Preparation Of YBa₂Cu₃O_7-δ Superconducting Materials By Sol-Gel Method

YBa₂Cu₃O_7-δ(YBCO)is one of the most promising high-temperature(high-Tc)materials. After its discorvering in 1987,great researches have been done on the high-Tc superconducting theory,and the development of fabrication process of the high-Tc superconducting powders, films,bulks and coatings.However,YBCO has complex compostions,anisotropic superconductivity.It is difficult to prepare YBCO single-crystal bulk,and both YBCO bulk and film should be of certain orientation.All of these bring out the difficulties and high cost in preparation of YBCO materials,thus preventing its larege-scale production.Therefere,it is necessary to put forward a low-cost chemical method for YBCO preparation,and it is also necessary to make clear its growth mechanism.In this paper,yttium,barium and copper organic salts are used as starting materials.By sol-gel method,YBCO sol,gel,powder,and film are prepared and corresponding mechanisms are explored.Results indicate that:The YBCO solution prepared using yttium,barium,and copper acetates as starting materials have better properties than that prepared using chlorides and nitrates as starting materials.When yttium,barium,and copper acetates are used as starting materials, diethylenetriamine,lactic acid and methacrylic acid can be used as complexing agents respectively,and thus three solutions(yttrium solution,barium solution,and copper solution) can be prepared.The three solutions are of good comparability.After the three solutions are mixed with certain ratio,transparent and stable YBCO sol is obtained.Dried at 70℃,the sol becomes gel.After the gel is rapidly pyrolyzed at 500℃,sintered at 850℃for 3 hours in oxygen,and annealled in oxygen at 500℃for 6 hours,superconducting YBCO powders with diameter of 0.5-1μm are obtained.By traditional metallurgical method,i.e.the powders are cold-compressed into bulk and sintered at high temperature,good YBCO superconduting bulks can be obtained.The YBCO sol mentioned above can be effectively used to prepare YBCO powders. Hower,the film prepared using this sol exhibits poor superconductivity due to the fact that the residue carbon,which exists as BaCO₃,is difficult to be removed out.To overcome this, some researchers use the yttrium,barium and copper trifluoroacetates as starting material to prepare YBCO sol(All-TFA method).However,in All-TFA method the trifluoroacetates will generate large amount of HF gas during the heat treatment,leading to the rough film surface,and bringing about the problem of environment pollution.To overcome this disadvantage,a low-fluorine sol-gel method is put forward in this paper using yttrium, barium,and copper acetates as starting materials.In the preparation of this low-fluorine sol, trifluoroacetic acid is only added into the barium solution,which also effectively prevents the formation of BaCO₃.Compared with All-TFA method,the fluorine-content is greatly decreased in this low-fluorine sol,and therefore shortening the duration of the heat treatment.Especially,the film surface is greatly improved using this low-fluoirne method. The film roughness is only 2nm,and the as-prepared YBCO films show high crictical transition temperature(90K),high critical current density(10⁶A/cm²).All of these factors reach a high level.By research on the film formation mechanism,it is found that four steps are involved in the heat treatment:firstly,the metal organic materials are pyrolyzed; secondly,inter-mediate phases are formed;thirdly,tetragonal YBCO phase is formed and lastly,orthorhombic YBCO phase is formed.