Correlations between the crystal structure and microwave dielectric properties in Ce2[Zr1-xMx]3(MoO4)9 (M = Mn1/3Nb2/3, Mn1/3Ta2/3) solid-solution ceramics

Ce₂[Zr_1-xM_x]₃(MoO₄)₉ (M = Mn_1/3Nb_2/3, Mn_1/3Ta_2/3; x = 0.02, 0.04, 0.06, 0.08 and 0.10) (abbreviated as CZ_1-xN_x and CZ_1-xT_x) ceramics were prepared through the solid-state reaction method. Effects of (Mn_1/3Nb_2/3)⁴⁺ and (Mn_1/3Ta_2/3)⁴⁺ ions on the sintering characteristics, crystal structures, microwave dielectric properties and infrared vibrational modes were studied in detail. X-ray diffraction (XRD) results reveal the formation of solid solutions for all components. Based on the chemical bond theory and Rietveld refinement, intrinsic structure parameters including the polarizability (P), the packing fraction (P.F.) and the octahedral distortion (Δ_octa.), and bond parameters including the lattice energy (U), bond energy (E) and thermal expansion coefficient (α) were calculated. Interestingly, the Ce–O bond plays a major role in the bond ionicity (f_i), while Mo–O bond dominates the contributions in the lattice energy (U), bond energy (E) and thermal expansion coefficient (α). In addition, these parameters are used to explain the variations of the microwave dielectric properties of ceramics either changing the doping contents or replacing different ions at x = 0.06. Furthermore, far infrared (FIR) spectra uncover that the phonon modes provide the major polarization contribution of 68.59% in the CZ_0.9T_0.1 ceramic, implying that the main contribution to ε_r stems from the ionic polarization instead of the electronic polarization. Typically, the optimum microwave dielectric properties are achieved for the CZ_0.9N_0.1 and CZ_0.9T_0.1 ceramics with ε_r = 10.76, Q × f = 85,893 GHz (at 9.52 GHz), τ_f = −14.83 ppm °C⁻¹ and ε_r = 10.72, Q × f = 87,355 GHz (at 9.81 GHz) and τ_f = −8.68 ppm °C⁻¹, respectively. Notably, the CZ_0.9T_0.1 ceramic has a markedly increased Q × f while maintaining a good τ_f = −8.68 ppm °C⁻¹ and a low sintering temperature of 700 °C.     

Temperature stable 0.35Ag2MoO4-0.65Ag0.5Bi0.5MoO4 microwave dielectric ceramics with ultra-low sintering temperatures

In this study, the novel temperature-stable (1-x)Ag₂MoO₄-xAg_0.5Bi_0.5MoO₄ microwave dielectric ceramics were prepared by a modified solid-state reaction method. The phase composition, microstructures and microwave dielectric properties of the (1-x)Ag₂MoO₄-xAg_0.5Bi_0.5MoO₄ ceramics were investigated. All the compounds can be sintered well at ultra-low temperatures (<540 °C). The XRD and SEM analysis indicate that the Ag₂MoO₄ and the Ag_0.5Bi_0.5MoO₄ can coexist with each other. When x = 0.65, the ceramics exhibit the best microwave dielectric properties with a relative permittivity of 23.9, a Q × f value of 16,200 GHz (at 7.3 GHz) and a near-zero TCF value of -2.4 ppm/°C at 520 °C. The results indicate that temperature-stable (1-x)Ag₂MoO₄-xAg_0.5Bi_0.5MoO₄ ceramics are promising candidates for low temperature co-fired ceramics (LTCC) applications.     

Novel ultra-low loss and low-fired Li8MgxTi3O9+xF2 microwave dielectric ceramics for resonator antenna applications

Low temperature sintered Li₈Mg_xTi₃O_9+xF₂ microwave dielectric ceramics with x = 2−7 were developed based on a newly designed pseudo ternary phase diagram of the Li₂TiO₃–MgO–LiF system. Dense solid solution ceramics (of relative density >96 %) with cubic rock-salt structure, accompanied by a small amount of secondary phase MgO, were obtained in the temperature range of 800−925 °C. With increasing Mg²⁺ content, the value of ε_r decreased, whereas that of τ_f remained nearly constant, and the Q × f increased to a maximum at x = 5. The Li₈Mg₅Ti₃O₁₄F₂ ceramic sintered at 875 °C exhibited superior microwave dielectric properties with ε_r = 16.8, Q × f = 119,700 GHz, and τ_f = −41.6 ppm/°C. The good compatibility with Ag electrodes highlights the promising prospects of this ceramic in low-temperature co-fired ceramic technology. Furthermore, a dielectric resonator antenna fabricated based on a Li₈Mg₅Ti₃O₁₄F₂ ceramic exhibited an outstanding S₁₁ of −34.7 dB and a broad bandwidth of 360 MHz at the desired resonant frequency of 5.98 GHz.     

Investigation of chemical bonds in the ordered Ba3Zn(Nb2-xMox)O9+x/2 ceramics and its effects on the microwave performance

1:2 ordered Ba₃Zn(Nb_2-xMo_x)O_9+x/2(BZNM) ceramics with space group $\text{P}\overline{3}\text{ml}$ were prepared by solid-state method. The nature of chemical bonds in Ba₃Zn(Nb_2-xMo_x)O_9+x/2(BZNM) ceramics was investigated for the first time. Firstly, the bond energy was closely related to the lattice vibration. High bond energy would lead to weak non-harmonic interactions, which were the main factors of improving Qf. Secondly, polarization of the chemical bond was the principal contributor to the relative permittivity of microwave ceramics. Compared with the traditional method, the calculated result based on the P-V theory reduced the error from 81% to 4.4%. Through the discussion, it was confirmed that the analysis method based on chemical bond was highly feasible and scientific in the microwave ceramics. At last, the system of Ba₃Zn(Nb_1.992Mo_0.008)O_9.004 sintered at 1435?°C for 6?h and annealed at 1300?°C for 10?h had excellent microwave dielectric properties: ε_r?=?38.9, Qf?=?102,931?GHz, τ_f?=?19.2?ppm/°C, which, to our best knowledge, provided a alternative for the application of millimeter-wave communications.     

(Cu1/3Nb2/3)4+ ionic substituting effects,low-temperature sintering behaviors,and improved temperature stability of Ba3Nb4Ti4O21 microwave dielectric ceramics

In this study, (Cu_1/3Nb_2/3)⁴⁺ complex cation and BaO–ZnO–B₂O₃ glass frit were adopted to solve the high sintering temperature and poor temperature stability of Ba₃Nb₄Ti₄O₂₁ ceramics. It is shown that pure Ba₃Nb₄Ti₄O₂₁ phase was formed when Ti site was partially replaced by (Cu_1/3Nb_2/3)⁴⁺ cation. The increasing number of dopants decreases the dielectric polarizability, correspondingly, the dielectric constant and temperature coefficient of the resonance frequency values are reduced consistently. The variation of the Q × f value is determined by internal ionic packing fraction and external sintering densification. The (Cu_1/3Nb_2/3)⁴⁺ cation effectively decreases the suitable sintering temperature from 1200 to 1050 °C while greatly improving the temperature stability. BaO–ZnO–B₂O₃ glass was used to further improve the low-temperature sintering characteristics of Ba₃Nb₄Ti₄O₂₁ ceramics. It is proven that the addition of glass frits effectively decreases the temperature to 925 °C with combinational excellent microwave dielectric properties: ε_r ～55.6, Q × f ～5700 GHz, τ_f ～3 ppm/^°C, making the Ba₃Nb₄Ti₄O₂₁ ceramics promising in the applications of low-temperature cofired ceramic technology.     

Fabrication of transparent Ce3+-doped (Gd,Lu)3Al5O12 ceramics by two-step spark plasma sintering

Transparent Ce³⁺:(Gd,Lu)₃Al₅O₁₂ with microstructure control was fabricated by two-step spark plasma sintering. In the two-step profile, the heating rate was changed from 50 to 5°C/min at the first step temperatures. During the initial stage of shrinkage, the holding time of the first step sintering could induce densification by suppressing the microstructure coarsening. As compared to the single-step profile, the two-step profile showed a smaller grain size, which decreased with a decrease in the first step temperature. The porosity of the two-step profile was lower than that of the single-step profile, and the lowest porosity was obtained at the first step temperature of 1000°C, which was the starting point of shrinkage. The TS-1000 specimen showed the highest transmittance among all specimens because of the microstructure control offered by the two-step profile. Thus, by employing the two-step profile, the transmittance could be increased from 50.1% (SS-1250) to 56.5% (TS-1000).     

Li+ enrichment to improve the microwave dielectric properties of Li2ZnTi3O8 ceramics and the relationship between structure and properties

Herein, Li⁺-enriched Li_(1+x)2ZnTi₃O₈ ceramics are prepared via the solid-phase methods. As x increases, the unit cell volume gradually increases, while the grain size initially increases and then decreases gradually. The Li_(1+0.06)2ZnTi₃O₈ ceramics exhibit the best dielectric properties: ε_r = 25.92, Q × f = 109534 GHz (@7.37 GHz, which is a 48 % increase compared with the stoichiometric counterpart.), and τ_f = ?8.21 ppm/°C. The complex chemical bond theory and Raman spectroscopy reveal that Ti-O bonds have a significant effect on the dielectric properties. An optimal Li⁺ enrichment leads to an overall reduction in the distortion of the Li/ZnO₄ tetrahedra, resulting in a reduction in τ_f. First-principles calculations demonstrate that a suitable excess of Li⁺ leads to an increase in the band-gap as well as an enhanced electron cloud density in the internal space of the Li1/ZnO₄ tetrahedra, thereby increasing the Q × f. In summary, Li⁺-enriched Li_(1+0.06)2ZnTi₃O₈ ceramics are promising for a wide array of applications in microwave communications.