a coupling constant that combines the effect of the two material parameters, r and a, increases as the semi-soft coefficient a decreases. Similar results are found for the scaled maximal spontaneous strains and the scaled strain concentration factor difference at small strains.

On the role of shock melting and anatexis in breccia formation: Southern Sudbury impact structure, Canada

The ca. 1.85 Ga Sudbury Igneous Complex (SIC) in Ontario, Canada, represents the partially eroded remnant of an impact melt sheet that overlies brecciated target rocks. At Crean Hill, along its southwest margin, it is underlain by contact metamorphosed basement rocks cut by breccias of various compositions. The breccias formed as melts with contact-parallel flow textures, which are defined by elongate wispy clasts that wrap around basalt clasts. The presence of melts in the footwall of large impact melt sheets may be due to shock melting during propagation of the shock wave, frictional melting along superfaults during modification of the crater, injections from the impact melt sheet, or anatexis during cooling of the impact melt sheet. The absence of nearby superfaults at Crean Hill precludes the formation of the breccia by frictional melting. Modeling of partial melt compositions during contact metamorphism produces chemical melts that are generally enriched in Zr and more felsic than the breccia matrices, negating the possibility of their formation by anatexis during contact metamorphism. The breccia matrices have significantly lower SiO2 and higher TiO2 than the SIC original melt composition, suggesting that they could not have formed by the injection of SIC melt into the fractured target rocks. Because the composition of the breccias mirrors that of their host rocks, the breccias are interpreted as pockets and dikes of locally-derived shock melt that formed during shock compression and were trapped in the basement rocks during cooling of the SIC. Thus, the breccias formed by shock melting during impact and were subsequently modified by contact metamorphism (T ≥ 750 °C) during cooling of the SIC, and by later syn-tectonic regional metamorphism at upper greenschist to amphibolite conditions.

Surface modified and functionalized graphene oxide membranes for separation of strontium from aqueous solutions

Graphene oxide-strontium (GO-Sr) composites prepared under different ambient conditions were characterized using morphological and spectroscopic techniques to optimize the uptake of Sr from aqueous solutions. These studies indicated that interactions among GO and Sr2+ ions are highly sensitive to size and aging of GO sheets, as well as pH of the ambience. Further, the extent of Sr uptake on GO sheets was found to be largely influenced by relative fractions of the associated –COOH, –OH, C–O–C functional groups and sp2-C domains. Membranes prepared using various forms of GO were evaluated for their Sr separation ability and, a window of parameters for optimum separation of Sr has been proposed. Among the variety of membranes studied, those made up of fresh and large GO sheets were found to exhibit superior Sr adsorption capacity (~296 mg/g) at limited GO mass. Further, adsorption efficiency of these GO membranes was observed to deteriorate with aging of GO sheets and rise of GO mass on membrane. The membrane based filtration procedure introduced in present work facilitates to provide a lamellar structure of GO sheets with abundant surface area, diverse and accessible sites for Sr2+ ion uptake and offer high Sr adsorption efficiencies.

Experimental and molecular dynamics studies of an ultra-fast sequential hydrogen plasma process for fabricating phosphorene-based sensors

Low concentration phosphorene-based sensors have been fabricated using a facile and ultra-fast process which is based on an exfoliation-free sequential hydrogen plasma treatment to convert the amorphous phosphorus thin film into mono- or few-layered phosphorene sheets. These sheets have been realized directly on silicon substrates followed by the fabrication of field-effect transistors showing the low leakage current and reasonable mobility for the nano-sensors. Being capable of covering the whole surface of the silicon substrate, red phosphorus (RP) coated substrate has been employed to achieve large area phosphorene sheets. Unlike the available techniques including mechanical exfoliation, there is no need for any exfoliation and/or transfer step which is significant progress in shortening the device fabrication procedure. These phosphorene sheets have been examined using transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Raman spectroscopy and atomic-force microscopy (AFM). Electrical output in different states of the crystallization as well as its correlation with the test parameters have been also extensively used to examine the evolution of the phosphorene sheets. By utilizing the fabricated devices, the sensitivity of the phosphorene based-field effect transistors to the soluble L-Cysteine in low concentrations has been studied by measuring the FET response to the different concentrations. At a gate voltage of − 2.5 V, the range of 0.07 to 0.60 mg/ml of the L-Cysteine has been distinguishably detected presenting a gate-controlled sensor for a low-concentration solution. A reactive molecular dynamics simulation has been also performed to track the details of this plasma-based crystallization. The obtained results showed that the imparted energy from hydrogen plasma resulted in a phase transition from a system containing red phosphorus atoms to the crystal one. Interestingly and according to the simulation results, there is a directional preference of crystal growth as the crystalline domains are being formed and RP atoms are more likely to re-locate in armchair than in zigzag direction.

Defining Patterns and Behaviours of Forward Spatter Gunshot Misting

The purpose of this research was to study forward-spatter misting patterns by shooting a firearm through a chamber of blood encased in ballistic gel to determine if there is a relationship between bloodstain pattern size as a function of distance and orientation. There is a lack of research on forward spatter, blood travelling in the direction of a bullet, as most studies focus on back spatter, blood travelling in the opposite direction of a bullet. A bullet was fired through ballistic gel containing a blood chamber, depositing bloodstains onto a large sheet of butcher paper as the target surface. In total, there were 34 trials. The distances observed were 10, 20, 40, and 80 cm, the angles tested were 30°, 60°, and 90°. The orientation between the ballistic gel and paper target varied. A criterion was established to observe the overall area and symmetry of the bloodstain patterns. Statistical analyses indicated a negative linear relation relationship between the bloodstain pattern size and the paper’s angle and distance (R2 = 0.78) and the vertical symmetry of the bloodstain (R2 = 0.87). The orientation between the ballistic gel and paper target can impact the bloodstain pattern’s symmetry and size.

The impacts of warming on rapidly retreating high-altitude, low-latitude glaciers and ice core-derived climate records

Alpine glaciers in the low- and mid-latitudes respond more quickly than large polar ice sheets to changes in temperature, precipitation, cloudiness, humidity, and radiation. Many high-altitude glaciers are monitored by ground observations, aerial photography, and satellite-borne sensors. Regardless of latitude and elevation, nearly all nonpolar glaciers and ice caps are undergoing mass loss, which compromises the records of past climate preserved within them. Almost without exception, the retreat of these ice fields is persistent, and a very important driver is the recent warming of the tropical troposphere and oceans. Here we present data on the decrease in the surface area of four glaciers from low- to mid-latitude mountainous regions: the Andes of Peru and northern Bolivia, equatorial east Africa, equatorial Papua, Indonesia, and the western Tibetan Plateau. Climate records based on oxygen isotopic ratios (δ 18 O) measured in ice cores drilled from several glaciers in these regions reveal that the records from elevations below ~6000 m above sea level have been substantially modified by seasonal melting and the movement of meltwater through porous upper firn layers. Fortunately, δ 18 O records recovered from higher altitude sites still contain well-preserved seasonal variations to the surface; however, the projected increase in the rate of atmospheric warming implies that climate records from higher elevation glaciers will eventually also be degraded. A long-term ice core collection program on the Quelccaya ice cap in Peru, Earth's largest tropical ice cap, illustrates that the deterioration of its climate record is concomitant with the increase in mid-troposphere temperatures. The melting ice and resulting growth of proglacial lakes presents an imminent hazard to nearby communities. The accelerating melting of glaciers, if sustained, ensures the eventual loss of unique and irreplaceable climate histories, as well as profound economic, agricultural, and cultural impacts on local communities. • Rising global temperature is an important driver of tropical glacier retreat. • As tropical glaciers melt, the unique climate records they preserve are disappearing. • Glacier lake outbursts in southern Peru have created hazards for local communities. • Ablation of ice from the glacier summit is observed in three low-latitude regions.

Cryo-electron tomography provides topological insights into mutant huntingtin exon 1 and polyQ aggregates

Huntington disease (HD) is a neurodegenerative trinucleotide repeat disorder caused by an expanded poly-glutamine (polyQ) tract in the mutant huntingtin (mHTT) protein. The formation and topology of filamentous mHTT inclusions in the brain (hallmarks of HD implicated in neurotoxicity) remain elusive. Using cryo-electron tomography and subtomogram averaging, here we show that mHTT exon 1 and polyQ-only aggregates in vitro are structurally heterogenous and filamentous, similar to prior observations with other methods. Yet, we find filaments in both types of aggregates under ~2 nm in width, thinner than previously reported, and regions forming large sheets. In addition, our data show a prevalent subpopulation of filaments exhibiting a lumpy slab morphology in both aggregates, supportive of the polyQ core model. This provides a basis for future cryoET studies of various aggregated mHTT and polyQ constructs to improve their structure-based modeling as well as their identification in cells without fusion tags.

Preventing colour fading in artworks with graphene veils.

Modern and contemporary art materials are generally prone to irreversible colour changes upon exposure to light and oxidizing agents. Graphene can be produced in thin large sheets, blocks ultraviolet light, and is impermeable to oxygen, moisture and corrosive agents; therefore, it has the potential to be used as a transparent layer for the protection of art objects in museums, during storage and transportation. Here we show that a single-layer or multilayer graphene veil, produced by chemical vapour deposition, can be deposited over artworks to protect them efficiently against colour fading, with a protection factor of up to 70%. We also show that this process is reversible since the graphene protective layer can be removed using a soft rubber eraser without causing any damage to the artwork. We have also explored a complementary contactless graphene-based route for colour protection that is based on the deposition of graphene on picture framing glass for use when the direct application of graphene is not feasible due to surface roughness or artwork fragility. Overall, the present results are a proof of concept of the potential use of graphene as an effective and removable protective advanced material to prevent colour fading in artworks.

Synthesis of Hydrochar Cassava peels with hydrothermal carbonization method and applications as hard water softener

Groundwater before being used is very important to pay attention to the quality of the water. Water has good quality if it is in accordance with several parameters, including chemical, physical, and biological parameters. Mineral salt content is one of the chemical parameters that can be used to determine the quality of water. In this study, the synthesis of cassava peel hydrochar was successfully carried out using the hydrothermal carbonization (HTC) method. This material was developed to reduce waste of cassava peel biomass and overcome the problem of water hardness. HTC cassava peel was developed as an alternative hard water softener adsorbent. The results of FTIR characterization on hydro-activation (activating hydrochar) and hydro-non (non-activating hydrochar) contain functional groups of O-H stretch, C-H stretch, C≡C stretch and C=C stretch. Observation of surface morphology by SEM method on hydro-non in the form of large sheets, while the surface on hydro-activities in the form of small sheets. The adsorption capacity of hydro-active against Ca2+ ions and Mg2+ ions were 85.240 mg/g and 87.210 mg/g, respectively. Meanwhile, the hydro-non adsorption capacity of Ca2+ and Mg2+ ions were 84,736 mg/g and 85,713 mg/g. The kinetics of hydro-active adsorption on Mg2+ ion took place on order 3 (0.000003 min-1) while when adsorption of Ca2+ ion took place on order 0 (0.0324 min-1).©2021 JNSMR UIN Walisongo. All rights reserved.

Development of Highly Sensitive Strain Sensor Using Area-Arrayed Graphene Nanoribbons.

In this study, a basic design of area-arrayed graphene nanoribbon (GNR) strain sensors was proposed to realize the next generation of strain sensors. To fabricate the area-arrayed GNRs, a top-down approach was employed, in which GNRs were cut out from a large graphene sheet using an electron beam lithography technique. GNRs with widths of 400 nm, 300 nm, 200 nm, and 50 nm were fabricated, and their current-voltage characteristics were evaluated. The current values of GNRs with widths of 200 nm and above increased linearly with increasing applied voltage, indicating that these GNRs were metallic conductors and a good ohmic junction was formed between graphene and the electrode. There were two types of GNRs with a width of 50 nm, one with a linear current–voltage relationship and the other with a nonlinear one. We evaluated the strain sensitivity of the 50 nm GNR exhibiting metallic conduction by applying a four-point bending test, and found that the gauge factor of this GNR was about 50. Thus, GNRs with a width of about 50 nm can be used to realize a highly sensitive strain sensor.

A Late Pleistocene channelized subglacial meltwater system on the Atlantic continental shelf south of Ireland

The study of palaeo‐glacial landforms and sediments can give insights into the nature and dynamics of ice sheets. This is particularly the case with regards to the subglacial record, which is challenging to observe in contemporary glaciated settings and hence remains only partially understood. The subglacial hydrological system is an essential component of ice dynamics, where increased water pressure enhances ice motion and sediment deformation, thus reducing ice‐bed contact. Tunnel valleys are large, sinuous, steep‐sided incisions that, together with smaller scale meltwater channels, indicate subglacial meltwater discharge beneath large ice sheets. Through the use of high‐resolution marine geophysical data, a system of buried and exposed tunnel valleys, possible subglacial or proglacial meltwater channels and palaeo‐fluvial valleys have been identified across the shelf of the Celtic Sea between Ireland and Britain. The presence of steep‐sided and overdeepened tunnel valleys is indicative of a large channelized meltwater drainage system beneath the former Irish Sea Ice Stream, the most extensive ice stream to drain the last British–Irish Ice Sheet. After the rapid ice expansion across the Celtic Sea shelf around 28–26 ka, the tunnel valleys were carved into both bedrock and glacigenic sediments and are associated with rapid ice stream retreat northwards into the Irish Sea Basin between 25.6 and 24.3 ka. The presence of a major subglacial meltwater system on the relatively shallow shelf suggests that significant erosive meltwater discharge occurred during the last deglaciation and highlights the important contribution of meltwater to the retreat of the British–Irish Ice Sheet on the continental shelf.