Microwave dielectric properties of glass-ceramic composites for low temperature co-firable ceramics

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Microwave dielectric properties of glass-ceramic composites

for low temperature co-ﬁrable ceramics

Chung-Chin Cheng

a

_{, Tsung-Eong Hsieh}

a

_{, I-Nan Lin}

b,

_*

a_{Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, 300, Taiwan, ROC} b_{Materials Science Center, National Tsing-Hua University, Hsinchu, 300, Taiwan, ROC}

Abstract

Ba–B–Si glass was added to Ba–Nd–Sm–Bi–Ti–O (BRT114) microwave dielectric material for LTCC applications. Conventional

one-step processing method for preparing glass-BRT114composite materials yields low dielectric constant, since the glass was easy

to react with BRT114and forms a low dielectric constant phase, Ba3B6Si2O16. A large proportion of pores appeared. The nature of

glass, whether it is sol-gel derived or fused, shows marked inﬂuence on the microstructure and microwave dielectric properties of the composites. A two-step process containing precoating the BRT114powders with a thin layer of glass, followed by conventional

samples preparation process, tremendously improved the densiﬁcation behaviour of the material. The formation of pores and interactions between glass and BRT114 was greatly suppressed such that materials with high dielectric constant ("r=40) were

achieved by sintering 9 wt.% glass-containing composite at 950_{C for 2.5 h.}

Keywords:Glass ceramics; Microwave ceramics; BRT114; LTCC materials

1. Introduction

Low temperature co-firable ceramics (LTCC) posses-sing good microwave dielectric properties have been widely investigated due to the necessity for miniaturiza-tion of devices in order to reduce the size of wireless communication system.1 3 _{However, the microwave} dielectric materials possessing high quality factor and large dielectric constant usually require very high sin-tering temperature and long soaking time to achieve high enough density. On the other hand, using Ag material as conducting materials for transmission lines and ground planes is needed in order to minimize the microwave absorption loss. Reduction of the sintering temperature of the microwave materials to a level cofir-able with Ag-electrode materials is thus called for. Generally, low softening temperature glass materials were mixed with the ceramic materials to reduce the fir-ing temperature.4 9 _{However, network formers} con-tained in the glass materials may absorb the microwave power profoundly in high frequency regime, degrading the quality factor of the materials.10_{In this article, the} microwave dielectric properties of the glass materials

and the eﬀect of processing parameters on the char-acteristics of the glass-to-ceramic composites were investigated. We show that a two-step processing method is able to raise the density of glass-BRT114 composites and to increase the dielectric constant of composite to a high value of 40.

2. Experimental

The glass-BRT114 specimens were prepared by con-ventional mixed oxide process. The glass component, which consists of BaO:B2O3:SiO2 in the ratio of 42:45:13 wt.% with Ts=619_{C, were prepared by either} hydrolysis of alkoxide mixture (designated as sol-gel glass) or direct fusion process (designated as fused

glass). The BRT114 was a commercial microwave

dielectric (MBRT-90B, Fujititan).

In the one-step process, the sol-gel (or fused) glass of 9–40 wt.% was mixed directly with BRT114 powders, followed by pelletization, and then sintering at 850– 950_{C for 2.5 h. For the two-step process, the BRT114} powders were mixed with a small proportion of either sol-gel glass or fused glass ( 5.5 wt.%) and were calcined at 700 _{C for 1 h. The glass coated BRT}

114 powders were then mixed with fused glass (4.5–34.5

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*Corresponding author.

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3. Results and discussion 3.1. One-step processed materials

The SEM micrographs shown inFigs. 1 and 2 illus-trate, respectively, the compositional dependence of microstructure with diﬀerent amount of glass for the glass-BRT114 composites incorporated with the sol-gel glass or fused glass (BaO:B2O3:SiO2 in the ratio of 51:45:4 wt.%). All the samples possess a large propor-tion of porosity, indicating that both the sol-gel glass and fused glass could not wet the BRT114powders good enough and, therefore, could not eﬀectively densify the ceramics.

The sol-gel glass derived materials contain pores with smaller size than the fused glass derived ones. The implication of this phenomenon is that the sol-gel glass powders, which are finer in size ( 0.5 mm), mix more uniformly with the BRT114-powders than the fused-glass powders, which are around 1.5. Moreover, the fused glass is more reactive such that large agglomerates were resulted prior to the interaction of the glass pow-ders with the BRT114 materials. Bridging phenomenon occurred during firing process, resulting in large pores. Bridging phenomenon that occurred in sol-gel glass derived materials is not as serious as that observed in fused glass. Increasing the glass-content from 9 to 23 wt.% only slightly increases the density of the samples, which can also be ascribed to the agglomeration and bridging phenomenon occurred prior to densification of the composite materials. Moreover, Fig. 3(a) and (b), the X-ray diffraction patterns for sol-gel glass derived and fused glass derived BRT114composites, respectively show that both kinds of glass material interact with the BRT114 powders markedly at firing temperatures, forming Ba3B6Si2O16phase.

That the fused glass can react with BRT114powders more readily than the sol-gel glass is further demon-strated by the compositional dependence of the micro-wave properties for composite materials [Fig. 4(a) and (b)].Fig. 4(a) shows that the dielectric constant ("r) and quality factor (Qf) of the sol-gel glass-derived materi-als ﬂuctuate profoundly with the concentration of glass

Fig. 1. SEM micrographs of polished and thermal-etched surface for the glass-BRT114composite materials containing sol-gel derived glass

of (a) 9 wt.%, (b) 16.7 wt.%, and (c) 23 wt.%, which were sintered at 950_{C for 2.5 h.}

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about er=12.5 for 23 wt.% glass containing samples. The quality factor (Qf) increases with proportion of glass added, reaching a maximum value, around 9000, for samples containing 23 wt.% glass and decrease again for further increase in glass content. The dielectric constant ("r) and quality factor (Qf) not only for the fused glass derived materials not only are of higher value, but also vary with the proportion of the glass more consistently, as compared with those for sol-gel glass derived ones.

3.2. Two-step processed materials

The above results imply that the prime factor, result-ing in high porosity and poor microwave dielectric properties for the one-step processed glass-BRT114 composites, is that the glass can not wet the BRT114 powders very well. The possible process to improve the wetting of the glass with BRT114materials is to precoat the BRT114materials with a thin layer of glass compo-sition, prior to the mixing with large proportion of glass.

Figs. 5(a) and 6(a) respectively show the tremendous improvement in densiﬁcation behaviour for the glass-BRT114composites due to the two-step process. For the specimens containing 9 wt.% fused BaBSiO glass, which were sintered at 950_{C for 2.5 h, very little pores were} Fig. 2. SEM micrographs of polished and thermal-etched surface for

the glass-BRT114composite materials containing fused glass of (a) 9

wt.%, (b) 16.7 wt.%, and (c) 23 wt.%, which were sintered at 950_C

for 2.5 h.

Fig. 3. X-ray diﬀraction patterns for the glass-BRT114composite

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observed. Whether the precoating layer was a sol-gel glass or fused glass seems to have no influence. Appar-ently, the pronounced improvement achieved in the two-step process can be ascribed to the increase in wet-ting ability of glasses with the BRT114 powders due to the presence of the thin glass coating. That the nature of coating layers does not influence the densification behavior is understandable, as the high temperature calcination (700 _{C) after coating will react completely} the glass layer with the BRT114 powders, regardless of the precoating materials.

Even for the two-step processed composites, the pores appear again when the materials contain a larger pro-portion of glasses, which are shown inFig. 5(b) and (c) for sol-gel glass coated materials and Fig. 6(b) and (c) for fused glass coated materials. Presumably, an excess of glass results in agglomeration and bridging phenom-enon, hindering the densification process. Lowering the firing temperature also markedly increases the porosity, which is presumed to be due to insufficient wetting ability between the glasses and glass-coated BRT114 powders. X-ray diffraction analysis (Fig. 7) indicates that interaction between glass and BRT114 powders is

signiﬁcantly suppressed for the materials prepared by two-step process.Fig. 8(a) and (b) show the "rand Qf-properties of these materials, which were, respectively, precoated with a thin layer of the sol-gel glass and fused glass, followed by mixing with the fused glass and the other sample preparation process. For a specimen

con-Fig. 4. Microwave dielectric properties (erand Qf ) for the

glass-BRT114composite sintered at 850–950C for 2.5 h (a) sol-gel derived

glass and (b) fused glass.

Fig. 5. SEM micrographs for two-step processed glass-BRT114

com-posite materials, which were precoated with a thin layer of sol-gel glass and then mixed with (a) 9 wt.%, (b) 16.7 wt.%, and (c) 23 wt.% fused glass, followed by sintering at 950_{C for 2.5 h.}

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taining 9 wt.% glass, the dielectric constant was still low for 850 or 900 _{C-ﬁred samples, but increased} markedly, to er=40, for 950C-ﬁred samples. The same phenomenon was observed, no mater the precoating

layer is sol-gel or fused glass. The dielectric constant decreases pronouncedly as the proportion of fused glass increased, which is apparently due to the presence of secondary phases that are rich in Bi2O3.

Fig. 6. SEM micrographs for two-step processed glass-BRT114

com-posite materials, which were precoated with a thin layer of fused glass and then mixed with (a) 9 wt.%, (b) 16.7 wt.%, and (c) 23 wt.% fused glass, followed by sintering at 950_{C for 2.5 h.}

Fig. 7. X-ray diﬀraction patterns for two-step processed glass-BRT114composites, which were precoated with a thin layer of fused

glass and then mixed with 9–23 wt.% fused glass, followed by sintering at 950_{C for 2.5 h.}

Fig. 8. Microwave dielectric properties for two-step processed glass-BRT114composites, which were precoated with a thin layer of (a)

sol-gel glass and (b) fused glass, and then were mixed with 9–23 wt.% of fused glass, followed by sintering at 850–950_{C for 2 h.}

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The characteristics of the glass-BRT114 composite materials were systematically investigated. For one-step processed composites, a large proportion of pores appear, which results in low dielectric constant for the materials, but the Q is large. The nature of glass, whe-ther it is sol-gel derived or fused, shows a marked influ-ence on the microstructure and microwave dielectric properties of the composite materials. Pores are large and interconnected, inferring the occurrence of bridging phenomenon which, in turn, can be ascribed to the agglomeration of glass powders during firing process. A two-step process, which precoats the BRT114 powders with a thin layer of glass, tremendously improves the densification behaviour for the materials. The formation of pores were almost completely suppressed such that

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