5,915 research outputs found
Polyhedral units and network connectivity in calcium aluminosilicate glasses from high-energy x-ray diffraction
Structure factors for Cax/2AlxSi1-xO2 glasses (x=0,0.25,0.5,0.67) extended to
a wave vector of magnitude Q= 40 1/A have been obtained by high-energy x-ray
diffraction. For the first time, it is possible to resolve the contributions of
Si-O, Al-O and Ca-O coordination polyhedra to the experimental atomic pair
distribution functions (PDF). It has been found that both Si and Al are
four-fold coordinated and so participate in a continuous tetrahedral network at
low values of x. The number of network breaking defects in the form of
non-bridging oxygens (NBO's) increases slowly with x until x=0.5 (NBO's ~ 10%
at x=0.5). By x=0.67 the network breaking defects become significant as
evidenced by the significant drop in the average coordination number of Si. By
contrast, Al-O tetrahedra remain free of NBO's and fully integrated in the
Al/Si-O network for all values of x. Calcium maintains a rather uniform
coordination sphere of approximately 5 oxygen atoms for all values of x. The
results suggest that not only Si/Al-O tetrahedra but Ca-O polyhedra, too, play
a role in determining the glassy structure
Probability for Primordial Black Holes in Multidimensional Universe with Nonlinear Scalar Curvature Terms
We investigate multi-dimensional universe with nonlinear scalar curvature
terms to evaluate the probability of creation of primordial black holes. For
this we obtain Euclidean instanton solution in two different topologies: (a)
- topology which does not accommodate primordial black holes and (b)
-topology which accommodates a pair of black holes. The
probability for quantum creation of an inflationary universe with a pair of
black holes has been evaluated assuming a gravitational action which is
described by a polynomial function of scalar curvature with or without a
cosmological constant () using the framework of semiclassical
approximation of Hartle-Hawking boundary conditions. We discuss here a class of
new gravitational instantons solution in the -theory which are relevant
for cosmological model building.Comment: 18 pages, no figure. accepted in Phys. Rev.
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