J. Cent. South Univ. Technol. (2011) 18: 1359-1364

DOI: 10.1007/s11771-011-0846-8

Microstructure and microwave dielectric properties of CaO-B₂O₃-SiO₂ glass ceramics with various B₂O₃ contents

WEI Peng-fei(韦鹏飞)^{1, 2}, ZHOU Hong-qing(周洪庆)¹, ZHU Hai-kui(朱海奎)¹,

DAI Bin(戴斌)¹, WANG Jie(王杰)¹

1. College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China;

2. College of Materials Engineering, Jinling Institute of Technology, Nanjing 211169, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: The effects of B₂O₃ addition on both the sintering behavior and microwave dielectric properties of CaO-B₂O₃-SiO₂ (CBS) glass ceramics were investigated by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that the increasing amount of B₂O₃ causes the increase of the contents of [BO₃], [BO₄] and [SiO₄], which deduces the increase of CaB₂O₄ and α-SiO₂ and the decrease of CaSiO₃ correspondingly. No new phase is observed throughout the entire experiments. A bulk density of 2.54 g/cm³, a thermal expansion coefficient value of 11.95×10^{-6 °C -1} (20-500°C), a dielectric constant ε_r value of 6.42 and a dielectric loss tan δ value of 0.000 9 (measured at 9.7 GHz) are obtained for CBS glass ceramics containing 35% B₂O₃ (mass fraction) sintered at 850 °C for 15 min.

Key words: CaO-B₂O₃-SiO₂; B₂O₃ content; Fourier transform infrared spectroscopy; microstructure; dielectric properties

1 Introduction

In the development of co-fired multilayer circuits (CMC), low temperature co-fired ceramic (LTCC) played a decisive role as a base material. The LTCC is crucial in the development of various modules and substrates. This technology combines many thin layers of ceramics and conductors resulting in multilayer LTCC modules and is generally used in the form of a three-dimensional (3D) wiring circuit board today, using low permittivity (usually ε_r<4-9) dielectric components. Additionally, it enables a versatile mixture of passive microwave components such as micro-strips, strip lines and dividers. Furthermore, these integrated components are interconnected with 3D strip line circuitry [1]. Calcium borosilicate glass (CaO-B₂O₃-SiO₂; CBS), as one of the most important LTCC substrate materials, was reported in many literatures [2-4]. It was sintered at temperatures below 900 °C, and offered low dielectric loss characters and low thermal expansion coefficient (close to Si and Ga/As). Additionally, it is very robust against environmental stress. Several studies had shown that the addition of B₂O₃ in microwave materials can perform the advantages of lowering sintering temperatures and improving the dielectric properties [5-8]. However, as one of the most common glass- formers [9], vitreous B₂O₃ has a structure consisting of a random network of boroxol rings and [BO₃] triangles connected by B-O-B linkages [10]. It was reported that the addition of a network modifier in borate glasses could produce the conversion of the triangular [BO₃] structural units to [BO₄] tetrahedral units with a coordination number of four [11]. Researchers concluded that B existed in glass with the coordination polyhedron of [BO₃] and [BO₄], and the “boron abnormality” phenomenon might arise [12-13]. The main objective of this work is to investigate the microstructure of CBS glass ceramics with B₂O₃ addition. The variations of the microwave dielectric properties were also investigated in terms of the microstructural changes.

2 Experimental

The composition of the glass was determined according to the phase diagram of the CBS systems. The glass contained 25%-40% CaO, 25%-40% SiO₂, 15%- 40% B₂O₃, P₂O₅ and 2% ZnO (all in mass fraction) in total. CaCO₃, SiO₂ and H₃BO₃ of reagent grade were chosen as the raw materials. So, the composition chosen for the experimental samples was listed in Table 1. They were dry-mixed in polyethylene jars with zirconia balls for 8 h. After being mixed uniformly, the powders were put into Pt crucible and molten in the temperature range of 1 400-1 700 °C in air. It was held at melting temperature for 30-120 min. Then, the molten glass was quenched into de-ion water. The cullets were dried and milled with d 5 mm zirconia balls to obtain an average particle size less than 5 μm. Furthermore, the powders prepared above were mixed with an amount of polyvinyl alcohol (12% PVA, mass fraction), pressed under a pressure of 100 MPa to obtain green compacts with diameter of 13 mm and thickness of 4-10 mm, and then followed by sintering at the temperatures between    800 °C and 925 °C for 15 min with a heating rate of    5 °C/min. In order to understand the effects of glass composition on the sintering characteristic, the powders were pressed under a pressure of 100 MPa to obtain green compacts with length of 40 mm, width of 4 mm and thickness of 5 mm. Dilatometric analysis was performed to characterize the shrinkage of the glass with respect to temperature. Experiments were performed with a DIL 402C dilatometer in air and at a heating rate of 5 °C/min.

Table 1 Composition of different CBS glasses

The IR spectra of the glass ceramic samples were recorded at room temperature using the KBr disc technique. The samples were ground into fine powders. Then, 2 mg glass powders and 200 mg KBr powders were accurately weighed for semiquantitative analysis of the glass structure. A Fourier transform infrared spectroscope (FTIR, Nicolet Nexus, Thermo Nicolet) was used and the wave number was ranged from 400-  1 500 cm^-1. The phase composition of the samples was determined by X-ray diffraction with Cu K_α radiation (XRD ARL X/TRA). The crystal structures were examined by scanning electron microscopy (SEM JSM-5900). Bulk density of the fired samples on the CBS glass ceramics were measured by Archimedean immersion method using water as media with the accuracy of ±0.001 g/cm³. The dielectric properties such as dielectric constant (ε_r) and dielectric loss (tan δ) were measured at 10 GHz by network analyzer (8722ET, Agilent). Cylindrical-shaped samples with an aspect ratio (diameter/height) ≥1 were used to chock off other interfering modes. The samples were positioned inside the circular cavity with good contact to the top and bottom plates. Dielectric constant and tan δ were calculated from the frequency of the TM₀₁₀ resonant mode.

3 Results and discussion

3.1 Effect of B₂O₃ on FTIR absorption spectra of CBS glass ceramics

FTIR spectroscopy was used to obtain the essential information concerning the arrangement of the structural units of the studied glass ceramics. The experimental FTIR spectra of the CBS glass ceramics with various contents of B₂O₃ are presented in Fig.1.

Fig.1 FTIR spectra of CaO-xB₂O₃-SiO₂ glass ceramics with 25%≤x≤40% (mass fraction)

It is known that in the vitreous B₂O₃ about 80% of the boron atoms are presented in the B₃O₆ boroxol rings, which are interconnected by independent [BO₃] groups [14]. Thus, the vibrational modes of the vitreous borate network are mainly active in three infrared spectral regions. The IR features located in the first region that ranges between 1 200 and 1 500 cm^-1 are due to the asymmetric stretching relaxation of B—O bonds from the [BO₃] trigonal units. The second region ranging between 800 cm^-1 and 1 200 cm^-1 and its spectral features are due to the B—O bond stretching of [BO₄] tetrahedral units. In the third region ranging from    400 cm^-1 to 800 cm^-1, there is an important band located around 700 cm^-1 assigned to the bending vibrations of various borate segments [14-15]. The bands observed in these FTIR spectra and their assignments are summarized in Table 2 [16-17]. The most obvious change at about 700, 1 080 and 1 440 cm^{-1 in Fig.1 is that the vibrating bands gradually become stronger. This indicates that the amount of [BO}₃] and [BO₄] increases with the increasing of B₂O₃ content. At the same time, the vibrating bands of the B40 sample at about 460 cm^-1 gradually become stronger. The addition of CaO as a network modifier, can offer free oxygen. So, when the free oxygen is full enough, [BO₃] has priority to associate itself with free oxygen to form [BO₄]. The rest of the free oxygen can associate with SiO₂, and the free oxygen mainly serves to destroy Si—O—Si and to form non-bridging oxygen. The amount of [BO₃] increases with the content of boron, and then, the amount of [BO₄] increases. The free oxygen in the systems is overused by [BO₃] trigonal units to form [BO₄] tetrahedral units, the amount of free oxygen decreases, and then the amount of [SiO₄] increases. This can explain the fact that the position of these bands are shifted to the higher frequencies.

Table 2 Assignment of IR absorbance spectra of samples

3.2 Effect of B₂O₃ on XRD results of CBS glass ceramics

The X-ray diffraction patterns of the sintered samples with different amounts of B₂O₃ are illustrated in Fig.2. The main crystalline phases are composed of CaB₂O₄, CaSiO₃ and α-SiO₂. The major crystalline phases of the four samples do not change, irrespective of the amount of B₂O₃ additions. But the amount of each crystalline phase varies greatly. It is noticed that the intensity of the main crystal diffraction peaks at 2θ=26.6° and 29.3° increase with increasing the B₂O₃ content. The position and intensity of the diffraction peaks are analyzed using powder diffraction files (PDF) and are found to be characteristic of CaB₂O₄ and α-SiO₂ crystalline phases. However, the intensity of diffraction peaks of the CaSiO₃ crystalline phases at 2θ=28.4° decreases with increasing the B₂O₃ content. The facts can be explained by FTIR spectra of the CBS glass ceramics systems with various contents of B₂O₃ in Fig.1. From the previous FTIR spectra analysis, [BO₃], [BO₄] and [SiO₄] increase with the increase of B₂O₃ content. Ca ions have priority to associate with [BO₃] and [BO₄] to form CaB₂O₄ crystalline phases due to its larger ionic strength. So, the rest of Ca ions combines with [SiO₄] to form CaSiO₃ crystalline phases. Without enough Ca ions, [SiO₄] associates with itself to form α-SiO₂ crystalline phases.

Fig.2 XRD results of CaO-xB₂O₃-SiO₂ glass ceramics with 25%≤x≤40% (mass fraction)

3.3 Effect of B₂O₃ on sintered and dielectric properties of CBS glass ceramics

Figure 3 shows the SEM micrographs of the fractured surface of CaO-xB₂O₃-SiO₂ glass ceramics with 25%≤x≤40% (mass fraction) sintered at 900, 870, 850 and 830 °C for 15 min, respectively. It is indicated that B₂O₃ is not only a glass networker, but also a good sintering aid, which is able to reduce the sintering temperature of the glass ceramics [18-19]. The B25 sample sintered at 900 °C is more porous in Fig.3(a). With the increasing addition of B₂O₃, the pore size and porosity decrease obviously (Figs.3(b)-(d)). At the same time, the grain size decreases with the increase of B₂O₃ amount. This is due to the structural changes from [BO₃] to [BO₄] as the content of the glass networker B₂O₃ increases. The [BO₃] groups in the borosilicate glasses prefer a coordination change to [BO₄] rather than producing non-bridging oxygen. This structural change in [BO₄] will increase the stability of the glasses. It can prevent the glass phases from converting to the crystalline phases. So, the grain size of the B40 sample in Fig.3(d) is smaller than others. The more the B₂O₃ is added, the more the glass phases form. The B40 sample has the most glass phases in all samples. This can be accompanied by the bulk density variation of the samples with different B₂O₃ contents.

Figure 4 shows the variation in the bulk density of the CBS glass ceramics as a function of the B₂O₃ content. When the content of B₂O₃ increases from 25% to 35%, the bulk density considerably increases up to 2.54 g/cm³ from 2.40 g/cm³. But the bulk density decreases slightly when the B₂O₃ content is 40%. The SEM images reveal that the microstructure of the samples becomes dense with the increasing addition of B₂O₃. However, when the amount of B₂O₃ content is more than 35%, the bulk density of the sintered samples declines to 2.47 g/cm³. This is attributed to the formation of too much glass phases.

 

Fig.3 SEM micrographs of CaO-xB₂O₃-SiO₂ glass ceramics with 25%≤x≤40% (mass fraction): (a) B25, 900 °C; (b) B30, 870 °C;   (c) B35, 850 °C; (d) B40, 830 °C

Fig.4 Sintering and thermal properties of CaO-xB₂O₃-SiO₂ glass ceramics with 25%≤x≤40% (mass fraction): (a) Bulk density;     (b) Coefficient of thermal expansion
 

The coefficient of thermal expansion (CTE) of CBS glass ceramics is also shown in Fig.4. The behavior of CTE is similar to that of the bulk density, indicating that microstructure is an important factor influencing the CTE of the samples. The maximum thermal expansion coefficient is observed on the B35 sample, and the value is 11.95×10^{-6 °C-1}. A further addition of B₂O₃ over 35% leads to a decrease in the CTE of the B40 sample. However, it is still higher than that of 30% B₂O₃. In the previous discussion, [BO₄] can prevent the glass phases from converting to the crystalline phases. It can be observed in Fig.3 that the crystalline phases of the B25 and B30 samples are more than those of the B35 and B40 samples. A large amount of glass phases in the B40 sample can be found. The increasing amplitude of vibration of atoms with increasing temperature causes thermal expansion. This increased amplitude generally causes an increase in the average interatomic distance, and then it increases the length or volume of a sample. The coefficient of linear thermal expansion is defined as the ratio of the change in length per unit length and per unit temperature [20]. In Fig.3, it can be realized that the structure of the B35 sample is more compact than other samples. So, the B35 sample has the highest thermal expansion coefficient. In the same system, the imporosity crystalline phases have higher thermal expansion coefficients than the porosity glass phases. The excessive [BO₄] causes more glass phases. When the content of B₂O₃ is more than 35%, a large number of glass phases lead to a less thermal expansion coefficient.

Figure 5 summarizes the ε_r and tan δ of the sintered CBS samples with different B₂O₃ contents. The values of ε_r and tan δ vary greatly as the amount of B₂O₃ content increases from 25% to 40%. Among these glass ceramics, the B25 sample has the lowest dielectric constant (ε_r= 6.18), and its dielectric loss is the highest (tan δ=0.034). The B35 sample has the highest dielectric constant (ε_r= 6.42), and its dielectric loss is the lowest (tan δ=0.000 9). The ε_r value of the well-sintered CBS samples are mainly influences by the major crystalline phases (CaSiO₃, CaB₂O₄ and SiO₂) and glass phases, whose ε_r values are about 5.4, 7, 3.8 and 7, respectively [4]. In the previous discussion, it could be known that CaB₂O₄ and α-SiO₂ crystalline phases increase with the increase of B₂O₃ content, while CaSiO₃ changes in a contrary tendency. The ε_r value of CBS samples rises firstly and reaches its maximum on 6.42 (the B35 sample), then drops to 6.28. The increasing of α-SiO₂ phases hampers the electronic transfer rate, and it gives rise to the decrease of dielectric loss. At the same time, the bulk density is closely related to the dielectric loss. Therefore, the samples with the highest bulk density have the lowest dielectric loss. For example, the B35 sample with 2.54 g/cm³ has dielectric loss of 0.000 9. Furthermore, in glass ceramics systems, glass phases as the main reason result in the increase of dielectric loss [4]. So, the tan δ value of CBS samples presents to drop first and reaches the minimum of   0.000 9 (the B35 sample), then rises to 0.014.

Fig.5 Dielectric properties of CaO-xB₂O₃-SiO₂ glass ceramics with 25≤x≤40 (mass fraction, %): (a) Dielectric constant;    (b) ielectric loss

4 Conclusions

1) The most obvious change at about 700, 1 080 and 1 440 cm^-1 on Fourier transform infrared spectra of CaO- Ba₂O₃-SiO₂ is that the vibrating bands gradually become stronger. The amount of [BO₃] and [BO₄] increases with the increase of B₂O₃ content. At the same time, the [SiO₄] vibrating bands of the B40 sample at about 460 cm^-1 gradually become stronger.

2) CaB₂O₄ crystalline phases increase with increasing the B₂O₃ content of the samples. Without enough Ca ions, CaSiO₃ crystalline phases decrease and α-SiO₂ crystalline phases increase for the CaO-xB₂O₃- SiO₂ glass ceramics.

3) As the amount of B₂O₃ increases, the bulk density, CTE and dielectric constant of the samples increase to a maximum and then decrease thereafter. A bulk density of 2.54 g/cm³, a CTE value of 11.95×10^{-6 °C-1} (20-500 °C), a ε_r value of 6.42 and a tan δ value of 0.000 9 (measured at 9.7 GHz) are obtained for 35% B₂O₃ (mass fraction) of CBS glass ceramics sintered at 850 °C for 15 min.

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(Edited by HE Yun-bin)

Foundation item: Project(2007AA03Z0455) supported by the National High-Technology Research and Development Program of China; Project(BE2009168) supported by the Natural Science Foundation of Jiangsu Province in China; Project supported by the Priority Academic Program Development of Jiangsu Higher Education Institution, China

Received date: 2010-07-15; Accepted date: 2010-10-08

Corresponding author: ZHOU Hong-qing, Professor; Tel: +86-25-86639976; E-mail: hqzhou323@yahoo.cn