tailieunhanh - Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 12

Tham khảo tài liệu 'encyclopedia of smart materials (vols 1 and 2) - m. schwartz (2002) episode 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | tions in power transmission nuclear magnetic resonance extremely strong magnets and high-speed computing. High-temperature superconductors such as YBa2Cu3O7 Bi2Sr2CaCu2O8 d and Tl2Ba2Ca2Cu3Ox are the focus of today s research 22 . Most ceramic superconductors are based on the perovskite structure. The structure of YBa2Cu3O7 will be used as an example to show the construction of its structure from anion-deficient perovskites see later section . A superconductor is characterized by three physical quantities. The first is the critical transition temperature Tc below which superconductivity appears. The second is the critical magnetic field Hc below which a superconducting body exhibits perfect diamagnetism and excludes a magnetic field. If the applied magnetic field is higher than Hc the materials revert to the normal state. Hc is temperature-dependent. The third is the critical current density Jc above which superconductivity is destroyed and the superconductor reverts to the normal state. Polycrystalline superconducting materials are limited mainly by low Jc due to the weak link between grain boundaries. Improving Jc in the field is the essential task of current research in superconductivity. THE FUNDAMENTAL STRUCTURAL CHARACTERISTICS OF ABO3 perovskite Perovskite and related structures cover a large portion of smart materials and their crystal structures can vary to a large extent. The key questions are do the perovskitetype structures have smart properties and are there any intrinsic connections among the structures The answers may be found in the following areas 1 nonstoichiometry structure 2 . Vertex Sharing of Oxygen Octahedra In the ABO3 type structure the cation B is 4 is usually a transition-metal elemen to form a six-coordinated octahedron with it oxygen anions and itself is located at the c tahedron is the basic unit of these structure ric configuration of the arrangement of the o has the lowest interactive energy is a linea sharing connection. If the .