Enhancement of Critical Current Density in MgB2 Bulk with CNT-coated Al Addition

Bulk MgB₂ sample with carbon nanotube (CNT)-coated Al addition was prepared by conventional solid-state reaction at 900 °C for 30 min. We investigated the effects of Al and C co-doping on the lattice parameter, the microstructure, and the critical current density of MgB₂. The substitution of Al and C atoms for the sites of Mg and B in the MgB₂ lattice resulted in dislocations in the MgB₂ grains, which makes great contributions, along with the nanoscale oxide particles, to the enhancement of critical current density at high field (10³ A cm^?2, 7 T, 5 K) in the co-doped sample. These results contrasted significantly with the measured values of the pure MgB₂ and Al- and C-doped samples. Co-doping introduced more electrons into MgB₂ and decreased both the parameters c and a of MgB₂ lattice, and the used coating technique delayed and shortened the oxidation process of Mg and Al, leading to the decrease in the size and the content of the oxide. These advantages should be responsible for the enhancement of the critical current density as well.     

Effect of Silver Addition on Mechanical Properties of Melt-Processed Sm-Ba-Cu-O Bulk Superconductor

We have studied mechanical properties of melt-processed single-grain Sm-Ba-Cu-O (Sml23+Sm211) bulks with and without silver doping. Tensile stress is induced by thermal stress during heat treatment, and also induced by applying magnetic fields. It is found that the silver addition was effective in decreasing the thermal stress and increasing the tensile strength of the Sm-Ba-Cu-O bulk. The tensile strength of the silver doped sample was 37.4MPa, which is about one and half times larger than that of the undoped sample.     

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.     

Evaluation of Microstructural and Mechanical Properties of Ag-Diffused Bulk MgB2 Superconductors

Electrical, microstructural, and mechanical properties of undiffused and Ag-diffused bulk MgB₂ superconductors are systematically studied using dc resistivity, scanning electron microscopy (SEM), and Vickers microhardness (H _V ) measurements. The resistivity (at room temperature), critical (onset and offset) temperature, variation of transition temperature, hole-carrier concentration, surface morphology, Vickers microhardness, elastic modulus, and yield strength values of the samples are obtained and compared with each other. One can see that all superconducting parameters given above depend on the Ag diffusion on MgB₂ system. The obtained results illustrate that the room temperature resistivity reduces with the increment of diffusion annealing temperature because of the hole filling when the onset ( $T_{c}^{\mathrm{onset}}$ ) and offset ( $T_{c}^{\mathrm{offset}}$ ) critical temperatures determined from the resistivity curves are obtained to enhance from 38.4 to 39.7 K and from 36.9 to 38.8 K, respectively. Further, SEM studies carried out for the microstructural characterization demonstrate that the surface morphology and grain connectivity also improve with the increase of the diffusion annealing temperature. In fact, the best surface morphology is observed for the Ag-diffused bulk MgB₂ superconductor exposed to 850 °C annealing temperature. Besides, it is obtained that the load-dependent microhardness values reduce nonlinearly as the applied load increases until 2 N, beyond which the curves shift to the saturation region, presenting that all the samples exhibit the indentation size effect (ISE) behavior. Further, the elastic modulus and yield strength values observed decrease with the enhancement of the applied load.     

High Critical Current Densities in MgB2 Films Grown on Hastelloy Tape by Hybrid Physical-Chemical Vapor Deposition

We report on the high critical current densities in MgB₂ films directly grown on Hastelloy tapes without any buffer layer by using the hybrid physical-chemical vapor deposition method. MgB₂ films were formed by reaction of Mg metal vapor with the incoming B₂H₆ gas on the heated substrates. In MgB₂ films grown for 10 min at 500 °C in total working pressure 100 Torr with gas mixing ratio H₂:B₂H₆=70:30, we observed the transport critical current density (J _c) was approximately 10⁶ A/cm² at 4 T and 20 K in magnetic fields applied parallel to the substrate plane. This value is higher than those observed in epitaxial MgB₂ films on sapphire substrates grown by using the same method. Magnetic field dependence of J _c of this sample was well explained by the grain-boundary pinning model. Our result opens up a possibility that the coated conductors made of MgB₂ films have a strong potential for high current applications.     

Effect of Silver Admixture on Transport Properties in Polycrystalline MgB2

The normal-state and superconducting properties of polycrystalline MgB₂ samples including various amounts of Ag powder were investigated by measurements of electrical resistivity. The temperature-dependent electrical resistivity roughly obeys the power law (ρ₀ + aT ⁿ with n > 1) in the normal state and becomes linear as we increase the Silver content. The critical superconducting temperatures remain almost unchanged. Silver, not reacting with MgB₂, reacts with excess magnesium to form intermetallic layers of AgMg that cluster at crystallite boundaries.     

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.     

Analysis of Indentation Size Effect on Mechanical Properties of Cu-Diffused Bulk MgB2 Superconductor Using Experimental and Different Theoretical Models

This study indicates the change of the electrical, microstructural, physical, mechanical and superconducting properties of Cu-diffused bulk MgB₂ superconductors by means of scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness and dc resistivity measurements. The samples are prepared at different annealing temperatures in the range from 650 to 850 ^°C. Electrical and superconducting properties of samples are estimated from the dc electrical resistivity measurements. Moreover, microhardness measurements are performed to investigate the mechanical properties. Further, phase composition, grain sizes and lattice parameters are determined from the XRD measurements. At the same time, the surface morphology and grain connectivity of the samples are examined by SEM investigations. The measurements conducted demonstrate that both the Cu diffusion into the MgB₂ system and the increment in the diffusion-annealing temperature increase the critical transition temperatures. Similarly, microstructure and grain size improve while the voids and porosity decrease with the increase of the diffusion-annealing temperature. In addition, the experimental results of the microhardness measurements are investigated using the Meyer’s law, PSR (proportional specimen resistance), modified PRS (MPSR), elastic-plastic deformation model (EPD) and Hays–Kendall (HK) approach. The obtained microhardness values of the samples decrease with the increase of the diffusion-annealing temperature up to 850 ^°C. The Hays–Kendall approach is found to be the most successful model describing the mechanical properties of the samples studied in this work.     

提高MgB2超导体临界电流密度的研究进展

临界电流是超导体载流能力大小的重要表征,论述了制备工艺、元素掺杂和物理场对MgB2超导体临界电流密度的影响.使用非晶态高纯硼粉制备致密的MgB2试样有利于样品临界电流密度的提高,添加SiC或者Ti也能起到提高临界电流密度的作用,质子照射使MgB2试样在高磁场条件下的临界电流密度增加,强磁场下烧结则有利于提高MgB2样品临界的传输电流密度.     

High Critical Current Density of c-Axis-Oriented MgB2 Thin Films

From the magnetic hysteresis loops (M-H) and the current-voltage (I-V) relations, we obtained the critical current density (J _c) for c-axis-oriented MgB₂ thin films with a transition temperature of 39 K. The temperature dependence of J _c obtained from the M-H and the I-V curves coincide, which indicates the strongly linked nature of the intergrains in these thin films. And, the high value of J _c was estimated to be 4×10⁷ A/cm² at 5 K and 1×10⁵ A/cm² at 37 K under a self field, indicating a promising candidate for technological applications.     

Effects of Metalloid B Addition on the Glass Formation,Magnetic and Mechanical Properties of FePCB Bulk Metallic Glasses

Low-cost Fe_(80)P_(12-x)C_8B_x(x = 0,1,2,3 and 4 at.%) bulk metallic glasses(BMGs) with good soft magnetic and mechanical properties were prepared,and effects of metalloid B addition on the glass-forming ability(GFA) as well as thermal,magnetic,and mechanical properties of the BMGs were investigated.It was found that the proper B substitution for P improves the GFA of the Fe-P-C BMGs.The alloy with2 at.%B addition manifests the highest GFA with critical diameter for glass formation of 2 mm.Besides,these BMGs exhibit good soft magnetic properties featured by high saturation magnetization of 1.35一1.57 T and low coercivity of 2.2-7.7 A/m as well as unique mechanical properties of high fracture strength of ~3.3 GPa and visible plastic strain of 0.4%-2.5%.The combination of high GFA,good soft magnetic and mechanical properties as well as low cost makes the present Fe-P-C-B BMGs promising as soft magnetic materials for industrial applications.     

Influence of Hydrocarbon Doping on Critical Current Density and Percolation Behavior of MgB2

Journal of Superconductivity and Novel Magnetism - In polycrystalline MgB2 samples, the crystal grains are randomly oriented, and the anisotropy of the upper critical field leads to different...     

Surface Barrier and Bulk Pinning in MgB2 Superconductor

We present a modified method of preparation of the new superconductor MgB₂. The polycrystalline samples were characterized using X-ray and magnetic measurements. The surface barriers control the isothermal magnetization loops in powder samples. In bulk as prepared samples we always observed symmetric magnetization loops indicative of the presence of a bulk pinning mechanism. Magnetic relaxation measurements in the bulk sample reveal a crossover of surface barrier to bulk pinning.     

Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

Ultrafine nanostructured MgB₂ bulks with an average grain size less than 10 nm have been fabricated by high-energy ball milling and subsequent high pressure sintering. Microstructural evolution in MgB₂ subjected to high-energy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB₂ powders of about 7 nm has been estimated from Rietveld refinement analysis of XRD data, which is confirmed by a transmission electron microscope (TEM) observation. There is almost no grain growth in the subsequent sintering at low temperature of 600?°C under pressure of 3?C5 GPa for 10?C30 min. The nanocrystalline MgB₂ bulks exhibit the lower onset critical transition temperatures (T _{c onset}) of 32?C33?K. The relative wider width of the magnetic hysteresis loops at high external magnetic field and the higher critical current density (J _c ) are obtained in nanocrystalline bulks. J _c is as high as 10⁵?A/cm² in 8?T at 10?K and 2.7×10³?A/cm² in 4?T at 20?K.     

Effects of High Pressure on the Physical Properties of MgB2

The synthesis of MgB₂-based materials under high pressure gave the possibility to suppress the evaporation of magnesium and to obtain near theoretically dense nanograined structures with high superconducting, thermal conducting, and mechanical characteristics: critical current densities of 1.8?C1.0×10⁶ A/cm² in the self-field and 10³ A/cm² in a magnetic field of 8 T at 20 K, 5?C3×10⁵ A/cm² in self-field at 30 K, the corresponding critical fields being H _c2=15 T at 22 K and irreversible fields H _irr=13 T at 20 K, and H _irr=3.5 T at 30 K, thermal conduction of 53±2 W/(m?K), the Vickers hardness H _V =10.12±0.2 GPa under a load of 148.8 N and the fracture toughness K _1C =7.6±2.0 MPa?m^0.5 under the same load, the Young modulus E=213 GPa. Estimation of quenching current and AC losses allowed the conclusion that high-pressure-prepared materials are promising for application in transformer-type fault current limiters working at 20?C30 K.     

纳米晶Mg2Si块体的制备及力学性能

采用机械激活固相反应及真空热压方法制备纳米晶Mg2Si金属间化合物块体材料.研究表明,过量Mg配量对获得纯相Mg2Si至关重要.在1.5GPa压力于450℃热压得到的纯相Mg2Si纳米块体(相对密度D～98%,晶粒度d～54nm)断裂韧性达1.67MPa·m1/2,较常规粗晶Mg2Si有明显提高.     

Magnetic Characterization of SiC Doped Bulk MgB2 Superconductor

Flux pinning is a very significant mechanism to improve critical current density in MgB₂ for many applications, such as developing the performance of wires and tapes. In this study, we have done a systematic study of SiC inclusions in the main matrix of MgB₂. Sample production in the form of MgB_2?x (SiC) _x was carried out by using solid state reaction method (x=0,0.01,0.02,0.04,0.06,0.10) in the argon atmosphere. The structural and magnetic properties of the prepared samples were investigated. Structural characterization of samples was performed by the X-ray powder diffraction (XRD) technique and the Scanning Electron Microscopy (SEM) measurements. Magnetic properties were determined by M?CH loops, magnetization-temperature (M?CT), and AC susceptibility measurements. The T _c of the samples was determined by the M?CT measurements while the J _c is determined through the M?CH measurements. SiC inclusions up to some ratios (x=0.06) increase the critical current density by up to about 50%, while the critical temperature T _c remains unchanged in compatible with the literature.