With the development of power electronic systems, energy storage capacitors have the advantages of large discharge power, fast charging and discharging speed and stable performance, and play an important role in power systems, electronic devices, pulse power supplies, etc.. They are widely used in civilian and military fields. Compared with fuel cells, the energy storage capacitors do not need to convert the Gibbs free energy of the chemical energy of the fuel into electrical energy through electrochemical reactions, but store the charge by pressurizing between the two plates of the capacitor with higher safety and reliability as well as environmentally friendness. The existing pulsed dielectric materials are mainly divided into ceramics, glass ceramics, thin films and other composite materials. Glass ceramics have energy storage advantages, compared with other materials. Increasing the dielectric constant of dielectric materials plays a crucial role in increasing the energy density.

A glass with the composition of 13K₂O-21SrO-32Nb₂O₅-5B₂O₃-4Al₂O₃-25SiO₂-xBi₂O₃ (x=0.0%, 1.0%, 2.0%, and 4.0%, in mole) was produced by a conventional melt annealing process. First, 40 g of raw materials were weighed and ground in a polypropylene ball mill in ethanol for 12 h. The resulting mixture was dried at 100 ℃ and subsequently melted in a corundum crucible in a resistance furnace at 1 500 ℃ for 2 h. The molten liquid was pressed into a sheet on a preheated copper plate and then annealed at 500 ℃ for 6 h to eliminate residual internal stresses. Finally, a niobate glass was obtained. The glass slices were crystallized at 850 ℃ and 950 ℃ for 2 h to obtain glass ceramics. The glass ceramic samples were ground, polished, and plated with electrodes for the structural and performance tests.

The effect of Bi₂O₃ concentrations (x=0.0%, 1.0%, 2.0%, and 4.0% in mole) on the phase evolution, microstructure, dielectric and energy storage properties of K₂O-SrO-Nb₂O₅-B₂O₃-Al₂O₃-SiO₂ glass ceramics was investigated by a controlled crystallization process. Based on the XRD analysis, KSr₂Nb₅O₁₅ is a main precipitated crystalline phase, and the crystallization promotes due to the addition of bismuth oxide. The A-site ions are adjusted by Bi³⁺ with a smaller radius (i.e., 1.03 Å) into the crystal lattice instead of Sr²⁺ with a larger radius (i.e., 1.18 Å), thus reducing the unit cell volume and increasing the lattice distortion. From the Raman spectra, the change of A-site occupancy leads to the distortion of the structure and enhances the spontaneous polarization. The microstructure shows that the grains are dense and evenly distributed, and the grain size is fine. The dielectric temperature spectroscopy indicates that the relaxation characteristics correspond to the polar nanoregion of the microstructure, and the DC breakdown strength firstly increases and then decreases, which is consistent with the change of the dielectric constant. The polarized electric field curve is slender with a large spontaneous polarization and a small residual polarization, improving the energy storage efficiency. The charge-discharge curve shows that the energy density firstly increases and then decreases with the addition of bismuth oxide, and the glass-ceramic sample B2-9 obtains the maximum energy density, discharge power density and fast discharge time, having a great application prospect in the field of dielectric energy storage.

Strontium niobate potassium glass ceramics were prepared by a conventional melting method and a controlled crystallization technology. At a crystallization temperature of 950 ℃, the unit cell volume of the crystalline phase decreased from 611.83 ų to 610.19 ų with the increase of Bi₂O₃ doping content. The grain size of the microstructure firstly decreased and then increased, and the optimal grain size of glass ceramic sample B2-9 was 57.72 nm. The dielectric and energy storage properties also firstly increased and then decreased, and the maximum dielectric constant of glass-ceramic sample B2-9 was 342 times greater than that of glass-ceramic sample B2-8, which was 1.9 times greater than that of glass-ceramic sample B2-8. According to the P-E curve, the glass ceramic sample B2-9 had the optimum performance (i.e., P_max=9.33 μC/cm², P_r=0.73 μC/cm², W_rec=1.27 J/cm³, W_total=1.55 J/cm³, η=82% at 350 kV/cm). The energy density of the glass ceramic sample B2-9 was 1.33 J/cm³ at 400 kV/cm, the maximum discharge power density was 124.13 MW/cm³, and the fastest discharge time was 22 ns. The breakdown strength firstly increased and then decreased, which were 742, 886, 923 kV/cm and 622 kV/cm, respectively. Glass-ceramic materials could have great application prospects in the field of dielectric energy storage.