Effect of MgO on microstructure and microwave dielectric properties of 0.84CaTiO3–0.16Sm0.9Nd0.1AlO3 ceramics

doi:10.1016/j.materresbull.2014.09.082

Materials Research Bulletin

Volume 67, July 2015, Pages 226-229

https://doi.org/10.1016/j.materresbull.2014.09.082 Get rights and content

Highlights

•
An optimal microwave dielectric property was obtained with ϵ_r = 64.21, Q × f = 30867 GHz and τ_f = +26.4 ppm/°C.
•
MgO inhibited the migration of grain boundaries, which suppressed the grain growth and promoted homogeneity in microstructures consequently.
•
MgAl₂O₄ secondary phase was formed when MgO addition was over 0.6 wt% due to the solubility limit of Mg.

Abstract

The effect of MgO content on the microstructure and microwave dielectric properties of MgO-doped 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ ceramics, prepared by solid-state reaction method, was investigated. The permittivity, ϵ_r, of the MgO-doped 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ ceramics was about 65, and it decreased slightly with increasing MgO content. And the temperature coefficient of resonant frequency, τ_f was tuned from +53.3 ppm/°C to +26.4 ppm/°C as the MgO addition increasing. In addition, it is found that a small amount of MgO could effectively promote the uniformity of the grain morphology, which is beneficial for the improvement of Q × f value. However, grain growth was restrained, and secondary phase MgAl₂O₄ was observed as MgO content increasing to 0.8 wt%; thus, Q × f value deteriorated seriously. Considering these three parameters of the microwave dielectric properties, the 0.4 wt% MgO-doped 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ ceramics exhibited the best performance of ϵ_r = 64.21, Q × f = 30867 GHz, and τ_f = 26.4 ppm/°C sintered at 1375 °C for 3 h.

Graphical abstract

Introduction

The rapid progress of the third generation to the fourth generation telecommunication has driven the improvement of microwave dielectric ceramics used for base station in cellular networks. Three key property parameters are paid much attention [1], [2], [3]. They include high permittivity, ϵ_r, to reduce the size of a circuit, high quality factor, Q × f, which is necessary for frequency selectivity, and near-zero temperature coefficient of the resonator frequency, τ_f, to prevent frequency drifting. Most commercial materials have a permittivity around 40, such as Ba(Co, Zn)_1/3Nb_2/3O₃ [4], [5], CaTiO₃–NdAlO₃ [6], [7], [8], and (Zr, Sn)TiO₄ [2], [9]. In the light of the demand to minimize the size of microwave devices, permittivity should be higher. However, materials with permittivity between 50 and 70 have not been extensively studied. This is because high Q × f and zero τ_f were hardly obtained simultaneously [10], [11], [12], [13].

Solid solution materials are formed between one positive τ_f and another negative τ_f material to tailor the dielectric properties. For example, the properties of xCaTiO₃–(1 − x)LnAlO₃ (Ln = La, Nd, Sm) solid solution can be tuned between two end compositions [14], [15], [16], [17], [18]. Considering that CaTiO₃ has ϵ_r = 170 and SmAlO₃ has ϵ_r = 20.4, the permittivity varies in a wide range from 20 to 70 with the ratio of two end members and it was found that 0.84CaTiO₃–0.16SmAlO₃ has permittivity around 65 [19], [20], [21]. Therefore, it is significant to design materials with permittivity between 50 and 70 in xCaTiO₃–(1 − x)SmAlO₃ solid solutions.

In this work, MgO doped 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ solid solution was prepared by conventional solid-state reaction method. The effect of Mg dopant on structure and dielectric properties was investigated.

Section snippets

Experimental procedure

To prepare MgO doped 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ (0.84CTSA) ceramics, CaTiO₃ and Sm_0.9Nd_0.1AlO₃ were synthesized respectively. Starting materials included Sm₂O₃, Nd₂O₃, Al₂O₃, CaCO₃, TiO₂, and MgO powders with high-purity (99.9%). CaTiO₃ and Sm_0.9Nd_0.1AlO₃ were weighed separately according to the stoichiometric composition. The blended powders were then milled with ZrO₂ balls in deionized water for 24 h, dried and calcined at 1170 °C for 3 h.

According to the formula of 0.84CaTiO₃–0.16Sm_0.9Nd_0.1

Results and discussion

Fig. 1 shows the XRD patterns of 0.84CTSA ceramics added with various amount of MgO sintered at 1375 °C. It can be seen that a perovskite phase was obtained. No apparent variation was observed.

Fig. 2 shows the SEM images of x wt% MgO-doped 0.84CTSA ceramics sintered at 1375 °C. All the samples are well sintered, and dense microstructure are obtained. The grains become more homogeneous as MgO amount is increased. However, for the specimen with x = 0.8 and x = 2, the grain size decreases to 2 μm. This

Conclusions

The MgO added 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ ceramics were prepared via solid state reaction method. The addition of MgO inhibited the migration of grain boundaries, which promoted homogeneity in microstructures and suppressed the grain growth consequently. When the addition was over 0.6 wt%, the solubility limit of Mg was reached and it induced the segregation of MgAl₂O₄ secondary phase. The combined effect of polarizablity and secondary phase lowered ϵ_r and τ_f as MgO amount increased. A

Acknowledgments

The work was supported by Ministry of Sciences and Technology of China through National Basic Research Program of China (973 Program 2009CB623301), National Natural Science Foundation of China for Creative Research Groups (Grant No. 51221291), National Natural Science Foundation of China (Grant No. 51272123), National Natural Science Foundation of China for distinguished young scholars (Grant No. 50625204), and CBMI Construction Co., Ltd. This work made use of the resources of the Beijing

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Effect of MgO on microstructure and microwave dielectric properties of 0.84CaTiO3–0.16Sm0.9Nd0.1AlO3 ceramics

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgments

Curr. Opin. Solid State Mater. Sci.

J. Eur. Ceram. Soc.

Mater. Res. Bull.

J. Eur. Ceram. Soc.

Mater. Lett.

J. Eur. Ceram. Soc.

Mater. Res. Bull.

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

J. Am. Ceram. Soc.

Dielectric Materials for Wireless Communications

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

Acta Chim. Slov.

Chem. Mater.

Effect of MgO on microstructure and microwave dielectric properties of 0.84CaTiO₃–0.16Sm_0.9Nd_0.1AlO₃ ceramics