Superconducting Properties of In Situ Mg1.05(B1−xCx)2 Doped with Melanin (C16H2O3N2) via Ultrasonication in Ethanol

In this paper, we have reported melanin (C₁₆H₂O₃N₂) as a dopant of MgB₂ for the first time. Here, the effects of melanin doping to the microstructures and superconducting properties of bulk MgB₂ are thoroughly studied from XRD, SEM, TEM, magnetization, and resistivity data. We have analyzed the critical current density (J _c), irreversibility field (H _irr), flux pinning, resistivity, lattice parameters, grain sizes, critical temperatures (T _c), and other microstructures of all the samples. We have varied the doping percentage according to the nominal atomic ratio of Mg_1.05(B_1?x C _x )₂, x=0,0.02,0.06,0.08,0.1. The J _c of all the melanin-doped samples are improved as compared to that of the undoped sample in high-field region (above 6 Tesla) at low temperature. The 8 and 10 % doped samples give the best results. The 8 % doped sample registers an enhancement of J _c by a factor of 3.6 at 7 T and 5 K as compared to that of the undoped one. But, in the low-field region, melanin doping reduces J _c. The H _irr shows remarkable enhancement at low temperatures below 20 K. The best value of H _irr was found for the 8 % doped sample. However, H _irr reduces at high temperatures above 20 K in all the melanin-doped samples. The volume pinning strength of all the doped samples is enhanced over the entire field range. Further improvement in superconducting properties can be achieved by further reducing the size of the melanin particles, increasing density, and improving the homogeneity of doping.     

Effect of Ag Addition on the Surface Topography and the Vibrational Dynamics of MgB<Subscript>2</Subscript>

We fabricated MgB₂ samples with Ag additions using in situ solid-state reaction via a single-step sintering to study the effect of Ag on the structural, vibration, and superconducting properties of MgB₂ samples. Ag addition to MgB₂ resulted in a significant improvement in J _c although no appreciable effect was observed in the lattice parameters and the superconducting transition temperature T _c. Dramatic increase in the grain size was observed with Ag addition and topographic measurements with atomic force microscopy revealed the formation of Ag–Mg nanoparticles 5–20 nm in size at 2 and 4 wt% Ag additions. The fact that these samples showed high J _c values suggests that the nanoparticles formed as a result of Ag addition are responsible for enhanced flux pinning. Raman spectroscopy measurements showed that Ag additions also increased disorder in the system and thereby affected the line width of the Raman active E _2g mode.     

Sintering mechanism of Ag-doped MgB<Subscript>2</Subscript> superconductor from low temperature to high temperature

Combined with thermal analysis and phase identification, the sintering process of Ag-doped MgB₂ superconductor was investigated. It is found that the Ag doping could form Mg–Ag liquid through the eutectic reaction at low temperature (about 470 °C) and then obviously accelerated the formation of MgB₂ phase. Moreover, a sintering model is also proposed to illustrate the liquid activated sintering mechanism present in the sintering process of Ag-doped MgB₂ samples. The sintering model is supposed to provide theoretic guidance for optimizing the sintering condition in the synthesis of doped MgB₂ superconductors.     

Comparative study on indentation size effect, indentation cracks and superconducting properties of undoped and MgB2 doped Bi-2223 ceramics

This study examines the evaluation mechanism of MgB₂ doped Bi_1.8Pb_0.4Sr₂(MgB₂)_xCa_2.2Cu_3.0O_y (0 ≤ x ≤ 1.0) superconducting ceramics prepared by conventional solid-state reaction method via dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and Vickers micro hardness (H_v) measurements. Variation of room temperature resistivity, critical transition temperatures (onset and offset), phase purity, cell parameter, texturing, grain connectivity, surface morphology, crystallinity and H _v values of the materials are deduced and compared with each other for the determination of the optimum doping level in the Bi-2223 system. It is found that all the properties given above depend strongly on the MgB₂ concentration. From dc resistivity investigations, each sample studied exhibits the superconducting behavior below their variable offset critical temperature values. The maximum onset (T _c ^onset ) and offset (T _c ^offset ) temperatures are found to be about 121.3 and 114.1 K, respectively, for the sample doped with x = 0.05. The minimum T _c ^onset of 118.6 K and T _c ^offset of 109.4 K are observed for the sample doped with x = 1.0. Similarly, XRD and SEM examinations indicate that there is an improvement in the crystal structures and surface morphologies of the superconducting materials with the increment of the MgB₂ inclusions in the Bi-2223 system up to x = 0.05 beyond which the crystallinity, grain connectivity and surface morphology start to degrade regularly and in fact reach to the worst structure appearance for the doping level of x = 1.0. Furthermore, the H_v measurement results being analyzed by Meyer’s law, proportional sample resistance (PSR), modified PSR, elastic–plastic deformation model, Hays–Kendall (HK) approach, Indentation-induced cracking model (IIC) allow us to derive the mechanical properties of the superconducting samples for the potential technological and industrial applications. According to the results obtained, HK approach, among the mechanical analysis methods, is determined as the most successful model for the samples (doped with x = 0, 0.1, 0.3, 0.5 and 1.0) exhibiting indentation size effect behavior whereas the IIC model is noted to be superior to other models for the other samples (doped with x = 0.01, 0.03 and 0.005) presenting reverse indentation size effect feature.     

Improved photodegradation activity of SnO<Subscript>2</Subscript> nanopowder against methyl orange dye through Ag doping

Silver doped tin oxide (SnO₂:Ag) nanopowders were synthesized by a simple soft chemical route with 0, 5, 10 and 15 wt% concentrations of Ag. The structural, morphological, optical, photoluminescence and photocatalytic properties of the synthesized samples were studied and the results obtained are reported in this paper. XRD studies confirm the polycrystalline nature of the synthesized samples. The undoped and doped samples exhibit a strong (1 0 1) preferential growth. Decreased crystallite size is observed with Ag doping. Nanosized grains were observed for the doped samples. Peak related to Sn–O–Sn lattice vibration is observed for both the undoped and doped samples in the FTIR spectra. Peaks related to oxygen vacancies were observed at 362 and 499 nm for all the samples in the PL spectra. Enhanced photocatalytic activity was observed for the doped samples and the SnO₂:Ag nanopowder with 10 wt% Ag doping concentration exhibited maximum photodegradation efficiency against the degradation of methyl orange dye.     

Superconducting properties and growth mechanism of layered structure in MgB2 bulks with Cu/Y2O3 co-doping

Bulk MgB₂ samples containing Cu and Y₂O₃ have been prepared by conventional solid state reaction at 850 °C, and the structure and superconducting properties have been investigated. Differing from the structure in previous studies, a type of novel, layered structure was obtained in Cu/Y₂O₃-doped MgB₂ sample. Furthermore, the critical current density (J _c) at high field (>4 T) was improved compared to undoped, and Cu-doped MgB₂ samples. After analyzing the phase composition, microstructure, and sintering process, it was found that Y₂O₃ and Cu are independent in providing effective pinning centers, and YB₄ impurity should be responsible for the enhancement of J _c as well as the increased irreversible magnetic field. However, J _c at low field was slightly worsened, but still maintaining 10⁵ A cm^?2 at 0 T, since the intercrystalline connectivity was not seriously deteriorated. Finally, a possible growth model was put forward to describe the formation sequences of the layered structure. It was supposed that the formation of steps at low temperature originated from the coherent relationship between Mg and MgB₂, while the steps formed at high temperature were related to the pinning effect of secondary phase during the migration of grain boundary.     

AC Susceptibility Study of Superconducting YBa2Cu3O7:Ag x Bulk Composites (x=0.0–0.20): The Role of Intra and Intergranular Coupling

We report the effect of silver addition on superconducting performance of bulk YBCO (YBa₂Cu₃O₇) superconductor. All the studied samples are prepared by conventional solid-state reaction method. Rietveld fitted X-ray diffraction data confirmed the single phase formation for all the studied samples. Detailed AC susceptibility measurements as a function of driven AC amplitude (1 Oe–17 Oe) of these samples revealed the enhancement of grains coupling with increasing Ag content in YBCO + Ag _x composite system. 10 wt% Ag added YBCO superconductors exhibited the optimal intergranular coupling. The Scanning Electron Microscopy (SEM) observations indicate an increase in the grains connectivity in terms of narrow grain boundaries for doped samples. The average grain size is found to increase with Ag doping. It is concluded that limited addition of Ag in bulk YBCO superconductor significantly improves the grains coupling and, as a result, the optimal superconducting performance. YBCO + Ag composites could prove to be potential candidates for bulk superconducting applications of the studied high-T _c system.     

Effect of Rare-Earth Oxides Doping on the Superconductivity and Flux Pinning of MgB<Subscript>2</Subscript> Superconductor

A series of rare-earth-oxide-doped MgB₂ bulks are prepared by in situ solid-state reaction with Pr₆O₁₁, CeO₂, Lu₂O₃, and Ho₂O₃ as the dopants. The superconducting properties are investigated and compared with the nano-Fe₃O₄ doped MgB₂. It is found that different from doping ferromagnetic nano-Fe₃O₄ which drastically suppresses superconductivity of MgB₂, doping the rare-earth-oxides has little effect on superconductivity of MgB₂ although most rare-earth elements have strong magnetic moment. In addition, some boride impurities formed during the reaction between rare-earth oxides and boron can work as effective pinning centers and significantly improve J _c and H _irr of MgB₂ when these fine nanoboride precipitates (<20 nm) are embedded into the MgB₂ intragrains.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Enhancement of Critical Current Density of MgB<Subscript>2</Subscript> by Glutaric Acid Doping: a Simultaneous Improvement on the Intrinsic and Extrinsic Properties

In this study, we report an enhancement of critical current density of bulk MgB₂ superconductors by glutaric acid (C₅H₈O₄) doping. The effects of glutaric acid doping on MgB₂ lattice resulted in a record self-field J _c of the order of 10⁶ A/cm². A simultaneous improvement in the connectivity, pinning force, and H _c2 is the major factor that determined excellent J _c performance. X-ray diffraction analysis showed that samples were single-phase MgB₂ with a minor trace of impurities. A dramatic change in grain morphology and homogeneity in grain distribution was found in the SEM images of doped samples. We observed that homogeneity in grain distribution played a crucial role in the connectivity and the upper critical field (H _c2) of the doped samples. We were able to introduce a new dopant through a two-step mixing approach which is suitable to overcome the degradation of low field and self-field J _c reported for carbon-doped MgB₂ superconductor samples.     

Comparison of Critical Current Density in Undoped,Glycine-doped,and Cu-and-Glycine-Co-doped MgB2 Synthetized from nm-Boron and μm-Boron

Nano-boron (800 nm) and μm-boron (25 μm) precursor were used to synthetize glycine-doped, Cu-and-glycine-co-doped, and undoped MgB₂ samples at 800 ^°C. The C substitution level caused by glycine doping, the MgO content, and the full width at half maximum of the (101) peak for MgB₂ phase were compared to evaluate the critical current density (J _c) of the six samples. The undoped sample from the nm-boron powder showed enhanced J _c over the entire field in contrast with those from 25-um boron, since the excess MgO in nm-boron prepared sample serves as effective pinning centers. On the contrary, due to the reduced MgO pinning centers as well as the increase of the grain size, the glycine-doped nm-boron sample only enhanced the J _c performance in the high-field region (H>4.5 T), while the low-field J _c values showed a considerable decrease. For the Cu-and-glycine-co-doped sample, the J _c performance is nearly without regard to the size of the boron precursor as the high-field J _c of the nm-B sample decreased a little, while the low-field J _c remained at the same level as that of the μm-B sample.     

The Effects of Structural,Thermal, and Magnetic Properties of Hexylbenzene-Doped MgB<Subscript>2</Subscript> Superconductor

The effects of the amount of hexylbenzene additive (C₁₂H₁₈) on the structural, thermal, and magnetic properties of MgB₂ superconductor are examined in this study. Pure and hexylbenzene-doped MgB₂ bulk samples were produced with in situ solid-state reaction method. X-ray diffraction patterns of MgB₂ doped with MgB₂ and hexylbenzene at different ratios were determined to have MgB₂ as the main phase and consisted of a small amount of MgO. Pure and different ratios of hexylbenzene-doped Mg and B starting powders were heat-treated by a differential scanning calorimeter between room temperature and 800 °C. It was determined from the differential scanning calorimetry curves obtained that the first exothermic peak pointed the MgB₂ phase emerging with a solid–solid (Mg–B) reaction, and this temperature shifted towards the low temperatures as the hexylbenzene addition rates increased. It was observed that there was dependency to the applied field in all samples from the ac susceptibility measurements as a function of the temperature in pure and hexylbenzene-doped MgB₂ superconductor materials, and shift towards the lower temperatures in T _c, superconducting transition temperature, with increasing content. It was observed that the changes occurred in in-phase (\(\chi ^{\prime })\) and out-off-phase (\(\chi ^{\prime \prime }\)) components of ac susceptibility both weakened the MgB₂ phase structure of hexylbenzene content and, as a result of this, led to changes in the pinning mechanism.     

Effect of Nano-AlN Addition on the Critical Current Density of MgB2 Superconductors

A series of MgB₂ polycrystalline pellets with and without aluminum nitride (nano)addition have been synthesized by a solid-state reaction. No detectable shift in the XRD peak position of MgB₂ has been observed with AlN nanoparticles addition indicating no substitution for Mg or B. Surface morphology displays randomly oriented well-defined grains. The presence of AlN nanoparticles between the grain boundaries and also on the grain surface is observed in AlN-added MgB₂. Addition of AlN nanoparticles decreases the superconducting transition temperatures from 38.5 to 37 K. Resistivity data confirm an improved connectivity of the grains in MgB₂ pellets for low-level addition of AlN nanoparticles. Magnetization data measured at various temperatures between ±6 T are used to estimate the critical current density (J _c ) of undoped and AlN nanoparticles doped MgB₂ pellets. J _c enhances significantly up to 4 T at 20 K for MgB₂ containing 0.5 wt.% AlN. Our analysis confirms excellent correspondence of the measured field dependence of critical current density (J _c ) in terms of collective pinning model. A new universal scaling behavior is established between the reduced critical current density, J _n (=J _c /J _c (0)) and the reduced field h _n (=H/H ₀) for both undoped and AlN-doped MgB₂ pellets at different temperatures, where J _c (0) and H ₀ are the best fit parameters obtained from the collective pinning model. Our analysis further confirms the dominance of δT _c -type pinning for pure MgB₂, and for n-AlN-added MgB₂ pellets at low temperatures the major contribution comes from δl-type pinning.     

Superconductivity in MgB2: Phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB₂, and superconducting transition in carbon doped MgB₂ are presented. The superconducting transition temperature in MgB_{2 x}C_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39-4 K forx = 0 to 26 K forx = 0.5. It is shown the changes in lattice volume, as obtained from x-ray diffraction measurements, can account only partially for the observed decrease inT _c . The observed variation ofT _c with carbon content is seen to correlate with the Debye temperatures, obtained from an analysis of the resistivity data. Investigation of the phonon modes in MgB₂, through infrared absorption measurements indicate three modes at 410,475 and 560 cm^-1. The former two are associated with the infrared active modes, and the third component is associated with the Raman mode, that gets activated due to disorder. A study of the temperature dependence of these modes indicates no changes across the superconducting transition. The mode at 560 cm^-1 shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity.     

Investigation of Lu effect on YBa2Cu3O7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7?δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.     

Enhanced Critical Current Properties of MgB<Subscript>2</Subscript> Bulks by Doping Amorphous Carbon Containing Magnetic Impurity

The doping effect of amorphous carbon (C) containing magnetic impurity in MgB₂ bulk has been studied. Structural characterization by means of X-ray diffraction and the superconducting transition temperature, T _c , measurement indicate that little C effectively enters the MgB₂ structure. This should be due to the lower sintering temperature. The upper critical field, H _c2, and irreversibility field, H _irr, of samples show no systematic evolution with C doping. However, critical current density J _c (H) performance is greatly improved with C doping at 4, 15, and 28 K, respectively. Corresponding to this case, scanning electron microscope (SEM) image indicates that the grain size in samples becomes very small and grain boundary is developing roundness with the increasing of C content. This should be intimately related with the increase of magnetic impurity along with C doping. The result is discussed.      

Studies on the Composite System of Bi-2212 Glass Ceramic and MgB<Subscript>2</Subscript> Superconductors

The composites of glass ceramic Bi-2212 and MgB₂ superconductors were prepared at ambient conditions. The transmission electron microscopy images of the composite samples illustrate the presence of glass ceramic inclusions in bulk MgB₂. Temperature-dependent magnetization of the composite samples shows two superconducting transitions: one at 80 K corresponding to the Bi-2212 phase and a second one at 39 K corresponding to the MgB₂ phase, suggesting that the two superconducting phases are separated with clear boundaries. The critical current density (J _c) and pinning force values are increased in composite systems by an order of magnitude compared to that of individual samples. The pinning mechanism in the composite sample is the same as in the matrix phase. Reduced field maxima (h _max) are observed at 0.15 for composite samples. A low value of h _max for composite samples indicates the random orientation of grain boundaries and repulsive pinning force in the composite samples.     

The Effect of Cu Addition on the Phase Formation and Critical Current Density in the Sugar Doped MgB2 Superconductor

With the aim of improving the critical current density (J _c ) in the MgB₂ superconductor, minor Cu (3?at%) was doped to the MgB₂ samples in-situ sintered with Mg powder and sugar-coated amorphous B powder. Combined with thermal analysis, phase identification, microstructure observation and J _c measurement, the effect of minor Cu addition on the sintering mechanism, microstructure and critical current density of sugar-doped MgB₂ superconductors were investigated. It is found that the minor Cu addition could obviously accelerated the MgB₂ phase formation and improve the growth of MgB₂ grains during the sintering process of sugar-doped MgB₂ due to the appearance of Mg?CCu liquid at low sintering temperature. On the other hand, the Mg?CCu liquid hindered the reactive C released from sugar entering in the MgB₂ crystal lattice. Hence, the connectivity between MgB₂ grains was improved accompanying with the C substitution for B is decreased. At 20?K, the J _c of co-doped samples at low fields was further increased whereas it is decreased at high fields, compared with the only sugar-doped samples.     

Structure and Superconductivity of (MgB<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.97</Subscript>Cu<Subscript>0.03</Subscript>

A series of samples of (MgB_2−x C _x )_0.97Cu_0.03 (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25) and MgB₂ were synthesized by a solid state reaction method. The structure, superconducting transition temperature and transport properties of the samples were studied by means of X-ray diffraction (XRD) and resistivity measurements. It is found that the c-axis of the lattice remains unchanged with increasing C doping, while the a-axis shows a small decrease. The T _c of the samples steadily decreases with increasing C doping. It is suggested that the chemical pressure effect plays a more important role influencing the normal state transport and T _c than the change of carrier concentration.     

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB₂ monofilamentary tapes are reported. Polycrystalline powders of MgB₂ doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H _c2), irreversibility field (H _irr) and critical current density (J _c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J _c), being determined from both magnetization (J _cm) and transport (J _ct) measurements, was improved significantly. In particular, the SiC doped MgB₂ tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J _ct of nearly 10⁴ A/cm² under applied fields of as high as 7 Tesla. Further, it was found that the J _ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB₂ tapes was responsible for decreased anisotropy and improved J _c(H) performance.