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J. Korean Ceram. Soc. > Volume 56(4); 2019 > Article
Hong, Kim, Kwon, and Yoon: Microwave Dielectric Properties of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 Ceramics

Abstract

The phase evolution, microstructure, and microwave dielectric properties of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 (0 ≤ x ≤ 0.05) ceramics were investigated. All compositions exhibited a 1:1 ordered perovskite structure. As the value of x increased, the dielectric constant (ɛr) exhibited a tendency to increase slightly. The quality factor reached the maximum value at x = 0.01. The temperature coefficient of resonant frequency (τf) increased from −19.32 ppm/°C to −5.64 ppm/°C in the positive direction as x increased. The dielectric constant (ɛr), quality factor (Q × f0), and temperature coefficient of resonant frequency (τf) of the composition x = 0.05, i.e., (Ba0.95Na0.05)(Mg0.4Y0.1W0.45Ta0.05)O3 were 19.9, 128,553 GHz, and −5.6 ppm/°C, respectively.

1. Introduction

According to the rapid growing of commercial wireless communication industry, many works of microwave dielectric ceramics used for mobile phone, wireless LAN (local area network), GPS (global position satellite), and ITS (intelligent transport system) are being actively conducted.1-3) For applications in resonators, filters, and oscillators at microwave frequencies, microwave dielectric ceramics should have high dielectric constant (ɛr) for miniaturization, high quality factor (Q × f0) for high frequency selectivity, and nearly zero temperature coefficient of resonant frequency (τf) for thermally stable circuits.4)
Among the various dielectric resonators at microwave frequencies such as Ba(M0.33Ta0.67)O3 (where M = Mg2+ and Zn2+) with 1:2 ordered structure of B-site cations in the perovskite, 5,6) Ba(Mg0.5W0.5)O3 (BMW) having the ordered perovskite structure, in which B-site cations are 1:1 ordered because their large difference in size and charge, has been investigated since Takahashi et al. reported that the dielectric properties of BMW are ɛr = 16.7, Q × f0 = 42,000 GHz, and τf = −33.6 ppm/°C.7-10) Bian et al. reported that the composition of x = 0.3 in the Ba[{Mg(1−x)/2Yx/3(VMg)x/6}W1/2]O3 system exhibited the dielectric properties ɛr = 21.9, Q × f0 = 133,000 GHz, and τf = −2.4 ppm/°C.8) Lin et al. investigated the microwave dielectric properties of the (Ba1−xSrx)-(Mg0.5W0.5)O3 system, and found that the composition of x = 0.25 had the dielectric properties ɛr = 20.6, Q × f0 = 152,600 GHz, and τf = +24 ppm/°C.9) Wu et al. reported that the composition of x = 0.02 in the (1−x)Ba(Mg0.5W0.5)O3-(x)Ba(Y0.67 W0.33)O3 system exhibited the dielectric properties ɛr = 20, Q × f0 = 160,000 GHz, and τf = −21 ppm/°C.10) In this paper, we investigate the phase evolution, microstructure, and microwave dielectric properties of Na2O-, Y2O3-, and Ta2O5-doped BMW ceramics, i.e., the (Ba1−xNax)-(Mg0.5−2xY2xW0.5−xTax)O3 system (0 ≤ x ≤ 0.05).

2. Experimental Procedure

Raw powders of BaCO3 (purity 2N5, Sakai Chem. Ind. Co., Ltd., Japan), MgO (purity 2N, High Purity Chem. Co., Ltd., Japan), Y2O3 (purity 4N, High Purity Chem. Co., Ltd., Japan), Na2CO3 (purity 2N5, Samcheon Chemical Co., Ltd. Korea), WO3 (purity 4N, High Purity Chem. Co., Ltd., Japan), and Ta2O5 (purity 3N, High Purity Chem. Co., Ltd., Japan) were mixed to prepare the (Ba1−xNax)(Mg0.5−2xY2x-W0.5−xTax)O3 system (0 ≤ x ≤ 0.05). The appropriate ratios of raw powders were ball-milled using zirconia balls and ethyl alcohol in a polyethylene container for 24 h. After drying in an oven, the powder mixture was calcined at 900°C for 10 h in an alumina crucible, followed by pulverizing and uniaxial pressing at 50 MPa to form disk-type specimens 15 mm in diameter. The disk-type specimens were sintered at 1700°C for 1 h. The crystalline phases of the sintered specimens were identified by a powder X-ray diffractometer (XRD, D/MAX-2500V/PC, Rigaku, Japan). The microstructure of the sintered specimens was characterized by a field emission scanning electron microscope (FE-SEM, Quanta 250 FEG, FEI, U.S.A.). Microwave dielectric properties of the specimens were determined using network analyzers. The dielectric constant was measured according to the Hakki-Coleman method using a network analyzer (E5071C, Keysight, U.S.A.). The quality factor was measured by the cavity method using the same equipment. The temperature coefficient of the resonant frequency was measured by the cavity method using a network analyzer (R3767CG, Advantest, Japan) at temperatures ranging from 20°C to 80°C.

3. Results and Discussion

The XRD patterns of Na2O-, Y2O3-, and Ta2O5-doped BMW, i.e., (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3, ceramics are shown in Fig. 1. All compositions show a 1:1 ordered perovskite structure, i.e., an ordered arrangement of MgO6 and WO6 octahedra in the B-site of the perovskite structure. For all compositions, BaWO4 with low melting point of 1475°C was detected as the secondary phase. It has been reported that BaWO4 is usually formed during the sintering process of Ba(Mg0.5W0.5)O3 owing to its structural instability at high temperatures.8-10) Khalyavin et al. proposed the mass balance reaction Ba2MgWO6 → BaWO4 + MgO + BaO as a potential mechanism for the evolution of BaWO4.11) The intensity ratio of the (112) plane for BaWO4 to the (220) plane for BMW is shown in Fig. 2. As x increased, the intensity ratio increased, indicating that the addition of the dopants may promote the formation of BaWO4.
The lattice parameters of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics are shown in Fig. 3. As the amount of dopants increased, the lattice parameters of the BMW ceramics increased linearly, indicating the occurrence of a substitutional solid solution. The substitution of Y3+ ions larger than that of Mg2+, where the ionic radii of Y3+ and Mg2+ ions are 0.9 Å and 0.72 Å, respectively, when the coordination number is 6, may lead to the increase of lattice parameters. The lattice parameter of undoped BMW ceramics was measured as 8.1092 Å; this value is reasonable because it was reported as between 8.1072 Å (sintered at 1650°C) and 8.1115 Å (at 1600°C).11)
The microstructure of the Na2O-, Y2O3-, and Ta2O5-doped BMW ceramics was observed by FE-SEM. The typical microstructures (compositions with x = 0.01, 0.03, and 0.05) are shown in Fig. 4. All the compositions exhibited a dense microstructure with polyhedron-shaped grains. In the case of x = 0.05 composition, large grains were observed. According to the EDS result for the x = 0.05 composition as shown in Fig. 4(d), the large grains were identified as BaWO4. At the sintering temperature of 1700°C, the liquid BaWO4 contributed to densification.
The variations of linear shrinkage, dielectric constant (ɛr), and quality factor (Q × f0) for the Na2O-, Y2O3-, and Ta2O5-doped BMW ceramics are shown in Fig. 5. As x increased, the linear shrinkage increased. The dielectric constant exhibited a tendency to increase slightly as the x value increased, i.e., with increasing dopant concentration. The dielectric constant is mainly influenced by the relative density and ionic polarizability. The slight increase of the dielectric constant can be attributed to the increase of ionic polarizability by Y2O3 and Ta2O5 doping; the ionic polarizability of the Y3+ ion (αY3+ = 3.81 (Å3)) and Ta5+ ion (αTa5+ = 4.73 (Å3) is larger than that of the Mg2+ ion (αMg2+ = 1.31 (Å3)) and W6+ ion (αW6+ = 3.2 (Å3), respectively.12) The quality factor (Q × f0), i.e., the inverse of the dielectric loss, of undoped BMW was measured as 59,738 GHz, whereas the quality factor of doped BMW has a maximum value of 237,188 GHz in the composition x = 0.01, which gradually decreased with increasing x.
The dielectric losses or the inverse of the quality factor were classified into intrinsic and extrinsic categories.13) The intrinsic dielectric losses depend on the crystal structure, ac field frequency, and temperature. The extrinsic losses are associated with the microstructure, e.g., pores, grain size, grain boundaries, and secondary phases. Reany and Iddles have pointed out that, in reality, extrinsic losses dominate the quality factor.3) The maximum Q × f0 value when x = 0.01 is attributed to increased density (Fig. 5(a)) and minimum amount of BaWO4 phase (Fig. 2). The temperature coefficient of resonant frequency (τf) as a function of the calculated tolerance factor (t) using Shannon ionic radii is illustrated in Fig. 6.14) The temperature coefficient of resonant frequency (τf) increased monotonically from −19.32 ppm/°C to −5.64 ppm/°C as x increased, i.e., tolerance factor decreased. It is generally considered that the temperature coefficient of resonant frequency of perovskite structures is related to the tolerance factor, i.e., the degree of oxygen octahedral tilting.15) In the perovskite structure, the temperature coefficient of resonant frequency was reported to be related to the tilting of BO6. The tilting of YO6 was mainly caused by the employment of Y3+ (0.9 Å) with ion radius larger than Mg2+ (0.72 Å). The results of linear shrinkage, lattice parameters, and dielectric properties of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics are summarized in Table 1.

4. Conclusions

The phase evolution, microstructure, and microwave dielectric properties of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 (0 ≤ x ≤ 0.05) ceramics were investigated. In addition to the 1:1 ordered perovskite structure of Ba(Mg0.5W0.5)O3, a secondary phase of BaWO4 was created. As x increased, the dielectric constant increased slightly and the quality factor showed a tendency to reach the maximum value at x = 0.01, and then decreased. The temperature coefficient of resonant frequency (τf) increased from −19.32 ppm/°C to −5.64 ppm/°C in the positive direction as x increased. The dielectric constant of the x = 0.05 composition was 19.9, the quality factor was 128,533 GHz, and the temperature coefficient of resonant frequency was −5.6 ppm/°C.

Fig. 1
Powder X-ray diffraction patterns of (Ba1−xNax) (Mg0.5−2xY2xW0.5−xTax)O3 ceramics.
kcers-2019-56-4-12f1.jpg
Fig. 2
IBaWO4(112)/IBMW(220) of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics.
kcers-2019-56-4-12f2.jpg
Fig. 3
Lattice parameters of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics as a function of x.
kcers-2019-56-4-12f3.jpg
Fig. 4
FE-SEM images of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics; (a) x = 0.01, (b) x = 0.03, (c) x = 0.05, and (d) EDS result of large grains for x = 0.05.
kcers-2019-56-4-12f4.jpg
Fig. 5
(a) Linear shrinkage, (b) dielectric constant, and (c) quality factor of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics as a function of x.
kcers-2019-56-4-12f5.jpg
Fig. 6
Temperature coefficient of resonant frequency of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 ceramics as a function of tolerance of x.
kcers-2019-56-4-12f6.jpg
Table 1
Linear Shrinkage, Lattice Parameters, and Dielectric Properties of (Ba1−xNax)(Mg0.5−2xY2xW0.5−xTax)O3 Ceramics
x Linear shrinkage (%) Lattice parameter, a0 (Å) ɛr Q × f0 (GHz) τf (ppm/°C)
0 18.3 8.1092(3) 17.3 59,738 −27.3
0.01 21.3 8.1260(4) 18.8 237,188 −19.3
0.02 22.0 8.1431(6) 19.0 183,600 −13.9
0.03 22.2 8.1601(14) 19.3 174,078 −12.8
0.04 22.4 8.1781(11) 19.7 160,964 −9.4
0.05 22.5 8.1942(6) 19.9 128,553 −5.6

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