Metallic Bonding, Alloys & Semiconductors

Metallic Bonding, Alloys & Semiconductors Presented By, Mark Langella, APSI Chemistry 2014 , PWISTA .com

Alloys • Alloyscontain 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 SolutionsSubstance 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? • 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 right-hand 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. Arrange the four materials in order of increasing electrical conductivity. Explain. Tell whether the conductivity of each material increases or decreases when the temperature increases. (b) (c) 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

Which of the following is a p-type semiconductor? • 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 Suggest an element that could be used to dope silicon to yield a p-type material. Which of the following elements, if doped into silicon, would yield an n-type semiconductor? Ga; As; C. 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

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

Solar Cells Presented By, Mark Langella, APSI Chemistry 2014 , PWISTA .com

Metallic Bonding, Alloys & Semiconductors