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Owing to the rapid development of wireless communication technologies, microwave dielectric ceramics have been proposed to facilitate an increase in the performance of functional ceramic components. Herein, CaTiO3-LnAlO3(Ln=La, Nd) ceramics were synthesized via reaction sintering to produce low-cost, high-efficiency, materials. The reaction-sintering method, which imparts excellent comprehensive properties, was used to fabricate these ceramics. The subtle variations in the densification behavior, phase transformation, phase composition, microstructural evolution, and performance exhibited by the CaTiO3-LnAlO3(Ln=La, Nd) ceramics were systematically evaluated. The X-ray diffraction patterns obtained from each sample indicate that the Ca0.61La0.39Al0.39Ti0.61O3 (CTLA) and Ca0.7Nd0.3Ti0.7Al0.3O3 (CTNA) phases were generated under certain environmental conditions. The as-synthesized ceramics exhibited excellent performance parameters: er = 39.1, Q × f = 45 635 GHz, τf = −1.48 ppm/°C and er = 44.03, Q × f = 46 269 GHz, τf = +2.63 ppm/°C, respectively. In addition, the ceramics retained their strong performance parameters irrespective of external temperature fluctuations. The reaction sintering method provides an economical, convenient, and facile method for the fabrication of CaTiO3-LnAlO3(Ln=La, Nd) ceramics which, in turn, imparts broad prospects for the application and development of these materials.

In the present work, a high quality (Q) Ca1.15Sm0.85Al0.85Ti0.15O4 (CSAT) ceramic was prepared via reaction sintering (RS) method. The phase structure, surface morphology, packing fraction (PF), and valence bond of the ceramic were systematically investigated. By studying the X-ray diffraction (XRD) pattern of the ceramic, it was determined to exhibit a single-phase tetragonal structure with the dimensions of a = b = 3.6943(13)A and c = 12.0320(23)A and volume of V = 164.22(10) A3. The influence of the intrinsic quality loss factor on the Q × f value was investigated by calculating the PF. Simultaneously, the bond valence of the samarium (Sm) sites was evaluated to elucidate the relationship with the temperature coefficient of resonant frequency (τf). The CSAT ceramic was sintered at 1550 °C for 6 h and exhibited exceptional characteristics in terms of the relative dielectric constant (er) = 17.5, quality factor (Q × f) = 66700 GHz, and τf = −6.93 ppm/°C. These results highlighted the excellent suitability of the RS method for preparing CSAT ceramics with outstanding microwave dielectric properties.

0.85(0.74CaTiO3–0.26SmAlO3)–0.15Ca1.15Sm0.85Al0.85Ti0.15O4 (CSC) composite ceramics were prepared by a reaction-sintering method. The phase structure, sintering behavior and microwave dielectric properties of CSC ceramics were investigated. The preparation process of reaction-sintering route (RS) is simple. XRD patterns show that Ca0.74Ti0.74Sm0.26Al0.26O3 (similar to CaTiO3), SmAlO3 and Ca1.15Sm0.85Al0.85Ti0.15O4 phases are formed in the CSC ceramics. The grain size and dielectric constant of CSC ceramics increase with rising the sintering temperature. As the sintering temperature increases, the quality factor (Q × f) and τf value of the CSC ceramics increase to the maximum values first and then decrease. The bulk density fluctuates up and down around a value, but little change. The ceramics sintered at 1500 °C got the best microwave dielectric performances with er = 35.9, Q × f = 49,221 GHz, τf = − 0.53 ppm/°C.

A novel La2MgGeO6 ceramic was synthesized through a solid-state reaction process within a sintering temperature range of 1450–1550 °C. By a combination of X-ray diffraction and Rietveld refinement analyses, the ceramics were found to have a pure hexagonal phase structure belonging to space group R3/146. The scanning electron microscopy images revealed that the ceramic grains were closely connected. The effects of internal (lattice energy, valence bond, and fraction packing) and external factors (density) on the microwave properties of ceramics were also studied. The ceramic exhibited excellent microwave dielectric performances, with a relative permittivity (Ɛr) of 21.2, a quality factor (Q × f) of 52 360 GHz, and a temperature coefficient of resonant frequency (τf) of −44.2 ppm/°C, when sintered at 1500 °C for 4 h. The τf value of the La2MgGeO6 ceramic doped with CaTiO3 could be adjusted to zero. Particularly, 0.2La2MgGeO6-0.8CaTiO3 ceramics have good microwave dielectric properties with τf = +2.1 ppm/°C, Q × f = 15 610 GHz, and Ɛr = 40.3.

Novel 0.695CaTiO3-0.305SmAlO3+xwt% CeO2 (x = 0, 0.5, 1.0, 1.5) ceramics were fabricated using a reaction-sintering (RS) approach. The crystal structure, morphology, and microwave dielectric properties of ceramics were systematically studied. The addition of CeO2 could effectively improve the sintering behavior of 0.695CaTiO3-0.305SmAlO3 (CTSA) ceramics. When x = 0.5 wt%, the ceramics exhibited optimal microwave dielectric properties, with er = 43.9, Q×f = 48 779 GHz, and τƒ = −0.24 ppm/°C, thereby indicating that the samples prepared via the RS route possess superior dielectric properties compared to those prepared by the conventional solid phase reaction. The results demonstrate that CaTiO3-SmAlO3 ceramics can be prepared simply and efficiently through a reaction-sintering process.

NaTaO3 ferroelectric ceramics sintered at 1550 °C exhibit high relative permittivity and outstanding microwave dielectric properties of er = 113.76, Q × f = 8824 GHz, and τƒ = +645 ppm/°C in the low-frequency band (3.5195 GHz). The high relative permittivity is advantageous for the miniaturization of modern wireless communication devices. A near zero temperature coefficient of the resonance frequency (τƒ = +1.05 ppm/°C) and good microwave dielectric properties (Q × f = 54,680 GHz, er = 19.42) were obtained for 0.92MgTiO3-0.08NaTaO3 ceramics. Moreover, the sintering temperature of MgTiO3 could be decreased from 1350 °C to 1200 °C. Hence, NaTaO3 is a high-potential microwave dielectric material for adjusting systems with a negative temperature coefficient of resonance frequency.

Sr1+xSm2Al2O7+x (0 ≤ x ≤ 0.05) ceramics were prepared by a conventional solid-state reaction method. Slight Sr2+ nonstoichiometry dramatically enhanced the microwave dielectric performance of the ceramics. Compared with the stoichiometric material, Sr-deficient ceramics show greatly enhanced microwave dielectric properties. For x = 0.03, the ceramics exhibited good microwave dielectric properties of er = 18.31, Q × f = 78,000 GHz and τf = 2.28 ppm/°C. ZnO and LiF sintering aids were added to the ceramic to reduce the presintering temperature and enhance the microwave dielectric properties of the ceramics. After 0.25 wt% ZnO and 0.25 wt% LiF were added, the ceramics exhibited microwave dielectric properties of er = 19.40, Q × f = 81,400 GHz and τf = 3.27 ppm/°C.

Microwave dielectric ceramics of Ln2MoO6(Ln = La and Y) were fabricated via a conventional solid-state reaction route. XRD, Rietveld refinement, SEM and vector network analysis were applied to analyse the crystal structural, microstructural and dielectric properties of the ceramics. The Y2MoO6 ceramic was crystallized in a monoclinic crystal structure, whereas the La2MoO6 ceramic could form a pure tetragonal structure. All components can be sintered at 1200–1500 °C with a relative permittivity of 14.1–17.1 and a high quality factor. The La2MoO6 ceramic exhibits satisfactory dielectric performance with Q × f = 67,090 GHz, er = 16.6 and τf = −50.1 ppm/°C, while Q × f = 27,760 GHz, er = 14.6, and τf = −37.4 ppm/°C for Y2MoO6; hence, these ceramics could be candidates for 5G technology.

Using the solid-state reaction method, novel series of 3CaO–RE2O3–2WO3 (RE = La, Nd and Sm) dielectric ceramics materials were fabricated, and the crystal structure and microstructure were studied through X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The Ca3La2W2O12 (CLW) ceramic exhibited a single pure phase with a rhombohedral structure, and Ca3Nd2W2O12 (CNW) and Ca3Sm2W2O12 (CSW) ceramics had similar crystal structures with CLW. 3CaO–RE2O3–2WO3 (RE = La, Nd and Sm) ceramics are packed tightly, and the grains exhibited clear boundaries and uniform size. The CLW, CNW, and CSW ceramics sintered at 1375 °C for 4 h possessed excellent microwave dielectric properties: Ɛr = 19.1, Q × f = 34370 GHz, τf = − 80 ppm/°C; Ɛr = 19.3, Q × f = 36610 GHz, τf = − 74 ppm/°C and Ɛr = 18.8, Q × f = 23240 GHz, and τf = − 68 ppm/°C, respectively. Moreover, the (1 − x)CNW–xTiO2 (0.3 ≤ x ≤ 0.6) and (1 − y)CNW–yCaTiO3 (0.4 ≤ y ≤ 0.7) ceramics were designed and prepared to get a thermally stable material. The addition of TiO2 and CaTiO3 had a significant impact on the performances of ceramics. With increasing the TiO2 and CaTiO3 content, the relative permittivity (Ɛr) and temperature coefficient of resonance frequency (τf) values of ceramics increased, but the quality factor (Q × f) values of ceramics decreased. Especially, 0.55CNW–0.45TiO2 and 0.45CNW–0.55CaTiO3 ceramics sintered at 1375 °C for 4 h had microwave dielectric properties of Ɛr = 22.4, Q × f = 32,490 GHz, and τf = − 0.5 ppm/°C and Ɛr = 23.4, Q × f = 6440 GHz, and τf = − 1.5 ppm/°C, respectively.

Low-permittivity SrMgSi2O6 microwave dielectric ceramics were synthesized via a conventional solid-state reaction method. Further, their sintering behavior as well as structural and microwave dielectric properties were studied. The x-ray diffraction patterns of the SrMgSi2O6 and Sr2MgSi2O7 ceramics were compared; both the samples were confirmed to be tetragonal. Meanwhile, the microwave dielectric properties of the samples were related to their microscopic morphology. In addition, the SrMgSi2O6 ceramics had a low sintering temperature of 1125°C; they exhibited good microwave dielectric performances with a relative permittivity of er = 6.7, a quality factor Q × f = 25,800 GHz, and temperature coefficient of the resonator frequency τf = −46 ppm/°C. Therefore, the ceramics exhibit potential for application in microwave devices.

Bi4B2+xO9+3x/2 (x = 1.5, 2, 2.5, 3) compounds with high relative permittivity and ultra-low sintering temperature were prepared using the conventional solid-state method. Due to the volatilisation of B2O3 during sintering, a Bi4B2O9 single phase was obtained in excess addition of boric acid for x = 2.5. The densest structure of the Bi4B2+xO9+3x/2 (x = 2.5) ceramic, sintered at 585 °C, exhibited good microwave dielectric properties of a relative permittivity (er) = 42.5, quality factor (Q × f) = 3240 GHz, and temperature coefficient of resonance frequency (τƒ) = −59 ppm/°C. As the value of x changed, the second phases of Bi2O3 and Bi3B5O12 appeared, and the (er) and (τƒ) values decreased. The Bi4B2+xO9+3x/2 (x = 2.5) ceramics did not react with either Ag or Al, indicating that they are good candidate materials for ultra-low temperature co-fired ceramic (ULTCC) applications.

Novel series of MLa2WO7(M = Sr, Ba) ceramics were prepared via a traditional solid-state reaction method. The sintering characteristic, crystal structures, micromorphology and microwave dielectric performances of systems were systematically investigated. X-ray diffraction (XRD) and Rietveld refinement display that both SrLa2WO7(SLW) and BaLa2WO7(BLW) ceramics are monoclinic structure with Fdd (No. 14) space group. SLW and BLW ceramics have high relative density (more than 94%). The SLW ceramics sintered at 1400 °C exhibited good microwave dielectric performances with Ɛr = 23.2, Q × f = 29,720 GHz, τf = − 127 ppm/°C and the BLW ceramics sintered at 1300 °C had excellent properties of Ɛr = 25.3, Q × f = 36,473 GHz, τf = − 108 ppm/°C. These results show that MLa2WO7(M = Sr, Ba) ceramics could be candidates for microwave devices.

Different content of CeO2 and Y2O3 (2 vol.% and 3 vol.%) were used as additives to restrain the Ti/Al2O3 interfacial reaction by the vacuum hot pressing sintering. Results showed that rare earth oxides effectively reduced the thicknesses of the reaction layers. Specifically, when 3 vol.% CeO2 was added, the thickness reached the minimum value of 38.4 μm at 1350 °C. Based on the analysis of XRD and SEM, the positive effect of the rare earth oxides could be ascribed to their combination with Al2O3 and pinning effect at the grain boundary. Furthermore, the superior effect of CeO2 to Y2O3 was explained by the reasonable mechanism which was focused on its smaller ionic radius, lower bond energy and stronger ability of combining with Al2O3. The diffusion mechanisms of various elements at the interfaces were also discussed in details.

LiSmTa4O12 ceramics were prepared via the conventional solid-state method, and their microwave dielectric properties were studied. The ceramics could be densified at 1425 °C–1525 °C. X-ray diffraction (XRD) analysis suggested that LiSmTa4O12 is single phase with A-site deficient perovskite structure a, whereas the impact from sintering temperature is weak. The excellent microwave dielectric performances of er = 59.60, Q × f = 7 760 GHz (at 5.065 GHz), and τf = +41.8 ppm/°C were achieved when the ceramics were sintered at 1500 °C for 4 h, which means that the ceramics are suitable for miniaturized electronic devices.