Search

The effects of rare earth site doping on the structural, superconducting and magnetic properties of SmFeAsO0.7F0.3 iron pnictide are investigated. Gd3+ and Ce3+ ions are chosen by virtue of their position being on either side of Sm3+. Doping of both smaller (Gd3+) and larger (Ce3+) ions at the Sm3+ site increases the TC up to 55 K. Doping with the smaller Gd3+ results in lattice contraction and thereby enhances TC. It is interesting to observe that though Ce3+ doping in SmFeAsO0.7F0.3 exhibits an increase in lattice parameters, a substantial enhancement of TC occurs. The enhancement in TC due to increased charge carrier concentration is also confirmed using Hall Effect measurement. Apart from TC enhancement, the simultaneous doping of both at oxygen and rare earth sites prominently increases the superconducting properties such as JC, HC1 and HC2. The co-doped samples also exhibit better magnetic field dependence of JC over the entire field of study. It is also observed that the Ce3+ doped sample shows higher JC in the high field region due to its enhanced flux pinning properties.

The influence of rare earth (RE – Ce, Gd) doping at the Nd site in the NdFeAsO0.7F0.3 superconductor wherein Ce and Gd have ionic radii in the order Ce > Nd > Gd is investigated. The structural and superconductivity characterization of the pure and doped samples show that Ce doping enhances the TC of Nd1−xRExFeAsO0.7F0.3 to a maximum of 53.6 K at x = 0.1 while Gd doping attains a TC of 55.1 K at x = 0.15. Interestingly, both Ce and Gd doping create neither secondary phases nor precipitates within the detection limit of XRD. However, the lattice defects due to Ce and Gd doping modifies NdFeAsO0.7F0.3 and assists in pinning the flux lines on these defects thereby exhibiting an enhanced JC(H) performance especially at high fields. It is also observed that the relatively small ionic size of Gd is more effective in TC enhancement, while the lattice defects due to larger ionic size of Ce favor the remarkable enhancement of JC(H).

Introduction of nanoparticles as flux pinning centers in yttrium barium copper oxide (YBCO/Y-123) superconductors has been extensively explored in order to improve high field current-carrying capabilities. A uniform distribution of these nanodopants throughout the superconducting matrix is the most essential and desirable. The present work provides a nonconventional approach of introducing nanoparticles into YBCO superconductors by introducing nanoparticles in Y2BaCuO5 (Y-211) preform and then subjecting the preform to infiltration and growth process. The Y-211 preform containing 2 wt.% ceria nanoparticles of the order of 60 nm is referred to as Ce-2 preform. The microstructure of Ce-2 preform shows a uniform distribution of nanoparticles without much interaction even to a temperature of 950 °C. At a higher concentration, a mild interaction was observed which in a way aided in keeping the nanoparticles firmly attached to Y-211 particles. Superconducting transition temperature (T c) of Ce-2 has reduced to 86 from 92 K of pure YBCO with a broad transition. Microstructures of Ce-2 composite suggest strong interaction of ceria nanoparticles with Y-211 as well as with liquid phases which resulted in the formation of fine particles of BaCeO 3 of the order of 200 nm, which has been observed by many investigators. It is possible that some amount of ceria might have entered into a superconducting matrix which can reduce T c as reported earlier. The presence of low T c phase along with a large number of fine particles have improved critical currents densities (J c ) to a field of 9 T at 65 K. The present approach has demonstrated substantial improvement in J c at high fields at the expense of low and broad T c. ∖AuthQuery{Q2}{The sentence “The present approach provides a scope for detail study on reaction mechanisms and infiltration process in IGP with nanoparticles” was modified to “The present approach provides a scope for detailed study on reaction mechanisms in infiltration and growth process (IGP) with nanoparticles.” Please check if the edit made appropriately retains the intended meaning of the sentence.}The present approach provides a scope for detailed study on reaction mechanisms in infiltration and growth process (IGP) with nanoparticles.

A bulk YBa 2 Cu 3 O 7− δ (YBCO)/Ag superconducting composite with a homogenous distribution of fine and spherical particles of metallic Ag and Y 2 BaCuO 5 (Y-211) has been fabricated by employing Directionally Solidified Preform Optimized Infiltration Growth Process (DS-POIGP). The effect of adding Ag into the liquid phase source (YBa 2 Cu 3 O 7− δ ) placed above the Y-211 preform on the microstructures and current densities is investigated. The addition of Ag led to a significant refinement of Y-211 particle size and, hence, enabled the enhancement of current densities at low fields.

The effect of compaction pressure applied to the Y 2 BaCuO 5 (Y-211) preform during its fabrication, on the final microstructures and current densities ( J c ) is investigated. The necessity for optimizing the sintering conditions in order to provide mechanical stability to the preform prior to the infiltration of liquid phases is demonstrated. The sample fabricated under optimized conditions showed J c values better than 10 3 A cm −2 up to applied magnetic fields of 6.5 Tesla at 77 K. The microstructural factors influencing the field dependence of J c ‘ J c ( H )’ at low and high fields are investigated. Extensive nano-twinning with large number of crossing twins observed in the optimized sample and the associated high defect densities are correlated to be the source of flux pinning to high fields. The fact that YBa 2 Cu 3 O 7− δ (YBCO, Y-123) superconductor obtained employing the present Preform Optimized Infiltration and Growth Process (POIGP) yields a homogenous and dense distribution of fine Y-211 particles, that creates suitable twin/defect densities to provide flux pinning at high fields, is of technical importance.

Bulk YBa2Cu3O7−δ (YBCO, Y-123) superconductors with reasonable critical current densities J c are successfully fabricated in relatively short-time durations employing Directionally Solidified Preform Optimized Infiltration Growth Process (DS-POIGP). The effect of preform compaction pressure, applied to Y2BaCuO5 (Y-211) preform prior to the infiltration of liquid phases, on the end microstructures and current densities is investigated. It is found that a preform compaction pressure of 460 MPa resulted in samples with superior microstructures and superconducting properties. YBCO sample fabricated by DS-POIGP under optimized conditions revealed presence of fine-sized Y-211 particles distributed uniformly in the matrix of Y-123 causing large interfacial defect density (Y-211/Y-123). Extensive twinning on a nano-scale with twins in the size range of 40–100 nm is observed in the optimized sample. These microstructural parameters enabled a considerable improvement in the field dependence of J c . Irreversibility fields greater than 5.5 Tesla even at 77 K are achieved in the optimized sample fabricated by DS-POIGP.

The importance of optimizing the fabrication of the Y 2 BaCuO 5 (Y-211) preform in achieving high current densities to high magnetic fields has recently been established. We report the effect of the choice of infiltration temperature 1040 °C (sample A) and 1100 °C (sample B) on the microstructure, magnetic properties and mechanical strength. Both the samples showed [1 0 3] texture after slow cooling through peritectic temperature. Infiltration at higher temperature is found to yield highly dense composites with minimal macrodefects and higher hardness of 18.73 GPa in sample B. Both the samples show uniform distribution of Y-211and comparable zero-field critical current density. High current densities are retained to a higher field of 7 T in sample B, unlike 2 T in sample A. The occurrence of [1 0 3] texture promoting higher hardness, simultaneous with retention of considerably high current density to high fields in sample B has definite advantages for trapped field applications.

Doping of non-interacting nanoparticles in YBCO superconductors in order to get fine microstructure defects has been studied. Y-211 inclusion, itself has large potential to create structural defects without affecting Y-123 matrix phase. Nanoparticles of grain refining agent like CeO2/Pt/PtO2 are interesting dopants, especially CeO2 for being cheaper than Pt. POIG process, as is one of the advance processing techniques to get uniform distribution of fine Y-211 particles of the order of 1-2 μm, throughout the YBCO composites. Finer Y-211 particles can drastically increase the pinning centers. Introduction of fine sized CeO2 as grain refining agent to reduce the size of Y-211 particles would create additional pinning centers. In the present work, the effect of CeO2 on Y-211size and distribution in POIG processed YBCO composite was studied. It was observed that with increase in CeO2 content Y-211 particle size decreases below 1μm and even less than 0.5μm for 10 weight percent CeO2 doping. The mechanism for refinement appears to be the formation of BaCeO3 on the surface of Y-211 during liquid phase infiltration that leads to splitting of Y-211 into smaller particles.

Multilayered YBa 2 Cu 3 O 7−δ (YBCO) thick films were coated on silver substrates by electrophoretic deposition (EPD) followed by heat treatment. A butanol-based YBCO suspension is used instead of the common acetone–iodine combination. Tests with several dispersing agents reveal that a branched polyethyleneimine (PEI) dispersant develops large positive surface charge on suspended YBCO particles. As a demonstration of the performance of this new suspension formulation, a 12-layer 100 µm-thick YBCO coating was deposited on an Ag tube. The superconducting transition is sharp with onset critical temperature at 92 K. The sample can shield a magnetic field of ~1.3 mT at 77 K, i.e., the best value so far for an YBCO coating on a metallic substrate.

We report the existence of a sub‐precipitate microstructure and the resulting fishtail effect in YBCO/Ag superconducting composites fabricated by Seeded Infiltration and Growth Processing. The SEM micrographs reveal sub‐precipitate microstructure in the form of precipitates of size less than 100 nm within the larger non‐superconducting Y‐211 precipitates that contributes to the enhancement of Jc in the form of secondary peak effect at lower fields.

We report record high current densities of 230 kA cm − 2 at zero field, and in excess of 10 kA cm − 2 up to 7 T at 77 K, in YBa2Cu3O7 − δ (Y-123) superconductors fabricated by a modified infiltration growth (IG) process. This was accomplished by optimizing the Y2BaCuO5 (Y-211) preform, into which liquid phases were infiltrated, through a combination of high pressure compaction and limiting the sintering temperature. The optimized sample yielded a Y-123 superconductor with a uniform distribution of fine-grained Y-211. Strong and almost invariant flux pinning observed to high fields up to 7 T, suggest a temperature independent flux pinning mechanism originating from defects in the size range 15–50 nm. Since the present sample has no added grain refiners, nano-sized dopants or mixed rare earths leading to low Tc solid solutions, a unique opportunity presents itself to investigate the cause of the enhanced flux pinning to high fields. We have therefore investigated our samples by transmission electron microscopy, and the studies revealed the presence of domains in the sample with nano-sized defects starting from the domain boundaries, as a possible source of enhanced flux pinning.