R.F. and microwave dielectric properties of (Zn0.95M0.05)2TiO4 (M = Mn2+, Co2+, Ni2+ or Cu2+) ceramics

R.F. and microwave dielectric properties of (Zn0.95M0.05)2TiO4 (M = Mn2+, Co2+, Ni2+ or Cu2+) ceramics

Materials Science and Engineering B 168 (2010) 151–155 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepag...

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Materials Science and Engineering B 168 (2010) 151–155

Contents lists available at ScienceDirect

Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

R.F. and microwave dielectric properties of (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ or Cu2+ ) ceramics Sandeep Butee a,c,∗ , Ajit R. Kulkarni a , Om Prakash a , R.P.R.C. Aiyar b , K. Sudheendran d , K.C. Raju James d a

Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology-Bombay (IIT-Bombay), Powai, Mumbai 400076, India CRNTS, Indian Institute of Technology-Bombay (IIT-Bombay), Powai, Mumbai 400076, India Department of Metallurgy and Materials Science, Govt. College of Engineering, Shivaji Nagar, Pune 411005, India d School of Physics, University of Hyderabad, Hyderabad 500046, India b c

a r t i c l e

i n f o

Article history: Received 29 July 2009 Received in revised form 29 October 2009 Accepted 3 November 2009 Keywords: Ceramics Oxides Permittivity Electrical measurements

a b s t r a c t Here, we report our results on the synthesis and properties of pure and 3d transition metal substituted zinc ortho-titanate (Zn1−x Mx )2 TiO4 dielectrics. Polycrystalline (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ or Cu2+ ) samples of sintered density ≥94%, were prepared by ceramic powder mixing and solid state reaction followed by sintering (in air) between 1060 and 1180 ◦ C. The XRD patterns of the furnace cooled samples revealed single ortho-titanate tetragonal inverse spinel phase. The SEM micrographs showed fairly uniform grains between 5 and 25 ␮m depending upon the composition. Dielectric measurements on the samples were made at low frequencies (1 kHz to 1 MHz) from 30 to 450 ◦ C. The dielectric constant (εr ) and dielectric loss (tan ı), at all the frequencies, were found to rise with progressive increase in sample’s temperature albeit the rise being less at higher frequencies (≥100 kHz). The room temperature values of εr were between 21 and 26 (depending on the 3d M2+ ion) and remained almost constant in the entire low frequency range ≤100 kHz. Both εr and tan ı were found to decrease with increasing frequency at higher temperatures (>200 ◦ C). At microwave frequencies (7.0–7.5 GHz), the room temperature values of εr for all the (Zn0.95 M0.05 )2 TiO4 samples were found to be in the range 18–20 and the unloaded quality factor (Qu ·f) values ranged between 2100 and 9650 GHz. The (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ or Cu2+ ), samples exhibited over four times improvement in quality factor vis-à-vis pure Zn2 TiO4 , which is attributed to relative increase in grain size and density in case of Cu2+ , and reduction in tetragonality of the unit cell for Mn2+ substitution. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In recent past, R.F. multilayer device structures have been developed to reduce the device size and weight. These often require low temperature co-fired ceramics (LTCCs) having internal-electrode metal such as silver or copper [1]. A few of the (multifunctional) electronic components thus produced include miniaturized band-pass filters and antennas, microtools for the biologist like a dielectrophoretic cell-sorter, microfluidic optical detector, sensors and actuators, integrated inductors, etc. [2–4]. Several LTCCs include Bi–Nb–O, Zn–Ti–O, CoNb2 O6 , LiNb3 O8 and (Mg,Ca)TiO3 with or without the sintering additives such as glass, among others [3]. Traditionally, fine chemicals of Zn–Ti–O system have been used as catalysts and pigments [5]. The low cost of oxides of Zn and

∗ Corresponding author at: Department of Metallurgy and Materials Science, Govt. College of Engineering, Shivaji Nagar, Pune 411005, India. Tel.: +91 20 25507263; fax: +91 20 25507299. E-mail addresses: [email protected]n, [email protected] (S. Butee). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.11.007

Ti, however, make this system appear attractive to be explored for dielectric applications vis-à-vis the Ta or Nb based established dielectrics [6,7]. Three different compounds are known to exist in the Zn–Ti–O system viz. Zn2 TiO4 (cubic/tetragonal), ZnTiO3 (hexagonal, though metastable), and Zn2 Ti3 O8 (cubic) [8,9]. Amongst these, zinc ortho-titanate (Zn2 TiO4 ) is the only stable compound in sintered form, and can easily be prepared by conventional solid state reaction between stoichiometric proportions of ZnO and TiO2 [8,10]. An attempt is made here to synthesize Zn2 TiO4 based compositions by partial substitution of 3d ions at the Zn site [5] and study their R.F. and microwave dielectric properties. Although, Zn2 TiO4 per se has not been very attractive as a microwave dielectric resonator material, nevertheless its low sintering temperature (∼1100 ± 100 ◦ C) and flexibility to allow dilute substitution [8,11,12] hold the possibility of making this system attractive to be explored for LTCC applications. Here, we report synthesis and microwave dielectric properties of (Zn0.95 M0.95 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ , or Cu2+ ), compositions as sintered ceramics. Relationships among crystal lattice, microstructure, frequency and temperature (besides the effects of

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divalent ion substitution) on the dielectric behaviour in the R.F. range are also discussed.

2. Experimental 2.1. Ceramic sample preparation Pure and the substituted Zn2 TiO4 monophasic dense samples were prepared by initially making the monophasic powders by ceramic solid state reaction route and subsequently compacting the powders into pellet form and sintered. The details of which are given in the following sections.

2.1.1. Preparation of a single phase material powder High-purity (>99%, AR grade) powders of ZnO, TiO2 , Co(NO3 )2 ·6H2 O, CuO, Mn2 O3 and NiO were finely mixed in the desired stoichiometry of nominal (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ , or Cu2+ ), using wet ball milling method. The finely mixed powders were solid state reacted in air at 1000 ◦ C for 3 h to get a single (ortho-titanate tetragonal) phase material. The reaction parameters were based on the differential thermal analysis (DTA) study of the mixture of stoichiometric proportions of ZnO and TiO2 powders. The powder XRD pattern of all the above composition powders were recorded on PanAnalytical XPERT-PRO Diffractometer in 2 range of 20–70◦ , using Cu K␣ radiation, to check the phase formation. The reacted powders were reground ¯ ≤ 1 ␮m) for to get fine active powders of average grain size (D making them into the desired pellet form for sintering.

2.1.2. Sintering of a single (ortho-titanate tetragonal) phase material powder The different ortho-titanate phase powders were uniaxially compacted at ∼100 MPa into green pellets (12 mm diameter and ∼1.5–6 mm thickness), which were dried and heated in a programmed manner and sintered in air for 4 h at ∼1180 ◦ C (except the (Zn0.95 Cu0.05 )2 TiO4 pellets, which were sintered at 1060 ◦ C as loss of CuO at higher temperatures was observed) and furnace cooled. The above sintering temperatures were established on the basis of initial exploratory runs that were conducted on a few compositions sintered at different temperatures from 1100 to 1200 ◦ C for 4 h [9,11,13,14]. Subsequently, only the samples that showed the maximum sintered densities were characterized further. In the case of (Zn0.95 Cu0.05 )2 TiO4 samples, when fired at temperatures greater than 1100 ◦ C loss of CuO was observed as revealed by SEM-EDX analyses (see Fig. 1(a) and (b) and Table 1). The samples fired at ∼1060 ◦ C for 4 h and assisted by some liquid phase sintering due to CuO were found to be well sintered and taken up for further studies.

Table 1 Elemental analysis done by EDX on a (Zn0.95 Cu0.05 )2 TiO4 pellet sintered at 1150 ◦ C. The regions of EDX are marked in the SEM photomicrographs given in Fig. 1(a) and (b). Elementa

Ti Zn a

Grain

Grain boundary

wt%

at.%

wt%

at.%

30.83 69.17

37.82 62.18

28.57 71.43

35.31 64.69

Corresponding to K line.

2.2. Characterization The powder XRD patterns for the sintered pellets were recorded and lattice parameters computed. The bulk density of the sintered samples was determined following the ASTM method C 373 1988 (2006) [15]. The microstructure investigation of polished and thermally etched surfaces of the sintered pellets was conducted using Hitachi S-3400N low vacuum SEM. For electrical characterizations, the sintered disks were polished to make both the faces flat and parallel and electroded with high-purity air-drying conducting silver paste. Relative dielectric constant (εr ) and dielectric loss (tan ı) in the temperature range 30–450 ◦ C for 1 kHz to 1 MHz frequency range were found using Solartron 1260 Impedance Gain Phase Analyzer with 1296 Dielectric Interface. Dielectric Post (DP) resonator technique was employed for the microwave characterization of the ceramic samples. The sample under test was placed over a low loss support material and was enclosed in a microwave cylindrical cavity, which acted as a resonating structure. The TE011 mode was identified for each resonator. The measurements were done using Agilent 8722ES Vector Network Analyzer (VNA). The real part (εr ) of relative permittivity was computed from the measured resonance frequency (f) of the resonator containing the sample under test. The loaded Q factor (QL ) of the resonator was measured directly from the VNA and the unloaded Q factor (Qu ) was calculated using a computer program which took into account the cavity losses and coupling conditions [16,17]. The quality factor is usually represented as Qu ·f. 3. Results and discussion The powder XRD patterns of the sintered (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ , or Cu2+ ) pellets, as shown in Fig. 2, match with JCPDS card no. 821438, revealing single ortho-titanate (tetragonal) phase formation having inverse spinel structure. The computed lattice parameters and cell volume besides sintering temperatures and sintered densities of all the samples are collected in Table 2. The densities of the different compositions ranged between 94% and 98% of theoretical density (TD) (estimated from the lattice volume). It is noted that the substitution of Cu2+ has led to good density

Fig. 1. (a) and (b) SEM photomicrographs of a thermally etched (Zn0.95 Cu0.05 )2 TiO4 pellet sintered at 1150 ◦ C for 4 h. The EDX of the sample at the marked positions of grain and grain boundary regions have not shown any peak due to Cu. Thus Cu loss is indicated due to high sintering temperature. (EDX analysis is given in Table 1.)

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Table 2 Ionic radii, sintering temperatures, bulk densities, lattice parameters and unit cell volumes of (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ or Cu2+ ) ceramics. M (ionic radius, Å)

Sinteringa temperature (◦ C)

d (%TD)

Zn2+ (0.74) Mn2+ (0.83) Co2+ (0.65) Ni2+ (0.69) Cu2+ (0.73)

1180 1180 1180 1180 1060

94.8 94.2 95.0 97.7 96.4

Refined lattice parameters (Å)b

Cell volume

a

c

(Å3 )

5.9839 6.0695 5.9635 6.0017 5.9822

8.4358 8.3756 8.4090 8.4755 8.4433

302.204 308.550 299.048 305.292 303.160

TD is the theoretical density calculated from X-ray data. In pure compound the ion is Zn2+ . a Sintering was done in air for 4 h. b The uncertainty in lattice parameters is ≤5 in the last digit. Table 3 Microwave dielectric parameters (Zn0.95 M0.05 )2 TiO4 , ceramics.

computed

from

experimental

data

for

M

¯ (␮m) D

εr

Qu ·f (GHz)

 f (ppm/K)

f (GHz)

Pure Mn2+ Co2+ Ni2+ Cu2+

20 22 12 14 17

20.0 18.2 19.0 19.5 18.0

2060 9555 2105 2180 9650

X −204.4 X X −166.3

7.1 7.5 7.3 7.3 7.4

¯ average grain size following Ref. [18]; X: values not measured; f: resonant freD: quency (GHz).

Fig. 2. XRD patterns of (Zn0.95 M0.05 )2 TiO4 , (M = Mn2+ , Co2+ , Ni2+ or Cu2+ ) ceramics sintered in air for 4 h at 1180 ◦ C (except Cu2+ - sintered at 1060 ◦ C); (a) Pure Zn2 TiO4 , other patterns are with substitution of 5 at% of: (b) Mn2+ , (c) Co2+ , (d) Ni2+ and (e) Cu2+ .

(∼96.4%) in the sintered samples through liquid phase sintering at relatively lower temperature (∼1060 ◦ C) vis-à-vis 1180 ◦ C for other compositions. It is seen that the unit cell volume has slightly increased for Mn2+ , Ni2+ and Cu2+ substitution, and decreased for Co2+ substitution as compared to pure Zn2 TiO4 (Table 2). The tetragonality parameter (ratio of c/a lattice parameters) in titanates remained almost constant ∼1.41 for pure and different 3d ion substitutions except for Mn2+ wherein c/a ∼1.38 is obtained.

SEM photomicrographs of the sintered (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ , or Cu2+ ), samples are shown in Fig. 3(a)–(e). It is observed that, microstructures are clean with fairly uniform grains and the average grain size ranges between 5 and 25 ␮m depending upon the composition. The results of grain size measurement, following the method of Mendelson [18], are given in ¯ is found to be higher for M = Mn2+ Table 3. The average grain size (D) 2+ (∼22 ␮m) and M = Cu (∼17 ␮m) as compared to M = Co2+ (∼ 12 ␮m) and M = Ni2+ (∼14 ␮m). The low frequency (1 kHz to 1 MHz) as well as temperature (30–450 ◦ C) dependent dielectric responses of the (Zn0.95 M0.05 )2 TiO4 samples (pure, Mn2+ and Cu2+ substituted ones) are shown in Fig. 4(a)–(c), respectively. It is seen that both εr and tan ı rise with progressive increase in sample’s temperature from 30 to 450 ◦ C and the rise is almost exponential above 200 ◦ C which could be ascribed to thermal ionic polarization becoming more dominant [19]. This trend is seen for all the frequencies (from 1 kHz to 1 MHz) albeit the rise being less at higher frequencies (>100 kHz). Further, both εr and tan ı, at a given temperature, have been found to progressively decrease with increasing frequencies (from 1 kHz to 1 MHz), which is typical of dielectric materials. Further, it is very

Fig. 3. SEM photomicrographs of (Zn0.95 M0.05 )2 TiO4 , sintered ceramics at 1000× magnification. (a) Pure Zn2 TiO4 , (b) M = Mn2+ , (c) M = Co2+ , (d) M = Ni2+ and (e) M = Cu2+ .

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Fig. 4. Variations of εr and tan ı with temperature and frequency for a few (Zn0.95 M0.05 )2 TiO4 ceramics. (a) Pure, (b) Mn substituted and (c) Cu substituted at the Zn site.

clearly seen from Fig. 4 that the profiles of both εr and tan ı are becoming flattish for almost all the samples studied (values for only three compositions are reported here) at frequencies >1 kHz and temperature <200 ◦ C indicating their suitability for practical applications. A marginal increase observed at frequencies close to 1 MHz at low temperatures (∼100–200 ◦ C), which is prominently revealed in Fig. 4(a), is attributed to higher noise level while measuring the dielectric properties of the high impedance ceramics (here, the high resistivity samples).The room temperature value of εr between 22 and 26 (depending on M2+ ion substituted, lowest ∼22 for Mn2+ substitution) remained almost constant in the entire low frequency

range ≤100 kHz. The marginal changes in dielectric constant values can be attributed to the changes in densities and lattice parameters due to 3d ion substitution [20]. The values of εr and Qu ·f at microwave frequencies (∼7 GHz) for (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ , or Cu2+ ), ceramics are given in Table 3. The εr values are found to lie between 18 and 20. This marginal spread in εr is attributed to variations in composition, grain size (contributing to changes in spontaneous polarization), electronegativities and densities [21,22] due to substituent 3d ions. The Qu ·f values are between ∼2100 GHz (for pure and M = Co2+ or Ni2+ ) and ∼9500 GHz (for M = Mn2+ or Cu2+ ). Thus, showing

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over four times improvement in Qu ·f. The microwave dielectric loss comprises of intrinsic losses caused by anharmonic phonon decay process occurring in the crystal lattice and the extrinsic losses caused by defect structures, grain boundaries, second phases, and pore structure [23]. The sources of Co2+ and Ni2+ ions viz. nickel and cobalt monoxide by their nature are both metal deficient (their formulas usually written as Ni1−y O and Co1−y O, respectively, where y is a small fraction of one (∼10−2 to 10−4 )). Both these oxides are predominantly associated with metal vacancies defects because of their deviation from stoichiometry [24]. While substituting Co2+ and Ni2+ ions in (Zn0.95 M0.05 )2 TiO4 , these defects are carried along in the end samples (since the samples are sintered in air) and are most likely responsible for the observed low Qu ·f values (∼2100 GHz). An apparent increase in grain size and/or density of Mn2+ or Cu2+ containing samples is seen, which is associated with less grain boundary area and decrease in number of pores leading to less lattice imperfections and thereby lower dielectric loss [6]. Further, in case of Mn2+ containing samples, a decrease in tetragonality parameter (c/a ratio) from ∼1.41–1.38 ought to result in modification of distances between the Ti and the surrounding O ions of TiO6 octahedron [22], which could contribute to the observed increase in the Qu ·f value. Furthermore, the M2+ and Zn2+ ionic size difference causes increase in the bond strain due to octahedral distortion leading to marginal increase in the Qu ·f value [25,26]. Temperature coefficient of resonant frequency ( f ) has been measured only for the Mn2+ and Cu2+ containing samples.  f is −204.4 ppm/K for M = Mn2+ and −166.3 ppm/K for M = Cu2+ . In the present spinel structured compounds, it appears that the difference in tilting of TiO6 -octahedra [27,28] because of partial substitution of Zn2+ with Mn2+ or Cu2+ , having different ionic radius, can be a probable cause for the changes in the observed  f values. 4. Conclusions Polycrystalline (Zn0.95 M0.05 )2 TiO4 (M = Mn2+ , Co2+ , Ni2+ or Cu2+ ) spinel ceramics were successfully synthesized. The room temperature values of εr (at 1 kHz to 1 MHz) were found between 21 and 26 (depending on the 3d M2+ ion) and remained almost constant in the entire low frequency range ≤100 kHz. At microwave frequencies (∼7 GHz), the room temperature values of εr for all the (Zn0.95 M0.05 )2 TiO4 samples were in the range 18–20 and the quality factor (Qu ·f) ranged between 2100 and 9650 GHz. For M = Mn2+

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and Cu2+ , the (Zn0.95 M0.05 )2 TiO4 samples exhibited over four times improvement in the quality factor vis-à-vis pure Zn2 TiO4 . Acknowledgements The authors wish to thank Dr. Subray L. Kamath and Mr. Dilip Agrahari (MEMS, IIT Bombay) for their help in SEM-EDS and measurements on Solartron impedance gain phase analyzer. References [1] Q.L. Zhang, H. Yang, J.L. Zou, H.P. Wang, Mater. Lett. 59 (2005) 880–884. [2] A. Baker, M. Lanagan, C. Randall, E. Semouchkina, G. Semouchkin, K.Z. Rajab, R. Eitel, Int. J. Appl. Ceram. Technol. 2 (2005) 514–520. [3] M.T. Sebastian, H. Jantunen, Int. Mater. Rev. 53 (2008) 57–90. [4] L.J. Golonka, Bull. Pol. Acad. Sci. Technol. 54 (2006) 221–231. [5] S.C. Souza, M.A.F. Souza, S.J.G. Lima, M.R. Cassia-Santos, V.J. Fernandes Jr., L.E.B. Soledade, E. Longo, A.G. Souza, I.M.G. Santos, J. Therm. Anal. Calorim. 79 (2005) 455–459. [6] X. Liu, F. Gao, C. Tian, Mater. Res. Bull. 43 (2008) 693–699. [7] H.T. Kim, J.D. Byun, Y. Kim, Mater. Res. Bull. 33 (1998) 963–973. [8] R.L. Millard, R.C. Peterson, B.K. Hunter, Am. Miner. 80 (1995) 885–896. [9] J. Yang, J.H. Swisher, Mater. Charact. 37 (1996) 153–159. [10] X. Liu, M. Zhao, F. Gao, L. Zhao, C. Tian, J. Alloy Compd. 450 (2008) 440–445. [11] S.K. Manik, S.K. Pradhan, Phys. E 33 (2006) 69–76. [12] R.B. Rankin, A. Campos, H. Tian, R. Siriwardane, A. Roy, J.J. Spivey, D.S. Sholl, J.K. Johnson, J. Am. Ceram. Soc. 91 (2008) 584–590. [13] M.V. Nikolic, N. Labus, M.M. Ristic, Ceram. Int. 35 (2009) 3217–3220. [14] A. Navrotsky, O.J. Kleppa, J. Inorg. Nucl. Chem. 30 (1968) 479–498. [15] ASTM Method C 373 1988, Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity and Apparent Specific Gravity of Fired Whiteware Products, ASTM Int., West Conshohocken, PA, United States, 2006, pp. 1–2. [16] E.L. Ginzton, Microwave Measurement, McGraw Hill Book Co., 1957. [17] M.V. Jacob, J. Mazierska, K. Leong, J. Krupka, IEEE Trans. Microwave Theory Technol. 49 (2001) 2401–2407. [18] M.L. Mendelson, J. Am. Ceram. Soc. 52 (1969) 443–446. [19] D. Zhou, H. Wang, X. Yao, X. Wei, F. Xiang, L. Pang, Appl. Phys. Lett. 90 (2007) 172910-1–172910-3. [20] R. Ratheesh, H. Sreemoolanadhan, M.T. Sebastian, J. Solid State Chem. 131 (1997) 2–8. [21] H.J. Lee, I.T. Kim, K.S. Hong, Jpn. J. Appl. Phys. Part 2 36 (1997) L1318–L1320. [22] Y.L. Chai, Y.S. Chang, L.G. Teoh, Y.J. Lin, Y.J. Hsiao, J. Mater. Sci. 43 (2008) 6771–6776. [23] H. Tamura, Am. Ceram. Soc. Bull. 73 (1994) 92–95. [24] P. Kofstad, Oxid. Methods 44 (1995) 03–27. [25] D. Shihua, Y. Xi, M. Yu, L. Puling, J. Eur. Ceram. Soc. 26 (2006) 2003–2005. [26] E.S. Kim, W. Choi, J. Eur. Ceram. Soc. 26 (2006) 1761–1766. [27] H.J. Lee, K. SunHong, J. Mater. Res. 12 (1997) 1437–1440. [28] E.L. Colla, I.M. Reaney, N. Setter, J. Appl. Phys. 74 (1993) 3414–3425.