For the detailed investigation of the effects of the starting materials and oxygen atmosphere in annealing, the temperature dependence of dc resistance, Meissner effect, and the X-ray diffraction patterns are measured for sintered body of Ba₂YCu₃O_x prepared by the thermal decomposition method of organic or inorganic acid salts. Consequently, it was found that the values of T_c were very sensitive with both factors, whereas diamagnetic behaviors at liquid nitrogen temperature were not. These results imply the excess amount of oxygen is necessary to obtain continuous superconducting phase in the thermal decomposition process of organic acid salts.

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The superconducting YBa₂Cu₃O_x ceramics were producted using oxides and carbonate powders by the hot-pressing method in various atmosphere. The superconducting properties such as critical temperature (T_c) and current density (J_c) were measured, and X-ray diffraction patterns were discussed. The hot-pressed sample in oxygen-atmosphere had an orthorhombic crystal structure and exhibited a T_c of 93K. The J_c was of 115A/cm², which was 1.5 times larger than that of ordinary sintered samples.

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The preparation of superconducting YBa₂Cu₃O_2y through sol-gel method was performed using an acetate aqueous solution. Conditions of gelation and conversion of gel to YBa₂Cu₃O_2y ceramics were investigated, applying thermal analysis, infrared absorption spectroscopy and X-ray diffraction techniques. Transparent gel was obtained from the acetate solution with cation mole ratio of Y:Ba:Cu=1:2:3 by adjusting the initial pH of the solution. Formation of Cu metal, Cu₂O and BaCO₃ below 400°C was found to occur during heat treatment of the gel. Above 600°C the heated sample turned into a mixture of Y₂O₃, BaCO₃ and CuO, which finally transformed into the single phase of YBa₂Cu₃O_2y at 910°C. The resultant material showed good high T_c superconductivity.

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Reactor irradiation effects on the superconducting transition temperature (T_c) have been studied by the in-pile measurements of electrical resistance and inductance of the YBa₂Cu₃O_x before and after as well as during neutron irradiation. The reactor irradiations were performed for the periods of one and two weeks at 25 and/or 360K using the low temperature irradiation loop facility at KUR. The resistance and the inductance have been measured continuously during the cooling down and the warming up of the loop before and after the irradiation. It was found that the irradiation at the ambient (360K) temperature enhances broadening of the transition in the superconductivity in Y-Ba-Cu oxides, compared with the irradiation at the low temperature (25K), up to the fluence of 10¹⁵-10¹⁶ fast neutron/cm².

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The superconducting compound, Y-Ba-Cu-O, was prepared and treated (transfer of the sample from the furnace, mounting in the equipment and measurements, etc.) in the pure oxygen atmosphere in order to prevent absorption of moisture and carbon dioxide, as well as to hinder oxygen removal from the oxide. The electrical resistivities were measured under the initial oxygen pressure of 2.02×10⁶Pa. The oxygen pressing and heating treatment at 2.02×10⁶Pa for 24h at 363K was conducted for several times, and the effects of the treatment on the superconducting transition temperature (T_c) and on the resistivities were investigated. By the oxygen pressing and heating treatment for the Y-Ba-Cu-O system, the on set and off set T_c decreased, and the resistivities increased. On the other hand, the oxygen pressing treatment at the room temperature gave a constant T_c, while the resistivities increased. The dry air pressing and heating treatment decreased the off set T_c, and enhanced the resistivities greatly. By the dry air pressing treatment at room temperature, on the contrary, the same T_c was kept and the resistivities was slightly increased.

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The formation mechanism of superconducting Ba₂YCu₃O_7-δ crystals in the melt quenching method and the superconducting properties of the annealed samples were examined. The following results were obtained. Ba₂YCu₃O_7-δ crystals were formed by annealing the melt quenching samples at around 750°C in the air. It was necessary to supply oxygen for the formation of Ba₂YCu₃O_7-δ crystals. The bulk density of the samples annealed at around 900°C was 5.7g/cm³ and the pores which were regarded to be the path of oxygen were observed. The critical temperature T_{c, zero} of superconducting Ba-Y-Cu-O oxides ranged from 85K to 93K. The critical current density at 77K in the zero magnetic field for the sample annealed at 950°C for 24h in oxygen atmosphere was 35A/cm².

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The fabrication of ceramics is subject to the variables which are determined to some extent by the processing technique being used. The most common parameters are traditional, synthesis of the powder including purity of the raw materials, particle size distribution, powder reactivity, organic additives and rheology before firing, as well as optimum firing profile and atmosphere. An effort is being made to optimize all the variables to put these materials into commercial use. This article illustrates the processing-property relationships focusing on powder reactivity. The superconducting ceramics of high density, YBa₂Cu₃O_7-x was prepared by very careful mixing with small size (1-2mmφ) zirconia balls in ethyl-alcohol. The sintered body density of 5.7g/cm³ has been obtained by this method. However, T_c depends on the thickness of the polished surface.

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Co-decomposition of the mixture of Y, Ba, and Cu nitrates in the air at the atomic ratio of Y:Ba:Cu=1:2:3 led to the formation of single-phase orthorhombic YBa₂Cu₃O_7-y powders, of which the sintered pellets had the superconducting transition width within 1K. Applying X-ray diffraction and the resistivity measurements, this material was compared with the one produced from Y₂O₃, BaCO₃, and CuO by the solid state reaction.

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A new preparation method for the high T_c superconductor, Ba₂Y₁Cu₃O_7-δ ceramics has been developed by the combination of the conventional and the chemical powder processing methods. A single phase of Ba₂Y₁Cu₃O_7-δ was prepared by sintering at around 800°C for more than 6h in the air, which indicated low a oxygen pressure. The superconducting transition temperature measured by the resistivity change was elevated from 93K to 104K with decreasing the sintering temperature.

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Ba₂Y₁Cu₃(O, F)_7-δ compounds were prepared by solid-state reaction using BaCO₃, Y₂O₃, CuO powders and one of BaF₂, YF₃ and CuF₂⋅2H₂O powders. It was found by thermal and X-ray analyses that YF₃ was better fluoridized material to make the Ba₂Y₁Cu₃(O, F)_7-δ compounds compared with BaF₂ and CuF₂. Ba₂Y₁Cu₃(O, F)_7-δ compounds with a small amount of fluorine were a single phase with the orthorhombic structure like the Ba₂Y₁Cu₃(O, F)_7-δ compound. The superconducting transition temperature, T_c, and the lattice constants a₀, b₀ and c₀ were slightly changed by adding the fluorine content. The T_c depends on the lattice constant. The maximum T_c was obtained for a sample with about 0.65 and 1.3 at % fluorine and a T_c onset was about 97K. The thermal stability of the Ba₂Y₁Cu₃(O, F)_7-δ compounds was increased compared with the Ba₂Y₁Cu₃O₇ compound.

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Phase equilibrium in the La-Sr-Cu-O system has been studied mainly by the X-ray diffraction analysis. The equilibrium phase diagram of the La₂O₃-SrO-CuO system was drawn up at 1173K in air. We found two new compounds; Sr₃Cu₅O_x and La₄SrCu₅O_x. Sr₃Cu₅O_x has an orthorhombic structure with lattice parameters; a=3.956, 6=13.412 and c=11.475 in A. U. It was confirmed that the K₂NiF₄-type oxide (La_1-αSr_α)₂CuO_x is a solid solution of 0≤α≤0.5 in which the terminal compounds are La₂CuO₄ and LaSrCuO₄, respectively. In contrast with an orthorhombic La₂CuO₄ (α=0), (La_1-αSr_α)₂CuO_x of 0.1≤α≤0.5 has a tetragonal structure. (La_1-αSr_α)₂CuO_x coexists with CuO, La₄SrCu₅O_x and La₂SrCu₂O₆ for 0≤α≤0.05, 0.05≤α≤0.13 and 0.13≤α≤0.5, respectively. The dependence of lattice parameters of the solid solution (La_1-αSr_α)₂CuO_x on α was also determined.

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Ten YBa₂Cu₃O_7-y (YBCO) perovskite samples with varying oxygen deficiency (0.15<y<1) were prepared by changing the quenching temperature or annealing conditions. With increasing quenching temperature, the transition temperature (T_c) for superconductivity decreased. This is accompanied by increasing oxygen deficiency and changes in the lattice constants. The oxygen deficiency of the sample annealed under nitrogen atmosphere at 700°C was more than that of the sample quenched at 950°C. There were no differences observed between the oxygen deficiency, lattice constants, nor superconductivity of the sample annealed under oxygen atmosphere and that cooled slowly from 350°C (0.5°C/min) in air. Annealing under high oxygen pressure caused destruction of the YBCO structure and disappearance of the superconductivity. Considering all the experimental results obtained in this study, YBCO perovskite can be classified into three types: Ortho-I (y<0.2), Ortho-II (0.2<y<0.5), and Tetra (0.5<y<1). Ortho-I type shows superconductivity with a T_c of around 90K, while the T_c onset of Ortho-II is below 70K. Although oxygen deficiency and lattice constants change continuously, a discontinuous T_c change is observed when YBCO changes from Ortho-I to Ortho-II. Tetragonal type is semiconductive and is not superconductive above 4K. The small difference in the oxygen deficiency between Ortho-I and Ortho-II indicates that it is necessary to minimize the oxygen deficiency in order to obtain a uniform material with a T_c of 90K. However, annealing under oxygen does not modify the superconductivity of samples cooled slowly.

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In terms of complex susceptibility and Mossbauer spectroscopy, we studied the superconducting and related properties of GdBa₂(Cu_1-xFe_x)₃O_y(x≤0.04). By the measurement of complex susceptibility, we find that the magnetic response of the specimens depends very much on the amount of Fe impurities, and there exists at least three kinds of coupling phases in the disk sample, suggesting serious inhomogeneity at the grain boundaries. For x=0.04, the onset temperature of superconducting transition is suppressed down to about 69K. Some experimental evidences found by the Mössbauer spectroscopy are also presented. These findings are to be under qualitative discussions.

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Single crystals of high-temperature superconductor YBa₂Cu₃O_x have successfully been prepared using a CuO or a BaO-CuO flux method. Several shapes of crystals are obtained in a single batch. Two types of crystals having the same YBa₂Cu₃O_x composition are foud: one is thin plates which are semiconductive but not superconductine above 4.2K, and another shows a very sharp superconducting transition above 90K in the as-grown state. The former is supposed to form under a solution growth condition, whereas the latter by the use of a peritectic reaction between Y₂BaCuO₅ and surrounding liquid phase. As the former is extremely thin whereas the latter is thick, a large volume of block crystals can possibly be obtained in the latter reaction. For the purpose of obtaining crystals of higher quality, an experiment was held at various growth-temperatures and compositions of flux. As a result, growth temperature should be set at a little higher than the peritectic temperature. If the max. temperature is too high, the separation of Y₂BaCuO₅ from the co-existing liquid phase occurs because of the coagulation of the liquid at high temperatures, and this separation prevents the progress of the peritectic reaction. An application of CuO-BaO flux in the place of pure CuO effectively prevents the crystals from taking in flux. Due to the low melting point, CuO-BaO flux can easily be discharged from the crystals and the strong impetus to advance the peritectic reaction.

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Thin films of the superconductor YBa₂Cu₃O_7-δ (YBCO) were prepared on the yttria stabilized zirconia (YSZ) substrates by the dipping-pyrolysis process. The starting solution, produced by dissolving barium and copper naphthenates and yttrium stearate in the organic solvent, was coated on the substrates and heated up to 800°C. After annealing the films in O₂ at 800°C for 80h, the resistivities of the films were measured by the dc four probe method resulting superconduction at T_c=90K (onset) and 60K (end).

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The phase stability and the decomposition behavior for the high T_c superconductor, YBa₂Cu₃O_x (x=6.92), at 20°-1200°C in O₂, air, N₂ and H₂ were studied by means of the thermogravimetry (TG), the differential thermal gas analysis (DTGA), the differential thermal analysis (DTA) and the liquid N₂ quenching. In the air, O₂ and N₂, the phase transition and the decomposition of YBa₂Cu₃O_x take place according to the following five steps.
Orthorhombic-YBa₂Cu₃O_x→Tetragonal-YBa₂Cu₃O_x(Tetra)→Tetra, BaCuO₂→Tetra, Y₂BaCuO₅, BaCuO₂→Y₂BaCuO₅, BaCuO₂, CuO
The temperature and the oxygen content of specimens, when the phase transition and/or the decomposition take place, decrease along with the lowering oxygen partial pressure. Under hydrogen atmosphere, the decomposition takes four steps commencing with TG measurement. At the final step (around 900°C), YBa₂Cu₃O_x decomposes to Cu metal, BaO and Ba₃Y₄O₉. At the second step (450°C), Y₂O₃, Ba(OH)₂⋅H₂O, and Cu were already found. At the third (600°C) and the forth (750°C) steps, a phase unknown was detected. The formation of Ba(OH)₂⋅H₂O can be regarded to occur due to the reaction of BaO and H₂O. The decomposition process under hydrogen atmosphere is found to be very complicated.

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Raw materials for sintering of superconductor were synthesized by the spray drying method. Three kinds of atomized solutions were used: (1) solution of Ba-, Y- and Cu-nitrates, (2) its ammoniacal alkaline solution and (3) slurry made by the mixing of Ba-, Y-, Cu-nitrates solution diluted by the water-ethanol solvents with oxalic acid solution. Resulting powders were calcined up to 900°C. They formed perovskite structure at 850°C ((1), (2)) and 800°C ((3)). Calcinated powders from (1) and (2) showed strongly aggregated or cortinuous structure with many pores, whereas that from (3) was flocculated structure and homogeneity in the mass and each particles. In conclusion, (3) was optimum method to produce the raw materials for sintering of superconductor.

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The crystal structures and magnetic characteristics of the superconducting substance in YBa₂(Cu_1-xM_x)₃O_y, where M is 3 dtransition element and x=0, 0.01, 0.03, 0.10, were investigated. Substitution of Ni and Zn left the orthorhombic structure at x=0.10, whereas substitution of Fe and Co induced an orthorhombic to tetragonal transition at x=0.03. The valency of substituted Co ion was divalence, whereas the valency of other substituted magnetic ions was not determined for certain.

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We found that densification was enhanced by rare-earth element mixing in LnBa₂Cu₃O_7-δ where rare-earth elements were Y_1-xEu_x, Yb_1-xEu_x and Yb_1-xDy_x. And at 0.5 for x, we obtained fairly dense samples with densities over 90% by sintering at 900°C, and they showed critical current densities as high as 500A/cm² at 77K. The mechanism for this densification enhancement has not been clear, but it seems to be related to the occurence of evaporation and condensation process at temperatures below the decomposition temperature.

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The thin films of the superconductor Yb-Ba-Cu-O were prepared by the 50Hz ac sputtering method. The resitivity of the film deposited on SrTiO₃ (100) substrate reached zero at 86K by annealing in oxygen atmosphere at 750°C. At annealing temperature as low as 600°C, the zero resistivity temperatures of the films prepared on SrTiO₃ (100) and (110) substrates were found to be 61K and 74K, respectively. The crystal structures of the films were analyzed by X-ray diffractometer and SEM and their variation depending on the substrate temperature and the orientation were noted. Some experimental data relating to the reduction of preparation temperature of the superconducting oxide films have been accumulated.

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High-T_c superconducting Y-Ba-Cu oxide thick films were produced on various substrates (quartz, sapphire, MgO and YSZ) by the screen-printing method. The reactions between the substrates and the screen-printed superconductor under several firing conditions were studied using scanning electron microscopy, EDX analysis and powder X-ray diffraction analysis. For the YSZ substrates, BaZrO₃ was generated at the interface between the superconductor and the substrate upon firing at 990°C in an oxygen atmosphere. As Ba ions diffused into the substrate, parts of printed ceramic changed to Y₂BaCuO₅. Thick-film densification and interfacial reaction occurred in the same temperature range. When the firing time at 990°C was made as short as possible, the greatest fraction of the screen-printed ceramic remained fully-dense YBa₂Cu₃O_7-δ. A superconducting thick film on YSZ with T_{c, onset}=89K, T_{c, end}=87K and J_c(77K)=30A/cm² was obtained. The rate of the thermal expansion of YBa₂Cu₃O_7-δ was also measured. The thermal expansion coefficient of YBa₂Cu₃O_7-δ was 18.7×10^-6K^-1 and had a maximum at around 600°C, near the orthorhombic-tetragonal phase transition temperature in air.

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The oxygen content in Ba-Y-Cu superconducting ceramics was determined by the charged particle activation analysis using the ¹⁶O(³He, p)¹⁸F reaction. Firstly, the accuracy of the method was evaluated by determining the oxygen contents in Al₂O₃, SrTiO₃ and BaTiO₃ with SiO₂ as the standard sample. The oxygen content in the materials of densities and average atomic numbers approximating those of SiO₂ was accurately determined by irradiation with ³He of various energies. Furthermore, with ³He of more than 15.2MeV, the determination of oxygen with 1% error was possible and independent of the density and the average atomic number. Secondly, thick target yields were measured for interfering nuclides produced by other constituent elements in the ceramics. The interference to the measurement of ¹⁸F radioactivity was significant with more than 15.2MeV of ³He, and the chemical separation of ¹⁸F was necessary for accurate determination of the oxygen content in the ceramics. The charged particle activation analysis with ¹⁸F chemical separation was used to determine the oxygen content in Ba-Y-Cu and Ba(Sr)-Y-Cu superconducting oxide ceramics. These ceramics were confirmed to be in a single phase by the X-ray diffraction method. The oxygen concentrations were determined to be 16.3% in Ba-Y-Cu and 16.5% in Ba(Sr)-Y-Cu; the molar ratio between Ba(Sr), Y, Cu and O was calculated to be 1.96:1.00:3.08:6.90 for the former, and 1.96:1.00:3.11:6.99 for the latter.

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YBCO superconducting ceramics prepared by the coprecipitation method, have been studied. It was found that the particle size of the ceramics is less than 0.5μm, and that a bulk density of more than 5.9g/cm³ is obtained by calcining at 850°C for 9h, followed by sintering at 920°C for 3h in O₂. The critical current density (J_c), measured by the standard four-probe resistive method was 200A/cm². This experimental value was comparable with the calculated J_c value from the magnetization curve using the critical state model. The result suggests that J_c is limited by the ratio of low superconducting phase and the weak flux pinning force in the ceramics, prepared by the coprecipitation method. These properties were compared with those ceramics prepared by the solid state reaction method, and has a particle size of more than 5μm.

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The phase stability and the change of the microstructure of the high-T_c superconducting oxide, YBa₂Cu₃O_7-x, at high temperatures in the air were investigated using two samples, a powder and a sintered material. On the basis of DTA analysis, which showed endothermic reactions at 977° and 1050°C in the air, the two samples were annealed at 950°, 990°C for the specified periods and, quenched into liquid nitrogen. The changes of the phase and the microstructure were determined by X-ray powder analysis and the SEM observation. It is confirmed that the tetragonal phase YBa₂Cu₃O_7-x can exist as a single phase at 950°C, and the phase and the microstructure change at 990°C. The YBa₂Cu₃O_7-x phase cannot exist, and decomposes to a crystalline phase of Y₂BaCuO₅ and a liquid phase of BaO⋅nCuO at 1070°C. On the basis of these results, the phase transformation and the phase diagram were discussed.

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A possible method for obtaining the superconducting YBa₂Cu₃O_y fiber, based on the sol-gel process, has been presented. The gel fibers were directly drawn from homogeneous viscous sol prepared from aqueous acetate solution of the composition, Y:Ba:Cu=1:2:3 in mole. The heat treatment of the gel fibers up to 910°C in air successfully resulted in the formation of black ceramic fibers of about 10μm in diameter. The scanning electron micrographs showed a rough and porous nature of the fired fibers.

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Microscopic observation has been conducted on newly discovered high Tc oxide superconductors using the polarized optical microscope. It was revealed that all the superconductors exhibited almost the same microstructure irrepective of the kind of the rare earth site. In the samples prepared in the usual sintering process, the grains are oriented at random and have rectangular shapes. In the sample with the orthorhombic structure, the grains contain twins, whereas no twin is observed in the tetragonal sample.

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Effects of fluorine treatment of YBa₂Cu₃O_7-x were investigated. The T_c was increased about 2 degrees by fleon treatment at 400°C. The T_c, however, was dropped about 2 degrees in a day of exposure in air at a room temperature into similar values for the sample without fleon treatment. The fluorine-doping to YBa₂Cu₃O_7-x have also been examined, but did not yield any higher T_c phases. Since the doped fluorides were hydrolyzed by H₂O contained in air at high temperature, the contents of fluorine in Y-Ba-Cu-O phase were gradually decreased during reaction, sintering, and/or annealing, and would become very small.

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The electrical properties and microstructures of the specimens, Y-Ba-Cu-O compound annealed in oxygen atmosphere after being fired in argon atmosphere at 800°-1000°C, were investigated. The electrical resistance at 77K showed zero to the specimens fired at temperature of 850°-950°C and then annealed at 900°C.

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The effects of MoO₃ addition on the electrical properties and the microstructures of Y-Ba-Cu-O system were investigated. The electrical resistivity at 300K grew along with the increasing addition of MoO₃ content, and the specimen without MoO₃ showed zero resistance but the specimens containing MoO₃ did not. The grain diameter was maximum at 1wt% MoO₃.

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The high T_c superconductors YBa₂Cu₃O_y: F were prepared from the mixtures of Y₂O₃, CuO, BaCO₃ and BaF₂, where the mixing ratio of BaCO₃ and BaF₂ was described as r=BaF₂/(BaCO₃+BaF₂). Substitution of oxygen by fluorine in YBa₂Cu₃O₇ caused the c-axis orientation in the composition range of r<0.2. Electrical resistivity of the samples annealed in O₂ showed the two step drops; the first small drop at about from 150 to 250K and the second sharp drop to zero at 90K. However, the sample of r=0.4 showed the zero resistivity in the temperature range from 200 to 250K. All these samples showed the diamagnetism below 90K.

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The effect of sintering temperature (890°-950°C), on the electric and the magnetic properties of the high-T_c superconducting ceramics in Y-Ba-Cu-O system was investigated. The specimens were sintered for 20 hours in the air. The highest susceptibility, -0.135emu/g was observed in the specimen sintered at 920°C. The highest T_c was obtained in the specimen sintered at 950°C, and its zero resistivity temperature, T_c-end was observed at 77K. Its apparent density was 5.8×10³kg/m³.

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The oxygen content y was measured in the superconductive ceramic Er₁Ba₂Cu₃O_y quenched at several temperatures. In the quenching process at 950°C the oxygen content went down from 6.85 to 6.24. The critical temperature where oxygen fugacity starts was about 500°C. The superconductive transition temperature was strongly affected by the oxygen content. In those results, no essential difference could be found between Er₁Ba₂Cu₃O_y and Y₁Ba₂Cu₃O_y systems.

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The effects of hydrogen ion shower on the properties of the superconducting Y-Ba-Cu-O ceramics thin films have been studied. The resistivity at the room-temperature, the superconducting transition temperature (T_c) and the crystalline structure changed by irradiation of the hydrogen ions as a function of ion dose. These changes can be considered to be attributed to the deoxidization by the hydrogen ions. Application of this ion shower technique will enable us to realize the fabrication of superconductive devices without serious damages.

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Fluorine (F) substitution effects on the superconducting properties are investigated with YBa₂Cu₃O_xF_y prepared by two different methods. At first, the sintered samples were prepared with the appropriate mixture of Y₂O₃, Ba₂O₃, CuO and several fluorides. Since the samples had strong peaks of BaF₂ phase together with the perovskite one in the X-ray diffraction patterns, it was difficult to obtain information on the F-substitution effects on the physical properties. Next, the samples were prepared by annealing YBa₂Cu₃O_x sintered pellets and CoF₃ powder in a sealed quartz tube. In this case, the crystalline state was a single phase of the orthorhombic perovskite structure. It was found that the diamagnetic susceptibility χ decreased concurrently to the increase of the F content and the tempreture dependence of χ was similar regardless of the presence of F-substitution. This indicates that the volume fraction of the superconductor phase decreased. In the resistivity measurement, the superconductivity was not observed down to 4.2K. These results suggests that the F atoms enter the crystalline grains from the surfaces and change the superconductivity state into a normal one in the vicinity of the crystalline grain surfaces.

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In this study, several samples with slight deviations from the metal ion stoichiometry of YBa₂Cu₃O_7-δ were synthesized at temperatures from 935° to 980°C. The extent of the single-phase region of the superconducting phase YBa₂Cu₃O_7-δ, and the effects of variation in composition on the electrical and the chemical properties were investigated. X-ray diffraction studies of samples with up to 10 at% excess or 1 at% deficit of Cu, and 1.5 at% excess or 5 at% deficit of Ba showed only the YBa₂Cu₃O_7-δ phase. The results of DTA measurements and resistivity measurements, however, demonstrated that solid solubility in YBa₂Cu₃O_7-δ was very limited: less than 0.5 at% excess or deficit of Cu or Ba could be incorporated. Samples deviating from YBa₂Cu₃O_7-δ stoichiometry differed sharply from stoichiometric material in their electrical behavior. Tailing or two-step superconductive transitions were found, as well as decreased T_cend. Composition variations affected sintering and grain growth. Sintered Cu-poor pellets were porous, having densities 60% or less of theoretical. For Cu-rich and Y-rich-Ba-poor pellets, densities about 90% of theoretical were obtained, and extensive grain growth was observed.

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The oxygen nonstoichiometry in the high T_c superconductor Ba₂SmCu₃O_7-x was determined over the temperature range of 350° to 900°C under the oxygen partial pressure between 10^-3 and 1atm, with the thermogravimetric measurement calibrated by the iodometric titration technique. The temperature dependency of the nonstoichiometry indicates that a phase transition occurs at a temperature between 600° and 700°C.

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The magnetic properties of the superconductive materials Ba₂YCu₃O_7-x, Ba₂DyCu₃O_7-x and Ba₂HoCu₃O_7-x have been measured and discussed. Each material had superconductive transition temperatures T_c in the zero magnetic field near 90K. The magnetic susceptibilities at the liquid nitrogen temperature of the Ho and Dy based materials extrapolated from normal conductive state were very near those of being extrapolated to the high magnetic field on the magnetization curves. This result suggests that the magnetic and superconductive properties independently coexist in this materials.

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A thin foil of a YBa₂Cu₃O_7-x sintered body was observed by the high voltage electron microscope operating at 1000kV. The sintered body is consisted of granular and lath-like grains of orthorhombic YBa₂Cu₃O_7-x. Some of the grains were revealed to have special orientation relationship to the adjacent grains. The observed orientation relations were (001)A//(100)B or (010)B and (001)A//(001)B. These grains were considered to link three-dimensionally together maintaining special orientation relations to one another.

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The melt-quenched products of the orthorhombic YBa₂Ca₃O_7-δ compound from 1200° and 1400°C were characterized by X-ray powder diffraction and DTA-TG methods. It was found that crystalline phases in the quenched samples from 1200°C were Y₂BaCuO₅ (major phase) and BaCuO₂, while those in the quenched samples from 1400°C were Y₂O₃ (major phase) and BaCuO₂. The products showed the weight gain and the exothermic peaks in the range of 200° to 300°C, corresponding to the oxidation of Cu(I) in BaCuO_2-x and amorphous compounds to Cu(II), and the endothermic peaks due to the partially melting in the range of 900° to 950°C. In addition, it was confirmed that superconducting YBa₂Cu₃O_7-δ compound began to form by annealing the melt-quenched products at 900°C.

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Element addition effects of YBa₂Cu₃O_7-y ceramics on the superconductivity have been studied. Fifty-two kinds of elements, and 163 compositions have been investigated but no element capable of improving the critical temperature (T_c) was found. Pb, Al and W were chosen additives and have been studied in detail about the changes in the superconducting properties and the crystal structures by the ratios of addition. Pb and W had almost no solubility in YBa₂Cu₃O_7-y. The crystal structure of YBa₂Cu₃O_7-y changed from orthorhombic to tetragonal by the addition of Al or W. T_c was lowered along with the change of the crystal structure.

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We fabricated Sr-Ba-Y-Cu-O ceramics that consists of Sr:Ba:Y:Cu=8x:8(1-x):2:10, where x=0, 0.125, 0.25, 0.375, 0.5 and 0.75, for the purpose of studying the effects of Sr substitution on Ba-Y-Cu-O crystal structure, the electrical and magnetic properties, and the microstructure. By observing the microstructure, it was learned that as the Sr component increases, the grain size of the samples decreases gaining density. The Sr concentration dependence of the lattice constants shows orthorhombic-tetragonal phase transition at x=0.5. The critical onset temperature, T_co decreases as the Sr component increases. However, at x=0.5, when the T_co increases, the resistivity at the room temperature shows the minimun value.

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