Engineering Physics

LESSON 24. High Temperature SuperConductor (HTS)

Introduction to High \[{T_c}\] superconductors

Basic concept of High Temperature Superconductors (HTS)

• High-temperature superconductors are materials that behave as superconductors at unusually high temperatures.

• Symbolically, it is denoted by high- \[{T_c}\] or HTS.

• The first high- \[{T_c}\] superconductor was discovered in 1986 by the researchers Karl Müller and Johannes Bednorz.

• The “ordinary” or “metallic” superconductors usually have transition temperatures (temperatures below which they exhibit the superconductivity phenomena) below 30 K (-243.2°C ).

• The HTS have been observed with transition temperatures as high as 138 K (-135°C ).

• The compounds of Copper and Oxygen (so-called "cuprates") were believed to have HTS properties, and the term high-temperature superconductor was used interchangeably with cuprites superconductor for compounds such as Bismuth Strontium Calcium Copper Oxide (BSCCO) and Yttrium Barium Copper Oxide (YBCO).

• However, several iron-based compounds are now known to be superconducting at high temperatures.

History

The phenomenon of superconductivity was discovered by Kamerlingh Onnes in 1911, in metallic Mercury below 4 K (-269.15°C ). For seventy-five years after that, researchers attempted to observe superconductivity at higher and higher temperatures.

In 970, superconductivity was observed in certain metal oxides at temperatures as high as 13 K (-260.2°C ), which were much higher than those for elemental metals.

In 1987, K Alex Müller and J. Georg Bednorz were exploring a new class of ceramics for superconductivity. Bednorz encountered a Barium-doped compound of Lanthanum and Copper oxide whose resistance dropped down to zero at a temperature around 35 K (-238.2°C).

The superconductor with the highest transition temperature that has been confirmed by more than one independent research groups is mercury barium calcium copper oxide ( \[HgB{a_2}C{a_2}C{u_3}{O_8}\] ) at around 133 K.

Crystal structures of high-temperature ceramic superconductors

• The structure of high- \[{T_c}\] Copper oxide or cuprate superconductors are often closely related to perovskite  structure [A perovskite structure is any material with the same type of crystal structure as Calcium Titanium Oxide ( \[CaTi{O_3}\] )].
• One of the properties of the crystal structure of oxide superconductors is an alternating multi-layer of \[Cu{O_2}\] planes with superconductivity taking place between these layers.
• This structure causes a large anisotropy in normal conducting and superconducting properties, since electrical currents are carried by holes induced in the oxygen sites of the \[Cu{O_2}\]  sheets.

\[YBaCuO\] Superconductors Fig 15(1)

 

The first superconductor found with \[{T_c}\] > 77 K (liquid nitrogen boiling point) is Yttrium Barium Copper Oxide ( \[YB{a_2}C{u_3}{O_{7 - x}}\] )

The proportions of the 3 different metals in the \[YB{a_2}C{u_3}{O_{7}}\] superconductor are in the mole ratio of 1:2:3 for yttrium to barium to copper, respectively. So it is referred to as the 123 superconductor.

The unit cell of \[YB{a_2}C{u_3}{O_{7}}\] consists of three pseudocubic elementary perovskite unit cells.

Each perovskite unit cell contains a Y or Ba atom at the center: Ba  in the bottom unit cell, Y in the middle one, and Ba in the top unit cell.

Thus, Y and  Ba a are stacked in the sequence [Ba - Y  - Ba ] along the c-axis.

All corner sites of the unit cell are occupied by Cu, which has two different coordination, Cu(1) and Cu(2) , with respect to oxygen.

There are four possible crystallographic sites for oxygen: O(1) , O(2) , O(3) and O(4) . The coordination polyhedral of Y  and Ba with respect to oxygen is different.

The tripling of the perovskite unit cell leads to nine oxygen atoms, whereas \[YB{a_2}C{u_3}{O_7}\] has seven oxygen atoms and, therefore, is referred to as an oxygen-deficient perovskite structure. The structure has a stacking of different layers: \[\left( {CuO} \right)\left( {BaO} \right)\left( {Cu{O_2}} \right)\left( Y \right)\left( {Cu{O_2}} \right)\left( {BaO} \right)\left( {CuO} \right)\] .

One of the key features of the unit cell of \[YB{a_2}C{u_3}{O_{7 - x}}\] is the presence of two layers of \[Cu{O_2}\] . The role of the Y plane is to serve as a space between two \[Cu{O_2}\] planes.

In \[YBCO\] ,  the \[Cu\] - \[O\] chains are known to play an important role for superconductivity.

\[{T_c}\] is maximal near 92 K when \[x\] \[ \approx\] 0.15 and the structure is orthorhombic.

Superconductivity disappears at \[x\] \[ \approx\]  0.6, where the structural transformation of   \[YBCO\]  occurs from orthorhombic to tetragonal.

\[{B_i}\] - ,  \[Tl\]  - and \[{H_g}\] - based high- \[{T_c}\] superconductors

• The crystal structure of  \[{B_i}\] - ,  \[Tl\]  - and \[{H_g}\] - based high- \[{T_c}\] superconductors are very similar. Like \[YBCO\] ,

• It is perovskite-type feature and the presence of  \[Cu{O_2}\]  layers also exists in these superconductors.

• However, unlike \[YBCO\] , - \[Cu\] - \[O\] chains are not present in these superconductors. The  superconductor has an orthorhombic structure, whereas the other high-   superconductors have a tetragonal structure.

The \[B{i_2}\]-\[Sr\] -\[Ca\] -\[Cu\] -\[O\]  -system has three superconducting phases forming a homologous series as \[B{i_2}S{r_2}C{a_{n - 1}}C{u_n}{O_{4 + 2n + x}}\]  (n = 1, 2 and 3)

These three phases are \[Bi\]-2201, \[Bi\]-2212 and \[Bi\]-2223, having transition temperatures of 20, 85 and 110 K, respectively, where the numbering system represent number of atoms for   \[Bi\], \[Si\] , \[Cu\]  Ca and Cu respectively.

The two phases have a tetragonal structure which consists of two sheared crystallographic unit cells. The unit cell of these phases has double \[Bi - O\]  planes which are stacked in a way that the Bi atom of one plane sits below the oxygen atom of the next consecutive plane. The Ca atom forms a layer within the interior of the \[Cu{O_2}\] layers in both   \[Bi\]-2212 and   \[Bi\]-2223; there is no \[Ca\]  layer in the   \[Bi\]-2201 phase. The three phases differ with each other in the number of    \[Cu{O_2}\] planes; \[Bi\]-2201, \[Bi\]-2212 and \[Bi\]-2223 phases have one, two and three \[Cu{O_2}\] planes, respectively. The c axis of these phases increases with the number of \[Cu{O_2}\] planes (see table below). The coordination of the \[Cu\] atom is different in the three phases. The \[Cu\] atom forms an octahedral coordination with respect to oxygen atoms in the 2201 phase, whereas in 2212, the \[Cu\] atom is surrounded by five oxygen atoms in a pyramidal arrangement. In the 2223 structure, \[Cu\]  have two co-ordinations with respect to oxygen: one \[Cu\] tom is bonded with four oxygen atoms in square planar configuration and another   \[Cu\] atom is coordinated with five oxygen atoms in a pyramidal arrangement.

\[Tl - Ba - Ca - Cu - O\]   Superconductor:

• The first series of the \[TL\] -based superconductor containing one  \[TL\]–\[O\] layer has the general formula \[TlB{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 3}}\] whereas the second series containing two \[TL\] –\[O\] layers has a formula of \[T{l_2}B{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 4}}\] with n = 1, 2 and 3

  

• In the structure of  \[T{l_2}B{a_2}Cu{O_6}\] ( \[TL\]-2201), there is one \[Cu{O_2}\] layer with the stacking sequence ( \[TL\]–\[O\] ) ( \[TL\]–\[O\] ) ( \[Ba\]–\[O\] ) ( \[Cu\]–\[O\] ) ( \[Ba\]–\[O\] ) ( \[TL\]–\[O\] )( \[TL\]–\[O\] ).

• In \[T{l_2}B{a_2}CaC{u_2}{O_8}\] ( \[TL\]-2212), there are two  \[Cu\]–\[O\] layers with a \[Ca\] layer in between. Similar to the \[T{l_2}B{a_2}Cu{O_6}\] structure,  \[TL\]–\[O\]  layers are present outside the   \[Ba\]–\[O\]  layers.

• In \[T{l_2}B{a_2}C{a_2}C{u_3}{O_{10}}\] ( \[TL\]-2223), there are three \[Cu{O_2}\] layers enclosing  \[Ca\] layers between each of these.

• In \[TL\] -based superconductors, \[{T_c}\]  is found to increase with the increase in \[Cu{O_2}\] layers. However, the value of \[{T_c}\] decreases after four layers in \[TlB{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 3}}\] and in the \[TlB{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 4}}\] compound, it decreases after three \[Cu{O_2}\] layers.

\[Hg - Ba - Ca - Cu - O\] Superconductor:

• The crystal structure of \[HgB{a_2}Cu{O_4}\] ( \[Hg\] -1201), \[HgB{a_2}CaC{u_2}{O_6}\] ( \[Hg\] -1212) and \[HgB{a_2}C{a_2}C{u_3}{O_8}\] ( \[Hg\] -1223) is similar to that of  \[TL\]-1201, \[TL\]-1212 and \[TL\]-1223, with  in place of  \[TL\] .

• It is noteworthy that the \[{T_c}\] of the Hg compound (\[Hg\]-1201) containing one \[Cu{O_2}\] layer is much larger as compared to the one- \[Cu{O_2}\] -layer compound of thallium (\[TL\]-1201).

• In the \[Hg\] -based superconductor,  \[{T_c}\] is also found to increase as the \[Cu{O_2}\] layer increases.

• For \[Hg\]-1201, \[Hg\]-1212 and \[Hg\]-1223, the values of  \[{T_c}\] are 94, 128 and the record value at ambient pressure 134 K, respectively.

Table-1 High temperature superconductors

Formula	Notation	Tc (K)	No. of Cu-O planes in unit cell	Crystal structure
\[YB{a_2}C{u_3}{O_7}\]	123	92	2	Orthorhombic
\[B{i_2}S{r_2}Cu{O_6}\]	Bi-2201	20	1	Tetragonal
\[B{i_2}S{r_2}CaC{u_2}{O_8}\]	Bi-2212	85	2	Tetragonal
\[B{i_2}S{r_2}C{a_2}C{u_3}{O_6}\]	Bi-2223	110	3	Tetragonal
\[T{l_2}B{a_2}Ca{O_6}\]	Tl-2201	80	1	Tetragonal
\[T{l_2}B{a_2}CaC{u_2}{O_8}\]	Tl-2212	108	2	Tetragonal
\[T{l_2}B{a_2}C{a_2}C{u_3}{O_{10}}\]	Tl-2223	125	3	Tetragonal
\[TlB{a_2}C{a_3}C{u_4}{O_{11}}\]	Tl-1234	122	4	Tetragonal
\[HgB{a_2}Cu{O_4}\]	Hg-1201	94	1	Tetragonal
\[HgB{a_2}CaC{u_2}{O_6}\]	Hg-1212	128	2	Tetragonal
\[HgB{a_2}C{a_2}C{u_3}{O_8}\]	Hg-1223	134	3	Tetragonal

 

Properties of High Temperature Superconductors.

• HTS are brittle in nature

• The properties of the normal state of these materials are highly anisotropic.

• The Hall Coefficient is positive indicating the charge carriers are hole.

• Their behavior can’t be explained by BCS theory.

• The isotopic effect is almost absent in these materials.

• The magnetic properties of these materials are highly anisotropic.

• The effect of pressure is different on different materials e.g the application of pressure increases the critical temperature of  \[LBCO\] compounds but decreases the critical temperature of   \[YBCO\]

 Application of HTSC

Commercial quantities of HTSC wire based on \[BSCCO\] are now available and applications include transformers, fault current limiters, power storage, motors and fusion reactors etc.

HTSC cables-

• Lower voltage at the same power rating

• Low impedance power guiding

• Lower life cycle costs

• No soil heating

• No electromagnetic spray fields.

Electrical Machines-

• High quality of generated power

• Improved voltage stability

• More reactive power is available at a given rating

• Low total harmonic distortion

• Low maintenance

• Size and Weight reduction

• High efficiency and low operating cost

Fault current limiters-

Over 100times faster response time

Time adjust response function

Full power protection cycles

Magnets-

• Higher operational temperature range up to 77 K

• Compact and high weight

• Higher magnetic field of the order of 100 Tesla

• Greater stability and thermal efficiency

• Less complex cryogenic system

• Higher signal to noise ratio

Transformers-

• HTS turns power transformer into compact, highly efficient and environment friendly performers.

RF and Microwave Filters-

Medical Diagnosis-