High Quality Factor, Ultralow Sintering Temperature Li₆B₄O₉ Microwave Dielectric Ceramics with Ultralow Density for Antenna Substrates

Dense Li₆B₄O₉ microwave dielectric ceramics were synthesized at low temperature via solid-state reaction using Li₂CO₃ and LiBO₂. Optimum permittivity ∼ 5.95, quality factor ∼ 41 800 GHz and temperature coefficient of resonant frequency ∼ - 72 ppm/°C were obtained in ceramics sintered at 640 °C with a ultrasmall bulk density ∼2.003 g/cm³ (∼95% relative density, the smallest among all the reported microwave dielectric ceramics). Li₆B₄O₉ ceramics were shown to be chemically compatible with silver electrodes but reacted with aluminum forming Li₃AlB₂O₆ and Li₂AlBO₄ secondary phases. A prototype patch antenna was fabricated by tape casting and screen printing. The antenna resonated at 4.255 GHz with a bandwidth ∼279 MHz at -10 dB transmission loss (S₁₁) in agreement with simulated results. The Li₆B₄O₉ microwave dielectric ceramic possesses similar microwave dielectric properties to the commercial materials but much lower density and could be a good candidate for both antenna substrate and low-temperature cofired ceramics technology.