High Tc Superconductors

arousedpodunkΠολεοδομικά Έργα

15 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

200 εμφανίσεις

High Tc Superconductors

Kevin W. Cooper

Sajida Khan

Yuan Zhang

Superconductors


A Brief Introduction

Properties



DC electrical resistivity goes to zero at a low temperature. (Called Tc:
critical temperature)




A superconductor in a weak magnetic field will work like a perfect
diamagnet.




Meissner effect: magnetic flux present in superconductor is ejected
when superconductor cooled through Tc.




Superconductivity destroyed by strong enough magnetic field, Hc.


Types of Superconductors

Type I Superconductors (soft superconductors)



many types of pure materials have this behavior



Zn, Cd, Hg, etc.



Hc is too low for applications in superconducting
magnets


Type II Superconductors



tend to be alloys



has a region that is a mixture of superconducting and
non
-
superconducting states (vortex state)



Hard superconductors (large magnetic hysteresis)
MRI’s


Applications for Superconductors

MagLev Trains


Source:
www.superconductors.org

MRI (non
-
invasive health imaging)

Possible applications could be in power
generation and transmission.

TBCO


Tl
2
Ca
2
Ba
2
Cu
3
O
10


Ceramic cuprate superconductor


T
c
=125 K (Beyers, R.) or 120K (Kittel)


(2223)


Three Cu perovskite
-
like units separated
by Tl
-
O bi
-
layers.


Tc seems to go up with number of CuO
2

layers.

TBCO nominal unit cell

Source: R. Beyers, et. al.

Crystallography and
microstructure of Tl
-
Ca
-
Ba
-
Cu
-
O
superconducting oxides

Tl
-
O layer

Tl
-
O layer

Ba layer

CuO
2

layer

Ca layer

CuO
2

layer

Ca layer

CuO
2

layer

Ba layer

Tl
-
O layer

Body centered (b.c.)
tetragonal cell

Lattice Parameters:

a = 3.822(4) angstroms

c = 36.26(3) angstroms

Source: R. Beyers, et. al.

Crystallography and microstructure of Tl
-
Ca
-
Ba
-
Cu
-
O superconducting oxides

a

c

a

Note:
c

~ 10
a

Naming Scheme

2223

# of CuO
2

planes
within a
conducting block.

# of insulating layers between
conducting blocks. (Tl
-
O layers)

# of layers separating
CuO
2

planes.

# of spacing layers
between CuO
2

blocks.

Source:
www.superconductors.org

Perovskite

Source: http://cst
-
www.nrl.navy.mil/lattice/
struk.picts/e2_1.s.png

Perovskite like structure

Source: J. Phillips.
“Physics of High
-
Tc
Superconductors”

In TBCO this structure is
deformed to the
tetragonal structure of
the CuO
2

units.

Oxygen

Copper

Ba or
Ca

Bonding

The conduction occurs along the CuO
2

planes.

The bond type between Cu and O is covalent.

The average Cu oxidation state being +2 for ideal Tl
-
O bilayer systems.

Source: R. Beyers, et. al., and R.
Vasquez, et. al.

Bonding


ctd.



Ba and Ca are in the same group, thus having similar properties.



Both would tend to act as electron donors, forming an ionic bond
with the O (as indicated in R. Vasquez, et. al.)

References

R. Beyers, et. al. “Crystallography and microstructure of Tl
-
Ca
-
Ba
-
Cu
-
O
superconducting oxides” Appl. Phys. Lett.
53

(5), 1 August 1988.


C. Kittel. “Introduction to Solid State Physics” 8
th

ed. John Wiley & Sons, Inc.
(2005).


J. C. Phillips. “Physics of High
-
Tc Superconductors”. Academic Press, Inc.
(1989).


S. Subramanyam, and E. Raja Gopal. “High Temperature Superconductors”.
John Wiley & Sons. (1989).


R. P. Vasquez, et. al. “Chemical bonding in Tl cuprates studied by x
-
ray
photoemission” Physical Review B
60

(6), 1 August 1999.


www.superconductors.org

www.nrl.navy.mil/lattice/struk.picts/e2_1.s.png