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ATOMIC STRUCTURA, CHEMICALBONDS AND PROPERTIES
ACCORDING OF BOND
MATERIAL ENGINEER I
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Size of structure
Atomic structureAtomic sizeAnstromg1 = 1 x 10-10 m
Crystalline structure(Nanostructure)
Molecular sizeNanometer1m = 1 x 10-9 m
SIZE OF STRUCTURE
Example of technology
Atomic arrangementChemical bonding
NanotechnologyNanomachine
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Size of structure
MicrostructureGrains (crystallites)Microns
1m = 1 x 10-6 m
MacrostructureDefects (pores)Milimeter1mm = 1 x 10-3 m
Example of technology
Metals and alloysMechanical properties
Fabrication defects
Corrosion
100 m
1 cm
SIZE OF STRUCTURE
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Metallic bond, Covalent bond, Ionic bond, van derWaals bond are the different types of bonds. Ductility refers to the ability of materials to be stretched
or bent without breaking
Van der Waals interactions: London forces, Debyeinteraction Glass temperature is a temperature above which many
polymers and inorganic glasses no longer behave as
brittle materials Intermetallic compound is a compound such as Al3V
formed by two or more metallic atoms
Atomic Bonding
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SORT OF BOND
The chemical bonding can be classificated on hardor primary and weak or secundary
Hard orprimary
Weak orsecundary
Ionic BondMetallic BondCovalent BondMixed Bond
Van der Waals Bond Dipolar Bond Hydrogen-bridge Bond
OscilatingFixed
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IONIC BOND
The bond is development by a ELECTROSTATIC ATRACTION of ionswhich exhibit opposed charge.
Characteristics Cede electrons High electronegativity difference
Groups VIA, VIIA y IA y IIA Potential energy decrease Non directional Must exhibits electroneutrality
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IONIC BOND
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2003 Brooks/Cole Publishing / Thomson Learning
When voltage is applied to an ionic material, entire ions must move tocause a current to flow. Ion movement is slow and the electricalconductivity is poor (for Example 2-8)
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COVALENT BOND
The bond is development by a SHARING OF PAIRS OF ELECTRONbetween atoms.
Characteristics Sharing electrons Low electronegativity difference
Groups close on periodic table Dont exhibit electroneutrality Directional Bond
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COVALENT BOND
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COVALENTE BOND
Direcctional Bond
Some covalent solids are brittle
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METALLIC BONDING
Electromagnetic interactionbetween delocalized electrons.
The positive nuclei of metallsare joined to the electrons sea
Charge atraction + and -
Characteristics
No direccionality No electroneutrality Free electrons libres
http://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Delocalized_electron8/3/2019 2 Atomo y Enlace Eng
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2003Brooks/Cole
Publishing/ThomsonLearning
When voltage is applied to a metal, the electrons in the electron sea caneasily move and carry a current
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WEAK OR SECUNDARY BONDS
Presents on compounds of low fusion temperature.Example: ICE
Characteristics
Between two permanent dipoles (Keesom force)Between a permanent dipole and a corresponding induceddipole (Debye force) Between two instantaneously induced dipoles (Londondispersion force)
IMPORTANT ON DIELECTRICS AND POLYMERSMATERIALS
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The Keesom interactions are formed as a result of polarization ofmolecules or groups of atoms. In water, electrons in the oxygen tend toconcentrate away from the hydrogen. The resulting charge differencepermits the molecule to be weakly bonded to other water molecules
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2003 Brooks/Cole Publishing / Thomson Learning
Illustration of London forces, a type of a van der Waals force,
between atoms
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2003 Brooks/Cole Publishing / Thomson
Learning
(a) In polyvinyl chloride(PVC), the chlorine atomsattached to the polymerchain have a negative chargeand the hydrogen atoms arepositively charged. Thechains are weakly bonded byvan der Waals bonds. This
additional bonding makesPVC stiffer, (b) When a forceis applied to the polymer, thevan der Waals bonds arebroken and the chains slidepast one another
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Interatomic spacing is the equilibrium spacing betweenthe centers of two atoms.
Binding energy is the energy required to separate two
atoms from their equilibrium spacing to an infinitedistance apart. Modulus of elasticity is the slope of the stress-strain
curve in the elastic region (E).
Yield strength is the level of stress above which amaterial begins to show permanent deformation. Coefficient of thermal expansion (CTE) is the amount
by which a material changes its dimensions when the
temperature changes.
Binding Energy and InteratomicSpacing
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2003 Brooks/Cole Publishing / T homson
Learning
Atoms or ions areseparated by andequilibrium spacing thatcorresponds to theminimum inter-atomicenergy for a pair of atoms
or ions (or when zeroforce is acting to repel orattract the atoms or ions)
Atraction
Repulsion
Sum
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Comparision of binding energyType of Bond Bonding Energy
(Kcal/mol)
Ionic 150-370
Covalent 125-300
Metallic 25-200
Van der Waals 10
Streching of bond
Breaking of bond
Elastic deformation
Fracture- StateTransfomration (Solid, -lquid)
High binding energy
The bond is stronger andrequires more energy to break it
Low thermal dilatation
Higher fusion temperature
Higher Young Module
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2003 Brooks/Cole Publishing / T homson
Learning
The inter-atomic energy (IAE)-separation curve for two atoms. Materialsthat display a steep curve with a deep trough have low linear coefficientsof thermal expansion
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Sort
Ionic
Covalent
Metallic
Secondary
Bingind energy
High
VariableHigh-DiamontLow-Bismut
VariableHigh-Tungsten
Low-Mercury
Low
Characteristics
Non directional (ceramics)Electroneutrality
DirectionalSemiconductors, ceramics
Polymers (Chain)
Non directional (metals)
DirectionalPolymers (Between Chain)
inter-molecular
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Materials, properties,Binding
Ceramics(Ionic & covalent)
Metals
(Metallic)
Polimeros(Covalent & Secondary)
High binding energyHigh TtvHigh ELow a
Variable binding energymoderate Tfmoderate Emoderate a
Variable binding energyLos Ttv
Low E
High a
Ttv Transition vitreous temperatureE Elastic modulus
a Thermal Expansion
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Silica is used for making long lengths of optical fibers(Figure 2-4). Being a covalently and ionically bondedmaterial, the strength of Si-O bonds is expected to be
high. Other factors such as susceptibility of silicasurfaces to react with water vapor in atmospherehave a deleterious effect on the strength of silicafibers. Give n this, what design strategies can you
think of such that silica fibers could still be bent to aconsiderable degree without breaking?
Design Strategies for Silica Optical Fibers
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SOLUTION
Based on the mixed ionic and covalent bonding insilica we know that the Si-O bonds are very strong.We also know that covalent bonds will be directionaland hence we can anticipate silica to exhibit limitedductility. Therefore, our choices to enhance ductility
of optical fibers are rather limited since thecomposition is essentially fixed. Most other glassesare also brittle. We can make an argument that silicafibers will exhibit better ductility at higher
temperatures. However, we have to use them formaking long lengths of optical fibers (most of whichare to be buried underground or under the sea) andhence keeping them at an elevated temperature isnot a practical option.
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SOLUTION (Continued)
Therefore, we need to understand, beyond what thenature of bonding consideration can offer us, why glass fibersexhibit limited ductility. Is this a property that is intrinsic to theglass or are there external variables that are causing a change inthe chemistry and structure of the glass? Materials scientists andengineers have recognized that the lack of ductility in opticalglass fibers is linked to the ability of the silica surface to react
with water vapor in the atmosphere. They have found that watervapor in the atmosphere reacts with the surface of silica leadingto micro-cracks on the surface.When subjected to stress thesecracks grow rapidly and the fibers break quite easily! They havealso tested silica fibers in a vacuum and found that the levels to
which one can bend fibers are much higher.
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Design of a Space Shuttle Arm
NASAs space shuttles have a long manipulator robot arm, alsoknown as the Shuttle Remote Manipulator System or SRMS(Figure), that permits astronauts to launch and retrieve satellites.It is also used to view and monitor the outside of the spaceshuttle using a mounted video camera. Select a suitable material
for this device.
NASAs Shuttle Remote
Manipulator System: SRMS.
Courtesy of Getty Images)
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SOLUTION
Lets look at two of the many materialchoices.First, the material should be stiff so thatlittle bending occurs when a load is applied; thisfeature helps the operator maneuver themanipulator arm precisely. Generally, materialswith strong bonding and high melting points alsohave a high modulus of elasticity,or stiffness.Second, the material should be light in weight topermit maximum payloads to be carried into orbit;
a low density is thus desired. It is estimated that itcosts about US $100,000 to take the weight of abeverage can into space! Thus, the density mustbe as low as possible.
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SOLUTION (Continued)
Good stiffness is obtained from high-melting-point metals (such asberyllium and tungsten), from ceramics, and from certain fibers (such
as carbon). Tungsten, however, has a very high density, whileceramics are very brittle. Beryllium, which has a modulus of elasticitythat is greater than that of steel and a density that is less than that ofaluminum, might be an excellent candidate. However, toxicity of Beand its compounds must be considered. The preferred material is a
composite consisting of carbon fibers embedded in an epoxy matrix.The carbon fibers have an exceptionally high modulus of elasticity,while the combination of carbon and epoxy provides a very low-density material. Other factors such as exposure to low and hightemperatures in space and on earth must also be considered. Thecurrent shuttle robot arm is about 45 feet long, 15 inches in diameterand weighs about 900 pounds. When in space it can manipulateweights up to 260 tons.