Based on structural and bonding features, a new classification scheme of superconductors is proposed to classify them into three classes： three-dimensional, two-dimensional and molecule-assembled superconductors. The ＇sandwich model＇ for the high-Tc euprates is extended to a ＇binary structure model＇： i.e., the crystal structure of most super- conductors can be partitioned into two parts, a superconducting active component and a supplementary component. Partially metallic covalent bonding is found to be a common feature in all superconducting active components, and the electron states of the atoms in the active components usually make a dominant contribution to the energy band near the Fermi surface. Possible directions to explore new superconductors are discussed based on the structural classification and the binary structure model.
The crystal structure and the superconductivity for samples Mg（B1-xCx）2 （0〈 x 〈0.09） prepared by a hybrid microwave synthesis have been investigated. The starting material B10C is also obtained by using the microwave method. The carbon can distribute uniformly in the Mg（B1-xCx）2 samples because boron and carbon are mixed on an atomic scale in the staring material B10C. The dependences of both lattice parameters and superconducting transition temperature Tc on carbon content accord with those reported in the literature. The upper critical field He2 at 20 K can be enhanced from about 4.3 T for x = 0 to 10 T for x = 0.05. The critical current density Jc of Mg（B0.95 C0.05）2 is 1.05×10^4 A/cm^2 at 20 K and 1 T.
