Bulk MgB2 Superconducting Materials: Technology, Properties, and Applications

Abstract

The intensive development of hydrogen technologies has made very promising applications of one of the cheapest and easily produced bulk MgB₂-based superconductors. These materials are capable of operating effectively at liquid hydrogen temperatures (around 20 K) and are used as elements in various devices, such as magnets, magnetic bearings, fault current limiters, electrical motors, and generators. These applications require mechanically and chemically stable materials with high superconducting characteristics. This review considers the results of superconducting and structural property studies of MgB₂-based bulk materials prepared under different pressure-temperature conditions using different promising methods: hot pressing (30 MPa), spark plasma sintering (16-96 MPa), and high quasi-hydrostatic pressures (2 GPa). Much attention has been paid to the study of the correlation between the manufacturing pressure-temperature conditions and superconducting characteristics. The influence of the amount and distribution of oxygen impurity and an excess of boron on superconducting characteristics is analyzed. The dependence of superconducting characteristics on the various additions and changes in material structure caused by these additions are discussed. It is shown that different production conditions and additions improve the superconducting MgB₂ bulk properties for various ranges of temperature and magnetic fields, and the optimal technology may be selected according to the application requirements. We briefly discuss the possible applications of MgB₂ superconductors in devices, such as fault current limiters and electric machines.

Keywords: effect of impurity oxygen; magnesium diboride bulk superconductors; pinning; structural study; superconducting properties.

Figure 1

Dependences of critical current density (magnetic measurement), J_c, on magnetic field, μ_oH, for MgB₂-based materials at 20 K (a) and 30 K (b) [108]. 1 HP—high-pressure synthesized under 2 GPa at 1050 °C for 1 h from Mg(I):2B(I) with 10% SiC addition; 2 HP—high-pressure synthesized (2 GPa, 1050 °C, 1 h) from Mg(I):2B(I); 3 HP—high-pressure-sintered (2 GPa, 1050 °C, 1 h) from MgB₂ (VII); 4 HP—high-pressure-synthesized (2 GPa, 600 °C, 1 h) from Mg (II):2B (II); 5 SPS—spark-plasma-synthesized under 50 MPa at 600 °C for 0.3 h and then at 1050 °C for 0.5 h from Mg(I):2B(III); 6 HotP—synthesized by hot pressing (30 MPa, 900 °C, 1 h) from Mg(I):2B(III) with 10% Ta addition; 7 HIP—synthesized under high isostatic (gas) pressure (0.1 GPa, 900 °C, 1 h) from mixture of Mg(I):2B(III) with 10% Ti addition, which was precompacted into a ring shape by broaching; 8 PL—pressureless sintering (in flowing Ar under 0.1 MPa at 800 °C for 2 h) from mixture of Mg(I):2B(III) with 10% Ti addition, which was precompacted into a ring shape by broaching.

Figure 2

(a,b)—Sample structures obtained by SEM in COMPO (compositional) contrast: (a)—Sample sintered from MgB₂ (Type VI) under 2 GPa at 1000 °C for 1 h; bright small zones in (a) seem to be inclusions (containing O, Zr, Nb, and possibly ZrO₂) appearing due to milling of initial MgB₂. (b)—Structure of sample synthesized from Mg(I):2B(I) under 2 GPa at 800 °C. (c,d)—X-ray patterns of these samples, respectively [109].

Figure 3

Critical current density, J_c, vs. magnetic field, μ_oH, of MgB₂ prepared (a) from Mg(I):2B(III) by SPS under 50 MPa at 600 °C for 0.3 h and then at 1050 °C for 0.5 h and (b) from Mg(I):2B(III) + 10 wt% Ti by HotP under 30 MPa at 1000 °C for 15 min [119].

Figure 4

Structures of MgB₂ materials prepared from Mg(I):2B(III) mixtures under 50 MPa at 600 °C for 0.5 h and then at 1050 °C for 0.5 h. Images were obtained using SEM at different magnifications [109]; (a–c)—SEI and, (d)—BEI.

Figure 5

Real (m’) part of the ac susceptibility (magnetic moment) vs. temperature, T [108]. Small samples for the study were cut from superconductors prepared under 2 GPa. 1—edge of block 63 mm in diameter, prepared from Mg(I):2B (I and III) + 2 wt% Ti, at 800 °C; 2—center of the same block; 3—block 63 mm in diameter, Mg(I):2B(III) at 950 °C, 4—tablet 9 mm in diameter, Mg(I):2B(V) + 10 wt% Ti, at 1050 °C; 5—tablet 9 mm in diameter, Mg(I):2B(III) + 10 wt% Ti, at 800 °C; 6—tablet 9 mm in diameter, Mg(I):2B(III) + 10 wt% Ti at 1050 °C; 7—tablet 9 mm in diameter, Mg(I):2B(III) + 10 wt% Ta, at 1050 °C; 8—tablet 9 mm in diameter, Mg(II):2B(II) at 600 °C.

Figure 6

Thermal dependence of the upper critical magnetic field, B_c2, of bulk MgB₂ [120,132], prepared from: 1—Mg(II):2B(II) under 2 GPa (HP) at 600 °C for 1 h; 2—Mg(I):2B(III) (30 MPa (HotP), 800 °C, 2h); 3—Mg(I):2B(III) (50 MPa (SPS), at 600 °C for 0.3 h and then at 1050 °C for 0.5 h); 4—Mg(I):2B(III) (2 GPa (HP), 900 °C, 1 h); 5—Mg(I):2B(V) + 10 wt% Zr (2 GPa (HP), 800 °C, 1 h); 6—Mg(I):2B(V) + 10 wt% Ti (2 GPa (HP), 800 °C, 1 h); 7—Mg(I):2B(I) + 10 wt% SiC (2 GPa (HP), 1050 °C, 1 h).

Figure 7

(a–d)—Microstructures obtained by SEM at different magnifications of MgB₂ material prepared from Mg(II):2B(II) mixtures under 2 GPa at 600 °C for 1 h [109]. (a)—SEI and, (b–d)—BEI.

Figure 8

The dependences of critical current density, J_c, at 20 K on a magnetic field. The MgB₂ samples were prepared from Mg(I):2B(I) and Mg(I):2B(III). The graph was composed using the data presented in [20,98,103,119].

Figure 9

(a,b)—SEM images in SEI mode of MgB₂ materials synthesized from Mg(I):2B(III) mixtures under 2 GPa, for 1 h at 800 and 1050 °C, respectively [109]. (c,d)—Schema of MgB₂-based material structures synthesized at low temperature of 800 °C (e) and high temperature of 1050 °C (f) [85]. (e,f)—X-ray patterns of samples shown in (a,b) [113].

Figure 10

Critical current density, J_c, vs. magnetic field, µ_oH, of MgB₂ materials prepared from Mg(I):2B(I) and Mg(I):2B(III) mixtures under 2 GPa, at 800 and 1050 °C for 1 h (a,b), respectively; additions of SiC (0.2–0.8 μm) to Mg(I):2B(I) mixture (c) and Ti (99%, 1–3 μm) to Mg(I):2B(III) (d) [103].

Figure 11

(a) Maximal pinning forces, BFp(max), and corresponding values of magnetic fields at 20 K vs. synthesis pressure for MgB₂-based materials synthesized from Mg(I) and B(III) at 800 (circles) and 1050 °C (stars); (b)—normalized pinning force, F_p, vs. magnetic field, B, calculated from the critical current density, J_c; and (c)—dependence of critical current density, J_c, on magnetic field. Designations: k = B_peak/B_n; PP—point pinning; GBP—grain boundary pinning; and MP—mixed pinning [128]. Curves: (1) Mg(I):2B(I) + 10% SiC, 2GPa, 1050 °C, 1 h, k = 0.51 (PP); (2) Mg(I):2B(III) + 10% Ti, 2 GPa, 1050 °C, 1 h, k = 0.42 (MP); (3) Mg(I):2B(III), 50 MPa, 600 °C for 0.3 h and then 1050 °C for 0.5 h, k = 0.63 (>PP?); (4) Mg(II):2B(II) with 3.5% C, 2 GPa, 600 °C, 1 h, k = 0.31 (GBP); (5) Mg(I):2B(III) + 10% Ti, 30 MPa for 1 h and then 1000 °C for 0.2 h, k = 0.42 (MP); (6) MgB₂, 16 MPa, 1150 °C, 0.3 h, k = 0.45 (PP); (7) Mg(I):2B(III), in flowing Ar atmosphere under 0.1 MPa, 800 °C, 4 h, k = 0.35 (GBP).

Figure 12

(a)—X-ray patterns of the material synthesized under 2 GPa at 1200 °C for 1 h from Mg(I):12B(III); (b)—dependences of J_c on the external magnetic fields, μ_oH, at 20 K for the materials synthesized under 2 GPa for 1 h from Mg(I) and B(III), taken in the ratio Mg:xB, and synthesized at temperature, T_S: curves 1—Mg:12B, T_S = 1200 °C; curve 2—Mg:10B, T_S = 1200 °C; curve 3—Mg:8B, T_S = 1200 °C; curve 4—Mg:6B, T_S = 1200 °C; curve 5—Mg:4B, T_S = 1200 °C; curve 6—Mg:12B, T_S = 800 °C; curve 7—Mg:20B, T_S = 1200 °C; (c)—backscattering SEM image of the material prepared under 2 GPa at 1200 °C for 1 h from Mg(I):12B(III); (d) HRT—EM microstructure (of a MgB₁₂ grain, the stoichiometry of which was estimated by HRTEM EDX); (e)—dependences of critical current density, J_c, on magnetic fields, μ_oH, at 10–35 K for the materials prepared under 2 GPa at 1200 °C for 1 h from mixtures of Mg(I):8B(III) [103].

Figure 13

Microstructures of the materials synthesized from Mg(I):B(III) with a 10 wt% of Ti (3–10 μm) addition under 2 GPa for 1 h at 800 (a,c) and 1050 °C (b,d) [108]. X-ray patterns of these materials (e,f). (c,d) show the places where Ti is absent [103,113].

Figure 14

(a) Image of microstructure of MgB₂ sample with 10 wt% of Ti (3–10 μm); image 16a was taken in the place where the Ti grains are absent. (b–d)—EDX maps of boron, oxygen, and magnesium distributions over the area of the image shown in 16e (the brighter the area looks, the higher the amount of the element under study) [103].

Figure 15

(a–c) SEM images of MgB₂ sample with 10 wt% of Ti powder (about 60 μm) synthesized under 2 GPa at 800 °C for 1 h: SEI (a–c) [113]. Notations: “I”—Mg-B-O inclusions, MgB_x—higher magnesium borides. In (c), the points marked by No. 1–6 are the points for which were made quantitative Auger analyses, the results of which are summarized in Table 6 [113].

Figure 16

(a,b)—Microstructure of magnesium diboride synthesized from Mg(I):B(III) with 10 wt% TiH₂ addition under 2 GPa at 950 °C for 1 h in SEI [84] (a) and COMPO (b) regimes.

Figure 17

Microstructure of materials with 10 wt% of SiC additions (0.2–0.8 μm) prepared from Mg(I):2B(I) under 2 GPa (HP) at 800 °C for 1 h (a–d) and at 1050 °C (e–h); (a,c,e,g)—SEI images; (b,d,f,h)—COMPO images; (a,b), (c,d), (e,f), and (g,h) are paired images of the same place under the same magnification but in different modes—SEI and COMPO [132].

Figure 18

Characteristics of MgB₂-based materials synthesized from Mg(I):2B(III) and Mg(II):2B(II) under 2 GPa for 1 h at different temperatures: (a–f)—dependences of critical current density, J_c, on magnetic field, B, of materials without (a) and with additions of titanium (Ti) (b,e), polyvalent titanium oxides (Ti-O) (c,f), and titanium carbide (TiC) (d); (g)—fields of irreversibility, B_irr, and (h) upper critical magnetic fields, B_C2, vs. temperature [85].

Figure 19

(a)—X-ray diffraction pattern, (b)—dependence of critical current densities, J_c, on magnetic field, µ_oH, at 10, 20, 25, 30, 33, and 35 K of the material, prepared from Mg(I):2B(I) under 2 GPa at 1050 °C for 1 h [117].

Figure 20

Calculated density of electronic states, N(E), for MgB₂ (a), MgB_1.75O_0.25 (b), MgB_1.5O_0.5 (c) per formula unit; (d)—calculated DOS at the Fermi level. N(E_F) depends on the oxygen concentration, x, in MgB_2-xO_x compounds (hollow squares). The total DOS and partial contributions of Mg, B, and O atoms are indicated by solid squares, solid triangles, and solid circles, respectively [132].

Figure 21

(a)—Dependence of the binding energy, E_b, on the oxygen concentration, x, in MgB_2-xO_x/C_x: 1, 3—homogeneous oxygen and carbon substitutions of boron atoms, respectively; 2, 4—the lowest binding energy vs. x for the ordered oxygen and carbon substitutions (for example, in nearby positions or in pairs), respectively. (b)—Z-contrast image of coherent oxygen-containing inclusions in [010] of MgB₂ obtained using HRTEM (high–resolution transmission microscopy). Bright atoms—Mg. The contrast increases in each second row and is due to the presence of oxygen in each second boron plane. The white arrows show the columns of atoms in which oxygen is present [117].

Figure 22

Maps of electron density distribution for: (a)—MgB₂ (z = 1/2, (001)), (b)—MgB_1.75O_0.25 (z = 1/4, (001) [108]), (c)—MgB_1.5O_0.5 (z = 1/4, (001)); z-coordinates of the plane of a 2 × 2 × 2 supercell, where z is given in units of the c parameter of a 2 × 2 × 2 MgB₂ supercell [132]; (d)—MgB_1.5O_0.5 in the transversal plane under an angle to the basal boron planes of the hexagonal unit cell to show the boron plane without imbedded oxygen atoms together with the Mg plane (the plane goes through the 7-B, 8-B, and 1′-B, 2′-B positions of a 2 × 2 × 2 supercell [132]); (e)—MgB_1.5C_0.5 (z = 1/4, (001)); (f)—MgB_1.5C_0.5 in the transversal plane under an angle to the basal boron planes (the plane goes through the 7-B, 8-B, and 1′-B, 2′-B positions of a 2 × 2 × 2 supercell) [117].

Figure 23

Examples of MgB2 bulk superconductors: (a)—obtained using HotP, (b) [120], (c) —obtained using HP and then the rings were cut mechanically, and (d)—obtained by machining a bulk cylinder manufactured using SPS [26].

Figure 24

High quasi-hydrostatic pressing (HP) in ISM NASU. Hydraulic 140 MN-effort press from the ASEA company (a), hydraulic 25 MN-effort press (b), cylinder piston high–pressure apparatus (HPA) (c), recessed-anvil type (HPA) for 25 MN press (d), and scheme of high–pressure cell of the recessed-anvil HPA (before and after loading) (e).

Figure 25

Hydraulic press DO 630 for hot pressing with generator and inductor (a,b); general view of inductor of hot press during heating (shining window—opening for temperature estimation by pyrometer) (c), scheme of assembled inductor (d).

Figure 26

Installation for spark plasma sintering (a) and, scheme of SPS heating chamber (b) [166].

Figure 27

(a)—The schemes of an SFCL model and a testing circuit for the simulation of a fault event. (b)—Typical oscilloscope traces of the current in a protected circuit (black, solid curve) and the voltage drop across the primary coil of the SFCL model (red, dashed curve) at 50 Hz and about 4 K (from [90]). The experiment details are described in [120]. “A”—is ammeter.

Figure 28

General view of azebra-type rotor of a 1300W/215V superconducting motor with MgB₂ bulk superconductor [9].

Figure 29

(a) Magnetic shield of MgB₂ in the shape of a cup (outer radius, R_o = 10.15 mm; inner radius, R_i =7.0 mm; external height, h_e = 22.5 mm; internal depth, d_i = 18.3 mm). The material is machinable by chipping. The shielding factors (i.e., the ratio between an outer applied magnetic field, H_appl, and an inner magnetic field measured by a Hall sensor at different z₁–z₅ positions (b)) at T = 30 K are shown in (c). The dashed lines represent the shielding factors computed in correspondence with the Hall probe positions, assuming the magnetic field dependence of J_c(B) at 30 K. (Figure 2 in [26] adapts the results obtained in [159]).