J. Cent. South Univ. Technol. (2011) 18: 1838-1843

DOI: 10.1007/s11771-011-0911-3

Microstructure and microwave dielectric properties of

lead borosilicate glass ceramics with Al₂O₃

WEI Peng-fei(韦鹏飞)^{1, 2}, ZHOU Hong-qing(周洪庆)^{1, 2}, WANG Jie(王杰)¹,

ZHANG Yi-yuan(张一源)¹, ZENG Feng(曾凤)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 Al₂O₃ addition on both the sintering behavior and microwave dielectric properties of PbO-B₂O₃-SiO₂ glass ceramics were investigated by Fourier transform infrared spectroscope (FTIR), differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that with the increase of Al₂O₃ content the bands assigned to [SiO₄] nearly disappear. Aluminum replaces silicon in the glass network, which is helpful for the formation of boron-oxygen rings. The increase of the transition temperature T_g and softening temperature T_f of PbO-B₂O₃-SiO₂ glass ceramics leads to the increase of liquid phase precipitation temperature and promotes the structure stability in the glasses, and consequently contributes to the decreasing trend of crystallization. Densification and dielectric constants increase with the increase of Al₂O₃ content, but the dielectric loss is worsened. By contrast, the 3% (mass fraction) Al₂O₃-doped glass ceramics sintered at 725 °C have better properties of density ρ=2.72 g/cm³, dielectric constant ε_r=6.78, dielectric loss tan δ=2.6×10^-3 (measured at 9.8 GHz), which suggest that the glass ceramics can be applied in multilayer microwave devices requiring low sintering temperatures.

Key words: PbO-B₂O₃-SiO₂ glass ceramics; Al₂O₃; Fourier transform infrared spectroscope; microstructure; dielectric properties

1 Introduction

The development of communication systems such as mobile systems requires the miniaturization of devices. Low temperature co-fired ceramic (LTCC) multilayer devices have been extensively investigated for the miniaturization of microwave dielectric components. For the application of LTCC multilayer devices, it is necessary to reduce the sintering temperature of the microwave dielectric ceramics so that they could be co-fired with metallic electrodes such as silver or copper [1]. Lead borosilicate glasses (PbO-B₂O₃-SiO₂, PBS) are of interest for application in preparing low-temperature, low-dielectric ceramic substrates, because they have a lot of advantages, such as low-softening point, low- dielectric constant and low thermal conductivity [2]. So, they can be useful in forming low-temperature, low- dielectric glass ceramic substrates for high frequency equipments, such as Dupont 951 and 9K7. The PBS structural study by SAWVEL et al [3] suggests that PbO shows a transition from ionic (Pb²⁺) to covalent bound. Lead species with increased PbO content in the borosilicate glasses and the addition of aluminum to the glass network further enhance the lead species transition in borosilicate glasses, resulting in a relative higher amount of covalent lead bonding. Al₂O₃ is an important component of the glass systems [4-7]. In tetrahedral coordination, it replaces silicon in the glass network.  But at larger concentrations, Al₂O₃ acts as a network modifier [8]. So, Al₂O₃ plays an important role in lead borosilicate glass systems. Unfortunately, few studies pay close attention to the Al₂O₃ effect on the microstructure of lead borosilicate glasses. The main objective of this study is to investigate the sintering behavior and microwave dielectric properties of lead borosilicate glass ceramics with Al₂O₃ addition. The relationships between the sintering temperatures, microstructure evolution and microwave dielectric properties of lead borosilicate glass ceramics with various amounts of Al₂O₃ were presented.

2 Experimental

Chemical compositions of the glasses were listed in Table 1. The composition of 10PbO-17B₂O₃-68SiO₂- 5CaO (mass fraction, %) was used as basic glass, and different amounts of Al₂O₃ (3%-9%, mass fraction) were introduced to replace SiO₂ and B₂O₃ in the basic glass according to the same mass ratio of SiO₂ to B₂O₃. Reagent-grade PbO, H₃BO₃, SiO₂, CaCO₃ and Al₂O₃ were chosen as the raw materials. After uniform mixing, the batches were melted at 1 400-1 700 °C for 30-120 min. Then the molten glass was quenched into de-ion water. The cullet was dried and milled with 5 mm- diameter 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 the green compacts with diameter of 13 mm and thickness of 4-10 mm, and then followed by sintering at the temperatures between 700 °C and 950 °C for 15 min with a heating rate of 5 °C/min. It was noticed that the glass ceramics deformed above 850 °C.

Table 1 Composition of different PBS 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 powder, 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 to 1500 cm^-1. Glass transition and crystallization temperatures of the bulk glass samples were determined by differential thermal analyses (DTA) with a heating rate of 10 °C/min (Netzch, α-alumina as reference material). Crystallization heat treatments were planned according to the DTA results. The phase composition of the samples was determined by X-ray diffraction with Cu K_α radiation (XRD, ARL, X/TRA). The microstructures of the samples were examined by scanning electron microscopy (SEM, JSM-5900) after glass samples were eroded in 2% HF acid (mass fraction) for 30 s. Bulk density measurements of the fired samples was conducted by Archimedean immersion method using water as media with the accuracy of ±0.01 g/cm³. The dielectric properties such as dielectric constant (ε_r), 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 many interfering modes. The samples were positioned inside the circular cavity with good contact to 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 Al₂O₃ on FTIR absorption spectra of lead borosilicate glasses

FTIR spectroscopy was used to obtain essential information concerning the arrangement of the structural units from the studied glasses. The experimental FTIR spectra of the lead borosilicate glasses with various contents of Al₂O₃ are presented in Fig.1. The IR spectra of the Al₂O₃ free glass W1 shows four main bands. The band at about 471 cm^-1 is due to the Si—O—Si asymmetric bending vibration [8-9] and the band in the region of 800–850 cm^-1 for α-SiO₂ is commonly assigned to the anti-symmetric stretching vibration of Si—O—Si in [SiO₄] tetrahedral [10]. The main intense band located at 850-1 200 cm^-1 is assigned to the stretching vibration of [BO₄] tetrahedral [11-13]. The band located at 1 402-  1 420 cm^-1 is attributed to the B—O bending vibration of [BO₃] triangles.

Fig.1 Infrared absorbance spectra of PBS glass samples

The introduction of Al₂O₃ causes significant change to the glass structure. With increasing the Al₂O₃ content, the relative intensity of the band at about 471 cm^-1 seems to decrease. Especially, the band of sample W4 in the same position nearly disappears. The band in the region of 800-850 cm^-1 is similar. Aluminum not only acts as a network modifier, but also can replace silicon in the glass network, which depends on its content. This indicates that when the content of Al₂O₃ is lower than 9%, a majority of aluminum acts as a network modifier and reduces the non-bridging oxygen in the glasses. When the Al₂O₃ content is 9%, aluminum breaks the [SiO₄] tetrahedral and replaces silicon in the glass network. The conversion between [BO₃] units and [BO₄] units can be confirmed by the change of the band at 850-1 200 cm^-1 (characteristic for the [BO₄] units) and the band at     1 402-1 420 cm^-1 (characteristic for the [BO₃] units). The equilibrium between [BO₃] and [BO₄] units mainly depends on the Al₂O₃ content [2, 8]. When the content of Al₂O₃ is lower than 9%, the amount of [BO₃] and [BO₄] units increases. While the content of Al₂O₃ is 9%, the strength of [BO₄] is weakened. The broad of the band at about 1 170 cm^-1 to the vibration of boron-oxygen rings is composed of [BO₃] and [BO₄] units. The increase of [BO₃] and decrease of [BO₄] units in glass W4 indicate that the content of Al₂O₃ is sufficient to satisfy the formation of maximum amount of [BO₄] units. This clearly suggests that Al₂O₃ is acting as network former in glass W4. The vibration band of boron-oxygen rings located at 1 170 cm^-1 turns clearly with the increase of Al₂O₃ which indicates that Al₂O₃ is helpful to the formation of boron-oxygen rings. This can result in a larger number of bridge oxygen in the network structure of the glasses, which leads to the increase of the continuity of glass network and consequently contributes to the decreasing trend of crystallization.

3.2 Effect of Al₂O₃ on thermal stability and phase of lead borosilicate glasses

A parameter usually employed to estimate the glass stability is the thermal stability, which is defined by ΔT= T_f-T_g [14]. Another parameter introduced by HRUBY [15-16] is the glass forming ability (K_gl) which is defined as

where T_f, T_g and T_l are the softing temperature, glass temperature and liquid temperature, respectively.

Both the increasing ΔT=T_f-T_g and a decreasing temperature interval T_l-T_f indicate an increasing glass stability and a lower tendency toward crystallization. Figure 2 shows the DTA curves of the glass samples. The characteristic temperatures and glass stability parameters are listed in Table 2. T_c is the crystallization temperature. It can be seen that an increase of Al₂O₃ leads to the decrease of T_g and increase of ΔT and K_gl. This means that the introduction of Al₂O₃ to the glass systems helps to improve the glass stability and decrease the trend to crystallization.

Fig.2 DTA curves of PBS glass samples

Table 2 Thermal properties of PBS glass samples

The X-ray diffraction patterns of the PBS glass ceramics with different amounts of Al₂O₃ additive are shown in Fig.3. Crystallization sequence of the alumina free composition W1 is quite simple, and consists of three steps: The XRD patterns of the lead borosilicate glass ceramics are basically composed of amorphous glassy phases below 870 °C, while the DTA samples sintered at 870 °C for 15 min are mainly composed of crystalline phases of CaSiO₃, and the crystallization of α-SiO₂ occurs at higher temperature. Thus, the first and the second exothermic heat peaks in DTA curves represent these crystallization stages, respectively. Comparing with the pure PBS glass ceramics, the addition of Al₂O₃ does not change the phase structure of the PBS samples and the ratio of two phases, which indicates that the Al₂O₃-doped PBS glass ceramics exhibit good phase stability for actual application.

This also can be seen in the SEM photographs of lead borosilicate glass ceramics with various Al₂O₃ contents sintered at different temperatures (Fig.4). It is known that the addition of Al₂O₃ into normal lead borosilicate glasses can decrease phase separation and restrain crystallization. The main reasons for this are that Al₂O₃ can decrease the non-bridging oxygen in glass and connect and reinforce the network configuration by [AlO₄] combined with [SiO₄]. But the influence on phase separation is quite different when Al₂O₃ is added into lead borosilicate glass. Here, when Al₂O₃ content is lower than 9%, Al₂O₃ has the main effect on decreasing the non-bridging oxygen. When the Al₂O₃ content is 9%, Al₂O₃ has the main effect on replacing silicon in the glass network and forming boron-oxygen rings. So the samples sintered below 870 °C are composed of amorphous glassy phases. Furthermore, boron-rich glass phases are easily eroded by HF acid, the samples of W3 and W4 remain many round holes.

3.3 Effect of Al₂O₃ on sintering and dielectric properties of lead borosilicate glass ceramics

Figure 5 represents the bulk density of the PBS glass ceramics with the addition of 0-9% (mass fraction) Al₂O₃ as a function of sintering temperature from 700 °C to 825 °C. For all lead borosilicate glass ceramics, the bulk density increases to the maximum values as the sintering temperature increases, and then drops slightly with further increasing sintering temperature. At a given sintering temperature, the higher the amount of Al₂O₃ is, the higher the bulk density of the sintered sample is. The dot on the vertical axis shows the density of the pure PBS glass ceramics (sample W1) sintered at 700 °C. The density of pure PBS glass ceramics is about 2.68 g/cm³. The density of the samples doped with 3% Al₂O₃ remains relatively low, which indicates that 3% Al₂O₃ is not enough for densifying the ceramics efficiently at the low sintering temperature. Then, when the Al₂O₃ content increases to 6%, the density of sample W3 reaches the highest value at around 750 °C, the value of the density is 2.77 g/cm³. But when the Al₂O₃ content increases to 9%, the density of samples reaches the highest value at around 800 °C. The density slightly increases initially with increasing the sintering temperature and then decreases slightly after reaching the maximum value. All these results show that Al₂O₃ is effective in enhancing the sintering ability of the PBS glass ceramics [5].

Fig.3 XRD patterns of PBS glass ceramics held at different temperatures for 15 min: (a) W1; (b) W2; (c) W3; (d) W4

Fig.4 SEM photographs of PBS glass ceramics eroded by HF acid at different temperatures: (a) W1, 700 °C; (b) W2, 725 °C; (c) W3, 750 °C; (d) W4, 800 °C

Fig.5 Bulk density of PBS glass ceramics with addition of 0-9% Al₂O₃

Figure 6 shows the dielectric constant (ε_r) curves of PBS glass ceramics doped with Al₂O₃ as a function of sintering temperature. The relationship between ε_r values of Al₂O₃-doped glass ceramics and sintering temperature presents a trend similar to that between density and sintering temperature since a higher density means a lower porosity. The dielectric constant increases slightly with increasing the sintering temperature and reaches the saturated value, and then drops slightly with further increasing the sintering temperature. The ε_r values of sintered samples W1, W2, W3 and W4 are 6.52, 6.78, 7.35 and 7.91, respectively. Many factors are believed to affect the microwave dielectric loss and these factors can be divided into the intrinsic loss and the extrinsic loss. Intrinsic losses are mainly caused by lattice vibration modes while extrinsic losses are dominated by second phases, oxygen vacancies, grain sizes and densification or porosity [1]. Here, all samples are basically composed of amorphous glassy phases below 870 °C, so the densification is the main factor to improve the dielectric constant. On the other hand, the dielectric constant of Al₂O₃ (ε_r=9.6) is also helpful to the increase of the dielectric constants of the samples.

Fig.6 Dielectric constant of PBS glass ceramics with addition of 0-9% Al₂O₃

Figure 7 depicts the dielectric loss (tan δ) curves of PBS glass ceramics doped with Al₂O₃ as a function    of sintering temperature. The tan values of sintered samples W1, W2, W3 and W4 are 0.001 2, 0.002 6, 0.004 2 and 0.003 7, respectively. The behavior of tan δ has crosscurrent to that of the bulk density, indicating that microstructure is an important factor influencing the  tan of the samples. In the lead borosilicate glasses investigated here, at least three types of dielectric loss for glasses have been distinguished: resonance-type vibrational loss at very high frequency, migration loss by the movement of mobile ions, and deformation loss by defects or deformation of the basic silicon oxide network [17]. When the content of Al₂O₃ is lower than 9%, a majority of aluminum acts as a network modifier, which gives rise to a higher electronic migration rate. So the  tan values are improved with the increase of Al₂O₃ content. In the lead borosilicate glasses, a stable glass network will consist of SiO₄, BO₃ and BO₄ network forming units linked through oxygen. Therefore B—O—B, Si—O—Si and B—O—Si bonds will be presented in the glass network. When the content of Al₂O₃ is 9%, aluminum breaks the [SiO₄] tetrahedral and replaces silicon in the glass network, it contributes to the polarization processes occurring in the glass ceramic materials as a whole. So it can be observed that the sample W1 has the lowest tan δ values.

Fig.7 Dielectric loss of PBS glass ceramics with addition of 0-9% Al₂O₃

4 Conclusions

1) With the increase of Al₂O₃ content, the band assigned to [SiO₄] nearly disappears. Aluminum replaces silicon in the glass network, which is helpful for the formation of boron-oxygen rings.

2) The increase of the transition temperature T_g and softening temperature T_f of PbO-B₂O₃-SiO₂ glass ceramics leads to the liquid phase precipitation temperature increasing and promotes the structure stability in the glasses, and consequently contributes to the decreasing trend of crystallization.

3) Densification and dielectric constant increase with the increase of Al₂O₃ content, but dielectric loss is worsened. By contrast, the 3% Al₂O₃-doped glass ceramics sintered at 725 °C have better properties of density ρ=2.72 g/cm³, dielectric constant ε_r=6.78, dielectric loss tanδ=2.6×10^-3 (measured at 9.8 GHz), which suggests that the glass ceramics can be applied in multilayer microwave devices requiring low sintering temperatures.

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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 supported by the Priority Academic Program Development of Jiangsu Higher Education Institution, China

Received date: 2010-11-25; Accepted date: 2011-03-07

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