MgB2　superconductor　bulks　were　fabricated　using　Mg　and　B　precursors.　The　physical　state　of　Mg　(solid,　liquid,　or　gas)　significantly　influences　the　final　product,　MgB2　bulk.　In　this　study,　the　Solid-Solid,　Liquid-Solid,　and　Gas-Solid　methods　were　employed　to　fabricate　MgB2　bulks.　The　superconducting　properties　and　microstructure　of　various　samples　were　systematically　investigated.　Notably,　for　the　first　time,　nano-CT　was　utilized　to　comprehensively　and　non-destructively　analyze　the　microstructure　of　MgB2　bulks,　particularly　the　size　and　distribution　of　voids,　which　serve　as　the　primary　barrier　affecting　the　superconducting　properties.　By　examining　the　microstructure　characteristics,　the　infiltration　effect　of　Mg　in　different　physical　states　during　the　reaction　was　analyzed,　and　a　feasible　reaction　model　was　established.　This　research　contributes　to　a　better　understanding　of　the　reaction　mechanism　of　MgB2,　which　may　aid　other　researchers　in　enhancing　the　properties　of　MgB2　superconductors.　©　2024　Elsevier　Ltd