32 research outputs found
Should there be more women in science and engineering?
Many people hold this truth to be self-evident, that there should be more female students in science and engineering. Typical arguments include possible benefits to women, possible benefits to the economy, and the unfairness of the current female under-representation. However, these justifications are never explicitly and thoroughly presented. Clearly stating and scrutinizing them, we show that they in fact have logical flaws. When made consistent, these arguments do not unconditionally justify enrolling more women in scientific disciplines. In particular, what women want must be taken into account. Outreach programs towards K-12 girls must therefore purport to allow them to choose a field freely, rather than try to draw as many of them to scientific disciplines as possible. This change of mindset must be accompanied by a close examination of the purpose and effects of these programs
Effect of grain shape on the agglomeration of polycrystalline thin films
Grain-boundary grooving is a general phenomenon occurring in all
polycrystalline materials at the intersection between the grain-boundary and
the interface or free surface. It has been studied theoretically for some time.
Grain-boundary grooving in the context of faceted interfaces in particular has
attracted some attention. However, these works did not consider the case of
thin films and the consequences on agglomeration of the shape of the interface.
In this Letter, we compare the agglomeration of thin films with rounded and
faceted interfaces. The shape of the grains can dramatically affect the
agglomeration of polycrystalline thin films by grain-boundary grooving.
Anisotropy plays a central role in the stability against agglomeration of
faceted films. Even a small difference between the interface energies of the
facets can destabilize faceted grains or, on the contrary, it can make them
perfectly stable at any thickness. keywords: grain-boundary grooving, dihedral
angle, faceting, energy, silicide, theory, model.Comment: 3 pages, 3 figure
Phase field simulations of coupled phase transformations in ferroelastic-ferroelastic nanocomposites
We use phase field simulations to study composites made of two different
ferroelastics (e.g., two types of martensite). The deformation of one material
due to a phase transformation can elastically affect the other constituent and
induce it to transform as well. We show that the phase transformation can then
occur above its normal critical temperature and even higher above this
temperature in nanocomposites than in bulk composites. Microstructures depend
on temperature, on the thickness of the layers, and on the crystal structure of
the two constituents -- certain nanocomposites exhibit a great diversity of
microstructures not found in bulk composites. Also, the periodicity of the
martensite twins may vary over 1 order of magnitude based on geometry.
keywords: Ginzburg-Landau, martensitic transformation, multi-ferroics,
nanostructure, shape-memory alloyComment: 8 pages, 15 figure
Grain-boundary grooving and agglomeration of alloy thin films with a slow-diffusing species
We present a general phase-field model for grain-boundary grooving and
agglomeration of polycrystalline alloy thin films. In particular, we study the
effects of slow-diffusing species on grooving rate. As the groove grows, the
slow species becomes concentrated near the groove tip so that further grooving
is limited by the rate at which it diffuses away from the tip. At early times
the dominant diffusion path is along the boundary, while at late times it is
parallel to the substrate. This change in path strongly affects the
time-dependence of grain boundary grooving and increases the time to
agglomeration. The present model provides a tool for agglomeration-resistant
thin film alloy design. keywords: phase-field, thermal grooving, diffusion,
kinetics, metal silicidesComment: 4 pages, 6 figure
Effect of lattice mismatch-induced strains on coupled diffusive and displacive phase transformations
Materials which can undergo slow diffusive transformations as well as fast
displacive transformations are studied using the phase-field method. The model
captures the essential features of the time-temperature-transformation (TTT)
diagrams, continuous cooling transformation (CCT) diagrams, and microstructure
formation of these alloys. In some materials systems there can exist an
intrinsic volume change associated with these transformations. We show that
these coherency strains can stabilize mixed microstructures (such as retained
austenite-martensite and pearlite-martensite mixtures) by an interplay between
diffusive and displacive mechanisms, which can alter TTT and CCT diagrams.
Depending on the conditions there can be competitive or cooperative nucleation
of the two kinds of phases. The model also shows that small differences in
volume changes can have noticeable effects on the early stages of martensite
formation and on the resulting microstructures.
-- Long version of cond-mat/0605577
-- Keywords: Ginzburg-Landau, martensite, pearlite, spinodal decomposition,
shape memory, microstructures, TTT diagram, CCT diagram, elastic compatibilityComment: 10 pages, 13 figures, long version of cond-mat/0605577. Physical
Review B, to appear in volume 75 (2007
Fermentation kinetics including product and substrate inhibitions plus biomass death: a mathematical analysis
Fermentation is generally modelled by kinetic equations giving the time
evolutions for biomass, substrate, and product concentrations. Although these
equations can be solved analytically in simple cases if substrate/product
inhibition and biomass death are included, they are typically solved
numerically. We propose an analytical treatment of the kinetic equations
--including cell death and an arbitrary number of inhibitions-- in which
constant yield needs not be assumed. Equations are solved in phase space, i.e.
the biomass concentration is written explicitly as a function of the substrate
concentration.Comment: 4 pages, 4 figure