A Criterion for Brittle Failure of Rocks Using the Theory of Critical Distances

This paper presents a new analytical criterion for brittle failure of rocks and heavily overconsolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as the critical distance. This fracture criterion is known as the Point Method, and is part of the Theory of Critical Distances, which is utilized in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, ó0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, óc and ót. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (óc/ót=3-50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low confining stresses.The work presented was initiated during a research project on “Structural integrity assessments of notch-type defects", for the Spanish Ministry of Science and Innovation (Ref.: MAT2010-15721)

Super-Precise Nanolithography Using Multilayer of Self-Assembled Monolayers

The combination of self-, directed and positional assembly techniques, i. e., “bottom up” fabrication, is demonstrated in this work because they will be essential for patterning and connecting future nanodevices. An array of polystyrene spheres was used instead of conventional lithographic techniques to make “parent” structures. A close-packed monolayer of polystyrene spheres (diameter∼400 nm) was used as a hard mask. Gold was vapor deposited through this mask and lift-off of the spheres produced arrays of triangular gold dots, which were used as “parent” structures. Multilayers of 16-mercaptohexadecanoic acid and Cu2+ ions were selectively deposited as a molecular resist onto these gold dots followed by chromium deposition creating a daughter structure on the substrate. After lift-off of the molecular resist, uniform spacings between “parent” and “daughter” structures are observed using a scanning electron microscope. Nanometer-scale spacing can be controlled around the nanostructures by simply changing the number of layers in the molecular resist or by using molecules of different lengths. In the near future, many applications using this method are expected to impact nanofabrication, such as nanoelectronic devices and micromachines

Exploiting intermolecular interactions and self-assembly for ultrahigh resolution nanolithography

The combination of self-, directed, and positional assembly techniques, i.e., “bottom up” fabrication, will be essential for patterning and connecting future nanodevices. Systematic exploration of local intermolecular interactions on surfaces will permit their exploitation for the rational design of molecular-scale surface structures. We use the scanning tunneling microscope to probe the local behavior of self-assembled films at the nanometer scale. The ability to control the molecular placement within and by self-assembled monolayers is a means of patterning surfaces. A monolayer with customized features can be produced by manipulating the dynamics of film formation, which are heavily affected by the selectable intermolecular interactions of adsorbates and the structural components naturally occurring within the films. Additionally, the controlled placement and thickness of self-assembled multilayers created from alternating strata of α,ω-mercaptoalkanoic acids and coordinated metal ions can be developed to form precise “molecular ruler” resists and to assist in the formation of tailored, lithographically defined metal contacts