Mei-Hui Liang

Effect of sintering process on microstructure characteristics of Ba(Mg1/3Ta2/3)O3 ceramics and their microwave dielectric properties

Abstract Microstructure of Ba(Mg1/3Ta2/3)O3, BMT, materials were examined using transmission electron microscopy. Selected area electron diffractions imply that the ordering of Mg2+ and Nb5+ cations transforms the BMT materials from cubic symmetry to P 3 m1 symmetry, which induces the formation of polar domains in BMT grains. Large grains contain wavy polar domain boundaries, dislocations and even the dislocation networks, which are detrimental to the microwave dielectric properties. Presumably, induction of abundant defects in association with large BMT grains is the prime factor degrading the microwave dielectric properties of the BMT materials, when they were sintered at too high temperature.

Influence of Zr-doping on the microstructure and microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 materials

The effect of Zr-species addition on the microwave dielectric properties of Ba(Mg 1/3 Ta 2/3 )O 3 , BMT materials was examined. The sintering behavior and the microwave dielectric properties of the BMT materials varies with the routes by which the Zr-species were incorporated. Two-step process leads to pronounced improvement, whereas single-step process results in markedly degradation on microwave Q-factor for the materials. SEM and X-ray diffraction analyses reveal that the main factor, resulting in deleterious effect due to addition of Zr-species via single-step process, in the induction on the formation of secondary phase, which is turn, is owing to the slow reaction kinetics of ZrO 2 and BaCO I for the formation of BaZrO 3 perovskite phase.

EFFECT OF BA5TA4O15 INCORPORATION ON SINTERING BEHAVIOR AND MICROWAVE DIELECTRIC PROPERTIES OF BA(MA1/3TA2/3)O3 MATERIALS

The characteristics of Ba(Mg1/3Ta2/3)O3, BMT materials synthesized by a two-step process, consisting of the formation of B-site compounds, e.g., MgTa2O6, followed by the formation of a BMT perovskite, were compared with the properties of BMT powders prepared by the conventional single-step mixed-oxide route. BMT powders prepared by the two-step process exhibits superior reactivity compared to those synthesized by the single-step process. The single-phase perovskite can be formed at a lower calcination temperature (900°C) and high density ceramic materials can be obtained at a lower sintering temperature with a short period of time (1500°C, 2 h). When the Ba5Ta4O15 is incorporated into pure BMT perovskite, the sintered density of BMT materials decreases significantly, and the dielectric constant (K) and temperature coefficient of resonance frequency (τf) are not altered significantly, but the Q-factor of the BMT materials decreases signicantly. These observations imply that the mechanism by which the two-step process significantly improves the densification process and the related microwave dielectric properties is the effective suppression of formation of the secondary phase, Ba5Ta4O15.

Terahertz spectroscopic technique for characterizing the microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 materials

Abstract We have measured the complex index of refraction of Ba(Mg 1/3 Ta 2/3 )O 3 , BMT, ceramics using coherent terahertz (THz) time domain spectroscopy. The dielectric properties of BMT ceramics, which were prepared via a two-step mixed oxide process or a hot isostatic press (HIP) process in 0.1–0.6 THz regime were studied. The real part of the index of refraction of BMT prepared via a two-step process was about 5.04, whereas that of BMT materials prepared via a HIP process was around 4.97. These values are similar to the real part of the index of BMT ceramics in the microwave range ( n =5.0). Hipped BMT materials exhibit much larger power loss and hence lower Q -factor than two-step mixed oxide processed BMT in 0.1–0.6 THz regime.

Terahertz Response of Bulk Ba(Mg1/3Ta2/3)O3

We have measured the complex index of refraction of Ba(Mg1/3Ta2/3)O3 bulk material using coherent THz time domain spectroscopy. The transmission spectrum of Ba(Mg1/3Ta2/3) in the 0.1–1.0 THz region is obtained. The real part of the index of refraction of Ba(Mg1/3Ta2/3) in this band is estimated (~4.87), and is comparable with the dielectric constant measured at microwave frequency. The measured Q-factor is around Qf=40 THz, which is slightly smaller than the Qf-values of the Ba(Mg1/3Ta2/3)O3 materials in the microwave regime (Qf=60,000 GHz in 10 GHz).

Effect of Ba5Ta4O15 on microstructural characteristics of Ba(Mg1/3Ta2/3)O3 ceramics and their microwave dielectric properties

Abstract Microstructures of the Ba(Mg 1 3 Ta 2 3 )O 3 , BMT, materials were examined using transmission electron microscopy. It is observed that small BMT grains usually contain very few defects, whereas large BMT grains always contain large proportion of dislocations, stacking faults and midribs of secondary phase. Incorporation of small amount (∼0.1 mol%) of Ba 5 Ta 4 O 15 particles improves the uniformity of grains. However, the density of dislocations increases tremendously, resulting in tangling of dislocations and even the formation of networks, whereas the proportion of stacking faults decreases pronouncedly for the Ba 5 Ta 4 O 15 added BMT materials. Presumably, it is through the hindrance on the growth of grains and modification on the formation of defects that incorporation of Ba 5 Ta 4 O 15 particles alters the microwave dielectric properties of BMT materials.

THZ Transmission spectroscopy applied to dielectrics and microwave ceramics

Abstract The room-temperature complex dielectric function of Ba(Mg1/3Ta2/3)O3 (BMT) high-permittivity microwave ceramics was measured in the sub-millimeter spectral range using the time-domain terahertz-transmission spectroscopy. The capabilities of the method and unambiguous evaluation of the dielectric function are also discussed.

ENHANCING THE DENSIFICATION PROCESS ON BA(MG1/3TA2/3)O3 MICROWAVE DIELECTRICS BY Y2O3 INCORPORATION

Abstract The relative density of Ba(Mg1/3Ta2/3)O3 (BMT) has been found to significantly increase due to the addition of Y2O3. A sintering condition of 1650°C (4 hrs) is necessary to densifie the undoped BMT specimens to 95% theoretical density (T.D.), while relatively lower sintering temperature, 1450°C (4h), is sufficient to densify the BMT materials containing 6 mol% Y2O3 to 95% T.D. However, the Q·f-value of Y2O3-doped materials increases monotonously with sintering temperature and reaches a maximum value (Q·f=650000) when 1600°C (4h) sintered. Microwave dielectric properties of the 1600°C -sintered BMT samples also increase with Y2O3-content. High Q·f value of the samples is intimately correlated with their large ordering factor (S). The temperature coefficient of resonant frequency (τf) of these samples is τf=13.2ppm/°C for the temperature ranging from 25 to 80°C.

Ba(Zn1/3Nb2/3)O3 ceramics synthesized by spray pyrolysis technique

Abstract Ba(Zn1/3Nb2/3)O3(BZN) powder has been prepared by pyrolyzing the polyester precursors, followed by heat treated at 800°C for 4 hrs in air to remove the residual organic materials. The as prepared powders are of hollow geometrical spheres, which inherit the morphology of the atomized droplets. The perovsike structure is indicated by X-ray diffraction pattern to have already formed. A second phase, identified as Ba5Nb4O15, is observed among the pellets sintered at 1250°C∼1350°C in open air, which is ascribed to the loss of ZnO. When the pellets were packed with ZnO powders and placed in a covered Al2O3crucibles, pure perovsike, free of Ba5Nb4O15 phase, was obtained when sintered at 1300°C. Q·f value as high as 180,000 GHz was achieved for thus obtained BZN.

Improvement on microwave dielectric properties of Ba(Mg⅓Ta⅔)O3 materials prepared via a two-step process

Abstract A two-step process, which includes the formation of B-site compounds, MgTa2O6, followed by the formation of Ba(Mg⅓Ta⅔)O3, BMT, perovskite by reacting BaCO3 with MgTa2O6, has been successfully applied for synthesizing BMT materials. The powders prepared by 2-step process exhibit superior reactivity and result in ceramics with larger Q-factor (Q·fo =600,000 GHz) than the properties obtainable for the materials prepared by the single-step process. Thermogravimetric and differential thermal analyses indicate that the mechanism is the effective suppression on the formation of secondary phase, Ba5Ta4O15