Chemistry of Materials: Bronze Age to Space Age

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Chemistry of MaterialsBronze Age to Space Age
Chapter Twenty-Four
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Metallurgy From Natural Sourcesto Pure Metals

• A mineral is a crystalline inorganic material in
  the Earths crust.
• An ore is a solid deposit containing a
  sufficiently high percentage of a mineral to make
  extraction of a metal economically feasible.
• Native ores contain free metals and include gold
  and silver.
• Oxides include iron, manganese, aluminum, and
  tin.
• Sulfides include copper, nickel, zinc, lead, and
  mercury.
• Carbonates include sodium, potassium, and
  calcium.
• Chlorides (often in aqueous solution) include
  sodium, potassium, magnesium, and calcium.

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Extractive Metallurgy

• Metallurgy is the general study of metals
• Extractive metallurgy focuses on the activities
  required to obtain a pure metal from one of its
  ores
• Mining from deep mines or open-pit mines.
• Concentration by physical separation from waste
  rock.
• Roasting is often used to convert metal compounds
  to the corresponding oxides.
• Reduction may be performed by simple heating to
  decompose an oxide, or with a reducing agent such
  as coke, or by electrolysis.
• Slag formation removes high-melting impurities.
• One or more final steps of refining may be
  required.

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Concentration of an Oreby Flotation

• In the flotation method, the ore is ground into a
  powder and mixed with water and additives.

Particles of ore are attached to air bubbles and
rise to the top.
Undesired waste rock, called gangue, falls to the
bottom.
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Hydrometallurgy

• Metallurgical methods that use ore concentration,
  roasting, chemical or electrolytic reduction, and
  slag formation are often called pyrometallurgy.
• In some cases, these methods are being replaced
  by hydrometallurgymethods that involve
  processing of aqueous solutions of metallic
  compounds. Operations include
• Leaching the metal ions from the ores with water,
  acids, bases, or salt solutions.
• Purification and/or concentration to remove
  impurities.
• Precipitation and reduction to the desired metal.

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Alloys

• In many metallurgical procedures the desired
  product is an alloya mixture of two or more
  metals, or of a metal with a nonmetal.
• Some alloys are heterogeneous mixtures, like the
  familiar leadtin alloy solder.
• Two other alloy types are homogeneous solid
  solutions
• In substitutional alloys, atoms of one metal
  substitute for another in the crystal lattice.
• In interstitial alloys, atoms of one substance
  occupy voids in the crystal lattice.
• A few alloys are actually intermetallic
  compounds, such as the amalgam NaHg2.

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Iron and Steel

• The iron formed in a blast furnace is an impure
  form called pig iron. It generally contains 34
  C, 0.53.5 Si, 0.51 Mn, 0.052 P, and
  0.050.15 S.
• The solid metal obtained from liquid pig iron is
  called cast iron and is used in automobile engine
  blocks, boilers, stoves, and cookware.
• Cast iron is brittle when cold but can be wrought
  by hammering at 800900 C.
• Most iron is converted to alloys known
  collectively as steel.

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A Modern Blast Furnace
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Steel

• Steel has more desirable properties (strength,
  malleability, corrosion resistance) for most
  purposes than does iron.
• Converting pig iron to steel requires
• reduction of the carbon content to less than
  1.5.
• removal of major impurities (Si, Mn, P, S) and
  some minor impurities.
• Low-carbon steel (about 0.25 C) is used for
  construction beams and girders and for
  reinforcing rods in concrete.
• Harder, high-carbon steel (more than 0.7 C)
  finds use in cutting tools and railroad rails.
• The remainder of steel production is in the form
  of alloy steel, ordinarily containing Cr, Ni, Mn,
  V, Mo, Co, and/or W as a major component.

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A Basic Oxygen Furnace

• The basic oxygen process has replaced most older
  methods of steel production in the U.S.
• Limestone, pig iron, and scrap steel are treated
  with high-pressure oxygen.
• The oxygen removes impurities as their
  corresponding oxides (CO2, SO2) or as slags
  MnSiO3, Ca3(PO4)2.

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Tin and Lead

• Tin occurs in nature mainly as the ore
  cassiterite, SnO2, which can be concentrated by
  flotation, then reduced to the metal with coke.
• Recycling is an important source of tin.
• Treatment of tin plate with Cl2 converts the tin
  to SnCl4, which is volatile. SnCl4 is converted
  to SnO2, which is then reduced to tin metal.
• Lead is found chiefly as galena, PbS. The ore is
  concentrated by flotation, roasted to the oxide,
  then reduced with coke.
• The recycling of used lead is an important
  alternative to the production of new lead.
  Currently, about 70 of manufactured lead is
  recycled lead.

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Copper, Zinc, Silver, and Gold

• Copper ores commonly contain iron compounds,
  which complicates copper production. These ores
  undergo a four-step process to produce blister
  copper which is 9799 pure Cu with entrapped
  bubbles of SO2.
• The recycling of used copper is now an important
  alternative to the production of new copper.
  Currently, nearly half of manufactured copper is
  recycled copper.
• Zinc occurs mainly as ZnS (sphalerite) and ZnCO3
  (smithsonite).
• Zinc ore is roasted to produce ZnO, followed by
  reduction with coke at high temperature, and
  distillation of zinc vapor.
• Alternatively, zinc ore can be processed by
  hydrometallurgy, with electrolysis as the last
  step.

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Copper, Zinc, Silver, and Gold (contd)

• Silver and gold are both found free in nature,
  but all easily accessible known deposits have
  been mined. A typical gold ore today contains
  only about 10 g Au per ton.
• A modern method of obtaining gold is by
  cyanidation. The ore is treated with cyanide,
  forming Au(CN)2. Gold is then displaced from
  the complex, using an active metal such as zinc.
• A similar method is used for obtaining silver.
  Air is blown through an aqueous solution of
  cyanide ions in which highly insoluble Ag2S is
  suspended.
• Sulfide ion is oxidized to sulfate ion, and the
  silver appears in the complex Ag(CN)2.

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• Example 24.1 A Conceptual Example
• Consider the electrolytic method of zinc
  metallurgy previously described. Why must the
  ions of metals less active than zinc (for
  example, Cd2) be removed before the electrolytic
  reduction of ZnSO4(aq) is carried out?

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The Free-Electron Modelof Metallic Bonding

• In the free-electron model, a metal consists of
  more-or-less immobile metal ions in a crystal
  lattice, surrounded by a gas of the valence
  electrons.

An applied electric potential causes the
free-moving electrons to travel from () to ().
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Deformation of a MetalCompared to an Ionic Solid
In the free-electron model, deformation merely
moves the positive ions relative to one another.
Metals are therefore malleable and ductile.
In contrast, deformation of an ionic solid brings
like-charged ions into proximity the crystal is
brittle and shatters or cleaves.
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Band Theory

• The free-electron model is a classical theory,
  which is less satisfactory in many ways than a
  quantum-mechanical treatment of bonding in
  metals.
• Band theory is a quantum-mechanical model.

The spacing between electron energy levels is so
minute in metals that the levels essentially
merge into a band.
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Band Theory

• When the band is occupied by valence electrons,
  it is called a valence band.
• In band theory, the presence of a conduction
  banda partially filled band of energy levelsis
  required for conductivity.
• Because the energy levels in bands are so closely
  spaced, there are electronic transitions in a
  partially filled band that match in energy every
  component of visible light.
• Metals therefore absorb the light that falls on
  them and are opaque.
• At the same time electrons that have absorbed
  energy from incident light are very effective in
  radiating light of the same frequencymetals are
  highly reflective.

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Band Overlap in Magnesium
The partially-filled band fulfills the
requirement for electrical conductivity.
The 3s band is only partially filled because of
overlap with the 3p band.
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Semiconductors
In an insulator, the energy gap between
conduction and valence band is large.
When the energy gap is small, some electrons can
jump the gap we have a semiconductor.
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Electrical Conductivityin Semiconductors
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n-Type and p-Type Semiconductors

• An n-type semiconductor is produced when a
  crystal (such as Si) is doped with an element
  (such as As) with more valence electrons.
• The energy levels of these donor atoms lie quite
  close to the conduction band and the extra
  electron(s) are lost to the conduction band.
• A p-type semiconductor is produced when a crystal
  is doped with another substance with fewer
  valence electrons.
• The energy levels of these acceptor atoms lie
  quite close to the valence band and electrons are
  easily promoted from the valence band into the
  acceptor level.

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n-Type and p-Type Semiconductors
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A Semiconductor DeviceThe Photovoltaic Cell

• A photovoltaic cell is a semiconductor device
  that converts light to electricity.
• The cell consists of a thin (1 x 104 cm) layer
  of p-type semiconductor, in contact with a piece
  of n-type semiconductor.

• Some of the electrons in the p-type semiconductor
  absorb energy from the sunlight and are promoted
  to the conduction band. These electrons can cross
  the pn junction and leave the cell as an
  electric current.

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Polymers

• Polymers, also known as macromolecules, are made
  from smaller molecules, much as a brick wall is
  constructed from individual bricks.
• The small building-block molecules are called
  monomers.
• Synthetic polymers are a mainstay of modern life,
  but nature also makes polymers they are found in
  all living matter.

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Natural Polymers

• Three types of natural polymers are
  polysaccharides, proteins, and nucleic acids.
• Modifications to cellulose (a polysaccharide) are
  of economic importance.
• Modifications of cellulose take place at the many
  hydroxyl (OH) groups using acidbase reactions.

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Addition Polymerization

• In addition polymerization, monomers add to one
  another in such a way that the polymeric product
  contains all the atoms of the starting monomers.
• The steps for addition polymerization include
• Initiation - often through the use of free
  radicals.
• Propagation - radicals join to form larger
  radicals.
• Termination - occurs when a molecule is formed
  that no longer has an unpaired electron.

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Molecular Models of a Segment of a Polyethylene
Molecule
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Condensation Polymerization

• In condensation polymerization, a small portion
  of the monomer molecule is not incorporated in
  the final polymer.
• Each monomer molecule contains at least two
  functional groups.
• The monomers are linked through the functional
  groups.
• Small molecules are formed as by-products as the
  monomers are linked.

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• Example 24.2
• Write a condensed structural formula for
  polypropylene, made by the polymerization of
  propylene (CH2CHCH3).

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Physical Properties of Polymers

• A thermoplastic polymer is one that can be
  softened by heating and then formed into desired
  shapes by applying pressure.
• Thermosetting polymers become permanently hard at
  elevated temperatures and pressures.
• High-density polyethylene (HDPE) consists
  primarily of linear molecules and has a higher
  density, greater rigidity, greater strength, and
  a higher melting point.
• Low-density polyethylene (LDPE) has branched
  chains and is a waxy, semi-rigid, translucent
  material with a low melting point.

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Organization ofPolymer Molecules
HDPE molecules are linear and can pack closely
together for increased strength.
LDPE molecules have branches that keep the
molecules from packing closely.
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A Small Segment of Bakelite
Numerous cross-links between chains produce an
extensive three-dimensional structure that is
highly rigid and strong.
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Flexibility and Elasticity

• A polymer is flexible if it can yield to force
  without breaking.
• A polymer is elastic if it regains its original
  shape after a distorting force is removed.
• Elastomers are flexible, elastic materials.
• The natural polymer rubber is the prototype for
  this kind of material.
• Natural rubber is soft and tacky when hot. It can
  be made harder in a reaction with sulfur, called
  vulcanization.

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Elasticity in Naturaland Vulcanized Rubber
Heating with sulfur cross-links the carbon chains
to one another.
In natural rubber the chains are separate and can
slide over one another when stretched, the
product may not return to its original shape.
When deformed as shown here, the cross-links tend
to return the product to its original shape.
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A larger distance between cross-links corresponds
to a softer, more elastic product.
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Synthetic Rubber

• Several kinds of synthetic rubber were developed
  during and after World War II. Neoprene
  (polychloroprene) is one example of this.
• Copolymerization is a process in which a mixture
  of two different monomers form a product in which
  the chain contains both monomers as building
  blocks.

SBR is a copolymer of styrene and butadiene.
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Fibers and Fabrics

• A fiber is a natural or synthetic material
  obtained in long, threadlike structures that can
  be woven into fabrics.
• Cotton, wool, and silk are natural fibers of
  great tensile strength that have long been spun
  and woven into cloth.
• Synthetic polymers have revolutionized the
  clothing industry.
• Polyacrylonitrile (Acrilan) is a synthetic
  addition polymer used for fibers.
• Polyesters (Dacron) are synthetic condensation
  polymers.
• Polyamides (nylon) are synthetic analogs to
  proteins, and have properties similar to silk.

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Biomedical Polymers

• One of the most interesting uses of polymers has
  been in replacements for diseased, worn out, or
  missing parts of the human body.
• Pyrolytic carbon heart valves are widely used.
• Knitted Dacron tubes can replace arteries
  blocked or damaged by atherosclerosis.
• Polymers of glycolic acid and lactic acid have
  been used in synthetic films for covering burn
  wounds, and are less likely to be rejected by the
  bodys immune system.
• The development of biomedical polymers has barely
  begun.

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Space Age Materials

• Physical properties of polymers can be designed
  through control of their composition and
  molecular structure.
• In a similar way, other new materials are being
  designed and developed to meet the needs of
  advancing technologies.
• Examples include new alloys, composite materials,
  and materials structured on the nanometer scale
  (nanomaterials).
• In each of these cases, we will see that the
  introduction of a specific chemical composition
  or structure produces materials with unique and
  novel properties.

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• Beryllium enhances the elasticity of copper.
• Waspaloy is usable to 600 C, well above the
  useful temperature of most other alloys.
• Shape-memory alloys, when deformed, return to
  their original shape when heated.

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Composites

• Composites are made of two or more physically
  distinct materials that, when combined, exploit
  the desired structural and mechanical properties
  of the individual components.
• One type of composite material widely used today
  is fiber-reinforced polymer (FRP), in which
  fiberglass is impregnated either with epoxy resin
  or polyester resin.
• FRP composites containing either graphite fiber
  or Kevlar in lieu of fiberglass are known for
  their extreme strength and rigidity.
• For high-temperature applications, a
  phenolformaldehyde resin may replace the epoxy
  resin.

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A Metal-Ceramic Composite 3M Companys Composite
Conductor
Aluminum oxide fibers provide strength needed for
long electrical cables.
Aluminum provides electrical conductivity.
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Nanomaterials

• A nanomaterial develops unique physical or
  chemical properties when the sample size is
  reduced to a nanometer scale.
• In order for the term nanomaterial to apply,
  there must be a change in some property or
  properties as the scale is reduced to the
  nanometer level.
• Coatings that incorporate nanometer-sized grains
  of one metal oxide in a matrix of a second metal
  oxide have been prepared. This inclusion has
  resulted in significantly increased hardness, an
  important property of protective coatings, by
  altering the mechanisms of mechanical deformation
  within the material.

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Nanomaterials (contd)

• In carbon nanotubes (page 464), the electrical
  conductivity depends on the way the sheets have
  been rolled, because of the confinement of
  electrons within the nanometer-sized structure.
• Nanometer-sized particles of semiconducting
  materials exhibit optical properties that are
  related to the perturbation of electronic states
  within a very small sample of a material.
• The rate for reactions involving extremely fine
  particles of aluminum, just a few nanometers in
  diameter, is greater than the predicted rate
  based solely on increased surface area. This
  nano form of aluminum is being investigated for
  use in high-performance rocket propellants.

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• Cumulative Example
• A polyurethane is a polymer involving the
  reaction of hydroxyl (OH) groups and isocyanate
  (NCO) groups
• OH NCO ? NH(CO)O
• Suppose a polyurethane is to be prepared using
  pure 1,6-diisocyanatohexane (hexane diisocyanate,
  HDI) for the isocyanate functional groups and a
  12 molar mixture of glycerol (1,2,3-trihydroxypro
  pane) and 1,4-dihydroxybutane for the hydroxyl
  functional groups.
• (a) How many grams of the hydroxyl mixture must
  be used for every 100.0 grams of HDI? (b) Based
  on Table 24.2, what general physical properties
  might be expected for the product?