Jc Characteristics Research Articles

Evolution of critical current density in CaKFe4As4 with La-doping

Abstract Single crystals of (Ca1-xLax)KFe4As4 (0 ≤ x ≤0.16) have been grown by using the self-flux method, and the evolution of physical properties including the critical current density (Jc) with La-doping has been investigated. Tc decreases monotonically with increasing x, while Jc at the same temperature and magnetic field increases initially and reach its maximum at x = 0.082. The increase in Jc is more obvious at low temperatures and high fields. At T = 5 K and H = 40 kOe, Jc reaches 0.34 MA/cm2, which is ~4 times larger than that for pure crystals. It is also found that anomalous temperature dependence of Jc in CaKFe4As4 is wiped away as the La content is increased. However, Jc shows non-monotonic field dependence (peak effect) at high fields in crystals with large x. In addition, we found that despite weak anisotropy of Hc2, there is extremely large anisotropy of Jc up to ~15, which is most likely caused by novel planar defects in the crystal, similar to CaKFe4As4. Jc characteristics in (Ca1-xLax)KFe4As4 with disorder outside FeAs planes is compared with that in CaK(Fe1-xCox)4As4 with disorder within FeAs planes.

Nanocomposite Beta Nb50Ti based SWCNTs by FAST-SPS-FCT

The synthesis of β-type phase NbxTi (x= 50 at%) / SWCNTs (Single Walled Carbon Nanotubes) intermetallic matrix nanocomposite by mechanical alloying to ensure the effective distribution of (SWCNTs) within the matrix. It has been stated by several researchers that during ball-milling of NbxTi (x =50 at%) powder mixtures, Nb-Ti intermetallic compound formation occurs either gradually along milling time of mechanical allowing (MA), or suddenly through amechanically self-propagating reaction (MSPR), which occurs after a ignition time of MA. For this purpose, 0.4and 0.8 at% of SWNTs was added to the powder mixture after the completion of reactionbetween Nb and Ti. The obtained powders Nb50Ti intermetallic compound mixed with SWCNTs powder and then was also ball-milled. Bulk samples were compacted and then sintered by (FAST-SPS-FCT) method (Field Assisted Sintering Technics-Sparck Plasma Sintering- Furnace, Advanced Ceramics for High-Temperature) at the temperature range (1273-1473 °K) with some time that retained the integrity of SWNTs in the intermetallic matrix.Structural and mechanical and vibronic changes of the nanocomposites were investigated by X-ray diffractometery (XRD). Field emission scanning electron microscopy (FESEM) micrographs showed that the offered MA approach caused the SWNTs to be uniformly embedded in the in situ synthesized NbTi intermetallic matrix. Meanwhile better distribution of SWCNTs resulted in higher density of FAST-SPS-FCT bulk nanocomposite as well higher hardness up to 2.75GPa compared to 2.4 of Nb50Ti intermetallic alloy obtained after MA time. The total porosity, compressive strength, and compressive elastic modulus of the FAST-SPS-FCT manufactured material were determined as 7%, 600 MPa, and 120 MPa, respectively. The alloy’s and its intermetallic nanocomposite have Young’s elastic modulus is comparable to that of healthy cancellous bone which makes it applicable in the biomedical field. The in vitro biocomptability will be performed in the near future. The comparable results for the FAST-SPS-FCT nanocomposites were 3%, 650 MPa, and 130 MPa. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. This difference in mechanical properties results from different porosity and phase composition of the bot β-phase NbxTi (x= 50 at%) and NbxTi (x= 50 at%)/SWCNTs intermetallic matrix nanocomposite. More other nanotechnologies applications of the nanocomposite will be focused in the study of the superconducting type I for the ITER Poloidal Field Coils by measuring of Jc (T, B) characteristics.

Improvement of critical current density of REBa2Cu3O7-δ by increase in configurational entropy of mixing.

REBa2Cu3O7-δ (RE123, RE: rare earth) is one of the high-temperature superconductors with a transition temperature (Tc) exceeding 90 K. Because of its high-Tc and large critical current density (Jc) under magnetic fields, RE123 superconductors have been expected to play a key role in superconductivity application. To accelerate application researches on RE123-based devices, further improvements of Jc characteristics have been desired. In this study, we investigated the effects of high-entropy alloying at the RE site on the superconducting properties, through the measurements of local (intra-grain) Jc () by a remanent magnetization method. We found that shows a trend to be improved when four or five RE elements are mixed at the RE site, which results in high configurational entropy of mixing (ΔSmix). All samples exhibited an order of few MA cm−2 which is a criterion for practical application and the highest resulted in a value of around 7.0 MA cm−2 at T = 2.0 K. Because high-entropy alloying can improve of RE123 superconductors, our entropy-engineering strategy introduced here would be useful for the development of RE123 superconducting materials available under high magnetic fields.

Enhanced critical current density and flux pinning of anthracene doped magnesium diboride superconductor

The present investigation reports the structural, superconducting and flux pinning properties of anthracene doped MgB2 superconductor prepared by powder-in-sealed-tube (PIST) method. The formation of the hexagonal structure of MgB2 is confirmed from the XRD analysis of all the synthesized samples. The microstructural changes by anthracene doping in the MgB2 superconductor are studied using FESEM, TEM and Raman analysis. Raman spectra show the change in the E2g peak frequency in all the doped samples. All the samples show a sharp superconducting transition and the critical temperature (Tc) of anthracene doped samples decreases. The Jc(B) behavior at 10 and 20 K shows a significant enhancement in the anthracene doped samples as compared to undoped MgB2. 3 wt% anthracene doped MgB2 shows maximum enhancement in Jc at 10 and 20 K, i.e., 7.03 × 105 and 2.5 × 105A∕cm2 at 2 T, while 5 wt% anthracene doped sample (MBA5) shows lower Jc(B) characteristics. The grain boundary pinning is the dominant pinning mechanism in all the synthesized samples.

Developments of (Ba,Na)Fe2As2 and CaKFe4As4 HIP round wires

With excellent superconducting properties such as high Tc, large Hc2, small anisotropy, and large Jc, developments of superconducting wires and tapes of iron-based superconductors (IBSs) have been undertaken with promising results. So far, K-doped 122-type IBSs have been mainly developed for such purposes. Here, efforts to fabricate Na-doped 122-type IBS, (Ba,Na)Fe2As2, are presented. With the help of hot isostatic pressing (HIP) process, excellent Jc characteristics of up to 44 kA cm−2 at 4.2 K under 100 kOe have been achieved. Detailed characterizations of (Ba,Na)Fe2As2 HIP round wire are presented. On the other hand, discoveries of new IBSs continue, with CaKFe4As4 as one of recent examples. CaKFe4As4 belongs to a new family of IBSs with 1144 structure, where Ca and K occupy the Ba site in BaFe2As2 alternately along the c-axis. Studies on CaKFe4As4 single crystals have demonstrated moderately large Jc with peculiar anisotropic properties related to the presence of novel planar defects. We have also fabricated HIP round wires of CaKFe4As4, and reported their basic characterizations. Despite the instability of CaKFe4As4 at low temperatures, Jc values at 4.2 K close to 100 kA cm−2 at self-field and 7.6 kA cm−2 at 100 kOe are achieved. Attempts to introduce defects into CaKFe4As4 single crystals, chemically by Co substitution and physically by proton irradiation, are attempted, and the effects are compared. Attempts to fabricate Co-doped CaKFe4As4 HIP round wires with limited success are also reported.

Microstructural study on Sn–Zn/Cu–Ti diffusion reaction for internal tin Nb3Sn conductor development

To take advantage of Zn and Ti doping effects on improvement of diffusion behavior in the Sn–Cu mixing step and the Nb3Sn layer formation for the internal tin process, a diffusion couple configuration of Nb/Cu–Ti/Sn–Zn has been proposed. In this work, a fundamental study on the diffusion behavior of Sn–Zn/Cu–Ti, with different heat treatments, was conducted to aid in a better understanding of Nb3Sn phase formation, via internal tin diffusion. The first feature is that at 210∘C, the γ-CuZn phase forms at the reaction interface of Sn-20 wt%Zn and Cu, following which the β-CuZn and η-CuSn phases form at 400∘C. Compared with the Sn/Cu-12 wt%Zn system, Kirkendall voids appear to be suppressed, as few ε-CuSn phases, in which many voids grow, form in the Sn–Zn/Cu system. At 550∘C, a dendritic mixed phase of α-CuZn and δ-CuSn phases form. At this temperature, Sn appears to diffuse faster than in the case of the Sn/Cu–Zn system. It is observed that, after heat treatment at 550∘C, very fine compound particles of Sn–Ti are homogeneously distributed in the outer regions originally known to be the Cu–Ti sheath area. In the multifilamentary wires consisting of Nb/Cu–Ti/Sn–Zn diffusion couple configuration, no Ti rich compound layer was formed at the boundary of the Nb sub-elements, which overcomes the problem to suppress a smooth Sn and Ti diffusion into the Nb sub-element, when Ti is doped to Sn cores. Thus, no Ti-rich compound layer segregation, few void formations in the Sn–Cu mixing step and distributed fine Sn–Ti particles in the matrix accounts for the improved Jc characteristics in Nb/Cu–Ti/Sn–Zn wires. With regards to the mechanical properties, alloying of 20 wt% Zn to Sn cores increases the Vickers hardness of the Sn core to approximately 20kgf/mm2, almost double that of pure Sn or Sn-alloys with a small amount of Ti.

Synthesis of β-type NbXTi (X= 50 at%)/Carbon nanotubes by FCT-FAST-SPS: For metallic orthopedic components

The synthesize of β-type phase NbxTi (x=50 at%)/SWCNTs intermetallic matrix nanocomposite by mechanical alloying to ensure the effective distribution of single walled carbon nanotubes (SWCNTs) within the matrix. It has been stated by several researchers that during ball-milling of NbxTi (x=50 at%) powder mixtures, Nb-Ti intermetallic compound formation occurs either gradually along milling time, or suddenly through a mechanically self- propagating reaction (MSPR), which occurs after a ignition time of MA. For this purpose, 0.4 and 0.8 wt% of SWNTs were added to the powder mixture after the completion of reaction between Nb and Ti. The resultant powders Nb50Ti intermetallic compound and with addition of powder of SWCNTs and then also ball-milled.Bulk samples were compacted and then sintered by field actived sintering technic spark plasma sintering method (FAST- SPS) at lower temperature in the range (1273 to 1473 K) with short time that retained the integrity of SWNTs in the intermetallic matrix. Structural and characteristics evolutions of the nanocomposites were investigated by X-ray diffractometery (XRD). Field emission scanning electron microscopy (FESEM) micrographs showed that the offered MA approach caused the SWNTs to uniformly embed in the in situ synthesized NbTi intermetallic matrix. Meanwhile better distribution of SWCNTs resulted in higher density of FAST-SPS-FCT bulk nanocomposite as well as higher hardness up to 2.75 GPa compared to2.4 of Nb50Ti intermetallic alloy obtained from the after MA time. The total porosity, compressive strength, and compressive elastic modulus of the FAST-SPS-FCT manufactured material were determined as 7%, 600 MPa, and 120 MPa, respectively. The alloy’s and its intermetallic nanocomposite have Young’s elastic modulus is comparable to that of healthy cancellous bone which makes it applicable in the biomedical field. The in vitro biocomptability will be performed in the near future. The comparable results for the FAST-SPS-FCT nanocomposites were 3%, 650 MPa, and 130 MPa. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. This difference in mechanical properties results from different porosity and phase composition of the bot β-phase NbxTi (x=50 at%) and NbxTi (x=50 at%)/SWCNTs intermetallic matrix nanocomposite. More other nanotechnologies applications of the nanocomposite will be focused in the study of the superconducting type I for the ITER Poloidal Field Coils by measuring of Jc (T, B) characteristics.

Evaluation of Layer Thickness Dependence of Critical Current Density using Longitudinal Magnetic Field Effect in Superconducting Coated Conductors

When the critical current density Jc for a superconductor is constant, the critical current Ic increases in proportion to the superconducting cross-sectional area. However, in the case of a superconducting coated conductor, when the thickness of the superconducting layer exceeds about 2 μm, there is no further increase in Ic. This is because Jc decreases due to the structural degradation of the superconducting layer. In this study, Jc for samples with superconducting layers of different thicknesses was measured using the longitudinal magnetic field effect in order to investigate the optimum layer thickness for maximizing Ic. Differences in the Jc characteristics due to the layer thickness appeared more clearly under a longitudinal magnetic field than a transverse magnetic field. This was particularly true at low temperatures, and the optimum layer thickness was found to be approximately 1 μm.

Synthesis of β-type NbXTi (X= 50 at %) /carbon nanotubes by FCT-FAST-SPS: For metallic orthopedic components

The synthesize of β-type phase NbxTi (x=50 at%)/SWCNTs intermetallic matrix nanocomposite by mechanical alloying to ensure the effective distribution of single walled carbon nanotubes (SWCNTs) within the matrix. It has been stated by several researchers that during ball-milling of NbxTi (x=50 at%) powder mixtures, Nb-Ti intermetallic compound formation occurs either gradually along milling time, or suddenly through a mechanically self- propagating reaction (MSPR), which occurs after a ignition time of MA. For this purpose, 0.4 and 0.8 wt% of SWNTs were added to the powder mixture after the completion of reaction between Nb and Ti. The resultant powders Nb50Ti intermetallic compound and with addition of powder of SWCNTs and then also ball-milled.Bulk samples were compacted and then sintered by field actived sintering technic spark plasma sintering method (FAST- SPS) at lower temperature in the range (1273 to 1473 K) with short time that retained the integrity of SWNTs in the intermetallic matrix. Structural and characteristics evolutions of the nanocomposites were investigated by X-ray diffractometery (XRD). Field emission scanning electron microscopy (FESEM) micrographs showed that the offered MA approach caused the SWNTs to uniformly embed in the in situ synthesized NbTi intermetallic matrix. Meanwhile better distribution of SWCNTs resulted in higher density of FAST-SPS-FCT bulk nanocomposite as well as higher hardness up to 2.75 GPa compared to2.4 of Nb50Ti intermetallic alloy obtained from the after MA time. The total porosity, compressive strength, and compressive elastic modulus of the FAST-SPS-FCT manufactured material were determined as 7%, 600 MPa, and 120 MPa, respectively. The alloy’s and its intermetallic nanocomposite have Young’s elastic modulus is comparable to that of healthy cancellous bone which makes it applicable in the biomedical field. The in vitro biocomptability will be performed in the near future. The comparable results for the FAST-SPS-FCT nanocomposites were 3%, 650 MPa, and 130 MPa. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. This difference in mechanical properties results from different porosity and phase composition of the bot β-phase NbxTi (x=50 at%) and NbxTi (x=50 at%)/SWCNTs intermetallic matrix nanocomposite. More other nanotechnologies applications of the nanocomposite will be focused in the study of the superconducting type I for the ITER Poloidal Field Coils by measuring of Jc (T, B) characteristics.

Passive magnetic shielding by machinable MgB2 bulks: measurements and numerical simulations

We report on a combined experimental and modelling approach towards the design and fabrication of efficient bulk shields for low-frequency magnetic fields. To this aim, MgB2 is a promising material when its growing technique allows the fabrication of suitably shaped products and a realistic numerical modelling can be exploited to guide the shield design. Here, we report the shielding properties of an MgB2 tube grown by a novel technique that produces fully machinable bulks, which can match specific shape requirements. Despite a height/radius aspect ratio of only 1.75, shielding factors higher than 175 and 55 were measured at temperature T = 20 K and in axially-applied magnetic fields μ0Happl = 0.1 and 1.0 T, respectively, by means of cryogenic Hall probes placed on the tube’s axis. The magnetic behaviour of the superconductor was then modelled as follows: first we used a two-step procedure to reconstruct the macroscopic critical current density dependence on magnetic field, Jc(B), at different temperatures from the local magnetic induction cycles measured by the Hall probes. Next, using these Jc(B) characteristics, by means of finite-element calculations we reproduced the experimental cycles remarkably well at all the investigated temperatures and positions along the tube’s axis. Finally, this validated model was exploited to study the influence both of the tube’s wall thickness and of a cap addition on the shield performance. In the latter case, assuming the working temperature of 25 K, shielding factors of 105 and 104 are predicted in axial applied fields μ0Happl = 0.1 and 1.0 T, respectively.

A new carbon source MgB2C2 for the synthesis of carbon-doped MgB2 materials

One of the major problems of superconducting MgB2 materials for extensive application is a large decrease in Jc with an increase of magnetic field. Carbon substitution for the boron site of MgB2 has been well known to reduce this problem and numerous carbon containing compounds have been proposed as carbon sources thus far. However, the actual carbon substitution level, which is dependent on carbon source materials and sintering conditions, tends to be lower than the nominal one. In addition, impurity phases or hydrogen gas generate from most of the carbon sources during heat-treatment except for pure carbon and B4C. In this paper, MgB2C2 is proposed as a promising new carbon source for preparation of carbon-doped MgB2. Although synthesis of MgB2C2 has been considered to be difficult through a simple solid state reaction under an ambient pressure, we found that this phase can be obtained from powder mixture of nano-carbon (carbon black), boron and magnesium by sintering in a sealed stainless tube. The carbon-doped MgB2 polycrystalline bulks using MgB2C2 as a carbon source contained almost the same carbon level as nominal one and exhibited improved Jc characteristics in magnetic field.

Study of Critical Current and n-Values of 2G HTS Tapes: Their Magnetic Field-Angular Dependence

Since many applications of YBCO tapes operate in external magnetic fields, it is necessary to investigate the magneto-angular dependence of critical current and n-values in coated conductors. In this paper, five commercial YBCO tapes with different microstructures produced by three different manufacturers are chosen. The selected samples have a width of 2.0, 4.0, 4.8, 6.0 or 12 mm, with copper, brass or stainless steel laminations. The critical current density dependence Jc(B, θ) and n-values characteristics n(B, θ) of the tapes are comprehensively measured under various magnetic fields and orientations. Afterwards, the obtained experimental data sets are successfully fitted using a novel multi-objective model which considers the material anisotropy. By using this approach, a fitting function Ic(B, θ) can always be obtained to accurately describe the experimental data, regardless of the fabrication and width differences of the superconducting tapes. Moreover, our experiment shows that when subject to different external magnetic fields, the angular dependence of n-values characteristics is directly correlated with the corresponding critical current profiles. Our results are helpful to predict the critical current of electromagnetically interacting 2G HTS wires, thereby improving the design and performance of the devices made from YBCO tapes.

Present status of PIT round wires of 122-type iron-based superconductors

Outstanding characteristics with high Tc and Hc2 and small anisotropy in iron-based superconductors (IBSs) have triggered the development of superconducting wires and tapes using these novel superconductors. In this short article, developments and present status of round wires of 122-type IBSs are reviewed. By introducing hot-isostatic pressing (HIP) technique, Jc in round wires of 122-type IBSs has been improved significantly. Further improvements have been realized by refining the fabrication process of the core material and introducing partial texturing of the wire core. The largest transport Jc for round wires at 4.2 K at self-field and 100 kOe are 2.0x105 A/cm2 and 3.8x104 A/cm2, respectively. We also compare the Jc characteristics of wires and tapes processed by HIP.

Numerical modelling of iron-pnictide bulk superconductor magnetization

Iron-based superconductors exhibit a number of properties attractive for applications, including low anisotropy, high upper critical magnetic fields (Hc2) in excess of 90 T and intrinsic critical current densities above 1 MA cm−2 (0 T, 4.2 K). It was shown recently that bulk iron-pnictide superconducting magnets capable of trapping over 1 T (5 K) and 0.5 T (20 K) can be fabricated with fine-grain polycrystalline Ba0.6K0.4Fe2As2 (Ba122). These Ba122 magnets were processed by a scalable, versatile and low-cost method using common industrial ceramic processing techniques. In this paper, a standard numerical modelling technique, based on a 2D axisymmetric finite-element model implementing the H-formulation, is used to investigate the magnetisation properties of such iron-pnictide bulk superconductors. Using the measured Jc(B, T) characteristics of a small specimen taken from a bulk Ba122 sample, experimentally measured trapped fields are reproduced well for a single bulk, as well as a stack of bulks. Additionally, the influence of the geometric dimensions (thickness and diameter) on the trapped field is analysed, with a view of fabricating larger samples to increase the magnetic field available from such trapped field magnets. It is shown that, with current state-of-the-art superconducting properties, surface trapped fields >2 T could readily be achieved at 5 K (and >1 T at 20 K) with a sample of diameter 50 mm. Finally, an aspect ratio of between 1 and 1.5 for R/H (radius/thickness) would be an appropriate compromise between the accessible, surface trapped field and volume of superconducting material for bulk Ba122 magnets.

Development of Ag-Barrier RHQT Nb3Al Wires

A new type of rapid-heating, quenching and transformation-processed Nb 3 Al wire incorporating Ag-barrier structure have been developed to realize both the interfilament decoupling and the low wire breakage risk during the drawing. The barrier has a three-layered structure of Nb/Ag/Nb. Ag does not react with Nb at all even during the rapid-heating and quenching (RHQ) treatment and not penetrate into the Nb 3 Al filaments, which is necessary for maintaining the critical current density (J c ) characteristics. Drawability of the Ag-barrier Nb/Al precursor composite was excellent; a prototype 200 m wire with O1.35 mm was successfully drawn without any breakage. Ag intermediate layer allocated between the Nb layers suppresses flux jumps at low fields even at 1.8 K. It is notable that the Ag intermediate layer acts as a role of cushioning, when the wire is subject to applied strain, mitigating stress concentration inside the wire and leading to enhancement of the irreversible strain up to about 0.6%: this would be a key factor especially for the “React & Wind” magnet applications. Cu stabilizer can be easily attached through a “tube-method” after RHQ. The filament size can be reduced down to about 25 μm. The hysteresis loss (±3 T@4.2 K) per nonstabilizer volume was reduced down to 614 mJ/cm 3 .

Another approach for controlling size and distribution of nanoparticles in coated conductors fabricated by the TFA-MOD method

A new process, which is conceptually different from that reported so far, was developed for positive and direct control of the size and distribution of BaZrO3 (BZO) nanoparticles acting as the effective magnetic flux pinning centers for coated conductors (CCs) fabricated on buffered metal substrates by the trifluoroacetate-based metal-organic decomposition method. The (Y,Gd)Ba2Cu3O6+δ (YGdBCO) phase crystallization heat-treatment process was applied one time to a multi-layered precursor prepared by repeating the coating/calcination process of the extremely thin layer of a few tens of nanometers. The size of BZO nanoparticles refined with thinning the once coated film thickness (donce) even in the same total film thickness of 0.6−0.7 μm, consequently the critical current density in magnetic-field (Jc(B)) was increased. A YGdBCO CC fabricated with donce of 30 nm revealed very high Jc(B∣∣c) characteristics of 0.26 and 1.60 MA cm−2 at 77 and 65 K, respectively, for 3.0 T.

鉄ニクタイド系高温超伝導多結晶バルク磁石の開発

This article reviews the recent development of bulk trapped field magnet made with iron-based superconductor (IBSC). A trapped field of over 1 T at 5 K and 0.5 T at 20 K has been measured between a stack of magnetized cylinders of bulk polycrystalline Ba0.6K0.4Fe2As2 superconductors 10 mm in diameter. Magneto-optical imaging revealed a trapped field distribution corresponding to uniform macroscopic current loops circulating through the sample. A standard numerical modelling technique using the measured Jc(B, T) characteristics of a small specimen reproduced the experimentally measured trapped fields well, again indicating the homogeneous current loops in the polycrystalline bulk. Given the untextured polycrystalline nature of the cylinders and their large irreversibility field, it is expected that larger IBSC bulks could trap much higher field.

Trapping a magnetic field of 7.9 T using a bulk magnet fabricated from stack of coated conductors

We have fabricated a bulk magnet using double stack, each 130 layers, of short segments of coated conductors (CCs). The bulk magnet is magnetized by field-cooling in a magnetic field of 9 T down to 4.2K. After reducing the magnetic field down to zero, we have successfully trapped a magnetic field of 7.9 T at the centre of the double stack. The magnetic field profile of the bulk magnet is calculated by fully considering the Jc(B) characteristics of the short segment of the CC. The trapped magnetic field values measured by Hall probes at three locations near the centre of the double stacks agree reasonably well with the calculated magnetic induction.

Numerical modelling and comparison of MgB2 bulks fabricated by HIP and infiltration growth

MgB2 in bulk form shows great promise as trapped field magnets (TFMs) as an alternative to bulk (RE)BCO materials to replace permanent magnets in applications such as rotating machines, magnetic bearings and magnetic separation, and the relative ease of fabrication of MgB2 materials has enabled a number of different processing techniques to be developed. In this paper, a comparison is made between bulk MgB2 samples fabricated by the hot isostatic pressing (HIP), with and without Ti-doping, and infiltration growth (IG) methods and the highest trapped field in an IG-processed bulk MgB2 sample, Bz = 2.12 at 5 K and 1.66 T at 15 K, is reported. Since bulk MgB2 has a more homogeneous Jc distribution than (RE)BCO bulks, studies on such systems are made somewhat easier because simplified assumptions regarding the geometry and Jc distribution can be made, and a numerical simulation technique based on the 2D axisymmetric H-formulation is introduced to model the complete process of field cooling (FC) magnetization. As input data for the model, the measured Jc(B,T) characteristics of a single, small specimen taken from each bulk sample are used, in addition to measured specific heat and thermal conductivity data for the materials. The results of the simulation reproduce the experimental results extremely well: (1) indicating the samples have excellent homogeneity, and (2) validating the numerical model as a fast, accurate and powerful tool to investigate the trapped field profile of bulk MgB2 discs of any size accurately, under any specific operating conditions. Finally, the paper is concluded with a numerical analysis of the influence of the dimensions of the bulk sample on the trapped field.

Doping effect of nano-Ho2O3 and naphthalene in MgB2 superconductor prepared by powder-in-sealed-tube method

The effect on crystal structure, critical temperature (TC), and critical current density (JC) of bulk MgB2 doped with nano-Ho2O3 and naphthalene was studied. Among all the samples studied, the sample doped with 2.5 wt. % nano-Ho2O3 have shown the best field dependent critical current density [JC(H)], i.e., 0.77 × 105 A/cm2 at 2 T and 10 K. While naphthalene doped MgB2 sample has shown the least JC(H) characteristics. The improved JC(H) characteristics in the nano-Ho2O3 doped MgB2 samples are attributed to improved flux pinning properties due to the formation of HoB4 and in naphthalene doped MgB2 samples. The slight lower TC value (37.01 K) in naphthalene doped samples is attributed to the occurrence of lattice defect by the substitution of carbon at boron site of MgB2 superconductor. Lower ΔTC value implies the lesser anisotropy in all the synthesized samples. The flux pinning force density (FP/FPmax) curves are theoretically analyzed using Dew-Hughes model. The result revealed that point pinning is the dominant pinning mechanism for nano-Ho2O3 doped MgB2 samples, while, surface and grain boundary pinning become dominant with increasing naphthalene addition in nano-Ho2O3 doped MgB2 samples.