简介概要

Microwave dielectric properties and microstructure of a new kind Eu-based Eu(Mg_0.5Ti_0.5)O₃ ceramic

来源期刊：Rare Metals2018年第11期

论文作者：Tao Shi Jing Zhu

文章页码：913 - 918

摘要：A new Eu-based microwave dielectric ceramic,Eu(Mg_0.5Ti_0.5)O₃（EMT for short）, was prepared through conventional solid-state reactions. The EMT ceramics were well densified at 1425 ℃ and exhibit good microwave dielectric properties with permittivity of 23.92, high quality factor（Q*f） of 66,957 GHz at 8.93 GHz and relatively low-temperature coefficient of resonant frequency of-20.3 10^-6/K. Additionally, the variations of dielectric constants among Ln(Mg_0.5Ti_0.5)O₃（Ln represents rare earth elements, LMT） compounds were analyzed. Ionic radii of Ln³⁺, associated with the tolerance factors of the perovskite structure of LMT, were investigated to be the key factor to influence the permittivity. These results indicate that EMT ceramics might be one of the promising candidates as microwave resonators.

详情信息展示

稀有金属(英文版) 2018,37(11),913-918

Microwave dielectric properties and microstructure of a new kind Eu-based Eu(Mg_0.5Ti_0.5)O₃ ceramic

Tao Shi Jing Zhu

National Center for Electron Microscopy in Beijing, Tsinghua University

School of Materials Science and Engineering, Tsinghua University

The State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University

Laboratory of Advanced Materials, Tsinghua University

作者简介：*Jing Zhu,e-mail: jzhu@mail.tsinghua.edu.cn;

收稿日期：4 January 2018

基金：financially supported by the National Basic Research Program of China (No. 2015CB654902);the National Natural Science Foundation of China (Nos. 51390471 and 51527803);the National Key Research and Development Program (No. 2016YFB0700402);

Microwave dielectric properties and microstructure of a new kind Eu-based Eu(Mg_0.5Ti_0.5)O₃ ceramic

Tao Shi Jing Zhu

National Center for Electron Microscopy in Beijing, Tsinghua University

School of Materials Science and Engineering, Tsinghua University

The State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University

Laboratory of Advanced Materials, Tsinghua University

Abstract：

A new Eu-based microwave dielectric ceramic,Eu(Mg_0.5Ti_0.5)O₃(EMT for short), was prepared through conventional solid-state reactions. The EMT ceramics were well densified at 1425 ℃ and exhibit good microwave dielectric properties with permittivity of 23.92, high quality factor(Q*f) of 66,957 GHz at 8.93 GHz and relatively low-temperature coefficient of resonant frequency of-20.3 10^-6/K. Additionally, the variations of dielectric constants among Ln(Mg_0.5Ti_0.5)O₃(Ln represents rare earth elements, LMT) compounds were analyzed. Ionic radii of Ln³⁺, associated with the tolerance factors of the perovskite structure of LMT, were investigated to be the key factor to influence the permittivity. These results indicate that EMT ceramics might be one of the promising candidates as microwave resonators.

Keyword：

Microwave dielectrics; Eu(Mg_0.5Ti_0.5)O₃; Complex perovskite; Solid-state reactions;

Received： 4 January 2018

1 Introduction

With the development of wireless communication systems including cell phones and GPS,dielectric materials used in these areas have new requirements [ 1, 2, 3] ,i.e.,proper dielectric permittivity (usually 10-100),low dielectric loss and a near to zero temperature coefficient of the resonant frequency (τ_f).The size of electric components is closely related to the dielectric permittivity,so a relatively higher dielectric constant can simply lead to a smaller size of components,satisfying the demand of the miniaturization of components.The high quality factor reduces the energy loss during information transports at extremely high frequencies,while a lowτ_f helps dielectric materials remain stable during temperature variations.Among various kinds of microwave dielectric materials,materials with complex perovskite structure [ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] (usually expressed as A( )O₃) have accepted wide attentions due to their high quality factors.

Because of similar physical and chemical properties among different rare earth elements,microwave dielectric materials made up of rare earth elements usually exhibit similar properties.Furthermore,due to their minor differences in the ionic radii,their compounds usually exhibit continuous variations in microwave dielectric properties such as permittivity andτ_f,which remains great space for researchers to manipulate these properties through doping proper contents of different rare earth elements [ 14, 15, 16, 17, 18, 19] .Recently,Ln(Mg_1/2Ti_1/2)O₃ (Ln represents rare earth elements,LMT for short) ceramics,one of the A( )O₃families,have attracted attentions due to their extremely high quality factor and tunable dielectric permittivity [ 20, 21] .Additionally,they are also important substrate materials for high temperature superconductive films and perovskite-type ferroelectric films due to their close lattice constants.Among them,some important compounds such as Eu(Mg_0.5Ti_0.5)O₃ (EMT for short) are still lack of research.However,Eu-based materials such as Eu_xBa_1-xTiO₃ [ 22, 23] and EuTiNbO₆ [ 16] are of excellent performances in both dielectrics and microwave dielectrics.Thus,in this work,EMT ceramics with pure phase were sintered through conventional solid-state reactions for the first time.Thereafter,the crystal structure and micros tructure were studied,while the microwave dielectric properties were measured and discussed associated with the microstructure.

2 Experimental

The EMT ceramics were prepared by conventional solidstate reactions.High-purity oxide powders (>99.5%) of Eu₂O₃,MgO and TiO₂ were used as raw materials.To ensure the chemical composition,a two-step process was used.A precursor MgTiO₃ was made before the final sintering of EMT ceramics.The powders of MgO and TiO₂were mixed by ball milling for 6 h in alcohol.After drying the slurry,the dried powder mixtures were pressed into disks with diameter of 10 mm and thickness of 2 mm under 4 MPa using 5%polyvinyl alcohol as a binder,followed by burning the binder at 400℃and sintering at1100℃to get the MgTiO₃ samples with pure phase.The sintered MgTiO₃ disks were then broken into powders and mixed with Eu₂O₃ powders,followed by the same ball milling and drying process.The dried mixture of MgTiO₃powders and rare earth oxides were then pressed into cylinders with diameter of 10 mm and height of 6 mm at6 MPa,followed by cold isostatic pressing at 200 MPa.The cylinders were final sintered at certain temperatures to get the EMT samples.

The phase structure of the ceramics was determined using X-ray diffractometer (XRD,Cu Kα1,D8advance,H2500).The bulk densities of sintered specimens were measured by Archimedes methods.The surfaces of the specimens were observed in a field-emission scanning electron microscopy (SEM,Merlin) operated at 15 kV.The microwave properties were measured by a network analyzer (Agilent HP8720ES).The quality factor (Q) at microwave frequencies was measured in TE₀₁₁ mode,while the permittivity and the temperature coefficient of resonant frequency were performed in the TE_01δmode [ 24] .

3 Results and discussion

3.1 Phase structure

XRD patterns recorded at room temperature of the EMT ceramics sintered at 1400℃are illustrated in Fig.1.All LMT compounds exhibit the same perovskite structure with similar XRD patterns [ 21] .Meanwhile,Sm³⁺has the closest ionic radii with Eu³⁺.Thus,the pattern of Sm(Mg_0.5Ti_0.5)O₃ (SMT for short) marked in blue is referenced as comparison with that of EMT and is coincided well with the reported PDF card no.37-0954 [ 25, 26] ,confirming the SMT pellet to be a pure phase.The distributions and shapes of the peaks of the EMT marked in red in Fig.1 are nearly the same with those of SMT,indicating that the EMT has a similar phase structure with SMT.Additionally,all the peaks of the EMT are at the right side compared with those of SMT,according to the enlarged spectrum in the range from 31°to 35°in the insert diagram in Fig.1.Therefore,the lattice constants of the EMT materials are slightly smaller than those of SMT,which is mainly induced by smaller ionic radius of Eu³⁺compared with that of Sm³⁺ [ 27] .Accordingly,the space group and lattice constants of the EMT are calculated and shown in Table 1.

3.2 Density related to sintering temperature in EMT ceramics

The EMT ceramics were sintered in the temperature range from 1400 to 1450℃.All the EMT samples exhibit the same XRD pattern with the samples calcined at 1400℃shown in Fig.1,indicating that all samples have formed a pure phase.The density of the EMT ceramics versus sintering temperature is shown in Fig.2a.The density firstly increases with the increase in the sintering temperature,and the optimized temperature is 1425℃at which the ceramic attains a maximum densification of 96.9%.Thereafter,further increasing the sintering temperature reduces the density of the ceramic instead.

Fig.1 XRD patterns of SMT and EMT (insert diagram illustrating details in 2θrange from 31°to 35°)

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Table 1 Space group and lattice constants of EMT

Fig.2 a Variation of bulk densities and relative densities at different sintering temperatures and b variation of permittivity and quality factor at different sintering temperatures

Figure 3a-c illustrates SEM images of the raw surface of the EMT calcined at 1400,1425 and 1450℃,respectively.Although the EMT ceramics sintered at 1400℃exhibit a pure phase according to the XRD pattern,there still exist numerous pores around grain boundaries.As the sintering temperature rises to 1425℃,the pores around grain boundaries disappear,which is consistent well with the enhancement of the densities.Figure 3d-i shows typical EDS analysis of the EMT samples.No segregation of any compositional elements can be seen,indicating that all samples have formed homogenous pure phases.However,when the sintering temperature rises to 1450℃,overheated areas marked as red circle in Fig.3c appear,leading to the final decrease in the density.

3.3 Microwave dielectric properties

The microwave dielectric properties of the EMT as a function of sintering temperature are shown in Fig.2.As the sintering temperature rises,the dielectric constant increases and then reaches a maximum value of 23.92 at1425℃.The increase in the permittivity is associated with the disappearance of the pores shown in Fig.3b.The influence of the porosity on the permittivity can be described by Bosman and Havinga’s correction [ 28] as shown in Eq.(1):

whereε_corrected is the corrected values of permittivity,whileε_m is the experimental values;P is the fractional porosity(0.031 in this work).The corrected permittivity of the EMT is about 25.03.

Tolerance factors [ 29, 30, 31, 32] (TFs for short) are widely discussed and accepted as one of the key factors to influence the structures and dielectric properties of materials with perovskite structure.TFs are usually expressed as Eq.(2):

where r_A,r_B and r_O represent the ionic radii of A-site ions,B-site ions and O^2-in perovskite structure,respectively.Figure 4 illustrates the variations of microwave dielectric constants and tolerance factors of Ln(Mg_0.5Ti_0.5)O₃ (Ln represents rare earth elements) ceramics as the atomic numbers of Ln.Here r_B is considered as the average ionic radii of Mg²⁺and Ti⁴⁺,because Mg²⁺and Ti⁴⁺are randomly occupied as B-site ions with the same contents.The effective ionic radii of rare earth elements decrease with the increase in the atomic number [ 27] ,resulting in the decrease in TFs.For dielectric materials with perovskite structures,the displacements of B-site ions are the main source of polarizations which directly determine the dielectric constants [ 33] .TFs greatly influence the volumes of the oxygen octahedrons which allow the displacements of B-site ions.Therefore,a dielectric material with a bigger TF usually exhibits more excellent dielectric properties.Particularly for the LMT system,the variation of the dielectric constants also follows the regularity.With the increase in the atomic number of Ln,the ionic radii of Ln³⁺reduce continuously,resulting in the reduction in TFs,as shown in Fig.4.Therefore,the permittivity decreases linearly with the decrease in TFs,as well as the increase in the atomic number.

Fig.3 SEM images of raw surfaces of samples of EMT sintered at a 1400℃,b 1425℃and c 1450℃;d SEM image,elemental mapping of e O Kα1,f Mg Kα1,g Eu La1,h Ti Kα1 and i EDS analysis

Fig.4 Variations of permittivity and tolerance factors of Ln(Mg_0.5Ti_0.5)O₃ with different atomic numbers of Ln,where data of permittivity being from literature [20,21] and this work

The changes of high quality factor (Q^*f) of the EMT ceramics sintered at various temperatures are also illustrated in Fig.2b.Based on the theories of microwave dielectric loss,it is widely accepted that the quality factor is sensitive to the defects including secondary phase,vacancy defects and density [ 34, 35] .Compared with other metal elements,Mg is a more volatile constituent due to a lower statured vapor pressure.Therefore,at higher sintering temperatures,the vacancy densities of the EMT material increase significantly due to the lack of Mg,leading to the final dramatic fall of quality factor at1425℃.

4 Conclusion

In a summary,a new Eu containing orthogonal perovskite oxide EMT with pure phase was synthesized and characterized.EMT ceramics are determined to be perovskite structure with the space group of Pun2₁.Additionally,the material exhibits a dielectric permittivity of 23.92,high quality factor (Q^*f) of 66957 GHz at 8.93 GHz and relatively low-temperature coefficient of resonant frequency of-20.3×10^-6/K at its maximum densification sintered at1425℃.These results indicate that the EMT is a promising candidate in microwave resonator.Besides,the dielectric constants of LMT are directly determined by TFs and decrease continuously with the increase in the atomic number of Ln.

Acknowledgements This work was financially supported by the National Basic Research Program of China (No.2015CB654902),the National Natural Science Foundation of China (Nos.51390471 and51527803) and the National Key Research and Development Program(No.2016YFB0700402).This work made use of the resources of the National Center for Electron Microscopy in Beijing.