The effect of LMBS glass on the microwave dielectric properties of the Mg₃B₂O₆ for LTCC

Abstract

The Mg₃B₂O₆ ceramics with lithium magnesium borosilicate (LMBS) glass were prepared at a lower sintering temperature. The effect of LMBS glass on the densification, phase development, microstructure and microwave dielectric properties of the Mg₃B₂O₆ ceramics was investigated. Adding the LMBS glass to Mg₃B₂O₆ ceramics lowered the sintering temperature of the ceramics to 950 from 1300 °C. The X-ray diffraction patterns showed that LMBS glass reacted with Mg₃B₂O₆, forming the Mg₂B₂O₅ and Mg₂SiO₄. Because of the high dielectric performance of these phases, the Mg₃B₂O₆ mixed with 35.0 wt% LMBS glass sintered at 950 °C had ε _r  = 6.5, Q × f = 21,000 GHz, and τ _f  = −49.5 ppm/°C at 6.55 GHz. Therefore, the Mg₃B₂O₆-LMBS composites are promising materials for LTCC applications.

1 Introduction

With the rapid progress in microwave telecommunication and satellite broadcasting, the information technology requires high-speed data transmission. The demands for miniaturization, integration, high frequency and modularity of microelectronics and information technology are getting more and more urgent [1]. Low temperature co-fired ceramics (LTCC) is regarded as a promising future technology for the integration of components and substrates that are compact, light, and offer high-speed and functionality for portable electronic devices for high frequency applications [2]. In LTCC, as microwave dielectric for substrates in electronic packaging or patch antennas, low dielectric loss or high quality factor Q × f (for selectivity), low relative permittivity ε _r (to increase signal speed) and near zero temperature coefficient of resonant frequency (−6 ppm/°C ≤ τ _f  ≤ +6 ppm/°C) are essential for practical applications [3–7]. The LTCC materials are co-fired with an inner electrode, so the sintering temperature must be lower than the melting point of the electrode such as Ag (961 °C) [8, 9].

Many materials with excellent microwave dielectric properties, such as Mg₃B₂O₆ and Mg₂B₂O₅ ceramics [10, 11], usually have high sintering temperature (above 1250 °C), which is too high to be applicable to LTCC [8]. As far as we know, there are several approaches to reduce the sintering temperature of the ceramics: (1) the usage of smaller particle size of staring materials synthesized by chemical process, (2) the addition of low-melting glasses, oxide or mixed-oxide [12–14], (3) the search for new low temperature firing ceramic compounds. Chemical synthesis complicates the manufacturing and increases the production cost. In contrast, the addition of low-melting glass or oxide, such as ZnO–B₂O₃ glass, Li₂O–B₂O₃–SiO₂ glass, Li₂O–MgO–ZnO–B₂O₃–SiO₂ glass or B₂O₃, is known to be cheaper and effective method for achieving densified ceramics [15–17]. And a number of glass–ceramic composites have been developed for LTCC application [18–20].

As the lithium magnesium borosilicate (LMBS) glass reacted with Mg₃B₂O₆, forming the Mg₂B₂O₅ and Mg₂SiO₄ phases with the high dielectric performance, in this paper, LMBS glass was used to lower the sintering temperature of Mg₃B₂O₆ ceramics. The densification, phase development, microstructure and microwave dielectric properties of Mg₃B₂O₆-LMBS composites were systematically investigated.

2 Experimental procedure

The Mg₃B₂O₆ ceramic samples were prepared by the conventional solid-state ceramic route, using the high purity reagent: MgO (98.5 %) and H₃BO₃ (99.5 %) as the starting materials. According to the desired stoichiometry, appropriate ratio of the powder samples were mixed and then calcined at 1000 °C for 10 h in air. The loss of B₂O₃, which occurred during the pre-reactions, was compensated by weighing 11.0 wt% excess of B₂O₃ [13]. The LMBS glass powders were synthesized from reagent-grade powders of Li₂CO₃ (97.0 %), MgO (98.5 %), H₃BO₃ (99.5 %) and SiO₂ (99.7 %). These chemicals were weighed in the mole percentage of 10, 40, 30 and 20 respectively and well mixed in a dry condition. Then they were melted in an alumina crucible above their melting point, quenched, and powdered. The Mg₃B₂O₆ with different weight percentages (15.0, 25.0, 35.0, 45.0 and 55.0 wt%) of LMBS glass were ball milled with zirconia balls in deionized water for 4 h. After drying at 110 °C, the mixed powders were ground, and then pressed to cylindrical disks of about 25 mm diameter and 14 mm thickness. The samples were then sintered in the temperature range of 900–1000 °C for 3 h in air.

Crystalline phases of the sintered samples were identified by the X-ray diffraction (XRD: PANalytical B.V., X’Pert PRO) using Cu Kα radiation. The microstructures were characterized by scanning electron microscope (SEM: Philip, XL30TM). The bulk densities of the sintered ceramics were measured using the Archimedes method. The microwave dielectric properties were measured by an Advantest network analyzer (E5071C). The ε _r and Q values were measured by using the Hakki–Coleman dielectric resonator method [21, 22]. The temperature coefficient of resonant frequency (τ _f ) was also measured by the same method using Eq. (1)

$$\tau_{f} (ppm/^\circ C) = \frac{{(f_{80} - f_{25} ) \times 10^{6} }}{{55f_{25} }}$$

(1)

where f ₈₀ and f ₂₅ are the resonant frequencies at 80 and 25 °C respectively.

3 Results and discussion

The effect of LMBS glass on the Mg₃B₂O₆ ceramics has been studied. Figure 1 shows the room temperature XRD patterns of Mg₃B₂O₆ mixed with 15.0, 35.0, 55.0 wt% LMBS glass sintered at 950 °C for 3 h. The powder diffraction pattern of Mg₃B₂O₆ is indexed based on JCPDS #75-1807 with the orthorhombic structure, and the lattice parameters are a = 5.398 Å, b = 8.416 Å, c = 4.497 Å. As shown in the figure, LMBS glass reacted with Mg₃B₂O₆ to form Mg₂B₂O₅ (JCPDS #15-0537, anorthic crystal system, lattice parameters: a = 6.155 Å, b = 9.22 Å, c = 3.122 Å) and Mg₂SiO₄ (JCPDS #85-1364, orthorhombic, lattice parameters: a = 4.749 Å, b = 10.190 Å, c = 5.980 Å), via the reactions:

Fig. 1

XRD patterns of Mg₃B₂O₆ mixed with a 15.0 wt%, b 35.0 wt% and c 55.0 wt%, LMBS glass sintered at 950 °C for 3 h in air

$${\text{Mg}}_{3} {\text{B}}_{2} {\text{O}}_{6} + {\text{ MgO }} + {\text{ B}}_{2} {\text{O}}_{3} = \, 2{\text{Mg}}_{2} {\text{B}}_{2} {\text{O}}_{5}$$

(2)

$${\text{Mg}}_{3} {\text{B}}_{2} {\text{O}}_{6} + {\text{ MgO }} + {\text{ SiO}}_{2} = {\text{ Mg}}_{2} {\text{B}}_{2} {\text{O}}_{5} + {\text{ Mg}}_{2} {\text{SiO}}_{4}$$

(3)

If the LMBS glass is superfluous, the MgO and SiO₂ crystallize into Mg₂SiO₄ (JCPDS #85-1364). And the rest of the LMBS glass as glassy state exists in the ceramics. The increasing glass content promotes the reactions: the peak intensity of Mg₃B₂O₆ decreases, the peak intensity of the Mg₂B₂O₅ and Mg₂SiO₄ phases increase with the addition of LMBS glass. At the glass content in excess of 35.0 wt%, the main phase of the glass–ceramics turns into Mg₂B₂O₅ and the diffraction peaks of Mg₃B₂O₆ continue to reduce. When the content of glass addition is 55.0 wt%, Mg₃B₂O₆ disappears and the diffraction peaks of Mg₂B₂O₅ and Mg₂SiO₄ increase significantly, which means that the transformation has fully completed and more precipitation phase separates out from the superfluous LMBS glass.

As there is no other phase existed except for Mg₃B₂O₆, Mg₂B₂O₅ and Mg₂SiO₄, the proportion of these phases in this composite can be calculated using starting material composition data according to the mass conservation. Theoretical results show that the Mg₃B₂O₆ completely converts into Mg₂B₂O₅ when the content of LMBS is about 50.0 wt%, close to the experimental value of 55.0 wt%.

The scanning electron micrographs (SEM) of the fracture surfaces of Mg₃B₂O₆ ceramics with LMBS glass (15.0, 25.0, 35.0, 45.0 and 55.0 wt%) sintered at 950 °C for 3 h are shown in Fig. 2a–e. With less glass, the samples have a dense and small grain size. The grain size of the sample with 15.0 wt% glass is about 1–5 µm. The grains became larger and denser with the increasing addition of LMBS glass. For 35.0 wt% glass, the ceramics turned much denser and less small pores with irregular shape were left. When the content of glass addition is 45.0 wt%, the grain sizes are inhomogeneous and the smaller grains form, which may be related to the reaction between Mg₃B₂O₆ and glass. In the 55.0 wt% glass composites, as the formation of liquid phase during melting of the glass, the boundaries of grains blur to obscurity and arrange closely together. Clearly, the addition of LMBS glass promoted the crystal phase transformation and grain growth.

Fig. 2

SEM images of the fracture surfaces of Mg₃B₂O₆ ceramics with a 15.0 wt%, b 25.0 wt%, c 35.0 wt%, d 45.0 wt% and e 55.0 wt% LMBS glass additives sintered at 950 °C for 3 h in air

Figure 3 shows that the relative density, permittivity ε _r and the Q × f values of Mg₃B₂O₆ ceramics with different weight percentage of LMBS glass addition sintered at 900–1000 °C for 3 h in air. Both glass content and sintering temperature are important factors in the densification process. When the temperature rises from 900 to 950 °C, the relative density and permittivity ε _r of the samples increased, which may be due to the decrease in porosity and the enlargement of grain sizes. At 950 °C, the composite with 35.0 wt% glass has a maximum relative density of 95 %. The change in permittivity ε _r has a similar tendency with relative density. Since the phases of Mg₃B₂O₆, Mg₂B₂O₅ and Mg₂SiO₄ have a similar dielectric constant (6.8, 6.6 and 6.8) [10, 11, 23], the changes of the permittivity can be mainly attributed to the densification. At the optimized sintering temperature 950 °C for the composites with 35.0 wt% glass, the permittivity ε _r is more than 6.5, which corresponds to the values of Mg₃B₂O₆ and Mg₂B₂O₅. The samples with low glass content have a slightly higher permittivity, this is because there is more amount of unreacted Mg₃B₂O₆ in them and Mg₃B₂O₆ has a slightly larger permittivity than Mg₂B₂O₅.

Fig. 3

The relative density, the permittivity ε _r and the Q × f values of Mg₃B₂O₆ ceramics with 15.0–55.0 wt% LMBS glass addition sintered at 900–1000 °C for 3 h in air

In the case of Mg₃B₂O₆ with 15.0 and 25.0 wt% LMBS glass, the Q × f values increased with the increasing temperature. At 950 °C, the Q × f values of Mg₃B₂O₆-35.0 wt% LMBS reached to the maximum of 21,000 GHz (f = 6.55 GHz). When the sintering temperature is higher than 950 °C, the Q × f values of Mg₃B₂O₆ with 35.0–55.0 wt% LMBS glass falls considerably. Comparing the relative density, permittivity ε _r and the Q × f values of the samples, we can conclude that the change of the permittivity ε _r and Q × f value is homologous to that of densification. A moderate amount of glass addition increases the density and microwave dielectric properties of the composite because of the liquid phase sintering. An excess amount of glass obviously deteriorates the microwave dielectric properties due to the decrease in densification and the higher dielectric loss of the LMBS glass.

The temperature coefficients of resonant frequency (τ _f ) values of Mg₃B₂O₆ ceramics with different weight percentage of LMBS glass addition sintered at 950 °C for 3 h in air are shown in Fig. 4. The τ _f values increased to the negative side with the increasing glass content from 15.0 to 55.0 wt%. This slight change of τ _f (from −48 to −51.5 ppm/°C) may be due to the increasing content of Mg₂SiO₄ (−65 ppm/°C), which has slightly higher τ _f values in the negative side than Mg₃B₂O₆ (−46 ppm/°C) [10, 11, 23].

Fig. 4

The τ _f values of Mg₃B₂O₆ ceramics with 15.0–55.0 wt% LMBS glass sintered at 950 °C for 3 h in air

4 Conclusion

Low-temperature sintering of Mg₃B₂O₆ ceramics with different weight percentages of LMBS glass has been investigated. The addition of LMBS glass lowered the sintering temperature of Mg₃B₂O₆ ceramics to 950 °C from 1300 °C. The XRD patterns reveal that the formation of the new crystallization phase and transformation between Mg₃B₂O₆ and Mg₂B₂O₅ are controlled by the content of glass. The crystalline phases with low loss, such as Mg₃B₂O₆, Mg₂B₂O₅ and Mg₂SiO₄, affect the microwave dielectric properties of Mg₃B₂O₆-LMBS composites dramatically. It is revealed that Mg₃B₂O₆ ceramics with 35.0 wt% LMBS glass sintered at 950 °C for 3 h have excellent microwave dielectric properties: ε _r  = 6.5, Q × f = 21,000 GHz, and τ _f  = −49.5 ppm/°C at 6.55 GHz. Therefore, Mg₃B₂O₆-LMBS composites are promising materials for LTCC applications.