Saturday, August 10, 2013

A new kind of solid - why "q-glass" really is weird and interesting

As long-time readers here know, I'm not a big fan of hype and press releases.  While it is very important to let people know what academic scientists and engineers are doing, not everything needs to be trumpeted from the rooftops as a huge breakthrough or a paradigm-altering thunderbolt.  However, this new result (the actual paper is here) is genuinely weird, unexpected, and exciting (at least to me).  The authors claim to have discovered a truly new kind of solid.  Let me break down what this means and why it's surprising.

A solid is a material that resists shear deformation - if you exert a certain force horizontally across the top surface of the material, the material will deform a bit until it's internal forces balance your applied force, and then deformation will reach some constant amount.  (In contrast, a fluid will keep deforming continuously!)  The most ordinary solids people know about are either crystalline (this includes polycrystalline materials made up of many crystal grains) or glasses.  In a crystalline solid, the atoms have taken on highly symmetric spatial arrangements.  That is, the atoms aren't separated by random distances, but integer multiples of certain particular spacings; similarly, crystals are not isotropic - there are particular directions along which atoms are arranged.   In contrast, simple liquids are isotropic, and except for some typical nearest-neighbor distance set by the atomic or molecular size, there is no other spatial ordered arrangement.   When a solid crystallizes from a liquid, it is a collective phenomenon, a phase transition, and this happens on cooling when the free energy of solid phase becomes lower than that of the liquid phase.  Quasicrystals (see 2011 Nobel for Chemistry) are in these senses crystals - their symmetries are just more subtle than those of ordinary crystals.

Glasses (including those made from polymers) are different.  They resist shear, too, but they do not have the long-range, periodic/anisotropic arrangement of constituents seen in crystals.  Instead, upon cooling, glasses become solid (meaning that their viscosity diverges toward infinity) because the constituents become "kinetically hindered".  At the risk of dragging up controversy, the simple description is that there is no true glass phase in the thermodynamic sense - glasses are rigid because the constituents can't readily move out of each others' ways, not because there is some true collective thermodynamic stability (involving free energies) at work. 

The authors of this new work have found something special that they have termed a "q-glass" while looking at what happens in the solidification of a molten mixture of aluminum, iron, and silicon.  In the resulting solids, they find nodules of a new material (Al91Fe7Si2, approximately) that is definitely not crystalline or polycrystalline (no preferred lattice spacings; completely isotropic).  At the same time, the material does form out of the melt through a genuine first-order phase transition (!), and therefore appears to be highly ordered in some sense (both distinguishing it from a glass).  It will be very interesting to learn exactly what is going on here, and whether there are other materials that have these peculiar features.

1 comment:

Zed said...
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