materials science ceramics materials science ceramics
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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.
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.
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