Microwave dielectric properties of Na+-substituted CaMg0.9Li0.2Si2O6 ceramics

Ceramics with low dielectric constant are widely used in high frequency substrates. The low temperature sintered CaMg0.9-xNa2xLi0.2Si2O6(x = 0-0.05 and 0.1) ceramics with low dielectric constant and dielectric loss were prepared by the traditional solid-state reaction method, with 0.5wt%LBSCA additive. The XRD patterns of the samples were obtained by X-ray diffraction and it was found that there were three ceramic components, CaMgSi2O6, CaSiO3 and Na2MgSiO4, which indicated that the experimental sample was a multiphase ceramic system. Through the trend of bulk density as functions of the content of substitution and the change of SEM morphology, it could be found that appropriate amount of Na+ substitution can promote the grain growing and the densification of ceramics. Results demonstrated that both the Q × f and r were relevant to bulk density and the second phase. The f was also affected by the second phase to some extent. In particular, the ceramics sintered at 925 for 3h possessed the desirable microwave dielectric properties for LTCC application: r = 7.03, Q × f = 17,956 GHz, and f = 79 ppm/ .


Introduction
Dielectric ceramics have been widely used in microwave devices because of their low dielectric loss, serialized dielectric constant and temperature coefficient of resonant frequency which can be adjusted to near zero. In the field of 5G communication and the Internet of Things, microwave dielectric ceramics with low dielectric constant are often used in microwave substrates, because according to Td = L ξ ߝ /c(where L is the distance travelled by the signal, and c is the speed of light), low dielectric constant can effectively reduce the time delay of high-frequency signals during transmission [1,2]. At the same time, LTCC technology, which requires sintering temperature below 950Ԩ, has been paid more attention in the research of microwave dielectric ceramics to follow the development trend of device miniaturization and integration [3][4][5].
CaMgSi2O6 ceramic (ɂr = 7.46, Q × f = 59,638 GHz, and Ĳf í SSPԨ) prepared by Sun et.al, is a promising candidate for LTCC substrate [6]. Thereafter, a variety kinds of researches have been performed on the diopside with monoclinic structure, reducing sintering temperature and improving Q × f values [7][8][9]. Lai et.al reported that Cu 2+ substituting for Mg 2+ could effectively reduce the sintering temperature as well as the dielectric loss [10]. Mn 2+ has also been proved to enhance the performance of CaMgSi2O6 ceramics when substituted in Mg site [11]. However, the influence of Na + on the properties and composition of CaMgSi2O6 ceramics has not been investigated yet. Recent reports have found that the substitution of Na + for Mg 2+ improved the properties and reduced the sintering temperature of MgWO4 ceramics [1].
In this work, Na + -substituted CaMg0.9Li0.2Si2O6(x = 0-0.05, 0.1) ceramics were synthesized with 0.5wt% LBSCA glass addition. Then the phase composition, microstructure and microwave dielectric properties of the prepared ceramic samples were further investigated.

Experimental and methods
CaMg0.9-xNa2xLi0.2Si2O6(x = 0-0.05 and 0.1) ceramics were synthesized via conventional solid-state reaction method. The raw material, CaCO3(99.5%), MgO(99%), SiO(99.9%), Na2CO3(99.8%) and Li2CO3(99%) was ball-milled with deionized water for 12h after weighing according to the stoichiometric ratio. The slurry was dried in air for 24h and then grinded, and calcined at 925 for 3h. The presintered powder was added with 0.5wt% LBSCA glass and ball-milled for a second time for 6h. The mixture was dried again before it was mixed with polyvinyl alcohol (PVA) at a concentration of 12% and pressed into cylindrical sheets of 12 mm in diameter and 6 mm in thickness under pressure of 10MPa. The cylinder were sintered from 900 °C to 950 °C for 3h.
The phase composition was detected by X-ray diffractometer (Bruker D8 advance). Based on Archimedean principle, the bulk densities were measured with distilled water as a buoyancy liquid. The morphology pictures of sintered ceramic samples were taken by a scanning electron microscope at an acceleration voltage of 15 kV. Microwave dielectric properties, resonance frequency f, ɂr and dielectric loss tgɁ were measured using the Hakki-Coleman dielectric resonator method combining a network analyzer (Agilent N5230A, USA). The temperature coefficient of resonant frequency was calculated using the equation: where f1 and f2 represent the resonant frequencies at temperatures T1 and T2 respectively.  (1)) when x value changes from 0 to 0.1; When x0.04, the phase Na2MgSiO4(PDF#19-0249) was produced, indicating the limited solid solubility of Na + in the CaMgSi2O6 ceramics that is resulted from the relative big radius difference between Na + (1.13Å) and Mg 2+ (0.72 Å).  shows the bulk density of CaMg0.9-xNa2xLi0.2Si2O6(x = 0-0.05 and 0.1) ceramics sintered at different temperatures. It could be easily seen from the curves that the bulk density changes with the sintering temperature which is in the range of 900-950 Ԩ due to the densification process. At 900 Ԩ and 925 Ԩ, the bulk density values of the ceramics reach their maximum at x=0.05; However, for samples sintered at 925 Ԩ and 950 Ԩ, there are two bulk density peaks at x=0.01 and 0.05. The peak valley at x=0.03 could be ascribed to different mechanisms of action of Na + in the CaMg0.9Li0.2Si2O6 ceramics: when x0.03, Na + is solidly dissolved in the lattice, while when x0.04, the phase Na2MgSiO4 is formed. The results prove that appropriate amount of Na + substitution for Mg 2+ could promote the densification of ceramics. are showed in Fig. 3. Fig. 3(a) shows that although there are few pores, most of the grains are round and have not grown fully. From Fig. 3(b)-(g), the grains gradually present a clear shape, but the grain size distribution is becoming more and more uneven. In addition, it could be seen that the changing trend of the microscopic morphology of ceramics is consistent with the one of the bulk density in Fig. 2. Results show that Na + could promote the growth of grain.  H H H [12]. Fig. 4(b) shows the Q × f values of CaMg0.9-xNa2xLi0.2Si2O6 ceramics as functions of Na + doped content and sintering temperatures. In general, the Q × f value is influenced by both intrinsic and extrinsic factors [13]. Extrinsic factors include second phase, density and grain size distribution, while the intrinsic factors consist of crystal structure and the lattice vibration. The Q × f values of sintered CaMg0.9-xNa2xLi0.2Si2O6 composites in this work are nearly dependent on the density and the second phase, which could be seen from the Fig.  4(b) when considering the XRD patterns and Fig. 2. With regard to the second phase, the XRD patterns demonstrate that when x=0.01 there is no Na2MgSiO4 phase appearing and the density of the certain samples is high when taking the density into account, leading to the maximum Q × f value at 900 and 925 Ԩ. So given the results of the trends, the Q × f value is determined by more second phase than the density when Na + substitution content exceeds 0.06mol. Fig. 4(c) shows the ߬f values of CaMg0.9-xNa2xLi0.2Si2O6 ceramics sintered at 925 Ԩ. Notably, the ߬f value changes non-linearly with the Na + substitution. However, it shows a slight upward trend in the entire sample range. So it could be speculated that the ߬f value is related to the second phase on the grounds of Lichtenecker empirical logarithmic rule: 1 2

Results and discussions
The optimal Q × f value samples sintered at 925Ԩ was obtained at x=0.01, corresponding to the ߬f value of -79. Therefore, the ߬f value need to be adjusted to meet the requirements of LTCC application in subsequent work.

Conclusion
In this work, low temperature sintering CaMg0.9-xNa2xLi0.2Si2O6 ceramics were synthesized via solid state reaction method. And the phase structure, bulk density, microstructure and the microwave dielectric properties were investigated in detail. There were totally three phases detected by the X-ray diffraction in the entire samples sintered at 925Ԩ and CaMgSi2O6 was the major phase as expected. From the bulk density and SEM pictures, it could be concluded that suitable Na + substitution promoted growth of the grains and eliminated the pores. The second phase and density play an important role on the properties of the ceramics and the excellent microwave dielectric properties was obtained at x=0.01: ɂr = 7.03, Q × f = 17,956 GHz, and Ĳf í79 ppm/Ԩ when considering the actual technological requirements.