Owing to its strong B–N covalent bonds and inferior dissolution–precipitation characteristics in selected liquid phases, densification of h-BN ceramics is generally achieved via pressure-assisted sintering techniques. Herein, high-density h-BN ceramics were fabricated by a secondary cold isostatic pressing (CIP(II))-assisted pressureless sintering process using commercial micron-sized h-BN powder as the raw material and deionized water as a green lubricant additive. The addition of water significantly promotes the particle sliding and rearrangement of lamellar structured h-BN during the forming process, and the relative density of the green body can reach 94.60%. A secondary cold isostatic pressing applied to the dried green body can effectively suppress cracking caused by water evaporation and further increase the relative density to 98.10%. The use of low-oxygen raw powder effectively inhibits void formation induced by volatilization, reduces density loss after high-temperature sintering, and enhances the thermal conductivity of pressureless-sintered h-BN ceramics. The internal flake-like h-BN grains retained a highly oriented layered structure, endowing the ceramic with excellent in-plane thermal conductivity (up to 57.16 W·m⁻¹·K⁻¹). This study offers an alternative approach for the industrial-scale production of high-density h-BN ceramics through pressureless sintering.

High dielectric loss and poor temperature stability are the current barriers to the application of dielectric materials. In present work, we synthesized a system of acceptor Gd³⁺ and donor Ta⁵⁺ co-doped (Gd_0.5Ta_0.5)_xTi_1-xO₂ (GTTOx, x = 0, 0.01, 0.02, 0.04, 0.06) ceramics to enhance dielectric response. It was found that a colossal permittivity (CP, 2.65 × 10⁴@1 kHz, 2.37 × 10⁴@1 MHz), a very low dielectric loss (tanδ, 0.007@1 kHz, 0.03@1 MHz), good stability of frequency (20–10⁶ Hz) and temperature (RT–250 ℃, Δε′ (T)/ε′₃₀ < ± 15%, at 1 kHz) were achieved simultaneously in GTTO0.01 ceramic. Complex impedance spectroscopy, XPS, SEM, and Raman spectroscopy were used to investigate the reasons for the improved dielectric properties. The result indicated that the main reasons for CP and low dielectric loss are the synergistic effect of the electron pinning defect-dipole (EPDD) model, the internal blocking layer capacitance (IBLC) mechanism, and electrode response. This work provides a promising approach for the design of defect-related high-performance giant dielectric ceramics.