Superconductor Ceramics - School Of Materials & Mineral

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Nov 15, 2013 (3 years and 1 month ago)

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Superconductor
Ceramics

EBB443
-
Technical Ceramics


Dr. Sabar D. Hutagalung

School of Materials & Min. Res. Eng.,

Universiti Sains Malaysia

What's a superconductor?

Superconductors have two outstanding features:

1). Zero electrical resistivity
.



This means that an electrical current in a
superconducting ring continues indefinitely until a force
is applied to oppose the current.

2). The magnetic field inside a bulk sample is zero
(the Meissner effect)
.



When a magnetic field is applied current flows in the
outer skin of the material leading to an induced magnetic
field that exactly opposes the applied field.


The material is strongly diamagnetic as a result.


In
the Meissner effect

experiment, a magnet floats
above the surface of the superconductor

What's a superconductor?


Most materials will only superconduct, at very low
temperatures, near absolute zero.


Above the critical temperature, the material may
have conventional metallic conductivity or may
even be an insulator.


As the temperature drops below the critical
point,T
c
, resistivity rapidly drops to zero and
current can flow freely without any resistance.

What's a superconductor?


Linear reduction in resistivity as temperature is
decreased:





=

o

(1 +

(T
-
T
o
))


where

: resistivity and

: the linear temperature
coefficient of resistivity.


Resistivity:

s

~ 4x10
-
23



cm

for
superconductor.


Resistivity:

m

~ 1x10
-
13



cm

for
nonsuperconductor metal.

Meissner Effect


When a material makes the transition from the normal to
superconducting state, it actively excludes magnetic fields
from its interior; this is called the Meissner effect.


This constraint to zero magnetic field inside a
superconductor is distinct from the perfect diamagnetism
which would arise from its zero electrical resistance.


Zero resistance would imply that if we tried to magnetize a
superconductor, current loops would be generated to
exactly cancel the imposed field (Lenz’s Law).

Non
-
superconductor

B
int

= B
ext

Superconductor

B
int
= 0

B
ext

Magnetic Levitation


Magnetic fields
are actively excluded from
superconductors (Meissner effect).


If a small magnet is brought near a
superconductor, it will be repelled becaused
induced supercurrents will produce mirror
images of each pole.


If a small permanent magnet is placed above
a superconductor, it can be levitated by this
repulsive force.









Magnetic Levitation

Types I Superconductors


There are 30 pure metals which exhibit zero
resistivity at low temperature.


They are called Type I superconductors (
Soft
Superconductors
).


The superconductivity exists only below their
critical temperature

and below a
critical magnetic
field strength
.

Mat.

T
c

(K)

Be

0

Rh

0

W

0.015

Ir

0.1

Lu

0.1

Hf

0.1

Ru

0.5

Os

0.7

Mo

0.92

Zr

0.546

Cd

0.56

U

0.2

Ti

0.39

Zn

0.85

Ga

1.083

Mat.

T
c

(K)

Gd*

1.1

Al

1.2

Pa

1.4

Th

1.4

Re

1.4

Tl

2.39

In

3.408

Sn

3.722

Hg

4.153

Ta

4.47

V

5.38

La

6.00

Pb

7.193

Tc

7.77

Nb

9.46

Type I
Superconductors

Types II Superconductors


Starting in 1930 with
lead
-
bismuth alloys
, were
found which exhibited superconductivity; they are
called Type II superconductors (
Hard
Superconductors
).


They were found to have much higher critical fields
and therefore could carry much higher current
densities while remaining in the superconducting
state.

Type II
Superconductors

The Critical Field


An important characteristic of all superconductors
is that the superconductivity is "quenched" when
the material is exposed to a sufficiently high
magnetic field.


This magnetic field, B
c
, is called the critical field.


Type II superconductors have two critical fields.


The first is a low
-
intensity field, B
c1
, which partially
suppresses the superconductivity.


The second is a much higher critical field, B
c2
,
which totally quenches the superconductivity.

The Critical Field


Researcher stated that the upper critical field of
yttrium
-
barium
-
copper
-
oxide

is
14 Tesla

at liquid
nitrogen temperature (77 degrees Kelvin) and at
least
60 Tesla

at liquid helium temperature.


The similar
rare earth ceramic oxide
, thulium
-
barium
-
copper
-
oxide, was reported to have a critical
field of
36 Tesla

at liquid nitrogen temperature and
100 Tesla

or greater at liquid helium temperature.

The Critical Field


The critical field, B
c
, that destroys the
superconducting effect obeys a parabolic law of the
form:





where B
o

= constant, T = temperature, T
c

= critical
temperature.


In general, the higher T
c
, the higher B
c
.



















2
1
c
o
c
T
T
B
B
BCS Theory of Superconductivity


The properties of type I superconductors were modeled by
the efforts of
John Bardeen, Leon Cooper,
and

Robert
Schrieffer

in what is commonly called the
BCS theory
.


A key conceptual element in this theory is the pairing of
electrons close to the Fermi level into Cooper pairs through
interaction with the crystal lattice.


This pairing results from a slight attraction between the
electrons related to lattice vibrations; the coupling to the
lattice is called a phonon interaction.

BCS Theory of Superconductivity


The electron pairs have a slightly lower energy and leave
an energy gap above them on the order of .001 eV which
inhibits the kind of collision interactions which lead to
ordinary resistivity.


For temperatures such that the thermal energy is less
than the band gap, the material exhibits zero resistivity.


Bardeen, Cooper, and Schrieffer received the Nobel
Prize in 1972 for the development of the theory of
superconductivity.

JOSEPHSON EFFECT


JOSEPHSON EFFECT,
the flow of electric current, in the
form of electron pairs (called Cooper pairs), between two
superconducting materials that are separated by an
extremely thin insulator.


A steady flow of current through the insulator can be
induced by a steady magnetic field.


The current flow is termed Josephson current, and the
penetration ("tunneling") of the insulator by the Cooper
pairs is known as the
Josephson effect
.


Named after the British physicist Brian D. Josephson, who
predicted its existence in 1962.

Superconductor Ceramics


The ceramic materials used to make
superconductors are a class of materials called
perovskites.


One of these superconductor is an yttrium (Y),
barium (Ba) and copper (Cu) composition.


Chemical formula is
YBa
2
Cu
3
O
7
.


This superconductor has a critical transition
temperature around 90K, well above liquid
nitrogen's 77K temperature.

High Temperature Superconductor (HTS)
Ceramics


Discovered in 1986, HTS ceramics are working at 77 K,
saving a great deal of cost as compared to previously
known superconductor alloys.


However, as has been noted in a Nobel Prize publication
of Bednortz and Muller, these HTS ceramics have two
technological disadvantages:


they are brittle and


they degrade under common environmental influences.

HTS Ceramics


HTS materials the most popular is
orthorhombic
YBa
2
Cu
3
O
7
-
x

(YBCO)

ceramics.


Nonoxide/intermetallic solid powders
including
MgB
2

or CaCuO
2

or other
ceramics while these ceramics still have
significant disadvantages as compared to
YBCO raw material.

Table I: Transition temperatures in inorganic
superconductors

Compound



T
c

(K)

PbMo
6
S
8

12.6

SnSe
2
(Co(C
5
H
5
)
2
)
0.33

6.1

K
3
C
60

19.3

Cs
3
C
60

40 (15 kbar applied pressure)

Ba
0.6
K
0.4
BiO
3

30

La
l.85
Sr
0.l5
CuO
4

40

Nd
l.85
Ce
0.l5
CuO
4

22

YBa
2
Cu
3
O
7

90

Tl
2
Ba
2
Ca
2
Cu
3
O
10

125

HgBa
2
Ca
2
Cu
3
O
8+d

133