409 research outputs found
Use of a nanoindentation fatigue test to characterize the ductile-brittle transition
When considering grinding of minerals, scaling effect induces competition
between plastic deformation and fracture in brittle solids. The competition can
be sketched by a critical size of the material, which characterizes the
ductile-brittle transition. A first approach using Vickers indentation gives a
good approximation of the critical size through an extrapolation from the
macroscopic to the microscopic scales. Nanoindentation tests confirm this
experimental value. According to the grain size compared to the indent size, it
can reasonably be said that the mode of damage is deformation-induced
intragranular microfracture. This technique also enables to perform cyclic
indentations to examine calcite fatigue resistance. Repeated loadings with a
nanoindenter on CaCO3 polycrystalline samples produce cumulative mechanical
damage. It is also shown that the transition between ductile and brittle
behaviour depends on the number of indentation cycles. The ductile domain can
be reduced when the material is exposed to a fatigue process.Comment: Journal of European Ceramic Society accept\'e pour publication (2008)
sous-press
Double walled carbon nanotube/polymer composites via in-situ nitroxide mediated polymerisation of amphiphilic block copolymers
Because of their unique physical, chemical, and structural properties, carbon nanotubes (CNT) are playing an increasingly important role in the development of new engineering materials [1]. Across many different applications, CNT/polymer composites have been extensively studied [2] S.B. Sinnot and R. Andrews, Carbon nanotubes: synthesis, properties, and applications, Crit Rev Solid State Mater Sci 26 (2001), pp. 145–249.[2]. The key problem for CNT/polymer composite elaboration is the dispersion, compatibilization, and stabilization of the CNT in the polymer matrix. To solve this problem, a structure with di-block copolymers, one with a good affinity to CNT (monomer M1), the other being the matrix (monomer M2), is proposed in this study, as shown on the two steps mechanism of Fig. 1
Consequences of anisotropy in electrical charge storage: application to the characterization by the mirror method of TiO2 rutile
This article is devoted first to anisotropic distributions of stored electric
charges in isotropic materials, second to charge trapping and induced
electrostatic potential in anisotropic dielectrics. On the one hand, we examine
the case of anisotropic trapped charge distributions in linear homogeneous
isotropic (LHI) insulators, obtained after an electron irradiation in a
scanning electron microscope. This injection leads to the formation of a mirror
image
Adhesion forces due to nano-triboelectrification between similar materials
Contact electrification and triboelectrification are well-known in the case
of dissimilar materials, however the case of charge exchange during friction
between nominally identical insulating materials is less documented. We
experimentally investigated the triboelectrification between two smooth
monocrystalline α-Al 2O 3 (sapphire) antagonists by surface force
measurements with a Surface Force Apparatus (SFA). The force between a sphere
and a plane, both in sapphire, was measured as a function of the sphere-plane
distance D, before and after nano-friction tests, under dry argon atmosphere.
Respective contributions of van der Waals, water meniscus and electrostatic
forces were determined. The estimated Hamaker constant was in good agreement
with the Lifshitz theory, and the dominant meniscus attraction at low
separation could be overcome with small radius sphere. We demonstrated that
electrostatic forces were generated by the nano-friction test and we quantified
the adhesion that results from this contact-electrification. In the first stage
of the unloading process, the short range electrostatic force was found to vary
both with time and distance D. Experimental results were correlated with
surface densities of mobile charges on the two surfaces, and the
time-dependence was related to classical surface transport phenomena on alumina
surfaces
Vacuolar hepatopathies in dogs
Vacuolar hepatopathies include a heterogeneous group of so-called overloading hepatopathies. Their
diagnosis is based on histological criteria, such as hepatocyte swelling resulting in cytomegaly with
vacuolated and reticulated cytoplasm. Vacuolar hepatopathy is often idiopathic. Its development can
be secondary to a simple reactive hepatopathy, and include numerous associated conditions.
Endogenous or exogenous hypercorticism is the most common cause of vacuolar hepatopathy in dogs.
The frequency and complications of idiopathic vacuolar hepatopathy in Scottish terriers suggest a breed
and familial predisposition.Les hépatopathies vacuolaires
désignent un groupe assez hétérogène d'hépatopathies dites de surcharge, dont le diagnostic
repose sur des critères histologiques comme la ballonisation des hépatocytes, qui entraîne
une cytomégalie, la vacuolisation et la réticulation de leur cytoplasme. Les hépatopathies
vacuolaires sont souvent idiopathiques. Elles peuvent être le prolongement d'une simple
hépatopathie réactive et de nombreuses affections peuvent s'associer à leur développement.
Un hypercorticisme endogène ou exogène est la cause la plus fréquente d'hépatopathie
vacuolaire chez le chien. Il existe chez le chien de race Scottish terrier une hépatopathie
vacuolaire idiopathique dont la fréquence et les complications permettent d'envisager une
prédisposition raciale et familiale
Modelization of flow electrification in a polymer melt
Flow electrification of polymer melts is an important side effect of polymer
processing. The studies dealing with this phenomenon are seldom and most of the
scientific work has been focused on flow electrification of aqueous and
insulating Newtonian liquids. From that prior art it is well established that
the flow electrification in Newtonian liquids is a consequence of the formation
of an ionic double layer. Convection of this layer induces the electrification
of the liquid at the outlet of the pipe. In those models, the key parameters
governing the flow electrification are thus the intrinsic electrical properties
of the polymer and the flow characteristics. In this work, we reconsider the
assumptions made previously and we propose a new approach to modelise the flow
electrification in the particular case of non-Newtonian polymer materials in
laminar flow conditions. We establish that, a key parameter for the
electrification quantification in the polymer melt is the shape of the velocity
profile. Additionally, in some cases, we show that a slip velocity at the
polymer/die wall interface must be considered to describe accurately the
electrification. As a consequence, we deduce that the slip velocity at the
interface can be calculated by measuring the electrification: this work gives
an alternative manner to measure the slip velocity during polymer flow
Chlorido[1-phenyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazol-2-ylidene]silver(I)
In the title compound, [AgCl(C24H24N2)], the terminal phenyl and tetramethylbenzene rings [which form a dihedral angle of 87.92 (14)°] make dihedral angles of 59.59 (11) and 83.19 (12)° with respect to the central benzimidazole ring system. The Ag—C and Ag—Cl single-bond lengths are 2.087 (3) and 2.3267 (9) Å. The C—Ag—Cl bond angle is 172.84 (7)°. C—H⋯π interactions contribute to the stabilization of the crystal structure. A very weak π–π stacking interaction between adjacent tetramethylbenzene rings [centroid–centroid distance = 4.0610 (18) Å] is also observed
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