domingo, 21 de marzo de 2010

union de los materiales

Samer Elatrache V-17810600 estudiante de CRF

Widely spaced isolated atoms condense to form solids due to the energy
reduction accompanying bond formation. Thus, if N atoms of type A in the
gas phase (8) combine to form a solid (s), the binding energy Eb is released
according to the equation
NA, + NA, -k Eb.
Energy Eb must be supplied to reverse the equation and decompose the solid.
The more stable the solid, the higher is its binding energy. It has become the
custom to picture the process of bonding by considering the energetics within
and between atoms as the interatomic distance progressively shrinks. In each
isolated atom, the electron energy levels are discrete, as shown on the
right-hand side of Fig. 1-8a. As the atoms approach one another, the individual
levels split, as a consequence of an extension of the Pauli exclusion principle,
to a collective solid; namely, no two electrons can exist in the same quantum
state. Level splitting and broadening occur first for the valence or outer
electrons, since their electron clouds are the first to overlap. During atomic
attraction, electrons populate these lower energy levels, reducing the overall
energy of the solid. With further dimensional shrinkage, the overlap increases
and the inner charge clouds begin to interact. Ion-core overlap now results in
strong repulsive forces between atoms, raising the system energy. A compromise
is reached at the equilibrium interatomic distance in the solid where the
system energy is minimized. At equilibrium, some of the levels have broadened
into bands of energy levels. The bands span different ranges of energy,
depending on the atoms and specific electron levels involved. Sometimes as in
metals, bands of high energy overlap. Insulators and semiconductors have
energy gaps of varying width between bands where electron states are not
allowed. The whys and hows of energy-level splitting, band structure evolution,
and implications with regard to property behavior are perhaps the most
fundamental and difficult questions in solid-state physics. We briefly return to
the subject of electron-band structure after introducing the classes of solids.
An extension of the ideas expressed in Fig. 1-8a is commonly made by
simplifying the behavior to atoms as a whole, in which case the potential
energy of interaction V(r) is plotted as a function of interatomic distance r in
Fig. 1-8b. The generalized behavior shown is common for all classes of solid
materials, regardless of the type of bonding or crystal structure. Although the
mathematical forms of the attractive or repulsive portions are complex, a
number of qualitative features of these curves are not difficult to understand.
For example, the energy at the equilibrium spacing r = a, is the binding
energy. Solids with high melting points tend to have high values of Eb. The
curvature of the potential energy is a measure of the elastic stiffness of the
solid. To see this, we note that around a, the potential energy is approximately
harmonic or parabolic. Therefore, V(r) = (1/2)K,r2, where K, is related to
the spring constant (or elastic modulus). Narrow wells of high curvature are
associated with large values of K,, broad wells of low curvature with small
values of K, . Since the force F between atoms is given by F = - dV/dr,
F = - Ksrr which has its counterpart in Hooke's law-i.e., that stress is
linearly proportional to strain. Thus, in solids with high K, values, correspondingly
larger stresses develop under loading. Interestingly, a purely
parabolic behavior for I/ implies a material with a coefficient of thermal
expansion equal to zero. In real materials, therefore, some asymmetry or
anharmonicity in V(r) exists.
For the most part, atomic behavior within a thin solid film can also be
described by a V(r)-r curve similar to that for the bulk solid. The surface
atoms are less tightly bound, however, which is reflected by the dotted line
behavior in Fig. 1-8b. The difference between the energy minima for surface
and bulk atoms is a measure of the surface energy of the solid. From the
previous discussion, surface layers would tend to be less stiff and melt at lower
temperatures than the bulk. Slight changes in equilibrium atomic spacing or
lattice parameter at surfaces may also be expected.
Despite apparent similarities, there are many distinctions between the four
important types of solid-state bonding and the properties they induce. A
discussion of these individual bonding categories follows.

Metallic
The so-called metallic bond occurs in metals and alloys. In metals the outer
valence electrons of each atom form part of a collective free-electron cloud or
gas that permeates the entire lattice. Even though individual electron-electron
interactions are repulsive, there is sufficient electrostatic attraction between the
free-electron gas and the positive ion cores to cause bonding.
What distinguishes metals from all other solids is the ability of the electrons
to respond readily to applied electric fields, thermal gradients, and incident
light. This gives rise to high electrical and thermal conductivities as well as
high reflectivities. Interestingly, comparable properties are observed in liquid
metals, indicating that aspects of metallic bonding and the free-electron model
are largely preserved even in the absence of a crystal structure. Metallic
electrical resistivities typically ranging from lop5 to ohm-cm should be
contrasted with the much, much larger values possessed by other classes of
solids.
Furthermore, the temperature coefficient of resistivity is positive. Metals
thus become poorer electrical conductors as the temperature is raised. The
reverse is true for all other classes of solids. The conductivity of pure metals is
always reduced with low levels of impurity alloying, which is also contrary to
the usual behavior in other solids. The effect of both temperature and alloying
element additions on metallic conductivity is to increase electron scattering,
which in effect reduces the net component of electron motion in the direction
of the applied electric field. On the other hand, in ionic and semiconductor
solids production of more charge carriers is the result of higher temperatures
and solute additions.
The bonding electrons are not localized between atoms; thus, metals are said
to have nondirectional bonds. This causes atoms to slide by each other and
plastically deform more readily than is the case, for example, in covalent
solids, which have directed atomic bonds.
Examples of thin-metal-film applications include A1 contacts and interconnections
in integrated circuits, and ferromagnetic alloys for data storage
applications. Metal films are also used in mirrors, in optical systems, and as
decorative coatings of various components and packaging materials.

tonic
Ionic bonding occurs in compounds composed of strongly electropositive
elements (metals) and strongly electronegative elements (nonmetals). The
alkali halides (NaCl, LiF, etc.) are the most unambiguous examples of
ionically bonded solids. In other compounds, such as oxides, sulfides, and
many of the more complex salts of inorganic chemistry (e.g., nitrates, sulfates,
etc.), the predominant, but not necessarily exclusive, mode of bonding is ionic
in character. In the rock-salt structure of NaC1, for example, there is an
alternating three-dimensional checkerboard array of positively charged cations
and negatively charged anions. Charge transfer from the 3s electron level of
Na to the 3p level of C1 creates a single isolated NaCl molecule. In the solid,
however, the transferred charge is distributed uniformly among nearest neighbors.
Thus, there is no preferred directional character in the ionic bond since
the electrostatic forces between spherically symmetric inert gaslike ions is
independent of orientation.
Much success has been attained in determining the bond energies in alkali
halides without resorting to quantum mechanical calculation. The alternating
positive and negative ionic charge array suggests that Coulombic pair interac
tions are the cause of the attractive part of the interatomic potential, which
varies simply as - 1 / r. Ionic solids are characterized by strong electrostatic
bonding forces and, thus, relatively high binding energies and melting points.
They are poor conductors of electricity because the energy required to transfer
electrons from anions to cations is prohibitively large. At high temperatures,
however, the charged ions themselves can migrate in an electric field, resulting
in limited electrical conduction. Typical resistivities for such materials can
range from lo6 to 1015 ohm-cm.
Among the ionic compounds employed in thin-film technology are MgF,,
ZnS, and CeF,, which are used in antireflection coatings on optical components.
Assorted thin-film oxides and oxide mixtures such as Y,Fe,O,, ,
Y3Al,01,, and LiNbO, are employed in components for integrated optics.
Transparent electrical conductors such as In,O,-SnO, glasses, which serve as
heating elements in window defrosters on cars as well as electrical contacts
over the light exposed surfaces of solar cells, have partial ionic character.

Covalent

Covalent bonding occurs in elemental as well as compound solids. The
outstanding examples are the elemental semiconductors Si, Ge, and diamond,
and the 111-V compound semiconductors such as GaAs and InP. Whereas
elements at the extreme ends of the periodic table are involved in ionic
bonding, covalent bonds are frequently formed between elements in neighboring
columns. The strong directional bonds characteristic of the group IV
elements are due to the hybridization or mixing of the s and p electron wave
functions into a set of orbitals which have high electron densities emanating
from the atom in a tetrahedral fashion. A pair of electrons contributed by
neighboring atoms makes a covalent bond, and four such shared electron pairs
complete the bonding requirements.
Covalent solids are strongly bonded hard materials with relatively high
melting points. Despite the great structural stability of semiconductors, relatively
modest thermal stimulation is sufficient to release electrons from filled
valence bonding states into unfilled electron states. We speak of electrons
being promoted from the valence band to the conduction band, a process that
increases the conductivity of the solid. Small dopant additions of group 111
elements like B and In as well as group V elements like P and As take up
regular or substitutional lattice positions within Si and Ge. The bonding
requirements are then not quite met for group III elements, which are one
electron short of a complete octet. An electron deficiency or hole is thus
created in the valence band.
For each group V dopant an excess of one electron beyond the bonding octet
can be promoted into the conduction band. As the name implies, semiconductors
lie between metals and insulators insofar as their ability to conduct
electricity is concerned. Typical semiconductor resistivities range from 10-
to lo5 ohm-cm. Both temperature and level of doping are very influential in
altering the conductivity of semiconductors. Ionic solids are similar in this
regard.
The controllable spatial doping of semiconductors over very small lateral
and transverse dimensions is a critical requirement in processing integrated
circuits. Thin-film technology is thus simultaneously practiced in three dimensions
in these materials. Similarly, there is a great necessity to deposit
compound semiconductor thin films in a variety of optical device applications.
Other largely covalent materials such as Sic, Tic, and BN have found coating
applications where hard, wear-resistant surfaces are required. They are usually
deposited by chemical vapor deposition methods and will be discussed at length
in Chapter 12.

van der Waals Forces

A large group of solid materials are held together by weak molecular forces.
This so-called van der Waals bonding is due to dipole-dipole charge interactions
between molecules that, though electrically neutral, have regions possessing
a net positive or negative charge distribution. Organic molecules such as
methane and inert gas atoms are weakly bound together in the solid by these
charges. Such solids have low melting points and are mechanically weak. Thin
polymer films used as photoresists or for sealing and encapsulation purposes
contain molecules that are typically bonded by van der Waals' forces.

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