High Q Microwave Dielectric Ceramics in (Ni1�x Znx)Nb2O6 System

Sandeep Butee,z,y Ajit Kulkarni,w,z Om Prakash,z Ramnath P.R.C. Aiyar,z Sumesh George,J andMailadi SebastianJ

zDepartment of Metallurgical Engineering and Materials Science, IIT-Bombay, Powai, Mumbai 400076, India

yDepartment of Metallurgy and Materials Science, Government College of Engineering, Shivaji Nagar, Pune 411005, India

zCentre for Research in Nanotechnology and Science (CRNTS), IIT-Bombay, Powai, Mumbai 400076, India

JNational Institute for Interdisciplinary Science & Technology, Thiruvananthapuram 695019, India

(Ni1�xZnx)Nb2O6, 0rxr1.0, ceramics with 497% densitywere prepared by a conventional solid-state reaction, followedby sintering at 12001–13001C (depending on the value of x). TheXRD patterns of the sintered samples (0rxr1.0) revealed sin-gle-phase formation with a columbite (Pbcn) structure. The unitcell volume slightly increased with increasing Zn content (x). Allthe compositions showed high electrical resistivity (qdc5 1.670.3� 1011 X . cm). The microwave (4–5 GHz) dielectric prop-erties of (Ni1�xZnx)Nb2O6 ceramics exhibited a significant de-pendence on the Zn content and to some extent on themorphology of the grains. As x was increased from 0 to 1, theaverage grain size monotonically increased from 7.6 to 21.2 lmand the microwave dielectric constant (e0r) increased from 23.6to 26.1, while the quality factors (Qu� f) increased from 18900to 103 730 GHz and the temperature coefficient of resonant fre-quency (sf) increased from �62 to �73 ppm/1C. In the presentwork, we report the highest observed values of Qu� f5 103 730GHz, and e0r 5 26.1 for the ZnNb2O6-sintered ceramics.

I. Introduction

FOR making low-noise oscillators, antenna duplexers, andnarrow-band microwave filters with low insertion loss, and

many other devices operating at centimeter and millimeter wavebands, a need for developing new low-cost dielectric resonator(DR) materials with a high Q (quality parameter5 1/tan d) hasalways been there.1–5 Moreover, a rapid growth of the mobilephone market in recent years has further led to extensive re-search and development in the area of microwave dielectrics.Most of the R&D work reported has mainly been on betterutilization of available frequency bandwidth (achievable by us-ing materials with extremely low dielectric losses at microwavefrequencies, i.e. Q45000, which also improves the performanceof devices such as DR filters by enhancing electromagnetic en-ergy storage capacity), besides using high-permittivity (e0r410)materials to reduce the component size and thereby reducing thecost.1,5–7 In addition to the need for a high Q, it is also desirableto ideally have a near-zero temperature coefficient of resonantfrequency (tfo720 ppm/1C) for better temperature stability ofthe functional elements.5

Over the years, a range of DR ceramics in different oxidesystems have been explored such as Ba2Ti9O20 (e0rB40,

Qu� fB40 000 GHz, and tfB2 ppm/1C), Zr0.8Sn0.2TiO4

(e0rB38, Qu� fB60000 GHz, and tfB0 ppm/1C), BaO–Nd2O3–TiO2–Bi2O3 (e0rB90, Qu� fB6000 GHz, and tfB0ppm/1C), Ba(Mg1/3Ta2/3)O3 (e0rB24, Qu� f4400000 GHz,and tfB0 ppm/1C), Ba(Zn,Ta)O3 (e0rB30, Qu� f4140 000GHz, and tfB0 ppm/1C),1 MgTiO3–CaTiO3 (e0rB21,Qu� f455 000 GHz, and tfB0 ppm/1C),8 A5B4O15 (A5Ba,Sr, Mg, Ca, and Zn; B5Nb, Ta) (e0rB11–51, Qu� fB2400–88 000 GHz, and tfB�73 to 1232 ppm/1C),9 (BaxSr1�x) (Zn1/3Ta2/3)O3 (x5 0–1.0) (e0rB30–35, Qu� fB3000–60 000 GHz,and tfB�64 to 1127 ppm/1C),10 and ZnO–TiO2 (Zn/Ti5 0.67–2.0) (e0rB20–43, Qu� fB15 000–40 000 GHz, andtfB�65 to 1130 ppm/1C)11, and the search is still ongoing.However, use of not all but many of the above dielectrics is oftenrestricted due to problems like unpredictable structures andproperties, low Qu� f values, high tf, high cost, need for highsintering temperatures (414001C), or difficulty in cosintering ofthe dielectric with the base metal (copper or silver) for low-tem-perature cofired ceramics applications.,1,5,9–14

Interestingly, another important class of dielectric ceramicscomprising of the 3d-transition metal niobates, MNb2O6 (whereM5Mn, Co, Ni, Cu, and Zn), has been found. These niobatessinter well in air atB11001–13001C,5,15,16 which is a much lowertemperature than that needed for many of the complex perovs-kites, and also exhibit a very high quality factor (Qu� fB104–105) with rf permittivity (e0r) ranging between 20 and 25.5,13,16

Further, niobates have also been found to be quite suitable forhigh-frequency (dielectric) applications5,16–18 and as resonatordielectrics.3–6,13,15–18

For NiNb2O6 ceramics, Pullar et al.5 have reported values ofe0r 5 21, Qu� f5 19 300 GHz (at 6.55 GHz), and tf 5�71.3ppm/1C for the samples sintered at 12001C, whereas samplesfired at 11501C by Lee et al.15 showed e0r 5 22.6, Qu� f5 40100GHz, and tf5�38.0 ppm/1C. Maeda et al.18 reported, for thefirst time, the dielectric properties of ZnNb2O6 samples, fired at12001C in air, having e0r 5 20 and Qu� f5 44 000 (at 4 GHz). Ina subsequent work on this material, considerably improved mi-crowave dielectric properties were reported. For example,e0r 5 25, Qu� f5 83700 (at 10 GHz), and tf 5�56.1 ppm/1Cfor the samples fired in air at 11501C,15 while Qu� f5 84 500 (at6.29 GHz) with e0r 5 23.2 and tf5�75.8 ppm/1C for the sam-ples fired at 12001C.5 Still higher Qu� f5 88 000 (at 6.98 GHz),e0r 5 22, and tf 5�73 ppm/1C were reported by Jawahar et al.,19

while Belous et al.20 reported e0r 5 20.9, Qu� f5 120 000 GHz,and tf 5�74 ppm/1C in a nonstoichiometric (Zn11xNb2O6,x5 0.01) composition. A significant spread seen among the re-ported values of e0r and Qu� f for the sintered ZnNb2O6 canpossibly be ascribed to the fine variations in the microstructure,presence of trace impurities, and/or deviations from stoichiome-try in the samples prepared at different laboratories. Studies onthe microwave dielectric properties of the NiNb2O6 system

N. Alford—contributing editor

Sandeep Butee—On leave from the parent organization and pursuing Ph.D. work at IITBombay.

wAuthor to whom correspondence should be addressed. e-mail: ajit.kulkarni@iitb.ac.in

Manuscript No. 25232. Received September 16, 2008; approved December 15, 2008.

Journal

J. Am. Ceram. Soc., 92 [5] 1047–1053 (2009)

DOI: 10.1111/j.1551-2916.2009.02955.x

r 2009 The American Ceramic Society

1047

through systematic substitution of other 3d ions at the Ni site, tothe best of our knowledge, have not been found in the accessibleliterature.

In the present work, the effects of progressive Zn substitutionon the resonator dielectric properties of the sintered(Ni1�xZnx)Nb2O6, 0rxr1.0, ceramics due to changes in thechemical composition, lattice volume, density, and microstruc-ture have been determined.3,5,13–18 The entire composition range(0rxr1.0) is expected to be single phase as the two extremecompositions NiNb2O6 (x5 0) and ZnNb2O6 (x5 1) have thesame columbite (Pbcn) structure and there is little difference inthe ionic radii of Ni21 (rNi

21 5 0.69 A, CN5 6) and Zn21

(rZn21 5 0.74 A, CN5 6; here, CN is the coordination number).

Further, the two extreme compositions have almost similar den-sities, electronegativities, and small differences in sintering tem-peratures.5,15 This makes the (Ni1�xZnx)Nb2O6 system veryinteresting to explore within the columbite structure. Only amarginal increase in lattice parameters is expected with increas-ing Zn21 substitution for Ni21.

II. Experimental Procedure

High-purity (� 99.9%) oxide powders of ZnO (Thomas Baker,Mumbai, India), NiO, and Nb2O5 (Research Lab Fine Chem.,Mumbai, India) were used as the starting constituents. Thesewere mixed in the desired stoichiometry of (Ni1�xZnx)Nb2O6,(0rxr1.0), and wet ground in polyethylene bottles using zir-conia balls and methanol as a medium for 48 h to achieve propermixing and a surface-active fine powder. The finely mixed pow-ders were solid state reacted in air at 10001C for 2 h to obtain anearly single-(Columbite) phase material. The reacted powderswere reground for another 60 h to obtain a fine powder of av-erage grain size ( �Dr1 mm) necessary for achieving dense sinte-ring. Three weight percent PVA solution (Burgoyoyne,Mumbai, India) in deionized water was used as a binder. Thepowders were uniaxially compacted atB100MPa to form greencylindrical-shaped pellets. After carefully debinding below4001C, the samples were fired in air at temperatures of 12001–13001C for 4 h based on the sigmoidal curves generated in ex-ploratory runs. Only the samples with the maximum sintereddensities were subsequently characterized. To prevent zinc loss,a sacrificial pellet of ZnO was used during sintering and the pel-lets were sintered in a well-covered alumina crucible. The pow-der XRD patterns for the solid-state-reacted powders and thesintered pellets were recorded on a PanAnalytical XPERT-PRODiffractometer system (Philips Analytical B. V. (PANalytical),

Almelo, the Netherlands) in the 2y range of 201–701 using CuKaradiations (l5 1.5406 A, generator voltage5 40 KV, tube cur-rent5 30 mA, scan step size5 0.01701, continuous scan type,and time per step5 24.6724 s). A slow scan was performed inthe 2y range of 31–301 (time per step5 59.4645 s, all the otherparameters remaining the same as mentioned above) to detectthe (040) superlattice reflections. The lattice parameters werecomputed from the XRD peak positions and using ‘‘Wincell1.1,’’ a Windows package of DOS routines for unit cell

20 30 40 50 60 70

(041

)

(023

) (3

32)

(802

)(1

23)

(313

)(7

12)

(602

)(6

21)(1

31) (

330)

(620

)(022

) (1

30) (

502)

(312

)(4

02)

(302

)(3

21)

(600

)

(002

)(0

21)

(020

)(3

11)

(111

) (3

10)

Inte

nsi

ty (

A.U

.)

2θ {Cu Kα}

(a)(b)(c)(d)

(e)

(f)

(400

)

Fig. 1. Powder XRD patterns of (Ni1�xZnx)Nb2O6, 0rxr1.0, sam-ples (a) NiNb2O6 sintered at 13001C, 4 h, (b) Ni0.8Zn0.2Nb2O6 sintered at12801C, 4 h, (c) Ni0.6Zn0.4Nb2O6 sintered at 12601C, 4 h, (d)Ni0.4Zn0.6Nb2O6 sintered at 12401C, 4 h, (e) Ni0.2Zn0.8Nb2O6 sinteredat 12201C, 4 h, and (f) ZnNb2O6 sintered at 12001C, 4 h.

Fig. 2. Superlattice (040) reflection observed in ZnNb2O6 (a magnifiedview).

Fig. 3. Variation of lattice parameters a, b, and c in the(Ni1�xZnx)Nb2O6, 0rxr1.0, system with the Zn content (x).

Table I. Sintering Temperature (T), Bulk Density (d)and Lattice Parameters of (Ni1�xZnx)Nb2O6, 0rxr1.0,

Sintered Samples

x T (1C) d (%TD)

Refined lattice parameters (A)w

Cell volume (A3)a b c

0.0 1300 98 14.0304 5.6788 5.0182 399.82930.2 1280 98 14.0707 5.6821 5.0193 401.29870.4 1260 98 14.1006 5.6915 5.0221 403.04140.6 1240 98 14.1270 5.7038 5.0262 404.99900.8 1220 98 14.1529 5.7079 5.0260 406.01701.0 1200 97 14.1790 5.7186 5.0305 407.8932

TD is the theoretical density calculated from X-ray data. Uncertainty 70.5%.wThe uncertainty in lattice parameters is r5 in the last digit.

1048 Journal of the American Ceramic Society—Butee et al. Vol. 92, No. 5

refinements.21 The bulk density of the sintered samples wasdetermined by the Archimedes method following the guidelinesof ASTM method C 373 1988 (2006)22. The microstructure in-vestigation of polished and thermally etched surfaces of the sin-tered pellets was conducted using Hitachi S-3400N (HitachiHigh-Technologies Corporation, Minatoku, Tokyo, Japan)low-vacuum SEM. Energy-dispersive spectroscopy (EDS) wasperformed by a ThermoNoran ultra dry detector (Madison, WI)at a few locations for qualitative chemical composition deter-mination and elemental mapping. Grain size measurement onthe SEM-grabbed images was performed using the linear inter-cept method using Olysia m3 software (Olympus Corporation,Tokyo, Japan). The dc electrical resistivity (r) of the sampleswas measured using a Keithley 6514 System Electrometer(Cleveland, OH). For resistivity measurements, the sampleswere of size B10 mm diameter� 1.0 mm thickness. Sinteredpellets of B15 mm diameter� 7.5 mm height were used as res-onator posts to measure the microwave dielectric properties.Dielectric properties at microwave frequencies were measuredby the resonance method using a Vector Network Analyzer

(Agilent 8753 ET, Agilent Technologies, Hewlett Packard, PaloAlto, CA). The TE01d mode was used for microwave measure-ments in the 4–5 GHz range.23 Transmission and reception ofelectromagnetic waves through the DR were functionalized bytwo loop antennas coupled with the resonator cavity and mi-crowave source. The coefficient of temperature variation of res-onant frequency was measured by noting the temperaturevariation of the TE01d resonant mode at every 21C interval inthe temperature range of 251–701C. The error in e0r was calcu-lated using the root sum of squares (RSS) method. The accuracyof e0r measurement was restricted to the accuracy in the mea-surement of the resonant frequency and dimensions of the sam-ple. The error in e0r measurement was o70.01. The errors inunloadedQ (Qu5 1/tan de) and tf were calculated using the RSSmethod.24

III. Results and Discussion

The powder XRD patterns of the sintered samples of(Ni1�xZnx)Nb2O6, 0rxr1.0 (Fig. 1), show single columbite

Fig. 4. SEM photomicrographs of the as sintered, polished, and thermally etched surfaces of (Ni1�xZnx)Nb2O6, 0rxr1.0, ceramics. (a) NiNb2O6,(b) Ni0.8Zn0.2Nb2O6, (c) Ni0.6Zn0.4Nb2O6, (d) Ni0.4Zn0.6Nb2O6, (e) Ni0.2Zn0.8Nb2O6, and (f) ZnNb2O6 ceramics at � 1000 magnification. (rular bargiven at the top corresponds to a size of 50 mm)

May 2009 (Ni-Zn)Niobate Microwave Resonator Ceramics 1049

phase formation for all the values of x. All the observed (hkl)reflections could be indexed satisfactorily to orthorhombic unitcells (JCPDS file No. 761827 or 720481). The (040) superlatticereflection associated with proper ordering of cations in the Col-umbite (Pbcn) structure is clearly observed only for the pureZnNb2O6 samples as shown in Fig. 2. This finding is furthersubstantiated from Raman spectra on the system (to be pub-lished elsewhere). Earlier reports3,25–27 suggested that orderingcould be quantified from the relative integrated intensity of the

superlattice lines or the ratio of ‘‘a’’ cell edge to the ‘‘c’’ cell edgeor plots of ‘‘c’’ versus ‘‘a’’. For all the samples, a narrow lineprofile and improved separation of Ka1 and Ka2 peaks (Fig. 2),indicative of good crystallization (i.e., existence of an orderedsublattice), are observed.2,7 In the columbite (AB2O6)-typestructure, although the 3d transition ions Zn21 (rZn

21 5 0.74 A)and Ni21 (rNi

21 5 0.69 A) mainly occupy the A site, there may besome degree of cation occupation disorder28,29 existing betweensites A and B (as Nb51 [rNb

51 5 0.64 A], mostly occupying the B

Fig. 5. Elemental mapping by energy-dispersive spectroscopy conducted on a typical Ni0.6Zn0.4Nb2O6 sample showing the uniform distribution of allthe constituting elements (a) oxygen, (b) nickel, (c) zinc, (d) niobium, and (e) the corresponding microstructure of the Ni0.6Zn0.4Nb2O6 sample at � 1000magnification.

1050 Journal of the American Ceramic Society—Butee et al. Vol. 92, No. 5

site, has an ionic radius similar to that of Zn21 and Ni21 ions).Further for Zn and Ni ions, both being bivalent, the cation or-dering is expected to be governed by their size differences withNb51 ion and accordingly a higher degree of ordering is ex-pected in the presence of Zn21 vis-a-vis Ni21.30

The measured % theoretical density (TD), lattice parameters,and cell volume of the various sintered compacts are presentedin Table I. The (Ni1�xZnx)Nb2O6, 0rxr1.0, samples showgood sintered density (497%) and the TD of all the sinteredcompacts is in the vicinity of 5.65 g/cm3. Both ‘‘a’’ and ‘‘c’’ lat-tice parameters (Fig. 3) show a small monotonic increase withincreasing Zn21 substitution as Ni21 ion has a smaller ionic ra-dius (0.69 A) than Zn21 ion (0.74 A). Further, there is a com-mensurate change in the cell volume and it increases fromB399.83 to 407.89 A3 with an increase in x from 0 to 1, clearlyindicative of the formation of a substitutional solid solution forthe cations (Ni,Zn) in the AB2O6-type columbite structure.7

The SEM images of the sintered (Ni1�xZnx)Nb2O6,0rxr1.0, samples, as shown in Fig. 4, show a uniform micro-structure characteristic of a single phase. For the Ni-rich (x-0)samples, grains are slightly elongated but the samples have afairly uniform grain size distribution, while, for Zn-rich (x-1)samples, the grains represent two different size ranges, with thefractions of the larger nearly rectangular-shaped grains morethan the fine grains. The elemental mapping reported here for arepresentative Ni0.6Zn0.4Nb2O6 sample by SEM-EDS shows auniform distribution of all the constituent elements (see Fig. 5),which is also observed for the other samples (not shown here).The analysis of the SEM-EDS results, given in Table II, indi-cates that each of the compositions qualitatively matches to therespective compound (Ni1�xZnx)Nb2O6, 0rxr1.0. The resultsof grain size measurement, following the method of Mendel-son,31 are given in Table III. The average grain size ( �D) is foundto increase with progressive Zn substitution; �D is B7.6 mm forNiNb2O6 (x5 0) and increases toB21 mm for ZnNb2O6 (x5 1)(also see Fig. 4).

(1) Microwave Dielectric Resonator Parameters

For the (Ni1�xZnx)Nb2O6, 0rxr1.0, sintered samples, the ob-served and calculated (using Shannon’s polarizability data32 and

the Clausius–Mosotti relation15) values of e0r as a function of x(Zn) are shown in Fig. 6(a). It can be seen that the general trendof a monotonic increase in both the theoretical and the exper-imental values (for e0r vs. x) is nearly the same, albeit the cal-culated values are systematically increased by r5%. The valuesof microwave resonator parameters (e0r, Qu� f, and tf), mea-sured in the frequency range 4–5 GHz and plotted as a functionof Zn content, are shown in Fig. 6. Assuming e0r to be less sen-sitive to small impurities and variations in grain size,15–20,25,33–38

the values of e0r for samples containing different amounts of Zn(x) in (Ni1�xZnx)Nb2O6, 0rxr1.0, are compared. As can beseen from the plot given in Fig. 6(a), the e0r value increases from23.6 to 26.1 as x (Zn) is increased from 0 to 1. The e0r valuesseem to follow a simple additive rule of mixtures and suggestthat e0r primarily depends on the composition and to someextent on the grain size, with the other factors, as referredto in the introduction, being nearly the same.39 The calculatedionic polarizability (a), following Shannon’s polarizabilitydata32 of (Ni1�xZnx)Nb2O6 (x5 0–1.0) samples, has increasedfrom 21.23 to 22.04 as x increases from 0 to 1 and experimentale0r has also increased from 23.6 to 26.1. This is in agreement withthe findings of Kan et al.40 Thus, ZnNb2O6 has shown the high-est e0r value; the likely additional contribution to e0r may be dueto its higher lattice volume (see Table I) providing cations withinthe oxygen octahedron a larger polarization space, resulting inlarger dielectric polarizability.34

The quality factor (Qu� f ) values of (Ni1�xZnx)Nb2O6

0rxr1.0 samples (see Fig. 6(b)) progressively increased from18 900 to 103 730 GHz as x (Zn) increased from 0 to 1.0. TheZnNb2O6-sintered samples showed the highest quality factor(also see Table III). Further, the Qu� f data over the entire

Table II. Compositional Analysis by SEM-EDS of(Ni1�xZnx)Nb2O6, 0rxr1.0, Sintered Samples

x

Qualitative analysis (at. %)

O Ni Zn Nb Total

0.0 66.4772.03 9.8471.02 00.00 23.6970.40 1000.2 64.9972.43 7.5371.08 3.0470.78 24.4470.46 1000.4 67.1672.35 5.8670.97 2.3570.82 24.6470.44 1000.6 63.3372.26 4.0970.53 7.6670.91 24.9370.45 1000.8 66.3972.11 2.4370.45 7.8470.92 23.3470.42 1001.0 64.3672.37 00.00 10.7271.15 24.9270.49 100

Fig. 6. Variation of (a) e0r, (b) Qu� f, and (c) tf in (Ni1�xZnx)Nb2O6

(x50–1.0) ceramics with x (Zn content) (The line joining the points is amere guide to the eye).

Table III. Observed and Computed (by Additive Rule of Mixture37) Dielectric Properties of (Ni1�xZnx)Nb2O6, 0rxr1.0,

Sintered Samples

x �Dw (mm)

Observed Applying additive rule between end members

e0r Qu� f (GHz) tf (ppm/1C) f z (GHz) e0r Qu� f (GHz) tf (ppm/1C)

0.0 07.6 23.6 18900 �62 4.36 23.6 18 900 �62.00.2 12.5 24.4 24250 �63 4.31 24.1 35 860 �64.20.4 14.0 24.9 35370 �66 4.27 24.6 52 830 �66.40.6 13.3 25.0 51330 �66 4.27 25.1 69 800 �68.60.8 17.8 25.5 90420 �68 4.22 25.6 86 760 �70.81.0 21.2 26.1 103 730 �73 4.18 26.1 103 730 �73.0

wAvgerage grain size.31

zResonant frequency at which measurements were recorded.

May 2009 (Ni-Zn)Niobate Microwave Resonator Ceramics 1051

composition range, 0rxr1.0, show that both (i) the simpleadditive rule41 (see Table III) as well as (ii) the Lever rule ofmixture42 are, however, not applicable here.

The adverse effect due to the small porosity on the qualityfactor (Qu� f) could be ignored for the dense (497% of theTD) (Ni1�xZnx)Nb2O6, 0rxr1.0, samples.41 However, thegrain size effect on Qu� f need not be insignificant. ForNiNb2O6 samples (with a narrow size distribution), an averagegrain size of B7.6 mm is obtained (see Fig. 6 and Table III),which incidentally falls within the same range of grain size B1–10 mm as reported by Pullar et al.,5 and the Qu� f5 18900 GHzobtained here is also comparable. The Qu� f reported by Leeet al.15 for NiNb2O6 is almost twice the value of what we haveobtained; however, the grain size in their samples was not re-ported. In the present ZnNb2O6 (x5 1) samples, for �DB21 mm,the Qu� f5 103 730 GHz obtained is much larger than thevalues reported by Pullar et al.5 for the same composition sam-ples having grain sizeB1–10 mm (with a few larger platelets andmany smaller grains) and also the Qu� f results of others.15,18,19

A relatively larger grain size means a less specific grain boundaryarea, which also meant lesser interfacial mismatch and grainboundry disorders in the material, and therefore lower dielectriclosses.41 It is seen that with the substitution of a marginallylarger Zn21 (rZn

21 5 0.74 A) for a smaller Ni21 (rNi21 5 0.69 A), the

lattice parameters increase via a small incremental change in theoxygen octahedral surrounding Zn, thereby possibly causing anincrease in the Qu� f values.2 It is interesting to observe that inthe XRD patterns recorded for the entire composition range0rxr1 (Fig. 1), only the ZnNb2O6 (x5 1) sample, having themaximum value of Qu� f (103 730 GHz), shows the (040) su-perlattice reflection (a magnified view is shown in Fig. 2), whichis a manifestation of the ordered arrangement of cations and isvery likely leading to lower microwave losses in the columbitephase.3,26,34 For the other (0oxo1) compositions, the effects ofZn–Ni in solid solutions can cause some local disorderedarrangements of cations leading to deviation from the perfectlattice periodicity at the unit cell scale, and thereby a declinein Qu� f values.14 Further, Ni-containing oxidic systems oftenhave point defects due to the presence of Ni1�yO,43,44 which ineffect could contribute to electrical losses, thereby adverselyaffecting the Qu� f values.

The tf for the (Ni1�xZnx)Nb2O6 system is found to graduallyincrease from �62 to �73 ppm/1C (Fig. 6(c)) as x is increasedfrom 0 to 1.0. This can be attributed to an increase in theunit cell volume from 399.82 A3 (for NiNb2O6) to 407.89A3

(for ZnNb2O6).15 tf also shows (as reported by others) a direct

correlation to the structural Schmitt tolerance factor (t) ascolumbite-type compounds are considered to be a sub-component of the complex perovskite A(B01/3B

002/3)O3 com-

pounds.15,45 Nevertheless, this aspect needs further investiga-tions.

In order to understand the factors that contribute to the highobserved value of Qu� f41.0� 105 GHz in ZnNb2O6, the dcelectrical resistivity (r) was measured at room temperature on allthe (Ni1�xZnx)Nb2O6, 0rxr1.0, sintered samples and r wasfound to be within 1.670.3� 1011 O � cm. It appears that inhighly resistive niobates, the contribution toward an increase inQu� f in ZnNb2O6 vis-a-vis NiNb2O6 is essentially not due toresistivity variations of the materials.

IV. Conclusions

The microwave DR properties, viz., e0r, Qu� f, and tf have beeninvestigated for the (Ni1�xZnx)Nb2O6, 0rxr1.0, sintered ce-ramics. For all the compositions, 0rxr1.0, the e0r valuesranged between 23.6 and 26.1, the Qu� f values were between18900 and 103730 GHz, and the tf values ranged between �62and �73 ppm/1C. The ZnNb2O6-sintered samples showede0r 5 26.1 and Qu� f5 103 730 GHz, which, to our knowledge,are the highest values obtained so far.

Acknowledgments

The authors wish to thank Dr. S. L. Kamat and Mr. Dilip Agrahari (MEMSDept., IIT, Bombay) for their help with the SEM-EDS and resistivity measure-ments-respectively.

References

1M. T. Sebastian, Dielectric Materials for Wireless Communication. ElsevierScience Publishers, Oxford, 2008.

2H. Tamura, T. Konoike, Y. Sakabe, and K. Wakino, ‘‘Improved High-Q Di-electric Resonator with Complex Perovskite Structure,’’ J. Am. Ceram. Soc., 67,C59–61 (1984).

3D.-W. Kim, D.-Y. Kim, and K. S. Hong, ‘‘Phase Relations and MicrowaveDielectric Properties of ZnNb2O6–TiO2,’’ J. Mater. Res., 15 [6] 1331–5 (2000).

4J. Wang, Z. Yue, Z. Gui, and L. Li, ‘‘Low Temperature Sintered ZnNb2O6

Microwave Dielectric Ceramics Doped with ZnO–V2O5 Additions,’’ J. Mater. Sci.Lett., 40, 6581–3 (2005).

5R. C. Pullar, J. D. Breeze, and N. M. Alford, ‘‘Characterization and Micro-wave Dielectric Properties of M21 Nb2O6 Ceramics,’’ J. Am. Ceram. Soc., 88 [9]2466–71 (2005).

6I. M. Reaney, ‘‘Microwave Dielectric Ceramics for Resonators and Filters inMobile Phone Networks,’’ J. Am. Ceram. Soc., 89 [7] 2063–72 (2006).

7T. Nagai, T. Inuzuka, and M. Sugiyama, ‘‘Contribution of Dielectric Constantto Change in Temperature Coefficient of Resonant Frequency in (Ba1�xSrx)(Mg1/3Ta2/3)O3 Compounds,’’ Jpn. J. Appl. Phys. Part I, 31 [9B] 3132–5 (1992).

8K.Wakino, T. Nishikawa, Y. Ishikawa, and H. Tamura, ‘‘Dielectric ResonatorMaterials and their Applications for Mobile Communication Systems,’’ Br.Ceram. Trans. J., 89, 39–43 (1990).

9I. N. Jawahar, P. Mohanan, and M. T. Sebastian, ‘‘A5B4O15 (A5Ba, Sr, Mg,Ca, Zn; B5Nb, Ta) Microwave Dielectric Ceramics,’’ Mater. Lett., 57 [24–25]4043–8 (2003).

10J. Venkatesh and V. R. K. Murthy, ‘‘Microwave Dielectric Properties of(Ba, Sr) (Zn1/3Ta2/3)O3 Dielectric Resonators,’’ Mater. Chem. Phys. (Mater. Sci.Commun.), 58 [3] 276–9 (1999).

11H. T. Kim, J. D. Byun, and Y. Kim, ‘‘Microstructure and Microwave Di-electric Properties ofModified Zinc Titanates (II),’’Mater. Res. Bull., 33 [6] 975–86(1998).

12R. C. Pullar, K. Okeneme, and N. McN. Alford, ‘‘Temperature CompensatedNiobate Microwave Ceramics with the Columbite Structure, M21Nb2O6,’’ J. Eur.Ceram. Soc., 23, 2479–83 (2003).

13S. d’Astorg, S. Marinel, O. Perez, and A. Veres, ‘‘Investigation of Some Nio-bate-Based Dielectrics in View of Base Metal Co-Sintering,’’ J. Eur. Ceram. Soc.,27, 4445–51 (2007).

14M. T. Sebastian and H. Jantunen, ‘‘Low Loss Dielectric Materials LTCCApplications—A Review,’’ Int. Mater. Rev., 53, 57–90 (2008).

15H.-J. Lee, K.-S. Hong, S.-J. Kim, and I.-T. Kim, ‘‘Dielectric Properties ofMNb2O6 Compounds (Where M5Ca, Mn, Co, Ni, or Zn),’’Mater. Res. Bull., 32[7] 847–55 (1997).

16D.-W. Kim, K. H. Ko, and K. S. Hong, ‘‘Influence of Copper(II) Oxide Ad-ditions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Di-electric Properties,’’ J. Am. Ceram. Soc., 84 [6] 1286–90 (2001).

17D.-W. Kim, K. H. Ko, D.-K. Kwon, and K. S. Hong, ‘‘Origin of MicrowaveDielectric Loss in ZnNb2O6–TiO2,’’ J. Am. Ceram. Soc., 85 [5] 1169–72 (2002).

18M. Maeda, T. Yamamura, and T. Ikeda, ‘‘Dielectric Characteristics of SeveralComplex Oxide Ceramics at Microwave Frequencies,’’ Jpn. J. Appl. Phys. Part 1,26–2 [Suppl.] 76–9 (1987).

19N. Jawahar, M. T. Sebastian, G. S. Binoy, and P. Mohanan, ‘‘A NovelMethod for Tuning the Microwave Dielectric Properties by Stacked DielectricResonators,’’ Proc. APSYM-2000, 173–5 (2000).

20A. Belous, O. Ovchar, B. Jancar, and J. Bezjak, ‘‘The Effect of Non-Stoic-hiometry on the Microstructure and Microwave Dielectric Properties of the Col-umbites A21Nb2O6,’’ J. Eur. Ceram. Soc., 27, 2933–6 (2007).

21F. A. Rajabalee, inWincell 1.1: A Windows Package of DOS Routines for UnitCell Refinements, Edited by F. A. Rajabalee. Bordeaux, France, 2000.

22ASTM Method C 373 1988, ‘‘Standard Test Method for Water Absorption,Bulk Density, Apparent Porosity, and Apparent Specific Gravity of FiredWhiteware Products,’’ ASTM Int., pp. 1–2, West Conshohocken, PA, UnitedStates, (2006).

23J. Krupka, K. Derzakowsky, B. Riddle, and J. B. Jarvis, ‘‘A Dielectric Res-onator for Measurements of Complex Permittivity of Low Loss Dielectric Mate-rials as Function of Temperature,’’ Meas. Sci. Technol., 9, 1751–61 (1998).

24J. B. Jarvis, G. Gayer Jr., J. H. Grosvenor, M. D. Janezic, C. A. Jones,B. Riddle, C. M. Weil, and J. Krupka, ‘‘Dielectric Characterisation of Low-LossMaterials—A Comparison of Techniques,’’ IEEE Trans. Dielectr. Electr. Insul., 5,571–7 (1998).

25Y. C. Zhang, L. T. Li, Z. X. Yue, and Z. L. Gui, ‘‘Effects of Additives onMicrostructures and Microwave Dielectric Properties of ZnNb2O6 Ceramics,’’Mater. Sci. Eng. B, 99, 282–5 (2003).

26M. Wenger, T. Armbruster, and C. Geiger, ‘‘Cation Distribution in PartiallyOrdered Columbite from the Kings Mountain Pegmatite, North Carolina,’’ Am.Mineral., 76, 1897–904 (1991).

27T. S. Ercit, M. A. Wise, and P. Cenry, ‘‘Compositional and Structural Sys-tematics of the Columbite Group,’’ Am. Mineral., 80, 613–9 (1995).

28C. A. dos Santos, L. I. Zawislak, V. Antonietti, E. J. Kinast, and J. B. M. daCunha, ‘‘Iron Oxidation and Order–Disorder in the (Fe21,Mn)(Ta,Nb)2O6-(Fe21,Mn)Fe31(Ta,Nb)2O8 Transition,’’ J. Phys.: Condens. Matter., 11, 7021–33(1999).

1052 Journal of the American Ceramic Society—Butee et al. Vol. 92, No. 5

29M. S. Augsburger, J. C. Pedregosa, G. M. Sosa, and R. C. Mercader,‘‘Mossbauer Assessment of Cation Disorder in Columbite–Tantalite Minerals,’’J. Solid State Chem., 143 [2] 219–23 (1999).

30R. Ratheesh, M. Wohlecke, B. Berge, Th. Wahlbrink, H. Haeuseler, E. Ruhl,R. Blachnik, P. Balan, N. Santha, and M. T. Sebastian, ‘‘Raman Study ofthe Ordering in Sr(B0.5Nb0.5)O3 Compounds,’’ J. Appl. Phys., 88 [5] 2813–8(2000).

31M. L. Mendelson, ‘‘Average Grain Size in Polycrystalline Ceramics,’’ J. Am.Ceram. Soc., 52 [8] 443–6 (1969).

32R. D. Shannon, ‘‘Dielectric Polarizabilities of Ions in Oxides and Fluorides,’’J. Appl. Phys., 73, 348–66 (1993).

33Y. C. Zhang, Z. X. Yue, Z. L. Gui, and L. T. Li, ‘‘Effects of CaF2 Addition onthe Microstructure and Microwave Dielectric Properties of ZnNb2O6 Ceramics,’’Ceram. Int., 29, 555–9 (2003).

34J. Wang, Z. X. Yue, Z. L. Gui, and L. T. Li, ‘‘Low Temperature SinteredZn(Nb1�xVx/2)2O6�2.5x Microwave Dielectric Ceramics with High Q Value forLTCC Application,’’ J. Alloy. Compd., 392, 263–7 (2005).

35Y. C. Zhang, Z. X. Yue, X. Qi, B. Li, Z. L. Gui, and L. T. Li, ‘‘MicrowaveDielectric Properties of Zn(Nb1�xTax)2O6 Ceramics,’’ Mater. Lett., 58, 1392–5(2004).

36Y. C. Zhang, Z. X. Yue, Z. L. Gui, and L. T. Li, ‘‘Microwave DielectricProperties of (Zn1�xMgx)Nb2O6 Ceramics,’’ Mater. Lett., 57, 4531–4 (2003).

37L. Tu, Z. Y. Chun, Y. Zhenxing, and G. Zhilun, ‘‘Microstructure and Mi-crowave Dielectric Properties of (1�x)ZnNb2O6–(x)ZnTa2O6 Ceramics,’’ Ceram.Trans., 167, 109–15 (2005).

38S. J. Penn, N. M. Alford, A. Templeton, X. Wang, M. Xu, M. Recce,and K. Schrapel, ‘‘Effect of Porosity and Grain Size on the MicrowaveDielectric Properties of Sintered Alumina,’’ J. Am. Ceram. Soc., 80 [7] 1885–8(1997).

39H.-J. Lee, I.-T. Kim, and K. S. Hong, ‘‘Dielectric Properties of AB2O6 Com-pounds at Microwave Frequencies (A5Ca, Mg, Mn, Co, Ni, Zn, and B5Nb,Ta),’’ Jpn. J. Appl. Phys. Part 2, 36 [10A] L1318–20 (1997).

40A. Kan, H. Ogawa, and H. Ohsato, ‘‘Influence of Microstructure on Micro-wave Dielectric Properties of ZnTa2O6 Ceramics with Low Dielectric Loss,’’J. Alloy. Compd., 337, 303–8 (2002).

41Y. C. Zhang, J. Wang, Z. X. Yue, Z. L. Gui, and L. T. Li, ‘‘Effects of Mg21

Substitution on Microstructure and Microwave Dielectric Properties of(Zn1�xMgx)Nb2O6 Ceramics,’’ Ceram. Int., 30, 87–91 (2004).

42H.-J. Lee and K. S. Hong, ‘‘Crystal Structure and Microwave DielectricProperties of M(NbxTa1�x)2O6 Solid Solution (M5Mg or Zn),’’ J. Mater. Res.,12 [6] 1437–40 (1997).

43P. Kofstad, ‘‘Defects and Transport Properties of Metal Oxides,’’ Oxid. Met.,44 [1/2] 3–27 (1995).

44K. Kenzo, ‘‘Thermogravimetric Study of the M–Ni–O system. I. M5Tanta-lum and Niobium,’’ J. Solid State Chem., 83 [1] 37–44 (1989).

45A. J. Moulson and J. M. Herbert, Electroceramics—Materials,Properties, Applications, Vol. 90, 2nd edition, John Wiley & Sons Ltd, U.K.,pp. 322–5, 2003. &

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