Sintering mechanism and microwave dielectric properties of BaTi₄O₉-BBZ composite for LTCC technology

Abstract

A low temperature co-fired ceramic (LTCC) material was fabricated by mixing BaTi₄O₉ ceramic with BaO-B₂O₃-ZnO (BBZ) glass. The sintering mechanism was further analyzed through the wetting behavior, activation energy, phase evolution, microstructure and microwave dielectric properties of the BaTi₄O₉-BBZ composite. The results show that the sintering temperature of the BaTi₄O₉ ceramics can be significantly lowered from 1300 to 925 °C by the BBZ glass. This is due to the three-stage partially reactive liquid assisted sintering process which consists of glass redistribution and local grains rearrangement, solution-reprecipitaion including glass crystallization and reactions between the glass and ceramic, and global grain rearrangement, closure of pores and viscous flow. XRD patterns exhibit that BaTi₄O₉ reacts with the crystallization phase of BBZ glass obviously during sintering to form two new phases BaTi(BO₃)₂ and Ba₄Ti₁₃O₃₀. The activation energy of BaTi₄O₉ ceramic is calculated to be 520.9 ± 40.46 kJ/mol, while that of BaTi₄O₉-BBZ composite is reduced to 330.98 ± 47.34 kJ/mol. With increasing sintering temperature, the dielectric constant (ε_r) and the quality factor (Q×f) value increases firstly and then decreases, and the temperature coefficient of resonant frequency (τ_f) value slightly decreases. Typically, the BaTi₄O₉ -BBZ composite sintered at 925 °C for 2 h displays excellent microwave dielectric properties of ε_r = 26.4, Q×f = 27300 GHz and τ_f = + 0.3 ppm/°C. In addition, the good chemical compatibility of this material with Ag electrode makes it as a potential candidate for LTCC technology.

Introduction

Low-temperature co-fired ceramics (LTCC) technology has played a more and more important role in the development of mobile and satellite communications, radar systems, global position systems (GPS) and wireless area network (WLAN) technology since it can meet the requirements of miniaturization, integration and high reliability of electronic devices [1], [2]. There are several features for ideal LTCC materials [1], [2], [3]: sintering temperatures lower than 950 °C, an appropriate dielectric constant (ε_r), high quality factor (Q×f), a near-zero temperature coefficient of resonant frequency(τ_f), and the excellent chemical compatibility with Ag inner electrodes. Unfortunately, most of the commercial dielectric ceramics with good microwave dielectric properties cannot be used for LTCC application due to the high sintering temperatures above 1200 °C. One of the most important focal problems for the development of LTCC materials is to lower the sintering temperature and maintain the excellent dielectric properties of the ceramics as much as possible.

BaTi₄O₉ is a well-known commercialized dielectric ceramic with dielectric constant of approximately 37–39 and quality factor (Q×f) of 21,000–37,000 GHz and sintering temperature of it is around 1300 °C [4], [5], [6]. On the other hand, BaTi₄O₉ ceramics have a large temperature coefficient of resonant frequency (+15 ppm/°C). To meet the requirements for LTCC, large amount of work has been done to lower the sintering temperature by adding low-melting glasses or compounds [7], [8], [9], [10], [11]. However, it is easy for BaTi₄O₉ to react with the low-melting additive, and the reaction products may have an effect on its microwave dielectric properties. Chu [8] et al. found that CuB₂O₄ and BaCuB₂O₅ additives can decrease the sintering temperature of the BaTi₄O₉ ceramics below 950 °C, but the new formed phases BaTi₅O₁₁ and Ba₄Ti₁₃O₃₀ deteriorate the Q×f value and cause the rising of τ_f. Gormikov and Belous [9], [10] reported that ZnO not only can effectively lower the sintering temperature of the BaTi₄O₉ ceramic to 1200 °C but also turn the positive τ_f value to near-zero due to the formation of the second phase BaZn₂Ti₄O₁₁. Kim [11] et al. pointed out that the microwave dielectric properties of low temperature fired BaTi₄O₉ ceramic are dependent on the content of the ZnO-B₂O₃ glass additive, which determines the crystalline phases present and the microstructure after processing. Therefore, selecting a proper sintering aid for BaTi₄O₉ ceramics to lower the sintering temperature below 950 °C as well as improve the τ_f is of critical importance for the LTCC application.

BaO-B₂O₃-ZnO glass system with a low softening temperature of 480–560 °C exhibits great potential as candidates for LTCC applications [12]. In our previous studies, the sintering temperature of Ba₂Ti₉O₂₀ ceramics has been successfully lowed from 1400 °C to around 900 °C by using a glass from BaO-B₂O₃-ZnO (BBZ) system [13]. It was found in these studies that the BBZ glass additive had an enhancement effect upon the quality factor (Q×f) of Ba₂Ti₉O₂₀ ceramics, instead of degradation one as found in usual cases when low melting glass additive is involved. The BBZ glass has also been proved to be a negative τ_f -tailoring material to Ba₂Ti₉O₂₀, and it can make the positive τ_f value of the ceramic shift to negative as the amount of the glass increased. Long [14] et al. reported a similar result, in which a BaO-B₂O₃-ZnO (BBZ) glass was used as a sintering aid of BaNd₂Ti₄O₁₂ ceramics, and the densification temperature of it had been lowered from 1450 to 900 °C, and the τ_f values decreases gradually with increasing the amount of BBZ glass. Therefore, it is believed that the BBZ glass might act as an effective τ_f-tailoring material for BaTi₄O₉ ceramic adjusting the τ_f value to near-zero as well as be a kind of effective sintering aid for reducing the sintering temperature of BaTi₄O₉ ceramic to below 950 °C. In this paper, a LTCC material based on BaTi₄O₉ ceramic powder with BBZ glass is fabricated, and the phase composition, sintering process, wetting behavior, activation energy, microstructure and microwave dielectric properties of the material are investigated.