Crystal structure and microwave dielectric properties of La2(Zr1−xTix)3(MoO4)9 (0 ≤ x ≤ 0.1) ceramics

La₂(Zr_1−xTi_x)₃(MoO₄)₉ (0 ≤ x ≤ 0.1) ceramics were prepared by the traditional solid-state reaction method. XRD analysis showed that La₂(Zr_1−xTi_x)₃(MoO₄)₉ (0 ≤ x ≤ 0.1) ceramics belonged to a trigonal system. Based on the chemical bond theory, the consequences of bond energy, bond ionicity, lattice energy, and thermal expansion coefficient of ceramics on microwave dielectric properties were discussed. As Ti⁴⁺ addition was increased, the reduction in dielectric constant was ascribed to the fact that the polarizability of Ti⁴⁺ is smaller than Zr⁴⁺, and the downward trend was related to the bond ionicity. Besides, the tendency of Q·f value depended on the packing fraction and the lattice energy. The improvement in τ_f value, the increase in bond energy, and the decrease in the coefficient of thermal expansion were all correlated. The far-infrared spectra implied that the absorptions of structural phonon oscillation were the main reason for the maximum polarization contribution. La₂(Zr_0.92Ti_0.08)₃(MoO₄)₉ ceramics sintered at 750°C for 4 hours exhibited the best dielectric properties (ε_r = 10.33, Q·f = 80 658 GHz, and τ_f = 3.48 ppm/°C).     

Intrinsic factors affecting the microwave dielectric properties of Mg2Ti1−x(Mg1/3Sb2/3)xO4 ceramics

The effect of crystal structure on the microwave dielectric properties of Mg₂Ti_1−x(Mg_1/3Sb_2/3)_xO₄ (0.025≤x≤0.15) ceramics was investigated. A single phase having a cubic inverse spinel structure formed over the entire range of compositions, in specimens sintered at 1450 °C for 4 h. The structural characteristics of these ceramics were quantitatively evaluated by applying the Rietveld refinement method to the X-ray diffraction data. The largest bond strength between the cation and the oxygen ion and hence the highest quality factor (Qf) of the specimens were obtained at x=0.05. Although the ionic polarizability (3.29 Å³) of (Mg_1/3Sb_2/3)⁴⁺ was larger than that (2.93 Å³) of Ti⁴⁺, the dielectric constant (K) of the specimens decreased owing to the decrease of rattling effect with increasing x. In addition, the temperature coefficient of resonant frequency (TCF) decreased with decreasing K values. Typically, a high Qf value of 229,000 GHz was obtained for the specimens with x=0.05.     

Preparation and microwave dielectric properties of Ca0.6La0.8/3(SnxTi1−x)O3 ceramics

Ca_0.6La_0.8/3(Sn_xTi_1−x)O₃ ceramics were prepared via a conventional solid state reaction method, and the effect of Sn doping on their crystal phase structure and microwave dielectric properties was investigated. Results showed that Sn doping could hinder the formation of the rutile TiO₂ detrimental phase of Ca_0.6La_0.8/3TiO₃ ceramic. Also, the Q×f₀ value was enhanced and the τ_f value was lowered by Sn doping. The best microwave dielectric properties, i. e. ε_r=113 and Q×f₀=8487 GHz were obtained for a Sn doping content of 0.02. The mechanism of the improved properties deriving by Sn doping is discussed.     

Microstructure and microwave dielectric properties of Li2Ti1−x(Zn1/3Nb2/3)xO3 ceramics

Li₂Ti_1?x(Zn_1/3Nb_2/3)_xO₃ (0≤x≤0.5) ceramics were prepared by a solid state ceramic route, and the phase purity, microstructure, and microwave dielectric properties were investigated. The XRD results suggest the formation of solid solutions for all studied compositions (0≤x≤5). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the samples. The Q×f value increases with x up to x=0.2 and then decreases with the further increase of x. The best microwave dielectric properties of ε_r=20.5, Q×f =75,257 GHz, and τ_f =15.4 ppm/°C could be obtained when x=0.2.     

Structure,spectral analysis and microwave dielectric properties of novel x(NaBi)0.5MoO4-(1-x)Bi2/3MoO4 (x = 0.2 ∼ 0.8) ceramics with low sintering temperatures

Herein, the x(NaBi)_0.5MoO₄-(1-x)Bi_2/3MoO₄ (xNBM-(1-x)BMO, x = 0.2 ∼ 0.8) microwave dielectric ceramics with low sintering temperatures were prepared via the traditional solid-state method to adjust the τ_f value and dielectric constant. The crystal structure was determined using X-Ray diffraction and Raman spectroscopy, the microstructure was investigated using scanning electron micrograph and energy disperse spectroscopy, and the dielectric properties were studied using a network analyser and infrared spectroscopy. For the xNBM-(1-x)BMO composite ceramics, the (NaBi)_0.5MoO₄ tetragonal phase coexisted with the Bi_2/3MoO₄ monoclinic phase. With the rise of x value, the permittivity increased from 23.7–29.8, and the τ_f value shifited from -53.3 ppm/°C to -13.7 ppm/°C. The 0.8NBM-0.2BMO ceramic sintered at 680 °C possessed excellent microwave dielectric properties with a ε_r = 29.8 (6.7 GHz), a Qf = 11,800 GHz, and a τ_f = -13.7 ppm/°C. These results made the xNBM-(1-x)BMO composite ceramics great candidates in low temperature co-fired ceramics technology.     

Structure and microwave dielectric characteristics of Sr2[Ti1−x(Al0.5Nb0.5)x]O4 (x ≤ 0.50) ceramics

Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ (x = 0, 0.10, 0.25, 0.30, 0.5) ceramics were synthesized by a standard solid-state reaction process. Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ solid solutions with tetragonal Ruddlesdon-Popper (R-P) structure in space group I4/mmm were obtained within x ≤ 0.50, and only minor amount (1-2 wt%) of Sr₃Ti₂O₇ secondary phase was detected for the compositions x ≥ 0.25. The temperature coefficient of resonant frequency τ_f of Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ ceramics was significantly improved from 132 to 14 ppm/°C correlated with the increase in degree of covalency (%) with increasing x. The dielectric constant ɛ_r decreased linearly with increasing x, while high Qf value was maintained though it decreased firstly. The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure. Good combination of microwave dielectric properties was achieved for x = 0.50: ɛ_r = 25.1, Qf = 77 580 GHz, τ_f = 14 ppm/°C. The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta.     

Effect of electronegativity on microwave dielectric properties of MgTi1−x(A1/3Sb2/3)xO3 (A = Mg2+, Zn2+) ceramics

Microwave dielectric properties of MgTi_1−x(A_1/3Sb_2/3)_xO₃ (A = Mg²⁺, Zn²⁺, 0 ≤ x ≤ 0.125) ceramics were investigated as a function of the electronegativity difference (X) between (A_1/3Sb_2/3)⁴⁺ ions (A = Mg²⁺, Zn²⁺) and O²⁻ ion. A single phase with an ilmenite structure was detected for the specimens sintered at 1450°C for 4 hours in the entire range of compositions. With the substitution of (A_1/3Sb_2/3)⁴⁺ ions (A = Mg²⁺, Zn²⁺) at Ti⁴⁺-sites of MgTiO₃, the quality-factor (Qf) values of the specimens increased up to x = 0.05 mol, and then decreased with the further substitution owing to the electronegativity difference (X) between the (A_1/3Sb_2/3)⁴⁺ ions (A = Mg²⁺, Zn²⁺) and Ti⁴⁺ ion. The specimens with (Mg_1/3Sb_2/3)⁴⁺ ions exhibited higher Qf values than those with (Zn_1/3Sb_2/3)⁴⁺ ions. These results could be attributed to the smaller electronegativity of (Mg_1/3Sb_2/3)⁴⁺ (1.80) than that of (Zn_1/3Sb_2/3)⁴⁺ (1.92). The dependences of the microwave dielectric properties on the structural characteristics of the MgTi_1−x (A_1/3Sb_2/3)_xO₃ ceramics were also discussed.     

Mixed-valent structure,dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics

Single-phase Ca_1−3x/2Tb_xCu₃Ti_4−xTb_xO₁₂ (0.025≤ x≤0.075) (CTCTT) ceramics with a cubic perovskite-like structure and a fine-grained microstructure (1.6‒2.3 µm) were prepared using a mixed oxides method. The results revealed that mixed valence states of Cu²⁺/Cu⁺, Ti⁴⁺/Ti³⁺, and Tb³⁺/Tb⁴⁺ coexisted in CTCTT. A multiphonon phenomenon in the Raman scattering at 1148, 1323, and 1502 cm⁻¹ was reported for undoped and doped CTTO. Tb was mainly incorporated in the interior of the CTCTT grains rather than on the surface. The dielectric permittivity of CTCTT (ε_r'_RT =3590‒5200) decreased relative to CCTO (ε_r'_RT =10240) at f =1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ=0.12 at RT) increased as a result of Tb doping. The defect chemistry of CTCTT is discussed. The internal barrier layers capacitance (IBLC) model was adopted for impedance spectroscopy (IS) analysis. The activation energies of the grain boundaries (E_gb) and semi-conductive grains (E_g) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to a decrease in conductivity in the semiconducting CTCTT grains caused by the acceptor effect of Tb⁴⁺ at the Ti site, which resulted in a decrease in the IBLC effect.     

Abnormal variation of microwave dielectric properties in A/B site co-substituted (Ca1−0.3xLa0.2x)[(Mg1/3Ta2/3)1−xTix]O3 complex perovskite ceramics

A/B site co-substituted (Ca_1?0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1?xTi_x]O₃ ceramics (0.1 ≤ x ≤ 0.5) were prepared by solid state reaction and the structures, microstructures and dielectric properties were investigated. B site 1:2 cation ordering and oxygen octahedra tilting lead to monoclinic symmetry with space group P2₁/c for x = 0.1. For x above 0.1, the ordering was destroyed and the crystal structure became orthorhombic with space group Pbnm. The B site 1:2 cation ordering tended to be destroyed to form 1:1 ordering by the A site La³⁺ substitution. The dielectric constant increased linearly with increasing content of Ti⁴⁺ as the increasing second Jahn–Teller distortion enhanced the B site cation rattling. The temperature coefficient of resonant frequency and Qf values showed abnormal variations, which were refined to be caused by the increasing A site cation vacancy and diffused distribution of small size ordering domains respectively. Good combination of microwave dielectric properties was obtained at x = 0.5, where ?_r = 48, Qf = 21,000 GHz and τ_f = 2.2 ppm/°C.     

New scheelite-type Cd1−3x⌷xGd2x(MoO4)1−3x(WO4)3x ceramics – their structure,thermal and magnetic properties

Polycrystalline samples of scheelite-type Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x solid solution with limited homogeneity (0<x≤0.25) and cationic vacancies (denoted as ⌷) have successfully prepared by a high-temperature annealing of CdMoO₄/Gd₂(WO₄)₃ mixtures composed of 50.00 mol% and less of gadolinium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA–TG, and SEM techniques. A phase diagram of the pseudobinary CdMoO₄–Gd₂(WO₄)₃ system was constructed. The eutectic point corresponds to 1404±5 K and ~70.00 mol% of gadolinium tungstate in an initial CdMoO₄/Gd₂(WO₄)₃ mixture. With decreasing of Gd³⁺ content in a CdMoO₄ framework, the melting point of Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x increases from 1406 (x=0.25) to 1419 K (x=0.0833), and next decreases to 1408 K (x=0). EPR method was used to identify paramagnetic Gd³⁺ centers in Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x for different values of x parameter as well as to select biphasic samples containing both Cd_0.25⌷_0.25Gd_0.50(MoO₄)_0.25(WO₄)_0.75 and Gd₂(WO₄)₃.     

Structural characteristics and microwave dielectric properties of Zn1−xBixVxW1−xO4-based ceramics for LTCC applications

Herein, the improvement of the microwave dielectric properties and sintering characteristics of Zn_1?xBi_xV_xW_1?xO₄(x = 0–0.15)-based ceramics is reported. The results showed that an appropriate amount of doping could not only reduce the optimum sintering temperature from 1100° to 900°C, but also enhance the densification of the microstructures and increase the Q×f value from 5351 to 42525 GHz. Additionally, various structural parameters including the phase composition, crystal structure, vibrational and chemical bond characteristics that are correlated with the dielectric properties were systematically investigated. By considering the chemical bond characteristics, the first-principles calculations and the acquired Raman spectra, the interaction between W-O is stronger than Zn-O in the ZnWO₄ structure, while the interaction between V-O is stronger than Bi-O in BiVO₄. Interestingly, when the Zn_0.97Bi_0.03V_0.03W_0.97O₄-based ceramics were sintered at 900 °C, improved microwave dielectric properties were acquired (ε_r =18.32, Q×f=42525 GHz, τ_f=?67.51 ppm/°C), which provides a promising candidate in low-temperature co-fired ceramics technology.     

Microwave dielectric properties of Bi2O3-doped Ca[(Li1/3Nb2/3)1−xTix]O3−δ ceramics

The effect of the additive on the densification, low temperature sintering, and microwave dielectric properties of the Ca[(Li_1/3Nb_2/3)_1−xTi_x]O_3−δ(CLNT) was investigated. Bi₂O₃ addition improved the densification and reduced the sintering temperature from 1150 to 900 °C of CLNT microwave dielectric ceramics. As the Bi₂O₃ content increased, the dielectric constant (ε_r) and bulk density increased. The quality factor (Q·f₀), however, was decreased slightly. The temperature coefficient of resonant frequency (τ_f) shifted to a positive value with increasing Bi₂O₃ content. The dielectric properties (ε_r, Q·f_0, τ_f) of Ca[(Li_1/3Nb_2/3)_0.95Ti_0.05]O_3−δ and Ca[(Li_1/3Nb_2/3)_0.8 Ti_0.2]O_3−δ with 5 wt.% Bi₂O₃ sintered at 900 °C for 3 were 20, 6500 GHz, −4 ppm/°C, and 35, 11,000 GHz, 13 ppm/°, respectively. The relationship between the microstructure and dielectric properties was studied by X-ray diffraction (XRD), and SEM.     

Dielectric properties of Ba1−xSrxTiO3 ceramics prepared by microwave sintering

A comparative study on the dielectric properties of Ba_1?xSr_xTiO₃ (x=0.1–0.6) ceramics prepared by microwave sintering (MS) and conventional sintering (CS) has been done. It was found that MS samples need lower temperature and much shorter time than CS samples to obtain the same degree of densification. Compared with CS samples, MS samples possessed smaller grain size, better densification and more uniform grain growth. The dielectric properties of the samples were measured as a function of temperature. It was observed that the dielectric constant was higher for MS samples compared with that of CS samples especially in the ferroelectric phase.     

Crystal structure and microwave dielectric behaviors of scheelite structured (1-x)BiVO4-xLa2/3MoO4 (0.0 ≤ x ≤ 1.0) ceramics with ultra-low sintering temperature

In the present work, a novel (1-x)BiVO₄-xLa_2/3MoO₄ scheelite related solid solution ceramics were prepared via solid state reaction method. As revealed by X-ray diffraction data, the crystal structure changed continuously from monoclinic to tetragonal phase at x = 0.10 and then the tetragonal solid solution was kept in a wide composition range up to x = 0.70. Both the Raman and far infrared spectra supported this phenomenon. When x value reached 0.9, the ceramic sample was found to composed of both scheelite tetragonal and monoclinic La_2/3MoO₄ phases. Large microwave permittivity value between 68 ± 0.2–73 ± 0.3 can be achieved in compositions with 0.02 ≤ x ≤ 0.10 with high Qf value about 10,000 ± 500 GHz. This series of solid solution ceramics might be candidate for both dielectric resonator and low temperature co-fired ceramic technology.     

Sintering behaviour and microwave dielectric properties of BaAl2−2x(ZnSi)xSi2O8 ceramics

BaAl_2?2x(ZnSi)_xSi₂O₈ (x = 0.2–1.0) ceramics were prepared using the conventional solid-state reaction method. The sintering behaviour, phase composition and microwave dielectric properties of the prepared compositions were then investigated. All compositions showed a single phase except for x = 0.8. By substituting (Zn_0.5Si_0.5)³⁺ for Al³⁺ ions, the optimal sintering temperatures of the compositions decreased from 1475 °C (x = 0) to 1000 °C (x = 0.8), which then slightly increased to 1100 °C (x = 1.0). Moreover, the phase stability of BaAl₂Si₂O₈ was improved. A novel BaZnSi₃O₈ microwave dielectric ceramic was obtained at the sintering temperature of 1100 °C. This ceramic possesses good microwave dielectric properties with ε_r = 6.60, Q × f = 52401 GHz (at 15.4 GHz) and τ_f = ?24.5 ppm/°C.     

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.     

Microwave dielectric properties of (1−x)ZnTa2O6−xMgNb2O6 ceramics

Single phase MgNb₂O₆ and ZnTa₂O₆ powders were synthesized by solid-state method, and the high quality factor composite ceramics of (1?x)ZnTa₂O₆?xMgNb₂O₆ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25 and 1.0) were prepared using the as-synthesized powders. The microwave dielectric properties, microstructure, phase transition and sintering behavior of the composite ceramics were investigated. The X-ray diffraction analysis revealed that solid solution between ZnTa₂O₆ and MgNb₂O₆ phases appeared in the composite ceramic. SEM results show that the grain sizes of the composite ceramics increased with increasing x values. The temperature coefficient of resonant frequency of (1?x)ZnTa₂O₆?xMgNb₂O₆ composite ceramics reaches near-zero of 1.02 ppm/°C with ε_r=35.58 and a high quality factor of 65500 GHz when x=0.20 and sintered at 1350 °C for 2 h.     

Synthesis and thermoelectric properties of (NaxCa1−x)3Co4O9 ceramics

(Na_xCa_1−x)₃Co₄O₉ (x=0.05−0.2) ceramics with a layered crystal structure were prepared by a sol–gel method followed by a low-temperature sintering procedure. The electrical conductivity and Seebeck coefficient of the complex oxide ceramics were measured from 400 to 900 °C. Their electrical conductivity and power factor increase with increasing temperature, while the thermal conductivity is very weakly dependant on the temperature. Na dopant amount has a remarkable effect on electrical and thermal transport properties. The figure of merit in the ceramic samples is smaller than that of traditional thermoelectric alloys.     

Low temperature sintering and microwave dielectric properties of (Mg3 − xZnx)(VO4)2 ceramics

This paper describes the effects of Zn substitution for Mg on the microwave dielectric properties of (Mg_3 − xZn_x)(VO₄)₂ ceramics. As for the XRPD patterns of (Mg_3 − xZn_x)(VO₄)₂ ceramics sintered at the sintering temperature of 750 °C, no secondary phase was detected over the whole composition range. However, in the case of the sample sintered at 850 °C, the Zn₄V₂O₉ and Zn₂V₂O₇ compounds were identified by using XRPD; this result was attributed to the decomposition of Zn₃(VO₄)₂ phase. From crystal structure analysis, it was found that the atomic distances of M(1)O (M = Mg and Zn) and M(2)O in MO₆ octahedra increased, though that of VO in VO₄ tetrahedron decreased. Moreover, the slight tilting of M(2)O₆ octahedron was observed by the Zn substitution. As for the covalency of cation–oxygen bond, the covalency of MO bond in M(1)O₆ and M(2)O₆ octahedra decreased because of the increase in the atomic distance of MO, whereas that of VO increased with increasing the Zn addition. However, as a result, the slight decrease in the covalency of cation–oxygen bond was recognized because the variation in the covalency of MO bond is predominant in this crystal structure. The dielectric constants of the samples range from 4.4 to 11.1. The decrease in the covalency may be related to the difference in the dielectric constant of each composition. The maximum Q · f value of bulk densities is effected by varying the chemical composition of (Mg_3 − xZn_x)(VO₄)₂ ceramics and it shifts toward lower sintering temperature with an increase in x within the temperature region of 800–1050 °C. The temperature coefficient of resonant frequency (τ_f) of the samples decreased with increasing of Zn, and then a variation in τ_f value was attributed to the tilting of M(2)O₆ octahedron caused by Zn substitution for Mg.