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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The density of most materials increases when they solidify. For this reason, the solid state is heavier and sinks to the bottom. However, the competition between van der Waals attractions and hydrogen‐bond‐driven expansion leads to solid ice being less dense than its melt (Brini et al. 2017). Ice is thus lighter than water and floats on top. This simple fact enables life to thrive in water under ice‐covered regions of the globe. This density difference also results in rather unique changes with the application of pressure. Due to the reduced density upon solidification, the slope of phase boundary between the solid and liquid on the pressure–temperature phase diagram is negative, whereas it is positive for most other materials. While applying pressure to most liquids freezes them into solids, applying pressure to ice transforms it into a liquid. For most engineering materials, this aspect may not be considered highly important, but as we go deep inside the earth, this becomes crucial and complicates the simplistic use of the melting point for defining a solid from its liquid state. This naturally leads to the use of viscous state for materials at great depths.

Schematic illustration of simplified phase diagram of water showing stable ice polymorphs.

      Source: Adapted from Chaplin (n.d.) and Brini et al. (2017).

Schematic illustration of snowflake and hexagonal ice.

      Source: International Glaciological Society (2009).

      (b) Arrangement of water molecules forming a hexagonal structure with surrounding water molecules in gray. (c) Positions of oxygen atoms in (0001) or basal plane. (d) Hexagonal prism with three equatorial axes: a1, a2, and a3, and one c‐axis or optic‐axis, ˂0001˃.

      As defined in Section 2.1, ceramics are inorganic materials primarily held by covalent or ionic bonds. The ceramic class of materials thus encompasses a broad range of materials, including the network glasses explored in Section 2.4. In this section, we focus on crystalline ceramics and ceramic composites.

      Ceramics include some of the earliest man‐made materials, such as brick and pottery produced from clay. Modern ceramics include the widely used asphalt concrete in roads and pavements, high dielectric materials used in electronics, and strong, lightweight, high‐temperature materials for engines and turbines.

      The crystalline structures of ceramic materials are generally more complex than those of metals due to their composition of atoms of widely different sizes and bonding capability. In ionic bonding, the development of negatively charged ions or anions and positively charged ions or cations strongly impacts the structural characteristics of the ceramic. Although ionic bonds are nondirectional, spatial considerations lead to positive ions being surrounded by negative ions in definite ordered structures that depend on the nature of the cation. The coordination number (CN), which is the number of atoms/ions bonded to a central atom, also depends on the relative sizes of the atoms involved. The structure of many ceramics, particularly those with large oxygen anions, often includes smaller ions occupying interstitial sites.

Schematic illustration of (a) NaCl structure with sixfold polyhedral coordination and (b) zincblende structure with tetrahedral coordination.

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