Superconductor 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,Tc, 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-To)) 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 Bint = Bext

Superconductor Be xt Bint = 0

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.

Type I Superconductors Mat. Tc (K) Be Rh W 0.015 Ir 0.1 Lu Hf 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 Gd* 1.1 Al 1.2 Pa 1.4 Th Re 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, Bc, is called the critical field. Type II superconductors have two critical fields. The first is a low-intensity field, Bc1, which partially suppresses the superconductivity. The second is a much higher critical field, Bc2, 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, Bc, that destroys the superconducting effect obeys a parabolic law of the form: where Bo = constant, T = temperature, Tc = critical temperature. In general, the higher Tc, the higher Bc.

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 YBa2Cu3O7. 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 YBa2Cu3O7-x (YBCO) ceramics. Nonoxide/intermetallic solid powders including MgB2 or CaCuO2 or other ceramics while these ceramics still have significant disadvantages as compared to YBCO raw material.

40 (15 kbar applied pressure) Table I: Transition temperatures in inorganic superconductors Compound     Tc (K) PbMo6S8 12.6 SnSe2(Co(C5H5)2)0.33 6.1 K3C60 19.3 Cs3C60 40 (15 kbar applied pressure) Ba0.6K0.4BiO3 30 Lal.85Sr0.l5CuO4 40 Ndl.85Ce0.l5CuO4 22 YBa2Cu3O7 90 Tl2Ba2Ca2Cu3O10 125 HgBa2Ca2Cu3O8+d 133