Low-Temperature Sintering and Microwave Dielectric Properties ofBa3(VO4)2–BaWO4 Ceramic Composites

Hao Zhuang, Zhenxing Yue,w Siqin Meng, Fei Zhao, and Longtu Li

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,Tsinghua University, Beijing 100084, China

New low-temperature co-fired microwave dielectric compositeswith compositions of (1–x)Ba3(VO4)2–xBaWO4 (x5 0–1) wereprepared by firing mixtures of Ba3(VO4)2 and BaWO4. Thermalmechanical analysis indicated the ceramic composites hadrelatively low densification temperatures and could be sinteredin the temperature range 9251B9501C. X-ray diffractionpatterns showed that Ba3(VO4)2 and BaWO4 coexisted andno secondary phase was detected in the sintered bodies, implyinggood chemical compatibility between the two phases. Near-zerotemperature coefficients of the resonant frequency (sf) could beachieved by changing the relative content of the two phases, dueto their positive and negative sf values, respectively. Withincreasing BaWO4 (x from 0.50 to 0.65), the sf value of thecomposites decreased from 110.9 to �1.9 ppm/1C, and thequality factor (Q� f value) increased from 66700 to 79 100GHz. The best microwave dielectric properties were obtained forx5 0.65 samples sintered at 9251C with a dielectric constant of11.1, a Q� f value of 79100 GHz, and a sf value of �1.9 ppm/1C. Chemical compatibility experiments showed the ceramicsare compatible with silver during cofiring process.

I. Introduction

RECENTLY, extensive attention has been paid to multilayermicrowave devices because of the rapid progress of mobile

communication systems.1 The development of low-temperaturecofired ceramics (LTCC) has been stimulated by the benefitsoffered for the fabrication of miniature multilayer devices in-volving the cofiring of dielectrics and highly conductive metals,such as silver and copper.2 The sintering temperatures of dielec-tric ceramics must be lower than the melting point of the elec-trode metals, for example, 9611C for silver or 10641C for copper;furthermore, chemical compatibility between the ceramic andmetal electrodes must be satisfied. However, most of the com-mercial microwave dielectric ceramics available exhibiting highquality factor (Q), high dielectric constant (er), and near-zerotemperature coefficient of resonant frequency (tf) are sintered athigh temperatures. In order to lower the sintering temperatureof dielectric ceramics, some low melting point oxides or glassesare generally added to promote the densification process byliquid-phase sintering. Unfortunately, the presence of a glassyphase in the sintered ceramics increases dielectric loss and de-creases dielectric constant, due to the high loss and low permit-tivity behavior of the glassy phases. Therefore, exploring newceramics with low firing temperature has been receiving increas-ing attention. Some low-temperature firing ceramic compoundssuch as bismuth- or tellurium-containing material systems have

been studied as glass-free LTCCs.3–7 Bi2O3- or TeO2-richcompounds show relatively low sintering temperatures;however, they have poor chemical compatibility with silverelectrodes,3–6 which limits their use in multilayer devices.

Tungsten- and vanadium-rich compounds have both lowsintering temperatures and excellent dielectric properties atmicrowave frequency.8,9 Among them, BaWO4, which can besintered at 11001C, was reported to have microwave dielectricproperties of erB8, Q� fB57 500 GHz, and tfB�78 ppm/1C.9

Ba3(VO4)2 has microwave dielectric properties of erB11, Q� fB40 000 GHz, and tfB160 ppm/1C.8 However, large tf valueslimited their applications in microwave devices. It is noted thatBa3(VO4)2 has a large positive tf value (B160 ppm/1C), andBaWO4 has a large negative tf value (B�78 ppm/1C). One canexpect that a dielectric material with near-zero tf value andhigh Q� f value might be obtained by combining Ba3(VO4)2with BaWO4. In the present work, (1–x)Ba3(VO4)2–xBaWO4

(x5 0–1) ceramic composites were prepared and theirmicrostructure and microwave dielectric properties wereinvestigated systematically.

II. Experimental Procedure

The starting materials used were high-purity (499.9%) powdersof BaCO3, V2O5, and WO3. Ba3(VO4)2 and BaWO4 powderswith stoichiometric compositions were synthesized using aconventional mixed oxide route by calcining at 8001C for 4 hand 7501C for 5 h, respectively. (1–x)Ba3(VO4)2–xBaWO4

(x5 0–1) mixtures were then prepared by pure Ba3(VO4)2 andBaWO4 by weight ratio. The mixtures were ball milled in alcoholfor 5 h using zirconia balls. The slurries were dried, mixed withan appropriate amount of PVA (5 wt%) as a binder, and thenscreened with a 60 mesh. The screened powders were pressedinto cylindrical pellets with a diameter of 10 mm and a height ofabout 5 mm under a pressure of B200 MPa. These pellets werepreheated at 6001C for 4 h to expel the binder and then sinteredat temperatures from 9001 to 10001C for 4 h in air.

The crystalline phases of the sintered samples were deter-mined by X-ray diffraction (XRD), using CuKa radiation(Rigaku D/Max-2500, Rigaku, Tokyo, Japan). Shrinkagecurves were measured using thermal mechanical analysis(TMA) (SETARAM TGA TMA DSC, Caluire, France) at aheating rate of 101C/min. The microstructures of sintered ce-ramics were observed using a scanning electron microscope(SEM, FEI QUANTA 200F, FEI Deutschland GmbH, Kassel,Germany). The microwave dielectric properties were measuredby the Hakki–Coleman method and cavity method10–12 using anHP8720ES network analyzer (Hewlett-Packard, Santa Rosa,CA). The temperature coefficient of the resonant frequency(tf) was obtained in the temperature range from 201 to 801C.

III. Results and Discussions

Figure 1 shows the XRD patterns for (1–x)Ba3(VO4)2–xBaWO4

(x5 0.50–0.65) ceramics sintered at 9251C for 4 h. All the mainpeaks could be indexed in terms of BaWO4 with the scheelite

N. Alford—contributing editor

This work was supported by the Natural Science Foundation of China (Grant Nos.50672043, 50621201, and 50632030), and the Ministry of Science and Technology of Chinathrough 973-Project under 2002CB613307.

wAuthor to whom correspondence should be addressed. e-mail: yuezhx@tsinghua.edu.cn

Manuscript No. 24726. Received May 23, 2008; approved July 23, 2008.

Journal

J. Am. Ceram. Soc., 91 [11] 3738–3741 (2008)

DOI: 10.1111/j.1551-2916.2008.02672.x

r 2008 The American Ceramic Society

3738

structure(�) and Ba3(VO4)2 with a hexagonal structure(.). TheXRD patterns indicate that Ba3(VO4)2 and BaWO4 can coexistin the sintered bodies and no secondary phase was detected,suggesting a diphasic composite. WO4 compounds have twostructure types: wolframite and scheelite structures, whichdepend on the ion radius of the A21 cation.13 BaWO4 tends tobe a scheelite structure due to the large ion radius of the Ba21

cation.13 In the scheelite structure, the coordination number ofW61 in the polyhedron is four, and tetrahedral [WO4] is linkedby eightfold coordinate Ba21 ions. However, Ba3(VO4)2 is ahexagonal structure ðR�32=mÞ in which the V51 ion is locatedin the center of tetrahedral [VO4] units linked by sixfold andtenfold coordinate Ba21 ions.14 The different crystal structures

of BaWO4 and Ba3(VO4)2 with the different linkages of the[WO4] and [VO4] units, and the different valences of W61 andV51 has limited the formation of solid solution between BaWO4

and Ba3(VO4)2. Therefore, BaWO4 and Ba3(VO4)2 can coexist inthe sintered bodies, suggesting good chemical compatibility be-tween BaWO4 and Ba3(VO4)2. A cofiring experiment of thecomposites with silver was performed by firing mixtures ofBa3(VO4)2–BaWO4 and 20 wt% Ag powder, and the XRDresult is also shown in Fig. 1. It reveals that no secondary phases

Fig. 1. X-ray diffraction patterns for (1–x)Ba3(VO4)2–xBaWO4 ceram-ics (x5 0.50–0.65) and the mixture of x5 0.65 with 20 wt% Ag sinteredat 9251C for 4 h.

Fig. 2. Shrinkage (solid line, dL/L0) and shrinkage rate (dotted line,dL/dT) of (1–x)Ba3(VO4)2–xBaWO4 ceramics as a function of temper-ature.

Fig. 3. Scanning electron microscopic photographs of the as-fired surface of (1–x)Ba3(VO4)2–xBaWO4 ceramics sintered at 9251C, (a) x50.50, (b)x5 0.55, (c) x5 0.65, (d) Backscattered electron image of (c).

November 2008 3739

are detected, implying good chemical compatibility betweenBa3(VO4)2–BaWO4 and silver.

Figure 2 shows the shrinkage and shrinking rate as a functionof temperature for the pressed samples. It demonstrates that themaximum densification rate occurs at B11451C and B9001Cfor Ba3(VO4)2 and BaWO4, respectively. The different sinteringbehavior of BaWO4 and Ba3(VO4)2 will influence the densificat-ion process of the composites. From Figs. 2(b) and (c), it isobserved that the temperature at which the maximum densifi-cation occurs, decreases from 10101 to 9901C when x increasesfrom 0.50 to 0.65. Additionally, the onset temperature of shrink-age decreases from 8301 to 8001C in this case. Thus, BaWO4

improved the sintering behavior of the composites, whichmay be due to the lower sintering temperature of BaWO4.Considering the high heating rate (101C/min) during the TMAmeasurement, the actual sintering temperature of the compositeswill be lower than the temperature at which the maximumdensification occurs.

Figure 3 shows SEM micrographs for the as-fired surface ofthe composites sintered at 9251C. Dense ceramics with a bimod-al grain size distribution were observed for all compositionsinvestigated. As shown in Figs. 3(a)–(c), the increase of BaWO4

content significantly promoted the grain growth of theBa3(VO4)2–BaWO4 ceramics. This suggests that the sinterabili-ty of Ba3(VO4)2–BaWO4 ceramics is improved with increasingBaWO4 content, which is in good agreement with the TMAanalyses (see Fig. 2). Figure 3(d) shows the backscatteredelectron image of x5 0.65. In accordance with the XRDpatterns, two different phases can be observed. Because ofthe different atomic weight of W (183.85) and V (50.94), thelight-colored grains are rich in W and the dark ones are rich in

V. Noticeably, almost all the large grains are composed ofBaWO4, which may be due to the lower sintering temperatureof BaWO4 compared with Ba3(VO4)2.

The relative densities and microwave dielectric properties ofthe Ba3(VO4)2–BaWO4 ceramics sintered at the optimizedtemperature are shown in Table I and Fig. 4. With x increasingfrom 0.50 to 0.65, the dielectric constant and tf values ofBa3(VO4)2–BaWO4 ceramics decrease, whereas Q� f increases.The variation in microwave dielectric properties is related to therelative content of Ba3(VO4)2 and BaWO4 in the mixtures, dueto the differences in dielectric constant and Q� f values of twophases. It is noted that the Q� f value of x5 0.65 is higher thanthat of pure BaWO4. An SEM photograph (not provided here)of the as-fired surface of BaWO4 showed abnormal graingrowth, which led to the low-relative density of BaWO4

(94.3%). This result is in agreement with Yoon et al.’s work.9

In contrast to pure BaWO4, fine grains are observed in thecomposite ceramics as shown in Fig. 3. The existence ofBa3(VO4)2 suppresses the grain growth of BaWO4, thus resultsin a higher relative density and higher Q� f values.

The relative density and microwave dielectric properties of(1–x)Ba3(VO4)2–xBaWO4 ceramics as a function of sinteringtemperature are shown in Fig. 5. The relative density initiallyincreases with increasing temperature but then decreases around9251B9501C. Maximum density is achieved at 9501C forx5 0.50 and 0.55, and 9251C for x5 0.60 and 0.65. The er in-creases with increasing temperature and then saturates. Q� finitially increases with increasing temperature but then decreasesaround 9251B9501C. The trend in Q� f is in accordance withthat of the relative density.

IV. Conclusions

Ceramic composites with (1–x)Ba3(VO4)2–xBaWO4 could beobtained by sintering mixtures of Ba3(VO4)2 and BaWO4 ataround 9251–9501C. Ba3(VO4)2 and BaWO4 coexist in the sin-tered bodies. SEM reveals the composites have a fine-grainedmicrostructure. The (1–x)Ba3(VO4)2–xBaWO4 composite withx5 0.65 sintered at 9251C has good microwave dielectric prop-erties of er 5 11.1, Q� f5 79 100 GHz, and tf5�1.9 ppm/1C.Chemical compatibility experiments show the ceramics to becompatible with silver during the cofiring process. The presentLTCC composites are promising materials for microwave andmillimeter wave applications.

Table I. Relative Density and Microwave Dielectric Proper-ties of (1–x)Ba3(VO4)2–xBaWO4 Ceramics Sintered at

Optimized Temperature

x

Sintering

Temperature

(1C)

Relative

Density (%)

Dielectric

Constant

(er)

Quality

Factor

(Q� f)

(GHz)

Temperature

coefficient of

resonant

frequency

(tf) (ppm/1C)

x5 0(pure Ba3(VO4)2)

1100 97.8 14.2 42,200 52.3

x5 0.50 925 97.4 11.9 63,600 10.9x5 0.55 925 97.9 11.7 69,200 6.7x5 0.60 925 97.9 11.3 75,000 3.3x5 0.65 925 98.4 11.1 79,100 �1.9x5 1(pureBaWO4)

1000 94.3 8.3 67,500 �54.4

Fig. 4. Microwave dielectric properties of the (1–x)Ba3(VO4)2–xBaWO4 ceramics as a function of x.

Fig. 5. Relative density and microwave dielectric properties of the (1–x)Ba3(VO4)2–xBaWO4 ceramics as a function of sintering temperature.

3740 Journal of the American Ceramic Society—Zhuang et al. Vol. 91, No. 11

References

1W. Wersing, ‘‘Microwave Ceramics for Resonators and Filters,’’ Curr. Opin.Solid State Mater. Sci., 1, 715–31 (1996).

2K.Wakino, T. Nishikawa, Y. Ishikawa, andH. Tamura, ‘‘Dielectric ResonatorMaterials and Their Applications for Mobile Communication Systems,’’Br. Ceram. Trans. J., 89, 39–43 (1990).

3D. Zhou, H. Wang, X. Yao, and L. Pang, ‘‘Dielectric Behavior and Cofiringwith Silver of Monoclinic BiSbO4 Ceramic,’’ J. Am. Ceram. Soc., 91 [4] 1380–3(2008).

4A. Feteira and D. C. Sinclair, ‘‘Microwave Dielectric Properties of LowFiring Temperature Bi2W2O9 Ceramics,’’ J. Am. Ceram. Soc., 91 [4] 1338–41(2008).

5D. Zhou, H. Wang, and X. Yao, ‘‘Microwave Dielectric Properties andCo-Firing of BiNbO4 Ceramic with CuO Substitution,’’ Mater. Chem. Phy.,104, 397–402 (2007).

6D. Kwon, M. Lanagan, and T. R. Shrout, ‘‘Microwave Dielectric Propertiesand Low-Temperature Cofiring of BaTe4O9 with Aluminum Metal Electrode,’’J. Am. Ceram. Soc., 88 [12] 3419–22 (2005).

7G. Subodh and M. T. Sebastian, ‘‘Glass-Free Zn2Te3O8 Microwave Ceramicfor LTCC Applications,’’ J. Am. Ceram. Soc., 90 [7] 2266–8 (2007).

8R. Umemura, H. Ogawa, A. Yokoi, H. Ohsato, and A. Kan, ‘‘Low-Temper-ature Sintering-Microwave Dielectric Property Relations in Ba3(VO4)2 Ceramic,’’J. Alloy Compd., 424, 388–93 (2006).

9S. H. Yoon, D. W. Kim, S. Y. Cho, and K. S. Hong, ‘‘Investigation of theRelations between Structure and Microwave Dielectric Properties of DivalentMetal Tungstate Compounds,’’ J. Eur. Ceram. Soc., 26, 2051–4 (2006).

10W. E. Courtney, ‘‘Analysis and Evaluation of a Method of Measuring theComplex Permittivity and Permeability of Microwave Insulators,’’ IEEE Trans.Microwave Theory Tech., MMT-18, 476 (1970).

11B. W. Hakki and P. D. Coleman, ‘‘A Dielectric Resonator Method ofMeasuring Inductive in the Millimeter Range,’’ IRE Trans. Microwave TheoryTech., MMT-8, 402–10 (1960).

12J. Krupka, K. Derzakowski, B. Riddle, and J. B. Jarbis, ‘‘A DielectricResonator for Measurements of Complex Permittivity of Low Loss DielectricMaterials as a Function of Temperature,’’ Meas. Sci. Technol., 9, 1751–6 (1998).

13L. L. Y. Chang, M. G. Scroger, and B. Phillips, ‘‘Alkaline-Earth Tungstates:Equilibrium and Stability in the M–W–O Systems,’’ J. Am. Ceram. Soc., 49 [7]385–90 (1966).

14M. H. Whitmore, ‘‘Electron-Paramagnetic-Resonance Spectroscopy ofManganese-Doped Ba3(VO4)2: Identification of Tetrahedral Mn51 and Mn41

Centers,’’ Phys. Rev. B, 47 [17] 11479–82 (1993). &

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