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 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.     

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。     

用8—羟基喹啉（oxine）制备Ba（Mg1／3Ta2／3）O3粉体

本文对用８－羟基喹啉（ｏｘｉｎｅ）为螯合剂来制备Ｂａ（Ｍｇ１／３Ｔａ２／３）Ｏ３（ＢＭＴ）粉料进行了研究。就ｏｘｉｎｅ的加入量，ｐＨ值的改变对生成钽镁沉淀的产率及其粉料性能的影响进行了研究比较，结果表明，当ｏｘｉｎｅ的加入量大于ＭｇＴａ２Ｏ６摩尔数的３倍，ｐＨ值保持在１０左右时制得的钽镁粉较为理想，其比表面积达３２ｍ＾２／ｇ，与ＢａＣＯ３混合后烧结温度比用传统制备法降低１８０～２５０℃。     

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.     

Ba1-3x/2Lax(Mg1/3Ta2/3)O3陶瓷的微波介电性能

采用固相合成法制备了Ba1-3x/2Lax(Mg1/3Ta2/3陶瓷,研究了La掺杂对钽镁酸钡的结构和微波介电性能的影响.结果表明:A位取代能改进其烧结性能.在x≤0.02时,烧结样品为单相的钙钛矿结构,B位离子1:2有序;当x>0.02时出现第二相Ba0.5TaO3.B位离子有序度随着x的增大先增加后减小,在x=0.04时出现最大值.x≤0.02时介电常数变化较小,而后其值逐渐增大.品质因数与谐振频率的乘积(Q×f)值随着x的增大先增大后减小,在x=0.02时取得最大值;谐振频率温度系数(τf)值随着x增大而增大.     

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.     

B位离子共沉-煅烧法制备Ba(Mg1/3Ta2/3)O3微波陶瓷材料

采用钽镁共沉淀法制备ＭｇＴａ２Ｏ６粉体,然后与ＢａＣＯ粉体混合制备Ｂａ（Ｍｇ１／３Ｔａ２／３）Ｏ３（简称ＢＭＴ）微波陶瓷材料,成功地解决了ＴａＣｌ３溶解问题。对钽镁共沉条件和粉体热分解工艺与粉体的平均粒径和钽镁比例关系;ＢＭＴ陶瓷制备工艺与材料的微观结构和微波性能关系进行了研究,制备的ＭｇＴａ２Ｏ６晶相粉体平均粒径为０．３μｍ,最大与最小粒径比为５。制备的ＢＭＴ陶瓷的几乎不含ＢａＴａ２Ｏ６等杂相,     

1:1 Ordered Domain Growth in Pb(Mg1/3Ta2/3)O3-La(Mg2/3Ta1/3)O3 Relaxor Ferroelectric Perovskites

Single-phase perovskite solid solutions are formed across the entire (1 - x )Pb(Mg_1/3Ta_2/3)O₃- x La(Mg_2/3Ta_1/3)O₃ (PLMT) pseudobinary system. Although as-sintered (1300°C, 3 h) samples with x lessthan equal to 0.1 adopt a phase-separated, PMN-type microstructure comprising small (2-3 nm) ordered domains dispersed in a disordered matrix, extensive domain growth and complete order can be induced by extended thermal annealing (1300°C, 24 h). These observations, and the alterations in the thermal stability and domain size across the system, can be interpreted using a charge-balanced random site model for the cation order. No evidence is found to support the space-charge models currently used to describe the cation ordering and properties of the corresponding niobate systems.     

Phase development in Ba(Mg1/3Ta2/3)O3 via conventional and B-site precursor routes

Phase-formation stages in Ba(Mg_1/3Ta_2/3)O₃, prepared by conventional and B-site precursor methods, were investigated and the results are compared. Appropriate powder mixtures were heat-treated at selected temperatures and characterized by X-ray diffraction (XRD) in order to examine the phase development. In the conventional route, monophasic perovskite, finally resulted at high temperatures, had several preliminary intermediate phases, whereas in the B-site precursor method the perovskite formed directly from the reactants. The perovskite formation using B-site precursors was completed at significantly lower temperatures.     

Signature of lattice defects in far infrared reflectivity of Ba(Mg1/3Ta2/3)O3

Far infrared reflectivity spectra of Ba(Mg_1/3,Ta_2/3)O₃ prepared at several sintering temperatures were measured, and the eigenfrequencies and damping constants of the TO modes were determined. The reflectivity spectra were fitted with the four-parameter semi-quantum model. The variation in the E_u(O_II) at 222 cm⁻¹ and A_2u(O_II) at 238 cm⁻¹ modes in well ordered ceramics was attributed to the variation in the concentration of the B site defects. It was also found that the change in the oxygen partial pressure of the sintering atmosphere causes a change in the seventh (316 cm⁻¹) and eighth (352 cm⁻¹) modes. We attribute these changes to the oxygen site defect although we cannot evaluate the concentration of this defect at this moment. From the behavior of the damping constants it is suggested that the Ba(Mg_1/3Ta_2/3)O₃ (BMT) attains equilibrium defect density at the heat treatment temperature of more than 1630 °C (120 h).     

(Ba1-xSrx)(Mg1/3 Ta2/3)O3微波陶瓷介电性能研究

Ba(Mg1/3Ta2/3)O3(简称BMT)陶瓷是A(B′1/3B″2/3)O3(A=Ba,Sr;B′=Zn,Mg;B″=Nb,Ta)型复合钙钛矿化合物中的一种,A位由Sr离子取代Ba离子,形成(Ba1-xSrx)(Mg1/3Ta2/3)O3(简称BSMT)固溶体型化合物,也具有复合钙钛矿结构。Sr含量x≥0．6时发生相转变,形成一种新的低温相,这是由于氧八面体畸变造成的。这种低温相结构与BMT六方晶系结构相比具有较低的对称性。低温相的形成．可显著降低BSMT陶瓷的烧结温度．在150℃即可烧结致密(BMT为160℃)。BSMT的微观结构和介电性能(如介电常数ε和介电常数温度系数aε)的变化也与此相转变有关。     

Temperature and frequency dependence of dielectric loss of Ba(Mg1/3Ta2/3)O3 microwave ceramics

Ba(Mg_1/3Ta_2/3)O₃ ceramic possessing extremely high Q × f value of more than 300 THz at microwave frequency was developed in our previous study. It is of great interest to understand the mechanism of microwave absorption in such a practical material. In the present study we report on the temperature dependence of the dielectric loss in the Ba(Mg_1/3Ta_2/3)O₃. The mechanism of the microwave absorption is discussed using two phonons difference process. The samples were prepared by conventional solid state reaction and sintered at 1893 K in oxygen atmosphere. Dielectric properties in the microwave range were measured by Hakki & Colemann and resonant cavity methods in the temperature range of 20–300 K. Whispering gallery mode technique was used for the measurement of the dielectric properties at the millimeter wave frequency. Dielectric loss of the Ba(Mg_1/3Ta_2/3)O₃ at the microwave frequency increases with temperature between 200 and 300 K in general agreement with the theory of intrinsic dielectric loss derived from the two phonon difference process. However below 200 K, the dielectric loss has shown a distinctive behavior with a loss peak at 40 K. It was inferred that the loss peak of the Ba(Mg_1/3Ta_2/3)O₃ was caused by the local orientation polarization having dispersion at the microwave frequency.     

Characteristics and thermal cycling behavior of plasma-sprayed Ba(Mg1/3Ta2/3)O3 thermal barrier coatings

Ba(Mg_1/3Ta_2/3)O₃ (BMT) powders were synthesized by the solid state reaction method. BMT thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS). The phase composition and microstructure of the BMT coatings were characterized. The thermal cycling behavior of the BMT coatings was investigated by the water quenching method from 1150 °C to room temperature. The results reveal that BMT powders have an ordered hexagonal perovskite structure, whereas the as-sprayed coating of BMT has a disordered cubic perovskite structure because of the different degree of structural order for different treatment conditions. During thermal cycling testing, the entire spalling of coatings occurred within the BMT coating near the bond coat. This is attributed to the following reasons: (1) the growth of a thermally grown oxides (TGO) layer, which leads to additional stresses in the coatings; (2) the coefficient of thermal expansion mismatch between the BMT coating and bond coat, which develops enormous stress in the coatings; (3) the precipitation of Ba₃Ta₅O₁₅ due to the evaporation of MgO during the spraying process, which changes the continuity of the coatings.     

Local structure study of phase transition behavior in Ba(Ti,Sn)O3 perovskite by X-ray absorption fine structure

In this work, BaTi_1−xSn_xO₃ (BST) powders (x=0–0.95) were synthesized by a conventional mixed-oxide method. The phase information was investigated by a combination of X-ray diffraction and X-ray absorption spectroscopy techniques. The XRD measurements indicated the global phase transition from tetragonal to cubic perovskite structure. From the synchrotron X-ray Absorption Near-Edge Structure (XANES) measurements at the Ti K-edge and Sn L₃-edge, it was seen that an increase of Sn content in BaTiO₃ affected the phase transition behavior and local structure of BST. In addition, the local structure of Ba(Ti,Sn)O₃ materials were experimentally determined and compared with that obtained from the simulation. The local structure obtained from the XANES technique provided additional structural information unavailable from XRD investigation. Finally, the phase transition behavior from relaxor ferroelectric to polar cluster in BST system was discussed and attributed mainly to the change in the local structure.     

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.     

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.     

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.