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In this article, garnet-type Y3Al5O12 (YAG) were doped with Mg2+-Si4+ ionic pairs and its effect on the structure and the microwave dielectric properties was investigated. It was revealed that in ceramics Y3MgxAl5-2xSixO12 (x = 0, 0.5, 1.0, 1.5, 2.0), increasing doping concentration decreased the sintering temperature and time for the optimal densification. X-ray diffraction studies showed that the samples are isostructural to Al5Y3O12 garnet. All sample were formed a single phase except for x = 2.0, where a secondary peak was detected. Excellent microwave dielectric properties of εr = 10.56, Q × f = 67,571 GHz and τf = -35.23 ppm/°C were obtained for Y3Mg0.5Al4Si0.5O12 ceramics sintered at 1625 °C for 5 h. The factors influencing the microwave dielectric properties of Y3MgxAl5-2xSixO12 ceramics were further discussed by considering the ionic polarization, packing fraction, average grain size and chemical bond valence theory. [ABSTRACT FROM AUTHOR]

In this study, MgxY3-xAl5-xSixO12 (x = 0–0.6) ceramics were prepared via a high-temperature solid state method. Mg2+ and Si4+ replacing Y3+ ions at A-site and Al3+ at C-site in garnet-type Y3Al5O12 (YAG) ceramics were designed to discuss the effects of crystal structure, microstructure on microwave dielectric properties for the first time. Liquid phase sintering induced by doping of Mg2+-Si4+, significantly decreased the sintering temperature and time for the densification of YAG ceramics. X-ray diffraction of the samples showed single phase for x ≤ 0.5, while a secondary phase of MgAl2O4 formed for x = 0.6. From the Rietveld analysis, a decrease in lattice dimensions, bond length and valence and rattling effect were observed, while Raman results showed a change in FWHM of the Eg mode. Based on these parameters, a structure–property relation was developed for MgxY3-xAl5-xSixO12 ceramics. Specially, Mg0.1Y2.9Al4.9Si0.1O12 showed a good set of microwave dielectric properties sintered at 1575 °C for 4 h with εr = 10.67, Q × f = 58,799 GHz, τf = -29.5 ppm/°C. [ABSTRACT FROM AUTHOR]

Novel low‐εr, thermal and phase stable of (1‐x)Mg3B2O6‐xBa3(VO4)2 (x mol% =51, 53, 55, 57, 59) microwave composite ceramics were firstly fabricated and reported using the conventional solid‐state reaction method. X‐ray diffraction, scanning electron microscopy, energy dispersive X‐ray, and Raman spectroscopy confirmed the coexistence of both phases without other phases. Near‐zero temperature coefficient of resonant frequency (τf ~ +1.2 ppm/°C) is obtained for the 0.43Mg3B2O6‐0.57Ba3(VO4)2 composite ceramic, with permittivity (εr) of 8.8 and high‐quality factor (Q×f) of 45 420 GHz, which is a promising candidate for 5G applications. [ABSTRACT FROM AUTHOR]

Cold sintered, Li2MoO4‐based ceramics have recently been touted as candidates for electronic packaging and low temperature co‐fired ceramic (LTCC) technology but MoO3 is an expensive and endangered raw material, not suited for large scale commercialization. Here, we present cold sintered temperature‐stable composites based on LiMgPO4 (LMP) in which the Mo (and Li) concentration has been reduced, thereby significantly decreasing raw material costs. Optimum compositions, 0.5LMP‐0.1CaTiO3‐0.4K2MoO4 (LMP‐CTO‐KMO), achieved 97% density at <300°C and 600 MPa for 60 minutes. Raman spectroscopy, X‐ray diffraction, scanning electron microscopy, and energy dispersive X‐ray mapping confirmed the coexistence of end‐members, LMP, CTO, and KMO, with no interdiffusion and parasitic phases. Composites exhibited temperature coefficient of resonant frequency ~ –6 ppm/°C, relative permittivity ~9.1, and Q × f values ~8500 GHz, properties suitable for LTCC technology and competitive with commercial incumbents. [ABSTRACT FROM AUTHOR]

In this work, (1-x)Li2TiO3-xLi4NbO4F ceramics were prepared by the conventional solid-state ceramic route. With the increase of Li4NbO4F content, the phase structure transformed from ordered monoclinic to disordered cubic. By increasing Li3NbO4F content, the temperature coefficient of resonant frequency (τf) was successfully adjusted closer to zero, while the dielectric constant (εr) and microwave quality factor (Qf) decreased to some degree. Outstanding microwave dielectric properties with a εr = 18.7, Qf = 61,388 GHz (6.264 GHz), and τf = 0.9 ppm/°C were obtained for 0.9Li2TiO3-0.1Li4NbO4F ceramics sintered at 1050 °C for 2 h, which indicated that these ceramics are suitable for practical applications in the field of microwave substrates and components. [ABSTRACT FROM AUTHOR]

In this work, Zn-Ni co-modified LiMg 0.9 Zn 0.1- x Ni x PO 4 (x = 0–0.1) microwave dielectric ceramics were fabricated using a solid state synthesis route. Rietveld refinement of the XRD data revealed that all ceramic samples have formed a single phase with olivine structure. SEM images showed that the samples have a dense microstructure, that agrees with the measured relative density of 97.73 %. Based on the complex chemical bond theory, Raman and infrared reflectance spectra, we postulate that ε r is mainly affected by the ionic polarizability, lattice and bond energy, while P-O bond plays a decisive role in Q×f and τ f value. Optimum properties of Q×f ~ 153,500 GHz, ε r ~ 7.13 and τ f ~ −59 ppm/°C were achieved for the composition LiMg 0.9 Zn 0.06 Ni 0.04 PO 4 sintered at 875 ℃ for 2 h. This set of properties makes these ceramics an excellent candidate for LTCC, wave-guide filters and antennas for 5 G/6 G communication applications. [ABSTRACT FROM AUTHOR]

5G and forthcoming 6G communication systems require dielectric ceramics with low relative permittivity (ε r) and near-zero temperature coefficient of resonant frequency (τ f) for the lower part of the microwave (MW) band and at sub-Terahertz. Mg 2 Al 4 Si 5 O 18 (MAS) ceramics are promising candidates due to their low ε r (~ 6) and high-quality factor (Q× f > 40,000 GHz) but have a large τ f. In this study, 5.5 wt% TiO 2 (MAS-T 5.5) was used to adjust τ f of MAS to −2.8 ppm/℃ whilst retaining low ε r (5.24) and good Q× f (33,400 GHz), properties consistent with those obtained by infrared reflectance. A demonstrator microstrip patch antenna with gain 4.92 dBi and 76.3% efficiency was fabricated from MAS-T 5.5. [ABSTRACT FROM AUTHOR]

In this work, the effects of divalent ions on the phase structure, microstructure, and microwave dielectric properties of Mg 1.8 R 0.2 Al 4 Si 5 O 18 (R = Mg, Ca, Sr, Ba, Mn, Co, Ni, Cu, Zn) cordierite ceramics were investigated. Complex chemical bond theory, Raman spectrum and infrared reflectance spectrum were employed to understand the relationship among structural characteristics, vibration modes and microwave dielectric properties for alkaline earth and transitional metal divalent elements doped cordierite ceramics. The optimal microwave dielectric properties were obtained for Mg 1.8 Ni 0.2 Al 4 Si 5 O 18 with ε r of 4.53, Q× f of 61,880 GHz and τ f of -32 ppm/℃. Finally, a 5G-Sub 6GHz patch antennas with a central frequency of 4.91 GHz was successfully designed and fabricated using a Mg 1.8 Ni 0.2 Al 4 Si 5 O 18 substrate. The antenna exhibited excellent performance with a gain of 5.83 dBi and an efficiency of 76 %, showing promise to be employed in 5 G/6G millimeter-wave communication technology. [ABSTRACT FROM AUTHOR]

• Graphene oxide/ZnO ceramic composites with highly preserved pristine structure of filler are prepared via cold sintering technology. • Moderate complex permittivity and good impedance matching lead to highly improved microwave absorbing performance in cold sintered composites compared to the spark plasma sintered ones. • Both flexural strength and hardness of the graphene oxide reinforced composites are enhanced remarkably compared to the cold sintered pure ZnO. The abruptly increased dielectric loss in graphene oxide (GO) induced by high-temperature sintering has prevented GO from being used as an absorbent in ceramic-matrix microwave absorbing composites (MACs). Here, we show the first example that the pristine structure of GO can be largely preserved in ceramic composite by using cold sintering technology. Compared with the composites densified by ordinary spark plasma sintering, cold sintered samples possess lower complex permittivity and superior impedance matching. Therefore, the cold sintered ZnO composite with 0.50 wt% GO loading shows the minimum reflection loss of -47.5 dB at thickness of 3.2 mm, and the effective absorption bandwidth of 4.4 GHz at thickness of 1.7 mm. Moreover, both flexural strength and hardness of the cold sintered composite are improved considerably compared to the monolithic cold sintered ZnO. These findings imply that cold sintering enables the thermally unstable materials to be applied in highly effective MACs with robust mechanical performance. [ABSTRACT FROM AUTHOR]