Papers by Keyword: MgB₂

Abstract: 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.

Abstract: MgB₂ has the higher critical temperature of superconducting transition (T_C : 39K) among the intermetallic compound superconductive materials, however, MgB₂ is hard for practical use because of its unworkable and lower critical current density (J_C) in a high magnetic field than Nb-based superconductive materials. We have developed the original method of three-dimensional penetration casting (3DPC) to fabricate the MgB₂/Al composite materials. In the composite material we made, MgB₂ particles dispersed to the matrix uniformly. Thus, these composite materials can be processed by machining, extrusion and rolling. The T_C was determined by electrical resistivity and magnetization to be about 37～39K. In this work, we made composite material with ground MgB₂ particle with the purpose of extruding thinner wires of composite material, successfully produced φ1mm wire and changed the matrix from pure Al to Al-In alloy. J_C of composite materials with the matrix of Al-In alloy was calculated from the width of the magnetic hysteresis based on the extended Bean model. The result was better than that of MgB₂/Al composite material without Indium. Microstructures of these samples had been confirmed by SEM observation.

Abstract: MgB₂ has the higher critical temperature of superconducting transition (T_C : 39K) among the intermetallic compound superconductive materials, however, MgB₂ is hard for practical use because of its unworkable and lower critical current density (J_C) in a high magnetic field than Nb-based superconductive materials. We have developed the original method of three-dimensional penetration casting (3DPC) to fabricate the MgB₂/Al composite materials. In the composite material we made, MgB₂ particles dispersed to the matrix uniformly. Thus, these composite materials can be processed by machining, extrusion and rolling. The T_C was determined by electrical resistivity and magnetization to be about 37～39K. In this work, we made composite material with ground MgB₂ particle with the purpose of extruding thinner wires of composite material, successfully produced φ1mm wire and changed the matrix from pure Al to Al-In alloy. J_C of composite materials with the matrix of Al-In alloy was calculated from the width of the magnetic hysteresis based on the extended Bean model. The result was better than that of MgB₂/Al composite material without Indium. Microstructures of these samples had been confirmed by SEM observation.

Abstract: In this work we present the technology of preparing multilayer MgB₂ superconducting films via CVD method. The first layer of the MgB₂ superconducting film was obtained by two-step method: growthing the precursor boron film on a polycrystalline Al₂O₃ substrate first, then post annealing the film in magnesium ambient, thus we got the first layer of MgB₂ film. After that a pure boron film was deposited on the MgB₂ film , acting as the medium insulating layer, finally the second layer of the MgB₂ superconducting film was formed by in-situ growth, evaporated magnesium atoms and boron atoms that decomposed from diborane met near the substrate and generated the second layer of the MgB₂ superconducting film. The square resistance of the boron medium insulation layer is higher than 20 MΩ. The transition temperature of the both superconducting films was above 38 K, and the temperature for zero resistance is above 37 K. We found that different thickness of the B layer made different I-V curve between two superconducting films, which is very important for the use of Josephson junction. More meaningful results can be expected as the experiment goes on.

Abstract: SiC added MgB2 polycrystalline samples were synthesized at low (650°C) and high (850°C) temperatures in order to study the sintering effect on the phase formation and superconducting properties. The MgB2 bulks with additions of 0wt%, 1wt%, 3wt% and 5wt% SiC were studied with powder X-ray diffraction technique. We observed that MgB2 remained as the primary phase for both sintering temperatures in all samples with the presence of MgO and Mg2Si as the main impurities. Some diffraction peaks associated with unreacted SiC is also noticeable. The relative intensity of the Mg2Si peaks was found to decrease in samples sintered at higher temperature. Temperature dependent magnetic moment measurements showed that the superconducting transition temperature, Tc decreases as the SiC addition level increases while lower sintering temperature degrades Tc to a greater extent. The changes in the physical properties is discussed based on the results of phase formation, full width half maximum (FWHM), lattice parameter and crystallite size.

Abstract: We present the fabrication and electric properties of MgB2 ceramic samples doped with nanosized spheres, 4-8 nm, of graphite with a metallic core. The samples were prepared using the spark plasma sintering technique. The size of the additive is comparable to the superconducting coherence length. The short processing time limits the diffusion of the carbon while keeping the core intact. Therefore, in addition to the doping with carbon, the metallic core, which has the size smaller than the superconducting coherence length, create pinning centers which might improve the dissipationless electric transport. The results are analyzed in the framework of different pinning models.

Abstract: The discovery of the superconductivity of MgB2 was of great importance, because this material is one of the few known binary compounds and has one of the highest critical temperatures (39° K). As MgB2 is a granular compound, it is fundamentally important to understand the mechanisms of the interaction of the defects and the crystalline lattice, in addition to the eventual processes involving the grain boundaries that compose the material. In this sense, the mechanical spectroscopy measurements constitute a powerful tool for this study, because through them we can obtain important information about phase transitions, the behavior of interstitial or substitutional elements, dislocations, grain boundaries, diffusion, instabilities, and other imperfections of the lattice. For this paper, the samples were prepared using the PIT method and were characterized by density, X-ray diffraction, scanning electron microscopy, electric resistivity, magnetization, and mechanical spectroscopy. The samples were measured in their as-cast condition and after an ultra-high-vacuum heat treatment. The results showed complex spectra, in which were identified relaxation processes due to dislocation movement, interaction among interstitial elements and dislocations, auto-diffusion, and movement of grain boundaries. Some of these processes disappeared with the heat treatment.

Abstract: Superconducting wires have been applied for the fabrication of superconducting magnets in nuclear magneto-resonance (NMR), Magneto-resonance imaging (MRI) and so on. MgB2 has the highest critical temperature of superconducting transition (TC39K) among intermetallic compound superconductive materials. This means that MgB2 Superconductive wire doesn’t need expensive liquid He for cooling. We used the original method of the three-dimensional penetration casting (3DPC) in this laboratory to fabricate the MgB2/Al composite. Our 3DPC method for fabricating composite materials can disperse particles in the matrix homogenously without any aggregation and control volume fractions of composites within the range of 4 – 40%, even when particle size is less than 1 m. Thus, these composite materials can be processed by machining, extrusion and rolling. In the composite material we made, MgB2 particles dispersed to the Al matrix uniformly. The TC was determined by electrical resistivity and magnetization to be about 37 – 39K. We succeeded in extruding MgB2/Al composite billet to 1mm wire. Microstructures of these samples have been confirmed by SEM method. MgB2/Al composite billet and extruded wire were showed there no cracks inside the materials.

Abstract: High purity MgB2 powders were synthesized by solid sintering reaction. The degradation behavior of MgB2 powders in deionized water was investigated. XRD and SEM analysis indicate that the changes of MgB2 are a function of time. XRD patterns show that the lattice constants are increasing with the time, and MgB2 samples mostly contain amorphous phase after water treated. The decomposition reaction equation was obtained based on the IR and mass spectrum results. The changes of MgB2 exposed to the 10%HCl, 10%H2SO4, 10%NaOH, 1%HCl, 1%H2SO4 and 1%NaOH (mole concentration) were also studied. The variation of phase composition and morphology indicate that the small amount of MgB2 has decomposed on some extent. The results show that acid can catalyze the decomposition of MgB2 but inversely alkali inhibits the decomposition. The decomposition reaction is limited in water, acid and alkali for less than 1h, and it could be said that the stability of MgB2 is better under such conditions.

Abstract: The superconductive MgB2/Al composite material with low and high volume fractions of particles were fabricated by our special 3-dimensional penetration casting (3DPC) method. The composite material showed homogeneous distribution of MgB2 particles in the Al-matrix with neither any aggregation of particles nor defects such as cracks or cavities. The critical temperature of superconducting transition (TC) was obtained via electrical resistivity and magnetization to be about 37 ~ 39K. The Meissner effect was also verified in the liquid He, in which a piece of the composite floated above a permanent magnet. Extruded rod and wire were successfully fabricated and they also showed onset TC of 39 K on their electrical resistivities as the same as the billet sample.