Research On Matrixes Of High Temperature Radar Absorbing Materials

In order to select matrixes for high temperature radar absorbing materials (HRAM), mullite, MAS and Si3N4 ceramics are studied.Employing aluminium powders and TEOS as raw materials, mullite precursor powders are prepared through sol-gel method. The relative density of silica-rich mullite ceramic sintered from these powders at 1550℃ for 1 hour with a pressure of 15MPa is 98.4%. DSC-TG and XRD show that these mullite precursor powders are amorphous before heat treatment and mullite begins to form at 975℃. Using aluminium sulfate and colloidal silica as raw materials, another kind of mullite precursor powders is made. The relative density of silica-rich mullite ceramic sintered from these powders at 1550℃ for 1 hour with a pressure of 15MPa is 97%. These powders transform to mullite completely at 1300℃. The dielectric constant of mullite ceramics is related with their composition. The dielectric constants of silica-rich, alumina-rich, and stoichiometric mullite ceramics are 5.5, 6.0 and 6.3 respectively.The dielectric constant of Mullite-Si/C/N composites is related with the sintering parameters. With the increase of the sintering temperature and dwelling time, the ε' ε" and tg of Mullite-Si/C/N composites are decreased, this phenomenon is related with the change of the microstructure and phase composition of nano Si/C/N powders. With the increase of the content of nano Si/C/N powders, the ε' ε" and tg5 of Mullite-Si/C/N composites are increased . There are great differences between the dielectric constant calculated by Maxwell Garnet formula and the one measured experimentally. This is caused by the possible interface reaction between absorbent and mullite matrix. The dielectric constant of Mullite-Si/C/N composite reduces obviously with the increasing of frequency, and its tg5 is quite high, so it is a possible candidate as the absorber layer of high temperature radar absorbing materials.MAS precursor powders are prepared using aluminium sulfate, colloidal silica, and magnesium nitrate as raw materials via sol-gel methods, DSC-TG and XRD show that the MAS precursor powders transform to cordierite completely at 1300℃. MAS precursor powders were uniaxially pressed at 100 MPa employing a lubricated steel cylindrical mold. After sintering at 1450℃ for 2 hours, Single-phase cordierite ceramic is prepared, and its relative density is 95.1%.With the increase of the sintering temperature, the e' and tgS of MAS-Si/C/N composites are increased. The ε" of MAS-Si/C/N composites changes irregularly with the increase of temperature due to the change of phase composition. With the increase of dwelling time or Si/C/N content, the ε' ε" and tg?of MAS-Si/C/N composites are increased. The dielectric constant of MAS-Si/C/N composites reduces obviously with the increase of frequency, and its tg is quite high, so it is a possible candidate for absorber layer of high temperature radar absorbing materials.Si3N4-Si/C/N composites are fabricated via hot pressing methods employing MgO and Y2O3 as sintering additives. The bending strengths of these materials are approximately 250MPa. With the increase of the content of Si/C/N nano powders, the e" and tgS of Si3N4-Si/C/N composites are increased. The ε' of Si3N4-Si/CYN composites increases with the increase of the content of Si/C/N nano powders when the weight percentage of Si/C/N nano powders is less than 10%. With the weight percentage of Si/C/N nano powders increase from 10% to 15%, the ε' of Si3N4-Si/C/N composites decreases.