lecture 2

how can physical properties be split

into mechanical electrical and thermal

(can be split further)

material structure

every body consists of large numbers of extremely small material particles: atoms ions and molecules

structure of materials defn

stable bonds between building blocks ensuring their integrity under external influences

simple substance

contain building blocks of a single element

complex substance

contain building blocks of more than 1 element

molecule

smallest particle of a substance that can exist on its own and retain its properties

chemical bonds

can be primary or secondary

primary chemical bonds

- chemical in nature that bind atoms together in a molecule

- characterised by the sharing of electrons

- covalent, metallic, ionic

secondary chemical bonds

- the result of the similarity of differently loaded sections of the molecule

- no sharing of electrons

- Van Der Waals, hydrogen

covalent bond

- 2 atoms share electrons

- outer electronic layer complete

- characteristic of atoms of the same element + non metals

- single, double or triple

- polar, non polar, coordinate

metallic bond

- typical for metals

- characterised by electron sharing

- electron cloud formed

- atoms are +ve

ionic bond

- results from transfer of electron from one atom to another

- characteristic of chemical elements that follow inert gases

Hydrogen bond

- formed by electrostatic attraction of oppositely charged ions

- need to be bonded in a molecule to an atom that is -vely charged

Van der Waals

- due to electron movement, electrostatic fields created

- dipoles are +vely and -vely charged in diff regions

- weaker than other bonds

- power of london and dipole-dipole are eg

cohesive bond

between atoms and molecules in composition of an entity

adhesive bond

between different substances

aggregate state

four states of matter

solid

liquid

gas

plasma

melting --> evaporation --> ionisation

deionisation --> condensation --> freezing

solid --> gas is sublimation

gas --> solid is deposition

crystal structure

with a clear arrangement of the building blocks

triclinic = alpha, beta gamma is not = 90 degrees

monoclinic (centred and simple) = alpha is not = 90 degrees but beta gamma is

orthorhombic (simple, base centred, face centred, body centred) = a is not = b which is not = c

rhombohedral = alpha, beta gamma is not 90 degrees

tetragonal (simple and body centred) = a is not = c

hexagonal= a is not = c

cubic/isometric ( simple, body centred, face centred) = a

allotropy/ polymorphism

when a crystalline substance has the ability of changing their crystal lattice depending on temp

eg diamond, graphite

eg oxygen, ozone

Amorphous structure

with disordered particles

structure of materials in solid state

- random arrangement of building blocks

- similar state to liquid

thermodynamically unstable and no fixed mpt

- soften on slow heating and gradual decrease in viscosity

some materials change with temp variations

eg wax ( above 52 deg, amporphous but below 36, crystalline

eg feldspathic porcelain ( 1100-1200 deg amorphous but crystallises after cooling)

isotropic

mechanical properties of materials can manifest equally depending on direction of applied external forces

gases and liquids

anisotropic

mechanical properties of the materials can manifest differently depending on the direction of applied external forces

crystalline substances

mpt

the point at which substances change from solid aggregate state to liquid

bpt

indicates the temp at which gas pressure of a liquid = pressure of surroundings --> gas

1g of water from liquid to gas at 100 deg needs 540 calories of heat to be added to system

denisty/ bulk density

the mass of a substance per unit of volume

relative density

ratio of specific gravity of substance of interest to another element, often water

(often used to compare individual substances)

thermal properties

response of a material to an applied temp

thermal conductivity

ability to conduct heat without changing aggregated state

the amount of heat in calories that passes thru 1cm of a material with a cross sectional area of 1cm^2

coefficient of thermal expansion

alpha = delta L divided by (L x delta T)

electric current

directed movement of electric charges whose carriers may be electrons or ions

electrochemical potential

the tendency to give up or receive electrons when placed in soln

usually compared to hydrogen (0.00V)

Galvanism

caused by presence of metal restorations in mouth and their different electrical potentials

rheology

study of material fluidity

eg alloys in casting, freshly mixed impression materials, dental cement, composites

creep

other materials that are in a solid state can flow over time. is a plastic deformation which develops gradually

viscosity

resistance a fluid exerts during motion

thixotropy

property of materials to change their viscosity when pressure is applied

colloid

when they consist of two or more phases with the building blocks of at least 1 of the phases being larger than the size of a simple molecule

colloidal systems

can be emulsion, fine gel dispersion or film/foam

difference between soln and gel

soln = composed of colloidal particles dispersed in a liquid

gel = on cooling or reaction with appropriate chemical elements the soln may pass into a gel: agar-agar

Adsorption, Absorption, Sorption

adsorption = on surface

absorption = inside

sorption = both

water sorption

amount of water adsorbed on surface and absorbed into its interior during manufacture and use

in acrylics, water sorption is measured gravimetrically in micrograms per cubic mm ( after 7 day immersion)

surface tension in liquids

atoms and molecules on surface of substances have more energy than those inside

greater strength of bond between individual building blocks, of a substance, the greater the surface energy

surface energy of metals and oxides > liquids

capillary and capillary penetration

degree of capillary rise in slits is important for retention

higher surface tension gives higher penetration coefficient

higher viscosity gives lower penetration coefficient

capillary rise (h) formula

h = 2ycosθ