Research On The Growth Law And Superconducting Properties Of Single-domain YBCO Superconducting Bulk Materials

Single domain YBCO bulk superconductor is one of the most important and practical potential high temperature superconductors(HTS)because of the high critical current density(Jc),trapped field(Btr)and the high magnetic levitation force(F).At the same time,YBCO bulks have the significant applications in hi-tech areas,such as the superconducting maglev,superconducting magnet and energy storage flywheel,et al.Therefore,it is very urgent for us to invest a lot of human and financial resources for the related research,so that we can occupy the dominant position in international competition of this hi-tech field.Nowadays,the fabrication technology of YBCO superconductor bulks has made great progress,but in the process of practical application still exist some key problems to be solved,such as how to optimize and improve the performance of the YBCO superconductor bulks,reduce costs and improve the efficiency of material fabrication,etc.Hence,for solving the above important problems,there are four aspects works which were carried on as followings.The firstly,in order to clarify the mechanism of the crystal growth of YBCO superconductor bulk,the self-developed crystal growth high temperature observation device was improved for real-time recording on the growth process of the YBCO bulks.At the same time,we further studied the isothermal growth of single domain YBCO superconductor bulks under the condition of different under-cooling,which laid the foundation for the rate of the final products.Secondly,the traditional top seeded infiltration growth(TSIG)process was modified for improving the fabrication efficiency and simplifying process.Meantime,the new fabrication method,top seeded melt oxygen melt texture growth(TS-MOMTG),was introduced based on the top seeded melt texture growth(TSMTG)process.This new method is kind of the high efficiency and low cost preparation process,which greatly shortens the preparation time of superconductor bulk and improves the preparation efficiency.Thirdly,the second particles,Y2Ba4CuNbOy(YNb2411)and Nb2O5,were successfully doped in the YBCO bulks respectively,which effectively improved the Jc of the YBCO bulk.And lastly,the effect of the trapped flux density on the YBCO superconductor bulks was investigated by the different pulse magnetic field and the same pulse magnetic field repeatedly magnetization.The structure and optical design of crystal growth observation was improved to adjust the key parameters,such as focal distance and contrast ratio in real-time,according to light intensity and height variation of crystal surface more conveniently,which enhances the overall quality of video during crystal growth as well as the stability of system work at the same time.Through crystal growth observation,the growth law of crystal can be investigated more intuitively,and the good conditions and methods are provided to explore the fabrication and the batch production of large grain YBCO superconductor bulks.The whole isothermal growth process of YBCO crystal under the different under-cooling temperatures was carefully observed by utilizing the improved crystal growth observation device and the growth law of YBCO superconducting crystals was studied.The determined temperature window of YBCO crystal is about 988-1008?.There is no strict linear relationship between growth length and growth time of YBCO bulk,and meanwhile the relationship between them can be described as an equation Ra = a(?T)b.Under a certain under-cooling temperature,the average growth rate of YBCO crystals is associated with growth time.When under-cooling ??15 ?,the average growth rate decreases gradually with growth time,and when under-cooling?T?11 ?,the average growth rate first decreases with time and then trends to a stable value.The instantaneous growth rate of YBCO crystal is between 0.01-0.72 mmh-1,but the fluctuation of that is very great,and the fluctuation is not strictly consistent with the linear relationship between growth length and growth time,which reflects the growth inhomogeneity and complexity of superconducting bulk more intuitively.However,the average of instantaneous growth rate increased linearly with the increasing of the under-cooling temperature.Through analysing the ternary phase diagrams of Y2O3-BaO-CuO,the new method,top seeded metal oxide melt textured growth method(TS-MOMTG),was proposed to fabricate the single domain YBCO superconductor bulk in this thesis.This new method directly adopted the mixture powder of three kinds of metal oxide(Y2O3,BaO and CuO)as the raw material powders,and we successfully obtained the single domain YBCO bulk superconductors by the melt textured growth process.Without sintering any kind of precursor powders(Y123,Y211 and Y011),this new method not only saved the preparation time and cost,energy resource,human and financial resource,but also improved the fabrication efficiency and reduced the discharge of wastewater containing rare earth elements.Therefore,the TS-MOMTG process is a high efficiency and low cost green preparation process.For getting single domain samples,we used three different precursor powders by the traditional TSIG process,which are Y2BaCuO5(Y211),BaCuO2(Y011)and YBa2Cu3O7-?(Y123)respectively.Because it is very difficult for us to fabricate pure Y123 powder,the new liquid phase(Y211+Y035)was successfully adopted to fabricate the single domain YBCO superconductor bulks instead of conventional liquid phase source(Y123+Y035).The single domain YBCO samples were fabricated by the two different process methods respectively,and the levitation force and the trapped filed were measured by the self-designed 3D magnetic force&field measuring device.The levitation force of the two samples by modified TSIG and traditional TSIG process are 33N and 31N respectively,and the trapped field of each sample are 0.296T and 0.28T.The above results showed that there are not many differences in the superconducting properties of the samples,but there are just two precursor powders(Y211 and Y035)should be used in the modified process which efficiently save the preparation time and simplify the fabrication process.The YBCO superconductors were fabricated by adopting YNb2411 or Nb2O5 powder with the modified TSIG process.The second phase particles of YNb2411 can be found during the crystal growth stage.The YNb2411 particle size can slightly enlarge with the increasing of doping concentration.The wrinkles region with step will be formed in the surface of the YBCO superconductor bulks.Furthermore,the second phase of YNb2411 nanoparticles can enhance the flux pinning capability,and improve the superconducting properties of single domain of YBCO superconductor bulks.These two different dopants on the YBCO superconductor bulks are different.For the YNb2411 doping,there is no obvious difference on the particle size and the distribution of the Y211 particles when the dopant of YNb2411 particle is greater than 10 wt%.The random nucleation will be very serious during the crystal growth process when the dopant is too much and the epitaxial growth along the NdBCO seed will also be inhibited.For the Nb2O5 doping,it seems that the Nb2O5 doping is helpful for the growth of the YBCO superconductor bulk and refinement of the Y211 particles.The best levitation force and the trapped field of the YBCO superconductor bulk is 42.5N and 0.313T when the dopant amount of the YNb2411 particles is 7 wt%.In the meantime,the superconducting properties of the samples can reach up to 36N and 0.263T when the dopant of Nb2O5 is 0.9 wt%which is the best one,and the Jc are 3.6×104 A/cm2 and 1×105 A/cm2 on the condition of 77K and 60K respectively.Using the pulsed magnetic field on YBCO superconductor,the trapped field density of the superconductor bulks increased first and then decrease with the increasing of pulse peak magnetic field.It is showed that there is an optimal magnetization field.At the same time,the distribution pattern of the trapped field on the superconductors also has the corresponding change from valley pattern to single peak pattern,and then to double or multi peak shape.For the YBCO superconductor bulk in 20 mm diameter,the optimum pulse magnetic field is about 4.8T.At the optimum magnetization peak magnetic field,continuous magnetization can improve the maximum trapped field of YBCO superconductor bulk and improve the symmetry of the trapped field density distribution.The maximum trapped field can be increased about 30%when applying a 7 times of continuous magnetization,which can improve the potential trapped field of the YBCO superconductor bulks containing the second phase nanoparticles doping.When the pulse magnetic field is stronger than the optimum pulse magnetic field value,the temperature of superconductor bulk will rise seriously and the flux pinning ability and the trapped field will be reduced.Relatively speaking,the trapped field of the superconductor bulk doped the YNb2411 nanoparticles will decrease more slowly.The distribution of the trapped field density shows more symmetrical when the permanent magnet is used to magnetization for the YBCO superconductor bulks.For the same central surface field of the permanent magnet,the trapped field is gradually increasing with the larger of the diameter of the magnet.No matter using the permanent magnet(diameter is 40mm and the top surface field is about 0.5T)or the pulse magnet(the peak field is 4.8T)repeated 3 times to magnetize on the YBCO superconductor bulk,the trapped field is basically the same(0.332T).