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Classification of materials
Solid materials have been conveniently grouped into three basic classifications: metals, ceramics, and polymers. This scheme is bad primarily on chemical makeup and atomic structure, and most materials fall into one distinct grouping or another, although there are some intermediates. In addition, there are three other groups of important engineering materials—composites, miconductors, and biomaterials. Composites consist of combinations of two or more different materials, whereas miconductors are utilized becau of their unusual electrical characteristics; biomaterials are implanted into the human body. A brief explanation of the material types and reprentative characteristics is offered next.
Metals
Metallic materials are normally combinations of metallic elements. They have large numbers of nonlocalized electrons; that is, the electrons are not bound to particular atoms. Many properties of metals are directly attributable to the electrons. Metals are extremely good conductors of electricity and heat and are not transparent to visible light;
a polished metal surface has a lustrous appearance. Furthermore, metals are quite strong, yet deformable, which accounts for their extensive u in structural applications. Ceramics
Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. The wide range of materials that falls within this classification includes ceramics that are compod of clay minerals, cement, and glass. The materials are typically insulative to the passage of electricity and heat, and are more resistant to high temperatures and harsh environments than metals and polymers. With regard to mechanical behavior, ceramics are hard but very brittle.
Polymers
Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically bad on carbon, hydrogen, and other nonmetallic elements; furthermore, they have very large molecular structures. The materials typically have low densities and may be extremely flexible.
Composites
humorA number of composite materials have been engineered that consist of more than one
material type. Fiberglass is a familiar example, in which glass fibers are embedded within a polymeric material. A composite is designed to display a combination of the best characteristics of each of the component materials. Fiberglass acquires strength from the glass and flexibility from the polymer. Many of the recent material developments have involved composite materials.
Semiconductors
Semiconductors have electrical properties that are intermediate between the electrical conductors and insulators. Furthermore, the electrical characteristics of the materials are extremely nsitive to the prence of minute concentrations of impurity atoms, which concentrations may be controlled over very small spatial regions. The miconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries (not to mention our lives) over the past two decades.
Biomaterials
Biomaterials are employed in components implanted into the human body for replacement of diad or damaged body parts. The materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cau adver biological reactions). All of the above mat
erials—metals, ceramics, polymers, composites, and miconductors—may be ud as biomaterials. For example, in Section 20.8 are discusd some of the biomaterials that are utilized in artificial hip replacements. Crystalline material
A solid is a material that retains both its shape and volume over time. If a solid posss long range, regularly repeating units, it is classified as a crystalline material. Crystalline solids are only produced when the atoms, ions, or molecules have an opportunity to organize themlves into regular arrangements, or lattices. For example, crystalline minerals found in nature have been formed through many years of extreme temperature and pressure, or slow evaporation process. Most naturally occurring crystalline solids compri an agglomeration of individual microcrystalline units; single crystals without significant defects are extremely rare in nature, and require special growth techniques.
Amorphous material
If there is no long-range structural order throughout the solid, the material is best described as amorphous. Quite often, the materials posss considerable shortrange order. However, the lack of long-range translational order (periodicity) parates this class of materials from their crystalline counterparts. Since the majority of studies have been addresd to study crystalline solids
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假期实践报告relative to their amorphous counterparts, there
is a common misconception that most solids are crystalline in nature. In fact, a solid product generated from many chemical reactions will be amorphous by default, unless special procedures are ud to facilitate molecular ordering (i.e., crystal formation). Although the crystalline state is more thermodynamically favorable than the disordered state, the formation of amorphous materials is favored in kinetically bound process (e.g., chemical vapor deposition, sol–gel, solid precipitation, etc.).
Quasicrystal material
In 1982, Shechtman discovered a new atomic structure when studying a rapidly cooled mix of aluminum and mangane. Unlike a regular crystal, which has an orderly, repeating structure, this material contained a pattern that never repeated. Many other kinds of quasicrystals have been discovered since then. Dan Shechtman, was awarded the 2011 Nobel Prize in Chemistry. Quasi-periodic materials have certain properties which are unique, such as electrical properties, optical properties, hardness and nonstick properties. The direction of light through this material is different. Electrically, they behave in a very peculiar way depending on temperature. Some of the properties have been put to u.
The Unit cell
The smallest repeating unit of an extended crystal lattice is known as the unit cell, which governs the symmetry and structure of the overall bulk crystal. Oftentimes, it is possible to define a number of possible repeat units for a crystal lattice (Figure 2.6). The proper lection of a unit cell reprents the smallest repeatable unit that posss the same symmetry elements of the bulk crystal, and if translated along the x, y, and z axes, will generate the entire extended crystal lattice.
Figure 2.7 provides a schematic of a general structure for a unit cell. It is convenient to describe the units as having three vectors (a, b and c) that may or may not be aligned along the Cartesian axes, depending on the values of unit cell angles. Depending on the geometry and volume of the unit cell, there are ven crystal systems that may be generated (Table 2.2).
b2c是什么意思
Since there are many different possible crystal structures, it is sometimes convenient to divide them into groups according to unit cell configurations and/or atomic arrangements. One such scheme is bad on the unit cell geometry, that is, the shape of the appropriate unit cell parallelepiped without regard to the atomic positions in the cell. Within this framework, an x, y, z coordinate system is established with its origin at one of the unit cell corners; each of the x, y, and z axes coincides with o
ne of the three parallelepiped
mstedges that extend from this corner, as illustrated in Figure 3.4. The unit cell geometry is completely defined in terms of six parameters: the three edge lengths a, b, and c, and the three interaxial angles α, β, and γ. The are indicated in Figure 3.4, and are sometimes termed the lattice parameters of a crystal structure.
On this basis there are ven different possible combinations of a, b, and c, and α, β, and γ, , and each of which reprents a distinct crystal system. The ven crystal systems are cubic, tetragonal, hexagonal, orthorhombic, rhombohedral,2 monoclinic, and triclinic. The lattice parameter relationships and unit cell sketches for each are reprented in Table 3.2.The cubic system, for
which and has the greatest degree of symmetry. Least symmetry is displayed by the triclinic system, since挚爱的英文
From the discussion of metallic crystal structures, it should be apparent that both FCC and BCC structures belong to the cubic
crystal system, whereas HCP falls within hexagonal. The conventional hexagonal unit cell really consists of three parallelepipeds
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nobelsituated as shown in Table 3.2.
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