Transcript (a) type I superconductors and

LECTURE 4
CONTENTS

SUPERCONDUCTORS
APPLICATIONS OF SUPERCONDUCTORS

DETAILED STUDY ON PHOTONIC
MATERIALS(LED
AND LCD)
SUPERCONDUCTOR
Introduction
Father of Super conductivity is H.Kamerlingh Onnes’
Duch Physist (1911).
In his experiments on the properties of metals in general
and on the electrical conductivity (thereby resistivity) of
Mercury (Hg).
He observed that, when pure mercury is cooled, its
resistivity vanished abruptly at 4.2 K. Above this temperature,
the resistivity is immensurable, while below this temperature
the resistivity is very small that it is essentially zero. ( is in
the order of 105 ohm cm).i.e., at 4.2 K, Hg is converted into a
superconductor.
This phenomenon of loosing resistivity absolutely when
cooled to a sufficiently, low temperature is called `super
conductivity’.
Transition Temperature (or) Critical
Temperature
The temperature at which the transition of a normal
conductor into a superconductor occurs is called as the
`Transition temperature or critical temperature [Tc].
 Above Tc- the substance is in the normal state, but
 Below Tc-The substance is in the super conducting state.
 For semiconductors -Tc varies from, 0.3K to 1.25K
 For metals
-Tc varies from 0.35K to 9.22K and
 For alloys
-Tc varies from 18.1K to 22.65K.
Resistance of mercury vs temperature
High Temperature Superconductors (HTS)
High-temperature superconductors (abbreviated high Tc) are
a family of superconducting materials containing copper-oxide
planes as a common structural feature. For this reason, the
term
is
often
used
interchangeably
with
cuprate
superconductors.
This feature allows some materials to support
superconductivity at temperatures above the boiling point of
liquid nitrogen (77 K) . Indeed, they offer the highest transition
temperatures of all superconductors. The ability to use
relatively inexpensive and easily handled liquid nitrogen as a
coolant has increased the range of practical applications of
superconductivity.
Some examples of HTS
Notations
Chemical formula
Tc (K)
123
YBa2 Cu3 O7
90
Tl-1212
Tl Ba2 CaCu2 O7
80
Characteristics of HTSC
 Superconductors are characterized by a materialdependent magnetic field H, above which the superconducting
state disappears.
 The critical field is a function of temperature. All the HTS
materials are type II superconductors .When the applied field
H < Hc1, the material is in the superconducting Meissner state
whereas in the mixed state, the magnetic field penetrates
partly into the material in the form of vortices.
 Type II superconducting materials have usually higher
critical fields than type I superconductors which makes them
suitable for many advanced applications.
(a)
(b)
Critical magnetic field as a function of temperature for (a)
type I superconductors and (b) type II superconductors.
Most of the HTS materials are layered cuprates, i.e.,
they consist of CuO2 planes separated by layers of other
elements or oxides. Because of the layered structure, HTS
materials exhibit strong anisotropy: the values of the
superconducting parameters are different in different
directions. In addition, charge transport is mainly confined to
the CuO2 planes.
IMPORTANT FEATURES HTS
• They have high Tc.
• They have PEROVSKITE crystal structure.
• They are direction dependent
• They are reactive, brittle and cannot be easily formed (or)
joined.
HTS Material - YBCO
 HTS materials usually have complicated crystal
structures.
 The compounds of HTS almost consists more than three
different chemical elements and the materials with the highest
Tc have seven elements in the crystal lattice.
 Ex: YBa2Cu3O7-d (YBCO).
YBCO has numerous
advantages compared to other ceramic superconductors
•
This is only known stable four-element compound with a Tc
above 77 K. Includes neither toxic elements nor volatile
compounds
Easy to make single-phase YBCO. Less anisotropic
than other HTS materials, carries higher current densities at
higher magnetic fields.
Structure of a single unit cell of YBCO
The critical temperature of YBCO is approximately 90
K and the critical magnetic field can be as high as 300 T. For
thin-film applications, critical current density (Jc) is an
important parameter and, in the case of YBCO, it is typically
Jc > 1 MA/cm².
The dimensions of a single unit cell of YBCO are a =
3.82 Å, b = 3.89 Å, and c = 11.68 Å. The lattice is composed
of so-called perovskite layers (ACuO3)
where A is a rare-earth or alkaline-earth element (e.g., Y
or Ba in YBCO). The term 7-d in the chemical formula implies
a slight deficiency of oxygen. If d=0, the lattice is in the
orthorhombic phase whereas in the case of d=1, the material
has a tetragonal structure. Only the orthorhombic
configuration is superconducting but it is stable only at
temperatures below 500°C.
APPLICATIONS OF SUPERCONDUCTORS
1. Superconducting Transmission Lines
Since 10% to 15% of generated electricity is
dissipated in resistive losses in transmission lines, the
prospect of zero loss superconducting transmission lines
is appealing.
Current experiments with power applications of
high-temperature superconductors focus on uses of
BSCCO in tape forms and YBCO in thin film forms.
Current densities above 10,000 amperes per square
centimeter are considered necessary for practical power
applications, and this threshold has been exceeded in
several configurations.
2. Superconducting Motors and Generators
Superconducting motors and generators could be
made with a weight of about one tenth that of conventional
devices for the same output. This is the appeal of making
such devices for specialized applications. Motors and
generators are already very efficient, so there is not the
power savings associated with superconducting magnets. It
may be possible to build very large capacity generators for
power plants where structural strength considerations place
limits on conventional generators.
3. Superconducting Magnetic Energy Storage
Superconducting magnetic energy storage (SMES)
stores electricity for long periods of time in superconductive
coils. SMES will be used by electrical utilities some day.
4. Computers
If computers used superconducting parts they
would be much more faster than the computers today. They
would much smaller because no space for heat would be
required. Computers of today need a great deal of space for
cooling.Computers are being developed today that use
Josephson junctions. The Josepson effect states that electrons
are able to flow across an insulating barrier placed between
two superconducting materials. Josephson junctions have a
thin layer of insulating materials squeezed between
superconductive material. Josephson junctions require little
power to operate, thus creating less heat.
5. Josephson Devices
Devices based upon the characteristics of a Josephson
junction are valuable in high speed circuits. Josephson
junctions can be designed to switch in times of a few
picoseconds. Their low power dissipation makes them useful in
high-density computer circuits where resistive heating limits the
applicability of conventional switches.
6. SQUID Magnetometer
The superconducting quantum interference device
(SQUID) consists of two superconductors separated by thin
insulating layers to form two parallel Josephson junctions.
The device may be configured as a magnetometer to detect
incredibly small magnetic fields -- small enough to measure
the magnetic fields in living organisms. Squids have been
used to measure the magnetic fields in mouse brains to test
whether there might be enough magnetism to attribute their
navigational ability to an internal compass.
Threshold for SQUID:
10-14 T
Magnetic field of heart:
10-10 T
Magnetic field of brain:
10-13 T
The great sensitivity of the SQUID devices is associated
with measuring changes in magnetic field associated with
one flux quantum
7. Superconductors in NMR Imaging
Superconducting magnets find application in magnetic
resonance imaging (MRI) of the human body. Besides
requiring strong magnetic fields on the order of a Tesla,
magnetic resonance imaging requires extremely uniform
fields across the subject and extreme stability over time.
Maintaining the magnet coils in the superconducting state
helps to achieve parts-per-million spacial uniformity over a
space large enough to hold a person, and ppm/hour stability
with time.
8. Fault-Current Limiters
High fault-currents caused by lightning strikes are a
troublesome and expensive nuisance in electric power grids.
One of the near-term applications for high temperature
superconductors may be the construction of fault-current
limiters which operate at 77K. The need is to reduce the fault
current to a fraction of its peak value in less than a cycle (1/60
sec).
9. Magnetically Levitated Trains
Perhaps
the
most
famous
and
fascinating
superconducting invention is magnetically levitated trains, or
"maglev" trains. Maglev trains have no wheels and friction.
The trains float silently on a magnetic field due to diamagnetic
behaviour.
Photonic materials – Light Emitting Diodes
LED (Light Emitting Diode) is a semiconductor p-n junction
diode which converts electrical energy to light energy under
forward biasing.
Principle
The diode is forward biased. Due to forward bias, the
majority carriers from ‘n’ and ‘p’ regions cross the junction
and become minority carriers in the other junction (i.e.)
Electrons, which are majority carriers in ‘n’ region cross the
junction and go to ‘p’ region and become minority carriers in
p-region
Similarly, holes which are majority carries in ‘p’ region
cross the junction and go to ‘n’ region and become minority
carriers in ‘n’ region and this phenomenon is called minority
carrier injection.
Radiative recombination
Now if the biasing voltage is further increased, these
excess minority carriers diffuse away from the junction and
they directly recombine with the majority carriers. (i.e.) the
electrons, which are excess minority carriers in p-region
recombine with the holes which are the majority carriers in
‘p’ region and emit light. Similarly, the holes which are
excess minority carriers in ‘n’ region recombine with the
electrons which are majority carriers in ‘n’ region and emit
light.
Thus radiative recombination events lead to photon
emission. The number of radiative recombination is
proportional to the carrier injection rate and hence to the
total current flowing through the device as given by

 eV  
I I 0 exp
 kT 
 1

 

where
I0 - the saturation current ; V- the forward bias
voltage; k - the Boltzmann constant ;  -varies from 1 and 2
depending on the semiconductor and temperature.
The optical photon emitted due to radiative recombination
has the energy very close to the bandgap energy Eg and
frequency of the emitted photon is given by
hc

 Eg
where
 - the photon wavelength;
constant; c - the velocity of light in vacuum.
h - Plancks
LED Construction
An LED must be constructed such that the light emitted by
the radiative recombination events can escape the structure.
Sketches of LEDs
LEDs can be designed as either surface or
edge emitters. Surface emitting LEDs can be made such
that the bottom edge reflects light back towards the top
surface to enhance the output intensity. The main advantage
of edge emitter LEDs is the emitted radiation is relatively
direct. Hence edge emitter LEDs have a higher efficiency in
coupling to an optical fibre.
Although the internal quantum efficiency of LEDs is 100%,
the external efficiencies are much lower. The main reason is
that most of the emitted light radiation strikes the material
interface at greater than critical angle and hence trapped
with in the device. The internal critical angle at the
semiconductor – air boundary is given by
sin c 
n2
n1
Where n1 is the refractive index of air = 1.0
n2 is the refractive index of the semiconductor
For group III semiconductor n2 = 3.5
Therefore
c = 16°
Critical angle
Therefore all rays of light striking the surface at an angle
exceeding 16° suffer total internal reflection and as a result
most of the emitted light is reflected back inside the
semiconductor crystal.
Two methods used to reduce reflection losses in LEDs
Hence to improve the external efficiency losses caused
bulk absorption has to be minimized and the surface
transmission has to be increased. One method to achieve this is
to give the semiconductor a dome structure.
Hemi spherical domes made from plastics are effective in
increasing the external efficiency by a factor 2 or 3. There will
be some losses at the plastic/ air interface but these are easily
minimized by molding the plastic into an approximately
hemispherical shape.
Materials
The choice of the materials for an LED is decided by the
spectral requirements for a particular application. The most
commonly used materials for LEDs are GaP, GaAs and their
related ternary compound Ga Asx P1-x
The bandgap radiation of GaP, GaAs and GaAsP. GaP
which gives a peak at 560 nm is very close to the wavelength
of maximum eye response.
This makes GaP one of the most useful of all visible
semiconductor light sources since in addition to green light
both red and other colours can be produced by appropriate
dopants.
Wavelength response of LED materials
Material Dopant
GaP
GaP
GaP
GaAs
AlGa
N
Zn0
N
P
As
Band
gap
(eV)
Wavelength
( Nm)
Quantum
efficiency
( %)
2.88
1.80
2.25
1.88
1.84
430
690
550
660
675
0.6
0.2
0.1
0.2
0.2
Photonic materials – Liquid Crystal Display
Liquid crystals are organic compounds that flow like a liquid
while maintaining a long range orderliness of a solid.The
molecules of liquid crystal compound are in the form of long cigar
shaped rods.
Liquid Crystal
Types
Based on the orientation of these rods–like polar molecules,
the liquid crystals are classified into three basic types. They
are smectic, nematic and chloesteric.
(i) Smectic
The Smectic phase consists of flat layers of cigar shaped
molecules with their long axes oriented perpendicular to the
plane of the layer. The molecules within each layer remain
oriented within each layer and do not move between layers.
This most ordered smectic mesophase structural model.
(ii) Nematic
The nematic phase also has molecules with their long
axes parallel to each other, but they are separated into layers.
In the nematic mesophase, while the molecules maintain their
orientation, the individual molecules can move freely up and
down.
The nematic liquid crystal molecule consists of two
benzene rings linked with a central group. A typical example is
4-methoxybenzenylidene-4-butylanaline
(MBBA).
The
nematic liquid
exhibits crystalline property over the
temperature range 20°C to 47°C.
(iii) Chloesteric
Chloesteric mesophase can be defined as a special type of
nematic in which the thin layers of mostly parallel molecules have
their longitudinal axes twisted (rotated) in adjacent layers at a
definite angle. This is the most ordered phase. Each layer is
basically nematic.
LIQUID CRYSTAL ORIENTATION
In LCD’s two preffered orientations of LC molecules are
used (i). Homeotrophic[With long axis of the molecules parallel to
the glass plates and electrodes] and
(ii). Homogeneous[With long axis of the molecules perpandicular
to the glass plates and electrodes]
LC molecular orientations
The dielectric layer near the electrodes has preferred
orientation which in turn aligns LC molecules. The top and
bottom dielectric layers are by 90° with respect to one
another. Therefore the direction of the crystal is rotated 90°
with respect to the bottom of the liquid crystal. The liquid
crystal thus acts like a set of polarisers whose optic axes are
parallel to each other in the presence of electric field and in
crossed position in the absence of electric field.
LC molecular orientations
The orientation of the LC molecules parallel to the glass
plates is achieved by the deposition of a layer of dielectric
over the transparent electrodes.
The dielectric layer near the electrodes has preferred
orientation which in turn aligns LC molecules. The top and
bottom dielectric layers are by 90° with respect to one
another. Therefore the direction of the crystal is rotated 90°
with respect to the bottom of the liquid crystal. The liquid
crystal thus acts like a set of polarisers whose optic axes are
parallel to each other in the presence of electric field and in
crossed position in the absence of electric field.
Effect of electric field
The fundamental property of LCs that makes them useful as
display device is that they are sensitive to an external electric
field.
The nematic liquid crystal finds its applications increasingly in
electro-optic devices since their molecules can be aligned by
electric and magnetic fields to produce sufficient change in their
optical properties. Liquid crystal molecules rotate as a result of
external electric field. The behaviour of initially ordered liquid
crystal material due to increase in electric field.
Behaviour of LC molecules in an electric field
Let ‘E’ be the electric field applied in a direction perpendicular
to the liquid crystal / solid interface. Also let ‘Ec’ be the electric
field strength at which LC molecules change from homogeneous
order to a homeotropic type.
When E<Ec the ordering existsFig-a[Above]. If E<Ec the liquid
crystal molecules away from the electrode begins align along the
field direction as in Fig-b[Above]. When E>>Ec then most of the
molecules align along the field direction as in Fig-c[Above].
LCD cell construction
The most important structure of twisted nematic
mesophase liquid crystal cell. It consists of a thin layer of
LC material between two glass plates that are fused
together. The thickness of the LC is 10 to 25 m. The two
glass plates have transparent electrodes on their inside
faces made of conducting material indium tin oxide.
LCD operation
The working of LC display device is shown in above fig.
The cell is assembled so that LC molecules undergo
90° twist from the top plate to the bottom plate. The cell is
sandwiched between two polarisers with their polarisation
direction is parallel to the LC direction of each plate.
The incident unpolarised light on the cell is polarised
linearly as indicated and undergoes 90° rotation as it passes
through the LC before exiting the bottom of the polariser. In
this mode of operation, frequently used in LCDs, the cell is
transmissive, in the absence of electric field.
When voltage is applied to the
electrodes the LC molecules will align with field.
Now the incident light do not undergo rotation in
polarization direction due to liquid crystal and
therefore absorbed by the exit polarizer. Thus
the twisted nematic liquid crystal cell is opaque
in driven state and transmissive in non driven
state.
The LC display device can be operated either in
transmission or reflection mode. In the reflector mode, a
reflector is placed below the bottom of the polarizer. With no
field, therefore, the device reflects the incident light and
appears bright. When the field is applied, the direction of
polarisation of light travelling across the cell is not rotated and
cannot pass through the second polarizer and the device will
appear dark.
Limitation of twisted nematic displays
(i) Viewing angle is restricted to 45°
(ii) Use of polarisers reduces the maximum amount of light
that can be reflected.
Super twisted nematic displays
They are basically TN displays only but they have a
twist of 90° to 270° from top to bottom plates, Super twisted
nematic displays have greater image contrast and wider
range of viewing angles. High resolution displays are
developed using STN.
Comparison between LED and LCD
LED
LCD
Demerits
Merits
S.No.
1
Cost is high compared to LCD
Cost is very low.
2.
Not suitable for large area display
Suitable for large areas display
3.
High consumption Power (milliwatts)
Low power consumption (microwatts)
Merits
Demerits
4.
Operating temperature is 0° to 70°C.
Operating temperature is 10°C to 47°C.
5.
Response time is in nano seconds
(10-9 sec)
Response time is in microseconds (10-6 sec)
6.
Intensity of light can be controlled
Intensity of light cannot be controlled.
7.
Different colour displays are available at low cost.
Colour displays will not be available at low cost.
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