Page 157 - Materials Chemistry, Second Edition

144 2 Solid-State Chemistry

[91]
dissolve more rapidly in body fluid than crystalline hydroxyapatite. It has been
shown that the plasma spraying procedure used to deposit hydroxyapatite coatings
onto orthopedic and dental implants may also result in co-deposits of amorphous
calcium phosphate and other resorbable phosphates. Increased concentrations of
these other phosphates are thought to result in premature resorption of the coating
before the bone may attach to the implant, drastically shortening its lifetime. [92]
Due to its inherent brittleness, hydroxyapatite is often combined with calcium
.
sulfate (CaSO 4 2H 2 O, denoted as plaster of Paris) to form a more durable ceramic
material. Calcium sulfate is biocompatible and bioactive, being resorbed after
30–60 days. Calcium sulfate is mostly used in its partially hydrated form; when
mixed with water, an exothermic reaction leads to recrystallization into the
dihydrate final form shared by the mineral gypsum (Eq. 57):
ð57Þ CaSO 4 1=2H 2 Oð Þ þ 3=2H 2 O ! CaSO 4 2H 2 O þ heat
Analogous to cement, plaster is used as a building material by hydrating a finely-
divided powder. However, unlike cement, plaster remains relatively soft following
drying, which limits its utility for structural applications. The hardening mechanism
of plaster is due to the recrystallization process. That is, when the hemihydrate
powder is mixed with water, the evolved heat evaporates the water, forming a
network of interlocked dihydrate needle-like crystals. For identical chemical com-
positions, the size and morphology of the starting powder will determine the amount
of water required to make a sufficiently strong ceramic material. Whereas plaster

crystals are irregular in size/morphology, formed by heating gypsum in air at 115 C,
stone crystals feature a much greater uniformity – a consequence of being fabricated
by heating gypsum under an applied pressure. The less uniform crystals of plaster
will pack relatively inefficiently, requiring more water for mixing relative to stone.
As a result, the evaporation of larger volumes of water will result in a greater
porosity, causing the hardness of set plaster to be much less than dried stone.

IMPORTANT MATERIALS APPLICATIONS I: FUEL CELLS

By definition, a fuel cell is any device that generates electricity by chemical reac-
tions that occur at the cathode (positively charged) and anode (negatively charged).
Figure 2.100 illustrates the general operating principle of fuel cells. As opposed to
batteries that store a limited amount of energy, fuel cells operate with a continuous
fuel flow that allows prolonged periods of electricity generation. In addition, these
systems may be easily scaled-up to power large electrical grids.
An electrolyte is an essential component within fuel cells, used to facilitate the
selective migration of ions between the electrodes. Fuel cells are typically classified
according to the electrolytes used: alkaline fuel cell (AFC), polymer electrolyte (or
proton exchange membrane) fuel cell (PEMFC), phosphoric acid fuel cell (PAFC),
molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). Typical
efficiencies, operating temperatures and output voltage for the various types of
fuel cells are shown in Table 2.14. It should be noted that none of these fuel cells