Delamination strengths (DSs) of ReBCO tapes were evaluated using a transverse tensile anvil test. This study first demonstrated that the electromechanical DS is nearly equivalent to the purely mechanical DS for ReBCO tapes exhibiting abrupt delamination behavior. Mechanical DS was then examined for tapes with and without stabilizer edges to assess the influence of stabilizer strength. Scanning electron microscopy/energy-dispersive x-ray spectroscopy analysis revealed the delamination sites and crack propagation in ReBCO tapes. Further investigation revealed that the DS of ReBCO tapes is primarily determined by two factors: (1) interfacial bonding strength between the layers and (2) mechanical strength of the stabilizer edges. These findings point to a potential approach to enhancing DS by increasing the strength of the stabilizer (e.g. by optimizing the Cu electroplating process).

Previous studies have shown that APC Nb3Sn strands based on the internal oxidation of Nb-Ta-Zr or Nb-Ta-Hf alloys had higher non-Cu critical current density (J(c)s) than the state-of-the-art strands at high fields (e.g., >= 12 T) while having lower non-Cu J(c)s at low fields (e.g., <= 5 T), which helps to reduce the undesired persistent-current magnetization. Moreover, APC strands reacted at lower temperature tend to have flatter J(c)(B) curves, possibly allowing the targeting of even better J(c) performance at high fields, while at the same time suppressing J(c)s at low fields. However, it is required that APC strands attain large fine-grain (FG) area fractions and suppressed coarse-grain (CG) area fractions at low reaction temperature. In this work, the influence of reaction temperature on the evolution of FG and CG Nb3Sn phases in APC Nb3Sn strands was investigated. Our goal is to find more effective methods to increase the FG area fraction as well as the overall performance of APC strands.

For the accelerator magnets of the next hadron collider, reducing superconductor persistent-current magnetization is not only important for achieving the desired field quality, but also crucial for its sustainability because the magnetization loss is the major heat load to the magnet cold mass. For conventional Nb 3 Sn conductors this requires reduction of effective subelement size ( D eff ). For the restacked-rod-process (RRP ® ) conductors a physical subelement size ( D sub ) as small as 35 µ m (corresponding to a D eff close to 45 µ m) can be reached, but at a significant price in J c . Another way to reduce the magnetization is by introducing artificial pinning centers (APC) using the internal oxidation approach. APC conductors outperform conventional Nb 3 Sn wires in two aspects: 1) higher J c at high fields, and 2) much lower J c and magnetization at low fields (e.g. below 5 T). In this work we explored the fabricability of APC wires with small D sub . A 180-stack APC wire was produced and drawn to 0.7- and 0.5 mm diameters with good quality, with D sub s of 34 and 24 µ m ( D eff s of 36 and 25 µ m), respectively. For the 34 µ m- D sub wire, its non-Cu J c is higher than that of an RRP ® wire used for the High-Luminosity Large Hadron Collider (HL-LHC) project above 13 T (e.g. 36% higher at 4.2 K, 18 T), while its non-Cu magnetization at 1 T, Δ M (1 T), is only 29% of the RRP ® wire. Its non-Cu hysteresis loss for a cycle between 1 and 14 T, Q h (1–14 T), is 37% of the RRP ® wire. For the 24 µ m- D sub wire, its non-Cu J c surpasses the HL-LHC RRP ® wire above 17.5 T, while its Δ M (1 T) and Q h (1–14 T) are only 17% and 23% of the RRP ® wire, respectively. Its non-Cu Q h (±3 T) even meets the specification of the International Thermonuclear Experimental Reactor project.

Vapor-solid reactions were applied to monofilament and 6-filament AIMI MgB2 wires. A layer J(c) of similar to 10(5) A cm(-2) was achieved by the generation of a highly dense MgB2 layer after reaction at 625 degrees C for 8 to 16 h. It was found that the critical current, I-c, and engineering current density, Je, reached maximum values of 92.3 A and 1.15 x 10(4) A cm(-2) at 4.2 K, 10 T when the multifilament AIMI wire was heat-treated at 625 degrees C for 14 h. Transport I(c)s were also measured at 10-20 K for the wire heat-treated at 625 degrees C for 16 h. Phase formation investigations suggested that even better performance of I-c and J(e) could be attained in multifilamentary AIMI MgB2 wires after further conductor optimization.

Synchrotron high-energy X-rays were used in an attempt to estimate nanoprecipitate size and size distribution in Nb 3 Sn powder-in-tube wires with ZrO 2 or HfO 2 artificial pinning centers via small angle X-ray scattering (SAXS). The effect of sample preparation was studied but measurements for as-received and for partially etched wires were not successful. Extracted sub-elements appeared to show more scattering contrast due to the lower number of phases, but no particle size could be extracted in this first attempt. Analysis of TEM data from the literature showed large particle size distribution (PSD) for different heat treatment conditions, suggesting that PSD could likely never be measured with SAXS due to a smeared signal. Experimental challenges with in-situ measurements revealed the sensitivity of infra-red furnaces and to changes in sample emissivity. The use of in-situ wide angle X-ray scattering data was successful in estimating the wire temperatures from the lattice parameter of copper and could be used in future heat treatment studies to better understand tin oxide dissolution.

A series of monofilamentary powder-in-tube MgB2 wires were fabricated with 2 mol. % C doping and co-additions of 0-3 wt. % Dy2O3. Irreversibility fields (μ 0 Hirr ), upper critical fields (μ 0 Hc 2), and transport critical currents were measured, and from these quantities, anisotropies ( γ ) and electronic diffusivities ( D π , σ ) were estimated. The addition of 1 wt. % Dy2O3 to already optimally C-doped MgB2 wires produced higher Hc 2//ab , Hc 2//c , and Hirr values at 4.2 K. In addition, the critical current density, Jc , increased with Dy2O3 concentration up to 1 wt. % where non-barrier Jc reached 4.35 × 104 A/cm2 at 4.2 K, 10 T. At higher temperatures, for example, 20 K and 5 T, co-additions of 2 mol. % C and 2 wt. % Dy2O3 improved non-barrier Jc by 40% and 93% compared to 2 and 3 mol. % C doping, respectively. On the other hand, measurements of Tc showed that C/Dy2O3 co-additions increase interband scattering rates at a lower rate than C doping does (assuming C doping levels giving similar levels of low-T μ 0 Hc 2 increase as co-addition). Comparisons to a two-band model for μ 0 Hc 2 in MgB2 allowed us to conclude that the increases in Hc 2//ab , Hc 2//c , and Hirr (as well as concomitant increases in high-field Jc ) with Dy2O3 addition are consistent with increases primarily in intraband scattering. This suggests C/Dy2O3 co-addition to be a more promising candidate for improving non-barrier Jc of MgB2 at temperatures above 20 K.

Multifilamentary MgB 2 strands (filament numbers 36 to 114) prepared by the in-situ power-in-tube (PIT) route with carbon doping contents of 0, 2, and 3.2% were wound on barrels for transport Jc and n -value measurement at 4.2 K in fields of up to 12 T. The strand and gauge lengths were 1 m and 0.5 m. Heat treatments at 675 °C and 650 °C centered around the melting point of Mg (650 °C) and both utilized the liquid-solid reaction. A pair of strands, with and without 2% C doping exhibited the Jc (B) crossover effect. Studied were the dependencies of Jc on field strength, dopant concentration, and cabling and the dependence of n -value on field strength.

Nb 3 Sn superconductors are promising for building accelerator magnets for future energy-frontier circular colliders. A critical factor for this application is the low-field persistent-current magnetization because it leads to several critical issues: e.g. low-field instability (including flux jumps), hysteresis loss, and field errors in magnet bores. Suppression of low-field magnetization requires reduction of low-field critical current density ( J c ) or effective subelement size ( d eff ). However, reduction of d eff of state-of-the-art Nb 3 Sn conductors—the restacked-rod-process (RRP ® ) type—below 40–50 μ m without a pronounced decrease in high-field J c is difficult. On the other hand, the internal oxidation method which forms artificial pinning centers (APC) in Nb 3 Sn offers an alternative approach to reducing the low-field magnetization. Compared with a conventional Nb 3 Sn conductor whose flux pinning force versus field ( F p – B ) curve peaks at ∼20% of its irreversibility field ( B irr ), the F p – B curve peaks of APC conductors shift to higher fields due to the point pinning effect, leading to flattening of the J c – B curves. The goal of this paper is to quantitatively study how much the APC approach can reduce the low-field magnetization. We measured the J c – B curves of an RRP ® conductor and two APC conductors (reacted at 700 °C) from zero field to B irr using a high-field vibrating sample magnetometer. The results showed that the APC conductors have higher non-Cu J c at high fields (e.g. 32%–41% higher at 16 T) and simultaneously lower non-Cu J c at low fields (e.g. 28%–34% lower at 1 T) compared with the RRP ® . This effect is due to a competition between their Nb 3 Sn layer fraction ratios and layer F p ratios. Suppose they reach the same 16 T non-Cu J c , then the 1 T non-Cu J c and magnetization of the APC conductors are only half or even less compared with the RRP ® conductor.

A possible approach to reducing Nb 3 Sn magnet training is to increase the energy margin of Nb 3 Sn conductors by enhancing their specific heat ( Cp ). We have been developing Nb 3 Sn conductors with increased Cp by incorporating substances with high Cp at 2-10 K based on a conductor design that is compatible with standard Nb 3 Sn strand production. In the past couple of years our efforts have been mainly focused on improving strand design (e.g., position of high- Cp filaments, thickness of the Cu tube for the high- Cp filaments, ratio of the Cu powder to the high- Cp substance, filament spacing, etc.) in order to obtain good strand drawability and to reduce degradation after rolling, which is needed for production of Rutherford cables. We also tried a new high- Cp substance, Gd 2 O 2 S, and verified that it has much higher Cp over the whole magnetic field range than the Gd 2 O 3 we used before. With that development work we can now produce high- Cp strands with good drawability and low levels of degradation after rolling. This paper reports our findings and the current status of the development of high- Cp Nb 3 Sn conductors.

In the last few years, a new type of Nb$_3$Sn superconducting composite, containing a high density of artificial pinning centers (APC) generated via an internal oxidation approach, has demonstrated a significantly superior performance relative to present, state-of-the-art commercial Nb$_3$Sn conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr alloy. On the other hand, our recent studies have shown that internal oxidation of Nb-Ta-Hf alloys can also lead to dramatic improvements in Nb$_3$Sn performance. In this work we follow up this latter approach, fabricating a 61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy, and compare its critical current density (Jc) and irreversibility field (Birr) with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to improve the filamentary design of APC wires in order to improve their wire quality and electromagnetic stability. Our new modifications have led to significantly improved RRR and stability in the conductors, while still keeping non-Cu Jc at or above the FCC Jc specification. Further improvement via optimization of the wire recipe and design is ongoing. Finally, additional work needed to make APC conductors ready for applications in magnets is discussed.

Iron-based superconducting wires and tapes hold great promise for high-field magnet applications. A promising design for 122-type wires and tapes based on the powder-in-tube method is using silver and copper double-layer sheaths. For this design a heat treatment temperature below -779 C is required to prevent Ag-Cu liquid for-mation. However, this may be below the optimal heat treatment temperature for the critical current density, and still cannot prevent Ag-Cu interdiffusion occurring in the solid state. In this work we propose adding a niobium or tantalum or vanadium (or their alloys) barrier layer between the Ag and Cu to solve the Ag-Cu interdiffusion issue, given that the group-VB metals (vanadium, niobium, tantalum) are relatively inert to both Ag and Cu. To investigate the effectiveness of this design, BaFe1.84Co0.16As2 wires and tapes with Ag/Cu and Ag/Ta/Cu sheaths, as well as Ba0.6K0.4Fe2As2 wires and tapes with Ag/Cu and Ag/Nb/Cu sheaths, were fabricated. It was found that both the Ta and Nb layers kept integral after wire drawing, but after a large flat-rolling reduction the Ta layer broke while the Nb layer kept integral. In the tapes with Ag/Cu sheaths (without the Ta or Nb layer) Cu diffused through the Ag layer and into the powder cores during 740 ? heat treatment, while in the tapes with Ag/Nb/Cu sheaths the Nb layer effectively blocked Ag-Cu interdiffusion even at 900 ?. This work demonstrates that Ta is a suitable barrier material for 122-type wires, while Nb is suitable for both wires and tapes. In this design using Ag/Nb (or Ta)/Cu sheaths, we can regard the outer Cu as the conductor matrix while the Ag and Nb (or Ta) serve as two layers of barriers that suppress reactions between the components. Thus, we call this design a "bi-layer barrier " design for 122-type wires and tapes.

we present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB2 strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB2 short (1.37 m length) MRI magnet based on the performance parameters of existing MgB2 wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

MgB2 superconducting wires made using a Mg infiltration method have reached a higher performance than either in-situ or ex-situ mixed powder based routes. Indeed, very high layer J c coupled with whole-strand J e (critical current per total strand cross section) exceeding 104 A cm-2 at 4.2 K, 10 T have been found for monocore MgB2 wires. However, previous multicore infiltration route wires have not reached their potential for J e due to partially reacted and non-uniform MgB2 layers. This study shows that 18-core MgB2 AIMI wires processed using a low temperature route can attain higher and more uniform J e values due to a more uniform MgB2 reaction layer. The formation of fully reacted, uniform MgB2 layers is attributed to the switch from a liquid-solid to a vapor-solid reaction route.

MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc(2) and unchanged B-irr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M H), transport critical current density (Jr) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density J(c)(B) of P-05 at 4.2 K and 8 T (6.0 X 10(4) A/cm(2)) was twice that of the control wire. The critical magnetic fields (Bc(2) and B-irr) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05' s transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc(2) or B-irr.

Research into in-situ MgB2 strand has been focused on improvements in JC through reduction of porosity. Both of cold-high-pressure-densification (CHPD) and advanced-internal-magnesium-infiltration (AIMI) techniques can effectively remove the voids in in-situ MgB2 strands. This study shows the nature of the reduced porosity for in-situ MgB2 strands lies on increases in transverse grain connectivity as well as longitudinal connectivity. The CHPD method bi-axially applying 1.0 GPa and 1.5 GPa yielded 4.2 K J CM║s of 9.6 × 104 A/cm2 and 8.5 × 104 A/cm2 at 5 T, respectively, with compared with 6.0 × 104 A/cm2 for typical powder-in-tube (PIT) in-situ strand. Moreover, AIMI-processed monofilamentary MgB2 strand obtained even higher J Cs and transverse grain connectivity than the CHPD strands.

(Ba,Sr)1-xKxFe2As2 superconductor is of great interest as a wire for practical applications. In this paper, Ba0.6K0.4Fe2As2 precursor powder was prepared by high-energy ball-milling aided sintering (12h/750°C). The prepared precursor was drawn to powder-in-tube round wire. Partial as-drawn wires were subjected to biaxial cold pressure densification (BCPD) at 0.5 and 1.0GPa, followed by an annealing for 1min/770°C. According to our results, the BCPD improved the grain connectivity of the BCPDed wires without degrading the superconductivity. A (Ba,Sr)1-xKxFe2As2 round-wire record transport critical current density, 1.14 × 105A/cm2 (4.2K, 2T), was observed in our BCPDed samples.

The 4.2 K non-barrier transport J(c) values of the powder-in-tube (PIT) in-situ MgB2 wires have been enhanced by cold high pressure densification (CHPD). With respect to the control wire, the 1.5 GPa pressure induced the Jc improvement of the wire at 4.2 K and 10 T by 25 %. The J(c) enhancement induced by the CHPD may result from two aspects. First, the CHPD resulted in the reduction of the transverse MgB2 core area. Second, the grain connectivity of the PIT in-situ MgB2 wire was improved by the CHPD, which is reflected by the reduced porosity.

In this work we studied the influence of SnO2 doping on the critical fields and temperatures of MgB2 bulk samples. Bulk samples were made by mixing ex-situ MgB2 powder with 5 wt% SnO2 powder and then pressing the mixed powders into pellets using a pressure of 2000 psi. The bulk pellets were sintered at 900°C in a furnace under flowing Ar. The samples were quenched to room temperature after dwelling at 900°C for 5h. XRD, SEM, and magnetic measurements were made on doped and control samples. XRD showed a decomposition of the SnO2 and very slight reductions in the a-axis and c-axis lattice parameters of the MgB2 phase. M-T (Magnetization-Temperature) curves showed a decrease in Bc2 of approximately 1 T in the temperature range of 24 K - 39 K with SnO2 additions as compared to the control samples.

Conventional methods of Ba 0.6 K 0.4 Fe 2 As 2 precursor preparation are complicated and expensive. In this paper, we describe the mechanical alloying of precursor by high-energy ball milling and its processing to wires and tapes. Our approach is to high-energy ball-mill a starting mixture of Ba and K pieces with Fe and As powders. The resultant powders are packed into an Ag sheath and reduced by groove rolling or flat rolling. Critical current density measurements were performed on unsintered wire and tape, and compared to the results of measurements on wire and tape sintered at 750 °C for 12 h. We achieved 6.98 × 10 4 A/cm 2 at 4.2 K and self-field in the sintered tape. Our results indicate that the mechanical alloyed precursor and the resulting Ag-sheathed Ba 0.6 K 0.4 Fe 2 As 2 wires and tapes are promising but require further development.

由于钠资源价格低廉、分布广泛等优点,钠离子电池及其关键电极材料的研究近年来引起了广泛的关注.然而,与锂相比,钠的离子半径大得多,使其在储钠材料中的迁移速度过慢而严重地限制了钠离子电池倍率性能的提升和储钠容量的表达,而且钠元素具有更高的相对原子质量,也在理论上限制了钠离子电池的能量密度.因此,开发先进的、利于钠离子脱嵌的电极材料是开发高性能钠离子电池的关键.本文在钠离子电池工作原理的基础上,着重介绍了几类典型的关键电极材料,并对它们的研究进展进行了简要综述.