Effects of BaCu(B2O5) on sintering characteristics and microwave dielectric properties of CaWO4 ceramics

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  • CERAMICSAvailable online at www.sciencedirect.com

    Ceramics International 40 (201

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    properties of r9.547, Q f6.07 104 GHz, and f41 ppm/1C.

    resonators, lters, and oscillators. Materials with low sintering

    increases the possibility of chemical interaction with the metalelectrode because of the presence of complicated phases in the

    f32 ppm/1C) [12]. BCB is a good ux former for low-

    ppm/1C).In this study, BCB was synthesized and added to CaWO4

    ceramics to investigate the effects of BCB on the sintering

    characteristics and microwave dielectric properties of CaWO4ceramics.

    http://dx.doi.org/10.1016/j.ceramint.2014.03.0280272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

    nCorresponding author. Tel.: +86 2883203793.E-mail address: uestcsh@163.com (H. Su).temperature, low dielectric constant, and high quality factorvalue are necessary for high-frequency and high-speed appli-cations [16]. Owing to their low permittivity and high Q fvalues, scheelite ceramics, which have the general formulaMWO4 (M=Ca, Sr, and Ba), are regarded as promisingmaterials for such applications [7].Low-melting-temperature glass is generally used to decrease

    the sintering temperature of microwave dielectric materials forLTCC applications [8,9]. However, the addition of such glassleads to poor microwave dielectric properties or signicantly

    ering the sintering temperature of many materials but withoutthe CaWO4 ceramic [1317], which sinters at 1100 1C for 3 hand exhibits low r (10) and a high Q f value (75,000GHz) [18]. In our previous work [19], we fabricated (1x)CaWO4xLi2WO4 (0rxr0.14) ceramics. The Li2WO4 cera-mic presented excellent microwave dielectric properties(r5.5 and Q f62,000 GHz) and a low melting point(741 1C) but had a signicant negative f value (f146)[20]. The best sample exhibited a very high Q f value(117,600 GHz) but with a relatively poor f value (55Keywords: BaCu(B2O5); CaWO4; Sintering characteristic; Microwave dielectric property

    1. Introduction

    The intensive development of wireless communicationsystems has increased the demand for microwave dielectricmaterials. Low temperature co-red ceramics (LTCCs) arewidely used in microwave communication devices, such as

    LTCC system. Therefore, glass-free LTCC materials arenecessary [10]. Kim [11] rst reported the BaCu(B2O5)(BCB) ceramic as a low-temperature sintering aid for micro-wave dielectric materials. This material not only has a lowmelting point (850 1C) but also exhibits favorable micro-wave dielectric properties (r7.4, Q f50,000 GHz, and& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Effects of BaCu(B2O5) on sintering cproperties of C

    Zhu Zhang, Hua Sun, Xiaoli Tang, Hua

    State Key Laboratory of Electronic Thin Films and Integrated Devices, Univ

    Received 15 January 2014; received in revisAvailable onlin

    Abstract

    The effects of BaCu(B2O5) (BCB) addition on the sintering characterthe solid-state ceramic route. BCB addition effectively decreased the sdegraded the microwave dielectric properties. BCB addition also grea(0.06rxr0.14) ceramics. Some pits existed on the grain surface po875 1C, Q f values rst increased and then decreased with the increasINTERNATIONAL

    4) 1053110535

    racteristics and microwave dielectricO4 ceramics

    u Zhang, Tingchuan Zhou, Yulan Jing

    ty of Electronic Science and Technology of China, Chengdu 610054, China

    orm 4 March 2014; accepted 4 March 20145 March 2014

    cs and microwave dielectric properties of CaWO4 were investigated byring temperature of CaWO4 from 1150 1C to 875 1C and only slightlyinuenced the densication and grain size of CaWO4x wt% BCBly because of the volatilization of BCB. For the ceramics sintered atx. With x0.12, the sample presented excellent microwave dielectric

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  • increase of density with BCB content was mainly due to thepromotion of CaWO4 sintering by the appropriate liquid phaseof BCB. However, the CaWO41.2 wt% BCB ceramic wasdenser than the CaWO41.4 wt% BCB ceramic, probablybecause of the minimal evaporation of BCB [13].Fig. 4 shows the permittivity of the BCB-doped CaWO4

    ceramics as a function of sintering temperature ranging from850 1C to 950 1C. As expected, the permittivity closelyfollowed the density trend in all cases. The rst increase ofr might have resulted from the density increase, as shown inFig. 3. The decrease of r with the increase of BCB contentwas due to the density decrease and the low r value of BCB(7.4) [21]. The similar behavior of permittivity and densityindicated that the permittivity was signicantly affected by thedensity.Fig. 5 shows the Q f values of the BCB-doped CaWO4

    Fig. 1. The XRD patterns of BCB and CaWO4+BCB (a) the XRD patterns ofBaCu(B2O5) ceramic sintered at 8001C/2 h; (b) the XRD patterns of CaWO4ceramics with various amount of BCB additions ((a) 0.6 wt% (b) 0.8 wt%(c) 1.0 wt% (d) 1.2 wt% (e) 1.4 wt%).

    rna2. Experimental procedure

    CaWO4 ceramic samples were prepared by solid-state reaction.WO3 (99%) and CaCO3 (99.9%) were used as starting materials.A stoichiometric amount of the powder samples was mixed andball-milled using zirconia balls in distilled water for 12 h. Theresultant slurry was then dried and calcined at 800 1C for 2 h. Thecalcined powders were ground to form ne powders. Tosynthesize the BCB ceramic, BaCO3 (99%), CuO (99%), andH3BO3 (99%) were mixed for 12 h in a nylon jar with zirconiaballs, dried, and calcined at 800 1C for 2 h. After subsequent ball-milling with 0.61.4% BCB (mass fraction), the powders weredried, pelleted, and uniaxially pressed into cylinders 12 mm indiameter and 56 mm in height under a pressure of 10 MPa. Thecylindroid specimens were subsequently sintered for 2 h in air at850950 1C.The crystalline phases of the sintered ceramics were

    identied by X-ray diffraction (XRD: DX-2700) using CuK radiation. Micrographs were obtained by scanning electronmicroscope SEM (SEM: JOEL JSM6490LV). Bulk densitywas measured by the Archimedes method. The microwavedielectric properties of the sintered samples were evaluated bythe HakkiColeman method and an Agilent N5230A(300 MHz20 GHz) network analyzer in a resonant cavity.The resonant frequency of the temperature coefcient (f) wasmeasured by the same method, which increased temperaturefrom 25 1C to 85 1C, and was calculated by f (f85 f25)/[f25(8525)] 106 (ppm/1C), where f85 and f25 represent theresonant frequency at 85 and 25 1C, respectively.

    3. Results and discussion

    Fig. 1(a) shows the XRD patterns of the BCB ceramicssintered at 800 1C/2 h in air. The result agreed well with thereport [13], indicating that the BCB phase was perfectlyformed.Fig. 1(b) shows the XRD patterns of CaWO4 ceramics with

    various amounts of BCB ((a) 0.6 wt%, (b) 0.8 wt%, (c) 1.0 wt%,(d) 1.2 wt% and (e) 1.4 wt%) sintered at 875 1C/2 h in air. Alladded BCB samples exhibited the CaWO4 phase (PDF #41-1431)without any second phase. Accordingly, the trace amount of BCBdid not chemically react with CaWO4, and the BCB liquid phasedid not crystallize during cooling [21].SEM micrographs of CaWO4x wt% BCB ceramics sin-

    tered at 875 1C for 2 h in air are shown in Fig. 2. The grainsize increased and became more uniform with the increase ofBCB. Therefore, the BCB liquid phase accelerated the graingrowth of CaWO4. Some pits also existed on the grain surfacepossibly because of the volatilization of BCB [22].Fig. 3 presents the bulk densities of the BCB-doped CaWO4

    ceramics as a function of sintering temperature ranging from850 1C to 950 1C. The bulk density of the sample could reachabove 97% of the theoretical density of CaWO4 ceramics whenthe mass fraction of BCB was 1.2. The bulk densities of allsamples rst increased and then gradually decreased with the

    Z. Zhang et al. / Ceramics Inte10532increase of sintering temperature. The density was maximizedat 875 1C for all BCB-doped CaWO4 ceramics. The rsttional 40 (2014) 1053110535ceramics as a function of sintering temperature ranging from850 1C to 950 1C. The Q f value rst increased and then

  • Fig. 2. SEM micrographs of CaWO4+x wt% BCB ceramics (a) x=0.06, (b) x=0.08, (c) x=0.1, (d) x=0.12, (e) x=0.14.

    Fig. 3. The bulk density of the BCB-doped CaWO4 ceramics as a function ofsintering temperature ranging from 8501C to 9501C.

    Fig. 4. The permittivity of the BCB-doped CaWO4 ceramics as a function ofsintering temperature ranging from 8501C to 9501C.

    Z. Zhang et al. / Ceramics International 40 (2014) 1053110535 10533

  • rnaFig. 5. The Qf values of the BCB-doped CaWO4 ceramics as a function ofsintering temperature ranging from 8501C to 9501C.Z. Zhang et al. / Ceramics Inte10534decreased with an increasing amount of BCB. This trend might berelated to the density variation of the samples, as shown in Fig. 3[23,24]. The Q f value reached its maximum value of approxi-mately 6.07 104 GHz (at 13.77 GHz) when x1.2. Thereafter,the Q f values gradually decreased with increasing sinteringtemperature, possibly because of extrinsic factors, such as theincrease of the liquid phase in the ceramics and the abnormalgrowth of grains [2528].Fig. 6 shows the f values of the CaWO4x wt% BCB

    ceramics sintered at 875 1C for 2 h in air. The TCF of thespecimens monotonically increased from 50.1 ppm/1C to41 ppm/1C with the increase of BCB content. This result canbe attributed to the relatively high TCF of BCB (E32 ppm/1C)[13].

    4. Conclusion

    In this work, BCB was synthesized and added to CaWO4ceramic to obtain low-temperature-sintered ceramics with only

    Fig. 6. The f values of the CaWO4+x wt% BCB (0.06 x 0.14) ceramicssintered at 875 1C for 2 h in air.minimal loss. BCB addition greatly inuenced the densicationand grain size of CaWO4x wt% BCB ceramics (0.06rxr0.14). With x0.12, the ceramic could be well sintered at875 1C/2 h (97% theoretical density) and presented excellentmicrowave dielectric properties with r9.547, Q f6.07 104GHz, and f41 ppm/1C. The proposed ceramic is a promisingcandidate material for LTCC applications.

    Acknowledgment

    This work was supported by the National Natural ScienceFoundation of China under Grant nos. 61171047, 51372031,51132003 and 61271038, Program for New Century ExcellentTalents in University, the Fundamental Research Funds for theCentral Universities and Science and Technology Departmentof Sichuan Province.

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    Effects of BaCu(B2O5) on sintering characteristics and microwave dielectric properties of CaWO4 ceramicsIntroductionExperimental procedureResults and discussionConclusionAcknowledgmentReferences

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