Zeolites: From Washing Powder to Strengthened Glass

New research on low-frequency vibrational modes of molecules in glass could lead to stronger, cheaper glasses for optical applications.

Researchers at the University of Wales (Aberystwyth, UK), in collaboration with the Ecole Nationale Superiéure de Chimie de Paris, the European Synchrotron Radiation Facility (Grenoble, France), and the ISIS Facility at the Rutherford Appleton Laboratory (Oxfordshire, UK), have identified the characteristic modes of zeolitic structures that are responsible for converting glass from a low-density structure to a high-density structure.

Zeolites are porous crystalline aluminosilicates with a strong open framework and regular cage arrangement. Their vast internal surfaces allow them to be exploited in industry, from components of washing powders, where the active chemicals can be captured within the structure, to their use in the cracking of petroleum to make gasoline.

Computer simulations show zeolites (left) and what could be the perfect glass (right).

Constructed from a variety of secondary building units (SBUs) - the most common being * cages, ß cages, and double six rings - low-density zeolites melt at much lower temperatures (about 900°C, or half that of silica). If heating is carried out at a slow rate, low-frequency vibrational modes cause the destabilization of the microporous structure. The resultant collapse of the cages produces a 60%-more-dense material. The outcome is a mechanically and chemically stronger glass than that available today.

The vibrations of crystals can be understood as phonon modes; however, the nature and origin of vibrational modes in glasses are less understood. Crystalline and amorphous solids differ in a number of ways, particularly in low- frequency vibrational modes. Anharmonic modes in silica glass can be modeled as librations (hindered rotations) that couple groups of tetrahedra. The broad band of harmonic modes in glass, called the Boson Peak (BP), is rarely seen in crystals. It is found to be greatest for strong glasses such as silica and to decline in strength as fragility increases.

Using high-resolution inelastic neutron scattering, resear-chers were able to characterize the low-frequency vibrations that appear in zeolites during heating. "We have discovered the triggering mechanism," explains Professor Neville Greaves of the University of Wales research group.

The broad band centered around 6 meV and sharp doublet at 4.5 meV in zeolite Y, prominent in the intermediate thermally amorphized states, was replaced in the glass phase by a BP with no sharp features. By identifying the dispersed modes within the characteristic vibrations of the SBUs, it was possible to attribute the BP to vibrations within connected rings of many different sizes. Librational in origin, the modes are responsible for destabilizing the microporous crystalline structure and converting the resulting glass from a low- to a high-density phase, changing the network topology, which affects the BP.

Asked whether these advances could lead to cheaper, stronger glass, John Taylor of the ISIS Facility explains, "of course, zeolites are very cheap to buy - the material can be purchased by the bucket load."

The final aim is to determine the conditions in which the strengthened glass forms, Taylor says. The roles of impurities within the zeolite structure at various stages of amorphization and the associate impact on the strength of the glass are yet to be determined, but the findings may make manufacturing strengthened glass cheaper, which has applications across many market sectors.