BMT Ceramics Research Articles

Effect of Nonstoichiometry on the Structure and Microwave Dielectric Properties of Ba(Mg0.33Ta0.67)O3

The effect of a slight A- and B-site cation nonstoichiometry on the structure, densification, and microwave dielectric properties of Ba(Mg1/ 3Ta2/3)O3 (BMT) was investigated. Magnesium and barium nonstoichiometric compositions based on Ba(Mg0.33-xTa0.67)O3 [x = −0.015, −0.010, −0.005, 0.0, 0.005, 0.010, 0.015, 0.020, 0.025, and 0.030] and Ba1-x(Mg0.33Ta0.67)O3 [x = −0.015, −0.010, −0.005, 0.0, 0.0025, 0.005, 0.0075, 0.010, 0.015, 0.020, 0.025, and 0.030] were prepared using the conventional solid-state ceramic route. The lattice distortion and cation ordering were determined using X-ray diffraction technique. The phase composition and surface morphology were studied by EDX and scanning electron microscopy techniques, respectively. The sintered samples were characterized in the microwave frequency range using the resonance technique. It is found that a slight barium or magnesium deficiency can improve density, microwave dielectric properties, and cation ordering, while the addition of excess ions deteriorated them. The improvement in microwave dielectric properties was more pronounced in barium nonstoichiometric samples. Microwave dielectric properties of Ba0.9925(Mg0.33Ta0.67)O3 [εr = 24.7, τf = 1.2 ppm/°C, Quxf = 152 580 GHz] and Ba(Mg0.3183Ta0.67)O3 [εr = 25.1, τf = 3.3 ppm/°C and Quxf = 120 500 GHz] were found to be better than stoichiometric BMT [εr = 24.2, τf = 8 ppm/°C and Quxf = 100 500 GHz]. Raman spectroscopy was employed to study the effects of nonstoichiometry and related lattice distortions in BMT ceramics on their vibrational modes. Raman results clearly showed the 1:2 ordered structures of these materials with all active modes assigned. The spectra showed variations in the normal modes as a function of the composition. Also secondary phases contributed to the changes in the Raman spectra observed in compounds with x ≥ 0.02.

Synthesis of Ba(Mg1/3Ta2/3)O3 Microwave Ceramics through a Sol–Gel Route Using Acetate Salts

A sol–gel process to unsophisticatedly synthesize Ba(Mg1/3Ta2/3)O3 (BMT) ceramics at low cost has been developed in the present work. This process involves the reaction of TaCl5 with acetates of Ba and Mg in the presence of citric acid. Pure BMT polycrystalline powders can be obtained by calcining the synthesized products at 1000°C. The BMT powders were found to have a primary particle size as small as 100 nm. BMT ceramics with favorable structural characteristics can be obtained from sintering of the sol–gel BMT at temperatures much lower than that for the conventional solid‐state BMT. Sintering the sol–gel BMT in pellet form at 1300°C resulted in an ordering parameter of 0.72 for the pellet, and a relative density of >95% was achieved with sintering at 1500°C. The grain size of the sintered sol–gel BMT was large and uniform in comparison with the products from the solid‐state method. Using the sol–gel route, sintering at temperatures as low as 1400°C gave ceramics with acceptable microwave dielectric properties (a dielectric constant of 16 and Qf factor of 14 400 GHz), while higher temperatures (>1600°C) are needed for the solid‐state route to give similar properties.

Solution liquid-phase coating processing and microwave characteristics of Ba(Mg 1/3Ta 2/3)O 3 dielectrics

A new technology was adopted to prepare Ba(Mg 1/3Ta 2/3)O 3 powder by using Ta 2O 5· nH 2O nano-particles as base material and magnesium citrate, barium citrate solutions as coating phase through the liquid-phase coating interfacial reaction. The product was investigated by use of TG, DTA, XRD and TEM analysis. The results show that approximately spherical Ba(Mg 1/3Ta 2/3)O 3 particles with high purity and good sintering activity can be synthesized at about 800 °C, the diameter is 20–30 nm. After sintering at 1400 °C for 4 h, the as-prepared BMT ceramics show grains of 2–5 μm and about 95.1% of the theoretical density. Preliminary microwave dielectric measurements reveal that the dielectric constant and the unloaded Q of the ceramics are 24.9 and 8792, respectively, at about 8 GHz.

Effect of glass doping for the sinterability of Ba(Mg 1/3Ta 2/3)O 3 ceramics

Sintering of undoped Ba(Mg 1/3Ta 2/3)O 3 (BMT) ceramics at 1500 °C gives BMT ceramics with 96.0% theoretical density and an ordering structure, but the satellite phases of Ba 5Ta 4O 15, Ba 4Ta 2O 9, Ba 7Ta 6O 22, and Mg 4Ta 2O 9 are difficult to eliminate completely. The addition of MgO–CaO–Al 2O 3–SiO 2 (MCAS) glass as a sintering aid not only decreases the satellite phases of BMT ceramics but also improves the sinterability of BMT ceramics. The densification of the BMT ceramics can be performed well at lower temperatures of 1300–1350 °C. The satellite phases of Ba(Mg 1/3Ta 2/3)O 3 powder can almost be eliminated at 1350 °C when 3 wt.% MCAS is used as a sintering aid or at 1300 °C when 6 wt.% MCAS is used as a sintering aid. When the sintering temperature is higher than 1350 °C and the amount of MCAS addition is equal to or more than 3 wt.%, the densities of BMT ceramics decrease with the increase of sintering temperature because of the increase of satellite phases and the melting of MCAS glass.

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

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.

Ba(Mg1/3Ta2/3)O3 ceramics with different raw materials

Pure Ba(Mg1/3Ta2/3)O3 ceramics were prepared by using different raw materials, Ta2O5xH2O and Ta2O5, (MgCO3)4Mg(OH)25H2O and MgO, as well as BaCO3, were employed as source of Ta, Mg and Ba, respectively. Ba(Mg1/3Ta2/3)O3(BMT) solid-state formation reaction, their sintering behavior, B-site ions ordering and dielectric properties have been studied. The experimental results indicated that the adoption of Ta2O5xH2O could retard the intermediate phase and a pure BMT phase could be synthesized at lower temperature. A pure and dense BMT ceramics with 98.6%theoretical density from Ta2O5xH2O could be obtained at 1600°C. The Q value for the BMT ceramics prepared by using Ta2O5xH2O is higher than that of the same material prepared from Ta2O5.

Nanocomposite versus sol-gel processing of barium magnesium tantalate

Barium magnesium tantalate, Ba(Mg1/3Ta2/3)O3 (BMT) was prepared by a nanocomposite processing route. Density, particle size and microstructure were determined on the BMT nanocomposite pellets sintered at various temperatures and compared with those derived from a sol-gel process. BMT ceramics prepared by the nanocomposite process showed fine grains of about 2 μm and higher densities compared to that of BMT prepared by sol-gel processing, apparently due to a decreased temperature interval between crystallization and densification in the nanocomposite process.

Dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low temperatures

Dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics (BMT ceramics) sintered at low temperatures with 2–3 wt% NaF additives were determined. A dielectric constant of 25 and extremely low dielectric loss (< 0.0001) were measured at 100 kHz and 1 MHz in BMT ceramics sintered under these conditions, and no frequency dependence of the dielectric constant was observed. This suggested that NaF as sintering additive had no harmful influence on the dielectric properties of the ceramics.