Well dispersed single-crystalline BaTiO₃ nanoparticles with controlled morphologies were synthesized using a thermohydrolysis route. The nanoparticles were tuned from spherical to cubic upon changing the NaOH concentration under a critical molar ratio of oleic acid to hydrazine. Density functional theory (DFT) and molecular dynamics (MD) calculations indicated that hydrazine molecules adsorbed preferably on the Ti position of the Ti–O terminated surface; meanwhile, oleic acid molecules tended to adsorb on the Ba position of the Ba–O terminated surface. The added hydrazine changed the formation mechanism of BaTiO₃ nanoparticles from an in situ growth to a dissolution–precipitation growth. Excellent dispersibility in aqueous solution was achieved for the BaTiO₃ nanoparticles under the assistance of hydrazine. Meanwhile, a high-quality self-assembled film with a stable dielectric constant of 30 in the frequency range from 0 Hz to 1 MHz was prepared using the well dispersed BaTiO₃ nanoparticles, providing a novel low-temperature route for the fabrication of perovskite films.

The development of dielectric materials with low permittivity and low loss is a great challenge in wireless communication. In this study, LiLn(PO₃)₄ (Ln = La, Sm, Eu) ceramic systems were successfully prepared using the traditional solid-state method. X-ray diffraction analysis indicated that the LiLn(PO₃)₄ ceramics crystallized in a monoclinic structure when sintered at 850–940 ℃. The characteristic peak shifted to high angles with variations in the Ln element, which was ascribed to a reduction in cell volume. Further analysis by structure refinement revealed that the reduction of cell volume resulted from the decline of chemical bond lengths and compression of [LiO₄] and [PO₄] tetrahedra. Remarkably, the LiLn(PO₃)₄ ceramic system displayed exceptional performances at low sintering temperatures (910–925 ℃), including high Q·f of 41,607–75,968 GHz, low t_f ranging from −19.64 to −47.49 ppm/℃, low ε_r between 5.04 and 5.26, and low density (3.04–3.26 g/cm³). The application of the Phillips–Van Vechten–Levine theory revealed that the increased Q·f value of the LiLn(PO₃)₄ systems can be attributed to the enhanced packing fraction, bond covalency, and lattice energy, and the stability of t_f was associated with the increase of bond energy. Furthermore, a microstrip patch antenna prototype using the LiEu(PO₃)₄ ceramics was fabricated. Measurement results demonstrated excellent antenna performances with a bandwidth of 360 MHz and a peak gain of 5.11 dB at a central frequency of 5.08 GHz. Therefore, the low ε_r LiLn(PO₃)₄ ceramic systems are promising candidates for microwave/millimeter-wave communication.

Percolative composites always suffer from their unstable and filler-loading dependent microstructures and negative electromagnetic parameters. Here, stable negative permittivity is achieved by in situ constructing a three-dimensional carbon network in the silica spherical matrix after a self-assembly and pyrolysis process. An electrical percolation phenomenon appears with the formation of a carbon network. Once the carbon network is formed, further increasing carbon loadings will only influence the porosity rather than the connectivity due to the nature of the porous carbon itself. Hence, the microstructure and plasma-like negative permittivity are not sensitive to carbon loading, leading to a negligible carbon loading dependent permittivity behavior. Moreover, negative permittivity with small values (−100 < ε′ < 0), beneficial for matching with permeability, was effectively adjusted by changing the carbonization temperature. The carbon composites with negative permittivity showed an extremely low dielectric loss (tan δ = 1–7) compared with metal composites (usually tan δ = 10–100). This work provides a convenient means to obtain stable negative permittivity properties. The carbon composites can be regarded as a promising candidate for metamaterials and will facilitate the applications of materials with negative electromagnetic parameters.