Model Synthesis for Ceramics: Superconductors, Magnets and Others
Presenter: Karl Muller
Published: July 2014
Age: 18-22 and upwards
Views: 736 views
Tags: ceramic; magnet; superconductor; cuprate
The discovery of superconductivity in hole doped La2CuO4 was motivated by the interest to find this phenomenon in an oxide. After the discovery near 35 K, copper oxides with transition temperatures of up to 131 K at normal pressure were found, i.e. above the boiling temperature of inexpensive liquid nitrogen. Therefore the interest in applications rose quickly. These occur in two different areas: In the weak current field as e.g. high Q resonators in communications or SQUIDS (Superconducting Quantum Interferometers) for accurate magnetic field detection. The other prospect was the high current field, as in generators of large magnetic fields, current transport in cables or current limiters to protect generators, to name just a few of the more important applications. Early on it became clear what the requirements for the superconducting cuprates are: pure phases, oriented crystallites for optimum superconducting current and the necessity to overcome the notorious brittleness of oxides. Over the past quarter of the last century this goal has been met by a global effort in many laboratories to such an extent that it is comparable to the quality achieved in semiconductor technology, especially for silicon. At the start, to reach such a high degree of technology, it appeared an impossible reachable goal. The products which meet these requirements have as a consequence a price. For the low current applications where there are no alternatives to the superconducting devices – such as square filters in communications or the sensitivity of SQUIDS – the price is not impeding their use. Also the HTS superconductor is deposited on a rigid surface and not prone to mechanical strain. The latter situation is considerably different in large current applications where the current is carried in more or less flexible cables. In the latter case two methods have been developed, the first one based on a nearly conventional drawing process and the other on thin film deposition. The first consists of introducing the HTS Material in powder form in a silver tube, which is then drawn in successive steps and annealed. The first generation wires were manufactured by this ‘Powder In Tube’ i.e. PIT method. It is restricted to the Bi_Cuprates because the crystals cleave easily. Cleavage takes place during drawing between neighboring BO planes weakly bound by van der Waals forces. Platelets having their large faces parallel to the CuO2 planes are formed. Current in the planes flows parallel to the platelets, and passes from one platelet to another through their larger faces. Multi-filamentary Bi-2223 and Bi-2212 wires and tapes are commercially produced in kilometer lengths. The second method consists of growing HTS films on suitably oriented substrates. It is used for YBCO or the ‘123’ family cuprates which do not cleave easily. The reduced anisotropy of YBCO makes it a more desirable superconductor than the Bi-2223 or 2212 because of the better vortex pinning. Here contact occurs between YBCO grains and is only through alignment of the grains present. It requires hetero-epitaxial growth on well-oriented substrates. Manufacturing these ‘coated conductors’ involves deposition of up to a dozen different layers starting with i.e. NiW alloy tapes or Haste alloy. The RABITS (Rolling Assisted Bi-axially Textured Substrate), or the IBAD (Ion Beam Assisted Deposition) methods to which various ‘buffer’ layers are added. To reach optimal current carrying properties in either, the powder in tube or the RABITS and IBAD processes requires a careful adjustment of the oxygen content of the doped cuprate used. This is carried out by letting the strand transverse a sequence of regions at various temperatures, and implies long-term (for tens of hours) heating of reactants in powder form at high temperatures (800-1200C) in a furnace, which is a highly time and energy consuming process and increases product costs. Therefore there is a significant effort to develop technologies to considerably reduce the solid-state reaction temperature and time. Here it appears that a novel formation procedure discovered at the Tiblisi State University in Georgia under the leadership of Prof. Alexander Shengelaya may be employed, especially in the production with the RABBIT or IBAD technologies: In a collaboration of the group of Shengelaya and the chemistry department in Tbilisi a solid-state synthesis of oxide materials was found, which enables a dramatic increase of the reaction speed along with lowering the temperature of the reaction. This method involves the irradiation of the mixture of starting oxides by light in a broad spectral range from infrared to ultraviolet with intensities sufficient for starting the solid-state reaction between the reagents contained in the powder mixture. It was shown that the rate of the resulting reaction exceeds the conventional thermal solid-state reaction rate in a furnace by about two orders of magnitude in thin film HTS materials. This novel reaction method can as well be used to produce thin film magnetic materials. Because oxides become more and more important in other applications e.g. in catalysis it may receive sufficient attention.