The effect of composition on Ba-Nd-Sm-Ti-O microwave dielectric materials for LTCC application

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The effect of composition on Ba-Nd-Sm-Ti-O microwave

dielectric materials for LTCC application

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_{Material Science Center, National Tsing-Hua University Hsinchu 300, Taiwan, ROC}

Abstract

Effect of composition of BaO·(R2O3)y·(TiO2)_z·0.06(2Bi2O3·3TiO2) materials,R = Nd_(1−x)Sm_(x), BRT, on the materials characteristics and microwave dielectric properties of the samples was systematically examined. The Ti/Ba ratio (z value) shows the most significant effect on the microwave dielectric properties of the materials. TheQ × f -value is small for z 3.82 materials, and showing maximum value forz = 4.32 samples, then decrease for z = 4.52 composition, which is possibly due to the presence of secondary phase for z = 3.82 and 4.52 materials. Diffusion couple experiments shows that the interaction diffusion zone between glass and BRT ceramic is not pronounced, so that Ba-B-Si glass and BRT could be a good low temperature co-firable ceramic system is inferable.

Keywords: BaO-Nd2O3-TiO2series; Dielectric properties; Low temperature co-firable ceramic (LTCC) composites

1. Introduction

BaO-Nd2O3-TiO2 series materials possess marvellous

microwave dielectric properties, such as high dielectric constant and high quality factor, and were extensively in-vestigated for the applications in microwave devices[1–6]. Processing of BaO-Nd2O3-TiO2 series materials is,

how-ever, extremely difficult due to complicated interaction between the constituents. Kolar et al., [1] incorporated 2Bi2O3·3TiO2, which possess positive coefficient of

resonance frequency (τ_f = 650 ppm◦C−1), with BaNd2

-Ti4O12, which possess negative ␶f (−l20 ppm◦C−1), to

achieve low τ_f composite materials. Durand [2] and Kawashima [3] improved the microwave properties of Bi2O3-BaO-Nd2O3-TiO2 materials by adding BaSm2

Ti5O14 into the materials, as soild solution [3]. Laffez [4] proposed the complicated formula Ba6−x(Sm1−y

Nd_y)8+2x/3Ti18O54 and Satheesh [5] assumed the

for-mula BaNd2(1−x)Sm2xTi5O14 for these series of materials.

The trend by which the composition influences the mi-crowave dielectric properties of the materials is quite contro-versial. In this paper, the Ti/Ba ratio and the Nd/Sm ratio in BaO·(R2O3)1.08·(TiO2)z·0.06(2Bi2O3·3TiO2) where

R = Nd(1−x)Sm_(x) were systematically examined to un-derstand the mechanism that the composition influence the

∗_{Corresponding author.}

E-mail addresses: [email protected] (C.-C. Cheng), [email protected] (T.-E. Hsieh), [email protected] (I.-N. Lin).

microwave dielectric properties of the materials. The inter-action between BRT and glass was also examined for the possibility of using these materials as LTCC materials.

2. Experimental

The BaO·(R2O3)1.08·(TiO2)z·0.06(2Bi2O3·3TiO2)

ma-terials with R = Nd_(1−x)Sm_(x), designated as (BRT)114

were prepared by conventional mixed oxide process. High purity oxides, BaCO3 (Kali, 99.8%), TiO2 (rutile, Bayor,

99.7%), Nd2(CO3)3 (Treibache or Rhodia, 99%), and

Sm2O3 (Rhodia, 99.5%), with the nominal composition

BaO·(R2O3)1.08·(TiO2)z·0.06(2Bi2O3·3TiO2) where R =

Nd_(1−x)Sm_(x)was mixed and then calcined at 1170◦C for 2 h, followed by pulverization, pressing and then sintering at 1330◦C for 2 h. In the first series of (BRT)114

mate-rials, the Nd/Sm ratio is controlled at x = 0.3 in R = Nd_(1−x)Sm_(x)and the Ti/Ba ratio is controlled atz = 4.0, 4.5 and 4.7, which are designated as (BRT)I, (BRT)II and

(BRT)III, respectively. In the second series of (BRT)114

materials, z = 4.5 and Nd/Sm = 5.67-2.33, which cor-responding to x = 0.15–0.30 in Nd_(1−x)Sm_(x) The den-sity of the sintered (BRT)114 materials was measured by

Archimedes method. The crystal structure and microstruc-ture of the samples were examined using X-ray diffractmeter (XRD, Simens D5000) and scanning electron microscopy (Hitach, 2500-s). The microwave dielectric properties of the

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Fig. 2a. So does the microwave dielectric constant of the (BRT)114 materials.Fig. 2b. While the dielectric constant

of the (BRT)114materials correlates with TiO2-content

inti-mately, the quality factor of the samples varies with the TiO2

content in a quite different trend.Fig. 2cindicates that for (BRT)114withR = Nd(1−x)Sm(x)andx = 0.2 the quality

factor (Q×f ) is small with value 5240 GHz when z = 3.82, (BRT)I, and reach its maximum 7440 atz = 4.32, (BRT)II,

but theQ × f -value decreases abruptly to Q × f = 6410 for higher TiO2-content materials at (z = 4.52, (BRT)III.

Fig. 1. X-ray diffraction patterns of BaO·(Nd1−xSmx)1.08·(TiO2)z·0.06(2Bi2O3·3TiO2) material whichz = Ti/Ba = (a) 3.82, (b) 4.32 or (c) 4.52 and x = 0.2.

Fig. 2. (a) Density, (b) dielectric constant and (c) quality factor of BaO·(Nd1−xSmx)1.08·(TiO3)z·0.06(2Bi2O3·3TiO2) materials withz = Ti/Ba = 3.82, 4.32 or 4.52.

surface shown in Fig. 4a–creveals that the (BRT)114

ma-terials contain rod-shaped grains. As Ti/Ba ratio increased to 4.7 some elongated (BRT)115 grains were distributed in

the (BRT)114 matrix. (Fig. 4c) Voids are observed for all

the three samples, which are easily formed for the materi-als containing rod-shaped grains with large aspect ratio. The unique feature of the (BRT)III materials, as compared with

the (BRT)Iand (BRT)IImaterials, is that in additional to the

grains with short-blunt-rod shaped (aspect ratio∼ 4:1), there appears numerous grains with long-thin-rod shape (aspect

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Fig. 3. SEM microstructure of the as-sintered surface of BaO·(Nd1−xSm_x)1_.08·(TiO3)_z·0.06(2Bi2O3·3TiO2) materials withz = Ti/Ba = (a) 3.82, (b) 4.32 or (c) 4.52 andx = 0.2.

Fig. 4. SEM microstructure of the BaO·(Nd1−xSm_x)1_.08·(TiO3)z·0.06(2Bi2O3·3TiO2) materials withz = Ti/Ba = (a) 3.82, (b) 4.32 or (c) 4.52, x = 0.2 after polishing and thermal etching.

Fig. 5. The EDS analysis of the (BRT)114 samples with z = 4.52, indicating that the long-thin-rod shaped grains are secondary phase, enriched in Ti-species and deficit in Sm- and Nd-species.

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Fig. 6. Line scan of (a) SiKα and (b) NdLα over the surface of diffusion couple composed of Ba-B-Si glass and BRT ceramic material.

ratio∼ 10:1). EDS analysis indicates that the long-thin-rod shaped grains are secondary phase enriched in Ti-species and deficit in Sm- and Nd-species (Fig. 5), (Table 1). It is be-lieved that the presence of these high aspect ratio rod-shaped grains is the main factor for the quality factor of the (BRT)114

materials decreasing. Whereas the nature of these unique grains is not quite understood yet.

Generally, the microwave dielectric materials were mixed with low melting temperature glass for synthesising low temperature co-firable ceramic (LTCC) composites, which

are important materials for the development of microwave multilayer ceramic devices. The interaction between glass

and dielectric materials is of concern, since such an inter-action may impose deleterious effect on the repeatability in microwave characteristic of the sintered composites, or even completely degrades the dielectric properties of the di-electrics. To examine the suitability of (BRT)114 materials

for using as LTCC materials, a diffusion couple experiment was performed to investigate the inter-diffusion between the glass and microwave dielectrics.

The BaO-B2O3-SiO2, BBS, glass with molar ratio of

27:18:55 is chosen, since the BBS glass possess highest

Q-factor in microwave regime, among the MO-B2O3-SiO2

and MO-Al2O3-B2O3-SiO2(M = Mg, Ca, Sr or Ba) glass

materials.Fig. 6reveals that there exists clear boundary be-tween BBS glass (labelled as G) and (BRT)114material. Line

scan of Si and Nd signals acrossing the glass-to-BRT bound-ary indicates the inter-diffusion is minimal. Restated, the BBS and (BRT)114 composites (with 50 wt.%), which can

Table 1

The comparison of EDS analysis between matrix and point A α

TiK BaL NdL SmL BiM

Matrix 53.76a _16.83 _18.34 _9.81 _1.63

A 61.76 19.54 9.99 7.53 1.16

a_{Values are in atomic percent.}

be densified to more than 97% T.D. by sintering at 900◦C for 2 h, could be a good LTCC materials for multilayer mi-crowave devices.

4. Conclusion

Effect of composition of BaO·(R2O3)y·(TiO2)z·0.06

(2Bi2O3·3TiO2), withR = Nd(1−x)Sm(x), on the materials

characteristics and microwave dielectric properties of the samples was systematically examined. Both the density (D) and dielectric constant (ε) of the materials increase with the Ti/Ba ratio and Nd/Sm ratio monotonously, revealing the close relationship between these characteristics. How-ever, the quality factor (Q × f ) of the materials varies with the Ti/Ba and Nd/Sm ratios in slightly different trends. TheQ × f -value is small for Ti/Ba 3.82 materials and increases to its maximun for Ti/Ba = 4.32 samples, then decrease abruptly for Ti/Ba = 4.52 samples. Such a phe-nomenon is more closely related to the microstructures rather than the density of materials. Excess TiO2 can

in-duce long- and -thin rod-shaped grains of large aspect ratio (α 10). The results of diffusion couple experimen-tal indicates that BRT and Ba-B-Si are insoluble to each other. However, an inter-diffusion zone was observed at the interface, which is good for LTCC application.

Acknowledgements

Financial support from the National Science Council, ROC, through the project no. NSC 89-2112-M-003-037 was gratefully acknowledged by the authors.

References

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