Abstract
The effects of B2O3–LiF addition on the phase composition, microstructures, and microwave dielectric properties of (Mg0.95Cu0.05)2SiO4 ceramics fabricated by a wet chemical method were studied in detail. The B2O3–LiF was selected as liquid-phase sintering aids to reduce the densification sintering temperature of (Mg0.95Cu0.05)2SiO4 ceramics. The B2O3 6%–Li2O 6%-modified (Mg0.95Cu0.05)2SiO4 ceramics sintered at 1200°C possess good performance of ∼ 4.37, ∼ 36,700 GHz and ∼ −42 ppm/°C.
1. Introduction
The rapid development in modern communications and Internet technology has created an urgent demand for the development of microwave ceramics [1–5]. Such microwave equipments require light weight, low loss, small size, and good temperature stability. Therefore, the materials used for the microwave substrates and dielectric resonators should have low dielectric constant (), high quality factor (), and near-zero temperature coefficient of resonator frequency (). Mg2SiO4 is an essential material with low and high . So it is considered to be a promising candidate as a low-dielectric-constant microwave material for applications in microwave substrates [2–9]. To our knowledge, the relation between the microwave dielectric properties and the structure for the Cu2+-doped Mg2SiO4 system has not yet been investigated. Moreover, the sintering temperature of (Mg1−xCux)2SiO4 is too high in the recently reported microwave ceramics. Thus, 12 wt.% B2O3–LiF was used as sintering aids to reduce the densification sintering temperature down to a lower temperature range. In this work, we have reported the (Mg1−xCux)2SiO4 ceramics by a wet chemical process. The effects of additive B2O3–LiF on the phase compositions, microstructures, and microwave dielectric behavior were studied.
2. Experimental Procedure
SiO2 nanospheres were prepared by the sol-gel method, and then the powders were used as raw material to synthesize the SiO2-based ceramics. The microwave dielectric ceramics with high performance were successfully prepared in our previous work. The (Mg1−xCux)2SiO4 ceramics belong to SiO2-based ceramics. Following the method of Hu et al. [1], it was quite expected to achieve success. All experiments were conducted in air. All reagents were analytical pure and used as received without further purification. (Mg1−xCux)2SiO4 powders were prepared using the sol-gel method. For this purpose, Mg(NO3)2·6H2O and CuSO4·5H2O were dissolved individually in 100 ml ethanol and 10 ml distilled water and then added into the mixture under magnetic stirring, marked as A. Tetraethyl orthosilicate (TEOS) and CuSO4 were used as received. The starting sol was prepared by hydrolysis of TEOS under magnetic stirring in the presence of 100 ml alcohol solution in 40°C water for 30 min, marked as B. Subsequently, the solution B was added to A by continuous vigorous magnetic stirring for 3 h at 40°C in water and then kept at 60°C overnight to allow gel formation. Finally, the obtained products were dried and calcined at 900°C for 4 h in an alumina crucible. The calcined powders were milled with ZrO2 balls in ethanol for 12 h and dried to obtain (Mg1−xCux)2SiO4 powders. The prepared powders were divided into two groups. The first group was not doped, while the second group was doped with B2O3–LiF. Both of them with polyvinyl alcohol water solution were pressed individually into cylindrical specimen with a diameter of 11.5 mm under a uniaxial pressure of 120 MPa. The undoped specimens were placed in an alumina crucible and heated to the sintering temperatures varying from room temperature to 1350°C with a rate of 5°C/min. After sintering for 4 h in air atmosphere, the specimens were freely cooled down to room temperature inside the furnace. The B2O3–LiF co-doped (Mg1−xCux)2SiO4 ceramics were sintered at 1000–1350°C for 4 h.
The crystalline-phase structure was identified by D/max-2550V/PC X-ray diffractometer (Rigaku, Tokyo, Japan) with Cu-κα radiation (at 40 kv and 20 mA) at a scan rate 2θ = 0.02 s−1. The morphology of the samples was characterized by a scanning electron microscope (SEM, FEI-quanta 200, USA) and a high-resolution transmission electron microscope (JEM-2100, Japan). The X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd., Japan) with a monochromatic Al was conducted to examine the microstructure for fracture surface of ceramics.
The bulk densities () of the samples were measured by the Archimedes method using distilled water as medium. The microstructures were observed on the as-sintered ceramic surfaces of samples by a scanning electron microscope (SEM, FEI-quanta 200, USA). The phase evolution of the (Mg1−xCux)2SiO4 ceramics was performed by XRD. The as-sintered cylindrical specimens with the thickness of about 5 mm and diameter of about 10 mm were used for evaluating the microwave dielectric properties. The dielectric constant and quality values were determined by the TE01δ shielded cavity method using a vector network analyzer (ZVB20, Rohde & Schwarz, Munich, Germany). The temperature coefficient of the resonant frequency () was calculated with the following formula:where and represent the TE01δ resonant frequency measured at 25°C and 80°C, respectively.
3. Results and Discussion
Figure 1(a) depicts the XRD patterns of (Mg1−xCux)2SiO4 (x = 0.05–0.20) ceramics sintered at 1350°C for 4 h. It could be seen that all the main diffraction peaks can be well indexed to the standard patterns of Mg2SiO4 (PDF#72–0296), indicating that the forsteritic-olivine solid solution with a single phase was formed. A very little protoenstatite Mg2SiO3 secondary phase appeared along with the main phase Mg2SiO4 in all compositions, which is similar to the results of Li et al. [10]. This result might be attributed to the fact that the amount of miscellaneous phase MgO is enough so that it could not react with MgSiO3 to form Mg2SiO4. Moreover, as shown in Figure 1(b), with increasing the Cu content (x), the main peaks of Mg2SiO4 shift slightly toward lower angles, indicating an increase of unit-cell volume. Furthermore, the refined lattice parameters were also plotted as a function of Cu content (x) as shown in Figure 1(c), a near linear dependence between the lattice parameters (except c) and x value can be found, which is in agreement with the Vegard’s law and also confirms the formation of a solid solution.
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Figure 2 showed the bulk density of the (Mg0.95Cu0.5)2SiO4 with 12% B2O3–LiF ceramics sintered at 1000°C was low, about 2.38 g/cm2, but increased with increasing sintering temperature to a maximum value of 2.48 g/cm2 for the specimen sintered at 1150°C and then reduced with a further increase in the temperature. The 12 wt.% H3BO3 added ceramics shows relative densities of 89%. The low density of magnesium silicate ceramics can be caused by two factors: the first is its crystallographic anisotropy, and the other is the hollow formed by the sintering agent at high temperature.
The SEM micrographs of the (Mg0.95Cu0.5)2SiO4 with 12% B2O3–LiF ceramics sintered at 1000–1200°C for 4 h in air are shown in Figures 3(a)–3(e). Figure 3 shows that the grains became lager and denser with increasing sintering temperature. Three types of grains were observed in the specimens of B2O3–LiF-doped (Mg0.95Cu0.5)2SiO4 ceramics. In order to understand the distribution of the elements in the sample, the EDS of (Mg0.95Cu0.5)2SiO4 with 12% B2O3–LiF ceramics sintered at 1150°C for 4 h is shown in Figure 3(f). The results presented the ratio of Mg : Si : O of rod-shaped grain (sport A and B) which is approximately 2 : 1 : 4, which agreed with the composition of forsterite. Li and coworkers also observed Mg2SiO4 with similar shape [10]. It seems impossible for the EDS detector to detect boron and lithium ions, which led to the restricted detection of B2O3–LiF. The small grains are to be Mg2Si and Li2B4O7, which is in accordance with the XRD analysis as shown in Figure 4.
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Figure 5 showed the dielectric constant, quality factor, and of the (Mg0.95Cu0.5)2SiO4 with 12% B2O3–LiF ceramics sintered at various temperatures for 4 h, respectively. The is mainly dominated by the structural characteristics, dielectric polarizability, and relative density of the ceramics [11–13]. It can be seen from Figure 5(a) that the variation trend of with sintering temperatures was not in agreement with that of relative density. It is noted that the sample had lower (4.37) when the sintering temperature was 1150°C. The values of the (Mg0.95Cu0.5)2SiO4 with 12% B2O3–LiF ceramics sintered at 1000°C were low (15,000 GHz at 12.36 GHz) due to the low density, small grain size, and porous microstructure (as shown in Figure 3(a)). But, it increased with increasing sintering temperature to a value of 36,700 GHz for the specimen sintered at 1150 then seldom decreased to 32,600 GHz. The optimum sintering temperature was reduced by 200°C compared to the results of Li et al. Based on the classical dielectric theory, the value should increase as the grain size increases, because a reduction in the number of grain boundaries per unit volume would result in materials with a lower loss [12–15]. In fact, the ceramics had the highest values when the mean grain size of the ceramics was about 8∼17 as shown in Figure 3. Li et al. also observed similar result in our previous work [10]. This could be due to the interaction of various factors, such as distribution of element, porosity, grain size, presence of liquid-phase, and so on, which made it too difficult to reveal definitive remarks on grain size loss relationships [15–18]. The values of all ceramics, on the other hand, appeared to be a slight change (from −60∼−40 ppm/°C) to sintering temperature, as shown in Figure 5. Generally, glass materials of lower melting temperature were mixed with the ceramic materials to degrade the sintering temperature. However, network formers contained in the glass ceramics may absorb the microwave power deeply at microwave frequency band, deteriorating the microwave performance for the material. Chang et al. reported that Li3BO3 ceramic possesses the microwave dielectric properties of ∼ 5, ∼ 37,200 GHz, and ∼ 3.1 ppm/°C [19]. In this work, the addition of 12 wt.% of B2O3–LiF effectively reduced the sintering temperature but worsened the microwave performance of the ceramic. It is an important task to explore the low sintering temperature, high density, and high-performance Mg2SiO4 ceramics in future work.
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4. Conclusions
The microwave performance of (Mg1−xCux)2SiO4 (x = 0.05–0.20) ceramics were studied in terms of their microstructure and structural characteristics, as well as the sintering behavior. The results show that the Cu substitution not only obviously enhances the sintering activity but also improves the microwave performance of the ceramics. The (Mg0.95Cu0.5)2SiO4 with 12% B2O3–LiF ceramics sintered at 1150°C for 4 h achieved excellent microwave dielectric properties of = 4.37, = 36,700 GHz, and = −42.6 ppm/°C. It can be used as a promising microwave substrate and LTCC material [20].
Conflicts of Interest
The authors declare that there are no conflicts of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (Project nos. 51272150, 51072110, and 51402235).