High-Q Microwave Dielectrics in the (Mg1−xCox)2TiO4 Ceramics

The microwave dielectric properties and the microstructures of (Mg_{1− x} Co _x )₂TiO₄ ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x . The formation of solid solution (Mg_{1− x} Co _x )₂TiO₄ ( x =0.02–0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q × f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.7, Q × f ∼286 000 GHz at 10.4 GHz, τ_f∼−52.5 ppm/°C) was achieved for (Mg_0.95Co_0.05)₂TiO₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured (Mg_0.95Co_0.05)TiO₃ was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x =0.04–0.05 at 1360°–1420°C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications.     

Low-Loss Microwave Dielectric Ceramics Using (Mg1−xMnx)2TiO4 (x=0.02–0.1) Solid Solution

Composite ceramics in a solid solution of (Mg_{1− x} Mn _x )₂TiO₄ ( x =0.02–0.1) have been prepared by the mixed oxide route. Formation of the solid solution was confirmed by the X-ray diffraction, the EDX analysis, and the measured lattice parameters, which varied linearly from Mg₂TiO₄ ( a = b = c =8.4410 Å) to (Mg_0.9 Mn_0.1)₂TiO₄ ( a = b = c =8.4445 Å). The XRD analysis also confirmed the co-existence of a cubic-structured (Mg_{1− x} Mn _x )₂TiO₄ and an ilmenite-structured second phase (Mg_{1− x} Mn _x )TiO₃. The composition expected to have a maximum Q × f (276 200 GHz at 10.5 GHz) is (Mg_0.95Mn_0.05)₂TiO₄ with ɛ_r∼15.69 and τ_f∼−52.6 ppm/°C. The existence of the second phase, however, would lead to no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase.     

A/B Site Modified CaTiO3 Dielectric Ceramics for Microwave Application

(Li_1/2Nd_1/2)²⁺ substitution into the A site and (Mg_1/3Ta_2/3)⁴⁺ substitution into the B site of CaTiO₃ ceramic were investigated, respectively. The modified CaTiO₃ dielectric ceramics prepared by conventional solid-state method exhibit single perovskite structure and improved dielectric properties. Optimal microwave dielectric properties of ɛ_r=112.6, Q × f =4480 GHz, τ_f=8.2 ppm/°C in [Ca_0.4(Li_1/2Nd_1/2)_0.6] TiO₃ and ɛ_r=60.2, Q × f =36900 GHz, τ_f=−10.1 ppm/°C in Ca[Ti_0.4(Mg_1/3Ta_2/3)_0.6] O₃ are obtained, which indicates their potential for microwave application. The effects of change of crystal structure on dielectric properties are also discussed.     

The Li2O–Nb2O5–TiO2 Composite Microwave Dielectric Ceramics with Adjustable Permittivities

A group of new y M-phase/(1− y ) Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ ( x =0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li₂TiO₃ss" composition ( x value). The best dielectric properties of ɛ_r=40.1, Q × f values up to 9318 GHz, τ_f=25 ppm/°C, were obtained for the ceramics composites at x =0.02. Based on the good dielectric properties, the suitable "Li₂TiO₃ss" composition with x =0.02 was mixed with the Li_1.0Nb_0.6Ti_0.5O₃ powder as the ratio of y "M-phase"/(1− y ) "Li₂TiO₃ss" ( y =0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from ∼20 to ∼60, while Q × f and τ_f values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li₂TiO₃ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.     

High-Q Microwave Dielectric Materials Based on the Spinel Mg2TiO4

Composite ceramics based on the spinel Mg₂TiO₄ were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO₃. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q × f value of around 150–160 THz was obtained for the undoped Mg₂TiO₄, whereas a reduced Q × f value was observed for an increase in the Co concentration in the system (1− x )Mg₂TiO₄− x Co₂TiO₄. Upon doping with 7 mol% CaTiO₃, the Q × f value passed through a maximum with increasing Co concentration. Adding ZnO–B₂O₃ to the composite system based on Co-doped Mg₂TiO₄ resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q × f value.     

Low-Loss Microwave Dielectrics Using Mg2(Ti1−xSnx)O4 (x=0.01–0.09) Solid Solution

Low-loss ceramics having the chemical formula Mg₂(Ti_{1− x} Sn _x )O₄ for x ranging from 0.01 to 0.09 have been prepared by the conventional mixed oxide route and their microwave dielectric properties have been investigated. X-ray powder diffraction patterns indicate the corundum-structured solid solutions for the prepared compounds. In addition, lattice parameters, which linearly increase from 8.4414 to 8.4441 Å with the rise of x from 0.01 to 0.09, also confirm the forming of solid solutions. By increasing x from 0.01 to 0.05, the Q × f of the specimen can be tremendously boosted from 173 000 GHz to a maximum 318 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.57, Q × f ∼318 000 GHz at 10.8 GHz, τ_f∼−45.1 ppm/°C) was achieved for Mg₂(Ti_0.95Sn_0.05)O₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured Mg(Ti_0.95Sn_0.05)O₃ (ɛ_r∼16.67, Q × f ∼275 000 GHz at 10.3 GHz, τ_f∼−53.2 ppm/°C) was detected as a second phase. The presence of the second phase, however, would cause no significant variation in the dielectric properties of the specimen, because the second phase properties are very similar to the primary phase. These unique properties, in particular, low ɛ_r and high Q × f , can be utilized as a very promising dielectric material for ultra-high-frequency applications.     

Modification of MgAl2O4 Microwave Dielectric Ceramics by Zn Substitution

MgAl₂O₄ microwave dielectric ceramics were modified by Zn substitution for Mg, and their dielectric characteristics were evaluated, along with their structures. Dense (Mg_{1− x} Zn _x )Al₂O₄ ceramics were obtained by sintering at 1550°–1650°C in air for 3 h, and the (Mg_{1− x} Zn _x )Al₂O₄ solid solution was determined in the entire composition range. With Zn substitution for Mg, the dielectric constant ɛ of MgAl₂O₄ just varied from 7.90 to 8.56, while the Q × f value had significantly improved up to a maximal value of 106 000 GHz at x =1.0. Moreover, the τ_f of MgAl₂O₄ ceramics had declined from −73 to −63 ppm/°C.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

Preparation and Dielectric Properties of Low-Temperature-Sinterable (Zr0.8Sn0.2)TiO4 Powder

Low-temperature-sinterable (Zr_0.8Sn_0.2)TiO₄ powders were prepared using 3 mol% Zn(NO₃)₂ additive and then compared with powders prepared using 3 mol% ZnO additive and no additives. Sintering at 1200°C for 2 h produced a dielectric ceramic with ρ= 98.6% of theoretical density (TD), ɛ_r= 38.4, Q × f (GHz) = 42000, and τ _f =−1 ppm/°C. Sintering at 1250°C resulted in an excellent dielectric with ρ= 99% of TD, epsilon_r= 40.9, Q × f (GHz) = 49000, and τ _f =−2 ppm/°C. Scanning electron microscopy showed a microstructure with grains measuring 0.5 to 1 μm, and transmission electron microscopy revealed secondary phase in the grain boundaries.     

Low-Dielectric Loss Characteristics of Nd(Co1/2Ti1/2)O3 Ceramics at Microwave Frequencies

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ (NCT) ceramics using starting powders of Nd₂O₃, CoO, and TiO₂ prepared by the conventional solid-state route have been researched. The dielectric constant values (ɛ_r) saturated at 24.8–27. Quality factor ( Q × f ) values of 37 900–140 000 (at 9 GHz) and the measured τ_f values ranging from −45 to −48 ppm/°C can be obtained when the sintering temperatures are in the range of 1410°–1500°C. The ɛ_r value of 27, the Q × f value of 140 000 (at 9 GHz) and the τ_f value of −46 ppm/°C were obtained for NCT ceramics sintered at 1440°C for 4 h. For applications of high selective microwave ceramic resonator, filter, and antenna, NCT is proposed as a suitable material candidate.     

Dielectric Properties of (1−x)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic System at Microwave Frequency

The microwave dielectric properties of the (1− x )CaTiO₃– x Ca(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°–1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ɛ_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13 030 GHz, and the temperature coefficient τ_f decreased from 336 to −28 ppm/°C. When x =0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f∼−6 ppm/°C, Q × f ∼10 860 GHz and ɛ_r∼51 is obtained.     

Microwave Dielectrics in the (La1/2Na1/2)TiO3–Ca(Fe1/2Nb1/2)O3 System

Dielectric properties of the system (1 − x)(La_1/2Na_1/2)TiO₃– _x Ca(Fe_1/2Nb_1/2)O₃, where 0.4 # x # 0.6, have been investigated at microwave frequencies. The temperature coefficient of resonant frequency (τ_f), nearly 0 ppm/°C, was realized at x = 0.58. These ceramics had perovskite structure and showed relatively low dielectric losses. A new dielectric material applicable to microwave devices having Q · f of 12000–14000 GHz and a dielectric constant (ε_r) of 59–60 has been obtained at 1300–1350°C for 5–15 h sintering.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Tailoring the Microwave Dielectric Properties of MgNb2O6 and Mg4Nb2O9 Ceramics

The columbites MgNb₂O₆, MgTa₂O₆, and corundum-type Mg₄Nb₂O₉ ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the sintered samples were investigated by X-ray diffraction and scanning electron microscopic techniques. The microwave dielectric properties of the samples were measured by the resonance method in the frequency range 4–6 GHz. The dielectric properties have been tailored by forming a solid solution between MgNb₂O₆ and MgTa₂O₆ and by the substitution of TiO₂ for Nb₂O₅ in both MgNb₂O₆ and Mg₄Nb₂O₉ ceramics. The Mg(Nb_0.7Ta_1.3)O₆ has ɛ_r=29, Q _u× f =67 800 GHz, and τ_f=0.8 ppm/°C and the MgO–(0.4)Nb₂O₅–(1.5)TiO₂ composition has ɛ_r=34.5, Q _u× f =81 300 GHz, and τ_f=−2 ppm/°C.     

Effects of Ca/Ti Cosubstitution upon Microwave Dielectric Characteristics of CaSmAlO4 Ceramics

(Ca_{1+ x} Sm_{1− x} )(Al_{1− x} Ti _x )O₄ (0≤ x ≤0.4) ceramics were synthesized by solid-state reaction method and their microstructures and microwave dielectric properties were investigated. X-ray diffraction analysis and energy-dispersive X-ray analysis indicated that the matrix phase was a solid solution with a composition represented by the chemical formula (Ca_{1+ x} Sm_{1− x} ) (Al_{1− x} Ti _x )O₄ and minor amount of (Ca,Sm)(Al,Ti)O₃ secondary phase was detected. Ca/Ti cosubstitution could significantly improve the microwave dielectric characteristics of CaSmAlO₄ ceramics, and the excellent microwave dielectric characteristics were obtained in the modified ceramics as ɛ_r=19–23, Q × f =49 100–118 700 GHz, and τ_f=−15–15 ppm/°C.     

Effects of Calcium Substitution on the Structural and Microwave Dielectric Characteristics of [(Pb1−xCax)1/2La1/2](Mg1/2Nb1/2)O3 Ceramics

The effects of calcium substitution on the structural and microwave dielectric characteristics of [(Pb_{1− x} Ca _x )_1/2La_1/2](Mg_1/2Nb_1/2)O₃ ceramics (with x = 0.01–0.5) were investigated. All the materials were observed to have an ordered A(B_1/2^'B_1/2^")O₃-type perovskite structure; however, the space group of the structure changed from Fm 3 m to Pa 3 as the calcium content increased to x = 0.1, and then from Pa 3 to R 3¯ at the x = 0.5 composition. During the structural evolution, the lattice parameter of the perovskite cell decreased linearly, and the dielectric constant ( k ) also decreased, from k = 80 to k = 38. However, the product of the quality factor and the resonant frequency ( Q × f ) increased from 50 000 GHz to 90 000 GHz as the calcium content increased. Also, the temperature coefficient of resonant frequency (τ_ƒ) gradually changed from 120 ppm/°C to −40 ppm/°C as the calcium content increased. At the x = 0.3 composition, a combination of properties— k ∼ 50, Q × f ∼ 86 000 GHz, and τ_ƒ∼ 0 ppm/°C—can be obtained.     

Microwave Dielectric Properties of Li2TiO3 Ceramics Doped with ZnO–B2O3 Frit

A type of new low sintering temperature ceramic, Li₂TiO₃ ceramic, has been found. Although it is difficult for the Li₂TiO₃ compound to be sintered compactly at temperatures above 1000°C for the volatilization of Li₂O, dense Li₂TiO₃ ceramics were obtained by conventional solid-state reaction method at the sintering temperature of 900°C with the addition of ZnO–B₂O₃ frit. The sintering behavior and microwave dielectric properties of Li₂TiO₃ ceramics with less ZnO–B₂O₃ frit (≤3.0 wt%) doping were investigated. The addition of ZnO–B₂O₃ frit can lower the sintering temperature of the Li₂TiO₃ ceramics, but it does not apparently degrade the microwave dielectric properties of the Li₂TiO₃ ceramics. Typically, the good microwave dielectric properties of ɛ_r=23.06, Q × f =32 275 GHz, τ_f = 35.79 ppm/°C were obtained for 2.5 wt% ZnO–B₂O₃ frit-doped Li₂TiO₃ ceramics sintered at 900°C for 2 h. The porosity was 0.08%. The Li₂TiO₃ ceramic system may be a promising candidate for low-temperature cofired ceramics applications.     

Origin of Microwave Dielectric Loss in ZnNb2O6-TiO2

(1− x )ZnNb₂O₆· x TiO₂ ceramics were prepared using both anatase and rutile forms of TiO₂. At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb₂O₈) and rutile was observed and the temperature coefficient of resonant frequency (τ_f) was ∼0 ppm/°C. We found that although ɛ_r and τ_f were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb₂O₆·0.58TiO₂ prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti⁴⁺. It is suggested that the increase of dielectric loss originates from an increase in Ti³⁺ and oxygen vacancies due to an anatase–rutile phase transition.