MgB2 Grain Size Research Articles

Enhanced critical current density of in situ processed MgB2 bulk superconductors with MgB4 additions

The effects of MgB4 addition on the superconducting properties and the microstructure of in situ processed MgB2 bulk superconductors were studied. MgB4 powder of 1-20 wt.% was mixed with (Mg + 2B) powder and then pressed into pellets. The pellets of (Mg + 2B + xMgB4) were heat-treated at 650 ℃ for 1 h in flowing argon. The powder X-ray diffraction (XRD) analysis for the heat-treated samples showed that the major formed phase in all samples was MgB2 and the minor phases were MgB4 and MgO. The full width at half maximum (FWHM) values showed that the grain size of MgB2 decreased as the amount of MgB4 addition increased. MgB4 particles included in a MgB2 matrix is considered to suppress the grain growth of MgB2. The onset temperatures (Tc,onset) of MgB2 with MgB4 addition (0-10 wt.%) was between 37-38 K. The 20 wt.% MgB4 addition slightly reduced the Tc,onset of MgB2 to 36.5 K. This result indicates that MgB4 addition did not influence the superconducting transition temperature (Tc) of MgB2 significantly. On the other hand, the small additions of 1-5 wt.% MgB4 increased the critical current density (Jc) of MgB2. The Jc enhancement by MgB4 addition is attributed not only to the grain size refinement but also to the possible flux pinning of MgB4 particles dispersed in a MgB2 matrix.

Effect of grain size on the electrical and magnetic properties of MgB2 thick films deposited on the Al2O3 single crystal substrates

MgB2 films having thicknesses of approximately 10 μm have been prepared on the Al2O3 single crystal substrates by using ultrasonic spray pyrolysis technique. The films fabricated were then analyzed by means of microstructural, structural, transport and magnetic properties. The influence of grain size, depending on the ex-situ and in situ heating, were investigated. The results obtained showed that the size of MgB2 grains has an effect on the transport and magnetic properties of the films. Films prepared with in situ heating have small particle size (<50 nm) and were found to be much better properties than the films prepared ex-situ heating, which have larger grains (~1 μm). The best Tc value and critical current density, Jcmag, were obtained to be ~36 K and 9.88 × 105 A cm−2 respectively for the films prepared with small grains. The results were showed that the smaller grain size and in situ heating cycles are essential points for quality thick film MgB2 fabrication.

Effect of Boron Purity on Superconducting Properties of SiC Doped MgB&lt;sub&gt;2&lt;/sub&gt;

The effect of purity of amorphous boron raw materials on properties of the hot pressed SiC doped MgB2 superconductor was investigated. MgB2 superconductors with magnetic Jc over 106 A/cm2 and remaining Jc of 105 A/cm2 at 4.2 K and 5 T were fabricated by hot pressing using both high purity (99.00%) and low purity (88.84%) boron powders. XRD analysis shows that purity of the boron powders has little effect on phase component of the MgB2 samples. If the main impurity in amorphous boron is Mg, low purity low cost boron powder is suitable as one of the raw materials for fabricating MgB2. Particle sizes of boron has significant effect on microstructure and properties of MgB2. Smaller boron particle size leads to smaller grain size of MgB2, higher density, higher lattice distortion, and thus higher magnetic Jc.

Enhanced Jc of B-rich and SiC doped MgB2 tapes fabricated by a modified in-situ PIT method with two stage heat treatment

B-rich and SiC doped MgB2 tapes were fabricated by a modified in-situ PIT method with two stage heat treatment. B composition ratio and an amount of SiC doping were systematically changed. The effect of pre-heating and final heating conditions on Jc-B properties was also studied. Mean grain size of MgB2.8 specimens reduced to about 100 nm by increasing SiC doping 5.7%. In low fields Jc slightly increased for a little SiC doped specimens. On the other hand, in high fields, Jc obviously increased with increasing SiC doping. Maximum Jc reached 1.8×103 A/cm2 at 3 T, 20 K for MgB2.8 specimen doped SiC 5.7 mol%. Birr also increased with increasing SiC doping. Jc systematically increased with the decrease of temperature of pre-heat treatment. In contrast Jc systematically increased with the increase of temperature of final heat treatment. Best Jc of 8.6×104 A/cm2 at 20 K, 0 T was achieved for specimen with 700 °C for 5 hours + 800 °C for 1 hour heating. Birr tended to increase with the increase of Jc.

Investigation on MgB2 Wires Doped by ZrH2

ZrH2-doped MgB2 wires, with the component of Mg:ZrH2:B= (1-x):x:2(x = 0, 5, 8, and 10%), were fabricated through thein situ powder-in-tube method by using low-carbon steel tube as the sheath material Samples were sintered at 750^C for 1h in a flow of high-purity argon.The lattice parameters a and c of MgB2exhibit a little decrease with the doping of ZrH2.Jc(B)property of MgB2wires are improved gradually with the increase of x, reaches the best whenx = 8%, but decreases significantlywhen x = 10%. The decrease of MgB2grain size, the excellent grain connection, and the substitution of Zr4+for Mg2+which is caused by the doping of ZrH2, are responsiblefor the enhancement of Jc(B)property forthe doped MgB2wires.

Improved critical current properties of MgB2 bulks by controlling microstructures

Effects of controlled grain size on the critical current properties of in-situ fabricated polycrystalline MgB2 bulks were systematically investigated. Grain sizes of MgB2 bulks were controlled by three different methods: changing the heating conditions, such as temperature and holding time, changing the molar ratio between Mg and B, and starting from B powders with different grain sizes. MgB2 bulks with smaller grain sizes were obtained when heating at low temperatures or for short time at high temperatures starting from fine B powder. Small MgB2 grains were also found in the bulks starting from Mg-deficient composition using fine B powder. Through the careful analyses on the grain size of MgB2 and critical current properties, we found that MgB2 bulks with smaller grain sizes exhibit higher irreversibility fields and larger maxima of pinning force density. Our present results supported the scheme that grain boundaries are the predominant pinning centres in polycrystalline MgB2 bulks.

Effect of nano-C doping on the in-situ processed MgB2 tapes

The effect of nano-C doping on the microstructure and superconducting properties of Fe-sheathed MgB2 tapes prepared through the in-situ powder-in-tube method was studied. Heat treatment was performed at a low temperature of 650°C for 1 h. Scanning electron microscopy investigation revealed that the smaller grain size of MgB2 in the samples with the C-doping. Further, the a-axis lattice parameter and transition temperature decreased monotonically with increasing doping level, which is due to the C substitution for B. High critical current density JC values in magnetic fields were achieved in the doped samples because of the very fine-grained microstructure of the superconducting phase obtained with C doping.

Fabrication of MgB2 thin films by electron beam evaporation technique

MgB2 thin films were prepared on polished sapphire C(001) single crystal substrates by using electron beam evaporation technique (EB) without any post-annealing. We controlled the background pressure less than 10−6Pa and the deposition conditions were varied experimentally to investigate the effects of the growth temperature and the chemical composition of MgB2 thin films on the pinning characteristics.We found that superconducting MgB2 thin films can be obtained even at very low substrate temperature of 120°C. With raising the deposition temperature up to 240°C, the Tc’s increased to 33K in the MgB2 thin films with the stoichiometric composition (Mg:B=1:2). However, ΔM in M–H curve of the film deposited at 240°C was smaller than that of the film at 210°C. We confirmed that the ΔM value was affected by the grain size of MgB2. We also clarified that the Mg poor compositions make the c-axis length short and that the some defects introduced by the Mg deficiency act as pinning centers.

Effects of sintering conditions on critical current properties and microstructures of MgB2 bulks

The effects of heating conditions on critical current properties and microstructures of undoped MgB2 bulks were systematically studied. Strong correlation was observed between Jc and microstructures. The network structure with an excellent inter-grain connectivity of MgB2 grains contributed to high-Jc under low magnetic fields, and small grain size of MgB2 enhanced the grain boundary flux pinning. Long time heating at low temperatures below the melting point of magnesium was discovered to be most effective for synthesis of MgB2 bulks having strongly connected MgB2 network structure with small grains. The sample heated at 550°C for 1200h recorded a high-Jc of 4.02×105Acm−2 at 20K in self-field, while high-temperature and long time heating brought a significant grain growth which resulted in low Jc.

High critical current density in Ti-doped MgB2/Ta/Cu tape by powder-in-tube process

Ti-doped MgB2/Cu tapes with Ta as a buffer layer were prepared through the in situ powder in tube method by using Mg, Ti, and B powders. The phase compositions, microstructure features, and superconducting properties were investigated by x-ray diffraction, scanning electron microscope, and superconducting quantum interference device magnetometer. It is found that TiB2 phase was formed in Ti-doped MgB2 tape. Magnetization measurement results show that the critical transition temperature of MgB2/Ta/Cu tape with Ti doping is around 38 K. The irreversibility field Hirr and critical current density Jc can be greatly enhanced by Ti doping. Hirr of the Mg0.9Ti0.1B2 tape is as high as 7.4 T at 10 K. The high critical current density Jc of 1.5×106 A/cm2 (10 K, self field) and 9.3×105 A/cm2 (20 K, self field) are obtained in the Mg0.9Ti0.1B2 tape. In addition, a suitable amount of Ti doping can lead to a high density and fine grain size of MgB2, which may be the reason for high Jc in Ti-doped MgB2 tapes.

Enhanced flux pinning in Zr-doped MgB2 bulk superconductors prepared at ambient pressure

We prepared a series of Mg1−xZrxB2 bulk samples with different x values by the solid-state reaction at ambient pressure and investigated the effects of the Zr doping on phase compositions, microstructure, and flux pinning behavior. The results indicate that the a axis increases with the increase of the Zr doping and the c axis remains unchanged. ZrB2 phases form and a small amount of Zr atoms may substitute Mg in Zr-doped samples. Critical temperature drops with increasing the Zr doping. It is found that critical current density Jc can be significantly enhanced by the Zr doping and the best data are achieved in Mg0.9Zr0.1B2. For this sample, Jc values are remarkably improved to 1.83×106 A/cm2 in the self field and 5.51×105 A/cm2 in 1 T at 20 K. Also, the irreversibility field is increased by the Zr doping. The high density, fine grain size of MgB2, and small ZrB2 particles in Zr-doped samples may be responsible for the enhancement of Jc. Also, the field dependence of Jc at different temperatures was briefly discussed.

As-grown MgB2 thin films deposited on Al2O3 substrates with different crystal planes

We present the results of an investigation of the crystal structures and superconducting properties of as-grown MgB2 superconducting thin films deposited on Al2O3 substrates having different crystal planes (C- and R-planes). The MgB2 thin films were prepared by a multiple-target sputtering method without performing a post-annealing process. The MgB2 films deposited on the C-plane Al2O3 substrates were found to have a c-axis orientation, from the results of 2θ/θ x-ray diffraction (XRD) measurements. However, there were no MgB2 peaks for the films grown on the R-plane Al2O3 substrates. Even though the maximum Tc values of both films deposited on the C- and R-plane Al2O3 substrates were nearly the same, Tc,onset ∼ 28 K and residual resistivity ratio ∼ 1.2, it is very interesting to note that the resistivity of the films on the C-plane Al2O3, ρ ∼ 70 μΩ cm at 40 K, was lower than that on the R-plane Al2O3, ρ ∼ 300 μΩ cm at 40 K. These results indicate that the grain size of MgB2 in the films deposited on the C-plane Al2O3 substrates was larger than that on the R-plane Al2O3, indicating a good agreement with the XRD measurements. The surface morphology of the as-grown MgB2 thin films was investigated using scanning electron microscopy. The surfaces of the films on the C- and R-plane Al2O3 substrates were found to be very flat and smooth. This result indicates the high potential for fabricating Josephson tunnel junctions using as-grown MgB2 thin films.Presented at the IUMRS-ICEM conference, Xi'an, China, 10–14 June 2002.