单畴GdBCO超导体的制备及物性研究

【作者】 裴佳良；

【导师】 杨万民；

【作者基本信息】 陕西师范大学 ， 凝聚态物理， 2008， 硕士

【摘要】 自从高温超导材料发现以来,人们为开发它的实际应用进行了大量的研究工作。其中钇系超导体是高温超导材料中研究最多和应用最为广泛的超导体之一,受到普遍关注。本文主要研究钆钡铜氧（GdBCO或Gd-Ba-Cu-O）超导体的熔化生长工艺及其物理性能。本文采用顶部籽晶熔融织构工艺（TSMTG）生长单畴钆钡铜氧块材。在恒温场条件下,通过调整生长过程中的工艺参数,基本找到了单畴钆钡铜氧块材在空气气氛下生长的工艺制度。目前,已经能小批量化生长出直径为18mm的单畴样品,同时对GdBCO超导块材的显微组织结构和磁悬浮力等物理性能作了初步的研究。在制备GdBCO超导体的过程中,需要用到GdBa₂Cu₃O_7-δ（Gd123）和Gd₂BaCuO₅（Gd211）等初始粉体。采用传统的固态烧结法制备GdBa₂Cu₃O_7-δ和Gd₂BaCuO₅初始粉体,采用多次研磨多次预烧的制备工艺。在制备过程中采用差热分析和X-射线衍射分析方法确定初始粉的预烧结温度,其中Gd123的烧结温度为940°,Gd211为900°。用顶部籽晶熔融织构工艺制备GdBCO超导体过程中,主要分析了温度参数的对块材生长的作用和第二相掺杂比例对样品性能的影响。其中制备工艺主要研究了高温区保温时间、起始慢冷温度、慢降温速率等对超导体形貌和性能的影响。实验结果显示,200°/h的快升温速率、1h-2h的高温区保温时间、1040℃的起始慢冷温度、0.2-1.0℃/h的慢降温速率有利于制备出表面带有十字花纹、畴区面积比较大的GdBCO超导体。另外,我们还研究了Gd211的加入对GdBCO超导体的形貌和性能影响.研究表明,0.3mol、0.4mol、0.5mol、0.6mol的Gd211添加都能制备出表面带有十字花纹、畴区面积比较大的GdBCO超导体。为了抑制样品底部的自发成核以及MgO对样品的诱导成核,制备单畴样品时,样品的底部加入Y123+0.4molY211的混合物从而避免了底部的自发成核以及MgO对样品的诱导成核。采用环境扫描电镜对样品的显微组织结构和元素比例成份进行分析。结果表明GdBCO的123相晶体都具有层状结构。在GdBCO的晶体中分布着细小而弥散的211粒子。细小而弥散的211粒子的分布是高性能块材所具备的微观组织特征。利用四引线法测量Gd123中添加0.4molGd211样品的起始转变温度达到91K。在自行设计研发的三维磁场与磁力测试系统上,我们对GdBCO超导块材的磁悬浮力进行了测量。发现超导块材与永磁体之间最大磁悬浮力随Gd211相不同含量的添加先增大后减小。其中在添加0.4mol Gd211的GdBCO超导块材的最大磁悬浮力达到5.64N。更多还原

【Abstract】 A great of works have been done on the fabrication and applications of the high Tc superconductors.Gd-Ba-Cu-O is one of the most promising materials,which can be widely used for applications such as magnetic bearing,energy storage system,and advance motors and magnets etc. The preparation,microstructure and levitation force of Gd-Ba-Cu-O bulk superconductor have been studied in this paper.Batch production of Gd-Ba-Cu-O single domain bulks have been fabricated in small scale by cold-seeding melt growth process with a box furnace.Through adjusting the heat treatment parameter in the growth process,batch of Gd-Ba-Cu-O single domain bulks can be produced by top seeded melt textured growth（TSMTG） technique in an isothermal condition.Meanwhile the preparation,microstructure and levitation force of the GdBCO bulk superconductor have been studied in this paper.GdBa₂Cu₃O_7-δ（Gd123） and Gd₂BaCuO₅（Gd211） powders were synthesized by the solid state reaction method,and all of them were prepared with several times milling and sintering process.The differential thermal analysis（DTA） and XRD analysis results indicate that the propriety reacting temperature of Gd123 powder is in 940℃,and Gd211 is in 900℃.Temperature is a very important factor to the top seeded melt textured growth method （TSMTG）.So we primary investigated the temperature parameters and the addition content of Gd211 phase on the quality of the GdBCO sample bulks.The top seed melt-textured growth process including the heating and cooling rate,the dwelling time at high temperature,the start cooling temperature etc..The experiment results show that single-domain GdBCO bulks can be obtained with some reasonable processing parameters:the heating rate is 200℃/h,the dwelling time is 1-2h, the start cooling temperature is 1040℃,the cooling rate during the GdBCO crystal growth process is less than 1℃/h.Large domain or single domain GdBCO bulk can be synthesized with 0.3mol-0.6mol additions of Gd211 particles.In order-to depress the random nucleation during the preparation of single domain GdBCO superconductor bulks,YBCO material with lower melting temperature（compared with GdBCO） is used to realize this goal.The YBCO material was just pressed on the bottom of the GdBCO bulks in a thin layer.The YBCO layer is able to depress the random nucleation during the preparation of single domain GdBCO superconductor bulks.The microstructure of the GdBCO bulk was observed by the scanning electron microscope.The results show that the microstructure of GdBCO bulks is of highly anisotropy.There are fine Gd211 particles dispersion distributed in the Gd123 crystals.The levitation force of single-domain GdBCO bulk has been studied at liquid nitrogen temperature under zero field cooling condition.It is found that the maximum levitation force of the bulk are increasing first and then decreasing with the increasing of the Gd211 particle additon.The maximum magnetic levitation force was reached 5.64 N.The critical temperature of the sample was reached 91K.更多还原