MgB2 Bulk Materials Research Articles

The quasiorder-disorder phase transition and peak effect in MgB2 type-II superconducting materials and thin films

The peak effect and the quasiorder-disorder first-order phase transition for Magnesium diboride, MgB2, superconducting bulk materials have been studied. The peak values of the critical current density Jc, and the exact peak positions together with its corresponding half-widths for a constant temperature as well as for a constant applied magnetic field have been calculated by considering the quantum, thermal as well as random fluctuations of the vortex lattice. The results for MgB2 bulk materials are in agreement with the experiment. The peak effect for MgB2 superconducting thin films is also predicted theoretically. The expected peak effect may be observed provided that doping or other experimental techniques are applied to improve the flux pinning of the MgB2 superconducting thin films.

The quasiorder‐disorder phase transition and peak effect in MgB2 type‐II superconducting materials and thin films

AbstractThe peak effect and the quasiorder‐disorder first‐order phase transition for Magnesium diboride, MgB2, superconducting bulk materials have been studied. The peak values of the critical current density Jc, and the exact peak positions together with its corresponding half‐widths for a constant temperature as well as for a constant applied magnetic field have been calculated by considering the quantum, thermal as well as random fluctuations of the vortex lattice. The results for MgB2 bulk materials are in agreement with the experiment. The peak effect for MgB2 superconducting thin films is also predicted theoretically. The expected peak effect may be observed provided that doping or other experimental techniques are applied to improve the flux pinning of the MgB2 superconducting thin films.

ＭｇＢ２超伝導体の臨界電流特性および材料化の現状と展望 ＭｇＢ２多結晶バルク体の力学特性

Mechanical properties of superconducting MgB2 materials are important for their practical uses. This paper reports relationships between mechanical properties (Vickers hardness and fracture toughness) and the microstructure in MgB2 bulk materials. MgB2 bulk specimens were prepared by a Premix-PICT (powder in closed tube)-diffusion method. It was found that Vickers hardness and fracture toughness were improved up to about 1.5 times by co-doping with SiC and C10H8. It is interpreted that (i) the improvement of Vickers hardness by the co-doping is mainly due to reducing the MgB2 grain size, and (ii) the improvement of fracture toughness by the co-doping is mainly due to decreasing the amount of amorphous phases.

Doping and effect of nano-diamond and carbon-nanotubes on flux pinning properties of MgB2

Doping effects of two types of nano-carbons: nano-diamond and carbon-nanotubes (CNTs), on the flux pinning properties of MgB2 bulk materials have been studied. Compared with nano-diamond, CNTs is prone to be doped into MgB2 lattice. Nano-diamond doping improves Jc(H) characteristics more significantly than CNTs doping in MgB2. TEM analysis reveals a unique microstructure in diamond-doped MgB2, which consists of tightly packed MgB2 nanograins (50–100nm) with dense distribution of diamond nanoparticles (10–20nm) inside the grains. Relatively, such a unique microstructure is not easy to form in CNTs-doped MgB2 due to an active reaction between CNTs and MgB2.

Effects of Nano-Al Doping on Superconductivity and Microstructure of MgB<sub>2</sub> Bulk Superconductors

Nano-size Al powders were dispersed by ultrasonic vibrations to avoid reuniting and could be doped into the MgB2 bulk materials uniformly. The effects and mechanism of doping were studied. The phase analysis and microstructure analysis show that, Al atoms can enter into the lattice of MgB 2 and substitute the site of Mg atoms. The crystal parameters a and c of the MgB 2 phase decrease gradually with increase of Al addition, and the value of c is depressed more. The critical temperature T c drops with the increase of Al addition. The value of T c is 35.5K for the superconductor with 8mol% addition, while 38.5K for the pure MgB 2 bulk superconductor. The properties of critical current density J c (B) can be improved at low level doping such as 1 mol%, 2mol% addition. The results demonstrate that the superconductivity of MgB 2 with nano-Al doping is improved at the conditions of low temperature, high magnetic field and low level addition.

Synthesis of bulk MgB2 superconductors by pulsed electric current

AbstractA preparation method to simultaneously synthesize and consolidate bulk MgB2 superconductors from Mg and B commercial elemental powders by means of the spark plasma sintering technique is reported. The influence of process parameters on sintering process dynamics as well as product characteristics, determined by transport and magnetic measurements, is investigated. The superconducting properties of the obtained samples, and particularly the critical current density, are comparable or better than those corresponding to other MgB2 preparation techniques. Thus, the superconductive properties of the bulk MgB2 materials synthesized in this work are suitable for selected applications, such as magnetic levitation, magnetic screening, and fault current limiters. It should be finally noted that the proposed method represents a particularly rapid preparation route as compared to other techniques. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Bulk MgB 2 superconductor with high critical current density synthesized by self-propagating high-temperature synthesis method

Pure MgB2 bulk samples are successfully synthesized by self-propagating high-temperature synthesis (SHS) method. The experiments show that the best preheating temperature is 250°C, the highest Jc values of the prepared MgB2 reach 1.5×106A/cm2 (10K, 0.5T) and 1.7×106A/cm2 (20K, 0T) and the MgB2 particle sizes range from 2 to 5μm. The advantages of this method are that it is simple, economical and suitable for the manufacture of bulk MgB2 materials on industrial scale.

Improved irreversibility behavior and critical current density in MgB2-diamond nanocomposites

MgB 2 -diamond nanocomposite superconductors have been synthesized by addition of nanodiamond powder. Microstructural analysis shows that the nanocomposite superconductor consists of tightly packed MgB2 nanograins (∼50–100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (∼10–20 nm) inside the grains. The Jc–H and Hiir–T characteristics have been significantly improved in this MgB2-diamond nanocomposite, compared to MgB2 bulk materials prepared by other techniques. Also, the Jc value of 1×104 A/cm2 at 20 K and 4 T and the Hirr value of 6.4 T at 20 K have been achieved.

Improved superconducting properties of MgB2 bulk materials prepared by sintering

We report here the improvement of superconducting properties of MgB2 bulk samples by sintering at ambient pressure, employing RF induction heating. It has been found that Tc (R = 0) transition temperature of the sintered sample gets enhanced to ∼40.00 K as compared to Tc (R = 0) ∼25 K for the unsintered MgB2 sample. A high critical current density of ∼5.45×104 A cm−2 (20 K) has been achieved for a sintered MgB2 sample whereas for an unsintered sample, the Jc is found to be ∼1.20×103 A cm2 (20 K). Detailed microstrucural investigations by transmission electron microscope (TEM) reveal that most of the intergrain interfaces correspond to low angle grain boundaries. Based on TEM explorations, these boundaries have been recognized to be dominantly low angle tilt boundaries. Surface microstructural features of the MgB2 sample as investigated by scanning electron microscopy reveal a well-connected uniform grain structure for the sintered samples.

Superconducting properties of MgB2 bulk materials prepared by high-pressure sintering

High-density bulk materials of a recently discovered 40 K intermetallic MgB2 superconductor were prepared by high-pressure sintering. Superconducting transition with the onset temperature of 39 K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M–H) curve shows the behavior of a typical type-II superconductor and the lower critical field Hc1(0) estimated from the M–H curve is 0.032 T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 5×104 A/cm2 at 20 K and 1 T. Upper critical field Hc2(0) determined by extrapolating the onset of resistive transition and assuming a dirty limit is 18 T.