Commonly used facets of sapphire crystals

Commonly used facets of sapphire crystals

In addition to the main crystallographic plane, dozens of other crystallographic planes also exist and have their own signatures. The major forms of these crystallographic planes and their spherical coordination are shown in Table 2.4, and some less common forms can be encountered in [38].

Those sapphire planes most often encountered in reality are shown in Figure 2.24.2 The (1012) plane forms an angle of 57º36′ with the (0001) plane and an angle of 32º24′ with the (112 0) plane. These normals The angles to the sapphire facets are listed in Table 2.43[1].

Equilibrium shape. At normal temperature, pressure and constant volume, the crystal shape with the smallest free surface energy is known as the equilibrium shape. The crystal face of the equilibrium shape may appear on the surface of the growing crystal. This is because when the crystal is oriented Considering these crystal planes may affect the roughness characteristics of the crystalline front (CF) [39].

There are several ways to form crystals of an equilibrium shape. One is to heat small crystals in a closed system at high temperature to speed up the transport process. This process is relatively Difficult to control. Another approach is to use external cavities, which actually have very slow lattice diffusion and "negative" crystals of constant volume in contact with a certain atmosphere. Condition of contact with its vapor and maintaining a certain volume The latter is easier to maintain than the former. Anytime a crystal deviates from its Wulff shape depends on the initial shape and the size of the crystal and the rate of kinetic processes that change its shape (Fig. 2.25) [36]. Doping has a considerable effect on the Wulff shape, even at very dilute concentrations (ppm) (Fig. 2.26) [39].

Experimentally, the equilibrium shape is obtained by crystallization or decomposition of spherical crystals, but solvents, impurities, and other factors often change the equilibrium shape. If the judgment is made on the basis of a given crystal in its own melt, the effect of the solvent here will be is excluded, so conclusions about the equilibrium shape become more reliable. However, the shape of the equilibrium crystal is rarely observed in the melt, because the shape is often affected by the temperature field. Under metastable conditions, in the melt itself The equilibrium crystal shape of a is first transformed into cellular, and then into framework and dendrite [40,41]. Depending on the growth conditions, the external conditions under which a piece of crystal shape is in equilibrium may vary. However, the shape of framework crystals is Certainly, because it is determined by the properties of the lattice lattice. Therefore, the equilibrium shape can be established by the analysis of the crystal skeleton and dendrites.

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