Partially-oriented MgB₂ superconducting bulks with addition of B₄C and cubic BN obtained by slip casting under high magnetic field and spark plasma sintering

Highlights

• Partially-oriented bulks of added MgB₂ were fabricated for the first time.

• Critical current density is similar or higher than in randomly-oriented bulks.

• Additives (c-BN and B₄C) and orientation synergetically influence superconductivity.

• The properties’ anisotropy depends on carbon substituting for boron in MgB_2.

• Partially-oriented samples show discrepancies not covered by pinning force models.

Abstract

Partially-oriented MgB₂ bulk discs (13 and 9 %) with the starting compositions of (MgB₂)_0.99(B₄C)_0.01 and (MgB₂)_0.99(c-BN)_0.01 were fabricated by slip casting under an H₀ = 12 T magnetic field (perpendicular to the disc surface) and subsequent spark plasma sintering. The maximum critical current density and irreversibility field are for H//H₀ (H=applied field). These values are higher or similar to the randomly-oriented samples with the same composition. The maximum volume pinning force (F_p) is lower in the partially-oriented ones than in the randomly-oriented samples. The pinning-force-related parameters depend on the additive and orientation. Assessment of the major pinning mechanism within the scaling and percolation models considering these parameters shows significant limitations. A method to scale F_p is proposed; for the randomly and partially-oriented samples (that show an extra peak in F_p), the single and double Gaussian functions fit well. The results suggest an anisotropic influence of carbon substituting for boron in the MgB₂.

Introduction

The superconducting properties of MgB₂ are widely investigated in relation to the additions, microstructures, and processing technologies. One has to pay attention to their synergetic and complex nature. In general, a high density and a nanostructured material is necessary to achieve enhancement of the critical current density, J_c, and of the irreversibility field, H_irr. Higher J_c and H_irr values can boost applications of the MgB₂ with significant commercial impact. The strategies for the enhancement of J_c and H_irr are concerned with improvement of the connectivity and pinning. In practice, they relate to the engineering of defects, to the creation of strained regions (through chemical substitutions or epitaxial interfaces), to the formation of nanoprecipitates, and to the generation of a higher density of grain boundaries and of their modification. All these elements can act as effective pinning centers if their size is comparable to the coherence length of MgB₂ [1], i.e., about 10−40 nm. In addition, the high density of MgB₂ is needed to improve the connectivity. The methods to enhance the connectivity and pinning are:

• Use of different thermomechanical processing procedures taking advantage of high pressures such as hot isostatic pressing [2,3], cold drawing, cold rolling [3], cold high pressing [4,5], and spark plasma sintering [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. One can also apply energetic methods such as high energy ball milling [2,17] or irradiation [18].

• Control of raw materials. The purity, morphology, type of the raw materials, and mixing degree [2,19] are typical features that will influence the quality of the superconductor. It is noteworthy that the MgB₂ fabrication processes that are using Mg- and B-based raw materials are called in situ because MgB₂ is formed by the chemical reaction between Mg and B [20]. The routes that are using the MgB₂ compound are called ex situ. The two routes have specific features and some comparative aspects have been presented by Yamamoto et al. in ref. [21].

• Introduction of additions. Among the most popular additives are those acting as a source of carbon substituting for boron in the crystal lattice of MgB₂ [2,5,[22], [23], [24], [25], [26]]. Other additives promote the formation of nanoprecipitates and grain boundary modification [2,21,27].

In our work, we fabricate high density bulk samples of MgB₂ by ex situ spark plasma sintering (SPS). The method applies to the MgB₂ powder, pristine or mixed with additives, a uniaxial pressure and an electrical current that has a pulsed component. The pulsed current is considered as the origin of the unconventional far-from-equilibrium activation processes that are mainly developing at the grain boundaries. Although debatable, the literature [[28], [29], [30], [31]] presents evidence for particle surface cleaning, electrodiffusion, formation of hot spots, occurrence of sparks possibly accompanied by the plasma states, and heating from the inside to outside similar to microwave sintering. The consequences are a fast and efficient consolidation of difficult-to-sinter materials and a decreased sintering time and temperature. The method is flexible, allowing high heating and cooling rates. These details are useful for hindering the particle growth. Hence, SPS is a valuable technique for the processing of high density nanomaterials in general, and of the MgB₂ superconductor in our particular case. The activation processes may also promote formation of the defects useful for pinning. They can also influence the reactive processes and development of precipitates, when, e.g., additions are introduced to the MgB₂ samples. Different additions to the spark plasma sintered MgB₂ were tested [32]. Among the most efficient for improvement of the superconducting parameters were B₄C [33] and cubic BN (c-BN) [34].

Recently, we successfully fabricated partially c-axis oriented pristine MgB₂ bulk samples [35]. The texture of the MgB₂ was obtained by combining slip casting under a high magnetic field of 12 T and further SPS processing. Our promising results enabled continuation of the studies targeting improvement of the superconducting functional parameters, such as J_c and H_irr, by taking advantage of the anisotropic nature of MgB₂. The details of pinning in the textured MgB₂ bulk also deserves attention. Hence, the fabrication and characterization of partially-textured MgB₂ bulks with additives is the next natural course of investigation which is the aim of this study. The criteria for the selection of additives, B₄C and c-BN, are not only their positive influence on pinning, J_c, and H_irr as already mentioned in the previous paragraphs, but also the fact that B₄C is anisotropic (rhombohedral) and it was demonstrated that it can be aligned by magnetic slip casting [36], while c-BN is isotropic (cubic).