Random close-packed glass

Samples can be concentrated beyond tire glass transition. If tliis is done quickly enough to prevent crystallization, tliis ultimately leads to a random close-packed stmcture, witli a volume fraction (j) 0.64. Close-packed stmctures, such as fee, have a maximum packing density of (]) p = 0.74. The crystallization kinetics are strongly concentration dependent. The nucleation rate is fastest near tire melting concentration. On increasing concentration, tire nucleation process is arrested. This has been found to occur at tire glass transition [82]. 

Random close packing (applicable to metallic glasses)... 

The use of confocal microscopy to study concentrated colloidal suspensions was pioneered by van Blaaderen and Wiltzius [83], who showed that the structure of a random-close-packed sediment could be reconstructed at the single-particle level. Confocal microscopy of colloidal suspensions in the absence of flow has been recently reviewed [13-16]. We refer the reader to these reviews for details and references. Here, we simply note that this methodology gives direct access to local processes, such as crystal nucleation [84] and dynamic heterogeneities in hard-sphere suspensions near the glass transition [34,37]. 

From a structural point of view, sulfate glasses are excellent examples in which packing considerations dominate as in ionic crystals. Sulfate ions are tetrahedral but with a spherical envelope of rotation. The random close packing volume of S04 spheres with the radii of 2.65 A is... 

Russel WB, Wagner NJ, Mewis J (2013) Divergence in the low shear viscosity for Brownian hard-sphere dispersions at random close packing or the glass transition J Rheol 57 1555-1567... 

The slight polydispersity in particle size allows the system to avoid the crystalline phase and reach the metastable glass state. Above ( ) = 0.58, the system is metastable with polydispersity, the random close-packing volume fraction shifts to higher values. 

Characteristically, glasses are brittle solids which in practice break only under tension. The ionic and directional nature of the bonds and the identification of electrons with particular pairs of atoms preclude bond exchange. This, coupled with the random nature of the atomic lattice, i.e. the absence of close-packed planes, makes gross slip or plastic flow impossible. 

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