Fig. 1 a Transition temperature versus doping level ©2001 The American Physical Society; b lattice parameter a for various carbon doping levels © 2006 IOP Publishing Ltd Printed in the UK; c the critical current density as a function of magnetic field at 20 K (Pure © 2006 IOP Publishing Ltd Printed in the UK, C nanotube doping © 2003 American Institute of Physics), d TEM image of elemental carbon-doped MgB2 © 2006 IOP Publishing Ltd Printed in the UK [11, 65, 66]

Fig. 2 a Critical temperature versus doping level of the SiC dopant and C dopant; b critical temperature versus crystal lattice unit cell volume [23, 66]. © 2003 American Institute of Physics

Fig. 3 a Critical current density (Jc) at 4.2 K versus magnetic field for the SiC-doped and pure MgB2 sintered at 650 and 1000 °C, and C doped sintered at 700 and 950 °C; b the critical temperature (Tc) versus sintering temperature; c the actual C substitution level, x, in Mg(B1-xCx)2 versus sintering temperature; d the a-axis lattice parameter versus sintering temperature[18]. © 2007 The American Physical Society

Fig. 4 Critical current density (Jc) at 20 K versus magnetic field with different SiC-doped and pure MgB2[16, 17, 23, 24, 72,73,74]

Fig. 5 XRD patterns a, critical current density b, TEM image c of the sugar-doped MgB2, respectively © 2007 WILEY-VCH Verlag GmbH & Co. KGaA; critical current density d, SEM images of the pure e, the malic acid-doped f MgB2, respectively © 2006 American Institute of Physics[30, 32]

Fig. 6 a Critical current density for pure and sugar-doped MgB2; b crystal lattice defects in organic-doped MgB2, respectively[28]. © 2008 IOP Publishing Ltd Printed in the UK

Fig. 7 X-ray tomogram restruction of the malic acid-doped MgB2a, undoped MgB2b, respectively; scanning transmission electron microscope image of MgB2c, electron energy loss spectrum (EELS) d, fast Fourier transform (FFT) pattern e for the impurity phase, color map of electron energy loss f of boron K (blue color) and carbon K (red color) in a selected area of a (marked with yellow box), respectively[26] © Nature Japan K.K. All rights reserved 1884-4057/12

Fig. 8 aHc2 and Hirr versus operating temperature for the malic acid-treated MgB2 sintered at 700 and 900 oC; b RHH ratio versus reduced operating temperature for the undoped and malic acid-treated MgB2 sintered at 700 and 900 ° C; c, d the reduced pinning force f(h) versus the field dependence in the temperature range of 10-30 K[79]. © 2013 IOP Publishing Ltd Printed in the UK & the USA

Fig. 9 Critical current densities (Jc) at 20 K versus magnetic field with organic-doped and pure MgB2[28, 30, 32, 77, 80,81,82]

Fig. 10 a Raman spectra of graphene-doped MgB2 © 2012 IEEE 1051-8223; b Raman spectra with Gaussian fitted E2g mode and PDOS distortion of the pure, rGO and rCCG-doped MgB2 © The Royal Society of Chemistry 2012; c TEM images of graphene-doped MgB2 © 2012 IEEE 1051-8223; d, e the crossover field Bsb as a function of temperature and critical current density of graphene-doped MgB2 © 2010 IOP Publishing Ltd Printed in the UK & the USA[35,36,37]

Fig. 11 a Temperature dependence of normalized Hirr and Hc2, b critical current density; c 3D atom probe tomography reconstruction of GO-doped MgB2[33]. © The Royal Society of Chemistry 2014

Fig. 12 Critical current density a and normalized pinning force density b of MgB2, respectively © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved; c XRD patterns of the Cu-doped MgB2 prepared with carbon-coated boron. The inset is TEM image of carbon-coated nanosize amorphous boron particles prepared by the pyrolysis technique and © 2015 IOP Publishing Ltd Printed in the UK; d critical current density of Cu-doped MgB2 prepared with carbon-coated boron. The inset is the corresponding normalized pinning force Fp/Fpmax as a function of reduced magnetic field h. ©2016 Elsevier B.V. All rights reserved

Fig. 13 AFM images of the MgB2 Prepared with boron powders encapsulated by a 0, b 2.8, c 4.5, d 7.8, e 12, f 16.5 wt% carbon, respectively; Grain size distributions in the samples prepared by boron powders with g 0 wt% and h 2.8 wt% carbon, respectively; i grain size distribution obtained from the samples prepared by boron powders with 4.5, 7.3, 12 and 16.5 wt% carbon[41]. © 2017 Elsevier B.V. All rights reserved

Fig. 14 a Schematic of the fabricating process of PMMA-derived carbon-coated boron; b the critical current density of MgB2 prepared by PMMA-derived carbon-coated boron; c normalized temperature dependence of Hc2 and Hirr for the MgB2 prepared by PMMA-derived carbon-coated boron[39]. © 2017 Elsevier B.V. All rights reserved

Fig. 15 Critical current density (Jc) at 20 K versus magnetic field with graphene-related material-doped and pure MgB2[33, 34, 36, 37, 92,93,94,95]

Fig. 16 Critical current density of malic acid-doped MgB2 wires fabricated by cold densification at 2 GPa[53]. © 2009 IOP Publishing Ltd Printed in the UK

Fig. 17 a, b Magnetic field dependence of Jc for in situ and diffusion method-processed MgB2 at 5 and 20 K, respectively; c, d two-dimensional remnant graphene dispersing within the MgB2 particles and coating on the surface serving as effective flux pinning centers[103]. © 2019 American Chemical Society。

Fig. 18 Phase and microstructures of MgB2 samples. a X-ray diffraction (XRD) patterns for MgB2 prepared with B@g-C3N4 with different coating contents. b Lattice parameters of MgB2 with different g-C3N4 coating contents derived from the refinement results. c TEM image for C#004 sample. d-f Liner scans of EDS spectrum for C#004 sample. g, h HRTEM images for C#004 sample[104]. © 2019 American Chemical Society。

Fig. 19 Critical current density and the flux pinning performance of the MgB2 samples. a, b Citical current density as a function of magnetic field for all the prepared MgB2 at 5 and 20 K, respectively and the inset shows the double logarithmic Jc(H) curve at 20 K. c Critical current density decreasing rates as a function of magnetic field for all the prepared MgB2. d Linear extrapolation to zero of the low-Jc segment of J0.5c × H0.25 to get the Hirr values and the inset displays the Hirr values. eHirr values of MgB2 with different g-C3N4 contents. f Normalized flux pinning force as a function of the normalized magnetic field (H/Hirr) for all the prepared samples, and the reference curve for two-dimensional flux pinning[104]. © 2019 American Chemical Society。