Preparation and characterization of new microwave dielectric ceramics Ba_3-nLa_1+nTi_nNb_3-nO₁₂

FANG Liang (方 亮)^1,2, MENG Sen-sen(孟森森)², ZHANG Hui (张　辉)², WU Bo-lin(吴伯麟)¹

1. Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, Guilin University of Technology, Guilin 541004, China；

2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology, Wuhan 430070, China

Received 10 April 2006; accepted 25 April 2006

Abstract: A series of new microwave dielectric ceramics Ba_3-nLa_1+nTi_nNb_3-nO₁₂ (n=0,1,2) were prepared in the BaO-La₂O₃-TiO₂-Nb₂O₅ system by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction, scanning electron microscope, and microwave dielectric properties measurements. These samples were identified as single phase and adopted A₄B₃O₁₂-type cation-deficient hexagonal perovskite structure, which can be described as consisting of identical perovskite-like blocks, three corner-sharing BO₆ octahedra thick, separated by layers of vacant octahedral. These ceramics have high dielectric constants in the range of 48-42, high quality factors (Q×f ) up to 38 000 GHz, and low temperature coefficient of resonant frequencies (τ_f) in the range of (-40-+6)×10^{-6 ℃-1}. With increasing content of La and Ti, the dielectric constants slightly decrease but τ_f gradually increases.

Key words: Ba_3-nLa_1+nTi_nNb_3-nO₁₂;  BaO-La₂O₃-TiO₂-Nb₂O₅; dielectrics; dielectric resonators; microwave ceramics; hexagonal perovskite

1 Introduction

The dramatic advances during the last two decades in the microwave integrated circuit technology have brought a revolution in telecommunication and satellite broadcasting system. Dielectric resonators (DRs) provide significant advantages in terms of compactness, light mass, temperature stability and relatively low cost in the production of high frequency devices. The important characteristics required for a DR are high dielectric constant (e_r >25) for miniaturization, high quality factor (Q>2 000) for selectivity and low temperature coefficient of resonant frequency (τ_f＜±20×10^{-6 ℃-1}) for stability. Although several DR materials such as Ba(Zn_1/3Ta_2/3)O₃, BaTi₄O₉, Ba₂Ti₉O₂₀, (Zr, Sn)TiO₄, and Ba_6-3xRE_8+2x- Ti₁₈O₅₄ (Re=Nd, Sm, La) systems have been investigated for practical applications[1, 2], the drive for further system miniaturization and improved filtering capabilities requires the development of new materials with higher dielectric constant and lower losses[3-5]. Recently, the microwave dielectric properties of some A_nB_n-1O_3n(n=5, 6, 7 and 8)cation-deficient hexagonal perovskites such as A₅B₄O₁₅ (A=Ba, Sr; B=Nb,Ta), Ba₄La₂Ti₃Nb₂O₁₈, Ba₇Ti₂Nb₄O₂₁ and Ba₈Ti₃Nb₄O₂₄ have been reported [6-9]. These ceramics are characterized by high dielectric constant up to 56, high quality factors with Q×f up to 88 000 GHz. However, there is no report so far on the microwave dielectric properties of any A₄B₃O₁₂-type cation-deficient hexagonal perovskites, then it is worthwhile to investigate whether other A₄B₃O₁₂ perovskites might have equivalent or superior properties. In this study, we report the synthesis, characterization and properties of the cation-deficient hexagonal perovskites Ba_3-nLa_1+nTi_nNb_3-nO₁₂(n=0,1,2) in the BaO-La₂O₃-TiO₂-Nb₂O₅ system.

The ceramics were prepared through the solid-state ceramics route. High purity raw powders BaCO₃ (99.90%), La₂O₃ (99.99%), TiO₂ (>99.95%) and Nb₂O₅ (99.90%), all Aldrich Chemicals were used. The powders were weighed according to the stoichiometry of Ba_3-nLa_1+nTi_nNb_3-nO₁₂(n=0,1,2) and ball milled in distilled water medium for 12 h in an plastic bottle using zirconia balls. The wet mixtures were dried and calcined at 1 200 ℃ for 4 h. The calcined powders were thoroughly reground and mixed with 5% solution of polyvinyl alcohol (PVA) as a binder. The slurries were dried, ground and then pressed into cylindrical disks of different thickness in the range of 5-7 mm and 11 mm in diameter under a pressure of 150 MPa. The green pellets were sintered at 1 300-1 350 ℃ for 6 h. The sintered samples were polished and the bulk densities were measured using the ARCHIMEDE’s method. The phase purity of the sintered samples were studied by XRD using a Rigaku D/MAX-RB X-ray diffractometer using Cu K_a radiation (λ=0.154 06 nm). The fracture surface morphology of the ceramics was studied using a JSM-5610LV scanning electron microscope (SEM).

The microwave dielectric properties such as dielectric constant and Q were measured  using an Agilent 8722ET network analyzer; the dielectric constants were calculated using TE₀₁₁ mode under the end-shorted condition using the method suggested by HAKKI et al[10] and modified by COURTNEY[11]. The τ_f was measured by noting the temperature variation of the TE₀₁₁ resonance in the temperature range of 25-85 ℃.

The Ba_3-nLa_1+nTi_nNb_3-nO₁₂(n=0,1,2) ceramics were sintered into dense bodies. The relative densities of the sintered samples are shown in Table 1. The ceramics show densities in the range of 96%-98% of their theoretical densities. The XRD patterns of ceramics are shown in Fig. 1. The patterns are similar to and match with that of Ba₃LaNb₃O₁₂ (JCPDS file No.44-929 and

Fig.1 XRD patterns of Ba_3-nLa_1+nTi_nNb_3-nO₁₂ ceramics

73-914). The unit cell parameters of the ceramics refined by the least-squares method are also listed in Table 1.

Those compounds crystallize in a cation-deficient hexagonal A₄B₃O₁₂ perovskite structure where Ba and La ions occupy the A sites with coordination number of 12, and Nb and Ti ions disorderly occupy the B sites with coordination number of 6. The crystal structure can be described as consisting of identical perovskite-like blocks, three corner-sharing BO₆ octahedra thick, separated by layers of vacant octahedral[8, 9]. The unit-cell parameters, volumes and tolerance factor of Ba_3-nLa_1+nTi_nNb_3-nO₁₂(n=0,1,2) slightly decrease with increasing content of La and Ti since SHANNON’s effective ionic radius[12] of La³⁺ (0.136 nm) is smaller than that of Ba²⁺ (0.161 nm) at the A site and the radius of Ti⁴⁺ (0.060 5 nm) is smaller than that of Nb⁵⁺(0.064 nm) at the B site.

Fig.2 shows the typical SEM micrographs of Ba₂La₂TiNb₂O₁₂ and BaLa₃Ti₂NbO₁₂. The microstructure indicates a monophase constitution with uniformly hexagonal packed grains in the range of 3-10 ?m size for both ceramics.

The microwave dielectric properties of Ba_3-nLa_1+nTi_nNb_3-nO₁₂(n=0,1,2) were measured under TE₀₁₁ mode, as shown in Table 2. They exhibit high e_r in the range of 42.8-48.3 and high quality factors with Q×f in the range of 31 668-38 000 GHz. Ba₂La₂-

TiNb₂O₁₂ and BaLa₃Ti₂NbO₁₂ may be suitable for

Fig.2 SEM micrographs of Ba₂La₂TiNb₂O₁₂ (a) and BaLa₃Ti₂NbO₁₂ (b)

Table 1 Unit cell parameters and tolerance factor of Ba_3-nLa_1+nTi_nNb_3-nO₁₂ (n=0,1,2)

Table 2 Microwave dielectric properties of Ba_3-nLa_1+nTi_nNb_3-nO₁₂ (n=0,1,2)

practical application as dielectric resonators due to their smaller τ_f close to zero. It is found that e_r gradually decreases from 48.3 to 42.8 with increasing content of La and Ti ions. Generally, the dielectric constant of the single-phase ceramics

Fig.1 Experimental, calculated and difference X-Ray pattern for Ba₅LaTi₂Nb₃O₁₈

Moreover, the values of τ_f of these ceramics increase from -40×10^-6℃-1 to +6×10^{-6 ℃-1} with increasing content of La and Ti ions. It has been reported that the τ_f could be effectively evaluated by tolerance factor in the perovskite-related structure[14]. It is clear that the tolerance factor of these ceramics gradually decrease in contrast to the increases in τ_f, with increasing content of La and Ti ions. Therefore, the τ_f might be controlled by adjusting the tolerance factor values of A₄B₃O₁₂ cation-deficient perovskites in the BaO-La₂O₃-TiO₂-Nb₂O₅ system, further, a near-zero τ_f might be achieved in the ceramics of intermediate compositions between Ba₂La₂TiNb₂O₁₂ and BaLa₃Ti₂NbO₁₂.

4 Conclusions

Ba_3-nLa_1+nTi_nNb_3-nO₁₂(n=0,1,2) ceramics were synthesized and identified as A₄B₃O₁₂ type cation-deficient hexagonal perovskites. The polycry- stalline samples of these perovskites exhibit high dielectric constants in the range of 48.3-42.8, high quality factors with Q×f in the range of 31 668-38 000 GHz, and low τ_f in the range of (-40-+6) ×10^{-6 ℃-1}. The dielectric constants of these ceramics gradually decrease in contrast to an increase in τ_f with increasing content of La and Ti ions, and this suggests the potential for microwave application of the present materials.

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(Edited by YANG You-ping)

Foundation item: Projects(50572078, 20571059) supported by the National Natural Science Foundation of China

Corresponding author: FANG Liang; Tel: +86-773-5896436; Fax: +86-773-5896671; E-mail:fangliang001@263.net