Simultaneously improved permittivity and breakdown strength, accompanied with reduced dielectric loss and conductivity, are difficult to achieve in polymer nanocomposites with conductive fillers by blending to construct a homogeneous structure. In this work, an MXene/fluoropolymer nanocomposite with a gradient sandwich structure has been prepared by casting various polymer solutions thrice, followed by annealing. Unfunctionalized Ti₃C₂T_x MXene nanosheets with lower conductivity and higher Young's modulus compared to graphene nanosheets have been utilized as fillers. Polyvinylidene fluoride (PVDF) has been employed as the polymer matrix. High interface compatibility between MXene and PVDF, as well as between the adjacent two sub-layers in the sandwich composite, has been obtained due to effects of p–π conjugation, hydrogen bond and similarity compatibility, leading to enhanced interface polarization and improved permittivity. The interface barrier effect between the neighboring two sub-layers restrains electric tree growth, resulting in elevated breakdown strength. Compared with mono-layered composites and PVDF, the sandwich composite shows improved comprehensive electrical properties. It has a permittivity of ∼26 at 100 Hz, a breakdown strength of ∼350 MV m⁻¹ and an energy density (efficiency) of ∼12.5 J cm⁻³ (∼64%) at breakdown strength. This work might open the way for large-scale fabrication of promising nanocomposites with a high energy storage density.

The inclusion evolutions after calcium treatment in Al-killed steel with different sulphur content were determined by chemical experiments and thermodynamics calculations. The results show that the inclusions are calcium aluminate with a little amount of calcium sulphide in low content sulphur steel after calcium treatment. As the sulphur content in steel increases, the typical inclusions generated in steel are layered or homogeneous irregular oxysulphides. A number of pure sulphides can be observed in higher content sulphur steel, and the morphology of sulphides with different CaS/MnS mass ratio has different shape. Thermodynamics calculations of Al–Ca–O–S–Mn system inclusions in steel at 1873 K and during solidification process were comprehensively conducted, considering all types of inclusions. The experimental results are in good agreement with thermodynamic calculations, which can predict the formation of the inclusions in Al-killed steel with different sulphur content.