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Materials Chemistry and Physics 148 (2014) 96e102

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Structural, dielectric and thermal properties of Ca9R2W4O24[ReNd, Sm] ceramics

Jobin Varghese, Mathieu Pr�esumey, Mailadil Thomas Sebastian,Kuzhichalil Peethambharan Surendran*

Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695019, Kerala, India

h i g h l i g h t s

� Synthesis and characterization of novel Ca9R2W4O24 [R ¼ Nd, Sm] ceramics.� Good dielectric properties εr ¼ 18, tan d ¼ 0.0037 and εr ¼ 19, tan d ¼ 0.0048 at 15 GHz.� Thermal expansivity is 4.2 and 4.1 ppm �C�1 at 25e600 �C.� Moderate thermal conductivity 1.6 and 1.8 W m�1 K�1 at room temperature.

a r t i c l e i n f o

Article history:Received 11 January 2014Received in revised form8 June 2014Accepted 11 July 2014Available online 1 August 2014

Keywords:CeramicsPowder diffractionMicrostructureDielectric propertiesThermal properties

* Corresponding author. Tel.: þ91 471 2515258; faxE-mail addresses: kpsurendran@niist.res.in,

(K.P. Surendran).

http://dx.doi.org/10.1016/j.matchemphys.2014.07.0170254-0584/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Dielectric ceramics based on Ca9Nd2W4O24 (CNW) and Ca9Sm2W4O24 (CSW) were synthesized usingsolid state reaction method. X-ray diffraction and EDAX analysis showed that phase purity could beachieved when ceramics were calcined at 1200 �C and sintered at 1450 �C. Dielectric properties of CNWand CSW ceramics were investigated with measurement performed at radio as well as microwave fre-quency range. The microstructure and thermal properties of the CNW and CSW ceramic samples werealso investigated.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The present state of microelectronics is both pleasing and con-cerning. Today's rapid growth of radio frequency (RF) and micro-wave wireless products has redefined the central focus to reducedsize and cost-effective packaging technologies. Design of RF circuitsrequires packaging that is highly reliable and superior in dielectricand thermal performance in order to enable wideband frequencyoperation [1]. An adequate solution is provided by “passive inte-gration” whose principal aim is to integrate the passive compo-nents such as inductors, capacitors, and resistors on a singlesubstrate. Embedded passives, especially embedded resistors andcapacitors have been a hot topic since the mid-to-late 1990's [2].

: þ91 471 2491712.drkpsurendran@yahoo.com

One of the major challenges of embedded passive is the lack ofqualified materials processed through optimal conditions. Ceramicmaterials have been considered as a solution to meet the stringenttechnological demands due to their better electrical and thermalproperties along with greater dimensional stability. One of themajor material requirements for the embedded passive circuits isthe matching coefficient of thermal expansion to that of silicon [3].In this context, tungstates solicit special attention due to theirinteresting dielectric and thermal properties. The dielectric prop-erties of several tungstate materials such as AWO4 (A ¼ Mg, Zn, Niand Co), RE (Ti0.5W0.5) O4 (RE ¼ Pr, Nd, Sm, Gd, Tb and Dy) andBiREWO6 [RE ¼ La, Nd, Pr, Sm, Gd, Dy, Tb, Yb, Y] were reported inthe microwave frequency range [4e6]. A couple of tungstates wereinvestigated for low temperature co-fireable ceramics (LTCC) ap-plications also owing to the low firing temperatures [7,8]. In recentyears, tungstates have attracted attention of diverse scientific andtechnological fields because of their wide potential for optoelec-tronic and microwave applications. For example, the optical

Fig. 1. Particle size analysis of CNW and CSW ceramics calcined at 1200 �C.

J. Varghese et al. / Materials Chemistry and Physics 148 (2014) 96e102 97

properties of trivalent rare-earth ions (RE3þ) in tungstate materialswith scheelite (CaWO4) structure have been widely investigated,especially due to their attractive third order nonlinear dielectricsusceptibilities [9,10]. In 1987, a similar structured tungstate with amolecular formula Ca9Sm2W4O24 has been reported by a group ofRussian researchers [11] which was later found to possess prom-ising luminescent properties when doped with Eu3þ [12]. To date,no focused effort was made to elucidate the crystal structure ofCa9RE2W4O24 [RE ¼ rare earth] ceramics. We believe that thermaland electrical properties of calcium neodymium tungstates need tobe investigated further as a suitable candidate for microwave pas-sive circuits [13]. To extend the knowledge about materials withscheelite structure, we have performed a detailed investigation onthe crystal structure, microstructure, dielectric and thermal prop-erties of Ca9Nd2W4O24 (CNW) and Ca9Sm2W4O24 (CSW) ceramics,which is being discussed in this paper.

2. Experimental

The Ca9Nd2W4O24 and Ca9Sm2W4O24 ceramics were preparedby the conventional solid state ceramic route. High purity powdersof CaCO3, WO3 (Aldrich Chemicals 99.9%), Nd2O3 and Sm2O3 (IRE,India 99.9%) were used as starting materials. They were weighedaccording to the stoichiometric compositions based on Ca9R2W4O24(R¼ Nd and Sm) and were ball milled in a plastic bottle using yttriastabilized zirconia balls as milling media in distilled water for 24 h.The resultant slurry was dried in an oven at 100 �C and calcined inplatinum crucible at 1200 �C for 8 h with intermediate grinding.The calcined powder was then ground for 2 h and 4 wt.% aqueoussolution of polyvinyl alcohol (PVA) was added to it as a binder. Thepowder was uniaxially pressed in to cylindrical compacts of 16 mmdiameter and 2 mm thickness under pressure of 150 MPa. Thesecompacts were fired at a rate of 5 �C per minute up to 600 �C andhold at 600 �C for 30 min to expel the binder before they weresintered in the temperature range 1300e1500 �C for 4 h in air atheating rate of 10 �C h�1. The sintered samples were then cooled to200 �C at a slow rate of 60 �C h�1. The bulk density of the sinteredsamples was measured using Archimedes method. The calcinedpowder samples of CNW and CSW ceramics were dispersed inacetone using ultrasonicator for particle size analysis (ZetasizerNanoseries: ZEN 3600, Malvern, Worcestershire, UK). The crystalstructure and phase purity of the calcined samples were charac-terized (PAN analytical X'Pert Pro Powder X-ray Diffractometer,Netherlands) with a Ni filtered Cu Ka radiation (l ¼ 1.5406 Å). Thesintered samples were thermally etched for 30 min at a tempera-ture of about 25 �C below the sintering temperature and surfacemorphology was studied using Scanning Electron Microscope(SEM) (JEOL-JSM 5600 LV, Tokyo, Japan). The powder samples ofCNW and CSW were dispersed in acetone using ultrasonication forfive minutes. The suspended particles were lifted on carbon-coatedcopper grid and the grids were dried on a filter paper under aninfrared lamp, followed by inspection in the high resolutiontransmission electron microscopy (HR-TEM). The stoichiometry ofCNW and CSW was evaluated by energy dispersive spectroscopy(EDS) analysis carried in a transmission electron microscope (FEITecnai G2 30S-TWIN, FEI Company, Hillsboro, OR) operated at300 kV.

The dielectric properties of CNW and CSW at 1 MHz weremeasured using a LCRmeter (Hioki 3532-50 LCRHiTESTER, Nagano,Japan). The error associated with the LCR measurement of relativepermittivity and dielectric loss is <±2%. The variation of relativepermittivity of CNW and CSW at 1 MHz was studied in the tem-perature range of 25e70 �C. The microwave measurements ofrelative permittivity and dielectric loss were performed by SplitPost Dielectric Resonator (SPDR) method [14] using a Vector

Network Analyzer (E5071C, Agilent Technologies, Santa Clara, CA).The polished ceramic pellets with thickness 0.5e0.6 mm anddiameter 16e20 mm are ideal for microwave measurements inSPDR method at 15 GHz. In this technique the total uncertainty ofreal permittivity does not exceed 0.5% and is possible to resolvedielectric loss tangents to approximately 5 � 10�5.

The coefficient of thermal expansionwasmeasured using a pushrod dilatometer (NETZSCH, Model DIL 402 PC, Germany) with ac-curacy < 1%. The dilatometer used an alumina tube and push rod.The thermal expansion tests were performed according to thespecifications of ASTM E228 test method. The sintered cylindricalsamples of diameter 8mm and height 10mmwere used tomeasurethe coefficient of thermal expansion (CTE) of the CNW and CSWceramics in the temperature range of 30e600 �C.

The thermal conductivity is computed from the thermal diffu-sivity (mm2 s�1), specific heat (J g�1 K�1) and density (g cm�3) [15]which was measured using a Thermal Diffusivity System (AnterCorporation, Flashline 2000, Pittsburgh, PA). The flashmethod useda high speed xenon discharge pulse source directed to the top faceof the specimen to increase the temperature of the specimen by DTas a function of time. The specimens used for thermal diffusivitytesting were in the form of a disc, with a diameter of 12.6 mm and athickness of 2 mm. Specimen preparation involves ensuring thesmoothness and flatness of surfaces using a 400 grit SiC grindingpaper, followed by coating both sides of the sample with graphitefor thermal contacts to avoid reflection of the xenon discharge lightbeam. Three specimens were tested each of at different tempera-ture range up to 300 �C. The error limit of thermal property mea-surement is ±1%.

3. Results and discussion

Fig. 1 shows the particle size distribution of CNW and CSW ce-ramics calcined at 1200 �C. The particle size distribution of CSWceramics is more uniform than for CNW. The average particle size ofCNW is calculated to be 655 nm with particle size distributionranges 136e1105 nmwhile that of CSW ceramics is 850 nm and itsdistribution rages from 440 to 960 nm. Fig. 2 shows the variation ofrelative density as a function of sintering temperature of CNW andCSW ceramics. It is found that relative density increases graduallywith sintering temperature up to 1450 �C, and further increase inthe sintering temperature decreases the relative density. The rela-tive density of sintered CSW ceramics was increased from 83 to 93%

Fig. 2. Relative density of CNW and CSW ceramics as a function of sinteringtemperature.

J. Varghese et al. / Materials Chemistry and Physics 148 (2014) 96e10298

while for CNW ceramic the variation is from 82 to 92%. Any furtherincrease in the sintering temperature (above 1500 �C) results in thedecrease of the relative density which may be due to the partialmelting and subsequent decomposition. However it should benoted that the densification of CNW is slightly less to that of CSWceramics sintered at the same temperature.

Fig. 3 shows the powder X-ray diffraction (XRD) patterns of CSWand CNW ceramics sintered at 1450 �C. The colour of the sample ispurple blue for CSW and orange brown for CNW ceramics aftersintering. All the peaks are indexed using ICDD file card No. 41-0186for Ca9Eu2W4O24, which is isostructural with Sr3(Sr0.5La0.5)4-La4Nb4O24. It has a tetragonal scheelite structure that belongs toI41/a space group [11] with 4 formula units per unit cell. Thehighest intensity peak was observed at 2q of 31.5326�

(d ¼ 2.83496 Å) for CSW ceramics and 2q of 31.4631�

(d ¼ 2.84107 Å) for CNW ceramics respectively. The sharp intensepeaks of the patterns testify the high crystalline nature of thesamples. The theoretical density of the CNW and CSW ceramicscomputed from the XRD pattern was 5.80 and 5.85 g cm�3

Fig. 3. Room temperature X-ray diffraction pattern of CNW and CSW ceramics sinteredat 1450 �C.

respectively. Fig. 4 shows the powder energy dispersive X-ray(EDAX) pattern of CSW and CNW ceramics sintered at 1450 �C. Thecompositional analysis is carried out using energy dispersive X rayspectrum (EDS) and all the elements are detected in EDS mappingas depicted in Fig. 4. EDAX spectra collected at various positions ofthe surface seems to confirm phase purity of the CSW and CNWsamples without any additional phase other than carbon coatedcopper grid which hold the ceramic particles. The EDAX analysis isconfirming the elemental composition of CSW and CNW ceramics.The variation in the atomic wt.% of W in the elemental analysis ofCSW and CNW ceramic is within the error limit of HR-TEM e EDAXanalysis.

The TEM micrographs of well dispersed ceramic particles isdepicted in Fig. 5(left), which clearly reveal a bigger particle size forCSW in comparison to CNW. To confirm this visible difference, wehave carried out a detailed image analysis of SEM micrograph [seeFig. 5(right)] using “Image J’ software [16]. It was found that theaverage grain size of the CNW ceramics is calculated to be4.3 ± 0.2 mm. On the other hand, CSW posses a relatively largergrain structure whose average size fall in the range 6.9 ± 0.8 mmwhen both the ceramics were sintered at 1450 �C. It is interesting tonote that the average particle size of CNW calculated using particlesize analyzer is 655 nmwhile that of CSW ceramics is 850 nm (seeFig. 1). The discrepancy between two methods of particle sizecalculation can be attributed to the random sampling errors asso-ciated with the solution dispersion techniques used for particle sizemeasurement. The larger grain size of CSW ceramics even afterseveral hours of post calcinations grinding could be correlated totheir inherent mechanical strength, which is beyond the scope ofpresent investigation.

It should be noted that the sintered CNW ceramic powder ismore electron transparent than CSW powder and the differencecould be attributed to the different thinning rate between CSW andCNWduring ionmilling. The high resolution image shown in Fig. 6arepresents the morphology of the single particle of CNW ceramicwhich is used for mapping the lattice fringes and Selected AreaElectron Diffraction (SAED) pattern. Fig. 6b shows the high reso-lution lattice image of CNW ceramic. It is well known that thelattice images are formed by interference between the direct beamand diffracted beams in HR-TEM. As a general rule, the width of aset of fringes is proportional to the lattice spacing of the originalcrystal lattice when the corresponding lattice planes meet the

Fig. 4. Energy dispersive X-ray analysis of sintered CNW and CSW sintered powdersample.

Fig. 5. TEM and SEM morphology of sintered CNW and CSW powder sample.

J. Varghese et al. / Materials Chemistry and Physics 148 (2014) 96e102 99

Bragg condition. The estimated interplanar distance of CNWceramic is about 2.8 Å within the experimental error limit, whichcorresponds to the plane (224). The lattice plane spacing wascomputed with the help of image processing software “Image J”.The SAED is shown in Fig. 6c and the circular diffraction spothighlights the polycrystalline nature of CNW ceramic. Based on atetragonal scheelite phase, the diffraction circles are indexed. Thediffraction circle enables to identify the different planes corre-sponding to each circle. The measured d spacing of different planesare d1 ¼ 2.8 Å, d2 ¼ 4.6 Å and d3 ¼ 3.0 Å respectively from Fig. 6c.The lattice planes (224), (202) and (312) that originally giving riseto the diffraction circles present in the SAED pattern are calculatedfrom the XRD pattern shown in Fig. 3.

Fig. 6. (a, b, c). High resolution TEM image of C

Fig. 7(a, b) shows the SEM microstructure of the densified andsintered CNW and CSW ceramics sintered at 1450 �C. The sinteredpellets show good agreement with the percentage density values(~92% and 93%). On sintering, ceramic powder begins to diffuse andshrink in volume resulting in densification. Grain growth also takesplace in parallel with densification [17]. The open porosity is visiblein both the micrographs with evident micro crack and fissureswhich have obvious effect on the dielectric properties of CNW andCSW ceramics. The presences of large grains could be the conse-quence of fusion of several grains [18].

Fig. 8 shows variation of relative permittivity (εr) and dielectricloss (tan d) of CNW and CSW ceramics as a function of sinteringtemperature. The best dielectric properties and densification are

NW ceramic powder sintered at 1450 �C.

Fig. 7. Microstructure of (a) CNW and (b) CSW ceramic sintered at 1450 �C.

J. Varghese et al. / Materials Chemistry and Physics 148 (2014) 96e102100

observed for the both the ceramics sintered at 1450 �C. Hence thedeterioration of dielectric properties of CNW and CSW at highersintering temperature could be due to the degradation of the ma-terial. A maximum relative permittivity and minimum dielectricloss are observed for the best dense samples. The dielectric prop-erties of CNW and CSW at 1 MHz after sintering at 1450 �C wereεr ¼ 19.8, tan d ¼ 0.026 for CNW and εr ¼ 24.1 and tan d ¼ 0.022 for

Fig. 8. (a, b). Variation of relative permittivity (εr), dielectric loss (tan d) of CNW andCSW ceramics as a function of sintering temperature.

CSW respectively. The porosity corrected relative permittivity isεr ¼ 22.3 for CNW ceramics while, εr ¼ 26.8 for CSW ceramics,which is calculated using the equation derived by Penn et al. [19].The measured relative porosity of the CNW and CSW ceramics are8% and 7% respectively. Fig. 9 shows the variation of relativepermittivity (εr) at 1 MHz as a function of operating temperature ofCSW and CNW ceramics sintered at 1450 �C. The relative permit-tivity shows a slight increase with change in temperature fromroom temperature to 70 �C. The CSW ceramics has temperaturecoefficient of relative permittivity (tε) ¼ 980 ppm �C�1 while CNWceramic has tε ¼ 1368 ppm �C�1 in the same operating tempera-ture. The temperature sensitivity of dielectric properties is essentialfor the device level integration of functional materials and thethermal stability of the above said materials could be improvedthrough doping with negative tε materials such as TiO2, SrTiO3,CaTiO3 [20].

It is worthwhile to note that several scheelite tungstates exhibitpromising microwave dielectric properties [4e6]. As a typical case,scheelite tungstates based on AWO4 [A ¼ Ca, Sr, Ba] have relativepermittivities 10.4, 8.1 and 8.1 respectively while that of wolframitetungstates MWO4 [M ¼ Mg, Zn, Mn] exhibits comparatively supe-rior relative permittivities of 13.5, 17.5 and 14.8 respectively [21].The present study reveals that the microwave dielectric propertiesof CNW and CSW ceramics are relatively higher as compared toscheelites and wolframites. The microwave dielectric properties ofCNW and CSW ceramics were characterized using SPDR method.Table 1 shows themicrowave dielectric properties of CNWand CSWceramics at room temperature. The CNW ceramic samples sinteredat 1450 �C have εr ¼ 16 and tan d¼ 0.0037, while CSW ceramic haveεr ¼ 17 and tan d ¼ 0.0048 (at 15 GHz) respectively. The porositycorrected relative permittivities are εpc ¼ 18 and εpc ¼ 19 respec-tively for CNW and CSW ceramics which are calculated usingthe equation of Penn et al. [19]. It should be noted that the mi-crowave dielectric properties of ceramics are influenced by anumber of factors such as processing conditions, phase transitions,purity of the raw materials, inherent crystal defects and porosity[22e25]. The present study reveals that porosity of the CNW(densification¼ 92%) and CSW (densification¼ 93%) ceramics havea pronounced influence on their apparent relative permittivities atsub-microwave frequencies. This is because at lower frequencies,the influence of interfacial polarization dominate, which iscontributed mainly by polar and volatile molecules present in the

Fig. 9. Variation of relative permittivity of CNW and CSW ceramic as a function ofoperating temperature.

Table 1Microwave dielectric properties of sintered CNW and CSW ceramics.

Material Sintering temperature (�C) Relative density (%) Microwavefrequency(15 GHz)

εr tan d εpc

CNW 1450 92 16 0.0037 18CSW 1450 93 17 0.0048 19

εr relative permittivity.tan d dielectric loss.εpc porosity corrected relative permittivity.

Fig. 11. Variation of (a) diffusivity (a) (b) specific heat (C) and (c) thermal conductivity(l) of CNW and CSW ceramic (sintered at 1450 �C) with respect to temperature.

J. Varghese et al. / Materials Chemistry and Physics 148 (2014) 96e102 101

pores. The influence of interfacial polarization may be more in CSWthan CNW, which also varies from sample to sample. The highrelative permittivity of CSW ceramics (say, εr ¼ 34 at 500 Hz) ascompared to CNW ceramics (say, εr ¼ 21.7 at 500 Hz) sintered at1450

�C, can be understood as the influence of interfacial polari-

zation. However in the microwave frequency region, influence oforientation polarization ceases, and the permittivity is mainlydictated by ionic polarization. It should be noted that in microwavefrequency region, the difference in permittivities of CNW and CSWare only marginal (εr ¼ 16 for CNW and 17 for CSW).

Fig. 10 shows the variation of the linear dimension of the CSWand CNW ceramic as a function of temperature in the range25e600 �C. It is found to have a positive but low CTE, ~4 ppm oC�1

in the measured temperature range. It can be seen that, except atvery low temperatures CTE plot remains nearly linear within thelimits of experimental error. Generally industrial electronic ce-ramics and several high temperature polymers possess low positiveCTE with values <10 ppm oC�1. The materials like polyimide andsilicon have an in-plane CTE of about 3 ppm oC�1 [26]. The averagecoefficient of thermal expansion of CSW and CNW ceramic iscalculated to be 4.1 and 4.2 ppm oC�1 respectively in the temper-ature range 25e600 �C. The matching coefficient of thermalexpansion of CSW and CNW ceramic with that of standard low kmaterials like polyimide, silicon etc. suggests a good thermalintegrability of developed materials in silicon based architecture,which is suitable for integration.

Fig. 11(a, b, c) shows the variation of diffusivity (a), specific heat(C) and thermal conductivity (l) of CNW and CSW ceramics sin-tered at 1450 �C as a function of temperature in the range of25e300 �C. The error associated with Flashline method was about±0.6% for thermal diffusivity measurement. Both CNW and CSW

Fig. 10. Linear thermal expansion of CNW and CSW ceramic sintered at 1450 �C.

ceramics show a similar trend as specific heat and thermal con-ductivity increases initially with temperature and then decreasesas shown in Fig. 11b and c respectively. The variation of specificheat and thermal conductivity are within the error limit ±1%.Majority of non-metallic compounds also exhibit a similarbehaviour with increasing temperature [27e29]. For CNW ce-ramics, the thermal conductivity consists in the range1.2e1.8 W m�1 K�1 and while it is in the range 1.4e2.1 W m�1 K�1

for CSW. Porosity in ceramic materials may have a dramatic in-fluence on thermal conductivity, since increase in the pore volumeeventually results in reduction of thermal conductivity. Internalpores contain air, which has an extremely low thermal conduc-tivity ~0.02 W m�1 K�1 [30]. Fig. 11a shows the thermal diffusivityof the CNW and CSW ceramics as a function of temperature. Mostof the ceramic samples, heat transport is contributed by phononscattering mechanism which is obviously inefficient in an envi-ronment with porosity. The non-monotonous behaviour of ther-mal conductivity of CNW and CSW with temperature could becontributed by porosity [29].

4. Conclusion

Ca9Nd2W4O24 (CNW) and Ca9Sm2W4O24 (CSW) ceramics wereprepared by the conventional solid state ceramic route. The struc-ture, microstructure and elemental analysis of CNW and CSW ce-ramics were investigated. The dielectric properties of CNW andCSW ceramic have been studied both at low frequency as well asmicrowave frequency region. The CNW and CSW ceramic sinteredat 1450 �C showed good microwave dielectric properties εr ¼ 18,tan d ¼ 0.0037 and εr ¼ 19, tan d ¼ 0.0048 respectively at 15 GHz.The room temperature thermal conductivity of CNW and CSWceramic was found to be 1.6 and 1.8 W m�1 K�1 respectively andcoefficient of thermal expansion of CNW and CSW ceramics were4.2 and 4.1 ppm �C�1 respectively measured in the temperaturerange of 25e600 �C.

Acknowledgements

Mr. Jobin Varghese is grateful to the Council of Scientific andIndustrial research, India for the research fellowship.

J. Varghese et al. / Materials Chemistry and Physics 148 (2014) 96e102102

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