Effects of Forming Pressure on Physicochemical Properties of YBCO Ceramics

This article details the solid-state synthesis of high-temperature superconducting YBa2Cu3O7−x. Tests were carried out on samples formed at different pressures (200, 400, 600 and 800 MPa) before being annealed under pure oxygen. The X-ray diffraction method showed that, regardless of the forming pressure, the samples contain about 97 wt.% of Y-123 phase. SEM images showed a polycrystalline structure of samples of similar grain size and number of pores (intergranular spaces). The values of critical temperatures (Tc0), determined from magnetoresistance measurements, are about 91.5 K for all samples, and the Tc0 temperatures do not depend on sample-forming pressure. Magnetoresistance measurements have shown that samples formed with higher pressures exhibit smaller changes in Tc0 and superconducting transition width ΔT due to the influence of the HDC magnetic field, than the samples formed with lower pressures. Values of specific resistance determined by the use of van der Pauw method at 300 K is about 2 mΩcm for all samples. The critical temperatures (T cintra) of grains and critical current densities at 77 K were determined from AC magnetic susceptibility measurements, and they are about 91.6 K and 400 A cm−2, respectively.

boiling point of liquid nitrogen. High-temperature superconductors are characterised by complex chemical composition. Their main ingredients are oxygen and copper; in smaller quantities, they contain bismuth, lead, thallium, mercury and lanthanides. In recent years, research has been focused on methods of obtaining and determining the physicochemical properties of YBa 2 Cu 3 O 7−x (YBCO). The YBa 2 Cu 3 O 7−x compound, also called Y-123 because of its stoichiometric composition, is one of the most important and most popular superconducting material [9].
Its advantage is a fairly high temperature of phase transition into superconducting state (about 93 K), which makes it possible to use it at liquid nitrogen temperatures [1]. The process of obtaining this superconductor is quite complex chemically [12].
It also requires maintaining a suitable temperature for a long time during the calcination processes in the air atmosphere and during annealing in pure oxygen atmosphere [12][13][14][15][16]. Currently, the work is focused on improving the efficiency of the process of obtaining this superconductor and reducing production costs. Many modifications of this process are also proposed, e.g., the use of weak microwaves during YBCO synthesis, the replacement of BaCO 3 with BaO [17].
In order to eliminate the formation of heterogeneous phase and loss of mass as a result of evaporation, several calcination methods are used: rapid [18], two-stage [19] and vacuum [20] calcinations. YBa 2 Cu 3 O 7−x can be obtained at temperatures below 800 • C from oxides and carbonates by low-pressure oxygen calcination [21,22]. In this way, it is possible to eliminate the non-superconductive phase that is formed at low temperatures. There has been virtually no works on the dependence of properties on the technological parameters of obtaining solid YBCO material. One of the few works is that of Ohmukai [17], which dealt with microwave annealing and studied the influence of pressure during YBCO molding in the range of 80-2000 kgf cm −2 (about 7. . He concluded that in order to minimise the undesirable phase of BaCuO 2 in YBCO, the pressure should be 700 kgf cm −2 (about 68.6 MPa) [17].
Ohmukai [17] studied the effect of molding pressure on properties of the YBCO ceramics obtained during the use of the microwave method. Al-Shibani [23] studied the effect of molding pressure on the structural and physical properties of YBCO obtained by conventional calcination. Al-Shibani's paper [23] discussed the effect of forming pressure of samples on their critical temperature T c of their superconducting transition. Additionally, the author discussed the studies of resistance of the samples pressed under different pressures. The tests show the increase of the sample's resistance with increasing forming pressure.
Ohmukai's and Al-Shibani's researches have inspired the research team of the Institute of Ceramics and Building Materials in Warsaw trying to obtain a high-temperature ceramic superconductor YBa 2 Cu 3 O 7−x and determine the dependence of its characteristics on the forming pressure of the material samples. To achieve this goal, a method of calcining a mixture of starting powders to obtain a superconducting material was used.
The purity of the oxides used in this synthesis is very important. Trace amounts of other elements, especially ferromagnetic ones, can greatly reduce the critical temperature. The surface morphology of the substrates needed to make YBCO ceramics was evaluated using a scanning electron microscope. The microscope images shown in Fig. 1 indicate that the grains of yttrium oxide are nonhomogeneous in shape and are cracked; the barium carbonate particles are longitudinal, while the copper oxide particles form very porous agglomerates.
The summary chemical equation (1) which describes the reaction of YBCO formation and which has been used to quantify the composition of raw materials is as follows: (1) Table 1 gives an example of the composition of the set of substrates needed to obtain 50 g of YBa 2 Cu 3 O 6.5 .
After weighing them with the accuracy of 0.0001 g, the Y 2 O 3 , BaCO 3 and CuO substrates were wet milled in the presence of isopropanol in a 1:1 powder/isopropanol ratio in a planetary mill for 75 min. The size of the grinders in 150 ml agate milling balls was between 15.0 and 19.7 mm, and the average size of the grinders was 17.24 ± 1.05 mm. Weights of single grinders in grinding balls ranged from 4.18 to 10.43 g, while the average weight was 7.25 ± 1.45 g. To analyse the dynamics of the milling process, slurry samples were taken for laser particle diffraction measurements every 15 min (Table 2). Based on own milling experience, it was found that the milling time of 75 min was sufficient both for the grinding of powders and their homogenisation.

Preparation of Test Samples
After milling and drying, the powder was placed in corundum crucibles and subjected to calcination processes for solid-phase reaction. Reactions in solid phase (first and second) were carried out in an electric muffle furnace (Fig. 2). Each of them consisted of the following stages: After the first solid-phase reaction in air atmosphere, the mixture was comminuted in the presence of isopropanol in a proportion of 1:1 powder/isopropanol in a planetary mill for 210 min. After the first reaction in solid phase in the air atmosphere, the following granulations were obtained: The mixture was also comminuted after the second reaction in solid phase, as before, in the presence of isopropanol in a ratio of 1:1 powder/isopropanol in a planetary mill for 210 min. After both first and second reaction processes in air atmosphere, we carried out a phase analysis of the powders obtained. Using the milled powders from first and second reactions in solid phase, we prepared two sets of test samples (A and B) obtained thanks to the axial pressing method with pressures of 200, 400, 600 and 800 MPa. The series A samples consisted of pellets with a diameter of about 12 mm, a weight of about 1 g and a height of about 2 mm, marked according to the pressures-200A, 400A, 600A and 800A. In contrast, the B series produced pellets with a diameter of about 12 mm, a weight of about 3 g and a height of about 6 mm, marked according to the pressures-200B, 400B, 600B and 800B. Each of the series was annealed in a separate process in an electric tube M, molar mass; M , molar mass multiplied by the stoichiometric coefficient; K, molar mass ratio (multiplied by the stoichiometric ratio) of a substrate or the total molar mass of the product; K YBCO , molar mass ratio (multiplied by the stoichiometric ratio) of the substrate or product to the total YBCO molar mass furnace. Series A pellets were stacked on three 10 × 10 cm alumina slabs, while batch B samples were stacked on one such plate. The plates were placed in the central part of the electric tube furnace (Fig. 3). The annealing process was carried out in an atmosphere of flowing oxygen, which provided adequate doping of the samples with O 2− ions. Oxygen was supplied to the furnace from the cylinder through a reducer and flowmeter (oxygen flow during the whole process was 20 l h −1 ). The annealing process followed the curve shown in Fig. 4.
After the annealing process, the samples were placed in a desiccator to prevent them from aging due to air humidity.

Applied Testing Methods
The analysis of the grain size of the used raw materials was carried out using the low-angle laser light scattering (LALLS) method (also known as the laser diffraction method), with the use of a laser particle analyser with an attachment for preparation of samples in the form of dispersion in aqueous solutions or organic solvents.  Open porosity P o , apparent density dp and water absorption N w samples of YBCO material were determined by hydrostatic weighing in water using an analytical balance with an attachment for determining the density of solids. The calculations were made according to the following formulas: where P o is open porosity (%), m ds is mass of dried sample (g), m inH 2 O is mass of sample immersed in water (g), m ms is mass of the moistened sample (g), V is sample volume (cm 3 ), dp is apparent density calculated on the basis of hydrostatic weighing (g cm −3 ), and N w is water absorption (%). Qualitative and quantitative phase analysis of the calcined powders and post-annealing materials was performed by X-ray diffraction (XRD) in Bragg-Brentano configuration on a diffractometer equipped with a copper anode lamp. The diffractometer's optical system was composed of a 0.3-in. divergence slot, a 1.50 • anti-scatter slot, two 2.5 • Soller slots, a Ni filter and a LynxEye strip detector with 2.94 • field of view. Quantitative analysis was carried out using the Rietveld method in the Topas v.50 software, based on published crystalline structures (COD and PDF+2014, including ICSD).
Initial powder morphology and material microstructure investigations were performed using SEM with field emission using a low-energy electron beam detector, with energies less than 50 keV, striking electrons from atoms near the surface of the sample. For documenting purposes, raw material photos were taken with magnifications of ×10,000 and samples tested with magnifications of ×5000.
Measurements of AC magnetic susceptibility in the function of temperature and intensity of the AC magnetic field H AC were made using an inductive bridge consisting of a transmission coil and two detector coils set up in the helium cryostat. An SRS830 Lock-In amplifier was used as a detector and the AC current source at 189 Hz. The LakeShore 330 temperature controller was used to control the temperature. The control of the measurements and data acquisition was performed with a PC with appropriate software. The above apparatus was also used to measure the resistance by the four-point method. Resistance was measured as a function of the temperature and the intensity of the external DC magnetic field H DC . DC magnetic field was produced in a classic copper coil powered by the Glassman Europe Limited LV 20-50 DC Power Supply. The direction of the magnetic field was parallel to the longest side of the parallelepipedal sample and parallel to the current direction.
Specific resistances of round-shaped pellets of thickness d were determined at room temperature using the van der Pauw method [24,25] in accordance with the following formula  6 7.3 ± 0.3 - where by M, N, O and P are voltage and current contacts that are arranged symmetrically (every 90 • ) on the circumference of the sample. The Zero Field Cooling (ZFC) temperature measurements of the dependence of resistance and the dispersive part of the magnetic susceptibility allowed for the determination of the critical temperatures of the superconductive transition which were marked, respectively, by T c0 and T intra c . T intra c was determined as the temperature at which the diamagnetic signal from the sample faded. T c0 was determined as the temperature at which the increase of the measured voltage drop on the sample meets the criterion E = 10 −6 V cm −1 (Fig. 5), where E is the intensity of the electric field. The superconducting transition width T was determined from the following relationship: wherein T 90% and T 10% are the temperatures where the resistance drop is equal to 90 and 10% of the value of resistance in the temperature at which the superconducting transition beginning was assumed and indicated by T 100% . For all the samples measured, the T 100% value was estimated at 94 ± 0.2 K. The superconducting transition width T increases with the increasing intensity of the external magnetic field and is described by the following relation [26,27]: where T 0 is the superconducting transition width in zero magnetic field and C is the coefficient that depends on the critical current density value in zero external magnetic field and the critical temperature value and m is the exponent whose value is also related to the properties of the tested superconductor. The density of the intergranular critical current was calculated from the measurements of the absorption part of the AC susceptibility on the basis of Bean's critical condition model [28,29] and the Ginzburg-Landau strong coupling limit approach [30]. Critical current density J c for a flat sample with thickness d, located in a AC magnetic field with intensity H AC and parallel to the long side, one can describe as: In the case of annealed superconductors, one can distinguish two types of critical currents, related to the   [31]. In AC magnetic susceptibility measurements, we typically measure the properties of weaker, intergranular connections. From Ginzburg-Landau approximation, the critical current density in the function of temperature is given by: where T c0 is the critical temperature of the intergranular connections for H AC = 0, J c0 is the critical current density  at 0 K, n is the exponent whose value can be associated with the value of exponent m in formula (9) with n = 1/m.

XRD Phase Analysis of Calcined Powders
The results of XRD phase analysis obtained after one and two reactions in solid phase are presented in Table 3. These results clearly indicate that the material after two reactions contained smaller amounts of some undesirable phases, such as BaCO 3 and Y 2 BaCuO 5 . On the other hand, otherslike tenorite and Ba 2 Cu(OH) 6 -were not present at all.  progressive increase in sample density with increasing forming pressure. In contrast, the increase in diameter is due to internal stresses of pressing and the phenomenon of expansion of the sample when it is pushed out of the mold. As the pressure of the pressing increases, the internal stresses in the sample increase, which results in a higher expansion. In order to assess the physical properties of samples annealed in oxygen, their apparent density, open porosity and water absorption were determined by hydrostatic weighing. The results of the study are shown in Table 5.

Study of Physical Properties
On the basis of the results in Table 5, it can be seen that with increasing forming pressure, the apparent density increases, while the open porosity and water absorption decrease. This proves that the forming pressure affects the degree of densification of the samples.

Meisner-Ochsenfeld Phenomenon
To verify that the samples obtained are high-temperature ceramic superconductors, a pre-test for the Meisner-Ochsenfeld phenomenon has been performed. By immersion in liquid nitrogen, the samples were cooled to the boiling point of liquid nitrogen (77 K). A magnet weighing about 1 g was placed above the sample. The observed levitation of the magnet over the sample of YBCO material confirmed the superconductivity of the samples obtained (Fig. 6).

Observations of Microstructure
Microscopic observations were made using a scanning microscope that did not show any significant differences  (Fig. 7).

Phase Analysis of the Material
The obtained YBCO superconductor samples were subjected to phase tests. The A series samples were selected for the study. The quantitative analysis of the phase content was carried out using the Rietveld method in the Topas v.50 software based on published crystalline structures (COD and PDF+2014, including ICSD). The results of the quantitative analysis are shown in Table 6. The analysis showed that the samples were mainly obtained with a rhombohedral phase of YBa 2 Cu 3 O 6.83 (where x = 0.17) and a small amount of BaCuO 2 phase. Oxygen factor x was the fitting parameter in Rietved refinement procedure utilised in the Topas v.50  software. Phase analysis did not show the dependence of the content of superconducting-rhombohedral-phase on the pressure of sample pressing. The X-ray diffraction patterns for the samples obtained at different pressures are given in Fig. 8. The diffractograms show clear peaks confirming the presence of the rhombohedral YBa 2 Cu 3 O 6.83 phase.

Specific Resistance and Dependence of Resistance on Temperature and Magnetic Field
The testing of the superconducting properties of the samples was carried out on the basis of the results of magnetoresistance measurements and of the AC magnetic susceptibility in the function of the temperature and intensity of the external magnetic field H DC .
The results of the measurement of resistivity in room temperature ρ (300 K) with the use of the van der Pauw method are presented in Table 7. The obtained values of ρ (300 K) are about 2 m cm (2000 μ cm) and are typical of polycrystalline superconductors prepared with the use of the solid-state reaction method [32,33]. There is no apparent relationship between the ρ (300 K) value and the sample forming pressure.
The dependence of the resistance in function of the temperature and the external magnetic field H DC is shown in Fig. 9. Based on the magnetoresistance measurements for each sample, the critical temperature T c0 and the superconducting transition width T were determined. The determined values are shown in the function of the field strength H DC in Fig. 9 and are summarised in Table 7.
Critical temperature T c0 for all samples decreases as the intensity of the external magnetic field H DC increases and the superconducting transition width T grows. A positive effect of forming pressure on the samples of T and T c0 can be observed. Samples that were pressed at a higher pressure have a higher critical temperature T c0 and a narrower superconducting transition T . With the increasing intensity of the H DC field, the differences between T c0 and T are increasing.
The result of the fit of the superconducting transition width T (H ) to relationship (9) is shown in Fig. 10b. The determined values of parameters C, m and T 0 are shown in Table 8. For all samples, the value of the exponent m is less than the theoretical value m = 2/3, which, according to the interpretation in Woch et al. [34] points to a more twodimensional structure of the vortexes in the samples tested and a high pinning force, which will present itself as a high critical current density.

The Dependence of Magnetic Susceptibility on Temperature
The results of the measurements of the AC magnetic susceptibility in the function of the temperature and amplitude of the magnetic field H AC from 26 mOe to 10.9 Oe are shown in Fig. 11.
The measurements of the dispersion component of the AC magnetic susceptibility allowed for the determination of the values of the critical temperatures T intra c of grains. Within the tested range of the magnetic field intensity H AC , these temperatures do not depend on the amplitude H AC , and they are 91.7, 91.6, 91.5 and 91.6 K, respectively, for samples 200, 400, 600 and 800 MPa ( Table 7). The uncertainty of the determined temperature was estimated to be 0.3 K. The indicated temperatures T intra c of the grains are consistent with literature reports obtained for good-quality YBCO samples.
In the measurements of the absorption part of AC magnetic susceptibility, a single absorption peak was observed for all samples. The peak appears due to energy losses connected with the magnetic field penetration into the intergranular regions. For small H AC field strengths, intergranular absorption peak appears approximately 1.5-2 K below T intra c of the sample. With the increase in H AC , the location of the peak on the temperature axis is shifted towards lower temperatures. For the maximum value of H AC = 10.9 Oe, the absorption peak were approximately 86 K for all samples. No absorption peak was observed for the magnetic field penetration into grains, which, if visible, is slightly below the critical temperature T intra c . Figure 12 shows the value of critical current density of intergranular connections in the function of temperature, calculated according to dependence (10) and their fit to dependence (11). The determined values of fitting parameters J c0 , T c0 , n and the estimated values of the critical current density at liquid nitrogen temperature J c (77 K) are shown in Table 9

Summary
By solid-state reaction method, single-phase samples of superconductive YBa 2 Cu 3 O 6.83 (rhombohedric phase) were obtained with a very small amount of BaCuO 2 phase. No dependence of the content of the superconducting (rhombohedral) phase on the sample-forming pressure was found. The microstructure of the samples observed using a scanning electron microscope did not show any significant differences in either grain size or intergranular spaces depending on the pressing conditions.
The pressing pressure has an effect on the degree of density of the samples. As the pressure increases, the apparent density of the samples increases, while their open porosity and water absorption decrease.
Resistivity at room temperature ρ (300 K) was determined by the van der Pauw method. The obtained values at ρ (300 K) are about 2000 μ cm and are typical of polycrystalline superconductors prepared with the use of the solid-state reaction method.
Value of critical temperature T c0 in self magnetic field (H DC = 0) does not depend on the forming pressure and is 91.4, 91.4, 91.5 and 91.6 K for samples 200, 400, 600 and 800 MPa, respectively. The T c0 uncertainty was estimated as 0.3 K.
In the external magnetic field H DC , all samples showed a decrease in the critical temperature T c0 and increase in the width of the superconducting transition T along with increasing intensity of the H DC filed increases. Samples pressed at higher pressures show smaller changes of T c0 and T caused by H DC field than the samples pressed at lower pressures.
The analysis of the superconducting transduction width T in the function of the external magnetic field showed that the samples are characterised by a more twodimensional structure of the vortexes.
The measurements of the dispersion component of the AC magnetic susceptibility allowed for the determination of the values of the critical temperatures T intra c of grains. Within the tested range of the varying magnetic field intensity H AC (0-10.9 Oe), these temperatures do not depend on the amplitude H AC , and they are 91.7, 91.6, 91.5 and 91.6 K for samples 200, 400, 600 and 800 MPa, respectively. The T intra c uncertainty was estimated as 0.3 K. The critical current density J c (77 K) varies within the range from 340 to 460 A cm −2 , does not depend on the forming pressure and is typical for polycrystalline superconductors prepared with the use of the solid-phase reaction method.
The determined critical temperatures T intra c and T c0 , critical current density J c (77 K) have values very similar to the results reported in the literature for good-quality YBCO samples prepared with the use of the solid-phase reaction method.
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