2 Atomo y Enlace Eng

8/3/2019 2 Atomo y Enlace Eng

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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_electron


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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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22

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.

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