Effects of composition on low temperature sinterable
Ba–Nd–Sm–Ti–O microwave dielectric materials
Chung-Chin Cheng
a,*
, Tsung-Eong Hsieh
a, I-Nan Lin
b aDepartment of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, Taiwan, 300bMaterials Science Center, National Tsing-Hua University, Hsinchu, Taiwan, 300
Abstract
This work investigated the effects of MgO and ZnO additives on the microwave properties of BRT114=[(BaO.R
2-O3.4TiO2).0.06(2Bi2O3.3TiO2)] materials. Incorporation of small amount of ZnO (<¼1 wt.%) markedly lowered the temperature coefficient of resonant frequency (f), to around f 61 ppm/C, slightly increased the density and dielectric constant ("r) of the
materials, but degraded the Qf factor . Doping 2.5 mol% of MgO, in addition to ZnO, further improved the f-value for the
BRT114 materials. The dielectric constant and the Qf factor of the materials degrade pronouncedly when doped with too
abundant ZnO. Microstructure and EDX analyses indicated that the main factor for degrading the microwave properties is the induction on formation of secondary phases. Moreover, sol–gel and fused Ba–B–Si glass reacted with BRT114in quite a different
way. Fused glass wets BRT114 materials more easily than the sol–gel derived glass, resulting in composite materials with higher
density and larger dielectric constant. Precalcining the glass-dielectrics mixture, greatly improved the wetting ability of the glass and markedly increased the microwave properties of the glass/dielectric composite, i.e. LTCC materials.
#2003 Elsevier Ltd. All rights reserved.
Keywords:Fused glass; Glass/dielectric composite; LTCC materials; Sol–gel derived glass
1. Introduction
BaO–R2O3–TiO2 series materials, where R is the
rare earth element, possess superior microwave
dielectric properties, such as high dielectric constant and high quality factor and low temperature coeffi-cient of resonant frequency.15 These materials have great potential for microwave device applications and
have been extensively investigated. Processing of
BaO–Nd2O3–TiO2 series materials is very complicated due to the complex crystal structure of the materials, which easily induces the formation of intermediate phase. Iso-valent ions incorporation such as Sm2O3 ,-which form solid solution with the BaO–R2O3–TiO2 materials, was observed to markedly modify the micro-wave dielectric properties of the materials.35 The alivalent ions addition, such as Bi2O3, can also pronouncedly alter the material’s properties.68 The
explanation on the corresponding mechanism is,
however, still quite controversial. The investigation on the effect of additives, which modify the microstructure
of the BaO–R2O3–TiO2 series materials, is thus even more difficult.911
On the other hand, the trends for miniaturization of the microwave devices requires the development of low temperature cofirable ceramic (LTCC) materials. To lower the sintering temperature of the microwave dielectric materials to a level cofirable with Ag electrode materials, glass materials with low softening tempera-ture were usually mixed with the microwave dielectric materials to form glass-ceramics composites.12,13
In this paper we develop a LTCC material consisting of BaO–R2O3–TiO2microwave dielectric materials and glass additives to result in high dielectric constant glass-ceramic composite materials. For the first, we improved the microwave dielectric properties of the BaO.(R2O3)1.08.(TiO2)4.24.0.06(2Bi2O3.3TiO2) with R= Nd0.72Sm0.28, which were reported to possess the best microwave dielectric properties,37through the addition of MgO or ZnO species, among the BaO–R2O3–TiO2 series materials. The MgO and ZnO additives were chosen because these dopants markedly improved the dielectric properties of other series of dielectric materi-als.14,15 Then we used the BaO–B
2O3–SiO2 glass to reduce the sintering temperature necessary for densifying the materials.
0955-2219/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00493-X
Journal of the European Ceramic Society 24 (2004) 1787–1790
www.elsevier.com/locate/jeurceramsoc
*Corresponding author.
2. Experimental methods
The BaO.(R2O3)1.08.(TiO2)4.24.0.06(2Bi2O3.3TiO2) with R=Nd0.72Sm0.28, designated as (BRT)114, were prepared by conventional mixed oxide process. High purity materials, including BaCO3(Kali, 99.8%), TiO2 (rutile, Bayor, 99.7%), Nd2(CO3)3(Rhodia, 99%), and
Sm2O3 (Rhodia, 99.5%), with the nominal
com-position BaO.(R2O3)1.08.(TiO2)4.24.0.06(2Bi2O3.3TiO2)
where R= Nd(0.72)Sm(0.28) were mixed and then
calcined at 1170C for 2 h, followed by pulverization, pressing, and then sintering at 1330 C for 2.5 h. In
the first series of (BRT)114, only ZnO with 0–3 wt.% was added. Whereas in the second series of (BRT)114, 2.5 mol% MgO was doped in addition to the ZnO of the same proportion. In the preparation of LTCC materials, BRT114 (with median particle size of 1.0 mm) was mixed with sol-gel derived or fused BaO– B2O3–SiO2 (51:45:4 wt.%) glass with median particle size of 1.5 2 mm with different proportion (9– 37.5wt.%). The glass–ceramic mixture were granulated, pelletized (500 kg/cm2), and then sintered at 850– 1000C for 2.5 h. These samples are designated as
one-step processed ones. To facilitate the comparison, some of the glass–ceramic mixture was calcined at 700C for
2 h, followed by pulverization, granulation, pelletization and sintering process (designated as two-step process).
The density of the sintered BRT114 materials was
measured by the Archimedes method. The crystal structure and microstructure of the samples were examined using X-ray diffraction method (XRD, Simens D5000 diffractometer) and scanning electron microscopy (SEM, Hitach 2500-S with Kevex EDX). The microwave dielectric properties of the materials were measured by a cavity method using a HP 8722ES network analyzer.
3. Results and discussion 3.1. BRT114dielectric ceramics
Incorporation of 2.5 mol% MgO into BRT114
materials markedly improved the sinterability of the materials, the sintered density increased from 92% T.D.(5.52 g/cm3) to 96.5% T.D. (5.75 g/cm3), when sintered at 1330 C for 2.5 h [Fig. 2(a)]. SEM
micro-structure is not markedly changed due to the incor-poration of a small proportion of MgO, but the number of pores seems greatly be depressed. (not shown). Addition of MgO beyond 2.5 mol% induced the formation of secondary phases, which monotonically degraded both the "r- and Qf-values (Fig. 1). Presumably, the Mg2+-species can form solid solution when their concentration is smaller than 2.5 mol%.
For the BRT114 specimens including only ZnO
additives [solid squares in Fig. 2(a)], the density of the samples increased markedly from 92% T.D. (5.52 g/ cm3) to 97% T.D (5.8 g/cm3), when doped with 1 wt.% ZnO. The density decreased monotonously with further
Fig. 1. Dielectric constant ("r) and Qf versus MgO doping content
for (Ba1xMgx)O.1.08(Nd0.72Sm0.28)2O3.4.24TiO2.0.6(Bi2O3.3TiO2),
BRT114 materials.
Fig. 2. The variation of microwave properties with amount of ZnO added into BRT114: (a) density, and (b) temperature coefficient of
resonant frequency, f.
increase in ZnO-dopants larger than 1 wt.%. Similar behavior was observed for samples containing 2.5 mol% MgO [open circles, in Fig. 2(a)], that is, the density of MgO-doped BRT114 first increases with the ZnO content, reaches a maximum value of 96.8%T.D. (5.78 g/cm3), and then decreases with further increase of ZnO addition.
Fig. 2(b) indicates the beneficial effect of MgO or ZnO addition on lowering the temperature coefficient of
resonance frequency (f) of BRT114 materials, a
characteristic of concerned for the microwave device applications. For the materials containing no MgO additives, the value of freduces monotonously from 8 to 1.2 ppm/C when the content of ZnO increases from
0 to 1.0 wt.% [solid squares inFig. 2(b)]. The effect of ZnO addition on improving fcharacteristic of BRT114 is even more pronounced when the specimen contains 2.5 mol% of MgO [open circles, inFig. 2(b)]. A f-value smaller than 0.8 ppm/C was reached for the BRT
114 co-doped with 2.5 mol% MgO, and 1 wt.% of ZnO. The f-value increased again for materials containing more than 1 wt.% ZnO, regardless of whether the samples contain MgO species or not. Incorporation of a small proportion of ZnO (<¼1 wt.%) to BRT114 materi-als, insignificantly alters their dielectric constant ("r=86 to 89), but moderately reduces the Qf-value of the materials from Qf=7000 to Qf=5000 (not shown).
To understand how the addition of dopants affects the related microwave dielectric properties of BRT114 materials, the SEM microstruture was examined and is shown inFig. 3, which reveals the secondary phases of equi-axis geometry emerged for the samples containing more than 1 wt.% of ZnO [Fig. 3(a) and (b)]. The other kind of secondary phase, stripe-shaped, emerged for those containing 2 wt.% of ZnO [Fig. 3(c)]. The EDX analyses reveal that the equi-axised phases contain low Sm Nd and are rich in Ti, but deficient in Zn element, whereas the stripe-shaped phase [cf.Fig. 3(c)] is a ZnO-rich intermediate compound. Apparently, the presence of these secondary phases is the main factor degrading the dielectric constant and Qf-value of the over-doped materials.
Fig. 4. The comparison of thermal etched SEM micrograph of samples, added with 33wt%, prepared by one-step process with (a) fused glass and (b) sol–gel derived glass; and by two-step process with (c) fused glass and (d) sol–gel derived glass.
Fig. 3. The SEM micrographs for BRT114with (a) 0.25 wt%, (b) 1
wt%, (c) 2 wt%, of ZnO additives.
3.2. Low temperature cofirable composite
To study the feasibility of using the composite of Ba–B–Si glass and BRT114dielectrics as LTCC materials, pellets of these composite materials were prepared and their characteristic were investigated. It is observed that the nature of glass, sol–gel derived or fused, and the processing route for composite materials markedly influences the microwave properties and density of the materials even for materials containing same composition of glass and dielectrics.
Generally, sol–gel derived glass possesses much more inferior reactivity than the fused-glass, such that the corresponding composites show a markedly lower density and lower dielectric constant. Fig. 4(a) and (b)
reveals that the feature size is smaller in sol–gel glass/ dielectrics composite materials, inferring that the wettability of sol–gel derived glass is inferior to that of fused glass. Precalcining the glass/dielectric mixture profoundly improved the sinterability of the composite
materials. The two-step processing increased the
sintered density of fused glass/BRT114 Composite
materials from 3.76 to 4.04 g/cm3, when sintered at 950C, resulting in higher dielectric constant, "
r increa-ses from 12.4 to 13.4. In contrast, for the composite materials prepared from sol–gel derived glass, the pre-calcination insignificantly improved the characteristics
of the samples. SEM micrographs shown in Fig. 4(c)
and (d) indicate that the two-step process method increases the wettability of the glass pronouncedly, no matter whether the glass are fused or sol–gel derived.
4. Conclusions
This work investigated the effects of MgO and ZnO additives on the microwave properties and microstructure of BRT114=[(BaO.Re2O3.4TiO2).0.06(2Bi2O3.3TiO2)]. Incorporation of small amount of ZnO pronouncedly improved the temperature coefficient of resonant frequency (f), markedly increased the density and dielectric constant ("r), but degraded the Qf factor for the BRT114 materials, no matter whether the materials contain MgO or not. Microstructure and EDX analyses indicated that the secondary phases, equi-axis or strip-like shape, were induced for materials containing too high concentration of ZnO.
Moreover, sol–gel derived and fused Ba–B–Si glass react with BRT114 in a different way, which results in marked characteristics for LTCC materials consisting of Ba–B–Si glass and BRT114microwave dielectrics.
References
1. Azough, F., Freer, R. and Tang, C. C., Determination of the structures of Ba-Ln-Ti-O microwave dielectric ceramics using synchrotron high resolution X-ray diffration. Br. Ceram. Proc., 1996, 57, 111–117.
2. Suvorov, D., Valant, M., Rawn, C. J. and Kolar, D., High permittivity microwave ceramics-correlation between structure, processing parameters and electrical properties. Ceram. Trans., 1997, 88, 195–213.
3. Chen, X. M. and Li, Y., A and B site cosubstituded B63xSm8+2xTi18O54 microwave dielectric ceramics. J. Am.
Ceram. Soc., 2002, 85, 3 579-584.
4. Satheesh, V., Synthesis and role of Nd and Sm on the microwave dielectric properties of BaNd2(1x)Sm2xTi5O14 dielectric
resonator. Mater. Sci. Eng., 1997, B48, 202–204.
5. Nishigaki, S., Kato, H., Yano, S. and Kamimura, R., Microwave dielectric properties of (Ba,Sr)O-Sm2O3-TiO2 ceramics. Ceram.
Bull., 1987, 66(9), 1405–1410.
6. Kolar, D., Gaberscek, S., Stadler, Z. and Suvorov, D., High stability, low loss dielectrics in system BaO-Nd2O3-TiO2-Bi2O3.
Ferroelectrics, 1980, 27, 260–272.
7. Durand, J. M., Microwave characteristics of BaO-Bi2O3-Nd2O3
-TiO2dielectric resonators. J. Mater. Sci. Lett., 1987, 6, 134–136.
8. Okawa, T., Imaeda, M. and Ohsato, H., Microwave dielectric properties of Bi-added Ba4Nd9+1/3Ti18O54solid solutions. Jpn. J.
Appl. Phys., 2000, 39, 5645–5649.
9. Laffez, P., Desgardin, G. and Raveau, B., Microwave dielectric properties of doped Ba6x(Sm1yNdy)8+(2x/3)Ti18O54 oxides.
J. Mater. Sci., 1995, 30, 267–273.
10. Yamada, A., Utsumi, Y. and Watarai, H., The effect of Mn addition on dielectric properties and microstructure of BaO-Nd2O3-TiO2ceramics. Jpn. J. of Appl. Phys., 1991, 30(9B), 2350–
2353.
11. Wakino, K., Minai, K. and Tamura, H., Microwave character-istic of (Zr,Sn)TiO4 and BaO-PbO-Nd2O3-TiO2 dielectric
resonators. J. Am. Ceram. Soc., 1984, 67(4), 278–281.
12. Wu, J. M. and Huang, H.-L., Microwave properties of zinc, barium and lead borosilicate glasses. J. Non-Crystalline Solids, 1999, 260, 116–124.
13. Jantunen, H., Uusimaki, A., Rautioaho, R. and Leppavuori, S., Temperature coefficient of microwave resonnancy of a LTCC system. J. Am. Ceram. Soc., 2002, 85(3), 697–699.
14. Caballero, A. C., Fernandez, J. F., Moure, C. and Duran, P., ZnO-doped BaTiO3: microstructure and electrical properties. J. Eur. Ceram. Soc., 1997, 17, 513–523.
15. Nagai, T., Iijima, K., Hwang, H. J. and Sando, M., Effect of MgO doping on the phase transformations of BaTiO3. J. Am.
Ceram. Soc., 2000, 83(1), 107–112. 1790 C.-C. Cheng et al. / Journal of the European Ceramic Society 24 (2004) 1787–1790
