Effect of bond valence on microwave dielectric properties of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 ceramics
Graphical abstract
This picture shows that the variation in A-site bond valence will affect the Q × f value. A smaller A-site bond valence indicates that the bond strength between oxygen and A-site cation is weaker and the anharmonic interaction is increased, so the Q × f value is decreased.
Highlights
► CaTiO3–(Li0.5La0.5)TiO3 ceramics were prepared by solid-state reaction method. ► Origins of microwave dielectric properties were analyzed via bond valence theory. ► XRD result indicates that solid solutions are formed in the present system. ► The composition of x = 0.6 shows a near-zero τf (0.75 ppm/°C) for practical use.
Introduction
With the rapid progress in wireless communication technology, the demand for microwave components with combined dielectric properties has been increasing. Materials for microwave applications should have three key factors, i.e., high dielectric constant (ɛr), low dielectric loss (tan δ) and near-zero temperature coefficient of resonant frequency (τf). Dielectric loss can be quantified in terms of tan δ or the Q-factor. For microwave dielectric materials, Q × f (quality factor) is considered to be a constant, where f is the microwave frequency. High dielectric constant ceramics make it possible to noticeably miniaturize passive microwave components since the size of microwave components is inversely proportional to the square root of the ɛr. Therefore, many efforts have been made to develop microwave ceramics with a high dielectric constant. For example, the microwave dielectric properties of CaTiO3 ceramics with perovskite structure were first investigated by Kell et al. [1] in the early 1970s. The combination of a high dielectric constant (ɛr = 170) and a moderate dielectric loss (Q × f = 3500 GHz) makes it can be used in electronic devices, such as filters, resonators, oscillators, and dielectric antennas. However, the τf value of CaTiO3 ceramics is too large (τf = +800 ppm/°C) for practical applications.
As a result, the primary task for developing any new CaTiO3-based microwave materials is to adjust its large and positive τf value to near-zero. Recently, a number of attempts on the modification of microwave dielectric properties of CaTiO3-based ceramics have been undertaken. A series of new dielectric ceramics with near-zero τf values have been developed by mixing CaTiO3 to those materials with negative τf values, including (Li1/2Nd1/2)TiO3, Ca(Mg1/3Ta2/3)O3, NdAlO3, Sm(Co1/2Ti1/2)O3, and Nd(Zn1/2Ti1/2)O3 [2], [3], [4], [5]. However, the dielectric constants for these new ceramic systems decrease inevitably since those materials with negative τf values usually have the much smaller ɛr than that of CaTiO3 ceramics. Moreover, the modification of the τf is mainly based on empirical approaches, such as forming solid solutions or mixed phases with two or more compounds.
In fact, the ɛr and τf of ABO3 perovskite compounds are strongly related to the structural characteristics of BO6 octahedra such as bond strength and bond length between octahedral-site cation and oxygen, respectively [6], [7]. On the other hand, the structural characteristics are mainly depended on the bond valence since the bond valence is a function of bond strength and bond length. Hence, the dielectric properties at microwave frequencies can be effectively evaluated by the theory of bond valence. To better understand the relationships between microwave dielectric properties and structural characteristics of ABO3 perovskite compounds, the theory of bond valence has been applied to some ceramic systems such as Ca1−xSm2x/3TiO3, (Pb0.45Ca0.55)[Fe0.5(Nb1−xTax)0.5]O3 and (1 − x)(Ca0.7Nd0.2)TiO3–x(Li0.5Nd0.5)TiO3 [8], [9], [10].
(Li0.5La0.5)TiO3 ceramics with perovskite structure is found to possess high ɛr and largely negative τf [11], [12]. It is expected that a near-zero τf, combined with high ɛr and Q × f, can be achieved by the substitution of (Li0.5La0.5)2+ for Ca2+. In this study, the microwave dielectric properties of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 ceramics were investigated as a function of (Li0.5La0.5)TiO3 content (0.2 ≤ x ≤ 0.8). The effects of A-site and B-site bond valences in ABO3 perovskite compounds on the microwave dielectric properties were also discussed.
Section snippets
Experimental procedure
(1 − x)CaTiO3–x(Li0.5La0.5)TiO3 (0.2 ≤ x ≤ 0.8) ceramics were prepared by a conventional solid-state reaction method using high-purity grade powders of CaCO3, Li2CO3, La2O3, and TiO2 as starting materials. These starting materials were weighed according to the desired stoichiometry of CaTiO3 and (Li0.5La0.5)TiO3, and then wet ball milled in ethanol for 24 h in polyethylene bottles with zirconia balls, respectively. After dried at 80 °C for 6 h, the two mixed powders were calcined at 1100 °C for 3 h in
Results and discussion
Fig. 1 shows the XRD patterns of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 (hereafter referred to as (1 − x)CT–xLLT, 0.2 ≤ x ≤ 0.8) ceramics sintered at 1300 °C for 3 h. All peaks can be indexed based on CaTiO3 (JCPDS #42-423) with an orthorhombic perovskite structure and no second phase was detected, indicating the formation of (1 − x)CT–xLLT solid solution. The (1 − x)CT–xLLT solid solution exhibits the same orthorhombic perovskite structure as CaTiO3 because CaTiO3 is orthorhombic and has a GdFeO3-type perovskite
Conclusions
Complete (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 solid solutions with orthorhombic perovskite structure were obtained by a conventional solid-state reaction method through the entire compositional range (0.2 ≤ x ≤ 0.8). With the increase of (Li0.5La0.5)TiO3 content, the dielectric constant (ɛr) enhanced due to the increase in ionic polarizability and the decrease in B-site bond valence of ABO3 perovskite compounds, resulting from the increase of unit cell volume. The quality factor (Q × f) was dependent not
Acknowledgments
This work was supported by National Science Foundation of China (21071003) and Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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