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Novel LiCa(PO 3) 3 and LiSr(PO 3) 3 phases were produced by conventional solid-state synthesis and formed into ceramic pellets followed by conventional sintering. The crystal structure, ceramic microstructure, microwave dielectric properties and ac conductivity were investigated. A triclinic space group P1- was obtained from Rietveld refinements, and the lattice vibration modes were determined by Raman spectroscopy. The optimum sintering conditions for LiCa(PO 3) 3 and LiSr(PO 3) 3 ceramics were 700 ℃ and 625 ℃, respectively, both for 10 h. Dense glass-free ceramic microstructures were confirmed by SEM. High relative densities of 94.6 % and 93.8 %, low microwave ε r values of 5.85 and 5.62 at 14 GHz, and high quality factors Q×f of 25,200 and 23,450 GHz at 14 GHz were detected. Large negative τ f of − 105 and − 106 ppm/°C were obtained, which are uncommon but highly valuable for offsetting positive τ f values of other systems. Moderate Li+-ion conduction was detected in both compounds by ac impedance spectroscopy. [ABSTRACT FROM AUTHOR]

Mg2SiO4 and MgSiO3 ceramics with superior microwave dielectric properties are considered to be promising candidates for 5G applications. However, a slight deviation from the stoichiometric Mg/Si ratio will significantly influence their microwave dielectric properties, which will hinder their practical applications. In this work, the xMgO-SiO2 (x = 1~2) ceramics were synthesized by a solid-state reaction method. The influence of the Mg/Si ratio x on the crystalline phase, microstructure, and microwave dielectric properties was investigated through X-ray diffraction (XRD), a scanning electron microscope (SEM), and the resonant cavity method. The XRD patterns revealed the coexistence of Mg2SiO4 and MgSiO3 within the x range of 1~2, which was further demonstrated by the energy-dispersive X-ray spectra. The SEM images show a typical polycrystalline morphology of ceramics with an inhomogeneous grain size distribution. It is found that the microwave dielectric properties fluctuate at both sides of the x range while those remain relatively stable with minor changes at the intermediate components, indicating an obvious low composition dependence helpful for practical applications. Further, a demonstrator of a microstrip patch antenna for 5G applications using the 1.5MgO-SiO2 ceramic was designed and fabricated, and a return loss of −16.2 dB was demonstrated, which demonstrated the potential applications. [ABSTRACT FROM AUTHOR]

In this investigation, the effects of Mg2+ substitution on structure, sintering temperature, microstructure, and microwave dielectric performance of CaV2O6 ceramics were studied. The (Ca0.95Mg0.05)V2O6 compounds were initially synthesized by the solid-state reaction route then sintered at 600–675°C for 4 h. All samples demonstrated a pure CaV2O6 phase without any additional phases. We found that the sintering temperature of CaV2O6 ceramic is decreased by substitution of Mg2+ ions for Ca2+, which leads to density improvement. We finally found and obtained excellent microwave dielectric properties, with εr = 10.25, Q × f ≈ 50,500 GHz (at 15.5 GHz), and τf = −29 ppm/°C, for (Ca0.95Mg0.05)V2O6 when sintered at 650°C for 4 h [ABSTRACT FROM AUTHOR]

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]

Microwave dielectric ceramics with low dielectric permittivity ε r combined with low ceramic sintering temperatures have raised considerable research interest recently due to their potential application in LTCC technology for 5G wireless communications. Here in this study, ceramic processing, crystal structure, and microwave dielectric properties of LiM(PO 3) 3 (M = Mg2+, Zn2+) ceramics were comprehensively investigated. Optimized LTCC compatible ceramic sintering processes were developed at 725 °C and 625 °C for 10 h, respectively, leading to high relative densities of 96.27 % and 94.09 %. Low microwave ε r values of 5.74 and 6.30 and Q × f values of 16,660 GHz and 9629 GHz (at 14 GHz) were obtained, where such values were found to be affected by the grain microstructure. LiMg(PO 3) 3 exhibited a small τ f of −11 ppm/°C, while LiZn(PO 3) 3 showed −58 ppm/°C. Complementary impedance spectroscopy measurements indicated modest Li+-ion conductivity with a higher value of 2.92·10−7 (Ωcm)−1 at 560 K (287 °C) in LiZn(PO 3) 3. Both ceramics were found to be dielectrically homogeneous without any perceptible contributions from the grain boundaries. [ABSTRACT FROM AUTHOR]

In this study, the (Ca0.95M0.05)V2O6 (M = Zn, Ba) and the CaV2O6 ceramics were synthesized through a solid‐state reaction method, and the effects of Zn2+ and Ba2+ substitution on the structure, sintering temperature, densification, microstructure and microwave dielectric properties of CaV2O6 ceramic were analysed. The XRD patterns of the sintered samples indicated a single‐phase of CaV2O6 in all temperatures. Substitution of Zn2+ caused a lower sintering temperature and improved the densification of the CaV2O6 ceramic. While the dielectric properties of the (Ca0.95Ba0.05)V2O6 compound were not desirable, the (Ca0.95Zn0.05)V2O6 sample sintered at 650°C for 4 hours showed significant dielectric properties, with εr = 10.29, Q × f ~  53 000 GHz (at 15.5 GHz) and τf = −72.37 ppm/°C. Moreover, the chemical compatibility of the CaV2O6 ceramic with Al electrode was examined. [ABSTRACT FROM AUTHOR]

This paper studies the microwave dielectric properties, microstructure, vibration and densification of Li2ZnTi3+xO8+2x (-0.04≤x≤+0.06) ceramics, manufactured via a conventional mixed oxide route. The X-ray diffraction and Raman spectroscopy revealed the unit cell parameter and cation ordering in LZT non-stoichiometry in their vibrational modes. The densification and phase composition were characterized by the EDX and SEM methods. It was found that a slight Ti vacancy can improve the relative density to the maximum value (96.2%). The XRD results showed that the second phase of TiO2 in the Li2ZnTi3.06O8.12 composition is formed. The sintered samples were detected in the microwave frequency range by using the resonance technique. The τf values of the ceramics within Ti excess adjusted to near zero. The Li2ZnTi2.96O7.92 ceramic showed the best relative density, single phase and best microwave dielectric εr=25.98, Q × f = 61,000 GHz, τf=-17.4ppm/∘C sintered at 1100 °C for 4 h. [ABSTRACT FROM AUTHOR]

In the present work, the Li2ZnTi3O8 ceramics were prepared via the solid-state reaction method, afterward annealed at 800 °C in a time variation from 4 to 20 h. The ordering, microstructures and dielectric properties were investigated using x-ray diffraction, scanning electron microscopy, network analyzer, and Raman spectroscopy. The most significant enhancement of quality factor is obtained in the sample annealed for 20 h, while the dielectric constant and temperature coefficient of resonant frequency change slightly. This result mainly attributes to the enhancement of ordering, which could be related to the increase in the Zn–O bond strength in ZnO4 tetrahedra. Meanwhile, the full-width at half-maximum of A1g mode decreased with higher annealing time, which suggested less variation in the Zn–O bond length and a higher degree of ordering. The best combination of microwave dielectric characteristic is obtained in the sample annealed at 800 °C for 20 h: Q × f = 112,400 GHz, εr = 24.500, and τf = −11 ppm/°C. [ABSTRACT FROM PUBLISHER]

In this study, high-entropy Mg 1.8 R 0.2 Al 4 Si 5 O 18 (R Ni, Ni 1/2 Co 1/2 , Ni 1/3 Co 1/3 Zn 1/3 , Ni 1/4 Co 1/4 Zn 1/4 Cu 1/4 , Ni 1/5 Co 1/5 Zn 1/5 Cu 1/5 Mn 1/5) cordierite ceramics based on equimolar ratios were designed and synthesized through the solid-state reaction technique at high temperatures. The effect of various R-ionic substitutions at the octahedral site and their relationship with ionic polarization, chemical bonding, and configurational entropy were explored. These factors have greatly affected the microwave dielectric properties of Mg 1.8 R 0.2 Al 4 Si 5 O 18 ceramics. The presence of cation radius mismatch and electronegativity mismatch at [MgO 6 ]-octahedron lattice sites revealed lattice distortion resulting from multiple cation substitutions. As the entropy values increased, the temperature stability of microwave dielectric properties of Mg 1.8 R 0.2 Al 4 Si 5 O 18 ceramics improved. For R Ni 1/2 Co 1/2 , a higher Q×f of approximately 73,658 GHz @ 13 GHz, with ɛ r ∼4.44 and τ f ∼ −23.9 ppm/°C was obtained. In the case of R Ni 1/5 Co 1/5 Zn 1/5 Cu 1/5 Mn 1/5 , a near zero τ f (−10.7 ppm/°C), ɛ r = 4.68 and Q×f = 35,478 GHz @13 GHz were achieved, making it potentially suitable for practical applications. [ABSTRACT FROM AUTHOR]

In this paper, different divalent ions (X = Ni, Cu, Mg, Co, Zn) are doped to replace Mg2+ at the tetrahedral sites in mixed spinel Mg 0.95 X 0.05 Al 2 O 4. These substitutions induce variations in the degree of inversion/preferential occupation at the A/B lattice site in MgAl 2 O 4 ceramics because the octahedral stabilization energies change as a function of dopant. According to crystal field theory, the number of d -orbital electrons has a significant impact on the stability of octahedral sites. Therefore, OSPE values vary between 0 Dq and 0.845 Dq for different elements. The higher the OSPE value, the greater the degree of inversion, and the higher the quality factor (Q × f). The dielectric constant (ε r) and temperature coefficient of resonant frequency (τ f) of Mg 0.95 X 0.05 Al 2 O 4 ceramics changes as a function of the ionic polarizability and B -site bond valence. Mg 0.95 Ni 0.05 Al 2 O 4 with a higher OSPE (8.45 Dq) and degree of inversion of A/B sites (λ = 0.221), has the better set of microwave dielectric properties (ε r = 8.39, Q × f = 78,338 GHz, τ f = −66.8 ppm/°C). The adjustment of stability energy of octahedral sites is favorable to improve the microwave dielectric properties of MgAl 2 O 4 spinel. [ABSTRACT FROM AUTHOR]

Non-stoichiometric Ba (Co 1 / 3 Nb 2 / 3) O 3 (BCN) ceramics were synthesized using the solid-state method at 1150 ℃. The heat treatment process was then followed by sintering at 1400–1550 ℃ and annealing at 1300 °C. Systematically varying the extent of the non-stoichiometry and the heat treatment conditions resulted in the division of the BaO − CoO − Nb 2 O 5 system into two categories of compounds: (A) high-order Co / Ba -deficient and Co / Nb -excess ceramics and (B) completely disordered Nb -deficient and Ba -excess ceramics. It was found that a slight deviation from stoichiometry toward the group (A) and subsequent annealing led to a significant improvement in the ordering from 82% to 100% accompanied with the formation of non-equidistant Nb − O bonds. The crystal structure based on the ultra-high order BCN − 0.02 Ba 5 Nb 4 O 15 was also designed to display non-equidistant Nb − O bonds. The dielectric characteristics demonstrated that high Q correlates with the 1:2 ordering and the highest Q × f values obtained from BCN − x Ba 5 Nb 4 O 15 ceramics (x = 0.02, 0.04, 0.06): 83, 84.95, and 79.71 THz, respectively. [ABSTRACT FROM AUTHOR]

In garnet phase ceramics Y 2.9 M 0.1 Al 4.9 Si 0.1 O 12 (M = Mg2+, Co2+, Zn2+, Mn2+, Ca2+, Sr2+, Ba2+), rattling effects occur when M = Mg2+, Co2+, Zn2+, Mn2+ ions with smaller ionic radius (<1Å) occupy the dodecahedron lattice lattices. This results in an abnormally high dielectric constant, in contrast to the larger ionic radius (>1 Å) of M = Ca2+, Sr2+ and Ba2+, which deviate from the theoretical values calculated using the Clausius-Mosotti formula according to each ion's polarizability. The optimal microwave dielectric properties for Y 2.9 Mg 0.1 Al 4.9 Si 0.1 O 12 was obtained by sintering at 1575 °C for 5 h (Q × f = 51,382 GHz, ε r = 9.74, and τ f = −30.12 ppm/°C). Additionally, a thermal stable Y 2.9 Mg 0.1 Al 4.9 Si 0.1 O 12 -6 wt%TiO 2 composite ceramic was developed. Based on this composite, a Wifi6E/7 patch antenna operating at 6.3 GHz and a millimeter-wave resonant antenna functioning at 27.1 GHz were designed and validated. This indicates the potential application prospects of garnet ceramics in 5G/6G radio communication technologies. [ABSTRACT FROM AUTHOR]

This study first reports the synthesis of Li 2 ZnTi 3 O 8 (LZTO) system by microwave (MW) and conventional (CS) sintering techniques. The effects of both synthesis and sintering technique on the structure, degree of ordering and microwave dielectric properties of LZTO ceramics were investigated. Based on XRD data analysis, a single phase Li 2 ZnTi 3 O 8 is obtained for both synthesis techniques. However, fully ordered LZTO system can only be obtained by the CS method. All the samples display nearly non-homogenous grain growth with an average grain size of 7 μm (MW sintering) and 38 μm (CS sintering), respectively. The calculated activation energy of the grain growth for LZTO ceramics was reduced from 329 kJ/mol (MW process) to 233 kJ/mol (CS process). The LZTO ceramics finally densified at 1075 °C display dielectric constants of ε r C S = 25.26 and ε r M W = 25.65, temperature coefficient of resonant frequencies of τ f C S = -13.6 ppm/°C and τ f M W = -12.0 ppm/°C and Q×f values of 76,000 GHz and 65,000 GHz for CS and MW samples, respectively. It is clear, therefore, that the synthesis and thermodynamics (sintering) can both play an important role in shaping the physical properties of LZTO materials. [ABSTRACT FROM AUTHOR]

Y 3- x R x Al (Oct)2 Al (Tet)3- x Si x O 12 (x = 0.1–0.5; R = Mg, Ca) ceramics were prepared via solid-state reaction method. The effects of Mg2+ (∼0.89 Å) and Ca2+ (∼1.12 Å) substitution at the dodecahedral site for Y3+ (∼1.019 Å) on the microstructure, crystal structure and microwave dielectric properties of Y 3- x R x Al (Oct)2 Al (Tet)3- x Si x O 12 garnet ceramics were investigated and discussed using the P–V-L theory, Raman spectrum, bond valence theory, Terahertz time-domain spectroscopy and infrared reflectance spectrum. Different from normal garnet structure of ceramics, the dielectric constant is abnormally increased in Y 3- x Mg x Al (Oct)2 Al (Tet)3- x Si x O 12 (x = 0.1–0.5) ceramics due to the increase of ion polarizability caused by the rattling effect. There is a dependence of bond energy, bond valence and dodecahedral distortion on τ f value, which provides new possibility for tuning the temperature coefficient of garnet ceramics. There are strong effects of grain size distribution, average lattice energy, packing fraction, FWHM of Raman peak on their Q×f values of Y 3- x R x Al (Oct)2 Al (Tet)3- x Si x O 12 isostructural ceramics caused by the difference of Mg2+ and Ca2+ ionic radius. Above work provides deeper insights into the understanding of the microstructural characteristics and performance relationship of garnet-type microwave ceramics, and lays a foundation for future applications in the 5G/6G frequency bands. [ABSTRACT FROM AUTHOR]

Garnet-typed ceramics of Y 3 Mg 1- x Mn x Al 3 SiO 12 (0 ≤ x ≤ 0.2) have been synthesized using the traditional solid-state reaction method. The optimal microwave dielectric properties (ε r = 10.73, Q׃ = 62,824 GHz, τ f = −34.8 ppm/°C) are obtained for Y 3 Mg 0.9 Mn 0.1 Al 3 SiO 12 (x = 0.1) sintered at 1575 °C for 5h. A millimeter-wave dielectric resonator antenna is designed and fabricated using Y 3 Mg 0.9 Mn 0.1 Al 3 SiO 12 as a dielectric unit due to its excellent characteristics of low dielectric constant and high Q׃. The designed DRA resonates at 24.94 GHz with a bandwidth ∼2.20 GHz (S 11 < −10 dB). The simulated gain and efficiency are 6.64 dBi and 91.08%, respectively. The results indicate that the Y 3 Mg 0.9 Mn 0.1 Al 3 SiO 12 ceramic has a potential application as an antenna for the 5G/6G millimeter wave frequency band. [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]