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Structure of Materials

Supanee Buranadham, DDS, PhD

Dept of Prosthetic Dentistry

Faculty of Dentistry, PSU

email: supanee.b@ratree.psu.ac.th

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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