haseeb ullah khan jatoi department of chemical engineering ... · degree depends on its...
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Haseeb Ullah Khan Jatoi
Department of Chemical Engineering
UET Lahore
Greek word Keramikos which means “Burnt Stuff”
indicating that desired properties of these materials
are normally achieved through a high temperature
treatment.
Ceramics are compounds between metallic and
nonmetallic elements; they are most frequently oxides,
nitrides, and carbides. For example, some of the
common ceramic materials include aluminum oxide
(or alumina,Al2O3), silicon dioxide (or silica, SiO2),
silicon carbide (SiC), silicon nitride (Si3N4).
The traditional ceramics are composed of clay
minerals such as porcelain, cement, and glass.
PROPERTIES
•Ceramic materials are relatively stiff and
strong—and comparable to those of the metals.
• Very hard.
• Extremely brittle and are highly susceptible
to fracture.
• Insulator of heat and electricity and are more
resistant to high temperatures and harsh
environments than metals and polymers.
Typical Ceramic Materials
GLASSES
• A familiar group of ceramics;
•containers, lenses, and fiberglass are typical
applications.
•They are non-crystalline silicates containing
other oxides, notably CaO, Na2O, K2O, and
Al2O3, which influence its properties.
• A typical soda–lime glass consists of
approximately 74 wt% SiO2, the balance being
mainly Na2O (soda) and CaO (lime).
•They may be fabricated with ease.
•Most inorganic glasses can be made totransform from a non-crystalline state tocrystalline state by the proper high-temperature heat treatment. This process iscalled crystallization.• The product is a fine-grained polycrystallinematerial which is called a glass–ceramic.•The most common uses for these materials areas ovenware, tableware, oven windows, andcooking range tops primarily because of theirstrength and excellent resistance to thermalshock.
CLAY PRODUCTS
One of the most widely used ceramic raw
materials is clay. Inexpensive ingredient, found
naturally in great abundance and ease with
which clay products may be formed; when
mixed in the proper proportions, clay and
water form a plastic mass that is very amenable
to shaping. The formed piece is dried to
remove some of the moisture, after which it is
fired at an elevated temperature to improve its
mechanical strength.
Most of the clay-based products fall within two
broad classifications:
Structural clay products include building
bricks, tiles, and sewer pipes.
White ware ceramics become white after the
high-temperature. e.g. porcelain, pottery,
tableware, china, and plumbing fixtures.
•A refractory material is one that retains its
strength at high temperatures. They are
important for their capacity to withstand high
temperatures without melting or decomposing,
and the capacity to remain unreactive and inert
when exposed to severe environments.
•Able to provide thermal insulation
•Typical applications include furnace linings
for metal refining, furnaces, kiln and reactor.
Glass manufacturing, metallurgical heat
treatment, and power generation.
Performance of a refractory ceramic, to a large
degree depends on its composition.
Porosity is one micro structural variable that
must be controlled to produce a suitable
refractory brick. Strength, load-bearing
capacity, and resistance to attack by corrosive
materials all increase with porosity reduction.
Fireclay Refractories
•The primary ingredients for the fireclay
refractories are high-purity fireclays,
alumina and silica mixtures usually
containing between 25 and 45 wt% alumina.
•Fireclay bricks are used principally in furnace
construction, to confine hot atmospheres, and
to thermally insulate structural members from
excessive temperatures.
•Highest temperature it can withstand is 1587
˚C
Acid or Silica Refractories
The prime ingredient for silica refractories is
silica, sometimes termed acid refractories.
These materials, well known for their high-
temperature load-bearing capacity, are
commonly used in the roofs of steel- and glass-
making furnaces; for these applications,
temperatures as high as 1650˚C may be
realized. Basic raw material is Ganister (sand
stone) and Quartzite (mineral rock)
Basic Refractories
The refractories that are rich in magnesia
(MgO), are termed basic; they may also contain
calcium, chromium, and iron compounds. Find
extensive use in some steel-making open
hearth furnaces. temperatures as high as 1500-
1700˚C may be realized. Basic raw material is
Dolomite {carbonate mineral Ca Mg(Co3)2 }
One chief concern in the application of ceramic materials is the
method of fabrication.
FABRICATION AND PROCESSING OF GLASSES AND
GLASS–CERAMICS
Glassy, or non-crystalline, materials do not solidify in the same
sense as do those that are crystalline. Upon cooling, a glass
becomes more and more viscous in a continuous manner with
decreasing temperature; there is no definite temperature at
which the liquid transforms to a solid as with crystalline
materials. One of the distinctions between crystalline and non-
crystalline materials lies in the dependence of specific volume
on temperature.
For crystalline materials, there is a discontinuous
decrease in volume at the melting temperature Tm
However, for glassy materials, volume decreases
continuously with temperature reduction; a slight
decrease in slope of the curve occurs at what is called
the glass transition temperature, or fictive
temperature Tg, Below this temperature, the material
is considered to be a glass; above, it is first a super
cooled liquid, and finally a liquid.
Glass Transition Temperature. It is a temperature at
which the viscosity is 1017 and viscous flow ceases.
Logarithm of viscosity versus temperature
Melting PointIt is the temperature at which the viscosity is 10Pa-s (100 P); the glass is fluid enough to beconsidered a liquid.Working PointIt is the temperature at which the viscosity is103 Pa-s ( 104P); the glass is easily deformed atthis viscosity.Softening PointIt is the temperature at which the viscosity is4*106Pa-s (4*107 P), is the maximumtemperature at which a glass piece may behandled without causing significantdimensional alterations.
Annealing Point
It is the temperature at which the viscosity is
1012Pa-s (1013P); at this temperature, atomic
diffusion is sufficiently rapid that any residual
stresses may be removed within about 15 min.
Strain Point
The strain point corresponds to the
temperature at which the viscosity becomes 3
*1013 Pa-s ( 3 * 10 14P); for temperatures below
the strain point, fracture will occur before the
onset of plastic deformation. The glass
transition temperature will be above the strain
point.
Glass is produced by heating the raw materialsto an elevated temperature above whichmelting occurs. It is essential that the glassproduct be homogeneous and pore free.Homogeneity is achieved by complete meltingand mixing of the raw ingredients. Porosityresults from small gas bubbles that areproduced; these must be absorbed into themelt or otherwise eliminated.Four different forming methods are used tofabricate glass products: pressing, blowing,drawing, and fiber forming
Heat Treating GlassesAnnealingWhen a ceramic material is cooled from anelevated temperature, internal stresses, calledthermal stresses, may be introduced as a resultof the difference in cooling rate and thermalcontraction between the surface and interiorregions. These thermal stresses are importantin brittle ceramics, especially glasses, sincethey may weaken the material or, in extremecases, lead to fracture, which is termed thermalshock. Normally, attempts are made to avoidthermal stresses, which may be accomplishedby cooling the piece at a sufficiently slow rate.
Once such stresses have been introduced,however, elimination, or at least a reduction intheir magnitude, is possible by an annealingheat treatment in which the glassware is heatedto the annealing point, then slowly cooled toroom temperature.Glass TemperingThe strength of a glass piece may be enhancedby intentionally inducing compressive residualsurface stresses. This can be accomplished bya heat treatment procedure called thermaltempering. Tempered glass is used forapplications in which high strength isimportant; these include large doors andeyeglass lenses. Used as a safety glasses
Glass is heated to the temperature of more than
600˚C. The glass then undergoes a high-pressure
cooling procedure called "quenching." During this
process, which lasts just seconds about 3 to 10
seconds, high-pressure air blasts the surface of the
glass. Quenching cools the outer surfaces of the
glass much more quickly than the center. As the
center of the glass cools, it tries to pull back from
the outer surfaces. As a result, the center remains
in tension, and the outer surfaces go into
compression, which gives tempered glass its
strength.