modern materials © 2009, prentice-hall, inc. chapter 12 modern materials john d. bookstaver st....

ModernMaterials

© 2009, Prentice-Hall, Inc.

Chapter 12Modern Materials

John D. Bookstaver

St. Charles Community College

Cottleville, MO

Chemistry, The Central Science, 11th editionTheodore L. Brown, H. Eugene LeMay, Jr.,

and Bruce E. Bursten

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Modern Materials & Material Revolution

Old: Stone, Wood, Copper, Bronze, IronNew: Plastics and other synthetic materials

• Polymer: Various Plastics Biomaterials (heart valve, atificial tissues, …)

• Semiconductors: Computer Chips, Light-emiting Diode (LED), Solar Cells

• Ceramic and Superconductors:

• Liquid Crystals: LCD (PC & TV Screen)

• Nanomaterials

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Types of Materials:Metals, Semiconductors, Conductors

Review:

Molecular Orbital Theory: p 369, 375

Recall that atomic orbitals mix to give rise to molecular orbitals.

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Silicon Crystal Lattice: Tetrahedral base

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Formation of Bands

As the number of atoms grows, so does the number of molecular orbitals,

then merge to form

bands

many MOs a BAND

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Semiconductors and Insuators

Rather than having molecular orbitals separated by an energy gap, these substances have energy bands.

Conduction Band: from antibonding MOs

Valence Band:

from bonding MOs

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Classification of Materials:Based on electrical conductivity

The gap between bands determines whether a substance is a metal, a semiconductor, or an insulator.

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Types of Materials

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Metals

• No energy separation between Valence Band and Conduction Band

Two bands merge to One

• Valence electrons are in

a partially-filled band.

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Metals

• 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.

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Semiconductors

Semiconductors have a gap between the valence band and conduction band of ~50-300 kJ/mol (0.5~3 eV).

1eV=1.602x10-19 J

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Figure 12.04

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Band Gap Energies

Diamond: 5.5 eV, (Insulator)

155pm,

larger overlap

larger splitting

Silicon: 1.1eV (Semicond.)

235pm

smaller overlap

smaller splitting

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Figure 12.05b

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Table 12.2 (p486)

Band gap decreases as atomic sizes increase(Group IV elements) C 5.5 eV largest Si 1.11 Ge 0.68 Sn 0.08 Pb zero

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Figure 12.06

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Figure 12.03

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Figure 12.07

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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).

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Doping• By introducing

very small amounts of impurities that have more (n-Type) or fewer (p-Type) valence electrons, one can increase the conductivity of a semiconductor.

n-Type : Group IV & Vp-Type : Group IV & III

• Transistor

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Insulators

• The energy band gap in insulating materials is generally greater than ~350 kJ/mol.

• They are not conductive.

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Figure 12.03

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Electronics

• Silicon is very abundant, and is a natural semiconductor.

• This makes it a perfect substrate for transistors, integrated circuits, and chips.

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Integrated Circuit (IC)(microcircuit, microchip, silicon chip, or chip)

• a miniaturized electronic circuit (consisting mainly of semiconductor devices) that is built in the surface of a thin semiconductor material.

• in almost all electronic equipment in use today and have revolutionized the world of electronics.

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Semiconductor:

Intrinsic : Pure semiconductor (Si)

Extrinsic: Simeconductor with a dopant (P, Ga, etc) n-type or p-type

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p-n Diode allows unidirectional flow of electrical currentCurrent flows only if p-type is positive, and N type is negativeNo current flows, if a voltage is applied the other way → Rectifier

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Transistor: basic unit of integrated circuita semiconductor device commonly used to amplify or switch electronic signals.

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Photovoltaic Effect & Solar Cell conversion of solar energy, producing electricity from light

(1) Light hits electrons in the VB (of n-type) excited it to CB (pumping e’s to higher energy level)

(2) The promoted electrons at the upper energy level of the CB can cross the junction to CB of p-type, then to it’s VB,

to complete a cycle.

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Conversion Efficiency of Solar Cell light energy to electrical energy

• Most efficient with 1.3eV of band Gap• Theoretical maximmm: 31%

Lab: 24%

Commercial: 15%

• Cost Comparison:

$0.25~$0.65/kWh

Coal-based Power plant

$0.04~$0.06/kWh

($0.045, Ga Power, 2003)

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Photovoltaic Effect & Solar Cells : n-p Diodes → produce electricity from light

(1) Light hits electrons in in the valence bands exciting it to conduction bands (pumping e’s to higher energy level)

(2) The excited electrons at the upper energy level of the CB moves to the lowest level of the CB.

This process is revered in LED to generate light from electricity

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Light Emitting Diode: a n-p diode - an opposite of photo voltaic process

(1) Small voltage is applied across the n-p junction

(2) Electrons in the CB of n-side are forced to the junction where they meets the holes at the p-side.

(3) Electron falls into the holes that is at a lower energy level,

thus generating a light.

(4) Color (wavelength) light emitted depends on the band gap energy

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Designing Light Emitting Diodes (LEDs)

• For LED above 550nm• GaP : 2.26 eV Green (549nm)• GaAs: 1.43 eV Infrared(IR, 867nm)

• Mix of GaP & GaAs: Red and others

• For LED below 550nm• GaN: 3.4 eV Violet (~360nm)• InN 2.4 eV Green (520nm)

• Mix of GaN and InN : Blue (~450nm)

• varying the composition of the elements in LEDs, lights of various color can be generated (Sample Exercise 12.3)

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Find the wavelength & color of the LED made of GaP (2.26 eV).

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Solution:

E = h x f = h x c / wavelength

Wavelength = hc / E

=(6.626E-34Js)(3.00E+8m/s) / 2.26eV

=(6.626E-34Js)(3.00E+8m/s) / 2.26eV x (1/1.602E-19eV/J) x (1E9nm/m)

= 549 nm (Green)

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Sample Exercise 12.3: GaP: 2.26eV(549nm), AlP: 2.43eV(510nm): Desired: 520nm.

What’s the composition of GaAlP ?

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Solution:E = hc / wavelength =

=(6.626E-34Js)(3.00E+8m/s) / 520nm = 2.38eV

2.38eV = E(GaP) + E (AlP) = 2.26x eV + 2.43(1-x) eV x: fraction of GaP (1-x): fraction of AlP 2.38 = 2.26x + 2.43(1-x) x= 0.294, for fraction of GaP 1-x= 0.706, for fraction of AlP.

Ga0.29Al0.71P

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Polymers

Polymers are molecules of high molecular mass made by sequentially bonding repeating units called monomers.

Natural: Rubber, Protein DNA, RNA, Cellulose, Starch

Artificial:

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Some Common Polymers

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Addition Polymers

Addition polymers are made by coupling the monomers by converting -bonds within each monomer to -bonds between monomers.

Ethylene Polyethylene

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Condensation Polymers

• Condensation polymers are made by joining two subunits through a reaction in which a smaller molecule (often water) is also formed as a by-product.

• These are also called copolymers.

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Synthesis of Nylon

- one example of a condensation polymer.

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Properties of Polymers

Interactions between chains of a polymer lend elements of order to the structure of polymers.

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Properties of Polymers

Stretching the polymer chains as they form can increase the amount of order, leading to a degree of crystallinity of the polymer.

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Properties of Polymers

Such differences in crystallinity can lead to polymers of the same substance that have very different physical properties.

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Cross-Linking

Chemically bonding chains of polymers to each other can stiffen and strengthen the substance.

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Cross-Linking

Naturally-occurring rubber is too soft and pliable for many applications.

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Cross-Linking

In vulcanization, chains are cross-linked by short chains of sulfur atoms, making the rubber stronger and less susceptible to degradation.

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ElectronicsIn 2000, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa won a Nobel Prize for the discovery of “organic semiconductors” like the polyacetylene below.

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

H H H H H H H H H

HHHHHHHHH

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Biomaterials

• Natural Biopolymers:

- Proteins: amino acids (monomers)

Polypeptide bonds

Enzymes ,

Structural material (Keratins)

- Polysaccharides (sugar polymers): Glycogens,Cellulose

- Polynucleotides: RNA, DNA

• Synthetic Biopolymers:

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Biomaterials

• Materials used in the body must– be biocompatible,– have certain physical

requirements, and– have certain chemical

requirements.

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Biomaterials• Biocompatibility

– The materials used cannot cause inflammatory responses.

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Biomaterials• Physical

Requirements– The properties of

the material must mimic the properties of the “real” body part (i.e., flexibility, hardness, etc.).

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Biomaterials• Chemical

Requirements– It cannot contain even

small amounts of hazardous impurities.

– Also it must not degrade into harmful substances over a long period of time in the body.

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Biomaterials

• These substances are used to make:

– Heart valves– Vascular grafts– Artificial skin grafts– Dental fillings

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Biomaterials

Dacron:

Polyethylene terephthalate Condensation polymer

ethylene glycol and terephthalic acid

- Heart valves, Vascular grafts

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Artificial Tissue: Skin graft

• suitable scaffolds on which cells can grow

- lactic acid / glycolic acid copolymer (p509)

has many C-O bonds along the chain providing many opportunities for H-bonding with cell

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Dental Fillings

• Old Method: Amalgam fillings (Hg with Ag, Cu)

• New Method: Composite fillings (white),

- has been around for last two to three decades

- composed of an organic polymer,

bisphenolaglycidyl methacrylate (BIS-GMA), &

inorganic particles such as quartz, borosilicate

glass, and lithium aluminum silicate.

(apply composite mixture to make a thin layer over a tooth; then polymerize them with UV)

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Polymer Adhesives:Super Glue, Krazy Glue

• Polycyanoacrylate

- consists of monomers of cyanoacrylate molecules.

- Methyl-2-cyanoacrylate (monomer)

CH2=C(CN)COOCH3 (or C5H5NO2)C

- polymerization can be initiated by H2O

OH (from H2O) helps breaking the double bond.

- requires air-tight container for storage

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Ceramics

• These are inorganic solids, usually hard and brittle.• They are highly resistant to heat, corrosion and wear.

– Ceramics do not deform under stress.– They are much less dense than metals, and so are used in place of

metals in many high-temperature applications.

pottery, china, cement, spark-plug insulators, etc.

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Hardness - Ability to scratch

Skin = 1.5 Gyspum = 2

Fingernail = 2.5 Calcite = 3

Penny = 3.0 =

Knife = 5.5 Orthoclase = 6

Streak Plate = 6.5 Quartz = 7

Talc = 1

Diamond = 10

Hardness < 5 ½ : “soft”

Corundum = 9

Topaz = 8

Fluorite = 4

Apatite = 5

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Superconductors

Substances that lose virtually all resistance to the flow of electrons below certain temperature,

- Transition Temperature

- a special property of excluding

magnetic lines,

levitating a magnet. (not for non-magnetic materials)

- can save much of electrical energy

- can make much stronger magnet

levitated train with high speed

(~250 MPH), MRI

Meissner Effect

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Superconductors (SC)

1911: Hg (Metal):

1987 , High Temp SC

BP of liq. Nitrogen: 77K

Much research has been done recently into the development of high-temperature superconductors.

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SuperconductorsThe development of higher and higher temperature superconductors will have a tremendous impact on modern culture.

Mechanism of Superconductivity:

still debated

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Applications of Superconductors

• superconducting wire can generate very strong magnets, whivh id s not possible with Cu wires

- Magnetically levitated high-speed train

(~200 MPH)

- MRI(Magnetic Resonance Imaging)

- Superconducting supercolliders

- Nuclear Fusion Power Plant

Not practical yet

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Liquid Crystals

• Some substances do not go directly from the solid state to the liquid state.

• In this intermediate state, liquid crystals have some traits of solids and some of liquids. High (179) Low (145)

Cholesteryl benzoate

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Liquid Crystals

Unlike liquids, molecules in liquid crystals have some degree of order.

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Liquid Crystals

In nematic liquid crystals, molecules are only ordered in one dimension, along the long axis.

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Liquid Crystals

In smectic liquid crystals, molecules are ordered in two dimensions, along the long axis and in layers.

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Liquid Crystals

In cholesteryl liquid crystals, nematic-like crystals are layered at angles to each other.

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Liquid Crystal Displays

Liquid Crystal can rotate a plane-polarized light.

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Liquid Crystals

These crystals can exhibit color changes with changes in temperature.