Ceramics: An Introduction  Comes from the Greek word-keramikos (burnt stuff) which indicates their desirable properties are achieved through high temperature heat treatment  Compounds between metallic & nonmetallic elements i.e. oxides, nitrides & carbides  Can be classified into clay minerals, cement & glass  Typically insulative to the passage of electricity & heat, & more resistant to high temperatures & harsh environment.  They are hard but very brittle

Classification of Ceramics

Ceramic Bonding Mostly ionic, some covalent. % ionic character increases with difference in electronegativity

Large vs small ionic bond character: Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

Two characteristics of the component ions in crystalline ceramic materials which influence the crystal structure:  The magnitude of the electrical charge on each of the component ions: The crystal must be balanced by an equal number of anion –ve charges  The relative sizes of cations & anions This involves the sizes or ionic radii, r C & r A respectively The ratio of r C /r A is less than unity due to cation size that is small. This is caused by the metallic elements give up electrons when ionized

Stable ceramic crystal structures form when those anions surrounding a cation are all in contact with the cation The coordination no. is related to r C /r A radius ratio For a specific coordination no., there is a critical or min r C /r A ratio for which this cation-anion contact is established This ratio maybe determined from pure geometrical considerations The coordination numbers and nearest neighbor for various r C /r A ratios are presented in the next table.

Example Problem 12.1 Show that the minimum cation-to-anion radius ratio for the coordination number 3 is Solution: The small cation is surrounded by 3 anions to form equilateral triangle.The centers of all four ions are coplanar AP = r A & AO = r A + r C Note: the side length ratio AP/AO = cos α The magnitude of α is 30 o, since line AO bisects the 60 o angle BAC: AP/AO =r A /r A +r C =30 o =√3/2 The cation-anion radius ratio; r A /r C =(1- √3/2) / √3/2 =0.155

AX-TYPE CRYSTAL STRUCTURES AX-TYPE CRYSTAL STRUCTURES  Some ceramic materials have equal number of cations & anions  These are referred as AX compounds: A-cation & X-anion Rock Salt Structure A common example for AX crystal structure. Coordination no. is 6 r C /r A ratio between & A unit cell is generated from an FCC (Face Centered Cubic) arrangement of anions with one cation situated at the cube center & one at the center of each of 12 cube edges An equivalent crystal structure results from a face-centered arrangement of cations

 The rock salt crystal is thought of a interpenetrating FCC lattices.  One composed of the cations, the other of anions  NaCl, MgO, MnS, LiF, FeO Cesium Chloride (CsCl) Structure Coordination no. is 8 for both ion types The anions are located at each of the corners of a cube The cube center is a single cation Interchange of anions with cations,vice versa, produce same structure

Zinc Blende (ZnS) StructureZinc Blende (ZnS) Structure Coordination no. is 4, all ions are tetrahedrally coordinated All corner and face positions of the cubic cell are occupied by S atoms The Zn atoms fill interior tetrahedral positions An equivalent structure results if Zn and S atom positions are reversed Most often the atomic bonding is highly covalent in compounds exhibiting this crystal structure

A m X p TYPE CRYSTAL STRUCTURE A m X p TYPE CRYSTAL STRUCTURE Charges of cations & anions are not the same, the compound can exist with chemical formula A m X p, m and/or p ≠ 1. Example:AX 2, a common crystal structure found in CaF 2 r C /r A is about 0.8 & coordination no. is 8 Ca 2+ ions are positioned at the centers of cubes with F - ions at the corners Half as many Ca2+ ions as F- ions Only half the center cube positions are occupied by Ca2+ ions *Note! One unit cell consists of eight cubes as in the figure!

A m B n X p -TYPE CRYSTAL STRUCTURE A m B n X p -TYPE CRYSTAL STRUCTURE  It is possible for ceramic compounds to have more than one type of cation as their chemical formula can be designated as A m B n X p  Example: Barium Titanate (BaTiO 3 ), which have both Ba 2+ & Ti 4+ cations  Ba2+ ions are situated at all 8 corners of the cube & a single Ti4+ is at the cube center, with O2- ions located at the center of each of the 6 faces.  Another name for this structure is perovskite crystal structure

Crystal Structures From the Close Packing of Anions A number of ceramic crystal structures maybe considered in terms of closed-packed planes of ions, as well as unit cells Closed packed planes are composed of large anions These planes are stacked atop each other, small interstitial sites are created between them, cations may reside between them

4 atoms (3 in 1 plane,& a single one in the adjacent plane) surround one type, labeled T- tetrahedral position 6 join spheres, 3 in each of 2 planes, denoted as 0 Because an octahedron is produced by joining these 6 sphere centers-octahedral position Coordination numbers for cations filling tetrahedral & octahedral are 4 & 6 respectively

Ceramic crystal structure depends on 2 factors:  The stacking of the close-packed anion layers (both FCC & HCP arrangements are possible)  The manner in which the interstitial sites are filled with cation Example: Example: The unit cell has cubic symmetry & each cation (Na + ) at the center has 6 Cl - ion nearest neighbor that reside at the centers of each of the cube faces The crystal structure having cubic symmetry is considered in an FCC array of close- packed planes of anions & all planes are {111} type The cations reside in octahedral positions because they have as nearest neighbors six anions All octahedral positions are filled, since there is a single octahedral site per anion and the ratio of anions to cations is 1:1

Question: On the basic of ionic radii, what crystal structure would you predict for FeO? Solution: FeO is an AX-type compound. Determine cation-anion radius ratio (refer to table 3.4), r Fe2+ /r O2- r Fe2+ /r O2- = 0.077nm/0.140 nm = 0.077nm/0.140 nm =0.550 =0.550 The coordination no. for Fe2+ ion is 6; also the coordination no. for O2- The predicted crystal structure will be rock salt, which is AX crsytal structure having a coordination no. as 6.

Density Computations- Ceramics Density Computations- Ceramics  This is the alternative way to compute the theoretical density of a crystalline ceramic material.  The density, ρ is determined as follows: ρ= n’ (ΣA C + ΣA A ) V C N A V C N A n’ = the number of formula units within the unit cell ΣA C = the sum of atomic weights of all cations in the formula unit ΣA A = the sum of atomic weights of all anions in the formula unit V C = the unit cell volume N A = Avogadro no., 6.023× formula units/mol

Question: On the basis of crystal structure, compute the theoretical density for sodium chloride. How does this compare with its measured density? Question: On the basis of crystal structure, compute the theoretical density for sodium chloride. How does this compare with its measured density? Solution: The theoretical density can be determined using: Solution: The theoretical density can be determined using: ρ= n’ (ΣA C + ΣA A ) ρ= n’ (ΣA C + ΣA A ) V C N A V C N AWhere n ’, the no. of NaCl units per unit cell = 4, (both sodium & chloride ions form FCC lattices) ΣA C =A Na = g/mol ΣA A =A Cl = g/mol V C = a 3, & a=2r Na+ + 2r Cl-, r Na+ =0.102 nm & r Cl- =0.181 nm

Thus, V c = a 3 = (2r Na+ + 2r Cl- ) 3, Finally, ρ= n’ (A Na + A Cl ) (2r Na+ + 2r Cl- ) 3 Na (2r Na+ + 2r Cl- ) 3 Na =4( ) [2(0.102×10 -7 ) + 2(0.181×10 -7 )] 3 (6.023×10 23 ) = 2.14 g/cm 3

Silicate Ceramics: Introduction  Composed primarily of silicon & oxygen  It is more convenient to characterized these materials in terms of various arrangement of SiO 4 4-  4 oxygen atoms at tetrahedron corners, a silicon atom at the center. Usually treated as a –ve charged entity  Si-O bonds are covalently bond which are directional and relatively strong. A silicon-oxygen tetrahedron

Silica/ Silicon Dioxide (SiO 2 )  The most simple silicate material  A three dimensional network. Generated when every corner O atom is shared by adjacent tetrahedra.  Electrically neutrally & all atoms have stable electronic structures  Three primary polymorphic crystalline forms: quartz, cristobalite & tridymite  Have a relatively complicated structure & the atoms are not closely packed together  This results in relatively low densities  The melting point is high:1710 o due to strong Si-O interatomic bond

Silica Glasses Fused/vitreous silica-a noncrystalline solid/glass, high degree of atomic randomness (character of liquid) SiO 4 4- tetrahedron is the basic unit as with crystalline silica. Beyond this structure, considerable disorder exists The common inorganic glasses that are used for containers, windows are silica glasses. Other oxides i.e. CaO & Na 2 O These oxides don't form polyhedral networks Their cations are incorporated within & modify SiO 4 4- network; these oxide additives- network modifiers Intermediates i.e. oxides like TiO 2 & Al 2 O 3 are not network former, substitute for Si & become part & stabilize the network Addition of modifiers & intermediates lowers melting point & viscosity of glass

The Silicates  For various silicate minerals, the corner oxygen atoms of the SiO 4 4- tetrahedra are shared by other tetrahedra to form complex structures (some represented below):  Positively charged cations i.e. Ca 2+, Mg 2+ & Al 3+ serve to neutralize –ve charges from SiO 4 4- units & bonding the SiO 4 4- tetrahedra together

 Simple Silicates Include the most structurally simple ones involve isolated tetrahedra For ex.; forsterite (Mg 2 SiO 4 ) has the equivalent of two Mg 2+ ions associated with each tetrahedron in such a way that every Mg 2+ ion has 6 oxygen nearest neighbor Si 2 O 7 6- ion is formed when two tetrahedra share a common oxygen atom Akermanite (Ca 2 MgSi 2 O 7 ) is a mineral having the equivalent of two Ca 2+ ions & one Mg 2+ ion bonded to each Si 2 O 7 6-

Layered Silicates  A 2D sheet or layered structure can be produced by sharing 3 oxygen ions in each tetrahedra  The repeating unit formula represented by (Si 2 O 5 ) 2-  The net negative charge is associated with the unbonded oxygen atoms projecting out of the plane of the page  Electroneutrality is ordinarily established by a 2 nd planar sheet structure having an excess of cations, which bond to these unbonded oxygen atoms from the Si 2 O 5 sheet  Such materials are called the sheet or layered silicates & their basic structure is characteristic of the clays & other minerals.

Kaolinite {Al 2 (Si 2 O 5 )(OH) 4 }as a relatively simple 2 layer silicate sheet structure. The silica tetrahedral layer represented by (Si 2 O 5 ) 2- is made neutral by Al2 (OH) 4 2+ The bonding within this 2 layered sheet is strong & intermediate ionic-covalent. Adjacent sheets are loosely bound by weak Van der Waals forces

Carbon  Diamond Metastable carbon polymorph at room temperature & atmospheric pressure Its crystal structure is a variant of zinc blende,carbon atoms occupy all positions Each carbon bonds to 4 other carbons. The bond is totally covalent. This crystal structure-diamond cubic crystal structure

Physical properties of diamond: hardest known material, very low electrical conductivity attributed to its crystal structure & strong interatomic covalent bonds Other properties: high thermal conductivity, optically transparent in the visible & infrared light, high index of refraction Applications: gem stones, grinding/cutting softer materials in industry (mostly man-made) Latest: diamond thin films has been produced

Graphite is another polymorph carbon; it has different crystal structure than diamond. More stable than diamond at ambient temperature & pressure The structure is composed of hexagonal layers arranged carbon atoms Within the layers, each carbon atom is bonded to 3 coplanar neighbor atoms by strong covalent bonds The fourth bonding electron participates in a weak van der Waals type of bond between layers Interplanar cleavage is facile, which gives rise to the excellent lubricative properties of graphite The electrical conductivity is relatively high in crystallographic directions parallel the hexagonal sheets

Properties of graphite:  High strength & good chemical stability at elevated temperatures & non oxidizing atmospheres  High thermal conductivity, low coefficient of thermal expansion & high resistance to thermal shock  High adsorption of gases & good machinability  Application: Heating element for electric furnace Electrodes for arc welding Casting molds for metal alloys and ceramics High temperature refractories and insulations Brushes, resistors

Fullerenes & Carbon Nanotubes Exist in discrete molecular form & consists of a hollow spherical cluster of 60 carbon atoms (C 60 ) Each molecule is composed of carbon atoms that are bonded to one another to form both hexagon and pentagon geometrical configuration The molecular surface exhibits symmetry of a soccer ball

Carbon atoms in C 60 (buckminsterfullerene) bond together to form spherical molecules In solid state, C 60 unit form crystalline structure & packed together in a face centered cubic array As a pure crystalline solid, the material is electrically insulating Can be highly conductive and semi conductive if impurity is added

Carbon Nanotubes Its structure consists of a single sheet of graphite rolled into a tube, both ends are capped with fullerene hemisphere The tube diameters can be 100 nm or less Each nanotube is a single molecule composed of millions of atoms; the length of the molecule might be thousand times greater than its diameter Carbon nanotubes are extremely strong, stiff, relatively ductile, & have low densities. It may behave electrically as metal or semiconductor

IMPERFECTIONS IN CERAMICS IMPERFECTIONS IN CERAMICS  Atomic Point Defects defect structure The expression defect structure is used to designate types & concentrations of atomic defects in ceramics Electroneutrality Electroneutrality is the state that exists when there are equal no. of +ve & -ve charges from ions Frenkel defect Frenkel defect a defect which involves cation-vacancy and cation-interstitial pair. This is formed by a cation leaving its normal position & moving into an interstitial site. There is no change in charge because the cation maintains the same positive charge as an interstitial.

Schottky defect Schottky defect found in AX materials is a cation vacancy-anion vacancy pair This defect might be created by removing one cation & one anion from the interior of the crystal & then placing them both at an external surface Since both cations & anions have the same charge, & since anion vacancy there exists a cation vacancy, the charge neutrality o the crystal is maintained. If no defects are present, the material is said to be stoichiometric. Stoichiometry Stoichiometry- a state for ionic compounds where is the exact ratio of cations to anions as predicted by chemical formula. Non stoichiometry Non stoichiometry exists in which two valence or ionic states exist for one of the ion types.

Impurities in Ceramics Impurities in Ceramics  Impurity atoms can form solid solutions in ceramic materials from both substitutional &interstitial types

Ceramics Application in Biomedical Engineering Maybe divided into 3 classes according to their chemical reactivity with the environment  Nearly inert (alumina, carbons)  Surface reactive (bioglass)  Completely resorbable (hydroxyapatite) Nearly inert ceramic show little chemical reactivity of long hours of exposure to physiological pH & show minimal interfacial bonds with living tissue. Fibrous capsule adjacent to implant is few cells thick Surface reactive ceramic show intermediate behavior which bond the soft tissue and cell membrane, producing tissue adherence Reactive material release ions from the surface & provide protein bond site over some time

Carbons  Carbon coating-applications in heart valves, blood vessel grafts, knee prosthesis Knee prosthesis featuring diamond-like carbon coating From- ?ArticleID=2568 Two glassy polymeric carbon ( GPC ) heart valves. From- /Activities/df.html

Alumina  Applications-hip prostheses & dental implants Dental implants From- /tech/t_acc06.html Alumina acetabular cup From

Surface Reactive Ceramics- Bioglass  Usually used as coatings on implant SEM image of Bioglass 45S5 after incubation in SBF. Brunner al, 2006

Resorbable Ceramics  Resorbable biomaterials commonly used are hydroxyapatite & β-tricalcium phosphate  Artificial bone & dental implants Rootform implant From- ce.co.uk/implants/HAScr ew.jpg/ Blade implant From- implants/ MandBlade.jpg