Microwave (MW) dielectric ceramics are used in numerous electronic components for modern wireless communication systems, including antennas, resonators, capacitors and filters. However, to date, MW ceramics are manufactured by an energy-intensive, conventional high-temperature (> 1000 °C) sintering technology and thus cannot be co-sintered with low melting point and base electrodes (Ag, Al, etc., < 1000 °C), nor directly integrated with polymers (< 200 °C). Cold sintering is able to densify ceramics at < 200 °C via a combination of external pressure and a transient liquid phase, reducing the energy consumed and facilitating greater integration with dissimilar materials. This review outlines the basics of MW ceramics alongside the mechanism of cold sintering. Recent developments in cold sintering of MW ceramics, composites and devices are described, emphasizing new materials and progress towards component/device fabrication. Future prospects and critical issues for advancing cold-sintered MW materials and devices, such as unclear mechanism, low Q × f values and poor mechanical properties, are discussed.

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Cold sintering of microwave dielectric ceramics and devices

Since its first introduction in 2016, cold sintering process (CSP) has gained worldwide interest from the scientific community as green and innovative fabrication route due to the dramatic reduction of processing time, energy, and costs. Cold sintering resembles the geological formation of rocks where a ceramic powder is densified with the aid of a liquid phase under an intense external pressure and limited heating conditions (below 350 °C). Up to date, tens of different materials, including composites, have been successfully processed through CSP and extraordinary results in terms of densification, microstructure and final properties have been achieved. In the present review, processing features and variables, possible densification mechanisms and issues also for the realization of ceramic-based composites are explored. Advantages with respect to existing techniques are analysed and current challenges are described to lay the ground for new processing opportunities to be faced in the near future.

The cold sintering process: A review on processing features, densification mechanisms and perspectives

Insights into the structure, photoluminescence and Judd-Ofelt analysis of red emitting SrLaLiTeO6: Eu3+ phosphors

Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) double perovskites were synthesized via solid-state ceramic route. Preliminary X-ray diffraction studies indicated a pseudocubic structure with lattice parameters ranging from 8.55 to 8.44 A for the substitution of rare earths from La to Eu. Raman spectra show the frequency dependence of various Raman bands with respect to rare-earth substitution and exhibit a significant shift in peaks to higher wavenumber region, which was observed only for symmetric stretching modes of LnO6 and TeO6 octahedra. In accordance with observed number of bands and group theoretical predictions, the most likely symmetry of all compounds in the Ba2Ln2/3TeO6 system was found to be monoclinic with P21/n space group. Rietveld refinement of the XRD patterns further confirmed the P21/n space group and also the 1:1 rock salt ordering of the B-site cations. Diffuse reflectance spectra of Ba2Ln2/3TeO6 showed the optical bandgaps of these compounds between 3.9 and 4.8 eV, indicating the suitabi...

Structural Characterization of B-Site Ordered Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu3+ Phosphor Hosts.

5G microstrip patch antenna and microwave dielectric properties of cold sintered LiWVO6–K2MoO4 composite ceramics

In the present study, a mineral garnet (Fe1.72Mg0.8Mn0.01Ca0.02)2+(Fe0.04Al2.36)3+Si2.934+O12 which decomposes above 600 °C was densified using cold sintering process. Composites of the garnet with different weight percentages of NaCl and Li2MoO4 were fabricated using water as a solvent. These composites heat treated at 120–250 °C under a uniaxial pressure of 450 MPa for 50 min showed 80–95 % of theoretical densities. The influences of amount of additives, cold sintering time and temperature on the microstructure and dielectric properties of the composites were investigated. As the amount of NaCl and Li2MoO4 increases, the grain boundaries become more evident and the garnet grains are segregated between the grains of additives due to the capillary force and pressure gradient. Garnet-NaCl and garnet-Li2MoO4 composites possess relative permittivity of 6–8 and quality factor (Qux f) > 2000 GHz (f ≈10  GHz) at microwave frequencies.

Garnet mineral based composites through cold sintering process: Microstructure and dielectric properties

A sustainable cold-sintering process was adopted to fabricate composites of (1–x)SrFe12O19–xLi2MoO4, x = 0.4, 0.5, 0.6, and 0.7 with density up to 91%, and their broad-band electromagnetic properti...

Cold Sintering: An Energy-Efficient Process for the Development of SrFe12O19–Li2MoO4 Composite-Based Wide-Bandwidth Ferrite Resonator Antenna for Ku-Band Applications

The exponential growth in microwave communications in recent years demands innovations and high yield microwave materials. Rare earth iron garnets are well known for their remarkable performance in microwave devices owing to their superior features. In this work we investigate the microstructure and dielectric properties of Bismuth substituted Gd3Fe5O12 ceramics prepared by the conventional solid state reaction route. The variation of lattice parameter, microstructure, permittivity, quality factor and temperature coefficient of resonant frequency (τf) of Gadolinium Iron Garnet (GIG) with several concentrations of bismuth was investigated. The Bi3+ ion is effectively incorporated into Gadolinium Iron Garnet, due to which the sintering temperature is considerably reduced, and the densification is improved significantly. Moreover, with increase in Bi concentration, the dielectric constant of GIG also increases, while the quality factor is slightly reduced, which is correlated with the higher ionic polarizability of bismuth. At an appreciably low sintering temperature of 1050°C, Gd2BiFe5O12 possess a porosity corrected relative permittivity value of 27.3 ± 0.2, unloaded quality factor (Qu × f) of 2990 ± 60 GHz (f = 7.3 GHz) and temperature coefficient of resonant frequency + 60 ± 1 ppm/°C.

Athira Rajan

Papers

Garnet mineral based composites through cold sintering process: Microstructure and dielectric properties

Cold Sintering: An Energy-Efficient Process for the Development of SrFe12O19–Li2MoO4 Composite-Based Wide-Bandwidth Ferrite Resonator Antenna for Ku-Band Applications

Influence of Bi Substitution on the Microstructure and Dielectric Properties of Gd 3 Fe 5 O 12 Ceramics

Crystal Structure and Luminescence Properties of Rare Earth Doped Ba2Bi2/3TeO6 Double Perovskites

Crystal structure, microstructure, and broadband electromagnetic response of Al3+-substituted Sr3YCo4O10+δ double perovskites