Remaining Material Science Terms

large-particle composite

A type of particle-reinforced composite in which particle-matrix interactions cannot be treated on an atomic level; the particles reinforce the matrix phase.

dispersion strengthening

A means of strengthening materials in which very small particles (usually 0.1 μm) of a hard, inert phase are uniformly dispersed within a load-bearing matrix phase.

rule of mixtures

The properties of a multiphase alloy or composite material are a weighted average (usually on the basis of volume) of the properties of the individual constituents.

cermet

A composite material consisting of a combination of ceramic and metallic materials. The most common cermets are the cemented carbides, composed of an extremely hard ceramic (e.g., WC, TiC), bonded together by a ductile metal such as cobalt or nickel.

concrete

A composite material consisting of aggregate particles bound together in a solid body by a cement.

reinforced concrete

Concrete that is reinforced (or strengthened in tension) by the incorporation of steel rods, wires, or mesh.

prestressed concrete

Concrete into which compressive stresses have been introduced using steel wires or rods.

fiber-reinforced composite

A composite in which the dispersed phase is in the form of a fiber (i.e., a filament that has a large length-to-diameter ratio).

specific strength

The ratio of tensile strength to specific gravity for a material.

specific modulus (specific stiffness)

The ratio of elastic modulus to specific gravity for a material.

longitudinal direction

The lengthwise dimension. For a rod or fiber, in the direction of the long axis.

transverse direction

A direction that crosses (usually perpendicularly) the longitudinal or lengthwise direction.

whisker

A very thin, single crystal of high perfection that has an extremely large length-to-diameter ratio. Whiskers are used as the reinforcing phase in some composites.

fiber

Any polymer, metal, or ceramic that has been drawn into a long and thin filament.

polymer-matrix composite (PMC)

A composite material for which the matrix is a polymer resin and fibers (normally glass, carbon, or aramid) are the dispersed phase.

metal-matrix composite (MMC)

A composite material that has a metal or metal alloy as the matrix phase. The dispersed phase may be particulates, fibers, or whiskers, which normally are stiffer, stronger, and/or harder than the matrix.

ceramic-matrix composite (CMC)

A composite for which both matrix and dispersed phases are ceramic materials. The dispersed phase is normally added to improve fracture toughness.

Ohm’s Law

The applied voltage is equal to the product of the current and resistance; equivalently, the current density is equal to the product of the conductivity and electric field intensity.

resistivity (ρ)

The reciprocal of electrical conductivity; a measure of a material's resistance to the passage of electric current.

conductivity, electrical

The proportionality constant between current density and applied electric field; also, a measure of the ease with which a material is capable of conducting an electric current.

metal

The electropositive elements and the alloys based on these elements. The electron band structure of metals is characterized by a partially filled electron band.

insulator (electrical)

A nonmetallic material that has a filled valence band at 0 K and a relatively wide energy band gap. Consequently, the room-temperature electrical conductivity is very low, less than about 10^-10 (Ωm)^-1

semiconductor

A nonmetallic material that has a filled valence band at 0 K and a relatively narrow energy band gap. The room-temperature electrical conductivity ranges between about 10^-6 and 10⁴ (Ωm)^-1

electron energy band

A series of electron energy states that are very closely spaced with respect to energy.

Fermi energy (Ef)

For a metal, the energy corresponding to the highest filled electron state at 0 K.

valence band

For solid materials, the electron energy band that contains the valence electrons.

conduction band

For electrical insulators and semiconductors, the lowest-lying electron energy band that is empty of electrons at 0 K. Conduction electrons are those that have been excited to states within this band.

energy band gap

For semiconductors and insulators, the energies that lie between the valence and conduction bands; for intrinsic materials, electrons are forbidden to have energies within this range.

free electron

An electron that has been excited into an energy state above the Fermi energy (or into the conduction band for semiconductors and insulators) and may participate in the electrical conduction process.

hole (electron)

For semiconductors and insulators, a vacant electron state in the valence band that behaves as a positive charge carrier in an electric field.

mobility (electron and hole)

The proportionality constant between the carrier drift velocity and applied electric field; also, a measure of the ease of charge carrier motion.

matthiessen’s rule

The total electrical resistivity of a metal is equal to the sum of temperature-, impurity-, and cold-work-dependent contributions.

intrinsic semiconductor

A semiconductor material for which the electrical behavior is characteristic of the pure material—that is, in which electrical conductivity depends only on temperature and the band gap energy.

extrinsic semiconductor

A semiconducting material for which the electrical behavior is determined by impurities.

donor state (level)

For a semiconductor or insulator, an energy level lying within and near the top of the energy band gap and from which electrons may be excited into the conduction band. It is normally introduced by an impurity atom.

acceptor state (level)

For a semiconductor or insulator, an energy level lying within yet near the bottom of the energy band gap that may accept electrons from the valence band, leaving behind holes. The level is normally introduced by an impurity atom.

doping

The intentional alloying of semiconducting materials with controlled concentrations of donor or acceptor impurities.