The processing methods, strengthening methods, water-proof coating procedures, and some ways to improve the transmission of microwaves have been reviewed in relation to fused silica ceramics and their composites for radome applications. Fused silica ceramics are characterized by a residual porosity (up to 18%), low dielectric constant (3.06-3.32), low loss tangent (0.00053-0.0065), excellent thermal properties, but low mechanical strength (37-65 MPa). To achieve higher mechanical strength and better transmission efficiency, new randome materials would be those engineered composites consisting, for instance, of a dense layer-porous core structure, a continuous fiber reinforcement, a multilayered structure, and/or frequency selective surface (FSS) layer (s)/metamaterials.

We investigated a novel artificial metamaterial that includes two plates of quartz glass dielectric material and a Ag microstructure sandwiched between the two plates. The Ag grid layer was designed and subsequently prepared by tape casting and screen printing. The transmission characteristics of this metamaterial were able to be controlled by adjusting the geometry parameters of the Ag grid such as the width of the strip and the size of the unit cell. Our work has demonstrated the possibility that the ceramic metamaterial can be used as a transmission material capable of work at high temperatures below the melting point of the metal.

A glass powder of the SiO2-Al2O3-K2O-Na2O system was added to fused silica particles to form composite green compacts by tape casting. This procedure was able to lower the sintering temperature of the fused silica particles making the composites suitable for being co-fired with the silver-palladium (Ag-Pd) paste commonly used as a conductive circuit in several microwave applications. The resulting new ceramic composite with the composition of 50 wt% fused silica and 50 wt% glass (brand name: TC-3) had a low dielectric constant and a low loss tangent of 2.7 and 2.5-3.7 x10-3 , respectively, and was able to be co-fired with the Ag-Pd conductive paste at the temperature of 895 °C, resulting in a potential low temperature co-fired ceramic (LTCC) system for microwave applications.

Laminated composites with a frequency selective surfaces (FSS) or more complex metamaterials are potential radome materials due to their unique characteristics of electromagnetic wave transmission. For making high-temperature resistant radomes, metamaterials or laminated composites with an FSS should be based on ceramic substrates. However, the processing methods for ceramic metamaterials are very limited and the conventionally used LTCC technique suffers from the shortcoming of large sintering shrinkage rates, which unfortunately impede the production of ceramic-based metamaterials. In this paper, a novel method of a low temperature co-fired ceramic (LTCC) technique combined with a technique of ceramic joining via green tapes was developed to fabricate the fused silica ceramic laminates sandwiched with the FSS. It was found that the newly developed composites with the FSS unit cells of the Ag-Pd strips exhibited near zero shrinkage of the unit cells, showed predictable transmission efficiencies of electromagnetic microwaves, and were able to overcome the poor transmission efficiencies below 11 GHz of the pure fused silica ceramic plates with an identical thickness.

Frequency selective surfaces (FSS) and recent metamaterials (MTM) have shown unique electromagnetic characteristics and are of potential benefits for radome applications. To make the radomes/windows high-temperature resistant, the substrates of the frequency selective surfaces or metamaterials should be made of ceramic/glass-based materials of a low dielectric constant and a low loss tangent. However, fabricating ceramic/glass-based FSS or MTM is always challenging. In this paper, a constrained low-temperature co-fired ceramic (LTCC) technique was used to produce quartz glass-based radome material consisting of a frequency selective surface (FSs) layer embedded in a surface laminate. Due to the constrained sintering shrinkage, the geometry and the dimensions of the unit cells of the FSs were not subjected to significant variations and thus the measured electromagnetic (EM) wave transmission spectra matched those of the computer simulation results quite well. This preliminary work marks the beginning of our long-term efforts toward the goal of achieving high-temperature resistant, highly electromagnetic wave transparent, as well as carefullydesigned and fabricated radome materials.