Page 145 - Materials Chemistry, Second Edition

132 2 Solid-State Chemistry

process. In general, the observed color is the complement of the color that is
absorbed by the ion. That is, the absorption of short wavelengths will result in an
observable red color. Decolorizing agents may also be added; for instance, to
remove a yellowish color (e.g., from the presence of Fe 3þ impurities), a slight
excess of manganese may be added that will yield the complementary color of
pale purple, effectively neutralizing the glass to a colorless state.
For colloidal dopants, the particle size must be smaller than a wavelength of
visible light, or an opaque glass will result. If one would prefer a cloudy glass, a
number of additives (e.g., SnO 2 , TiO 2 , CaF 2 ) may be used that result in a suspension
that changes the overall index of refraction. Colloidal metals yield a deep red color,
with colloidal gold first used in the late seventeenth century. Alternatively, a metal
salt such as AuCl 3 may be added to glass followed by thermal or chemical (e.g.,
using NaBH 4 ) reduction to metallic Au. It is important to note that a red color will
only result if an agent is also added to prevent particle agglomeration. In general, the
observed color will shift toward the blue portion of the spectrum as the average
particle size decreases (e.g., blue color results from diameters of <50 nm). Chapter 6
will provide more details related to the scattering properties and other applications
of nanoparticles.
In comparison, transition metals are added to a molten glass matrix as soluble
oxides. As you may see from Table 2.13, the observed color is a consequence of the
metal ion type/concentration, as well as its oxidation state. To obtain a desired color,
oxidizing agents such as NaNO 3 , or reducing agents such as carbon powder may be
added to afford the desired oxidation state. An intriguing form of glass, referred to as
vaseline glass contains UO 2 and is slightly radioactive. Since UV radiation is
sufficient to excite the weakly bound outer electrons of U, this additive results in a
fluorescent green color. Although this is observable under normal light, it is most
pronounced upon irradiation with a UV lamp. Interestingly, UO 2 was also added to
ceramic glaze to yield bright orange dinner plates and tableware in the 1930s.
However, it was later discovered that heat and acidic foods caused uranium to
leach from the glaze, resulting in an immediate disband of this application. As one
might expect, UO 2 -doped materials are not currently manufactured for decorative
applications, making such acquisitions a collector’s item. [80]
In order to achieve opacity, tiny bubbles may be purposely introduced within the
viscous melt – a process that dates back to ancient preparations. The resultant
dispersion of light gives rise to an opalescent glass; however, it is now more
prevalent to use opalizing agents. Earliest examples, dating back to 1,400 B.C.,
used M 2 Sb 2 O 7 (M ¼ Pb, Ca) for opaque white glass; mixtures of Cu/Cu 2 O are
used to yield opaque red glass, and opaque white/blue glass often uses
CaF=CaF 3 þ NaF=SnO 2 combinations.
Thus far, we have considered the varying chemical compositions and properties of
glasses. In this last section, we will examine an important architecture – glass fibers,
of paramount importance in our society. The synthesis of glass fibers dates back to
the early eighteenth century, and applications for surgical lamps were prevalent as
early as the nineteenth century. We are all familiar with the bright pink bags of