estructura de los minerales.
Transcript of estructura de los minerales.
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Structure of Materials
Supanee Buranadham, DDS, PhD
Dept of Prosthetic Dentistry
Faculty of Dentistry, PSU
email: [email protected]
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“ Properties of materials
originate from their internalstructures.”
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Classes of DentalMaterials
Polymers
Metals
Ceramics and Glasses
Composites
General Properties?
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Polymers (Plastics)
Low density
Both thermal and electrical
insulators
Poor reflectors of light
Some are flexible and deformable.
WHY?
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Metals
High density
High thermal and electrical
conductivity
Opaque
Polished to high luster
Relatively heavy and deformable
WHY?
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Ceramics and Glasses
Hard and brittle
Thermally resistant
Inert (chemically resistant)
WHY?
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Structures
Atomic level Atomic bonding
Bonding distance/ energy
Structural level Crystalline structure
Noncrystalline structure (Amorphous)
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Atomic Level
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Bonding
Primary bonds Ionic, Covalent, Metallic Strong and stiff, do not easily melt with
increasing temperature e.g. Metals, Ceramics
Secondary bonds Hydrogen, van der Waals Weak
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Ionic Bonding
Involves the transfer of one or more electrons between atoms of different types
The atom becomes charged ions (e.g. Na+ and Cl-) which attract oneanother and form a chemical bond due to their opposite electrostaticcharges.
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e.g. salts, oxides, NaCl, MgO,gypsum
Tend to be hard and brittle
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Covalent Bonding
Involves sharing of electrons by adjacent atoms.
The tight covalent bonds make such simplemolecules relatively independent of one another, so that collections of them tend to form liquids or gases at ambient temperatures.
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Precise bond orientations
e.g. H2,, H
2O, Cl
2,
Methylmethacrylate
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Metallic Bonding
The outer shell of electrons of most metals is lessthan half full. Each atom donates one or more electrons to a “cloud” of electrons.
These electrons are shared in common by all of themetal atoms, which have become positivelycharged ions as a result of giving up electrons.
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The metal ions are thus heldtogether by their mutual attraction
to the electron cloud. Excellent electrical and thermal
conductivity
Ability to deform plastically
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B. Covalent bond
formation•e- sharing•very precise bondorientations
A. Ionic bond formation•e- transfer from one
element to another
C. Metallic bondformation
•e- sharing•formation of a “gas” of e that bonds the atomstogether in a lattice
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Discussion of PrimaryBonds
Covalent bonds vs. Ionic and Metallic bonds? Covalent bonds Sharing of electrons with specific
neighboring atoms
Ionic and Metallic bonds electrostatic attractioninvolving all neighboring ions
Covalent: Bonding being strongly directional to forma three-dimensional network to make a solid
The crystal is very hard and stiff e.g. carbon in the form of diamond.
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Many solids, such as SiO2and other
ceramics have a mixed ionic-
covalent character. (more commonthan pure bonding)
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Secondary Bonds
Occur due to the presence of anelectrostatic dipole, which can be
induced by a primary bond. Hydrogen bond
van der Waals bond
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Hydrogen Bonding
“Permanent dipole bonds”Differences in electron densities resultin charge variations along the molecule.
H2O
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van der Waals Forces
“Fluctuating dipole bonds”Fluctuating positions of electronsrelative to an atom’s nucleus ( the dipole is not fixed in direction)
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Role of Secondary Bonds inPolymers
Covalent bonds form the chain molecules andattach hydrogen and other atoms to the carbon backbone.
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• Hydrogen bonds and other secondary bondsoccur between the chain molecules and tendto prevent them form sliding past one
another.
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The relative weakness of the secondarybonds accounts for the low meltingtemperatures and the low strengths andstiffnesses of polymers.
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Bond DistanceIntermolecular Force
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Bond distance
Factors that control bond distance Diameter of the atoms involved
Repulsion and attraction forces
“Disturbing” force e.g. mechanical, thermal, orelectrical force
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Interm
olecularForc
es
InteratomicDistance
Attractive Force
Repulsive Force
Force Balance
Equilibrium Position
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Stiffness
The stiffness of thematerial is proportionalto the rate of change of
the force with a changein displacement.
Measured by the slopeof the net force curve n
ear equilibriumposition.
Intermolecular
Forces
Resultant Force
Equilibrium Position,Resultant force = 0
InteratomicDistance
slope
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Generalizations based onatomic bonding
Properties
Density
Stiffness
Thermal
expansion
Conductivity
Factors
Atomic wt, radius,packing
Interatomic forces
Bond strength and Tm
Freedom of electronmovement
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Structure Level
Crystalline structure
Noncrystalline structure
(Amorphous)
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Materials can be amorphous orcrystalline (or mixtures of both, as
in some plastics) in solid state.Crystallinity, or the lack of it, oftendetermines the use properties of a
material.
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Metals and ceramics are composed of aggregations of small grains, each of which isan individual crystal.
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Glasses have an amorphous or noncrystallinestructure.
Polymers are composed of chain-like
molecules, which are sometimes arranged in regular arrays in a crystalline manner.
Amorphous structure Crystalline structure
POLYM
ER
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Crystalline Structure
When a solid has acrystalline structure,the atoms are arrangedin repeating structures
call unit cells. The cells form a larger3-D array called alattice or a crystal.
Possess a periodicity
that produces longrange order
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A Unit Cell: examples
Body-centered cubic Face-centered cubic
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Arrangements of atoms in a cubic unit cell Simple cubic (SC) or Primitive cubic (PC) Body-centered cubic (BCC): e.g. Cr, Fe, Mo, Na, Tungsten
Face-centered cubic (FCC): e.g. Ag, Al, Pb, Cu, Ni
Hexagonal closed-pack (HCP): e.g. Be, Mg, Ti, Zn
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In the solid state, a crystallinepolymer is stiff and rigid with a
very high “viscosity”.When heated to the melting point(Tm), the viscosity drops
significantly and abruptly to amuch lower level. (~ ice, when itmelts).
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Atoms in a space lattice are constantly invibration about their centers. Theaverage kinetic energy of vibration over
the entire crystal is related to thetemperature. All the atoms do not posses the same amount
of energy.
If the energy of an atom exceeds the bondingenergy, it can move to another position in thelattice. “Diffusion”
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Noncrystalline Structure
e.g. glasses, amorphous polymersDo not have regular, repeating 3-D arraysof atoms
Do not represent low internal energiesDo not have a definite Tm but graduallysoften as the temperature is raised. The temperature at which there is an
abrupt increase in the thermal expansioncoefficient, indicating increased molecularmobility, is called the glass transition tem perature (Tg)
Gl T i i T
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Glass Transition Temperature(Tg)
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How does the structure effectmaterial properties?
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Strong primary bonds are resistant tostretching High Elastic modulus (E)
High E stiff/rigid material
Examples: Covalent bonds in diamond: E ~ 1000 GPa Metallic bonds in metals: E ~ 3x100 GPa Covalent and secondary bonds in polymers
(glassy state): E ~ 3 GPa 1 Pa (Pascal) = 1 N/m2
C t lli A h
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Crystalline vs. AmorphousStructure
Polymer properties are significantly affectedby the degree of crystallinity.
Crystallinity tends to Increase mechanical properties e.g. tensile strength
and hardness
Diminish ductility, toughness, and elongation
Decrease chemical permeability and solubility
Generally optically opaque or translucent (whileamorphous polymers optical clarity)
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The Nature of Metals
solids composed of atoms held togetherby a matrix of electrons
Strong
Good ductility and malleability due to theability of the atomic centers to slide against each other into new positions within the same crystal lattice (natureof metallic bond) Ductility = ability to be drawn into a wire Malleability = ability to be pounded into a thin sheet
Good electrical and thermal conductivity
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The Nature of Ceramics
a combination of one or more metals with anonmetallic elementvery rigid covalent or ionic bonds between
adjacent atomsStrongBrittle Both ionic and covalent bonds involve very strongbonds between neighboring atoms. Thus crystalline ceramicswith this type of bond tend to be very brittle. Loading tends toresult in crystal cleavage or separation.
Tends to be electrical insulators because theelectrons are tied up in bonding and are notfree to move throughout the crystal.
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The Nature of Polymers
Composed of long-chain repeating molecules
The atoms in the repeating molecule are stronglybonded (usually covalent), and the bonds betweenmolecules may be weaker secondary bonds or
similar covalent bonds. Tend to form long chains and the degree of
crystallinity depends on chain alignmentUsually weaker than most ceramics and metalsbecause the molecular chains are bonded to eachother only with secondary bond.
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The Nature of Composites
A combination of two or morematerials that has properties that
the component materials do nothave by themselves.
Examples: Wood, Fiberglass (fiber-
reinforced plastic), Sportequipment,..etc
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General Characteristics of Materials
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Summary
Atomic level Atomic bondings: Primary, Secondary Bonding distance/ energy
Structural level Crystalline vs. Noncrystalline structures Tg
How does the structure effect thematerial properties?
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End of
Structure of Materials