Structure,microwave dielectric performance,and infrared reflectivity spectrum of olivine-type Mg2Ge0.98O4 ceramic

An ultra-low dielectric loss ceramics Mg₂Ge_0.98O₄ with olivine structure was fabricated by conventional solid-state route. The phase composition, crystal structure, and microwave dielectric properties were investigated. The phase of Mg₂Ge_0.98O₄ is formed to the orthorhombic forsterite structure with a space group Pmnb (62). The dense microstructure and excellent microwave dielectric properties of Mg₂Ge_0.98O₄ ceramic were obtained at 1360°C for 4 hours, with relative density ~96.4%, ε_r ~ 7.3, Q × f = 112 400 GHz, and τ_f ~ −64.6 ppm/°C. The conductive mechanism of Mg₂Ge_0.98O₄ in the low frequency (<1 MHz) was studied by the dielectric spectroscopy and the result with E_dc = 0.93 eV demonstrates that the defect was contributed to the double ionized oxygen vacancies. The intrinsic dielectric properties of Mg₂Ge_0.98O₄ in the microwave region were obtained by infrared reflectivity spectra with ε_r ~ 7.13, Q × f = 120 400 GHz. And, acceptable τ_f (~+2.6 ppm/°C) of 0.92Mg₂Ge_0.98O₄–0.08CaTiO₃ composite ceramic was obtained by adding the CaTiO3.     

New insights on the structural and optical-vibration properties of noncentrosymmetric lanthanides pyrogermanates

A complete series of noncentrosymmetric Ln₂Ge₂O₇ materials (Ln = lanthanides) were obtained by the molten-salts processing route. Triclinic (P1 space group) as well as tetragonal (P4₁2₁2 space group) structures were observed, respectively, for the La–Eu compounds (larger ions) and for the Gd–Lu ones (smaller ions). The optical-vibration properties of both groups of samples were well investigated by Raman spectroscopy besides group-theory calculations. All the predicted phonon modes were discerned for the triclinic polar phase, while the most significant phonon modes of the tetragonal phase were determined. Beside the well-known classical dependence of the phonon wavenumbers with the ionic radii of the lanthanide (hardening of the higher-frequency modes from La to Lu), it was observed an opposite softening of the low-frequency modes within the triclinic phase (La–Eu samples). This result could be related to the contribution of the increasing lanthanide masses, added to the high lattice anharmonicity near the phase boundary between the triclinic and tetragonal phases, which can be a consequence of the strong changes in the coordination numbers of the Ln-polyhedra. Once the polar triclinic phase and the noncentrosymmetric tetragonal structure are thermally stable, and owing to the low-temperature magnetic features already shown by of non-cubic Ln₂Ge₂O₇ compounds, we believe that some of these materials could present magnetoelectric or related properties.     

Crystal structure,chemical bonding and infrared reflectivity of microwave dielectric SrIn2O4 ceramics

In this work, the orthorhombic structured SrIn₂O₄ ceramics with a space group Pnam were synthesized by a solid-state reaction method. A high relative density (96.5%) coupled with excellent microwave dielectric properties (ε_r ∼ 12.3, Q × f = 96,900 GHz, τ_f ∼ −61.6 ppm/°C) were obtained as sintered 1300 °C for 4 h. The bond valence analysis demonstrates that the large sized cation Sr²⁺ exhibits a compressed state, while the In³⁺ exhibits a weaken rattling effect. The P-V-L chemical bond theory analysis indicates that the In-O bonds play a key role in affecting the dielectric loss. The thermally conductivity activation energy E_dc (0.94 eV) of SrIn₂O₄ was obtained by the dielectric spectroscopy, indicating that the E_dc was contributed to the double ionized oxygen vacancies. Furthermore, the intrinsic dielectric properties (ε_r ∼ 11.2, Q × f = 148,900 GHz) of SrIn₂O₄ were obtained by infrared reflectivity spectroscopy.     

Raman spectra,bond characteristics,and microwave dielectric properties of Gd2Mo3O12 ceramics

Gd₂Mo₃O₁₂ ceramics were prepared using the traditional solid-phase reaction method. All samples were found to possess an orthorhombic crystal structure with a space group of Pba2, as revealed by refined XRD results. The ceramic sintered at 1000 °C exhibited a high relative density of 96.95 % and superior microwave dielectric properties, including ε_r of 9.42 ± 0.05,  × f of 49258 ± 1200 GHz, and τ_f of −71.8 ± 0.7 ppm/°C. The results suggest a correlation between an increase in ε_r and higher relative density, and a more compact and uniform microstructure can lead to higher  × f value. Chemical bonding theories and Raman spectroscopy analysis reveal that Mo-O bonds, rather than Gd-O bonds, dominate the microwave dielectric properties. Furthermore, the ε_r of Gd₂Mo₃O₁₂ ceramic was closely related to bond ionicity, while  × f and τ_f were mainly determined by lattice energy and bond energy, respectively.     

Highly transparent polycrystalline Gd2O3 ceramic attained via ZrO2 stabilization effect

The transition from the cubic to monoclinic phase of Gd₂O₃ at high temperatures poses a significant challenge to the preparation of transparent Gd₂O₃ materials. In this work, we presented a straightforward yet effective method for fabricating transparent Gd₂O₃ ceramics. Via ZrO₂ stabilization effect for phase structure, highly transparent Gd₂O₃ ceramics were successfully fabricated by vacuum sintering at 1850 °C for 8 h. The effect of different Zr (0 ∼ 13 at%) concentrations on phase transition, grain growth, fracture mode and optical properties of Gd₂O₃ transparent ceramics was investigated. As the Zr content increases, the transition from the cubic (C) to monoclinic (M) phase is effectively suppressed, which is crucial for achieving Gd₂O₃ transparent ceramics. Moreover, the results indicate that the addition of ZrO₂ has a significant effect on grain growth by not only impeding the migration of grain boundaries but also affecting the phase composition. In addition, the 11 at% Zr-doped Gd₂O₃ ceramic exhibits the best optical properties, of which transmittance is about 76% at 850 nm and about 80% in the 2.5 µm ∼ 6 µm mid-infrared range. This work provides an illustrative example for the development of other ceramics with phase transition. The obtained Zr-doped Gd₂O₃ transparent ceramics with high optical quality are potential candidates for optical window, scintillator host and mid-infrared transmission materials.     

Synthesis of β-Bi2O3 nanoparticles via the oxidation of Bi nanoparticles: Size,shape and polymorph control,anisotropic thermal expansion,and visible-light photocatalytic activity

Bismuth trioxide (Bi₂O₃) is known for its simple composition but rich polymorphism that allows a number of phase-dependent physicochemical properties and technical applications. We here report our controllable synthesis of highly discrete β-Bi₂O₃ (tetragonal) nanoparticles (NPs) via the thermal oxidation of nearly-monodisperse Bi NPs. The size and shape (spherical and tear- or rod-like) of β-Bi₂O₃ NPs are tunable by the size of parent Bi NPs whereas β and α (monoclinic) polymorphs can be selectively achieved by tailoring the oxidation temperature. The phase stability of β polymorph from room temperature to 450 °C enables us to perform an in situ high-temperature XRD study on the temperature dependence of its lattice parameters, which reveals a marked thermal expansion anisotropy in β-Bi₂O₃ with a linear thermal expansion coefficient of +35.1 × 10^?6 °C^?1 in the c axis, fifteen times higher than that in the a axis. Meanwhile, the narrow band gap (2.27 eV for β vs. 2.77 eV for α) and strong visible-light absorption endow β-Bi₂O₃ NPs with a good photocatalytic activity for the visible-light Rhodamine B dye degradation. We expect that our work could be a valuable reference for the studies on the size, shape and polymorph control, thermal property, and photocatalytic application of Bi₂O₃.     

Optical-vibration properties and pressure-induced phase transition in (In,Sc)2Ge2O7 pyrogermanates

Thortveitite-type (T-type) scandium and indium pyrogermanates were obtained through a solid-state reaction route. The crystal structures, morphologies, and chemical compositions of these compounds were investigated by X-ray diffraction and transmission electron microscopy techniques. The optical-vibration properties were evaluated by Raman scattering and infrared spectroscopy. Reflectivity far-infrared data were obtained for sintered Sc₂Ge₂O₇ samples, which exhibited good surfaces after polishing, allowing the determination of the infrared optical functions, the complete polar phonon characteristics, the intrinsic dielectric constant and the unloaded quality factor. Because of the poor surface reflectivity of In₂Ge₂O₇ samples, only mid-infrared absorbance data could be collected by using loose powders in an attenuated total reflectance accessory. By using polarized Raman spectroscopy on both ceramic materials, at room temperature and ambient pressure, all the 15 active Raman modes predicted by group theory for the monoclinic C2/m T-type structure were identified and assigned. Further, the phase stabilities of In₂Ge₂O₇ and Sc₂Ge₂O₇ were studied by high-pressure Raman spectroscopy. In the limit of pressures studied here (up to 10 GPa), a reversible structural phase transition (SPT) for In₂Ge₂O₇ and Sc₂Ge₂O₇ was induced by pressures of 4.2 GPa and 2.4 GPa, respectively. These would correspond to the reported C2/m (#12) ↔ P2₁/c (#14) SPT. Representative Raman spectra of the high-pressure phase for both materials were fitted and 24 first-order Raman modes were depicted for each material, a number greater than compatible with a proposed distorted and oxygen-deficient fluorite-like structure, but explained by an analysis of the gtroup-subgroup constraints of this SPT.     

Crystal structures and electrical properties of Sr/Fe-modified KNbO3 ferroelectric semiconductors with narrow bandgap

In the process of exploring ferroelectric semiconductors, a new system of (1−x) KNbO₃–xSrFeO_3−δ (x = 0.00-0.20) was successfully synthesized via solid-state reaction. The crystal structures, ferroelectric, dielectric, optical, and electrical properties were systematically characterized. The orthorhombic phase with Amm2 space group is detected in all the ceramics. In addition, the orthorhombic and tetragonal phases coexist in 0.80KNbO₃-0.20SrFeO_3-δ ceramic. The decrease in oxygen octahedron distortion induces a weak ferroelectric polarization. The existence of long-range ferroelectric polarization order in all the ceramics is verified and the bandgap of the ceramics can be tuned to ~2.18 eV. The improved short-circuit photocurrent density (J_sc) and open-circuit voltage (V_oc) of the poled 0.95KNbO₃-0.05SrFeO_3−δ ceramic at 30 kV/cm are ~6.90 nA/cm² and 0.04 V, respectively. The activation energies for electrical conductivity of the grains and grain boundaries from 0.90KN–0.10SF ceramic are 0.67 and 0.77 eV, respectively, which indicate the doubly ionized oxygen vacancies. This work provides a new way to tune the optical bandgap/ferroelectric properties of KNbO₃-based ceramics for potential application in ferroelectric photovoltaic and energy fields.     

Low temperature synthesis and characterization of nano‐crystalline Mg(Zr0.05Ti0.95)O3 ceramics

Mg(Zr_0.05Ti_0.95)O₃ (MZrT) ceramics nanoparticles have been synthesized by polyol method for the first time. The phase evaluation of the MZrT nanoparticles was confirmed using thermo gravimetric analysis and the phase purity of the samples were analyzed using X‐ray diffraction and Raman spectroscopy. The transmission electron microscopy (TEM) images revealed the average particle size between 30 and 40 nm. The optical bandgap is in the range of 3.66‐3.82 eV and is attributed to the quantum confinement effect. Interestingly, the nanopowders sintered at 950°C for 3 hours exhibit the maximum density of 97.52% of the theoretical density which is attributed to the higher sintering velocity of the smaller particles. The obtained microstructure of the ceramics reveals porous free uniform microstructure with prominent grain boundaries. A best combination of microwave dielectric properties (ε_r ~18.04, Q × f_o ~175 THz at 9.5 GHz) are obtained for MZrT ceramics sintered at 950°C for 3 hours. The non‐Debye‐like relaxation process is found to exist inside the sample confirmed by impedance spectroscopy. The AC conduction mechanism is explained on the basis of Correlated Barrier Hopping model. Thermal conductivity of the MZrT ceramics is found to be 10 W/mK. The obtained properties of MZrT ceramics are suitable for resonator, microwave integrated circuit and LTCC applications.     

Crystal structure,far-infrared spectra,and microwave dielectric properties of bazirite-type BaZr(Si1-xGex)3O9 ceramics

Novel BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) microwave dielectric ceramics were prepared by solid-state reaction sintering at 1200–1450 °C for 5 h Ge⁴⁺ ions occupied the Si⁴⁺ positions, and BaZr(Si_1-xGe_x)₃O₉ solid solutions were obtained. The BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics exhibited hexagonal structures with P-6c2 space groups and octahedral layers and [Si/Ge₃O₉]^6- rings. Owing to these structural characteristics, the ceramics exhibited low permittivity. With an increase in x, the relative permittivity (ε_r) values of the BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics increased from 7.68 (x = 0) to 9.45 (x = 1.0), while their quality factor (Q × f) values first increased and then decreased. The Q × f value (10,300 GHz at 13.43 GHz) of the BaZrSi₃O₉ (x = 0) ceramic improved with the substitution of Si⁴⁺ by Ge⁴⁺. A high Q × f value (36,100 GHz at 13.81 GHz) was obtained for the BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic, and the Q × f values of the BaZr(Si_1-xGe_x)₃O₉ ceramics could be controlled by varying the Si/Ge-site bond valence. The temperature coefficient of resonance frequency (τ_f) values of the BaZr(Si_1-xGe_x)₃O₉ ceramics were mainly affected by the O2-site bond valence, and the optimum τ_f value (?22.8 ppm/°C) was achieved for the BaZrSi₃O₉ ceramic. The BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic showed the optimum microwave dielectric properties (ε_r = 8.36, Q × f = 36,100 GHz at 13.81 GHz, and τ_f = ?34.5 ppm/°C).     

Thermal and structural modification in transparent and magnetic germanoborate glasses induced by Gd2O3

A series of new transparent and magnetic germanoborate glasses in the system (100-x)[60GeO₂–25B₂O₃–10Na₂O–4Al₂O₃–1PbO] – (x) Gd₂O₃, with x = 0, 1, 2, 5, 10, 15 and 20 mol%, was prepared and studied with respect to their thermal and structural changes in the presence of Gd₂O₃. Based on Differential Scanning Calorimetre (DSC) analysis, a glass with 5% of Gd₂O₃ showed a high thermal stability, which progressively decreases for samples with higher content of Gd₂O₃. By the analysis of Raman and Fourier Transform Infrared (FTIR) spectra, it was possible to identify that by increasing the amount of Gd₂O₃, a progressive depolymerization of 6-membered Ge^[IV] rings is promoted, concomitant with an increase of Ge^[IV] tetrahedra units with non-briding oxygens. The structural analysis through the local-sensitive techniques EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure) showed that the short-range structural modification around the elements Ge and Gd³⁺ does not change with the addition of Gd₂O₃ and the presence of germanium four-fold coordination [Ge^IV] and Gd³⁺ states, respectively. A simulation of the coordination number (N), the interatomic distance (R) of Ge–O and Gd–O bonds and the Debye-Waller factor was also carried out. The microstructure_, after crystallization, of the sample with 15 mol% of Gd₂O₃ was evaluated using optical and electron microscopes. Finally, the paramagnetic behaviour and ion probe quantification of Gd³⁺ ions were obtained based on magnetic susceptibility measurements.     

Microwave dielectric properties of Co2P2O7 ceramics

Co₂P₂O₇ ceramics were prepared through the traditional solid-state sintering technique. The phase composition, grain size distribution, and densification were researched via X-ray diffraction and scanning electron microscope. The influence of pores on permittivity was described by various models. The dielectric loss was found highly dependent on porosity. Moreover, the low ε_r (<10) values of Co₂P₂O₇ ceramics were explained by the covalent feature of P–O bonds. Raman spectroscopy was used for exploring the relationship between polar phonon modes and dielectric properties in terms of intrinsic factors. The optimum dielectric properties (ε_r = 6.76, Qf = 36,400 GHz and τ_f = ?23.9 ppm/°C) were obtained at 1160 °C for 4 h.     

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li₄Mg₃Ti₂O₉ dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li₄Mg₃Ti₂O₉ at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450 °C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε^’ and ε^’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li₄Mg₃Ti₂O₉ ceramics sintered at 1450 °C exhibited excellent properties of ε_r=15.97, Q·f=135,800 GHz and τ_f=−7.06 ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li₄Mg₃Ti₂O₉−3 wt% LiF ceramics sintered at 900 °C possessed suitable dielectric properties of ε_r=15.17, Q·f =42,800 GHz and τ_f=−11.30 ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).     

Structural dependence of microwave dielectric properties in ilmenite-type Mg(Ti1-xNbx)O3 solid solutions by Rietveld refinement and Raman spectra

Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were prepared by the conventional solid-state reaction method. The phase composition, sintering characteristics, microstructure and dielectric properties of Ti⁴⁺ replacement by Nb⁵⁺ in the formed solid solution Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb⁵⁺ doping in Mg(Ti_1-xNb_x)O₃ were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics crystallised into an ilmenite-type with R-3 (148) space group. The replacement of the low valence Ti⁴⁺ by the high valence Nb⁵⁺ can improve the dielectric properties of Mg(Ti_1-xNb_x)O₃ (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics. The obtained results proved that 1350 °C is the best sintering temperature. The permittivity and Q × f initially increased and then decreased mainly due to the effects of porosity caused by the sintering temperature and the doping amount of Nb₂O₅, respectively. Furthermore, the increased Q × f is correlated to the increase in Ti–O bond strength as confirmed by Raman spectroscopy, and the electrons generated by the oxygen vacancies will be compensated by Nb⁵⁺ to a certain extent to suppress Ti⁴⁺ to Ti³⁺, which was confirmed by XPS. The increase in τ_f from ?47 ppm/°C to ?40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τ_f is the reduction in the distortion degree of the [TiO₆] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti_0.95Nb_0.05)O₃ ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of ε_r = 18.12, Q × f = 163618 GHz and τ_f = ?40.1 ppm/°C.     

Effects of LiF addition on the densification and microwave dielectric properties of LiInO2 ceramics

Low-temperature co-fired ceramics (LTCC) LiInO₂ + xwt% LiF (x = 0, 1, 2, 3, 4, 5) was synthesized by a traditional solid-state reaction method. XRD, TEM, and SEM show that all specimens form a pure-phase tetragonal structure with a space group of I4₁/amd. The addition of LiF can effectively optimize the microstructure and improve the relative density of LiInO₂ ceramics. As x value increased, the ε_r increased from 9.6 to 13.6, Q×f increased from 39,600 GHz to 52,500 GHz, and all the specimens exhibit a positive τ_f value at the range of 9.6–19.1 ppm/°C. The low-ε_r and positive τ_f in LiInO₂ ceramics was investigated by bond valence and P–V-L chemical bond theory, indicating that it was closely related to the rattling effect of In and Li. The concentration of oxygen vacancies was studied by dielectric spectroscopy, indicating that doping LiF can compensate for the volatilization of Li during high temperature sintering. Notably, excellent microwave dielectric properties (ε_r ~13.6, Q×f = 52,500 GHz, and τ_f ~ 18.1 ppm/°C) were achieved in the LiInO₂ + 3 wt% LiF sintered at 890 °C.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Structural and kinetic study of phase transitions in LaYSi2O7

Phase transitions in LaYSi₂O₇ have been investigated as a function of temperature using XRD, NMR and TEM. Previously described empirical crystal structure guidelines based on average cation radius in rare-earth RE₂Si₂O₇-type disilicates predict a stable tetragonal A-LaYSi₂O₇ polymorph at temperatures below 1500 °C. This study demonstrates that A-LaYSi₂O₇ is not thermodynamically stable at these temperatures and suggests that guidelines based on average cation size do not accurately describe the equilibrium behaviour of this silicate system. The A to G-type polymorph transition is extremely sluggish; complete transformation requires ～250 h at 1200 °C, and more than 3 weeks of calcination at 1100 °C. This observation is important when this type of material is used as environmental barrier coating (EBC) of advanced ceramics. Analysis of XRD and TEM data reveal complete substitution of Y and La on the rare-earth cation sites in both LaYSi₂O₇ polymorphs, but indicate preferential site occupancies in the G-type polymorph.     

Microwave dielectric properties,low-temperature sintering mechanism,and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li₂Zn₃Ti₄O₁₂-xSr₃(VO₄)₂ (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO₃ and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τ_f value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti³⁺, and V⁴⁺, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li₂Zn₃Ti₄O₁₂-0.3Sr₃(VO₄)₂ ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of ε_r = 17.8, Q × f = 41,891 GHz, and τ_f = ?4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.     

Investigation on phase structure,spectral characteristics,microstructure and microwave dielectric properties of Li2Zn[Ti1-x(Co1/3Nb2/3)x]3O8 (0.0 ≤ x ≤ 0.4) ceramics

In this paper, a series of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ (0.0 ≤ x ≤ 0.4) ceramics were prepared via the conventional solid-state method. The influences of (Co_1/3Nb_2/3)⁴⁺ complex ions on the phase composition, spectral characteristics, microstructure, and microwave dielectric properties of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ ceramics were studied systematically. XRD analysis accompanied with Rietveld refinements showed that pure Li₂ZnTi₃O₈ solid solution ceramics with the cubic spinel structure were obtained at x = 0.2–0.4. New Raman-active mode of about 858 cm⁻¹ should be attributed to the vibrations of NbO₆ due to the high bond energy of Nb–O bonds, exerting a certain impact on the structure and performance of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ ceramics. XPS results indicated that Nb⁵⁺ ion donor suppressed the deoxidation process and therefore resulted in the disappearance of Ti³⁺ ion and oxygen vacancy. The downward trend variation in the ε_r value with the increase of (Co_1/3Nb_2/3)⁴⁺ content could be explained by the presence of “compressed” cations and “rattling” cations effect. In addition, the Q × f of the current ceramics was closely dependent on relative density, grain size, FWHM, and oxygen vacancy. Good combined microwave dielectric properties of ε_r = 24.5, Q × f = 91,250 GHz, and τ_f = −16.8 ppm/°C were achieved for the Li₂Zn[Ti_0.8(Co_1/3Nb_2/3)_0.2]₃O₈ ceramic sintered at 1120 °C. High quality factor gives evidence that the Li₂Zn[Ti_0.8(Co_1/3Nb_2/3)_0.2]₃O₈ ceramic is an appealing candidate for highly selective microwave devices.     

Correlation between structural characteristics and microwave dielectric properties of walstromite BaCa2M3O9 (M = Si,Ge) ceramics

Novel BaCa₂M₃O₉ (M = Si, Ge) microwave dielectric ceramics were prepared via solid-state reaction with sintering at 1125°C–1275°C for 5 h. Single-phase BaCa₂M₃O₉ (M = Si, Ge) ceramics were obtained according to stoichiometry. The single-phase BaCa₂Ge₃O₉ ceramic was confirmed through Rietveld refinement and high-resolution transmission electron microscopy/selected area electron diffraction and synthesized for the first time. The BaCa₂M₃O₉ (M = Si, Ge) exhibited a triclinic structure with a P$\overline{1}$ space group and good microwave dielectric properties. The ε_r, Q × f, and τ_f values of BaCa₂M₃O₉ (M = Si, Ge) ceramics are mostly dominated by the relative density, ionic polarizability, relative covalence, and bond energy of M–O bond, respectively. A high Q × f value (61 800 GHz at 16.3 GHz) was obtained in BaCa₂Ge₃O₉ ceramic due to its high r_c (Ge–O) and low intrinsic dielectric loss. The BaCa₂Si₃O₉ ceramic exhibited small |τ_f| value (‒36.4 ppm/°C) due to its large E_Si-O. Excellent microwave dielectric properties (ε_r = 8.31, Q × f = 61 800 GHz, and τ_f = ‒58.7 ppm/°C) were obtained for the BaCa₂Ge₃O₉ ceramic.