Ceram Micros

8/11/2019 Ceram Micros

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High Temperature Materials

Course KGP003

By

Docent. N. Menad

Dept. of Chemical Engineering

and Geosciences

Div. Of process metallurgy

Lule University of Technology( Sweden )

Ch. 12

Structures and Properties of Ceramics


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Ceramic materials are inorganic and non-metallic materials. Most ofthem are compounds between metallic and non-metallic elements for

which the inter-atomic bonds are either totally ionic, or predominantly

ionic but having some covalent character.

Ceramic materials are inorganic and non-metallic materials. Most ofthem are compounds between metallic and non-metallic elements for

which the inter-atomic bonds are either totally ionic, or predominantly

ionic but having some covalent character.

Ceramics used in electronic, computer, communication,

aerospace and a host of other industries

Ceramics used in electronic, computer, communication,

aerospace and a host of other industries

Ch. 12


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CERAMIC STRUCTURES

Their crystal structures are generally more complex than those for metals.

_________________

CaF2 89

MgO 73NaCl 67

Al2O3 63

SiO2 51

SiN4 30

ZnS 18

SiC 12

_________________

% Ionic characterof inter-atomic Bonds

Ch. 12

The atomic boning ( purely ionic totally covalent)

Many ceramicsCombination of these bonding

% ionic character = 1- exp[-0.25)(Xa - Xb)2] X 100

Xa and Xb are the electro-negativities for the

respective elements from table 2.7 p. 19


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Crystal structures

atomic bonding ( ionic )

Ch. 12

For Ceramic materials

Crystal structures Electrically charged ions

Metallic ions or cations+ charge ( the valence electrons moved to

non-metallic ions or anions (- charge)

The magnitude of the electrical

charge on each component ions

The relative sizes of cations and

anions

Influence the crystal structure

1

2

1 The crystal must be electrically neutral (total cations (+ charge) = total

anions (- charge) ex CaF2 (Ca+2,2 F-1 ), Al2O3 (2Al

+3, 3O-2)

2 rA and rCSize of rA > rC rc/rA < 1


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Crystal structures

Stable Stable unstable

Ch. 12

cation anion

Stable ceramic crystal structure

Critical or minimum rc/rA ratio

Cation-anion contact


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2 < 0.155

3 0.155 0.225

4 0.225 0.414

6 0.414 0.732

8 0.732 1.0

CoordinationNumber Cation - AnionRadius Ratio CoordinationGeometry

Ch. 12


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B C

A

rc

rA

P

O

Ex. Pr. 2.1 p. 387

Show that the minimum cation to anion radius ratio

for the coordination number 3 is 0.155

rc

rA = 0.155

o

A

P

CB

AO = rc+ rA

AP = rA

Cos =AO rc+ rA

AP=

rA

Cos = 30o =3

2

rc+ rA

rA

=

3

2 rA

rc

= 0.155


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AX-TYPE CRYSTAL STRUCTURES

Some common ceramics Number of cations = number of anions

Cl- Na+ Cl- Cs+

Coordination number is 8

This is not BCC crystal structure because

(cation and anion are involved).

ZnS, CaF2, p. 389 and 390

Ch. 12 Structures and Properties of Ceramics


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Electron micrograph of

Kaolinite crystals. They are inthe form of hexagonal plates

Structures and Properties of CeramicsCh. 12

St t d P ti f C i


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The silica structure is the basic structure for many

ceramics, as well as glass. It has an internal arrangement

consisting of pyramid (tetrahedral or four-sided) units.

Four large oxygen (0) atoms surround each smaller silicon

(Si) atom. When silica tetrahedrons share three corner

atoms, they produce layered silicates (talc, kaolinite clay,

mica).

Clay is the basic raw material for many building products

such as brick and tile. When silica tetrahedrons share four

comer atoms, they produce framework silicates (quartz,

tridymite). Quartz is formed when the tetrahedra in thismaterial are arranged in a regular, orderly fashion.

If silica in the molten state is cooled very slowly it

crystallizes at the freezing point. But if molten silica is

cooled more rapidly, the resulting solid is a disorderlyarrangement which is glass.

Silicate Ceramics

Ch. 12



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Cement (Portland cement) is one of the main ingredients of concrete.

There are a number of different grades of cement but a typical

Portland cement will contain :

19 - 25% SiO25 - 9% Al2O3

60 - 64% CaO

2 - 4% FeO.

Cements are prepared by grinding the clays and limestone in proper

proportion, firing in a kiln, and regrinding. When water is added, the

minerals either decompose or combine with water, and a new phase

grows throughout the mass.

The reaction is solution, re-crystallization, and precipitation of a silicate

structure. It is usually important to control the amount of water to prevent

an excess that would not be part of the structure and would weaken it. The

heat of hydration (heat of reaction in the adsorption of water) in setting ofthe cement can be large and can cause damage in large structures

CementCh. 12Structures and Properties of Ceramics



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Nitrides combine the superior hardness of ceramics with high

thermal and mechanical stability, making them suitable for

applications as cutting tools, wear-resistant parts and

structural components at high temperatures. TiN has a cubicstructure which is perhaps the simplest and best known of

structure types. Cations and anions both lie at the nodes of

separate fcc lattices. The structure is unchanged if the Ti and

N atoms (lattices) are interchanged.

Nitride Ceramics

Depending on the crystal structure, in some crystal lattices,

the centers of the positive and negative charges do not

coincide even without the application of external electric field.In this case, it is said that there exists spontaneous

polarization in the crystal. When the polarization of the

dielectric can be altered by an electric field, it is called

ferroelectric. A typical ceramic ferroelectric is barium

titanate, BaTiO3. Ferroelectric materials, especiallypolycrystalline ceramics, are very promising for varieties of

application fields such as piezoelectric/electrostrictive.

Ferroelectric Ceramics

Ch. 12



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Ch. 12Structures and Properties of Ceramics



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CERAMIC DENSITY

= nA

VcNA

For metals

For crystal ceramic material

= n ( Ac + AA )

VcNA

n is number of formula units

within the unit cell, ex: CaF2,

one ion Ca, two ions F.

Ac is the total of the atomicweight of all cations in

the formula unitsAA is the total of the atomic

weight of all anions in

the formula units

Vc is the volume of the unit cel l

NA is Avogadros number

6.023X1023 formula units/mol



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Ex. Pr. 12.3, p 393

Based on crystal structure, write the theoritical density for NaCl. How it can

compared with its measured density?

rCl-

rNa+

a

Cl-

Na+

2rCl- + 2rNa+


= n ( Ac + AA )

VcNA

Ac =ANa

AA =ACl

n Is 4 because both ions form FCC

= 22.99 g/mol

= 35,45 g/mol

Vc = a3

= ( )2rCl- + 2rNa+ 3

From Table 12.3 p. 387, rNa+ = 0.102, and rCl- = 0.181 nm

=

4 (22.99 + 35.45)

2(0.102X10-7) + 2(0.181X10-7)[ ]3 (6.023X1023)= 2.14 g/cm3

Experimental = 2.16 g/cm3


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Silicate Ceramics


These materials contain silicon and oxygen

Si4+

O2-

-

-

-

-

Crystal structure of silicate

Various arrangement of SiO4 4- (tetrahedron)

O

OOOSi

Often the silicates are not ionic because the covalent

character is significant to Si-O bonds (51 form table12.1

Different silicate structures arise from different ways

SO4 Units can be combined into 1, 2 and 3 dimensional arrangements.4-

St t d P ti f C i


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Silica SiO2

Crystal structure: 3-dimentsional

The material is electrically neutral, all atoms have stable

electronic structures Si/O = 1/2

If the structure is organised, a crystalline structure is formed

3 primary polymorphic crystalline forms of silica

1. Quartz, 2. cristobalite, 3. tridymite ( structures are complicated)

They have low density, quartz at room temperature ( 2.65 g/cm3 )

Tm of Si-O is 1710oC

Silica glasses

High degree of atomic randomness (characteristics

of l iquid, called fused sil ica or vitreous silica)

SO4 is the basic unit in the disorder4-

B2O3, GeO2

Ch 12 St t d P ti f C i


19/22


The silicates

SiO4

4-Si

2

O7

6-Si

3

O9

7-

Si6O1812-

Ex: Mg2SiO4

(forsterite)

Ca2MgSi2O7

(Akermanite)

Simple silicates

(SiO3

)n

2n-

Si 4+O

2-

Ch 12 Str ct res and Properties of Ceramics


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Layered sil icates

It is represented by (SiO5)2-

These materials called the sheert or layered silicates

Ex: Clay, Kaolinite clay Al2(Si2O5)(OH)4

Tetrahedral layer is Si2O52-

Al2(OH)4Adjacent layer is2+

Carbon

Diamond

Graphite

Carbon nano-tubes

Ch 12 Structures and Properties of Ceramics


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Imperfections in Ceramics

Atomic point defects

Impurities in Ceramics

Impurity atoms can form SS in ceramic materials as in metals

SS: substitution

Interstitial types are possible

Interstitial

impurity atom

Substitution

impurity atom

Ex: NaCl,

impurity Ca 2+ would likely

substitute for Na+ and O2- ions

would likely substitute for Cl- ions

Ch 12 Structures and Properties of Ceramics


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Mechanical properties

Brittle Fracture of Ceramics

Ceramicscrystalline

Non-crystallineFracture before any plastic deformation

At room temperature

Britt le fracture process

Formation

Propagation of cracks in crosssection of ceramics

The growth of the cracks is through grains

The degree of stress amplification

depends on cracks length and tip

radius of curvature

K lc= Y a

Y

is applied stress

Is a dimensionless

parameters

a is length of surface crack

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