Sintering Characteristic, Structure, Microwave Dielectric Properties and Compatibility with Ag of Novel 3MgO-B2O3-xwt% Ba(CuB2)O5-ywt% H3BO3 Ceramics

In this study, 3MgO-B 2 O 3 -xwt%BCB-ywt%H 3 BO 3 (2 ≤ x ≤ 8, 0 ≤ y ≤ 20) ceramics were sintered at the optimum temperature to form Mg 3 B 2 O 6 and MgO phases. The effects of H 3 BO 3 and BCB on product characteristics, phase transition, microstructure, and microwave properties of 3Mg-B 2 O 3 ceramics were investigated. The intensities of diffraction peaks of two phases varied with changing the x and y values. After sintering at 950 °C, the ceramics with x = 6 and y = 15 achieved excellent microwave properties with a ε r of 6.72, Q × f of 83205 GHz, and τ f of −65.05 ppm/ ℃ . Besides, the ceramic with x = 8 and y = 5 sintered at 925 ℃ also achieved good microwave dielectric properties with a ε r of 6.64, Q × f of 78173 GHz, and τ f of −57.27 ppm/ ℃ . The sintering temperatures of above both ceramics are well lower than the melting point of Ag, showing promising applications in low temperature cored ceramic devices. In particular, these two ceramics can be used as potential candidate materials for microwave ceramics for 5G technology, provided that τ f can be further optimized.


Introduction
In recent years, LTCC technology is widely used in microwave circuits manufacturing and has become the research hot topic for passive component integration [1][2][3][4][5][6][7]. In particular, with the emergence of 5G largescale antenna (Massive MIMO) technology, the number of antennas will increase exponentially. The demand of lters for signal frequency selection and processing will grow with the passage of time, thus the demand for low-temperature co-red ceramics (LTCC) will also increase signi cantly [8][9][10][11][12][13]. LTCC technology requires the dielectrics co re with high conductivity material electrodes. Because of the low melting point of electrodes (e.g., Ag melting point: 961°C, Al melting point: 660°C), microwave dielectric materials is desired to have low sintering temperatures [14][15][16][17].
In the MgO-B 2 O 3 system, Zhou et al [18][19][20] reported that the sintering temperatures of MgO-xB 2 O 3 ceramics could be reduced by adding BaCu(B 2 O 5 )(BCB). MgO-2B 2 O 3 -4wt%BCB ceramics manufactured at 925°C have excellent microwave properties and is a candidate for LTCC components. Based on the above research, they found that MgO-2B 2 O 3 -40wt%H 3 BO 3 -4wt%BCB and MgO-2B 2 O 3 -10wt%H 3 BO 3 -4wt%BCB manufactured at 900°C and 925°C also had excellent microwave dielectric properties. MgO-rich 3MgO-B 2 O 3 ceramics with good microwave properties have also been of interest to scholars. Dosler reported that Mg 3 B 2 O 6 ceramics with a grain size of 1000 µm could reach Q×f values up to more than 220,000 GHz [21]. Gu et al. [22] found that pure phase Mg 3 B 2 O 6 ceramic could be obtained when the Mg/B molar ratio is 1.2. An appropriate excess of MgO could increase its Q×f value, but it did not contribute to its densi cation. Kan et al. [23] found that an appropriate amount of B 2 O 3 doping effectively lowered the sintering temperature of this ceramic and improved the dielectric properties of MgO compound. As x = 0.99, xMgO−(1-x)B 2 O 3 ceramic sintered at 1,350°C for 4 hours had a Q×f value of 773,300 GHz. Appropriate ion substitution has been reported to favor densi cation and dielectric properties of ceramics. For example, Gu et al. [24] found that (Mg 0.8 Ca 0.2 ) 3 B 2 O 6 ceramics manufactured at 1,250°C showed ε r = 6.8, Q × f = 103556 GHz, and τ f = −34.5 ppm/°C. Furthermore, they [22] also reported the excellent microwave properties of (Mg 0.998 Sr 0.002 ) 3 B 2 O 6 ceramics sintered at 1250°C: ε r = 6.9, Q × f = 110820 GHz, and τ f = −32.4 ppm/°C .   Though 3MgO-B 2 O 3 material system has high Q×f values, it is manufactured at high sintering   temperatures, which not only hinder their incorporation with low melting electrode and polymer based substrates, but also lead to huge energy consumption and volatile components evaporation. Su et al [25] found that Ni 2+ can be used as a substitute of Zn 2+ to optimize the sintering behavior and microwave dielectric properties of Zn 3 B 2 O 6 ceramics. In addition, they [26] reported that a mixture of Mg 3 B 2 O 6 and Zn 3 B 2 O 6 was used to obtain ceramics with excellent dielectric properties at 950 o C. Dou et al. [27] investigated the excellent microwave dielectric properties in Mg 3 B 2 O 6 ceramics with 35% lithium magnesium borosilicate glass sintered at 950°C for 3 h: ε r = 6.5, Q × f = 21000 GHz, τ f = −49.5 ppm/°C. Hu et al. [28] found that 55 wt% lithium magnesium zinc borosilicate glass addition reduced the sintering temperature of Mg 3 B 2 O 6 ceramics to ~ 950°C and achieve the excellent microwave dielectric properties with a ε r of6.8, Q × f of 50,000 GHz, and τ f of −64 ppm/°C. Usually, materials (such as H 3 BO 3 , CuO, and V 2 O 5 ) with low melting point are often added to lower the sintering temperature for liquid-phase sintering to obtain dense sintered ceramics [29][30][31][32][33]. However, the microwave properties of ceramics doped with low melting point materials are usually much deteriorated [16, 33,34]. The addition of BaCu(B 2 O 5 ) (BCB) with low melting temperature, good wettability and microwave dielectric properties could contribute to the densi cation of Mg 3 B 2 O 6 ceramics [35][36][37]. However, it is di cult to obtain pure Mg 3 B 2 O 6 ceramics because high-temperature sintering will lead to the volatilization of B 2 O 3 [22]. Although some studies have been reported, the relationship between structure and microwave properties of ceramics with addition of BCB and H 3 BO 3 in 3MgO-B 2 O 3 needs to be further investigated.
In this study, a series of ceramics of 3MgO-B 2 O 3 -xwt%BCB-ywt%H 3 BO 3 (where x = 2, 4, 6, 8; y = 0, 5,10,15,20) were prepared by a conventional solid-phase reaction method. The sintering behavior, microstructure, microwave dielectric properties and compatibility with Ag are reported in detail. An X-ray diffraction spectroscopy (Model X'Pert Pro, PANalytical, Almelo, Netherlands) was used for structure analysis of the specimens. The microstructure of the ceramic surfaces was observed with a scanning electron microscope (SEM, JSM6380-LV SEM, JEOL, Tokyo, Japan). The bulk density of the sintered samples was measured by Archimedes method. Microwave dielectric properties at microwave frequencies were measured using TE01 δ dielectric resonator method and network analyzer (E5071C, Agilent Co., CA, USA) over a frequency range of 300 kHz to 20 GHz at room temperature. τ f values were obtained over a temperature range of 25 ℃ to 85 ℃ as shown below.

Experimental
where, f T and f T0 represent the resonant frequencies at 85℃ (T) and 25℃ (T 0 ), respectively.

Results And Discussion
As shown in Fig. 1, the XRD patterns of 3MgO-B 2 O 3 -xwt%BCB (x = 2, 4, 6, 8; y = 0) ceramics sintered at the optimum temperature. As x increased from 2 to 8, two major phases, indexed as Mg 3  , the bulk density rst increased slightly and then decreased with the increase of the sintering temperature. With changing the sintering temperature, the variation of ε r is consistent with that of bulk density. The higher the bulk density is, the higher the permittivity is. As x value increased from 2 to 8, the bulk density increased but the value of ε r decreased, which may be attributed to the addition of BCB with a low ε r (ε r ~7 .4) [35].
The change of Q×f with sintering temperature for 3MgO-B 2 O 3 -xwt%BCB (x = 2, 4, 6, 8) ceramics is similar to that of bulk density, as shown in Fig. 3(d). The Q×f is mainly affected by ceramic densi cation. Higher density leads to a lower porosity and lower losses. A moderate particle size is associated with higher quality factors and lower grain boundary losses [38]. However, desired moderate particle size cannot be obtained for ceramics at lower sintering temperatures. The bulk density of samples rst increased and then decreased with the increase of x, indicating that the addition of appropriate amounts of BCB not only decreased the sintering temperature of ceramics but also resulted in denser ceramic, which are consistent with the analysis of the SEM images, as shown in Fig. 2    ceramics as a function of the sintering temperature. Clearly, the bulk density of ceramic rst increased and then decreased with the increase of sintering temperature. This phenomenon indicates that the ceramic was effectively sintered denser initially with the sintering temperature increase. Further temperature increase would cause over-sintering of ceramic and resulted in lower density. The bulk density of 3MgO-B 2 O 3 -2wt%BCB-ywt%H 3 BO 3 (y = 0, 5, 10, 15, 20) ceramics showed an overall increasing trend with the H 3 BO 3 content increase, which may be attributed to the decrease in porosity, as shown in Fig. 5. As y increased from 5 to 15, the bulk density varids from 3.017 g/cm 3 to 3.128 g/cm 3 , indicating that the addition of appropriate H 3 BO 3 could make the ceramic samples sintered more densely. However, with further increasing x value to 20, the bulk density reached the lowest at ρ of 3.04 g/cm 3 , which is due to the over-sintering by addition of too much H 3 BO 3 .  the sintering temperature. The dielectric constants of ceramic materials are generally closely related to bulk density, phase composition, and crystal structure [39,40]. ε r rst increased to the maximum and then decreased with the increase of temperatures. As shown in Fig. 6, the change in ε r with temperature is similar to that of Q×f and bulk density. However, as the H 3 BO 3 content increased, the maximum values of dielectric constant for 3MgO-B 2 O 3 -2wt%BCB-ywt%H 3 BO 3 ceramics increased rst and then decreased (from 6.95 to 7.05 and then to 6.67), which is similar to that of the bulk density of 3MgO-B 2 O 3 -2wt%BCB-ywt%H 3 BO 3 ceramics with sintering temperature. The dielectric constant increased with the increase of bulk density. As the bulk density of ceramic increased, the number of active particles inside the ceramic was relatively high, and the dielectric constant increased and vice versa.  [41][42][43]. In Fig. 6(d), the Q×f of 3MgO-B 2 O 3 -2wt%BCB-ywt%H 3 BO 3 (y = 0, 5,10,15,20) ceramics showed a trend of increasing and then decreasing with the increase of sintering temperature, which is similar to that of the bulk density and relative permittivity. From the analysis of phase structure and microstructure, the microstructure of 3MgO-B 2 O 3 -2wt%BCB-ywt%H 3 BO 3 (y = 0, 5,10,15,20) ceramics is clearly relatively denser, and a second phase is present in addition to the main phase 3MgO-B 2 O 3 . As y = 15, the ceramics possessed relatively high Q×f and maximum density ρ of 3.128 g/cm 3 . With increasing the y to 20, the optimum sintering temperature was 975°C. Combined with the XRD analysis, it is found that the Q×f of 3MgO-B 2 O 3 -2wt%BCB-xwt%H 3 BO 3 ceramic also reached the maximum value of 113,645 GHz as x increased.The content of the main phase of the ceramic increases as the content of the second phase MgO in the crystal structure decreases. Table 1   ceramics could reacedt with silver electrodes or not, the two calcined powders were mixed with 20 wt% Ag powder and sintered at 950°C and 925°C for 4 hours, respectively. Figure 7 shows the XRD patterns and backscattered electron diagrams of the samples. The XRD shows that Ag was present as a single phase and the EDS shows that the bright particles in the main ceramic matrix were silver, which further con rms that there is no reaction between the silver and 3MgO-B 2 O 3 -8wt%BCB-5wt%H 3 BO 3 . All these results evidence the potential application of both ceramics in LTCC technology.

Conclusion
In this study, 3MgO-B 2 O 3 -xwt%BCB-ywt%H 3 BO 3 (where x = 2, 4, 6, and 8; y = 0, 5, 10, 15, and 20) ceramics was synthesized using solid-phase reaction method. When y = 0, the ceramics consist of two phases, namely Mg 3 B 2 O 6 and MgO. As x increased, the main diffraction peak of MgO phase gradually decreased while the diffraction peak of Mg 3 B 2 O 6 gradually increased. As x was constant, the diffraction peak intensity of Mg 3 B 2 O 6 phase increased with the increase of y while the main diffraction peak of MgO Q×f of 113,645 GHz, ε r of 6.67, τ f of −53.19 ppm/°C, and ρ of 3.04 g/cm 3 of 3MgO-B 2 O 3 ceramic were achieved at 975°C at x = 2 and y = 20. In particular, comprehensive comparison of the ceramic properties sintered at same low sintering temperature revealed that the ceramics with x = 6, y = 15 sintered at 950°C achieved good microwave dielectric properties with a ε r of 6.72, Q × f of 83,205 GHz, and τ f of −65.05 ppm/°C. The ceramic with x = 8, y = 5 sintered at 925 °C also achieved good microwave dielectric properties (ε r = 6.64, Q×f = 78173 GHz, and τ f = −57.27 ppm/℃). 3MgO-B 2 O 3 -x wt%BCB-y wt%H 3 BO 3 ( x = 6, y = 15 or x = 8, y = 5) had excellent microwave dielectric properties and can also be produced at low cost. Both ceramics can be co-red with Ag, suggesting that they are promising for 5G applications, provided that τ f can be further optimized to zero.    Bulk density, εr, Q×f, and τf of 3MgO-B2O3-2wt%BCB-ywt%H3BO3 (0 ≤y ≤ 20) ceramics sintered at optimal temperatures as a function of y value.