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UDC 539.21 (06)
Vestnik SibGAU Vol. 16, No. 3, P. 763-769
INFLUENCE OF PLASMA SYNTHESIS OF ZrO2 NANOPARTICLES ON MAGNETIC FLUX PINNING IN GRANULAR YBa2Cu3O7-y
A. V. Ushakov1,2, I. V. Karpov1,2, A. A. Lepeshev2,3, L. Yu. Fedorov2*, A. A. Shaihadinov2
1 Reshetnev Siberian State Aerospace University 31, Krasnoyarsky Rabochy Av., Krasnoyarsk, 660037, Russian Federation 2 Siberian Federal University 79, Svobodny Av., Krasnoyarsk, 660041, Russian Federation
3 Krasnoyarsk Scientific Center, SB RAS 50, Akademgorodok, Krasnoyarsk, 660036, Russian Federation * E-mail: sfu-unesco@mail.ru
In this work, the effect of ZrO2 nanoparticles prepared in a low-pressure arc discharge plasma on magnetic flux pinning of granular YBa2Cu3O7-y/nanoZrO2 composites has been studied. It has been shown that the ZrO2 nanoparticles do not change the superconducting transition and the microstructure of superconductors. At a temperature of 5 K, the addition of 0.5 and 1 wt % of ZrO2 nanoparticles may lead to the additional effect of magnetic flux pinning and the increase in the critical current density Jc. The Jc value for composites with 1 wt % is two times larger than that for the reference sample. The fishtail effect is observed for YBa2Cu3O7-y/nanoZrO2 composites at the temperatures of 20 and 50 K. The problems associated with the additional effect of magnetic flux pinning of granular YBa2Cu3O7-y/nanoZrO2 composites and the appearance of the fishtail effect have been discussed.
Keywords: nanoparticles, low-pressure arc discharge plasma, superconductors, magnetic flux pinning, fishtail effect.
Вестник СибГАУ Т. 16, № 3. С. 763-769
ИССЛЕДОВАНИЕ ВЛИЯНИЯ НАНОЧАСТИЦ ZrO2 ПЛАЗМОХИМИЧЕСКОГО СИНТЕЗА НА ПИННИНГ МАГНИТНОГО ПОТОКА В ГРАНУЛЯРНЫХ YBa2Cu3O7-y
А. В. Ушаков1'2, И. В. Карпов1'2, А. А. Лепешев2,3, Л. Ю. Федоров2*, А. А. Шайхадинов2
1 Сибирский государственный аэрокосмический университет имени академика М. Ф. Решетнева Российская Федерация, 660037, г. Красноярск, просп. им. газ. «Красноярский рабочий», 31
2 Сибирский федеральный университет Российская Федерация, 660041, г. Красноярск, просп. Свободный, 79
3 Красноярский научный центр СО РАН Российская Федерация, 660036, г. Красноярск, Академгородок, 50
* E-mail: sfu-unesco@mail.ru
Изучено влияние наночастиц Тг02, полученных в плазме дугового разряда низкого давления, на пиннинг магнитного потока гранулярных УВа2Си307.у/ нано-2г02 композитов. Показано, что наночастицы 2г02 не меняют сверхпроводящего перехода и микроструктуры сверхпроводников. При температуре 5 К добавление 0,5 и 1 % по массе наночастиц 2г02 может привести к дополнительному эффекту пиннинга магнитного потока и увеличению плотности критического тока Зс. Для композитов с 1 масс. %. Зс в два раза больше, чем у эталонного образца. При температуре 20 и 50 К наблюдается fishtail-эффект для композитов УВа2Си307_/нано-1г02. Обсуждаются вопросы, связанные с дополнительным эффектом пиннинга магнитного потока гранулярных УВа2Си307_/нано-1г02 композитов и возникновение fishtail-эффекта.
Ключевые слова: наночастицы, плазма дугового разряда низкого давления, сверхпроводники, пиннинг магнитного потока, fishtail-эффект.
Introduction. High critical current densities Jc are of considerable significance for multiple prospective applications of high-temperature superconductors such as wires for electric energy transmission and magnets [1-3].
As a rule, the limitation of Jc is connected with two main factors [4]. First, Jc is limited by the thermally activated magnetic flux creep; i. e., it decreases considerably under the action of a magnetic field much less than the critical
field Hc(T). This phenomenon is caused by the well-known properties of high-temperature superconductors, e. g., the high anisotropy and small coherence length, which lead to weak magnetic flux pinning. Second, Jc in poly-crystalline high-temperature superconductors is limited by the insufficient ordering of crystallites and their chemical in homogeneity, which leads to weak coupling with low critical current densities. This problem is overcome by different methods of growing the texture such as oxide-powder-in-tube, OPIT [5]. Owing to this technology, the value Jc > 80 kA/cm2 was reached in short multiwire Bi-2223/Ag strips at a temperature of 77 K [6].
In the last years, there appeared many new methods of creating pinning centers and, accordingly, increasing the critical current Jc of high-temperature superconductors. Of the highest interest among them are the following. Amorphous cylindrical tracks with a diameter of about 10 nm and length from 1 to 10 were prepared by heavy-ion bombardment of YBa2Cu3O7-y single crystals [7]. Tracks in amorphous Bi2Sr2CaCu2O8 were prepared by proton irradiation with the subsequent decay of Bi nuclei [8]. The magnetization hysteresis and critical temperature of HgBa2CaCu2O6 were increased by its neutron irradiation [9].
However, the application of these technologies is accompanied by considerable difficulties: serious hindrances in the application of accelerators, radioactive elements, and neutron irradiation.
Obviously, it is necessary to study further the methods of the incorporation of defects or impurities into the superconducting matrix. Apparently, the most realistic approach is the preliminary mixing of nanoparticles and high-temperature superconducting powder and then the performance of the standard technological procedures, i. e., annealing, sintering, and oxygenation (if necessary). The following advantages will be achieved: the possibility of industrial application, the considerable decrease in the anisotropy of magnetic properties, and the considerable increase in the critical current when the contact between the grains is enhanced.
The nanoparticles should meet the following main requirements: first, their size should be comparable with the coherence length of the high-temperature superconductors and, second, they should be stable in a chemically aggressive medium at elevated temperatures. This is necessary for the optimization of superconductivity in the matrix material. In this work, we chose ZrO2 nanoparticles for studies in the form of inclusions in a polycrystalline superconducting YBa2Cu3O7-y matrix. The melting temperature of ZrO2 reaches 2400 °C. They are chemically stable. The aim of this work is to study the effect of ZrO2 nanoparticles prepared in a low-pressure arc discharge plasma on the magnetic flux pinning of the granular YBa2Cu3O7-y/nanoZrO2 composites.
Experimental. The powder of the precursor YBa2Cu3O7-y was prepared using the conventional solidphase synthesis. The YBa2Cu3O7-y/nanoZrO2 composite was synthesized according to the technique described in detail in [10-12]. The material was synthesized under the following conditions. Technically pure zirconium was used as the sputtering cathode. Prior to the evaporation, the cathode was heated to a working temperature of 800 K. The purification in the glow discharge was performed at
the voltage on the substrate of 1000 V for 1 min. The ion bombardment activation was performed for 1 min at an arc discharge current of 20 A and a voltage on the substrate of 1000 V. The rotation frequency of the mixing device was 8 min-1. The vibration amplitude was 1 mm. The vibration frequency was 50 Hz. The direct deposition of the ZrO2 nanoparticles on the high-temperature superconducting microgranules was performed at a discharge current of 500 A. The longitudinal magnetic field strength created by the focusing coil of the cathode surface was 6366.2 A/m. To implement the plasma-chemical reaction, a 5 % O2 + 95 % He gas mixture was inserted in the chamber using a two-channel regulator of the gas flow rate after the preliminary evacuation to a pressure of 1 mPa. The synthesis was performed at a pressure above 120 Pa. After the deposition of the nanoparticles, the samples were passivated in a pure oxygen atmosphere for a day. The prepared samples of the material contained from 0.1 to 1 wt % of nanoparticles.
The prepared mixture was preliminarily heated to 940 °C and kept at this temperature for 30 h. The preliminarily heated powder was ground and then pressed into tablets with a diameter of 1 mm and a thickness of 5 mm at a pressure of 1.2 x 105 N/cm2. Finally, the granules were sintered at 940 °C for 24 h and then cooled to room temperature in an oven in air.
Magnetization was recorded by differential Hall mag-netometry with the use of two semiconductor Hall sensors switched opposite to the Hall potential outputs. The first Hall sensor was far from the sample and measured the external magnetic field H. The second sensor was placed on the sample surface and measured the magnetic flux density. As a result of the apparatus subtraction of the Hall potential of the first Hall sensor from the potential of the second Hall sensor, the resultant signal appeared corresponding to the magnetization M(H). According to the Bean formula including the demagnetization factor and the dependence of the critical current on the magnetic field, Jc(H) = 30M(H)/d, where M is the width of the magnetic hysteresis loop and d is the average size of the crystallite. We used the value d = 6 obtained from the electron microscopy results. The hysteresis magnetic loops were measured at 5, 20, and 50 K. The pinning force was calculated using the equation Fp(B) = Jc(B)B [13].
The phase composition of the high-temperature superconducting samples were studied on an XRG-6000 dif-fractometer using CuKa radiation. The phase composition and the size of the coherent scattering regions were analyzed using the PCPDFWIN database.
The sample structure was studied by scanning electron microscopy on a JEM-100CX electron microscope with an ASID-4D scanning device at an accelerating voltage of 40 keV.
Results and discussion. Fig. 1 shows the X-ray diffraction patterns of all YBa2Cu3O7-y/nanoZrO2 composites. Peaks related to the admixture of the ZrO2 phase or other oxides are absent. The characteristic peaks of YBa2Cu3O7-y do not shift. The study of the temperature dependence of the magnetic susceptibility showed that the
superconducting transition temperature Tc does not change and is 90 K for all samples. No large aggregates of the ZrO2 nanoparticles were detected using scanning electron microscopy. Thus, the ZrO2 nanoparticles are homogeneously distributed over the superconducting matrix. The field dependences Fp(B) and Jc of the YBa2Cu3O7-y/nanoZrO2 composite are shown in fig. 2-4.
Fig. 2 shows the magnetic field dependences of the calculated critical current density J c and the pinning force Fp at a temperature of 5 K for all studied composites. It is seen in Fig. 2, a that, for all applied magnetic fields, Jc is much higher for composites with 0.5 and 1.0 wt % of ZrO2 nanoparticles than that for the sample without the addition of ZrO2 nanoparticles. For the composite with 1.0 wt % of ZrO2 nanoparticles, the Jc value is two times higher than that for the samples without nanoparticles. It is seen in fig. 2, b that the pinning force Fp for all composites increases with the applied magnetic field. In the whole range of the applied magnetic field, the pinning force Fp of composites with 0.5 and 0.1 wt % of ZrO2 nanoparticles is higher than that of the pure sample YBa2Cu3O7-y. These plots show that the addition of the
ZrO2 nanoparticles leads to the formation of additional effective pinning centers. These centers can increase the pinning force of YBa2Cu3O7_y superconductors and improve their critical current densities Jc at a temperature of 5 K.
Fig. 3 shows the magnetic field dependences of the calculated critical current density Jc and the pinning force Fp at the temperature of 20 K for all studied composites. It is seen that, for the samples with 0.5 and 1.0 wt % of ZrO2 nanoparticles at a magnetic field below 1.5 T, Jc and Fp are less than the respective values for samples without the ZrO2 nanoparticles. However, in the field range from 1.5 to 5 T, the Jc value of the samples with 0.5 and 1.0 wt % of ZrO2 nanoparticles is comparable with that for the pure sample YBa2Cu3O7-y. This fact indicates that the ZrO2 nanoparticles enhance the weak intergranular bonds of YBa2Cu3O7-y in the specified magnetic field range. As the magnetic field increases above 2 T, the critical current density for the YBa2Cu3O7-y/nanoZrO2 composites with 0, 0.5, and 1.0 wt % of ZrO2 nanoparticles increases, i. e., the fishtail effect is observed.
Fig. 1. X-ray diffraction patterns of YBa2Cu3O7.y/ZrO2 composites with 0, 0.5, and 1.0 wt % of ZrO2 nanoparticles
й v
.ü <
S 4
- Ы
1
- \ '-.
1 ОгЫ.% " ———*--- i i i
В
ë
b
Fig. 2. Magnetic field dependences of Jc (a) and Fp (b) for YBa2Cu3O7-y/ZrO2 composites with 0, 0.5, and 1.0 wt % of ZrO2 nanoparticles at 5 K
a
Fig. 3. Magnetic field dependences of Jc (a) and Fp (b) for YBa2Cu3O7.y/ZrO2 composites with 0, 0.5, and 1.0 wt % of ZrO2 nanoparticles at 20 K
Fig. 4 shows the magnetic field dependences of the calculated critical current density Jc and the pinning force Fp at a temperature of 50 K for all studied composites. It can be seen that Jc and Fp for the samples with 0.5 and 1.0 wt % of ZrO2 nanoparticles are less than those for the samples without ZrO2 nanoparticles at a magnetic field below 0.8 T. This fact means that the additives of ZrO2 nanoparticles in the specified magnetic field range favor the weakening of the intergranular bonds of the superconductor. However, for a magnetic field in the range from 0.8 to 3 T, Jc of composites is higher than that of the reference sample. For a field above 3 T, Jc of composites is less than that of the reference sample; i. e., the fishtail effect is also observed.
An analogous behavior is observed for the magnetic field dependence of the pinning force. The results obtained show that the addition of the nonsuperconducting ZrO2 nanoparticles homogeneously dispersed in the matrix in the YBCO superconducting systems can affect magnetic flux pinning and creep. The role of the ZrO2 nanoparticles in the increase in the critical current density is similar to the role of inclusions of the phase 211 [14]. It assumes that the decreased size of these particles directly conditions the increased pinning. At the same time, rather large particles of the normal (nonsuperconducting) phase 211 naturally decrease the superconducting properties of the sample. Therefore, to optimize the properties of the ceramic, it is necessary to control the concentration and size of the ZrO2 nanoparticles in the precursor powder used for the preparation of YBCO [15].
No general understanding has been achieved yet concerning the mechanism of the effect of Y211 on pinning in high-temperature superconducting fused textured ceramic materials on the basis of YBa2Cu3O7-y. In [16], it was assumed that the Y211 particles may act as an effective flux of the pinning centers. In [17], there was an attempt to semi quantitatively theoretically interpret the behavior of Jc(T, H) at high temperatures in high magnetic fields. The authors of [18] considered the defects associated with the interface of the Y211 particles and YBa2Cu3O7-y as effective pinning centers, which was confirmed quite successively by many experimental results. An alternative explanation of the increase in the current
density J c may be the transition from the ordered state of the vortex lattice to the disordered state because of the interaction of the lattice with ZrO2 nanoparticles. It is assumed [19] that the order-disorder transition is implemented if the transverse deformations of the vortex filaments u satisfy the Lindemann criterion: u = cLa0, where cL is the Lindemann number, a0 = (®0/B)12 is the intervortex distance, and ®0 is the magnetic flux quantum. These deformations lead to the increase in the elastic energy of the order-disorder transition. Therefore, the order-disorder transition is implemented when the increase in the elastic energy Eel is compensated by the pinning energy Ep. The energy Ep does not depend on the angle at pinning on point defects.
Many studies show that the oxygen vacancies lead to the appearance of the fishtail effects in YBa2Cu3O7-y samples [16]. It was established that the oxygen saturation of samples under its controlled isostatic pressure and high temperatures (under the condition of the initial heating to high temperatures in a nitrogen medium) makes it possible to considerably decrease the crack formation in the YBCO ceramics, increase the mechanical characteristics of the material and accelerate the saturation process, and achieve the record high values of the critical current density.
The average size of the ZrO2 nanoparticles is considerably larger than the coherence length of YBa2Cu3O7-y superconductors. Therefore, the statement that the ZrO2 nanoparticles act as effective pinning centers seems to be incorrect. In addition, the average size of the ZrO2 nanoparticles is comparable with the penetration depth of the system of YBaCuO superconductors, so that the role of the interphase pinning in ZrO2/ YBa2Cu3O7-y systems is limited. In our opinion, ZrO2 nanoparticles in the YBa2Cu3O7-y superconductor are responsible for the pinning expansion. They lead to the distortions of the crystal structure on the interface surface, affect the distribution of the oxygen-deficient regions, and increase the number of microregions with low Jc. In this respect, ZrO2/YBa2Cu3O7-y composites in comparison with pure samples manifest the enhancement of pinning in wide temperature and magnetic field ranges. As a result, they have a higher critical current density.
НО
с
JO
20
—~ *
0.J -
Г , i 1 (b) .....
2 3 Ml// СП
b
Fig. 4. Magnetic field dependences of Jc (a) and Fp (b) for YBa2Cu3O7.y/ZrO2 composites with 0, 0.5, and 1.0 wt % of ZrO2 nanoparticles at 50 K
a
At a low temperature (in our case, T = 5 K), the role of the localized microregions is minimal. For this reason, we did not observe the fishtail effects in ZrO2/YBa2Cu3O7-y composites with additives of 0, 0.5, and 1.0 % of ZrO2 nanoparticles. At higher temperatures (T = 20 and 50 K), the role of the localized microregions is manifested, the additional pinning center flux appears, and the fishtail effect can be observed for all ZrO2/YBa2Cu3O7-y composites.
Conclusion. Thus, ZrO2 nanoparticles homogeneously dispersed in the matrix of YBa2Cu3O7-y superconductors can lead to the existence of additional effective magnetic flux pinning centers. As a result, the critical current density Jc in the ZrO2/YBa2Cu3O7-y composite with 1.0 % of ZrO2 nanoparticles is improved twofold in comparison with that of the pure YBa2Cu3O7-y superconductor. At temperatures of 20 and 50 K, the fishtail effect was observed in the magnetic field dependences of the current density and pinning force in all studied ZrO2/YBa2Cu3O7-y composites. ZrO2 nanoparticles cause the distortions of the crystal structure on the interface surface and affect the distribution of the oxygen-deficient regions. In addition, they are responsible for an increase in the number of microregions with low density Jc.
Acknowledgments. This work was supported in part by the Ministry of Education and Sciences of the Russian Federation (projects no. 11.370.2014/K, 11.1287.2014/K).
Благодарности. Данная работа выполнена при частичной финансовой поддержке Министерства образования и науки Российской Федерации (проекты № 11.370.2014/K, 11.1287.2014/K).
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© Ushakov A. V., Karpov I. V., Lepeshev A. A., Fedorov L. Yu., Shaihadinov A. A., 2015
