钇钡铜氧单畴超导块的多籽晶制备方法

顶部籽晶技术与熔融织构生长工艺的结合已经被证明是制备单一晶体取向的YBaCuO单畴超导体的成功方法.由于YBaCuO晶体的生长速度慢,使大尺寸单畴超导快的制备受到限制.采用多籽晶的方法可以减少生长大块材料的时间,但多籽晶诱导生长的大晶粒的边界通常是弱耦合的,材料的磁浮力和冻结磁场性能明显低于单籽晶生长的材料.本研究在采用多籽晶法制备YBCuO超导块时,通过在先驱物坯料的顶部增加成分梯度过渡层的方法成功地生长出单畴结构的超导材料.冻结磁通的分布为单峰状,证实材料为单畴结构,其磁浮力也达到单籽晶样品的水平.这些结果表明成分梯度层和多籽晶结合的工艺可以改善多籽晶生长样品的晶界耦合,是生长大尺寸单畴超导材料的一种可行方法.     

单畴熔融织构YBCO/Ag样品的生长工艺研究

本文研究了用冷籽晶技术制备掺Ag单畴熔融织构YBCO超导块材的工艺.通过引入有效的隔离层避免了籽晶的熔化.使用纯Y1.8粉作为隔离层的材料,并探索出同时施加两个隔离层的方法.对施加隔离层后大块单畴YBCO/Ag样品生长窗口的研究发现,加入隔离层后使慢降温上限温度升高,970℃到955℃是Y123/Ag系单畴生长的主要窗口.制备的直径26mm单畴超导块材的磁浮性能已达到12N/cm2(77K,0.5T).认为如果进一步提高熔化温度或延长保温时间将有助于抑制自发成核.     

钇钡铜氧单畴超导块的多籽晶制备方法

顶部籽晶技术与熔融织构生长工艺的结合已经被证明是制备单一晶体取向的YBaCuO单畴超导体的成功方法．由于YbaCuO晶体的生长速度慢，使大尺寸单畴超导快的制备受到限制．采用多籽晶的方法可以减少生长大块材料的时间，但多籽晶诱导生长的大晶粒的边界通常是弱耦合的，材料的磁浮力和冻结磁场性能明显低于单籽晶生长的材料．本研究在采用多籽晶法制备YBCuO超导块时，通过在先驱物坯料的顶部增加成分梯度过渡层的方法成功地生长出单畴结构的超导材料．冻结磁通的分布为单峰状，证实材料为单畴结构，其磁浮力也达到单籽晶样品的水平．这些结果表明成分梯度层和多籽晶结合的工艺可以改善多籽晶生长样品的晶界耦合，是生长大尺寸单畴超导材料的一种可行方法．     

多籽晶技术生长大尺寸YBCO单畴超导块材的研究

本文利用多籽晶技术成功地生长出直径53mm的大尺寸YBCO单畴超导块材.与传统的单籽晶引导生长相比,该多籽晶技术在大尺寸YBCO单畴生长方面更有效.通过同时使用四个SmBCO籽晶引导生长,大尺寸YB-CO单畴的生长时间得以缩减.实验结果显示多籽晶生长的大尺寸YBCO单畴块材比传统的单籽晶生长的同尺寸块材在超导性能上有所提高.磁通俘获场测量显示多籽晶样品的磁通俘获场比单籽晶样品的峰值更高,均匀性更好.在77K零场冷的环境下,多籽晶的磁悬浮力密度可达到14.6N/cm2,相比单籽晶的12.5N/cm2的磁悬浮力密度有了很大的提高.     

NdBCO厚膜籽晶生长YBCO块体材料

本研究采用液相外延生长(Liquid phase epitaxy,简称LPE)的NdBa2Cu3O7-δ(NdBCO)高温超导厚膜作籽晶,通过冷籽晶熔融织构的方法生长YBa2Cu3O7-δ(YBCO)块体材料.与通常使用的籽晶相比,NdBCO厚膜籽晶具有如下优点:相对于非超导材料籽晶(MgO 等),NdBCO具有和YBCO相似的晶体结构和晶格常数,并对体系不会造成污染;与单晶体及熔融织构获得的籽晶相比,LPE获得的大尺寸NdBCO厚膜,有可能得到大单畴结构的YBCO块体材料;与NdBCO薄膜籽晶相比,NdBCO厚膜具有更高的结晶性能.本文讨论NdBCO厚膜籽晶生长YBCO块材过程中籽晶的影响及其热力学稳定性等问题.     

同质冷籽晶技术生长SmBCO/Ag超导单畴块材的研究

在前期的研究中,利用NdBCO123冷籽晶技术成功制备出具有理想织构的SmBCO/Ag单畴块材.但由于高质量的Nd籽晶的供应难以得到保证,从而限制了SmBCO单畴块材的批量化制备.Ag可以降低SmBCO坯体的熔点,这为用SmBCO冷籽晶技术引导SmBCO/Ag坯体生长单畴块材提供了可能性.然而,实验中我们发现,坯体里面的Ag容易向籽晶内部扩散,导致籽晶熔融,对单畴块材的生长产生不利影响.为克服这困难,我们通过在坯体和籽晶之间加入一个SmBCO211缓冲层,有效地阻止坯体里面的Ag向籽晶扩散,成功实现了同质冷籽晶技术生长SmBCO超导单畴块材.     

直径53 mm熔融织构单畴YBCO块材制备研究

实验表明:通过对Y123/Y211前驱粉体粒度的调整以及采用Y123∶Y211=1∶0.3摩尔比的配制方案,在顶部籽晶诱导熔融织构生长条件下,可以提高单畴YBCO块材的起始生长温度Ts,从而有效地抑制了大尺寸单畴高温超导块材生长过程中的非籽晶成核现象. 利用这一技术,我们成功地生长出多块直径53 mm、厚18mm单畴YBCO高温超导块材,其中最大磁浮力达到302牛顿,最小为274牛顿,平均磁浮力密度达到12.9 N/cm2(77K、0.55T),达到和超过了该类材料的实用化国际标准.从宏观形貌上看,样品沿径向呈完全织构生长状态,沿轴向也基本呈完全生长状态. XRD和极图分析结果显示,整块样品具有较理想的织构化程度.这一结果意味着近期有望实现更大直径单畴YBCO超导块材的织构生长.     

二次单畴化制备GdBCO超导块材的方法及其性能

高质量单畴REBa_2Cu_3O_(7-δ) (REBCO)超导块材具有广泛的应用前景,但制备过程中极易产生大量多畴样品,致使成功率明显下降、成本显著提高,并制约其进一步批量化和实用化的进程.受顶部籽晶熔渗生长(TSIG)方法的启发,本文提出了一种对生长失败的GdBCO超导块材重新单畴化织构生长的新方法,即将生长失败的样品进行预处理后作为固相源坯块,然后采用改进后的TSIG法进行二次单畴化织构生长,并成功地制备出了一系列二次单畴化的GdBCO超导样品,同时,对样品的超导性能及微观结构进行了研究.结果表明,所制备的二次单畴化GdBCO超导样品的磁悬浮力均大于30 N,样品的捕获磁通密度均在0.3 T以上,捕获磁通效率高达60%以上,该结果为进一步发展低成本、高效率制备REBCO超导体的新方法提供了科学依据和新思路.     

Y2BaCuO5相含量对准单畴YBCO块材生长及其性能的影响

本文采用Y-123 Y-211合成粉体作为前驱物粉体,利用顶部籽晶熔融织构技术制备了最大顶角间距为34 mm正六边形YBCO准单畴块材,研究了Y-211含量对准单畴块材生长过程,以及对成材样品磁浮力性能的影响.实验结果发现:Y-211摩尔含量为30%样品的最大磁悬浮力明显高于Y-211含量为40%的样品.实验结果还发现,在同一烧结程序下,Y-211含量为40%样品出现单畴区未完全生长状态,而Y-211含量30%样品则呈现完全生长状态.这一结果可以解释两样品之间磁悬浮力性能的差异,而造成这一结果的原因可能是不同Y-211含量样品单畴区的起始生长温度不同所致.     

Y211相粒径对YBCO单畴块材的生长及磁通俘获场的影响

本文研究了Y211相粒径对YBCO单畴块材的生长及磁通俘获场性能的影响.能量色散谱(EDS)分析结果显示:YBCO熔体中的Y离子浓度随着Y211相粒径减小而增加.差示扫描量热法(DSC)热分析结果表明:YBCO熔体的起始结晶温度随着Y211相粒径减小而升高.前驱粉Y123/Y211中使用粒径细化的Y211相能使YBCO单畴块材的磁通俘获场明显增强.上述实验分析结果意味着使用细化的Y211粉体将有助于大尺寸、高性能单畴样品的制备.     

Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10^?4 atm (argon + 100 ppm O₂). A nickel film of ～35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter (αD _gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.     

Observation of defects and growth orientations of YBa2Cu3Ox thin films with YSZ buffer layers on Si

We have investigated defects and in-plate orientations of YBa₂Cu₃O_x thin films prepared by pulsed laser deposition (PLD) with YSZ as a buffer layer. The films showed c-axis oriented growth with the transition temperature T_co up to 87 K. Several types of defects including thermally induced cracks, grain boundaries and outgrowths were observed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The grain boundary provided a favorable path for crack propagation. The outgrowths nucleated on the YSZ surface grew with stoichiometric composition. According to X-ray diffraction (XRD) and HRTEM studies the YSZ buffer layer grew with the orientation relationship, YSZ110//Si110 and YSZ(001)//Si(001) up to the YBCO/YSZ interface. The superconducting YBCO films on top grew mainly with YBCO100//Si110 and YBCO(001)//Si(001), with some minor portions of YBCO110//Si110 and YBCO(001)//Si(001).     

The Relation Between Barrier Structure and Current Uniformity in YBCO Josephson Junctions

Electromagnetic transport measurements were combined with high-resolution electron microscopy observations to study the relation between structure and local critical currents in YBa₂Cu₃O_7–x (YBCO) Josephson junctions. The spatial variation of the critical current J(x) along the length of the boundary for interface engineered Josephson junctions and bicrystal grain boundary Josephson junctions was determined using a phase retrieval algorithm. The current distribution solutions were found to be highly uniform along the length of interface engineered junctions in contrast to solutions for grain boundary junctions. The latter showed significant spatial oscillations in the critical current as well as areas along the boundary that carried no current. Microstructural evaluation of interface engineered junctions fabricated using identical processing parameters to the junctions used for transport measurements suggest that the uniform current distribution is controlled by a highly uniform barrier layer formed between the superconducting electrodes. Microstructural evaluation of grain boundary junctions similar to the junctions used for transport measurements show considerable variations of the grain boundary structure within a single junction.     

Nb/Au/(1 1 0)YBa2Cu3O7−δ Josephson junctions: evidence against atomic scaled “corner junctions”

We report on I–V characteristics for in situ formed Nb/Au/(1 1 0)YBa₂Cu₃O_7−δ (YBCO) Josephson junction, where the homoepitaxial (1 1 0)YBCO film shows ultra-smooth surface morphology. The field dependence of critical supercurrent I_c shows anisotropic large junction behavior with normal Fraunhofer patterns expected from BCS model of d_x²−y² wave superconductors. This strongly suggests that the Nb/Au/(1 1 0)YBCO junctions cannot be regarded as atomic scaled corner junctions, in contrast with (0 0 1)/(1 1 0)YBCO grain boundary junctions to show “π-junction” with a pronounced dip near zero fields in field modulation of I_c.     

Single domain generation in YBCO textured samples

This paper discusses the partial melting technique applied to texture cylindrical YBa₂Cu₃O_6+x samples with Y₂BaCuO₅ additions and the experimental procedures developed for obtaining large single domains. The thermal texturing methods produce liquid migration which hinders the stable and continuous solidification process. In the present work different methodologies for avoiding capillary liquid absorption are discussed. The mechanism of single domain selection is studied and an interpretation proposed explaining the oblique angle that the (001) plane of the single domain forms with the sample axis. This mechanism is also compatible with the shape of the recrystallization front shown by the samples studied. In order to impose axial orientation of the (001) crystallographic planes, solidification experiments with textured YBCO seeds are carried out.     

电泳法制备ZnO/YBCO异质结及电学性质研究

采用电泳方法及高温煅烧工艺制备ZnO/YBCO异质结,XRD图谱显示ZnO具有c轴方向的择优取向。在SEM的截面图中可观察到ZnO与YBCO结合致密,放大500倍的表面形貌图呈现织构(textured)的微结构特征,放大5000倍的表面形貌图中可观察微米量级的六方晶粒。通过对ZnO/YBCO异质结的电学性质进行测试,结果显示整流特性。     

脉冲场励磁YBCO块材俘获磁通特性

研究了熔融织构法制备的单畴高温超导YBaCuO块材在脉冲场下的俘获磁通特性。分别测试了YBCO块材在不同强度脉冲场下的俘获磁场,并对实验结果进行了分析。实验发现:YBCO块材在脉冲场输入电压小于300V时的俘获磁场,随着外加脉冲场强度的增加而增加;而脉冲输入电压大于300V时,俘获磁场并没有继续增加。实验获得的超导块材俘获磁场图形呈现出明显的十字结构,这是由于块材采用顶部籽晶熔融织构生长技术、存在五个生长区域、不同生长区域及边界处临界电流密度的不同而导致的。     

金属有机物化学气相法制备YBCO超导薄膜研究

运用金属有机物化学气相沉积法(MOCVD),在LaAlO3(LAO)单晶上沉积YBa2Cu3O7-x(YBCO)超导薄膜。通过使用优化的进液装置,使金属有机源连续、稳定地输送至蒸发皿进行闪蒸。通过优化总气压、氧分压等生长条件,获得高质量的YBCO薄膜。在固定的温度条件下,调节反应总气压和氧分压,在总压为380Pa,氧分压为180Pa获得YBCO薄膜临界电流密度Jc=0.6MA/cm2。随着氧分压增大,YBCO薄膜产生a轴取向,(005)峰向左偏移,且薄膜中的Cu/Ba由1.0变化至1.63。在Cu/Ba=1.48时,YBCO薄膜结构与性能较优。     

Zn doping of YBa2Cu3O7 in melt textured materials: peak effect and high trapped fields

The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density j_c in the j_c vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.