January 25, 2016
Modern materials
How have computers gotten so much faster?
Today’s Keywords
Strength, composite materials,
electrical conductor, resistance, insulator,
semiconductor, superconductivity, doping, microchip
Contents
n  Introduction: materials and modern world
n  Three different properties of materials
1. Strengths of materials
2. Electrical properties of materials
3. Magnetic properties of materials
Introduction:
materials and the modern world 1
n  Look around your room now. How many different
materials do you see?
n  Glass, wood, metal, woven fabric, …
n  Thanks to the discoveries of chemists, the number of
everyday materials has increased by a thousandfold
in the past two centuries!
Introduction:
materials and the modern world 2
n  Chemists take natural elements and compounds of
earth, air, and water, and devise thousands of useful
materials.
n  Materials display many different properties: color,
hardness, smell, flexibility, density, texture, luster,
melting point, strength, and so on.
Introduction:
materials and the modern world 3
n  Three essential features of properties of each material,
based on the properties of atoms and their chemical
bonding, are:
1. the
of which it is made;
2. the way those
; and
3. the way the
to each other.
Three different properties of materials
Related to their atomic architecture.
(Recall: every material is held together by the
bonds between its atoms.)
1. The strength of materials
2. The ability of materials to conduct electricity
3. The magnetic property of materials
1.Strengths of materials
Think about a plastic bag carrying a heavy load of
groceries. How can something as light, flexible, and
inexpensive as a piece of plastic be so strong?
Strength:
n  The ability of a solid to resist changes in shape.
n  A strong material must be made with strong chemical
bonds!
n  There is no type of bond universally stronger than
others, but the strongest materials incorporate long
chains and clusters of carbon atoms bonded by
covalent bonds
- Diamond, plastic shopping bags, spider webs are
bonded by covalent bonds
Strengths of materials n  The strength of a material is related to the size of the
force it can withstand when it is pushed or pulled.
n  There are three very different kinds of strength
characterizing materials:
1. its ability to withstand crushing (compressive strength);
2. its ability to withstand pulling apart (tensile strength);
3. its ability to withstand twisting (shear strength).
n  Elastic limit: the point at which a material stops resisting
external forces and begins to bend, break, or tear
Ex) Breaking an egg, crush an aluminum can, or
fold a piece of paper
Strengths of materials Let us think about what determines a material’s
strength!
Strengths of materials n  Examples of materials with different strengths
Diamond: strong under all three kinds of stress (left picture)
Plastic shopping bag: strong under stretching, but weak
under twisting or crushing (right picture)
Strengths of materials n  Examples of materials with different strengths
Diamond: strong under all three kinds of stress because
of its 3-dimensional framework of strong
carbon-carbon bonds (left picture)
Plastic shopping bag: strong under stretching, but weak
under twisting or crushing because
of strong bonds in only one direction (right picture)
Strengths of materials What determines a material’s strength?
è A material’s strength is a result of the <type>
and <arrangements> of chemical bonds.
Composite materials: n  Combined materials of two or more substances.
n  The weakness of a material is compensated by the
strength of other materials.
n  Examples …
Composite materials: n  Combined materials of two or more substances.
n  The weakness of a material is compensated by the
strength of other materials.
n
Ex) Plywood 合板
Reinforced concrete 鉄筋コンクリート
Fiberglass ガラス繊維
Carbon-fiber
, … , and so on.
The body of cars: formed from a fiberglass or other
molded lightweight composite.
Composite materials:
Ex) Plywood
One of most common composite materials, consists of thin
wood layers glued together with alternating grain direction.
The weakness of a single thin sheet of wood is compensated by
the strength of the neighboring sheets.
Composite materials:
Ex) Reinforced concrete
A common composite materials in which steel rods are
embedded in a concrete mass.
(Tensile strength + compressive strength)
http-//www.specialist-foundations.co.uk
2. Electrical properties of
materials
A large number of different kinds of materials contribute
to any electrical device. Some should be efficient to carry
the electrical energy without much loss, but some should
not conduct electricity so that we will not be endangered
by electricity.
Conductors: n  Conductors: any material capable of carrying electrical
currents – electrons can flow freely through it.
Metals: the most common conductor - such as copper
Saltwater: containing ions of sodium (Na+) and chlorine (Cl-)
– free to move when they are part of an electric circuit
Conductors: n  Conductors: any material capable of carrying electrical
currents – electrons can flow freely through it.
Metals: the most common conductor - such as copper
Saltwater: containing ions of sodium (Na+) and chlorine (Cl-)
– free to move when they are part of an electric circuit
n  The arrangement of electrons in a material determines its
ability to conduct electricity.
Metals: loosely bonded electrons shared by many atoms, and
they are free to move in an electric circuit.
Metallic bonds.
Conductors: n  Electrical resistance: the property by which materials drain
the energy away from a current.
Under normal circumstances, electrons moving through a
metal collide continuously with heavier ions, then their
energies are converted to heat.
n  Electrical conductance: the inverse of electrical
resistance
Insulators: n  Insulators: materials that don’t conduct electricity
(unless they are subjected to an extremely high voltage)
Ex) Rocks, ceramics and many biological materials:
their electrons are bounded tightly to one or more
atoms by ionic or covalent bonds.
Insulators: n  The primary use of insulators in electric circuits:
- to channel the flow of electrons
- to keep people from touching wires carrying current
Ex)* Plastic materials made for power outlets, casings
for car batteries, light switches
* Protective rubber boots, gloves for electrical workers
* Glass, ceramic components for isolating the current
in high-power lines
Semiconductors:
n  Many materials in nature are neither good conductors
nor perfect insulators.
- A semiconductor carries electricity but not carry it very
well
- Ex) Silicon: It has much higher resistance than that of
a conductor such as copper, but some of
its electrons flow in an electric circuit.
Semiconductors:
n  In a silicon crystal,
- a regular pattern of silicon atoms bonded by covalent
bonds is displayed in the picture
Semiconductors:
n  In a silicon crystal, (cont’d)
- some of its electrons are taken loose by atomic
vibrations, and these are free to move around and
conduct electricity.
electrons are free
to move by
atomic vibrations,
leaving holes
http://hyperphysics.phy-astr.gsu.edu/hbase/
solids/intrin.html
Semiconductors:
n  In a silicon crystal, (cont’d)
à A defect after an electron leaves in the crystal is
called a hole (a missing electron).
electrons are free
to move by
atomic vibrations,
leaving holes
http://hyperphysics.phy-astr.gsu.edu/hbase/
solids/intrin.html
Semiconductors: n  In semiconductors, the effects of the successive
jumping of electrons from one to another conduct
electricity.
(equivalently those of the hole moving through the
material)
•  Doped semiconductors
In semiconductor production, doping intentionally
introduces impurities into an extremely pure
semiconductor for the purpose of modulating its
electrical properties.
N-type
P-type
Electron
impurity
Hole
impurity
electrapk.com/
n  Doped semiconductors
Doping: the addition of a minor impurity to an element or
compound. p-type, n-type.
n  Diodes: one way gate of electrical currents
Ex) AC à DC in electronic devices
n  Transistor: device to amplify and switch electronic signals
and power. Simplest ex) pnp-type, npn-type
n  Microchips: incorporate hundreds or thousands of
transistors in one integrated circuit
n  Digital electronic devices such as Computers and mobile
phones are made possible using microchips
Superconductors:
n  Materials that exhibit a property known as
superconductivity - the complete absence of any
electrical resistance -, when they are cooled to within
a few degrees of absolute zero
Superconductors:
n  If we make an electromagnet out of superconducting
material and keep it cold, the magnetic field will be
maintained at no energy cost except for refrigeration.
à Superconducting magnets
Examples …
Superconductors:
n  If we make an electromagnet out of superconducting
material and keep it cold, the magnetic field will be
maintained at no energy cost except for refrigeration.
à Superconducting magnets
Ex) - Magnetic Resonance Imaging (MRI) systems for
medical purpose
- Magnetic levitation transportation system
- Particle accelerators such as LHC at CERN in
Switzerland
Superconductors:
n  How can a superconducting material allow electrons to pass
through without losing energy?
Superconductors:
n  How can a superconducting material allow electrons to pass
through without losing energy?
Fast moving
electron
www.quora.com
At very low temperature
Superconductors:
n  How can a superconducting material allow electrons to pass
through without losing energy?
- At very low temperatures, heavy ions in a material don’t
vibrate very much.
- As a fast moving electron passes between two positive ions,
ions are attracted to the electron, but the electron is long
gone at the time.
- Nevertheless, the positive ions create a positively charged
region so that the region can attract a second electron.
- As far as the temperature is kept cold enough, the
phenomenon is also kept forever. 3. Magnetic Properties of
materials
Q: Why some materials such as iron display strong
magnetism, while other substances seem to be
unaffected by magnetic fields?
Magnet field in an atom
n  We have learned that every magnetic field is due to
the presence of electrical currents, thus each electron
in an atom acts like a little electromagnet and the
total magnet field of the atom arises by adding those
of all the tiny electron electromagnets.
à Each atom in the material can be thought of as a
tiny dipole magnet.
à The magnetic field of a solid material like a piece
of lodestone arises from the combination of all
these tiny magnetic fields
Magnet field in an atom
•  Each electron in an atom acts like a little electromagnet
•  Magnetic field of a solid material arises from combination
of all these tiny magnetic fields
An atom
in a material
Atomic nucleus
+
-
electron
Magnetic field
originated from
orbital and
spinning motion of
an electron
n  Q: However, why most materials do not have
magnetic fields?
Magnetic Properties of materials
n  In ordinary situation,
atomic dipole magnets point in random direction (Fig. 1),
so at a place outside the material their acts tend to cancel.
à nonmagnetic materials
Fig.1
Magnetic Properties of materials
n  In a few materials,
including iron, cobalt, and nickel metals, the atomic magnets
line up – an effect called ferromagnetism, but orientation of
‘domains’ is random.
à No magnetic field is measured outside the material,
because the small magnetic fields in different domains
cancel each other. (Fig. 2-a) Fig.2-a
Domains
Magnetic Properties of materials
n  In a few materials,
à In special cases, when iron cools from very high
temperature in the presence of a strong magnetic field,
all neighboring domains may line up. Then the material
exhibits an external magnetic field – permanent
magnet! (Fig. 2-b)
Fig.2-b
Magnetic field
Magnetic Properties of materials
n  In ordinary situation, à nonmagnetic materials (Fig. 1)
n  In a few materials,
à No magnetic field is measured outside the material (Fig. 2-a)
à In special cases, in the presence of a strong magnetic field,
the material exhibits an external magnetic field – permanent
magnet! (Fig. 2-b)
Fig.2-a
Fig.1
Domains
Fig.2-b
Fig.3-b
Magnetic field
http-//www.ece.auckland.ac.nz/~kacprzak/notes.htm
Next topic is,
Nuclear power: chapter 8
www.sci.hokudai.ac.jp/~epark/ekpark_e.html