Weldability

Ionic solids

Ionic bonds are non-directional  Each ion is surrounded by as many ions as possible in order to reduce potential energy – close packed structures are formed  But different from metallic structures because the sizes of the ions are different here  Cation assumed to be smaller  C.N. is the number of anions surrounding the cation and depends on the sizes of the ions 

For a stable configuration, the following conditions must be satisfied simultaneously  (i) Anions and cations are considered to be hard spheres and always touch each other  (ii) Anions generally will not touch, but may be close enough to be in contact with one another in a limiting situation  (iii) As many anions as possible surround the cation for the maximum reduction in electrostatic energy 

Radius ratio – C.N.3

a 0 < rc/ra <0.155

b rc/ra = 0.155

c 0.155
Three Cases Case (i) Fig(a) rc/ra < 0.155 – the cation is very small when compared to anion (a), the cation touches only two anions, thus only two neighbors, and it will rattle in the hole first condition is not satisfied and hence unstable – only way to satisfy all the three conditions at this rc/ra value is to reduce the number of anions to 2 Case (ii) Fig.(b) 0.155< rc/ra<0.225

Three Cases When rc/ra = 0.155, limiting condition, critical value because the cation touches all the three anions – all the thre conditions satisfied- maximum stability – Case (iii) Fig.(c) rc/ra > 0.155 All the anions touch the cation but do not touch one another All the three conditions of stability are still satisfied – this condition prevails till the radius ratio increases to 0.225

C.N.4 Tetrahedral coordination rc/ra = 0.225; at this value all the three conditions for stability are satisfied

C.N.5  Does not exist because it does not satisfy all the three conditions for stable configuration  Reason: it is always possible to have six anions as an alternative to any arrangement that contains five anions, without change in the radius ratio

C.N.6  Octahedral coordination  rc/ra = 0.414

Radius ratio - table Ligancy

Radius ratio

2

0.0 – 0.155

3

0.155–0.225

B2O3

Triangular

4

0.225-0.414

ZnS

Tetrahedral

6

0.414-0.732

NaCl

Octahedral

8

0.732-1.0

CsCl

Cubic

12

1.0

Example

Shape Linear

FCC or HCP

Examples NaCl: rNa+/rCl- = 0.98/1.81 = 0.54 Lies between 0.414 and 0.732, ∴C.N.=6 MgO: rMg2+/rO2- = 0.78/1.32 = 0.59 Lies between 0.414 and 0.732, ∴C.N.=6 CsCl:rCs+/rCl- = 1.65/1.81 = 0.91 Lies between 0.732 and 1.0, ∴C.N.=8 CaF2: rCa2+/rF- = 0.94/1.33 =0.73 –border Difficult to predict whether C.N = 6 or 8 Obsd. C.N. =8; each Ca2+ is surrounded by 8 F-

Radius ratio and hybridization 



When a structure satisfies the radius ratio rules & hybridization, structure expected by radius ratio rules will result Ex: silica, SiO2 rSi4+/rO2- = 0.29, lies in the range, 0.225-0.414, C.N. = 4; SiO bonds in silicates are 50% ionic and 50% covalent; tetrahedral coordination is predicted both from radius ratio rules and sp3 hybridization

Contd. • If directional characteristics of bonding happens to be more significant, then radius ratio rules alone will not lead to the correct ligancy (C.N.) • Ex: ZnS: Bond is more covalent than ionic • rZn2+/rS2- = 0.83/1.74 = 0.48 • Suggests octahedral coordination, but tetrahedral coordination is oberved

Formation of a crystal Formation of a crystal Local packing around a central atom Long range arrangement of ions in the crystal

Contd. 





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Local packing: depends up on the ligancy rules Local arrangement: depends on the following factors 1. The overall electrical neutrality should be maintained, whatever be the net charge on a local group of cation and surrounding ions Ex: NaCl; each Na+ is surrounded by six Cl-. The net charge on NaCl is 5. This has to be neutralized in the long range arrangement 2. ionic bond – non-directional. Therefore, ions are packed as closely as possible in the crystal, consistent with the local

Contd. • 3. If small cations with a large net charge are present in the crystal, the cationcation repulsion will be high – therefore, long range arrangement must maximize the cation-cation distance, even if close packing is not possible – such a situation arises when the charge on the cation increases to three or four. • Voids filling:The fraction of octahedral voids that are filled depends on the number of cations to anions in the chemical formula

Cation: Anion

Crystal

rc/ra

Anion Packing

1:1

NaCl

0.54

FCC

Fraction O-voids With cations All

1:1

MgO

0.59

FCC

All

1:2

CdCl2

0.57

FCC

Half

2:3

Al2O3

0.43

HCP

Twothirds

Alumina,Al2O3 ► Cation

positions in the neighboring planes of octahedral voids are staggered such that the mutual repulsion of the trivalent cations is minimized ► Due to multivalent ions, the bond strength in Al2O3 is high – hence a hard crystal is produced with a high m.p. – therefore, v.good electrical insulating properties

Alumina,Al2O3(Contd.) ► Use:

in building integral circuits and in spark plugs of automobiles ► Al3+ can be replaced by ions of similar size –substitutional solid solution – ► Replacement of a small amount of Al3+ by Cr3+ gives ruby – by Fe3+ gives blue sapphire

Silica & silicates ► Silica

tetrahedron – basic repeating unit is SiO44Si Oxygen

► Silica,

SiO2 cannot form a close packed layer of oxygen anions with cations in the tetrahedral voids

Silica & silicates (Contd.) ► Such

a structure brings neighboring silicon cations so close to one another that it results in an appreciable increase in potential energy due to repulsion between them ► Forms a three dimensional network of tetrhedra, each one of which shares all its four corners with other tetrhedra ► Oxygen anions at the corners are common to two tetrahedra

Silica & silicates (Contd.) ► Effective

number of silicon cation per tetrahedral unit is 1 ► Effective number of oxygen per unit is 4x½ = 2 ► Each corner oxygen is shared by two tetrahedra – thus electrical neutrality of the network is maintained

Quartz crystal ► The

tetrahedra are arranged in a periodically repeating manner

Quartz (Contd.) Used in optical components  Piezoelectric – mechanical stress applied to the crystal displaces the ions in the crystal and induces electric polarization  Electric field will cause the crystal to be electrically strained  Used in watches and clocks and for accurate frequency control in electronic circuits 

Silica glass Non-crystalline: tetrahedra are randomly bonded to other tetrahedra

Silica glass (Contd.)  Fused silica glass is used in applications

requiring low thermal expansion  Highly viscous even in the molten state because of the Si-O bond between the tetrahedra

Soda lime glass  Addition of Na2O introduces weaker bonds in

the in the frame work  In three dimensional network of silica, other oxides can be dissolved to yield a number of both crystalline and non-crystalline silicates  Soda lime glass is a non-crystalline silicate with Na2O and CaO added to silica. The alkali cations break up the network of the silicate terahedra as shown in the figure

Na+

+ Na

+

Na+

Na

Contd. • For each Na2O introduced, one Si-O bridge is disrupted and the extra oxygen atom from Na2O splits up one common corner into separate corners • The two sodium ions stay close to the disrupted corner due to the electrostatic attraction • The network at the corner is bonded through the Na-O bonds. The Na-O bond being weaker than Si-O bond, the viscosity of the glass is drastically reduced as a result of the alkali addition

Classification of minerals No. of oxygens Shared 0

Structural Structural Charge Unit Formula Balance (SiO4)4Island ortho

1

(Si2O7)6pyro

Si +4 4O -8 Net -4 2Si +8 7O -14 Net -6

Ex. Of minerals Olivine (Mg,Fe)2SiO4

Hemimorh ite Zn4Si2o7(OH) 2H2O

2

(SiO3)2Single chain

Ring (SiO3)22½

Double Chain (Si4O11) Sheet (Si2O5)2-

Si +4 3O -6 Net -2 Si +4 3O -6 Net -2 4Si +16 11O -22 Net -6

6-

3

2Si +8 5O -10 Net -2

Enstatite MgSiO3 Beryl Be3Al2(SiO3)6

Tremolite Asbestos Ca2Mg5(OH)2 (Si4O11)2 Muscovite (Mica) KAl2(OH)2(Si3 Al)O10

4

Three (SiO2)0 Dimensio nal

Si +4 2O -4 Net 0

Quartz

Engineering requirements of materials The most important properties expected in diff. engg. Materials are: Mechanical – strength, stiffness, ductility, elasticity, plasticity, toughness, brittleness, hardness, malleability Electrical – conductivity, resistivity, dielectric permitivity, dielectric strength Magnetic – permeability, coercive force, hysteresis Thermal – specific heat, thermal expansion, conductivity Chemical – corrosion resistance, acidity or alkalinity, composition Physical – dimensions, density, porosity, structure Acoustical – sound transmission, sound reflection Optical – color, light transmission, light reflection