Solid materials are formed from densely-packed atoms, with intense interaction forces between them. These interactions are responsible for the mechanical (e.g. hardness and elasticity), thermal, electrical, magnetic and optical properties of solids. Depending on the material involved and the conditions in which it was formed, the atoms may be arranged in a regular, geometric pattern (crystalline solids, which include metals and ordinary water ice) or irregularly (an amorphous solid such as common window glass).
The forces between the atoms in a solid can take a variety of forms. For example, in a crystal of sodium chloride (common salt), the crystal is made up of ionic sodium and chlorine, and held together with ionic bonds. In others, the atoms share electrons and form covalent bonds. In metals, electrons are shared in metallic bonding. Other solids, particularly including most organic compounds, are held together with van der Waals forces resulting from the polarisation of the electronic charge cloud on each molecule. The differences between the types of solid result from the differences between their bonding.
Crystal and glass
In crystalline solids, the atoms or molecules that compose the solid are packed closely together. These constituent elements have fixed positions in space relative to each other. This accounts for the solid's structural rigidity. In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A specific symmetry or crystal structure is composed of a Bravais lattice which is typically represented by a single unit cell. The unit cell is periodically repeated in three dimensions on a lattice. The spacings between unit cells in various directions are called lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its physical properties, such as cleavage, electronic band structure, and optical properties.
Glasses do not exhibit the long-range order exhibited by crystalline substances. Strongly supercooled liquids behave partly as liquids, partly as glasses, depending on the time scale of observation (see glass transition).
Much work has been done to elucidate the primary microstructural features of glass forming substances (e.g. silicates) on both small (microscopic) and large (macroscopic) scales. One emerging school of thought is that a glass is simply the "limiting case" of a polycrystalline solid at small crystal size. Within this framework, domains, exhibiting various degrees of short-range order, become the building blocks of both metals and alloys, as well as glasses and ceramics. The microstructural defects of both within and between these domains provide the natural sites for atomic diffusion and the occurrence of viscous flow and plastic deformation in solids.[1]
Classes of solids
Metals
The study of metallic elements and their alloys makes up a significant portion of the fields of solid-state chemistry, physics, materials science and engineering. Generally speaking, metals have delocalized electrons and an electronic band structure containing partially filled bands. The resulting large number of free electrons (often referred to as a "sea of electrons") gives metals their high values of electrical and thermal conductivity. The free electrons also prevent transmission of visible light, making metals opaque, shiny and lustrous.
When considering the electronic band structure and binding energy of a metal, it is necessary to take into account the positive potential caused by the specific arrangement of the ion cores, which is periodic in crystals. The most important consequence of the periodic potential is the formation of a small band gap at the boundary of the Brillouin zone. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the nearly free electron model.
Mechanical properties of metals include their ductility, which is largely due to their inherent capacity for plastic deformation. Thus, elasticity in metals can be described by Hooke's Law for restoring forces, where the stress is linearly proportional to the strain. Larger forces in excess of the elastic limit may cause a permanent (irreversible) deformation of the object. This is what is known in the literature as plastic deformation -- or plasticity. This irreversible change in atomic arrangement may occur as a result of either (or both) of the following factors:
- The action of an applied force (or work)
- A change in temperature (or heat).
In the former case, the applied force may be tensile (pulling) force, compressive (pushing) force, shear, bending or torsion (twisting) forces. In the latter case, the most significant factor which is determined by the temperature is the mobility of the structural defects such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. The movement or displacement of such mobile defects is thermally activated, and thus limited by the rate of atomic diffusion.
Viscous flow near grain boundaries, for example, can give rise to internal slip, creep, fatigue in metals. It can also contribute to significant changes in the microstructure like grain growth and localized densification due to the elimination of intergranular porosity. Screw dislocations may slip in the direction of any lattice plane containing the dislocation, while the principal driving force for "dislocation climb" is the movement or diffusion of vacancies through a crystal lattice.
Polymers
Other than metals, polymers and ceramics are also an important part of materials science. Polymers are the raw materials (the resins) used to make what we commonly call plastics. Plastics are the final product, created after one or more polymers or additives have been added to a resin during processing, which is then shaped into a final form. Polymers which have been around, and which are in current widespread use, include carbon-based polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylons, polyesters, acrylics, polyurethane, and polycarbonates, and silicium-based silicones. Plastics are generally classified as "commodity", "speciality" and "engineering" plastics.
Ceramics
A ceramic material may be defined as any inorganic polycrystalline solid or mineral. Mechanically speaking, ceramic materials are brittle, hard, strong in compression, weak in shearing and tension. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness indicates how much energy a material can absorb before mechanical failure, while fracture toughness (denoted KIc ) describes the ability of a material with inherent microstructural flaws to resist fracture via crack growth and propagation. If a material has a large value of fracture toughness, the basic principles of fracture mechanics suggest that it will most likely undergo ductile fracture. Brittle fracture is very characteristic of most ceramic and glass-ceramic materials which typically exhibit low (and inconsistent) values of KIc.
Ceramic solids are chemically inert (or stable), and often are capable of withstanding chemical erosion that occurs in an acidic or caustic environment. Ceramics generally can withstand high temperatures ranging from 1,000 °C to 1,600 °C (1,800 °F to 3,000 °F). Exceptions include non-oxide inorganic materials, such as nitrides, borides and carbides.
Ceramic engineering is the science and technology of creating solid-state devices from inorganic, non-metallic materials. This is done either by the action of heat, or, at lower temperatures, using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components, and the study of their structure, composition and properties. Ceramic materials may have a crystalline or partly crystalline structure, with long-range order on a molecular scale.
Glass ceramics may have an amorphous or glassy structure, with limited or short-range molecular order. They are typically formed from a molten mass that solidifies on cooling, or formed and matured by the action of heat. Glass by definition is not a ceramic because, although it may be identical in chemical composition (e.g. glassy SiO2 vs. crystalline quartz) it is an amorphous solid.
Traditional ceramic raw materials include clay minerals such as kaolinite, more recent materials include aluminium oxide (alumina). The modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance, and hence find use in such applications as the wear plates of crushing equipment in mining operations. Advanced ceramics are also used in the medicine, electrical and electronics industries.
Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque materials, as opposed to transparent materials. Recent nanoscale (e.g. sol-gel) technology has, however, made possible the production of polycrystalline transparent ceramics such as transparent alumina and alumina compounds for such applications as high-power lasers.
Composites
Composite materials are structured materials composed of two or more macroscopic phases. While there is considerable interest in composites with one or more non-ceramic constituents, the greatest attention is on composites in which all constituents are ceramic. These typically comprise two ceramic constituents: a continuous matrix, and a dispersed phase of ceramic particles, whiskers, or short (chopped) or continuous ceramic fibers.
The challenge, as in wet chemical processing, is to obtain a uniform distribution of the dispersed particle or fiber phase. Applications range from structural elements such as steel-reinforced concrete, to the thermally insulative tiles used to protect the surface of NASA Space Shuttles from the heat of re-entry into the Earth's atmosphere. Domestic examples can be seen in the "plastic" casings of television sets, cell-phones and so on. These plastic casings are usually a composite made up of a thermoplastic matrix such as acrylonitrile butadiene styrene (ABS) in which calcium carbonate chalk, talc, glass fibers or carbon fibers have been added for strength, bulk, or electro-static dispersion. These additions may be referred to as reinforcing fibers, or dispersants, depending on their purpose.
Biomaterials
Most natural (or biological) materials are complex composites whose mechanical properties are often outstanding, considering the weak constituents from which they are assembled. These complex structures, which have risen from hundreds of million years of evolution, are inspiring materials scientists in the design of novel materials. Their defining characteristics include structural hierarchy, multifunctionality and self-healing capability. Self-organization is also a fundamental feature of many biological materials and the manner by which the structures are assembled from the molecular level up.
The basic building blocks often begin with the 20 amino acids, and proceed to polypeptides, polysaccharides, and polypeptides–saccharides. These compose the basic proteins, which are the primary constituents of ‘soft tissues’ and are also present in most biominerals. There are over 1000 proteins, including collagen, chitin, keratin, and elastin. The ‘hard’ phases of biomaterials are primarily strengthened by minerals, which nucleate and grow in a biomediated environment that determines the size, shape and distribution of individual crystals. The most important mineral phases hydroxyapatite, silica, and aragonite.
Thus, the principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials are being investigated. Molecular self-assembly is found widely in biological organisms and provides the basis of a wide variety of biological structures. For example, the crystallization of inorganic materials in nature generally occurs at ambient temperature and pressure. Yet the vital organisms through which these inorganic materials form are able to create extremely precise and complex structures. Understanding the process in which living organisms control the growth of inorganic materials could lead to significant advances in materials science, opening the door to novel synthesis techniques for nanoscale composite materials.
One system which has been under intense scientific scrutiny by several major research groups is the microstructure of the mother-of-pearl (or nacre) portion of the abalone shell. This natural material exhibits the highest mechanical strength and fracture toughness of any non-metallic substance known. Electron microscopy has revealed neatly stacked (or ordered) mineral tiles separated by thin organic sheets along with a macrostructure of larger periodic growth bands which collectively form what scientists are currently referring to as a hierarchical composite structure. (The term hierarchy simply implies that there is a range of structural features which exist over a wide range of length scales). Early work showed that the overall nacre composite consists of only 5 wt.% organic material. Yet the work necessary to fracture the body was increased by up to 3000 times over inorganic CaCO3 crystals as a result of the intricate hierarchy of structural organization.[2][3]
Self-assembly is also emerging as a new strategy in chemical synthesis, nanotechnology and biotechnology. Technical ceramics are in a very dynamic stage of development because of the increasingly diverse nature of ceramic needs and opportunities. This introduces an increasing need for improved properties, greater uniformity, reproducibility and reliability. This is coupled with the need for larger scale, more efficient production. All of these demands can benefit from further development in both basic science and the engineering aspects of the field.
Semiconductors
Semiconductors are materials that have an electrical resistivity (and conductivity) between that of metallic conductors and non-metallic insulators. They can be found in the periodic table moving diagonally downward right from boron. They separate the electrical conductors (or metals, to the left) from the insulators (to the right).
Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, etc. Semiconductor devices include the transistor, solar cells, diodes and integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light energy into electrical energy.
In a metallic conductor, current is carried by the flow of a "sea of electrons". In semiconductors, current can be carried either by the flow of electrons or by the flow of positively charged "holes" in the electronic band structure of the material. Silicon is used to create most semiconductors. Other semiconductor materials of commercial interest include germanium (Ge) and gallium arsenide (GaAs).
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