A dual-modification strategy combining (Li 0.5 Bi 0.5 ) 2+ substitution and BaCu(B 2 O 5 )/LiF atmosphere-assisted sintering was developed to reduce the sintering temperature of MgGa 2 O 4 ceramic. The strategy achieved a 360 °C reduction in sintering temperature (from 1610 to 1250 °C) while maintaining a high performance. The optimal sample (Li 0.5 Bi 0.5 ) x Mg 1- x Ga 2 O 4 ( x = 0.02) exhibited ε r = 9.41, Q × f = 127,159 GHz, and τ f = -39.6 ppm/°C. XRD and Rietveld refinement confirmed that the spinel structure was preserved upon doping, alongside a lattice expansion. Consistent with the enhanced Q × f , a Raman spectroscopy analysis revealed a narrowing of the T 2 g mode, indicating an improved structural ordering. Furthermore, τ f was improved to −39.6 ppm/°C, directly related to the induced lattice distortion and expansion. The enhanced performance was attributed to the synergistic effects of composite ion doping and atmosphere-assisted sintering. The former modified the intrinsic structure and polarization characteristics, while the latter facilitated the densification at lower temperatures.
Spinel MgGa 2 O 4 ceramic materials were synthesized by solid-state method. MgGa 2 O 4 powders were investigated with X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectrum, Raman spectrum, X-ray photoelectron spectrum (XPS), and high resolution transmission electron microscopy (HRTEM). The spinel-structured MgGa 2 O 4 ceramics sintered at 1410 °C exhibited excellent microwave dielectric properties: a dielectric constant ( ɛ r ) of 9.54, a quality factor ( Q × f ) of 117,000 GHz (at 14.7 GHz frequency) and a temperature coefficient of resonant frequency ( τ f ) of −4.0 ppm/°C. MgGa 2 O 4 ceramics have a wide sintering temperature region (∼150 °C) and nearly zero τ f value. The large relative density, uniform microstructure and cation ordering resulted in high Q × f values. Crystal distortion from the oriented growth was advantageous to produce nearly zero τ f value. MgGa 2 O 4 ceramics are promising candidate materials for millimeter-wave devices.
This study demonstrates the successful synthesis of (1- x )MgGa 2 O 4 - x LiF ( x = 0.1–0.4, mass fraction) ceramics via solid-state reaction, achieving a significant reduction in sintering temperature from 1410 to 850 °C. The XRD and Raman testing results showed the formation of a new phase of LiGaO 2 , which affected the dielectric properties. Variations in the sintering density and ionic polarizability cause changes in the permittivity. In addition, the packing fraction analysis of MgGa 2 O 4 , which influences the crystal structure stability, elucidates the intrinsic mechanisms affecting the dielectric loss. The composite effect makes the τ f value of the ceramic shift toward the negative value, owing to the large negative frequency temperature coefficient of LiF. Finally, (1- x )MgGa 2 O 4 - x LiF ( x = 0.3) sintered at 850 °C possesses the superior dielectric properties: ε r = 9.45, Q × f = 32,790 GHz, τ f = −28.5 ppm/°C (at 14 GHz). In addition, (1- x )MgGa 2 O 4 - x LiF ( x = 0.3) ceramics exhibit good cofiring compatibility with Ag electrodes promoting the ceramic to be a suitable candidate for LTCC applications.
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