Microwave Dielectric Properties of CaB₂O₄-CaSiO₃ System for LTCC Applications

Abstract

A novel composite ceramic with low densification temperature was fabricated using the conventional solid-state method. The XRD and Rietveld refinement results indicated that the two phases of CaB₂O₄ and CaSiO₃ can coexist in all compositions. Furthermore, a phase transition of CaSiO₃ ceramic from α-phase to β-phase was observed. A dense ceramic with excellent microwave dielectric properties (ε_r~6.4, Q × f~75,600 GHz, and a negative τ_f~26.9 ppm/°C) was obtained at x = 0.5 when sintered at 925 °C at the frequency of 14.2 GHz. A good chemical compatibility between the composite ceramic and Ag electrode was improved by elemental mapping results. A patch antenna was fabricated based on the simulated result. All results indicated that the 0.5 CaB₂O₄ + 0.5 CaSiO₃ ceramic has large application potential in the LTCC field.

1. Introduction

In recent years, the multilayer ceramic substrate has attracted much attention owing to its high capacity and low cost. The LTCC (low-temperature co-fired ceramics) technology has been widely used as a new method to achieve multilayer structures [1,2,3,4]. The LTCC technology can embed the passive components into a multilayer ceramic substrate with precious metals, such as Ag Al, Cu, and interconnections [5,6,7].

Although the LTCC technology exhibits many advantages, there are also many preconditions that most ceramics cannot meet for use in the LTCC technology. For example, the substrate materials should have high strength, good insulation, and suitable thermal expansion coefficient [5]. Meanwhile, high relative density and good crystallization are obligatory for achieving good mechanical and dielectric properties. The low densification leads to low mechanical and poor dielectric properties. Scholars explored the glass composite by dividing it into high-softening-point glass and low-softening glass, which can meet the above requirements of the LTCC technology [8]. The composite ceramic has the superiority of avoiding the consequence caused by the interaction between refractory filler and glass, achieving high densification, optimizing the dielectric properties, and tailoring mechanical properties. As one of the attractive materials, calcium borosilicate glass (CaO-B₂O₃-SiO₂, CBS) has attracted much attention owing to its low dielectric and dielectric loss, and similar thermal expansion coefficient to the silicon chips [9,10,11,12]. In this system, the CaB₂O₄ and CaSiO₃ phase was easily crystallized at different B₂O₃ contents or the Ca/Si ratio [13]. For example, the CBS ceramic with a Ca/Si ratio of ~1.2 and a B₂O₃ content of 16.5% showed an excellent dielectric property (ε_r~6.1, Q × f~11,700 GHz) when sintered at 850 °C for 30 min [14]. As is well known, the sintering aids can significantly promote the densification procedure and decrease densification temperature. However, the unsuitable sintering-aid dosage caused the liquid residue to deteriorate the dielectric properties at the microwave range. Hence, this work selected the major composition of the CBS glass of CaB₂O₄ and CaSiO₃ to design the composition with (1 − x)CaB₂O₄ + xCaSiO₃ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1). The objective is to analyze the phase composition, sintering characteristics, microstructure, microwave dielectric properties, and mechanical behavior.

2. Experimental Procedure

The CaCO₃, H₃BO₃, and SiO₂ were weighed according to the chemical formulas CaB₂O₄ and CaSiO₃, respectively. All materials had a high purity, above 99.8%. The powders were ball-milled for 12 h with alcohol and deionized water as the medium, respectively. The slurries were dried at 70 °C for 12 h and then calcinated at 700 °C for 3 h (CaB₂O₄) and 1175 °C for 12 h (CaSiO₃), respectively. The resulting powders were weighted based on the ratio of (1 − x) CaB₂O₄ + x CaSiO₃ (0 ≤ x ≤ 1) and then remilled in the same condition as mentioned above. After drying, the powders were mixed with 5 wt.% PVA as the binder and pressed into a cylinder of Φ12 × 6 mm. The green samples were firstly sintered at 550 °C for 2 h and then sintered at 900~1350 °C for 5 h.

X-ray diffraction with CuKα radiation (XRD, XRD-7000, Shimadzu, Kyoto, Japan) was employed to analyze the phase composition. The crystal structure was determined using Rietveld refinement with GSAS and EXPGUI software. The thermal etching surfaces were observed using a scanning electron microscope (SEM, Sirion 200, Eindhoven, The Netherlands). The thermal etching temperatures were carried out at 50 °C lower than the optimum temperature. The flexural strength was measured using a universal testing machine (CMTll04NB). The dielectric properties of the ceramics were measured at microwave frequencies in the TE₀₁₁ mode with the Hakki and Coleman method by using a vector network analyzer (Agilent E8362B, Agilent Technologies, St. Clara, CA, USA) [15]. Equation (1) was used to calculate τ_f over the temperature range of 25 °C to 80 °C:

$${\tau }_f=\frac{1}{f(T_0)}\frac{[f(T_1)-f(T_0)]}{T_1-T_0}$$

(1)

where f(T₁) and f(T₀) represent the resonant frequencies at T₁ (80 °C) and T₀ (25 °C), respectively. The CST Microwave Studio software was employed to simulate the geometrical parameter of a patch antenna.

3. Results and Discussion

Figure S1 (see supplementary materials) exhibits the X-ray patterns of the CaB₂O₄ and CaSiO₃ ceramics calcined at 700 °C and 1175 °C, respectively. The CaB₂O₄ peaks accompanied by a small amount of unknown phase can be detected and matched well with PDF # 075-0640, whereas the α-CaSiO₃ phase (PDF # 089-6585) coexists with a small amount of the SiO₂ phase. Figure 1 exhibits the X-ray patterns of (1 − x)CaB₂O₄ + xCaSiO₃ (0 ≤ x ≤ 1) sintered at their optimum temperature.

For the x = 0 composition, the second phase disappeared, indicating that the unknown phase was an interphase, and a pure CaB₂O₄ phase was obtained at 950 °C. When 0.2 ≤ x ≤ 0.8, the coexistence of CaB₂O₄ and CaSiO₃ was detected with an interesting phenomenon; that is, the α-CaSiO₃ phase was transformed to the β-CaSiO₃ phase with the CaB₂O₄ doped. Meanwhile, the SiO₂ phase was not determined for all compositions. No phase transition was detected when the α-CaSiO₃ was sintered at 1350 °C for 3 h, indicating that the CaB₂O₄ ceramic can promote the CaSiO₃ phase transformation from α to β phase. Figure S2 shows the Rietveld refinement results to further conform the phase composition of (1 − x)CaB₂O₄ + xCaSiO₃. The red circle shows the measured data, whereas the black line represents the calculated data. The good agreement between them indicated that the results were reliable. All results indicated that only the CaB₂O₄ and CaSiO₃ phase existed, which further confirms the deficiency of chemical reaction between them. This case is an important factor in achieving component controllability.

Microstructure analysis gives further evidence for the chemical compatibility of the CaB₂O₄ and CaSiO₃ phases. Figure 2 exhibits the thermal etching microstructure and elemental mapping analysis of 0.5 CaB₂O₄ + 0.5 CaSiO₃ ceramics sintered at their optimum temperature. A dense microstructure with two shapes of grain was observed. The elemental mapping analysis result indicated that the large grains can be indexed to the CaSiO₃ phase, whereas the small ones are identified as the CaB₂O₄ phase.

Figure 3 shows the density and flexural strength of (1 − x)CaB₂O₄ + xCaSiO₃ ceramics as a function of the x value. The bulk density shows a rising trend that increased from 2.6 g/cm³ at x = 0 (with a relative density of ~94.8%) to a maximum value of 2.7 g/cm³ at x = 0.5 (with a relative density of ~97.3%), then it decreased evidently as the content of CaSiO₃ further increased. As is well known, although the intrinsic characteristic of the material determines the theoretical flexural strength, the flexural strength is also proportional to the relative density. In other words, a high relative density means a high flexural strength, whereas a low relative density can result in a low flexural strength. As shown in Figure 3, the CaB₂O₄ ceramic has a low flexural strength, which can be attributed to the low relative density. By contrast, a high flexural strength with 170 MPa was obtained at the x = 0.5 composition, but the flexural strength decreased as the content of CaSiO₃ further increased. These results indicate that the suitable addition of CaSiO₃ can significantly enhance the flexural strength of the composite ceramic.

Figure 4 shows the variation of microwave dielectric properties of (1 − x)CaB₂O₄ + xCaSiO₃ ceramics with different content of CaSiO₃. With CaSiO₃ doping, the τ_f values were decreased. from −21.1 ppm/°C to −34.3 ppm/°C. Generally, τ_f is sensitive to the phase transition and phase composition [16,17,18,19]. The linear decrease of τ_f can be attributed to the lower value of CaSiO₃ (τ_f~−34.3 ppm/°C) ceramic compared with the CaB₂O₄ ceramic (τ_f~−21.1 ppm/°C). The Q × f significantly increased from 23,600 GHz at x = 0 to 75,600 GHz at x = 0.5 (tanδ = 1.64 × 10⁻⁴) and then decreased evidently with further doping of CaSiO₃, which shows a similar variation trend to the relative density. This result indicates that density is crucial in this system.

The dielectric permittivity showed a slight growth trend from x = 0 to 0.8 and sharply increased at the x = 1 composition. This case can be attributed to the relatively low permittivity and high volume fraction of the CaB₂O₄ ceramic in this composite ceramic system. The Bosman and Having’s equation can be employed to estimate the effect of porosity on the permittivity [20,21,22]:

$${\epsilon }_{\mathrm{corr}}={\epsilon }_{\mathrm{r}}(1+1.5P)$$

(2)

As shown in Figure 4, the measured values were lower than the measured ones and matched a minimum deviation at the x = 0.5 composition. A dense composite ceramic with comprehensive microwave dielectric properties was obtained at the x = 0.5 composition sintered at 925 °C.

In recent years, the LTCC technology has attracted much attention owing to its advantages of high-frequency characteristics, high assembly density, thermal stability, and multifunctionality. The primary condition of the LTCC technology is the densification temperature of the ceramic lower than the melting point of the electrode, such as Ag at ~961 °C and Cu at ~1060 °C. In this work, the 0.5 CaB₂O₄ + 0.5 CaSiO₃ ceramics can be densified at 925 °C, which meets the requirement of the LTCC technology. Figure 5 shows the elemental mapping results collected on the fracture surface of the 0.5 CaB₂O₄ + 0.5 CaSiO₃ ceramic co-fired with the Ag electrode to estimate the application potential of this composite ceramic in the LTCC field. The clear boundary and distinct distribution of different elements indicate that the composite ceramic had good chemical compatibility with the Ag electrode.

A patch antenna was handmade using the 0.5 CaB₂O₄ + 0.5 CaSiO₃ composite ceramic to confirm the application potential. Initially, the optimal configuration was simulated using the CST Microwave Studio software. Figure 6a shows the simulated geometry parameters of the patch antenna. Figure 6b presents the simulated and measured results of the S₁₁ parameter. The center frequency of the simulated result was located at 6.6 GHz (impedance bandwidth of 80 MHz), whereas that of the measured result was localized at 6.62 GHz with a higher bandwidth of 94 MHz. Figure 6c,d show the radiation. The E and H planes showed nearly similar orientation radiation patterns. These results indicated that the 0.5CaB₂O₄ + 0.5CaSiO₃ ceramic has potential applications in wireless communication systems.

4. Conclusions

In this work, (1 − x)CaB₂O₄ + xCaSiO₃ composite ceramics with low densification temperature were prepared. The XRD and Rietveld refinement results indicate that the CaB₂O₄ and CaSiO₃ can coexist in all compositions. Furthermore, the CaB₂O₄ can promote the CaSiO₃ transformation from α-phase to β-phase. A dense ceramic with excellent microwave dielectric properties (ε_r~6.4, Q × f~75,600 GHz, and a negative τ_f~26.9 ppm/°C) was obtained at x = 0.5 when sintered at 925 °C. The SEM image and elemental mapping results indicate that the 0.5CaB₂O₄ + 0.5CaSiO₃ ceramic has good chemical compatibility with the Ag electrode. A patch antenna with a center frequency of 6 GHz and a high bandwidth was fabricated using the 0.5CaB₂O₄ + 0.5CaSiO₃ ceramic. All results indicate that the 0.5CaB₂O₄ + 0.5CaSiO₃ ceramic has large application potential in the LTCC field.