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用化学溶液法快速制备YBCO超导薄膜的研究
【作者】 严武卫；
【作者基本信息】 陕西师范大学 ， 材料物理与化学， 2011， 硕士
【摘要】 在超导材料中,YBa2Cu3O7-x (YBCO)涂层导体以其独特的电性能被认为是当今最有前途能够实用化的第二代高温超导材料,因此引起了全球人员多年的深入研究。在YBCO超导薄膜的制备技术中,物理法和化学法各有千秋,但是化学法相对操作简单、成本低廉,而且更容易实现工业化。在传统的三氟乙酸-金属有机沉积方法(TFA-MOD)中,由于低温分解时间过长,影响了YBCO薄膜的制备速率,不利于实现工业化。因此,本论文开展了对快速制备YBCO超导薄膜方法的研究。在TFA-MOD法中,研究了水气压对前驱膜形态和YBCO薄膜的形态以及电性能的影响。水汽含量的大小不仅影响低温分解后前驱膜的形貌,而且影响YBCO薄膜的形貌和组织结构以及电性能的高低。在低温分解过程中,通入水汽量过大,薄膜容易吸湿,从而造成膜局部收缩、变形和开裂。在高温晶化过程中,水汽不充分,YBCO成核就会不完全,从而有中间相出现和大的孔洞出现,造成非超导相存在,引起电流路径的减少和电子场密度的降低；水汽量过大,会造成膜开裂和较大的孔洞存在,两者都能造成临界电流降低。实验中,水汽量为8%时,制备的薄膜形貌、组织和电性能较好。选用稳定无氟的铜盐来代替前驱液中的三氟乙酸铜成为快速制备YBCO的一个重要方向。实验中我们用硝酸铜来替代三氟乙酸铜,并加入螯合剂聚乙二醇(PEG)和蔗糖,研究了两种不同螯合剂对YBCO薄膜的形貌、结构和电性能的影响。PEG在低温能够保持溶液的稳定均一,而蔗糖在高温发生的交联作用不仅使膜表面平整、光滑,而且两种螯合剂螯合硝酸铜后形成的新型铜盐分解速度较快,只需80min.,分解后的氧化铜颗粒比加入单一螯合剂的要小。在760℃,晶化1h后,加入蔗糖和PEG的YBCO薄膜致密、几乎为c轴晶粒；YBCO(00l)峰趋向明显,没有其他杂相峰,而且薄膜的电性能较好。高分子辅助沉积法(PAD)是近年发展起来的一种新型制备薄膜的化学沉积方法,通常前驱液是水相溶液。我们采用硝酸钇、硝酸钡和硝酸铜前驱物,在水溶液中加入螯合剂蔗糖、PEG和聚乙烯醇配置前驱液。DSC/TG分析表明螯合剂有利于硝酸盐的分解温度降低,证实了前驱膜能够实现低温快速分解。通过优化工艺后,在100ppmO2/N2氧分压下,制备的YBCO超导薄膜致密,YBCO(00l)峰趋向明显,膜的转变起始温度TC=91K,转变宽度△Tc=2K；临界电流密度为IMA/cm2 (在77K、自场下)。DEA/YBCO改进型前驱液制备YBCO超导薄膜具有快速、简单和易操作的特点。DEA不仅能螯合三氟乙酸铜抑制三氟乙酸铜原子升华,而且有利于膜内应力释放均匀,保持薄膜性质的稳定。实验证实：涂敷后的薄膜在空气中,低温分解温度325℃,保温90s,制备的前驱膜形貌较好,接着在晶化温度800℃和晶化时间70min制备的YBCO膜致密和电性能较好,该方法为涂层导体的长带生产提高了实验依据。对DEA/YBCO前驱液采用高温烧结一步法快速制备YBCO超导薄膜有利于简化制备工艺过程。我们省掉了低温分解这一独立过程,而是直接经行高温烧结阶段。因为低温分解时间较短,可以通过高温过程中400℃以前的缓慢加热和低氧气氛来补偿低温分解这一过程。通过DSC/TG热解曲线分析表明,形成的-Cu-DEA-Cu-大分子链使三氟乙酸铜分解温度得到提高,分解范围变宽,而且DEA大分子链有利于减少铜原子的挥发和过快分解。添加DEA后的溶液性质得到了改善,通过高温烧结一步法使整个制备时间缩短至少10h以上,而且在350℃通入水汽,晶化温度820℃和晶化时间60min的条件下,制备出的YBCO超导薄膜性能较好。高温烧结一步法制备的优点是制备过程得到简化,影响制备过程的参数更加容易控制,制备时间缩短,有利于工业化长带连续生产。
【Abstract】 During superconducting materials, YBa2Cu3O7-xYBCO) coating conductor is widely considered to be the most promising high temperature superconductor of the second generation for its unique properties. Many researchers study its scale production and applications for a couple of decades. The physical method and chemical method are two important approaches for the preparation technology of YBCO superconducting film. Chemical method is more relatively simple operation, low cost, and more easily opertaion to implement. In traditional TFA-MOD (Metal Organic Deposition using Trifluoroacetates), the long calcinations time of YBCO film leads to a low production and needs to be solved in order to carry out a fast long tape production. Therefore, we studied the fast preparation methods of preparing the YBCO superconducting film in this thesis.In TFA-MOD, we studied the influences of the water partial pressure on properties, such as precursor film morphologies, YBCO films microstructure and the performance of the YBCO film. It was found that the range of P (H2O) not only affected the film morphologies after calcinations, but also influenced morphology, structure and performance of YBCO films. If the P (H2O) was too large to be moisture absorption during the decomposition process, it caused partial shrinkage, deformation and cracking in film. If the P (H2O) was lower in the crystallization process, the YBCO nucleation didn’t grow completely, thus the non-superconducting-intermediate phases and large pores appeared, which led to not only the reduction of a current path area, but also the concentration of an electric field. If the films were treated under higher P (H2O), small pores and cracking film are existed, which suggested to reduce Ic. It was found that the YBCO films with better morphologies, microstructure and Jc were attained in P (H2O) of 8%.It was an important direction of higher YBCO preparation rates to choose stabile Cu-salt without F instead of TFA-Cu precursor. We studied the influences of two different chelating agents on the appearance, structure and performance of YBCO film, by replacing TFA-Cu whith nitric copper nitrate added chelating agent consisting of polyethylene glycol (PEG) and sucrose. PEG can help to maintain solution uniformity during decomposition. On the other hand, a cross-linking reaction during sucrose caramelization made the film surface smoother and flatter. The new copper salt, that is, two kinds of chelating agent coordinates with the copper nitrate, decomposed faster, just only 80 min., and smaller sizes of the copper oxide particles after decomposition appeared, compared with a single chelating agent added. After the crystallization process at 760℃, 1h, the YBCO films after adding the sugar and PEG had a better characteristics, such as a low pore, no a-axis grains, stronger YBCO(00l) peak, non-superconducting phases and better superconducting properties.A polymer-assisted deposition (PAD) process developed in recent years is a new solution deposition technique of preparing film. Generaly, the precursors were resolved in aqueous solution. A precursor solution was prepared by dissolving nitrates of Y, Ba and Cu in aqueous solution with the chelating agent of sucrose, PEG and polyvinyl alcohol. The DSC/TG thermal analysis indicated the presence of polymers and nitrates in the precursor resulted in a suppression of the nitrate decomposition temperatures. After identifying the optimal process condition of O2 pressure, crystallization at 100ppmO2/N2 resulted in high-quality YBCO films with denser morphologies, high phase purity, stronger YBCO(00l) peak, Tc～91K and Jc～1MA/cm2(77K, OT).The improved precursor solution DEA/YBCO for preparing YBCO superconducting film showed rapid, simple and easy fabrication characteristics. DEA (Diethanolamine) not only chelates TFA-Cu resulted in avoilding sublimation of Cu-TFA, but also prevents film buckling by relaxing stress gradients and make film stable. The as-deposited films pyrolyzed at 325℃in air for 90s showed smooth surface morphologies, and then, the YBCO film with preferable density and performance after crystallization temperature of 800℃,60min, was successfully obtained. This YBCO solution deposition method provides an experimental basis for production of commercializable coated conductor wires.Crystallization one-step via a diethanolamine-modified trifluoroacetic precursor solution was a simple process for solution deposition method of YBCO superconducting film. Because the time of low temperature decomposition was relatively short, we can compensate for the process of the low temperature decomposition through high temperature process before 400℃at slow heating and low oxygen atmosphere. It was found that DEA appeared to extend the thermal decomposition range of the -Cu-DEA-Cu- molecular polymer chain and prevented Cu-TFA from sublimating through the DSC/TG analysis. The YBCO precursor solution was improved by DEA, which made the preparation time shorten 10h and above at least, and the YBCO superconducting film prepared by water gas at 350℃, the crystallization at 820℃for 60 min exhibted good performance. The crystallization one-step for preparation is the advantage of simple process, easier control parameters and shortened preparation time, and is helpful for fabrication of long-length YBCO tapes with high performance using the reel-to-reel system.