【作者】 陈源清；

【导师】 赵高扬；

【作者基本信息】 西安理工大学 ， 材料科学与工程， 2008， 博士

【摘要】 钇钡铜氧YBa₂Cu₃O_7-δ（YBCO）被认为是最有应用前景的高温超导材料,自1987年YBCO被发现以来,人们除了在高温超导理论方面进行了大量探讨以外,在粉体、薄膜、块材以及涂层等的制备方法方面也进行了大量研究,但由于YBCO成份复杂,具有导电的各向异性,又难以形成大块单晶,无论是块材还是薄膜,都要制成特定取向的织构,导致材料制备难度增大、成本过高,难以适应产业化应用需求。因此探讨一种廉价的化学制备方法及其形成机理具有重要的理论意义和实用价值。本研究以含钇、含钡、含铜的金属无机盐为出发原料,探讨溶胶-凝胶法制备YBCO溶胶-凝胶、粉体,薄膜的可行性及材料的形成机理。研究结果表明:以Y、Ba、Cu的醋酸盐为起始原料所制备的溶胶,比采用Y,Ba,Cu的氯化盐,硝酸盐为起始原料所制备的溶胶具有更好的溶解特性和成膜特性。对于钇、钡、铜的醋酸盐,分别以二乙烯三胺、乳酸以及丙烯酸等作为络合剂,形成3种溶液,这三种溶液之间具有良好的相容性,将这3种溶液按照一定比例混合就会得到透明清亮、性能稳定的YBCO溶胶,将该溶胶在70℃下保温可使其凝胶化,进一步将该凝胶加热到500℃快速热分解,850℃氧气氛下烧结3小时后,再在500℃通氧气退火6小时,即可获得具有超导特性、粒径为0.5-1μm的超导粉体。采用通常的粉末冶金方法,即将该粉体冷压成块,高温烧结就可获得具有超导特性YBCO块体材料。上述溶胶在YBCO粉体制备方面展示了良好的性能,但在薄膜制备过程中由于易残留BaCO₃,影响了薄膜的超导特性,已有的研究采用三氟乙酸盐法来避免这一现象,但由于三氟乙酸盐法在热处理过程中产生大量HF气体导致薄膜热处理周期过长,薄膜表面质量差,污染环境的等问题。针对这一问题,该研究开发了一种低氟的溶胶-凝胶法,即以醋酸钇、醋酸钡、醋酸铜为出发原料,在含钡溶液制备过程中添加适量的三氟醋酸,能有效抑制BaCO₃的形成。和三氟乙酸盐法比较,该方法大幅减小了溶液中的氟含量,从而大幅缩短了薄膜的热处理时间,特别是该方法获得YBCO薄膜的表面质量大幅提高,其表面粗糙度达到2nm,临界转变温度Tc达到90K,临界电流密度达到10⁶ A/cm²,这些指标都达到了较高的水平。通过薄膜形成机理的研究,发现该方法制备的YBCO薄膜经历了有机物热分解、中间相形成,四方相YBCO的形核与长大和正交YBCO形成等4个阶段。更多还原

【Abstract】 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.更多还原