162 research outputs found
Early Tithonian Saturnalidae (Radiolaria) from the Solnhofen area (Southern Franconian Alb, Southern Germany)
In order to complete the study of the very rich early Tithonian
(Hybonoticeras hybonotum Zone) radiolarian fauna from the Muhlheim
Member of the Mornsheim Formation outcropping in the Solnhofen area, the
taxa of the family Saturnalidae are described. Although rather rare, the
Saturnalidae of this member contain 14 species, ten of which are new.
These species belong to four genera, one of which is new (Moebicircus n.
gen.), and two subfamilies (Hexasaturnalinae and Saturnalinae). The
taxonomy at generic level of these late Jurassic radiolarians is founded
on the basis of the position of the blades along the ring and number and
morphology of the spines. Type of spines (simple or forked) has either
species level value or none, depending on species. Special attention was
given to anomalies, which sometimes are rather frequent, since they can
give information of paleobiological and paleoecological orders. Among
them frequent cases of open ring and additional spines with
Dicerosaturnalis and Siamese twins skeletons with Spongosaturninus and
Dicerosaturnalis are to be noted. The authors hope that this new
taxonomy will give a better image of the evolution and radiation of the
Saturnalidae during the Tithonian
Radiolarian biostratigraphic evidence for a Late Jurassic age of the El Tambor Group ophiolites (Guatemala)
We present a radiolarian biostratigraphic study of the metacherts of the El Tambor Group ophiolites (South Motagua Unit), Guatemala. The ophiolite sequence comprises MOR pillow metabasalts, massive metabasalts, metacherts and micaschists. The age of the studied metacherts is referable to the Late Jurassic (Oxfordian - Kimmeridgian). The radiolarian assemblage described in this paper is the first Jurassic finding in the ophiolitic MOR succession of the Motagua zone and represents a valuable tool to constrain the geodynamic evolution of the Caribbean area. A review of the ages of Jurassic rocks associated with the ophiolites from the Caribbean area is also reported
Diversité des familles de radiolaires au cours du temps
L’examen de la biodiversité des radiolaires, au niveau de la famille au cours du Phanérozoïque révèle quelques
tendances générales connues chez d’autres groupes d’organismes, surtout dans le plancton, alors que d’autres tendances
leur sont particulières. La crise permo-triasique, l’une des plus importantes dans l’évolution des organismes
marins, est marquée chez les radiolaires par l’extinction de deux familles (Albaillellaria et Latentifistularia) vers la fin
du Permien, mais surtout par une énorme diversification des spumellaires et nassellaires au Trias inférieur et moyen. La
diversité des radiolaires s’accroît du Cambrien au Jurassique, reste relativement stable au Crétacé et décroît légèremen
Enhancement of Friction between Carbon Nanotubes: An Efficient Strategy to Strengthen Fibers
Interfacial friction plays a crucial role in the mechanical properties of
carbon nanotube based fibers, composites, and devices. Here we use molecular
dynamics simulation to investigate the pressure effect on the friction within
carbon nanotube bundles. It reveals that the intertube frictional force can be
increased by a factor of 1.5 ~ 4, depending on tube chirality and radius, when
all tubes collapse above a critical pressure and when the bundle remains
collapsed with unloading down to atmospheric pressure. Furthermore, the overall
cross-sectional area also decreases significantly for the collapsed structure,
making the bundle stronger. Our study suggests a new and efficient way to
reinforce nanotube fibers, possibly stronger than carbon fibers, for usage at
ambient conditions.Comment: revtex, 5 pages, accepted by ACS Nano 10 Dec 200
Dislocations and Grain Boundaries in Two-Dimensional Boron Nitride
A new dislocation structure-square-octagon pair (4|8) is discovered in
two-dimensional boron nitride (h-BN), via first-principles calculations. It has
lower energy than corresponding pentagon-heptagon pairs (5|7), which contain
unfavorable homo-elemental bonds. Based on the structures of dislocations,
grain boundaries (GB) in BN are investigated. Depending on the tilt angle of
grains, GB can be either polar (B-rich or N-rich), constituted by 5|7s, or
un-polar, composed of 4|8s. The polar GBs carry net charges, positive at B-rich
and negative at N-rich ones. In contrast to GBs in graphene which generally
impede the electronic transport, polar GBs have smaller bandgap compared to
perfect BN, which may suggest interesting electronic and optic applications
The influence of size effect on the electronic and elastic properties of diamond films with nanometer thickness
The atomic structure and physical properties of few-layered oriented
diamond nanocrystals (diamanes), covered by hydrogen atoms from both sides are
studied using electronic band structure calculations. It was shown that energy
stability linear increases upon increasing of the thickness of proposed
structures. All 2D carbon films display direct dielectric band gaps with
nonlinear quantum confinement response upon the thickness. Elastic properties
of diamanes reveal complex dependence upon increasing of the number of
layers. All theoretical results were compared with available experimental data.Comment: 16 pages, 5 figures, 3 table
Radial Corrugations of Multi-Walled Carbon Nanotubes Driven by Inter-Wall Nonbonding Interactions
We perform large-scale quasi-continuum simulations to determine the stable cross-sectional configurations of free-standing multi-walled carbon nanotubes (MWCNTs). We show that at an inter-wall spacing larger than the equilibrium distance set by the inter-wall van der Waals (vdW) interactions, the initial circular cross-sections of the MWCNTs are transformed into symmetric polygonal shapes or asymmetric water-drop-like shapes. Our simulations also show that removing several innermost walls causes even more drastic cross-sectional polygonization of the MWCNTs. The predicted cross-sectional configurations agree with prior experimental observations. We attribute the radial corrugations to the compressive stresses induced by the excessive inter-wall vdW energy release of the MWCNTs. The stable cross-sectional configurations provide fundamental guidance to the design of single MWCNT-based devices and shed lights on the mechanical control of electrical properties
Hexasaturnalis nakasekoi nov. sp., a Jurassic saturnalid radiolarian species frequently confounded with Hexasaturnalis suboblongus (Yao)
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