Ceramic-ceramic composites intended for applications as substrates for microwave
MW antennas have been fabricated and investigated. BaTi4O9 – BaFe12O19 (BT4-BF12) and
BF12 – TiO2 composites were initially prepared using conventional sintering but it became
obvious early on in this study that interaction between the end-member phases resulted in
a significant deterioration of properties. As a consequence, cold sintering was attempted in
which ceramics densify at <200 OC and therefore interdiffusion between the end members is
negligible. To facilitate cold sintering, Li2MoO4 (LMO) was used instead of BT4 as one end
member since it has been shown to densify at 120 OC using a pressure mediated
hydrothermal route. Dense cold sintered ceramics have competitive MW properties, with
quality factor, Qf = 30,000 and relative permittivity, er = 5.5.
(1-x)LMO-xBF12 (0.00≤x≤0.15) were cold sintered at 120oC and their structure and
properties characterized. X-ray diffraction (XRD), scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) confirmed that compositions were dual phase and
had a dense microstructure. Composites in the xBF12-(1-x)LMO (0.0≤x≤0.15) series
resonated at MW frequencies (~5-6GHz) with 5.6≤ permittivity(er) ≤5.8 and 16,000≤Qf≤22,000 GHz,
despite the black colour of compositions with x > 0. The permeability (Mu) of the composites
was measured in the X band (~8 GHz) and showed an increase from 0.94 (x=0.05) to 1.02
(x=0.15). Finite element modelling revealed that the volume fraction of BF12 dictates the
conductivity of the material, with a percolation threshold at 10 vol.% BF12 but changes in permittivity (er) as a function of x were readily explained using a series mixing model. In summary, these
composites are considered suitable for the fabrication of dual mode or enhanced bandwidth
microstrip patch antennas. Conventionally sintered LMO-BF12 composites failed to produce
high quality ceramics and were brittle with unwanted second phases.
Cold sintering of several further composites was also performed, including LMO-TiO2
and LMO-BaTiO3. Of particular note however, were dense (≃95%) (1-x)K2MoO4-xBF12
composites which resonated at MW frequencies with a 5.6≤ permittivity(er)≤5.8, temperature coefficient
factor of resonance, -66≤TCF≤-39 ppm/°C and Qf ≃14,000 GHz for 0.05≤x≤0.15. No
interaction was noted between the two end members according to XRD, SEM and chemical
mapping using energy dispersive X-ray spectroscopy. (1-x)LMO-xLi1.5Al0.5Ge1.5P3O12 (LAGP)
were also fabricated but as anticipated this composite became conducting for low values of
x. TCF varied from -169 to -43 ppm/°C for x=0.05≤x≤0.15 but high relative density (94%)
could be achieved.
Overall, cold sintering proved a successful route for the fabrication of ceramic-ceramic
composites for MW applications with a number of systems showing great promise.
However, tuning TCF to zero proved problematic within the systems described due to the
limited range of x (<0.2) for which dense ceramics could be achieved.
