Page 149 - Materials Chemistry, Second Edition

136 2 Solid-State Chemistry

conduction band. Charge balance is maintained through the interstitial placement of
þ
Li ions in the lattice. Since the electron becomes delocalized, metallic behavior is
induced in the tungsten oxide layer changing the transparent layer to a dark reflec-
tive color (ca. 10% transmittance of incoming light through the device). The dark
color will remain even if the applied voltage is removed, since the reverse reactions
are not spontaneous. If the reverse bias is applied to the device, lithium ions flow
from the WO 3 layer reoxidizing the W 5þ ions and restoring transparency. We will
discuss more details regarding the electrical properties and band structure of semi-
conductive oxides such as TiO 2 , SnO 2 , and WO 3 in Chapter 4.
More recently, thin films of Ni/Mg hydride alloys have also been developed for
light attenuation using electrochromic or gas-chromic (injection of H 2 and O 2 gases)
technology. [85] Although they can technically be classified as electrochromic mate-
rials, the new reflective hydrides that are being developed behave in a noticeably
different way. Instead of absorbing light, they reflect it. Thin films made of
nickel–magnesium alloy are able to switch back and forth from a transparent to a
reflective state. The switch can be powered by low voltage electricity (electrochro-
mic technology) or by the injection of hydrogen and oxygen gases (gas-chromic
technology). Furthermore, this material has the potential to be even more energy
efficient than other electrochromic materials.
By comparison, SPDs operate through the behavior of rod-like particles (e.g.,
liquid crystals, see Appendix C.3) toward an applied voltage (Figure 2.95b). When
no voltage is applied, the particles are randomly aligned, and do not allow light to
pass through the device. However, an electric charge will polarize the particles to
align with the field. We will describe the molecular behavior of polarizable particles
in more detail later (Chapter 4), related to dielectric materials placed in a parallel
plate capacitor.

2.4.3. Cementitious Materials

The use of cementitious materials for structural applications dates back to ancient
Egypt. A type of cement was used to hold together the limestone blocks of the great
pyramids that still stand today. During the time of the Roman Empire, an improve-
ment of the cement formulation was developed, which used a fine, volcanic ash
known as Pozzolana found in various parts of Italy. Although they did not realize it
at the time, the hardening process occurred due to the reaction of the aluminosili-
cate-based ash with Ca(OH) 2 in the presence of water to yield a calcium–silicate–
hydrate (CSH) rigid gel. Amazingly, thousands of years later, the CSH structure is
not yet completely understood – it is likely a disordered form of the hydrated
calcium silicate mineral tobermorite (Figure 2.96).
The last major development in cement technology occurred in the early nine-
teenth century in England. Bricklayer Joseph Aspdin first made a variety of cement
known as Portland cement – not in a laboratory, but on his kitchen stove! His patent
in 1824 changed the world forever, as this form of cement is the basic ingredient in
concrete – essential for the erection of virtually all buildings and many roads