Abstract: The Bi_2(Sr,Ca)_3Cu_2O_x system superconductor was prepared by different melting procedures and after treatment.The effects of cooling rate,annealing temperature and time,as well as melting procedures on phase transformation were investigated.The structure and properties were examined by X-ray diffraction,optical microscopy,EPMA,DTA,TGA and measurements of electric and magnetic properties.When the melt of nominal composition Bi_2(Sr,Ca)_3Cu_2O_x solidified in air,the Bi_2(Sr,Ca)_3Cu_2O_(8+δ)(2212)superconducting phase is stable,the easily solidified product including Bi_2(Sr,Ca)_2CuO_(6+δ)(2201)phase is metastable, and another may be amorphous if rapidly quenched.The last two may transform into 2212 phase by after treatment at an optimal annealing temperature ranging 780—870℃.While 2201 phase,prepared by the laser floating zone melting method,may easily transform into 2212 phase at layer or filament in shape which is favourable to improve the links among superconducting grains and critical current density.

Key words: Bi system superconductor, melt, stability, phase transformation