Transcript Metallic Bonding

Metallic Bonding, Alloys &
Semiconductors
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Alloys
• Alloys contain more than one element and have the
characteristic properties of metals.
• Solid Solution alloys are homogeneous mixtures.
• Heterogeneous alloys: The components are not
dispersed uniformly (e.g., pearlite steel has two
phases: almost pure Fe and cementite, Fe3C).
• Pure metals and alloys have different physical
properties.
• An alloy of gold and copper is used in jewelry (the
alloy is harder than the relatively soft pure 24 karat
gold).
• 14 karat gold is an alloy containing 58% gold.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Metal Alloys-Solid Solutions
Substance has mixture of element and
metallic properties.
1.Substitutional Alloy: some metal atoms replaced by
others of similar size. Electronegativities usually are
similar. The atoms must have similar atomic radii.
The elements must have similar bonding
characteristics.
• brass = Cu/Zn
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Metal Alloys
(continued)
2.Interstitial Alloy: Interstices (holes) in closest packed
metal structure are occupied by small atoms. Solute
atoms occupy interstices “small holes” between solvent
atoms. One element (usually a nonmetal) must have a
significantly smaller radius than the other (in order to fit
into the interstitial site).
steel = iron + carbon
3.Both types: Alloy steels contain a mix of substitutional
(Cr, Mo) and interstitial (Carbon) alloys.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Substitutional
Alloy
Interstitial Alloy
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Alloys vs. Pure Metal
• The alloy is much harder, stronger, and less ductile than the pure metal
(increased bonding between nonmetal and metal).
An example is steel (contains up to 3% carbon).
mild steels (<0.2% carbon) - useful for chains, nails, etc.
medium steels (0.2-0.6% carbon) - useful for girders, rails, etc.
high-carbon steels (0.6-1.5% carbon) - used in cutlery, tools, springs.
Other elements may also be added to make alloy steels.
Addition of V and Cr increases the strength of the steel and improves its
resistance to stress and corrosion.
The most important iron alloy is stainless steel. It contains C, Cr (from
ferrochrome, FeCr2), and Ni.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Which two substances are most likely
to form an interstitial alloy?
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Nickel and titanium
Silver and tin
Tin and lead
Copper and zinc
Tungsten and carbon
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Which two substances are most likely
to form an interstitial alloy?
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Nickel and titanium
Silver and tin
Tin and lead
Copper and zinc
Tungsten and carbon
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Bonding Models for Metals
APSI 2014 PWISTA.com
Bonding Models for Metals
•Electron Sea Model: A regular
array of metals in a “sea” of
electrons. The electron-sea model
is a qualitative interpretation of
band theory (molecular-orbital
model for metals).
•Band (Molecular Orbital) Model:
Electrons assumed to travel
around metal crystal in MOs
formed from valence atomic
orbitals of metal atoms.
•Conduction Bands: closely
spaced empty molecular orbitals
allow conductivity of heat and
electricity.
APSI 2014 PWISTA.com
Molecular Orbital Theory
Recall that atomic
orbitals mix to give
rise to molecular
orbitals.
APSI 2014 PWISTA.com
Molecular-Orbital Model for Metals
• Delocalized bonding requires the atomic orbitals on one
atom to interact with atomic orbitals on neighboring atoms.
• Example: Graphite electrons are delocalized over a whole
plane, while benzene molecules have electrons delocalized
over a ring.
• Recall that the number of molecular orbitals is equal to the
number of atomic orbitals. Each orbital can hold two
electrons.
• In metals there are a very large number of orbitals.
• As the number of orbitals increases, their energy spacing
decreases and they band together.
• The available electrons do not completely fill the band of
orbitals.
APSI 2014 PWISTA.com
APSI 2014 PWISTA.com
Molecular-Orbital Model for Metals
• Therefore, electrons can be promoted to unoccupied
energy bands.
• Because the energy differences between orbitals are small
the promotion of electrons requires little energy.
• As we move across the transition metal series, the
antibonding band starts becoming filled.
• Therefore, the first half of the transition metal series has
only bonding-bonding interactions and the second half has
bonding–antibonding interactions.
• We expect the metals in the middle of the transition metal
series (group 6B) to have the highest melting points.
• The energy gap between bands is called the band gap.
APSI 2014 PWISTA.com
Molecular Orbital Theory
In such elements, the
energy gap between
molecular orbitals
essentially
disappears, and
continuous bands of
energy states result.
APSI 2014 PWISTA.com
Formation of Bands
When atoms come together to form a compound, their atom orbital energies mix to
form molecular orbital energies. As more atoms begin to mix and more molecular
orbitals are formed, it is expected that many of these energy levels will start to be very
close to, or even completely degenerate, in energy. These energy levels are then said
to form bands of energy remember each orbital only holds two electrons.
APSI 2014 PWISTA.com
The electronic band structure of nickel.
The left side of the figure shows the electron configuration of a single Ni atom, while the righthand side of the figure shows how these orbital energy levels broaden into energy bands in bulk
nickel. The horizontal dashed gray line denotes the position of the Fermi Level, which separates
the occupied molecular orbitals (shaded in blue) from the unoccupied molecular orbitals.
APSI 2014 PWISTA.com
Types of Materials
Rather than having molecular orbitals separated by an
energy gap, these substances have energy bands.
The gap between bands determines whether a substance is
a metal, a semiconductor, or an insulator.
Presented By, Mark
Langella, APSI
Chemistry 2014 ,
PWISTA .com
Energy bands in metals, semiconductors, and insulators.
Metals are characterized by the highest-energy electrons occupying a partially filled band.
Semiconductors and insulators have an energy gap that separates the completely filled band
(shaded in blue) and the empty band (unshaded), known as the band gap and represented by
the symbol Eg. The filled band is called the valence band (VB), and the empty band is called
the conduction band (CB). Semiconductors have a smaller band gap than insulators.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Metals
• Valence electrons are in
a partially-filled band.
• There is virtually no
energy needed for an
electron to go from the
lower, occupied part of
the band to the higher,
unoccupied part.
• This is how a metal
conducts electricity.
Presented By, Mark
Langella, APSI
Chemistry 2014 ,
PWISTA .com
Insulators
• The energy band gap
in insulating materials
is generally greater
than ~350 kJ/mol.
• They are not
conductive.
Presented By, Mark
Langella, APSI
Chemistry 2014 ,
PWISTA .com
Semiconductors
Semiconductors have a
gap between the valence
band and conduction
band of ~50-300 kJ/mol.
Presented By, Mark
Langella, APSI
Chemistry 2014 ,
PWISTA .com
An intrinsic semiconductor is a semiconductor in its pure state. For
every electron that jumps into the conduction band, the missing
electron will generate a hole that can move freely in the valence band.
The number of holes will equal the number of electrons that have
jumped. The higher the temp more electrons into conduction band.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
The following pictures show the electron populations of the bands of MO energy
levels for four different materials:
(a) Classify each material as an insulator, a semiconductor, or a metal.
(b) Arrange the four materials in order of increasing electrical conductivity.
Explain.
(c) Tell whether the conductivity of each material increases or decreases when
the temperature increases.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Semiconductors
• Among elements, only silicon,
germanium and graphite
(carbon), all of which have 4
valence electrons, are
semiconductors.
• Inorganic semiconductors (like
GaAs) tend to have an average of
4 valence electrons (3 for Ga, 5
for As).
Presented By, Mark
Langella, APSI
Chemistry 2014 ,
PWISTA .com
Doping
By introducing very small
amounts of impurities
that have more valence
electrons (n-Type) or
fewer (p-Type) valence
electrons, one can
increase or decrease the
conductivity of a
semiconductor.
Presented By, Mark
Langella, APSI
Chemistry 2014 ,
PWISTA .com
The addition of controlled small amounts of impurities (doping) to a semiconductor changes the electronic
properties of the material.
•Left: A pure, intrinsic semiconductor has a filled valence band and an empty conduction
band (ideally).
•Middle: The addition of a dopant atom that has more valence electrons than the host
atom adds electrons to the conduction band (i.e., phosphorus doped into silicon). The
resulting material is an n-type semiconductor.
•Right: The addition of a dopant atom that has fewer valence electrons than the host
atom leads to fewer electrons in the valence band or more holes in the valence band
(i.e., aluminum doped into silicon). The resulting material is a p-type semiconductor.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Which of the following is a p-type semiconductor?
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Sulfur-doped carbon
Boron-doped germanium
Phosphorus-doped silicon
Ultra-pure silicon
Carbon-doped copper
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Which of the following is a p-type semiconductor?
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Sulfur-doped carbon
Boron-doped germanium
Phosphorus-doped silicon
Ultra-pure silicon
Carbon-doped copper
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Practice Exercises
Which of the following elements, if doped into silicon, would yield an n-type
semiconductor? Ga; As; C.
Suggest an element that could be used to dope silicon to yield a p-type material.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Diode- Used to switch and convert between
electromagnetic radiation and electric current
• Semiconductor created that has p-type on one
half and n-type on the other half
• Known as “p-n junction”
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Light emitting diodes.
The heart of a light emitting diode is a p-n junction where an applied voltage drives
electrons and holes to meet. Bottom: The color of light emitted depends upon the band
gap of the semiconductor used to form the p-n junction. For display technology red,
green, and blue are the most important colors because all other colors can be made by
mixing these colors.
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com
Color
Red
λ
Voltage Drop
610 < λ < 760 1.63 < ΔV < 2.03
Orange
590 < λ < 610 2.03 < ΔV < 2.10
Yellow
570 < λ < 590 2.10 < ΔV < 2.18
Green
500 < λ < 570 1.9[63] < ΔV < 4.0
Blue
450 < λ < 500 2.48 < ΔV < 3.7
Violet
400 < λ < 450 2.76 < ΔV < 4.0
Composition
Aluminium gallium arsenide (AlGaAs)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Traditional green:
Gallium(III) phosphide (GaP)
Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
Pure green:
Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
Zinc selenide (ZnSe)
Indium gallium nitride (InGaN)
Silicon carbide (SiC) as substrate
Silicon (Si) as substrate—under development
Indium gallium nitride (InGaN)
Presented By, Mark Langella, APSI Chemistry 2014 ,
PWISTA .com
Solar Cells
Presented By, Mark Langella, APSI
Chemistry 2014 , PWISTA .com