C hapter 4 Material Classes, structure, and properties
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coefficient is the strain per unit of electric field, and although it is
very small, it is a true linear effect, which makes it useful: When
you want to position or move a probe with nanoscale precision, it
is just what you need. Pyroelectric materials contain molecules with
permanent dipole moments that, in a single crystal, are aligned,
giving the crystal a permanent polarization. When the temperature
is changed, the polarization changes, creating surface charges or, if
the surfaces are connected electrically, a pyroelectric current—the
principle of intruder-detection systems and of thermal imaging.
Ferroelectric materials, too, have a natural dipole moment; they
are polarized to start with, and the individual polarized molecules
line up so that their dipole moments are parallel, like magnetic
moments in a magnet. Their special feature is that the direction of
polarization can be changed by applying an electric field, and the
change causes a change of shape.
the physics of electrical properties
electrical conductivity
An electric field, E (volts/m), exerts a force Ee on a charged particle,
where e is the charge it carries. Solids are made up of atoms containing electrons that carry a charge −e and a nucleus containing
protons, each with a positive charge +e. If charge carriers can move,
the force Ee causes them to flow through the material—that is, it
conducts. Metals are electron conductors, meaning that the charge
carriers are the electrons. In ionic solids (which are composed of
negatively and positively charged ions such as Na
+ and Cl
− ), the diffusive motion of ions allows ionic conduction, but this is only possible at temperatures at which diffusion is rapid. Many materials have
no mobile electrons, and at room temperature they are too cold to
be ionic conductors. The charged particles they contain still feel a
force in an electric field, and it is enough to displace the charges
slightly, but they are unable to move more than a tiny fraction of
the atom spacing. These are insulators; the small displacement of
charge gives them dielectric properties.
How is it that some materials have mobile electrons and some do
not? To explain this we need two of the stranger results of quantum
mechanics. Briefly, the electrons of an atom occupy discrete energy
states or orbits, arranged in shells (designated 1, 2, 3, and so on,
from the innermost to the outermost); each shell is made up of
subshells (designated s, p, d, and f), each of which contains 1, 3, 5,
or 7 orbits, respectively. The electrons fill the shells with the lowest
energy, two electrons of opposite spin in each orbit; the Pauli exclusion principle prohibits an energy state with more than two. When
122
coefficient is the strain per unit of electric field, and although it is
very small, it is a true linear effect, which makes it useful: When
you want to position or move a probe with nanoscale precision, it
is just what you need. Pyroelectric materials contain molecules with
permanent dipole moments that, in a single crystal, are aligned,
giving the crystal a permanent polarization. When the temperature
is changed, the polarization changes, creating surface charges or, if
the surfaces are connected electrically, a pyroelectric current—the
principle of intruder-detection systems and of thermal imaging.
Ferroelectric materials, too, have a natural dipole moment; they
are polarized to start with, and the individual polarized molecules
line up so that their dipole moments are parallel, like magnetic
moments in a magnet. Their special feature is that the direction of
polarization can be changed by applying an electric field, and the
change causes a change of shape.
the physics of electrical properties
electrical conductivity
An electric field, E (volts/m), exerts a force Ee on a charged particle,
where e is the charge it carries. Solids are made up of atoms containing electrons that carry a charge −e and a nucleus containing
protons, each with a positive charge +e. If charge carriers can move,
the force Ee causes them to flow through the material—that is, it
conducts. Metals are electron conductors, meaning that the charge
carriers are the electrons. In ionic solids (which are composed of
negatively and positively charged ions such as Na
+ and Cl
− ), the diffusive motion of ions allows ionic conduction, but this is only possible at temperatures at which diffusion is rapid. Many materials have
no mobile electrons, and at room temperature they are too cold to
be ionic conductors. The charged particles they contain still feel a
force in an electric field, and it is enough to displace the charges
slightly, but they are unable to move more than a tiny fraction of
the atom spacing. These are insulators; the small displacement of
charge gives them dielectric properties.
How is it that some materials have mobile electrons and some do
not? To explain this we need two of the stranger results of quantum
mechanics. Briefly, the electrons of an atom occupy discrete energy
states or orbits, arranged in shells (designated 1, 2, 3, and so on,
from the innermost to the outermost); each shell is made up of
subshells (designated s, p, d, and f), each of which contains 1, 3, 5,
or 7 orbits, respectively. The electrons fill the shells with the lowest
energy, two electrons of opposite spin in each orbit; the Pauli exclusion principle prohibits an energy state with more than two. When