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.     

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.     

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.     

La(Mg1/2Ti1/2)O3–La2/3TiO3 microwave dielectric ceramics

Structure and microwave dielectric properties were studied in the (1−x)La(Mg_1/2Ti_1/2)O₃–xLa_2/3TiO₃ system. Ceramics with this composition in the 0⩽x⩽0.5 range were processed from powders obtained by a citrate-based chemical route. Structure of these perovskite solid solutions changed from orthorhombic for x=0.1 and 0.3 to pseudocubic for x=0.5. Microwave and radio frequency measurements revealed increase in permittivity and temperature coefficient of the resonant frequency τ_f with increasing of La_2/3TiO₃ content. Close to zero τ_f value was found near to x=0.5 composition of (1−x)La(Mg_1/2Ti_1/2)O₃–x La_2/3TiO₃.     

Effect of Ga3+ substitution on the microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics

0.67CaTiO₃–0.33La(Al_1−xGa_x)O₃ (0≤x≤0.4) (CTLAG) ceramics with pure perovskite structure were prepared by a conventional two-step solid-state reaction process. The effect of Ga³⁺ substitution for Al³⁺ on the microwave dielectric properties of the ceramics was subsequently investigated. As Ga content increased, the ionic polarizability increased and led to an increase of the dielectric constant (ε_r). Meanwhile, both the tolerance factor (t) of CTLAG ceramics and A-site bond valence were considered to have effect on the temperature coefficient of the resonant frequency (τ_f) with the increase of Ga content. Results also showed that the quality factor (Q×f ) varied with increasing Ga³⁺ content because of not only intrinsic factor but also extrinsic factors such as the bimodal grain size distribution, the variation of relative density, and the packing fraction. Excellent microwave dielectric properties with ε_r≈45.81, Q×f≈34,152 GHz, and τ_f≈3.09 ppm/°C were achieved for 0.67CaTiO₃–0.33La(Al_0.9Ga_0.1)O₃ ceramics sintered at 1420 °C for 4 h.     

Effect of ZnO on Mg2TiO4–MgTiO3–CaTiO3 microwave dielectric ceramics prepared by reaction sintering route

ZnO-doped Mg₂TiO₄–MgTiO₃–CaTiO₃ microwave dielectric ceramics were successfully prepared by the reaction sintering route. The compact samples consisted of MgTiO₃, Mg₂TiO₄ and CaTiO₃, which was confirmed by X-ray diffraction and energy-dispersive spectra. ZnO efficiently lowered the sintering temperature and promoted the densification, as well as the improvements in the dielectric constant and the quality factor. At the level of ZnO?=?1 wt-%, the ceramics exhibited optimum microwave dielectric properties: a dielectric constant of 20.3, a high quality factor of 64,740 GHz (at 9.9 GHz) and a near-zero temperature coefficient of resonant frequency (–1.3 ppm/^oC) after sintering at 1320^oC for 4 h.     

Effects of Magnesium–Tungsten co-substitution on crystal structure and microwave dielectric properties of CaTi1-x(Mg1/2W1/2)xO3 ceramics

CaTi_1-x (Mg_1/2W_1/2)_xO₃ (x = 0, 0.02, 0.04, 0.06, 0.08) dielectric ceramics were synthesized via the traditional solid-state reaction method. Crystal structure and microwave dielectric properties of CaTi_1-x (Mg_1/2W_1/2)_xO₃ system were systematically investigated based on chemistry bond theory (P–V-L theory) for the first time. The pure perovskite phase was obtained for all doped samples, as confirmed through the XRD and Rietveld refinement results. The lattice characteristics were closely related to the microwave dielectric properties. The bond ionicity, lattice energy, and bond energy affected the dielectric constant, quality factor, and temperature stability of the ceramic material. Through the use of (Mg_1/2W_1/2)⁴⁺ doped on B-site, the CaTi_1-x (Mg_1/2W_1/2)_xO₃ system can maintain a high dielectric constant (ε_r > 100) while effectively reducing the τ_f value from 800 ppm/^°C to less than 300 ppm/^°C and improving the Q × f value to 9650 GHz (at 3.76 GHz).     

Preparation and microwave dielectric properties of BaLi1+xF3+x (x = 0–0.01) ceramics

BaLi_1+xF_3+x (x = 0–0.01) were successfully mechanosynthesized by a simple ball-milling process. The effects of excessive LiF and sintering method and/or annealing atmosphere on its sintering behavior, microstructure, and microwave dielectric properties have been investigated in this paper. The mechanosynthesized powder can be densified with relative densities of ∼95 % after sintering at 750–800 °C/2 h in N₂. The obtained ceramics exhibit excellent optimized microwave dielectric properties with ε_r of ∼11.46 ± 0.06, Q×f values of 83175 ± 1839 GHz and τ_f of ∼ − 70 ± 3 ppm/°C at the x = 0.006 composition. Its Q×f value could be improved to 94603 ± 2037 GHz) by post-annealing in N₂ after post annealing at 700 °C/2 h. The Q×f value could be further improved to (120,098 ± 2344 GHz) by hot-pressed sintering (HPS). Sintering in the ambient atmosphere or O₂ leads to lower Q×f values than those of the counterparts sintered in N₂ due to the introduction of F-vacancies by oxidation_, while little variation in ε_r andτ_f.     

Influence of cation order on crystal structure and microwave dielectric properties in xLi4/3Ti5/3O4-(1-x)Mg2TiO4 (0.6 ≤ x ≤ 0.9) spinel solid solutions

Effect of order/disorder transition on microwave dielectric characteristics is reported to develop a deeper understanding of structure-property relationship in spinel ceramics. Dense xLi_4/3Ti_5/3O₄-(1-x)Mg₂TiO₄ (0.6≤x≤0.9) spinel ceramics were synthesized and characterized for structural and dielectric properties. The critical order/disorder structural transition was induced when x < 0.8, resulting in the ceramic crystallized into a primary cubic spinel phase, while when x> 0.8, the ceramic crystallized into a disordered face-centered cubic phase. The cation occupation caused this order-disorder transition, which directly influenced the variation in microwave dielectric properties. At x = 0.75 the maximum degree of order was achieved resulting in a maximum quality factor of 55,000 GHz and a near-zero τ_f = 2.9 ppm/^oC. Dielectric properties decreased sharply after x = 0.8 when the disorder face-centered cubic phase started to crystallize. All the results indicated that cation ordering/disordering plays a critical role in determining the optimum microwave dielectric properties in spinel ceramics.     

Low-loss microwave dielectric of novel Li1-2xMxVO3 (M = Mg,Zn) (x = 0–0.09) ceramics for ULTCC applications

Novel Li_1?2xM_xVO₃ (M = Mg, Zn) (x = 0–0.09) microwave dielectrics suitable for ULTCC (ultra-low temperature co-fired ceramics) applications were synthesized. The X-ray diffraction patterns revealed that all samples were monoclinic structured with a space group of C2/c. Microstructures, lattice parameters, and Raman spectra of the ceramics were also studied. Q×f (Q: quality factor) was mainly controlled by intrinsic and extrinsic loss, while the variation of τ_f (temperature coefficient of resonant frequency) was related to ceramic bond valence. Poor microwave dielectric properties of pure LiVO₃ can be tremendously enhanced by substituting a minute amount of Mg or Zn in place of Li. Excellent properties could be obtained for specimens sintered at 520 °C with an ε_r of 9.78, a Q×f of 45,600 GHz and an τ_f of –45 ppm/°C for Li_0.98Mg_0.01VO₃, in addition to an ε_r (dielectric constant) of 9.25, a Q×f of 33,100 GHz and an τ_f of –53.6 ppm/°C for Li_0.98Zn_0.01VO₃. Furthermore, the Li_0.98Mg_0.01VO₃ specimen was found to be chemically comparable with the Al electrode. With 2 mol% of TiO₂ added, the specimen at 520 °C achieved excellent characteristics which include an ε_r of 9.2, a Q×f of 30,000 GHz, and an τ_f of –2.8 ppm/°C, making it a very promising ULTCC dielectric for high-frequency 5 G applications.     

Relationships between crystal structure and microwave dielectric properties of Li2(Mg1−xCox)3TiO6 (0 ≤ x ≤ 0.4) ceramics

Li₂(Mg_1−xCo_x)₃TiO₆ (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared using the conventional solid-state reaction method. Effects of Co-substitution on the crystal structure, sintering characteristics, micro–structures, microwave dielectric properties and Raman spectra were investigated. Co-substitution would affect the crystal structure and further influence the microwave dielectric properties. On the basis of the chemical bond theory and Rietveld refinement, some intrinsic factors such as the lattice energy, bond ionicity were calculated to explain the variations of the microwave dielectric properties. Ti–O and Li–O bonds played major role in influencing intrinsic factors and correlations between intrinsic factors of Ti–O, and Li–O bonds and microwave dielectric properties were established. With the increase of x values, the dielectric constant (ε_r) gradually increased, which could be explained by the variations of the polarizability, bond ionicity of Ti–O bonds, and Raman vibration modes. The decrease of the quality factor (Q·f) could be predicted by the decrease of the bond valence, packing fraction and lattice energy of Ti–O bonds. The temperature coefficient of resonant frequency (τ_ƒ) significantly correlated with the bond energy of Li–O bonds and thermal expansion coefficient of Ti–O bonds.     

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.     

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.     

Effects of Ti-substitution on the crystal structures,micro-structures and microwave dielectric properties of Li2Mg3Zr1-xTixO6 (0 ≤ x ≤ 1) ceramics

Li₂Mg₃Zr_1?xTi_xO₆ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) ceramics were prepared via a solid-state reaction method. Crystal structures, sintering behaviors, micro-structures and microwave dielectric properties of Li₂Mg₃Zr_1?xTi_xO₆ (0 ≤ x ≤ 1) ceramics were investigated by XRD, SEM and chemical bond theory. XRD results showed that a single phase with the rock-salt structure was formed in all ranges. On the basis of the Rietveld refinement and chemical bond theory, several intrinsic parameters were calculated and connections between intrinsic parameters and microwave dielectric properties were investigated. The substitutions of Ti⁴⁺ for Zr⁴⁺ obviously increased the relative density and improved the quality factors. Variations of ε_r could be explained by changes of the polarizability. Q·f values showed the similar trend with the packing fractions, average bond valences and lattice energy of Zr–O bonds. τ_? values significantly correlated with the bond energy of Zr–O bonds.     

Effect of Sn4+–Isovalent doping concentration on giant dielectric properties of SnxTa0.025Ti0.975-xO2 ceramics

The Sn_xTa_0.025Ti_0.975-xO₂ (x%Sn(TTO)) ceramics with x = 2.5–10% were prepared using a standard mixed-oxide method and sintered at 1450 °C for 3 h to achieve a dense microstructure. The effects of the isovalent–Sn⁴⁺ doping concentration on the crystal structure, microstructure, giant dielectric behavior, and electrical properties were systematically investigated. Continuously enlarged lattice parameters and bond lengths with a single rutile–TiO₂ phase were observed as x% increased. The mean grain size was slightly reduced (～17.3–14.6 μm) due to an increased oxygen vacancy and the solute drag effect. The dielectric permittivity (ε′) decreased with increasing x%, whereas the loss tangent (tanδ) was remarkably reduced. The semiconducting grain resistance of the x%Sn(TTO) ceramics remained unchanged owing to the same Ta⁵⁺ donor concentration. The insulating grain boundary (GB) resistance was extremely increased by more than two orders of magnitude when x% increased from 2.5 to 5.0%, leading to the significantly improved giant dielectric properties. The optimized low tanδ～0.02 and high ε′～10⁴ with temperature coefficient less than ±15% in the range of -60–210 °C were reasonably described by the internal barrier layer capacitor model. Improved dielectric properties can be obtained by engineering GB by varying the Sn⁴⁺–isovalent doping concentration. This study provides an important approach for improving the dielectric properties of co–doped TiO₂ without the creation of complex defect clusters inside the grains.     

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).     

Phase composition,crystal structure and microwave dielectric properties of Mg2−xCuxSiO4 ceramics

In this work, the Mg_2-xCu_xSiO₄(x = 0–0.40) microwave dielectric ceramics were prepared using solid-state reaction method. Compared with the Mg₂SiO₄ sample, the Cu-substituted Mg samples could be sintered at a lower temperature. The Mg_2?xCu_xSiO₄ ceramics exhibit the composite phases of Mg₂SiO₄ and a small quantity of MgSiO₃. The Cu²⁺ ion presented a solid solution with the Mg₂SiO₄ phase and preferentially occupy Mg(1) site. The distortion of MgO₆ octahedron was modified by Cu²⁺ ions, resulting in a positive change in the temperature coefficient of resonance frequency (τ_f) values. Excellent microwave dielectric properties of ε_r = 6.35, high Qf of ～ 188,500 GHz and near zero τ_f = ?2.0 ppm/°C were achieved at x = 0.08 under sintering at 1250 °C for 4 h. Thus, the fabricated ceramics were considered as possible candidates for millimeter-wave device applications.     

Investigation of the microwave dielectric properties of the Li4x/3Zn2−2xTi1+2x/3O4 system by P–V–L theory and vibration spectroscopy

Li_4x/3Zn_2–2xTi_1+2x/3O₄ microwave dielectric ceramics with a spinel phase were prepared via a high-temperature solid-phase method. P–V–L theory, vibration spectra, and XPS were utilized to establish the links between the intrinsic and extrinsic factors and the microwave dielectric properties. According to the characterization, the change in permittivity (ε_r) was ascribed to the increase in the average bond ionicity of Ti–O(Af_iTi-O) and the polar mode of the lattice vibration; the change in quality factor(Q × f) resulted from the change in the Ti–O lattice energy (AU_Ti-O) and existence of oxygen vacancy; the increase in temperature coefficient of the resonance frequency (τ_f) was triggered by the increase in the Ti–O bond energy. The Li_0.6Zn_1.1Ti_1.3O₄ ceramics (x = 0.45) sintered at 1125 °C finally obtained optimal microwave dielectric constants of ε_r = 17.3, Q × f = 76,318 GHz and τ_f = -58 ppm/°C.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.