Lecture: 2
Structure Levels:
• Subatomic level
Electronic structure of individualatoms that
defines interaction among atoms (interatomic
bonding).
• Atomic level
Arrangement of atoms in materials(for the same
atoms can have different properties, e.g. two forms
of carbon: graphite and diamond)
• Microscopic structure
Arrangement of small grains of material that can
be identified by microscope.
• Macroscopic structure
Structural elements that may beviewed
with the naked eye.
Properties:
Properties are the way the material responds to the environment and external forces.
Mechanical properties – response to mechanical forces,strength, etc.
Electrical and magnetic properties - response electrical and magnetic fields, conductivity, etc.
Thermal properties are related to transmission of heat and heat capacity.
Optical properties include to absorption, transmission and scattering of light.
Chemical stability in contact with the environment - corrosion resistance.
Types of Materials:
Let us classify materials according to the way the atoms are bound together.
Metals: valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Strong, ductile, conduct electricity and heat well, are shiny
if polished.
An alloy is a combination of two or more metals usuallyproduced to have new materials with improved properties.
Alloys like : Cu-Zn (brass) , Fe-C (steel) , Sn-Pb (solder) .
Semiconductors:the bonding is covalent (electrons are shared between atoms). Their electrical properties depend strongly on minute proportions of contaminants.
Examples: Si, Ge, GaAs.
Ceramics:atoms behave like either positive or negative ions, and are bound by Coulomb forces. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Hard, brittle, insulators.
Examples: glass, porcelain.
Polymers:are bound by covalent forces and also by weak van der Waals forces, and usually based on C and H. They decompose at moderate temperatures (100 – 400 C), and are lightweight. Examples: plastics rubber.
Composite Materials: Consist of more than one material type (such as fiberglass) , display a combination of the best characteristics of each of the individual material.
Examples: Continuous carbon fibers in one or more orientations in apolymer matrix.
Concrete is another structural composite obtained by combiningcement, sand, gravel.
Composites are classified according to their matrices into:
1- polymer-matrix composites : used for lightweight structures
(aircraft, sporting goods, wheelchairs, etc.) in addition to vibrationdamping.
2- Cement matrix composites
3-Metal-matrix composites : with aluminum as the matrix are used for lightweightstructures, but they have a high cost of fabrication.
4 - Carbon-matrix composites : relatively expensive because
of the high cost of fabrication and they are important for lightweightstructures (like the Space Shuttle) and components that need towithstand high temperatures (such as aircraft brakes).
5 - Ceramic-matrix : superior to carbon-matrix composites in oxidationresistance, but they are not as well developed.
Biomaterials:Implanted in the human body foreplacement of diseased or damaged body parts. They are must be:
1-Not produce toxic.
2- Compatible with body tissues.
Future of materials science :
Design of materials having specific desired characteristics directly from our knowledge of atomic structure.
• Miniaturization or "Nanotechnology ": “Nanostructured" materials, with microstructure that has length scales between 1 and 100 nanometers with unusual properties. Electronic components, materials for quantum computing.
• Smart materials:
Materials that are able to sense changes in their environments and then respond to these changes in predetermined manners.
■ Airplane wings that deice themselves
■photosensitive glass,which darkens in response to sunlight. The process is similar to theway that photographic films darken, but is reversible. The glasscontains dispersed silver halide crystals doped with copper.
■ Buildings that stabilize themselves in earthquakes .
■Actuators : able to change shape , position , natural frequency , or mechanical characteristics in response to change in temp. , electric field , and / or magnetic field .
Four types of materials used for actuators:
1- Shape memory alloys : ∆T ∆ Shape
Ex: Ni/Ti alloys such as Nitinol, Cu/Zn/Al Alloys, and Cu/Al/NiAlloys.
2- Piezoelectric ceramics : ∆ Voltage expand & contract, also ∆ dimensions generate an electric field.
Ex:
Piezoceramics (Lead Zirconate Titanate,PZT) are used extensively as actuators and sensors, for a wide range of frequency including ultrasonic applications.
3-Magnetostrictive materials : Magnetic field
4- Electrorheological & Magentorheological Fluids : Electric & Magnetic fields ∆ Viscocity .
■Sensors
Smart System as used in helicopters to reduce noise.
• Advance Materials :( High-Tech.)
-Leaser
-Integrated Circuits (IC)
-Magnetic information Storage
-Liquid Crystal Display (LCDs)
-Fiber Optics.
-Thermal Protection System for the Space Shuttle Orbiter.
• Environment-friendly materials:biodegradable or photodegradable plastics, advances in nuclear waste processing, etc.
• Learning from Nature:shells and biological hard tissue can be as strong as the most advanced laboratory-produced ceramics, mollusces produce biocompatible adhesives that we do not know how to reproduce…
• Materials for lightweight batteries with high storage densities, for turbine blades that can operate at 2500°C, room-temperature superconductors? chemical sensors (artificial nose) of extremely high sensitivity, cotton shirts that never require ironing…