Properties and Characteristics of Materials

Thermal Properties

Magnetic Properties

Optical Properties

Electrical Properties

Thermal Properties

Why Study the Thermal Properties of Materials?

Of the three primary material types, ceramics are the most susceptible to thermal shockbrittle fracture resulting from internal stresses that are established within a ceramic piece as a result of rapid changes in temperature (normally upon cooling). Thermal shock is normally an undesirable event, and the susceptibility of a ceramic material to this phenomenon is a function of its thermal and mechanical properties (coefficient of thermal expansion, thermal conductivity, modulus of elasticity, and fracture strength).

Introduction

Thermal property refers to the response of a material to the application of heat. As a solid absorbs energy in the form of heat, its temperature rises and its dimensions increase. The energy may be transported to cooler regions of the specimen if temperature gradients exist, and, ultimately, the specimen may melt. Heat capacity, thermal expansion, and thermal conductivity are properties that are often critical in the practical use of solids.

Heat Capacity

A solid material, when heated, experiences an increase in temperature, signifying that some energy has been absorbed. Heat capacity indicates a materials ability to absorb heat from the external surroundings; it represents the amount of energy required to produce a unit temperature riseDefinition of heat capacityratio of energy change (energy gained or lost) and the resulting temperature change.

Heat capacity represents the quantity of heat required to produce a unit rise in temperature for 1 mole of a substance; on a per-unit-mass basis, it is termed specific heat.Most of the energy assimilated by many solid materials is associated with increasing the vibrational energy of the atoms.Only specific vibrational energy values are allowed (the energy is said to be quantized); a single quantum of vibrational energy is called a phonon.

Thermal Expansion

Solid materials expand when heated and contract when cooled. The fractional change in length is proportional to the temperature change, the constant of proportionality being the coefficient of thermal expansion.Thermal expansion is reflected by an increase in the average interatomic separation, which is a consequence of the asymmetric nature of the potential energyversus- interatomic spacing curve trough. The larger the interatomic bonding energy, the lower is the coefficient of thermal expansion.

Values of coefficient of thermal expansion for polymers are typically greater than those for metals, which in turn are greater than those for ceramic materials.

Thermal Conductivity

The transport of thermal energy from high to low temperature regions of a material is termed thermal conduction.For solid materials, heat is transported by free electrons and by vibrational lattice waves, or phonons.The high thermal conductivities for relatively pure metals are due to the large numbers of free electrons and the efficiency with which these electrons transport thermal energy. By way of contrast, ceramics and polymers are poor thermal conductors because free-electron concentrations are low and phonon conduction predominates.

Thermal StressesThermal stresses are stresses induced in a body as a result of changes in temperature. An understanding of the origins and nature of thermal stresses is important because these stresses can lead to fracture or undesirable plastic deformation.The capacity of a material to withstand this kind of failure is termed its thermal shock resistance. For a ceramic body that is rapidly cooled, the resistance to thermal shock depends not only on the magnitude of the temperature change, but also on the mechanical and thermal properties of the material. The thermal shock resistance is best for ceramics that have high fracture strengths and high thermal conductivities, as well as low moduli of elasticity and low coefficients of thermal expansion.

Formulas

Materials of ImportanceOne type of thermostat - a device that is used to regulate temperature uses the phenomenon of thermal expansion - the elongation of a material as it is heated. The heart of this type of thermostat is a bimetallic strip - strips of two metals having different coefficients of thermal expansion are bonded along their lengths.A change in temperature causes this strip to bend; upon heating, the metal having the greater expansion coefficient elongates more, producing the direction of bending shown in the figure.

Some Applications that Require Dimensional Stability with Temperature Fluctuations Compensating pendulums and balance wheels for mechanical clocks and watches. Structural components in optical and laser measuring systems that require dimensional stabilities on the order of a wavelength of light. Bimetallic strips that are used to actuate microswitches in water heating systems. Shadow masks on cathode-ray tubes that are used for television and display screens; higher contrast, improved brightness, and sharper definition are possible using low-expansion materials. Vessels and piping for the storage and piping of liquefied natural gas.

Magnetic Properties

Why Study the Magnetic Properties of Materials?

An understanding of the mechanism that explains the permanent magnetic behavior of some materials may allow us to alter and in some cases tailor the magnetic properties.

Introduction

Magnetism - the phenomenon by which materials exert an attractive or repulsive force or influence on other materials - has been known for thousands of years. However, the underlying principles and mechanisms that explain magnetic phenomena are complex and subtle, and their understanding has eluded scientists until relatively recent times.Many modern technological devices rely on magnetism and magnetic materials; these include electrical power generators and transformers, electric motors, radio, television, telephones, computers, and components of sound and video reproduction systems.

Basic Concepts

Magnetic DipolesMagnetic forces are generated by moving electrically charged particles; these magnetic forces are in addition to any electrostatic forces that may exist. Often it is convenient to think of magnetic forces in terms of fields. Imaginary lines of force may be drawn to indicate the direction of the force at positions in the vicinity of the field source.

Magnetic Field VectorsThe externally applied magnetic field, sometimes called the magnetic field strength.The magnetic induction, or magnetic flux density, represents the magnitude of the internal field strength within a substance that is subjected to a magnetic field.The permeability, is a property of the specific medium through which the magnetic field passes and in which magnetic induction is measured.Magnetic flux density - as a function of magnetic field strength and magnetization of a material. Magnetization of a material - dependence on susceptibility and magnetic field strength.

DiamagnetismDiamagnetism is a very weak form of magnetism that is nonpermanent and persists only while an external field is being applied. It is induced by a change in the orbital motion of electrons due to an applied magnetic field. The magnitude of the induced magnetic moment is extremely small and in a direction opposite to that of the applied field.

ParamagnetismFor some solid materials, each atom possesses a permanent dipole moment by virtue of incomplete cancellation of electron spin and/or orbital magnetic moments. In the absence of an external magnetic field, the orientations of these atomic magnetic moments are random, such that a piece of material possesses no net macroscopic magnetization.These atomic dipoles are free to rotate, and paramagnetism results when they preferentially align, by rotation, with an external field. These magnetic dipoles are acted on individually with no mutual interaction between adjacent dipoles.

FerromagnetismCertain metallic materials possess a permanent magnetic moment in the absence of an external field and manifest very large and permanent magnetizations. These are the characteristics of ferromagnetism, and they are displayed by the transition metals iron, cobalt, nickel, and some rare earth metals such as gadolinium (Gd).The maximum possible magnetization, or saturation magnetization, of a ferromagnetic material represents the magnetization that results when all the magnetic dipoles in a solid piece are mutually aligned with the external field; there is also a corresponding saturation flux density.

AntiferromagnetismMagnetic moment coupling between adjacent atoms or ions also occurs in materials other than those that are ferromagnetic. In one such group, this coupling results in an antiparallel alignment; the alignment of the spin moments of neighboring atoms or ions in exactly opposite directions is termed antiferromagnetism.

FerrimagnetismSome ceramics also exhibit a permanent magnetization, termed ferrimagnetism. The macroscopic magnetic characteristics of ferromagnets and ferrimagnets are similar; the distinction lies in the source of the net magnetic moments

Formulas

Materials of ImportanceTransformer cores require the use of soft magnetic materials, which are easily magnetized and demagnetized (and also have relatively high electrical resistivities).

Optical Properties

Why Study the Optical Properties of Materials?

When materials are exposed to electromagnetic radiation, it is sometimes important to be able to predict and alter their responses. This is possible when we are familiar with their optical properties and understand themechanisms responsible for their optical behaviors.

Introduction

Optical property refers to a materials response to exposure to electromagnetic radiation and, in particular, to visible light. This chapter first discusses some of the basic principles and concepts relating to the nature of electromagnetic radiation and its possible interactions with solid materials. Then it explores the optical behaviors of metallic and nonmetallic materials in terms of their absorption, reflection, and transmission characteristics.

Basic Concepts

Electromagnetic RadiationIn the classical sense, electromagnetic radiation is considered to be wavelike, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation. Light, heat (or radiant energy), radar, radio waves, and x-rays are all forms of electromagnetic radiation.Sometimes it is more convenient to view electromagnetic radiation from a quantum mechanical perspective, in which the radiation, rather than consisting of waves, is composed of groups or packets of energy, which are called photons.

Light Interactions with SolidsMaterials that are capable of transmitting light with relatively little absorption and reflection are called transparent - one can see through them.Translucent materials are those through which light is transmitted diffusely; that is, light is scattered within the interior to the degree that objects are not clearly distinguishable when viewed through a specimen of the material.Materials that are impervious to the transmission of visible light are termed opaque.

Optical Properties of Nonmetals

RefractionLight that is transmitted into the interior of transparent materials experiences a decrease in velocity and, as a result, is bent at the interface; this phenomenon is termed refraction. The index of refraction of a material is defined as the ratio of the velocity in a vacuum to the velocity in the medium

Reflection

When light passes from one transparent medium to another having a different index of refraction, some of it is reflected at the interface.When light radiation passes from one medium into another having a different index of refraction, some of the light is scattered at the interface between the two media even if both are transparent.

Absorption

Pure nonmetallic materials are either intrinsically transparent or opaque.Some light absorption occurs in even transparent materials as a consequence of electronic polarization.

Color

Transparent materials appear colored as a consequence of specific wavelength ranges of light that are selectively absorbed.The color discerned is a result of the distribution of wavelength ranges in the transmitted beam.

Applications of Optical Phenomena

LuminescenceWith luminescence, energy is absorbed as a consequence of electron excitations, which is subsequently reemitted as visible light. When light is reemitted less than 1s after excitation, the phenomenon is called fluorescence. For longer reemission times, the term phosphorescence is used.Electroluminescence is the phenomenon by which light is emitted as a result of electronhole recombination events that are induced in a forward-biased diode.The device that experiences electroluminescence is the light-emitting diode (LED).

PhotoconductivityPhotoconductivity is the phenomenon by which the electrical conductivity of some semiconductors may be enhanced by photo-induced electron transitions, by which additional free electrons and holes are generated.

LasersCoherent and high-intensity light beams are produced in lasers by stimulated electron transitions.Light Amplification by Stimulated Emission of Radiation

Optical Fibers in CommunicationUse of fiber-optic technology in modern telecommunications provides for the transmission of information that is interference-free, rapid, and intense.An optical fiber is composed of the following elements:A core through which the pulses of light propagate.The cladding, which provides for total internal reflection and containment of the light beam within the core.The coating, which protects the core and cladding from damage.

Formulas

Materials of ImportanceThe following schematic diagram illustrates the operation of a photovoltaic solar cell. The cell is made of polycrystalline silicon that has been fabricated to form a pn junction. Photons that originate as light from the sun excite electrons into the conduction band on the n side of the junction and create holes on the p side. These electrons and holes are drawn away from the junction in opposite directions and become part of an external current.

Electrical Properties

Why Study the Electrical Properties of Materials?

Consideration of the electrical properties of materials is often important when materials selection and processing decisions are being made during the design of a component or structure. For example, when we consider an integrated circuit package, the electrical behaviors of the various materials are diverse. Some need to be highly electrically conductive (e.g., connecting wires), whereas electrical insulativity is required of others (e.g., protective package encapsulation).

Introduction

The prime objective of this topic is to explore the electrical properties of materials, that is, their responses to an applied electric field. We begin with the phenomenon of electrical conduction: the parameters by which it is expressed, the mechanism of conduction by electrons, and how the electron energy-band structure of a material influences its ability to conduct. These principles are extended to metals, semiconductors, and insulators. Particular attention is given to the characteristics of semiconductors and then to semiconducting devices.

Electrical Conduction

Ohms LawOne of the most important electrical characteristics of a solid material is the ease with which it transmits an electric current. Ohms law relates the current or time rate of charge passage to the applied voltage.Electrical resistivity is the dependence on resistance, specimen cross-sectional area, and distance between measuring points.

Electrical Conductivity The ease with which a material is capable of transmitting an electric current is expressed in terms of electrical conductivity or its reciprocal, electrical resistivity. The relationship between applied voltage, current, and resistance is Ohms law. An equivalent expression, relates current density, conductivity, and electric field intensity. On the basis of its conductivity, a solid material may be classified as a metal, a semiconductor, or an insulator.

Electronic and Ionic Conduction For most materials, an electric current results from the motion of free electrons, which are accelerated in response to an applied electric field. In ionic materials, there may also be a net motion of ions, which also makes a contribution to the conduction process.

Within most solid materials a current arises from the flow of electrons, which is termed electronic conduction. In addition, for ionic materials a net motion of charged ions is possible that produces a current; this is termed ionic conduction.

Energy Band Structures in Solids The number of free electrons depends on the electron energy band structure of the material. An electron band is a series of electron states that are closely spaced with respect to energy, and one such band may exist for each electron subshell found in the isolated atom. Electron energy band structure refers to the manner in which the outermost bands are arranged relative to one another and then filled with electrons. An electron becomes free by being excited from a filled state to an available empty state at a higher energy.

Electron Mobility Free electrons being acted on by an electric field are scattered by imperfections in the crystal lattice. The magnitude of electron mobility is indicative of the frequency of these scattering events. In many materials, the electrical conductivity is proportional to the product of the electron concentration and the mobility.

Materials of ImportanceThe functioning of modern flash memory cards (and sticks) that are used to store digital information relies on the unique electrical properties of silicon, a semiconducting material.Flash memory is also used in cell phones to store programs required for making and receiving calls, as well as frequently-called telephone numbers.The modern cell phone may also have other functionalities that necessitate information storagefor texting, for games, as a camera, and/or as a video recorder.