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Register a new userMechanochemical synthesis of pnictide compounds and superconducting Ba0.6K0.4Fe2As2 bulks with high critical current density
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BaFe2As2 (Ba-122) and (Ba0.6K0.4)Fe2As2 (K-doped Ba-122) powders were successfully synthesized from the elements using a reaction method, which incorporates a mechanochemical reaction using high-impact ball milling. Mechanically-activated, self-sustaining reactions (MSR) were observed while milling the elements together to form these compounds. After the MSR, the Ba-122 phase had formed, the powder had an average grain size < 1 {mu}m, and the material was effectively mixed. X-ray diffraction confirmed Ba-122 was the primary phase present after milling. Heat treatment of the K-doped MSR powder at high temperature and pressure yielded dense samples with high phase purity but only granular current flow could be visualized by magneto optical imaging. In contrast, a short, low temperature, heat treatment at ambient pressure resulted in global current flow throughout the bulk sample even though the density was lower and impurity phases were more prevalent. An optimized heat treatment involving a two-step, low temperature, heat treatment of the MSR powder produced bulk material with very high critical current density above 0.1 MAcm-2 (4.2 K, 0 T).
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A safe, simple and easily scaleable one-step sintering method is proposed to fabricate newly discovered superconductors of SmO1-xFxFeAs. Superconducting transition with the onset temperature of 54.6 K and high critical fields Hc2(0) >=200 T were confirmed in SmO1-xFxFeAs with x = 0.3. At 5 K and self field, critical current density Jc estimated from the magnetization hysteresis using the whole sample size and the average particle size reached 8.5x10^3 and 1.2x10^6 A/cm^2, respectively. Moreover, the Jc exhibited a very weak dependence on magnetic field. Microstructural characterizations revealed that the whole sample Jc improvement could be achieved by either perfect texture or optimization of fabrication process in this strongly-layered superconductor. Our results clearly demonstrated that one-step synthesis technique is unique and versatile and hence can be tailored easily for other rare earth derivatives of REFeAsO superconductors.
The K- and Co-doped BaFe2As2 (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (Hc2) of well over 50 T, Hc2 anisotropy Gamma < 2, and thin film critical current densities exceeding 1 MAcm-2 at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behavior for [001] tilt misorientations of more than about 5 degrees, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba0.6K0.4)Fe2As2 bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MAcm-2 (SF, 4.2 K), more than 10 times higher than that of any other ferropnictide wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (Gamma ~ 1-2) compared to YBa2Cu3O7-x (Gamma ~ 5).
A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can pin the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.
The high resistivity of many bulk and film samples of MgB2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence the supercurrent (Jc) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, Jc ~ 1/{Rho(300K) - Rho(50K)}, where Rho(300K) - Rho(50K) is the change in the apparent resistivity from 300 K to 50 K. We report measurements of Rho(T) and Jc for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the reduced effective area argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB2.
Critical current density (Jc) is one of the major limiting factors for high field applications of iron-based superconductors. Here, we report that Mn-ions are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe films. Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm^2 under 33 T, and the vortex pinning force density monotonically increases up to 106 GN/m^3, which is the highest record so far among all iron-based superconductors. Our results demonstrate that Mn incorporation is an effective method to optimize the performance of (Li,Fe)OHFeSe films, offering a promising candidate for high-field applications.
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