C. Marlière

Glass Breaks like Metal, but at the Nanometer Scale

We report in situ atomic force microscopy experiments which reveal the presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in glass. Their presence might explain the departure from linear elasticity observed in the vicinity of a crack tip in glass. Such a ductile fracture mechanism, widely observed in the case of metallic materials at the micrometer scale, might be also at the origin of the striking similarity of the morphologies of fracture surfaces of glass and metallic alloys at different length scales.

Two fractal structures in aerogel

Abstract Composite aerogels have been prepared by the addition of silica soot (aerosil) in the solution before gelation. The fractal network previously described which results from the aggregation mechanism of the organosiloxane is strongly affected by the influence of the aerosil soot. Ultra small angle X-ray scattering measurements at ESRF show that besides the fractal network built up by the organosiloxane, the silica soot is forming another fractal structure at a higher scale. The fractal dimension characterizing the inorganic network could be interpreted as the signature of the linkage between the polymeric clusters. From a mechanical point of view, for aerosil content lower than 40 wt% , the elastic modulus is not dependent on the aerosil fractal network and the mechanical features are those of the polymeric gel.

Surface fracture of glassy materials as detected by real-time atomic force microscopy (AFM) experiments

We have studied the low speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully mastered surrounding atmosphere. By using optical and atomic force microscopy (AFM) techniques we tracked in real-time the crack tip propagation at the nanometer scale, on a wide velocity range (10 � 3 to 10 � 10 ms � 1 and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments revealed also—for the first time—that the crack advance proceeds from nucleation, growth and coalescence of nanometric damage cavities inside the amorphous phase, which generate large velocity fluctuation. Implications of the existence of such a nano ductile fracture mode in glass are discussed. # 2003 Elsevier Science B.V. All rights reserved.

In-situ determination of the mechanical properties of gliding or non-motile bacteria by atomic force microscopy under physiological conditions without immobilization.

We present a study about AFM imaging of living, moving or self-immobilized bacteria in their genuine physiological liquid medium. No external immobilization protocol, neither chemical nor mechanical, was needed. For the first time, the native gliding movements of Gram-negative Nostoc cyanobacteria upon the surface, at speeds up to 900 µm/h, were studied by AFM. This was possible thanks to an improved combination of a gentle sample preparation process and an AFM procedure based on fast and complete force-distance curves made at every pixel, drastically reducing lateral forces. No limitation in spatial resolution or imaging rate was detected. Gram-positive and non-motile Rhodococcus wratislaviensis bacteria were studied as well. From the approach curves, Young modulus and turgor pressure were measured for both strains at different gliding speeds and are ranging from 20±3 to 105±5 MPa and 40±5 to 310±30 kPa depending on the bacterium and the gliding speed. For Nostoc, spatially limited zones with higher values of stiffness were observed. The related spatial period is much higher than the mean length of Nostoc nodules. This was explained by an inhomogeneous mechanical activation of nodules in the cyanobacterium. We also observed the presence of a soft extra cellular matrix (ECM) around the Nostoc bacterium. Both strains left a track of polymeric slime with variable thicknesses. For Rhodococcus, it is equal to few hundreds of nanometers, likely to promote its adhesion to the sample. While gliding, the Nostoc secretes a slime layer the thickness of which is in the nanometer range and increases with the gliding speed. This result reinforces the hypothesis of a propulsion mechanism based, for Nostoc cyanobacteria, on ejection of slime. These results open a large window on new studies of both dynamical phenomena of practical and fundamental interests such as the formation of biofilms and dynamic properties of bacteria in real physiological conditions.

Very large-scale structures in sintered silica aerogels as evidenced by atomic force microscopy and ultra-small angle X-ray scattering experiments

During the last few years the bulk structure of silica aerogels has been extensively studied mainly by scattering techniques (neutrons, X-rays, light). It has been shown that small silica particles aggregate to constitute a fractal network. Its spatial extension and fractal dimension are strongly dependent on the synthesis conditions (e.g., pH of gelifying solutions). These typical lengths range from 1 to 10 nm. Ultra-small angle X-ray scattering (USAXS) and atomic force microscopy (AFM) experiments have been carried out on aerogels at different steps of densification. The results presented in this paper reveal the existence of a spatial arrangement of the solid part at a very large length scale. The evolution of this very large-scale structure during the densification process has been studied and reveals a contraction of this macro-structure made of aggregates of clusters.

Crack fronts and damage in glass at the nanometre scale

We have studied the low-speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully controlled surrounding atmosphere. By using optical and atomic force microscopy techniques, we tracked, in real-time, the crack tip propagation at the nanometre scale over a wide velocity range (10−3–10−12m s−1 and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments reveal also—for the first time—that the crack progresses through nucleation, growth and coalescence of nanometric damage cavities within the amorphous phase. This may explain the large fluctuations observed in the crack tip velocities for the smallest values. This behaviour is very similar to that involved, at the micrometric scale, in ductile fracture. The only difference is very probably due to the related length scales (nanometric instead of micrometric). The consequences of such a nano-ductile fracture mode observed at a temperature far below the glass transition temperature, Tg, in glass is also discussed.

ELECTRON TRANSMISSION THROUGH ULTRA-THIN METAL LAYERS AND ITS SPIN DEPENDENCE FOR MAGNETIC STRUCTURES

Abstract We present a new set of experiments in which the attenuation of a ‘monoenergetic’, possibly spin-polarized, free-electron beam is measured by direct transmission through an ultra-thin metal layer. The self-supported metal target is either a reference gold sample or a ferromagnetic structure. The overall thickness is of the order of 25 nm. The magnetic structure consists of a 1 nm thick cobalt film sandwiched between 21-2 nm thick gold layers, with perpendicular magnetization. Measurements are performed throughout a wide energy range, with incident electron energies 2–1000 eV above the Fermi level. The transmission of the gold layer is found to be substantially higher than that of the magnetic structure. In the latter case, at low energy, close to the clean surface vacuum level, we find that the majority spin electrons are more easily transmitted than the minority spin electrons. Cesium deposition on the exit side or on both sides of the target increases the overall transmitted current by almost an order of magnitude. In the case of the magnetic structure, this also increases the transmission spin asymmetry from 16 to about 40%. Such structures appear to be well-suited to the construction of convenient and compact spin-detectors.

Perpendicular magnetic behavior of ultra-thin Co sandwiches

Abstract In situ polar Kerr effect measurements have been used to study the properties of MBE-grown X/Co/Y trilayers, where X and Y are combinations of the non-magnetic metals Au, Cu or Pd. Polar hysteresis curves were measured in situ for systematically-varied Co and overlayer Y layer thicknesses 2 A ≤ t Co ≤ 20 A and 0 A ≤ t Y ≤ 100 A. We recently reported that for base layers of Pd or Au that induce a large perpendicular anisotropy, the magnitude of the X/Co/Y perpendicular coercivity is strongly peaked at ≌ 1 atomic layer overlayer coverage. However, in Pd overlayers the peak in coercivity is much less pronounced than for the other materials. Here we report further measurements on Pd overlayers confirming the existence of a small peak in coercivity. In addition, measurements have been performed on a ‘reversed’ layered Cu/Co/Au sandwich where the Cu base layer induces only a weak perpendicular anisotropy. We find similar peaked behavior similar to that in our earlier measurements on Au/Co/Cu.

Damage Mechanisms and Fracture of Glass at the Nanometer Scale

The morphology of fracture surfaces of glass and metallic alloys have been shown to exhibit amazing similarities, but at very different scales of observation. Both kinds of surfaces present two self-affine regimes. At small length scales/low average crack velocities, they are characterized by a roughness exponent close to 0.5. At larger length scales/higher average crack speeds, this exponent is close to 0.8. Although the values of these exponents do not vary with the material, the range of length scales where these regimes appear is very sensitive to its structure. In the case of glass, the 0.5 regime extends up to 10 nm while the 0.8 regime extends up to some 100 nm. In the case ofxi some metallic alloys, the first regime may extend from 5 nm to 30 µµm, and the second one up to 0.5 mm. Is this similarity in fracture surfaces morphologies at different length scales due to a similarity in the damage and crack propagation mechanisms?