Lecture 1 cont.. Atomic Bonding & Material Properties Bonding Forces and Energies Consider two isolated atoms separated by inter-atomic dist r r At large r, atoms do not interact. As r gets smaller, an attractive force F A starts to act pulling atoms closer. As R 0, a repulsive force F R begin to act preventing atoms from getting too close.. Page 1
Resultant force is F F F N A R Force (F) Repulsive force At r = r o, F R = F A and F N = 0 O r o Resultant force r Attractive force r o is the equilibrium inter-atomic separation dist (r o 0.3 nm) at which atoms enter into bonding. 3 F R gives rise to a +ve Potential Energy (V Rep ) while F A gives rise to a ve P.E (V Att ) where V Rep r F dr R r Z Z e 1 2 4 o r 2 2 dr and V Att r F dr A V Rep 1 m r B m r and V Att 1 n r A n r 2 Z Z e 1 2 A o Where 4, Z 1 & Z 2 are the Atomic Numbers. o o e = 1.6 x 10-19 C, o = 8.85 x 10-12 F/m o A, B, and n are constants. n 8. Page 2
The net potential V N V Att V Re p A n R B R m The NET Force F N dv dr na r n 1 mb m 1 r Fig shows variation of V N and F N with r called the Condon-Morse curves NB. At r = r o, V N = E 0 = Bonding energy E 0 = (Potential Energy Well) or min energy required to separate two atoms to an infinite separation. 5 V N Repulsive 0 r 0 r Potential Energy well E o Attractive r R r > r 0 ; V N increases gradually to 0 as R. The force is attractive r < r 0 ; V N increases rapidly to at small separation. The force is repulsive Page 3
Force vs. Separation Distance Energy vs. Separation Distance 7 Bonding Energies & Material properties Material properties depend on Depth of Energy well, E 0 Shape of the P.E well Type of bonding The deeper the well, the higher the bonding energy E 0, and the stronger the bonding High MP and material exists as solid Shallow well Low MP and material is gaseous e.g., H 2 8 Page 4
Energy r o smaller MP r larger MP MP is larger if E o is larger. 9 (a) Mechanical properties Elastic Modulus (E) = measure of resistance to separation of atoms i.e., inter-atomic bonding forces E Stress Strain df slope dr of Force vs dist curve at r o 10 Page 5
df the steeper the slope of dr, the deeper the well, higher the E stronger material Smaller E (Weaker material Large E (Stronger material) 11 (b) Thermal properties Linear thermal expansion coefficient ( ) L T T L o The trough at E o corresponds to equilibrium inter-atomic spacing at OK. 2 1 When a material is heated from T 1 to T 5, vibrational energy increases thereby increasing the width of the curve. 12 Page 6
length, L o unheated, T 1 heated, T 2 L Energy E o E o r o larger smaller r is larger if E o is smaller. ~ symmetric at r o 13 Curve is asymmetric Page 7
When E o is small (shallow well), and the curvature is very assymmetric, then, the interatomic spacing increase with temp rise indicating high. is small when E o is large & the well is deep and narrow is due to the asymmetric curvature of the P.E trough, rather than the increased atomic vibration amplitudes with rising temp. 15 If P.E curve were symmetric, there would be no net change in inter-atomic separation with rise in temp and consequently, no thermal expansion Metals >> Ceramics >> Polymers Because in metals, the vibrational transfer is through atoms and in ceramics it is through atoms and in polymers, it is due to the rotation and vibration of long chain molecules. 16 Page 8
Activity Question 1: a) Explain the thermal expansion of a material on the basis of the P.E -interatomic distance curve. b) On the same plot sketch the P.E-distance curve of a material with i) higher thermal expansion. Give example ii) lower thermal expansion. Give example c) How can the Young s modulus be determined from the energy-distance curve? 17 Activity -2 Question 2: Why do ceramics exhibit much lower strength than their theoretically expected strength of E/10? 18 Page 9
Atomic Bonding When atoms combine they form compounds that are unique both chemically & physically from its parent atoms. E.g., Na is a metal that reacts violently with water while Cl is a very poisonous greenishcolored gas BUT Na + Cl = Salt + = Bonding between the atoms is due to electrostatic interaction between nuclei and electrons. Atoms enter into bonding to achieve atomic stability determined by Hund s rule which favours closed electron shelf or half-shells in the atom. Type of bonding is influenced by the atom s position in the periodic table 20 Page 10
inert gases 7 horizontal rows are called periods. Elements Periodic in a Table given column or group have similar valence electron structures, as well as chemical and physical properties. give up 1e - give up 2e - H Li Be Na Mg give up 3e - accept 2e - accept 1e - O F S Cl He Ne Ar K Ca Sc Se Br Kr Rb Sr Y Te I Xe Cs Ba Po At Rn Fr Ra 22 Page 11
inert gases The Periodic Table Columns: Similar Valence Structure give up 1e - give up 2e - H Li Be Na Mg give up 3e - K Ca Sc Rb Sr Cs Ba Fr Ra Y accept 2e - accept 1e - O Se Br Kr Te He F Ne S Cl Ar I Xe Po At Rn Adapted from Fig. 2.6, Callister & Rethwisch 8e. Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. 23 Electronegativity Ranges from 0.7 to 4.0, Large values: tendency to acquire electron Smaller electronegativity Larger electronegativity Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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. 24 Page 12
Types of Atomic & Molecular Bonds Primary Atomic Bonds Ionic Bonds Covalent Metallic Secondary Atomic & Molecular Bonds Permanent Dipole (Van der Waals) bonds Fluctuating Dipole Bonds (a) Ionic Bonding Occurs between atoms lying at the two extreme ends of the periodic table. Atoms tend to lose or gain valency electrons to achieve complete outer shells thereby forming ions +ve ions (cations) or -ve ions (anions) Ionic Bonding results from the electrostatic attractions between +ve and ve ions Predominant bonding in Ceramics 26 Page 13
Examples: Ionic Bonding NaCl MgO CaF 2 CsCl Give up electrons Acquire electrons Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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. 27 Ionic Bonding in NaCl Page 14
Properties of ionic bonding Nondirectional - has same strength in all directions ST cations sorround themselves with as many anions as possible forming a giant molecule Low electrical & thermal conductivity No free electrons. Entire ion must move to conduct electricity Transparent Hard and Brittle - because the ions are bound strongly to the lattice and aren't easily displaced. High MP and BP - large amt of thermal energy is required to separate the ions which are bound by strong electrical forces. 29 (b) Covalent Bonding Takes place between atoms with small differences in electronegativity which are close to each other in periodic table (i.e., between non-metals and non-metals lying in the central column of the periodic table ). Bonding results from sharing of outer s and p electrons so that each atom attains the noble-gas electron configuration. 30 Page 15
column IVA H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 H2 Ti 1.5 Cr 1.6 Fe 1.8 H2O C(diamond) SiC Ni 1.8 Zn 1.8 Ga 1.6 C 2.5 Si 1.8 Ge 1.8 Sn 1.8 Pb 1.8 As 2.0 GaAs O 2.0 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 He - Ne - Ar - Kr - Xe - Rn - F2 Cl2 Number of ē -pair bonds that an atom can form is determined by the 8-N rule where N = No of the column in the periodic table containing the atom. Thus, F can form 1 bond, O can form 2 bonds etc 31 Properties of Covalent bonding Directional strength of bond not equal in all directions Low electrical & thermal conductivity Since electrons cannot move through the lattice. Very strong (diamond) or very weak (bismuth). High MP and BP -because each atom is bound by strong covalent bonds. E.g., Diamond, silicon, CH 4, H 2 O, HNO 3, H 2, Cl 2, F 2, etc., 32 Page 16
(c ) Metallic Bonding In metals, all valence electrons in a metal combine to form a sea of electrons that move freely between the atom cores. A metallic bond results from the electrostatic force of attraction between +ve ions and delocalized outer electrons. The free electrons act as the bond (or as a glue ) between the +ve ions. As a result we have a high ductility (plastic deformation) of metals - the bonds do not break when atoms are rearranged. The more electrons, the stronger the attraction. High MP and BP and the metal is stronger and harder. 34 Page 17
Properties of Metallic bonding Non-directional bond High Thermal & electrical conductivity Due to free electrons Ductile, opaque The metallic bond is weaker than the ionic and the covalent bonds. E.g., Na, Cu, Al, Au, Ag, etc. 35 NB. Transition metals (Fe, Ni, etc.) form mixed bonds, comprising of metallic and covalent bonds in-volving their 3d-electrons. As a result the transition met-als are more brittle (less ductile) than Au or Cu 36 Page 18
increases Bond type Example Bond Energy Optical Property Electrical Property Thermal Property Mechanical Property Ionic NaCl, ZnS Transparent Semiconductor High MP Hard & Brittle Covalent Diamond, Graphite Transparent & Coloured Insulators V. High MP & BP V. Hard Metallic Na, Fe, Cu, Ag Opaque & Reflecting Conductors Good heat conductors Tough & Ductile Molecular ( Van der Waals) Ne, Ar, Xe, Phenol, Transparent Insulators Low MP Soft and brittle Hydrogen Bonding Ice, Organic solids, H2, CH4 Transparent Insulators Low MP Soft and brittle 37 Activity Explain the general properties of ionic, covalent and metallic bonding giving examples in each case 38 Page 19
Secondary Bonds (Van der Waal) They are physical bonds involving no electron movement Secondary bonds are as a result of the interaction of the electric dipoles contained in atoms or molecules A dipole exists in a molecule if there is asymmetry in its electron density distribution due to large difference in electronegativities between atoms, S.T. there is some separation of positive and negative portions of an atom or molecule. Special case: Hydrogen bonding. 39 Can be divided by: (1) Fluctuating Dipoles (2) Permanent Dipoles Fluctuating dipoles are due asymmetrical electron charge distribution within the atoms that changes in both direction and magnitude with time. symmetric asymmetric 40 Page 20
E.g Electron charge cloud distribution in a noble-gas atom Idealized symmetrical electron charge cloud distribution Real case with asymmetrical electron charge cloud distribution that changes with time, creating a Fluctuating electric dipoles Permanent Dipoles Polar Molecules have Permanent dipole and can induce dipoles in adjacent non-polar molecules and bonding can take place between the permanent and induced dipoles. E.g. Hydrogen bonding Page 21
Examples of Hydrogen Bonding: o HF, o HCl o H 2 O, o Polymers 43 In hydrogen bonding, the H end of the molecule is positively charged and can bond to the negative side of another H 2 O molecule (the O side of the H 2 O dipole) Hydrogen bond secondary bond formed between two permanent dipoles in adjacent water molecules. 44 Page 22
The bigger a molecule is, the easier it is to polarise (to form a dipole), and so the van der Waal's forces get stronger, so bigger molecules exist as liquids or solids rather than gases. Physical Bonds (no electron involvement). 45 The ability of geckos to hang on vertical or upside down on flat surface has been attributed to the van der Waals forces between these surfaces and the spatulae on their toes. 46 Page 23
Questions How can the high electrical and thermal conductivities of metals be explained by the electron gas model of metallic bonding? Ductility? SOLUTION The high electrical and thermal conductivities of metals are explained by the mobility of their outer valence electrons in the presence of an electrical potential or thermal gradient. The ductility of metals is explained by the bonding electron gas which enables atoms to pass over each other during deformation, without severing their bonds. 48 Page 24
Summary A deep and narrow trough in the curve indicates large bond energy, high MP, large elastic modulus and small 49 Lecture -Evaluation 1. Explain ionic, covalent and metallic bonding 2. Explain secondary bonding and differentiate between permanent and fluctuating induced dipole bonds giving examples of each FWN_UoN 50 Page 25
General Properties of Materials Metals Composed of one or more metallic elements e.g., Iron, Copper, Aluminum. Have crystalline structure with metallic bonding Valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Metals and Alloys Ferrous Eg: Steel, Cast Iron Nonferrous Eg:Copper Aluminum FWN_UoN 52 Page 26
General Properties Strong in Tension & ductile with high fracture toughness Good conductors of electricity & heat Reflective (Shinny if polished) and Opaque to light FWN_UoN 53 Ceramics Properties & applications Classification Page 27
Ceramics means burnt stuff properties achieved through high-temperature heat treatment (firing). Ceramics are inorganic, non-metallic materials i.e., a combinations of metals or semiconductors with oxygen, nitrogen or carbon (e.g., Al 2 O 3, NaCl, SiC, SiO 2 ) Typically produced using clays and other minerals or chemically processed powders FWN_UoN 55 Bonding and structure bonds are mixture of ionic & covalent i.e., atoms behave like +ve or ve ions, and are bound by Coulomb forces. Type of bonding results in either crystalline (with atoms arranged in regular repetitive pattern) or amorphous (non-crystalline) e.g., glass FWN_UoN 56 Page 28
crystalline SEM of ceramic showing mullite crystals Amorphous 57 Diversity in properties ( Mechanical, Optical, Thermal, Electrical and Magnetic properties) stems from type internal structure and bonding Material properties are influenced by microstructural features viz: grain size Porosity & secondary phases grain boundaries Imperfections such as micro-cracks, defects FWN_UoN 58 Page 29
Flexural Strength (MPa) e.g. Elastic modulus of ceramics decreases with increase in Porosity 59 25 20 15 10 5 0 5 10 15 20 Volume Porosity (%) Depence of Flexural strength (MOR) on porosity 60 Page 30
General Properties Brittle with low fracture Toughness Extreme hardness & wear resistant - Everlasting!!! Corrosion resistant Heat resistance Low Thermal Conductivity Low Electrical Conductivity Wide range of applications High heat capacity (high MP upto 1,600 C ) FWN_UoN 61 Applications Thermal insulator Abrasives Construction materials Cookery Examples - Porcelain, Glass, Silicon nitride. Insulation in brick walls Thermal insulators 62 Page 31
Classification of Ceramics Classified according to major functions i.e. Bonded Clay ( Traditional ) ceramics & Advanced ceramics FWN_UoN 63 Classification of Ceramics (a) Bonded Traditional ceramics Are Clay-based porous ceramics They include These include: (a) Structural Clay Products pottery, porcelain, tiles & Whitewares (Wall tiles, Electrical porcelain & Decorative ceramics) Bricks FWN_UoN 64 Page 32
(b) Refractory Ceramics High temp applications (d) Cement, glass FWN_UoN 65 Advanced Ceramics Exhibits superior mechanical, electrical, optical, properties and corrosion or oxidation resistance. Classified according to: Oxides: alumina, zirconia, Have low thermal conductivity & Used as thermal barriers to protect metals surfaces from wearing out Non-oxide ceramics: carbides and nitrides -SiC, Si3N4 etc. Extremly hard & used as polishing tools Composites: reinforced materials for high toughness e.g., bioceramics FWN_UoN 66 Page 33
SiC polishing tools zirconia Ceramic Matrix Composite (CMC) rotor FWN_UoN 67 Bioceramic implants Silicon carbide is used for inner plates of ballistic vests FWN_UoN 68 Page 34
Lecture -Evaluation 1. Explain bonding and structure in ceramics 2. Explain general properties of ceramics & their applications FWN_UoN 69 Page 35