Transcript stoichiometric defects

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Point Defects These are irregularities or deviations from
ideal arrangement around a point or an atom in a crystalline
substance.
Line Defects These are irregularities or deviations from ideal
arrangement in entire rows of lattice points
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STOICHIOMETRIC DEFECTS.
IMPURITY DEFECTS.
NON- STOICHIOMETRIC DEFECTS
These are the point defects that do not disturb the stoichiometry
ofthe solid. They are also called intrinsic or thermodynamic
defects.
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Vacancy Defects : When some of the lattice sites are vacant, the
crystal is said to have vacancy defect (Fig. 1). This results in
decrease in density of the substance. This defect can also develop
when a substance is heated.
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Interstitial Defect: When some constituent particles(atoms or
molecules) occupy an interstitial site, the crystal is said to have
interstitial defect(Fig). This defect increases the density of the
substance
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Frenkel Defects.
This defect is shown by ionic solids. The smaller ion (usually
Cation) is dislocated from its normal site to an interstitial site.
Hence it creates a vacancy defect on its original site and an
interstitial defect in its new site.
Frenkel defect is also called dislocation defect. It does not
change the density of the solid. Frenkel defect is shown by ionic
substance in which there is a large difference in the size of ions,
for example, ZnS, AgCl, AgBr and AgI due to small size of Zn2+
and Ag+ ions.
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Schottky Defects It is basically a vacancy defect in ionic solids. In
order to maintain electrical neutrality, the number of missing
cations and anions are equal.
Like simple vacancy defect, Schottky defect also decreases the
density of the substance.
Schottky defect is shown by ionic substances in which the cation
and anion are of almost similar sizes.For example, NaCl, KCl,
CsCl and AgBr.
AgBr shows both Frenkel and Schottky Defects.
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If molten NaCl containing a little amount of SrCl2 is crystallised,
some of the sites of Na+ ions are occupied by Sr2+.
Each Sr2+ replaces two Na+ ions. It occupies the site of one ion
and the other site remains vacant.
The cationic vacancies thus produced are equal in number to that
of Sr2+ ions.
Another similar example is the solid solution of CdCl2 and
AgCl.
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Inorganic solids which contain the constituent elements in nonstoichiometric ratio due to defects in their crystal structures.
These defects are of two types.
METAL EXCESS DEFECT.
METAL DEFICIENT DEFECT.
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Alkali halides like NaCl and KCl show this type of defect. When crystals of
NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are
deposited on the surface of the crystal.
The Cl– ions diffuse to the surface of the crystal and combine with Na
atoms to give NaCl. This happens by loss of electron by sodium atoms to
form Na+ ions. The released electrons diffuse into the crystal and occupy
anionic sites. As a result the crystal now has an excess of sodium.
The anionic sites occupied by unpaired electrons are called F-centres. (from
the German word Farbenzenter for colour centre).
They impart yellow colour to the crystals of NaCl.
The colour results by excitation of these electrons when they absorb energy
from the visible light falling on the crystals.
Similarly, excess of lithium makes LiCl crystals pink and excess of
potassium makes KCl crystals violet (or lilac).
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A typical example of this type is FeO which is mostly found with
a composition of Fe0.95O.It may actually range from Fe0.93O to
Fe0.96O.
In crystals of FeO some Fe2+ cations are missing and the loss of
positive charge is made up by the presence of required number
of Fe3+ ions.
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In case of semiconductors, the gap between the valence band and
conduction band is small. Therefore, some electrons may jump to
conduction band and show some conductivity.
Electrical conductivity of semiconductors increases with rise in
temperature, since more electrons can jump to the conduction band.
Substances like silicon and germanium show this type of behaviour
and are called intrinsic semiconductors.
The conductivity of these semiconductors are too low for practical use. Their
conductivity can be increased by adding a suitable amount of impurity. This
process is called DOPING.
Doping can be done with an impurity which is electron rich or
electron deficient as compared to the intrinsic semiconductor silicon
or germanium. Such impurities introduce electronic effects.
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Si and Ge belong to group 14 of the periodic table having four valence
electrons.
When doped with elements of Group 15 such as P or As which
contains 5 valence electrons, they occupy some of the lattice sites of Si
and Ge.
Four of the five valence electrons form covalent bonds with the
neighbouring Si or Ge.
The fifth electron is free and is delocalised. These delocalised
electrons increase the conductivity of Si and Ge.
Here the increase in conductivity is due to thenegatively charged
electron.
Hence such type of semiconductors are called n type semiconductor.
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Silicon or germanium can also be doped with a group 13 element like B, Al
or Ga which contains only three valence electrons.
The place where the fourth valence electron is missing is called electron hole
or electron vacancy.
An electron from a neighbouring atom can come and fill the electron hole,
but in doing so it would leave an electron hole at its original position.
If it happens, it would appear as if the electron hole has moved in the
direction opposite to that of the electron that filled it.
Under the influence of electric field, electrons would move towards the
positively charged plate through electronic holes, but it would appear as if
electron holes are positively charged and are moving towards negatively
charged plate.
This type of semi conductors are called p-type semiconductors.
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Various combinations of n-type and p-type semiconductors are used for
making electronic components.
Diode is a combination of n-type and p-type semiconductors and is used as a
rectifier.
Transistors are made by sandwiching a layer of one type of
semiconductor between two layers of the other type of
semiconductor. npn and pnp type of transistors are used to detect or
amplify radio or audio signals.
The solar cell is an efficient photo-diode used for conversion of light
energy into electrical energy.
Gallium arsenide (GaAs) semiconductors have very fast response and
have revolutionised the design of semiconductor devices.
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It is a semiconductor device that emits light when electric current is
passed through it.
The light is not particularly bright and in most LEDs it is
Monochromatic (single wavelength).
The output from an LED can range from red (at a wavelength of
approximately 700 nanometers) to blue-violet (about 400
nanometers). Some LEDs emit infrared (IR) energy (830 nanometers
or longer); such a device is known as an infrared-emitting diode (IRED).
An LED or IRED consists of two elements of processed material
called P-type semiconductors and N-type semiconductors.
These two elements are placed in direct contact, forming a region
called the P-N junction.
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Low power requirement: Most types can be operated with
battery power supplies.
High efficiency: Most of the power supplied to an LED or IRED
is converted into radiation in the desired form, with minimal
heat production.
Long life: When properly installed, an LED or IRED can
function for decades.
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Indicator lights: These can be two-state (i.e., on/off), bar-graph, or
alphabetic-numeric readouts.
LCD panel backlighting: Specialized white LEDs are used in flat-panel
computer displays.
Fiber optic data transmission: Ease of modulation allows wide
communications bandwidth with minimal noise, resulting in high speed
and accuracy.
Remote control: Most home-entertainment "remotes" use IREDs to transmit
data to the main unit.
Optoisolator:- Stages in an electronic system can be connected together
without unwanted interaction.
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Isamu Akasaki
Meijo University, Nagoya, Japan and Nagoya University, Japan
Hiroshi Amano
Nagoya University, Japanand
Shuji Nakamura
University of California, Santa Barbara, CA, USA
“for the invention of efficient blue light-emitting diodes which has enabled
bright and energy-saving white light sources"
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A transistor is a semiconductor device used to amplify or switch
electronic signals and electrical power. It is composed of
semiconductor material usually with at least three terminals for
connection to an external circuit.
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A transistor is a miniature electronic component that can do two
different jobs. It can work either as an amplifier or a switch.
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When it works as an amplifier, it takes in a tiny electric
current at one end (an input current) and produces a much
bigger electric current (an output current) at the other.
Its useful in making hearing aids.
The Hearing aid is a tiny microphone that that turns the sound
around the world into electric signals. These are fed into a
transistor that boosts them and powers a tiny loudspeaker.
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Transistors can also work as switches.
A tiny electric current flowing through one part of a transistor can make a much bigger
current flow through another part of it.
A tiny electric current flowing through one part of a transistor can make a much bigger
current flow through another part of it.
This is essentially how all computer chips work.
For example, a memory chip contains hundreds of millions or even billions of transistors,
each of which can be switched on or off individually.
Since each transistor can be in two distinct states, it can store two different numbers, zero
and one. With billions of transistors, a chip can store billions of zeros and ones, and almost
as many ordinary numbers and letters (or characters, as we call them). More about this in a
moment.
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Transistors are made from silicon, a chemical element found in
sand, which does not normally conduct electricity (it doesn't
allow electrons to flow through it easily).
If we treat silicon with impurities (DOPING) with the chemical
elements arsenic, phosphorus, or antimony, the silicon gains
some extra "free" electrons—ones that can carry an electric
current—so electrons will flow out of it more naturally.
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Making sandwiches of p-type and n-type material, we can make different kinds of
electronic components that work in all kinds of ways.
Suppose we join a piece of n-type silicon to a piece of p-type silicon and put electrical
contacts on either side. Exciting and useful things start to happen at the junction between
the two materials.
If we turn on the current, we can make electrons flow through the junction from the n-type
side to the p-type side and out through the circuit.
This happens because the lack of electrons on the p-type side of the junction pulls electrons
over from the n-type side and vice-versa.
But if we reverse the current, the electrons won't flow at all.
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Logic gates let computers make very simple decisions using a mathematical technique called Boolean algebra.
Using AND, OR, and other operators called NOR, XOR, NOT, and NAND, computers can add up or compare binary
numbers. That formed the basis of computer programming.
Normally, a junction transistor is "off" when there is no base current and switches to "on" when the base current flows.
That means it takes an electric current to switch the transistor on or off.
But transistors like this can be hooked up with logic gates so their output connections feed back into their inputs.
The transistor then stays on even when the base current is removed.
Each time a new base current flows, the transistor "flips" on or off. It remains in one of those stable states (either on or
off) until another current comes along and flips it the other way.
This kind of arrangement is known as a flip-flop and it turns a transistor into a simple memory device that stores a zero
(when it's off) or a one (when it's on). Flip-flops are the basic technology behind computer memory chips.
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Transistors were invented at Bell Laboratories in New Jersey in
1947 by three brilliant US physicists:
John Bardeen (1908–1991),
Walter Brattain (1902–1987),
William Shockley (1910–1989).
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Magnitude of this ELECTRON SPIN magnetic moment is very small
and is measured in the unit called Bohr magneton, Μb.
On the basis of their magnetic properties, substances can be classified
into five categories.
Paramagnetic.
Diamagnetic.
Ferromagnetic
Antiferromagnetic.
Ferrimagnetic
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Paramagnetic substances are weakly attracted by a magnetic
field.
They are magnetised in a magnetic field in the same direction.
They lose their magnetism in the absence of magnetic field.
Paramagnetism is due to presence of one or more unpaired
electrons which are attracted by the magnetic field.
Examples. O2 Cu2+ Fe3+ Cr3+
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Diamagnetic substances are weakly repelled by a magnetic field.
They are weakly magnetised in a magnetic field in opposite direction.
Diamagnetism is shown by those substances in which all the electrons
are paired and there are no unpaired electrons.
Pairing of electrons cancels their magnetic moments and they lose
their magnetic character.
Examples: H2 O, NaCl, C6H6.
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A few substances like iron, cobalt, nickel, gadolinium and CrO2 are attracted very strongly by a
magnetic field. Such substances are called ferromagnetic substances.
Besides strong attractions, these substances can be permanently magnetised.
In solid state, the metal ions of ferromagnetic substances are grouped together into small regions
called domains.
Thus, each domain acts as a tiny magnet.
In an unmagnetised piece of a ferromagnetic substance the domains are randomly oriented and their
magnetic moments get cancelled.
When the substance is placed in a magnetic field all the domains get oriented in the direction of the
magnetic field and a strong magnetic field is produced.
This ordering of domains persist even when the magnetic field is removed and the ferromagnetic
substance becomes a permanent magnet.
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Substances like MnO showing antiferromagnetism have domain
structure similar to ferromagnetic substance, but their domains
are oppositely oriented and cancel out each other's magnetic
moment.
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Ferrimagnetism is observed when the magnetic moments of the
domains in the substance are aligned in parallel and anti-parallel
directions in unequal numbers.
They are weakly attracted by magnetic field as compared to
ferromagnetic substances.’
Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4 are
examples of such substances.
These substances also lose ferrimagnetism on heating and become
paramagnetic.