Ceramic and Glass Materials

4 R.H. Doremus

The time and temperature required to form the desired degree of porosity in the
dense solid depend mainly on the particle size of the alumina powder. The usual
sintering sequence is imagined to be: (1) neck formation between powder particles,
(2) formation of open porosity with a continuous solid phase (intermediate stage),
and (3) removal of closed pores imbedded in the dense solid. In the usual practical
sintering of alumina, stage one is rapid, and the final density or porosity is deter-
mined mainly by stage three.
Various other oxides have been added to alumina to reduce the porosity of the final
sintered solid. An especially valuable finding by Coble [9] was to add MgO to pure alumina
powder; the resulting sintered alumina can be translucent (partial transmission of light).
Usually sintered ceramics are opaque because of light scattering from residual pores, but
in the translucent alumina, called Lucalox™, the porosity is low enough to reduce this
scattering, so that Lucalox tubes are used in street lamps for containing a sodium arc.
Because the lamp can be operated at high temperature, it is quite efficient.
Dense alumina can also be made by melting, but the high-melting temperature of
2,054°C makes this process expensive and difficult to control. High-value materials
such as gem stones and laser hosts can be made by adding various colorants such as
chromium, titanium, iron, cobalt, and vanadium to the melt.
In the sintering of alumina powders, the desired shape is formed in the green state
before drying and firing. Various other constituents can be added to the starting powder.
The density of the final product can be increased by hot-pressing, that is, by carrying
out the firing under pressure. This method is expensive, so it is used only for high value
polycrystalline products.
Alumina refractories for use in high temperature applications such as glass
melting furnaces are usually made by the fusion-cast process. Various other
oxides, such as SiO 2 , MgO, Cr 2 O 3 , and ZrO 2 are added to the alumina powder to
lower its melting point, and the resulting mixture is melted in an electric furnace
and cast into the desired shapes for refractory applications. See the section on
phase diagrams for the melting temperatures and compositions of a few mixtures
of alumina with other oxides.

3 Structures of Pure and Hydrated Alumina

The structures of aluminas and hydrated aluminas are given in Tables 2 and 3. The only
stable phase of Al 2 O 3 is corundum at all temperatures and up to at least 78 GPa pressure
(see earlier discussion). The corundum structure is shown in Fig. 1. It consists of oxy-
gen ions in a slightly distorted close-packed hexagonal (rhombohedral) lattice, space
group R3c. The aluminum ions occupy two-thirds of the octahedral sites in the oxygen
lattice. The lattice parameters for corundum in Table 2 are for a hexagonal unit cell
containing 12 Al 2 O 3 molecules. The rhombohedral lattice parameters are a = 5.128 Å
andα = 55.28°. The ionic porosity Z of a solid is given by the formula

Z = 1 - Va / V (3)

in which Va is the volume of atoms in a molecule (or in the unit cell) and V is the
specific volume, or the volume of the unit cell. For alumina Z = 0.21 with the radius