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Original Articles

Synthesis and Dielectric Properties of a Temperature Stable Ceramic: Ba4La3.73Sm4.66Bi0.93Ti18O54

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Pages 31-37 | Received 14 Jan 2012, Accepted 30 May 2012, Published online: 11 Sep 2012

Abstract

A temperature stable high dielectric constant Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic was synthesized by conventional solid state route. The structural and micro-structural properties were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The compound crystallizes in the orthorhombic system with unit cell parameters as a = 12.56Å, b = 22.25Å, c = 3.83Å and v = 1070.33Å3. The microwave dielectric properties were measured in the frequency range 0.3 to 3.0 GHz using a network analyzer and it shows a high dielectric constant of 86.1 and low loss tangent of 4.3 × 10–3 at 3.0 GHz along with a very low temperature coefficient of resonant frequency of 4.3 ppm/°C. It could be a promising candidate to be used as dielectric resonator in microwave communication.

1. Introduction

The dramatic progress during the last two decades in the wireless communication and satellite broadcasting system has demanded the need for high quality dielectric ceramics. A high dielectric constant for miniaturization, low loss for selectivity and a near zero temperature coefficient of resonant frequency are the important characteristics required for these materials. Although several materials have been investigated for practical applications, the urge for further miniaturization and improved stability requires the development of new temperature stable high dielectric constant materials.

Microwave dielectric ceramics based on BaO-R2O3-TiO2 (R: Rare earth: Sm, Nd, Pr, La, Gd) ternary system have been studied extensively by various researchers [Citation1Citation4]. Many studies concentrate on the optimization of the dielectric properties by adjusting ionic substitution and modifying structure in a wide range [Citation5, Citation6]. The effects of Sm/Bi co-substitution on the microstructure and microwave dielectric characteristics of Ba6–3xNd8+2xTi18O54 ceramics were studied by Wu and Chin [Citation7] and it was reported that co-substitution could improve the dielectric properties. Similar studies have been carried out on Ba6–3xLa8+2xTi18O54 ceramics [Citation8Citation10].

In the present paper, we report the synthesis and dielectric properties of a new temperature stable high dielectric constant Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic.

2. Experimental Procedure

Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic was synthesized by conventional solid state route where reagent grade BaCO3 (99.5%), La2O3 (99.9%), Sm2O3 (99.9%), Bi2O3 (99.9%) and TiO2 (99.5%) powders were used as raw materials. The weighted raw materials were mixed in methanol for 12 h, dried and calcined in air at 1100°C for 2 h. The calcined powder was re-milled for 12h, dried and mixed with 3–5 wt% PVA (Polyvinyl alcohol). It was then pressed into pellets of different shapes under a load of 98 kN and sintered at 1300°C for 2h in air in a linearly programmable furnace. Finally samples were polished with fine emery paper to make the surfaces flat, smooth and parallel for measurements.

The crystalline phases were analyzed from x-ray powder diffraction pattern obtained using CuKα radiation for 2θ range from 20°−80° (model PWQ 1729, Philip). The microstructure of the freshly fractured surface of the sintered sample was observed by scanning electron microscopy (model JSM 6100, JEOL Japan). The bulk density of the sintered sample was measured by Archimedes method. The microwave dielectric properties were measured by open ended coaxial probe method using a network analyzer (model 8714ET, Agilent Technologies) in the frequency range 0.3 – 3.0 GHz at room temperature.

Thin disc coated with air drying silver paste was used as capacitor to determine dielectric constant in the frequency range 10 kHz – 10 MHz with temperature variation from 25°C to 150°C using an impedance analyzer (model 4192A, Agilent Technologies) and a programmable temperature chamber interfaced with a PC. The temperature coefficient of resonant frequency (τf) was estimated in the temperature range between 25°C and 90°C by using the relation[Citation8]: τf = –((τϵ)/2 + α), where τϵ is temperature coefficient of relative permittivity and α is thermal expansion coefficient which is about 10 ppm/°C [Citation8] in present ceramics. Room temperature field induced polarization was recorded using Radiant Technology's Precision Material Analyzer Workstation (RT6000). The external field was applied at 50 Hz frequency.

3. Results and Discussion

shows the indexed X-ray diffraction pattern of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic. Tungsten bronze type phase was recognized as main phase from the XRD pattern. However, due to presence of Sm and La contents in the synthesized samples, it was difficult to completely interpret XRD pattern because of overlapping of main lines of two tungsten bronze type phases namely BaSm2Ti4O12 and Ba4La8Ti17O50. This solid solution has perovskite structure with orthorhombic symmetry. The lattice parameters obtained by least square refinement method were as follows: a = 12.56Å, b = 22.25Å, c = 3.83Å and v = 1070.33Å3. It was observed that the synthesized ceramic has high bulk density (5.35 g/cm3) and high relative density (94%) and low apparent porosity (6%).

Figure 1 Indexed X-ray diffraction pattern of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic.

Figure 1 Indexed X-ray diffraction pattern of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic.

SEM micrograph of freshly fractured surface of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic coated with gold film is shown in . It possesses typical columnar grain morphology usually observed for tungsten bronze phase [Citation11]. This microstructure evolution confirms the observation of phase identification in XRD pattern. It was also noted that the synthesized sample has a close microstructure with few pores consistent with high measured relative density.

Figure 2 SEM micrograph of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic.

Figure 2 SEM micrograph of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic.

shows the variation of dielectric constant and loss tangent of Ba4La3.73Sm4.66 Bi0.93Ti18O54 ceramic with frequency in the range 0.3 – 3.0 GHz at 25°C. It was observed that dielectric constant decreases from 88.91 at 0.3 GHz to 86.10 at 3.0 GHz and loss tangent increases from1.97 × 10–4 at 0.3 GHz to 4.3 × 10–3 at 3.0 GHz. This is due to phenomenon of frequency dispersion [Citation12]. At higher frequencies, the frequency of hopping between the ions could not follow the applied field frequency and it lags behind the applied field frequency, hence dielectric constant normally decreases and loss tangent increases at higher frequencies.

Figure 3 Variation of dielectric constant and loss tangent of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency at 25°C.

Figure 3 Variation of dielectric constant and loss tangent of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency at 25°C.

The variation of dielectric constant and loss tangent of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency in the range 10 kHz–10 MHz and temperature in the range 25°C − 150°C has been shown in and respectively. The dielectric constant and loss tangent of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic significantly decreases from 109.34 to 108.12 and 5.11 × 10–2 to 4.02 × 10–2 respectively at 25°C with increasing frequency from 10 kHz to 100 kHz, which suggests that at low frequencies the electronic, ionic, dipolar and interfacial/surface polarizations contribute to the dielectric constant. However, above 100 kHz the contribution from the interfacial/surface polarization is minimized, and then the dielectric constant and dielectric loss changed to 95.15 and 4.75 × 10–2 at 10 MHz. This feature is similar to that observed in similar dielectric oxides such as Ba4LaMNb3O15 (M = Ti and Sn) [Citation13]. This study also revealed that no dielectric anomaly was observed for this material in the temperature range from 25°C to 150°C. It is assumed that there is no structural transition over the temperature range of interest. This investigation is in accordance with the works of Belous et al. [Citation14, Citation15] on Ba6–3xR8+2xTi18O54 solid solutions where R = rare earth.

Figure 4 Variation of dielectric constant of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency and temperature.

Figure 4 Variation of dielectric constant of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency and temperature.

Figure 5 Variation of loss tangent of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency and temperature.

Figure 5 Variation of loss tangent of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic with frequency and temperature.

The temperature coefficient of resonant frequency of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic was estimated to be 4.3 ppm/°C in the temperature range from 25°C to 90°C at 1 MHz. It should be noted, however, that the estimated value of temperature coefficient of resonant frequency was only approximate. Nevertheless, this data could be useful for improving the temperature characteristics of resonant frequency of the material under study. The d.c. conductivity of Ba4La3.73Sm4.66Bi0.93Ti18O54 solid solution was very low (5.12 × 10–9 S/cm), which was expected from materials of very high dielectric constant.

The a.c. conductivity of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramics increased from 2.85 × 10–6 S/cm to 5.95 × 10–6 S/cm with increase in frequency from 0.3 GHz to 3.0 GHz. The hopping of electrons between Ti3+ and Ti4+ ions on the octahedral sites could be responsible for conduction. As the frequency of the applied field is increased, the conductive grains become more active by promoting the hopping between ions, thereby increasing the a.c. conductivity.

shows the PE curve of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic at 25°C. The linear nature of polarization curve indicates that the material under study is paraelectric, that is, it polarizes under the influence of external electric field and removal of field results in polarization of material returning to zero.

Figure 6 PE curve of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic at 25°C.

Figure 6 PE curve of Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic at 25°C.

4. Conclusions

A new temperature stable Ba4La3.73Sm4.66Bi0.93Ti18O54 microwave dielectric ceramics was prepared by conventional solid state route sintered at 1300°C. X-ray diffraction analysis shows the formation of tungsten bronze phase in the orthorhombic crystal structure at room temperature. SEM micrographs of synthesized solid solution shows closely packed, uniform, elongated grain distribution on the surface.

Studies related to temperature dependent dielectric properties shows that the studied ceramic possess very high dielectric constant with low loss and low temperature coefficient of resonant frequency.

This material has great potential for applications in wireless communication at microwave frequencies in the region from 0.3 to 3.0 GHz. It can be used to improve the performance of resonators and filters used in cellular phones.

Acknowledgments

Communicated by Dr. Deborah J. Taylor

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