Properties Of MgB2 Research Articles

Refinement and carbon incorporation effects on the superconducting properties of MgB2 through wet milling process of low purity boron powder

The wet milling effects of boron (B) powder on the full width at half maximum (FWHM) value of X-ray reflexes, grain size, carbon (C) substitution, critical temperature (Tc), critical current density (Jc), irreversibility field (Hirr), and upper critical field (Hc2) have been investigated for MgB2 bulk superconductors. The semi-crystalline B powder with a low purity of 95–97% was refined with different milling times of 1 to 15h using a toluene medium. As a result of B powder milling, the particle size of the B powder decreased and the semi-crystalline phase became an amorphous phase. As the milling time increased, Tc of MgB2 bulk decreased by the deterioration of the crystallinity and Jc increased over the applied magnetic fields. With the refinement of the B powder, the grain boundary pinning increased due to smaller MgB2 grain size. In addition, it was found that a C substitution for the B site occurred from a toluene medium during wet milling process. The increased electron scattering due to an increase of the defects like grain boundary and lattice disorder by C substitution led to enhance the superconducting properties with an increase of the FWHM value of the MgB2 peaks and a decrease of the Tc. The MgB2 prepared from B powder mechanically milled for 7h showed the highest magnetic Jc of approximately 104A/cm2 at 5K and 8T. The Hc2 and Hirr were also improved due to the increase of grain boundary pinning and lattice disorder as a result of milling in toluene.

Mechanical properties of MgB2 superconducting wires fabricated by internal Mg diffusion process

Mechanical properties of MgB2 monocore superconducting wires fabricated by an internal Mg diffusion (IMD) process and a powder-in-tube (PIT) process were investigated. The uniaxial tensile strain dependence of critical current (Ic) and the transverse compressive stress dependence of Ic were investigated at 4.2 K, 10 T. The IMD-processed MgB2 wire showed a large irreversibility tensile strain limit (ϵirr = 0.67%) and transverse compressive stress tolerance of 206 MPa, whereas the PIT-processed wire had 0.51% and 160 MPa. Furthermore, the tensile strain sensitivity of Ic for the IMD-MgB2 wire was smaller than that for the PIT-MgB2 wire. Consequently, mechanical properties of IMD-processed MgB2 superconducting wires are better than those of PIT-processed wires.

Elastic Properties of MgB2 and SiC-Doped MgB2 Superconductors

We investigated the elastic properties of MgB2 and 5 wt.% SiC-doped MgB2 superconductors utilizing the pulse-echo overlap technique. Longitudinal and shear ultrasound velocities were measured for each sample at 80 K and 300 K. The measured velocities at 80 K were used to calculate the various elastic moduli, i.e. longitudinal (CL), shear (G), bulk (B) and Young’s (Y) modulus, and the Debye temperature D, for both samples. The high D at 694 K and 706 K obtained for MgB2 and SiC-doped MgB2, respectively, provide strong evidences via direct acoustic measurements to support numerous theoretical and non-acoustic data-based calculations. At 300 K, the higher longitudinal velocity of the pure MgB2 compared to the doped MgB2 but converging towards the same value of longitudinal modulus at 80 K, seems to suggest that the SiC-doped MgB2 undergoes greater elastic stiffening through temperature range of 300 K down to 80 K. Using the D values, the electron-phonon coupling constant  was calculated within the BCS framework and the two dimensional van Hove scenario. The results led us to conclude that MgB2 is a moderately-strong coupled superconductor.

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB2 superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T c ) increase with increasing the diffusion-annealing temperature from 650 to 850 °C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850 °C. As for the XRD results, all the samples contain the MgB2 phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81 × 10−8 to 4.69 × 10−7 cm2 s−1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D = 3.75 × 10−3 exp (−1.15 ± 0.10 eV/k B T) and the corresponding activation energy of copper in MgB2 system is found to be about 1.15 eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.

Influence of Ni addition on the process of phase formation in MgB2 bulk

Nickel is an important sheath material for the fabrication of MgB2 wires. However, the effects of Ni doping on the phase formation and superconducting properties of MgB2 remain controversial. In this work, Ni powder is selected for doping in MgB2 bulk in order to examine the corresponding changes. Combining with the DSC analysis and in-situ XRD results, we find indications that the Ni powder reacted with Mg and B, forming MgNi2.5B2 at 600°C. The ternary compound began to decompose at a temperature above 800°C. The reactive phase, MgNi2.5B2, acted as an obstacle to the supercurrent flow, creating weak links among the MgB2 grain boundaries. However, it is found that the added Ni formed a eutectic liquid phase with Mg at 506°C. The liquid phase helps the formation of MgB2 at low temperature, which not only increases the density of the sample, but also improves the grain connectivity. Consequently, the presence of Ni in the MgB2 sample is not necessarily a disadvantage; it depends on the desired application.

Effect of high-energy ball milling time on superconducting properties of MgB2 with low purity boron powder

MgB2 bulks and tapes were fabricated using ball-milled precursor powders. The selected B powders have the β-rhombohedral crystalline structure as a main phase, with a small amount of MgO impurity. Milling time was tested from 0.5 to 120 h to get an optimized value. It is found that ball milling can facilitate MgB2 phase formation, refine the MgB2 grains and increase the defect density. Moreover, the active area, Bc2 and flux pinning are improved, which is of benefit to the enhancement of Jc performance. The 80 h milled MgB2 bulk sample has a Jc value of 2.8 × 104 A cm−2 at 5 K and 6 T, 41 times larger than that of the 0.5 h sample. At the same time, the transport Jc of the 80 h MgB2 tape reaches 2.3 × 103 A cm−2 at 4.2 K and 9 T, 32 times larger than that of the 0.5 h one. Due to the increase of impurities and the release of lattice deformation and strain, the Jc was decreased for the sample milled up to 120 h.

Optimization of Phase Formation and Superconducting Properties in MgB2 Prepared by Phase Transformation from MgB4

In this work, polycrystalline MgB4 was prepared, and then the presence of MgO in the powder was reduced by acid leaching. To synthesize MgB2, the MgB4 powder was reacted with Mg at temperatures from 650°C to 950°C for 4 h and 8 h. Using the Rietveld method, the MgB2 phase was estimated to be in greater proportion over the sintering temperature range of 650°C to 750°C, above which it decreased rapidly, accompanied by the formation of more of the MgB4 phase. Scanning electron microscopy showed that the samples have reduced porosity compared with those synthesized by the direct reaction of (Mg + 2B). Upon increasing the sintering temperature, the superconducting transition temperature decreased, which is attributed to lattice distortion. The results of this work demonstrate that control of heat treatment is essential to optimize the weight fraction of the MgB2 phase and thereby increase the critical current density, Jc. The value of the magnetic Jc (5 K, 1 T) for the sample sintered at 750°C for 4 h is estimated to be 1.00 × 105 A/cm2.

Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi

Increasing dissipation-free supercurrent has been the primary issue for practical application of superconducting wires. For magnesium diboride, MgB2, carbon is known to be the most effective dopant to enhance high-field properties. However, the critical role of carbon remains elusive, and also low-field critical current density has not been improved. Here, we have undertaken malic acid doping of MgB2 and find that the microscopic origin for the enhancement of high-field properties is due to boron vacancies and associated stacking faults, as observed by high-resolution transmission electron microscopy and electron energy loss spectroscopy. The carbon from the malic acid almost uniformly encapsulates boron, preventing boron agglomeration and reducing porosity, as observed by three-dimensional X-ray tomography. The critical current density either exceeds or matches that of niobium titanium at 4.2 K. Our findings provide atomic-level insights, which could pave the way to further enhancement of the critical current density of MgB2 up to the theoretical limit. Hiroaki Kumakura, Shi Xue Dou and co-workers have uncovered the role of carbon doping at the microscopic scale on magnesium diboride (MgB2). This inexpensive superconductor — a material with no electrical resistance below a certain temperature — is also sensitive to magnetic fields. The researchers show that incorporating an organic molecule into MgB2 (‘carbon doping’) improves its critical current density under low magnetic field — an effect previously known only for high fields. Detailed characterizations revealed that the number of voids within the material increased, yet their size decreased, so that MgB2 was denser after doping. The team suggests that a boron deficiency causes microscopic defects within the superconductor's structure, improving its properties, while a particular arrangement of the doping molecules can explain the increased density. These findings may lead to further enhancement of the properties of MgB2, which has recently attracted interest in various fields. In medical magnetic resonance imaging, for example, MgB2-based magnets are promising alternatives to the current NbTi-based technology that relies on expensive cooling with liquid helium. Where does carbon go when it is doped into magnesium diboride (MgB2) and why the superconducting properties are improved? In this work, malic acid-doped MgB2 was investigated and it was shown that carbon encapsulates boron powder, prevents agglomeration and as a result reduces void fraction as was confirmed by the first detailed X-ray tomogram analysis. It was also found that carbon induces a lot of stacking faults within MgB2 grains. The critical current density is now comparable to commercial niobium titanium (NbTi) wire and further improvements are expected.

Superconducting Properties of MgB2 with Addition of Other AlB2–type Diborides and Carbon Sources, Prepared Using High Energy Ball Milling and HIP

Abstract In the present work it is described the production of MgB2 samples by using the mixture of MgB2 with other diborides, (TaB2, VB2, and AlB2) which have the same C32 hexagonal structure as the MgB2, and simultaneous addition with the diborides and SiC, that contribute with C, to replace B in the crystalline structure of the matrix. As an important result, the critical current density ( Jc ) was improved at low magnetic fields when just the diborides are added. However, when SiC is added simultaneously with the diborides, the result is the improvement of Jc at high fields. The critical temperature ( Tc ) was maintained high.

The effect of reduced graphene oxide addition on the superconductivity of MgB2

Reduced graphene oxide (rGO) and highly reduced chemically converted graphene (rCCG) samples were prepared under different processing conditions and were doped into MgB2 by a diffusion process at 800 °C for 10 hours. It was demonstrated that a small addition rCCG can significantly improve the superconducting properties of MgB2. Doping of MgB2 with 1 mol% of rCCG resulted in a Jc of 5.45 × 105 A cm−2 at 20 K in self-fields, which is nearly 32% improvement over that of the undoped sample. This is a significant improvement as most carbon sources adversely affect the Jc performance at the zero field.

The effect of Ti3SiC2 doping on the microstructure and superconducting properties of MgB2

Abstract Doping of MgB2 by Ti3SiC2 and its potential for improving the flux pinning were studied for MgB2(1-x)(Ti3SiC2)x with x = 0, 0.025, 0.05, 0.075 and 0.1, respectively. These bulks were pressed at 20 - 50 MPa and heated to 650 - 950°C for 1.5 - 3 h. It was found that optimal samples were obtained, which were pressed at 40 MPa and sintered at 850°C for 2 h. The transition temperature (Tc) was measured on the optimal series samples using dc susceptibility and a reduction of Tc from 38 K to 36 K was observed. The critical current density (Jc) which was deduced from the hysteresis loop was found to increase at doping levels x = 0.025 - 0.075. With a higher doping level x = 0.1, Jc was suppressed lower than that in the pure one due to non-superconducting precipitates collection at the grain boundaries.

Enhanced superconducting properties ofMgB2 tapes achieved by treating B powder with glycolic acid

MgB2 tapes were fabricated by the in situ powder-in-tube method usingC2H4O3-treated B powder as a source material. It is found that theJc valuesof MgB2 tapes are significantly improved by treating B powder withC2H4O3. At 4.2 K,the Jc of theC2H4O3-treated samplesintered at 800 °C reaches 5 × 103 A cm − 2 at12 T. The C2H4O3-treated B improves not only the upper critical field through enhancing intraband scatteringbut also the flux pinning by increasing grain boundaries and C substitution effects. A largeramount of intragranular structural defects and grain boundaries were found in theC2H4O3-treatedMgB2 matrix. Benefiting from the good crystallinity, theC2H4O3-treated tapesintered at 800 °C shows a bigger active area fraction and a higherJc than the samplesintered at 700 °C. These results suggest that treating B powder withC2H4O3 is an effective way to enhance the critical current density ofMgB2 superconductors.

Thermal–electrical transport of high purity, melt impregnated, polycrystallineMgB2

In the polycrystalline superconductorMgB2 the fraction of cross section that carries electrical current was previouslyshown to be obtained from the normal state electrical resistivity. Thelatest single crystal measurements show the thermal transport properties ofMgB2 to be dominated by electrical transport. For the first time we apply the electrical crosssection to the thermal conductivity of three polycrystalline samples with different grain sizeand porosity. It is found that the results can be readily discussed in the context of granularthermal–electrical properties consisting of common temperature dependent transportproperties independent of grain size and porosity. The result gives an alternativeinterpretation to the temperature dependent thermal transport than given in previouspublications.

Flux pinning mechanisms in graphene-doped MgB 2 superconductors

The effects of graphene doping on the superconducting properties of MgB2 were studied. We found that small addition of graphene significantly improves the superconducting properties of MgB2, with only a small reduction in Tc. Low resistivity, high critical fields and enhanced flux-flow activation energy were observed for the optimally doped bulk sample. The spatial fluctuation in the transition temperature (δTc pinning) is the flux pinning mechanism in graphene-doped MgB2.

Effect of the Powder Strain State on the Mechanical Properties of MgB$_{2}$ Tapes

Starting from commercial MgB2 powder of 325 mesh, we further reduced the agglomerates by attrition milling in nitrogen atmosphere to minimize oxidation. Using different milling media (tungsten carbide, yttria stabilized zirconium oxide and stainless steel balls), we obtained different milling energies and, consequently, powders with different strain states. Crystallite size and micro-strain within the grains were estimated by x-ray diffraction. We investigated the changes in the superconducting properties with the use of different milling media by measuring the magnetic moment as a function of temperature in a SQUID magnetometer, after zero field cooling. Single-filament stainless steel-sheathed wires and tapes were prepared with the obtained powders. The flexural properties of the tapes were determined by 4-point bending micro-tests. The use of a precursor powder with a higher strain resulted in more flexible wires and tapes, with lower flexural strength. Three different regions were determined for the maximum normal stress-strain curves. Some in-situ monel-sheathed hollow wires and ex-situ titanium-sheathed tapes and monel-sheathed 7-filament tapes were measured for comparative purposes. We discuss the correlation between sheath yielding and superconducting core cracking with the possible critical current degradation due to mechanical failure of the wires and tapes.

Critical current density in MgB2 bulk samples after co-doping with nano-SiC and poly zinc acrylate complexes

The co-doped MgB2 bulk samples have been synthesized using an in situ reaction processing. The additives is 8wt.% SiC nano powders and 10wt.% [(CH2CHCOO)2Zn]n poly zinc acrylate complexes (PZA). A systematic study was performed on samples doped with SiC or PZA and samples co-doped with both of them. The effects of doping and co-doping on phase formation, microstructure, and the variation of lattice parameters were studied. The amount of substituted carbon, the critical temperature (Tc) and the critical current density (Jc) were determined. The calculated lattice parameters show the decrease of the a-axis, while no obvious change was detected for c-axis parameter in co-doped samples. This indicates that the carbon was substituted by boron in MgB2. The amount of substituted carbon for the co-doped sample shows an enhancement compared to that of the both single doped samples. The co-doped samples perform the highest Jc values, which reaches 3.3×104A/cm2 at 5K and 7T. It is shown that co-doping with SiC and organic compound is an effective way to further improve the superconducting properties of MgB2.

Effect of W-addition on pinning property of MgB2

The 5d transition metal W was added into the MgB2 superconductor. The Mg, B and W were sintered at 1173K for 30min under H2/Ar atmosphere in the electric furnace. The Wx(MgB2)1−x samples were prepared in the W concentration range of 0⩽x⩽0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of Jc at 20K were analyzed by the extended critical state model. The enhancement of Jc became maximum for the x=0.02 sample.

Comparative Experimental and Density Functional Theory (DFT) Study of the Physical Properties of MgB2 and AlB2

In the present study, we report an intercomparison of various physical and electronic properties of MgB2 and AlB2. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB2 shrinks in c-direction for AlB2 with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB2. Superconductivity is not observed for AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is −ve for AlB2. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB2 to Debye–Einstein model and estimated the value of Debye temperature (Θ D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E F) is calculated. From the ratio of experimental N(E F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below T c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB2, and hence it is not superconducting. DFT calculations demonstrated that for MgB2, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB2 and AlB2.

ChemInform Abstract: Binary Boron‐Rich Borides of Magnesium: Single‐Crystal Investigations and Properties of MgB7 and the New Boride Mg≈5B44.

AbstractDark red single crystals of MgB7 are obtained from a Cu/Mg/B melt (3:1:0.75; BN crucible in a sealed Ta tube, 1600 °C, 40 h).

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 1.8–1.0×106 A/cm2 in the self-field and 103 A/cm2 in a magnetic field of 8 T at 20 K, 5–3×105 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⋅m0.5 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 20–30 K.