Materials ScienceMaterials Science

Ceramics

Materials ScienceMaterials Science

Ceramics

The word ceramic can be traced back to the Greek term keramos, meaning

"a potter" or "pottery". Keramos in turn is related to an older Sanskrit root

meaning "to burn". Thus the early Greeks used the term to mean "burned

stuff" or "burned earth" when referring to products obtained through the

action of fire upon earthy materials.

Definition of ceramicsDefinition of ceramics

The art and science of making and using solid articles formed by the action

of heat on earthy raw materials.

→ Conventional ceramics (Traditional ceramics)

i.e. clay product, glasses, and cement

The art and science of making and using solid articles with have as their

essential components, and are composed in large part of inorganic

nonmetallic materials.

→ Conventional ceramics & New ceramics (ceramics which

have either unique and outstanding properties, they have been

developed in order to fulfill a particular need. Ex. electronic ceramics,

bioceramics, etc.)

Ceramic structuresCeramic structuresCeramic structuresCeramic structures Two or more different elements Ionic and/or covalent bonds

Ceramic Compound

Melting Point °

% Covalent character

% Ionic character

Magnesium Oxide 2798° 27% 73%

Aluminum Oxide 2050° 37% 63%

Silicon Dioxide 1715° 49% 51%

Silicon Nitride 1900° 70% 30%

Silicon Carbide 2500° 89% 11%

Ionic bonds most often occur between metallic and

nonmetallic elements that have large differences in their

electronegativities. Ionically-bonded structures tend to have

rather high melting points, since the bonds are strong and

non-directional.

The other major bonding mechanism in ceramic structures is

the covalent bond. Unlike ionic bonds where electrons are

transferred, atoms bonded covalently share electrons. Usually

the elements involved are nonmetallic and have small

electronegativity differences.

Electronegativity: The attraction of an atom for shared electrons.

Ceramic structuresCeramic structures

Ceramic materials can be divided into two classes:

crystalline and amorphous (noncrystalline).

In crystalline materials, atoms (or ions) are arranged in a regularly

repeating pattern in three dimensions (i.e., they have long-range

order).

In amorphous materials, the atoms exhibit only short-range order.

Some ceramic materials, like silicon dioxide (SiO2), can exist in

either form. A crystalline form of SiO2 results when this material is

slowly cooled from a high temperature (Tm>1723˙C). Rapid

cooling favors noncrystalline (amorphous) formation since time is

not allowed for ordered arrangements to form.

Crystalline form of SiO2Crystalline form of SiO2Amorphous form of SiO2Amorphous form of SiO2

The type of bonding (ionic or covalent) and the internal

structure (crystalline or amorphous) affects the properties

of ceramic materials.

Classification of ceramicsClassification of ceramics

Ceramic materials

Glasses

Clay products

Refractories

Abrasives

Cements

Advanced ceramics

Structural clay product

Whitewares

Glasses

Glass ceramics

Industry Segment Common Examples

Structural clay products

Brick, sewer pipe, roofing tile, clay floor and wall tile (i.e., quarry tile), flue linings

WhitewaresDinnerware, floor and wall tile, sanitaryware, electrical porcelain, decorative ceramics

RefractoriesBrick and monolithic products are used in iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries

GlassesFlat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical fibers)

AbrasivesNatural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure blasting of materials

CementsUsed to produce concrete roads, bridges, buildings, dams, and the like

Advanced ceramics

Structural Wear parts, bioceramics, cutting tools, and engine components

Coatings Engine components, cutting tools, and industrial wear parts

Chemical and environmental

Filters, membranes, catalysts, and catalyst supports

ElectricalCapacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets and superconductors

Typical properties of ceramicsTypical properties of ceramics

Light weight

Corrosion resistance

Very brittle

Low and variable tensile strengths

High compressive strengths - generally much higher than tensile strength

Very high hardness, high wear and abrasion resistance

High heat capacity and low heat conductance

Electrically insulating, semiconducting, or superconducting

Nonmagnetic and magnetic

Chemical PropertiesChemical Properties

Chemical properties describe the chemical stability of materials.Chemical properties describe the chemical stability of materials.

The high chemical durability of the great majority of ceramic

products makes them resistant to almost all acids, alkalis, and

organic solvents.

Ceramics are more resistant to corrosion than plastics and

metals are. Of further importance is the fact that ceramic

materials are not affected by oxygen.

Most ceramics have very high melting points, and certain

ceramics can be used up to temperatures approaching their

melting points.

Ceramics also remain stable over long time periods.

Mechanical Properties : OverviewMechanical Properties : OverviewMechanical properties describe the way that a material

responds to forces, loads, and impacts.

The following characteristics are commonly tested:

• Tensile strength - failure under tension

• Compression strength – failure under compression

• Stiffness – resistance to bending (elastic deformation)

• Hardness – resistance to surface penetration or scratching

• Impact (Toughness) – resistance to abrupt forces

• Fatigue failure – resistance to continued usage (cyclic

deformation)

DeformationDeformation

When materials are put into use, they undergo

changes in dimensions in response to the forces

they are exposed to. This is called deformation.

Elastic deformation: the object reverts to its

original size and shape when the load is removed.

Plastic deformation: when load is removed, object

has permanent change in shape

Fracture occurs when the load causes the object to

break into two or more pieces.

Elastic Modulus(Young’s modulus)

Stress-Strain diagrams for typical (a) brittle and (b) ductile materials

Mechanical behavior is dependent on many factors: e.g.Mechanical behavior is dependent on many factors: e.g.

• • TemperatureTemperature – – the ratio of the service or test temperature to the melting point is known as the homologous temperature.

• • CompositionComposition

• • MicrostructureMicrostructure – – minuscule structural and fabrication flaws

The ability to deform reversibly is measured by the elastic

modulus. Materials with strong

bonding require large forces to

increase space between

particles and have high values

for the modulus of elasticity.

Temp E

Ceramics are strong, hard materials. The principal limitation of

ceramics is their brittleness, i.e., the tendency to fail suddenly

with little plastic deformation. - In ionic solids because ions of

like charge have to be brought into close proximity of each other

forming large barrier for dislocation motion, the slip is very

difficult. Similarly, in ceramics with covalent bonding slip is not

easy (covalent bonds are strong).

→ High yield stress and hardness

Brittle fracture occurs by the formation and rapid propagation of

cracks. Tensile stress would be needed

to break the bonds between atoms

in a perfect solid and pull the object

apart.

→ Ceramics are weak in tension.

Compressive (crushing) strength is important in ceramics used

in structures such as buildings or refractory bricks. The

compressive strength of a ceramic is usually much greater than

their tensile strength. Ceramics are generally quite inelastic and do not bend like metals.

The fracture toughness is the ability to resist fracture when a

crack is present. Ceramics have low fracture toughness.

Fracture of ceramics highly sensitive to the presence of defects

e.g. pores. Highly resistant to wear and erosion (compression loading

phenomena)

Thermal PropertiesThermal Properties The most important thermal properties of ceramic materials are

heat capacity, thermal expansion coefficient, and thermal

conductivity.

In solid materials at T > 0 K, atoms are constantly vibrating.

Thermal conductivity : The ability to carry thermal energy (heat).

Thermal energy can be either stored or transmitted by a solid.

The ability of a material to absorb heat from its surrounding is its

heat capacity (The ability of a material to absorb heat).

Thermal expansion coefficient

Fractional change in length divided by change in temperature, a

measure of a materials tendency to expand when heated.

The potential energy between two bonded atoms is related to their

bond length. Ceramics generally have strong bonds and light

atoms. The result is that they typically have both high heat

capacities and high melting temperatures.

The conduction of heat through a solid involves the transfer of

energy between vibrating atoms. The vibration of each atom

affects the motion of neighboring atoms, and the result is elastic

waves that propagate through the solid.

Amorphous ceramics which lack the ordered lattice undergo

even greater scattering, and therefore are poor conductors.

Those ceramic materials that are composed of particles of

similar size and mass with simple structures (such as

diamond or BeO) undergo the smallest amount of scattering

and therefore have the greatest conductivity.

Thermal expansionThermal expansion

Materials change size when heating.

Atomic view: Mean bond length increases with T.

The heat transmission is interrupted by imperfection of structure,

i.e. grain boundaries and pores, so that more porous materials

are better insulators.

Heat Capacity

Coefficient of Linear Expansion

Thermal Conductivity

(J/kg · K) 1/ ° Cx10-6 (W/m K)

Aluminum metal 660 900 23.6 247

Copper metal 1063 386 16.5 398

Alumina 2050 775 8.8 30.1

Fused silica 1650 740 0.5 2

Soda-lime glass 700 840 9 1.7

Polyethylene 120 2100 60-220 0.38

Polystyrene 65-75 1360 50-85 0.13

MaterialMelting

Temp (°C)

One of the most interesting high-temperature applications

of ceramic materials is their use on the space shuttle.

Ceramics are strong, hard materials that are also resistant

to corrosion (durable). These properties, along with their

low densities and high melting points, make ceramics

attractive structural materials, e.g. automobile engines,

armor for military vehicles, and aircraft structures.

Electrical PropertiesElectrical Properties

Ceramics exhibit the largest possible diversity in electrical

conductivity [ (-cm)-1], in terms of the type and magnitude of the

conductivity:

insulators, < 10-22 (-cm)-1 (such as alumina)

ionic conductors, ~ 10-2 (-cm)-1 (such as AgI)

electronic semi-conductors, ~ 100 (-cm)-1 (such as SiC)

electronic conductors, >103 (-cm)-1 (such as TiN)

electronic superconductors, (such as YBa2Cu3O7-x)

Electrical current in solids is most often the result of the flow

of electrons (electronic conduction).

Electrical current in solids is most often the result of the flow of

electrons (electronic conduction).

Type Material Resistivity (w- cm)

Copper 1.7 x 10- 6

CuO2 3 x 10- 5

SiC 10

Germanium 40

Fire- clay brick 108

Si3N4 > 1014

Polystyrene 1018

Superconductors: YBa2Cu3O7- x < 10- 22 (below Tc)

Metallic conductors:

Semiconductors:

I nsulators:

Resistivity = 1/conductivity

A dielectric material is a substance that is a poor conductor of

electricity, but an efficient supporter of electrostatic fields. If the

flow of current between opposite electric charge poles is kept to a

minimum while the electrostatic lines of flux are not impeded or

interrupted, an electrostatic field can store energy. This property is

useful in capacitors, especially at radio frequencies.

Primary applications of dielectric ceramics include resistors,

insulators, and capacitors.

An important property of a dielectric is its ability to support an

electrostatic field while dissipating minimal energy in the form of

heat. The lower the dielectric loss (the proportion of energy lost

as heat), the more effective is a dielectric material. Another

consideration is the dielectric constant, the extent to which a

substance concentrates the electrostatic lines of flux.

MaterialDielectric constant

at 1 MHz

Dielectric

strength (kV/cm)

Air 1.00059 30

Polystyrene 2.54 - 2.56 240

Glass (Pyrex) 5.6 142

Alumina 4.5 - 8.4 16 - 63

Porcelain 6.0 - 8.0 16 - 157

Titanium dioxide 14 - 110 39 - 83

Magnetic PropertiesMagnetic Properties

Atoms are composed of protons, neutrons and electrons.

Electrons carry a negative electrical charge and produce a

magnetic field as they move through space. A magnetic field is

produced whenever an electrical charge is in motion.

Magnetism is a phenomenon by which materials exert an

attractive or repulsive force on other materials. There are two

types of magnetic poles, conventionally called north and south.

Unlike electric charges, magnetic

poles always occur in North-South

pairs; there are no magnetic

monopoles.

When a material is placed within a magnetic field, the magnetic

forces of the material's electrons will be affected.

In most atoms, electrons occur in pairs. Each electron in a pair

spins in the opposite direction. So when electrons are paired

together, their opposite spins cause there magnetic fields to

cancel each other. Therefore, no net magnetic field exists.

Alternately, materials with some unpaired electrons will have a

net magnetic field and will react more to an external field. Most

materials can be classified as diamagnetic, paramagnetic, or

ferromagnetic.

When a current flows through

a conductor, a magnetic field

surrounds the conductor. As

current flow increases, so

does the number of lines of

force in the magnetic field

The right hand rule helps demonstrate the

relationship between conductor current and

the direction of force. Grasp a wire conductor

in the right hand, put your thumb on the wire

pointing upward, and wrap your four fingers

around the wire. As long as the thumb is in the

direction that current flows through the wire,

the fingers curl around the wire in the direction

of the magnetic field.

Diamagnetic materials have a very weak and negative susceptibility to

magnetic fields. Diamagnetic materials are slightly repelled by a

magnetic field and the material does not retain the magnetic properties

when the external field is removed. Diamagnetic materials are solids

with all paired electron and, therefore, no permanent net magnetic

moment per atom. Diamagnetic properties arise from the realignment of

the electron orbits under the influence of an external magnetic field.

Most elements in the periodic table, including copper, silver, and gold,

are diamagnetic.

Paramagnetic materials have a small and positive susceptibility to

magnetic fields. These materials are slightly attracted by a magnetic

field and the material does not retain the magnetic properties when the

external field is removed. Paramagnetic properties are due to the

presence of some unpaired electrons and from the realignment of the

electron orbits caused by the external magnetic field. Paramagnetic

materials include magnesium, molybdenum, lithium, and tantalum.

Ferromagnetic materials have a large and positive susceptibility to an

external magnetic field. They exhibit a strong attraction to magnetic fields

and are able to retain their magnetic properties after the external field has

been removed. Ferromagnetic materials have some unpaired electrons

so their atoms have a net magnetic moment. They get their strong

magnetic properties due to the presence of magnetic domains. In these

domains, large numbers of atoms moments (1012 to 1015) are aligned

parallel so that the magnetic force within the domain is strong. When a

ferromagnetic material is in the unmagnitized state, the domains are

nearly randomly organized and the net magnetic field for the part as a

whole is zero. When a magnetizing force is applied, the domains become

aligned to produce a strong magnetic field within the part. Iron, nickel,

and cobalt are examples of ferromagnetic materials. Components with

these materials are commonly inspected using the magnetic particle

method.

Ceramics containing iron oxide (Fe2O3) can have magnetic

properties similar to those of iron, nickel, and cobalt magnets.

These iron oxide-based ceramics are called ferrites. Other

magnetic ceramics include oxides of nickel, manganese, and

barium. Ceramic magnets, used in electric motors and electronic

circuits, can be manufactured with high resistance to

demagnetization. When electrons become highly aligned, as they

do in ceramic magnets, they create a powerful magnetic field

which is more difficult to disrupt (demagnetize) by breaking the

alignment of the electrons.

Optical PropertiesOptical Properties

An optical property describes the way a material reacts to

exposure to light. When light strikes an object it may be

transmitted, absorbed, or reflected.

There are three primary ways to describe the optical quality of a

material:

Transparent

Translucent

Opaque

The absorption of energy results in the shifting of electrons from

the ground state (Lowest electron energy state) to a higher,

excited state (An energy state to which an electron may move by

the absorption of energy ). The electrons then fall back to the

ground state, accompanied by

the reemission of electromagnetic

radiation.

The energy range for visible light is from 1.8 to 3.1 eV. Materials

with band gap energies (Eg) in this range will absorb those

corresponding colors (energies) and transmit the others. They

will appear transparent and colored. Materials with band gap

energies less than 1.8 eV will be opaque because all visible light

will be absorbed by electron

Light that is emitted from electron transitions in solids is called

luminescence. If it occurs for a short time it is fluorescence, and if

it lasts for a longer time it is phosphorescence.

Conversions between light and electricity are the basis for the

use of semiconducting materials such as gallium arsenide in

lasers and the widespread use of LED's (light-emitting diodes) in

electronic devices. Fluorescent and phosphorescent ceramics

are used in electric lamps and television screens.