Sintering behavior and microwave dielectric properties of Ca1-xBixW1-xVxO4 ceramics

Structure, sintering behavior and microwave dielectric properties of ceramics have been investigated by x-ray powder diffraction (XRD) and scanning electron microscopy (SEM) in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 6–11 GHz. The sintering temperature and microwave dielectric properties could be successfully tuned in a wide window simultaneously by adjusting the A–O bond characteristics. The sintering temperature of CaWO₄ was successfully reduced from 1100 °C to about 950 °C by BiVO₄ addition. Approximately 95%–96% theoretical density could be obtained after sintering at 950 °C for 2 h. All samples exhibit single Scheelite structure (I4₁/a) phase. The dielectric constant increased, whereas the Q×f value decreased, with the increase of x. The τ_f value changed from negative to positive with the increases of x. Combined excellent microwave dielectric properties with ε_r=22. 1, Q×f=16,730 GHz and τ_f=2.39 ppm/°C could be obtained after sintered at the 950 °C for 2 h for x=0.3 compositions.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Preparation and microwave dielectric properties of low-loss MgZrNb2O8 ceramics

A novel low-loss microwave dielectric material MgZrNb₂O₈ was reported for the first time. Single-phase MgZrNb₂O₈ was prepared by a conventional mixed-oxide route and sintered in the temperature range of 1280–1360 °C. The microstructure and microwave dielectric properties were investigated systematically. The X-ray diffraction results showed that all samples exhibit a single wolframite structure. When the sintering temperature was lower than 1340 °C, the Q×f value mainly depended on the relative density. However, when the sintering temperature was above 1340 °C, the Q×f value mainly relied on the grain morphology in addition to the density. The MgZrNb₂O₈ ceramic sintered at 1340 °C for 4 h exhibited excellent microwave dielectric of ε_r=26, Q×f=120,816 GHz (where f=6.85 GHz), and τ_f=?50.2 ppm/°C. These results demonstrate that MgZrNb₂O₈ could be a promising candidate material for the application of highly selective microwave ceramic resonators and filters.     

Phase structure,band gap and microwave dielectric properties of Ba8Ti3Nb4−xSbxO24 ceramics

High dielectric constant and low loss ceramics in the Ba₈Ti₃Nb_4?xSb_xO₂₄ (x=0–2) system were prepared by conventional solid-state ceramic route. As x increased from 0 to 1.5, a single phase with hexagonal 8H perovskite structure was formed and the band gap values increased from 3.38 to 3.47 eV. However, the Sb₂O₃ secondary phase was detected as the x reached 2. The optimum sintering temperature was reduced from 1460 to 1380 °C, the quality factors (Q×f) were effectively enhanced from 22,900 to 38,000 GHz and τ_f was significantly lowered from 110 ppm/°C to 2 ppm/°C, whereas the dielectric constant decreased from 49 to 35. A good combined microwave dielectric properties with ε_r=37.5, Q×f=38,000 GHz, τ_f=15 ppm/°C were obtained for x=1.5.     

Dielectric properties of high-Q (Mg1–xZnx)1.8Ti1.1O4 ceramics at microwave frequency

(Mg_1?xZn_x)_1.8Ti_1.1O₄ (x = 0.03–1.00) ceramics were prepared via the conventional solid-state method and their dielectric properties were investigated in the microwave frequency region. Formation of solid solutions was confirmed by the X-ray diffraction analysis, the EDX analysis, and the measured lattice parameters. A small amount of Zn substitution for Mg produced a large increase in Q × f due to a high packing fraction as well as a high relative density of the ceramics. By increasing x from 0.00 to 0.06, the Q × f of the specimen could be tremendously improved from 141,000 to a maximum of 210,700 GHz (at 10.52 GHz), demonstrating an unique potential for low-loss microwave applications. The τ_f values were found to retain in the range from ?54.2 to ?62.3 ppm/°C for all compositions because the resultant change of unit cell volume was small. Also, a remarkable lowering of the sintering temperature down to ～300 °C was observed when Mg was totally substituted by Zn, thereby making it possible for low firing applications.     

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.     

Two novel ultralow temperature firing microwave dielectric ceramics LiMVO6 (M = Mo,W) and their chemical compatibility with metal electrodes

Low temperature cofired ceramics technology (LTCC) has been widely studied and used in wireless communication because of their outstanding capability for device miniaturization and integration. However, many commercial microwave dielectric materials have high sintering temperatures that pose challenge for cofiring with inner electrodes. Herein, two brannerite vanadate LiMVO₆ (M = Mo, W) ceramics with intrinsically low sintering temperatures were prepared. Dense and stable LiMVO₆ (M = Mo, W) ceramics could obtained at 640 °C for LiMoVO₆ and 700 °C for LiWVO₆. Favorable microwave dielectric properties were also obtained with ε_r = 13.3, Q × f = 12,460 GHz, and τ_f = +101.0 ppm/°C for LiMoVO₆ and ε_r = 11.5, Q × f = 13,260 GHz, and τ_f = +163.8 ppm/°C for LiWVO₆. Moreover, the relationship between crystal structure and microwave dielectric properties was studied by means of packing fraction, bond valence, and octahedral distortion. Their chemical compatibility with the metal electrodes were confirmed.     

Low-temperature sintered MgWO4–CaTiO3 ceramics with near-zero temperature coefficient of resonant frequency

The phases, microstructure, composition analysis and microwave dielectric properties of (1 ? x)MgWO₄–xCaTiO₃ ceramics with Li₂CO₃–4H₃BO₃ additions prepared by solid-state reaction method have been investigated by using X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and advantest network analyzer. The τ_f of (1 ? x)MgWO₄–xCaTiO₃ were dependent on phase constitutions. The microwave dielectric properties of 0.91MgWO₄–0.09CaTiO₃ ceramics with Li₂CO₃–4H₃BO₃ were characterized, the results indicated that the ?_r and Q × f were associated with the sintering temperature and amount of Li₂CO₃–4H₃BO₃. The sintering temperature of ceramics was reduced to 950 °C from 1150 °C and τ_f was modified to 0 ppm/°C with good Q × f. Addition of 5.0 wt% Li₂CO₃–4H₃BO₃ in 0.91MgWO₄–0.09 CaTiO₃ ceramics sintered at 950 °C showed excellent dielectric properties of ?_r = 15.5, Q × f = 20,780 GHz (f = 7.1 GHz) and τ_f ～ 0 ppm/°C. The material has a chemical compatibility with silver, making it a very promising candidate materials for LTCC applications.     

Structure and properties of low temperature sintered ZnTa2O6 microwave dielectric ceramics

ZnTa₂O₆ microwave dielectric ceramics have been prepared using ZnTa₂O₆ nano-powders synthesized by sol–gel processing in this study. The crystal structure and microstructure of the ZnTa₂O₆ powders and ceramics were characterized by XRD and SEM techniques. ZnTa₂O₆ ceramics can be densified at a lower sintering temperature of 1200 °C. Microwave dielectric properties show that both of Q × f and ?_r values are lower than those of ceramics prepared by solid state route, and the τ_f values do not show different from that of solid state route. ZnTa₂O₆ ceramics sintered at 1200 °C exhibit good microwave dielectric properties: Q × f = 50,600 GHz, ?_r = 35.12 and τ_f = 9.69 ppm/°C.     

Microwave dielectric characteristics of SrLaGaO4 and SrNdGaO4 ceramics

SrLnGaO₄ (Ln = La and Nd) ceramics with K₂NiF₄ structure were prepared by solid-state reaction approach, and the microwave dielectric properties and microstructures were characterized. The SrLaGaO₄ and SrNdGaO₄ ceramics with minor secondary phase, Sr₃Ga₂O₆, were obtained by sintering at 1250–1350 °C for 3 h, and good microwave dielectric characteristics were achieved: the ceramics had (1) ɛ = 20.3, Q × f = 16,219 GHz, and τ_f = −33.5 ppm/°C for SrLaGaO₄; and (2) ɛ = 21.4, Q × f = 16,650 GHz, and τ_f = 7.1 ppm/°C for SrNdGaO₄.     

Microwave dielectric properties of Re3Ga5O12 (Re: Nd,Sm, Eu,Dy and Yb) ceramics and effect of TiO2 on the microwave dielectric properties of Sm3Ga5O12 ceramics

Re₃Ga₅O₁₂ (Re: Nd, Sm, Eu, Dy and Yb) garnet ceramics sintered at 1350–1500 °C had a high quality factor (Q × f) ranging from 40,000 to 192,173 GHz and a low dielectric constant (ɛ_r) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (τ_f) in the range of −33.7 to −12.4 ppm/°C. In order to tailor the τ_f value, TiO₂ was added to the Sm₃Ga₅O₁₂ ceramics, which exhibited good microwave dielectric properties. The relative density and grain size increased with addition of TiO₂, resulting in the enhancement of Q × f value. The τ_f increased with the addition of TiO₂. Excellent microwave dielectric properties of ɛ_r = 12.4, Q × f = 240,000 GHz and τ_f = −16.1 ppm/°C were obtained from the Sm₃Ga₅O₁₂ ceramics sintered at 1450 °C for 6 h with 1.0 mol% TiO₂. Therefore, Re₃Ga₅O₁₂ ceramics, especially TiO₂-added Sm₃Ga₅O₁₂ ceramics are good candidates for advanced substrate materials in microwave integrated circuits (MICs) applications.     

Structure and microwave dielectric properties of Ba1−xSrxMg2V2O8 ceramics

The Ba_1−xSr_xMg₂V₂O₈ (0≤x≤0.4) microwave dielectric ceramics were fabricated by a standard solid-state reaction method. The formation of a continuous solid solution within the whole composition range was identified. The ceramic samples could be well densified in the temperature range of 885–975 °C in air for 4 h. The permittivity ε_r was found to increase with increasing ionic polarizabilities. The Q×f values were believed to be closely related with packing fraction and grain refinement. The Sr²⁺ substitution contributed to a monotonous increase of the A-site bond valence, such that the τ_f value experienced a considerable variation from negative to positive values. The optimum microwave dielectric properties of an ε_r of 13.3, a high Qxf of 86,640 GHz (9.6 Hz) and a near-zero τ_f of −6 ppm/°C could be yielded in the x=0.15 sample when sintered at 915 °C for 4 h.     

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A = Zn,Co)

The microwave dielectric properties of low-loss A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics were discussed systematically. The Zn_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛ_r = 37.4, Q × f = 194,000 (GHz), and τ_f = −58 ppm/°C and Co_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as CTN) had ɛ_r = 64, Q × f = 65,300 (GHz), and τ_f = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τ_f are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A_0.5Ti_0.5NbO₄ (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.     

The effect of non-stoichiometry on the microstructure and microwave dielectric properties of the columbites A2+Nb2O6

Both the microstructure and microwave dielectric properties of sintered columbites with the composition A²⁺_1+xNb₂O₆ (A²⁺ = Mg, Zn, Co) were analysed. The slight changes in the Nb/A ratio in studied systems have been found to noticeably affect the microwave quality factor (Q) of the ceramics. Regardless of the A²⁺ ion the dependencies of a Q-factor magnitude on chemical composition demonstrate rather a non-linear trend. Very low products Q × f measured for all studied systems at x < 0 can be attributed to the presence of secondary phase Nb₂O₅. At x ≥ 0 the Q × f magnitude passes through the maxima subject to the A²⁺ ion. The highest products Q × f ≥ 110,000 GHz have been found in the Mg and Zn-containing columbites at x = 0.03 and 0.01, respectively, whereas in the case of Co-containing analogues the maximum Q × f = 82,000 GHz corresponds to stoichiometric composition.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Microwave dielectric properties of La3Ti2TaO11 ceramics with perovskite-like layered structure

The first characterization of the microwave dielectric properties of the La₃Ti₂TaO₁₁ ceramics is presented. An ordinarily sintered ceramic at 1560 °C exhibits good microwave dielectric properties with ?_r = 46, Q × f = 7500 GHz and τ_f = ?47 ppm/°C. An alternative approach to tailor the temperature coefficient of resonate frequency of La₃Ti₂TaO₁₁ ceramics is also presented. Textured La₃Ti₂TaO₁₁ ceramics were fabricated using spark plasma sintering (SPS). By controlling the sintering temperature, orientation degree increased together with the steadily increase in ?_r and Q × f. A noteworthy change in τ_f from ?43.1 ppm/°C to ?13.6 ppm/°C with increasing orientation degree was observed. These results suggest that grain-orientation control was an effective way to tailor the microwave dielectric properties of La₃Ti₂TaO₁₁ ceramics.     

Structure and properties analysis for low-loss (Mg1−xCox)TiO3 microwave dielectric materials prepared by reaction-sintering method

(Mg_1?xCo_x)TiO₃ (x = 0.00–0.09) ceramics were prepared by reaction sintering method. The correlations of crystal structure and the properties were analyzed on the basis of Rietveld refinement for specimens with different x. For x ≤ 0.5, solid solution phases with the ilmenite structure were obtained, which can be confirmed by the X-ray diffraction patterns and the lattice parameters measured. Whereas for x > 0.05, secondary phase was detected besides the main phase. Dielectric properties were closely related with theoretical dielectric polarizabilities, packing fraction and average octahedral distortion. By increasing x from 0.00 to 0.09, the Q × f value of the specimen decreased from a maximum of 289,400 GHz to 228,100 GHz. A fine combination of microwave dielectric properties (?_r ～ 16.1, Q × f ～ 289,400 GHz, τ_f  ～ ?54.4 × 10^?6/°C) was achieved for MgTiO₃ ceramics sintered at 1350 °C for 4 h.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co1/3Nb2/3)O3 ceramics

The effect of B-site cation deficiency on the structure and microwave dielectric properties of Ba(Co_1/3Nb_2/3)O₃ (BCN) was investigated. Stoichiometric and co-deficient compositions based on Ba(Co_1/3−xNb_2/3)O₃ [x = 0.0, 0.01, 0.02, 0.03 and 0.04] were prepared using the conventional mixed oxide route. Small amounts of V₂O₅ (0.1 wt%) were added to promote densification. The dielectric loss is very sensitive to the composition; it was found that co-deficiency degraded the microwave dielectric properties. The stoichiometric formulation (x = 0) exhibited the best microwave properties. The improvements in the microwave dielectric properties were achieved by increasing the degree of 1:2 cation ordering. The highly ordered, stoichiometric BCN ceramics showed a relative permittivity (ɛ_r) of 32, quality factor (Q × f) of 66,500 GHz and a negative temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C at 4 GHz.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Microwave dielectric properties of low-fired Li2MnO3 ceramics co-doped with LiF–TiO2

Li₂MnO₃ ceramics co-doped with 2 wt% LiF and x wt% TiO₂ (x=0, 3, 5, 7, 10) were prepared by solid-state reaction for low-temperature co-fired ceramics (LTCC) applications. The sintering temperatures of Li₂MnO₃ ceramics were successfully lowered to 925°C due to the formation of a LiF liquid phase. Their temperature stability was improved by doping with TiO₂. A typical Li₂MnO₃-2 wt% LiF-5 wt% TiO₂ sample with well-densified microstructures displayed optimum dielectric properties (ε_r=13.8, Q×f= 23,270 GHz, τ_f=1.2 ppm/°C). Such sample was compatible with Ag electrodes, which suggests suitability of the developed material for LTCC applications in wireless communication systems.