Engineering Physics of High-Temperature Materials. Nirmal K. Sinha. Читать онлайн. Newlib. NEWLIB.NET

Автор: Nirmal K. Sinha
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119420460
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efficiency in packing leads to the abundance of three main crystal structures. While these structures do not describe all materials, their prevalence makes them important to dive into and enables understanding of similar structures. Two of these structures are based on a simple cubic arrangement. The simple cubic structure consists of atoms placed on each corner of a cube. The simple cubic unit cell is thus a cube containing 1/8 of each corner atom for a total of one atom per unit cell. The distance between the corners of the cube is called the unit cell dimension, which is commonly denoted as the lattice constant, “a.” Assuming the highest packing efficiency in the simple cube and the model of an atom as incompressible spheres of radius “r” we have a = 2r. Three of the most common crystal structures in nature are the body‐centered cubic (BCC) structure, the face‐centered cubic (FCC) structure, and the hexagonal close‐packed (HCP) structure. As can be seen in Figure 2.1, the first two structures are based on the simple cube.

Schematic illustration of basic crystal lattice structures: (a) BCC, (b) FCC, and (c) HCP crystals.

      The BCC structure takes the simple unit cell and increases the packing efficiency by having an additional atom at the center of the cube. Thus, there are two atoms per unit cell. The diagonal passing through the center of the BCC unit cell is called the close‐packed direction because atoms along the diagonal touch each other. The length of the diagonal is thus equal to 4r and a equals StartFraction 4 Over square-root 3 EndFraction r. Common BCC materials include iron, chromium, and tungsten.

      The HCP structure, which is the last of the most common structures, also gives rise to the same packing efficiency as does the FCC structure and thus the closest packing of spheres in a cube. As shown in Figure 2.2, it can be seen to have an ABA stacking sequence. The HCP unit cell is not based on a simple cube, but rather, as its name implies, on a hexagonal structure with atoms at each corner of the hexagon. An additional atom is located at the center of each face of the hexagon and three more atoms are located in a plane between the hexagonal faces. The hexagonal unit cell contains within it three primitive unit cells (the smallest repeating unit). This primitive cell is represented as the shaded region in the top‐down view of the HCP unit cell in Figure 2.1c. The larger hexagonal unit cell is easier to visualize, and it contains six atoms per cell. Common HCP metallic materials include magnesium, zinc, and titanium. Snow and ice, which are among the most abundant, but nonmetallic, materials on Earth's surface, also belong to the hexagonal family system, though these materials do not have the close‐packed structure at normal pressures and temperatures on earth’s surface. Ice is discussed in greater detail in Section 2.6.

Schematic illustration of stacking sequence in (a) FCC and (b) HCP crystals.

      Amorphous solids can best be described by what they are not. They are materials that exhibit elastic response like crystalline solids, but possess no crystal‐like long‐range order in the spatial arrangements of their atoms. Amorphous materials could be inorganic or organic and are also known as glasses and vitreous solids. Glasses are increasingly being used in wide‐ranging branches of science and technology. Bulk amorphous metals (aka metallic glasses) are increasingly being used as engineering materials owing to their extraordinary isotropic properties, including high strength, corrosion resistance, formability, and even biocompatibility (Khan et al. 2017; Jafary‐Zadeh et al. 2018). Molten minerals quenched to an amorphous state are used as analogs for magma (molten rocks) deep inside the surface of Earth (Davidovits 2005).


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Coal Graphite Diamond Buckminsterfullerene (C60) Carbon nanotubes
Schematic illustration of the structure of coal. Schematic illustration of the structure of graphite. Schematic illustration of the structure of diamond. Schematic illustration of the structure of Buckminsterfullerene. Schematic illustration of the structure of Carbon nanotubes.
Structure/hybridization Amorphous (varies) Trigonal planar (sp2)