Resonant Revelations
Towards smarter glass
January 1997

The Borate Glasses, Crystals and Melts conference at Abingdon, UK was held in honor of Philip J. Bray, Hazard Professor of Physics Emeritus at Brown University, Rhode Island, for his outstanding contributions to the study of borate glasses over 40 years. This article is condensed from his plenary lecture at the conference.

After a century of research and experiment, glassmakers know a great deal about what boric oxide brings to glass formulations - but rather less about the whys and wherefores. Why, for example, does borosilicate fiberglass become so extremely resistant to attack by water if, after being cooled, it is re-heated but not re-melted? Answers to such questions and pointers to the further improvement - or 'tailoring' - of glass properties, can often be provided by nuclear magnetic resonance spectroscopy (NMR) and now by the associated technique of nuclear quadrupole resonance spectroscopy (NQR).

NMR depends on the fact that in a magnetic field, the nuclei of different elements resonate (i.e. absorb applied electromagnetic radiation) at different radio frequencies. The precise frequency depends on the strength of the magnetic field as well as the type of atomic nucleus. Figure 1 depicts the NMR spectrum and resonance of a boron atom (in crystalline sodium borohydride).

In addition to interacting with an applied magnetic field, an atomic nucleus can also interact with electrons in the atom or molecule, and with other adjacent nuclei. One of these electrical interactions can broaden and create structure in the NMR spectrum. When a magnetic field of moderate strength is employed, the effect is seen in the spectrum for vitreous boric oxide (B2O3) shown in Figure 2. This type of resonance arises from boron atoms which are linked to three rather than four oxygen atoms. Illustrated in Figure 3, where all the boron atoms are linked to three oxygens and all the oxygens to two borons, these flat or 'planar' BO3 units control some of the glass properties.

When sodium oxide (Na2O) is added to the glass composition, two significant changes can occur to the structure displayed in Figure 3. In the first, oxygen from the sodium oxide converts some of the BO3 units into BO4 units, where the boron atom is linked to four oxygen atoms, and forms a tetrahedron (Figure 4). These tetrahedra tighten the glass network by bonding in three dimensions as contrasted with the two dimensional bonding of the planar units. The resulting glass is harder and more resistant to water attack; it also has different optical qualities and is much less prone to expansion or contraction caused by changes in temperature.

The NMR spectrum in Figure 5 shows a narrow line from the BO4 units and a broad line from BO3 units in a sodium borate glass with a relatively low Na2O content. Also revealed by NMR was the answer to the question posed at the beginning of this article: the heating of insulation fiberglass causes the conversion of many BO3 triangles into BO4 tetrahedra, thus tightening the network and hindering water penetration.

However, the second change that sodium oxide (and certain other metallic oxides) can make when incorporated in a glass is the formation of non-bridging oxygens (NBOs) - oxygens that bond only to one boron, not two. The presence of NBOs disrupts the boron-oxygen linking and weakens the glass, thus having the opposite effect to that produced by the formation of three dimensional boron-oxygen units. This is sometimes called the boron anomaly.

In practice, NMR shows that increases in the sodium oxide content up to about 30 percent of the batch continue to promote the occurrence of BO4 tetrahedra. After that the concentration of BO4 units falls, and if the sodium oxide content is greatly increased, the number of BO4 units eventually goes down to zero. As the BO4 units are destroyed, BO3 units with NBOs are produced, again changing the glass properties.

NMR is particularly helpful in showing the inner workings of the sodium borosilicate glass system which is the basic prototype for commercially important glasses such as Pyrex. It has shown that the sodium oxide content is used exclusively for the formation of BO4 units until it exceeds a certain threshold value. Above that threshold - which varies according to the glass's silicon content - sodium oxide interacts with both the boron and silicon components, forming NBOs. This behavior was not understood until the NMR study was made.

Combining this type of knowledge with the known effects on glass of producing BO4 configurations or creating NBOs facilitates a search throughout the region of glass formation in the sodium borosilicate system for new glasses having particular desired properties.

The boron NMR studies of many glasses containing boron oxide support a model in which the glasses contain not just randomly arranged BO3 triangles and BO4 tetrahedra, but the structural groupings found in borate crystalline compounds. The groupings are clusters of BO3 and BO4 units linked together by the oxygens in specific ways into particular geometries. This is useful knowledge, since the various groupings influence glass properties in different ways, and the amounts of the groupings can be controlled to some extent by the rapidity of quenching of the glass from the liquid melt and/or the time and temperature of the annealing of the glass.

A more recent NMR technique is called nuclear quadrupole resonance (NQR). When the quadrupole interaction* is sufficiently strong, no magnetic field is needed and the borons will resonate at specific frequencies determined by the particular quadrupolar interaction. Figure 6 displays the NQR spectrum for vitreous B2O3. There are two responses, revealing a situation not apparent from the NMR spectrum (Figure 2); there are two sites for boron in B2O3 glass, not just one, and they have somewhat different quadrupole interactions.

This discovery has been followed by studies showing that the addition of metallic oxides such as sodium oxide selectively changes borons in one site from the BO3 to BO4 units before starting to convert the other BO3 units. This selectivity may have uses in fabricating glasses with special characteristics.

Clearly, the knowledge of chemical bonding, atomic arrangements, and structural groupings gained from NMR and NQR, added to information obtained using other techniques (e.g. x-ray and infrared spectroscopics) provides a basis for developing and manufacturing glasses tailored to specific properties and uses.

*The quadrupole interaction is one of the possible interactions between the electrical charge in an atomic nucleus and charges external to the nucleus (i.e. electrons and other nuclei).