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1- ADVANCED CERAMIC POWDERS:

The applications for ceramic materials are diverse, from bricks and tiles

to electronic and magnetic components. These applications use the wide

range of properties exhibited by ceramics. The functions of ceramic

products are dependent on their chemical composition and

microstructure, which determines their properties. It is the

interrelationship between structure and properties. Ceramics can divide

according to their properties and applications into traditional or

advanced.

Traditional ceramics include high-volume items such bricks and tiles,

toilet bowls (white wares, and pottery.

!dvanced ceramics include newer materials such as laser host materials,

pie"oelectric ceramics, and ceramics for dynamic random access

memories (#$!%s, often produced in small &uantities with higher

prices. There are other characteristics that separate these categories.

Traditional ceramics are usually based on clay and silica. There is

sometimes a tendency to e&uate traditional ceramics with low technology'

however, advanced manufacturing techni&ues are often used. Competition

among producers has caused processing to become more efficient and

cost effective. Complex tooling and machinery is often used and may be

coupled with computer-assisted process control.

Advanced ceramics are also referred to as special,) technical,) or

engineering) ceramics. They exhibit superior mechanical properties,

corrosion*oxidation resistance, or electrical, optical, and*or magnetic

properties.

+hile traditional clay-based ceramics have been used for over ,

years, advanced ceramics have generally been developed within the last

/ years. The following 0igure compares traditional and advanced

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1-1.1Alumina

!luminum oxide (!l12, alumina, corundum is the most widely used

inorganic chemical for ceramics and is produced from the mineral bauxite

using the 3ayer process. 3auxite is a mixture of hydrated aluminum

oxide with iron oxide (0e12, silica (4i1, and titania (Ti1 impurities.

It results from the decay and weathering of aluminous rocks, often

igneous, under tropical conditions. 5ike kaolin, bauxite occurs as both

primary deposits and secondary deposits. The 3ayer process produces a

nominal 66.7 !l12product. The alumina can be prepared in a range of

grades to suit specific applications. The grades differ by the si"e and

shape of the crystals and the impurity content.

&a'#e 1: (ig)- a#umina engineering "erami"s *graes A1-A+, a! #eas!

A#/O0 an !)eir ")ara"!eris!i".

8rade!l12

Content7,

Type of product and

applications

9orosity

7

#ensity

g*cm2!pplications

!/:66.;Translucent?/2.6-2.66@a lamp

!2:66.Aot-pressed?/2.>-2.66%achine tools

!B:66.;4intered

$ecrystallised2-;2.=-2.>$efractory

!66.-66.;5ow dielectric/-2.=;-2.6B%icrowave

&a'#e /: Engineering a#umina gras A2 - A *34 5 A# /O0 5 asre6uiremen! an !)eir ")ara"!eris!i"s.

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8rade!l12

Content7,

Type of product and

applications

9orosity

7

#ensity

g*cm2!pplications

!;66.-66.;.-6B..->;.

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Washing: The precipitate is filtered and washed to reduce the sodium

content.

Calcination: The powder is calcined at temperatures in the range //F

/GC to convert the hydroxide to the oxideE

!t this stage the alumina is in the form of agglomerates of small grains

about -/ Dm in diameter.

Milling: The powder is then milled to give the desired particle si"e and

particle si"e distribution. The alumina produced in this way contains

K66.7 !l12and, as mentioned earlier, the maor impurity is @a1. The

powder may also contain small amounts, on the order of ./7, 4i1.

This level of purity is sufficient for many applications. Table 3

gives typical compositions of the main forms of calcined

aluminas.

Table 3: Composition of Calcined Aluminas.

0igure E 4hows the 3ayer 9rocess.