臺灣博碩士論文加值系統

本研究探討滲透法(Infiltration Growth, IG)製程及添加利用溶膠凝膠法(sol-gel)製備的Y2Ba4CuAgOy(Y2411(Ag))奈米等級添加物對Y-Ba-Cu-O(YBCO)超導體的影響，並與傳統頂端熔融製程(Top-Seeded Melt-Textured Growth, TSMG)的 YBCO超導體比較。研究結果指出，將CeO2添加進入IG法製備的YBCO超導體內能有效的細化超導體內Y2BaCuOy(Y211)相，能提升其低場下的臨界電流密度(critical current density，Jc)，其和傳統TSMG製程樣品的結果相似。另一方面，添加CeO2的IG製備YBCO樣品(IG with CeO2)出現了峰效應(peak effect)，即高磁場下的臨界電流密度獲得增加，此在YBCO系統是不常見的。微結構的結果指出IG with CeO2的樣品具有較高含量的Sm，Sm是高溫製程時從SmBCO晶種擴散進入YBCO超導塊材中形成(Y, Sm)BCO成份差異區導致峰效應的出現；然而峰效應在IG with CeO2樣品中呈現不均勻的分佈，主要是因為Sm在塊材中呈現不均勻的分佈所導致，其Sm在樣品內的分佈範圍從0.007 ~ 0.058wt%。Sm在樣品內不均勻分佈主要是受到液相在高溫時流動的影響，此外，可以發現當樣品中Sm含量大於0.015wt%時會有峰效應的出現。此峰效應分佈不均的現象可以藉著添加奈米級的Sm2O3於塊材中改善，使其內部不同位置樣品的Sm含量均高於0.015wt%而能都有峰效應的出現。IG製備的樣品因其收縮率較小的特性及具有峰效應，因此，樣品的擄獲磁場的範圍及最大值都能高於同樣生胚製備的TMSG樣品。
本研究利用溶膠凝膠法製備出相對於固態法較低溫度合成且尺寸較小的Y2411(Ag)，並將其添加進入YBCO超導體中，研究結果指出，超導性質，如Jc及擄獲磁場，均能有效的藉著添加Y2411(Ag)而增加，並會隨著其添加量的增加而增加，主要是因為SEM結果指出Y2411(Ag)能以奈米等級(10-20nm)分佈於Y123的基地相中形成有效的釘扎中心而提升YBCO超導塊材的釘扎能力，因而提升其超導性質。

This study investigates the superconductivity of single grain bulk Y-Ba-Cu-O (YBCO) superconductors via infiltration growth (IG) method and with the addition of nano-scale second phases of sol-gel derived Y2Ba4CuAgOy (Y2411(Ag)) and then compared with that of the undoped YBCO bulk by traditional top-seeded melt textured growth (TSMG) method. It was found that the Y211 phases in the CeO2 doped IG-YBCO sample were smaller and well-distributed, comparing with those of other samples. Therefore, the enhancement of Jc in the self-field was observed. The result was the same as that in TSMG-YBCO samples. On the other hand, the CeO2 doped IG-YBCO sample showed a superior critical current density Jc(H,T) with a peak effect. Microstructure analysis indicated that higher concentration of Sm was found in the CeO2 doped IG-YBCO sample. The Sm which dissolved from the SmBCO seed diffused into the bulk to form compositional fluctuations of (Y, Sm)BCO and was correlated to the effective pinning in high field regions (or peak effect) to improve the Jc(H, T ) in high fields. However, it was found that the peak effect was strongly spatial dependent. The ICP-MS results showed that the concentration of Sm ranged from 5.8x10-2 to 7.0x10-3wt% within the bulk. Peak effect was only observed in regions where the concentration of Sm was higher than 1.5x10-2 wt%. The spatial distribution of Sm was attributed to the way liquid (BaCuO2 and CuO) flowed during the melting process of the IG technique. In order to suppress the spatial dependence of the composition, nano-sized Sm2O3 particles were added to the precursor powders in this study. Microstructural analysis demonstrated that with the addition of nano-sized Sm2O3, the concentration of Sm could be enhanced to be higher than 1.5x10-2 wt% within the bulk. Therefore, peak effect was obtained throughout the bulk material. In addition, the maximum trapped field value and trapped field profile of the CeO2 doped sample grown by IG were larger than that of samples grown by TSMG using the same diameter of precursor pellets.
The sol-gel process was used to fabricated smaller Y2411(Ag) precursors at a relative low reaction temperature than the solid state reaction. It was seen that the enhanced Jc and trapped fields were obtained in bulk YBCO superconductors with the addition of Y2411(Ag) particles. Jc was observed to increase with an increasing Y2411(Ag) content, which was the same as for the trapped fields results. Microstructural observations showed nano-scale of 10-20 nm Y2411(Ag) particles distributed homogeneously throughout the sample. These nano-scale Y2411(Ag) particles resulted in the enhancement of Jc and the trapped fields in YBCO samples doped with sol-gel Y2411(Ag) particles. This demonstrated that the potential of the addition of Y2411(Ag) to the precursor composition for fabricating YBCO bulk superconductors with high performance.

摘要 ……………………………………………………………………………I
Abstract ……………………………………………………………………III
誌謝 ………………………………………………………………………………V
目錄 ……………………………………………………………………………VII
表目錄 ……………………………………………………………………………XI
圖目錄 ……………………………………………………………………………XII

第一章 緒 論…………………………………………………………………………1
1-1 前 言……………………………………………………………………………1
1-2研究目的……………………………………………………………………………2
第二章 理論基礎與文獻回顧…………………………………………………………3
2-1超導體的發展歷程與基礎理論……………………………………………………3
2-1.1 超導體的發展歷程……………………………………………………………3
2-1.2超導體的特性及分類……………………………………………………………6
2-1.3 Bean Model[8]……………………………………………………………………7
2-1.4 BCS和Ginzburg-Landau理論………………………………………………9
2-1.4.1BCS theory…………………………………………………………………………9
2-1.4.2 Ginzburg-Landau理論…………………………………………………11
2-1.5 界面能 (Energy of a Normal Metal - Superconductor Interface)……………………………………………………………………………………………12
2-2 Y-Ba-Cu-O高溫超導材料晶體成長……………………………………………15
2-2.1 熔融織構製程 …………………………………………………………………15
2-2.2 包晶凝固成長(Peritectic solidification)…………17
2-2.3 123晶體成長模式……………………………………………………………19
2-2.4.滲透成長法……………………………………………………………………21
2-3 Y-Ba-Cu-O高溫超導體…………………………………………………………22
2-3.1 晶體結構………………………………………………………………………22
2-3.2 Y-Ba-Cu-O系統相圖………………………………………………………23
2-3.3 Y-Ba-Cu-O系統超導特性…………………………………………………24
2-4 單晶粒Y-Ba-Cu-O熔融製程…………………………………………………25
2-4.1 頂端接種技術：晶種的影響…………………………………………………25
2-4.2製程溫度曲線的影響…………………………………………………………27
2-4.2.1過冷度的影響………………………………………………………………27
2-4.2.2最高製程溫度(Tmax)的影響………………………………………………29
2-5 超導體的釘扎特性及其性質的提升……………………………………………31
2-5.1 第二類超導體的釘扎………………………………………………………31
2-5.2 臨界電流密度(critical current density，Jc)…32
2-5.3 超導體內渦旋線的釘扎能力(pinning force of vortex)…33
2-5.3 非超導RE211相的釘扎能力…………………………………………………34
2-5.4 奈米添加物的引入…………………………………………………………36
2-5.4.1 添加不同種類的奈米RE211於超導體中…………………………………36
2-5.4.2 新穎、奈米級的第二相Y2Ba4CuMOy的引入……………………………38
2-5.4 擄獲磁場………………………………………………………………………39
第三章 實驗方法及步驟………………………………………………………………70
3-1 實驗材料…………………………………………………………………………70
3-2 實驗流程…………………………………………………………………………72
3-2.1 奈米前驅物(Y2Ba4CuAgOy )之製備……………………………………72
3-2.2 頂端接種熔融製程及滲透法成長塊材(Top-seeded melt-textured growth, TSMG and infiltration growth, IG)……………73
3-3 性質分析…………………………………………………………………………76
3-3.1 粉末之熱性質分析……………………………………………………………76
3-3.2 相成分之鑑定 …………………………………………………………………76
3-3.3 微結構觀察……………………………………………………………………76
3-3.4 超導性能的量測………………………………………………………………77
3-4 儀器設備…………………………………………………………………………78
第四章 結果與討論……………………………………………………………………83
4-1 前驅物粉末合成…………………………………………………………………83
4-1.1 Y123、Y211及BuCuO2粉末XRD繞射結果…………………………………83
4-1.2 溶膠凝膠法製備Y2411(Ag)奈米粉體……………………………………84
4-1.2.1溶膠凝膠法的優點…………………………………………………………84
4-1.2.2Polyacrylic acid (PAA)高分子前驅物法……………………………84
4-1.2.3熱差分析(DTA)結果………………………………………………………85
4-1.2.4XRD結晶相鑑定……………………………………………………………86
4-1.2.5粒徑形貌及分析……………………………………………………………87
4-2 滲透法成長YBCO單晶粒超導塊材………………………………………………92
4-2.1 塊材成長的表面形貌…………………………………………………………92
4-2.2 塊材的超導性質………………………………………………………………93
4-2.2.1 擄獲磁場……………………………………………………………………93
4-2.2.2 臨界溫度……………………………………………………………………93
4-2.2.3 臨界電流密度………………………………………………………………94
4-2.2.4 釘扎力 (Pinning Force, Fp)…………………………………95
4-2.3 微結構觀察……………………………………………………………………97
4-2.3.1 孔隙率………………………………………………………………………97
4-2.3.2 Y211相分布………………………………………………………………97
4-2.3.3 Sm含量……………………………………………………………………98
4-2.4 結論……………………………………………………………………………99
4-3單晶粒YBCO超導塊材的均勻性…………………………………………………109
4-3.1 塊材的不同位置……………………………………………………………109
4-3.2塊材不同位置的超導性質……………………………………………………109
4-3.2.1 臨界溫度…………………………………………………………………109
4-3.2.2 臨界電流密度……………………………………………………………110
4-3.2.3 釘扎力 (Pinning Force, Fp)………………………………………111
4-3.3 微結構觀察…………………………………………………………………111
4-3.3.1 不同位置樣品Y211第二相的分佈 ………………………………………111
4-3.3.2 不同位置樣品Sm含量……………………………………………………112
4-3.4 IG with CeO2塊材均勻性的改善………………………………………113
4-3.4.1 擄獲磁場…………………………………………………………………114
4-3.4.2 臨界電流密度與釘扎力…………………………………………………114
4-3.4.3 微結構的分析：Sm在塊材內的分佈……………………………………115
4-3.5 結論…………………………………………………………………………115
4-4 添加sol-gel製程的Y2Ba4CuAgOy粉末……………………………………133
4-4.1塊材成長的表面形貌…………………………………………………………133
4-4.2 超導性質的量測……………………………………………………………134
4-4.2.1 擄獲磁場…………………………………………………………………134
4-4.2.2 臨界溫度Tc……………………………………………………………………135
4-4.2.3 臨界電流密度Jc……………………………………………………………135
4-4.3 微結構的觀察 ……………………………………………………………………………136
4-4.3.1 XRD繞射結果……………………………………………………………136
4-4.3.2 SEM…………………………………………………………………………………136
4-4.4 結論…………………………………………………………………………137
第五章 結論…………………………………………………………………………148
5-1 低磁場下Jc……………………………………………………………………………148
5-2 高磁場下Jc……………………………………………………………………………149
5-3 塊材均勻性的改善……………………………………………………………149
5-4擄獲磁場能力的增加……………………………………………………………150
5-5 結論……………………………………………………………………………150
Reference…………………………………………………………………………………………153
 
表目錄
Table 2-1 Approximate size of Y2Ba4CuMOy phase particles observed in Y123 matrix of bulk superconductor.……………………………42
Table 2-2 Maximum trapped field Bo of bulk HTSC at 77K…………43
Table 3-1 List of raw materials and related manufactory companies…………………………………………………………………………………………………………………………………70
Table 4-1 Superconductivity and compositional analysis of TSMG and IG-YBCO samples………………………………………………………………………………………101
Table 4-2 The number of particles/103m2 and volume fraction of Y211 in CeO2 doped IG and TSMG-YBCO samples with different regions……………………………………………………………………………………………118
Table 4-3 Superconductivity and microstructural analysis of the CeO2 doped IG samples with and without the addition of nano-sized Sm2O3 and CeO2 doped TSMG-YBCO samples……………………119
Table 4-4 Summary of this work and prior study on Y2411(Ag) powders……………………………………………………………………………………………………………………………………139

 
圖目錄
Fig. 2-1 (a) The discovery of superconductivity of Hg by H.K. Onnes in 1911, (b) The evolution of critical temperature of superconductor.…………………………………………………………………………44
Fig.2-2 Flux penetration under various magnetic fields.………45
Fig.2-3 Typical hysteresis loops of real and ideal type II superconductor in magnetic measurement.…………………………………………………45
Fig. 2-4 Critical current density (Jc) can be deduced by the magnetization difference (DM) of the hysteresis loop………………………………………………………………………………………………………………………………………………46
Fig. 2-5 Density of the Gibbs free energy GSH in the vicinity of a normal metal-superconductor interface.………………46
Fig. 2-6 Spatial variations of the order parameter  and the magnetic field H in the vicinity of the NS interface for (a)  〈〈 1 and (b)  〉〉 1……………………………………………………………………………47
Fig. 2-7 Illustration of Top-seeded melt-textured growth (TSMG) method………………………………………………………………………………………………………………………48
Fig.2-8 Solidification methods for producing bulk high temperature superconductor crystals.…………………………………………………………48
Fig.2-9 The melting point of RE123 under 10-2 and 10-3 atm P(O2).……………………………………………………………………………………………………………………………………………49
Fig.2-10 Two kinds of solidification model near the interface during peritectic reaction (a) solutes transport through the solid (b) solutes transport through the liquid [21]………………………………………………………………………………………………………………………………………………50
Fig.2-11 (a) Sources of composition difference: C1 caused by the curvature of RE211, C2 due to undercooling and C3 due to temperature gradient around interfaces. (b) Composition difference profile indicated the driving force for the solute during solidification.………………………………………………………51
Fig. 2-12 The arrangement of the sample prior to infiltration and growth.…………………………………………………………………………………………52
Fig. 2-13 Microstructures of YBCO bulk samples grown by (a) IG and (b) conventional melt processing techniques.…………………52
Fig. 2-14 The final products with different shapes of bulk superconductors fabricated by IG technique.………………………………………53
Fig.2-15 Crystal structure of YBa2Cu3O7-：(a) Tetragonal phase ( =1 )；(b) Orthorhombic phase ( =0 ).………………………………54
Fig.2-16 Temperature dependence of the resistivity for orthorhombic and tetragonal phases in Y123.………………………………………55
Fig.2-17 Calculated oxygen contents in YBa2Cu3O7- compared with experimental data (open squares) at various temperatures and oxygen partial pressures.cBroken line indicates the solid phase orthorhombictetragonal transition.……………………………………………………………………………………………………………………………55
Fig.2-18 Dependence of Tc on oxygen content (7-) in YBa2Cu3O7-.………………………………………………………………………56
Fig.2-19 Liquidus lines of RE elements in the Ba3Cu5O8 melt under air atmosphere. (RE=Y ,Yb, Dy, Gd, Sm and Nd).………………56
Fig.2-20 Temperature-time-transformation (T-T-T) diagram for a solidifying phase.…………………………………………………………………………………………57
Fig.2-21 Different kinds of thermal profile in MTG process (a) Continuous cooling method (b) Isothermal growth (c) Two step undercooling method.………………………………………………………………………………………58
Fig.2-22 Correlation between critical current density, Jc and Y211 particle size in melt processed YBCO.………………………………59
Fig.2-23 TEM micrograph show the normal pinning centers (δl pinning) originated from non-superconducting crystalline defects, which results in the enhancement of Jc in low-field regions.……………………………………………………………………………………………………………………59
Fig.2-24 SEM morphology and the histograms of Y211 size distribution of YBCO bulk superconductors (a) without and (b) with CeO2 addition. It is clear seen that CeO2 additives is effective to reduce the size of the 211 second phase inclusions and make them well-distributed in 123 matrix.………………………………………………………………………………………………………………………………………60
Fig.2-25 (a) TEM and (b) particle size distribution of sol-gel derived Sm211; (c) TEM and (d) particle size distribution of sol-gel derived Nd422.……………………………………………………61
Fig.2-26 Jc(H, T) results of SmBCO bulk superconductors with the addition of different type additives at different temperatures.………………………………………………………………………………………………………………………62
Fig.2-27 TEM micrograph show the normal pinning centers originated from non-superconducting crystalline defects, which results in the enhancement of Jc in low-field regions.…………………………………………………………………………………………63
Fig.2-28 TEM bright-field image of nm RE211 doped samples. (a) A nanoscale periodic structure was observed in the matrix. The insert figure showed the SAD result of the periodic nano-structure, which is along the (001) direction. (b) TEM bright field image of another region. Periodic nano-structure and twin can be seen simultaneously.…………………………………………………………………………………………63
Fig.2-29 XRD for solid state reacted Y2Ba4CuMOy for M = U, Bi, W and Nb. All peaks are indexed to a cubic double perovskite structure as showed by insert illustration, suggesting that all these phases are iso-structural.………………64
Fig.2-30 Measured values of Tc (discrete, unconnected points) and Jc (points connected by the solid and dashed lines) as a function of Y2Ba4CuMOy phase content in the superconducting nanocomposite.…………………………………………………………………………64
Fig.2-31 The measured mean size of Y2Ba4CuNbOy and Y211 phase particles in Ba–Cu–O liquid as a function of time.……65
Fig.2-32 Scanning electron micrographs of superconducting nanocomposites with starting composition YBa2Cu3O7- + 10 mol% Y2Ba4CuMOy for (a) M = Ag, (b) M = Ru, (c) M = W and (d) M = Hf.……………………………………………………………………………………………………………………………65
Fig.2-33 Jc as a function of Y2Ba4CuAgOy phase inclusion volume fraction in a Y123/ Y2Ba4CuAgOy nanocomposite. For comparison, Jc of single grains containing only Y211 phase inclusions is included in the figure.………………………………………………………66
Fig.2-34 (a) Jc(B) curves for single grains with initial compositions 70 wt% Sm-123 + 30 wt% Sm-211 + 2wt% BaO2 and 70 wt% Sm-123 + 20 wt% Sm-211 + 10wt% Sm-2411 + 2 wt% BaO2. (b) Comparison of Jc(B) for GdBCO processed with andwithout Gd2411 addition………………………………………………………………………………………………………………67
Fig.2-35 Schematic of HTSC disks of (a) zero-field cooling and (b) field cooling.……………………………………………………………………………………………68
Fig.2-36 The effect of temperature on trapped-field distribution. The field was trapped between two 26.5mm-diameter YBCO disks with carbon fiber wrapping, resin impregnation and embedded Al.……………………………………………………………………………69
Fig. 3-1 Experimental flow chart of synthesis of Y2Ba4CuAgOy precursor powders by sol-gel method.…………………………80
Fig. 3-2 Schematic illustration of the (a) top-seeded melt-textured growth (TSMG) and (b) infiltration growth (IG) technique.………………………………………………………………………………………………………………………………81
Fig. 3-3 Thermal profile for the continuous cooling method.…………………………………………………………………………………………82
Fig. 4-1 The XRD results of Y123, Y211 and BaCuO2.……………………88
Fig. 4-2 (a) Repeating structure of PAA (b) Decomposition reaction of PAA at different temperature stage.……………………………89
Fig. 4-3 DTA curve of sol-gel derived Y2411(Ag) precursor powders.……………………………………………………………………………………………………………………………………90
Fig. 4-4 XRD for sol-gel derived Y2411(Ag) precursors calcined in air at different temperatures.…………………………………………90
Fig.4-5 Particle size distribution for Y2411(Ag) powder specimens. (D75：0.96m means 75% particles are smaller than 0.96m)………………………………………………………………………………………………………………………91
Fig.4-6 SEM for Y2411(Ag) precursor powders.……………………………………91
Fig 4-7 The morphology of grown samples of (a)TSMG, (b)TSMG with CeO2, (c)IG and (d)IG with CeO2.……………………………………………………102
Fig. 4-8 Counter plot of trapped field for (a) TSMG, (b) TSMG with CeO2, (c) IG, and (d) IG with CeO2 YBCO single grain samples grown from precursor pellets of 25 mm in diameter, and (e) trapped magnetic field profiles for these samples at liquid nitrogen temperature.………………………………………………103
Fig. 4-9 Normalized DC susceptibility, as a function of temperature for TSMG, TSMG with CeO2, IG, and IG with CeO2 samples……………………………………………………………………………………………………………………………………104
Fig. 4-10 Jc–H curves at 65 and 77 K for un-doped and CeO2 doped YBCO samples fabricated by both the TSMG and IG processes.……………………………………………………………………………………………………………………………105
Fig. 4-11 Pinning force, Fp(H), curves at 77 K for un-doped and CeO2 doped YBCO samples fabricated by both the TSMG and IG processes.……………………………………………………………………………………………………………………106
Fig. 4-12 Microstructures of YBCO bulk samples by (a) IG and (b) TSMG processes.…………………………………………………………………………………………107
Fig 4-13 SEM morphology and the histograms of Y211 size distribution of (a) TSMG, (b) TSMG with CeO2,(c) IG, and (d) IG with CeO2. (Y211: white area, Y123: dark area.)………108
Fig. 4-14 A schematic diagram illustrating the regions of top (T), bottom (B), center top (CT), center bottom (CB), side top (ST), and side bottom (SB) in the sample from where specimens to measure various properties are collected.…………………………………………………………………………………………121
Fig. 4-15 Normalized DC susceptibility, as a function of temperature for (a) IG with CeO2 and (b) TSMG with CeO2 samples with different regions.…………………………………………………………………122
Fig 4-16 Jc-H curves of specimens with different regions in the IG with CeO2 sample at 65K and 77K.………………………………………………123
Fig 4-17 Jc-H curves of specimens with different regions in the TSMG with CeO2 sample at 65K and 77K.…………………………………………124
Fig 4-18 Pinning force, Fp(H), curves at 77K for specimens with different regions in the (a) IG with CeO2 and (b) TSMG with CeO2 samples. The dash line is the position with the magnetic field of 2.5T.…………………………………………………………………………………………125
Fig. 4-19 SEM micrographs of the IG with CeO2 and TSMG with CeO2 samples with different regions by the IG and TSMG methods. The size distribution of Y211 particles corresponding to the position is shown in the insert of histograms. (Y211：White area, Y123：Gray area).………………………126
Fig. 4-20 Schematic drawing of the [Sm] distribution in the specimens with different regions in the IG with CeO2 and TSMG with CeO2 samples.…………………………………………………………………………………………127
Fig. 4-21 Schematic drawing of liquid flow and distribution of Sm in the YBCO sample for the IG steps.………………………………………128
Fig. 4-22 (a) The trapped magnetic field distribution at 77K of IG with CeO2 sample with the addition of nano-sized Sm2O3 addition and (b) the trapped file profiles at 77K for TSMG with CeO2, IG with CeO2 and IG with CeO2 sample with the addition of nano-sized Sm2O3.………………………………………………………………129
Fig. 4-23 Jc-H curves of specimens with different regions in the IG with CeO2 sample with the addition of nano-sized Sm2O3 at different temperatures.…………………………………………………………………130
Fig. 4-24 Pinning force, Fp(H), curves at 77K for specimens with different regions in the IG with CeO2 sample with the addition of nano-sized Sm2O3.…………………………………………………………………………131
Fig 4-25 Schematic drawing of the [Sm] distribution in different specimens in the Sm2O3 doped IG with CeO2 sample.…………………………………………………………………………………………131
Fig. 4-26 Sm concentration with different regions within the TSMG with CeO2, IG with CeO2, and IG with CeO2 sample with the addition of nano-sized Sm2O3 addition.…………………………132
Fig. 4-27 H (Fp,max) with different regions within the TSMG with CeO2, IG with CeO2, and IG with CeO2 sample with the addition of nano-sized Sm2O3 addition.…………………………………………………132
Fig. 4-28 The morphology of YBCO samples with the addition of various amounts of sol-gel derived Y2411(Ag).………………………140
Fig. 4-29 The trapped field distribution of (a) Y123/Y211, (b) Y123/Y211/CeO2 and samples with the addition of (c) 1mol%, (d) 2mol% and (e) 4mol% Y2411(Ag).…………………………………………141
Fig. 4-30 (a) The trapped field profile of YBCO samples with different compositions, and (b) Bt,max as a function of Y2411(Ag) concentration in the Y123/Y2411(Ag) nanocomposite.…………………………………………………………………………………………………………………142
Fig. 4-31 Normalized magnetic moment as a function of temperature of the samples with different compositions.……143
Fig. 4-32 Jc as a function of external magnetic field (applied parallel to c-axis) at 65K and 77K for single grain YBCO bulks with different compositions.………………………………144
Fig. 4-33 XRD results for single grain YBCO bulks with different compositions.…………………………………………………………………………………………145
Fig. 4-34 SEM morphology and the histograms of Y211 size distribution of (a) Y123/Y211 (b) Y123/Y211/CeO2 (c) Y123/Y2411(Ag), and (d) high resolution SEM of Y123/Y2411(Ag).…………………………………………………………………………………………………………………………………………146
Fig. 4-35 The particle size distribution of second phases of (a) sol-gel derived and (b) solid state reacted Y2411(Ag) in the YBCO bulks.…………………………………………………………………………………………147

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