S. K. Chen

Room temperature ferromagnetism in bulk Mn-Doped Cu2O

Bulk Mn-doped Cu2O samples were produced by reacting Cu2O and Mn2O3 powders in Ar gas at 650 and 800°C to give a nominal composition of 1.7at.% Mn-doped Cu2O. From x-ray energy dispersive spectrum analysis, the actual doping level was lower at 0.3–0.5at.% Mn. Room temperature ferromagnetism with a coercive field of 50Oe was found in the 650°C samples. The Curie temperature (TC) of samples sintered at 650°C was above 300K, whereas for 800°C samples it was 215±5K. Using the nominal doping level, the magnetization saturation value was calculated to be ∼0.4μB∕Mn at 10K.

Strong pinning enhancement in MgB2 using very small Dy2O3 additions

0.5–5.0wt% Dy2O3 was in situ reacted with Mg+B to form pinned MgB2. While Tc remained largely unchanged, Jc was strongly enhanced. The best sample (only 0.5wt% Dy2O3) had a Jc∼6.5×105Acm−2 at 6K, 1T and 3.5×105Acm−2 at 20K, 1T, around a factor of 4 higher compared to the pure sample, and equivalent to hot-pressed or nano-Si-added MgB2 at ⩽1T. Even distributions of nanoscale precipitates of DyB4 and MgO were observed within the grains. The room temperature resistivity decreased with Dy2O3 indicative of improved grain connectivity.

Strong influence of boron precursor powder on the critical current density of MgB2

The influence of the nature of the boron precursor on the superconducting properties of polycrystalline MgB2 was studied. Critical current densities (Jc) for MgB2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower Jc compared to those made from amorphous precursors. X-ray, Tc and resistivity studies indicate that this is as a result of reduced current cross-section due to the formation of (Mg)B–O phases. The samples made from amorphous B contain far fewer Mg(B)–O phases than crystalline B despite the fact that the amorphous B contains more B2O3. The different reactivity rates of the precursor powders accounts for this anomaly.

Effect of heating rates on superconducting properties of pure MgB2, carbon nanotube- and nano-SiC-doped in situ MgB2/Fe wires

The influence of heating rates and annealing temperatures on the transition temperatures (Tc) and critical current densities (Jc) of pure MgB2, carbon nanotube- and nano-SiC-doped in situ monofilamentary MgB2∕Fe wires was investigated. It was found that higher Jc was obtained for pure MgB2 samples when heat treated with slower heating rates. SiC-doped samples also have higher Jc with slower heating rates, but the Jc is less sensitive to annealing temperatures. However, the Jc of the carbon nanotube-doped wire was found to be insensitive to heating rates. The variation in Tc and Jc with heating rate, and the different behaviors of differently doped MgB2∕Fe wires, make it essential to carefully select the optimum heating rates for heat treatment.

Minute pinning and doping additions for strong, 20K, in-field critical current improvement in MgB2

Minute additions of a combination of Dy2O3 and B4C have been used to enhance both pinning and upper critical field in MgB2. A delicate balance of Dy2O3 and B4C additions is required to improve properties. The Dy2O3 nanoparticles react with B to form 10–15nm DyB4 nanoparticles, while B4C supplies carbon into the MgB2 crystal lattice and increases the upper critical field. The optimum level of Dy2O3 and B4C additions is ∼0.5wt% of Dy2O3 and 0.04wt% of B4C, yielding a Jc (20K) of 105Acm−2 at 2.7T.

Improved current densities in MgB2 by liquid-assisted sintering

Polycrystalline MgB2 samples with GaN additions were prepared by reaction of Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic phase which allowed liquid phase sintering and produces platelike grains. For low-level GaN additions (⩽5at.%), the critical transition temperature, Tc, remained unchanged and in 1T magnetic field, the critical current density, Jc was enhanced by a factor of 2 and 10, for temperatures of ∼5K and 20K, respectively. The values obtained are approaching those of hot isostatically pressed samples.

Influence of in situ and ex situ ZrO2 addition on the properties of MgB2

The effect of ZrO2 addition on the properties of MgB2 has been studied using in situ and ex situ processes. The in situ process was performed by introducing ZrO2 from the milling tools into MgB2 throughout the planetary ball milling, whereas the ex situ process was accomplished by mixing ZrO2 from the milling tools with MgB2 by hand grinding in a mortar. A detectable amount of ZrO2 was present in MgB2 after 4 h of milling during the in situ process and its content increased with milling time as expected. The 400 h milled powder was partially amorphized and showed the formation of a minority ZrB2 phase. For milling up to 100 h, diamagnetism of MgB2 was significantly reduced while Tc remained unchanged. Superconductivity was totally destroyed after 148 h of milling. The loss of superconductivity is attributed to the effect of disordering induced by mechanical milling. As a result of in situ ZrO2 addition, the initial Tc and crystal structure of MgB2 could not be restored upon annealing. With increasing milling time, the expansion of lattice parameters in both the a-axis and c-axis may be due to possible substitution of Mg or B by Zr. The result from the magnetic measurement shows that Jc of MgB2 is deteriorated by in situ ZrO2 addition. On the other hand, ex situ ZrO2 addition with annealing did not degrade the Tc of MgB2.

The influence of unidirectional copper alloying on the critical current density of MgB2

Inward unidirectional copper diffusion into a pellet and the resulting alloying of Cu–Mg during in situ reaction of MgB2 were studied. The reaction between the copper and magnesium occurred at as low a temperature as 660 °C in a flowing argon atmosphere giving rise to the formation of a MgCu2 layer at the interface between the Cu and the MgB2. The kinetics of copper alloying in the core sample was much enhanced at 800 °C and above, as shown by energy dispersive x-ray analysis. Consequently, Jc was degraded drastically due to the decrease in the superconducting volume fraction. However, the alloying effect did not lower the Tc onset, but broadened the transition. A sharp Tc transition and Jc(6 K,4 T)~2 × 104 A cm−2 was achieved when annealing was conducted at the lower temperature of 750 °C. Direct comparison between the pure and copper alloyed samples shows that optimization of the heat treatment and hence Jc rests on the competing effects of phase formation and grain connectivity versus MgCu2 formation.