Microwave and Infrared Dielectric Response of TemperatureStable (1�x)BaMoO4–xTiO2 Composite Ceramics

Jing Guo,‡,§ Di Zhou,‡,§ Hong Wang,‡,§,† Yuehua Chen,‡,§ Yi Zeng,‡,§

Feng Xiang,‡,§ Ying Wu,‡,§ and Xi Yao‡,§

‡Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education,Xi’an Jiaotong University, Xi’an, 710049, China

§International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, 710049, China

The (1�x)BaMoO4–xTiO2 (x = 0.0, 0.2, 0.3, 0.338, 0.4, 0.5,0.66) ceramics were synthesized by the conventional mixed-

oxide process. The sintering behaviors, phase composition,

chemical compatibility with silver, and microwave dielectric

properties of pure (1�x)BaMoO4–xTiO2 ceramics and0.662BaMoO4–0.338TiO2 ceramic with H3BO3–CuO addition

were studied. Infrared reflectivity spectra of (1�x)BaMoO4–xTiO2 (0.2 � x � 0.4) composites were measured in the range

of 50–4500 cm�1

at room temperature. X-ray diffraction anal-ysis reveals that scheelite BaMoO4 and rutile TiO2 phase

coexist with each other at 1275°C and both of them do not

react with silver (Ag) at 850°C. When the mole fraction of

TiO2 (x value) is 0.4, a temperature stable microwave dielec-tric material is obtained, with er = 13.8, Q 3 f = 40 500 GHz

(f = 8.02 G), and τf = �6.13 ppm/°C. Complex dielectric

spectra gained from the infrared spectra were extrapolateddown to microwave range, and they were in good agreement

with the measured microwave permittivity and dielectric losses.

With 5 wt% H3BO3 and 1 wt% CuO addition, the 0.662

BaMoO4–0.338TiO2 ceramics can be sintered well below 900°C,and possess good microwave dielectric properties with er = 14,

Q 3 f = 48 360 GHz, and τf = +13.9 ppm/°C.

I. Introduction

WITH the development of high-frequency wireless com-munication technology, low temperature cofired cera-

mic (LTCC) technology, which can integrate the passivecomponents to a function module, has attracted much scien-tific and commercial attention. For microwave (MW) appli-cations, dielectric materials should have a low sinteringtemperature, a high quality factor (Q 9 f) value, a near-zerotemperature coefficient of resonant frequency (τf), and chemi-cal compatibility with metal electrodes.1–5 Recently, MWdielectric properties of AMoO4 ceramics with scheelite(A = Ca, Sr, Ba) and wolframite (A = Mg, Mn, Zn) struc-ture have been reported.6–8 All of them exhibit good MWdielectric properties (permittivity er = 7–11, Q 9 f = 37 000–90 000 GHz, τf = �57 to �87 ppm/°C). The BaMoO4 hasthe largest negative τf value (�79.24 ppm/°C) and the lowestsintering temperature (900°C/2 h) in the series of AMoO4

ceramics with scheelite structure. Therefore, it is interesting

to compensate the τf value of BaMoO4 using ceramics withlarge positive τf values. Many successful examples have beenachieved by mixing two or more materials with opposite τfvalues, such as Zn2Te3O8–TiO2, (Mg0.95Zn0.05)TiO3–SrTiO3,MgTiO3–ZnAl2O4–TiO2, Mg4Nb2O9–ZnAl2O4–TiO2, ZnNb2O6–TiO2, Mg2TiO4–Co2TiO4–CaTiO3, CaWO4–TiO2, and Bi2MoO6–TiO2.

9–15 Our previous study on ZnMoO4–TiO2 system alsodemonstrated that the rutile TiO2 (er = 105, Q 9 f =46 000 GHz, τf = +465 ppm/°C) is an effective material tocompensate the negative τf value of ZnMoO4, which is also aMoO3-rich compound similar to BaMoO4 but with a wol-framite structure.16 For this reason, rutile TiO2 is chosen tomix with BaMoO4 as a compensator.

Dielectric losses include intrinsic part and extrinsic part.Intrinsic losses are determined by polar optical phonons andextrinsic losses are caused by impurity, pores, size and shapesof grains, etc. Wakino et al.17 and Tamura et al.18 havereported the relation between intrinsic loss and infrared (IR)spectra. A large amount of research on IR reflective spectraof rutile TiO2 was done.19–21 But the investigation on IRspectra of BaMoO4 was few and the complex dielectricresponse was not discussed.22–24 Therefore, it is important tostudy the intrinsic losses of (1�x)BaMoO4–xTiO2 com-pounds from the infrared spectra and it helps to understandthe dielectric loss at microwave region.

In this work, the sintering behaviors, phase composition,chemical compatibility with silver, microwave dielectric prop-erties, and complex dielectric response between 50 and4500 cm�1 of the (1�x)BaMoO4–xTiO2 ceramics were stud-ied. To lower the sintering temperature, H3BO3–CuO wasadded to the composites and its influence on the compoundswas also investigated.

II. Experimental Procedure

The (1�x)BaMoO4–xTiO2 composite ceramics were preparedby the conventional solid-state synthesis. Reagent-grade pow-ders BaCO3 (>99%, Sinopharm Chemical Reagent Co. Ltd,Shanghai, China), MoO3 (>99%, Fuchen Chemical Reagents,Tianjin, China), rutile TiO2 (>99%, Linghua Co. Ltd.,Zhaoqing, China), H3BO3 (>99.5%, Sinopharm ChemicalReagent Co. Ltd), and CuO (>99%, Sinopharm ChemicalReagent Co. Ltd) were used as starting materials. TheBaCO3 and MoO3 powders were milled with ethanol andZrO2 milling media (2 mm in diameter) for 4 h using a plan-etary ball-mill. After being dried, the mixture was calcined inair at 650°C for 4 h. Then, some BaMoO4 powder was mixedwith TiO2 according to the following formula (1�x)BaMoO4–xTiO2 (0 � x � 0.66). Furthermore, the otherpowders were mixed with TiO2, 5 wt% H3BO3, and 1 wt%CuO. The mixture was milled with zirconia balls in ethanolfor 4 h and then dried. After being mixed with PVA binder,

D. W. Johnson Jr—contributing editor

Manuscript No. 29623. Received April 20, 2011; approved June 21, 2011.This work was supported by the National 973-project of China (2009CB623302),

NSFC projects of China (61025002, 109790365), and National Project of InternationalScience and Technology Collaboration (2009DFA51820).

†Author to whom correspondence should be addressed. e-mail: hwang@mail.xjtu.edu.cn

232

J. Am. Ceram. Soc., 95 [1] 232–237 (2012)

DOI: 10.1111/j.1551-2916.2011.04760.x

© 2011 The American Ceramic Society

Journal

the final powders were pressed into cylindrical disks (12 mmin diameter and 5–6mm in height) using uniaxial pressing.For (1�x)BaMoO4–xTiO2 and samples with H3BO3–CuOaddition, pellets were sintered at 1150°C–1300°C and 700°C–900°C for 2 h with a heating rate of 3°C/min, respectively.Chemical compatibility tests with silver were performed byadding 20 wt% Ag to (1�x)BaMoO4–xTiO2 composite withH3BO3–CuO addition.

The phase compositions of samples were identified by X-raydiffraction with CuKa radiation (Rigaku D/MAX-2400 X-raydiffractometry, Tokyo, Japan). A scanning electron micro-scope (JSM-6460, JEOL, Tokyo, Japan) was used to examinethe microstructures of the surfaces. The bulk densities of thecomposites were determined by Archimedes’ method. Thedielectric constant and quality factor at microwave frequencywere measured using the TE01d-shielded cavity method witha network analyzer (8720ES, Agilent, Palo Alto, CA). TheTCF (τf) value was measured in the temperature range of25°C–85°C, and was calculated as

sf ¼ f85 � f25f25ð85� 25Þ � 106ðppmÞ=�C (1)

where, f25 and f85 were the resonant frequencies at 25°C and85°C, respectively. The IR reflectivity spectra were measuredbetween 50 and 4500 cm�1 (This measurement was done inthe IR beamline of NSRL), by a Fourier transform IR(FTIR) spectrometer (IFS 66v/S Vacuum, Bruker OptikGmbh, Germany).

III. Results and Discussions

Figure 1 presents the XRD patterns of calcined BaMoO4

powders, (1�x)BaMoO4–xTiO2 composites with various val-ues of x sintered at 1275°C/2 h, specimens with H3BO3–CuOaddition sintered at 850°C/2 h, and samples cofired with20 wt% Ag sintered at 850°C/2 h. The pure scheeliteBaMoO4 phase was formed at 650°C for 4 h. No secondaryphase can be observed when x varies from 0 to 0.66 and theintensity of the diffraction peaks of TiO2 rises with x value.Hence, it can be concluded that scheelite BaMoO4 phasecoexists with rutile TiO2 phase below 1300°C. The additionof 5 wt% H3BO3 and 1 wt% CuO do not bring significantinfluence on the phase composition and the silver seems notto react with (1�x)BaMoO4–xTiO2 composites.

Figure 2 shows the backscattered electron images (BEI) ofthe as-fired surfaces of 0.6BaMoO4–0.4TiO2 compounds

sintered at 1250°C/2 h, samples with H3BO3–CuO addition,and cofired samples with 20 wt% Ag, sintered at 850°C/2 h.It is observed from Fig. 2(a) that two types of grains withdifferent shapes coexist. From the EDS analysis, in Fig. 2(b),it can be found that the light-color grains with polygonalshape marked as A belong to BaMoO4 phase and the dark-color ones with bar shape marked as B belong to rutile TiO2

phase. The grain size of 0.6BaMoO4–0.4TiO2 ceramics lies inthe range of 2–10 lm and for samples with H3BO3–CuOaddition sintered at 850°C/2 h, the grain size is in the rangeof 0.5–2.5 lm, which is much smaller than that of the pure0.6BaMoO4–0.4TiO2 ceramics. This may be attributed to thefact that the H3BO3–CuO addition lowers the sintering tem-perature of (1�x)BaMoO4–xTiO2 ceramics and that thegrains have not fully grown. From Fig. 2(d), clear silver par-ticles can be observed, which confirms that Ag does not reactwith (1�x)BaMoO4–xTiO2 composites combined with theXRD analysis.

Densities of (1�x)BaMoO4–xTiO2 ceramics as a functionof x values are shown in Fig. 3(a). The theoretical densitiesof the compounds are calculated by the following formula:

qtheo ¼ W1 þW2

W1=q1 þW2=q2(2)

where, ρ1 and ρ2 are the theoretical densities of BaMoO4 andTiO2; W1 and W2 are the weight fractions of BaMoO4 andTiO2, respectively. The variation tendency of measured densi-ties is in accordance with that of theoretical densities and therelative densities of the samples are all above 96%. The den-sity of 0.662BaMoO4–0.338TiO2 ceramics with H3BO3–CuOaddition is shown in Table I. Figure 3(b) plots the micro-wave dielectric constant of (1�x)BaMoO4–xTiO2 composites.In composite ceramics, the dielectric constant is determinedby the permittivity, volume fraction, and the complex formof the component material. Therefore, the theoretical permit-tivity (eth) of the (1�x)BaMoO4–xTiO2 compounds followsthe Lichtenecker empirical logarithmic rule15:

lgeth ¼ y1lge1 þ y2lge2 (3)

where, e1 and e2 are permittivities of BaMoO4 and TiO2; y1and y2 represent the volume fractions of BaMoO4 and TiO2

in the composites, respectively. As x value is the mole frac-tion of TiO2, it is proportional to y2 value, i.e., x = (y2 9 ρ2/M2)/(y1 9 ρ1/M1 + y2 9 ρ2/M2), where, M1 and M2 are themolar mass of BaMoO4 and TiO2, respectively. Figure 3(b)shows that the measured permittivity is consistent with thetheoretical one. When x shifts from 0.2 to 0.66, the permit-tivity of the compound increases from 10.6 to 21.3.

The theoretical τf value is also obtained by the Lichteneckerlogarithmic rule15:

sf ¼ y1sf1 þ y2sf2 (4)

where, τf1 and τf2 are the τf values of BaMoO4 and TiO2,respectively. There is a good agreement between the mea-sured τf values and the theoretical ones as plotted inFig. 3(c). When x varies from 0.2 to 0.66, the τf value shiftsfrom �56.0 to 116.6 ppm/°C and the τf approaches near-zerovalue (�6.13 ppm/°C) when x = 0.4. Figure 3(d) presents therelationship between the Q 9 f values and the TiO2 content.It can be seen that the Q 9 f values reach a maximum about52 630 GHz at x = 0.3. When the mole fraction of TiO2

rises, the sintering temperature of the (1�x)BaMoO4–xTiO2

composites increases due to the high sintering temperature ofpure TiO2. The BaMoO4 phase changes to liquid phase andsecondary recrystallization appears at higher temperature,which may result in the worsening of Q 9 f values when xexceeds 0.3.

Fig. 1. XRD results of calcined BaMoO4 powder, (1�x)BaMoO4–xTiO2 ceramics sintering at 1275°C/2 h, specimens with H3BO3–CuOaddition sintered at 850°C/2 h, and samples cofired with 20 wt% Agsintered at 850°C/2 h (R, rutile TiO2; B, BaMoO4; A, Ag).

January 2012 Microwave and Infrared Dielectric Response of (1-x)BaMoO4-xTiO2 Ceramics 233

Figure 4 shows the microwave dielectric properties of0.6BaMoO4–0.4TiO2 and 0.662BaMoO4–0.338TiO2 withH3BO3–CuO addition as a function of sintering temperature.It is observed that the permittivity of 0.6BaMoO4–0.4TiO2 isstable from 1175°C to 1275°C, and decreases sharply from1275°C to 1300°C. It is probably caused by the BaMoO4

phase change to liquid phase, and secondary recrystallizationappears at high sintering temperature. The 0.6BaMoO4–0.4TiO2 ceramics have a maximum Q 9 f value(40 500 GHz) at 1285°C and the 0.662BaMoO4–0.338TiO2

with H3BO3–CuO addition possess a maximum Q 9 f value(48 360 GHz) at 875°C.

Figure 5 presents the IR reflectivity spectra of (1�x)BaMoO4–xTiO2 composite ceramics with the lowest amountof TiO2 and that have near-zero τf value. The spectra havebeen fitted according to the standard Lorentzian model25

[Eq. (5)] and the Fresnel formula [Eq. (6)]:

e�ðxÞ ¼ e1 þXn

j¼1

x2pj

x2oj � x2 � icjx

(5)

RðxÞ ¼ 1� ffiffiffiffiffiffiffiffiffiffiffie�ðxÞp

1þ ffiffiffiffiffiffiffiffiffiffiffie�ðxÞp

�����

�����

2

(6)

where, e*(ω) is complex dielectric function; e∞ is the dielec-tric constant caused by the electronic polarization at high fre-quencies; γj, ωoj, and ωpj are the damping factor, the

Fig. 3. (a) Densities, (b) microwave dielectric constant, (c) Q 9 fvalues, and (d) τf values of BMT ceramics as a function of x values.

Fig. 2. Backscattered electron image (a) and EDS spectra (b) of the surfaces of 0.6BaMoO4-0.4TiO2 compounds sintered at 1250°C/2h,backscattered electron images of (c) samples with H3BO3-CuO addition and (d) cofired samples with 20 wt% Ag sintered at 850°C/2h.

Table I. Densities and Microwave Dielectric Properties of (1�x)BaMoO4–xTiO2 Ceramics

ST(°C) Density (g/cm3) Permittivity Q 9 f (GHz) τf (ppm/°C)

BaMoO4 900 4.821 9.0 37 110 �90.10.8BaMoO4 + 0.2TiO2 1275 4.754 10.6 51 840 �56.00.7BaMoO4 + 0.3TiO2 1285 4.715 11.3 52 630 �34.80.662BaMoO4 + 0.338TiO2 1285 4.712 12.1 41 620 �25.40.6BaMoO4 + 0.4TiO2 1285 4.710 13.8 40 500 �6.130.5BaMoO4 + 0.5TiO2 1285 4.649 16.5 25 520 +45.90.34BaMoO4 + 0.66TiO2 1300 4.592 21.3 20 705 +119.60.662BaMoO4 + 0.338TiO2 + BCu 875 4.625 14.0 48 360 +13.9

234 Journal of the American Ceramic Society—Guo et al. Vol. 95, No. 1

transverse frequency, and oscillator strength of the j-thLorentz oscillator, respectively; n is the number of transversephonon modes; and R(ω) is the IR reflectivity.

The BaMoO4 has a tetragonal scheelite structure withsymmetry C6

4h at room temperature. The optical phononmodes are described as follows24:

C ¼ 3Ag þ 5Bg þ 5Eg þ 5Au þ 3Bu þ 5Eu (7)

where, Ag, Bg, and Eg are Raman active; 4Au and 4Eu can beonly observed in IR spectra; Bu is silent mode; the left 1Au

and 1Eu are acoustic vibrations. There are 15 vibrationmodes in rutile TiO2 among which 1Au and 3Eu (Eu is two-fold degenerate) are IR active.19,20 In this study, 13 reflectionbands are identifiable. Among the vibration modes analyzedabove, six peaks belong to rutile TiO2 and seven peaksbelong to scheelite BaMoO4. Table II lists the fitted-mode

Fig. 4. Microwave dielectric properties of 0.6BaMoO4–0.4TiO2 and0.662BaMoO4–0.338TiO2 with H3BO3–CuO addition as a functionof sintering temperature.

Fig. 5. Infrared reflectivity spectra of the (1�x)BaMoO4–xTiO2

composite ceramics.

Table II. Mode Parameters of a Lorentzian Fit in (1�x)BaMoO4–xTiO2 Ceramics

x = 0.2 e∞ = 2.77 x = 0.4 e∞ = 3.31

Mod ωoj ωpj γj Dej ωoj ωpj γj Dej

1 95.42 168.91 15.68 3.13 97.66 147 14.54 2.272 130.01 89.41 10.48 0.47 130.91 59.81 6.23 0.213 143.6 179.53 18.23 1.56 147.97 234.86 25.2 2.524 235.14 180.96 93.68 0.59 223.43 299.68 121.45 1.85 284.29 184.04 37.08 0.42 283.01 228.44 37.9 0.656 318.5 134.46 11.87 0.18 318.94 168.77 12.07 0.287 354.91 70.43 13.54 0.04 353.88 110.25 17.79 0.18 371.96 75.97 14.49 0.04 371.21 66.62 11.28 0.039 413.65 174.17 61.73 0.18 410.54 317.3 85.81 0.610 552.81 329.98 202.91 0.36 549.1 479.7 160.72 0.7611 670.13 246.35 134.89 0.14 650.55 394.99 140.85 0.3712 817.41 374.13 28.7 0.21 803.53 541.45 14.84 0.4513 848.83 332.87 56.23 0.15 855.8 116.23 47.24 0.02

January 2012 Microwave and Infrared Dielectric Response of (1-x)BaMoO4-xTiO2 Ceramics 235

parameters. The measured reflectivity spectra match well withthe fitted ones. The fitted vibration bands of BaMoO4 are at97–100, 147–151, 284–287, 321–323, 371–373, 807–823, 841–856 cm�1 and the others belong to TiO2. It is seen that theintensity of the reflectivity of TiO2 increases and most modesof TiO2 are more damped when x varies from 0.2 to 0.4 asshown in Fig. 5 and in Table II. This result indicates thatBaMoO4 coexists with TiO2 and the contribution of TiO2 tovibration rises when x increases.

The IR reflectivity data are transformed to complex dielec-tric data by the Kramers–Kroning relationship as plotted inFig. 6. Compared with Table II, it is seen that vibrationmodel 1 and model 3 make the most contribution to e.Extrapolation of e’ from the IR spectra down to MW rangeshows that the measured permittivity corresponds with thecalculated one which demonstrates that the MW dielectricconstant of (1�x)BaMoO4–xTiO2 composite is caused mostlyby the polar optical phonons.26 The IR dielectric loss ismainly caused by intrinsic processes. Consequently, the e’’extrapolated from the IR spectra should be slightly lowerthan the measured MW dielectric loss. From Fig. 6(b), it isseen that a good agreement between the measured andcalculated e’’ is obtained which indicates that the intrinsiclosses are the main MW dielectric losses of (1�x)BaMoO4–xTiO2.

IV. Conclusion

A new series of composite MW dielectric materials of (1�x)BaMoO4–xTiO2 were prepared via the solid-state reactionmethod. The X-ray diffraction analysis indicates that scheeliteBaMoO4 and rutile TiO2 phase coexist with each other andboth of them do not react with silver (Ag). When the molefraction of TiO2 (x value) rises from 0 to 0.66, the MW dielec-tric properties varies as follows: er = 9.0–21.3, Q 9 f =20 705–52 630 GHz, and τf = �90.1 to +13.9 ppm/°C. Themeasured MW dielectric losses are in consistent with the calcu-lated e’’ obtained from IR spectra. With H3BO3–CuO addi-tion, a low-firing (below 900°C) and temperature stablemicrowave dielectric material (x = 0.662) has been obtained,with er = 14, Q 9 f = 48 360 GHz, and τf = +13.9 ppm/°C.

Acknowledgments

The authors thank the administrators in IR beamline workstation of NationalSynchrotron Radiation Laboratory (NSRL) for their help in the IR measure-ment.

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