The development of lead-free bulk ceramics with high recoverable energy density (W_rec) is of decisive importance for meeting the requirements of advanced pulsed power capacitors toward miniaturization and integration. However, the W_rec (<2 J cm⁻³) of lead-free bulk ceramics has long been limited by their low dielectric breakdown strength (DBS < 200 kV cm⁻¹) and small saturation polarization (P_s). In this work, a strategy (compositions control the grain size of lead-free ceramics to submicron scale to increase the DBS, and the hybridization between the Bi 6p and O 2p orbitals enhances the P_s) was proposed to improve the W_rec of lead-free ceramics. (K_0.5Na_0.5)NbO₃–Bi(Me_2/3Nb_1/3)O₃ solid solutions (where Me²⁺ = Mg and Zn) were designed for achieving large P_s, and high DBS and W_rec. As an example, (1 − x)(K_0.5Na_0.5)NbO₃–xBi(Mg_2/3Nb_1/3)O₃ (KNN–BMN) ceramics were prepared by using a conventional solid-state reaction process in this study. Large P_s (41 μC cm⁻²) and high DBS (300 kV cm⁻¹) were obtained for 0.90KNN–0.10BMN ceramics, leading to large W_rec (4.08 J cm⁻³). The significantly enhanced W_rec is more than 2–3 times larger than that of other lead-free bulk ceramics. The findings in this study not only provide a design methodology for developing lead-free bulk ceramics with large W_rec but also could bring about the development of a series of KNN-based ceramics with significantly enhanced W_rec and DBS in the future. More importantly, this work opens a new research and application field (dielectric energy storage) for (K_0.5Na_0.5)NbO₃-based ceramics.

Lightweight structures with multi-functions such as electromagnetic wave absorption and excellent mechanical properties are required in spacecraft. A three-dimensional metamaterial absorber consisting of honeycomb and resistive films was proposed and fabricated through 3D printing and silk-screen printing technology. According to simulation and experiment results, the present three-dimensional metamaterial absorber can realize an absorptivity of more than 90% in a wide band of 3.53–24.00 GHz, and improve absorbing efficiency for transverse magnetic (TM) waves of oblique incidence angle from 0° to 70°. The compression test results reveal that compressive strength of the 3D printed honeycomb can reach 10.7 MPa with density of only 254.91 kg/m³, and the energy absorption per volume W _v and per unit mass W _m are 4.37 × 10³ KJ/m³ and 17.14 KJ/Kg, respectively. The peak compressive strength and energy absorption per mass are at least 2.2 and 3 times comparing to metallic lattice cores with the same density. Outstanding electromagnetic wave absorption and mechanical performance make the present three-dimensional metamaterial absorber more competitive in engineering applications.

Lossless coding metasurfaces and metamaterial absorbers have been widely used for radar cross section (RCS) reduction and stealth applications, which merely depend on redirecting electromagnetic wave energy into various oblique angles or absorbing electromagnetic energy, respectively. Here, an absorptive coding metasurface capable of both the flexible manipulation of backward scattering and further wideband bistatic RCS reduction is proposed. The original idea is carried out by utilizing absorptive elements, such as metamaterial absorbers, to establish a coding metasurface. We establish an analytical connection between an arbitrary absorptive coding metasurface arrangement of both the amplitude and phase and its far-field pattern. Then, as an example, an absorptive coding metasurface is demonstrated as a nonperiodic metamaterial absorber, which indicates an expected better performance of RCS reduction than the traditional lossless coding metasurface and periodic metamaterial-absorber. Both theoretical analysis and full-wave simulation results show good accordance with the experiment.

Using coordinate transformation stated earlier by Pendry et al. [Science 312, 1780 (2006)], we deduced the material parameter equation for elliptic cylinder cloaks and reported the numerical simulations as well. The theoretical and numerical results indicate that, an elliptic cylinder cloak, which is constructed with metamaterials possessing the permittivity and permeability ruled by our equation, can exclude the electromagnetic fields from the interior region without perturbing the exterior fields and therefore the objects in it can be effectively invisible to the outer region.

A two-dimensional (2D) coding phase gradient metasurface (CPGM) is proposed for radar cross section (RCS) reduction in this work. The 2D phase gradient super cell is employed to serve as the coding element. The primary pattern of the coding element will be modulated by the designed 2D phase gradient. Thus, a more flexible method of scattering manipulation will be achieved by both the 2D phase gradient and coding sequences. The specific scattering patterns of the 2D CPGM under the modulation of phase gradient and coding sequence were analyzed. A controllable backward diffusion scattering coding phase gradient metasurface was realized based on Pancharatnam–Berry phase by modulating both the phase gradient and coding sequence. Both simulated and measured results demonstrate its excellent performance on RCS reduction.

The design of perfect electromagnetic cloaks is usually based on the coordinate transformation method. However, for cloaks with arbitrary shapes, it is difficult to express analytically such coordinate transformation. Thus, we have to resort to numerical methods to design approximate cloaks. In this paper, we propose a numerical method of designing cloaks with arbitrary shapes. According to this method, spatial compressions are treated as homeomorphous mappings and described in numerical forms. The corresponding coordinate transformations are therefore expressed numerically. As an application of this method, we deduced the discrete constitutive parameters of a two-dimensional polygon cloak, and verified the results by numerical simulations. This method can be also used in cases with acoustic waves and matter waves.