Structure and dielectric properties of perovskite ceramics in the system Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3

Ceramics in the system Ba(Ni_1/3Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ (BNN–BZN) were prepared by the mixed oxide route. Powders were mixed and milled, calcined at 1100–1200 °C then pressed and sintered at temperatures in the range 1400–1500 °C for 4 h. Selected samples were annealed or slowly cooled after sintering. Most products were in excess of 96% theoretical density. X-ray diffraction confirmed that all specimens were ordered to some degree and could be indexed to hexagonal geometry. Microstructural analysis confirmed the presence of phases related to Ba₅Nb₄O₁₅ and Ba₈Zn₁Nb₆O₂₄ at the surfaces of the samples. The end members BNN and BZN exhibited good dielectric properties with quality factor (Qf) values in excess of 25,000 and 50,000 GHz, respectively, after rapid cooling at 240 °C h⁻¹. In contrast, mid-range compositions had poor Qf values, less than 10,000 GHz. However, after sintering at 1450 °C for 4 h and annealing at 1300 °C for 72 h, specimens of 0.35(Ba(Ni_1/3Nb_2/3)O₃)–0.65(Ba(Zn_1/3Nb_2/3)O₃) exhibit good dielectric properties: τ_f of +0.6 ppm °C⁻¹, relative permittivity of 35 and quality factor in excess of 25,000 GHz. The improvement in properties after annealing is primarily due to an increase in homogeneity.     

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.     

High-Q microwave dielectrics in low-temperature sintered (Zn1−xNix)3Nb2O8 ceramics

The effects of Ni substitution for Zn on microwave dielectric properties of (Zn_1−xNi_x)₃Nb₂O₈ (x = 0.02–0.08) ceramics were investigated in this study. The XRD patterns of the sintered samples reveal single-phase formation with a monoclinic structure. The tremendous improvement of Q × f value can be achieved by a small level of Ni substitution (x = 0.05). The τ_f value was found to decrease with a decreasing A-site bond valence. In addition, B₂O₃ and CuO were used as a sintering aid to lower the sintering temperature from 1180 to 900 °C. Excellent microwave dielectric properties (ɛ_r ∼ 20.7, Q × f ∼ 98,000 GHz and τ_f ∼ −85.2 ppm/°C) and a chemical compatibility with Ag electrodes can be obtained for 4 wt% B₂O₃–CuO doped (Zn_0.95Ni_0.05)₃Nb₂O₈ ceramics sintered at 930 °C for 2 h. This constitutes a very promising material for LTCC applications.     

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.     

Low-temperature sintering and microwave dielectric properties of Ba5Nb4O15 with ZnB2O4 glass

The Influence of ZnB₂O₄ glass addition on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using dilatometry, X-ray diffraction, scanning electron microscopy and network analyzer. It was found that a small amount of glass addition to Ba₅Nb₄O₁₅ lowered the sintering temperature from 1400 to 900 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases such as Ba₃B₂O₆. The Ba₅Nb₄O₁₅ ceramics with ZnB₂O₄ glass, sintered at a low temperature, exhibited good microwave dielectric characteristics, i.e., a quality factor (Q × f) = 12,100 GHz, a relative dielectric constant (ɛ_r) = 40, a temperature coefficient of resonant frequency (τ_f) = 48 ppm/°C. The dielectric properties were discussed in terms of the densification of specimens and the influence of glassy phases such as Ba₃B₂O₆ and ZnB₂O₄.     

Microwave dielectric properties of Ca(Li1/4Nb3/4)O3–CaTiO3 ceramic systems

The microwave dielectric properties of Ca(Li_1/4Nb_3/4)O₃–CaTiO₃ ceramics have been investigated with regard to calcination temperature and the amount of CaTiO₃ additive. Ca(Li_1/4Nb_3/4)O₃ ceramics with an orthorhombic crystal structure can be synthesized by the conventional mixed oxide method by calcining at 750 °C and sintering at 1275 °C. The dielectric constant (ɛ_r), quality factor (Q × f₀) and temperature coefficient of resonant frequency (τ_f) for Ca(Li_1/4Nb_3/4)O₃ ceramics are 26, 13,000 GHz and −49 ± 2 ppm/°C, respectively. With increase in the CaTiO₃ content, ɛ_r and τ_f are increased and the quality factor decreased due to the solid-solution formation between Ca(Li_1/4Nb_3/4)O₃ and CaTiO₃. The 0.7Ca(Li_1/4Nb_3/4)O₃–0.3CaTiO₃ ceramic exhibits ɛ_r of 44, quality factor (Q × f₀) of 12,000 GHz and τ_f of −9 ± 1 ppm/°C.     

Novel microwave dielectric LTCCs based uponV2O5 doped M2+Cu2Nb2O8 compounds (M2+ = Zn,Co, Ni,Mg and Ca)

The copper-niobates, M²⁺Cu₂Nb₂O₈ (M²⁺ = Zn, Co, Ni, Mg or Ca) have good microwave dielectric properties when sintered between 985–1010 °C and 1110 °C for CaCu₂Nb₂O₈. Therefore, they would be potential dielectric LTCC materials if they could be made to sinter below 960 °C (melting point of silver). To this end, additions of 3 wt.% V₂O₅ were made to ZnCu₂Nb₂O₈, CoCu₂Nb₂O₈, NiCu₂Nb₂O₈, MgCu₂Nb₂O₈ and CaCu₂Nb₂O₈, and their sintering and dielectric behaviour was investigated for samples fired between 800 and 950 °C. Doping lowered sintering temperatures to below the 960 °C limit in all cases. Doping had the general effect of reducing ɛ_r, density, Qf and τ_f, although doped CaCu₂Nb₂O₈ had a Qf value of 9300 GHz, nearly four times that of the best undoped sample. Doped ZnCu₂Nb₂O₈ fired to 935 °C had Qf = 10,200 GHz, and for doped CoCu₂Nb₂O₈ fired to 885 °C Qf = 7500 GHz. When doped and undoped samples all fired to 935 °C were compared, all doped samples had greater ɛ_r and density, and all except ZnCu₂Nb₂O₈ had a smaller τ_f. All doped samples had a more linear relationship between frequency and temperature in the range 250–300 K.     

Microwave dielectric properties of BaO–Ta2O5–TiO2 system

Microwave dielectric properties of the BaO–Ta₂O₅–TiO₂ system were investigated by the solid-state reaction method. It was recognized that the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions have the higher Q · f value in comparison with the Ba₈(Ta_4 − xNb_x)Ti₃O₂₄ solid solutions. The limit of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions was approximately x = 0.75; the lattice parameter c of the solid solutions, which is related to the change in the B(1)O₆ octahedron, was significantly increased in the composition range from 0 to 0.75. The Q · f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions are remarkably improved by the Sn substitution for Ti; the highest Q · f value of 59,100 GHz is obtained at x = 0.75. Moreover, the ɛ_r and τ_f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions at x = 0.75 were 25.6 and 30.3 ppm/°C, respectively.     

Property control of microwave dielectric materials using self-flux compositions

In order to improve the sinterability and controllability of dielectric properties, we focused on self-flux composition. In the case of Ba(Mg_1/3Nb_2/3)O₃, we selected Ba_(1 − β)Nb_βO_δ as self-flux system and investigated correlations between Q-factor and the β value. Interestingly, high Q-value was obtained only at the β = 0.45. Moreover, the dielectric constant (ɛ_r) and temperature coefficient of resonant frequency (τ_f) change linearly with the quantity of Ba_0.55Nb _0.45O_δ by keeping up high Q-value. As a result, it was indicated that the dielectric properties could be controlled by the assumption of stoichiometric composition and the liquid phase consisted of “self-flux”. In a similar way of thinking, it seems that this concept of property designing will be applied to the complex-perovskite-type materials.     

Mixture behavior and microwave dielectric properties of (1 − x)CaWO4–xTiO2

The microwave dielectric characteristics and mixture behavior of (1 − x)CaWO₄–xTiO₂ ceramics prepared with conventional solid-state route were studied using a network analyzer and X-ray power diffraction, respectively. The CaWO₄ compound had good properties (low permittivity and high quality factor) for microwave applications, but it had a high negative temperature coefficient of resonant frequency (τ_f = −53). Hence, in order to tune the dielectric properties, (1 − x)CaWO₄–xTiO₂ were prepared for different values of x. X-ray powder diffraction and SEM analysis revealed that CaWO₄ and TiO₂ coexisted as a mixture. The mixture formation and dielectric properties could be explained by mixture rule. In particular, at x = 2.6, good microwave dielectric properties were obtained: Qf = 27,000, ɛ_r = 17.48, and τ_f = ∼0 ppm/°C.     

Temperature-stable and high-ɛ dielectric ceramics based on Ag (Nb1−xTax)O3

The dielectric properties of novel dielectric system AgNb_1−xTa_xO₃ (ANT) have been studied in this paper. In this system, the temperature coefficient of capacitance (TCC) can be adjusted to 0 ± 30 × 10⁻⁶/°C by choosing proper molar ratio of Nb⁵⁺ to Ta⁵⁺. When 2 wt% glass is added to the ceramics, the sintering temperature is reduced to 960 °C, which restrains Ag⁺ decomposition in ambient atmosphere. It is noted that the dielectric loss reduces further after adding 2.5 wt% Sb₂O₅. The dielectric properties of the resultant samples are as follows: dielectric constant ɛ ≈ 512, loss tangent tan δ ≈ 5.2 × 10⁻⁴, and TCC ≈ 10 × 10⁻⁶/°C.     

Correlation between infrared,THz and microwave dielectric properties of vanadium doped antiferroelectric BiNbO4

Terahertz (THz) transmissivity and infrared (IR) reflectivity spectra of orthorhombic microwave (MW) ceramics Bi(Nb_1−xV_x)O₄ (0.002 < x < 0.032) were measured between 4 and 3000 cm⁻¹ (0.09–90 THz) at room temperature. A well underdamped mode, presumably the ferroelectric soft mode, was observed at 25 cm⁻¹. Complex permittivity spectra obtained from the fits to our data were extrapolated down to the MW range and compared with the dielectric data near 5 GHz. The linear extrapolation of dielectric losses from THz down to the MW range is in agreement with the experimental MW losses. Addition of 3.2% of vanadium reduces the sintering temperature to 850 °C and the dielectric properties (ɛ′ = 42.2, Q·f = 14,000 GHz, τ_f = +10 ppm/°C) remain at a level satisfactory for MW applications. Somewhat lower MW losses were observed in a sample sintered in the N₂ atmosphere.     

Hydrothermal synthesis of nanocrystalline BaSnO3 using a SnO2·xH2O sol

A BaSnO₃ powder with a crystallite size of 27.6 nm has been prepared through a hydrothermal reaction of a peptised SnO₂·xH₂O and Ba(OH)₂ at 250 °C and the following crystallization of this hydrothermal product at 330 °C. The peptisation of the SnO₂·xH₂O gel is dependent on the pH value. Through peptisation the mean particle size of SnO₂·xH₂O in the aqueous solution has been decreased by a factor of 100 to 8 nm. A limited agglomeration in the sol-prepared powder has been observed under the microscope. The structure evolution and crystallisation behaviours of the sol-prepared powders were investigated by TG-DTA, IR and XRD. The BaSn(OH)₆ phase in the as-prepared powder transforms into an amorphous phase at 260 °C, from which the BaSnO₃ particles nucleate and grow with an increase in temperature. The single-phase BaSnO₃ powder has been obtained at a temperature as low as 330 °C. This sol-prepared powder is more sinter-reactive than the gel-prepared powder and can be sintered to a ceramic with 90.7% of the theoretic density.     

Microwave dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

TiO₂ bulk ceramics were fabricated by using both spark plasma sintering (SPS) and the conventional sintering method (CSM). Starting materials were ultra fine rutile powders (<50 nm) prepared via the sol–gel process. CSM achieved the relative sintering density of 99.2% at 1300 °C. The grain size of 1300 °C sintered specimen was 6.5 μm. However, the sintering temperature of SPS for the density of 99.1% was as low as 760 °C, where the grain size was only 300 nm. In order to re-oxidize the Ti³⁺ ions due to the reducing atmosphere of the SPS process and the high temperature of the CSM process, the prepared TiO₂ specimens were annealed in an oxygen atmosphere. The dielectric constant (ɛ_r) and quality factor (Q × f) of SPS-TiO₂ re-oxidized specimens in a microwave regime were 112.6 and 26,000, respectively. These properties were comparable to those of 1300 °C sintered CSM specimens (ɛ_r ∼ 101.3, Q × f ∼ 41,600). These microwave dielectric properties of nanocrystalline TiO₂ specimens prepared using SPS were discussed in terms of grain size variation and Ti⁴⁺ reduction.     

Characterization and dielectric behavior of willemite and TiO2-doped willemite ceramics at millimeter-wave frequency

Willemite ceramics (Zn₂SiO₄) have been successfully prepared in the temperature range from 1280 to 1340 °C. It is found that willemite ceramics possess excellent millimeter-wave dielectric properties: a dielectric constant ɛ_r value of 6.6, a quality factor Q × f value of 219,000 GHz and a temperature coefficient of resonant frequency τ_f value of −61 ppm/°C. By adding TiO₂ with large positive τ_f value (450 ppm/°C), near zero τ_f value can be achieved in a wide sintering temperature range. With 11 wt% of TiO₂, an ɛ_r value of 9.3, a Q × f value of 113,000 GHz, and a τ_f value of 1.0 ppm/°C are obtained at 1250 °C. The relationships between microstructure and properties are also studied. Our results show that willemite with appropriate TiO₂ is an ideal temperature stable, low ɛ_r and high Q × f dielectric for millimeter-wave application.     

Microwave dielectric properties of Ba8Ta6(Ni1 − xMx)O24 (M = Zn and Mg) ceramics

The influence of M (M = Zn and Mg) substitution for Ni on the microwave dielectric properties and the crystal structure of Ba₈Ta₆(Ni_1 − xM_x)O₂₄ ceramics was investigated in this study. The Ba₈Ta₆(Ni_1 − xZn_x)O₂₄ (BTNZ) solid solutions showed a single phase in the composition range of 0–1, whereas the limit of Ba₈Ta₆(Ni_1 − xMg_x)O₂₄ (BTNM) solid solutions was approximately x = 0.75; the lattice parameters of both solid solutions increased linearly, depending on the composition x. Although the dielectric constants (ɛ_r) of BTNZ were almost constant over the whole composition range, those of BTNM slightly decreased from 27.8 to 24.3; the decrease in the dielectric constant of BTNM is due to the change in relative density of the sample. The quality factors (Q × f) of both solid solutions were improved by the M substitution for Ni; the maximum Q × f values of BTNZ and BTNM were 91729 and 93127 GHz, respectively. Moreover, the temperature coefficients of resonant frequency (τ_f) of BTNZ and BTNM varied from 33 to 40 ppm/°C and from 33 to 26 ppm/°C, respectively.     

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.     

Fabrication and dielectric properties of barium titanate-based glass ceramics for tunable microwave LTCC application

Low-fired ferroelectric glass ceramics were fabricated from glass powders with a basic composition of 0.65BaTiO₃·0.27SiO₂·0.08Al₂O₃. The combined addition of SnO₂ (or ZrO₂) and SrCO₃ was conducted to modify the dielectric properties of the glass ceramics. The Sr-component could be incorporated preferentially in the perovskite structure after heating at 1000 °C. The bulk and thick film samples obtained by sintering glass powder with a starting composition of 0.65(Ba_0.7Sr_0.3)(Ti_0.85Sn_0.15)O₃·0.27SiO₂·0.08Al₂O₃ at 1000 °C for 24 h showed a broadened ɛ_r–T relation with T_c ≈ 10 °C and ɛ_r(max) ≈ 280 and microwave tunability of 32% at 3 GHz, respectively.     

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 BaxLa4Ti3 + xO12 + 3x (x = 0.0–1.0) ceramics

In the BaO–La₂O₃–TiO₂ system, the Ba_nLa₄Ti_3 + nO_12 + 3n homologous compounds exist on the tie line BaTiO₃–La₄Ti₃O₁₂ besides tungstenbronze-type like Ba_6 − 3xR_8 + 2xTi₁₈O₅₄ (R = rare earth) solid solutions. There are four kinds of compounds in the homologous series: n = 0, La₄Ti₃O₁₂; n = 1, BaLa₄Ti₄O₁₅; n = 2, Ba₂La₄Ti₅O₁₈; n = 4, Ba₄La₄Ti₇O₂₄. These compounds have the layered hexagonal perovskite-like structure, which has a common sub-structure in the crystal structure. These compounds have been investigated in our previous studies. In this study, we have investigated the phase relation and the microwave dielectric properties of Ba_xLa₄Ti_3 + xO_12 + 3x ceramics in the range of x between 0.2 and 1.0. With the increase in x, the dielectric constant ɛ_r locates around 45, the quality factor Q × f shows over 80,000 GHz at x = 0.2 and the minimum value of 30,000 GHz at x = 0.9, and the temperature coefficients of resonant frequency τ_f is improved from −17 to −12 ppm/°C. At x = 0.2, the ceramic composition obtained has dielectric constant ɛ_r = 42, the temperature coefficient of the resonant frequency τ_f  = −17 ppm/°C and a high Q × f of 86,000 GHz.