Powder Mixture Of Mg Research Articles

ＰＩＴ法による線材開発−高ＪＣ線材の開発−

The effects of various impurity additions have been investigated for MgB2 tapes fabricated by the in-situ process of the powder-in-tube method. As impurities, we tried SiO2, SiC and ZrB2. These powders were added to powder mixtures of Mg and B and MgH2 and B. The powder mixtures were then stuffed into Fe tubes and cold worked into tapes using a combination of grove rolling and flat rolling. Heat treatment was performed at the relatively lower temperature of 600 °C for 1 h with argon flowing and in an Ar gas atmosphere in a closed chamber after evacuating up to 10−5 Torr. From the XRD observation of each tape, Mg2Si was formed for tapes to which SiO2 and SiC were added. ZrB2 did not react with Mg and B. We obtained JC enhancements for all of the additives in the Mg + B in-situ processed tapes. Much higher JC values were obtained using the MgH2+B powder. A high JC value of 1.6 x 104 A/cm2 at 4.2 K and 10 T was obtained for the MgH2+B powder mixture to which 10 at%SiC was added. JC values in the high magnetic fields were increased, while the JC values in the low magnetic fields were decreased by the addition of SiC. The irreversibility field of MgB2 tape was enhanced from 17 T to 23 T by the addition of SiC. However, at low fields of ∼4T, the JC enhancement resulting from the addition of SiC seemed to be very small. The highest JC value, 2.1 x 104 A/cm2 at 10 T, was achieved with the 5at% SiC-doped tape heated in an atmosphere with very small oxygen impurity.

ＭｇＢ２超伝導材料の現状と展望

Recent studies on MgB2 superconductors are reviewed. MgB2 single crystals show Bc2 anisotropy factors of 2.6—4.5. The Bc2 of a single crystal at 4.2 K is ∼15 T when the field is parallel to the ab plane. MgB2 bulks, wires and tapes show a higher Bc2 than single crystals. This Bc2 enhancement can be explained by the introduction of defects that decrease the coherence length. MgB2 thin films show a much higher Bc2 than bulk materials. The highest Bc2, 49 T at 0 K, was reported for highly resistive MgB2 thin film. One of the effective pinning centers in MgB2 seems to be grain boundary. Effective pinning centers are also introduced by nanometer-size impurity additions such as SiC. SiC addition significantly increased Jc values in the high field regions, and hence Birr. Most tapes and wires are now fabricated by a powder-in-tube (PIT) method. The ex-situ PIT method employs MgB2-reacted powder and hard sheath materials such as stainless steel. The Jc of ex-situ tapes sensitively depends on the packing density of the MgB2 core. A maximum Jc of ∼4.5×105 A/cm2 is obtained at 4.2 K and zero fields. Ex-situ processed tapes show Jc anisotropy with respect to the field orientation due to the grain alignment. The in-situ PIT method employs a powder mixture of Mg and B. High-energy ball milling and mechanical alloying of the starting powder mixture enhanced the Jc values. The replacement of MgH2 with Mg is effective for improving the reactivity and enhancing Jc values. The Jc at 4.2 K and zero magnetic field reached 2—3×106 A/cm2. The highest Jc, 2.5×104 A/cm2, was obtained at 10 T and 4.2 K when SiC was added to in-situ processed tape prepared with MgH2+B powder. This Jc is comparable to that of a commercial Nb-Ti conductor.

ＭｇＢ２の生成と超伝導特性に及ぼすボロン粉末粒度の影響

MgB2 superconductors were prepared by sintering of a mixture of Mg powder and born powder with different particle sizes at 1073K for 12h. The single MgB2 was obtained at particle sizes below 20μm and showed a superconducting onset temperature of 38K and zero resistance temperature of 36K. The amorphous boron with particle sizes below 1μm makes it easier to form a MgB2. The single MgB2 was obtained by sintering at 1073K for 1h. Even by the sintering at a temperature as low as 873 K, the MgB2 with a zero resistance temperature of 36K was obtained.

Effect of grinding on synthesis of MgAl 2O 4 spinel from a powder mixture of Mg(OH) 2 and Al(OH) 3

Dry grinding of the powder mixture composed of magnesium hydroxide and gibbsite using a planetary ball mill followed by its heating was conducted to prepare MgAl 2O 4 spinel. Both the starting materials are easily converted to amorphous phase by intensive grinding within 60 min. The monolithic MgAl 2O 4 spinel phase can be obtained from the mixtures ground over 15 min by calcination at 900°C. The reactivity of the mixtures ground over 30 min is not improved. Prolonged grinding promotes the aggregation of ground fine particles, but the aggregates deflocculate into fragments after calcination.