Phase-microstructure evolution and microwave dielectric characteristic of (1−x)(Sr0.5Ce0.5)TiO3+δ−xNdAlO3 solid solution
Introduction
Microwave (MW) materials have gained, increasing research attention because of their ability of miniaturization of MW components and devices [1]. The performance of MW device is characterized by dielectric constant (εr), Q × f value, and temperature coefficient of resonant frequency (τf) [2]. Ti-based oxide compounds exhibit high dielectric constant (εr) for state-of-the art miniaturization because of their ability to form a solid solution using a wide range of substituting atoms [3], [4], [5], [6]. Among Ti-based perovskites, doped SrTiO3 (STO) [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], and CaTiO3 (CTO) [3], [4], [15], [16], [17], [18], [19] ceramics have been rarely investigated. Compared with complex perovskites, these ceramics system possess relatively large dielectric loss together with high τf value. For example, the substitution of the A-site (Sr and Ca-site) [3], [4], [10], [11], by rare earth (RE-ions) trivalent ions (Sm3+, Nd3+, and La3+) form A-site vacancies, which consequently affected the dielectric characteristics [15], [16], [17], [18], [19]. Interestingly, the charge compensation mechanism of cation vacancies still, remained disputable; which arises from the charge unbalance caused by trivalent ions substituting divalent A-site ions (Sr and Ca sites) [3], [4], [10], [11], [15], [16], [17], [18], [19]. Hence, researchers aim to modify MW dielectric properties through substitution of B-site, rather than A-site [20], [21], [22].
Recently, Ti-based ceramics system has been reported with promising MW dielectric properties (Table 3) [13], [14]. Conversely, most Ti-base ceramics with a larger εr and τf value exhibits a generally smaller Q × f value due to the contribution of increasing anharmonic terms [12], [16]. For instance, a high εr (113–250, Table 3) value is obtained in the Sr(1-1.5x)CexTiO3 (x ≤ 0.4), and Sr9-xPbxCe2Ti2O36 (x ≤ 0.9) ceramics system by Subodh and Kamba et al. [6], [23]. However, their τf value (+306 ppm/°C) is still too large for direct applications in MW devices. At the same time, a high εr value (78–103, Q×f = 13,000–15340 GHz, and τf = 200–247 ppm/°C) is obtained in the Ca(1-x)Nd2x/3TiO3 (x ≤ 0.9) ceramics system by Lowndes et al. [3], and Fu et al. [4]. All these results show that, however, A-site vacancy is introduced to maintain charge neutrality caused by substitution of RE-ions (Ce and Nd) in the Sr [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], and Ca-site [15], [16], [17], [18], [19]. The more RE-ions content, the more cation vacancies. This makes the STO [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] and CTO structure more distorted, and thus limited the solubility [15], [16], [17], [18], [19]. Early research by Kim et al. [19] focused on the MW dielectric properties of (1-x)(Ca0.85Nd0.1)TiO3–xLnAlO3 (Ln = Sm, Dy, and Er) ceramics. The solid solution is characterized by εr = 58, Q × f = 14,600 GHz and τf = 12.8 ppm/°C. Fu et al. studied the MW dielectric properties of (1−x)(Sr0.4Na0.3Nd0.3)TiO3–xLnAlO3 (Ln = Sm and Nd) system; the solid solution exhibits εr = 45, Q × f = 38,000 GHz and τf = 1.2 ppm/°C [11]. However, substitution of A-site ions by La and Na decreases the εr of the material, which manifests as increased Q × f value. Besides this, there also has been some research in achieving a high εr value (εr = 115–245, τf = 65–0.75 ppm/°C, and Q × f = 2500–2750 GHz) by mixing CaTiO3-based and (Li1/2Ln1/2)TiO3 (Ln = La, Nd) ceramics [24], [25].
In summary, all these results indicate interesting and distinct results but still, there are specific difficulties in investigating the crystal chemistry and dielectric properties of ceramics. For example, when Sr2+ and Ca2+ is substituted by excess-RE-ions (La, Ce, Nd, Pr and Sm) on the A-site (Sr and Ca-site), unwanted phases, cation-deficient formulation due to the large amount of vacancies defects tend to destabilize the crystal structure, leading to high dielectric loss [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Hence, for better understanding the property structure relationship of doped Ti-base perovskite; it should be an important issue to stimulated further research in this direction for its intrinsic and extrinsic mechanism.
In our previous work, Sr(1-1.5x)CexTiO3 (0.1 ≤ x ≤ 0.67, sintered at 1300–1350 °C) ceramics system were investigated systematically [5], [8], [12]. The X-ray photoelectron spectroscopy (XPS) analyses revealed that all the samples displayed a cluster of Ce3+ ≥ 95%, rather than Ce4+ ≤ 5% [5], [6], [9]. Taking into account the XPS analysis on (1-x)(Sr0.5Ce0.5)TiO3+δ − xNdAlO3 together with earlier reported data [6], [9], the existence of minor amount of Ce4+ (Ce4+ ≤ 2%, Ce3+ ≥ 98%) sintered at 1550 °C for 4 h) on the Sr-sublattice should not change the defect chemistry mechanism to a significant extent [9]. Besides this, the charge compensating mechanism, is still, remain disputable. However, incorporation of Ce3+ on the Sr-site (Sr0.5Ce0.5TiO3+δ, Ce2O3 ↔ + 3Oo + 2e′) and the oxygen excess in the system will tend to maintain the charge neutrality. Further, the influences of NdAlO3 on the structure, microstructure, and dielectric properties of the present ceramics system are investigated and their relationships are discussed in detail.
Section snippets
Powder synthesis
Ceramic samples of (1-x)(Sr0.5Ce0.5)TiO3+δ−xNdAlO3, where x = 0.1–0.4, abbreviated (Sr,Ce)T-NdA, were prepared by conventional solid state route by using SrCO3 (≥99.9% purity; Dongying J & M, Shandong, China), CeO2 (≥99.99% purity; SCR, Shanghai, China), Nd2O3 (≥99.99% purity; Huizhou Si Ruier, Guangdong, China), TiO2 (≥99.8% purity; Xian-Tao, Hubei, China), and Al2O3 (≥99.9% purity; SCR, Shanghai, China). The starting powders were heated and dried at 200 °C for 10 min and cooled to room
Results and discussion
The (Sr,Ce)T-NdA solid solution (x = 0.1–0.4) at room temperature exhibits a tetragonal structure (T-phase, space group P4/mmm; Fig. 1). The doping of NdAlO3 stabilized (Sr0.5Ce0.5)TiO3+δ and formed a tetragonal solid solution.
The X-ray diffraction spectra were refined based on the tetragonal space group P4/mmm (Fig. 2). The final agreement factors (Rwp = 8.45, Rp = 6.42, and χ2 = 2.65) for x = 0.4 and the refined lattice parameters suggested that (Sr,Ce)T-NdA crystallized into the tetragonal P4/mmm
Conclusions
For (Sr, Ce)T-NdA ceramics, a solid solution of tetragonal perovskite structure is identified within x = 0.1–0.4. This finding indicates that the addition and/or doping of NdAlO3 stabilizes (Sr0.5Ce0.5)TiO3+δ ceramics and maintains the single phase. Furthermore, the addition of NdAlO3 facilitates large grain sizes with tetragonal-like profile. The dielectric constant is strongly dependent on the solidity and ionic polarizability of the ceramics. The decline in the values of τf with increasing x
Acknowledgements
This work was supported by national natural science foundation of China (NSFC-51572093) and Chinese Scholarship Council (CSC).
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