Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 84

There are seven different unit cell shapes, called crystal systems, defined by their unit cell lengths and interaxial angles (Figure 3.6). When packed together, each of these seven unit cell shapes fills up three‐dimensional space completely. The unit cell contents are specified in terms of where the atoms are located. Consequently, depending on the location of the atoms in the unit cell, some crystal systems can generate more than one crystal lattice. The cubic system, for example, can generate three different crystal lattices, the simple cubic, BCC and FCC lattice. In total, there are 14 different crystal lattices, called Bravais lattices (Figure 3.7).

Figure 3.6 The seven crystal systems and their parameters.

Figure 3.7 The 14 Bravais lattices.

Different planes and directions in crystalline solids can have a different arrangement of atoms as, for example, in a pure metal such as Ti. Depending on the composition of the solid, different planes and directions can also have a different combination of atoms. Some planes in sodium chloride (NaCl), for example, have different ratios of sodium to chlorine atoms. Consequently, some solids can show different properties in different planes or directions. Often, it is necessary to describe specific atomic positions, lattice planes or lattice directions in a succinct way. How this is done is described in Section 3.6.