SPH 203: STRUCTURE AND PROPERTIES OF MATTER

Course Instructor:

Dr Justus Simiyu

ATOMIC AND MOLECULAR BONDING

Bond Strength

measure of the energy required to break a bond, that is, the amount of energy required to

vapourize the solid and hence separate the constituent atoms. From the measure of heat

required to vapourize 1 Kg mole

Bond Strength and Melting Point.Material Atomic Number Melting Point

(C)Carbon(diamond)

6 3,750

Silicon 14 1,421

Germanium 32 937

Tin 50 232

Thus melting point occurs when the vibration becomes so great

that the bonds are broken and the atoms become mobile.

PHASES OF MATTER

GasesIdeal Gase (Assumptions)• behave as small elastic spheres• The molecules

•are all alike-hence the homogeneity of a gas.•are in constant motion-hence the diffusion.•exert no force on each other except when in actual contact.

• volume occupied by the molecules is small compared to the total volume

• exerted Pressure is due to collisions of the molecules with the walls of the container.

Mean Free Path

• average distance between successive

collisions of molecules u

vw

u

u

-u

v

w

-w

w

u

c

c

c

Gas Mean free path (m)

Hydrogen 16.3x10-8

Nitrogen 8.5

Oxygen 9.6

important for gas transport properties e.g. thermal conductivity

Liquids

Surface tensionelastic tendency of a fluid surface which makes it acquire

the least surface area possibledecreases with temperature rise

Composed of molecules with much less free space

Substance Surface tension (Nm-1)

Water 0.070

Alcohol 0.025

Molten aluminum 0.500

Molten iron 1.500

Eg. Water striders

Eg. Water striders

Solids

array of atoms whose mean positions do not change with time

Characterization of solids

• x-ray diffraction

sin2dn

METALS/NON-METALSInter-atomic Forces in Solids

r

F

rro

U

Young’s Modulus

Mat

erial

Bonding Young’s modulus (Nm-2)

MgO ionic 30 x 1010

Copper Metallic 13 x 1010

Diamond Covalent 54 x 1010

MECHANICAL TESTSTensile tests

• Subject specimen to tension

Extension

Load

A

BC

E

F

O D

Hardness test

Classified into:

• Resistance to indentation by a particular shape of indenter.

• Resistance to scratching.

• The rebound of a steel ball from the surface.

Brinell Hardness (HB), test

• Steel ball pressed onto the surface under steady load, maintained for few seconds and indenter removed

Hardness:• Ratio of applied weight to diameter of indentation

22 11

2

DdD

WHB

Vickers Hardness (HV).

• Similar to HB but the indenter is always Diamond

• Hardness: ratio of weight to surface area of indent

2854.1

d

WHW

Rockwell Hardness: (HR)A, B, C

• Applied on soft materials

• Using different types of indenter

• A: cone shaped diamond indenter (tungsten)

• B: Steel sphere (Soft steels, Al, Brass)

• C: cone shaped diamond indenter (Harder steels)

• Hardness:

• d is the depth, N & s are scale factorss

dNHR

NON-METALSInorganic materials (Ceramics)

Eg. Magnesia (MgO)

aluminum oxide

(Alumina) – Al2O3,

Silicon Carbide (SiC)

Mostly ionic bonded e.g. MgO, or covalently bounded e.g. SiC.

Unique properties

• high melting points, • high corrosion resistance • chemical stability (chemically resistant to most acids

and alkalis), and resistance to abrasion. • Most ceramics do not have free conducting electrons

as a result have relatively low thermal and electrical conductivities,

• have high compressive, and mechanical strengths at high temperatures.

• Low densities of ceramic materials are also low in comparison to those of most metals; hence they have strength-weight ratios (Light but strong)

Ceramics

Property

Melting Point

(0C)

Density x

103Kgm-3

Thermal

Conductivity

(Wm-1K-1)

Tensile Strength

(MPa)

Young’s Modulus

(GPa)

Magne

sia

(MgO)

Sintered

Alumina

Al2O3

Vitreous

Silica

(SiO2)

Hot pressed

SiC

Hot pressed

Si2N4

Hotpressed

(Si2N4)

2800

3.6

36-45

100

210-

310

2040

3.9

12-32

400-500

350-380

1710

2.5

1-2

80-160

70

Decompose

s at 2300

3.2

100

350-800

350-470

Sublimes at

1900

3.2

10-16

500-900

150-320

Sublimes

1900

3.2

10-16

500-900

150-320

Organic Materials (Polymers)

Polymers

• atoms aggregate together into molecules,

• held together by either Van der Waal or hydrogen bonds

• Have long chain or repeating units that make up its molecule.

• The smallest unit is known as a monomer

• repeating network unit forms the polymer.

•

Composites

• consists of two or more components that can be distinguished from one another under a microscope.

• Target: superior material whose property of interest is superior to those of the individual components.

• Example combination of strength & ductility may be realized in solids that comprise fibres or precipitated particles embedded in a ductile host material.

• Polymers are most suitable materials to host fibres. Example fibreglass roof covers of pick-up trucks or car-bumpers that are made of composites.

Plywood

Composites

Examples

Natural composite

materials

Wood, bone, bamboo, muscle and

other tissues

Microscopic

materials

Metallic alloys: eg., steels,

toughened thermoplastics e.g.

impact polystyrene

Macrocoposites Galvanised steel, reinforced

concrete, helicopter blade, car

bumpers, pick-up truck roofs, skis

etc

FRACTURE AND OXIDATION

Fracture

Break in continuity of a surface of a materialEg: wood, bone, stone etc

Fracture vs Failure

• Fracture is a major mechanical property of metals• The study of this property aims at improvement of

mechanical properties of metals• For safety (to prevent fracture due to stress)• Fracture is caused by Failure• Failure is for specific purpose eg.

Fractures Types Ductile

• Necking before fracture

• Cup-&-cone shapes of the two new surfaces created

• Most metals are ductile

Brittle

• No necking

• E.g. ceramics, glass, extra cold metals

Scientific explanation of Metal Failure

• Metals contain voids

• Some only seen through microscopy

• When subjected to tress, voids increase in size

• Get close to each other

• Weakens the bond

• Leads to breaking (fracture)

Prevention of Brittle Fracture

• Etching (Chemical process of filling cracks and voids)

• Polishing (Physical process of filling the cracks)

• Introduction of ductile fibers in micro cracks

– Mainly composites (commonest: concrete)

– Prevents crack growth

– Construction industry

Causes of Failure

• No single cause of failure

• Important for safety (construction, transport – air etc)

• Causes:

• thermal shock,

• wear,

• corrosion,

• stress corrosion cracking,

• fatigue

Corrosion

• Chemical process of conversion of a metal from a refined state to a more chemically stable

• Formation of compounds on a metal surface when exposed to air, water, or an electrolyte

• Common terminology: Rusting

• Its an irreversible process

The Process

• It’s a chemical process• one of the reactants must be

at a higher energy state• natural process involves

transition from high to low level

• Hence energy absorbed: +ve• energy released: –ve• For spontaneous reaction: G = –ve.

fb eeG

e: Activation energy (energy required to overcome a barrier)

(Free Energy)

Factors affecting corrosion

• Temperature of the surrounding

• The diffusion rates of the reaction products

• The equilibrium concentration of the ions or reaction products in the solution;

• The pH value of the solution

• Electrolyte velocity

• Solid or dissolved pollutants

• Relative humidity.

Types of Corrosion

• Direct corrosion by dry gases

• electrochemical corrosion

• galvanic or bimetallic) corrosion

Manifestation of corrosion• Uniform attack

• Pitting-

• Crevice corrosion-

• Intergranular

• Stress corrosion-

• Corrosion fatigue-

• Fretting

Corrosion hazards

Prevention/Control of Corrosion

• Painting

• Galvanization

• Environmental control (pH, humidity, air quality)

• Appropriate choice of materials

• Cathodic and anodic protection

• Sydney Opera House

Sydney Opera House

THERMODYNAMICS

a b c

d e

f g h i

(Adiabatic )

Definitions System

Phase

Surrounding (Kitchen)

Rigid boundary

isolated system

Equilibrium state

one state in which all the bulk physical properties of a system are uniform

throughout the system & do not change with time.

• e.g. Temp, Pressure, concentration, volume, magnetic field, etc

• Min. of two variables required for specification

Zeroth Law (Temperature)

Diathermal wallAdiabatic wall

A B C

A AB C

REVERSIBLE, IRREVERSIBLE, QUASISTATIC AND ADIABATIC

PROCESSES

Thermodynamic reversibility

1. the process must be quasi-static (equilibrium state is always maintained)

2. there must be no hysteresis (i.e., no dissipation of energy).

Reversible

(P1,V1)

P2,V2

P

Work & Volume changes(for reversible Process)

Atmospheric thermodynamicsP

V

1

2

3

V1 V2

P2

P1

Isotherm at T

FP

AGas

CSA=A

dx

PAF

PdVPAdxdW

nRTPV

work done is path dependent 12231 VVPW

2

1

2

11

2

21 lnV

V

V

V V

VnRT

V

dVnRTPdVW

INTRODUCTION TO QUANTUM THEORY

Classical vs Quantum

• Newtonian mechanics:

• Interested in

x, v, a, p

• Light as a wave

• Quantum mechanics:

• Interested in

• Pos, energy, momentum

• Light as a particle

FmaF , txt

itxxm

,,2 2

22

Photoelectric Effect

• Proof of Quantum (or particle) nature of light

ejection of electrons from the surface of a metal when a beam of monochromatic light of some frequency () is shone on the surface.

Other phenomena• Heat Capacity of Solids

• The Atomic Spectra (The Hydrogen Atom)

Wave-Particle duality & the Complementarity’s Principle

the particle and wave models are complementary; if a measurement confirms

the wave nature of radiation or matter, then it is impossible to prove the particle nature in

the same experiment, and vice-versa.

WAVE MECHANICS AND SCHROEDINGER’S EQUATION

wave function (),

• , a complex function, that it is, it has a real part and an imaginary part.

• Is used to find probability that a particle is somewhere (in volume element dV)

Schrödinger Equation

• Time – dependent

• In a potential (V(x))

txt

itxxm

,,2 2

22

txVxmti

,2 2

22

mE

xEti

e2

2

22

2

222

82 mL

hn

mL

nEn

Wave - Particle

Position - Wave (Continuous)

Energy – Particle (quantized)

mE

xEti

e2

2

22

2

222

82 mL

hn

mL

nEn

PHYSICS CAREER DAY WORKSHOP

DATE: JULY 6 2018

VENUE: MH1

TIME 8.00am

KEY NOTE ADDRESS:

Rose M. Mutiso, Ph.D.

Co-Founder and CEO The Mawazo Institute

Rose M. Mutiso, Ph.D.

• Co-Founder and CEO of The Mawazo (“Ideas”) Institute, a Nairobi-based think tank.

• Scientist, researcher & practitioner working on technology & policy aspects of energy, environment & innovation issues in North America, South Asia, & Sub-Saharan Africa.

• Materials Scientist by training with research experience in the fields of nanotechnology & polymer physics.

• Passionate about harnessing science & technology to improve lives