Ceramics - PowerPoint Presentation

Slide1

Ceramics

Haseeb

Ullah

Khan

Jatoi

Department of Chemical Engineering

UET LahoreSlide2

Ceramics

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,Al

2

O

3

), silicon dioxide (or silica, SiO

2

), silicon

carbide (SiC), silicon nitride (Si

3

N

4

).

The

traditional

ceramics are composed

of clay minerals

such as porcelain, cement

, and glass.Slide3

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

. Slide4

Typical Ceramic Materials Slide5
Slide6
Slide7
Slide8
Slide9

Types of CeramicsSlide10

Glasses

GLASSES

A

familiar group of ceramics;

containers

, lenses, and fiberglass

are typical applications.

T

hey

are

non-crystalline silicates

containing

other oxides, notably CaO, Na

2

O, K

2

O, and Al

2

O

3

, which influence

its properties.

A typical soda–lime glass consists of approximately

74

wt%

SiO

2

, the

balance being mainly Na

2

O (soda) and CaO (lime

).

T

hey

may be

fabricated with ease.Slide11
Slide12

The viscosity of glasses varies with temperature according to Arrhenius Type Relationship

μ

= A exp (B/T)

A and B are constants, T is temperature,

μ

is viscosity.

Creep rate equation is

d

ε

/dt = B

σ

n

n = 10 for glasses.Slide13

GLASS–CERAMICS

Most inorganic glasses can be made to transform from a

non-crystalline

state

to crystalline state

by the proper high-temperature heat treatment. This process

is called

crystallization

.

The

product is a fine-grained polycrystalline

material which is called a

glass–ceramic.

The

most common uses

for these

materials are as ovenware, tableware, oven windows, and

cooking range tops primarily because

of their strength and excellent resistance to thermal

shock.Slide14

CLAY PRODUCTSOne

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

.Slide15

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.Slide16

REFRACTORIES

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. Slide17

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

.Slide18

Classification on basis of Composition

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 ˚CSlide19

Acid or Silica RefractoriesThe 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)Slide20

Basic RefractoriesThe 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(Co

3

)

2

}Slide21

Fabrication and Processing of Ceramics

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.

O

ne

of the distinctions

between crystalline

and

non-crystalline

materials lies in the dependence of specific

volume on temperature.Slide22

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 10

17

and viscous flow ceases. Slide23
Slide24

Logarithm of viscosity versus temperatureSlide25

Melting PointIt is the temperature at which the viscosity is 10 Pa-s (100 P); the glass is fluid enough to be considered a liquid.

Working Point

It is the temperature at which the viscosity is 10

3

Pa-s ( 10

4

P); the glass is easily deformed at this viscosity.

Softening Point

It is the temperature at which the viscosity is 4*10

6

Pa-s (4*10

7

P), is the maximum temperature at which a glass piece may be handled without causing significant dimensional alterations.Slide26

Annealing PointIt is the temperature at which the viscosity is 1012

Pa-s (10

13

P); 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 *10

13

Pa-s ( 3 * 10

14

P); 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.Slide27

Glass FormingGlass is produced by heating the raw materials to an elevated temperature

above which

melting occurs

. It

is essential that the glass

product be

homogeneous and pore free. Homogeneity is achieved by complete melting

and mixing

of the raw ingredients. Porosity results from small gas bubbles that are

produced; these

must be absorbed into the melt or otherwise

eliminated.

Four different forming methods are used to fabricate glass products:

pressing, blowing

, drawing, and fiber formingSlide28

Heat Treating GlassesAnnealingWhen a ceramic material is cooled from an elevated temperature, internal

stresses, called

thermal stresses, may be introduced as a result of the difference in

cooling rate

and thermal contraction between the surface and interior

regions. These thermal stresses are important in brittle ceramics, especially glasses, since they may weaken the material or, in extreme cases, lead to fracture, which is termed thermal shock. Normally, attempts are made to avoid thermal stresses, which may be accomplished by cooling the piece at a sufficiently slow rate. Slide29

Once such stresses have been introduced, however, elimination, or at least a reduction in their magnitude, is possible by an annealing heat treatment in which the glassware is heated to the annealing point, then slowly cooled to room temperature.Glass Tempering

The strength of a glass piece may be enhanced by intentionally inducing

compressive residual

surface stresses. This can be accomplished by a heat treatment

procedure called

thermal tempering.

Tempered

glass is used for applications in which high strength is

important; these

include large doors and eyeglass lenses

.

Used as a safety glassesSlide30

How Glass Tempering is done

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.