Cold High Pressure Densification Research Articles

Interplay between cold densification and malic acid addition (C4H6O5) for the fabrication of near-isotropic MgB2 conductors for magnet application

Abstract The effect of cold high pressure densification (CHPD) on anisotropy of the critical current density (Jc) in « in situ » single core binary and alloyed MgB2 tapes has been determined as a function of temperatures at 4.2 K, 20 K and 25 K as well as at applied magnetic fields up to 19 T. The study includes binary and C4H6O5 (malic acid) doped MgB2 tapes before and after CHPD. It is remarkable that the CHPD process not only improved the Jc values, in particular at the higher magnetic fields, but also decreased the anisotropy ratio, Γ = Jc///Jc┴. In binary MgB2 tapes, the anisotropy factor Γ increases with higher aspect ratios, even after applying CHPD. In malic acid (C4H6O5) doped tapes, however, the application of CHPD leads only to small enhancements of Γ, even for higher aspect ratios. This is attributed to the higher carbon content in the MgB2 filaments, which in turn is a consequence of the reduced chemical reaction path in the densified filaments. At all applied field values, it was found that CHPD processed C4H6O5 doped tapes exhibit an almost isotropic behavior. This constitutes an advantage in view of industrial magnet applications using wires with square or slightly rectangular configuration.

The Role of CHPD and AIMI processing on enhancing JC and transverse connectivity of in-situ MgB2 strand

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.

Pressure-induced property improvement of magnesium diboride wire

The 4.2 K non-barrier transport Jc 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 Jc 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.

Viewpoint on fast creation of dense MgB2 phase in wires made by internal Mg diffusion process

Md Shahriar Al Hossain, University of Wollongong, shares his views on a letter by P KovaAcA○ and colleagues on fast creation of dense MgB phase in wires made by internal Mg diffusion process. According to him, In situ and ex situ powder metallurgical processes are currently used for the industrial fabrication of mono- and multi-filamentary MgB wires. The mass density in the MgB filaments of the wires after the final heat treatment is quite low with respect to the theoretical density. The typical effective superconductor area in the wire is only about 10%. Cold high pressure densification (CHPD) has been suggested as an alternative to enhance filament mass density. The authors showed that 8 min sintering time is sufficient for the formation of a uniform MgB2 phase 90 μm in thickness and without cracks, which is nearly identical to samples produced under the same sintering temperature conditions but longer sintering time of 3?5 h that were previously reported by the NIMS group. This finding is important for economical industrial production, but a question still remains as to how the round wires with a hollow filament will perform in magnet design and application, where enhancement of the filling factor, reduction of filament size, and mechanical and electrical stability and reliability are key factors.

Superior transport Jc obtained in in-situ MgB2 wires by tailoring the starting materials and using a combined cold high pressure densification and hot isostatic pressure treatment

The best Jc performance (8×104A·cm−2 at 10T, 4.2K) has been obtained in our work among all the in-situ powder-in-tube (PIT) MgB2 wires reported so far by tailoring their boron and magnesium precursors, as well as employing appropriate high pressure during the manufacturing process. Moreover, the second stage densification using the special HIP has been applied to commercial long-length wire. Our work suggests that the economical starting precursors and the combination of cold and hot densification techniques could represent a promising alternative for industrial and economical production of practical MgB2 wires with excellent Jc.

The roles of CHPD: superior critical current density and n-value obtained in binary in situ MgB2 cables

A binary magnesium diboride (MgB2) cable has been assembled by braiding six Nb/Monel sheathed monofilament strands around a central copper stabilizer for improving the operational environment. The total critical current (Ic) of the braided cable is obtained by multiplying the Ic of six single wires, without any dissipation. In this work, various mechanical deformations, i.e., swaging, two-axial rolling, groove rolling, and cold high-pressure densification (CHPD) at 1.8 GPa have been applied to the 6-stranded cable to obtain additional densification. The highest critical current density at both 4.2 and 20 K has been achieved in this work through the CHPD treated cable due to higher filament mass density. The present results are promising in view of the cable, particularly in power applications at industrial lengths that pave the way to seeking an optimal protocol to meet a practical functionality.

Intrawire resistance, AC loss and strain dependence of critical current in MgB2 wires with and without cold high-pressure densification

The intrawire resistance and alternating current (AC) loss of two MgB2 wires with filaments surrounded by Nb barriers have been measured and analyzed. Relatively high values of filament-to-matrix contact resistivity are found in the MgB2 wires; the values are two or three orders higher than those commonly found in NbTi or Nb3Sn wires. Considering the high porosity of the MgB2 filaments, cold high-pressure densification has been applied on the two MgB2 wires to investigate its impact on intrawire resistance and AC loss. The intrawire resistance is measured with a direct four-probe voltage-current method at various temperatures. The AC loss is measured by vibrating sample magnetometer measurements at 4.2 K. In addition to the intrawire resistance measurements, the critical current of MgB2 wires before and after densification is measured with a U-shaped bending spring at 4.2 K as function of axial strain. The critical current in densified MgB2 wires is found to be higher than that in the same wire without densification; it is also less sensitive to the applied axial strain.

Effect of cold high pressure deformation on the properties of ex situ MgB2 wires

A substantially different behaviour was observed between MgB2 wires produced either by in situ or ex situ processing after applying the recently developed technique of cold high pressure densification (or CHPD). In contrast to in situ wires, where densification at 1.5 GPa on binary and ternary alloyed in situ MgB2 wires causes an enhancement of mass density and a strong enhancement of Jc, ex situ wires up to 2 GPa show only a negligible enhancement of the MgB2 mass density, while a considerable enhancement of Jc is still observed. In both cases, this reflects an enhancement of grain connectivity, however in ex situ wires, the enhancement of Jc is connected to the partial disruption of the oxide layer around each MgB2 powder particle, in contrast to the enhancement of Jc in situ wires, which is correlated to a smaller void fraction and a larger contact area between neighbouring grains. It is well known that Jc of ex situ wires decreases after longer exposition times when exposed to air prior to annealing; after several months Jc falls to values ≤50% of the original value. After cold pressing, we have found that Jc of the same wire exceeded the original values, even after exposing the unreacted wire for >1 year to air. A lower electrical resistivity is measured on pressed ex situ MgB2 wires, which confirms the improvement of grain connectivity due to the breakage of the oxide layers. Thus, the application of high pressure at room temperature allows us to recover the values of Jc for ex situ wires even after they have been degraded either by low quality MgB2 precursors or by prolonged ageing of the already formed wire prior to the final heat treatment. The limits of the Jc enhancement in ex situ wires are discussed.

Synergetic Combination of LIMD With CHPD for the Production of Economical and High Performance $\hbox{MgB}_{2}$ Wires

We propose an economical fabrication concept, the localized internal magnesium diffusion (IMD) method. Instead of using a single magnesium (Mg) rod in the center of a metal sheath tube, we use large-sized Mg particles (20-50 mesh) mixed well with cheap 97% crystalline boron powder to fill the metal sheath tube. After a repeated drawing process, the coarse Mg is elongated along the core wire axis of the metal sheath tube. Textured MgB2 grains are then formed during the sintering process. In the localized IMD process, however, there is still a need to improve the overall density. In order to increase the density of the composite, a modified cold high pressure densification (CHPD) technique has been applied before the reaction. It is found that the critical current density (Jc) of the sample made from large-sized Mg with crystalline boron powder and treated by CHPD is increased significantly, so that it is quite comparable with the Jc values of samples made from expensive small magnesium and nanosized amorphous boron powder. At 4.2 K and 8 T, the Jc value of the wire in this work with the cheapest starting materials reaches 10 000 A/cm2 , which is similar to reported values for samples made by the powder-in-tube and IMD processes with expensive nanosized amorphous boron powder. A possible mechanism is proposed, and the microstructure is analyzed to explain this interesting feature. The main goal of this work is to develop a novel and cost-effective fabrication technique by combining the localized IMD process with CHPD and using cheap crystalline boron powder to manufacture MgB2 superconductor wires with electromagnetic performance superior to that of low-temperature Nb-Ti superconductors.

Effects of cold high pressure densification on Cu sheathed Ba0.6K0.4Fe2As2 superconducting wire

Cu sheathed polycrystalline Ba0.6K0.4Fe2As2 superconducting wire was prepared by a two-step powder-in-tube method. A pressure of 2GPa was applied to a short sample before heat treatment. Magnetization and transport measurements were performed to investigate the effects of cold high pressure densification on the microstructure and superconductivity. The cold pressed sample shows an improved self-field transport critical current density of 2.8×104A/cm2 (Ic=83A) at 4.2K, which is nearly as twice as the unpressed sample. However, both samples manifest pronounced weak-link behavior, suggesting the technique need to be further optimized. The comparison of properties between pressed and unpressed sample and related mechanism was discussed.

Enhanced Connectivity and Percolation in Binary and Doped In Situ ${\rm MgB}_{2}$ Wires After Cold High Pressure Densification

The cold high pressure densification technique (CHPD) was recently developed in Geneva for improving the in-field critical current density Jc of in situ binary and alloyed MgB2 wires and tapes,. Jc of CHPD treated square wires alloyed with malic acid (C4H6O5) was enhanced by a factor 2 at 10 T and 4.2 K. In order to understand the fundamental mechanism behind this strong improvement of Jc, the properties of binary and alloyed MgB2 wires have been investigated without and with CHPD, using resistivity and specific heat measurements in the temperature range from 5 to 35 K in magnetic fields up to 15 T. In particular, a deconvolution of the specific heat data was used to determine the distribution of Tc in the samples. We have found that the effect of the densification process on the electrical and transport properties is related to the improved grain connectivity and percolation. By combining the results arising from the analysis of the Tc distribution and those from resistivity measurements, it is concluded that the minimum superconducting volume fraction needed for the percolation of a superconducting path is strongly reduced in samples treated by CHPD.

A New Generation of In Situ ${\rm MgB}_{2}$ Wires With Improved $J_{\rm c}$ and $B_{\rm irr}$ Values Obtained by Cold Densification (CHPD)

By means of Cold High Pressure Densification (CHPD), the critical current density, Jc, of binary and alloyed MgB2 wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB2 mass density of binary MgB2 wires was enhanced to ~ 54% after applying 2.5 GPa at 300 K before reaction. In C4H6O5 (malic acid) alloyed wires, densification also caused the enhancement of Birr, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropic Jc values were obtained for C4H6O5 added wires of 1 × 0.6 mm2 cross section, the values of Jc(4.2 K)=1 × 104 A/cm2 for parallel and perpendicular fields being obtained at 13.8 and 13.4 T, respectively (1 μV/cm criterion). The corresponding values for 20 K were both close to 6.2 T. The value of Birr// at 20 K was 11 T. The positive effects of cold densification on Jc and Birr on MgB2 was also observed on 150 mm long wires alloyed with C4H6O5 (malic acid) or with SiC, by the succession of 6 overlapping pressure steps. This process can be extended to long wire lengths: by means of a newly developed prototype machine with an automatic press/release/advance sequence, a first wire length of 1 m was densified at 1.5 GPa, yielding Jc(4.2 K) = 1 x 104 A/cm2 at 13.1 T. Further improvements are expected after optimization.

Improvement of Jc by cold high pressure densification of binary, 18-filament in situ MgB2 wires

Cold high pressure densification (CHPD) has recently been foundto be a promising way to improve the critical current density,Jc, insingle-core MgB2 wires prepared by in situ processing. In this work, the CHPD process was also applied to multifilamentary,binary MgB2 wires, leading again to a strong enhancement of the transportJc. The fields and where the transport Jc at4.2 K reaches the value 1 × 104 A cm − 2 for parallel and perpendicular fields were determined as 8.5 and 8.2 T, respectively (0.1 µV cm − 1 criterion). Thebehaviour of Jc versus B at 20 and 25 K was almost isotropic, the corresponding values being 4.7 and 3 T, respectively. The observed enhancement ofJc by a factor 2.3 at 4.2 K at all applied fields up to 10 T in densified samples isdirectly correlated with the observed enhancement of the mass density andmicrohardness. The decrease of the electrical resistivity of the densified wirereflects improved grain connectivity in the filaments. The improvement ofJc by CHPD was still observed after sequential pressing with overlapping regions, up to atotal wire length of 150 mm. This result is promising as regards practical applications.

Improved transport properties and connectivity of in situ MgB2 wires obtained by Cold High Pressure Densification (CHPD)

The critical current density, Jc, of in situ MgB2 wires with C4H6O5 (malic acid) has been strongly enhanced by means of Cold High Pressure Densification (CHPD) at pressures up to 2.5GPa prior to reaction. An increase in Jc by factors 2 and 8 was observed at 4.2 and 20K, regardless of the applied field. Densified wires were found to exhibit a higher homogeneity due to enhanced connectivity. In addition, a higher C content was found, as shown by the lattice parameter change, the decrease of Tc, the shift of the calorimetric Tc distribution and a reduced anisotropy in tapes obtained from the same wires. The higher C content in densified wires is responsible for the observed enhancement of Birr by more than 1T, up to 11T at 20K. Almost isotropic Jc values were obtained for C4H6O5 added square wires of 1×0.6mm2 cross section, the values of Jc(4.2K)=1×104A/cm2 for fields // and ⊥ to the wider surface being obtained at 13.8 and 13.1T (1μV/cm criterion), the values for 20K being 6.4 and 6.2T, respectively. The method was successfully applied to mono- and multifilamentary wires with lengths up to several meters, suggesting applicability for industrial lengths.

The enhanced Jc and Birr of in situ MgB2 wires and tapesalloyed with C4H6O5 (malic acid) after cold high pressure densification

Cold high pressure densification, a method recently introduced at GAP inGeneva, was applied for improving the transport critical current density,Jc, and the irreversibilityfield, Birr, of monofilamentaryin situ MgB2 wires and tapesalloyed with 10 wt% C4H6O5 (malic acid). Tapes densified at 1.48 GPa exhibited after reaction an enhancement ofJc from 2to 4 × 104 A cm−2 at4.2 K/10 T and from0.5 to 4 × 104 A cm−2 at20 K/5 T, whilethe Birr was enhanced from 19.3 to 22 T at 4.2 K and from 7.5 to 10.0 T at 20 K.Cold densification also caused a strong enhancement ofB(104), the field atwhich Jc takesthe value 1 × 104 A cm−2. For tapes subjected to 1.48 GPa, and at 4.2 K were found to increase from 11.8 and 10.5 T to 13.2 and 12.2 T, respectively.Almost isotropic conditions were obtained for rectangular wires with aspect ratiosa/b<2 subjected to 2.0 GPa, where and T were obtained. At 20 K, the wires exhibited an almost isotropic behavior, with T and T, Birr(20 K) being ∼10 T. These values are equal to or higher than the highest values reported so far for isotropicin situ wires with SiC or other carbon based additives. Further improvements are expected onoptimizing the cold high pressure densification process, which has the potential for fabrication ofMgB2 wires of industrial lengths.

Strong enhancement of Jc and Birr inbinary in situ MgB2 wires after cold high pressure densification

Cold high pressure densification was found to substantially enhance the critical current density of binaryin situ Fe/MgB2 wires. A wire densified at 1.85 GPa exhibited at 20 K and 5 T an increase ofJc by 300% with respect to same wire without the application of pressure. At 4.2 K and 10 T,Jc was found to be increased by 53%. The decrease of the electrical resistancefor densified wires reflects an improved connectivity. The values ofBirr at 4.2 and 20 K were enhanced up to 0.7 T for densified wires.After applying pressures up to 6.5 GPa at 300 K, the relative mass densitydm of theunreacted (B+Mg) mixture inside the filament increased up to 96% of thetheoretical density. This corresponds to a relative mass densitydf in thereacted MgB2 filaments of 73%. A quantitative correlation between filament mass density and criticalcurrent density was established.