Abstract
Among the iron-based superconductors, the so-called 1144 family has, in recent years, attracted significant interest due to its stoichiometric nature, with materials robust towards chemical ...inhomogeneities and characterized by a well-defined critical temperature. The most studied 1144 compounds are characterized by the A
1
AE
1
Fe
4
As
4
chemical composition, where A and AE constitute an appropriate combination of alkaline and alkaline-earth metals, respectively. The 1144 structure is in fact formed only when the A and AE elements respect specific requirements in terms of relative size and parent compound structure. The stoichiometric aspect, one of their strong points, has represented, however, up to today a restriction, limiting the conceptualization of 1144 structures to quaternary compounds. In this work, we demonstrate that to obtain the 1144 crystalline phase it may be sufficient to maintain a 1:1 ratio between ions of different size that intercalate the Fe-As planes, and that in selected conditions an opportunely tailored cation substitution is possible. Using a simple mechanochemically assisted synthesis route 1144 compounds where Ca is substituted by Na, K by Ba, and both simultaneously, are obtained. We demonstrate that the critical temperature of doped compounds is not simply related to the substitution amount or to the resulting Fe valence. We show that the superconducting transition is in fact linked to the structural distortion induced by the chemical composition variation: by tailoring the chemical composition we obtain doubly substituted samples—with substitution levels up to 20%—characterized by a tetragonality ratio
c
/
a
similar to the pristine compound and critical temperatures of approximately 34 K.
Taking advantage of the large experience of the ENEA Superconducting Laboratory in the manufacture and characterization of large scale superconducting systems, a project was launched, aimed at using ...High Temperature Superconductor (HTS) 2G wires for the manufacture of a cable-in-conduit conductor (CICC). In particular, the main aim was the definition of a conductor design fully compatible with existing cabling technologies, to be promptly transferred to an industrial scale production. The considered layout is based on 150 HTS tapes, arranged as five layered structures of twisted tapes wound on a helically slotted core with external round jacket. All manufacturing steps (slotted core production, tape stacking and insertion into the ducts, external wrapping and jacketing) are fulfilled by using equipments and technologies available at TRATOS Cavi S.p.A. These CICCs are intended for operation using forced flow of Helium. A 2D local thermal model has been developed for the optimization of the cooling configuration. This conductor is designed to target 20 kA at 4.2 K and 15 T (or, alternatively, higher temperature, in self-field and LN2 cooling) corresponding to a J e of about 55 A/mm 2 . The production of a short dummy sample revealed that the exploited industrial production process is very promising for the development of HTS CICC.
In this paper we study the feasibility of Fe(Se, Te) coated conductors on a simple rolling-assisted biaxially textured substrate (RABiTS) template. Starting from commercially available Ni-5 at% W ...tapes which show an out-of-plane orientation of about 7° and an in-plane orientation around 5°, we realized a RABiTS template for Fe(Se, Te) coated conductor fabrication by depositing CeO2 thin films on the metallic tape. The oxide buffer layers, deposited via pulsed laser ablation, exhibit an out-of-plane and an in-plane orientation suitable for Fe(Se, Te) deposition and act as a chemical barrier against Ni diffusion. Fe(Se, Te) thin films deposited on such a simple template show a superconducting transition at 16 K and very high upper critical field values with a ΔTc,0 of only 3 K in 18 T. The transport critical current reaches values of 1.7 × 105 A cm−2 at 4.2 K in self-field and is reduced by less than one order of magnitude up to 18 T, remaining isotropic with respect to the magnetic field direction.
The nucleation of YBa2Cu3O7−δ (YBCO) film deposited via metal organic decomposition (MOD) approach is a complex process that has a key role in YBCO film crystalline orientation and, consequently, ...superconducting properties. Up to now, several studies have been carried out to clarify this step in the fluorine based MOD route which employs only or partially metals trifluoroacetate for the precursor solution preparation. However, these previous works provided only qualitative indications, whereas a quantitative theoretical investigation seems to be still lacking. In this paper, the free energy for critical nucleus formation of fluorine based MOD YBCO has been determined according to the classical nucleation theory. The volume contribution to the free energy is computed using thermodynamic data on (Y, Ba, Cu, O) systems available from the literature. The free energy barrier for nucleation depends upon nucleus orientation, via the work of adhesion and surface energies of vacuum/nucleus interfaces. The model is applied to describe heterogeneous nucleation of YBCO on (001) SrTiO3 single crystal and provides quantitative information on nucleation barriers as a function of temperature and both precursor and gas phase compositions. Through a kinetic approach, the energy barriers are further employed to estimate the fraction of c-oriented nuclei as a function of temperature and water partial pressure. Comparison with experimental data from literature lends support to the proposed computations. Therefore, the results shown in this work may be useful in designing the YBCO deposition process in order to obtain films with superior superconducting properties avoiding expensive and time-consuming experimental optimizations.
Abstract
Since the discovery of high temperature superconductors, a possible cryogen-free scenario has always been wished. Nowadays, liquid Helium is running out, and it is likely that the cooling by ...will be a large part of the costs of any superconducting system. Bi-2212 wires at temperature higher than 4.2 K still show a very high irreversibility field and thus a deep investigation of their properties in such a range of temperature is very useful in order to assess the applicability in high field cryogen-free magnets. Here electrical transport and magnetic properties characterization at variable temperature and magnetic field on our “GDG—processed” wires are reported together with a well-described original approach to calculate the irreversibility field
H
irr
. This study is devoted to provide reference data on the behaviour of the only isotropic wire for high field application with an eye to the performances at temperatures above 4.2 K.
Abstract
The fabrication of a Fe-based coated conductor (CC) becomes possible when Fe(Se,Te) is grown as an epitaxial film on a metallic oriented substrate. Thanks to the material’s low structural ...anisotropy, less strict requirements on the template microstructure allow for the design of a simplified CC architecture with respect to the REBCO multi-layered layout. This design, though, still requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, Fe(Se,Te) films are grown on chemically-deposited, CeO
2
-based buffer layers via pulsed laser deposition, and excellent properties are obtained when a Fe(Se,Te) seed layer is used. Among all the employed characterization techniques, transmission electron microscopy proved essential to determine the actual effect of the seed layer on the final film properties. Also, systematic investigation of the full current transport properties
J
(
θ
,
H
,
T
) is carried out: Fe(Se,Te) samples are obtained with sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm
−2
at 4.2 K in self-field. The in-field and angular behavior of the sample are in line with data from the literature. These results are the demonstration of the feasibility of a Fe-based CC, with all the relative advantages concerning process simplification and cost reduction.
In recent years, iron-based superconductors have attracted attention due to their interesting electromagnetic properties that open wide application prospects. Among these, FeSe is particularly ...studied since it is both a superconductor with a bulk critical transition temperature, T c , of 8 K and a semiconductor used in solar cells. For the design of an iron-based superconductor, the preparation of larger samples is envisaged, and the possibility of maintaining low manufacturing costs is extremely appealing. One possible technique suitable for this purpose is electrodeposition, which combines inexpensive equipment to the easy scaling towards larger/longer samples. In this work, we study the possibility of using electrodeposition as a method for FeSe film growth. We start by analyzing the electrochemical behaviour of the precursor solution via cyclic voltammetry before attempting electrodeposition. We find out that the main drawback of this technique is the microstructure of the deposited film, which is made mostly of unconnected grains. This issue does not rule out this technique for the preparation of superconducting thin films, but it has to be addressed in view of a potential application of this technique to the preparation of an iron-based coated conductor.
An outstanding current carrying performance (namely critical current density,
J
c
) over a broad temperature range of 10-77 K for magnetic fields up to 12 T is reported for films of YBa
2
Cu
3
O
7−
x
...with Ba
2
Y(Nb,Ta)O
6
inclusion pinning centres (YBCO-BYNTO) and thicknesses in the range of 220-500 nm.
J
c
values of 10 MA cm
−2
were measured at 30 K - 5 T and 10 K - 9 T with a corresponding maximum of the pinning force density at 10 K close to 1 TN m
−3
. The system is very flexible regarding properties and microstructure tuning, and the growth window for achieving a particular microstructure is wide, which is very important for industrial processing. Hence, the dependence of
J
c
on the magnetic field angle was readily controlled by fine tuning the pinning microstructure. Transmission electron microscopy (TEM) analysis highlighted that higher growth rates induce more splayed and denser BYNTO nanocolumns with a matching field as high as 5.2 T. Correspondingly, a strong peak at the
B
||
c
-axis is noticed when the density of vortices is lower than the nanocolumn density. YBCO-BYNTO is a very robust and reproducible composite system for high-current coated conductors over an extended range of magnetic fields and temperatures.
Tailoring of self-assembled nanoinclusions in the YBCO matrix improves in-temperature and in-field performances.
REBCO tapes are currently being produced by various companies, each employing unique combinations of materials and deposition processes. To effectively design and optimize their use in power devices, ...it is crucial to ascertain their critical field characteristics and understand their flux pinning properties. In this work, we conducted a comprehensive investigation involving electrical transport and magnetization measurements on coated conductors (CCs) produced by Shanghai Superconductor Technology Co., Ltd. (SST). Our findings reveal that the flux pinning energy of these CCs exhibits a collective pinning behavior across the entire range of magnetic fields under consideration. Moreover, the behavior of these CCs in high magnetic fields suggests that their design is well-suited for low-temperature, high-power applications.
The 1144 Iron-Based Superconductors (IBSC), characterized by the A 1 AE 1 Fe 4 As 4 chemical composition (A=Alkaline, AE=Alkaline-Earth), has gained significant interest in the recent years due to ...their crystalline structure character-ized by an intrinsic modulation of the strain along the c-axis and by the proneness in forming crystalline defects with a positive impact on flux pinning. Moreover, it has been proved that the compound is compatible with the cost-effective powder-in-tube (PIT) manufacturing process. In our recent experiments, we have showed that the A 1 AE 1 Fe 4 As 4 structure can be tailored to obtain a 1144 com-pound characterized by different (A x AE 1-x )(AE y A 1-y )Fe 4 As 4 chemical formulae without any depression in the critical temper-ature value. On the other hand, it has been recently shown that the doping with selected elements on both A and AE sites has a clear influence on the pinning and the grain boundary properties of poly-crystalline samples. In this work we report the results of the extensive magnetic characterization performed on pristine Ca 1 K 1 Fe 4 As 4 and doped (A x Ca 1-x )(AE y K 1-y )Fe4As4 with Ba as dopant on the K site and ei-ther La or Na as dopant on the Ca site. In particular, the magnet-ization hysteresis loops recorded at different temperatures M(H, T) and the critical current density dependences extracted from the M(H) using the Bean model, J c (B, T), have been analyzed in order to assess the effect of aliovalent doping on the quality of the produced samples in terms of grain boundary properties and pinning efficiency.
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