Boron-doped mullite derived from single-phase gels

Mullite with low dielectric constant and high transparency in infrared and microwave range has potential applications in communication industry. To improve the above properties of mullite, boron-doped mullite single-phase gels with a constant molar ratio of Al/Si = 3/1 and various B/Al ratios (B/Al = 0–0.4/3) were prepared in this study by slow hydrolysis of aluminum nitrate, boric acid and tetraethoxysilane. It was found that boron reduces the mullite formation temperature and suppresses spinel formation. The cell unit lattice parameters and cell volume in boron-doped mullite generally decrease with the increase of boron amount. The SEM observation shows that a small amount of boron reduces the grain sizes of mullite sintered bodies while a large amount of boron facilitates the formation of elongated grains and the amorphous glass phase. Boron decreases the transmittance of mullite ceramic and produces additional intensive absorption bond at 3.9 μm and also reduces the dielectric constants in the frequent range of 1 M–1 GHz.