Crystallization Phase Regulation of BaO-CaO-SiO₂ Glass-Ceramics with High Thermal Expansion Coefficient

Abstract

In this work, the influence of Ba/Ca ratios on the BaO-CaO-SiO₂ (BCS) glass network structure, crystallization phases, and coefficient of thermal expansion (CTE) was investigated. As the Ba/Ca ratio increases, the Qⁿ units in the glass network structure have undergone significant changes. The Q⁴ units in the BCS glass network transform into Q³ units, indicating the reduction of the glass network connectivity. The variation in the Ba/Ca ratio leads to a change in the crystallization phases of BCS glass-ceramics sintered at a temperature higher than Tc (crystallization temperature). The addition of α-SiO₂ (quartz) could regulate the crystallization phases and their ratio of the barium silicates (BaSi₂O₅, Ba₂Si₃O₈, and Ba₅Si₈O₂₁) in the BCS glass-ceramics. An abundant orthorhombic BaSi₂O₅ phase can be obtained in the BCS glass-ceramics with 15 wt% α-SiO₂ calcinated over 875 °C. The α-SiO₂ modified BCS glass-ceramics exhibited excellent properties (CTE = 12.10 ppm/°C, ε_r = 7.49 @ 13.4 GHz, tanδ = 4.96 × 10⁻⁴, Q × f = 27,034 GHz) sintered at optimized conditions, making it a promising candidate material for RF module and electronic packaging substrate.

1. Introduction

Multilayered ceramic technology has been widely used in electronic packaging, radio frequency (RF) components, and integrated modules [1,2,3]. The key properties of ceramic materials for these applications include low sintering temperature, low dielectric constant (ε_r), and low dielectric loss (tanδ) [4,5]. Low sintering temperature enables the material to cofire with the metal electrode (Cu/Au/Ag) for multilayer integration [6]. A low dielectric constant leads to a small propagation delay of electromagnetic signals. The low dielectric loss would reduce the decay of electromagnetic signals and the thermal effect in packaging [7]. Furthermore, when substrates or modules are mounted onto printed circuit boards (PCBs) through ball grid arrays (BGAs), the difference in the coefficient of thermal expansion (CTE) between ceramic materials and PCBs (CTE~12–20 ppm/°C) may result in potential solder joint fatigue [8,9]. On the other hand, the high-throughput wireless data communication of 5G/6G systems would induce more pronounced thermal effects in integrated modules. To enhance the reliability of integrated systems necessitates the use of multilayered ceramic substrates with a high CTE [10,11,12].

Strategies to enhance the CTE of ceramic materials for packaging or integrated modules primarily focus on adjusting the composition of glass-ceramics and the crystallized phase to regulate their thermal expansion behavior. Studies on various glass-ceramics systems such as BaO-Al₂O₃-B₂O-SiO₂ [13,14,15,16,17], CaO-BaO-Al₂O₃-B₂O₃-SiO₂ [18], CaO-B₂O₃-SiO₂ [19], and Li₂O-Al₂O₃-SiO₂ [20] have demonstrated that the incorporation of phases with higher CTE (e.g., barium silicate and calcium barium silicate; see Table 1) could effectively increase the CTE of these materials. However, many studies revealed a trade-off between CTE and dielectric loss because of the complicated crystallization process, uncontrollable crystallization phase, and microstructure of glass-ceramic [13,14,17]. Previous works suggest that network-modifying ions (e.g., Ca²⁺, Sr²⁺, Ba²⁺, Mg²⁺) doping in glass can adjust the network connectivity and introduce the mixed alkaline earth effect to regulate both thermal expansion and dielectric properties [14,16,21,22,23,24]. Fillers and some additives could also modify the crystallization kinetic process and tune the crystallization phase of glass-ceramics [17]. It would be favorable for obtaining glass-ceramic with both high CTE and low dielectric loss if a large amount of high CTE low loss phase can be achieved by composition and thermal process control.

The BaO-CaO-SiO₂ (BCS) glass-ceramics system could precipitate high thermal expansion coefficient phases, such as Ba₅Si₈O₂₁, Ba₃Si₅O₁₃, Ba₂Si₃O₈, and BaSi₂O₅ [25,26,27]. Ghosh et al. [28] reported that the thermal expansion coefficient of the BCS glass-ceramics system can reach 9.5–13.0 ppm/°C by adjusting the composition to modify crystallization. Li et al. [29] found that the addition of Al₂O₃ and B₂O₃ to the BCS glass-ceramics system can achieve low dielectric loss (tanδ < 0.001). However, the addition of Al₂O₃ and B₂O₃ suppresses the precipitation of BaSi₂O₅, and the α-cristobalite/β-cristobalite phase transition leads to nonlinear thermal expansion, which can also cause thermal mismatch in electronic packaging substrates. The BaSi₂O₅ phase has garnered considerable attention due to its high CTE and low dielectric loss. Therefore, achieving a high concentration of the BaSi₂O₅ crystallization phase in the BCS glass-ceramics system is key to realizing both high thermal expansion and low dielectric loss.

Table 1. CTE and dielectric properties of high thermal expansion calcium silicate and barium silicate Phases.

Compounds	CTE (ppm/°C)	Q × f (GHz)	εr	Reference
CaSiO3	11.2	29,300	6.9	[30,31]
Ca2SiO4	8.5	26,100	8.6	[30,31]
BaSiO3	12.5	6600	11.1	[32,33]
Ba2Si3O8	11.7	29,800	8.2	[32]
Ba5Si8O21	10.6	16,700	7.3	[32]
Ba3Si5O13	11.8	12,500	6.9	[32]
BaSi2O5	14.0	59,500	6.7	[32,33]

Table 1. CTE and dielectric properties of high thermal expansion calcium silicate and barium silicate Phases.

Compounds	CTE (ppm/°C)	Q × f (GHz)	εr	Reference
CaSiO3	11.2	29,300	6.9	[30,31]
Ca2SiO4	8.5	26,100	8.6	[30,31]
BaSiO3	12.5	6600	11.1	[32,33]
Ba2Si3O8	11.7	29,800	8.2	[32]
Ba5Si8O21	10.6	16,700	7.3	[32]
Ba3Si5O13	11.8	12,500	6.9	[32]
BaSi2O5	14.0	59,500	6.7	[32,33]

In this work, the BaO-CaO-SiO₂ glass-ceramics with different Ba/Ca ratios were investigated. The effects of Ba/Ca ratios on glass microstructure, crystallization, and CTE of BCS glass-ceramics were systematically investigated. α-SiO₂ was incorporated into BCS glass-ceramics to regulate the phase compositions and dielectric properties using a solid-state process. An abundant orthorhombic BaSi₂O₅ phase can be obtained in the BCS glass-ceramics modified by α-SiO₂. Excellent performance was obtained from the modified BCS glass-ceramics.

2. Materials and Methods

BCS glasses with compositions of xBaO-yCaO-45SiO₂ (mol%, where x + y = 55, and x/y ratios are 30/25, 35/20, 40/15, 45/10, and 50/5) were synthesized using a traditional melt-quenching method [26]. Reagent-grade BaCO₃ (99%), CaCO₃ (99%), SiO₂ (99%), and α-SiO₂ (99.9%) powders (China National Pharmaceutical Group Corporation, Beijing, China) were used as raw materials. The resulting glass fragments were ball-milled in ethanol for 24 h. The BCS glass powders with 5 wt%, 10 wt%, 15 wt%, and 20 wt% α-SiO₂ addition were prepared. The BCS glass-ceramics powders with 15 wt% α-SiO₂ addition, respectively, were prepared by calcining the mixed powders at 875 °C for 2 h and then subjected to a second round of ball-milling for 24 h. Polyvinyl alcohol (PVA) was added to the glass or composites to prepare the sintered bulk samples, and then, the granulated mixture was uniaxially pressed at 50 MPa into disks (13 mm in diameter) and rods (5 mm × 5 mm × 30 mm). After the removal of the PVA binder by heating at 450 °C for 2 h, the disks and rods were sintered at temperatures ranging from 700 °C to 1000 °C for 2 h, with a heating rate of 2 °C/min.

The volume density of the sintered glass-ceramics was determined using the Archimedes displacement method. Differential scanning calorimetry (DSC, NETZSCH 404, Selb, Germany) was employed to analyze the endothermic and exothermic peaks of the glass. The crystallization phases were characterized by X-ray diffraction (XRD, BRUKER D8 ADVANCE, Ettlingen, Germany). The microstructure and elemental distribution of the sintered samples were observed using a scanning electron microscope equipped with energy disperse spectroscopy (SEM&EDS, Verios G4, Waltham, MA, USA). Raman spectroscopy (InVia, Renishaw, Gloucestershire, UK) was utilized to analyze the glass microstructure. The network structure of the glass was analyzed using magic angle spinning nuclear magnetic resonance (MAS-NMR, BRUKER AVANCE IIIHD 500M, Ettlingen, Germany). The coefficient of thermal expansion (CTE) of the samples was measured using a thermomechanical analyzer (NETZSCH 402 F3, Selb, Germany). The dielectric constant (ε_r) and low dielectric loss tangent (tanδ) of sintered ceramics were measured at room temperature using the Hakki–Coleman method by a PNA Series Network Analyzer (AGILENT E8363A, Santa Clara, CA, USA). The Q × f value can be calculated using the Equation (1):

$$\mathrm{Q}\times f={\displaystyle \frac{f}{\tan \mathrm{\delta }}}$$

(1)

where Q is the quality factor, f is the resonant frequency, and tanδ is the dielectric loss tangent.

3. Results and Discussion

3.1. Effect of Ba/Ca Ratio on Structure and Crystallization of BCS Glasses

The Raman spectra of BCS glasses with different Ba/Ca ratios (Figure 1a) primarily exhibit three main bands at 300–500 cm⁻¹, 550–650 cm⁻¹, and 850–1200 cm⁻¹. The bands with peaks at 334 cm⁻¹ and 468 cm⁻¹ are attributed to the [Si–O] rocking and bending vibrations [34]. The band peaked at 606 cm⁻¹ is ascribed to the [Si–O–Si] bending vibration in depolymerized structural units [35], and the band at 850–1200 cm⁻¹ originates from the [Si–O–Si] bending vibration of 3- or 4-membered silica rings in the glass [36,37]. As shown in Figure 1a, the Ba/Ca ratio in the BCS glass composition has minimal effect on the low-frequency bands. The variation is observed in the peak intensity within the 900–1200 cm⁻¹ range. Specifically, the enhanced peak at around 1062 cm⁻¹ indicates that changes in the Ba/Ca ratio affect the connectivity of the glass network, which is closely associated with the Qⁿ units [38].

The local environment of the Si element was examined using MAS-NMR spectra to further confirm the effect of the Ba/Ca ratio on the connectivity of the glass network. The ²⁹Si NMR spectra (Figure 1b) of BCS glasses all exhibit a broad peak in the range of −120 ppm to −70 ppm, indicating that there is a distribution of Qⁿ units (where Qⁿ refers to the [SiO₄] tetrahedra with n bridging oxygens, n = 0, 1, 2, 3, 4). This broad chemical shift peak observed in the ²⁹Si NMR spectra (−120 ppm to −70 ppm) can be Gaussian-fitted into three peaks (Figure 1c). The Q⁰ and Q¹ units are primarily located at −66 ppm to −62 ppm and −76 ppm to −68 ppm with very low detected intensities [37]. In contrast, the Q², Q³, and Q⁴ units are distributed across the spectra: the peak at −110 ppm to −95 ppm corresponds to the Q⁴ unit in the glass network, the peak at −95 ppm to −85 ppm corresponds to the Q³ unit, and the peak at −85 ppm to −75 ppm corresponds to the Q² unit [39,40]. The proportion of Qⁿ units can be determined by calculating the area of the Gaussian-fitted peaks (Figure 1d). As the Ba/Ca ratio increases, the content of Q² units remains nearly unchanged. While, the content of Q³ units increases from 32% to 56%, and the content of Q⁴ units decreases from 60% to 40%. This demonstrates that an increase in the Ba/Ca ratio induces the conversion of Q⁴ units to Q³ units in the BCS glass network, leading to a decrease in the number of bridging oxygens. It has been reported that different Qⁿ units form various anionic structures [35,41,42]. Different anionic structures would result in different nucleation, crystallization phenomena, and crystallization phases after heat treatment.

The effect of the Ba/Ca ratio on the thermal behavior of BCS glass was further investigated using differential scanning calorimetry (DSC). Figure 2a shows the DSC curves of glass powders with different Ba/Ca ratios, measured at a heating rate of 10 K/min. The DSC curves of various BCS glass compositions exhibit one or two distinct exothermic peaks, which correspond to the crystallization peaks of glasses with different compositions. As the Ba/Ca ratio increases, the crystallization temperature of the BCS glass decreases from 925.7 °C to 830 °C. This is attributed to the reduction in the connectivity of the glass network, which releases more “free oxygen”, disrupting the Si-O bonding network and relaxing the glass structure [15]. The Ba²⁺ and Ca²⁺ ions have a depolarizing effect on the Si-O bonds in the network, weakening the strength of the Si-O bonds. Ba²⁺ has a stronger weakening effect than Ca²⁺ [12,43]. According to the relationship between the sintering temperature and the volume density of BCS glass-ceramics (Figure S1), BCS glasses with different Ba/Ca ratios can all achieve their maximum bulk density at a sintering temperature of no more than 775 °C. Since the DSC curves show that the crystallization temperatures of all BCS glass compositions are above 775 °C, the sintering process was performed above the crystallization temperature of all samples to obtain BCS glass-ceramics.

The crystallization temperatures of different glass compositions are determined by the temperature of the first crystallization peak in their DSC curves. The crystallization temperatures for the Ba/Ca ratios of 30/25, 35/20, 40/15, 45/10, and 50/5 are 925 °C, 875 °C, 875 °C, 850 °C, and 850 °C, respectively. Figure 2b shows the main crystallization phase changes of BCS glass-ceramics with different Ba/Ca ratios at their crystallization temperatures. Overall, the main crystalline phases formed in the glass of each composition include BaSi₂O₅ (JCPDS#72-0171), Ba₅Si₈O₂₁ (JCPDS#35-0766), Ba₂Si₃O₈ (JCPDS#27-1035), BaSi₄O₉ (JCPDS#83-0958), and Ca₂SiO₄ (JCPDS#36-0642). In BCS glasses with Ba/Ca ratios of 30/25, 35/20, and 40/15, the main crystallization phases include BaSi₂O₅, Ba₂Si₃O₈, and BaSi₄O₉, along with the formation of minor amounts of Ca₂SiO₄ phases. As the Ba/Ca ratio increases, Ca₂SiO₄ phases gradually disappear, and the barium silicate phases in the BCS glass also undergo transformations. At a Ba/Ca ratio of 45/10, the primary crystallization phases are BaSi₂O₅ and Ba₂Si₃O₈. At a Ba/Ca ratio of 50/5, monoclinic Ba₅Si₈O₂₁ becomes the dominant crystallization phase.

The phases precipitated in glass are closely related to the glass network structure. It has been reported that all lattice structures of barium silicates contain [SiO₄] tetrahedra. Both Ba₂Si₃O₈ and Ba₅Si₈O₂₁ have tetrahedral chains or one-dimensional structures, while BaSi₂O₅ has layered or two-dimensional structures [44]. For both one-dimensional and two-dimensional structures, the tetrahedral chains are zwieier, with two [SiO₄] tetrahedra in the repeating unit [45]. The structure of barium silicate can be described as Ba_1+1/M [Si₂O_5+1/M]. In the case of BaSi₂O₅, its layered structure is composed of an infinite number of single chains (M = ∞) [32]. This structure is similar to the short-range or medium-range ordered structure composed of Q⁴ units [1]. Therefore, when Q⁴ units dominate the glass network structure, BaSi₂O₅ preferentially nucleates and precipitates. As the Ba/Ca ratio increases, Q³ units replace Q⁴ units, and the corresponding crystalline phase Ba₅Si₈O₂₁ becomes the preferred nucleation phase. This is also attributed to the structural similarity between the Ba₅Si₈O₂₁ phase and the glass composition. The CTE value of these types of barium silicates is closely related to the complexity of the crystal structure, phase composition, and the tilting and distortion of [SiO₄] tetrahedra [32].

The thermal expansion property was investigated to clarify the effect of the Ba/Ca ratio and crystallization phase composition on the properties. The thermal expansion of BCS glasses with different Ba/Ca ratios and sintered at different conditions are illustrated in Figure 3. The thermal expansion curves (Figure 3a) for all compositions demonstrate good linearity. After sintered at 775 °C, the CTE of sintered BCS glass increases with the increase of Ba/Ca ratio, rising from 8.8 ppm/°C to 10.7 ppm/°C at the range of −50 °C to 400 °C. This increase is primarily attributed to the change in the chemical bond strength and reduced network connectivity in the BCS glass as the Ba/Ca ratio increased [46]. Alkali earth meta ions exert effects on the Si-O bonds within the network and increase the availability of “free oxygen” to break the Si-O network and relax the glass structure, thereby weakening the strength of the Si-O bonds [44,45]. Notably, Ba²⁺ has a stronger weakening effect on Si-O bonds than Ca²⁺ due to its larger ionic radius and lower field strength compared to Ca²⁺. The increase in the Ba/Ca ratio would also result in an increased concentration of non-bridging oxygen (NBO). Raman spectroscopy and solid-state NMR analyses confirm that this alteration is closely linked to the Qⁿ units. The increase in the Ba/Ca ratio induces the transition from Q⁴ units to Q³ units, thereby reducing the connectivity of the glass network. Lower network connectivity in glass results in a higher thermal expansion coefficient [46].

Upon increasing the sintering temperature to above the crystallization temperature, the precipitation of the low-expansion phase Ca₂SiO₄ (CTE = 8.5 ppm/°C) results in a decrease in CTE of the BCS glass-ceramics for these with low Ba/Ca ratios. Obviously, the CTE of the glass-ceramic is higher than its glass counterpart for those with Ba/Ca ratios higher than 40/15. The highest CTE value, 11.3 ppm/°C (−50 to 400 °C), is observed from glass-ceramic with a Ba/Ca ratio of 45/10. The CTE of BCS glass-ceramics exhibits a nonlinear variation. This is primarily attributed to the changes in the crystallization phases. As the Ba/Ca ratio increases, the low-expansion phase Ca₂SiO₄ (CTE = 8.5 ppm/°C) gradually disappears, which contributes to the increase in the CTE of the BCS glass-ceramics. Concurrently, as the Ba/Ca ratio continues to rise, the crystalline phases of barium silicate (BaSi₂O₅, Ba₅Si₈O₂₁, Ba₂Si₃O₈, BaSi₄O₉) undergo structural changes. At a Ba/Ca ratio of 45/10, the primary crystallization phases are BaSi₂O₅ and Ba₂Si₃O₈. At a Ba/Ca ratio of 50/5, monoclinic Ba₅Si₈O₂₁ becomes the dominant crystallization phase. Since the CTE of Ba₅Si₈O₂₁ (10.6 ppm/°C) is lower than that of BaSi₂O₅ (14.0 ppm/°C) and Ba₂Si₃O₈ (11.7 ppm/°C), this results in a reduction in the CTE of BCS glass-ceramics with a Ba/Ca ratio of 50/5.

3.2. Phase Regulation of BCS Glass-Ceramics by α-SiO₂

The phase composition of glass-ceramics plays a crucial role in determining their microwave dielectric and thermal properties. To develop material with both a high thermal expansion coefficient and low dielectric loss, we added α-SiO₂ (quartz) to BCS glass (Ba/Ca ratio is 45/10) to tune the sintering temperature and properties. Interestingly, it was found that a substantial amount of orthorhombic BaSi₂O₅ (CTE = 14.0 ppm/°C, Q × f = 59,500 GHz) precipitates after sintering at 875 °C when the α-SiO₂ content exceeds 15 wt% (Figure S2), which is beneficial for obtaining high thermal expansion and low dielectric loss. So, more detailed work was carried out to check the phase regulation effect of α-SiO₂ on the BCS glass-ceramics.

XRD was used to confirm the crystallization phases of the 15 wt% α-SiO₂ modified BCS glass powders after sintered at temperatures ranging from 400 °C to 900 °C (Figure 4). The BCS glass did not exhibit any phase transitions below 600 °C. After reaching 700 °C, Ba₂Si₃O₈ began to precipitate, and the diffraction peaks of α-SiO₂ gradually diminished. Ba₅Si₈O₂₁ began to form after 750 °C, and at 775 °C, Ba₅Si₈O₂₁ reacted with SiO₂ to form BaSi₂O₅. This suggests that the barium silicate phases undergo transformations as outlined in Equations (2)–(4) by reacting with SiO₂. Above 800 °C, a small amount of Ba₂Si₄O₁₀ (a high-temperature form of BaSi₂O₅) begins to form. This is attributed to the higher tetrahedral orientation diversity in the structure of Ba₂Si₄O₁₀ compared to orthorhombic BaSi₂O₅, which results in a lower energy barrier for the precipitation of Ba₂Si₄O₁₀ [47,48]. However, as the temperature increases, Ba₂Si₄O₁₀ gradually transforms into orthorhombic BaSi₂O₅. When the sintering temperature exceeds 875 °C, the phase composition of the BCS glass-ceramics does not change anymore. The composites predominantly consisted of BaSi₂O₅ with a minor amount of BaCa₂Si₃O₉.

$$T<750 °\mathrm{C}: B{a}_{2}S{i}_3{O}_8+Si{O}_2$$

(2)

$$T\ge 750 °\mathrm{C}: 2.5B{a}_{2}S{i}_3{O}_8+0.5Si{O}_2=B{a}_{5}S{i}_8{O}_{21}$$

(3)

$$T\ge 775 °\mathrm{C}\text{:} B{a}_{5}S{i}_8{O}_{21}+2Si{O}_2=5BaS{i}_2{O}_5$$

(4)

The types and concentrations of crystalline phases in glass-ceramics are key factors in determining their coefficient of thermal expansion (CTE). The phase composition of the BCS glass-ceramics after the addition of α-SiO₂ was further evaluated using Rietveld refinement analysis (Figure S3). As shown in Figure 5, the residual glass phase content in the BCS glass-ceramics is similar across all samples (ranging from approximately 34 wt% to 41 wt%). Additionally, when the α-SiO₂ content exceeds 15 wt%, a substantial amount of orthorhombic BaSi₂O₅ (over 50 wt%) precipitates. Considering the effects of all phases in the glass-ceramics, the CTE can be calculated using the Equation (5) [49]:

$$\alpha ={\alpha }_g{w}_g+{\alpha }_1{w}_1+{\alpha }_2{w}_2+\cdots +{\alpha }_n{w}_n$$

(5)

where α is the coefficient of thermal expansion (CTE) of the glass-ceramics, α_g, α₁, α₂, …, α_n are the CTEs of the glass phase and crystalline phase in the composition, ω_g, ω₁, ω₂, …, ω_n are the weight fractions of the glass phase and crystalline phase in the composition. The CTEs of the crystalline phases are listed in Table 1, and the CTE of the residual glass phase can be considered as the BCS glass (with a Ba/Ca ratio of 45/10) before crystallization (CTE = 10.65 ppm/°C; see Figure 3b). The comparison between the calculated and experimental CTE values is shown in Figure 5, which exhibits similar trends. The CTE of the BCS glass-ceramics increases with the addition of α-SiO₂. This is attributed to the precipitation of the crystalline phase BaSi₂O₅, which has a higher CTE. As the content of α-SiO₂ increases, the content of the BaSi₂O₅ crystalline phase also increases. In addition, there are discrepancies between the calculated CTE values and the experimental values. These discrepancies may arise from the presence of pores and defects within the glass-ceramics. The addition of α-SiO₂ makes it more difficult for the BCS glass-ceramics to achieve dense sintering. The higher the addition of α-SiO₂, the more pores are present in the BCS glass-ceramics (Figure S4).

3.3. Properties of the Modified BCS Glass-Ceramics

The sintering process was further optimized to obtain BCS glass-ceramics with superior properties and high density (selecting the BCS glass-ceramics composition with 15wt% α-SiO₂ addition). The SEM images presented in Figure 6 illustrate the microstructure and elemental composition distribution of the α-SiO₂ modified BCS glass-ceramics sintered at 1000 °C. As shown in Figure 6a, crystallized ceramic grains have precipitated, encircled by the residual BCS glass phase. On the polished surface (Figure 6c), three distinct phases with varying contrasts are discernible. The elemental composition and distribution of these phases were further investigated using energy-dispersive X-ray spectroscopy (EDS) (Figure 6d–h). Point-scan EDS analysis reveals that the three phases correspond to BaSi₂O₅, BaCa₂Si₃O₉, and the residual glass phase, which is consistent with the XRD results. Additionally, the EDS surface scan demonstrates an enrichment of calcium in certain regions. While most crystallized areas exhibit a lack of calcium, further substantiating the significant precipitation of BaSi₂O₅.

The dielectric constant and loss of the sintered modified BCS glass-ceramics were measured and shown in Figure 7. As the sintering temperature increases, the dielectric constant (ε_r) initially increases and then slightly decreases. This trend can be attributed to the reduction in porosity of the glass-ceramics composites. The highest dielectric constant (ε_r = 7.49) is observed at the sample with the maximum volume density (ρ = 3.66 g/cm³). Figure 7b shows the variation of the Q × f value and dielectric loss (tanδ) with sintering temperature. The dielectric loss decreases with increasing sintering temperature, reaching a minimum value at 1000 °C. The α-SiO₂ modified BCS glass-ceramics exhibited the best dielectric properties (ε = 7.49 @ 13.4 GHz, tanδ = 4.96 × 10⁻⁴, Q × f = 27,034 GHz) sintered at 1000 °C. The precipitation of BaSi₂O₅ crystals and the reduction in the glass phase can significantly reduce the dielectric loss of the composites. The decrease of porosity and other possible defects with the increase of sintering temperature may also contribute to the low dielectric loss.

As shown in Figure 7c, the α-SiO₂ modified BCS glass-ceramics exhibits a high coefficient of thermal expansion (CTE = 12.10 ppm/°C) after sintered at 1000 °C. This represents a notable increase compared to the CTE of the BCS glass-ceramics. The enhancement in CTE can be attributed to the incorporation of α-SiO₂, which modifies the phase composition of the BCS glass-ceramics and promotes the precipitation of orthorhombic BaSi₂O₅. Compared with commercial ceramic packaging substrate materials (

Table 2. Thermal expansion and dielectric properties of commercial ceramic packaging substrate materials and the material studied in this work.

Materials	CTE (ppm/°C)	tanδ × 10−4	εr @ 10 GHz	Reference
Dupont 951	5.8	15	7.5	[50]
Kyocera GL771	12.3	38	5.2	[50]
Kyocera GL773	11.7	25	5.8	[50]
Heraeus CT708	10.6	30	6.4	[6]
This work	12.1	4.96	7.5

), the high CTE and excellent dielectric performance make the α-SiO₂ modified BCS glass-ceramics a promising candidate for RF module and electronic packaging substrate applications.

4. Conclusions

In summary, the effect of the Ba/Ca ratios on the network structure of BCS glasses and their crystallization behavior was investigated. As the Ba/Ca ratio increases, the transformation from Q⁴ to Q³ units in the BCS glass network reduces the connectivity of the network, leading to an increase in CTE. The increase in the Ba/Ca ratio leads to a change in the crystallization phases of BCS glass-ceramics sintered at a temperature higher than Tc (crystallization temperature). The highest CTE was achieved from BCS glass-ceramics with Ba/Ca = 45/10. The crystallization phase composition of BCS glass-ceramics can be regulated by α-SiO₂. Orthorhombic BaSi₂O₅ becomes the main phase treated at 875 °C with 15 wt% α-SiO₂ addition. The α-SiO₂ modified BCS glass-ceramics exhibited excellent properties (CTE = 12.10 ppm/°C, ε = 7.49 @ 13.4 GHz, tanδ = 4.96 × 10⁻⁴, Q × f = 27,034 GHz) sintered at optimized condition, demonstrating its potential for RF module and electronic packaging substrate applications.