Tailoring the microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramics

Ba(Mg_1/3Ta_2/3)O₃ (BMT) is a very important microwave dielectric resonator material with a high dielectric constant of about 25, high quality factor and small temperature variation of resonant frequency. The preparation temperature of BMT ceramics is relatively high, about 1650 °C. The microwave quality factor of BMT depends on the ordering of Mg–Ta–Ta ions on the perovskite B-site. In the present paper we report how one can tailor the properties of BMT ceramics by glass addition, slight non-stoichiometry and by dopant addition. (a) It is found that addition of small amount of glasses such as B₂O₃, ZnO–B₂O₃, ZnO–2B₂O₃, ZnO–B₂O₃–SiO₂ reduces the sintering temperature, increases density, order parameter and quality factor. (b) Slight Mg or Ba deficiency improves densification, order parameter and quality factor whereas excess Mg or Ba deteriorated the properties (c) Small amounts of dopants such as Sb₂O₅, MnCO₃, ZrO₂, WO₃, SnO₂ and ZnO improve the microwave dielectric properties. It is found that the quality factor is maximum when the ionic radii of the dopant ions are close to the weighted average ionic radii of B site ions (i.e. Mg_1/3Ta_2/3), which is 0.653 Å.     

High frequency dielectric properties of Ba(Mg1/3Ta2/3)O3 complex perovskite ceramics

The dielectric properties of Ba(Mg_1/3Ta_2/3)O₃ and Ba(Mg_1/3Nb_2/3)O₃ ceramics were investigated using Fourier transformed infrared and Raman spectroscopies. The real part of dielectric constant of these materials decreases when the heavier Ta atoms replaced the lighter Nb atoms, which is closely related to the shift of mode-frequency. The Q-factor of the materials depends weakly on the composition. The decrease in Q-factor with the frequency is due to the presence of a resonance mode at around 3 THz. Larger width of the breathing Raman mode, A_1g[O], correlates very well with the lower Q-value of the BMN materials, as compared with the BMT materials.     

Co-optimization of microwave dielectric properties in Ba(Mg1/3Ta2/3)O3 complex perovskite ceramics through ordered domain engineering

It is an important subject to improve the temperature coefficient of resonant frequency (τ_f) and thermal conductivity (κ) of microwave dielectric ceramics without reducing the Qf value. Ordered domain engineering was applied to realize the previous objectives in Ba(Mg_1/3Ta_2/3)O₃ ceramics. With the increasing ordering degree from 0.835 to 0.897, the optimized Qf value was obtained. Meanwhile, near zero τ_f from 11.9 to 5.6 ppm °C⁻¹ was achieved, together with increased κ from 5.5 to 7.6 W m⁻¹ K⁻¹, and enhanced dielectric strength from 801 to 921 kV cm⁻¹. The noticeable ordered domain structure with large ordered domains (∼100 nm) and low-energy domain boundaries was revealed in Ba(Mg_1/3Ta_2/3)O₃. The consequent weakened phonon scattering rises the thermal conductivity. The increased bond covalency and oxygen distortion in ceramics with higher ordering degree were suggested as a cause of enlarged bandgap, which enhanced the dielectric strength. The reduced τ_f is dominated by the less “rattling” space of the cations in the ordered state by inducing more positive τ_ε. The reduced τ_f, optimized thermal conductivity, and Qf value in the present work indicate that the ordered domain engineering could open up a new direction for the optimization of microwave dielectric ceramics.     

Origin of dielectric loss in Ba(Co1/3Nb2/3)O3 microwave ceramics

The microwave dielectric loss of stoichiometric and non‐stoichiometric Ba(Co_1/3Nb_2/3)O₃ ceramics have been measured from 2 to 300 K in magnetic fields ranging from 0 to 5 T using a dielectric resonator (DR) technique. The microwave absorption from spin excitations of unpaired d‐electrons in exchange coupled Co²⁺ ions dominate the loss of the Ba(Co_1/3Nb_2/3)O₃ ceramics at cryogenic temperatures. Two peaks in the loss tangent (tan δ) vs temperature relation from a distinctly different origin occur at 25‐30 K and 90 K, which increase in magnitude with increasing Co content in the bulk dielectric samples. Evidence that these peaks result from polaron conduction from hopping between Co²⁺ and Co³⁺ ions includes (i) the peak's observed temperature range; (ii) the decrease in peak intensity of approximately a factor of two in a large applied magnetic fields (5 T); and (iii) a strong correlation between the peak's magnitude and both the fraction of the minority Co³⁺ in the dominant Co²⁺ matrix and D.C. conductivity at elevated temperatures. A magnetic‐field independent high temperature peak with a maximum at 250 K dominates the room temperature microwave loss whose magnitude correlates with those of the low temperature peaks. This suggests that the defects responsible for carrier conduction play an important role in establishing the loss tangent at room temperature.     

Effect of grain size and secondary phase on microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 and Ba([Mg,Zn]1/3Ta2/3)O3 systems

Here we report the influence of the grain size of the Ba(Mg_1/3,Ta_2/3)O₃ (BMT) and secondary phase formed in Ba([Mg_0.8Zn_0.2]_1/3,Ta_2/3)O₃ (BMZT) ceramics on the microwave dielectric properties. BMT and BMZT ceramics were prepared by conventional mixed-oxide reaction method using high purity reagents. Samples with different grain size and secondary phase content were obtained by controlling the sintering time of the BMT and BMZT. The secondary phase comprised of BaTa₂O₆, which includes small amount of Zn, was formed due to the vaporization of ZnO from BMZT by the high temperature sintering. The Qf value of BMT increased up to 400 THz with increasing the grain size of ceramics, whereas the Qf value of BMZT decreased with increase of the BaTa₂O₆ content. This second phase also caused an increase in the temperature coefficient of the resonant frequency of BMZT.     

Structural,chemical and dielectric properties of ceramic injection moulded Ba(Zn1/3Ta2/3)O3 microwave dielectric ceramics

We report the development of a ceramic injection moulding (CIM) process to produce complex-shaped structures using high-performance microwave ceramic materials. In particular, we describe the synthesis methods and the structural, chemical and dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) doped with Ni and Zr ceramics produced using ceramic injection moulding. Sintering the ceramic injection moulded Ba(Zn_1/3Ta_2/3)O₃ to a relative density of ∼94% was possible at a temperature of 1680 °C and a time of 48 h. The best samples to date exhibit a dielectric constant, ɛ_r, of ∼30, a Q value, of ∼31,250 (i.e. tan δ < 3.2 × 10⁻⁵) at 2 GHz, and a temperature coefficient of resonance frequency, τ_f, of 0.1 ppm/°C.     

Dielectric loss and damping constants of lattice vibrations in Ba(Mg1/3,Ta2/3)O3 ceramics

Far-infrared reflectivity spectra for Ba(Mg_1/3,Ta_2/3)O₃ ceramics (prepared from high quality oxide reagents) sintered at 1600 °C for 4 and 50 h were measured at room temperature, to determine eigen frequencies and damping constants of lattice vibration in order to understand why the Q·f value at microwave frequency was improved by sintering time. The observed reflectivity spectra were fitted to 16 IR active modes predicted by factor group analysis in order to estimate the fitting parameters. Differences in reflectivity and damping constants were confirmed between the two samples by observation of the measurement spectra and their spectrum fitting. The fitting parameters were used to simulate dielectric loss at low frequency. Results of the simulation suggested a contribution of 3rd, 4th and 5th vibration modes to dielectric loss at low frequency.     

Correlation of the phonon characteristics and microwave dielectric properties of the Ba(Mg1/3Ta2/3)O3 materials

The Ba(Mg_1/3Ta_2/3)O₃, BMT, materials possess the highest quality factor (Q × f) in microwave frequency regime among the microwave dielectric materials and can potentially be used for high frequency communication application. To understand the mechanism that determines microwave dielectric properties of the BMT materials, spectroscopic techniques including Raman and Fourier transform infrared (FTIR) analyses are used for investigating the phonon characteristics of the materials. The Raman-shift (Δω_0j) of the Raman peaks and the resonance frequency (ω_0j) of the FTIR peaks vary insignificantly among the samples, which correlate very well with the phenomenon that the K-values for these materials are similar with one another. In contrast, the full-width-at-half-maximum (FWHM) of the Raman peaks and the damping coefficient (γ_j) of the FTIR peaks vary markedly among the samples. The high-Q materials possess sharpest vibrational modes, viz., smallest FWHM value for Raman peaks and smallest γ_j value for FTIR peaks and vice versa. The intimate relationship between the phonon characteristics and the fine structure of the materials is confirmed.     

Microwave properties of Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Co1/3Nb2/3)O3 ceramics revealed by Raman scattering

Five Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperatures (form 1350 to 1550 °C), one Ba(Mg_1/3Ta_2/3)O₃ and a Ba(Mg_1/3Nb_2/3)O₃ sample were examined by Raman scattering to reveal the correlation of the 1:2 ordered perovskite structure with the microwave properties, such as dielectric constant and Q factors. The Ba(Co_1/3Nb_2/3)O₃ sample sintered at 1400 °C, which possesses the highest microwave Q value and the lowest dielectric constant among five Ba(Co_1/3Nb_2/3)O₃ samples, has the narrowest width and the highest frequency of the stretch mode of oxygen octahedron (i.e. A_1g(O) near 800 cm⁻¹). We found that the dielectric constant is strongly correlated with the Raman shift of A_1g(O) stretch modes, and the width of A_1g(O) stretch mode reflects the quality factor Q × f value in the 1:2 ordered perovskite materials. This concludes that the oxygen octahedron play an important role of the material's microwave performance. Based on the results of Q × f values and the lineshapes of A_1g(O) stretch mode, we found that the propagation of microwave energy in Ba(Mg_1/3Ta_2/3)O₃ and Ba(Mg_1/3Nb_2/3)O₃ shows weak damping behavior, however, Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperature exhibit heavily damped behavior.     

Effect of nonstoichiometry on the microstructure and microwave dielectric properties of Ba(Mg1/2W1/2)O3 ceramics

Effects of nonstoichiometry on crystal structure and the microstructure of double perovskite Ba(Mg_1/2W_1/2)O₃ ceramics have been investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results show that small deviation from stoichiometric composition has little influence on the crystal structure such as B-site 1:1 ordering degree. Evaporation of BaO was confirmed during the sintering of BMW ceramics, which in turn produce more BaWO₄ phase. Ba-deficiency or W-excess in BMW could improve the sinterability and Q×f value, while Ba-excess or W-deficiency could suppress the formation of BaWO₄ at the expense of increase in sintering temperature and decrease in Q×f value. Mg nonstoichiometry has little effect on the variation of BaWO₄ content and Q×f value. Maximum Q×f value of about 140,000 GHz could be obtained for the Ba-deficient or W-excessive samples after sintering at 1500 °C/2 h or 1550 °C/2 h, respectively. All Mg-nonstoichiometric compositions exhibit high Q×f value of about 120,000 GHz after sintering at 1550 °C/2 h. All well-densified samples have dielectric permittivity of about 19–20 and τ_f value varied within the range of ?21～?28 ppm/°C.     

Correlation between the 1:2 atomic order and microwave dielectric loss in the off-stoichiometric Ba(Zn1/3Ta2/3)O3

We report close correlation between the 1:2 atomic order and microwave dielectric loss in the off-stoichiometric Ba(Zn_1/3Ta_2/3)O₃ (BZT). Small off-stoichiometric deviations have a large effect on the ordering and Q?×?f values. The Ba₄Ta₂O₉-BZT-‘Zn₄Ta₂O₉’ pseudo tie line separating the Ta-rich and Ta-poor regions also demarcates the 1:2 ordered and disordered BZT. The low-loss off-stoichiometric BZT ceramics are located in the 1:2 ordered region of the BaO-ZnO-Ta₂O₅ phase diagram. Without exception the disordered BZT ceramics show high dielectric loss. Stoichiometric BZT with 1:2 atomic order shows Q?×?f?=?113?THz. The off-stoichiometric 1:2 ordered BZT ceramics with the highest Q?×?f?>?200?THz are located in the ZnO-deficient part of the ternary phase diagram.     

Ultra-high Q Ba(Mg1/3Ta0.675)O3 microwave dielectric ceramics realized by slowly cooling step process and the simulation design for hairpin dielectric filters

With the development of 5G technology, it would inevitably lead to the problem of energy consumption owing to the massive construction of 5G base stations and the increase in the number of antenna channels. Dielectric filters as the core component in the base station, the energy consumption was closely related to its insertion loss, which could be achieved by reducing the dielectric loss (tanδ = 1/Q) of microwave dielectric ceramics. In this work, in order to further enhance the Q × f value, the slowly cooling step process was innovatively introduced to the Ba(Mg_1/3Ta_0.675)O₃ ceramic. The optimal microwave dielectric properties were obtained in Ba(Mg_1/3Ta_0.675)O₃ ceramic with the step temperature of 1500 °C: ε_r = 24.767, Q × f = 298,051 GHz, τ_f = ?0.66 ppm/°C. Based on ultra-high Q Ba(Mg_1/3Ta_0.675)O₃ ceramic, the hairpin dielectric filter with high integration and low insertion loss was designed and simulated. Compared with traditional RT/duroid5870 dielectric substrate, the circuit area was reduced by more than 6 times and the passband insertion loss (|S₂₁| < 1 dB) was decreased by more than 50%. The current work could provide a solution for the low-power applications of microwave dielectric devices in 5G base stations.     

Effect of dopants on microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been prepared with various amounts of different dopants such as oxides of monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent elements. Effect of these dopants on microwave dielectric properties of BZN is investigated. Some of the dopants are found to increase quality factor Q × f and slightly alter the temperature coefficient of resonant frequency (τ_f). Annealing undoped BZN increased the quality factor. Small amounts of dopants such as oxides of Ni, In, Al, Ga, Zr, Ce, Sn, Ti, Sb, and W increased the quality factor. The doped ions substitute for the ordered B ions decreasing the order parameter. Annealing increased the quality factor for all doped BZN samples. Doping BZN with In₂O₃, Al₂O₃, WO₃ and SnO₂ decreased the order parameter but at the same time increased the quality factor indicating that order parameter alone is a poor indicator of quality factor. The quality factor is found to depend on the dopant ionic radii and its concentration. The quality factor increased when the ionic radius of the dopant is close to the ionic radius of the B site ions Zn or Nb. Microstructure studies using SEM showed that the doped high Q ceramics contained large grains.     

Improvement of microwave dielectric properties for Ba(Ni1/3Nb2/3)O3 ceramics by Zr-substitution

Effects of Zr-substitution on the structure, microstructure and microwave dielectric properties of Ba(Ni_1/3Nb_2/3)O₃ ceramics have been investigated. A small amount of Zr-substitution facilitates the densification of Ba(Ni_1/3Nb_2/3)O₃ ceramics. Within x≤0.05, the densification temperature decreases with increasing x in Ba[(Ni_1/3Nb_2/3)_1−xZr_x]O₃, while it turns to increase for x>0.05. With increasing x, the grains become more homogeneous and closely contacted, and significantly increase in size for x=0.15–0.20. The B-site cations 1:2 ordering is destroyed by Zr-substitution, and only stabilizes for x≤0.04. B-site cations 1:1 ordering starts to form in x=0.04, and the 1:1 ordering degree first increases and then decreases with increasing x. Qf value decreases slightly in x=0.01 and then increases monotonously with x increasing from 0.02 to 0.20. The destroyed 1:2 ordering structure is responsible for the decreased Qf value in x=0.01, while the improved grain configuration dominates the increase of Qf value for x=0.02–0.20. The dielectric constant ε_r increases monotonously with increasing x, due to the higher polarizability of Zr ion than the average value of Ni/Nb ions. The temperature coefficient of resonant frequency τ_f shifts from negative to positive through zero with increasing x, which is ascribed to the highly positive τ_f value of the end member BaZrO₃. The significant improvement of microwave dielectric properties has been achieved for x=0.10, higher ε_r, higher Qf as well as near zero τ_f value have been obtained: ε_r=31.8, Qf=36,100 GHz, τ_f=7.8 ppm/°C.     

Preparation of phase pure Ba(Zn1/3Ta2/3)O3 nanopowders for microwave dielectric resonator applications

Nanosized phase pure Ba(Zn_1/3Ta_2/3)O₃ (BZT) was prepared by decomposition of a citrate polymer precursor and subsequent pyrolysis. The powders were pyrolysed at different elevated temperatures and the effect of temperature on particle size was investigated. The sinterability of BZT ceramics made from nanopowders was very poor. Sintering at high temperatures led to formation of barium tantalite (BaTa₂O₆) due to vaporization of zinc. The samples were sintered by muffling with calcined BZT powder. Microwave dielectric properties of sintered dielectric resonators prepared from nanopowders of BZT are discussed.     

复合钙钛矿型Ba（Mg1／3Ta2／3）O3中B位有序结构的研究

本文讨论了复合钙钛矿型Ｂａ（Ｍｇ１／３Ｔａ２／３）Ｏ３中Ｂ位离子的有序程序同材料的介电损耗之间的关系。用Ｘ射线谱和Ｒａｍａｎ光谱研究了陶瓷烧结温度对Ｂａ（Ｍｇ１／３Ｔａ２／３）Ｏ３中Ｂ位离子的有序程度的影响，当适当提高烧结温度可增大有序程度。     

Effect of Ni substitution on the dielectric properties and lattice vibration of Ba(Mg1/3Ta2/3)O3

Far infrared reflectivity spectra for Ba(Mg_1/3,Ta_2/3)O₃ and Ba([Mg_1−xNi_x]_1/3,Ta_2/3)O₃ ceramics were measured and eigenfrequencies and damping constants of transverse optical modes were estimated in order to investigate the variations in the dielectric properties with Ni substitution. These ceramics were prepared by the conventional solid phase reaction method using high purity chemicals. The observed reflectivity spectra were fitted as a function of the vibration frequencies and the damping constants. The permittivity and Qf value decreased with Ni substitution. The decrease in permittivity is due to the decrease in the damping constant of 4th vibration mode of the oxygen layer vibration. The decrease in Qf value is due to the increase in the damping constant of 5th vibration mode of the oxygen layer vibration. It was inferred that the increase in the Qf value might be due to the substitution of the Ni ion for the Ta ion in the Ta layer in the BMT.     

Measurements of dielectric constant and quality factor of Ba(Mg1/3Ta2/3)O3 at X band frequencies

Dielectric properties of Ba(Mg_1/3Ta_2/3)O₃ in forms of ceramic, single crystal fiber, and polyethylene composite at microwave frequencies were measured. The dielectric constants 23.33, 24.30, and 26.50 and the quality factors 8,050, 6,430, and larger than 2,000 at 10 GHz were measured for the ceramic, hot pressed ceramic and the fiber respectively. The powder was studied by forming composites of Ba(Mg_1/3Ta_2/3)O₃ -Polyethylene. It is found that the experimental dielectric constant values at X band frequencies fit reasonably well to the logarithmic mixture rule. A newly reported mixing equation by Wakino et al. was studied by comparing with our experimental results.