Microwave Dielectric Properties of Sr-Substituted Ba(Mg_0.5W_0.5)O₃ Ceramics

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Abstract

1. Introduction

Motivated by the rapid growth of the commercial wireless communication industry, research on microwave dielectric ceramics used for mobile phones, wireless local area networks (LAN), global positioning system (GPS), and intelligent transport systems (ITS) is being actively conducted.1,2) For a ceramic to be usable as a dielectric resonator/filter, three key properties need to be optimized; dielectric constant, ɛ_r, should fulfill the condition 20 < ɛ_r < 50, dielectric loss or its inverse, the quality factor, Q > 30,000 at 1 GHz, and the temperature coefficient of resonant frequency, τ_f ~ ± 3 ppm/°C.3) Structural stability and microwave dielectric properties of Ba(Mg_0.5W_0.5)O₃ systems with a 1 : 1 ordered perovskite structure have been investigated widely4–14) ever since Takahashi et al. reported the dielectric properties of Ba(Mg_0.5W_0.5)O₃ ceramics with ɛ_r = 16.7, Q × f₀ = 42,000 GHz, and τ_f = − 33.6 ppm/°C.4) Wu et al. reported that for x = 0.02, the (1-x)Ba(Mg_0.5W_0.5)O₃-(x)Ba(Y_0.67W_0.33)O₃ system exhibited dielectric behavior with ɛ_r = 20, Q × f₀ = 160,000 GHz, and τ_f = − 21 ppm/°C.10) Bian and Wu reported that for x = 0.30, the Ba[{Mg_(1-x)/2Y_x/3(□_Mg)_x/6}W_1/2]O₃ (□ refers to the vacancy) system exhibited dielectric behavior with ɛ_r = 21.9, Q × f₀ = 133,000 GHz, and τ_f = − 2.4 ppm/°C.13) Lin et al. investigated the microwave dielectric properties of the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ system with the compositions at wide intervals of x = 0, 0.25, 0.50, and 0.75 and found that at x = 0.25, the system exhibited the following dielectric properties: ɛ_r = 20.6, Q × f₀ = 152,600 GHz, and τ_f = +24 ppm/°C.12) In this study, we have investigated the phase evolution, microstructure, and microwave dielectric properties of the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ system with 0 ≤ x ≤ 0.30 at intervals of 0.05 mol.

2. Experimental Procedure

Raw powders of BaCO₃ (purity 2N5, Sakai Chem. Ind. Co., Ltd., Japan), SrCO₃ (3N, High Purity Chem. Co., Ltd., Japan), MgO (2N, High Purity Chem. Co., Ltd., Japan), and WO₃ (3N, High Purity Chem. Co., Ltd., Japan) were mixed to prepare the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics. Appropriate ratios of the 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 specimen 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 using a powder X-ray diffractometer (XRD, D/MAX-2500V/PC, Rigaku, Japan). The apparent lattice parameter, a, of the cubic structure was calculated using the equation of a² = [(h² + k² + l²)λ²]/(4sin²θ), where θ is the angle of diffraction of the X-rays from the plane of (hkl) and l is the wave length of the X-rays. The lattice parameter was determined by plotting the calculated a values versus the Nelson-Riley function f(θ) ( f(θ) = 1/2(cos²θ/sinθ + cos²θ/θ)) and determining the y-intercept where the Nelson-Riley function went to zero, i.e., at θ = π/2. Microstructure of the sintered specimens was characterized by a field emission scanning electron microscope (FE-SEM, Quanta 250 FEG, FEI, U.S.A.). The microwave dielectric properties of the specimens were determined using network analyzers. The dielectric constant was measured by Hakki-Coleman method using a network analyzer (E5071C, Keysight, U.S.A.). The quality factors of the sintered specimens were measured by the cavity method using the same equipment. The temperature coefficient of resonant frequency was measured by the cavity method using a network analyzer (R3767CG, Advantest, Japan) in the temperature range of 20°C to 80°C.

3. Results and Discussion

The powder X-ray diffraction patterns of (Ba_1−xSr_x)-(Mg_0.5W_0.5)O₃ (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) ceramics sintered at 1700°C 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 B-site of the perovskite structure. In all the compositions, BaWO₄ is detected as the secondary phase. It was reported that BaWO₄ is usually formed during the sintering of Ba(Mg_0.5W_0.5)O₃ due to its structural instability at high temperatures.5,8–10) Khalyavin et al. suggested a mass balance reaction of Ba₂MgWO₆ → BaWO₄ + MgO + BaO as a possible mechanism for the formation of BaWO₄.5) The peak at 2θ ~ 42.9° may correspond to the (200) plane of MgO (ICDD number 45-0946) and/or the (204) plane of BaWO₄ (ICDD number 43-0646). There was no inter-dependency between the amount of BaWO₄ calculated from the intensity ratio of I_{(110) BaWO4}/I_{(220) Ba(Mg0.5W0.5)O3} and value of x in (Ba_1-xSr_x)-(Mg_0.5W_0.5)O₃. The minimum value of the intensity ratio I_(110)BaWO4/I_{(220) Ba(Mg0.5W0.5)O3} is obtained for the composition with x = 0.10. The presence of MgO or Mg-rich phases in the sintered specimens of Ba(Mg_0.5W_0.5)O₃ ceramics has been previously reported.5,8) Similar to other studies, BaO is not detected in any of the compositions in this study. It was previously suggested that BaO might exist in the amorphous state or as a solid solution with Ba(Mg_0.5W_0.5)O₃ or BaWO₄.6,8)

Fig. 1

Powder X-ray diffraction patterns of the (Ba_1-xSr_x)-(Mg_0.5W_0.5)O₃ ceramics.

The lattice parameters of (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics are shown in Fig. 2. With increasing x in (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃, the lattice parameter decreases linearly indicating that a substitutional solid solution occurs. The substitution of Ba²⁺ ion by a smaller Sr²⁺ ion, where the ionic radii of Sr²⁺ ion and Ba²⁺ ion are 1.44 Å and 1.61 Å, respectively, when the coordination number is 12, may be the cause for the decrease in the value of lattice parameter.15) The lattice parameter of Ba(Mg_0.5W_0.5)O₃ sintered at 1700°C in this study is 8.1092(3) Å and it was reported as between 8.1072 Å (sintered at 1650°C) and 8.1115 Å (sintered at 1600°C).5) The typical FE-SEM images of (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics (the compositions with x = 0, 0.10, and 0.30) are shown in Fig. 3. All the compositions exhibit a dense microstructure and the grain shape is certainly a polyhedron.

Fig. 2

Lattice parameter of the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics as a function of x.

Fig. 3

FE-SEM images of the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics; (a) x = 0, (b) x = 0.10, and (c) x = 0.30.

The linear shrinkage, the dielectric constant (ɛ_r), and the quality factor (Q × f₀) are illustrated in Fig. 4. The linear shrinkage of the composition for x = 0, i.e., Ba(Mg_0.5W_0.5)O₃, is much lower than that of other compositions. The linear shrinkage increases with the increase of x except for the composition with x = 0.25, implying that the sinterability of Ba(Mg_0.5W_0.5)O₃ ceramics is improved by the Sr substitution. In addition, it is considered that its sinterability is improved by the liquid phase sintering due to the formation of BaWO₄ having a low melting point of 1430°C.12)

Fig. 4

Linear shrinkage, dielectric constant (ɛ_r), and quality factor (Q × f₀) of the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics as a function of x.

The variation of ɛ_r is similar to that of the linear shrinkage. The value of ɛ_r for the composition with x = 0 is 17.3 and it was reported as 16.7 (sintered at 1500°C.4)), 17.6 (sintered at 1600°C5)), and 18.9 (sintered at 1575°C12)) earlier. The value of ɛ_r increases slightly with increasing x except for the composition with x = 0.10. It is considered that the relatively high ɛ_r of the composition with x = 0.10 may be due to the presence of small amount of BaWO₄ having a low ɛ_r of 8.116) as mention above. The value of Q × f₀ for Ba(Mg_0.5W_0.5)O₃ ceramics is 59,738 GHz, which is higher than that reported by Takahashi et al.4) and lower than that reported by Lin et al..12) The value of Q × f₀ increases with increase in x up to x = 0.10 and reaches a saturated value of about 100,000 GHz. It is considered that the increase in Q × f₀ may be related to the improvement in sinterability owing to the substitution of Ba²⁺ ion by Sr²⁺ ion and the liquid phase sintering as mentioned above. The dielectric losses or the inverse of the quality factor are generally classified into intrinsic and extrinsic categories. 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 boundaries, and secondary phases.9) Reany and Iddles have pointed out that, in reality, extrinsic losses dominate the quality factor.3)

The temperature coefficient of resonant frequency (τ_f) as a function of the calculated tolerance factor (t) using Shannon ionic radii is illustrated in Fig. 5.15) The value of τ_f for the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramics increases from a negative value to a positive one with increasing x. It was reported that τ_f is related to the temperature coefficient of the dielectric constant (τ_ɛ) and the linear thermal expansion coefficient (α_L) by τ_f = −(τ_ɛ/2 + α_L).3) In addition an overall empirical correlation between τ_ɛ and the tolerance factor of the perovskite structure (t) is given by t=(RA+RO)/[2(RB+RO)] where R_A, R_B, and R_O are the ionic radii of A, B, and oxygen ions in the ABO₃ perovskite structure, has been established by Reaney et al..17) The value of τ_f in this study ranges from − 27.3 ppm/°C for the composition with x = 0, which is similar to that reported by Lin et al.,12) to 10.3 ppm/°C for the composition with x = 0.30 and zero value of τ_f is obtained for the composition with x = 0.20. The composition with x = 0.20, i.e., (Ba_0.80Sr_0.20)(Mg_0.5W_0.5)O₃, sintered at 1700°C for 1 h, exhibits superior microwave dielectric properties of ɛ_r = 19.6, Q × f₀ = 99,358 GHz, and τ_f = 0.0 ppm/°C.

Fig. 5

Temperature coefficient of resonant frequency (τ_f) of the (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ ceramic as a function of tolerance factor.

4. Conclusions

The phase evolution, microstructure, and microwave dielectric properties of Sr-substituted Ba(Mg_0.5W_0.5)O₃ ceramics, i.e., (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃ (0 ≤ x ≤ 0.30), sintered at 1700°C for 1 h were investigated. All compositions showed a 1 : 1 ordered perovskite structure. In all the compositions, BaWO₄ was detected as the secondary phase. With increasing x in (Ba_1-xSr_x)(Mg_0.5W_0.5)O₃, the lattice parameter increased linearly, indicating that a substitutional solid solution occurred. All compositions exhibited a dense microstructure and the grain shape was confirmed to be a polyhedron. The linear shrinkage increased with the increase in x except for the composition with x = 0.25, implying that the sinterability of Ba(Mg_0.5W_0.5)O₃ ceramics was improved by the substitution of Sr. The value of ɛ_r increased slightly with increasing x except for the composition with x = 0.10. The value of Q × f₀ increased with increase in x up to x = 0.10 and reached a saturated value about 100,000 GHz. The composition with x = 0.20, i.e., (Ba_0.80Sr_0.20)(Mg_0.5W_0.5)O₃, sintered at 1700°C for 1 h exhibited superior microwave dielectric properties with ɛ_r = 19.6, Q × f₀ = 99,358 GHz, and τ_f = 0.0 ppm/°C, respectively.

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