A broadband metamaterial absorber intended for applications at elevated temperatures was designed and fabricated. Its performance was measured over a temperature range from room temperature to approximately 400 °C. The absorber consists of a lossy resistive frequency selective surface (RFSS) on a dielectric substrate and a metallic ground plane with high-temperature stability scalable to approximately 400 °C. The top RFSS layer, made of conductive composites, was prepared using a mixture of graphite microflakes and water glass, and it showed good chemical and thermal stability up to 400 °C. Its sheet resistance was adjustable over a wide range with different proportions of graphite flakes and decreased gradually with increasing temperature. The metamaterial absorber demonstrated broadband absorption with a reflection coefficient of less than −10 dB in the X-band frequency range (8.2–12.4 GHz), resulting in the absorption of more than 90%. The influence of testing temperature on the reflection coefficient was correlated with the variation in the sheet resistance. The proposed metamaterial absorber is a promising candidate for use in high-temperature microwave absorption applications.

Hollow yttria-stabilized zirconia (YSZ) microspheres with a diameter of 1.2-5.1 μm and a wall thickness of about 125 nm were synthesized, and their thermal stability was revealed for a hold time of 1 h at temperatures of 800°C, 1000°C and 1200°C. The microspheres were then used as raw materials to prepare bulk porous ceramics by gelcasting processing. The effects of the solid loading contents and sintering temperatures of the hollow YSZ microspheres on the microstructures, porosity, compressive strength and room-temperature thermal conductivity of the porous ceramic materials were investigated. Increases in the solid contents and sintering temperatures were found to decrease the bulk porosity and number of larger pores. Porous materials made with hollow spherical powders exhibited porosities ranging from 61.50% to 90.17%, with a peak value of 90.17% obtained under a solid loading of 5 vol% at a sintering temperature of 1100°C. The experimental thermal conductivity as a function of porosity was within the ranges predicted by one form of the Maxwell-Eucken model (Maxwell-Eucken 1) and the effective medium theory (EMT), reaching the lowest value of 0.052 W/(m•K) at a porosity 90.17%.

Lanthanum orthophosphate (LaPO 4 ) powder was prepared by a solid-liquid precipitation reaction of lanthanum carbonate powder with H 3 PO 4 in order to fabricate porous LaPO 4 materials. The phase transition and morphology evolution of the reaction products were found to be dependent mainly on the mass ratios of reactants and calcination temperatures. Porous materials using as-synthesized single-phase LaPO 4 powders and TiO 2 additives via direct foam-gelcasting exhibit large spherical-shaped cells without preferred orientation. The bulk density increases with the increase of sintering temperatures, as evidenced by bigger strut thickness, smaller cell and window sizes and lower inter-connectivity. Porous LaPO 4 bulk ceramic with 5 wt% TiO 2 addition sintered from 1100°C to 1500°C shows a porosity within 66.80-82.03% and a compressive strength from 4.0 to 17.2 MPa. The compressive strength dependence on the relative density follows a power-law relationship with an exponent value of 2.45. While imaginary parts of dielectric permittivity of the porous LaPO 4 materials measured over the X-band frequencies (8.2-12.4 GHz) depend weakly on the frequency and TiO 2 addition, the real parts are affected strongly by TiO 2 addition and sintering temperatures and fall within the range between 1.9 and 3.5 in this study. ARTICLE HISTORY