Microwave dielectric properties and low-temperature sintering of Ba3Ti4Nb4O21 ceramics with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ɛ_r), high quality factor (Q × f) and small temperature coefficient of resonant frequency (τ_f) are required for the application of chip passive components in the wireless communication technologies. In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₄Nb₄O₂₁ ceramics were investigated as a function of B₂O₃–CuO content. Ba₃Ti₄Nb₄O₂₁ ceramics with B₂O₃ or CuO addition could be sintered above 1100 °C. However, the additions of both B₂O₃ and CuO successfully reduced the sintering temperature of Ba₃Ti₄Nb₄O₂₁ ceramics from 1350 to 900 °C without detriment to the microwave dielectric properties. From the X-ray diffraction (XRD) studies, the sintering behaviors and the microwave dielectric properties of low-fired Ba₃Ti₄Nb₄O₂₁ ceramics were examined and discussed in the formation of the secondary phases. The Ba₃Ti₄Nb₄O₂₁ sample with 1 wt% B₂O₃ and 3 wt% CuO addition, sintered at 900 °C for 2 h, had the good dielectric properties: ɛ_r = 65, Q × f = 16,000 GHz and τ_f = 101 ppm/°C.     

Effects of SrTiO3 additives on the structure and microwave dielectric properties of Ba4.2Sm9.2Ti18O54 ceramics

The effects of SrTiO₃ additives on the microstructure and the dielectric properties of Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) materials have been investigated. The microstructure of BST doped with SrTiO₃ was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA). The XRD, SEM and EPMA results showed that small amounts of SrTiO₃ (less than 4 wt.%) could be accommodated in BST while a secondary metatitanate phase appeared in the samples with 8 wt.% SrTiO₃ added, the addition of SrTiO₃ increased the lower solubility limit (x values) of Ba_6?3xSm_8+2xTi₁₈O₅₄. The relative permittivities and the TC_f values showed an approximately linear increase with increasing additions of SrTiO₃. It was observed that near zero TC_f values could be achieved with the addition of SrTiO₃ to BST ceramics. With 4 wt.% SrTiO₃ addition and sintered at 1360 °C for 2 h, BST ceramics exhibited excellent dielectric properties: ?_r = 81.2, Q_f = 8470 GHz and TC_f = ?1.5 ppm/°C.     

Effect of dopants on synthesis of BaTi4O9 and Ba2Ti9O20 ceramics prepared by reaction-sintering process

The effect of dopants on BaTi₄O₉ (BT4) and Ba₂Ti₉O₂₀ (B2T9) ceramics by the reaction-sintering process was investigated. CuO addition is more effective in lowering the sintering temperature of BT4 and B2T9 ceramics. MnO₂ and CuO addition are effective to obtain temperature stable BT4 ceramics. With MnO₂ addition, Q × f of BT4 ceramics could be raised. ZrO₂ addition is effective to obtain B2T9 ceramics with higher dielectric constant. With CuO addition, τ_f of B2T9 ceramics shifted toward negative values and 0 ppm/°C could be obtained. Optimum properties in BT4 doped with MnO₂ of ɛ_r = 37.1, Q × f = 51,200 GHz (at 7 GHz) and τ_f = 0 ppm/°C and in B2T9 doped with ZrO₂ of ɛ_r = 37.9, Q × f = 39,700 GHz (at 7 GHz) and τ_f = 5.9 ppm/°C were obtained.     

Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 with ZnO–B2O3 glass additions for LTCC applications

The effects of ZnB₂O₄ glass additions on the sintering temperature and microwave dielectric properties of Ba₃Ti₅Nb₆O₂₈ have been investigated using dilatometer, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and a network analyzer. The pure Ba₃Ti₅Nb₆O₂₈ system showed a high sintering temperature (1250 °C) and had the good microwave dielectric properties: Q × f of 10,600 GHz, ɛ_r of 37.0, τ_f of −12 ppm/°C. It was found that the addition of ZnB₂O₄ glass to Ba₃Ti₅Nb₆O₂₈ lowered the sintering temperature from 1250 to 925 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases. Also the addition of ZnB₂O₄ glass enhanced the microwave dielectric properties: Q × f of 19,100 GHz, ɛ_r of 36.6, τ_f of 5 ppm/°C. From XPS and XRD studies, these phenomena were explained in terms of the reduction of oxygen vacancies and the formation of secondary phases having the good microwave dielectric properties.     

Microwave dielectric properties of Ba2Ti9O20 materials prepared by reaction sintering process

We synthesized the Ba₂Ti₉O₂₀ materials by direct reacting the nano-sized BaTiO₃ with TiO₂ powders. Pure Holland-like structure was obtained when the mixture of the nano-powders were calcined at 1100 °C. The influence of the processing method on the characteristics of the Ba₂Ti₉O₂₀ powders and the related microwave dielectric properties for the sintered Ba₂Ti₉O₂₀ materials was investigated. The combined high-energy-milling and ball-milling (HeM/BM) process results in pronounced improvement for the sinterability of the Ba₂Ti₉O₂₀ powders such that the samples attain a density higher than 96.2% T.D. when sintered at 1200 °C/4 h. Such a sintering temperature is significantly lower than the one needed for densifying the BM-processed samples to the same high density (1350 °C/4 h). However, the HeM/BM-processed samples show pronouncedly inferior microwave quality factor (Q × f-value) to the BM-processed ones. Raman spectroscopic and SEM microstructural analyses imply that the prime factor resulting in lower Q × f-value for HeM/BM-processed samples is the incomplete development of the granular structure.     

Low temperature sintering of BaCu(B2O5)-added BaO–Re2O3–TiO2 (Re = Sm,Nd) ceramics

The sintering temperature of BaSm₂Ti₄O₁₂ (BST) and BaNd₂Ti₅O₁₄ (BNT) ceramics was approximately 1350 °C and decreased to 875 °C with the addition of BaCu(B₂O₅) (BCB) ceramic powder. The presence of the liquid phase was responsible for the decrease of the sintering temperature. The liquid phase is considered to have a composition similar to the BaO-deficient BCB. The bulk density and dielectric constant (ɛ_r) of the specimens increased and reached saturated value with increasing BCB content. The Q-value initially increased with the addition of BCB, but decreased considerably when a large amount of BCB was added, because of the presence of the liquid phase. Good microwave dielectric properties of Q × f = 4500 GHz, ɛ_r = 60 and τ_f = −30 ppm/°C were obtained for the 16.0 mol% BCB-added BST ceramics sintered at 875 °C for 2 h. Moreover, the BST and BNT ceramics containing BCB show good compatibility with silver metal.     

The effect of sintering temperature and time on microwave properties of Sm(Zn1/2Ti1/2)O3 ceramics for resonators

The microwave dielectric properties of Sm(Zn_1/2Ti_1/2)O₃ ceramics have been investigated. Sm(Zn_1/2Ti_1/2)O₃ ceramics were prepared by conventional solid-state route with various sintering temperatures and times. The prepared Sm(Zn_1/2Ti_1/2)O₃ exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. Higher sintered density of 7.01 g/cm³ can be produced at 1310 °C for 2 h. The dielectric constant values (ɛ_r) of 22–31 and the Q × f values of 4700–37,000 (at 8 GHz) can be obtained when the sintering temperatures are in the range of 1250–1370 °C for 2 h. The temperature coefficient of resonant frequency τ_f was a function of sintering temperature. The ɛ_r value of 31, Q  ×  f value of 37,000 (at 8 GHz) and τ_f value of −19 ppm/°C were obtained for Sm(Zn_1/2Ti_1/2)O₃ ceramics sintered at 1310 °C for 2 h. For applications of high selective microwave ceramic resonator, filter and antenna, Sm(Zn_1/2Ti_1/2)O₃ is proposed as a suitable material candidate.     

Low temperature sintering of the Ba2Ti9O20 ceramics using B2O3/CuO and BaCu(B2O5) additives

The effects of B₂O₃/CuO and BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of Ba₂Ti₉O₂₀ ceramics were investigated. The B₂O₃ added Ba₂Ti₉O₂₀ ceramics were not able to be sintered below 1000 °C. However, when both CuO and B₂O₃ were added, they were sintered below 900 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the Ba₂Ti₉O₂₀ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the Ba₂Ti₉O₂₀ ceramics. Good microwave dielectric properties of Qxf = 16,000 GHz, ɛ_r = 36.0 and τ_f = 9.11 ppm/°C were obtained for the Ba₂Ti₉O₂₀ ceramics containing 10.0 mol% BaCu(B₂O₅) sintered at 875 °C for 2 h.     

Effect of B2O3 on the sintering and microwave dielectric properties of M-phase LiNb0.6Ti0.5O3 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics have been investigated. It is found that low-level doping of B₂O₃ (≤2 wt.%) can significantly improve the densification and dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics. Due to the liquid phase effect of B₂O₃ addition, LiNb_0.6Ti_0.5O₃ ceramics could be sintered to a theoretical density higher than 95% even at 880 °C. No secondary phase was observed for the B₂O₃-doped ceramics. There is no obvious degradation in dielectric properties for the ceramics with B₂O₃ additions. In the case of 1 wt.% B₂O₃ addition, the ceramics sintered at 880 °C show good microwave dielectric properties of ɛ_r = 70, Q × f = 5400 GHz, τ_f = −6.39 ppm/°C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) applications.     

Effect of Al2O3 on the structure and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3

The effect of excess Al₂O₃ on the densification, structure and microwave dielectric properties of Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) was investigated. CTLA ceramics were prepared using the conventional mixed oxide route. Excess Al₂O₃ in the range of 0.1–0.5 wt% was added. It was found that Al₂O₃ improved the densification. A phase rich in Ca and Al was found in the microstructure of Al₂O₃ doped samples. Additions of Al₂O₃ coupled with the slow cooling after sintering improved the microwave dielectric properties. CTLA ceramics with 0.25 wt% Al₂O₃ cooled at 5 °C/h showed high density and a uniform grain structure with ɛ_r = 46, Q × f = 38,289 and τ_f = +12 ppm/°C at 4 GHz. XRD and TEM examinations showed the presence of (1 1 2) and (1 1 0) type twins arising from a⁻a⁻c⁺ tilt system with the presence of anti-phase domain boundaries from the displacement of A-site cations of the orthorhombic perovskite structure.     

Microwave dielectric properties of tungstenbronze type like (Ba1−αSrα)6−3xR8+2xTi18O54 (R = Sm,Nd) solid solutions

Tungstenbronze type like Ba_6−3xR_8+2xTi₁₈O₅₄ (R = Sm or Nd) dielectric ceramics reveal high quality factor Q·f as well as high dielectric constant ɛ_r. We have investigated the effect of Sr substitution for Ba ions on the microwave dielectric properties of the compounds. (Ba_1−αSr_α)_6−3xR_8+2xTi₁₈O₅₄ (R = Sm or Nd) ceramics were prepared in the composition ranges of x = 0–0.2 and α = 0–0.312 and the microwave dielectric properties were investigated. (Ba_1−αSr_α)_6−3xSm_8+2xTi₁₈O₅₄, where x = 0.1 and α = 0.298, and (Ba_1−αSr_α)_6−3xNd_8+2xTi₁₈O₅₄, where x = 0.2, α = 0.296 revealed remarkably higher Q·f value among the solid solutions, indicating that Q·f increased with substituting Sr ions into Ba ions at the rhombic A1-site. This fact suggests that relaxation of local distortions at the A1-sites is closely related to improvement of Q·f.     

Microwave dielectric properties and crystal structure of the tungstenbronze-type like (Ba1 − αSrα)6(Nd1 − βYβ)8Ti18O54 solid solutions

A study of the dielectric properties, especially the Q × f value, of the tungstenbronze-type like (Ba_1 − αSr_α)_6 − 3xNd_8 + 2xTi₁₈O₅₄ solid solutions in x = 0 system was carried out. These compositions near x = 0 have very low Q × f values. To improve the Q × f value of these materials, we tried two substitutional systems, which are (Ba_1 − αSr_α)₆Nd₈Ti₁₈O₅₄ and Ba₄Sr₂(Nd_1 − βY_β)₈Ti₁₈O₅₄. In the former composition, the Q × f value was increased from 206 to 5880 GHz in the range of 0 ≤ α ≤ 0.5. And we found that Sr ions substituted for Ba ions in A1 sites have good effect on increasing the Q × f value, but Sr ions substituted for Ba ions in A2 sites have poor effect on increasing it. The latter composition also has a small effect on increasing the Q × f value.     

Low-temperature sintering and microwave dielectric properties of Nd(Co1/2Ti1/2)O3 ceramics using glass addition of oxides

The microstructures and microwave dielectric characteristics of complex perovskite Nd(Co_1/2Ti_1/2)O₃ ceramics with 60P₂O₅–15ZnO–5La₂O₃–5Al₂O₃–5Na₂O–5MgO–5Yb₂O₃ (PZLANMY) additions (1–4 wt%) prepared through the conventional solid-state route were investigated. It was found that Nd(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1210 °C owing to the sintering aid of PZLANMY-glass addition. At 1300 °C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 1 wt% of PZLANMY-glass addition possess a dielectric constant (ε_r) of 27, a Q×f value of 64,000 GHz and a temperature coefficient of resonant frequency (τ_f) of ?29 ppm/°C. The PZLANMY-glass doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices that require low sintering temperature.     

Effect of reaction sintering process on the microwave dielectric properties of Ba2Ti9O20 materials

The pre-reaction step, which is the holding time at 1000 °C in reaction-sintering process, was observed to influence the characteristics of the Ba₂Ti₉O₂₀ materials profoundly. For the materials possessing high sintered density and of pure Hollandite-like structure, the uniformity of granular structure is the prime factor, altering the microwave dielectric properties of the samples. Utilizing the BaTi₄O₉ + BaTi₅O₁₁ mixtures to replace for the 2BaTiO₃ + 7TiO₂ ones pronouncedly improved the uniformity of the microstructure and thus markedly increased the Q × f-value for the Ba₂Ti₉O₂₀ materials. The possible explanation for such a phenomenon is that the B-series mixture forms the Ba₂Ti₉O₂₀ Hollandite-like phase in a simpler way and results in more uniform microstructure then the A-series mixture. The microwave properties of B-series materials are superior to those of A-series ones, achieving K = 38.2 and Q × f-value = 36,000 GHz for the samples, which were pre-reacted at 1000 °C for 6 h and were sintered at 1410 °C for 4 h by reaction-sintering processes.     

“Dark hole” cure in Ba4.2Nd9.2Ti18O54 microwave dielectric ceramics

Large size Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT) ceramics doped with MnCO₃, CuO and CoO were prepared by the conventional solid-state method. Only a single BaNd₂Ti₄O₁₂ phase was formed in all samples. No second phase was found in the XRD patterns. The bulk density increases slightly because of the dopants. The SEM results showed that the grain size of Mn²⁺and Cu²⁺-doped BNT ceramics became larger with the increasing amount of dopants. The permittivity of all samples stays the same. However, the Q×f value of BNT ceramics increases by doping, especially with Mn²⁺ ions. The conductivity of BNT ceramic doped with Mn²⁺(0.5 mol‰) under high temperature is lower than that without doping. There are fewer defects in Mn²⁺-doped BNT ceramics. The XPS results indicated that Ti reduction was suppressed in BNT ceramics doped with 0.5 mol‰ Mn²⁺. BNT ceramics doped with 0.5 mol‰ Mn²⁺ ions sintered at 1320 °C for 2 h exhibited good microwave dielectric properties, with ε_r=88.67, Q×f=7408 GHz and τf = 82.98 ppm/°C.     

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.     

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.     

Effect of Li2O–V2O5 on the low temperature sintering and microwave dielectric properties of Li1.0Nb0.6Ti0.5O3 ceramics

Li₂O–Nb₂O₅–TiO₂ based ceramic systems have been the candidate materials for LTCC application, due to their high dielectric constant and Q × f value and controllable temperature coefficient in the microwave region. However, the sintering temperature was relatively higher (above 1100 °C) for practical application. In this study, dielectric properties of Li_(1+x−y)Nb_(1−x−3y)Ti_(x+4y)O₃ solid solution were studied with different x and y contents and among them, the Li_1.0Nb_0.6Ti_0.5O₃ composition (x = 0.1, y = 0.1) was selected, due to its reasonable dielectric properties to determine the possibility of low temperature sintering. The effects of 0.17Li₂O–0.83V₂O₅, as a sintering agent, on sinterability and microwave dielectric properties of Li_1.0Nb_0.6Ti_0.5O₃ ceramics were investigated as a function of the sintering agent content and sintering temperature. With addition of 0.17Li₂O–0.83V₂O₅ above 0.5 wt%, the specimens were well densified at a relatively lower temperature of 850 °C. Only slight decrease in apparent density was observed with increasing 0.17Li₂O–0.83V₂O₅ content above 0.75 wt%. In the case of 0.5 wt% 0.17Li₂O–0.83V₂O₅ addition, the values of dielectric constant and Q × f reached maximum. Further addition caused inferior microstructure, resulting in degraded dielectric properties. For the specimens with 0.5 wt% 0.17Li₂O–0.83V₂O₅ sintered at 850 °C, dielectric constant, Q × f and TCF values were 64.7, 5933 GHz and 9.4 ppm per °C, respectively.     

Microwave dielectric properties and crystal structure of homologous compounds ALa4Ti4O15 (A = Ba,Sr and Ca) for base station applications

Homologous compounds ALa₄Ti₄O₁₅ (A = Ba, Sr and Ca) with high dielectric constant ϵ_r and quality factor Q × f are candidate materials for dielectric resonators in base station of telecommunication systems. We have investigated the relationship between the microwave dielectric properties and crystal structure of these new materials. Single crystals of CaLa₄Ti₄O₁₅ were synthesized to analyze the crystal structure precisely. The ceramic discs of the ALa₄Ti₄O₁₅ (A = Ba, Sr and Ca) were also synthesized and the microwave dielectric properties were measured. Ba-analogy showed the highest ϵ_r of 44.4 due to the large cationic movement. Ca-analogy showed the highest Q × f of 50,246 GHz due to resemblance in ionic radius between Ca²⁺ (r = 1.34 Å: 12-coordination) and La³⁺ (r = 1.36 Å: 12-coordination) ions. Sr-analogy showed near zero temperature coefficient of resonant frequency τ_f of −8.4 ppm/°C compared with the others. The relationships between their crystal structures and properties were discussed.     

Structural ordering and dielectric properties of Ba3CaNb2O9-based microwave ceramics

Ba₃CaNb₂O₉ is a 1:2 ordered perovskite which presents a trigonal cell within the D_3d³ space group. Dense ceramics of Ba₃CaNb₂O₉ were prepared by the solid-state reaction route, and their microwave dielectric features were evaluated as a function of the sintering time. From Raman spectroscopy, by using group-theory calculations, we were able to recognize the coexistence of the 1:1 and 1:2 ordering types in all samples, in which increasing the sintering time tends to reduce the 1:1 domain, leading to an enhancement of the unloaded quality factor. We concluded that this domain acts as a lattice vibration damping, consequently raising the dielectric loss at microwave frequencies. The best microwave dielectric parameters were determined in ceramics sintered at 1500 °C for 32h: ε′ ~ 43; Q_u×f_r = 15,752 GHz; τ_f ~ 278 ppm °C⁻¹.